Phase transitions of pure and Ba-doped BiFeO3 under high pressure

In order to evaluate the effect of impurity substitution on phase transition of BFO, we report high-pressure X-ray diffraction experiments on pure and Ba-doped BiFeO₃ powders in diamond-anvil cells up to 18.7 GPa. A low-temperature hydrothermal synthesis route was utilized to fabricate these two samples. The experimental results reveal that two structural phase transitions take place in these two different samples in the investigated pressure range. As two ADXRD data sets are compared, it is shown that the intermediate phase II is not detected in Ba-doped sample. In addition, modifications in phase transition behavior and compressibility due to a minor Ba incorporation highlight the high sensitivity of BFO to variations of chemical composition.     

New collection systems for multi LED light engines

Light emitting diodes (LEDs) have numerous advantages as light sources in projectors. LEDs are more compact, exhibit a larger color gamut, have a longer lifetime, and need a lower supply voltage. However, there is still one important disadvantage: the optical power per unit of étendue (luminance) of an LED is significantly low. As a result of the étendue limitations of LEDs, the projected flux on the screen will not be high. Despite this shortcoming, LED’s are still of great interest for low power applications because of their other superior properties. Thus we collect the available light flux optimally and combine multiple high luminance LEDs within the system. In this study we discuss three collection systems designed to collect the LED flux with high optical efficiency while retaining small device size. The best collection efficiency attained with our collection systems is 96%. The fabrication tolerance and cost of our collection systems are also analyzed.     

Enhancing the resonance quality factor in membrane-type resonant grating waveguides

In this Letter, we present a method of reducing the spectral width of guided-mode resonance (GMR) in air-bridged resonant grating-waveguide structures to enhance the Q factor. The posttreatment of adding a dielectric film to the bottom of the membrane to manipulate the resonance behavior is practicable. The introduced underlayer is shown to be capable of effectively reducing the coupling and enhancing the resonant Q factor. The proposed method provides an effective means of adjusting the resonance property without varying the original GMR structure. The results also imply that TM resonance is more feasible for achieving narrow resonance and potentially in sensing applications, because it has higher sensitivity than TE resonance.     

Silicon-based and suspended-membrane-type guided-mode resonance filters with a spectrum-modifying layer design

The concept of a spectrum-modifying layer is proposed for the design of a silicon-based guided-mode resonance filter. To realize such a novel device, a grating and waveguide structures are fabricated simultaneously in a suspended silicon nitride membrane. The cladding layer of the silicon substrate is replaced by the silicon dioxide membrane to reduce the absorption loss of the bulky substrate. Moreover, the silicon dioxide membrane plays a role in modifying the spectral response. According to the experimental results of the proposed structures, symmetrical line shapes and improved sidebands of nonresonance are demonstrated.     

Effect of heat treatment on electrochromic properties of TiO<Subscript>2</Subscript> thin films

Electrochromic titanium oxide (TiO₂) films were deposited on ITO/glass substrates by chemical solution deposition (CSD). The stock solutions were spin-coated onto substrates and then heated at various temperatures (200–500 °C) in various oxygen concentrations (0–80%) for 10 min. The effects of the processing parameters on the electrochromic properties of TiO₂ films were investigated. X-ray diffraction measurements demonstrated that the amorphous TiO₂ films were crystallized to form anatase films above 400 °C. The electrochromic properties and transmittance of TiO₂ films were measured in 1 M LiClO₄–propylene carbonate (PC) non-aqueous electrolyte. An amorphous 350 nm-thick TiO₂ film that was heated at 300°C in 60% ambient oxygen exhibited the maximum transmittance variation (ΔT%), 14.2%, between the bleached state and the colored state, with a ΔOD of 0.087, Q of 10.9 mC/cm², η of 7.98 cm²/C and x in Li _x ClO₄ of 0.076 at a wavelength (λ) of 550 nm.     

