Fabrication and Characterization of a Porous Clad Optical Fiber Gas Sensor. Brian L. Scott. Master of Science In Materials Science and Engineering

Transcription

1 Fabrication and Characterization of a Porous Clad Optical Fiber Gas Sensor By Brian L. Scott Thesis submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Master of Science In Materials Science and Engineering Gary Pickrell, Chair Carlos Suchicital Anbo Wang December, Blacksburg, Virginia Optical fiber gas sensor, Spinodal phase separable glass, Pore consolidation, Pore coarsening Copyright 2009

2 Fabrication and Characterization of a Porous Clad Optical Fiber Gas Sensor By Brian L. Scott (Abstract) An optical fiber has been developed that can be used as a chemical gas sensor. Fabrication of the optical fiber produces a fiber that has a solid core with a porous cladding. The porous cladding region is made from a spinodally phase separable glass where the secondary phase is removed through dilute acid leaching. A non-phase separable glass composition is used for the core region. The properties of the phase separable glass are dependent on the processing conditions and the thermal history of the glass after the porosity has been achieved. Investigation of how processing conditions affected the pore structure was conducted to determine what pore characteristics are achievable for the glass composition used. Phase separation temperature, removal of silica gel deposited in the pores, and the post fabrication heat treating were used as experimental processing conditions. A maximum useable average pore size of approximately 29 nm was achieved. Maximum pore volume in the experimental groups was cc/g. Most heat treatments of the porous glass caused consolidation of the pore structure, with some conditions producing pore coarsening.

3 Dedication I would like to dedicate this work to the late Sensei J. Friedless. iii

4 Acknowledgments I would like to thank all of the people that have helped me to make it this point. Many people have helped me along the way and for that I am grateful. I would like to first and foremost thank my advisor Dr. Gary Pickrell, whose patience, understanding and advice have been without end. I would like to thank Dave Berry for all of the technical and other assistance, my friend Niven Monseque for his support and help, and my current Sensei, James Huston for helping me stay sane through all of the stress. iv

5 Table of Contents Abstract Dedication Acknowledgments Table of Contents List of Figures List of Tables ii iii iv v vii ix Chapter 1-Introduction 1 Chapter 2-Background Optical Fibers Gas Detection with Optical Fibers Spinodal decomposition Morphology History and Vycor Thermodynamics of phase separation Kinetics Processing Leaching process Spinodal glass systems Porous glass systems Silica gel removal Pore consolidation and pore growth 22 Chapter 3- Materials and Methods Fiber fabrication Preform manufacture Fabrication of the initial glass fiber Phase Separation Leaching process to produce the porous clad fiber Experiments to affect pore size of cladding region Removal of colloidal silica with an alkaline solution to increase pore size Phase separation temperature and post heat treatment of bulk glass experiment Characterization Materials characterization Optical characterization 30 v

6 Chapter 4- Results and Discussion Initial fiber fabrication Removal of colloidal silica to increase pore size with alkaline solution Gas sensing Phase separation temperature and post heat treatment of bulk glass experiment 39 Chapter 5-Conclusion 59 Chapter 6-Future work 60 References: 61 vi

7 List of Figures Figure 1. Schematics of experimental fiber design (a) pore pathway perpendicular to core (b) Tortuosity of pores...1 Figure 2. Cross section of a typical optical fiber design with refractive index n 1, n 2 where n 2 <n Figure 3. Refraction of light at a material interface...3 Figure 4. Representation of total internal reflection at the core-cladding interface...3 Figure 5. Schematic of optical power distribution in an optical fiber...4 Figure 6. Schematics of ordered hole (a) and random hole (b) optical fibers...5 Figure 7. Etched borosilicate glass...6 Figure 8. Scanning Electron Micrograph of porous Vycor prior to reconsolidation...7 Figure 9. Free Energy of Mixing- Enthalpy and Entropy contributions...9 Figure 10. Free energy reduction through decomposition...10 Figure 11. Schematic of phase diagram metastable region...11 Figure 12. Schematic of perform manufacture...25 Figure 13. Phase separation temperature and post heat treatment experimental schematic...29 Figure 14. The transmission measurement system...31 Figure 15. transmission measurement system...32 Figure 16. Far field pattern measurement setup...32 Figure 17. Optical micrograph of a fracture surface of the end face of a leached only optical fiber...33 Figure 18. SEM micrograph of leached only optical fiber surface...34 Figure 19. SEM micrograph of core-cladding interface of leached only optical fiber...34 Figure 20. Pore size distribution of leached only optical fiber...35 Figure 21. SEM micrograph of core-cladding interface of fiber treated with 0.5 N NaOH for 15 minutes...36 Figure 22. SEM micrograph of core-cladding interface of fiber treated with 0.5 N NaOH for 30 minutes...36 Figure 23. Pore size distribution for fibers treated with NaOH...37 Figure 24. Average pore size of fibers treated with NaOH...37 Figure 25. Transmission of the porous clad fiber...38 Figure 26. Far field pattern of the porous clad fiber...38 Figure 27. Absorption peaks of acetylene...39 Figure 28. Average and median pore size for different phase separation temperatures...41 Figure 29. Pore volume for different phase separation temperatures...42 Figure 30. Surface area for different phase separation temperatures...42 Figure 31. Pore size distribution widths for different phase separation temperatures...43 Figure 32. Average pore width range for major treatment groups...45 Figure 33. Pore volume range for major treatment groups...45 Figure 34. Surface area range for major treatment groups...46 Figure 35. Pore size distribution width range for major treatment groups...46 Figure 36. Average pore width and pore volume of 500N group...48 Figure 37. Surface area of 500N group...48 Figure 38. Average pore width and pore volume of 500Y group...49 Figure 39. Surface area of 500Y group...50 vii

8 Figure 40. Average pore width and pore volume of 550N group...51 Figure 41. Surface area of 550N group...51 Figure 42. Average pore width and pore volume of 550Y group...52 Figure 43. Surface area of 550Y group...53 Figure 44. Average pore width and pore volume of 600N group...54 Figure 45. Surface area of 600N group...54 Figure 46. Average pore width and pore volume of 600Y group...55 Figure 47. Surface area of 600Y group...56 viii

9 List of Tables Table 1. Phase separating binary oxide glasses...18 Table 2. Porous glass systems...20 Table 3. Experimental sample notation and processing conditions...43 Table 4. Percent change of pore characteristics of the 500N group...48 Table 5. Percent change of pore characteristics for 500Y group...50 Table 6. Percent change of pore characteristics for 550N group...51 Table 7. Percent change of pore characteristics for 550Y group...53 Table 8. Percent change of pore characteristics for 600N group...54 Table 9. Percent change of pore characteristics for 600Y group...56 ix

10 Chapter 1-Introduction The objective of this research was to develop an optical fiber sensor that can be used in the detection of various gaseous species through the use of evanescent wave interaction and adsorption. The evanescent wave results from the propagation of some of the light transmitted through the fiber core into the fiber cladding. Design parameters require that the sensor be robust in harsh environments, have a response time faster than that of current optical fiber sensors, and be tailorable to different applications. A design that may meet these criteria is an optical fiber that has a completely porous cladding region with the pore structure orientated normal to the optical axis of the fiber. Having the pore structure normal to the fiber axis reduces the distance the gas species has to diffuse in order to sufficiently interact with the evanescent wave. With the cladding made from a high silica glass, the sensor will be robust in a variety of harsh environments. A schematic of the current design is shown in Figure 1. A fiber of this design can be fabricated using any cladding glass that can be spinodally phase separated. Spinodal phase separation will produce a 3-dimensionally interconnected pore structure and when used for the cladding glass will orient the pore structure perpendicular to the fiber core. Porosity achieved through this type of material allows for the alteration of pore morphology through changes in initial processing conditions and through additional processing after the pore structure is fabricated. Radial oriented pores Porous cladding Core Solid core (a) (b) Figure 1. Schematics of experimental fiber design (a) pore pathway perpendicular to core (b) Tortuosity of pores The content of this work details the design, development and fabrication of a new optical fiber sensor and its characterization. This work is focused on the materials aspect of the fabrication of the sensor, manipulation of the material systems and the materials characterization of the resultant sensor. Testing of the sensor for gas sensing capability and other optical qualities was conducted through a cooperative effort with the electrical engineering department at Virginia Tech. All materials related to the testing of the optical fiber for the sensing of gas is provided by Cheng Ma., the graduate student who has tested the fabricated sensors. Testing of the fiber for gas sensing is included to demonstrate the functionality of the device and to provide a framework for the experiments conducted. 1

