Miroslaw Miller

Chromium Vapor Species over Solid Oxide Fuel Cell Interconnect Materials and Their Potential for Degradation Processes

Alloys protected from corrosion by a chromia scale and LaCrO{sub 3}-based perovskites are used as materials for the interconnect of solid oxide fuel cells (SOFCs). The chromium vaporization of these materials was studied by thermochemical modeling. Partial pressures of the vaporizing species were determined for different O{sub 2} and H{sub 2}O concentrations in the oxidizing gas. CrO{sub 2}(OH){sub 2}(g) and CrO{sub 3}(g) are the most abundant species in air with and without humidity, respectively. The potential of the Cr-containing vapor species for the degradation of the electrical properties of an SOFC was analyzed by thermodynamic computations. The electrochemical reduction of the Cr-containing vapor species at the cathode/electrolyte/gas phase boundary can lead to polarization losses.

Vaporization of Sr- and Mg-Doped Lanthanum Gallate and Implications for Solid Oxide Fuel Cells

Vaporization of the La0.85Sr0.15Ga0.85Mg0.15O2.85, and La0.90Sr0.10Ga0.80Mg0.20O2.85 perovskite phases was investigated by the use of Knudsen effusion mass spectrometry in the temperature range of 1618-1886 K. The partial pressures of the gaseous species O2, Mg, Sr, SrO, Ga, GaO, Ga2O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of thermodynamic activities of the components at 1800 K. The results are compared with thermodynamic data of LaGaO3~s! without additives. Implications of the present data for the potential use of the material in solid oxide fuel cell technology are discussed as well.

Phase diagram studies in the SrO-Cr2O3-La2O3 system in air and under low oxygen pressure

Abstract Phase equilibria were determined in the complete composition range of the CaO–Cr 2 O 3 –La 2 O 3 system in air at 1223 K and under low oxygen pressure (high vacuum, p (O 2 )≈10 −9 bar). XRD, DTA and electron probe micro analysis of quenched samples were used. The (La, Ca)CrO 3 perovskite phase is the only ternary phase stable in air. The Ruddlesden-Popper phases (La, Ca) n +1 Cr n O n +1 ( n =1, 2, 3) were identified in addition to the perovskite phase in the samples annealed under low oxygen pressure. The solubility limit of Ca in the La 1− x Ca x CrO 3 phase was determined in air as x =0.31±0.01 at 1223 K and under low oxygen pressure as x =0.22±0.02 at 1873 K. In addition, investigations of the pseudo binary CaO–Cr 2 O 3 phase diagram were performed which clarify contradictions in the literature.

Vaporization and thermodynamics of La1−xCaxCrO3−δ investigated by Knudsen effusion mass spectrometry

Abstract Vaporization of the La 1− x Ca x CrO 3− δ perovskite phase, x =0.047–0.21, was investigated by the use of Knudsen effusion mass spectrometry in the temperature range of 1950–2050 K. Ion intensities were assigned to the neutral precursors by the isothermal evaporation method in addition to literature data. The partial pressures of Cr(g), CrO(g), CrO 2 (g), CaO(g), and LaO(g) were determined for all the samples investigated. The partial pressure of O 2 (g) was evaluated from the gaseous equilibria. The experimental data were interpreted using the phase equilibria in the CaO–Cr 2 O 3 –La 2 O 3 system under low oxygen pressure we previously studied. The ion intensities were used to calculate the thermodynamic activities of the system components at 2000 K. The Gibbs energy of formation of La 0.79 Ca 0.21 CrO 3 at 2000 K was estimated from the thermodynamic activities as −81.5 kJ mol −1 . The results were compared with thermodynamic data of the undoped LaCrO 3 (s). Implications of the present data for the potential use of the material in SOFC technology were discussed as well.

