The thermoelectric properties of the double perovskite-type oxides Sr_2−xK_xFeMoO₆ were investigated in terms of K-doping at the A site of the oxides. The electrical conductivity, σ, of the oxides showed a metallic behavior, decreasing monotonically from ca. 10³ S/cm at room temperature to ca. 10² S/cm at 1250 K. The Seebeck coefficient, S, showed that the oxides are of n-type over the measured temperature range. The absolute value of S increased with increasing temperature. The absolute value of S at 300 K increased up to x = 0.2 and then decreased dramatically for x = 0.3–0.4. The Rietveld refinement of the XRD patterns of the oxides suggested that the increase in orbital degeneracy due to the eased distortion of FeO₆ and MoO₆ octahedra at x = 0.2 was responsible for the |S| maximum at this composition. The power factor, S²σ, of the oxides increased sharply with increasing K-doping level; the value for Sr_1.6K_0.4FeMoO₆ was ca. 4.2 × 10⁻⁴ W/mK², which is the highest among all the samples in this study. The thermal conductivity, κ, of the oxides generally decreased from ca. 3–5 W/mK at room temperature to ca. 2–4 W/mK at 1100 K. Because the power factor of the oxides increased above 900 K, the dimensionless figure of merit, ZT = S²σT/κ, increased dramatically above 900 K. The largest ZT value of 0.24 was observed for Sr_1.6K_0.4FeMoO₆ at 1250 K.

View full abstract

The effect of hetero-template grains on texture development in Bi₄Ti₃O₁₂ (BiT) was examined. The hetero-template was SrBi₈Ti₇O₂₇ (S_0.5BT) grains with a large platelike shape. Texture developed in pure BiT made by solid-state reaction without template grains. The origin of texture development was attributed to the growth of small platelike grains with an aspect ratio of approximately 3 (homo-template) present in the BiT powder. The hetero-template grains added to the BiT powder promoted texture development, and the degree of the promotion was dependent on the amount of hetero-template. When the concentration of hetero-template was 1 vol %, the hetero-template and homo-template acted as the origin of texture development. When the concentration was increased to 5 vol %, the orientation of grains was almost only caused by the hetero-template. When the concentration was further increased to 10 vol %, texture did not develop. SrTiO₃ in the hetero-template dissolved in the BiT matrix grains, thus reducing the grain growth rate. These results indicate that grain growth is essential for texture development in BiT.

View full abstract

Precursor films based on poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and P(VDF-TrFE) blended with Pb(Zr,Ti)O₃ (PZT) were spin-coated on Si substrates and subsequently annealed at 150, 170, or 190°C. X-ray diffraction studies showed that the crystallization from the amorphous precursor films to the γ phase starts at higher annealing temperatures without involving the formation of other polymorphs when the P(VDF-TrFE) is blended with PZT, and the PZT content increases, resulting in an amorphous phase and/or crystalline γ phase. Nevertheless, a larger memory window width and much higher accumulation capacitance are induced by the blended PZT within the low operating voltage ranges from −0.5 to 2.0 V and from −2.0 to 6.0 V for 76.7 and 96.7 wt % PZT blending, respectively. Furthermore, these improvements in the hysteretic characteristics in the capacitance–voltage measurements are also directly correlated with the degree of P(VDF-TrFE) crystallization and the presence of PZT. This approach enables viable routes toward the commercialization of nonvolatile ferroelectric memory devices and their market extension to potential applications as functional devices.

View full abstract

Zirconium oxide fiber was prepared by electrospinning technique and sol–gel processing from zirconium acetate and poly(methyl methacryate) (PMMA) as precursor. Calcination of these structures between temperature of 450 and 1000°C resulted in the formation of zirconium oxide fiber whose diameters ranged from 1000 to 500 nm. The samples of calcination at 450 and 500°C were observed of only in the tetragonal phase. The tetragonal phase and monoclinic mixed phase appeared between 600 and 900°C. After calcination at 1000°C, only the monoclinic phase was identified in the XRD patterns. Grain size increased with increasing calcinations temperature. Pure monoclinic phase with grain size of 25.70 nm was obtained from the calcination temperature of 1000°C. The activation energy for grain growth of electrospun zirconium oxide fiber was estimated to be 20.48 kJ/mol.

View full abstract

The influence of B₂O₃ additives on the sintering behavior, microstructure, and dielectric properties of Li₂MgSiO₄ (LMS) ceramics was investigated. The experimental results showed that doping of LMS ceramics with 2 wt % B₂O₃ caused a decrease in the sintering temperature from 1250 to 810°C. The ceramics produced were dense with a well-crystallized microstructure where predominantly fine grains of Li₂MgSiO₄ were formed. The dielectric properties appeared to be dependent on microstructure. The measured dielectric values of 2% B₂O₃-doped LMS ceramics (ε_r = 6.61 and tan δ = 1.6 × 10⁻⁴ at 1 MHz and ε_r = 5.84 and tan δ = 7.43 × 10⁻⁵ at 8 GHz) indicate their potential for use in applications which require the involvement of low temperature co-fired ceramics (LTCC).

