Accurate Values Of Thermal Conductivity Research Articles

Analysis and improvement of the hot disk transient plane source method for low thermal conductivity materials

The hot disk transient plane source (TPS) method is a widely used standard technique (ISO 22007–2) for characterizing the thermal properties of materials, especially the thermal conductivity, k. Despite its well-established reliability for a wide variety of common materials, the hot disk TPS method is also known to suffer from substantial systematic errors when applied to low-k thermal insulation materials, because of the discrepancies between the idealized model used for data analysis and the actual heat transfer process. Here, we present a combined numerical and experimental study of the influence of the geometry of the hot disk sensor on the measured k value of low-k materials. We demonstrate that the error is strongly affected by the finite thickness and thermal mass of the sensor's insulation layer as well as the corresponding increase of the effective heater size beyond the radius of the embedded metal heater itself. We further numerically investigate the dependence of the error on the sample thermal properties, confirming that the errors are worse in low-k samples. A simple polynomial correction function is provided based on the numerical error analysis, which converts the apparent (erroneous) result from a standard hot disk TPS measurement to a more accurate value of k. To experimentally validate the conclusions from numerical simulations, standard polyimide (Kapton) sensors are systematically optimized (thinned) by etching and used to measure low-k materials, including a standard polystyrene foam, a commercial Airloy® x56 aerogel, and a commercial Hydrophobic Silica Disk aerogel from Aerogel Technologies, LLC. The experimental results clearly demonstrate the strong influence of the sensor thickness. The k results of these samples obtained using either the optimized sensor or the pristine sensor then corrected with the corresponding polynomial correction functions are in good agreement with the values measured independently using a steady-state heat flowmeter (HFM) method: whereas the raw values measured with the pristine sensor are in error by 35% and 40% compared to the HFM reference values for the Airloy® x56 and the hydrophobic aerogel, respectively, the correction function greatly reduces those errors to <2% and <4%. This study reveals the detailed mechanisms of the systematic error in the hot disk TPS method for low-k samples, and shows that both the numerical correction to a pristine sensor and/or using an optimized (thinned) sensor are capable of providing highly accurate values of thermal conductivity for such materials.

Mean Free Path Effects on the Experimentally Measured Thermal Conductivity of Single-Crystal Silicon Microbridges

Accurate thermal conductivity values are essential for the successful modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure the thermal conductivity of these systems, as well as the thermal conductivity itself, varies with the device materials, fabrication processes, geometry, and operating conditions. In this study, the thermal conductivities of boron doped single-crystal silicon microbridges fabricated using silicon-on-insulator (SOI) wafers are measured over the temperature range from 80 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and either 50 or 85 μm wide. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry (SSERT) and optical time-domain thermoreflectance (TDTR). A thermal conductivity of 77 Wm−1 K−1 is measured for both microbridge widths at room temperature, where the results of both experimental techniques agree. However, increasing discrepancies between the thermal conductivities measured by each technique are found with decreasing temperatures below 300 K. The reduction in thermal conductivity measured by TDTR is primarily attributed to a ballistic thermal resistance contributed by phonons with mean free paths larger than the TDTR pump beam diameter. Boltzmann transport equation (BTE) modeling under the relaxation time approximation (RTA) is used to investigate the discrepancies and emphasizes the role of different interaction volumes in explaining the underprediction of TDTR measurements.

On-line temperature measurements for polymer thermal conductivity estimation under injection molding conditions

The determination of reliable and accurate values of thermal conductivity of semi-crystalline polymers is a challenging task because of the intimate coupling existing between crystallization and the conditions of molding process. In this paper, we describe an experimental device that allows to the molding of the polymer and to submit it to conditions close to those of the injection process (high pressure and shear). It permits to identify the thermal conductivity according to the temperature, both in melted and solid phases by solving an inverse heat conduction problem. Taking into account the shrinkage of the piece and its anisotropy, our results demonstrate that this conductivimeter is a reliable experimental apparatus.

Thermal conductivity of gas mixtures

In continuation of our earlier work on binary mixtures (1992 J. Phys. B: At. Mol. Opt. Phys. 25 679), an empirical relationship to estimate the higher-order contributions to thermal conductivity of dilute monatomic multicomponent gas mixtures has been developed. This relationship is able to reproduce the exact contributions to within 0.2%. Finally, a method of estimating accurate values of thermal conductivity is outlined.

PCB glass-fibre laminates: Thermal conductivity measurements and their effect on simulation

Accurate values of thermal conductivity are required for the simulation of temperature phenomena in electronic circuits. This paper presents the results of measurements carried out to determine the thermal conductivity along and normal to the plane of fibre glass laminates used in the manufacture of printed circuit boards. It has been found that the reinforced fibre-glass substrates used in PCBs are strongly anisotropic with the conductivity normal to the boards being much smaller than tangential to it. The test samples were type FR4 epoxy/glass laminates. An experiment has been designed which determines the thermal conductivity in-the-plane of the laminates by matching the measured temperature distribution along a heated specimen with a finite difference solution. An electrically heated Lees’ disc apparatus is also used to measure the thermal conductivity of these boards in a direction normal to their plane. The samples tested yielded values of 0.343 W/mK and 1.059 W/mK for thermal conductivity through and along the plane of the boards, respectively.

Accurate thermal conductivity coefficients for argon based on a state-of-the-art interatomic potential

An accurate interatomic potential was constructed by fitting a realistic function, constrained by theory, to a set of judiciously selected macroscopic and microscopic properties but not including thermal conductivity. Using this potential, accurate values of thermal conductivity of argon are obtained for the temperature range 100 to 6600 K. These are presented in tabular form as well as in terms of a correlation function.