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A helium-free cryostat for DC flashover voltage measurement was constructed. The G-11CR film specimens were tested in vacuum at the temperatures of 293 K, 77 K and 9 K and Weibull distribution was obtained. The results show that the surface flashover voltage of G-11CR film increases as the temperature decreased. The surface flashover voltages did not decrease after 80 times successful flashover and slightly increased as the flashover time increased. Further analysis was performed to investigate the change of G-11CR surface after flashover was characterized by atomic force microscopy (AFM). G-11CR film presented different surface morphologies after 80 times successful flashover at different temperatures.

The insulation structure which is mainly composed of epoxy resin is a key part of the superconducting magnet system. Due to the special operating environments, superconducting coils must address the challenges of cryogenic liquids and various kinds of high voltages. In this paper, the electrical tree characteristics of epoxy resin at 77 K were investigated. An experimental setup with a partial discharge (PD) device and a cryostat filled with liquid nitrogen (LN2) was set up. Optical microscopy and confocal Raman spectrometer were applied to investigate the accumulation of degradation products within the tree channels. Each group of samples was tested at a range of AC voltages (50 Hz) from 8 kV rms (root mean square) to 20 kV rms and the PD experiments were carried out at both room temperature and 77 K. The results indicated that the color of electrical trees was darker and the tree structures were more complicated at 77 K compared with those at room temperature. The weak reaction between oxygen and the degradation products within the tree channels was responsible for the darker tree channels at 77 K. The density of electrical trees increased and the damage to materials caused by electrical trees was more serious at 77 K. Meanwhile, the tree inception probability decreased and the tree growth was inhibited obviously at 77 K. It’s also revealed that the degradation products within the tree channels were confirmed to be graphitic carbon based on the corresponding Raman spectra. The size of graphitic domains within the tree channels at 77 K was more than 2 times of that at room temperature, which contributed to the tree growth acceleration in the later growth stage at 77 K.

A cryogenic system has been established to measure electrical properties of insulation materials. The cryostat in the system contains two operating modes: 4.2 K and 1.65 K. A G-M cryocooler is used as cold source in these two modes. In 4.2 K mode, samples were directly cooled by the copper plates connected to the second stage of the cryocooler. In the 1.65 K mode, the helium gas is liquefied. The superfluid 4He (He-Ⅱ) of 1.65 K was obtained by throttling and depressurizing the liquid 4He (He-Ⅰ). The thermal switch in the 1.65 K mode can reduce the precooling time by 11.6 h and saved 22.5 L of helium. The transforming of the two modes is controlled by four threaded rods which are connected to the first and the second stage flange. The electrical test unit is designed, including the breakdown strength test and the flashover characteristics test with the voltage from 0 to 100 kV, which realizes the coupling of low temperature and high voltage.

Sealing performance under cryogenic temperatures should be a consideration in the development of a cryogenic valve for use in cryogenic engineering. In the present work, a cryogen-free leak detection system at 20 K to conduct leakage testing of cryogenic valves was developed. The experimental system was composed of a cooling system, a leak detection system, and a PLC (Programmable Logic Controller) system. Compared with the traditional cryogenic leak detection systems, this system using the GM cryocooler instead of cryogen is beneficial for the application for its low operating cost, simple structure, and safe handling. Five valves can be installed simultaneously and tested in sequence. The helium mass spectrometer leak detector is used for its sensitive response and high precision. The reliability of this system was verified by testing the leak rate of the system itself. Three high-pressure cryogenic solenoid valves were tested using helium gas under the pressure varied from approximately 2.5 bar to 80 bar and at both room temperature (300 K) and liquid hydrogen temperature (20 K). The tested valves can be cooled from room temperature to 18 K within 6 h. The test results of the internal leakage rate and the external leakage rate of the samples were discussed. This system will aid in the inspection of many critical cryogenic components such as liquid oxygen and liquid hydrogen valve assemblies for which leakage may lead to serious consequences.

In the present work, a cryogenic mechanical property testing system conduction-cooled by two G-M cryocoolers was developed. The testing sample can be cooled from room temperature to 2.7 K within 7.5 h. The sample was first cooled down to 11.1 K directly by the two G-M cryocoolers and then cooled down to 2.7 K by decompressing the chamber. Instead of liquid helium, the cooling process is characterized by cooling with recycled helium gas as heat transfer medium. The heat load of the system was analyzed and optimizations were adopted in terms of material selections and design. The static load capacity of the system reaches 200 kN and the fatigue load capacity can reach 50 kN. This system can be installed onto an electronic universal testing machine or a fatigue testing machine to characterize static tension, fracture mechanics or fatigue properties at tunable low temperatures. Tensile properties of 316L austenitic stainless steels at 4.2 K were tested with the system and the results were compared with those obtained by cooled using liquid helium, which demonstrates high reliability.

