Electronic, magnetic and thermodynamic properties of magnetic shape memory alloys from first principles

Der magnetische Formgedächtnis-Effekt ist eine funktionale Eigenschaft die von wissenschaftlicher, aber auch industrieller Bedeutung ist. In magnetischen Formgedächtnis-Legierungen lassen sich durch das Anlegen moderater magnetischer Felder makroskopisch messbare Verzerrungen erzielen. Die Physik magnetischer Formgedächtnis-Materialien wird durch die Wechselwirkung magnetischer und struktureller Phasen bestimmt, da die Materialien sowohl einen ferromagnetischen als auch einen martensitischen Phasenübergang zeigen. Die Heusler-Legierung Ni2MnGa ist ein vielversprechendes magnetisches Formgedächtnis-Material, dass allerdings eine für industrielle Anwendungen zu niedrige martensitische Übergangstemperatur aufweist und vergleichsweise spröde ist. In der vorliegenden Arbeit wurden die bisherigen in der Literatur erwähnten Untersuchungen an Ni2MnGa durch Ab Initio Untersuchungen zum Einfluss der Beiträge von Gitterschwingungen, magnetischen Anregungen, elektronischen Korrelationen und Fermiflächennesting auf den Martensit-Übergang erweitert. Es wird gezeigt, dass der martensitische Phasenübergang von mehreren Beiträgen getrieben wird. In einem nächsten Schritt wurden Mangan-reiche Zusammensetzungen der Legierungssysteme Ni-Mn-Z (Z = Ga, In, Sn, Sb) untersucht, die hohe martensitische Übergangstemperaturen aufweisen. Es zeigt sich, dass ein direkter Ansatz, der die Energiedifferenzen zwischen Austenit und Martensit, die in den Dichtefunktionalrechnungen auftreten, mit der Übergangstemperatur identifiziert zu qualitativen und teilweise sogar quantitativen Übereinstimmungen mit experimentellen Ergebnissen führt. Der Ansatz, der nicht-stöchiometrische Zusammensetzungen über Superzellen berücksichtigt, ermöglicht die Berechnung vollständiger Phasendiagramme mittels Ab Initio Methoden. Die dazu erforderliche Rechenleistung steht in einem angemessenen Verhältnis zu den erzielbaren Resultaten. In einem weiteren Schritt wurden die vorgestellten Methoden auf die Legierungssysteme Ni-Pt-Mn-Z (Z = Ga, Sn) angewandt, die noch nicht ausführlich in der Literatur behandelt wurden. Die partielle Substitution von Nickel durch Platin ist von besonderem Interesse, da diese die Valenzelektronenkonzentration nicht verändert. Es zeigt sich, dass durch das Hinzufügen von Platin höhere Übergangstemperaturen erwartet werden. Kürzlich veröffentlichte experimentelle Arbeiten bestätigen die vielversprechenden Eigenschaften dieser Legierungssysteme. Eine weitere magnetische Formgedächtnis-Legierung die im Rahmen dieser Arbeit untersucht wurde ist Co-Ni-Ga. Dieses System zeichnet sich durch die Abwesenheit weicher Phononen und antiferromagnetischer Wechselwirkungen aus, die in Ni-Mn-Ga beobachtet werden. Die Tendenz, dass die erwarteten martensitischen Übergangstemperaturen bei Erhöhung der Valenzelektronenkonzentration ansteigen, lässt sich hingegen auch in Co-Ni-Ga beobachten.The magnetic shape memory effect (MSME) is a functional property which is of outmost interest from a scientific point of view but also regarding industrial applications. Magnetic shape memory alloys (MSMA) allow to obtain large magnetic field induced strains (MFIS) in moderate magnetic fields and hence qualify for actuator and sensor devices. The underlying physics of MSMAs is governed by an interplay between different structural and magnetic phases as a ferromagnetic and a martensitic transition are required for the appearance of the MSME. Several alloys are known to show the MSME. From these Ni$_{2}$MnGa is promising with regard to industrial applications but still shows too low operating temperatures and is rather brittle. In this thesis the comprehensive investigation of Ni2MnGa which was undertaken by a lot of scientific groups in the past is expanded by an investigation of the different contributions to the martensitic transformation (MT) using first-principles calculations. In particular the influence of lattice vibrations, magnetic excitations, correlation effects and Fermi surface nesting on the MT are investigated. It is shown that the structural phase transition that appears in Ni2MnGa is driven by several martensitic driving forces. In a further step the Mn-rich compositions of the alloys systems Ni-Mn-Z (Z = Ga, In, Sn, Sb) have been investigated. The Mn-rich compositions are of particular interest as they show increased MT temperatures. It turns out that a simple approach which relates the transformation temperature to the energy difference between the austenite and martensite structure which appears in first-principles calculations yields good qualitative and in some cases even quantitative agreement with experimental results. The approach which models off-stoichiometric compositions by using supercells makes it possible to calculate the transformation temperatures of off-stoichiometric compositions within feasible computational demand. This gives rise to the calculation of complete phase diagrams by ab initio methods. In the next step, the established methods which have been applied to phase diagrams which are already known experimentally have been applied to the alloy systems Ni-Pt-Mn-Z (Z = Ga, Sn) which has not been discussed extensively in the literature before. The (partial) substitution of Ni by Pt is interesting as it is an isoelectronic substitution which sustains the valence electron concentration e/a. It turns out that higher martensitic transformation temperatures are expected when adding platinum to the alloys. The promising properties of such alloys have recently been confirmed experimentally. Another magnetic shape memory alloy which has been studied within this thesis is Co-Ni-Ga. This system is of special interest as the phonon softening that is present in Ni2MnGa as well as antiferromagnetic interactions which are found in Mn-rich compositions of the latter alloy are absent in Co-Ni-Ga. Nevertheless, the basic trend that the martensitic transformation temperature increases with e/a is also observed in this alloy system

