Normal state properties of lithiated YBa2Cu3O7−x samples

Measurements of the resistivity in the normal state of lithiated YBa₂Cu₃O_7−x are reported. At low x_Li-values (<0.06), lithium substitutes copper of the CuO₂ planes. The samples are metallic; a progressive localization of the charge carriers is observed. In the intermediate concentration range, lithium occupies both copper sites planes and chains which leads to a two-phased system; the slope dϱ/dT is nearly independent of temperature. In the concentrated range, lithium is in the chain site. The resistivity behaviour is well described by the simple relation with ρ₀T₀=10Ω.cm.K, a result which is consistent with a single YBCO₆-type phase.     

Specific contact resistance and metallurgical process of the silver-based paste for making ohmic contact structure on the porous silicon/p-Si surface of the silicon solar cell

Porous silicon has been considered as a promising optoelectronic material for developing a variety of optoelectronic devices and sensors. In the present study, the electrical properties and metallurgical process of the screen-printed Ag metallization formed on the porous silicon surface of the silicon solar cell have been investigated. The contact structure consists of thick-film Ag metal contact patterned on the top of the porous silicon surface. The sintering process consists of a rapid firing step at 750–825 °C in air ambient. It results in the formation of a nearly perfect contact structure between the Ag metal and porous silicon/p-Si structure that forms the top metalization for the screen-printed silicon solar cells. The SEM picture shows that Ag metal firmly coalesces with the silicon surface with a relatively smooth interfacial morphology. This implies that high temperature fire-through step has not introduced any signs of adverse effect of junction puncture or excessive Ag indiffusion, etc. The three-point probe (TPP) method was applied to estimate the specific contact resistance, ρ _c (Ω-cm²) of the contact structure. The TPP measurement shows that contact structure has excellent ohmic properties with ρ _c = 1.2 × 10⁻⁶ Ω-cm² when the metal contact sintered at 825 °C. This value of the specific contact resistance is almost three orders of magnitude lower than the corresponding value of the ρ _c = 7.35 × 10⁻³ Ω-cm² obtained for the contact structure sintered at 750 °C. This improvement in the specific contact resistance indicates that with increase in the sintering temperature, the barrier properties of the contact structure at the interface of the Ag metal and porous silicon structure improved which in turn results a lower specific contact resistance of the contact structure.     

Scaling of the Temperature Dependent Resistivity and Hall Effect in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)As<Subscript>2</Subscript>

We show that the temperature-dependent resistivity ρ(T), Hall number n _H(T) and the cotangent of the Hall angle cot θ _H(T) of Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2) can be scaled using a recently proposed model-independent scaling method (Luo et al. in Phys. Rev. B 77:014529, 2008). The zero field normal-state resistivity above T _c can be reproduced by the expresion r(T) = r₀ +cTexp(- \frac2\varDelta T )\rho(T) = \rho_{0} +cT\exp(- \frac{2\varDelta }{T} ) and scaled using the energy scale Δ, c and the residual resistivity ρ ₀ as scaling parameters. The scaling parameters have been calculated and the compositional variation of 2Δ and ρ ₀ has been determined. The 2Δ(x) dependence show almost linear decreasing in underdoped regime, minimum corresponding to the T _c maximum and increasing in overdoped regime. The latter is different from that reported for cuprates. The existence of a universal metallic ρ(T) curve which, however, is restricted for the underdoped compounds to temperatures above a structural and antiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in Ba(Fe_1−x Co _x )As₂ (x=0.0–0.2).     

SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si₃N₄) deposited over the n⁺-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si₃N₄ layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n⁺-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n⁺-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρ _c (in Ω-cm²) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρ _c  = 2.53 × 10⁻⁵ Ω-cm² is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρ _c is two orders of magnitude lower than the typical value of ρ _c  = 3 × 10⁻³ Ω-cm² reported previously for the Ag contacts of the solar cell. Such low value of ρ _c for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρ _c versus impurity doping level (N _s ) shows that measured value of the ρ _c follows a linear relationship with the N _s as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n⁺-Si surface of the silicon solar cell.

