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Thermal expansion of TiO2SiO2 and TiO2GeO2 glasses
Journal of Non-Crystalline Solids 52 (1982) 357-363 North-Holland Publishing Company
357
T H E R M A L E X P A N S I O N OF T i O 2 - S i O 2 A N D T i O 2 - G e O 2 G L A S S E S K. K A M I Y A and S. S A K K A
Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie, 514, Japan
The thermal expansion coefficients of TiO2-GeO 2 glasses containing up to 12.7 mol% TiO2 were determined and compared with those of TiO2-SiO2 glasses. The TiO2-GeO2 glasses were prepared by a conventional melting technique. The TiO2-SiO2 glasses were prepared by heating the gels formed as a result of hydrolysis and polycondensation of the alkoxide solutions consisting of Ti(OC3HT) 4 and Si(OC2Hs)4. The thermal expansion coefficient of the alkoxy-derivedTiO2SiO2 glasses decreased with increasing TiO2 content similarly to the TiO2-SiO2 glasses prepared by the flame hydrolysis technique. It was found that the addition of TiO2 markedly decreased the thermal expansion coefficient of the GeO2 glass. It was concluded on the basis of the contribution of the TiO2 component to the expansivity of the glasses that the Ti 4+ ions were present in the 4-coordination state with respect to oxygen atoms in both TiO2-GeO2 and TiO2-SiO2 glasses.
1. Introduction T i O 2 - S i O 2 glasses c o n t a i n i n g up to 10 wt% TiO 2 prepared by the flame hydrolysis technique above 1750°C are k n o w n to show ultra-low thermal expansivity [ 1,2]. G e O 2 glass has been revealed to be similar to SiO 2 glass in structure [3], that is, all the G e atoms in G e O 2 glass are 4-coordinated with oxygen atoms. In the present study, TiO 2 was added to G e O 2 glass to form glasses of the b i n a r y T i O 2 - G e O z system. The thermal expansion coefficients of those T i O z - G e O 2 glasses were determined a n d compared with those of the T i O 2 - S i O 2 glasses prepared by heating the gels o b t a i n e d from the metal alkoxides c o n t a i n i n g Ti a n d Si atoms through hydrolysis a n d p o l y c o n d e n s a t i o n . The effect of the TiO 2 c o m p o n e n t on the thermal expansion coefficients of b o t h G e O 2 and SiO 2 glasses is discussed on the basis of the c o o r d i n a t i o n n u m b e r of Ti atoms in the glasses.
2. Experimental 2.1. Preparation of the glasses Quartz type G e O 2 powder of 5-nine purity (supplied by Mitsuwa Chemicals Co.) a n d reagent grade anatase type T i O 2 powder (supplied by Merck Chemicals Co.) were used as starting materials for the T i O 2 - G e O 2 glasses. The 30 g 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
K. Kamiya, S. Sakka / Thermal expansion of glasses
358
Table 1 Compositions and melting temperatures of the TiO 2 - G e O 2 glasses Glass
Temperature
ZiO 2 content
State
(°C) 1 2 3 4 5 6 7
(tool%)
(wt%)
0 3.2 6.4 9.6 12.7 15.9 19.1
0 2.5 5.0 7.5 10.0 12.5 15.0
1550 1550 1550 1500 1500 1500 1500
glass glass glass glass glass crystallized crystallized
batches of oxide mixtures corresponding to the compositions listed in table 1 were melted in platinum crucibles at 1500-1550°C in air for 2 or 3 h. The compositions containing up to 12.7 mol% TiO 2 (samples Nos. 1-5) provided us with the clear glasses. X-ray diffraction revealed no trace of crystalline TiO 2 in those five glasses. For the compositions containing more than 12.7 mol% TiO 2, the small amount of crystalline TiO 2 precipitated during cooling the melts. The melts were too viscous at 1500-1550°C to be poured out of the crucibles to form plate glasses. The melts, therefore, were fabricated into rods the diameters of which were 2 or 3 ram, by immersing a fused silica rod into the melts and pulling it up slowly by hand. The glass rods thus obtained were annealed at temperatures slightly lower than their glass transition temperatures and were used for determining the thermal expansion coefficients. The titanium tetraisopropoxide Ti(OC3HT) 4 and silicon tetraethoxide Si(OC2Hs) 4 (supplied by Wako Chemicals Co.) were used as raw materials in the preparation of the TiO2-SiO 2 glasses. The alkoxide mixtures for the TiO2-SiO 2 system given in table 2 were diluted with ethanol. The mixture of
