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.

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.

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 in the development of RE_(2)TMTM’O_(6)double perovskite oxides for cryogenic magnetic refrigeration

The magnetic functional materials play a particularly important role in our modern society and daily life.The magnetocaloric effect(MCE)is at the basis of a solid state magnetic refrigeration(MR)technology which may enhance the efficiency of cooling systems,both for room temperature and cryogenic appli-cations.Despite numerous experimental and theoretical MCE studies,commercial MR systems are still at developing stage.Designing magnetic solids with outstanding magnetocaloric performances remains therefore a most urgent task.Herein,recent progresses on characterizing the crystal structure,magnetic properties and cryogenic MCE of rare earths(RE)-based RE_(2)TMTM’O_(6)double perovskite(DP)oxides,where TM and TM’are different 3d transition metals,are summarized.Some Gd-based DP oxides are found to exhibit promising cryogenic magnetocaloric performances which make them attractive for active MR ap-plications.

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.

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.

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.

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.

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.

Magnetic entropy change and magnetic properties of LaFe_(11.5)Si_(1.5) after controlling the Curie temperature by partial substitution of Mn and hydrogenation

Magnetic properties and magnetic entropy changes of La（Fe_（1-x）Mn_x）_（11.5）Si_（1.5）H_y compounds are investigated. Their Curie temperatures are adjusted to room temperature by partial Mn substitution for Fe and hydrogen absorption in 1-atm（1 atm = 1.01325×10~5Pa） hydrogen gas. Under a field change from 0 T to 2 T, the maximum magnetic entropy change for La（Fe_（0.99）Mn_（0.01））_（11.5）Si_（1.5）H_（1.61）is-11.5 J/kg. The suitable Curie temperature and large value of ？S_m make it an attractive potential candidate for the room temperature magnetic refrigeration application.

Magnetic properties of Fe_(3(1-x))Cr_(3x)C alloys

The magnetic properties of Fe3（1-x）Cr3xC alloys with x=0.05, 0.1, 0.15, and 0.2, which crystallize in the cementite Fe3C-type structure with space group Pnma, were investigated by means of magnetization measurements. These alloys show temperature-induced second-order magnetic phase transitions. The Curie temperature （Tc） of these alloys decreases with increasing x. The isothermal magnetic-entropy changes of these alloys were derived from the magnetic isotherms measured with increasing temperature and increasing field. The maximum values of the magnetic-entropy change are about 0.9 and 3.6 J·kg^-1·K^-1 at Tc =360 K for x = 0.05 in a magnetic field change from 0 to 1 T and 0 to 5 T, respectively.

Magnetocaloric effect of (Gd_(1-x)Nd_x)Co_2 alloys in low magnetic field

The phases and magnetocaloric effect in the alloys （Gd1-xNdx）Co2 with x = 0, 0.1, 0.2, 0.3, and 0.4 were investigated by X-ray diffraction analysis and magnetization measurement. The samples are single phase with a cubic MgCu2-type structure. The To decreases obviously with increasing Nd content from 404 K of the alloy with x = 0 to 272 K of the alloy with x = 0.4; forx = 0.3, the To is 296 K, which is near room temperature. In the samples （Gd1-xNdx）Co2 with x = 0.0, 0.1, 0.2, 0.3, and 0.4, the maximum magnetic entropy change is 1.471, 1.228, 1.280, 1.381 and 1.610 J·kg^-1·K^-1, respectively, in the applied field range of 0-2.0 T. The results of Arrott plots confirmed that the transition type were second order magnetic transition forx = 0, 0.3, and 0.4.

Magnetocaloric effect of (Gd_(1-x)Ce_x)Co_2 compounds in low magnetic fields

The phases in the compounds （Gd1-xCex）Co2 with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 were investigated by X-ray diffraction, and the magnetocaloric effect for x = 0-0.4 was studied by magnetization measurements. The samples are almost single phase with a cubic MgCu2-type structure for x = 0-0.5. The magnetization decreases with an increase in Ce content. There is almost no magnetic transition for x = 0.5 at 100-350 K. The Curie temperature （To） of the （Gd1-xCex）Co2compounds with x from 0.1 to 0.4 are 350, 344, 340, and 338 K respectively. The maximum magnetic entropy change is 2.34 J·kg^-1·K^-1 when x = 0.3. The results of Arrott plots show that the magnetic phase transition is second-order magnetic phase transition in these compounds.

Magnetic properties and magnetocaloric effect in RE_(55)Co_(30)Al_(10)Si_(5)(RE=Er and Tm)amorphous ribbons

The magnetic and magnetocaloric effects(MCE)of the amorphous RE_(55)Co_(30)Al_(10)Si_(5)(RE=Er and Tm)ribbons were systematically investigated in this paper.Compounds with R=Er and Tm undergo a second-order magnetic phase transition from ferromagnetic(FM)to paramagnetic(PM)around Curie temperature T_(C)~9.3 K and 3 K,respectively.For Er_(55)Co_(30)Al_(10)Si_(5) compound,an obvious magnetic hysteresis and thermal hysteresis were observed at low field below 6 K,possibly due to spin-glass behavior.Under the field change of 0 T–5 T,the maximum values of magnetic entropy change(-△S_(M)^(max))reach as high as 15.6 J/kg·K and 15.7 J/kg·K for Er_(55)Co_(30)Al_(10)Si_(5) and Tm_(55)Co_(30)Al_(10)Si_(5) compounds,corresponding refrigerant capacity(RC)values are estimated as 303 J/kg and 189 J/kg,respectively.The large MCE makes amorphous RE_(55)Co_(30)Al_(10)Si_(5)(RE=Er and Tm)alloys become very attractive magnetic refrigeration materials in the low-temperature region.

