MASTER. Pore size distribution and surface group analysis a study of two electrical grade carbon blacks. Geraedts, S.

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1 MASTER Pore size distribution and surface group analysis a study of two electrical grade carbon blacks Geraedts, S. Award date: 2002 Link to publication Disclaimer This document contains a student thesis (bachelor's or master's), as authored by a student at Eindhoven University of Technology. Student theses are made available in the TU/e repository upon obtaining the required degree. The grade received is not published on the document as presented in the repository. The required complexity or quality of research of student theses may vary by program, and the required minimum study period may vary in duration. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain

2 Pore Size Distribution and Surface Group Analysis A study of two electrical grade carbon blacks Saskia Geraedts M. Sc. Thesis October 2002, Eindhoven Supervisors: Dr. J.M.C. Brokken-Zijp (DPI) Dr. R. Neffati Graduation Professor: Prof. Dr. M.A.J. Miehels (Physics department, DPI) TU/e technische universiteit eindhaven

3 Technology assessment Often in elastomers, thermoplastic and thermoset coatings, pigments or fillers are used as essential part in order to enhance properties and/ or to add certain functionalities to the polymer matrix. Carbon black is the most frequently used filler in polymer technology as reinforcing agent or as conductive filler to obtain permanent antistatic and electro-magnetic interterenee shielding properties. Therefore, many interesting applications from polymer- carbon black nanocomposites and coatings were found. When increasing the carbon black concentration, mechanica! properties as wellas the conductivity of the composite change dramatically at a certain critica! volume fraction of carbon black. Some macroscopie properties scale in a power law fraction when increasing the volume fraction of the filler. After the percolation threshold, an infinite cluster of particles spans the sample inducing a drastic change in macroscopie properties such as the volume conductivity. Theoretica! estimations of the critica! volume fraction range from 0.16 to 0.34 (volume fraction), but experimental values up to three orders of magnitude lower (10. 4 ) can be observed insome cases. Therefore, both from an industrial as well as fundamental point of view a low critica! volume fraction are of special interest. In this respect the carbon black characteristics (particle size, porosity and specific surface, surface groups, etc.), the dispersion, and processing conditions are the key parameters. In this master thesis the carbon black characteristics are studied of the two carbon blacks, which are known to give in certain polymer matrixes the lowest percolation threshold.

4 Summary In electrical conductive polymer composites carbon black particles are often used as a conductive filler material Above a critica} carbon black concentration, the percolatien threshold, the particles form a continuous network through the polymer matrix resulting in electrical conductivity. A low percolatien threshold ( <0.0 1 vol. %) can be achieved with special electrical grade carbon black powders in certain polymer matrices. According to literature a high specific surface area, small primary particles, high porosity, low content of surface groups and high aggregate structure are the most important characteristics of the electrical grade carbon black powders, which enhance low percolatien thresholds. The aim of this study is to gain more insight in the achievement of the low percolatien threshold for electrical grade carbon blacks. Characterizing the pore size distribution and the surface groups of two electrical grade carbon blacks, Printex XE-2 and Ketjenblack EC 600JD, is of high importance. The pore size distribution measurements were done with N 2 adsorption, thermoporosimetry and NMR relaxivity. With N 2 adsorption for both blacks a mean pore size of about 40 nm was measured. Thermoporosimetry is a new technique to study the pore size distribution in porous materials. The mean pore sizes of Printex XE-2 and Ketjenblack EC 600JD, obtained with thermoporosimetry, are 72 with a full width at half maximum (FWHM) of 48 nm and 36 with a FWHM of 38 nm, respectively. These sizes are of the order of magnitude of the size of the voids within the aggregates. However, these values differ from N 2 adsorption. This could be explained if the interfacial energy is different for both blacks. Nuclear magnetic resonance relaxation measurements were also attempted todetermine the pore size distribution. Although no pore sizes could be extracted these measurements show a stronger interaction of the cyclohexane molecules with the surface of Printex than with the surface of Ketjenblack. The functional groups present at the surface and the pore size distribution of two electrical grade carbon black powders were studied with three different techniques: Fourier Transferm infrared spectroscopy, X-ray photoelectron spectroscopy and Low energy ion scattering. No significant differences in surface groups and contarninations were found with FT -IR and LEIS. XPS shows different oxygen content for Ketjenblack. Part of this oxygen can be attributed to oxidation and part to adsorbed water. The difference in interaction of cyclohexane with the surface of carbon black may be caused by the differences in oxygen content and by differences in surface structure of Printex XE-2 and Ketjenblack EC 600JD. To elucidate the subject of the liquid - carbon black interaction, NMR relaxometry is needed below the melting temperature of cyclohexane. Also inverse gas chromatography can be used.

5 Table of Contents Table of Contents t~ Introduetion Motivation Carbon Black Qy~~~!t?~ _ c:k!~~.r~<?j~~_t ~ Pore Size Distribution in Carbon Black Powders Introduetion Analytica! Methods- Theory Nitrogen Adsorption Thermoporosimetry Nuclear Magnetic Resonance Relaxometry Experimental Procedure Materials and Instrumentation Measurements Results I Discussion Surface Group Characterization Introduetion Experimental Procedure Materials and Instrumentation Measurements Results I Discussion Conclusions Pore Size Distribution 51 -::1:} ~-t!~f~~-t:: _ÇJ_r_~~.R~ ~? Recommendations 53 6 Appendix 55 A Nitrogen Adsorption 55 B Melting Enthalpy 56 C Fourier transfarm Infrared Spectroscopy 57 D X -Ray Photoelectron Spectroscopy 64 -~ ~<?~ -~~~_rgy -~<?~ ~~~-t~~_rj~g_ ~? - - References 73 1

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7 Words ofthanks Words of thanks First I would like to thank all the people that made my stay at the university a success, especially Erik, my parents and all my friends. Also I would like to thank them for supporting me in the last year of my study, during which this masterthesis was realized. Further I would like to thank the people at the university that helped with the measurements: Thanks to José Brokken-Zijp for supervising my work and Riadh Neffati for the interesting discussions. Thanks tothijs Miehels for the interesting project. Thanks to Otto van Asselen for helping with the interpreting of the FT -IR spectra. Thanks to Tiny Verhoeven for doing the XPS-measurements. Thanks to Amoud Denier van der Gon for helping with the LEIS measurements. Thanks to Roland Valckenborg, Jelena Petkovic and Henk Huinink for helping with the NMR measurements and for the discussions on the results. Thanks to Ton Sommen for doing the nitrogen adsorption measurements. Thanks to Joachim Loos for making the SEM pictures. And last butnotleast I would like to thank Ruud Sturme from Océ and Henk Vinke from Akzo Nobel for the conversations and for supplying information. 3

8 Chapter 1 Introduetion Figure 1-1 TEM pictures [Akzo Nobel] ofprintex XE-2 (left) and Ketjenblack EC 600JD (right). The width of the opper pictures is 1250 nm, the lower ones have a width of 250 nm. 4