Table 2 Oxide compositions and amounts of raw materials used for the TiO 2 -SiO 2 glasses Glass
TiO 2 (mol%)
Ti(OC 3 H 7 )4 (g)
Si(OC 2 H 5 )4 (g)
State a
11 12 13 14 15
2.3 3.8 6.2 7.7 11.5
0.65 1.08 1.73 2.16 3.24
20.42 20.02 19.31 18.95 17.88
glass glass glass glass crystallized b
State after heating the gels to 900°C. b Anatase crystal precipitated. a
K. Kamiya, S. Sakka / Thermal expansion of glasses
359
water and glacial acetic acid as a catalyst was added to the alkoxide solution for hydrolysis. The molar ratios of the added water and acetic acid to the alkoxide was 50 and 0.01, respectively. The mixed solutions were kept standing at room temperature (about 20°C) and at 40°C to undergo hydrolysis and polycondensation. The resultant monolithic gels were heated up to 900°C with a heating rate of 6 ° C / h . The gels for the compositions containing up to 7.7 mol% TiO 2 could be converted to clear glasses. In the composition containing 11.5 mol% TiO 2, precipitation of anatase crystal was observed at 900°C. The other details of the preparation procedures have been published elsewhere [4,51. 2.2. Measurement of the thermal expansion coefficient
The thermal expansion was measured using glass bars of about 1.5 cm in length. A dilatometer manufactured by Rigaku Denki Co. was used. A fused silica rod was used as a reference. The average linear thermal expansion coefficient a was determined from the slope of the expansion curve over the temperature range from 25 to 400°C for the TiO2-GeO 2 glasses. The thermal expansion coefficients averaged over the temperature range from 25 to 700°C were measured for the TiO2-SiO 2 glasses. The glass transition and softening temperatures of the TiO2-GeO 2 glasses were read from the inflection points of the thermal expansion curves.
3. Results
The thermal expansion curves of the TiO2-GeO 2 glasses containing up to 12.7 mol% TiO 2 are shown in fig. 1. The linear thermal expansion coefficients, and transition and softening temperatures of those glasses are given in table 3. Both glass transition and softening temperatures can be seen to increase with increasing TiO 2 content with one exception. The thermal expansion coefficient of the GeO 2 glass was 74.8 × I0-7/°C, agreeing well with that reported by Mackenzie [6]. The thermal expansion curves of the alkoxy-derived TiO2-SiO 2 glasses are shown in fig. 2. It can be seen that the glasses show negative expansions relative to the fused silica. Fig. 3 shows the thermal expansion coefficient of the TiO2-GeO 2 glasses plotted as a function of the TiO 2 content. It can be seen that the thermal expansion coefficient of the glass decreases proportionally to the TiO 2 content. The thermal expansion coefficients of the TiO2-SiO 2 glasses are plotted against the TiO 2 content in fig. 4. The open marks in fig. 4 correspond to the glasses prepared from the metal alkoxides through hydrolysis and polycondensation. The two different open marks correspond to the two different hydrolysis temperatures. The thermal expansion data of the TiO2-SiO 2 glasses produced by the flame hydrolysis technique in Corning Glass Works are shown
K. Kamiya, S. Sakka / Thermal expansion of glasses
360
Fused silica -I x=O
"__ez o
32
x
=
m
.
