Xiaotao Wang

Microwave response of FeCo/carbon nanotubes composites

Complex permittivity and permeability of FeCo/carbon nanotubes (CNTs)-paraffin composites have been investigated at microwave frequencies between 2 and 18 GHz. Percolation threshold PC of about 34 wt. % is determined in the present system by AC conductivity measurements. Microwave absorption of FeCo/CNTs-paraffin composites is enhanced when the mass ratio of the fillers approaches PC. Meanwhile, the maximum absorption shifts to thinner thicknesses and lower frequencies by increasing the filler content. The maximum absorption was found to be -37.5 dB at 11.2 GHz with a layer thickness of 8 mm in the composites with 30 wt. % FeCo/CNTs nanocomposites

A high efficiency hybrid stirling-pulse tube cryocooler

This article presented a hybrid cryocooler which combines the room temperature displacers and the pulse tube in one system. Compared with a traditional pulse tube cryocooler, the system uses the rod-less ambient displacer to recover the expansion work from the pulse tube cold end to improve the efficiency while still keeps the advantage of the pulse tube cryocooler with no moving parts at the cold region. In the meantime, dual-opposed configurations for both the compression pistons and displacers reduce the cooler vibration to a very low level. In the experiments, a lowest no-load temperature of 38.5 K has been obtained and the cooling power at 80K was 26.4 W with an input electric power of 290 W. This leads to an efficiency of 24.2% of Carnot, marginally higher than that of an ordinary pulse tube cryocooler. The hybrid configuration herein provides a very competitive option when a high efficiency, high-reliability and robust cryocooler is desired.

Magnetic anisotropy and transport properties of 70 nm SrRuO3 films grown on different substrates

Magnetic and transport properties of 70 nm SrRuO(3) films grown on (001) SrTiO(3), (001) LaAlO(3) and (001) MgO have been investigated. A perpendicular magnetic anisotropy is observed in compressive strained films grown on SrTiO(3). A weaker perpendicular magnetic anisotropy and a weak in-plane magnetic anisotropy are found in strain-free films grown on MgO and LAO, respectively, possibly due to different growth mechanisms. In addition, metallic behavior is observed in all the as-grown films and the resistivity of the film grown on MgO is lowest (230 mu Omega cm at 300 K), which is close to that of bulk single crystal SrRuO(3) (about 195 mu Omega cm). The relation between structure and properties indicates that the magnetic anisotropy, as well as the magnitude of resistivity of SrRuO(3) films, can be effectively tailored by taking advantage of different strains and growth mechanisms induced by growth on different substrates

Influence of acoustic pressure amplifier tube on a 300 Hz thermoacoustically driven pulse tube cooler

High frequency thermoacoustically driven pulse tube coolers (PTCs) have the advantages of high energy density, compact structure, and high reliability. Through a series of improvements, the 300 Hz thermoacoustically driven PTC in this paper achieved a fourfold increase in the cooling power compared with that in our last report. A cooling power of 1.04 W at 80 K and a no-load temperature of 63 K are obtained with 500 W heating power. In the system, the acoustic pressure amplifier tube (APAT) plays an important role in coupling the thermoacoustic engine and the PTC. The effects of APAT on the system performance are investigated here through both numeric simulations and experiments. The simulation indicates that length of the APAT is limited due to impedance match requirement between the PTC and the engine, which is partly evidenced by experiments. Calculation also gives a good agreement between calculated pressure wave amplification ratio and experimental values. APAT with different lengths and diameters ar...

CHARACTERIZATION OF A 100 Hz MINIATURE PULSE TUBE COOLER DRIVEN BY A LINEAR COMPRESSOR

Pulse tube coolers driven by linear compressors usually operate between 30 Hz and 60 Hz. A higher frequency could lead to a higher specific power. This article presents a miniature pulse tube cooler with a working frequency at 100 Hz. Due to an impedance mismatch between the cooler and the compressor, the overall system efficiency is not good. The theoretical model considering the compressor and pulse tube cooler is described. Influences of various parameters, including frequency and inertance tube length, have been investigated both experimentally and analytically. A no‐load temperature of 59.6 K was achieved with a mean pressure of 3.55 Mpa, a pressure ratio of 1.23 and a frequency of 100 Hz. The cooling power at 80 K was 0.8 W.

