Journal of Sol-Gel Science and Technology, 2, 371-375 (1994) Q 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands.

Preparation of A I 2 T i O s - Z r O 2 M i x e d P o w d e r s via Sol-Gel Process Code: DP7 L. BONHOMME-COURY, N. LEQUEUX, S. MUSSOTTE AND P. BOCH Laboratoire C~ramiques et Matdriaux Min~raux, UA CNRS 1466, ESPCI, 10 rue Vauquelin, Paris, France

Abstract. Sol-gel routes in the ternary system A1203-TiOs-ZrO2 were investigated to prepare A12TiOs-ZrO2 mixed powders. The preparation of ZrTiO4 and A12TiO5 was studied before going on with the ternary system. Zirconium titanate precursor gels were prepared from Ti(OPr~)4 and Zr(OPr'~)4 mixtures. The crystallization of ZrTiO4 develops at T < 700°C. AI2TiO5 was prepared by different ways, using mixtures of Al(OBuS)2(C6HaO3) with Ti(OPr~)4 (i), or with acetic acid addition (ii). Route (i) leads to a separate crystallization of TiO2 and c~-A1203, with subsequent formation of p-A12TiO5 at T ~ 1360°C. Although the pseudobrookite/3-A12TiO5 is thermodynamically unstable below 1280°C, route (ii) leads to the crystallization of metastable/3-A12TiO5 at T 800°C. At increasing temperature,/3-A12TiO5 decomposes into TiO2 and a-A1203, then the two compounds react to form stable/3-A12TiOs. For the ternary system, all the preparation routes which were studied lead to ZrTiO4 and a-A1203 with subsequent reaction (at T ~ 1500°C) to give/3-A12TiO5 and ZrO2. Keywords: sol-gel, aluminium titanate, zirconium titanate, A12TiOs-ZrO2, crystallization 1. Introduction

2. Experimental

Ceramics composed of highly anisotropic crystals such as aluminium titanate are attractive for their low average thermal expansion, which allow the materials to resist severe thermal shocks [1-2]. Sol-gel syntheses in the ternary system A12Oa-TiO2-ZrO2 were investigated to prepare A12TiOs-ZrO2 powders as precursor for ceramic materials. Syntheses of definite compounds ZrTiO4 and A12TiO5 were first studied. It is known that sol-gel syntheses of zirconium titanate usually lead to ZrTiO4 at temperatures < 700°C [3-4]. For aluminium titanate, the sol-gel syntheses usually lead to a separate crystallization of TiO2 and A1203, and A12TiO5 forms at temperatures higher than the eutectoid one (1280°C) [5-11]. However,/3-A12TiO5 can form at temperature lower than 1280°C when organometallic precursors are used [12]. In the present study, aluminium titanate was obtained at T < 800°C. Correlation between R.T. reactions and crystallization behaviour of the gel was explored.

Commercial alkoxides were used in their as-received state: titanium isopropoxide (Aldrich), a solution of zirconium n-propoxide (70%) in n-propanol (Aldrich), and aluminium sec-butoxide modified with ethyl acetoacetate (Alfa). Schematic diagrams of the preparative routes are presented in Figure 1. After hydrolysis, the oxide content is 5 wt% in colloidal solutions and gels as well. The various ratios were Zr:Ti:H20 = 1:1:8 for ZrTiO4, AI:Ti:CHaCOOH:H20 = 2:1:0:10 or 2:1:2:10 for AI2TiOs, and Al:Ti:Zr:CHaCOOH:H20 = 2:1:1:0:14 or 2:1:1:2:14 for A12TiOs-ZrO2. Preparations of A12TiO5 without or with acetic acid are labelled as route (i) and (ii), respectively. Crystallization was examined by X-ray diffraction (Philips 1710 diffractometer, Cu-Ka radiation) after heat treatment of gels (heating rate of 4°C • rain -1 from R.T. to the soaking temperature, soaking time of 2 h). The thermal behaviour of gels was followed by simultaneous differential thermal and thermogravimetric analysis (TA Instrument, SDT 2960) at 5°C. min-1