-2
~/////
6.4 K
o',o
.=
-E
o;"
'
26o
'
,;o
Temperature (*CI
'
66o
i
200 400 Temperature (*C)
600
Fig. 1. Thermal expansion A L / L of the xTiO2.(100- x)GeO 2 glasses. Fig. 2. Thermal expansion A L / L of the x TiO 2. ( 1 0 0 - x ) S i O 2 glasses relative to fused silica.
in the same figure by full circles for comparison. The ultra-low thermal expansion is achieved in the alkoxy-derived TiO2-SiO 2 glasses. It can be seen that the thermal expansion coefficient of the TiO2-SiO 2 glasses decreases linearly with increasing TiO 2 content.
70
~
60
0 50
' 5
I0
TIO= content, real%
5
j~
TiOE content (mole%)
Fig, 3. Change of the linear thermal expansion coefficient of the TiO 2 - G e O 2 glasses with the TiO 2 content. Fig. 4. Change of the linear thermal expansion coefficient of the TiO 2 -SiO2 glasses with the TiO 2 content. O , prepared from alkoxides hydrolyzed at 40°C; [:], prepared from alkoxides hydrolyzed at room temperature; O, prepared by flame hydrolysis technique [1 ].
K. Kamiya, S. Sakka / Thermalexpansion of glasses
361
Table 3 The linear thermal expansion coefficient a, glass transition temperature Tgand softening temperature T~of the TiO2-GeO/glasses Glass 1 2 3 4 5
TiO2 (mol%)
a ( × 10- 7/°C)
Tg (°C)
(°C)
0 3.2 6.4 9.6 12.7
74.8 70.8 63.7 58.4 54.9
537 555 552 573 587
577 624 600 647 645
4. Discussion 4.1. The effect of 7702 component on thermal expansion The empirical additive law has been found to be valid between thermal expansion coefficients of the oxide glasses and their compositions. That is, the linear thermal expansion coefficient of a particular oxide glass is expressed by the following equation,
a=~anen,
(l)
where a n is the expansion coefficient factor and Pn is the molar fraction of a particular constituent. The aTio2 was evaluated using eq. (1) as - 9 9 × 1 0 - 7 / ° C for the TiO2-GeO 2 glasses. The expansion coefficient factor of 75 × 10 7 / ° C was assumed for the GeO 2 component. A similar value of - 1 0 4 × 10 7 / ° C for the avio2 in the TiO2-SiO 2 glasses was obtained, using the value of 5 x 1 0 - 7 / ° C for the expansion coefficient factor of the SiO 2 component. The Ti 4 + ions can be present either in the 4-, 5- or 6-coordination state in the Ti 4÷ containing crystalline compounds, although only a few examples have been reported for the 4-coordination state. The coordination number of Ti 4+ ions in the TiO 2 containing oxide glasses has attracted much attention [7]. It has been shown that the Ti n+ ions are also present in the 4- or 6-coordination state in the oxide glasses. It is assumed that the Ti 4"- ions in the different coordination states should make different contributions to the thermal expansion and other physical properties of the glasses [7]. The fact that the contribution of the TiO 2 component to the thermal expansion of the TiO2-GeO 2 glasses is equivalent to that of the TiO2-SiO 2 glasses suggests that the coordination state of Ti 4÷ ions is the same in both of the TiOz-GeO 2 and the TiO2-SiO 2 glasses.
