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This paper presents the experimental investigation of the effect of Nb2O5 content on the phase composition, microstructure, and physical and mechanical performance of the zirconia-toughened alumina (ZTA) ceramics, aimed to optimize the additional amount of Nb2O5, refine the ceramic grains, and improve the densification, hardness and fracture toughness. ZTA ceramics with various Nb2O5 contents were prepared using the spark plasma sintering method. The addition of trace amounts of Nb2O5 did not produce a new Nb-rich phase; the Nb atoms were dissolved in the ZrO2 grains. The addition of trace amounts of Nb2O5 led to grain refinement and an increase in density, but the addition of excessive Nb2O5 resulted in grain growth and a decrease in relative density. Furthermore, the addition of Nb2O5 promoted uniformity of the microstructure. The sample with 0.25 wt.% Nb2O5 had the maximum Vickers hardness and indentation fracture toughness, which were 1228.40 HV and 9.17 MPa √m, respectively. Mechanical properties of the composite mainly depended on the grain size, which was influenced to some extent by the relative density. [ABSTRACT FROM AUTHOR]

The paper reviews the wear behaviour of oxide ceramics. Wear maps are considered and consequently mild and severe wear are defined. Since the use of ceramics in engineering applications require operation in the mild wear regime, the paper concentrates on mild wear mechanisms, but also considers factors which control the transition to severe wear. Within the mild wear regime, the formation of tribofilms are discussed and the manner in which dislocation activity leads to the wear transition is considered. The wear of so-called ceramic nanocomposites, for which no time dependent wear transition has yet been observed, is considered and the reasons for enhanced performance discussed. The role of transformation toughening in zirconia ceramics is considered in detail, and reasons for the generally poor wear response of these materials defined. [ABSTRACT FROM AUTHOR]

Designing tissue engineering scaffolds with the required mechanical properties and favourable microstructure to promote cell attachment, growth and new tissue formation is one of the key challenges facing the tissue engineering field. An important class of scaffolds for bone tissue engineering is based on bioceramics and bioactive glasses, including: hydroxyapatite, bioactive glass (e.g. Bioglass®), alumina, TiO2 and calcium phosphates. The primary disadvantage of these materials is their low resistance to fracture under loads and their high brittleness. These drawbacks are exacerbated by the fact that optimal scaffolds must be highly porous (>90% porosity). Several approaches are being explored to enhance the structural integrity, fracture strength and toughness of bioceramic scaffolds. This paper reviews recent proposed approaches based on developing bioactive composites by introducing polymer coatings or by forming interpenetrating polymer-bioceramic microstructures which mimic the composite structure of bone. Several systems are analysed and scaffold fabrication processes, microstructure development and mechanical properties are discussed. The analysis of the literature suggests that the scaffolds reviewed here might represent the optimal solution and be the scaffolds of choice for bone regeneration strategies. [ABSTRACT FROM AUTHOR]

A study of the structural characteristics of the composites [Pb(Fe0.5Nb0.5)O3(PFN)] x -[Cr0.75Fe1.25O3(CRFO)]100− x ( x = 0 (CRFO100), 10, 50, 90, 100) was performed in this work. The compounds PFN100 and CRFO100 were prepared by conventional solid-state method and investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and 57Fe Mössbauer Spectroscopy techniques. The X-ray analysis shows that PFN100 is tetragonal and the CRFO100 phase has a trigonal symmetry. The refinement of all the composites was also performed and discussed in this paper. The Mössbauer spectrum for the composite samples shows a paramagnetic doublet and a sextet probably assigned to a magnetic phase associated to Fe+3. For the sample PFN100, only a magnetic field of 49.5 T (isomer shift (δ) = 0.21 mm/s) was detected. For the composite sample, the δ and Δ are typical of Fe ions at sites of octahedral coordination. [ABSTRACT FROM AUTHOR]

Gd3+ was chosen as a substitute for Bi3+ in BiNbO4 ceramics, and the substitution effects on the sintering performance and microwave dielectric properties were studied in this paper. The high temperature triclinic phase was observed only in the Bi0.98Gd0.02NbO4 ceramics when sintered at 920 °C. Both bulk densities and dielectric constant (εr) increased with the sintering temperature, while decreased with the Gd content. The quality factor ( Q) exhibited a correlation to the Gd content and the microstructures of Bi1− x Gd x NbO4 ceramics. At the sintering temperature of 900 °C, Bi0.992Gd0.008NbO4 ceramics exhibited microwave dielectric properties of εr ∼ 43.87, Q × f ∼ 16,852 GHz (at 4.3 GHz), and its temperature coefficient of resonant frequency (τf) was found to be near-to-zero. [ABSTRACT FROM AUTHOR]

