Phase transformation and indentation response in the surface of MoSi2 exposed to vacuum at high temperature

A MoSi₂ sample contains a small amount of Mo₅Si₃C phase was exposed to vacuum at 1300 °C for 100 h. The surface of the sample was examined by XRD, SEM and indention test. Two layers were revealed in the surface of the MoSi₂ sample. The outer layer consists of (002) textured Mo₅Si₃ phase and porosities. The Mo₅Si₃ phase is formed through the decomposition of the MoSi₂ phase. The Mo₅Si₃C phase, which is a constituent phase in the matrix, disappears in the inner layer. The phase transformation process was analyzed in the light of assuming that silicon is the predominant diffuser. Indentation test on the surface revealed that the fracture mode of Mo₅Si₃ is completely transgranular.     

Room temperature mechanical properties and high temperature oxidation behavior of MoSi2 matrix composite reinforced by adding La2O3 and Mo5Si3

MoSi₂ matrix composites containing 0.8 wt.%La₂O₃ and different volume fractions of Mo₅Si₃ were synthesized by self-propagating high temperature synthesis. The room temperature mechanical properties and high temperature oxidation behavior at 1200 °C were studied. Results showed that La₂O₃ and Mo₅Si₃ caused the grain size to decrease of the MoSi₂ matrix composite. The flexure strength and fracture toughness are improved compared with pure MoSi₂. The strengthening mechanism of La₂O₃–Mo₅Si₃/MoSi₂ is fine-grain strengthening, and its toughening mechanisms are fine-grain toughening, crack deflection, crack branching and crack bridging. With an increase in Mo₅Si₃ content, the oxidation resistance gradually decreased. This is attributed to the poor oxidation resistance of Mo₅Si₃, grain refinement and relative density decrease of the composites. In this experiment, a La₂O₃–Mo₅Si₃/MoSi₂ composite was found to have optimal mechanical properties and high temperature oxidation resistance after adding 0.8 wt.%La₂O₃ and 16.3 wt.% Mo₅Si₃.     

Synthesis of molybdenum silicides by the self-propagating combustion method

The synthesis of the silicides of molybdenum (Mo₃Si, Mo₅Si₃, and MoSi₂) by the self-propagating combustion method is investigated. Only the reactants corresponding to the last phase can be reacted in a self-sustaining mode without preheating. The product of such a reaction is single-phase MoSi₂. Although reactant mixtures corresponding to the other two silicides can react in a self-sustaining mode with prior heating, the products of combustion were multi-phase. The dependence of the combustion temperature and velocity on the stoichiometry of the silicide was determined. The activation energy for the combustion synthesis of MoSi₂ determined in this investigation, 139.4 kJ mol^–1, is considerably lower than activation energies reported for the diffusion of silicon in molybdenum or in MoSi₂.     

Effect of the W addition content on valence electron structure and properties of MoSi2-based solid solution alloys

Based on the empirical electron theory (EET) of solids and molecules, the valence electron structures (VES) of MoSi₂-based solid solution alloys have been analyzed using the average atom model. The results showed that with the increase of the W addition content, the hybridization steps of Mo and Si atom of the alloys occurred in C3 and 1, respectively. The hybridization step of W was always C5. The bond energy of the main bond branch, the covalence electron number on the strongest bond and the percentage of the total covalent electron numbers accounting for the total valence electron number of (Mo_1−x, W_x)Si₂ solid solutions increased with the increase of W addition content. These suggested that the addition of W would increase the melting point, hardness and strength and decrease the fracture toughness of (Mo_1−x, W_x)Si₂ solid solutions. Based on those results, MoSi₂-based solid solution alloys were manufactured, and the results of the experiments were in accordance with those of the theory.     

Impurity effects in molybdenum silicide formation

The reaction between molybdenum thin films and single-crystal Si〈111〉 substrates was studied as a function of the concentrations of impurities (mainly oxygen) in the metal film. At a low oxygen concent (1–2 at.%), only the silicide phase MoSi₂ was observed, and a thickness proportional to the square root of time corresponding to an average activation energy of 3 eV in the temperature range 545–600 °C was found. During the formation of the silicide the oxygen originally present in the molybdenum films accumulates at the interface between the silicon and the MoSi₂.In contrast, a higher oxygen content (4–5 at.%) prevents the formation of any silicide phases in the above temperature range and leads to the formation of MoSi₂ and Mo₅Si₃ phases at temperatures near 800 °C. MoSi₂ was always observed at the inner interface with Mo₅Si₃ on the surface.The oxygen segregates from the silicides and accumulates at the Si-MoSi₂ and MoSi₂-Mo₅Si₃ interfaces to form a non-uniform layer of SiO_x(x ? 2).     

