boldmath Characterization of the BaBiO3-doped BaTiO3 positive temperature coefficient of a resistivity ceramic using impedance spectroscopy with Tc=155℃

BaBiO₃-doped BaTiO₃ (BB-BT) ceramic, as a candidate for lead-free positive temperature coefficient of resistivity (PTCR) materials with a higher Curie temperature, has been synthesized in air by a conventional sintering technique. The temperature dependence of resistivity shows that the phase transition of the PTC thermistor ceramic occurs at the Curie temperature, T_c=155 ℃, which is higher than that of BaTiO₃ ( ≤ 130 ℃). Analysis of ac impedance data using complex impedance spectroscopy gives the alternate current (AC) resistance of the PTCR ceramic. By additional use of the complex electric modulus formalism to analyse the same data, the inhomogeneous nature of the ceramic may be unveiled. The impedance spectra reveal that the grain resistance of the BB-BT sample is slightly influenced by the increase of temperature, indicating that the increase in overall resistivity is entirely due to a grain-boundary effect. Based on the dependence of the extent to which the peaks of the imaginary part of electric modulus and impedance are matched on frequency, the conduction mechanism is also discussed for a BB-BT ceramic system.     

高温无铅BaTiO_3-(Bi_(1/2)Na_(1/2))TiO_3正温度系数电阻陶瓷阻抗和介电谱分析

采用固相反应法制备了Y2O3施主掺杂的92 mol%BaTiO3-8 mol%(Bi1/2Na1/2)TiO3(BBNT8)高温无铅正温度系数电阻(positive temperature coe?cient resistivity,PTCR)陶瓷.利用透射电镜观察材料的显微结构,发现陶瓷的显微结构主要包括晶粒和晶界两部分,观察不到明显的壳层结构.进一步利用交流阻抗谱研究了陶瓷的宏观电学性能,发现陶瓷的总电阻是晶粒和晶界两部分的贡献,而晶粒电阻很小,在居里温度以上变化不大,材料的PTCR效应主要是晶界部分的贡献.当温度高于居里温度时,随着温度的升高,晶界介电常数逐渐减小,导致势垒增加,晶界电阻增大,从而产生正温度系数效应.最后,通过测试材料的介电频谱特性,研究计算了陶瓷的室温电阻率.     

Combined effects of Li content and sintering temperature on?polymorphic phase boundary and electrical properties of Li/Ta co-doped (Na, K)NbO3 lead-free piezoceramics

Crystallographic structure, phase transition and electrical properties of lead-free (Na_0.535K_0.485)_1−x Li _x (Nb_0.942Ta_0.058)O₃ (x=0.042–0.098) (NKL _x NT) piezoelectric ceramics were investigated. The experimental results show that both Li content and sintering temperature strongly affect the orthorhombic–tetragonal polymorphic phase boundary (PPB), which results in remarkable differences of the piezoelectric property and its temperature stability in the NKL _x NT ceramics. Chemical analysis indicates that sodium volatilizes more seriously than potassium and lithium with increasing sintering temperature. Due to the comprehensively optimized effects of Li content and sintering temperature, an enhanced piezoelectric constant d ₃₃ (276 pC/N) was obtained at room temperature in the ceramics with x=0.074 sintered at 1000°C. In the same composition, a further high d ₃₃ up to 354 pC/N was obtained at 43°C, which is close to its T _o−t temperature. Furthermore, better temperature stability can be obtained when x=0.082 sintered at 1000°C, whose piezoelectric constant d ₃₃ (236 pC/N) keeps almost constant from room temperature to 100°C. Such a temperature-independent piezoelectric property is available in the NKL _x NT ceramics with high Li content because its T _o−t was moved below room temperature.     

