Remarkable improvement of hydrogen sorption kinetics in magnesium catalyzed with Nb2O5

Kinetics of hydrogen absorption and desorption reactions was investigated on the MgH₂ composite doped with 1 mol% Nb₂O₅ as a catalyst by ballmilling. The composite after dehydrogenation at 200 °C absorbed gaseous hydrogen of 4.5 mass% even at room temperature under lower pressure than 1 MPa within 15 s and finally its capacity reached more than 5 mass%. On the other hand, the catalyzed MgH₂ after rehydrogenation desorbed 6 mass% hydrogen at 160 °C under purified He flow, which followed the first order reaction. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be 71 kJ/mol H₂, indicating the product was significantly activated due to the catalytic effect of Nb₂O₅.     

A facile route to VN and V2O3 nanocrystals from single precursor NH4VO3

V₂O₃ and VN nanocrystals have been synthesized by the decomposition of the precursor NH₄VO₃ and following nitridation in an autoclave with metallic Na flux at 450–600 °C. X-ray powder diffraction (XRD) recorded the evolution process of the reaction from precursor NH₄VO₃ to hexagonal V₂O₃ and then to NaCl-type VN. In addition, the products were characterized by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).     

Structure and hydrogen storage properties of MgH2 catalysed with La2O3

The catalytic effect of the addition of lanthanum oxide (La₂O₃), in the range 0.5–2.0 mol%, on the hydrogen storage properties of MgH₂ prepared by ball milling has been studied. The addition of La₂O₃ reduces the formation during milling of the metastable orthorhombic γ-MgH₂ phase. The desorption rate of samples with 1 and 2 mol% La₂O₃ comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La₂O₃. The presence of LaH₃ after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La₂O₃ gives a lower hysteresis factor compared with sample with 2 mol%.     

Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

C3N4 as a precursor for the synthesis of NbC, TaC and WC nanoparticles

While there already exit some routes to prepare carbides, highly efficient and facile routes are still desired to meet the increasing demand on carbides. By a facile solid-state reaction process using graphite-like phase of C₃N₄ (g-C₃N₄) as the carbonizing reagent, we synthesized three technologically important carbides including cubic NbC and TaC, and hexagonal WC nanoparticles at relatively low temperature (1150 °C). The products were characterized by power X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The results show that g-C₃N₄ is a highly efficient carbonizing reagent and the oxides Nb₂O₅, Ta₂O₅ and WO₃ are completely converted into the corresponding carbides at 1150 °C, which is significantly lower than that reported for the commercial preparation of the carbides, typically >1600 °C. The NbC, TaC and WC nanoparticles are found to have an average particle size of 4, 35 and 60 nm, respectively. An important feature of this solid-state reaction process is that g-C₃N₄ plays double roles as both efficiently reducing and carbonizing reagent.     

Hydrogen storage properties of MgH2/SWNT composite prepared by ball milling

A systematic investigation was performed on the hydrogen storage properties of mechano-chemically prepared MgH₂/single-walled carbon nanotube (SWNT) composites. It is found that the hydrogen absorption capacity and hydriding kinetics of the composites were dependent on the addition amount of SWNTs as well as milling time. A 5 wt.% addition of SWNTs is optimum to facilitate the hydrogen absorption and desorption of MgH₂. The composite MgH₂/5 wt.% SWNTs milled for 10 h can absorb 6.7 wt.% hydrogen within about 2 min at 573 K, and desorb 6 wt.% hydrogen in about 5 min at 623 K. Prolonging the milling time over 10 h leads to a serious degradation on hydrogen storage property of the MgH₂/SWNT composite, and property/structure investigations suggest that the property degradation comes from the structure destruction of the SWNTs.     

Crystallization of magnesium niobate from mechanochemically derived amorphous phase

The effect of high-energy ball milling and subsequent annealing on the mixture of MgO and Nb₂O₅ has been investigated. X-ray diffraction (XRD) measurement indicates that an amorphous phase is produced after milling for 5 h, while traces of MgNb₂O₆ crystallized from the amorphous phase during prolonged milling. Significant crystallization of MgNb₂O₆ from the amorphous state is observed after annealing at 500 °C, while the reaction of the remaining MgO and Nb₂O₅ does not take place at this temperature. Single phase MgNb₂O₆ can be achieved for all the milled samples at 700 °C. No significant grain growth is observed when the milled powders were annealed at temperature below 900 °C. Almost fully dense MgNb₂O₆ ceramics are obtained after annealing at 1100 °C from the as-milled powders.     

