Article updated on 18 February 2021
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Assessing the thermal conductivity of various nanomaterials is essential to material engineering at the nanometric scale. Nanomaterials that conduct heat effectively for example, could be used to design future circuits and electronics, as removal of heat is one crucial challenge in electronic devices, partly because of the increasing need to dissipate power.
A nanomaterial’s capacity to conduct heat is based in its atomic structure. By understanding these thermal qualities, it can also reveal other essential characteristics of a material. When materials are structured on a nanometre scale, their thermal attributes are different from the same materials without these specific nanostructures. For instance, research studies of heat conduction in two-dimensional and one-dimensional crystals show that they have unique qualities that result in a substantial thermal conductivity.
Processes determining thermal conductivity can be split into two groups: steady-state and transient. In transient techniques, a thermal gradient is defined relative to time, allowing for quick measurements of the thermal diffusivity for large temperature ranges. In general, the specific heat and mass density need to be established independently of these procedures to determine thermal conductivity.
Although many procedures depend on electrical methods to heat a sample and determine temperature increase, there are other tactics where the rising temperature is supplied with a high-intensity light. In most steady-state techniques, the temperature is assessed by using thermocouples.
Laser Flash Technique
Thermal conductivity can be measured with the transient “laser flash” technique. This process typically uses a xenon flash lamp to heat a sample at one end via bursts of predetermined energy. The temperature increase of the sample is gauged at the far end of the sample using an infrared detector. The results generated by the detector are adjusted to compensate for the ambient conditions. The result of this test is a rising temperature curve depicting the change in temperature due to the flash lamp.
Thermal conductivity is determined based on the thermal diffusivity and specific heat of a sample. The shape of the rising temperature curve is used to determine thermal diffusivity. To ascertain specific heat, the temperature rise of the sample is contrasted to that of a reference sample.
Hot Disk Technique
The thermal conductivity and diffusivity of materials can be assessed simultaneously using a transient plane source, or “Hot Disk,” technique.
To carry out this test, a nickel sensor is put between two identical samples, to act as both a source of heat for the samples and as a temperature probe. Thermal qualities of the samples are determined by assessing the increase of their temperatures over time. The time and the heat range must be set so that the sample can tolerate the amount of heat and the temperature rise of the sensor is not affected.
Optothermal Technique
The laser flash technique described previously in this article requires a temperature drop in the sample film. Being one atom thick and having a relatively high thermal conductivity, graphene cannot provide such a temperature drop. Therefore, scientists at the University of California Riverside used a localized laser as a localized heating source with an optothermal Raman method, to find out the thermal conductivity of graphene for the first time.
The UCR scientists suspended graphene over a trench and heated it at the center. The heat then spread in-plane through the layer in the direction of a heat sink. The minimal cross-section of the heat conduction channel in graphene allowed for the detection of the temperature increase. The quantity of heat dissipated in graphene can be established by a detector placed directly under the sample.
The UCR team’s process for gauging the thermal conductivity of graphene has been expanded for use with other nanomaterial films, like graphene films and other materials with significant temperature-dependent Raman signatures.
Sources and Further Reading
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