TEM analysis of the microstructure in TiF3-catalyzed and pure MgH2 during the hydrogen storage cycling

We utilized transmission electron microscopy (TEM) analysis, with a cryogenically cooled sample stage, to detail the microstructure of partially transformed pure and titanium fluoride-catalyzed magnesium hydride powder during hydrogenation cycling. The TiF₃-catalyzed MgH₂ powder demonstrated excellent hydrogen storage kinetics at various temperatures, whereas the uncatalyzed MgH₂ showed significant degradation in both kinetics and capacity. TEM analysis on the partially hydrogen absorbed and partially desorbed pure Mg(MgH₂) revealed a large fraction of particles that were either not transformed at all or were completely transformed. On the other hand, in the MgH₂+TiF₃ system it was much easier to identify regions with both the hydride and the metal phase coexisting in the same particle. This enabled us to establish the metal hydride orientation relationship (OR) during hydrogen absorption. The OR was determined to be (1 1 0)MgH₂ || (?1 1 0 ?1)Mg and [?1 1 1]MgH₂ || [0 1 ?1 1]Mg. During absorption the number density of the hydride nuclei does not show a dramatic increase due the presence of TiF₃. Conversely, during desorption the TiF₃ catalyst substantially increases the number of the newly formed Mg crystallites, which display a strong texture correlation with respect to the parent MgH₂ phase. Titanium fluoride also promotes extensive twinning in the hydride phase.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Improvement in hydrogen sorption kinetics of MgH2 with Nb hydride catalyst

Nanocrystalline MgH₂ with fine and evenly dispersed Nb hydride was prepared by ball milling a mixture of MgH₂ and 1 mol.% NbF₅. This NbH-catalyzed MgH₂ desorbed 6.3 wt.% H₂ in 15 min and absorbed more than 90% of its initial hydrogen capacity within 5 min at 573 K. Moreover, this fast sorption kinetics was maintained after 10 cycles. Based on X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy analyses, it is suggested that NbF₅ melts during high-energy ball milling and this promotes the formation of extremely fine, film-like Nb hydride preferentially along the grain boundaries of nanocrystalline MgH₂ by a liquid/solid reaction. This unique nanostructured Nb hydride is believed to suppress the grain growth of MgH₂ quite effectively and thus maintain its initial catalytic effect throughout repeated hydrogenation–dehydrogenation cycles.     

Characterization of mechanically alloyed nanocomposites in TiO2–B2O3–Mg–C quaternary system

The influence of the simultaneous presence of magnesium and graphite on mechanosynthesis of various nanocomposite powders in TiO₂–B₂O₃–Mg–C quaternary system was investigated. A mixture of boron oxide and titanium dioxide powders along with different amounts of magnesium and graphite was milled using a high-energy planetary ball mill to provide necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). In the absence of C (100 wt.% Mg), TiB₂ nanopowder was formed as a result of combustion reaction after 34 min of milling. In the presence of both Mg and C, the mechanochemical reaction was completed after different milling times depending on the weight fraction of the reducing agents in the powder mixture. In the presence of x wt.% Mg–y wt.% C (x = 85 and 90; y = 100 − x), the mechanosynthesized composites contained TiB₂ and TiC as major compounds as well as MgO and Mg₃B₂O₆ as unwanted phases. With further increasing the graphite content to 30 wt.%, no mechanical activation was observed after 90 min of milling. The nanocomposite powders showed a bimodal particle size distribution characterized by the presence of several coarse particles (≈ 250 nm) along with finer particles with a mean size of about 75 nm. Formation mechanism of nanocomposites was explained through the analysis of the relevant sub-reactions.     

Hydrogen sorption properties of MgH2–LiBH4 composites

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH₂ + 2LiBH₄ ↔ MgB₂ + 2LiH + 4H₂ was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH₂ + 2LiBH₄ ↔ Mg + 2LiBH₄ + H₂ ↔ MgB₂ + 2LiH + 4H₂. However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH₄ and MgH₂ are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250–300°C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.     

