Shiro Shimada

A thermoanalytical study on the oxidation of ZrC and HfC powders with formation of carbon

Abstract The oxidation of ZrC and HfC powders was thermoanalytically investigated by simultaneous thermogravimetry (TG), differential thermal analysis (DTA) and mass spectrometry (MS) at various oxygen pressures (PO2) between 0.5 and 40 kPa in the temperature range 20–1000 °C. TG results showed that the oxidation of ZrC and HfC begins at a fixed temperature of 380 and 400 °C, respectively, independent of PO2 and the type of sample. The degree of reaction, defined for the complete carbide–oxide transformation, reached a maximum of 103–160%, then gradually returning to 100% at higher temperatures. For the oxidation of ZrC, a sharp exothermic DTA peak appeared at PO2≥10 kPa with the corresponding CO2 evolution but broad DTA and CO2 evolution peaks occurred at PO2≤5 kPa. The oxidation of HfC gave two exothermic DTA peaks at all pressures, the higher temperature peak agreeing with the CO2 evolution. Oxidation exceeding 100%, related with the formation of carbon, is discussed from the results of TG, DTA, MS and X-ray analysis.

Oxidation of TiC at low temperatures

The isothermal oxidation of TiC powders was carried out at low temperatures of 350–500 °C at oxygen pressures of 3.9, 7.9 and 16 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electro-microbalance. The oxidation kinetics are described by the one-dimensional diffusion equation. It was found that oxidation consists of four steps, I (fast step), II (slow step), III (fast step) and IV (slow step), at all the pressures. Two activation energies were obtained in steps II–IV: 125–150 kJ mol−1 below about 420 °C and 42–71 kJ mol−1 above that temperature. The low- and high-temperature oxidation mechanisms are discussed in connection with the formation of oxycarbide/titanium suboxides and the crystallization of anatase, followed by the generation of cracks in the grains.

Interfacial reaction on oxidation of carbides with formation of carbon

Abstract The oxidation of carbides (ZrC, HfC, TiC) using single crystals with the 200 or 220 face was carried out at temperatures of 500–1500°C and oxygen partial pressures (PO2) of 0.02–80 kPa. The oxidation proceeded linearly with time. The preferred {200} or {220} orientation of c/t- or m-ZrO2, m-HfO2 and Ti3O5 was recognized on the oxidized surfaces of the crystals or near the reaction interface. The oxide scale consisted of inner dense dark scale (zone 1) and outer porous white/gray scale (zone 2). Zone 1 contained much carbon at around 23–25 at.%. Simultaneous TG–DTA–MS analysis of HfC powders showed that the oxidation of Hf component in HfC occurs to produce carbon at 380–600°C above which remaining carbon is oxidized with evolution of CO2. From HRTEM observation at the ZrC/zone 1 interface, it was found that c-ZrO2 crystallites are formed directly on the ZrC lattice while maintaining the above preferred orientation. The interfacial reaction with the formation of carbon on oxidation of the carbides was discussed from the above results.

Preparation and properties of antimony-doped SnO2 films by thermal decomposition of tin 2-ethylhexanoate

Transparent Sb-doped SnO2 films were prepared at 600° C on glass substrates by thermal decomposition of tin 2-ethylhexanoate and antimony tributoxide. The films 100 to 300 nm thick, which are composed of fine particles, were very smooth. The films showed no preferred orientation. The minimum resistivity (2.1×10−2Ω cm) was attained at a concentration of 8 at% Sb on the substrate precoated with SiO2. The transmission of these films was about 80% over a wavelength range from 0.4 to 2.0 μm.

