12-month post doctoral position in cryogenic turbulence   

Domain: development and microfabrication of cryogenic temperature sensors.

Title: NbN microfabricated hot films for the characterization of turbulent cryogenic flows.

The Low Temperature Laboratory at CEA-Grenoble (SBT) operates two major cryogenic facilities able to produce highly turbulent cryogenic flows: the HeJET facility [1] – a superfluid helium free jet - and the SHREK facility [2] – the world largest Von Karman flow, able to work with both superfluid and supercritical helium. These two facilities need a panel of advanced sensors to characterize the turbulence in these flows. For this purpose, CEA-SBT has started for two years a program of microfabrication of hot wires. Platinum hot films (or wires) have already been fabricated, according to the Princeton process [3], adapted and optimized for low temperature operation. Hot wires are hydrodynamical sensors – also named anemometers – which allow the measurement of the turbulent velocity, thanks to the cooling effect of the flow on the heated wire. However, it is necessary to heat the platinum film above 25 K to measure the velocity. This is a severe drawback, as this temperature could affect the flow properties. Therefore, we wish to develop NbN anemometers, based on a insulator/metal transition of the NbN, as it is known that such a material, according to the stoechiometry, may exhibit either a Superconducting/metal transition (not used in this work) or an insulator/metal transition, which we plan to use in this work. We believe that such a hot wire would allow measurements at a much lower temperature of the film (typically 5 K), perhaps lower, which will reduce the thermal perturbation of the flow.

In this work we expect (i) to establish the conditions for a reproducible insulator/metal transition of NbN; (ii) to modify the fabrication process of the anemometer to include the NbN film; (iii) to measure the turbulent velocity thanks to this sensor.

A second year of post doc could follow (European grant possible). It would be more dedicated to temperature measurements thanks to this NbN material.

The skills required in this post doc are basically an experience in microfabrication (at CEA-Grenoble the Plateforme Technologique Amont (PTA) is available for microfabrication on 100 mm Si wafers). Photolithography, PVD, etching, DRIE are the most used processes, and MEB is used for the characterization of the sensor. Experience with NbN would be appreciated. Skills in cryogenics and fluid mechanics are not required, but would be welcome.

Contact:alain.girard@cea.fr SBT, CEA Grenoble, 17 rue des Martyrs 38054 Grenoble Cedex France

[1] D. Duri et al, Rev. Sci. Instrum. 82, 115109 (2011).

[2] B. Rousset et al, Review of Scientific Instruments 85, 103908 (2014)

[3] M. Vallikivi & A.J. Smits, "Fabrication and Characterization of a Novel Nanoscale Thermal Anemometry Probe", Journal of Microelectromechanical Systems, 99, (2014).

Post_doc_position_at_CEA.pdf (75 Ko)
A. Girard, 2015-09-30


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