Calcium Hydride Reduction Research Articles

Synthesis of nano- and ultra-fine refractory carbide powders by low-temperature calcium-carbothermic process and their grain growth during sintering

Metal carbides are widely used in various applications as catalysts, components of composite materials, cutting tools, and abrasive materials where the particle/grain size has a great impact on target properties. Here, we report a facile way to synthesize nano- and ultra-fine carbides of IVb-Vb groups, including high-entropy carbides, which are of interest nowadays due to their superior properties, by the combination of calcium-hydride reduction of metal oxides and the direct reaction of the reduced metals with carbon in one production step. By tailoring the synthesis parameters, it is possible to obtain the carbide powders of a size in the range of 50–1500 nm with high crystallinity. The average particle size is found to be dependent on the homologous synthesis temperature and follows a power law. We have also shown the possibility of obtaining ultrafine-grained bulk carbides (d = 0.24–0.9 μm) via spark-plasma sintering at a temperature of 1500–1650 °C with a relative density >95 % without using additives. The HEC (Ti,Zr,Nb,Hf,Ta)C has a higher activation energy of grain growth (collective recrystallization) compared to binary TaCx (x = 0.8 and 1), which confirms the effect of lattice distortion on diffusion coefficients. The suggested fabrication route can be widely used for nano and ultrafine carbide powder preparation on the industrial scale.

Fabrication of high-entropy carbide ceramics (Ti,Zr,Hf,Nb,Ta)C through low-temperature calcium-hydride reduction of oxides

Fabrication of high-entropy carbide ceramics (Ti,Zr,Hf,Nb,Ta)C through low-temperature calcium-hydride reduction of oxides

The Influence of Electron-Beam Treatment on the Structure of a TiNi Powder Alloy Obtained by Calcium-Hydride Reduction

The Influence of Electron-Beam Treatment on the Structure of a TiNi Powder Alloy Obtained by Calcium-Hydride Reduction

Preparation and Magnetic Properties of Nano-Sized Iron Powder Particles Coated with Silica Film by Calcium Hydride Reduction Method

Preparation and Magnetic Properties of Nano-Sized Iron Powder Particles Coated with Silica Film by Calcium Hydride Reduction Method

Preparation and Magnetic Property of Nano-sized Iron Powder Particles Covered with Silica Film by Calcium Hydride Reduction Method

Preparation and Magnetic Property of Nano-sized Iron Powder Particles Covered with Silica Film by Calcium Hydride Reduction Method

Synthesis of Powder Alloys Based on the Ti–Ni–Hf System via a Calcium Hydride Reduction Process

We have demonstrated the feasibility of preparing powder alloys based on intermetallic phases in the Ti–Ni–Hf and Ti–Ni–Hf–Zr systems by a calcium hydride reduction process and investigated the physicochemical and technological properties of Ti40Ni50Hf10, Ti28Ni50Hf22, and Ti28Ni50Hf12Zr10 powders thus prepared. Experimental data show that the synthesis of the powders typically involves the formation of intermediate phases inherent in the binary phase diagrams of the constituent components of the alloys. According to X-ray diffraction data, the synthesized powders consist predominantly of B19' martensite, whose content depends significantly on the synthesis temperature. At given process parameters, the alloy with the lowest Hf dopant concentration is the most homogeneous.

Plasma Spheroidization of Micropowders of a Heat-Resistant Alloy Based on Nickel Monoaluminide

Spheroidization of micropowders of a heat-resistant alloy based on nickel aluminide with a particle size of 20 to 45 μm was studied. The alloy was produced by calcium hydride reduction (CHR) and elemental synthesis (SHS). The spheroidization was carried out in a flow of argon–hydrogen thermal plasma generated by a direct-current (DC) plasma torch. The degree of spheroidization of the micropowders reached 98.5%, and the flowability was 20 s/50 g.

Calcium hydride synthesis of Ti–Nb-based alloy powders

The metallothermic (calcium hydride) synthesis of Ti–Nb alloy powders alloyed with tantalum and zirconium is experimentally studied under various conditions. Chemical, X-ray diffraction, and metallographic analyses of the synthesized products show that initial oxides are completely reduced and a homogeneous β-Ti-based alloy powder forms under the optimum synthesis conditions at a temperature of 1200°C. At a lower synthesis temperature, the end products have a high oxygen content. The experimental results are used to plot the thermokinetic dependences o formation of a bcc solid solution at various times of isothermal holding of Ti–22Nb–6Ta and Ti–22Nb–6Zr (at %) alloys. The physicochemical and technological properties of the Ti–22Nb–6Ta and Ti–22Nb–6Zr alloy powders synthesized by calcium hydride reduction under the optimum conditions are determined.

