In Situ Measurement Of Transport Properties For Conductive Materials Under High Pressure And High Temperature

In situ measurement of transport properties for materials under high pressureand high temperature (HPHT) has been widely used in physics, materialogy,geophysics, etc. This method can be used in study on phase transition, P-T phasesdiagram, thermoelectric properties, minerals, and so on. Pressure-inducedmetallization for semiconductors has been successfully found by the in situmeasurement technique.In situ measurements of resistivity and Seebeck coefficient under HPHT weremost on diamond anvil cells (DACs) equipment. Compared to multiple anvilequipment, DAC has higher pressure and temperature. However, there are alsodisadvantages for DAC during the in situ measurement, for example: small capsule,higher pressure and temperature gradient, difficult to reuse of the sample, hard tooperate, and so on. While, multi-anvil apparatus has larger cell (the sample volumescan increased to a few cubic millimeters) which is enough to measure two or morekinds of transport properties simultaneously. Besides the HPHT environments in themulti-anvil cell can be stable for a long time which is necessary for accuracymeasurement. So far, in situ measurements of resistivity and Seebeck coefficient werealways at high pressure and room temperature, for HPHT in situ measurementtechnique is more intricacy and hard. Furthermore, the temperature effect is morecomplex and the phenomenon of the experiment is hard to explain. In this paper, wepresent the development of techniques for performing simultaneous measurement ofelectrical resistivity and Seebeck coefficient of the thermoelectrical material Ag2Te atHPHT in a cubic anvil apparatus.The samples used in this experiment were β-Ag2Te, the process of synthesis asfollows. High pure tellurium (99.999%) and silver (99.9%) powders in astoichiometric ratio1:2were mixed in an agate motar. HPHT synthesis experiment was carried on a cubic anvil apparatus (SPD6×600). The experiment was performedat pressure up to5.0GPa and temperature up to1273K for1hour. The density of thesynthesized β-Ag2Te was8.335g/cm3, the relative density was get to99%of thetheoretic density, the density can be enhanced by about10%than traditional vacuumsintering method. Differential thermal analysis indicated that an endothermic peak at428K. This result indicates that there is a temperature-induced phase transition. TheXRD patterns of Ag2Te showed that the phase transitions, which were induced byhigh pressure and high temperature, were reversible.To exploit the special design of HPHT sample assembly, the test circuit andtemperature gradient of in situ measurement have been successfully obtained. Theresults of the in situ measurement of electrical resistivity and Seebeck coefficient forAg2Te under HPHT verified that this method is accurate and reliable. Under HPHT,the resistivity was measured by van der Pauw method and the measurement resultsshown as follows: at selected pressure1,2,3GPa, the curves of temperaturedependence of the electrical resistivity are similar. Under these pressures, Ag2Te isshown semiconductor behavior. The transition temperature of thetemperature-induced phase transition increases with the pressure increasing. Thisresult may be attributed that the molar volume of Ag2Te is greater for theface-centered-cubic phase than for the monoclinic phase. With the pressure increasingfurther, the resistivity slightly increase with the increase of temperature. This resultindicates that β-Ag2Te exhibits a metal behavior. So pressure-induced metallization ofβ-Ag2Te occurred at3~3.8GPa. The temperature gradient produced by the HPHTsample assembly was used for the measurement of Seebeck coefficient. Based on theresults of Seebeck coefficient measurement, the same temperature of phase transitionat selected pressure was confirmed. After phase transition, the Seebeck coefficient andpower factor of α-Ag2Te were higher than β-Ag2Te. This result indicates that theα-Ag2Te may be more suitable as the thermoelectric material than β-Ag2Te. Used theresults of in situ measurement under HPHT, the maximum value of power factor of Ag2Te is6.49×10-4W/mK2obtained at2GPa and690K. This value is superior to theresults of the Ag2Te obtained at ambient pressure.