BISMUTH AND LEAD BASED GLASSES

Transcription

1 BISMUTH AND LEAD BASED GLASSES CHAPTER III This chapter deals with discussion on heavy metal oxide glasses. Glass systems based on borate and silicate containing oxides of Bi and Pb are discussed. Further a brief review from the literature on the optical, DSC and ultrasonic properties for bismuth and lead based glasses are given. 3.1 Importance of Bismuth and Lead Glasses Heavy metal oxide glasses (HMOG) can be defined as those glasses which contain more than 50 mol% of bismuth or lead cations. Heavy metal oxide glasses are generally characterized by their properties of high density, high refractive index, low transformation temperature and good transparency in the infrared region up to 8µm (Dumbaugh and Lapp 1992). Heavy metal oxide glasses are studied for their excellent infrared transmission properties as compared with the conventional glasses (Baccaro et al. 2002). Recently the heavy metal oxide glasses (HMOG) containing oxides of Bi and Pb have been proposed as suitable candidates for the applications in the field of scintillation detectors and medical imagers. Bi and Pb are placed next to each other on the periodic table. Atomic number of lead is 82 and of bismuth the atomic number is 83. Both Bi 3+ and Pb 2+ ions have the same 6s 2 electronic configurations. Therefore, a number of similarities have been confirmed in various properties. In toxicity, however, Bi is much safer than Pb. For these reasons, Bi-based glasses have been expected as substitutes for the Pb-containing glasses, and the novel glass systems have been explored in glass industries. 3.2 Glasses containing oxide of Lead Lead containing glasses find a lot of applications in the field of solder glasses, table wares and optical lenses due to their high refractive index and low melting point. The glasses based on oxides of lead have a high percentage of lead oxide (at least 20% of the batch). The surface of lead glasses is relatively softer, and refractive index is high thus giving a brilliance that may be exploited by cutting. Due to soft surface of lead glasses they are easy to decorate by grinding, cutting and engraving. These glasses are more 27

2 expansive than soda-lime glass and are favoured for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made up of lead-alkali glasses also known as lead glasses. These glasses do not withstand high temperatures or sudden change in temperature. Glasses with much higher lead content are used as radiation shielding processes because of their ability to absorb gamma rays and other forms of harmful radiations. 3.3 Lead Silicate Glass Lead Silicate glasses can be prepared at low temperature generally less than 1000 o C. It is also observed that lead silicate glasses are stable even when PbO content is more than 65 mol%. Lead Silicate glasses as compared to the silicate glasses containing modifier oxides of groups I and II of the Mendeleev periodic system of elements have a wider region of glass formation.(see table 3.1). This is determined by the high polarization capacity of the valence non-coupled electrons 6p 2 in the Pb 2+ ion, which are capable of forming directed, predominantly covalent bonds, for instance, with O 2 ions contained in [SiO 4 ] tetrahedrons, which are the main structural elements of the glass forming frame in silicate glasses. The high ploarizability of Pb 2+ ions has considerable effect on the glass forming capability of lead oxide ions. The high polarizability of Pb 2+ ions results in asymmetric coordination of oxygen atoms around a lead atom which leads to the chain structure of lead glasses when the three dimensional silicon-oxygen network is destroyed. Different authors (Bessada et al. 1994, Imaoka et al. 1986, Mydlar et al. 1970, Piriou and Arashi 1980, Rabinovich 1976) have deduced different structural units which consist of SiO 4 tetrahedara and PbO 4 pyramids. Authors Bessada et al have reported that at a high SiO 2 content, the SiO 4 are linked and form three dimensional structure to which PbO 4 units are connected. At high PbO content, the PbO 4 are linked and form PbO 4 polymeric chains to which SiO 4 units are connected. It is suggested by several researchers that lead is a glass former as well as a glass modifier. The behavior of the lead oxide as a glass modifier is not clearly detected. The chemical environment or bonding state for the lead atoms is different depending upon its role as network former or as network modifier. It is known that Pb 2+ ions in lead silicate glasses (as distinct from silicate glasses containing only modifying cations of groups I and II of the periodic system) are essentially not modifying cations in their traditional meaning, since their introduction to 28