Experimental study of a pulse tube cold head driven by a low frequency thermal compressor

Cryocoolers operating at liquid helium temperature span a number of application domains, such as cooling of superconducting magnets, SQUID devices etc. GM type cryocoolers are widely used at liquid helium temperature but with shortcomings of using an oil-lubricated compressor that require regular maintenance and rotary valves that reduces the efficiency of the cryocooler. We are developing an alternative system that makes use of a Vuilleumier type thermal compressor. The system consists of a Stirling type pulse tube cryocooler that provides a cold heat sink to a thermal compressor. The thermal compressor generates pressure wave to drive a second pulse tube cold head. We experimentally studied the influence of pre-cooling temperature and frequency on the performance of the pulse tube cold head. The lowest recorded temperature is 24.3 K with a pressure ratio of 1.18 and a frequency of 3 Hz. In this paper, the design of the cooling system and preliminary experimental results are presented

Advances in a high efficiency commercial pulse tube cooler

The pulse tube cryocooler has the advantage of no moving part at the cold end and offers a high reliability. To further extend its use in commercial applications, efforts are still needed to improve efficiency, reliability and cost effectiveness. This paper generalizes several key innovations in our newest cooler. The cooler consists of a moving magnet compressor with dual-opposed pistons, and a co-axial cold finger. Ambient displacers are employed to recover the expansion work to increase cooling efficiency. Inside the cold finger, the conventional flow straightener screens are replaced by a tapered throat between the cold heat exchanger and the pulse tube to strengthen its immunity to the working gas contamination as well as to simplify the manufacturing processes. The cold heat exchanger is made by copper forging process which further reduces the cost. Inside the compressor, a new gas bearing design has brought in assembling simplicity and running reliability. Besides the cooler itself, electronic controller is also important for actual application. A dual channel and dual driving mode control mechanism has been selected, which reduces the vibration to a minimum, meanwhile the cool-down speed becomes faster and run-time efficiency is higher. With these innovations, the cooler TC4189 reached a no-load temperature of 44 K and provided 15 W cooling power at 80K, with an input electric power of 244 W and a cooling water temperature of 23 ℃. The efficiency reached 16.9% of Carnot at 80 K. The whole system has a total mass of 4.3 kg.

Numerical and experimental study of an annular pulse tube used in the pulse tube cooler

Multi-stage pulse tube coolers normally use a U-type configuration. For compactness, it is attractive to build a completely co-axial multi-stage pulse tube cooler. In this way, an annular shape pulse tube is inevitable. Although there are a few reports about previous annular pulse tubes, a detailed study and comparison with a circular pulse tube is lacking. In this paper, a numeric model based on CFD software is carried out to compare the annular pulse tube and circular pulse tube used in a single stage in-line type pulse tube cooler with about 10 W of cooling power at 77 K. The length and cross sectional area of the two pulse tubes are kept the same. Simulation results show that the enthalpy flow in the annular pulse tube is lower by 1.6 W (about 11% of the enthalpy flow) compared to that in circular pulse tube. Flow and temperature distribution characteristics are also analyzed in detail. Experiments are then conducted for comparison with an in-line type pulse tube cooler. With the same acoustic power input, the pulse tube cooler with a circular pulse tube obtains 7.88 W of cooling power at 77 K, while using an annular pulse tube leads to a cooling power of 7.01 W, a decrease of 0.9 W (11.4%) on the cooling performance. The study sets the basis for building a completely co-axial two-stage pulse tube cooler.

Performance of a 260 Hz pulse tube cooler with metal fiber as the regenerator material

Pulse tube coolers operating at higher frequency lead to a high energy density and result in a more compact system. This paper describes the performance of a 300 Hz pulse tube cooler driven by a linear compressor. Such high frequency operation leads to decreased thermal penetration, which requires a smaller hydraulic diameter and smaller wire diameter in the regenerator. In our previous experiments, the stainless steel mesh with a mesh number of 635 was used as the regenerator material, and a no-load temperature of 63 K was obtained. Both the numerical and experimental results indicate this material causes a large loss in the regenerator. A stainless steel fiber regenerator is introduced and studied in this article. Because this fiber has a wide range of wire diameter and porosity, such material might be more suitable for higher frequency pulse tube coolers. With the fiber as the regenerator material and after a series of optimizations, a no-load temperature of 45 K is acquired in the experiment. Influenc...

An air-cooled pulse tube cryocooler with 50 W cooling capacity at 77 K

A pulse tube cryocooler with 50 W cooling capacity at 77 K is developed to cool superconducting devices mounted on automobiles. The envisioned cryocooler weight is less than 40 kg, and the input electric power is less than 1 kW. To achieve these requirements, the working frequency is increased to 75 Hz, and the dual-opposed pistons use gas bearings to reduce compressor weight and volume. The heat from the main heat exchanger is rejected by forced convective air instead of water. The compressor and the cold finger are carefully matched to improve the efficiency. The details of these will be presented in this paper. After some adjustment, a no load temperature for the pulse tube cryocooler of 40 K was achieved with 1 kW input electric power in surroundings at 298 K. At 77 K, the cooling capacity is 50 W. If the main heat exchanger is cooled by water at 293 K, the cooling capacity increases to 64 W, corresponding to a relative Carnot efficiency of 18%.