Apparatus for Two-Probe Conductivity Measurements on Compressed Powders Fred Wudl and Martin R. Bryce' Institute for Polymers and Organic Solids, Department of Physics, University of California, Santa Barbara, CA 93106 Electrical conductivity is a fundamental property of solids. The conductivity value, a, of a material is expressed in the units (fl cm)-' or S cm-1; the resistivity, p, is the inverse of this. Thevalue of afor solidsvaries over about 25 ordersof magnitude between a good insulator and a metal. For examole. at room temoerature, aromatic hvdrocarbons. ex.. anihricene and pe;ylene (1.)have cond;ctivity values the range u+. = 10-'a-10-22 S em-', whereas comer .- has a conduciiviti value (2) art = 6 X 105Scm-'. Materials are often loosely classified as insulators, semiconductors, or metals depending upon the value of a*. However, to define correctly a material in one of these classes, variable temperature conductivity measurements must be performed as well as thermopower measurements. As the temperature decreases, the electrical conductivity of a metal increases while that of a semiconductor decreases. Thus a solid with ud = 50 S em-' mav he a metal or a semiconductor. As a generalguideline, if a* 2 1 S cm-', the material will not show metallic behavior on coolina. A suoerconductor is defined as a material that has zero resistance, i.e., infinitely high conductivity. The last 15 years has seen a revolution in the area of electrically conductive organic materials. Traditionally organic solids aregood insulators but "organicmetals" entered the arena in the early 1970's with the discovery that crystals of the charge-transfer complex TTF-TCNQ, formed hetween donor molecule tetrathiafulvalene and acceptor molecule tetracyanoq"inodimethAe (TCNQ). showed o, values as hirh as 500 S &-I, with a increasing with coiling of the = lo4 S cm-I at -60 K sample to reach a,. (2). More recently, superconducting organic salts have been discovered, notably salts of the donors tetramethyltetraselenafulvalene (TMTSF) and bis(ethy1enedithiolato)-TTF (BEDT-TTF); presently the highest temperature for the onset of organic superconductivity is T,= 10.4 K for the salt (BEDTTTF)zCu(NCS)2 (3). Many organic polymers that have extensive a-electron delocalization along their backbones are also highly conducting in their doped (partially oxidized) states, e.g., polyacetylene, a, = lo4 S cm-', and poly-3-methylthiophene, a* = 450 S cm-'. For information on the chemistry and physics that underlies conducting organic compounds several review articles are avail-
-I..t is,.the. purpose of this article to descrihe the simple two-prohe apparatus that we use for measurine- t h e room-temoerature d.c. conductivity of solids in the form of com-
niques (5),but these measurements are often not possible to perform, as many materials can be obtained only in the form of microcrystalline powders. Our apparatus enables rapid evaluation of the room-temperature couductivity of very small quantities of any solid material that can be comoressed into a small oellet. As little as 1-2 me of samole is beeded for each measurement. The apparatus ys not sui'table for variable-temoerature measurements. It must be stressed that the condu&ivity values of an organic material as a compressed pellet is typically 1-3orders of magnitude lower than the value for single crystals of the same sample. This is due to the anisotropy of the conductivity exhibited by almost all organicmetals and semiconductors, i.e., the value of ovaries when measured along the different crystal axes. For TTF-TCNQ the ratio of a along the three principle crystal axes is -500:5:1 (4b),while for many other organic conductors., ex.. - . triethvlammonium-(TCNQL (6) . . the anisotroov may exceed lo3: Clearly, a compacted pellet will comprise random soatial orientations of the microcrvstals. so the bulk conductihty of the pellet is a weighted av&agedetermined largely by the value of a along the least conducting axis of the crystal and by interparticle resistance. Nonetheless, reliable data can be obtained on compressed pellets because couductivity is determined mainly by short-range order, not
..
ohln (A\, "
Apparatus for two-probe Wnductivily measurements on compressed powders
pressed pellets. Undoubtedly, better quality data are ohtained for single-crystal samples using four-probe techPermanentaddress: Department of
ham, DH1 3LE. U.K.
Chemistry.University of ~ u r -
long-range order, which is present only in single crystals. Our apparatus is shown in the figure. The sample holder (A) is a segment of thick-walled glass capillary tubing inside which the sample is compressed between two steel pins (B). Correct alignment of the pins and a good fit of the pins inside the capillary is necessary for accurate results. The pins (B) Volume 67
Number 8 August 1990
717
Conductlvlty Data for Selected Organic Metals and Semiconductors on(S cm-')
(thiswork)lb
Samples TTF-TCNQ TTF-TCNQB~Z TTF-TCNQCII CU-TCNQ (TMTSF~PFB Paly(3MT),fFs
8. 10.3 1.0.4, 1.3 lo-$, 5 X 10P. 3 X lo-' 2X10~2.5X10~2.4X10~3 12.5 4. 10. 1
Literature
lob7. 500" 2-lob7 3.4 x lo-'" 5X10-2Lb 540" 450de
where a is the cross-sectional area of the sample and 1 is the length of the sample. For our apparatus a = 0.0137 cm2and 1 is typically 0.5 mm. The conductivity values we have measured using our apparatus for a range of samples that are metallic or semiconducting are collated in the table and compared with literature values for those compounds. In general the values for different batches of each sample are reproducible to within a factor of five or better. Acknowledgment .
We thank E. Southwick and M. Kaplan for assistance and MHH thanks N.A.T.O. for financial support to visit Santa Barbara. Literature Clted
can be easily removed for cleaning, or replaced, by the use of screws ( C ) . The supporting block is made of material that has high hulk resistivity, e.g., Teflon. A micrometer is built into the apparatus to enable the length of the compressed sample to be measured as accurately as possible. The diameter of the sample holder, and hence the cross-sectional area of the sample, is fixed by the bore of the capillary tube that is used. Avoltage ( V )is applied t o the system by attachment of current leads to the screws C. The resultant current (0is measured in the external circuit. The conductivity value, a, for the sample is obtained from the formula
718
Journal of Chemical Education
309,119-126.
5. Coleman, L. R. Re". Sci.Insfrum. 1976,46,112&1126. 6. Me1hy.L. R.;Harder,R. J.;Hertler, W.R.:Mshler, W.;Benson,R.E.:MocheL,W E . J Am. Chem Soc. 1962.84,337b3367.
7. Wheland, R. C.: Gillson. J. L. J. Am. Chem. Sm. 1976.98,39163925. 3. Becheeard. K.: Jaeobsen. C. 8.: Mortensen, K.; P e d e m , H. J.: Thomp. N. SolidStnfe c ~ ~ ~ 1980.33. u o . 1119-1126.
9. hmaire, M.; Garreeu, R.; Gamier, F.;Roncali, &New J. Cham. 1987,ll. 705-708
