Site: Institut für Festkörper und Werkstofforschung (IFW)
PF 27 00 16
D-01171 Dresden, Germany
Tel: (49) 351-4659 0; Fax: (49) 351-4659 540
Date Visited: 14 October 1997
WTEC: C. Koch (report author), R.W. Siegel
The Institute of Solid State and Materials Research, IFW Dresden, was founded in 1992. As an institute of the Wissenschaftsgemeinschaft Blaue Liste (WBL) it is funded by the Free State of Saxony and the Federal Republic of Germany. It has a staff of over 400 employees and is devoted to applications-oriented fundamental research. It is a member of the Materials Research Association, Dresden, and is associated with the Dresden University of Technology and the Fraunhofer Institute. In 1996 the staff consisted of 235 permanent and 180 temporary employees, of which 220 were scientists. The budget for 1996 consisted of about DM 30 million basic financing and about DM 11 million in projects. The scope of the research includes studies of the relationships between fundamental and applications-related characteristics of metallic and nonmetallic functional materials and thin films, investigation of structural properties and failure mechanisms, and studies of processing variables for property optimization. The WTEC team was hosted by Prof. Dr. Ludwig Schultz, who is director of the Institute for Metallic Materials. Research on nanostructures and nanostructure science is carried on in many of the groups in IFW, with about one-third of the groups partly or mostly involved.
Investigators in several fields of nanostructure science made presentations to the WTEC team, after which we toured the laboratory facilities. Brief descriptions of the presentations are given below, with scientific and/or technical highlights.
Professor Leo gave a presentation on studies of electronic transport through single molecules in epitaxially grown organic heterostructures. This involves single-electron tunneling effects with single molecules. Common approaches using metal structures of ~ 30 nm size with capacitance of a few 10-17F require temperatures of < 20K. IFW's approach is to use molecules providing stable and reproducible "bricks" with a typical size of 1-2 nm. Organic molecular beam epitaxy (OMBE) was used to deposit molecules of perylene-tetra-carboxylic-dianhydride (PTCDA) on Au (111), which is on a cleaved mica substrate. PTCDA orients on lines on the Au lattice. Coulomb blockade measurements are made at room temperature with an STM tip. This can only be accomplished on an "ordered" molecular lattice that is locked into the Au lattice so it does not move. An insulating layer of deconethiol is placed between PTCDA and the Au, and the S in the deconethiol binds to the Au. Preliminary I-V measurements have been made successfully.
Dr. Golden presented a review of his work on the electronic structure of fullerenes, nanotubes, and metal/fullerene multilayers. Spectroscopic methods are used for these studies and include X-ray absorption spectroscopy (XAS), angle resolved photoelectron spectroscopy (ARPES), X-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS). The facility for high resolution EELS measurements is a dedicated machine, i.e., not part of a transmission electronic microscope. Among the measurements made are charge states, bonding, plasmon dispersions, optical properties, and core level excitations. Various materials studied include C60/metal multilayers, nanotubes, and doped fullerenes (off-ball doping-intercalation, on-ball doping such as C59N, and in-ball doping-endohedral metallofullerenes such as Tm in C82).
Dr. Brückner described his work on the electrical and mechanical properties of a resistive CuNi(Mn) thin film with a nanocrystalline structure. The CuNi(Mn) films, sandwiched between Ni-Cr films, had columnar grains about 30 nm in dimension, and were twinned. The temperature coefficient of resistivity (TCR) was a function of composition x of the Cu1-xNix films and of the temperature of thermal cycling. The initial negative TCR changed to positive after heating to > 500°C. This was explained by the changes in the mechanical stresses in the films, which were influenced by formation of NiO and grain growth at the higher temperatures.
The four major thrusts of this group are: (1) basic principles of mechanically alloyed nanocrystalline materials, (2) high-strength lightweight nanostructured alloys, (3) mechanically alloyed superconducting borocarbides, and (4) bulk metallic glasses.
In the first thrust the relationship to nanostructured research is that ball milling is a nonequilibrium processing method for preparation of nanoscale materials. In this regard the formation of nanocrystalline materials is studied by determining grain size as function of milling conditions such as temperature, milling intensity, and alloy composition. In terms of high strength lightweight alloys, Al and Mg alloys with mixed phases of nanocrystalline, amorphous, and/or quasicrystalline nanoscale microstructures are studied. Of special interest are Al-base (> 90 at.% Al) alloys with nanoscale quasicrystalline phases of 20-100 nm diameter surrounded by fcc Al phase of 5-25 nm thickness. The quasicrystalline phase comprises 60-80% volume fraction of the alloys. These alloys combine high strength (1,000-1,300 MPa fracture strength) and good ductility (6-25%). The suggested mechanisms for these excellent mechanical properties include the thin fcc Al layer around the quasicrystalline particles, a high density of phason defects and approximant crystalline regions with subnanoscale size, and the spherical morphology of the quasicrystalline particles with random orientations. The research is aimed at a better understanding of the mechanical behavior of these promising materials.
