High-Temperature Thermoelectric Power of The Metal Oxides: La2?xSrxCuO4 and Bi1?xSrxMnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35) and Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO < T < 700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO ∼ 520 K for x = 0.5 to T _CO ∼ 435 K for x = 0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

NMR studies of Nd2−x Ce x CuO4

Copper NMR has been studied as a function of temperature in a number of superconducting Nd_2−x Ce _x CuO₄ samples. The electric field gradient is very small and the Knight shift is 2380 ppm at room temperature, both of these implying that the copper is in a Cu⁺ state. The Knight shift decreases with temperature particularly belowT _c . The spin contribution to the Knight shift is estimated to be ∼ 200 ppm (about a factor ten smaller than in YBa₂Cu₃O₇) indicating thatN _s (E _F ) at the copper sites is small in this material.     

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.     

Power generation of the touching Cu/Bi-Te/Cu composites under the periodically alternating temperature gradients

The thermo-emf ΔV of the touching p- and n-type Cu/Bi-Te/Cu composites with different thicknesses of t _Bi-Te and t _Cu was measured as a function of time by alternating the temperature difference ΔT at periods of T = 20, 60, 120, 240 and ∞ sec, where t _Bi-Te was varied from 0.1 to 2.0 mm and t _Cu from 0 to 4.0 mm. As a result, ΔV changes significantly with t _Bi-Te, t _Cu and T. The effective thermo-emf ΔV _eff increases significantly with an increase of 1/T and exhibited a local maximum at 1/T = 1/240 s⁻¹. The resultant | α | and the effective temperature difference ΔT _eff were increased significantly by optimizing t _Bi-Te and t _Cu at 1/T = 1/240 s⁻¹. The power generation ΔW _eff (= ΔV _eff²/4R _calc) estimated using the measured ΔV _eff and calculated R _calc also exhibited a local maximum at 1/240 s⁻¹ for an optimum combination of t _Bi-Te = 0.1 mm and t _Cu = 2.0 mm, so that the maxima ΔW _eff at 1/T = 1/240 s⁻¹ for the p- and n-type composites were 2.28 and 2.92 times higher than those obtained at 1/T = 0 s⁻¹. This significant increase in ΔW _eff is owing to both the increase in ΔT _eff and the increase in ZT due to the increase in |α|. The power generation was thus found to be enhanced significantly by imposing the alternating temperature gradients on touching Cu/Bi-Te/Cu composites.     

Effect of simultaneous substitution of La and Mn on dielectric behavior of barium titanate ceramic

A few compositions in the valence compensated system Ba_1−x La _x Ti_1−x Mn _x O₃ were synthesized by solid-state ceramic method to study the effect of co-doping lanthanum and manganese in equimolar amounts on dielectric behavior of BaTiO₃. Compositions with x ≤ 0.10 have shown solid solution formation. Compositions with x ≤ 0.05 are found to have tetragonal structure at room temperature while composition with x = 0.10 is cubic. Plots of relative dielectric constant, ε_r versus temperature for composition with x = 0.01 show dielectric anomalies around 376 ± 2, 264 ± 2 and 179 ± 2 K which correspond to cubic to tetragonal (T _C–T), tetragonal to orthorhombic (T _T–O) and orthorhombic to rhombohedral (T _O–R) transition, respectively, similar to BaTiO₃. Curie temperature has been found to decrease with increasing concentration of lanthanum and manganese simultaneously in barium titanate. The broadening in the dielectric peak at cubic to tetragonal (T _C–T) transition temperature increases with increasing x. For x = 0.10, only one anomaly at 100 K is observed in its ε_r versus T plots. The observation of this single anomaly may be due to pinching effect of the substitutions on the three phase transitions.     

Thermal decomposition of trimethylamine borane as a precursor to nanocrystalline CVD BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> films

We have studied the kinetics of BC _x N _y chemical vapor deposition through trimethylamine borane decomposition at atmospheric pressure. The rate constant of the heterogeneous interaction between trimethylamine borane and an adsorption center has been determined to be k _s ⁰ = 2.7 × 10⁷exp(−10560/T) cm/s. The obtained kinetic parameters of the reaction fully determine the growth rate of nanocrystalline carbonitride films under kinetic control. The film thickness has been determined as a function of time, temperature, reactant concentration, and reactor dimensions.     

Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Tc as a Function of Electron Doping in Mg10B2 Using Sc for Mg Substitution

We have studied the variation of superconducting critical temperature T _c as a function of electron doping in the Mg¹⁰B₂ system using Sc for Mg substitution. The critical temperature in the¹⁰B isotope substituted system Mg_{1− x} Sc _x ¹⁰B₂ increases by increasing the scandium content x in the range 0<x<0.012 up to 41.4 K, while the T _c of the natural boron system Mg_{1− x} Sc _x B₂ is nearly constant. The overall difference of T _c in Mg_{1− x} Sc _x B₂ as function of x between the natural B and ¹⁰B isotope system seems to indicate that the isotope effect shows large variations with electron doping as expected for the T _c enhancement driven by a shape resonance.     

