Tetragonal zirconia powders from the zirconium n-propoxide-acetylacetone-water-isopropanol system

J O U R N A L OF

Journal of Non-Crystalline Solids 147&148 (1992) 542-547 North-Holland

NON-CRYSTALLIN SO ELIDS

Tetragonal zirconia powders from the zirconium n-propoxide-acetylacetone-water-isopropanol system R. Guinebreti~re,

A. Dauger, A. Lecomte

and H. Vesteghem

Laboratoire de Cdramiques Nouvelles, UA 320 CNRS, ENSCI, 47 Avenue Albert Thomas, 87065 Limoges, France

As a part of a work concerning densification and toughening of silicate ceramic products, this paper describes the preliminary characterization of a zirconia precursor fabricated through a sol-gel route in the zirconium n-propoxideacetylacetone-water-isopropanol system. When the molar ratio R =[acac]/[Zr] increases from R = 0 to R = 0.8, the precursor changes from a colloidal precipitate to a polymeric gel with an increasing gelation time. Drying and firing the precipitates leads to the monoclinic stable form of zirconia while the first crystalline phase obtained beyond 500°C from the gel is the metastable tetragonal one. Successive steps of the reactions are investigated by small angle X-ray scattering, differential thermal analysis, thermogravimetry and X-ray diffraction.

1. Introduction

2. Experimental

It is well e s t a b l i s h e d t h a t d i s p e r s i n g z i r c o n i a p a r t i c l e s in an oxide m a t r i x can i m p r o v e t h e m e c h a n i c a l p r o p e r t i e s of t h e c e r a m i c b o d y [1]. T h e size a n d d i s p e r s i o n state of Z r O 2 p a r t i c l e s , t o g e t h e r with t h e i r c r y s t a l l o g r a p h i c s t r u c t u r e , a r e the main parameters controlling the toughening [2]. T h e s o l - g e l p r o c e s s is a s t r a i g h t f o r w a r d r o u t e to o b t a i n h o m o g e n e o u s l y d i s p e r s e d fine particles. D i r e c t hydrolysis o f a z i r c o n i u m n - p r o p o x i d e solution l e a d s to small Z r O 2 p r e c i p i t a t e s [3]. T r a n s p a r e n t m o n o l i t h i c gels can also b e a c h i e v e d [4,5], using a s u i t a b l e a d d i t i v e such as a c e t y l a c e t o n e to c o n t r o l r e a c t i o n kinetics. We describe here the preliminary characterization of a z i r c o n i a p r e c u r s o r u s e d to i n c o r p o r a t e a Z r O 2 s e c o n d p h a s e in v a r i o u s oxide ceramics. W e discuss t h e i n f l u e n c e o f t h e a l k o x i d e c o n c e n t r a tion in t h e sol a n d of t h e a m o u n t of a c e t y l a c e t o n e a d d e d . T h e n a n o s t r u c t u r e a n d crystalline s t a t e of z i r c o n i a p a r t i c l e s a r e s t u d i e d by small angle X - r a y s c a t t e r i n g ( S A X S ) , t h e r m a l analysis ( D T A , T G A ) and X-ray diffraction (XRD).

Zirconium n-propoxide (Alfa Products, Danvers, M A 01923, U S A ) was u s e d as p r e c u r s o r a n d i s o p r o p a n o l as solvent to p r e p a r e t h e sols. M o d i fication of Z r - n - p r o p o x i d e was a c h i e v e d by a d d i tion o f an a c e t y l a c e t o n e (acac) s o l u t i o n in isop r o p a n o l a n d t h e m o l a r r a t i o R = [ a c a c ] / [ Z r ] was s t u d i e d b e t w e e n 0.4 a n d 0.8 d e p e n d i n g on t h e c o n c e n t r a t i o n , C, o f Z r in t h e sol (0.1 < C < 1.5). T h e w a t e r u s e d for hydrolysis in s o l u t i o n with i s o p r o p a n o l was a d d e d u n d e r m e c h a n i c a l stirring o f t h e sol, a n d t h e m o l a r r a t i o W = [ H z O ] / [ Z r ] was fixed to 5. Small angle X - r a y s p e c t r o s c o p i c d a t a w e r e obt a i n e d with a p o i n t - l i k e c a m e r a . Two m o n o c h r o m a t o r s , a q u a r t z m o n o c h r o m a t o r giving a p o o r r e s o l u t i o n b u t high i n c i d e n t intensity a n d a doub l e - c h a n n e l cut g e r m a n i u m m o n o c h r o m a t o r with high r e s o l u t i o n a n d low intensity, w e r e available following t h e level of s c a t t e r e d intensity. T h e d e t e c t o r was a p o s i t i o n - s e n s i t i v e p r o p o r t i o n a l c o u n t e r with an effective l e n g t h o f 55 m m a n d a s a m p l e to d e t e c t o r d i s t a n c e of 500 ram. T h e s c a t t e r i n g v e c t o r H = 4"rrA-a sin0, w h e r e 0 is the

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

R. GuinebretiOre et aL / Tetragonal Zn powders

Bragg angle and A the X-ray wavelength, ranged from 0.06 to 2 nm -1. Experimental data were corrected for parasitic scattering and normalized to constant sample thickness, direct beam intensity and counter efficiency. Thermal analysis experiments ( T G A and DTA) were carried out in air flowing with 'Rigaku' equipment up to 1000°C at a heating rate of 10 K / m i r a The X-ray diffraction apparatus was equipped with a forward quartz monochromator, a plate sample holder and a curved position-sensitive proportional counter I N E L - CPS 120. NiO

543

powders were incorporated in samples as a standard.

