Synthesis,growth, optical,mechanical and electrical properties of <Emphasis Type="Italic">L</Emphasis>-lysine <Emphasis Type="Italic">L</Emphasis>-lysinium dichloride nitrate (<Emphasis Type="Italic">L</Emphasis>-LLDN) single crystal

Semi-organic nonlinear optical material, L-lysine L-lysinium dichloride nitrate (2C₆H₁₅N₂O₂⁺_{2}^{+} · H⁺ · NO₃^-_{3}^{-} · 2Cl⁻) was synthesized at room temperature. Single crystals of L-LLDN were grown by slow cooling solution growth technique. The grown crystal was confirmed by powder X-ray diffraction analysis. The crystalline perfection of the grown single crystal was characterized by high-resolution X-ray diffraction (HRXRD) studies. The cut-off wavelength was determined by UV-vis transmission spectral analysis. The frequency doubling of the grown crystal was confirmed by powder second harmonic generation (SHG) measurement. The refractive index and birefringence of the crystal were determined using He–Ne laser source. Mechanical property of the crystal was determined by Vickers hardness tester. The frequency and temperature dependence of dielectric constant (ε _r), dielectric loss (tan δ) and a.c. conductivity (σ _ac) were also measured.     

Synthesis,dielectric, and hysteresis behaviour of Ba(Nb<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics and their solid solutions with BaZrO<Subscript>3</Subscript>

Solid state reaction technique was employed to synthesize Ba(Nb_0.2Ti_0.8)O₃ [BNT], and 0.9Ba(Nb_0.2Ti_0.8)O₃ + 0.1BaZrO₃ [BNT + BZ] samples. Sintered pellets were investigated for its dielectric (ε_r and tanδ) properties in the temperature range 100 K–380 K and in the frequency range of 100 Hz–1 MHz. The variation of ε_r and tan δ may be attributed to hopping of trapped charge carriers, which resulted in an extra dielectric response in addition to the dipole response. Hysteresis loop measurements were studied in the temperature regime 295 K–423 K. Loop area shrunk with the increase of temperature that may be due to phase transition from ferroelectric to paraelectric state.     

Properties of neodymium-doped Ca<Subscript>0.15</Subscript>Sr<Subscript>1.85</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>18</Subscript> ferroelectric ceramics

Bismuth-layered compound Ca_0.15Sr_1.85Bi_4−xNd_xTi₅O₁₈ (CSBNT, x = 0–0.25) ferroelectric ceramics samples were prepared by solid-state reaction method. The effects of Nd³⁺ doping on their ferroelectric and dielectric properties were investigated. The remnant polarization Pr of CSBNT ceramics increases at beginning then decreases with increasing of Nd³⁺ doping level, and a maximum Pr value of 9.6 μC/cm² at x = 0.05 was detected with a coercive field Ec = 80.2 kV/cm. Nd³⁺ dopant not only decreases the Curie temperature linearly, but also the dielectric constant (ε_r) and dielectric loss tangent (tan δ). The magnitudes of ε_r and tan δ at the frequency of 100 kHz are estimated to be 164 and 0.0083 at room temperature, respectively.     

Studies on a.c. properties of Ca<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> perovskites

A.c. measurements were preformed on bulk samples of Ca_1−x Sr _x TiO₃ (CST) perovskites with x = 0, 0.1 and 0.5 as a function of temperature range 300–450 K and frequency range 10³–10⁵ Hz . The experimental results indicate that the a.c. conductivity σ_a.c.(ω), dielectric constant ε′ and dielectric loss ε′′ depend on the temperature and frequency. The a.c. conductivity as a function of frequency is well described by a power law Aω ^S with s the frequency exponent. The obtained values of s > 1 decrease with increasing temperature. The present results are compared to the principal theories that describe the universal dielectric response (UDR) behavior.     

Microstructure,electrical properties of CeO<Subscript>2</Subscript>-doped (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

CeO₂-doped K_0.5Na_0.5NbO₃ lead-free piezoelectric ceramics have been fabricated by a conventional ceramic fabrication technique. The ceramics retain the orthorhombic perovskite structure at low doping levels (<1 mol.%). Our results also demonstrate that the Ce-doping can suppress the grain growth, promote the densification, decrease the ferroelectric–paraelectric phase transition temperature (T _C), and improve the dielectric and piezoelectric properties. For the ceramic doped with 0.75 mol.% CeO₂, the dielectric and piezoelectric properties become optimum: piezoelectric coefficient d ₃₃ = 130 pC/N, planar electromechanical coupling coefficient k _p = 0.38, relative permittivity ε_r = 820, and loss tangent tanδ = 3%.     

