Dielectric properties of SrTiO3 single crystals subjected to high electric fields and later irradiated with X-rays orγ-rays

The dielectric constant (K), loss (tanδ), and hence conductivity (σ) of SrTiO₃ single crystals have been measured in the frequency region 10²–10⁷ Hz and in the temperature range 30°–350° C. Quenching, subjecting the crystals to high electric fields (a.c. or d.c.) and X-ray orγ-ray irradiation, or a combination of these treatments, is found to bring about interesting changes in these properties. An attempt is made to understand the results.     

Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites

Composites of epoxy resin having different amounts of graphite particles have been prepared by solution casting method. Temperature dependence of dielectric constant, tan δ and a.c. conductivity was measured in the frequency range, 1–20 kHz, temperature range, 40–180°C for 0.99, 1.96 and 2.91 wt% graphite filled and unfilled epoxy composites. It was observed that the dielectric constant, tanδ and a.c. conductivity increase with increasing temperature. Near the transition temperature the materials show anomalous behaviour for the observed properties. Peaks of dielectric constant, tan δ and a.c. conductivity were observed to shift towards lower temperature with increasing frequency. Clear relaxation (tan δ) peaks around 169°C were observed in epoxy resin, which shifted to lower temperature side on increasing the frequency. Addition of 2.91 wt% graphite shifted the tan δ peaks towards higher temperature side by creating hindrances to the rotation of polymer dipoles. Addition of 2–91 wt% graphite leads to an increased relaxation time τ of dipoles in polysulphide epoxy from 1.44 × 10⁻⁵− 3.92 × 10⁻⁵ (s) at 90°C by creating the hindrance to the rotation of dipoles.     

Dielectric properties of LiF-B2O3 glasses doped with certain rare earth ions

Dielectric constantɛ, loss tanδ and a.c. conductivityσ of LiF-B₂O₃: Ln³⁺ (where Ln=Ce, Pr, Nd and Tb) glasses are studied as functions of frequency (in the range 10²–10⁶ Hz) and temperature (range 30–200°C). The dielectric breakdown strength of these glasses was also determined at room temperature in an air medium. The rate of increase ofɛ and tanδ with temperature decreases with decrease in the ionic radius of RE³⁺ ion whereas the dielectric breakdown strength, the activation energy for a.c. conduction in the high temperature region decreases with increase in the ionic radius of RE³⁺ ion. An attempt has been made to explain the a.c. conduction in these glasses on the basis of quantum mechanical tunnelling (QMT) model.     

High-temperature electrical conductivity and thermal expansion of SmBa2(Cu1-xFex 3O6+δ (x = 0–0.2)

The electrical conductivity and thermal expansion of SmBa₂(Cu_1-xFe_{x 3}O_6+δ (x = 0-0.2) were measured in air in the temperature range from 20 to 900°C. The linear thermal expansion coefficient was determined to be (12.8–13.5) × 10^-6 K^-1 in the temperature range from 20 to 350°C and (16.2–17.8) × 10^-6 K^-1 in the range from 350 to 800°C. Between 400 and 900°C, the conductivity of SmBa₂(Cu_1-xFe_x)₃O_6+δ was found to decrease with increasingx, mainly in the rangex = 0–0.1. This finding was interpreted in terms of the Fe occupancies on the Cu(1) and Cu(2) sites.     

Electric and dielectric properties of solution-gas interface-grown PbS films

Dielectric properties andI–V characteristics of solution-gas interface-formed PbS thin-film capacitors (Al/PbS/Al) of various thicknesses have been studied in the frequency range 10-10⁶ Hz at various temperatures (300–443 K). Current-voltage (I–V) characteristics show space-charge-limited conduction. Dielectric constant (ε) increases with increasing film thickness and temperature and decreases with increase of frequency. The loss factor (tanδ) peaks observed in tanδ vs frequency and tanδ vs temperature reveal relaxation effect from dipolar orientation. These maxima shift to higher-temperature region with increasing frequency. The large increase in capacitance (C) and dielectric constant (ε) towards low-frequency (f) region indicates the possibility of an interfacial polarization mechanism in this region.     