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Quench-condensed indium-hydrogen films. II. Changes in electric and superconducting characteristics near the metal-insulator transition

Electrical and superconducting properties of indium films condensed in a H₂ atmosphere (pressurep _{H 2}=6×10⁻⁶ to 1.4×10⁻⁴ Torr) onto a substrate cooled with liquid helium are investigated. As hydrogen content is increased, a continuous increase in residual resistivity ρ* is observed, permitting systematic study of the resistance vs. temperature dependenceR(T) and the superconducting transition temperatureT _c on approaching the metal-insulator transition (MIT). With regard to ρ*, four regimes of conductivity can be observed: (1) conductivity with a positive temperature resistance coefficient (TRC), (2) conductivity with a small, constant, negative TRC, (3) conductivity under weak localization with ΔR (T) ∼ln T or type corrections, (4) hopping conductivity.T _c rises continuously with ρ* and reaches its peak (∼5.2K) in the second regime. A further increase of ρ* leads to a decrease ofT _c and complete suppression of superconductivity. The experimental dependenceR(T) is compared with theory. TheT _c variation on approaching the MIT and the relation between Mooij's rule and the superconducting properties are discussed.     

Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρ_c) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρ_c of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10⁻⁵ Ω−cm ² is measured for the Ag metal contacts formed on the n⁺ silicon surface. This value is much lower than the ρ_c data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρ_c of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

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Specific contact resistance measurements of the screen-printed Ag thick film contacts in the silicon solar cells by three-point probe methodology and TLM method

The specific contact resistance of the screen-printed Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screen-printed Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ _c  = 2.15 × 10⁻⁶ Ω-cm² indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ _c  = 8.34 × 10⁻⁵ Ω-cm². These results indicate that the ρ _c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ _c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screen-printed Ag contacts are also discussed.     

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.      

Van der Waals chain-molecule fluid in self-consistent field approximation: Some thermal properties

The van der Waals equation for a monomer is used to derive the equation of state for a fluid consisting of chain molecules of equal length. The evolution of a part of the diagram of state pertaining to liquid-vapor equilibrium is treated for the case of an increase in the number of links in a molecule. The dependences on the number of links n are found for the following properties of polymer fluid: the critical temperatureT _c , the critical pressurep _c , the critical density p_c, the critical compressibilityz _c , the temperature of normal boiling, the Riedel parameter of similarity a, the acentric factor Ω, and the enthalpy of vapor formation δH_V, A comparison with the experimental data forn-alkanes and 1-alkanols reveals that the obtained dependences reflect qualitatively correctly the variation of the above-identified properties with an increase of the number of links in a molecule. For long chains(n ≪ 1), the scaling dependences are determined for the properties of a chain-molecule fluid:T _c ∽n ^−1/2,p _c ∽n ^−3/2ρ_c∽n ^−1/2,z _c∽n ⁻¹,α∽n, ω∽n ^2/3,ΔH _ν∽n.     

Turbidity Data of Weightless SF<Subscript>6</Subscript> Near its Liquid–Gas Critical Point

Light transmission measurements performed in SF₆ close to its liquid–gas critical point are used to obtain turbidity data in the reduced temperature range (T is temperature, T _c is the critical temperature). Automatic experiments (ALICE 2 facility) were made at a near critical density, i.e., , in the one-phase homogeneous region, under the microgravity environment of the Mir Space Station ( is the average density, ρ _c is the critical density). The turbidity data analysis verifies the theoretical crossover formulations for the isothermal compressibility and the correlation length ξ. These latter formulations are also used to analyze very near T _c thermal diffusivity data obtained under microgravity conditions by Wilkinson et al. (Phys. Rev. E 57 436, 1998).     

Free vibrations of a circular plate with attached concentrated mass,spring and dashpot

Summary The natural vibrations of a circular plate with attached concentrated mass, spring and dashpot have been obtained by means ofYoung's analysis [1]. The results are presented in terms of eigen-functions of the plate alone. The case of a plate carrying two masses and resting on elastic foundation has also been studied. Some particular cases have been deduced.

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Zusammenfassung Die Eigenschwingungen einer Kreisplatte mit lokal befestigter Einzelmasse, Feder und Dämpfer werden nach derYoungschen Methode [1] ermittelt. Die Ergebnisse werden als Entwicklung nach den Eigenfunktionen der reinen Plattenschwingung dargestellt. Der Fall der elastisch gebetteten Platte mit zwei Einzelmassen wird ebenfalls studiert. Einige Sonderfälle werden hergeleitet.

