Relative Partial Molar Entropy Research Articles

Thermodynamic Properties of Ni–Nd Intermetallic Compounds Measured Electrochemically in Molten CaCl2–NdCl3

The electrochemical formation of Ni–Nd intermetallic compounds has been investigated in a molten CaCl2–NdCl3 system. For further understanding the thermodynamic properties of different Ni–Nd intermetallic compounds, an electrochemical investigating method was used. The temperature dependences of the Nd3+/Nd potential and two-phase coexisting potentials of the Ni–Nd intermetallic compounds were determined via open-circuit potentiometry using Mo and Ni flag electrodes, respectively. Expressing the two-phase coexisting potentials with respect to the Nd3+/Nd potential, the relative partial molar Gibbs energies of Nd were calculated. The Nernst and Gibbs-Duhem equations were used to determine the activities of Ni and Nd, respectively. The relative partial molar Gibbs energies of Ni were calculated using its activities. Thereafter, the relative partial molar enthalpies and entropies of Ni and Nd were obtained using the temperature dependences of the relative partial molar Gibbs energies. Finally, the standard Gibbs energies, enthalpies, and entropies of formation of the various Ni–Nd intermetallic compounds were calculated using the relative partial molar Gibbs energies of Nd and Ni. The obtained thermodynamic parameters were compared with previously reported values obtained by the CALPHAD method and modeling calculations.

Thermodynamic Properties of Ni–Dy Intermetallic Compounds Measured Electrochemically in Molten CaCl2–DyCl3

The temperature dependence of the coexisting phase potentials of various Nickel–Dysprosium (Ni–Dy) intermetallic compounds in a molten CaCl2–DyCl3 (1.0 mol% added) system was determined between 1073 K and 1173 K at 25 K intervals. The coexisting phase potentials of (NiDy + Ni2Dy), (Ni2Dy + Ni3Dy), (Ni3Dy + Ni5Dy), and (Ni5Dy + Ni) were measured via open-circuit potentiometry using a Ni flag electrode. The Dy activities and the Dy relative partial molar Gibbs energies in Ni–Dy intermetallic compounds were calculated from these potentials. Then, the Ni activities and the Ni relative partial molar Gibbs energies were calculated using the Gibbs-Duhem equation. Thereafter, the relative partial molar enthalpies and entropies of Dy and Ni were obtained by comparing the temperature dependence equations of the relative partial molar Gibbs energies. Finally, the relative partial molar Gibbs energies of Dy and Ni were used to calculate the standard Gibbs energies of formation for various Ni–Dy intermetallic compounds and compared with reported values.

Investigations on Defect Equilibrium, Thermodynamic Quantities, and Transport Properties of La0.5Sr0.5FeO3-δ

In this work, relationship between defect equilibrium and thermodynamic quantities is investigated to elucidate mass and charge transport properties of La0.5Sr0.5FeO3-δ (LSF55). The oxygen nonstoichiometry (δ) was measured as a function of oxygen partial pressure in 10−19 ≤ ≤ 0.21 and 750 ≤ (T/°C) ≤ 900 range. The relationships indicated an electronic n-p transition point at δ = 0.25 which moved to a higher value with increasing temperature. The relative partial molar enthalpy () and entropy () of oxygen indicated that across the electronic stoichiometric point stabilized around −86.9 ± 3.6 kJ/mol in p-type and −402.2 ± 12.6 kJ/mol in n-type regimes, whereas values were kept changing because of the contribution from configuration entropy (SO(Conf)). From the DC 4-probe conductivity and electrical conductivity relaxation (ECR) measurement, the oxygen-ion conductivity at 900°C was 0.29 S.cm−1 with activation energy of 0.72 ± 0.04 eV and the oxygen self-diffusivity (DO) for LSF55 with δ = 0.114 during oxidation increased from (8.27 ± 0.05) × 10−8 cm2⋅s−1 at 800°C to (2.55 ± 0.01) × 10−7 cm2⋅s−1 at 900°C with an activation energy of 1.22 ± 0.14 eV. Dilatometry measurements indicated an isothermal chemical expansion as function of , which was explained on the basis of the relative change in mean ionic radius of transition metal cation Fe against δ. This model showed that transition metal cation existed as a mixture of high-spin and low-spin states and made a transition from low-spin to high-spin state with increasing δ.

