Testing the Accuracy of the GHP 456 and HFM 446 with the New ERM-FC440 Reference Material

1. Results Obtained with the GHP 456

The guarded hot plate (GHP) technique is an absolute method without any calibration of the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity. In the two-plate mode, the Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity λ is calculated from the power Q flowing into the hot plate with metering area A, the temperature gradient across the two samples ΔT as well as the mean sample thickness d as follows:

GHP measurements on ERM-FC440 samples were carried out using a NETZSCH GHP 456 HT Titan^® equipped with liquid nitrogen cooling. ERM-FC440a specimens with serial numbers 001, 002, 003, and 005 were used for the GHP tests. The pair of specimens 001+002 and the pair 003+005 were each measured simultaneously in the two-plate mode. The temperature gradient across the samples was 30 K at temperatures below -10°C, and 20 K at 10°C and higher temperatures. The specimens had rigid spacers in their corners with a length identical to the nominal sample thickness in order to ensure a defined sample thickness.

Figure 3 depicts the GHP measurement results: In the temperature range between -150°C and 70°C, the relative deviation from the nominal Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity values calculated from equation 1 is less than ± 1.3% for all measurement data except for one point at -150°C which deviates by -2.2%. These results are in accordance with the expected accuracy of the GHP 456.

2. Results Obtained with the HFM 446

The heat flow meter (HFM) technique is a relative method based on calibration of the heat-flux sensors using a reference material with known thermal conductivity. The unknown thermal conductivity λ of a sample is calculated from the heat flux per area Q/A and the temperature gradient ΔT across the sample with mean thickness d according to the Fourier‘s equation for onedimensional heat flow as follows:

The HFM measurements on the ERM-FC440 samples were carried out using the NETZSCH HFM 446 Lambda Eco-Line Small, Medium and Large instruments. Two HFM 446 Large devices with serial numbers 0009 and 0010 at different locations were used to test the ERM-FC440c specimens with serial numbers 004 and 005. Two HFM 446 Medium devices with serial number 0007 and 0009 were used to test the ERM-FC440a specimens with serial numbers 001, 002, 003 and 005. Three HFM 446 Small devices with serial numbers 0086, 0087 and SOA-002 were used to test ERM-FC440 specimens with the size 20 cm x 20 cm cut out of the ERM-FC440c board with serial number 005 after the measurements in the HFM 446 Large were completed. For documentation purposes, each 20-cm board was assigned an identification number with five digits, which was lasered on the front face (see also figure 1 on the right). All devices were calibrated with either NIST SRM 1450d or IRMM440. The measurements were performed at mean temperatures between -10°C and 70°C with a temperature gradient across the samples of 20 K and a contact pressure of maximum 2 kPa.

Figures 4 to 6 depict the measurement results from all HFM instruments. Across the entire temperature range, the relative deviation from the nominal thermal conductivity values calculated using equation 1 is within ± 1.5% for most measurement results, except for some measurement points at the highest temperature of 70°C. All results are in accordance with the expected accuracy of ± 2% for the HFM 446 heat flow meters.