Dielectric properties of cubic bismuth based pyrochlores containing lithium and fluorine

In this work dielectric properties of Bi_1.5Zn_1?xLi_xNb_1.5O_7?xF_x with x = 0.25 were investigated in a 20 Hz–12 GHz frequency and 120–500 K temperature range and compared to that of regular cubic BZN (when x = 0). Measurements showed that both ceramics have dipolar glass type dielectric dispersion with wide relaxation time distributions. Mean relaxation time follows Arrhenius law in the investigated frequency range, although Vogel–Fulcher law was anticipated.     

Effect of magnesium content on structure and dielectric properties of cubic bismuth magnesium niobate pyrochlores

Structure and dielectric properties of cubic pyrochlore Bi_1.5Mg_NNb_2.5−NO_8.5−1.5N (BMN) compositions with N=0.6–1.3 have been studied. X-ray diffraction (XRD), infrared reflectivity spectra and Raman spectra were employed to analyze the crystal structures and phonon vibration modes of BMN compositions. The vibration spectra were sensitive to the content of Mg²⁺ ions, which is caused by the randomness of Mg²⁺ ions partially filling both the cubic pyrochlore A and B sites. The intensity of A3O stretching vibrations became stronger with increasing Mg²⁺ content, but B3O stretching vibrations were quite opposite. With the increase of Mg²⁺ content, the dielectric constant and loss tangent both increased. Temperature dependent dielectric constant was observed in the samples with N>0.8. The tendency of the dielectric constant with the increasing temperature showed a quick drop in the samples with higher Mg²⁺ content, which seems to be associated with the disorder in the A₂O′ network.     

Structural characterization of Bi–Zn–Nb–O cubic pyrochlores

Structure and dielectric properties of oxide dielectrics with the composition of Bi_1.5Zn_NNb_2.5−NO_8.5−1.5N (with N=0.73 to 1.20) have been studied. These samples were treated at 1050 °C for 4 h. The cubic pyrochlore phase was found to be predominant as from X-ray diffraction and Raman spectra analysis. Lattice constant of the cubic pyrochlore and dielectric constant of the sample have been found to increase with the increase of Zn content. A model of the structural defects has been proposed to explain the stabilization of the pyrochlore structure. The limitation of composition for the formation of single cubic pyrochlore phase has been attributed to the distribution of oxygen defects.     

Growth and optical properties of thulia-doped cubic yttria stabilized zirconia single crystals

Single crystals of yttria stabilized zirconia (YSZ) doped with different thulia (Tm₂O₃) contents (0.2–3.0 mol%) (abbreviated to Tm₂O₃: YSZ) were grown by the optical floating zone method. The crystals were transparent and inclusion free. These samples were then analyzed by X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DSC), and Raman spectroscopy, and their optical properties were determined with Ultraviolet–visible (UV–Vis) and Photoluminescence spectroscopy (PL). The Tm₂O₃: YSZ single crystals were in the cubic phase, and the lattice parameters first increased and then decreased with increasing Tm₂O₃ content. The absorption spectra showed four peaks at around 356 nm (³H₆→¹D₂), 460.5 nm (³H₆→¹G₄), 678.5 nm (³H₆→³F_2,3) and 784 nm (³H₆→³H₄) in the visible region, and the optical bandgap energy increased with increasing Tm₂O₃ content, as a result of the Moss-Burstein effect. Measurements of PL spectra indicated a strong blue emission peak at 458 nm (¹D₂→³F₄), and three weak emission peaks at 487 nm (¹G₄→³H₆), 497 nm (¹D₂→³H₅), and 656.5 nm (¹G₄→³F₄) when the crystals were excited by light with a wavelength of 356 nm. The intensities of the emission peaks were strongly affected by the Tm₂O₃ content of the YSZ single crystals; the intensity increased with Tm₂O₃ content at low doping levels, reached the maximum at 0.5 mol% Tm₂O₃, then decreased with further increase in Tm₂O₃ content due to the concentration quenching effect. Additionally, the color changed from blue to cyan as the Tm₂O₃ content was increased. Overall, this work demonstrates that the cubic YSZ single crystal is a suitable host material for solid state luminescence.     

