Fluctuation Conductivity of RE1−2x Ca x M x Ba2Cu3O7−δ (RE=Nd,Y and M=Pr,Th) Superconductors

Polycrystalline samples of RE_1?2x Ca _x M _x Ba₂Cu₃O_7?δ with RE=Nd, Y and M=Pr, Th (with 0.0≤x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping.     

尖晶石LiMn1.98RE0.02O4(RE=Ce,Nd)及其性能研究

采用传统的高温固相法合成了稀土掺杂改性的锂离子电池用正极材料LiMn1.98 RE0.02O4(RE=Nd,Ce).用XRD、激光粒径和恒电流充放电测试对材料的结构和性能进行了表征.从材料的晶体结构、充放电和循环性能等方面分析了掺杂元素在稳定材料性能中的作用.研究结果表明,掺杂后的材料仍为尖晶石结构,常温和高温下的循环性能都得到了明显改善.常温下50次循环后LiMn1.98Ce0.02O4和LiMn198Nd0.02O4的放电比容量分别维持在114.2 mAh·g-1和117.5mAh·g-1.     

Effect of synthesis pressure on hydride phases in Mg–M systems (M=Mn,Y)

High-pressure synthesis of the hydrides in Mg–M (M=Mn, Y) systems and the influence of applied pressures during synthesis on present phases and their crystal structures have been studied. In Mg–Mn system, it was found that the crystal structure of Mg₃MnH_y changed from hexagonal structure (a=0.47107(4) nm and c=1.0297(1) nm) to monoclinic structure (a=0.8819(8) nm, b=0.4658(4) nm, c=0.4678(5) nm and β=105.6(1)°) in a pressure range of 3–3.5 GPa. This crystal structural change was reversible with respect to pressure. The Mg₃MnH_y synthesized under 5 GPa was stable up to around 620 K. From thermogravimetric and fusion extraction analyses, the hydrogen content was determined as Mg₃MnH_5.0–5.6. In Mg–Y system, the high-pressure hydride (MgY₂H_y) with yellowish color was synthesized at 1073 K for 2 h under 3 GPa or higher. This phase exhibited an FCC-type structure with a cell parameter of a=0.516 nm. Its hydrogen content was determined to be about 3.7 mass%, corresponding to a chemical formula of MgY₂H_7.8. The hydride was partially dehydrogenated at around 600 K, and the amount of hydrogen partially desorbed was 1.4 mass%. The FCC-type structure was stable even after the partial dehydrogenation.     

Effect of synthesis pressure on hydride phases in Mg–M systems (M = Mn,Y)

High-pressure synthesis of the hydrides in Mg–M (M = Mn, Y) systems and the influence of applied pressures during synthesis on present phases and their crystal structures have been studied. In Mg–Mn system, it was found that the crystal structure of Mg₃MnH_y changed from hexagonal structure (a = 0:47107(4) nm and c = 1:0297(1) nm) to monoclinic structure (a = 0:8819(8) nm, b = 0:4658(4) nm, c = 0:4678(5) nm and b= 105:6(1)8) in a pressure range of 3–3.5 GPa. This crystal structural change was reversible with respect to pressure. The Mg₃MnH_y synthesized under 5 GPa was stable up to around 620 K. From thermogravimetric and fusion extraction analyses, the hydrogen content was determined as Mg₃MnH_5.0–5.6. In Mg–Y system, the high-pressure hydride (MgY₂H_y) with yellowish color was synthesized at 1073 K for 2 h under 3 GPa or higher. This phase exhibited an FCC-type structure with a cell parameter of a = 0:516 nm. Its hydrogen content was determined to be about 3.7 mass%, corresponding to a chemical formula of MgY₂H_7.8. The hydride was partially dehydrogenated at around 600 K, and the amount of hydrogen partially desorbed was 1.4 mass%. The FCC-type structure was stable even after the partial dehydrogenation.     

稀土离子RE(RE=Pr,Nb,Eu)在Sr2CeO4基质中的发光性质

通过燃烧法合成了Sr2CeO4:RE材料,研究了Sr2CeO4材料的结晶过程和发光性质及Pr3+,Nd3+和Eu3+稀土离子在Sr2CeO4基质中的发光性能.实验通过燃烧法制得的前驱体在1200℃焙烧2 h可得均一的Sr2CeO4相,较传统方法的合成时间大为降低.稀土离子Pr3+,Nd3+和Eu3+的在基质中的少量掺杂(0.02%)均可使Sr2CeO4在475 nm左右的特征发射谱峰明显变宽增强,且Eu3+离子的掺杂可使材料在510 nm,540nm,610 nm左右产生多个明显的稀土离子的特征发射峰.实验合成的发光材料具有良好的发光性能,证明Sr2CeO4材料可作为一种优良的发光材料的基质使用,为寻找新型的发光材料提供了一条新的途径.     

