Refractory Silicides Research Articles

Dielectric Materials As Optical Emitters for Thermophotovoltaics

Thermophotovoltaics (TPV) are a promising route for converting heat generated as a byproduct into usable electricity through a clean energy paradigm. A primary challenge in the field is that the material options used to date for the optical emitters substantially constrain the power conversion efficiency of this process. Thus, we screened the optical response (i.e. permittivity) of >2,800 material combinations with melting point >2,000 oC, comprising refractory silicides, borides, carbides, nitrides, oxides, and metals [1]. Overall, the mismatch in permittivity allowed for emission control, key for the development of high performing TPV. We found a handful of emitters for TPV with theoretical efficiency >60% at 1800 oC, for GaSb solar cells. To complement our analysis, we also identified optimal material combinations for InGaAsSb, InGaAs, Ge, GaSb, and Si photovoltaic devices. The final down selection of materials is also based on their thermal expansion and thermochemical stability, which are frequently overlooked. The discussion of the best material options will be accompanied by the optical characterization of selected materials up to 1500 oC, using in situ reflection and transmission measurements [2]. Finally, we will discuss the specific case of SiC/AlN emitters for the abovementioned photovoltaic options [3].

Simultaneous preparation of high-purity silicides, eutectic Si/MSi2 (M: Nb, Ta, Mo, or W) alloys, and silicon through the separation of Si–M solvents using electromagnetic directional crystallization

• Refractory silicides and eutectic Si/MSi 2 (M: Nb, Ta, Mo, or W) alloys were prepared by separation Si–M solvent. • High-purity silicon can be obtained by acid leaching of eutectic Si/MSi 2 alloy. • A new approach for simultaneous preparation of high-purity refractory silicides, eutectic Si/MSi 2 alloys, and Si. • Mechanism separation and impurity removal of Si–M alloy was discussed. Refractory silicides, eutectic Si/MSi 2 (M: Nb, Ta, Mo, or W) alloys, and high-purity silicon are materials that have been used in many high-tech industries. Today, they are prepared using different technologies, i.e., each of them is prepared using an independent technology. This study proposes a new method to simultaneously prepare three high-purity materials (refractory silicides, eutectic Si/MSi 2 alloys, and silicon) using electromagnetic directional crystallization. The Si–M (Si–Nb, Si–Ta, Si–Mo, or Si–W) solvents are separated using electromagnetic directional crystallization, and then the MSi 2 (NbSi 2 , TaSi 2 , MoSi 2 , or WSi 2 ) and eutectic Si/MSi 2 (Si/NbSi 2 , Si/TaSi 2 , Si/MoSi 2 , or Si/WSi 2 ) alloys are precipitated in sequence because the crystallization temperature of MSi 2 is higher than that of the eutectic Si/MSi 2 alloys. Additionally, owing to the segregation behavior of impurities between the solid and liquid phases, impurities in the Si–M solvents can be eliminated efficiently to prepare high-purity MSi 2 and eutectic Si/MSi 2 alloys. In addition, as the eutectic Si/MSi 2 alloys consist of Si and MSi 2 phases, high-purity Si powders (>99.98%) can be obtained by separating the Si and MSi 2 phases using acid leaching. The results indicate the feasibility of the new approach in preparing refractory silicides, eutectic Si/MSi 2 alloys, and high-purity Si.

Effect of Additives on Physicomechanical Properties of High-Strength AS-Materials

Results are provided for studies of the effect of high temperature on highly-refractory unmolded materials based on refractory oxides and silicides, and also the effect of mineral additives on their characteristics and physicomechanical properties. It is stablished that unmolded aluminosilicate materials based on electromelted or tabular material are able to operate without loss of their properties at a working temperature not lower than 1600°C. Introduction of carbon or basalt microfiber into the material makes it possible to reduce the weight and density of corundum refractories, but at the same time it increases linear shrinkage under high temperature action.

Effects of Pt and Si on the low temperature hot corrosion of aluminide coatings exposed to Na2SO4-60 mol% V2O5 salt

Hot corrosion of pack (zero/medium) Pt-modified aluminide and slurry (zero/low/medium) Pt-modified silicon-aluminide coatings exposed to a Na2SO4-60 mol%V2O5 deposit at 700 °C was evaluated using the deposit recoat technique. Thickness loss metrology was utilized to study corrosion penetration into the coatings. The corrosion products and remaining coatings were characterized by means of SEM, EDX, XRD and EPMA. The aluminide coating and medium Pt-modified silicon-aluminide coating showed the worst and the best corrosion resistance, respectively. Enhanced hot corrosion resistance of the Pt-modified silicon-aluminide was attributed to microstructure of the coating with a two-phase PtAl2-NiAl outer layer and formation of refractory silicides.

