杂质对Gd_5(Si_xGe_(1-x))_4(x≈0.5)合金结构与磁熵变的影响

采用纯度分别为99.94%和99.2%的稀土金属Gd,配制了Gd5Si1.9Ge2.1和Gd5Si1.72Ge2.28两组合金,研究杂质对Gd5(SixGe1-x)4合金磁熵变的影响原因。粉末衍射结构分析表明,所研究的合金中都有Gd5Si2Ge2相,而采用低纯Gd配制的Gd5(SixGe1-x)4合金中还出现了明显的Gd5(Si,Ge)3相。磁性测量表明,杂质不改变合金中主相的居里温度,即没有改变合金中主相磁性原子的相互作用,但由于低温反铁磁性Gd5(Si,Ge)3相对室温Gd5Si2Ge2相的磁熵变没有贡献,导致Gd5(SixGe1-x)4合金在室温附近的磁熵变下降。     

Sn合金化对Gd5Si2Ge2磁致冷材料结构和性能的影响

以商业纯Gd为原料,采用非自耗电弧炉氩气保护下熔炼了Gd5Si2Ge2-xSnx(x=0.2,0.5,1)和Gd5Si2-yGe2Sny(y=0.1,0.2,0.5)系列合金,研究Sn合金化对Gd5Si2Ge2晶体结构和磁热性能的影响.粉末XRD结果表明Sn代Ge样品具有正交Gd5Si4型结构;Sn少量代Si(y=0.1,0.2)的样品具有单斜Gd5Si2Ge2型结构;Gd5Si1.5Ge2Sn0.5则为单斜和正交的混合结构.用超导量子磁强计(SQUID)测定了样品的M-T和不同温度的M-H曲线,结果表明Gd5Si2Ge2-xSnx(x=0.2,0.5,1)不具有巨磁热效应;Gd5Si1 9Ge2Sn01合金的最大磁熵变达15.3 J/kg·K(0 T～5 T),具有巨磁热效应.     

Gd60Co26Al14大块非晶合金的磁熵变

利用铜模吸铸法制备了Gd60Co26Al14非晶合金。采用示差扫描量热法（DSC）,X射线衍射（XRD）和超导量子磁强计（SQUID）研究了其结构与磁热性能。XRD分析表明：铸态Gd60Co26Al14合金是完全的非晶结构;DSC测试显示Gd60Co26Al14合金在加热过程中在571K发生玻璃化转变,并且出现了两个结晶温度,分别是602K和642K。SQUID测试结果表明：Gd60Co26Al14合金出现两个居里温度,分别是82K和128K;合金在外磁场5T下82K处的磁熵变达到7J．（kg·k^-1）。     

Gd60Co26Al14大块非晶合金的磁熵变

利用铜模吸铸法制备了Gd60Co26Al14非晶合金.采用示差扫描量热法(DSC),X射线衍射(XRD)和超导量子磁强计(SQUID)研究了其结构与磁热性能.XRD分析表明:铸态Gd60Co26Al14合金是完全的非晶结构;DSC测试显示Gd60Co26Al14合金在加热过程中在571 K发生玻璃化转变,并且出现了两个结晶温度,分别是602 K和642 K.SQUID测试结果表明:Gd60Co26Al14合金出现两个居里温度,分别是82 K和128 K;合金在外磁场5 T下82 K处的磁熵变达到7 J·(kg·K-1).     

Gd1-xVx系列合金的磁热效应研究

采用真空电弧熔炼方法制备Gd1-xVx(x=0.01,0.03,0.05,0.07,0.09)系列合金.研究发现:Gd1-xVx合金完全保持了纯Gd的六方型晶体结构,其在居里温度附近的磁特性符合二级相变规律;合金居里温度比纯Gd低1～2 K,并且随x的增加变化很小;在低磁场下Gd1-xVx合金具有较大的磁熵变、绝热温变以及较宽的ΔSM-T 曲线峰,并且所有样品的相对制冷能力都明显优于纯Gd.     

