Oxidation/Corrosion of Polymer-Derived SiAlCN Ceramics in Water Vapor

Fully dense polymer-derived SiAlCN ceramics were exposed to pure water vapor flowing at the rate of 4.4 cm/s at 1100°C. Weight changes as a function of annealing time were recorded to illustrate the oxidation/corrosion behavior of the materials. The paralinear kinetic model was used to determine the parabolic and linear rate constants from the experimental data. The results show that the polymer-derived SiAlCN ceramics have a much better water-vapor corrosion resistance than pure silicon-based ceramics.     

Electrical property of joints made of polymer-derived SiAlCN ceramic via adhesive joining

Electrical property of joints made with polymer-derived silicoaluminum carbonitride (SiAlCN) ceramics was investigated. SiAlCN ceramics were successfully joined using polycarbosilane precursor as joining medium at 1123 K and 250 kPa. The maximum tensile strength of obtained joints reached ~3 MPa, meeting strength requirement of sensor fabrication. DC and AC conductivity of joints were measured in temperature range of 373–823 K, and were shown to follow band tail hopping mechanism. Conductive behavior of joints was evaluated via comparison with free SiAlCN ceramics. Unique amorphous structures of SiAlCN substrate and joining layer played important role in overall electrical properties. Therefore, the prepared joints presented practical application in high-temperature sensors based on polymer-derived ceramics.     

Correlation between crystallinity and thermal expansion in LiAlSiO4 ceramics prepared by spark plasma sintering

LiAlSiO₄ (LAS) ceramics are prepared by using the sol-gel method followed by spark plasma sintering. XRD patterns and SEM images verify that the ceramics contain amorphous and LAS phases and that microcracks appear in the sample prepared at 900 °C due to its larger grain size. Compared with applied pressure and soaking time, sintering temperature has a greater impact on the crystallinity and density of the ceramics during sintering. High-temperature XRD results reveal that the LAS phase exhibits its intrinsic negative thermal expansion independently in all samples regardless of crystallinity. The coefficients of thermal expansion (CTE) measured by the dilatometric method change from positive values in samples prepared at 600 and 650 °C to near zero in samples prepared at 700 and 800 °C and then to a negative value in the sample prepared at 900 °C. The combined effects of an amorphous phase with a positive CTE and the LAS phase with a negative CTE are responsible for the observed transformation of thermal expansion in the samples. The calculated total CTEs of the glass-ceramic bulks are in agreement with the results measured through the dilatometric method in samples prepared at 650–800 °C. Microcracks in the sample prepared at 900 °C cause a more negative bulk CTE than the calculated CTE.     

Preparation of dense La0.7Ca0.3MnO3 ceramics from freeze-dried precursors

Sinterability of La_0.7Ca_0.3MnO₃ precursors, obtained by the freeze-drying method, is studied in order to develop a technique for preparation of dense (>95%) ceramics for CMR measurements and sputtering applications. Single phase powders, obtained by thermal decomposition at 650°C, were subjected to deagglomeration by ultrasonic or mechanical treatment. Sintering of deagglomerated powders for several hours at T=1200–1300°C allowed to achieve densities up to 97–98%. The best sinterability is demonstrated by mechanically processed powder, but further sintering of ceramics, obtained from this precursor, results in significant dedensification (up to 85% at 1300°C). Analysis of precursors and dedensified samples shows at high temperature decomposition of carbonates in closed pores to be the most probable reason for the observed process.     

Dense additive-free bulk boron nitride ceramics developed by self-densification of borazine

Boron nitride (BN) ceramics are promising candidates for high-temperature structural and functional materials. However, it is difficult to sinter dense additive-free bulk BN monoliths because of the covalent bond and flake structure. Here, we report dense bulk BN with relative density higher than 95% without sintering additives achieved via a self-densifying approach of borazine within a wide temperature range from 800 °C to 1800 °C. A polyborazylene with a controlled degree of cross-linking was synthesized and press-molded into shaped green bodies, which were pressureless pyrolyzed into porous frameworks and then densified through repeated borazine infiltration and pyrolysis method. The microstructural and crystalline evolution of the bulk BN ceramics, as well as the corresponded mechanical, thermal and dielectric properties were investigated.     