3 Table 3.1. Various silicate glass systems along with their upper boundary of glass forming region. System Upper Boundary of glass-forming region, R x O molar content, % Li 2 O-SiO Na 2 O-SiO K 2 O-SiO Rb 2 O-SiO MgO-SiO CaO-SiO SrO-SiO BaO-SiO PbO-SiO

4 the glass composition does not lead to the formation of non-bridge oxygen ions, but is accompanied by the formation of pseudo-bridge bonds O Pb O between the [SiO 4 ] tetrahedrons in the glass structural lattice. The data in reported by several authors (Gotz et al. 1976, Imaoka et al. 1986, Leventhal and Bray 1965, Mydlar et al. 1970, Smets and Lommen 1982, Shakhmin and Tyutikov 1990) suggest the possibility of the existence of such pseudo-bridge bonds. The probability of this type of structural lattice in lead silicate glasses is further corroborated by the data on the structure of lead silicates formed in the PbO SiO 2 system (Toropov et al. 1969). The existence of the following compounds in this system has been established: PbO.SiO 2, 3PbO.2SiO 2, 2PbO.SiO 2, and 4PbO.SiO 2 (Toropov et al. 1969). The simplified schemes of possible coupling of [SiO 4 ] tetrahedrons in the specified lead silicates constructed according to the data in Penkalya 1974, Vinchell and Vinchell 1967, neglecting the angles at which the tetrahedrons are oriented toward each other in actual structures, are represented in Fig Lead metasilicate (alamosite, PbO.SiO 2 ) correlates with the structural formula 6Pb 1/2 [SiO 3 ] 3. This silicate belongs to the group of silicates with ring silicon-oxygen anions of finite dimensions formed by metasilicate groups [SiO 3 ] 2. The anion groups have a two-tier structure: the anion groups arranged in different planes are cross-linked by pseudo-bridge bonds O Pb O created by Pb 2+ ions 4 6, whereas the anion groups located in the same plane are joined by pseudobridge bonds formed by Pb 2+ ions 1 3 (Fig. 3.1 a ). Trilead disilicate (barisilite, 3PbO.2SiO 2 ) correlated with the structural formula 6Pb 1/2 [Si 2 O 7 ]. The anion groups of this silicate formed by diortho groups [Si 2 O 7 ] 6 have the double-tier structure as well. Cross-linking of anion groups positioned in different planes is implemented through pseudobridge bonds O Pb O created by Pb 2+ ions 5 and 6, and anion groups located in the same plane are joined by pseudobridge bonds formed by Pb 2+ ions 1 4 (Fig. 3.1 b ). Lead orthosilicate 2PbO.SiO 2 correlated with the structural formula 4Pb 1/2 [SiO 4 ]. The two-tier anion groups of this silicate formed by ortho groups [SiO 4 ] 4. Cross-linking of anion groups positioned in different planes is implemented through pseudo-bridge bonds O Pb O created by Pb 2+ ions 4, and anion groups located in the same plane are joined by pseudo-bridge bonds formed by Pb 2+ ions 1 3 (Fig. 3.1 c ). Tetralead silicate 4PbO.SiO 2 correlates with the structural formula Pb 4 [SiO 4 ]O 4/2. The two-tier structural patterns of this silicate formed by ortho groups 30

