Magnetocaloric properties of phenolic resin bonded La(Fe,Si)13-based plates and its use in a hybrid magnetic refrigerator

We present a simple hot press-based method for processing La(Fe,Si)13-based compounds consisting of La–Fe–Co–Si–C particles and phenolic resin. The magnetic entropy change △S per unit mass for the La Fe_(10.87)Co_(0.63)Si_(1.5)C_(0.2)/phenolic resin compounds have nearly the same magnitude with the base materials. With the content of phenolic resin of 5.0 wt%, the compound conductivity is 3.13 W·m^(-1)·K^(-1). In order to measure the cooling performance of La(Fe,Si)13-based compounds,the La(Fe_(11.6-x)Co_(x))Si_(1.4)C_(0.15)(x =0.60, 0.65, 0.75, 0.80, 0.85)/phenolic resin compounds were pressed into thin plates and tested in a hybrid refrigerator that combines the active magnetic refrigeration effect with the Stirling cycle refrigeration effect. The test results showed that a maximum cooling power of 41 W was achieved over a temperature span of 30 K.

Magnetic and magnetocaloric effect of Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass

Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass exhibited excellent magnetic refrigeration material with a wide temperature range and high refrigeration capacity(RC)was reported.Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass was observed with typical spin glass behavior around 15.5 K.In addition,we find that the magnetic entropy change(-△S_(M))originates from the sample undergoing a ferromagnetic(FM)to paramagnetic(PM)transition around 20 K.Under a field change from 0 T to 7 T,the value of maximum magnetic entropy change(-△S_(M)^(max))reaches 12.5 J/kg·K,and the corresponding value of RC reaches 487.7 J/kg in the temperature range from 6 K to 60 K.The large RC and wide temperature range make the Er_(20)Ho_(20)Dy_(20)Cu_(20)Ni_(20)high-entropy metallic glass be a promising material for application in magnetic refrigerators.

Development of 400-μW cryogen-free dilution refrigerators for quantum experiments

We have successfully developed cryogen-free dilution refrigerators with medium cooling power that can be applied to quantum experiments. Breakthroughs have been made in some key technologies and components of heat switches and dilution units. Our prototype has been running continuously and stably for more than 100 hours below 10 m K, with a minimum temperature of 7.6 m K and a cooling power of 450 μW at 100 m K. At the same time, we have also made progress in the application of dilution refrigerators, such as quantum computing, low-temperature detector, and magnet integration. These indicators and test results indicate good prospects for application in physics, astronomy, and quantum information.

Development of permanent magnetic refrigerator at room temperature

A reciprocating magnetic refrigerator was developed based on the active magnetic regeneration technology. Rare earth metal Gd and intermetallic compound LaFe11.2Co0.7Si1.1 were used as the magnetic operating materials in the machine. The particles of the magnetic operating materials, with diameter of 0.5-2 mm and total mass of 950 g, were mounted in the cooling bed. A magnetic field was assembled using NdFeB rare earth permanent magnets. It had the magnetic field space of Φ 34×200 and the magnetic induction of 1.5 T. The water at pH=10 is used as a heat transfer fluid. When the ambient temperature is 296 K, a temperature span of 18 K was achieved after operation of 45 min at a frequency of 0.178 Hz. The temperature span and the output power increase significantly with the increasing velocity of heat transfer.

Structure,room-temperature magnetic and optical properties of Mn-doped TiO_2 nano powders prepared by the sol-gel process

TiO2 nano powders with Mn concentration of 0 at%-12 at% were synthesized by the sol-gel process, and were annealed at 500 ℃ and 800 ℃ in air for 2 hrs. X-ray diffraction （XRD） measurements indicate that the Mn-TiO2 nano powders with Mn concentration of 1 at% and 2 at% annealed at 500 and 800 ℃ are of pure anatase and rutile, respectively. The scanning electron microscope （SEM） observations reveal that the crystal grain size increases with the annealing temperature, and the high resolution transmission electron microscopy （HRTEM） investigations further indicate that the samples are well crystallized, confirming that Mn has doped into the TiO2 crystal lattice effectively. The room temperature ferromagnetism, which could be explained within the scope of the bound magnetic polaron （BMP） theory, is detected in the Mn-TiO2 samples with Mn concentration of 2 at%, and the magnetization of the powders annealed at 500 ℃ is stronger than that of the sample treated at 800 ℃. The UV-VIS diffuse reflectance spectra results demonstrate that the absorption of the TiO2 powders could be enlarged by the enhanced trapped electron absorption caused by Mn doping.

