As-milled Samples Research Articles

Improved hydrogen storage performance of as-milled Sm–Mg–Ni alloy by adding CeO2

To investigate the influence of adding CeO2 on the hydrogen storage characteristics of Sm–Mg–Ni-based SmMg11Ni-type alloy, mechanical milling was utilized to synthesize SmMg11Ni and SmMg11Ni + 5 wt.% CeO2 (named SmMg11Ni–5CeO2) alloys. The microstructure of as-cast and as-milled samples was measured via X-ray diffractometer and transmission electron microscope. Sieverts device was utilized to measure the isothermal hydriding and dehydriding kinetics. The non-isothermal dehydrogenation performance was explored by thermogravimetry and differential scanning calorimetry. The hydrogen desorption activation energy of the compound metal hydride can be computed by both Arrhenius and Kissinger methods. The related data show that adding CeO2 can engender a very slight influence on the hydrogen storage thermodynamics, but it can result in an obvious reduction in hydrogen absorption and desorption capacities. Furthermore, the hydrogen desorption performance of experimental alloys is conspicuously ameliorated by the addition of CeO2, viz. lowering the initial hydrogen desorption temperature and enhancing hydrogen desorption rate. The hydrogen desorption activation energies with and without CeO2 addition are 84.28 and 100.31 kJ/mol, respectively, with an obvious decrease of 16.03 kJ/mol. This is thought to be responsible for the ameliorated hydrogen desorption kinetics by adding CeO2.

Microstructure and Properties of Mg-Zn-Y Alloy Powder Compacted by Equal Channel Angular Pressing.

Mg97Zn1Y2 (at %) alloy with a long period stacking ordered (LPSO) phase has attracted a great deal of attention due to its excellent mechanical properties. It has been reported that this alloy could be fabricated by warm extrusion of rapid solidified alloy powders. In this study, an alternative route combining mechanical milling and equal channel angular pressing (ECAP) was selected to produce the bulk Mg97Zn1Y2 alloy. Microstructural characterization, mechanical properties and corrosion behavior of the ECAP-compacted alloys were studied. The as-cast alloy contained α-Mg and LPSO-Mg12Zn1Y1 phase. In the as-milled powder, the LPSO phase decomposed and formed Mg24Y5 phase. The ECAP-compacted alloy had identical phases to those of the as-milled sample. The compacted alloy exhibited a hardness of 120 HV and a compressive yield strength of 308 MPa, which were higher than those of the as-cast counterpart. The compacted alloy had better corrosion resistance, which was attributed to the reduced volume fraction of the secondary phase resulting in lower microgalvanic corrosion in the compacted alloy. The increase in Y content in the α-Mg matrix also contributed to the improvement of corrosion resistance.

Cluster-glass-like behavior in zinc ferrite nanograins

Structural and magnetic properties of ZnFe2O4 nanograins, prepared by high-energy ball milling and annealed, were systematically studied by X-ray diffraction, 57Fe Mössbauer spectroscopy and AC magnetic susceptibility measurements. Disordered spinel-like structure, with a grain size of 12 nm, is established after 200 h of milling. While the 300 K Mössbauer spectrum of the as-milled sample (200 h) displays broad magnetic absorption lines, characteristic of a disordered system, the magnetization data do not show a magnetic phase transition between 4 and 300 K. At low temperatures, the Mössbauer spectra suggest the presence of two distinct ferrite magnetic phases: one attributed to the grain core (crystalline-like phase), with magnetic ordering temperature of about 90 K, and one showing a magnetic hyperfine field distribution; the latter is associated with a chemically disordered phase (grain boundary contributions). Annealing the 200 h sample at 973 K leads to an improvement of atomic ordering of the spinel structure (reduction of cationic inversion) and average grain size of about 17 nm. AC magnetic susceptibility shows a cusp at about T≈30K, whilst Mössbauer experiments in the same sample reveal magnetic blocking in the same temperature range. The frequency dependence of susceptibility suggests the formation of a cluster-glass-like state. High temperature susceptibility can be described with a Fulcher law of interacting magnetic clusters.

