Extended D.C. Electrical Transport Measurements on the Mixed Conductor Cu 3 Cs 2
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Mat. Res. Soc. Symp. Proc. Vol. 500 © 1998 Materials Research Society
transport mechanisms, and also addresses the stability of these materials. It is also shown that the data can be analyzed simply following Yokota [8]. EXPERIMENTAL The experimental set-up consisted of a CTI-Cryogenics closed cycle refrigerator, a Keithley 220 Programmable Current Source, a Keithley 196 Digital Multimeter, a Keithley 705 Programmable Scanner, a Lakeshore Cryotronics Model 330 Autotuning Temperature controller, all connected to a computer controller through a GPIB 488 bus. The routines for data acquisition and device control were home written in the National Instruments LabVIEW environment. Trial measurements were conducted under computer control to verify the overall software and instrument operation as well as the accuracy of the timed intervals. Confi-mation of the timing intervals was necessary since a run of the sample from 250 K to 450 K required 28 days of mostly unattended operation. The sample holder comprised of a mica - copper base, and spring loaded platinum current and voltage probes. For good thermal contact as well as electrical insulation between the sample and the cold finger of the closed cycle refrigerator, the sample was mounted on mica which in turn was mounted on a copper base. The sample was 4.57 mm in length and 1.92 X 0.54 mm in cross section. The current density was kept below approximately I XlI A/cm2 to prevent the plating out of the conducting ions on the electrodes. The distance between the voltage probes was 2.16 mm and they were approximately centered between the current probes to allow for a more accurate four point voltage measuring technique and to avoid the areas close to the current electrodes which may exhibit any unusual effects. Voltage versus time measurements were carried out for an applied constant current. After the voltage had reached steady state, the applied current was shut off and the decay voltage characteristics versus time were measured. Measurements were taken from 200 K to 450 K in 10 degree increments on the sample of Cu3 CS 2. The sample was allowed to equilibrate at each temperature set point for one hour prior to any measurements being taken. The sample used was one of the same samples produced by Kuo's group in 1983.
RESULTS AND DISCUSSION There are a number of well established methods for studying transport mechanisms in ionic conductors. The one used in this project is the polarization technique based on a theory originally developed by Yokota [8]. In this method one can measure the chemical diffusion coefficient, the total conductivity and the partial conductivities of the mobile species. The technique also lends itself readily to automatic control which is especially indispensable for the study of rapidly diffusing ions. Electronic probes (platinum) were used instead of ionic probes since ionic probes are susceptible to decomposition and often lead to large errors in the measured quantities [9]. Furthermore, measurement of the ionic conductivity component using
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