Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

LaNi5-NdNi5,LaNi5-CeNi5赝二元系相图和吸氢性能的研究

本文用X射线衍射和差热分析的方法测定了LaNi₅-NdNi₅,LaNi₅-CeNi₅两个赝二元系相图。用X射线衍射线宽化法测定了活化后LaNi₅-NdNi₅体系的样品晶粒尺寸的各向异性。考查了吸氢性能和第二相存在对LaNi₅吸氢性能的影响。测定和计算了两个体系中样品的点阵常数、单胞体积、原了间距吸氢前后的变化。从结构的观点,解释了LaNi₅-NdNi₅体系中活化后样品晶粒尺寸各向异性的原因。用本实验的结果,解释了RmNi₅(Rm代表混合稀土金属)的吸氢量和平台压力的差异。 关键词：     

Magnetic properties of Zr(Cr1−xCox)2 alloys and their hydrides

Magnetic properties of Zr(Cr_1?xCo_x)₂ alloys, with 0 ? x ? 0.75, and their hydrides were determined over the temperature range 4 to 275 K. All systems were paramagnetic. The susceptibility (?) is significantly enhanced both by increase of the Co content and by hydrogenation in all cases except cubic ZrCr₂, for which ? is virtually unchanged by hydrogenation. The rise in ? is ascribed to electron transfer from Co to Cr and to H. This transfer increases the local density of states for Co at the Fermi energy. The temperature dependence of the susceptibility can be represented by an equation of the form ? = ?₀ + C/(T ? θ), indicating Pauli and Curie-Weiss contributions to ?. The contributions are attributed to the ZrCr₂ matrix and the solute Co atoms, respectively. The Co atoms on Zr sites are weakly coupled antiferromagnetically. Pressure-composition isotherms for the ZrCrCo-H₂ system are given at 80, 120 and 165°C. At 80°C this alloy hydrogenates to the composition ZrCoCrH_3.25 at a pressure of 50 atm.     

Measurements of the electrical resistance and the hydrogen depth distribution for Ni60Nb20Zr20 amorphous alloy before and after hydrogen charging

A Ni₆₀Nb₂₀Zr₂₀ amorphous alloy was prepared by the single-roller melt-spinning technique. The change in the electrical resistance of the alloy after electrochemical hydrogen charging in 6 N KOH solution was investigated. The change in the hydrogen depth distribution in the alloy was also investigated by elastic recoil detection. As a result, we found that the electrical resistance of the alloy increases with increasing the hydrogen content in the alloy and that a large number of hydrogen atoms are remained in the surface area of the hydrogen-charged alloy.     

氢对非晶态Y5Ni95合金膜的磁性和电子输运性质的影响

采用电解法实现了非晶态Y,Ni₉₅合金的加氢。在1.5—400K温度范围内测量了a-Y,Ni₉₅H_x(x=0—15.1)合金的磁矩、电阻率和霍耳电阻率随氢含量的变化关系。结果指出,随氢含量增加,样品的0K磁矩、居里温度和电阻率温度系数显著下降,而高场磁化率、电阻率和反常霍耳系数则迅速增加。借助现行的理论对加氢引起的上述影响进行了简要的讨论。 关键词：     

A comparative study of surface oxidation behaviour of amorphous and crystallized Ni36Fe32Cr14P12B6 metallic glass using Auger Electron Spectroscopy

Native oxide and in situ oxidation of amorphous Ni₃₆Fe₃₂Cr₁₄P₁₂B₆ have been investigated at room temperature using Auger Electron Spectroscopy. The Ni₃₆Fe₃₂Cr₁₄P₁₂B₆ sample has been annealed as ～ 800 K in UHV and its oxidation behaviour has been studied at room temperature. A comparison of in situ oxidized amorphous and crystallized samples has been carried out.     

On the bimodality of the hyperfine field distribution in amorphous Fe32Ni36Cr14P12B6 alloy

The bimodality of the hyperfine field distribution in amorphous Fe₃₂Ni₃₆Cr₁₄P₁₂B₆ alloy reported by Chien et al. (Phys. Rev. B19 (1979) 81) is shown to be the result of the bimodality in the distribution of the values of the strengths of the bonds within the alloy. The bimodality in the distribution of the bond strength values arises from the presence of two types of dense random packed cells (one rich in Cr atoms and the other deficient in Cr) in the alloy.     

