Increase In Milling Time Research Articles

Investigation of Microhardness, Microstructural, Tribological, and Thermal Properties of Al7075/TiO2/Kaoline Hybrid Metal Matrix Composites Produced by Powder Metallurgy Process

The shift from traditional, single‐component metals and alloys continues, and researchers are currently investigating alternative materials. This evolution has led to the creation of innovative materials, including hybrid metal‐matrix composites. The present study aims to investigate the microstructural, surface morphological, and thermal properties of a novel Al7075/TiO2/Kaoline hybrid metal‐matrix composite prepared using high‐energy ball milling and sintering methods. In this study, phases related to the composite components are formed depending on the milling time, no undesirable phase is formed, but the peak intensities decreas as the milling time increases. Particle size increases from 63 to 215 μm with increasing milling time. Increasing the kaoline reinforcement ratio and sintering temperature increases the microhardness from 62.27 ± 2 to 75.83 ± 2 HV, and reduces the friction coefficient from 0.82 ± 0.01 to 0.62 ± 0.01. The wear rate of the composite without kaoline addition is 2.1 (mm3 m−1) × 10−3, while with 6 wt% kaoline addition, it decreases to 1.5 (mm3 m−1) × 10−3. There are no cracks in the composite other than plastic deformation due to sintering and wear. Peaks indicating endothermic and exothermic reactions during continuous heating occurr in the 635–750 °C temperature range.

Mechanical processing of wet stored fly ash for use as a cement component in concrete

Wet storage effects on fly ash mean that processing may be necessary to achieve the physical properties required for use in concrete. This paper considers drying, de-agglomeration and milling of various wet stored fly ashes at laboratory and pilot/benchtop scales, towards meeting these. In the laboratory, different batch quantities and milling times with as-received/pre-screened materials were examined using a ball mill. Greater particle size reductions were obtained with increased milling time but at gradually reducing rates. Pre-screening and batch quantity had relatively minor effects on particle size reductions, with small differences also found between wet and dry stored fly ash. Extended milling time resulted in a darkening of colour; slight increases in loss-on-ignition, the main oxides content and crystalline components; reductions in water requirement (to a point); and greater reactivity. Similar effects were generally noted in concrete for the superplasticising admixture dose to achieve a target slump and compressive (cube) strength. At pilot/benchtop scale, a dryer-pulveriser and spiral jet mill were used, which gave general agreement with the behaviour noted in the laboratory, but with the effects tending to be less. Fineness levels in Standards were achievable, with subsequent performance in concrete, appearing to depend on the milling process used.

The influence of ball milling conditions on the powder characteristics and sintering densification of Mo[sbnd]Cu alloy

Mechanical alloying was utilized to produce Mo-30Cu(wt%) composite powder. The effects of milling conditions on the powder characteristics and sintering densification were investigated. The morphological evolution during the mechanical alloying process can be categorized into four stages: 1-individual Mo and Cu particles, 2-coexistence of Mo particles and blocky MoCu composite particles, 3-coarse irregular composite particles, and 4-refined near-spherical composite particles or lamellar MoCu composite particles, depending on whether process control agent (PCA) is added. The mechanical alloying degree of the MoCu composite powder was deepened gradually from stage 1 to 4, which is influenced by the ball milling parameters. As the milling speed and milling time increase, the lattice strain and the alloying degree of the MoCu composite powders increase while the grain size of molybdenum particles decreases, which is conducive to accelerating the sintering density. Among these ball milling parameters, an elevated milling speed notably accelerates the mechanical alloying process and the sintering density. Under the milling speed of 600 r/min, ball to powder ratio (BPR) of 10:1, and milling time of 4 h, the grain size, lattice strain, and average particle diameter of the composite powder were measured to be 48.4 nm, 0.247 %, and 21.04 μm, respectively, with the particle morphology being nearly spherical. The sintered density of the MoCu composite reached 98.1 %, larger than the other composites.

High Milling Time Influence on the Phase Stability and Electrical Properties of Fe50Mn35Sn15 Heusler Alloy Obtained by Mechanical Alloying.

