Impact of the initial field on the thermodynamic performance of room-temperature magnetic refrigeration cycle

The thermodynamic cycle performance of Gadolinium (Gd) and Gd_0.87Dy_0.13 used as the working substance in regeneration magnetic Brayton and Ericsson refrigeration cycles are investigated under different external magnetic field conditions. Based on the experimental iso-field heat capacities of Gd with different magnetic fields, the effects of magnetic field change on thermodynamic performances including the magnetic entropy change, cooling quantity, non-perfect regeneration, net cooling quantity, and coefficient of performance (COP) are analyzed and discussed. The present work shows the possibility of reducing the regenerative losses and thereby improving the net cooling quantity for a given field change by selecting optimal initial and final magnetic field values. The similar analysis and calculation of the related thermodynamic performances are further applied to the magnetic material Gd_0.87Dy_0.13 which exhibits better net cooling quantities when compared to Gd at low temperature.     

The magnetocaloric effect in (Gd0.74Tb0.26)5(SixGe1−x)4 alloys

The magnetic phase transition and magnetocaloric effect of the alloys of Gd_0.74Tb_0.26 and (Gd_0.74Tb_0.26)₅(Si_xGe_1−x)₄ (x = 0.43, 0.50, 0.505) have been investigated by magnetization measurement. Experimental results show that partial substitution of the Gd by Tb in Gd₅(Si_xGe_1−x)₄ system keeps the first order magnetic transition. Although the values of transition temperature decrease, the as-cast (Gd_0.74Tb_0.26)₅(Si_0.43Ge_0.57)₄ and annealed (Gd_0.74Tb_0.26)₅(Si_0.50Ge_0.50)₄ alloys display large magnetic entropy change up to 18.89 J kg^− 1 K^− 1 and 13.79 J kg^− 1 K^− 1 near their transition temperatures in the low magnetic field change of 0–2.0 T, respectively.     

Development and parametric study of the convection-type stationary adiabatic demagnetization refrigerator (ADR) for hydrogen re-condensation

The adiabatic demagnetization refrigerator (ADR) system in this paper is composed of a conduction-cooled current cycling high-temperature superconducting (HTS) magnet system, a magnetic bed assembly, its heat exchange parts and an auxiliary precooling stage (a commercial GM cryocooler and a liquid nitrogen vessel). The whole magnetic refrigeration system including the conduction-cooled HTS magnet is cooled by the precooling stage to absorb the rejection heat of the ADR cycle. The packed bed type magnetic bed consists of tiny irregular powders of Dy_0.9Gd_0.1Ni₂ enclosed in a thin walled stainless steel container (22.2 mm in O.D., 0.3 mm in thickness and 40.0 mm in height). The precooled heat transfer fluid (helium) travels through the magnetic material when heat rejection is required; otherwise the helium stagnates within its pores (pseudo-adiabatic process). Flow of the heat transfer fluid substitutes for the function of a traditional heat switch, creating, essentially, a forced-convection type heat switch. The magnetic bed assembly is periodically magnetized and demagnetized at the center of the conduction-cooled HTS magnet which can stably generate both strong and alternating magnetic field from 0 T to 3.0 T (0–130 A) with an average ramp rate of 0.24 T s⁻¹. The cooling capacities of the ADR system at 20 K which is the normal boiling point (NBP) of hydrogen, are 11.1 J cycle⁻¹, 6.3 J cycle⁻¹ and 1.9 J cycle⁻¹ when the temperature spans are 1 K, 2 K and 3 K, respectively. We describe the detailed construction of the ADR system and discuss the test results with the operational parameters (the entrained helium pressure, the mass flow rate of helium and the operating temperature span) in the 20 K region.     