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Nomenclature a radius of circular plate - h plate thickness - k ₁ spring constant - k _c generalized spring constant - modulus of elastic foundation - decay constant - c dashpot strength - D , flexural rigidity of plate - E Young's modulus - v Poisson's ratio - p natural frequency of plate alone - natural frequency of composite system - w deflection mode of plate - r, cylindrical coordinates - mass density - r - (l/D)^1/4 - - - F _m ,L _m ,G _m ,M _m unknown constants With 5 Figures     

Vacancy concentration in strain ageing of substitutional f c c alloys

The relation between vacancy concentration, C _v and tensile plastic strain, ɛ, has been constantly expressed as C _v ∞ ɛ ^m . To take into account the grain-size effect, we have recently proposed that C∞ ɛβ_v ∞ϱ_m, where ϱ_m is the mobile dislocation density. With the conventional expression that ϱ_m d ⁻ⁿ , where d is the average grain size, the strain and grain-size dependence of vacancy concentration appears to be C _v ∞ ɛ^βγ d ^-nγ. This equation has proved effective in rationalizing the onset strain, ɛ_c, and stress amplitude, Δσ, of flow instability associated with the Portevin-LeChatelier effect of substitutional f c c alloys, where ɛ_c and Δσ are described by

Isochoric Heat Capacity of CO2 + n-Decane Mixtures in the Critical Region

The isochoric heat capacity of two binary (CO₂+n-decane) mixtures (0.095 and 0.178 mole fraction of n-decane) have been measured with a high- temperature, high-pressure, nearly constant volume adiabatic calorimeter. Measurements were made at nineteen near-critical liquid and vapor densities between 87 and 658 kg·m⁻³ for the composition of 0.095 mole fraction n-decane and at nine densities between 83 and 458 kg·m⁻³ for the composition of 0.178 mole fraction n-decane. The range of temperatures was 295 to 568 K. These temperature and density ranges include near- and supercritical regions. The measurements were performed in both one- and two-phase regions including the vapor + liquid coexistence curve. The uncertainty of the heat- capacity measurements is estimated to be 2% (coverage factor k=2). The uncertainty in temperature is 15 mK, and that for density measurements is 0.06%. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures (T _S), and densities (ρ_S) at saturation were measured by using the well-established method of quasi-static thermograms for each filling density. The critical temperatures (T _C), the critical densities (ρ_C), and the critical pressure (P _C) for the CO₂+n-decane mixtures were extracted from the isochoric heat-capacity measurements on the coexistence curve. The observed isochoric heat capacity along the critical isochore of the CO₂+n-decane mixture exhibits a renormalization of the critical behavior of C _{V X} typical for mixtures. The values of the characteristic parameters (K ₁, K ₂), temperatures (τ₁ ,τ₂), and the characteristic density differences were estimated for the CO₂+n-decane mixture by using the critical-curve data and the theory of critical phenomena in binary mixtures. The ranges of conditions were defined on the T-x plane for the critical isochore and the ρ-x plane for the critical isotherm, for which we observed renormalization of the critical behavior for the isochoric heat capacity.     

Microwave and Terahertz Surface Resistance of MgB2 Thin Films

The knowledge of the surface resistance R _s of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the R _s of MgB₂ films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between R _s and normal-state resistivity right above T _c, ρ₀, i.e., R _s decreases almost linearly with the decrease of ρ₀. (2) A low residual R _s, less than 50 μΩ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB₂ has the lowest R _s compared with two other superconductors Nb₃Sn and the high-temperature superconductor YBa₂Cu₃O_7−δ(YBCO). (3) From THz measurement it is found that the R _s of MgB₂ up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of R _s follows ω ⁿ , where ω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB₂ thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.     

Diffusivity and solubility of oxygen in solid palladium

The solid solubility c _O of oxygen in palladium in equilibrium with gaseous oxygen has been determined from absorption-desorption experiments for temperatures T of 1123 and 1173 K and oxygen partial pressures between 2.7 × 10³ and 4.0 × 10⁴ Pa. The relationship between c _O, and T is given by , where R = 8.314 JK⁻¹ mol⁻¹ is the universal gas constant, ΔH _s = −13.55 kJ/mol denotes the heat of solution of oxygen in palladium and the constant a amounts to or . The diffusion coefficient D _O of oxygen in solid palladium has been determined by incomplete isothermal internal oxidation of Pd–Fe alloys using the data on the oxygen solubility in palladium. The temperature dependence of D _O obeys the Arrhenius equation D _O = D ⁰ exp(−E _d/RT) with pre-exponential factor D ⁰ = 2.33 × 10⁻⁷ m²/s and activation energy of diffusion of oxygen in palladium, E _d = 102.76 kJ/mol     

Effect of the self-organized cluster structure in LaSrMnO films on the slope of the temperature dependence of resistivity

