Disilicide Powders Research Articles

Preparation and microstructural evolution of spherical B-modified MoSi2 powders by induction plasma spheroidization

In order to improve and consolidate the properties of boron (B)-modified molybdenum disilicide (MoSi2) powders applied in selective laser melting and make them attractive candidate for selective laser melting, in this study, induction plasma spheroidization (IPS) was conducted to spheroidize Mo–62Si–5B (at.%) powders. Mo–62Si–5B powders prepared by mechanical crushing exhibited polygonal morphology and consisted of MoSi2 and MoB2 phases. Subsequently, spherical B-modified MoSi2 powders with the size in the range of 45–100 μm were obtained via IPS. The effect of plasma power on the spheroidization rate, flowability, and oxygen content of the powders was investigated, which improved obviously with the increase of the plasma power. Powders spheroidized at the largest plasma power of 40 kW exhibited the highest spheroidization rate (89%) and the best flowability (25 s/50 g). Furthermore, morphological evolution and microstructural evolution of particles were systematically explored based on melting and solidification processes of powders. With the increase in heat absorption, the morphology of particles gets gradually converted from polygonal to spherical. The microstructure of spherical particles transforms from original hypoeutectic structure to cellular eutectic structure with the decrease in the powder size.

A novel sulfur‐emission free route for preparing ultrafine MoSi2 powder by silicothermic reduction of MoS2

AbstractThe extraction of metal from sulfide mineral is usually accompanied by the emission of sulfur‐containing gas (e.g., SO2), which will cause serious pollution to the environment. In this work, a sulfur‐emission free route for preparing ultrafine molybdenum disilicide (MoSi2) powder by silicothermic reduction of MoS2 with the desulfurization agent of lime was proposed. The internal MoS2‐Si mixture is wrapped in an external desulfurization layer composed of lime. After the reaction is completed, the prepared MoSi2 can be easily separated from the desulfurization layer by using a non‐chemical method. In addition, when the reaction temperature is higher than 1000°C, almost all S in MoS2 is transformed to sulfur‐containing gas SiS, which can be fully captured by lime to generate CaS, Si, and CaSiO3. For the raw material with a MoS2:Si molar ratio of 1:4, after reacting at 1000, 1100, and 1200°C for 2 h, the average grain sizes of the obtained MoSi2 powder are approximately 100, 300, and 800 nm, respectively. Moreover, when the reaction time is prolonged from 2 to 6 h at 1000°C, the average grain size of the acquired MoSi2 powder is about 200 nm, and the residual sulfur content is about 0.12%. This work provides a new insight to extract metals or metal compounds from sulfide ores without releasing sulfur‐containing gas.

Synthesis of a MoSi2-Based Powder Material Under the Influence of Pressure and Shearing

The SHS-grinding method was employed to prepare molybdenum disilicide powder material through the reduction of molybdenum oxide by aluminum and silicon. It is shown that the mechanical action applied to the material after the passing of an SHS-combustion wave through it can destroy the synthesized briquette created by the conventional self-propagating high-temperature synthesis (SHS) method. The increase in the intensity of mechanical action in the course of SHS-grinding breaks the agglomerated particles up into finer fractions, and also changes the morphology of the synthesized powder.

The synthesis of the MoSi<sub>2</sub> material under the influence of the shift pressure

The self-propagating high-temperature synthesis (SHS) was used to prepare the molybdenum disilicide powder material by means of the aluminum and silicon reduction. It was shown that the mechanical action applied to the material after the SHS combustion wave has passed it through, can destroy the synthesized pellets which the conventional self-propagating high-temperature synthesis has created. The mechanical action's increasing in the course of the SHS-crushing breaks the agglomerated particles up to the fner fractions, as well; as it changes the morphology of the synthesized powder.Ill.3. Ref. 8.

Activation Energy and Mechanism of the Molybdenum Disilicide Sintering Process

This paper presents dilatometric analysis data for the sintering of off-the-shelf molybdenum disilicide (MoSi2) powder prepared by magnesiothermic synthesis. We have obtained continuous shrinkage curves for MoSi2 powder compacts with an initial relative density of 70% at different heating rates: 5, 10, 20, and 30°C/min. From a quantitative analysis of densification curves for the compacts, the activation energy for the initial stage of sintering has been determined to be Q = 695 kJ/mol. It has been shown that the dominant process in the initial stage of MoSi2 powder sintering is volume diffusion from grain boundaries and surfaces.

