THE POWDER METALLURGY OF RUTHENIUM

Abstract

An investigation of the powder metallurgy of ruthenium is described, from the reduction of ammonium ruthenium chloride to the working of sintered compacts. The powder properties measured were specific surface area, by a simplified BET method, and tap density. The dependence of these properties on the conditions of reduction has been determined. The surface area of powders varies from 1 to 10 m²/g in the temperature-of-reduction range 700-350°C. The tap density is also variable (1–3 g/c.c.) and is generally related to the surface area. The effects of compacting pressure and temperature on sintering are described, the progress of sintering being observed by measurements of the “open” and “closed” porosity present in samples. Compact densities up to 95% of theoretical can be obtained by sintering at 1500°C. The selection of powder properties and compacting pressures to be used in the production, by vacuum sintering at 1500°C, of high-density compacts for working, is governed by the necessity to maintain open porosity during the heating cycle up to at least 1200°C, as considerable gas evolution occurs at this temperature; at the same time it is essential that good densification shall have occurred even at this stage. These conditions can be met by using powder with a surface area of 2–5 m²/g and compacting pressures in the range 0·5–25 tons/in ².

Observations on the hot working of sintered compacts indicate that ease of working is related to the surface area of the powder.     

Heat Treatment of Tantalum and Niobium Powders Prepared by Magnesium-Thermic Reduction

Changes in the specific surface area and porous structure of tantalum and niobium powders, which were prepared by magnesium-thermic reduction of Ta₂O₅, Mg₄Ta₂O₉, and Mg₄Nb₂O₉ oxide compounds and subjected to heat treatments at temperatures of 600–1500°C, have been studied. It is noted that, owing to the mesoporous structure of the magnesium-thermic powders, the decrease in the surface area during heat treatment, first of all, is related to a decrease in the amount of pores less than 10 nm in size. The heat treatment of a reacting mass is shown to allow us to correct the specific surface area of the powder without any increase in the oxygen content in it. Data on the effect of heat treatment conditions on the specific charge of capacitor anodes are reported.     

Effect of heat treatment on the characteristics of sodium-reduced niobium powders

The effect of the heat-treatment conditions on the bulk density, flowability, and electrical properties of the sodium-reduced niobium powders prepared using two versions of reduction is studied. These versions include (i) the supply of liquid sodium on the surface of a melt containing potassium heptafluoniobate K₂NbF₇ (liquid-phase reduction) and (ii) the supply of solid K₂NbF₇ on the surface of liquid sodium (heterophase reduction). Heat treatment of a bulk niobium powder in the temperature range 900–1300°C is shown to result in a substantial loss in the specific surface area without increasing the bulk density. To produce a powder with a specific capacitance higher than 90 mCV/g, a bulk density of 1.2 g/cm³, and a good flowability, the initial pelleted heterophase-reduction powder should be sintered at 1200°C.     

Structure formation patterns in carbamide synthesis of nanocrystalline graphite-like boron nitride

We have used chemical analysis, x-ray diffractometry, transmission and scanning electron microscopy, and also measurement of the specific surface area to study the characteristic patterns of nitriding and structure formation in graphite-like BN powders synthesized by the carbamide method under various conditions. We have studied the effect on the synthesis process from the ratio of boric acid to carbamide in the starting mixture, preliminary grinding of the charge, the nitriding temperature and time. We show that the major factor determining the composition and structure of the product synthesized at 900–1200 °C is the ratio of the starting components. Carbon-free BN of stoichiometric composition is formed for a 1: 3 ratio of boric acid to carbamide as a result of two-hour nitriding at 1200 °C. The graphite-like BN obtained is characterized by a turbostratic nanocrystalline structure with a specific surface area of 100 m²/g and high nanoporosity of the powder particles.     

Electrochemical synthesis of an iridium powder with a large specific surface area

The synthesis of iridium powder in a molten NaCl–KCl medium at 700°C is carried out for the first time. The influence of the ratio of the cathode to the anode current density (i _c/i _a) on the structure and the morphology of the iridium powder is investigated. Single-phase and polycrystalline iridium powders with a specific surface of 16.8 m²/g are produced. The phase composition and the surface texture of the deposits are studied. The specific surface and the particle size of iridium powders as functions of the ratio i _c/i _a are analyzed.     

Application of mechanochemical activation in production of zircon ceramics

The processes of preliminarily treatment of powders of various zircon concentrates when obtaining zircon ceramics are investigated. Based on the results of Raman spectroscopy, it is shown that, even at an insignificant content of impurities in the zircon concentrate, samples of powders with a small specific surface are prone to decomposition in the course of smelting at t = 1600°C. Mechanochemical activation with additives of surfactant species yields an increase in the milling efficiency and sinterability of the powder. However, resistance to the thermal decomposition of the samples increases. Based on an activated powder without the addition of any sintering activators, almost single-phase zircon ceramics with a residual porosity of <10% and a temperature of thermal decomposition over 1600°C was obtained.     

