A high-temperature ultrasonic interferometer

Abstract

The interferometer was built as part of a general study of the liquid state. Velocity and absorption measurements of Boron Trioxide-Potassium Oxide mixtures were made between 344 and 1500°C and 1 to 15 MHz. Of special interest was the "Boron anomaly." The unusual peak in its thermal expansion coefficient and its very large viscosity range, suggest that B₂O₃, would be an interesting liquid for visco-elastic studies. In particular, the viscosity apparently behaves differently at high and low temperatures. Above about 800°C it is Arrhenius, whereas below, it is not.
High temperature ultrasonic measurements have been made by generating the sound at room temperature and transmitting it into the high temperature section of an oven using fused silica or polycrystalline sapphire rods. Such can not be used with corrosive liquids like oxides. Also, the desired high temperatures are above the softening temperature of fused silica (1100°C). Sapphire and silica are soluble in molten oxides. Also, polycrystalline sapphire has large grain boundary loss at high temperature. To avoid these problems here, molybdenum acoustic transmission lines (rods) were used.