ABSTRACT: 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 823–1273 K 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 700–900 K was measured by drop calorimetry. The energetic parameters allow for the correlation of the structural and electrical stability with the defect structures.
Journal of Solid State Chemistry 04/2013; 200:354–362. · 2.16 Impact Factor
ABSTRACT: The high absorption energies of partially wetted particles at fluid interfaces allow the production of macroporous composite materials from particle-stabilized foams. Competition between the different particle types determines how they are distributed in the foam lamella and allow the phase distribution to be controlled; a technique that is useful in the design and engineering of porous composites. Here, we report details on the effects of preferential and competitive adsorption of poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles at the foam interfaces on the consolidated macroporous composite materials. By varying the relative composition and surface energies of the stabilizing particles, macroporous composite materials with a broad range of phase distributions are possible.
Journal of Colloid and Interface Science 06/2012; 383(1):1-12. · 3.07 Impact Factor
ABSTRACT: During the growth of metal thin films on dielectric substrates at a given
deposition temperature T, the film's morphology is conditioned by the magnitude
and asymmetry of up- and downhill diffusion. Any severe change of this
mechanism leads to a growth instability, which induces an alteration of the
thin film morphology. In order to study this mechanism, ultra-thin Pt films
were deposited via pulsed laser deposition (PLD) onto
yttria-stabilized-zirconia single crystals at different deposition
temperatures. The morphological evolution of Pt thin films has been
investigated by means of scanning electron microscopy (SEM), atomic force
microscopy (AFM) and standard image analysis techniques. The experimentally
obtained morphologies are compared to simulated thin film structures resulting
from a two-dimensional kinetic Monte Carlo (KMC) approach. Two main
observations have been made: i) Thin Pt films deposited onto zirconia undergo a
growth transition from two-dimensional to three-dimensional growth at T > 573
K. The growth transition and related morphological changes are a function of
the deposition temperature. ii) A critical cluster size of i\ast = 4 in
combination with an asymmetric Ehrlich-Schwoebel (ES) barrier favoring the
uphill diffusion of atoms allows for a computational reproduction of the
experimentally obtained film morphologies.
Acta Materialia 06/2012; 61(9):3297-3303. · 3.76 Impact Factor
ABSTRACT: This work presents a theoretical study of the forces established between colloidal particles connected by means of a concave
liquid bridge, where the solid particles are partially wetted by a certain amount of liquid also possessing a dry portion
of their surfaces. In our analysis, we adopt a two-particle model assuming that the solids are spherical and with the same
sizes and properties and that the liquid meniscus features an arc-of-circumference contour. The forces considered are the
typical capillary ones, namely, wetting and Laplace forces, as well as the van der Waals force, assuming the particles uncharged.
We analyze different parameters which govern the liquid bridge: interparticle separation, wetting angle, and liquid volume,
which later determine the value of the forces. Due to the dual characteristic of the particles' surfaces, wet and dry, the
forces are to be determined numerically in each case. The results indicate that the capillary forces are dominant in most
of the situations meanwhile the van der Waals force is noticeable at very short distances between the particles.
KeywordsLiquid bridge-Meniscus-Capillary forces-van der Waals
Colloid and Polymer Science 04/2012; 288(2):133-139. · 2.33 Impact Factor
ABSTRACT: Experimental data on the thermodynamics and the phase diagram of the Mn-O system were reviewed, and by application of the
CALPHAD method, a consistent set of thermodynamic model parameters was optimized. The phases pyrolusite (MnO2), bixbyite (Mn2O3), and hausmannite (Mn3O4) were described as stoichiometric compounds. Manganosite (Mn1−x
O) was described using the compound-energy model and the liquid described using the two-sublattice model for ionic liquids.
Journal of Phase Equilibria 04/2012; 24(1):21-39.
ABSTRACT: In situations and processes where finely divided solids are in contact with small amounts of liquid, capillary effects influence
the behavior of such systems. If the quantity of liquid is rather limited, it arranges as individual liquid bridges connecting
the solid particles just wetting a portion of the solids surface. These bridges develop forces which drive the cohesion and
motion of the solid particles, further determining in many times the final structure or even the quality of the material.
Since the liquid is not able to fully cover the solid particles like in a proper suspension, this liquid adopts a shape which
is determined by the principle of constant mean curvature. A rigorous determination of such a shape, which in turn determines
the capillary forces, must be carried out by solving the Young–Laplace equation. Due to the difficulties in such calculation,
it was proposed to approximate the meniscus profile by an arc-of-circumference, the so-called toroidal approximation. Here
it is quantitatively studied the suitability of such approximation for the most general geometry of liquid bridges, finding
that the error of the approximation is below 10% for concave menisci and 30% for convex ones.