Low-temperature synthesis of α-Al2O3

The paper considers the production of nanocrystalline α-Al₂O₃ powder at a temperature below 900 °C. It is shown that α-Al₂O₃ forms after high-temperature treatment of aluminum nitrate mixed with a mineralizer at 650 °C (holding time of 6 h). The primary particles of α-Al₂O₃ are 25 to 30 nm in size and the specific surface area is 15.2 m²/g. The powder is used to produce self-reinforced composites in the ZrO₂-Y₂O₃-CeO₂-Al₂O₃ system.     

Al-Fe-Ce alloys based on water-atomized powders for high-temperature applications

The microstructure and mechanical properties of Al-Fe-Ce alloys based on water-atomized powders between 20 and 300 °C are examined in comparison with the properties of similar alloys produced by other rapid crystallization techniques. Changes in atomization parameters vary both the cooling rate (from 10⁴ to 10⁶ K/sec) and powder size distribution (from 5 to 100 µm). The excellent compactability of water-atomized powders facilitates powder consolidation, which is based on hot extrusion and cold pressing of degassed powders. The mechanical properties are examined by tensile tests. The ultimate tensile strength is 500 to 550 MPa at 20 °C and 270 to 300 MPa at 300 °C at adequate plasticity. The properties achieved are comparable with those of similar alloys known from the literature.     

Microstructure and Shape Memory Characteristics of Powder-Metallurgical-Processed Ti-Ni-Cu Alloys

Even though Ti-Ni-Cu alloys have attracted a lot of attention because of their high performance in shape memory effect and decrease in thermal and stress hysteresis compared with Ti-Ni binary alloys, their poor workability restrains the practical applications of Ti-Ni-Cu shape memory alloys. Consolidation of Ti-Ni-Cu alloy powders is useful for the fabrication of bulk near-net-shape shape memory alloy. Ti₅₀Ni₃₀Cu₂₀ shape memory alloy powders were prepared by gas atomization, and the sieved powders with the specific size range of 25 to 150???m were chosen for this study. The evaluation of powder microstructures was based on a scanning electron microscope (SEM) examination of the surface and the polished and etched powder cross sections. The typical images showed cellular/dendrite morphology and high population of small shrinkage cavities at intercellular regions. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis showed that a B2-B19 one-step martensitic transformation occurred in the as-atomized powders. The martensitic transformation start temperature (M_s) of powders ranging between 25 and 50???m was 304.5?K (31.5?°C). The M_s increased with increasing powder size. However, the difference of M_s in the as-atomized powders ranging between 25 and 150???m was only 274?K (1?°C). A dense cylindrical specimen of 10?mm diameter and 15?mm length were fabricated by spark plasma sintering (SPS) at 1073?K (800?°C) and 10?MPa for 20?minutes. Then, this bulk specimen was heat treated for 60?minutes at 1123?K (850?°C) and quenched in ice water. The M_s of the SPS specimen was 310.5?K (37.5?°C) whereas the M_s of conventionally cast ingot is found to be as high as 352.7?K (79.7?°C). It is considered that the depression of the M_s in rapidly solidified powders is ascribed to the density of dislocations and the stored energy produced by rapid solidification.     

Progress of Sm-Fe-N Anisotropic Magnets

A reduction and diffusion method (R/D) is used to make a mother alloy of Sm-Fe-N anisotropic magnets. Reduction of 0.5wt% of samarium content compared to the conventional powder increases magnetization. Milling condition and surface treatment improve the squareness of demagnetization curve, the aging property and the heat resistance. The maximum energy product of 292 kJ/m³ is obtained with the powder. High coercive force is maintainable even if the powder is exposed for 300h in 80 °C 90%RH. The maximum energy product of 141 kJ/m³ is obtained with an injection molded anisotropic magnet. The aging property estimated by irreversible flux loss is comparable to the conventional MQP-B magnets. The heat resistance temperature (T_−5%) at which die initial irreversible flux loss becomes −5% is 125-more than 150 °C for Sm-Fe-N magnets and 150–170 °C for hybrid magnets. The magnetic properties of bonded HDDR Nd-Fe-B magnet were improved by substituting for Nd-Fe-B powder with Sm-Fe-N powder. A new technology to make anisotropic bonded Sm-Fe-N thin cylinder magnets by an injection molding using unsaturated polyester (UP) resin was developed.     

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.     

