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.     

Preparation of WC nanoparticles by twice ball milling

In order to improve the ball milling efficiency of WC powders and thus to fabricate nano-grained WC–Co cemented carbides with high mechanical properties, WC nanoparticles were prepared by twice ball milling in nylon vessels. The best technology to disperse WC powders in alcohol was investigated at first. Based on the dispersion results, 2 wt.% PEG was used with La₂O₃ as additive to improve ball milling efficiency. The particle size, crystal structure, surface morphology and surface properties were tested by a laser particle sizer, XRD, FE-SEM and FT-IR, respectively. During the first ball milling, sample d achieved the best milling performance, including average particle size (168 nm) and grain size (27.2 nm) among samples a (pure WC), b (with PEG), c (with La₂O₃) and d (with PEG and La₂O₃). La₂O₃ could greatly decrease particle size and grain size while PEG could narrow particle size distribution. During the second milling, the particle size and grain size of sample d reached 89 nm and 13.2 nm at 96 h, respectively. The results indicated that twice ball milling can greatly improve particle size and grain size compared with the first ball milling, and further narrow the size distribution. In conclusion, multiple ball milling can reduce the particle size of certain powders with suitable milling technology.     

Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3

MgH₂ nanoparticles were obtained by hydriding ultrafine magnesium particles which were prepared by hydrogen plasma–metal reaction. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the obtained sample is almost pure MgH₂ phase, without residual magnesium and with an average particle size of ∼300 nm. Milled with 5 wt.% TiF₃ as a doping precursor in a hydrogen atmosphere, the sample desorbed 4.5 wt.% hydrogen in 6 min under an initial hydrogen pressure of ∼0.001 bar at 573 K and absorbed 4.2 wt.% hydrogen in 1 min under ∼20 bar hydrogen at room temperature. Compared with MgH₂ micrometer particles doped with 5 wt.% TiF₃ under the same conditions as the MgH₂ nanoparticles, it is suggested that decrease of particle size is beneficial for enhancing absorption capacity at low temperatures, but has no effect on desorption. In addition, the catalyst was mainly responsible for improving the sorption kinetics and its catalytic mechanism is discussed.     

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.     

Nanocrystalline Fe–Nb–(B,Ge) alloys from ball milling: Microstructure,thermal stability and magnetic properties

Fe₇₅B₂₀Nb₅, Fe₇₅Ge₁₀B₁₀Nb₅ and Fe₇₅Ge₂₀Nb₅ alloys were prepared by ball milling from pure powders and their microstructure and magnetic properties were studied. A nanocrystalline solid solution of α-Fe type is the main phase formed, although traces of some intermetallics were found in the Fe–B–Nb alloy. The local arrangements of Fe atoms in Ge containing alloys continuously evolve with milling time. The obtained powders are thermally stable even heating up to 773 K. After heating up to 1073 K, intermetallic compounds are detected. The best soft magnetic properties are achieved after heating up to 773 K, due to stress relaxation of the nanocrystalline microstructure (for Fe–Ge–Nb alloy, coercivity ∼ 600 A/m).     

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.     

Synthesis of nanocrystalline Bi2Te3 via mechanical alloying

Bi₄₀Te₆₀ thermoelectric compound was fabricated via mechanical milling of bismuth and tellurium as starting materials. Effect of the milling time and heat treatment temperatures were investigated. In order to characterize the ball milled powders, the X-ray diffraction (XRD) was used. Thermal behavior of the mechanically alloyed powders was studied by differential thermal analysis (DTA). The morphological evolutions were studied by scanning electron microscopy (SEM). Results showed that the nanocrystalline Bi₂Te₃ compound was formed after 5 h of milling. Further milling (25 h) and heating to 500 °C showed that the synthesized phase was stable during these conditions. Nanocrystalline Bi₂Te₃ with 9–10 nm mean grain size and flaky morphology (lamellar structure) was obtained at the end of milling.     

