Gd-Co二元非晶及晶态合金的磁性和大磁热效应

通过熔体快淬制备了Gd59Co41、Gd56Co44非晶条带,并对Gd56Co44非晶条带进行588K、10min的晶化处理。利用X射线衍射仪(XRD)分析了合金的结构,通过综合物性测量系统(PPMS)研究了合金的磁性及磁热效应。结果表明,Gd59Co41和Gd56Co44非晶条带的初始晶化温度分别为523和544K;Gd56Co44非晶条带晶化处理后获得了Ho4Co3型六方单相。非晶态和晶态合金在居里温度附近都发生铁磁到顺磁的二级相变。随着Gd/Co比例的降低,Gd59Co41和Gd56Co44非晶合金的居里温度(TC)从198K提高到217K;晶化处理后Gd56Co44合金的居里温度为218K,与非晶态合金相比变化甚微。在ΔH=5T时,Gd59Co41和Gd56Co44非晶合金的最大磁熵变(-ΔSM)和制冷能力(RC)分别为7.7J/kg·K、525J/kg和6.6J/kg·K、544J/kg;而Gd56Co44晶态合金的最大磁熵变(-ΔSM)和制冷能力(RC)分别为5.6J/kg·K和528J/kg。大的磁熵变和制冷能力,几乎可以忽略的矫顽力和热滞/磁滞效应,表明Gd-Co二元非晶和晶态系列合金是200K温区附近一类具有潜在应用价值的磁制冷工质。     

放电等离子烧结Gd/Gd5Si2Ge2复合材料磁热效应研究

以高纯钆和Gd5Si2Ge2合金为原料,采用放电等离子烧结技术制备了两组元Gdx(Gd5Si2Ge2)1-x(x=0,0.33,0.5,0.7,1)层状复合磁制冷材料.通过自制的磁热效应测量仪器直接测量了复合材料在外加磁场1.5 T下的磁热效应(ΔTad).随着复合比例的变化,材料的最大绝热温变(ΔTad)从x=0.3时的1.6 K增加到x=0.7时的2.0 K,而最大绝热温变峰的位置从286K变到了293 K.同时,与单组元的Gd5Si2Ge2合金相比,随着钆的含量增加时,复合材料的最大绝热温变峰变宽.当x=0.7时,层状复合磁制冷材料在外加磁场1.5 T下的最大绝热温变(ΔT)在260-310K范围里从1.1 K变到2.0 K,这种材料非常适合作为室温磁制冷材料.     

TmNiIn化合物的磁性和磁致热效应

利用电弧熔炼技术得到TmNiIn稀土金属间化合物,研究其磁性和磁热效应。研究表明TmNiIn金属间化合物的晶体结构是ZrNiAl型的六方密堆积结构,是一种具有可逆磁热效应的材料。TmNiIn是反铁磁材料,其相变温度为3.5K,在液氦温度(4.2K)附近。根据等温磁化曲线并利用麦克斯韦关系式计算可以得到磁熵变与温度的关系。当磁场变化为5T时,最大磁熵变和制冷能力分别为12.1J/kg·K和138J/kg。当磁场变化为2T时,最大磁熵变5.3J/kg·K,同时在TmNiIn金属间化合物中未观察到热滞现象和磁滞损耗。大的可逆磁致热效应表明TmNiIn是一种在低温磁制冷技术中有应用前景的材料。     

快淬低纯Gd5Si2Ge2合金的结构和磁熵变

采用纯度为99%的低纯稀土金属Gd经电弧炉熔炼获得Gd5Si2Ge2 母合金,然后通过快淬研究快淬工艺对低纯Gd5Si2Ge2 合金结构与磁熵变的影响。粉末衍射结构分析表明,铸态Gd5Si2Ge2 合金由Gd5Si2Ge2 相、Gd5 (Si、Ge)3 相和Gd(Si、Ge)相所组成;而甩带速度为25、40m/s的合金均为Gd5Si4 型正交相。用最小二乘法计算得两种不同快淬速度合金的晶格常数没有明显差异。甩带速度为40m/s 的Gd5Si2Ge2 合金的居里温度(Tc=305K)在室温区间,等温磁熵变为7.25J/kg·K(0~5T)。     

Gd5SixSn4-x合金的结构与磁热性能

利用XRD和振动样品磁强计研究了Gd5SixSn4-x(x=2.4、2.6、2.8)合金的结构与磁热性能.结果表明,在x=2.8时合金具有Gd5Si4形正交结构,x=2.4和2.6时则为Gd5Si4和Gd5Si2Gi2型两种结构,此外均存在少量其它杂相.随着Si含量的增大,居里温度分别为276、290.5和301.5K,逐渐升高,1.8T外加磁场变化时的磁熵变分别为1.88、2.26和1.69J/(kg·K),x=2.6时最大.低温XRD分析表明合金发生磁相变的同时没有结构相变发生.     