9 Chapter 1 Introduetion Chapter 1 Introduetion 1.1 Motivation Carbon black was used already by the ancient Chinese and Egyptians as a pigment for the production of inks and paints. The usage of carbon blacks increased enormously since the discovery of the reinforcing effect of carbon blacks on to natura! rubber around Nowadays the rubber industry accounts for approximately 90% of the total carbon black sales, of which the major portion goes to the automotive industry. Another important application of carbon black nowadays is to produce electrically conductive and permanently antistatic polymer composites [5, 6]. For the production of electrical conductive polymer composites carbon black particles are mixed into an isolating polymer melt. Above a critica! carbon black concentration, the percolation threshold, the particles form a continuous network through the polymer matrix resulting in electrical conductivity. Theoretica! estimations of the percolation threshold predict a critica! volume fraction of 0.16, using a model of solid spherical particles randornly dispersed within a matrix [4]. Experimentally, however, in some systems the percolation threshold is found to be very low, as low as about 0.01 vol% [2, 3]. This low percolation threshold is of large importance in the industry. The problem is the production of a low percolation threshold. The percolation threshold of carbon black particles in a polymer matrix is a complex function of several parameters. The most important parameters are: Carbon black properties Polymer type, e.g. viscosity and surface tension Processing method, e.g. dispersion method, mixing speed/ time The aim of the DPI (Dutch Polymer Institute) project (# 294) is to explain when, why and how the carbon black network forms, as well as to explain the low value of the percolation threshold. The focus of this study is on characterization of the properties, in particular the pore size distri bution and the surface groups, of two grades of electrically conducting carbon black powders. The secondary goal is the use of relative new techniques to determine these characteristics. The following section will go deeper into the manufacturing, the different grades and the microstructure of carbon black powders. 5

10 Chapter 1 Introduetion 1.2 Carbon black Manufacture The manufacturing process, the origin of the feedstock, and the storage of the carbon black determine the specific properties of a carbon black grade. At present, nearly all carbon blacks, including the carbon black powders used in this study, are made from oil by the furnace process. In this process a flame is generated, usually from natural gas and air, and a liquid feedstock, generally a residual oil from petroleum refining, is injected into the flame. In the flame the carbon black particles are produced. For further details one is referred to the book of Donnet et all. [ 6]. Different grades Different grades of carbon black are distinguished by the following typical properties: size of the primary partiel es, size of the aggregates shape of the aggregates porosity specific surface area surface groups I contarninations specific resistivity In table 1-1 literature data about the specific surface area, measured with N 2 adsorption, and the diameter of the partiele itself of a selection of carbon black powders are shown. Also the applications for the typical blacks are mentioned. In general the carbon blacks applied in electrical conductive polymer materials are characterized by high specific surface area, small primary particles, high porosity, surface contarnination and high aggregate structure[5, 6]. Table 1-1 Characteristics of some carbon black powders, important in this study. Cabon black manufacturer BET Partiele diameter Application [mz/g] [om] Printex 30 Degussa Printing inks Printex XE-2 Degussa Electrical conductive coatings and plastics Vulcao XC-72 Ca bot Industry standard for conductive and anti-static applications Black pears 2000 Ca bot Discharge and anti-static properties in thermoplastics Ketjenblack EC AkzoNobel Electrical conductive 600JD coatings and plastics Microstructure The primary carbon black partiele is the smallest distinguishable particle. lndividual graphitic layers are the basic building blocks of carbon black particles (figure 1-2). In general the graphitic layers are concentric, surface parallel oriented with larger, more ordered groupings near the surface. The primary particles are chernically bonded into aggregates. The layers continue from one partiele to the next within an aggregate. The primary aggregates of different blacks vary in shape and size. The presence 6

11 Chapter 1 Introduetion of more complex shapes creates internal voids within a sample of carbon black powder. These voids may be larger than the voids found in a packing of round spheres. From the SEM pictures (Scanning Electron Microscopy, figure 1-3) it can be seen that the Printex aggregates are more homogeneously dispersed than the Ketjenblack aggregates. The Ketjenblack aggregates are more lumped and the lumps are separated by voids of about one micron. From the SEM pictures it can also be seen that the primary particles of Printex are larger than that of Ketjenblack (table 2-9, section 2.4). In the TEM pictures (Transrnission Electron Microscopy, figure 1-1) primary particles are shown. The pictures show that the graphitic layering structure is different: In Printex the graphite layers are concentric and in Ketjenblack the layers are more disordered. According to Akzo Nobel (personal communication) the shape of the primary particles of Ketjenblack EC 600JD is like a half hollow ball with a cavity diameter of about 25 nm. The Printex XE-2 particles are solid round particles. '" Figure 1-2 Graphitic layering structure of a carbon black partiele [5, 7]. 7

12 Chapter 1 Introduetion Figure 1-3 SEM picturesof different enlargements of Printex XE-2 (left) and Ketjenblack EC 600 JD. 8

13 Chapter 1 Introduetion 1.3 Overview of the project The aim of this master thesis project is to characterize the surface groups and to determine the pore size distribution of two electrical grade carbon blacks. The sub goal is the use of relative new techniques to determine these characteristics. For this study two different electrical conductive grades, Printex XE-2 form Degussa AG [8] supplied by Océ, and Ketjenblack EC600JD from Akzo Nobel [9] were selected. To determine the pore size distribution in the carbon black powders a new method, thermoporosimetry, was employed. In this metbod Differential Scanning Calorimetry (DSC) was used to measure the depression of the melting point of a solvent in pores relative to the bulk values. In principle the pore size distribution can be determined from these measurements, using the Gibbs Thornson equation. These measurements were compared with N 2 -adsorption measurements. The behaviour of liquid near the carbon black surface was studied with NMR relaxation measurements. Section 2.1 gives an introduetion on porous media and the techniques used to determine the pore size distribution. Section 2.2 treats some theory used to determine the pore size distribution with the techniques mentioned above. The experimental procedure and the discussion on the results are treated insection 2.3 and 2.4 respectively. The analytica} techniques Fourier Transfarm Infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS) were employed to determine the functional groups on the surface of the carbon black particles I aggregates. Low energy ion scattering (LEIS) was used to determine elements present in the outer layers of the particles. In section 3.1 an introduetion is given on surface groups on carbon black powders and the different techniques that were used. In section 3.2 and 3.3 the way of measuring is explained and the results are discussed. In Chapter 4 the conclusions of this study are summarized and Chapter 5 gives recommendations on thermoporosimetry measurements and to gain more insight in the surface differences of the two carbon black powders. The appendix contains the treatment of the techniques used in surface group determination. Here also some other additional information is given. 9

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15 Chapter 2 Pore Size Distribution in Carbon Black Powders Chapter 2 Pore Size Distribution Powders ID Carbon Black 2.1 Introduetion A porous material consists of a solid matrix and empty pores. If the length scale of the sample is large compared to the typical pare size, and the pores are distributed reasonably uniformly throughout the material, then the material is called a porous material Pores have a variety of shapes, they twist, turn, broaden, narrow and have rough walls and other distorted shapes. Pores can be isolated and interconnected and they are categorized into three size ranges: pores larger than 50 nm are termed macropores, pores between 2 nm and 50 nm mesopores, and pores smaller than 2 nm are termed micropores. In carbon black powders the open porosity, i.e. the pores that are accessible from the outside, can be in the farm of small pores of the order of nanometers. These pores are present on the surface of the partiele and have an undefined shape. They may be connected to intemal voids. When the intemal voids are nat accessible to the surface, they represent isolated porosity. Intemal porosity may occur as a result of oxidative hollowing-out of the cores of individual particles of the primary aggregates [6]. These pores are accessible to small gas molecules and their surface will show up in surface area measurements. The measured surface area also depends on the number of primary aggregates present per unit weight. Besides the open and intemal pores in the particles, the carbon black powder contains voids within and between the aggregates, formed by the particles. The size of these voids is in the range of the size of meso- and macropores. There are various methods to determine the surface area, the pare size and the pore-size distribution of a material The methods all have different measurement principles and some result in different values. The methods used in this study are nitrogen adsorption, thermoporosimetry and nuclear magnetic resonance relaxometry. Nitrogen adsorption, a standard technique, has been used as a reference method. This technique uses the volume of molecules adsorbed as a monolayer on the solid's surface to calculate the specific surface area. It can also be used to determine the pare size distribution. Secondly, thermoporosimetry has been used in this study. It was confirmed by many authors [13-16] to be a suitable technique to determine the pare size distribution. Thermoporosimetry uses the melting point depression of a solvent confined in a pare todetermine the pare size distribution. 11