Building a Continuing Education Program for Embedded Systems with Labs and Distance Support

This presentation was part of the session : Technology for the Continuing Engineering EnterpriseIACEE 11th World Conference on Continuing Engineering EducationIn spring 2000 the University of Colorado Electrical Engineering department and CAETE (Center for Advanced Engineering and Technology Education) launched an Embedded Systems Certificate program with two new courses taught by two adjunct professors from industry. Since this time the program has become self funding, has added three additional courses, and has built out a shared lab for embedded systems using funding from the CU Engineering Excellence fund and grants from local industry. This paper reports on how the successful certificate program was built and innovative new approaches being investigated for distance lab support in a fifth class added this spring 2007, Real Time Digital Media and Control Systems. The approach taken and lessons learned at CU are reported to assist other institutions designing similar programs.Distance Learning and Professional Education ; International Association for Continuing Engineering Educatio

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.Comment: 15 pages, 12 figure

Familie - Generation - Institution. Generationenkonzepte in der Vormoderne

Dieser Band versammelt Beiträge zu Generationenbeziehungen und Generationenkonzepten in der Vormoderne, die auf eine Tagung des DFG-Graduiertenkollegs 'Generationenbewusstsein und Generationenkonflikte in Antike und Mittelalter' in Bamberg zurückgehen. Die behandelten Untersuchungsgegenstände reichen von den antiken Diadochenreichen über die ottonische Königsfamilie des 10. und 11. Jahrhunderts bis zum frühneuzeitlichen Landadel Westfalens. Dabei werden historische, literaturwissenschaftliche und soziologische Fragestellungen aufgegriffen, um den Erkenntniswert des Konzepts 'Generation' interdisziplinär zu diskutieren.This volume brings together contributions on generational relations and concepts of generation in Early Modernity given on the occasion of a conference organized at Bamberg by the Research Training Group of the German Research Foundation (DFG) 'Generational Awareness and Generational Conflicts in Antiquity and the Middle Ages'. The topics ranged from the ancient Diadoch kingdoms to the history of the Ottonian royal family of the 10th and 11th centuries and the early modern landed gentry in Westfalia. Addressing questions from historical, literary and social studies the validity of the concept of 'generation' was discussed among scholars and students from a variety of disciplines

N-type organic thermoelectrics : demonstration of ZT > 0.3

The 'phonon-glass electron-crystal' concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly 'phonon glasses'. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being 'electron crystals'. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic 'PGEC' thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10Scm(-1) and an ultralow thermal conductivity of 0.3 for organic thermoelectrics