Electrical properties of thin films of Eu2O3 substituted compounds

Europium oxide (Eu₂O₃) substituted compound has been prepared by solid-solid reaction of the powders of Eu₂O₃, BaCO₃ and CuO at 950°C for 16 hours. The thin films have been deposited by high vacuum evaporation technique (vacuum ≈ 10⁻⁶ torr). The variation of current (I) with voltage (V) at room temperature (RT) i.e. 294 K and in ice (273 K) are found to be linear. The variation of electrical resistivity (ρ) with temperature (T) by heating the sample above RT has been determined. Resistivity is found to decrease with increase in temperature. Further the variation of electrical resistivity (ρ) with temperature (T) from 77 K, liquid nitrogen temperature (LNT), to 270 K has also been determined. It is observed that resistivity suddenly becomes zero at around 87 K. Thus the prepared material has superconducting properties with superconducting transition temperature, T _c at 87 K.      

Electrical and galvanomagnetic properties of biocarbon preforms of white pine wood

The electrical and galvanomagnetic properties of high-porosity biocarbon preforms prepared from white pine wood by pyrolysis at carbonization temperatures T _carb = 1000 and 2400°C have been studied. Measurements have been made of the behavior with temperature of the electrical resistivity, as well as of magnetoresistance and the Hall coefficient in the 1.8–300-K temperature interval and magnetic fields of up to 28 kOe. It has been shown that samples of both types (with T _carb = 1000 and 2400°C) are characterized by high carrier (hole) concentrations of 6.3 × 10²⁰ and 3.6 × 10²⁰ cm⁻³, respectively. While these figures approach the metallic concentration, the electrical resistivity of the biocarbon materials studied, unlike that of normal metals, grows with decreasing temperature. Increasing T _carb brings about a decrease in electrical resistivity by a factor 1.5–2 within the 1.8–300-K temperature range. The magnetoresistance also follows a qualitatively different pattern at low (1.8–4.2 K) temperatures: it is negative for T _carb = 2400°C and positive for T _carb = 1000°C. An analysis of experimental data has revealed that the specific features in the conductivity and magnetoresistance of these samples are described by quantum corrections associated inherently with structural characteristics of the biocarbon samples studied, more specifically with the difference between the fractions of the quasi-amorphous and nanocrystalline phases, as well as with the fine structure of the latter phase forming at the two different T _carb.     

Phase transition and electrical properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)(Nb<Subscript>1-x</Subscript>Ta<Subscript>x</Subscript>)O<Subscript>3</Subscript> lead-free piezoelectric ceramics

(K_0.5Na_0.5)(Nb_1-xTa_x)O₃ lead-free piezoelectric ceramics have been prepared by an ordinary sintering technique. The results of X-ray diffraction reveal that Ta⁵⁺ diffuses into the K_0.5Na_0.5NbO₃ lattices to form a solid solution with an orthorhombic perovskite structure. Because of the high melting temperature of KTaO₃, the (K_0.5Na_0.5)(Nb_1-xTa_x)O₃ ceramics can be sintered at higher temperatures. The partial substitution of Ta⁵⁺ for the B-site ion Nb⁵⁺ decreases both paraelectric/cubic–ferroelectric/tetragonal and ferroelectric/tetragonal–ferroelectric/orthorhombic phase transition temperatures, T_C and T_O-T. It also induces a relaxor phase transition and weakens the ferroelectricity of the ceramics. The ceramics become ‘softened’, leading to improvements in d₃₃, k_p, k_t and ε_r and a decease in E_c, Q_m and N_p. The ceramics with x=0.075–0.15 become optimum, having d₃₃=127–151 pC/N, k_p=0.43–0.44, k_t=0.43–0.44, ε_r=541–712, tanδ=1.75–2.48% and T_C=378–329 °C. PACS 77.65.-j; 77.84.Dy; 77.84.-s     

Ferroelectric phase transition in LaBSiO5 crystals from results of thermal and dielectric measurements

