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High-strength ZrC ceramics with relative density above 98% were prepared by reactive hot pressing of ZrC and Al at 1900°C. The reaction between ZrC and Al resulted in the formation of ZrC1−x, Zr3Al3C5 and Zr–Al compound such as AlZr3 and Al–C–Zr. The intermediate product AlZr3 below 1600°C and remained Al–C–Zr phase could form liquid phase and promoted the first stage of densification process. The improvement in densification behavior at higher temperatures (1800°C–1900°C) could be attributed to the formation of nonstoichiometric ZrC1−x. Adding 5 wt% and 7.5 wt% Al to ZrC, the formed ZrC0.85–Zr3Al3C5 and ZrC0.80–Zr3Al3C5 based ceramics had 3-point bending strength as high as 757 ± 79 MPa and 967 ± 50 MPa, respectively, with hardness and fracture toughness being 16.2–18.3 GPa and 3.3–3.5 MPa m1/2, respectively.

Composites of ZrC–SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC1−x, SiC, and ZrSi. Low-temperature densification of ZrC−SiC ceramics is attributed to the formed nonstoichiometric ZrC1−x and Zr–Si liquid phase. Adding 5 wt% Si to ZrC, the three-point bending strength of formed ZrC0.8–13.4 vol%SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m1/2, respectively.

Local dopant compositions within individual α-Sialon grains were measured by analytical electron microscopy (AEM) in hot-pressed CaxSi12−3xAl3xOxN16−x (x = 0.3–1.4) ceramics. The reduction of local x values from the nominal dopant compositions is about 40% in general, and it reaches 60% for the end member (x = 1.4) which contains inclusions of AlN-based 21R phase. This results exhibit stronger departures from x than the previous report of 30% dopants missing in α-Sialon phase by electron probe micro-analysis (EPMA) [J Eur Ceram Soc 19:1637, 1999]. Amorphous films of ~1 nm thick were commonly found at grain boundaries (GBs), which could only take a small fraction of undetected dopants while the film composition exhibits a quite different behavior. The general presence of GB films can rationalize the discrepancy between AEM and EPMA results by their differences in probe size and detection geometry, while the much larger gap in the end member suggests the existence of Ca-rich glasses in the intergranular regions. By excluding this end member, a linear relation between dopant solution and lattice expansion is restored in α-Sialon structure, which leads to 20 and 80% increases of the expansion coefficients from those given in the previous and original reports, respectively. This study not only demonstrated the necessity of solubility study in ceramics by AEM refinement, but also opens a new front to correlate the solution behavior with the intergranular glass/amorphous structures, both were regarded so far as largely independent.

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 (

Transmission electron microscopy study of pressureless sintered ZrB2–20 vol% SiC composites with WC (5–10 vol%) additions was carried out in this work. Two independent chemical reactions in which the ZrO2 impurities were removed from ZrB2 grain surface were confirmed. Three new formed phases, (W, Zr)ssB, (Zr, W)ssC, and (W, Zr)ssSi2, apart from the plate-like ZrB2 and SiC grains, were identified in the samples. Microstructure observations strongly suggest that the liquid phases in terms of (W, Zr)ssB and (W, Zr)ssSi2, appear in this system during the sintering process below 2200°C, and the former could well wet the ZrB2 grain boundaries. The mass transport process between ZrB2 and (W, Zr)ssB was also confirmed by scanning-transmission electron microscope analysis. The above results supports that the densification at 2200°C is assisted by liquid phase and the elongation of ZrB2 platelets resulted from the Ostwald ripening during the dissolution–diffusion–precipitation process.

WC powder was synthesized by a nitridation–carburization method with WO3 prepared by the citrate sol–gel route as the starting material, and the reaction mechanism was investigated. It was found that WO3 was converted to nanosized W2N after the nitridation process in NH3. Then, the W2N was transformed to W metal, W2C, and WC in sequence when it was carburized in CH4–H2 gas mixture. The WC powder obtained possessed a high specific surface area of 24.6 m2/g, and showed a particle size of 20–30 nm, which was much finer than the WC powder prepared by direct carburization of WO3 in CH4–H2 gas mixture. The reason was also discussed.

The influence of the spark plasma sintering (SPS) heating rate on densification, microstructure and strength of ZrB2–20 vol.% SiC composites with or without 5 vol.% Yb2O3 addition was assessed. A low heating rate promoted densification, induced grain growth and slightly increased the strength in the Yb2O3-free ceramics, whereas the low heating rate inhibited the densification, maintained the grain size and greatly decreased the strength in the Yb2O3-doped composites.