16 Chapter 2 Pore Size Distribution in Carbon Black Powders Thermoporosimetry is a metbod that is cheap and easily implemented and will be interesting for industrial applications. However, up to now most studies in thermoporosimetry were carried out calorimetrically on standard materials. Here this technique is used to deterrnine the pore size distri bution on carbon black powders, containing a variety of pore types and sizes. The tbird metbod used in tbis study tbat can be used to deterrnine tbe pore sizes is relaxometry witb nuclear magnetic resonance. Tbis metbod uses tbe random motions of tbe molecules of a liquid in tbe pores by studying relaxation times of tbe protons of tbe liquid. R. Valekenborg [22] used NMR relaxometry in deterrnining tbe pore sizes of technologkal poreus materials. However, we used tbis technique to study tbe interaction of liquid witb the carbon black surface. In section 2.2 descriptions of tbe used metbods are given. Tbe experimental procedure and sample information is given in section 2.3. Tbe results of tbe measurements are reported in sec ti on

17 Chapter 2 Pore Size Distribution in Carbon Black Powders 2.2 Analytical Methods - Theory Nitrogen Adsorption Nitrogen sorption is the standard metbod for determining the pore-size distribution and surface area of porous materials with pores smaller than 100 nm. The first step in the procedure is to investigate an adsorption or desorption isotherm. This is a measure of the molar quantity of gas taken up or released, at a constant temperature (77 K) by a solid surface as a function of the equilibrium pressure in the gas phase. In the isotherm the quantity of gas adsorbed is expressed as its volume at standard conditions of temperature and pressure (0 C, 1 atm), while the pressure is expressedas a relative pressure which is the actual gas pressure at equilibrium divided by the vapor pressure of the adsorbing gas at the liquid nitrogen temperature (77 K). To obtain these adsorption I desorption isotherms the following steps are taken. First the free space volume V is determined. This is done by measuring the quantity of helium gas in the sample-holder at varying pressure and at constant liquid nitrogen temperature. Helium is used in this process because it is not adsorbed in discernible quantities by most samples and because it behaves as anideal gas [19]. When the free space has been determined, the helium is evacuated from the sample. Then nitrogen gas is added into the sample holder from a manifold of which volume, temperature and pressure are known. The quantity of gas, which is moved from the manifold to the sample bolder, is determined when the equilibrium in the gas phase is established. This amount minus the quantity of helium at the same pressure gives the amount of adsorbed N 2 -gas. The amount of nitrogen is corrected by a nonideality factor to give an accurate quantity of nitrogen F==========;; ,.. Ketjenblack EC 600JD adsorption 1600 Ketjenblack EC 600JD desorption til.a. Printex XE-2 adsorption 1400., Printex XE-2 desorption 1200 : : ~.",":.:....,.. " "... l 1..a..a.. c;; l(xxj.,... E ~ 800 > =--~ 200 =~ ~-.,.,.a..a P/P 0 Figure 2-1 Adsorption I desorption hysteresis of nitrogen in Printex XE-2 and Ketjenblack EC 600JD. 13

18 Chapter 2 Pore Size Distribution in Carbon Black Powders Adsorbing molecules that are close to the solid's walls in pores experience attractive forces that lead to the beginning of gas condensation at a lower pressure, compared to bulk nitrogen. Molecules interact first with the most energetic regions of the solid surface and then with the less energetic regions. The isotherms of Printex XE-2 and Ketjenblack EC 600JD are shown in figure 2-1. In a microporeus material, where the walls are very close to each other, attractive forces make sure that the adsorption starts in the micropores. The smaller the pores are, the quicker the pores are filled. In an adsorption isotherm this is shown by an almost vertical curve at low relative pressure. A more horizontal region fellows this, where a surface layer is formed. As can be seen in figure 2-1, the adsorption and desorption curves show hysteresis in part of the measured domain. This is where bulk condensation begins to occur. An isotherm containing a hysteresis loop is typical of mesopores and rnacropores. The hysteresis occurs because the adsorbing process, where kinetics is involved, is different with the desorbing process, which is a pure thermodynamica} proces. Surface area The surface area can be deterrnined from the volume of molecules that are adsorbed as a monolayer on the surface of the solid. The specific surface area [m 2 /g] can be calculated by multiplying the number of adsorbed molecules by the cross-sectional area of the adsorbent (for N * 10 2 nm 2 ) and dividing by the actual weight of the sample in grarns. Because it is rarely the case that adsorption is limited to a monolayer, as assumed in the Langmuir model [18, 19], Brunnauer, Emmett, and Teller developed a model that takes into account multilayer adsorption. This model is known as the BET model, and yields accurate results for surface area deterrnination. lt is a widely accepted metbod for deterrnining the surface area of porous materials including carbon black powders surface area deterrnination via multipoint gas adsorption [6]. It has to be taken into account that in this model the surface area measured is the sum of the surfaces of the pores and the outside surface of the particles. Pore size distribution The micropores are assumed to be cylindrical or slit shaped. The latter because of the graphite like structures in the carbon black powders. For calculating the pore size distri bution of the micropores, the model of Horvath-Kawazoe [18, 19] is applied. This is a model of calculating the mieropere size distribution of slit-shaped pores in molecular-sieve carbon from the adsorption isotherm. Saito and Foley extended the Horvath-Kawazoe approach to cylindrical pores [18, 19]. To calculate the mean pore size distribution the meso and macropores in the carbon black samples are assumed to be cylinders with infinite lengths. Forthese calculations the model of Barrett, Joyner and Halenda [20] is the most commonly used. lt is valid in the range of nm. This BJH-model is based on the Kelvin equation (appendix A). It is applicable for the adsorption branch as well as the desorption branch. In general the desorption branch is used. The model considers that all the pores are filled at a relative pressure of lt is also assumed that a layer of adsorbed molecules is stripped off a bit at a time with each pressure step. The relationship between the thickness of the layer and the relative pressure is defined by the thickness equation. In the software, used in our measurements, the thickness is calculated by the Halsey thickness equation (appendix A). More details on the modeland equation can be found in the hooks of P.A Webband C. Orr [19] and T. Allen [18]. Lamond [21] discusses differences in t-values for different absorbate I adsorbent combinations. However, when the t-curve is being used for pore size distribution these errors are small enough to be neglected [18]. 14

19 Chapter 2 Pore Size Distribution in Carbon Black Powders Thermoporosimetry The melting point depression of liquids confined in the pores of a porous material can be used to characterize the pare size distribution. The lowered melting temperature of a crystal in the pores is ascribed to a reduced crystal size in the pare and the large surface to volume ratio of the crystals. The formation of small crystals was first described theoretically by Gibbs [ref. in 13] and results from the effect of surface curvature on the equilibrium state of a pure substance. Thomson derived a related theory for the effect of surface curvature on the vapor pressure of liquid droplets. When this theory is applied to a small crystal, it gives the same equation as that of Gibbs. Descrihing the melting behavior it is assumed that the crystal surface is completely wetted by the liquid. The Gibbs - Thomson equation for the temperature shift of melting in confined cylindrical shaped geometries is: (2.1) where!ltm is the melting-point depression, r;: and Tm (d) are the bulk melting temperature and the melting temperature of a crystal with diameter d, respectively, ac1 is the corresponding crystalliquid interfacial energy of the solvent, Pc is the density of the crystal, and fl.jl';. is the bulk enthalpy of melting. lt is assumed that ac1 is isotropie and that the crystal size is sufficiently large for the confined liquid to retain its bulk physical properties w; and Pc The assumptions made are discussed in [13]. Liquid - Solid interaction As most parameters in equation (2.1) are constants, it can he rewritten as: fl.t = ~ m d (2.2) When the liquid is confined in a porous material, however, the interfacial energy ac1 may he influenced by the porous material [13, 14]. In this case the value of k depends on the properties of the prohing liquid and on the interaction with the carbon black surface. The total melting enthalpy also depends on the influence of the porous material on the liquid (appendix B). Determining ll.t m The phase transition from solid to liquid is norrnally purely determined by thermodynamics, but in the phase transition from liquid to solid nucleation will he involved, which is a kinetic phenomena. This is the reason for using the melting peak instead of the crystallization peak depression in the pare size distribution calculations. The melting point depression is measured by means of Differential Scanning Calorimetry (DSC). The measuring system in power compensation DSC, which we used, consists of two microfumaces of the same type, each of which contains a temperature sensor and a heating resistor. One of the ovens contains a pan with the sample and the other an empty reference pan. During heating up or cooling down, the same heating power is supplied to bath micro fumaces. This is controlled via a circuit in order to change their mean temperature in accordance with a preset heating rate. When deviation 15

