Kazunori Kijima

Optical Band Gap of Barium Titanate Nanoparticles Prepared by RF-plasma Chemical Vapor Deposition

The size dependence of the band gap for barium titanate nanoparticles prepared by RF-plasma chemical vapor deposition was investigated by measuring the diffuse reflectance spectra. The band gap of the indirect transition gradually increased, as the particle sizes decreased to less than 11.5 nm. Consequently, the band gap for particles of 6.7 nm in size was measured to be 3.47 eV, which was approximately 0.25 eV larger than that of BaTiO3 bulk. This phenomenon was discussed on the basis of quantum confinement model.

Sintering of ultrafine SiC powders prepared by plasma CVD

Ultrafine SiC powders with a nanometre particle size were synthesized by r.f. plasma chemical vapour deposition (CVD) using a chemical system of SiH4−C2H4−Ar. The powder was also ultrapure with a grade of 99.999% purity. The product was polytype 3C−SiC and black in colour, in spite of its high purity, because of its ultrafine size. Silicon carbide is a difficult ceramic to sinter; it is possible to sinter it to full density with the aid of sintering additives. Ultrafine and ultrapure SiC powders were hot-pressed without sintering additives in the present study, in order to investigate the sintering behaviour. The CVD powders proved sinterable to 88% theoretical density without sintering additives. The present experiments revealed that powder treatment before firing was a key technology when using ultrafine powders as starting materials in the sintering process. The sintering behaviour of the powder was characterized by a large shrinkage. Phase transformation was negligible after hot pressing at 2200°C for 30 min.

Dielectric properties of BaTiO3 films prepared by RF-plasma chemical vapor deposition

The size dependence of the dielectric properties of BaTiO3 nano-grains was investigated using films prepared by RF-plasma chemical vapor deposition. BaTiO3 nanoparticles were directly deposited on Pt/Al2O3/SiO2/Si substrates; then they were annealed at temperatures between 600 and 900°C in order to control the grain size. The dielectric constants of BaTiO3 nano-grains smaller than 30 nm were measured to be 200–300, and reached approximately 1000 for grain sizes of 50–60 nm. According to capacitance–voltage (C–V) curves and hysteresis loops, the phase transformation from paraelectric to ferroelectric occurred at a grain size of approximately 40 nm. The dielectric constants of the films were considerably stable against the temperatures ranging from 5 to 160°C, even though the dielectric constants were approximately 1000. The breakdown electric fields of the films increased with decreasing grain sizes and a value of 649 kV/cm was achieved at a grain size of 27 nm.

High Energy-Resolution Electron Energy-Loss Spectroscopy Study of Electronic Structures of Barium Titanate Nanocrystals

High energy-resolution electron energy-loss spectra were obtained from barium titanate (BaTiO3) nanocrystals (BTNCs), which were synthesized by chemical vapor deposition using inductively coupled plasma. Onset energies of spectral intensities were 3.2 eV for 34 nm BTNCs and 3.5 eV for 6 nm BTNCs. This indicates an increase in the bandgap energy of BaTiO3 with a decrease in crystal sizes. Those onset energies obtained from 90 nm specimen areas showed an excellent agreement with those estimated by previously reported optical measurements. Volume plasmon peaks were observed at 26.5 eV for 34 nm BTNCs and 25 eV for 6 nm BTNCs. Dielectric functions of the BTNCs were derived from loss-functions by Kramers–Kronig analysis using the refractivity of bulk BaTiO3 (=2.4). The peaks of O 2p→Ti 3d (t2g) transitions in the imaginary part of dielectric function (e2) were observed at 3.3 eV for 34 nm BTNCs and 3.8 eV for 6 nm BTNCs. Those of O 2p→Ti 3d (eg) transitions were at 7.8 eV for 34 nm BTNCs and 6.6 eV for 6 nm BTNCs.

Thermal Change of Unstable Stacking Faults in β-SiC

The thermal change of unstable stacking faults (USFs) in ultrafine and highly pure β-SiC powder synthesized by the plasma chemical vapor deposition (plasma-CVD) method was studied by high-resolution transmission electron microscopy (HR-TEM). The number of USFs, which were frequently observed in as-synthesized powder, decreased at elevated temperatures in an Ar atmosphere. In contrast, the number of general stacking faults (GSFs) increased with decreasing number of USFs. Moreover, a type of stacking fault that was geometrically more unstable than USF was observed in the specimen after heat treatment. In the present study, the annihilation process of USFs observed in the specimen before heat treatment is discussed.

