Marvin Schmidt

Thermal Stabilization of NiTiCuV Shape Memory Alloys: Observations During Elastocaloric Training

The paper presents novel findings observed during the training process of superelastic, elastocalorically optimized Ni–Ti-based shape memory alloys (SMA). NiTiCuV alloys exhibit large latent heats and a small mechanical hysteresis, which may potentially lead to the development of efficient solid-state-based cooling processes. The paper starts with a brief introduction to the underlying principles of elastocaloric cooling, illustrating the effect by means of a typical thermodynamic cycle. It proceeds with the description of a custom-built testing platform that allows observation of temperature profiles and heat transfer between SMA and heat source/sink during high-loading-rate tensile tests. Similar to other SMA applications, a training process is necessary in order to guarantee stable performance. This well-known mechanical stabilization affects the stress–strain hysteresis and the cycle-dependent evolution of differential scanning calorimetry results. In addition, it can be shown here that the training is also accompanied by a cycle-dependent evolution of temperature profiles on the surface of an SMA ribbon. The applied training leads to local temperature peaks with intensity, number, and distribution of the temperature fronts showing a cycle dependency. The homogeneity of the elastocaloric effect has a significant influence on the efficiency of elastocaloric cooling process and is a key aspect of the specific material characterization.

Elastocaloric cooling processes: The influence of material strain and strain rate on efficiency and temperature span

This paper discusses the influence of material strain and strain rate on efficiency and temperature span of elastocaloric cooling processes. The elastocaloric material, a newly developed quaternary Ni-Ti-Cu-V alloy, is characterized at different maximum strains and strain rates. The experiments are performed with a specially designed test setup, which enables the measurement of mechanical and thermal process parameters. The material efficiency is compared to the efficiency of the Carnot process at equivalent thermal operation conditions. This method allows for a direct comparison of the investigated material with other caloric materials.

On the widths of the hysteresis of mechanically and thermally induced martensitic transformations in Ni–Ti-based shape memory alloys

Abstract It is well known that a good crystallographic compatibility between austenite and martensite in Ni–Ti-based shape memory alloys results in narrow thermal hystereses (e.g. Ball and James, Arch. Ration. Mech. Anal., 1987). The present work suggests that a good crystallographic fit is moreover associated with a small mechanical hysteresis width, observed during a forward and reverse stress-induced transformation. Furthermore, shape memory alloys with a good crystallographic fit show smaller transformation strains. The results obtained in the present study suggest that these correlations are generic and apply to binary Ni–Ti (with varying Ni contents) and quaternary Ni–Ti–Cu–X (X = Cr, Fe, V) alloys. For binary Ni–Ti, it was observed that Ni-rich compositions (good lattice fit) show a lower accummulation of irreversible strains during pseudoelastic cycling.

Elastocaloric cooling: From fundamental thermodynamics to solid state air conditioning

This article discusses fundamental thermodynamic concepts, as well as experimental investigations of elastocaloric cooling processes and presents a concept of a potential elastocaloric air conditioning device. Various cooling cycles suitable for elastocaloric cooling are introduced and the process efficiencies are determined based on a graphical approach. The graphical method is validated experimentally with a specially designed scientific test setup, which enables the measurement of mechanical and thermal process quantities. The material, a newly developed quaternary Ni-Ti-Cu-V alloy, is investigated in various thermodynamic cycles and an advanced cycle control is applied to increase the process efficiency. In addition, the influence of the thermal boundary conditions on the material and system efficiency is investigated. The results are compared with the values predicted by the graphical approach. Furthermore, a concept of a continuously operating elastocaloric air cooling device is introduced.

Cooling Efficiencies of a NiTi-Based Cooling Process

Energy saving and environmental protection are topics of growing interest. In the light of these aspects alternative refrigeration principles become increasingly important. Shape memory alloys (SMA), especially NiTi alloys, generate a large amount of latent heat during solid state phase transformations, which can lead to a significant cooling effect in the material. These materials do not only provide the potential for an energy-efficient cooling process, they also minimize the impact on the environment by reducing the need for conventional ozone-depleting refrigerants.Our paper, presenting first results obtained in a project within the DFG Priority Program SPP 1599 “Ferroic Cooling”, focuses on the thermodynamic analysis of a NiTi-based cooling system. We first introduce a suitable cooling process and subsequently illustrate the underlying mechanisms of the process in comparison with the conventional compression refrigeration system. We further introduce a graphical solution to calculate the energy efficiency ratio of the system. This thermodynamic analysis method shows the necessary work input and the heat absorption of the SMA in stress/strain- or temperature/entropy-diagrams, respectively. The results of the calculations underline the high potential of this solid-state cooling methodology.Copyright

Experimental Investigation and Numerical Simulation of the Mechanical and Thermal Behavior of a Superelastic Shape Memory Alloy Beam During Bending

Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents).In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermo-mechanically coupled process.As a part of the DFG (German Research Association) Priority Programme SPP1599 “Ferroic Cooling” which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation.In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework.Copyright

Experimental Investigation on the Efficiency of a Control Dependent NiTi-Based Cooling Process

Energy efficient systems and environmentally friendly solutions are the focus of many commercial development projects. Current refrigeration technology carries a significant share of global energy consumption and exploring alternative refrigeration principles has become increasingly important. Shape memory alloys (SMA’s), especially Nickel-Titanium (NiTi) alloys, generate a large amount of latent heat during solid-state phase transformations, which can lead to a significant cooling effect in the material. These materials not only provide the potential for an energy efficient cooling process, they also minimize the impact on the environment by reducing the need for conventional ozone-depleting refrigerants.This paper presents the first experimental results obtained in a project within the DFG Priority Programme SPP 1599 “Ferroic Cooling”. It focuses on the performance of a control-dependent process of a NiTi-based cooling system. First, a suitable cooling process is introduced and the underlying mechanisms of the process are explained. Then different process variations are developed, which influence the efficiency of the cooling process. These process variations are systematically analyzed with a novel, experimental testing system capable of tuning process parameters independently. The testing system is able to measure force, displacement, temperature distribution and heat simultaneously. The coefficient of performance (of the cooling process) can then be determined by which the influence of the control process on the efficiency can be observed.Copyright

Experimentelle Untersuchung elastokalorischer Kühlprozesse

Zusammenfassung Die im Rahmen dieser Arbeit durchgeführten Untersuchungen beinhalten die Charakterisierung von Formgedächtnislegierungen hinsichtlich ihrer elastokalorischen Eigenschaften sowie die Analyse des Einflusses von Prozessführung und thermischen Randbedingungen auf die Kühlleistung. Diese Untersuchungen wurden mit einer speziell entwickelten wissenschaftlichen Testplattform durchgeführt, die es ermöglicht, die Legierungen zu charakterisieren und in einem Kühlprozess zu erforschen. Die Materialcharakterisierung umfasst Zugversuche bei verschiedenen Dehnraten, wobei simultane Messungen von Spannung und Dehnung sowie des Temperaturfeldes durchgeführt werden. Basierend auf diesen Messungen lassen sich Aussagen über die Ratenabhängigkeit und Homogenität des elastokalorischen Effektes sowie die Effizienz des Kühlprozesses insgesamt treffen. Zusätzlich kann die Langzeitstabilität der Materialien in Abhängigkeit von den Prozessparametern und dem Materialtraining, d. h. dem Einfahrprozess, untersucht werden. Basierend auf diesen Untersuchungen erfolgt eine Variation der Prozessparameter unter veränderlichen thermischen Randbedingungen. Die Parameterstudie zeigt, dass eine von Wärmesenken- und Wärmequellentemperatur abhängige Anpassung der Prozessführung zu einer Steigerung der Kühlleistung führt.

Experimental Methods for Investigation of Shape Memory Based Elastocaloric Cooling Processes and Model Validation.

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMAs mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects - both important for efficient heat transfer from the medium to be cooled. The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

Elastocaloric Cooling With Ni-Ti Based Alloys: Material Characterization and Process Variation

Solid state refrigeration processes, such as magnetocaloric and electrocaloric refrigeration, have recently shown to be a promising alternative to conventional compression refrigeration. A new solid state elastocaloric refrigeration process using the latent heats within Shape Memory Alloys (SMA) could also hold potential in this field. This work investigates the elastocaloric effects in Ni-Ti-based superelastic Shape Memory Alloy (SMA) systems for use in an elastocaloric cooling processes. Ni-Ti alloys exhibits large latent heats and a small mechanical hysteresis, which may potentially lead to the development of an efficient environmentally friendly solid-state cooling system, without the need for ozone-depleting refrigerants. A systematic investigation of the SMA is conducted using a novel custom-built scientific testing platform specifically designed to measure cooling process related phenomena. This testing system is capable of performing tensile tests at high rates as well as measuring and controlling the solid-state heat transfer between SMA and heat source/heat sink.Tests are conducted following a cooling process related training cycle where the material has achieved stabilized behavior. First, a characterization of the elastocaloric material properties is performed followed by an investigation of the material under cooling process conditions.A comprehensive monitoring of the mechanical and thermal parameters enables the observation of temperature changes during mechanical cycling of the SMA at high strain rates. These observations can be used to study the rate dependent efficiency of the elastocaloric material.The measurement of the temperature of both the heat source/heat sink and the SMA itself, as well as the required mechanical work during a running cooling process, reveals the influence of the operating conditions on the elastocaloric effect of the material.Furthermore investigations of the process efficiency at different thermal boundary conditions (temperature of heat source/heat sink), indicates that the process is dependent on the boundary conditions which have to be controlled in order to optimize the efficiency.Copyright