Joining of reaction-bonded silicon carbide using a preceramic polymer

Ceramic joints between reaction-bonded silicon carbide (RBSiC) were produced using a preceramic polymer (GE SR350 silicone resin) as joining material; samples were heat treated in an argon flux at temperatures ranging from 800–1200°C without applying any pressure. The strength of the joints was determined by four-point bending, shear and indentation tests. Microstructural and microchemical analyses were performed by optical microscopy, SEM, TEM and AEM. The room-temperature strength of the joints increased with the joining temperature. Maximum values as high as 220 MPa in bending and 39 MPa in shear tests were reached for samples joined at 1200°C. No detectable residual stresses were observed both in the joining material and the joined parts, and the fracture mechanism was nearly always cohesive. The joint thickness was shown to depend on the processing temperature, and ranged from about 2–7 m. The joining material was a silicon oxycarbide amorphous ceramic, with no oxygen diffusion occurring between this and the RBSiC joined parts. The lack of compositional gradients, precipitates or reaction layers indicate that the SiOC ceramic acted as an inorganic adhesive, and that the joining mechanism involved the direct formation of chemical bonds between the RBSiC parts and the joining material. © 1998 Chapman & Hall     

Silicon dioxide thin film derived from polyphenylcarbosilane under an oxidizing atmosphere

A polyphenylcarbosilane (PPCS) precursor was heated under an oxidizing atmosphere to form a silicon dioxide thin film, although the PPCS precursor coated on silicon carbide fiber is usually heated under a reducing atmosphere for high strength ceramics. The PPCS phase decomposed and then recombined with numerous oxygen and phenyl groups. Finally, it was converted from hybrid to inorganic material in a temperature range of 400 °C to 550 °C under an oxidizing atmosphere. Based on our results, the PPCS precursor can be changed into silicon dioxide via heat treatment under an oxidizing atmosphere, and can serve as an alternative to SiO₂ for high performance electric devices.     

New heat treatment methods for glass removal from silicon nitride and sialon ceramics

Sialon ceramics were discovered simultaneously (but independently) in late 1971 at Newcastle University and also at the Toyota Research Laboratories in Japan. During the 30 years since their original discovery, the Newcastle laboratory has made a significant contribution to current understanding of the science and technology of these materials. Sialons are of interest as engineering materials for high temperature (>1000°C) applications because they can be pressureless-sintered to high density and be designed to retain good mechanical properties even up to 1350°C, whereas competing metallic materials are weaker and prone to corrosion. A characteristic disadvantage of all nitrogen ceramics is that an oxide additive is always included in the starting mix to promote densification, and this remains in the final product as a glassy phase distributed throughout the grain boundaries of the final microstructure. Since the glass melts at 1000°C, the high temperature properties of the final ceramic are in fact determined by the properties of the grain-boundary glass. The most common method of improving high-temperature performance is to heat-treat the material at temperatures of 1100–1350°C in order to devitrify the glass into a mixture of crystalline phases. More specifically it is desirable to convert the glass into a sialon phase plus only one other crystalline phase, the latter having a high melting point and also displaying a high eutectic temperature (max 1400°C) in contact with the matrix sialon phase. Previous studies have shown that there are a limited number of possible metal-silicon-aluminium-oxygen-nitrogen compounds which satisfy these requirements. The present paper gives an overall review of this subject area and then summarises recent work at Newcastle aimed at total removal of residual grain boundary glass. This has been achieved by: (1) a post-preparative vacuum heat treatment process to remove the grain boundary glass from silicon nitride based ceramics in gaseous form, (2) above-eutectic heat-treatment (AET) of sialon-based ceramics to crystallize grain-boundary liquid into five-component crystalline sialon phases.     

