PREPARATION OF Al2O3 - TiC CERAMIC BODIES BY THE ALKOXIDE PROCESS

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PREPARATION OF Al2O3 - TiC CERAMIC BODIES BY THE ALKOXIDE PROCESS

M. Hoch

To cite this version:

M. Hoch. PREPARATION OF Al2O3 - TiC CERAMIC BODIES BY THE ALKOXIDE PROCESS.

Journal de Physique Colloques, 1986, 47 (C1), pp.C1-37-C1-40. �10.1051/jphyscol:1986106�. �jpa- 00225490�

JOURNAL D E PHYSIQUE

Colloque C 1 , suppl6ment au n02, Tome 47, fkvrier 1986 page c1-37

PREPARATION OF A1,0,-Tic CERAMIC BODIES BY THE ALKOXIDE PROCESS

M. HOCH

Department of Materials Science and Metallurgical Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, U.S.A.

Re'sumC - Une solution d'isopropoxide d1Aluminium et de Titane est d6compos6epar l'eau pour obtenir une poudre ultrafine d'oxydes. Le de'pst de carbone s'effectue par chauffage de la poudre 2 800°C dans du methane pendant quatre heures; la carburation se fait 1200°C sous atmosph$re d1hydrogSne. La poudre de A1 0 -Tic superrgactive est

2 3

pressge isostatiquement 0.3 GPa, et frittge 2 1350°C sous atmosphsre d1hydrog;ne. On obtient des pisces de densit6 th'eorique, avec des grains plus petits que 1 ym. La maille du Tic est de 0.4307 nm.

Abstract - An A1 0 /Tic mixed ceramic was prepared, starting from a 2 3

solution of A1 and Ti isopropoxides. These were decomposed in H20 to an hydroxides ultrafine powder of mixed oxides. Carbon was deposited on the powder by heating it to 800°C in methane for four hours. Carburization was carried out in a hydrogen atmosphere at 1200°C. The highly reactive A1 0 -Tic powder was pressed isostatically under .3 GPa pressure and

2 3

sintered to full density at 1350°C in a hydrogen atmosphere. The sintered material is made up of ultrafine (<<I pm) A1 0 and Tic grains.

2 3 The lattice parameter of the Tic phase is 0.4307 nm.

1 - INTRODUCTION

In several earlier papers Hoch and coworkers (1-7) applied the alkoxide process to prepare very homogeneous, highly reactive, high purity, ultrafine powders.

In the case of oxides, these powders were compacted, isostatically pressed at room temperature, and then sintered in air at temperatures up to 1500°C. The results were very dense (above 99%) bodies with a grain size of 1-2 pm. The oxides thus prepared were ferroelectric materials such as BaTi03, pure and doped with LapOg, Nd203, and Nb205; PLTZ,lead-lanthanum-titanium-zirconate (1);

and structural ceramics (2,3), such as Z y t t r i t p (yttria stabilized zirconia), alumina-zirconia eutectic and mullite.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986106

~ 1 - 3 8 J O U R N A L DE PHYSIQUE

The difference between alkoxide powder and normal powder was studied by Young and Cutler (8) who found that in sintering iyttrit@"the results (~ig. 2 ) substantiate the concl.usion that no surface diffusion occurs in sintering of this zirconia. This behavior is surprising, since surface diffusion would be expected to be enhanced by decreasing particle size; this zirconia had the smallest particles of the material studied (-100

1)"

The next step in the development of the alkoxide process was to convert the oxides or hydroxides to carbides and nitrides by carburizing them with methane to obtain silicon carbide and tungsten carbide (5) or nitriding them with ammonia to obtain alumina nitride, silicon nitride, and sialon (6,7). The powders were characterized by particle-size analysis, X-ray diffraction and, to see the bonding in sialon, examined with infrared radiation (9).

In the present study we prepared a sintered compact of aluminum oxide - ^30%

titanium carbide with greater than 99% density, starting with comrnerically available aluminum and titanium isopropoxides. We decomposed a solution of these substances in isopropyl alcohol with water to a homogeneous solution of the hydroxide, carburized the titanium oxide to titanium carbide, and sintered the powder to a dense compact.

2 - THERMODYNAMICS A&_KINETIC CONSIDERATIONS The reduction-carburization reaction can be written

Me0 + 2C + MeC + CO

.

This formula assumes that, if methane is used, the methane will decompose to carbon and hydrogen. As A1 C is not very stable, only the reaction that forms

4 3

Tic has to be considered. Table 1 contains the thermodynamic data for the various possible reactions, computed from Barin and Knacke (11). The CO pressure when reducing Ti0 should be the lowest; it is not, however, because the entropy of Ti0 is too small; we had already noted this earlier (101.

Table 1 shows that the CO pressure is high enough at 1250°C to carry out the reaction in a flowing hydrogen stream at 1 atm pressure.