Dielectric relaxation behaviour of Sr<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>SbMnO<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript> ceramics fabricated from nanocrystalline powders prepared by molten salt synthesis

Double perovskite polycrystalline single phase and dense Sr ₂ SbMnO ₆ (SSM) ceramics, fabricated using the nanocrystalline powders synthesized by molten salt method, exhibited high dielectric constant with low dielectric loss as compared to that of SSM ceramics obtained from the powders prepared by solid-state synthesis method. The dielectric data obtained over a wide frequency (100 Hz–1 MHz) and temperature (190 K–300 K) ranges exhibited distinct relaxations owing to both the grain and grain boundary. The dielectric dispersion was modeled using the Cole–Cole equation consisting of two separate relaxation terms corresponding to the grain and grain boundary. The grain and grain boundary relaxations observed in the Nyquist plots (Z ^′ and Z ^″) were modeled by an equivalent circuit consisting of two parallel RC circuits connected in series with each other. A careful analysis of both the impedance (Z ^″ vs ω) and modulus (M ^″ vs ω) behaviour corroborated the conclusions drawn from the dielectric data.     

Study of the structural and dielectric properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> and PbO addition on BiNbO<Subscript>4</Subscript> ceramic matrix for RF applications

In this paper, the structural and dielectric properties of BNO (BiNbO₄) was investigated as a function of the external RF frequency and temperature. The BNO Ceramics, prepared by the conventional mixed oxide method and doped with 3, 5 and 10 wt. % Bi₂O₃–PbO were sintered at 1,025 °C for 3 h. The X-ray diffraction patterns of the samples sintered, shown the presence of the triclinic phase (β-BNO). In the measurements obtained at room temperature (25 °C) was observed that the largest values of dielectric permittivity (ε′ _r ) at frequency 100 kHz, were for the samples: BNO5Bi (5 wt. % Bi₂O₃) and BNO5Pb (5 wt. % PbO) with values ε′ _r ~ 59.54 and ε′ _r ~ 78.44, respectively. The smaller values of loss tangent (tan δ) were for the samples: BNO5Bi and BNO3Pb (3 wt. % PbO) with values tan δ ~ 5.71 × 10⁻⁴ and tan δ ~ 2.19 × 10⁻⁴, respectively at frequency 33.69 MHz. The analysis as a function of temperature of the dielectric properties of the samples, obtained at frequency 100 kHz, showed that the larger value of the relative dielectric permittivity was about ε′ _r ~ 76.4 at temperature 200 °C for BNO5Pb sample, and the value smaller observed of dielectric loss was for BNO3Bi sample at temperature 80 °C, with about tan δ ~ 5.4 × 10⁻³. The Temperature Coefficient of Capacitance (TCC) values at 1 MHz frequency, present a change of the signal from BNO (−55.06 ppm/°C) to the sample doped of Bi: BNO3Bi (+86.74 ppm/°C) and to the sample doped of Pb: BNO3Pb (+208.87 ppm/°C). One can conclude that starting from the BNO one can increase the doping level of Bi or Pb and find a concentration where one have TCC = 0 ppm/°C, which is important for temperature stable materials applications like high frequency capacitors. The activation energy (H) obtained in the process is approximately 0.55 eV for BNO sample and increase with the doping level. These samples will be studied seeking the development ceramic capacitors for applications in radio frequency devices.     

Nernst Signal in High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductors

The time-dependent Ginzburg–Landau equation with thermal noise is used to calculate the Nernst signal e _N , describing the Nernst effect, in type-II superconductor in the vortex-liquid regime. The Gaussian method used is an elaboration of the Hartree–Fock method. An additional assumption often made in analytical calculations that only the lowest Landau level significantly contributes to physical quantities of interest in the high-field limit is lifted by including all the Landau levels. The values of e _N are in good quantitative agreement with experimental data for temperature close to T _c on Bi₂Sr₂CaCu₂O_8+δ and Bi₂Sr₂Ca₂Cu₃O_10+δ .     

High-<Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> phase transition in K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript> lead-free ceramic synthesised using solid-state reaction

Solid-state reaction synthesised K₂Ti₆O₁₃ lead-free ceramic was characterized using XRD, SEM, and X-band EPR, at room temperature. EPR-spectra showed the presence of ( \textFe_\textTi^￠ - V_\textO ^·· ) \left( {{\text{Fe}}_{\text{Ti}}^{\prime } - V_{\text{O}}^{ \bullet \bullet } } \right) defect associate dipoles, in orthorhombic phase, responsible for the broadening of the dielectric anomaly identified in the ε _r (T) plots at T _C  ~ 300 °C. This anomaly resembled a ferroelectric–paraelectric type phase transition following Curie–Weiss type trend. Besides, dielectric loss mechanism jointly represented electrical conduction, dipole orientation, and space charge polarization.     