Tunable luminescence and energy transfer properties of Sr₃AlO₄F:RE³+ (RE = Tm/Tb, Eu, Ce) phosphors

Sr(3)AlO(4)F:RE(3+) (RE = Tm/Tb, Eu, Ce) phosphors were prepared by the conventional solid-state reaction. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), photoluminescence (PL) spectra, as well as lifetimes were utilized to characterize samples. Under the excitation of UV light, Sr(3)AlO(4)F:Tm(3+), Sr(3)AlO(4)F:Tb(3+), and Sr(3)AlO(4)F:Eu(3+) exhibit the characteristic emissions of Tm(3+) ((1)D(2)→(3)F(4), blue), Tb(3+) ((5)D(4)→(7)F(5), green), and Eu(3+) ((5)D(0)→(7)F(2), red), respectively. By adjusting the doping concentration of Eu(3+) ions in Sr(3)AlO(4)F:0.10Tm(3+), 0.10Tb(3+), zEu(3+), a white emission in a single composition was obtained under the excitation of 360 nm, in which an energy transfer from Tb(3+) to Eu(3+) was observed. For Sr(3)AlO(4)F:Ce(3+),Tb(3+) samples, the energy transfer from Ce(3+) to Tb(3+) is efficient and demonstrated to be a resonant type via a dipole-quadrupole interaction by comparing the experimental data and theoretical calculation. Furthermore, the critical distance of the Ce(3+) and Tb(3+) ions has also been calculated to be 9.05 ?. The corresponding luminescence and energy transfer mechanisms have been proposed in detail. These phosphors might be promising for use in near-UV LEDs.     

Dy(Fe,M)_(12)(M=Si,Ti,V)合金的Mssbauer谱学研究

DyFe_(10)Si_2合金的~(57)Fe变温Mssbauer谱学研究,发现温度在210K处,超精细场随温度有不连续变化。此温度对应于合金的自旋重取向温度。Si,Ti和V作为形成ThMn_(12)型结构的稳定化元素,造成各相应合金的自旋重取向温度有一些差别。分析了Si,Ti和V元素造成合金磁晶各向异性差别的原因,它使得DyFe_(10)V_2,DyFe_(11)Ti和DyFe_(10)Si_2合金的自旋重取向温度不同。DyFe_(10)Si_2合金的8i晶位四极劈裂在温度210K处改变符号。     

Fluctuation Conductivity of La0.5RE0.5BaCaCu3O7−d (RE=Y, Sm, Dy) Superconductor

We report production, conductivity fluctuation and magnetoconductivity fluctuation measurements of the system La_0.5RE_0.5BaCaCu₃O_7?δ RE=(Y,Sm,Dy) high-temperature superconductors. Samples were synthesized by the standard solid state reaction recipe. By Rietveld-like refinement of X-ray diffraction patterns we determined that the material crystallizes in a tetragonal structure. DC resistivity measurements show a bulk critical temperature T _c (71, 72 and 72 K for RE=Y, Sm, Dy, respectively) for the analyzed samples. In order to examine the effect of substitution of rare earth into the lanthanum structural sites on the pairing mechanism close to T _c , we performed conductivity fluctuation analysis by using the method of logarithmic temperature derivative of the conductivity excess. We found the occurrence of Gaussian and genuinely critical fluctuations. Our results are in agreement with reports on non-doped systems, but we observe minor values of the coherence length of the fluctuation regimes for the doped case. Correlations of critical exponents with the dimensionality of the fluctuation system for Gaussian regimes were made by using the Aslamazov–Larkin theory. The genuinely critical exponent was interpreted by the 3D-XY.     

Production,Properties and Microstructures of Mg—RE—Zn—Zr（RE=MM,Nd）Alloy

The Mg-MM and Mg-Nd master alloys were prepared through the ingot metallurgy method under the protection of a special flux.The thermodynamic behaviors of different rare earth elements in the molten Mg alloys were investigated.Two experimental alloys,Mg-3.0 wt pct MM-0.7wt pct Zn-0.7wt pct Zr and Mg-2.8wt pct Nd-0.7wt pct Zn-0.7wt pct Zr,were prepared.The hardness and tensile properties of experimental alloys were measured and the microstructures were observed.The results showed that the rare earth elements could react greatly with the Mg chloride in the flux.The Mg-Nd-Zn-Zr alloy displayed a good aging hardening effect.The dispersed metastable phase βˊ(Mg3Nd) is an important strengthening phase to improve the tensile properties of this alloy.So the M-g-Nd-Zn-Zr alloy has better tensile properties and elongation than those of the Mg-MM-Zn-Zr alloy,and the ductile fracture character could be observed.The microvoid coalescence is the dominant fracture mechanism in this alloy.     

