Chii Shyang Hwang

Effects of the S ∕ Zn Ratio on the Photoluminescence Properties of Color-Tunable ZnS:Mn Nanophosphors

In this study, color-tunable nanocrystalline phosphors of ZnS:0.5 mol % Mn 2+ (S/Zn ratio = 1, 0.85, 0.75, 0.65, 0.5) were synthesized at a low temperature using the solid-state reaction method. The emission spectra of ZnS:0.5 mol % Mn 2+ samples excited at 325 nm exhibited two emission peaks. The blue-light emission is due to the donor-acceptor recombination in the ZnS host, and the orange-light emission is attributed to the 4 T 1 → 6 A 1 transition of Mn 2+ ions. The chromaticity coordinates of the phosphors changed from orange, near white, to blue with decreasing S/Zn ratio. The local strain and the quantum efficiency of the ZnS:Mn 2+ phosphors are also investigated.

PHASE FORMATION AND COMPOSITION OF MN-ZN FERRITE POWDERS PREPARED BY HYDROTHERMAL METHOD

Mn–Zn ferrite powders were prepared by hydrothermally aging the coprecipitates of compositional metal ions using ammonium hydroxide as a precipitant. R value (alkalinity) = (moles of added OH − )/[(moles of added Zn 2+ ) × 2 + (moles of added Mn 2+ ) × 2 + (moles of added Fe 3+ ) × 3] was introduced to adjust the amount of added ammonia. The results show that the R value of starting suspension and hydrothermal time have similar and dominant effects on the composition, spinel ratio, and crystallite size of synthesized powders. From the analyses of x-ray diffraction (XRD) and inductively-coupled plasma (ICP), it notes that no α–Fe 2 O 3 peak in the XRD patterns of powders synthesized at R = 2–3, 150 °C × 2 h, may be due to lower degree of crystallinity and less amount of α–Fe 2 O 3 existing in these powders. Both the increase of hydrothermal time and of R value can promote the crystallinity of powders and also cause a significant loss of zinc, hinting that in the hydrothermal process, the loss of zinc may play a crucial role in the crystallinity of hydrothermally synthesized powders.

Characteristics of powder and sintered bodies of hydrothermally synthesized Mn-Zn ferrites

To improve the sinterability of powders fabricated by the conventional mixed-oxides method, ultrafine Mn-Zn ferrite powders were hydrothermally synthesized from metal nitrates solution using ammonia as a precipitant. The R value (alkalinity) was introduced to adjust the amount of added OH− in the reaction suspension. The characteristics of the powders synthesized at different hydrothermal conditions and the properties of the sintered bodies were investigated. The results show that the R value and hydrothermal time have a great effect on the compositions and phases of hydrothermally synthesized Mn-Zn ferrite powders. Powders synthesized from a starting suspension with a higher content of Zn ions (or lower content of Mn2+) may approach to a stable spinel structure with a lower Mn/Zn ratio as the hydrothermal time is longer. Factors affecting the position of the diffraction angle (2θ) of the spinel Mn-Zn ferrite (311) of powders may include both the compositions of spinel ferrite structure and crystallite sizes (or particle sizes) of powders. Some possible reasons were suggested to explain the dependence of composition and phase of hydrothermally synthesized Mn-Zn ferrite powders on the R value and hydrothermal time. The temperature that the green compact begins to shrink at increases with increasing R value, and ranges from 510°C (R = 2) to 650°C (R = 6). After being sintered at 950°C for 2 h in N2 atmosphere, the relative sintered density of each specimen reaches a value of 94.5–99.8%.

Nanostructures and capacitive properties of manganese oxide film synthesized by hydrothermal electrochemical process

For application in a supercapacitor as electrodes, nano-scaled manganese oxide films were deposited on titanium substrates in manganese acetate solution using the hydrothermal electrochemical process. The effects of synthesis temperature, pH, and the concentration of manganese acetate on the material characteristics and capacitive behavior of manganese oxide film were investigated. The complex tri- and divalent hydrous manganese oxides examined using X-ray photoelectron spectroscopy (XPS) and Raman scattering spectroscopy were obtained at 60 °C in 0.2 M neutral solution. A maximum specific capacitance (SC) of 244 F g-1 was measured using cyclic voltammetry (CV) at 25 °C in 0.1 M Na2SO4 within the range of 0–1 V. The nanometer scale surface morphology of the film was altered by varying the synthesis temperature, pH, and the concentration of the solution. In addition to the surface morphology, the extent of hydration in the manganese oxide film also affected the capacitive properties.

Effect of synthesis temperature on the characteristics of hydrous manganese oxide deposited using the hydrothermal electrochemical method

Nanometer-scale manganese oxide films were synthesized at various temperatures in the range of 60–150 °C using the hydrothermal electrochemical deposition method. Field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were used to verify that the deposits were tri- and divalent hydrous manganese oxides. The electrochemical performance of manganese oxide films was examined by cyclic voltammetry (CV) in the range of 0.0–1.0 V [versus saturated calomel electrode (SCE)] in 0.1 M Na2SO4 solution at 25 °C. The specific capacitance (SC) increased with decreasing synthesis temperature, with a maximum value of approximately 244 Fg-1 at 60 °C. Crystalline manganese oxide prepared at 150 °C improved stability of capacitive performance, which only decreased by 4% after 800 cycles. The SC depended not only on the surface morphology but also on the amount of hydration in the manganese oxide structure. The deposited manganese oxide has the chemical formula Mn3O4nH2O (n~1.2), as established using thermogravimetric and differential thermal analysis (TG/DTA).

