Study on phase formation and sintering kinetics of BaTi0.6Zr0.4O3 powder synthesized through modified chemical route

BaTi_0.6Zr_0.4O₃ powder was prepared from barium oxalate hydrate, zirconium oxy-hydroxide and titanium dioxide precursors. Barium oxalate hydrate and zirconium oxy-hydroxide were precipitated from nitrate solution onto the surface of suspended TiO₂. Phase formation behaviour of the materials was extensively studied using XRD. BaTiO₃ (BT) and BaZrO₃ (BZ) start forming separately in the system upon calcinations in the temperature range 600–700 °C. BT–BZ solid solution then forms by diffusion of BT into BZ from 1050 °C onwards. The precursor completely transforms into BaTi_0.6Zr_0.4O₃ (BTZ) at 1200 °C for 2 h calcination. The activation energy (AE) of BT (134 kJ mol⁻¹) formation was found to be less than that of BZ (167.5 kJ mol⁻¹) formation. BTZ formation requires 503.6 kJ mol⁻¹ of energy. The sintering kinetics of the powder was studied using thermal analyzer. The mean activation energy for sintering was found to be 550 kJ mol⁻¹.     

Influence of Al- and Cu-doping on the thermodynamic properties of the LaNiIn–H system

The Pressure–Composition–Temperature (P–C–T) relations for the LaNiIn, LaNi_0.95Cu_0.05In and LaNiIn_0.98Al_0.02–H systems were measured by a volumetric Sieverts’ method at 398–423 K. All isotherms show plateau pressure regions indicating equilibria between two hydride phases. The replacements of Ni by Cu and In by Al affect the P–C–T diagrams, stability of the hydrides, homogeneity regions of the hydrides formed, slope of the isotherms and critical temperatures of the β–γ transition. In addition, the Cu-doping induces a significant hysteresis between the hydrogen absorption and desorption processes. The relative partial molar thermodynamic properties for the studied systems are: ΔH_H = −34.6 ± 2.1 kJ (mol_H)⁻¹, ΔS_H = −70.7 ± 3.6 J (K·mol_H)⁻¹ for LaNiIn–H; ΔH_H = −34.1 ± 0.5 kJ (mol_H)⁻¹, ΔS_H = −74.9 ± 1.0 J (K·mol_H)⁻¹ for LaNi_0.95Cu_0.05In–H; ΔH_H = −33.2 ± 0.8 kJ (mol_H)⁻¹, ΔS_H = −68.3 ± 1.2 J(K·mol_H)⁻¹ for LaNiIn_0.98Al_0.02–H.     

Effects of heat treatment on the properties of powder injection molded AlN ceramics

The effects of two different heat-treatment atmospheres,nitrogen atmosphere and reducing nitrogen atmosphere with carbon,on the properties of Y2O3-doped aluminum nitride (AlN) ceramics were investigated.The AlN powder as a raw material was synthesized by self-propagating high-temperature synthesis (SHS) and compacts were fabricated by employing powder injection molding technique.The polymer-wax binder consisted of 60 wt.% paraffin wax (PW),35 wt.% polypropylene (PP),and 5 wt.% stearic acid (SA).After the removal of binder,specimens were sintered at 1850℃ in nitrogen atmosphere under atmospheric pressure.To improve the thermal conductivity,sintered samples were reheated.The result reveals that the heat-treatment atmosphere has significant effect on the properties and secondary phase of AlN ceramics.The thermal conductivity and density of AlN ceramics reheated in nitrogen gas are 180 W·m-1 K-1 and 3.28 g,cm-3 and the secondary phase is yttrium aluminate.For the sample reheated in reducing nitrogen atmosphere with carbon,the thermal conductivity and density are 173 W.m-1.K-1 and 3.23 g·cm-3,respectively,and the secondary phase is YN.     