Electrical properties of Gd-modified Pb(SnTi)O3 ferroelectric ceramics

The Polycrystalline samples of (Pb_1−x Gd _x )(Sn_0.45Ti_0.55)_1−x/4O₃ (PGST) (x = 0, 0.05, 0.07, 0.1) were prepared using a standard mixed-oxide method. The XRD and SEM studies of the samples reveal that PGST samples have tetragonal symmetry with uniform grain distribution. Measurements of dielectric constant, dielectric loss and ac electrical conductivity were made in a wide temperature (300–650 K) and frequency (100 Hz to 1 MHz) range. These materials undergo a ferroelectric–paraelectric phase transition at 569, 582, 598, and 603 K for x = 0, 0.05, 0.07, 0.1, respectively. The ferroelectric property of some samples at room temperature was confirmed through the study of hysteresis loops. The Cole–Cole plots (ε′′ vs. ε′) at 125 and 150 °C form semicircular arcs with the center lying underneath the abscissa, deviated from the ideal Debye relaxation model to some extent. In all the compounds, the slope of ln σ_ac ~ 1/T shows a distinct variation for temperature below 450, 450 K to T _c and for T > T _c. The room temperature ac conductivity at 10 kHz lies in the range of 10⁻⁵–10⁻⁶ (Ω⁻¹ m⁻¹).     

Electrical properties of Y-modified Pb(SnTi)O<Subscript>3</Subscript> ferroelectric ceramics

Y-modified Pb(SnTi)O₃ polycrystalline materials with a general formula (Pb (Sn (PYST) (x = 0, 0.05, 0.07, 0.1) have been synthesized using the standard mixed oxide method. Crystallographic and surface morphology studies are carried out by the powder X-ray diffraction and scanning electron microscopy techniques. X-ray diffraction studies of the samples showed the tetragonal structure of the unit cell. Uniform grain distributions with porosity throughout the surface of the sample pellets are observed. The T _C (transition temperature) increases with increase in the mole fraction of the substituent (x). ɛ^/ _max is found to decrease with increase in x. The slope of lnσ_ac∼ 1/T shows distinct variations for temperature below 410 K, 410 K to T _c and for T > T _c. The room temperature ac conductivity at 10 kHz lies in the range of 10⁻⁵–10⁻⁶ (ohm⁻¹ m⁻¹).     

Ferroelectric transitions in valence compensated Ba1? x La x Ti1? x Cr x O3 ceramics

An attempt has been made to investigate the solid solubility limit between barium titanate, BaTiO₃ and lanthanum chromite, LaCrO₃. Compositions with x ≤ 0.15 have shown solid solution formation in the system Ba_1−x La _x Ti_1−x Cr _x O₃. Compositions with 0.01 and 0.05 are found to have a tetragonal structure, whereas those with x = 0.10 and 0.15 are found to have a cubic structure at room temperature. Dielectric measurements have been carried out at different frequencies in the temperature range 100–550 K. Plots of ε_r versus temperature for composition with x = 0.01 show dielectric anomalies at 376 K (T _C-T) and 275 (T _T-O) where T _C-T and T _T-O denote the temperature for cubic to tetragonal and tetragonal to orthorhombic transition. Dielectric anomaly corresponding to the ferroelectric to paraelectric transition has been found to shift to lower temperatures with increasing concentration of lanthanum and chromium. For compositions with x ≤ 0.05, T _C has been found to be frequency independent while for x = 0.10 and 0.15, T _C has been found to be frequency dependent in a manner similar to relaxor ferroelectrics.     

Dielectric response of PLZT ceramics <Emphasis Type="Italic">x</Emphasis>/57/43 across ferroelectric–paraelectric phase transition

The dielectric properties of lead lanthanum zirconate titanate (PLZT) ceramics [Pb(Zr_0·57Ti_0·43)O₃ + x at% of La, x = 3, 5, 6, 10 and 12] have been measured in the frequency range 1 Hz–1 MHz using the vector impedance spectroscopy (VIS) at different temperatures. All the compositions show both non-dispersive and dispersive dielectric responses in different temperature regions. The non-dispersive region obeys the universal dielectric response. A low frequency (<1 kHz) relaxation phenomenon with a high value of distribution parameter ‘h’ (~0·4 to 0·6) has been observed in all the compositions around the temperature corresponding to the maximum dielectric constant (T _m). The activation energies as calculated from the relaxation and d.c. conduction processes are comparable. The ferroelectric phase transition is diffuse in nature and broadening of the peak increases with La content.     