THE PHYSIOLOGICAL AND HYGIENIC EVALUATION OF THE OPERATOR’S WORKING CONDITIONS IN THE SYNTHESIS OF CHROMIUM DISILICIDE NANOCRYSTALLINE POWDERS BY HIGH-ENERGETICAL MECHANOACTIVATION METHOD

Objective: to carry out the physiological and hygienic evaluation of the working conditions of operators producing chromium disilicide nanopowders by high-energetical mechanoactivation method and to develop the preventive recommendations. Material and methods. The object of research was the technological process of producing nanocrystalline chromium disilicide powder within a planetary ball mill. The hygienic assessment of the technological process, technological equipment and psycho-physiologic evaluation of the working environment of the operators were carried out using the generally accepted psychophysiological, hygienic, and chronometer methods of study. The concentration of nanoparticles in the working area was measured using the diffusion aerosol spectrometer DAS-2702 («Aeronanoteh», Russia), the nanopowder particle size was measured by the device Analysette 12 DynaSizer («Fritsch», Germany), the chemical composition of air samples was determined by atomic emission spectrometry with inductively coupled plasma (ICP-AES) using the device «Ortima 2100 DV» («Perkin-Elmer», USA). Results. It was found out for the first time that the mechanical activation process was accompanied by emission of nano-sized chromium into the air of the working area, which had not been detected before the beginning of the work. The total concentration of nanoparticles in the main room was 1.6-1.9 times higher than that in the working area of the planetary ball mill and exceeded the test levels recommended for nanomaterials in European countries. Conclusion. The basic adverse factors in case of producing nanopowder of chromium disilicide by mechanoactivation method are presence of nanoparticles of metals in the workplace air and intensity of work. We have proposed hygienic recommendations which are aimed at improving the plant design for the high-energy mechanical activation in the direction of ensuring tightness, reduction of manual work operations, audible and visual signaling during the technological process.

Nanostructured titanium disilicide powders: Preparation by self-propagating high-temperature synthesis and mechanochemical processes and physicochemical properties

Nanostructured titanium disilicide (TiSi2) powders with semiconducting properties have been prepared via cold fusion of silicon and titanium nanopowders and mechanochemical activation of TiSi2 powders prepared by self-propagating high-temperature synthesis. The semiconducting properties of TiSi2 have been shown to be determined by the nanocrystallite size. Basic to the formation of TiSi2 as a semiconductor material is a change in its band structure upon the conversion of the conductor to a semiconductor. The transformation takes place when the crystallite size decreases from microns to the nanometer range (≤70 nm). Such crystallites are nanoclusters with distorted order in the arrangement of the silicon and titanium atoms.

Formation of semiconductor titanium disilicide

Nanostructured titanium disilicide powders with semiconductor properties are synthesized and studied. The optical and electrophysical properties of TiSi2 are found to be controlled by its crystallite size.

Phase Formation During Nitriding of Vanadium Disilicide

The evolution of microstructural and phase transformations during nitriding of mechanically preactivated vanadium disilicide powder is investigated by X-ray diffraction, chemical analysis, and transmission electron microscopy. It is established that, in the initial stage of nitriding (1000–1100°C), the phase formation is accompanied by the dispersion of near-surface zones of VSi2 particles and the formation of V2N and α-modification silicon nitride. With increase in the nitriding temperature, the phase formation is accompanied by the delamination of particles and the formation of mainly VN and silicon nitride of α- and α-modifications. Nitriding of a mechanically activated vanadium disilicide powder at 1400°C enables synthesizing a fine silicon nitride–vanadium nitride composite powder in a single process. The synthesized powder is formed as loose aggregates consisting of 50 nm particles.

Hot-pressing of MoSi2 reinforced B4C composites

This paper reports on the effect of molybdenum disilicide (MoSi2) addition on the densification and mechanical properties of boron carbide (B4C). Molybdenum disilicide powder (10 and 30wt%) was added to boron carbide powder and consolidated by hot pressing at various temperatures (1700–1900°C) and pressures (30–50MPa) in 0.001Pa vacuum. MoSi2 was found to react with B4C and form reaction products SiC, MoB2 and Mo2B5. Slight improvement in density was observed with respect to MoSi2 addition. High density compact has been obtained with 30wt% MoSi2 at 1900°C under a pressure of 50MPa. The hardness, elastic modulus and fracture toughness of high dense B4C+30wt% MoSi2 compact was found to be 35.1GPa, 555GPa and 4.8MPam1/2 respectively.

