Chung-Hsin Lu

Influence of hydrothermal conditions on the morphology and particle size of zinc oxide powder

Abstract Crystalline zinc oxide powder has been successfully prepared by adopting ammonia as the base source via the hydrothermal process at temperature ⩾100°C. The temperature for synthesizing ZnO powder is around 200°C lower than that of conventional heating process. The formation and growth ZnO powder under hydrothermal environment progress rapidly as soon as the temperature reaches 100°C. Prolonging the reaction time at 100°C does not significantly influence the characteristics of obtained powder; however, raising the reaction temperature slightly reduces the particle size and the production yield of ZnO powder. On the other hand, as the pH of the starting solution increases from 9 to 12, the morphology of ZnO powder markedly varies from an ellipsoidal shape to a rod-like shape. In addition, the crystallinity and particle size of ZnO powder increase with rise in the pH of solutions, but the yield of production decreases. The reason for the variation of morphology and characteristics of obtained powder with pH mainly depends on the nucleation states occurring in the hydrothermal reaction.

Hydrothermal synthesis of nonlinear optical potassium niobate ceramic powder

Abstract Potassium niobate KNbO3 particles having nonlinear optical properties were successfully prepared via a hydrothermal process. When KOH, with a concentration of 8 M, reacted with Nb2O5 particles at 200°C, monophasic KNbO3 was obtained. The high concentration of KOH was found to be a critical requisite for the formation of KNbO3. Compared with the traditional solid-state reaction, the hydrothermal process significantly lowered the temperature required for synthesizing KNbO3. The added amounts of Nb2O5 in the hydrothermal reaction greatly influenced the morphology and particle size of KNbO3, as well as the intensity of the second harmonic generation.

Sol–gel synthesis and photoluminescent properties of cerium-ion doped yttrium aluminium garnet powders

Luminescent Y3Al5O12∶Ce3+ (YAG∶Ce3+) ceramics with nanometer- to micronmeter-sized particles were prepared by a new sol–gel pyrolysis method. Notwithstanding thermochemical considerations, the combined fuel system of urea and poly(vinyl alcohol) (PVA) yielded nanoparticles exhibiting single-crystal features. This can be attributed to the pyrolysis of sols trapped within the cages formed by the cross-linking of PVA chains in the presence of urea. Another salient feature of this investigation is the higher luminescence yield for YAG∶Ce3+ samples prepared using only urea as the fuel at much lower temperature (1000 °C) compared to YAG∶Ce3+ samples prepared by the conventional solid-state reaction method at 1450 °C. In addition, the partial aliovalent substitution of Y3+ by Ba2+ in this important luminescent ceramic results in new defect complexes serving as precursors for the generation of excitons. Under band-edge photoexcitation, the excitons thus generated can resonate with excited levels of Ce3+ which have a profound dependence on the particle size.

Synthesis of cerium hydroxycarbonate powders via a hydrothermal technique

Abstract CeOHCO 3 powders have been directly synthesized using a hydrothermal process at temperatures as low as 160°C. The well-dispersed powders are obtained in a short period of reaction time during hydrothermal reaction via the hydrolysis of urea. For synthesizing CeOHCO 3 , the concentration of urea is found to be a crucial determinant, which has significant effects on the morphology of the derived powders. When low urea concentrations are provided, the formed particles are rhomboidal platelets. On the other hand, the high urea concentrations cause the shape of the powders to become prismatic. Increasing the concentration of urea tends to increase the particle size as well as the aspect ratio of CeOHCO 3 powders. After further heating at 500°C, a phase transformation from orthorhombic CeOHCO 3 to cubic CeO 2 takes place. The crystallinity and size of CeO 2 strongly depend on the particle size of CeOHCO 3 .

Emulsion precipitation of submicron zinc oxide powder

Abstract Ultrafine zinc oxide powder was successfully synthesized through the emulsion precipitation process. The aqueous solution containing zinc cations was well emulsified in n -heptane solution, and then precipitated by adding ammonia. After calcination of the precipitates, submicron zinc oxide powder with a near spherical morphology was obtained. On comparison with the conventional precipitation method, this developed process was confirmed to be superior in preparing uniform ZnO powder and reducing its particle size. The emulsion stability was significantly affected by the ratio of the volume of the oil phase to that of the aqueous phase, and this ratio also influenced the mean particle size of ZnO powder.