362
K. Kamiya, S. Sakka / Thermal expansion of glasses
4.2. Coordination state of
Ti 4÷
ions
It was shown that the thermal expansion coefficient factor of the TiO 2 component obtained in the present work for the T i O 2 - G e O 2 and TiO2-SiO 2 glasses is considerably smaller than those reported so far for the 6-coordinated Ti 4+ ions. Values of - 3 5 X 1 0 - 7 / ° C and 20 X 1 0 - 7 / ° C have been reported [8]. This indicates that the Ti 4+ ions in the present glasses are in a coordination state other than 6-coordination. The 4-coordination state of the Ti 4+ ions with respect to oxygen atoms in the TiO2-SiO 2 glasses prepared by the flame-hydrolysis technique has been suggested by several authors to elucidate the low thermal expansion and other properties of those glasses. Evans [9] reported that the cristobalite crystal in which some part of the Si 4+ ions are replaced by Ti 4+ ions is precipitated when the TiO2-SiO 2 glass is heated at 1450°C for 2h, suggesting that the Ti 4+ ions occupy the sites equivalent to Si 4+ ions even in the glassy state. This means that the Ti 4+ ions are in a 4-coordination state like Si 4÷ ions to form a three-dimensional network structure. Smith et al. [10] measured the different infrared spectra of the TiO2-SiO 2 glasses using the SiO 2 glass as a reference. They observed the absorption peak at 735 c m - t due to the T i - O stretching vibration in the groups containing 4-coordinated Ti 4+ ions. Recently, Sandstrom et al. [11] analyzed the EXAFS spectra of the Ti 4+ ions in the TiO2-SiO 2 glasses and showed that almost all the Ti 4+ ions were in the 4-coordination state in those glasses. The present authors [12] have also shown that the low thermal expansion coefficients and densities of TiO2-SiO 2 glasses prepared from metal alkoxides could be explained by the presence of the 4-coordinated Ti 4+ ions. The intense infrared absorption peak around 950 cm-1 was interpreted as showing the presence of the 4-coordinated Ti 4+ ions. Some consideration should be given to the coordination state of Ge atoms in the T i O 2 - G e O 2 glasses. It is known that the coordination number of Ge atoms is 4 in GeO 2 glass and the concentration of the 6-coordinated Ge atoms increases on the addition of an alkali to GeO2 glass [13]. It was observed in our measurement of the infrared spectra [14] that the absorption peak ascribed to the G e - O stretching vibration shifts from 875 cm - l to the lower wave numbers when the alkali oxide was added to GeO 2 glass, while such a shift is not observed in the TiO2-GeO 2 glasses. This indicates that the Ge atoms remain 4-coordinated on addition of the TiO2 component to GeO 2 glass. One of the authors (KK) is grateful for the support of the Ohkura Kazuchika Memorial Fund. References [1] P.C. Schultz and H.T. Smyth, Amorphous Materials (John Wiley, New York and London, 1972) p, 453.
K. Kamiya, S. Sakka / Thermal expansion of glasses [2] [3] [4] [5] [6] [7] [8]
363
P.C. Schultz, J. Am. Ceram. Soc. 59 (1976) 214. J. Zarzycki, Verres Refract. 11 (1957) 3. K. Kamiya and S. Sakka, J. Mat. Sci. 15 (1980) 2937. S. Sakka and K. Kamiya, J. Non-Crystalline Solids, 42 (1980) 403. J.D. Mackenzie, J. Am. Ceram. Soc. 42 (1959) 310. Bh.V.J. Rao, Phys. Chem. Glasses, 4 (1963) 22. S. Naruse, Glass Handbook, eds., S. Sakka, T. Sakaino, K. Takahashi (Asakura Shoten, Tokyo, 1975) p. 696. [9] D.L. Evans, J. Am. Ceram. Soc. 53 (1970) 418. [10] C.F. Smith Jr., R.A. Condrate, Sr. and W.E. Votava, Appl. Spectrosc. 29 (1975) 79. [11] D.R. Sandstrom, F.W. Lytle, P.S.P. Wei, R.B. Greegor, J. Wong and P.C. Schultz, J. Non-Crystalline Solids 41 (1980) 201. [12] K. Kamiya and S. Sakka, Nippon Kagaku Kaishi (J. Chem. Soc. Japan, Chem. Ind.) (1981) 1571. [13] S. Sakka and K. Kamiya, Rev. Chim. Min. 16 (1979) 293. [14] K. Kamiya and S. Sakka, to be published.