3. Results

Transparent monolithic gels are obtained for a range of R values which depend on the concentratiom C (see table 1). For a given C, when R is too low, precipitation instantaneously occurs. When R is increased, we observe successively cloudy gels, transparent gels and stable sols.

5.000

20.000

5.000

20.000

3.000

10.000 f A , ~ , 2.000

,ooof

\

4.000

.~1.000

15.000

10.000

°iii

500

3.000 300

200

~ .

1%11

200

_r

0,2 0,3

0,5

2.000

1°°o,1 H(nm-1)

1

H(nrn-1) 5.000 1.000

I

(a)

0,5

~

~

~

1 H(nm-1)

'~

1,5

-

~

0

2

0 (b)

0,5

1 H(nm-1)

1,5

Fig. 1. (a) SAXS spectra of wet gels (C = 0.1, W = 5). Scattering curves, normalized intensities versus H and log-log plots, show the molecular nature of scattering entities. (b) SAXS spectra of wet gels (C = 1.5, W = 5) with intensity versus H and log-log plots. A broad m a x i m u m appears on the scattering curves, revealing some interparticle ordering effect.

R. GuinebretiOre et al. / Tetragonal Zn powders

544

Table 1 Range of R = [acac]/[Zr] values needed to obtain transparent gels as a function of the Z r - n - p r o p o x i d e concentration, C C

R

0.1 0.5 1 1.5

0.4-0.55 0.5-0.75 0.55-0.8 0.7-0.85

The polymeric nature of particles in transparent gels is clearly shown by SAXS (fig. l(a)), in the case C = 0 . 1 , even though the linear behaviour of the scattered intensity versus H, in a log-log plot, is rather poor. The observed slope, limited to the 0.2-1 nm -1 range, is near 2 and seems to decrease with increasing R. Particle size obviously decreases when R increases at given C, or when C is increased. For example, the radius of gyration of particles, as measured in the gel C = 0:5, goes from R g = 9 to 3.5 nm when R increases from 0.55 to 0.75. The main feature observed on the scattering curves is a single broad maximum (fig. l(b)) which is more and more evident as the particle size decreases, that is when C, and R, increase. This interparticle effect shows that particles are distributed not randomly, but in a more or less ordered manner. Such a short-range order between scattering entities makes evaluation of the radius of gyration and fractal dimension difficult. After gelation, the product is dried for 12 h at 110°C, and then fired. Figure 2 shows the thermal behaviour of the dried gels, C = 0.5, through D T A diagrams. The well known 'glow p h e n o m e n o n ' [6] occurring at about 430°C, is characteristic of the sudden crystallization of zirconia and gives rise to a sharp exothermic peak which disappears when R increases. For higher values of R, the crystallization kinetics become quite different with a broader peak occurring at higher temperatures (525°C). This effect clearly goes with the drastic increase of gelation time when R increases and gives evidence of a different crystallization mechanism associated with the chemical modification of zirconium-n-propoxide by acetylacetone.

An additional mass loss is associated with the new crystallization peak as shown by fig. 3, here in the case of C = 1.5 and R = 0.8. The same effects are observed whatever the concentration C in the explored range. Calcination of dried gels, for 1 h at 600°C, leads to crystalline powders. SAXS experiments performed on compacted xerogels of initial concentration C = 0A are shown in fig~ 4. The colloidal nature of particles appears on the log-log plot where Porod's law is approximately obeyed, for calcined gels as well as for precipitates (R = 0). The scattering curves exhibit a sharp increase of intensity towards small H values which is ascribed to the sample surface and powder effects. More surprising is the fact that the ordered arrangement observed in wet gels is maintained

530°C

525°C

440°C 530°C

430°C

425°C

0

200

400 600 Temperature (°C)

800

1000

Fig. 2. Modification of D T A traces with the R ratio (C = 0.5,

w = 5).

R. Guinebreti~re et a L / Tetragonal Zn powders

in xerogels, with characteristic distances varying from 15 to 25 nm very close to the mean particle diameter. The spatial distribution of elementary particles is quite different in the precipitates which appear to be large and non-dense aggregates, since no interference effect is observed. The size of these aggregates is much larger than the resolution limit of our apparatus. The schematic representation of fig. 5 shows the corresponding proposed nanostructures. X-ray diffraction measurements (fig. 6) show that the first crystalline phase observed in the xerogels is a fine-grained tetragonal zirconia (about 6 nm after firing at 500°C). The transformation towards the stable monoclinic phase begins rapidly. The perfect crystallinity of powders

ii

5.000

30.000

,°.°°° I

.ooob.. ......,\ \

4.000 ,'-' V',

100 ~ 3.000

20

40

E