Fida Rehman

Grains and grain boundaries contribution to dielectric relaxations and conduction of Bi5Ti3FeO15 ceramics

Dielectric relaxation behaviors of Aurivillius Bi5Ti3FeO15ceramics were investigated in a wide range of frequency and temperature via dielectric and impedance spectroscopies. We distinguished two dielectric relaxations using the combination of impedance and modulus analysis. Resistance of the grain boundary was found to be much larger than grains, whereas capacitance was at the same level. The kinetic analysis of dielectric data was carried out to evaluate the contributions of microstructure and defects to the relaxation and conduction. The possible relaxation-conduction mechanism in the ceramics was discussed. The results enable deep understanding of microstructure-defect-relaxation behaviors in Bi5Ti3FeO15ceramics.

Dual-band tunable negative refractive index metamaterial with F-Shape structure

This paper presents a negative refractive index tunable metamaterial based on F-Shape structure which is capable of achieving dual-band negative permeability and permittivity, thus dual-band negative refractive index. An electromagnetic simulation was performed and effective media parameters were retrieved. Numerical investigations show clear existence of two frequency bands in which permeability and permittivity both are negative. The two negative refractive index bandwidths are from 23.8 GHz to 24.1 GHz and from 28.3 GHz to 34.9 GHz, respectively. The geometry of the structure is simple so it can easily be fabricated. The proposed structure can be used in multiband and broad band devices, as the band range in second negative refractive index region is 7 GHz, for potential applications instead of using complex geometric structures and easily tuned by varying the separation between the horizontal wires.

Hydrothermal growth of VO2 nanoplate thermochromic films on glass with high visible transmittance

The preparation of thermochromic vanadium dioxide (VO2) films in an economical way is of interest to realizing the application of smart windows. Here, we reported a successful preparation of self-assembly VO2 nanoplate films on TiO2-buffered glass by a facile hydrothermal process. The VO2 films composed of triangle-shaped plates standing on substrates exhibit a self-generated porous structure, which favors the transmission of solar light. The porosity of films is easily controlled by changing the concentration of precursor solutions. Excellent thermochromic properties are observed with visible light transmittance as high as 70.3% and solar modulating efficiency up to 9.3% in a VO2 film with porosity of ~35.9%. This work demonstrates a promising technique to promote the commercial utilization of VO2 in smart windows.

Self-assembly process of China rose-like β-Co(OH)2 and its topotactic conversion route to Co3O4 with optimizable catalytic performance

China rose-like structured β-Co(OH)2 was fabricated by a facile and surfactant free solvothermal method. The self-assembly process of the 3D flower-like structure was preliminarily explored via time-dependent observation. The chemical transformation achieved by thermal treatment from brucite-like β-Co(OH)2 to spinel Co3O4 was investigated in detail, while the original flower-like framework of the precursor was preserved during the entire process. With increase in the temperature of heat treatment, the specific surface area decreased correspondingly. Meanwhile, propagation of cracks and the following cavitation progress were realized on petal-like flakelets. Topotactic nucleation and crystallization in the [111] direction of spinel-structured Co3O4 inheriting the [0001] direction of hexagonal structured β-Co(OH)2 were analyzed in detail. The catalytic performance of the obtained Co3O4 materials on the thermal decomposition of ammonium perchlorate (AP) was analyzed. It was fortunately found that samples annealed at 280 °C could lower the thermal decomposition temperature of AP from 450 °C to 245 °C, when the obtained Co3O4 made up 4% w/w of the mixture. This excellent performance is of great practical significance in the development of solid rocket fuels.

Effect of reduction/oxidation annealing on the dielectric relaxation and electrical properties of Aurivillius Na0.5Gd0.5Bi4Ti4O15 ceramics

Bismuth layer-structured Na0.5Gd0.5Bi4Ti4O15 ceramics were synthesized via a high temperature solid state reaction. The samples were annealed under pure-argon and pure-oxygen atmospheres respectively. The dielectric relaxation and electrical properties were investigated in a wide range of frequencies and temperatures by dielectric/impedance spectroscopies. Two anomalies were observed in the temperature dependent dielectric spectra, one occurred in the range of ∼200–400 °C and the other was at ∼590 °C due to the occurrence of a ferroelectric phase transition. The low temperature peak was suppressed significantly by oxygen-annealing and enhanced by argon-annealing, suggesting it is oxygen-vacancy related. The ceramics exhibited a single dielectric relaxation behavior, which was attributed to the bulk contribution according to the combination analysis of impedance and electric modulus frequency spectra. Kinetic analyses of ac dielectric data were carried out to probe the possible conduction–relaxation mechanisms. Above ∼400 °C, the relaxation and conduction of the compound was assigned to the motion of ionized oxygen vacancies. The experimental results indicated that the Gd-doping suppressed the leakage and improved dielectric properties of Na0.5Bi4.5Ti4O15 ceramics.