Electrical properties of V<Subscript>2</Subscript>O<Subscript>5</Subscript>-CaO-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses exhibiting majority charge carrier reversal

The thermoelectric power and d.c electrical conductivity of x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses were measured. The Seebeck coefficient (Q) varied from +88 μ V K⁻¹ to −93 μV K⁻¹ as a function of V₂O₅ mol%. Glasses with 10 and 15 mol% V₂O₅ exhibited p-type conduction and glasses with 25 and 30 mol% V₂O₅ exhibited n-type conduction. The majority charge carrier reversal occurred at x = 20 mol% V₂O₅. The variation of Q was interpreted in terms of the variation in vanadium ion ratio (V^{5 +}/V^{4 +}). d.c electrical conduction in x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses was studied in the temperature range of 150 to 480 K. All the glass compositions exhibited a cross over from small polaron hopping (SPH) to variable range hopping (VRH) conduction mechanism. Mott parameter analysis of the low temperature data gave values for the density of states at Fermi level N (E_F) between 1.7 × 10²⁶ and 3.9 × 10²⁶ m⁻³ eV⁻¹ at 230 K and hopping distance for VRH (R_VRH) between 3.8 × 10⁻⁹m to 3.4 × 10⁻⁹ m. The disorder energy was found to vary between 0.02 and 0.03 eV. N (E_F) and R_VRH exhibit an interesting composition dependence.     

Specific Heat Capacity and Thermal Conductivity of Foam Glass (Type 150P) at Temperatures from 80 to 400 K1

Low-temperature specific heat capacities of foam glass (Type 150P) have been measured from 79 to 395 K by a precision automated adiabatic calorimeter. Thermal conductivities of the glass foams have been determined from 243 to 395 K with a flat steady-state heat-flow meter. Experimental results have shown that both the specific heat capacities and thermal conductivities of the 150P foam glass increased with temperature. Experimentally measured specific heat capacities have been fitted by a polynomial equation from 79 to 395 K: C _p /J · g⁻¹· K⁻¹=0.6889+0.3332x− 0.0578x ²+0.0987x ³+0.0521x ⁴− 0.0330x ⁵− 0.0629x ⁶, where x=(T/K − 273)/158. Experimental thermal conductivities as a function of temperature (T) have been fitted by another polynomial equation from 243 to 395 K: λ/ W · m⁻¹· K⁻¹=0.14433+0.00129T − 2.834 × 10⁻⁶ T ²+2.18 × 10⁻⁹ T ³. In addition, thermal diffusivities (a) of the form glass sample were calculated from the specific heat capacities and thermal conductivities and have been fitted by a polynomial equation as a function of temperature (T): a/m² · s⁻¹=−1.68285+0.01833T − 5.84891 × 10⁻⁵ T ²+8.11942 × 10⁻⁸ T ³ − 4.24975 × 10⁻¹¹ T ⁴.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Magnetic Studies of Spin Wave in Fe/Ag Multilayer Films

Magnetic properties of Fe/Ag multilayer films are investigated and examined versus Fe layer thickness t _Fe. As a result, spontaneous magnetization M(T) temperature dependence has been revealed to be well described by a T ^3/2 law in all multilayer films (7 ?≤t _Fe≤60 ?). Spin-wave theory based on anisotropic ferromagnetic system has been also used to explain magnetization temperature dependence. For Fe layer thickness, approximate values for J ₀ bulk exchange interaction and J _s surface exchange interaction have been estimated. First principle calculations based on density functional theory (DFT) and Korringa–Kohn–Rostoker (KKR)—coherent potential approximation (CPA) method—combined with Local Spin Density Approximation (LSDA), are performed as well. Magnetic moment, in fcc Ag_1−x Fe _x and bcc Fe_1−x Ag _x systems, versus x is presented and discussed in terms of Fe content on magnetic coupling. Reasonable agreement between experimental data and theoretical calculations is highlighted.     

Superconductivity in ZrxNb1−2xMox alloys: Possible dominance of a soft phonon mode