Production of a sintered alloy based on the TiAl intermetallic compound: Part 2. Investigation into forming and sintering processes

A complex production technology of electrodes from the powder alloy based on titanium aluminide TiAl, which includes the stages of fabrication of the powder alloy by calcium hydride reduction, treatment of the powder in a ball hard alloy mill with the addition of Y2O3 as a structural modifier, and the hydrostatic formation and sintering of a billet, is proposed. Formation and sintering processes are investigated for experimental samples and the alloy microstructure is investigated at all stages of process flowchart. The electrode for the plasma centrifugal spraying of the granules is fabricated using this technology.

Forms of presence and contents of light elements in finely divided Nb3Al powders

Three batches of finely divided Nb3Al intermetallic powders obtained by calcium hydride reduction were studied for the contents of light elements (H, O, N, C, and S). The oxygen-containing phases present in the Nb3Al powder samples were determined. The oxygen contained in the powder was qualitatively and quantitatively divided into oxygen as a part of adsorbed water, oxygen as a part of surface organic compounds, and oxygen as a part of niobium oxides (NbO x ) and aluminum oxides (Al2O3).

Low temperature synthesis of LaNi5 nanoparticles for hydrogen storage

LaNi5 nanoparticles as small as 170 and 250 nm were prepared by combustion and precipitation methods followed by calcium hydride reduction, respectively. The formation of such small particles was due to the effective reduction of the nanoscale lanthanum/nickel oxide precursors at low temperatures (600 °C). LaNi5 nanoparticles were found to be stable and hydrogen cycling did not change their morphology but led to a significant decrease in crystallite size reflecting the formation of lattice defects. The reduction of particle size enhanced hydrogen kinetics with full desorption occurring in a few minutes. Particle size effects on thermodynamics were also investigated by measuring the equilibrium plateau pressure (Peq) of the LaNi5/H2 reaction at various temperatures. Nanosized LaNi5 showed a relatively large hysteresis between Peq for hydrogen absorption and desorption because of lattice defects. However, no significant changes in enthalpy and entropy were observed for these nanoparticles.

Calcium Hydride Catalyzed Highly 1,2-Selective Pyridine Hydrosilylation.

Reaction of the calcium hydride complex (DIPPnacnac-CaH⋅THF)2 with pyridine is much faster and selective than that of the corresponding magnesium hydride complex (DIPPnacnac = [(2,6-iPr2 C6 H3 )NC(Me)]2 CH). With a range of pyridine, picoline and quinoline substrates, exclusive transfer of the hydride ligand to the 2-position is observed and also at higher temperatures no 1,2→1,4 isomerization is found. The heteroleptic product DIPPnacnac-Ca(1,2-dihydropyridide)⋅(pyridine) shows fast ligand exchange into homoleptic calcium complexes and therefore could not be isolated. Calcium hydride reduction of isoquinoline gave well-defined homoleptic products which could be characterized by X-ray diffraction: Ca(1,2-dihydroisoquinolide)2 ⋅(isoquinoline)4 and Ca3 (1,2-dihydroisoquinolide)6 ⋅(isoquinoline)6 . The striking selectivity difference in the dearomatization of pyridines by Mg or Ca complexes could be explained by DFT theory and was utilized in catalysis. Whereas hydroboration of pyridine with pinacol borane with a calcium hydride catalyst gave only minor conversion, the hydrosilylation of pyridine and quinolines with PhSiH3 yields exclusively 1,2-dihydropyridine and 1,2-dihydroquinoline silanes with 80-90 % conversion. Similar results can be achieved with the catalyst Ca[N(SiMe3 )2 ]2 ⋅(THF)2 . These calcium complexes represent the first catalysts for the 1,2-selective hydrosilylation of pyridines.

Properties of the intermetallic phase Nb3Al prepared by a calcium hydride reduction process

This paper demonstrates the feasibility of and examines the main steps in preparing the Nb3Al compound by a metallothermic process: by heating a mixture of niobium and aluminum oxides and calcium hydride (calcium hydride reduction process). Our calculations indicate that, when thermodynamic equilibrium sets in during the reaction between the components of a starting mixture, the oxygen content of the intermetallic phase may be very low. We obtained experimental Nb3Al intermetallic powders of nearly 100% purity and investigated their physicochemical and technological properties.

