Christoph Bechtold

High cyclic stability of the elastocaloric effect in sputtered TiNiCu shape memory films

The elastocaloric effect that occurs during the stress-induced martensitic transformation in shape memory alloys is a promising mechanism in view of solid state cooling applications. It also allows for downscaling to feature sizes in the μm range, thus, being attractive for micro-cooling applications using thin film materials. In this study, elastocaloric properties of TiNi and TiNiCu films and their relation to functional fatigue were investigated. Both materials show similar effect sizes, their fatigue behavior is however different. While the temperature change in TiNi degrades by a factor of two within 150 cycles, no significant elastocaloric fatigue was found in TiNiCu.

Artificial Single Variant Martensite in Freestanding Fe70Pd30 Films Obtained by Coherent Epitaxial Growth

2010 WILEY-VCH Verlag Gmb Microactuators and sensors based on magnetic shape-memory (MSM) alloys will benefit from the large strain close to 10% obtained in these materials. These strains exceed the values obtainable by magnetostriction or piezoelectricity by more than one order of magnitude. Thus, they can be used directly for most applications, avoiding additional complications of mechanical amplification. As the highest strains to-date are obtained in bulk single crystals, the use of epitaxial films is most promising for microsystems, owing to their single-crystal-like microstructure. With reduced actuator size, however, the influence of interfaces, and in particular of oxidation, becomes more important. Though the prototype Ni2MnGa system is relatively inert, an oxide surface layer may hinder the martensitic transformation in thin films. For the Fe70Pd30 system [8] oxidation is expected not to be critical due to the high content of a noble element. In Fe70Pd30 the martensitic transformation occurs around room temperature (RT). First reports indicate that epitaxial growth of thin Fe70Pd30 films can be obtained already at RT. For epitaxial growth of the NiMnGa system, a minimum temperature of 350 8C is required. Recently it was shown that epitaxial growth of Fe70Pd30 is possible on various metallic buffers. Due to coherent growth, huge tetragonal distortions were stabilized in 50 nm thick films, covering most of the Bain transformation path from face-centered cubic (fcc) to bodycentered cubic (bcc) structure. Here, we show how this approach can be extended to obtain freestanding films of micrometer thickness, thus fulfilling both key requirements for the integration into microsystems as well as prerequisites for MSM films, that is, martensitic, ferromagnetic at RT, freestanding, and single-crystalline-like. In addition, our experiments reveal that the transformation behavior in these films differs from in the bulk. While this topic has been an extensive playground for theory, experiments are rare, since a detailed analysis often requires epitaxial films. Recently, experiments on epitaxial films, for example, have revealed a variant selection by the rigid interface to the substrate or by reduction of the magnetic stray field energy in freestanding films. The present experiments are more fundamental since they show that circumventing the forward martensitic transformation by forming the martensitic structure directly at RT hinders the nucleation of the reverse transformation to austenite. This remarkable suppression of the transformation not only gives a better understanding of the martensitic transformation but also opens innovative routes for microsensors. Films of Fe70Pd30, 1.2mm thick, were deposited with a low deposition rate of 0.024 nms 1 at 30W sputtering power in a magnetron sputtering system on Au-buffered MgO(001) oriented, epi-polished substrates. The crystal structure of the Fe70Pd30 films was analyzed by four-circle and temperature-dependent two-circle X-ray diffraction (XRD), where x and w denote the tilt and rotational angles, respectively. The u–2u scans (see Fig. 1a) show the 200 Au reflection of the buffer layer (2u1⁄4 44.448) as well as the Fe70Pd30 002 reflection (57.388). Assuming a constant volume of the Fe70Pd30 unit cell compared to the cubic austenite, [15] the lattice parameters a and c were calculated to be 0.287 nm and 0.321 nm ( 0.001 nm), respectively, which constitutes a c/a ratio of 1.12. Temperature-dependent XRD measurements showed no change in the crystal structure in the accessible temperature range between 150 and 375K. The pole figure of the 101 reflection reveals a four-fold symmetry (Fig. 1b). The maximum intensity is obtained at an average of 47.278 at w1⁄4 458. The peak in the w direction is rather sharp with a small full width at half maximum (FWHM), indicating well-oriented growth of the body-centered tetragonal (bct) unit cell rotated by 458compared to the edges of the MgO cell. The larger FWHM in the x direction indicates relaxation of the lattice. There are no indications that twinning has occurred in the film. The sample for transmission electron microscopy (TEM) investigations was prepared by focused ion beam (FIB) lift-out

