Microwave irradiation effects on reversible hydrogen desorption in sodium aluminum hydrides (NaAlH4)

The effect of microwave irradiation on the reversible desorption reaction in sodium aluminum hydride (NaAlH₄) is explored. NaAlH₄ is doped with 2 mol% TiCl₂ and pre-activated by high energy ball milling and aging to show the presence of metallic aluminum phase. As a catalyst, Ti²⁺ has been used to improve desorption kinetics in sodium alanate. X-ray diffraction was performed on the samples exposed to microwave irradiation for 10, 20, 30, 40 and 50 min. Results show that when the powders show the presence of aluminum, a steady increase in the formation of the hexahydride (Na₃AlH₆) phase and Al occurs during microwave irradiation; and is accompanied by a steady reduction in the NaAlH₄ phase XRD peak (h k l) intensities. This data suggests that microwave irradiation drives the reversible H₂ desorption reaction in NaAlH₄. NaAlH₄ doped with 2 mol% TiCl₂ which does not show the presence of Al phase, undergoes a reduction in NaAlH₄ peak intensities with increasing microwave exposure (and no reversible product phases are detected in this case). Dielectric studies on NaAlH₄ indicate that microwave penetration is low. Therefore, it is proposed that microwave irradiation heating of the Al particulate phase is responsible for the hydrogen desorption reaction pathway which is similar to that of conventional heating.     

TEM characterization of pure and transition metal enhanced NaAlH4

Possibilities and limitations in using transmission electron microscopy to characterize pure NaAlH₄ and transition metal enhanced NaAlH₄ have been investigated in detail. NaAlH₄ is extremely sensitive to O₂ and H₂O and must be handled under inert atmosphere at all times. Furthermore, it is highly unstable under the electron beam and only basic techniques such as diffraction contrast imaging and selected area diffraction that can be performed with a low flux electron beam can be used without the NaAlH₄ decomposing. By comparison, phases containing transition metal additive are very stable under the electron beam. The latter are investigated by a combination of high resolution imaging, electron diffraction and spectroscopy to determine distribution, composition, crystal structure and defect content in ball milled and hydrogen cycled, TiCl₃ and FeCl₃ enhanced NaAlH₄. It is demonstrated that a large amount of the added Ti or Fe is located at the surface of the NaAlH₄ grains as a combination of crystalline and amorphous Al_1−xTM_x (TM = Ti, Fe) nanoparticles.     

Structural and magnetic properties of nanostructured Fe50(Co50)-6.5 wt% Si powder prepared by high energy ball milling

This work investigates the effects of 6.5 wt% Si addition and milling times on the structural and magnetic properties of Fe₅₀Co₅₀ powders. For this purpose, at first the elemental Fe and Co powders were milled for 10 h to produce Fe₅₀Co₅₀ alloy and then Si was added and the new product was milled again for different times. The microstructural and magnetic properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The results show that the minimum crystallite size of the as-milled powders (∼12 nm) has been achieved after introducing Si and milled for 8 h (total milling time of 18 h). Also an amount of 188 emu/g has been achieved for M_s. This amount of M_s is higher than most of those which have been already reported for M_s of different Fe-Si systems.     

Synthesis of nanocomposite hydrides for solid-state hydrogen storage by controlled mechanical milling techniques

The following composite hydride systems: NaBH₄–MgH₂, MgH₂–LiAlH₄, MgH₂–VH_0.81 and MgH₂–NaAlH₄, were synthesized in a wide range of compositions by controlled reactive/mechanical (ball) milling in a magneto-mill. In effect, composites having nanometric grain sizes of the constituent phases (nanocomposites) were produced. It is shown that the hydrogen desorption temperature of the composite constituent with the higher desorption temperature in the systems such as NaBH₄ + MgH₂, MgH₂ + VH_0.81 and MgH₂ + LiAlH₄ substantially decreases linearly with increasing volume fraction of the constituent having lower desorption temperature according to the well-known composite rule-of-mixtures (ROM). It is also shown that the ROM behavior can break down due to an ineffective milling of a composite. The composite system MgH₂ + NaAlH₄ does not obey the ROM behavior.     

Synthesis behavior of nanocrystalline Al-Al2O3 composite during low time mechanical milling process

In this work, four different volume fractions of Al₂O₃ (10, 20, 30 and 40 vol.%) were mixed with the fine Al powder and the powder blends were milled for 5 h. Scanning electron microscopy analysis, particle size analysis and bulk density measurements were used to investigate the morphological changes and achieving the steady state conditions. The results showed that increasing the Al₂O₃ content can provide the steady state particle size in 5 h milling process. It was found that increasing the volume fraction of Al₂O₃ leads to increasing the uniformity of Al₂O₃. Standard deviations of microhardness measurements confirmed this result. The XRD pattern and XRF investigations depicted that increasing the Al₂O₃ content causes an increase in the crystal defects, micro-strain and Fe contamination during 5 h milling process of nanocrystalline composite powders while the grain size is decreased. To investigate the effect of milling time, Al-30 vol.% Al₂O₃ (which achieved steady state during 5 h milling process) was milled for 1-4 h. The results depicted that the milling time lower than 5 h, do not achieve to steady state conditions.     

