GB2257532A

GB2257532A - Conductivity measurement cell

Info

Publication number: GB2257532A
Application number: GB9213190A
Authority: GB
Inventors: Duncan James Westland; Vladimir Skarda
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 1991-06-26
Filing date: 1992-06-22
Publication date: 1993-01-13
Also published as: GB9113740D0; GB9213190D0

Abstract

In an apparatus for measuring conductivity, a cell (1) receives a powdered or malleable material under test between two insulating discs (4, 5), and rams (2, 3) are provided for compressing the material. Probes (6) pass current through the material as it is compressed, thereby enabling a determination of the conductivity of the material. <IMAGE>

Description

CONDUCTIVITY MEASUREMENT CELL The present invention relates to a cell for the measurement of the conductivity of powders and malleable materials.

Recently there has been much interest in novel electrically conducting materials such as doped polymers and 7,7,8,8 tetracyanoquinodimethane complexes, for applications including, but not limited to, radio frequency yielding, solid state electrolytic capacitors, power transmission, and electrostatic discharge prevention.

Examples of doped polymers which might be used with the cell include Polythiophene, Polyaniline, Polypyrole or Polyacetylene.

Materials of this sort have limited processibility and are commonly formed as powders or malleable solids of very low rigidity. The physical nature of such materials makes the preparation of samples for electrical conductivity determination difficult. One technique which is frequently used is to compress a fine powder into a pellet and subsequently to measure the conductivity of the pellet using an in-line four-point probe technique. However although this method allows some determination of the conductivity, in practice is suffers severe disadvantages.

The contact resistance with the pellet is frequently found to be inconveniently high, and, more importantly, conductivity is limited by poor contact between the powder grains within the pellet and by cracks and fissures within the pellet, making repeatable determination of the conductivity difficult.

According to the present invention an apparatus for measuring conductivity comprises a cell for receiving a powdered or malleable material, means for compressing the material received within the cell, and a plurality of probes arranged to make contact with the material within the cell and to pass current through the material as it is compressed, thereby enabling a determination of the conductivity of the material.

The present inventors have found that by leaving the material in its powdered or malleable state, and compressing the material as the conductivity is measured a marked improvement is realised in the accuracy and repeatability of the conductivity measurement. By contrast with the prior art, spurious measurements are not encountered as a result of cracks and fissures in the sample. Moreover, the range of data that can be obtained from the material can be increased by measuring the conductivity as the pressure applied to the sample is varied, thereby allowing the effects of inter-grain contact resistance to be quantified.

Preferably the cell comprises one or more walls defining a hollow chamber for receiving the material, means for applying pressure dimensioned to fit within the hollow chamber with an inner face of the said means in contact with the material to be tested, the plurality of probes extending through the side walls into the chamber to make contact with the material in a region away from the said means for applying pressure.

Preferably the means for applying pressure comprise a pair of rams arranged to make a sliding fit with the inside of the chamber, and the material to be tested is supported within a region between the inner ends of the rams.

Preferably the material under test is sandwiched between a pair of non-conducting discs arranged between the inner ends of the rams.

An apparatus in accordance with the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a side elevation of an embodiment of the present invention; and Figure 2 is a plan view of the apparatus of Figure 1.

An apparatus for measuring the electrical conductivity of a powdered or malleable material comprises a non conducting cylinder 1 into which are placed two metal rams 2,3 which make a close fit within the cylinder 1. Two insulating discs 4,5 are placed between the rams and a sample of the material to be measured is placed between the discs. In use, the material is aligned with the tips of four test probes 6. The probes are placed at 900 intervals around the cylinders in the plane of the sample and make electrical contact with the edge of the sample. The probes are held into the cylinder by screw threads which allow their position to be adjusted. Pressure is applied to the sample by loading the two metal rams.The particular pressure applied will vary according to the nature of the sample, but for a typical powdered electrically conducting polymer would be in the range 107 to 2 x 108 Pa.

In this example, the cylinder part 1 of the cell is of perspex or polycarbonate (other materials are possible) and the cylinder is 25mm high with a 25mm outside diameter and a 6mm bore. The rams 2,3 are of steel 6mm diameter and 12.5mm (lower ram 3) and 25mm (upper ram 2) long. The insulating discs 4,5 are nylon 6mm diameter and lmm thick.

The probes 6 are stainless steel with an M3 thread and with cylindrical tips (which contact the sample) of O.25mm diameter and 5mm length. The tips could optionally be made of gold or platinum (or coated in these metals), which would reduce the contact resistance and also the possibility of the tips reacting chemically with the sample.

As pressure is applied to the sample, the resistance is measured using a four-point probe technique. A known current is injected into the sample through one of the probes and extracted via an adjacent probe. A voltage is then measured across the other two probes. The conductivity of the sample can then be found using the relationship

where a is the conductivity, I is the injected current, V is the measured voltage, and t is the sample thickness. For optimum accuracy, the thickness of the sample should be much less than its diameter.

Although described above in relation to a four-point measurement technique, the present invention is equally applicable to two-point conductivity measurement.

Claims

1. An apparatus for measuring conductivity comprising a cell for receiving a powered or malleable material, means for compressing the material received within the cell, and a plurality of probes arranged to make contact with the material within the cell and to pass current through the material as it is compressed, thereby enabling a determination of the conductivity of the material.

2. An apparatus according to claim 1, in which the cell comprises one or more walls defining a hollow chamber for receiving the material, means for applying pressure dimensioned to fit within the hollow chamber with an inner face of the said means in contact with the material to be tested and in which the plurality of probes extend through the side walls into the chamber to make contact with the material in a region away from the said means for applying pressure.

3. An apparatus according to claim 2, in which the means for applying pressure comprise a pair of rams arranged to make a sliding fit with the inside of the chamber, and the material to be tested is supported within a region between the inner ends of the rams.

4. An apparatus according to claim 3, in which in use the material under test is sandwiched between the pair of nonconducting discs arranged between the inner ends of the rams.

5. A method of measuring the conductivity of a powdered or malleable material comprising; receiving the material within a cell; compressing the material and simultaneously contacting the material with a plurality of probes and passing current through the material as it is compressed; and determining the conductivity in accordance with the magnitude of the current.

6. A method according to claim 5, in which the material is received in the cell inside a hollow chamber and is contacted by an inner face of pressure applying means dimensioned to fit within the chamber, and in which the probes make contact with the material in a region away from the pressure applying means.

7. A method according to claim 6, in which the pressure applying means comprise a pair of rams arranged to make a sliding fit with the inside of the chamber, and in which the material is supported within a region between the inner ends of the rams and is compressed by a load being applied to the rams.

8. A method according to claim 7, in which the material under test is sandwiched between a pair of non-conducting discs arranged between the inner ends of the rams.

9. A method according to any one of claims 5 to 8, in which the pressure applied to the material is in the range from 106 Pa to 109 Pa.

10. A method according to claim 9, in which the pressure applied to the material is in the range 107 to 2 x 108 Pa.

11. A method according to any one of claims 5 to 10, in which the material is a doped electrically conducting polymer.

12. A method according to any one of claims 1 to 10, in which the material is a 7,7,8,8 tetracyanoquinodimethane complex.

13. An apparatus substantially as described with respect to the accompanying drawings.

14. A method substantially as described with respect to the accompanying drawings.