Yoshinori Kanno

New gel-casting process for alumina ceramics based on gelation of alginate

Abstract Sodium alginate—a natural innoxious polymer, which can be dissolved in cold water by mechanical stirring and in which irreversible gelation takes place when enough calcium ions are added—is applied for in situ forming process of ceramics by using chelator to control its gelling behavior. In the present work, the viscosity of sodium alginate solution and factors influencing on it were analyzed. How to control the gelation behavior was discussed. The rheology property of the slurry was examined. An appropriate method to apply the gelation of sodium alginate in order to fabricate alumina ceramic green body was systematically investigated. The viscosity of alumina slurry with 50 vol.% Al2O3 and 1.8 vol.% Ca3(PO4)2 dispersed by 1 wt.% sodium alginate solution is less than 400 MPa s when the shear rate is higher than 50 s−1. The green body has uniform structure and smooth surface, and bending strength is about 8 MPa. After being sintered at 1550xa0°C for 2 h the linear shrinkage of ceramics composed of 95% alumina is about 15.7% during firing and the relative density of the final product is about 98.7%. Bending strength reaches 320 MPa. SEM images show compact and uniform microstructure.

Fabrication of alumina green body through gelcasting process using alginate

Abstract In the present work, we studied the addition of sodium alginate, a natural innoxious polymer, to coagulate alumina slurry in the fabrication of an Al 2 O 3 green body. Calcium iodate was added into the alumina suspension that was dispersed in a sodium alginate solution. The slurry was stable at room temperature but it solidified with increasing temperature. A slurry of 50 vol.% alumina ceramic powder, 1 wt.% calcium iodate and 1 wt.% sodium alginate solidified after being heated at 60 °C for 1 h. In addition to this process, a chelator was used to control the gelation behavior. Associated with the effect of the chelator, the consolidation reaction rate was delayed to longer than 30 min. The gelling properties of the sodium alginate solution and the resultant alumina slurry were analyzed. The rheological behavior of the slurry was examined. Appropriate forming processes were determined. Near-net-shaped green bodies were produced. The resultant green body has uniform structure and smooth surface, and the bonding strength is as high as 8.0 MPa.

Cation Capturing Ability and the Potential Response of a Poly(o-aminophenol) Film Electrode to Dissolved Ferric Ions

Poly(o-aminophenol) (PoAP) film was electrodeposited on an indium-tin oxide electrode. The PoAP film electrode captured dissolved cations such as Fe 3+ because of the coordination ability of the film. The cation capturing ability was confirmed by the X-ray photoelectron spectroscopy and the cyclic voltammograms of the PoAP film. The redox potential of PoAP was lower than that of the Fe 3+ /Fe 2+ redox couple and the Fe 3+ ions captured in the film were partially reduced to form Fe 2+ ions The PoAP film electrode had the Fe 2+ ions, and the electrode showed a Nernstian potential response to dissolved Fe 3+ ions. The response time was less than 10 s, and the response was observed in the concentration range above 10 -5 M. Although the PoAP film electrode captured Zn 2+ , Ni 2+ , and Cu 2+ , it showed no potential response to the corresponding cations because the corresponding reduced forms (Zn, Ni, Cu) were absent in the film.