Effect of surface microstructure of aluminum coating on corrosion properties of magnetic refrigerant gadolinium

AI coatings with different microstructures were prepared on the surface of Gd using the magnetron sputtering technique to improve its corrosion resistance. The corrosion behaviors for the pure Gd and Gd with Al coating in distilled water were studied using the mass loss and electrochemical performance. As a result, pure Gd without coating shows a certain amount of surface cracks under water flow conditions, whereas the polygonal Al coating decreases the path of the corrosive medium to body due to the existence of eroding pits structure. Compared with the polygonal structure Al coating and pure Gd, the lamellar structure of Al coating exhibits a higher electrochemical protection performance （e.g., a lower corrosion current and higher self-corrosion potential） and no occurrence of pitting corrosion. Due to an effective physical shield, the formation of the lamellar structure protected the inner Gd part from being corroded, and prolonged the duration of cathodic protection.

Rare Earths and Magnetic Refrigeration

Magnetic refrigeration is a revolutionary, efficient, environmentally friendly cooling technology, which is on the threshold of commercialization. The magnetic rare earth materials are utilized as the magnetic refrigerants in most cooling devices, and for many cooling application the Nd2Fe14B permanent magnets are employed as the source of the magnetic field. The status of the near room temperature magnetic cooling was reviewed.

Development of magnetocaloric materials in room temperature magnetic refrigeration application in recent six years

Some magnetocaloric materials were used successfully in magnetic refrigeration application and became one of the critical parts of magnetic refrigeration technology whose delightful progresses were made worldwide in the past 30 years. At the same time, the research on giant magnetocaloric materials will accelerate the development of room temperature magnetic refrigeration. In this paper, the new theoretical and experimental investigations on magnetic materials in room temperature application were described, including Gd and its binary and ternary intermetallic compounds, Mn-based compounds, La(Fe13-xMx)-based compounds and manganites. Based on the analysis of hysteresis, corrosion, cost and heat process, the comparison between different families of magnetic materials was discussed. Further research of room temperature magnetic refrigerant was suggested.

Magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys near Martensitic transformation

This paper studies the magnetic hysteresis and refrigeration capacity of Ni-Mn-Ga alloys in detail during heating and cooling isothermal magnetisation processes. The Ni-Mn-Ga alloys show larger magnetic hysteresis when they trans-form from austenite to martensite, but smaller magnetic hysteresis when they transform from martensite to austenite. This behaviour is independent of either the pure Ni-Mn-Ga alloys or the alloys doped with other elements. Because of the existence of the magnetic hysteresis, the relation between the magnetic entropy change and refrigeration capacity is not simply linear. For practical consideration, magnetocaloric effect of Ni-Mn-Ga alloys should be investigated both on cooling and heating processes.

High room-temperature magnetization in Co-doped TiO_(2) nanoparticles promoted by vacuum annealing for different durations

To clarify the contribution of oxygen vacancies to room-temperature ferromagnetism(RTFM)in cobalt doped TiO_(2)(Co-TiO_(2)),and in order to obtain the high level of magnetization suitable for spintronic devices,in this work,Co-TiO_(2) nano-particles are prepared via the sol-gel route,followed by vacuum annealing for different durations,and the influence of vacu-um annealing duration on the structure and room-temperature magnetism of the compounds is examined.The results reveal that with an increase in annealing duration,the concentration of oxygen vacancies rises steadily,while the saturation magnetiza-tion(Ms)shows an initial gradual increase,followed by a sharp decline,and even disappearance.The maximum Ms is as high as 1.19 emu/g,which is promising with respect to the development of spintronic devices.Further analysis reveals that oxygen va-cancies,modulated by annealing duration,play a critical role in tuning room-temperature magnetism.An appropriate concentra-tion of oxygen vacancies is beneficial in terms of promoting RTFM in Co-TiO_(2).However,excessive oxygen vacancies will result in a negative impact on RTFM,due to antiferromagnetic superexchange interactions originating from nearest-neighbor Co^(2+)ions.

