Speaker
Description
Cryocooler-based conduction cooling has gained traction in replacing liquid He baths for cooling in applications such as superconducting radiofrequency cavities owing to their compact design and simplified construction. Conduction cooling near room temperature is also being explored for dissipating heat from electronic chips. One of the primary governing factors in conduction cooling is thermal contact resistance between two different metals, which, for example, varies from 0.1 to 0.35 K/W between 3.5 and 6 K for a bolting force of 3 to 7 kN between high-purity aluminum and niobium. However, the contact resistance is generally far from the intrinsic thermal contact resistance expected of metal contact, thus leading to losses at the contact.
With the present study, we aim to measure the intrinsic thermal contact resistance between two distinct materials using time-resolved EUV diffraction measurements. The intrinsic value will serve as a benchmark for evaluating the thermal contact's effectiveness, which is currently lacking.
EUV diffraction measurements rely on depositing periodic arrangement of the top layer on another material and instantaneous laser heating the top layer using a pulsed laser source. The resulting heating profile is measured via diffraction from another pulsed source in the extreme ultraviolet (EUV) range synchronized with the pulse used for laser heating. To demonstrate the efficacy of the proposed approach, we deposit 10 nm of Nickel (Material 1) in a periodic arrangement on top of Silicon (Material 2) with a width of 250 nm and a period of 1000 nm using electron beam lithography and electron beam evaporator. The deposition provides a near-ideal contact between the two materials for characterization. We heat the periodic Nickel grating using a pulse of 50 femtosecond duration centered at 800 nm. The subsequent heat transfer from Nickel to silicon is measured using a high-harmonic EUV pulse of 30 to 40 nm wavelength. The EUV pulse diffracts from a periodic Nickel grating and is detected using a multi-channel plate coupled with EMCCD. The resulting measured change in diffraction profile with time is fitted to an effective Fourier model to extract the thermal contact resistance, which is found to be the order of 10^(-9) m^2K/W at room temperature for the studied thermal contact. The proposed approach offers an extremely high sensitivity in measuring thermal contact resistance. For comparison, if we assume the contact area is 1 cm^2 between aluminum and nickel, the thermal contact resistance will be 10^(-5) m^2K/W, which is nearly four orders of magnitude higher than the sensitivity of the present setup. The above-demonstrated experimental setup for nickel/silicon at room temperature is a proof of concept. However, it could be extended to evaluate thermal contact effectiveness for superconducting RF cavities or thermomechanical heat switches used in dilution refrigerators. The precise measurements will also allow us to assess various surface treatment techniques further to improve thermal conduction.
D.B. acknowledges the financial support from the Science & Engineering Research Board (SERB) under project no. CRG/2022/001317.
| Submitters Country | India |
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