Low‐Temperature Synthesis and Thermodynamic and Electrical Properties of Barium Titanate Nanorods

Studies regarding the morphology dependence of the perovskite‐type oxides functional materials properties are of recent interest. With this aim, nanorods (NRs) and nanocubes (NCs) of barium titanate (BaTiO3) have been successfully synthesized via a hydrothermal route at temperature as low as 408 K, employing barium acetate, titanium isopropoxide, and sodium hydroxide as reagents without any surfactant or template. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X‐ray powder diffraction (XRD), used for the morphology and structure analyses, showed that the NRs were formed by an oriented attachment of the NCs building‐blocks with 20 nm average crystallites size. The thermodynamic properties represented by the relative partial molar free energies, enthalpies, and entropies of the oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressure of oxygen, indicated that NRs powders have lower oxygen vacancies concentration than the NCs. This NRs characteristic, together with higher tetragonallity of the structure, leads to the enhancement of the dielectric properties of BaTiO3 ceramics. The results presented in this work show indubitably the importance of the nanopowders morphology on the material properties.

Thermodynamic data of Ba0.6Sr0.4Co0.8Fe0.2O3−δ SOFC cathode material

The compound Ba0.6Sr0.4Co0.8Fe0.2O3−δ with perovskite structure has been studied focusing mainly on the thermodynamic stability in correlation to the change in the oxygen stoichiometry. The thermochemical behavior has been investigated from room temperature to 1300K by thermal gravimetry and differential thermal analysis in air and by calorimetry in scanning mode in Argon. The temperature dependence of enthalpy (ΔHT298) in the temperature range of 300–900K was measured by drop calorimetry. Thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained in the temperature range of 823–1273K using solid electrolyte electrochemical cells (EMF). The influence of the oxygen stoichiometry on the thermodynamic properties was examined using a coulometric titration technique coupled with EMF measurements. The results are discussed based on the strong correlation between the energetic parameters and the charge compensation of the material system.

Hydrogen storage properties and microstructures of Ti0.7Zr0.3(Mn1−xVx)2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys

Hydrogen storage properties, activation performance and thermodynamics of Ti0.7Zr0.3(Mn1−xVx)2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys and associated microstructures and surface chemical states were investigated by hydrogenation measurements and relevant structure and surface characterization methods. The results showed that the phase composition of the alloy changed from single C14 Laves phase (x ≤ 0.2) to coexistent Laves phase and V-based BCC solid solution phase with increasing V content (x ≥ 0.3). The V in the alloys catalyzed hydrogen dissociation and improved resistivity to oxygen poisoning, so that the alloys could be easily and quickly activated at 293 K even after being exposed in air for a long time. The hydrogen storage capacity of the alloy increased and the plateau pressure decreased with increasing V content. The x = 0.2 and 0.3 alloys exhibited the best reversible hydrogen storage capacities of above 1.8 wt% at 1 kPa–4 MPa and 293 K. The relative partial molar enthalpy |ΔH| increased but the relative partial molar entropy |ΔS| decreased with increasing V content, and deviated from the linear relationship for x = 0.4 and 0.5 alloys due to coexisted BCC phase in the alloys.