KeywordsLiquid bridges–Toroidal approximation–Meniscus–Young–Laplace equation
Granular Matter 04/2012; 13(4):487-492. · 1.75 Impact Factor
ABSTRACT: A comprehensive compilation and evaluation of experimental and thermodynamic data for the Cr−O system is presented and, by
application of the CALPHAD method, a consistent set of thermodynamic model parameters is optimized based on new experiments.
Nonstoichiometry of eskolaite (Cr2O3-δ) is described using the compound energy model, and the liquid, is described using the two-sublattice model for ionic liquids.
Cr3O4 is described as a stoichiometric compound. Also the magnetic transition in Cr2O3 and the oxygen solubility in Cr are modeled.
Journal of Phase Equilibria and Diffusion 04/2012; 27(4):353-362. · 0.62 Impact Factor
ABSTRACT: The particular relevance of the La-Mn-O system is due to the perovskite phase La1−x
, which, especially when doped with alkaline earth metals, is of interest both as cathode material for solid oxide fuel cells
and its unusual giant magnetoresistive properties. Here, a complete thermodynamic description of all phases in the oxide part
of the La-Mn-O system is presented. Particular focus is placed on modeling the defect chemistry of the perovskite phase. We
used the compound energy model with the sublattice occupation (La3+, Mn3+, Va)(Mn2+, Mn3+, Mn4+, Va)(O2−, Va)3. On reducing Mn3+ to Mn2+, O vacancies are formed. On oxidation of Mn3+ to Mn4+, equal numbers of vacancies are formed on the two cation sublattices while the O sublattice remains fully occupied. La-deficient
perovskites have some Mn3+ substituting for La3+ on the A-site under reducing conditions. Under oxidizing conditions, more A-site vacancies are formed than B-site vacancies.
Mn deficiency in perovskites can only be achieved by the formation of more vacancies on the B-sites than on the A-sites as
La3+ does not substitute for Mn on the B-site. The ionic liquid is modeled using the two-sublattice model for ionic liquids. The
phase La2MnO4 that is only stable above 1650 K and at low O partial pressures is described as a stoichiometric phase. Model parameters
for the Gibbs energy functions are optimized according to the CALPHAD approach. No interaction parameters are necessary to
give a good reproduction of all experimental data of the system.
Journal of Phase Equilibria and Diffusion 04/2012; 26(2):131-151. · 0.62 Impact Factor
ABSTRACT: The data on the thermodynamic properties of La2O3 have been reviewed and optimized using the CALPHAD method. A consistent set of parameters is presented. Data on this system
are scarce and, with the exception of a few datapoints on substoichiometric La2O3−x
and one measurement of oxygen solubility in La metal, limited to the properties of pure La and pure La2O3. Using the optimized parameters, a tentative phase diagram and stability diagram have been calculated.
Journal of Phase Equilibria 04/2012; 22(2):105-113.
ABSTRACT: The thermodynamic properties of CoO, Co3O4, and the liquid phase were assessed, and an optimized set of parameters of Gibbs energy functions is proposed. The two stable
solid oxides, CoO and Co3O4, were both treated as stoichimetric compounds. The paramagneticantiferromagnetic transition of CoO is well represented by
a magnetic ordering model. The Co3O4 spinel phase was described as a normal spinel at room temperature and with cation redistribution at high temperatures. A
high-temperature anomaly of Co3O4 was interpreted as a normal-inverse spinel transition. An ionic two-sublattice model was used to model the liquid phase.
A calculated phase diagram is presented, and values for the thermodynamic properties are compared with experimental data.
Journal of Phase Equilibria 04/2012; 24(3):212-227.
ABSTRACT: AbstractThis article focuses on perovskite materials for application as cathode material in solid oxide fuel cells. In order to develop
new promising materials it is helpful to classify already known perovskite materials according to their properties and to
identify certain tendencies. Thereby, composition-dependent structural data and materials properties are considered. Structural
data under consideration are the Goldschmidt tolerance factor, which describes the stability of perovskites with respect to
other structures, and the critical radius and lattice free volume, which are used as geometrical measures of ionic conductivity.
These calculations are based on the ionic radii of the constituent ions and their applicability is discussed. A potential
map of perovskites as a tool to classify simple ABO3 perovskite materials according to their electrical conduction behavior is critically reviewed as a structured approach to
the search for new cathode materials based on more complex perovskites with A and/or B-site substitutions. This article also
covers the approaches used to influence electronic and the ionic conductivity. The advantage of mixed ionic electronic conductors
in terms of the oxygen exchange reaction is addressed and their important properties, namely the oxygen-exchange coefficient
and the oxygen diffusion coefficient, and their effect on the oxygen reduction reaction are presented.