Composition and structure of functional groups on the surface of tungsten disilicide powder

X-ray photoelectron spectroscopy and infrared spectroscopy are used to establish that functional groups of three types form in air in the presence of water traces on the surface of WSi2 powder: silanol groups and residues of H2WO4 tungsten acid and H4[Si(W3O10)4] tungstate–silicate heteropolyacid (with the dominating role of silanol groups present on the surface in the greatest amount). It is shown that the surface properties of WSi2 are similar to those of SiO2 and SiC.

Low-temperature oxidation behavior of MoSi 2 powders

The oxidation behavior of molybdenum disilicide (MoSi 2) powders at 400, 500, and 600°C for 12 h in air were investigated by using X-ray diffraction (XRD) and transmission electron microscopic (TEM) techniques. Significant changes were observed in volume, mass, and color. Especially at 500°C, the volume expansion was found to be as high as 7–8 times, the color changed from black to yellow-white, and the mass gain was about 169.34% after 8 h, with SiO 2 and MoO 3 as main reaction products. The gains in volume and mass were less at 400 and 600°C compared with those at 500°C, probably due to the less reaction rate at 400°C and the formation of silica glass scale at 600°C, which would protect the matrix and restrain the diffusion of oxygen and molybdenum. Thus, the accelerated oxidation behavior of MoSi 2 powder appeared at 500°C and the volume expansion was the sign of accelerated oxidation.

Biomineralization of calcium disilicide in porous polycaprolactone scaffolds

The incorporation of CaSi(2) grains within a polycaprolactone (PCL) framework results in bioactive and biodegradable scaffolds which may be used in bone tissue regeneration. Porous PCL scaffolds were prepared via a combination of salt-leaching and microemulsion methods. To provide markedly different structural environments for the inorganic phase, calcium disilicide powder was either added to a mixed-composition porogen during a given scaffold preparation, or alternatively added to pre-formed scaffolds. Selective fluorescent labeling, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis were employed to assess scaffold calcification in vitro. The process of CaSi(2)/PCL scaffold calcification under zero bias, during which calcium phosphate growth is significantly dependent on the structural degradation of CaSi(2) grains, has a similar mechanism as the calcium phosphate growth on bioactive glasses/ceramics. The biomineralization of these scaffolds is initiated solely by the silicide phase and can be accelerated by the degradation of the polymer matrix.

Thermoelectric properties of mechanically alloyed iron disilicides consolidated by various processes

Mn doped p-type iron disilicide powders have been produced by a mechanical alloying process. As-milled powders were of metastable state and mostly transformed to β-FeSi2 phase by subsequent isothermal annealing. As-milled powders were consolidated by various processes such as sintering of the cold compact in vacuum, vacuum hot pressing (VHP), and spray drying/atmospheric plasma thermal spraying. Phase transitions during the processes were investigated using XRD, EDS, and SEM. As-consolidated specimens consisted of a mixture of α-Fe2Si5 and e-FeSi phases, which were gradually transformed into a thermoelectric semiconducting α-FeSi2 phase by subsequent isothermal annealing in the vicinity of 845°C in vacuum. However, some residual α and e phases remained even after prolonged annealing. Thermoelectric properties were measured as a function of temperature and correlated with phase transformation. They showed optimum values in the vacuum hot pressed specimen due to a higher fraction of β phase and/or higher density.

In-Flight Carburization of Molybdenum Disilicide Powders by an Induction Plasma

In-flight carburization of MoSi2 powders has been performed in an argon–H2–CH4 induction plasma. CH4 was used as the powder carrier gas, and it reacted with MoSi2 powders under the high-temperature conditions of the plasma. Carbon and α-SiC were the major reaction products of the in-flight carburized MoSi2 powders. The silicon carbide formed through nucleation and subsequent growth in the liquid phase. The influence of the induction plasma power level, reactor pressure, and quantity of CH4 on the carburization efficiency was investigated through a Box–Behnken experimental design. Under the optimal conditions achieved in this investigation, ∼8.0 wt% of carbon was incorporated into the MoSi2 powder particles.

