Short-time Milling Research Articles

Coupling rare-earth metal introduction and alkali oxide addition to achieve efficient regeneration of LiBH4 by reacting hydrous LiBO2 with Al

The lack of cost-effective regeneration techniques for LiBH4 limits its large-scale applications for supplying hydrogen via hydrolysis. Now, we report a facile method to regenerate LiBH4 by ball milling its real hydrolytic byproduct (LiBO2·xH2O) with cheap Al based reducing agents, instead of traditional Mg. Effective strategies are developed to promote the formation of LiBH4 by introducing Li2O to enable the dense Al2O3 to convert into LiAlO2 on Al surface and break up passivation layers, and by partially replacing Al with Al-RE alloy to improve grinding and contacting efficiency of raw materials and facilitate the conversion from initial H+ in LiBO2·2H2O to final H¯ in LiBH4. A yield of 10.4 % can be obtained for a relatively short ball milling time (10 h) for the Al-Al2Ce-Li2O-LiBO2·2H2O system. Another advantage of this method is that the utilization of H+ source in LiBO2·2H2O improves energy efficiency and reduces the cost. This work broadens the species of reducing agents for borohydride regeneration and enriches its regeneration methods.

Fast Hydrogen Sorption Kinetics in Mg-VCl3 Produced by Cryogenic Ball-Milling.

Hydrogen storage in Mg/MgH2 materials is still an active research topic. In this work, a mixture of Mg-15wt.% VCl3 was produced by cryogenic ball milling and tested for hydrogen storage. Short milling time (1 h), liquid N2 cooling, and the use of VCl3 as an additive produced micro-flaked particles approximately 2.5-5.0 µm thick. The Mg-15wt.% VCl3 mixture demonstrated hydrogen uptake even at near room-temperature (50 °C). Mg-15wt.% VCl3 achieved ~5 wt.% hydrogen in 1 min at 300 °C/26 bar. The fast hydriding kinetics is attributed to a reduction of the activation energy of the hydriding reaction (Ea hydriding = 63.8 ± 5.6 kJ/mol). The dehydriding reaction occurred at high temperatures (300-350 °C) and 0.8-1 bar hydrogen pressure. The activation energy of the dehydriding reaction is 123.11 ± 0.6 kJ/mol. Cryomilling and VCl3 drastically improved the hydriding/dehydriding of Mg/MgH2.

Labeling and Probing the Silica Surface Using Mechanochemistry and 17 O NMR Spectroscopy*.

In recent years, there has been increasing interest in developing cost‐efficient, fast, and user‐friendly 17O enrichment protocols to help to understand the structure and reactivity of materials by using 17O NMR spectroscopy. Here, we show for the first time how ball milling (BM) can be used to selectively and efficiently enrich the surface of fumed silica, which is widely used at industrial scale. Short milling times (up to 15 min) allowed modulation of the enrichment level (up to ca. 5 %) without significantly changing the nature of the material. High‐precision 17O compositions were measured at different milling times by using large‐geometry secondary‐ion mass spectrometry (LG‐SIMS). High‐resolution 17O NMR analyses (including at 35.2 T) allowed clear identification of the signals from siloxane (Si−O−Si) and silanols (Si−OH), while DNP analyses, performed by using direct 17O polarization and indirect 17O{1H} CP excitation, agreed with selective labeling of the surface. Information on the distribution of Si−OH environments at the surface was obtained from 2D 1H−17O D‐HMQC correlations. Finally, the surface‐labeled silica was reacted with titania and using 17O DNP, their common interface was probed and Si−O−Ti bonds identified.

Effect of ball-milling on the phase formation and enhanced thermoelectric properties in zinc antimonides

We report the phase formation mechanism and the enhanced thermoelectric properties of zinc antimonide (ZnSb) thermoelectric material. The phase pure ZnSb thermoelectric material is achieved using high-energy ball milling of Zn and Sb in a shorter span of time. The ZnSb phase formation is explained by the kinetic energy transferred to the powders during milling for the solid-state reaction between Zn and Sb to form the desired ZnSb phase. The repeatability in transport properties up to three thermal cycles corroborates the thermal stability of the processed samples. The thermoelectric figure of merit obtained at 600 K is ~ 0.76 for the processed phase pure ZnSb sample, the highest value in binary ZnSb reported so far. Our results address the ZnSb phase evolution in a shorter milling time and the enhanced thermoelectric properties of the ZnSb materials. The observations will help to scale up the processing of high-performance ZnSb thermoelectric materials.

