Schwellwert für Ermüdungsrißausbreitung: Bestimmungsmethoden,Kennwerte, Einflußgrößen

Threshold for Fatigue Crack Propagation: Experimental Procedure, Characterizing and Influencing Parameters For the determination of a fatigue crack propagation threshold there exists no “Standard” or other generally accepted experimental procedure. The threshold behaviour of “long” cracks can be characterized by threshold ΔK_th determination as a function of R as well as a function of K_max. The experimental determination of ΔK_th with constant K_max is simpler and a plot of the results versus K_max represents the threshold behaviour of a material more accurate. The whole threshold behaviour is characterized by three parameters. These three parameters can be obtained with relatively little experimental expenditure. The influence of certain conditions, i. e. material, microstructure, environment, ect., on the threshold ΔK_th are discussed. It seems that for a particular material and environment only two of these parameters can be considered as material properties. Test results show that with large change of amplitude ΔK fatigue cracks can grow still at ΔK smaller than ΔK_th. But after some growth, fatigue crack propagation ceases.     

Brannerite,UTi<Subscript>2</Subscript>O<Subscript>6</Subscript>: Crystal chemistry,synthesis, properties,and use for actinide waste immobilization

The host materials suggested for immobilization of actinide waste of military or civil origin often contain the secondary (U,Pu)Ti2O6 phase of brannerite structure. For example, the materials for incorporation of excess plutonium, mainly consisting of pyrochlore, contain up to 30% brannerite. This is a usual phase in titanate host materials for isolating spent nuclear fuel (SNF) and products of its reprocessing, including waste from production of ^99mТс for medical purposes and other kinds of waste with high U and Pu content. Despite simple ideal stoichiometry, brannerite can contain large amounts of rare earths. This feature is due to the presence of uranium not only in the 4+ oxidation state, but also in the 5+ and 6+ states, which favors the exchange of rare earth elements (REE), e.g., in accordance with the scheme 2U⁴⁺ ? U⁵⁺ + REE³⁺. The REE amount in brannerite reaches 0.5–0.7 atom per formula unit. Therefore, brannerite is of interest as a host material for the rare earth–actinide fraction of high-level waste (HLW). To evaluate the prospects for such use of brannerite, data on the radiation resistance of brannerite and its behavior in aqueous solutions are analyzed. In these properties, brannerite is inferior to pyrochlore and zirconolite. The rate of actinide leaching from brannerite is higher by an order of magnitude than from these phases, but lower by 3–4 orders of magnitude than from glass host materials. Natural brannerite is stable in media with weakly alkaline and reducing waters. Therefore, brannerite seems suitable for immobilization of rare earth–actinide waste. This host material can be synthesized by sintering or cold crucible induction melting followed by crystallization.     

Photoluminescence of ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> doped with Mn,Yb, Sm,and Tb

ZnGa₂O₄ samples doped with Mn, Yb, Sm, and Tb have been prepared by a ceramic processing technique, and their 20°C photoluminescence spectra were measured. The results demonstrate that the four dopants have a significant effect on the photoluminescence of ZnGa₂O₄. Doping with Mn and Yb allows the peaks in the red spectral region to remain narrow and increases the PL intensity.     

LiNbO<Subscript>3</Subscript> films: Potential application,synthesis techniques,structure, properties

The main directions of application and synthesis of lithium niobate (LiNbO₃) and the advantages and drawbacks of the synthesis techniques are considered. The results of structural and morphological characteristics appropriate for use in the creation of devices are shown. The tasks for successful implementation of LiNbO₃ films into novel devices are formulated.     

Antiferromagnetic,Neutral, and Superconducting Band in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The symmetry of the Bloch functions in the conduction band of tetragonal and orthorhombic La₂CuO₄ is examined for the existence of symmetry-adapted and optimally localizable (usual or spin-dependent) Wannier functions. It turns out that such Wannier functions do not exist in the tetragonal phase. In the orthorhombic phase, on the other hand, the Bloch functions can be unitarily transformed in three different ways into optimally localizable Wannier functions: they can be chosen to be adapted to each of the three phases observed in the pure or doped material, that is, to the antiferromagnetic phase, to the superconducting phase or to the phase evincing neither magnetism nor superconductivity. This group-theoretical result is proposed to be interpreted within a nonadiabatic extension of the Heisenberg model. Within this model, atomic-like states represented by these Wannier functions are responsible for the stability of each of the three phases. However, all the three atomic-like states cannot exist in the tetragonal phase, but are stabilized by the orthorhombic distortion of the crystal. A simple model is proposed which may explain the physical properties of La_2−x Sr _x CuO₄ as a function of the Sr concentrationx.     

Thermoelectric properties of transition metals-doped Ca<Subscript>3</Subscript>Co<Subscript>3.8</Subscript>M<Subscript>0.2</Subscript>O<Subscript>9+δ</Subscript> (M = Co,Cr, Fe,Ni, Cu and Zn)

The thermoelectric properties of the Ca₃Co₄O_9+δ and the transition metals-doped Ca₃Co_3.8M_0.2O_9+δ (where M = Cr, Fe, Ni, Cu and Zn) ceramics were reported. Ca₃Co₄O_9+δ single phase was checked by using X-ray diffraction analysis performed for the Ca₃Co_3.8M_0.2O_9+δ samples. The scanning electron micrographs showed some degrees of grains alignment in the compacted direction. The resistivity of the samples measured from 100 up to 700 °C varies in magnitude for different transition metals substitution. The variation of resistivity was explained by a change of carrier concentration induced by the doped ions. The thermopower increased with increasing temperature but showed no obvious change for any transition metals doping. The thermal conductivities changed for the doped samples but were relatively independent of temperature. The ZT was calculated to be the highest for the Fe substitution for the whole measurement temperature with the maximum value of 0.12 at 700 °C.