Properties of W–Cu composite powder produced by a thermo-mechanical method

In order to improve the process of co-reduction of oxide powder, a new thermo-mechanical method was designed to produce high-dispersed W–Cu composite powder by high temperature oxidation, short time high-energy milling and reduction. The properties of W–Cu composite powder are analyzed in terms of oxygen contents, BET specific surface (BET-S), particle size distributions, morphology of final powder and their sintering behaviors. The results show that the oxygen content of W–Cu composite powder decreases with the increase of milling time, while the BET-S of final powder increases with the milling time. The distributions of final powder are more uniform after reduction at 630 °C than at 700 °C. After milling of the oxide powder for about 3–10 h, W–Cu composite powder with very low oxygen content can be achieved at the reduction temperature of 630 °C owning to the increasing of BET-S of W–Cu oxide powder. The particle size of W–Cu powder after reduction is lower than 0.5 μm and smaller than that reduced at 700 °C. After sintering at 1200 °C for 60 min, the relative density and thermal conductivity of final products (W–20Cu) can attain 99.5% and 210 W m⁻¹ K⁻¹ respectively.     

Direct synthesis of La9.33Si6O26 ultrafine powder via sol–gel self-combustion method

Single phase La_9.33Si₆O₂₆ ultrafine powder, as a kind of highly activated precursor to prepare medium-to-low temperature electrolyte for solid oxide fuel cells (SOFCs), has been successfully synthesized via a non-aqueous sol–gel and self-combustion approach from the starting materials: lanthanum nitrate (La(NO₃)₃·6H₂O), citric acid, ethylene glycol (EG), tetraethyl orthosilicate (TEOS) and ammonium nitrate. The details of gel's self-combustion were investigated by DTA–TG and the structural characterization of as-synthesized powder from self-combustion was performed by XRD and SEM. The results show that La_9.33Si₆O₂₆ single phase of apatite-type crystal structure can be directly synthesized by sol–gel self-combustion method without further calcinations on the condition that the molar ratio (R) of NO₃⁻ to citric acid and ethylene glycol being 6:1. Such powders composed of well-dispersed particles with an average size of 200 nm and a specific surface area of 5.54 m²/g. It can be sintered to 90% of its theoretical density at 1500 °C for 10 h, about 200 °C lower than the sintering temperature for the powder derived from traditional solid reactions. The sintered material has a thermal expansion coefficient of 9.2 × 10⁻⁶ K⁻¹ between room temperature and 800 °C.     

High coercivity in Sm5Fe17 melt-spun ribbon

Sm–Fe binary alloy with the hard magnetic Sm₅Fe₁₇ phase was produced by melt-spinning followed by heat treatment. The magnetic properties of the Sm–Fe melt-spun ribbon depended on the annealing temperature. The highest coercivity, _iH_c = 1.90 MA m⁻¹, was obtained by annealing the Sm–Fe melt-spun ribbon for 1 h at 923 K. The coercivity of the Sm₅Fe₁₇ melt-spun ribbon was attributed to the grain size of the hard magnetic Sm₅Fe₁₇ phase.     

Synthesis and characterization of doped LaCrO3 perovskite prepared by EDTA–citrate complexing method

The La_0.85Sr_0.15Cr_0.95Ni_0.02Co_0.02O₃ (LSC) interconnect materials for solid oxide fuel cells (SOFCs) were synthesized by EDTA–citrate complexing method. Thermal decomposition behavior of the gel, phase formation and morphology of LSC powders were characterized by thermogravimetry/differential thermal (DSC/TG) analysis, X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. It appeared that lower pH value (pH 4.9) of the precursor solution resulted in a transient liquid phase, SrCrO₄, in the calcined LSC powder. The sintering characteristics, electrical conductivity and thermal expansion properties of sintered bars were investigated. La_0.85Sr_0.15Cr_0.95Ni_0.02Co_0.02O₃ prepared in the condition of pH 4.9 showed an electrical conductivity of 15.6 S cm⁻¹ at 800 °C and a thermal expansion coefficient (TEC) of 10.8 × 10⁻⁶ K⁻¹ (20–900 °C), which is suitable for use as interconnect materials for SOFCs.     

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.     