Phase interaction and oxygen transport in La0.8Sr0.2Fe0.8Co0.2O3–(La0.9Sr0.1)0.98Ga0.8Mg0.2O3 composites

Composite ceramics made of two perovskite-type compounds, (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3−δ (LSGM) and La_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (LSFC) mixed in the ratio 60:40 wt.%, possess relatively high oxygen permeability limited by both bulk ionic conduction and surface exchange at 700−950 °C. Sintering at elevated temperatures (1320–1410 °C) necessary to obtain dense materials leads to fast interdiffusion of the components, forming almost single perovskite phase ceramics with local inhomogeneities. This phase interaction decreases the oxygen ionic transport in the composites, where the level of ionic conductivity is intermediate between those of LSGM and LSFC. The scanning electron microscopy (SEM) suggests a presence of Ga-enriched domains, probably having a high ionic conductivity. The size and concentration of these domains can be increased by decreasing sintering temperature or using preliminary coarsened LSGM powders. The maximum oxygen permeability is thus observed for the composite prepared under minimum sintering conditions sufficient to obtain gas-tight ceramics, including the use of LSGM, preliminary passivated at 1150 °C, and sintered at 1320 °C. The activation energy values for total conductivity, which is predominantly p-type electronic and slightly decreases due to component interaction, vary in the narrow range from 24.0 to 26.2 kJ/mol at 25–575 °C. The average thermal expansion coefficients (TECs) of LSGM-LSFC composites, calculated from dilatometric data in air, are (12.4–13.5)×10⁻⁶ K⁻¹ at 100–650 °C and (17.8–19.8)×10⁻⁶ K⁻¹ at 650–1000 °C.     

Preparation study for the low thermal expansion spodumene/mullite composites

In this work, spodumene/mullite ceramics with low thermal expansion were successfully prepared from spodumene, quartz, talc, and clay. The effects of spodumene content and sintering temperature on the mechanical properties of spodumene/mullite ceramics were investigated. The formed phases were then detected by X-ray diffraction analysis and the microstructures of the sintered bodies were determined by scanning electron microscopy. The interaction effects of the spodumene content and sintering temperature on the apparent porosity and bulk density were studied by response surface methodology. The results demonstrate that an appropriate sintering temperature and spodumene content can promote densification, improve the mechanical properties, and reduce the coefficient of thermal expansion (CTE) of spodumene/mullite ceramics. At the spodumene content of 40 wt.%, the sintering temperature of 1270°C, and the holding time of 90 min, the bending strength was 60.45 MPa, the CTE was 1.73 × 10^–6/°C (α_{[25–650°C]} < 2 × 10^–6/°C), the bulk density was 2.28 g cm^-3, and the apparent porosity was 0.43%. Therefore, this study was of guiding significance for reducing the production cost of spodumene low thermal expansion ceramics and improving product quality.     

Low temperature synthesis of dense MoAlB thin films

While bulk synthesis of MoAlB requires temperatures larger than 1000 °C with up to 40 % of excess Al in the feedstock, here we report the temperature range for the formation of single phase, orthorhombic MoAlB synthesized by magnetron sputtering from a stoichiometric target is 450–650 °C. Lower synthesis temperatures yield the formation of amorphous films, while at 700 °C, impurity phases form in addition to orthorhombic MoAlB. Amorphous MoAlB films were observed by in-situ X-ray diffraction to crystallize between 545 and 575 °C. Hence, we infer that the formation of orthorhombic MoAlB thin films is surface diffusion mediated below 545 °C. As bulk diffusion is activated between 545 and 575 °C the synthesis of fully dense MoAlB films with a maximum hardness of 15 ± 2 GPa and a Young’s modulus of 379 ± 30 GPa at 600 °C is surface and bulk diffusion mediated.     

