Examining And Measuring Microscopic Thermal Expansion

It is a well known fact that when a current flows through a resistor it must heat up. The power loss is often stated as proportional to the current squared, which is why electricity grids are engineered to have the highest voltage considered safe, as this reduces the current in the wires which significantly reduces the energy loss. It is also true that in most cases resistors gain resistance the hotter they get (there are some exceptions but in general the greater the temperature the greater the resistance). Now normally these troubles can be accounted for or worked out of designs by ever cleverer material scientists, but with electronic components becoming smaller, passing through microscopic into nanoscopic, thermal effects return with a vengeance.

In order to understand thermal characteristics on a microscopic level devices with great special resolution are required. In 1986, such a machine was created called the scanning thermal microscope. It can be seen as a very small thermometer with a probe coming to a spike only 300 nanometres wide. This can be moved carefully a minute distance above the specimen being observed and a sensitive thermocouple integrated into the tip will record the temperature changes.

This study has aimed to look at the thermal expansion due to the ohmic heating in a resistor. The opening simulation combines aspects of electric, elastic and thermal elements. The electrical to thermal part of the simulation was that of a direct current flowing through the resistor dissipating power. This provides the heat source for the thermal analysis. This part of the analysis is iterative with the non linear problem of the material’s electrical properties changing with temperature being calculated over small time intervals. The elastic part of the simulation takes the output of the thermal simulation and uses it to calculate the displacement vectors of the particles in the structure. After the simulation the resistor was created and studied experimentally under the same conditions as the model. The 3D temperature profile produced by the model was accurately found within the resistor. This agreement validates this model as accurate and so hopefully it can be properly employed in the future.

Paper links: Diffraction phase microscopy imaging and multi-physics modeling of the nanoscale thermal expansion of a suspended resistor


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