41 research outputs found
Micro-solid oxide fuel cells: status, challenges, and chances
Abstract: Micro-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 abstract: Graphical Abstract tex
Dealloying of Platinum-Aluminum Thin Films Part II. Electrode Performance
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.Comment: 5 pages, 4 figure
Dealloying of Platinum-Aluminum Thin Films Part I. Dynamics of Pattern Formation
Applying focused ion beam (FIB) nanotomography and Rutherford backscattering
spectroscopy (RBS) to dealloyed platinum-aluminum thin films an in-depth
analysis of the dominating physical mechanisms of porosity formation during the
dealloying process is performed. The dynamical 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 analysis yields that the porosity formation
is mainly caused by a linearly propagating diffusion front, i.e. the
liquid/solid interface, with a uniform speed of 42(3) nm/s when using a 4M
aqueous NaOH solution at room temperature. The experimentally observed front
evolution is captured by the normal diffusive
Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP) equation and can be interpreted
as a branching random walk phenomenon. The etching front produces a gradual
porosity with an enhanced porosity in the surface-near regions of the thin film
due to prolonged exposure of the alloy to the alkaline solution.Comment: 4 pages, 5 figure
Hillock formation of Pt thin films on Yttria stabilized Zirconia single crystals
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.Comment: 8 pages, 7 figure
Temperature-dependent 2D-3D growth transition of ultra-thin Pt films deposited by PLD
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.Comment: 7 pages, 6 figures, 1 tabl
Scanning transmission electron microscopy strain measurement from millisecond frames of a direct electron charge coupled device
A high-speed direct electron detection system is introduced to the field of transmission electron microscopy and applied to strain measurements in semiconductor nanostructures. In particular, a focused electron probe with a diameter of 0.5 nm was scanned over a fourfold quantum layer stack with alternating compressive and tensile strain and diffracted discs have been recorded on a scintillator-free direct electron detector with a frame time of 1 ms. We show that the applied algorithms can accurately detect Bragg beam positions despite a significant point spread each 300 kV electron causes during detection on the scintillator-free camera. For millisecond exposures, we find that strain can be measured with a precision of 1.3  × 10−3, enabling, e.g., strain mapping in a 100×100 nm2 region with 0.5 nm resolution in 40 s