Advanced Engineering Materials

Cover image for Vol. 7 Issue 10

Special Issue: TU Darmstadt

October, 2005

Volume 7, Issue 10

Pages 867–969

    1. Editorial: Adv. Eng. Mater. 10/2005 (pages 867–868)

      Ralf Riedel

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200590018

    2. Perspectives and Trends in Modern Materials Analysis (pages 875–881)

      H. Ortner

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500123

      The beginning of our being humans is usually characterized by the initial conscious use of materials and tools produced thereof. Today, complex materials are the basis for key technologies which in turn are of major significance for the economic success and the social development in modern society. Materials analysis produces information on the composition and structure of materials without which a successful materials development as well as a stringent materials control would be impossible. Modern materials analysis gives answers to four basic questions by use of an astonishing multitude of methods:

      – What and how much? Bulk analysis

      – How structured? Structure analysis

      – How bound? Speciation

      – How distributed? Topochemical analysis

      These four questions are further elucidated and discussed in some detail. The materials analyst in industry faces a spectrum of further responsibilities which are also discussed.

    3. Fatigue In Bulk Lead Zirconate Titanate Actuator Materials (pages 882–898)

      D. Lupascu and J. Rödel

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500117

      Fatigue in ferroelectric ceramics is the gradual change of material properties with cyclic loading. It is caused by microscopic material modifications of mechanical or electrical origin. Due to the electromechanical coupling, both mechanisms can yield similar or even identical changes in properties. Macroscopically, a reduction of switchable polarization is anticipated and asymmetries in the macroscopic hysteresis curves arise due to charge carrier migration. This review elaborates on the multiple loading scenarios that lead to asymmetries in material response and loss in performance. The disparities between unipolar, bipolar, and mixed electromechanical loading are displayed. Possible microscopic origins are categorized. The strong similarities in the roles of microcracks, dielectric layers, and grain boundaries are worked out.

    4. Structural Characterization of Automotive Catalysts (pages 899–913)

      F. Haaß and H. Fuess

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500120

      Scarcely any other technology is of such a global interest due to economical and ecological reasons like automotive catalysis, and world wide efforts are exerted to improve and optimize automotive emission control systems enclosing new engine concepts, better fuel quality, and novel exhaust catalysts. Especially the continuous improvement of the exhaust catalysts over the last 30 years brought major advances in saving ressources and fulfilling present and future emission directives. This paper deals with current developments in automotive exhaust catalysts. After an introductorily overview, the principles of spark ignition engines and diesel engines will be described, followed by selected fields of recent research of our and others groups. The focus is brought upon Pd-only catalysts and new oxygen storage systems for three-way catalysis, as well as on new concepts and characteristics of diesel oxidation catalysts based on platinum and palladium.

    5. Thin Film Solar Cells: Materials Science at Interfaces (pages 914–920)

      J. Fritsche, A. Klein and W. Jaegermann

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500126

      Interfaces are important for the efficiencies of thin film solar cells. In particular for polycrystalline chalcogenide semiconductors as Cu(In,Ga)(S,Se)2 and CdTe the existing physical concepts, which describe the electronic properties of semiconductor interfaces, are not sufficient. The increased complexity is mostly due to the non-abruptness of the interfaces and the strong tendency for the formation of defects. For the CdTe thin film solar cell a very relevant interface for their operation and efficiency is the CdTe/CdS semiconductor hetero junction. The properties of the semiconductor interfaces have been characterised systematically with photoelectron spectroscopy (XPS/UPS) in integrated ultra high vacuum (UHV) systems for sample preparation and analysis. Withal the key topic is the experimental determination of the band alignment at the semiconductor interfaces. For high efficiency CdTe solar cell production CdCl2 activation is of major importance. The effects of the CdCl2 treatment step on CdTe solar cells had been not completely understood so far. To investigate its influence the activation process has been transferred into the integrated UHV system. We will report about chemical and electronic modifications of the CdTe/CdS hetero interface due to in-situ CdCl2 activation performing sputter depth profiles in combination with x-ray photoelectron spectroscopy (XPS).