Crystal growth of bismuth titanates and titanium oxide from melts in the system Bi2O3-V2O5-TiO2

Abstract Single crystals of Bi 4 Ti 3 O 12 , Bi 2 Ti 2 O 7 , Bi 2 Ti 4 O 11 and TiO 2 were grown from the melt with various compositions in the system Bi 2 O 3 -V 2 O 5 -TiO 2 . The crystal growth regions can be divided into three parts designated I, II, and III in the ternary diagram of the system: Bi 4 Ti 3 O 12 growth in the region I, Bi 2 Ti 4 O 11 in II, Bi 2 Ti 2 O 7 on the boundary between I and II and TiO 2 in region III. The V 2 O 5 content in the Bi 2 O 3 , V 2 O 5 and TiO 2 mixture was increased according to the sequence I → II → III. The increase in V 2 O 5 content reduced the Bi content in the grown crystal to 0% in the case of the Bi-free crystal of TiO 2 . It was concluded that the increase in V 2 O 5 content leads to complexing between bismuth oxide and vanadium oxide in the melt and thus reduces the Bi content in the grown crystal.

A thermoanalytical study of oxidation of TiC by simultaneous TGA-DTA-MS analysis

The oxidation of TiC powders was non-isothermally carried out by heating up to 900 °C at a rate of 5°C min−1 and at an oxygen pressure of 2–60 kPa, using a simultaneous thermogravimetric analysis-differential thermal analysis-mass spectroscopy (TGA-DTA-MS) apparatus. It was found that oxidation was divided into four stages, I to IV. Stage I, covering the oxidation fraction, α, 0–20%, was probably due to formation of oxycarbide (TiCxO1−x, 0 < x<0.5) with slight heat and CO2 evolution. Oxidation in stages II and III at 20–60% was affected by oxygen pressure; the higher pressure giving rise to very sharp exothermic and CO2 evolution peaks in stage II, which are correlated with the formation of anatase. Oxidation in stage III proceeded gradually with increasing rutile phase. The sample amount also exerted a similar effect on oxidation as the oxygen pressure.

Soft mode spectroscopy of bismuth titanate single crystals

Abstract The 948 K ferroelectric phase transition of Bi 4 Ti 3 O 12 single crystals has been studied by Raman scattering measurements in the temperature range between 293 and 958 K. The lowest mode at 28 cm −1 shows remarkable softening as the temperature increases from room temperature. The peak frequencies of bands above 100 cm −1 reveal no remarkable change.

On the adsorption of CO2 by molecular sieve carbons—Volumetric and gravimetric studies

Abstract Two microporous carbons, derived from the same phenolic resin at 700 and 1000 °C, were investigated by volumetric and gravimetric adsorption of CO2 and N2 at different temperatures. The combination with immersion calorimetry into liquids of different molecular sizes reveals the presence of constrictions near 0.4–0.45 nm leading into relatively large micropores. The size of the constrictions and the micropore volume are smaller for the solid prepared at 1000 °C.

Crystallization of amorphous bismuth titanate

We studied the transformation of amorphous bismuth titanate by heat treatments. After an as-quenched amorphous sample was annealed at 500 °C, the lowest Raman peak became intense like a boson peak for glass. This fact indicates the formation of intermediate range order. The medium range correlation length of 7 nm calculated from the boson peak frequency is in agreement with the mean cluster size measured by atomic force microscopy (AFM). The differential thermal analysis (DTA) shows the two-step crystallization at 608 and 830 °C on heating. It suggests the existence of a metastable state. The samples annealed at 770 and 1000 °C are identified as the pyroclore and the layered perovskite structures, respectively. It strongly suggests that at first a three-dimensional (3D) crystalline state appears by nucleation process from a 3D amorphous state and secondly it transforms into the pseudo-2D layered perovskite structure.

Fabrication and Electrical Properties of Bi4Ti3O12 Ceramics by Spark Plasma Sintering

Spark plasma sintering (SPS) was applied to the production of non-lead ferroelectric Bi4Ti3O12 ceramics. A powder compact of a calcined oxide mixture was embedded in a mixture of ZrO2 and Sb-doped SnO2 powders in a carbon die and heated for a very short duration under a constant current of 450 A. This process led to the rapid densification of the sample with the simultaneous elongation and coarsening of grains in the sintered body. When a coprecipitated precursor was used as the starting powder, a grain-oriented sample was readily obtained by the SPS method. A strong anisotropy in electrical properties was observed for the grain-oriented Bi4Ti3O12 ceramics.