Effects of noble metal addition on microstructure and thermoelectric properties of NaxCo2O4

The synthesis of Na_xCo₂O₄/Ag and Na_xCo₂O₄/Au composites was tried by mechanical milling and subsequent sintering. Ag and Au particles were added to the Na_xCo₂O₄ powder prior to the mechanical milling. The microstructure and thermoelectric properties of the Na_xCo₂O₄/Au composite were compared to those of the Na_xCo₂O₄/Ag composite and the Na_xCo₂O₄ single phase, and the effects of the Ag and Au addition on the thermoelectric performance of Na_xCo₂O₄ were discussed. Au particles around 2 μm or smaller in size, which were significantly smaller than Ag particles around 10 μm in size, were dispersed in the Na_xCo₂O₄ matrix. The Seebeck coefficient and the electrical resistivity of Na_xCo₂O₄ were slightly enhanced and significantly reduced by these noble metals addition, resulting in the large power factor of these composites. On the other hand, the Na_xCo₂O₄/Au composite showed the electrical resistivity larger than that of the Na_xCo₂O₄/Ag composite. Ag and Au addition markedly increased the thermal conductivity, and the dimensionless figure of merit of Na_xCo₂O₄ could not be improved by these noble metals addition.     

X-ray diffraction study on formation of ErNbO4 powder

The formation of ErNbO₄ powder, prepared by calcining an Er₂O₃ (50 mol%) and Nb₂O₅ (50 mol%) powder mixture at 1100 and 1600 °C for different durations, was investigated by using X-ray diffraction. The experimental results have displayed that although the solid-state reaction had started to some extent when the mixture was pre-calcined at 1100 °C for a duration of 13 h, the two original phases Er₂O₃ and Nb₂O₅ still dominated the mixture. When the duration of the calcination reaction was increased to 120 h at the same temperature, the resultant mixture experienced a nearly complete phase transformation. Accordingly, the ErNbO₄ phase was dominant phase in the mixture. Nevertheless, a small portion of the raw powder still existed in the mixture. When the calcining temperature was elevated to 1600 °C, ErNbO₄ powder with higher purity could be obtained for a relatively much shorter duration (only up to several tens of hours). A simple formation mechanism of ErNbO₄, an elevated-temperature-assisted solid-state chemical reaction: Er₂O₃+Nb₂O₅2ErNbO₄, is suggested. In addition, the present experimental results offer important evidence for the formation of the additional phase ErNbO₄ induced in Er:LiNbO₃ crystals by vapour transport equilibration (VTE) treatment.     

Microstructure and magnetic properties of Co/Al2O3 nanocomposite powders

Nanocomposite powders of magnetic cobalt nanoparticles dispersed by nonmagnetic Al₂O₃ particles have been prepared by planetary ball milling. Ball milling of the CoO and Al mixture powder after a certain milling duration reduces CoO to (fcc and hcp) Co completely and oxidizes Al to -Al₂O₃ simultaneously. The average grain sizes of the nanocomposite powders are 19 nm for Co and 28 nm for -Al₂O₃ after the completion of the reduction reaction. By direct ball milling of the mixture of Co and Al₂O₃, the allotropic phase transformation of Co was observed and the average grain size of Co is reduced to 5 nm. For both the samples of the mechanochemical series and the direct milling series, the saturation magnetizations of the nanocomposite powders decrease with decreasing average grain size of Co. This may be due to the enhancement of the interface effects and the increase of the superparamagnetic particles with decreasing Co grain size. The coercivities of the Co/Al₂O₃ nanocomposite powders increase up to 380 Oe. The increasing grain boundaries with decreasing Co grain size result in the domain wall pinning which predicts the coercivity enhancement. In addition to the grain size effects, the reduction of the particle size toward the size region of single domain also contributes to the increase of coercivity.     

Relationship between dielectric properties and structural long-range order in (x)Pb(In1/2Nb1/2)O3:(1 − x)Pb(Mg1/3Nb2/3)O3 relaxor ceramics

The dielectric and structural order–disorder properties of as-sintered complex perovskite (x)Pb(In_1/2Nb_1/2)O₃:(1 − x)Pb(Mg_1/3Nb_2/3)O₃ ceramics are highly influenced by the quantity of Pb(In_1/2Nb_1/2)O₃ (PIN). A high PIN quantity causes the relative permittivity maxima (ε_max) to decrease and the temperature (T_max) to increase. Also, strong frequency dispersion is dominant in the relative permittivity when plotted against the temperature. In ferroelectric hysteresis loop measurements, the maximum values of electric displacements (D_max) decrease with increasing PIN. The ceramics in the composition range x = 0.1–0.8 behave as ferroelectric relaxors and exhibit very slim hysteresis loops for all these compositions. Transmission electron microscopy (TEM) studies show that the size of the 1:1 structural ordered domains is influenced by the PIN quantity. The relationship between the dielectric properties and the long-range 1:1 order in these relaxors appear to be in conflict with the commonly accepted order–disorder behavior in complex perovskite ferroelectrics, in which large structural domains correspond with the tendency to depart from the relaxor state. TEM observations show that individual 1:1 ordered domains in (x)PIN:(1 − x)PMN ceramics are composed of numerous nano-sized ordered domains, separated by fine antiphase boundaries.     