Superconducting magnet is a key component in magnetic resonance imaging (MRI). In general, vacuum pressure impregnation (VPI) technology is used for insulating system of the superconducting magnet. Performance of the superconducting magnets is affected by the adhesion strength between superconducting wires and resins at cryogenic temperature. In this study, experimental superconducting coil was prepared by modified bisphenol-A type epoxy resin and the VPI technology. Tensile, shear, and compressive properties of the samples cut from the superconducting coil were measured at both room temperature (RT) and liquid nitrogen temperature (77 K), and the bonding properties between the superconducting wire and the resin were discussed. The results show that the resin exhibits low viscosity, long pot life and higher mechanical strength, and it is suitable for impregnating superconducting magnets by VPI technology. Moreover, the shear and compressive strength of the impregnated superconducting magnet at 77 K are significantly higher than that at room temperature, with the increase of 132% in radial compressive strength, 170% in axial compressive strength, 63% in radial shear strength and 70% in axial shear strength, respectively. This study provides references for the design of insulation for superconducting magnet system and the manufacture of insulating materials for MRI superconducting magnets.

In the present work, fatigue crack propagation behavior of 316LN tungsten inert gas (TIG) weldment and weldment with post weld heat treatment (PWHT) was investigated at liquid helium temperature. Results indicates that the weldment with PWHT results in the highest fatigue crack propagation resistance, whereas the base material demonstrates the lowest fatigue crack propagation resistance. Microstructure evolution reveals that the fatigue crack propagation is characterized as transgranular fracture for the base material as well as the as-welded weldments and the weldment with PWHT. Moreover, the crack propagation path of the weldments shows a zigzag crack extension, especially for the weldment with PWHT, whereas the crack extension of the base material is relative straight. Electron backscattered diffraction reveals more amount of strain-induced martensite near the crack tip in the base material compared to in the as-welded weldment and in the weldment with PWHT, which is considered to be a crucial factor in the fatigue crack propagation behavior at cryogenic temperature.

In the present work, microstructural and microchemical of Nb3Sn strands with the internal tin process produced by the Western Superconductor Technology co., Ltd (WST) and Oxford Instruments co., Ltd (OST) were analyzed. The internal 3D-view of two Nb3Sn strands were detected with a (sub-)micrometric resolution in a non-invasive, non-destructive way, i.e. high-resolution 3D X-ray micro tomography. The distribution of filaments and voids induced by the functional heat treatment of two strands were investigated. The elements distribution after heat reaction of two strands was investigated through elemental mapping by scanning electron microscope (SEM) and energy dispersive X-ray analysis. The filaments of the WST Nb3Sn strand distributed uniformly, whereas the filaments separated with clear copper boundary among filaments. However, the filaments of the OST Nb3Sn strand have serious overlap phenomenon and the copper isolation interface was not clear among filaments. In order to investigate the distinction of microstructure, the selected area electron diffraction was also performed by transmission electron microscopy (TEM) and the results indicated that obvious impurities were observed in the Nb3Sn phase of the WST strand compared to the OST strand.

The resistivity of metal materials usually increases as the temperature rises and the temperature coefficient of resistivity (TCR) could be a few thousand (ppm/K). In fact, this would cause trouble to the accurate measuring instrument used in varying temperature conditions. Thus, it is a great challenge to develop materials with a low-temperature coefficient of resistivity (L-TCR). In this paper, we synthesized LaFe13-xSix compounds showing L-TCR behavior. The X-ray powder diffraction (XRD) patterns revealed that the crystal structure of LaFe13-xSix compounds changes from a cubic structure to a tetragonal structure as the Si component rises, and new phases like LaSi2 and FeSi appear. By testing electrical property, we find that the local extremum of the TCR is obtained in LaFe5Al8 compound, which is −54.4 ppm/K over 5–300 K.

In the present work, influences of room temperature tensile pre-strain on cryogenic tensile properties of 316LN austenitic stainless steel (SS) were investigated. The tensile properties of the 316LN SS with various tensile pre-straining amounts of 15%, 25% and 35% were measured at both room temperature (RT) and liquid helium temperature (4.2 K). Results indicated that both 0.2% proof strength (Rp0.2) and ultimate tensile strength (Rm) at both RT and 4.2 K of the pre-strained 316LN SS significantly increased compared to those of the as-received material. The Rp0.2 and Rm at 4.2 K for 35% pre-strained specimens increased around 86% and 18%, respectively, compared to those of the as-received material. However, the elongation at fracture (A) at both RT and 4.2 K decreased for the pre-strained 316LN SS compared to those of the as-received material. The fractured morphologies of the pre-strained 316LN SS were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Dimples in the surface fractured at both RT and 4.2 K of the as-received and pre-strained 316LN exhibited a ductile fracture mode. Moreover, fractographic results indicated that the pre-strain treatment considerably decreased the ductility at both RT and 4.2 K. TEM analyses revealed that pre-strain treatment at RT led to dislocation accumulation and stress-induced martensite phase occurred at 4.2 K, which interpreted the cryogenic tensile properties of the pre-strained 316LN SS.

