Plastic-bonded magnets were prepared by magnetic field injection molding, using a mixture of Nd2Fe14B and SrO·6Fe2O3 pellets. The magnetic characteristics and temperature coefficients of these magnets were investigated. The hysteresis loop for the temperature change of the plastic-bonded magnet was measured. An important phenomenon was revealed pertaining to the physical properties of the magnetic material: under ordinary temperature conditions, the magnetic temperature coefficient of magnetic flux density and coercive force for the Nd2Fe14B and SrO·6Fe2O3 plastic-bonded magnet for the case of adequate SrO·6Fe2O3 content attained a positive value at a SrO·6Fe2O3 content ranging from about 20 to 60 mass% at 297–323 K, and from around 18 to 43 mass% at 323–373 K. Moreover, it was proven that the temperature coefficients of the magnetic flux density at operating points of permeance coefficients Pc(B⁄H)=1 and 2 could be controlled from 0 to 0.22%/K at 297–323 K and from 0 to 0.1%/K at 323–373 K, respectively.
The possibility of recycling silicon nitride ceramic by hydrothermal treatment was investigated. A Si3N4 sintered compact with 0.9 mass% SrCO3, 3.6 mass% MgO and 4.5 mass% CeO2 as sintering aids was used as a specimen for treatment. Hydrothermal treatment was performed using aqueous HF solution at 70–150°C for 0–120 h. The dense sintered compact was easily disintegrated into powdery particles and a core fragment by the combination of hydrothermal treatment and subsequent sonication treatment. The treated powder was identified by X-ray diffraction (XRD) analysis as β-Si3N4 containing CeF3 and MgF2, which were formed in aqueous HF solution during hydrothermal treatment. These fluorides were completely removed by rinsing in HNO3 solution. Then, a powder consisting only of needle like β-Si3N4 crystals was recovered by a series of these treatments. X-ray photoelectron spectroscopy (XPS) revealed that the surface of the recovered Si3N4 powder was oxidized very slightly. Hydrothermal treatment at 120°C for 24 h gave approximately 30% yield of powdery β-Si3N4 particles, excluding the amount of sintering aid components. In the case of hydrothermal treatment for shorter terms or at lower temperatures, the corrosion behavior of the Si3N4 ceramic is linearly dependent on treatment time and is controlled by surface chemical reaction. The activation energies for the dissolution of the Si3N4 ceramic, and the grain boundary components of strontium, magnesium and cerium were estimated as 31.9±0.7, 54.4±3.6, 74.3±1.4 and 51.7±3.9 kJ/mol, respectively.
Hydroxyapatite (HA) coatings prepared by plasma spraying method, as precipitated HA and calcined HA (2 h at 850°C) were immersed in Ca-free simulated body fluids (Ca-free SBF) for different periods to investigate the surface dissolution at 37°C. The amount of released Ca dissolved into the SBF and zeta potential of the HA samples were monitored. In addition, the surface of immersed coating was analyzed with SEM, XRD and FT–IR methods. The results showed that the maximum amounts of zeta potential and released calcium are found in plasma sprayed HA, while the minimum amounts are observed in the calcined. Furthermore, the surface of coating samples was dissolved in the SBF and consequently the amount of calcium concentration increased, immediately after the starting of immersion. This phenomena and the increase of zeta potential showed that the surface of the HA coated samples were unstable. After 21 d of immersion a new HA precipitate formed on the surface. The formation mechanism of HA on the coating surface could be interpreted in terms of the ionic exchange between H+ within the solution and Ca2+ in the coating, which caused an increase of the zeta potential of the HA samples. The mentioned increase provided a specific surface with lower interface energy, which introduces the formation of HA nucleus. The results, in a general view, confirm the occurrence of the diffusion, dissolution and precipitation reactions.
Electrochemical NOx reduction system composed of three-layered laminate, 8 mol%Y2O3–ZrO2 (YSZ) // 70 vol%YSZ/BaMnAl11O19 (BMA) // 30 vol%YSZ/BMA, could be successfully synthesized at 1550°C by taking into account the shrinkage mismatch between YSZ and YSZ/BMA layers. Two types of alumina, α- and γ-phase, were used as an alumina source for the in-situ processing of the laminate. The total shrinkage of the precursor powder for the composite using γ-alumina was closer to that of YSZ than that using α-alumina. The shrinkage rate of the precursor powder was adjusted to that of YSZ at the heating rate of 2°C/min. The shrinkage curve of YSZ/BMA composite consisted of the first shrinkage region, a plateau region and the second shrinkage region. The plateau region, in which the formation of BMA crystallites inhibited the growth and sintering of YSZ, decreased with increasing YSZ/BMA ratio. The control of heating rate and the use of the intermediate layer with compositional gradient lead to the development of the three-layered laminate tightly bonded with each other without any separation, which can be applied for the electrochemical NOx reduction system.