Magnetic refrigerator for hydrogen liquefaction

This paper reviews the status of magnetic refrigeration system for hydrogen liquefaction. There is no doubt that hydrogen is one of most important energy sources in the near future. In particular, liquid hydrogen can be utilized for infrastructure construction consisting of storage and transportation. When we compare the consuming energy of hydrogen liquefaction with high pressurized hydrogen gas, FOM must be larger than 0.57 for hydrogen liquefaction. Thus, we need to develop a highly efficient liquefaction method. Magnetic refrigeration using the magneto-caloric effect has potential to realize not only the higher liquefaction efficiency >50%, but also to be environmentally friendly and cost effective. Our hydrogen magnetic refrigeration system consists of Carnot cycle for liquefaction stage and AMR (active magnetic regenerator) cycle for precooling stages. For the Carnot cycle, we develop the high efficient system with >80% liquefaction efficiency by using the heat pipe. For the AMR cycle, we studied two kinds of displacer systems, which transferred the working fluid. We confirmed the AMR effect with the cooling temperature span of 12 K for 1.8 T of the magnetic field and 6 s of the cycle. By using the simulation, we estimate the efficiency of the hydrogen liquefaction plant for 10 kg/day. A FOM of 0.47 is obtained for operation temperature between 20 K and 77 K including LN₂ work input.     

The influence of Dy additions on the magnetocaloric effect in Gd0.97V0.03 alloys

The influence of Dy on the magnetocaloric effect in Gd_0.97 ? xDy_xV_0.03 (x = 0.1, 0.2, 0.3) alloys has been studied. These alloys were prepared by arc melting on a water-cooled copper hearth under an argon atmosphere. The magnetization behavior has been analyzed by X-ray diffraction and a vibrating sample magnetometer. Results indicate that the Curie points of Gd_0.97 ? xDy_xV_0.03 alloys decrease linearly with increasing content of Dy. The values of maximum magnetic entropy change (ΔS_M) and relative cooling power (RCP) for x = 0 ～ 0.2 is larger than that of Gd alone over a wider temperature range. The Gd_0.97 ? xDy_xV_0.03 alloys have promising potential as working substance candidates for magnetic refrigeration due to their tunable Curie temperature and the favorable properties of the magnetocaloric effect.     

Influence of short time milling in R5(Si,Ge)4, R = Gd and Tb,magnetocaloric materials

The effect of the short milling times on R₅(Si,Ge)₄ R = Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5 h), a reduction of the Gd₅Si_1.3Ge_2.7 and Tb₅Si₂Ge₂ particles size was achieved down to approximately 3.5 μm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd₅Si_1.3Ge_2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 min of milling. Interestingly, an opposite effect is observed in Tb₅Si₂Ge₂ where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process.     

Thermodynamic study of a combined double-way solid–gas thermochemical sorption refrigeration cycle

A combined double-way thermochemical sorption refrigeration thermodynamic cycle was proposed and tested. Both adsorption refrigeration and resorption refrigeration processes were combined in order to improve the system performance. Two different consolidated composite materials were used as the reactive sorbents and ammonia was used as the refrigerant. Experimental results showed that a system operating with such proposed cycle can have two useful cold productions during one cycle at the expense of only one heat input at high temperature. The average specific cooling power (SCP) during the adsorption refrigeration phase was 301 W kg^?1. Analysis of the experimental data showed that the driving equilibrium drop during the resorption process was much lower than that during the adsorption process, when the cold production temperature was similar. The proposed combined double-way sorption cycle has a larger cooling capacity per unit of heat input and the maximum theoretical coefficient of performance (COP) is 1.24 when MnCl₂ and BaCl₂ are used as the reactive sorbents.     

Structural,magnetic and magnetocaloric properties of La0.65Sr0.35V0.1Mn0.9O3 perovskite

A new complex magnetic material La_0.65Sr_0.35V_0.1Mn_0.9O₃, suitable for the magnetic refrigeration, has been investigated. X-ray diffraction result showed that this compound had rhombohedral structure. The substitution of manganese with V leads to a decrease in the Curie temperature, T_C from 378 K to 353 K. Using Arrott plots; it was found that the phase transition for this compound is of the second-order. The magnetocaloric study exposed a quite large value of the magnetic entropy change ～ 1.56 J/kg K and the relative cooling power value of 67 J/kg at magnetic field variation of 1 T.     

