Lumped Element Model for Thermomagnetic Generators Based on Magnetic SMA Films

This paper presents a lumped element model (LEM) to describe the coupled dynamic properties of thermomagnetic generators (TMGs) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a freely movable cantilever, caused by a large abrupt temperature-dependent change of magnetization and rapid heat transfer inherent to the MSMA films. The LEM is validated for the case of a Ni-Mn-Ga film with Curie temperature TC of 375 K. For a heat source temperature of 443 K, the maximum power generated is 3.1 µW corresponding to a power density with respect to the active material’s volume of 80 mW/cm3. Corresponding LEM simulations allow for a detailed study of the time-resolved temperature change of the MSMA film, the change of magnetic field at the position of the film and of the corresponding film magnetization. Resonant self-actuation is observed at 114 Hz, while rapid temperature changes of about 10 K occur within 1 ms during mechanical contact between heat source and Ni-Mn-Ga film. The LEM is used to estimate the effect of decreasing TC on the lower limit of heat source temperature in order to predict possible routes towards waste heat recovery near room temperature

Large superplastic strain in non-modulated epitaxial Ni-Mn-Ga films

The phase transformation and superplastic characteristics of free-standing epitaxial Ni-Mn-Ga stripes are reported. The stripes are prepared by micromachining a 1 μm thick Ni-Mn-Ga film sputter-deposited on a single crystalline MgO (100) substrate using optical lithography and a Chromium-based sacrificial layer technology. The stripes are oriented at angles of 0 and 45 degrees with respect to the Ni-Mn-Ga unit cell. Electrical resistance versus temperature characteristics reveal a reversible thermally induced phase transformation between 169°C and 191°C. Stress-strain measurements are performed with the stress applied along the [100]Ni-Mn-Ga as well as [110]Ni-Mn-Ga direction. Depending on the orientation, the twinning stress ranges between 25 and 30 MPa, respectively. For the [100] Ni-Mn-Ga and [110]Ni-Mn-Ga directions, superplastic behaviour with a strain plateau of 12 % and 4% are observed, respectively, indicating stress-induced reorientation of non-modulated martensite variants

Magnetic domain structure of epitaxial Ni-Mn-Ga films

For the magnetic shape memory effect, knowledge about the interaction between martensitic and magnetic domain structure is essential. In the case of Ni-Mn-Ga bulk material and foils, a staircase-like magnetic domain structure with 90{\deg}- and 180{\deg}-domain walls is known for modulated martensite. In the present paper we show that the magnetic domain pattern of thin epitaxial films is fundamentally different. Here we analyze epitaxial Ni-Mn-Ga films by atomic and magnetic force microscopy to investigate the correlation between the twinned martensitic variants and the magnetic stripe domains. The observed band-like domains with partially perpendicular outof-plane magnetization run perpendicular to the microstructure domains defined by twinning variants. These features can be explained by the finite film thickness, resulting in an equilibrium twinning period much smaller than the domain period. This does not allow the formation of a staircase domain patter. Instead the energies of the magnetic and martensitic microstructures are minimized independently by aligning both patterns perpendicularly to each other. By analyzing a thickness series we can show that the observed magnetic domain pattern can be quantitatively described by an adapted band domain model of Kittel.Comment: 12 pages, 4 figure

Development of Microactuators Based on the Magnetic Shape Memory Effect

The giant magneto-strain effect in Ni-Mn-Ga alloys is particularly attractive for actuator applications. Two different approaches are being pursued to develop MSM microactuators. To observe large deflections of Ni-Mn-Ga microactuators, the material should be exhibiting low twinning stress and large magnetic anisotropy. In addition, design rules and boundary conditions for operating the Ni-Mn-Ga actuator material are having significant importance for evolution of performance characteristics