Thermodynamic and structural characterization of the Mg–Li–N–H hydrogen storage system

The Mg–Li–N–H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. Some of thermodynamic and structural properties for this system are characterized here. Pressure-composition isotherms are measured and presented in this paper for absorption–desorption at 220, 200 and 180 °C. Powder X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) analysis were carried out for samples at various degrees of hydrogenation. These results provide information about the structural changes during absorption/desorption. The mixture of (2LiNH₂ + MgH₂) partially converts to (Mg(NH₂)₂ + 2LiH) when heated at 220 °C and 100 bar of hydrogen without undergoing desorption. Based on two distinct parts which appear in all of the pressure-composition isotherms (180–220 °C), two reactions taking place isothermally in hydrogen absorption/desorption are proposed for the material starting with (2LiNH₂ + MgH₂) or (Mg(NH₂)₂ + 2LiH). These reactions include a single solid-phase reaction, corresponding to the sloping region for hydrogen weight percent (Hwt%) smaller than 1.5%, and a multiple-phase reaction, corresponding to a plateau region for Hwt.% > 1.5 in the isotherms. During hydrogen absorption/desorption, the single-solid-phase reaction corresponds to the forming/consuming of NH₂ which is bonded to Li and the multiple-solid-phase reaction corresponds to forming/consuming Mg(NH₂)₂ and LiH. A mechanism for the sorption reactions has been proposed.     

Synthesis and hydrogen storage of carbon nanofibers

Carbon nanofibers were synthesized by catalytic decomposition of methane using Ni–MgO catalyst and hydrogen adsorption experiments were carried out by a Sievert’s apparatus under 120 bar at 25 °C. Hydrogen adsorption capacity was elevated up to 1.4 wt.% after heat treatment at 1200 °C in N₂ atmosphere. CO and CO₂ were detected by gas chromatography (GC) during heat treatment which promoted active surface suitable for hydrogen adsorption of carbon nanofibers. High resolution transmission electron microscopy (HRTEM) and XRD analysis revealed that the structure of carbon nanofibers was durable after hydrogen uptake even at high pressures.     

Mechanochemical transformations in Li(Na)AlH4–Li(Na)NH2 systems

Mechanochemical transformations of tetrahydroaluminates and amides of lithium and sodium have been investigated using gas volumetric analysis, X-ray powder diffraction, solid-state nuclear magnetic resonance (NMR) and transmission electron microscopy. In a transformation of LiAlH₄ and LiNH₂ taken in an 1:1 molar ratio, the amount of released hydrogen (6.6 wt.% after 30 min ball milling) was higher than in any known one pot mechanochemical process involving a hydrogen-containing solid. A total of 4.3 wt.% of hydrogen is released by the NaAlH₄–NaNH₂ system after 60 min ball milling; and 5.2 wt.% H₂ is released when LiAlH₄ and NaNH₂ or NaAlH₄ and LiNH₂ are ball milled for 90 min and 120 min, respectively. All transformations proceed at room temperature. The mechanism of the overall transformation MAlH₄(s) + MNH₂(s) → 2MH(s) + AlN(s) + 2H₂(g) was identified based on detailed spectroscopic analysis of the intermediate (M₃AlH₆) and final products of the ball milling process.     

Effects of Al2O3 addition on the microstructure and properties of Ni activated sintered W matrix composites

In the present investigation, fabrication of high dense (> 97.8%) W matrix composites with increased microhardness values were investigated. W and W–1 wt.% Ni powders were mechanical alloyed for 18 h and sintered at 1400 °C for 1 h under Ar, H₂ gas flowing conditions in order to investigate the effects of 1 wt.% Ni addition on the densification and properties of W. The effects of Al₂O₃ particles additions on the microstructural and physical properties of the sintered W–1 wt.% Ni sample were investigated. A 92.59% relative density value of the sintered W sample increased to 99.47% with the addition of 1 wt.% Ni. Moreover, despite the observed grain growth, microhardness values significantly increased from 2.81 ± 0.34 GPa to 4.07 ± 0.16 GPa with the addition of 1 wt.% Ni. The relative density values of the sintered W–1 wt.% Ni sample slightly decreased with increasing Al₂O₃ additions, a relative density value of 97.81% was measured for the W–1 wt.% Ni sample reinforced with 2 wt.% Al₂O₃ particles. As the average grain size of W in the sintered W–1 wt.% Ni sample decreased from 4.41 ± 1.71 μm to 1.29 ± 0.39 μm with the addition of 2 wt.% Al₂O₃ particles, the microhardness of the sample increased to 5.98 ± 0.31 GPa.     

