C/C–SiC composite prepared by Si–10Zr alloyed melt infiltration

A high performance and low cost C/C–SiC composite was prepared by Si–10Zr alloyed melt infiltration. Carbon fiber felt was firstly densified by pyrolytic carbon using chemical vapor infiltration to obtain a porous C/C preform. The eutectic Si–Zr alloyed melt (Zr: 10 at.%, Si: 90 at.%) was then infiltrated into the porous preform at 1450 °C to prepare the C/C–SiC composite. Due to the in situ reaction between the pyrolytic carbon and the Si–Zr alloy, SiC, ZrSi₂ and ZrC phases were formed, the formation and distribution of which were investigated by thermodynamics. The as-received C/C–SiC composite, with the flexural strength of 353.6 MPa, displayed a pseudo-ductile fracture behavior. Compared with the C/C preform and C/C composite of high density, the C/C–SiC composite presented improved oxidation resistance, which lost 36.5% of its weight whereas the C/C preform lost all its weight and the high density C/C composite lost 84% of its weight after 20 min oxidation in air at 1400 °C. ZrO₂, ZrSiO₄ and SiO₂ were formed on the surface of the C/C–SiC composite, which effectively protected the composite from oxidation.     

A simple and efficient method for the synthesis of SiBNC ceramics with different Si/B atomic ratios

The high-temperature durability of SiBNC ceramics is significantly influenced by Si/B ratios and the synthetic procedures. Single-source synthetic routes can yield homogeneous ceramics at the atomic level, but the Si/B ratio cannot be efficiently adjusted. In this paper, a simple and efficient method for the synthesis of SiBNC precursor polyborosilazanes (PBSZs) with different Si/B ratios has been established via a one-pot reaction involving boron trichloride, dichloromethylsilane and hexamethyldisilazane in different molar ratios. The Si/B ratios of the derived SiBNC ceramics were consistent with that of the precursor PBSZs. When pyrolysed at 1000 °C, PBSZs with 0.52, 0.94 and 2.12 Si/B ratios transformed into SiB_2.6N₅C_2.2, SiB_0.9N_2.7C_1.3 and Si₂BN₃C_1.4 ceramics respectively. The polymer-to-ceramic process was also studied and featured ceramic yields of 43.2 wt%, 50.1 wt% and 62.2 wt%, respectively. The derived ceramic SiB_0.9N_2.7C_1.3 resisted crystallization up until 1700 °C, whereas the SiB_2.6N₅C_2.2 and Si₂BN₃C_1.4 could remain amorphous up to 1600 °C only. Using the precursor with 0.94 Si/B ratio, the SiBNC ceramic fibres were also obtained.     

Oxidation behavior and microstructure evolution of SiC-ZrB2-ZrC coating for C/C composites at 1673 K

To protect carbon/carbon (C/C) composites against oxidation, a SiC-ZrB₂-ZrC coating was prepared by the in-situ reaction between ZrC, B₄C and Si. The thermogravimetric and isothermal oxidation results indicated the as-synthesized coating to show superior oxidation resistance at elevated temperatures, so it could effectively protect C/C composites for more than 221 h at 1673 K in air. The crystalline structure and morphology evolution of the multiphase SiC-ZrB₂-ZrC coating were investigated. With the increase of oxidation time, the SiO₂ oxide layer transformed from amorphous to crystalline. Flower-like and flake-like SiO₂ structures were generated on the glass film during the oxidation process of SiC-ZrB₂-ZrC coating, which might be ascribed to the varying concentration of SiO. The oxide scale presented a two-layered structure ~130 µm thick after oxidation, consisting of a SiO₂-rich glass layer containing ZrO₂/ZrSiO₄ particles and a Si-O-Zr layer. The multiphase SiC-ZrB₂-ZrC ceramic coating exhibited much better oxidation resistance than monophase SiC, ZrB₂ or ZrC ceramic due to the synergistic effect among the different components.     

