Wein-Duo Yang

Nanostructured Iron Oxide Films Prepared by Electrochemical Method for Electrochemical Capacitors

Nanostructured iron oxides were electrodeposited anodically onto nickel substrate in aqueous solution at room temperature. Deposited films after annealing at 300°C are characterized as rhombohedral Fe 2 O 3 . The morphology of the porous film deposited at high current density shows nanosheets of 10-16 nm in thickness. A film deposited at low current density has aggregates of nanorods. Electrochemical impedance results show that the film of randomly distributed nanosheets has a higher percentage of large pores than the film of aggregated nanorods. Consequently, specific capacitance of the film with nanosheets is higher than that of the film with aggregated nanorods at galvanostatic charge/discharge.

Preparation of barium strontium titanate powder from citrate precursor

TiCl 4 or titanium isopropoxide reacted with citric acid to form a titanyl citrate precipitate. Barium strontium citrate solutions were then added to the titanyl citrate reaction to form gels. These gels were dried and calcined to (Ba,Sr)-TiO 3 powders. The gels and powders were characterized by DSC/TGA, IR, SEM and XRD analyses. These results showed that, at 500 °C, the gels decomposed to Ba,Sr carbonate and TiO 2 , followed by the formation of (Ba,Sr)-TiO 3 . The onset of perovskite formation occurred at 600 °C, and was nearly complete at 1000 °C. Traces of SrCO 3 were still present. The cation ratios of the titanate powder prepared in the pH range 5-6 were closest to the original stoichiometry. Only 0.1 mol% of the free cations remained in solution. The titanyl citrates were precipitated in either ethanol or acetone. The acetone-derived precipitates were always viscous, but those with a sufficient quantity of alcohol were powdery. The specific surface areas of the ceramic powders obtained by air-, vacuum- and freeze-drying methods were 8.3 × 10 3 , 10.2 × 10 3 and 12.5 × 10 3 m 2 kg -1 , respectively. The powder obtained by freeze-drying had the lowest degree of agglomeration. The precipitated powders of titanyl citrate which were freeze-dried and calcined at 1100°C were compacted and sintered at 1300 °C to obtain dense ceramic bodies with 95% of the theoretical density.

Preparation and electrical characterisation of strontium titanate ceramic from titanyl acylate precursor in strong alkaline solution

Abstract Strontium titanate powder can be obtained from titanyl acylate precursor by using acetic acid chelating with titanium isopropoxide in a strong alkaline solution (pH > 13) at room temperature. Infrared spectroscopy, differential scanning calorimetry/thermogravimetric analysis and X-ray diffraction were used to investigate the effects of the size and properties of the precipitated precursor on the properties of the powder and the sinterability for ceramics. It was found that the strontium ion diffused into the gel made by the hydrolysis of the titanyl acylate precursor to form strontium titanate powder. The reaction rate of forming powder at higher temperature such as 100 °C and higher water content (the ratio of water to titanium alkoxide is 20) or higher acetic acid (the ratio of acetic acid to titanium alkoxide is 10), is rapid and the particle size is less than 0.1 μm. Without adding any reagent the density of the sintered strontium titanate at 1300 °C from powder after freeze drying was over 95% of its theoretical density. The dielectric constant and dissipation factor of the strontium titanate body sintered at 1250, 1300, 1400 and 1450 °C, were also determined at a frequency of 1 kHz to 10 MHz at constant temperature.

PZT/PLZT ceramics prepared by hydrolysis and condensation of acetate precursors

Titanyl zirconyl acetate was prepared by chelating titanium isopropoxide with acetic acid, and was used as a precursor in a sol-gel process to obtain PZT and PLZT powders. The combined results of the DTA/TGA, FTIR and SEM studies indicate that the PLZT-precursor contained a higher mol fraction of acetate groups and that polycondensation was slower, resulting in a small particle size for the PLZT powder. X-ray photoelectron spectroscopy (XPS) was used to detect Ti oxidation states on the surface of the PZT and PLZT bulk ceramics sintered at various temperatures. The Ti binding energy of the PLZT powders prepared at various temperatures were all lower than those of the corresponding PZT powders, indicating a relatively higher Ti3+ concentration on the PLZT surface. This higher Ti3+ surface concentration is responsible for the higher relative dielectric constant measured for the PLZT bulk ceramics.

Preparation and electrical properties of fine strontium titanate powder from titanium alkoxide in a strong alkaline solution

Abstract The particle size of the forming ceramic powder is related to the morphology of forming precipitated precursor. The rates of hydrolysis and condensation of titanyl acylate gel were slow because of the chelating action of acetic acid. Therefore, the particle size of the precursor gel was uniform and finer, and the formation rate of the powder was quicker. The powder obtained contained fine particles of size ∼ 0.04–0.05 μm and was readily sintered at 1423–1523 K. The dielectric constant and dissipation factor of strontium titanate were determined.