Structural, Thermal, Optical, and Photoacoustic Study of Mechanically Alloyed Nanocrystalline SnTe

A nanostructured SnTe phase was produced by mechanical alloying after 2 h of milling. Part of the as-milled powder was annealed and its X-ray diffraction (XRD) pattern was recorded. The XRD patterns of the as-milled and annealed samples were refined using the Rietveld method. After annealing, partial decomposition of the SnTe phase was observed and corroborated by estimating the mean crystallite size using a Willianson-Hall plot. The Cowley-Warren parameter αSnTeCWfor the first coordination shell was calculated, showing a preference for homopolar pairs. This preference is consistent with the partial decomposition observed. According to the optical absorbance spectra, the band gap energy is inversely proportional to crystallite size, following a decaying exponential function. From the photoacoustic absorption spectroscopy measurements, the thermal diffusivity parameter and the transport properties of as-milled and annealed SnTe powder were determined.

Influence of milling media on the structure and agglomeration behaviour of some hardmetal powder

ABSTRACTThis paper presents the influence of milling media on the structure and morphology of WC–Co powders for sintered tools. WC–Co powders have been milled in two different organic media, acetone and isohexane. The characterisation of the as-milled samples has been done by X-ray diffraction, scanning electron microscopy (SEM) and differential scanning calorimetry. The mean crystallite size decreases with milling time for the samples processed in acetone, while in the case of isohexane, the crystallite size stays at a relatively constant value when increasing the milling duration. The calculated mean crystallite size is below 50 nm for any milling duration, with larger values for acetone (16–46 nm) as compared to isohexane samples (10–12 nm). The SEM revealed that the dried powder is composed of agglomerated submicronic particles, and at high magnification, the presence of nanoparticles is noticed. The density of the aggregates was linked to the polarity of the solvents, which influences the sedimentation kinetics.

GaN decomposition after high-energy ball milling and its transformation to BN hollow spheres in annealing samples doped with B

In this work, GaN powders doped with B and Si were treated by high-energy ball milling, and subsequently, annealing at different temperatures. Transmission electron microscopy (TEM) observations showed the in-situ GaN decomposition in the undoped GaN specimen milled for 5 h. TEM also revealed that the chemical decomposition of GaN was more affected in samples doped with B. Thermo-gravimetric analysis indicates that an undoped GaN specimen decomposes at 1120 °C, Si-doped GaN samples disintegrate at 1150 °C, and finally, B-doped GaN samples decompose at the lower temperature of 940 °C. TEM and SEM results show that the annealing treatment of the as-milled B-doped GaN samples at 1150 °C for 15 min leads to the GaN decomposition and BN hollow nanostructures formation under an N2 atmosphere. The synthesized product consists of mesoporous powders with variable particle sizes from 500 nm to 1 μm having variable cavities in the range from 60 nm to 800 nm.

Comparison of the Hydrogen Release Properties of Zn(BH4)2-Added MgH2 Alloy and Zn(BH4)2 and Ni-Added MgH2 Alloy

