Effect of LiAlSiO4 particle size on the properties of LAS/SiC porous ceramics with near zero thermal expansion

Near zero thermal expansion porous ceramics were fabricated by using SiC and LiAlSiO₄ as positive and negative thermal expansion materials, respectively, bonded by glassy material. The microstructure, mechanical properties, and thermal expansion behavior of LAS/SiC porous ceramics with different particle sizes of LiAlSiO₄ were investigated. The results indicated that the coefficient of thermal expansion of the LAS/SiC porous ceramics decreased from 0.5206×10⁻⁶ to −1.1053×10⁻⁶ K⁻¹ with increasing the LiAlSiO₄ particle size from ~45 µm to ~125 μm. It was attributed to the reduction in the reaction between LiAlSiO₄ and SiO₂ as the particle size of LiAlSiO₄ increased. Young’s modulus increased from 36 MPa to 54 MPa as the sintering temperature increased from 850 °C to 950 °C because of the good bonding between the SiC grains and the glass materials.     

Crystal structure,mechanical and thermal properties of Yb4Al2O9: A combination of experimental and theoretical investigations

The key requirements for a successful thermal and environmental barrier coating (T/EBC) material include stability in high temperature water vapor, low Young's modulus, close thermal expansion coefficient (TEC) with mullite, low thermal conductivity and weak mechanical anisotropy. The current prime candidates for top coat are ytterbium silicates (Yb₂SiO₅ and Yb₂Si₂O₇). A major weakness of these two silicates is the severe anisotropy in mechanical properties and thermal expansion that would lead to cracking of the coating. Thus, searching for new materials with weak mechanical and thermal anisotropy is of signification. In this work, the crystal structure, mechanical and thermal properties of a promising T/EBC candidate, Yb₄Al₂O₉, are investigated theoretically and experimentally. Good ductility, low shear deformation resistance, low Young's modulus (151 GPa) and low thermal conductivity (0.78 W m⁻¹ K⁻¹) is underpinned by heterogeneous bonding characteristic and distortion of the structure. Close TEC (6.27 × 10⁻⁶ K⁻¹) with mullite and weak mechanical anisotropy highlight the suitability of Yb₄Al₂O₉ as a prospective T/EBC.     

Effect of post-sintering heat-treatment on thermal and mechanical properties of Si3N4 ceramics sintered with different additives

To improve the thermal conductivity of Si₃N₄ ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si₃N₄ ceramics containing SiO₂–MgO–Y₂O₃-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO₂ and MgO could be eliminated from the ceramics by vaporization during post-sintering heat-treatment at 2223 K for 8 h under a nitrogen gas pressure of 1.0 MPa. Thermal conductivity of 3 mass% SiO₂, 3 mass% MgO and 1 mass% Y₂O₃-added Si₃N₄ ceramics increases from 44 to 89 Wm⁻¹ K⁻¹ by the decrease in glassy phase and lattice oxygen after the heat-treatment. Relatively higher fracture toughness (3.8 MPa m^1/2) and bending strength (675 MPa) with high hardness (19.2 GPa) after the heat-treatment were achieved in this specimen. Effects of heat-treatment on microstructure and chemical composition were also observed, and compared with those of Y₂O₃–SiO₂-added and Y₂O₃–Al₂O₃-added Si₃N₄ ceramics.     

Thermal properties of single-phase Y2SiO5

Y₂SiO₅ is a promising candidate for oxidation-resistant or environmental/thermal barrier coatings (ETBC) due to its excellent high-temperature stability, low elastic modulus and low oxygen permeability. In this paper, we investigated the thermal properties of Y₂SiO₅ comprehensively, including thermal expansion, thermal diffusivity, heat capacity and thermal conductivity. It is interesting that Y₂SiO₅ has a very low thermal conductivity (～1.40 W/m K) but a relatively high linear thermal expansion coefficient ((8.36 ± 0.5) × 10^?6 K^?1), suggesting compatible thermal and mechanical properties to some non-oxide ceramics and nickel superalloys as ETBC layer. Y₂SiO₅ is also an ideal EBC on YSZ TBC layer due to their close thermal expansion coefficients. As a continuous source of Y³⁺, it is predicted that Y₂SiO₅ EBC may prolong the lifetime of zirconia-based TBC by stopping the degradation aroused by the loss of Y stabilizer.     