Structural and electrical properties of SrBi2(Ta0.5Nb0.5)2O9 thin films

SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBTN) thin films were obtained by polymeric precursor method on Pt/Ti/SiO₂/Si(1 0 0) substrates. The film is dense and crack-free after annealing at 700 °C for 2 h in static air. Crystallinity and morphological characteristic were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The films displayed rounded grains with a superficial roughness of 3.5 nm. The dielectric permittivity was 122 with loss tangent of 0.040. The remanent polarization (P_r) and coercive field (E_c) were 5.1 μC/cm² and 96 kV/cm, respectively.     

Preparation of nano-sized SrAl2O4 using an amorphous hetero-nucleus complex as a precursor

Nano-crystalline SrAl₂O₄ with spinel structure was successfully prepared at 700 °C using amorphous SrAl₂(diethylenetriaminepentaacetic acid (DTPA)_1.6)(H₂O)₄ as precursor. The precursor was synthesized by a simple inorganic reaction and decomposed into SrAl₂O₄ at temperatures above 500 °C, which was proved by DTA–TGA and X-ray photoelectron spectroscopy (XPS) analysis. X-ray diffraction (XRD) results illustrated that a crystalline SrAl₂O₄ phase can form at 700 °C, which is about 600 °C lower than that used in the traditional method. The crystalline SrAl₂O₄ prepared at 900 °C for 2 h had a crystal size of about 28 nm and a grain size of about 80 nm, and its BET surface area can reach 28.056 m²/g. Calcination temperature and time had a weak effect on crystal size.     

Direct synthesis of A2(Ti(1 − y)Zry)2O7 (A = Gd, Y) solid solutions by ball milling constituent oxides

Different compositions in two solid solutions, A₂(Ti_(1 − y)Zr_y)₂O₇ (A = Gd³⁺, Y³⁺), with high oxygen ion conductivity, have been successfully prepared at room temperature via mechano-chemical synthesis. Stoichiometric mixtures of the constituent oxides were milled in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by using X-ray diffraction showing that in all cases and after milling for 19 h, the powders consisted of a single phase. Powders were also examined by scanning electron microscopy (SEM) finding out that they basically consist of sub-micron size agglomerates and aggregates of nanoparticles.     

Solid state reactions in highly dispersed praseodymium oxide–SiO2 system

This paper reports results of studies on the interaction of praseodymium oxide nanocrystals with an amorphous silica. Nano-sized (3–4 nm) amorphous precursor of praseodymium oxide synthesized using a microemulsion technique were supported onto a high surface SiO₂ or occluded into SiO₂ matrix. Solid state reactions occurring in these binary systems upon heat treatment in air, argon or hydrogen at 800–1100 °C were studied by TEM, XRD, FT-IR and UV–vis spectroscopy. It has been found that morphology of the sample as well as annealing atmosphere influence greatly the phase evolution. At temperatures above 900 °C, nanocrystalline praseodymium silicates of various morphology and crystal structure were obtained. In particular, a new polymorph of Pr₂Si₂O₇, isostructural with I-type Ln₂Si₂O₇ (Ln₆[Si₄O₁₃][SiO₄]₂) Ln = Ce, La, has been identified.     

Formation and chemical leaching effects of nonequilibrium Al0.6(Fe25Cu75) alloy powder produced by rod milling

The formation and chemical leaching effects of a nonequilibrium Al_0.6(Fe₂₅Cu₇₅)_0.4 powder produced by rod milling is described. X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and vibrating sample magnetometry were used to characterize both the as-milled and leached specimens. After 400 h of milling, only the bcc AlFe phase with an amorphous phase was detected in the XRD patterns. The crystallite size for the bcc AlFe phase (110) after 400 h of milling was about 5.3 nm. The peak temperature and the crystallization temperature of the as-milled powders were 448.7 and 428.0 °C, respectively. Al atoms leaching from the as-milled bcc AlFe powders in the L1 condition did not alter the diffraction pattern significantly, even though Al atoms had been removed. After the L1 specimen was annealed at 500 °C for 1 h, the bcc AlFe phase transformed to the fcc Cu, Fe, and CuFe₂O₄ phases. The peak widths of L1 and L2 specimens were similar, but became broader than that of the as-milled powder. The saturation magnetization decreased with increasing milling time, and a value of 10.4 emu/g was reached after 400 h of milling. After cooling the specimen from 750 °C, the magnetization slowly increased at approximately 491.4 °C, indicating that the bcc AlFe phase had transformed to the fcc Cu and Fe phases.     