Samples with compositions of 99 w/o MgH2+1 w/o Zn(BH4)2 (designated MgH2-1Zn(BH4)2) and 96 w/o MgH2+2 w/o Zn(BH4)2+2 w/o Ni (designated MgH2-2Zn(BH4)2-2Ni) were prepared by milling in a planetary ball mill in a hydrogen atmosphere (reaction-involving milling). The hydrogen release properties of MgH2- 1Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni were compared. A larger quantity of additives and the addition of Ni decreased the temperature at which MgH2 decomposes in the as-milled samples. Activation processes were not required for these two samples. A larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased the quantity of hydrogen absorbed in 60 min (U (60 min)), the initial hydrogen release rate, and the quantity of hydrogen released in 60 min (R (60 min)). MgH2-2Zn(BH4)2-2Ni had an effective hydrogen storage capacity of about 5.5 w/o at the cycle number, CN, of one (CN=1). A ZnH2 phase, which has not been reported to be formed, was found in the MgH2-1Zn(BH4)2 sample and the Zn(BH4)2 and Ni-added MgH2 sample after hydrogen uptake-release cycling. Mg2Ni was formed in the Zn(BH4)2 and Niadded MgH2 sample after hydrogen uptake-release cycling. The rapid hydrogen release rate of the Mg2NiH4 formed in MgH2-2Zn(BH4)2-2Ni is believed to increase the initial hydrogen release rate of MgH2-2Zn(BH4)2-2Ni. The Mg2Ni decomposed from Mg2NiH4 might facilitate the nucleation of a Mg-H solid solution in the MgH2-2Zn(BH4)2-2Ni sample. There is also a slight possibility that the contraction due to the relatively rapid hydrogen release of the Mg2NiH4 provides passages for the hydrogen released from neighboring MgH2. (Received December 20, 2017; Accepted January 9, 2018)

Synthesis of LiNiO2 by two-step solid-state method

Abstract LiNiO2 was prepared through two-step solid-state reaction by mechanochemical method and heat treatment, using LiOH (Li2CO3) and Ni(OH)2 as starting materials. The influence of grinding speed and time, heat treatment time, and starting materials on the structure of LiNiO2 was studied. The as-milled samples and products were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that uniform as-milled samples can be obtained at grinding speed of 580 rpm for 0.5 h, using LiOH and Ni(OH)2 as raw materials. Perfect crystal LiNiO2 has been prepared by calcining the as-milled samples at 700 °C for 15 h. Composite material powders consisting of Li2Ni8O10 and LiNiO2 have been obtained using Li2CO3 as lithium source.

Processing and Characterization of Ag-Cu-Sn Brazing Alloy Prepared by a Mechanical Alloying Method

Mechanical alloying was used to prepare an Ag-Cu-Sn brazing alloy with the low amounts of intermetallic compounds. X-ray diffraction and differential scanning calorimetry were carried out to investigate the phase transformation of the as-milled samples. It seemed that the intermetallic compounds experienced the stage of synthesis-decomposition-resynthesis in the ball milling process. By controlling the ball milling time and tin contents, the solid solution phases of Ag (Cu, Sn) and Ag (Cu) were achieved in the 60Ag30Cu10Sn sample after milling for 40 h. Although the intermetallic compound will be generated during the brazing process, the improvement of their distributed morphology is beneficial for the good bonding interface of the joint with stainless steel substrate.

Mechanical Alloying and Hot Pressing of Ti-Zr-Si-B Powder Mixtures

This work discusses microstructure evolution during ball milling and hot pressing of Ti-xZr-10Si-5B (x = 2 and 5 at. %) and Ti-xZr-20Si-10B (x = 5, 7, 10, 15 and 20 at. %) powder mixtures. Mechanical alloying was carried out in a ball mill using stainless steel balls and vials, 300 rpm and a ball-to-powder ratio of 10:1. Powders milled for 600 min were then hot-pressed (25 MPa) under vacuum at 1100 °C for 60 min. As-milled and hot-pressed samples were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and energy dispersive spectrometry (EDS). Peaks of Si and Zr disappeared in powders milled for 60 and 180 min, respectively, while the lattice parameters and cell volume of α-Ti were varied during ball milling up to 300 min indicating that supersaturated solid solutions were achieved. Ti6Si2B dissolving up to 10 at. % Zr was found in microstructure of hot-pressed Ti-xZr-10Si-5B (x = 2 and 5 at. %) and Ti-xZr-20Si-10B (x = 2, 5, 7 and 10 at. %) alloys. The amount of TiB and Ti5Si3 was preferentially increased whereas the Ti3Si formed in microstructure of the hot-pressed Ti-15Zr-20Si-5B and Ti-20Zr-20Si-10B alloys.