Fe50Mn35Sn15 Heusler alloy, obtained by mechanical alloying, was subjected to larger milling times in the range of 30-50 h to study the phase stability and morphology. X-ray diffraction studies have shown that the milled samples crystallise in a disordered A2 structure. The A2 structure was found to be stable in the milling range studied, contrary to the computation studies performed on this composition. Using Rietveld refinements, the lattice parameter, mean crystallite size, and lattice strain were computed. The nature of the obtained phases by milling was found to be nanocrystalline with values below 50 nm. A linear increase rate of 0.00713 (h-1) was computed for lattice strain as the milling time increased. As the milling time increases, the lattice parameter of the cubic Heusler was found to have a decreasing behaviour, reaching 2.9517 Å at 50 h of milling. The morphological and elemental distribution-characterised by scanning electron microscopy and energy-dispersive X-ray spectroscopy-evidenced Mn and Sn phase clustering. As the milling time increased, the morphology of the sample was found to change. The Mn and Sn cluster size was quantified by elemental line profile. Electrical resistivity evolution with milling time was analysed, showing a peak for 40 h of milling time.

An assessment of the mechanically alloyed equiatomic FeCoNiMnSi high entropy amorphous alloy for non-isothermal crystallization kinetics and magnetocaloric refrigeration application

We studied an equiatomic FeCoNiMnSi high entropy amorphous alloy successfully processed via mechanical alloying technique. The structural, magnetic, and magnetocaloric characteristics of the resulting materials were examined using X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and a vibrating sample magnetometer. The structural analysis showed a prominent amorphous phase formation as the milling time increases from t = 0–35h of mechanical alloying. The differential scanning calorimetry was employed to investigate the non-isothermal crystallization kinetics of the 35-h milled sample. The results revealed that the milled powder consists of a single exothermic peak with its apparent activation energy (Eg, Ex, Ep) being determined using the Kissinger, Ozawa, and Augis-Bennett equations. The findings exhibit Ex > Ep > Eg, representing that nucleation is more complicated than the growth mechanism during the crystallization process. Meanwhile, under non-isothermal conditions, the Kissinger-Akahira-Sunose, Friedman, and Flynn-Wall-Ozawa models were calculated using the local activation energies that agree with the apparent activation energy. Furthermore, the Johson-Mehl-Avrami-Kolmogorov exponent (n) and Avrami-Ozawa combined [F(T)] models exhibit high-dimensional nucleation and growth with an increasing nucleation rate enabled by lowering the local activation energies as a function of degree of conversion with respect to temperature. In magnetic measurements, a feasible mathematical model was proposed as a novel strategy for predicting the value of saturation magnetization as a milling time function. The model has an exceptional predictive capability, confirmed by fitting other research outcomes. Finally, the proposed system also delivers the best magnetocaloric properties with a maximum magnetic entropy change of 3.70 Jkg−1 K−1 at 150 K curie temperature and a refrigeration capacity of 252.86 J/kg with 1000 Oe applied magnetic field.

Structure, mechanical, and neutron radiation shielding characteristics of mechanically milled nanostructured (100-x)Al-xGd2O3 metal composites

Metallic substances are favored for storing and transporting spent nuclear fuels (SNFs) due to their thermal stability, mechanical strength, and corrosion resistance. Adding gadolinium (Gd) significantly enhances neutron shielding, improving efficiency with reduced thickness and dimensions. In this study, Al–Gd2O3 metal nanocomposites were synthesized using powder-processed mechanical milling. The crystal structure features of (100-x)Al-x%Gd2O3 samples with varying percentages of Gd2O3 (5 %, 10 %, and 15 % by weight) were analyzed by X-ray diffraction. Microstructural properties were studied using scanning and transmission electron microscopy (SEM and TEM). The synthesized composites' neutron and γ-radiation shielding characteristics were investigated theoretically using the MCNP6.2 transport code. Analysis of the XRD data revealed prominent peak shifts with increased milling time, indicating reduced particle sizes. Differential scanning calorimetry (DSC) indicated Gd2O3-induced changes in temperature behavior, and the Vickers microhardness tests demonstrated enhanced mechanical strength with increasing gadolinium concentration. Improved neutron radiation shielding was observed with higher Gd2O3 content. Our study highlights the potential of Gd2O3-enriched metal matrix composites for advanced industrial applications.

