Synthesis of submicron size silver powder for passive components application

Submicron silver powder was prepared from AgNO₃ by a chemical reduction method in the presence of a mixture of caprylic acid and triethanolamine as a surfactant. Hydrazine hydrate (N₂H₄·H₂O) is preferred as a reducing agent. A spherical silver powder with an average particle size of about 150 nm was achieved. Effort was also made to correlate the crystal structure and microstructure evolution of the prepared powders with the resultant thick film characteristics.     

Luminescence enhancement of rare earth ions by metal nanostructures

Well-ordered metal structures, i.e. arrays of nanosized tips on silver surface for studies of the luminescence enhancement of absorbed media with rare earth ions were used. These arrays were prepared by the metal evaporation on track membranes. Calculations of resonance frequencies of tips regarded as semispheroids were done taking into account the interaction between dipoles of tips. They were used to discuss experimental results for media with Eu(NO₃)₃·6H₂O salt basing on data for bulk silver dielectric function.     

Preparation and characterization of Y3Al5O12:Ln (Ln=Eu,Ce) phosphor powders by ultrasonic atomization and co-precipitation process

A novel technique for YAG:Ln (Ln=Eu, Ce) phosphor powder synthesis with a nanocrystalline structure was developed. Nanocrystalline YAG:Ln powder was prepared by an ultrasonic atomization and co-precipitation method using a mixture solution of ammonium hydroxide(NH₃·H₂O) and ammonium hydrogen carbonate(NH₄HCO₃) as precipitant. The as-prepared nano-powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and fluorescence spectrometer. The obtained phosphor powders were homogenous and in size of 50-70 nm. The results demonstrated that by using ultrasonic atomization and co-precipitation process, we could synthesize a good quality YAG:Ln (Ln=Eu, Ce) phosphor powder that had many potential applications.     

Preparation of Ce0.65Zr0.35O2 by co-precipitation： The role of hydrogen peroxide

The effect of H2O2 on the properties of Ce0.65Zr0.35O2 was explored by treating cerium nitrate and zirconium nitrate with a mixed aqueous solution of ammonia and ammonia-carbonate in the presence/absence of H2O2 . The resultant products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption/desorption, oxygen storage capacity (OSC) and H2-temperature-programmed reduction (H2-TPR). The presence of H2O2 was found to have profound effect on powder properties such as surface area, crystallite size of the samples. It was also shown that the addition of H2O2 favored the incorporation of Zr4+ into CeO2 lattice, which facilitated the formation of CeO2-ZrO2 solid solution, and enhanced the thermal stability of the samples. OSC and H2-TPR studies indicated that the use of H2O2 enhanced the OSC and redox properties. Catalytic activity tests showed that as a support, the Ce0.65Zr0.35O2 prepared in the presence of H2O2 was more suitable for three-way catalyst. The corresponding Pd-only three-way catalyst demonstrated outstanding performance: wide air to fuel operation window, low light-off and total conversion temperature for the conversion of C3H8, NO and CO.     

Mechanochemical processing of nanocrystalline Ti-6Al-4V alloy

Synthesis of nanocrystalline Ti-6Al-4V was explored using mechanochemical processing. The reaction mixture was comprised of CaH₂, Mg powder, anhydrous AlCl₃, anhydrous VCl₃, and TiCl₄. The milled powder (reaction product) primarily consisted of nanocrystalline alloy hydride having a composition (Ti-6Al-4V)H_1.942, along with MgCl₂ and CaCl₂ as by-products. Aqueous solutions of nitric acid, sulfuric acid, and 1 pct sodium sulfite were found to be very effective in leaching of the chlorides from the milled powder. The (Ti-6Al-4V)H_1.942 on dehydrogenation at 375°C resulted in nanocrystalline Ti-6Al-4V alloy powder.     

