How Reliable is the Thermal Conductivity of Biobased Building Insulating Materials Measured with Hot Disk Device?

Thermal conductivity is of high importance for insulating materials since it strongly influences the thermal performance of the building. Generally, it is recommended to measure this property with steady-state methods like guarded hot plate (GHP) or heat flow meter (HFM). These methods are reliable, but steady-state condition can take a long time to be reached. Therefore, transient methods were developed to speed-up the measurements. For instance, the hot disk transient plane source method is a widely used standard technique (ISO 22007-2) for measuring thermal conductivity of various materials. In the last 20 years, this technique has been applied also to bio-based insulating materials. However, overestimated thermal conductivity (compared to steady state method) are frequently measured. More generally, such differences are also observed for low thermal conductivity materials. The aim of this work is to evaluate the influence of numerous factors to explain the origin of these differences. The factors include the experimental setting parameters, the measurement analysis parameter or even the discrepancies between the theoretical model and the real experimental set-up. The analysis is performed for a light-earth biobased concrete made of raw earth and hemp shiv. Recommendations are proposed in conclusion.

Thermal properties of wood measured by the hot-disk method: comparison with thermal properties measured by the steady-state method

The hot-disk method is a transient method for the measurement of thermal properties. This method can measure both the thermal conductivity and thermal diffusivity in a short time for isotropic materials. To establish a method for measuring the thermal properties of wood by the hot-disk method, the relationship between the thermal properties of wood obtained by the hot-disk method and those obtained by the steady-state method was investigated. The thermal properties were measured by the hot-disk method using small pieces of kiri (Paulownia tomentosa), sugi (Cryptomeria japonica), hinoki (Chamaecyparis obtusa), yachidamo (Fraxinus mandshurica), and buna (Fagus crenata) when the hot-disk sensor was in contact with the cross section, radial section, and tangential section. The thermal conductivities in the longitudinal, radial, and tangential directions were also measured by the comparison method using the same specimen. The thermal properties obtained by the hot-disk method and the steady-state method were compared, based on the assumption that the thermal diffusivity measured by the hot-disk method was the geometric mean of that in the two main directions in the plane of the sensor, and the thermal conductivity measured by the hot-disk method was a power of that in three main directions. As a result, the thermal conductivity obtained by the hot-disk method was 10–20% higher than that obtained by the steady-state method; the thermal diffusivity measured by the hot-disk method was equal to that obtained by the steady-state method on average, while in the former thermal diffusivity varied widely. These results were found to be explainable in terms of the Dufour effect, which is the heat flow induced by the mass flow caused by the heating of the sensor, and the existing findings on the time dependence of the sensitivity coefficient in the hot-disk method. The present study proposed two methods for calculating the thermal properties of wood from the hot-disk method were proposed, and it was found that the errors between the obtained thermal properties and those obtained by the steady-state method differed depending on the calculation method.

Analysis of Different Thermal Conductivity Measurements for Al-Si Casting Alloys

Three types of methods, including Laser flash, Hot-Disk, and Wiedemann-Franz law, have been applied for thermal conductivity measurements of Al-Si casting alloys. The first two methods can obtain the thermal parameter directly for specific samples, while the third one calculates the target value by formula containing the electrical conductivity. Thus, the latter is widely used in the foundries because of its convenient and rapid characteristics. The purpose of this paper was to make a polite comparison among them and optimize the key constant C in the Wiedemann-Franz law to improve the calculation accuracy for Al-Si alloys. Measurements were conducted on the same set of specimens of Al-Si-xCu alloys (x ranges from 0.1 wt.% to 2.0 wt.%) at room temperature. The results showed that the measured value of Laser Flash method was well consistent with Hot Disk. While that of the Wiedemann-Franz law was different with them, the average deviation percentages were 2.17% and 2.36% when using empirical constant C (12.6 W/m·K) in the formula. Then, the constant C was modified to 8.4 W/ m·K and the average deviation percentage were decreased to 0.4% and 0.2% respectively. The reason for the differences was analyzed and a thermal conductivity evaluation model was proposed.

Effect of Different Firing Temperature on Thermal Conductivity of Ceramic Tiles

The aim of this study is to investigate the effect of different firing temperature on thermal conductivity of ceramic tiles. The body formulation powders of ceramic tiles were made according to the formulation given by company and compacted at 18 MPa using pressing machine in order to obtain button shape specimen with 50 mm diameter. The button shape specimen was fired at different firing temperature which 1150°C, 1175°C, 1200°C and 1225°C. Then, the thermal conductivity of fired specimens was measured by using Hot-Disk Thermal Constant Analyzer. Thermal conductivity result shows that the ceramic tile body fired at 1150 °C producing the lowest thermal conductivity values (0.97 W/mK) in comparison with other specimens. This low thermal conductivity performance is due to the high porosity value in the specimen as a result of more trapped air and implies delaying the heat transfer either inward or outward from the ceramic tiles. Therefore, this study proved that by altering firing temperature, different thermal conductivity values of ceramic tiles were obtained.

Study of Ultrasonic Attenuation and Thermal Conduction in Bimetallic Au/Pt Nanofluids

Here, we report the frequency dependent ultrasonic attenuation of monometallic Au and bimetallic Au/Pt based aqueous nanofluids (NFs). The as synthesized bimetallic nanofluids (BMNFs) revealed less resistance to ultrasonic wave compared to the monoatomic NFs. Thermal conductivity of both nanofluids taken at different concentrations, measured by the Hot Disk Thermal Constant Analyzer (TPS-500) revealed substantial conductivity improvement when compared to the base fluid, although Au/Pt showed lesser improvement compared to Au. We rationalized our obtained results with thorough characterization of the as synthesized nanoparticles/fluids with techniques such as XRD, UV-Vis, TEM, EDS etc. and some of the important information revealed were about the distinct two-phase bimetallic nature of Au/Pt, its two plasmonic band optical absorption feature and the spherical morphology of the particles. The findings were correlated with the observed thermal and ultrasonic behaviour and proper rationalization was provided. It was revealed that the comparatively lesser thermal conductivity of Au/Pt had direct implication on its attenuation property. The findings could have important repercussions in both industrial applicational aspects and mechanistic approach towards the field of ultrasonic attenuation in nanofluids