Creating a Protective Shell for Reactive MoSi2 Particles in High‐Temperature Ceramics

Alumina encapsulated molybdenum silicide (MoSi₂) intermetallic particles were synthesized using a simple precipitation method followed by calcining at temperatures of 800°C–1000°C, to prevent the premature oxidation of MoSi₂ at high temperatures. The shell composition and the influence of the calcining temperature on microcapsule integrity were investigated by means of X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The results demonstrate that the composition and the mechanical stability of the alumina shell can be tuned by the annealing temperature. After calcining at 800°C and 850°C the alumina shell remains intact. Calcining at higher temperature promotes the formation of mullite, which leads to cracking of the shell. However, when annealed at 1000°C for 24 h these cracks were filled with mullite and preserved the molybdenum silicide particles. Furthermore, the mechanical stability of the shell was improved by applying an intermediate calcining treatment at 450°C prior to the annealing process at 1000°C.     

Preparation and self-healing performance of spherical α-Al2O3@MoSi2 particles using the precipitation method

α-Al₂O₃ was coated on the powder surface using the precipitation method to improve the pre-oxidation resistance of molybdenum disilicide in thermal barrier coatings (TBCs). The coating effects of four different aluminum sources (Al(NO₃)₃·9H₂O) to MoSi₂ mass ratios were evaluated by viscosity and micro-morphology. The shell-forming effects on the powder size and post-treatment were thoroughly analyzed to meet the requirements for practical application in TBCs. The results showed that the precipitation method was superior to the sol-gel process in terms of the shell thickness obtained. Optimal shell-forming was generated at 1200 °C in an inert atmosphere. A bonding layer (Al₆Si₂O₁₃) would be formed at the core-shell interface, further preventing oxidation penetration. The self-healing particles MoSi₂@α-Al₂O₃ can effectively seal micro-cracks (width below 300 nm) because of the fluidity of SiO₂ at working temperatures.     

Preparation of MoSi2@ZrO2 core-shell powders by hydrothermal-calcination and evaluation of low-temperature oxidation resistance

MoSi₂ is a promising candidate for high-temperature, structural materials. However, their oxidation resistance is poor below 600 °C. To prevent MoSi₂ from being oxidized at low temperatures, core-shell structured MoSi₂@ZrO₂ powder were prepared by hydrothermal-calcination approach. First, MoSi₂@Zr(OH)₄ core-shell composite powder with nano-sized Zr(OH)₄ shell particles were synthesised using a hydrothermal method. Subsequently, the MoSi₂@ZrO₂ core-shell structured powder were obtained by calcination of the MoSi₂@Zr(OH)₄ powders at 900 °C for 2h. Finally, microstructure and oxidation behaviour of the MoSi₂@ZrO₂ powder at 400 °C–600 °C in air were investigated systematically. Microstructural analysis revealed that all samples had a distinct core-shell structure, and the ZrO₂ shells coated the surface of the MoSi₂ core. Oxidation behaviour studies showed that the dense ZrO₂ shell layer could isolate the MoSi₂ surface from oxygen, improving the low-temperature oxidation resistance and providing better low-temperature antioxidant properties than those of the MoSi₂ and MoSi₂@Zr(OH)₄ core-shell structures.     

High-temperature mechanical properties of NextelTM 610 fiber reinforced silica matrix composites

Novel Nextel? 610 fiber reinforced silica (N610f/SiO₂) composites were fabricated via sol-gel process at a sintering temperature range of 800–1200?°C. The sintering-temperature dependent microstructures and mechanical properties of the N610f/SiO₂ composites were investigated comprehensively by X-ray diffraction, nanoindentation, three-point bending etc. The results suggested a thermally stable Nextel? 610 fiber whose properties were barely degraded after the harsh sol-gel process. A phase transition in the silica matrix was observed at a critical sintering temperature of 1200?°C, which led to a significant increase in the Young's modulus and hardness. Due to the weak fiber/matrix interfacial interaction, the 800?°C and 1000?°C fabricated N610f/SiO₂ composites exhibited quasi-ductile fracture behaviors. Specially, the latter possessed the highest flexural strength of ≈ 164.5?MPa among current SiO₂-matrix composites reinforced by fibers. The higher sintering-temperature at 1200?°C intensified the SiO₂ matrix, but strengthened the interface, thus resulting in a brittle fracture behavior of the N610f/SiO₂ composite. Finally, the mechanical properties of this novel composite presented good thermal stability at high temperatures up to 1000?°C.     

