Ceramic Industry

SPECIAL REPORT/THERMAL ANALYSIS: Measuring Thermal Conductivity

June 1, 2008
A recent development of the modified transient plane source (MTPS) technique offers an innovative solution to the traditional challenges of thermal conductivity measurement.



Whether for development or quality control purposes, obtaining accurate thermal conductivity data for ceramics presents numerous challenges to industry. Traditional methods are cumbersome, time-consuming and offer little flexibility for integration with more advanced production processes due to sample format restrictions.

By contrast, a recent development of the modified transient plane source (MTPS) technique offers an innovative solution to these challenges while providing a rapid, nondestructive test method that requires no advanced training requirements to operate. The MTPS technique offers a significant advancement in the measurement of thermal conductivity of ceramics, and offers flexibility for the testing of solids, liquids, powders and pastes as well. The MTPS-based sensor can be incorporated as an in-line quality control measurement for better process control in the manufacturing of ceramics.

Figure 1. The sensor includes a central heater/sensor element in the shape of a spiral surrounded by a guard ring.

Technique

The MTPS technique employs a new system that is comprised of a sensor, control electronics and computer software.* The sensor includes a central heater/sensor element in the shape of a spiral surrounded by a guard ring. The guard ring generates heat in addition to the spiral heater, thus approximating a one-dimensional heat flow from the sensor into the material under test in contact with the sensor (see Figure 1).

*C-Therm TCiTM thermal conductivity analyzer.

Figure 2. Voltage drop chart.

The voltage drop on the spiral heater is measured before and during the transient (see Figure 2), and the voltage data is then translated into the effusivity value of the tested material. The conductivity is calculated from the voltage data by a patented iterative method. Effusivity is defined as the square root of the product of thermal conductivity (k), density (r) and specific heat capacity (cp), and has units of KmsW2:

e = (kρcp)1/2

The system measures effusivity directly and determines conductivity from this measurement as well. The system automatically compensates for variations in sensor temperature, thus enabling reliable measurements over a wide range of temperatures (-50 to 200ºC).

Table 1.

Sample Results for Solids

For illustrative and comparative purposes in testing a solid ceramic, Pyroceram® was tested with the MTPS sensor. Pyroceram was originally developed for military applications and is now commonly used for various domestic purposes (e.g., cooking utensils). The thermal conductivity of the material is a key product performance attribute. Results obtained with the MTPS technique at ambient operating conditions are presented in Table 1. These results were obtained in under five minutes.

As a comparison point offered from a third-party lab, the University of Maryland’s Center for Advanced Life Cycle Engineering (CALCE) reports a thermal conductivity for Pyroceram of 3.96 W/mK for approximate ambient conditions via the traditional flash method (following ASTM E1461-92 Standard Test Method for Thermal Diffusivity of Solids by the Flash Method). The difference in the two methods’ reported values is less than 1.5%. The flash method employed in CALCE’s characterization of the material is well-known and provides a highly accurate measurement. The technique is advantageous in a number of circumstances, particularly in testing samples above 200ºC.

The principle points of difference between the flash and MTPS methods are the greater convenience and flexibility that the MTPS technique offers. The flash technique requires the sample to be reduced to specific geometry. In addition, if the sample is transparent, it must be coated with either a gold or graphite coating. The flash measurement also relies on inputted sample bulk density in calculating the thermal conductivity from the the direct measurement of the sample’s diffusivity and calibrated determination of heat capacity.

Other traditional methods of thermal conductivity measurement, such as the guarded hot plate (GHP) technique, also require specific sample dimensions, which is often expensive and destructive to accommodate. The GHP technique is also susceptible to errors arising from the non-realization of the assumed boundary or steady-state conditions. In contrast, the flash and MTPS methods for measuring thermal conductivity both remove the steady-state condition at the expense of measuring the temperature as a varying function of time.1 The MTPS technique is unique in its ability to nondestructively test any flat surface greater than 17 mm in diameter. Test times per measurement via the MTPS method are as short as one second.

Figure 3. MTPS test procedure.

Furthermore, since the samples are not altered by the MTPS method, other equipment can still be used after testing is complete (an important feature for researchers). The speed and small sample size gives the technology an advantage in studying experimental formulations. Overall, the simplified process and flexibility demonstrated by the MTPS technique in measuring thermal conductivity, as illustrated in Figure 3, offers a striking contrast to the traditional GHP and flash methods.

Due to the upper temperature limitation of 200ºC for MTPS equipment, the technique is employed in a complementary fashion to the traditional flash technique in maximizing asset utility and operator time in some quality control scenarios. The MTPS technique is applied to shoulder the bulk of high-throughput testing as a screening tool, while the flash technique is reserved for the more labor-intensive high-temperature testing requirements.

Figure 4. Small-volume test kit accessory.

Sample Results for Powders

The MTPS technique also offers the flexibility to test the thermal conductivity of powders. With use of a small-volume test kit (minimum sample volume 1.8 mL), such as the one shown in Figure 4, the MTPS method can accurately measure the thermal conductivity of a wide range of powder materials. In highlighting this capability, powdered zirconium oxide was tested with the MTPS sensor.

Table 2.

Zirconium oxide is an advanced ceramic used in the manufacture of opacifiers, electronics, sensors, abrasives, catalysts, high-temperature filler and insulation, and wear-resistant ceramic products.2 The material offers the advantages of having a very low thermal conductivity while remaining chemically inert up to very high temperatures. The results for the testing of the zirconium oxide powder are shown in Table 2.

Conclusion

The new MTPS-based analyzer offers researchers and technicians significant latitude in the study and characterization of a wide range of sample formats in the ceramic field. Both solid samples and powders can easily be tested, with results available in minutes. The MTPS’ operating temperature range is -50 to 200ºC, and the technique offers accuracy and precision comparable to traditional methods for thermal conductivity testing.

The key advantages of the MTPS method include a significantly shorter test time, greater ease of measurement and wider sample flexibility. As thermal conductivity takes on greater importance in the development and production control of ceramics, the MTPS technique provides researchers and manufacturers with a powerful new approach.

For additional information regarding thermal conductivity testing options, contact C-Therm Technologies Ltd. at 863 Union St., Fredericton, New Brunswick, Canada E3A 3P7; (877) 827-7623 or (506) 457-1515; fax (506) 454-7201; e-mail sales@ctherm.com; or visit the website at www.ctherm.com.

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