Dielectric properties of LaPO4 ceramics

Dielectric constant (K) and loss (tan), and hence conductivity (), have been measured for LaPO₄ ceramics prepared by a molten urea process in the frequency region 10² to 10⁷ Hz and in the temperature range –193 to 280° C. At room temperature ( 30°C),K decreases with frequency up to 10⁵ Hz, beyond which it attains a constant value; tan behaves in a similar way. The values ofK and tan at 30°C and 10⁶ Hz for this material are 14 and 2.7 × 10^–3, respectively. As a function of temperature,K exhibits two stages of increase: (i) a slow stage up to –30° C and (ii) a fast one beyond –30° C which is considerably frequency dependent,K having larger values at lower frequencies. Similar behaviour is exhibited by tan. Plots of against 1/T for LaPO₄ exhibit the usual extrinsic and intrinsic regions; the activation energy for conduction in the intrinsic region is calculated to be 0.61 eV. It seems possible to understand the results through space charge polarization effects due to lattice defects in LaPO₄.     

Electrical properties of ion irradiated polypropylene films

The effect of high-energy (50 MeV) Li³⁺ ion beam irradiation on polypropylene (PP) film has been studied in the fluence range 2.4 × 10¹²−l.5 × 10¹⁴ ions/cm². The a.c. electrical properties of PP films were measured in the frequency range from 0.05– 100 kHz, and at temperature range between 30 and 140°C. This study indicates two peaks at 60°C and 120°C with comparatively high magnitudes. There is an exponential increase in conductivity with log of frequency and the effect is significant at higher fluences. The loss factor (tan δ) vs frequency plot suggests that PP film based capacitors may be useful below 10 kHz. The capacitance is constant over a wide temperature range up to 130°C. FTIR spectra of the PP films before and after irradiation indicate that intensity of C-H stretching vibration at 2900 cm⁻¹ is modified. The presence of many new peaks with the increase of fluence suggests the formation of alkanes and alkynes which might be responsible for the observed changes in the dielectric and electrical properties of PP films.     

Study of dielectric properties of RbNO3 single crystals

Dielectric properties, dielectric constant (k), dielectric loss (tan δ) and a.c. conductivity (σ) in the solution-grown single crystals of RbNO₃ are presented from room temperature to about 200°C covering the frequency range 10² to 10⁵ Hz. A broad peak observed in tan δ-frequency data between 10³ and 10⁵Hz is thought to be due to impurity-vacancy dipoles. The sudden rise of three parameters near 160°C is attributed to the known phase transition from trigonal to CsCl structure.     

Ferroelectric phase transition in Ba5RTi3Nb7O30 [R=Nd, Eu, Gd] ceramics

Polycrystalline samples of Ba₅RTi₃Nb₇O₃₀ [R=Nd, Eu, Gd], were prepared using high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the compounds shows the formation of single phase compounds (orthorhombic crystal system) at room temperature. Detailed studies of dielectric properties (ɛ, tanδ,σ) as a function of frequency (400 Hz to 10 kHz) and temperature (30° to 380°C) show that these compounds exhibit diffuse ferroelectric phase transition.     

Synthesis,structure and characterization of ceramic Ca4Bi2Ti4Nb6O30

The polycrystalline samples of Ca₄Bi₂Ti₄Nb₆O₃₀ (herein designated CBTN) were synthesized by the conventional ceramic method. Preliminary X-ray structural study of the compound showed the formation of a single phase solid solution having orthorhombic structure in the paraelectric phase. Measurements of the dielectric constant (ε) and dielectric loss (tan δ) as a function of temperature (−180–200°C) at 1 kHz and 10 kHz and also as a function of frequency (10² Hz to 10⁴ Hz) at five different temperatures [−180°C, −40°C, − 10°C 26°C (room temperature) and 75°C] have shown a dielectric anomaly and a phase transition at − 13 ±1°C in CBTN.     

Microstructure and microwave dielectric characteristics of CaO–B2O3–SiO2 glass ceramics

CaO–B₂O₃–SiO₂ (CBS) glass powders are prepared by traditional glass melting method, whose properties and microstructures are characterized by Differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the pure CBS glass ceramics possess excellent dielectric properties (ε _r = 6.5, tan δ = 5 × 10⁻³ at 10 GHz), but a higher sintering temperature (>900 °C) and a narrow sintering temperature range (about 10 °C). The addition of a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) could greatly decrease the sintering temperature of CBS glass to 820 °C and significantly enlarge the sintering temperature range to 40 °C. The CBS glass ceramic with 30 wt% LG glass addition sintered at 840 °C exhibits better dielectric properties: ε _r ≈ 6, tan δ < 2 × 10⁻³ at 10 GHz, and the major phases of the sample are CaSiO₃, CaB₂O₄ and SiO₂.     