11 Chapter 2-Background The literature review begins with an overview of the conventional structure of optical fibers and then the use of optical fibers for gas sensing applications. The review of the fiber structure, the optical properties of the fibers and the propagation of light in the fibers is described in the simplest manner. An in depth discussion of the optical components of the research is beyond the scope of the research and is unnecessary for the understanding of the materials research that is presented. Following the discussion of the structure of the optical fibers is the materials background on spinodally phase separable glasses. Glass production, kinetics and the thermodynamics of producing a spinodally phase separated glass is detailed. 2.1 Optical Fibers Conventional optical fibers have a structure composed of essentially two parts in general, a core section and a cladding section that surrounds the core. The material in the core region has a higher index of refraction than the material in the cladding region of the fiber. The typical structure for an optical fiber is shown in Figure 2, where the core and cladding is indicated with notation for the index of refraction, n, for each region. Light transmission in the optical fiber relies on total internal reflection within the core. This is achieved through a difference in the refractive index between the core and the cladding region that surrounds the core. Total internal reflection is set up by a refraction condition between the core and the cladding regions.[1] Cladding-n 2 Core-n 1 Figure 2. Cross section of a typical optical fiber design with refractive index n 1, n 2 where n 2 <n 1 Propagation of light between two different materials behaves according to Snell s law, which describes the condition of refraction of light as it travels from one material to another. As light travels between materials, of different refractive indices, it is bent or refracted in relation to the angle of travel relative to the interface. The amount that the light is bent is described by Snell s law 2

12 and is a function of the light s propagation angle to the interface normal and the indices of refraction of the two materials. Snell s law is given by equation 1 with a schematic of the refractive condition in Figure 3.[1] n = θ (1) 1 sinθ 2 n2 sin 2 n 2 θ 2 n 1 θ 1 Material interface Figure 3. Refraction of light at a material interface Total internal reflection occurs when the angle of light relative to the core-cladding interface satisfies the following condition given by equation 2 and schematically represented in Figure 4.[1] n Sin = 2 θ min (2) n1 Cladding-n 2 n 2 <n 1 Core-n 1 Φ θ Reflected light Figure 4. Representation of total internal reflection at the core-cladding interface The evanescent wave in an optical fiber arises due to the fact that not all of the light injected into the core is confined within the core region. A percentage of the optical power propagates into the cladding region. This is known as the evanescent wave and the amount that propagates into the cladding region is determined by the optical fiber structure.[2] The optical power that leaks out into the cladding region can interact with a chemical species in the cladding, which will show up as an additional loss of optical power at specific wavelengths depending on the chemical species. Figure 5 shows a schematic of the intensity distribution in an optical fiber as a function of the radius from the center of the optical fiber core. 3

13 I R-Core R-Core Evanescent Wave R Figure 5. Schematic of optical power distribution in an optical fiber Different fiber structures can also be used to confine the light in the core. The relevant fiber structures in this case are called micro-structured optical fibers and cover both ordered hole and random hole optical fibers.[3-6] A schematic representation of an ordered hole and random hole fiber is given in Figure 6. Ordered hole optical fibers have a regular arrangement of holes in the fiber that run longitudinally parallel to the core. This structure confines light in the core in one of two ways, either through a photonic band gap effect or a modified total internal reflection.[4, 5, 7] The photonic band gap effect is where the periodicity of the ordered arrangement produces a band gap where specific wavelengths of the spectrum can t propagate into the cladding region.[3] On the other hand, for average index guiding fibers, modified total internal reflection occurs where the index of refraction of the cladding is modified by the absence of material. The refractive index will be the weighted average of the cladding material and the air that fills the hole structure. In both of the micro-structured fiber types the same glass composition can be used for the core region and the cladding region with the light being confined solely through the processes described. Random hole optical fibers (RHOF) confine light through an average index guiding mechanism. The structure surrounding the core region in the RHOF consists of longitudinally oriented holes that are arrayed in a random pattern surrounding the core. This random pattern of holes lowers the effective refractive index in the region and confines the light to the core region.[8] Ordered longitudinal holes Core (a) (b) 4

14 Figure 6. Schematics of ordered hole (a) and random hole (b) optical fibers Gas Detection with Optical Fibers Several types of optical fibers have been used in conjunction with the evanescent wave to detect gas species. An optical fiber with a section of the core exposed so that the core will interact directly with a gas was shown to work.[9] The fiber was heated and stretched so that a section existed were the evanescent wave was exposed to a gas that absorbed light at µm.[9] Optical fibers have been coated with a sol-gel structure to produce a porous cladding about a core. These fibers have also used the evanescent wave-gas interaction in the detection of gases.[10] The use of micro-structured fibers for evanescent wave detection has been demonstrated in both ordered hole and random hole fibers. These fibers have also been used in the detection of gas through the use of an evanescent wave-gas interaction.[2, 6, 11] Porous glass has been used to make gas sensors by fabricating optical fibers that are used in optical systems. Previous optical systems have used a porous glass optical fiber that is treated with an indicator. The indicator reacts with a specific environmental condition, which then produces a transmission change in the system. These fibers are drawn from a glass that phase separates and is treated to make a section porous.[12] Other porous optical fibers have been used to detect solvent vapors through a change in transmission without the use of an indicator. These fibers were sensitive to a few thousand parts per million of toluene with response time on the order of a few minutes.[13] Both of these fibers were made so that the fiber was completely porous and without a core structure. 2.2 Porous Glass Porous objects can be made by using a material that spinodally phase separates. Producing the object out of this type of phase separating material allows, once the phase separation has taken place, for one of the phases to be removed. With one of the phases removed, the remainder will be a porous material in the shape of the object as it was originally produced. Phase separation by spinodal decomposition occurs in many different materials. The phases that separate can be of differing compositions or different crystal structures. A glass that spinodally decomposes is the basis for the production of porous Vycor, a product that is made by Corning, Inc. Potentially any material that spinodally phase separates can be used to create a porous object. The attractive nature of this process is that the material can be shaped and then made porous. A limiting factor is the slow rate of removal of the second or soluble phase. This slow rate of removal requires cross section to be on the order of millimeters Spinodal decomposition Spinodal decomposition is one of the process by which a material separates into two different phases. This separation occurs because the material is unstable in the homogenous state at a given temperature. Phase separation will take place simultaneously throughout the material in such a manner as to produce two phases that are essentially 5

15 continuous and interpenetrating. Each phase is continuous throughout the material with few or no isolated regions and is completely intertwined within the other phase that is present. Spinodal decomposition occurs when the components of a mixture are immiscible. This immiscibility causes the material to phase separate into the different phases. Separation occurs when the parent material is within a given composition range and the temperatures are sufficiently high to allow for material transport. While a material that will phase separate is unstable at low temperatures, the separation will not occur readily until it is at a temperature where sufficiently high diffusion rates can take place.[14] Most materials that are intentionally phase separated are produced by subjecting the material to a heat treatment process. The material can be produced quickly and efficiently if a temperature is chosen where diffusion will occur at a reasonable rate and the material is in a compositional region where it is still unstable Morphology Porous glasses can be made by utilizing the spinodal decomposition phase separation process that occurs within some glass and ceramic systems. Since the phase separation occurs simultaneously, the scale of the resultant phase is typically in the nanometer range. The development of the morphology of the phase separation results in two three dimensionally interpenetrating phases. An SEM micrograph of etched borosilicate glass is shown in Figure 7. One of the phases is removed to highlight the interconnected nature of the phase separation. Figure 7. Etched borosilicate glass 6

16 The high silica glass Vycor that is made by Corning is a well known and commercially successful glass that is produced using spinodal phase separation. The glass is produced, phase separated, acid leached until porous, and then reconsolidated at high temperatures. The glass is also sold in the porous condition in addition to the reconsolidated state. Figure 8 shows an SEM micrograph of porous Vycor prior to being reconsolidated. It can be seen that the porosity of the Vycor is due to the removal of a second phase that forms in the glass upon heat treatment. Figure 8. Scanning Electron Micrograph of porous Vycor prior to reconsolidation History and Vycor One of the first phase separating glasses to be identified was a sodium borosilicate glass. In the 1930s two glass scientist that worked at Corning glass works filed patents in which a process was described that would produce a high silica glass from a glass in the sodium borosilicate system. The process involves heat treating a borosilicate glass of a given composition range to temperatures between 550 C and 650 C for around twenty hours. Heat treating the glass causes it to phase separate into a silica rich glass phase and a sodium borate glass phase. Phase separation in this glass occurs through a spinodal decomposition mechanism. Once the phase separation has been completed, the glass is then subjected to an acid bath treatment. While in the acid bath the sodium borate glass phase is dissolved while at the same time the high silica phase remains relatively untouched. Since the phase separation produces a three dimensionally interconnected morphology the acid treatment will 7

