查看更多>>摘要：The micro-orifices during the Joule-Thomson process are used in a variety of energy applications such as microfluidic systems and large-scale cryogenic helium systems。 The structure of the micro-orifice impacts the cooling capacity by adapting the mass flow rate and pressure drop in the refrigeration system。 The pressure drops and flow characteristics in micro-orifices with diameters of 20-40 mu m and thicknesses of 50 mu m were numerically and experimentally investigated。 Helium was used as the working fluid， and the simulations using CFD (Computational Fluid Dynamics) method were conducted with upstream pressure (pu) of 0。5-2。0 MPa (MegaPascal) and downstream pressure (pd) of 0。1-1。5 MPa at 293 K (Kelvin)。 The simulations found that the completely choked pressure ratio of each micro-orifice is smaller than the theoretical critical pressure ratio lambda*=0。487 and revealed the flow field characteristics in micro-orifices flow under different working conditions。 Experiments were carried out with four micro-orifices with the thickness of 50 mu m and the effective diameter of 23。88 mu m， 26。53 mu m， 33。30 mu m， 40。69 mu m under different pressure conditions (pu 0。5-2。0 MPa， pd 0。1-1。5 MPa)， respectively。 The errors between the numerical and experimental mass flow rates in different micro-orifices corresponding to the pressures are within 10 %。 The W。 B。 Brower model was used to predict the mass flow rates that are consistent with the experiments and simulations。 The semi-empirical model is developed using the data set for the modifications to W。 B。 Brower model and with an error range of +/- 0。08 mg/s in the calculated flow rate compared to the experimental value。
查看更多>>摘要：Optimizing the design of a two-stage Pulse Tube Cryocooler (PTC) usually affects its geometry and regenerator filling。 Here， this is demonstrated based on a recently developed small scale two-stage PTC， driven by a 1 kW Helium compressor with an initial base temperature of 5 K。 The improvements are aimed at reducing the minimum temperature and maximizing the cooling performance around 4。2 K。 By resealing the geometry， while keeping the overall length constant， and implementing Gd2O2S (GOS) in the regenerator filling， the coefficient of performance (COP) can be doubled from 7 x 10(-5) to 1。5 x 10(-4)。 The simulations generated with the software Sage agree with the experimental results and describe the cooling performance accurately down to 4。2 K。 This demonstrates that such simulations are important to optimize PTCs around liquid Helium temperatures。
查看更多>>摘要：To promote the commercial use of high-temperature superconducting devices， the ability to detect temperature anomalies easily and reliably and thereby protect the devices is important。 In particular， a sudden local temperature increase due to a superconducting-to-normal transition can damage a device。 In the present study， we propose a method to detect explosive boiling of superheated liquid nitrogen sealed in a long thin tube using a pressure gauge connected to the end of the tube。 Experiments using a stainless-steel tube with an inner diameter of 1 mm and a maximum length of 50 m under various conditions showed that the local temperature rise caused explosive boiling at a temperature slightly below the superheat limit of 110 K for liquid nitrogen; the results also showed that the rapid pressure increase was transmitted to the end of the tube at approximately the speed of sound (similar to 830 m/s)。 The proposed method of placing a thin tube along high-temperature superconducting conductors or cryogenic structures can be used to easily detect a local temperature rise。
查看更多>>摘要：Further enhancement of critical current density of Nb3Sn superconductors is still of great interest and challenging towards realization of leading-edge magnets such as the future circular collider (FCC) and demonstration Power Station (DEMO) fusion reactors。 In this study， the influence of the Ti-doping position on the Nb3Sn layer formation in internal Sn-processed Nb3Sn superconducting wires， which has not been sufficiently clarified yet， was investigated using the simplest diffusion pair structure that consists of a single Nb/Cu/Sn layer。 Three types of specimens were prepared， in which Ti was doped into the Sn core， Nb， and Cu matrix (denoted as ST， NT， and CT， respectively)。 The NbCuSnTi compound phase appeared as a diffusion barrier at the boundary between the Nb3Sn and Cu-Sn phases in the ST and CT samples and remained during the Nb3Sn layer formation， although it was absent in NT。 The electron backscatter diffraction analysis of the formed Nb3Sn grain morphologies showed no significant difference in terms of the average grain size and area fraction of the equiaxed grains regardless of the doping position。 However， the average Nb3Sn layer thickness of NT was at least 1。6 times larger than that of ST and CT。 The absence of the NbCuSnTi phase promotes Sn diffusion into Nb， which accounts for the thicker Nb3Sn layer and much higher I-c-B characteristics in NT。
