2019, Vol.29, No.5

Journal of the Korean Crystal Growth and Crystal Technology 2023 KCI Impact Factor : 0.19

pISSN : 1225-1429 / eISSN : 2234-5078
https://journal.kci.go.kr/jkcgct

Perovskite-type oxides have consistently attracted considerable attention for their applications in high-temperature electrochemical devices, such as electrolytes and electrodes of solid oxide fuel cells, oxygen permeating membranes and sensors etc. Among them, the electrical conductivity of 10 % Sr and 10 % Mg doped LaAlO3 (LSAM9191) was measured using impedance spectroscopy and 4-probe d.c. method. Below 550°C, the grain boundary resistance mostly determined the overall conductivity; however, it nearly disappeared above 800°C. Using the defect model and curve fitting, the ionic and electronic conductivity contributions were also separated. In the temperature region where the sample resistance is mostly determined by the grain volume property, LSAM9191 was an oxygen ion conductor at low Po2 and a mixed conductor at high Po2. With increasing temperature, the ionic conduction region only slightly increased. Thus, LSAM9191 is a promising material as an oxygen ion conductor at high temperature and in low Po2.

Physicochemical properties and sintering characteristics of pottery stone (Taebaek, Haenam, Aphae, Haengnam) were evaluated as a raw material for porcelain products. Due to acid leaching procedure, the concentration of iron oxide (Fe2O3) was decreased to < 1.0 wt%, which affects the whiteness of sintered samples. Mean particle size of acid leached samples is 5.7~10 μm with narrow particle size distribution (PSD), which is lower than that of the pristine (8~18 μm) with broad PSD. According to phase analysis by X-ray diffraction, most of pottery stones (PS) have Quartz phase as a main phase with Pyrophyllite as a second phase, however, Haenam PS shows halloysite phase. The absorption rate was in order of Taebaek (A, B, C)~Aphae (A, B) < Taebaek (Special A) < Haengnam < Haenam, and the samples sintered in reductive atmosphere showed lower absorption rate. This result might be due to the concentration of feldspar contained in PS, working as a flux in sintering process. Comparing the color of the sintered samples, the whiteness of refined PS (Taebaek special A, Haenam, Hangnam) is higher than acid leached PS (Taebaek A/B/C, Aphae A/B). The whiteness (L * ) for refined PS is 95~97 %, which is higher than acid leached (82~96 %). This might be due to lower iron oxide concentration of the refined PS (0.11~0.58 %) than those of the acid leached PS (0.41~1.91 %) even though most of iron oxide was removed by acid leaching.

Calcium phosphates such as hydroxyapatite (HAp), tricalcium phosphate (β-TCP), and biphasic calcium phosphate (BCP, HAp/β-TCP) have been prepared via hydrothermal treatment. The synthesis was conducted by reacting (Ca(OH)2) aqueous solution with phosphoric acid (H3PO4) under different hydrothermal synthesis conditions (temperatures up to 150°C and pH lower than 12). The effects of initial precursor Ca/P ratio (1.30, 1.50 and 1.67) and post heat treatment on the phase evolution behavior of the powders and sintered ceramics were investigated. The phases of resulting powders and sintered ceramics were controllable by adjusting the initial Ca/P ratio. A single HAp phase without any noticeable second phase was obtained for the initial Ca/P ratio of 1.67 in the overall heat treatment range. Pure β-TCP and biphasic calcium phosphate (HAp/β-TCP) were synthesized from precursor solutions having Ca/P molar ratios of 1.30 and 1.50, respectively, after having been heat treated at 900°C or higher. Dense ceramics with translucency were obtained at considerably lower sintering temperatures