Effects of Ni2+ substitution on the structure and dielectric properties of Bi1.5MgNb1.5O7 cubic pyrochlores

The dielectric properties of bismuth-based cubic pyrochlores strongly depend on the environment of the A-site ions, e.g. the Ni²⁺ ions doped into Bi_1.5MgNb_1.5O₇ (BMN) pyrochlores for tailoring dielectric properties. Both BMN and Bi_1.5NiNb_1.5O₇ (BNN) ceramics exhibit a cubic pyrochlore structure with preferential (222) planes. However, {442} reflections are observed in BNN pyrochlores, revealing an off-center displacement of A and O' ions. The dielectric constant of BNN pyrochlores is lower than that of BMN pyrochlores, besides BNN pyrochlores have a larger dielectric loss (0.002) than BMN pyrochlores (0.0007). Ni-doping results in a loose and flexible structure contributing positively to the dielectric tunability, besides creating a large amount of oxygen vacancies. The higher amount of oxygen vacancies increases the dielectric loss of BNN pyrochlores. However, BNN pyrochlores exhibit enhanced temperature stability, with a temperature coefficient of –57 ppm/^oC, which is significantly better than that of BMN pyrochlores (–362 ppm/^oC).     

Structural,optical, and electrical properties of tin iodide-based vacancy-ordered-double perovskites synthesized via mechanochemical reaction

Over the recent past, lead-based halide perovskite materials have drawn significant attention due to their excellent optical and electrical properties for solar cells and optoelectronics applications. However, the toxicity of lead elements and instability under ambient conditions leads to develop alternative compositions. Herein, we report a novel mechanochemical synthesis of tin iodide-based double perovskites (A₂SnI₆; A = Rb⁺, Cs⁺, methylammonium, and formamidinium), and their structural, optical, and electrical properties are investigated. Importantly, we found that the hydrogen iodide (HI) addition during the ball-milling process minimizes secondary phase formation in the synthesized A₂SnI₆ powders. The effects of HI addition and the A-site substitution are investigated with respect to the lattice parameters, optical bandgaps, and electrical properties of the synthesized perovskite materials. Our results demonstrate essential information to improve the understanding of halide perovskite materials and develop efficient lead-free perovskite photoelectric devices.     

Structural,magnetic and photocatalytic properties of Sr-doped BiFeO3 nanoparticles based on an ultrasonic irradiation assisted self-combustion method

A novel ultrasonic irradiation assisted self-combustion method was developed to prepare single-phase Bi_1−xSr_xFeO_3−δ (BSFO) nanoparticles, which were charactered by XRD, SEM, TEM and UV–vis spectra. The results show that structure, as well as magnetic and photocatalytic properties of BSFO are influenced by the particle size and the Sr²⁺ dopant content. Regarding smaller particles, even if small amount of Sr²⁺ substitution content change can result in the phase transition from the rhombohedral distorted perovskite to the cubic. The doping of heterovalent Sr²⁺ ions in BiFeO₃ (BFO) nanoparticles improves the ferromagnetic property. As ultrasonication can generate particles with larger surface area and more defections, BSFO nanoparticles exhibit efficient photocatalytic activity as a promising photocatalyst.     

First-principles study of structural,mechanical, and thermodynamic properties of cubic Y2O3 under high pressure

The structural, mechanical, and thermodynamic properties of cubic Y₂O₃ crystals at different hydrostatic pressures and temperatures are systematically investigated based on density functional theory within the generalized gradient approximation. The calculated ground state properties, such as equilibrium lattice parameter a₀, the bulk modulus B₀, and its pressure derivative B₀′ are in favorable agreement with the experimental and available theoretical values. The pressure dependence of a/a₀ and V/V₀ are also investigated. Furthermore, the elastic constants C_ij, bulk modulus B, shear modulus G, Young's modulus E, the ductile or brittle (B/G), Vickers hardness H_v, isotropic wave velocities and sound velocities are calculated in detail in a pressure range from 0 to 14 GPa. It was found that the Debye temperature decreases monotonically with an increase in pressure, the calculated elastic anisotropic factors indicate that Y₂O₃ has low anisotropy at zero pressure, and that its elastic anisotropy increases as the pressure increases. Finally, the thermodynamic properties of Y₂O₃, such as the dependence of the heat capacities C_V and C_P, the thermal expansion coefficient α, the isothermal bulk modulus, and the Grüneisen parameter γ on temperature and pressure, are discussed from 0 to 2000 K and from 0 to 14 GPa, respectively, applying the non-empirical Debye model in the quasi-harmonic approximation.     

