Influence of Ti/TiAlN-multilayer designs on their residual stresses and mechanical properties

In this research work, Ti/TiAlN multilayers of various designs were deposited onto substrates pretreated by different etching procedures. The influence of multilayer design and substrate pretreatment on multilayers adhesion, hardness, wear and friction coefficients was systematically analyzed and correlated with residual stresses of these multilayers as well as with residual stresses on the coating-near substrate region, which were analyzed by synchrotron X-ray diffraction at HZB-BESSYII. These investigations show that the adhesion can be improved by a specific etching procedure, which cause increased compressive stress in the coating-near the substrate region. Additionally, it was found, that the multilayer with the thickest ceramic layers has the highest hardness and the lowest wear coefficients as well as the lowest compressive residual stress within studied multilayers.     

Graphene as a reversible spin manipulator of molecular magnets

One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S=1→S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.     

Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles

First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field.     

Surface Aggregation of Candida albicans on Glass in the Absence and Presence of Adhering Streptococcus gordonii in a Parallel-Plate Flow Chamber: A Surface Thermodynamical Analysis Based on Acid-Base Interactions

Adhesive interactions between yeasts and bacteria are important in the maintenance of infectious mixed biofilms on natural and biomaterial surfaces in the human body. In this study, the extended DLVO (Derjaguin-Landau-Verwey-Overbeek) approach has been applied to explain adhesive interactions between C. albicans ATCC 10261 and S. gordonii NCTC 7869 adhering on glass. Contact angles with different liquids and the zeta potentials of both the yeasts and bacteria were determined and their adhesive interactions were measured in a parallel-plate flow chamber.Streptococci were first allowed to adhere to the bottom glass plate of the flow chamber to different seeding densities, and subsequently deposition of yeasts was monitored with an image analysis system, yielding the degree of initial surface aggregation of the adhering yeasts and their spatial arrangement in a stationary end point. Irrespective of growth temperature, the yeast cells appeared uncharged in TNMC buffer, but yeasts grown at 37 degrees C were intrinsically more hydrophilic and had an increased electron-donating character than cells grown at 30 degrees C. All yeasts showed surface aggregation due to attractive Lifshitz-van der Waals forces. In addition, acid-base interactions between yeasts, yeasts and the glass substratum, and yeasts and the streptococci were attractive for yeasts grown at 30 degrees C, but yeasts grown at 37 degrees C only had favorable acid-base interactions with the bacteria, explaining the positive relationship between the surface coverage of the glass by streptococci and the surface aggregation of the yeasts. Copyright 1999 Academic Press.     

High-spin structure of yrast-band in 78Kr

Lifetime of levels up to 22⁺, have been measured in ⁷⁸Kr and an oblate shape is assigned to the ground state using the CSM and the configuration dependent shell correction calculations. Calculations further show that ⁷⁸Kr is highly γ-soft nucleus. The experimental Q _t values coupled with theoretical calculations indicate an oblate shape for ⁷⁸Kr at low spins and triaxial shape at higher spins     

Ab Initio Study of Iron and Cr/Fe(001)

In the present paper interfacial mixing of a thin chromium overlayer on bcc iron is studied. The calculations are performed in the framework of a pseudo-potential technique using the generalized gradient approximation for the exchange-correlation functional. Although Fe and Cr do not alloy in the bulk system at low temperatures, strong intermixing effects have been observed with Auger spectroscopy, if Cr is epitaxially grown on bcc Fe(001). Besides these structural effects we discuss the magnetic structure of the interface. It can be shown that the results are in good agreement with the experimental findings of Pfandzelter et al. ( Phys. Rev. B , 54 , 4496, 1996), provided we allow for lattice relaxation. Additionally, the structural and magnetic properties of iron investigated by the pseudo-potential method are compared to our former full potential results.     

A first-principles study aided with Monte Carlo simulations of carbon doped iron-manganese alloys

We have investigated the complex magnetic properties of Fe_1?x Mn _x C _y alloys by using an iterative combination of ab initio calculations and Monte Carlo simulations. The latter gives insight into finite temperature magnetism and allows to determine the critical temperature of magnetic phase transitions. We restrict the investigation to ordered systems with 25, 50 and 75% manganese and study the influence of carbon at octahedral interstitial sites on the magnetic properties. The combination of ab initio calculations with Monte Carlo simulations turns out to be a powerful tool to determine the complex magnetic structures, which originate from the competition of ferro- and antiferromagnetic interactions in the FeMn alloys.     

Structure of 72,74Se at high spin

Lifetimes of high spin states up to { }=22⁺ in the yrast positive parity bands have been measured to investigate the shape evolution with increasing spin in ^{72, 74}Se. The Q _t values derived from these measurements indicate that prolate shape stabilizes for ⁷²Se, while a triaxial shape develops for ⁷⁴Se at higher spins. Comparison of the observed trend in Q _t with spin for ^{72, 74}Se with that of the corresponding kryptones isotones emphasizes the stability provided by N=38 prolate shell gap even at high rotational frequency.