Diffusion coefficients of l-menthone and l-carvone in mixtures of carbon dioxide and ethanol

The molecular diffusion coefficients of l-menthone and l-carvone in supercritical carbon dioxide (SCCO₂) and carbon dioxide containing 5 and 10 mol% ethanol as a modifier were measured by the Taylor-Aris chromatographic peak broadening (CPB) method over the ranges of temperature from 308.15 to 338.15 K and pressure from 15 to 30 MPa. It was found that the correlation relationships between diffusion coefficients and the temperature, pressure, viscosity, and density, such as the linear correlation between the D₁₂ and ρ, and between the D₁₂ and T/η, which were valid in binary systems, were also suitable for ternary systems of carbon dioxide containing modifier. The diffusion coefficients in modified SCCO₂ decreased with increasing the ethanol mole fraction due to the chemical association between the two solutes and ethanol. Of several models used to predict experimental data in pure carbon dioxide, the two models of Funazukuri-Ishiwata-Wakao and He-Yu-1998 were the best with the AAD less than 3.2%. Furthermore, the models of modified Wilke-Chang, Scheibel, Reddy-Doraiswamy, Lusis-Ratcliff, Hayduck-Minhas, Tyn-Calus, and Lai-Tan overestimated the diffusion coefficient in ethanol modified SCCO₂ with the AAD values increaseing with the percentage of ethanol, which were probably due to the increase of the volume of solvaton sphere as a true diffusion unit with the percentage of ethanol. Moreover, the free volume model of Dymond is good for predicting the experimental data in pure carbon dioxide and ethanol modified SCCO₂ with the AAD values range from 3.21 to 1.90%.     

Crossover Equation of State for Selected Hydrocarbons (C4–C7)

The organic Rankine cycle (ORC) has attracted attention for waste heat recovery and renewable energy systems. An accurate prediction for thermodynamic properties of working fluids is of great importance for cycle performance evaluations and system design. Particularly, hydrocarbons are promising for their good performance and low global warming potentials. Moreover, the thermal efficiency of the ORC is higher when the evaporation temperature is closer to the critical temperature, which makes the properties in the critical region rather important. Recent research has shown that using mixture as working fluid can achieve better temperature matches. Therefore, an equation of state (EoS) that can be extended to mixture calculations is more attractive. Specific EoS for selected hydrocarbons is precise, but very complex. Cubic EoSs, such as widely used Peng–Robinson EoS and Soave–Redlich–Kwong (SRK) EoS, fail to accurately predict liquid densities over wide pressure ranges or pressure–density–temperature (pρT) properties in the near-critical region. This work combines the volume translation approach and the crossover method to provide better prediction for thermodynamic properties in the critical region and in regions far from the critical point. A crossover volume translation SRK EoS is developed and used for n-butane, i-butane, n-pentane, i-pentane, n-hexane, i-hexane and n-heptane. The volume translation term is set as a constant to ensure the accuracy of the saturated liquid density at low reduced temperatures. Then, the crossover method is introduced into the volume translation EoS to improve the predictions of thermodynamic properties in the critical region. Six crossover parameters are used, which are constants or functions of acentric factor and critical parameters. Therefore, none of the parameters in the crossover volume translation SRK EoS is adjustable, which makes the crossover EoS totally predictive and easily extend to mixtures. Comparisons show that the crossover EoS is in much better agreement with experimental data than the original SRK EoS.     

An analytical equation of state for water and alkanols

An analytical equation of state extended from statistical associating fluid theory (SAFT) is modified to describe the thermodynamic properties of fluids with high polarity such as water and alkanols up to the region close to the critical point. Five terms are used in the equation of state: the term for hard convex body, the term for dispersion energy, the term for the chain formation of hard convex body, the term for the change of the dispersion energy because of chain formation, and the term for dipole-dipole interaction. This equation of state is still called SAFT-CP (across critical points). Six fluids water, methanol, ethanol, 1-propanol, 1-butanol and 1-pentanol are used as examples. The new equation of state reproduces saturated pressures and densities in vapor-liquid equilibrium, critical properties (as temperature, pressure and density), and densities in the one-phase region with rational accuracies. The comparison of the calculated critical exponent β with the experimental one for methanol shows the improvement up to the region only with 20 K difference of temperature to the critical point. The result coincides with the estimation of the critical region on the basis of Ginzburg number and also with the available opinion that crossover methods and renormalization theories are necessary in the near-critical region.     