Kinetics and Products of Molybdenum Disilicide Powder Oxidation

In this study, we investigated the kinetics and products of the oxidation of MoSi2 powder with an average particle size of 1.6 μm at 900°, 1000°, and 1100°C, using a small sample size of 0.5 g. Such a small sample size allowed us to minimize the effect of oxygen transportation through the powder volume, while maintaining a good relative weighing accuracy. X‐ray diffraction of oxidized samples indicated the formation of Mo5Si3 and Mo metal. Analysis of the oxidation kinetics suggested that gaseous MoO3 formed initially and amorphous SiO2 film later. The oxidation kinetics and products observed in this study differ from those reported in an early study, in which a larger sample size was used.

High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders

Intermetallics like silicides are useful for protective coatings against high-temperature corrosion. Especially molybdenum disilicide which has a great potential as protective coating e.g. in aircraft engines and gas turbines in the temperature range between 1400 and 1800°C due to its high melting point and its low brittle-ductile transition temperature of approximately 800–1100°C. Four types of coatings were produced by high velocity oxyfuel spraying (HVOF): unreinforced MoSi 2 with low porosity, unreinforced MoSi 2 with high porosity, with silicon carbide reinforced MoSi 2 and with alumina reinforced MoSi 2. The coatings as sprayed were characterized by XRD, SEM and EDX. Microhardness and porosity were measured. The oxidation behavior of the coatings was determined at 500, 1000 and 1500°C. The influence of the heating rate was investigated during oxidation tests at 1000°C. The tests at 500°C showed that the pesting depends on the porosity of the coating. SiC as reinforcing phase seems to accelerate pesting, while alumina reduces this reaction. Unreinforced MoSi 2 coatings form a protective SiO 2 layer on the surface with a thickness below 10 μm during oxidation at 1500°C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline two-phase system of α-MoSi 2 and hexagonal Mo 5Si 3.

Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor

The present study reveals some of the important parameters which control the in-flight nitridation of molybdenum disilicide (MoSi 2) powders when carried out in an induction thermal plasma reactor. Initially, gradients of temperature, velocity and concentration were evaluated, using an enthalpy probe system, for the plasma flow without injection of MoSi 2 powders. Radial profiles were then measured at the torch exit to examine the mass and energy transfer mechanisms occurring under different nitridation conditions. These measurements were performed using an induction plasma torch connected to a 50 kW radio-frequency (r.f.) power supply, the torch being attached to a water cooled cylindrical reactor. The process operating conditions studied were plasma plate power, chamber pressure, sheath gas composition, composition and flow rate of quench gas. The effect of last named parameter on the nitridation of the powders was found to be the most important parameter in the nitridation process. The results show that there is an optimum flow rate value for each type of quench gas and the temperature and concentration mapping demonstrates that the combination of high temperatures and high concentrations of N 2 are necessary to reach maximum nitridation levels in MoSi 2.

Preparation and physical properties of nanosized semiconducting CrSi 2 powders

Nanosized chromium disilicide (CrSi 2) powder was prepared by the arc plasma method and a subsequent purification process. The as-prepared CrSi 2 powder was characterized by X-ray powder diffraction and transmission electron microscopy. The crystal structure of the powder was also confirmed with infrared spectroscopy. The thermal stability of powders were investigated from room temperature to 1000°C in air and in a helium atmosphere, indicating that even in the nanosize region this disilicide powder was stable up to 720°C in air. Analysis of its photovoltage spectra shows that there exist two direct interband transitions in nanosized CrSi 2 grains at 0.782 and 1.148 eV, respectively.

Micropyretic synthesis of MoSi2 powders through an aluminothermic reaction

An aluminothermic reaction starting with inexpensive MoO3, SiO2, and Al powders was utilized to prepare molybdenum disilicide (MoSi2) powders by the micropyretic/combustion synthesis process and leaching. The combustion-synthesized product was porous and could readily be crushed into powders. X-ray diffraction (XRD) analysis revealed that the product of such a reaction consisted of α–Al2O3, MoSi2, and a small amount of Mo(Si,Al)2 and Mo5Si3. The reason for the formation of Mo(Si, Al)2 phase is discussed. MoSi2 powders were obtained by leaching out the Al2O3 from the synthesized powder mixtures in boiling phosphoric acid solution. The synthesized MoSi2 powders, including a small amount of Mo(Si, Al)2 and Mo5Si3, were very fine with an average particle size of about 1 μm.