Accurate Computation of Electric Field Enhancement Factors for Metallic Nanoparticles Using the Discrete Dipole Approximation

We model the response of nanoscale Ag prolate spheroids to an external uniform static electric field using simulations based on the discrete dipole approximation, in which the spheroid is represented as a collection of polarizable subunits. We compare the results of simulations that employ subunit polarizabilities derived from the Clausius–Mossotti relation with those of simulations that employ polarizabilities that include a local environmental correction for subunits near the spheroid’s surface [Rahmani et al. Opt Lett 27: 2118 (2002)]. The simulations that employ corrected polarizabilities give predictions in very good agreement with exact results obtained by solving Laplace’s equation. In contrast, simulations that employ uncorrected Clausius–Mossotti polarizabilities substantially underestimate the extent of the electric field “hot spot” near the spheroid’s sharp tip, and give predictions for the field enhancement factor near the tip that are 30 to 50% too small.     

Structural aspects of mechanical properties of iPP‐based composites. I. Composite iPP fibers with VGCF nanofiller

Nanocomposite fibers consisting of isotactic polypropylene (iPP) as a matrix filled with vapor grown carbon nanofibers (VGCF) have been prepared and their fine crystalline structure and mechanical properties characterized. The obtained results point out that the VGCF oriented along the fiber extrusion direction induce crystallization in the surrounding iPP matrix in a special way leading to the formation of oriented iPP α‐transcrystallite layers. The VGCF content and the draw ratio (DR) affect the textural properties of the composite material and lead to the formation of an anisotropic structure. The improvements of the mechanical properties of the composite fibers in both undrawn and drawn states are attributed to the VGCF aligning effect during extrusion, which produces highly oriented iPP crystalline structure, rather than to the reinforcing effect of the nanofibers. A new detailed scheme explaining the changes in tensile strength from the structural point of view is proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41865.     

基于CFD模拟的甲烷裂解太阳能管式反应器结构优化

太阳能裂解甲烷具有产物纯度高且环保的优点。对湍流条件下的甲烷高温裂解太阳能管式反应器进行计算流体力学（CFD）模拟,为提高太阳能甲烷裂解反应器的转化率,通过调节反应器结构对流场进行优化。为了更加准确计算太阳能辐射的加热效应,在湍流反应扩散模型中引入碳颗粒的生成和聚集模型,并采用离散坐标（DO）模型进行辐射模型求解。然后,在太阳能管式反应器中引入射流及挡板进行流场调节,并对挡板高度、射流流速及角度进行优化,达到强化反应过程的目的。优化后的反应器中,甲烷转化率可以提高约8%。以反应转化率和代表强化成本的黏性耗散为指标,筛选出不同离散条件下的Pareto最优解,并用支持向量机回归（SVR）算法对离散的Pareto最优解进行插值,得到操作曲线和与之相对应的最优射流角度及流速。     

First-principle study of electronic structures and optical properties of chromium and carbon co-doped anatase TiO2

The band structure, density of states, electron density difference, and optical properties of Cr and C co-doped anatase TiO₂ are studied using first principles calculations under the framework of the density functional theory. We mainly discuss three possible Cr–C adjacent co-doped configurations, where one Cr atom and one C atom substitute for one Ti atom and O atom respectively. The band structures show that the sub-bands induced mainly by C-2p state and Cr-3d state narrow the effective band gap down to ~0.86 eV and ~1.19 eV for different doped configurations. Doped Cr and C ion have different degree polarization, which will promote the electrons and holes separating. The calculated optical absorption spectrum exhibits shifts of the absorption edges of the three Cr–C co-doped TiO₂ samples towards the visible light region.     

Structural,optical and magnetic behavior of (Pr,Fe) co-doped ZnO based dilute magnetic semiconducting nanocrystals