Development of nano-structure Cu-Zr alloys by the mechanical alloying process

Cu-Zr alloys have many applications in electrical and welding industries for their high strength and high electrical and thermal conductivities. These alloys are among age-hardenable alloys with capability of having nano-structure with high solute contents obtainable by the mechanical alloying process. In the present work, Cu-Zr alloys have been developed by the mechanical alloying process. Pure copper powders with different amounts of 1, 3 and 6 wt% of commercial pure zirconium powders were mixed. The powder mixtures were milled in a planetary ball mill for different milling times of 4, 12, 48 and 96 h. Ball mill velocity was 250 rpm and ball to powder weight ratio was 10:1. Ethanol was used as process control agent (PCA). The milling atmosphere was protected by argon gas to prevent the oxidation of powders. The milled powders were analysed by XRD technique and were also investigated by SEM observations. Lattice parameters, crystal sizes and internal strains were calculated using XRD data and Williamson-Hall equation. Results showed that the lattice parameter of copper increased with increasing milling time. The microstructure of milled powder particles became finer at longer milling time towards nano-scale structure. SEM observations showed that powder particles took plate-like shapes. Their average size increased initially and reached a maximum value then it decreased at longer milling times. Different zirconium contents had interesting effects on the behavior of powder mixtures during milling.     

Hydrogen storage properties of cold rolled magnesium hydrides with oxides catalysts

In this work, we compared ball milling and cold rolling as a mean to add transition metal oxides to magnesium hydride. We found that irrespective to the mixing technique the oxides NiO and Nb₂O₅ gave the fastest desorption kinetics. In general, sorption kinetics were slightly slower for cold rolled samples compared to their ball milled counterparts. Ball milling 30 min is more effective to get a nanocrystalline structure than rolling 5 times. However, as rolling was performed in air for a limited number of times, it could be expected that rolling under inert atmosphere for a larger number of times will be as effective as ball milling to produce nanocrystalline structure and enhance hydrogen storage properties.     

The effects of graphite on the reversible hydrogen storage of nanostructured lithium amide and lithium hydride (LiNH2 + 1.2LiH) system

The addition of 5 wt.% of graphite was incorporated into the (LiNH₂ + 1.2LiH) hydride system in order to study its effect on the prevention of LiH from hydrolysis/oxidation which leads to the escape of NH₃. The composite hydride system was processed by ball milling for 25 h. Thermal behavior in DSC up to 500 °C and isothermal desorption in a Sieverts-type apparatus were carried out. XRD was used to obtain information about phase changes. It is found that after ball milling graphite becomes amorphous. DSC analysis shows that for the mixture ((LiNH₂ + 1.2LiH) + 5 wt.% graphite) graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH since no melting peak of retained LiNH₂ is observed. Both the DSC and Sieverts-type tests show that the addition of graphite increases the apparent activation energy of desorption from the ∼57-58 to ∼85-90 kJ/mol range. On the other hand, the graphite additive increases measurably the desorbed/absorbed capacity of hydrogen at 275, 300 and 325 °C. The ((LiNH₂ + 1.2LiH) + 5 wt.% graphite) system is fully reversible desorbing/absorbing ∼5 wt.% H₂ at 325 °C in the following reaction: (LiNH₂ + LiH ↔ Li₂NH + H₂). Step-wise pressure-composition-temperature (PCT) tests show that the enthalpy and entropy change of this reversible reaction is −62.4 and −61.0 kJ/mol H₂ and 117.8 and 115.8 J/mol K for undoped and 5 wt.% G doped (LiNH₂ + 1.2LiH) system, respectively. It shows that within an experimental error there is no measurable effect of graphite additive on the thermodynamic properties. The Van’t Hoff analysis of the obtained thermodynamic data shows that the equilibrium temperature at atmospheric pressure of hydrogen (1 bar H₂) is 256.8 and 253.9 °C for the undoped and 5 wt.% G doped (LiNH₂ + 1.2 LiH) system ball milled for 25 h, respectively. Such high equilibrium temperatures render it rather obvious that both of these hydride systems cannot be employed for hydrogen desorption/absorption below 100 °C as required by the DOE targets for the automotive hydrogen storage materials.     

