Magnetic cooling could be a radically different energy solution that could replace conventional vapour compression refrigeration in the future. It is now shown that a Heusler-type magnetocaloric alloy exhibits a remarkable cooling capability due to the effect of a sharp structural transformation at a specific temperature. The finding may be of relevance beyond Heusler alloys and represents an important step towards the implementation of cooling systems based on magnetocaloric materials.

Giant magnetocaloric effect driven by structural transitions

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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Caloric materials near ferroic phase transitions

Finding new materials with targeted properties has traditionally been guided by intuition, and trial and error. With increasing chemical complexity, the combinatorial possibilities are too large for an Edisonian approach to be practical. Here we show how an adaptive design strategy, tightly coupled with experiments, can accelerate the discovery process by sequentially identifying the next experiments or calculations, to effectively navigate the complex search space. Our strategy uses inference and global optimization to balance the trade-off between exploitation and exploration of the search space. We demonstrate this by finding very low thermal hysteresis (ΔT) NiTi-based shape memory alloys, with Ti50.0Ni46.7Cu0.8Fe2.3Pd0.2 possessing the smallest ΔT (1.84 K). We synthesize and characterize 36 predicted compositions (9 feedback loops) from a potential space of ∼800,000 compositions. Of these, 14 had smaller ΔT than any of the 22 in the original data set.

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Accelerated search for materials with targeted properties by adaptive design.

Vapor compression (VC) is by far the most dominant technology for meeting all cooling and refrigeration needs around the world. It is a mature technology with the efficiency of modern compressors approaching the theoretical limit, but its environmental footprint remains a global problem. VC refrigerants such as hydrochloroflurocarbons (HCFCs) and hydrofluorocarbons (HFCs) are a significant source of green house gas emissions, and their global warming potential (GWP) is as high as 1000 times that of CO2 [Buildings Energy Data Book (Building Technologies Program, Department of Energy, 2009)]. There is an urgent need to develop an alternative high-efficiency cooling technology that is affordable and environmentally friendly [A. D. Little, Report For Office of Building Technology State and Community Programs, Department of Energy, 2001]. Here, we demonstrate that elastocaloric cooling (EC), a type of solid-state cooling mechanism based on the latent heat of reversible martensitic transformation, can have the coefficient of performance as high as ≈11, with a directly measured ΔT of 17 °C. The solid-state refrigerant of EC completely eliminates the use of any GWP refrigerants including HCFCs/HFCs.

Demonstration of high efficiency elastocaloric cooling with large ΔT using NiTi wires

The enhanced reversibility (stable transition temperature even at high strain under a solid-to-solid phase transition), low hysteresis and unusual riverine microstructure (ranging through thermal cycles) of the martensitic material Zn45Au30Cu25 makes it attractive for applications from eco-friendly fridges to medical sensors. Martensitic transformations are diffusionless, solid-to-solid phase transformations characterized by a change of crystal structure that can often be very useful. Applications include medical sensors, eco-friendly refrigerators and energy conversion devices. Repeated transformation cycles, however, can cause thermal hysteresis that modifies the material's properties and can cause permanent damage. Here Richard James and colleagues report the development of a martensitic alloy of zinc, gold and copper that maintains near-reproducible macroscopic properties despite drastic changes in its microstructure during each cycle. As well as providing a system that throws new light on the effects of hysteresis on reversible martensitic phase transformations, this work could help to extend applications for the materials in new areas — towards shape memory alloys for instance. Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators1, eco-friendly refrigerators2,3 and energy conversion devices4. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed ‘reversibility’) is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown2,5,6,7,8,9,10,11,12,13,14 to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling6,9,12,15. Recently, stronger compatibility conditions called the ‘cofactor conditions’5,15 have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles9,16. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

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Enhanced reversibility and unusual microstructure of a phase-transforming material