The specific heat of the ceramic and the permittivity of a single-crystal sample of LaBSiO₅, a new ferroelectric in the stilwellite family, were measured in a temperature range which includes the phase transition point (T _C=140 °C). The excess entropy of the phase transition ΔS=1.05 J/mol · K and the Curie-Weiss constant C _C-W=3.2×10³ K were determined. The results indicate that the phase transition in this crystal is of a “mixed” nature and exhibits features of a displacement-type transition and an order-disorder transition. Fiz. Tverd. Tela (St. Petersburg) 40, 1310–1312 (July 1998)     

Calibration of berlinite (AlPO4) as Raman spectroscopic pressure sensor for diamond‐anvil cell experiments at elevated temperatures

Changes in the band position of the 462 and the 1111 cm^–1 A₁ modes of berlinite (AlPO₄) with temperature and pressure were determined in situ to 500°C and to 10 GPa using Raman spectroscopy and diamond‐anvil cells. These bands shift in opposite directions with pressure and, likewise, with temperature. At a known temperature, the relative difference of both band positions (Δν)_P,T can therefore be used as a pressure gauge that does not require calibration of the spectrometer. At ambient pressure, the observed temperature dependence of this relative difference of the line positions is very close to linear and can be described by (Δν)_{T, 0.1 MPa} (cm^–1) = 0.0181 T – 0.46 where 23 ≤ T (°C) ≤ 500. Along the 23°C isotherm to 10 GPa, pressure and relative wavenumber difference (Δν)_{P, 23°C} are related by the equation P (GPa) = 0.00083 [(Δν)_{P, 23°C}]² – 0.062 (Δν)_{P, 23°C}. Both equations can be combined to determine pressures at higher temperatures under the assumption that the change in (Δν)_P,T with pressure is insensitive to temperature. Copyright © 2011 John Wiley & Sons, Ltd.     

Zero‐field μ+SR study of the colossal magnetoresistance material La0.67Ca0.33MnO3

Zero‐field μ⁺SR and resistivity experiments on La_0.67Ca_0.33MnO₃ powder show that the ferromagnetic transition temperature (T_C=274\ K) and resistivity peak temperature coincide to within 1 K, about 10 K higher than T_C determined from the bulk magnetization. The sublattice magnetization \nu_μ(T) is well described for T ≤ T_C by (1-T/T_C)^β , where β =0.345 ± 0.015. Unusual relaxation dynamics suggest a wide distribution of Mn‐ion correlation times. These results are discussed in terms of the effects of polarons on the spin and charge dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Internal friction in directed-crystallization (Cu-Sn)-Nb alloys

The distinctive features of the low-frequency internal friction Q ⁻¹(T) of (Cu-Sn)-Nb composites at high temperatures (up to 400°C) are investigated for strains in the range 10⁻⁵–10⁻⁴. Considerable hysteresis of Q ⁻¹(T) in the heating-cooling cycle is recorded, including the presence of a minimum at ∼175°C when the sample is heated to 400°C and two peaks P ₂ (at 280°C) and P ₁ (at ∼100°C) when the sample is cooled from 400°C. The activation energy of the anomalous internal friction background (up to 175°C), the oxygen diffusion parameters, and the oxygen concentration in the niobium fibers (all of which govern the peak P ₂) are calculated, and the value and temperature dependence of the yield point of the bronze matrix (which govern the peak P ₁) are estimated. Zh. Tekh. Fiz. 68, 114–117 (November 1998)     

Piezoelectric and ferroelectric properties of [(K0.4725Na0.4725)Li0.055]NbO3–(Ag0.5Li0.5)TaO3 lead‐free ceramics

New lead‐free piezoelectric (1 – x)[(K_0.4725Na_0.4725)Li_0.055]NbO₃– x (Ag_0.5Li_0.5)TaO₃ [(1 – x)KNNL–x ALT] ceramics were prepared by conventional sintering. Piezoelectric and ferroelectric properties and Curie temperature of the ceramics were studied. The (1 – x)KNNL–x ALT (x = 0.04) ceramics exhibit good properties (d₃₃ ～ 252 pC/N, k_p ～ 41%, T_C ～ 471 °C, T_o–t = 47 °C, P_r = 33.1 μC/cm², E_c = 10.6 kV/cm). These results show that (1 – x)KNNL–x ALT (x = 0.04) ceramic is a promising lead‐free piezoelectric material for high temperature application. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrolyte interaction with DPPC vesicles: An ESR study