ENTROPY CHARACTERISTICS OF MAGNETIC REFRIGERANTS

Mathematical models for predicting magnetic entropy,lattice entropy and electron entropy of magnetic refrigerants are carried out from the molecular field appoximation,the Debye model and the electron energy level theory respectively of quantum mechanics.Using theabove models the temperature and magnetic field dependences of the calculated magnetic entropy,lattice entropy,electron entropy and the total entropy of gadolinium,which is a typical magnetic refrigerant near room temperature,are obtained.The Influences of kinds of entropy on the performance of magnetic refrigeration are discussed,and some general rules for selecting magnetic refrlyerants are drawn to hely the develoyment of highly economic magnetic refrigerators.

Effect of Aluminum Coating on Corrosion and Heat Transfer Performance of Magnetic Refrigerant Gadolinium

The surface of gadolinium was covered with a layer of Al film(10-15 μm) by hot-press.Corrosion resistance, thermal conductivity and diffusivity of the composites were studied. As a result,temperature and pressure are the main factors that influence the combination of Gd and Al, and the effect of pressure is especially obvious. When the pressure exceeds 700 MPa, the binding force reaches 9 MPa. After coating, corrosion resistance of the composites is significantly improved, Corrosion rate is significantly reduced and mass loss reduced to one-tenth of pure Gd after 15 days. The current density decreases by an order of magnitude and corrosion potential increases by 0.3 V. The thermal conductivity of the composites is 11.12 W·m^-1·K^-1. Due to good thermal diffusion coefficient of aluminum, the magnetic work piece coated with Al film might show better heat transfer performance.

Study on Gd-Al-Dy System Magnetic Refrigeration Materials

d-Al-Dy system materials were prepared by the technique of powder sintering. Twolayers gradient function materials with compositions of (Gd_0.9Dy_0.1)_3Al_2 and Gd_3Al_2 respectively were studied. The results show that the Curie temperature (Tc) of the monolayer material decreases with the increment of Dy content. The Tc values of the twolayer gradient function material agree well with the layer numbers and corresponding to Dy content. For the Tc gradiently changed twolayers Gd-Al-Dy system material, its ΔSm changes smoothly with temperature. Therefore, the magnetic refrigeration is improved.

Estimation on Magnetic Refrigeration Material (Gd_(1-x)RE_x)_5Si_4 (RE=Dy,Ho)

A systematic （Gd1-xREx）sSi4 （RE=Dy, Ho） alloys are investigated to estimate their magnetocaloric effect. The Curie points of （Gd1-xREx）Si4 alloys can tunable from 266 K to 336 K when RE=Dy, Ho; z=0N0.35 and 0-0.15, respectively, and decrease nearly linearly with increasing x. These alloys keep orthorhombic structures GesSm4 and exhibit second order transition when they experience in a change magnetic field at about Curie points. The weight and voluminal magnetic entropy changes are about 3.5 J/（kg.K） and 23-29 mJ/（cm^3.K） when magnetic field changes 0-2 T. The adiabatic temperatures changes （△Tad） of these alloys at Curie points are larger than 1 K in a field change 0-1.4 T, the curve of ATad is wide as that of Gd. The relative cooling power is about 0.8-0.9 J/cm^3 when field changes 0-2 T, 55% of that of Gd. Comparing with Gds（Si1-xGex）4, these alloys do not contain expensive element Ge, so that their cost are lower than the former. Because they could work at temperature region 260-340 K due to their Curie points can be tuned, which is an advantage comparing with Gd, these alloys are potential magnetic refrigerants working in a magnetic refrigerator with a low magnetic field at room temperatures.

Recent Progress of Magnetic Refrigeration Technique Near Room Temperature

The recent progress of magnetic refrigeration technique at room temperature, especially in magnetic refrigerant materials with respect to Gd-Si-Ge, La-Fe-Si, Mn-Fe-P-As which has GMCE was reported. Also the recent progress in magnetic refrigerator design was reviewed.

