Thermal conductivity is a measure of the ability to transfer heat in a material. It is temperature dependent, and hence influenced by for example material defects and free electron concentrations. It can also be nonlinear over a broad temperature range. Furthermore, it may vary considerably between different samples/batches of the same material. To theoretically predict how thermal conductivity depends on temperature is challenging, and experimental methods are therefore essential.
The temperature dependence of the thermal conductivity is governed by the temperature dependence of the dominating heat transfer mechanism. In pure metals, heat is mainly transferred by free electrons, resulting in a decrease in thermal conductivity with increasing temperature. In electrically insulating materials the energy transport is dominated by lattice vibrations, by which instead generally follows an increase in thermal conductivity with increasing temperature.
Hot Disk AB offers various solutions with a wide range of instruments based on the TPS technique and temperature control units (TCU) as accessories to assist customers in tackling all kinds of temperature-dependent measurement situations. Each TCU can be controlled by Hot Disk software, which allows users to perform transient measurements scheduled with targeted temperatures.
In this application note, the unique ability of Hot Disk instruments to study thermal transport properties in materials at high temperatures is demonstrated by thermal conductivity measurements on fused quartz between -20°C and 1000°C. This was enabled by using a TPS instrument with suitable Hot Disk sensors and TCU accessories. Fused quartz is a material with thermal conductivity having a well-characterized temperature dependency; it is also a suitable demonstration material for elevated temperatures due to its chemical stability.
| Sample: | Fused quartz (diameter 60 mm, thickness 20 mm) |
Momentive (certified 99.99% purity) |
|---|---|---|
| TCU: | Bath circulator (temp range of operation: -20 ~ 180°C) |
Thermofisher |
| Convection Oven (temp range of operation: RT ~ 300°C) |
Carbolite | |
| Tube Furnace (temp range of operation: 100 ~ 1000°C) |
Entech | |
| Hot Disk instrument: | TPS 3500 | |
| Hot Disk sensor: | 4922, Kapton insulated (for temperatures < 300°C) |
|
| 4922, Mica insulated (for temperatures > 300°C) |
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| Measurement transient time: |
80 s | |
| Measurement heating power: |
400 mV |
Figure 1. Thermal conductivity of fused silica as a function of temperature, measured by the Hot Disk instrument (in red), and for comparison data obtained from Guarded Hot Plate measurements1 (in blue) and data obtained from TDTR measurements more recently2 (in grey).
In the measurements, the bath circulator and Kapton insulated sensor were used for temperatures -20 to 100°C, the convection oven and Kapton insulated sensor were used for temperatures RT to 300°C, and a tube furnace and Mica insulated sensor were used for temperatures above 300°C. Mica insulated sensors allow measurements up to 1000°C. For the tube furnace tests, nitrogen gas was purged during the measurements to suppress oxidation of – respectively – the sample, sensor and contact wiring.
For each programmed temperature, the TPS instrument was set to perform three repeated transient recordings with a 20-minute interval in-between, to ensure isothermal conditions in the sample before a new transient recording was initiated. Figure 1 (in red) shows the measured thermal conductivity as a function of temperature, taken as the average value of the three values extracted from the three transient recordings, respectively.
The Hot Disk Analysis software incorporates a special feature allowing users to calibrate the TCR as a function of temperature for an individual Hot Disk sensor, and to apply this TCR data when determining the thermal transport properties from a transient recording. This as an alternative to using the default TCR data provided for Hot Disk sensors, which has been factory calibrated by averaging over a series of Hot Disk sensors. More information on calibrating TCR for a specific Hot Disk sensor using a Hot Disk instrument, is available upon request.
In summary, the ability to measure thermal conductivity up to 1000°C is demonstrated on a fused quartz sample. Three different TCU accessories were employed to cover a broad temperature range between -20 to 1000°C. Measurement results proved in good agreement with literature data, with less than 5% deviation up to 500°C. For higher temperatures the deviation was slightly larger, which may have been caused by oxidation of the sample, sensor and/or contact wiring, as well as by reduced temperature stability in the test chamber.
The Hot Disk AB company continuously develops their instruments and accessories especially for higher temperature applications, and all feedback from customers is appreciated. For more information, questions or comments, please contact us at: moc.stnemurtsniksidtoh@ofni.
References
- A. Sugawara, Precise determination of thermal conductivity of high purity fused quartz from 0 to 650 °C, Physica 41 (1969) 515-520↩︎
- http://users.mrl.illinois.edu/cahill/tcdata/tcdata.html↩︎