Nanosized tetragonal β-FeSe phase obtained by mechanical alloying: structural, microstructural, magnetic and electrical characterization.

Nanocrystalline tetragonal β-FeSe phase was prepared mechanochemically using ball milling procedures in an inert atmosphere, starting from FexSe powder mixtures with x = 1.00, 1.25 and 1.50, with x = 1.25 and 1.50 leading to more than 93% of pure phase after annealing at 400 °C for 1 hour under vacuum. X-ray powder diffraction provides information on phase formation and phase transitions with milling time and temperature. The Rietveld method was used to refine the crystal structure, including the z coordinate of Se and occupancies, to determine the microstructure and to assess the amount of contaminant phases observed. Lattice contraction is found in the ab-plane more than along the c-axis, the small average size of crystalline domains (<22 nm) and the high microstrain (>1%) indicate the formation of highly strained nanoparticles. Magnetic and electrical characterization showed a poor superconductivity at 4 K and semiconducting properties only for thermally treated samples. These observations are explained by the presence of ferromagnetic impurity phases (residual Fe, hexagonal δ-FeSe phase and monoclinic Fe3Se4), but other effects caused by the mechanochemical synthesis must be considered, such as small average size, large/non-uniform size distribution and high microstrain of the nanosized tetragonal β-FeSe phase. The increase of the β-FeSe phase content with increasing storage time (ageing) above a few days to months in air, at RT and in the dark was observed for all as-milled samples. Preliminary data on the ageing effect are shown while a systematic study on this is in progress and will be presented elsewhere.

Structural and Magnetic Properties of Ordered/Disordered Sendust Powders Prepared by Mechanical Alloying

In this study, Sendust alloy (Fe85Si9.6Al5.4) was fabricated by mechanical alloying process, and its crystal structure and magnetic properties were investigated. Sendust alloy has excellent soft magnetic properties and is widely used in different applications. Raw materials (Fe, Si, and Al) were milled by a planetary ball mill at different milling times, from 5 to 20 h. The resulted alloys were annealed at 1373 and 1473 K for 2 h, and the effect of annealing temperature was investigated on the magnetic properties. Crystal structure of the samples was investigated by X-ray diffractometery method. The results indicated that saturation magnetization of 10-h milled sample was at maximum, and it decreased both by increasing milling time and by annealing. These variations were discussed based on phase transformation from ordered to disordered phases and/or vice versa. The maximum saturation magnetization obtained, 156 emu/g, was comparable by those reported for Sendust alloy, which was prepared by other methods. Magnetization–temperature curves of the alloys and annealed ones were recorded by a Faraday balance. Two clear drops were observed on each M-T curve of the as-milled samples that displayed the existence of two different magnetic phases. On the other hand, there is just one drop on the M-T curves of the annealed samples, indicating that these samples were in a less disordered phase.

High temperature stabilization of a nanostructured Cu-Y2O3 composite through microalloying with Ti

The effects of minor additions (0.2, 0.4 and 0.8wt%) of Ti on the thermal stability of both Cu nanograins and Y2O3 particles in nanostructured Cu-5vol%Y2O3 composite powder particles were investigated via 1h isochronal annealing at temperatures ranging from 300 to 1000°C. It was found that a small amount addition of 0.4wt%Ti effectively inhibits the coarsening of the Y2O3 particles during annealing at a very high homologous temperature of 0.87Tm, where Tm is the melting point of Cu, which, in turn, stabilizes Cu nanograins and retains the hardness value of the as-milled powder sample. However, this is in clear contrast with the significant decrease in hardness of the Ti-free and 0.2wt%Ti doped milled powder samples annealed at the same condition, resulting from the coarsening of the Y2O3 particles and growth of Cu nanograins. The energy dispersive X-ray spectrometry elemental analysis shows that the stabilizing mechanisms responsible for the improved thermal stability of the Y2O3 particles are associated with the chemical reactions between Ti, O and Y2O3.

