Solid State Reaction Sintering of Zirconium Tungstate

Zirconium tungstate is a special kind of negative thermal expansion material with isotropic negative thermal expansion property. It reaches thermodynamic stability in the temperature range of 1378K-1530K, and exists in metastable state at room temperature. At present, three kinds of phases have been found: low temperature cubic phase a-ZrW₂O₈, high temperature phase beta-ZrW₂O₈ and high pressure phase gamma-ZrW₂O₈. Their thermal expansion coefficients are aalpha=-8.7x10^-6/K, abet=-4.9x10^-6/K and agamma=-1x10^-6/K.

The preparation of negative expansion material zirconium tungstate by solid state reaction sintering can be carried out in the following steps:

1.Ingredients: Zirconia powder and tungsten oxide powder are mixed in the proportion of 1:3.5-4 to obtain the mixture material. Then the mixture material, zirconia spherulite and distilled water are added into the ball mill tank according to the ratio of 1:2-5:0.5-2.

2.Ball mill: Place the ball mill in the fast ball mill or planetary ball mill in step 1. The average particle size of the mixture is less than 1 micron. Then add polyvinyl alcohol as adhesive and ball mill for 5 minutes before extracting the slurry.

3.Drying slurry: The slurry obtained in step 2 is passed through 40 target quasi-sieves, then dried in an air-blast drying box at 85-100 ℃ for 14-16 hours, and then crushed with a mortar and passed through 60 target quasi-sieves to obtain powder.

4.Obsolescence: Add distilled water to the powder obtained in step 3, mix evenly and pack it for 24 hours.

5.Die forming: the stale powders in step 4 are moulded in the pressure range of 110-140 MPa with a tablet press and used as samples.

6.Sintering: The sample is placed in a sealed SiC crucible, discharged in a muffle furnace at 500 ℃, heated to 600 ℃ at the heating rate of 200 ℃/h for 1 h, then heated to T at the heating rate of 300 ℃/h, of which 1105 ℃ < T < 1260 ℃ for 2 h, and finally cooled to room temperature by air or water, that is, the negative expansion material zirconium tungstate is obtained.

Among them, the air cooling and water cooling methods can well inhibit the eutectoid Decomposition Phase Transformation of ZrW₂O₈ and maintain a high yield of ZrW₂O₈. The yield of ZrW₂O₈ can reach more than 80%. The yield of ZrW₂O₈ can be increased to about 95% by process improvement and adding proper amount of phase change inhibitor catalyst.

• < Prev
• Next >