Prediction of solute solubility in supercritical carbon dioxide: A novel semi-empirical model

Solubility data of solutes in supercritical fluids (SCF) are crucial for designing extraction processes, such as extraction using SCF or micronization of drug powders. A new empirical equation is proposed to correlate solute solubility in supercritical carbon dioxide (SC CO₂) with temperature, pressure and density of pure SC CO₂. The proposed equation is ln y₂ = J₀ + J₁P² + J₂T² + J₃ ln ρ where y₂ is the mole fraction solubility of the solute in the supercritical phase, J₀ − J₃ are the model constants calculated by least squares method, P (bar) is the applied pressure, T is temperature (K) and ρ is the density of pure SC CO₂. The accuracy of the proposed model and three other empirical equations employing P, T and ρ variables was evaluated using 16 published solubility data sets by calculating the average of absolute relative deviation (AARD). The mean AARD for the proposed model is 7.46 (±4.54) %, which is an acceptable error when compared with the experimental uncertainty. The AARD values for other equations were 11.70 (±23.10), 6.895 (± 3.81) and 6.39 (±6.41). The mean AARD of the new equation is significantly lower than that obtained from Chrastil et al. model and has the same accuracy as compared with Bartle et al. and Mèndez-Santiago–Teja model. The proposed model presents more accurate correlation for solubility data in SC CO₂. It can be employed to speed up the process of SCF applications in industry.     

One‐ and two‐phase isochoric heat capacity measurements of aqueous solution of nacl near the critical point of pure water

Isochoric heat capacities of H₂O+NaCl solutions (0.0031 and 0.0063 mole fraction of NaCl) were measured with a high temperature and high pressure adiabatic calorimeter near the sub‐critical and near the supercritical point of pure water. Temperatures ranged from 361 to 677 K. Measurements were made at six densities, namely: 367.07 and 485.59 kg·m^?3 for x = 0.0031 mol fraction of NaCl and 388.65, 560.03, 607.05, and 973.70 kg·m^?3 for x = 0.0063 mol fraction of NaCl. Measurements were conducted in the two‐and one‐phase regions including near phase transition temperatures T_S(ρ). The phase transition temperatures T_S(ρ) and saturated isochoric heat capacity values C_VX have been determined for each isochore. The uncertainty in heat capacity measurements is estimated to be 3.5% to 4.5% near the phase transition and critical points. Present and previous results of heat capacity measurements were compared with predictions from the crossover (CREOS) and Pitzer—Tanger–Hovey equations of state (PTH EOS). Our previous heat capacity measurements were found to deviate systematically from crossover‐model predictions. The present results show good agreement with the crossover model for the composition x = 0.0031 mol fraction.     

Non-uniform polarizations in SOFC oxygen electrodes

The polarization resistance (R_p) of Ag/(ZrO₂)_0.9(Ln₂O₃)_0.1 (Ln = lanthanides) oxygen electrodes was measured as a function of the resistivity (ρ) of the electrolyte, which was systematically changed by using different Ln dopants. It was found that R_p increases with ρ in contrast with the expectation from the usual electrode theory assuming the adsorption and diffusion of O₂ molecules on the electrode surface. We attempt to explain the ρ-dependence of R_p based on a non-uniform electrode model which assumes locally variable polarization resistances in combination with the interfacial electrolyte resistance. According to this model, as ρ is higher, the current distribution becomes spatially more uniform, therefore making the polarizations more non-uniform. The more non-uniform is the polarization, the larger is R_p, because it is determined by the largest local polarization. The model can thus explain why R_p increases with ρ, without assuming any dependence of the local polarization resistance (r_p) on ρ. If this mechanism dominates R_p, a negative slope of t₀ (time constant of decay curve) versus R_p plots is expected while a positive slope from the conventional electrode theory. In fact, R_p decreases with t₀ in a high ρ region, thus proving that R_p is controlled by the non-uniform polarization mechanism proposed. R_p versus ρ experiments were also carried out for Au/(ZrO₂)_0.9(Ln₂O₃)_0.1 electrodes. It was found that R_p decreases with ρ in contradiction to the Ag electrodes. This ρ-dependence of R_p was explained in terms of the unusual decrease of the interfacial electrolyte resistance, which might be due to a highly concentrated current density near the 3 phase boundary.     