Synthesis of green-emitting (Gd,La, Tb)2O(WO4)2 phosphors

Green-emitting (Gd_1−x−yLa_xTb_y)₂O(WO₄)₂ (0 ⩽ x ⩽ 0.05, 0.05 ⩽ y ⩽ 0.15) phosphors were synthesized in a single phase form by the conventional solid-state reaction method, and their photoluminescent properties were characterized. The (Gd_1−x−yLa_xTb_y)₂O(WO₄)₂ phosphors showed strong and broad excitation bands from 230 to 350 nm, corresponding to the energy transition from the 4f⁸ to 4f⁷5d configuration of Tb³⁺ and the charge-transfer (CT) transition of O²⁻−W⁶⁺. The oxytungstate phosphors exhibited typical emission peaks assigned to the transition from ⁵D₄ to ⁷F_J (J = 6, 5, 4, and 3) of Tb³⁺, and the luminescence emission intensity was effectively enhanced by the La³⁺ doping into the host Gd₂O(WO₄)₂ lattice. The highest green emission intensity was obtained for (Gd_0.87La_0.03Tb_0.10)₂O(WO₄)₂, where the relative emission intensity was 63% that of a commercial green-emitting (La_0.52Ce_0.31Tb_0.17)PO₄ phosphor.     

Experimental study of one-stage VM cryocooler operating below 8 K

The Vuilleumier (VM) refrigerator, known as heat driven refrigerator, is one kind of closed-cycle Stirling type regenerative refrigerator. The VM refrigerator with power being supplied by liquid nitrogen was proposed by Hogen and developed by Zhou, which shows great potential for development below 10 K. This paper describes the experimental development of a VM cryocooler operating below 8 K, which was achieved by using liquid nitrogen as a heat sink of middle cavity. The regenerator was optimized by using a part of metallic magnetic regenerator material Er₃Ni to replace the lead sphere and a no-load temperature of 7.8 K was obtained. Then all the lead spheres were replaced by Er_0.6Pr_0.4 material and a no-load temperature of 7.35 K was obtained, which is the lowest temperature for this kind of refrigerator reported so far. The cooling power at 10 K is about 500 mW with a pressure ratio near 1.6 and a charge pressure of 1.8 MPa. Especially, the magnetic material Er_0.6Pr_0.4 was found to be a potential substitution for the conventional lead.     

Development of a proof of concept low temperature 4He Superfluid Magnetic Pump

We describe the development and experimental results of a proof of concept Superfluid Magnetic Pump in this work. This novel low temperature, no moving part pump can replace the existing bellows-piston driven ⁴He or ³He-⁴He mixture compressor/circulators used in various sub Kelvin refrigeration systems such as dilution, Superfluid pulse tube, Stirling, or active magnetic regenerative refrigerators. Due to the superior thermal transport properties of sub-Lambda ⁴He this pump can also be used as a simple circulator to distribute cooling over large surface areas. Our pump was experimentally shown to produce a maximum flow rate of 440 mg/s (averaged over cycle), 665 mg/s (peak) and produced a maximum pressure difference of 2323 Pa using only the more common isotope of helium, ⁴He. This pump worked in an “ideal” thermodynamic state: The experimental results matched with the theoretical values predicted by a computer model. Pump curves were developed to map the performance of this pump. This successful demonstration will enable this novel pump to be implemented in suitable sub Kelvin refrigeration systems.     

Investigation of energy absorption and transfer process of Tb3+ or Eu3+ excited Na3Gd(PO4)2 in the VUV region

Na₃Gd(PO₄)₂, Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ and Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ are prepared by solid-state reaction and their photoluminescence (PL) properties are investigated in the ultraviolet (UV) and vacuum ultraviolet (VUV) region. The obtained results show that Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ has an efficient emission under 147 nm excitation, but the emission efficiency of Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ is low under 147 nm excitation. We discuss the energy absorption and transfer process in the VUV region to solve the special phenomenon.     