Effects of A-site composition and oxygen nonstoichiometry on the thermodynamic stability of compounds in the Ba–Sr–Co–Fe–O system

The thermodynamic stability of the perovskite-type oxides in the BaxSr1−xCo0.8Fe0.2O3−δ (BSCF) system was investigated with varying Ba:Sr ratios (x=0.2, 0.4, 0.5, 0.6, 0.8) and correlated with the charge compensation mechanism and the change in the oxygen stoichiometry of the materials. Thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained in the temperature range of 823–1273K using solid electrolyte electrochemical cells (EMF) method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using a coulometric titration technique coupled with EMF measurements. The temperature dependence of enthalpy increment (HT−H298) in the temperature range of 700–900K was measured by drop calorimetry. The energetic parameters allow for the correlation of the structural and electrical stability with the defect structures.

Thermodynamic quantities and oxygen nonstoichiometry of undoped BaTiO 3− δ by thermogravimetric analysis

Thermogravimetry analysis was performed to determine the oxygen nonstoichiometry of oxides by using a home-made experimental set-up with Cahn D200 microbalance for an undoped BaTiO 3− δ specimen. The relative partial molar enthalpy and entropy of oxygen for undoped BaTiO 3− δ were also calculated from the slope and intercept of the δ − P O 2 − T relation, respectively. The negative signs of partial molar enthalpy and entropy in the oxygen deficient regime indicated that the incorporation of oxygen is an exothermic process. The values for K i and K Re of the undoped BaTiO 3− δ are best described by the following equations: K i / cm − 6 = ( 1.69 ± 1.51 ) × 10 46 exp − 2.93 ± 0.23 eV k T K Re / cm − 9 = ( 7.15 ± 5.55 ) × 10 73 exp − 5.80 ± 0.15 eV k T The difference in n– p transition P O 2 compared with reference should be further understood by identifying the cation nonstoichiometry and the value of impurity defects for the clarification of defect structure.

Microstructure and hydrogenation thermokinetics of ZrTi 0.2V 1.8 alloy

The microstructures and phase composition of the pseudobinary ZrTi 0.2V 1.8 alloy were examined by scan electron microscope (SEM) and X-ray diffraction (XRD). Before hydrogenation, the hypoeutectic structure accompanied with ZrV 2 + (ZrV 2 + Zr) spherical-like texture has been observed in ZrTi 0.2V 1.8 and the dominant phase could be ascribed to the C15 Laves phase. Hydrogen absorption pressure–composition isotherms ( P– C isotherms) and hydriding kinetics of ZrTi 0.2V 1.8 were investigated by pressure reduction method using Sievert apparatus from 673 to 823 K. At hydrogen concentration 0.65 (H/A), the relative partial molar enthalpy and entropy calculated by Van’t Hoff equation are −60 ± 1 kJ mol −1 and −119 ± 1 J mol −1 K −1, respectively. In addition, two stages in the hydrogen absorption reaction between 673 and 823 K could be attributed to the different hydrogen absorption mechanisms including redistribution of the hydrogen atoms in the hydride phase and the diffusion of hydrogen in the β-phase. The activation energy E a of the alloy is ∼3.6 kJ mol −1 for the first absorption stage and ∼61.9 kJ mol −1 for the second one.

Influence of Composition and Particle Size on Spin Dynamics and Thermodynamic Properties of Magnetoresistive Perovskites

The thermodynamic behavior and spin dynamics of the colossal magnetoresistive (CMR) perovskites of general formula La(1-x)(A)xMn(1-y)(B)yO3 (where A is an alkaline earth, and B = Al, In) have been studied in order to evidence the effect of composition and the influence of nanocrystallinity on the thermodynamic and magnetic characteristics. By using electron paramagnetic resonance (EPR) spectroscopy, the behavior of the exchange coupling integral (J) between Mn spins and the polaron activation energy (Ea) have been investigated. The thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained by using solid electrolyte electrochemical cells method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with measurements of the electromotive force (EMF). The results were correlated with the average Mn valence values as determined by redox titration. The properties of the rare-earth manganites are strongly affected by the A- and B-site substitution and by the oxygen nonstoichiometry. New features related to the modifications in properties connected with the nanocrystalline state were evidenced. The correlation existing between the magnetic and thermodynamic characteristics were discussed in relation to significant changes in the overall concentration of defects.