La_1－zRz（Fe_1－x－yCoxAly）_（13）金属间化合物磁熵变特性研究

本文对用Ｆｅ、Ａｌ替代Ｃｏ的Ｌａ（Ｆｅ（０．７３）－ｘＣｏｘＡｌ（０．２７））（１３）系和用Ｒ（Ｃｅ，Ｐｒ，Ｎｄ，Ｓｍ，Ｄｙ，Ｇｄ）替代Ｌａ的Ｌａ１－ｚＲｚ（Ｆｅ（０．７１）Ｃｏ（０．０２）Ａｌ（０．２７））（１３）系的结构、磁性及磁熵变特性进行了研究。实验发现，Ｌａ（Ｆｅ（０．７８）－ｘＣｏｘＡｌ（０．２７））（１３）系的晶体结构均保持ＬａＣｏ（１３）的立方结构，每３ｄ原子的饱和磁矩与其电子数ｎｄ的关系在富Ｃｏ侧符合刚性能带模型，而在富Ｆｅ侧则与该模型不符。当ｘ＝０．０２时该化合物的居里温度在室温并有较高的磁熵变△ＳＭ，在１．１１ＭＡ／ｍ（１．４Ｔ）磁场下的（△ＳＭ）ｍａｘ为１３ｋＪ／ｍ３·Ｋ。对于Ｌａ１－ｚＲｚ（Ｆｅ（０．７１）Ｃｏ（０．０２）Ａｌ（０．２７））（１３）系，实验发现除Ｃｅ外，其它稀土元素均不能与Ｌａ、Ｆｅ、Ｃｏ、Ａｌ形成１：１３相，而是形成了２：１７相。     

Er2-xPrxFe17磁致冷合金在室温区的磁熵变

在氩保护气氛中用熔炼法制备了Er2-xPrxFe17系列合金，通过X射线衍射和SQUID磁强计研究了样品的结构和磁熵变。结果表明，Er2-xPrxFe17系列合金具有六方Th2Ni17型结构，通过成分微调使其居里温度处在室温附近，Er2-xPrxFe17合金有较大的磁熵变，且致冷温区较宽，是一类有很大应用潜力的室温磁致冷材料。     

室温磁致冷合金Pr2Fe17-xCox的结构、磁性和磁熵变

在氩气气氛中用熔炼法制备了Pr2Fe17-xCox系列合金，通过粉末X射线衍射和SQUID磁强计研究了样品的结构、磁性和磁熵变。结果表明：Pr2Fe17-xCox系列合金具有菱方Th2Zn17型结构；通过成分微调使其居里温度处在室温附近：Pr2Fe17-xCox，系列合金有较大的磁熵变，在低场下的磁熵变是金属Gd的65％～73％，而高场下的磁熵变则为金属Gd的63％～68％；但其成本约为金属Gd的1／10，具有很高的性价比，是一类有很大应用潜力的室温磁致冷材料。     

快速凝固Gd5Si2Ge2合金的显微组织与相组成

研究了在真空电弧炉和快淬炉2种快速凝固条件下制备的Gd5Si2Ge2合金的显微组织与相组成。结果表明，Gd5Si2Ge2合金亿电弧炉熔炼条件下，具有不同质蟹的样品可以获得不同的显微组织与相组成。不同速度下，快淬甩带实验表明，在小于40m／s的快淬速度下，Gd5Si2Ge2合金为单斜Gd5Si2Ge2-型晶体结构：在50m／s的快淬速度下Gd5Si2Ge2合金包含Gd5Si4-型和Gd5Si2Ge2-型两相。证实了冷却速度对Gd5Si2Ge2合金的相组成有重要影响。     

Nb掺杂Gd合金的大磁热效应和显微硬度增强

在氩气气氛中用熔炼法制备了Gd100-xNbx(x=0,1,2,3,5)系列合金,铸锭经1273K均匀化退火96h后水淬至室温。结果表明:Gd100-xNbx系列合金仍保持纯Gd的六方相结构;Nb掺杂合金的居里温度比纯Gd均低2K,在居里点附近发生的磁性转变为二级相变,5T外场下的最大磁熵变约为纯Gd的85%。通过少量Nb(≤5at%)掺杂后,Gd100-xNbx系列合金的显微硬度明显得到提高,与纯Gd相比,显微硬度最大提高幅度达～53%(x=5)。含少量Nb的Gd100-xNbx合金具有较大的磁熵变和较好的加工性能,是一类有很大应用潜力的室温磁致冷材料。     

（LaxCe1—x）0.9（PrNd）0.1（NiCoMnAl）5 贮氢合金结构和电化学性能的研究

研究了四元混合稀土（LaxCe1-x)0.9(PrNd）0.1（Ni3.55Co0.75Mn0.4Al0.3）（x=0.4～0.9）贮氢合金中La,Ce的不同含量和比例对合金结构和电化学性能的影响。结果表明：合金晶胞的α轴和晶胞体积随La含量x的增加而增大,而c轴则在小幅度内波动;合金电极的最大放电容量随x的增加而增大,并在x=0.90时达到最大值（328.9mAh/g）,但平均每循环容量衰减率提高,充放电循环稳定性下降。     