Structural and H2 sorption properties of MgH2�C10?wt%ZrCrM (M?=?Cu, Ni) nano-composites

Magnesium and its hydride MgH₂ are widely regarded as promising candidates for hydrogen storage materials due to its benefits of high gravimetric and volumetric capacity, excellent reversibility, abundance in the earth and a low cost. Much attention has been paid to improve its absorption/desorption kinetics, trying to make it useful for practical applications. To make composite of MgH₂ with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties. In this study nano-composite of MgH₂ with ZrCrCu alloy was prepared using high energy ball-milling for 5 h under Ar atmosphere. Microstructure and morphology of the composites were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM). XRD patterns show that no alloy formation between MgH₂ and elements of the alloys takes place during milling. Different morphology of the powders as-milled and after cycling was observed by SEM. Pressure?Ccomposition isotherms of these composites have been obtained in the pressure range 0.1?C10 bar at 275 and 300 °C. The absorption/desorption kinetics data have been analyzed using pressure 0.1?C5.0 bar at 275 and 300 °C to understand the mechanism of the hydriding/dehydriding reaction processes. A comparison of these results has been attempted with our previous published results of MgH₂?C10 wt%ZrCrNi in order to find the better composite for storage applications. It is observed by DSC curves that the onset temperature of hydrogen desorption is decreased for MgH₂?C10%ZrCrNi in comparison to MgH₂ which further decreased for MgH₂?C10%ZrCrCu. However, little loss in hydrogen absorption/desorption capacity is also observed for ZrCrCu composite in comparison to that of ZrCrNi composite.     

Fast hydrogen sorption kinetics for ball-milled Mg2Ni alloys☆

The sorption kinetics of Mg₂Ni alloys are strongly improved using two different surface treatments. One consists of the preparation of tailor-made Mg₂Ni/C composites by ball-milling with previously ground carbons. The strong reducing character of carbon, allowing for the reduction of NiO initially present at the alloy surface, leads to the partial removal of the oxide layer that strongly hinders the hydrogen migration throughout the alloy surface on desorption, whereas resulting Ni particles act as catalysts during the absorption process. The second surface treatment deals with the deposition of Pd particles on the alloy surface using the polyol process. The catalytic effect of Pd is responsible for an important enhancement of the absorption kinetic, whereas Pd particles probably act as hydrogen pumps, during the desorption process, leading to a faster hydrogen release. By combining both techniques, desorption rates as high as 2.7 wt% in 60 min and 2.9 wt% in 30 min are obtained at 150 and 200 °C, respectively.     

Studies of two-dimensional condensation of water on hydroxylated ZnO,SnO2, and Cr2O3: Determination of two-dimensional critical temperature

The water physisorption isotherms on fully hydroxylated ZnO, SnO₂ and Cr₂O₃ were measured in a wide temperature range (248–298 K for ZnO, 268–333 K for SnO₂ and Cr₂O₃). The two-dimensional critical temperature of water was found to be 305 K on Cr₂O₃ and 236 K on ZnO, but could not be determined on SnO₂ because of poor uniformity of the surface. The entropy of the two-dimensionally condensed water on Cr₂O₃ was found to be 9.3 cal K^?1 mol^?1, which suggests that the condensed phase of water on Cr₂O₃ is ice-like.     

Superconductivity of hydrides Ti3SbHx

The effect of interstitially dissolved hydrogen on the transition temperature T_c of superconductivity has been investigated in cubic A-15 type (Cr₃Si type) hydrides Ti₃SbH_x, 0 ? x ? 2. The transition temperature T_c decreases with increasing hydrogen content x from 5.6 K in Ti₃Sb to 1.0 K in Ti₃SbH_1.0.     

