Structure and Giant Inverse Magnetocaloric Effect of Epitaxial Ni-Co-Mn-Al Films

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be 14M. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent X-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that Ni$_{41}$Co$_{10.4}$Mn$_{34.8}$Al$_{13.8}$ films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5\,J\,kg$^{-1}$K$^{-1}$ for $\mu_0 \Delta H=5\,\text{T}$.Comment: 8 pages, 8 figure

Electrochemical Properties of LaNi5–xGex Alloys in Ni-MH Batteries

Electrochemical studies were performed on LaNi5–xGex metal hydride alloys with 0 <= x <= 0.5. We carried out single-electrode studies to understand the effects of the Ge substituent on the hydrogen absorption characteristics, the electrochemical capacity, and the electrochemical kinetics of hydrogen absorption and desorption. The electrochemical characteristics of the Ge-substituted alloys are compared to those of the Sn-substituted alloys reported earlier. LaNi5–xGex alloys show compositional trends similar to LaNi5–xSnx alloys, but unlike the Sn-substituted alloys, Ge-substituted alloys continue to exhibit facile kinetics for hydrogen absorption/desorption at high solute concentrations. Cycle lives of LaNi5–xGex electrodes were measured in 300 mAh laboratory test cells and were found to be superior to the Sn-substituted LaNi5 and comparable to a Mm(Ni,Co,Mn,Al)5 alloy. The optimum Ge content for LaNi5–xGex metal hydride alloys in alkaline rechargeable cells is in the range 0.4 <= x <= 0.5

Role of disorder when upscaling magnetocaloric Ni-Co-Mn-Al Heusler alloys from thin films to ribbons

Research in functional magnetic materials often employs thin films as model systems for finding new chemical compositions with promising properties. However, the scale-up of thin films towards bulk-like structures is challenging, since the material synthesis conditions are entirely different for thin films and e.g. rapid quenching methods. As one of the consequences, the type and degree of order in thin films and melt-spun ribbons are usually different, leading to different magnetic properties. In this work, using the example of magnetocaloric Ni-Co-Mn-Al melt-spun ribbons and thin films, we show that the excellent functional properties of the films can be reproduced also in ribbons, if an appropriate heat treatment is applied, that installs the right degree of order in the ribbons. We show that some chemical disorder is needed to get a pronounced and sharp martensitic transition. Increasing the order with annealing improves the magnetic properties only up to a point where selected types of disorder survive, which in turn compromise the magnetic properties. These findings allow us to understand the impact of the type and degree of disorder on the functional properties, paving the way for a faster transfer of combinatorial thin film research towards bulk-like materials for magnetic Heusler alloys

Carbon nanotubes growth from camphoric carbon sources using transition metal catalysts / Azira Abd Aziz

This thesis covers the analysis of catalytic growth of carbon nanotubes (CNTs) under well-defined conditions, the optimization of the catalyst and introduces model for the growth mechanism based on the experimental results. Experimental investigations are presented to obtain a comprehensive picture on the catalytic growth of CNTs. The overall aim of this thesis is to deposit CNTs by the seeded catalyst method and the modified fluidized floating catalyst method by Chemical Vapour Deposition (CVD) and to investigate the effects of starting material and catalysts on the morphology and structure of the deposited CNTs. Camphor (C10H16O, crystalline state) and camphor oil (liquid state) are the precursor materials used as the source of CNTs. Transition metal (Fe, Ni, Co, Mn, Al, Mg) catalysts were prepared and the effect on their catalytic behavior were studied. Metal catalysts have been prepared by sol-gel method with or without support catalyst. Correlation between the catalyst particle size and CNT diameter has been the motivation to reduce the catalyst particle size down to nanoparticle size

Lithium containing layered high entropy oxide structures

Layered Delafossite-type Lix(M$_{1}$M$_{2}$M$_{3}$M$_{4}$M$_{5}$…M$_{n}$)O$_{2}$ materials, a new class of high-entropy oxides, were synthesized by nebulized spray pyrolysis and subsequent high-temperature annealing. Various metal species (M = Ni, Co, Mn, Al, Fe, Zn, Cr, Ti, Zr, Cu) could be incorporated into this structure type, and in most cases, single-phase oxides were obtained. Delafossite structures are well known and the related materials are used in different fields of application, especially in electrochemical energy storage (e.g., LiNi$_{x}$Co$_{y}$Mn$_{z}$O$_{2}$ [NCM]). The transfer of the high-entropy concept to this type of materials and the successful structural replication enabled the preparation of novel compounds with unprecedented properties. Here, we report on the characterization of a series of Delafossite-type high-entropy oxides by means of TEM, SEM, XPS, ICP-OES, Mössbauer spectroscopy, XRD including Rietveld refinement analysis, SAED and STEM mapping and discuss about the role of entropy stabilization. Our experimental data indicate the formation of uniform solid-solution structures with some Li/M mixing