Effect of Ca/Si ratio on the microstructures and properties of CaO–B2O3–SiO2 glass-ceramics

CaO–B₂O₃–SiO₂ glass-ceramics were synthesized by sol–gel method, and the effect of Ca/Si ratio on the microstructures, electrical properties and mechanical characteristics of this ternary system was investigated. The results showed that the increase of CaO content is favorable for the crystallization of CaO–B₂O₃–SiO₂ system and formation of the desired glass-ceramics. The bending strength of the sintered glass-ceramics increases with CaO content by increasing of crystalline phases. When the Ca/Si ratio increases, the dielectric constant (ε _r) decreases and loss (tanδ) increases gradually. The thermal expansion coefficient (TEC) enhances by increasing CaO contents due to the formation of other crystal phases with large TEC value. The glass-ceramics exhibit low dielectric constant and loss (ε _r < 4.7, tanδ < 5 × 10^-4 at 1 MHz), high resistivity (ρ > 10¹² Ω · cm), as well as excellent mechanical properties (σ ≈ 160 MPa, α ≈ 3.6 × 10⁻⁶/°C).     

Electrical properties of thermally evaporated tellurium thin films

Results of dielectric and conduction properties of vacuum evaporated tellurium (Te) thin film capacitors (Al-Te-Al) have been reported in the frequency range 1–100 kHz at various temperatures (303–423 K). Loss factor (tanδ) which shows a maximum with frequency increases with rise of temperature and tanδ _max shift towards high frequency region. The large values of capacitance and dielectric constant (ɛ′) in the low frequency region indicate the possibility of an interfacial polarization mechanism.I-V characteristics show ohmic, space charge limited (SCLC) and thermionic emission conduction mechanisms to operate at low, intermediate and high voltages respectively. Various transport parameters have been calculated. It has been observed that the Schottky type of conduction is predominant in the high field region and the Schottky barrier height has been determined. The Hall coefficient, Hall mobility and carrier concentration are also discussed.     

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.     

Studies of structural and electrical properties of Pb(Li1/4Eu1/4Mo1/2)O3 ceramics

Polycrystalline sample of Pb(Li_1/4Eu_1/4Mo_1/2)O₃ was prepared by usual ceramic route. X-ray diffraction, SEM and EDAX were used to check the formation, homogeneity and elements present in the compound. The measurement of dielectric constant (ɛ), dielectric loss (tanδ), hysteresis loop parameters (e.g. spontaneous polarization) as function of frequency and temperature suggest that the compound is a ferroelectric one and has a ferro-paraelectric phase transition at 112 ± 2°C. The measurement of dc resistivity both as function of biasing electric field and temperature of the compound suggests that the compound has negative temperature coefficient (NTC) of resistance above 100°C.     

Synthesis and properties of LaNi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−δ</Subscript> as cathode materials in SOFC

The synthesis of LaNi_{1− x} Fe _x O_3−δ (LNF) perovskites with x = 0.0–1.0, for use as cathode materials for an IT-SOFC, was investigated using four combustion methods, Water Citrate (WC), Modified Water Citrate (MWC), Nitric Citrate (NC), and Modified Nitric Citrate (MNC). The structures and homogeneities of the synthesized powders were examined using an XRD, and the particle sizes were examined using an SEM and a particle size analyzer. All four combustion methods gave the single phase perovskites with the same structure. The main difference was shown in a particle size that the smallest to the largest sizes were obtained from MNC, MWC, NC, and WC, respectively. In this LNF series, as x is 0–0.5, the crystal structure is cubic and rhombohedral at the calcination temperature of 700 and 900 °C, respectively. Further investigation indicated that the cubic structure changed to rhombohedral structure at 900 °C, and was stable up to 1200 °C. As x is 0.6–1.0, the crystal structure is in orthorhombic phase when calcined between 700 and 1000 °C. This orthorhombic phase decomposed above 1100 °C. From the XRD and SEM–EDX results, LaNi_0.6Fe_0.4O_3−δ (LNF64) has a good chemical compatibility with 8YSZ from room temperature up to 900 °C. In addition, its thermal expansion coefficient is 13.2 × 10⁻⁶ K⁻¹ close to that of 8 mol% Y₂O₃ (8YSZ). Therefore, LNF64 also has a good physical compatibility with 8YSZ.     