We have studied the effect of atomic ordering in a nanodimensional clustered structure of amorphous LaSrMnO films on the slope of the temperature dependence of resistivity ρ(T) in the regions where dρ/dT > 0. It is established that a high concentration of clusters of a “metallic” phase (C _met = 9%) capable of fertromagnetic ordering leads to a giant temperature coefficient of resistivity (TCR), with its value (normalized slope) reaching (dρ/dT)/ρ = 1.6 × 10⁴% K⁻¹. Samples with a small concentration of such clusters (C _met = 0.1%) possess paramagnetic properties and their normalized TCR decreases by three orders of magnitude to (dρ/dT)/ρ = 1.7 × 10¹% K⁻¹. This behavior is explained by self-consistent changes in the atomic, magnetic, and electron subsystems.     

Oxidation state and site selection of gold in (YBa2Cu3O7 −δ)(1 − x)Au x

Recent experiments on the effect of Au substitution on the superconductivity of YBa₂Cu₃O_{7 –} (YBCO) have revealed that Au, unlike most other dopants, enhances the superconducting transition temperature, T _c, by 2 K at 10 mole % Au. In the present study. X-ray photoelectron spectra, nominally of 4, 6, 8 and 10 mole % Au-doped YBCO, have been obtained. The Au spectra display a relatively intense peak which is the oxidized component associated with the Au 4f_7/2 core level, and an additional component that is most probably due to unreacted gold. The binding energy of the oxidized component is consistent with Au⁺. This observation, together with previous studies, leads us to postulate a structural model in which Au⁺ substitutes into a two-ligand c-axis co-ordinated Cu(1) site at a Ba-O (4) surface layer.     

Influence of BaZrO<Subscript>3</Subscript> Amount on Microstructure and Properties in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films Prepared by TFA-MOD Process

Epitaxial YBa₂Cu₃O_7−x /BaZrO₃ films were prepared by complex metal–organic solutions including Y, Ba and Cu anhydrous trifluoroacetate precursors and Zr acetylacetonate precursors. The influence of the amount of BaZrO₃ (BZO) on the structure and properties of YBa₂Cu₃O_7−x (YBCO) films was systematically investigated. The YBCO films having 7 mol % BZO have a maximum critical current density (J _c) value (77 K, 0 T) of 6.0 MA/cm². The enhanced J _c derives from a high density of BZO nanoparticles existing in the YBCO matrix. With increasing BZO amount, J _c of the YBCO films begins to decrease due to larger particles.     

Specific heat and enthalpy of lattice disordering of LaF3 superionic crystals

Specific heat (C _p ) data obtained near the phase transition of LaF₃ are used to estimate the activation enthalpy for anion disordering, H _d. At the critical point T _c = 263 K, the concentration of disordered fluoride ions is n _c = 2.86 × 10²⁰ g⁻¹. In the dielectric phase of LaF₃ (T < T _c), H _d is 0.24–0.26 eV. In the superionic phase (T ≥ T _c), where the concentration of disordered fluoride ions exceeds n _c, the enthalpy of disordering drops to H _d ≃ 0.04 eV.     

Electrical and Optical Properties of (n)ZnO/(p)CdTe Heterojunction and Its Performance as a Photovoltaic Converter

Thin film heterojuctions of the type (n)ZnO/(p)CdTe with different doping concentration were prepared by vacuum evaporation, and their electrical and optical properties, both in dark and under illumination at room temperature as well as elevated temperatures, were studied. Different junction parameters such as ideality factors, barrier heights, Richardson constant, short-circuit current, etc. were determined from I–V characteristics. These parameters were found to change significantly on hydrogenation and annealing of the junctions and also on variation of temperature. The structures showed the change of the photovoltaic (PV) effect, giving a fill factor of 0.57 for hydrogen (H)-treated with an open-circuit voltage of 345 mV and a short-circuit current density of 75.72× 10⁻⁴ mA·cm⁻² and 0.42 for untreated with an open-circuit voltage of 244 mV and a short-circuit current density of 52.00× 10⁻⁴ mA·cm⁻² for doping concentrations of N _a=2.43× 10¹⁶/cm³(2.53% Sb doped CdTe) and N _d=3.88× 10¹⁵/cm³(2.74% Al doped ZnO). The junctions were found to possess a series resistance as high as 437 Ω for an untreated sample and 95 Ω for H-treated samples even under illumination. Proper doping, annealing, and hydrogenation are necessary to reduce the series resistance so as to achieve an ideal solar cell.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.