The effects of boron compound additive on the thermoelectric properties of α-SiC ceramics were studied. Porous SiC ceramics with 57–62% relative density were fabricated by sintering the pressed α-SiC powder compacts with B, B4C, and BN at 2000–2100°C for 3 h in Ar and/or N2 atmosphere. The sintered bodies were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameter measurements revealed incorporation of a certain amount of added B into the SiC lattice and negligible phase transformation during sintering. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured at 600–900°C in Ar and/or vacuum atmosphere. The kind of additives, the amount of addition and sintering atmosphere had significant effects on the thermoelectric properties. On the whole, the excess addition had a harmful influence upon electrical conductivity. The thermoelectric figure of merit of B-doped SiC was lower than that of n-type SiC.
Surface groups were evaluated to clarify the effects of sonochemical reactions on acid-washed diamond powder (5–12 μm) treated with high power ultrasound (18 W·cm−2) at 28 kHz in water under three different gas bubbling conditions; argon, oxygen and no bubbling gas. The raw and modified powder surfaces were analyzed by diffused reflectance infrared Fourier transform (DRIFT) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The results show that ultrasound can modify the surface of acid-washed diamond. Active oxygen species, were formed during the sonochemical treatment, reacted with the diamond surface. DRIFT results show that ether (C–O–C), carbonyl (C=O) and small amounts of peroxy (C–O–O–C) functional groups were formed on the diamond surface after the ultrasonic treatment. XPS analysis confirmed that oxygen is bonded to the diamond surface carbon after the ultrasonic treatment. There is good agreement between DRIFT and XPS results, showing that using argon as bubbling gas produces the highest relative surface oxygen content on the acid-washed diamond surface. Argon gas as monoatomic gas has higher effects in diamond surface modification by ultrasonic treatment than oxygen even for oxidation reaction, due to higher γ (the ratio of the isobaric to isochoric heat capacities) for monoatomic gases and low thermal conductivity.
Pressure loss in gas flow in three-dimensionally interconnected macropores in silica gel rods was investigated. In the preparation of the silica gel, the continuous macropores are formed by inducing phase separation in a solution containing tetraethoxysilane (TEOS) and poly (ethylene oxide) (PEO), and subsequent freezing of its transitional structures by gelation. Average macropore size of the silica gels was systematically controlled from 25 to 0.6 μm by changing PEO content in the solution without affecting porous morphology. The pressure loss in gas flow in the continuous macropores can be approximated with a simple straight channels model, and well reproduced with the Hargen–Poiseuille’s equation without any correction, in contrast to that in columns packed with micrometer-size particles, which show substantial higher pressure loss than the continuous macropores. The low flow resistance in the macroporous silica rods would be attributed to the absence of necks in flow pathways.
In order to study a synthesis of mullite whiskers using Na2SO4 flux, powder mixtures consisting of Al2(SO4)3, amorphous SiO2 and Na2SO4 were heated in alumina crucibles. After heating, the reaction product in the crucible was treated with hot hydrochloric acid to get mullite whiskers. Optimum conditions such as heating temperature, duration time and mixing ratio of the starting materials to get a large amount of mullite whiskers with high aspect ratio were established. When a powder mixture consisting of Al2(SO4)3:amorphous SiO2:Na2SO4=1:1:4 (molar ratio) was heated at 1000°C for 2 h, 10–25 μm length and 0.5–2 μm diameter (aspect ratio: 15–20) mullite whiskers were formed.
The half-wave potentials of the reduction of tin ion in Na2O–CaO–SiO2 glass melts were examined by differential pulse voltammetry. In all the melts, the half-wave potential shifted to the positive side with an increase in temperature. At low temperatures, the voltammograms exhibited two peaks or the main peak joined to a shoulder. These correspond to the successive reductions of Sn4+ to Sn2+ and Sn2+ to Sn0. At high temperatures, the two peaks unified and only a single peak was observed. The half-wave potential of the reduction of Sn4+ to Sn2+ increased with increasing the concentration of tin ion doped in the melt.
Infrared (IR) light microscopy with liquid immersion technique was applied to examine the internal structure of granules and green compact of ZrO2 nano-powders, whose refractive index is 2.14. Internal structure could be clearly observed despite the high refractive index by IR microscopy. This high transparency of the compact to IR light was ascribed to an increased ratio between the particle size 0.1 μm and wavelength λ=1.3μm. Granule deformation is directly observed with this method for green bodies compacted under various pressures. IR microscopy combined with the liquid immersion technique is very effective for understanding the internal structure of compacted nano-powders.
Erosion wear properties of pressureless sintered TiC/Al2O3 composites were measured by a collision test between Al2O3 or SiC particles accelerated in an air stream for the purpose of the comparison with various pressurized sintered ceramics composites, SiC, Si3N4, ZrO2, Al2O3 monolithic ceramics, and cemented carbides. In the case of collision by Al2O3 particles, the pressureless sintered TiC/Al2O3 composites had the highest wear resistance, which wear rate was almost the same as that of commercial pressurized sintered ceramics composite. On the other hand, in the case of collision by SiC particles, cemented carbides (WC+4Co) had the highest wear resistance. The wear rate of pressureless sintered TiC/Al2O3 composites was smaller than that of Al2O3, SiC and ZrO2/Al2O3 composites; however, it was almost the same as that of commercial pressurized sintered ceramics composites, Si3N4 and ZrO2.