Structural Evolution in Wet Mechanically Alloyed Co-Fe-(Ta,W)-B Alloys

In the present study, the effect of wet mechanical alloying (MA) on the glass-forming ability (GFA) of Co43Fe20X5.5B31.5 (X = Ta, W) alloys was studied. The structural evolution during MA was investigated using high-energy X-ray diffraction, X-ray absorption spectroscopy, high-resolution transmission electron microscopy and magnetic measurements. Pair distribution function and extended X-ray absorption fine structure spectroscopy were used to characterize local atomic structure at various stages of MA. Besides structural changes, the magnetic properties of both compositions were investigated employing a vibrating sample magnetometer and thermomagnetic measurements. It was shown that using hexane as a process control agent during wet MA resulted in the formation of fully amorphous Co-Fe-Ta-B powder material at a shorter milling time (100 h) as compared to dry MA. It has also been shown that substituting Ta with W effectively suppresses GFA. After 100 h of MA of Co-Fe-W-B mixture, a nanocomposite material consisting of amorphous and nanocrystalline bcc-W phase was synthesized.

The effect of ball milling on birch, pine, reed, walnut shell enzymatic hydrolysis recalcitrance and the structure of the isolated residual enzyme lignin

Methodologies for the high-yield recovery of lignin with retention of its native CO bonded structure is an essential prerequisite for many novel high-end lignin applications. Enzymatic residual lignin isolation is such a methodology that leaves the lignin untouched by using enzymatic desaccharification. Thus, a series of representative lignocellulose substrates (birch, pine, walnut shell and reed) were evaluated for effective native lignin isolation, with emphasis on the effect on the lignin structure and purity. The effect of enzyme loading and ball milling severity were studied by tracking residual saccharides and the structural integrity of the isolated lignin. Prolongation of ball milling time could achieve a higher carbohydrate removal and avoid the loading of extra enzyme. However, the application of two or more steps of enzymatic hydrolysis with higher enzyme loading and short ball milling time was shown as an alternative to long ball milling time to achieve similar carbohydrate removal and avoid extensive decrease of the lignin molecular weight (MW). This MW decrease was caused by breaking of some linkages, but not too a large enough extent to cause significant differences in the 2D HSQC NMR spectra. The recalcitrance towards increased enzyme hydrolysis activity by ball milling was different for the four representative biomasses and followed an order of walnut shell > reed ≈ pine > birch by comprehensive analysis the obtained data. Overall, the results showed a clear two-way synergy between enzymatic treatment and ball milling efficiency to isolate lignin with high yield, high native linkage content, purity and minimal MW reduction.

An uncomplicated method for growing nano-quasicrystalline structures in the AlCuFeB quaternary alloy system: A short-time milling

Quasicrystals have been comprehensively studied since their discovery in 1984 by Daniel Shechtman to dissolve their complex structures concerning their physical properties [1–5]. The quasi-periodic well-ordered atomic structures made QCs as a novel type of solids differing from the crystalline or amorphous materials [5–7]. The QC alloys have non-crystallographic rotational symmetries, such as five-fold, eight-fold, and even ten-fold rotational axes that are prohibited in ordinary periodic crystals [5–7]. The QC alloys have been recently considered for several applications owing to a series of the unusual combination of exclusive physicomechanical properties for metallic-based alloys [5–7]. Accordingly, the properties of these alloys can be pointed out to their high strength, elevated hardness, desirable corrosion and wear resistance, minute adhesion, low values of thermal and electrical conductivity, and superior optical properties [5–7].•This paper focuses on the synthesis, structural and microstructural evolutions, thermal stability, microhardness, and electrical and optical properties of the Al59Cu25.5Fe12.5B3 nanoquasicrystalline alloy for solar selective absorber usages. Accordingly, there are various advanced techniques for synthesizing the nanostructured quasicrystals, such as laser ablation, sol-gel, electros pining, mechanical alloying, and hydrothermal methods.•The structural and microstructural evolutions of the mechanically alloyed AlCuFeB powders were investigated by X-ray diffractometry and field-emission scanning electron microscopy. The thermal stability of the AlCuFeB powders was recorded by differential thermal analysis.•The nanostructured Al59Cu25.5Fe12.5B3 stable quasicrystalline phase was synthesized by short-time milling procedure in 1 h. It was found that the presence of the quasicrystalline phase in the AlCuFeB alloy prominently improves the microhardness, electrical resistivity, and sunlight absorptance.