The development of ZnO-based dilute magnetic semiconductor nanostructures co-doped with rare-earth and transition metals has attracted substantial attention for spintronics application. In this work, Pr (1%) and Fe (1%, 3%, and 5%) co-doped ZnO nanoparticles (NPs) were synthesized via co-precipitation method, and their structural, morphological, optical, photoluminescence, and magnetic properties were investigated. The single-phase wurtzite hexagonal crystal structure of all samples was detected via X-ray diffraction. Morphological analysis revealed spherical shape of the NPs with an average size in 20–50 nm range. The ultraviolet (UV)–visible measurements showed a redshift in the UV band and a slight change in the bandgap of the co-doped NPs. Fourier transform infrared analysis proved the existence of different functional groups in all synthesized NPs. X-ray photoelectron spectroscopy confirmed that Pr and Fe ions incorporated in the host ZnO lattice exhibit Pr³⁺ and Fe³⁺ oxidation states, respectively. Photoluminescence analysis showed that incorporated ions induce characteristic emission bands and structural defects in the synthesized NPs. Magnetic characterization indicated that the ZnO NPs exhibit a diamagnetic nature. However, the (Pr, Fe) co-doped NPs exhibit ferromagnetism at room temperature because of the interactions between Pr³⁺ and Fe³⁺ ions and trapped electrons mediated by bound magnetic polarons. Excellent optical and magnetic properties of synthesized samples may render them promising candidates for spintronics applications.     

Structural,morphological and mechanical insights from La2O3 doped machinable silicate glass ceramics for biomedical applications

Machinable glass ceramics attracted much attention in recent years due to its improved mechanical and therapeutic performances. La₂O₃ doped SiO₂–Al₂O₃–MgO–K₂CO₃–CaO–MgF₂ glass and glass ceramics (GCs) were synthesized using melt-quenching and solid-state reaction methods. Herein, doping impact of La₂O₃ on physical, optical, morphological, mechanical, and biological properties were studied. XRD reveals the major phase formation of monoclinic cuspidine, Ca₄F₂Si₂O₇ with some minor phases. 3 mol% of La₂O₃ GCs shows a new major crystalline phase of akermanite, Ca₂MgSi₂O₇. FTIR study shows that La₂O₃ acts as a network modifier and non-bridging oxygens presented in the glassy structure tends to be increased. Optical band gap and particle size were lies in the range of 4.18–4.10 eV, and 50–57 nm, respectively. Rod-like morphology and their elemental distributions were confirmed via SEM and EDS techniques. TEM studies suggested that the lattice planes agreed with the XRD results and confirmed a major phase formation of Ca₂MgSi₂O₇. Enhanced mechanical properties were observed using Universal Testing Machine. The cell viability and cell cytotoxicity, were performed by MTT and ALP assay.     

Gamma irradiation of graphene quantum dots with ethylenediamine: Antioxidant for ion sensing

Due to the low consumption of chemicals, the absence of toxic residual side products, the procedure simplicity and time-saving aspects, gamma irradiation offers advantages over the classical chemical protocols. We successfully employed gamma irradiation in order to introduce N-atoms in Graphene Quantum Dots (GQDs). By irradiating GQDs water dispersions in the presence of isopropyl alcohol and ethylenediamine, at doses of 25, 50 and 200 kGy, we attached amino groups onto GQDs in a single synthetic step. At the same time, a chemical reduction is achieved, too. Selected conditions induced incorporation of N-atoms within GDQs atomic lattice (around 3 at%), at all applied doses. Additionally, the C-atoms percentage was highly increased, from 63 to 79 at% or higher. The zeta potential of dots changed from −34.6 to +9.1 mV, due to the modification of functionalizing groups localized at the surface. Produced chemical changes lead to the desired alteration of the GQDs optical properties, such as an increased photoluminescence intensity, a higher photoluminescence quantum yields (from 2.07 to 18.40%) and a narrowing of the spectral features in the emission spectra. The ability of gamma-irradiated GQDs to quench free radical species was investigated and positively assessed; additionally, non-enzymatic optical detection of Cu(II) ions using GQDs as a sensor was studied and the detection limits are herein reported. These results suggest that GQDs can be potentially applied as smart photoluminescent sensors for metal cations.     

