Crystallization Kinetics in BaTiO3 Synthesis from Hydrate Precursors via Microwave-Assisted Heat Treatment.

The crystallization kinetics in BaTiO3 synthesis from hydrate precursors via microwave-assisted heating (MWH) were investigated. The structural and chemical features of powders synthesized via MWH and conventional heating (CH) were compared. The charged radicals generated under microwave irradiation were identified by chemical analysis and real-time charge flux measurements. Using Ba(OH)2∙H2O (BH1), Ba(OH)2 (BH0), and BaCO3 (BC) as the precursors for a Ba source, and TiO2∙4H2O (TH) for a Ti source, three different mixture samples, BH1TH (BH1 + TH), BH0TH (BH0 + TH), and BCTH (BC + TH), were heat-treated in the temperature range of 100-900 °C. BaTiO3 powders were synthesized at temperatures as low as 100 °C when sample BH1TH was subjected to MWH. Based on the growth exponent (n), the synthesis reactions were inferred to be diffusion-controlled processes (3 ≤ n ≤ 4) for MWH and interface-controlled processes (2 ≤ n ≤ 3) for CH. Current densities of approximately 0.073 and 0.022 mA/m2 were measured for samples BH1TH and BH0TH, respectively, indicating the generation of charged radicals by the interaction between the precursors and injected microwaves. The radicals were determined as OH- groups by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.

Effects of Microwave-Assisted Heat Treatments on the Nanostructural Evolution of Amorphous Silicon Oxycarbide Thin Films.

This study investigated the effects of heat treatment on changes in the nanostructure of amorphous silicon oxycarbide thin films. Hydrogenated amorphous silicon oxycarbide (a-Si0.6C0.3O0.1:H) thin films were prepared via plasma-enhanced chemical vapor deposition. The films were subjected to post-deposition heat treatments via microwave-assisted heating, which resulted in the formation of nanocrystals of SiC and Si in the a-Si0.6C0.3O0.1:H matrix at temperatures as low as ~800 °C. The crystallization activation energies of SiC and Si were determined to be 1.32 and 1.04 eV, respectively lower than those obtained when the sample was heat-treated via conventional heating (CH). Microwaves can be used to fabricate nanocrystals at a temperature approximately ~300 °C lower than that required for CH. The optical and nanostructural evolutions after post-deposition heat treatments were examined using photoluminescence (PL) and X-ray diffraction. The position of the PL peaks of the nanocrystals varied from ~425 to ~510 nm as the annealing temperature was increased from 800 to 1000 °C. In this study the optical band gap of SiC and Si varied from ~2.92 to ~2.40 eV and from ~2.00 to ~1.79 eV, as the size of the SiC and Si nanocrystals varied with respect to the heating temperature and isothermal holding time, respectively.

Hierarchically Structured Core-Shell Design of a Lithium Transition-Metal Oxide Cathode Material for Excellent Electrochemical Performance.

Tuning geometrical parameters of lithium-mixed transition-metal oxide (LiTM) cathode materials is a promising strategy for resource-efficient design of high-performance Li-ion batteries. In this paper, we demonstrate that simple and facile geometrical tailoring of the secondary microstructure of LiTM cathode materials without complex chemical modification or heterostructure engineering can significantly improve overall electrochemical performance of the active cathode materials. An optimized LiTM with a bimodal size distribution of primary particles inside the secondary particles exhibits a 53.8% increase in capacity at a high discharge rate (10 C) compared to a commercially available reference and comparable rate capability after 100 charge/discharge cycles. The key concept of this approach is to maximize the beneficial effects arising from the controlled sizes of primary particles. Multimodal/multiscale microscopic characterizations based on electron tomography and scanning transmission electron microscopy, combined with electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy from the atomic level to the microscale level, were employed to elucidate structural origins of enhanced battery performance. This study paves the way for the resource-efficient microstructure design of LiTM cathode materials to maximize capacity and stability via simple adjustment of processing conditions, which is advantageous for mass-production applications.

Identification of local phase of nanoscale BaTiO3 powders by high-resolution electron energy loss spectroscopy.

The electron energy loss spectroscopy (EELS) technique was applied to investigate the local variation in the phase of barium titanate (BaTiO3) ceramics. It was found that the fine structure of the titanium L2,3 edge and their satellite peaks were sensitively varied with the tetragonal-cubic phase transition. The peak splitting of Ti-L3 edge of tetragonal-phased BaTiO3 ceramics was widened because of the increased crystal field effect compared with that of cubic-phased BaTiO3. In case of nanoscale BaTiO3 powders, the L3 edge splitting of the core region was found to be smaller than that of the shell region. The energy gap between peaks t2g and eg varied from 2.36 to 1.94 eV with changing the probe position from 1 to 20 nm from the surface. These results suggest that the EELS technique can be used to identify the local phase of sintered BaTiO3 ceramics.

Variation in the nanostructural features of Nc-Si:H films with radio frequency power conditions.

The effect of radio frequency (RF) power on the variation in the nanostructures, chemical features and surface morphology of nc-Si:H thin films was investigated. SiH4 and H2 gases were used as source materials for the films. The films were prepared by plasma enhanced chemical vapor deposition (PECVD) techniques and the power ranged from 100 to 300 W. The average crystal size of the films varied from -1 to -12 nm and the highest crystalline volume fraction reached up to -33% when the applied RF power was 300 W. At the RF power of 300 W, the relative fraction of Si-H bond in the films (R(MONO) = Si-H/sigma[Si-H(n)]n = 1,2,3) was increased up to -29%. The variation in the nanostructures and surface features of the films with applied RF power can be interpreted by the change in the collision impact of the precursor on top of the growing films.

Nanostructural, optical, and chemical features of Al-added nanocrystalline-Si thin films.

The nanostructural and optical features of Al-added nanocrystalline Si (nc-Si) thin films, which were prepared by co-sputtering Al-chips and a Si main target, were investigated in terms of Al-addition and post-deposition annealing conditions; the optical features like photoluminescence (PL) and electroluminescence (EL) were related with the variation in the nanostructure of the films. The PL intensity as well as the relative volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the PL spectra of the films which were prepared with 4 Al-chips and then annealed at 800 degrees C for 60 min exhibit a significant increase in intensity at a wavelength of approximately 620 nm, compared to those of the films prepared without Al-addition. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume fraction of the approximately 3.0 nm-sized nanocrystallites in the films. It was found that the addition of Al as well as the post-deposition annealing can allow adjustment and control of the nanostructural and light-emission features of the nc-Si films.