Bonhomme-Coury, Lequeux, Mussotte and Boch

372

AI(OBuS)2(C6H903) priOH

AI(OBuS)2(C6H903) priOH I

I

Ti(OPri)4

Zr(OVrn!4

I solution J

Ti(OPri)4 I $ [ solution 1

I

1 I CH3COOH

Zr(OPrn)4 priOH I Ti(OPri)4 I

I

I

I

[ CH3COOH1 [ I

[ limpidsol ] g

[ whiteprecipitate ]

I opaquegel

I opaquesol

translucentgel

translucentgel a

b

c

Fig. 1. Preparation of ZrTiO4 (a), A12TiO5 (b), and A12TiOs-ZrO2 powders (c).

a double monochromator spectrometer (Jobin-Yvon U 1000, A = 514.5 nm, P = 200-280 mW). Infra-red spectra were recorded on a Perkin Elmer spectrophotometer in the 4000--200 cm -1 range.

100.

80 600

3.

1400

Results

3.1. ZrO2-TiO 2 System

6 0 -

z= 2

In the ZrO2-TiO2 system (Figure la), the gel is amorphous at room temperature. Crystallization of ZrTiO4 is observed after 2 h at 600°C. The DTA curve exhibits a slight endothermic peak near R.T., due to the evaporation of solvent, and a complex exothermic peak from 200°C to 450 °C, due to the combustion of organic residues. An exothermic peak at 705°C is related to the crystallization of ZrTiO4.

10C 80-

/

6040

600

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3.2. A1203-TiO 2 System .

0

.

.

,

400

800

.

,

-

.

.

1200

.

,

.

1600

Temperature (°C) Fig. 2. TG and DTA curves of A12TiO5 precursor gels elaborated

via route [i] (a and b), and via route [ii] (c and d). under air flux. Raman experiments were recorded on

3.2.1. Route (i) without Acetic Acid.

In the AI203TiO2 system (Figure lb), the DTA-TG experiments on an amorphous gel elaborated without acetic acid (route (i)) are presented in Figure 2. A slight endothermic peak near R.T. and a complex exothermic peak from 210°C to 530°C are due to the evaporation of solvent

Preparation of A12TiOh-ZrO2 Mixed Powders

373

a.u.

,, T i O 2 ruffle o a-Al203

.

v 0

[3-AI2TiO5

0

V

0

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(20 °)

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do

Fig.

3. X R D o f the A I i T i O 5 precursor gel (ii): dried (a), after 2 h at 8 0 0 ° C (b), ll00°C, and 1500°C (d).

and the combustion of residual organic components, respectively. The weight loss of the gel is 50%. Two exothermic peaks at 772°C and 950°C are attributed to the crystallization of rutile-TiO2 and c~-A1203, respectively. A subsequent reaction between TiO2 and A1203 to give A12TiO5 is associated to an endothermic peak at 1360°C [13]. This is in the stability domain of ~-AI2TiO5 (stable over 1280°C) [14].

3.2.2. Route (ii) with Acetic Acid. Route (ii) also leads to an amorphous gel. However, the DTA-TG curves are not the same as those corresponding to route (i) (Figure 2). The main points here are i) a complex exothermic peak at 730-745°C and ii) two very broad endothermic peaks at T ~ 930°C and 1230°C. The XRD patterns of dried and heated gels prepared using route (ii) are presented in Figure 3. The exothermic peak is related to the crystallization of a pseudo-brookite aluminium titanate phase, similar to fl-A12TiO5. The broadening of XRD peaks, due to either the very small size of particles or residual stresses does not allow us to differentiate this phase from fl-A12TiOh. The similarity between both phases is confirmed by Raman spectroscopy. Spectra of a gel heated at 800°C and fl-A12TiO5 elaborated at 1500°C via a powder route are shown in Figure 4. The intense

400.0

600.0

flO0,O RCM-i

1000,0

b IEO0.O

Fig.4. Ramanspectraof the A12TiO5phase, obtainedat 800°C (a), and of fl-Al2TiO5obtainedat 1500°C via the powderroute (b). band at 143 c m - : can be attributed to small amounts of TiO2 (anatase) [15-16], which are present in route (ii) at 800°C. This band is the only noticeable difference between the two spectra. As temperature increases, the 800°C-crystallized aluminium titanate decomposes into rutile-TiO2 and ce-Al203. Above the eutectoid temperature, the two oxides react to form fl-A12TiOh. The two broad endothermic peaks shown in ATD experiment could be related to these reactions.