20 Chapter 2 Pore Size Distribution in Carbon Black Powders occurs, for example as a result of a sample reaction or a phase transition, a temperature difference results between the micro-fumaces. The temperature difference is the measurement signal and at the same time the input signal of a second control circuit, which tries to compensate the reaction heat flow rate by proportional control by increasing or decreasing an additional heating power. The compensating heating power is proportional to the remaining temperature difference, whereas the time integral over the compensating heating power is proportional to the heat, which was consumed or released in the sample. The total compensating energy is equal to the heat of trans i ti on, in this case of melting. :-a-d I - I I I heaters I I I I I I I ~ ~ ~ ~ computer to monitor temperature T and regulate heat flow lq Figure 2-2 Schematic overview of the Differential Scanning Calorimetry apparatus. The reaction heat flow rates are rapidly compensated by an electrical heating power so only small temperature differences occur between the fumaces of sample and reference sample. In the crystal size determination, however, an accurate melting temperature is needed and the measured distribution peak has to be "desmeared" first. How to do this is described in the hook of G.Höhne [11] Nuclear Magnetic Resonance Relaxometry NMR Relaxometry uses the random motions of the molecules of a liquid in the pores by studying relaxation times of the protons of the liquid. This analytica! technique was chosen to locate the cyclohexane molecules in the undersaturated carbon black powders. Basic principles [23] Every partiele with a nuclear spin contains a magnetic moment fl. All the spin moments of an ensemble of nuclei add up to a macroscopie magnetization vector M. When a static magnetic field B 0 is applied, which is usually taken aligned with the z-axis, the magnetic moment of the particles will start to precess around this field with a frequency equal to the Larmor frequency k The Larmor frequency is proportional to the field strength, with a proportionality constant y/21t, where y is called the gyromagnetic ratio (figure 2-3). (2.3) When a radio frequent (RF) field B 1 is applied (for example along the x-axis), with the RF frequency equal to the Larmor frequency resonance is achieved. The magnetic moments of the nuclei are reoriented i.e. to the y-axis (figure 2-4). Then, after the RF field is tumed off, the magnetization will relax back to the original magnetization (aligned with the z-axis). The decreasein magnetization along an axis can be measured as function of time. 16

21 Chapter 2 Pore Size Distribution in Carbon Black Powders Two relaxation mechanisms can be defined, characterized by the longitudinal relaxation time T 1 and the transverse relaxation time T 2 The longitudinal relaxation time is the time the spins need to exchange energy with the surrounding "lattice" and is therefore also called the spin-lattice relaxation time. The transverse relaxation time is the time in which spins dephase due to interactions with their neighbouring molecules of the liquid. It describes the return to equilibrium of the transverse magnetization. This is called the spin-spin relaxation time. Magnetic relaxation occurs when a nucleus is subjected to a magnetic disturbance that has an oscillating component at the Larmor frequency. One such disturbance is the fluctuation of local magnetic fields arising from the random motion of neighbouring nuclei in the bulk. The transverse relaxation time T 2,bulk for bulk solvent is on the order of seconds. The molecules in a pore will move randornly due to Brownian motion, characterized by the self-diffusion coefficient. Diffusion gives a molecule the opportunity to collide with nearby solid surfaces, and each callision of a molecule with the surface has a finite probability to relax. The presence of a solid surface increases the rate of relaxation by reducing the frequency of molecular motion. The relaxation time near the surface is decreased to a value on the order of milliseconds. z Figure 2-3 Larmor precession of a nuclear magnetic moment in a magnetic field [22]. x Liquid - Solid interaction The measured relaxation time T 2.meas can he used to determine the surface-to-volume ratio [22, 17] by: 1 1 A --= --+-P2,suif T2,meas T2,bulk V (2.4) V the volume and A the surface area of the pore. T 2,bulk is the transverse relaxation time of the protons in the bulk liquid and P2.suif is the surface relaxivity. This equation applies when molecules in the bulk exchange fast with molecules near the surface of the solid. The surface to volume ratio A/V of aporous material can only be determined when the pore space is fully saturated with the wetting phase fluid. Because the bulk relaxation time is much larger than the surface relaxation time, the first term at the right hand si de of equation (2.4) can be neglected. 17

22 Chapter 2 Pore Size Distribution in Carbon Black Powders The surface relaxivity is a constant, which depends on the strength of the interactions between fluid nuclear spins and the solid surfaces they encounter. It is specific for each combination of solid and fluid. Relaxivities can be experimentally determined by camparing NMR relaxation measurements to surface-to-volume measurements made by other techniques on standard samples [17]. For standard materials with uniform pare sizes the surface relaxivity cao be determined in such a way. lt is much more difficult to determine Pz.s for natura} materials. Just as in carbon blacks pare sizes cao vary by orders of magnitude. In this study NMR relaxation measurements were done with under saturated carbon black powders. lf liquid is removed, the transverse relaxation time is typically shifted to shorter times, caused by the fact that the volume of fluid in a pare is decreased while the solid surface area remains the same. Finely the transverse relaxation time at the surface will be measured. This surface relaxation time rnay be related to the surface relaxivity in one way or the other. The relaxation mechanisms of a liquid in a porous material depend on the nuclear spin in the molecules that are fixed at the surface. These nuclear spins cao relax the spins in the fluid molecules for instanee by dipole-dipole interactions. As the magnetic moments of electron spins are 10 3 times larger than those of nuclear spins and therefore tend to dominate nuclear relaxation, paramagnetic i ons at the solid surface cao have a large effect on the relaxation of pare fluids. So the surface relaxivity depends on the type of molecules and groups present at the surface. The relaxation mechanism cao also be applied to lattice defects possessing an odd number of electroos and the resulting electron spin cao relax nuclear spin of adsorbed fluid molecules. The surface relaxivity also depends on the structure I roughness of the surface of the solid, in this case of the carbon black powder. Different geometries leadtonon-uniform magnetic fields. Especially sharp corners and wedges will give large magnetic field gradients [ 17]. Determining T2 When it is assumed that only the interconnected open pores are probed and that molecules are able to sample all pare sizes in the pare space befare relaxing, then the relaxation is single exponential with the decay rate. The time constant T 2, is now direct related to the surface-to-volume ratio A/V of the pare: (2.5) Mr is the transverse magnetization and t is the time. However, depending on the porous material multi-exponential decay is observed and a spread of transverse relaxation times is measured. This spread indicates a pare size distribution. 18

