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Densifications of hot-pressed ZrC ceramics with Zr and graphite additives were studied at 1800–2000 °C. ZrC with 8.94 wt% Zr additive (named ZC10) sintered at 1900–2000 °C achieved higher relative densities (>98.4%) than that of additive-free ZrC (

ZrB 2 –20 vol% SiC composites with 3 vol% Re 2 O 3 rare-earth oxide (Re = La, Nd, Y or Yb) were hot pressed to near full-density at 1900 °C. The La 2 O 3 and Nd 2 O 3 additions not only caused the formation of an amorphous grain boundary phase and enhanced densification, but also resulted in substantial ZrB 2 and SiC grain growth. In contrast, the Y 2 O 3 and Yb 2 O 3 additions resulted in the formation of crystalline (Y/Yb) 2 Zr 2 O 7 and enhanced densification without ZrB 2 and SiC grain growth. The hardness was improved by the rare-earth oxide addition, especially with Y 2 O 3 and Yb 2 O 3 . The La 2 O 3 and Nd 2 O 3 only had a minor effect on the fracture toughness, whereas the Y 2 O 3 and Yb 2 O 3 additions increased the fracture toughness. The type of Re 2 O 3 addition was found to influence the nature of the grain boundary and the concomitant fracture and toughening mechanisms.

Neodymium-modified bismuth titanate (BNT) powder with compositions of Bi 4− x Nd x Ti 3 O 12 ( x = 0, 0.2, 0.5 and 0.75) were prepared through a hydrolysis method. The influence of Nd addition on the powder characteristics, sintering, microstructure and properties were investigated. It was found that the absorption band of Ti–O stretching vibration at 589 cm −1 was shifted to a higher wave number in the IR spectra, and the sintering as well as grain growth was significantly retarded. The Curie temperature, dielectric loss and Curie peak intensity were steadily decreased with an increase in Nd concentration, and the dielectric constant and loss tangent became much less dependent on temperature and frequency. At the same time, the polarization properties of the material were greatly improved.

A stabiliser coating technique was applied to prepare mixed Nd2O3 +Y 2O3 coated ZrO2 nanopowder from an alcohol/water based suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia nanopowder. The mixed stabiliser starting powders could be fully densified into (Nd,Y)TZP ceramics by pressureless sintering in air at 1450 ◦ C. The obtained ceramics revealed an excellent fracture toughness up to 12.8 MPa m 1/2 in combination with a hardness of 11 GPa, a toughness that is comparable to that of the traditional 12 mol CeO2-stabilised TZP but with higher hardness.

Nano-powders of BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3, a mixture of the composition (BaTiO3)0.6(SrTiO3)0.4 with particle sizes in the range of 60 to 80 nm, and Bi4Ti3O12 with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO3)0.6(SrTiO3)0.4 mixture is 125 oC, ~75 oC for Bi4Ti3O12, ~25oC for BaTiO3 and SrTiO3, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba0.6Sr0.4TiO3. During the densification of the (BaTiO3)0.6(SrTiO3)0.4 mixture the reaction 0.6BaTiO3+0.4SrTiO3 → Ba0.6Sr0.4TiO3 takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures.The dielectric properties of selected samples of (Ba, Sr)TiO3 ceramics have been studied. The results are correlated with the microstructural features of these samples, e.g. to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region.The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi4Ti3O12 (BIT) and CaBi2Nb2O9 (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism.A new processing strategy for obtaining highly textured ceramics is described. It is based on a directional dynamic ripening mechanism induced by superplastic deformation. The new strategy makes it possible to produce a textured microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties.The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi4Ti3O12 ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi4Ti3O12 ceramics. Textured CaBi2Nb2O9 ceramics exhibited a very high Curie temperature, d33-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.

A suspension stabilizer-coating technique was employed to prepare x mol% Yb 2 O 3 ( x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y 2 O 3 co-doped ZrO 2 powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb 2 O 3 and Y 2 O 3 co-doped zirconia ceramics. Fully dense ZrO 2 ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb 2 O 3 content and the sintering parameters. Polycrystalline tetragonal ZrO 2 (TZP) and fully stabilised cubic ZrO 2 (FSZ) were achieved in the 1.0 mol% Y 2 O 3 stabilised ceramic, co-doped with 1.0 mol% Yb 2 O 3 and 4.0 mol% Yb 2 O 3 , respectively. The amount of stabilizer needed to form cubic ZrO 2 phase in the Yb 2 O 3 and Y 2 O 3 co-doped ZrO 2 ceramics was lower than that of single phase Y 2 O 3 -doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m 1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb 2 O 3 and 1.0 mol% Y 2 O 3 ceramic at 1450 °C for 1 h.