Summary The partition of the spin probe TEMPO between the fluid lipid phase of single-walled vesicles of dipalmitoylphosphatidylcholine and the aqueous bulk solution have been used to investigate the interaction of monovalent ions with polar head of neutral phospholipids. The study has been performed by electron spin resonance (ESR) spectroscopy in the temperature range of (20÷60)°C and in the presence of (0÷3) M 1∶1 electrolyte. In the absence of electrolyte the spin probe TEMPO reveals the characteristic order→disorder DPPC main phase transition atT _m≈37°C, while the pretransition occurs atT _p≈27.5°C. On increasing the ionic strength of the dispersion medium it results for the partition coefficient,P _C, that, at each temperature,P _C(3)>P _C(2)>P _C(1)>P _C(0). Correspondingly, the pretransition disappears and theT _m value downshifts from ≈37°C with 0 M electrolyte to ≈34°C with 3M salt in the order:T _m(3)>T _m(2)>T _m(1)>T _m(0). The results suggest an increase in the net surface charge density of vesicles due to high ionic-strength values. The alteration of the electric interactions occurring into the polar zone of DPPC bilayer reduces the hindrances which, in turn, favour the enhancement of TEMPO partitioning in the hydrophobic core of phospholipid bilayers. The authors of this paper have agreed to not receive the proofs for correction.     

Preparation and characteristics of?a?BaO–Al2O3–B2O3–SiO2–La2O3 glass ceramic via spray pyrolysis

The characteristics of a BaO–Al₂O₃–B₂O₃–SiO₂–La₂O₃ glass ceramic prepared by spray pyrolysis were studied. Glass powders with spherical shape and amorphous phase were prepared by complete melting at a preparation temperature of 1 500°C. The mean size and geometric standard deviation of the powders prepared at the temperature of 1 500°C were 0.6 μm and 1.3. The glass powders had similar composition to that of the spray solution. The glass transition temperature (T _g) of the glass powders was 600.3°C. Two crystallization exothermic peaks were observed at 769.3 and 837.8°C. Densification of the specimen started at a sintering temperature of 600°C, in which Ba₄La₆O(SiO₄)₆ as main crystal structure was observed. Complete densification of the specimen occurred at a sintering temperature of 800°C. The specimens sintered at temperatures above 800°C had main crystal structure of BaAl₂Si₂O₈.     

Intrinsic inhomogeneities of low-doped lanthanum manganites in the paramagnetic temperature range

The nature of the electrical resistivity for low-doped lanthanum manganites is elucidated. The electrical resistivity is described by the Efros-Shklovskii law (lnρ √ (T ₀/T)^−1/2, where T ₀ √ 1/R _ls) in the temperature range from T* ≈ 300 K ≈ T _C (T _C is the Curie temperature for conducting manganites) to their T _C and is explained by the tunneling of carriers between localized states. The magnetoresistance is explained by a change in the size of localized states R _ls in a magnetic field. The patterns of change in R _ls with temperature and magnetic field strength determined from magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a paramagnetic matrix. The sizes R _ls and their temperature dependence have been estimated through magnetic measurements. The results confirm the existence of a Griffith phase. The intrinsic inhomogeneities produced by thermodynamic phase separation determine the electrical resistivity and magnetoresistance of lanthanum manganites.     