Microstructure and desorption properties study of catalyzed NaAlH4

NaAlH₄ catalyzed by Ce(SO₄)₂ and LaCl₃ have been studied by PCT (Pressure-Content-Temperature) experiment and SEM (Scanning Electron Microscope) test method. The results show that doping with Ce(SO₄)₂ and LaCl₃ increases markedly the desorption amount of NaAlH₄. In the first desorption stage, NaAlH₄ doped with LaCl₃ display larger amount of hydrogen release than NaAlH₄ doped with Ce(SO₄)₂, while, the desorption rate of the latter is obviously faster than the former. SEM analysis shows that heating could make NaAlH₄ form a kind of porous structure. The further study indicates that different dopants have different effects on the microstructure of NaAlH₄.     

Role of milling time and Ni content on dehydrogenation behavior of MgH2/Ni composite

The effects of ball milling time and Ni content on the dehydrogenation performance of MgH₂/Ni composite were systematically investigated. The structural evolution of ball milled MgH₂+x%Ni (x=0, 2, 4, 8, 20, 30, mass fraction) samples during mechanical milling process and dehydrogenation properties were investigated by a series of experimental techniques. The results show that the desorption kinetics is independent of particle size, grain size and defects as the temperature is above 380 ^oC. The desorption kinetics is improved by prolonged milling time due to refined and uniformly distributed Ni. The formation of Mg₂Ni after dehydrogenation is proposed to explain the degradation of hydrogen storage properties of MgH₂ during de-/hydrogenation cycling process. The desorption activation energy of MgH₂ decreases with the increase of Ni content due to the catalytic effect of Ni. It is found Ni favors the nucleation of magnesium phase and accelerates the recombination of hydrogen atoms.     

Addition of catalysts to magnesium hydride by means of cold rolling

In this paper we compare the techniques of cold rolling and ball milling as a mean to synthesized nanocomposites MgH₂ + 2 at.%X where X = Co, Cr, Cu, Fe, Mn, Nb, Ni, Ti, V. High energy ball milling was performed under argon for 30 min while cold rolling was done in air with a number of roll limited to five. Particle and crystallite sizes are smaller in the ball milled compounds than in the cold rolled ones. The hydrogen sorption kinetics of the ball milled compounds was also faster than the cold rolled samples but not by a wide margin. Because cold rolling was done in air, this limited the number of rolls that could be performed. It is expected that rolling under protective atmosphere will enable a higher number of rolls and better sorption kinetics.     

Effects of mechanical activation of MoO3/C powder mixture in the processing of nano-crystalline molybdenum

Nano-crystalline molybdenum with a mean crystallite size of 45 nm was synthesized through reduction of MoO₃ by carbon using high energy ball milling and subsequent heat treatment. XRD, DTA/TG and SEM techniques were employed to evaluate the powder particle characteristics. It was found that initial mechanical activation at an ambient temperature did not result in the reduction of MoO₃ to molybdenum. DTA/TG results showed that in a sample milled for 25 h, MoO₂ and Mo₂C were formed at 485 and 925 °C, respectively during carbo-thermal reduction of MoO₃. At the final stage, Mo₂C reacts with the remaining MoO₂ at 1030 °C to produce nano-crystalline molybdenum.     

Mechanochemical synthesis and XPS analysis of sodium alanate with different additives

NaAlH₄ was mechanochemically synthesized under high hydrogen pressure. Additives can be added to NaAlH₄ directly during the milling process. In situ monitoring of the pressure and temperature was used to provide an insight into the reactions occurring in the vial during synthesis—a process so far based purely on empirical considerations. This method was then applied to directly compare different additives during synthesis. The following trend was obtained for the effectiveness of the additives: TiCl₃ > CeCl₃ > ScCl₃ ? Ti. This trend was also verified for desorption by differential scanning calorimetry and by monitoring decomposition at room temperature during long-term storage. In addition, the direct comparison by X-ray photoelectron spectroscopy of the chemical state of the additive after milling illustrated indirectly the importance of the reduction of the chloride—and the formation of NaCl—during synthesis. These reactions can be linked to the formation of vacancies necessary for fast diffusion of hydrogenated species within the material.     

Bulk Mg-3Al-Zn alloy with ultrafine grain size produced by powder metallurgy

Ultrafine-grained Mg-3Al-Zn alloys with an average grain size of 180 nm have been made by powder metallurgy. First, the nanocrystalline powders with mean grain size of 45 nm were produced by ball milling under argon atmosphere, and then through vacuum hot pressing at 633 K for 40 min and warm extrusion at 373 K, bulk solid samples were compacted successfully from the mechanically milled powders, and the relative density of the samples was about 98.87% (1.8003 g/cm³). XRD, SEM and TEM analysis showed that the microstructure of the samples consists of homogeneous equiaxed grains and grain growth has taken place during the consolidation process.     