Distribution profile of water and suppression of methanol crossover in sulfonated polyimide electrolyte membrane for direct methanol fuel cells

We have developed novel cross-linked sulfonated polyimide (c-SPI) membrane as an electrolyte for direct methanol fuel cells (DMFCs). When the DMFC using the c-SPI membrane (thickness = 155 μm), Pt-Ru dispersed on carbon black (Pt-Ru/CB) anode and Pt/CB cathode with a Nafion^® ionomer was operated at 80 °C and 0.1 A cm⁻² with 1 M CH₃OH and oxygen (oxidant), the methanol crossover rate, j(CH₃OH), was suppressed to about 1/2 compared with that of the Nafion^® 117 membrane (thickness = 180 μm) with the same electrodes. It was found for both cells that the j(CH₃OH) was not so small as expected from the membrane thickness. In order to obtain a clue for the suppression of j(CH₃OH), the distribution profiles of water (containing CH₃OH) in thickness direction were investigated by measuring the specific resistances (ρ) between Pt probes inserted into the electrolyte membrane. Values of ρ at the anode side were low irrespective of the discharge current density, because such a part of the membrane was humidified thoroughly by liquid water (1 M CH₃OH) allowing free penetration of CH₃OH into the swollen polymer. In contrast, the values of ρ at the cathode side were high at the low current density due to drying of the membrane contacting with oxidant gas (O₂ or air) in low humidity. We have succeeded to suppress the j(CH₃OH) further (about 1/2 at 0.2 A cm⁻²) by using bilayer c-SPI, having a low ion exchanging (low swelling) barrier layer at the anode side without increasing the ohmic resistance, compared with that of the single c-SPI.     

Interfacial tensions of ethanol-carbon dioxide and ethanol-nitrogen. Dependence of the interfacial tension on the fluid density—prerequisites and physical reasoning

Interfacial tensions in the systems ethanol-carbon dioxide and ethanol-nitrogen are measured. In both systems the interfacial tension decreases with increasing pressure. The interfacial tension in the system ethanol-nitrogen decreases with increasing temperature. In contrast to this, in the system ethanol-carbon dioxide at high pressures an isobaric increase in temperature provokes an increase in interfacial tension. In the system ethanol-carbon dioxide at elevated pressure the density of the carbon dioxide phase is the only influence parameter concerning the interfacial tension. This phenomena can be observed for various other systems with one near critical or supercritical component and a high solubility of this component in the liquid phase and at temperatures above the critical temperature of the gas. For this fact a physical reasoning, which is based on a new concept of partial interfacial tensions, is given. Furthermore, it is stated that the interfacial tension of fatty systems in contact with carbon dioxide has approximately the same dependence on the reduced density (ρ_r=ρ/ρ_c) of the supercritical phase as the interfacial tension of the same systems in contact with ethane. The knowledge of this fact can help to avoid experiments with ethane, which is flammable.     

