ZrO2-SiO2 materials prepared by sol-gel

330

Journal of Non-Crystalline Solids 100 (1988) 330-338 North-Holland, Amsterdam

ZrO2-SiO 2 MATERIALS PREPARED BY S O L - G E L I.M. M I R A N D A S A L V A D O Dpt. Engenharia Ceramica e do Vidro, Universidade de Aveiro, Portugal

C.J. S E R N A Instituto de Ciencia de Materiales (CSIC), Madrid, Spain

J.M. F E R N A N D E Z N A V A R R O lnstituto de Cer6rnica y Vidrio (CSIC), Arganda del Rey, Madrid, Spain

Gels with composition xZrO2-(100-x)SiO2, x =10-55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors. Gels treated at different temperatures up to 1100 °C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment. Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.

1. Introduction The incorporation of Z r O 2 to glass and ceramic materials has a marked influence on their properties. The use of ZrO2 is well k n o w n to enhance the mechanical properties of traditional ceramics and has led to the development of new materials with special applications [1]. In glasses, Z r O 2 has a great influence on the chemical durability. The addition of a few percent has been reported to substantially increase the resistance to alkaline attack [2]. The sol-gel process allows the preparation of glass compositions with a high content of Z r O 2 at low temperatures, and for ceramics the preparation of powders with great reactivity and controlled size. F o r the system Z r O 2 - S i O 2 different authors obtained monolithic glasses [3], glass fibers [4,5], A Z S C refractories [6], and glass-coatings [7-9]. The main purpose of this work is to study the preparation of materials in the system ZrO2-SiO2, characterize the materials obtained and study their thermal evolution. In this way, samples with composition x Z r O 2 • 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Pubfishing Division)

( 1 0 0 - x)SiO2, x = 10-55, were prepared. Samples of composition 100ZrO 2 and mixtures of SiO 2 and Z r O 2 solutions were m a d e in order to clarify whether, after thermal treatment, a physical mixture of SiO 2 and Z r O 2 or a h o m o g e n e o u s vitreous material would be obtained.

2. Experimental 2.1. S a m p l e preparation

Gels in the composition range x Z r O 2 • (100x ) S i O 2, x = 1 0 - 5 5 (mol%), were prepared utilizing the procedure outlined in fig. 1. The specific compositions and the experimental conditions are listed in table 1. Gels of composition 100ZrO 2 and mixtures (in the p r o p o r t i o n of 30 mol Z r O 2 to 70 mol SiO 2) of solutions of SiO 2 and Z r O 2 were prepared under the same experimental conditions of samples 30Zr-3. The SiO 2 and Z r O 2 solutions were mixed for zero and 48 h after their preparation (samples ( Z r / S i ) 0 and (Zr/Si)48).

LM. Miranda Salvado et al. / ZrO2-SiO 2 materials prepared by sol-gel

331

]

TEOS (Partiolhy~,~ ~2~C-2h,| stirring) |

J Zr AcAc

SOL

J[

GEL

Material

?

(diss, in EtOh and HCl)[

Fig. 1. P r e p a r a t i o n of s a m p l e s of the c o m p o s i t i o n x Z I O 2 (100-x)SiO2, x = 1 0 - 5 5 (mol%).

Tetraethylorthosilicate and zirconium acetylacetonate (both Merck) were used. All the reactions were carried out at room temperature with stirring. The solutions were put in glass containers covered with plastic foil and left at 30 ° C. After gelification and to allow the separation of gels from moulds and an initial slow drying the gels were maintained for 1-2 months at that temperature.

