Thermal-conductivity measurement by time-domain thermoreflectance
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uction This article will not attempt to delve into the technical details of time-domain thermoreflectance (TDTR), but will instead provide an overview of the key operating principles and innovations in measurement capability. Three examples of the use of TDTR in materials discovery and the development of new materials will be discussed. The first example is high thermal conductivity in small zincblende crystals of boron phosphide (BP) and boron arsenide (BAs).1 The second example is the highest and lowest thermal conductivities achieved within the class of amorphous polymer molecular solids.2,3 The final example is of liquid-crystal networks4 and illustrates a theme of significant current interest in the community—the discovery of materials that have enhanced functionality in their thermal-transport properties. In other words, we seek materials that have more than a static thermal conductivity value and can instead respond to temperature changes or to an external stimulus.5 Advances in techniques for measuring thermal-transport properties have a long history. Ångström was a key innovator in material characterization nearly 160 years ago.6 He realized that he could obtain measurements of thermal
diffusivity by carrying out measurements with oscillating temperature fields. Ångström was likely the first to use the frequency domain to measure thermal-transport properties. One illustration in his 1861 paper6 is my favorite scientific drawing from the 19th century. The sample is in the shape of a square cross-section bar that sticks out of the plane of the drawing. The end of the bar is exposed to a temperature boundary condition set by flowing either steam or ice water across the end of the bar. The amplitude and phase of the temperature oscillations were measured by observing thermometers at fixed distances from the end of the bar. The data that Ångström collected using this method were good to within a few percent, as good as any measurement we have now of the thermal conductivity of Fe and Cu.7
TDTR fundamentals In developing TDTR, in one sense, all we have done is to use modern optical instrumentation to extend the frequencydomain measurements pioneered by Ångström from frequencies on the order of mHz to GHz. The basic layout of a TDTR apparatus is shown in Figure 1 and has not changed significantly since we first assembled the original
David G. Cahill, Department of Materials Science and Engineering, Materials Research Laboratory, University of Illinois at Urbana-Champaign, USA; [email protected] doi:10.1557/mrs.2018.209
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Thermal-conductivity measurement by time-domain thermoreflectance
pump-probe instrument in 2000.8 The two TDTR systems currently in use at the University of Illin
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