Using three elementary substances, Mo, Ni, and amorphous B powders as raw materials, the ternary boride based hard alloy Mo2NiB2–Ni was prepared successfully. The formation mechanism of the ternary phase was verified by XRD. It was indicated that the formed Mo2NiB2 particles at lower temperatures could be as seeds for the further formation of Mo2NiB2 directly from the raw materials of Mo, Ni and B at higher temperatures. The appearance of liquid phases between 1100 and 1200 °C was important for the densification of hard alloy. The microstructures of the hard alloys were observed by SEM. The main fracture modes for the hard alloys were different for the samples sintered at different temperatures. The maximum bending strength and the Rockwell hardness reached 1.85 ± 0.04 GPa and 85.7 ± 0.1 HRA, respectively. These values were comparable to those reported in literature.

The ball-milling process is a usual route employed to break up agglomerates in the powder or mix the powder with additives when preparing ceramics. Although it is well known that a powder can be “contaminated” by the wear particles from the milling balls, the study dealing with how the preparation and properties of a hard material would be affected by additives just introduced by scrape of milling balls has been scarce. In the present work, sintering behavior and mechanical properties of hot-pressed B 4 C with additives derived from milling balls were investigated. Polyoxymethylene, ZrO 2 , Al 2 O 3 and Si 3 N 4 were selected as different ball materials. The results show that the sinterability of B 4 C could be significantly enhanced because of the incorporation of one of these additives, i.e. ZrO 2 , Al 2 O 3 and Si 3 N 4 (approximately 3–6 vol%). As a result of improvement in density, excellent mechanical properties of B 4 C ceramics were obtained. Among them a flexural strength of B 4 C added by ZrO 2 reached 630 MPa.

Alterable roles of VC addition on the densification, microstructure development, and mechanical properties of the ZrB2–SiC ultrahigh temperature ceramics (UHTCs) were comprehensively investigated based on utilizing the thermodynamic analysis and experimental results. Chemical reactions of VC with the inevitably existed oxide (ZrO2) impurity on the particle surfaces of the ZrB2 starting powder, or with the ZrB2 matrix phase could be controlled by adjusting the atmospheres during hot pressing. Different chemical reactions during hot pressing also resulted in much different material microstructures and then different material properties. Benefiting the VC addition and proper sintering atmosphere, refined grain size of ZrB2 and improved mechanical properties were developed in ZrB2–SiC systems. By properly controlling these reactions, the microstructures and mechanical properties of ZrB2–SiC based UHTCs could be tailored.

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.

This work reported the microstructural evolution and grain growth kinetics of ZrB2–SiC composites containing 10, 20, and 30 vol% SiC during heat treatment at 2000°C. The coarsening of ZrB2 occurred in the three systems, whereas the obvious coarsening for SiC appeared only in the composite with 30 vol% SiC. The kinetics analysis showed the ZrB2 grain growth rate in the ZrB2–30 vol% SiC was 25 times lower than that for ZrB2–10 vol% SiC during heat treatment. Furthermore, the grain growth controlling mechanisms of ZrB2 and SiC were discussed. In addition, it was found that the heat treatment had little effect on Vickers hardness and fracture toughness of ZrB2–SiC.

The perovskite-like layer-structured (PLS) Nd2Ti2O7 and La2Ti2O7 have possibly the highest Curie points of any materials. To pole these ceramics, highly textured, dense ceramics with high DC electrical resistivity are required. The ferroelectric and piezoelectric properties of lead-free Nd2Ti2O7 and La2Ti2O7 grain-oriented ceramics prepared by spark plasma sintering using a two-step method are reported. The Tc of Nd2Ti2O7 and La2Ti2O7 are 1482±5° and 1461±5°C, respectively. The measured piezoelectric constant of the textured La2Ti2O7 was d33=2.6 pC/N. These results now open up the possibility of studying the ferroelectric/piezoelectric properties of the PLS family of ceramics with super-high Curie points.

Using Si and BN powders as raw materials, silicon nitride/hexagonal boron nitride (Si 3 N 4 /BN) ceramic composites were fabricated at a relatively low temperature of 1450 °C by using the reaction bonding technology. The density and the nitridation rate, as well as the dimensional changes of the specimens before and after nitridation were discussed based on weight and dimension measurements. Phase analysis by X-ray diffraction (XRD) indicated that BN could promote the nitridation process of silicon powder. Morphologies of the fracture surfaces observed by scanning electron microscopy (SEM) revealed the fracture mode for Si 3 N 4 /BN ceramic composites to be intergranular. The flexural strength and Young's modulus decreased with the increasing BN content. The reaction-bonded Si 3 N 4 /BN ceramic composites showed better machinability compared with RBSN ceramics without BN addition.