Conceptual Protection Model for Especially Heat-Proof Materials in Hypersonic Oxidizing Gas Flows

This article is a continuation of the publication cycle of the authors on the subject matter “Multifunctional Protective Coatings for Especially Heat-Loaded Constructional Elements of Hypersonic Systems.” A conceptual physicochemical operation model of the protective coating in a high-speed high-enthalpy oxidizing gas flow taking into account and leveling main surface fracture sources by the gas flow is proposed. The model is successfully implemented when developing a whole series of alloys of the Si–TiSi2–MoSi2–B–Y system intended to form thin-layer coatings from them by any method of the stratified deposition providing the reproduction of the structure, phase composition, and morphological features of the deposited material in the coating. During the deposition, the formation of a microcomposition layer is provided. This layer is a refractory silicide framework with the cells filled by a low-melting (relative to the melting point of framework-forming phases) eutectic structural component. This layer transforms into a multilayer system with a series of functional layers (anticatalytic, reradiative, antierosion, heat-proof, and barrier-compensation layers) of micron and submicron thicknesses during high-temperature interaction with oxygen-containing media (the synergetic effect). The protection ability is provided by the formation of self-restoring oxide vitreous film based on alloyed silica. The self-restoring effect consists of rapid filling of random defects with a viscoplastic eutectic component and protective film formation accelerated when compared with known coatings. The high resistance to the erosion carryover is provided by the presence of a branched dendritic-cellular refractory framework. Coatings MAI D5 and MAI D5U, designed in the scope of the proposed concept, are successfully approved in high-speed high-enthalpy oxygen-containing gas flows affecting the samples and constructional elements made of especially heat-proof material of various classes (niobium alloys, carbon–carbon and carbon–ceramic composite materials, and graphitized carbon materials). The protective ability of coatings of 80–100 μm in thickness in flows with the Mach number of 5–7 and enthalpy of 30–40 MJ/kg is no shorter than 600 s at Tw = 1800°C, 200 s at 1900°C, and 60 s at 2000°C, including the constructive elements with sharp edges.

Environmentally Resistant Mo-Si-B-Based Coatings

High-temperature applications have demonstrated aluminide-coated nickel-base superalloys to be remarkably effective, but are reaching their service limit. Alternate materials such as refractory (e.g., W, Mo) silicide alloys and SiC composites are being considered to extend high temperature capability, but the silica surfaces on these materials require coatings for enhanced environmental resistance. This can be accomplished with a Mo-Si-B-based coating that is deposited by a spray deposition of Mo followed by a chemical vapor deposition of Si and B by pack cementation to develop an aluminoborosilica surface. Oxidation of the as-deposited (Si + B)-pack coatings proceeds with partial consumption of the initial MoSi2 forming amorphous silica. This Si depletion leads to formation of a B-saturated Mo5Si3 (T1) phase. Reactions between the Mo and the B rich phases develop an underlying Mo5SiB2 (T2) layer. The T1 phase saturated with B has robust oxidation resistance, and the Si depletion is prevented by the underlying diffusion barrier (T2). Further, due to the natural phase transformation characteristics of the Mo-Si-B system, cracks or scratches to the outer silica and T1 layers can be repaired from the Si and B reservoirs of T2 + MoB layer to yield a self-healing characteristic. Mo-Si-B-based coatings demonstrate robust performance up to at least 1700 °C not only to the rigors of elevated temperature oxidation, but also to CMAS attack, hot corrosion attack, water vapor and thermal cycling.