357
T H E R M A L E X P A N S I O N OF T i O 2 - S i O 2 A N D T i O 2 - G e O 2 G L A S S E S K. K A M I Y A and S. S A K K A
Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie, 514, Japan
The thermal expansion coefficients of TiO2-GeO 2 glasses containing up to 12.7 mol% TiO2 were determined and compared with those of TiO2-SiO2 glasses. The TiO2-GeO2 glasses were prepared by a conventional melting technique. The TiO2-SiO2 glasses were prepared by heating the gels formed as a result of hydrolysis and polycondensation of the alkoxide solutions consisting of Ti(OC3HT) 4 and Si(OC2Hs)4. The thermal expansion coefficient of the alkoxy-derivedTiO2SiO2 glasses decreased with increasing TiO2 content similarly to the TiO2-SiO2 glasses prepared by the flame hydrolysis technique. It was found that the addition of TiO2 markedly decreased the thermal expansion coefficient of the GeO2 glass. It was concluded on the basis of the contribution of the TiO2 component to the expansivity of the glasses that the Ti 4+ ions were present in the 4-coordination state with respect to oxygen atoms in both TiO2-GeO2 and TiO2-SiO2 glasses.
1. Introduction T i O 2 - S i O 2 glasses c o n t a i n i n g up to 10 wt% TiO 2 prepared by the flame hydrolysis technique above 1750°C are k n o w n to show ultra-low thermal expansivity [ 1,2]. G e O 2 glass has been revealed to be similar to SiO 2 glass in structure [3], that is, all the G e atoms in G e O 2 glass are 4-coordinated with oxygen atoms. In the present study, TiO 2 was added to G e O 2 glass to form glasses of the b i n a r y T i O 2 - G e O z system. The thermal expansion coefficients of those T i O z - G e O 2 glasses were determined a n d compared with those of the T i O 2 - S i O 2 glasses prepared by heating the gels o b t a i n e d from the metal alkoxides c o n t a i n i n g Ti a n d Si atoms through hydrolysis a n d p o l y c o n d e n s a t i o n . The effect of the TiO 2 c o m p o n e n t on the thermal expansion coefficients of b o t h G e O 2 and SiO 2 glasses is discussed on the basis of the c o o r d i n a t i o n n u m b e r of Ti atoms in the glasses.
2. Experimental 2.1. Preparation of the glasses Quartz type G e O 2 powder of 5-nine purity (supplied by Mitsuwa Chemicals Co.) a n d reagent grade anatase type T i O 2 powder (supplied by Merck Chemicals Co.) were used as starting materials for the T i O 2 - G e O 2 glasses. The 30 g 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
K. Kamiya, S. Sakka / Thermal expansion of glasses
358
Table 1 Compositions and melting temperatures of the TiO 2 - G e O 2 glasses Glass
Temperature
ZiO 2 content
State
(°C) 1 2 3 4 5 6 7
(tool%)
(wt%)
0 3.2 6.4 9.6 12.7 15.9 19.1
0 2.5 5.0 7.5 10.0 12.5 15.0
1550 1550 1550 1500 1500 1500 1500
glass glass glass glass glass crystallized crystallized
batches of oxide mixtures corresponding to the compositions listed in table 1 were melted in platinum crucibles at 1500-1550°C in air for 2 or 3 h. The compositions containing up to 12.7 mol% TiO 2 (samples Nos. 1-5) provided us with the clear glasses. X-ray diffraction revealed no trace of crystalline TiO 2 in those five glasses. For the compositions containing more than 12.7 mol% TiO 2, the small amount of crystalline TiO 2 precipitated during cooling the melts. The melts were too viscous at 1500-1550°C to be poured out of the crucibles to form plate glasses. The melts, therefore, were fabricated into rods the diameters of which were 2 or 3 ram, by immersing a fused silica rod into the melts and pulling it up slowly by hand. The glass rods thus obtained were annealed at temperatures slightly lower than their glass transition temperatures and were used for determining the thermal expansion coefficients. The titanium tetraisopropoxide Ti(OC3HT) 4 and silicon tetraethoxide Si(OC2Hs) 4 (supplied by Wako Chemicals Co.) were used as raw materials in the preparation of the TiO2-SiO 2 glasses. The alkoxide mixtures for the TiO2-SiO 2 system given in table 2 were diluted with ethanol. The mixture of