Thermodynamic study of the CsCl–CeCl3 system by Knudsen effusion mass spectrometry

The vaporization of samples of different chemical and phase compositions covering the complete concentration range of the CsCl–CeCl3 system was investigated in the temperature range between 749 K and 1098 K by the use of Knudsen effusion mass spectrometry. The gaseous species CsCl, Cs2Cl2, CeCl3, Ce2Cl6 and CsCeCl4 were identified in the vapor and their partial pressures determined. The thermodynamic stability of CsCeCl4(g) was evaluated by second law treatment of the equilibrium partial pressures. The thermodynamic activities of CsCl and CeCl3 were obtained at 950 K in the two phase fields {liquid+Cs3CeCl6(s)} and {CsCe2Cl7(s)+CeCl3(s)} from the partial pressures of the vapor components. The Gibbs free energies of formation from the constituent halides of the pseudobinary phases Cs3CeCl6(s) and CsCe2Cl7(s) were evaluated from the thermodynamic activities of the components. The results are discussed with other literature data for the studied system.

Mass spectrometric and theoretical study of the mixed complex NaNdCl4(g)

Abstract The vaporisation of the NaCl–NdCl3 system was investigated at 833–1116 K using Knudsen effusion mass spectrometry. The vapour species NaCl, NdCl3, and NaNdCl4 were found in the equilibrium vapour and their partial pressures were evaluated. Theoretical calculations of the NaNdCl4(g) structure were performed and the thermodynamic functions of the complex species were evaluated. The enthalpy change of the dissociation reaction NaNdCl4(g)=NaCl(g)+NdCl3(g) was computed by the second and third law methods. The value of ΔdH°(298)=256.5±7.0 resulted from this computation. The volatility enhancement of NdCl3 by the formation of the vapour complex NaNdCl4 was estimated from obtained thermodynamic data of gaseous and condensed phases.

Vaporization of LnCl3 and thermochemistry of Ln2Cl6(g), Ln=Ce, Pr, Nd, Dy

Abstract The vaporization of LnCl 3 , Ln=Ce, Pr, Nd, Dy, was investigated by Knudsen effusion mass spectrometry. The monomer and dimer partial pressures in the equilibrium vapor of the compounds were determined. Molecular parameters of LnCl 3 (g) and Ln 2 Cl 6 (g) were determined theoretically and the thermodynamic functions of these species were computed applying statistical thermodynamics. The enthalpies of sublimation of LnCl 3 (g) were evaluated according to the second- and third-law methods. Recommended values of Δ sub H o (298)=334.8±7.2 kJ mol −1 (CeCl 3 ), 323.3±3.5 kJ mol −1 (PrCl 3 ), 317.3±6.4 kJ mol −1 (NdCl 3 ), and 305.3±6.3 kJ mol −1 (DyCl 3 ), resulted for the sublimation enthalpies of gaseous species given in brackets by compilation of the present study with the data available in the literature. The enthalpies of sublimation Δ sub H o (298)=443.2±12.0 kJ mol −1 (Ce 2 Cl 6 ), 418.7±12.1 kJ mol −1 (Pr 2 Cl 6 ), 414.8±11.3 kJ mol −1 (Nd 2 Cl 6 ), and 354.1±11.3 kJ mol −1 (Dy 2 Cl 6 ), were obtained for dimeric species by the use of the third law method.

The thermodynamic properties of the gaseous dimer of CdI2

The vaporization of CdI2(s) was investigated in the temperature range between 534 and 613 K by the Knudsen effusion mass spectrometry. The Cd2I4(g)-dimer content in the equilibrium vapor of CdI2(s) was determined for the first time. The enthalpies of sublimation and dissociation of Cd2I4(g) were evaluated according to the third-law method using the experimental p(CdI2)/p(Cd2I4) ratio. Molecular parameters and the thermodynamic functions of Cd2I4(g) were determined theoretically. Enthalpy of sublimation, 2 CdI2(s)=Cd2I4(g), was obtained as: ΔsubH0(298.15 K)=222.5±6.2 kJ mol−1, and enthalpy of dissociation, Cd2I4(g)=2 CdI2(g), was determined as ΔdH0(298.15 K)=70.7±7.0 kJ mol−1. The standard enthalpy of formation of Cd2I4(g) was obtained as ΔfH0(298.15 K)=−188.1±6.3 kJ mol−1.