17 produce a porous glass with a high silica skeleton. The porous glass skeleton that remains will have a porosity of around 28% by volume and a pore radius between four and twenty nanometers.[15] The amount of porosity and pore radius will be dependent on the initial glass composition, the heat treatment schedule, leaching conditions, and any post processing of the porous material to enlarge the existing pore structure Thermodynamics of phase separation Spinodal phase separation is process were a solid solution demixes into other solid solutions. The materials that exhibit this type of phenomenon are determined by the thermodynamics of the system. The phase separation will occur if the resultant separation of the material will lower the overall energy of the system. This can occur either by nucleation and growth of secondary particles within a matrix or through spinodal decomposition Gibbs Free Energy For material systems, the main thermodynamic quantity that is considered is the Gibbs free energy. This is used since it is concerned with systems at constant pressure and temperature. For material systems that spinodally decompose, the process is typically done at atmospheric pressure (constant pressure) and at a constant temperature. The Gibbs free energy (G) is defined as: G = H-TS (3) Where H is the enthalpy, T is the temperature, and S is the entropy of the system. When materials are mixed, there is an associated change in the free energy from that of a pure substance that is brought about by the change in entropy and enthalpy due to the mixing of the components of the solution.[16] This can be expressed as G mix = H mix -T S mix (4) Where G mix is the free energy change of the solution, H mix is the change in enthalpy due to mixing of the solution and S mix is the change in entropy due to mixing. Changes in the Gibbs free energy are compositionally dependant. The additive nature of a large positive enthalpy is shown in Figure 9. The free energy of mixing will be different depending on the relative amounts of the constituent parts of the mixture. Changes in enthalpy due to mixing can be zero, positive or negative depending on the type of bonding between the constituents of the solution. If the bond between different components is stronger than the bonds between like components then the change in enthalpy will be negative. If the bond between like components is stronger, then the enthalpy term will be positive upon mixing. Where the bond strengths are equal then the change in enthalpy will be zero.[17] 8

18 H G 0 S A Figure 9. Free Energy of Mixing- Enthalpy and Entropy contributions The entropy term will always be negative upon mixing of components and at high enough temperatures this term will dominate the Gibbs free energy equation and a solution that is completely intermixed will be stable at all compositions. If the enthalpy term of the free energy of mixing is positive and the temperature of the solution is decreased, the mixed solution will become unstable at and below the temperature and composition range where the enthalpy term is larger than the entropy term. When this happens the free energy of mixing becomes positive and the solution can decrease the total free energy of the system by separating into two or more phases were the enthalpy term is lower than the entropy term. Typically the free energy becomes more positive over a composition range. As the enthalpy term becomes positive with increasing amounts of one of the components then the free energy slowly becomes more positive. At a critical composition value the free energy will reach a maximum. This is detailed in Figure 9. As can be seen in Figure 9, the free energy is more positive within a given composition range than solutions of compositions on either side of that range. Within the composition range of positive free energy there will be two regions were different phase separation mechanisms take place. Where the free energy starts its positive deviation to where the free energy curve has its inflection point, the phase separation occurs by the process of nucleation and growth. On the free energy diagram the inflection points occur were the second derivative of the free energy with respect of 2 composition is zero( G = 0 ).[14] Nucleation and growth produces highly spherical 2 C particles of one composition within a surrounding matrix of another composition. As the 9 B

19 particle size increases, the surrounding matrix is depleted of the composition that the particle has and the free energy of the solution is lowered. This process requires a particle of a composition that has a lower free energy to be nucleated before any appreciable phase separation can take place. In the area where the free energy curve is between the inflection points the solution will begin to phase separate due to small fluctuations in the local composition. Any deviation from the original composition will lead to a decrease in the overall free energy of the solution. As the free energy is decreased the solution will become more stable and the phase separation will continue until the solution has separated into the compositions with the lowest free energy. Since most solutions have small compositional differences locally, solutions that are within the spinodal region will have regions throughout the solution that will begin the phase separation, which will take place simultaneously throughout the material.[18] This change in free energy of the system is shown schematically in Figure 10. G Gint GT Spinodal Region A 2 2 G X0 G = 0 = C C Figure 10. Free energy reduction through decomposition B The change in free energy changes with the temperature of the system and the compositions of the resultant phase will become increasingly different as the temperature is altered. This is shown schematically in Figure

20 Tc T1 T T2 Spinodal region A Figure 11. Schematic of phase diagram metastable region B Kinetics Kinetics for the process can be broken down into two categories: kinetics of the phase separation and the kinetics of the leaching process. Spinodal decomposition will occur within a system that exhibits metastable immiscibility; however the process of phase separation will only occur at temperatures below the consolute temperature and at temperatures that are high enough to allow for diffusion of the constituent materials to take place at a reasonable rate. The consolute temperature is the temperature above which the components are miscible and no phase separation will take place Spinodal decomposition The phase separation of a material that has a composition within the spinodal will proceed from the infinitesimal fluctuations that are inherent in the material. Within the spinodal region these fluctuations are unstable due to the reduction of the materials free energy if these fluctuations become larger. The discussion of the kinetics of spinodal decomposition is adapted from a chapter in the work by Kingery.[19] For a material with a composition in the spinodal region the change in the free energy is given by: = 2 G 2 C + 2κβ 2 G A 2 V 4 C0 (5) where β is the wave number of the fluctuation, κ is a positive constant and A is the amplitude of the fluctuation. The system is unstable for all wave numbers less than a critical wave number β c which is given in the following form: 11

21 2 2 1 G β 2 2 c = (6) κ C C 0 This can also be looked at using the wavelength as the descriptor, where the wave length is given by: 1 2π λ = (7) β And the critical wavelength, which all wavelengths that are above it will grow, is given by: π κ λ c = 2 (8) G 2 C C o The increase in energy due to an increase in interfacial surface energy is part of the gradient energy term, 2κβ 2. From this it can be seen that with a larger change in the free energy, the system will be able to have fluctuations with more surface energy. This leads to a finer scale of the separation as there will be a greater surface area with many small regions as opposed to a few large regions. The growth of the regions or the changes in compositions from the initial to the final compositions is accomplished through an uphill diffusion process of the constituents of the material to the different phase regions. Spinodal compositions will have a sinusoidal composition fluctuation that is given by: C-C 0 = A cos βx (9) With R (β) being the amplification factor. A (β, t) = A (β, 0) exp[r (β) t] (10) ~ 2 2 Mβ G 2 R ( β ) = + 2κβ (11) 2 N v C C 0 In equations (8) and (9) A (β, t) is the amplitude of a fluctuation with a wave number β, M ~ is the mobility of the constituents, and N v is the number of molecules per unit 12

22 volume. The maximum of ( β ) are given by: R occurs at the wavelength which grows the fastest. These R m 2 1 ~ G M 2 2 C = (12) C 0 λm = 2λ c (13) When a spinodal composition is within the temperature range of the spinodal and at a sufficient temperature for M ~ to be high enough the microstructure will be dominated by the growth of a structure with a wavelength of λ m. [20] Leaching Process The most used porous glass is produced from the sodium borosilicate system. This system is also the most extensively studied and as such will be used as a model system in describing the kinetics of the leaching process. In order to produce a porous glass one of the resultant phases has to be removed from the bulk material. This is typically done using a heated dilute acid solution to leach out the acid soluble phase. Solution temperatures are typically around 100 C but can also be done at room temperature.[15] The amount of time that the article needs to stay in solution will be dependent upon the solubility of the phase being removed in the solvent, acid type, solution temperature, and article size. The leaching process can be broken down into a reaction component and a diffusion component that produces the phase dissolution. Depending on the leaching parameters the process can be reaction controlled, diffusion controlled or a mixed diffusion-reaction process. Normally, the leaching variables that are adjusted are acid concentration, acid temperature, and acid type. These variables will have an effect on both the reaction process and the diffusion process. Depending on the level chosen for each variable the process will be either a reaction controlled or diffusion controlled process.[21] Reaction controlled process The leaching process begins by the dissolution of the sodium borate phase that exists within the silica matrix. As this phase is dissolved through the action of the acid a divot will form in the surface of the glass. The longer the glass is in contact with the acid solution, the greater the amount of the secondary phase that will be removed. Since the sodium borate phase and the silica phase are interconnected, a prolonged time in the acid solution will result in the formation of pores that penetrate into the glass. Surface morphology of a leached glass will show a silica skeleton with nanometer sized pores. Figure 8 shows an SEM image of the surface of commercially available porous Vycor. As can be seen in the image, the pore size is in the nanometer range and the pores exist across the entire surface. The pore formation process begins with the dissolution of the borate phase and ends when all of the phase is dissolved in the acid solution. The area where dissolution of the borate phase takes place is the interface between the acid 13

