Radiative heat transfer
Since thermal radiation presents one of the basic modes of heat transfer, comprehensive knowledge of thermal absorptivity and emissivity are important for calculations of heat flows in cryogenic devices. Although detection of weak thermal radiation of low-emitting metallic samples at cryogenic temperatures presents a challenging issue, our apparatus allows us to determine both total hemispheric emissivity and absorptivity of highly reflecting surfaces at temperatures ranging from 320 K down to 10 K.
Our group focused on the issue of the influence of condensed water on absorptivity properties of metallic surfaces at cryogenic temperatures (ASEP 352298, 351426). We prepared in situ very thin layers of condensed water and found that the coating with a thickness of 85 nm increases the absorptivity of a cold aluminium surface almost twice. Thus, surface contamination by volatile deposits can substantially affect thermal radiative properties of the exposed surfaces and increase the heat load of cryogenic devices.
We also evaluated the emissivity and absorptivity of samples with various types of surface finishing and coatings in the temperature range of 40 K – 300 K (ASEP 426241, 437370). Our research will enable us to optimize the production and the surface finishing of some parts of a cryostat designed for space operation with respect to its required thermal radiative properties.
In the last work (ASEP 434010) we used our experience, gained over 15 years, with measurement of total hemispherical emissivity and absorptivity of various materials at temperatures ~20 K – 300 K, to assess individual factors influencing their thermal radiative properties. The effects of the material and its purity, finishing and of protective coatings or accidental layers (water, oxidation) on the thermal radiative properties were analyzed. Summing up the results, it can be concluded that chemical treatment (polishing, etching) provides surfaces with the lowest absorptivity and without accidental impurities. In addition, a thin layer of water ice on the surface can have the same or stronger influence on the absorptivity enhancement than the natural oxide layer. Likewise mechanical stress applied on the surface by cutting tools or by abrasion has significant impact on the absorptivity, which could be further reduced by annealing.
Nanotechnology is a hot issue and has become a subject of fundamental research of all world-leading universities and research institutes. The application potential is huge, and hence the need for superior technology, characterization tools and related unique products is growing. Our team is engaged in the AMISPEC (Advanced Microscopy and Spectroscopy Platform for Research and Development in Nano and Microtechnologies) project, which is supported by the Technology Agency of the Czech Republic (Grant No.TE01020233, 2012-2019). This project forms a research and development platform bringing together three Czech companies and two research organizations, all of them are well established on the topic of the proposal. The goal of the project is to build an ultrahigh vacuum scanning electron microscopy (UHV SEM) and a scanning probe microscopy (UHV SPM) modular system (UHV SEM/SPM) for in situ fabrication and characterization of nanostructures and surface analysis. It will also include complex systems combining more technological and analytical techniques (e.g., SEM/FIB, SEM/SPM, SEM/nanoprobes).
We have developed the low temperature parts of an UHV scanning probe microscope (SPM), working at variable temperatures in the range of 20 K – 700 K. To achieve the required temperature range, a flow cooling system using liquid helium (~5 K) or liquid nitrogen (~77 K) as coolants was designed. A new helium/nitrogen flow cryostat is used for cooling, which allows low-loss transfer of cryoliquids from a Dewar vessel to the SPM system placed in a vacuum chamber. The cryostat consists of an inlet and an outlet, heat exchangers and copper strands (braids) for the thermal connection between the heat exchangers and both the sample holder/SPM and the radiation shield around the SPM. A compact sample holder support with low thermal conductivity and high mechanical stiffness was developed and experimentally tested in the vacuum chamber. The thermal and mechanical criteria are fulfilled by using three sets of silica glass balls as a mounting of the sample holder. Every set consists of four balls placed in vertices of a tetrahedron. Low thermal conductivity of the four-ball supports (FBS) is based on the small contact areas between the glass balls (ASEP 395139). Such a solution has the lowest thermal conductance of all feasible supporting structures. The unique low conductive pad (InBallPad) for thermal insulation of a sample holder of a variable temperature UHV SEM/SPM is characterised by low thermal conductance with heat flow of 140 mW between the top plate at 25 K and the bottom plate at 290 K, and by high mechanical stiffness of 1.0×106 N/m. The dimensions of the sample holder with a total mass of 34 g are small – 30mm diameter and 12mm height.
