Jeong-Hyun Park

Effect of polymer molecular weight variations on PZT slip for tape casting

Abstract Ceramic slips were formulated based on the PZT/plasticized-polymer system. The total volume of polymer in slip was held constant, while the relative amount of plasticizer and binder was varied. Experiments were performed to elucidate the effects of the polymer molecular weight and its end-to-end distance on the properties of green sheet as well as on the rheological properties of casting slips. Tapes(thickness=200 μm) were cast from these slips and their properties were evaluated. The properties of polymers such as their average molecular weight(end-to-end distance) and molecular weight distribution had a dramatic effect both on slip rheology and green sheet properties. The optimum ratio of PVB to DPB for the PZT tape casting process was 75 : 25. The properties of green sheet agreed well to viscosity behavior of PZT slip. And the burn-out behaviors of each green sheet were quite different.

Effect of multilayer coating on mechanical properties of Nicalon-fibre-reinforced silicon carbide composites

Nicalon-fibre-reinforced SiC composites were fabricated by combining polymer solution infiltration (PSI) and chemical vapour infiltration (CVI). Effect of multilayer coating on mechanical properties of the composites was investigated. The coatings consisted of chemically vapour deposited (CVD) C and SiC and were designed to enhance fibre pull-out in the composites. It was found that the flexural strength and fracture toughness of the composites were increased with the number of coating layers and was a maximum for 7 coating layers which consisted of C/SiC/C/SiC/C/SiC/C. Typical flexural strength and fracture toughness of the composites were 300 MPa and 14.5 MPa m1/2, respectively.

The possibility of the production of reaction bonded aluminium oxide using microwave energy

Recently many investigations of the use of microwaves in the area of material processing have been conducted. One of them involves the use of microwave energy to make metal-ceramic composites [1, 2]. Metal is known to reflect microwaves, but when fine metal powders are used, metal-ceramic powder mixtures can be heated due to the surface ohmic current of metal powder. Conventional ceramic sintering usually results in large shrinkage (15-20%), which causes problems such as fracture, and leads to difficulty in predicting the size of final products and in forming complicated shapes. Many processes have been developed to reduce this large shrinkage, one of which is the production of reaction bonded aluminium oxide (RBAO) [3]. Using this process, it is possible to produce low-shrinkage alumina, because the volume expansion of A1 powders during oxidation in A1-A1203 mixture can reduce the shrinkage during sintering. Since the starting powder of the RBAO process is metal-ceramic powder mixture which is suitable for heating in the microwave cavity, we investigated the possibility of making RBAO using microwave energy and the effects of the powder characteristics on the oxidation and sintering behaviour. Microwave processing experiments were carried out in air using a domestic microwave oven (Daewoo KOR111, 2.45 GHz, 650 W). The alumina powder used was reagent grade (99.6 wt% purity, A-12, IKEI Co., Japan) agglomerate, with average particle size of 16/xm, consisting of small alumina grain ( -0 .9 /xm) . The A1 powder was reagent grade (99.9 wt % purity, Junsei Chemical Co. Ltd, Japan) with average particle size of 23/~m and flake-like shape. The AI:A1203 ratio was 35:65 (vol %). After weighing, the A1-AI203 mixture was stirred by magnetic bar, then mixed using a mortar and pestle for 1 h. In order to prevent hydration and oxidation, ethanol was used as the mixing solvent. After drying, the powders were pressed into 4 g cylindrical pellets under 1.5 tonnes in a 10 mm diameter cylindrical die. For the oxidation experiment, a low temperature insulating brick was placed on the bottom of the microwave cavity, with a high temperaute insulating brick placed above. A hole a little larger than the size of the specimen was drilled in the middle of the top surface of the insulating brick, and another insulating brick was placed as a cover. For the sintering experiment an SiC plate was placed above the high temperature insulating brick as a heater,

Influence of stabilizers on Na-β-Al2O3 phase formation in Li2O(MgO)-Na2O-Al2O3 ternary systems

The influences of stabilizers on β- and β′′-Al2O3 phase formations in Li2O(MgO)-Na2O-Al2O3 systems were investigated. When stabilized with 4MgCO3ċMg(OH)2ċ5H2O, most of the β′′-Al2O3 phase formed below 1200°C and further β- to β′′-Al2O3 transformation with an increase of temperature was not observed. On the other hand, when stabilized with Li2CO3,β′′-Al2O3 formation occurred by two steps. First, β′′-Al2O3 was partly formed below 1200°C, and, second, noticeable transformation from β-Al2O3 to β′′-Al2O3 occurred at higher temperature ranges. It was shown that transient eutectic liquid in the Li2O-Na2O-Al2O3 system promoted the β- to β′′-Al2O3 transformation at higher temperatures. Uniform distribution of both Mg2+ and Li+ stabilizing ions enhanced β′′-Al2O3 formation at low temperatures. In the Li-stabilized systems, however, homogeneous distribution of Li+ ions hindered both the formation of transient eutectic liquid and the second β- to β′′-Al2O3 phase transformation at high temperatures.

Fabrication of β- and β′′-Al2O3 tubes by pressureless powder packing forming and salt infiltration

A new forming method, pressureless powder packing (PLPP), was studied to fabricate the β- and β′′-Al2O3 tubes. Alkali sources were infiltrated into the pores of α-Al2O3 tube preforms that had been prepared by the PLPP forming method. The composition for the synthesis of β′′-Al2O3 phase was Na2O · 0. 138Li2 · 4.4Al2O3. The β′′-Al2O3 fraction of calcined and sintered bodies was increased with the increase of calcination temperature, and phase formation was largely affected by the type of starting α-Al2O3. Large particle size and narrow size distribution of fused α-Al2O3 resulted in uniform green microstructure that enhanced the homogeneity of alkali salts after infiltration, which was very important for the β′-Al2O3 formation. Sintered microstructure was uniform in all specimens but further development was required for density improvement.