23 solution and the borate phase. This interface is termed the reaction front. Depth of penetration of the dissolution into the glass is described in terms of the pore length. As pore formation continues, the length of the pore structure will increase while the radius remains roughly unchanged since the pore is surrounded by the non-soluble silica phase. With the increasing length of the pores, dissolution of the borate phase will have an added diffusion component to the process. At the reaction front, the acid solution reacts with the soluble components in which they dissolve into solution. As more components are dissolved, the ph of the solution increases. With an increase in the ph, the rate of dissolution of the soluble phase decreases.[22] With an increase in the concentration of dissolved components in the solution surrounding the reaction front, a concentration gradient of these dissolved components is formed. The gradient decreases toward the surface of the glass and the bulk acid solution. An acid concentration gradient exists that points toward the reaction front. Due to the establishment of the concentration gradients, the components that are dissolved into solution will flow out of the interior of the glass and acid will flow in toward the reaction front Diffusion controlled process Without any diffusion taking place, the acid that surrounds the reaction front will eventually dissolve enough of the of the borate phase that the ph will increase to a point where the reaction will stop. If the solution is alkali enough, the silica surrounding the reaction front will begin to be dissolved.[22] If the diffusion rate in the solution is approximately equal to or greater than the rate of the reaction, the speed at which the reaction front moves will be controlled by the rate of the dissolution. If, however, the rate of diffusion is significantly lower than the reaction rate, then the speed that the reaction front moves will be controlled by the diffusion rate. Diffusion of the components out and more importantly, the diffusion of acid to the reaction front are necessary for the reaction to continue. Rates of diffusion can be controlled by temperature of the solution and pore size. Pore sizes in the range of 1-10 nm will result if the diffusion coefficient is an order of magnitude less than that of the free solution.[23] Sodium borosilicate glasses can be acid leached using dilute solutions of H 2 SO 4, HCl, or HNO 3.[15] When acid concentrations are above one normal, the type of acid used will affect the leaching rate.[21] For acid concentrations that are below one normal, the leaching process is of the mixed diffusion-reaction type when carried out at room temperature.[21] During low acid concentration leaching, the dissolution of the unstable phase will proceed at a rate that will allow the dissolved components to diffuse out from the interface and acid to diffuse to the interface. The speed of the reaction will occur at a rate close to that of the mass transport of the components. For acid concentrations above one normal, the rate of dissolution of the borate phase will proceed faster than the rate of mass transport. Leaching of these glasses with acid concentrations greater than one normal will result in a diffusion controlled process.[21] Borates that are dissolved during the reaction process are barely soluble in the acid solution. When the concentration of these borates in the acid solution reaches a high 14

24 enough level, they will be deposited on the pore walls. This is more common in the area around the reaction front where the chance for the solution to become super saturated with borates is greatest. Deposits will disappear if the glass is leached completely through and the deposits can be dissolved and transported out of the glass. The sodium borate phase that separates out during the heat treatment will contain some silica. During the dissolution of the sodium borate phase the solution will begin to become alkaline. This alkalinization of the solution will enable the silica within the borate phase to be almost completely dissolved within the acid solution. The silica that is dissolved in solution will become polymerized and deposited on the pore surfaces as a gel. Deposition of the silica gel is referred to as secondary silica and influences the microstructure of the porous glass.[22] As the length of the pores increase, the leaching process will become more and more dominated by diffusion. The transport of both dissolved components and acid solutions will have farther to travel and will take more time. In addition to the time it takes for the components to diffuse in or out of the pore, the acid solution that is diffusing in may be utilized to dissolve some of the borate deposits that are in the pores. As this happens it takes longer for the dissolution of the phases to occur as the length or thickness of the pores in the glass increases.[21, 22, 24] The length of a pore is mainly determined by the time that the glass is in the acid solution. Lengths of pores can be predicted by the following equation:[22] 2 D t h = (14) Co 1 C 2 eq Where C o is the compositional concentration of the dissolved phase and C eq is the equilibrium concentration of the components in solution Leaching products During the dissolution of a soluble phase, the components of the phase will be dissolved in the solvent solution and transported to the surface of the article. The products of the leaching process are dependent on the composition of the soluble phase that is removed. In the sodium borosilicate glasses, the products are borates and sodium ions. In some of the other systems that have been shown to phase separate, the products are mostly borates Processing In the family of glasses, there are many compositions that exhibit phase separation and several of these have the occurrence of phase separation by spinodal decomposition. While the occurrence of spinodal decomposition is the first requirement of a glass or ceramic to be able to produce a porous material by this method other problems arise upon manufacturing of porous materials. The process of producing a porous material follows the general process of first producing the bulk material, forming of the material into the 15

25 desired shape, heat treating of the item to produce the phase separated structure, leaching of the phase separated item to remove one of the phases, washing of the item to remove any contamination from the leaching process, and then careful drying of the item to prevent stress from developing as the washing solution evaporates Manufacture of bulk material Typically for a glass, this is done by fusion melting or sol-gel processing. Once the material is formed the heat treatment process can be done as the material is cooled or anytime thereafter. The process of heat treating is carried out by holding the material at a constant temperature for times appropriate for that particular material. Sodium borosilicate is typically heat treated for around 20 to 30 hours in order to ensure complete phase separation and to reduce the stresses that can develop during the leaching process.[25, 26] Heat treatment process Since most porous materials that are produced through spinodal decomposition are made from melted materials they need to undergo an annealing process to allow the phase separation process to occur. Heat treatment conditions will be particular to the system that is being used and on the specific microstructure desired. Most materials are heat treated in a furnace in ambient air and held at times ranging from a few hours to days. In heat treating the article to make it porous, the microstructure is going to be determined by the original composition, temperature of the heat treatment and the time that the article is at that temperature Temperature considerations Spinodal decomposition is achieved by the diffusion of the components of the glass to form regions of differing composition. The size and composition of the resultant phases will be determined by the heat treatment temperature. Size of the phase is related to the rate of diffusion that exists at that temperature. Composition is the result of the free energy reduction that occurs upon phase separation. As seen in the section , at a given temperature a material will lower its free energy by separating into the compositions on either side of the miscibility gap. In the sodium borosilicate system, a higher heat treatment temperature within the miscibility gap will produce larger size phases for a given heat treatment time. These phases will have different compositions than that of the phases separated from the same parent composition that is heat treated at a lower temperature. At higher temperatures, the miscibility gap becomes narrower and therefore the phases on the outside of the gap will be much closer in composition. Determination of the resultant phase compositions can be done by doing tie line determinations using a phase diagram that details the metastable immiscibility region. 16

26 Time considerations The length of time to heat treat a particular article is dependent on the microstructure that is desired. Since diffusion is the process that produces the rearrangement of the constituents of the material, the temperature of the heat treatment will determine the dwell time of the annealing process. A consideration as to the process of crystallization of a glass material should be made when determining the dwell time for the annealing process. If the temperature is sufficient to nucleate crystals within the material, a prolonged heat treatment will produce a phase separated material Strains due to phase separation The patents issued to Hood and Nordberg in the 1930s and 40s detailed the process of making a high silica glass through phase separating a borosilicate glass and then a subsequent leaching. Also covered in the patents, was the affect that the initial composition and heat treatment temperature had in producing strain in the resultant glass. Initial compositions based on a certain heat treatment temperature are given in the patents and these will produce a glass that develops no strain in the leached layer during the leaching process.[25-27] The development of the strain and stress in the glass due to the heat treatment process is due to the differences in the thermal expansion of the different phases.[28] As the composition of the resultant phases becomes increasingly different, the formation of stress will become more prevalent as differences in the thermal expansion of the phases become greater Leaching process After the products are heat treated and cooled, they can then be leached to remove one of the phases. Typically the material is leached using a hot dilute acid solution. The acids used are HCl, H 2 SO 4, or HNO 3.[15] Depending on the material, leaching can also be done using boiling water as the main leaching process or as one of the steps in leaching.[29-40] The amount of time it takes to leach the material to a porous condition is dependent upon the thickness of the material and the concentration and temperature of the acid solution. Acid leaching is typically done around C with acid solutions around 3N. With these conditions the leaching rate for the Vycor glass compositions is around 1mm per 24 hours.[41] Washing and drying Once the material has been completely leached it should be washed to remove any of the by products produced by the leaching process. The material can be washed in distilled water were the water is replaced on a continuous basis to ensure the steady diffusion of the leaching by-products from the pores within the material. The amount of time that is required to clean the leached material will be dependent on the pore size and the temperature of the water used. After the washing has been complete, care needs to be taken during the drying process to prevent stress buildup due to the pressure that can be 17