The structural and mechanical properties of polysulfide‐based polyurea

A series of polysulfide‐based polyureas was synthesized, which were based on isophorone diisocyanate (IPDI), liquid polysulfide and 2,5‐diamino‐3,6‐dimethylmercaptotoluene (Ethacure‐300). The structure and mechanical properties of these elastomers were investigated using dynamic mechanical thermal analysis, differential scanning calorimetry, stress–strain analysis, water‐resistance and oil‐resistance tests. The results showed that there was phase segregation between soft and hard segments in polysulfide‐based polyurea. With the increase in number‐average relative molar mass and content of liquid polysulfide, the tensile strength decreased while the ultimate elongation increased. The effect of number‐average relative molar mass of polysulfide and hard‐segment content on oil resistance is also discussed. Copyright © 2003 Society of Chemical Industry     

Molecular simulation of diblock copolymers; morphology and mechanical properties

Molecular modeling and computer simulations were used to construct, visualize, control and predict nanostructures with specific morphologies, self-assembling regions and mechanical properties associated to poly(styrene)-poly(isoprene) and poly(styrene)-poly(methyl methacrylate) diblock copolymers. Molecular structures of each diblock copolymer were constructed and used to obtain a Gaussian chain constituted of beads. Segment-segment interactions representing the chemical nature of the systems were obtained by means of numerical simulations. The numerical simulations for the two diblock systems predict structures with classic morphologies like bcc, hex, lamellar or gyroids and also other partial structure like islands and labyrinths. Young, bulk and shear modulus were also predicted from the structure and composition of the copolymer generating these morphologies. The excellent agreement between numerical and available experimental results opens a new strategy to modify existing diblock copolymer synthetic chemical processes to obtain products with specific morphologies.     

Effect of composition on phase assemblage,microstructure, mechanical and optical properties of Mg-doped sialon

Mg-doped sialon (Mg_m/2Si_12−m−nAl_m+nO_nN_16−n) ceramics with different compositions of m = 2n = 0.6, 0.84, 1.0, 1.2, 1.6 were hot pressed at 1850 °C for 1 h. Phase assemblage, microstructure, mechanical and optical properties of these samples were investigated. All samples achieved/approached full densification. However, the densification of Mg-doped sialon ceramics with higher MgO/AlN content becomes more difficult. Additionally, the anisotropic growth of β-sialon grains was significantly inhibited. The unique characteristics of Mg-doped sialon ceramics intrinsically derive from the formation of Mg-containing AlN polytypoids, which consumed most of the high-temperature liquid. Furthermore, their high stability at high temperatures accounts for the difficulty in preparing single-phase Mg-α-sialon., The hardness of these samples gradually increases while indentation fracture toughness gradually decreases with increasing m = 2n value. Due to little residual glassy phase, high infrared transparency/translucency was more readily achieved in Mg-doped sialon. The m = 1.2 sample possesses the maximum transmittance of ∼50% at ∼2 μm.     

Structure,dielectric and optical properties of Bi1.5+xZnNb1.5O7+1.5x pyrochlores

Non-stoichiometric pyrochlore ceramics with formula Bi_1.5+xZnNb_1.5O_7+1.5x were systematically investigated. Crystal structures of the compounds were studied by X-ray diffraction (XRD) technique. The structures were identified as pure cubic pyrochlores when |x| < 0.1. Dielectric and optical properties of the compositions when x = −0.1, 0 and 0.1 were studied. All samples have high resistivities and low dielectric loss. With increasing x in Bi_1.5+xZnNb_1.5O_7+1.5x, the lattice constant, permittivity, temperature coefficient of permittivity and thermal expansion coefficient increased, while dielectric loss decreased. Raman spectra indicated that the intensity of Bi–O stretching become stronger with increasing x. A vibration mode emerging at 861 cm⁻¹ when x = −0.1 means that the B–O coordination environment is significantly more disordered. Absorption spectra suggested that the bandgap energy become lower from 2.86 to 2.70 eV as lattice constants increased. Strong absorption occurs at wavelengths from 433 to 459 nm, shows that samples have the ability to respond to wavelengths in the visible light region.     

Molecular modeling for calculation of mechanical properties of epoxies with moisture ingress

Atomistic models of epoxy structures were built in order to assess the effect of crosslink degree, moisture content and temperature on the calculated properties of a typical representative generic epoxy. Each atomistic model had approximately 7000 atoms and was contained within a periodic boundary condition cell with edge lengths of about 4 nm. Four atomistic models were built with a range of crosslink degree and moisture content. Each of these structures was simulated at three temperatures: 300 K, 350 K, and 400 K. Elastic constants were calculated for these structures by monitoring the stress tensor as a function of applied strain deformations to the periodic boundary conditions. The mechanical properties showed reasonably consistent behavior with respect to these parameters. The moduli decreased with decreasing crosslink degree and with increasing temperature. The moduli generally decreased with increasing moisture content, although this effect was not as consistent as that seen for temperature and crosslink degree.