Bifunctional Perovskite-BiVO4 Tandem Devices for Uninterrupted Solar and Electrocatalytic Water Splitting Cycles

Photoelectrochemical (PEC) fuel synthesis depends on the intermittent solar intensity of the diurnal cycle and ceases at night. Here, an integrated device that does not only possess PEC water splitting functionality, but also operates as an electrolyzer in the nocturnal period to improve the overall capacity factor is described. The bifunctional system is based on an “artificial leaf” tandem PEC architecture that contains an inverse-structure lead halide perovskite protected by a graphite epoxy/parylene-C coating (conferring 96 h stability of operation in water), and a porous BiVO₄ semiconductor. The light-absorbers are interfaced with a H₂ evolution catalyst (Pt) and a Co-based water oxidation catalyst, respectively, which can also be directly driven by electricity. Thus, the device can operate in PEC mode during irradiation and switch to an electricity-powered mode in the dark through bypassing of the semiconductor configuration. The bifunctional perovskite-BiVO₄ tandem provides a solar-to-hydrogen efficiency of 1.3% under simulated solar irradiation and an onset for water electrolysis at 1.8 V. The compact design and low cost of the proposed device may provide an advantage over other technologies for round-the-clock fuel production.     

Ab initio study of mono-layer 2-D insulators (X-(OH)2 and h-BN) and their use in MTJ memory device

The paper presents an ab initio study of the 2-D insulators and their effect on the performance of a magnetic tunnel junction memory (MTJ) device. MTJ devices has been considered as an alternate to the charge based data storage cells due to its spin-polarised operation and high scaling probability. The use of 2-D insulators like X-(OH)₂ (X: Ca and Mg) and h-BN (hexagonal-Boron Nitride) in such device would be interesting. The authors have calculated the band structures, density of states and effective mass of electrons and holes for the mono-layer of these three non-conventional 2-D insulators using the first principle calculations in density functional theory framework using Quantumwise ATK tool. The ab initio calculation yielded band gap (E_g) of 4.633, 4.685 and 4.249 eV for h-BN, Ca(OH)₂ and Mg(OH)₂, respectively. The effective mass of electrons was calculated as 0.621, 0.604 and 0.478 for single layer h-BN, Ca(OH)₂ and Mg(OH)₂, respectively. While for holes it is 0.834, 0.446 and 0.407, respectively for h-BN, Ca(OH)₂ and Mg(OH)₂. The MTJ device properties as tunneling-magneto resistance, differential TMR, parallel and anti-parallel resistance, differential resistance and spin transfer torque components (in-plane and out-of-plane) with these materials as composite dielectric has been reported in this paper using MTJ Lab tool. The performance of MTJ memory device with h-BN based composite dielectric is found better.

     

Epoxy-carbon nanotubes as matrix in glass fiber reinforced laminated composites

A manufacturing process using vacuum-assisted resin infusion molding was used to produce glass fiber-reinforced laminates with the epoxy matrix and with the 0.5 weight % NH₂ functionalized-MWCNT based epoxy matrix. Images obtained from the TEM and SEM indicates that MWCNTs are well dispersed into the epoxy matrix. Microstructures observations of the composites from SEM show the better interaction between CNTs and epoxy. The mechanical and thermo-mechanical behavior of the glass fiber-epoxy system and glass fiber-CNT/epoxy system was characterized through flexural test and Dynamic Mechanical Analysis (DMA).     

Polyimide‐carbon nanotubes nanocomposites: electrical conduction behavior under cryogenic condition