23 Chapter 2 Pore Size Distribution in Carbon Black Powders z' -... '... )... / :;,... "..// M x I. y' Figure 2-4 Rotation of the magnetization, induced by a {3 pulse [22]. The transverse relaxation measurement consists of a series of pulses. The pulse is applied in such a way that the magnetization is rotated over an angle rt/2 from the B 0 direction. (2.6) Here, {3 is the angle over which the magnetic moments of the nuclei are rotated, /L the Larmor frequency and IRF is the period of time in which the RF pulse is applied. The resulting magnetization as function of time is called a Free Induction Decay (FID). The magnetic field in a poreus material can deviate largely from the magnetic field applied externally. The magnetic field inhomogeneities are large, which dominates the decay of the signa! (figure 2-5). The decay of the FID signa! is thus much faster than exp ( -t!t 2 ), which makes it difficult to deduce T 2 Therefore in this case the Hahn spin-echo pulse sequence is applied. When the Free Induction Decay signa! has decayed significantly a series of 1t pulses is applied that flip the phase of all individual spins and will result in a rephasing effect where the transverse components of all individual spins are aligned and give a maximum signa! intensity, which is called spin echo. The transverse magnetization is monitored by measuring the amplitudes of the Hahn spin echoes during this sequence. The characteristic decay time for echo amplitude equals the transverse relaxation time. 90\ RF FJD spin-echo signa! time Figure 2-5 Hahn pulse sequence and resulting spin. 19

24 Chapter 2 Pore Size Distribution in Carbon Black Powders 2.3 Experimental Procedure Materials and Instrumentation Materials In table 2-1 and 2-2 the characteristics of solid and liquid materials, important in this project, are reported. The width of the bulk melting peak must be as small as possible to make accurate calculations on the pore size distribution. The DSC curves of the solvents 1,2 dichloro-ethane and ethylene-glycol were not as good as the curves of cyclohexane and tetrachloroethylene. This is the reason that measurements with those solvents havenotbeen continued yet. Table 2-1 Overview of the properties of the solid materials used in this study. Material Pore size [nm] Printex XE-2 Ketjenblack EC - 600JD Graphite EG15 Silica-gel Partiele size [nm] Company (Manufacturer) Océ (Degussa) Akzo Nobel DSM (Nucleosil ) Remarks A networkof cyclindrical pores is assumed Table 2-2. Overview of the liquids used. The important values reported by the manufacturer and in literature are given. Liquid p* Tm* Merck Eurolab A.G. [glml] [K] Cyclohexane Tetrachloroethylene ,2 dichloroethane Ethyleneglycol CRC Handhook of chemistry and physics [1] Mlm* [J/g] Purity % ~99.9 ~99. 5 ~99.9 ~99.0 Manufacturer LiChrosolv Uvasol Uvasol Merck Samples and instrumentation The samples and the instrumentation used are reported in the tables below. To avoid water in the carbon black and graphite sampleswedried them. Presence of water can, however, not be completely ruled out due to contact with environmental air during transportation and sample preparation. The carbon black powders and the graphite were dried in an oven (Heraeus, type T6120, no vacuum) for over 48 hours at a temperature of 80 oe. The silica-gels were not dried before measuring. Nitrogen adsorption The balance used to weight the amount of the samples was from Satorius (BP 2105, max 210 g, resolution g). The samples used in nitrogen adsorption measurements and type of the apparatus, used in these measurements, are reported in table 2-3. Both apparatuses, ASAP 2010 and Tristar 3000, are obtain from Micromeritics Ltd. In the ASAP 2010 from Micromeritics Ltd., the samples can be dried under vacuum in the apparatus itself. In Tristar 3000 no drying procedure was 20

25 Chapter 2 Pore Size Distribution in Carbon Black Powders possible and the samples can contain a small amount of moisture. With this apparatus micropores cannot be determined. To decrease measuring time most samples were measured with the Tristar Table 2-3 The samples used in nitrogen adsorption measurements and the apparatuses used. Sample Material Weightm [g] Apparatus Remarks 1 Printex XE Tristar 3000 In the ASAP 2010 it was 2 Printex XE ASAP2010 possible to dry the samples 3 Ke~enblack EC 600JD Tristar 3000 just before measuring. This 4 Ketjenblack EC 600JD ASAP2010 was done at 80 C, under 5 Graphite EG Tristar 3000 vacuum, for more than 48 6 Silica-gel (5 nm) Tristar 3000 hours. 7 Silica-gel (10 nm) Tristar Silica-gel (30 nm) Tristar 3000 Thermoporosimetry The samples were prepared as follows. First the dried carbon black powder was weighted into the sample pan. Then the liquid was added. The weighing was done on a balans, Sartorius (type 4504 I , MP8-1). The weighing error was ± mg. For the DSC measurements two kinds of sample pans have been used: small high-pressure aluminium pans (8 ~I. Perkin-Elmer # ) and larger aluminium pans (50 ~1. Perkin Elmer B ). In the small ones always 0.5 ± 0.1 mg carbon black powders was put. The larger pans always contained carbon black powder with a weight of 3.5 ± 0.4 mg. The silica-gel I liquid and carbon black I liquid mixtures in the closed pans were kept for at least two hours at room temperature befare measuring. Befare analysis the samples were reweighed to insure that the pans were sealed properly. The measurements have been performed with two differential scanning calorimeters (DSC) measuring units. Both are a Pyris 1 DSC from Perkin Elmer inc. The nitrogen supply is controlled by the Cryofill liquid nitrogen cooling system, Perkin-Elmer CCA 7. The block temperature was kept at the constant temperature of -150 oe. Measurements were done in the range between -100 oe and 15 oe. The DSC 1 measuring unit is property of the group SKT of the department of chemical engineering and chemistry of the Technische Universiteit Eindhoven. The DSC 2 measuring unit is property of the group SPC of the same department Calibration Calibration of the DSC 1 was done with indium, cyclohexane and hexatriacontane by the person responsible for the apparatus. The calibration of the DSC 2 was done with indium and cyclohexane by myself. Here the two transitions (solid-liquid and solid-solid) of cyclohexane were used in this calibration. For both calibrations the heating I cooling rates were 1 K/min. The melting point temperature and enthalpy of melting for each solvent determined by DSC measurements as well as the literature values are given in table 2-5. The melting point of the bulk liquid is determined by taking the onset of the melting peak. The experimental values, agree with the literature values [1], except for tetrachloroethylene in DSC 1. In the calculations invalving the pore melting peak temperature the experimentally measured values of the bulk Tm (K) were used, thus the discrepancy is accounted for. Because of limiting measurement time the melting temperature of the liquid was measured only befare and after the measuring period. 21

26 Chapter 2 Pore Size Distribution in Carbon Black Powders Table 2-4 The samples measured in thermoporosimetry measurements and the DSC used. Sample Material Weight Liquid Weight Vnquld DSC Remarks material liquid lolmaterial [Dl2] [mg] [ml!g] 9 Printex XE cyclohexane Printex XE cyclohexane Bad meas. 11 Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE tetrachloroethylene Printex XE tetrachloroethylene Printex XE tetrachloroethylene Ketjenblack EC 600JD cyclohexane Bad meas. 20 Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD tetrachloroethylene Bulk and confined liquid in plot l q vs. d not distinguishable 23 Ketjenblack EC 600JD tetrachloroethylene Ketjenblack EC 600JD tetrachloroethylene Ketjenblack EC 600JD tetrachloroethylene Silicagel 5 nm cyclohexane Silicagel10 nm cyclohexane Silicagel 30 nm cyclohexane Silicagel 5 nm tetrachloroethylene Bad meas. 30 Silicagel 10 nm tetrachloroethylene Silicagel 30 nm tetrachloroethylene Table 2-5 Measurements by DSC 1 and DSC 2 of the melting temperatures and melting enthalpies of bulk cyclohexane and tetrachloroethylene after calibration. Solvent Cyclohexane Tetrachloroethylene Literature values [1] Tm (K) Experimental (peak) values DSC 1 DSC2 Tm (K) Mlm (J/g) Tm (K) t}.hm (J/g) Nuclear magnetic resonance relaxometry The amount of the dried carbon black powder and of cyclohexane has been weighted on a Satorius balance (MC 210P, max 210 g, resolution g). Here also the amount of liquid per weight of carbon black powder was varied. These mixtures were put in cylindrical cups of glass (10 mi) and the cups were closed with a plastic cover. The measurements were made on the 0.7 T 31 MHz apparatus positioned in the physics department of the Tehnische Universiteit Eindhoven. This apparatus uses a conventional electromagnet, generating a 22