Shape memory Heusler alloys for thin film applications

Teichert N. Shape memory Heusler alloys for thin film applications. Bielefeld: Universität Bielefeld; 2016.This thesis addresses the examination of shape memory Heusler alloy thin films for applications in spintronics and magnetocalorics. In the first experimental chapter, we investigate the potential of Ni-Mn-Sn films as pinning layers in magnetic tunnel junctions and the second chapter we elucidate the potential of Ni-Co-Mn-Al films for magnetic refrigeration. The underlying physical phenomenon for the first project is an intrinsic exchange bias effect (EB) caused by a cluster spin-glass state at low temperature, whereas for the second project the giant inverse magnetocaloric effect corresponding to the magnetostructural martensitic phase transformation is decisive. We integrated a Ni52Mn34Sn14 Heusler compound film on into an MgO (substrate)/Ni-Mn-Sn/CoFeB/MgO/CoFeB magnetic tunnel junction and have shown that the intrinsic exchange bias causes a shift on the switching field of the magnetic electrode. For the study of magnetocaloric Ni-Co-Mn-Al films we fabricated a series of films with different composition in order to obtain a set of different transformation temperatures. With this we compared the structural, magnetic and magnetocaloric properties of substrate constrained and freestanding films. The structural examination reveals an adaptive 14M martensite which coarsens into mesoscopic variants of tetragonal ‘NM’ martensite for substrate constrained films. In contrast to that, freestanding films exhibit only NM martensite with the peculiarity that the c-axis is exclusively in-plane oriented. Therefore, the martensite is not self-accommodating and a large misfit between the austenite and martensite film area is present. To compensate for this the film bulges out and as a consequence the martensitic film shows high waviness. Magnetocaloric measurements were conducted on one substrate constrained film (Ni40Co9.3Mn32.9Al17.8) and one freestanding film (Ni39.4Co9.2Mn32.3Al19.1) where different compositions were chosen because the martensitic transformation shifts to higher temperatures in freestanding films. Giant inverse magnetocaloric effects were found with up to DeltaS_maxwell=7.3 J/(kg K) for both substrate constrained and freestanding films. The entropy difference between austenite and martensite DeltaS_cc was found to decrease with increasing magnetic field which leads to saturation of the field induced entropy change and increasing field dependence of the transformation temperatures (dT_M/A/ dH) at high field. DeltaS_cc decreases with decreasing temperature which causes kinetic arrest and suppression of the martensitic transformation in films with low expected transformation temperature. The most striking limitations that should be addressed in future work are the temperature dependence of the intrinsic EB in Ni-Mn-Sn and the structural hysteresis in Ni-Co-Mn-Al

Poisoning-tolerant metal hydride materials and their application for hydrogen separation from CO2/CO containing gas mixtures

Metal hydride materials offer attractive solutions in addressing problems associated with hydrogen separation and purification from waste flue gases. However, a challenging problem is the deterioration of hydrogen charging performances resulting from the surface chemical action of electrophilic gases. In this work, the feasibility study of poisoning tolerance of surface modified AB5-type hydride forming materials and their application for hydrogen separation from process gases containing carbon dioxide and monoxide was carried out. Target composition of La(Ni,Co,Mn,Al)5 substrate was chosen to provide maximum reversible hydrogen capacity at the process conditions. The selected substrate alloy has been shown to be effectively surface-modified by fluorination followed by electroless deposition of palladium. The surface-modified material exhibited good coating quality, high cycle stability and minimal deterioration of kinetics of selective hydrogen absorption at room temperature, from gas mixtures containing 10% CO2 and up to 100 ppm CO. The experimental prototype of a hydrogen separation unit, based on the surface-modified metal hydride material, was tested and exhibited stable hydrogen separation and purification performances when exposed to feedstocks containing concentrations of CO2Web of Scienc

Electrochemical Studies on LaNi5–xSnx Metal Hydride Alloys

Electrochemical studies were performed on LaNi5–xSnx with 0 <= x <= 0.5. We measured the effect of the Sn substituent on the kinetics of charge-transfer and diffusion during hydrogen absorption and desorption, and the cyclic lifetimes of LaNi5–-xSnx electrodes in 250 mAh laboratory test cells. We report beneficial effects of making small substitutions of Sn for Ni in LaNi5 on the performance of the metal hydride alloy anode in terms of cyclic lifetime, capacity, and kinetics. The optimal concentration of Sn in LaNi5–xSnx alloys for negative electrodes in alkaline rechargeable secondary cells was found to lie in the range 0.25 <= x <= 0.3