Table 2 Oxide compositions and amounts of raw materials used for the TiO 2 -SiO 2 glasses Glass
TiO 2 (mol%)
Ti(OC 3 H 7 )4 (g)
Si(OC 2 H 5 )4 (g)
State a
11 12 13 14 15
2.3 3.8 6.2 7.7 11.5
0.65 1.08 1.73 2.16 3.24
20.42 20.02 19.31 18.95 17.88
glass glass glass glass crystallized b
State after heating the gels to 900°C. b Anatase crystal precipitated. a
K. Kamiya, S. Sakka / Thermal expansion of glasses
359
water and glacial acetic acid as a catalyst was added to the alkoxide solution for hydrolysis. The molar ratios of the added water and acetic acid to the alkoxide was 50 and 0.01, respectively. The mixed solutions were kept standing at room temperature (about 20°C) and at 40°C to undergo hydrolysis and polycondensation. The resultant monolithic gels were heated up to 900°C with a heating rate of 6 ° C / h . The gels for the compositions containing up to 7.7 mol% TiO 2 could be converted to clear glasses. In the composition containing 11.5 mol% TiO 2, precipitation of anatase crystal was observed at 900°C. The other details of the preparation procedures have been published elsewhere [4,51. 2.2. Measurement of the thermal expansion coefficient
The thermal expansion was measured using glass bars of about 1.5 cm in length. A dilatometer manufactured by Rigaku Denki Co. was used. A fused silica rod was used as a reference. The average linear thermal expansion coefficient a was determined from the slope of the expansion curve over the temperature range from 25 to 400°C for the TiO2-GeO 2 glasses. The thermal expansion coefficients averaged over the temperature range from 25 to 700°C were measured for the TiO2-SiO 2 glasses. The glass transition and softening temperatures of the TiO2-GeO 2 glasses were read from the inflection points of the thermal expansion curves.
3. Results
The thermal expansion curves of the TiO2-GeO 2 glasses containing up to 12.7 mol% TiO 2 are shown in fig. 1. The linear thermal expansion coefficients, and transition and softening temperatures of those glasses are given in table 3. Both glass transition and softening temperatures can be seen to increase with increasing TiO 2 content with one exception. The thermal expansion coefficient of the GeO 2 glass was 74.8 × I0-7/°C, agreeing well with that reported by Mackenzie [6]. The thermal expansion curves of the alkoxy-derived TiO2-SiO 2 glasses are shown in fig. 2. It can be seen that the glasses show negative expansions relative to the fused silica. Fig. 3 shows the thermal expansion coefficient of the TiO2-GeO 2 glasses plotted as a function of the TiO 2 content. It can be seen that the thermal expansion coefficient of the glass decreases proportionally to the TiO 2 content. The thermal expansion coefficients of the TiO2-SiO 2 glasses are plotted against the TiO 2 content in fig. 4. The open marks in fig. 4 correspond to the glasses prepared from the metal alkoxides through hydrolysis and polycondensation. The two different open marks correspond to the two different hydrolysis temperatures. The thermal expansion data of the TiO2-SiO 2 glasses produced by the flame hydrolysis technique in Corning Glass Works are shown
K. Kamiya, S. Sakka / Thermal expansion of glasses
360
Fused silica -I x=O
"__ez o
32
x
=
m
.