ZrB2–20 vol.% MoSi2 composites with elongated ZrB2 grains were prepared by reactive hot pressing using Zr, Si, B and Mo as raw materials. The ductile behavior of MoSi2 at high temperature helped the rod-like ZrB2 grains align under the applied pressure during sintering, resulting in a partly textured composite. The anisotropic growth of ZrB2 grains improved the toughness of the composite remarkably.

Phase pure hafnium diboride (HfB 2 ) powder was synthesized by borothermal reduction of hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum. The synthesized HfB 2 powder had an average particle size of 1.37 μm with an equiaxed shape, and a low oxygen content of 0.79 wt%. Using the as-synthesized HfB 2 powder and a commercial SiC, HfB 2 monolithic, and HfB 2 -20 vol%SiC composite were hot pressed at 2000°C to relative densities of 95.7% and 99.2%, respectively. With the addition of SiC, the grain size decreased and the fracture behavior changed from intergranular to a mixed mode, which resulted in a high flexural strength of 993 ± 90 MPa for the composite. Fracture toughness of the composite was 6.29 ± 0.65 MPa m 1/2 , which was significantly higher than that of the HfB 2 monolithic and the reported values in literature.

Three ZrB2–SiC composites were prepared from polycarbosilane (PCS)-coated ZrB2 powders. Powders with or without additions of Si or Zr were densified by hot pressing at 1800 °C. Finally, the oxidation resistance of the composites was studied at 1500 °C in air. The phase composition and microstructure were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with energy-dispersive spectroscopy (EDS). Results showed that Si addition, which reacted with free carbon coming from the pyrolyzed PCS to produce the SiC, improved the oxidation resistance of ZrB2–SiC composites with PCS as a precursor, whereas Zr addition, which reacted with the carbon to produce the ZrC, decreased it.

The isothermal oxidation of ZrB 2 powder was carried out in the range of 650–800 °C in a flowing air using thermogravimetric analysis (TGA). The evolution of the phase characterization and morphology of ZrB 2 powder oxidized at 700 °C for varying durations was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The excellent fit of TG curves by multiple-law model suggests that the oxidation of ZrB 2 powder in air follows para-linear kinetics, and based on the fitted results, oxidation mechanisms can also be obtained. The reaction product of ZrB 2 powder with oxygen is metastable tetragonal ZrO 2 at 700 °C, and the tetragonal phase transforms to the monoclinic phase with oxidation. The oxidation of ZrB 2 powder is associated with surface microcrack formation, which is attributed to volume expansion resulting from oxidation of ZrB 2 to tetragonal ZrO 2 and tetragonal-to-monoclinic phase transformation of ZrO 2 . In the last stage of oxidation, each ZrB 2 particle breaks into fragments.

A suspension stabilizer-coating technique was employed to prepare x mol% Yb2O3 or Gd2O3 (x = 1.0, 2.0, 3.0 and 4.0) co-stabilized 1.0 or 2.0 mol% Y2O3 doped ZrO2 nanopowders (xYb1Y, xYb2Y, xGd1Y). Fully dense Yb2O3/Gd2O3 and Y2O3 co-doped ZrO2 ceramics were obtained by means of pressureless sintering in air at 1450 °C. Phase assemblage, microstructure measurements indicated that polycrystalline tetragonal ZrO2 (TZP) was achieved in the 1Yb1–2Y and 1Gd1–2Y grades, whereas the 4Yb1–2Y ceramic was almost fully stabilized cubic ZrO2 (FSZ) and all other investigated stabilizer combinations resulted in partially stabilized zirconia (PSZ). Complex impedance analysis showed that the conductivity of the xGd1Y and xYb1–2Y co-doped zirconia ceramics increased with increasing Gd2O3 and Yb2O3 content. The xGd1Y grade had a lower conductivity than the xYb1Y and xYb2Y ceramics. The conductivity behaviour was assessed and explained in terms of the cubic phase content and dopant ionic radius.