Influence of different coating structures on the oxidation resistance of MoSi2 coatings

Two different structures of MoSi₂ coatings were prepared on Niobium based alloys by using a two step process. The as-deposited type(a) MoSi₂ coating structure consists of a MoSi₂ layer on the surface and a NbSi₂ layer underneath, while the type(b) MoSi₂ coating consists of an outer MoSi₂ layer and an inner unsiliconized Mo layer. The oxidation behaviors of the two different types MoSi₂ coatings were examined at 1200 °C for 100 h in air, and the mass gains of type(a) and type(b) MoSi₂ coated specimens were 0.64 mg/cm² and 0.59 mg/cm² respectively. The excellent oxidation resistance of both type(a) and type(b) MoSi₂ coated samples at 1200 °C was due to the formation of a dense and continuous SiO₂ scale during oxidation. As the CTE mismatch between the outer MoSi₂ coating and the inner layer, cracks distributed within both type(a) and type(b) MoSi₂ coating structures.     

Low-temperature oxidation resistance of the silica-coated MoSi2 powders prepared by sol-gel preoxidation method

MoSi₂ has been regarded as one of the most promising structural materials, due to its excellent high-temperature oxidation resistance and high emissivity. However, as the most commonly used emittance agent in high emissivity coatings, it can be easily oxidized when the operating temperature is below 600 °C. To address this issue, the silica-coated MoSi₂ powders with good low-temperature (400–600 °C) oxidation resistance were fabricated by a sol-gel method followed by a preoxidation process. Here in, by the combination of the non-isothermal and cyclic-isothermal oxidation analysis, we demonstrate that the silica gel film coated on MoSi₂ can prevent oxidation to some extent, but the preoxidation treatment plays a more critical role in improving its oxidation resistance at low temperature. Both the sol-gel and preoxidation processes enable the integrated and compact encapsulation of the silica coating. The synthesized silica-coated MoSi₂ exhibits excellent oxidation resistance with slightly weight gains (<1 wt%) after cyclic-isothermal oxidation at 400–600 °C for 12 h.     

In situ single-step reduction and silicidation of MoO3 to form MoSi2

Nanocrystalline molybdenum disilicide (MoSi₂) is synthesized in a specially designed autoclave at 900°C. The XRD results revealed that the formation of MoSi₂ is favorable with the blend of MoO₃, Si, and Mg powders. The HR-TEM and SAED patterns confirm the formation of MoSi₂ phase. The structural parameters (crystallite size, strain, stress, and deformation energy density) are calculated using the Williamson-Hall (W-H) analysis. The formation mechanism involved in the synthesis of MoSi₂ is proposed. The nonisothermal oxidation kinetics (~1200°C) of MoSi₂ phase is examined through the thermal analysis techniques. The activation energy is determined by the Kissinger-Akahira-Sunsose isoconversional kinetic model. Finally, the reaction mechanism involved during the oxidation of MoSi₂ phase is identified using the integral master-plots method.     

Damage evolution in a self-healing air plasma sprayed thermal barrier coating containing self-shielding MoSi2 particles

A self-healing thermal barrier coating (TBC) system is manufactured by air plasma spraying (APS) and tested by thermal cycling. The ceramic topcoat in the self-healing APS TBC system consists of an yttria stabilised zirconia (YSZ) matrix and contains self-shielding aluminium containing MoSi₂ healing particles dispersed close to the topcoat/bond coat interface. After spraying the healing particles the material was annealed to promote the formation of an oxygen impermeable Al₂O₃ shell at the MoSi₂-TBC interfaces by selective oxidation of the aluminium fraction. The samples were subsequently thermally cycled between room temperature and 1100°C. The study focussed on the spontaneous formation of the Al₂O₃ shell as well as the subsequent damage evolution in the APS produced TBC during thermal cycling. Experimental evidence showing characteristic signs of crack healing in the topcoat is identified and analysed. The study shows that while the concept of the self-healing APS TBCs containing self-shielding MoSi₂ particles is promising, future study is needed to improve the protectiveness of the Al₂O₃ shells by further tailoring the aluminium content in the MoSi₂ and the particle shape to avoid the premature oxidation of the healing particles and maximise crack healing efficiency.     