Mechanochemical preparation and investigation of properties of magnesium,calcium and lithium–magnesium alanates

Ball milling of MgCl₂ and CaCl₂ with NaAlH₄ or LiAlH₄ can be used for the preparation of magnesium, calcium and lithium–magnesium alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it was possible to obtain these alanates in nearly pure state. The alanates were characterized by X-ray diffractometry, solid-state ²⁷Al NMR and IR spectroscopy and thermovolumetric (TV) and differential scanning calorimetry (DSC) measurements. Mg(AlH₄)₂ dissociates thermally in one step to MgH₂, Al and hydrogen; at a higher temperature, MgH₂ and Al transform to Mg–Al alloy and hydrogen. Thermal dissociations of Ca(AlH₄)₂ and of LiMg(AlH₄)₃ (in mixture with NaCl or LiCl) proceeds in several steps, of which the first two can be assigned to the formation of CaH₂ and of a MgH₂/LiH mixture, respectively, in addition to Al and H₂. Possible intermediates of these two steps are CaAlH₅ and LiMgAlH₆. Higher temperature dissociations include formation of MgH₂ (LiH) and Ca–Al alloys from CaH₂, CaH₂ and Al, respectively. Upon ball milling of MgCl₂ or CaCl₂ with NaAlH₄ or LiAlH₄ in the presence of Ti catalysts, only the thermal dissociation products of the expected alanates are obtained. This indicates that dehydrogenation discharge of earth alkali metal alanates can be catalyzed by Ti. According to DSC measurements, the thermodynamic stability of Mg(AlH₄)₂ (ΔH = 1.7 kJ/mol) is too low for the purpose of reversible hydrogen storage. Determination of ΔH values for the second, endothermal step of calcium and lithium–magnesium alanate dissociations gave values of around 31.6 and 13.1 kJ/mol, respectively.     

Catalytic effect of Ni nanoparticles on the desorption kinetics of MgH2 nanoparticles

Mg and Ni nanoparticles were prepared by hydrogen plasma-metal reaction (HPMR). MgH₂ nanoparticles were obtained by hydriding the Mg nanoparticles. Hydrogen storage kinetics of the MgH₂ nanoparticles doped with different amount of Ni nanoparticles was investigated by differential scanning calorimetry (DSC) and hydrogen desorption rate measurements. The obtained samples show superior hydrogen storage kinetics. 6.1 wt% hydrogen is desorbed in 10 min at 523 K under an initial pressure of 0.01 bar of H₂ when the proportion of Ni nanoparticles is 10 wt%. The desorption rate increases when enhancing the amount of catalyst. However, the activation energy of desorption does not decrease any more when the amount of Ni exceeds a value. The enhanced desorption kinetics are mainly attributed to the accelerated combination process of hydrogen atoms by the Ni nanoparticles on the surface of MgH₂.     

Effects of the lanthanum content on the microstructure and properties of the molybdenum alloy

The effects of the lanthanum content on the microstructure and properties of molybdenum alloy were investigated. The molybdenum powders with various lanthanum contents were prepared by a solid-liquid doping method and reduction under hydrogen atmosphere, which could be processed into sintered molybdenum and rotary swaged molybdenum. The results indicated that the grain sizes of the alloys became finer and the tensile strength increased with increasing La content. The La₂O₃ particles could adsorb impurity elements that existed on the grain boundary and generate the amorphous structure around the particle. The rotary swaged Mo with 0.1 wt.% La was the highest tensile strength, and the rotary swaged Mo with 0.03 wt.% La possessed the highest elongation to failure of 42%. In addition, the electrical resistivity of the rotary swaged Mo increased at first and later decreased with increasing La content.     