Thermodynamic properties of bioleaching liquid mixtures with and without mesophilic bacteria at different temperatures

In this work, low-grade copper sulfide mine has been treated by the bioleaching process using native cultures of Acidithiobacillus ferrooxidans. The bioleaching experiments were carried out in shake flasks at pH 2.0, 180 r·min −1 and 30°C for mesophilic bacteria. The conductivity of copper bioleaching liquid was determined by the electric conductivity method at temperatures ranging from 298 K to 313 K. The ionic activity coefficients were estimated using Debye-Hücker and Osager-Falkenlagen equations. Meanwhile, the effects of temperature and concentration on the mean ionic activity coefficients were discussed. The relative partial molar free energies, enthalpies and entropies of copper leaching solution at above experimental temperatures were calculated. The behaviors of change of relative partial molar quantities were discussed on the basis of electrolytic solution theory. Simultaneously, the thermodynamic characters of bioleaching solution with and without mesophilic bacteria were compared. The existence of mesophilic bacteria changed the Fe 3+/Fe 2+ ratio, which resulted in the difference of ionic interaction. The experimental data show that the determination of the thermodynamic properties during the bioleaching processes should be important.

Structural and thermodynamical investigations of La 0.23Ni 0.34Co 0.33Nd 0.08Ti 0.01Al 0.01 hydrogen storage alloy

Hydrogen storage properties of the La 0.23Ni 0.34Co 0.33 Nd 0.08Ti 0.01Al 0.01 have been systematically studied in the present work. Pressure composition ( P – C – T ) isotherm has been investigated in the temperature and pressure ranges of 308 ⩽ T ⩽ 338 K and 10 ⩽ P ⩽ 140 psi , respectively. The studies show the presence of two single α and β phases with one mixed α + β phase. The maximum H / M ratio was found to be around 0.94 at 308 K and 90 psi. The introduction of interstitial hydrogen particles into a metallic lattice causes several changes in respect to its electrical resistance. Hence an attempt has been made to relate change in the resistance of material with the content of hydrogen. The resistance of the specimen was found to increase with increasing H / M . Thermodynamical parameters viz., the relative partial molar enthalpy ( Δ H ) and relative partial molar entropy ( Δ S ) of dissolved hydrogen are found to be 14.50755 ± 0.8 K J / mol H 2 and 47.01152 ± 2.42 J / K / mol H 2 , respectively. The variation of enthalpy and entropy with hydrogen concentration has also been calculated to confirm the existence of different phases.

Thermodynamics and structural aspects of hydrogen absorption in [formula omitted] alloys

The hydrogen storage properties of Zr 1 - x Cr x Fe 2 ( x = 0.2 , 0.3, 0.4, 0.5) alloys have been obtained in the pressure and temperature range of 0.5 ⩽ P ⩽ 60 bar and 303 K ⩽ T ⩽ 373 K . The alloys have been characterized by XRD and SEM. The results show that all the alloys were having C14 laves phase hexagonal structure. The substitution of Cr in Zr site leads to the contraction of their lattice. The P –C– T isotherms of the alloys showed the existence of α , α + β , and β regions. The maximum hydrogen intake capacity is around 3.131 H/f.u. at 303 K and at 48 bar pressure for Zr 0.8 Cr 0.2 Fe 2 alloy. The average relative partial molar enthalpy and entropy of hydrogen at plateau region have been calculated for all the alloys. The presence of α , α + β , and β region as seen in P –C isotherm has been confirmed by the variation of thermodynamical parameters with hydrogen content.