3.3.

Al203-TiO2-ZrO 2 System

In the ternary system (Figure lc), ZrTiO4 and c~A1203 are the first crystalline phases to develop at T 870°C and T ~-, 1045°C, respectively. The obtention of A12TiOh-ZrO2 powders requires thermal treatments at T ~ 1500°C. It involves the following reaction [17]: ZrTiO4 + A1203 --~ AI~TiO5 + ZrO 2

4.

Discussion

Any chemical modification of alkoxides is known to change the reactivity of the precursors by reducing their functionality [18]. This can be used to slow down the

374

Bonhomme- Coury, Lequeux, Mussotte and Boch

hydrolysis-condensation process for the most reactive precursors in order to conserve the homogeneity in a mixture of alkoxides having very different reactivities. In the present study, the role of acetic acid in aluminium titanate preparation is essential. In gel (i), hydrolysis of alkoxide mixture leads to a separate condensation of titanium oxide gel and alumina gel. In gel (ii), acetic acid modifies the chemical species that forms. When added to a titanium alkoxide, CHzCOOH binds to titanium and seems to form a bridge between two Ti atoms [19-20]. When added to aluminium alkoxides, CHaCOOH can also react [21]. The synthesis of gels from Ti and Zr alkoxides with acetic acid leads to complex compounds with acetate ligands [22]. In the present work, the true nature of CH3COOH bonding in gel (ii) remains an open question. AI(OBuQ 2(CrH903)/Ti(OPri)4/CH3COOH mixture was investigated by IR spectroscopy. The IR spectrum (not presented here) exhibits bands around 1632, 1610, 1590 and 1570 cm- 1. These bands are very close to the ones observed for AI(OBuS)2(CrH903), because of the chelating ethylacetoacetate ligand (uCO: 1635-1615 cm-1; uC = C: 1530 cm-1), and for the Ti(OPri)4/CH3COOH mixture, because of the bridging acetate ligands (uasCOO-: 1570-1535 cm-1; u~COO-: 1445 cm-1). A possible assumption is that acetate ligands may form a bridge between aluminium and titanium atoms: -Ti-O-C(CH3)--O-A1. Under hydrolysis or thermal decomposition acetate bridges could yield a mixed A1-O-Ti oxide network. This gel crystallises at T > 600°C into metastable/3-A12TiO5, then decomposes into the thermodynamically stable mixture of TiO2 + A1203. In the ternary system, no crystalline phase appears at 800°C and it seems that the mixture is still very homogeneous, with no demixing. The first phase which crystallises is ZrTiO4, as in the case of a powder route [17]. The reaction between ZrTiO4 and A1203 is complete after two hours at 1500°C. Attempts to form/3A12TiO5 and ZrO2 at 800°C, in the ternary mixture, gave no results: ZrTiO4 always forms first. Efforts are now devoted to the control of chemical reactions occuring in the early stage of synthesis, in order to prepare A12TiOs-ZrO2 powders at low temperatures.

5.

Conclusion

Sol-gel syntheses in the A1203-TiO2-ZrO2 system were investigated to elaborate A12TiOs-ZrO2 partic-

ulate composites. Preparations in the ZrO2-TiO2 system lead to crystallized ZrTiO4 at T ,,~ 650°C. Preparations in the AI203-TiO2 system lead to different resuits depending on whether acetic acid wits used (ii) or not (i). Route (i) leads to a separate crystallization of TiO2 and A I 2 0 3 which ulteriorly react at T ~ 1360°C to form fl-A12TiOs; route (ii) leads to a metastable /3- A12TiO5 phase, well crystallized at T ~ 800°C, that decomposes into TiO2 and A1203 under further heating. The two oxides react at high temperature to form fl-A12TiOs. Preparations in the AI2Oa-TiO2ZrO2 system lead to the crystallization of ZrTiO4 at temperature higher than 800°C, with ulterior reaction with A1203 (at about 1500°C) to give A12TiO5 +ZrO2 mixed powders.

Acknowledgments The authors are grateful to C. Bonhomme and G. Chottard for the Raman experiments and to R. Ollitrault for the DTA-TGA analyses.

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