Direct measurement of magnetization isotherms of Fe64Ni36 Invar alloy in a diamond anvil cell

Magnetization isotherms of the Fe₆₄Ni₃₆ Invar alloy have been measured under pressure up to 5.3?GPa in magnetic field up to 5?T using a diamond anvil cell and SQUID magnetometer. The unambiguous change of the pressure parameter dlnM_S/dP (from ?9 to ?13×10^?2?GPa^?1) has been observed in a narrow pressure interval from 2.5 to 3.5?GPa at all temperatures in the range from 5 to 300?K. The pressure interval, where the sharp decrease in magnetization was observed, coincides with the critical pressures of the pressure-induced decrease in Fe-moment that were determined by the X-ray Magnetic Circular Dichroism and the X-ray Emission Spectra studies, recently. The pronounced decrease in the Curie temperature of the Fe₆₄Ni₃₆ alloy under pressure, dT_C/dP = ?44 ±2?K/GPa, has been confirmed.     

Ni83Cr7Fe3Si4B3非晶态合金在常压及高压下的晶化过程

本文研究了Ni₈₃Cr₇Fe₃Si₄B₃非晶态合金在1bar及100kbar压力下的晶化过程,得到了“时间-温度-变态”图。其结果表明:经高压热处理后的非晶合金其晶化温度降低,fcc-Ni相区域加宽及亚稳Ⅱ相没有形成。另外,还分别计算了常压及100kbar压力下的晶化激活能。 关键词：     

Magnetoresistance and magnetostriction of Co2Cr0.6Fe0.4Al Heusler alloy

We report a study of the magnetotransport properties of the Co₂Cr_0.6Fe_0.4Al Heusler alloy grown by means of the arc-melting technique. X-ray diffraction and energy dispersive X-ray spectroscopy have been used to analyze the crystallographic structure and the sample stoichiometry. Temperature-dependent magnetization measurements have been carried out in the range 300-850 K. Co₂Cr_0.6Fe_0.4Al is theoretically predicted to have full positive spin polarization at the Fermi level, and as a consequence its spin-dependent transport properties are being intensively studied. Low field magnetoresistance exceeding 30% has been observed very recently in Co₂Cr_0.6Fe_0.4Al compact pellets. We have performed magnetoresistance and magnetostriction measurements in both the as-grown alloy and compact pellets made of mechanically milled Co₂Cr_0.6Fe_0.4Al. The as-grown and the milled sample show negligible anisotropic magnetostriction (25 μst at saturation), whilst only the milled sample exhibits magnetoresistance (0.65% at 300 K). These results permit us to discard the magnetostrictive effects as the magnetoresistance source.     

A LEED/AES study of the oxidation of Fe0.84Cr0.16 (100) and (110)

The interaction between single crystalline Fe_0.84Cr_0.16 (100) or (110) and oxygen gas in the pressure range 10^?9 to 10^?7 torr was studied at room temperature and at 800 K, using LEED and AES. The interaction starts with a chromium-oxygen reaction next to the alloy surface, followed by an iron—oxygen reaction outside the chromium-oxygen layer. At 800 K these reactions are connected with redistribution of cations between the interior of the alloy and the surface region, whereas at room temperature only a redistribution of cations within the surface region is observed. Different symmetries and lattice parameters of oxides which grow epitaxially on Fe_0.84Cr_0.16 (100) are compared with the corresponding surface compositions. It is found that the formation of spinel-like oxide layers is favoured by lower values of the Cr/Fe surface ratio.     

Advanced electrochemical performance of LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method

Spinel LiMn₂O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials were successfully synthesized by the citric-acid-assisted sol-gel method with ultrasonic irradiation stirring. The structure and electrochemical performance of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometer, cyclic voltamogram (CV) and the galvanostatic charge-discharge test in detail. XRD shows that all the samples have high phase purity, and the powders are well crystallized. SEM exhibits that LiMn_1.4Cr_0.2Ni_0.4O₄ has more uniform cubic-structure morphology than that of LiMn₂O₄. EDX reveals that a small amount of Mn³⁺ still exists in LiMn_1.4Cr_0.2Ni_0.4O₄. The galvanostatic charge-discharge test indicates that the initial discharge capacities for the LiMn_1.4Cr_0.2Ni_0.4O₄ and LiMn₂O₄ at 0.15 C discharge rates are 130.8 and 130.2 mAh g⁻¹, respectively. After 50 cycles, their capacity are 94.1% and 85.1%, respectively. The CV curve implies that Ni and Cr dual substitutions are beneficial to the reversible intercalation and deintercalation of Li⁺, and suppress Mn³⁺ generation at high temperatures and provide improved structural stability.     