Phase relationships and crystallography in the pseudobinary system Gd5Si4---Gd5Ge4

A study of phase relationships and crystallography in the pseudobinary system Gd₅(Si_xGe_1−x)₄ revealed: (1) that both terminal binary compounds Gd₅Si₄ and Gd₅Ge₄ crystallize in the Sm₅Ge₄-type orthorhombic structure, and (2) the appearance of an intermediate (ternary) phase with a monoclinic crystal structure which is similar to both Gd₅Si₄ and Gd₅Ge₄. The formation of the monoclinic phase at 0.24≤x≤0.5 [between Gd₅(Si_0.96Ge_3.03)Gd₅(Si₁Ge₃) and Gd₅(Si₂Ge₂)] is probably due to the large difference in bonding characteristics of Si and Ge in the Gd₅Si₄-Gd₅Ge₄ pseudobinary system which limits the ability of the mutual substitution of Si for Ge and vice versa without a change of the crystal structure. For the composition Gd₅(Si₂Ge₂) the lattice parameters of the monoclinic structure (space group P112₁/a) are a=7.580865), b=14.802(1), c=7.7799(5)Å, γ=93.190(4)°. A distinct difference in the magnetic behaviors of the alloys from three different phase regions in this system follows the distinct difference in the crystal structures observed for the alloys from the three phase regions.     

退火对(Mo1-xNbx)5Si3高温合金组织与力学性能的影响

采用电弧熔炼结合高温退火技术制备(Mo1-xNbx)5Si3(其中,x=0.0,0.2,0.4,0.6,0.8,1.0)系高温合金,研究了退火处理对其组织和性能的影响.结果表明:退火使合金组织趋于均匀化,Mo的加入使Nb5Si3合金的组织由α型向β型转变;退火前后试样的硬度均呈先增高后降低的抛物线规律,退火后试样的硬度高于退火前的硬度;断裂韧性呈先降低后升高的规律:断口主要以脆性断裂为主.     

Kinetic study on Li2.8(V0.9Ge0.1)2(PO4)3 by EIS measurement

Kinetics for lithium ion transfers in the fast ionic conductor Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ prepared by solid-state reaction method has been studied by electrochemical impedance spectroscopy (EIS) at various temperatures and the results were correlated with observed cathodic behavior. The specific conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ (x = 0.9–2.8) versus temperatures were analyzed from blocking-electrodes by Wagner's polarization method and the activation energy was calculated. It was observed that electronic conductivities of Li_x(V_0.9Ge_0.1)₂(PO₄)₃ increased with lithium contents in the materials. The compounds show a reversible capacity of 131 mAh g⁻¹ at low current density (13 mA g⁻¹). Modeling the EIS data with equivalent circuit approach enabled the determination of charge transfer and surface film resistances. The Li ion diffusion coefficient (D_Li⁺) versus voltage plot shows three valleys during the first charge cycle coinciding with the irreversible plateau of the voltage versus lithium content profiles reflecting the irreversible phase change in the compound. The obtained D_Li⁺ from EIS varies within 10⁻⁸ to 10⁻⁷ cm² s⁻¹, so Li_2.8(V_0.9Ge_0.1)₂(PO₄)₃ shows excellent chemical diffusion performance.     

Changes in the phase structure and magnetic characteristics of Gd5Si2Ge2 when alloyed with Mn

Gd₅Si₂Ge₂ parent compounds were alloyed with Mn in order to understand the underlying relation between the structural phases and the magnetic behavior of the pseudo ternary compounds formed. The alloying mechanism in Gd₅Si₂Ge₂ causes simultaneous substitution of the nonmagnetic Si and Ge atoms from the (Si + Ge) sublattice in equal amounts. No subsequent heat treatment was made on alloyed compounds. X-ray powder diffraction, magnetization versus temperature and isothermal magnetization measurements were carried out. X- ray diffraction patterns were used to qualitatively determine the existence of different structural phases in the alloys. It was observed that the starting, as-melted alloy with z = 0 has Gd₅Si₄-type orthorhombic structure at room temperature with traces of 1:1 stoichiometry phase which transforms totally to a Gd₅Si₂Ge₂-type monoclinic phase when heat treated. Similarly, increase in the Mn content leads to an increase in the monoclinic phase content of the originally orthorhombic compounds. Curie temperatures were determined from M(T) measurements and the magnetocaloric characterization was made using M(H) measurements by plotting the magnetic entropy change values against temperature. No giant magnetocaloric effect was observed for non heat treated samples.     