CONCEPTUAL PROTECTION MODEL FOR STRONGLY HEAT-RESISTANT MATERIALS IN HYPERSONIC OXIDIZING JET FLOWS

The article is a continuation of authors’ publications in the field of multi-function protective coatings for strongly heat loaded structural elements of hypersonic systems. The paper suggests a new physical and chemical model of heat-proof coating operation in a high-enthalpy oxidizing gas jet flow. The model considers and eliminates the main causes of surface destruction by the gas flow. The concept is efficiently used to produce a number of Si–TiSi2–MoSi2–B–Y system alloys intended for thin-layer coating formation using any layer deposition method capable of reconstituting the structure, phase composition and morphology of the deposited material. Deposition involves forming a microcomposite layer constructed from the refractory silicide framework with cells filled with a fusible (as compared with the framework phase) eutectic component. This layer transforms into a multilayer system during the high-temperature interaction with oxidizing media (synergetic effect). This multilayer structure contains anti-catalytic, reradiative, anti-erosion, heat-proof, barrier compensating function layers of micron and sub-micron thicknesses. Protection is ensured by a self-healing oxide glassy film formed based on alloyed silica. The self-healing effect consists in the rapid filling of incidental defects by the viscous plastic eutectics and faster (as compared with the known coatings) protection film forming. The branched dendrite cellular refractory framework ensures high resistance to erosion mass loss. The MAI D5 and MAI D5U protective coatings created as part of the presented concept were tested successfully in high-enthalpy oxygen-containing gas flows. The various specimens made of strongly heat-resistant materials were used to depose the coating such as niobium alloys, carbon-carbon and carbon-ceramic composites as well as graphitized carbon materials. The 80–100 μm thick coatings subjected to jet flows with M = 5÷7 and enthalpy 30–40 MJ/kg have shown the protection capacity above 600 s (Tw = 1800 °С), 200 s (Tw = 1900 °С), and 60 s (Tw = 2000 °С) for structural components with sharp edges as well.

Multi-layered silicides coating for vanadium alloys for generation IV reactors

The halide-activated pack-cementation technique was employed to fabricate a diffusion coating that is resistant both to isothermal and to cyclic oxidation in air at 650°C on the surface of the V–4Cr–4Ti vanadium alloy that is a potential core component of future nuclear systems. A thermodynamic assessment determined the deposit conditions in terms of master alloy, activator, filler and temperature. The partial pressures of the main gaseous species (SiCl4, SiCl2 and VCl2) in the pack were calculated with the master alloy Si and the mixture VSi2+Si. The VSi2+Si master alloy was used to limit vanadium loss from the surface. The obtained coating consisted of multi-layered VxSiy silicides with an outer layer of VSi2. This silicide developed a protective layer of silica at 650°C in air and was not susceptible to the pest phenomenon, unlike other refractory silicides (MoSi2, NbSi2). We suggest that VSi2 exhibits no risk of rapid degradation in the gas fast reactor (GFR) conditions.

Effective Refractory Metal Alloy Barrier Layer for High Temperature Microelectronic Device Application

ABSTRACTThe oxidation and diffusion of Molybdenum layer sputter-deposited on 2μm CVD diamond grown on silicon substrate has been studied. The Mo layer was protected by refractory metal silicide barrier layer. The samples were annealed in air ambient at 500°C over 30 hours. The oxidation of the samples was monitored with Rutherford Backscattering Spectroscopy (RBS). The effect of reactive sputtering of refractory silicide target in argon-nitrogen gas mixture (5% nitrogen by flow rate) on the barrier characteristics was investigated. The sheet resistivity of the barrier layer on SiC substrates as a function of annealing time in air at 500°C is reported. The surface structure and morphology of the refractory silicide films was determined with X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM).

Sintering of powders in the CrSi 2–Ti(Ta)Si 2 systems depending on the methods for synthesis

Composite materials based on refractory silicides, in particular their solid solutions, are widely used in many areas of engineering for parts of heating elements and heat resistance protective coatings. This paper presents the findings obtained in the study of peculiarities of solid-phase interaction during synthesis of Cr 0.9Ta 0.1Si 2 and Cr 0.5Ti 0.5Si 2 solid solutions depending on the synthesis conditions. The effect of the powder dispersity on compaction of targets from Cr 0.9Ta 0.1Si 2 and Cr 0.5Ti 0.5Si 2 powders has been studied, and it has been established that the use of nanosized powders complicates the process of pressing. Also, sintering of targets from nanosized Cr 0.9Ta 0.1Si 2 and Cr 0.5Ti 0.5Si 2 powders was studied. The sintered targets were established to have a small grain size and uniform porosity all over the volume. Thanks to small closed porosity, the targets exhibit high heat resistance under thermal shock.

Prediction of structural stabilities of transition-metal disilicide alloys by the density functional theory

Transition metal silicide based refractory composite alloys have great potential for applications as the next generation of turbine airfoil materials, due to their significantly higher operating temperatures than advanced Ni-based superalloys. In order to achieve the most of refractory silicide based materials, multi-component alloying has been considered to be the most useful route for developing alloys of a good balance of creep resistance, fracture toughness, oxidation resistance, and room-temperature ductility. In this work, structural stabilities of disilicides of the group IVB to VIB transition metal elements are studied using the ab initio density functional theory, shedding light on alloying/bonding behaviour at the electronic level. The results are also useful for developing full thermodynamic databases using the CALPHAD approach, where lattice stabilities of counter-phases are critical for accurate thermodynamic descriptions of multi-component systems.