查看更多>>摘要：The integrated refrigerator composed of sorption cooler and adiabatic demagnetization refrigerator (ADR) is proposed for sub-kelvin cooling without vibration。 In this configuration， the demagnetization process of the ADR is supposed to start after the termination of the sorption cooling。 The liquid helium-4 inside the thermosiphon of the sorption cooler should evaporate as quickly as possible。 This paper presents the results of preliminary test of the sorption cooling process。 The developed sorption cooler primarily consists of the sorption pump filled with activated charcoal of 5。2 g and the helium thermosiphon。 The adsorption capacity of the sorption pump is experimentally confirmed。 Furthermore， the empirical correlation of adsorbate concentration which predicts the experimental results within 15% error in the temperature range of 5-70 K is proposed。 The sorption pump is thermally connected to the 4。5 K heat sink through a pure tin strap， and regulates its temperature by the heater installed on it。 Sorption cooling experiment is conducted under two experimental conditions。 The experiment shows that the cooler can refrigerate the thermal capacitor to 2。8 K from 4。5 K heat sink， and provides the cooling capacity of 2。29 J。 Moreover， it is confirmed that the pumping speed of the sorption pump is dominantly affected by its transient cool-down process。 The new configuration of the thermosiphon condenser is proposed to enhance the evaporation characteristic of liquid helium while the pumping speed of the sorption pump is not changed。 Since the new thermosiphon has smaller gas volume， liquid helium evaporates more quickly due to the rapid reduction of the vapor pressure inside the thermosiphon。
查看更多>>摘要：High performance and reliable joints between 2G HTS coated conductors play a necessary and extremely important role in fabricating superconducting devices。 Therefore， developing stable joints and achieving low joint resistance have become an important issue。 Long overlap length and low resistivity solder were effective ways to achieve low resistance。 In this paper， we fabricate several soldered joints at different temperature and in different lengths， and test their electrical performances at 77 K， self-field。 According to experimental and simulation results， we found that quality of soldered joint is influenced not only by solder resistivity， but also by interfacial resistance。 In addition， the internal resistance test shows an unusual distribution。 The analysis of the overlap area shows that current sharing inside the joint is highly non-uniform along the length and width direction between two connected HTS tapes due to the influence of interfacial resistance and self-field， which is special as "U"-shaped。
查看更多>>摘要：The insulation failure is a bottleneck to restrict the safe operation and development of direct current (DC) superconducting power devices。 The solid-liquid flashover test is an important evaluation index， but the previous researches mostly focus on engineering application， and less involve in the mechanism analysis。 As a characterization method for defects existing in the dielectric， the trap has been applied to the study of flashover to occur polymer surface in liquid nitrogen (LN2) in recent years。 However， the mechanism of trap action in solid-liquid flashover is still confusing。 Therefore， this paper carries out the following research。 Firstly， the flashover voltage and current of glass epoxy laminate sheet (G10) in LN2 were obtained through experiments and analysis using the Weibull model in this paper。 Secondly， the flashover process of G10 was monitored。 The changes of G10 surface morphology and chemical bond before and after flashover were also observed。 Finally， the relationship between polymer flashover and trap in LN2 was analyzed based on the surface trap distribution test and theoretical calculation of the charge transport equation。 The results are beneficial to guide the selection and performance improvement of dielectric materials and the design of insulating structures in superconducting devices。