Structural and electrical properties of Ta<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>O<Subscript><Emphasis Type="Bold">5</Emphasis></Subscript> thin films prepared by photo-induced CVD

Tantalum oxide (Ta ₂ O ₅ ) films and Al/Ta ₂ O ₅ /Si MOS capacitors were prepared at various powers by ultraviolet photo-inducing hot filament chemical vapour deposition (HFCVD). Effects of ultraviolet light powers on the structure and electrical properties of Ta ₂ O ₅ thin films were studied using X-ray diffraction (XRD) and atomic force microscopy (AFM). The dielectric constant, leakage current density and breakdown electric field of the samples were studied by the capacitance–voltage (C–V) and current–voltage (I–V) measurements of the Al/Ta ₂ O ₅ /Si MOS capacitors. Results show that the Ta ₂ O ₅ thin films grown without inducement of UV light belong to amorphous phase, whereas the samples grown with inducement of UV-light belong to δ-Ta ₂ O ₅ phase. The dielectric constant and leakage current density of the Ta ₂ O ₅ thin films increase with increasing powers of the UV- lamps. Effects of UV- lamp powers on the structural and electrical properties were discussed.     

High-speed preparation and dielectric properties of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript> film by laser chemical vapor deposition

BaTi₄O₉ film was prepared on Pt/Ti/SiO₂/Si substrate by laser chemical vapor deposition. The microstructure and dielectric properties were investigated. The single-phase BaTi₄O₉ film with random orientation was obtained. The surface consisted of round and rectangular grains, and the cross-section was columnar microstructure. The deposition rate (R _dep) was 135 μm h⁻¹. The dielectric constant (ε _r) and loss (tanδ) were 35 and 0.01, respectively, at 1 MHz. With increasing temperature, ε _r increased and showed a broad peak around 736 K, which indicated there might be a phase transition.     

Influence of specimen geometry and size-effect on the <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">R</Emphasis></Subscript>-curve based on the cohesive stress in concrete

Comparative study for determining the K _R -curves associated with the cohesive stress distribution for complete fracture process for two standard specimen geometries i.e., three-point bending test and compact tension test specimen geometries of concrete using analytical method and weight function approach is reported in the paper. The laboratory size specimen (100 ≤  D  ≤  400 mm) with initial-notch length/depth ratios 0.3 and 0.5 are considered in the investigation. The load-crack opening displacement curves for these specimens are obtained using well known version of Fictitious Crack Model (FCM). It is found from the numerical results that the weight function method improves computational efficiency without any appreciable error. The stability analysis on the K _R -curves and the influence of specimen geometry and the size-effect on the K _R -curves, the CTOD-curves and the process zone length during crack propagation of complete fracture process are also described.     

The Activation Energy <Emphasis Type="Italic">U</Emphasis>(<Emphasis Type="Italic">T</Emphasis>,<Emphasis Type="Italic">H</Emphasis>) in Y-based Superconductors

Based on the Arrhenius equation, a method to calculate the activation energy from the resistance transition is proposed for high temperature superconductors. This method is applied to the Y-based superconductors. The activation energy is found to be U(T,H)∼(1−T/T _c )^4.8(H/H ₀)^−3.8 of YBCO crystal, and U(T,H)∼(1−T/T _c )^3.3(H/H ₀)^−2.2 of Er doped MTG YBCO crystal, respectively. With the obtained activation energy U(T,H), the lower part of the experimental curve ρ(T,H) and its derivative can be reproduced.      

Synthesis and dielectric properties of MXTi<Subscript>7</Subscript>O<Subscript>16</Subscript> (M = Ba and Sr; X = Mg and Zn) hollandite ceramics

MXTi₇O₁₆ (M = Ba and Sr; X = Mg and Zn) ceramics have been synthesized by the conventional solid state ceramic route. The dielectric properties such as dielectric constant (ε_r), loss tangent (tan δ) and temperature variation of dielectric constant (τ_εr) of the sintered ceramic compacts are studied using an impedance analyser up to 13 MHz region. The strontium compounds have relatively high dielectric constant and low loss tangent compared to the barium analogue. The phase purity of these materials has been examined using X-ray diffraction studies and microstructure using SEM method.     

