Key Engineering Materials Vols. 336-338

Abstract: Ni0.5Zn0.5Fe2O4-BaTiO3 composites were prepared by calcining Ni0.5Zn0.5Fe2O4 with BaTiO3 at 1220°C for 3 h. Both μ′ and μ″ of the composites decrease rapidly, but the cut-off frequency shifts to higher level with increasing BaTiO3 content. The dielectric constant of the composites increases quickly with increasing BaTiO3 content below 50% and keeps a high and nearly unchangeable value with increasing BaTiO3 above 50%. The composites with BaTiO3 content between 20% and 40% are suitable to use for the magneto-electric devices under high applied frequency due to its high permeability and low dielectric loss.

Abstract: In this paper, three types of Al2O3 powders with different sizes (1.6μm, 0.7μm, and 0.3μm) were used in a LTCC system of Al2O3 and borosilicate glass (BSG). Al2O3 and BSG are selected on account of their coefficient of thermal expansion (CTE) similar to silicon, which can reduce thermal stress. It was found that the dielectric constant, ε, of substrates drops with the decrease of Al2O3 powder size. ε increases with the increase of Al2O3 content for the sample with 1.6 μm Al2O3 powder, but decreases with the increase of Al2O3 content for the samples with 0.7 μm and 0.3 μm Al2O3 powders. SEM observations revealed that more pores with low permittivity appeared for the samples with smaller Al2O3 powders, resulting in the drop of dielectric constant of the sample finally.

Abstract: BN-AlN-TiB2 compound conductive ceramics from powder mixtures of BN, Al, and TiB2 was fabricated by self-propagating high temperature synthesis (SHS) and hot isostatic pressing (HIP). The powder mixtures were shaped by isostatic cool pressing at 5-10MPa and the combustion reaction was carried at 100-200 MPa N2 by an ignitor. XRD experiments confirmed that the reaction was complete and only AlN, BN and TiB2 were detected. Optical microscopy as well as SEM with an electron probe microanalysis was used for microstructural analysis and revealed a relatively uniform distribution of particulates. The temperature-dependence and composition-dependence of the electrical resistivity of BN-AlN-TiB2 ceramics were studied. The results showed that the optimum composition was 5-10wt% BN, 30-55wt% Al and 60-40wt% TiB2, and the products had the density of 90% of the theoretical, resistivity of 80-1000 μ⋅cm and bending strength of 100-200 MPa.

Abstract: High thermal conductivity, along with good electrical and mechanical properties, makes aluminum nitride a very promising material for electronic packaging and substrates. Much work has been reported on the influence of additions and sintering aids on densification and thermal conductivity. In this paper, the influence of raw materials on the sintering behavior and thermal conductivity characteristics of pressureless-sintered AlN has been investigated. The highest thermal conductivity is 248W/m.K.

Abstract: The electrical properties and microstructures of SrTiO3 based voltage-sensing and dielectric dual-functional ceramics with nanometer donor and acceptor additives were studied. The La2O3 nanopowders and MnO-SiO2-Al2O3 (or CuO-SiO2-Al2O3) nano-composite powders were incorporated into SrTiO3 as donor, acceptor and liquid-phase sintering aids. Then semiconducting SrTiO3-based ceramics were sintered at 1360-1440oC for 2 h in a reducing atmosphere. The effects of the nanometer donor and acceptor additives and the sintering temperature on the electrical properties and microstructures of materials were discussed. The results showed that SrTiO3-based varistor ceramics with 1.1 mol% La2O3 and 0.1 mol% MnO nano-additives sintered at 1360-1420oC in graphite and N2 reducing atmosphere have excellent voltage-sensing and dielectric characteristics. The varistor voltage ranges from 2.3 to 5.3 V/mm, the nonlinear coefficient from 3.0 to 3.8, and the dielectric constant from 215,600 to 413,000.

Abstract: In this study, hexagonal Boron Nitride-Aluminium Nitride composites were prepared by spark plasma sintering at 1700°C
for 4 min under the pressure of 25 MPa, varying the amount of BN from 0 to 30 wt.%. The dielectric constants and losses of the samples were measured from 100 Hz to 1 MHz at room temperature, and the results show that with the increment of BN content, the dielectric constants and losses of AlN-BN composites decrease. When the BN content is 20 wt.%, the dielectric constant and loss of the composites at 1 MHz are 6.18, 2.1 × 10-3 respectively.

Abstract: Ni-Si-O thin films on Si substrate have been prepared by a sol-gel method. The microstructure and phase composition of the films were investigated by XRD, SEM, FTIR. The XRD results showed that the films are amorphous while annealed at 800oC for 10 min, the SEM imagines proved that the films are smooth and thickness is about 190 nm. Electrical property of the film indicated that the leakage current was as low as 10-6A/cm2 at an electric field of 1MV/cm. The dielectric properties of Ni-Si-O thin films can be improved as increasing the annealed temperature.

Abstract: The K, Ca, Sr and Ce doped and non-doped γ-NaxCo2O4 samples were prepared by the solid-state reaction method. Results show that the Na-site doping might lead to an increase in the Seebeck coefficient and a decrease in the electrical conductivity of the samples. The maximum power factors of the K, Ca and Sr doped samples are respectively 2.04, 1.93 and 1.9 W·m-1·K-2, corresponding to an increase by 58%, 50% and 47% compared with that of the non-doped samples. Thus, the Na-site doping can improve the thermoelectric properties of γ-NaxCo2O4 oxides.

Abstract: [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 polycrystalline samples were prepared using sol-gel method followed by spark plasma sintering. Thermoelectric properties of the samples were examined from room temperature to 1000K. The temperature dependence of electrical conductivity shows that the hole hopping conduction mechanism is dominant for the samples. The activation energy of hopping conduction is 0.11 eV. At the temperature of about 1000K, the electrical resistivity is 7.1m
cm, the thermopower is 160 μVK-1, the thermal conductivtity is 2.94Wm-1K-1 and dimensionless figure of merit reaches 0.12. These results indicate [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 is a potential material for high temperature thermoelectric energy conversion.

Abstract: Nb-content dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on (100)-face of LaAlO3 single crystal substrates by a pulsed-laser deposition. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT reached 0.37 (20%-Nb-doped SrTiO3 epitaxial film) at 1000 K, which is the largest value among n-type oxide semiconductors ever reported.