A near zero coefficient of thermal expansion ceramic: Tantalum oxyfluoride

Cubic tantalum oxyfluoride (TaO2F) powders were synthesized and consolidated by hot pressing. The effects of sodium fluoride (NaF) and lithium fluoride (LiF) sintering aids on the densification and microstructure of TaO2F were investigated. Hot pressing conditions employed in this study appeared to change the stoichiometry of TaO2F, and most likely created an oxygen deficient TaO2−δF. It was shown that TaO2F decomposes in air at temperatures ≥500 °C but that the kinetics of its decomposition is very slow up to 750–800 °C. Hot-pressed TaO2F had a near-zero coefficient of thermal expansion over 25–600 °C that did not change after exposure to air for extended periods of time (24 h) at 600 °C. TaO2F has no inherent absorption bands in the mid-infrared (mid-IR) region up to about 5 μm suggesting that polycrystalline TaO2F ceramic can be IR transparent provided that it can be densified without altering its chemistry.