Dah-Shyang Tsai

Growth and characterization of well-aligned densely-packed rutile TiO2 nanocrystals on sapphire substrates via metal?organic chemical vapor deposition

Well-aligned densely-packed rutile TiO(2) nanocrystals (NCs) have been grown on sapphire (SA) (100) and (012) substrates via metal-organic chemical vapor deposition (MOCVD), using titanium-tetraisopropoxide (TTIP, Ti(OC(3)H(7))(4)) as a source reagent. The surface morphology as well as structural and spectroscopic properties of the as-deposited NCs were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAED), x-ray diffraction (XRD) and micro-Raman spectroscopy. FESEM micrographs reveal that vertically aligned NCs were grown on SA(100), whereas the NCs on the SA(012) were grown with a tilt angle of ∼33° from the normal to substrates. TEM and SAED measurements showed that the TiO(2) NCs on SA(100) with square cross section have their long axis directed along the [001] direction. The XRD results reveal TiO(2) NCs with either (002) orientation on SA(100) substrate or (101) orientation on SA(012) substrate. A strong substrate effect on the alignment of the growth of TiO(2) NCs has been demonstrated and the probable mechanism for the formation of these NCs has been discussed.

Field emission from vertically aligned conductive IrO2 nanorods

We report on the preparation and field-emission properties of vertically aligned conductive IrO2 nanorods. The unique geometrical features of IrO2 nanorods, including nanosized structure and self-assembled sharp tip, exhibit a strong effect on field enhancement (β∼40 000), which result in a low threshold field (Eth∼0.7 V/μm) defined at the beginning of emission. A low turn-on field for driving a current of 10 μA/cm2 is about 5.6 V/μm, which is comparable with the carbon nanotube, diamond, and amorphous carbon. The potential of using IrO2 nanorods as an emitter material has been demonstrated.

Electrochemical capacitors of RuO2 nanophase grown on LiNbO3(100) and sapphire(0001) substrates

The electrochemical properties of RuO2/LNO and RuO2/SA electrodes made of RuO2 rods and plates of nanometre size on LiNbO3(100) and sapphire(0001) substrates are investigated. Both rods and plates are grown vertically using chemical vapor deposition. The RuO2/LNO electrode has a higher electrolyte/solid surface area compared with the RuO2/SA electrode. When immersed in H2SO4 acid, voltammograms of both electrodes exhibit chemisorption and pseudocapacitive characteristics of the RuO2 single crystal. The chemisorption features decrease after repeated cyclic voltammetry (CV) sweeps, and the voltammograms become more mirror-like. The specific capacitance of the RuO2/LNO electrode measured in CV is 569 F g−1, that of RuO2/SA 357 F g−1. These values are reconfirmed in charging–discharging measurements. The measured capacitance decreases with the sweep rate, and the decreasing trend of RuO2/LNO is higher than that of RuO2/SA. Reduction of accessible charge at high sweep rates results from higher internal resistance of the RuO2/LNO electrode which is distributed because of its porous nature. The impedance spectrum of the RuO2/LNO electrode confirms its higher internal resistance. It also indicates the electrode is a nearly ideal capacitor below a knee frequency 200 Hz. Since the knee frequency is less than 1 Hz for most electrochemical capacitors, both RuO2/LNO and RuO2/SA are electrodes for fast charging–discharging applications.

Preparation and characterization of iridium dioxide–carbon nanotube nanocomposites for supercapacitors

A thin film of novel hierarchical structure, suitable for supercapacitor applications, has been developed through combining conductive multi-wall carbon nanotubes (MWCNTs) and square IrO(2) nanotubes (IrO(2)NT) of nanometer size. Synthesis of this hierarchical structure with open porosity is performed by depositing IrO(2) short tubes densely along the long wires of carbon nanotube on a substrate of stainless steel. A IrO(2) tube of rutile structure grows in the [001] direction, with an opening at its top, surrounded by very thin walls. The IrO(2) addition on the MWCNT template increases the capacitance of the CNT thin film effectively, because of pseudocapacitance of the IrO(2) surface. For this particular composite, featured with two tubular nanostructures, the specific capacitance increases from 15 F g(-1) (MWCNT) to 69 F g(-1) (IrO(2)NT/MWCNT), measured using the galvanostatic discharge experiment. Its property of fast retrieval of the stored charge is assured in the impedance measurement, showing that the internal resistance of the IrO(2)NT/MWCNT nanocomposite electrode is lower than that of the bare MWCNTs.

Effective conductivities of random fiber beds

Upper and lower reciprocal variational principles have been used to estimate the effective conductivity (or diflusivity)across beds of randomly dispersed fibers with orientations distributed in two and three dimensions. Estimated thermal conductivities of fiber glass insulation, graphite-epoxy fiber reinforced composite materials, as well as the diffusivities of several fibrous materials agree well with measured values.

