a) Breakthrough shape-memory metals
Through the combined effort of high resolution XRD and TEM investigations the mechanism behind novel ultra-long lasting shape memory alloys has been uncovered. High structural compatibility and an epitaxial stabilization of high and low temperature phases allow the Nickel-Titanium based alloy to withstand tens of millions of cycles without fatigue.
*C. Chluba, W. Ge, R. L. de Miranda, J. Strobel, L. Kienle, E. Quandt, and M. Wuttig, “Ultralow-fatigue shape memory alloy films,” Science, vol. 348, no. 6238, pp. 1004–1007, May 2015.
b) Characterization of elastocaloric Ni-Ti based thin films via TEM (in cooperation with Prof. Quandt, CAU Kiel).
Ni-Ti based alloys are known for their unique characteristics such as shape memory effect and pseudoelasticity. In the last decade such alloys were envisaged for ferroic cooling systems since the structural changes during the shape memory/pseudoelastic transitions are related to entropy changes. Here, we investigated fatigue mechanisms of Ni-Ti based alloys, which showed a small temperature change during mechanical cycling, via state-of-the-art TEM techniques. In the case of binary Ni-Ti alloys, SAED and HRTEM imaging revealed that the film consists of Ni4Ti3 precipitates, R-Phase, and austenite. The average size of the Ni4Ti3 precipitates is determined to be around 20 nm and they are coherently intergrown with the austenitic matrix. The distribution of the precipitates in the austenitic matrix was analyzed via automatic texture measurements (ASTAR) in TEM for the first time. This method works quite well; however, the sizes of the precipitates represent the determining factor for quantitative evaluations. After ex-situ mechanical cycling the alloys show strong distortions of the austenitic matrix which are interrelated to residual strain accumulating during cycling. Another focus of research dealt with Cu doped Ni-Ti alloys, since the addition of Cu to the Ni-Ti system leads to decreasing martensite start temperatures. Various structures with different compositions have been detected via diffraction studies as well as ASTAR measurements accompanied by STEM-EDX nanoprobe investigations. In this case, and in contrast to the binary materials, the microstructure did not change after ex-situ mechanical cycling thus giving rise to 2000 stable pseudoelastic cycles.
c)Telluride based Thermoelectrics: Stability and defects in Bi2Te3/Sb2Te3 superlattices
Bi2Te3/Sb2Te3 superlattices are known for their superior thermoelectric properties. However, a detailed investigation of the structure and its defects and the influence on the thermoelectric properties is still missing. Hence to close this gap Bi2Te3/Sb2Te3 superlattices have been grown by MBE and characterized in detail. Due to the difference in growth rates of the a and c direction and favorable nucleation at edges, conical shaped domains separated by threading dislocations were observed. Close to the threading dislocations the superlattice was strongly bowed. HRSTEM micrograph reveal step-like in grown Bi2Te3 stripes (see figure 1). In situ and ex situ XRD and TEM measurements have been performed on these samples. The in situ XRD measurements showed an onset of interdiffusion around 200 °C by decreasing intensity of the superlattice reflections (see figure 2). 200 °C is remarkably low, considering the melting temperatures of Sb2Te3 and Bi2Te3. In situ and ex situ TEM measurements reveal that at bowed superlattices the diffusion is preferred (Figure 3). It was concluded that the diffusion rate is increased due to the facile diffusion paths in a- and b-direction thus lowering the overall stability.
*A.-L. Hansen, T. Dankwort, M. Winkler, J. Ditto, D.C. Johnson, J.D. Koenig, K. Bartholomé, L. Kienle, and W. Bensch, Chem. Mater. 26, 6518 (2014).Copyright 2014 American Chemical Society.
d) Morphology-property relations for AlN thin films (collaborative research centre SFB 855, in cooperation with Prof. Wagner, ISIT Fraunhofer, and Prof. Kohlstedt, CAU Kiel).
In the framework of SFB 855 magnetoelectric (ME) materials are developed as strain-mediated piezoelectric-magnetostrictive nanocomposites for sensor applications. These sensors seem to be excellent candidates for biomagnetic interfaces, enabling non-invasive medical imaging such as magneto-encephalography or cardiography (MEG, MCG). For the piezoelectric component, non-toxic III-V aluminium nitride (AlN) thin films proved their outstanding performance. The piezoelectric properties of AlN are directly associated with the crystal orientation and thus the morphology. In optimum cases the films are strongly c-textured, i.e. the film normal is parallel to the polar crystal axis [0 0 0 1]. Modern TEM techniques, particularly when introducing automated electron diffraction (ASTAR, see above), allow a detailed investigation of the morphology on different length scales ranging from nano- to micron size. The interrelation of these results with piezo-response force microscopy (PFM, Figure 2) demonstrates the relationship between morphology and property. The colour code (Fig. 2b) and the diffraction patterns of the marked areas prove columnar growth of AlN. However, several columns are misaligned from the [0 0 0 1] orientation. These morphology defects can also be identified by PFM measurements from similar regions. The topological view in Fig. 2d reveals only minor differences in altitude, nevertheless, the magnitude (Fig. 2e) and phase (Fig. 2f) signals of the piezo-response are marginal, as indicated by the dark areas. According to the strict interrelation of morphology and piezoelectric properties the dark areas should correlate with misaligned columns. Furthermore, a closer inspection of the orientation maps of Figs. 2b-d shows that the azimuthal rotation of the columns around the [0 0 0 1] direction is not subject to any restriction. Intrinsic features of the rotation at the grain boundaries and defect structures such as stacking mismatch boundaries can be analyzed quantitatively by high resolution microscopy and (precession) electron diffraction.
Fig. 2: Crystallite orientation mapping of AlN thin films (plane view). (a) Virtual bright field image. (b) Orientation map along the out-of-plane direction (z-direction), and for two in-plane directions (c) x-direction, (d) y-direction. (Right) Colour code of the stereographic triangle (hexagonal point group: 6mm) for all three maps. Diffraction patterns of area (i) and (ii) verifying the presence of misaligned grains. PFM measurements on an AlN thin film; (d) topographic image (e) magnitude signal (vertical) and (f) phase signal (vertical).