Co0.525Fe0.475MnP化合物的室温磁热效应和磁电阻效应

采用多步骤固态烧结方法合成了具有单一Co₂P相的Co_0.525Fe_0.475MnP化合物,其反铁磁有序温度在室温附近。在升温过程中,这种化合物经历两个连续的磁转变：在285 K发生反铁磁到铁磁的一级相变,在375 K发生由铁磁到顺磁的二级相变。在0~5 T的外磁场中,两个相变点温度对应的最大磁熵变分别为1.1 J/(kg·K)(303 K)和-2.0 J/(kg·K)(383 K)。外磁场为零时,随着温度的降低电阻率曲线在铁磁到反铁磁转变温度附近出现极小值,是铁磁有序与反铁磁有序的竞争所致。在35 K再次出现的电阻率极小值,可归因于由Fe替代Co引起的自旋无序所导致的金属-绝缘体转变。在5 T磁场中磁电阻率的最大值对应温度为200 K时的-2.5%,在反铁磁温度以上磁电阻率迅速减小。这表明,这种化合物的磁电阻效应源于外磁场对反铁磁有序的影响。     

Fe2-xCoxZr材料磁热效应的研究

制备了Fe_2-xCo_xZr(x=0.8,0.9和1.0)系列材料并研究了材料的磁热性能,通过X射线衍射分析确定了材料为MgCu₂结构的Laves相,空间群为Fd-3mS。通过GSAS精修确定了材料的晶格常数,发现材料的晶格常数随Co元素含量增加而线性减小。通过麦克斯韦方程计算了材料的磁熵变,磁熵变最大值随Co元素含量的增加而下降。通过Co元素含量的调控可以实现材料居里温度在236～320K之间进行调节。研究发现Fe_2-xCo_xZr材料相变类型均为二级磁相变。Fe_1.2Co_0.8Zr材料在0～3T磁场下磁熵变的最大值为0.27J/kg/K。Fe_2-xCo_xZr材料具有居里温度可调和制冷区间较大的特点,为具有较大应用潜力的无稀土磁致冷剂。     

C对Gd5Si4磁致冷材料结构和性能的影响

以商业蒸馏Gd为原料,采用非自耗电弧炉在氩气保护下熔炼了Gd5Si4Cx(x=0、0.24、0.35、0.5)系列合金,研究了C对Gd5Si4磁致冷合金组织结构与磁热性能的影响.粉末XRD结果表明,Gd5Si4Cx系列合金主相均为正交的Gd5Si4型结构,此外C(x=0.24、0.35、0.5)加入后合金中出现了少量的GdSi相.利用振动样品磁强计测量的合金的磁性能的结果表明,Gd5Si4Cx(x=0、0.24、0.35、0.5)系列合金的居里温度Tc在344～297K连续可调.在1.5T外加磁场变化时居里温度附近的最大磁熵变分别是2.79、2.47、2.03、1.65J/(kg·K).     

Al(Ga)原子择优占位对Gd2Co17化合物磁晶各向异性的影响

用磁性测量和磁取向粉末X射线衍射方法,研究了Gd2Co17-xMx(M=Al,Ga;x≤4)化合物的磁晶各向异性与Al(Ga)原子浓度的依赖关系.随着x的增加,Gd2Co17xAlx(x≥1)和Gd2Co17-xGax(x≥2)化合物的室温磁晶各向异性由易面转变为易轴;在Al、Ga原子的浓度分别为x=0.9和x=1.8附近时,室温各向异性常数K1由负变正,在x=3时达到最大值,且含Al化合物的K1最大值约为含Ga化合物的四倍.根据Al(Ga)原子在Gd2Co17化合物不同Co晶位择优占位的事实,讨论了Co亚点阵磁晶各向异性的演变结果表明,Al原子择优占据6c晶位是导致Al原子对Co亚点阵各向异性正贡献大于Ga原子的主要原因.     

