Shaping ceramic materials is a research area which has continuing innovation and excellence. Dry processing routes gave way to colloidal techniques enabling the preparation of more complex shapes for a wide and growing field of applications. Both dense and porous ceramics with improved properties can be produced. The development of colloidal processing led to the creation of the near-net-shaping concept, performed through slip casting, gelcasting and freeze casting. Even though these concepts have been used for the last few decades, different trends and combinations of these techniques are refreshing the processing strategies in recent years. New chemistries and concepts continue to emerge. This paper creates an overview of the recent trends in near-net-shaping routes, having a deeper look into gelcasting and freeze casting, since both processes are used in the preparation of tailored and custom dense and porous ceramics.

View full abstract

The consolidation behavior of two kinds of colloidal SiC particles (30 and 330 nm diameters) with and without phenylalanine dispersant (C₆H₅CH₂CHNH₃⁺COO⁻) at pH 7 was examined using a developed pressure filtration apparatus at 1 MPa of applied pressure. The height of 5 vol %-30 nm SiC suspension with and without phenylalanine as a function of filtration time was well simulated by a filtration model developed for a flocculated suspension rather than a traditional filtration model for a dispersed suspension. The result with phenylalanine is related to the colloidal phase transition from well dispersed particles to flocculated particles under an applied pressure of 1 MPa. The consolidation time became longer as the amount of phenylalanine added exceeded 0.1 mg/m²-SiC. However, the packing density of 30 nm SiC particles after consolidation increased from 35% for no addition of phenylalanine to 37–42% for 0.1–1 mg phenylalanine/m²-SiC. The consolidation behavior of 20 vol %-330 nm SiC suspension with and without phenylalanine was also well expressed by the filtration model for a flocculated suspension. A longer consolidation time was also measured for the addition of phenylalanine above 1 mg/m²-SiC. However, the packing density (49%) of 330 nm SiC particles increased scarcely to 52% by the addition of phenylalanine (2.68 × 10⁻²–2.85 mg/m²-SiC).

View full abstract

Desiccant air conditioning and humidity control systems, which utilize the water adsorption/desorption by porous materials, have attracted much attention as an environment-friendly and energy-efficient air conditioning system. Micelle-templated mesoporous silicas (MPS), which have been extensively studied in recent years, are considered as the optimum adsorbent for adsorption/desorption-based air conditioning systems because of their excellent water adsorption property due to their highly ordered porous structure; i.e., a uniform nanopores with an ordered arrangement. We first reported the formation of a thick coating of MPS powder on stainless steel by an electrophoretic deposition (EPD) method and then demonstrated the preparation of the MPS coating on aluminum substrates, which has the advantages of lightweight properties and high thermal conductivity. In this study, we investigated the dispersion stability of an EPD bath, and the influence of the EPD condition on the deposition amount in detail. The EPD bath composed of spray-dried MPS powder and acetone was stable 3 min after the suspension was prepared, and the deposition amount could be easily controlled by the EPD conditions by changing the MPS powder concentration, the applied dc voltage and the deposition time. The obtained MPS thick coating had the same porous structure and adsorption properties with respect to nitrogen and water vapor as the parent spray-dried MPS powder.

View full abstract

Polyorganosilsesquioxanes (RSiO_3/2) (R: organic group) particles with different organic groups such as alkyl and aryl groups were prepared using a two-step acid–base catalyzed sol–gel process. Spherical particles were formed at room temperature using the two-step acid–base catalyzed sol–gel process in which trifunctional organoalkoxysilanes with R = methyl, n-propyl, n-butyl, phenyl, and benzyl were used as starting materials. Polyorganosilsesquioxanes with longer alkyl groups or phenethyl group prepared by the two-step acid–base catalyzed sol–gel process were oily material at room temperature. Polyorganosilsesquioxanes with shorter alkyl or aryl groups exhibited the glass transition. In contrast, polyorganosilsesquioxanes with longer alkyl groups showed no glass transition behavior. The occurrence of glass transition phenomena of polyorganosilsesquioxanes was concluded to be strongly dependent on the chain length of organic substituents.

View full abstract

Herein a novel approach to assemble single-walled carbon nanotube (SWCNT) microcapsules using spray-drying and solvent exchange techniques is proposed. Spray drying a colloidal dispersion of SWCNTs containing a nonvolatile lactose species formed hollow-shell microspheres involving highly dispersed SWCNTs in the precipitated lactose shell. The spray-dried particles were then dispersed in absolute ethanol which is a poor solvent against lactose, and the ethanol was gradually exchanged with water through dialysis. The lactose and dispersing agent (sodium deoxycholate) were almost completely removed from the shell matrix under the solvent exchange, leaving SWCNTs non-covalently assembled in the hollow-shell microcapsule structures, held together by van der Waals interactions. Thus, the SWCNT microcapsules can be assembled using the spray-dried particles as “templates”.