The samples dried at 120°C for 48 h were treated at different temperatures: 550, 850 and 1100 ° C, the heating rate being in all cases equal to 75 ° C / h . The temperature was maintained at 250 ° C for 4 h for all thermal treatments and the maximum temperature was maintained for 5.5 h, except for 1100°C (1 h). The materials obtained were subsequently

Table 1 P r e p a r a t i o n of gels of c o m p o s i t i o n x Z r O 2 ( 1 0 0 - x ) S i O 2. E x p e r i m e n t a l c o n d i t i o n s Composition

1 0 Z r O 2. 90SiO 2

3 0 Z r O 2 • 70SiO 2

5 5 Z r O z. 45SIO 2

Sample

10Zr-1 10Zr-2 10Zr-3 10Zr-4 10Zr-5 10Zr-6 30Zr-1 30Zr-2 30Zr-3 30Zr-4 30Zr-5 30Zr-6 55Zr-1 55Zr-2 55Zr-3 55Zr-4 55Zr-5 55Zr-6

Mol% H 20

a

ETOH

Teos + ZrAcAc

Teos + ZrAcAc

Teos + ZrAcAc

11.6 11.6 11.6 4.0 10.0 40.0 11.6 11.6 11.6 4.0 10.0 40.0 11.6 11.6 11.6 4.0 10.0 40.0

a = HC1

a = NH 3

0.35 0.35 2.03 0.35 0.35 0.35 0.55 0.55 2.19 0.55 0.55 0.55 0.70 0.70 2.30 0.70 0.70 0.70

0.097 0,090 0.090 -

2.3 2.3 2.3 2.3 2.3 2.3 4.2 4.2 4.2 4,2 4,2 4,2 5,7 5,7 5,7 5.7 5.7 5.7

LM. Miranda Salvado et a L / Zr02-SiO 2 materials prepared by sol-gel

332

Zr (T)

3 0 Zr - 1

,

30 Zr - 3

IlO0°C - 1 h Zr(T)

[I ]/

I lO0°C - 1 h

Zr(T)

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8Zr~gff~C- 5"5 h

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,

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Zr(T) ,

,

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.

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.

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.

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.

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,

42

ZrlM) Zr(M) Zr(M) Zr(T) .

.

38 2g

.

.

.

.

34

.

,

30

26

Fig. 2. X-ray diffraction results for samples 30Zr-1 and 30Zr-3 treated at 550 0,850 o and 1100 o C.

studied by X-ray diffraction, IR spectroscopy and TEM. 2.2. General observations

Gels of composition xZrO2- (100 - x ) S i O 2 , immediately after their preparation, were clear, light-yellow. During the drying at 30 °C this colour changed to brown. In samples with a greater content of ZrO 2 and/or with higher ratios HC1/(TEOS + ZrAcAc)

1OO Zr - I

zrl(m I z~(M)

850°C -5.5h

Zr(M) .

~r!~,).|:]r(") zr~,t)zr(a)

550°C-

I

Zr(T) A

54

50

5.5 h

a surface segregation with the appearance of a powdered film (which, as will be seen later, is m a i n l y Z r O 2) appeared. Samples of composition 100ZrO 2 never gelifled, instead a powdered material was obtained. The individual solutions of S i O 2 w e r e clear, colourless and the Z r O 2 solutions were clear, light-yellow. After these were mixed and aged, at 30 o C, the colour changed to brown and again, the segregation phenomenon was observed. After heating above 450°C transparent and

100 Z r - 3

zrl(M} I

850°C-5.5 h

~

Zr(M)e.(M~i

Zr(l.l~ 11

....

H

Zr(M) Zr(M) J Zr~..)j Zr(M),..,Zr(M) Zr(M) Zr(l~j)j R

II za'): r"'

Z~T)

~ Zr(T) zr(l~ I Zr,(M)

46 '4;2 ' 3~ " 54 2e

Zr(M}

~

26

22

Zr(M) 54

50

46

42

Zr~M)

ZrlM) Zr(M) Zr(,) ~ ]1

550°C-5"5 h

[

JJ

Jl

58 2e

54

30

26

Fig. 3. X-ray diffraction results for samples 100Zr-1 and 100Zr-3 treated at 550 o and 850 ° C.