Pressure increase and “dynamic effective diffusivity” within a powder bed during reaction-reduction of iron oxide powders by hydrogen at 900°C

α-Hematite powder having an average diam of 8.1 μm was packed in a vertical quartz reactor (I.D. 14.6 mm) and was reduced by feeding hydrogen toward the top of the packed bed at 900°C under atmospheric pressure. A large increase in pressure as high as 118 mmHg was observed within the powder bed during the course of reaction. The stoichiometric condition that the diffusion rate of the reactant gas, H₂, should be equal to that of the product gas, H₂O, gave rise to this pressure increase within the bed. When the pressure within the bed were uniform, the ratio of the diffusional fluxes of the reactant gas, H₂, and of the product gas, H₂O, would be equal to the square root of the ratio of the molecular weight of H₂O and of H₂. An effective diffusivity which includes not only the diffusional term but also the hydrodynamic term was introduced into the equations for the flow of gases with chemical reactions. The pressure variations within the powder bed calculated from the proposed flow equations agreed quite well with those observed during the course of the three-step reduction of hemattte (Fe₂O₃) to magnetite (Fe₃O₄), then to wustite (Fe_1?y O), and finally to iron. It was also found that, during the reduction, the presence of pressure gradient within the bed reduced the effective diffusivity of hydrogen to about one third of that without the pressure gradient.     

Kinetics of silver leaching from manganese-silver associated ores in sulfuric acid solution in the presence of hydrogen peroxide

Kinetics of silver leaching from a manganese-silver associated ore in sulfuric acid solution in the presence of H₂O₂ has been investigated in this article. It is found that sulfuric acid and hydrogen peroxide have significant effects on the leaching rate of silver. The reaction orders of H₂SO₄ and H₂O₂ were determined as 0.80 and 0.68, respectively. It is found that the effects of temperature on the leaching rate are not marked, the apparent activation energy is attained to be 8.05 kJ/mol within the temperature range of 30 °C to 60 °C in the presence of H₂O₂. Silver leaching is found to be diffusion-controlled and follows the kinetic model: 1−2x/3−(1−x)^2/3=Kt. It is also found that particle size presents a clear effect on silver leaching rate, and the rate constant (k) is proportional to d ₋₂ ⁰ .     

Direct Hydrothermal Precipitation of Pyrochlore-Type Tungsten Trioxide Hemihydrate from Alkaline Sodium Tungstate Solution

Pyrochlore-type tungsten trioxide hemihydrate (WO₃·0.5H₂O) powder with the average particle size of 0.5 μm was prepared successfully from the weak alkaline sodium tungstate solution by using organic substances of sucrose or cisbutenedioic acid as the acidification agent. The influences of solution pH and acidification agents on the precipitation process were investigated. The results showed that organic acidification agents such as sucrose and cisbutenedioic acid could improve the precipitation of pyrochlore WO₃·0.5H₂O greatly from sodium tungstate solution compared with the traditional acidification agent of hydrochloric acid. In addition, the pH value of the hydrothermal system played a critical role in the precipitation process of WO₃·0.5H₂O, and WO₃·0.5H₂O precipitation mainly occured in the pH range of 7.0 to 8.5. The precipitation rate of tungsten species in the sodium tungstate solution could reach up to 98 pct under the optimized hydrothermal conditions. This article proposed also the hydrothermal precipitation mechanism of WO₃·0.5H₂O from the weak alkaline sodium tungstate solution. The novel method reported in this study has a great potential to improve the efficiency of advanced tungsten trioxide-based functional material preparation, as well as for the pollution-reducing and energy-saving tungsten extractive metallurgy.     