热处理对Gd5Si4Cx组织结构与磁热性能的影响

以商业蒸馏Gd为原料,采用非自耗电弧炉在氩气保护下熔炼了Gd5Si4Cx(x=0.35、0.5、0.7)系列合金,随后将合金在钼丝炉进行1300℃,1h热处理.研究了高温热处理对Gd5Si4Cx磁致冷合金组织结构与磁热性能的影响.粉末XRD结果表明,热处理后的Gd5Si4Cx系列合金主相均为正交的Gd5Si4型结构,并含有少量的GdSi相.利用振动样品磁强计测量的合金的磁性能的结果表明,Gd5Si4Cx(x=0.35、0.5、0.7)系列合金的居里温度分别为302、294、217K.相对于铸态合金分别下降了5、3、25K.在1.5T外加磁场变化时居里温度附近的最大磁熵变分别是2.36、1.87、0.72J/(kg·K).相对于铸态合金分别提高了16%、13%、24%.     

Al2O3-SiO2纤维增强ZL108合金复合材料的强度特性

用低成本的Al₂O₃-SiO₂系纤维作为增强相,通过加压铸造法制作ZL108合金复合材料,并对该复合材料和ZL108合金进行不同温度下的时效处理和压缩试验。通过DSC、EPMA和TEM分析认为:经488K、0.5h时效处理(T6处理)的V_f 20%的复合材料在573K以下的压缩屈服强度低于ZL108合金,是由于基体中的Mg与Al₂O₃-SiO₂纤维在加压铸造过程中起化学反应而生成MgAl₂O₄,损耗了基体中的大量Mg,导致基体铝合金时效硬化效果很差,所以压缩屈服强度低下。623K、720h保温后的V_f 20%的复合材料的压缩屈服强度比ZL108合金要高得多,是由于在这种温度环境下对ZL108合金来说是过时效,所以纤维的增强怍用显得明显。在高温(673K)下V_f 20%的复合材料的屈服强度比ZL108台金高一倍左右。不论在什么温度场合下V_f5%的复合材料的屈服强度比V_f 20%的复合材料都低。     

Estimation on Magnetic Refrigeration Material （Gd1-xREx）5Si4 （RE=Dy, Ho）

A systematic (Gd1-xREx)5Si4 (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; x=0～0.35 and 0～0.15, respectively, and decrease nearly linearly with increasing x. These alloys keep orthorhombic structures Ge5Sm4 and exhibit second order transition when they experience in a change magnetic field at about Curie 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 △Tad is wide as that of Gd. The relative cooling power is about 0.8～0.9 J/cm3 when field changes 0～2 T, 55% of that of Gd. Comparing with Gd5(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.     

Y2O3与Gd2O3共掺杂SrZrO3热障涂层材料的热物理性能

采用固相反应法合成了5mol%Y2O3与5mol%Gd2O3共掺杂SrZrO3(Sr(Zr0.9Y0.05Gd0.05)O2.95,SZYG)粉末.采用X射线衍射(XRD)和差示扫描量热仪(DSC)分别研究了SZYG粉末在1450℃长期热处理后以及200~1400℃范围内的相稳定性.采用高温热膨胀仪测量了SZYG块材的热膨胀系数,结果表明:通过Y2O3与Gd2O3共掺杂改性可以明显抑制SrZrO3的相转变.在1000℃下SZYG块材的热导率是～1.36 W/(m.K),与SrZrO3和8YSZ块材相比降低~35%SZYG分别与8YSZ和Al2O3在1250℃热处理24 h表现出很好的化学相容性.     

Magnetic Properties of GdCoAsO

The magnetic properties of GdCoAsO have been investigated as a function of temperature and magnetic field. Complicated magnetism consists of antiferromagnetic, ferromagnetic, and paramagnetic has been observed. Most interestingly, magnetization measurements performed under low field, exhibit negative magnetization. Ferrimagnetic transition occurs at 137 K, which results from antiferromagnetic exchange interaction between Gd and Co magnetic moments, and then followed by transition to antiferromagnetic state. There exist three antiferromagnetic transitions at 12 K, 43 K, and 75 K, respectively. Antiferromagnetic transition at 75 K is independent of the magnetic field and at 43 K decreases with increasing field. Antiferromagnetic transition at 12 K corresponds to the antiferromagnetic ordering of Gd. A metamagnetic transition between two antiferromagnetic orders is observed just below 75 K. With increasing temperature, the metamagnetic transition field decreases. The magnetic phase diagram has been established.     

