Magnetocaloric materials are promising as solid state refrigerants for more efficient and environmentally friendly cooling devices. The highest effects have been observed in materials that exhibit a first-order phase transition. These transformations proceed by nucleation and growth which lead to a hysteresis. Such irreversible processes are undesired since they heat up the material and reduce the efficiency of any cooling application. In this article, we demonstrate an approach to decrease the hysteresis by locally changing the nucleation barrier. We created artificial nucleation sites and analyzed the nucleation and growth processes in their proximity. We use Ni-Mn-Ga, a shape memory alloy that exhibits a martensitic transformation. Epitaxial films serve as a model system, but their high surface-to-volume ratio also allows for a fast heat transfer which is beneficial for a magnetocaloric regenerator geometry. Nanoindentation is used to create a well-defined defect. We quantify the austenite phase fraction in its proximity as a function of temperature which allows us to determine the influence of the defect on the transformation.

A method is developed for cutting up sheets with defective areas into given pieces while minimizing waste. The sheets, the pieces, and the defects are all rectangles, these latter to be identified by the coordinates of two opposite corners in a coordinate system attached to the sheet. The cutting is done in in three stages. If the length of the sheet is along the x-axis, the first cuts are made parallel to the y-axis, obtaining “sections.” The sections are then cut into “strips” parallel to the x-axis, and, finally, the strips into “pieces” parallel to the y-axis. The procedure uses dynamic programming, which requires a value to be attached to each size. The computer program senses the defects and fits pieces into the clear portion of the sheet in such a way that the total value is a maximum. In order to shorten machine time, some simplifying shortcuts are made.

Epitaxial films have the potential to be used as model systems for fundamental investigations on the martensitic transformation in binary NiTi. In this paper, we discuss growth of binary NiTi thin films on single crystalline MgO substrates. Sputter deposition is used to grow NiTi films. Films prepared by complementary preparation routes (with different deposition temperatures and subsequent heat treatments) are investigated by X-ray diffraction, electron microscopy, atomic force microscopy, and electrical resistivity measurements, with the aim of optimizing film properties, particularly to obtain a well defined orientation of the austenitic unit cell and smooth surfaces. Our results show that deposition at elevated temperatures and carefully controlled subsequent heat treatments allow to produce epitaxially grown and smooth NiTi films that exhibit reversible one-or two-step martensitic transformations.

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This paper reports on concepts, technology, integration aspects, and results of a MEMS-based test platform for reliability tests of micro and nano objects.Building blocks of this platform to be used in in-plane push-pull-configurations are a thermal actuator, a force sensor, and a displacement sensor as well as several types of samples to be integrated directly or indirectly. The technology concept is proven for aluminum wires as a first demonstrator for an integrated specimen. The modular concept enables the adoption of the platform for further types of samples and test configurations. The building blocks are simulated using a Finite-Element approach and the results are compared to experimentally obtained data sets. According to the specifications, the thermal actuator is capable of providing displacements up to 1.5 μm. The displacement and the force sensor are specified to provide sensitivities of 3 fF nm À1 and 2.5 μV nN À1 , respectively. SEM image of a piezoresistive force sensor attached to an integrated Al wire as test sample À building blocks of MEMS-based test platforms.

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A new reference spring for the simultaneous calibration of probing force and displacement has been developed. The spring consists of two single silicon springs, which are placed at a distance of 3 μm from each other. Each single spring consists of a moveable shaft, which is suspended and guided by four double-folded silicon springs. This leads to a much higher stiffness of the spring perpendicular to the direction of movement than in the direction of movement. The area of contact of the double spring has a size of 50 μm  ×  60 μm. However, measurable changes in the calibration parameters could not be observed when we varied the location of the loading point within this area. Furthermore, it could be shown for measurements at different temperatures that the calibration parameters also show a very small dependence on temperature (<0.4%/K between 22 °C and 23 °C). A further outstanding property of this new reference spring is its small non-linearity of the force deflection curve of 0.1%. The spring can be used for the calibration of force and the displacement of atomic force microscopes, nanoindenters, and stylus instruments in the micro-Newton range up to 12 μN and up to 3 μm displacements.