Characterization and sintering of BaZrO3 nanoparticles synthesized through a single-step combustion process

Barium zirconate (BaZrO₃) nanoparticles synthesized by a self-sustained single-step combustion process is reported in this paper. In this process, a phase pure nanopowder of BaZrO₃ has been obtained by the combustion of an aqueous solution containing Ba and Zr ions by using citric acid as complexing agent and liquor ammonia as fuel, thus giving rise to phase pure BaZrO₃ nanopowder in a single-step combustion without any further calcination. The X-ray diffraction studies have shown that the as-prepared powder was single phase, crystalline, and has a cubic perovskite structure (ABO₃) with a lattice constant a = 4.19 Å. The average particle size calculated from FWHM is 30 nm. The phase purity of BaZrO₃ nanopowder has been examined using differential thermal analysis (DTA), thermo gravimetric analysis (TGA) and Fourier transform of infrared spectroscopy (FTIR). The transmission electron microscopic investigation has shown that the particle size of the as-prepared powder was in the range 30–50 nm with a mean size of 40 nm. The nano BaZrO₃ has been sintered to a density of 99% of the theoretical density at 1650 °C in 2 h without the use of any sintering aids. The morphology of the sintered pellets has been studied with scanning electron microscopy (SEM). The dielectric constant (_r) and loss factor (tan δ) values obtained at 10 MHz for a well-sintered barium zirconate pellet has been found to be 32.2 and 1 × 10⁻⁴, respectively, at room temperature.     

Isotope effects on hydrogen absorption by Pd–4at.%Pt alloy

Isotope effects on hydrogen absorption were investigated for a Pd–4at.%Pt alloy by using a high vacuum microbalance. The absorption kinetics were well explained by a model assuming comparative contributions of the dissociative adsorption and associative desorption on the surface, and the diffusion into the bulk. The activation energies for adsorption were determined to be 29.1 and 32.8 kJ mol⁻¹(H₂, D₂) for protium and deuterium, respectively. The activation energies for desorption were 48.1 kJ mol⁻¹(H₂) and 49.0 kJ mol⁻¹(D₂). Accordingly, the heat of absorption was evaluated to be −19.0 kJ mol⁻¹(H₂) for protium and −16.2 kJ mol⁻¹(D₂) for deuterium. The activation energies for diffusion were determined to be 28.7 kJ mol⁻¹(H, D) for both protium and deuterium, but the frequency factor for deuterium was about 1.5 times greater than that for protium.     

Thermal and electrical properties of perovskite-type strontium molybdate

Polycrystalline samples of perovskite-type strontium molybdate, SrMoO₃, have been prepared and the thermal and electrical properties have been measured from room temperature to about 1000 K. The electrical resistivity is of an order of magnitude of 10⁻⁵ to 10⁻⁶ (Ω m) in the whole temperature range. The Seebeck coefficient is around 4–9 μV K⁻¹. At room temperature, the thermal conductivity is about 30 W m⁻¹ K⁻¹, and it decreases with increasing temperature.     

Characterization of crystallization kinetics of a Ni- (Cr, Fe, Si, B, C, P) based amorphous brazing alloy by non-isothermal differential scanning calorimetry

The thermal stability and crystallization kinetics of a Ni- (Cr, Si, Fe, B, C, P) based amorphous brazing foil have been investigated by non-isothermal differential scanning calorimetry. The glass transition temperature T_g, is found to be 720 ± 2 K. The amorphous alloy showed three distinct, yet considerably overlapping crystallization transformations with peak crystallization temperatures centered around 739, 778 and 853 ± 2 K, respectively. The solidus and liquidus temperatures are estimated to be 1250 and 1300 ± 2 K, respectively. The apparent activation energies for the three crystallization reactions have been determined using model free isoconversional methods. The typical values for the three crystallization reactions are: 334, 433 and 468 kJ mol⁻¹, respectively. The X-ray diffraction of the crystallized foil revealed the presence of following compounds Ni₃B (Ni₄B₃), CrB, B₂Fe₁₅Si₃, CrSi₂, and Ni_4.5Si₂B.     