Terahertz time-domain spectroscopy as a novel tool for crystallographic analysis in cellulose: tracking lattice changes following physical treatments

The authors’ series of studies aimed to explore the potential of terahertz time-domain spectroscopy (THz-TDS) in cellulose crystallographic studies, since THz radiation can detect most intermolecular vibrations and respond to lattice phonons. In this study, we tracked changes in four types of cellulose after ball milling. As the planetary ball milling time increases, it is observed through electron microscopy that the four types of cellulose particles are gradually destroyed into finer particles, while gel permeation chromatography can prove that the molecular weight gradually decreases after ball milling and the dispersity gradually approaches one, which indicates that the dispersion of the material was reduced. The most fascinating observation was made by THz-TDS, that is we have confirmed that after ball milling, the absorption characteristics of cellulose I and II in cellulose I treated with 10% NaOH (crystalline partial transition from cellulose I to II) exhibited an opposite trend. Specifically, the absorption of cellulose II at 2.40THz and 2.77THz increased, while the absorption of cellulose I at 2.11THz and 3.04THz decreased after ball milling, which suggests an increased conversion rate of cellulose I to cellulose II post-milling. Cellulose with different crystalline allomorphs shows different characteristic absorption in the THz region, and the peak position will not change even after the ball milling, only the absorption intensity changes. Although it can be observed through the most traditional X-ray diffraction method that the crystallinity index of all cellulose samples gradually decreases after ball milling. However, different from the THz results, the change after ball milling of cellulose I treated with 10% NaOH is only reflected in very subtle pattern changes, that is, the peak close to the 200 crystalline plane position is slightly shifted after ball milling.

New structural features and pinning-evolved coercivity mechanism: A potential route for developing high coercivities in anisotropic Sm-Fe-N material

Sm-Fe-N material holds immense promise as new material for developing next generation permanent magnets. However, the achievement of high coercivity in this material remains a big challenge, and moreover new coercivity theories beyond the commonly considered nucleation-type mechanism needs exploration. This study describes both the yielding of anisotropic Sm-Fe-N powders with high coercivities and the discovery of new coercivity mechanism by a systematic study using a designed milling preparation method. As a result, a random-direction cleavage behavior with unexpected XRD peak shifts and formation of new micron-sized flower-like particles were found in the yielded Sm-Fe-N powders. The grain sizes of the powders are of nanoscale dimension and they refine gradually as the milling time increases. The measured coercivity is up to 15.0 kOe, which is a substantial increase of approximately 36.4% compared to the original commercial powders and among the highest reported values via the milling preparation method (commonly below 13 kOe). The yielded powders also demonstrate a new dual-type coercivity mechanism (combing both nucleation-type and pinning-type control effect), and the improved pining effect contributes to the high coercivities. This study opens the door of utilizing pinning effect for achieving high coercivities in anisotropic Sm-Fe-N material.

Effect of ball milling on the structure and electrochemical hydrogen storage properties of a RE-Mg-Ni alloy

RE-Mg-Ni alloy is considered a highly promising anode material for the next generation of MH-Ni batteries. However, the low cycle life, fast performance, and high manufacturing cost of this ideal MH-Ni battery anode material have become obstacles to its optimization. In this work, an as-cast La0.7Sm0.3MgNi3.6Co0.4 superlattice alloy was prepared by vacuum medium frequency induction melting. Then, La0.7Sm0.3MgNi3.6Co0.4 + 10 wt% Ni (0–2 h) composite hydrogen storage alloys with excellent properties were prepared by adding a catalyst (Ni) and conducting ball milling. XRD, SEM, and HRTEM were conducted to analyse the phase compositions, microstructures and macrostructures of the samples. The experimental results showed that with the increase in ball milling time, the microstructural characteristics of the alloy changed, such as the phase abundance, grain refinement, particle size reduction, and amorphous structure increase, directly leading to the change in electrochemical performance. The discharge capacity (Cmax) of the sample decayed from 264.0 mAh/g to 219.1 mAh/g, the cycle stability (Sn) gradually increased with the extension of the grinding time, and the rate discharge performance first increased and then decreased. When the grinding time was 1 h, the best performance was HRD900 = 36.53%. The critical factors affecting the dynamic performance, such as the charge transfer resistance (Rct), limiting current density (IL), and hydrogen diffusion coefficient (D), all exhibited the same trend and reached their respective optimal values at 1 h.