Combined thermodynamic and mass transport modeling for material processing from the vapor phase

The computational modeling of vapor phase processes like CVD involves different routes. For complex chemical systems, thermodynamic analysis can be first performed. Three examples on the CVD of refractory metal disilicides and ternary silicides show the process guidelines which can be obtained from an a priori thermodynamic analysis even in feature scale technology. However, it is a static analysis. Linking the thermodynamic approach with mass transport modeling allows the description of the dynamic chemical system in local thermochemical equilibrium (LTCE). Two examples on the deposition of titanium silicide and silicon-germanium alloys illustrate the framework provided by this approach keeping in mind the assumptions that underlie the computations. Thermodynamic and LTCE transport modeling are to be seen as an a priori computational methods based on the access of only thermodynamic and transport database.

Fabrication of silicide materials and their composites by reaction sintering

The paper reports on a new powder metallurgical technique applicable for the preparation of refractory silicides, silicide composites, and other poorly sinterable materials. Starting from the fundamentals of sintering a general approach for reaction sintering of elemental powders—termed controlled reaction sintering (CRS)—will be given. The sintering behaviour of pre-alloyed powders, coarse elemental powders, and the mechanically treated elemental powder mixtures are compared. A detailed study of powder preparation, consolidation, CRS, and properties will be pointed out for MoSi 2 and MoSi 2 –SiC composites. Several silicides (MoSi 2 , MoSi 2 –SiC, TiSi 2 , Ti 5 Si 3 , FeSi 2 , ReSi 2 ) were produced by CRS. Properties such as fracture toughness, minimum creep rate, bending strength, and oxidation resistance were determined for MoSi 2 -based materials. The paper presents an outlook for different fabrication routes for silicides and silicide composites, as pressureless and pressure assisted sintering, metal injection moulding (MIM), and tape casting.

Mechanical Properties of C40-based Ternary Mo(Si,Al) 2 and Quaternary (Mo,Zr)(Si,Al) 2 Silicides

Refractory silicides with transition metals are of interest as structural materials operating at very high temperatures to improve energy efficiency. MoSi{sub 2} is particularly attractive because of its high melting point (2,030 C), relatively low density (6.24 g/cm{sup 3}), superior oxidation resistance and high thermal conductivity. Nevertheless, MoSi{sub 2} still has several problems which must be overcome before structural application. In this paper an attempt to improve the ductility, toughness and high-temperature strength of C40-based MoSi{sub 2} silicides was made by controlling additional Al and Zr contents in order to change the ductility, species of the constituent phase and the volume fraction of each phase.

Microstructural analysis and high-temperature strength of a directionally solidified Er 2Mo 3Si 4-MoSi 2 eutectic

Recent investigations have shown that Er[sub 2]Mo[sub 3]Si[sub 4] is an effective reinforcement for strengthening MoSi[sub 2]. When present in hot-pressed MoSi[sub 2] compacts, the Er[sub 2]Mo[sub 3]Si[sub 4] improved both hot hardness and creep resistance of the MoSi[sub 2]. The crystal structure of Er[sub 2]Mo[sub 3]Si[sub 4], as reported by Bodak et al., is monoclinic with lattice parameters of 0.667 nm, 0.689 nm, and 0.681 nm. The point group and space group of this phase have been identified as 2/m and P2[sub 1]/c, respectively. A theoretical density for Er[sub 2]Mo[sub 3]Si[sub 4] was calculated to be 8.25 g/cm[sup 3] and this compares favorably with other refractory silicide compounds (e.g., Mo[sub 5]Si[sub 3] = 8.25 g/cm[sup 3]). Because of the low crystal symmetry, this intermetallic compound is expected to exhibit limited plasticity, but may possess good high temperature mechanical properties. Mechanical property data on Er[sub 2]Mo[sub 3]Si[sub 4] is still quite limited; however, preliminary creep results from a decremental step-strain rate test show that the directionally solidified (DS) Er[sub 2]Mo[sub 3]Si[sub 4]-MoSi[sub 2] eutectic has excellent creep resistance at 1,300 C. At this temperature a flow stress of 625 MPa was observed for a strain rate of 5 [times] 10[supmore » [minus]5]s[sup [minus]1], but failure occurred as the strain rate was reduced to 10[sup [minus]5]s[sup [minus]1]. In this paper the authors present further results obtained from constant engineering strain rate compression tests on the same DS eutectic bar.« less

Combustion synthesis of single phase Ti5Si3

Ti[sub 5]Si[sub 3] is an important compound of the Ti-Si binary system with potential usage in high temperature aerospace applications. Silicides are important because of their superb oxidation resistance. Other characteristics for structural service temperatures of up to 1,600 C include low density, good strength at high temperature, good creep resistance, and microstructural stability over the temperature range between 25 C and the maximum service temperature. Because they are high temperature materials, the processing of the refractory silicides may not be easy. Typical processes for producing the silicides include chemical/ physical vapor deposition, rapid thermal processing for thin films and traditional casting or powder metallurgical processing. The drawbacks of these processes include high capital investments and an extended amount of time at high temperatures. As detailed in several reviews, Combustion Synthesis (CS) processing, provides several benefits, including the generation of substantial heat in situ, use of a simple reactor, ease of scaling up, tailoring of the composition, and reduction of impurities.