27 exerted by the evaporation of the water within the pores. Usually drying in air is sufficient to ensure breakage is avoided. 2.3 Spinodal glass systems Many glass systems have been identified that exhibit phase separation. These systems typically will display both nucleation and growth separation and spinodal decomposition.[28] An appropriate heat treating schedule and the proper leaching mechanism for the desired material would need to be developed in order to produce a porous material. A list of some of the metastable phase separating systems excluding the glass systems that already have been used to make porous glasses is given in Table 1. Table 1. Phase separating binary oxide glasses Components Composition range (mol %) Consolute Temperature ( C) Binary with silica L 2 O Na 2 O BaO Al 2 O Ga 2 O Binary with B 2 O 3 L 2 O Na 2 O K 2 O Rb 2 O Cs 2 O Consolute Composition (mol %) Ref: [42] [19] Porous glass systems The glasses that can be made into porous materials can be grouped into sodium borosilicates, other silicate systems, silica replacing systems, and non-silicate systems Sodium borosilicate systems The sodium borosilicate composition that phase separates is within the composition range of SiO 2, 10-(0.1(SiO 2-55)) Na 2 O with the balance being B 2 O 3.[43] Various other components can be added to the basic sodium borosilicate composition to alter the phase separation characteristics of the glass. Adding less than 3 mol % alumina to the composition reduces the tendency to phase separate, and additions above 3 mol % prevent phase separation.[44] Addition of Alumina also reduces the tendency of the glass to crystallize.[43] Phase separation is also significantly decreased with the addition of zirconia above 3.56 mol % while additions below this limit only reduce the pore volume 18

28 slightly.[45] By adding CaO in quantities greater than 3 wt %, the zirconia content of the silica-rich phase can be increased. Increasing the zirconia content in the silica phase improves the alkali durability of the porous glass structure.[46] Once the glass has been heat treated and leached it has a porous skeleton of high silica glass. This porous glass structure can withstand continuous service temperatures up to 900 C and intermittent use up to 1200 C. At temperatures above 900 C the glass will begin to reconsolidate and will eventually become solid.[15] Other silicate systems Silicate glass systems that have three or more components were also found to spinodally decompose. This is not surprising due to the fact that the Vycor system is a ternary glass in the silicate system. There are four spinodally phase separating silicate glass systems with three or more components that have produced porous structures. The first glass system, Na 2 O-B 2 O 3 -SiO 2 -Ta 2 O 5 undergoes phase separation permitting one of the phases to be leached out with boiling water. The remaining phase has a composition of Na 2 Ta 6 O 15 Si 2 where tantalum comprises 76 wt %, oxygen 16.8 wt %, sodium 3.2 wt % and silicon 3.9 wt % of the insoluble phase. The leached glass has a pore structure in the range of 3-10 nm. A sintering temperature of 1520 C for 5 minutes produces a heterophase glass ceramic.[47] The second and third systems, TiO 2 -SiO 2 -Al 2 O 3 -B 2 O 3 -CaO-MgO[48] and Na 2 O-CaO- TiO 2 -P 2 O 5 -SiO 2 [49], produce a porous TiO 2 -SiO 2 glass-ceramic and a TiO 2 -SiO 2 ceramic material, respectively. A starting glass of TiO 2 -SiO 2 -Al 2 O 3 -B 2 O 3 -CaO-MgO produces a spinodal phase separated structure when heat-treated between 700 and 850 C. After leaching, the porous structure that remains is composed of mol % TiO 2 and mol % SiO 2 depending on the initial compositions. The average pore radius produced is between 3 and 9 nm. Na 2 O-CaO-TiO 2 -P 2 O 5 -SiO 2 also spinodally decomposes upon casting of the ceramic material. Leaching develops a porous structure which leaves a skeleton of silica (33 mol %) and titania (67 mol %). The titania is held together by the silica, which acts as a binder. The median pore diameter for this material is 1.2 µm which remains unchanged when heated to 1000 C.[49] The fourth system is in the sodaphosphate-silica system and the immiscibility region is similar to that of sodium borosilicate system.[50, 51] Silica replacing systems Other phase separable glasses can be created by using various substitutes in place of the SiO 2 component from the ternary sodium borosilicate system. Replacing silica with Ce 2 O 3 -Nb 2 O 3 and Ce 2 O 3 -Nb 2 O 3 -Al 2 O 3, and using stepped heat treatments ranging from 650 C to 800 C produces a spinodally phase separated glass.[36] The average pore radius ranged from nm for a composition that contained alumina, while the non-alumina containing glass has a pore radius of around 55 nm. The non-alumina material also displays crystallinity while the alumina containing material showed very little 19

29 crystallization of the base material after leaching. The porous glass shows signs of consolidation when sintered at 1350 C for 30 minutes. Silica has also been replaced with combinations of the alkali resistant oxides of thorium, zirconium, cerium and yttrium.[40] Initial compositions within this glass system are predominantly borate with the component comprising around 66 wt % of the melt. When the silica is replaced with these components it produces a spinodally phase separated glass upon heat treatment. The heat-treated glass can be leached in boiling water, which results in glasses with an average pore radius range of nm. All of these leached glasses are brittle, but posses good alkali resistance. The porous structure is sintered at 1520 C for 30 minutes. Replacing silica with only CeO 2, the glass will spinodally separate into a CeO 2 rich region and a SiO 2 rich region when heat-treated between 500 and 700 C. Leaching is done with boiling water for hours. Average pore radius after leaching was between 5 and 17 nm and 0.5 nm when the melt was done in an alumina crucible. The sintering process begins at 1300 C, and can be accomplished in 30 minutes at 1500 C. Silica has also been replaced by CeO 2 -HfO 2, which has results similar to the CeO 2 only glass.[34] The leachable phase in the CeO 2 -HfO 2 glass is water-soluble and results in an average pore radius between 1 and 3 nm. This glass sinters at temperatures between 950 and 1250 C. In addition to these glasses, several other porous glasses have been produced by replacing the silica with a combination of oxides. These oxides include a combination of Y 2 O 3 and ZrO 2, CeO 2 and Ta 2 O 5, Sc 2 O 3 and HfO 2, and the combination of Sb 2 O 3, V 2 O 5, Ta2O5, La 2 O 5, TiO2, and ZrO 2. These glasses are summarized in Table Non-silicate systems Porous glasses have also been made from non-silicate based compositions. In the CaO- TiO 2 -P 2 O 5 system a porous glass-ceramic can produced with a pore radius of 13 nm.[52] A summary of the non-silicate porous glass systems and properties is included in Table 2. Table 2. Porous glass systems Glass system Initial comp. Range (Wt %, unless mol% noted) Na 2 O-B 2 O 3 - Na 2 O: SiO 2 B 2 O 3 : SiO 2 :55-70 CeO 2 -Ta 2 O 5 - Na 2 O- B 2 O 3 [30] CeO 2 : Ta 2 O 5 : Na 2 O:15,20 B 2 O 3 :55-70 Skelton comp Heat treat condition( C) Pore radius range (nm) Notes B 2 O 3- SiO :20 hr 1-20 Vycor rangeup to 4% alumina can be added B 2 O 3 - CeO 2 - Ta 2 O 5 650:2hr 700:3hr 19-42: no Al 2 O 3 <1-3: trace Al 2 O 3 Trace alumina reduces pore size Composition in mol% Na 2 O- CeO 2 - Na 2 O: B 2 O 3 CeO 2 : [34] B 2 O 3 : B 2 O 3 - CeO 2 500:2hr 550 :2hr 600:2hr C sintering temp 20