Simultaneously enhanced toughness and strain tolerance of SiC-based ceramic composite by in-situ formation of VB2 particles

SiC-30vol%VB₂ ceramic composite was pressureless densified at 2150 °C with excess B₄C and C as sintering aids after in-situ formation of VB₂ in SiC matrix. The sintered bulk gained a considerably high fracture toughness of 7.0 ± 0.4 MPa m^1/2, which was ∼2.4 times as high as that of the monolithic SiC ceramic, owing to the existences of weak heterophase boundaries, thermal residual stresses and microcracks. Meanwhile, since the VB₂ particle has a lower elastic modulus than SiC and significantly suppressed the grain growth of SiC, the composite exhibited a high flexural strength of 458 ± 36 MPa and a relatively low Young’s modulus of 356 ± 6 GPa, resulting in an increase of ∼59.3% in mechanical strain tolerance (1.29 × 10⁻³) compared with that of single-phase SiC ceramic. Besides, the residual stresses and microcracks also induced a lower-than-expected Vickers hardness of 20.8 ± 0.5 GPa in the composite.     

Glasses for laser joining of zirconia ceramics

Laser joining of ZrO₂ ceramics using glasses and glass ceramics as sealing components requires optimized systems. The ternary systems SiO₂–BaO–B₂O₃ and BaO–SrO–SiO₂ were selected as a basis for development of suitable glass compositions for the laser joining process. Additives such as CaO, TiO₂, Al₂O₃, and MgO were used to control the crystallization processes and hence the thermal expansion coefficients during glass synthesis. The glass viscosity, the strength of the ceramic-glass-ceramic joint, and the joint tightness are other important glass properties which were optimized for the laser process. For glass G018-345, this yielded strengths of up to 225 MPa (Weibull modulus of m = 8.6) and He leak rates of up to 4.3 × 10⁻⁵ mbar l s⁻¹. Because of the varying viscosities obtained, the optimized glass systems could be used selectively in a temperature range of 700–900 °C.     

Physical,thermal and ablative performance of CVI densified urethane-mimetic SiC preforms containing in situ grown Si3N4 whiskers

A two-step process has been developed for silicon carbide (SiC) coated polyurethane mimetic SiC preform containing silicon nitride (Si₃N₄) whiskers. SiC/Si₃N₄ preforms were prepared by pyrolysis/siliconization treatment at 1600 °C, of powder compacts containing rigid polyurethane, novolac and Si, forming a porous body with in situ grown Si₃N₄ whiskers. The properties were controlled by varying Si/C mole ratios such as 1–2.5. After densification using a chemical vapour infiltration, the resulting SiC/Si₃N₄/SiC composites showed excellent oxidation resistance, thermal conductivity of 4.32–6.62 Wm⁻¹ K⁻¹, ablation rate of 2.38 × 10⁻³ − 3.24 × 10⁻³ g cm⁻² s and a flexural strength 43.12–55.33 MPa for a final density of 1.39–1.62 gcm⁻³. The presence of a Si₃N₄ phase reduced the thermal expansion mismatch resulting in relatively small cracks and well-bonded layers even after ablation testing. This innovative two-step processing can provide opportunities for expanded design for using SiC/Si₃N₄/SiC composites being lightweight, inexpensive, homogeneous and isotropic for various high temperature applications.     

Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.     

Application of BaO–CaO–Al2O3–B2O3–SiO2 glass–ceramic seals in large size planar IT-SOFC

BaO–CaO–Al₂O₃–B₂O₃–SiO₂ (BCAS) glass–ceramics can be used as sealant for large size planar anode-supported solid oxide fuel cells (SOFCs). BCAS glass–ceramics after heat treatment for different times were characterized by means of thermal dilatometer, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the coefficients of thermal expansion (CTE) of BCAS glass–ceramics are 11.4×10⁻⁶ K⁻¹, 11.3×10⁻⁶ K⁻¹ and 11.2×10⁻⁶ K⁻¹ after heated at 750 °C for 0 h, 50 h, and 100 h, respectively. The CTE of BCAS matches that of YSZ, Ni–YSZ and the interconnection of SOFC. Needle-like barium silicate, barium calcium silicate and hexacelsian are crystallized in the BCAS glass after heat-treatment for above 50 h at 750 °C. The glass–ceramics green tape prepared by aqueous tape casting can be directly applied in sealing the cell of SOFCs with 10 cm×10 cm. The open circuit voltage (OCV) of the cell keeps 1.19 V after running for 280 h at 750 °C and thermal cycling 10 times from 750 °C to room temperature. The maximum power density is 0.42 W/cm² using pure H₂ as fuel and air as oxidation gas. SEM images show no cracks or pores exist in the interface of BCAS glass–ceramics and the cell.     