22

I.M. Miranda Salvado et aL / Zr02-SiO 2 materials prepared by sol-gel

(Zr/Si) 0

333

(Zr/Si)48

Zr OM) Zr(M)

8 5 0 ° - 5.5 h Zr(M)

.~.,

Zr(M) Zr(M) 1 I . ._,.tZl'~) l A Zr(M) l Zr(M) = W J ~ k ::/-" I

5 5 0 ° C - 5 5h I

Zr(M'

54

50

Zr

46

42

38 2e

34

30

26

_ ,.Zr(M) ~ ZrtMl, II

Zr(M)

~I II

550°C-5.5 h

)

(T Zr(M) Zr(M), ,

•

Zr(M) ZrIM) -- "

ZrlM) ,I

Zr(M)

i

22

54

50

46

42

38 20

34

30

26

22

Fig. 4. X-ray diffraction results for samples (Zr/Si)o and (Zr/Si) 48 treated at 550 ° and 850 ° C.

3. Results and d i s c u s s i o n

colourless (x = 10) or white and opaque (x = 30, 55, 100) materials were obtained. In general for x = 10 the samples had a minor shrinkage, showed fewer cracks and had larger dimensions than for higher values of x. For samples with x >/30 white aggregates with low mechanical strength were present and were easily pulverized by subsequent handling. For this reason a sintering process should be used.

3.1. X-ray diffraction In the series 10ZrO 2 all samples were amorphous after heating for 5.5 h at 550 ° C. At 850 o C crystallization of tetragonal ZrO 2 was indicated for all samples excepting 10Zr-4. ZrO2(t) was present in all 10ZrO 2 samples treated at 1100 o C.

55 Z r - 4 Zr(T)

t

oo*c- ] h

A

I

55Zr-6

~ Zr(T)

I IOOOC - I h

I[

Zr(T)

II

850°C - 5.5 h ~_~00C_5.5 h

/ Zr(T) Zr (T) ~t

Zr (T)

Zr(T)

Zr(T)

Jk.

A

I

\ - 2L_---._LJ t " 550,C-5.5h

5500C-5.5h

Zr(T)

I

Zr ('r)

Zr (r) 54

50

. . . . _,,ilL. 46

42

38

2e

34

30

. 26

54

50

46

42

38

34

30

2e

Fig. 5. X-ray diffraction results for samples 55Zr-4 and 55Zr-6 treated at 550 o C, 850 o and 1100 o C.

I.M. Miranda Salvado et aL / Zr02-SiO 2 materials prepared by sol-gel

334

crystallization of monoclinic zirconia was observed at 850°C, this crystallization being more pronounced for (Zr/Si)48 at the same temperature. This behaviour can be explained if it is assumed that in (Zr/Si)0 it was possible to obtain a greater homogeneity and reactivity between the two oxides which prevents the growth of the ZrO 2 crystals and stabilizes almost all the ZrO 2 in the tetragonal phase (fig. 4). The effect of water content can be observed (fig. 5) in all samples 4, 5 and 6 of table 1.

For higher ZrO 2 content (30ZrO 2 and 55ZrO 2 mol%) the crystallization of ZrO2(t ) was detected at 550°C, except for 30Zr-4 that remained amorphous. At 850 ° C, ZrOz(t) was present for all samples of compositions 30 and 55 ZrO 2 (including 30Zr-4). For samples 30 Zr-3 and 55Zr-3 (higher ratios of HC1/(TEOS + ZrAcAc))crystallization of monoclinic ZrO 2 occurs simultaneously (fig. 2) at all temperatures, this fact being related to the segregation phenomenon observed in those samples. The influence of the ratio HC1/(TEOS + ZrAcAc) in the formation of ZrO2(m ) is confirmed by the results obtained for the samples of composition 100 ZrO2 treated at the same temperatures. As described earlier these samples were obtained under the same experimental conditions as for tests 30Zr-1, 30Zr-2, 30Zr-3 and by analysis of figs. 2 and 3 the previous results are confirmed: an increase in HC1 increased the ZrO2(m) formation (fig. 3). In sample (Zr/Si)0 (mixture of SiO 2 and ZrO 2 solutions for zero hours after preparation) the