30 Na 2 O-CeO 2 - Nb 2 O 3 - B 2 O 3 [36] Na 2 O:5-20 CeO 2-3Nb 2 O 3 :20-30 B 2 O 3 :55-70 Not reported (CeO 2 - Nb 2 O 3) 650:2hr 700:2hr 750:2hr C sintering temp Composition in mol% CeO 2 - HfO 2 - Na 2 O- B 2 O 3 [29] Sc 2 O 3 -HfO 2 - Na 2 O- B 2 O 3 [33] Na 2 O- Sc 2 O 3 - B 2 O 3 [32] Na 2 O- La 2 O 3 - TiO 2 - B 2 O 3 [39] Na 2 O- La 2 O 3 - B 2 O 3 - Ta 2 O 5 [38] ZrO 2 -Y 2 O 3 - Na 2 O- B 2 O 3 [31] Al 2 O 3 - Na 2 O- TiO 2 - B 2 O 3 [37] P 2 O 5 - Na 2 O- TiO 2 -SiO 2 - CaO [49] CeO 2 : HfO 2 :10-20 Na 2 O:10-15 B 2 O 3 :55-65 Sc 2 O 3 :6-20 HfO 2 :10-24 Na 2 O:20 B 2 O 3 :50 Na 2 O:15.82 Sc 2 O 3 :26.41 B 2 O 3 :57.77 Na 2 O: La 2 O 3 : TiO 2 :15-23 B 2 O 3 :49-56 Na 2 O: La 2 O 3 : B 2 O 3 : Ta 2 O 5 : ZrO 2 : 12 Y 2 O 3 : 12 Na 2 O: 12 B 2 O 3 : 64 Al 2 O 3 :trace-17 Na 2 O:9-18 TiO 2 :17-20 B 2 O 3 :56-70 P 2 O 5 :15 Na 2 O:15 TiO 2 :19 SiO 2 :19 CaO:32 B 2 O 3 - HfO 2 - CeO 2 Na 2 O- Sc 2 O 3 - B 2 O 3 - HfO 2 B 2 O 3 - Sc 2 O 3 B 2 O 3 -TiO 2 - La 2 O 3 Not reported B 2 O 3 - Y 2 O 3 - ZrO to cool 600:2hr 600 to cool 500:2hr 550:2hr 600:2hr 650:2hr 600:2hr 650:2hr 700:2hr 750:2hr 600:2hr 650:2hr 700:2hr 750:2hr 650:2hr 700:2hr 750:2hr C sintering temp High sintering temp >1400 C 1.2 Trace alumina reduces pore size Alumina from crucible smaller pore size and better strength Glass ceramic skeleton Mechanically weak Not reported :3hr Mechanically weak SiO 2 - TiO 2 none 1200 No consolidation at 1000 C Composition in mol% MgO-B 2 O 3 - Al 2 O 3 -TiO 2 - CaO-SiO 2 [48] Na 2 O- TiO 2 - P 2 O 5 -CaO [52] MgO:1.5-5 B 2 O 3 :5-7.5 Al 2 O 3 :10-16 TiO 2 :17-26 CaO: SiO 2 :26-39 Na 2 O:1 TiO 2 :24.9 P 2 O 5 :33 CaO:43.1 CeO 2 : CeO 2 -ThO 2 - Y 2 O 3 -ZrO 2 - ThO 2 : TiO 2 - SiO Glass ceramic structure CaO- P 2 O 5 - TiO 2 Not reported Composition in mol% 700:10 min 13 Composition in mol% 600:2hr 650:2hr Resultant glass is brittle 21

31 Na 2 O-B 2 O 3 [40] Y 2 O 3 : ZrO 2 : Na 2 O: B 2 O 3 : :2hr Composition in mol% V 2 O 5 -Sb 2 O 3 - ZrO 2 -TiO 2 - Na 2 O-Ta 2 O 5 - La 2 O 3 -B 2 O 3 [35] V 2 O 5 : Sb 2 O 3 : ZrO 2 : TiO 2 : Na 2 O: Ta 2 O 5 : La 2 O 3 : B 2 O 3 : Na 2 O: P 2 O 5 : Na 2 O-P 2 O 5 - SiO 2 [50] SiO 2 : Reduced: B 2 O 3 ; Ta 2 O 5 Enriched: TiO 2 ; ZrO 2 ; La 2 O 3 600:2hr 650:2hr 700:2hr 750:2hr 790:2hr Sintering Temp of 1450 C for 30 minutes P 2 O 5 - SiO Not reported High silica skeleton 2.4 Silica gel removal Porous glasses that are made from the sodium borosilicate glass system have a silica gel that is deposited in the pore during the leaching process depending on the previous processing conditions. The heat treatment temperature will determine the amount of silica that is in the secondary phase and the acid concentration and acid to glass ratio will affect how much of that silica will be transported out of the pores during leaching.[53] After the initial leaching process this silica gel will reduce the effective pore size of the glass. The gel that is contained in the pores is easily removed by treating the glass with an alkaline solution. Typical concentrations of 0.5N NaOH or KOH are used.[28, 54] This solution will initially dissolve the gel, followed by etching of the surface of the pores and then finally etching of the silica skeleton.[55] Times between steps will be dependent upon the thickness of the article and the amount of secondary silica deposited in the pores. 2.5 Pore consolidation and pore growth Originally the production of porous glass was an intermediate step in the production of a high silica glass. The solid glass article was heated to above 900 C to fully consolidate the pore structure. At temperatures above this the glass has a low enough viscosity for the pores to collapse where the main driving force is the reduction in the surface energy of the pores.[56] A mathematical model has been developed to describe the consolidation of a porous glass where the pore structure has only one pore width and no pore width distribution. Rates of sintering proposed are a function of viscosity of the glass structure. The models cover sintering and consolidation of optical fiber preforms, sol-gel structures and porous glasses with the models being supported by some experimental evidence.[57-59] The same author also developed a model to describe consolidation of porous glass that is constrained. The example given is of a silica sol-gel impregnated in a fiber mat. The modeling for a constrained environment describes a coarsening of the pores in which smaller pores become part of the larger pores in the material. The driving force is proposed to be the balancing of the reduction of surface energy with strain energy in the material.[60] Several studies have been done with a thermal treatment of non alkaline treated porous glasses at temperatures less than 900 C. These studies indicated that 22

32 coarsening occurs.[61-64] However, these studies do not track the evolution of the pore structure as a function of treatment time, and only report the change in the average pore size as a function of temperature for a single heat treatment time. 23

33 Chapter 3- Materials and Methods Fabrication of the porous optical fiber can be broken down into two parts, fabrication of the porous clad fiber and alteration of the morphology of the pore structure through additional processing. Fabrication of the fiber is described in two parts, with the first section describing the preform fabrication and fiber drawing and the second section describing the phase separation and leaching of the fiber cladding. Two experiments were done to explore the achievable pore sizes in the cladding. The first of these experiments looked at the use of an alkaline treatment to remove the silica gel from the pore structure. Removal of the silica gel allows for the maximum pore size as determined by the phase separation temperature while leaving the silica skeleton intact. Retaining as much of the skeleton as possible ensures that the fiber maintains as much mechanical integrity as possible. In the second experiment three processing variables are investigated. The first variable is the temperature at which the phase separation is carried out. Second is the use of an alkaline treatment on the porous glass. The third variable investigated is a heat treatment on the porous glass, where different temperatures and times were used in the heat treatment. Due to the extent of the second experiment, samples were prepared from bulk samples of the phase separating glass. These experiments are detailed in the second section with each experiment treated independently. Characterization of the fibers and bulk glass samples is treated in the last section and includes both material characterizations and optical and functional characterization. The methods to do the material characterizations were the use of scanning electron microscopy, nitrogen adsorption, and optical microscopy. 3.1 Fiber fabrication Preform manufacture The fibers that have been fabricated have a two part structure, a porous cladding surrounding a solid core. Making of the fiber begins with construction of a preform from which the fibers are pulled. To make the preform, a glass tube of a phase separable sodium borosilicate composition is collapsed around a rod made of a non-phase separable borosilicate glass composition. The phase separable glass portion of the preform was obtained in the form of tubes from the Corning glass plant in Danville, Virginia. The starting glass composition is proprietary, but is within the spinodal phase separation region of the sodium borosilicate system. The tubes were pulled from the Vycor production line prior to the phase separating heat treatment. The core region of the fiber is made from a different glass composition that does not phase separate and has been made using a borosilicate comparable to the Corning code 7740, although many other non-phase separating glass compositions can be used. A borosilicate glass was chosen so that the glass softening points of the core and cladding region would be close enough to allow for reduction of both regions during the pulling of the fibers. Figure 12 details the beginning process to make the preform. First a length of glass tubing is taken and a section on either end of the tube is collapsed on a glass lathe so that there is opening at the center slightly larger than the diameter of the core rod to be used. The initial diameter 24

