2015, Vol.25, No.6

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

Sapphire single crystals have been highlighted for epitaxial of gallium nitride films in high-power laser and light emitting diode industries. Among the many crystal growth methods, vertical Bridgman process is an excellent commercial method for growing high quality sapphire crystals with c-axis. In this study, the thermally induced stress in Sapphire during the vertical Bridgman crystal growth process was investigated using a finite element model. A vertical Bridgman process of 2-inch Sapphire was considered for the model. The effects of vertical and transverse temperature gradients on the thermal stress during the process were discussed based on the finite element analysis results.

Lead zirconate titanate (PZT) thick films with thickness of 10~20 μm were fabricated on silicon substrate by aerosol deposition method. As-deposited films on silicon were annealed at the temperatures of 700 o C. The electrical properties of films deposited by PZT powders were characterized using impedance analyzer and Sawyer-Tower circuit. The PZT powder was prepared by both conventional solid reaction process and sol-gel process. The remanent polarization, coercive field, and dielectric constant of the 10 μm thick film with solid reaction process were 20 μC/cm 2 , 30 kV/cm and 1320, respectively. On the other hand, the PZT films by sol-gel process showed a poor dielectric constant of 635. The reason was probably due to the presence of pores produced from organic residue during annealing.

We have applied mechanical alloying (MA) to produce soft magnetic composite material using a mixture of elemental Fe2O3-Mg powders. An optimal milling and heat treatment conditions to obtain soft magnetic α-Fe/MgO composite with fine microstructure were investigated by X-ray diffraction, differential scanning calorimetry (DSC) and vibrating sample magnetometer (VSM) measurement. It is found that α-Fe/MgO composite powders in which MgO is dispersed in α-Fe matrix are obtained by MA of Fe2O3 with Mg for 30 min. The saturation magnetization of ball-milled powders increases with increasing milling time and reaches to a maximum value of 69.5 emu/g after 5 h MA. The magnetic hardening due to the reduction of the α-Fe grain size by MA was also observed. Densification of the MA powders was performed in a spark plasma sintering (SPS) machine at 800~1000 o C under 60 MPa. X-ray diffraction result shows that the average grain size of α-Fe in α-Fe/MgO nanocomposite sintered at 800 o C is in the range of 110 nm.