Diffusion and desorption of submonolayer platinum deposited on the multi-faceted surface of a tungsten micro-crystal

Diffusion and desorption of platinum on the tungsten micro-crystal in the form of the W(1 1 1) oriented emitter tip has been studied using the field electron microscopy (FEM) technique. Diffusion of small dose of platinum (average thickness about 0.18 geometrical ML after spreading) on the thermally clean W emitter tip was studied at temperatures 648-742 K. Average activation energy for diffusion E_diff was found to lie between 1.16 ± 0.08 eVand 1.30 ± 0.16 eV. During annealing at the diffusion temperatures Pt-induced faceting of the emitter surface was visible in the neighbourhood of the {1 1 1} pole. The layer equilibrated in the diffusion process was stable at temperatures up to 1100 K where reduction of the high voltage at a fixed emission current, characteristic of alloying of Pt with W, was detected. Submonolayer of platinum (Θ_Pt = 0.18 ML) started to desorb at tip temperature ≥1780 K. The measurements of average activation energy for desorption of ‘zero coverage’ Pt (0.03 ML ≤ Θ_Pt ≤ 0.06 ML) from the entire W emitter surface were carried out at temperatures 1990-2170 K and yield the value of E_des = 5.19 ± 0.22 eV to 5.33 ± 0.19 eV. The results are compared with data for diffusion of individual Pt atoms and small clusters and with data for adsorption of Pt atoms on a planar W(1 1 0) surface. In discussion the atomic surface structure of the substrate, modified by the strong interaction of Pt with the W micro-crystal, is also taken into account.     

Adsorption of iso-butane on ZnO(0 0 0 1)-Zn

Presented are thermal desorption spectroscopy (TDS) and adsorption probability measurements of iso-butane on the Zn-terminated surface of ZnO. The initial adsorption probability, S₀, decreases linearly from 0.57 to 0.22 (±0.02) with impact energy, E_i = 0.74-1.92 eV, and is independent of adsorption temperature, T_s = 91-114 K (±5 K), indicating non-activated molecular adsorption. The coverage, Θ, dependent adsorption probabilities, S(Θ), show a cross-over from adsorbate-assisted adsorption (S increases with Θ) to Kisliuk-like dynamics at about the desorption temperature of iso-butane bi-layers (∼110 K). Thus, the adsorption dynamics are precursor-mediated. The enhanced (gas-surface) mass-match, caused by forming a second layer of the alkane, leads to adsorbate-assisted adsorption. A direct fitting procedure of the TDS data yields a pre-exponential factor of 2.5 × 10¹³/s and a coverage dependent heat of adsorption of E_d(Θ) = 39 − 6 ∗ Θ + 2.5 ∗ exp(−Θ/0.07) kJ/mol.     

Microstructure and piezoelectric properties of Bi0.5(Na0.82K0.18)0.5TiO3−NaSbO3 ceramics

Lead-free piezoelectric ceramics (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ have been prepared by a conventional ceramics technique, and their microstructure and electrical properties have been investigated. The addition of NaSbO₃ has no remarkable effect on the crystal structure within the studied doping content; however, an obvious change in microstructure took place. With increase in NaSbO₃ content, the temperature from a ferroelectric to antiferroelectric phase transition increases, and the temperature for a transition from antiferroelectric phases to paraelectric phases changes insignificantly. Simultaneously, the temperature range between the rhombohedral phase transition point and the Curie temperature point becomes smaller. The piezoelectric properties significantly increase with increase in NaSbO₃ content and the piezoelectric constant and electromechanical coupling factor attain maximum values of d₃₃=160 pC/N and k_p=0.333 at x=0.01. The results indicate that (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ ceramic is a promising lead-free piezoelectric candidate material.     

Preparation of activated carbon from a renewable bio-plant of Euphorbia rigida by H2SO4 activation and its adsorption behavior in aqueous solutions

The use of activated carbon obtained from Euphorbia rigida for the removal of a basic textile dye, which is methylene blue, from aqueous solutions at various contact times, pHs and temperatures was investigated. The plant material was chemically modified with H₂SO₄. The surface area of chemically modified activated carbon was 741.2 m² g⁻¹. The surface characterization of both plant- and activated carbon was undertaken using FTIR spectroscopic technique. The adsorption process attains equilibrium within 60 min. The experimental data indicated that the adsorption isotherms are well described by the Langmuir equilibrium isotherm equation and the calculated adsorption capacity of activated carbon was 114.45 mg g⁻¹ at 40° C. The adsorption kinetics of methylene blue obeys the pseudo-second-order kinetic model and also followed by the intraparticle diffusion model up to 60 min. The thermodynamic parameters such as ΔG°, ΔH° and ΔS° were calculated to estimate the nature of adsorption. The activation energy of the system was calculated as 55.51 kJ mol⁻¹. According to these results, prepared activated carbon could be used as a low-cost adsorbent to compare with the commercial activated carbon for the removal textile dyes from textile wastewater processes.     