3.2. Infra-red spectroscopy In all samples of composition 100ZrO 2 an amorphous gel was formed at low temperature (120 ° C) with the characteristics absorption bands at 450, 1595, 1625 and 3400 cm -1 (fig. 6). The band at 450 cm -1, that is still present at 5 5 0 ° C but disappears for samples treated at 850 ° C, was more intense for 100Zr-1 than for 100Zr-3. Since this sample had a higher degree of crystallization of ZrO2(t ), this band was attributed to that crys-

100 Zr- 1

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hi 0

--.J~

lib I]1: o In ,,~

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'z°°c 5~ooc

.....

........... /....."'--\,............. 85ooc

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zeoo

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I

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%~

;y

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i~oo idoo e6o

66o

460

2oo

-I]

lOOZr-3

i

/~

'z°°c

I \ ,...........I----.. \

""~:::.:

,~

55o°c o

............ 8 5 0 c

...

i 3ooo 2~o

I

.;:~

I I

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, ~"'... "'--.. ~

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,~

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i

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2obo ~ o ,e'oo .'oo , ~

~

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~

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Mo ~o 4~o 2o0

9[c~.-'.I Fig. 6. IR spectra of samples 100Zr-1 and 100Zr-3 treated at 120 o, 550 o and 850 o C.

I.M. Miranda Saloado et al. / Zr02-SiO 2 materials prepared by sol-gel

talline phase. With an increase in temperature the other bands disappeared. In agreement with X-ray diffraction results the crystallization of ZrO2(m ) in all samples of composition 100ZrO 2, treated at 550 ° C and 850 ° C is evident by the presence of the narrow bands located at 350, 412, 495, 568 and 729 cm -1 attributed to this phase. These values agree with results obtained by Phillippi and Mazdiyasni [10]. The greatest differences in infra-red spectra are observed for 120°C: the intensity of the bands located at 1020, 1395, 1440-1450 and 1535 cm -] decreased considerably with the increase in HC1 content. These bands correspond to the vibration of organic groups from ZrAcAc (not hydrolyzed). Therefore, in a media with low HC1 content, the hydrolysis of ZrAcAc is incomplete. After thermal treatment at 120 ° C for 48 h those organic groups are still present (this was confirmed by an IR spectra of ZrAcAc not hydrolyzed). In sample 100Zr-1 three other bands at 2800,

335

3030 and 3130 cm -1, due to the modes of vibration of N - H bonds are observed. Their presence is attributed to the addition of N H 4 O H when the sample was prepared. For (Zr/Si)48, treated at 850°C, the well defined bands of ZrO2(m ) were present (fig. 7 similar to the spectra of 100Zr-1 and 100Zr-3. The ZrO: bands of sample (Zr/Si)0 were less defined than for (Zr/Si)48 (fig. 7). This fact, as already mentioned, is attributed to the greater interaction of SiO 2 and ZrO: in sample (Zr/Si)0. The presence of SiO2 mixed physically with ZrO2 seems to decrease the formation of monoclinic zirconia. N o other differences between these spectra are observed. All the bands characteristic of SiO 2 were similar for the two samples. The band at 980 cm -] attributed by D'yakonov et al. [11] to bonds S i - O - Z r is more intense in samples at 1 2 0 ° C and is practically absent for higher temperatures. For this reason this band is assigned to the stretching modes of terminal Si-O- groups. In

{Zr/Si)0

M I \

"'

120°C

A

5~ooc .............

(~in"'"""

i'...

L; ",~

/~i

=oc

"'..

......:

r--

" ~ Y - " ! ' ~ " "

•x ~ o o

~oo zsbo ~ o

"

,sbo ,~o Nbo ,zbo ,doo 8~o ~o 46o 2c ~Co~-']

(Zr/Si)48

::

.............f -.. \

o; '.