5 [SiO 4 ] 4 which are cross-linked to the adjacent ortho groups by pseudo-bridge bonds Pb O Pb created by Pb 2+ ions 1 4 (Fig. 3.1 d ). The above schemes of the structural patterns of various lead silicates do not claim absolute certainty and are illustrative; however, they make it possible to demonstrate that the structural lattice of lead silicate glasses can have a high degree of cohesion even at R *(depolymerization degree of the structural lattice of glass) > 1, which ensures the possibility of producing lead silicate glasses with the molar content of PbO exceeding 66%. 3.4 Lead Borate Glass Lead oxide is supposed to be capable of behaving as a glass former in suitable circumstances. At low content of PbO its bond is ionic, while the formation of BO 4 groups proceeds at the rate of two tetrahedral for each added oxygen. FTIR studies in lead borate glasses have confirmed the existence of ionic and covalent PbO bonds. Tawansi et al found that at low PbO content the formation of four coordinated borons proceeds the rate of formation of two tetrahedral for each added oxygen. At low PbO content, the lead is assumed to enter the glass as modifier Pb 2+ ions. The ability of Pb 2+ to share as glass former Pb 4+ increase with the higher PbO content. In the binary alkali metal borate systems, a transition into three-dimensional cross-linking takes place with high alkali metal content through the coordination shift (BO 3 ) (BO 4 ). The tetrahedral BO 4 groups are strongly bonded than the triangular BO 3 groups and hence a compact structure is expected leading to a higher density. The borate glasses containing lead has the high density. In borate glasses containing low proportion of lead oxide, it would be expected that some of the lead ions will exist as PbO 4 groups, while the remaining lead ions can exist as bridges between BO 3 and BO 4 groups or enclosed in the interstices in the glass structure as Pb 2+ ions. Raman spectroscopic studies by authors (Lorosch et al. 1984, Witke 1995, Zahra et al )on binary and ternary lead borate glasses have revealed that PbO may get incorporated into the network in four-coordinated positions. Several properties of molybdophosphate and tungstophosphate glasses can be explained by incorporation of PbO in the network (Selvaraj and Rao 1988). Ganguly and Rao 1999 investigated the structural role of PbO in Li 2 O-PbO-B 2 O 3 glasses.acoording to authors the various structural units present in the studied system is shown in table

6 (a) 1 2 (b) (c) (d) Fig.3.1 Simplified patterns of possible coupling of [SiO 4 ] tetrahedrons in the structure of various lead silicates; Pb 2+ ( ); O 2- ( ); a) metasilicate; b ) trilead disilicate; c) orthosilicate; d ) tetralead silicates (Si 4+ ions located in tetrahedron centers are not indicated in the figure). 32

7 Table3.2 Different Structural Units Present in Li 2 O-PbO-B 2 O 3 Glasses along with their notations (Ganguly and Rao 1999). Sr. No. Structural Unit Notation 1 [BO 3/2 ] 0 B [BO 2/2 O] - B 2-3 [BO 1/2 O 2 ] - B [BO 4/2 ] - B 4-5 [PbO 2/2 ] 0 P [PbO 4/2 ] 2- P

8 Shelby 1982 examined xpbo (1 - x)b 2 O 3 glasses containing up to 0.67 mol % of PbO, using thermal expansion and viscosity measurements. The existence of a phase separation region up to mol % of PbO was confirmed: two glass transition temperatures, Tg, are observed corresponding to the B 2 O 3 -rich phase (270 C) and to the 0.2PbO glass (445 C). The DTA curves of Tawansi et al still exhibited two glass transition temperatures (Tg) at mol % of PbO = 0.2. The highest Tg is observed at about 0.28 (Shelby 1982) which is a composition with the most negative enthalpy of glass formation (Shartsis and Newman 1948). The highest elastic constants and hardnesses are, however, observed for compositions around 0.4 mol % of PbO (Shinkai 1983). 3.5 Glasses containing oxides of Bismuth It is well known that Bi 2 O 3 does not vitrify without additives, but the addition of other oxides enables its vitrification. It has been reported that Bi ions in oxide glasses have 5- or 6-fold coordination state (Miyaji et al. 1994). Bi ions in trivalent state possess lone pair electrons in the outer most 6s 2 shell. It has been accepted that the 6s 2 lone pair electrons are localized on Bi ions and they do not partake in Bi O bonds. It has been also accepted that BiO n polyhedra were distorted due to the lone pair electrons, and the distortion in BiO n units would make the structural analyses for the Bi-containing glasses difficult. According to radial distribution analyses, Watanabe et al revealed that Bi 2 O 3 Li 2 O glass had a similar structure to a Bi 2 O 4 crystal. In Bi 2 O 4 crystal, Bi ions are in a mixed valence state of 3 + and 5 +, and in the Bi 2 O 3 Li 2 O glass, however, it was confirmed that Bi ions were all in a trivalent state and large amount of defects, such as positive electron holes on oxide ions were present in the glass. It was expected that Bi ions played a role as a network former, NWF in the Bi 2 O 3 Li 2 O glass. While coordination number of Bi ions has been reported in the combination of some typical NWFs such as B 2 O 3 and P 2 O 5 (Baia et al. 2002, Chowdari and Rong 1996, Shaim et al. 2002), structural roles of Bi ions have not been clarified. 3.6 Borate glasses containing oxides of Bi 34

