Hydrogen sorption properties of Ti–Cr alloys synthesized by ball milling and cold rolling

Three Ti–Cr alloys with nominal compositions of TiCr_x (x = 2, 1.8 and 1.5) were synthesized by cold rolling and ball milling of as-cast ingots, and their microstructures and hydrogenation properties were studied. X-ray diffraction showed that TiCr_x transformed from a mixture of C14 and C15 Laves phases to a metastable BCC phase after 5 h of ball milling under argon. Cold rolling did not lead to the formation of a metastable BCC phase but only to the reduction of TiCr_x size particles under 20 nm. Surprisingly, the hydrogen absorption/desorption curves of cold rolled and ball milled samples at 323 K were quite similar. This result proves that hydrogen storage properties do not depend only on microstructure and that cold rolling could be an interesting method to synthesize hydrogen storage materials.     

Microstructure and Physical Properties of Tb<Subscript>2</Subscript>TiO<Subscript>5</Subscript> Neutron Absorber Synthesized by Ball Milling and Sintering

Tb₂TiO₅ neutron absorber was synthesized by ball milling and sintering. Microstructure character of ball-milled Tb₄O₇-17.605%TiO₂ (mass fraction, %) powders and sintered bulks was analyzed using XRD, SEM and TEM. The microhardness, coefficient of thermal expansion and thermal conductivity of sintered bulks were measured. The experiment results showed that the nanocrystalline solid solution was obtained during ball milling. After 96 h of ball milling, TiO₂ was completely solved in Tb₄O₇ and the crystal size of Tb₄O₇ was up to 37 nm. The bulk materials prepared by cold isostatic pressing were sintered at 1300 °C. Tb₂TiO₅ bulks with an orthorhombic structure were obtained. The microhardness of sintered bulks, as well as the thermal conductivity, increased firstly with increasing ball milling time and then decreased. The coefficient of thermal expansion decreased initially and then increased with increasing ball milling time. For the sintered bulk with powder milled for 48 h, the highest values of both microhardness and thermal conductivity were observed, whereas the lowest coefficient of thermal expansion was exhibited. In addition, with increasing testing temperature, the thermal conductivity of sintered bulks initially fell and then rebounded while an opposite trend was found in the coefficient of thermal expansion.     

Mechanical alloying characteristic and thermal stability of Al<Subscript>78</Subscript>Fe<Subscript>20</Subscript>Zr<Subscript>2</Subscript> amorphous powders

The powders of pure Al, Fe, and Zr for preparing Al₇₈Fe₂₀Zr₂ were subject to a high-energy planetary ball milling. The microstructure evolution of the mixtures at the different intervals of milling was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was found that a nearly complete amorphization could be achieved in the mixtures after ball milling for 23 h. Further ball milling led to the crystallization of the amorphous powders. A long time ball milling, e.g., 160 h, led to a complete crystallization of the amorphous powders and the formation of Al₃Zr and Al1₃Fe₄. The crystallization products caused by ball milling are almost the same as that produced by isothermal annealing of the amorphous powders in vacuum at 800 K for 1 h.     

Microstructure and hardness of Mg-based composites reinforced with Mg<Subscript>2</Subscript>Si particles

Magnesium powders were mechanically alloyed with SiO₂ powder particles having different particle sizes using high-energy ball milling techniques under Ar atmosphere for 1 h. The powders were consolidated with cold pressing under 560 MPa. They were then sintered at 550°C for 45 min under Ar atmosphere. The composites obtained on the Mg-SiO₂ system were investigated using the Archimedes principle, a differential scanning calorimeter, X-ray diffraction, optic microscopy, and scanning electron microscopy. For the mechanically alloyed powders, the solid-state reaction of the synthesis of Mg₂Si and MgO progressed further during sintering of the materials. The results showed that the strengthening mechanisms were dependent on dispersion hardening of fine Mg₂Si and MgO particulates dispersed homogeneously in the matrix.     

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.     