Fabrication of grooves on polymer-derived SiAlCN ceramics using femtosecond laser pulses

In this study, grooves were fabricated on the surface of fully dense polymer-derived SiAlCN ceramics. Industrial femtosecond laser source was used at wavelength of 1030 nm with pulse duration of 290 fs and repetition rate of 100 kHz. Moreover, comprehensive study was carried out to evaluate the influence of scan speed and energy fluence on grooves quality, including the heat-affected zone around the laser-machined grooves, microstructures of laser-irradiated surface, and cross-section morphology of grooves. A series of grooves with width in the range of 30–80 μm and depth below 280 μm was successfully fabricated using femtosecond laser pulse. Laser parameters were optimized to obtain grooves with satisfying surface quality. Furthermore, formation and disappearance of laser-induced periodic surface structures were systematically investigated. This study proposes fabrication of grooves on SiAlCN ceramics via laser processing, which provides precise method for fabrication of microstructure with fascinating properties.     

Shaped MgO–Al2O3–SiO2 refractory ceramics from recycled materials

Abstract

The aim of the present work is to prepare, characterise and assess MgO–Al₂O₃–SiO₂ refractory ceramics; namely, spinel, mullite and cordierite from chemically recycled precipitates. These precipitates include pure and fine magnesium and aluminium hydroxides as well as water treated fumed silica. Corresponding batches of the aimed oxide ceramics were coprecipitated from these precipitates and subsequently processed up to firing using the proper techniques. The processed bodies were investigated for their chemical and phase composition as well as morphology, microstructure and physical properties. According to the results of these investigations, the processed ceramics could be recommended for the adequate applications. It is concluded that dense, direct bonded and highly refractory spinel and mullite–corundum bodies could be obtained after firing their coprecipitated batches up to 1700°C. On the other side, dense, porous and refractory cordierite–spinel bodies could be processed from its batch after firing up to 1350°C. All of these bodies are refractory oxide ceramics with a very wide range of thermo-chemical, physical and mechanical applications.     

Microwave dielectric properties of low-temperature sinterable α-MoO3

The α-MoO₃ ceramics were prepared by uniaxial pressing and sintering of MoO₃ powder at 650 °C and their structure, microstructure, densification and sintering and microwave dielectric properties were investigated. The sintering temperature of α-MoO₃ was optimized based on the best densification and microwave dielectric properties. After sintering at 650 °C the relative permittivity was found to be 6.6 and the quality factor was 41,000 GHz at 11.3 GHz. The full-width half-maximum of the A_1g Raman mode of bulk α-MoO₃ at different sintering temperatures correlated well with the Qf values. Moreover, the sintered samples showed a temperature coefficient of the resonant frequency of ?25 ppm/°C in the temperature range from ?40 to 85 °C and they exhibited a very low coefficient of thermal expansion of ±4 ppm/°C. These microwave dielectric properties of α-MoO₃ will be of great benefit in future MoO₃ based materials and their applications.     

Characterisation of transparent hydroxyapatite nanoceramics prepared by spark plasma sintering

Abstract

Hydroxyapatites (HA) have good biocompatibility and are used as bioceramics for artificial bones. The application areas can be extended further if transparent and dense HA ceramics can be prepared. The preparation of dense and transparent HA ceramics were attempted using a spark plasma sintering technique at relatively low temperatures (900–1000°C) under a pressure of 80 MPa for a short time of 10 min. The sintered body was almost fully dense (>99%) and transparent with a transmittance >70%. The microstructure was examined by SEM, TEM, STEM and EDX. The HA ceramics exhibited a microstructure with grains, approximately 100 nm size. A number of intragranular voids, 5–10 nm in size, with flat boundaries were also observed. The voids were believed to have been generated by evaporation during spark plasma sintering and were stabilised during cooling. The grain boundaries were clean without a glassy phase.     