......

/,,,,"l~

....

55o°c

•............ ~0ooo ..........

/i

I

-...:

t'.......i ~..-\\

"---'-"--- "--~"- .....'- • ............... o_ 4000 ~

I

3o'oo 25t>o

i " i

21o:,o ,.oo ' ,d~ .too , ~

..-

.

.--:"~'"t ,;oo "

do

I

,,oo

1 2o0

~[~'m-'] Fig. 7. IR spectra of samples (Zr/Si)0 and (Zr/Si)48 treated at 120 o, 550 o and 850 o C.

336

I.M. Miranda Salvado et aL

/ Z r 0 2 - S i O 2 materials prepared by sol-gel

effect at 120 ° C the S i - O H groups are still present in the gel but disappear with an increase in temperature. For all temperatures the bands due to the presence of molecular adsorbed water (3420 cm -1) and O H groups (1625 cm -1) are present. By analysis of fig. 8 it can be seen that the characteristic bands of silica are present in the spectra of samples 30Zr-1 and 30Zr-3. N o n e of the bands attributed to ZrO2(m ) is observed (in contrast to the X-ray results). This fact is due to the low crystallinity of this phase. Also for these samples the bands assigned to the organic groups of zirconium acetylacetonate (not hydrolyzed) disappear with the increase in HC1 content and treatment temperature. The band at - 800 cm -1, shifts to higher frequencies in all samples with increasing temperatures, which is attributed to an increase of the S i - O bond strength. The two main bands located at 1080 and 1220 cm - ] become narrower with the increase in HC1 content. This fact is in agreement

50Zr-

1

with the previous conclusions: segregation of ZrO 2 is favoured in a strong acid media enhancing the formation of monoclinic zirconia. In this way, a network richer in silica is formed. This structure is less distorted and a more homogeneous SiO 2 network is obtained with a more uniform energy distribution. Then it may be said that in samples with high HC1 content the proportion of ZrO2 segregated after heat treatment leads to the formation of monoclinic zirconia. The influence of the molar ratio H 2 0 / ( T E O S + ZrAcAc) in samples of composition 10ZrO 2 • 90SiO 2 and 55ZrO 2 • 45SIO 2 was also studied (fig. 9). In sample 10Zr-4, the width of bands located at 450 and 1060 cm -1 decreases with the rise in temperature and the bands are also displaced to higher frequencies. This behaviour m a y be explained as an effect of the water added during the hydrolysis process. With a low content of water the hydrolysis is not complete. When the temperature is raised the network cohesion

120°C 550°C

i

~

............... 8500C

,moo

3--~oo

~moo--

2500 '

2000 ' 1800 ' 1600 ' 1400 ' 1200 ' I000 ' 800 ' 7 ~ m -I]

600 '

400 ' 200

~Zr-5 120°C . . . . . . . . . . . . . . .

55ooc ,, ooc

/

#

,~

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.

,

,

3000

2500

,

,

2 0 0 0 1800

I

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,s

,

,

,

,

,

1600

1400

1200

I000

800

z

- - - -

600

400

200

~l~m-']

Fig. 8. IR spectra of samples 30Zr-1 and 30Zr-3 treated at 120 o, 550 o and 850 o C.

I.M. Miranda Saloado et al. / ZrOe-SiO 2 materials prepared by sol-gel

IOZr-4

I

120°C 550°C I

~oo35o0

3ooo

2~o

~doo

A

]~

18010600 I

337

I

I

I

,4bo 12OO ,ooo 8~o 8do

do

20o

l 400

200

IOZr-6

~ 4000

. 850oc d 3 O0

I 3000

I 2600

I 2000

I 1800

../ ~... ~ I 1600

I 1400

l 1200

I I000

I 800

I 600

Fig. 9. IR spectra of samples 10Zr-4 and 10Zr-6 treated at 120 o, 550 o and 850 o C.