9 The role of Bi 3+ in glasses may be compared with that of Pb 2+ ions, due to similarity of Bi 3+ and Pb 2+ ions, in atomic weight, ionic radius and electronic configuration and in possessing extensive glass forming regions. Similar to the lead borate system it has relatively low liquidus temperature. Relatively low melting point of the compound is a consequence of the high polarizability of the ion. It has been pointed out by Heynes and Rawson 1957 that a compound in which the ratio of the bond strength to the melting point is high is more likely to form a glass. Strong short range interaction in the bismuth borate glasses results from the triple charged Bismuth ion. They are highly polarizable and cations may exist in the glass network in [BiO 3 ] pyramidal units in the presence of conventional glass forming cations like Boron. The shift of the 1380 cm 1 absorption band in the IR spectra to lower frequencies with increasing bismuth oxide shows the perturbation of the B-O-B linkage by the Bismuth ion by increasing the interaction of the network oxygen with Bismuth (Hirayama and Subbarao 1962). Bi 2 O 3 does not form glass by itself. The melt becomes transparent at C, but on cooling it fully crystallizes. It has been observed that if Bi 2 O 3 is melted in silica crucible, it can dissolve enough silica to form a stable glass, therefore it can be assumed that in this case it behaves as a network former (Heynes and Rawson 1957). However, bismuth oxide forms glasses with known glass formers as SiO 2, B 2 O 3, P 2 O 5, GeO 2, but not with TiO 2 or TaO 2. Most of the crystals in the Bi2O3-B2O3 system are congruently melting, at the respective stoichiometric composition, therefore they can be grown from stoichiometric melts. Borate glassy melts are characterized by high viscosity and a steep negative slope of its temperature dependence (Liebertz 1983). Therefore, it is expected that the rate of nucleation and crystal growth processes to be very small. The reduced glass transition temperature T gr = T g /T m, can give us an indication about the glass ability to nucleate homogeneously or heterogeneously. Becker 2003 have determined experimentally the glass transition temperature, T g, liquidus temperature T liq. and the crystallization onset, T x, for bismuth borate glasses. These values are plotted in Fig.3.2. The variation of the reduced glass transition temperature with bismuth oxide content in the glass is also shown in fig The high values of T gr > 0.72, indicate that only heterogeneously nucleation (on surfaces or nucleating agents) will take place in these glasses. 35

10 T ( o C) Bismuth oxide (mole fraction) Fig. 3.2 Glass transition temperature(t g, ), liquidus temperature(t liq, ) and crystallization onset(t x, ) for bismuth borate glasses (Becker 2003). 36

11 Tg/Tliq Bismuth oxide (mole fraction) Fig. 3.3 Reduced glass transition temperature (Tgr = Tg/Tliq.) variation with bismuth borate glass composition calculated using data from (Becker 2003). 37

12 3.7 Silicate glasses containing oxides of Bi Bismuth silicate glasses find a lot of industrial and special applications as materials for low loss fiber optics, IR transmitting materials or as active media of Raman optical amplifiers (Dimitriev et al. 1995, Nassau et al. 1987). Another interesting property of these glasses is that in the course of reduction in the hydrogen atmosphere they develop an electronic conducting layer at their surface. In Bi 2 O 3 SiO 2 glasses the Bi ions are found to be present in the trivalent state. X-ray radial distribution and 29 Si NMR studies have shown the similarities in local structure around bismuth and oxide ions ( Hayakawa et al. 2000). The structural similarities have been observed between Bi 2 O 3 SiO 2 glasses corresponding to 50mol% of bismuth oxide and 50 mol% of silicon dioxide and Bi 2 SiO 5 crystal. In bismuth silicate glasses containing 50mol% of bismuth oxide and 50 mol% of silicon dioxide the Bi ions occupies BiO 6 sites, and the BiO 6 units are surrounded by eight BiO 6 units sharing their edges. The oxygen sites are classified into three groups, Si O Si, Si O Bi and Bi O Bi, whose respective populations were estimated as 20%, 40% and 40% in bismuth silicate glasses corresponding to 50 mol% of bismuth oxide and 50 mol% of silicon oxide. The Bi O Bi bridges form layers, the Si O Si bridges form chains of Q2 units, and the bismuthate layers are joined with the silicate chains. In case of bismuth silicate glasses the Bi O Bi networks exist over the whole glass forming region, on the other hand in case of lead silicate glasses the Pb O Pb networks exist only in the PbO-rich region. The population of Bi O Bi bridges is higher than that of Pb O Pb bridges, indicating that Bi ions prefer to act as a network former than Pb ions. In Bi 2 O 3 SiO 2 system, however, Bi O Bi networks alone could not form glass, and Si O Si networks were indispensable for the glass formation. 3.8 Review of optical, DSC and ultrasonic properties of Bi and Pb based glasses The UV absorption edge of vitreous borate has been reported to be 186 nm by Kordes 1959 and 172 nm by Mc et. al Optically the short wavelength cut off shifts towards the red with increasing heavy metal oxide content (Stehle et al. 1998). The transmission window lies between 430 and 3850 nm for the high density lead bismuth borate glass system ( Kirck and Martin 1992). This change is linked to structural 38

