Sandra Kauffmann-Weiss

Magnetic nanostructures by adaptive twinning in strained epitaxial films.

We exploit the intrinsic structural instability of the Fe(70)Pd(30) magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials.

Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain

Rare earth free alloys are in focus of permanent magnet research since the accessibility of the elements needed for nowadays conventional magnets is limited. Tetragonally strained iron-cobalt (Fe-Co) has attracted large interest as promising candidate due to theoretical calculations. In experiments, however, the applied strain quickly relaxes with increasing film thickness and hampers stabilization of a strong magnetocrystalline anisotropy. In our study, we show that already 2 at. % of carbon substantially reduces the lattice relaxation leading to the formation of a spontaneously strained phase with 3% tetragonal distortion. In these strained (Fe0.4Co0.6)0.98C0.02 films, a magnetocrystalline anisotropy above 0.4 MJ/m3 is observed while the large polarization of 2.1 T is maintained. Compared to binary Fe-Co, this is a remarkable improvement of the intrinsic magnetic properties. In this paper, we relate our experimental work to theoretical studies of strained Fe-Co-C and find a very good agreement.

Nucleation and growth of hierarchical martensite in epitaxial shape memory films

Abstract Shape memory alloys often show a complex hierarchical morphology in the martensitic state. To understand the formation of this twin-within-twins microstructure, we examine epitaxial Ni-Mn-Ga films as a model system. In-situ scanning electron microscopy experiments show beautiful complex twinning patterns with a number of different mesoscopic and macroscopic twin boundaries between already twinned regions. We explain the appearance and geometry of these patterns by constructing an internally twinned martensitic nucleus, which can take the shape of a diamond or a parallelogram, within the basic phenomenological theory of martensite. These nucleus contains already the seeds of different possible mesoscopic twin boundaries. Nucleation and growth of these nuclei determines the creation of the hierarchical space-filling martensitic microstructure. This is in contrast to previous approaches to explain a hierarchical martensitic microstructure. This new picture of creation and anisotropic, well-oriented growth of twinned martensitic nuclei explains the morphology and exact geometrical features of our experimentally observed twins-within-twins microstructure on the meso- and macroscopic scale.

TiNi-based films for elastocaloric microcooling— Fatigue life and device performance

The global trend of miniaturization and concomitant increase of functionality in microelectronics, microoptics, and various other fields in microtechnology leads to an emerging demand for temperature control at small scales. In this realm, elastocaloric cooling is an interesting alternative to thermoelectrics due to the large latent heat and good down-scaling behavior. Here, we investigate the elastocaloric effect due to a stress-induced phase transformation in binary TiNi and quaternary TiNiCuCo films of 20 μm thickness produced by DC magnetron sputtering. The mesoscale mechanical and thermal performance, as well as the fatigue behavior are studied by uniaxial tensile tests combined with infrared thermography and digital image correlation measurements. Binary films exhibit strong features of fatigue, involving a transition from Luders-like to homogeneous transformation behavior within three superelastic cycles. Quaternary films, in contrast, show stable Luders-like transformation without any signs of degradation. The elastocaloric temperature change under adiabatic conditions is −15 K and −12 K for TiNi and TiNiCuCo films, respectively. First-of-its-kind heat pump demonstrators are developed that make use of out-of-plane deflection of film bridges. Owing to their large surface-to-volume ratio, the demonstrators reveal rapid heat transfer. The TiNiCuCo-based devices, for instance, generate a temperature difference of 3.5 K within 13 s. The coefficients of performance of the demonstrators are about 3.

Finely-tuned NIR-to-visible up-conversion in La2O3:Yb3+,Er3+ microcrystals with high quantum yield

Up-conversion (UC) materials whose emission color can be finely-tuned while a high UC quantum efficiency is maintained are desirable for many applications. Herein, we report near-infrared-to-visible La2O3:Yb3+,Er3+ (LYE) UC materials with a high internal quantum yield (UCQY) of 3.8%, external UCQY (brightness) of 1.6% and tunable emission color. UC emission colors from pure green to reddish-orange can be precisely tailored by simply controlling synthesis conditions, whilst maintaining the high UCQY. The internal UCQY and external UCQY of LYE yield better performance than both commercially available and other record UC phosphors reported in the literature under the same excitation conditions. The facile preparation combined with the color-tuning and high UCQYs make these materials attractive candidates for solar energy harvesting, sensors, 3D volumetric displays, solid state lasers and bio-imaging.