    6. Advances in Gallium Oxonitride Ceramics: A New Class of Materials in the System Ga-O-N (pages 921–927)

      I. Kinski, F. Scheiba and R. Riedel

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500127

      This research news highlights the history, synthesis and new developments of gallium oxonitride phases. Many practical applications are expected of this class of materials because of its analogy to aluminium oxonitride phases. A new approach to gallium oxonitride ceramics is a synthesis pathway starting from a precursor that provides gallium simultaneously bonded to nitrogen and oxygen on a molecular level. One suitable precursor is the dimethylamine adduct of gallium tris-(t-butoxide), Ga(OtBu)3·HNMe2. The optimization of the pyrolysis process was controlled by analyzing the nitrogen to oxygen content and the reduction of carbon content in the ceramic. The samples were investigated using FTIR spectroscopy and elemental analysis. The degree of crystallinity was determined by transmission electron microscopy in combination with an electron energy loss spectrometer. The precursor-derived gallium oxonitride ceramic remains nanocrystalline up to 600 °C and reacts to GaN with the wurtzite type structure by loss of all oxygen in an ammonia atmosphere at temperatures above 600 °C.

    7. Characterization of Pearl Luster Pigments (pages 928–931)

      M. Knoth, G. Miehe and H. Fuess

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500115

      This paper presents current developments in pearl luster pigments. After a short introduction on the application of pigments, the synthesis and characterization of a ZrO2/SiO2 based pearlescent pigment will be described. The applied methods for characterization were ex situ x-ray diffraction (XRD) of dried and calcined pigment powders, in situ XRD in a temperature range from 100 °C up to 850 °C and transmission electron microscopy (TEM) of thin cross-sections. The XRD measurements were used for phase analysis, determination of cell parameters and average crystal grain sizes. A special preparation method was developed for TEM cross section specimens for platy like pigment particles. The microstructure, the interface between substrate and coating material and the particle sizes are described with high resolution TEM pictures of cross section specimens. The basic interest is the behaviour of the zirconium dioxide coating during sintering in order to achieve a condensation of the film without any further crystal grain growth.

    8. Conditioning of Li(Ni,Co)O2 Cathode Materials for Rechargeable Batteries During the First Charge-Discharge Cycles (pages 932–935)

      H. Ehrenberg, K. Nikolowski, N. Bramnik, C. Baehtz, T. Buhrmester and T. Gross

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500116

      As-prepared Li(Ni,Co)O2 with good electrochemical performance is an insulator with low degree of cation disorder, i.e. Li and (Ni,Co) are distributed on different and alternating layers. Lithium extraction in the first cycle induces an irreversible first-order phase transition into a metallic phase with a discontinuous change in the c/a ratio by 3.6 % and an accompanied partial occupation of some of the vacant Li-sites by Ni-ions. The specific arrangement of those Ni-ions on Li-layers is proposed as a key feature for the good cycling behaviour of Li(Ni,Co)O2 based cathodes in rechargeable batteries.

    9. Molecular Dynamics Simulations of Gas Phase Condensation of Silicon Carbide Nanoparticles (pages 937–945)

      P. Erhart and K. Albe

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500119

      Gas phase condensation of silicon and silicon carbide nanoparticles is studied by molecular-dynamics simulations. By using a recently developed bond-order potential for Si, C and SiC we investigate the fundamental processes governing nucleation and growth of SiC nanoparticles. For the case of elemental silicon particles we show that variations in the binding energy of dimers, which represent stable nuclei for the condensation process, significantly affect the long time evolution of the cluster formation process. A detailed analysis of the molecular reactions during the early stages of SiC particle growth is presented. Reactions, in which silicon monomers are formed, are dominant in case of stoichiometric composition of the precursor gas. Moreover, we find the formation of carbon-dominated species to be preferred and a sensitive dependence of the particle composition and morphology on the processing conditions, especially the cooling and precursor gas composition.

    10. In-Situ Preparation and Analysis of Functional Oxides (pages 945–949)

      D. Ensling, A. Thißen, Y. Gassenbauer, A. Klein and W. Jaegermann

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500125

      Performance degradation of functional oxide materials and interfaces is – besides structural changes – caused by changes of the chemical composition, the electronic structure and the surface and interface potentials. In-situ preparation and analysis for LiCoO2 thin films and for ITO/ZnPc interfaces are presented in this work. Sample stoichiometries, electronic structure and surface and interface potentials with photoelectron spectroscopy are analysed under well defined conditions.

    11. Continuum Mechanical Approach to Sintering of Nanocrystalline Zirconia (pages 949–952)

      R. Zuo, E. Aulbach and J. Rödel

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500121

      Nanocrystalline 3 mol % yttria-stabilized zirconia was sinter-forged isothermally under varying external uniaxial stresses. The applied uniaxial stresses were relatively low, compared to the intrinsic sintering stress of the material studied. Uniaxial sintering stresses and uniaxial viscosities were experimentally determined as function of density by means of a continuum mechanical approach which involves measuring the sintering rate of a free-sintered specimen, and a specimen sintered under the application of an external uniaxial stress. The uniaxial viscosity increased strongly with density only in the final stage sintering regime. The magnitude of the uniaxial sintering stress exhibited a decrease with density.