Growth control and characterization of vertically aligned IrO2 nanorods

Iridium dioxide (IrO2) nanorods with pointed tips have been grown on Si(100) and transition-metal-coated-Si(100) substrates, via metal–organic chemical vapor deposition (MOCVD), using (MeCp)Ir(COD) as the source reagent. The as-deposited nanorods were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). FESEM micrographs revealed that the majority of the nanorods are a wedge shape in cross section and converge at top; occasionally several of them pack into a column of a spiral tip. The vertical alignment and packing density are significantly improved by prior deposition of a thin layer of Ti on Si. TEM and XRD results indicate that the sputtered Ti thin layer erects the nanorods in the c-axis direction. XPS spectra show that iridium in IrO2 nanorods also exist in a higher oxidation state.

One-dimensional conductive IrO2 nanocrystals

We review the results of the synthesis of IrO2 nanocrystals (NCs) on different substrates via metal-organic chemical vapour deposition (MOCVD) using (MeCp)(COD)Ir as the source reagent. The surface morphology, structural and spectroscopic properties of the as-deposited NCs were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffractometry (XRD), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The roles of different substrates for the formation of various textures of nanocrystalline IrO2 are studied. Several one-dimensional (1D) nanostructures have evolved by decreasing the degree of interface instability. The morphological evolution occurs from triangular/wedged nanorods via incomplete/scrolled nanotubes to square nanotubes and square nanorods (NRs), with increasing morphological stability. The results show that the three-dimensional (3D) grains composing traditional film belong to the most stable form as compared to all the 1D NCs, and the sequential shape evolution has been found to be highly correlated to a morphological phase diagram based on the growth kinetics. In addition, area selective growth of IrO2 NRs has been demonstrated on sapphire(012) and sapphire(100) substrates which consist of patterned SiO2 as the nongrowth surface. The initial growth of IrO2 nuclei is studied. Selectivity, rod orientation, and other morphological features of the nanorod forest can find their origins in the nucleation behaviour during initial growth. XPS analyses show the coexistence of higher oxidation states of iridium in the as-grown IrO2 NCs. The usefulness of the experimental Raman scattering together with the modified spatial correlation (MSC) model analysis as a residual stress and structural characterization technique for 1D IrO2 NCs has been demonstrated. The field emission properties of the vertically aligned IrO2 NRs are studied and demonstrated as a high-performance and robust field emitter material owing to its low work function, low resistivity and excellent stability against oxygen.

Solvent Debinding Kinetics of Alumina Green Bodies by Powder Injection Molding

Solvent debinding of alumina green bodies shaped by powder injection molding is investigated. Debinding is carried out in normal hexane (n-C6), normal heptane (n-C7) and normal octane (n-C8), at 50, 60 and 80 °C. Wax is the major constituent being leached. A shrinking core is observed at the early stage of 50 °C solvent debinding. Debinding kinetics is adequately described by a single-parameter (effective diffusivity De) model except in the region of the core. The effective diffusivity decreases with the increase of carbon number of solvent from 5.2 × 10−6 (n-C6) to 3.5 × 10−6 cm2/s (n-C8) at 50 °C and from 6.2 × 10−6 (n-C6) to 3.8 × 10−6 cm2/s (n-C8) at 60 °C. For each solvent, the effective diffusivity is proportional to (leaching temperature/solvent viscosity). A core of clear edge not removed by solvent leaching could cause cracks in the subsequent processing steps. Cracks are observed around the edge of core after thermal debinding.

Microstructure of Ba(Mg1/3Ta2/3)O3-BaSnO3 microwave dielectrics

Abstract Ceramics of Ba(Mg 1/3 Ta 2/3 )O 3 -BaSnO 3 (BSMT) are synthesized by a two-stage calcination method. Microstructures are analyzed, using X-ray diffractometry, transmission electron microscopy, Raman spectroscopy, and scanning electron microscopy. The addition of BaSnO 3 reduces the cation ordering of Ba(Mg 1/3 Ta 2/3 )O 3 and its sinterability. Variations in ordered structure with BaSnO 3 content are revealed by the reduction in intensity of the superlattice reflections, the domain size, the c/a ratio, and the shifting and broadening in Raman lines. The relations between microstructure and quality factors of BSMT are discussed. ©

The growth and characterization of well aligned RuO2 nanorods on sapphire substrates

Self-assembled and well aligned RuO2 nanorods (NRs) have been grown on sapphire (SA) substrates via metal–organic chemical vapour deposition (MOCVD), using bis(ethylcyclopentadienyl)ruthenium as the source reagent. The surface morphology, structural, and spectroscopic properties of the as-deposited NRs were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and micro-Raman spectroscopy. FESEM micrographs reveal that vertically aligned nanorods (NRs) were grown on SA(100), while the NRs on the SA(012) were grown with a tilt angle of from the normal to the substrates. TEM and SAD measurements showed that the RuO2 NRs with square cross-section have the long axis directed along the [001] direction. The XRD results indicate that the RuO2 NRs are (002) oriented on SA(100) and (101) oriented on SA(012) substrates. A strong substrate effect on the alignment of the RuO2 NRs growth has been demonstrated and the probable mechanism for the formation of these NRs has been discussed. XP spectra show the coexistence of higher oxidation state of ruthenium in the as-grown RuO2 NRs. Micro-Raman spectra show the red-shift and peak broadening of the RuO2 signatures with respect to that of the bulk counterpart which may be indicative of a phonon confinement effect for these NRs.