Hajime Kiyono

Cation Substitution for BaTiO3 Phase in Low-Fired Glass-Ceramics for Tunable Microwave Applications

Cation substitution for the Ba2+ and Ti4+ sites in the BaTiO3 structure was attempted in low-fired glass-ceramics derived from glass powders that have perovskite phase compositions corresponding to (BaTiO3:REAlO3) = 9:1 and 8:2 (RE=La, Nd and Sm) to control the Curie temperature (Tc) of the ferroelectric glass-ceramics. The La3+-substituted (9:1) sample heated at 950°C for 24 h showed a broadened er–temperature relationship with substantial lowering in Tc to ~-55°C. The Ba:La ratio in the perovskite particles was estimated to be in the range of 4:1–6.7:1, whereas very little Al3+ was incorporated into the same particles. It was found that the Tc of the glass-ceramics fabricated in our serial study could be adjusted by A-site cation substitution in BaTiO3. Tailored glass-ceramic samples with room temperature er of 200–300 and Tc of around 0°C had good microwave tunability of 30–40% under dc bias voltage of 10–50 kV/cm. The high tuning sensitivity of the low-fired glass-ceramics was confirmed by comparative examination of the dielectric properties of well-crystallized ceramics and thin films.

Synthesis and Dielectric-Magnetic Properties of Rare-Earth (La, Nd, Sm)-Modified Bi4Ti3O12

Rare-earth (RE = La, Nd, Sm)-substitution into BiT (Bi4-XREXTi3O12) was conducted in a composition range of 0 ≤X ≤3.0. BiT powders, into which fixed amounts of RE cations were incorporated, were directly synthesized by low-temperature calcination of coprecipitated precursors. Complete substitution with RE was achieved for samples up to X = 2.5 at 1100 °C. Dielectric permittivity (er)–temperature curves of the La-2.0, Nd-2.0, and Sm-2.0 ceramics showed er maxima in a frequency-dependent manner. The er maxima were located at around 540–560 °C (at 1 kHz), which corresponded to the transition from the ferroelectric to paraelectric phase. Increasing Nd-substitution from X = 2.0 to 2.5 resulted in a much suppressed er maximum at 200–300 °C, which may be due to substantial reduction in the structural distortion giving rise to ferroelectricity of the BiT-based phase. The appearance and enhancement of magnetization was found for the Nd- and Sm-substituted BiT samples. They revealed specified magnetization–magnetic field curves with a small hysteresis loop, suggesting the presence of a weak canted antiferromagnetic interaction.

TEM observation of oxide scale formed on TiC single crystals with different faces

Abstract Cross-sectional observation of the oxide scale formed by oxidation of TiC single crystals with (100), (110) and (111) faces at 1500 °C for 30 min in a mixed gas of Ar/O 2 ( P O2 =0.08 kPa) was performed by transmission electron microscope (TEM). The oxide scale was composed of outer (zone 2) and inner (zone 1) subscales. TEM and selected area electron diffraction combined with X-ray energy dispersion analysis showed that the zone 2 consists of rutile and pores and the zone 1 of carbon and titanium oxide, identified as Ti 3 O 5 in the oxide scale formed on the (110) face. Zone 1 formed on the (100) and (111) faces showed crisscross patterns, in contrast to the (110) with the wavy lamellar pattern.

Microstructure and properties of various fluorine-containing SiAlON ceramics synthesized by HIPing

Abstract An attempt was made to prepare various F-doped β-, O-, X-, and α-SiAlONs from a mixture of Si 3 N 4 , SiO 2 , Al 2 O 3 , AlN, or Y 2 O 3 using AlF 3 or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z , x , and m/n values determined for β-, O-, and α-SiAlONs by X-ray diffraction. When AlF 3 was used, a single phase ceramic (O-SiAlON) was produced from a mixture of α-Si 3 N 4 and SiO 2 at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of α-Si 3 N 4 , AlN, and Y 2 O 3 with AlF 3 produced β-/Y-α-SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of α-Si 3 N 4 and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from α-Si 3 N 4 and SiO 2 at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y-α-SiAlON mixture showed the growth of O- and β-SiAlON and Y-α-SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.