13 modification of the network resulting from the addition of excess oxygen and the subsequent destruction of compact cross linked four coordinated and dipentaborate units. It has been reported that the addition of Bi 2 O 3 trasforms the structure of B 2 O 3 from boroxol groups to the formation of BO 4 tetrahedra which is present mainly as tetraborate and diborate groups ( Becker 2003). These grous can be charactrised by the presence of an infrared absorption band at 940 cm -1 and a shoulder at 1000 cm -1 (El Batal et.al. 2002).Vibrational spectra of heavy metal oxide glasses by authors Ford and Holland 1987 show that infrared absorption is dominated by the lighter elements in the glass system, whereas in the far infrared region a strong band occurs at about 100 to 190 cm -1 and this band shifts to higher wave numbers with increasing PbO content of glass (Harder et al. 1995). Heavy metal oxide glasses possesses good transparency in the IR region up to 10-15µm (Pan and Ghosh 2000) and infrared cut off shifts to longer wavelength with increase in bismuth to lead ratio ( Kirck and Martin 1992).. The infrared measurement by Capelletti et al show that the addition of Bi 2 O 3 to Ge 2 O 3 causes a shift and splitting into two components of band associated with the stretching mode occurring in the GeO 4 tetrahadra. Sidek et. al studied the optical properties of lead bismuth phosphate glasses over the composition range given in table 3.3. The optical absorption spectra of the glass samples are shown in Fig.3.4. From the investigations carried on lead bismuth phosphate glass systems the authors concluded glassy state of the samples due to absence of any sharp absorption edges in the optical absorption spectra. With the increase in the content of glass modifier, PbO and Bi 2 O 3, the position of the fundamental absorption edge shifts to longer wavelengths. The shift in the absorption edge of the lead bismuth phosphate glasses was found to be lesser than lead phosphate glass system. It was suggested by the authors Sidek et. al that the introduction of Pb 2+ ion into the phosphate network in the form of PbO disrupts two bridging oxygens, or alternatively creates two non-bridging oxygens (NBOs). On the other hand, the introduction of Bi 3+ ion into the phosphate network in the form of Bi 2 O 3 probably disrupts the activity on creating the NBOs. Thus, creation of smaller number of NBOs seems to be the reason for absorption edge shifting towards shorter wavelengths with an increase in Bi 2 O 3 content. El-Adawy and Moustafa 1999 studied the elastic properties of some bismuth borate glasses in the composition 39

14 Table 3.3. Glass composition, and optical band gap for lead-bismuth phosphate glass system (Sidek et al. 2005). Glass Sample Weight % PbO Bi 2 O 3 P 2 O 5 E opt (ev) A A A A B B B B C C C C

15 Absorbance (arbitrary units) Absorbance (arbitrary units) Absorbance (arbitrary units) a) b) c) 2 1 A1 A2 A3 A Wavelength,λ(nm) B1 B2 B3 B Wavelength,λ(nm) 2 1 C1 C2 C3 C Wavelength,λ(nm) Fig. 3.4 Optical absorbance of (a) glass sample A1-A4, (b) glass sample B1-B4, (c) glass sample C1-C4 (Sidek et al. 2005). 41

