CSE Newsletter - March 2020

J. Liberadzka, “Heat transfer across dielectric-metallic interfaces and thin layers at low and ultra-low temperatures”, 24 January 2019, University of Twente
Supervisor: Prof.dr.ir. H.J.M. ter Brake, Co-supervisor: Dr.rer.nat. T. Köttig
Graduation Committee members: Prof. dr. ing. B. van Eijk, Prof.dr.ir. H.H.J. ten Kate, Prof.dr.rer.nat. R. Nawrodt, Prof.dr.ir. T.H. Oosterkamp
Special expert: Dr. G. Vermeulen
Link: https://research.utwente.nl/en/publications/heat-transfer-across-dielectric-metallic-interfaces-and-thin-laye
Short abstract:
The goal of the AEgIS experiment at CERN is the first direct measurement of the Earth's gravitational acceleration on antimatter within 1 % precision. In the framework of this project, a thermalisation strategy for a set of ultra-cold electrodes forming a Penning trap for antimatter has been investigated.
Two sandwich setups have been analysed. The investigation of the first sandwich consisting of copper, indium and sapphire has demonstrated the importance of the oxide layers on the interface thermal resistance. An important conclusion has been drawn that with indium in the normal conducting state the compression force can be removed without changing the total thermal resistivity of the sandwich. The total thermal resistivity of the second setup with additional thin layers of gold and titanium shows, that the sputtering of titanium and the mechanism of its
adhesion to sapphire can have a significant influence on the interface thermal resistivity. The low temperature and ultra-low temperature thermal diffusivity of the sandwich has also been measured. The results are consistent with respect to the built model over the whole temperature range.
The new electrode has been designed and manufactured. A new way of clamping provides more uniform pressure distribution on the bottom surface of the electrode and increases the chances for a formation of a robust indium bond. The thermal performance of the electrode has been tested in steady state and transient conditions and it proved to be over five times higher than the performance of the old design.

Thomas Kochenburger, "Kryogene Gemischkältekreisläufe für Hochtemperatursupraleiter-Anwendungen.", Karlsruher Institut für Technologie, 2019.

ISBN: 978-3-8439-3987-4.
Remark: The PhD thesis is written in German language and is only available as a hardcopy.
Abstract
High Temperature Superconductors (HTS) have already demonstrated their capabilities in cables, busbars, electric motors, generators and transformers in several pilot projects. Due to the lack of closed-cycle cooling processes in the relevant cooling capacity range of some kW, they usually have to be cooled by vacuum evaporation of liquid nitrogen at 65-77 K in open cycles.
Cryogenic Mixed Refrigerant Cycles (CMRCs), that are currently used for large-scale LNG production, allow efficient, cost-effective and reliable cooling in the temperature range of approximately 80-150 K with almost unlimited scalability. To take advantage of closed-cycle CMRC cooling processes for HTS applications, their cooling temperatures must be reduced, which has previously been associated with a significant efficiency drop. For safety reasons, non-flammable refrigerant mixtures are also preferred to the commonly used nitrogen-hydrocarbon mixtures. This work aims to contribute to the tackling of these challenges.
At first, suitable components for wide-boiling, non-flammable refrigerant mixtures with a low freezing point are identified. In addition to the previously used fluorinated aliphatic refrigerants, the refrigerant R1234yf, which has only been available for several years, is also considered as a new possible mixture component. Based on an overview of previous work, a two-stage cooling process is devised for HTS applications. The first stage is distinguished by the use of a non-flammable refrigerant mixture, the second stage by a significantly increased pressure level to raise efficiency and the use of oxygen to reduce the freezing point. A modified Peng-Robinson equation of state is chosen for the use in calculations during the later process design, and fitted to thermophysical pure substance data and binary phase equilibrium data from the literature to reproduce the phase behaviour. Since no literature data exist for some binary systems at low temperature, own phase equilibria measurements were conducted.
To this end, an existing setup was first modified to allow operation with the selected refrigerants in compliance with process and safety requirements. Phase equilibrium measurements were then made in binary mixtures of R1234yf with R14, R23, R218, N2 and Ar in the temperature range of 153-273 K and at pressures up to 50 bar.
In the process simulation part of the work, the mixture compositions and process parameters are defined and optimized in Aspen Plus for maximum process efficiency in each case. On the basis of a detailed examination of the two stages of the developed cooling process, solutions for two HTS cooling scenarios are developed and evaluated . The efficiencies achieved are significantly higher than in previous CMRC processes and comparable to those of other cooling technologies for the temperature range under consideration. Also, the use of R1234yf as a high-boiling mixture component proves to be advantageous compared to the alternative R218. A CMRC test stand was set up and put into operation to validate the process simulation. Initial measurement results with three simple mixtures are in excellent agreement with the process calculations in Aspen Plus, which suggests a high predictive quality of the modeling. The experimental validation of the complete cooling process was not possible in the scope of this work, but is being pursued in follow-up projects.

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