Electrical conductivity and dielectric properties of cadmium thiogallate CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> thin films

Cadmium thiogallate CdGa₂S₄ thin films were prepared using a conventional thermal evaporation technique. The dark electrical resistivity calculations were carried out at different elevated temperatures in the range 303–423 K and in thickness range 235–457 nm. The ac conductivity and dielectric properties of CdGa₂S₄ film with thickness 457 nm has been studied as a function of temperature in the range from 303 to 383 K and in frequency range from 174 Hz to 1.4 MHz. The experimental results indicate that σ_ac(ω) is proportional to ω ^s and s ranges from 0.674 to 0.804. It was found that s increases by increasing temperature. The results obtained are discussed in terms of the non overlapping small polaron tunneling model. The dielectric constant (ε′) and dielectric loss (ε″) were found to be decreased by increasing frequency and increased by increasing temperature. The maximum barrier height (W _m) was estimated from the analysis of the dielectric loss (ε″) according to Giuntini’s equation. Its value for the as-deposited films was found to be 0.294 eV.     

Structure and dielectric properties of (1 − <Emphasis Type="Italic">x</Emphasis>)BiFeO<Subscript>3</Subscript> · <Emphasis Type="Italic">x</Emphasis>(KBi)<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript> perovskite solid solutions

(1 − x)BiFeO₃ · x(KBi)_1/2TiO₃ ceramics have been prepared by solid-state reactions. The system has been shown to contain a continuous series of perovskite solid solutions. In the composition ranges x < 0.4, 0.4 < x < 0.9, and x > 0.9, the solid solutions have rhombohedral, orthorhombic, and tetragonal structures, respectively. The observed compositional phase transitions are accompanied by sharp changes in unit-cell volume. We describe the dielectric properties of the orthorhombic solid solutions, which demonstrate that these materials exhibit relaxor behavior.     

Electrical and thermal properties of PTFE-Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> composites

A new polymer-ceramic composite was prepared using PTFE and low loss Sr₂ZnSi₂O₇. The dielectric properties of the composite were studied in the microwave and radiofrequency ranges. The relative permittivity (ε_r) and dielectric loss (tan δ) increased with the filler loading from 0.10 to 0.50 volume fractions (v_f). The observed values of ε_r, thermal conductivity and coefficient of thermal expansion (CTE) were compared with the corresponding theoretical predictions. The ability of the composite towards moisture absorption resistance was studied as a function of filler loading. It was also found that the variation of ε_r was less than 2% in the temperature range 25–90 °C, at 1 MHz. For a filler content of 0.50 v_f, the PTFE/Sr₂ZnSi₂O₇ composite exhibited ε_r = 4.4_, tan δ = 0.003 (at 4–6 GHz), CTE = 38.3 ppm/°C, thermal conductivity = 2.1 W/mK and moisture absorption = 0.09 wt%.     

Microwave Response of Perfect YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films Deposited on CeO<Subscript>2</Subscript>-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z _s(T), of epitaxial YBCO thin films deposited on CeO₂-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R _s(T) at low temperatures obeys the exponential law: R _s(T) = R _res+R ₀⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T _c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R _s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R _s(T) behavior.     

Superconducting State Parameters of MgB<Subscript>2</Subscript>: <Emphasis Type="Italic">Ab Initio</Emphasis> Study

Harrison’s first principle pseudopotential (HFPP) technique in conjunction with BCS theory and McMillan’s formalism has been used for the investigation of superconducting state parameters viz., Coulomb pseudopotential μ ^∗, electron–phonon coupling strength λ, SC transition temperature T _C , interaction strength N ₀ V, semi band gap Δ, energy or mass renormalization parameter Z ₀ and isotope effect exponent δ. The ground state properties of MgB₂ have also been calculated employing full-potential linearized augmented plane wave (FLAPW) method. This enables us to estimate the equilibrium values of bulk modulus and its pressure derivative through optimization of the crystal structure of the system. We have also described the total density of state (DOS) and the partial DOS (PDOS) around the Fermi energy.     

Effect of La substitution on structural and electrical properties of Ba(Fe<Subscript>2/3</Subscript>W<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> nanoceramics

The nanocrystalline fine powders (∼80 nm) of (Ba_1−x La _x )(Fe_2/3W_1/3)_1−x/4O₃, (BLFW) (x = 0.0, 0.05, 0.10 and 0.15) were synthesized with a combined mechanical activation and conventional high-temperature solid-state reaction methods. Preliminary X-ray structural analysis of pellet samples (prepared from fine powders) showed formation of a single-phase tetragonal system. Detailed studies of dielectric properties (ε_r and tan δ) exhibit that these parameters are strongly dependent on frequency, temperature and La composition. The La-substitution increases the dielectric constant and decreases the tan δ up to 10% substitutions of La at the Ba-site, and then reversed the variation, and hence this composition is considered as a critical composition. This observation was found valid for structure, microstructures, dielectric constant, electrical conductivity, J–E characteristics and impedance parameters also. Like in other perovskites (PZT, BZT), La substitution plays an important role in tailoring the properties of Ba(Fe_2/3W_1/3)O₃ ceramics.