The phase assembly of 1.0–5.0 mol% Nd 2 O 3 -doped ZrO 2 sintered at 1400 °C revealed that the tetragonal ZrO 2 phase could not be completely stabilised. Co-stabilising of 0.5–2.5 mol% Nd 2 O 3 with 0.5–1.0 mol% Y 2 O 3 , however, allowed the preparation of fully dense (Nd,Y)-TZP ceramics by pressureless sintering in air at 1450 °C. The mixed stabiliser monoclinic zirconia nanopowder starting material was synthesized from a suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder in an alcohol/water mixture. A HV 30 hardness of 10 GPa combined with an excellent indentation toughness of 13 MPa m 1/2 could be achieved for the (1.0Nd,1.0Y)- and (1.5Nd,1.0Y)-TZP ceramics. The influence of the mixed stabiliser content on the phase stability and mechanical properties are investigated and discussed.

CeO2-doped ZrO2, (8 mol%) starting powder was sintered by means of spark plasma sintering (SPS) at 1300 degrees C without holding time. The stability of the tetragonal ZrO2 phase in the Ce-ZrO2 ceramic sintered under strongly reducing conditions was investigated. The SPS sample consisted of monoclinic and tetragonal ZrO2 phase, with a volume ratio of two to one, as well as a trace amount of a Zr-Ce-O cubic solid solution phase. In contrast, the same powder sintered by hot-pressing in nitrogen at 1300 and 1500 degrees C for 1h showed no tetragonal ZrO2. Microstructural observation of the SPS ceramic by SEM and TEM revealed grains with and without twins. The reason for the appearance of the tetragonal phase in the SPS sample sintered under strongly reducing conditions is discussed. (c) 2004 Elsevier Ltd. All rights reserved.

By spark plasma sintering (SPS) and post-sintering thermal treatment at 1700 °C for 7 and 17 h, respectively, the infrared transmissions of Dy-α-sialon with three compositions, Dy 0.33 Si 9.3 Al 2.7 O 1.7 N 14.3 , Dy 0.4 Si 10.2 Al 1.8 O 0.6 N 15.4 and Dy 0.67 Si 9 Al 3 O 1 N 15 , (abbreviated as Dy1017, Dy1206 and Dy2010, respectively), in 1500–4000 cm −1 wave number (6.6–2.5 μ) regions were investigated. The results showed that the second crystallized phase, the sizes of α-sialon grains, the distribution of cations in the grains and the existence of holes on the surface of ceramics were main factors that affected the infrared transmittance of Dy-α-sialon. Among the three SPS-ed specimens, Dy2010 (0.5 mm in thickness) possessed the highest infrared transmission 65%, while those are 58 and 44% for Dy1206 and Dy1017, respectively. Through thermal treatment at 1700 °C for 7 h, the infrared transmittance of Dy2010 and Dy1206 decreased greatly, while that of Dy1017 nearly kept invariable. When thermal treatment time was prolonged from 7 to 17 h, the corresponding infrared transmittance of Dy1206 increased from 37 to 65%. Based on XRD analysis and microstructure observation, the influence of second phase and concentration distribution of cations in the grains on the transmissions of Dy–α-sialon ceramics were discussed.

In the present work, the mechanical properties and erosion-resistance ability of hot-pressed and pressureless sintered slag α-sialon ceramics, which were started from slag α-sialon powders prepared by SHS technique based on slag powders, [J. Mater. Chem. 12 (2002) 1199] were measured. The hardness and toughness of pressureless sintered slag α-sialon samples reached 15.53 GPa and 4.72 MPam 1/2 respectively. The strength of hot-pressed slag α-sialon ceramics reached 599 MPa as well. In comparison with Al 2 O 3 and ZrO 2 ceramics, as-sintered slag α-sialon ceramics showed much higher erosion-resistance ability. These properties are very close to that of normal α-sialon ceramics prepared by expensive chemical agents as raw materials.

The synthesis of Ce α-Sialon powder by self-propagating high-temperature synthesis (SHS) and the sintering behaviors of Ce α-Sialon material by hot-pressing derived from SHS-ed powder with the addition of CeO 2 or Y 2 O 3 were performed in the present work. The results have shown that the phase assemblages of synthesized Ce α-Sialon powder consisted of β-Sialon, as the major crystalline phase, and small amount of α-Sialon, Ce 2 Si 6 O 3 N 8 and AlN-polytypoid. Ce α-Sialon phase was meta-stable and would decompose to β-Sialon and JEM phase [Ce(Si 6− z Al z )(N 10− z O z , z ≈1)] with the disappearance of Ce 2 Si 6 O 3 N 8 and AlN-polytypoid after hot-pressing. The addition of CeO 2 further encouraged the transformation of Ce α-Sialon phase, but the addition of Y 2 O 3 did stabilize the Ce α-Sialon structure and (Ce,Y) α-Sialon phase would be as the major crystalline phase in the sintered material when 0.375 molar Y 2 O 3 was added, in which most α-Sialon grains having elongated morphology were observed. As a result, the hardness (H v ) and fracture toughness (K IC ) of sintered (Ce,Y) α-Sialon material being 15.23 GPa and 4.90 MPa m 1/2 respectively were obtained.