Mekanik alaşımlama ile üretilen Al4.5Cu/TiO2 kompozitlerinin mikroyapı, sertlik ve termal özellikleri

Al4.5Cu/TiO2 composites were fabricated from their elemental powders by the mechanical alloying method. Microstructural and thermal properties of the composites were investigated by a combination of differential thermal analysis (DTA), scanning electron microscopy with energy dispersive X-ray detection (SEM-EDX), and X-ray diffraction (XRD). Microstructural evolutions, phase transformations, and crystallite size changes were investigated depending on the milling time. XRD and SEM results showed that there were a more homogeneous structure and shrinkage in grain size due to the increased milling time. The DTA results showed an endothermic peak of around 650 oC which indicates the melting temperature of Al. Besides, the mechanical properties of the pressed and sintered composites were investigated by Vickers micro-hardness testing. The results showed that microhardness values significantly increased as milling time increased from 5h to 10h. The maximum microhardness value of 173±10 HV was obtained for Al4.5Cu with 20 wt% TiO2 composite after milling for 10h.

Mechanical hydroxyapatite coatings on PEO-treated Ti–6Al–4V alloy for enhancing implant's surface bioactivity

In this study, to reactivate the oxide film and to secure biocompatibility, plasma electrolytic oxidation (PEO) treatment and hydroxyapatite (HA) deposition via mechanical coating (MC) processes were investigated. This experimental approach involved subjecting the Ti–6Al–4V alloy specimens to PEO treatment, followed by MC with HA powder for different hours as HA has the potential to increase bioactivity and surface characteristics due to its chemical composition and structural properties similar to natural bone. Therefore, the study aimed to examine the effects of MC with HA powder for 0, 1, 3, 5, 7, and 9 h on PEO-coated Ti–6Al–4V alloy discs for analyzing the surface characterization, phase analysis, surface roughness, wettability, adhesion strength, hardness, corrosion behavior, and cell proliferation properties for dental implant use. The findings demonstrate that prolonged HA milling enhances the substrate's mechanical strength, corrosion resistance, and adhesion properties. The presence of prominent anatase and HA peaks, particularly in the 5 MC-PEO, 7MC-PEO, and 9 MC-PEO samples, validates the synergistic impact of the PEO oxide layer and HA milling, resulting in outstanding corrosion resistance and biocompatibility. Cell culture analysis reveals a positive correlation between increased milling time and enhanced cell proliferation, particularly towards small-sized HA particles.

Preparation of serpentine fiber/poly (vinyl alcohol) aerogel with high compressive strength to stabilize phase change materials for thermal energy storage

The applications of paraffin used as phase change materials (PCMs) in energy saving buildings have been greatly limited due to its inherent leakage problem. Herein, a novel serpentine/poly(vinyl alcohol) (SER/PVA) composite aerogel was fabricated by incorporation of SER fibers treated with ball milling into PVA via self-assembly and freeze-drying, which has high compressive strength and can be used as a supporting material to encapsulate paraffin and prevent its leakage. The results show that the aerogel has excellent mechanical properties, the shrinkage of aerogels prepared with SER after ball milling was reduced from 42.8% to 5.0%, and its compressive strength can reach 6.30 MPa at 80% strain. With the increase of the ball milling time, the compressive strength of the composite aerogel increase. Then, the SER/PVA@P composite PCMs were prepared by vacuum impregnation with paraffin (P) into SER/PVA. The loading mass of paraffin in the SER/PVA@P composite PCMs is up to 89.14%, resulting in a superior latent heat storage capacity of 184.8 J/g in the melting process. The leakage-proof test shows that the SER/PVA@P appears no melting paraffin trace evenly heating at 60 °C for 10 min. The composite PCMs have excellent thermal reliability over 400 thermal cycles. The thermal conductivity of SER/PVA@P reached 0.237 W/(m∙K), which was lower than that of pure paraffin. Therefore, the prepared SER/PVA@P composite PCMs with high latent heat storage capacity, lower thermal conductivity, excellent thermal reliability, and good leakage-proof demonstrate a potential application in energy saving buildings.