Power Generation by Resonant Self-Actuation

Die Forschung im Bereich der Mikro-Energiegewinnung wurde durch den Bedarf an au-tarken sowie stabilen Energiequellen für vernetzte und drahtlose Sensoren vorangetrieben. Abwärme, insbesondere bei Temperaturen unter 200 °C, stellt eine vielversprechende, aber mit den derzeitigen Umwandlungstechnologien schwer zu gewinnende Energiequelle dar. Der Fortschritt von thermomagnetischen Generatoren (TMGs) mit hoher Leistung wurde durch den Mangel an Weiterentwicklungen von thermomagnetischen Materialien behindert. Diese Arbeit stützt sich auf frühere Forschungsarbeiten zu TMGs im kleinen Maßstab. Die Hauptziele sind: • Entwicklung eines LEM-Modells (Lumped Element Model) zur Simulation des TMG, um die Leistung zu analysieren und zu optimieren. • Nutzung von LEM und Experimenten, um die Auswirkungen verschiedener De-signparameter zu verstehen. • Die Hochskalierung des Volumens des aktiven Materials eines TMG, um die absolute Ausgangsleistung eines einzelnen Generators zu erhöhen. • Die Hochskalierung des TMG durch Parallelbetrieb mehrerer TMGs zur Vergrößerung der lateralen Größe. • Erweiterung des Betriebsbereichs der Wärmequelle auf Temperaturen nahe der Raumtemperatur, ohne die resonante Selbstaktivierung zu verlieren. Zunächst werden mittels experimenteller Messungen und LEM-Simulationen TMGs, die auf verschiedenen Materialien wie dem Ni-Mn-Ga Heusler-Legierungsfilm, Gadolinium und La-Fe-Si-H basieren, grundsätzlich erforscht. Die Auswirkung verschiedener Designparameter auf die Leistung des TMGs wird untersucht. Dabei beschreiben LEM-Simulationen die gekoppelten dynamischen Eigenschaften von TMGs, die Filme aus magnetischen Formgedächtnislegierungen (MSMA) verwenden. Die TMG nutzen Selbstaktivierung, indem ein temperaturabhängige Magnetisierungsänderungen und einen schnellen Wärmetransfer durch thermomagnetische Dünnschichten ausgenutzt wird. Detaillierte LEM-Simulationen zeigen die Temperaturänderung, die Magnetfeldänderung und die daraus resultierende Magnetisierung der TM-Filme über Zeit und Position. Opti-male Bedingungen für eine resonante Selbstaktivierung werden durch sorgfältiges Design der TMG-Parameter erreicht, was zu einer kontinuierlichen, ungedämpften Oszillation des TMG-Ausleger führt. In dieser Arbeit werden verschiedene Design-Parameter erörtert, die sich auf die resonante Selbstaktivierung im Falle von Ni-Mn-Ga-Dünnschichten