Thermodynamic properties of B2-AlFeNi Alloys. Part I: Investigation by knudsen effusion mass spectrometry

The vaporization of Al-Fe-Ni alloys has been investigated in the temperature range 1180 to 1508 K by Knudsen effusion mass spectrometry (KEMS). Fourteen different compositions were examined in the B2 region: 10 compositions at two fixed Al concentrations,x Al=0.45 andx Al=0.50 plus four extra compositions at constantx Ni/x Fe=1. For the first time, reliable partial pressures and thermodynamic activities of Al, Fe, and Ni have been evaluated from the measured ion intensities for both the alloy and the pure element. Gibbs energies, partial molar enthalpies, and entropies of formation for all the components have also been obtained. The relative partial molar enthalpies and entropies were found to be nearly temperature independent over the wide temperature ranges investigated. At 1400 K, the Gibbs energy of formation of Al0.50Fe0.25Ni0.25 and Al0.45Fe0.275Ni0.275, with Al(liq), Fe(fcc), and completely paramagnetic Ni(fcc,cpm) as reference states, are −37.9±0.42 kJ/mol and −38.1±0.42 kJ/mol, respectively. At the same temperature, the enthalpies of formation of Al0.50Fe0.25Ni0.25 and Al0.45Fe0.275Ni0.275, with the same reference states, are −51.5±1.7 kJ/mol and −49.2±1.7 kJ/mol, respectively.

Hydrogen uptake characteristics of mischmetal based alloy

Hydrogen storage properties of Mm 39.2Ni 42.1Mn 4.9Al 1.25Co 10.2Fe 2.35 alloy have been systematically studied in the present work. An attempt is made to relate the content of hydrogen with change in resistance. It is found that the resistance of material increases with the increase in value of H/ M due to hydrogen absorption. Pressure composition (P-C-T) isotherm using water displacement method has been investigated in the temperature and pressure ranges of 308 ≤ T ≤ 338 K and 0.5 ≤ P ≤ 10 bar, respectively. The P-C isotherms show the presence of two single α and β regions one mixed α + β phase. The maximum H (wt%) was found to be around 1.53 at 308 K and around 6 bar. Since enthalpy is an index of thermochemical stability of metal hydride the thermo dynamical parameters viz., the relative partial molar enthalpy (Δ H) and relative partial molar entropy (Δ S) of dissolved hydrogen have been calculated by plotting the Van’t Hoff plot. The variation of Δ H and Δ S with the hydrogen concentration confirm the phase boundaries.

Thermodynamics of Hydrogen Solution and Hydride Formation in Pd−Mn Alloys. 2. Ordered Alloys

There are marked differences in H(2) solubilities between ordered and disordered Pd-Mn alloys with the largest difference found between the L1(2) and the disordered form of the Pd(3)Mn alloy. The thermodynamics of H(2) solution have been determined for the L1(2) form, the long-period superstructure (lps), and the disordered forms of the Pd(0.80)Mn(0.20) and Pd(0.75)Mn(0.25)(Pd(3)Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from p(H)()2 approximately 10 Pa to approximately 0.3 MPa (303 K). The enthalpies for absorption of H(2) are more exothermic over most of the range of H contents for the L1(2) forms of the Pd(3)Mn and Pd(0.80)Mn(0.20) alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1(2) form of the Pd(0.80)Mn(0.20) and Pd(3)Mn alloys indicating that there are two-phase coexistence regions (303 K). The H-H attractive interaction, which leads to hydride formation, is much greater for the L1(2) than for the other forms of the Pd(3)Mn alloy and for Pd itself. It has been found that the H-H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.(1) The L1(2) ordered Pd(3)Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.