Sol–gel synthesis and photoluminescence of M2Gd8(SiO4)6O2: RE3+ (M = Ca, Sr; RE = Tb, Eu) phosphors by different silicate sources

Eight kinds of silicate sources were adopted to prepare M₂Gd₈(SiO₄)₅O₂: RE³⁺ (M = Ca, Sr; RE = Eu, Tb) phosphors by the sol–gel method. Scanning electronic microscope (SEM) was used to compare the different configuration of patterns rely on eight different silicon sources. X-ray powder diffraction (XRD) also has been employed to analyze their microstructures and particle sizes further. All the phosphor show the characteristic emission ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) of Tb ions and ⁵D₀ → ⁷F_J (J = 1, 2) of Eu ions, indicating that the emission intensity is affected by the silicate sources.     

Phase Relations in the Systems M–Mn–P (M = Cr, Mo, W, Re)

The 1070-K phase relations in the M–Mn–P (M = Cr, Mo, W; 67 at. % P) and Re–Mn–P (50 at. % P) systems were studied by x-ray diffraction. The existence of the known phosphides Mo₄Mn₆P₅ and (Mn,W)₁₂P₇ was confirmed, and a new phase with the general formula (Mn,Mo)₁₂P₇ and stoichiometry range Mo_4.6–1.8Mn_7.4–10.2P₇ was identified. Its structure was determined by powder diffraction profile analysis (R = 0.074): Cr₁₂P₇ structure type, sp. gr. P6, a = 0.94929(8) nm and c = 0.33565(3) nm for Mn_7.4Mo_4.6P₇. The Mo and Mn atoms in the structure of this phase were shown to occupy all positions at random. In addition to the known solid solutions (Cr,Mn)₃P (Ni₃P structure type), (Cr,Mn)₁₂P₇ (Cr₁₂P₇), (Cr,Mn)P, and (Mn,Mo)P (MnP), we revealed new solid-solution series: (Cr,Mn)₂P (Cr₂P), (Mn,Cr)₂P (Fe₂P), (Mn,Mo)₃P (Ti₃P), (Mo,Mn)₃P (-V₃S), (Mo,Mn)P (WC), (W,Mn)P (MnP), (Mo,Mn)P₂, and (W,Mn)P₂ (MoP₂). The Re–Mn–P system was found to contain neither ternary phosphides nor solid solutions.     

Solvothermal synthesis of well-dispersed MF(2) (M = Ca,Sr,Ba) nanocrystals and their optical properties

MF(2)?(M = Ca,Sr,Ba) nanocrystals (NCs) were synthesized via a solvothermal process in the presence of oleic acid and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, UV/vis absorption spectra, photoluminescence (PL) excitation and emission spectra, and lifetimes, respectively. In the synthetic process, oleic acid as a surfactant played a crucial role in confining the growth and solubility of the MF(2) NCs. The as-prepared CaF(2), SrF(2) and BaF(2) NCs present morphologies of truncated octahedron, cube and sheet in a narrow distribution, respectively. Possible growth mechanisms were proposed to explain these results. The as-prepared NCs are highly crystalline and can be well dispersed in cyclohexane to form stable and clear colloidal solutions, which demonstrate strong emission bands centred at 400?nm in photoluminescence (PL) spectra compared with the cyclohexane solvent. The PL properties of the colloidal solutions of the as-prepared NCs can be ascribed to the trap states of surface defects.     

Electrical resistivity and Seebeck coefficient of La1−x M x MnO3 (M = Ca,Sr) single crystals

The temperature dependence of the electrical resistivity and Seebeck coefficient was measured on single crystals of La_1–x Ca _x MnO₃(0 <x 0.3) and La_1–x Sr _x MnO 3 (0 <x 0.4) grown by the arc-image floating zone method. The electrical conduction for La_1–x crystals withx 0.2 was of the activation type aboveT _c and of the degenerate type belowT _c, while that for the crystal withx = 0.1 was of the activation type over the whole measured temperature range between –170 and 400°C. The conduction behaviour of La_{1– x} Sr _x MnO₃ was essentially the same as that of La_1–x Ca _x MnO₃ except that the conduction of the crystals withx = 0.3 and 0.4 was of the degenerate type aboveT _c. A distinct difference in Seebeck data was observed between the calcium and the strontium compounds.     

