氢化镁放氢动力学的研究

无烟煤经碱熔-酸洗-碳化处理后制得了微晶碳,以微晶碳及肥煤作助磨剂,在氢气气氛下机械球磨金属镁3h,制得储氢材料氢化镁。在等温条件下用体积法进行放氢测试,根据放氢数据用Arrhenius方程计算出的放氢活化能为104.85kJ/mol。在变温条件下用程序升温脱附法测得材料的TPD曲线,用Kissinger方程和Sharp方程分别对材料的活化能进行计算。Kissinger方程求得的活化能为105.87kJ/mol;当升温速率为5,10和15℃/min时,Sharp方程求得的活化能分别为108.33,98.70和113.19kJ/mol。等温法和非等温法均可用于放氢活化能的计算,由于公式、原理、采用数据等方面的不同,求得的活化能有差异。     

Kinetics of aluminum hydride thermal decomposition

The kinetics of A1H₃ decomposition were studied as a function of temperature and hydrogen pressure. Analysis of the experimental data in the formal heterogeneous kinetic approach suggests that the rate of the process is limited by the breaking of Al-H bonds.     

The kinetics of decomposition of solid solutions

The influence of solute–solute interactions on segregation processes in multi-component solid solutions leading to the formation of a new phase with a given stoichiometric composition, has been investigated for the first time. Expressions for the nucleation and growth rates have been derived. Estimates were developed for the time required to establish a steady-state nucleation rate in the system and the time interval for which such a steady-state can be sustained. Based on these results, it is anticipated that a method for an experimental determination of the parameters describing the interaction of the solute components, could be developed. The kinetic equation describing the evolution of the cluster size distribution function has been generalized to account for stochastic effects due to both fluctuations in the growth rate and possible spatial correlations of the evolving clusters in the matrix. The possible influence of such stochastic effects (thermal noise and random coalescence) on coarsening described by such additional terms has been discussed briefly. This revised version was published online in November 2006 with corrections to the Cover Date.     

Kinetics of hydrogen liberation from stoichiometric and nonstoichiometric magnesium hydride

We have carried out a systematic study of the kinetics of decomposition of stoichiometric and nonstoichiometric magnesium hydride. Using the thermal-desorption methods in barometric modification and mathematical modeling, we have shown that hydrogen desorption from stoichiometric MgH₂ proceeds in two stages: the formation of nuclei of the metal phase and hydrogen liberation through surface islands of the metal phase under the limiting influence of desorption rate. In the course of hydrogen liberation from partially hydrogenated magnesium, the first stage is usually absent. We have established that the influence of other reactions on the overall degasification rate is at least much smaller than the effect of desorption from the surface of α-Mg. We have obtained estimates of the parameters affecting the desorption kinetics. Finally, we have shown that, for analysis of the kinetics of hydrogen liberation from MgH₂, the choice of physically substantiated models has serious advantages as compared with the Avrami-Erofeev approach. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 25–35, September–October, 2007.     

The interaction of hydrogen with oxidic promoters of hydrogen storage in magnesium hydride

The present work is devoted to monitor the reactivity with hydrogen of various oxide systems (Nb₂O₅, WO₃ and a mixed Mg/Nb/O oxide) in the aim of understanding the role of oxides as promoters for hydrogen storage in MgH₂. The reactivity of the oxides has been tested using either molecular hydrogen or “nascent” hydrogen produced by reaction of zinc with hydrochloric acid. Thermal desorption-mass spectrometry experiments indicate that hydrogen adsorbed in Nb₂O₅ and Mg/Nb/O mixed oxides is, in part, reversibly released and, for a second fraction, released as water. By contrast, water is the only product desorbed by WO₃ after contact with hydrogen. This could explain the higher performances of Nb₂O₅ as kinetic promoter of hydrogen storage in comparison with WO₃.     

Activation of magnesium hydride by pressing with catalytic additives

We have studied the activation of magnesium hydride decomposition by means of its pressing with a catalyst. It is established that pressing leads to the formation of metal nuclei, which favor a decrease in the temperature threshold of magnesium hydride decomposition. The introduction of catalytic additives also reduces the temperature of dehydrogenation. The most effective in this respect was found to be the addition of nickel powder.     

泡沫铝制备过程中泡沫体形成机制研究

采用粉末冶金法制备泡沫铝材料,对发泡过程中泡沫体膨胀的演变过程及动力学机制进行了研究．通过对发泡过程的动态观测,获得了泡沫体体积膨胀与发泡时间的关系曲线．结果表明：泡沫体体积膨胀经历3个阶段：发泡初期的泡沫铝体积缓慢增大到迅速膨胀达到最大膨胀点,随后开始塌缩,体积保持在一定数值．对发泡过程动力学机制的分析表明,泡沫体膨胀过程与发泡剂分解产生的气体量和从熔体中排出气体的量有直接关系,由此推导出泡沫体体积-时间的动力学方程,该方程与实验获得的体积变化曲线相吻合,为发泡过程的控制提供理论依据．     

