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[en] Magnetic properties of Ni81Fe19/Al2O3/Ni81Fe19 tunneling junctions are studied for different Al thicknesses and plasma oxidation times. A maximal magnetoresistance of 34% is obtained with Al thickness of 20 A. Magnetometry reveals large exchange bias fields (∼400 Oe) over a wide range of barrier thicknesses, indicating junctions of high quality. Transport measurements conducted on junctions before and after thermal annealing show a dramatic improvement in barrier quality after annealing. Interlayer coupling fields have been measured as a function of barrier thickness for different oxidation times
[en] In this paper, epoxy resin reinforced by negative thermal expansion material, ZrW2O8, was fabricated. The surface modification of ZrW2O8 particles was performed via plasma enhanced chemical vapor deposition (PECVD) process. As a result, a thin film was uniformly deposited on the surfaces of the ZrW2O8 particles, leading to an improvement of compatibility and dispersion of ZrW2O8 fillers inside epoxy matrix. Moreover, the coefficients of thermal expansion (CTEs) of the composite material containing 0-40 vol. % fillers were studied under cryogenic temperatures. The results showed that a significant reduction in thermal expansion with increasing ZrW2O8 content. The cryogenic mechanical properties of ZrW2O8/epoxy composites were also investigated, showing the properties were improved by adding ZrW2O8 to certain content. In addition, the mechanical strength and modulus of the composite were observed significantly higher at cryogenic temperature than that at room temperature because of the thermal shrink effect and the frozen of epoxy matrix. (author)
[en] An analytical model for one-turn helium pulsating heat pipes (PHPs) with single liquid slug and vapor plug is established in present study. When an additional heat power takes place in the evaporating section, temperature and pressure will increase. The pressure wave travels through vapor and liquid phases at different speed, producing a pressure difference in the system, which acts as an exciting force to start up the oscillating motion. Results show that the start-up power of helium PHP is related to the filling ratio. The start-up power increases with the filling ration. However, there exist an upper limit. Furthermore, the start-up power also depends on the inclination angle of PHP. When the inclination angle increases, the heat input needed to start up the oscillating motion decreases. But for one-turn helium PHP, it can not be started up when the inclination angle is up to 90°, equalling to horizontal position,. While the inclination angle ranges between 0° (vertical position) and 75°, it can operate successfully. (paper)
[en] A refrigeration cryostat has been developed to produce continuous cooling to a sample below 1.8 K over 100 hours by using a cryocooler. A two-stage 4 K G-M cryocooler is used to liquefy helium gas from evacuated vapor and cylinder helium bottle which can be replaced during the cooling process. The liquid helium transfer into superfluid helium in a Joule-Thomson valve in connection with a 1000 m3/h pumping unit. The pressure of evacuated helium vapor is controlled by air bag and valves. A copper decompression chamber, which is designed as a cooling station to control the superfluid helium, is used to cool the sample attached on it uniformly. The sample connects to the copper chamber in cryostat with screw thread. The cryostat can reach the temperature of 1.7 K without load and the continuous working time is more than 100 hours. (paper)
[en] The thermophysical properties of matters are extremely important for engineering and materials science. This paper describes a multifunctional apparatus based on GM cryocooler for measurement of thermal conductivity and thermal expansion via steady-state longitudinal heat flow method and strain gauge technique respectively. The apparatus consists of a removable sample test rod on which bulk samples can easily be mounted and placed in the measurement device. Besides, the sample holder is easy to be replaced so that it suits various needs. All measurements are efficiently and accurately carried out at different temperatures by following a set of stability criteria the setup of the apparatus has been calibrated with sample stainless steel and copper, which gives an error within 6% around the published results in the literatures.
