Sintering In Hydrogen Research Articles

Influence of copper salts on the physical and mechanical properties of copper–graphite composite materials

We investigated composite materials based on electrolytic copper powder containing 1 and 5 wt. % powder of colloidal graphite the addition of trace amounts of copper sulfate and acetate. The materials were obtained through double cold pressing in a mold at a pressure of 600 MPa, intermediate sintering (annealing) in hydrogen at a temperature of 870 °C, and final sintering in vacuum at the copper premelting temperature. To analyze the influence of copper salts on the density, porosity, electrical resistivity, and strength of copper–graphite composite materials, we employed X-ray phase analysis, scanning electron microscopy, conducted strength tests in three-point bending, and determined electrical resistivity. We established that higher graphite content results in increased porosity and electrical resistivity of composite materials, along with decreased strength. In the materials containing copper sulfate, copper is reduced from the salt in the form of nanodispersed particles on the surfaces and inside graphite flakes, leading to a decrease in electrical resistivity compared to copper–graphite composites without salt additives. When copper acetate was added to the composite material, copper is reduced from the salt mainly on the surfaces of graphite particles in the form of microdispersed particles and their aggregations, as the copper acetate solution does not wet the graphite. In this case, the electrical resistivity was somewhat higher than that of the composite with sulfate but lower than that of the material without salts. The bending strength of the studied materials decreased as salts were introduced due to increased porosity and emerging defects in the crystal structure of graphite during its intercalation with copper.

Effect of High-Energy Ball Milling Time on the Density and Mechanical Properties of W-7%Ni-3%Fe Alloy

The present work was aimed at the investigation of the effect of high-energy ball milling (HEBM) time on the sintering kinetics, structure, and properties of the heavy tungsten alloy (HTA) W-7%Ni-3%Fe. The HTA samples were obtained from nanopowders (20–80 nm) using conventional liquid-phase sintering (LPS) in hydrogen and using spark plasma sintering (SPS) in vacuum. The HTA density was shown to depend non-monotonously on the HEBM time that originates from the formation of nonequilibrium solid solutions in the W-Ni-Fe systems during HEBM. The SPS kinetics of the HTA nanopowders was shown to have a two-stage character, the intensity of which depends on the Coble diffusion creep rate and on the intensity of diffusion of the tungsten atoms in the crystal lattice of the γ-phase. The kinetics of sintering of the initial submicron powders has a single-stage character originating from the intensity of the grain boundary diffusion in the γ-phase. The dependencies of the hardness and of the yield strength on the grain sizes were found to obey the Hall–Petch relation. The hardness, strength, and dynamic strength in the compression tests of the fine-grained tungsten alloys obtained using SPS and LPS were studied.

Obtaining and properties of rolled products from clad powders NPG-80 and Nibon-20

Antifriction tape sealing materials (TSM) are used in the manufacture of turbines. This work studied the mechanism of the increase in thickness of rolled products. The study showed that internal oxides of powder particles, which are reduced during sintering and annealing in hydrogen, cause a change in the size of tapes and compacts from NPG-80 and Nibon-20 clad powders. The distinctive feature of powders used in the work is the presence of a nickel shell around the particle of the solid lubricant (graphite or boron nitride). It was shown that an increase in sintering temperature and heating time to isothermal holding intensifies the growth of the tapes and samples The studies carried out point to a relationship between reduction processes occurring in hydrogen during heating, sintering, and annealing of NPG-80 and Nibon-20 powders with an increase in the thickness of the tape containing the solid lubricant in its composition. An increase in the duration of heating to isothermal holding at 1150 °C is accompanied by an increase in the thickness of rolled stock. Sintering of TSM Nibon-20 in the first mode (4 h to 1150 °C) gives an increase in thickness by 5-7 %, whereas sintering in the second mode (9 h to 1150 °C) gives an increase of 12-13 %. For NPG-80, the increase in thickness is 3-7 % and 8-11 %, respectively. This leads to some decrease in the physical and mechanical properties of TSM. Lower temperatures and higher heating rates are recommended for the sintering of sheet materials from NPG-80 and Nibon-20 in the hydrogen atmosphere. The increase in thickness of TSM from clad powders NPG-80 and Nibon-20 can be avoided if the sintering (heating) is carried out under pressure. By the method of electric rolling and subsequent hot rolling TSM is obtained from Nibon-20.