Processing, microstructure, and properties of laser remelted Al2O3/Y3Al5O12（YAG） eutectic in situ composite

Laser remelting and rapid solidification were performed in preparing the high-performance Al2O3/Y3Al5O12（YAG） eutectic in situ composite. The microstructure characteristic and solidification behavior were studied using scanning electron microscopy（SEM）, energy dispersive spectroscopy（EDS）, X-ray diffractometry（XRD） and simultaneous thermal analysis（STA）. The hardness and fracture toughness were obtained using an indentation technique. The results show that the laser remelted Al2O3/YAG composite has a homogeneous eutectic microstructure without microcrack and pore. The component phases of Al2O3 and YAG are three-dimensionally and continuously reticular connected, and finely coupled without grain boundaries, colonies and amorphous phases between interfaces. The eutectic interspacing is greatly refined with increasing the scanning rate and average is only l μm. The synthetically thermal analysis indicates that the eutectic temperature of Al2O3-YAG is 1 824 ℃, well matching the phase diagram of Al2O3-Y2O3 system. The maximum hardness reaches 19.5 GPa and the room fracture toughness is 3.6 MPa.m^1/2.     

Microstructure and thermoelectric properties of NaxCo2O4/Ag composite synthesized by the polymerized complex method

The synthesis of a thermoelectric Na_xCo₂O₄/Ag composite was attempted by the polymerized complex (PC) process using AgNO₃ as an Ag source and subsequent sintering at 1153 K for 72 ks. The effects of the PC process and Ag addition to Na_xCo₂O₄ on microstructure and thermoelectric properties of the Na_xCo₂O₄/Ag composite were investigated. Ag was hardly substituted for Na and Co sites, and the sintered sample was composed of the Na_xCo₂O₄ and Ag phases. The electrical resistivity of the composite was smaller than that of the Na_xCo₂O₄ single phase and the Seebeck coefficient was slightly enhanced by Ag addition, resulting in the significantly large power factor. However, most of precipitated Ag particles in the Na_xCo₂O₄ matrix were coarse, 5–8 μm in size, and the thermal conductivity of the composite was high as compared to the Na_xCo₂O₄ single phase. From these results, the dimensionless figure of merit of the composite was almost the same as that of the Na_xCo₂O₄ single phase.     

Microstructural modifications induced by hydrogen absorption in Mg5Ga2 and Mg6Pd

We have recently proposed a new method to design one-dimensional structures of MgH₂ in the nano- and micrometer ranges by hydrogen-induced disproportionation of bulk Mg₂₄Y₅. The present study confirms the same behavior in hydrogenated Mg₅Ga₂ and Mg₆Pd. Single-crystalline one-dimensional structures and microparticles of MgH₂ are formed by hydrogen absorption and subsequent partial disproportionation of Mg₅Ga₂ and Mg₆Pd. The MgH₂ whiskers and particles grow with different morphologies for different alloying partners. Growth mechanisms are proposed in relation to the morphology and the chemical surface composition of original compounds.     

Synthesis and characterization of LiGaxMn2−xO4 (0 ≤ x ≤ 0.05) by triethanolamine-assisted sol–gel method

Spinel LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) cathode materials with phase-pure particles and nano-sized distribution were synthesized by sol–gel method using triethanolamine as the chelating agent. The effects of heat treatment on the physicochemical properties of the spinel LiGa_xMn_2−xO₄ powders were examined with thermogravimetric and differential thermal analysis (TG/DTA), powder X-ray diffraction (XRD) and scanning electron micrograph (SEM). The LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) electrodes were characterized electrochemically by charge/discharge experiments under a current rate of 0.5C at 55 °C. Although the Ga-doped spinel electrode showed smaller initial discharge capacity, it exhibited better cycling performance than the undoped-LiMn₂O₄ electrode. The dQ/dV versus potential plots at 55 °C revealed that the improvement in cycling performance of the Ga-doped spinel electrode is attributed to stabilization of the spinel structure by the presence of gallium ion.