National Repository of Grey Literature 31 records found  1 - 10nextend  jump to record: Search took 0.00 seconds. 
SMV-2018-22: Optimization of helium recycling in the National NMR Centre of J. Dadok
Srnka, Aleš ; Hanzelka, Pavel ; Krejčí, Pavel ; Heinige, Vladimír
Experimental research included all aspects of ensuring optimal and effective helium recycling. There were particularly optimized procedures of liquid helium refilling to the individual apparatuses and were identified and eliminated undesirable losses on separate parts of technological chain including transport Dewar vessels.
Effect of thermal radiation on low temperature measurement for various types of temperature sensor installations
Urban, Pavel ; Hanzelka, Pavel ; Králík, Tomáš ; Vlček, Ivan ; Srnka, Aleš
We investigated an effect of thermal radiation (300 K) on precision of low temperature measurement of a sample holder (or sample) in an UHV SEM/SPM microscope working at range down to 20 K for various ways of temperature sensor mounting. We designed a special copper casing for installation of Lake Shore\ntemperature sensors (CernoxTM and Si diodes) encapsulated in SD packages. The sensor body is attached direct to the casing using indium solder. The current leads pertaining to the sensor are thermally anchored but electrically insulated from the casing. Quality of sensors installation in the casing was tested for various ways of mounting on a cooled plate. Insufficient thermal anchoring of electrical wires and thermal shielding were imitated. Comparative measurement was also performed with sensors in SD package mounted without\ncasing. The experiments proved that our special design of temperature sensors casing is adequate for intended application of temperature measurement of sample holder in UHV SEM/SPM microscope. In contrary, measurement precision of unprotected sensors was absolutely unsuitable.
SMV-2015-23: Development of a low temperature system for a spectroscopy
Urban, Pavel ; Hanzelka, Pavel
The system for measurement of temperature of solid and liquid samples for spectroscopic measurement in the range of 77 K - 300 K was designed. For the above mentioned system, the pump vacuum line was designed and analysed for pumping of vacuum spaces of two types of liquid nitrogen cryostats used for spectroscopic measurements. The part of the solution is design of temperature sensor positions and an electrical resistance heater for the sample temperature stabilisation. Furthermore, an electrical resistance heater for heating/outgassing of the vacuum pump line was designed.
SMV-2013-27: Thermal radiation absorptivity for development of multilayer insulation for alternating electromagnetic fields
Králík, Tomáš ; Hanzelka, Pavel
Super insulations composed of many layers of metalized plastic foils are one of the ways of significant heat flow reduction to low temperature parts of the apparatus. The metal layer was modified to reduce heat generation by alternating electromagnetic fields (e.g. in superconducting motors, generators or transformers). The modifications of the metallisation always worsen the thermal radiative properties of the foils and it was needed to evaluate its influence. The absorptivity of thermal radiation was evaluated in the experimental study on two foils coated with metallic film divided in segments and on the foil coated with metallic particles dispersed in a binder.
Design and Testing of Low Temperature Part of an UHV – SPM Microscope
Frolec, Jiří ; Vonka, J. ; Hanzelka, Pavel ; Králík, Tomáš ; Musilová, Věra ; Urban, Pavel
We have developed low temperature part of an ultra high vacuum scanning probe microscope (UHV-SPM) working at variable temperature within the range from 20 K to 700 K. To achieve the required temperature range, a flow cooling system using cryogenic helium (approximately 5 K) as a coolant was designed. The system consists of a flow cryostat and a flexible low-loss transfer line connecting a Dewar vessel with the flow cryostat. We have also tested liquid nitrogen (approximately 77 K) as an alternative low-cost coolant. Using nitrogen, the microscope can operate at temperatures of about 100 K and higher. As the flow of the coolant through the cryostat can cause thermally induced two-phase flow fluctuations resulting in instability in temperatures, preliminary tests were done in order to find ways of avoiding the temperature oscillations and optimising the cooling process.
Factors Influencing Thermal Radiative Properties of Metals
Králík, Tomáš ; Hanzelka, Pavel ; Musilová, Věra ; Srnka, Aleš
Data on thermal absorptivity and emissivity are important for calculations of heat flows in cryogenic devices. Although thermal radiative properties of materials used in cryogenics are measured, published and used for more than sixty years we have noticed that sometimes users of that data don’t well understand the significance of individual factors influencing thermal radiative properties. The sensitivity of the thermal radiative properties of metals to the condition of the surface and the difficulty of measurement, especially at low temperatures is probably the reason of dispersion of the published values. The effect of the material and its purity, finishing (machining, mechanical or chemical polishing) and of the protective coatings or accidental layers (water, oxidation) on the thermal radiative properties is not always obvious. Influence of the material, surface treatment, roughness and layers on the surface is discussed on the basis of our 20 year experience with measurement of the emissivity and absorptivity in the range from 20 K to 300 K.
Temperature Inversion and Heat Transport Against the Temperature Drop in Two-Phase Convection of Cryogenic Helium
Urban, Pavel ; Schmoranzer, D. ; Hanzelka, Pavel ; Skrbek, L.
In the system of two bodies A and B connected by a two-phase vapour-liquid system consisting only of cryogenic helium, we observed anomalous heat transport from the colder but heated body B to the warmer but cooled body A. We noticed this effect during the process of liquid evaporation inside a cylindrical pressure convection cell used for study of Rayleigh-Bénard convection. This anomalous heat transfer occurs under non-equilibrium conditions, owing to the phase transitions that take place inside the cell while temperature inversion develops between the heated but cooler bottom plate and the cooled but warmer top plate. This process is, by virtue of the system being open with respect to heat transport, in no contradiction to the second law of thermodynamics, and a numerical model was formulated that describes the observations with good agreement.
Cryogenic He experiment on Natural Turbulent Convection
Králík, Tomáš ; Urban, Pavel ; Musilová, Věra ; Hanzelka, Pavel ; Srnka, Aleš ; Repisky, Andrej
Cryogenic helium gas is a suitable fluid for study of natural turbulent convection at very high Rayleigh numbers. We present experimental method and new result on Reynolds and Péclet numbers characterizing large scale circulation (LSC) of fluid at Rayleigh numbers within the range from 1011 to 1015. Observation of LSC is based on measurement of local temperature fluctuations of convecting helium. For experiments we used a specially designed cylindrical cell of height L = 0.3 m and diameter D = 0.3 m with minimized parasitic effects on studied convection, previously published on CryoPrague 2006 [1].
Radiative Heat Transfer at Low Temperatures over Microscopic Distances in Vacuum
Králík, Tomáš ; Musilová, Věra ; Hanzelka, Pavel ; Horák, Michal ; Srnka, Aleš
We present experimental results on heat transfer between plane parallel surfaces of samples separated with microscopic vacuum gap. Radiative heat flux between metallic samples exceeding 50 times heat flux between black surfaces was observed for gaps of several micrometres at temperatures below 50 K. Comparison with near field theory of radiative heat transfer is presented.
Calibration of germanium temperature sensors of helium cryostat for experimental study of natural turbulent convection
Urban, Pavel ; Hanzelka, Pavel ; Musilová, Věra
Experimental study of Nusselt number on Rayleigh number dependence with using of cryogenic 4He gas requires precise measurement of the temperature differenceT between the top and the bottom plates of the convection cell. A special calibration device was adapted and used for comparative calibration of germanium temperature sensors. This device enables to compare up to four sensors within the temperature range of 4.3 K – 40 K.

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1 Hanzelka, Petr
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