Si3N4 rodlike crystal-reinforced MoSi2 matrix composites

A MoSi₂-based composite reinforced with 20 vol.% Si₃N₄ rodlike crystals was fabricated by Spark Plasma Sintering (SPS) process. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite were above 95%. No reactions between Si₃N₄ and MoSi₂ were observed. The composite containing Si₃N₄ rodlike crystal had higher hardness than monolithic MoSi₂. The room-temperature flexural strength increased 60% compared to that of pure MoSi₂. Especially the room-temperature fracture toughness of the composites was higher than that of MoSi₂, from 3.6 MPa m^1/2 for MoSi₂ to 5.1 MPa m^1/2 for composites with 20 vol.% Si₃N₄, respectively. Besides, the yield strength at elevated temperature and the low-temperature oxidation resistance for 20 vol.% Si₃N₄–MoSi₂ composite exhibited considerable improvement over that of monolithic MoSi₂. These results showed that Si₃N₄ rodlike crystal is an effective reinforcement for MoSi₂.     

Elevated temperature mechanical properties of MoSi2/Si3N4, MoSi2/SiC composites produced by self-propagating high temperature synthesis

MoSi₂/Si₃N₄ and MoSi₂/SiC composite powders were produced via a novel self-propagating high temperature synthesis technique (SHS) and consolidated both by hot-pressing and plasma spraying. Mechanical testing subsequently was performed under four-point bending at a variety of elevated temperatures and strain rates. Damage accumulation in the hot-pressed materials, either by particle cracking or particle/matrix separation, resulted in composite mechanical properties that were reduced below the level of the unreinforced material. Plasma sprayed materials showed power-law hardening behaviour caused by Mo₅Si₃ precipitates and increased strain tolerance because of the fine grain size and reduced yield strength of the matrix.     

Combustion synthesis of (Mo1 − xCrx)Si2 (x = 0.00–0.30) alloys in SHS mode

Combustion synthesis was adopted to successfully synthesize molybdenum–silicon–chromium (Mo?Si?Cr) alloys by the mode of self-propagating high-temperature synthesis (SHS). The experimental study of combustion synthesis of Mo?Si?Cr alloys was conducted on elemental powder compacts. Powder compacts with nominal compositions including MoSi₂, (Mo_0.95Cr_0.05)Si₂, (Mo_0.90Cr_0.10)Si₂, (Mo_0.85Cr_0.15)Si₂, (Mo_0.80Cr_0.20)Si₂, (Mo_0.75Cr_0.25)Si₂ and (Mo_0.70Cr_0.30)Si₂ were employed in combustion synthesis experiments. The combustion mode, combustion temperature, flame-front propagation velocity and product structure were investigated. The results showed that Mo?Si?Cr alloys were synthesized by an unsteady state combustion mode with a spiral-trajectory reaction front. The peak combustion temperature reduced with the addition of Cr to Mo–Si system. The flame-front propagation velocity decreased with an increase in Cr content of the powder compact. The X-ray diffraction (XRD) results showed that the crystal structure of the combustion product changed from Cll_b-type structure (Mo_0.90Cr_0.10)Si₂ to C40-type structure (Mo_0.85Cr_0.15)Si₂ with increase in Cr content of Mo–Cr–Si alloys. The intensities of diffraction peaks of the C40-type phase gradually increased with increase in Cr content.     

SiC晶须与Si_3N_4颗粒强韧MoSi_2复合材料

采用湿法混合和热压工艺制备了不同Si3N4(p)和SiC(w)体积含量的MoSi2基复合材料,研究了复合材料的显微组织,晶粒大小,硬度、断裂韧性和抗弯强度.结果表明,复合材料的晶粒比纯MoSi2明显细化,且随着强化相添加量的增加而减小,抗弯强度和断裂韧性均大幅度提高,其中MoSi2-20%SiC(w)-20%Si3N4(p)复合材料具有较好的综合力学性能,断裂韧性和抗弯强度分别427 Mpa和10.4 Mpa·m1/2.复合材料的强化机制为细晶强化和弥散强化,韧化机制为细晶韧化和裂纹偏转与分支韧化.     

