SU1749806A1 - Device for determination of specific electrical resistances of carbon-graphite materials - Google Patents

Abstract

Use: study of the electrophysical properties of carbon graphite and other current-conducting materials used in machine building, including fibrous materials (such as felt, tows, etc.) to the measuring electrodes and potential probes mounted in the top cover, made in the form of a removable insulating sleeve. The lower current lead is made up of rigidly interconnected supports and a rod of graphite. On the side surface of the tubular heater opposite potential probes, potential taps in the form of carbon filaments are installed. 2 ill., 1 tab. (L

Description

The invention relates to a measurement technique using thermal means and can be used to study the electrophysical properties of carbon graphite and other conductive materials used in engineering, including fibrous materials (such as felt, tows, etc.).

A device based on a two-probe measuring cell is known for determining the electrical resistance of ceramic materials. The sample is placed inside a heated muffle on a ceramic stand. Metal surfaces are placed between the surfaces of the sample and the ends of the current leads.

spacers for leveling the potential across the sample surface. The temperature inside the muffle is measured using a thermocouple

The main disadvantages of this device are: the large error in determining the electrical resistivity of materials with low resistance due to the significant contact resistances at the boundaries of the sample-gasket-current-supply system; the inability to study soft and fibrous materials due to the violation of the sample geometry under the action of the force applied to the electrical leads to ensure good electrical contact; limitation

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about

the upper temperature limit of the test temperature of 1800-2000 ° C.

It is also known a device (prototype), widely used to determine the electrical resistivity ρ of graphites, carbides and other conductive materials in the high temperature range (from 1000 to 2000-2500 ° C, depending on the thermal conductivity of the sample).

However, the large temperature differences in the sample radius (up to 1000 ° C and above) arising from the testing of carbon graphite materials due to their relatively low thermal conductivity lead to significant errors due to the uncertainty of the reference temperature, as well as to a decrease in the temperature level of research.

The reliability of the contact of potential probes with the sample is not necessary, since the sample can be deformed during heating,

The device does not allow testing of soft carbon-graphite materials due to violation of the sample geometry, as well as due to temperature differences that are even greater than on solid samples.

The aim of the invention is to provide the possibility of determining the electrical resistivity of soft carbon materials (bundles, felt, etc.), increasing the accuracy in the temperature range up to 3000 ° C.

The essence of the invention is that the elements of the four-electrode measuring cell, including the lower graphite (including the upper) and potential current leads, are all fixed to the upper removable insulating cover, made in the form of a sleeve. The sample heater to high temperatures is itself a sealed chamber, made in the form of a tubular graphite furnace, inside of which a measuring cell is located. To eliminate the potential difference between the sample and the heater, a pair of potential carbon fiber taps was additionally installed on the camera body against the potential electrodes of the cell.

Figures 1 and 2 show a device for determining the electrical resistivity of carbon-graphite materials.

Inside the tubular graphite heater 1, located vertically, a four-electrode measuring cell is placed, consisting of an upper current supply 2, installed for testing sample 3, a lower current supply

4, rigidly interconnected circular graphite stand 5 and a supporting graphite rod 6. The upper current lead and the bearing graphite rod are fixed in

upper removable cover 7 of electrically insulating material. Potential probes 8 cells are made of thin graphite rods with carbon filaments 9 attached to them, the ends of which are fixed

in the same lid 7. A pyrometric sensor 10 in the form of a graphite rod with an axial bore (black body model) is installed on a graphite stand. On the heater surrounding the measuring

The cell, opposite its potential probes, has additional potential taps 11 for eliminating the potential difference between the sample and the heater. The device works as follows.

In the current leads of the measuring cell, the sample is clamped (in the collet clamps), the potential probes are inserted into it at a certain distance from each other (the distance is controlled by a caliper). The cell is inserted into the furnace inside a tubular graphite heater, on which it is supported by a centering electrical insulator sleeve. Make the necessary electrical connections. Bake

closed, pumped out with forevacuum pumps and filled with inert gas. The voltage is applied to the heater from the secondary winding of the power transformer, one end of which is grounded. Regulation

power on the heater brings the system to the desired temperature. A voltage is applied to the measuring cell circuit. A separate autotransformer, the winding of which is grounded, provides the voltage

to the lower potential probe cell. Independent regulation of the current in the circuit of the measuring cell and the potential on the lower potential probe ensures equal potentials in the same

the level of the heater points (at potential taps) and the sample (at potential probes). Then, the current I in the circuit of the measuring cell (sample) and the voltage drop U U at the working section of the sample are measured.

(between potential probes). The electrical resistivity of the sample material is calculated from the ratio

55

R

A us

where S is the cross-sectional area of the sample;

E is the distance between the potential probes.

Similar measurements are made at other temperatures.

The device was tested to determine the electrical resistivity of carbon felt with an apparent density of 0.1 g / cm3 (based on carbonized viscose fiber). For comparison, measurements were also carried out on unit A-3000 to determine the thermal conductivity of carbon-graphite materials with direct heating of the sample with an electric current fundamentally similar to the prototype (special transitional devices were made to secure the sample of the felt in the current leads).

The results of the definitions are presented in the table.

As the results show, the presence of large temperature differences in the thickness of the sample in the installation I-3000, similar to the prototype, leads to a noticeable difference between the results and significant

reduce the temperature range of research.

Claims (1)

The invention of the device for determining the specific electrical resistivity of carbon-graphite materials containing a sealed chamber, upper and lower graphite current leads to the measuring electrodes, fixed in the upper and lower covers chambers, and lateral graphite topsides to potential probes, m, characterized in that, in order to increase the accuracy of determining soft fibrous carbon-graphite materials in the interval temperatures up to 3000 ° С, hermetic chamber is made in the form of a tubular graphite heater, the lower current lead is formed from rigidly interconnected supports and a rod from graphite, the potential and lower current leads are fixed in the upper cover, made in the form of a removable insulating sleeve, and on the side surface of the tubular heater opposite potential probes are installed potential taps in the form of carbon filaments. Fie.2