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.     

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.     

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.     

Production of niobium carbide ceramic composites derived from polymer/filler mixtures: preliminary results

Studies have shown that Al₂O₃–NbC composites present a good potential to be used for metalworking. Manufacturing of composite ceramic material derived from polymer reactive filler mixtures were investigated. The present study reports the preliminary results of reaction bonded niobium carbide derived from polymer (polysiloxane), inert filler (Al₂O₃) and reactive filler (Nb). Niobium powder, alumina and polysiloxane mixtures were homogenized in a planetary ball milling and pressureless sintered in inert atmosphere at temperatures up to 1600 °C. Depending on the niobium content and pyrolysis conditions, ceramic materials with a porosity of 20–40%, a weight loss of 5–15%, a linear shrinkage of 2–4% and a flexural strength of maximum 80 MPa were obtained. X-ray diffraction (XRD) of a sample containing 60 wt% polymer + 40 wt% Nb showed the presence of new crystalline phases such as NbC, Nb₃Si and Nb₅Si₃.     

Sm0.5Sr0.5CoO3 cathode material from glycine-nitrate process: Formation, characterization, and application in LaGaO3-based solid oxide fuel cells

Cathode material Sm_0.5Sr_0.5CoO₃ (SSC) with perovskite structure for intermediate temperature solid oxide fuel cell was synthesized using glycine-nitrate process (GNP). The phase evolution and the properties of Sm_0.5Sr_0.5CoO₃ were investigated. The single cell performance was also tested using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) as electrolyte and SSC as cathode. The results show that the formation of perovskite phase from synthesized precursor obtained by GNP begins at a calcining temperature of 600 °C. The single perovskite phase is formed completely after sintering at a temperature of 1000 °C. The phase formation temperature for SSC with complete single perovskite phase is from 1000 to 1100 °C. The SrSm₂O₄ phase appeared in the sample sintered at 1200 °C. It is also found that the sample sintered at 1200 °C has a higher conductivity. The electrical conductivity of sample is higher than 1000 S/cm at all temperature examined from 250 to 850 °C, and the highest conductivity reaches 2514 S/cm at 250 °C. The thermal expansion coefficient of sample SSC is 22.8 × 10⁻⁶ K⁻¹ from 30 to 1000 °C in air. The maximum output power density of LSGM electrolyte single cell attains 222 and 293 mW/cm² at 800 and 850 °C, respectively.     

Formation of Al–Y oxide during processing of γ-TiAl

While processing Y₂O₃ dispersed γ-TiAl, Y₂O₃ particles which dissolved during hot isostatic pressing (HIP’ing) were found to precipitate during the heat treatment in the form of a mixed Al–Y oxide. To understand the chemical reaction that occurs between Y₂O₃ and γ-TiAl during the heat treatment cycle, a powder mixture comprising of γ-TiAl and 10 wt.% Y₂O₃ was mechanically alloyed (MA’d) for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 °C prior to analyzing it using X-ray diffraction technique. The present study clearly demonstrates that aluminum in the combined form either as γ-TiAl or Al₂O₃ reacts in a similar manner with Y₂O₃ when milled and heat treated at 1150 °C. In either case there is formation of Al₂Y₄O₉ (2Y₂O₃.Al₂O₃).     

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.     

A study of the boron-rich corner of the Er---Al---B system

The ternary phase equilibria of the Er---Al---B system have been investigated and determined at 1600 °C. The ternary phase Er_1—xAl_1—yB₁₄ forms equilibria with ErB₄, ErB₁₂, ErB₆₆, -AlB₁₂ and ErAlB₄. X-Ray diffraction technique, electron microprobe analysis and X-ray powder Rietveld refinements were used to study the phase equilibria. The presence of the ternary phase Er_1—xAl_1—yB₁₄ with a homogeneity range around the composition Er_0.57Al_0.62B₁₄ has been established.     