Improved photochromic stability in less deficient cesium tungsten bronze nanoparticles

Hexagonal cesium tungsten bronze (Cs0.32WO3) nanoparticles are well known as near-infrared (NIR) shielding materials. However, one of the critical issues for industrial applications involves a photochromic instability, i.e., a blue color change of this material under ultraviolet (UV) irradiation. This color change is attributed to the formation of HxWO3 phase through doping of H+ ions into Cs-deficient sites present at the surface of Cs0.32WO3 nanoparticles. Therefore, a new approach was adopted to prevent the color change by synthesizing a Cs0.32WO3 nanoparticle with less Cs-deficient sites using spray pyrolysis followed by heat treatment and short-time milling. The Cs0.32WO3 nanoparticles of spray pyrolysis route exhibited an improved photochromic stability under UV irradiation compared to those synthesized through a solid-state-reaction route. Detailed analysis using atomic resolution transmission electron microscopy revealed that the Cs0.32WO3 nanoparticles of spray pyrolysis route retain Cs ions up to a depth of less than 1 nm from the surface. This work demonstrates that the less-Cs-deficient Cs0.32WO3 nanoparticles are beneficial to improve the photochromic stability.

Microstructure homogeneity of milled aluminum A356–Si3N4 metal matrix composite powders

A metal matrix composite was produced by co-milling an A356 aluminum alloy powder obtained by rotating electrode off-equilibrium solidification, with different mass fractions (10, 20 and 30%) of Si3N4. The structural and microstructural modifications occurring during the milling were investigated with X-ray powder diffraction (XRPD). Whole powder pattern modeling (WPPM) of the XRPD reveals the inhomogeneous nature of the material in terms of silicon content and allows the crystallite size distribution and dislocation content to be followed in detail for all phases present in the powder. Neither microscopy nor the traditional Scherrer equation can reveal such a detailed picture in this case. Short milling times are sufficient to homogenize the microstructure and to obtain nanoscale crystallites. Long milling times are advantageous to increase the dislocation density that might be favorable for subsequent sintering.

The formation and evolution mechanism of amorphous layer surrounding Nb nano-grains in Nb-Al system during mechanical alloying process

The microstructure and phase evolution in Nb-Al system during MA are systemically studied. The existence of Nb-Al amorphous layer surrounding Nb nano-grains after short-time ball milling was confirmed by the HRTEM observation as well as the crystallization behavior in the annealing experiment. It was also found that after short time milling, only very little Al can enter the crystal lattice of Nb and substitute for Nb, whereas most of Al mainly concentrate in this amorphous layer surrounding Nb nano-grains. What's more, interface thermodynamic analysis and calculation were employed to investigate the phase formation and evolution of this amorphous layer in Nb-Al binary system during MA process. Accordingly, positive driving force (≡-ΔG) definitely exists for the formation of the amorphous layer {NbAl} surrounding Nb nano-grains. Thus, these amorphous layers can form and exist stably in the short-time milled Nb-Al system. With the milling time prolonging, this amorphous layer becomes unstable and can crystallize into Nb(Al)ss solid solution when its thickness increases beyond a critical thickness. The critical thickness of this amorphous layer is also predicted by the interface thermodynamics calculation and the corresponding calculated value (approximately 1.9 nm) is consistent with the observed thickness (about 2.2 nm) by the HRTEM within the error range. More importantly, the solubility of Al in the lattice of Nb can be significantly increased resulting from the crystallization of this amorphous layer {NbAl}. Our work provides a new and rational perspective to well understand the complicated microstructure and phase evolution in various nano-sized systems during MA process.

The effect of process control agents and milling atmosphere on the structural changes of Ti50Cu27,5Ni10Zr10Co2,5 master alloy during short time milling

Properties of powders made from Ti50Cu27,5Ni10Zr10Co2,5 alloy, milled under different conditions for only 3600 s in total were investigated. For this purpose two different milling atmospheres and two different Process Control Agents (PCA) were tested: argon without PCA, air without PCA, argon combined with stearic acid, argon combined with petroleum spirit. Results showed that the size and shape of particles, and the microstrucutre and hardness of partcicles were highly depended on the milling conditions. According to XRD analysis, the maximal amorphous fraction has been achieved in the air atmosphere withouth PCA milled sample (57 wt. %) powder. However the extent of Fe contamination and weight loss during milling powder resulted bigger values. As a result, the optimal combination of morphology, contamination amount and weight lost level has been observed in the case of powder milled in argon atmosphere using stearic acid.