An approach in exploring the fundamental dosimetric characteristics for a long shelf life irradiated acrylamide-based gel

In this study, a long shelf life irradiated gel made from a low-toxicity N-isopropylacrylamide (NIPAM) monomer was used to study the dosimetric characteristics associated with magnetic resonance imaging (MRI). Specifically, the Taguchi method was utilized to investigate the influence of MRI scanning parameters on the performance objectives of NIPAM gels, and the priorities of the parameters that influence the process were determined using Delta statistics. The influence of different echo spacing on the relaxation rate–dose response of the gel and the relationship among the dose resolution (D _Δ ^P ), dose accuracy, and reproducibility were also examined. The results indicated that the repetition time (TR) and echo time (TE), having Delta values of 3.81042 and 4.52752, respectively, were the most influential factors. Analysis of variance further confirmed that TE was the major factor, with a percentage contribution of 58.36 %, and TR was the second factor with a percentage contribution of 41.34 %. The maximum sensitivity was 0.1506 s^?1 Gy^?1 from 0 to 15 Gy, and the best D _Δ ^P of the NIPAM gel was 0.017 Gy. Two months after the irradiation, no obvious changes in linearity and sensitivity were observed, indicating that the NIPAM gels are highly stable. The gel dosimeters were also applied in comparing the planar dose distributions of the gel dosimeters and treatment planning system. The comparison showed good agreement of isodose lines on selected planes in the transverse plane. This result indicates the great potential of the NIPAM dosimeter has a clinical dose verification tool.     

Phonon behavior of CaSnO3 perovskite under pressure

Raman scattering and x-ray diffraction studies of CaSnO(3) perovskite were performed under high-pressure conditions. This high-pressure study was motivated by a recent theoretical study predicting a phase transition in CaSnO(3) from GdFeO(3)-type perovskite to CaIrO(3)-type structure occurred at 12 GPa. Despite no obvious structure change up to a pressure of 26 GPa based on the x-ray diffraction data, high pressure Raman measurements revealed that some Raman modes disappeared upon compression; either merging into neighboring bands or vanishing. The signals for these Raman peaks were recovered during decompression. The measured pressure derivative of Raman shift (?ν∕?P) of CaSnO(3) ranged from ~1.29 to ~4.35, up to 20 GPa. Due to the lack of lattice dynamic study for CaSnO(3) perovskite, the mode symmetry for CaSnO(3) was tentatively assigned based on the empirical relation among Ca-bearing perovskites. The pressure derivative of the Raman shifts was found to be related to their mode vibrations: modes related to Ca and O shifts had a strong pressure dependence compared with those associated with oxygen octahedral rotation.     

Thermokinetics simulation for multi-walled carbon nanotubes with sodium alginate by advanced kinetics and technology solutions

To accomplish an effective analysis of adsorption, the strong acid dye from aqueous solution of sodium alginate (SA) and multi-walled carbon nanotubes (MWCNTs) composite gel beads were used as important parameters. Differential scanning calorimetry (DSC) was used to measure the heat of breakdown reaction. The experimental conditions were set at 0.5, 1, 2, 4, and 8 °C min^?1, and the temperature range was 30–300 °C. The heating rates and the temperature range were set as follows: Four kinds of proportion in this experiment contained 2 SA % w/v (SA), 0.03, 0.09, 0.18, 0.36 % w/v (MWCNTs), and 10 % w/v calcium chloride, respectively. Four samples, 5, 6, 7, 8, and 9 mg, were used to detect the experimental data. It contributed to understanding the reaction for the distinctive MWCNTs. With the thermokinetic data by isoconversional approach obtained from advanced kinetics and technology solutions (AKTS), the related thermal safety information can be obtained from the thermal reaction of MWCNTs. Valuable parameters, such as activation energy (E _a) and heat of decomposition, can be applied in operation, including adsorption and desorption processes. After DSC tests, and under the four compositions of SA/MWCNTs, at different heating rates of 0.5, 1, 2, 4, and 8 °C min^?1, primarily we found that when the heating rate was increased, exothermic onset temperature would increase gradually. After analyzing E _a value by isoconversional kinetics, we learned that in four different adsorption compositions, SA/MWCNTs_0.03 (161.20 kJ mol^?1) was the minimum. Among them, the highest value was SA/MWCNTs_0.18 (220.48 kJ mol^?1). However, in this study, for SA/MWCNTs compositions we found that E _a value will drop in the final material SA/MWCNTs_0.36. Accordingly, if the ratio of SA and calcium chloride was fixed, then different compositions of the MWCNTs would affect adsorption efficiency of SA/MWCNTs and E _a variation.     

The effect of molecular conformation on single molecule conductance: measurements of pi-conjugated oligoaryls by STM break junction

Measurements of molecular break junction reveal quantitatively the correlation between the single-molecule conductance and the conformation of pi-conjugated molecules with 6-18 conjugated double bonds.