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).     

Double SHS of ZrB2 powder

Production of ZrB₂ powder through self-propagating high-temperature synthesis (SHS) from ZrO₂, Mg and H₃BO₃ mixture often leads to incomplete conversion. A new technique, called DSHS (double SHS) has been developed, wherein the reaction product of the first SHS is mixed with calculated amounts of Mg and H₃BO₃ powder and subjected to a second SHS. The ZrB₂ powder produced by DSHS technique yields increased conversion. The NaCl is used as a diluent during SHS to control the particle size of the product. The average particle size of SHS ZrB₂ powder found to be 75–125 nm in range, which decrease to 25–40 nm after DSHS.     

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%.     

Characterization of NH3 formation in desorption of Li–Mg–N–H storage system

Hydrogen energy may provide the means to an environmentally friendly future. One of the problems related to its application for transportation is “on board” storage. Hydrogen storage in solids has long been recognized as one of the most practical approaches for this. Recently the hydrogen storage system, (Li₃N + 2H₂  LiNH₂ + 2LiH), was introduced by Chen et al. [P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302–304. [1]]. This type of material has attracted a great attention of the researchers from the metal hydride research community due to its high reversible storage capacity, up to 11.5 wt%. Currently the Li–Mg–N–H system has been shown to be able to deliver 5.2 wt% reversibly at a H₂ pressure of 30 bar and temperature of 200 °C. The accessibility of the capacity beyond 5.2 wt% is being actively explored. One of the issues related to the application of the metal–N–H storage systems is NH₃ formation that takes place simultaneously with H₂ release. NH₃ formation will not only damage the catalyst in a fuel cell, but also accelerate the cyclic instability of the H-storage material since the metal–N–H system turns into a metal–H system after loosing nitrogen and, therefore, it would not function at the temperature and pressure range designed for the metal–N–H system. The accurate determination of the amounts of NH₃ in the H₂ is, therefore, very important and has not been previously reported. Here a novel method to quantify NH₃ in the desorbed H₂, the Draeger Tube, is reported as being suitable for this purpose. The results indicate that the concentration of NH₃ in desorbed H₂ increases with the desorption temperature. For the (2LiNH₂ + MgH₂) system the NH₃ concentration was found to be 180 ppm at 180 °C and 720 ppm at 240 °C.     

Mechanochemical reduction of MoO3/SiO2 powder mixtures by Al and carbon for the synthesis of nanocrystalline MoSi2

In this investigation, MoSi₂ intermetallic compound has been synthesized by reducing of MoO₃/SiO₂ powder mixtures by Al and carbon via mechanical alloying (MA). Powder mixtures were ball milled for 0–100 h and structural evolutions have been monitored by X-ray diffraction. In the Al system, both β-MoSi₂ (high temperature phase) and -MoSi₂ (low temperature phase) were obtained after 3 h of milling and after 70 h of milling the β-phase transformed to -phase. The crystallite size of -MoSi₂ and Al₂O₃ after milling for 100 h was 12 and 17 nm, respectively. In reducing with carbon, two different compositions with nominal carbon content of 13.7 and 24 wt.% were used that in both compositions, -MoSi₂ forms during 10 h of milling. Higher carbon content increases the amount of MoSi₂.     

The effect of sequential and continuous high-energy impact mode on the mechano-chemical synthesis of nanostructured complex hydride Mg2FeH6

The effect of sequential and continuous high-energy impact mode in the magneto-mill Uni-Ball-Mill 5 on the mechano-chemical synthesis of nanostructured ternary complex hydride Mg₂FeH₆ was studied by controlled reactive mechanical alloying (CRMA). In the sequential mode the milling vial was periodically opened under a protective gas and samples of the milled powder were extracted for microstructural examination whereas during continuous CRMA the vial was never opened up to 270 h duration. MgO was detected by XRD in sequentially milled powders while no MgO was detected in the continuously milled powder. The abundance of the nanostructured ternary complex hydride Mg₂FeH₆, produced during sequential milling, and estimated from DSC reached 44 wt.% after 188 h, and afterwards it slightly decreased to 42 wt.% after 210 and 270 h. In contrast, the DSC yield of Mg₂FeH₆ after continuous CRMA for 270 h was 57 wt.%. Much higher yield after continuous milling is attributed to the absence of MgO. This behavior provides strong evidence that MgO is a primary factor suppressing formation of Mg₂FeH₆. The DSC hydrogen desorption onset temperatures are close to 200 °C while the desorption peak temperatures for all powders are below 300 °C and the desorption process is completed within the range 10–20 min. Within the investigated nanograin size range of 5–13 nm, the DSC desorption onset and peak temperatures of β-MgH₂ and Mg₂FeH₆ do not exhibit any trend with nanograin (crystallite) size of hydrides. TPD hydrogen desorption peaks from the powders containing a single ternary complex hydride Mg₂FeH₆, are very narrow, which indicates the presence of small but well-crystallized hydride particles. Their narrowness provides good evidence that the phase composition, bulk hydrogen distribution and hydride particle size distribution are very homogeneous. The overall amount of hydrogen desorbed in TPD from single-hydride Mg₂FeH₆ powders is somewhat higher than that observed in DSC and TGA desorption.