View full abstract

Au doped silica glass was fabricated by use of SiO₂–PVA nanocomposite. Porous inorganic–organic nanocomposite, which has nanometer sized pores, was prepared from fumed silica and poly(vinyl alcohol) (PVA). HAuCl₄ solution impregnated the porous SiO₂–PVA nanocomposite, and then Au ion was reduced and Au metal nanoparticles were formed by heat treatment. After combustion of PVA, SiO₂ nanoparticles were sintered and Au particle growth were occurred above 800°C, and then bulk Au nanoparticles doped silica glasses were obtained by sintering at 1100°C for 6 h in air. Au particle size and concentration in the silica glass are increased with the concentration of the HAuCl₄ solution. Au doped bulk silica glass was obtained without gold escape from the surface.

View full abstract

Porous SiC ceramics were prepared at 1900–2000°C without holding time at 50–200°C/min under a pressure of 15 MPa in Ar with Al₂O₃, Y₂O₃, and AlN additives by spark plasma sintering. The effects of heating rate, heating temperature, and sintering additive on the porosity, pore size, microstructure, and flexural strength of the porous SiC ceramics were investigated. As the heating rate increased, the size of SiC grains in the samples decreased, the porosity increased, and the flexural strength decreased. The strength of the samples increased with an increase in heating temperature. The growth of SiC grains was suppressed in the sample prepared at 1950°C, and the sample had both high porosity (42%) and high strength (95 MPa). In the samples prepared with AlN–Y₂O₃ and AlN–Al₂O₃ additives at 1950°C the porosities were 32–39%. The addition of AlN to Y₂O₃ and Al₂O₃ additives resulted in the inhibition of grain growth and a decrease in the pore size of the samples. The maximum strength was 167 MPa in the prepared porous SiC ceramics.

View full abstract

To improve the oxidation resistance of Cu nanoparticles chemically, silica-coated Cu nanoparticles were obtained by a multi-step process. First, Cu nanoparticles were produced by reducing copper inorganic salts in the presence of citrate acid in aqueous solution. Then these particles were undergone an in-situ surface modification treatment with 3-aminopropyl-trimethoxysilane (APS). And then they were coated subsequently with a silica layer by adding tetraethyl orthosilicate (TEOS) ethanol solution into the above solution. The molar ratio of citrate: Cu²⁺: hydrazine is 1:1:10, which can guarantee the basic environment of aqueous solution, in which the preparation of Cu particles and the latter coating of silica can be carried out simultaneously and controllably. The stability of Cu particles through coating was improved. Citrate acid and APS had essential effects on the antioxidation of Cu nanoparticles during their synthesis and coating with silica, respectively.

View full abstract

Four types of commercially available SiC powders were hot-pressed with 1 wt % Dy₂O₃–AlN additives in a 3:2 molar ratio. Three submicron-sized SiC powders could be densified so as to have a relative density higher than 98%, whereas a micron-sized SiC powder showed limited densification up to 93.5%. SiC ceramics fabricated from β-SiC powders exhibited electrical resistivities (∼10⁻¹ Ω·cm) that were lower than those from α-SiC powders (∼10¹ Ω·cm). The lower electrical resistivity of the SiC ceramics fabricated from β-SiC powders could be attributed to a higher carrier density and enhanced charge mobility when compared to the SiC ceramics from α-SiC powders.

View full abstract

Low-frequency Raman scattering and TEM observations were conducted in 1Li₂O–2SiO₂ glass subjected to annealing at maximum homogeneous nucleation temperature (∼460°C) to elucidate the annealing effect on Boson-peak behavior and its relation to the structural evolution in nanoscale. It is suggested that the 1Li₂O–2SiO₂ glass possesses a successive medium-range ordering through phase separation, and the Boson-peak frequency was largely affected by the ordering/phase-separation.

View full abstract

Single-crystalline CdWO₄ nanorods and nanowires have been controllably synthesized via a simple surfactant-free hydrothermal route. The obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction (ED). The results showed that the starting material played a crucial role in the controllable preparation of CdWO₄ nanorods and nanowires. The synthesis mechanism was also discussed.

View full abstract

An easy processing route has been developed for a nano-sized crystalline tungsten mono-carbide (WC) material. Starting materials for WC synthesis were made by simply mixing tungstic acid (H₂WO₄) and polyacrylonitrile (PAN) powders. The starting powders were applied to heat treatment at 1200°C for 30 min with an argon gas flow in an alumina tube. Tungsten mono-carbide has been successfully synthesized in PAN/H₂WO₄ weight ratios of more than 0.6. The acquired WC powder had a nano-sized crystalline and high-specific surface area. Crystalline size and specific surface area were estimated to approximate 50 nm and 17.2 m² g^–1, respectively.

View full abstract