27 Chapter 2 Pore Size Distribution in Carbon Black Powders field of 0.7 T (31 MHz). lt bas the possibility to accommodate cylindrical samples with a diameter of 40mm. Because of a limited measurement time, we chose to measure the transverse relaxation time T 2 with Hahn pulse sequence. The echo spacing was 1.6 ms and 1000 echoes were collected. The time of the 90 degrees RF pulse was 8.5 IJ.S. The time of the 180 degree pulse was 1.6 ms. This last pulse was repeated 1000 times. To measure the whole sample a slice of half a centimeter was selected by applying a gradient in the magnetic field. This magnetic field gradient was 0.16 T/m (8000 Hz/mm). The chemical shift between the protons in cyclohexane and the water cannot be distinguished because the magnetic field inhomogenity was larger than the maximum resolution of 3 khz. Table 2-6 The samples measured in NMR relaxation measurements. Sample Material Weight Liquid Weight Vuquid Remarks material liquid /mmaterlal [g] [g] [ml/g] 32 Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Precipitation of carbon black 3S Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane Printex XE cyclohexane S Printex XE cyclohexane Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane Precipitation of so carbon black Ketjenblack EC 600JD cyclohexane S1 Ketjenblack EC 600JD cyclohexane S2 Ketjenblack EC 600JD cyclohexane S3 Ketjenblack EC 600JD cyclohexane S4 Ketjenblack EC 600JD cyclohexane ss Ketjenblack EC 600JD cyclohexane S6 Ketjenblack EC 600JD cyclohexane S7 Ketjenblack EC 600JD cyclohexane S8 Ketjenblack EC 600JD cyclohexane S9 Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane Ketjenblack EC 600JD cyclohexane

28 Chapter 2 Pore Size Distribution in Carbon Black Powders Measurements Nitrogen adsorption The resulting isotherms of the nitrogen adsorption measurements on the carbon black powders are shown in figure 2-1. From these isotherms the specific surface area and the pore size distri bution can be determined. To calculate the surface area the BET equation [18, 19] is used. To calculate the pore size distribution, invalving meso- and macropores, the BJH-desorption model is used. The BJH-model assumes a layer of nitrogen to stay adsorbed on the carbon black surface during desorption. The interaction of the molecules with the walls is included in the thickness layer. Differences in the t-values for different porous materials may be neglected in pore size determination [18]. In the calculations on the silica-gels and on both carbon black powders the same thickness layer is used. The thickness equation used is the Halsey equation [appendix A, 18, 19]. The pore size of the micropores was calculated with Horvath-Kawazoe model for cylindrical shaped pores ~-= , om' ~\ : c.\ ' ~ I 1\ \, N 2 -adsorption - c- 5nm - 10 nm 30 nm IJ 0 ~~\.. ~~ ~ ~~~~~~~~ ~~ ~ ~~ ~c~ ~ ~~~~ ~ Pore Diameter (nm) Figure 2-6 Pore size distribution of the three silica-gel samples obtained from N2-adsorption measurements. The sizes given in the legend are the nomina! pore size specified by the manufacturer. The specific surface area and the pore size of the silica-gels are also calculated with the BET -model and the BJH-desorption model respectively, [18, 19]. Figure 2-6 shows the pore size distribution ofthe silica-gels (see also table 2-7). The pore sizes of two silica-gels were found to deviate slightly from those specified by the manufacturer, but for the sample with a nomina} pore size of 30 nm a value of 14.8 nm was found (table 2-7). The same deviations were found in earlier experiments [22]. The calculated specific surface areas and the pore sizes of the carbon black powders and the graphite are given in table 2-9, section 2.4. The given errors are the full widths at half maximum (FWHM) of the peaks. 24

29 Chapter 2 Pore Size Distribution in Carbon Black Powders Thermoporosimetry The measurements have been done with cooling and heating rate of 1 K/min to conserve thermodynamical equilibrium as good as possible. Measurements at lower rates would take too much time to perfarm them. Because only the melting peaks are of importance the background of the heating curves was always set to zero. The error in the measured temperature, caused by setting the slope of the background to zero, can be neglected. The melting point depression, as predicted in paragraph 3.2, of the liquids, cyclohexane and tetrachloroethylene, in the different carbon blacks can be seen in figure 2-7 and figure 2-8. To compare, also the melting curves of the pure solvents are shown. The temperature shift is caused by the melting of small crystals. Also a peak broadening is observed, what is indicative of the distri bution in crystal and thus pore sizes as discussed previously by many authors on other porous media such as porous glass, e.g. [13] and [15]. In the DSC curve of Printex with cyclohexane the peak at the right is the bulk peak. This is the melting peak of cyclohexane not confined in the pores but present outside the powder. The DSC curve of Printex with tetrachloroethylene has a small shoulder at the right, which means that the sample is a little bit oversaturated, but not as much as the Printex sample with cyclohexane. The weight of cyclohexane and tetrachloroethylene added to the carbon black in the pans was varied. This was done in such a way that the bulk melting peak of the solvent was eliminated from the melting peak of the crystals ins i de the pores. For further sample in formation see table cyclohexane - Printex XE-2 I cyclohexane ~ < Temperature (K) ~ - tetrachloroethylene. - Printex XE-2/ tetrachloroethylene.. ",, ",, '' ' '.' '.. '' '' ' ' '' ' '' ' ' ~ i: ' ' Temperature (K) Figure 2-7 Melting point depression of cyclohexane in Printex. Figure 2-8 Melting point depression of tetrachloroethylene in Printex. 25

30 Chapter 2 Pore Size Distribution in Carbon Black Powders k-value determination In this study the k-value of cyclohexane and tetrachloroethylene were determined by correlating Nr adsorption measurements with thermoporosimetry measurements. Silica-gels with known pore sizes (table 2-1) were used as a reference. The k-values of cyclohexane and tetrachloroethylene in silica-gel were calculated with (equation 2.2). The temperature shift!! for both liquids in the silica-gels were measured with thermoporosimetry. The bulk melting temperature Tm (equation 2.2, table 2-5) obtained from bulk measurements was used to determine the temperature shift. The pore size distributions of the silica-gels were obtained from Nradsorption measurements (BJH - model). The results are presented in table 2-8. The k-value of cyclohexane in silica-gels is in the range nm reported by literature [ref. in 22]. For the value of tetrachloroethylene no literature value was found. The thermoporosimetry measurements with cyclohexane did not measure a signa! from the silica-gel with a nomina! pore size of 5 nm. Due to the lower bulk!lh.m of cyclohexane compared to tetrachloroethylene, the cyclohexane measurements are less sensitive, causing the signa! to be undetected. Another explanation may be that the pores are too small to form a crystal with cyclohexane, but large enough to form a crystal with tetrachloroethylene. Table 2-7 Overview of the mean pore size reported from the manufacturer and measured by Nz-adsorption for the three silica-gels. The indicated error values correspond to the full width at half maximum of the pore size distribution curves. Manufacturer [nm] Nitrogen adsorption [nm] 7.4± ± ± 13.2 Table 2-8 Overview of the k-values of cyclohexane and tetrachloroethylene reported in literature and deduced from thermoporosimetry measurements on silica-gels. Solvent cyclohexane tetrachloroethylene k[km] (1.95 ± 0.14)"10" 7 (0.96 ± 0.14)"10" 7 Literature [22] Pore sizes [nm] (2.0 ± 0.3)"