Phase Formation in the Systems α(δ)-FeOOH–M(OH)2–H2O (M = Mn, Co, Zn)

The sequence of chemical transformations induced in the systems ()-FeOOH–M(OH)₂–H₂O (M = Mn, Co, Zn) by heat treatment in the range 125–200°C is studied. The results demonstrate that the kinetics of M(II) ferrite formation, the chemical and phase compositions of the product, and its particle size distribution depend on the heat-treatment temperature, the pH of the suspension (or NaOH concentration), the FeOOH polymorph used as a starting reagent, and the starting-mixture composition.     

(Y3-x-yREx)Al5O12:Cey(RE=Tb,Gd)荧光粉晶体结构与光致发光

采用化学共沉淀法合成具有Tb、Gd稀土掺杂的Y3Al5O12:Ce3 荧光粉,通过XRD和荧光光谱研究其晶体结构和光致发光.结果表明,(Y2.95-xTbx)Al5O12:Ce3 和(Y2.95-xGdx)Al5O12:Ce3 系列荧光粉具有与Y3Al5O12:Ce3 相同的石榴石晶体结构,但晶胞参数随Tb、Gd取代Y而略有增加;随Tb、Gd掺杂量增加,荧光粉发射波长逐渐红移,当x=2.95时,发射波长λmax分别从532nm红移至551和542nm;根据晶体场理论解释了荧光粉发射波长红移现象.这种发射波长红移的荧光粉能够显著地增强蓝光LED芯片与荧光粉组合形成的白光中红光成分,进而改善白光LED光源质量.     

Giant Magnetoresistance Effect of [bcc-Fe(M)/Cu](M=Co,Ni) Multilayers

1. IlltroductionGiant magnetoresistance (GMR) effect of metallic multilayers has been widely investigated after thefinding by Baibich et al.11], as a new phenomenon tobreak through the memory density in ultra high density magnetic recording, high sensitivity in magnetichead, and so on. Metallic multilsyers of 3d transition elements could be classified into three groups of[bee/bcc], [fee/fccl and [bee/fcc] from the standpointof combination of crystal structure of constituting elements of metal…     

Dielectric properties and crystal structure of Ba6−3x(Nd, M)8+2xTi18O54 (M = La, Bi, Y) microwave ceramics

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa₂O₃4TiO₂ and Ba_6-3x (Nd, M)d_8+2x Ti₁₈O₅₄ (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd_1-xBi_x)₂O₃4TiO₂ were _r = 89–92, Qf = 5855–6091 GHz, and _f = –7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd_1-xY_x)₂O₃4TiO₂ reduces the dielectric constant _r. Both the Y and Bi substitutions make _r positive. The relative dielectric constant _r and temperature coefficient _r are 109.5 and –180 ppm/°C in BaOLa₂O₃4TiO₂, 76 and +40 ppm/°C in BaO(Nd_0.77Y_0.23)₂O₃4TiO₂, respectively. The crystal structures were refined by Rietveld method using x-ray and neutron diffraction data. The most reliable results were obtained by refining the cation positions using the x-ray data and the oxygens from the neutron with a superlattice structure model Pnam(c-axis 7.6 Å). The refined structures show that the a/c ratios are related to the apical oxygen displacements of the Ti–O octahedra. The substitution of the small radius atom, Y, produced a structure of severely tilted and distorted Ti–O octahedra and large a/c ratio, while the large radius atom, La, small a/c ratio. Differential scanning calorimetry analysis showed heat anomaly indicating suspected phase transition in these materials. The relation between _r and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.     

Synthesis and Properties of (Sr1 – xMx)2Nb2O7 (M = Cu, Ni) Solid Solutions

(Sr_{1 – x} M _x )₂Nb₂O₇ (M = Cu, Ni) layered perovskite solid solutions were synthesized, and their stoichiometry ranges were determined. The electrical properties of the solid solutions were compared with those of Sr₂Nb₂O₇.     

Co-doping effect on electrical conductivity in the fluorite-type solid-solution systems Zr0.7(Sc1-x M x )0.3O2-δ (M=Ca, Mg, Al, Gd, Yb)

Electrical conductivity of the fluorite-type solid-solution system Zr_0.7(Sc_1-x M _x )_0.3O_2- (M=Ca, Mg, Al, Gd, Yb) was systematically investigated from the viewpoint of a stabilization effect of the cubic phase. The rhombohedral phase of Zr_0.7Sc_0.3O_1.85 transformed to the cubic phase by the partial substitution of M for Sc. At the same time, the electrical conductivity abruptly increased. The electrical conductivity of the cubic phase decreased linearly and gradually with increase in oxygen vacancy content, but nevertheless the kind of dopants, and the extrapolated value of the conductivity to =0.25 agreed with that of a pyrochlore-type La₂Zr₂O₇. This fact was understood by considering that the partial substitution of M ions in place of Sc produced a micro-cluster having a pyrochlore-type structure with cubic symmetry.