查看更多>>摘要：The over-current withstanding ability of the silicon carbide (SiC) based metal-oxidesemiconductor field effect transistor (MOSFET) is really important for the survival during over-load and short-circuit disturbances。 This article investigates the experimental characterizations of the steady-state over-current safe operation area (SOA) of SiC MOSFET at both the cryogenic temperature (77 K) and room temperature (300 K)。 Benefiting from the sufficient cooling power by the liquid nitrogen around devices， the cryogenic SiC MOSFET can have over 10 times longer than the over-current withstanding time at room temperature。 For practical applications in highperformance SiC-based power conversions and superconductor-semiconductor-coupled systems， 4 over current-dependent functions have been derived to predict the withstanding time and power dissipation at both 77 K and 300 K。 Considering the lowest on-state resistance during the transient over-current process of first increasing and then decreasing， a new SOA concept is further used to judge the safety threshold of the SiC MOSFET and the subsequent switch-off action。 The new method and procedure can effectively predict the potential safety issue of over heat， and make the switching-off actions in advance， which can prevent the late protection action due to the hardware/software delays。 Our experimental/theoretical investigations and the new analysis can also be regarded as general methods which are applicable to study other types of MOSFETs and build corresponding protection schemes。
查看更多>>摘要：A 60 nm gate length， n-type Gate all around (GAA) metal oxide semiconductor field effect transistor (MOSFET) is simulated at different temperature。 The temperature dependent electrical characteristics for various temperature are investigated extensively。 The improved gate controllability in lower technology node as well as immunity against short channel effects are thoroughly examined。 This paper also includes the analog performance analysis of GAA MOSFET along with the DC performances。 The performance matrix of the device for different temperature ranging from 70 K to 800 K were illustrated clearly。 Results exhibit that the drain current， transconductance， device gain and transconductance to drain current ratio (gm/Ids) improves when temperature is decreased。
查看更多>>摘要：Cryogenic pipeline chilldown is an inevitable process in cryogenic liquid transfer operation， but the researches on chilldown performance through a thick-wall tube are still insufficient。 In the present study， a high-pressure cryogenic chilldown test platform is established to study the tube wall thickness influence on chilldown characteristics。 Two stainless steel tubes， with 2 mm thickness wall and 5 mm thickness wall， respectively， are manufactured and tested under different inlet Reynolds numbers (Re) conditions。 The temperature decrease curves and heat transfer variations are analyzed and compared。 The test results show that a thicker wall tube requires an apparently longer chilldown time and a larger liquid consumption to reach the chilldown purpose。 In the present experiments， the mass ratio of the two tubes is about 3。0， but the time cost ratio varies within 3。5-6。0 and the liquid consumption ratio is about 4。5。 This performance indicates that in a thicker-wall pipe chilldown event， a lower efficient utilization of liquid cold energy could be speculated。 Moreover， increasing inlet Re produces temperature rises both at critical heat flux (CHF) and Leidenfrost point (LFP)。 For the 2 mm wall tube cases， increasing inlet Re from Re = 5500 to Re = 43000 gives rise to a 39 K temperature rise in LFP and a 32 K temperature rise in CHF。 Under the similar inlet Re variation， the temperature rises of LFP and CHF in the 5 mm wall tube cases are 28 K and 46 K， respectively。 Comparatively， the 5 mm wall tube experiences a shorter transition boiling-dominated range when increasing the inlet Re。 In addition， in the present test cases， the chilldown thermal efficiency varies with 8%-23%， and the 5 mm wall tube cases experience lower thermal efficiency than the 2 mm wall tube cases。 To realize a high-efficient chilldown purpose， the approaches of increasing chilldown thermal efficiency should be significantly concerned in the chilldown scheme design， especially when a thick wall tube situation is encountered。
NSTL
Elsevier