This paper deals with the effects of preparation methods on the texture development and densification in textured SrBi4Ti4O15 (SBT) ceramics. The specimens were prepared by the oriented consolidation of anisotropic particles (OCAP), templated grain growth (TGG) and reactive templated grain growth (RTGG) methods. The specimens with extensive texture were obtained by all methods except for TGG with small template and large matrix particles. The origin of extensive texture development was the growth of aligned platelike particles. A characteristic of microstructure was the formation of colonies, in which platelike grains contacted face-to-face. The size of colonies was dependent on the preparation method, and determined the size of pores between colonies, which had a large effect on the sintered density. The OCAP and RTGG methods gave the green compacts composed of only platelike particles and large colonies and pores formed during sintering. The TGG method gave the green compacts composed of platelike and equiaxed particles and prevented the formation of large colonies and pores during sintering. Thus, the dense, highly-textured SBT ceramics were obtained by the TGG method. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Zirconia toughened alumina (ZTA) ceramics are commonly used in various industries. Adding suitable additives is an effective way to improve the performance of ZTA ceramics. However, the common reinforcement in ZTA, chromia (Cr2O3), tends to volatilize at high temperature in the presence of oxygen, limiting ceramic densification. Consequently, additional amounts of Cr2O3 are typically limited to below 1 wt%. In this study, spark plasma sintering was utilized to inhibit the volatilization of Cr2O3. Dense ZTA ceramics with high Cr2O3 content (from 1 to 10 wt%) were prepared at 1350 °C. The impact of the solid solution on the structure and properties of the ceramics was investigated. The findings demonstrate that the high Cr2O3 solution can regulate the grain size in the matrix, leading to an enhancement in both hardness and fracture toughness. The optimum performance of the sample was obtained with a 5 wt% dosage of Cr2O3, where the maximum values of Vickers hardness and fracture toughness were 20.66 GPa and 6.37 MPa m1/2, respectively. [ABSTRACT FROM AUTHOR]

Gasping the controlling mechanism of multi-core-rim structure, like the ratio of multi-cores and the rim thickness, is a key to develop Ti(C, N)-based cermets with high performances. In this work, three types of cermets with the same nominal composition were fabricated by doping self-made (Ti, W, Ta) (C, N) pre-solid solution powders with different compositions, (Ti, 61.3 W, 8.6Ta) (C, N), (Ti, 53.3 W, 7.5Ta) (C, N) and (Ti, 47.1 W, 6.6Ta) (C, N). Effect of the composition of (Ti, W, Ta) (C, N) pre-solid solutions on the multi-core-rim microstructure and mechanical properties of cermets was investigated. Results showed that the decrease of W and Ta contents in the pre-solid solutions leads to an increase of white cores with thinner rims and a decrease of black cores with thicker rims. Thermodynamic calculations indicated that the decrease of W and Ta contents strengthens the thermal stability of the pre-solid solutions. With the decrease of the thermal stability difference between black core Ti(C, N) and white core (Ti, W, Ta) (C, N), more rimless black particles are reserved via Ostwald ripening process. The rim thickness and core size were statistically analyzed, which indicates that the cermet doping medium-solid-solution (Ti, 53.3 W, 7.5Ta) (C, N) exhibits smallest ceramic grains with thinnest rims. An optimal comprehensive mechanical performance was obtained in the cermet doping (Ti, 53.3 W, 7.5Ta) (C, N). It implies that an appropriate pre-solid solution of W and Ta in (Ti, W, Ta) (C, N) promises fine multi-core-rim structure and thus sound mechanical properties of Ti(C, N)-based cermets can be obtained under the same condition of alloy composition and processing parameters. [ABSTRACT FROM AUTHOR]

The stability of closed pores in two and three dimensions has been discussed and it is found that the stability of pores in two dimension can be determined mathematically from their particle coordination number and dihedral angle; while those in three dimension can be approximately determined by a spherical pore model. This model is set up by first excluding the effect of interface tension, so the pore was supposed to be spherical, and then the tensile stress arising from the interface tension was allowed to act on this hypothesized spherical pore. On the basis of the spherical pore model, pore microstructure models for real powder compacts were set up and the densification equations for the intermediate and final stages of sintering were derived. The criterion for pore shrinkage, and the effect of pore size distribution and green density were discussed according to the derived equations. The densification equations for pressureless solid state sintering can be easily extended to describe the densification behaviour during hot-pressing or hot-isostatic-pressing. Densification characteristics in liquid state sintering were also considered from the result of solid state sintering. [ABSTRACT FROM AUTHOR]

Acicular mullite-reinforced porous self-bonded ceramics were fabricated using mullite powders as principal raw materials, alumina and silicon dioxide powders as starting synthetic source of self-bonded mullite phase, and 2 wt% AlF3·3H2O as an accelerant. The influences of self-bonded mullite weight percent on the phase composition, microstructures, mechanical performance and thermal conductivities of acicular mullite-reinforced porous self-bonded samples were investigated. As self-bonded mullite weight percent varied between 0 and 50 wt%, linear shrinkage, bulk density and thermal conductivities of fired porous self-bonded mullite samples reduced, whereas their porosity increased. Addition of appropriate amounts of self-bonded mullite decreased the mean pore size and window size in porous sample, and enhanced the mechanical strength. When self-bonded mullite weight percent reached 50 wt%, the compressive and flexural strength of acicular mullite-reinforced porous self-bonded ceramics was, respectively, kept as 19.9 MPa and 8.8 MPa, with a porosity of 74.7%, a diminutive mean pore size of 159 μm and a window size of 44 μm. Moreover, the linear shrinkage and thermal conductivity of the materials at 50 °C were only 7.4% and 0.23 W m K−1, respectively. [ABSTRACT FROM AUTHOR]

Magnesium potassium phosphate ceramics are chemically bonded ceramics employed as biomaterials, in nuclear waste encapsulation and for concrete repair. The microstructure dictates material performance and depends on the raw mix composition. Synchrotron X-ray computed microtomography was employed to describe the 3D microstructure and its time evolution during hardening and gain insights into the reaction mechanisms. Any excess water with respect to the stoichiometry of the reaction brought about an increase in porosity, but, notably, a reduction in the average pore size. Crystals filled the water 'pockets' in the ceramic volume by growing larger, although less densely packed, increasing the complexity of the pore shape. Platelet over elongated crystal habit was favoured. Such a change in shape is likely related to a change in reaction mechanism, as crystallization from a gel-like amorphous precursor is hindered and progressively substituted by a through-solution mechanism. It is proposed that the time evolution of the microstructure is dictated by the balance between crystallization from amorphous precursor, prevailing in relatively 'dense' systems (with stoichiometric water or in low excess), and water segregation, prevailing at higher water contents. The former mechanism was shown to produce an increase in porosity with time, because of the density mismatch between the amorphous and the crystalline phase. [ABSTRACT FROM AUTHOR]