Phase separation and toughening of SiC-AIN solid-solution ceramics

SiC-AIN solid-solution ceramics were prepared by pressureless sintering using Al₂O₃ and Y₂O₃ as the sintering additives. The resulting ceramics were subjected to annealing treatments over a range of temperatures from 1400 °C to 1800 °C in the spinodal region. The fracture toughness of the annealed ceramics was examined, by the indentation method, in relation to the annealing temperature and annealing time. X-ray diffraction profiles revealed that phase separation occurred during annealing. In ceramics containing 50 mol % SiC annealed at 1800 °C, the morphology of the phase separation is the characteristic modulated stratiform structure. Energy-dispersive X-ray spectroscopy (EDS) showed that the structure consisted of alternations of silicon-rich and aluminium-rich composition. The fracture toughness of the annealed ceramics increased compared to the as-sintered solid-solution ceramics. The phase separation is expected to contribute to the toughening of ceramics with nanometre-scale texture.     

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca₁₀(PO₄)₆(OH)₂ or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m²/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 m were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.     

Effect of active metal coatings on the mechanical properties of silicon nitride-based ceramics

The effects of titanium, zirconium, hafnium and tantalum coatings on the mechanical properties of three silicon nitride ceramics were studied. The titanium coatings was found to cause a 50% decrease in the four-point bend strength of one of the silicon nitride ceramics while the effects of the zirconium, hafnium and tantalum coatings on all three silicon nitride ceramics were moderate. The reactions at a high temperature (940–980°C) between titanium and the grain-boundary glassy phase was the major cause for the degradation of the ceramic properties by the titanium coating. Residual tensile stress developed at the reaction interface replaced the glassy grain-boundary phase. Analytical electron microscopy showed the formation of a 180 nm thick Ti₅Si₃ layer and the crystallization of the amorphous grain-boundary phase. An indentation technique was used to measure qualitatively the residual stress developed at the reaction interface.     

Synthesis and dielectric characterization of Ba0.6Sr0.4TiO3 ferroelectric ceramics

Ferroelectric ceramics Ba_0.6Sr_0.4TiO₃ (BST 40) were prepared, by solid-state reaction in the temperature range 1210-1450 °C. Maximum values of the ceramic densities were around 94% of their theoretical value. X-ray diffraction techniques (XRD) and scanning electron spectroscopy (SEM) were used to analyze the structure and the surface morphology of ceramics. Rounded, well defined or abnormal granular growth was observed in the SEM images, vs. sintering conditions and purity of the raw materials. In all samples, BST 40 ceramic is the major phase, but there are also present small amounts of secondary phases, as revealed in XRD diffraction patterns. Permittivity and dielectric loss measurements were performed in the temperature range − 150 to + 150 °C, and 150 Hz-5 MHz frequency values. Permittivity values rising from 1200 to 12,500, with increasing sintering temperatures, were recorded. Narrow and well defined transition peaks were noticed at higher sintering temperatures. Curie temperature was around 2 °C, for samples with the mentioned composition. Permittivity and losses vs. frequency show different behavior whether BST ceramics are in polar or non-polar state and with the distance toward phase transition. Microwave measurements performed at room temperature have shown lower values of permittivity, compared with similar data at low frequency, and dielectric losses lower than 1% at 0.7 GHz. The sintering conditions (temperatures, sintering time, etc.) and purity of the raw materials lead to important changes of transition temperatures in the polymorphic diagram, which we have built—for the other Ba1−xSrxTiO3 compositions (x = 0.25-0.90) sintered at 1260 °C for 2 h.     

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB2 ceramics at elevated temperatures

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB₂ ceramics with various TiN/TiB₂ molar ratios of 2/1, 1/1 and 1/2 was evaluated in the temperature range of 500–800 °C in air. TiN–TiB₂ ceramics have a relative density of 97–98.6%. The oxidation weight gains of TiN–TiB₂ ceramics depend upon the composition, oxidation temperature and exposure time. The structure and morphology of oxidized layers of TiN–TiB₂ ceramics were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). During isothermal oxidation of TiN–TiB₂ ceramics, anatase and rutile-TiO₂ form as the oxidized products at 500 °C. However, phase transformation from anatase to rutile occurs at temperatures between 500 and 600 °C, and therefore rutile-TiO₂ becomes the only crystalline phase after oxidation at temperatures of 600–800 °C for 10 h. The oxidation mechanism was proposed with reference to thermodynamically feasible oxidation reactions. The influence of composition on oxidation behavior of TiN–TiB₂ ceramics varies with temperature.     