This study is aimed to investigate the electrical conduction behavior of polyimide (PI)/multiwall carbon nanotubes (CNTs) nanocomposites in cryogenic environment (temperature from 10 to 300 K) prepared by in situ polymerization technique. The experimental results of direct current (DC) electrical conductivity have been fitted with different theoretical models to check their applicability and to understand the conduction behavior for the present nanocomposite system. The PI/CNT nanocomposites show low electrical percolation threshold. Negative temperature coefficient effect of resistivity is observed for all the composites under investigation. The analysis shows that Mott's variable range hopping (VRH) model is more applicable compared to Arrhenius and Kivelson models for the present composites over the entire range of measurement temperature. The electronic transport behavior in each composite at temperature above 70 K can be ascribed to thermally activated tunneling of charge carriers through insulating barriers between CNTs; however, the electronic transport behavior at temperature below 70 K can be attributed to three dimensional VRH of charge carriers through the networks of CNTs in the polymer composite. The current–voltage characteristics of the composite show non‐ohmic behavior for temperature below 60 K and become ohmic in nature as temperature rises to 300 K. POLYM. ENG. SCI., 57:291–298, 2017. © 2016 Society of Plastics Engineers     

Robocasting of silicon nitride with controllable shape and architecture for biomedical applications

Silicon nitride (Si₃N₄) implants are used in spinal fusion surgery and are under development for use in other biomedical applications. The ability to create Si₃N₄ implants with anatomically relevant shapes and controllable architecture can be beneficial in these applications. In the present study, an aqueous paste composed of Si₃N₄ powder and sintering additives was prepared with the requisite rheology and formed into structures with different geometry and architecture using a robocasting technique. Sintering and hot isostatic pressing produced an almost fully dense Si₃N₄ phase (density=3.23±0.01 g/cm³) with a fibrous microstructure. Four‐point bending tests of as‐fabricated dense beams showed a flexural strength of 552±68 MPa. Together, these results indicate that robocasting combined with sintering and hot isostatic pressing can provide a viable manufacturing approach to create Si₃N₄ implants with controllable shape and architecture for applications in orthopedic and dental surgery.     

Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

SiC-Whisker-Reinforced Al2O3 Composites by Free Sintering of Coated Powders

SiC whiskers were coated with a thick cladding of finegrained Al₂O₃ powder by controlled heterogeneous precipitation in a concentrated suspension of whiskers. After calcination, the coated whiskers were compacted by cold isostatic pressing and sintered at a constant heating rate of 5°C/min in a helium atmosphere. The parameters which control the coating process and the sintering characteristics of the consolidated powders are reported. Starting with an initial matrix density of 40–45% of the theoretical, composites containing up to ≅20 vol% whiskers (aspect ratio ≅15) were sintered freely to nearly theoretical density below 1800°C. By comparison, a similar composite formed by mechanical mixing of the whiskers and the precipitated Al₂O₃ powder reached a density of only 68% of the theoretical after sintering under identical conditions. For a fixed whisker content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio.     

Sintering of Spherical Glass Powder under a Uniaxial Stress

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×10⁹ P (∽1×10⁸ Pa·s) compared to ∽3×10⁹ P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.     

Sintering, Creep, and Electrical Conductivity of Model Glass-Matrix Composites

The sintering, creep, and electrical conductivity of model glass-matrix composites were investigated as functions of inclusion content and size. The composites consisted of spherical soda–lime glass particles and spherical nickel inclusions. At inclusion volume fractions below 10 vol% there was no inclusion size effect on the creep viscosity. Above 10 vol% there was an inclusion size effect, with the viscosity increasing significantly with decreasing inclusion size. The increase in electrical conductivity commenced at lower inclusion volume fractions as the inclusion size decreased. The data indicate that interactions between the inclusions might be responsible for the observed inclusion size effect. The sintering rates of the composites were compared with the predictions of Scherer's self-consistent model. Good agreement was obtained when the measured creep viscosity was used in the model equations.     

A power law approach to orifice flow rate calibration

Although standards for orifice flow meter design, installation, and calibration are supported herein, noncompliant devices exist in many pilot-, lab-scale, and on-board applications. For these, a common calibration practice is to preserve the ideal square root relation and determine a device specific discharge coefficient value. This work provides theoretical and empirical analyses to support relaxing the square root relation between orifice pressure drop and flow rate for noncompliant devices. The resulting power law relation is shown to improve accuracy, precision, and rangeability. Whether a device specific square root or power law model is used, it requires off-line or in-line calibration data. As such, a power law calibration model may only be useful for on-board and small-scale applications.