Influence of debinding and sintering conditions on the composition and thermal conductivity of copper parts printed from highly loaded photocurable formulations

Metal 3D printing based on the photopolymerization reaction (Digital Light Processing—DLP) of an organic matrix in which metal particles are embedded is a developing technology. This technology requires a step of resin removal and densification by sintering to obtain a metal part. This process has been applied to copper. Photocurable formulations with a high loading rate of copper powder of 60 vol.% were developed and suitable for DLP printing with thicknesses > 25 µm. Debinding and sintering cycles were investigated on specimens fast cured by gamma irradiation to save materials and time. A debinding in air at 400 °C and sintering in hydrogen lead to a C content of 0.018 wt.%, similar to the raw copper powder and slightly higher oxygen content. The low thermal conductivity of 250 W·m−1·K−1 highlighted the harmful effect of phosphorus from the powder and photoinitiators such as BAPO. The C and O contents and the thermal conductivity measured on copper parts printed by DLP confirm the results obtained on specimens cured by gamma irradiation.

Study of Molybdenum-Tungsten Powder Compaction Processes by Spark Plasma Sintering

This paper presents the results of studying the production of molybdenum-tungsten alloys by spark plasma sintering at various process conditions, as well as the processes of material compaction and structure formation. A comparative analysis of compactability of molybdenum-tungsten alloys produced by spark plasma sintering and sintering in hydrogen was performed. It was shown that a relative density of > 95% can be achieved at a temperature of 1,800°C and external pressure of 50 MPa.

Nickel–Ceramic Electrodes with High Nickel Content for Solid Electrolyte Electrochemical Devices

Composite powders (66 wt % NiO + 34 wt % Zr0.84Y0.16O1.92 were obtained according to the solution combustion synthesis technique by crystallization of nickel oxide particles on the surface of Zr0.84Y0.16O1.92particles. The samples pressed from powders after sintering and reduction in moist hydrogen had high electrical conductivity, more than 16 × 103 S cm–1 at room temperature. Impedance spectroscopic studies have shown that in a humid atmosphere of Ni – Zr0.84Y0.16O1.92, electrodes formed on the surface of a dense Zr0.84Y0.16O1.92 electrolyte have low electrochemical activity. The impregnation of the electrodes with a solution of cerium nitrate followed by thermolysis made it possible to increase the electrochemical activity of the electrode by two to three orders of magnitude. An analysis of the electrochemical impedance spectra through the calculation of the distribution function of the relaxation time showed that the rate of hydrogen oxidation at the electrodes is limited by surface–adsorption processes. After impregnation, the gas diffusion resistance significantly limits the electrode reaction rate.

Электропроводный материал на основе порошковой меди с повышенными механическими свойствами

The methods of pressing, rolling, and sintering in hydrogen at 800°C have been applied to production of powdered materials based on copper of the Cu–Fe–Al system. The influence of single components and their blends on compaction, structure, and main functional properties of composite materials—electrical conductivity, hardness, and strength (yield stress)—has been analyzed as a function of process variables: compacting pressure of 200–700 MPa, deformation by rolling of 40–85%, dispersity of iron powders having particle sizes <50 and 63–100 μm, contents of iron powder of 5–10 wt % and Fe2Al5 intermetallic powder of 2.5–5 wt %. The best combination of physicomechanical properties has been found for composite material with the composition (wt %) 92.5 Cu + 2.5 Fe2Al5 + 5 Fe, together with iron particle sizes of 63–100 μm. It has been demonstrated that dispersion strengthening of copper by Fe2Al5 particles and filling of its structure with iron fibers formed by rolling provides high relative density of 98–99% and subsequent strength properties of the composite material: hardness HB of 980 MPa, yield stress of 150 MPa at electrical conductivity of 71% of copper conductivity.