The effect of combined diffusion and kinetic transport barriers on multi-phase solid state reactions with a vapour reactant

Analytical models are presented for the rates of layer thickness growth of MoSi₂ and of Mo₅Si₃ that form by reaction of vapour-supplied Si with Mo or with partially silicided Mo. The models are applicable to other systems. Coupling of the diffusive flux of the reactive species Si with the rate of the chemical reactions determines the growth kinetics. The rate of chemical reaction is assumed to be proportional to the magnitude of a discontinuity in the Si activity at the physical boundary where the silicide reaction is occurring. Various combinations of diffusive versus chemical-kinetics-dominated transport in the two phases which grow in tandem are found to affect the functional dependence of the growth kinetics on time. Models include cases in which the host solid is heterogeneous, as occurs when the average composition of the host lies in a poly-phase region of the phase diagram.Nomenclature p porosity, (For examplep ₁ is the porosity of the MoSi₂ layer.) - q 1 — p - v _i volume fraction ofi in the unsilicided compact - V _i theoretical molar volume ofi - _i theoretical volume per gram atom of Si ini - _i (1 —p _i/_i ing atoms of Si per unit volume ini - D _i Diffusion coefficient of Si ini - K _i Kinetic rate constant for conversion of phasei to phasei+ 1 in g.At cm^–2s^–1 per C - J _i Flux of Si through phasei in g.Atcm^–2s^–1 - C ₀ in MoSi₂ at outer surface in equilibrium with Si vapour - C ₁ in MoSi₂ at boundary in equilibrium with Mo₅Si₃ - C ₂] in Mo₅Si₃ at boundary in equilibrium with MoSi₂ - C ₃ in Mo₅Si₃ at boundary in equilibrium with Mo - C ₄ in Mo at boundary in equilibrium with Mo₅Si₃ - C ₁ in MoSi₂ at boundary with Mo₅Si₃, but at a higher Si activity than the equilibrium value - C ₃ in Mo₅Si₃ at boundary with Mo, but at a higher Si activity than equilibrium value - C_ij C _i —C _j - A(Si) Atomic weight of Si - T elapsed time since start of siliciding in seconds - X, Y thicknesses of MoSi₂, Mo₅Si₃ layers in cm     

Characterization of mechanically alloyed and pressureless sintered Al-7 wt% Si-2 wt% LaB6-2 wt% (MoSi2, WSi2) hybrid composites

The purpose of this research is to investigate the dual effect of silicide (MoSi₂ or WSi₂) and LaB₆ reinforcing particles on the microstructural and mechanical properties of Al-7 wt% Si (Al7Si) matrix. Hypoeutectic Al7Si blends prepared from elemental Al and Si powders were mechanically alloyed (MA’d) for 12 h in a planetary ball mill (at 300 rpm). Afterwards, 2 wt% silicide reinforcements (MoSi₂ or WSi₂) with various particle size distributions (micron, bimodal, submicron) were separately added into these MA’d Al7Si powders together with 2 wt% of LaB₆ particles. Powders having compositions of Al7Si, Al7Si-2 wt% LaB₆, Al7Si-2 wt% LaB₆-2 wt% MoSi₂ and Al7Si-2 wt% LaB₆-2 wt% WSi₂ were milled for 30 min using a high-energy ball mill (at 1200 rpm) in order to obtain homogeneous distribution throughout the microstructure. Compositional, microstructural and mechanical characterization studies were performed on the sintered samples. The results showed that high-energy ball milling ensured the homogeneous distribution of micron-sized MoSi₂ and WSi₂ particles within the matrix rather than those of bimodal and submicron-sized ones. Micron-sized MoSi₂ and WSi₂ reinforced hybrid composites displayed dramatically higher hardness and wear resistance than those of composites reinforced with different size of MoSi₂ and WSi₂ particles. The striking point of the study was the remarkably higher hardness and wear resistance properties of the hybrid composites compared to those of un-reinforced and only LaB₆-reinforced ones. As a conclusion, hybrid composites extremely displayed promising mechanical properties.     