Preparation of neodymium sulphides by the reaction of Nd2(So4)3 with carbon disulphide

The reaction to synthetize neodymium sulphides from neodymium sulphate octahydrate in a stream of carbon disulphide gas was studied. The dehydration of the octahydrate in vacuum was finished at 300 °C. At 1050–1100 °C in air neodymium oxysulphide, Nd₂O₂SO₄, was formed. Neodymium oxysulphide, Nd₂O₂S, was formed upon heating with a reducing agent such as annealed carbon. The reaction of neodymium sulphate with carbon disulphide commenced at 500–600 °C, resulting in formation of the disulphide, NdS₂. The crystal structure of NdS₂ heated at 500 °C was, however, different from that of the sample heated at 600 °C. In the temperature range 800–900 °C -Nd₂S₃ was obtained as a single phase after heating for at least 3 h in high flow rates of gas mixtures of nitrogen and high concentrations of carbon disulphide. The sesquisulphide, γ-Nd₂S₃ (or Nd₃S₄), was formed at temperatures as high as 1100 °C. The reaction conditions for the compounds mentioned above are discussed together with the analysis of their crystal structures by X-ray powder diffractometry.     

Synthesis and characterization of molybdenum aluminide nanoparticles reinforced aluminium matrix composites

Aluminium matrix composites reinforced with molybdenum aluminide nanoparticles were synthesized by ball milling and reactive sintering of the mixture of aluminium and 10 wt% hydrated molybdenum oxide powders. Sintering the as milled powder in air below 750 °C produced MoAl₁₂ intermetallic compound nanoparticles, at 750 °C produced a mixture of MoAl₅ and MoAl₄ nanoparticles and at 800 °C under Argon atmosphere produced predominantly MoAl₄ intermetallic nano-particles in the Al matrix. The powder compacts sintered in air below 750 °C produced MoAl₁₂ whereas at 750 °C or above formed the Al matrix composite reinforced with the MoAl₅ nanoparticles. These nanoparticles become agglomerated to take up some irregular shaped flakes in the metal matrix. The reaction between Al and hydrated Mo oxide powders was found to be a favorable way to produce predominantly a particular Mo–Al intermetallic compound at a particular temperature. The Al₂O₃ particles formed as another reaction product, in all the above reactions, remain distributed in these composites. The composites thus formed were characterized by SEM-EDS, DTA, XRD and TEM analysis.     

Preparation and luminescent properties of europium-activated YInGe2O7 phosphors

Effects of precursor milling on phase evolution and morphology of mullite (3Al₂O₃·2SiO₂) processed by solid-state reaction have been investigated. Alumina and silica powders were used as starting materials and milling was taken place in a medium energy conventional ball mill and a high-energy planetary ball mill. Milling in a conventional ball mill although decreases mullite formation temperature by 200 °C, but does not considerably change mullite phase morphology. Use of a planetary ball mill after 40 h of milling showed to be much more effective in activating the oxide precursors, and mullitization temperature was reduced to below 900 °C. Whisker like mullite was formed after sintering at 1450 °C for 2 h and volume fraction of this structure was increased by increasing the milling time. XRD results showed that samples mechanically activated for 20 h in the planetary ball mill were fully transformed to mullite after sintering at 1450 °C, whereas Al₂O₃ and SiO₂ phases were still detected in the samples milled in the conventional ball mill for 20 h and then sintered at the same conditions.     

Microstructural evolution of some MgO–TiO2 and MgO–Al2O3 powder mixtures during high-energy ball milling and post-annealing studied by X-ray diffraction

High-energy dry ball-mill and post-anneal processing were applied to synthesize MgTiO₃ and Mg₂TiO₄ single crystalline phases from the predetermined compositions of MgO–TiO₂ powder mixtures. Also, the experiments were performed to show that it is possible to prepare MgAl₂O₄ single crystalline phase from the predetermined composition of MgO–Al₂O₃ powder mixture only by employing high-energy dry ball milling, i.e. without post-annealing the milled samples. In contrast, fully developed single crystalline powders of MgTiO₃ and Mg₂TiO₄ were obtained after post-annealing the milled samples for 1 h at 900 and 1200 °C, respectively.     

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%.     