The novel positive colossal electroresistance in PbPdO2 thin film with (002) preferred orientation

Doped PbPdO₂ materials have attracted much attention as a spin gapless semiconductor (SGS) with the important properties of colossal electroresistance (CER) and giant magnetoresistance (GMR). In this study, the PbPdO₂ thin films with (002) preferred orientation were prepared by pulsed laser deposition (PLD), and the temperature dependences of resistance and resistivity, R (T) and ρ_I (T), were measured under different applied DC currents. Remarkably, a positive CER effect induced by the current was firstly observed in the PbPdO₂ films. In particular, it is novelty found that the positive CER value of PbPdO₂ with I = 10 μA and T = 10 K reached about 300%. Moreover, the cyclic ρ_I (T) curves were also measured for I = 0.01 and 10 μA going back and forth between 10 K and 400 K. The time dependences of ρ_t/ρ₀ ratio with T = 100 K, 300 K, 400 K and I = 0.01 and 0.1 μA were also obtained. A critical temperature T_c with about T = 260 K for all applied currents was found. As T > T_c，the band gap of the film is enhanced by the combined effect of the temperature and current. At the same time, Pb and O vacancies, and the evolution of oxygen valence states in the PbPdO₂ film were observed by energy dispersive spectrometer (EDS), electron paramagnetic resonance (EPR) and in-situ x-ray photoelectron spectroscopy (XPS). Especially, the charge transport between O¹⁻ and O²⁻ was confirmed by in-situ XPS. Finally, based on first-principles calculation, an internal electric field model and its induced potential barrier were established, which well explains the positive CER effect and the critical temperature T_c.     

The effects of reaction conditions on the electrical conductivity of PAAG hydrogels

The effects of reaction conditions: concentration of crosslinker, monomer, initiator and gelatin and neutralization degree (ND) of acrylic acid (AA), of a graft radical crosslinking polymerization of AA and gelatin, on the specific electrical conductivity and the primary structural parameters of synthesized poly(acrylic acid)-g-gelatin (PAAG) hydrogels were investigated. It was established that: (a) electrical conductivity of all of the investigated hydrogels is higher than of distilled water; (b) the increasing concentration of crosslinker, monomer and initiator leads to the two linear distinct increases of electrical conductivity with different slopes (c) the increasing ND of AA leads to linear decrease in electrical conductivity and (d) gelatin concentration does not significantly influence the electrical conductivity of hydrogel. An analyses of the primary structural parameters of synthesized PAAG xerogels reveals that: (a) values of molar mass between crosslinks and distance between polymer chains are power form function on crosslink density (ρ_c); (b) the critical value of ρ_c corresponds to the percolation threshold of H⁺/K⁺ ions through the hydrogels network and (c) electrical conductivity of PAAG hydrogels is a power form function on ρ_c. Fractal model of conductivity of hydrogel is suggested and explained.     

Microstructural and electrical characterization of bulk YBa2Cu3O7−δ ceramics

The value of critical current density at 77 K in “zero” applied field (J_c) characterizing the superconducting state for YBa₂Cu₃O_7−δ ceramics is closely related to the microstructure.The interrelationships between the microstructural factors such as pore volume fraction, oxygen content, average grain size, are complex. However, these factors also influence the normal state resistivity measured at room temperature (ρ₃₀₀). We demonstrate how the current carrying cross section influences Jc and ρ₃₀₀ in a similar way. Data, reported for two classes of YBa₂Cu₃O_7−δ: small grain porous ceramics and larger-grain denser ceramics, reveal an approximate linear relation between ρ_300 K and J_c. Extrapolation of this relation to a fully dense small grain YBa₂Cu₃O_7−δ ceramic yields values of ρ₃₀₀ = 0.4 mΩ cm and J_c = 10³ A cm⁻².     

Geometry and microstructure parameters to predict fracture at notches in a polycrystalline material

The critical stress intensity factor of notched specimen K_C^V,ρ is influenced by notch geometry (root radius and opening angle) and material microstructure. In this paper, by analyzing 12 groups of measurements on specimens made from materials with average grain size G from 22 to 500 μm and with different notches, a unified model is proposed to describe notch geometry effect on K_C^V,ρ related to G. Additionally, normal distribution theory is incorporated to consider inevitable scatter of measurements. The results show that the model can probabilistically predict K_C^V,ρ of notched specimens made from polycrystalline materials with 96% reliability.     

