Papers by Author: K.H. Wong

Abstract: Epitaxially graded barium strontium titanate (BaxSr1-x)TiO3 (x = 0.75, 0.8, 0.9, 1.0, abbreviated as BST75, BST80, BST90 and BTO respectively) thin films were fabricated by pulsed laser deposition method on the (La0.7Sr0.3)MnO3 (LSMO)/LaAlO3 (LAO) single crystal substrate. Scanning probe microscopy with a contact mode was used to characterize the temperature dependence of polarization from room temperature to 140°C. Results indicated that the piezo-response signal of the BST graded films had an obvious change with temperature, and that the graded structures had a flatter temperaturedependence of permittivity. Furthermore, the contrasts of the SPM images were lower for the ferroelectric – paraelectric (F-P) phase transition temperatures of BST 75, BST 80, and BST90, but higher for the F-P transition temperature of BTO.

Abstract: Bilayer Ba0.6Sr0.4TiO3 - Ba0.4Sr0.6TiO3 and Ba0.4Sr0.6TiO3 - Ba0.6Sr0.4TiO3 thin films were deposited on the LaNiO3-buffered Pt/Ti/SiO2/Si substrates using pulse laser deposition method. A (100)preferred orientation was obtained. The structure was characterized using x-ray diffraction (XRD) and Raman spectroscopy. The leakage current, and dielectric permittivity versus temperature were characterized. Results indicated that the (100) preferred bilayer structure had less leakage current and smaller loss tangent, which was in favor of enhancing the quality of thin film used as microwave dielectrics.

Abstract: Ba2Ti9O20 (B2T9) and BaTi4O9 (BT4) ceramics have been fabricated using sol-gel technique. The sol-gel derived powder was produced from barium acetate and butyl titanate using acetic acid as solvent. The as-prepared powders were then calcined at different temperatures ranging from 600° to 1200°C. After that, the powder was pressed into pellets and further sintered at different temperatures ranging from 1250°C to 1380°C. XRD characterization and Raman spectroscopy showed that after calcining the sol-gel powder at 900°C for 2 h and sintering the pressed pellets at 1330°C for 4 h, triclinic B2T9 and orthorhombic BT4 ceramics could be obtained. The dielectric constant and Q-value are 35 and 1910 at 6.1 GHz.