Mechanical synthesis and rapid consolidation of a nanocrystalline 3.3Fe0.6Cr0.3Al0.1-Al2O3 composite by high frequency induction heating

Nanopowders of 3.3Fe_0.6Cr_0.3Al_0.1 and Al₂O₃ were synthesized from Fe₂O₃, Cr, and Al powders by high-energy ball milling. A high density nanocrystalline 3.3Fe_0.6Cr_0.3Al_0.1-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within 3 min from mechanically synthesized powders of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1. The average grain sizes of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1 were 84 and 32 nm, respectively.     

A general hydrothermal route to synthesis of nanocrystalline lanthanide stannates: Ln2Sn2O7 (Ln = Y, La–Yb)

A general hydrothermal process with use of lanthanide (III) nitrates and Na₂SnO₃ as precursors has been proposed for synthesizing nanocrystalline lanthanide stannates with general formula: Ln₂Sn₂O₇ (Ln = Y, La–Yb). Stannates of all lanthanides except for the radioactive promethium were successfully synthesized. Characterization by XRD and TEM revealed that all the products were phase-pure nanocrystalline lanthanide stannates with pyrochlore-type structure. Photoluminescent properties of three samples (i.e. Tb₂Sn₂O₇, Dy₂Sn₂O₇ and Yb₂Sn₂O₇) are also presented. The mole ratio of Ln(NO₃)₃:Na₂SnO₃ and hydrothermal temperature were two key factors for this general hydrothermal route.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

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.     

Fast consolidation of a nanocrystalline Al2O3 reinforced 3.7Fe0.54Cr0.18Al0.26Si0.016 composite from mechanical synthesized powder through pulsed current activated heating

Nanopowders of Fe_0.54Cr_0.18Al_0.26Si_0.016 and Al₂O₃ were synthesized from Fe₂O₃, Cr, Si, and Al powders using high-energy ball milling. A high-density nanocrystalline 3.7Fe_0.54Cr_0.18Al_0.26Si_0.016-Al₂O₃ composite was consolidated with mechanically synthesized powders of Al₂O₃ and 3.7Fe_0.54Cr_0.18Al_0.26Si_0.016-Al₂O₃ through a pulsed current activated sintering (PCAS) method within 1 min. The hardness of the composite and the average grain sizes of Al₂O₃ and Fe_0.54Cr_0.18Al_0.26Si_0.016 were investigated.     

Synthesis of MoSi2/WSi2 nanocrystalline powder by mechanical-assistant combustion synthesis method

MoSi₂/WSi₂ nanocrystalline powder has been successfully synthesized by the mechanical-assistant combustion synthesis method. This method includes a ball-milling process followed by combustion synthesis. The composition and microstructure of the as-milled powder mixture were detected by X-ray diffraction and scanning electron microscopy analyses. Their results show that the Mo(W) solid solution and Si nanocrystals could be obtained during the ball-milling process. Compared with normal powder mixture (Mo + Si + W), it could be easily ignited and high maximum combustion temperature was achieved. It was also confirmed that MoSi₂/WSi₂ solid solution powder with nanometric structure could be prepared through combustion synthesis method from the mechanical activated powder mixture.     

Nano-sized Y2O3:Eu hollow spheres with enhanced photoluminescence properties

Nano-sized Y₂O₃:Eu³⁺ hollow spheres were fabricated via a facile strategy including preparation of the hollow precursor and a later calcination. Moreover, the growth process of these hollow spheres was monitored by time-dependent experiments and their luminescence properties were also intensively studied. The products exhibit strong red emitting at 613 nm under ultraviolet excitation and control experiments were carried out to optimize the synthetic conditions. It was found 850 °C calcination with 9 mol% doping level could give out the best photoluminescence performance. Moreover, a possible mechanism for the enhanced PL performance was also proposed based on the FT-IR investigation.     

New low thermal expansion ceramics: Sintering and thermal behavior of Ln1/3Zr2(PO4)3-based composites

Materials with the general formula M_xZr₂(PO₄)₃ are known to possess low coefficients of thermal expansion (CTE). The present work investigates the thermal properties of new composite materials issued from the decomposition at high temperature of Ln_1/3Zr₂(PO₄)₃ (Ln=La, Gd). The decomposition process was studied and showed that the resulting powder was a LnPO₄, Zr₂P₂O₉ and ZrO₂ mixture. Composite materials made of that mixture were sintered and characterized. The effect of sintering aids such as ZnO was considered. Final densities of the composites were about 90% of theoretical density and these materials presented low CTE in the 10⁻⁶ °C⁻¹ range.     

