Yen-Hwei Chang

High color purity phosphors of LaAlGe2O7 doped with Tm3+ and Er3+

Phosphors of LaAlGe2O7 doped with Tm3+ and Er3+ of high color purity, exhibiting a narrow band emission in the blue and green regions, were obtained. (La1−xLnx)AlGe2O7 (Ln=Tm,Er) powders are bright emitters, with chromaticity color coordinates that are comparable to or better than those of standard phosphors for display or lighting devices. The blue emission of the Tm3+-doped phosphor had CIE chromaticity coordinates (0.151, 0.033) with a dominant wavelength of 455nm and a color purity of 94%. The Er3+-doped phosphor had color coordinates (0.249, 0.718), a dominant wavelength of 542nm, and 92% purity.

Cubic to tetragonal phase transformation of ultrafine BaTiO3 crystallites at room temperature

The critical crystallite size for the cubic (c-) to tetragonal (t-) phase transformation of ultrafine BaTiO3 crystallites was examined. Crystallites were prepared by calcination of barium titanyl oxalate tetrahydrate (BTOT) at various temperatures as to obtain crystallites with different sizes. The c- BaTiO3 crystallite which is thermodynamically considered as a high-temperature form can exist at room temperature if the crystallite is smaller than certain size. The critical size for the c→t phase transformation is ~30 nm. The size may become larger if the crystallite suffers higher lattice strain which can be induced by different powder preparation methods.

Effects of lanthanum doping on the dielectric properties of Ba(Fe0.5Nb0.5)O3 ceramic

The relaxor type of ferroelectric material Ba(FeNb)1∕2O3, which has a partially disordered perovskite structure, has been characterized and shown a maximum plateau of the dielectric permittivity depending upon the temperature. The dielectric constant at a low frequency is up to 30 000 at room temperature. While the single-phase La-doped Ba(FeNb)1∕2O3, Ba1−xLax(Fe1∕2Nb1∕2)1−x∕4O3(0⩽x⩽0.2) has a monoclinic structure solid solutions up to x⩽0.2, the lattice parameters decrease with an increasing La concentration. The temperature dependence of the dielectric constant of the La-doped Ba(FeNb)1∕2O3 was measured with different La contents, showing that the dielectric permittivity is higher than 105 at an 8mol% La concentration. The present dielectric dispersion was interpreted on the basis of the interfacial polarization at the grain-boundary region.

The structure, electrical and sensing properties for CO of the La0.8Sr0.2Co1−xNixO3−δ system

Abstract The perovskite structure of La 0.8 Sr 0.2 Co 1− x Ni x O 3− δ film can be formed in a wide range between 0≦ x ≦0.6 by the dipping method. The distortion of the perovskite structure is described with changing Ni content. The influence of Ni ions which replaced the Co ions on B-sites can be directly observed from its electrical and sensing properties to CO gas. In the range from room temperature to 600°C, the La 0.8 Sr 0.2 Co 1− x Ni x O 3− δ films exhibited semiconductor behavior. The energy barrier for conduction can be reduced by Ni doping. We studied the sensitivity to 50 ppm CO in a temperature range from 150 to 300°C. The influence on sensitivity with different Ni content has also been investigated. The best sensitivity to CO was observed with Ni content equal to 0.5 mol ratio at an operating temperature of 200°C. From the life cycle test at 200°C for La 0.8 Sr 0.2 Co 0.5 Ni 0.5 O 3− δ film, the sensitivity decays to a constant value, but it can be repeatedly used by reheating the film to over 300°C.

Synthesis and characterization of zinc titanate doped with magnesium

Zinc titanate crystals doped with magnesium have been grown by conventional solid state reaction technique using metal oxides. It is shown that they are semiconductors. The characteristics of zinc titanate samples were found to depend on the heating conditions and the amounts of additions. Our studies revealed that magnesium can replace the zinc ion and forms a solid solution in the ZnTiO3 phase. The electrical resistivity of (Zn,Mg)TiO3 varied with sintering temperature, and has a minimum when sintered at 900 8C. Increasing amounts of magnesium will also decrease the resistivity. A V-shaped temperature dependence of resistivity was observed. Furthermore, the dielectric constant increased with sintering temperature and decreased with increasing amounts of magnesium. It also shows a maximum Q factor at a frequency of 8 GHz for the sample of (Zn0.9, Mg0.1)TiO3 sintered at 900 8C. q 2003 Elsevier Ltd. All rights reserved.