Synthesis of (Mo1−x–Crx)Si2 nanostructured powders via mechanical alloying and following heat treatment

MoSi₂–CrSi₂ nanocomposite powder was successfully synthesized by ball milling of Mo, Si and Cr elemental powders. Effects of the Cr content, milling time and annealing temperature were studied. X-ray diffraction (XRD) was used to characterize the milled and annealed powders. The morphological and microstructural evolutions were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). High temperature polymorph (HTP) of MoSi₂ begins to form after 50 h of milling and completes after 70 h of milling. MoSi₂–CrSi₂ composite powder was also prepared with a combination of short milling time (50 h) and low temperature annealing (850 °C). Annealing led to the HTP to low temperature polymorph (LTP) transformation of MoSi₂. MoSi₂–CrSi₂ nanocomposite powder with the mean grain size less than 50 nm was obtained at the end of milling. This composite maintained its nanocrystalline nature after annealing. A spherical morphology was procured for 50 h milled powder with 0.25 mole Cr.     

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.     

Preparation of Mo(Si, Al)2–ZrO2 nanocomposite powders by mechanical alloying

Phase transformations and the final formation of Mo(Si, Al)₂–ZrO₂ nanocomposite during high-energy ball milling of a series of Mo–Si–Al–ZrO₂ powders were investigated. Mechanical alloying led to phase transformations from the initial Mo–Si–Al powders mixture to Mo_ss (2 h) → C40 Mo(Si, Al)₂ (4, 8 h) → Mo_ss (12 h) phases. The phase transformations studied by XRD are discussed considering the alloying and second phase effects. Finally, the Mo_ss matrix reinforced with ZrO₂ particles nanocomposite structure was studied by means of TEM. The Mo_ss matrix phase formed was revealed to be strongly inhomogeneous even after 12 h of mechanical alloying and Mo-, Si- and Al-enriched regions were observed. The ZrO₂ nanostructured phase, evenly distributed in the Mo_ss matrix, had grain size of about 5–20 nm.     

Synthesis and characterization of MoSi2-Mo5Si3 nanocomposite by mechanical alloying and heat treatment

The nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi₂ was produced. However with continued milling for 48 h α-MoSi₂ and Mo₅Si₃ phases were formed. DTA results of 24 h and 48 h as milled mechanical alloyed specimens showed a well-defined peak at 852 °C and 920 °C relating to the formation of α-MoSi₂. The activation energy for 24 h and 48 h milled specimens were –128.6 KJ/mol and –121.4 KJ/mol respectively. Annealing the milled specimens at 1000 °C for 2 h revealed the phase transformation of β-MoSi₂ to α-MoSi₂ and the formation of Mo₅Si₃. The crystallite size of α-MoSi₂ and Mo₅Si₃ were about 9 nm and 12 nm after 48 h mechanical alloying. These values increased slightly to 18 nm and 14 nm after annealing at 1000 °C.     

Nanoscale characterisation and clustering mechanism in an Fe–Y2O3 model ODS alloy processed by reactive ball milling and annealing

Reactive ball milling and annealing is proposed as a new production method for oxide dispersion strengthened (ODS) steels. A highly concentrated Fe–38 atm.% Y₂O₃ ODS model alloy was processed by reactive ball milling and annealing of YFe₃ and Fe₂O₃ powders so as to induce the chemical reaction 2YFe₃ + Fe₂O₃ → 8Fe + Y₂O₃. The model alloy was characterised after milling and annealing by complementary techniques, including atom probe tomography. Ball milling up to the stationary state results in the formation of two metastable nanometric interconnected phases: super-saturated α-iron and an yttrium and oxygen rich phase. Annealing leads the system towards equilibrium through: (i) a chemical evolution of each phase to nearly pure α-Fe and Y₂O₃ oxide slightly sub-stoichiometric in oxygen; and (ii) growth of the phases. A pure iron matrix reinforced by nanometric Y₂O₃ particles was successfully synthesised by reactive ball milling and annealing.     

A novel approach to in situ synthesis of WC-Al2O3 composite by high energy reactive milling

In-situ synthesis of WC-Al₂O₃ composite by milling and its subsequent heat treatment were investigated. Mixtures of Al, W, and C with stoichiometric ratio of W₃AlC₂ were ball milled up to 20 h. Then, the 20-hour ball milled powder was heat treated at different temperatures of 900 and 1200 °C. The reaction path was investigated by X-ray diffractometry (XRD). The particle size and microstructure of powders after milling was investigated by field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). Also, in order to analyze the heating behavior of 20 h ball milled powder mixture during heat treatment, simultaneous thermal analysis (STA) was used. The results showed that after milling for 20 h, the reactants reacted together and new phases including W₂C and (W,Al)C_1 − x were formed. After heat treatment, the semi-stable compounds synthesized at the milling stage, were transferred to more stable compounds including WC and Al₂O₃.     