31 Chapter 2 Pore Size Distribution in Carbon Black Powders Nuclear magnetic resonance relaxometry In relaxation measurements on pure carbon black samples no significant signa! was found. In figures 2-9 and 2-10 the NMR signa! intensity is plotted logarithrnically as a function of time for bath bulk cyclohexane and cyclohexane confined in bath Printex and Ketjenblack. In these measurements the amount of liquid per weight of carbon black powder was also varied. The legend gives the ratio Il1carbonblackN cyclohexane in g/ml. The relaxation times of the confined cyclohexane are much smaller than the relaxation time of the bulk cyclohexane, what was expected in pores. Single exponential behaviour would show up as a straight line in this plot. The relaxation behaviour of the bulk liquid is single exponential. The relaxation of the confined cyclohexane in Ketjenblack and in Printex is nat single exponential. This indicates a distribution in relaxation time T 2, which could be converted into a pare size distribution. This would take some more research and is nat done in this study. This, however, was done by R. Valekenborg [22]. I()()()() afrr' 0.09Jiml l.isj!ml cyclohexane O.Degfml 0.09gfml 1.57glml ;;; $ ;; ~ 100 i l time(s) ;;; $ -;; = ~ ', 10 -l-~,...,,...,~--.-~-,.--~-,-~,..--.~,...;--t time (s) Figure 2-9 NMR signa! as a function of spin-echo time, determined for bulk cyclohexane and cyclohexane present in Printex XE-2. The noise level is around 500 a.u. Figure 2-10 NMR signa! as a function of spin-echo time, determined for bulk cyclohexane and cyclohexane present in Ketjenblack EC 600JD. The noise level is around 1000 a.u. 27

32 Chapter 2 Pore Size Distribution in Carbon Black Powders 2.4 Results I Discussion The k-values of the two solvents used in thermoporosimetry were determined with the standard silicagel samples with well defined pore diameter (section 2.3.2). Thesek-values were used to calculate the pore size distribution for the two carbon black powders using equation (2.2). However, in section it was mentioned already that the k-value depends on the liquid and on the interaction of the liquid molecules with the porous material This should be taken in mind. For comparison, the pore size distribution was also determined using N 2 -adsorption. Table 2-9 gives the specific surface area measured with nitrogen adsorption and the pore size distribution determined with the adsorption measurements and with thermoporosimetry for the carbon blacks. The error, given in the pore size distribution, is full width at half maximum (FWHM) of the distribution. The specific surface areas found are in the same order of magnitude as the ones reported in literature [8, 9]. With nitrogen adsorption measurements smaller pore sizes can be measured than with thermoporosimetry. The calculations of the diameter of these smaller pores were done with the Horvath Kawazoe model [18, 19] that assumes cylindrical shaped pores. The errors in these smallest pores are the measurement uncertainties. The smallest pore sizes are around 0.75 nm. This size is camparabie with the distance between two graphite layers when one in between them was oxidized away. The fourth type of pore has a size around 1 nm. Both are very small pores, which can be reached by nitrogen (d- 0.2 nm). They are likely to be too small to contain crystals of the solventand hence are not determined by thermoporosimetry with cyclohexane and tetrachloroethylene molecules (d nm). In tigure 2-11 the pore size distributions in Printex and Ketjenblack with varying amounts of cyclohexane are plotted against the heat flow rate lq [mw]. These pore size distributions were calculated as follows: For every point at the temperature axis, the temperature shift of the confined liquid was calculated by subtracting the bulk melting temperature (table 2-5). With equation 2.2 and the k-value of the liquid confined in silica-gel the pore size distribution was calculated. Figure 2-12 shows the pore sizes determined with varying amounts of tetrachloroethylene. The same procedure was applied, using the k-value of tetrachloroethylene. The black solid lines in both figures are the curves of the just over- or just under-saturated samples. Due to "smearing" of the melting peak by the apparatus this is not exactly the right way to determine the pore size distribution. In [11] it is explained how to do this correctly. The shape of the bulk melting peak depends on several parameters. It depends on the heating rate, the mass of the sample, impurities in the sample and on the temperature differences between sample and heating probe. This difference requires a certain time interval. So the temperature and the timing of processes in the sample are not exactly reproduced but are distorted. Ho wever, the error in the pore size distri bution ( < 10%) can be neglected, because the used k-value may be incorrect. By decreasing the amount of liquid per unit weight carbon black, VsolventiiDcarbonbiack. the measured mean pore size (figure 2-11 and 2-12) decreases. The decreasein crystal size suggests that the liquid prefers to wet the inside surface of the pores with smaller diameters first. This is caused by capillary farces. The maximum of the peaks shifts to lower pore sizes, hence, the peak broadening is caused by a distribution in pore sizes. Figure 2-13 shows the pore size distributions of Printex and Ketjenblack respectively, determined by thermoporosimetry and by Nradsorption. The samples that are just over- or just under-saturated are 28

33 Chapter 2 Pore Size Distribution in Carbon Black Powders claimed to give the pore size distribution in carbon blacks. The integrals of the peaks are normalized. This is a real pore size distribution when indeed the right k-value is used and the peaks are "desmeared" [11]. The pore size distributions of the largest pores of Printex XE-2 and Ketjenblack EC 600JD, found by thermoporosimetry, are 72 with a full width at half maximum (FWHM) of 48 nm and 36 with a FWHM of 38 nm respectively (table 2-10). When these results are compared with the pore size distribution determined with Nradsorption measurements, we see a large difference for Printex (figure 2-13). lt is remarkable that both cyclohexane and tetrachloroethylene measurements result in the same pore size per black. An explanation can be that the chemical interaction of the different liquids with carbon black is more or less the same. The large difference between the N 2 -adsorption and thermoporosimetry measurements may depend on different interaction of the liquids with the different surface structure of both carbon blacks. The interfacial energy O'c1 and thus the k-value must be different for the liquids in the blacks. Another explanation concerns the assumptions (e.g. cylcindrical shaped pores) made in the BJH-model that uses the Kelvin equation (section 2.2.1, appendix A). These assumptions may be incorrect in the materials used in this study. As can beseen in figure 2-12 and 2-13 for both Printex and Ketjenblack a shoulder is present at the left of the bigger peak. By decreasing the amount of liquid this shoulder will appear as a sole peak (figure 2-12). It seems that in both carbon black powders two types of pores can be measured with thermoporosimetry. The signa} coming from these smaller pores is only measured in the samples of the carbon black powders when tetrachloroethylene is used as prohing liquid. This peak does not show on the cyclohexane measurements, what can be explained by the larger melting enthalpy of tetrachloroethylene, what makes the tetrachloroethylene measurements more sensitive. Another explanation may be if one assumes that a larger layer of molecules of cyclohexane stays unfrozen than of tetrachloroethylene. This will be discussed in the next section. The nitrogen adsorption measurements also show two types of pores in the two carbon black powders in this range. However, more measurements have to be done to besure that these peaks indeed come from smaller pores. The larger pore sizes are camparabie with the size of voids within aggregates, the smaller ones may be open porosity within the carbon black particles, In the introduetion it was mentioned that the shape of Ketjenblack particles should be a half hollow ball. Because no distinguishable pore size peak around nm is found, there is no indication that a substantial part of Ketjenblack primary particles should have this typical shape. Bes i des the decrease in crystal size in decreasing the volume to mass ratio, V tiquicv'lllcarnonblack. also the total (bulk and pore) enthalpy of melting decreases (figure 2-14). Below this phenomenon will be discussed in more detail. 29