View full abstract

In this study, we have proposed “drain time” (DT) measurement to be used for the characterization of particle dispersion states in the multi-component slurry, which are much more complicated than those in single-component one. The DT is related to drainage characteristics of a solvent from ceramic slurry cake, of which particle packing is correlated in principle to particle dispersibility in the slurry. For mixed slurry of alumina and silica powders, higher DT values at acidic and basic conditions indicated relative good dispersion but lower DT values around neutral pH region implied hetero-coagulation of alumina and silica particles. These dispersion and coagulation patterns were confirmed by electron microscopy for thin section of in-situ solidified slurry, and hence the DT measurement was found to be feasible for the characterization of particle dispersibility in multi-component slurry.

View full abstract

2-layered all-solid-state battery, which is made up of 3DOM (3dimensionally ordered macroporous) porous layer and dense layer, is one of the promising structures for all-solid-state Li battery with Li metal anode. In this paper, an influence of precursor powder size of solid electrolyte on formation of 2-layered electrolyte which is a framework of all-solid-state battery was studied using Li_0.35La_0.55TiO₃ (LLT) electrolyte. The particle size strongly affected on morphology of prepared 2-layered structure. In the case of mixture of milled powder with below 1 µm diameter and non-milled powder with sub µm in weight ratio = 3:1, completely sintered 2-layered LLT could be obtained. This would be due to high bulk density of the mixture. Li ion conductivities of obtained 2-layered LLT at bulk and grain boundary were 8.0 × 10⁻⁴ and 2.6 × 10⁻⁵ S cm⁻¹, respectively, which were well consistent with reported values. These findings imply the 2-layered structure can be prepared by many kinds of ceramics if size of precursor powder would be controlled properly.

View full abstract

The sintering shrinkage behavior of gel-cast body was studied to be clarify what has influence to the green property, and causes the deformation in sintering, beside the slurry property in gel-casting method. The alumina sintered body was fabricated by gel-cast method. The two type mold, Flat-mold and Vertical-mold, were used for investigating the difference between the outside, central, upper and bottom of the sintered body. The measurement of sintering shrinkage shows that sintering shrinkage occurred toward the center. And, from the observation of cross-section SEM in sintered body, there were long grains along the mold wall in Flat-mold, and the grain size was larger in bottom of the body in Vertical-mold. It is suggested that the effect of interaction from the mold-wall, and effect of sedimentation causes the deformation of sintered body.

View full abstract

A highly textured alumina green compact was obtained from well dispersed non-aqueous slurry by drying it in a high magnetic field. Compatibility of poly(vinyl butyral) (PVB) as a dispersant and organic solvent was optimized in order to control the conformation of PVB which affects the dispersion state by steric repulsion. A novel observation method was developed to analyze the structure of dispersed particles in concentrated slurry with an optical microscope. This method clarified that well dispersed slurry based upon good compatibility of PVB and organic solvent led to a highly particle-oriented green compact.

View full abstract

Fluidity of wet-jet milled and ball-milled ZnO slurries containing polyvinyl alcohol (PVA) was investigated from rheological behaviors and centrifugal separation. The viscosities of wet-jet milled ZnO slurries were much small as compared to ball-milled ones irrespective of the solids loading and PVA contents. At 30 vol % of high solids loading, the viscosity of wet-jet milled slurry with PVA was decreased to one-ninth of ball-milled one. From the creep-recovery measurements, the ball-milled slurry with PVA showed the elastic property, while the wet-jet milled one with PVA showed the liquid-like property. Therefore, the fluidity of wet-jet milled ZnO slurry with PVA was much higher than that of ball-milled one, indicating that the adsorption affinity between wet-jet milled particle and PVA was weak as compared to ball-milled one with PVA. From the centrifugal separation test, it was found that the relative packing densities of the precipitates prepared from wet-jet milled particles was high as compared to those prepared from ball milled ones due to the difference of adsorption affinity. Hence, the fabrication of weak adsorption affinity between ZnO particle and PVA leads to high fluidity of high solids loaded slurries with an organic binder like a chain polymer.

View full abstract

The CoAl₂O₄ pigment commonly used for coloring ceramic products was synthesized by microwave hydrothermal processing (M-H), and ink-jet printing with this aqueous pigment ink was performed to decorate porcelain. First, a precursor was prepared by adjusting the solution to pH 12. Then, M-H reaction was applied to this precursor at 240°C for 2 h, yielding particles of CoAl₂O₄. In addition, the precursor concentration of the suspension was shown to greatly affect the color tone of the resulting particles. As the precursor concentration increased, the synthesized particles that were bluish-green at lower concentrations took on a darker blue color. The synthesized CoAl₂O₄ particles were regular octahedrons measuring approximately 70 nm. The particles synthesized by M-H reaction were used to prepare an aqueous suspension, which was then used for printing on tiles by an ink-jet printing system. The synthesized particles were well dispersed in the prepared aqueous suspension, as little sediment was formed in this suspension left for 1 week. The printing system experienced no clogging and the printed image was of good quality.

