Key Engineering Materials Vols. 368-372

Abstract: Aluminum oxynitride powders were synthesized by carbothermal reduction, using nano-sized alpha-alumina, gamma-alumina and carbon as raw materials. Effects of the weight percentages of alpha-alumina in alumina (R) and synthesizing temperatures (T) on the preparation of single-phase AlON powders have been studied. The results showed that proper R can reduce the shrinkage and increase the formation rate of AlON phase, but a high R will reduce the formation rate of AlON phase, because the activity of alpha-alumina was lower than that of gamma-alumina. Single-phase AlON powder with excellent properties can be obtained under the condition of R=0.15, T=1700°C. Moreover, this single-phase AlON powder was well-dispersed with an average size of about 2 μm.

Abstract: Transparent AlON ceramics are prepared with single-phase AlON powder synthesized from micron-sized aluminum and nano-sized alumina. The ceramics are prepared through sintering at the temperature of 1880°C for 10, 20 and 40 hours in flowing-nitrogen atmosphere, respectively. The effects of powder and temperature holding-time on transparent ceramic preparation and microstructure and transparency are investigated. The results show that all samples are transparent and of a single-phase AlON and, with increasing holding-time, the average pore size and porosity decreases and consequently, the ceramics are dense and their transparency is improved.

Abstract: Single-phase AlON powder was prepared by carbothermal reduction in flowing nitrogen atmosphere, using nano-sized alumina and carbon powder as raw materials. Transparent AlON ceramic samples using the single-phase powder were prepared by sintering at the temperature of 1880°C for 10h, 20h and 40h in flowing nitrogen atmosphere, respectively. It showed that the transparency and the density of the ceramic samples increased with increasing temperature holding-time. And the microstructures of the obtained samples were observed.

Abstract: The single-phase AlON powder used to prepare transparent ceramics was synthesized from solid-state-reaction using nano-sized Al2O3 and AlN powders with high purity. An appropriate amount of the powder was molded into pellets and isostatically pressed using polyvinyl alcohol as the binder. The AlON ceramic samples were obtained by sintering the pellets at 1880°C for 10 h, 20 h and 40 h, respectively. The transparency of the samples increased as the temperature holding time was prolonged. The density, phase compositions and microstructures of the sintered samples were discussed in detail.

Abstract: AlON with a composition of Al23O27N5 was prepared by hot pressing at temperatures lower than 1900 °C. The microstructures and final properties, including both mechanical properties and optical properties, of the sintered specimens were studied. The results showed that sintering temperature had a great influence on the densification of specimens and could lead to very different properties, especially the optical transmittance and the maximum infrared transmission.

Abstract: BaTiO3 nanopowders prepared by sol-gel process were used for multilayer PTCR ceramics in order to utilize grain boundary effect and lower sintering temperature. The precursor gel was calcined at different temperatures and the powders were characterized by XRD and TEM. The average grain size was about 26nm when calcined at 800°C for 2h. Effects of acceptor/donor concentration and sintering temperature on PTCR ceramics were also investigated. The optimal concentration of the donor was found to be 0.6mol with the acceptor concentration being 1/8 of the donor. Multilayer PTCR elements were fabricated by tape-casting technique. The jump ratio of PTCR chips sintered at 1240°C was above 103 with the average grain size smaller than 1~2 μm, which is suitable for the multilayer PTCR elements.

Abstract: Dense PTC ceramics were prepared with BaTiO3 nanopowders synthesized by hydrothermal method. BaCO3 and Ti(OC4H9)4 were used as barium and titanium sources, and Y(NO)3·6H2O as the donor dopant respectively. The average grain size of the powders obtained after hydrothermal treatment at 160°C for 9h was about 30nm with cubic structure. Mn(NO3)2 was introduced to the as-prepared nanopowders in order to improve the PTC effect. After sintered at 1280°C, the PTC ceramic samples exhibited sufficient resistance jump ratio(1.086×103) around Curie temperature, the density of which was 5.81g/cm3(96.5% of the theoretical density).

Abstract: PTCR ceramic samples of two systems, Bi4Ti3-xNbxO12+x/2(BTNO, 0≤x≤0.01) and Bi4Ti3O12 + (1-z)BaTi1-xNbxO3+x/2+2.0mol%TiO2 (BTO+(1-z)BTNO, 0≤x≤0.01, 0≤z≤1.0), were prepared by mixed oxide method. The properties of the sapmples were characterized by XRD, AFM and electrical properties testing. The results showed that single BaBi4Ti4O15 was obtained when x=0.05 and z=0.5. The roomtemperature resistivity of BTO+(1-z)BTNO ceramic sintered in air was 102-1045·cm. The minimum resistivity occurred at Nb content of about 0.3mol%.

Abstract: The influence of liquid phase on grain growth of Ba0.998La0.002TiO3+xmol%TiO2 (x=0~5.0) ceramics sintered at 1350°C was investigated. The result showed that the liquid phase must present during grain growth; on the other hand, BaTiO3 grains must be dissolved, and then, precipitated from the liquid phase during the process of dissolution-precipitation. Otherwise, the grain growth was inhibited. The liquid phases of Ba6Ti17O4 and Ba2TiSi2O8 promoted grain growth due to high solution of BaTiO3 grains in the liquid phases. Ba2Ti2SiP2O13 liquid phase inhibited grain growth since BaTiO3 grains cannot dissolve into the phase, consequently the samples showed insulating behaviour.

Abstract: The ceramic samples of BaTiO3+½xLn2O3+2mol%TiO2 (0.001≤x≤0.01) were prepared, where Ln is Y, Dy, Ho, Er or Y. The influence of rare earth on the resistivity and microstructure of the samples was investigated by the means of XRD, SEM and electric properties testing. The results showed that the resistivity of La-doped sample decreases when x= 0.003 compared with the undoped sample. However, the range of dopant concentration with Dy, Ho, Er or Y for semiconducting samples is wider, especially for Y-doping. Minimum resistivity at room temperature was observed when x=0.003, which is named after the critical concentration. The experimental results indicated that below the critical concentration of Dy, Ho, Er or Y, the substitution took place in the barium sublattice with electronic compensation, and the rare earth ions began to substitute for titanium gradually above the critical concentration. For Ho-doped BaTiO3 ceramic, the resistivity jump (PTCR effect) near the Curie temperature was the highest in the case of x=0.003. Fine-grained structure of the sample doped with 0.6mol% Dy led to the increase of breakdown field strength and dielectric constant of ceramic samples.