auswirken, wobei die Bedeutung der Feinabstimmung jedes Parameters für eine maximale Ausgangsleistung hervorgehoben wird. Die Auswirkungen von Faktoren wie Magnet, Spulenwindungen, Auslegersteifigkeit, Lastwiderstand (RL), Curie-Temperatur (Tc), Wärmeübergangskoeffizient (hf) und Wärmewiderstand (Rb) werden untersucht, um ihren Einfluss auf die TMG-Leistung zu verstehen. LEM-Simulationen zeigen kritische Werte für hf und Rb, die eine stabile Energieerzeugung mit signifikantem Hub und Frequenz ermöglichen, was zu einer deutlichen Steigerung der elektrischen Leistung führt. Die Hochskalierung des TMG mit Ni-Mn-Ga-Dünnschicht zeigt gegensätzliche Auswir-kungen auf die Leistungsabgabe und die Grundfläche, wobei eine verbesserte elektrische Leistung pro Grundfläche durch eine Erhöhung der Schichtdicke von 5 auf 40 µm erreicht wird. Bei einer Temperaturänderung von nur 3 °C und einer Frequenz von 146 Hz wer-den Werte von 50 µW/cm2 erreicht. Die parallelen Architekturen sind entscheidend für die Erzeugung ausreichender Energie für die direkte Anwendung. Die thermische Kreuz-kopplung beeinträchtigt die dynamische Leistung und die Leistungsabgabe von parallel betriebenen TMGs. Thermische Effekte machen sich vor allem bei geringen Abständen zwischen den Bauelementen und hohen Temperaturen der Wärmequelle bemerkbar, wobei jedoch keine magnetischen oder mechanischen Wechselwirkungen zwischen den parallel arbeitenden TMGs beobachtet werden. Bei Verwendung von Gadolinium als aktiver TM-Schicht ist ein Betrieb bei einer niedrigen Wärmequellentemperatur (Tsource) von 40 °C möglich. Der TMG kann bei dieser Tsource eine Leistung von 1,3 µW bei einer Frequenz von 54 Hz erzeugen, was einer Ausgangs-leistung von 10 µW/cm2 pro Fläche entspricht. Bei einer Tsource von 65 °C steigt dieser Wert sprunghaft auf 24 µW/cm2 bei einer Frequenz von 117 Hz an. Obwohl für eine opti-male Leistung eine Tamb von 11 °C erforderlich ist, kann das Bauelement die resonante Selbstaktivierung bis zu einer Umgebungstemperatur (Tamb) von 19 °C aufrechterhalten und dabei immer noch 8,7 µW/cm2 Leistung bei einer Tsource von 50 °C erzeugen. Außer-dem werden die scharfen Grenzen der Betriebstemperaturen in Bezug auf Tsource und Tamb untersucht und vorgestellt. Ein TMG, bei den hydrierten La-Fe-Si-Legierungen als aktiven TM-Film verwendet, kann 38 µW/cm2 aus einer Tsource von 90°C erzeugen, während es mit einer Frequenz von 137 Hz arbeitet