A New Type of Stacking Fault in β-SiC

A new type of stacking fault was found in ultrapure and ultrafine β-SiC in images obtained by high-resolution transmission electron microscopy (HR-TEM). The β-SiC particles were prepared by the plasma chemical vapor deposition (plasma-CVD) method and quenched rapidly from the gas phase. This type of stacking fault disappeared in the specimen heated at 1850°C for 10 min in Ar atmosphere. The proposed stacking fault was examined from the viewpoints of energy and synthetic condition.

Comparison of .ALPHA.-Alumina Crystallization Behavior in the Solution Method between with and without the Addition of Citric Acid.

アルミナ前駆体繊維を得るための溶液系(1)NCS溶液:N(OPri)3-A1(NO3)3-クエソ酸-H20系(A1(OPri)3mol/A1(NO3)3mo1=2,H2Omol/total AImo1=50,total A1 mol /クエン酸mol=10),(2)NS溶液:A1(OPri)3-N(NO3)3-H2O系(A1(OPri)3mol/A1(NO3)3mol=2,H2Omol/total A1 mol=50),に関し,クエン酸の有無が,溶液構造に如何なる影響を与えるのかを,小角X線散乱測定より検討した.これより,NCS溶液及びNS溶液何れの溶液の場合も濃縮に伴い慣性半径は低下したが,その割合はクエン酸を含むNCS溶液の方がクエン酸を含まないNS溶液より大きく,紡糸可能濃度溶液における慣性半径値は前者が50Å,後者が100Å であった.これより,A1(NO3)3に対するA1(OPri)3の量が同一であっても,系にクエン酸を添加した場合と添加しなかった場合で,溶液を構成していると考えられるA1のクラスター構造が異なると共に,その後の濃縮に対する溶液構造の変化に対しても異なった挙動を示すことが明らかとなった.これらの溶液より得られたゲル試料を焼成した場合のα-アルミナへの結晶化速度を定量的X線回折測定で検討したところ,950,1000℃ 及び1050℃ で恒温処理した場合の誘導期間は,NCS試料の方がNS試料より短いことが示された.また,誘導期間を差し引いた時間と転換率との間で成り立つAvrami式より得られる反応速度定数と1/T(処理絶対温度)とのArrheniusプロットより得られた活性化エネルギーは,NCS試料の方がNS試料より若干大きかった.これらは何れも,クエン酸の有無による炭素量における違い,及び溶液構造における違いを反映していると考えられる乾燥ゲル試料での構造の違いが原因していると考えられた.また,Avrami指数は何れの試料の場合も1.6であり,溶液構造を反映すると考えられる試料における構造の違いは核形成一成長の様式には影響しないことがわかった.

Temperature measurement of Y2O3 doped ALN during plasma sintering

The temperature (Ts) of Y2O3 doped AlN during plasma sintering was measured using a (W-5%Re)-(W-26%Re) thermocouple buried in the specimen. Ts below 1373 K could be measured directly in-situ using a low pass filter. Ts above 1373 K could not be measured in-situ due to interference of plasma noise. Hence, Ts was measured by extrapolation of a cooling curve after extinguishment of plasma. The thermal conductivity and emissivity of the specimen could be also estimated from simulation of the cooling curves. It was found that rapid re-heating and re-densification after AlN evaporation occurred by the increase in heating from plasma.

Preparation of ceramic fiber by precursor method employing polyfunctional hydroxy acid

The continuous ceramic fiber such as Na 1-x Ti 2+x Al 5-x O 12, YBa 2 Cu 3 O y and alumina fibers were made by a modified method which is composed of metal ion-polyfunctional hydroxy acid-H 2 O system. The spinnable solution could be obtained easily when the solution, in which metal elements were adjusted to the aimed composition, was concentrated. It was indicated to be important in this method to take into consideration the cluster structure of metal element in the aqueous solutionin order to control the appearance of spinnability.

Catalytic behavior of silicon carbide for hydrogen activation

The catalytic activities for hydrogen-deuterium exchange were examined at 473 K for three kinds of SiC powders with different particle sizes. Three SiC powders showed a high activity after outgassing at high temperatures above 873 K. The thermodesorbed gases from SiC were mainly H2 and CO, and they were produced from the thermal decomposition of adsorbed water on SiC surfaces. The extent of decomposition was found to be parallel to the catalytic activity at 473 K. Therefore, the sites of the décomposition are responsible for the catalytic activity.