Influence of Grain Growth Inhibitors and Powder Size on the Properties of Ultrafine and Nanostructured Cemented Carbides Sintered in Hydrogen

The influence of grain growth inhibitors and powder size on the microstructure and mechanical properties of ultrafine and nanostructured cemented carbides was researched. Three different WC powders, with an addition of different type and content of grain growth inhibitors GGIs, VC and Cr3C2 and with dBET grain sizes in the range from 95 to 150 nm were selected as starting powders. Four different mixtures with 6 and 9 wt. % Co were prepared. The consolidated samples are characterized by different microstructural and mechanical properties with respect to the characteristics of starting powders. Increased sintering temperatures led to microstructural irregularities in the form of a discontinuous WC growth, carbide agglomerates and abnormal grain growth as a consequence of coalescence via grain boundary elimination. The addition of 0.45% Cr3C2 contributed to microstructure homogeneity, reduced discontinuous and continuous grain growth, and increased Vickers hardness by approximately 70 HV and fracture toughness by approximately 0.15 MN/m3/2. The reduction of the starting powder to a real nanosize of 95 nm resulted in lower densities, and significant hardness increase, with a simultaneously small increase in fracture toughness. The consolidation of real nanopowders (dBET &lt; 100 nm) solely by conventional sintering in hydrogen without isostatic pressing is not preferred.

Effect of sintering conditions on mechanical strength and gas permeability of long porous Inconel 625 tubes for hot gas filtration

The sintering responses of Inconel 625 prealloyed powder (powder size: 80–180 μm accounting for 96 wt-%) were first assessed using dilatometry. Then, sintering of Inconel 625 tubes (Φ54 mm × 1000 mm), shaped by cold isostatic pressing from prealloyed powder at 170 MPa, was studied at temperatures between 1180 and 1260°C in vacuum and hydrogen. Sharp, rapid shrinkage of Inconel 625 prealloyed powder was observed at 1240°C when sintered in vacuum, while this temperature was postponed to 1260°C when sintered in hydrogen due to the cooling effect of the large hydrogen gas flow. It is shown that sintering in hydrogen is better suited to the fabrication of high performance porous Inconel 625 tubes than sintering in vacuum. The preferred isothermal sintering conditions were determined to be sintering in hydrogen at 1240°C for 120 min, which resulted in balanced mechanical strength and gas permeability. The as sintered porous Inconel 625 tubes (Φ54 mm × 1000 mm) have been used to filter hot (up to 400°C) gas mixtures of SiHCl3 and HCl in continuous industrial operations for 6 months and demonstrated outstanding performance.

Influential factors on hardness uniformity of Vickers hardness blocks for high hardness range

Reference Vickers hardness blocks for high hardness range were developed from WC 9–Co and WC 6–Co mixtures by two different processes of powder metallurgy, conventional liquid phase sintering in hydrogen and vacuum-sintering with subsequent hot isostatic pressing in one cycle, sinter-HIP, in order to analyse the appropriateness of the manufacturing process and the influence of starting characteristics of the mixtures on hardness uniformity and overall metrological characteristics. WC powder of 150nm grain size and 2.5m2/g specific surface area with the addition of grain grow inhibitors was used as starting material. The emphasis of the research was placed on hardness uniformity of the test surface as it is the most important property placed on hardness blocks. For that purpose the surface of the blocks was divided into radial and circumferential divisions, forming different sections on the block surface. Hardness measurements were performed in each section with HV1 measuring method. A total of 40 indentations were performed, based on which the conclusions about hardness uniformity of Vickers hardness blocks were drawn. Hardness measurements and uniformity were tested by analysis of variance, ANOVA, for single factor in order to determine if significant hardness variations across the block surface were present. From the research conducted it was concluded that hardness distribution across test surfaces of Vickers hardness blocks had a trend according to a process of manufacturing and the amount of binder in the WC–Co mixture. The best material and metrological characteristics were obtained on the block manufactured by sinter-HIP with 9wt.% Co.