Effect of calcination on characteristics and sintering behaviour of Al2O3-ZrO2 composite powders

The effect of calcination on the characteristics and sintering behaviour of zirconia-toughened alumina (ZTA) composite powders has been investigated. TiO2 was selected as an additive to promote the sinterability of ZTA powders. The starting materials were Al2O3 powder, Zr(OC3H7)4 and Ti(OC3H7)4, and homogeneous ZTA powder containing Zr-O-Ti bonding was prepared. Calcination affected the tetragonal→monoclinic phase transformation temperature of ZrO2 crystallizing from the gels. Calcination improved the densification rate of ZTA powder compact during sintering, which was attributed to the optimal ZrO2 particle size and distribution on the surface of alumina. A ZTA specimen with high bulk density and high tetragonal ZrO2 content was obtained under the conditions of 850°C/1 h calcination and 1500°C/1 h sintering.

Photoluminescence of Nano-scaled YAG:Ce Phosphor Powders

Four series of YAG:Ce phosphor powders were synthesized by solid state, co-precipitation, and sol-gel (with two different growth inhibitors, HMDS and PAA respectively) methods, which results in the average particle sizes of 800, 56, 43, and 32 nm respectively. All the nano-scale YAG powders were heat treated below 1000_. Both TEM and XRD results of these nano-scale YAG powders show high degree of crystalline structure. Photoluminescence (PL) characterization shows that the nano-scale YAG phosphor powders emit a higher intensity of luminescence than that of sub-micro sized samples. The effect of different Ce doping level on the PL of (Y 3-x Ce x )Al 5 O1 2 shows a maximum at x ∼ 0.01. The Commissiom International de I.Eclairage (CIE) chromaticity coordinates were determined and show different trends of shifts in the resulting PL spectra. Systematic study of the processing parameters characterized by DTA/TGA, XRD, FTIR, and BET will be discussed in detail in the report.

Promoting porcelain–zirconia bonding using different atmospheric pressure gas plasmas

OBJECTIVES To evaluate the effects of different atmospheric-pressure plasma (APP) on the physicochemical properties of yttria-stabilized zirconia, and promoting the adhesion of veneering porcelain. METHODS Cercon base zirconia disks were prepared to receive different treatments: as-polished, three APPs (oxygen, OP; argon, AP; and CF4, CP), and grit-blasted (GB). Their surface roughness and hydrophilicity were measured, and surface morphology was examined either after treatments, after simulated porcelain firing, or additional thermal etching. X-ray photoelectron spectroscopy (XPS) analysis characterized the surface chemical compositions. Shear bond strength (SBS) tests examined the adhesion between veneering porcelain and zirconia either before or after thermocycling. The layered ceramic disks were also sectioned to inspect the porcelain-zirconia interfaces. Statistical analysis was performed with one-way ANOVA and post hoc Duncans test. RESULTS Grit-blasting caused surface damage and increased roughness. All APP-treated disks exhibited deeper grain boundaries and enlarged grain sizes after thermal etching, while CP disks revealed additional particle dispersions. Three APPs rendered the zirconia surface superhydrophilic. XPS spectra of three APP groups revealed increased hydroxyl groups and reduced C-C contents, and CP group especially showed the existence of Z-F bonds. CP exhibited the highest SBS both before and after thermocycling, while AP and GB also showed improved SBSs compared to the as-polished. OP presented reduced SBS, and its cross-sections showed increased microporosities in the veneering porcelain. SIGNIFICANCE APP did not change surface morphology but enhanced wettability. CP and AP improved porcelain-zirconia SBSs, primarily through surface hydroxylation. OP induced the microporosities in porcelain and adversely affected the adhesion.

Spark plasma sintering of PbTe thermoelectric bulk materials with small grains

PbTe is a conventional thermoelectric material for thermoelectric generator at intermediate temperature. Small grain size effect has been reported to improve PbTe ZT values (figure of merit). We report a combination process of attrition milling and spark plasma sintering (SPS) for preparing PbTe bulk materials with small grain sizes. The PbTe powders were milled by attrition under 600 rpm for 6–96 h and followed by SPS process under the sintering temperature of 573–773 K, the heating rate of 100 K/min, and the sintering pressure of 50 MPa. The powders and bulk materials as-prepared were then studied by X-ray diffraction patterns, scanning electron microscopy images, and transmission electron microscopy images. Transport properties of polycrystalline PbTe bulks were evaluated through temperature dependent thermal conductivity measurements.Copyright

Fabrication and Mechanical Properties of Al2O3 Reinforced NiAl Composites

Al2O3 reinforced NiAl composites can show better mechanical propertie s than monolithic NiAl. For enhancing the properties of NiAl/Al 2O3 composites NiAl powders were oxidized and/or ball milled to form Al 2O3 into NiAl matrix. The as-received and oxidized/ball milled NiAl powders were studied to determine the effects of oxidation and ball milling . Al2O3 particles are dispersed in the grain boundaries and within the NiAl grains of hot-pressed NiAl/ Al 2O3 composites. As the milling time and the oxidation temperature of NiAl powder incre ased, αAl2O3 contents increase, and fracture mode of the composite transforms from intergranular to transgranular cleavage which accounts for the enhancement of mechanical properties of NiAl/Al 2O3 composites.