-2
~/////
6.4 K
o',o
.=
-E
o;"
'
26o
'
,;o
Temperature (*CI
'
66o
i
200 400 Temperature (*C)
600
Fig. 1. Thermal expansion A L / L of the xTiO2.(100- x)GeO 2 glasses. Fig. 2. Thermal expansion A L / L of the x TiO 2. ( 1 0 0 - x ) S i O 2 glasses relative to fused silica.
in the same figure by full circles for comparison. The ultra-low thermal expansion is achieved in the alkoxy-derived TiO2-SiO 2 glasses. It can be seen that the thermal expansion coefficient of the TiO2-SiO 2 glasses decreases linearly with increasing TiO 2 content.
70
~
60
0 50
' 5
I0
TIO= content, real%
5
j~
TiOE content (mole%)
Fig, 3. Change of the linear thermal expansion coefficient of the TiO 2 - G e O 2 glasses with the TiO 2 content. Fig. 4. Change of the linear thermal expansion coefficient of the TiO 2 -SiO2 glasses with the TiO 2 content. O , prepared from alkoxides hydrolyzed at 40°C; [:], prepared from alkoxides hydrolyzed at room temperature; O, prepared by flame hydrolysis technique [1 ].
K. Kamiya, S. Sakka / Thermalexpansion of glasses
361
Table 3 The linear thermal expansion coefficient a, glass transition temperature Tgand softening temperature T~of the TiO2-GeO/glasses Glass 1 2 3 4 5
TiO2 (mol%)
a ( × 10- 7/°C)
Tg (°C)
(°C)
0 3.2 6.4 9.6 12.7
74.8 70.8 63.7 58.4 54.9
537 555 552 573 587
577 624 600 647 645
4. Discussion 4.1. The effect of 7702 component on thermal expansion The empirical additive law has been found to be valid between thermal expansion coefficients of the oxide glasses and their compositions. That is, the linear thermal expansion coefficient of a particular oxide glass is expressed by the following equation,
a=~anen,
(l)
where a n is the expansion coefficient factor and Pn is the molar fraction of a particular constituent. The aTio2 was evaluated using eq. (1) as - 9 9 × 1 0 - 7 / ° C for the TiO2-GeO 2 glasses. The expansion coefficient factor of 75 × 10 7 / ° C was assumed for the GeO 2 component. A similar value of - 1 0 4 × 10 7 / ° C for the avio2 in the TiO2-SiO 2 glasses was obtained, using the value of 5 x 1 0 - 7 / ° C for the expansion coefficient factor of the SiO 2 component. The Ti 4 + ions can be present either in the 4-, 5- or 6-coordination state in the Ti 4÷ containing crystalline compounds, although only a few examples have been reported for the 4-coordination state. The coordination number of Ti 4+ ions in the TiO 2 containing oxide glasses has attracted much attention [7]. It has been shown that the Ti n+ ions are also present in the 4- or 6-coordination state in the oxide glasses. It is assumed that the Ti 4"- ions in the different coordination states should make different contributions to the thermal expansion and other physical properties of the glasses [7]. The fact that the contribution of the TiO 2 component to the thermal expansion of the TiO2-GeO 2 glasses is equivalent to that of the TiO2-SiO 2 glasses suggests that the coordination state of Ti 4÷ ions is the same in both of the TiOz-GeO 2 and the TiO2-SiO 2 glasses.