High temperature oxidation behavior of ZrB2-SiC added MoSi2 ceramics

In this paper, MoSi₂, MoSi₂-20?vol% (ZrB₂-20?vol% SiC) and MoSi₂-40?vol% (ZrB₂-20?vol% SiC) ceramics were prepared using pressureless sintering. The oxidation behaviors of these MoSi₂-(ZrB₂-SiC) ceramics were investigated at 1600?°C for different soaking time of 60, 180 and 300?min, respectively. The oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics were studied through weight change test, oxide layer thickness measurement, and microstructure analysis. Further investigation of the oxidation behaviors of the MoSi₂-(ZrB₂-SiC) ceramics was conducted at a higher temperature of 1800?°C for 10?min. The microstructure evolution of the ceramics was also analyzed. It was finally found that the oxidation resistance of MoSi₂ was improved by adding ZrB₂-SiC additives, and the MoSi₂-20?vol% (ZrB₂-20?vol% SiC) ceramic exhibited the optimal oxidation resistance behavior at elevated temperatures. From this study, it is believe that it can give some fundamental understanding and promote the engineering application of MoSi₂-based ceramics at high temperatures.     

In situ reactive spark plasma sintering of WSi2/MoSi2 composites

MoSi₂ based materials have the potential for use in high temperature structural parts. In this work, WSi₂ reinforced MoSi₂ composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi₂ at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi₂, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi₂/MoSi₂ was as high as 16.47 GPa.     

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

The 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.     

Densification of silicon dioxide formed by plasma-enhanced atomic layer deposition on 4H-silicon carbide using argon post-deposition annealing

Post-deposition annealing (PDA) was used to improve gate oxide physical and electrical properties. Deposition was accomplished by plasma-enhanced atomic layer deposition (PEALD). We investigated the densification silicon dioxide (SiO₂) formed by PEALD on 4H-silicon carbide (SiC) using PDA without oxidation and nitridation. PDA was conducted at 400–1200?°C in argon (Ar) ambient. The thickness of the SiO₂ was reduced by up to 13.5% after Ar PDA at 1000?°C. As the temperature of the Ar PDA increased, the etching rate of SiO₂ decreased. At temperatures greater than 1000?°C, the SiO₂ etching rate was low compared with that of thermal SiO₂. After PDA, the SiO₂/4H-SiC interface was smoother than that of thermal SiO₂/4H-SiC. The current density versus oxide field and capacitance versus voltage of the SiO₂/4H-SiC metal oxide semiconductor (MOS) capacitors were measured. Sufficient densification of SiO₂ formed by PEALD on 4H-SiC was obtained using Ar PDA at 1200?°C.     

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part I: Processing and microstructure

This study systematically correlates processing with quantitative microstructural information over an extended compositional range for ZrB₂-MoSi₂ ceramics, with MoSi₂ contents ranging from 5 to 70 vol% and diboride starting particle sizes ranging from 3 to 12 μm. Fifteen different ceramics were hot pressed between 1750 and 1925 °C. Plastic deformation of MoSi₂ contributed to initial densification, but some of the MoSi₂ decomposed during the later stages of hot pressing. Finer diboride particles required lower temperatures to densify (1750 to 1850 °C) compared to coarser diboride particles (1900 °C). Increasing MoSi₂ content led to a decrease in sintering temperature. As MoSi₂ content increased, ZrB₂ grain size decreased and MoSi₂ cluster size increased. Starting powders with lower impurity contents and isothermal vacuum holds contributed to lower oxide impurity contents in the final ceramics. A diboride core-shell microstructure involving (Zr_1-x,Mo_x)B₂ solid solutions formed in all compositions with Mo contents in the solid solution shells ranging from 3 to 6 at%.This work identified specific relationships between starting composition, processing conditions and final microstructure, showing how microstructure and properties could be tailored by processing. The outcomes of this extensive study will serve as guidelines for the design of other structural ceramics that have to attain determinate thermo-mechanical properties for targeted applications.     

Thermal stability of MoSi2-TiB2-CrB2 composites at 900–1400°C

Slip casting procedure with subsequent firing in air is used to synthesize composites (100 ? x)MoSi₂?x MB₂, where M = Ti, Cr, (Ti, Cr) and x = 10, 20, and 30 mol % at 900–1400°C. The MoSi₂-CrB₂ and MoSi₂-(Ti, Cr)B₂ composites containing 20 and 30 mol % of borides are found to possess the highest protective properties. The optimum temperature range for sintering is 1100–1400°C.     