Effect of quenching wheel speed on the structure, magnetic properties and magnetoimpedance effect in Co64Fe4Ni2B19−xSi8Cr3Alx (x=0, 1 and 2) melt-spun ribbons

Melt-spun ribbons of Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al_x (x=0, 1 and 2) alloy at wheel speed of 25 m/s and Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al₁ alloy at different wheel speeds (25, 30, 35 and 40 m/s) have been prepared and investigated for structural and magnetic properties and magnetoimpedance effect. Based on the results obtained, it was shown that replacement of B by Al can improve the magnetoimpedance ratio (MIR) and the highest value of MIR (191%) was observed for the sample with X_Al=1 at wheel speed of 25 m/s. Further, it was seen that the higher quenching wheel speed gives rise to a higher coercivity and lower magnetic permeability/MIR value.     

Electron transport properties of amorphous Mg80.4Cu19.6 alloy

The Hall coefficient, thermoelectric power and electrical resistivity of the liquid quenched amorphous Mg_80.4Cu_19.6 alloy have been measured over a wide temperature range below room temperature. The 2k_F value deduced from the observed Hall coefficient agrees well with the free electron value. The thermoelectric power is positive and increases linearly with increasing temperature above approximately 180 K. The downward deviation is evident in the lower temperature range. This behaviour is in sharp contrast with the negative thermoelectric power observed in the amorphous Mg-Zn alloy. Nevertheless, the overall temperature variation of the electrical resistivity resembles the previously reported data of the amorphous Mg_72.5Zn_27.5 alloy in various aspects; (1) the room temperature resistivity value, (2) the negative TCR and its magnitude, (3) the appearance of a broad maximum at about 50 K and a minimum at about 10 K and (4) T²-dependence in the range of 10～30 K and $\text{-(T-T}{}_{\max }\text{)}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$-dependence in the range of 50～220 K. The results are discussed in connection with $\text{2k}{}_{\mathrm{F}}\text{K}{}_{\mathrm{P}}$ criterion and Mooij correlation.     

Cr3+:Al2O3透明多晶陶瓷光谱特性分析

对透光性良好的Cr³⁺:Al₂O₃透明多晶陶瓷的光谱性能 进行了研究，其吸收光谱中吸收峰与单晶红宝石相一致，按吸收光谱和Tanabe-Sugano能级 图，算出其晶场强度参数D_q及Racah参数B分别为1792cm^-1， 689cm ^-1，D_q/B=2.6，陶瓷中Cr³⁺离子所处格位的晶体场强 比单晶弱一些，但Cr³⁺:Al₂O₃透明陶瓷仍属于强场晶 体材料；当Cr³⁺掺杂浓度到达0.8wt%时，陶瓷的发射谱仍保持较好的R线发射 ；随Cr³⁺掺杂浓度的增大，激发峰位发生“红移”.在Cr³⁺:Al₂O₃透明多晶陶瓷的荧光谱上，发现一个波长为670nm的发射峰，经激发 谱确认为Cr³⁺的发射峰. 关键词： 氧化铝 透明陶瓷 离子格位 光谱性质     

Existence of both spin-glass and mictomagnetic behaviour in the amorphous Fe10Ni70P14B6 alloy

Results of the accurate magnetization measurements performed on the amorphous Fe₁₀Ni₇₀P₁₄B₆ alloy in the temperature range 20–77 K in fields upto 1 kOe are reported. The complex magnetic behaviour exhibited by this alloy has been analyzed to show that a ferromagnetic ordering, occuring on a localized microscopic scale at a temperature T₀ very close to that given by the earlier Hall effect measurements, is accompanied by a superparamagnetic behaviour which below T₀ causes at first a spin-glass freezing of the magnetic spins when they interact with one another on a long range scale and then a mictomagnetic freezing of the giant superparamagnetic clusters at a lower temperature as a result of exchange interaction between their moments and the frozen spin-glass matrix. In addition, the present results, besides providing a clear physical insight into the widely different ordering temperatures obtained for this alloy from previous Mössbauer and resistivity measurements, on one hand and from our magnetization measurements on the other, strongly suggest a magnetic origin for the observed resistivity-minimum phenomenon. In conclusion, the present alloy represents a composition in the amorphous (Fe_xNi_1?x)₈₀P₁₄B₆ system well below the percolation limit.