Bone-like matrix formation on magnesium and magnesium alloys

Mg metal and its alloys have promise as a biocompatible, degradable biomaterials. This work evaluates the potential of in vitro cell culture work with osteoblast-like cells on Mg based materials, and investigates cell differentiation and growth on Mg alloyed with various non-toxic or low-toxicity elements. Mg based substrates support the adhesion, differentiation and growth of stromal cells towards an osteoblast-like phenotype with the subsequent production of a bone like matrix under in vitro conditions. No significant difference in the final tissue layer is observed on pure Mg, an AZ21 alloy or a 0.5 wt% Ca alloy. Only a 0.8 wt% Ca alloy which shows complete structural disintegration shows minimal cell growth. Due to association of non-soluble degradation products formed when Mg is incubated in physiological-like fluid, mass changes typically used to report Mg degradation are not viable estimates of degradation. Methods quantifying the time dependent change in the mechanical integrity of samples as a function of incubation time are required for a proper assessment of Mg degradation. We conclude that in vitro cell culture of bone cells on Mg substrates is expected to be a viable screening technique to assess the relative biological activity of Mg-based materials.     

Phase formation mechanism and kinetics in solid-state synthesis of undoped and calcium-doped lanthanum manganite

LaMnO_3+δ and La_0.7Ca_0.3MnO₃ were synthesized from La₂O₃(La(OH)₃), CaCO₃ and MnO₂ powder mixture with solid-state reaction technique. X-ray diffraction and thermal analysis were employed in the present study on the process of synthesizing of the two compound powders. The kinetic study on solid-state reaction between La₂O₃ and MnO₂ in the powder mixture was isothermally carried out for LaMnO_3+δ formation. The result showed that the reaction process was controlled by three-dimensional solid-ionic diffusion. Both Jander and Ginstling-Brounstein model can be used to describe the reaction kinetics satisfactorily. The relevant apparent activation energy values obtained were as great as 205 ± 11 and 189 ± 8 kJ/mol. The reaction process of La_0.7Ca_0.3MnO₃ preparation from La₂O₃, SrCO₃ and MnO₂ powder mixture was also studied using X-ray diffraction and thermal analysis. The result indicated that the following steps composed the overall reaction: the decomposition of the reactant; the formation of La_1−xCa_xMnO_3+δ; La₂O₃ and Mn₃O₄ reacted with La_1−xCa_xMnO₃ to form final La_0.7Ca_0.3MnO₃ phase. The latest step was the most time-consuming one among others in the overall reaction.     

Amelioration of the hydriding and dehydriding kinetics of Mg by pulverization and the formation of Mg2Ni hydride

The addition of an oxide Fe₂O₃ is considered to increase the hydriding rate of Mg by pulverization, and the addition of Ni to increase the hydriding and dehydriding rates by the formation of Mg₂Ni hydride. The sample Mg-10 wt.% (Fe₂O₃, Ni) was prepared by grinding Mg mechanically under H₂ (reactive mechanical grinding) with nano-structured Fe₂O₃ particles and Ni. The as-milled sample absorbed 4.24 wt.% hydrogen at 593 K under 12 bar H₂ for 60 min. Its activation was accomplished after two hydriding-dehydriding cycles. The activated sample absorbed 4.05 wt.% hydrogen at 593 K, 12 bar H₂ for 60 min and desorbed 3.05 wt.% hydrogen at 593 K, 1.0 bar H₂ for 60 min. After hydriding-dehydriding cycling, Mg₂Ni is formed by the reaction of Mg with Ni, Fe₂O₃ is reduced, and a small fraction of Mg is oxidized.     

Agglomeration of magnesium oxide particles formed by the decomposition of magnesium hydroxide

The agglomeration process of MgO powder derived from Mg (OH)₂ was investigated at fixed temperatures of 600, 800, 900 and 1200° C; these temperatures were chosen as representative of four regions, i.e. below 600° C, 650 to 850° C, 850 to 1050°C and 1050 to 1200° C previously reported. At 600° C, coherent crystallites coalesced within the heating time of 60 min; on further heating till 300 min, the primary particles which consisted of crystallites grew rapidly. The original Mg (OH)₂ framework or pseudomorphs, composed of minute crystallites and primary particles, still remained in the powder. At 800° C, the pseudomorphs had disintegrated into fragments. The crystallite growth and primary particle growth were accelerated with increasing the heating times beyond 60 min. At 900° C, a further fragmentation of agglomerates occurred with increasing the heating times; the crystallite and primary particle growth in fragments brought about the pore coalescence. At 1200° C, the crystallite and primary particle growth proceeded with the coarsening of pores; on heating beyond 240 min, the crystallites and primary particles grew rapidly due to the entrapment of pores within them.     