Crystallization and microstructure of glasses with the molar compositions 1MgO·1.2AlO·2.8SiO·1.2TiO· xLaO ( x = 0.1 and 0.4) were thermally treated at different temperatures in the range from 950 to 1250 °C and then analyzed by X-ray diffraction and scanning electron microscopy, in combination with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. It was found that the microstructure is first homogeneous with the precipitation of randomly distributed crystals and then indialite domains with embedded perrierite and rutile crystals are formed. For higher temperatures or prolonged times, more domains appear and expand into the bulk of the sample. Finally, the entire sample consists of the indialite domains and the boundaries that are enriched in rutile, perrierite, and magnesium aluminotitanate. Nevertheless, very distinct differences are observed between the samples with different LaO concentrations. For the sample with x = 0.4, the domains were detected at lower temperatures, while the quantity and size of the domains increase faster due to the promoted precipitation of indialite. For the sample with x = 0.1, in addition to the domain boundaries, secondary boundaries between the 'regions' (assemblages of the domains) are observed in a larger length scale. The average size of the crystalline phases found between the 'regions' is larger than that typically observed at the domain boundaries. The sizes of the crystals at the boundaries decrease with higher concentrations of LaO, and the crystals (especially perrierite) within the domains become larger, resulting in a more homogeneous microstructure. This results in better dielectric properties, i.e., much higher quality factor for the sample with x = 0.4 in comparison to that with x = 0.1 after heat-treatment at 1150 or 1250 °C. [ABSTRACT FROM AUTHOR]

Vitrified bonded SiC composite ceramics were prepared using a simple powder blending, forming, and sintering process. The effect of SiC particle size on the mechanical and dielectric properties of vitrified bonded SiC composite ceramics was investigated. Vitrified bonds wet and coat SiC particles during sintering and form continuous bonding networks upon cooling. The vitrified bonds render the ceramics high flexural strength and act as impedance mismatching modification layer between SiC particles and air. With varying SiC particle size, the microstructure, and properties of the vitrified bonded SiC composite ceramics change accordingly and at 7 μm, it reaches optimal microstructure, a highest flexural strength of 67.85 MPa and highest reflection loss. However, the dielectric loss of composite ceramics decreases as the SiC particle size increases. [ABSTRACT FROM AUTHOR]

By gel casting with polymethylmethacrylate microbeads (PMMA) as pore-forming agent, porous boron nitride/silicon nitride (BN/SiN) composite ceramics were successfully prepared. The obtained ceramic shows bimodal hierarchical structures that composed of spherical-shaped micro pores depending on PMMA content and irregular sub-micro pores formed by the stacking of ceramic particles. Porosity of the porous BN/SiN ceramics can be well controlled from 53.0 to 60.6 % by the PMMA content from 10 to 40 wt%, as well as the mechanical and dielectric properties. Effect of PMMA content on phase composition and the relationship between microstructure and the basic properties of the porous BN/SiN ceramics was discussed in detail. Microstructure analysis reveals that the sub-micro pores acted as channels between micro pores. BN particles have a relatively denser distribution on the wall of spherical-shaped micro pores with a window between micro and sub-micro pores, and resulting in a half-closed micro pore structure, which is meaningful for material design with concentration of BN particles on the wall of pore structure. [ABSTRACT FROM AUTHOR]

A method of manufacturing polymer-infiltrated ceramics (PICs) is to pre-infiltrate a porous ceramic with a liquid monomer and subsequently polymerize the organic component inside the ceramic structure. The volume reduction during polymerization leads to the formation of pores (defects), which has a detrimental effect on the mechanical properties of PICs. To avoid the generation of defects, a new polymerization method that uses pressure during polymerization was developed. To investigate the influences of pressure and heating rate on strength and microstructure, both parameters were varied. The influences of both parameters on the strength of PICs were studied using a biaxial test. The influence on the microstructure was investigated through microscopy. Fracture toughness and R-curve behavior of the manufactured PICs were determined with the SEVNB method. The process parameters have a strong influence on strength and microstructure of PICs. Defect-free PICs with improved strength could be manufactured using elevated pressure during polymerization. As expected, a distinct R-curve behavior and enhanced fracture toughness relative to composites manufactured using conventional methods was found. The developed manufacturing method leads to defectless PICs with increased mechanic behaviors. [ABSTRACT FROM AUTHOR]

The mechanically alloyed amorphous SiBCN powders were hot pressed at 1500, 1600, 1700, 1800, and 1900 °C under a pressure of 80 MPa in the nitrogen atmosphere for 30 min. The crystallization, the microstructural evolution, and the properties of the prepared ceramics were carefully studied by XRD, TEM, HRTEM, and property testing. Results show that the crystallization of β-SiC, turbostratic BN(C), and α-SiC in the amorphous matrix starts at about 1500, 1600, and 1700 °C, respectively. When the powder is hot pressed at the temperatures higher than 1700 °C, the prepared ceramics always consist of nano β-SiC, α-SiC, turbostratic BN(C), and amorphous body. With the increase of the sintering temperature, the ceramic crystallinity becomes higher, the grains get larger, and the amorphous content becomes lower. At the temperatures lower than 1800 °C, the bulk density, the relative density, the flexural strength, the Young's modulus, and the fracture toughness of the prepared ceramics show persistent but insignificant increase. However, when the ceramic is sintered at 1,900 °C, these properties are rapidly improving to 2.6 g/cm, 91.8 %, 331.0 MPa, 139.4 GPa, and 2.8 MPa m. [ABSTRACT FROM AUTHOR]