Small-scale helium liquefiers using regenerative cryocoolers with cooling power up to 1.5 W at 4.2 K could be used to re-liquefy evaporated helium gas of small- and medium-sized cryogenic devices such as MEG and PPMS. A serial–parallel-path helium liquefier with a liquefaction rate of 83 Litres per day (L/d) using five 4 K G-M cryocoolers is developed, and has been applied to the Wuhan National High Magnetic Field Center (WHMFC) in China. Different from parallel-path helium liquefier, the helium gas is effectively, stepwise precooled by heat exchangers on multi-cold flanges, and thus the additional purifier and precooling coil heat exchangers on the thinner part of the cold head cylinder containing the 2nd stage displacer could be removed to simplify the construction. Through theoretical calculation and conclusive analysis, an optimum configuration is proposed and makes a reference to the design of serial–parallel-path helium liquefier with multi-cryocoolers.

Epoxy resin insulating materials used in superconducting feeder system of fusion device are required to be low thermal expansion coefficient (TEC). In this paper, negative thermal expansion (NTE) material ZrW 2 O 8 filled epoxy resins were fabricated. To improve the dispersion of fillers in epoxy matrix, plasma polymerization was performed on the surface of ZrW 2 O 8 powders. Transmission electron microscope (TEM) and surface wettability analysis were performed before and after the surface modification of ZrW 2 O 8 powders. The TEC of ZrW 2 O 8 /epoxy composites were measured from 77 K to room temperature. The results show the doping of ZrW 2 O 8 can significantly reduce the TEC of epoxy resins. The sedimentation rate of ZrW 2 O 8 before and after modified in epoxy was compared by density measurement. It can be seen that the ZrW 2 O 8 surface modified by plasma polymerization can enhance its dispersion properties in epoxy matrix.

Boron-free glass fiber reinforced isopropylidenebisphenol bis[(2-glycidyloxy-3-n-butoxy)-1-propylether]/triglycidyl-p-aminophenol (IPBE/TGPAP) epoxy matrix composite cured by diethyl toluene diamine (DETD) was prepared by vacuum press impregnation (VPI). The thermal behaviors of the composite, such as thermal expansion coefficient and thermal conductivity, between room temperature (RT) and liquid nitrogen temperature (77 K) were investigated before and after 1 MGy of 60 Co γ-ray radiation. In addition, thermogravimetric analysis and Fourier transform infra-red spectroscopy were used to evaluate thermal stability and chemical structural changes of epoxy matrix. Results revealed that the thermal properties of the composites and the chemical structure of epoxy resin matrix was not affected by the γ-ray radiation with a dose of 1MGy.

Antiperovskite manganese nitride Mn 3 Cu 0.6 Ge 0.4 N was fabricated by spark plasma sintering (SPS) at different temperatures and its negative thermal expansion behavior was investigated. It is observed that the width of negative thermal expansion (NTE) operation-temperature window becomes broader when the sintering temperature decreases. Moreover, it is significantly larger than that of other Mn 3 CuN-based antiperovskite manganese nitrides prepared by solid-state reaction. More interestingly, the Mn 3 Cu 0.6 Ge 0.4 N sintered at 650 °C shows near zero thermal expansion (ZTE) behavior in the temperature range of 220–170 K. The average linear coefficient of thermal expansion (CTE) is estimated to be −0.9 × 10 − 6 K − 1 . Magnetic measurement shows that the process of the magnetic transition becomes slow when the sintering temperature decreases. This antiperovskite manganese nitride Mn 3 Cu 0.6 Ge 0.4 N with ZTE behavior is much useful for applications in the fields of cryogenics and applied superconductivity.

Anti-perovskite manganese nitrides Mn 3 Cu 0.6 Ag x Sn 0.4− x N ( x = 0–0.3) were synthesized by mechanical ball milling followed by solid state sintering. Their negative thermal expansion (NTE) coefficient and electrical resistivity were investigated in the temperature range of 80–300 K. It is found that the transition temperature of NTE gradually moves toward lower temperature and the NTE operation-temperature window (Δ T ) becomes narrower with increasing Ag content within the testing temperature ranges. Interestingly, though the electrical resistivity of the samples shows a metallic behavior, the variation of electrical resistivity appears to be nearly independent of temperature above the transition temperature of NTE. The present discovery highlights the potential application of NTE materials in cryogenic engineering.