Microwave-assisted synthesis of Pr–ZrSiO4, V–ZrSiO4 and Cr–YAlO3 ceramic pigments

An innovative two-step route was successfully applied to the microwave-assisted synthesis of Pr–ZrSiO₄, V–ZrSiO₄ and Cr–YAlO₃ ceramic pigments. It is able to reduce the total synthesis time from many hours down to a few minutes. Industrial batches were pelletized and underwent a short time pre-heating in conventional electric furnace (from 300 up to 1000 °C) prior to dielectric heating in microwave oven (2.45 GHz, 800 W continuous or paused). Pre-heating is necessary to reach a thermal level at which pigment precursors are activated under microwave irradiation (high enough tan δ of dielectric properties). Activation kinetics was followed by measuring (optical pyrometer) the external temperature of crucible during MW irradiation and the internal temperature at the end of heating process. Pigments were characterized by colourimetry (CIE Lab) and XRPD with Rietveld refinement. The V–ZrSiO₄ system is easily activated, already at pre-heating of 300 °C, with good reaction yield and colour performance; nevertheless it is difficult to keep under control, as temperatures exceeding 1200 °C are rapidly reached. The Pr–ZrSiO₄ system is activated after pre-heating over 600 °C and exhibits a slow kinetics, resulting in a low reaction yield and unsatisfactory colour. The Cr–YAlO₃ pigment is formed with colour and reaction yield close to those of the correspondent industrial product, even though after pre-heating at 800 °C or more. A peculiar behaviour of silica phases was observed in zircon systems, particularly after lower pre-heating temperatures. It consists in an early transformation of quartz (present as precursor) into cristobalite and abundant amorphous phase, before the SiO₂ + ZrO₂ → ZrSiO₄ transformation occurred.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

TaB2–SiC–Si multiphase oxidation protective coating for SiC-coated carbon/carbon composites

To improve the oxidation resistance of the carbon/carbon (C/C) composites, a TaB₂–SiC–Si multiphase oxidation protective ceramic coating was prepared on the surface of SiC coated C/C composites by pack cementation. Results showed that the outer multiphase coating was mainly composed of TaB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The coating could protect C/C from oxidation for 300 h with only 0.26 × 10^?2 g²/cm² mass loss at 1773 K in air. The formed silicate glass layer containing SiO₂ and tantalum oxides can not only seal the defects in the coating, but also reduce oxygen diffusion rates, thus improving the oxidation resistance.     

Structural,chemical and dielectric properties of ceramic injection moulded Ba(Zn1/3Ta2/3)O3 microwave dielectric ceramics

We report the development of a ceramic injection moulding (CIM) process to produce complex-shaped structures using high-performance microwave ceramic materials. In particular, we describe the synthesis methods and the structural, chemical and dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) doped with Ni and Zr ceramics produced using ceramic injection moulding. Sintering the ceramic injection moulded Ba(Zn_1/3Ta_2/3)O₃ to a relative density of ∼94% was possible at a temperature of 1680 °C and a time of 48 h. The best samples to date exhibit a dielectric constant, ɛ_r, of ∼30, a Q value, of ∼31,250 (i.e. tan δ < 3.2 × 10⁻⁵) at 2 GHz, and a temperature coefficient of resonance frequency, τ_f, of 0.1 ppm/°C.     

Structural characteristics and ablative behavior of C/C–ZrC–SiC composites reinforced with “Z-pins like” Zr–Si–B–C multiphase ceramic rods

