In the present study, the effect of synthesis processes on the microstructure of Y-Ba-Cu-O superconducting system has been investigated. And DIGM (diffusion-induced grain boundary migration), DIR(diffusion-induced recrystallization), and thermal faceting phenomena in pure silver have been studied.
The recent studies on the processing of bulk $YBa_2Cu_3O_x$ superconductors have been largely focused on the melt-growth method in order to obtain the structures with aligned grains. The solid state sintering process, however, can still be important for certain bulk and thick film products such as targets for the deposition of thin films. In chapter 2, the calcination treatment of $Y_2O_3$, CuO, and $BaCO_3$ powder mixtures is shown to be critical for obtaining the maximum density after the subsequent sintering treatment of bulk $YBa_2Cu_3O_x$. The calcination has been carried out at 850$^\circ$C in $N_2$ atmosphere for various times and the sintering at 950$^\circ$C in $O_2$ atmosphere for 30 min and 20 h. The calcined powders have been ground with a mortar and pestle in order to break up the agglomerates of grains. The sintered density is observed to increase initially with the calcination time until it reaches the maximum at the calcination time of 10 h. The low sintered density with the calcination times shorter than 10 h is attributed to the incomplete decomposition of $BaCO_3$ as shown by the X-ray diffraction analysis. The low sintered density with the calcination times longer than 10 h is attributed to the large grain size. There is thus an optimum calcination time of 10 h for obtaining the highest sintered density of this material.
In Chapter 3, the shape of $YBa_2Cu_3O_x$ grains dispersed in liquid matrix and the effect of heat-treatment atmosphere on it have been studied. When $YBa_2Cu_3O_x$ is liquid-phase sintered at 925$^\circ$C in $O_2$ atmosphere for 16 h, liquid pockets are entrapped within the grains. The liquid pockets show a thin parallelepiped shape with short edge lengths in the c axis, even after reannealing at 925$^\circ$C in $O_2$ for 16 h. All grains in contact with the liquid matrix show the same shape. However, when liquid-phase sintered at 925$^\circ$C in $N_2$ atmosphere for 16 h, the grains dispersed in the liquid matrix show a thicker parallelepiped shape than in $O_2$, and their shape remains intact even after reannealing at 925$^\circ$C in $N_2$ for 16 h. The effect of atmosphere on the grain shape is expected to be due to the variation of oxygen vacancy concentration in CuO basal plane of the tetragonal unit cell. In this chapter, the effect of sintering atmosphere on isolated pores during the liquid phase sintering has been also investigated.
In chapter 4, I report a new isothermal peritectic reaction process(IPRP) that enables us to make a textured growth of the $YBa_2Cu_3O_x$ grains parallel to the a-b conduction planes ($CuO_2$ planes) without a temperature gradient and, at the same time, to introduce fine $Y_2BaCuO_5$ inclusions, which act as pinning centers, homogeneously within the grains, and show its possibility. The process is that liquid phase is externally infiltrated into $Y_2BaCuO_5$ specimen during a heat treatment below the peritectic temperature to form the $YBa_2Cu_3O_x$ phase. As a model experiment of IPRP, the sintered $Y_2BaCuO_5$ pellet has been packed with the liquid-forming powder of composition $3BaCuO_2$.2CuO and isostatically pressed into a cylindrical-shaped compact. These compacts have been then isothermally annealed below the peritectic temperature (at 950$^\circ$C to 965$^\circ$C), infiltrating the melt into the $Y_2BaCuO_5$ pellet. When the liquid-forming powders surrounded $Y_2BaCuO_5$ pellet is completely melted and the melt is supplied enough into $Y_2BaCuO_5$ pellet(at 940$^\circ$C in $N_2$ for 20 h and at 965$^\circ$C in $O_2$ for 20 h), in the surface region of specimen small $YBa_2Cu_3O_x$ grains are formed and near the center extremely large grains are formed. When the liquid-forming powders is partially melted and the insufficient melt is supplied (at 940$^\circ$C in $O_2$ for 20 h), the textured structure with grain alignment is obtained in the surface region of the specimen. Fine $Y_2BaCuO_5$ inclusions with particles size of submicron are observed to disperse uniformly within the formed $YBa_2Cu_3O_x$ grains, whether the liquid-forming powders is melted completely or partially during the heat treatment.
In chapter 5, the growth behavior of $YBa_2Cu_3O_x$ grains during the sintering and the effect of the chemical instability on the grain growth has been studied. Compact of the $YBa_2Cu_3O_x$ powder has initially been sintered at 950$^\circ$C for 20 h in $O_2$. The sintered specimen has been packed in $DyBa_2Cu_3O_x$ powder and annealed at 950$^\circ$C for 20 h in $O_2$. In the specimen $YBa_2Cu_3O_x$ grains have been observed to grow abnormally into grain size of 1 mm.
In chapter 6, DIGM and DIR phenomena in pure silver have been investigated. When oxygen atoms diffuse in or out from silver specimen, grain boundaries migrate, leaving behind them new equilibrium solid solutions of Ag-O and recrystallization also occurs at grain boundaries. In chapter 7, the thermal faceting phenomenon in pure silver has been studied.
