Date of Publication

6-1-1967

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Applied Science

Department

Applied Science

Language

English

Subjects

Metals -- Thermal properties, Electric resistance

DOI

10.15760/etd.325

Physical Description

1 online resource (2, iv, 31 leaves)

Abstract

In engineering practice it is important to know which factors affect the thermal and electrical resistance of metal contacts. This thesis is to investigate some of these factors such as surface roughness and contact pressure. Thermal electrical contact resistance ratios for metal contacts were calculated from the experimental data. The technical literature was searched, and several papers were found in which either thermal or electrical contact resistance was studied separately. However, none of the papers recorded data for both thermal and electrical resistances for the same samples. The information found in these papers has been used as a background for understanding the nature of thermal and electrical contact resistance. Both of these contact resistances are primarily a function of the load on the contact and the condition of the surfaces. At low pressures only a small fraction of the total gross area of the contacts is in metal-to-metal contact. Increasing the load, flattens the “hills” and reduces both the thermal and electrical contact resistance. This phenomenon is called “spreading resistance” since the flow of heat or electrical current must spread out after they pass through the restricted areas that are actually in contact. Another type of thermal and electrical resistance, which is called “interface resistance", is caused by a film of foreign material such as an oxide, etc. on the surfaces of the contacting “hills”. If the space between the “hills” of a contact is filled with air, there is a heat flow by convection currents. The literature indicates this quantity of heat flow is approximately one thousandth of the total heat flow through metal contacts. Since the only electrical current conduction mechanism acting between areas not in actual metallic contact is that due to thermionic emission, the electrical resistance for these areas will be extremely high at room temperature for which thermionic emission is negligible. The experimental apparatus to measure both the thermal and electrical contact resistances consists mainly of a bellows-actuated press which is operated remotely under a vacuum bell. The press pressure loads the sample metal wafers. A thin film heat meter is used to indicate the quantity of heat flowing through the metal contacts. The temperature drop caused by the contacts is measured with thermocouples. The temperature difference and the quantity of heat flowing is used to calculate the thermal contact resistance. A strain gage on the bellows-press stem measures the loading on the contact surfaces. Electrical probes are used to measure the electrica1 resistance across the contact surfaces. The thermocouples and electrical resistance probes are permanently installed in the outer two smooth copper wafers. This makes it possible to quickly change to other sets of sample wafers of other metals and finishes. In order to use this permanent arrangement, it is necessary to finish two mating surfaces of the particular set of metal wafers to be tested, similar to the permanent smooth copper wafers so that these two extra mating contact resistances can be found and thus be subtracted from the overall contact resistance. The data indicates that the thermal-electrical contact resistance ratio can be changed by changing the load on the contacts. The heat meter had performed very well, and this new method of measuring heat flow will undoubtedly become a standard method of measuring heat flux.

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Comments

Portland State College. Dept. of Applied Science

Persistent Identifier

http://archives.pdx.edu/ds/psu/8227

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