In order to improve the ablation and oxidation resistance of C/C–ZrC–SiC composites in wide temperature domain, “Z-pins like” Zr–Si–B–C multiphase ceramic rods are prepared in the matrix. The influence of different sintering temperatures on the microstructure of ceramic rods and the ablative behavior of heterogeneous composites are studied. The results showed that the ZrB₂ and SiC phases are formed in the sintered matrix, and the increase of sintering temperature is beneficial to improve the density of the ceramic rods. The ablation properties of samples have been greatly improved. The mass and linear ablation rate are 0.8 mg/s and 3.85 μm/s, respectively, at an ablation temperature of 3000 °C and an ablation time of 60 s. After ablation, the matrix surface is covered with SiO₂ and ZrO₂ mixed oxide films. This is due to the preferential oxidation of “Z-pins like” Zr–Si–B–C multiphase ceramic rods in the ablation process, and B₂O₃ melt, SiO₂ melt, borosilicate glass, ZrSiO₄ melt and ZrO₂ oxide film can be generated successively from the low-temperature segment to the ultra-high temperature segment. These oxidation products can be used as compensation oxide melts for the healing of cracks and holes on the matrix surface in different temperature ranges and effectively prevent the external heat from spreading into the matrix. Therefore, C/C–ZrC–SiC composites with “Z-pins like” Zr–Si–B–C multiphase ceramic rods achieve ablation resistance in wide temperature domain.     

High-temperature alloy/honeycomb ceramic composite materials for solar thermal storage applications: Preparation and stability evaluation

SiC_w/Al₂O₃ honeycomb ceramics were engaged as sensible shell materials for encapsulating Al-Si alloys (latent heat materials) in the honeycomb holes to obtain alloy/ceramic composite materials with a high thermal storage capacity for high-temperature solar thermal storage applications. The stability evaluation between the sensible honeycomb ceramics and the latent alloys had been conducted and the failure mechanism for the latent alloys was investigated. Results indicated that the addition of the latent alloys could improve the thermal storage capacity of the sensible honeycomb ceramics significantly by >114% and the thermal storage densities of honeycombs containing Al-12Si and Al-20Si alloys were 1141.3 kJ/kg and 1106 kJ/kg (400–900 °C), respectively. The composite materials exhibited excellent physical and chemical stability. No cracks formed in the honeycomb ceramics and no leakage of alloys was discovered after the composite materials were exposed to 100 thermal cycles in a high-temperature testing environment. The oxidation of Al at >600 °C would lower the latent heat of alloys and the thermal storage densities decreased to 1039.9 kJ/kg and 1013.2 kJ/kg after enduring 100 thermal cycles. This study not only provides a sensible-latent system of thermal storage materials with excellent stability but also gives an insight into the protection of metal containers against the corrosion from Al-based alloys.     

High compressive strength in nacre-inspired Al−7Si−5Cu/Al2O3–ZrO2 composites at room and elevated temperatures by regulating interfacial reaction

Al−7Si−5Cu/Al₂O₃−ZrO₂ composites with nacre-like structures were prepared via ice-templating and gas pressure infiltration techniques. The composites were subsequently heat-treated at 850 °C for 0, 30, 60, 90 and 120 min to regulate the interfacial reaction between Al and ZrO₂. The yield of larger (Al_1−m, Si_m)₃Zr and ZrSi₂ phases increased with longer dwell times. The compressive strength initially increased and then decreased. The highest strength was observed in composites treated for 60 min and reached 1600±40, 1261±30 and 1033±22 MPa at temperatures of 20, 150 and 300 °C, respectively. These values increased by 30−40% as compared to those of the non-treated counterparts and were 2-, 5- and 12-fold more than those of the matrix alloy, respectively, which is demonstrative of the material's excellent load-bearing capacity, particularly at elevated temperatures.     

Bonding ZrB2-SiC-G ceramics using modified organic adhesive for engineering applications at ultra high temperatures in air

It is a challenge to bond ceramics for engineering applications at ultrahigh temperatures in air. In this paper, a high temperature organic adhesive (HTOA) was prepared using methylphenylsilicone resins (MPSR) as the matrix, trisilanolisobutyl-methylsilicone resin/polyhedral oligomeric silsesquioxane (POSS) as the modifier, ZrB₂, SiO₂ and Si powders as the inorganic fillers, and γ-aminopropyltriethoxysilane (KH550) as the curing agent. The synthesized HTOA was used to bond ZrB₂-SiC-G ceramic (ZSG). The ceramic yield of MPSR was increased from 71% to 91% after being modified by trisilanolisobutyl-POSS. The average shear strength of ZSG joints bonded by HTOA was 13.2 MPa at room temperature. After 1500 °C/1 h processing, the bonding strength between HTOA and ZSG ceramic was 53.8 MPa. The inter-diffusion of elements between the HTOA and the ZSG occurred at 1500 °C and ZrSiO₄ compound was formed via the interface reaction. The excellent high-temperature performance of the prepared HTOA makes it one of the convenient and effective organic adhesive for joining ZSG for engineering applications at ultrahigh temperatures in air.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