Mo particles have been introduced into Ag-Cu-Ti brazing alloy for the joining of SiN ceramic. Effect of brazing temperature on microstructure and mechanical properties of the joints were investigated. The result shows that a continuous reaction layer which is composed of TiN and TiSi was formed at the SiN/braze interface. The central part of the joint was composed of Ag-based solid solution, Cu-based solid solution, Mo particles, and Cu-Ti intermetallic compounds. By increasing the brazing temperature, both the thickness of the reaction layer and amount of Cu-Ti intermetallic compounds in the joint increased, being beneficial for the joint strength. Whereas, the reaction between Ti and SiN ceramic proceeded excessively and more Cu-Ti intermetallic compounds were precipitated in the joint while elevating the brazing temperature to 950 °C, leading to deterioration of the bending strength. The maximal bending strength reached 429.4 MPa at 900 °C for 5 min when the SiN ceramic was brazed with Ag-Cu-Ti + Mo composite filler. [ABSTRACT FROM AUTHOR]

In this study, the high temperature mechanical behavior of cordierite porous disks prepared by the starch consolidation forming method was evaluated. In this method, due to the swelling and gelatinization properties of starch in aqueous suspension at a temperature between 55 and 85 °C, the starch granules perform as both consolidator/binder of the green body and pore former at high temperature. Aqueous suspensions (29.6 vol.%) of a cordierite precursor mixture (talc, kaolin, and alumina) with the addition of potato or cassava starches (11.5 vol.%) were prepared by intensive mechanical mixing, homogenization, and vacuum degasification. Green disks were formed by thermogelling of the aqueous suspensions at 85 °C for 4 h followed by additional drying at 50 °C for 24 h. They were characterized by bulk density and apparent porosity measurements, and microstructural analysis by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Porous cordierite materials were obtained by calcination at 650 °C for 2 h and reaction-sintering at 1330 °C for 4 h, employing specific, controlled heating schedules in both treatments. Cordierite disks were characterized by bulk density and apparent porosity measurements, and microstructural analysis by SEM. Mechanical behavior was evaluated in diametral compression using a servohydraulic testing machine at room temperature ( RT), 800, 1000, and 1100 °C. Apparent stress–strain relationships were obtained from load–displacement curves, and mechanical parameters, such as fracture strength (σF), apparent Young modulus ( Ea), and yield stress (σY), were determined. Moreover, crack patterns were also evaluated. The obtained results were analyzed in function of the developed microstructures, considering the presence of a silicate glassy phase and a complex porosity, and the testing temperature. [ABSTRACT FROM AUTHOR]

Porosity and microstructure control of the strut are essential for tailoring the properties of porous SiC ceramics. This study examined four different strategies for engineering the porosity of SiC ceramics: adjusting the template content, processing parameters, filler content, and sintering additive content. The suggested strategies offer substantial flexibility for producing SiC ceramics with engineered porosity, whereby the total porosity can be controlled effectively from 35 to 95%. These results suggest that combinations of the proposed strategies will be useful for the manufacture of porous SiC ceramics with engineered porosity. [ABSTRACT FROM AUTHOR]

The effect of thermal mismatch induced residual stresses on grain boundary microcracking in titanium diboride (TiB2) ceramics has been studied by finite element method. A cohesive zone model was used to simulate the microcracking initiation in four-point bending specimens. In particular, the microcracking was assumed to occur at a grain boundary which is located in the center of the specimen, surrounded by a thermally anisotropic area. The predicted failure strength appears to be significantly reduced by the presence of residual stresses when the cohesive energy of the microstructure is small. The failure load from experiments has been used to determine the critical damage parameters for microcracking initiation in both pristine and aluminum-infiltrated TiB2. A viscous regularization technique is employed in the simulations to improve the rate of convergence of the solution and the effect of the value of the viscosity parameter on the simulation results, has been investigated. The effect of grain size, grain orientation, and number of employed thermally anisotropic grains, on the microcracking is also discussed. [ABSTRACT FROM AUTHOR]

The potential of a ceramic nanocomposite technique employing a simple bimodal particle size packing and a pressureless sintering process as a low-cost and simple ceramic processing to obtain perovskite ferroelectric ceramics in the PMN/PT system was demonstrated. Attention was focused on relationships between chemical composition, densification, microstructure, and electrical properties. It has been found that the phase formation, microstructures, and dielectric properties of ceramic nanocomposites are totally different from those of typical solid solutions. [ABSTRACT FROM AUTHOR]