On the relative rates of CO2—CO and H2O—H2 reactions with liquid silver at 1283 K and a liquid silicate at 1373 K

Measurements have been made of the steady-state oxygen activity in liquid silver at 1283 K in flowing CO₂—H₂ and H₂O—CO gas mixtures. It is shown that the results are consistent with the consecutive reactions CO₂ (g) → O (ad) + CO (g) and H₂ (g) + O (ad) → H₂O (g) as the rate determining steps under the limiting conditions of low coverage by adsorbed oxygen. Measurements of the steady-state oxygen activity in two iron oxide-containing lithium silicates under flowing CO₂-H₂ gas mixtures at 1373 K are in agreement with a similar mechanism. The study indicates that the rate of dissociation of H₂O is about 4.6 times higher than the rate of dissociation of CO₂ on the liquid silver surface at 1283 K and about 2.1 times higher on the lithium silicate surface at 1373 K. From the present and previous work, it appears that the ratio of the rates of dissociation of H₂O and CO₂ at high temperatures is not strongly influenced by the nature of the reaction surface, despite significant differences in absolute reaction rates.     

Hydrochloric acid-hydrogen peroxide leaching and metal recovery from a Greek zinc-lead bulk sulphide concentrate

A zinc-lead bulk sulphide concentrate from Kirki (Greece) was leached in aqueous solution with HClH₂O₂at atmospheric pressure and 95°C to extract up to 97% zinc, 40% lead, 80% silver and less than 12% iron after 6 h. Highly pure PbCl₂ crystallized from the leach filtrate on cooling. Sulphur was oxidized to the elemental form; its loss as sulphate ion in solution and residue was 7.5%. During leaching no emission of H₂S or SO₂ was detected. Conditions were determined for producing a pregnant solution (130 g/L Zn) low in iron and lead, to facilitate zinc extraction by electrolysis. Leaching experiments were conducted at 40% solids, 0.55 g HCl and 0.26 g H₂O₂ per gramme concentrate in a 1-L reactor. After a second leaching of the residue in aqueous solution with HCl (1 M) for 1 h at 90°C, the residual lead sulphate was extracted, so that total lead recovery was over 98%. Other values, such as silver and sulphur, could be recovered by additional treatment of either the leach solution or leach residue.     

Leaching of metallic silver with aqueous ozone

The recycling of silver from metallic scraps can be performed through O₃ leaching at an ambient temperature and low (∼0.1 M) H₂SO₄ concentration. The main by-product is O₂, which can be recycled to the O₃ generation or used as leaching agent in a pretreatment step. The stoichiometry and the effects of the stirring speed, ozone and acid concentration and temperature on the leaching of silver were investigated. Silver dissolved as Ag²⁺_(aq) in the range 10⁻³–1 M H₂SO₄, but for pH ≥4, insoluble Ag₂O₂ was the main reaction product. Kinetics appeared to be controlled by mass transfer of O_3(aq) to the solid–liquid interface, showing first order dependency with respect to [O₃]_aq and P_O3. Specific rates were only slightly dependent on the temperature in the interval 10–50 °C, but decreased at 60 °C due to the fall in O₃ solubility. The mass transfer coefficients showed an average activation energy of 17 kJ/mol. No significant effect of [H₂SO₄] on mass transfer coefficients was observed for 10⁻²–1 M. Leaching rate gradually diminished for pH >2, as a consequence of the influence of the [H⁺] in the transport control.     

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO₄, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag₂S or (Ag, Cu)₂S rims on sphalerite although (Ag, Cu)₂S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag₂S, AgCl, and Cu₂S. In the low-silver flotation concentrate, silver occurs mostly as Ag₂S although traces of silver-bearing CuS and Cu₂S also are present. The Ag₂S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.     

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO₄, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag₂S or (Ag, Cu)₂S rims on sphalerite although (Ag, Cu)₂S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag₂S, AgCl, and Cu₂S. In the low-silver flotation concentrate, silver occurs mostly as Ag₂S although traces of silver-bearing CuS and Cu₂S also are present. The Ag₂S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.     

