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1.
Perovskite oxides with the composition La 1−xNd xFe 0.5Cr 0.5O 3 ( x = 0.10, 0.15 and 0.20) have been studied. The samples have been prepared using the standard solid-state reaction method in air. The X-ray and neutron diffractograms indicates that the samples crystallize in the orthorhombic perovskite structure with space group Pnma (No. 62). The Nd-substitution causes minor changes in the cell parameters and bond lengths but the nuclear structure remains orthorhombic in the whole range of substitutions and in the temperature range of 10–700 K. The magnetic structure of the samples at room temperature and below is G-type antiferromagnetic with an average magnetic moment of the Fe/Cr ions of 3.29(3) μ B/atom at 10 K independent of the Nd content. At room temperature the average magnetic moment of the Fe/Cr ions reduces to 1.23 μ B/atom. At temperatures below 250 K a weak but increasing uncompensated spontaneous magnetic moment develops that reaches a magnitude of about 0.5 emu/g (or 0.02 μ B per Fe/Cr site) at 10 K. This moment saturates rather rapidly in the magnetization versus magnetic field curve and is at higher field superposed on the response of the antiferromagnetic spin system. 相似文献
2.
The catalytic effect of the addition of lanthanum oxide (La 2O 3), in the range 0.5–2.0 mol%, on the hydrogen storage properties of MgH 2 prepared by ball milling has been studied. The addition of La 2O 3 reduces the formation during milling of the metastable orthorhombic γ-MgH 2 phase. The desorption rate of samples with 1 and 2 mol% La 2O 3 comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La 2O 3. The presence of LaH 3 after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La 2O 3 gives a lower hysteresis factor compared with sample with 2 mol%. 相似文献
3.
Single crystals of UNi 0. 5Sb 2 were investigated by means of Seebeck coefficient and Hall effect measurements in the temperature range 5–300 K. The results corroborated the occurrence of two magnetic phase transitions: from para- to antiferromagnetic state at TN = 161.5 K and a spin-reorientation near Tt = 64 K. The first-order character of the latter feature was proved by studying in detail the electrical resistivity and the magnetic susceptibility of single-crystalline UNi 0. 5Sb 2 in the vicinity of Tt. 相似文献
4.
Mono-disperse Ni 0.5Zn 0.5Fe 2O 4 spinel ferrite particles have been synthesized directly via the hydrothermal method using sodium dodecyl sulfate (SDS) as surfactant. Particle size could be varied from 6 to 19 nm by changing the experiment parameters. X-ray diffraction, high resolution TEM images confirmed the high crystallinity of ferrite nanocrystals. The effects of precursor suspension pH value, reaction temperature and surfactant (SDS) concentration on phase purity, particle size and dispersed property were discussed. The results indicated that mono-disperse Ni 0.5Zn 0.5Fe 2O 4 spinel ferrite nanoparticle had been obtained at pH value range (8–9), reaction temperature (90 °C) and moderate SDS concentration (>0.2 mM). The magnetic measurement shows that as prepared Ni 0.5Zn 0.5Fe 2O 4 nanoparticle possesses good super-paramagnetic behavior. We also put forward a primary experimental model to shed light on the controllability of the monodispersity of the nanosized particles. 相似文献
5.
SrBi 2(Ta 0.5Nb 0.5) 2O 9 (SBTN) thin films were obtained by polymeric precursor method on Pt/Ti/SiO 2/Si(1 0 0) substrates. The film is dense and crack-free after annealing at 700 °C for 2 h in static air. Crystallinity and morphological characteristic were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The films displayed rounded grains with a superficial roughness of 3.5 nm. The dielectric permittivity was 122 with loss tangent of 0.040. The remanent polarization ( Pr) and coercive field ( Ec) were 5.1 μC/cm 2 and 96 kV/cm, respectively. 相似文献
6.