Dielectric properties of NaF-B2O3 glasses doped with certain transition metal ions

Dielectric constant ε, loss tan δ, a.c. conductivity Σ and dielectric breakdown strength of NaF-B₂O₃ glasses doped with certain transition metal ions (viz. Cu²⁺, VO²⁺, Ti⁴⁺ and Mn⁴⁺) are studied in the frequency range 10²-10⁷ Hz and in the temperature range 30–250°C. The values of ε, tan δ, Σ_a.c. are found to be the highest for Cu²⁺ doped glasses and the lowest for Mn⁴⁺ doped glasses. Activation energy for a.c. conduction and the value of dielectric breakdown strength are found to be the lowest for Cu²⁺ doped glasses and the highest for Mn⁴⁺ doped glasses. With the help of infrared spectra, increase in the values of ε and tan δ of these glasses with frequency and temperature are identified with space charge polarization. An attempt has been made to explain a.c. conduction phenomenon on the basis of quantum mechanical tunneling model (QMT)/carrier barrier hopping model.     

Impact of metal filler on the dielectric properties of Nylon 11

Dielectric measurements of pure Nylon 11 in comparison with metal (Zn) filled Nylon 11 have been carried out using an impendence analyzer in the frequency range of 10²–10⁷ Hz and temperature range 20–120 °C. Two different concentrations (1% and 5% (w/w)) of metal (Zn) fillers were used. It was observed that at low frequencies and particularly at high temperature dielectric permittivity (ε′) for 1% Zn filled sample is more than that of pure Nylon 11 whereas ε′ for 5% Zn filler is less as compared to that for pure Nylon 11. But at very high frequencies dielectric permittivity (ε′) for pure Nylon 11 is less than Zn filled samples. Also it is found that for all frequencies and particularly at high temperature ε′′ as well as tan δ are maximum for pure Nylon 11 and decrease for filled Nylon 11 samples. The Cole–Cole arcs have also been plotted for these samples. Using these plots the static and instantaneous values of dielectric permittivity and orientation polarization parameter ‘S’ have been calculated.     

Dielectric dispersion and piezoelectric resonance in benzil single crystals grown by Bridgman-Stockbarger technique

Large single crystals of an organic nonlinear optical (NLO) material, benzil, have been grown by employing an indigenously built Bridgman-Stockbarger crystal grwoth system. The dielectric constant (ε _r), dielectric loss tangent (tanδ) and impedance (Z) of these crystals, monitored along the crystallographica-axis, as a function of frequency in the 100kHz-10MHz range, exhibit sharp resonance effects in the 100–300kHz region. These strong resonance effects are attributed to the piezoelectric nature of the crystal. The origin of the appearance of the sharp peaks in the frequency variation ofε _r and tanδ is in response to the elastic resonances induced via the externally applied electric field. The resonance peak positions shift towards lower frequencies with increase in temperature as a consequence of the decrease in the stiffness coefficient (C ₁₁) of the crystal.     

Microstructure evolution of delta-processed IN718 during holding period after hot deformation

Two IN718 alloys with different initial δ phase contents have been used to study the microstructure evolution during the holding period after the hot deformation by the heat treatment at 950–1040 °C after deformation at temperature of 950 °C and strain rate of 0.005 s⁻¹. The results indicate that the grain growth is restrained due to the pinning effect of δ phase. The activation energy of grain growth for the alloys with initial δ phase content of 0 and 8.14 wt% in the heat treatment after hot working were 273.8 and 882.8 kJ/mol, respectively. In addition, the dissolution of δ phase occurred during the heat treatment after the hot deformation, and the δ phase dissolved largely as the temperature is higher than 980 °C. Therefore, the heating and deformation temperature should be in the range of 950–980 °C for the Delta Process and multi-stage hot working processes.