Ni-Mn-Ga films in the austenite and the martensite structures at room temperature: Uniaxial texturation and epitaxial growth

Ni-Mn-Ga films in the austenite and the martensite structures at room temperature have been obtained using the DC magnetron sputtering technique. Two elaboration processes were studied. A first batch of samples was deposited using a resist sacrificial layer in order to release the film from the substrate before vacuum annealing. This process leads to polycrystalline films with a strong (022) fiber texture. The martensitic phase transformation of such polycrystalline freestanding films has been studied by optical and scanning electron microscopy. A second batch of samples was grown epitaxially on (100)MgO substrates using different deposition temperatures. The texture has been analyzed with four-circle X-ray diffraction. Epitaxial films crystallized both in the austenite and the martensite structures at room temperature have been studied

Desenvolvimentos de estruturas muitiferroicas de filmes finos de ligas Ni-Mn-Ga e PMN-PT

Mestrado em Ciência e Engenharia de MateriaisLigas de forma ferromagnética em sistemas Ni-Mn-Ga são uma classe recente de materiais activos que podem gerar deformações de até 10% induzidas por um campo magnético por um rearranjo de maclas. Esta e outras propriedades físicas destas ligas têm importância tecnológica. Este trabalho investiga as propriedades de filmes finos de ligas de Ni-Mn-Ga sobre diferentes substratos, incluindo substratos activos (piezeléctricos). Para estudar as propriedades de filmes finos da liga, heteroestruturas sob a forma de Ni-Mn-Ga/substrato foram produzidas por RF sputtering com magnetrão utilizando temperaturas de deposição de 3200C, 3700C, 4000C sobre substratos de Al2O3, MgO, SrTiO3 e PMN-PT. A influência da temperatura do substrato durante a deposição nas propriedades estruturais e magnéticas de filmes finos foi estudada. Além disso, o acoplamento magnetoeléctrico entre Ni-Mn-Ga como filme fino material ferromagnético e PMN-PT como material piezoeléctrico foi investigada. O efeito magnetoeléctrico foi investigado apenas em filmes depositados a temperatura do substrato de 3700C e 4000C. As propriedades estruturais foram estudadas por difração de raios-X, as propriedades magnéticas foram investigadas por VSM, SQUID, e MFM, e o efeito magnetoeléctrico foi estudado por técnica lock-in. A medida estrutural mostrou que os filmes depositados são parcialmente cristalinos e o grau de cristalinidade aumenta como o aumento da temperatura do substrato. Fases austenita e martensita foram observadas nesses filmes. Os resultados da medição magnética mostram que todos os filmes depositados exibem comportamento ferromagnético e o comportamento ferromagnético é favorecido com o aumento da temperatura do substrato. Todos os filmes depositados na temperatura do substrato de 400ºC apresentam temperaturas dev Curie acima da temperatura ambiente: 337K para Ni-Mn- Ga/PMN-PT, 345K para Ni-Mn-Ga/STO e 348K para Ni-Mn-Ga/Al2O3. Nenhuma evidência separada de temperatura de transição estrutural foi observada para nos filmes. Os resultados das medições magnetoeléctricas mostram que as heteroestruturas multiferróicas Ni-Mn-Ga/PMN-PT apresentam efeito magnetoelétrico. O valor máximo medido para a tensão magnetoeléctrica induzida para filmes depositados à temperatura do substrato de 3700C e 4000C são 3.16mV/cmOe e 3.02mV/cmOe, respectivamente.Ferromagnetic shape memory alloys (FSMAs) in Ni-Mn-Ga systems are a recent class of active materials that can generate large magnetic field induced strains up to 10% by twin rearrangement. This and other physical properties these alloys have many technological importance. This work investigates the properties of Ni-Mn-Ga alloy thin films on different substrates including active substrate (piezoelectric). To study the properties of thin films of the alloy, the heterostructures in the form of Ni-Mn-Ga/substrate were produced by RF magnetron deposition system using substrate deposition temperatures of 3200C, 3700C, and 4000C, where the substrates used were Al2O3, MgO, SrTiO3 and PMN-PT. The influences of deposition substrate temperature on structural and magnetic properties of sputtered thin films on the aforementioned substrates were studied. Moreover, magnetoelectric coupling between Ni-Mn- Ga thin film as ferromagnetic material and PMN-PT as piezoelectric material was investigated. The magnetoelectric effect was investigated only on films deposited at substrate temperature of 3700C and 4000C. The structural properties were studied by x-ray diffraction, magnetic properties were investigated by VSM, SQUID, and MFM, and the magnetoelectric effect was studied by lock-in technique. The structural measurement has shown that asdeposited films are partially crystalline and degree of crystallinity increases as substrate temperature increase. Austenite and martensite phases have been observed in these films. The magnetic measurement results show that all films as-deposited display ferromagnetic behaviour and ferromagnetic behaviour improvements are observed as substrate temperature increases. All films deposited at substrate temperature of 4000C exhibit Curie temperatures above room temperature which are 337K for Ni-Mn-Ga/PMN-PT, 345K for Ni-Mn- Ga/STO, 348K for Ni-Mn-Ga/Al2O3. No separate signature of structural transition temperature was observed for all these films. The magnetoelectric measurement results show that a heterostructure of Ni-Mn-Ga/PMN-PT multiferroic exhibit magnetoelectric effect. The measured maximum induced magnetoelectric voltage for films deposited at substrate temperature of 3700C and 4000C are 3.16mV/cmOe and 3.02mV/cmOe, respectively

Modulated Martensite: Why it forms and why it deforms easily

Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale. Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could follow from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows retaining modulated martensite as a metastable phase at room temperature. In this metastable state elastic energy is released by the formation of a 'twins within twins' microstructure which can be observed from the nanometre to millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries which are diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.Comment: 34 pages, 8 figure