Synthesis and characterization of silicon nitride powders produced in a d.c. thermal plasma reactor

Ultra-fine silicon nitride powder was synthesized from the SiCl₄-NH₃-H₂-Ar system using a d.c. plasma torch reactor (production rate 150–400 g h⁻¹). The powder produced is pure white, fluffy and amorphous. The particles are spheroidal in shape with a mean diameter between 30–60 nm forming aggregates of 0.1–0.4 μm depending on the operational conditions. Chemical analysis on the crude powder handled at ambient atmosphere revealed: N(−NH₄Cl):37–39%, O:3–5% and Cl:2–3%. The amorphous powder can be crystallized around 1500 °C under nitrogen to give an α-phase content in excess of 90%. Infrared spectra can be used to semi-quantitatively determine the NH₄Cl content of the crude powder. That proportion is between 2.5 and 4%. The influence of some process parameters e.g. (N/Si and H₂/N molar ratios, internal pressure) on powder properties was also investigated. The N/Si molar ratio was found to be the most important parameter for the powder composition whereas the internal pressure plays a major role on the powder morphology.     

In situ nitridation of titanium–molybdenum alloys during laser deposition

In situ nitridation during laser deposition of titanium–molybdenum alloys from elemental powder blends has been achieved by introducing the reactive nitrogen gas during the deposition process. Thus, Ti–Mo–N alloys have been deposited using the laser engineered net shaping (LENS^TM) process and resulted in the formation of a hard α(Ti,N) phase, exhibiting a dendritic morphology, distributed within a β(Ti–Mo) matrix with fine scale transformed α precipitates. Varying the composition of the Ar + N₂ gas employed during laser deposition permits a systematic increase in the nitrogen content of the as-deposited Ti–Mo–N alloy. Interestingly, the addition of nitrogen, which stabilizes the α phase in Ti, changes the solidification pathway and the consequent sequence of phase evolution in these alloys. The nitrogen-enriched hcp α(Ti,N) phase has higher c/a ratio, exhibits an equiaxed morphology, and tends to form in clusters separated by ribs of the Mo-rich β phase. The Ti–Mo–N alloys also exhibit a substantial enhancement in microhardness due to the formation of this α(Ti,N) phase, combining it with the desirable properties of the β-Ti matrix, such as excellent ductility, toughness, and formability.     

Fragment distribution as an aid to forensic failure investigations at the scene of explosions

Twelve pipe bombs were exploded, and the fragments were collected and weighed. The distribution of fragment masses was shown to follow a Weibull-type form M (n)=M₀ [1 − exp (−Bn^λ)] when RDX-based military explosives were used and a bimodal distribution when commercial explosives were used. The constant, B, was a function of the mass of fragments recovered, making its use inappropriate in a forensic context where complete collection of the fragments would not be possible. For RDX-based explosives with added Mg or Al powder, the value of the constant, λ, was reduced compared to that of the RDX-based explosive without metal powder additions. An alternative fragment distribution formula, log [(100 M (N))/M₀]=FWDM [m(N)/ M₀], can also be used to distinguish between explosives, although the correlation coefficient is inevitably somewhat lower and the scatter between the results of nominally identical tests can be greater. The coefficient, FWDM, has a dependence on the mass of fragments collected, restricting its utility in forensic contexts.     

Microstructure and thermal decomposition property of Ni-P/Cr2N composite powder by electroless plating

Cr₂N is the most promising blowing agent for the preparation of steel foam using melt foaming method. In this work, to obtain a blowing agent with suitable density and gas decomposition characteristics for steel melt foaming, Ni-P/Cr₂N composite powder was prepared by electroless plating. The surface morphology, phase, coating thickness, density and decomposition characteristics of Ni-P/Cr₂N composite powder were analyzed. The results indicate that the surface of Ni-P/Cr₂N composite coating powder is covered by the high nickel and low phosphorus layer which has a dense and uniform cell structure. The decomposition rate of the Ni-P/Cr₂N composite powder is 7.46?mW/mg slower than that of the uncoated Cr₂N powder at 1107.4?°C. When the plating time is 30?min, the thickness of Ni-P layer reaches 2.86?μm, the density of the Ni-P/Cr₂N composite powder is 7.45?g/cm³, and maximal decomposition rate temperature reaches 1500?°C. These findings suggested that Ni-P/Cr₂N composite powder meets the requirements of decomposition temperature and density of the blowing agent used to produce steel foam with a uniform pore structure by the melt foaming method.     