Fine hafnium diboride (HfB2) powders have been prepared by modified carbothermal/borothermal reduction of hafnium dioxide (HfO2) at relatively low temperatures (1500°–1600°C) for 1–2 h. The XRD patterns could be indexed as hexagonal HfB2 and no evidence of HfC, HfO2, or other impurities was observed. Glow discharge mass spectrometer analysis indicates that the synthesized HfB2 powders had high purity. The synthesized HfB2 powders had small average crystallite size (around 1 μm) and low oxygen content (

Adding a small amount of vanadium carbide as sintering aids, nearly fully dense ZrB2–SiC composites were obtained by pressureless sintering methods at 2000–2100 °C. Thermodynamic calculations and experimental results proved that VC is better at removing the surface oxide impurities of ZrB2 than WC, and played a very important role during the densification process. The mechanical properties of the sintered ceramics were also reported.

Cr 2 AlC powders prepared by the molten salt method, using chloride as the fluxes, were successfully obtained. The influences of composition, weight ratio of powder to flux and temperature on phase assembly and morphology of Cr 2 AlC powders were investigated. The results indicated that the amount of Cr 2 AlC increases as powder/flux ratio decreases from 2:1 to 1:1 while the further decrement of powder/flux ratio to 1:2 has little effect on its amount. As the excessive Al in elemental powders increases from 0 to 10 at.%, the amount of Cr 2 Al increases greatly and the Cr 7 C 3 content decreases obviously. The amount of Cr 2 AlC increases evidently with the obvious decrease of Cr 2 Al and the slight reduction of Cr 7 C 3 as temperature raises from 900 to 1000 °C while a further increase in temperature has limited effect on their amount. The results of scanning electron (SE) observation on fabricated Cr 2 AlC powders indicate that the liquid phase content in composition is the critical influencing factor on the morphology. Predominantly single-phase powders of Cr 2 AlC are fabricated and its sintering ability is also studied.

A ZrB 2 -SiC-ZrC ceramic was produced by reactive hot pressing using Zr, Si, and B 4 C as raw materials. The kinetics of the reaction process was studied. The reduction of powders by ball milling is of crucial importance for the sintering. The self-propagating high-temperature synthesis reaction between the raw powders can be ignited by controlling the sintering conditions, which leads to a sintering temperature as low as 1600°C, the lowest sintering temperature reported thus far. The relative density is 97.3%, with an open porosity of 0.6%, and the mechanical properties are comparable to the composites that sintered at higher temperatures. The depletion of oxygen impurities during the sintering was discussed.

ZrB 2 –20 vol%SiC ceramics without and with additive of 2 wt% carbon nanotubes (CNTs) were fabricated by hot-pressing at 1900 °C for 1 h under 30 MPa using ZrB 2 , SiC and CNTs as starting materials. The fracture toughness increased about 15%, but hardness, flexural strength as well as thermal conductivity did not vary significantly from the addition of CNTs.

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, kp, for oxidation at 1,100 and 1,250 °C were 1.1 × 10−12 and 7.1 × 10−10 kg2 m4 s−1, respectively.

Bi 3.15 Nd 0.85 Ti 3 O 12 ceramic, which is a three-layer ferroelectric Aurivillius phase, was prepared by spark plasma sintering. The effect of texture on the anisotropy of dielectric, ferroelectric and piezoelectric properties was studied. X-ray diffraction showed that samples perpendicular to the hot-pressing direction had a – b -axis preferred texture, whereas, samples parallel to hot-pressing direction had c -axis preferred orientation. The dielectric constant, remanent polarization and piezoelectric constant of samples with orientation close to a -axis are larger than those of samples with orientation close to c -axis. Their Curie points are all about 410 °C.

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.

The investigation of bulk Cr2AlC ceramic fabricated by Spark Plasma Sintering (SPS) from coarse powders (CAC10) and fine powders (NCAC10) in the temperature range of 1100–1400 °C was carried out. The XRD results indicate that Cr2AlC, as major phase, always appears with minor and trace amount of Cr7C3 and Cr2Al respectively in both NCAC10 and CAC10 samples and the amounts of later two phases decrease with increase in temperature. However, the Cr2AlC phase content in NCAC10 is higher than that of CAC10 sintered at the same temperature. The micrographs of back-scattered SEM show that grains with smaller size and pores with fewer amounts appear in SPSed NCAC10 in comparison to that of CAC10. As consequence, the higher hardness (5.6 GPa) of NCAC10 than that (3.9 GPa) of CAC10 was obtained. The patterns of XRD, microstructure and hardness of samples HPed at 1400 °C for the same composition were also presented for comparison. © 2007 Elsevier B.V. All rights reserved.