Contribution of polaron hopping to the electron paramagnetic resonance linewidth in La1−xCaxMnO3 and related materials

We analyze the contribution of polaron hopping to the electron paramagnetic resonance linewidth in La_1−xCa_xMnO₃ and related materials. The material is assumed to be in the paramagnetic phase and the conductivity is associated with the activated polaron hopping. It is also assumed that the adiabatic, small polaron picture is appropriate so that the conductivity varies as exp[−E_a/T]/T, where E_a denotes the polaron activation energy. The polaron contribution to the linewidth is given by the expression C[χ₀(T)/χ(T)]exp[−E_a/T] where χ₀(T) is the Curie susceptibility (∼1/T), χ(T) is the measured susceptibility and C is a material-dependent parameter. Various experimental studies reporting polaron contributions to the linewidth are discussed. It is pointed out that fitting the linewidth to the functional form ΔH₀+(A/T)exp[−E_a/T] is not physically justified. In the high temperature–mean field regime, the exchange narrowed width, (1−Θ/T)k(∞), where Θ is the paramagnetic Curie temperature, replicates the exponential functional form with reasonable values for the activation energy. From previous measurements of the conductivity that showed activated polaron hopping as the leading transport mechanism, we concluded that the linewidth in La_0.7Ca_0.3MnO₃ is a sum of exchange narrowing and one-phonon spin–lattice terms with no evidence of a contribution from polaron hopping or band transport as had been previously proposed. A similar conclusion is reached for La_0.8Ca_0.2MnO₃, nanometer-sized La_0.9Ca_0.1MnO₃, and La_0.9Te_0.1MnO₃.     

Electrical and electromechanical properties of lead-potassium-yttrium-niobate ceramics—piezoelectric applications

Tungsten bronze (TB)-type oxide ceramic Pb_0.74K_0.13Y_0.13Nb₂O₆ (PKYN) has been synthesized by the standard solid state reaction method. Single phase formation, orthorhombic crystal structure was confirmed by X-ray diffraction (XRD). The substitution of Y³⁺ in Pb_0.74K_0.52Nb₂O₆ (PKN) decreased the unit cell volume and T_C=260 °C. PKYN exhibited the remnant polarization, P_r=8.5 μC/cm², and coercive field, E_c=28.71 kV/cm. Electrical spectroscopy studies were carried out over the temperature (35-595 °C) and frequency (45 Hz-5 MHz) ranges, and the charge carrier phenomenon, grain-grain boundary contribution and non-Debye-type relaxation were analyzed. The relaxation species are immobile charges in low temperature and oxygen vacancies at higher temperature. The theoretical values computed using the relations, ε′=ε_∞+sin(n(T)π/2)(a(T)/ε₀)(ω^n(T)−1); σ(ω)=σ_dc+Aωⁿ are fitted with the experimental one. The n and A parameters suggested that the charge carrier's couple with the soft mode and become mobile at T_C. The activation enthalpy, H_m=0.38 eV, has been estimated from the hopping frequency relation ω_p=ω_e exp(−H_m/k_BT). The piezoelectric constants K_t=35.4%, d₃₃=69×10⁻¹² C/N, d₃₁=−32×10⁻³ mV/N, S₁₁^E=17.8 pm²/N, etc., achieved in PKYN indicate the material is interesting for transducer applications. The activation energies from different formalisms confirmed the ionic-type conduction.     

A kinetic study of the thermal decomposition process of potassium metabisulfite: Estimation of distributed reactivity model

The thermal decomposition kinetics of potassium metabisulfite was studied by thermogravimetric (TG) and differential thermogravimetric (DTG) techniques using non-isothermal experiments. The apparent activation energy (E_a) is determined using the differential (Friedman) isoconversional method. The results of the Friedman's isoconversional analysis of the TG data suggests that the investigated decomposition process follows a single-step reaction and the observed apparent activation energy was determined as 122.4±2.1 kJ mol⁻¹. A kinetic rate equation was derived for the decomposition process of potassium metabisulfite with contracting area model, f(α)=2(1−α)^1/2, which is established using the Malek's kinetic procedure. The value of pre-exponential factor (A) is also evaluated and was found to be A=1.37×10¹² min⁻¹. By applying the Miura's procedure the distributed reactivity model (DRM) for investigated decomposition process was established. From the dependence α versus E_a, the experimental distribution curve of apparent activation energies, f(E_a), was estimated. By applying the non-linear least-squares analysis, it was found that the Gaussian distribution model (with distribution parameters E₀=121.3 kJ mol⁻¹ and σ=1.5 kJ mol⁻¹) represents the best reactivity model for describing the investigated process. Using the Miura's method, the A values were estimated at five different heating rates and the average A values are plotted against E_a. The linear relationship between the A and E_a values was established (compensation effect). Also, it was concluded that the E_a values calculated by the Friedman's method and estimated distribution curve, f(E_a), are correct even in the case when the investigated decomposition process occurs through the single-step reaction mechanism.     