A study of S-doped TiO<Subscript>2</Subscript> for photoelectrochemical hydrogen generation from water

Sulfur-doped titanium dioxide (TiO₂) was investigated as a potential catalyst for photoelectrochemical hydrogen generation. Three preparation techniques were used: first ballmilling sulfur powder with Degussa P25 powder (P25), second, ball milling thiourea with P25, and third a sol–gel technique involving titanium (IV) butoxide and thiourea. The resulting powders were heat-treated and thin-film electrodes were prepared. In all three cases, the heat-treated powders contained small amounts of S (1–3%). However, Rietveld analysis on X-ray diffraction (XRD) measurements revealed no significant changes in lattice parameters. For the samples prepared using thiourea, X-ray photoelectron spectroscopy (XPS) measurements indicated the presence of N and C in the heat-treated powders in addition to S. In all cases, visible-ultraviolet spectroscopy performed on bulk powders confirmed the extension of absorption into the visible region. However, the same spectroscopic technique performed on thin-film electrodes (∼0.5 μm) suggests that the absorption coefficients were very small in the visible region (≤10⁴ m⁻¹). The first and third methods yielded powders with substantially smaller photocatalytic activity relative to P25 powder in the UV region. The electrodes prepared from powders obtained using the second method yielded photocurrents comparable to those prepared from P25 powder.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

Spillover effect in the hydrogen absorption by the polycrystalline powder alloy FeNi1.75Cu1.5Mo0.5 and electric conductivity of the pressed powder

The process of hydrogen absorption by the FeNi_1.75Cu_1.5Mo_0.5 alloy in the polycrystalline form was investigated for both the pure and palladized forms (0.008 76% Pd) at temperatures from ambient to 600 °C in a hydrogen flow. Using differential scanning calorimetry, (DSC) thermogravimetry (TG) and X-ray diffraction, the influence of palladization on the hydrogen absorption was demonstrated. Kinetic analysis of the DSC thermograms, the kinetic and thermic parameters of hydrogen absorption were determined. The TG thermograms showed that on hydrogen absorption a weight change took place due to water formation and reduction of the oxide film at the surface of the powder particles. The activation energies of hydrogen absorption were 170 kJ mol⁻¹ for the original powder and 32 kJ mol⁻¹ for the palladized one. The enthalpies of the absorption ranged from ΔH = −5 to ΔH = −380 J g⁻¹ for the original and palladized powder, respectively. The rate constants of the absorption depended on the palladization and were found to be 0.03 and 1.20 s⁻¹, respectively, at 162 °C. The electric conductivity of the pressed powder (9 kbar) increases on heating in both air and a hydrogen atmosphere up to 600 °C, tending to a constant value. The changes of the parameters characteristic of the palladized form are ascribed to the mechanism of a hydrogen spillover effect due to the presence of palladium.     

The hydroxyl concentration and the dielectric properties of barium titanate nano-powder synthesized by water-based ambient condition sol process

Nanocrystalline barium titanate, BaTiO₃, powders have been successfully synthesized via water-based ambient condition sol (WACS) process, using barium hydroxide and titanium isopropoxide. The properties of the powder were investigated as a function of various processing parameters such as the concentration of Ba²⁺ ions ([Ba²⁺]), the concentration of base, and reaction time. The concentration of hydroxyl (OH⁻) groups in the BaTiO₃ powder was greatly affected by changing the values of each processing parameter. The dielectric constant and tetragonality of the powders which contain lattice OH⁻ more than 0.35 wt% were little affected with further increase in the lattice OH⁻ concentration. The dielectric loss was highly varied with the concentration of the OH⁻ groups, and it was increased with increasing OH⁻ concentration. Calcination treatment significantly improved the dielectric properties of the powder. With higher calcination temperature, the dielectric constant increased and the dielectric loss decreased.     