Improving the functional stability of shape memory alloys (SMAs), which undergo a reversible martensitic transformation, is critical for their applications and remains a central research theme driving advances in shape memory technology. By using a thin-film composition-spread technique and high-throughput characterization methods, the lattice parameters of quaternary Ti-Ni-Cu-Pd SMAs and the thermal hysteresis are tailored. Novel alloys with near-zero thermal hysteresis, as predicted by the geometric nonlinear theory of martensite, are identified. The thin-film results are successfully transferred to bulk materials and near-zero thermal hysteresis is observed for the phase transformation in bulk alloys using the temperature-dependent alternating current potential drop method. A universal behavior of hysteresis versus the middle eigenvalue of the transformation stretch matrix is observed for different alloy systems. Furthermore, significantly improved functional stability, investigated by thermal cycling using differential scanning calorimetry, is found for the quaternary bulk alloy Ti 50.2 Ni 34.4 Cu 12.3 Pd 3.1 .

Identification of Quaternary Shape Memory Alloys with Near‐Zero Thermal Hysteresis and Unprecedented Functional Stability

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

In the present work, a computer-controlled test rig for simultaneous fatigue testing of several pseudoelastic NiTi wires through bending rotation is described. Bending rotation fatigue (BRF) testing represents a displacement-controlled experiment where a straight wire is bent into a semi-circle und forced to rotate around its axis. Thus, each point on the wire surface is sub- jected to alternating tension and compression. A test rig, which allows to control loading ampli- tudes, rotation frequencies and temperatures is described. We report preliminary results of an experimental program, which aims for a better understanding of fatigue lives, crack initiation, and crack growth in pseudoelastic NiTi wires. It was found that a good surface quality is of utmost importance to avoid early crack initiation. Whler curves of pseudoelastic NiTi wires typically show two different regimes depending on the maximum imposed surface strain during bending rotation fatigue testing. Larger strain amplitudes, which are associated with macro- scopic formation of stress-induced martensite, result in relatively low fatigue lives (LCF regime). In contrast, cycle numbers exceeding 10 7 were obtained for strain amplitudes where no large scale stress-induced formation of martensite occurred (HCF regime). In der vorliegenden Arbeit wird ein computergesteuerter Umlaufbiege-Versuchsstand beschrie- ben, mit welchem Ermdungsversuche an mehreren Drahtproben gleichzeitig durchgefhrt wer- den knnen. Bei der Umlaufbiegung wird eine zunchst gerade Drahtprobe zu einem Halbkreis gebogen und dann um die Lngsachse gedreht. Jeder Punkt auf der Oberflche erfhrt so eine wechselnde Zug- und Druckbelastung. Mit dem Umlaufbiegeprfstand knnen Dehnungsampli- tude, Drehzahl und Temperatur variiert werden. In der vorliegenden Arbeit werden erste Er- gebnisse aus einem Versuchsprogramm gezeigt, welches auf ein besseres Verstndnis von Ermdungslebensdauern, Rissbildungs- und Risswachstumsprozessen in pseudoelastischen NiTi- Formgedchtnislegierungen abzielt. Es wurde beobachtet, dass die Oberflchengte einen star- ken Einfluss auf die Ermdungslebensdauer hat. Whlerkurven von pseudoelastischem NiTi zeigen zwei Bereiche. Im Fall von hheren Dehnungsamplituden, bei denen eine spannungsindu- zierte martensitische Phasenumwandlung erfolgt, werden nur relativ geringe Ermdungslebens- dauern erreicht (LCF Bereich). Hhere Zyklenzahlen, die den 10 7

Bending rotation HCF testing of pseudoelastic Ni‐Ti shape memory alloys

Shape Memory Materials: Identification of Quaternary Shape Memory Alloys with Near‐Zero Thermal Hysteresis and Unprecedented Functional Stability (Adv. Funct. Mater. 12/2010)

Mustafa Rahim

Papers

Identification of Quaternary Shape Memory Alloys with Near‐Zero Thermal Hysteresis and Unprecedented Functional Stability

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

Bending rotation HCF testing of pseudoelastic Ni‐Ti shape memory alloys

Shape Memory Materials: Identification of Quaternary Shape Memory Alloys with Near‐Zero Thermal Hysteresis and Unprecedented Functional Stability (Adv. Funct. Mater. 12/2010)