The influence of gallium on the magnetocaloric properties of Gd5Si2Ge2

Gd₅Si₂Ge₂ was alloyed with varying amounts of Ga to study its influence on the giant magnetocaloric effect. Investigations on Gd₅(Si_2−xGe_2−x)Ga_2x with 2x = 0.03, 0.05 and 0.13 were carried out using X-ray powder diffraction, temperature and magnetic field dependent magnetization measurements, and differential scanning calorimetry. We observe that as the Ga content increases, the temperature stability range of the monoclinic phase narrows, and the orthorhombic structure gains stability. This is expected to be related to the decrease in the (Si/Ge)(Si/Ge) bond distance in the monoclinic phase. The maximum entropy change for the parent compound at 270 K was found to be 9.8 J kg⁻¹ K⁻¹ in an applied field of 5 T. For 2x = 0.03, this value reduces to 8.5 J kg⁻¹ K⁻¹, and the temperature corresponding to the maximum entropy change shifts marginally to 278 K. For other 2x values, the maximum entropy change further decreases.     

Thermal expansion studies on Th(MoO4)2, Na2Th(MoO4)3 and Na4Th(MoO4)4

Thermal expansion behavior of Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ was studied under vacuum in the temperature range of 298–1123 K by high temperature X-ray diffractometer. Th(MoO₄)₂ was synthesized by reacting ThO₂ with 2 mol of MoO₃, at 1073 K in air and Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were prepared by reacting Th(MoO₄)₂ with 1 and 2 mol of Na₂MoO₄, respectively at 873 K in air. The XRD data of Th(MoO₄)₂ was indexed on orthorhombic system where as XRD data of Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were indexed on tetragonal system. The lattice parameters and cell volume of all the three compounds, fit into polynomial expression with respect to temperature, showed positive thermal expansion (PTE) up to 1123 K. The average value of thermal expansion coefficients for Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were determined from the high temperature data.     

Structural and thermal studies on Na2U(MoO4)3 and Na4U(MoO4)4

In Na–U(IV)–Mo–O system, two quaternary compounds Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄ were prepared by solid state reactions of Na₂MoO₄, UMoO₅ and MoO₃ in the required stoichiometric ratio at 500 °C in evacuated sealed quartz ampoules. The crystal structure of both the compounds were derived from X-ray powder diffraction data in the tetragonal system by Rietveld profile method. Na₂U(MoO₄)₃ has scheelite structure, whereas Na₄U(MoO₄)₄ has scheelite superlattice structure.

TG curves of Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄ did not show any significant weight change up to 750 °C in an inert atmosphere. During the heating cycle in an inert atmosphere, DTA curves of Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄ showed endothermic peaks due to the melting of the compounds at 740 °C and 730 °C, respectively. Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄, when heated in air atmosphere at 1200 °C, decomposed to form Na₂U₂O₇ which was confirmed by weight loss calculation and XRD.     

The studies of phase relation, microstructure, magnetic transition, magnetocaloric effect in (Gd1−xErx)5Si1.7Ge2.3 compounds

The phase relation, microstructure, Curie temperatures (T_C), magnetic transition, and magnetocaloric effect of (Gd_1−xEr_x)₅Si_1.7Ge_2.3 (x = 0, 0.05, 0.1, 0.15, and 0.2) compounds prepared by arc-melting and then annealing at 1523 K (3 h) using purity Gd (99.9 wt.%) are investigated. The results of XRD patterns and SEM show that the main phases in those samples are mono-clinic Gd₅Si₂Ge₂ type structure. With increase of Er content from x = 0 to 0.2, the values of magnetic transition temperatures (T_C) decrease linearly from 228.7 K to 135.3 K. But the (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds display large magnetic entropy near their transition temperatures in a magnetic field of 0-2 T. The maximum magnetic entropy change in (Gd_1−xEr_x)₅Si_1.7Ge_2.3 compounds are 24.56, 14.56, 16.84, 14.20, and 13.22 J/kg K⁻¹ with x = 0, 0.05, 0.1, 0.15, and 0.2, respectively.