Exact solutions for a forced Burgers equation with a linear external force

We investigate the solutions of the Burgers equation , where F(x,t) is an external force and Φ(x,t) represents a forcing term. This equation is first analyzed in the absence of the forcing term by taking F(x,t)=k₁(t)−k₂(t)x into account. For this case, the solution obtained extends the usual one present in the Ornstein-Uhlenbeck process and depending on the choice of k₁(t) and k₂(t) it can present a stationary state or an anomalous spreading. Afterwards, the forcing terms Φ(x,t)=Φ₁(t)+Φ₂(t)x and Φ(x,t)=Φ₃x−Φ₄/x³ are incorporated in the previous analysis and exact solutions are obtained for both cases.     

Optical investigation of charge-transfer transitions in spinel Co3O4

Optical transitions in normal-spinel Co₃O₄ have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co₃O₄ by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution (Zn_xCo_3−xO₄) can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d-d charge-transfer transition between the t_2g states of octahedral Co³⁺ ion and t₂ states of tetrahedral Co²⁺ ion, t_2g(Co³⁺)→t₂(Co²⁺). The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O²⁻ ion: p(O²⁻)→t₂(Co²⁺) for the 2.4 eV absorption and p(O²⁻)→e_g(Co³⁺) for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for Zn_xCo_3−xO₄ can be explained in terms of the substitution of the tetrahedral Co²⁺ sites by Zn²⁺ ions. The crystal-field splitting Δ_Oh between the e_g and the t_2g states of the octahedral Co³⁺ ion is estimated to be 2 eV.     

Ruthenium adsorption and diffusion on the GaN(0 0 0 1) surface

We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2×2geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T₁, T₄ and H₃ special sites. We find that the most energetically favorable structure corresponds to the Ru- T₄ model or the ruthenium adatom located at the T₄ site, while the ruthenium adsorption on top of a gallium atom (T₁ position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2×2 surface presents a lateral relaxation of some hundredth of Å in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail.     

Chemical diffusion in molybdenum disulphide

Ceramic molybdenum disulphide (MoS₂) was equilibrated at an ambient sulphur vapour partial pressure p(S₂), 10 Pa<p(S₂)<1000 Pa. After the step change of p(S₂) to a new value, the equilibration kinetics was monitored by measuring electrical conductivity. The application of the solution of Fick's second law (with the initial condition: no concentration gradient in specimen and the boundary condition: surface concentration constant) to the kinetic data gave the chemical diffusion coefficient. The chemical diffusion coefficient, D_chem, determined at 1273 K, was D_chem=(3.20±0.32)*10⁻⁷ cm² s⁻¹ and was found to be independent of sulphur vapour partial pressure. The usefulness of transient electrical conductivity method for determining real values of diffusion data was discussed in terms of defect structure of the studied material.     

Type I FA (Rb, Cs) and II FA (Li, Na) tunable laser activities and donor-acceptor properties of O and O at KCl (001) surface: first principle calculations

Type I F_A (Rb⁺, Cs⁺) and II F_A (Li⁺, Na⁺) tunable laser activities, adsorptivity and donor-acceptor properties of O and O⁻ adsorbates at the flat surface of KCl crystal were investigated using an embedded cluster model and ab initio methods of molecular electronic structure calculations. Ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field of the host surface, and the nearest neighbor ions to the defect site were allowed to relax to equilibrium. Based on the calculated Stokes shifted optical transition bands, F_A tunable laser activities were found to be inversely proportional to the size of the dopant cation (Li⁺, Na⁺, Rb⁺, Cs⁺) relative to the host cation (K⁺). This relation was explained in terms of the axial perturbation of the impurity cation. The probability of orientational bleaching attributed to the RES saddle point ion configuration along the 〈110〉 axis was found to be inversely proportional to the size of the dopant cation, with activation energy barriers of ca. 0.44-3.34 eV. Surface relaxation energies of type II F_A centers were more important than those of type I F_A centers. In terms of defect formation energies, the products of type II F_A center imperfection were more stable than those of type I F_A. The difference between F or F_A band energies and exciton bands depended almost exclusively on the size of the positive ion species. As far as the adsorptivity of O and O⁻ is concerned, the results confirm that surface imperfection enhances the adsorption energies by ca. 4.38-16.37 eV. O and O⁻ penetrate through the defect-containing surface. The energy gap between the adsorbate and the defect containing surface and the donor-acceptor property of adsorbate play the dominant role in the course of adsorbate substrate interactions and the results were explained in terms of electrostatic potential curves and Mulliken population analysis.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

