Holger Rumpf

Study of the tetragonal-to-cubic phase transition in PbTiO3 nanopowders

The CPP (combined polymerization and pyrolysis) preparation route, in its enhanced liquid-precursor-based version, was combined with consecutive soft milling. For studies of temperature- and size-dependent structural changes occurring in ferroelectric lead titanate, this combined route yields a nanopowder series covering the relevant particle-size region at target quality. This material basis enables consistent SEM, TEM, XRD, Raman, EPR and dielectric measurements, which furnish a comprehensive picture of the cooperation between temperature rise and size reduction to eliminate tetragonality and concomitant ferroelectricity. Our previous original EPR studies on nanosized barium titanate are now extended to the lead titanate case. Furthermore, as compared to the pertinent literature standard, the materials basis is extended to powder samples of smaller mean particle sizes, comprising the critical size at which a PbTiO3 particle undergoes a transition into cubic paraelectric phase. Thus, the size-driven phase transition can be observed in a direct way (at 7 nm, which compares to 40 nm for BaTiO3), and the EPR data suggest a much less spacious gradient shell at the particle surfaces (thickness ≈ 2 nm) than in previous analogous investigations on BaTiO3 (15 nm).

Shape memory effect and magnetostriction of sputtered NiMnGa thin films

NiMnGa thin films have been deposited by magnetron sputtering on Mo substrates using a Ni50Mn30Ga20 powder metallurgical target. Independent from variation of substrate temperature during the sputtering process the deposited films are found to be polycrystalline. X-ray diffraction patterns show a decreasing peak width and a shift to slightly higher Bragg angles with increasing substrate temperature during sputtering, which is even amplified when subsequent rapid thermal annealing is applied. Annealing temperatures above 500°C lead to a remarkable enhancement of the shape memory effect as well as of the magnetostriction. Temperature induced martensitic transformations have been measured by a cantilever deflection technique and a cantilever resonance method. Martensitic start temperatures (MS) range between 50 and 90°C depending on composition and annealing temperature. Stress relief upon the martensitic transformation ranges between 200 and 300 MPa whereas the magnetostrictive coupling constant b is about 2 MPa. Magnetization measurements and Curie temperature determination reveal ferromagnetic behavior within the temperature range of the martensitic transformation.

Thermal decomposition of (NH4)2[PtCl6] - an in situ X-ray absorption spectroscopy study

The thermal decomposition of di-ammonium hexachloroplatinate ((NH4)2[PtCI6]) to metallic platinum has been investigated by time-resolved in situ XAS measurements at the CI Kand Pt Lmedge. Spectra at the CI K-edge were recorded utilizing an energy dispersive monochromator (EDM), while the Pt Lm-edge spectra were recorded using a double crystal monochromator. By analysing the energy shift of the CI K-edge a reaction intermediate has been observed at about 310°C. Fingerprinting arguments indicate that this intermediate could be cis-platin or some other platinum amine complex.

High-Throughput Characterization of Shape Memory Thin Films using Automated Temperature-Dependent Resistance Measurements

Shape memory alloy (SMA) thin films are used as actuator materials in MEMS due to their unique properties. Binary thin films with a composition close to Ni 50 Ti 50 are well-established materials, whereas ternaries like NiTiCu, NiTiPd, NiTiHf are less studied. Furthermore, new alloys are being developed which show a magnetic shape memory effect, e.g. Ni 2 MnGa. For the optimization of known, and the development of new, SMA thin films, a fast and reliable characterization technology is needed, which rapidly identifies the transformation temperatures (i.e. martensite and austenite start and finish temperatures) for a range of material compositions deposited on a whole wafer. In this paper, automated temperature-dependent resistance measurements are discussed as a means which yields the thermal hysteresis of the investigated thin films. Results of monitoring the uniformity of shape memory film depositions on the wafer level, as well as results on the use of this method as a tool for screening for new SMA films by characterization of materials libraries are reported.

SMA-thin film composites providing traveling waves

Actuators providign traveling waves are attractive for several industrial applications, like active skins for turbulent drag reduction or transport devices for assembling processes. Traveling waves require a flexible structure in contrast to standing waves which contain knots without vertical motion. Therefore, different concepts to realize these waves have been developed. This work presents the functional principle of wave generation by means of shape memory allow (SMA) thin film composites and the conditions that have to be considered for the performance of traveling waves with continuous wave flow. Devices using temperature inhomogeneities, an arrangement of separately addressed SMA composites as well as structures using different SMAs have been investigated and their feasibilty is discussed.

Fabrication and characterization of freestanding NiMnGa films

Freestanding Ni 50 Mn 30 Ga 20 films of 13 μm thickness were fabricated by DC magnetron sputtering and analyzed. Magnetic measurements revealed the influence of the post deposition rapid thermal annealing process on the ferromagnetic hysteresis. Ferromagnetic properties evolved after annealing at 400°C. Thermal annealing of at least 600°C led to polycrystalline films that transformed reversibly and martensitically as shown by structural analysis and differential scanning calorimetry. Transformation temperatures and enthalpies of transformation of the martensitic transformation were strongly influenced by the temperature of the rapid annealing process. It is proposed that the annealing data reflect the evolution of the crystalline state.

Smart motion control by phase-coupled shape memory composites

This contribution describes a new functional principle for the motion of walking robots by means of shape memory composites. It illustrates the phase-coupled motion of actuator components with different shape memory alloys allowing wave-like or inch-worm behavior, respectively. The thermo-mechanical properties of the corresponding thin film composites with Ti(NiCu) and (TiHf)Ni alloys are discussed with the focus on their hysteresis. The shape memory effect of the actuators is characterized and samples are performed to prove their capabilities. The contribution also presents a bistable shape memory composite in which a PMMA polymer fixes the bending states of the metallic composite to reduce the energy consumption.

Superelastic Thin Film NiTi-Polymer Composites and Sputtered Thin-walled Tubes

Superelastic shape memory materials are of special interest in medical applications due to the large obtainable strains, the constant stress level and their biocompatibility. Superelastic NiTi-polymer-composites have the potential to be used for novel applications in orthodontics and medical instrumentation as well as in certain areas of mechanical engineering. Especially, using NiTi thin films these composites have the potential to substantially reduce those forces compared to conventional NiTi wires and tubes. In orthodontic applications lowering the forces during archwire treatment is of special importance due to tooth root resorption, which can be caused by the application of oversized forces. Furthermore, the use of superelastic materials or composites enables the application of constant forces independent of diminutive tooth movements during the therapy due to the superelastic plateau. Superelastic NiTi thin films have been fabricated by magnetron sputtering using extremely pure cast melted targets. Special heat treatments were performed for the adjustment of the superelastic properties and the transformation temperatures. A superelastic strain exceeding 4% at 36°C was obtained. In this paper the fabrication of superelastic NiTi thin walled tubes by magnetron sputtering is presented and their mechanical properties are compared to conventional wires and tubes in view of orthodontic applications.