The Bain library: A Cu-Au buffer template for a continuous variation of lattice parameters in epitaxial films

Smallest variations of the lattice parameter result in significant changes in material properties. Whereas in bulk, lattice parameters can only be changed by composition or temperature, coherent epitaxial growth of thin films on single crystals allows adjusting the lattice parameters independently. Up to now only discrete values were accessible by using different buffer or substrate materials. We realize a lateral variation of in-plane lattice parameters using combinatorial film deposition of epitaxial Cu-Au on a 4-in. Si wafer. This template gives the possibility to adjust the in-plane lattice parameter over a wide range from 0.365 nm up to 0.382 nm.

Superconducting Properties of Thick Films on Hastelloy Prepared by the Aerosol Deposition Method With Ex Situ MgB2 Powder

We present a new high-deposition-rate coating technique that allows us to produce at room temperature long thick films of MgB₂ on flexible steel substrates. Such a technique might give rise to new tapes with higher filling factors compared to the standard processed tapes. With the so-called aerosol deposition technique, MgB₂ films were prepared on Hastelloy substrates with commercially available <italic>ex situ</italic>-prepared MgB ₂ powder (<italic>T</italic>_C,onset of 38 K, <italic>J</italic>_C of 3.8·10³ A/cm² at 4 K and 1 T). Microscopic analyses yield nanocrystalline dense films with high film stresses. The as-deposited films have so far a superconducting transition temperature <italic>T</italic>_C0 of 18.1 K and a critical current density <italic>J</italic>_C up to 5·10³ A/cm² at 4 K and self-field obtained.

Orientation relationship of eutectoid FeAl and FeAl2

The orientation relationship and interface plane of eutectoid FeAl and FeAl2 lamellae are investigated in detail and a crystallographic model is proposed.

Wide-range non-contact fluorescence intensity ratio thermometer based on Yb3+/Nd3+ co-doped La2O3 microcrystals operating from 290 to 1230 K

Non-contact ratiometric thermometry has applications ranging from in situ physiological measurements to industrial process monitoring. The technique is based on the optical detection of the fluorescence intensity ratio (FIR) of two thermally-coupled levels (TCLs). Here, we report a Yb3+/Nd3+ co-doped La2O3 microcrystal ratiometric thermometer based on upconverted emission in the near-infrared (NIR) after excitation with a 980 nm laser diode, which operates effectively over the wide temperature range from 290–1230 K. The thermometer uses the TCLs of Nd3+:4F7/2 (emission peak 762 nm) and Nd3+:4F5/2 (emission peak 825 nm). The chosen levels combine desirable characteristics to act as a sensitive temperature sensor over a wide range of elevated temperatures: namely a suitable energy gap (ΔE = 950 cm−1); and weak thermal quenching effects (maximum photoluminescence at 803 and 853 K respectively for two Nd3+ emission peaks). This leads to a high relative sensitivity (SR) of 1334/T2, and low temperature uncertainty ΔTmin of 0.1 K (<400 K), 1 K (400–853 K) and 3 K (853–1233 K). In addition to these characteristics, the excellent repeatability of FIR of the two Nd3+ emission peaks makes Yb3+/Nd3+ co-doped La2O3 microcrystals a promising non-contact NIR ratiometric thermometer for temperatures up to 1230 K.

The Aerosol Deposition Method: A Modified Aerosol Generation Unit to Improve Coating Quality

Owing to its ability to produce dense thick-films at room temperature directly from a ceramic powder, the Aerosol Deposition Method (AD) possesses a unique feature in ceramics processing. For this technology, the aerosol generation of particles is a decisive part of reliable process control. However, there has only been a small amount of work published addressing this topic. In this work, we compare the aerosolization and deposition behavior of a fluidized bed generator with an aerosol generator with the rotary brush principle. While film properties very much depend on deposition time for the fluidized bed generator, films produced with the brush generator show a constant film profile, and their film thickness correlates with the controllable aerosol concentration and the duration of deposition. This type of aerosol generation may improve the setup towards a more reliable AD process.