A semi-empirical equation of state for fluids

An eight parameter equation of state for calculating the volumetric behaviour of fluids has been derived on the basis of three main assumptions:—The thermodynamic equation of state P +(?E|?V)_T = T(?S|?V)_T as a general theoretical frame.—The Maxwell relation (?P|?T)_V = (?S|?V)_T as a thermodynamic link between the empirical parameters of the equation.—A Van der Waals-type potential for attractive forces.The new equation has been applied to ten fluids, five polar and five apolar, in an experimental range up to three times the critical density and up to five times the critical temperature.The new equation compares favorably in accuracy with the BWR equation.     

Compressibility and sinterability of CeO2–8YSZ powders synthesized by a wet chemical method

The compressibility and sinterability of CeO₂–8YSZ powders prepared by co-precipitation were investigated in detail. It was shown that the compressibility curves are characterized by three linear parts at low, middle and high pressures. The middle and high regions of the applied pressure, as least investigated, were studied in detail. The specific values of the compaction pressure (P_Y2) and density (ρ_Y2) at the intersection point of the compressibility curves were determined for all investigated powders. It was shown that the compressibility curves for all investigated powders can be described by two straight lines by using special coordinates ρ_G/ρ_Y2 and log(P/P_Y2).The sinterability curves of powders after drying and after calcination at 350 °C have a pronounced maximum. The optimum compaction pressures (P*) corresponding to the pressure at the maximal value of sintered density were determined for all investigated powders. It was shown that the region of optimal pressures is in the upper part of the middle-pressure region, whereas the P*/P_Y2 ratio varies between 0.7 and 0.9. The influence of the powder fractionation on the sinterability of the powders was also studied in detail as a function of the compaction pressure and calcination temperature.     

Thermal expansion behavior of composites based on non-axisymmetric ellipsoidal particles

A new model for calculating the coefficients of thermal expansion, CTE, in all three coordinate directions is developed for composites containing non-axisymmetric, ellipsoidal particles, i.e., three-dimensional ellipsoids (a₁ ≠ a₂ ≠ a₃) characterized by two aspect ratios, ρα=a₁/a₃ and ρβ=a₁/a₂. The model makes use of Eshelby's equivalent tensor for the particles and utilizes the methodology from recent papers by Lee et al. The effects of the primary, ρ_α, and secondary, ρ_β, aspect ratios and filler volume fraction (0-6%) on the various CTE for nanocomposites containing aligned inclusions are demonstrated by calculations for a matrix of nylon 6 and a filler of montmorillonite single platelets. The CTE in the longitudinal direction, α11, decreases, as both aspect ratios increase. The CTE in the transverse direction, α22, decreases as ρα increases but increases as ρβ increases. The CTE in the normal direction, α33, is greater than that of the matrix and increases as ρα increases but decreases as ρβ increases.     

Strictly two-dimensional self-avoiding walks: Density crossover scaling

The density crossover scaling of thermodynamic and conformational properties of solutions and melts of self-avoiding and highly flexible polymer chains without chain intersections confined to strictly two dimensions (d = 2) is investigated by means of molecular dynamics and Monte Carlo simulations of a standard coarse grained bead-spring model. We focus on properties related to the contact exponent set by the intrachain subchain size distribution. With R ～ N ^ν being the size of chains of length N and ρ the monomer density, the interaction energy e _int between monomers from different chains and the corresponding number n _int of interchain contacts per monomer are found to scale as with ν = 3/4 and θ₂ = 19/12 for dilute solutions and ν = 1/d and θ₂ = 3/4 for N? g(ρ) ≈ 1/ρ². Irrespective of ρ, long chains thus become compact packings of blobs of contour length with d _p = d ? θ₂ = 5/4 being the fractal line dimension. Due to the generalized Porod scattering of the compact chains, the Kratky representation of the intramolecular form factor F(q) reveals a non-monotonous behavior approaching with increasing chain length and density a power-law slope $F(q)q^d /\rho \approx 1/(qR)^{\theta _2 } $ in the intermediate regime of the wavevector q. The specific intermolecular contact probability is argued to imply an enhanced compatibility for polymer blends confined to ultrathin films. We comment briefly on finite persistence length effects.     