Effect of Gd3+ substitution on dielectric properties of nano cobalt ferrite

CoGd_xFe_2 ? xO₄ (x = 0.0, 0.1, 0.3, 0.5) nano magnetic ferrite particles were synthesized by chemical co-precipitation method. The variation of dielectric parameters like dielectric constant, dielectric loss, capacitance and resistance for different Gd³⁺ compositions has been measured at room temperature for frequency dependence in the range of 100 Hz to 10 MHz using impedance analyzer. Results of measurements reveal strong dependence of dielectric parameters on frequency and Gd³⁺ ion content. Dielectric constant, dielectric loss, capacitance and resistance decrease with increasing frequency for all the CoGd_xFe_2 ? xO₄ compositions. Increase in Gd³⁺ ion composition in material, increases the values of dielectric constant, dielectric loss and capacitance while decreases the electrical resistance of nano-particles. A qualitative explanation is given for the composition and frequency dependence of dielectric parameters.     

Microstructure,luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering

This paper reports the microstructure, luminescence and thermal stability properties of the NaSr_1−xPO₄:xTb³⁺ powders (x = 0.008, 0.01, 0.02, 0.04 and 0.06) via the conventional solid-state sintering at 1200 °C for 5 h. The X-ray diffraction result verifies all diffraction peaks are pure phase of NaSrPO₄. The luminescence results show that the NaSrPO₄:xTb³⁺ powders mainly excited at 370 nm have a series of the emission-states, related to the typical 4f → 4f intra-configuration forbidden transitions of Tb³⁺, and a major emission peak of around 546 nm. The concentration quenching of the NaSr_1−xPO₄:xTb³⁺ phosphors is appeared at x = 0.02. The decay time values of the NaSr_1−xPO₄:xTb³⁺ phosphors for the ⁵D₄ state of the Tb³⁺ are around 3.30 ms to 3.60 ms. It is also found the chromaticity coordinate of NaSrPO₄:Tb³⁺ phosphor varies with the increase of the concentration of Tb³⁺ ions from blue to green. Moreover, the thermal stability of the NaSrPO₄:xTb³⁺ phosphors is slightly better than that of conventional YAG phosphors.     

Luminescence properties of ZnMoO4:Tb3+ green phosphor prepared via co-precipitation

Tb³⁺-doped ZnMoO₄ green phosphor was synthesized by a co-precipitation method. The morphology and structure of the phosphor were characterized by Scanning electron microscopy (SEM) and X-ray diffraction (XRD). Photoluminescence (PL) spectra were also used to characterize the ZnMoO₄:Tb³⁺ samples. The results show that ZnMoO₄:Tb³⁺ phosphor has triclinic structure with diameters ranging from 1.0 to 2.0 μm. The obtained ZnMoO₄:Tb³⁺ phosphor emits green light emission centered at 541 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺ when excited by 378 nm or 488 nm. The optimized concentration of Tb³⁺ is 15 mol.% for the highest emission intensity at 541 nm, and the concentration quenching occurs when the Tb³⁺ concentration is beyond 15 mol.%. The concentration quenching mechanism can be interpreted by the quadrupole–quadrupole interaction of Tb³⁺ ions. The present work suggests a convenient, cost-effective method for green phosphor, which may lead to potential applications in white light-emitting diodes (WLED).     

Attractive performance of a Gifford–McMahon cryocooler by co-axial layout of regenerator materials

A novel filling method of regenerator materials, which we name a co-axial layout, is experimentally investigated. The 2nd stage regenerator of a Gifford–McMahon cryocooler was packed with 50% by volume of the co-axial layout (low temperature side) that consisted of 20% Gd₂O₂S and 30% HoCu₂ spheres. The warm temperature side was packed with 50% lead spheres as a single layer. The experimental results show that the cooling power of co-axial layout is almost the same as that of a three-layer layout, 20% Gd₂O₂S, 30% HoCu₂ and 50% lead, at temperatures between the ultimate low temperature (3.0 K) and 6 K. Moreover, at the temperatures above 10 K, the co-axial layout has 1.7–2.0 times the cooling power of the three-layer layout. Thus the co-axial layout produces a major improvement in cooling power. In this paper, the performance of five types of material assortments is discussed.     