Tuning the bandgap of 2D metallic Zn nanostructures

The semiconducting behavior of two-dimensional (2D) metal nanostructures has recently attracted much interest for their possible applications in optoelectronics and others. In particular, tuning the bandgap of such nanostructures can open up a new avenue for fabricating functional nano-devices. In the present article, we report the synthesis of 2D metallic Zn nanosheets at room temperature using a ball mill, which is capable of producing large-scale materials in a single run. Initially, nanoplates were formed for ball milling the octahedral-shaped Zn nanoparticles for the time of milling of 6 h. Subsequent ball milling for another 6 h leads these nanoplates to nearly uniform nanosheets. The thickness of these 2D nanostructures was found to decrease with an increase in the time of milling. Visible photoluminescence (PL) emissions centered at ∼3, ∼2.9, and ∼2.75 eV were observed from all the Zn particles showing semiconductor behavior. The origin of such semiconductor behavior was explained based on the radiative transition of electrons from the sp band to the upper states of the 3d band. This argument was confirmed through the studies of photoelectron spectroscopy and the first principle calculations employing density functional theory (DFT). Furthermore, excitation-dependent PL studies indicated that the bandgap of the 2D Zn nanostructures decreased with the increase in the ball milling time. Therefore, a redshift in the bandgap was observed with the increase in the ball milling time. Such changes in the bandgap with the thickness of 2D Zn nanostructures were also verified from the studies of DFT. Thus, the present study demonstrated that the bandgap of 2D metallic Zn nanostructures could be effectively tuned by reducing the thickness.

Processing and characterization of porous composites based on CaAl4O7/CaZrO3

According to previous research, CaAl4O7/CaZrO3 composites present good refractoriness, mechanical resistance and low thermal expansion coefficient (good thermal shock resistance), characteristics that insulating materials and lightweight structural components require. In this work, porous ceramics based on CaAl4O7/CaZrO3 were prepared from commercial powder combinations of m-ZrO2, Al(OH)3 and CaCO3. The powder mixture was processed by high-energy milling at different times (from 3 to 24 h). Different composites were obtained by solid state sintering at 1400 °C. The reaction was studied using thermal analysis techniques (TG-DTA), and the crystalline phases were analyzed by X-ray diffraction (XRD) and the Rietveld method. The resulting composites were also characterized. CaAl4O7 and CaZrO3 were the main phases present in these composites, with c-ZrO2 as secondary phase, and some scarce m-ZrO2. Porosity values varied from 54 to 59%, and flexural strengths ranged from 16.5 to 23.6 MPa. The increasing milling time produced an effective particle size reduction that accelerated the reactivity of the starting mixtures and reduced the grain size. The composite obtained by 24 h milling and sintered at 1400 °C (ACZ2414) presented the highest flexural strength and the lowest coefficient of linear thermal expansion. The mean thermal expansion coefficient of CaAl4O7/CaZrO3 composites was ca. 6 10−6 °C −1. This indicated that it is possible to manufacture ceramic bodies with expansion properties comparable to those of mullite, making it a suitable for insulation.

Electrochemical properties of mechanically milled Ni-20Cr for H2 oxidation in alkaline fuel cells

The electrochemical properties of Ni-20Cr alloy subject to high-energy mechanical milling were investigated to analyze its potential as a catalyst in fuel cells for hydrogen oxidation reaction (HOR) in an alkaline medium. The kinetic properties of Ni-20Cr alloy decorated with Pt are also reported. Results showed that as the milling time increases, the HOR kinetics improve. Pt particles grow mainly on the surface of highly deformed Ni-20Cr particles. Ni-20Cr and Ni-20Cr/Pt with 12 and 8 h of milling, respectively, are good candidates for HOR in an alkaline medium.

Preparation of α-Al2O3 by low-temperature calcining γ-Al2O3 with α-phase seed in-situ obtained by ball milling

Due to the high activation energy barrier and the sporadic nature of nucleation, the temperature required for the transition from cubic dense γ-Al2O3 to tripartite α-Al2O3 is as high as 1100–1200 °C. In this work, a two-step strategy was employed to reduce the phase transformation temperature, and prepare α-Al2O3 particles with good dispersion. As the first step, 10 nm sheet γ-Al2O3 was obtained by ammonium aluminum carbonate (AACH) decomposition, which was more favorable to the crystal transformation in the next ball-milling processing. In the second step, γ-Al2O3 powder with partial α-Al2O3 produced in situ by ball-milling γ-Al2O3 was calcined, and α-Al2O3 nanoparticles were obtained at just 850 °C. The content of the α-Al2O3 is determined by the Rietveld Refine method. The relationship between ball-milling time and α-Al2O3 content was clarified, and the effect of α-Al2O3 content on the transformation temperature and morphology of γ-Al2O3 was discussed. The results showed that the α-Al2O3 content rises with the increase of ball milling time, and spherical α-Al2O3 nanoparticles can be obtained when the α-Al2O3 content is 5 %. The addition of α-Al2O3 seeds reduces the phase transition temperature but agglomeration exists. Our proposed two-step low-temperature calcination strategy is an important method for the preparation of alumina nanoparticles