High-pressure synthesis, electrical and magnetic properties of new filled skutterudites LnOs4P12 (Ln = Eu, Gd, Tb, Dy, Ho, Y)

New filled skutteudites LnOs₄P₁₂ (Ln: Eu, Gd, Tb, Dy, Ho and Y) have been prepared at high temperatures and at high pressures. X-ray diffraction of these compounds is studied at room temperature. The relationship between lattice constants and atomic numbers of lanthanide (including Y) is obtained for LnOs₄P₁₂ (Ln: lanthanide). Electrical and magnetic properties of the new filled skutterudites with heavier lanthanide have been investigated at low temperatures. EuOs₄P₁₂ and GdOs₄P₁₂ show ferromagnetic transitions at around 15 and 22 K, respectively. The valence states of both compounds are +2 for the Eu compound and +3 for the Gd compound. DyOs₄P₁₂ does not show the magnetic transition down to 2 K. However, a small electrical anomaly is found at around 10 K. YOs₄P₁₂ exhibits a superconducting transition at around 3 K. This compound is a new superconductor. Electrical and magnetic anomalies of new filled skutterudites with heavier lanthanide LnOs₄P₁₂ (Ln: Eu, Gd, Dy and Y) are discussed. We have also found the electrical anomaly based on the magnetic transition at around 22 K for GdFe₄P₁₂.     

MlNi_4Co_(0.6)Al_(0.4)储H_2合金表面吸H_2的TDS研究

用热脱附谱 (TDS)对不同表面处理的富La混合储H2 合金MlNi4 Co0 6Al0 4 粉末样品进行H2 气吸附和脱附特性的比较和研究。未经表面处理的粉末样品 ,只测到一个H2 脱附峰 (α峰 ) ,脱附温度在 40 0K左右 ;经 6molKOH溶液 ,在 80℃下处理 6h的MlNi4 Co0 6Al0 4 粉末样品 ,有 2个H2 热脱附峰 (β峰和γ峰 ) ,脱附温度分别在 5 40和 6 30K处 ;而用 6molKOH 0 0 2molKBH4 溶液处理后 ,则有 3个H2 热脱附峰 (α峰 ,β峰和γ峰 ) ,脱附温度分别在 40 0 ,5 30和 6 40K处。TDS研究表明 ,热碱加还原的处理使材料表面对H2 气吸附的活性和容量提高 ,并使各个吸附态的扩散和转变更加容易。     

Search for thermometers with low magnetoresistive effects: platinum-cobalt alloy

Measurements performed on thermometers made of dilute ferromagnetic alloys of Pt with 0.45, 0.50, 0.75, 1.06 and 2.15 at% Co, in the temperature range 2–28 K and in magnetic fields up to 6 T showed a large region of negative magnetoresistivity that limits the magnetic error at low temperatures and field values. With 0.5 at% Co or less, the change of the magnetic error with T and B was smooth, while a magnetic transition has been observed with higher cobalt concentrations. Curie temperature has been found to occur at 6.0 K, 14.2 K and, by extrapolation, 32 K for alloys with 0.75, 1.06, and 2.15 at%Co respectively. The transition resulted in a second order discontinuity of the R-T characteristics at B = 0 and B = 3.8 ± 0.2 T with 0.75 at%Co and 8.0 ± 0.5 T with 1.06 at%Co, while for other field values the discontinuity was first-order. Hence, this alloy can be used for thermometry only with very low cobalt concentrations: with 0.3 at%Co it would be possible to limit the magnetic error within ±0.2 K up to 2 T and done to 5 K.     

The structure and magnetocaloric effect of rapidly quenched Gd5Si2Ge2 alloy with low-purity gadolinium

A magnetic refrigerating material Gd₅Si₂Ge₂ was prepared by arc-melting method under argon atmosphere with low-purity commercial gadolinium. The structure and magnetocaloric effect (MCE) of the as-cast and rapidly quenched Gd₅Si₂Ge₂ alloys were investigated by means of X-ray diffraction and magnetic measurements. The as-cast Gd₅Si₂Ge₂ consists of Gd₅Si₂Ge₂-type, Gd₅Si₄-type, Gd₅(Si,Ge)₃ and Gd(Si,Ge) phases from powder XRD results. The alloys quenched at a velocity of 25 m/s or 40 m/s both adopt in a single phase with orthorhombic Gd₅Si₄-type structure. The lattice parameters under different quenched velocities calculated by least-squares method have no significant difference. After being rapidly quenched at 40 m/s, the Curie temperature of Gd₅Si₂Ge₂ is about 303 K and its maximum magnetic entropy change is 7.25 J/kg K (0-5 T).