CHAPTER-2 HEAVY METAL OXIDE GLASSES CONTAINING BISMUTH AND LEAD There is an increasing interest in the scientific community regarding heavy metal oxide (HMO) glasses containing bismuth and lead oxide. Since lead and bismuth has high atomic weight, these are placed together in the periodic table. The atomic number of lead is 82 and bismuth is 83. The electronic configuration of both elements is same for s orbit i.e. 6s 2. Therefore, they have many similar properties. HMO glasses have high density, low transformation temperature, high refractive index, large thermal expansion, good gamma-ray shielding properties and transparency to the visible light. HMO glasses also have good chemical durability and excellent infrared transmission properties as compared to other glasses. 2.1 Heavy metal oxide glasses containing lead Lead based glasses are used for several applications. For example, these glasses can be used for industrial applications such as optical glass, crystal glasses, glass ceramics and low melting glasses. These glasses have high refractive index and high density. The excellent property of lead oxide glass is in terms of phase diagram which indicate that it is possible to make a glass containing 90 mol% of lead oxide. These glasses are softer in nature. Therefore, cutting, grinding and polishing is easy. Lead oxide glasses also have good chemical durability. 2.1.1 Lead borate glasses Lead oxide can behave as network former as well as network modifier. It acts as glass former, at higher values of PbO and behaves as network modifier at lower concentration of PbO. The PbO-B 2 O 3 glasses have very wide range of glass formation (20-80 mol% PbO). These glasses have high density and excellent transmittance values. The lead 9
borate glasses containing 40-90 mol% PbO was studied by infrared spectroscopy by Brekhoviskikh and Cheremisinov in 1960. All the reported glasses have shown spectra containing bands. These bands are very strong by nature. This indicates that the molecular structure of B 2 O 3 was retained in high lead glasses. These bands produced low softening point and viscosities of the glasses. Later Bray et al. 1963 studied the lead borate glasses by nuclear magnetic resonance (NMR) technique. It was concluded that the boron atoms are situated in BO 3 triangles. At low content of PbO, the Pb-O bond becomes ionic and Pb2 + ions are considered as network modifiers whereas the formation of BO 4 units proceeds at the rate of two tetrahedral for each oxygen atom. The formation rate of tetrahedral is reduced above 20 mol% of PbO because some lead atoms behave as pyramids. These pyramids contain BO 3 units rather than BO 4 units. The change in BO 3 units was discovered at 30 mol% PbO. This was speculated due to change in electronic distribution in B 3 -O units which results from replacement of B 3 -O-B 4 by B 3 -O-Pb bonds. When the content of PbO and B 2 O 3 are same, the fractional content of boron B 4 units attains its maximum value (~0.5). Laird and Bergeron in 1970 studied the structure of lead borate glasses and melts. They stated that BO 3 units were formed as a result of destruction of boron-oxygen network. De Luca and Bergeron in 1971 provided the structure of PbO-2B 2 O 3 glass which contains 1/3 boron atoms in four co-ordination (B 4 O 7 ) -2 states with one Pb 2+ ion linked with each state. Figure 2.1 has shown that each lead atom co-ordinates with four oxygen atoms. The lead borate glasses have higher values of densities (Saini et al. 2009) as shown in figure 2.2. George et al. 1999 suggested that the variations occur in the density values of PbO-B 2 O 3 glasses due to the use of different sample preparation techniques and crucible materials. Binary and ternary lead borate glasses have been studied by Zahra et al. 1993 and Witke et al. 1995 by using Raman spectroscopy technique. It was found that PbO is placed into four co-ordinated positions in the network. Lead borate glasses are known to form homogeneous and single phase glasses in 20-80 mol% PbO. Bray, 1985 has pointed out that the glass transition temperature is highest at 27 mol% PbO. 10
Fig. 2.1: Structure of PbO-2B 2 O 3 glass. 11
Fig. 2.2: Density of lead borate glasses (Saini et al. 2009). 12