Mechano-thermal treatment of TiO<Subscript>2</Subscript>-Al powder mixture to prepare TiAl/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite

In this research, a composite comprising an intermetallic matrix and dispersed Al₂O₃ particles was processed. A mixture of TiO₂ and Al was mechanically activated in the presence of a process control agent and/or without it, in a high-energy planetary ball mill. As a subsequent process, the sample was sintered at various temperatures. The phase composition and morphology of the samples were evaluated by XRD and SEM techniques, respectively. The thermal behavior of the samples milled for 8 h with PVA and/or without it, were also assessed by the DTA technique and compared with one another. The DTA results revealed that addition of PVA shifted the aluminothermic reduction of TiO₂ to higher temperatures; therefore, final composite phases were developed at higher temperatures. The results also showed that addition of PVA during milling caused the final microstructure to coarsen. The XRD pattern of the sample sintered at 700 °C exhibits the existence of TiAl, Ti₃Al, and Al₂O₃ phases. In the sample sintered at 850 °C, the remaining Ti₃Al peak was attenuated and completely disappeared at 1000 °C.     

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.     

Effect of Milling Time on Electrical Breakdown Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

In order to clarify the relationship between the microstructure and the arc erosion behavior of metal-matrix composite, Al₂O₃/Cu composites with different distributions of Al₂O₃ particles were prepared by high energy ball milling and powder metallurgy. The effect of milling time on microstructure, properties, and arc erosion behavior of Al₂O₃/Cu composite was investigated. The results show that the distribution of Al₂O₃ particles improves significantly with increase of milling time, but Al₂O₃ particles will be aggregated if milling time is too long. The optimal milling time is 24 h in the range of experiments. A uniform distribution of Al₂O₃ particles in copper matrix can improve the hardness, electrical conductivity, average breakdown strength, chopping level, and arc life. With improvement in the distribution of Al₂O₃ particles, the erosion area becomes larger, and the erosion pits become shallower and are dispersed more uniformly.     

吸、放氢循环对V及V0.9Cr0.1合金储氢性能的影响

通过添加少量LaNi5对V以及V0.9Cr0.1合金进行有控制的机械合金化处理,改善了材料的活化性能.吸、放氢循环测试表明,吸、放氢循环超过100次并脱氢后,V和V0.9Cr0.1合金仍然保持单一bcc结构.随着吸、放氢循环的进行,V的吸、放氢平台压力不断增加,相同压力下吸氢量减小,且吸氢动力学有所下降.这与吸、放氢过程中晶格应变的降低和晶胞体积的减小有关.SEM观察表明,随着吸、放氢循环次数的增加,材料表面和内部形成大量的深裂纹,这对提高放氢平台压力和改善放氢动力学有促进作用.吸、放氢循环过程中,V比V0.9Cr0.1合金放氢容量衰减更快的原因是:吸、放氢循环前、后V的晶格应变降低和晶胞体积下降程度均比V0.9Cr0.1合金的要大,导致吸、放氢循环后V的吸氢量下降更大.     

Mechanically Assisted Solid-State Mixing and Spark Plasma Sintering for Fabrication of Bulk Nanocomposite (WC/7(10Co/4Cr))-Based ZrO<Subscript>2</Subscript> Systems

Mechanically induced solid-state mixing, using high-energy ball milling technique, was employed for preparing WC/7 wt.% (10Cr/4Cr) solid-solution powders. The solid-solution powders obtained after 50 h of milling were mechanically mixed for 50 h together with small weight fractions (0-7 wt.%) of (ZrO₂ + 1.5 wt.% Y₂O₃) powders. The powders were then consolidated in vacuum under a uniaxial pressure of 30 MPa at 1250 °C, using spark plasma sintering. The consolidated bulk samples were nearly full dense and maintained their nanocrystalline structure after this consolidation step. The results showed that the consolidated samples over the entire range of ZrO₂ concentrations (0–7 wt.%) had low values for Young’s modulus (297–318 GPa) due to their nanocrystalline structures. Moreover, the WC/7 wt.% (10Cr/4Cr)/7(ZrO₂-1.5 mol.% Y₂O₃) showed excellent wear resistance, indexed by its low-value friction coefficient (~0.29).     