Fabrication of Intermetallic Compounds by Spark Plasma Sintering

The characteristics of spark plasma sintering, a new method of powder processing, were investigated. Four systems of intermetallic compounds—Ti-Al, Ti-Al-Cr, Mo-Si, and Mo-Si-Nb—were fabricated, and the formation process of compounds, the formed phases, and the microstructure of samples were observed. During the sintering of all the compositions of mixed powders, most of the compounds were formed by combustion reaction which occurred at almost the same temperature as the conventional combustion reaction temperature. The fabricated samples were well densified, however, the relative densities of the Mo-Si samples were lower than the Ti-Al samples. Ultrasonic images show that no internal defects were found in any sample and the grain size became finer with the increase in the Cr content in the Ti-Al system and Nb content in the Mo-Si system. The formed phases of Ti:Al=1:1 composition samples were TiAl and Ti₃Al phases, and Ti-Al added Cr samples consisted of TiAl, Ti₃Al, Cr₂Al, and Cr₉Al₁₇ phases. The sample synthesized with Mo:Si=1:2 mixed powders had only MoSi₂ phases, and Mo-Si samples with added Nb consisted of four phases: MoSi₂ with a small amount of Mo₅Si₃ phases in the matrix and Nb₅Si₃ with unreacted Nb for dispersed phases.     

Thermal expansion studies of Gd2Mo3O12 and Gd2W3O12

Simultaneous thermogravimetric/differential thermal analysis of Gd₂Mo₃O₁₂ showed an irreversible phase transition at 1178 K where as Gd₂W₃O₁₂ showed reversible phase transition at 1433 K, which were confirmed by powder X-ray diffraction. The thermal expansion behavior of α-Gd₂Mo₃O₁₂ (room temperature phase), β-Gd₂Mo₃O₁₂ (phase obtained by heating Gd₂Mo₃O₁₂ at 1223 K) and Gd₂W₃O₁₂ have been investigated using high temperature X-ray diffractometer. The cell volume of α-Gd₂Mo₃O₁₂, β-Gd₂Mo₃O₁₂ and Gd₂W₃O₁₂, fit into polynomial expression with respect to temperature, showed positive thermal expansion up to 1073, 1173 and 1173 K, respectively. The average volume expansion coefficients for α-Gd₂Mo₃O₁₂, β-Gd₂Mo₃O₁₂ and Gd₂W₃O₁₂ are 39.52 × 10⁻⁶, 21.23 × 10⁻⁶ and 37.96 × 10⁻⁶ K⁻¹, respectively.     

Electronic properties of semiconducting silicides: fundamentals and recent predictions

This review emphasizes progress in theoretical simulation and experiments that have been performed in the past years for semiconducting silicides. New fundamental electronic and optical properties of Ca₂Si and BaSi₂, recently found RuSi₂ phase, ternaries in Fe-Os-Si and Ru-Os-Si systems, β-FeSi₂, Mg₂Si and CrSi₂ with stretched and compressed lattices as well as transport properties of β-FeSi₂, ReSi_1.75, Ru₂Si₃ are presented. Prospects for practical applications of semiconducting silicides are discussed.     

Autowave synthesis of cast Mo-W-Si silicides

This work examines general trends in the autowave synthesis of cast Mo-W-Si ceramics in a reactor at an elevated gas pressure using highly exothermic mixtures of molybdenum oxide, tungsten oxide, aluminum, and silicon. The results indicate that the gas (N₂, Ar) pressure in the reactor and reactant ratio have a strong effect on the combustion process. We have obtained cast solid solutions with a controlled MoSi₂: WSi₂ ratio.     