Table 1

Gibbs energies of reactions and CO pressures for the formation of Tic from titanium oxides

1) T i O + 2 C -t T i C + C O AGO/R = 25.527 - 17.568T kK 2) Ti203 + 5C +2TiC + 3C0 AG"/R = 93.825 - ^58.221T

3) Ti02 + 3C -t Tic + 2CO AG"/R = 63.371 - ^40.765T

Lowest CO pressure from 2 at 600°C 7.4 x atm 800°C 5.9 x l f 5 atm 1250°C 3.4 x atm

The kinetics presents another problem: in equation (1) two solids react;

however, the Ti0 may behave as a liquid [in the nitridation of the A1203-Si02 ultrafine system (71, A1203 behaved thermodynamically as a liquid]. What is not known is whether the oxide particles and the carbon particles can be brought close enough together to carry the reaction (1) to completion, or whether the reaction stop at some point before completion.

3 - EXPERIMENTAL METHOD AND RESULTS

We used commercially available aluminum isopropoxide and titanium isopropoxide as the precursors to prepare the oxides. We mixed the precursors in a toluene isopropyl alcohol solution and hydrolyzed them in an ammoniacal aqueous solution: we washed the precipitate with deionized water, then three times with isopropyl alcohol and finally with toluene. The hydrolysis reactions are

A1(OC3H7)3 + 3H20 + A ~ ( o H ) ~ + 3C3H70H (2) T ~ ( o c ~ H ~ ) ~ + 4H20 + T ~ ( o H ) ~ + 4C3H70H

.

We then dried the hydroxides in an oven at 125'C for two hours. This drying removed most of the excess isopropyl alcohol. To determine how much oxide is present in the system we heated an aliquot part to 800°C in air for four hours:

the material left was the oxide.

We carried out the conversion of the titanium oxide into titanium carbide in two steps; first we heated the dried hydroxides, to 80OoC in a stream of methane for 12 hours, and cooled them to room temperature in a methane stream.

We heated an aliquot part of the black powder in air to 800°C, to determine the amount of carbon in the sample from the weight loss. In general, there was about 1.8 to two times as much carbon present on the sample as needed to convert the titanium oxide ( ~ i 0 ~ ) to titanium carbide. An appropriate amount of dried hydroxide was therefore added to the carbon-containing powder to obtain the correct carbon-to-titanium-oxide ratio for the conversion to stoichiometric Tic. The powder was mixed in a vibratory mill for four hours, using a plastic bottle and plastic balls to avoid contamination. The mixed powder was heated for four hours in a hydrogen stream at 1 atm at 1200 to 1250°C, cooled in flowing hydrogen, and stored it in tight plastic bottles.

X-ray diffraction of the powder revealed the presence of A1203 and Tic, with a lattice constant at 0.4307 nm. Electron micrographs of the powder resembled those of the nitride and sialon powders (6,7).

Considerable effort was made to find alternatives to the carbon deposition using methane; for example mixing the hydroxide powder with various types of carbon (such as carbon black), using the vibratory mill up to 12 hours. In all cases, however, the X-ray diffraction pattern showed the formation of Ti203, without further reaction. Apparently it was impossible to obtain good contact between the hydroxide powder and the solid carbon.

J O U R N A L D E PHYSIQUE

The powder was either pressed isostatically at .3 GPa at room temperature and sintered in a hydrogen atmosphere at 1350°C for two to three hours, or hot pressed at 1300°C.

Figure 1 shows the microstructure typical of an A1203-30%TiC compact, i.e. fine, evenly divided Tic particles, about 0.5pm in diameter in an A1203 matrix.

(a) ( b )

Fig. 1 A1203-30wt.XTiC compact prepared by the alkoxide process: (a) 200X, (b) 1500X.

REFERENCES

1. M. Hoch and K. S. Mazdiyasni, Proceedinzs of IV Interamerican Conference on Materials Technology, Centro Regional De Azuda Technica, Mexico, Buenos Aires, 1975, p. 322.

2. M. Hoch and K. M. Nair, Ceramurgia Int. 2, No. 2, 131-137 (1976).

3. M. Hoch, A. L. Thompson, J. Houck, and K. M. Nair, Journal of Powder and Bulk Solids Technology, pp. 34-38 (1977).

4. M. Hoch and K. M. Nair, Processing of Crystalline Ceramics, pp. 33-40, 1978; Plenum Publishing Corporation.

5. M. Hoch and K. M. Nair, Bull. Amer. Cer. Soc. 5 6 , 289 (1977).

6. M. Hoch and K. M. Nair, J. Amer. Cer. Soc. 8(2), 187-190 (1979).

7. M. Hoch and K. M. Nair, J. Amer. Cer. Soc. 58(2), 191-193 (1979).

8. W. S. Young and I. B. Cutler, J. Amer. Cer. Soc. 53, 659-663 (1970).

9. M. Hoch, T. Vernardakis, and K. M. Nair, Science of Ceramics 10, 227-233 (1980); Nederlandse Keramische Vereniging.

10. M. Hoch, A. S. Iyer, and J. Nelken, J. Phys. Chem. Solids 23, 1463-1471 (1962).

11. I. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances, Springer Verlag, Berlin, 1973; Thermochemical Properties of Inorganic Substances, Supplement, Springer Verlag, Berlin, 1977.