In this work, the effects of Nb2O5 addition on the dielectric properties and phase formation of BaTiO3 were investigated. A core–shell structure was formed for Nb-doped BaTiO3 resulted from a low diffusivity of Nb5+ ions into BaTiO3 when grain growth was inhibited. In the case of 0.3–4.8 mol% Nb2O5 additions, two dielectric constant peaks were observed. The Curie dielectric peak was determined by the ferroelectric-paraelectric transition of grain core, whereas the secondary broad peak at lower temperature was due to strong chemical inhomogeneity in Nb-doped BaTiO3 ceramics. The dielectric constant peak at Curie temperature was markedly depressed with the addition of Nb2O5. On the other hand, the secondary dielectric constant peak was enhanced when sintered above 1280 °C for higher Nb2O5 concentrations (≥1.2 mol%). The Curie temperature was shifted to higher temperatures, whereas the transition temperature corresponding to the secondary peak moved to lower temperatures as increasing the amount of Nb2O5 more than 1.2 mol%. The decrease of this lower transition temperature was assumed to be closely related with the secondary phase formation when Nb concentration greater than 1.2 mol%. From XRD analyses, a large amount of secondary phases was observed when Nb2O5 amount exceeded 1.2 mol%. The coefficients of thermal expansion of Nb-doped BaTiO3 were increased with increasing Nb2O5 contents, resulting in large internal stress between cores and shells. Therefore, the shift of Curie temperature to higher temperatures was attributed to internal stress resulting from the formation of a core–shell structure and a large amount of secondary phase grains. [ABSTRACT FROM AUTHOR]

Incorporation of LiSbO3 into the lead-free piezoceramic composition 0.95Na0.5K0.5NbO3–0.05LiTaO3 produced a change from an orthorhombic to tetragonal crystal system in samples produced by reaction-sintering. The inferred limit of solid solution along the compositional join, (0.95 − x)Na0.5K0.5NbO3–0.05LiTaO3– xLiSbO3, occurred at x ~ 0.06. Differential scanning calorimetry indicated broad peaks at temperatures associated with ferroelectric–paraelectric transitions. The transition temperatures decreased with increasing values of x, up to x = 0.06. Microstructures showed secondary grain growth; a slight decrease in grain-size with increasing LiSbO3 modification was identified. [ABSTRACT FROM AUTHOR]

Zinc-titanate ceramics were obtained by initial mechanical activation in a high-energy planetary mill for 15 min followed by sintering at temperatures 900–1100 °C for 2 h. Room temperature far infrared reflectivity spectra for all samples were measured in the range 100–1200 cm−1. The same ionic oscillators were present in the measured spectra, but their intensities increased with the sintering temperature in correlation with the increase in sample density and microstructure changes. Optical parameters were determined for seven oscillators belonging to the spinel structure using the four-parameter model of coupled oscillators. Born effective charges were calculated from the transversal/longitudinal splitting. [ABSTRACT FROM AUTHOR]

The isothermal oxidation behavior of Cr2AlC ceramics oxidized in air at 1,100 and 1,250 °C for 20 h was studied. The phase compositions and microstructure of the oxidized surface were identified and observed by XRD and electron probe microanalysis (EMPA), respectively, while the cross sections of oxidized samples were also examined by EMPA equipped with energy dispersive spectrum capabilities. The results indicated that the oxidation of Cr2AlC samples was carried out by the outward diffusion of Al, together with small amounts of Cr, and the inward diffusion of O to form a surface layer of α-Al2O3, while carbides (Cr7C3 and Cr3C2), rather than oxides (Cr2O3), were formed in a layer under the surface. The mass gain per unit surface area of oxidized Cr2AlC followed a parabolic relation with oxidation time, and the parabolic rates, k p, for oxidation at 1,100 and 1,250 °C were 1.1 × 10−12 and 7.1 × 10−10 kg2 m4 s−1, respectively. [ABSTRACT FROM AUTHOR]

Si3N4/ Al–Mg composites reinforced by ceramic interpenetrating network structure had been fabricated via pressureless infiltration technology. The matrix and the reinforcement phase, form an interconnected interpenetrating network structure. The Al–Mg/Si3N4 system exhibits an excellent wettability under moderate conditions. The increasing of Mg content (2–10 wt%) resulted in an increased amount of infiltration, once Mg content beyond 10 wt% has an adverse effect. Light chemical reaction occurs in the interface of Al–Mg/Si3N4 system and the reaction productions reduce the surface tension of melt and impulse the advance of infiltration. Infiltration temperature and infiltration time were the key parameters, which turn into the infiltration impetus. The appropriate infiltration temperature is 1050 °C and the corresponding infiltration time is 15 min, prolonging the infiltration time continuously has no significance. [ABSTRACT FROM AUTHOR]

Effects of oxygen partial pressure ( $$ {\hbox{P}}_{{\hbox{O}}_{\hbox{2}} } $$ ) control on the electrical properties and microstructural development of (Ba1-xHox)0.997TiO3 were studied. An oxidation condition ( $$ {\hbox{P}}_{{\hbox{O}}_{\hbox{2}} } $$ ∼ 1.0 atm) was maintained during the heating process, and then the specimen was sintered in a reducing atmosphere ( $$ {\hbox{P}}_{{\hbox{O}}_{\hbox{2}} } $$ < 10−9 atm) at 1350 °C, followed by the annealing process at 1000 °C and $$ {\hbox{P}}_{{\hbox{O}}_{\hbox{2}} } $$ = 1 atm. The switching temperature (TS) from the oxidation atmosphere to the reducing condition was changed from 1100 to 1350 °C. A significant decrease in the room-temperature resisitivity (ρ25) was observed as TS was increased. The temperature coefficient of resistance (TCR) was independent of the change in TS, and closed pores decreased with increasing TS. [ABSTRACT FROM AUTHOR]