The powder milled sequentially for 270 h and desorbed in a Sieverts-type apparatus at 250 and 290 °C, yielded about a half of the hydrogen content obtained during DSC and TGA tests. No desorption of hydrogen was detected in a Sieverts-type apparatus at 250 and 290 °C after 128 and 70 min, respectively, from the powder continuously milled for 270 h. The latter easily desorbed 3.13 and 2.83 wt.% hydrogen in DSC and TGA tests, respectively.     

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.     

Facile autocombustion synthesis of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) perovskite via a modified complexing sol–gel process with NH4NO3 as combustion aid

Ammonia nitrate was applied as an oxidizer and combustion trigger to modify the normal combined EDTA-citrate complexing method into a process with autocombustion and low ignition temperature properties. Therefore, the synthesis procedure was greatly simplified. The effect of NH₄NO₃/metal ions to organic mole ratios and the heating temperature on the autocombustion behavior and the properties of the powders derived were investigated in detail. The critical amount of NH₄NO₃ for the autocombustion to occur was identified at the NO₃⁻ to citric acid to EDTA mole ratio of around 10:2:1. After the experimental optimization, well-crystallized nanostructured La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) powder with a specific surface area as high as 21 m²/g was obtained; it is comparable with that obtained from a normal complexing process. By adjusting the combustion parameters, the properties of the powders then derived can be tailored for different applications, such as nanograined dense membrane for oxygen separation membrane, and porous cathode for fuel cells and sensors.     

Photoelectrochemical study of lanthanide zirconium oxides, Ln2Zr2O7 (Ln = La, Ce, Nd and Sm)

Photocatalytic activities for hydrogen evolution of lanthanide zirconium oxides, Ln₂Zr₂O₇ (Ln = La, Ce, Nd and Sm) prepared by a solution reaction method were investigated. Under the illumination of 500 W Xenon lamps, hydrogen gas was clearly evolved in a distilled water suspension of La₂Zr₂O₇, Sm₂Zr₂O₇ and Nd₂Zr₂O₇. Under the visible-light illumination, hydrogen gas was evolved in a distilled water suspension of Nd₂Zr₂O₇ and Sm₂Zr₂O₇. From the photoelectrochemical measurements, the values of the flat band potential were estimated to be −0.64, −0.52, −0.31 and +0.04 eV for La₂Zr₂O₇, Sm₂Zr₂O₇, Nd₂Zr₂O₇ and Ce₂Zr₂O₇, respectively, versus the normal hydrogen electrode (NHE). The values of the band gap energy were calculated to be about 3.52, 2.86, 2.67 and 2.53 eV for La₂Zr₂O₇, Sm₂Zr₂O₇, Nd₂Zr₂O₇ and Ce₂Zr₂O₇, respectively. Due to the effect of 4f orbital electrons, the band gap energy of these compounds becomes narrower than in ZrO₂ and as a consequence, Sm₂Zr₂O₇ and Nd₂Zr₂O₇ show the photocatalytic activity under the visible-light.     

Ce(Au,Sb)2 with UHg2 structure type, a new member of the AlB2 family

A new ternary compound Ce(Au,Sb)₂, with a homogeneity range has been observed from X-ray powder diffraction of as cast alloys, a = 4.743–4.712 Å, c = 3.567–3.768 Å. Its crystal structure was investigated by X-ray diffraction from Ce(Au_1−xSb_x)₂ (x = 0.266) single crystal: CAD-4 automatic diffractometer, Mo K radiation, a = 4.7256(6) Å, c = 3.6711(6) Å, P6/mmm space group, V = 70.997(17) Å³, Z = 1, ρ = 10.732 Mg/m³, μ = 76.369 mm⁻¹, R₁ = 0.0415, wR₂ = 0.0793 for 99 reflections with I > 2σ(I₀). The coordination polyhedron of X (X = 0.734Au + 0.266Sb) atom is a full-capped trigonal prism [XCe₆X₃X₂]. Ce atom is coordinated by 14 atoms: [CeX₁₂Ce₂]. The compound is isotypic with UHg₂ structure, a deformation derivative of AlB₂ structure type. It forms isostructural compounds with La and Pr.     