(Ca,Mg)-α-sialon powders were previously [J. Mater. Chem. 12 (2002) 1199] prepared by Self-propagating high-temperature synthesis (SHS) technology using slag as a raw material (abbreviated as Slag α-sialon powder). In this study, phase assemblages of the Slag α-sialon samples hot-pressed from 1450 to 1700 °C were investigated. Results indicated that α-sialon was the main crystalline phase in the whole sintering process, however, a small amount of β-sialon appeared in the samples sintered at 1500 °C and above. In addition, the SHS technique was also used to prepare α-sialon powders using chemical agents as raw materials (named Chem α-sialon). Samples made from the Chem α-sialon powder were hot-pressed under the same conditions for a comparison. Similar phase assemblages were obtained for the samples starting from Chem α-sialon powders, except for even more amount of βsialon phase contained. Material with a single α-sialon phase could be got in both samples when 10 wt.% additional slag and CaCO 3 were introduced into the Slag α-sialon and Chem α-sialon starting powders respectively.

Textured bismuth titanate (Bi 4 Ti 3 O 12 ) ceramics were fabricated by templated grain growth (TGG), using plate-like Bi 4 Ti 3 O 12 particles prepared by a molten salt method as the templates. The templates were aligned in the fine-grained matrix by aqueous tape casting with their major surface parallel to the casting plane. Effect of sintering conditions on the grain orientation in the material was investigated. It was found that the degree of grain orientation (Lotgering factor, f ) increased with the increase in sintering temperature, soaking time and heating rate. High Lotgering factor ( f ⩾0.92) can be obtained through careful control of the sintering parameters. The textured Bi 4 Ti 3 O 12 ceramics showed a high anisotropy in its dielectric properties in the directions parallel and perpendicular to the casting plane.

A thermodynamic estimation of the ZrO 2 –CeO 2 and ZrO 2 –CeO 1.5 systems, as well as the cubic phase in the CeO 1.5 –CeO 2 system has been developed and the complex relation between the nonstoichiometry, y , in Ce z O 2–y and the oxygen partial pressure at different temperatures is evaluated. The behavior of the nonstoichiometry phase Zr 1– z Ce z O 2– x is described based on the thermodynamic estimation in the ZrO 2 –CeO 2 , CeO 1.5 –CeO 2 and ZrO 2 –CeO 1.5 systems. Additionally, the interdependence among miscellaneous factors, which can be used to describe the change in oxidation states of cerium such as the oxygen partial pressure, the CeO 1.5 fraction in CeO 1.5 –CeO 2 in the quasi-ternary system, the nonstoichiometry y and the difference between the activity of CeO 2 and CeO 1.5 are predicted. The calculated results are found to be very useful to explain the influence of pressureless sintering at different O 2 partial pressures on the mechanical properties of CeO 2 -stabilised ZrO 2 ceramics

The influence of hydrothermal conditions (including starting materials, reaction temperature and time) on the crystal structure and the morphology of Bi4Ti3O12 particles are discussed in this paper. Bi4Ti3O12 nanocrystalline particles were hydrothermally synthesized at temperatures in the range of 180–230 °C for 4–12 h, from Bi(NO3)3·5H2O, TiCl4 and NaOH solution. The XRD results revealed that a typical bismuth layered perovskite structure Bi4Ti3O12 was obtained. The TEM showed that the Bi4Ti3O12 nanoparticles are tabular, and the sizes are about 200 nm.

Dual modifying cation (Ca+Nd) α-sialons were hot pressed for the x=1.0 composition ((Ca,Nd)xSi12-3nAl3nOnN16-n) with mixtures of CaO:Nd2O3 in the molar ratios of 0:1, 0.3:0.7, 0.5:0.5, 0.7:0.3 and 1:0. Experimental results showed that except for pure Nd- and Ca-α-silaon compositions, the main crystalline phase in samples hot-pressed at 1750°C for 1 h was α-sialon together with a small amount of melilite solid solution and AlN polytypoid. The higher CaO:Nd2O3 ratio in the starting composition, the higher α-sialon content and the lower melilite solid solution content in the final phase assembly. The α-sialon lattice also expanded with increasing substitution of Ca for Nd. Elongated grain morphology was observed in (Ca+Nd) α-sialon. EDS analysis revealed higher solubility of Ca than Nd in the α-sialon lattice and lower solubility of Ca than Nd in the intergranular phase. The hardness increased, ranging from 15.87 to 18.79 MPa, when the amount of α-sialon phase in the material became higher. The fracture toughness varied from 4.2 to 5.1 MP am1/2 for (Ca+Nd) α-sialon compositions, in which the composition with CaO:Nd2O3=0.5:0.5 possessed the highest value.

Silicon nitride has two polymorphous structures, α-Si 3 N 4 and β-Si 3 N 4 . In this study three different Si 3 N 4 starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca 1.8 Si 6.6 Al 5.4 O 1.8 N 14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si 3 N 4 resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si 3 N 4 remained until 1800° in samples prepared with ∼100%β-Si 3 N 4 powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si 3 N 4 starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.