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SMV-2019-10: Characterisation of Cu alloys at low temperatures
Králík, Tomáš
The goal of the project was to measure the electrical conductivity of eight samples made of pure or alloyed copper at the temperatures 4.2 K, 77 K and 300 K for the as received, annealed and hardened state. The electrical conductivity is correlated with the thermal conductivity and the ratio between electrical conductivity at 300 K and 4.2 K (RRR value) gives the purity of the metal including mechanical defects in the material. Pure copper is excellent conductor of heat and electric current but it does not have required strength properties compared to the alloyed coppers. The measured temperature dependence of the electrical conductivity enables to compare the copper materials and to choose one with optimal properties for the current leads of the fusion reactor. \nThe samples with a cross section 0.15-0.3 mm^2 and length 33-70 mm were cut by slow milling. One of the samples of each material was annealed at 980°C for 1.5 hour and the second was hardened at 480°C for 5 hours. All samples (24 pcs) were soldered with Indium solder on a special holder with contacts for the four wire electric resistance measurement. The electrical resistance was measured by a AC resistance bridge Lake Shore 372.\n
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SMV-2019-09: New materials for cryogenics
Králík, Tomáš
The aim of the study was evaluation of the emission of thermal radiation emitted from a set of five commercially available foils which thermal radiative properties unknown till now. These materials are used for insulations of low temperature devices for medicine, industry or space exploration. The emissivity of the materials was measured at conditions truly simulating its operational environment. The measurement was done in a dedicated apparatus cooled with liquid helium. The emissivity of the sample was measured in vacuum at temperatures in the range from 20 K to 300 K.
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SMV-2019-76: Maintenance and commissioning of liquid nitrogen Dewar vessel
Králík, Tomáš
The contractor needs a handy storage of liquid nitrogen for some analytical apparatuses in his laboratory. The contractor decided to utilize an older LN2 Dewar vessel with unknown state of its insulation and evaporation rate. A professional commissioning of a Dewar vessel need special know how and specialised equipment. It was done cleaning of the pumping valve of the vessel from old and hardened sealing grease, change of the O-ring sealing, visual check of the insulation inside the vessel, application of new silicone grease on the pumping valve plug, pumping out of the vacuum space and its outgassing, leak test with a helium leak detector, closing of the pumping valve and its filling with silicon grease, cool down and filling with the liquid nitrogen, long term measurement of the evaporation rate. The measured evaporation rate is 0.3 litres per day and the time of the cold state of the Dewar vessel is nearly hundred days.
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SMV-2018-23: Experimental estimation of the influence of the contamination on thermal radiative properties of adhesive tapes for cryogenic applications
Králík, Tomáš
The goal of this research cooperation, was to estimate an influence of possible contamination of the outer side of aluminium adhesive tapes on its thermal radiative properties in the temperature range from 10 K to 300 K. The aluminium adhesive tapes are commonly used for joining of multilayer insulations, for covering of spots with high emission or absorption of thermal radiation or holes in radiation shields. They are also used for fixation of electric wires. As low as possible emission or absorption of thermal radiation of the tapes is one of the important requirements. This fulfils only clean metallic surfaces. There are two types of adhesive tapes used in the industry. The first type has a special treatment of the outer side so the adhesive do not visibly stick on the outer side after unwinding of the tape. The second type has a lining made of silicon paper, which covers the adhesive side of the tape and protect the outer side. The lining is removed just before the application of the tape.\nThe tapes and an aluminium foil laminated on a PET foil were applied on copper plates suitable for special apparatus Emister developed at ISI Brno. The absorption of thermal radiation by the sample was measured in this apparatus. The source of the thermal radiation was a black reference sample with temperature in the range from 10 K to 300 K. The temperature of the sample of the tape in the range from 5 K to 33 K. The vacuum in the cryogenic part of the apparatus is kept by cryopumping so the parasitic heat transfer by the residual gas is negligible. \nThe conditions in the apparatus are very similar to those in common cryogenic devices and they enable to gain thermal absorptivity of the tapes and very precise comparison of the samples.\n
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SMV-2017-26: Upgrade of the apparatus Spaceman – capacitive system for the measurement of the sample thickness
Králík, Tomáš
The goal of this research cooperation, which is follow- up of the previous cooperation, was an upgrade of the apparatus Spaceman. The upgrade enables to obtain new data of the spacers used in the multilayer insulations. The spacer can be woven or non-woven textile made of glass, plastic or natural fibres. The apparatus Spaceman measures heat flow through a sample of the spacer in the range from 10 K to 290 K in vacuum. The temperature drop on the sample is typically 5 K. The apparatus was enhanced by a system for measurement of the in-situ sample thickness from evaluation of the electrical capacity between electrically conductive contact surfaces. The upgrade was done by particular disassembling of the apparatus, modification or exchange of some parts, welding of a new vacuum feed trough in the apparatus lid, development and installation of a new electronic in the vacuum space. The electronic is placed in the upper part of the apparatus at room temperature and the electrical capacitance is measured by a special cryogenic coaxial cable. The electronic is able to cancel a parasitic capacitance of the cable. Some modifications were done on the Labview control software developed for the apparatus. The apparatus now enables to measure not only the thermal conductivity at various temperatures and loads but also changes of the sample thickness due to thermal dilatations and loads. It is also possible to evaluate an elastic behaviour of the sample at temperatures from 10 K to 290 K.
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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.
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Role of the superconductivity in radiative heat transfer
Králík, Tomáš ; Musilová, Věra ; Srnka, Aleš ; Fořt, Tomáš ; Frolec, Jiří
The absorption and emission of thermal radiation by metals is related to their electrical properties that are influenced by impurities, defects and temperature. We show by some examples of measurements on common metals how different can be the thermal radiative properties. When the heat is transferred across a microscopic vacuum gap in the near field regime, it is possible to achieve a step-like change of the heat\ntransfer based on a superconducting transition. An interesting feature of this effect is the nonlinear thermal conductivity effecting a change of the sign of the differential thermal conductivity across the vacuum gap.
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Effect of gold coating on emissivity and absorptivity of Ti-6Al-4V alloy
Frolec, Jiří ; Králík, Tomáš ; Musilová, Věra ; Urban, Pavel
The article provides experimental analysis of total hemispherical emissivity and absorptivity of Ti-6Al-4V alloy before and after deposition of 1-2 .pí.m gold layer. We measured their dependences on the temperature of thermal radiation (varying from 20 K to 320 K) and confronted these results with mechanically polished copper, i.e. with material known for extremely low emissivity and absorptivity. Galvanic deposition of pure gold resulted in significant lowering of original emissivity of Ti-6Al-4V bare surfaces from 15.8 percent to 3.4 percent at 300 K and from 4.1 percent to 1.1 percent at 20 K. Our findings about thermal-radiative properties have practical relevance for an improvement of a low temperature part of an ultra-high vacuum scanning probe microscope\n(UHV-SPM) designed by our group and could also be useful for other applications and low temperature systems.
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