Fig. 10. T E M photographs of sample 30Zr-1 treated at 850 o and 1100 o C.

338

LM. Miranda Salvado et al. / Zr02-SiO 2 materials prepared by sol-gel

Fig. 11. TEM photographs of sample 30Zr-3 treated at 850 ° and 110 o C. enhances, the S i - O b o n d s b e c o m e s t r o n g e r a n d the d i s p e r s i o n of b o n d energy decreases. F o r water in excess (10Zr-6) the degree of h y d r o l y s i s is greater a n d the p o l y c o n d e n s a t i o n is c o m p l e t e at lower t e m p e r a t u r e s . This explains the similarity of s p e c t r a o b t a i n e d at various t e m p e r a tures. The o n l y c h a n g e o b s e r v e d is the d i s l o c a t i o n of the b a n d at 795 c m - a as a result of the cond e n s a t i o n of S i - O H g r o u p s to f o r m S i - O - S i bonds. A similar b e h a v i o u r is also o b s e r v e d for the 5 5 Z r O 2 sample, the o n l y difference b e i n g the i n c o m p l e t e h y d r o l y s i s of the z i r c o n i u m acetylacetonate. 3.3. Transmission electron microscopy

By the analysis of s a m p l e 30Zr-1 (fig. 10) the f o r m a t i o n of ZrO2(t) crystals in a h o m o g e n e o u s m a t r i x is o b s e r v e d for that s a m p l e t r e a t e d at 8 5 0 ° C a n d 1 1 0 0 ° C . F o r 1 1 0 0 ° C the crystals are m o r e defined. I n fig. 11 a crystal o f Z r O 2 ( m ) is o b s e r v e d in s a m p l e 30Zr-3 also t r e a t e d at 850 ° C a n d 1100 ° C. T h e s e o b s e r v a t i o n s agree with the p r e v i o u s results of X - r a y d i f f r a c t i o n a n d I R spectroscopy.

4. Conclusions T r a n s p a r e n t glassy m a t e r i a l s were o b t a i n e d for a Z r O 2 c o n t e n t of 10 mol%.

F o r p e r c e n t a g e s higher or equal to 30 mol% the m a t e r i a l s o b t a i n e d were o p a q u e a n d showed a strong t e n d e n c y to c r y s t a l l i z a t i o n of Z r O 2. H y d r o l y s i s in a s t r o n g acid m e d i u m favours the Z r O 2 segregation which l e a d s after h e a t i n g to the f o r m a t i o n of the m o n o c l i n i c phase. I n contrast, with h y d r o l y s i s in a low HC1 m e d i u m the Z r O 2 (t) is stabilized.

References [1] N. Claussen and M. Riihle. in: Advances in ceramics, Vol. 3, Science and technology of zirconia, eds. A.H. Heuer and L.W. Hobbs (The Amer. Ceram. Soc., Columbus, OH, 1981). [2] A. Paul, Chemistry of glasses (Chapman and Hall, London, 1982). [3] M. Nogami and M. Tomozawa, J. Amer. Ceram. Soc. 69 (1986) 99. [4] K. Kamiya et al., J. Mater. Sci. 15 (1980) 1765. [5] M. Guglielmi et al., J. Mater. Sci. Lett. 4 (1985) 123. [6] V. Gottardi et al., J. Non-Cryst. Solids 43 (1981) 105. [7] A. Lino and A. Mizuike, Bull. Chem. Soc. Japan 52 (1979) 2433. [8] H. Dislich, J. Non-Cryst. Solids 57 (1983) 371. [9] I. Salvado, A. Durfin and J.M. Fernfindez Navarro, Curr. Topics on non-cryst, solids, Proc. First Int. Workshop on Non-Cryst. Solids (World Sci. Publ. Co., 1986). [10] C.M. Phillippi and K.S. Mazdiyasni, J. Amer. Ceram. Soc. 54 (1971) 254. [11] D'yakonov et al., Inorg. Mater. 20 (1984) 79.