View full abstract

Characterization of particle dispersibility in slurry as raw materials for fabricating ceramics is very important since the dispersibility gives great effects directly on qualities of final ceramic products. In this study, a direct observation of particles dispersed in liquid media using an in-situ solidification technique, which has been developed by our group and examined only in aqueous systems so far, has been extended to non-aqueous systems. In this paper, radical polymerization with several combinations of monomer/cross-linker and initiator was attempted to find out gelling agent that was able to solidify slurry without any influence on particle dispersion. The direct observations of yttria or alumina particles in non-aqueous solvents were carried out with the selected gelling agent. The observations showed clearly dispersion patterns of the particles in the solvents such as pearl chain-like or island structures. The particles tended to be coagulated strongly in solvents with low dielectric constant, which agreed with results of conventional characterization methods such as apparent viscosity of the alumina slurries.

View full abstract

Silicon carbide (SiC) was first industrially synthesized in 1894 and has been used as refractories, abrasives and high temperature furnace parts. The basic fabrication and sintering technologies were almost established by about 1970. Recently, sintered SiC has become a key material as it has been widely used in advanced industries of semiconductors and high precision machines. New fabrication methods for modern SiC powders and sintering methods are being developed on the bases of traditional methods. In this review, the original studies and recent developments have been given maximum possible emphasis, and in particular, basic points of view are introduced.

View full abstract

With a new concept of reducing the spinel particles in ZnO varistor ceramics (ZVC), the molar ratio of Sb₂O₃ (M_S)/Bi₂O₃ (M_B) and the effect of the sintering temperature (T_S) on densification behavior and varistor voltage (V_b: V_1mA/mm) were investigated. The densification behavior of ZVC during the sintering process was significantly affected by the M_S/M_B, and the densification curve was shifted to a lower temperature with a decreasing M_S/M_B. Based on these experimental results, four types of spinel-reduced ZnO varistor ceramics (SRZVC) were fabricated to investigate the effect of transition elements such as Ni and Cr on electric stability of the resistive leakage current (I_r). These experiments revealed that electric stability of I_r depended on the contained transition elements, and was influenced by the segregating of transition elements in spinel particles.

View full abstract

Mesoporous silicas (MPS) are potential materials for adsorption of antibodies on their surface as they have the following features: a) an ordered pore network and b) a high surface area. These unique features make MPS an excellent candidate as an immobilizing carrier for immunoassays. We present here the relationship between antibody molecules and pore diameters of various MPS; moreover, the thermal stability and the organic solvent stability of antibodies immobilized on MPS are evaluated. From six MPS, the entire amount of antibodies (44 µg) was adsorbed on four MPS (3 mg; pore sizes: 3.4, 11.0, 15.0, and 23.1 nm) but not on MPS with 5.8 and 8.1 nm pores. However, the binding activities of antibodies to antigens were high only on MPS with 3.4 and 5.8 nm pores. Regarding the thermal stability of antibodies immobilized on MPS, the antibody immobilized on 23.1 nm pore MPS kept about 30% activity even after 3 h at 80°C, but that on 3.4 nm pore MPS was markedly decreased (<10% activity) after that time. These results indicated that the Fab fragment, which is an antigen binding site of the antibody, was incorporated inside larger pores but remained outside smaller pores.

View full abstract

A series of K₂NiF₄-type compounds, La_2−xPr_xNiO₄ (x = 0, 0.6, 1.0, 1.4, and 2.0), were synthesized by solid state reaction for use as cathodes in an intermediate-temperature (500–700°C) solid oxide fuel cell (IT-SOFC). La_2−xPr_xNiO₄ (x = 0 and 0.6) crystallize in the Fmmm space group, while La_2−xPr_xNiO₄ (x = 1.0, 1.4, and 2.0) belong to the Bmab space group. The electrical conductivity increased by Pr doping and La_2−xPr_xNiO₄ (x = 0.6, 1.0, and 1.4) exhibited electrical conductivities similar to one another while Pr₂NiO₄ had the highest. Single test-cells consisting of samarium oxide doped ceria (SDC) as an electrolyte, Ni–SDC cermet as an anode and La_2−xPr_xNiO₄ as a cathode, were fabricated for measurements of cell performance at 500–700°C. Current interruption measurements revealed that the overpotential losses mainly decreased with increasing Pr content. Pr₂NiO₄ was found to exhibit the best cathode characteristics; maximum test-cell power densities of 14.1, 45.1, and 104.4 mW/cm² were obtained at 500, 600, and 700°C, respectively.

View full abstract

Nitridation behaviors of silicon powder doped with various rare-earth oxides were studied in order to assess the nitridation enhancing effect of the rare-earth oxides on the formation of reaction bonded silicon nitride. Thermo-gravimetric analysis revealed that the addition of CeO₂ and Eu₂O₃ to Si powder reduced the temperature of the main nitridation reaction and increased the amount of silicon converted to silicon nitride at a given temperature.

View full abstract