ZnO-based varistor ceramics were prepared at sintering temperatures ranging from 900 °C to 1,300 °C, by subjecting the mixed oxide powders to high-energy ball milling (HEBM) for 0, 5, 10 and 20 h, respectively. Varistor ceramics prepared by HEBM featured denser body, better electrical properties sintered at low-temperature than at traditional high-temperature. The high density is due to the refinement of the crystalline grains, the enhanced stored energy in the powders coming from lattice distortion and defects as well as the promotion of liquid-phase sintering. Good electrical properties is attributed to proper microstructure formed at low-temperature and improved grain boundary characteristics resulting from HEBM. With increasing sintering temperatures, the electrical properties and density became worse due to the decrease in amount of Bi-rich phase. Temperature increased up to 1,200 °C or above, the Bi-rich phase vanished and the ceramics exhibited very low nonlinear coefficient. [ABSTRACT FROM AUTHOR]

<001>-textured KSr2Nb5O15 (KSN) ceramics were fabricated in a matrix of SrNb2O6 and KNbO3 powders via reactive templated grain growth (TGG), using <001>-oriented acicular KSN particles as the template for grain growth. KSN phase formation started at 1100 °C and was complete at 1300 °C. Template particles grew at the expense of matrix grains with increasing sintering time and temperature, resulting in improved texture fraction, f. A maximum fraction of 0.98 was obtained in 1.5 wt% Nb2O5 and 10 wt% template containing samples sintered at 1450 °C for 6 h. A template content of 10 wt% resulted in a high quality of texture with a narrow distribution of elongated grains in [001]. The highly textured ceramics ( f ≥ 0.9) had a remanent polarization of 0.19 C/m2 in the polar c direction, [001], as compared to 0.03 C/m2 in the non-polar a or b direction and 0.04 C/m2 as for the random sample. The estimated saturation polarization ( P sat) was 0.25 C/m2 which is comparable to reported KSN single crystal value of 0.25 C/m2. This indicates that TGG is a viable method to fabricate textured ceramics having ferroelectric properties close to the single crystals. [ABSTRACT FROM AUTHOR]

The crystal structure and dielectric properties as a function of temperature for Ba-based with Bi-layered structure BaBi2Nb2O9 (BBN) ceramics were investigated. The obtained results confirmed the relaxor ferroelectric behavior of the studied ceramics, including a strong frequency dispersion of the permittivity maximum and a visible shift of its temperature with frequency. Analysis of the real and imaginary part of permittivity allowed us to determine the values of Burn’s temperature and of the freezing temperature characterizing the relaxor ferroelectrics. The physical processes, responsible for the relaxor behavior of the studied ceramics are discussed. The additional low frequency dielectric dispersion at high temperatures in the paraelectric phase range was also observed. Correlation between this dispersion and the thermally stimulated depolarization current was ascertained. [ABSTRACT FROM AUTHOR]

In facing the design of new processing strategies for ZnO based ceramic varistors, a precise control of its microstructural development during sintering is demanded. Addition of dopants to zinc oxide results in the formation of secondary phases that to a large extent determine the macroscopic electrical properties of the ceramic. In a varistor system based on ZnO with small additions of Bi2O3 and Sb2O3 these three oxides govern the reactions at high temperature that give place to the secondary phases. These reactions become then the head point from which the functional microstructure is configured. In this way the present work deals with the thermal evolution of the ZnO-Bi2O3-Sb2O3 system in the region of interest for varistors, revealing the existence of two simultaneous reactions paths during sintering these ceramics. [ABSTRACT FROM AUTHOR]

The gas permeability and mechanical properties of mullite-alumina ceramics for potential use as filters in hot gas separation environments are examined. The mullite-alumina ceramics with different levels of induced porosity and pores sizes were fabricated by slip casting and characterised in terms of microstructure and strength properties at ambient and elevated temperatures. Permeability to nitrogen gas flow of the porous structures at ambient temperature was investigated over a range of flow velocities to quantify and assess the permeability. The strength at high temperatures is equivalent to ambient data signifying no discernible degradation. Nitrogen gas permeability tests reveal dramatic reductions in the pressure drop–gas velocity curves with increasing porosity. It is shown that the gas permeability increases with the level of porosity and pore size, with maximum Darcian permeability constant of k = 2.5×10−14 m2 for a porosity of 71%. [ABSTRACT FROM AUTHOR]

3Y-TZP/Al2O3 composites were pressureless sintered with the addition of TiO2-MnO2 and CaO-Al2O3-SiO2 glass. The densification, microstructure and mechanical properties of the composites were investigated. It was found that the composites could be densified at a temperature as low as 1400^°C by liquid phase sintering. The majority of the grain sizes for both Al2O3 and ZrO2 were below 1 µ m because of the lower sintering temperature. A bending strength of 934 ± 28 MPa and fracture toughness of 7.82 ± 0.19 MPam1/2 were obtained for 3Y-TZP/Al2O3 (20 vol%) composite. Transformation toughening is considered the responsible toughening mechanism. [ABSTRACT FROM AUTHOR]

The use of normalized porosity in models for the porosity dependence of mechanical properties is addressed first for the frequently used power law expression for such dependence, i.e.,E/E0 = (1-P)n whereEis the property of interest at any volume fraction porosity (P) andE0 is the value ofEatP= 0. NormalizingPbyPC, the value ofPat which the property of interest inherently goes to zero, givingE/E0 = (1-P/PC)n, clearly calls attention to the importance ofPC values<1 (e.g., potentially as low as ~ 0.2), a fact long known but inadequately recognized. Serious problems from the arbitrary use of bothnandPC as fitting parameters with little or no guidance as to the dependence thatnandPC (which is microstructurally sensitive) have on the type of porosity are shown. Further, porosity normalization of the power law model indicates at best limited compression of different porosity dependences into a single universal porosity dependence function and little distinguishing of property dependences as a function of the type of porosity. However, normalized porosity of minimum solid area (MSA) models gives a single universal porosity dependence. The difference in responses toPnormalization of the two modeling approaches is attributed to their being based respectfully on little or no pore character and on detailed pore character. Thus,Pnormalization may be a valuable tool for evaluating porosity models, but must be applied in a more rigorous fashion, i.e.,PC determined primarily by measurement and correlation with the type of porosity (as with MSA models) and not as an arbitrary fitting parameter as used in the evaluations of the power law model. [ABSTRACT FROM AUTHOR]