16 range for bismuth varying from 5 mol% to 45 mol %. From their studied authors concluded (i) The properties like density, molar mass, molar volume, longitudinal and shear ultrasonic velocities, elastic moduli,, Poisson s ratio, and longitudinal and shear internal elastic energies, shows the change in behaviour at around 25 mol% Bi 2 O 3. (ii) The density increases with an increase in mol% Bi 2 O 3 due change in crosslink density, greater mol wt. of the bismuth atom and the coordination numbers of Bi 3+ ions. (iii) The longitudinal and shear wave velocities and the elastic moduli of the bismuth borate glass system increase with increasing Bi 2 O 3 composition, up to 25 mol%, and then decreases as Bi 2 O 3 composition is increased further. (iv) The variations of the elastic internal energies of longitudinal and shear strains per unit mole of the glasses show that at compositions less than 35 and 30 mol% Bi 2 O 3, the network former B 2 O 3 is soft and easily deformed by stress while the modifier Bi 2 O 3 resists deformation more strongly. At 35 and 30 mol% Bi 2 O 3, the network former and the modifier equally resist deformation. At compositions of more than 35 and 30 mol% Bi 2 O 3, the network former becomes hard and resists deformation more strongly than the modifier. (v) The variation of Poisson s ratio with composition is exactly the reverse of the elastic moduli variation. It decreases with increasing Bi 2 O 3 composition up to 25 mol%, and then increases with increasing Bi 2 O 3 composition over 25 mol%. Sudarsan et. al.2002 studied the effect of replacement of PbO by borate in the lead borosilicate glasses (PbO) 0.5 x (SiO 2 )0.5(B 2 O 3 ) x glasses with 0.0 x 0.4 and the effect of replacement of SiO 2 by borate in the (PbO)0.5(SiO 2 ) 0.5 y (B 2 O 3 ) y glasses with 0.0 y 0.5 system. From the glass transition temperature measurements authors concluded that replacement of PbO by borate results in the formation of Q 4 structural units of Si along with Si-O-B linkage. Furter authors concluded that the initial increase in the tetrahedral boron structural units and its subsequent decrease with increasing boron concentration arise due to change in behaviour of PbO from network former to network modifier. 42

17 Watanabe et. al investigated the 20PbO-xBi 2 O 3 -(80-x)B 2 O 3 glass system for the hardness and elastic properties. The glass transition temperature decreased rapidly with the increase in Bi 2 O 3 content at the expense of B 2 O 3. The low values of Tg indicated that the strength of Bi-O chemical bond in PbO-Bi 2 O 3 -B 2 O 3 glasses is considerably weaker than B-O bonds. Chowdari and Rong 1996 studied the effect of Bi 2 O 3, on the glass transition temperature, electrical conductivity and structure of LiBO 2, glasses. From the T g vs. composition curve authors obtained three different linear regions with an overall decrease in T g, with the increase in Bi 2 O 3 content. The slope of these three straight lines is in a decreasing order. These results were interpreted in terms of the increase in the number of non-bridging oxygen atoms, substitution of Bi-O bond in place of B-O bond and change in Li + ion concentration. Avramov et al studied the glass transition temperature of borate and silicate glasses.according to authors the glass transition temperature for lead silicate glasses decreases with increase in mole fracton of PbO. Fig. 3.5 illustrates the dependence of the glass transition temperature of borosilicate glass-enamels on lead composition. The two points on the right are for pure lead-borates with the corresponding Pb content. It is seen that in the low PbO content range the glass transition temperature drops sharply. On the other hand in the high lead concentration region Tg changes slowly and the presence of Na ions has no effect on it. This can be explained on the basis that in the low concentration range a part of Pb 2+ ions tend to break the BO 3 triangles. At high concentration range, lead ions tends to associate with the four-fold coordination [BO 4 ] - groups, thus behaving in the same way as sodium cations. At the highest concentration range the glass transition temperature increases again. This is a particular case of invert glasses where a new continuous network of NM cations is created. 43

18 740 Glass transition temperature, Tg(K) Lead oxide (mole fraction) Fig. 3.5 Dependence of the glass transition temperature of borosilicate glassenamels on the mole fraction of lead oxide (Avramov et al. 2005). 44