Optimizing the hot-forging process parameters for connecting rods made of PM titanium alloy

In order to optimize the processing parameters of a new low-cost titanium alloy connecting rod made of powder forging, the deformation behavior of an α + β type Ti–1.5Fe–2.25Mo (wt%) alloy produced by elemental powder metallurgy (PM) route was studied using isothermal compression tests. The constitutive equations and a processing map were established to characterize the flow behavior and predict the optimum deformation parameters. The calculated apparent activation energy was 257.73 kJ/mol for deformation in the α + β phase region and 378.01 kJ/mol in the β phase region. Two deformation mechanism domains were found: α + β → β phase transformation and dynamic recrystallization. The results show that the optimum deformation parameters for the present alloy are (700–800 °C, 10^−1.7–1 s⁻¹) and (800–900 °C, 10⁻²–10 s⁻¹). Based on these results, a finite element method (FEM) simulation of the hot-forming of a connecting rod was conducted, and the simulated results have been successfully used in an industrial forging of the connecting rod.     

Wear behavior of laser metal deposited 17-4 PH SS-W composite at varied tungsten powder flow rate

This research studies the wear behavior of laser metal deposition of 17-4 PH SS-W composite using a 6 mm alumina-stainless steel ball under a load of 10 N, for 16 minutes, 40 seconds and with acquisition rate of 100 Hz conducted at 25 °C. The effect of laser power of between 2600 W and 1500 W; and powder flow rate of between 0.5 min⁻¹ and 2.0 min⁻¹ on wear resistance is investigated. Other processing parameters are constant throughout the experiments. The results show that the 17-4 PH SS-W composite produced at a high laser power of 2600 W exhibits a higher wear resistance as compared to the 17-4 PH SS-W composite samples produced at low laser power of 1500 W. The 17-4 PH SS-W composite sample produced at high laser power of 2600 W with tungsten powder flow rate of 2.0 min⁻¹ has the highest wear resistance with wear volume of 0.0276 mm³ and wear rate of 8.8 ⋅ 10⁻⁵ mm³/N m while the 17-4 PH SS-W composite sample produced at a low laser power of 1500 W with tungsten powder flow rate of 1.0 min⁻¹ has the wear volume of 0.02834 mm³ and wear rate of 9.0 ⋅ 10⁻⁵ mm³/N m.     

Influence of powder surface area on the magnetoelectric effect in barium lead zirconate titanate/nickel ferrite composite ceramics

This paper examines the influence of powder surface area on the magnetoelectric properties of a ferrite + piezoelectric composite. The use of fine powders with a specific surface area in the range 3.1–7.6 m²/g improves the magnetoelectric parameters of bulk composites by 17 to 44% in comparison with a coarse powder with a specific surface area of 0.8 m²/g. This is due to the formation of a more homogeneous composite material, improvement of its dielectric properties, increase in contact area between phases, reduction in internal demagnetizing factor, and increase in the density of the material.     

Effect of mechanical activation on the morphology and structure of hydroxyapatite

We have studied the effect of grinding in planetary mills on the phase composition, morphology, and water content of hydroxyapatite powder. The results indicate that milling for even relatively short times, which reduces the average particle size by a factor of 2, causes the monetite present in the unmilled powder to disappear and reduces the crystallite size of the hydroxyapatite. The fraction of nanoparticles in the powder is then 98% and remains constant during further milling. Milling for longer times leads to hydroxyapatite amorphization. For an average size of large particles R ≥ 1 μm, the surface area of the particles per unit volume, E (cm⁻¹), is determined only by R (E ∼ 1/R).     