Dehydriding and rehydriding processes of well-crystallized Mg(BH4)2 accompanying with formation of intermediate compounds

Dehydriding and rehydriding properties of well-crystallized Mg(BH₄)₂ were systematically investigated by thermogravimetry (TG) and pressure–composition–temperature (PCT) measurements. The dehydriding reaction of Mg(BH₄)₂ starts at approximately 500 K, and about 14.4 mass% of hydrogen is desorbed according to the following multi-step reaction:Mg(BH₄)₂ → some intermediate compounds → MgH₂ + 2B + 3H₂ → Mg + 2B + 4H₂The apparent enthalpy change in the dehydriding reaction from Mg(BH₄)₂ to MgH₂ is estimated to be 57 ± 5 kJ mol⁻¹ H₂ based on the result of the PCT measurement. It is proved that approximately 6.1 mass% of hydrogen can be reversibly stored for the sample of Mg(BH₄)₂ after the dehydriding reaction, through the formation of a possible intermediate compound such as MgB₁₂H₁₂.     

Analyses of hydrogen sorption kinetics and thermodynamics of magnesium–misch metal alloys

The multi-component Mg–x wt.% Mm alloys are synthesized using the mechanical ball-milling technique and their hydrogen storage capacities, absorption/desorption kinetics, and thermodynamic parameters are quantified. The analysis of kinetic properties presented here is based on the method of exponential peeling. It is seen that the presence of misch metal (Mm) in the alloy samples dramatically decreases their rate of decrepitation and increases their cyclic stability. However, the increased concentration of misch metal in the samples has an adverse effect on their hydrogen storage capacity and their reaction rate. The hydrogen storage properties also vary with reaction temperature. The best hydrogen absorption kinetics are observed at temperatures around 300 °C and the desorption kinetics are quite fast at temperatures of 400 °C and above. The hydrogen desorption activation energy of Mg–x wt.% Mm hydride is much lower than that of MgH₂. The pressure–composition–isotherm (P–C–T) plots of the samples at 300–420 °C indicate that the alloys possess good cyclic stability but very poor reversibility, making the study of their thermodynamic properties difficult. The P–C–T plots also indicate that the increase of the concentration of the misch metal's rare earths leads to an increase of the hydrogen equilibrium pressure and decrease of hydrogen storage capacity.     

Investigation the effect of B4C addition on properties of TZM alloy prepared by spark plasma sintering

TZM alloy is one of the most important molybdenum (Mo) based alloy which has a nominal composition containing 0.5–0.8 wt.% titanium (Ti), 0.08–0.1 wt.% zirconium (Zr) and 0.016–0.02 wt.% carbon (C). It is a possible candidate for high temperature applications in a variety of industries. However, the rapid oxidation of TZM alloys at high temperature in air is considered to be one of the drawback. In this study, TZM alloys with additions of 0–5 wt.% B₄C were prepared by spark plasma sintering (SPS) at 1420 °C utilizing 40 MPa pressure for 5 min under vacuum. The effects of B₄C addition on oxidation, densification behavior, microstructure, and mechanical properties were investigated. The TZM alloy with 5 wt.% B₄C have exhibited an approximately 66% reduction in mass loss under normal atmospheric conditions in oxidation tests made at 1000 °C for 60 min. And an increase from 1.9 GPa to 7.8 GPa has been determined in hardness of the alloy.     