Fullerene based materials for ultra-low-k application

Fullerene-based materials are considered to be a candidate for ultra-low-k material applications. We have incorporated fullerene C60 into a siloxane material by means of the sol-gel method. Thickness of obtained film was investigated by atomic force microscope, dielectric constant was measured by the capacitance-voltage characterization (CV). Interactions between the components within the films were investigated by using X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy. We found that the ratio of carbon, oxygen and silicon atoms within obtained film equals 2.7∶1.9∶1. The microscopic and CV investigations show that the samples composition is inhomogenous although the fullerenes concentration within the material is low. However, dielectric constant is in the range of 2.3 to 2.5.

Thermodynamic and Cation Diffusion Studies of Perovskites on the Basis of LaGaO3 and Implications for SOFC

Solid phase equilibria in the quasi quaternary system La2O3-Ga2O3-SrO-MgO near the solubility lobe of the (La,Sr)(Ga,Mg)O3−δ perovskite phase were determined by XRD, SEM/EDX and EPMA of annealed and quenched samples. Equilibration of the samples during these measurements was ensured by considering diffusion of the cations which are the slowest moving species and determine all equilibration processes. Vaporization studies of the perovskite phase La1−xSrxGa1−yMgyO3−(x+y)/2 and of the Ga2O3-La2O3 system were carried out by the use of Knudsen effusion mass spectrometry in the temperature range between 1600 K and 1900 K. The gaseous species O2, Ga, GaO, Ga2O, and LaO, as well as in part also Mg, Sr and SrO were detected over the samples investigated. Thermodynamic activities and Gibbs energies of formation of the quasi binary compounds LaGaO3(s) and La4Ga2O9(s) resulted from the measurements. The data obtained are used to compute the vaporization of La0.90Sr0.10Ga0.80Mg0.20O2.85 being considered as SOFC electrolyte by thermodynamic modeling for different SOFC operating conditions. A high volatility results for the anode side. Diffusion of the impurities Y, Fe and Cr was investigated by SIMS at 1673 K for different compositions of symmetrically doped LSGM, La1−xSrxGa1−xMgxO3−x (x = 0.0, 0.02, 0.10, 0.20) showing that diffusion occurs via bulk and grain boundaries. Implications for the kinetic stability of LSGM in a SOFC are discussed. Thermodynamik und Kationendiffusion in Perowskiten auf der Basis von LaGaO3 und Bedeutung fur SOFC Im quasi-quaternaren System La2O3-Ga2O3-SrO-MgO wurden mit Hilfe von XRD, REM/EDX und ESMA an getemperten und abgeschreckten Proben Phasengleichgewichte sowie das Stabilitatsfeld der Perowskitphase La1−xSrxGa1−yMgyO3−(x+y)/2 bestimmt. Die Gleichgewichtseinstellung wahrend der Messungen wurde durch Berucksichtigung der Diffusion der Kationen sichergestellt, da diese als langsamste Spezies die Gleichgewichtseinstellung bestimmen. Mit Hilfe der Knudsen-Effusionsmassenspektrometrie wurden im Temperaturbereich von 1600 bis 1900 K Verdampfungsuntersuchungen an der Perowskitphase La1−xSrxGa1−yMgyO3−(x+y)/2 sowie am System Ga2O3-La2O3 durchgefuhrt. Dabei wurden die gasformigen Spezies O2, Ga, GaO, Ga2O und LaO sowie Mg, Sr and SrO nachgewiesen. Aus diesen Messungen ergeben sich thermodynamische Aktivitaten und Bildungs-Gibbs-Energien der quasi-binaren Verbindungen LaGaO3(s) und La4Ga2O9(s). Die gemessenen Daten wurden benutzt, um die Verdampfung von La0.90Sr0.10Ga0.80Mg0.20O2.85, welches als Elektrolyt fur SOFC diskutiert wird, thermodynamisch zu modellieren. Fur die Anodenseite ergibt sich eine hohe Fluchtigkeit. Die Diffusion der Fremdionen Y, Fe und Cr wurde mit Hilfe von SIMS bei T = 1673 K in symmetrisch dotierten Proben La1−xSrxGa1−xMgxO3−x (x = 0.0, 0.02, 0.10, 0.20) untersucht. Die Diffusion verlauft im Bulk und uber die Korngrenzen. Die Bedeutung der Ergebnisse fur die thermodynamische und kinetische Stabilitat von LSGM in SOFC-Anwendungen wird diskutiert.