Four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2–SiC–AlN, were synthesized in situ via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), using Zr, Si, B4C, BN and Al as raw materials. The synthesis process plays a critical role in the microstructural features of the composites obtained. The R-SPS process can lead to a more homogeneous and finer microstructure due to its high heating rates and short holding time, while the RHP process is likely to result in coarse microstructures due to a long enough holding time for grains growth.

Dense and predominantly single-phase samples of Cr2AlC, together with a trace amount of Cr7C3, were fabricated by hot pressing of a mixture of chromium, aluminum, and graphite powders at 1400°C for 1 h. The hardness, Young's modulus, flexural strength, and compressive strength of Cr2AlC samples were 5.2, 288 GPa, 483±29, and 1159±23 MPa, respectively, which are comparable with those of Ti3AlC2 and Nb2AlC. The material exhibits good damage tolerance. For indentation loads up to 50 N, the post-indentation flexural strengths decrease by approximately 10% and by 31% for a load of 100 N. The flexural strengths of Cr2AlC samples quenched from 300°C to room temperature decrease from 483 to 455 MPa, while the retained strengths decrease quickly to 199 MPa when the quench temperature increases to 500°C. A further increase in quench temperature to 700°, 900°, and 1100°C results in a small reduction of strength.

Nd/V co-doped Bi4Ti3O12 ceramics, Bi4−yNdyTi2.98V 0.02O12.01 (BNTV, y = 0.16 , 0.26, 0.36 and 0.56), were prepared using a solid-state reaction method and their phase assemblages, microstructure, dielectric and ferroelectric properties were investigated. The Curie temperature of BNTV ceramics decreases with increasing Nd concentration. The loss tangent of the materials is much lower (1.8% at 400 ∘C/100 kHz for BNTV0256) than that of BIT sample (77%). Besides, BNTV ceramics exhibit saturated ferroelectric hysteresis loops with 2Pr of 26–31 μC/cm2 and Ec of 41–57.5 kV/cm.

ZrB 2 –SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB 2 and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also 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.

The reaction process of Cr 2 AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr 2 AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr 5 Al 8 , Cr 2 Al and Cr 7 C 3 were the intermediate phases appearing in turn in the heating process. The amount of Cr 2 AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.

Cr2AlC ceramics were fabricated by hot-pressing using Cr, Al and C powders as starting materials. The phase assemblages of the samples consisted of Cr2AlC, as a major crystalline phase, together with a very small amount of Cr7C3 and an unknown phase. Its thermal and electrical as well as mechanical properties were determined.

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.

Three kinds of standard specimens, including low-alloy steels (Japanese iron and steel CRM), glass from NIST and Chinese geological reference materials were analyzed by wavelength dispersive X-ray fluorescence spectrometry as unknown samples. The fundamental parameter method was used to correct the matrix effect and different specimens including pure element specimens and type specimens were employed as calibration samples. The quantitative results were compared in order to study the influence of calibration samples on the accuracy of quantitative results. The study showed that the fundamental parameter method could correct the matrix effects very well and relatively accurate quantitative results could be obtained, corrected by simple specimens, in case of type calibration specimens’ absence.

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.

Theoretical equations to calculate fluorescence intensity enhanced by scattering effects for coating samples were developed based on fundamental parameter models. The secondary enhancement by scattering radiation from the same layer or between layer and substrate and the primary fluorescence that was scattered into the direction of detector by atoms in layer and substrate were included in calculations. The contributions of different scattering effects to fluorescence intensity were calculated for a hypothetical Zn coating on infinite Fe substrate sample. The results show that the contributions of scattering effects to fluorescence intensity are related to the thickness of coating and are up to several percents of primary fluorescence intensity.

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.

The phase and microstructure relationship of 12 mol% CeO2-stabilized ZrO2 ceramics prepared from coated powder was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersed Xray spectroscopy (EDS). As compared with the sample prepared with co-precipitated method, which exhibited a similar grain size distribution, the EDS analysis revealed that the powder coating induced a wide distribution of CeO2 solubility, which decreases monotonically with the increase of grain size. This variation of stabilizer content from grain to grain rendered many large grains in the monoclinic phase. Stronger cerium segregation to grain boundaries was observed between large grains, which often form thin amorphous films there. The inhomogeneous; CeO2 distribution keeps more tetragonal ZrO2 grains close to the phase boundary to facilitate the transforming toughness. Addition of an Al2O3 precursor in coated powders effectively raises the overall CeO2 stabilizer content in the grains and preserves more transformable tetragonal phase in the microstructure, which further enhanced the fracture toughness. The dependence of CeO2 solubility on grain size may be explained in a simple coating-controlled diffusion and growth process that deserves further investigation.