Effect of CuO/MnO additives on microstructure and microwave dielectric properties of 3ZrO2-3TiO2-ZnNb2O6 ceramics

The effect of CuO/MnO additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 3ZrO₂-3TiO₂-ZnNb₂O₆ (3Z-3T-ZN) ceramics prepared by conventional solid-state route were systematically investigated. CuO/MnO doped ceramics exhibited a main phase of α-PbO₂-structured ZrTi₂O₆ and a secondary phase of rutile TiO₂. SEM results showed that the grain size of MnO doped ceramics became larger with increasing amount of dopants. The presence of CuO/MnO additives effectively reduced the sintering temperature of 3Z-3T-ZN ceramics to 1220 °C. MnO doped into ceramics could enhance the Q×f values significantly. The 0.5 wt% CuO doped 3Z-3T-ZN ceramics with 0.5 wt% of MnO, sintered at 1220 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 41.02, a Q×f value of 44,230 GHz (at 5.2 GHz), and τ_f value of +2.32 ppm/°C.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

The B₂O₃ added Ba(Zn_1/3Nb_2/3)O₃ (BBZN) ceramic was sintered at 900 °C. BaB₄O₇, BaB₂O₄, and BaNb₂O₆ second phases were found in the BBZN ceramic. Since BaB₄O₇ and BaB₂O₄ second phases have an eutectic temperature around 900 °C, they might exist as the liquid phase during sintering at 900 °C and assist the densification of the BZN ceramics. Microwave dielectric properties of dielectric constant (ɛ_r) = 32, Q × f = 3500 GHz, and temperature coefficient of resonance frequency (τ_f) = 20 ppm/°C were obtained for the BZN with 5.0 mol% B₂O₃ sintered at 900 °C for 2 h. The BBZN ceramics were not sintered below 900 °C and the microwave dielectric properties of the BBZN ceramics sintered at 900 °C were very low. However, when CuO was added, BBZN ceramic was well sintered even at 875 °C. The liquid phase related to the BaCu(B₂O₅) second phase could be responsible for the decrease of sintering temperature. Good microwave dielectric properties of ɛ_r = 36, Q × f = 19,000 GHz and τ_f = 21 ppm/°C can be obtained for CuO doped BBZN ceramics sintered at 875 °C for 2 h.     

High temperature strength of hot pressed ZrB2–20 vol% SiC ceramics based on ZrB2 starting powders prepared by different carbo/boro-thermal reduction routes

ZrB₂–20 vol% SiC (ZS) ceramics based on ZrB₂ starting powders obtained by different boro/carbo-thermal reductions involving ZrO₂ + B₄C, ZrO₂ + B₄C + C, and ZrO₂ + B, were fully densified by hot pressing at 1900–2000 °C. The flexural strength of these ZS ceramics was measured from room temperature up to 1600 °C. At 1600 °C, the flexural strength of the ceramics is 460 ± 31, 471 ± 32 and 345 ± 11 MPa, respectively. The evolution of the strength as function of temperature is explained in terms of the differences in oxygen content, nature of fracture, grain sizes, grain boundary phases and microstructural defects.     