Thermodynamical Evaluation on Magnetocaloric Effect of Magnetic Refrigerating Materials Near Room Temperature

The relationship between isothermal magnetic entropy change DELTA S andadiabatic temperature change DELTA T_(ad) was deduced according to the principles of thermodynamics.The MCE and the engineering application were discussed for Gd and several new kinds of magneticrefrigerating materials near room temperature, Gd_5Si_2Ge_2, MnFeP_(0.45)As_(0.55) and LaFe_(11.2)Co_(0.7)Si_(1.1). Isothermal entropy change is proportional to adiabatic temperature change with afactor of T/C (T is temperature, C is heat capacity). When the comparison of magnetacoloric effectis made for two different materials, we should consider isothermal entropy change as well asadiabatic temperature change.

Magnetic Field Enhancement in Ammonia-Water Absorption Refrigeration Systems

Absorption enhancement has been considered as an effective way of improving coefficient of performance (COP) of refrigeration systems and magnetic enhancement is one of these methods. A model of magnetic field enhancement in ammonia-water absorption systems is presented in this paper. A numerical model using finite difference scheme was developed based on the conservation equations and mass transport relationship. Macroscopic magnetic field force was introduced in the momentum equation. The model was validated using data obtained from the literature. Changes in the physical properties of ammonia solution while absorbing both in the direction of falling film and across its thickness were investigated. The magnetic field was found to have some positive effect on the ammonia-water falling film absorption. The results indicate that absorption performance enhancement increased with magnetic intensity. The COP of simple ammonia solution absorption refrigeration system increased by 1.9% and 3.6% for magnetic induction of 1.4 and 3.0 Tesla respectively.

Room-temperature ferromagnetism induced by Cu vacancies in Cu_x(Cu_2O)_(1-x) granular films

Cux（Cu2O）1-x（0.09 x 1.00） granular films with thickness about 280 nm have been fabricated by direct current reactive magnetron sputtering. The atomic ratio x can be controlled by the oxygen flow rate during Cux（Cu2O）1-x deposition. Room-temperature ferromagnetism（FM） is found in all of the samples. The saturated magnetization increases at first and then decreases with the decrease of x. The photoluminescence spectra show that the magnetization is closely correlated with the Cu vacancies in the Cux（Cu2O）1-x granular films. Fundamentally, the FM could be understood by the Stoner model based on the charge transfer mechanism. These results may provide solid evidence and physical insights on the origin of FM in the Cu2O-based oxides diluted magnetic semiconductors, especially for systems without intentional magnetic atom doping.

Room-temperature ferromagnetism and piezoelectricity in metalfree 2D semiconductor crystalline carbon nitride

Two-dimensional(2D)materials that combine ferromagnetic,semiconductor,and piezoelectric properties hold significant potential for both fundamental research and spin electronic devices.However,the majority of reported 2D ferromagnetic-semiconductor-piezoelectric materials rely on d-electron systems,which limits their practical applications due to a Curie temperature lower than room temperature(RT).Here,we report a high-crystallinity carbon nitride(CCN)material based on sp-electrons using a chemical vapor deposition strategy.CCN exhibits a band gap of 1.8 eV and has been confirmed to possess substantial in-plane and out-of-plane piezoelectricity.Moreover,we acquired clear evidences of ferromagnetic behavior at room temperature.Extensive structural characterizations combined with theoretical calculations reveal that incorporating structural oxygen into the highly ordered heptazine structure causes partial substitution of nitrogen sites,which is primarily responsible for generating room-temperature ferromagnetism and piezoelectricity.As a result,the strain in wrinkles can effectively modulate the domain behavior and piezoelectric potential at room temperature.The addition of RT ferromagnetic-semiconductor-piezoelectric material based on sp-electrons to the family of two-dimensional materials opens up numerous possibilities for novel applications in fundamental research and spin electronic devices.

Nanocrystalline and Nanocomposite Magnetic Materials and Their Applications

Nanocrystalline materials can possess bulk properties quite different from those commonly associated with conventional large-grained materials. Nanocomposites, a subset of nanocrystalline materials, in addition have been found to possess magnetic properties which are similar to, but different from, the properties of the individual constituents. New magnetic phenomena, unusual property combinations, and both enhanced and diminished magnetic property values are just some of the changes observed in magnetic nanocomposites from conventional magnetic materials. Here, a description will be presented of some of the exciting new properties discovered in nanomaterials and the magnetic applications envisioned for them.