Hafnia and hafnia-toughened ceramics

Hafnia (HfO₂) and hafnium-based materials are traditionally regarded as technologically important materials in the nuclear industry, a consequence of their exceptionally high neutron absorption coefficient. Following the discovery of transformation toughening in the mid 1970s, a considerable research effort has been devoted to zirconia (ZrO₂)-toughened ceramics (ZTCs). They are considered to be potentially useful materials for structural applications at low and intermediate temperatures (T<1000 °C). Their unsuitability for high-temperature structural applications (T>1000 °C) is related to the low temperature of the tetragonal to monoclinic transformation in ZrO₂. On the basis that HfO₂ exhibits a similar crystal structure and in particular that its tetragonal to monoclinic transformation temperature (1700 °C) is approximately 700 °C higher than that for ZrO₂, it has been suggested that high-temperature transformation toughening could be possible in HfO₂-toughened ceramics (HTCs). Although the concepts behind this suggestion are universally appreciated, only a limited success has been made of the fabrication and the microstructural and mechanical property evaluation of these materials. The fracture toughness values obtained so far in HfO₂ toughened ceramics are, in fact, considerably lower than those obtained in their ZrO₂ counterparts. A great deal of further research work is therefore required in order to understand fully and to exploit toughened ceramics in the HfO₂-based and HfO₂-containing systems. This review covers the science and technology of HfO₂ and HfO₂-toughened ceramics in terms of processing, phase transformation, microstructure, and mechanical properties.     

Effect of gadolinia on phase structure and thermal conductivity of ZrO2-4.5 mol%Y2O3 ceramics

This paper deals with the effect of gadolinia on the phase structure and thermal conductivity of ZrO₂-4.5 mol%Y₂O₃ (YSZ) ceramics for thermal barrier coatings. The YSZ-Gd₂O₃ ceramics were synthesized by solid state reaction at 1500 °C for 2 h in air. The relative density, structure of different YSZ-Gd₂O₃ ceramics and thermal diffusivity in a temperature range of room temperature to 1200 °C were investigated by the Archimedes method, X-ray diffraction and laser-flash method. The ZrO₂-4.5 mol%Y₂O₃ (YSZ) ceramics consist of tetragonal, cubic and a small amount of monoclinic phase, and the YSZ-1.5 mol%Gd₂O₃ ceramics consist of both tetragonal and cubic phases. However, the YSZ-3.0 mol%Gd₂O₃, YSZ-4.5 mol%Gd₂O₃ and YSZ-6.0 mol%Gd₂O₃ ceramics only exhibit a cubic structure. The thermal conductivity of YSZ-Gd₂O₃ ceramics decreases with the increase of gadolinia content under identical temperature conditions. The thermal conductivities of the YSZ and YSZ-1.5 mol%Gd₂O₃ ceramics first decrease gradually with the increase of temperature below 800 °C and then increase slightly above 800 °C. The thermal conductivities of the YSZ-3.0 mol%Gd₂O₃, YSZ-4.5 mol%Gd₂O₃ and YSZ-6.0 mol%Gd₂O₃ ceramics are almost constants from room temperature to 1200 °C.     

Formation of intergranular amorphous films during the microstructural development of liquid phase sintered silicon carbide ceramics

The microstructure of liquid phase sintered SiC ceramics was characterised by means of high resolution transmission electron microscopy (HRTEM). The SiC ceramics were pressureless sintered with the additions of Al₂O₃ and Y₂O₃ at sintering temperatures of 1800 and 1950°C, respectively. At a sintering temperature of 1800°C the microstructure of the SiC ceramics has no crystallised secondary phase and the SiC grains are separated by an intergranular amorphous film. In contrast, in the case of the microstructure of SiC ceramics sintered at 1950°C a clean interface without any amorphous layer between the SiC grains was observed. The secondary phase is crystallised into the Y₃Al₅O₁₂ phase and exhibits a clean interface between the SiC grains. An explanation for the existence or the absence of the intergranular glass films are given by an extended Clarke's model of the force balance of attractive van der Waals forces and repulsive steric forces. The chemical decomposition of the intergranular glass film at elevated temperature was considered.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Positive temperature coefficient of resistance effect in hot-pressed cristobalite-silicon carbide composites