Solvation of ions. Some applications. IV A novel process for the recovery of pure silver from impure silver chloride

Silver chloride is exceptionally soluble in dimethylsulfoxide saturated with calcium chloride at 25°C. Solutions containing up to 196 gl^?1 silver as Ca(AgCl₂)₂ have been prepared. If 30–40% by volume of water is added to such solutions, then pure silver chloride is precipitated almost quantitatively. The silver chloride can be easily converted to pure (99.99%) silver metal either by melting at 1100°C with excess sodium carbonate as flux, or by reducing with hydrogen at 300–400°C, or by reducing an aqueous suspension with zinc dust. The water added to the DMSO solution can be recovered by distillation and both it and the CaCl₂-DMSO bottoms are recycled for further leaching. This leads to a fast and cheap process for obtaining silver from crude silver chloride, or from materials containing silver in a form that can be converted to silver chloride. Applications to an anode slimes leach residue, a silver halide teaching laboratory residue, and the silver chloride cake from a gold refinery are demonstrated.     

Synthesis of ultrafine WC/Co powder by mechanochemical process

Abstract

A new approach to produce ultrafine WC/Co powder by a mechanochemical process was made to improve the mechanical properties of advanced hardmetals and to cut production costs. For powder preparation, the water soluble salts containing W and Co components were used as starting materials. After synthesis of the precursor powder from an aqueous solution by a spray drying technique, a salt removing heat treatment in air atmosphere was carried out to prepare the oxide powder. The oxide powder was mixed with carbon black by ball milling and this mixture was converted at 800^oC to the nanophase WC/Co powder in H₂ and N₂ atmospheres. The average size of the WC particle was 100-150 nm. The possibility of achieving high density sintered material with an ultrafine and homogeneous microstructure using grain growth inhibitors, such as tantalum and vanadium carbides, has been shown.     

Reduction kinetics of Ni(OH)2 to nickel powder preparation under hydrothermal conditions

The reduction kinetics from Ni(OH)₂ to fine Ni metal powder under hydrothermal and H₂ pressure conditions using anthraquinon as an activator were investigated. The reduction ratio increased with temperature up to 225 °C. The H₂ molecule was activated by anthraquinone. This activated hydrogen acts as a reduction reagent from Ni²⁺ to Ni⁰. The process may proceed by heterogeneous reaction of Ni²⁺ ion and solid anthraquinon as follows: (1) anthraquinon is hydrated, (2) Ni²⁺ ion is absorbed on anthraquinon hydride, and (3) this complex decomposes to Ni⁰ powder, anthraquinon, and H₂O. The particle size of nickel powder obtained increased with increasing temperature and with decreasing pH. The average particle size was about 350 nm. The reduction kinetics were in good agreement with a core model equation with surface reaction, that is, 1 − (1 −x)^1/3 =kt, wherex is a reaction ratio andt is reaction time. Arrhenius plots showed two slopes with activation energies 65.9 kJ/mol at higher temperature and 377.2 kJ/mol at lower one. This result shows that the hydration of anthraquinon and adsorption of Ni²⁺ ion onto anthraquinon hydride and decomposition to Ni metal and anthraquinone proceed by consecutive reactions and rate determining steps change in each temperature range.     

Development of Ti–Nb and Ti–Nb–Fe beta alloys from TiH2 powders

Ti–Nb β alloys are a promising alternative as an implant material due to their good properties and low Young’s modulus, compared to other Ti-alloys currently employed as biomaterials. In this study, three materials of the Ti–Nb and Ti–Nb–Fe systems were produced by powder metallurgy techniques starting from TiH₂ (TH) powder. Several sintering cycles were employed to evaluate the H₂ elimination and the effect of sintering temperature on densification and fraction of β-Ti phase. Also, the influence of alloying element size using two kinds of Fe powder was evaluated. The highest loss of H₂ was achieved by decreasing heating rate at the temperature range of hydride decomposition. SEM images and XRD results show mainly a β-Ti phase for TH–40Nb and TH–5Fe–25Nb samples. The TH–12Nb sample shows (α?+?β) microstructure. Fe addition with smaller particle size seems to improve the diffusion of Nb into Ti which promotes a higher β-phase fraction and sample homogeneity.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.