Magnetic moment investigation by frequency mixing techniques

Gas turbines and other large industrial equipment are subjected to high-temperature oxidation and corrosion. Research and development of efficient protective coatings is the main task in the field. Also, knowledge about the depletion state of the coating during the operation time is important. To date, practical nondestructive methods for the measurement of the depletion state do not exist. By integrating magnetic phases into the coating, the condition of the coating can be determined by measuring its magnetic properties. In this paper, a new technique using frequency mixing is proposed to investigate the thickness of the coatings based on their magnetic properties. A sensor system is designed and tested on specific magnetic coatings. New approaches are proposed to overcome the dependency of the measurement on the distance between coil and sample that all noncontact techniques face. The novelty is a low cost sensor with high sensibility and selectivity which can provide very high signal-to-noise ratios. Prospects and limitations are discussed for future use of the sensor in industrial applications.

Effect of crystallographic compatibility and grain size on the functional fatigue of sputtered TiNiCuCo thin films.

The positive influence of crystallographic compatibility on the thermal transformation stability has been already investigated extensively in the literature. However, its influence on the stability of the shape memory effect or superelasticity used in actual applications is still unresolved. In this investigation sputtered films of a highly compatible TiNiCuCo composition with a transformation matrix middle eigenvalue of 1±0.01 are exposed to thermal as well as to superelastic cycling. In agreement with previous results the thermal transformation of this alloy is with a temperature shift of less than 0.1 K for 40 cycles very stable; on the other hand, superelastic degradation behaviour was found to depend strongly on heat treatment parameters. To reveal the transformation dissimilarities between the differently heat-treated samples, the microstructure has been analysed by transmission electron microscopy, in situ stress polarization microscopy and synchrotron analysis. It is found that good crystallographic stability is not a sufficient criterion to avoid defect generation which guarantees high superelastic stability. For the investigated alloy, a small grain size was identified as the determining factor which increases the yield strength of the composition and decreases the functional degradation during superelastic cycling. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

Method for Fabricating Miniaturized NiTi Self-Expandable Thin Film Devices with Increased Radiopacity

Nitinol is the material of choice for many medical applications, in particular for minimally invasive implants due to its superelasticity and biocompatibility. However, NiTi has limited radiopacity which complicates positioning in the body. A common strategy to increase the radiopacity of NiTi devices is the addition of radiopaque markers by micro-riveting or micro-welding. The recent trend of miniaturizing medical devices, however, reduces their radiopacity further, and makes the addition of radiopaque markers to these miniaturized devices difficult. NiTi thin film technology has great potential to overcome such limitations and to fabricate new generations of miniaturized, self-expandable NiTi medical devices with additional functionalities, such as structured multilayer devices with increased radiopacity. For this purpose, we have produced superelastic thin film NiTi samples covered locally with Tantalum structures of different thickness and different shape. These multilayer devices were characterized regarding their mechanical and corrosion properties as well as their X-ray visibility. The superelastic behavior of the underlying NiTi layer is impeded by the Ta layer, and shows therefore a dependence on the Tantalum patterning geometry and thickness. No delamination was observed after mechanical and corrosion tests. The multilayers reveal excellent corrosion resistance, as well as a significant increase in radiopacity.

Reconstruction of Magnetization Curve Using Magnetic Spectroscopy

A new measurement principle based on the frequency mixing technique for investigating the shape of the magnetization curve of soft non-hysteretic magnetic materials is introduced. Based on Taylor expansion of the magnetization curve and spectral investigation of an inductively detected signal, a mathematical model for the reconstruction of M(H) is proposed, [7]. Here, the model is experimentally verified using a nanocrystalline soft magnetic material with defined properties. It is shown that the magnetization curve can be reconstructed very accurately and the influence of an additional parameter, i.e. strain, can be investigated in detail as well.