34 Chapter 2 Pore Size Distribution in Carbon Black Powders Table 2-9 Overview of specific surface area, specific pore volume and diameter of Printex XE-2, Ketjenblack EC 600JD and Graphite EG15, reported by the manufacturer and deduced by N2"adsorption and t h ermoporos1metry. Carbon black BET Specific pore volume Diameter primary partiele Specific Surface area [ml/g] [nm] [ml/g] Incl. Man u- Nr Thermo- SEM Manufacturer Micropores facturer adsorption porosimetry Printex XE ± ± ± Ketjenblakc EC 1415 ± ± ± JD Graphite EG ± ± Table 2-10 Overview of the pore size, including the distribution, deduced by N2"adsorption and thermoporosimetry. The two smallest pore sizes for both blacks are given with an error of measurement. The errors given in the pore size distributions (two larger pores) are given with FWHM. Carbon black Pore size Equation [nm] N radsorption Thermoporosimetr y Printex XE ± 0.03 Horvath-Kawazoe [18, 19] for 1.0 ± 0.3 cylindrical pore shape 718FWHMI7 I 6 FWHM BJH with the thickness equation of FWHM 72 48FWHM Halsey [appendix A] Ketjenblack EC 0.70 ±0.03 Horvath-Kawazoe for cylindrical pore 600JD 1.1± 0.3 shape 5 14 FWHM FWHM BJH with the thickness equation of 35 36FWHM 36 38FWHM Halsey 30

35 Chapter 2 Pore Size Distribution in Carbon Black Powders , Printex XE-2/ cyclohexane ~ 06..., Cystal Size (nm) Ketjenblack EC 600 JD I cyclohexane ~ g , Crystal Size (nm) Figure 2-11 Crystal sizes in Printex XE-2 and Ketjenblack EC 600JD respectively, measured by thermoporosimetry with varying amounts of cyclohexane. The legend gives the sample numbers (table 2-4). 31

36 Chapter 2 Pore Size Distribution in Carbon Black Powders 0.8 Printex XE-2 I tetrachloroethylene ~, ~ Crystal Size (run) 6 Ketjenblack I tetrachloroethylene i i i i i i i'', 'I ' i / / / i,.,.,. / /..., ,.- "., ""' Crystal Size (nm) Figure 2-12 Crystal sizes in Printex XE-2 and Ketjenblack EC 600JD respectively, measured by thermoporosimetry with varying amounts of tetrachloroethyleneo The legend gives the sample numbers (table 2-4)o 32

37 Chapter 2 Pare Size Distribution in Carbon Black Powders Printex XE-2 O.D20 --cyclohexane tetrachloroehty1ene 16 ~---.. N 2 -adsorption I ~' :0 i''\ il:' rr~ Î \ I ~ i ' ~~ \ ' ' ' ' ' Pore Diameter d (run) Ketjenblack EC 600JD O.D20 --cyclohexane tetrachloroethylene 23 - N 2 adsorption : il:' #::...T"T"T...,""T"T""r'T"T"~~:;::;:;:::;:;::;:;:;:;:;:;::;::;::;:::r:;:;=rr'i"T=;;=;=;=;=;=;o.,..l Pore Diameter (nm) Figure 2-13 Pore size distribution ofprintex XE-2 (up) and Ketjenblack EC 600JD (down) deterrnined by N 2 -adsorption measurements and by thermoporosimetry. In the legend sample numbers are given (table 2-4). 33

38 Chapter 2 Pore Size Distribution in Carbon Black Powders Decrease in melting enthalpy Mlm In figure 2-14 the melting enthalpy Mlm of the crystallized liquid is plotted against the volume of liquid per unit weight of carbon black powders. The dasbed curves are the bulk melting enthalpies of cyclohexane (Mlm = 31 J/g) and of tetrachloroethylene (Mlm = 65 J/g). The samples that are oversaturated approach these values ofthe melting enthalpy. By decreasing the volume ofliquid that is added to the carbon black, the measured melting enthalpy is decreased. Although there is some error in the calculated pare melting enthalpy, especially in the melting peaks of the smallest crystals, the error is not large enough to account for the observed reduction in the melting enthalpy. An explanation of the large decrease can be found when assurning that liquid molecules, adsorbed at the surface or present in the smallest pores of the carbon black powders stay unfrozen. When the integration of the total heat flow rate (lq) is divided by the weight of the amount of liquid (mliquict), a total melting enthalpy Ml 101 at lower than the bulk melting enthalpy is found. This can be expressed in an equation as follows: f Jq -~--=M-/total = ~m mliauid 1 + IE;_ me Here Mlm is the enthalpy of bulk melting, mne the weight of solvent that is not crystallized and me the weight of solvent that is crystallized. The ratio of non-crystallized and crystallized solvent is dimensionless so its volume can substitute the weight of the liquid. (2.7) 70~ ~ Printex I cyclohexane JO Ä Printex I tetrachloroethylene Ketjenblack /tetrachloroethylene Ketjenbleek I cyclohexane 0~~~~~~~~~~~~~~~~~~ 0 9 JO IJ 12 Figure 2-14 dhm measurements for the different Vsolven!ITlcarbonblack ratios of cyclohexane and tetrachloroethylene in Printex and Ketjenblack. The dashed lines are the measured bulk melting enthalpies. The straight curves are the fits of equation (2.7). 34

39 Chapter 2 Pare Size Distribution in Carbon Black Powders The experiments are well described assuming that part of the liquid, probably a (mono-) layer at the surface, does not crystallize. The model can be improved by including the contribution of the interfacial energy. This factor decreases the melting enthalpy with decreasing crystal diameter (equation B.2, appendix B). The fit for Printex with tetrachloroethylene is notaccurate because there are only three measurement points. In general the amount of liquid that stays unfrozen is larger for Ketjenblack than for Printex. This is in agreement with the larger specific surface area of Ketjenblack (table 2-9). Extrapolation of the curves to tlllm = 0 gives the amount of solvent that is not crystallized. When the contribution of the interfacial energy is included this can give the surface area, if it is assumed that unfrozen volume forms a monolayer. The volume of liquid that stays unfrozen is larger for cyclohexane than for tetrachloroethylene. If the layer is smaller for tetrachloroethylene than with cyclohexane smaller pores can be measured with tetrachloroethylene than with cyclohexane. NMR results on the carbon blacks Although the curves of confined cyclohexane in carbon black (figure 2-9 and 2-10) were found not to be strictly single exponential, Tz.meas is determined by fitting the relaxation curves, with a single exponential decay curve. The multi-exponential decay is taken into account by averaging the smallest and largest slope that can be determined from one curve. The spread is displayed in the error bars. In figure 2-15, 1/Tz,meas is plotted against the ratio mcarbonbtac/vcyclohexane since 1/Tz.meas is a measure of the pore sizes: 1 A -T-- = V Pz,surf 2,meas (2.4) Here A is the surface area and V is th volume of a pore. In our case the samples were undersaturated and no pore size distribution could be determined. It is shown again, that pores are filled due to capillary forces. The less amount of liquid per weight of carbon black powder the smaller is the relaxation time, the smaller are the pores that are filled with the liquid, the smaller is the adsorbed layer at the surface. It seems that when adding less liquid T 2,meas approaches a certain value: the relaxation time at the surface. The surface relaxation time, which somehow depends on the surface relaxivity (section 2.2.3), is larger for Ketjenblack than for Printex. From this observation it can be concluded that the surface relaxivity is different for cyclohexane in the two carbon black powders. The cyclohexane molecules move slower near the surface of Printex than near the surface of Ketjenblack. A possible explanation will be the difference in interfacial energy CJ'c1 and thus a difference in the k-value of cyclohexane in the two blacks. This could explain the difference in crystal sizes of Printex and Ketjenblack measured by thermoporosimetry, where the k-value was used to calculate the crystal diameter with the measured temperature difference. Different amounts and different types of surface groups containing a dipole moment can cause the difference in relaxation. The presence of paramagnetic molecules and lattice defects can also be responsible for the difference in surface relaxivity [ 17]. From TEM pictures, shown in the introduetion of this thesis, it is clear that the layering structure of the graphite layers is different in the two carbon blacks. In Printex the graphite layers are concentric and in Ketjenblack the layers are more disordered, which suggests a different surface structure, which 35