    12. Development of an In-Situ Cell for X-ray Absorption Measurements During Fuel Cell Operation (pages 952–956)

      C. Roth, N. Benker, M. Mazurek, F. Scheiba and H. Fuess

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500122

      A novel in-situ cell was designed to allow measurements during real fuel cell operation. Thus, potential and fuel dependent changes in the catalyst structure can be pursued in-situ. The X-ray absorption cell was built first in transmission geometry with an inherent graphite beam window, and spectra were recorded at the Pt L3-edge. This design, however, was not feasible for measurements at the Ru K-edge due to the strong absorption of Pt in the Pt-Ru anode catalyst and the low ruthenium content. Therefore, the cell was modified in a second step using either a Be or a Kapton® foil window to allow for fluorescence measurements. The fluorescence set-up offers the additional advantage that Pt at the anode can be separated from Pt at the cathode side without the need to remove part of the cathode in the beam window region. The optimum set-up for both the Pt L3- and the Ru K-edge was found to be the cell hardware modified with a Kapton® window in fluorescence geometry. Spectra of different supported and unsupported electrocatalysts were recorded at different potentials at both edges in hydrogen, H2/CO and methanol operation. But only measurements at a carbon-supported and an unsupported Pt-Ru catalyst in hydrogen will be presented here. Upon first contact with the fuel, both Pt and Ru (surface) oxides present in the as-received state are reduced. Potential-dependent changes, however, are less evident and can be revealed by data fits only.

    13. Electrochemical Interface Doping in Organic Light Emitting Field Effect Transistors (pages 957–960)

      A. Hepp, H. Heil, R. Schmechel and H. von Seggern

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500118

      Electrochemical interface doping is introduced as a new concept for the realization of organic light-emitting field-effect transistors (O-LEFTs). In a bottom contact tetracene transistor a 2–4 nm poly(ethylene-oxide) layer with lithium triflate was inserted as interface layer between the gate-oxide and the tetracene layer. For the first time a polymeric electrolyte is combined with “small molecule” functional material. The injection of both, holes and electrons allows for radiative recombination in the transistor channel. The onset of light emission occurs at a source-drain-voltage below 10 V and can be controlled by the gate voltage.

    14. Formation, Thermal Stability and Deformation Behavior of High-Strength Cu-Based Bulk Glassy and Nanostructured Alloys (pages 960–965)

      M. Calin and J. Eckert

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500114

      Cu47Ti33Zr11Ni8X1 (X = Fe, Si, Sn) bulk glass-forming alloys were prepared using arc melting and copper mold casting. Thermal analysis reveals an obvious glass transition and large supercooled liquid regions (ΔTx = 50 – 57 K) for all alloys. The bulk glassy alloys possess good mechanical properties, including high yield stress (1818 – 2102 MPa) and fracture (1943 – 2186 MPa) strengths, Young's moduli of 67 – 109 GPa and large elastic strains of about 2.0 – 2.7 %. Only the Si-containing alloy exhibits distinct strain hardening and plasticity (fracture strain of 4.4. %). This is due to a unique nanocomposite microstructure consisting of a uniform distribution of Cu-rich nanoparticles embedded in the bulk amorphous matrix. The nanocomposite microstructure facilitates the formation of multiple shear bands that make the bulk glassy alloy resistant to crack propagation.

    15. High Melting Pb-Free Solder Alloys for Die-Attach Applications (pages 965–969)

      M. Rettenmayr, P. Lambracht, B. Kempf and M. Graff

      Version of Record online: 3 NOV 2005 | DOI: 10.1002/adem.200500124

      Bi-Ag alloys with Ag contents of up to 12 wt.% Ag are potential substitutes for Pb-containing alloys in the high temperature range (T > 260 °C) of soft solders. Bi solders were produced in form of wires and can therefore be used in the same kind of soldering equipment as conventional Pb-rich solder wires. Bi-Ag alloys were characterized in terms of melting range and microstructure, mechanical properties (yield strength and elongation to fracture), wetting behavior, thermal fatigue behavior and thermal conductivity. The thermal conductivity is lower than that of the conventional Pb-containing solder alloys, but all other properties tested are comparable or superior to those of Pb-rich conventional solder alloys. It thus appears to be possible to use Bi-Ag alloys as ‘drop-in-replacement' for Pb-rich alloys.