Mapping the thermal structure of solid-media pressure assemblies

The refractory oxides MgO and Al2O3 are the most commonly used insulator, filler materials in solid-media pressure assemblies. These oxides react with one another at high temperatures and pressures, forming a well-defined layer of spinel (similar toMgAl(2)O(4)) at the contact. The spinel layer widens in proportion to the square root of time at a rate that also depends systematically upon temperature and pressure. On the basis of 44 piston-cylinder runs spanning 1.200-2,000 degreesC and 1.0-4.0 GPa. we present a general relationship describing the width (AY) of the spinel layer as a function of time (t. in s). temperature (T. in K) and pressure (R in GPa): DeltaX = [8.58 x 10(11) (.) exp(-48865/T - 2.08 (.) p(1/2))(.) t] (1/2). If the pressure and duration of an experiment are known (as is usually the case) this calibration makes it possible to calculate the temperature to within a few degrees at any location in a solid-media assembly where MgO and Al2O3 are in contact (at T above similar to1,200 degreesC - simply by measuring the width of the spinel layer with an optical microscope. Application of this "reaction-progress" thermometer to the 13- and 19-mm diameter piston-cylinder assemblies used in the RPI lab confirms generally parabolic axial T gradients with acceptably broad hot spots. Three-dimensional maps of the 19-mm assembly reveal a radial component to the thermal field. with somewhat higher temperatures near the graphite heating element (i.e., a saddle-shaped hot region). Two exploratory experiments in a multi-anvil apparatus at 14 GPa (1,700 and 1,975 degreesC) confirm that the reaction-progress technique will work at pressures well above 4 GPa. The piston-cylinder-based calibration predicts DeltaX to within a factor of two in the two multi-anvil runs, and relative changes in T along the assembly can be readily mapped. However, additional high-pressure calibration points will be needed before the thermometer can be used in quantitative multi-anvil applications. The spinel reaction-progress thermometer is easily implemented, and should allow other researchers to map the thermal structures of their own assemblies in a single experiment with one thermocouple.