Bulk metallic glasses (e.g., Mg55Y15Cu30) are prepared by solidification and mechanical alloying methods. The mechanically alloyed bulk metallic glass powders are consolidated at temperatures above Tg. Again some studies of mixed amorphous and nanocrystalline phases are carried out in these systems. That is, the nanocrystalline precipitates are used to strengthen the amorphous matrix.
Dr. Müller described the research program on the hydrogen-assisted preparation of fine-grained rare earth permanent magnets. The technique used is "hydrogenation disproportionation desorption recombination" (HDDR). The final structure is fine-grained, 100-500 nm, rather than nanoscale, but during the disproportionation and desorption steps the structures can be ~ 100 nm in size. An example of HDDR for Nd-Fe-B is as follows: Original cast alloy of Nd16Fe76B8 with Nd-rich and Nd2Fe14B phases is processed in four steps: (1) hydrogenation forms NdH2.7 and Nd2Fe14BH2.9; (2) disproportionation reaction results in a fine mixture of Fe, NdH2.2, and Fe2B; (3) desorption provides a very fine mixture of Fe, Nd, Fe2B + Nd2Fe14B nuclei; and (4) recombination yields fine grained Nd2Fe14B. Several rare earth permanent magnet alloys are studied at IFW using HDDR, including Sm2Fe17N3 and Sm2Fe17-xGax.
Dr. Neu described NbFeB magnet powders prepared by mechanical alloying. The goal of this work is to obtain high remanent, isotropic Nb-Fe-B powders for polymer-bonded permanent magnets. Mechanical alloying is used to obtain a nanoscale mixture of Nd2Fe14B and aFe which provides for remanence enhancement via exchange coupling when the grain sizes are < 30 nm. The mechanical alloying provides an amorphous + nc Fe structure which on annealing forms nc aFe + nc Nd2Fe14B which behaves as a single magnetic phase. The powders can be bonded with polymers and form isotropic magnets with high remanence. In addition some Fe can be replaced by Co which increases the remanence (as well as the Curie temperature) and has provided (BH)max values up to about 150kJ/m2.
Dr. Mattern described work on soft ferromagnetic materials such as the "finemet"-like alloys (FeNiSiBNbCu) and FeZrB alloys. These materials are made by rapid solidification to obtain amorphous alloys, which are then partially recrystallized to give nanoscale (~ 50 nm) aFe particles in the amorphous matrix. Studies have included composition variations to influence nc grain size and studies of the crystallization kinetics. High nucleation rates and slow growth rates are desired and influenced by the alloy dopants. This research is funded by the federal government and by Vacuumschmelze and Siemens.
Dr. Heilmaier described several projects involving dispersion hardening with nanoscale dispersoids. One project has the goal of dispersion strengthening of Ag to be used as casings for the BiSrCaCuO high TC superconductor. Mechanical alloying of Ag and Cr2O3 powders is followed by cold pressing, annealing in dry hydrogen, hot pressing at 500°C, and finally hot extrusion at 700°C. The mechanical alloying times were apparently too short in the initial study to provide a uniform distribution of the 40 nm Cr2O3 particles. A bimodal grain structure was observed with mean sizes of 10 mm and 0.3 mm composed of 60% pure Ag grains and 40% Ag grains with the nc oxide dispersoids. Even so, significant Hall-Petch hardening was observed at room temperature, along with increased creep resistance at 500°C in the dispersion-hardened Ag. Another project focuses on mechanical alloying of Ll2-(Al,Cr)3 Ti intermetallic with Y2O3 nanoscale dispersoids of 5 nm size.
Dr. Scholl described an in situ data acquisition and monitoring system for a planetary ball mill. This is important for studies of the mechanical alloying/milling processes used for formation of nanocrystalline materials. The device measures the temperature and pressure via a transmitter in the lid of the milling vial. This was done for a Fritsch "pulverisette 5" planetary mill; the work was partially supported by Fritsch. An example was given for milling of Ti and C powders to form TiC. Good time resolution, about 10 ms, is available to monitor the reactions, which can occur during milling and provide feedback to optimize the milling parameters.
After the formal presentations and discussions, the WTEC was given a tour of the IFW laboratories. We observed very impressive state-of-the-art facilities for processing, characterization, and property testing. The dedicated EELS facility referred to in the work of Dr. Golden (above) was particularly noteworthy.