Cathode material Sm 0.5Sr 0.5CoO 3 (SSC) with perovskite structure for intermediate temperature solid oxide fuel cell was synthesized using glycine-nitrate process (GNP). The phase evolution and the properties of Sm 0.5Sr 0.5CoO 3 were investigated. The single cell performance was also tested using La 0.9Sr 0.1Ga 0.8Mg 0.2O 3−δ (LSGM) as electrolyte and SSC as cathode. The results show that the formation of perovskite phase from synthesized precursor obtained by GNP begins at a calcining temperature of 600 °C. The single perovskite phase is formed completely after sintering at a temperature of 1000 °C. The phase formation temperature for SSC with complete single perovskite phase is from 1000 to 1100 °C. The SrSm 2O 4 phase appeared in the sample sintered at 1200 °C. It is also found that the sample sintered at 1200 °C has a higher conductivity. The electrical conductivity of sample is higher than 1000 S/cm at all temperature examined from 250 to 850 °C, and the highest conductivity reaches 2514 S/cm at 250 °C. The thermal expansion coefficient of sample SSC is 22.8 × 10 −6 K −1 from 30 to 1000 °C in air. The maximum output power density of LSGM electrolyte single cell attains 222 and 293 mW/cm 2 at 800 and 850 °C, respectively. 相似文献
7.
Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb 2FeV 3O 11 iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie-Weiss temperature, TCW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at TN = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb 2FeV 3O 11 at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at geff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4-300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb 2FeV 3O 11. 相似文献
8.
采用Al-Si钎料对经过Ag-Cu-Ti粉末活性金属化处理的Al 2O 3陶瓷与5005铝合金进行了真空钎焊,研究了钎焊接头的典型界面组织,分析了钎焊温度对接头界面结构特征及力学性能的影响. 结果表明,接头典型界面结构为5005铝合金/α-Al+θ-Al 2Cu+ξ-Ag 2Al/ξ-Ag 2Al+θ-Al 2Cu+Al 3Ti/Ti 3Cu 3O/Al 2O 3陶瓷. 钎焊过程中,Al-Si钎料与活性元素Ti及铝合金母材发生冶金反应,实现对两侧母材的连接. 随着钎焊温度的升高,陶瓷侧Ti 3Cu 3O活化反应层的厚度逐渐变薄,溶解进钎缝中的Ag和Cu与Al反应加剧,生成ξ-Ag 2Al+θ-Al 2Cu金属间化合物的数量增多,铝合金的晶间渗入明显;随钎焊温度的升高,接头抗剪强度先增加后降低,当钎焊温度为610 ℃时,接头强度最高达到15 MPa. 相似文献
9.
Magnetic refrigeration, an emerging new technology for cooling and gas liquefaction, needs magnetic materials with specific thermomagnetic behavior. Depending on the thermodynamic cycle selected, the isothermal magnetic entropy change or the adiabatic temperature change upon field application needs to be a preselected function of temperature. In double perovskite Ba 2CrMo 1−xW xO 6 ( x = 0, 0.2 and 0.5) prepared by the sol–gel method, the experimental results show the observation of a large magnetocaloric effect (MCE) near the Curie temperature TC which decreases with the increasing of the substitution of Mo by W. The maximum of the magnetic entropy change peaks at the magnetic ordering temperature TC, and a large magnetic entropy change (|Δ SM| ≈ 1.6 J kg −1 K −1) is obtained at 285 K in the sample Ba 2CrMo 0.5W 0.5O 6 under an applied magnetic field of 10 kOe. 相似文献
10.
Single crystals of UNi 1−xSb 2 have been grown from an Sb-rich melt and studied by means of X-ray diffraction, magnetic and electrical transport measurements. Crystal structure refinements indicated significant deficiency on the transition metal sites in the tetragonal HfCuSi 2-type unit cell, yielding the actual composition UNi 0.5Sb 2. The single crystals studied order antiferromagnetically below TN=161 K and exhibit another phase transition at Tt=60 K, presumably caused by a spin-reorientation. No crystal structure distortion could be detected at 10 K. Above TN the electrical resistivity is dominated by a Kondo effect, whereas at lower temperatures it shows a behavior characteristic of antiferromagnets. The overall magnetic and electrical transport properties of UNi 0.5Sb 2 are highly anisotropic both in the ordered and paramagnetic states. 相似文献
11.
We present a study of the 503, 880, 890, 901, 920, and 1475 cm −1 Raman active κ-ET 2Cu[N(CN) 2]Br single crystal phonons, around T=40 K, where an anomaly in the longitudinal sound velocity and antiferromagnetic spin fluctuations have been detected. Doublets, narrowing of phonon lines and non-monotonic intensity variations, as a function of temperature, are observed indicating a possible interaction between phonon and correlated electrons. 相似文献
12.
