Formation and structure of titanate glasses

Journal of Non-Crystalline Solids 80 (1986) 135-140 North-Holland, Amsterdam

135

Section II. Glass structure and phase transformation FORMATION A N D S T R U C T U R E OF T I T A N A T E GLASSES

CHENG Jijian and CHEN Wei East China Institute of Chemical Technology, Shanghai 201107, PRC The formation of high titanium oxide (30-60 wt%) containing glasses was studied. Stable titanate glasses with high content of TiO2 and BaO could be obtained even without other glass-forming oxides. It is demonstrated that the coloration of titanate glasses with high content of TiO2 is due to oxygen loss during the melting. A systematic study of controlling melting and heat treatment conditions led to the successful decoloration of titanium oxide containing glasses. Infrared spectra and X-ray diffraction studies showed that Ti 4~ in titanate glasses is in sixfold coordination. Phase separation was observed by electron microscopy when the titanate glasses were heat-treated at the temperature above Tg. The crystallization of titanate glasses is generally preceded by phase separation. The obtained crystalline phases are mainly different titanates.

1. Introduction

The titanium oxide containing glasses are of interest for both basic research and technological applications because of their unique optical properties and good chemical durability. Titanium oxide has already been studied as a component of silicate, borate or even more complex oxide glasses. The TiO2 content in these glasses may be as high as 30 wt%. It is well-known that the glasses with high content of TiO2 increase the tendency of crystallization and coloration. Nevertheless, until now little research has been conducted in the field of the systematic investigation of the high TiO2 containing glasses [ 1-3]. In this paper the formation and structure of titanate glasses with high refractive index are presented.

2. Experimental

2.1. Glass composition and melting A series of compositions of the TiO2-BaO system was selected with the addition of appropriate amount of ZrO2, ZnO, La203 etc. About 30 glass compositions in the following region (wt%) have been studied: TiO2 30-64, BaO 30-50, SiO2 0-10, B203 0-6, A1203 0-2, ZrO2 0-7, La203 0-8, ZnO 0022-3093/86/$03.50 O Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Cheng Jijian, Chen Wei I Formation and structure of titanate glasses

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0-10.5, C a O 0-2, MgO 0-3, SrO 0-2, K 2 0 0-1.5, N a 2 0 0-1.0, Li20 0-0.5, WO3 0-0.5. Glass samples of different compositions were prepared by melting the well-mixed raw materials at temperatures of 1360-1400°C. As the TiO2 content increased, the crystallization tendency of the glasses became more and more pronounced. It is interesting to note the effect of the addition of ZrO2, ZnO, La203 etc in glasses on the improvement of resistance to devitrification. T h e stable titanate glasses were successfully produced by adding more than 50 wt% TiO2 in the glass composition. In order to avoid the reduction of Ti 4+ ions, it is desirable to introduce a small amount of nitrate in the batch and shorten the melting time. T h e refractive index of prepared glasses was 2.1-2.2. 2.2. Decoloration treatment of titanate glasses

T h e coloration of titanate glasses may be attributed to the redox equilibrium of titanium oxide:

(1)

TiO2 ~ TiO2-~ + ½xO2 + XDo 2- •

According to eq. (1), it is quite evident that the oxygen ion vacancy occurs in glass structure, so the realization of decoloration is possible by diffusing the oxygen in titanate glasses. In fact, the experiments demonstrated the efficiency of the decoloration treatment at temperatures above Tg (figs. 1 and 2). As can be seen, after about 1 h, the further heat treatment does not favour the decoloration of glass samples. As expected, this process of decoloration is due to the relaxation of glass structure at Tg which leads to the diffusion of oxygen and the migration of oxygen vacancies.

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O6

•

"".". ~ ~ ~ 2 %

0.8 E3 06 0.4 ~

' 3

0.2

o2

0~

660''

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Fig. 1. Effect of temperature of decoloration treatment on the reflectance density D of different glass samples.

i

0

O

i

1.0

2.0

Fig. 2. Effect of duration of decoloration treatment on the reflectance density D of different glass samples.

Cheng Jifian, Chen Wei / Formation and structure of titanate glasses

137

Table 1 Increase in density after decoioration treatment Sample number

Density before treatment (g/cm 3)

Density after treatment (g/cm~)

Increase in density (%)

1 2

4.2227 4.0880

4.3307 4.1111

2.25 0.56

T h e i n c r e a s e in d e n s i t y a f t e r d e c o l o r a t i o n t r e a t m e n t p r o v e d the o c c u r r e n c e of t h e d i f f u s i o n p r o c e s s of o x y g e n in t i t a n a t e glasses (table 1). It s h o u l d be m e n t i o n e d t h a t the c o l o r a t i o n b e c a m e v e r y s e r i o u s if A s 2 0 3 was a d d e d to the b a t c h . T h i s is p r o b a b l y d u e to t h e f o l l o w i n g r e d o x r e a c t i o n : TiO2-x + As203 ~ TiO2 + As203 - x. C o n s e q u e n t l y , c a r e f u l a t t e n t i o n m u s t b e p a i d to the c h o i c e of refining a g e n t for the m e l t i n g of t i t a n a t e glasses.