Synthesis and high temperature oxidation of Mo–Si–B–O pseudo in situ composites

In this paper, the new concept of ‘pseudo in situ composites’ is introduced into artificial composites for ultra-high temperature applications, composed of five phases, Mo, Mo₃Si, Mo₅Si₃, Mo₅SiB₂ and SiO₂. Among these phases, Mo and Mo₅Si₃ are not thermodynamically stable with each other, but they are locally equilibrated in the composites due to the formation of Mo₃Si as their reactant. Using the spark plasma sintering (SPS) technique, the Mo–Si–B–O pseudo in situ composites are successfully synthesized from Mo₃Si/Mo₅Si₃/Mo₅SiB₂ in situ composite powder, Mo and/or SiO₂ powders. The consolidated compacts are sound and fully dense, indicating that the SPS is a promising technique to synthesize the Mo–Si–B–O pseudo in situ composites. High temperature oxidation properties of the composites were examined up to 1673 K. The temperature range is divided into three with respect to the oxidation behavior; i.e. (I) below 1000 K, (II) between 1000 and 1400 K, and (III) above 1400 K. In the range II, the oxidation resistance of the composites is significantly improved by SiO₂ addition. In the range III, the oxidation resistance of the composites is good enough even at 1673 K in spite of the existence of Mo, displaying high potential for ultra-high temperature applications.     

Thermochemical modelling of interfacial reactions in molybdenum disilicide matrix composites

The thermal and environmental stabilities of molybdenum disilicide have been evaluated using thermochemical modelling. The chemical reactivity of molybdenum disilicide with oxygen indicates that various molybdenum compounds and silica are formed, depending on oxygen pressures. The structure and properties of the silica films play an important role in the oxidation reaction and the reactions of water vapour (moisture) with molybdenum disilicide at high temperatures. The thermodynamic stabilities of various potential reinforcements, e.g. carbon, silicon carbide, silicon nitride, alumina, and some refractory compounds (borides, carbides, and oxides of titanium, zirconium and hafnium) in molybdenum disilicide matrix have been evaluated. Based on the results of thermochemical computations, SiC, Si₃N₄, TiC, ZrC, HfC, TiB, TiB₂, ZrB₂, HfB₂, ZrO₂ and HfO₂ were found to be stable, but carbon and TiO₂ were found to be unstable in MoSi₂.The Al₂O₃/MoSi₂ system was found to be stable below 1800 K. At temperatures above 1800 K, significant mass losses could occur due to the high vapour pressures of gaseous species (Al₂O, SiO). These thermodynamic predictions are in agreement with available experimental data.     

Molybdenum silicide schottky contacts to GaAs

The thermal stability of MoSi _x Schottky contacts to GaAs has been studied. The contacts were analysed by Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD), sheet resistance, and current-voltage measurements. We have studied MoSi _x with the compositionsx=0, 0.3, 0.6, 1, and 2. The films were annealed at various temperatures up to 850°C. The thermal stability of the films is found to bex-dependent. The best composition for thermally stable Schottky contacts is Mo₅Si₃. Good Schottky characteristics were retained with annealing temperatures up to 850°C for 30 min.     

Effect of starting composition on formation of MoSi2–SiC nanocomposite powder via ball milling

MoSi₂?CSiC nanocomposite powders were successfully synthesized by ball milling Mo, Si and graphite elemental powders. Effects of milling time and annealing temperature were also investigated. The composite formation and phase transformation were monitored by X-ray diffraction. The microstructure of milled powders was studied by SEM, TEM and XRD peak profile analysis. Formation of this composite was completed after 10 and 20?h of milling for 25%SiC and 50%SiC, respectively. High temperature polymorph (HTP) of MoSi₂ was obtained at the end of milling (20?h). On the other hand, annealing led to transformation of HTP to low temperature polymorph (LTP) of MoSi₂. Mo₅Si₃ was formed during annealing as a product of a reaction between MoSi₂ and excess graphite. Mean grain size <50?nm was obtained for 20?h milled sample on the basis of peak profile analysis and TEM images.     

Synthesis and electrical properties of mixed tetrahedral cluster phases,A(Mo2Re2)S8, A=Fe,Ni,Zn,Cu and Ga

Quaternary phases, A(Mo₂Re₂)S₈, A=Fe,Ni,Zn,Cu and Ga containing mixed tetrahedral (Mo₂Re₂) clusters have been synthesized and electrical properties examined in the range 77–300K. The compounds possess cubic symmetry and are n-type semiconductors except when A=Cu, which is metallic exhibiting p-type behavior. Isostructural GaMo₄S₈ is an n-type semiconductor.The type of bonding in these materials is discussed.