The Pr α-sialon powders prepared by self-propagating high-temperature synthesis (SHS), consisting of 55 wt% Pr α-sialon and 45 wt% of β-sialon (abbreviated as α′ and β′), were hot-pressed at 1800°C for 1 h. The results showed that Pr α′ phase would transfer to β′ with the appearance of JEM phase (Pr(Si6−zAlz)(N10−zOz)) after sintering, thus resulting in the increase of β′ phase to 86 wt%. The addition of Y2O3 into SHS-ed Pr α′ powders as the starting materials restrains the transformation of α′ to β′ and prevents the formation of JEM phase as well. The nucleation mechanism of Pr α′ grain during hot-pressing was investigated in terms of transmission electron microscope and energy-dispersive spectrometer analysis. Two nucleation modes of Pr α′ grains were found, i.e., nucleating on the undissolved Pr α′ grains and on the nuclei of (Pr, Y) α′ grains precipitated from liquid phase.

Effective grain alignment in Bi4Ti3O12 ceramics by superplastic-deformation-induced directional dynamic ripening

The characteristics of Eu-stabilized α-sialon ceramics derived from self-propagating high-temperature synthesis (SHS) Eu α-sialon powders without and with the addition of Y 2 O 3 are investigated. The results showed that the amount of α-sialon phase formed in sintered Eu α-sialon composition was much less than that in SHS-ed powder when the composition was hot-pressed at 1800 °C for 1 h, while the transformation of α-sialon to β-sialon phase did occur at the same time, which could be attributed to the metastability of SHS-ed powder because of the high heating and cooling rate during the SHS process and the reduction of Eu 3+ to Eu 2+ under the reduction conditions during hot pressing. By addition of Y 2 O 3 into SHS-ed Eu α-sialon powder, thus to form (Y,Eu) α-sialon phase in the sintered sample, the stability of α-sialon phase was improved, as the ratio of α-sialon to α-sialon was increased from 70 wt.% in SHS-ed powder to 83 wt.% in the sintered product by 50 mol% of Y 2 O 3 added into SHS-ed powder.

Different ratios of Ce to Y were selected to stabilize (Ce,Y) α-sialon during self-propagating high-temperature synthesis (SHS). The phase assemblages were then studied for both SHS-ed (Ce,Y) α-sialon powders and sintered ceramics. The results showed that the amount of α-sialon phase increased as a main crystalline phase in SHS-ed powders when 20 at.% of Ce was replaced by Y in the composition, and further increase in the substitution of Y for Ce did not obviously increase the α-sialon content, whereas β-sialon was the major phase in the SHS-ed powders without the introduction of Y in the composition. The transformation of α-sialon to β-sialon did occur when the SHS-ed powders were hot-pressed for the compositions without Y and substitution of Y for Ce less than 40 at.%. The 50 at.% replacement of Ce by Y was enough to stabilize (Ce,Y) α-sialon structure as the (Ce,Y) α-sialon phase was kept in the sample with similar amount to that in the SHS-ed powders.

A novel stabilizer coating technique was applied to prepare 1.0 mol% Nd2O3+1.0 mol% Y2O3-co-coated ZrO2 nanopowder from an alcohol/water-based suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder. The low stabilizer content starting powder could be fully densified into a (1Nd,1Y)-TZP ceramic when pressureless sintering in air at 1450 8C. The obtained ceramic revealed an excellent fracture toughness of 12.5 MPa m 1/2 in combination with a hardness of 10 GPa, a combination of properties that is comparable with that of the traditional 12 mol CeO2-stabilised tetragonal zirconia polycrystalline (TZP). The toughness could be further increased up to 14.3 MPad m 1/2 by the addition of 2 wt.% Al2O3 to the starting powder.

A Dy-α-sialon with a composition of Dy 0.33 Si 9.3 Al 2.7 O 1.7 N 14.3 was prepared by hot pressing at 1800 °C with an intermediate dwelling at 1550 °C or 1650 °C during the sintering. The microstructures and optical properties of the sintered specimens were studied. The results showed that an intermediate dwelling stage in the hot pressing was beneficial to densification of the specimens and could suppress the development of elongated α-sialon grains, but it had a negative effect on hardness and fracture toughness. Besides, the intermediate dwelling process also improved the optical transmittance and the maximum infrared transmission, which is attributed to the increase in bulk density and the development of a homogeneous equi-axed microstructure.