Effect of dopants on the formation of hafnon via a sol-gel route

Hafnon (hafnium silicate, HfSiO4) ceramics are important candidates for high-temperature refractory materials because of their high thermal-shock resistance; this originates from their thermalexpansion coefficient which is lower than any other oxide ceramic. It is, furthermore, well-known that the thermalexpansion coefficient of hafnon (3.6 × 10 -6, 150~ 1300°C) is smaller than that (5.1 x 10 -6, 25 ~ 1500 °C) of zircon (ZrSiO4) [1], and each of the three forms of silica quartz, cristobalite and vitreous silica when heated with HfO2 at 1550 °C yields hafnon [2]. Nevertheless, the formation mechanism of hafnon is still not well-known. This letter reports the effect of dopants on the formation of hafnon by a sol-gel method because this method has attracted attention as a lowtemperature route to oxide materials. An equimolar solution of hafnium oxychloride (HfOC12.8HeO) dissolved in water containing hydrochloric acid and Si(OC:Hs)4 (tetraethoxyorthosilane, TEOS) was mixed by magnetic-stirrer agitation at room temperature in the presence of a small amount of divalent metal ions (M~÷/TEOS ~ 3 x 10 -~ molmol-~). Thence, the water contents (molar ratio, r ~-H:O/TEOS) was nearly equal to 30. The molar ratio teaches us that the siloxane polymer formed is composed of a three-dimensionalladder-chain structure during hydrolysis and condensation of TEOS [3, 4]. All reagents were of the highest purity grade and they were employed without further purification. Finally, the rotation of the stirrer was terminated due to the increase in viscosity with the agitation time. The gel was allowed to air dry, and then it was dried to a powder and calcined at the increasing temperature rate of 300 °C h -~ with a clean air carrier, kept for 1 h at the desired temperatures (1100-1400 °C), and then cooled in the furnace under air flow. The heat-treated powders were characterized by X-ray diffraction (XRD, Rigaku Denki Company) using CuK~ radiation. Table I shows the starting reagents doped as a solution. Fig. 1 shows representative XRD patterns of powders calcined at 1100 °C. The XRD peaks of the sample in the absence of a dopant suggest the beginning of crystallization, and correspond to badly developed monoclinic hafnium dioxide (m-HfO;) with a = 0.512 nm, b = 0.518 nm, c = 0.525 nm, an extremely small amount of hafnon (that is a zircon

Stable free-standing aramid resin film containing vanadium pentoxide and new colour electrochromism of the film by electrodeposition of gold

Abstract Vanadium pentoxide (V 2 O 5 ) was electrodeposited on a poly( p -phenylene terephtalamide) (PPTA)-film coated electrode. The cyclic voltammogram of the film had a reversible redox current peak. The film was dark green in the reduced state and yellow in the oxidized state. To obtain new colour, gold was further electrodeposited on the film. Not only the redox current peak but also a new redox current shoulder appeared in the cyclic voltammogram of the obtained film, and it exhibited a multicoloured electrochromism: blackish greenxa0↔xa0dark greenxa0↔xa0greenxa0↔xa0 bright red. The red colour in the oxidized state was first obtained for the V 2 O 5 film. The new redox current shoulder and the colour were probably due to Au y V 2 O 5 partially formed during electrodeposition of the gold. The redox of the Au y V 2 O 5 was accompanied by egress and ingress of Li + ions and the new colour change.

Synthesis and sintering of KTiOPO4, via mechanochemical mixing route

Abstract KTiOPO 4 (KTP) powders have been synthesized by a solid-solid reaction method from two series of starting reagents activated mechanochemically (A, KH 2 PO 4 +TiO 2 ; B,KNO 3 +TiO 2 +NH 4 H 2 PO 4 ). The formation mechanism of KTP crystal is discussed on the basis of the evolution of the crystal phase detected by X-ray diffraction and IR absorption peak measurements on powders calcined at various temperatures. The sintering behaviour of the calcined powders has been investigated by density and weight loss measurements and microstructure observations. The final sinterability of series B is superior to that of series A at higher temperatures; the maximum density of the former has been obtained as 93% of the theoretical density.

Electron microscopic and X-ray diffraction studies on the fine structure of ground β-SiC powders

Abstract Powders of β-SiC were ground by a vibration ball mill, the pot and balls of which were made of SiC ceramics, in a purified methanol medium. TEM observation showed that the ground powders possess lattice moire patterns caused by the residual strain, dislocations, and also stripe-like patterns indicating surface heterogeneity. The electron diffraction pattern of ground powders is composed of halo rings and many Bragg reflection spots. The initial sinterability of the ground powders, at a comparatively low temperature, is enhanced by the mechanochemical activation of surface layers. The activation promoted the β→α phase transformation in SiC during heat treatment.

Synthesis of Bi-Sr-Ca-Cu-O system superconducting oxide by ultrasonic spray pyrolysis

On decrit un processus continu de preparation doxydes du systeme Bi-Sr-Ca-Cu-O, chaque element etant distribue de facon homogene sous la forme de particules submicroniques. On examine la structure des poudres obtenues, leur pressage et frittage et les proprietes electriques des ceramiques supraconductrices obtenues