Structural study of methyl isonicotinate by gas phase electron diffraction combined with ab initio calculations

Abstract The molecular structure of methyl isonicotinate was studied by gas phase electron diffraction combined with ab initio calculations. The molecular skeleton was assumed to be planar. The determined values of principal structure parameters ( r g and ∠ α ) are as follows: r( NC ) = 1.343(5) A , r( C…C ) ring = 1.401(3) A , r( C γ  C ) = 1.499(9) A , r( C  O ) = 1.205(5) A , r( C ( O ) O ) = 1.331(8) A , r( OC Me ) = 1.430(8) A , 〈r( CH )〉 = 1.103(10) A , ∠CNC = 117.6(9)°, ∠C β C γ C β = 118.7(9)°, ∠C β ,trans C γ C(O) = 118.6(12), ∠C γ CO = 121.4(12)°, ∠C γ CO = 114.2(10)°, ∠COC = 115.4(15)°, where angled brackets denote average values and C γ,trans denotes the carbon atom which is trans to the carbonyl oxygen atom. Values in parentheses are the estimated limits of error (3σ) referring to the last significant digit. The structure of the ring in methyl isonicotinate agrees with that of pyridine within experimental error. In contrast, the structure parameters of the COOCH 3 group are significantly different from those of methyl acrylate and methyl acetate. These differences have been discussed in terms of hyperconjugation and steric effects.

Highly ordered boron nitride nanotube arrays with controllable texture from ammonia borane by template-aided vapor-phase pyrolysis

An efficient approach for the preparation of good-quality boron nitride nanotubes (BNNTs) is developed. BNNTs with specific texture were prepared from ammonia borane (BH3NH3) by vapor-phase pyrolysis with the aid of a template in two independent temperature-controlled furnaces. Two kinds of BNNTs, 200-300nm wide ×60 µm long and 70-80nm wide ×40 µm long, were produced after removal of the templates. The as-produced BNNTs were heated at different temperatures in the range of 1300-1700°C in NH3. FT-IR and XPS results confirmed the formation of BN from BH3NH3. Ordered arrays of BNNTs without cracks on the surface were seen using microstructural observations. The diameter and length of the BNNTs are controlled using templates with different pore sizes and thickness. The wall thickness of the nanotubes was increased by increasing the number of deposition cycles. The crystallinity of the BNNTs was improved by heating at a high temperature (1700°C) in NH3.

Thermal Oxidation of Sintered Beta-Sialon (z=3) Ceramics in Atmospheres with Water Vapor

Thermal oxidation of β-sialon (Si6-zAlzOzN8-z, z = 3) ceramic was investigated at 1300 to 1600 ̊C in dry (Ar/O2 = 90/10 kPa), wet O2 (Ar/O2/H2O = 80/10/10 kPa) and wet (Ar/H2O = 90/10 kPa) atmospheres. Oxidation was followed from weight gains using thermogravimetry and the oxidized sample was characterized by scanning electron microscope, electron probe microanalysis and X-ray powder diffraction. Oxidation kinetics in the three atmospheres were represented by a parabolic rate model except for at 1600 ̊C. The wet and wet O2 oxidation rates were greater than the dry oxidation, the degree decreasing with rising temperature from 1300 to 1500 ̊C, while wet and wet O2 oxidation at 1600 ̊C was even faster again than the dry oxidation. It was found that the scale formed by the dry, wet and wet O2 oxidation consisted of mullite and cristobalite at 1300 1500 ̊C and of mullite and SiO2 rich aluminosilicate glass at 1600 ̊C. Introduction β-sialon is expected to be useful for engineering ceramics such as cutting tools, refractory materials and metal-forming tools, because of their high hardness, low thermal expansion, high strength, and high chemical stability. [1, 2] In high temperature conditions, oxidation becomes important for their practical uses. Water vapor has been reported to accelerate oxidation of Si-based ceramics, such as SiC, Si3N4. [3, 4] Oxidation of sintered β-sialon ceramics in dry atmosphere has been reported in detail by Perrson et al. [5] However, there is a very few reports of oxidation in atmosphere containing water vapor. [6, 7] The present work describes the effect of water vapor on oxidation of sintered β-sialon (z = 3) ceramics at 1300 to 1600 ̊C from viewpoints of the kinetics and microstructural observation of oxidized samples. Experimental Sintered β-sialon was fabricated by hot pressing of β-sialon (z = 3) powder without additive at 1850 ̊C and 24 MPa for 2 h in N2 atmosphere. The relative density of the sintered body was about 95%. The β-sialon powder was commercially purchased (Ube Industry); Purity of the powder was not reported by the supplier. X-ray diffraction pattern (XRD) of the β-sialon powder and sintered body showed the major peaks of β-sialon with a trace of 15R sialon. The polished sample of about 15 × 5 × 1 mm was oxidized in a flowing mixture of Ar/O2 (90/10 kPa, dry), Ar/H2O (90/10 kPa, wet) and Ar/O2/H2O (80/10/10 kPa, wet O2) at a temperature range of 1300 1600 ̊C for up to 100 h. The sample was hanged with an alumina wire in a fused quartz tube (at 1300 ̊ 1500 ̊C) or in an alumina tube (at 1600 ̊C). The weight change during oxidation was monitored by using an electronic microbalance (CAHN 2000). After the TG experiments, the oxidized sample was characterized by XRD, scanning electron microscope with energy dispersion analysis unit Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 893-896 doi:10.4028/www.scientific.net/KEM.264-268.893