13C CP/MAS NMR study of a wood/phenol–formaldehyde resin bondline

The dynamics and interactions of a wood powder/phenol–formaldehyde (PF) resin composite are evaluated by solid-state ¹³C NMR. A ¹³C labeled and perdeuterated PF resin, with low molecular weight distribution, is synthesized and cured in the neat state and also in combination with yellow-poplar wood powder. The ¹³C NMR spectral features and cross-polarization dynamics of the PF hydroxymethyl and methylene nuclei are compared in the neat resin and in the wood powder composite. In the composite, a downfield shift of the PF hydroxyl bearing carbons suggests secondary interactions between the PF resin and wood. In addition, the PF resin methylene and hydroxymethyl carbons exhibit slower CP dynamics compared to the neat resin. Lower resin CP rates in the composite indicate lower molecular rigidity of the resin in presence of wood compared to the neat cured PF resin.     

Diffusivity of silver ions in the low temperature co-fired ceramic (LTCC) substrates

Diffusion of silver inner-electrode occurred during sintering of commercial low temperature co-fired glass ceramic substrate made the dielectric surface become light yellow. The samples added with silicon oxide (SiO₂) powder, however, maintained white color. Silicon-oxide powder was used to modified the sintering behavior and inhibit the silver ions diffusion for the LTCC ceramics. The alumina particles in the LTCC substrates could be regarded as the diffusion barrier of silver ions. The activation energy for silver ions diffusion in the LTCC substrates was 101 kJ/mol. When 5 wt% SiO₂ powder was added into the LTCC substrate, the diffusion activation energy of silver ions became 145 kJ/mol. At sintering temperature of 1180 K, the diffusion coefficient of silver ion in the LTCC ceramic substrates with and without additional SiO₂ were 8.88 × 10⁻¹³ cm²/s and 1.08 × 10⁻¹² cm²/s, respectively.     

Properties of barium-strontium titanate PTCR ceramics sintered on different powder beds

Barium-strontium titanate (BST) ceramics, co-doped with Sb and Mn oxides, were sintered on different powder beds: Sb-doped BST; Al₂O₃; or Sb,Mn-codoped BST powder. Phase formation, microstructure and the electrical properties of the samples were analysed. The PTCR behavior depended significantly on the type of powder bed used. The BST ceramic pellet sintered on the Sb-BST powder displayed the largest PTCR effect, with a ρ_max/ρ_RT ratio of ∼10⁶. This was an order of magnitude greater than for samples sintered on the other two powders. Complex impedance analysis confirmed that this was due to a large increase in grain boundary resistance at 250 °C.     

The influence of combustion synthesis conditions on the α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powder preparation

Fuel type and fuel/aluminium nitrate molar ratio proved to be of great importance during the preparation of α-Al₂O₃ powders. A stoichiometric amount of urea (U) enabled the formation of α-Al₂O₃ with a surface area of 24 m²/g directly from the combustion reaction. Monoethanolamine, triethylenetetramine, trishydroxymethylaminomethane, and triethanolamine yield amorphous powders. This behaviour was explained by the reaction mechanism, which requires the simultaneous decomposition of metal nitrate and fuel, as shown by thermal analysis. The use of 50% of the stoichiometric amount of U was unable to trigger a combustion reaction. The resulting powder was amorphous and had a surface area of 424 m²/g. A parabolic correlation between the surface area of combustion-synthesized powder and the U/aluminium nitrate molar ratio was found. Due to U consumption during the hydrolysis side-reaction, 50% of U excess above the stoichiometric ratio is required in order to maximize the exothermic effect of the combustion reaction. The use of U excess higher than 150% of the stoichiometric ratio not only increases the surface area of the powder, but also changes the phase composition: as the U excess increases the proportion of α-Al₂O₃ decreases and the amount of γ-Al₂O₃ increases.     