Structural, electrical and thermodynamical aspects of hydrogenated La-Ni-Si alloy

The structural, electrical and thermodynamic properties of a La-Ni-Si [La = 28.9%, Ni = 67.5%, Si = 3.6%] alloy have been investigated. Powder XRD results show that the lattice constants and unit cell volume of the alloy increase after hydrogen storage. It was also found that the resistance of the alloy increased with dissolved hydrogen concentration. Hydrogen absorption pressure composition isotherms have also been investigated which show the presence of two single a and β regions and one mixed (α + β) phase. The thermo-dynamic parameters viz. the relative partial molar enthalpy (ΔH) and relative partial molar entropy (ΔS) of dissolved hydrogen, are found to be in the range 8–18 kJ (mol H)-1 and 25–63 JK-1 (mol H)-1. From the dependence of ΔH on the hydrogen concentration,X, the different phases [α, α + β, β] and phase boundaries of the alloy-H system are identified. Thermal conductivity and diffusivity of La-Ni-Si and its hydride have been measured at room temperature by using TPS technique. Thermal conductivity was found to decrease due to absorbed hydrogen in the alloy.

Thermodynamic properties and spin dynamics of some micro and nanostructured magnetoresistive lanthanum manganites

The spin dynamics and thermodynamic properties of micro- and nanostructured lanthanum manganites La 0.67Ca 0.33Mn 1− x Me x O 3 (Me = Al; x = 0–0.05) have been studied in order to evidence new features related to the effect of different compositional variables on the thermodynamic and magnetic behaviour. By electron paramagnetic resonance (EPR) spectroscopy, the influence of the grain size and the Mn substitution effects on the spin dynamics were investigated. A solid state electrochemical method was used to obtain the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen. The influence of the oxygen stoichiometry change on the thermodynamic properties has been investigated. The results were correlated with the average Mn valence values as determined by redox titration. In this paper an alternative method is proposed to estimate the contribution made by oxygen vacancies in balancing the local charge in CMR manganites.

Vapor pressures and derived thermodynamic properties of aqueous Zn(NO3)2 solutions from 423.15 to 623.15 K

Vapor pressures of aqueous Zn(NO3)2 solutions (0.321, 0.499, 0.900, 1.400, 2.000, and 3.002 mol . kg–1) have been measured by the static method in the temperature range from 423.15 to 623.15 K with a constant‐volume piezometer immersed in a precision liquid thermostat. The total uncertainty of temperature, pressure, and composition measurements were estimated to be less than 15 mK, 0.2%, and 0.014%, respectively. The vapor pressures of pure water were measured to confirm the accuracy of the method for aqueous Zn(NO3)2 solutions taken using the apparatus and procedure in this study. The resulting measurements of PS ‐ TS for pure water were compared with values calculated from IAPWS formulation. Useful thermodynamic functions (water and electrolyte activities, osmotic coefficient, excess relative partial molar entropy, and relative partial molar enthalpy values of solvent) were derived using measured values of vapor pressure for the solutions and pure water.

Electrode Reaction of Cerium into Liquid Bismuth in the Eutectic LiCl-KCl

The electrode reaction of Ce(III)/Ce couple in the eutectic LiCl - KCl, on a Bi liquid electrode, was investigated in the temperature range of 673 - 823 K. Cyclic voltammetry, using a Bi pool electrode, suggest a quasi - reversible behaviour of the system, and the values of the kinetic parameters, k0 and α, as well as the reversible half wave potential, Er 1/2, have been obtained. The differences between the equilibrium potential adopted by a Ce electrode and the Er1/2 observed with the same Ce(III) solution at the liquid electrode, were consistent with the activity coefficients of Ce in the liquid Bi phase taken from the literature. Because of the ease of application of cyclic voltammetry, it was suggested this to be used as a general method to obtain approximate values of the excess Gibbs energy change of Ce in liquid metal, and hence the partial molar enthalpy of mixing and the partial molar excess entropy. Electromotive force, emf, measurements for various intermetallic compounds in two - phase coexisting states were carried out using a Bi coated tungsten electrode. The activities and relative partial molar Gibbs energies of Ce were obtained for CeBi2 and CeBi. The relative partial molar entropies and enthalpies of Ce as well as the standard Gibbs energies of formation for the Ce - Bi intermetallic compounds were calculated.