362
K. Kamiya, S. Sakka / Thermal expansion of glasses
4.2. Coordination state of
Ti 4÷
ions
It was shown that the thermal expansion coefficient factor of the TiO 2 component obtained in the present work for the T i O 2 - G e O 2 and TiO2-SiO 2 glasses is considerably smaller than those reported so far for the 6-coordinated Ti 4+ ions. Values of - 3 5 X 1 0 - 7 / ° C and 20 X 1 0 - 7 / ° C have been reported [8]. This indicates that the Ti 4+ ions in the present glasses are in a coordination state other than 6-coordination. The 4-coordination state of the Ti 4+ ions with respect to oxygen atoms in the TiO2-SiO 2 glasses prepared by the flame-hydrolysis technique has been suggested by several authors to elucidate the low thermal expansion and other properties of those glasses. Evans [9] reported that the cristobalite crystal in which some part of the Si 4+ ions are replaced by Ti 4+ ions is precipitated when the TiO2-SiO 2 glass is heated at 1450°C for 2h, suggesting that the Ti 4+ ions occupy the sites equivalent to Si 4+ ions even in the glassy state. This means that the Ti 4+ ions are in a 4-coordination state like Si 4÷ ions to form a three-dimensional network structure. Smith et al. [10] measured the different infrared spectra of the TiO2-SiO 2 glasses using the SiO 2 glass as a reference. They observed the absorption peak at 735 c m - t due to the T i - O stretching vibration in the groups containing 4-coordinated Ti 4+ ions. Recently, Sandstrom et al. [11] analyzed the EXAFS spectra of the Ti 4+ ions in the TiO2-SiO 2 glasses and showed that almost all the Ti 4+ ions were in the 4-coordination state in those glasses. The present authors [12] have also shown that the low thermal expansion coefficients and densities of TiO2-SiO 2 glasses prepared from metal alkoxides could be explained by the presence of the 4-coordinated Ti 4+ ions. The intense infrared absorption peak around 950 cm-1 was interpreted as showing the presence of the 4-coordinated Ti 4+ ions. Some consideration should be given to the coordination state of Ge atoms in the T i O 2 - G e O 2 glasses. It is known that the coordination number of Ge atoms is 4 in GeO 2 glass and the concentration of the 6-coordinated Ge atoms increases on the addition of an alkali to GeO2 glass [13]. It was observed in our measurement of the infrared spectra [14] that the absorption peak ascribed to the G e - O stretching vibration shifts from 875 cm - l to the lower wave numbers when the alkali oxide was added to GeO 2 glass, while such a shift is not observed in the TiO2-GeO 2 glasses. This indicates that the Ge atoms remain 4-coordinated on addition of the TiO2 component to GeO 2 glass. One of the authors (KK) is grateful for the support of the Ohkura Kazuchika Memorial Fund. References [1] P.C. Schultz and H.T. Smyth, Amorphous Materials (John Wiley, New York and London, 1972) p, 453.
K. Kamiya, S. Sakka / Thermal expansion of glasses [2] [3] [4] [5] [6] [7] [8]
363
P.C. Schultz, J. Am. Ceram. Soc. 59 (1976) 214. J. Zarzycki, Verres Refract. 11 (1957) 3. K. Kamiya and S. Sakka, J. Mat. Sci. 15 (1980) 2937. S. Sakka and K. Kamiya, J. Non-Crystalline Solids, 42 (1980) 403. J.D. Mackenzie, J. Am. Ceram. Soc. 42 (1959) 310. Bh.V.J. Rao, Phys. Chem. Glasses, 4 (1963) 22. S. Naruse, Glass Handbook, eds., S. Sakka, T. Sakaino, K. Takahashi (Asakura Shoten, Tokyo, 1975) p. 696. [9] D.L. Evans, J. Am. Ceram. Soc. 53 (1970) 418. [10] C.F. Smith Jr., R.A. Condrate, Sr. and W.E. Votava, Appl. Spectrosc. 29 (1975) 79. [11] D.R. Sandstrom, F.W. Lytle, P.S.P. Wei, R.B. Greegor, J. Wong and P.C. Schultz, J. Non-Crystalline Solids 41 (1980) 201. [12] K. Kamiya and S. Sakka, Nippon Kagaku Kaishi (J. Chem. Soc. Japan, Chem. Ind.) (1981) 1571. [13] S. Sakka and K. Kamiya, Rev. Chim. Min. 16 (1979) 293. [14] K. Kamiya and S. Sakka, to be published.