The effect of particle size,sintering temperature and sintering time on the properties of Al–Al2O3 composites,made by powder metallurgy

Al₂O₃ is a major reinforcement in aluminum-based composites, which have been developing rapidly in recent years. The aim of this paper is to investigate the effect of alumina particle size, sintering temperature and sintering time on the properties of Al–Al₂O₃ composite. The average particle size of alumina were 3, 12 and 48 μm. Sintering temperature and time were in the range of 500–600 °C for 30–90 min. A correlation is established between the microstructure and mechanical properties. The investigated properties include density, hardness, microstructure, yield strength, compressive strength and elongation to fracture. It has been concluded that as the particle size of alumina is reduced, the density is increased followed by a fall in density. In addition, at low particle size, the hardness and yield strength and compressive strength and elongation to fracture were higher, compared to coarse particles size of alumina. The variations in properties of Al–Al₂O₃ composite are dependent on both sintering temperature and time. Prolonged sintering times had an adverse effect on the strength of the composite.     

Pressureless sintering of internally synthesized SiC-TiB2 composites with improved fracture strength

SiC-TiB₂ particulate composites were fabricated by converting TiO₂ to TiB₂ through the reaction between TiO₂, B₄C and C. The presence of initially very fine, in-situ created, TiB₂ particles increased driving force for sintering and enabled fabrication of a dense composite utilizing pressureless sintering and the liquid phase created between Al₂O₃ and Y₂O₃ additives. The effect of volume fraction of the in-situ formed TiB₂ on density, microstructure and flexural strength was discussed. It was found that the presence of TiB₂ particles suppressed the growth of SiC grains and enhanced fracture strength. The fracture strength of samples containing 12 vol% TiB₂ was more than 30% higher than that of the monolithic SiC. The effect of SiC grain size on fracture strength was also analyzed.     

Preparation of nanocrystalline Lu2O3:Eu phosphor via a molten salts route

The paper reports on the course of decomposition of hydrated lutetium nitrate and lutetium chloride to Lu₂O₃ in the eutectic mixture of NaNO₂ and KNO₂. It was shown that a crystallographically pure phase of the cubic Lu₂O₃ is formed at temperature as low as 250 °C. IR spectra revealed that the recovered powder contains some OH-contamination, however. The powders are characterized by crystallites sizes in the range of 18–30 nm in average. Emission and excitation spectra of Eu-doped powders show characteristic features for Eu³⁺ ion in an oxide host, which indicates that the procedure is appropriate for making activated nanoparticulate oxide phosphors. Most profound emission appears around 611 nm and the luminescence from the powder made starting with Lu(NO₃)₃ was noticeably higher compared to the product obtained from LuCl₃. The excitation spectrum of Eu³⁺ emission at 611 nm contains a band related to the fundamental absorption of the lutetia host lattice, which indicates an existence of the host-to-activator energy transfer.     

Synthesis and consolidation of TiN/TiB2 ceramic composites via reactive spark plasma sintering

The simultaneous synthesis and densification of TiN/TiB₂ ceramic composites via reactive spark plasma sintering (RSPS) was investigated. Different component ratios (TiH₂/BN (TiN, B)) and heating rates (112.5-300 °C/min) were used to initiate the chemical reaction for TiN/TiB₂ synthesis. The omit RSPS process was revealed to have three stages, which are described separately. The relationships between the RSPS conditions, the microstructure and the properties of sintered ceramic composites were established. A Vickers hardness of 16-25 GPa and a fracture toughness of 4-6.5 MPa m^1/2 were measured for various compositions. Sintered ceramic composites containing 36 wt% TiB₂ with the highest relative density of 97.4 ± 0.4% and an average grain size of 150-550 nm have been obtained.     

AlN ceramics prepared by high-pressure sintering with La2O3 as a sintering aid

The magnetic and dielectric properties of Bi–Zn codoped Y-type hexagonal ferrite was investigated. The samples with composition of Ba_2−xBi_xZn_0.8+xCo_0.8Cu_0.4Fe_12−xO₂₂ (x = 0–0.4) were prepared by the solid-state reaction method. Phase formation was characterized by X-ray diffraction. The microstructure was observed via scanning electron microscopy. The magnetic and dielectric properties were measured using an impedance analyzer. Direct current (dc) electrical resistivity was measured using a pA meter/dc voltage source. Minor Bi doping (x = 0.05–0.25) will not destroy the phase formation of Y-type hexagonal ferrite, but lower the phase formation temperature distinctly. Bi substitution can also promote the sintering process. The Bi-containing samples (x > 0.05) can be sintered well under 900 °C without any other addition. The sintering temperature is about 200 °C lower than that of the Bi-free sample. The Bi–Zn codoped samples exhibit excellent magnetic and dielectric properties in hyper frequency. These materials are suitable for multi-layer chip-inductive components.     