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(9):985-999. · 1.53 Impact Factor
ABSTRACT: AbstractMicro-solid oxide fuel cells (micro-SOFC) are predicted to be of high energy density and are potential power sources for portable
electronic devices. A micro-SOFC system consists of a fuel cell comprising a positive electrode-electrolyte-negative electrode
(i.e. PEN) element, a gas-processing unit, and a thermal system where processing is based on micro-electro-mechanical-systems
fabrication techniques. A possible system approach is presented. The critical properties of the thin film materials used in
the PEN membrane are discussed, and the unsolved subtasks related to micro-SOFC membrane development are pointed out. Such
a micro-SOFC system approach seems feasible and offers a promising alternative to state-of-the-art batteries in portable electronics.
Graphical abstractGraphical Abstract text
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(9):975-983. · 1.53 Impact Factor
ABSTRACT: The stability of a metal thin films on a dielectric substrate is conditioned
by the magnitude of the interactive forces at the interface. In the case of a
non-reactive interface and weak adhesion, the minimization of free surface
energy gives rise to an instability of the thin film. In order to study these
effects, Pt thin films with a thickness of 50 nm were deposited via ion-beam
sputtering on yttria stabilized zirconia single crystals. All Pt films were
subjected to heat treatments up to 973 K for 2 h. The morphological evolution
of Pt thin films has been investigated by means of scanning electron microscopy
(SEM), atomic force microscopy (AFM) and standard image analysis techniques.
Three main observations have been made: i) the deposition method has a direct
impact on the morphological evolution of the film during annealing. Instead of
hole formation, that is typically observed as response to a thermal treatment,
anisotropic pyramidal shaped hillocks are formed on top of the film. ii) It is
shown by comparing the hillocks' aspect ratio with finite element method (FEM)
simulations that the hillock formation can be assigned to a stress relaxation
process inside the thin film. iii) By measuring the equilibrium shapes and the
shape fluctuations of the formed Pt hillocks the anisotropy of the step free
energy and its stiffness have been derived in addition to the anisotropic kink
energy of the hillock's edges.
Physical Review B 03/2012; 85(125408). · 3.69 Impact Factor
Advanced Functional Materials 01/2012; · 10.18 Impact Factor
ABSTRACT: Microstructures of yttria-stabilized zirconia (YSZ) thin films deposited by spray pyrolysis at 370 °C on sapphire are investigated. The as-deposited films are predominantly amorphous and crystallize upon heating at temperatures above 370 °C, developing grains in the range of 5 nm to several 100 nm. During post-deposition heat treatment up to 800 °C, ∼ 50 vol% porosity develops in the center of the films with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin films consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 °C. In gadolinium-doped-ceria (CGO) thin films similar microstructures and porosity are found after low-temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin films is observed in both the YSZ and in CGO thin films. Stagnating grain growth in the thin films is rather caused by reduced grain boundary mobility, here predominately due to a “secondary phase”, i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densification has taken place.
Advanced Functional Materials 01/2012; · 10.18 Impact Factor
Journal of the European Ceramic Society 01/2012; 32(8):1701-1709. · 2.35 Impact Factor
ABSTRACT: Highly porous Pt/Al thin film electrodes on yttria stabilized zirconia
electrolytes were prepared by dealloying of co-sputtered Pt/Al films. The
oxygen reduction capability of the resulting electrodes was analyzed in a solid
oxide fuel cell setup at elevated temperatures. During initial heating to 523 K
exceptionally high performances compared to conventional Pt thin film
electrodes were measured. This results from the high internal surface area and
large three phase boundary length obtained by the dealloying process. Exposure
to elevated temperatures of 673 K or 873 K gave rise to degradation of the
electrode performance, which was primarily attributed to the oxidation of
remaining Al in the thin films.
Physical Review B 11/2011; 84(184111). · 3.69 Impact Factor
ABSTRACT: LaFe3/4Ni1/4O3 was subjected to oxygen near edge x-ray absorption fine
structure (NEXAFS) spectroscopy for 300 K < T < 773 K. The spectra show in the
pre-edge a small hole doped peak originating from Ni substitution. The relative
spectral weight of this transition to the weight of the hybridized O(2p) -
Fe(3d) transitions scales with T and has a maximum at around 600 K. The
characteristic energies of the thermal activated spectral intensity and
conductivity suggest that the concentration of charge transferred electrons
from O(2p) to Ni(3d) increases and that the pre-edges account in part for the
polaron activated transport.
ABSTRACT: The application of focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films allows for an in-depth analysis of the dominating physical mechanisms of nanoporosity formation during the dealloying process. The porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS and FIB analysis yields that the porosity evolution has to be regarded as superposition of two independent processes, a linearly propagating diffusion front with a uniform speed and a slower dissolution process in regions which have already been passed by the diffusion front. The experimentally observed front evolution is captured by the Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP). The slower dissolution is represented by a zero-order rate law which causes a gradual porosity in the thin film.
Physical Review Letters 11/2011; 107(22):225503. · 7.37 Impact Factor