Ac-conductivity and dielectric relaxations above glass transition temperature for parylene-C thin films

45% semi-crystalline parylene-C (–H₂C–C₆H₃Cl–CH₂–) _n thin films (5.8 μm) polymers have been investigated by broadband dielectric spectroscopy for temperatures above the glass transition (T _g =90°C). Good insulating properties of parylene-C were obtained until operating temperatures as high as 200°C. Thus, low-frequency conductivities from 10⁻¹⁵ to 10⁻¹² S/cm were obtained for temperatures varying from 90 to 185°C, respectively. This conductivity is at the origin of a significant increase in the dielectric constant at low frequency and at high temperature. As a consequence, Maxwell–Wagner–Sillars (MWS) polarization at the amorphous/crystalline interfaces is put in evidence with activation energy of 1.5 eV. Coupled TGA (Thermogravimetric analysis) and DTA (differential thermal analysis) revealed that the material is stable up to 400°C. This is particularly interesting to integrate this material for new applications as organic field effect transistors (OFETs). Electric conductivity measured at temperatures up to 200°C obeys to the well-known Jonscher law. The plateau observed in the low frequency part of this conductivity is temperature-dependent and follows Arrhenius behavior with activation energy of 0.97 eV (deep traps).     

Phase formation and electrical properties of BNLT–BZT lead-free piezoelectric ceramic system

Phase formation study in lead-free piezoelectric ceramics based on lanthanum doped bismuth sodium titanate (Bi_0.4871Na_0.4871La_0.0172TiO₃:BNLT) and zirconium doped barium titanate (BaZr_0.05Ti_0.95O₃:BZT), has been carried out in the system of (1−x)BNLT–xBZT where x = 0.0–1.0, by two-step mixed oxide method. It was observed that the addition of BZT in the BNLT ceramics developed the dielectric and piezoelectric properties of the ceramics with the optimum piezoelectric constant (d₃₃) and dielectric constant (ε_r) at room temperature of about 138 pC/N and 1651, respectively, from the 0.2 BNLT to 0.8 BZT ceramic sample. The Curie temperature (T_C) of this ceramic was found at 295 °C which is 195 °C higher than that of pure BZT ceramics, promising the use of this ceramic in a higher range of temperature.     

Double-peak resistivity transport properties of La0.67Ca0.33MnO3 ceramics

Polycrystalline samples of La_0.67Ca_0.33MnO₃ were prepared by solid-state reactions by varying the pelletization force and the sintering temperature. Lowering the sintering temperature gave rise to smaller grains and increased the overall resistivity of the ceramic. Partial melting was observed in the ceramics sintered at higher temperatures (1400-1500 °C). Additionally, these ceramics showed two distinct resistivity peaks. The resistivity peak near the magnetic transition (T_C) was sharp, whereas the second peak was a broad one observed below T_C.     

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at T _carb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T ^1.5 scaling. The results obtained are analyzed.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Electrical performance of Ti/Pt thin films on glass-ceramic substrates after high temperature loading

In this study, the influence of post-deposition annealings (PDA) up to temperatures of T _PDA=700°C on the room-temperature resistivity of e-beam evaporated titanium/platinum (Ti/Pt) bi-layers on low temperature co-fired (LTCC) substrates covered with a glass encapsulate is investigated. The thickness of the platinum top layer is varied between 24 and 95 nm (titanium film thickness: 5 nm) and between 23 and 90 nm (titanium film thickness: 15 nm), respectively. In the “as-deposited” state and up to post-deposition annealing temperatures of T _PDA=450°C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. When applying, however, solely the Fuchs-Sondheimer model for evaluation, the effective mean free path for electrons is substantially above the value reported for crystalline platinum at room temperature. Compared to similar investigations on smooth Si/SiO₂ substrates yielding interpretable results within this theoretical approach, this is due to the increase of the thickness-dependent fraction in film resistivity which is strongly affected by the enhanced LTCC/glass surface roughness. At T _PDA>600°C, diffusion of titanium into the platinum top layer and the roughening of the LTCC/glass substrate dominate the electrical behavior, both causing an increase in film resistivity above average. In contrast to Si/SiO₂ substrates, thermal induced grooving effects in the Pt top layer play a minor role as the temperature coefficients of expansion of metallization and glass-ceramic substrate match better and the effective temperature difference for stress generation is lower due a glass softening temperature of about 450°C.