Novel water-assisting low firing MoO3 microwave dielectric ceramics

MoO₃ ceramics can not be well densified via conventional solid state method and a low relative density (ρ) was obtained (˜64.5% at 680 °C) with a permittivity (ε_r) ˜ 7.58, a quality factor (Qf) ˜ 35,000 GHz and a temperature coefficient of resonant frequency (TCF) ˜ − 39 ppm/°C. However, cold sintering at 150 °C using 4 wt. % H₂O at 150 MPa enhanced densification and give a relative ρ ˜76.8% and ε_r ˜ 8.31 but with a Qf of only ˜ 900 GHz. The addition of (NH₄)₆Mo₇O₂₄·4H₂O further improved densification to give a relative ρ ˜ 83.7% after annealing at 700 °C, resulting in a ε_r ˜ 9.91 with a Qf ˜ 11,800 GHz. We conclude therefore that oxides that are difficult to be sintered via a conventional solid state route may benefit from cold sintering but despite the higher density, lower Qf cannot be avoided due to the impurities and grain boundary phases that are introduced.     

Correlation of Zeno (Z = 1) Line for Supercritical Fluids with Vapor-Liquid Rectilinear Diameters

For a wide range of substances, extending well beyond the regime of corresponding states behavior, the contour in the temperature-density plane along which the compressibility factor Z = P/ρkT is the same as for an ideal gas is nearly linear. This Z = 1 contour, termed the Zeno line, begins deep in the liquid region and ascends as the density decreases to the Boyle point of the supercritical fluid, specified by the temperature T_B for which (dZ/dρ)_T = 0 as ρ → 0; equivalently, at T_B the second virial coefficient vanishes. The slope of the Z = 1 line is — B₃/(dB₂/dT), in terms of the third virial coefficient and the derivative of the second, evaluated at T_B. Previous work has examined the Zeno line as a means to extend corresponding states and to enhance other practical approximations. Here we call attention to another striking aspect, a strong correlation with the line of rectilinear diameters defined by the average of the subcritical vapor and liquid densities. This correlation is obeyed well by empirical data for many substances and computer simulations for a Lennard-Jones potential; the ratios of the intercepts and slopes for the Zeno and rectilinear diameter lines are remarkably close to those predicted by the van der Waals equation, 8/9 and 16/9, respectively. Properties of the slightly imperfect fluid far above the critical point thus implicitly determine the diameter of the vapor-liquid coexistence curve below the critical point.     

The optoelectronic properties of Eu/F-codoped tin oxide,an experimental and DFT study

Tin oxide (TO), F-doped SnO₂ (FTO), and Eu/F-codoped SnO₂ (EFTO) films were deposited by spray pyrolysis. Additionally, for the DFT calculations, different superstructures were considered as models for TO (SnO₂), FTO (Sn₃₆O₇₁F), and EFTO (EuSn₃₅O₇₀F₂). Electrical and optical measurements showed that the EFTO films maintain the characteristics of a transparent conducting oxide (TCOs), i.e., high optical transparency and low electrical resistivity (ρ). The plasma wavelength for EFTO thin films was red-shifted when compared to FTO, which is related to the diminishing of free electron density (n_e) and the increase of ρ; such effects were promoted by the incorporation of Eu impurities which act as electron acceptors. Photoluminescence (PL) studies revealed that the EFTO films did not show electronic transitions related to Eu states in the visible part of the spectra (vis). On the other hand, the DFT calculations confirmed the experimental results related to the incorporation of Eu/F impurities in TO, such as cell expansion, the creation of direct bandgap TCOs, the appearance of the Burstein-Moss effect, the absence of emission lines in the vis, and the diminishing of n_e due to the presence of Eu acceptor states.