Key Engineering Materials Vols. 512-515

Abstract: Porous yttria-stabilized zirconia (YSZ) ceramics were fabricated by tert-butyl alcohol (TBA)-based gelcasting method using high solid loading slurry. Micron pores were generated by using low-cost carbon powder as the pore-forming agent. The TBA-based slurries of 50% solid loading appropriate for gelcasting were successfully prepared by using commercial dispersant BYK-163. The rheological behavior of the slurries were measured and discussed. The porosity, microstructure, compressive strength and thermal conductivity of the porous ceramics were tested and analyzed. The results show that this low-cost technology is significant for fabricating porous ceramics with both high compressive strength and low thermal conductivity.

Abstract: A kind of porous SiO2 bulk materials with pore size distributed in nano and micron scales has been fabricated by freeze-casting of SiO2 nano-size powders in TBA (tert-butyl alcohol)/H2O suspensions. The content of TBA in the well mixed TBA/H2O binary co-solvent plays an important role in size and morphology control of the pores in the obtained SiO2 monolithic materials, since the ratio of TBA/H2O determines the competition of crystallization of TBA and ice, therefore, the microstructure of the monolithic SiO2. It was found that the content of TBA in the range from 30 to 40% was suitable for formation of more nano-pores in the SiO2 monolith. The insulation materials freeze-casted from the suspension of SiO2 in 13wt% and TBA in 40wt% has the density of 0.17g/cm3, and the specific surface area of 282m2/g. The pore size has a trimodal size distribution with three mean diameters of 15nm, 100nm and 4μm, respectively.

Abstract: Nano-porous SiO₂ insulation material was made from foamed aqueous slurry of nano-silica powder by filter-pressing. The foamed aqueous slurry mainly consists of hydrophobic SiO2 nano-particles, xonotlite whiskers and glass fiber. The latter two will enhance strength of the insulation material. Rheological properties of the foamed slurry with varying solid mass concentration were studied. The effect of forming pressure on the density and pore size distribution of the nano-porous silica material was investigated. It was found that the foamed slurry with solid concentration less than 17% behaves pseudoplastic as decreasing its apparent viscosity with increasing of shear rate during rheological testing. Homogeneous foamed slurry can be obtained when solid concentration in it is below 7%. Such slurry should be dewatered into solid concentration being of 10-15% before filter-pressing. Rigorously stirring of the concentrated slurry is required before pouring it into a mould, which leads to reducing the viscosity of the slurry and helps uniformly filling mould. During pressing the loading speed should be strictly controlled in order to reaching designed pore size distribution in SiO₂ insulation material. The SiO2 insulation material with apparent density about 0.45g/cm3 and pore size mainly ranging from 10 to 50nm was obtained.

Abstract: Aqueous tape casting process was used to prepare the thin cordierite sheets, the influences of dispersant (PAAS), the rate of plasticizer (PEG),and binder (PVA),content (R) and drying temperature on the tape casting process were investigated, the sintering character of the sheets were studied after drying. It was showed that when the content of PAAS was 1wt%, the slurries were stable uniform and the shear thinning was reversible. The green sheets with the thickness of 177μm showed good flexibility, lubricious surface and compact microstructure when R was 0.7 and the drying temperature was 25°C. A compact α-cordierite sheet was obtained at 1000°Csintering for 2h.

Abstract: Tape casting play a significant role in industrial area, such as multilayered ceramic (MLC) packages, functionally graded materials (FGM), low temperature co-fired ceramics (LTCC) and so on. For the complexity of the rheology for slurry during tape casing process, the control of tape thickness by experience was unstable. Although few numerical and analytical studies on predicting the tape thickness have been done, but these efforts have focused on Newtonian, Bingham, Power law, respectively. There is no unified equation to calculate the tape thickness among different rheological models. In this paper, the calculation results are characterized by wide adaptability; the blade gap, the casting speed and the slurry rheological property are incorporated into calculation; the effect of parameters in the result is studied; Parameter P_nd can be used as a guide to check which mark patterns of the flow velocity profiles in the channel. The results proposed and the experimental measurements from existing publications are in close agreement. Compared with the prediction of the existing models, the calculation results proposed has good agreement with them.

