Showing papers by "Alfred Ludwig published in 2008"

Development of multifunctional thin films using high-throughput experimentation methods

Abstract: This paper describes the use of thin film high-throughput experimentation methods for the efficient development of multifunctional materials, using Ni – Ti – X and ferromagnetic shape memory alloys as examples. The thin films were fabricated in the form of binary, ternary, and quaternary materials libraries by special magnetron sputter deposition processes. These materials libraries were subsequently processed and characterized by high-throughput experimentation methods in order to relate compositional information with structural and functional properties. For this, appropriate visualization of the data is necessary. Results show that the martensitically transforming regions in ternary thin films are generally larger than was known from literature. Within these regions, the variation of the functional properties can be mapped with respect to the composition and microstructure, and thus the most suitable materials for applications can be effectively selected.

Combinatorial fabrication and high-throughput characterization of a Ti–Ni–Cu shape memory thin film composition spread

Abstract: The phase transformation properties of Ti–Ni–Cu shape memory thin films prepared in the form of a continuous composition spread were investigated. The thin film materials library was fabricated from elemental targets using an ultra-high vacuum combinatorial magnetron sputter-deposition system. Alternating wedge-type layers of Ti, Ni, and Cu were deposited on a thermally oxidized Si wafer and subsequently annealed at 500 °C for 1 h in situ . Automated temperature-dependent resistance measurements ( R ( T )), energy dispersive X-ray analysis and X-ray diffraction measurements revealed the compositional region in the ternary phase diagram where thermoelastic transformations occur. The transformation temperatures and the thermal hysteresis were determined from R ( T ) measurements. Within the composition spread an extended transformation region was found. For Ni-rich compositions, a two-stage B2 → R → B19′ phase transformation was observed. Among others, compositions of Ti 51 Ni 49− x Cu x (at.%) show a single-stage transformation for a Cu-content 14 at.% (B2 → B19). In the intermediate composition range two-stage transformations (B2 → B19 → B19′) were found. The dependency of the thermal hysteresis on the Cu-content as described in the literature was confirmed.

Micro-hotplates for high-throughput thin film processing and in situ phase transformation characterization

Abstract: This paper presents the use of micro-hotplates (MHPs) as thermal processing and in situ characterization platforms for phase transformations in thin films. MHPs are fabricated by microsystem technology processes and consist of a SiO2/Si3N4 membrane (app. 1 μm) supported by a bulk Si frame. Several embedded Pt thin films serve as heater and electrical measurement electrodes. It is shown that the MHPs have unique properties for the controlled annealing of thin film materials (up to 1270 K), as the annealing temperature and heating/cooling rates can be precisely controlled by in situ measurements. These rates can be extremely high (up to 104 K/s), due to the low thermal mass of MHPs. The high cooling rates are especially useful for the fabrication of metastable phases (e.g. Fe70Pd30) by quenching. By measuring the resistivity of a thin film under test in situ as a function of the MHP temperature, microstructural changes (e.g. phase transformations) can be detected during heating and cooling cycles. In this paper, examples are presented for the determination of phase transitions in thin films using MHPs: the solid–liquid–gas phase transition (Al), the ferromagnetic–paramagnetic phase transition (Fe–Pt) and martensitic transformations (Ni–Ti–Cu, Fe–Pd). Furthermore, it is demonstrated that crystallization processes from amorphous to crystalline (Ni–Ti–Cu) can be detected with this method. Finally the application of MHPs in thin film combinatorial materials science and high-throughput experimentation is described.

Magnetic properties of epitaxial Fe-Pd films measured at elevated temperatures

Abstract: Magnetic properties of disordered Fe–Pd films with 28 and 33at.% Pd epitaxially grown at room temperature were measured at room and elevated temperatures up to 900 and 600K, respectively. Thermomagnetic measurements of the film with 28at.% Pd having a body-centered-tetragonal structure in the as-deposited state show that, due to their metastability, disordered Fe–Pd phases can decompose at elevated temperatures. For a film with 33at.% Pd having a face-centered-tetragonal (fct) structure in the as-deposited state, no indication of decomposition or first order transformation is observed up to 600K, suggesting that the fct phase required for the magnetic shape memory effect is quite stable in epitaxial films.

Investigation of Thin Coatings from Mn-Co-Fe System Deposited by PVD on Metallic Interconnects for SOFC Applications

Abstract: Ternary Mn-Co-Fe metallic thin films were deposited by RF-magnetron co-sputtering on SiO2/Si wafers and on ZMG232L (Hitachi Metals®), a special ferritic stainless steel for Solid Oxide Fuel Cell applications. The deposition was followed by heat treatment in an oxidizing atmosphere in order to convert the metallic thin films to (Mn,Co,Fe)3O4 spinel oxides. Coated and uncoated steel samples were analyzed after 1 h heat treatment in order to confirm the presence of the spinel structure on top of the steel, as well as to investigate and characterize the growth of oxides, namely (Mn,Cr)3O4 and Cr2O3, at the internal steel/coating interface. From Grazing Incidence X-ray Diffraction (GI-XRD) investigations together with Energy Dispersive X-ray analysis – Scanning Electron Microscopy and Time of Flight – Secondary Ions Mass Spectroscopy sputtering depth profiling the presence of well adherent (Mn,Co,Fe)3O4 coatings with approximately 500 nm thickness and a grain size of about 150 nm was confirmed. After the preparation annealing, some samples were heat-treated in simulated cathodic atmospheres at 800 °C for 500 h in order to assess the stability of the coatings. GI-XRD spectra still showed the presence of the protective coatings, however sputtering depth profile analysis indicated the presence of Cr on the surface.