2.1.2 Lead silicate glasses Lead silicate glasses have been studied in wide range of composition. The melting temperature of these glasses has been achieved below 1000 0 C. These glasses have many commercial applications in technical areas of electronic multipliers, glass to melt seals, colored TV tubes etc.. These glasses have also excellent physical properties such as good values of micro-hardness as well as electrical and thermal conductivity. The X-ray diffraction of lead silicate glasses was undertaken by Bair in 1936. It was found that Si- O-Pb angle is smaller than 180 0 and bond length of Pb-Si is 3.80A 0. Bair stated that bond angles are separated from each other by silicon oxygen tetrahedral. He suggested that the Pb-Pb distance is variable (4 ~ 6.50A 0 ). It was concluded that the exact value depends upon the composition of PbO. Pb-Pb interaction was neglected by him. Krogh-Moe in 1958 proved the accuracy of Bair s results. PbO-SiO 2 glass system was studied. The random network theory could not describe the structure of lead silicate glasses with high content of lead. Bagdyk yants and Alekseev in 1960 studied the lead silicate glasses by electron diffraction method. It was found that in high silicate glasses, lead atoms are randomly distributed at three dimensional Si-O network and each lead atom is attached to two oxygen atoms. Rabinovich 1976 and Pirious and Arash 1980 have studied the different structural units which have SiO 4 tetrahedra and PbO 4 pyramids. Bessada et al. 1994 has given the structure of PbO-SiO 2 glasses at high content of PbO. He stated that PbO 4 are linked with each other to form polymeric chains. Toropov et al. 1969 has established the results of PbO-SiO 2, 2PbO-SiO 2, 3PbO-2SiO 2 and 4PbO-SiO 2 glass systems. Brosset 1963 has shown the position of Pb-Pb peak which describes that the lead groups have a definite structure. In alkali metal cations, the vitreous lead orthosilicate and high lead glasses are completely stable commercial products. The lead ion has capability to form glass because it has high polarizablity. Bobovich and Tulub in 1958 have provided the Raman spectra of lead silicate glasses. It was concluded that the lead containing glasses gives the excellent intense spectrum. This indicates that Pb-O bond is covalent as per Pauling- electro-negativity scale. This covalency occurs due to strong mutual polarizablity of Pb 2+ and O 2- ions and their participation in glass forming network. The density values increase with increasing mole fraction of lead oxide content as shown in table 2.1 by Singh et al. 2008. 13
2.2 Heavy metal oxide glasses containing bismuth Glasses containing bismuth have special applications in the field of industry such as low loss fibers, infrared transmitting materials and active medium of Raman fiber optical amplifiers and oscillators ( Donald et al. 1978, Dumbaugh 1986, Vogal et al. 1991 and Pan et al. 1994). These glasses can be used for production of electron multipliers after reduction of hydrogen atmosphere. Batal 2007 stated that reducing glasses show very high surface conductivity (up to 8-10 orders of magnitude higher than unmodified glasses). These glasses cannot vitrify individually but addition of other oxides can vitrify them. Miyaji et al. 1994 reported that bismuth ions have five or six fold coordination state. Bismuth ions always play the role of network former. Bismuth containing glasses have high refractive index and chemical durability. The electronic configuration of bismuth is 6s 2. Bishay and Maghrabi 1969 concluded that BiO 3 groups are formed in bismuth glasses. But some authors (Dimitriev et al. 1992, Gattaf 1997 Milankov et al. 2005 and Stehle 1998) claimed that Bi ions can participate as network former and network modifier ( ratio of these two participants depends upon the type of other glass constituents). Milankov et al. 2005 provided the theory that Bi ions are present in the network forming units as BO 6 groups but other authors (Trzeblatowski et al. 2000, Witkowska et al. 2002 and Stentz et al. 2000) stated that BiO 3 are network former groups and BiO 6 are network modifier groups. But some other authors, for example, Egili et al 1998, Dimitriev et al. 2001, Baia et al. 2002 and Culea et al. 2004 suggested that BiO 3 and BiO 6 units are present and their ratio changes with the other oxide glass components. 2.2.1 Bismuth borate glasses Bismuth borate glasses have shown large number of applications due to their many extraordinary properties such as wide range of glass formation, high density and high refractive index. The glass formation range is possible from 20-80 mol% of bismuth 14