Effect of Cu on the magnetic properties and reducibility of Fe2TiO5

Ferrites have been studied for several years due to their wide use as magnetic materials for telecommunications, audio and video, power transformers and many other applications.

Equimolar mixtures of Fe₂O₃ and TiO₂ were fired in a muffle furnace at 1200 °C for 4 h. Mixed samples were prepared by replacing TiO₂ with calculated amounts of CuO (x = 0.2, 0.4, 0.6, 0.8 and 1 mol). The synthesized samples were characterized with X-ray diffraction and their magnetic properties were measured using vibrating-sample magnetometer. The microstructure of the sample was examined using reflected light microscope and scanning electron microscope. The formation of Fe₂TiO₅, Fe₅CuO₈, Cu₂TiO₃ and CuFeO₂ phases were detected whereas their magnetic properties increased with increasing the added mole ratio of Cu²⁺ ions. The isothermal reduction kinetics of synthesized nanocrystallites Ti–Cu mixed ferrite compacts were studied at 500 °C using hydrogen gas. It was found that the reduction rate and the reduction extent increased with increasing the extent of Cu²⁺ (0.2–1) whereas the maximum reduction extent (100%) was detected for pure Cu ferrite (Cu²⁺) while the minimum reduction extent (12%) was detected for pure iron titanate (Cu²⁺ = 0). The magnetic properties showed a drastic improvement upon reduction with hydrogen gas.     

The characteristics of Li(CoxNi1 − x)O2 cathode powders formed from the fine-sized Co3O4/NiO precursor powders

Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 1) cathode powders were prepared by solid state reaction method using Co₃O₄/NiO precursor powders obtained by spray pyrolysis. The effect of the ratios of cobalt and nickel components on the characteristics of Co₃O₄/NiO precursor and Li(Co_xNi_1 − x)O₂ cathode powders were investigated. The Co₃O₄/NiO precursor powders with the ratios of cobalt and nickel components as 1/0, 0.75/0.25 and 0.5/0.5 had submicron size and regular morphologies. On the other hand, the Co₃O₄/NiO powders with the high contents of nickel component had aggregated morphologies of submicron size primary powders. The fine-sized precursor powders formed the fine-sized LiCoO₂ and Li(Co_0.75Ni_0.25)O₂ cathode powders by solid state reaction with LiOH powders. However, the high contents of the nickel component of the Co₃O₄/NiO precursor powders formed the Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 0.5) cathode powders with aggregated morphologies and large sizes. The discharge capacities of the powders increased with increasing the nickel content into the Li(Co_xNi_1 − x)O₂ cathode powders up to 188 mAh/g.     

Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er)

The REFe₆Sn₄Ge₂ (RE = Y, Gd–Er) compounds have been synthesized and studied by powder X-ray diffraction and magnetisation measurements. These compounds crystallize in the hexagonal HfFe₆Ge₆ structure although the parent ternary compounds REFe₆X₆ (X = Ge, Sn) display more complicated orthorhombic crystal structure. This evolution is discussed and interpreted on the basis of the relaxation of some RE–X contacts in the quaternary compounds. The iron sublattice order antiferromagnetically above room temperature (554 ≤ T_N ≤ 560 K) while the paramagnetic RE compounds display a second transition at low temperature (7.3 ≤ T_t ≤ 42.7 K). The magnetisation versus field curves display a metamagnetic behaviour at 4.2 K. The corresponding value of the magnetisation suggests a non-collinear ordering of the RE sublattice.     

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.     

Microstructure and mechanical properties of mechanically alloyed and spark plasma sintered amorphous–nanocrystalline Al65Cu20Ti15 intermetallic matrix composite reinforced with TiO2 nanoparticles

Multiphase Al₆₅Cu₂₀Ti₁₅ intermetallic alloy matrix composite, dispersed with 10 wt.% of TiO₂ nanoparticles, has been processed by mechanical alloying, followed by spark plasma sintering under pressure in the temperature range of 623–873 K. Differential scanning calorimetry and X-ray diffraction suggest that equilibrium crystalline phases evolve from the amorphous or intermediate crystalline phases. Transmission electron microscopy shows that the composite sintered at 873 K has partially amorphous microstructure, with dispersion of equilibrium, crystalline, intermetallic precipitates of Al₅CuTi₂, Al₃Ti, and Al₂Cu of 25–50 nm size, besides the TiO₂. The composite sintered at 873 K exhibits little porosity, hardness of 5.6 GPa, indentation fracture toughness in the range of 3.1–4.2 MPa√m, and compressive strength of 1.1 GPa. Indentation crack deflection by TiO₂ particle aggregates causes increase in fracture resistance with crack length, and suggests R-curve type behaviour. The study provides guidelines for processing high strength amorphous–nanocrystalline intermetallic composites based on the Al–Cu–Ti ternary system.     

Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.8Ti0.2O3−δ solid solutions

Series of perovskite-type compounds La_1−aCa_aCr_0.8Ti_0.2O_3−δ (a=0–1.0) were synthesized by the ceramic technique in air (final heating 1350 °C). The crystal structure of the compounds after cooling in air to room temperature was characterized as orthorhombic in space group Pbnm. Analysis of the lattice constants shows a noticeable decrease with increasing Ca content. All compounds prepared were stable in air and in a stream of Ar/1 Pa O₂ at 20–1400 °C, as also in Ar/5% H₂ (pH₂O/pH₂=0.01) at 850–1000 °C. Oxygen stoichiometry and electrical conductivity of the solid solutions with a=0.0–1.0 are investigated. Increasing Ca contents decrease the stability of the oxides in respect to the thermal dissociation of oxygen. All compounds are p-type semiconductors in the temperature range 20–1000 °C at oxygen partial pressures of 10⁻¹⁵ to 0.21×10⁵ Pa. A maximum conductivity of about 30 S/cm in air at 1000 °C is observed for the composition with a=0.6 corresponding to a ratio of Cr³⁺/Cr⁴⁺=1 at an oxygen stoichiometry near 3.0, and oxidation states of La, Ca, Ti, and O ions of 3+, 2+, 4+, and 2−, respectively.     

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.     

Magnetocrystalline anisotropy of Nd3(Fe1−xCox)27.7Ti1.3Ny compounds

The effect of Co substitution for Fe in Nd₃(Fe_1−xCo_x)_27.7Ti_1.3N_y (0 ≤ x ≤ 0.4) compounds on the magnetocrystalline anisotropy has been investigated. The anisotropy constants K's and the anisotropy field H_A have been deduced from the magnetization curves measured on magnetically aligned powder (4–7 μm) samples. The obtained results show that at RT the anisotropy is uniaxial and H_A (about 10 T) does not change substantially upon the substitution. At 5 K the results for K's give evidence for the presence of easy-cone-type anisotropy. The cone angle as well as the anisotropy field decrease upon the substitution from 21.6° to 11.8° and from 22.8 to 18.6 T, respectively.     

An investigation of the structures of the compounds in the tin-rich part of the Dy–Sn system: Crystal structure of Dy5Sn11 and Dy5Sn13

The structural properties of the compounds in the tin-rich part of the dysprosium–tin system have been studied by X-ray powder diffraction. The crystal structures of six compounds DySn_2+x (0 < x < 1) have been characterized. There are four compounds with known structural types: DySn₂ with the ZrSi₂ structure, Dy₃Sn₇ with the Gd₃Sn₇ structure, Dy₂Sn₅ with the Er₂Ge₅ structure, DySn₃ with the DyGe₃ structure and two compounds characterized by new body-centred orthorhombic types (Immm): Dy₅Sn₁₁ (a = 4.411 Å, b = 42.50 Å and c = 4.328 Å) and Dy₅Sn₁₃ (a = 4.341 Å, b = 48.05 Å and c = 4.405 Å) which result from various insertions of AuCu₃ and Po slabs into the ZrSi₂ structure. The relationships and structural evolution are discussed.     

Photoelectrochemical study of lanthanide titanium oxides, Ln2Ti2O7 (Ln = La, Sm, and Gd)

Photocatalytic activities for hydrogen evolution of lanthanide titanium oxides, Ln₂Ti₂O₇ (Ln = La, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb) prepared by a solid-state reaction were studied. Hydrogen gas was clearly evolved in distilled water suspension of La₂Ti₂O₇ and Sm₂Ti₂O₇. From the photoelectrochemical measurements, the values of the flat band potential were estimated to be −0.04, −0.02, +0.27 eV for La₂Ti₂O₇, Sm₂Ti₂O₇, Gd₂Ti₂O₇, respectively, versus the normal hydrogen electrode (NHE). The values of the band gap energy were calculated to be about 3.29, 2.79, 2.82 eV for La₂Ti₂O₇, Sm₂Ti₂O₇, Gd₂Ti₂O₇, respectively. The photocatalytic activities of Ln₂Ti₂O₇ (La, Sm, and Gd) were discussed along with detailed band structures estimated in this study. From the band structures, Sm₂Ti₂O₇ is a possible candidate of photocatalyst responding to visible light.