Effect of mechanical activation on the production of SiC from silica sand

In this study the effect of 100 and 200 h low energy ball milling on the carbothermic reduction of SiO₂ and C powder mixture was investigated. Microstructure studies of the mixture by SEM revealed that the particle size had been decreased and the SiO₂ particles had been covered by C particles due to the milling. The results of thermal analysis (TG-DTA) of milled and unmilled mixtures clearly showed that the reduction temperature decreased due to milling process. XRD pattern of 200 h activated mixture proved that β-SiC had been formed almost completely after reduction at 1500 °C.     

The TiCl3 catalyst in NaAlH4 for hydrogen storage induces grain refinement and impacts on hydrogen vacancy formation

TiCl₃ acts as an efficient catalyst for NaAlH₄ (sodium alanate), altering its hydrogen sorption kinetics and reversibility considerably. In order to clarify its role, we performed in situ neutron diffraction experiments on protonated catalysed and uncatalysed NaAlH₄. The phase transformations were monitored in the first two reaction steps during hydrogen release and in the second step during reloading. Our study for the first time provides clear indications that both Ti_xAl_1−x and NaCl formed act as grain refiner for Al and NaH, respectively, preventing particle growth. Particle sizes generally stay small upon desorption and reloading of TiCl₃ catalysed NaAlH₄, while significant particle growth is observed for uncatalysed NaAlH₄. The small crystallite sizes and observed hydrogen vacancy formation greatly facilitate the mass transfer during loading and unloading. This study underlines the importance of grain refining for achieving reversibility and faster kinetics of the hydrogen sorption processes, with a crucial double role played by the catalyst.     

The cycle stability of Mg-based nanostructured materials

Two nanostructured Mg-based composites MgH₂–5 wt.% LaNi₅–5 wt.% Ti and MgH₂–5 wt.% TiFeMn–5 wt.% V have been synthesised by ball milling with a milling time of 40 h. The influence of prolonged cycling on the hydriding/dehydriding properties has been investigated by more than 500 absorption–desorption cycles using the isobaric method. Upon cycling, MgH₂–5 wt.% LaNi₅–5 wt.% Ti showed very good stability both in kinetics and in storage capacity. For the material MgH₂–5 wt.% TiFeMn–5 wt.% V, the absorption kinetics slowed down continuously during cycling, whereas the desorption kinetics hardly changed.     

Synthesis of α-Mo-Mo5SiB2-Mo3Si nanocomposite powders by two-step mechanical alloying and subsequent heat treatment

A two-step mechanical alloying process followed by heat treatment was developed as a novel approach for fabrication of Mo-12.5 mol%Si-25 mol%B nanocomposite powders. In this regard, a Si-43.62 wt.% B powder mixture was milled for 20 h. Then, Mo was added to the mechanically alloyed Si-B powders in order to achieve Mo-12.5 mol%Si-25 mol%B powder. This powder mixture was further milled for 2,5,10 and 20 h. All of the milled powders were annealed at 1100 °C for 1 h. After first step of milling, a nanocomposite structure composed of boron particles embedded in Si matrix was formed. On the other hand, an α-Mo/MoSi₂ nanocomposite was produced after second step while no ternary phases between Mo, Si and B were formed. At this stage, the subsequent annealing led to formation of α-Mo and Mo₅SiB₂ as major phases. The phase evolutions during heat treatment of powders can be affected by milling conditions. It should be mentioned that the desirable intermetallic phases were not formed during heat treatment of unmilled powders. On the other hand, α-Mo-Mo₅SiB₂-Mo₃Si nanocomposites were formed after annealing of powders milled for 22 h. With increasing milling time (at the second step), the formation of Mo₃Si during subsequent heat treatment was disturbed. Here, an α-Mo-Mo₅SiB₂-MoSi₂ nanocomposite was formed after annealing of 30 and 40 h milled powders.     

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.     

The aluminothermic reduction of boric acid

The effect of metallic aluminium powder on the production of boron carbide–alumina composite was studied. Boric acid, carbon and aluminium powders were mixed in stoichiometric ratio, ball milled and heat treated at temperatures between 1300 and 1650 °C for 1–5 h in the presence of argon flow. Depending on the ratio of boron oxide to carbon, the formation of boron carbide by the carbothermal reduction, was possible at a temperature of around 1500 °C, but with the addition of metallic aluminium to the mixture of boric acid and carbon, the carbide formation temperature was reduced at least 300 °C. At 1300 °C, B₄C was the major phase with alumina in the reaction products. The liquid–solid reaction mechanism, which occurred during the aluminothermic process, had a specific influence on the formation of boron carbide.