The influence of microstructure and deposition methods on CO gas sensing properties of La0.8Sr0.2Co1−xNixO3−δ perovskite films

Abstract The sensing characteristics to CO of the La 0.8 Sr 0.2 Co 1− x Ni x O 3− δ perovskite sensor films were strongly dependent on the deposition methods. By the dipping method, the maximum sensitivity was obtained on the film treated at 800°C for 30 min. By the RF sputtering method, only the heat treated film showed response to CO. The maximum sensitivity was obtained in the film treated at 700°C for 5 h. The sensitivity variation was affected by their grain morphologies. It is found that more porous films exhibit a larger sensitivity. Comparing these two techniques, the film made by the dipping method showed both larger sensitivity and lower operating temperature (180°C). This phenomenon can be attributed to the larger surface reaction area on sensitivity. In addition, different binding energy states of oxygen can also influence the sensing properties to CO.

Green-emitting phosphor of LaAlGe2O7:Tb3+under Near-UV irradiation

As the Tb 3+ -doped LaAlGe 2 O 7 phosphors were synthesized by solid-state method, their characterization and luminescent properties were also investigated. The extraordinary excitation spectra showed only intense f-f transition of Tb 3+ ions around near-UV region, while the 4f-5d transition could be neglected. Under excitation, the as-obtained powders emitted bright green light at 544 nm ( 5 D 4 → 7 F 5 ). By analyzing the decay curves, the energy migration between Tb 3+ ions is conspicuous in 5 D 3 → 7 F 5 transition due to the cross-relaxation in LaAlGe 2 O 7 . One of the interesting results of this work is that the excitation spectrum of the phosphor and the emission spectrum of the near-UV light-emitting diode (LED) have closely overlapped, which provides the potential as a near-UV LED converted phosphor in solid state lighting technology.

The structure and properties of zinc titanate doped with strontium

Abstract Doped and undoped zinc titanate powders were prepared by a conventional solid state reaction technique using metal oxides. The characteristics of zinc titanate samples were found to depend on the heating conditions and the amounts of additions. It is shown that they are semiconductors, and the formation of SrTiO3, α-Zn2TiO4, rutile phase, morphology roughness and open porosity increase with increasing the amounts of doped Sr. The electrical resistivities of doped and undoped zinc titanate reveal a transition from semiconductor behavior to metal-like behavior as the temperature increased. All samples represent the V-type resistivity–temperature characteristic and possess the typical PTCR characteristics above room temperature.

Nanostructured red-emitting MgGa 2 O 4 :Eu 3+ phosphors

Nano-grained phosphors of Eu 3+ -doped MgGa 2 O 4 crystallites were prepared by sol-gel technique. The characterization and optical properties of luminescent MgGa 2 O 4 :Eu 3+ powders have been investigated. The dried sol-gel powders were calcined in air at different temperature from 600 to 1000 °C for 5 h. The x-ray diffraction profiles showed that the MgGa 2 O 4 :Eu 3+ powders began to crystallize around 600 °C and formed stable MgGa 2 O 4 phase in the temperature range of 600–900 °C. The transmission electron microscopy morphology observations revealed that the fired powders exhibit small grain size less than 20 nm. In the PL studies, under ultraviolet (394 nm) excitation, the calcined powders emitted bright red luminescence (615 nm, 5D 0 →7F 2 ), and the powders fired at 900 °C were found to have the maximum photoluminescence intensity. The quenching concentration of Eu 3+ in MgGa 2 O 4 crystallites was also indicated to be about 5∼6 mol%.

Synthesis of La0.8Sr0.2Co0.5Ni0.5O3-δ thin films for high sensitivity CO sensing material using the Pechini process

Abstract A sol–gel technique including the Pechini process followed by a spin-coating procedure has been employed for the preparation of nano-sized La 0.8 Sr 0.2 Co 0.5 Ni 0.5 O 3-δ (LSCN) thin films used as high sensitivity CO sensing materials. In the characterization of the material, X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis–thermogravimetric analysis (DTA–TG) and surface area analyses (BET equation) were used. The results showed that the molar ratio ( R c ) of the citrate/metal ions in the gelation process is the critical factor to obtain a thin film with smooth surfaces, low porosity and homogeneous grain sizes. It is found that an R c value of 3.3 can produce the best results compared to other tested values of 4.0, 2.5, and 1.7. Taking this R c value to prepare the precursor powder and calcined at the temperature of 650°C, a thin film of perovskite structure with average grain sizes of ∼60 nm can be obtained.