An Effective Electrodeposition Mode for Porous MnO2/Ni Foam Composite for Asymmetric Supercapacitors

Three kinds of MnO2/Ni foam composite electrode with hierarchical meso-macroporous structures were prepared using potentiodynamic (PD), potentiostatic (PS), and a combination of PS and PD(PS + PD) modes of electrodeposition. The electrodeposition mode markedly influenced the surface morphological, textural, and supercapacitive properties of the MnO2/Ni electrodes. The supercapacitive performance of the MnO2/Ni electrode obtained via PS + PD(PS + PD(MnO2/Ni)) was found to be superior to those of MnO2/Ni electrodes obtained via PD and PS, respectively. Moreover, an asymmetric supercapacitor device, activated carbon (AC)/PS + PD(MnO2/Ni), utilizing PS + PD(MnO2/Ni) as a positive electrode and AC as a negative electrode, was fabricated. The device exhibited an energy density of 7.7 Wh·kg−1 at a power density of 600 W·kg−1 and superior cycling stability, retaining 98% of its initial capacity after 10,000 cycles. The good supercapacitive performance and excellent stability of the AC/PS + PD(MnO2/Ni) device can be ascribed to its high surface area, hierarchical structure, and interconnected three-dimensional reticular configuration of the nickel metal support, which facilitates electrolyte ion intercalation and deintercalation at the electrode/electrolyte interface and mitigates volume change during repeated charge/discharge cycling. These results demonstrate the great potential of the combination of PS and PD modes for MnO2 electrodeposition for the development of high-performance electrodes for supercapacitors.

Determination of optimal experimental conditions for the preparation of lead zirconate titanate via sol-precipitate process by applying statistical methods

Abstract A fractional factorial design was implemented to optimize the experimental conditions for the preparation of lead zirconate titanate (PZT) from acetate precursors. The effects of such preparatory conditions as the molar ratio of acetic acid to metal alkoxide, the water to alkoxide ratio, the pH value, reaction temperature and the stirring speed, were systematically studied by using a Taguchi orthogonal array design. Results indicated that the molar ratio of acetic acid to metal alkoxides and the reaction temperature were the key variables influencing the average particle size of the powders obtained. By combining the optimal setting of these two influential processing variables, a fine powder with a particle size of ≈2200 A was obtained. Fourier transfer infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to investigate the effects of the chelation on the rate of formation of the precursor and PZT powder. The rates of hydrolysis and polycondensation of the acetate precursors were slow because of the chelation of a high ratio of acetic acid with the alkoxide. The particle size of the obtained PZT powder was small and a homogeneous distribution resulted.

Solvothermal synthesis of TiO 2 photocatalysts in ketone solvents with low boiling points

The titanium dioxide (TiO2) photocatalysts were synthesized by a solvothermal process in highly alkaline 70 : 30 water : ketone solutions with a TiO2-P25 precursor and calcined at different temperatures. The ketone solvents, such as acetone and methyl ethyl ketone (MEK), had low boiling points (<100°C). The as-prepared samples were characterized by XRD, TEM, FTIR, UV-vis and Raman spectroscopy. Theeffects of the different solvents on the nanostructure, the morphology, and the photocatalytic performance of the TiO2 productswere investigated. Nanotubes formed in water and water-MEK, while nanoparticle/nanowires formed inwateracetone and water-acetone-MEK. The ketone solvents played an important role in the improving nanostructure properties of these products, which affected their photocatalytic reactions. The results indicated that samples synthesized in solvents such as water and MEK had high adsorption and photocatalytic behaviors. The photocatalytic reactivity was the greatest for the TiO2 prepared in MEK and calcined at 100°C, which was even more reactive than the sample prepared in water and TiO2-P25 powder.

Optimization of the experimental conditions for the preparation of a thin strontium titanate film by hydrothermal process

A Taguchi orthogonal array design was implemented to optimize the experimental conditions for the preparation of a thin strontium titanate film on a Ti substrate using a hydrothermal process. The effects of such preparatory conditions as the strontium-concentration, pH-value, reaction temperature, reaction time and the substrate surface treatment were systematically studied. Results indicate that the effects of the reaction temperature and reaction time are the key variables influencing the relative crystal intensity (intensity of (110) SrTiO3 peaks in XRD spectrum/intensity of (101) Ti peak in XRD spectrum) of the films obtained. By combining the optimal settings of the two most influential process variables, a polycrystalline, SrTiO3-films was obtained, with improved crystallinity and a grain size of 0.2–0.4 μm and a dielectric constant of 360 at I MHz.

Alkyl groups effect on the formation of ultrafine Nb2O5-doped SrTiO3 powders by sol-precipitation process

Abstract Nb 2 O 5 -doped SrTiO 3 can be obtained from titanyl acylate precursors by using acetic acid chelating titanium alkoxides in a strong alkaline solution (pH>13) without external heating. Fourier transfer infrared spectroscopy (FTIR), differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to investigate the effects of the alkyl chain length of the alkoxide on the morphology of the powder and its sinterability. It was found that a longer alkyl chain length (Ti(O-n-C 4 H 9 ) 4 ), and a more electronegative alkoxide group, leads to a slower rate of hydrolyzation, and a slower shift rate in the condensation. Furthermore, less cross-linking of smaller particles is observed. In contrast, a shorter alkyl chain, Ti(OC 2 H 5 ) 4 , and a less electronegative alkoxide group, cause much more network cross-linking in the structure of larger particles. Therefore, the particle size of the precursor gel obtained from the longer alkyl chain length alkoxide was smaller, and the formation rate of the powder was quicker. The powder obtained from Ti(O-n-C 4 H 9 ) 4 at boiling contained ultrafine particle sizes (0.03–0.04 μm) and readily sintered at 1150–1250°C.