Transmission electron microscopy study on the superstructure and the precipitation of γ′-Fe4N in initially homogeneous ɛ-iron nitride powders

Annealed and quenched ɛ-Fe_2-3N powders with an initially homogeneous composition of Fe₃N_1.0 were studied by transmission electron microscopy (TEM). TEM specimens were successfully prepared from the powder using a sandwiching technique. The superstructure in ɛ-powders was identified as ɛ′-Fe₃N type (space group P6₃22), and no other type of superstructure was observed. γ′-Fe₄N nitride precipitated from ɛ powders as individual grains during the annealing process, which is different from the typical fine lamellar structure observed in bulk iron-nitride samples. The observed orientation relationships between γ′ and ɛ grains are also different from that reported in the lamellar structure of bulk iron-nitride samples. This suggests that in the powder investigated by us there is no one specific orientation of the precipitated γ′ grains with respect to the parent ɛ grains.     

Combustion synthesis of oxide materials for nuclear waste immobilization

Oxide materials like perovskite, zirconolite, hollandite, pyrochlore, NASICON and sphene which are used for nuclear waste immobilization have been prepared by a solution combustion process. The process involves the combustion of stoichiometric amount of corresponding metal nitrates and carbohydrazide/tetraformyl trisazine/diformyl hydrazide at 450°C. The combustion products have been characterized using powder X-ray diffraction, infrared spectroscopy, and²⁹Si MAS-NMR. The fine particle nature of the combustion derived powders has been studied using density, particle size, BET surface area measurements and scanning electron microscopy. Sintering of combustion derived powder yields 85–95% dense ceramics in the temperature range 1000°–1300°C.     

On the relations between ISE and structure in some RE(Mg)SiAlO(N) glasses

Six oxide and oxynitride glasses were synthesized in the Y–Mg–Si–Al–O–N, Nd–Mg–Si–Al–O–N and La–Mg–Si–Al–O–N systems. As already known, nitrogen introduction increases the T _g, packing factor and mechanical properties of the glasses. Cationic substitution also has an influence on the glasses’ behavior, particularly in terms of sensitivity to indentation load/size effect (ISE). The structure of the yttrium-containing glasses was investigated by mean of ²⁷Al and ²⁹Si MAS-NMR. Al is found to occur for 2/3 as a network former and for 1/3 as a modifier. The oxide glass mainly contains Q² and Q³ silicate units and SiO₃N and SiO₂N₂ units are created when nitrogen is introduced into the glass network. The average number of rigid bonds per network former was calculated from the glasses’ composition. A discrepancy between and the Raman spectra of the glasses suggests that parts of the magnesium behaves as a former in the network. seems to be a key parameter governing hardness and sensitivity to ISE and can be linked to normal/abnormal behavior of glasses regarding indentation.     

纳米Si/C/N复相粉体的微波吸收特性

采用双反应室激光气相合成纳米粉体装置,以六甲基二硅胺烷((Me₃Si)₂NH)(Me:CH₃)为原料合成了纳米Si/C/N复相粉体,粒径为20 nm~30 nm。研究了纳米Si/C/N复相粉体在8.2 GHz~18 GHz的微波吸收特性,结果表明:纳米Si/C/N复相粉体介电常数的实部(ε')和虚部(ε″)在8.2 GHz~18 GHz随频率增大而减小,介电损耗(tgδ=ε″/ε ')较高,是较为理想的微波吸收材料;纳米Si/C/N复相粉体在不同基体中的微波吸收特性出现很大差异。纳米Si/C/N复相粉体中的SiC微晶固溶了大量的N原子,形成大量带电缺陷,极化弛豫是吸收微波的主要原因。根据纳米Si/C/N复相粉体与石蜡复合体的实测介电参数,设计出多组在8 GHz~18 GHz范围内微波反射系数R≤-8dB的吸波涂层结构。