Effect of the graphitization level of the free carbon on the temperature sensitivity of silicon carbonitride-based pressure sensors

Polymer-derived ceramics (PDCs) have recently attracted an increasing attention because of their applications for wireless passive pressure sensors in the harsh environment. However, due to the effect of temperature on the frequency of PDC-based wireless passive pressure sensors, it is not beneficial to accurate measurement of pressure. In this paper, a dense polymer-derived silicon carbonitride (SiCN) ceramic was prepared by precursor infiltration and pyrolysis (PIP) technique to reduce the temperature sensitivity of PDC–SiCN-based pressure sensor. The open porosity and density of SiCN ceramics varied from 13.34% and 1.89 g/cm³ without PIP process to 3.24% and 2.09 g/cm³ after three PIP cycles, respectively. Raman spectroscopy revealed that the level of graphitization of free carbon in dense SiCN ceramics is higher than that in porous SiCN ceramics, which would lead to an increase in the conductivity of dense SiCN ceramics. After three PIP cycles, the conductivity increased by almost two orders of magnitude from 3.01E − 10 to 1.28E − 08 S/cm. In addition, SiCN ceramic discs after PIP cycles and without PIP were applied to wireless passive pressure sensor based on resonator, which were tested at high temperature, respectively. Results confirmed that the temperature sensitivity of PDC–SiCN-based pressure sensor decreased from 220.5 to 50.8 kHz/°C by PIP process.     

Synthesis and Characterization of a Novel Preceramic Polymer for SiBNC-Ti Ceramics

A ceramic precursor (PBSZ-Ti) with different Ti content for preparing SiBNC-Ti ceramics was synthesized by the polymer-derived method, with dicyclopentadienyltitanium dichloride, trichlorosilane, hexamethyldisilazane and boron trichloride as starting materials. FT-IR, NMR, EA and XPS were employed to characterize the compositions and structures of PBSZ-Ti and SiBNC-Ti ceramics, which indicated that PBSZ-Ti contained Si–N–B and B–N six-membered ring structures, and Ti element was successfully introduced into the polymeric structure. The micro-morphology and high temperature crystallization behavior of SiBNC-Ti ceramics were studied by XRD, Raman and SEM. The results show that the surface of the SiBNC-Ti ceramic is smooth and dense. The amorphous state of SiBNC-Ti ceramics could be maintained to 1500 °C in a N₂ atmosphere and 1300 °C in air.

     

Microstructural and optical properties of the ZnS ceramics sintered by vacuum hot-pressing using hydrothermally synthesized ZnS powders

ZnS nanopowders annealed at low temperatures (≤550?°C) have a pure cubic structure, while a small amount of hexagonal phase formed in specimens annealed at temperatures ≥700?°C. The particle sizes of the ZnS nanopowders increased with the annealing temperature. ZnS ceramics that were sintered using ZnS nanopowders annealed at low temperatures (≤550?°C) exhibited low transmittance, because of their porous microstructure. ZnS ceramics that were synthesized using ZnS powders annealed at high temperatures (≥800?°C) containing large agglomerated particles, also exhibited low transmittance, due to the presence of a liquid phase. A carbonate absorption band was found from the ZnS ceramics with small grains, because carbon ions diffused from the graphite mold into the ZnS ceramics during sintering, probably through the grain boundaries, and formed carbonates. A ZnS ceramic that was sintered at 1020?°C using the nanopowders annealed at 750?°C exhibited dense microstructure, with a large transmittance, 68%, in the wavelength range 6.0–12?μm.     

Na0.5Bi2.5Nb2O9 lead-free piezoelectric ceramics with high Curie temperature derived from sol–gel route

An Aurivillius phase bismuth layer-structured compound, Na_0.5Bi_2.5Nb₂O₉ (NBN), has been prepared by an economical aqueous sol–gel technique. The results indicated that pure NBN nano-particles can be obtained at a low temperature of 650°C and the morphology are platelike with grain sizes of about 40?nm. Using these nano-particles, the dense NBN ceramics were fabricated at 1100°C for 2?h. The Curie temperature T_c and piezoelectric coefficient d₃₃ at room temperature for the present ceramic were found to be 770°C and 11?pC/N, respectively. Thermal annealing studies indicate that the NBN ceramic possesses stable piezoelectric properties, demonstrating it is a promising candidate for high-temperature applications.     

