The effect of the crystallinity of boehmite powders on the temperatures of c-Al 2 O 3 formation and the h-to a-Al 2 O 3 transformation was investigated using boehmite powders of varying crystallite size prepared under various hydrothermal conditions. With increasing crystallite size of the boehmite powders, the specific surface area decreased and the expanded (020) d-spacing approached the reported value. Thermogravimetric ( TG) profiles of more poorly crystalline boehmite indicated the presence of excess water molecules of different binding energy located on the surface or in the interlayer. The crystallite size of boehmite also showed a strong correlation with the formation temperature of c-Al 2 O 3 and the phase transition temperature of h-to a-Al 2 O 3 . Since both these temperatures are increased with increasing the crystallite size of boehmite, this is an important factor in determining the conditions for obtaining c-, h-and a-Al 2 O 3 from boehmite. By relating the TG weight loss to the water and OH contents in boehmite and c-Al 2 O 3 , respectively, the crystallite size of boehmite can be related to the water content. This indicates that the c-Al 2 O 3 transformed from boehmite of smaller crystallite size contains larger amounts of OH groups, implying more poorly crystalline c-Al 2 O 3 . The reported topotactic transformation of boehmite via c-to h-Al 2 O 3 indicates a relationship between the crystallite size of the boehmite and that of the resulting transition alumina, which could explain the change of the phase transition temperature from h-to a-Al 2 O 3 .been performed to clarify the relation between the boehmite
Strongly c-axis oriented Pb(ZrxTi1−x)O3 (PZT) thin films with tetragonal perovskite structure (0.45≤x≤0.52) were epitaxially grown on (100)Pt/(100)MgO substrates using metalorganic chemical vapor deposition. Film thickness could be varied by altering the growth time. The electrical properties of PZT thin films sharply change below a thickness of 0.5 μm: the dielectric constant and remanent polarization decrease while the coercive field increases. These phenomena are explained by a model in which a layer with low dielectric constant exists in series with the normal PZT layer. The origins of this layer are systematically studied and found to be the intrinsic stress produced by the coalescence of crystal grains.
Strongly c-axis oriented Pb (ZrxTi1-x) O3 thin films were successfully grown on (100) Pt/(100) MgO substrate using the metalorganic chemical vapor deposition (MOCVD) method . The electrical properties of PZT thin films with thickness of 2ƒÊm are almost the same values of PZT single crystal . But they sharply change below film thickness of 0.5ƒÊm: dielectric constant and remanent polarization decrease , and coercive field increases. These phenomena are explained by the model that low dielectric constant layer exist in series with normal PZT layer.The origins of this layer is considered to be the intrinsic stress produced by the coalescence of crystal grains .
CeO2 films were epitaxially grown on Si(111) substrates by reactive sputtering following the single-crystal CeO2 seed layer formation by oxygen-reactive solid-phase epitaxy. Formation of metallic Ce layers and oxidation of the layers prior to reactive sputtering was found to be the key process for epitaxial growth of CeO2 on Si substrates. An Auger electron spectroscopy depth profiling analysis showed that the Ce–Si interlayer was formed after the Ce metal deposition. Reflection high-energy electron diffraction and transmission electron microscopy proved that epitaxial CeO2 films were grown by reactive sputtering on the seed layers formed by oxidation of Ce metal layers with thickness of 5 to 10 nm. Crystalline quality of the films depended on the sputtering conditions, especially on total sputtering pressure and oxygen concentration. The optimization of the conditions for seed layer formation by oxygen-reactive solid-phase epitaxy and reactive sputtering was an important factor for improving the crystalline quality of epitaxially grown CeO2 film.
Lithium niobate [LiNbO3 (LN)] thin films with good surface morphology and crystallinity have been epitaxially grown on lithium tantalate [LiTaO3 (LT)] substrates by chemical-vapor deposition using lithium dipivaloylmethane [Li(C11H19O2)] and niobium pentaethoxide [Nb(OC2H5)5] without postannealing. The Li/Nb ratio of LN thin films was easily controlled by adjusting the Nb source temperature, and it is possible to prepare LN thin films with stoichiometric composition (Li/Nb=1). The crystallinity of LN thin films greatly depends on substrate temperature and surface orientation of the LT substrate. It becomes better with increasing substrate temperature and is equivalent to a single crystal at 700 °C. It also becomes better in the order of (012), (110), and (001) planes of the LT substrate. The properties of surface acoustic wave propagation can be improved by preparing the LN thin film on the LT substrate. The electromechanical coupling factor k2 of LN/(012)LT is 0.52%, and is greater than that of the (012)LT substrate alone, 0.081%. The sound velocity of LN/(012)LT calculated from the central resonant frequency is 3200 m/s, and this value is greater than that of the (012)LT substrate by itself, 3140 m/s.
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