Deformation and heat treatment of NiAl-Y2O3-based powder alloys: II. fabrication of parts

Hydrostatic pressing of mechanically activated calcium hydride reduction (CHR) powders of plain intermetallic compound NiAl is used to fabricate parts of a complex shape that have a density and strength that allow the application of mechanical processing. The sections of a nozzle blade from an aviation gas turbine engine are produced from mechanically activated CHR mixtures of NiAl-2.5% Y2O3 powders by isothermal forging.

Deformation and heat treatment of NiAl-Y2O3-based powder alloys: I. Deformation and production of various pressed sections

The effect of a hot extrusion regime for workpieces produced by calcium hydride reduction (CHR) of mechanically activated powders of an NiAl intermetallic alloy and an NiAl-Y2O3 alloy composite on their deformability is studied to form various (round, flat, lens-shaped) sections. The extrusion regimes that make it possible to produce defect-free samples of the NiAl-Y2O3 alloy composite with a metal-deformed structure without traces of dynamic recrystallization are developed. Deformed semifinished samples are formed.

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: II. Mechanical activation

Effect of mechanical activation of NiAl powders produced by calcium hydride reduction in an attritor and a ball mill on the specific surface, the oxygen concentration, the strain hardening, and the coherent domain size (CDS) of the powders is studied. It is found that the powder specific surface milled in the attritor for 10–15 h is larger by a factor of 1.7–1.8 and the oxygen concentration in a powder is lower by a factor of 1.35 as compared to the its milling in the ball mill for 150 h. The powders milled in the attritor for 15 h have the level of microstresses higher by a factor of ∼2.4 and the CDS smaller by a factor of 2 as compared to the powder treated in the ball mill for 150 h. When milling a powder in the attritor, the milling time decreases by a factor of 10 and the degree of powder refinement increases, which improves the technological characteristics of the powders. As a result of the combination (in one operation) of mechanical activation of an NiAl intermetallic matrix powder in the attritor and the introduction of dispersed particles of a refractory oxide Y2O3 powder, the produced composite alloy has a density close to the theoretical one and has no aggregates of dispersed oxide particles at grain boundary junctions. Submicro- and nanosized oxide particles are homogenously distributed in the intermetallic matrix volume, which is characterized by a homogeneous distribution of nickel and aluminum.

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y2O3 produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ∼0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y2O3 powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y2O3 added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y2O3 interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y2O3 lattice and to the formation of submicroscopic particles of (Fe,Al)5Y3O12-type oxides.

NiAl powder alloys: II. Compacting of NiAl powders produced by various methods

The technological properties of granulated NiAl powders produced by gas spraying of melts and NiAl powders produced by calcium hydride reduction (CHR) of mixtures of nickel and aluminum oxides are compared. The possibilities of production of compact workpieces from these powders using hydrostatic pressing, hot pressing, hot isostatic pressing, and hot extrusion are estimated. To improve compressibility, preliminary milling and/or mechanical activation of the powders are proposed. The strength properties of NiAl rods with a diameter of 20 mm extruded from a temperature of 1100°C and made from the granulated powders are slightly higher than those made from the CHR powders. At temperatures higher than 800°C the properties becomes similar. Transition point td.b from the ductile to brittle state of samples made from powders sprayed in nitrogen and argon is 100–150°C higher than those made from the CHR powders. The difference in the mechanical properties is caused by the structural and chemical microheterogeneity of granules (microingots), which is inherited in the rods after hot deformation and annealing at 1200–1400°C and is (0.67–0.88)Tm NiAl (Tm is the melting point, K).

Preparation and Fine Purification of SiF4 and 28SiH4

Silicon tetrafluoride was obtained by the thermal decomposition of pure-grade sodium hexafluorosilicate and purified by low-temperature distillation. Next, SiF4 was 28Si-enriched by centrifugation and converted to silane by calcium hydride reduction. To remove hydrocarbons, the silane was distilled.

Chromium ― Copper System: Adhesion Characteristics, Doping, the Structure of Phase Boundary and Composites

The adhesive characteristics and the formation of phase boundaries in the Cr ― Cu system in a vacuum of (2-4)·10−3 Pa in the temperature range 1100-1300°C have been studied. The microstructure of composites (50:50 by mass) made by copper impregnation of porous samples of Cr electrolytic powders and Cr obtained by calcium hydride reduction. The effect of Cr, Fe, Co, Ni, Si additions on above characteristics has been investigated.