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10^-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min^-1) to temperatures above the melting point of Nb₂O₅ (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME     

Effects of Solute and Surfactant Addition on the Crystallization and Morphology of Hydroxyapatite Powders Synthesized by Hydrolysis of Calcium Hydrogen Phosphate Dehydrate (DCPD)

The effects of the addition of alcohol and cetyltrimethylammonium bromide (CTAB) on the crystallization and the morphology of hydroxyapatite (HA) powders synthesized by hydrolysis of calcium hydrogen phosphate dehydrate (DCPD) in the 2.5 M NaOH solutions at 348 K (75 °C) for 1 hour have been studied. The values of zeta potential have large differences between the sums of DCPD with CTAB (Z _DCPD+CTAB) minus the sum of DCPD and CTAB (Z _DCPD + Z _CTAB), and of HA with CTAB (Z _HA+CTAB) minus the sum of HA and CTAB (Z _HA + Z _CTAB), respectively. When the hydrolysis of DCPD occurred in the 2.5 M NaOH solutions at 348 K (75 °C) for 1 hour both with and without alcohol and CTAB, XRD results show the only one phase of HA in the as-dried powders. When the NaOH solution does not contain CTAB, the crystallite size of HA powders decreased from 23 ± 1 to 16 ± 1 nm as the alcohol content was more than 50 pct. The crystallite size of HA powders obtained from DCPD synthesized in the 2.5 M NaOH solution with 1.0 × 10^?3 M CTAB decreased when the alcohol content was increased to 70 pct, whereas the crystallite size increased when the alcohol concentration was greater than that of 70 pct. SEM images show that the HA powders have a rod-like shape when DCPD was synthesized in the 2.5 M NaOH solution without CTAB or alcohol. When the NaOH solution had 1.0 × 10^?3 M CTAB and various alcohol concentrations, the morphology of HA powder still maintained a rod-like or needle-like shape. The HA powder had a maximum specific surface area of 180.25 m²/g when the hydrolysis of DCPD occurred in a 2.5 M NaOH solution containing 1.0 × 10^?3 M CTAB and 70 pct alcohol at 348 K (75 °C) for 1 hour.     

Effect of Cu3P addition on sintering behaviour of elemental powders in the composition of 465 stainless steel

Abstract

The addition of Cu₃P for developing the high strength 465 maraging stainless steel from elemental powders was studied. The sintering parameters investigated were sintering temperature, sintering time and wt-%Cu₃P. In vacuum sintering, effective sintering took place between 1300 and 1350°C. The maximum sintered density of 7·44 g cm^?3 was achieved at 1350°C for 60 min with 4–6 wt-%Cu₃P. More than 6 wt-%Cu₃P content and temperature >1350°C caused slumping of the specimens. The sintered specimens were heat treated and a maximum ultimate tensile strength (UTS) of 767 MPa was achieved with 4 wt-%Cu₃P content. The maximum hardness of 45·5 HRC was achieved in heat treated condition with 4 wt-%Cu₃P content. Above 4 wt-%Cu₃P content increase in density was observed whereas the response to heat treatment decreased. Fracture morphologies of the sintered specimens were also reported. A comparison of sintering behaviour and mechanical properties of elemental powders with prealloyed powders was also given in the present study.     

Analysis of Particle and Crystallite Size in Tungsten Nanopowder Synthesis

Tungsten nanopowders were synthesized by a low-temperature technique and then heat treated in a gaseous reductive atmosphere in order to study the phase evolution, crystallite size, and particle size of the powders as the heat treatment temperature was modified. Synthesis of the powders was carried out in aqueous media using NaBH₄ as a reducing agent using careful control of the pH of the solutions. The XRD patterns of the as-synthesized powders showed an amorphous phase. After washing, energy dispersive spectroscopy showed that the powders had peaks for oxygen and tungsten. In order to promote crystallization and eliminate the oxygen, the powders were heat treated at 773 K, 923 K, and 1073 K (500 °C, 650 °C, and 800 °C) in a H₂/CH₄ reducing atmosphere for 2 hours. XRD after heat treatment showed α-W peaks for the powders treated at 1073 K and 923 K (800 °C and 650 °C) and a mixture of β-W and α-W for the powders treated at 773 K (500 °C). The crystallite sizes determined from X-ray peak broadening were 12, 16, and 20 nm, whereas the average particle sizes from dynamic light scattering were 260, 450, and 750 nm, for heat treatment temperatures of 773 K, 923 K, and 1073 K (500 °C, 650 °C, and 800 °C), respectively. The average crystallite size and particle sizes increased proportionally with the treatment temperature, in contrast to what has been found for some ceramics, in which as the heat treatment temperature is increased, the crystallite size increases, but the particle size stays constant.     