34 of the core rod will partially determine the core diameter of the fiber. Once the tubing section is collapsed, a borosilicate core rod is inserted into the tube which is then fully collapsed, beginning at one end and working toward the other collapsed section. The preform manufacture is carried out on a glass lathe with the glass being worked using an oxygen-hydrogen torch. Partially collapse tube sections Vycor green glass tube Corning 7740 core Figure 12. Schematic of perform manufacture Fabrication of the initial glass fiber With the preform manufactured the fibers are pulled from the end of the preform. The preform is worked with an oxygen-hydrogen torch to lower the viscosity of the preform tip to a suitable range to pull fibers. Random lengths and diameters were produced using this process. The following procedure was used for pulling fibers. I. The preform is secured in both chucks on a glass lathe and rotated while the one of the necked-down section is flame worked to ensure even heating. II. After the preform has reached a sufficient viscosity the chucks are moved apart drawing the necked-down section out into a fiber A. This step prepares the preform end for fiber pulling III. The end of the preform is then heated using a hydrogen-oxygen torch until the end of the preform has a low enough viscosity to begin pulling fibers A. Viscosity is approximated visually IV. After the end of the tube is at a sufficient viscosity a rod of borosilicate is attached to the molten mass of glass at the end of the preform. A. Prior to the rod being attached the following steps are taken. 1. The end of the fused silica rod to be attached is heated to ensure bonding to the borosilicate glass. 2. The rotation of the lathe is stopped to prevent any twisting of the glass during the fiber drawing process. V. With the borosilicate rod attached to the molten, glass the flame from the torch is removed from the vicinity of the preform to prevent differential heating of the glass during the fiber drawing process. 25

35 VI. Once the flame is removed from the end of the preform, a fiber is drawn by pulling the borosilicate rod away from the end of the tube. A. The speed of the pull will determine the diameter of the fiber produced B. Viscosity of the glass at the end of the preform will also determine the diameter and length of the fiber produced Phase Separation In order to produce the porosity within the fibers, the glass needs to be heat treated to induce the phase separation. This phase separation allows for one of the phases to be preferentially leached out of the glass, leaving the cladding porous. The fibers were heat treated as follows: I. Fibers produced from the heating and pulling process using the lathe were separated into fiber lengths of approximately 9 inches to accommodate the size of the furnace that was used. The furnace that was used is a Thermolyne II. Fiber lengths were placed on an alumina plate that was then placed in the furnace. III. The furnace was set at a temperature of 565 C and the fibers were held at this temperature for 20 hours followed by a furnace cool. These were the heat treatment conditions given by Corning for this glass. IV. After the dwell time was reached the furnace was switched off and allowed to cool to room temperature Leaching process to produce the porous clad fiber Fibers that are produced and heat treated as previously described are leached using a surface etching treatment and then an acid bath leaching treatment. The surface etching treatment is done using a 5% solution of ammonium biflouride. Fibers are surface etched for a duration of 5 minutes. Fibers are then subjected to the acid bath immediately upon completion of the surface etching treatment. The acid bath is done with a 3N concentration of HNO 3 at a temperature of 80 C. Fibers are processed according to the following procedure: I. The fibers are immersed into a 5% ammonium biflouride solution bath for five minutes. II. After a fiber is in the surface etching solution for its allotted time the etching solution is drained off. III. The fiber is then immediately immersed in a bath of a 3N solution of nitric acid that is at a temperature of 80 C. Fibers are left in the acid solution over night to ensure complete dissolution of the secondary phase. 26

36 IV. After the fibers have been in the acid solution for a sufficient time, the container is emptied of the nitric acid. V. Fibers are rinsed in the containers which then filled with distilled water and left overnight to wash any contaminates from the pores within the fibers. Fibers are left in the container during the rinsing process. VI. Once the washing process was completed, the fibers were removed from the containers and placed on paper towels and allowed to dry in air under ambient conditions. 3.2 Experiments to affect pore size of cladding region Removal of colloidal silica with an alkaline solution to increase pore size This portion of the research was conducted to determine the treatment time necessary to remove all of the silica gel from the pore structure. Treatment times for fibers in the solution ranged from 15 minutes to 60 minutes due to the small diameter of the fibers. Solution to glass ratio was well over 10:1 to ensure a stable alkaline environment. Experimental procedure was as follows: I II II. Surface etching -Fiber immersed in 5% ammonium biflouride solution 1) Immersed for 30 seconds Leaching of fibers 2) Placed in glass test tube containing 3N HNO 3 directly from surface etching process 3) Tubes placed in beaker of water that is heated to 90 C 4) Fibers left in solution for hours to ensure complete dissolution of secondary phase 5) Fibers washed by empting HNO 3 from tubes and replaced with 18.2 MΩ deionized water and replaced in heated beaker 6) Fibers washed for hours 7) Fibers removed from water Removal of colloidal silica 1) Fibers immersed in 0.5N solution of NaOH a. Immersion times of 15, 30, 45, and 60 minutes 2) Fibers removed from NaOH bath and placed in 3N solution of HNO 3 for 3 hours 3) Fibers were removed from the HNO 3 solution and rinsed in 18.2 MΩ deionized water for hours 4) Fibers removed from water and air dried. 27

37 3.2.2 Phase separation temperature and post heat treatment of bulk glass experiment Glass samples for this experiment were all prepared for processing on the same day from the same batch of tubes that were received from Corning. The processing of the tubes involved crushing the tubes into large fragments of about 1 cm 2. Glass fragments were placed in an alumina crucible and heat treated at one of the three phase separation temperatures. The temperature of the crucible was monitored through the placement of a thermocouple at the crucible location in the furnace. All three phase separation temperature groups were heat treated in the same location in the furnace with the phase separation temperature monitored and adjusted so that the thermocouple was touching the alumina crucible. Once the glass had cooled it was then leached to make the glass porous. Fragments were initially placed in a 5% solution of ammonium biflouride for 5 minutes. This initial immersion removes the silica rich layer that forms during the making of the tubes and heat treatment process. Without this initial step, the leaching process is inhibited and there is no visible porosity in the glass. After removal of the surface layer the glass was immediately placed in a 3N solution of nitric acid. The fragments were left in solution for 72 hours to ensure that complete leaching of the secondary phase is accomplished. A volume ratio of 50 ml glass to 500 ml of nitric acid solution was used during the leaching of the glass fragments to provide a more adequate supply of H + during the leaching. Experimental groups were classified by the initial phase separation temperature and were grouped and processed accordingly. All surface etching, leaching, and washing was done for a phase separation temperature group. After the group was leached and washed the total amount of fragments in a group were divided in half, with one group receiving the post heat treatment as the next step and the other group undergoing a treatment with a 0.5 N NaOH solution. After the fragments underwent the NaOH treatment they were washed in a 3N nitric acid solution for 24 hours to remove any contamination and residue during the NaOH treatment. Following the acid wash the fragments were washed in DI water for 24 hours and then air dried. The NaOH treated fragments received the post heat treatments after the air drying. A flow chart of the experimental process is shown in Figure 13. Treatment time for the NaOH was determined by assuming that the diffusion condition of the NaOH was the same for both the fibers as for the bulk glass pieces. If the conditions are the same then the diffusion distance is the only adjustment that needs to be taken into account. Since the NaOH treatment involves the dissolution of the silica gel in the pore structure and its transport out of the glass then the diffusion distance affects the time needed for the treatment. Time in solution was calculated as follows with D being the diffusion distance, t is the time in solution and α all other factors effecting diffusion. Diffusion in the fibers is denoted by an f subscript and in the bulk by b. Glass thickness is 1.4 mm and the average fiber diameter is 200 µm. 28