Magnequench magnets status overview

The advent of neodymium-iron-boron materials having excellent magnetic properties and potential economic advantages has initiated a new era in permanent magnet technology. One method of making these magnets is by the rapid solidification process. It is typically carried out by melt spinning, which produces a highly stable, dmagnetically hard microstructure powder, directly from the melt. This can be used for bonded magnet applications. Alternatively, this powder can be hot pressed to produce fully dense isotropic magnets with energy products up to 15 MGOe. Anisotropic magnets with energy products ranging up to 50 MGOe can be produced by thermomechanical orientation or hot deformation process. Current processing and properties of Magnequench (General Motors) materials are reviewed, das well as the applications and advances of these materials. The advances include high-temperature bonded magnet and high-energy product anisotropic bonded and fully dense magnets.     

Behavior of residual carbon in Sm(Co, Fe, Cu, Zr)z permanent magnets

When sintered Sm(Co, Fe, Cu, Zr)_z permanent magnets are prepared by metal injection molding, some organic binders are added in alloy powder, which leads to much residual carbon in the magnets. The residual carbon decreases magnetic properties and destroys the microstructure of the magnets. In this paper, the behavior of carbon in Sm(Co, Fe, Cu, Zr)_z permanent magnets has been studied. The results indicate that Sm(Co, Fe, Cu, Zr)_z magnets can keep excellent magnetic properties when the carbon content is below 0.1 wt.%: Br ≥ 10 kGs, H_cj ≥ 22 kOe, BH_max ≥ 25 MGOe. When the carbon content is above 0.1 wt.%, Br, H_cj and BH_max decrease with increasing carbon content evidently. Carbon consumes Zr content and forms ZrC, which reduces the volume fraction of the lamella and Sm(Co, Cu)₅ phases. Thus, the cell size increases and the cellular microstructure deteriorates. When the carbon content reaches 0.43 wt.%, there is not enough Sm(Co, Cu)₅ phase to form a uniform cellular microstructure. Br, H_cj and BH_max are approximate to zero. Since carbon has little influence on the content of Sm₂(Co, Fe)₁₇ phase, Ms can keep a high value (≥100 emu/g). ZrC has high melting point (3420 °C) and acts as dispersion particle in the magnets, which prevents the grains of SEM structure growing and reduces the liquid content of green compacts during sintering. Therefore, the density of the magnets decreases.     

The ternary system: silicon–titanium–uranium

Phase equilibria in the system Si–Ti–U were established at 1000 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed and were characterised by X-ray powder data refinement: (1) stoichiometric U₂Ti₃Si₄ (U₂Mo₃Si₄-type) with a small homogeneity region of about 3 at.% exchange U/Ti and (2) U_2−xTi_3+xSi₄ (Zr₅Si₄-type) extending at 1000 °C for 0.7<x<1.3. Mutual solubility of U-silicides and Ti-silicides was found to be below about 1 at.%. The Ti,U-rich part of the diagram was also investigated at 850 °C establishing the tie-lines to the low temperature compounds U₂Ti and U₃Si. U₂Ti₃Si₄ is weakly paramagnetic following a Curie–Weiss law above 50 K with μ_eff.=2.67 μ_B/U, Θ_P=−150 K and χ₀=1.45×10⁻³ emu/mol (18.2×10⁻⁹ m³/mol).     

