Jan Dupák

Ultra-Micro-Thermal Field-Flow Fractionation

Abstract Micro-thermal field-flow fractionation, proposed conceptually and implemented experimentally several years ago, has developed rapidly in terms of theory, instrumentation, and numerous applications for the analysis and characterization of polymers and particles of synthetic, natural, and biological origin. Although the advances have been important, achieving the ultimate limits of miniaturization imposed by the physics as well as by recent technologies represents a challenge that was explored. The result of the reported experimental study is a new separation channel for ultra-micro-thermal field-flow fractionation, which was compared, in terms of performance, with the existing compact micro-thermal field-flow fractionation unit. The limits of the miniaturization are experimentally demonstrated with the use of suspensions of colloidal particles, which represent a more difficult case of separation than polymers in solution.

Plasma ion source with hollow cathode

Abstract The paper describes a plasma ion source for ionization of low-volatile elements of a high ionization potential. The source is characterized by a high operation temperature and relatively effective ionization. Plasma in a closed volume of the hollow cathode is produced by thermoemission electrons and ionized atoms dosed to the source or diffused from the walls of heated cathode. The construction of the source makes its operation possible in electromagnetic isotope separators on-line, e.g. on a beam of highly energetic protons of YASNAPP facility (Nucl. Instrum. Methods B70 (1992) 62).

Temperature controlled cathode heating in electron beam welding machine

We describe in our paper how the lifetime of the directly heated tungsten cathode in electron beam welding machine can be substantially extended by automatic stabilisation of its temperature measured by its luminance. Further we show that the end of the life (the break) of the cathode can be predicted, giving the operating personnel the chance to avoid unwanted effects of its unexpected break during the welding process.

SOLDERING WITH DUCTILE ACTIVE SOLDERS

Ductile active solders enable production of the vacuum-tight joints of metals with such brittle nonmetallic materials as quartz, ceramics, silicon, and carbon. This article describes the techniques of the preparation of solders and the examples of the practical application of joints in the vacuum and cryogenic devices.

The new nuclear orientation facility at charles university prague

The Nuclear Orientation facility for solid state physics investigations was installed at the Department of Low Temperature Physics of the Faculty of Mathematics and Physics, Charles University. The small “top loaded” dilution refrigerator is used for cooling radioactive metallic samples to 10 mK in 4 T magnetic field. The construction and thermodynamic parameters of the “French type” refrigerator working without 1 K precooling stage are described.

Thermal destruction of antiferromagnetic Fe-Fe exchange bonds and mechanism of the transition to the spin-glass state in (Fe0.65Ni0.35)1−xMnx systems with antiferromagnetic competing exchange interactions

The thermal evolution of the competition between the ferro-and antiferromagnetic exchange interactions in (Fe0.65Ni0.35)1−xMnx alloys, which display different magnetic properties, depending on composition and temperature, is investigated. The distribution functions of the magnetic hyperfine fields P(Bhf) for 57Fe are determined by Mössbauer spectroscopy in the temperature range 5–300 K for the alloys with x=0, 0.024, 0.082, 0.136, 0.195, and 0.252. The temperature dependence of the integrated intensity Is(T) is analyzed for the low-and high-field portions of P(Bhf). The features found in the behavior of Is(T) are interpreted as results of variation of the ratio between the competing exchange interactions of different signs as a result of the thermal destruction of antiferromagnetic Fe-Fe exchange bonds. It is shown that the changes in the spin structure in the low-temperature range are due to the thermal destruction of Fe-Fe exchange bonds. One of the consequences of this destruction is “reentrance” (an increase in the hyperfine field with increasing temperature for some of the Fe atoms). The relationship between the thermal destruction of Fe-Fe exchange bonds and the magnetic transitions of the Fe-Ni-Mn system to the spin-glass state is considered.

Low temperature nuclear orientation of54Mn in FeNiMn

Magnetic moment behaviour of54Mn in the (Fe0.65Ni0.35)1-xMnx alloy (x=0, 0.024 and 0.082) was studied by low-temperature nuclear orientation in the temperature range of 9–50 mK in the external magnetic field (Bext) up to 3.6 T.54Mn hyperfine magnetic fields and a misalignment of the Mn magnetic moments were extracted from data using fitting procedure. A nonzero misalignment of the Mn magnetic moments with respect to Bext has been found.