O przewodności cieplnej meteorytu Jezersko

The thermal conductivity (K) of Jezersko H4 meteorite was predicted by various models of rocks, using literature data on the chemical composition, porosity (P), and by relationships between thermal conductivity and porosity, and between thermal conductivity and thermal diffusivity (D). The results confirm that the porosity of the chondrite and air pressure significantly affect thermal conductivity. The thermal conductivity of the chondrite skeleton/matrix predicted by the modal composition of the meteorite and by the geometric mean model is equal to 4.35 W m−1 K−1, and by arithmetic and harmonic mean models: 4.9 W m−1 K−1at 300 K. Bulk thermal conductivity of the meteorite predicted by the geometric mean model is equal to 2.6 W m-1 K-1 for air pressure of 1 atm, and 1.0 W m−1 K−1in vacuum at 300 K. The Hashin–Shtrikman model predicts the values: 2.4 and 1.9 W m−1 K−1, the Clausius–Mossotti model: 2.2 and 1.9 W m-1 K-1, and the mean of two-layer models: 2.1 and 2.0 W m−1 K−1 at 300 K, for air pressure of 1 atm, and in vacuum, respectively. The relationships between thermal conductivity and porosity based on experimental data for ordinary chondrites indicate a mean K value for bulk thermal conductivity of the Jezersko meteorite in vacuum: 1.18 W m−1 K−1, and between thermal conductivity and thermal diffusivity the mean value: 1.12 W m−1 K−1at 200–300 K. The mean value for all predictions for bulk thermal conductivity of the meteorite for air at 1 atm is equal to 2.45 ± 0.30 W m−1 K−1 (range: 2.0–2.9 W m−1 K−1) at 300 K, and in vacuum: 1.40 ± 0.40 W m−1 K−1 (range: 0.95–2.0 W m−1 K−1) at 200–300 K. Predicted values of bulk thermal conductivity of the Jezersko meteorite, for air and in vacuum, are in the range of values recently reported by Soini et al. (2020) for the H4 group of chondrites: 2.8 ± 0.6 W m−1 K−1, mean K for air at 1 atm, and 1.9 ± 1.0 W m−1 K−1 mean K value in vacuum at 200–300 K.

Transient Determination on the Bulk Thermal Conductivity of Sub-Millimeter Thin Films of Composite Phase Change Thermal Interfacial Materials

The bulk thermal conductivity of thin films having a sub-millimeter thickness, made of composite phase change materials (PCM) and utilized as an emerging thermal interfacial material (TIM) for thermal management of electronics, was determined using the transient plane source (TPS) technique. The actual bulk thermal conductivity of the thin film samples was obtained by deconvoluting the thermal contact resistance (TCR) during the measurement process, according to the linear relationship between the nominal bulk thermal resistance and the thickness. The slope of the correlation curve is the reciprocal of film sample thermal conductivity and the intercept is the overall TCR. For the PCM35 thin film samples (which melt at around 35 °C) having three nominal thicknesses of 271±1 μm, 460±2 μm and 511±2 μm, the corrected results in the solid and liquid state were found to be approximately 0.487 W/m·K and 0.186 W/m·K, respectively. It was shown that the corrected values are greater than the direct readings from the TPS instrument as the latter involves the effect of TCR across multiple interfaces. The results obtained in this work could serve as reference property data for design of thermal management systems involving such phase change TIM.

Developments for Copper-Graphite Composite Thermal Cores for PCBs for High-Reliability and High-Temperature RF Systems

Summary System designs that have only conduction cooling available, that must operate in harsh or challenging environments, present significant challenges to the system thermal engineer. A second thermal design challenge is continued miniaturization of semiconductor devices and increased functionality per square centimeter of semiconductor die, resulting in continued increases in device heat flux. Elimination of packaging materials allows more efficient heat transfer as thermal resistances from one material to another are reduced or designed out. When possible, concurrent elimination of package materials that have low bulk thermal conductivity and replacement with high thermal conductivity materials will improve heat transfer efficiency. Attachment of the resulting unpackaged semiconductor device can then be made directly to the circuit carrier; however, care must be taken regarding increases in potential for damage or failure due to mismatched coefficient of thermal expansion (CTE). Continuing reductions in die size that result in higher heat flux exacerbate this potential failure mechanism at the die-to-substrate level. This is further worsened in harsh environment (i.e., vibration, shock, high moisture, rapid power cycling) and/or high operating temperature conditions. For aerospace, military, geothermal, and other applications where increasingly high heat flux radio frequency (RF), microwave, and processor semiconductors are attached directly (with solders, silver sintering pastes, or other joining materials) to an organic or ceramic printed circuit card, efficient and rapid heat transfer becomes critical. These are frequently also applications where forced convection (air or liquid) may be unavailable to the system design engineer. One solution for thermal management design problems of this type has traditionally been the incorporation of one or more heavy copper layers within a complex multilayer printed circuit board (PCB). This solution, however, has come under increasing scrutiny in recent years due to concerns for weight (especially in airborne and space applications) and the potential for severe CTE mismatch between semiconductor die materials with relatively low thermal expansion values and the relatively very high value of copper. Therefore, development of CTE-matched alternative materials to replace a heavy copper layer has been a focus for development activities. A suitable selection must, however, have a bulk thermal conductivity that is as close to that of copper as is practicable. Recent developments of a copper-graphite composite material in sheet form that can be employed in standardized PCB manufacturing processes are described in this presentation.