2.3. Crystallization of titanate glasses As shown in fig. 3, the D T A curve of titanate glasses has two or three exothermic effects attributed to the formation of different crystalline phases. For the glass sample No. 1, there are three exothermic effects on the D T A curve. X-ray diffraction studies of the crystallized glass showed that the main

735

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832

t 0 a z

LLI

I

I

I

I

600

700

800

900

% (°el

Fig. 3. DTA curves of some titanate

glasses.

Cheng Jijian, Chen Wei / Formation and structure of titanate glasses

138

÷

*

***

1

I

*

I

60

I

i

50

I

¢

I

40

I

t

* ¢

I

I

30

I

20 2e

Fig. 4. X-ray diffraction spectra of titanate glass crystallized at different temperature: (1) 735°C, (2) 780°C, (3) 838°C. + BaTi205, o Ba4Ti13030, * Ba6Tit704o.

crystalline phase consists only of BaTi205 and the other two crystalline phases are Ba4Til303o and Ba6Til704o. In addition, it is quite possible that there is formation of a solid solution of different titanates which causes the complexity of characterization of crystalline phases. Fig. 4 shows the X-ray diffraction spectra of some crystallized titanate glasses. A series of electron micrographs of initial and crystallized titanate glasses is shown in fig. 5. It should be mentioned that when the glasses have been heat-treated at Tg for 1 h, an evident phase separation is observed. In this case, phase separation may play the role of the precursor of bulk crystallization. 2.4. Glass structure

Ti 4÷ may be in fourfold or sixfold coordination [4]. As noted earlier, when the total amount of TiO2 and BaO is about 80 wt%, the stable titanate glasses can be obtained even without the addition of SiO2, B203 or other glass network formers. Figs. 6 and 7 show the infrared absorption spectra of different initial, heat-treated and crystallized glasses. The band at 550560 cm -1 assigned to the stretching mode of the bond Ti-O resembles closely that of Ti in BaTiO3, in which Ti is octahedrally coordinated by oxygen. In general, the infrared spectra of the titanate glasses do not change a great deal in the range 30-50 wt% TiO2. Two Si-O bond-stretching frequencies appear at 960 and 730 c m -1 for titanate glasses containing a small amount of SiO2. Similarly, three bands at 1210, 1030, 930 cm -t due to the B-O bond occur in the IR absorption spectra for glasses containing boron oxide. No apparent differences are observed in the IR spectra of the various titanate glasses before and after decoloration treatment, but the absorption bands become

Cheng Jijian, Chen Wei / Formation and structure of titanate glasses

139

a

b

C

1

2

Fig. 5. Electron micrographs of titanate glasses containing 5 0 w t % TiO2: (1) a, without heat treatment; b, heat treated at 698°C for 1 h; c, heat treated at 730°C for 2 h; (2) a, heat treated at 660°C for 1 h; b, heat treated at 705°C for 1 h; c, heat treated at 710°C for 1 h (bar = 1 /~m).

140

; Jijian, Chen Wei / Formation and structure of titanate glasses

CJ

l oo . . . . . IOO0 . . . . . . 6OO

. 1400 . . . . . . 1000 . C iq1-'

Fig. 6. Infrared absorption spectra of titanate glasses containing SiO2 and B203: a, initial; b, after decoloration treatment; c, crystallized.

oo' C rn -~

Fig. 7. Infrared absorption spectra of titanate glass without SiO2 and B203: a, initial; b, after decoloration treatment; c, crystallized.

m o r e p o i n t e d which is due to the structure r e a r r a n g e m e n t during the heat t r e a t m e n t of glasses. A s expected, the difference b e t w e e n the spectra of the initial glasses and those of the crystallized glasses is the a p p e a r a n c e of fine structure after crystallization treatment. Conclusively, the structure of titanate glasses is postulated to consist oF a t h r e e - d i m e n s i o n a l n e t w o r k of (TiO6) o c t a h e d r a sharing c o r n e r s with one another.

References [1] [2] [3] [4]

J.M. Stevels, Phys. Chem. Glasses 1 (1960) 107. R.C. Turnbull et al., J. Amer. Ceram. Soc. 35 (1952) 48. D.R. Sandstrom et al., J. Non-Cryst. Solids 41 (1980) 201. P. Tarte, in: Physics of Non-Crystalline Solids (Interscience, New York, 1965) p. 549.