Milling induced amorphisation and recrystallization of α-lactose monohydrate

Preprocessing of pharmaceutical powders is a common procedure to condition the materials for a better manufacturing performance. However, such operations may induce undesired material properties modifications when conditioning particle size through milling, for example. Modification of both surface and bulk material structure will change the material properties, thus affecting the processability of the powder. Hence it is essential to control the material transformations that occur during milling. Topographical and mechanical changes in surface properties can be a preliminary indication of further material transformations. Therefore a surface evaluation of the α-lactose monohydrate after short and prolonged milling times has been performed. Unprocessed α-lactose monohydrate and spray dried lactose were evaluated in parallel to the milled samples as reference examples of the crystalline and amorphous lactose structure. Morphological differences between unprocessed α-lactose, 1 h and 20 h milled lactose and spray dried lactose were detected from SEM and AFM images. Additionally, AFM was used to simultaneously characterize particle surface amorphicity by measuring energy dissipation. Extensive surface amorphicity was detected after 1 h of milling while prolonged milling times showed only a moderate particle surface amorphisation. Bulk material characterization performed with DSC indicated a partial amorphicity for the 1 h milled lactose and a fully amorphous thermal profile for the 20 h milled lactose. The temperature profiles however, were shifted somewhat in the comparison to the amorphous reference, particularly after extended milling, suggesting a different amorphous state compared to the spray-dried material. Water loss during milling was measured with TGA, showing lower water content for the lactose amorphized through milling compared to spray dried amorphous lactose. The combined results suggest a surface-bulk propagation of the amorphicity during milling in combination with a different amorphous structural conformation to that of the amorphous spray dried lactose. The hardened surface may be due to either surface crystallization of lactose or to formation of a low-water glass transition.

Study of phases evolution in high-coercive MnAl powders obtained through short milling time of gas-atomized particles

Abstract Gas-atomized Mn 54 Al 46 particles constituted nominally of only e- and γ 2 -phases, i.e. no content of the ferromagnetic L1 0 -type τ-phase, have been used to study the evolution of phases during short time of high-energy milling and subsequent annealing. Milling for 3 min is sufficient to begin formation of the τ-MnAl phase. A large coercivity of 4.9 kOe has been obtained in milled powder after annealing at 355 °C for 10 min. The large increase in coercivity, by comparison with the lower value of 1.8 kOe obtained for the starting material after the same annealing conditions, is attributed to the combined formation of the τ-MnAl and β-Mn phases and the creation of a very fine microstructure with grain sizes on the order of 20 nm. Correlation between morphology, microstructure and magnetic properties of the rapidly milled MnAl powders constitutes a technological advance to prepare highly coercive MnAl powders.

Effect of short milling time and microwave heating on phase evolution, microstructure and mechanical properties of alumina–mullite–zirconia composites

Abstract Alumina–mullite–zirconia composites were prepared using alumina and zircon powders pressed uniaxially at 250 MPa and sintered in a microwave furnace held at 1 550 °C for 90 min. The effects of short milling and sintering time on the density, phase evaluation and mechanical strength of the sintered composites were analyzed and compared with composites sintered in a conventional furnace. The goal was to decrease sintering time and temperature over that for conventional heating. The results showed that, although the densities were similar for both methods, the hardness, mechanical strength and fraction of the tetragonal zirconia phase of the microwave-sintered composites were much higher. The milling time yielded better densification and higher mechanical properties. It was found that the shorter sintering time in a microwave furnace requires longer milling time of the powders to obtain the same composite properties.

High-resolution neutron diffraction study of microstructural changes in nanocrystalline ball-milled niobium carbide NbC0.93

High resolution neutron diffraction was applied for elucidating of the microstructural evolution of nanocrystalline niobium carbide NbC0.93 powders subjected to high-energy ball milling. The diffraction patterns were collected with the high resolution Fourier diffractometer HRFD by using the reverse time-of-flight (RTOF) mode of data acquisition. The traditional single diffraction line analysis, the Rietveld method and more advanced Whole Powder Pattern Modeling technique were applied for the data analysis. The comparison of these techniques was performed. It is established that short-time milling produces a non-uniform powder, in which two distinct fractions with differing microstructure can be identified. Part of the material is in fact milled efficiently, with a reduction in grain size, an increase in the quantity of defects, and a corresponding tendency to decarburize reaching a composition NbC0.80 after 15h of milling. The rest of the powder is less efficiently processed and preserves its composition and lower defect content. Larger milling times should have homogenized the system by increasing the efficiently milled fraction, but the material is unable to reach a uniform and homogeneous state. It is definitely shown that RTOF neutron diffraction patterns can provide the very accurate data for microstructure analysis of nanocrystalline powders.