The results of various physical properties namely, dielectric properties (dielectric constant, loss tan δ, ac conductivity σ, over a wide range of frequency and temperature and dielectric breakdown strength in air medium at room temperature), optical absorption, electron spin resonance (ESR) at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li 2O–CaF 2–P 2O 5:TiO 2 glass-ceramics have been reported. The optical absorption and magnetic susceptibility studies indicated that the titanium ions exist in Ti 3+ state in addition to Ti 4+ state in these samples. However, the reduction seems to be the lowest in the sample containing 0.6 mol% of TiO 2. The dielectric constant and loss variation with the concentration of TiO 2 have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analyzed by a pseudo Cole–Cole plot method and the spreading of dielectric relaxation has been observed. The ac conductivity in the high temperature region seems to be related both with electronic and ionic movements. The low temperature (or the nearly temperature independent) part of conductivity could be explained on the basis of quantum mechanical tunneling model. The studies on dielectric breakdown strength indicated the highest insulating strength for the sample containing 0.6 mol% of TiO 2. 相似文献
13.
Crystals of Ba 3NaRu 2O 9−δ ( δ≈0.5) and Ba 3(Na, R)Ru 2O 9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6 3mmc. Ba 3NaRu 2O 9−δ and Ba 3(Na, R)Ru 2O 9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba 3NaRu 2O 9−δ has an effective magnetic moment Peff of 0.91 μ B, while Peff of Ba 3(Na, R)Ru 2O 9−δ (except Ce) reflects the large free-ion moment
of the rare earth ions. Ba 3(Na, Ce)Ru 2O 9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba 3(Na, R)Ru 2O 9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV. 相似文献
14.
The PrBa 2−xSr xCu 3O 7±δ solid solution was investigated by means of X-ray powder diffraction in combination with Rietveld analysis. The Sr-doped Pr123 single phase could be synthesized at 950 °C in air. The solubility of PrBa 2−xSr xCu 3O 7±δ solid solution is 0.2≤ x≤0.6. The structure of PrBa 2−xSr xCu 3O 7±δ is orthorhombic for x=0.2. The structure transforms into tetragonal for 0.3≤ x≤0.6. In the PrBa 2−xSr xCu 3O 7±δ structure, Sr ions can replace Ba ions, the highest value is x=0.6 under our experimental condition. But Sr ions could not replace Pr ions. Furthermore Pr ions could not occupy the sites of Ba ions in the PrBa 2−xSr xCu 3O 7±δ system. Both ionic radii and chemical properties play an important role in the mutual substitution of Pr, Ba and Sr ions in the Pr123 structure of the PrBa 2−xSr xCu 3O 7±δ system. 相似文献
15.
Multiphase Al 65Cu 20Ti 15 intermetallic alloy matrix composite, dispersed with 10 wt.% of TiO 2 nanoparticles, has been processed by mechanical alloying, followed by spark plasma sintering under pressure in the temperature range of 623–873 K. Differential scanning calorimetry and X-ray diffraction suggest that equilibrium crystalline phases evolve from the amorphous or intermediate crystalline phases. Transmission electron microscopy shows that the composite sintered at 873 K has partially amorphous microstructure, with dispersion of equilibrium, crystalline, intermetallic precipitates of Al 5CuTi 2, Al 3Ti, and Al 2Cu of 25–50 nm size, besides the TiO 2. The composite sintered at 873 K exhibits little porosity, hardness of 5.6 GPa, indentation fracture toughness in the range of 3.1–4.2 MPa√m, and compressive strength of 1.1 GPa. Indentation crack deflection by TiO 2 particle aggregates causes increase in fracture resistance with crack length, and suggests R-curve type behaviour. The study provides guidelines for processing high strength amorphous–nanocrystalline intermetallic composites based on the Al–Cu–Ti ternary system. 相似文献
16.
Chemically vapor deposited Al 2O 3 coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al 2O 3 coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al 2O 3. To overcome this limitation, alternate systems to deposit Al 2O 3 at moderate temperatures have been investigated. Systems using NO–H 2, H 2O 2, NO 2–H 2 and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al 2O 3 in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates. 相似文献
17.