SiAlCN型PDC陶瓷的研究进展

综述了SiAlCN型PDC（Polymer Derived Ceramics）陶瓷的制备、性能和应用。SiAlCN陶瓷有四类制备方法：粉末混合型：聚硅氮烷陶瓷前驱体与氧化铝粉末直接混合;粉末溶解型：含铝化合物粉末溶解于聚硅氮烷前驱体溶液中;单源前驱体型：铝原子通过适当的含铝化合物接枝在聚硅氮烷主链上,生成一种单源陶瓷前驱体聚铝硅氮烷;聚合物混合型：两种聚合物即聚硅氮烷与含铝聚合物共混;然后交联裂解制备陶瓷。与无Al的Si/C和Si/C/N体系相比,SiAlCN陶瓷具有优异的抗蠕变性、更好的抗氧化性和耐腐蚀性以及更好的导热性。因此,聚合物衍生的硅铝碳氮化物(SiAlCN)陶瓷是在高温和恶劣环境中应用很有潜力的材料。     

Refractory glass–ceramics based on alkaline earth aluminosilicates

Glass–ceramics that can be used at temperatures of 1200–1500 °C are found in the alkaline earth aluminosilicate field, and are generally nucleated internally with titania. These glass–ceramics have good strength (>100 MPa, abraded), can be tailored to produce high fracture toughness (2–5 MPa m^1/2), and have good dielectric properties. Coefficients of thermal expansion (CTEs) are low to moderate ((25–45) × 10^?7 °C^?1, from 25 to 1000 °C).The major crystalline phase in the glass–ceramics exhibiting the lowest CTEs is hexagonal cordierite (indialite), while important toughening accessory phases are enstatite and acicular magnesium dititanate.The most refractory glass–ceramics that are easily melted at 1650 °C, yet when crystallized do not deform at 1450 °C, are based on strontium and barium monoclinic feldspars of the celsian type. CTEs range from 35 to 45 × 10^?7 °C^?1. Acicular mullite is an important accessory phase aiding fracture toughness in these materials.Mullite glass–ceramics which contain considerable siliceous residual glass are probably the most refractory of these glass–ceramics, but they require melting above 1700 °C. Nevertheless, they can be used at temperatures near 1600 °C.Potential applications for refractory glass–ceramics include improved radomes, engine components, substrates for semiconductors and precision metallurgical molds.     

Pressureless sintering of PMN–PT ceramics

PMN–PT ceramics with PMN to PT ratios of 60:40, 65:35, and 70:30 were prepared from PMN–PT powders synthesized by the columbite precursor method, and their sintering and grain growth characteristics at temperatures less than 1000°C were investigated. Results indicate that the PMN–PT ceramics can be pressureless-sintered to a relative density of approximately 96% at 950°C. However, full densification was prevented by the onset of abnormal grain growth. The addition of 0·5 wt% PbO to 65:35 PMN–PT ceramics lowered their sintering temperature to 900°C, but caused abnormal grain growth at lower temperatures. Preliminary TEM analyses indicate the presence of submicron-sized MgO particles at some ceramic microstructure triple points. Further studies will be required to understand abnormal grain growth behavior and to devise means for full densification.     

Dielectric and magnetic properties of BiFeO3 ceramics prepared by hydrothermal synthesis

Single-phase BiFeO₃ powders were prepared at a temperature of 200 °C by a hydrothermal synthesis. BiFeO₃ ceramics were prepared with the powders by a conventional ceramic process. The BiFeO₃ ceramics with no impurity phase were prepared at the sintering temperature of 650–800 °C. The dense microstructure was observed in the BiFeO₃ ceramics sintered at a temperature of 700 °C and higher. BiFeO₃ ceramics show linear M–H curves in low H, which are antiferromagnetic behaviors. The dielectric dispersion was observed at the frequency range of 10 kHz to 1 MHz in the BiFeO₃ ceramic sintered at 700 °C or lower. The dielectric constant and loss of the BiFeO₃ ceramics sintered at 750 °C or higher were about 85 and 0.4 at 100 kHz, respectively.