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

Synthesis of nano-structured La0.8Ba0.2MnO3 perovskite via a mechano-thermal route

In this study, barium-doped lanthanum manganite, La_0.8Ba_0.2MnO₃, was synthesized via a mechano-thermal route employing high energy ball milling and subsequent heat treatment. The structural evolution, morphology and thermal behaviour of the powders were evaluated using XRD, FESEM, and DTA/TGA, respectively. DTA/TGA results showed that the calcination temperature of the carbonates significantly decreased by increasing the milling time. The results revealed that single phase perovskite was formed at 900 °C in a milled sample for 2 h and this temperature decreased to 600 °C by increasing the milling time to 30 h. The mean crystallite size also decreased from 32 to 20 nm by increasing the milling time from 2 to 30 h. The reaction sequence of La_0.8Ba_0.2MnO₃ formation via the mechano-thermal route is proposed using XRD and DTA/TGA results. FESEM micrographs showed that the mean particle size of the perovskite phase is increased slightly from 30 to 40 nm by increasing the heat treatment temperature from 600 to 900 °C.     

Synthesis, structural and magnetic characterization of nanocrystalline nickel ferrite—NiFe2O4 obtained by reactive milling

Nanocrystalline nickel ferrite (NiFe₂O₄) has been synthesized from a stoichiometric mixture of oxides NiO and α-Fe₂O₃ in a high energy planetary mill. An annealing at 350 °C, after milling, was used to improve the solid state reaction. The obtained powders were investigated by X-ray diffraction, magnetic measurements, scanning electron microscopy, X-ray microanalysis and differential scanning calorimetry. The particles size distribution was analyzed using a laser particle size analyser. The nickel ferrite begins to form after 4 h of milling and continuously form up to 16 h of milling. The obtained nickel ferrite has many inhomogeneities and a distorted spinel structure. The mean crystallites size at the final time of milling is 9 ± 2 nm and the lattice parameter increases with increase the milling time. DSC measurements revealed a large exothermic peak associated with cations reordering in the crystalline structure. The magnetization of the obtained powder depends on the milling time and annealing. After the complete reaction between the starting oxides the milling reduces the magnetization of the samples. The magnetization increases after annealing, due to the reorganization of the cations into the spinel structure.     

The Key Role of Ball Milling Time in the Microstructure and Mechanical Property of Ni-TiC<Subscript>NP</Subscript> Composites

Titanium carbide nanoparticle-reinforced nickel-based alloys (Ni-TiC_NP composites) with ball milling time ranging from 8 to 72 h were prepared by ball milling and spark plasma sintering. Transmission electron microscopy (TEM) and scanning electron microscopy equipped with electron backscatter diffraction were used to characterize the microstructures. Their hardness and tensile properties were measured using the Vickers pyramid method and tensile tests. TEM results showed that a slight coarsening of TiC_NP occurred during the ball milling process. The grain sizes of the Ni-TiC_NP composites with various ball milling times were different, but they were all much smaller than those of the pure Ni. In all cases, the Ni-TiC_NP composites showed higher strengths and hardness values than the unreinforced pure nickel. Furthermore, the strength of the Ni-TiC_NP composites increased initially and then decreased as a function of ball milling time. The maximum strengths occurred in the 24-h ball milling sample, which presented the lowest average grain size. The Hall-Petch strengthening was suggested to be the main reason responsible for such variations in mechanical properties. Additionally, the elongation percentage of the Ni-TiC_NP composites decreased gradually with ball milling time. This may be caused by the change of microvoids in the composite as the ball milling time varies, which is also related to their fracture behavior.     

Microstructure and Adhesion Strength of Ni<Subscript>3</Subscript>Ti Coating Prepared by Mechanical Alloying and HVOF

In the present work we report the development of Ni₃Ti intermetallic compound by high energy ball milling of Ni and Ti powders. The ball milled powders were taken at various intervals (4, 6, 8, 10, and 11 h) to analyze the formation of Ni _x Ti _x intermetallic compounds. The ball milled powders were analyzed using scanning electron microscopy and X-ray diffraction. The layered shaped powder particles of Ni₃Ti phase were formed after 11 h of ball milling, which was confirmed by X-ray peaks. Further High-Velocity Oxy-Fuel (HVOF) process was used to coat Ni₃Ti and Ni₃Ti + (Cr₃C₂ + 20NiCr) on MDN 420 steel. Both the coated materials displayed excellent cohesion with minimal porosity less than 2%. The tensile adhesion strength test was carried out on these coatings to check the bond strength. Out of the two the Ni₃Ti coating showed excellent bond strength of 41.04 MPa compared to that of Ni₃Ti + (Cr₃C₂ + 20NiCr) coating.     