Studies the control of microstructure on multiphase ceramic composites. Observation of flexural strength and fracture toughness in ceramic composites; Higher strength of four-component composites compared with the three-component composite; Attribution of the improved strength to the decreased grain size by grain growth retardation in the multiphase composite.

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. [ABSTRACT FROM AUTHOR]

Electrophoretic deposition (EPD) of colloidal ZrO2 ceramic powder was examined with respect to the internal colloidal forces and the external electrical field. The influence of electrolytic dissociation of water close to the deposition electrode (cathode) on the electrostatic interaction between the particles and the local electric field is discussed. The discrete element method (DEM) was used to get an insight into the kinetics determining particle packing and density gradient microstructures. The simulation indicates that high particle concentrations combined with low electric field strength result in coagulated flocs and a low packing density in the deposit layer. Tentative phase diagrams for various colloidal forces and electrical field strengths were established. [ABSTRACT FROM AUTHOR]

Critical issues of electrophoretic deposition (EPD) and its potential for fabricating high-performance ceramics are reviewed. Deposition kinetics under constant-current and constant-voltage conditions are discussed. The process of submonolayer formation of mono-sized silica particles as a function of deposition time during EPD has been discussed and is shown to have a remarkable similarity with the atomic thin-film growth (10000 times smaller scale) process of Molecular Beam Epitaxial (MBE) process. Ceramic coatings by EPD are described and strategies to avoid cracking are outlined. Discussion on avoiding cracking using the Reaction Bonded Aluminium Oxide (RBAO) process is also described. The versatility of the EPD process towards fabrication of a wide spectrum of composite microstructures is discussed together with references. Potential use of EPD in the fabrication of Micro-Solid Oxide Fuel Cell (μSOFC) has also been discussed. [ABSTRACT FROM AUTHOR]

The microwave dielectric properties and the microstructures of LaAlO3 ceramics with B2O3 additions (0.25–1 wt%) prepared with conventional solid-state route have been investigated. Doping with B2O3 (up to 0.5 wt%) can effectively promote the densification of LaAlO3 ceramics. It is found that LaAlO3 ceramics can be sintered at 1400°C due to the liquid phase effect of B2O3 addition. The Q × f value as well as the dielectric constant decreases at higher B2O3 doping level (1 wt%) due to the increase of boundary phases. At 1460°C, LaAlO3 ceramics with 0.5 wt% B2O3 addition possesses a dielectric constant (ℇr) of 22.9, a Q × f value of 44700 (at 9 GHz) and a temperature coefficients of resonant frequency (τf) of -36 ppm/°C. The B2O3-doped LaAlO3 ceramics can find applications in microwave devices requiring low sintering temperature. [ABSTRACT FROM AUTHOR]

The crystalline structure, microstructure and dielectric properties of the (Sr1-1.5xBix)TiO3 (0 ≤ x ≤ 0.267) ceramics were studied. Cubic solid solutions were determined for x ≤ 0.2 at room temperature. However, lattice distortion was detected by Raman spectra. A dense microstructure with the grain sizes of 2-4 µm was obtained for (Sr1-1.5xBix)TiO3 (0 ≤ x ≤ 0.2) ceramics. The Bi concentration was examined and found to be in agreement with the nominal composition and overall uniformly distributed in the sample. Different from the observations in the earlier literature for other doped quantum paraelectrics, where only an induced dielectric anomaly was reported, there are three Bi induced dielectric modes A, B, and C in the Bi doped SrTiO3 samples. The occurrence of the "impurity modes" and the "ferroelectric relaxor mode" and their evolution are demonstrated as a function of Bi concentration. [ABSTRACT FROM AUTHOR]

Different microstructures of the 70%Si3N4-30%BAS self-reinforced composite, fine, coarse and bimodal, are obtained by pressureless sintering at 1920°C. Flexural strength, fracture toughness and crack-growth resistance ( R-Curve) behavior of each microstructure are characterized by three-point bending, indentation and modified compact tension methods respectively, at room temperature. The crack deflection, whisker bridging and pullout are considered as major toughening mechanisms in the composite. It is found that coarsening β-Si3N4 whiskers of this composite can improve the toughness/fracture resistance but deteriorate the strength. When limited large abnormally grown whiskers are introduced into the microstructure, the composite shows an improved toughness/fracture resistance behavior and concurrently sustains a high strength. [ABSTRACT FROM AUTHOR]

A proper powder preparation was used to develop α-Sialon single crystals as seeds. The microstructure and fracture toughness of seed-containing α-Sialon ceramics sintered by hot-pressing were investigated. The specimen without seeding consisted of fine grains and a small amount of coarse grains. Specimens seeded with α-Sialon single crystal particles presented a large amount of elongated α-Sialon grains. The aspect ratio and the amount of elongated α-Sialon grains can be tailored by using different sizes and amounts of the seeds. The fracture toughness of seed-containing α-Sialon ceramics is improved, which is attributed to grain pullout and bridging of elongated grains. [ABSTRACT FROM AUTHOR]

