This study presents a thermal contact conductance (TCC) model that improves predictive accuracy across a wide temperature range, particularly in the region above the peak in thermal conductivity. In conduction cooled cryogenic systems that include current leads or resistive thermal components, heat is generated internally or transferred from external sources, requiring accurate estimation of TCC. Copper, a commonly used thermal conductor in such systems, exhibits nonlinear thermal conductivity with temperature, which makes precise prediction of TCC challenging, especially near and beyond the conductivity peak. The proposed model builds upon the Cooper-Mikic-Yovanovich (CMY) formulation by incorporating mechanical contact parameters along with thermophysical influences, such as size effects and heat carrier behaviour, through a modified thermal conductivity expression. The model was assessed using previously reported experimental data for oxygen-free high conductivity copper interfaces. Compared to conventional approaches, it demonstrates a significant reduction in deviation from measured values across the full temperature range, with improved agreement in the post-peak region of thermal conductivity.
A transient liquid phase sintering (TLPS) brazing method for high-temperature thermoelectric devices was investigated through the development of Ni-Sn-based pastes. A pressureless brazing process was also developed to address the challenge of uniform pressure application in complex structures of thermoelectric devices. A brazing evaluation method using a simplified thermoelectric module structure was established to assess paste performance. The tensile strength of joints brazed with the 43Ni-57Sn paste, corresponding to the stoichiometric composition of Ni3Sn4, was lower than expected. To improve tensile strength, a new TLPS paste with excess Sn (30Ni-70Sn) was formulated based on a mechanistic study. As a result, the tensile strengths of the Ni-Ni and Cu-Ni joints were significantly improved to 29.08 kgf/cm(2) and 14.49 kgf/cm(2), representing 1.81-fold and 3.5-fold increases, respectively. These results demonstrate that applying a Sn-excess composition in TLPS paste effectively enhances the tensile strength of pressureless TLPS brazing by promoting the formation of robust adhesive intermetallic compound (IMC) layers.
High-precision vacuum sensing is an essential element in various industrial fields such as semiconductor processing and vacuum based packaging. Accordingly, the advancement in vacuum sensing technology that enables both miniaturisation and low power operation is required. Here, we manufactured highly miniaturised Pirani vacuum gauge using the micro-electro-mechanical systems (MEMS) process and verified its performance using various signal processing methods. First, the fabricated gauge was used to measure the pressure response in the range of 10(2) Torr to 10(-5) Torr, and its reliability was verified. Furthermore, using the same measurement setup, we conducted a comparative analysis in the pressure range of 800 mTorr to 15 mTorr between the fabricated gauge and a commercial sensor. Subsequently, the gauge was operated using the unipolar square-wave heating method and its characteristics were compared with those obtained from the sine-wave heating method. Although the unipolar square-wave heating method exhibited slightly lower precision than the sine-wave heating method, it demonstrated reliable performance suitable for practical implementation, along with the benefit of low power operation. Therefore, this method can be effectively applied to various practical vacuum measurement systems due to its advantages such as simple circuit configuration and low power consumption. The fabricated MEMS Pirani vacuum gauge, featuring miniaturisation and low-power characteristics and integrated with the unipolar square-wave heating-based measurement technique, demonstrates potential for applications in industrial fields such as semiconductor processing and vacuum-based packaging.
The heat transfer characteristics in cryogenic environments are critical in various fields such as semiconductors, aerospace, and superconducting applications. Minimising heat loss and ensuring efficient cooling performance are essential in cryogenic systems, necessitating the optimal design of the stage in terms of shape and material. In particular, maintaining temperature uniformity and reliability is crucial in experimental apparatus and precision equipment, requiring careful consideration of thermal shielding and conduction properties. This study aims to identify the optimal stage shape and material for effective heat transfer in cryogenic environments. Three stage designs and two materials were analysed through experiments. The experimental setup incorporated a cryocooler as the cooling source, along with multi-layer insulation (MLI) and a thermal link to reduce heat invasion and enhance thermal conduction. Temperature distribution and thermal performance were evaluated to identify the most efficient stage configuration. This study provides fundamental insights for optimising the thermal design of cryogenic stages and contributes to the advancement of cryogenic system performance.