Flexible TiO2 ceramic fibers near-infrared reflective membrane fabricated by electrospinning

Titanium oxide is a potential high temperature reflective material due to its high melting point, large refractive index, and suitable band gap. The flexible TiO₂ ceramic fibers membrane was successfully fabricated by sol–gel method using the polyacetylacetonetitanium (PAT) as the precursor. In order to obtain high-quality TiO₂ fibers, the PAT precursor with good stability and good spinnability was optimized by adjusting the molar ratio of acetylacetone to Ti to 1:1. The TiO₂ fibers heat-treated at 700?°C had a diameter of 400–500?nm. The crystal phase of TiO₂ fibers was anatase, and the surface of fibers was smooth without obvious defects. In addition, the TiO₂ ceramic fibers membrane heat-treated at 700?°C had good flexibility and tensile strength, and the average reflectance in the wavelength range of 500–2500?nm was up to 91.3%. The fibers membrane exhibits a significant reflection effect in the practical experiments and maintained good morphology of the fibers after 1200?°C test.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.     

The improvement of the self-healing ability of MoSi2 coatings at 900–1200 °C by introducing SiB6

The brittleness of MoSi₂ ceramic and the thermal mismatch between MoSi₂ coating and C / C composite lead to brittle cracking of the coating at 900−1200 °C. This problem has been overcome in this studyby introducing submicron-SiB₆ into the coating. The pre-fabricated cracks and a kinetics model of hot-pressed SiB₆-MoSi₂ ceramic could quantitatively predict the glass growth and crack healing. As expected, enhancing temperature and SiB₆ content increased the growth rate of the borosilicate glass and the crack healing ability of MoSi₂ ceramic, which was ascribed to the lower oxidation activation energy and larger specific surface area of submicron-SiB₆. For the plasma sprayed coating, SiB₆ with submicron structure was benefit for cracking inhibition and formation of borosilicate glass during oxidation, reducing the oxygen permeability and the consumption of inner coating. Hence, the 15 % SiB₆-MoSi₂ coatings raised the protection times to 84 and 120 h at 900 and 1200 °C respectively, presenting favorable oxidation protective performance.     

Oxidation behaviour of a pressureless sintered HfB2–MoSi2 composite

The thermal stability of a 80-vol.% HfB₂ + 20 vol.% MoSi₂ composite is tested under oxidizing environment. Oxidation tests are carried out in flowing synthetic air in a TG equipment from 1000 to 1400 °C with exposure time of 30 h. At temperatures ≥1200 °C the silica resulting from oxidation of molybdenum disilicide seals the sample surface, preventing hafnium diboride from fast degradation. Analysis of the kinetics is carried out through fitting of the thermogravimetric curves. Between 1200 and 1400 °C, the kinetic curves deviate from a parabolic behaviour, being more close to a logarithmic–parabolic behaviour.     

Phase stability and microstructure evolution of MgO-ZrO2 and MgO-6YSZ ceramic fibers

In this work, MgO-ZrO₂ and MgO-6YSZ ceramic fibers were prepared with sol-gel method via electrospinning. Polymorph stability and microstructure evolution of zirconia fibers were fully characterized by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, and Scanning electron microscope. The results indicated that tetragonal zirconia for MgO-ZrO₂ was obtained and cubic zirconia could be fully stabilized for MgO-6YSZ with MgO molar fractions varying from 0.1 to 0.5 at 800°C. Monoclinic phase appeared with MgO molar fractions even up to 0.5 for MgO-ZrO₂ system and partially or fully stabilized zirconia could be achieved for MgO-6YSZ at 1000°C and 1200°C. Grain size was gradually decreased with increasing of MgO content at 800°C both for MgO-ZrO₂ and MgO-6YSZ ceramic fibers. The grain size of both systems increased with MgO molar fractions varying from 0.1 to 0.2 and then decreased at higher contents at 1000°C and 1200°C. A discussion on relationship among MgO state and the phase stability and grain size was presented. This work shows surface excess and solid solution of MgO predominantly controlled the phase stability and microstructure evolution of zirconia fibers.     

Fabrication of ferroelectric lead bismuth tantalate nanoparticles by colloid emulsion route

Abstract

Ferroelectric lead bismuth tantalate (PbBi₂Ta₂O₉ nanoparticles were successfully synthesised using a colloid-emulsion process. Monophasic PbBi₂Ta₂O₉ was obtained through calcining the precursor powder at 750°C for 2 h. The precursor powders are soft agglomerates with nanosized primary particles. After calcination at 750°C, the average primary particle size is about 40 nm and the shape of the particle is near spherical. A rise in calcination temperature results in an increase in the particle size as well as the crystallinity. The collid emulsion technique as developed was found to be superior to the conventional solid state method for preparing PbBi₂Ta₂O₉ powder with finer particles and a narrow size distribution.