Magnetic properties and magnetocaloric effects of Tm_(1-x)Er_(x)CuAl(x=0.25,0.5,and 0.75)compounds

We investigate the structural,magnetic,and magnetocaloric effects(MCE)of Tm_(1-x)Er_(x)CuAl(x=0.25,0.5,and 0.75)compounds.The compounds undergo a second-order phase transition originating from the ferromagnetic to paramagnetic transition around 3.2 K,5 K,and 6 K,respectively.The maximum magnetic entropy changes(-△S_(M)^(max))of Tm_(1-x)Er_(x)CuAl(x=0.25,0.5,and 0.75)are 17.1 J·kg^(-1)·K^(-1),18.1 J·kg^(-1)·K^(-1),and 17.5 J·kg^(-1)·K^(-1)under the magnetic field in the range of 0-2 T,with the corresponding refrigerant capacity(RC)values of 131 J·kg^(-1),136 J·kg^(-1),and 126 J·kg^(-1),respectively.The increase of-△S_(M)^(max)for Tm0.5Er0.5CuAl may be relevant to the change of magnetic moment distribution of Er and stress coming from element substitution.This work provides several compounds that can enrich the family of giant MCE materials in the cryogenic region.

Magnetic Entropy Change of (Gd_(1-x)RE_x)_5Si_4(RE=Dy, Ho) Alloys

The magnetic properties, including Curie points, magnetic phases transition and magnetic entropy changes, of （Gd1-xREx）5Sin（RE = Dy, Ho） alloys were systematically studied. The results show that the alloys keep the Sm5Ge4 orthorhombic structures as Gd5Si4, and the Curie points of the alloys almost linearly decrease with increasing content of x, so that the Curie points can be adjusted by adding different concentrations of Dv or Ho in the alloys. The magnetic properties of these alloys obey second order transition. The costs of these alloys are cheaper than that of Gd- Si-Ge alloys because there is not expensive element such as Ge. The large magnetic entropy change at low fields （ 〈 2 T） and wide temperature ranges of these alloys suggest that they are suitable to be the gradient function materials and candidates of magnetic refrigerants at room temperature with low fields.

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 Transition and Magnetic Entropy Change of Gd_5Si_(1.75)Ge_(1.75)Sn_(0.5)

Gd5Si1.75 Ge1.75 Sn0.5 was prepared by arc melting method. The crystal structure and magnetic properties were investigated by XRD and VSM, respectively. The magnetization of the Gd5Si1.75 Ge1.75 Sn0.5 alloy changes abruptly near its corresponding Curie temperature 269 K, possesses a typical first which means that the alloy order phase transition. The Gd5Si1.75Ge1.75 Sn0.5 adopts in Gd5Si2Ge2-type monoclinic structure at room temperature, the maximal magnetic entropy change at a magnetic field change of 1.8 T is as large as 16.7 J·kg^-1·K^-1, exceeding that of Gd about two times and is a little larger than that of Gd5Si2Ge2.

Structure, magnetism and magnetocaloric effects in Er_(5)Si_(3)B_(x)(x=0.3,0.6) compounds

We investigate the structure, magnetic properties, magnetic phase transitions and magnetocaloric effects(MCEs) of Er5Si3Bx(x=0.3,0.6) compounds. The Er5Si3Bx(x = 0.3, 0.6) compounds crystalize in a Mn5Si3type hexagonal structure(space group: P63/cm) and exhibit a successive complicated magnetic phase transition. The extensive magnetic phase transitions contribute to the broad temperature range of MCEs exhibiting in Er_(5)Si_(3)B_(x)(x=0.3,0.6) compounds, with maximum magnetic entropy change(-ΔSM_(max)) and refrigeration capacity of 10.2 J·kg^(-1)·K^(-1), 356.3 J/kg and 11.5 J·kg^(-1)·K^(-1),393.3 J/kg under varying magnetic fields 0–5 T, respectively. Remarkably, the δTFWHMvalues(the temperature range corresponding to 1/2×|-ΔSM_(max)|) of Er5Si3Bx(x=0.3,0.6) compounds were up to 41.8 K and 39.6 K, respectively. Thus, the present work provides a potential magnetic refrigeration material with a broad temperature range MCEs for applications in cryogenic magnetic refrigerators.

Refrigeration effect of La(FeCoSi)_(13)B_(0.25) compounds and gadolinium metal in reciprocating magnetic refrigerator

The LaFe11.9–x Cox Si1.1 B0.25 with x=0.9 and x=0.82 compounds were synthesized from commercial purity raw materials.The magnetic property of LaFe11.9–x Cox Si1.1 B0.25 and Gd particles were tested on the reciprocating refrigerator at the same condition in order to compare the cooling capacity of the two materials.The results showed that the cooling velocity of Gd was obviously higher than that of LaFe11.9–x Cox Si1.1 B0.25.The maximum temperature span was 12.7 oC for LaFe11.0 Co0.9 Si1.1 B0.25,14.9 oC for Gd metal whose mass is the same as that of LaFe11.0 Co0.9 Si1.1 B0.25,8.1 oC for Gd metal whose volume is the same as that of LaFe11.0 Co0.9 Si1.1 B0.25.Series connection of LaFe11.0 Co0.9 Si1.1 B0.25 and LaFe11.08 Co0.82 Si1.1 B0.25 had the maximum cooling temperature span of 15.3 oC.