Important for the team is the membership (one person since 2003) in the COMPASS Collaboration at CERN (Project NA58, http://wwwcompass.cern.ch/). This membership brings the team not only additional experience in the field of cryogenics but also in the wide field of materials technology. During the period of 2010-2014 one member of our team worked in the COMPASS Polarized Target (PT) Group with a 10% part-time job. The contribution to the collaborative results corresponds to this percentage value. The PT group is responsible for the development and installation of the polarized target and also for the control of the target during the whole time of the experiment. The low-temperature polarized target (solid 6LiD or NH3 target cooled by dilution refrigerator, two superconducting magnets for target polarization, NMR system for polarization measurement and very high frequency dynamic polarization system) is the heart of the COMPASS experiment and has major influence on the quality of many results obtained in the field of particle physics. During the evaluated period a set of experiments to study hadron spectroscopy and structure of hadrons was performed and important results were published (e.g. ASEP 351513, 351515, 385194, 390817, 428202, 431897 and others).
In summary, the period of 2010-2014 was very important for the Cryogenics Group for its future scientific progress. We have successfully expanded our scientific interest into two new areas of cryogenics research – cryogenic natural convection and cryogenic near-field effect. These areas will be the most important tasks in our group for several subsequent years.
The Cryogenics group has received substantial support from the Czech Ministry of Education, Youth and Sports via grant ALISI (2009-2013), No. CZ.1.05/2.1.00/01.0017. With this support we could install a new helium liquefier and other parts of a closed helium recovery system in 2013. The vast reconstruction suspended our possibilities to perform cryogenic experiments for more than half a year, but this new installation ensures us long-term independence in experimental cryogenics research, which is a great advantage in view of the permanent liquid helium price growth.
Thermal conductivity, thermal radiative properties and their temperature dependence are the base of thermal design in cryogenics. Thermal conductivity of materials is well known and can be found in literature. On the other hand, thermal radiative properties of metals are very sensitive to the state of the surface, and the use of published data requires caution. However, they can be measured exactly for the specific surface intended for a cryogenic system. In some specific materials used in cryogenics, heat transfer is composed of the conduction across the weak mechanical contact and radiation, which influence each other. This effect can be found in components of multilayer insulations (MLI) like fabrics or stacks of foils. Heat transfer through such material is also sensitive to its compression. Last year we started, in scientific collaboration with an industrial partner, the design and development of a unique cryogenic apparatus enabling us to measure heat transfer across the components of MLI under such conditions (temperature, compression) that can be found in real systems. The influence of temperature on the material boundary and its compression will be investigated for various materials. The measurement method will be further developed to simplify and shorten the measurement procedure. The data will be used for improving the existing models and for the generalisation of the thermal properties of materials for MLI. We would like to continue enlarging our material database, which now contains the emissivity and absorptivity of almost eighty types of surfaces and, according to our knowledge, is globally unique. Our new description of the measuring process of emissivity and absorptivity with our apparatus along with its thorough uncertainty analysis will soon be published. We also intend to continue our collaboration with various subjects as well as to use our measuring method for the needs of the ISI Brno.
In the field of cooling low temperature parts of the SPM one additional problem has to be solved. The flow of a coolant throughout the cryostat can cause thermally induced two-phase flow oscillations resulting in destabilisation of the cooling system. More detailed tests will be run in order to find ways of avoiding the instabilities and to optimize the cooling performance. A new version of InBallPad with improved mechanical and thermal properties will be designed as well.
In 2010, an extension for the COMPASS program was approved by the CERN Research Board. It consists of a set of measurements for the study of the structure of hadrons in Deep Virtual Compton Scattering, Hard Exclusive Meson Production, Polarized Drell-Yan and Primakoff reactions. Further investigations in the field of hadron spectroscopy were envisaged as well. On January 2013 a new Memorandum of Understanding was signed, in order to fulfil this program. The membership in the Polarized Target Group for one person of our team is ensured till June 30, 2017 and may also be extended.