Plastic Behavior and Deformation Structure of Silicide Single Crystals with Transition Metals at High Temperatures

AbstractThe mechanical and plastic behaviors of refractory silicide single crystals with Cllb (MoSi2), C40 (CrSi2, TaSi2 and NbSi2), D88 (Ti5Si3) and Cl (CoSi2 and (Co0.9Ni0.1)Si2) structures were investigated. The C40–type silicides were deformed by (0001)<1120> slip. Their yield stress decreased sharply with increasing temperature but NbSi2 and TaSi2 which were deformable even at low temperatures, exhibited anomalous strengthening around 1350°C. Deformation of Ti5Si3 whose ductile-brittle transition occurred around 1300°C was controlled by twins and the brittle fracture occurred on the basal plane. In CoSi2 the {001}<100> slip was only activated at ambient temperatures but addition of Ni activated {110}<110> slip as secondary slip system and improved the ductility. The creep behavior of MoSi2 and CrSi2 single crystals were also investigated and was found to be controlled by the viscous and glide motion of dislocations.

Simulation of the microstructure of chemical vapor deposited refractory thin films

A ballistic deposition model (SIMBAD) has been extended to provide qualitative cross-sectional depictions of the microstructure present in chemical vapor deposited (CVD) films. The model qualitatively depicts the pronounced columnar structure typical of refractory metal, nitride, and silicide films−especially when deposited over integrated circuit topography. The important factors affecting thin film microstructure are seen to be flux shadowing, precursor surface diffusion, and a nonunity sticking coefficient. While the conformal step coverage typical of refractory CVD films is primarily due to a low sticking coefficient, the detailed columnar structure is the result of all three of these mechanisms. The angular distribution of the incident precursor flux is important to the shadowing mechanism, and a sticking coefficient-dependent angular distribution relevant to CVD is presented. Variations of the model to represent selective deposition (including selectivity loss) are also shown.

Fabrication and characterization of some novel reaction-bonded silicon carbide materials

Attempts have been made to produce modified reaction-bonded silicon carbide (RBSC) ceramics by incorporating a dispersion of other phases into the initial powder mix. ZrC, TiC, TaC and B4C were chosen as additives together with TiB2 as a phase likely to produce microcrack toughening in the final compact. During fabrication an important factor appears to be the possible reactions of the added phase with liquid silicon during the infiltration stage of the process. Thus, while all the carbides react with liquid silicon to form refractory silicides and new silicon carbide, this only significantly affected the reaction-bonding process if the dissolution/reaction kinetics were so fast as to disrupt the formation of the new silicon carbide framework which grows epitaxially to bond the existing silicon carbide particles together. As with conventional RBSC, the initial SiC grits play no part in any reaction except to act as nucleation sites for the new SiC. The microstructures of the various new materials have been characterized by reflected light microscopy, scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. This has led to an appraisal of the high-temperature reactions observed to have occurred and the unreliability of the high-temperature thermochemical data used to predict their occurrence. The mechanical properties of the new materials have been investigated by indentation testing (hardness and fracture toughness), including temperature-variant tests. Results are presented and the possibility for improving the properties of RBSC are discussed.

Device physics and technology of complementary silicon MESFET's for VLSI applications

The development of a complete complementary MESFET technology is presented. The state-of-the-art, fully implanted, CMOS-like process uses Shannon implants together with a refractory silicide Schottky-gate material to combine high gate barrier heights with ease of fabrication. To minimize parasitic resistances, a unique sidewall structure and sidewall spacers are utilized to allow for self-aligned implantation of the source/drain regions. A self-aligned titanium silicidation technique is employed to minimize sheet and contact resistance of the source/drain regions. The SUPREM process simulator was employed extensively. The performance and modeling of device parameters (e.g., threshold voltage, gate leakage, and short-channel effects) and circuit parameters (e.g. standby current, noise margin, and speed) were accomplished through analytic formulations, the PISCES two-dimensional device simulator, and the SPICE circuit simulator. >