Table 2.1: Chemical composition and density of PbO-SiO 2 glass system (Singh et al. 2008). Glass samples Composition (Mole Fraction) Density (g/cm 3 ) (±0.04) PbO SiO 2 G1 0.45 0.55 5.60 G2 0.50 0.50 G3 0.55 0.45 G4 0.60 0.40 G5 0.65 0.35 G6 0.70 0.30 5.99 6.22 6.45 6.65 6.82 15
oxide. Levin and Daniet 1962 gave the phase diagram of bismuth borate glasses. Egoryshwa et al. 2005 have prepared the single crystals of all bismuth borate phases and detected all of them by mid infrared absorption spectroscopy. George et al. 1999 gave the experimental procedure of formation of bismuth borate glasses which contain 88 mol% of bismuth oxide. This process was popularly known as roller quenching method. Glasses which contain high concentration of Bi 2 O 3 have very high density value (~9 gcm -3 ). The glass transition temperature, Tg values found were very high at 23 mol% of bismuth oxide. Bajaj et al. 2009 reported that density and molar volume increases with the increasing concentration of bismuth oxide. These values are provided in table 2.2. Murata and Mouri 2007 have obtained the UV-Visible absorption of bismuth borate glasses which show the optical absorption band around 440nm. Further, Sanz et al. 2006 stated that highest melting temperature of bismuth borate glasses effects the optical properties of these glasses. The optical absorption spectra of bismuth borate glasses is shown in figure 2.3. Becker 2003 has measured the glass transition temperature, liquidus temperature and crystalline onset values of bismuth borate glasses. Liebertz 1983 proved that borate glassy melts are recognized by high viscosity. Therefore, their rate of nucleation and crystal growth have very small values. 2.2.2 Bismuth silicate glasses Bismuth silicate glasses can be prepared by the reaction and melting of the components. The bismuth silicate glasses after reduction in hydrogen atmosphere possess high secondary emission and therefore, they can be used in electron multiple devices. In bismuth silicate glasses, the Bi ions are present in trivalent state. The crystal structure of Bi 2 SiO 5 glass is provided in figure 2.4. The structural similarities in local structure around bismuth and oxide ions have been confirmed by X-ray radial distribution and 29 Si NMR studies. In 50 mol% Bi 2 O 3 and 50 mol% SiO 2 glass, bismuth ions are substituted at BiO 6 sites and BiO 6 units are occupied by eight BiO 6 units, the edges are joined with 16
Table 2.2: Chemical composition, density and molar volume in bismuth borate glasses (Bajaj et al. 2009). Sample No. Composition (mol %) Density (gm /cm 3 ) Molar Volume (cm 3 mol -1 ) Bi 2 O 3 B 2 O 3 1 20 80 4.457±0.002 33.40 2 25 75 4.815±0.002 35.04 3 30 70 5.470±0.003 34.46 4 33 67 5.636±0.003 35.56 5 37.5 62.5 6.006±0.003 36.34 6 40 60 6.246±0.001 36.51 7 41 59 6.359±0.006 36.50 8 42 58 6.389±0.003 36.95 9 47 53 6.737±0.004 37.98 10 50 50 6.874±0.005 38.96 11 53 47 7.074±0.005 39.54 12 55 45 7.214±0.004 39.85 13 60 40 7.550±0.011 40.72 14 66 34 7.765±0.012 42.65 17
18
Fig. 2.4: Crystal structure of Bi 2 SiO 5. 19
each other. The oxygen sites are presented into three groups; Bi-O-Bi, Si-O-Bi and Si-O-Si, whose populations are defined as 20%, 40% and 40% in 50 mol% of bismuth and 50 mol% of silicate glass. Q n distributions represent the amounts of oxide ions present in the structural groups which was obtained from 29 Si-NMR analysis. The Bi-O-Bi bridges form layers; Si-O- Si bridges form chains of Q 2 units and the bismuthate layers are joined with silicate chains. IR and Raman spectra of -Bi 2 O 3 and bismuthate glasses contain the selenite spectra (Witkowska et al. 2005). But Witkowska et al. 2002 stated that no single commonly accepted diagram for bismuth silicate glasses has been reported. Later, Witkowska in 2005 have undertaken extended X-ray absorption fine structure (EXAFS) and molecular dynamics (MD) studies of bismuth silicate glasses with bismuth oxide in the composition range of 3-5 mol %. It was concluded that BiO 5 units are present in all glasses and one or two Bi-O distances are much larger than others within BiO 5 groups. The density and molar volume of bismuth silicate glasses increases with the increase of bismuth oxide content ( Batal 2007). Their values are provided in table 2.3. 20
Table 2.3: Chemical composition, density and molar volume in bismuth silicate glasses (Batal 2007). Sample No. Glass Composition (mol %) Density (gm /cm 3 ) Molar Volume V m (cm 3 mol -1 ) Bi 2 O 3 SiO 2 1 90 10 7.284 58.39 2 85 15 7.210 56.18 3 80 20 7.155 53.77 4 75 25 7.071 51.54 5 70 30 6.921 49.72 6 65 35 6.813 47.56 7 60 40 6.716 45.20 8 55 45 6.657 42.55 21