Energetic hybrid polymer network (EHPN) through facile sequential polyurethane curation based on the reactivity differences between glycidyl azide polymer and hydroxyl terminated polybutadiene

To improve the thermo-mechanical properties of glycidyl azide polymer (GAP) and hydroxyl terminated polybutadiene (HTPB) based propellants, a facile sequential polymerization approach has been conducted to prepare an energetic hybrid polymer network (EHPN) through stepwise curation. The detailed curing conditions for the EHPN formation were determined using an in situ FTIR kinetic study. The effect of curing ratio (NCO/OH) on the mechanical properties of the polyurethane networks of GAP and HTPB was investigated, wherein hexamethylene diisocyanate biuret trimer (Desmodur N100) and isophorone diisocyanate (IPDI) were used as mixed curative agents. A series of EHPNs were prepared by varying the relative weight ratios of GAP and HTPB with a single poly-isocyanate mixed curing system (IPDI/N100). A remarkable mechanical strength of up to 5.83 MPa and an elongation at break of 359% were achieved with a 50:50 weight ratio of GAP to HTPB, which is the maximum mechanical strength reported thus far for a binder system of GAP and HTPB, which has a thermally more stable cross-linked network. The thermal properties of the as-synthesized PU networks of GAP, HTPB and GAP–HTPB EHPNs with different weight ratios were characterized using the DMA and DSC techniques. Thermal degradation behavior and morphological studies were also investigated with TGA-DTG and scanning electron microscopy (SEM), respectively. The facile sequential polyurethane curation polymerization technique can be potentially used for advanced solid composite propellants.

Effect of Nd3+ substitution for Bi3+ on the dielectric properties and conduction behavior of Aurivillius NdBi4Ti3FeO15 ceramics

NdBi4Ti3FeO15 and Bi5Ti3FeO15 ceramics were prepared by solid state reaction. The effect of Nd3+ on dielectric and electrical properties was investigated in a wide range of frequencies and temperatures by dielectric/impedance spectroscopies. The combination of impedance and electric modulus analysis enabled the distinguishing of two relaxation behaviors that were assigned to originate from grains and grain boundaries. Kinetic analysis of temperature-dependent dielectric data was performed to probe the defect-related conduction. The results demonstrated that the Nd3+ substitution for Bi3+ significantly suppressed the conductivity and the ferroelectric transition temperature was decreased to ∼250 °C in NdBi4Ti3FeO15.

Effect of oxygen vacancy on the improved photocatalytic activity of Cr-doped TiO2

We employed ab initio calculations to study the effect of various defects on the structural, electronic and optical properties of anatase TiO2. Single Cr doping at Ti sites introduced isolated Cr 3d states in the band gap of TiO2. Charge compensated Cr doped model was introduced by simultaneously doping two Cr atoms at Ti sites along with one oxygen vacancy (Ti14O31Cr2). It has been found that Ti14O31Cr2 can narrow the band gap significantly and passivate gap states. Removal of the isolated Cr 3d states from the band gap and improved visible light absorption would effectively enhance the photocatalytic activity of Cr doped TiO2. Our calculations provide reasonable explanation for the previously reported experimental findings.

Evolution of Structural and Electrical Properties of Oxygen-Deficient VO2 under Low Temperature Heating Process

Structural stability and functional performances of vanadium dioxide (VO2) are strongly influenced by oxygen vacancies. However, the mechanism of metal-insulator transition (MIT) influenced by defects is still under debate. Here, we study the evolution of structure and electrical property of oxygen-deficient VO2 by a low temperature annealing process (LTP) based on a truss-structured VO2 nanonet. The oxygenation process of the oxygen-deficient VO2 is greatly prolonged, which enables us to probe the gradual change of properties of the oxygen-deficient VO2. A continuous lattice reduction is observed during LTP. No recrystallization and structural collapse of the VO2 nanonet can be found after LTP. The valence-band X-ray photoelectron spectroscopy (XPS) measurements indicate that the oxygen deficiency strongly affects the energy level of the valence band edge. Correspondingly, the resistance changes of the VO2 films from 1 to 4.5 orders of magnitude are achieved by LTP. The effect of oxygen vacancy on the electric field driven MIT is investigated. The threshold value of voltage triggering the MIT decreases with increasing the oxygen vacancy concentration. This work demonstrates a novel and effective way to control the content of oxygen vacancies in VO2 and the obvious impact of oxygen vacancy on MIT, facilitating further research on the role of oxygen vacancy in structure and MIT of VO2, which is important for the deep understanding of MIT and exploiting innovative functional application of VO2.

Numerical Study of an A-Shape Negative Refractive Index Material

Metamaterial designing is an important issue in order to reduce the complexity and for the extensive use in various potential applications. This paper presents an A-shape novel metamaterial structure which simultaneously exhibits negative permittivity and negative permeability. An electromagnetic wave simulation is performed, and the results show that the proposed metamaterial unit cell is capable of achieving a negative refractive index. Furthermore, a left hand prism was designed composed of metamaterial unit cells. Simulation of the left hand prism proves the presence of a negative refractive index. The proposed design is easy to fabricate compared to the complex structures, due to the simple geometry, and could be used in various applications.