A wet ball milling treatment of Zr-based AB2 alloys as negative electrode materials

A wet ball milling fluorination treatment was successfully developed to improve rate discharge capabilities of Zr-based AB₂ alloys. In this work, treatment conditions such as constituents in the treating solution, ball milling speed and time were optimized. It was found that the Ni-rich surface layer formed through the Ni²⁺ reduction by alloy component elements played an important role in the initial activation property. The size of alloy powder particles can be easily adjusted by changing the ball milling speed and time. Although a small size of alloy powder particle is beneficial for quick kinetics, excessive pulverization resulted in a decreased capacity at low discharge rates.     

Thermal Spray Deposition,Phase Stability and Mechanical Properties of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>/LaAlO<Subscript>3</Subscript> Coatings

This paper deals with the deposition of La₂Zr₂O₇ (LZO) and LaAlO₃ (LAO) mixtures by air plasma spray (APS). The raw material for thermal spray, single phase LZO and LAO in a 70:30 mol.% ratio mixture was prepared from commercial metallic oxides by high-energy ball milling (HEBM) and high-temperature solid-state reaction. The HEBM synthesis route, followed by a spray-drying process, successfully produced spherical agglomerates with adequate size distribution and powder-flow properties for feeding an APS system. The as-sprayed coating consisted mainly of a crystalline LZO matrix and partially crystalline LAO, which resulted from the high cooling rate experienced by the molten particles as they impact the substrate. The coatings were annealed at 1100 °C to promote recrystallization of the LAO phase. The reduced elastic modulus and hardness, measured by nanoindentation, increased from 124.1 to 174.7 GPa and from 11.3 to 14.4 GPa, respectively, after the annealing treatment. These values are higher than those reported for YSZ coatings; however, the fracture toughness (K _IC) of the annealed coating was only 1.04 MPa m^0.5.     

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.     

Nanocrystalline ZrO<Subscript>2</Subscript> preparation and kinetics research of phase transition

The precursor of nanocrystalline ZrO2 was synthesized by solid-state reaction at low heat using ZrOCl2·8H2O,and Na2CO3·10H2O as raw materials.The nanocrystalline ZrO2 was obtained by calcining the precursor.The precursor and its calcined products were characterized using TG/DTA,FT-IR,XRD,and SEM.The results showed that the precursor dried at 353 K was a zirconyl carbonate compound.When the precursor was calcined at 673 K for 150 min,highly crystallization ZrO2 with tetragonal structure (space group P42/nmc (137)) was obtained with a crystallite size of 24 nm.However,when the precursor was calcined at 1023 K for 150 min,highly crystallization ZrO2 with monoclinic structure (space group P21/c (14)) was obtained with a crystallite size of 20 nm.The mechanism and kinetics of the thermal process of the precursor were studied using DTA and XRD techniques.Based on the Kissinger and Arrhenius equation,the values of the activation energies associated with the thermal process of the precursor were determined to be 26.80 and 566.73 kJ·mol-1 for the first and third steps,respectively.The mechanism of ZrO2 phase transition from tetragonal to monoclinic structure is the random nucleation and growth of nuclei reaction.     

球磨Mg-Ni非晶在热处理过程中的相转变规律

采用差热分析仪（DTA）、带高温附件的X-射线衍射仪（XRD）等研究了球磨Mg-Ni样品的组织结构、热稳定性能及其在加热过程中的相转变规律。结果表明,经400r／min球磨40h后,样品由非晶体组成,其热稳定性能随着对磨过程传递到样品中能量的增加而降低。当球磨Mg-Ni非晶样品的能量较低时,其DTA曲线上只出现一个放热峰,在该温度加热时将发生晶化反应生成Mg2Ni和MgNi2相;当样品的能量较高时则出现两个放热峰,在其对应的初始温度之间加热时则逐步晶化生成Mg2Ni相,在第二个放热峰加热时,样品中的非晶体转变成MgNi2晶体相。