Normal-state specific heat and superconducting transition temperature measurements were performed on pure polycrystalline Nb and isoelectronic bcc Zr_xNb_1−2xMo_x alloys (x = 0.02, 0.05, and 0.10). Measured T _c, γ and θ _D all decline nearly linearly from the pure Nb values with increasing x. The changes are 15, 10, and 6%, respectively, for x = 0.1. T _cH calculated by Hopfield's theory, using data and linearly interpolated values of η and θ _D /〈θ²〉_linear ^1/2, increases with x by 28% for x = 0.1. By relaxing the linear interpolation of θ_D/〈θ²〉^1/2, Hopfield's theory can be used to evaluate an average phonon frequency 〈θ²〉_emp ^1/2 from data. It is found to be nearly constant at about 229 K, in contrast to 〈θ²〉_linear ^1/2, which declines by 8% at x = 0.1. We suggest that one possible explanation for this behavior is the dominance of incipient soft-phonon longitudinal modes which occur in Nb and presumably in the alloys. Miedema's method has been used to calculate the variation of both γ and T _c with alloying. The calculated γ is nearly constant, while the calculated T _c declines more quickly than the measured variation. The band-structure electron density of states calculated with the aid of McMillan's equation for these isoelectronic alloys is found to decline by 8% at x = 0.1, in contrast to the prediction of the rigid band model. Normal-state low-temperature specific heat measurements on a pure annealed polycrystalline sample of Nb yield γ = 7.80 ± 0.02 mJ mole⁻¹ K⁻² and θ_D = 276 ± 2 K, in excellent agreement with previous measurements. The break in slope of C _p/T vs. T ² at 3.2 K is discussed. Work supported in part by a National Science Foundation Departmental Science Development Grant No. NSF-GU2603.     

The Doping Effect on the Lattices and Electronic Structure in?Superconducting Fe-based Compounds Sr1?xKxFe2As2

The lattices and density of states (DOS) for the novel superconductor Sr_1−x K _x Fe₂As₂ (x=0–1) are calculated based on the density functional theory with the scheme of the linearized augmented plane wave and the improved local orbital (APW+lo). The effects of K-doping on the lattice indicate that the K substitution induces a modification of the FeAs₄ tetrahedron into the regular one gradually. DOS calculations reveale that Fe 3d and As 4p states play a key role to the Fermi surface, the peak at x=0.5 for the total N^l(E _F) DOS at Fermi surface which would be favor of the transition of electrons, and leads to the maximal T _c in Sr_1−x K _x Fe₂As₂ system. This result was considered as a possible origin of the observed T _c behavior in the experiment of Sr_1−x K _x Fe₂As₂ system.     

Electronic Structure of BaFe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript> Revealed by Angle-Resolved Photoemission Spectroscopy

We report an angle-resolved photoemission spectroscopy study of BaFe_2−x Co _x As₂. For x=0, above the structural and magnetic transition temperature (T _s ), the spectral weight near the Fermi level is considerably suppressed around the Γ point where the Fe 3d yz/zx orbital degeneracy is expected. This observation suggests that the Jahn–Teller type instability is playing an important role in the tetragonal phase above T _s . Below T _s , the spectral weight of 0–100 meV is reconstructed to form flat bands at 70–100 meV and Fermi surfaces, consistent with the orbital-dependent excitonic coupling. In the optimally doped and overdoped regimes, the hole pocket around the Γ point and the electron pocket around the M point are apparently nested, indicating that the doping dependence of the superconducting transition temperature cannot be explained by the nesting scenario and that the unusual electron–lattice fluctuation due to the orbital degeneracy is important.     

Microstructure,phase transition and electrical properties of LiSbO<Subscript>3</Subscript>-doped (K<Subscript>0.49</Subscript>Na<Subscript>0.51</Subscript>)NbO<Subscript>3</Subscript> lead-free piezoelectric ceramics

Microstructure, phase transition and electrical properties of (1 − x)K_0.49Na_0.51NbO₃ − xLiSbO₃ (x = 0–0.08) lead-free piezoceramics prepared by the conventional solid-state sintering method were investigated with an emphasis on the effects of LiSbO₃ doping amount x. SEM results showed that the ceramic became denser by increasing LiSbO₃ doping amount x. Being indexed by XRD profiles, the ceramics changed from an orthorhombic perovskite structure to a tetragonal one across a composition region of 0.04 ≤ x≤0.05. The sample of LiSbO₃ doping amount x = 0.05 in tetragonal side of the region had the maximum values of piezoelectric constant (d ₃₃ = 256 pC/N) and planar electromechanical coupling coefficient (k _p = 42.7%). Meanwhile, this ceramic sample showed other good properties such as ε _r = 1,463, tgδ = 0.036, Q _m = 48, P _r = 19.8 μC/cm², E _c = 1.9 kV/mm and T _c = 340 °C, which indicated it was a promising lead-free piezoelectric material for ultrasonic transducer applications.     

Pressure Effect on Superconductivity and Magnetism in α-FeSe x

In this paper, the pressure effect on superconductivity and magnetism has been investigated in FeSe _x (x=0.80,0.88). The magnetization curves display anomaly at T _s1∼106 K and T _s2∼78 K except for the superconducting diamagnetic transition around T _c ∼8 K. The magnetic anomaly at T _s1 and T _s2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises T _c , but also increases both T _s1 and T _s2.