Expression of chemokine receptor CXCR4 in esophageal squamous cell and adenocarcinoma

BACKGROUND: Prognosis of esophageal cancer is poor despite curative surgery. The chemokine receptor CXCR4 has been proposed to distinctly contribute to tumor growth, dissemination and local immune escape in a limited number of malignancies. The aim of our study was to evaluate the role of CXCR4 in tumor spread of esophageal cancer with a differentiated view of the two predominant histologic types – squamous cell and adenocarcinoma. METHODS: Esophageal cancer tissue samples were obtained from 102 consecutive patients undergoing esophageal resection for cancer with curative intent. The LSAB+ System was used to detect the protein CXCR4. Tumor samples were classified into two groups based on the homogeneous staining intensity. A cut-off between CXCR4w (= weak expression) and CXCR4s (= strong expression) was set at 1.5 (grouped 0 – 1.5 versus 2.0 – 3). Long-term survival rates were calculated using life tables and the Kaplan-Meier method. Using the Cox's proportional hazards analysis, a model of survival prediction was established. RESULTS: The overall expression rate for CXCR4 in esophageal squamous cell carcinoma was 94.1%. Subdividing these samples, CXCR4w was found in 54.9% and CXCR4s in 45.1%. In adenocarcinoma, an overall expression rate of 89.1% was detected with a weak intensitiy in 71.7% compared to strong staining in 29.3% (p = 0.066 squamous cell versus adenocarcinoma). The Cox's proportional hazards analysis identified the pM-category with a hazard ratio (HR) of 1.860 (95% CI: 1.014–3.414) (p = 0.045), the histologic tumor type (HR: 0.334; 95% CI: 0.180–0.618) (p = 0.0001) and the operative approach (transthoracic > transhiatal esophageal resection) (HR: 0.546; 95% CI: 0.324–0.920) (p = 0.023) as independent factors with a possible influence on the long-term prognosis in patients with esophageal carcinoma, whereas CXCR4 expression was statistically not significant (>0.05). CONCLUSION: Expression of the chemokine receptor CXCR4 in esophageal cancer is of major relevance in both histologic entities – squamous cell and adenocarcinoma. Though with lack of statistical significance, strong CXCR4 expression revealed a poorer long-term prognosis following curative esophagectomy in both histologic subtypes. Thus, the exact biological functions of CXCR4 in terms of tumor dissemination of esophageal cancer is yet undetermined. Inhibition of esophageal cancer progression by CXCR4 antagonists might be a promising therapeutic option in the future

Повышение эффективности проведения геолого-технических мероприятий на месторождениях Западной Сибири

Цель работы - анализ проведенных геолого-технических мероприятий, предложение нового инновационного подхода к повышению эффективности проведения ГТМ на месторождениях Западной-Сибири. В процессе исследования проводился анализ имеющихся данных, на основе которых были выделение наиболее эффективные геолого-технические мероприятия, проводимые на месторождениях Западной Сибири.The purpose of the work is to analyze the geological and technical measures carried out, to propose a new innovative approach to improving the efficiency of GTM in the fields of Western Siberia. In the course of the study, the available data were analyzed, on the basis of which the most effective geological and technical measures carried out in the fields of Western Siberia were identified

CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump–probe experiments at the FLASH free-electron laser

The non-monochromatic beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics, and a new permanent end-station, CAMP, was installed. This multi-purpose instrument is optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments at free-electron lasers. It can be equipped with various electron- and ion-spectrometers, along with large-area single-photon-counting pnCCD X-ray detectors, thus enabling a wide range of experiments from atomic, molecular, and cluster physics to material and energy science, chemistry and biology. Here, an overview of the layout, the beam transport and focusing capabilities, and the experimental possibilities of this new end-station are presented, as well as results from its commissioning

自称表現に見られる日中両言語の人称代名詞と親族語彙

千葉大学社会文化科学研究科研究プロジェクト報告書第84集『日中両言語における代名詞及び親族語彙の対照研究-琉球方言との比較研究も含めて』所