Conductive silicon carbide particles were incorporated into an insulating cristobalite ceramic matrix to produce composite materials with a sizeable positive temperature coefficient (PTC) effect. A large drop in resistivity with silicon carbide content at room temperature, or percolation behaviour, was observed. The PTC effect of the composites, which resulted from the thermal expansion of the cristobalite ceramic matrix, was a maximum with five orders of magnitude for the specimen with 25 vol% silicon carbide. The PTC transition temperature of the composites was at 260 °C, which coincided exactly the reversible high-low phase inversion temperature of the cristobalite ceramic matrix.     

Mechanical behaviour of MgO-partially stabilized ZrO2 ceramics at elevated temperatures

Transformation toughened partially stabilized zirconia ceramics containing magnesia exhibit quite high fracture toughness (K _lc 8 MPa m^1/2) at temperatures of up to 500° C. The observed temperature dependences of the toughness and the fracture strength are consistent with that of the transformation behaviour. The high toughness of these materials results in a significant reduction in the sensitivity of the flexure strength to crack size increases. Exposure of these materials at 1000° C for prolonged periods results in flexure strength changes associated with the generation of the monoclinic phase by tetragonal precipitate destabilization and eutectoid decomposition. However, when exposed at 500° C, neither the phase contents nor the flexure strength are altered for exposures of up to 1000 h.     

Refinement of Cast Microstructure of Hypereutectic Al-Si Alloys Through the Addition of Rare Earth Metals

Microstructural observation and thermal analysis of Al-21 wt % Si alloys with different rare earth metals were performed to examine the effect of rare earth metal on the refinement of primary silicon phase. Simultaneous refinement of both primary and eutectic silicon morphology is achieved with the addition of rare earth and its effect increases with the amount of rare earth addition and cooling rate. Depression of 12–17 °C in primary reaction temperature and 2–7 °C in eutectic temperature is measured with the addition of rare earth. Rare earth bearing compounds were not believed to act as a nucleation agent of primary silicon phase. Some rare earth bearing compounds determined to AlCe were around primary silicon in the matrix. The twin density of eutectic silicon remains same regardless of the addition of rare earth. The refinement of silicon in rare earth treated hypereutectic Al-Si alloys is supposed to be due to the suppression of the nucleation temperature of silicon phase.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Microstructure and Superconducting Properties of TaC-Doped Bi-2223 Ceramics

The microstructure, phase composition, and superconducting properties of Bi-2223 ceramics doped with 0.05–0.5 wt % TaC were investigated. The materials were heat-treated at 840°C in three steps (10 + 24 + 24 h). By optimizing the TaC content and heat-treatment conditions, the 77-K critical current density of the Bi-2223 ceramics in zero field was raised by a factor of 2.     

Nanocrystalline SiC films prepared by direct deposition of carbon and silicon ions

The method of direct deposition of carbon and silicon ions was used for preparation of nanocrystalline silicon carbide films. The deposition energy of carbon and silicon ions was 90 eV. The effect of substrate temperature in the range of 500-1150 °C on the structure of SiC films was studied by means of X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). According to XPS data, the films contained heterobonded Si-C atoms and homobonded Si-Si and C-C atoms, the relation between which varied as the function of substrate temperature. The data of XRD showed a noticeable growth of a nanocrystalline phase of cubic silicon carbide in the films at a temperature of about 700 °C. The content of 3C-SiC nanocrystalline phase reached 80 at.% at 950 °C. There was an established change from cubic polytype to rhombohedral polytype of silicon carbide α-SiC-21R at a substrate temperature higher than 1000 °C. The size of SiC crystal grains depended on the substrate temperature and changed from 4-5 up to 8-10 nm over the range of 700-950 °C. Besides, silicon unbonded with carbon also crystallized in nanocrystalline form with similar sizes of crystal grains. A possible model of the change of the polytypic composition of SiC film under the conditions of direct ion deposition was discussed.