Mechanical and tribological properties of aluminium based metal matrix composites: An overview

This paper presents a comprehensive review of open literatures to segregate the effects of composite fabrication process parameters, reinforcement types and its contents on mechanical and tribological properties of the developed metal matrix composites (MMCs). Effects of various wear factors (i.e. load, sliding speed and sliding distance) on wear rate (WR) and coefficient of friction (COF), along with different dominating wear mechanisms are emphasized. Outcomes of the review revealed that increase in stirring time affects the mechanical properties adversely, whereas increase in ball milling time improves these. Incorporation of ceramic and carbonaceous reinforcements in Al alloy improves its mechanical and wear resistance properties. Effects of wear process parameters on WR and COF depend upon the types of matrix-reinforcement combinations, reinforcement content and test conditions.

Structural, mechanical and tribological performance of a nano structured biomaterial Co–Cr–Mo alloy synthesized via mechanical alloying

The influence of milling time on the tribological behavior of a Co–Cr–Mo alloy designed for biomedical applications, synthesized via mechanical alloying is investigated. Elemental Co, Cr and Mo powders are milled using different milling times (2, 6, 12 and 18 h) in a high-energy ball mill. The resulting powders were subjected to cold uniaxial and hot isostatic pressing respectively, followed by sintering to obtain cylindrical samples, which were evaluated for their structural, mechanical and the wear behavior. Results showed that the grain and crystallite sizes of the powders decreased with increasing milling time, reaching low values of <10 μm and 32 μm respectively, at higher milling times. Furthermore, the wear rates and the coefficients of friction were lower, at higher milling times due to high densities (96%), and higher elasto-plastic resistance, as presented by the H/E and H3/E2 values of 0.026 and 0.0021 GPa, respectively. Increased milling time enables the refinement of grains and reduction in porosity in the Co–Cr–Mo alloy, which in turn increases the alloy's elasto-plastic resistance and enhances its wear resistance.

Crystallite size reduction of Cr doped Al2O3 materials via optimized high-energy ball milling method

α-Al2O3 based compounds have large crystals and it is very difficult to reduce the crystallite size because they are very stable and hard. One way of reducing the crystallite size of the materials is by using high-energy ball milling method. Pure and single-phase micron-sized α-Al2−xCrxO3 (x = 0.1, 0.2, 0.3) materials were successfully obtained via self-propagating combustion method. These materials were then subjected to a simple milling process from their microcrystalline powders. Comparisons between the micron-sized and milled samples in terms of their phase, structure, morphology and crystallite size were discussed. The XRD results reveal that all the milled samples were pure with no impurity or other phases present. Structural parameters are extracted via the Rietveld method, revealing that the cell constant, a, of the milled samples is higher than that of the micron-sized materials by 0.09 % to 0.11 %, resulting in a 0.28 % to 0.39 % increase in cell volume. FESEM results show a gradual decrease in crystallite size with increased milling time. Notably, the method successfully reduces the crystallite size without changing the phase of the materials and preserving the stoichiometry of the Al2−xCrxO3 materials which may offer improved properties in various applications.

Hydrogen generation from the hydrolysis of LaMg12H27 ball-milled with LiH

The LaMg12H27 (LMH) and LMH-5 wt% LiH samples were prepared by different ball milling times, and the hydrogen generation performances of samples were investigated and compared. X-ray diffraction and scanning electron microscopy techniques were adopted to elucidate the performance improvement mechanisms. For the LMH, with the increase of ball milling time, the hydrogen generation yield and kinetics can be improved significantly. This may be caused by the change in particle size and crystallite size of the LMH sample after ball milling. However, for the LMH-5 wt% LiH, with the increase of the ball milling time, the initial kinetics increases firstly, and then decreases. This may be due to the gradual movement of LiH from the surface to the interior of LMH and to the covering of LMH after ball milling.