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150nm and a specific surface area of 2.5m2/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr3C2 in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.

Sintering of Cu–Sn–P–MoS2 Powder Samples at 780°C

The influence of initial porosity and MoS2 content of pressed Cu + 9Sn + 1.5P + (5, 10, 15%) MoS2 powder samples on their growth during sintering in hydrogen at 780°C is studied. The growth of samples with the same MoS2 content decreases with higher initial porosity and the growth of samples of the same size and same porosity after pressing increases with greater MoS2 content. Water vapors generated during the hydrogen reduction of oxides present on the starting tin powder and phosphorous-copper alloy powder cause swelling of the samples. Moreover, phase transformations in the material during sintering can increase the samples as new phases are formed (copper sulfide Cu2S) with a larger crystal lattice that that of the matrix and molybdenum disulfide.

Synthesis and characterization of macrocomposites based on nickel-coated quasi-crystalline Al-Cu-Fe powder

Disperse composite materials consisting of quasi-crystalline Al-Cu-Fe particles covered by nickel nanolayers have been obtained using a dusty plasma trap coating technology. These powders were processed into macrocomposites by cold pressing with subsequent sintering in hydrogen. The macrocomposites were studied by the electron microscopy, electron-probe microanalysis, and X-ray diffraction techniques. It is established that sintering at ∼750°C yields a high-strength macrocomposite that is not fractured under tribological testing conditions. Despite the fact the samples sintered at 750°C contain about 50 wt % of β phase, their friction coefficient is close to that of Al-Cu-Fe quasi-crystals.

Fabrication of injection moulded translucent alumina ceramics via pressureless sintering

In this work, injection moulding and pressureless sintering in hydrogen were applied to manufacture translucent alumina ceramics. Excellent rheological properties of the feedstocks for injection moulding were obtained through a method of powder pretreatment with stearic acid induced by ball milling. Optimum solid loading was confirmed in terms of rheological data and porous properties. The average grain size of the sintered body is 30–50 μm, with no significant pores and abnormal grain growth observed. The real inline transmission is higher and more stable than those via other forming technologies and pressureless sintering reported in previous literatures. As a result, translucent alumina components with small size and high precision were fabricated in such a stable, efficient and low cost route.

Free machining PIM 316L stainless steels

The machinability of austenitic stainless steel has been problematic due to its high work hardening rate, excellent ductility and poor thermal conductivity. In this study, the effects of copper additions of 2, 3 and 4 wt-%, and the sintering atmosphere, hydrogen and cracked ammonia, on the machinability of injection moulded 316L specimens were investigated. The results show that the copper addition improved the machinability and slightly reduced the oxygen content of sintered compacts. The weight loss data show comparable corrosion resistance with copper added. The compacts sintered in dissociated ammonia had a lower cutting force but a higher corrosion rate than those sintered in hydrogen. Considering the machinability and the corrosion resistance, the optimum results were attained by adding 3 wt-% copper and sintering in pure hydrogen.

Gelcasting and Pressureless Sintering of Translucent Alumina Ceramics

Gelcasting is a very useful technique for producing ceramic parts with complex shapes. In this work, gelcasting in air and pressureless sintering in hydrogen were applied to manufacture translucent alumina ceramics. A stable suspension with 75 wt% solids was prepared by ball milling. Trapped bubbles were removed completely before casting to obtain homogeneous green bodies. Final sintering was carried out in a hydrogen furnace at 1850°C for 3 h. Homogeneous microstructures of both calcined and sintered bodies were obtained. The in‐line transmission of the sample with a thickness of 0.5 mm increased gently from 9% to 10% in the wavelength range of 190–1100 nm.