A study on the effects of silica particle size and milling time on synthesis of silicon carbide nanoparticles by carbothermic reduction

Silicon carbide nanoparticles were produced by a carbothermic reduction of nano and micro size silica with graphite at 1450 °C for 1 h. The SiC nanoparticles were characterized by XRD, SEM and TEM. The results showed that in the case of nano silica, milling up to 20 h could develop SiC particles of 5–25 nm with some residual SiO₂ particles. By extending milling time to 40 h, more energy was provided and produced Fe contamination, which could act as catalyst and increased SiC yield as well as Fe₂Si phase formation after heat treatment. Coarser particles of micro silica caused higher Fe erosion, more SiC formation with 20–70 nm size and presence of Fe₂Si phase at shorter milling times after heat treatment. Leaching treatment could purify SiC nanoparticles. Increase of milling from 20 to 40 h changed the morphology from polygonal shape to spherical with some reduction in the particle size.     

Effect of temperature and Ar flow rate on Ti3SiC2 formation from TiSiO4 powders coated with pyrolytic carbon

In this study, equilibrium thermodynamic analysis was initially carried out for TiO₂:SiO₂:C molar ratio of 1:1:4 at 1600 K, 1700 K and 1800 K as a function of Ar/solid reactant ratio. It was predicted that single phase Ti₃SiC₂ is formed when a critical Ar/solid reactant ratio is exceeded. This behavior is ascribed to the reduction of partial pressures of gaseous reaction products of SiO and CO. Subsequently, formation of Ti₃SiC₂ phase from carbon coated TiSiO₄ powders by carbothermal reduction was investigated as a function temperature, isothermal holding time and Ar flow rate. Carbothermal reduction experiments at 1800 K and at a Ar flow rate of 250 cm³/min for 60 min showed that the optimal C content was determined to be 27.47 wt.%. The ternary carbide compound was not detected within 120 min at 1600 K and 1700 K, but a major TiOC phase along with a minor SiC phase. Whereas at 1800 K, the ternary carbide phase was observed and its amount increased from 6.80 wt.% at 0 min to 38.91 wt.% at 75 min above which it gradually decomposed into the binary carbides. The experiments carried out for various Ar flow rate at 1800 K for 75 min revealed that the highest ternary carbide content (47.84 wt.%) was obtained at a Ar flow rate of 425 cm³/min. The thermodynamic and experimental results indicate that Ti₃SiC₂ formation takes place via the reaction of pre-formed TiC and SiC phases with the remaining SiO₂.     

Preparation of cobalt nanoparticles by direct current arc plasma evaporation method

In order to prepare cemented carbides with excellent mechanical properties, the cobalt nanoparticles were prepared by direct current (DC) arc plasma evaporation method for the first time and the effect of three adjustable parameters (current, filling pressure and the pressure ratio of hydrogen to argon (P_H2/P_Ar)) was investigated on the average diameter and the productivity through L₁₆(4³) orthogonal experiment in this paper. The crystalline structure, chemical composition, morphology, and particle size distribution of the samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray fluorescence analysis (XRF) and selected area electron diffraction (SAED), respectively. The results indicated that the cobalt nanoparticles achieved a high purity of 99.933 wt.% with a spherical shape, and the cobalt nanoparticles had a cubic crystal structure with a slight shrinkage in the lattice parameters. Both the average diameter and the productivity were increased with the increase of the three parameters, but the average diameter decreased when the value of P_H2/P_Ar varied from 2/3 to 1. Under different technical parameters, the average diameter varied from 28 nm to 70 nm and the productivity varied from 43 g/h to 324 g/h.     

Carbothermal synthesis of ZrC powders using a combustion synthesis precursor

ZrC powders were synthesized by carbothermal reduction of a combustion synthesized precursor derived from zirconium nitrate, urea, and glucose mixed solution. The results showed that the obtained precursor was comprised of polyporous blocky particles. The precursor powders were subsequently calcined under argon at 1200–1600 °C for 3 h. The transformation of ZrO₂ to ZrC, by adopting this route, occurred at 1300 °C. The preparation of ZrC experienced an intermediate phase of ZrOxCy. ZrC powders synthesized at 1500 °C are characterized by the spherical shape, small particle size (120–180 nm in diameter), low oxygen content (1.4 wt.%) and non-aggregated particles.