Fractal process and particle size distribution in a TiH2 powder milling system

The titanium powder metallurgy with TiH2 powders is a hot topic at present, and it is essential to control the size and distribution of the TiH2 powders during mechanical milling. The fractal behavior of the TiH2-milling is investigated in this paper based on the fractal fragmentation of rock mechanics, characterized by SEM, particle size analysis and calculation. The TiH2-milling system shows multiple-valued fractal dimensions during the different length scales. The particle size distribution of the milled TiH2 powders is limited strictly by the fractal process during short time milling and the weight percentage of a selected particle size range keeps an extremely low rate of change. The results show that the fractal process is useful to explain or estimate the phenomena of the particle size distribution during milling.

Influence of short time milling in R5(Si,Ge)4, R = Gd and Tb, magnetocaloric materials

The effect of the short milling times on R5(Si,Ge)4 R =Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles size was achieved down to approximately 3.5 DSm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 minutes of milling. Interestingly, an opposite effect is observed in Tb5Si2Ge2 where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process.

Catalytically Enhanced Hydrogen Sorption in Mg-MgH2 by Coupling Vanadium-Based Catalyst and Carbon Nanotubes

Mg (MgH2)-based composites, using carbon nanotubes (CNTs) and pre-synthesized vanadium-based complex (VCat) as the catalysts, were prepared by high-energy ball milling technique. The synergistic effect of coupling CNTs and VCat in MgH2 was observed for an ultra-fast absorption rate of 6.50 wt. % of hydrogen per minute and 6.50 wt. % of hydrogen release in 10 min at 200 °C and 300 °C, respectively. The temperature programmed desorption (TPD) results reveal that coupling VCat and CNTs reduces both peak and onset temperatures by more than 60 °C and 114 °C, respectively. In addition, the presence of both VCat and CNTs reduces the enthalpy and entropy of desorption of about 7 kJ/mol H2 and 11 J/mol H2·K, respectively, as compared to those of the commercial MgH2, which ascribe to the decrease of desorption temperature. From the study of the effect of CNTs milling time, it is shown that partially destroyed CNTs (shorter milling time) are better to enhance the hydrogen sorption performance.

Mechanical and Thermal Dehydrogenation of the Mechano-Chemically Synthesized Calcium Alanate (Ca(AlH4)2) and Lithium Chloride (LiCl) Composite

LiAlH4 and CaCl2 were employed for mechano-chemical activation synthesis (MCAS) of Ca(AlH4)2 and LiCl hydride composite. After short ball milling time, their X-ray diffraction (XRD) peaks are clearly observed. After ball milling for a longer duration than 0.5 h, the CaAlH5 diffraction peaks are observed which indicates that Ca(AlH4)2 starts decomposing during ball milling into CaAlH5+Al+1.5H2. It is estimated that less than 1 wt % H2 was mechanically dehydrogenated in association with decomposition reaction. After 2.5 h of ball milling, no Ca(AlH4)2 diffraction peaks were observed on XRD patterns which suggests that Ca(AlH4)2 was decomposed. Thermal behavior of ball milled powders, which was investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), indicates that a certain fraction of Ca(AlH4)2 could have been disordered/amorphized during ball milling being undetectable by XRD. The apparent activation energy for the decomposition of Ca(AlH4)2 and CaAlH5 equals 135 kJ/mol and 183 kJ/mol, respectively.

Mg2NiH4 synthesis and decomposition reactions

A ternary Mg2NiH4 hydride was synthesized using method that relies on a relatively short mechanical milling time (one hour) of a 2:1 MgH2–Ni powder mixture followed by sintering at a sufficiently high hydrogen pressure (>85 bar) and temperature (>400 °C). The ternary hydride forms in less than 2.5 h (including the milling time) with a yield of ∼90% as a mixture of two polymorphic forms. The mechanisms of formation and decomposition of ternary Mg2NiH4 under different hydrogen pressures were studied in detail using an in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high pressure DSC. The obtained experimental results are supported by morphological and microstructural investigations performed using SEM and high resolution STEM. Additionally, effects occurring during the desorption reaction were studied using DSC coupled with mass spectrometry.