THE INFLUENCE OF PRESSURE,TEMPERATURE, AND TIME ON THE HOT PRESSING OF COMMERCIAL BERYLLIUM POWDER

Abstract

For a particular batch of Brush Super-Pure – 200-mesh beryllium powder the hot-pressability (defined as the length of a standard compact of < 98% theoretical density), increased: (1) continuously with pressure over the range 0–1·25 ton/in² for compacts pressed for 1 h at 1100°C;(2) with temperature from 1000°C to a maximum at 1150°C when pressed for 1 h under 1 ton/in²; and (3) to a lesser extent with time over the range 10–120 min when pressed at 1050°C under 0·25 ton/in² and at 1100°C under 1·0 ton/in². Differences in hot-pressability between various batches of the same powder were small compared with the effects of temperature and pressure.

Compaction during hot pressing occurs in two stages: first, collapse of the powder column causing bulk powder flow; followed, secondly, by sintering of particles forced into close contact. The latter is accompanied by a diminution in both the number of pores and their average size and is associated with grain growth, particularly above 1100°C; after pressing at 1200°C any remaining porosity assumes a thermally stable, spherical configuration.     

Nanocrystalline Powders Based on ZrO2 for Biomedical Applications and Power Engineering

An 80 mass% ZrO₂ ― 20 mass% Al₂O₃ powder was produced using a complex method which integrates sol-gel technology and hydrothermal synthesis. The specific surface areas of the powder varied from 39 to 5.3 m²/g depending on the thermal treatment conditions. Metastable F-ZrO₂ formed after powder annealing at 400°C. The phase transformation F-ZrO₂ → T-ZrO₂ (traces of M-ZrO₂) occurred under powder thermal treatment from 700 to 1000°C. Only Θ-Al₂O₃ was detected under experimental conditions. The powder was characterized by sintering activity. Operating the processes under powder thermal treatment in the ZrO₂ ― Y₂O₃ ― CeO₂ ― Al₂O₃ system will allow one to produce a variety of ceramic microstructures from fine-grained to “self-reinforced.” These powders can be used in manufacturing surgical cutting tools as well as in ceramic passive bioimplants and solid electrolytes for fuel elements.     

Ultra Fast Cooling of a Hot Steel Plate by Using High Mass Flux Air Atomized Spray

In the current research, the ultra fast cooling (UFC) of a hot stationary AISI‐304 steel plate has been investigated by using air atomized spray at different air and water flow rates. The initial temperature of the plate, before the cooling starts, is kept at 900°C or above. The spray was produced from a full cone internal mixing air atomized spray nozzle at a fixed nozzle to plate distance; and the average spray mass flux was varied from 130 to 370 kg m^?2 s by selecting different combinations of air and water flow rates. The surface heat flux and surface temperature calculations have been performed by using INTEMP software and the calculated results have been validated by comparing with the measured thermocouple data. The heat transfer analysis indicates that the cooling occurs in the transition boiling regime up to surface temperature of 500°C and thereafter it changes to nucleate boiling regime. The superposed flow of air on the hot plate enhances the cooling in the temperature range of 900–500°C by sweeping the partially evaporated droplets from the hot surface. However, due to the high percentage of fine water droplets in the resultant spray produced at higher air flow rates, the maximum cooling rate is achieved at the medium air flow rate of 30 N m³ h^?1. The cooling rate (182°C s^?1) produced by an air atomized spray is found to be in the UFC regime of a 6 mm thick steel plate. The findings of this research can be considered as the basis for the fabrication of cooling system in the run‐out table of a hot strip mill.     

Synthesis of TiC–TiB–TiB2 composite powders by solid reaction of powders

In the present work, TiC–TiB–TiB₂ diffusion-layer-coated B₄C composite powders were synthesised via a powder immersion method using Ti and B₄C powders as reactants. The phase compositions and microstructure of the treated powders were characterised by employing X-ray diffraction and scanning electron microscopy. No significant reaction between B₄C and Ti could be detected at 800°C. After treatment at 900°C, the products generated were composed of TiC and TiB. After treatment at 1000°C, the products generated were primarily composed of TiC and TiB, with a small amount of TiB₂. The composition and proportions of the produced phases varied with process temperatures and the composition of the initial powders used. Powder mixtures with a Ti/B₄C molar ratio of 3.5:1 and treated at 1000°C for 14?h were more suitable for synthesis of TiC–TiB–TiB₂-coated B₄C composite powders.     

Preparation and Properties of Magnetically Soft Powder Metallurgy Alloy 79NM

The effect of production conditions and heat treatment on the magnetic properties and structure of powder metallurgy soft magnetic alloy 79NM were investigated. Optimal structure and magnetic properties were attained after heat treatment in hydrogen at 1150°C and a final heat treatment at 1300°C for maximum time.