VUV–UV luminescence of Ce3+, Tb3+, Eu3+, and Dy3+ doped GdOCl

The vacuum ultraviolet spectroscopic properties of GdOCl:Re³⁺ (Re³⁺ = Ce³⁺, Tb³⁺, Eu³⁺, and Dy³⁺) are investigated in detail for the first time. The host absorption band is determined to be around 179 nm, and the f–d transition bands as well as the charge transfer bands are assigned. Upon 179 nm excitation, Re³⁺ (Re³⁺ = Ce³⁺, Tb³⁺, Eu³⁺, Dy³⁺) ions shown their characteristic emissions. Energy transfers from Gd³⁺ to Re³⁺ ion were observed. A broad band ranging from 350 to 400 nm corresponding to the d–f transition of Ce³⁺ is observed. Eu³⁺ has typical red emission with the strongest peak at 620 nm; Tb³⁺ shows characteristic transition of ⁵D_3,4 → ⁷F_j, and its spin-forbidden and spin-allowed f–d transitions in VUV region are calculated with Dorenbos’ equations, these calculated values agree well with the experimental results. Dy³⁺ presents yellow emission (⁴F_9/2 → ⁶H_13/2) with the strongest peak at 573 nm.     

不可逆回热式玻色布雷顿制冷循环性能分析

基于玻色气体的热力学性质和回热式布雷顿制冷循环的不可逆模型,导出循环的一些重要性能参数,如循环的制冷量、回热量、输入功和性能系数的一般表达式。通过数值计算获得了循环的一些重要的性能特性曲线,分析了循环的不可逆性和回热特性对玻色布雷顿制冷机性能的影响。     

Forced flow He vapor cooled critical current testing facility for measurements of superconductors in a wide temperature and magnetic field range

As superconducting materials find their way into applications, there is increasing need to verify their performance at operating conditions. Testing of critical current with respect to temperature and magnetic field is of particular importance. However, testing facilities covering a range of temperatures and magnetic fields can be costly, especially when considering the cooling power required in the cryogenic system in the temperature range below 65 K (inaccessible for LN₂). Critical currents in excess of 500 A are common for commercial samples, making the testing of such samples difficult in setups cooled via a cryocooler, moreover it often does not represent the actual cooling conditions that the sample will experience in service. This work reports the design and operation of a low-cost critical current testing facility, capable of testing samples in a temperature range of 10–65 K, with magnetic field up to 1.6 T and measuring critical currents up to 900 A with variable cooling power.     

Above room temperature magnetocaloric effect in perovskite Pr0.6Sr0.4MnO3

The dependence of magnetization M on the applied magnetic field H and temperature T was measured carefully, near the Curie temperature T_C for the perovskite manganite sample Pr_0.6Sr_0.4MnO₃. The experimental results indicate that this specimen exhibit ferromagnetic (FM) to paramagnetic (PM) transition at T_C ~ 320 K. In the 200 K–45 K temperature range the spontaneous magnetization was decreasing, probably due to spin canted state between manganese and praseodymium spin systems. At 46 K the magnetization presents a second little transition, which can be ascribed to very weak traces of secondary Mn₃O₄ phase, and remains constant between 10 K and 46 K. The maximum value of the magnetic entropy change obtained from the M(H) plot data is |ΔS_M^max| = 2.3 Jkg^? 1 K^? 1 for applied magnetic field of 2.5 T. At this value of magnetic field the relative cooling power (RCP) is 34.5 Jkg^? 1. The temperature corresponding to ΔS_M maximum value is almost equal to T_C. The large entropy change can be attributed to the fact that the ferromagnetic transition enhances the effect of the applied magnetic field greatly. It is suggested by the results that this compound can be used as the working material in an active magnetic regenerative refrigerator above room temperature.