We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with oxides Cr 2O 3, Al 2O 3 and CeO 2. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH 2 during reactive mechanical grinding. The Mg+10wt.%Cr 2O 3 powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2O 3. The Mg+10wt.%Cr 2O 3 powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al 2O 3 and Mg+10wt.%CeO 2. Mg+10wt.%Cr 2O 3 absorbs 5.87wt.% H at 573 K, 11 bar H 2 during 60 min at the first cycle. The Mg+10wt.%Cr 2O 3 powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2O 3. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H 2 during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr 2O 3, Mg+10wt.%Al 2O 3 and Mg+10wt.%CeO 2. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr 2O 3, Mg+10wt.%Al 2O 3 and Mg+10wt.%CeO 2. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr 2O 3 in the Mg+10wt.%Cr 2O 3 powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr 2O 3 after cycling. 相似文献
18.
The superconducting YBa 2Cu 3O 7−x samples were prepared by an Arc-Cast-Annealing (ACA) and Arc-Quench-Powder-Growth (AQPG) processes as modifications of QMG and MPMG techniques. Pe'lets of YBa 7Cu 3O 7−x were quenched by arc-casting in a water cooled copper mould and then the solidified rods were annealed at different temperatures and times to store the superconductivity. Annealed at an appropriate temperature the cast rods showed rising superconducting properties with increasing the annealing time. Some of the rods after solidification were crushed to give powder which was compacted and then subjected to a melt growth process. As a result of this processing, large grained textured YBCO superconductors with dispersed 211 inclusions in the superconducting grains were produced. The microstructure and physical properties of these ACA and AQPG samples were investigated when subject to various temperature cycles. It was found that the volume fraction and size distribution of the second phase inclusions were dependent upon the maximum temperature during the melt growth process. The critical current density ( Jc) for ACA and AQPG samples was estimated from magnetization loops using Bean's critical state model. It was found that the value of Jc of AQPG sample was much higher than that of ACA sample. 相似文献
19.
In the present work, the spectroscopic and magnetic properties of heteronuclear Cu:Pr squarate are reported. Single crystals of [Pr 2Cu(C 4O 4) 4(H 2O) 16]·2H 2O were obtained by reaction of squaric acid, praseodymium chloride and copper chloride in water solution according to the procedure described earlier. The crystals of title compound are isomorphic with [La 2Cu(C 4O 4) 4(H 2O) 16]·2H 2O crystal, where squarate anions participate as bridging ligands between metal ions. The UV region of absorption spectra of the title compound is dominated by C–T band of Cu(II), f–d transition of Pr(III) and internal π–π*(A1g→Eu) and π–π*(A1g→Eg) ligand transitions. In visible and IR regions, t2g–eg of copper Cu(II) as well as 3H4→3PJ, 1D2, 1G4, 3FJ, 3H6 Pr(III) transitions at 293 and 4 K were recorded. At low temperature splitting given by Jahn–Teller effect can be observed. Significant anisotropy of d–d transitions intensities confirms well the Jahn–Teller effect, too. Unexpectedly high intensity of 3H4→1G4 transition is probably due to the intensity borrowing from the Cu (II) d–d transition. The 3P0 and 1D2 emission of Pr(III) in the [Pr2Cu(C4O4)4(H2O)16]·2H2O crystals is quenched even at 77 K. Whereas emission of appropriate polynuclear europium squarate was detected. The pathways of excited state quenching by eg levels of Cu(II), multhiphonon relaxation and concentration quenching can be considered in the system under studies. Magnetic susceptibility measurements were carried out in 300–1.7 K temperature range and are discussed in relation to the structure. Effect of the polymeric structure on spectroscopic behaviour is presented. Selectivity of polymeric europium squarate in vitro test for different tumor cells is shown. 相似文献
20.
The magnetic properties of Pb 2Sr 2PrCu 3O 8 were determined using X-ray absorption, inelastic neutron spectroscopy and magnetic susceptibility measurements. X-ray absorption on the Pr L 3-edge strongly indicates a trivalent oxidation state. Inelastic neutron scattering results are modeled assuming a 3H 4 ground-state multiplet split by a crystalline electric field potential similar to PrBa 2Cu 3O x. This potential correctly predicts the value of the Curie–Weiss moment, 2.72 μB, obtained from magnetic susceptibility measurements, and so explains the reduction from the free-ion value. The very broad magnetic response found in the inelastic neutron scattering experiments indicates a strong interaction between the Pr 4f electrons and the CuO 2 bands. 相似文献
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