Preparation and characterization of porous,biomorphic SiC ceramic with hybrid pore structure

Bio-carbon template (charcoal) was prepared by carbonizing pine wood at 1200 °C under vacuum, and was impregnated with phenolic resin/SiO₂ sol mixture by vacuum/pressure processing. Porous SiC ceramics with hybrid pore structure, a combination of tubular pores and network SiC struts in the tubular pores, were fabricated via sol–gel conversion, carbonization and carbothermal reduction reaction at elevated temperatures in Ar atmosphere. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were employed to characterize the phase identification and microstructural changes during the C/SiO₂ composites-to-porous SiC ceramic conversion. Experimental results show that the density of C/SiO₂ composite increases with the number of impregnation procedure, and increases from 0.32 g cm⁻³ of pine-derived charcoal to 1.5 g cm⁻³ of C/SiO₂ composite after the sixth impregnation. The conversion degree of charcoal to porous SiC ceramic increases as reaction time is lengthened. The resulting SiC ceramic consists of β-SiC with a small amount of α-SiC. The conversion from pine charcoal to porous SiC ceramic with hybrid pore structure improves bending strength from 16.4 to 42.2 MPa, and decreases porosity from 76.1% to 48.3%.     

Transport properties of sealants for high-temperature electrochemical applications: RO–BaO–SiO2 (R = Mg,Zn) glass–ceramics

The electrical properties and oxygen permeability of glass–ceramics 55SiO₂–27BaO–18MgO, 55SiO₂–27BaO–18ZnO and 50SiO₂–30BaO–20ZnO (%mol), which possess thermal expansion compatible with that of yttria-stabilized zirconia (YSZ) solid electrolytes, were studied between 600 and 950 °C in various atmospheres. The ion transference numbers, determined by the modified electromotive force (e.m.f.) technique under oxygen partial pressure gradients of 21 kPa/(1–8) × 10² Pa and 21 kPa/(1 × 10⁻¹⁸–2 × 10⁻¹²) Pa, are close to unity both under oxidizing and reducing conditions. The electronic contribution to the total conductivity increases slightly on increasing temperature, but is lower than 2% and 7% for the Zn- and Mg-containing compositions, respectively. The conductivity values measured by impedance spectroscopy vary in the range (1.4–7.8) × 10⁻⁶ S/cm at 950 °C under both oxidizing and reducing conditions, with activation energies of 122–154 kJ/mol and a minor increase in H₂-containing atmospheres, indicating possible proton intercalation. In agreement with the electrical measurements which indicate rather insulating properties of the glass–ceramics, the oxygen permeation fluxes through sintered sealants and through sealed YSZ/glass–ceramics/YSZ cells are very low, in spite of an increase of 15–40% during 200–230 h under a gradient of air/H₂–H₂O–N₂ due to slow microstructural changes.     

Synthesis of glass–ceramics in the CaO–MgO–SiO2 system with B2O3, P2O5, Na2O and CaF2 additives

Glass–ceramics based on the CaO–MgO–SiO₂ system with limited amount of additives (B₂O₃, P₂O₅, Na₂O and CaF₂) were prepared. All the investigated compositions were melted at 1400 °C for 1 h and quenched in air or water to obtain transparent bulk or frit glass, respectively. Raman spectroscopy revealed that the main constituents of the glass network are the silicates Q¹ and Q² units. Scanning electron microscopy (SEM) analysis confirmed liquid–liquid phase separation and that the glasses are prone to surface crystallization. Glass–ceramics were produced via sintering and crystallization of glass-powder compacts made of milled glass-frit (mean particle size 11–15 μm). Densification started at 620–625 °C and was almost complete at 700 °C. Crystallization occurred at temperatures >700 °C. Highly dense and crystalline materials, predominantly composed of diopisde and wollastonite together with small amounts of akermanite and residual glassy phase, were obtained after heat treatment at 750 °C and 800 °C. The glass–ceramics prepared at 800 °C exhibited bending strength of 116–141 MPa, Vickers microhardness of 4.53–4.65 GPa and thermal expansion coefficient (100–500 °C) of 9.4–10.8 × 10⁻⁶ K⁻¹.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

Pressureless sintering of HfB2–SiC ceramics doped with WC

Using WC as sintering aid, nearly full dense (～99%) HfB₂–20 vol% SiC ceramics were sintered at 2200 °C for 2 h without external pressure. The densification mechanism, microstructure evolution, mechanical properties and oxidation resistance were investigated. The results indicated that complex chemical reactions of WC in HfB₂–SiC system strongly related to the densification, microstructure and properties. The Young's modulus, fracture toughness and 3-pt bending strength of HfB₂–20 vol% SiC with 10 wt% WC were 511 GPa, 4.85 Mpa m^1/2 and 563 MPa, respectively, which were comparable to some hot pressed HfB₂–SiC ceramics in literature. The oxidation of HfB₂–20 vol% SiC with 10 wt% WC at 1500 °C in air exhibited parabolic kinetics. After oxidation at 1500 °C for 10 h, its weight gain and SiC-depleted layer thickness were 3.7 mg/cm² and 43 μm, respectively, and its residual flexural strength was comparable to or even a little higher than the value before oxidation.     

