A simple method for synthesis of acicular β-Si3N4 seed crystals

Acicular silicon nitride (β-Si3N4) seed crystals have been synthesized using a moderately low temperature heat-treatment process, with the aid of a low volume fraction of multiple sintering aids (i.e. MgO and CaO, in combination with either Y2O3 or Nd2O3). The effects of the heat-treatment cycle and the additive composition on the α- to β-Si3N4 phase transformation were examined, for both cold-isostatically pressed pellets and simple packed powder. In the case of the packed powder, full transformation was achievable following heat-treatment at 1625 °C for four hours, in ∼0.1 MPa nitrogen. The use of multiple oxide additives allows seed liberation from the heat-treated materials using a simple dilute HF soak at room temperature. The resulting β-Si3N4 seed crystals exhibit a narrow size distribution, with mean aspect ratios of ∼4:1.