An evaluation of Al2O3-ZrO2 composites produced by coprecipitation method

The objective of this work is to produce Al₂O₃-ZrO₂ composite from nano-sized powders processed by coprecipitation method. Al₂O₃ and mixture of Al₂O₃ + 10 wt.% ZrO₂ precipitated successfully by chemical route from aluminum sulfate and zirconium sulfate were pressed under uniaxial compression of 170 MPa and sintered at 1600 °C for 1 h. SEM investigations revealed that, pure alumina sample has a microstructure with coarse grains which anisotropically grown up to 30-40 μm in size. In alumina-zirconia composite, the structure consists of very fine equiaxed grains of typically 2 μm in which zirconia precipitates were uniformly dispersed. By adding zirconia to alumina, hardness and indentation fracture toughness were increased from 11.6 GPa to 16.8 GPa and from 3.2 MPa m^1/2 to 4.9 MPa m^1/2, respectively. Improvement in fracture toughness was attributed to bridging effects of zirconia particles as well as transformation toughening.     

Simple hydrothermal synthesis and sintering of Na0.5Bi0.5TiO3 nanowires

Single-crystalline Na_0.5Bi_0.5TiO₃ (NBT) nanowires, with diameters of 100 nm and lengths of about 4 μm, were synthesized by using a simple hydrothermal method. Phase composition, morphology and microstructure of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The effects of reaction temperature and reaction time on precipitation of the NBT nanowires were investigated. It was found that reaction time significantly influenced the growth behavior of the powders in the hydrothermal system. Based on the experimental results, the one-dimensional (1D) growth mechanism of the NBT was governed by a dissolution-recrystallization mechanism. NBT ceramics derived from the nanowires showed typical characteristics of relaxor ferroelectrics, with diffuseness exponent γ of as high as 1.73.     

Synthesis of dense WSi2 and WSi2-xvol.%SiC composites by high-frequency induction heated combustion and its mechanical properties

A method to synthesize the dense silicide WSi₂ and WSi₂-xvol.%SiC composites within two minutes in one step from W, Si and C elemental powders was investigated. Simultaneous combustion synthesis and densification were carried out under the combined effect of an induced current and mechanical pressure. Highly dense WSi₂ and WSi₂-xvol.%SiC (x=10, 20, 30) with relative density of up to 97% were produced under the simultaneous application of a 60 MPa pressure and the induced current on the reactant powders. The average grain size of WSi₂ in WSi₂-xvol.%SiC (x=0, 10, 20, 30) were in the region of 6.9, 6.1, 5.2, and 5.0 μm, respectively. The respective Vickers hardness values for these materials were 1375, 1540, 1710, and 1845 kg/mm². From indentation crack measurements, the fracture toughness values were calculated to be 3.3, 3.8, 4.4, and 5.1 MPa·m^1/2, 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.     

Properties and rapid low-temperature consolidation of nanocrystalline Fe-ZrO<Subscript>2</Subscript> composite by pulsed current activated sintering

Nanopowders of Fe and ZrO₂ were synthesized from Fe₂O₃ and Zr by high-energy ball milling. The powder sizes of Fe and ZrO₂ were 70 nm and 12 nm, respectively. Highly dense nanostructured 4/3Fe-ZrO₂ composite was consolidated by a pulsed current activated sintering method within 1 minute from the mechanically synthesized powders (Fe-ZrO₂) and horizontal milled Fe₂O₃+Zr powders under the 1 GPa pressure. The grain sizes of Fe and ZrO₂ in the composite were calculated. The average hardness and fracture toughness values of nanostuctured 4/3Fe-ZrO₂ composite were investigated.     

Effects of sintering on composite metal hydride alloy of Mg2Ni and TiNi synthesized by mechanical alloying

As-milled composite metal hydrides composed of Mg₂Ni and TiNi phases were cold-pressed under a pressure of 490 MPa and sintered for 1 h at 5×10⁻⁶ Torr and 300 °C. Electrochemical characteristics of the sintered composite metal hydride electrode were investigated. The maximum discharge capacity of the sintered composite alloy electrode was 125 mAh/g at a discharge current density of 100 mA/g. This value was similar to that of the as-milled one before sintering. However, the sintered electrode retained 80% of the maximum discharge capacity after 150 cycles, while the as-milled electrode retained only 55%. This is because after the sintering process an interface between Mg₂Ni and TiNi plays a role similar to a diffusion layer of hydrogen. In the sintered composite electrode, when a discharging step proceeds, hydrogen absorbed in a Mg₂Ni particle can move into a TiNi phase through the bonded-interface between Mg₂Ni and TiNi, then discharges at the interface between TiNi and the electrolyte. Also, the electrochemical impedance spectroscopy (EIS) tests showed that the composite alloy electrodes had a lower charge-transfer resistance and a higher hydrogen diffusion coefficient than those in single-phase Mg₂Ni. This indicates that TiNi particles in the composite are the active sites for redox reaction of hydrogen and the pathway for the diffusion of hydrogen