Enhanced field emission from printed CNTs by high-temperature sintering and plasma bombarding in hydrogen

A novel post-treatment, including high-temperature sintering and plasma bombarding in hydrogen ambient, was performed intentionally to improve the field emission capability and stability of screen-printed carbon nanotubes (CNTs). With this treatment, the organic bonding material covering on the CNTs was preferentially removed. And the CNTs were wetted and filled by silver at the interface between the CNTs and silver electrode so that a reliable ohmic contact was achieved. These effects improved the field emission capability and stability of CNT cathode. Moreover, the equal-high CNT emitters, after the removal of the excessively protruded CNT tips/or arches, would improve the luminescence uniformity. It is suggested that a high-temperature sintering and plasma bombarding in hydrogen may be a key technology for the commercialization of CNT field emission display.

Structurization in sintering of antifriction powder materials based on iron-copper alloys

The paper studies the contact interaction of the components in powders Fe-(Cu + Sn), Fe-(Cu + Sn + P + Pb), and Fe + B-(Cu + Sn + P + Pb) during sintering in hydrogen at 920 °C. It is shown that this interaction is responsible for the formation of both the interphase boundary and the general structure that defines the performance characteristics of an antifriction material. The interface and the phase and chemical composition of the products of interaction are examined. It is established that the powder composition Fe + Cu + Sn + P + B + Pb sintered in hydrogen at 920 °C is a microheterogeneous material whose matrix, which takes up the major load during friction, includes two phases: one based on iron alloyed with boron (Fe2B), copper, tin, and phosphorus and the other based on copper including tin in the form of α-solid solution, phosphorus in the form of Cu3P, and iron. Lead uniformly distributed over the matrix volume is the antifriction component of the material.

Evaluation of porous 430L stainless steel for SOFC operation at intermediate temperatures

In this paper a 430L porous stainless steel is evaluated for possible SOFC applications. Recently, there are extensive studies related to dense stainless steels for fuel cell purposes, but only very few publications deal with porous stainless steel. In this report porous substrates, which are prepared by die-pressing and sintering in hydrogen of commercially available 430L stainless steel powders, are investigated. Prepared samples are characterized by scanning electron microscopy, X-ray diffractometry and cyclic thermogravimetry in air and humidified hydrogen at 400 °C and 800 °C. The electrical properties of steel and oxide scale measured in air are investigated as well. The results show that at high temperatures porous steel in comparison to dense steel behaves differently. It was found that porous 430L has reduced oxidation resistance both in air and in humidified hydrogen. This is connected to its high surface area and grain boundaries, which after sintering are prone to oxidation. Formed oxide scale is mainly composed of iron oxide after the oxidation in air and chromium oxide after the oxidation in humidified hydrogen. In case of dense substrates only chromium oxide scale usually occurs. Iron oxide is also a cause of relatively high area-specific resistance, which reaches the literature limit of 100 mΩ cm 2 when oxidizing in air only after about 70 h at 800 °C.

Electroless coating of Permalloy powder and DC-resistivity of alloy composites

A high-resistivity coating of Permalloy (Ni–17.81Fe–1.97Mo wt.%) powders for soft magnetic composites, SMC, was developed and characterized. The coating was made using the electroless plating method employing an aqueous solution of sodium and nickel phosphates. The chemical process led to the formation of a layer up to 1 μm thick deposited on each particle surface and composed of a mixture of Ni(OH) 2, P 2O 5 and NaPO 3 compounds, as shown by X-ray photoelectron spectrometry and X-ray powder diffraction studies. The effect of coating on electrical DC resistivity was evaluated on SMC obtained mixing the metal, coated or uncoated, and thermoplastic polymer PEEK (polyetheretherketone) powders, pressing the mixture at 800 MPa and heating in air up to melting the polymer (380–410 °C). The SMC made with coated powders showed an increase in resistivity of more than two orders of magnitude compared to that obtained with uncoated powders when the polymer ranged between 0.5 and 1.5 wt.%. This difference prevails in comparison with sintered materials processed at the same pressure and sintering in hydrogen at 1350 °C. Thus, this process would restrict the eddy currents which affect the performance of soft magnetic products for AC applications.