Investigation of high-temperature reactions within the ZrSiO4–Al2O3 system

Solid state reactions between ZrSiO₄ and αAl₂O₃ in powders of stoichiometric composition 3Al₂O₃·2SiO₂ were studied by X-ray diffraction and electron scanning microscopy with energy dispersive X-ray analysis (SEM + EDX). Data were obtained at temperature ranging from 1400 °C to 1600 °C for a period of time ranging from 30 min to 60 h. The results indicate that ZrSiO₄ and αAl₂O₃ react and form crystalline ZrO₂, crystalline mullite (almost 3Al₂O₃·2SiO₂ composition) and non-crystalline silicon–alumina phase (pre-mullite). At the temperature of 1600 °C the fastest stage of reaction is dissociation of ZrSiO₄. Obtained results show that dissociation of zircon is a first-order reaction. The dissolution of Al₂O₃ particles and diffusion of Al into non-crystalline phase seem to be the slowest step of the reaction.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Large E-field induced strain and polar evolution in lead-free Zr-doped 92.5%(Bi0.5Na0.5)TiO3–7.5%BaTiO3 ceramics

Structure, dielectric permittivity, strain, electric (E) polarization, and piezoelectric responses of (Bi_1/2Na_1/2)_0.925Ba_0.075(Ti_1−xZr_x)O₃ (BNT7.5BT-100xZr; x = 0–0.04) ceramics were investigated as functions of poling E field and temperature. The BNT7.5BT ceramic reveals a phase transition from P4bm nanodomains to long-range-ordered P4mm domains. The Zr-doped BNT7.5BT ceramic reveals a reversible change of unit cell with dynamically fluctuating polar nanoregions, which are responsible for the large strain. The poled BNT7.5BT ceramic displays a depolarization temperature of T_d = 90 °C, which correspond to a phase transition from ferroelectric to relaxor states. The Zr-doped BNT7.5BT ceramics have Burns temperatures (T_B) in the region of 400–435 °C, below which polar nanoregions begin to develop. The Zr-doped BNT7.5BT ceramics display wide diffuse phase transitions, suggesting a transition from R + T to T phases. BNT7.5BT-2Zr ceramic shows a temperature dependent linear large strain of 0.482% at 150 °C and can be a potential candidate for lead-free actuator.     

Fabrication and properties of reactively hot pressed ZrB2–SiC ceramics

ZrB₂–SiC ceramics with relative densities >99% were fabricated by ‘in situ’ reactive hot pressing from ZrH₂, B₄C and Si. The reaction was studied using two processes, (1) powder reactions at temperatures from 1150 to 1400 °C and (2) reactive hot pressing between 1600 and 1900 °C. The products from the reaction of a 2ZrH₂:1B₄C:1Si molar mixture were ZrB₂, SiC, ZrO₂ and ZrC. Modification of the composition to 2ZrH₂:1.07B₄C:1.16Si resulted in the elimination of the undesired ZrO₂ and ZrC phases. The final composition was approximately ZrB₂–27 vol% SiC with no undesired phases detected by X-ray diffraction, and only low concentrations of B₄C detected by scanning electron microscopy. Elimination of the undesired phases was accomplished by removing surface oxides through chemical reactions at elevated temperatures. Reactively hot pressed samples consisting of ZrB₂ with 27 vol% SiC had a Young's modulus of 508 GPa, a flexure strength of 720 MPa, a fracture toughness of 3.5 MPa m^1/2 and a Vickers’ hardness of 22.8 GPa.     

High quality microwave dielectric ceramic sintered at extreme-low temperature below 200° and co-firing with base metal

Ultra-low temperature co-fired ceramics technology (ULTCC) requires the microwave dielectric ceramics with lower intrinsic sintering temperature than the melting point of inner electrodes. In the present work, a novel HBO₂ ceramic was found to be densified at extreme-low temperature below 200 °C, with pores, residual H₃BO₃, amorphous B₂O₃ inside, with a relative permittivity ∼2.12 ± 0.02, a Qf value ∼32,700 ± 300 GHz and a temperature coefficient of resonant frequency value ∼ − 43 ± 3 ppm/°C. This material can be easily obtained by dehydration from H₃BO₃ by sintering at low temperature below 200 °C. Its extreme-low sintering temperature and water solubility also provides the possibility to achieve some novel multi-functional inorganic-organic composite in the future.