[en] High field superconducting magnets made of Nb3Al will be a suitable candidate for future fusion device which can provide magnetic field over 15 T without critical current degradation caused by strain. The higher magnetic field and the larger current will produce a huge electromagnetic force. Therefore, it is necessary to develop high strength cryogenic structural materials and electrical insulation materials with excellent performance. On the other hand, superconducting magnets in fusion devices will experience significant nuclear radiation exposure during service. While typical structural materials like stainless steel and titanium have proven their ability to withstand these conditions, electrical insulation materials used in these coils have not fared as well. In fact, recent investigations have shown that electrical insulation breakdown is a limiting factor in the performance of high field magnets. The insulation materials used in the high field fusion magnets should be characterized by excellent mechanical properties, high radiation resistivity and good thermal conductivity. To meet these objectives, we designed various insulation materials based on epoxy resins and cyanate ester resins and investigated their processing characteristic and mechanical properties before and after irradiation at low temperature. In this paper, the recent progress of the radiation stable insulation composites for high field fusion magnet is presented. The materials have been irradiated by 60Co γ-ray irradiation in air at ambient temperature with a dose rate of 300 Gy/min. The total doses of 1 MGy, 5 MGy and 10 MGy were selected to the test specimens. (paper)
[en] Mesoporous silica materials have attracted great attention in many fields. However, facile and tunable synthesis of hierarchical mesoporous silica structures is still a big challenge, and thus the development of them still lags behind. Herein, well-defined mesoporous silica flower structure with wrinkled edges and mesoporous silica hollow structure with coarse surface were synthesized simply by using poly(vinylpyrrolidone) and hexadecylamine as cotemplates in different water/ethanol solvent systems. The shape evolution from flower to hollow can be easily realized by tuning the volume ratio of water to ethanol, and the yields of both materials can reach gram scale. The formation mechanisms of mesoporous silica flower and hollow structures were also experimentally investigated and discussed. These novel hierarchical structures having unique physicochemical properties may bring many interesting insights into scientific research and technological application.
[en] In this research, the cubic and tetragonal crystal structure coexisted LaFe10.0Si3.0 compound and its hydride have been prepared by the arc-melting method and the novel electrolytic hydriding. Furthermore, the crystal structure, magneticand negative thermal expansion (NTE) properties of LaFe10.0Si3.0 and its hydride were investigated by the X-ray diffraction (XRD) and physical property measurement system (PPMS). Intriguingly, the NTE operation-temperature window of LaFe10.0Si3.0 has been broadened significantly after introducing interstitial hydrogen atoms. The further magnetic characterizations combined with theoretical analysis reveal that the increase of ferromagnetic to paramagnetic transition temperature due to stronger Fe-Fe magnetic exchange interactions contribute to the broad NTE operation-temperature window in the LaFe10.0Si3.0 hydride. (paper)
[en] Impregnating resins as electrical insulation materials for use in ITER magnets and feeder system are required to be radiation stable, good mechanical performance and high voltage electrical breakdown strength. In present ITER project, the breakdown strength need over 30 kV/mm, for future DEMO reactor, it will be greater than this value. In order to develop good property insulation materials to satisfy the requirements of future fusion reactor, high voltage breakdown strength measurement system at low temperature is necessary. In this paper, we will introduce our work on the design of this system. This measuring system has two parts: one is an electrical supply system which provides the high voltage from a high voltage power between two electrodes; the other is a cooling system which consists of a G-M cryocooler, a superfluid chamber and a heat switch. The two stage G-M cryocooler pre-cool down the system to 4 K, the superfluid helium pot is used for a container to depress the helium to superfluid helium which cool down the sample to 1.8 K and a mechanical heat switch connect or disconnect the cryocooler and the pot. In order to provide the sufficient time for the test, the cooling system is designed to keep the sample at 1.8 K for 300 seconds. (paper)
[en] Low-temperature abnormal thermal expansion (ATE) materials have been recently developed because of their significant applications for cryogenic engineering. However, the challenge still remains for the control of ATE effect at cryogenic temperature and adjustable ATE is of fundamental interest. In this paper, we report the isotropic ATE in La(Fe, Al)13 compounds over a wide adjusting temperature range by partially substituting Fe by Mn. It is found that all samples crystallize in the cubic NaZn13-type structure with the space group. The introduction of nonmagnetic Mn atoms reduces the Fe-Fe exchange interaction, therefore, the itinerant electron system needs less energy to break the magnetic order in ferromagnetic (FM) state at low temperature. The negative thermal expansion (NTE) operation-temperature window moves towards lower temperatures accompanied with the decrease of Curie temperature (T C) by increasing Mn elements. Moreover, the composite combining Mn 0 and Mn 57 broadens the zero thermal expansion (ZTE) behavior occurring in the whole tested temperature range. The present studies could be useful to control the thermal expansion, and indicate the potential applications of ATE materials in cryogenic engineering. (paper)