High Temperature Oxidation of SiC Powder in Oxidizing Atmosphere Containing Water Vapor

Oxidation of SiC powder was studied at 1373 K to 1873 K in Ar-O2, Ar-H2O, and Ar-O2-H2O using thermogravimetry. At 1373 K to 1573K, the weight gain increased with increasing water vapor pressure. The oxidation rate was evaluated on the basis of the Ginstering-Brounshtein kinetic model. In this temperature region, the apparent activation energy for the oxidation was almost the same (139-191 kJmol-1) independent of the atmosphere, suggesting that the same oxidation process proceeds. On the other hand, at temperatures >1673 K, the weight gain in the dry O2 (Ar-O2) was greater than that in the wet and wet O2 (Ar-H2O and Ar-O2-H2O). The apparent activation energy in the dry O2 (442 kJmol-1) was much greater than that in the wet and wet O2. We propose that water molecule diffused in silica layer in the wet and wet O2 atmosphere at 1373 K to 1873 K.

Preparation and Corrosion of Mullite Thin Film on ß-SiAlON Ceramics

Mullite film was formed on ß-SiAlON (Si6-zAlzOzN8-z, z = 3) ceramics by reaction of Al2O3 films with silica formed on oxidation to improve a high temperature water vapor corrosion resistance. Sintered ß-SiAlON was fabricated by hot pressing of ß-sialon (z = 3) powder without additive at 1900°C and at 24 MPa for 4 h in N2. The Al2O3 film was deposited on polished ß-SiAlON ceramics by a dip coating process of Al2O3 precursor sols prepared from the alkoxide solution and aluminum sol. The Al2O3 coated sample was heated in Ar/O2 (95/5 vol.%) at 1300°C for 2 h, resulting in the formation of mullite by reaction with silica produced from sialon. The resulting mullite-coated sample was corroded in H2O/Ar/O2 (90/8/2 vol.%) at 1200°C for 2 ~ 100 h. The corroded samples were characterized by XRD and SEM-EDS for the evaluation of the corrosion resistance of mullite-coated sialon.

Dry and Wet Oxidation of Y-α-SiAlON Powder at 1000° - 1300°C

Oxidation of Y-α-sialon powder was investigated in and Ar/O 2 (dry) and Ar/O2/H2O (wet) atmospheres at the temperature range 1000° ~ 1300°C . Oxidation products were identified by X-ray powder diffraction, Si magic angle spinning nuclear magnetic resonance and transmission electron microscope. The weight change by isothermal oxidation was followed by thermogravimetry. Oxidation product was composed of mu llite, y-Y2Si2O7 and amorphous SiO 2 in both atmospheres. Amorphous SiO 2 is crystallized to cristobalite at 1300°C in the we t atmosphere. The weight changes were represented by the firs order reaction equation. The rate constants for oxidation were greater 4 ~ 10 times in th e wet atmosphere than in the dry atmosphere.