Porous AlN ceramic substrates by reaction sintering

A novel approach was undertaken in producing porous AlN microelectronics tapes with high thermal conductivity and low dielectric constant. This method essentially utilised polymer micro-spherical powders that were used as a sacrificial mould to introduce controlled porosity into the green tapes during pyrolysis. The Al₂O₃-rich porous green tapes were then reaction sintered at 1680 °C for 12 h to achieve porous AlN tapes. This work builds upon the previously developed novel reaction sintering process that densified and converted Al₂O₃-rich tapes (Al₂O₃–20 wt.% AlN–5 wt.% Y₂O₃) to AlN tapes at a relatively low sintering temperature of 1680 °C. The sintering behaviour of the porous tapes was investigated, and the effects of the microspheres particle size and volume addition were studied. The microspheres successfully contributed to the significant reduction of tape density by porosity, and this contributed to lowering its dielectric constant. Dielectric constant of the AlN tapes were reduced to about 6.8–7.7 whilst thermal conductivity values were reasonable at about 46–60 W/m K. Coefficient of thermal expansion (CTE) values showed a linear trend according to phase composition, with the porous AlN tapes exhibiting CTE values of (4.4–4.8)×10⁻⁶ °C⁻¹, showing good CTE compatibility with silicon, at 4.0×10⁻⁶ °C⁻¹. The added porosity did not significantly affect the CTE values.     

Gibbs free energy of formation of the ternary oxide Nd3RuO7

The Gibbs free energy of formation of Nd₃RuO₇(s) has been determined using solid-state electrochemical cell employing oxide ion conducting electrolyte. The electromotive force (e.m.f.) of the following solid-state electrochemical cell has been measured, in the temperature range from 929.3 to 1228.6 K.

Cell: (−)Pt/{Nd₃RuO₇(s) + Nd₂O₃(s) + Ru(s)}//CSZ//O₂(p(O₂) = 21.21 kPa)/Pt(+)

The Gibbs free energy of formation of Nd₃RuO₇(s) from elements in their standard state, calculated by the least squares regression analysis of the data obtained in the present study, can be given by:

{Δ_fG°(Nd₃RuO₇, s)/(kJ mol⁻¹) ± 1.6} = −3074.3 + 0.6097(T/K); (929.3 ≤ T/K ≤ 1228.6).

The uncertainty estimate for Δ_fG°(T) includes the standard deviation in e.m.f. and the uncertainty in the data taken from the literature. The intercept and the slope of the above equation correspond to the enthalpy of formation and entropy, respectively, at the average experimental temperature of T_av. = 1079 K.     

Lanthanide(III) halides: Thermodynamic properties and their correlation with crystal structure

Temperatures and enthalpies of phase transitions of 17 lanthanide(III) halides determined experimentally are reported. Correlations were made between temperature of fusion of lanthanide(III) halides, on the one hand, and enthalpy of fusion, on the other, versus atomic number of lanthanide. According to this classification, the lanthanide(III) halides split into groups, as also do the corresponding crystal structures. A correlation between the crystal structure of lanthanide(III) halides and their respective entropy of fusion (or entropy of fusion + entropy of solid–solid phase transition) was inferred from the aforementioned features. Fusion in those halides with hexagonal, UCl₃-type and orthorhombic, PuBr₃-type, structures entails an entropy of fusion change (or entropy of fusion + entropy of solid–solid phase transition change) by 50 ± 4 J mol⁻¹ K⁻¹. The homologous entropy change within the group of halides having the rhomboedric, FeCl₃-type, structure, is smaller and equals 40 ± 4 J mol⁻¹ K⁻¹. Halides with monoclinic, AlCl₃-type, crystal structure constitute a third group associated to an even smaller entropy change upon fusion, only 31 ± 4 J mol⁻¹ K⁻¹. The halides with lower entropies of fusion also have a lower S_1300 K − S_298 K indicating either a higher degree of order in the liquid or a higher entropy in the solid at room temperatures.     

Enthalpies of formation of solid Sm---Al alloys

A direct isoperibolic differential calorimeter was used to measure the formation heats of the Sm---Al intermetallic compounds. X-ray powder diffraction, optical and scanning electron microscopy and electron probe microanalysis were used to check the composition of the samples. The following values of Δ_fH⁰ for the different compounds were obtained in the solid state at 300 K: Sm₂Al, = −38.0 ± 2 kJ (mol atoms)⁻¹; SmAl, −49.0 ± 2 kJ (mol atoms)⁻¹; SmAl₂, −55.0 ± 2 kJ (mol atoms)⁻¹; SmAl₃, −48.0 ± 2 kJ (mol atoms)⁻¹. The results are discussed and compared with earlier experimental data.