Effect of temperature gradient on the erosion behavior of SiC coating for carbon/carbon composites in a combustion environment

SiC coating with a thickness of 50–70 µm was prepared on the surface of C/C composites by in-situ reaction method. The SiC coated C/C composites were then tested in a wind tunnel where a temperature gradient from 200 to 1600 °C could be obtained to investigate their erosion behavior. The results of wind tunnel test indicated that the service life of C/C composites was prolonged from 0.5 to 44 h after applying the SiC coating. After the wind tunnel test, three typical oxidation morphologies, including glassy SiO₂ layer, porous SiO₂ layer and clusters of honeycomb-like SiO₂ grains, were found on the SiC coated C/C composites. With the decrease of oxidation temperature, the amount of glassy SiO₂ declined and the thermal stress increased, which induced the cracking followed by the degradation of the SiC coating.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Influence of SiC particle addition on the nucleation density and adhesion strength of MPCVD diamond coatings on Si3N4 substrates

It is generally accepted that SiC layers are often involved in the adhesion efficiency of chemical vapour deposition (CVD) diamond films on Si-containing substrates. Si₃N₄–SiC composite substrates with different amounts of SiC particles (0–50 wt%) were then used for diamond deposition. Samples were produced by pressureless sintering (1750°C, N₂ atmosphere, 2–4 h). The diamond films were grown on a commercial MPCVD reactor using H₂/CH₄ mixtures. Despite there being no special substrate pre-treatment, the films were densely nucleated when SiC was added (N_d≈1×10¹⁰ cm⁻²) with primary nanosized (∼100 nm) particles, followed by a less dense (N_d≈1×10⁶ cm⁻²) secondary nucleation. Indentation experiments with a Brale tip of up to 588 N applied load corroborated the benefit of SiC inclusion for a strong adhesion. The low thermal expansion coefficient mismatch between Si₃N₄ and diamond resulted in very low compressive stresses in the film, as proved by micro-Raman spectroscopy.     

Microstructural evolution and dielectric properties of SiO2-doped CaCu3Ti4O12 ceramics

The abnormal grain growth (AGG) behavior of undoped and SiO₂-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. With the addition of 2 wt.% SiO₂, the AGG-triggering temperature decreased from 1100 to 1060 °C, and the temperature for obtaining a uniform and coarse microstructure decreased from 1140 to 1100 °C. The lowering of the AGG temperature by SiO₂ addition was attributed to the formation of a CuO-SiO₂-rich intergranular phase at lower temperature. The apparent dielectric permittivity of coarse SiO₂-doped CCTO ceramics was ∼10 times higher than that of fine SiO₂-doped CCTO ceramics at the frequency of 10³–10⁵ Hz. The doping of SiO₂ to CCTO ceramics provides an efficient route of improving the dielectric properties via grain coarsening. The correlation between the microstructure and apparent permittivity suggests the presence of a barrier layer near the grain boundary.     

The phase stability and thermophysical properties of InFeO3(ZnO)m (m = 2, 3, 4, 5)

The phase stability and thermophysical properties of InFeO₃(ZnO)_m (m = 2, 3, 4, 5) compounds were investigated, which are a general family of homologous layered compounds with general formula InFeO₃(ZnO)_m (m = 1–19). InFeO₃(ZnO)_m (m = 2, 3, 4, 5) ceramics were synthesized using cold pressing followed by solid-state sintering. They revealed an excellent thermal stability after annealing at 1450 °C for 48 h. No phase transformation occurred during heating to 1400 °C. InFeO₃(ZnO)₃ exhibited a thermal conductivity of 1.38 W m⁻¹ K⁻¹ at 1000 °C, which is about 30% lower than that of 8 wt.% yttria stabilized zirconia (8YSZ) thermal barrier coatings. The thermal expansion coefficients (TECs) of InFeO₃(ZnO)m bulk ceramics were in a range of (10.97 ± 0.33) × 10⁻⁶ K⁻¹ to (11.46 ± 0.35) × 10⁻⁶ K⁻¹ at 900 °C, which are comparable to those of 8YSZ ceramics.