Influence of aqueous milling duration on the sintered WC–10Co hard metal powders

This paper presents the effects of aqueous processing of WC–Co powders on their sinterability and mechanical properties. WC–Co powders have been milled in aqueous media (distilled water + corrosion inhibitor). The as-milled samples have been compacted and sintered in vacuum at 1380 °C. The investigations performed on the two components, carbide and binder, reveal different oxidation behaviour in water, with a further effect on the properties of the sintered product. The decrease of the saturation magnetisation of the compacts corroborated with the microstructure reveals the oxidation of the samples milled for a longer duration. The magnetic properties, hardness and microstructure suggest that the used corrosion inhibitor is effective for short milling durations.

MnFe2O4 nanopowder synthesised via a simple hydrothermal method for promoting hydrogen sorption from MgH2

The effects of MnFe2O4 nanopowder synthesised via a simple ‘hydrothermal’ method on the hydrogen storage properties of MgH2 are investigated for the first time. The particle size of the as-synthesised MnFe2O4 nanoparticles is determined to be about 10 nm. We observe that MnFe2O4 catalyst decreases the decomposition temperature of MgH2 and enhances the sorption kinetics. Interestingly, the onset hydrogen desorption temperature of 10 wt% MnFe2O4-doped MgH2 sample gets lowered from 350 °C to 240 °C with faster kinetics, and the sample shows an average dehydrogenation rate 8–9 times faster than that of the as-milled MgH2 sample. By adding 10 wt% of as-prepared MnFe2O4 to MgH2, approximately 5.5 wt% hydrogen can be absorbed in 10 min at 200 °C. In contrast, the un-doped MgH2 sample absorbed only 4.0 wt% hydrogen in the same period of time. From the Kissinger analysis, the apparent activation energy for hydrogen released in the MnFe2O4-added MgH2 composite is found to be 108.42 kJ/mol, which is much lower than the activation energy for hydrogen released in the as-milled MgH2 (146.57 kJ/mol). It is believed that the in situ formed Fe particle and Mn-containing phases together play a synergistic role in remarkably improving MgH2 storage properties.

Synthesis by High-Energy Ball Milling of MgH&lt;sub&gt;2&lt;/sub&gt;-TiFe Composites for Hydrogen Storage

The aim of this work is to investigate the influence of some processes variables on the microstructure and hydrogen absorption kinetics of MgH2 - X wt.% TiFe composites. Samples were synthesized by high-energy ball milling in a planetary (X = 40, 50, 60) and shaker mill (X = 40) under high-purity argon atmosphere. Commercial MgH2 instead of Mg powder was used in order to reduce adherence on the vial and balls. TiFe powder was previously produced by ball milling a mixture of TiH2 and Fe powders followed by a reaction synthesis at 600oC. Milled composites samples were characterized by XRD and SEM analysis. Milling time was preliminary investigated (X = 40) in the planetary ball mill (6 to 36h). TiFe particle size reduction was shown to be difficult since they are surrounded by MgH2 matrix. Strong particle reduction was obtained by using a shaker mill only for 2 hours and adding cyclohexane as process control agent. No reaction between MgH2 and TiFe compound was observed in any milled sample. Hydrogen absorption kinetics measurements of the as-milled samples were conducted on an Sieverts' type apparatus at room temperature after hydrogen desorption at 350oC under vacuum. The best hydrogen kinetics (3 wt% at the first hour) was attained by the planetary milled sample (36 h). Higher hydrogen capacity was observed for the sample milled in the shaker mill (4.0 wt.%), but only after 13h.