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.     

Mechanical properties of Mo-Nb-Si-B quaternary alloy fabricated by powder metallurgical method

Mo-Si-B alloys composed of two intermetallic compound phases (Mo₅SiB₂ and Mo₃Si) and a molybdenum solid solution matrix phase have been investigated for use as high-temperature structural materials due to their high melting point and good creep resistance. However, despite these advantages, Mo-Si-B alloys are difficult to use in practical applications because they have insufficient fracture toughness at room temperature. So, in many researches, microstructure control and the addition of other elements in the α-Mo matrix phase are conducted as an effective way to improve the fracture toughness.In this study, niobium (Nb) was added to a Mo-Si-B alloy by a powder metallurgical method to improve the mechanical properties. First, the Mo and Nb powders were pulverized by high-energy ball milling. Then, the synthesized intermetallic compound powders, which were fabricated by continuous heat treatment under a H₂ atmosphere, were mixed with ball-milled Mo and Nb powder. Pressureless sintering was conducted at 1400 °C for 3 h under a H₂ atmosphere. The Vickers hardness and fracture toughness were measured to investigate the mechanical properties of the sintered Mo-Si-B and Mo-Nb-Si-B alloy. The Vickers hardness was about 425 Hv for a Mo-Nb-Si-B alloy, which was lower value of 165 Hv compared to Mo-Si-B alloy (590 Hv). On the other hand, the fracture toughness of the Mo-Nb-Si-B alloy (14.5 MPa·√m) greatly increased compared to that of the Mo-Si-B alloy (12.6 MPa·√m).     

Mechanochemical synthesis of TiN/TiB2/Ti-silicide nanocomposite powders and their thermal stability

Extremely fine and homogeneous TiN/TiB₂/Ti-silicide composite powders have been synthesized from mixtures of Ti, BN and Si₃N₄ powders by high-energy ball milling through a mechanically activated self-sustaining reaction. They have a microstructure consisting of TiN and TiB₂ crystallites of less than 15 nm embedded in amorphous TiSi₂ or Ti₅Si₃ matrix. When these nanocomposite powders were annealed at high temperatures, the microstructure did not change significantly and TiN and TiB₂ mutually suppressed the grain growth of both phases effectively.     

Reduction of hydrogen desorption temperature of ball-milled MgH2 by NbF5 addition

Enhanced sorption properties of ball-milled MgH₂ are reported by adding NbF₅. Among various catalyst amounts, 2 mol% of NbF₅ reveals to be the optimum concentration leading to significant reduction of the desorption temperature as well as faster kinetics of ball-milled MgH₂. At 200 °C, temperature at which MgH₂ does not show any activity, MgH_{2NbF5/2 mol%} composite desorbs 3.2 wt.% of H₂ in 50 mins. Interestingly, the addition of NbF₅ is also associated with an increase in the desorption pressure. At 300 °C, MgH_{2NbF5/2 mol%} composite starts to desorb hydrogen at 600 mbar in comparison with 1 mbar for MgH₂. Further improvements were successfully achieved by pre-grinding NbF₅ prior to ball-milling the catalyst with MgH₂. Such pre-ground NbF₅ catalyzed MgH₂ composite desorbs 3 wt.% of H₂ at 150 °C. Improved properties are associated with smaller activation energies down to values close to the enthalpy of formation of MgH₂. Finally, the mechanism at the origin of the enhancement is discussed in terms of catalyst stability, MgF₂ formation and electronic density localization.     

LaMg11 with a giant unit cell synthesized by hydrogen metallurgy: Crystal structure and hydrogenation behavior

We have successfully applied a hydrogen metallurgy synthesis to yield LaMg_12?x intermetallic at 500 °C. During a vacuum thermal desorption from the LaH₃–MgH₂ nanocomposite prepared by reactive ball milling in H₂, hydrogen desorption from the La dihydride occurred via a mechanism of cooperative phase transformation at temperatures being 400° lower than the desorption from pure LaH₂. The crystal structure of the intermetallic was determined by synchrotron X-ray diffraction (SR XRD). The orthorhombic LaMg_12?x has a giant unit cell, the volume of which exceeds 8000 Å³. In situ SR XRD studies showed the fine details of the hydrogen desorption process with several parallel and consecutive transformation steps.     

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.