Agglomeration of magnesium oxide particles formed by the decomposition of magnesium hydroxide

Agglomeration of magnesium oxide (MgO) particles was studied by decomposing magnesium hydroxide (Mg(OH)₂). The properties of agglomerates varied according to the decomposition temperature region: (i) below 650° C, (ii) 650° C to 850° C, (iii) 850° C to 1050° C, and (iv) 1050° C to 1200°C. In region (i), the original Mg(OH)₂ frameworks or pseudomorphs remained in the powder and showed agglomeration. The strength of agglomerates containing the pesudomorphs was about 50 MPa; the primary particles in pseudomorphs are bonded chemically by the interaction of MgO and residual water. In region (ii) the pseudomorphs began to show some fragmentation: the bonding strength of these pseudomorphs reduced rapidly. In region (iii), both crystallite and primary particles were grown by the sintering; this growth may be due to an increase in contact area based on the collapse of pseudomorphs. The primary particles whose necks were grown by the sintering could be easily pulled apart by grinding. In region (iv) pore growth due to the rearrangement of primary particles caused the suppression of both densification rate and crystal growth of MgO.     

The relationship between thermal decomposition properties of titanium hydride and the Al alloy melt foaming process

Using a new temperature programmed decomposition (TPD) theory and related experimental technique, a set of thermal decomposition kinetics equations of titanium hydride can be acquired by separating and simulating its TPD spectrum. According to these equations, the relation curve of decomposition quantity and time for titanium hydride at temperature of 940 K is obtained and the result coincides well with the Al alloy melt foaming process, which provides a scientific basis for controlling the Al alloy melt foam and then the Al alloy foams with different pore structure are successfully prepared.     

AZ91D镁合金锌系磷化膜成膜机理和生长过程的研究

以磷酸盐化学转化膜为研究体系,采用SEM、XRD、OCP等分析方法及检测手段,研究AZ91D镁合金系磷酸盐化学转化膜的成膜机理、膜层结构及生长过程。研究发现AZ91D镁合金在磷化液中成膜过程分5个阶段:初始成核(1～5s)、基体快速溶解(5～60s)、晶体快速生长(1～2min)、膜层稳态生长(2～10min)和膜层沉积溶解平衡阶段(10min以后)。AZ91D镁合金表面的磷酸盐晶核的形成并非在金属进入溶液的最初时刻一次形成,是分批形成。最先形成的晶核逐渐长大,新的晶核不断生成,磷酸盐晶粒对其表面的覆盖度逐渐增大,直至各个晶粒逐渐长大相互接界,将其表面完全覆盖,结晶过程结束。晶核的形成未优先发生在基体金属的晶界上,随着晶核的生长和外延而形成磷化膜。     

Deposition of magnesium hydride thin films using radio frequency reactive sputtering

Reactive sputter deposition of MgH_x thin films was performed using mixed hydrogen-argon plasma. This technique allows in-situ deposition of metal hydride films in contrast to the commonly applied ex-situ hydrogenation of metallic films. Partly transparent films were obtained and the formation of crystalline MgH₂ could be observed for thicknesses above about 200 nm. The formation of some metallic Mg in the films could not be avoided. Increased hydrogen loading by increased pressure, H₂:Ar ratio or reduced power produced films of porous structure that easily oxidise. More densely packed films remain stable for several months of air exposure. Post-deposition treatments in H-plasma showed evidence of hydrogenation of deposited films without the use of a catalysing capping film. Film properties are studied by X-ray diffraction, scanning electron microscopy and by optical and resistivity measurements.     

The oxidation kinetics of magnesium at low temperatures and low oxygen partial pressures

The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10^− 8 Pa and 1.3 × 10^− 5 Pa and at temperatures just above room temperature has been investigated in situ with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Quantitative analysis of the XPS spectra showed that the initially formed oxide has a higher Mg/O ratio (> 1.3) than bulk MgO. Ellipsometry measurements indicated that the band gap values of the oxide layers are considerably smaller than the value expected for bulk MgO ( 2.5 eV vs. 7.8 eV). From the XPS and ellipsometry data recorded as a function of oxidation time the oxidation kinetics have been determined. The kinetics has been described quantitatively with a coupled currents model, involving simultaneous transport of electrons and ions through the oxide layer.     

碳质中间相形成机理研究

碳质中间相是制备高性能炭材料制品的优质前驱体，其产品已在高性能复合材料、超高功率石墨电极和二次锂离子电池用负极材料等方面得到了成功应用。碳质中间相产品的结构复杂、形态各异，造成了生产这些产品涉及的原料和处理过程的多样化。为了分析和预测一定条件下获得的碳质中间相产品的性能，进而确定其具体的用途，需要弄清碳质中间相的形成机理。通过系统地探讨碳质中间相的形成过程，对以往文献中的解释给予了归纳和分析。分析认为传统解释中存在许多不合理的地方，不能对一些现象给予合理的解释；“微域构筑”理论在传统解释的基础上有了很大进步，但由于该理论中引入了实际上并不存在的片层微晶单元而使其存在缺陷；“颗粒基本单元构筑”理论摒弃了以上两种理论中的不合理成分，能够对碳质中间相的形成过程进行很好的解释。此外，对“颗粒基本单元构筑”理论的一些应用也进行了讨论。