Domain and hull-enclosed area distributions in two-dimensional coarsening

We obtain the exact distribution of the areas enclosed by domain boundaries (“hulls”) during the coarsening dynamics of a two-dimensional nonconserved scalar field. We prove that the number of hulls per unit area that enclose an area greater than A has the form N_h(A,t)=2c/(A+λt), where . This result is also a demonstration of the validity of the dynamical scaling hypothesis in this system. We also show that domain areas (regions of aligned spins) have a similar distribution.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-SrTiO3 ceramics

In this article, we report successful preparation of dense [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ (x=0.005-0.100) ceramics by ordinary sintering in air. The dependence of phase structure on doping content of SrO and TiO₂ has been determined by the X-ray diffraction technique. It was found that the crystal structure changed from orthorhombic to tetragonal at x≈0.040. Dielectric study revealed that the dielectric relaxor behavior was induced by doping of SrO and TiO₂ into (Na_0.5K_0.5)NbO₃. The samples in the composition range from x=0.005 to 0.020 exhibited excellent electrical properties, piezoelectric constant of electromechanical planar and thickness coupling coefficients of k_p=26.6-32.5% and k_t=39.8-43.8%. The results show that the [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics are one of the promising lead-free materials for electromechanical transducer applications.     

Low-temperature property investigation of the lead indium-niobate-lead nickel-niobate solid solution

The complex perovskite solid solution (1−x) Pb(In_1/2Nb_1/2)O₃-(x) Pb(Ni_1/3Nb_2/3)O₃ has been successfully prepared by the Columbite precursor method. The temperature dependencies of the dielectric constant and pyroelectric coefficient were measured between −261 and 200 °C. Relaxor ferroelectric behavior has been noticed in all compositions across the solid solution. The room-temperature electrostrictive coefficient, Q₃₃, was 1.83×10⁻² C²/m⁴ for x=0.10. No room-temperature piezoelectric activity was detected; however, upon cooling to −261 °C the maximum coupling coefficients k_p=29%, k_t=11%, and k₃₃=31% were observed for the composition x=1.00.     

Exact solutions for a diffusion equation with a nonlinear external force

This work is devoted to investigate the solutions of the one-dimensional diffusion equation by taking the nonlinear external force F(x,t;ρ)=−k(t)x+K/x+κx|x|^α−1η[ρ(x,t)] into account. Our investigation is first performed by considering the case α=0 and η=1, which results in a Burgers like equation with a spatial and time dependent external force. After, we consider the case α≠0 and η=α+1 and show that the solution found may be expressed in terms of the q-exponential functions present in the Tsallis formalism. In addition, we also discuss the stationary solution for α and η arbitraries.     

Structural, electronic and optical calculations of CaxZn1−xO alloys: A first principles study

First principles density functional calculations, using full potential linearized augmented plane wave (FP-LAPW) method, have been performed in order to investigate the structural, electronic and optical properties of Ca_xZn_1−xO alloy in B1 (NaCl) phase. Dependence of structural parameters as well as the band gap values on the composition x have been analyzed in the range 0?x?1. Calculated electronic structure and the density of states of these alloys are discussed in terms of the contribution of Zn d, O p and Ca p and d states. Furthermore, optical properties such as complex dielectric constants ε(ω), refractive index including extinction coefficient k(ω), normal-incidence reflectivity R(ω), absorption coefficient α(ω) and optical conductivity σ(ω) are calculated and discussed in the incident photon energy range 0-45 eV.     

Adsorption dynamics of CO2 on Cu(1 1 0): A molecular beam study

Molecular beam scattering measurements have been conducted to examine the adsorption dynamics of CO₂ on Cu(1 1 0). The initial adsorption probability, S₀, decreases exponentially from 0.43 ± 0.03 to a value close to the detection limit (∼0.03) within the impact energy range of E_i = (0.12-1.30) eV. S₀ is independent of the adsorption temperature, T_s, and the impact angle, α_i, i.e., the adsorption is non-activated and total energy scaling is obeyed. The coverage, Θ, dependent adsorption probability, S(Θ), agrees with precursor-assisted adsorption dynamics (Kisliuk type) above T_s ∼ 91 K. However, below that temperature adsorbate-assisted adsorption (S increases with Θ) has been observed. That effect is most distinct at large E_i and low T_s. The S(Θ) data have been modeled by Monte Carlo simulations. No indications of CO₂ dissociation were obtained from Auger Electron Spectroscopy or the molecular beam scattering data.