38 D f = α t f D b = α t b D b = 7 D f = 7α t f = α t b t b =49 t f = 49*30 min t b =1470 min = 24.5 hours Heat treated ( C)/ leached/ washed glass BET/SEM analysis Treatment of glass with 0.5N NaOH for 24.5 hours to remove colloidal silica from pore structure Post leaching/ NaOH processing heat treatment. Temps ( C): 600/ 700/ 800/ 900 Times (min): 5, 15,25,1440 BET/SEM analysis Figure 13. Phase separation temperature and post heat treatment experimental schematic 3.3 Characterization Characterization of the fibers was done in two parts, materials characterization and optical characterization. The optical component was done by a counterpart in the Center for Photonic Technology in the electrical engineering department at Virginia Tech. A brief description of the methods used for characterize the fibers will be given. 29

39 3.3.1 Materials characterization Materials characterization done on the fibers investigated the overall structure of the fiber and the morphology and pore characteristics of the cladding region. The methods used were primarily scanning electron microscopy (SEM, LEO 1550) and nitrogen adsorption (BET, Quantachrome autosorb 1-c). Samples prepared for the SEM were done in two different ways depending on which type of sample was imaged. Fibers were prepared by mounting onto a sample stub with a colloidal silver paint. Mounting with the silver paint ensures an ample conduction path that is usually absent due to the size of the fiber and the curvature of the surface. Bulk glass samples were mounted onto the stubs with carbon tape. Both the fiber and bulk samples were coated with 10 nm of gold using a sputter coater. When imaged in the SEM the accelerating voltage was set to 5 KV so that only the surface of the glass would be imaged. Pore structure was characterized through the use of a nitrogen adsorption technique. The samples for the analysis were placed in a 9mm sample holder. Samples were then out gassed for a minimum of 24 hours at a heating cuff setting of 300 C. Time and temperature of the outgas step was chosen to ensure that all adsorbed water was removed from the glass samples. Weights of the samples were taken before and after out gassing and the samples were placed into the machine immediately after the final weight was taken. The produced isotherms were analyzed through the use of a Monte Carlo analysis program supplied with the instrument. Parameters chosen for the analysis were nitrogen adsorption onto oxygen and the analysis was applied to the adsorption branch of the isotherm. The model used was for nitrogen adsorption onto silica with a cylindrical pore structure Optical characterization Transmission Measurement The function of the porous clad solid core gas sensor is to locate the absorbance peaks of the gas under test spectrally by its transmission. The fiber transmission measurement system is made up of the following parts: i. optical components ii. lead in and lead out fibers iii. signal acquisition and processing unit (CTS and PC) Figure 14 gives a conceptual sketch of the transmission measurement system. 30

40 Lead-in fiber y x θ z ϕ Lead-out fiber CTS Figure 14. The transmission measurement system PC The optical parts consist of four components. The two in the middle hold the fiber under test, and they ensure that the fiber is as straight as possible; the outer two parts are multi-dimensional tunable stages. The stage is able to be tunable in the x, y, z directions, as well as in the θ and φ directions. The adjustability in the above five spatial directions ensures an optimal alignment among the fibers. The CTS has to be calibrated each time it is turned on. The optical signals are injected into the lead-in fiber, go through the fiber, and return back to the CTS from the lead-out fiber. The optimum transmission is obtained by adjusting the position of the 3-D optical stages. A Matlab program getcts.m running on the personal computer (PC) helps to capture the data from the CTS. Figure 15 shows a photo of the transmission measurement system. 31

41 Far field pattern measurement Figure 15. Transmission measurement system Far field pattern measurements of the fiber helps to understand the mode characteristic inside the fiber, and allows us to determine if the fiber is multimode or single mode. The light from the visible laser source is injected into the holey fiber from a lead-in fiber, transmitted through the holey fiber and projected onto a screen. The system consists of four parts: optical source (Argon Laser), lead in fiber (single mode fiber), optical mechanical components (tunable stages), and a screen (wall). The picture is shown in Figure 16. Lead-in Fiber Laser Optical Stage Figure 16. Far field pattern measurement setup Chapter 4- Results and Discussion 32

42 4.1 Initial fiber fabrication Some fibers that have been fabricated and subjected to the leaching process have not been able to sense gas. However fibers that have been treated with a 0.5 N NaOH solution for 30 minutes will sense acetylene gas. As a starting point the initial fiber characterization is presented for comparison to the remainder of the experimental results. An optical micrograph of the end face of the fiber is shown in Figure 17. The cladding region is visible and distinct from the core in the micrograph. Figure 18 is an SEM micrograph of the surface of the fiber and Figure 19 is an SEM micrograph of the interface between the core and the cladding of the fiber. Both of the micrographs show porosity on the surface of the fiber and the interface between the core and cladding regions. The interface between the core and cladding can be clearly seen in the SEM micrograph in Figure 19. Nitrogen adsorption characterization is detailed in Figure which shows the pore distribution profile for the initial fibers. The fiber sample amounts were small and were close to the resolution of the scale used for weighing. An accurate measure of the sample weight was difficult. This produces an incorrect specific pore size distribution and the amounts on the y-axis should be taken only as a relative measure. However, the actual distribution of the pore sizes is not affected by an incorrect sample weight. Nitrogen adsorption shows that the mode 1 pore size is 6 nm with an average pore size of 10.9 nm. Figure 17. Optical micrograph of a fracture surface of the end face of a leached only optical fiber 33

43 Figure 18. SEM micrograph of leached only optical fiber surface Figure 19. SEM micrograph of core-cladding interface of leached only optical fiber 34

44 dv (cc/g/ angstrom) pore width- angstroms Figure 20. Pore size distribution of leached only optical fiber 4.2 Removal of colloidal silica to increase pore size with alkaline solution The processing of fibers to remove the colloidal silica from the pore structure resulted in a maximum average pore size of approximately 30 nm after a processing time of 30 minutes. This is supported by SEM micrographs of the fibers. In Figure 21and Figure 22 the SEM micrographs show the interface between the core and cladding region of the fibers treated for 15 and 30 minutes respectively. The increase in the pore size is visible in the micrograph as the processing time changes. Pore size distributions from characterizing the fibers through nitrogen adsorption is shown in Figure 23 and a plot of the average pore size as a function of processing time is in Figure 24. From Figure 23, it can be seen that the maximum shift in the pore distribution occurs at 30 minutes and that the 45 and 60 minutes treated fibers have the same distribution as the fiber that was treated for 30 minutes. The nitrogen adsorption characterization shows that maximum average pore size is achieved after 30 minutes of treating the fibers with a NaOH solution. Enlargement of the pore structure is confirmed by the SEM micrographs in Figure 21and Figure 22 when compared against the SEM micrograph of the as-leached fiber in Figure

45 Figure 21. SEM micrograph of core-cladding interface of fiber treated with 0.5 N NaOH for 15 minutes Figure 22. SEM micrograph of core-cladding interface of fiber treated with 0.5 N NaOH for 30 minutes 36

46 dv (cc/g/angstrom) untreated fiber 15 min 30 min r 45 min 60 min pore width (angstrom) Figure 23. Pore size distribution for fibers treated with NaOH Pore width (nm) treatment time (min) 4.4 Gas sensing Figure 24. Average pore size of fibers treated with NaOH In the measurement of the transmission, it is required that the fiber ends be carefully treated before putting into the system. Otherwise, the raw surface ends will cause severe interference and contaminate the transmission spectrum. Typically, standard treatment to the fiber end face includes polishing or cleaving. It is demonstrated that cleaving is an effective and convenient way to treat the porous clad fiber end face. A well cleaved fiber together with a well adjusted system will result in a high quality transmission spectrum 37

47 that is high, smooth and flat, suitable for gas sensing. A transmission spectrum typical of the porous clad fiber is shown in Figure FIBER TRANSMISSION TRANSMISSION(dB) WAVELENGTH(nm) Figure 25. Transmission of the porous clad fiber The porous clad fiber is designed to be multimode operation. Given the dimension of the fiber and its structure, the fiber s mode characteristic can be theoretically predicted if some simplifying assumptions are used. Understanding the light guidance and distribution inside the fiber helps us verify our design and theoretical analysis on one hand, and gives valuable fiber characteristics guiding our future fiber parameter optimization on the other. A typical far field pattern of the porous clad fiber is shown in Figure 26. The spotty pattern shows that the fiber under test is strongly multimode. Figure 26. Far field pattern of the porous clad fiber By blowing acetylene gas (which has absorption lines in the range of ) directly onto the porous fiber, the presence of acetylene gas is clearly shown in the characteristic absorption peaks of acetylene. The response time is very quick (~ms) which is indicated by the time slot between the gas turning on/ off and appearing/ disappearing of the 38