CALIBRATION OF A HIGH TEMPERATURE X-RAY DIFFRACTION STAGE BY DIFFERENTIAL THERMAL EXPANSION, A.R. Drews, pp. 44-49

A new method for determining the temperature calibration function for a high-temperature X-ray diffraction stage is described. This method utilizes the relative thermal expansion of two diffraction peaks, either for peaks from a mixture of phases, or from a single phase that has anisotropic thermal expansion. A calibration function is derived from data at many temperatures and collected over a narrow angular range. Because only the relative separations of closely spaced peaks are used, this method is fast and insensitive to geometric aberrations. Materials used for calibration can include phase transition standards for an independent check on the calibration function. A new type of single-point temperature calibration standard can also be constructed that relies only on the observation of peak-crossing events at specific temperatures that are caused by relative thermal expansion. INTRODUCTION The thermocouple calibration in high-temperature X-ray diffraction (HTXRD) stages is accurate only for situations where the specimen is in direct thermal contact with the thermocouple. Any thermal impedance between the specimen and the thermocouple in the presence of a temperature gradient will lead to a discrepancy between the specimen temperature and the thermocouple temperature. In strip-heater HTXRD furnaces where the thermocouple is welded to the underside of the primary heating strip, there is reasonably good thermal contact between the thermocouple and small, thin specimens that can be placed directly on the heating strip above the thermocouple. For reactive specimens that may damage the heating strip, protective barriers between the specimen and the heating strip are necessary, but at the expense of invalidating the thermocouple calibration. The preferred method of calibration in HTXRD is to use the temperature dependence of the X-ray diffraction from a standard material. These reference materials are generally either single point calibrants, (phase transition standards), or multipoint calibrants, where direct measurement of the thermal expansion of the reference provides a calibration curve. The most frequently used materials for single-point temperature calibration are melting point standards. There are numerous examples of common materials with very well established melting temperatures that are adequate for this technique, such as NaCl, Ag or Au. These materials can be used for a very rapid calibration by monitoring a prominent peak (narrow angular range) at a small number of temperatures that span the melting transition. The melting Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol.44 1 44