Catalytic effect of SrFe12O19 on the hydrogen storage properties of LiAlH4

The effects of a SrFe12O19 addition on the hydrogen storage properties of LiAlH4 were investigated for the first time, to the best of our knowledge. The addition of the SrFe12O19 show improved dehydrogenation performance compared with that of undoped LiAlH4. In fact, the onset desorption temperatures decreased from 140 °C to 80 °C in the first reaction stage and from 180 °C to 135 °C in the second reaction stage. In terms of the dehydrogenation kinetics, the 10 wt% SrFe12O19-added LiAlH4 sample released approximately 5.54 wt% hydrogen at 130 °C after 20 min desorption, whereas the as-milled LiAlH4 sample released less than 0.3 wt% hydrogen in the same period of time and temperature. Kissinger analysis indicates that the apparent activation energies of the first two dehydrogenation reactions of LiAlH4 are decreased by approximately 27 and 15 kJ/mol, respectively, after the addition of SrFe12O19. It is reasonable to conclude that the formation of an in situ active species of Fe and LiFeO2 and amorphous Sr or Sr-containing phases could play a catalytic role in enhancing the dehydrogenation performance of LiAlH4.

Metallothermic reduction of zinc sulfide induced by ball milling

Stoichiometric mixtures of ZnS + Al and ZnS + Mg were milled for different times in a planetary ball mill. The XRD traces of the as-milled samples showed the presence of zinc, MgZn2, and MgS after 30-min milling in the ZnS–Mg system. The traces of MgZn2 disappeared after 1-h milling and the reduction reaction seemed to have been completed after 5-h milling. The ZnS–Al system was somewhat different with only slight reduction to zinc after 1 h and ZnS peaks still present after 10 h of milling. Isothermal heating under argon atmosphere of 3-h-milled samples showed the presence of hexagonal ZnAl2S4 and mixtures of MgS and Zn0.68Mg0.32S in the ZnS–Al and ZnS–Mg systems, respectively. These results show that the reaction in the ZnS–Al system progressed gradually during milling. The decrease in the crystallite size of reactants materials (especially ZnS) during milling operation led to decrease in the formation temperature of hexagonal ZnAl2S4 phase and decrease in the transformation temperature of sphalerite (ZnS) to hexagonal wurtzite.

Microstructures and properties of equiatomic CuZr and CuZrAlTiNi bulk alloys fabricated by mechanical alloying and spark plasma sintering

Equiatomic CuZr and CuZrAlTiNi bulk alloys were designed and fabricated by mechanical (MA) alloying and spark plasma sintering (SPS). The results show that during the MA process, the phases of as-milled samples transform from solid solutions to amorphous phases with increasing milling time. Moreover, the as-milled CuZrAlTiNi amorphous high entropy powders possess higher thermal stability and smaller particle sizes than the as-obtained CuZr powders. After SPS, the bulk CuZrAlTiNi samples are comprised of three solid solutions and a AlNi2Zr phase, and have good phase stability. The CuZrAlTiNi sample sintered at 950 °C shows the higher microhardness value of 1007 HV0.2 due to the fine precipitates, the existence of AlNi2Zr phase and solid solution strengthening. In addition, the chemical corrosion behavior of the as-sintered samples has been investigated in the seawater solution. The 950 °C-sintered CuZrAlTiNi sample exhibits excellent passivity and higher pitting resistance as compared to other sintered samples. All the CuZr and CuZrAlTiNi bulk samples show better corrosion resistance than 304 stainless steel revealed by the reduced corrosion rate.

Post-anneal effect on the structural and Li+ conduction properties in NaI - LiBH4 system

ABSTRACTIn the present work, NaI – LiBH4 system fabricated by ball milling were post annealed and their variation of Li+ ion conductivity were investigated. From the change of lattice parameters by post annealing, it was suggested that unreacted LiBH4 existed in as-milled sample further dissolved into NaI, which implied an enhancement of the sample homogeneity. On the other hand, the segregation of LiI was anticipated when ball milled 15NaI·LiI (BH4 free sample) was annealed at 423 K. Li+ conductivity was decreased by post anneal process and compositional dependence of an activation energy for Li+ conduction was indicated for the homogeneous NaI – LiBH4 system.