Microwave-hydrothermally synthesized (Sr1−x−yCexTby)Si2O2−δN2+μ phosphors: efficient energy transfer, structural refinement and photoluminescence properties

A facile microwave-assisted hydrothermal method was developed to prepare oxynitride-based (Sr1−x−yCexTby)Si2O2−δN2+μ phosphors. The microwave-assisted hydrothermal process led to lower synthesis temperatures and shortened heating duration. The obtained powders exhibited a narrow size distribution. The Rietveld refinement reveals the obtained phosphors to have a triclinic crystal system. For Ce3+ doped SrSi2O2N2 phosphors, the emission spectrum exhibited an asymmetric band at 447 nm (λex = 322 nm), corresponding to the blue hue. As Ce3+ and Tb3+ ions were co-doped into SrSi2O2N2, the energy transfer occurred via a dipole–dipole interaction. With increasing the Tb3+ concentration, the Tb3+ emission intensity enhanced and reached a maximum with the Tb3+ concentration of 2 mol%, then decreased due to the concentration quenching effect. It is also observed that the intensity of the Ce3+ emission peak decreased gradually. This indicates the effective energy transfer from Ce3+ to Tb3+ ions. The CIE coordinate of (Sr1−x−yCexTby)Si2O2−δN2+μ phosphors shifted from the blue region towards the green region with increasing Tb3+ concentration. The above findings indicate that the emitting colors of the microwave-hydrothermally derived (Sr1−x−yCexTby)Si2O2−δN2+μ phosphors can be tuned over a wide range under UV excitation.

New non-fatigue ferroelectric thin films of barium bismuth tantalate

Abstract New non-fatigue ferroelectric thin films of barium bismuth tantalate (BaBi 2 Ta 2 O 9 ) were synthesized in this work. These films were prepared on Pt/Ti/SiO 2 /Si substrates by the metalorganic decomposition method. As-deposited films were amorphous, and became well-crystallized after annealing at 700°C. The annealed films exhibited fairly smooth surface and small grain size (around 10 nm). The measured dielectric constant and dissipation factor of BaBi 2 Ta 2 O 9 films at 10 kHz were 97.7 and 0.0257, respectively. The polarization–electric field hysteresis loops revealed the ferroelectric characteristics of BaBi 2 Ta 2 O 9 films. Furthermore, the fatigue test indicated that these films hardly degraded in the polarization after 10 9 switching cycles. Because of its ferroelectric properties and excellent fatigue resistance, BaBi 2 Ta 2 O 9 has great potential in becoming a new candidate material for the applications of ferroelectric random access memories.

Synthesis of nano-sized LiNi0.8Co0.2O2via a reverse-microemulsion route

Nano-sized LiNi0.8Co0.2O2 powders utilized in lithium-ion secondary batteries have successfully been synthesized using a newly developed reverse-microemulsion process. This route not only reduces the essential calcination time to 2 h, but also effectively decreases the average particle size of LiNi0.8Co0.2O2 powders to the order of nanometers. In addition, the extent of capacity degradation of LiNi0.8Co0.2O2 at room temperature is markedly reduced.

Glycothermal preparation of potassium niobate ceramic particles under supercritical conditions

Abstract Potassium niobate KNbO 3 particles were synthesized via a newly developed glycothermal process using isopropanol as the reaction medium in supercritical environment. Increasing the concentration of KOH and the molar ratio of K + /Nb 5+ substantially facilitated the formation of KNbO 3 . When the concentration was 0. 5 M and the molar ratio of K + /Nb 5+ was greater than 2:1, well crystallized monophasic KNbO 3 was successfully produced at as low as 250 °C. In comparison with the traditional hydrothermal process, the supercritical glycothermal process greatly reduced the required concentration of KOH. This supercritical glycothermal process provides a new prospective approach for synthesizing electronic ceramic powders at low temperatures.

Effects of additives and secondary phases on the sintering behavior of zinc oxide-based varistors

AbstractThe influence of additives and secondary phases on the sintering behavior of zinc oxide-based varistors has been investigated in thisstudy. Addition of Bi 2 O 3 and Sb 2 O 3 to ZnO results in rapid shrinkage of the specimens at around 10008C. This rapid shrinkage is attributedto the reaction between the pyrochlore phase and ZnO. This reaction has been verified by investigating the system of ZnO and a synthesizedpyrochlore phase ZnBi 1.5 Sb 1.5 O 7 . This pyrochlore phase has a cubic symmetry, with a lattice parameter a o ‹10.4438 A˚ . ZnBi 1.5 Sb 1.5 O 7 isfound to react with ZnO at 10008C to lead to the formation of Bi 2 O 3 , which accelerates the sintering rate of ZnO through a liquid-phasesintering mechanism. On the other hand, the spinel phase a-Zn 7 Sb 2 O 12 hinders the sintering process as well as the grain growth of ZnO.# 2000 Elsevier Science S.A. All rights reserved. Keywords: Zinc oxide; Pyrochlore phase; Sintering; Microstructure 1. IntroductionZinc oxide has been widely used to fabricate varistorssince 1970s because of its excellent non-ohmic property [1].Varistors are important protection devices against voltagesurges in solid-state devices and electrical power generationsystems [2]. To improve the electrical characteristics ofvaristors, considerable research efforts have been devotedto study the effects of adding small amounts of variousadditives to ZnO [3–7]. The most commonly used additivesare Bi