Structural alumina ceramics used in ballistic armour and wear applications with varying alumina contents and manufactured using both slip casting and dry pressing techniques, have been investigated and characterized in terms of their hardness, elastic modulus, fracture toughness, and microstructural characteristics. For a given alumina content, fracture toughness decreases with increasing hardness. Dry pressed samples show slightly higher hardness, and lower fracture toughness for the same alumina content. The hardness, elastic modulus and fracture toughness are higher for the 98% alumina samples while the differences between the lower alumina samples (95 and 91%) are negligible. The grain sizes are bimodal with the majority ≤3 μm and the size range narrows with decreasing alumina content. The microstructures are composed of a matrix phase, corundum (α-Al2O3), grain boundary phases consisting of a glassy phase with varying Al2O3, SiO2, and CaO contents, a crystalline phase, triclinic anorthite (CaAl2Si2O8), and an additional phase, spinel (MgAl2O4), in the lower alumina samples. The proportion of the boundary phase increases with decreasing alumina content and no effect of fabrication method is observed. [ABSTRACT FROM AUTHOR]

Self-reinforcement of Li-α-sialon ceramics by in situ growth of elongated α and β-sialon grains has been explored and analysed. Properties of Li-α-sialon ceramics are mainly determined by the overall starting composition and the crystalline modification of the starting Si3N4 powder which in turn determine the final microstructure of the materials. Both the morphology and crystalline phase of the elongated sialon grains have strong effects on the toughening mechanism. The results indicate that α-sialons reinforced by elongated β-sialon grains have advantages over similar materials reinforced by elongated α-sialon grains because of the type of crack deflection toughening mechanism involved. [ABSTRACT FROM AUTHOR]

A unique porous ceramic with complex pore structure was synthesized by the freeze-dry process. A water-based ceramic slurry was frozen while controlling the growth direction of ice, and sublimation of the ice were generated by drying it at a reduced pressure. By sintering this green body, a porous ceramic with complex pore structure was obtained, where macroscopically aligned open pores exceeding 10 μm in size contained minute pores of about 0.1 μm in their internal walls. Wide control of the porosity was possible by changing the concentration of the starting slurry. The pore size distribution as well as the microstructure were substantially affected by the freezing and sintering temperatures. Optimization of the synthesis conditions was investigated in order to obtain the desired pore structure. [ABSTRACT FROM AUTHOR]

The purpose of this work was to study the effect of CuO as a versatile sintering agent for BaTiO3 based ceramics. The response is shown to exist in three directions. Firstly, as liquid-phase former at comparatively low temperatures. Hence, the influence of various CuO-based flux formers on the shrinkage behaviour and the microstructure development of the ceramics was investigated. Secondly, the aptitude as an internal susceptor for secondary phase initiated microwave sintering. The use of CuO as a lossy secondary phase leads to an accelerated microwave heating of the ceramics. Thirdly, the addition of CuO to BaTiO3 powders with Ba-excess results in a phase transition cubic → hexagonal at about 1300°C, which is a indication for the incorporation of Cu2+ ions into the BaTiO3 lattice at Ti sites under this conditions. [ABSTRACT FROM AUTHOR]

The effects of β-spodumene (Li2O·Al2O3·4SiO2) and zirconia additives on the physical and thermal characteristics aluminium titanate (AT) ceramics have been investigated. XRD, DTA, SEM, TEM and dilatometry have been used to characterise the influence of these additives on phase relations, densification, microstructure, thermal expansion, thermal decomposition and thermal shock behaviour of AT. The results show that the presence of both β-spodumene and zirconia significantly reduces porosity, lowers thermal expansion, improves densification, hardness and thermal stability, without degradation of thermal shock resistance. Microstructural examinations have revealed the presence of glassy phase due to the phase separation of β-spodumene and spontaneous microcracks due to thermal expansion anisotropy of AT. [ABSTRACT FROM AUTHOR]

The influence of processing on microstructural development of Ca α-sialon composition (Ca xSi12−3 xAl3 xO xN16− x) with x = 1.8 was studied by dwelling at different intermediate temperatures before reaching the final sintering temperature. The microstructural observation results have showed the different aspect ratios of elongated grains obtained by the various processing conditions, reflecting the effect of the number of nuclei of α-sialon on morphology of grains during sintering. Improved toughness was achieved by applying low temperature dwelling for Ca α-sialon compositions with low x values. The toughness showed an increase of 33% and 16% for x = 0.6 and 1.0 compositions respectively with middle dwelling processing at 1350°C for 3 h before reaching 1750°C for 1 h by hot-pressing. [ABSTRACT FROM AUTHOR]

The influences of V2O5 and CuO additives on the sintering behavior and microwave dielectric properties of BiNbO4 ceramics were investigated. The V2O5 and CuO additives lowered the sintering temperature of BiNbO4 ceramics to the range 875 °C–935 °C. All BiNbO4 compounds with additives had the orthorhombic structure. The dielectric constant εr was not significantly changed, while the unloaded Q value was affected with additives. The Qf value was found to be a function of the sintering temperatures and the amount of additives. It varied from 4500 to 15800 (GHz) and 1000 to 8000 (GHz) with additives V2O5 and CuO, respectively. The τ f values were increased in positive values with V2O5 doped, while decreased in negative values with CuO addition. V2O5 and CuO additives effectively improved the densification and dielectric properties of BiNbO4 ceramics. The correlation between the microstructure and the Qf value was observed with different additives. [ABSTRACT FROM AUTHOR]