Subsolidus phase relations in the ZnO–MoO3–B2O3, ZnO–MoO3–WO3 and ZnO–WO3–B2O3 ternary systems

The subsolidus phase relations in the ZnO–MoO₃–B₂O₃, ZnO–MoO₃–WO₃ and ZnO–WO₃–B₂O₃ ternary systems have been investigated by the means of X-ray powder diffraction (XRD). There is no ternary compound in all the systems. There are five binary compounds and five tie lines in the ZnO–MoO₃–B₂O₃ system. This system can be divided into six 3-phase regions. There are three binary compounds and three tie lines in the ZnO–MoO₃–WO₃ system. This system can be divided into four 3-phase regions. There are four binary compounds and four tie lines in the ZnO–WO₃–B₂O₃ system. This system can be divided into five 3-phase regions. The possible component regions for ZnO single crystal flux growth were discussed. The phase diagram of Zn₃B₂O₆–ZnWO₄ pseudo-binary system has been constructed, and the result reveals this system is eutectic system. The eutectic temperature is 1007 °C and eutectic point component is 70 mol% Zn₃B₂O₆.     

Effects of TiC addition on formation of Ti3SiC2 by self-propagating high-temperature synthesis

Formation of Ti₃SiC₂ was conducted by self-propagating high-temperature synthesis (SHS) from both the elemental powder compacts of Ti:Si:C = 3:1:2 and the TiC-containing samples compressed from powder mixtures of Ti/Si/C/TiC with TiC content ranging from 4.3 to 33.3 mol%. The effect of TiC addition was studied on combustion characteristics and the degree of phase conversion. For the elemental powder compacts, with the progress of combustion wave the sample experiences substantial deformation, including axial elongation and radial contraction. The extent of sample deformation and flame-front propagation velocity were considerably reduced for the samples with TiC addition, because the dilution effect of TiC lowered the reaction temperature. Two reaction mechanisms were adopted to explain the formation of Ti₃SiC₂, one involving the reaction of a Ti–Si liquid phase with solid reactants for the elemental powder compact and the other dominated by the interaction of solid-phase species for the TiC-containing sample. For all products synthesized in this study, the XRD analysis identifies formation of Ti₃SiC₂ along with a major impurity TiC and a small amount of Ti₅Si₃. The resulting Ti₃SiC₂ is typically elongated grains. Based upon the XRD profile, the Ti₃SiC₂ content at a level of 71.5 vol.% was obtained in the product from the elemental powder compact. With the addition of TiC, an improvement in the yield of Ti₃SiC₂ was observed and an optimal conversion reaching 85 vol.% was achieved by the sample with 20 mol% of TiC. However, further increase of the TiC amount led to a decrease in the Ti₃SiC₂ content, because of the low reaction temperature around 1150 °C.     

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.     

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.     

Achieving tensile superplasticity in 8 mol% Y2O3 cubic stabilized ZrO2 through the addition of intergranular silica

The addition of 5 wt.% SiO₂, a viscous second phase, to 8 mol% Y₂O₃ cubic stabilized ZrO₂ (8Y-CSZ) made superplastic 8Y-CSZ. This material had a fine grain size of 0.4 μm and exhibited deformations in tension as large as 520% at 1430 °C with a strain rate of 1.0 × 10⁻⁴ s⁻¹.     

Properties of ZrO2–Al2O3 composite as a function of isothermal holding time

Zirconia and alumina based ceramics present interesting properties for their application as implants, such as biocompatibility, good fracture resistance, as well as high fracture toughness and hardness. In this work the influence of sintering time on the properties of a ZrO₂–Al₂O₃ composite material, containing 20 wt% of Al₂O₃, has been investigated. The ceramic composites were obtained by sintering, in air, at 1600 °C for sintering times between 0 and 1440 min. Sintered samples were characterized by microstructure and crystalline phases, as well as by mechanical properties. The grain growth exponents, n, for the ZrO₂ and Al₂O₃ were 2.8 and 4.1, respectively, indicating that different mechanisms are responsible for grain growth of each phase. After sintering at 1600 °C, the material exhibited a dependency of hardness as function of sintering time, with hardness values between 1500 HV (120 min) and 1310 HV (1440 min) and a fracture toughness of 8 MPa m^1/2, which makes it suitable for bioapplications, such as dental implants.     

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).