Low Temperature Synthesis of Negative Thermal Expansion Y₂W₃O₁₂

The effect of variation of transmission ratio (r) in the pulverisette-4 (P4) ball-milling machine on the synthesis of Y₂W₃O₁₂ is reported. Y₂O₃ and WO₃ powders have been milled in a P4 planetary ball mill with different r values, i.e., −1.5, −1.75, −2, −2.25, −2.75, and −3, at a disk revolution speed of 300 rpm for 10 h with toluene as the process control agent. Differential thermal analysis results suggest that the reaction temperature of as-mixed powder is 1000 °C. It decreases down to 845 °C with an increase in the r value up to −2.25. However, a further increase in the r value results in an increase in the reaction temperature. The average particle size for different r values varies in similar manner and it is found to be around 65 nm for r = −2.25. XRD analysis of 10 h milled powders with the r value of −2.25, heat treated at different temperatures confirms the formation of Y₂W₃O₁₂ at 800 °C. The low temperature synthesis leads to retention of finer grain size and hence, helps in good densification and sinterability. The above material shows a negative thermal expansion coefficient of the order of −7.1 × 10⁻⁶/°C in the temperature range 150-650 °C.