Abstract: Zn-doped SnO2 seaflower like hierarchical architectures have been synthesized via a solvothermal synthesis route in the mixed solvents of ethanol and deionized water. The observations of field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) showed that Zn-doped SnO2 seaflower like hierarchical architectures were assembled by the nanowires. The preferential growth direction of nanowires was determined based on the analysis of high resolution transmission electron microscopy (HRTEM). The products were also characterized by X-ray diffraction (XRD) and X-photoelectron spectrum (XPS), and the results indicated that Sn4+ ions were successfully substituted by Zn2+. The photocatalytic activity of Zn-doped SnO2 seaflower like hierarchical architectures was evaluated by the degradation of RhB aqueous solution and the results showed that the product had high photocatalytic activity efficiency.

Abstract: Various textured alumina ceramics were prepared by colloidal processing in high magnetic field and heating from alumina powder as raw material in this study. The effects of the magnetic field strength, heating time and heating temperature on the particle orientation were systematically examined for these samples with scanning electron microscope (SEM) and X-ray diffraction (XRD) etc. The experimental results showed that alumina grains are polyhedral ball shape of, no textured structure exists in the sintered bodies without magnetic field treatment; the alumina grains align with the c-axis parallel the magnetic field direction under high magnetic field, the grains present strip shape in the sintered alumina bodies with the magnetic field treatment; the particle orientation degree increases with increasing the heating time and heating temperature; when the sintering temperature achieves about 1823K, the textured microstructure can be obviously observed in the sintered alumina bodies.

Abstract: By increasing addition amount of (CrO₃ + Al) subsystem in (B₄C + Ti) system, the solidified TiC-TiB₂ composites with a series of mass fraction of Cr binder were achieved by combustion synthesis in high-gravity field. The microstructures of the solidified ceramics presented a number of fine TiB₂ platelets embedded in TiC grains, Cr-Ti alloy or between TiC grains and Cr-Ti alloy. The increased Cr binder in the ceramic not only brought about the enhanced densification of the ceramic due to fill-up of Cr-Ti liquid in shrinkage cavities between solidified TiC and TiB₂ phases, but more importantly made fine-grained even ultrafine-grained microstructure achieved because of the accelerated nucleation and the decelerated growth of TiB₂ at initial stage of material solidification. As a result of the achievement of fine-grained even ultrafine-grained microstructure along with the enhanced toughneing mechanisms contributed by the refined TiB₂ platelets and the increased plastic phases of Cr-Ti alloy, the solifidied TiC-TiB₂ composite containing 20.7 % Cr binder presented the maximun values of 1045 ± 25 MPa and 21.5 ± 1.6 MPa • m ^0.5, simultaneously, in flexural strength and fracture toughness along with the moderate hardness of 17.5 ± 2.2 GPa.

Abstract: Both single-phase and multiphase bulk ceramics were prepared by high-gravity combustion synthesis via melt solidification instead of conventional powder sintering. The synthesis process included three steps, namely, combustion reaction, phase separation, and melt solidification. In these steps, phase separation played a key role in producing bulk ceramic materials with high purities and low porosities. It was demonstrated that, in a high-gravity field the phase separation was greatly accelerated, compared with the case under common gravitational condition. In comparison with powder sintering, high-gravity combustion synthesis requires no furnace and can reduce the processing time for the fabrication of bulk ceramic materials.

Abstract: Based on taking combustion synthesis to prepare high-performance TiC-TiB₂ composite in high-gravity field, joint of TiC-TiB₂ with stainless steel and Ti-6Al-4V alloy were achieved respectively by the introduction of stainless steel and Ti alloy plates at the bottom of combustion system. Instant melting at surface part of stainless steel and Ti alloy, followed by atom diffusion between liquid TiC-TiB₂ ceramic and the molten metal was considered a reason for the achievement of the joint. Because of the presence of both coarsened Al₂O₃ inclusions nearby the ceramic and the partition layers of Al₂O₃ between the ceramic and the intermediate, shear strength of joint between the ceramic and stainless steel presented a low value of 120 ± 30 MPa. In contrast, because of the interdiffusion of C, B and Ti atoms between the liquid ceramic and the molten Ti alloy, ultrafine-grained microstructure develops in the ceramic nearby the joint, while the joint characterized by constitutional continuous gradient is also achieved from the ceramic to Ti alloy, resulting in high shear strength of 650 ± 25 MPa between TiC-TiB₂ ceramic and Ti-6Al-4V alloy.