A novel class of non-crystalline materials with controllable atomic/electronic structures corresponding to the structures of modern technologically utilized crystalline materials.
Group Members (Left to Right): Herbert Gleiter, Mohammad Ghafari, Zbigniew Sniadecki, Anna Stößer, Chaomin Wang, Vladimir Šepelàk, Harsha Nandam, Ralf Witte, Robert Kruk, Julia Ivanisenko, Arne Fischer, Horst Hahn, Thomas Reisinger, Ascar Kilmametov
Not in Picture: Cahit Benel, Shiv Prakash Singh, Reda Chellali
Metallic nanoglasses represent a novel development in the field of disordered materials, which was initiated by our research group. In contrast to the metallic glasses, which have been widely reported in the literature and are synthesized by rapid quenching from the molten state, nanoglasses are prepared by consolidation (compaction) of nanoparticles, i.e. completely in the solid state.
It has been demonstrated that nanoglasses exhibit a different structure and as a consequence also different properties compared to rapidly-quenched glasses. Clearly, the presence of the internal nanostructure is the prerequisite of the modified properties.
Metallic nanoglasses have been initially synthesized using the well-established Inert Gas Condensation (IGC) technique, using thermal evaporation of an alloy. As the evaporation is performed in an inert gas atmosphere, nanoparticles are formed, typically in the size range of 5 to 20 nm (average) controlled by the synthesis parameters (total pressure of the inert gas, partial pressure of the evaporating alloy components and type of inert gas). The nanoparticles are transported by the convective inert gas flow from the hot source to a liquid nitrogen cooled cylindrical collector. The nanoparticulate powder is collected and then scraped from the surface of the cold finger, and transferred to an in situ (vacuum) compaction device. The nanopowder is compacted at pressures up to 2 GPa, and subsequently, can be compacted further in an ex situ compaction device, typically up to pressures of 6 GPa.
Alternatively, a sputtering source can be used instead of the thermal evaporation source, thus allowing the preparation of alloys consisting of elements with different vapor pressures. A third alternative is the use of laser ablation in order to vaporize the master alloy. In our group all three systems are employed to prepare a variety of metallic nanoglasses of different material systems.
Fig. NG1: Photos of the Inert Gas Condensation System (IGC) using thermal evaporation (left) and the IGC using laser ablation (right)
In order to prepare even smaller building blocks for the controlled assembly of nanoglasses, we have developed a cluster ion beam deposition system. In this system, clusters containing typically 50 to 1000 atoms, i.e., smaller than 3 nm in diameter, are prepared using a sputter source, and subsequently formed into a cluster ion beam, which can be manipulated inside a UHV beamline by means of electrical and magnetic fields. Using a 90° bending magnet, size selected clusters can be prepared. The impact energy of the cluster ions can be controlled by means of electric fields, from soft landing to hard landing on the substrates, resulting in a variety of different amorphous structures.
Fig. NG2: Schematic (left) and photo (right) of the cluster ion beam deposition system. In the schematic: (a) sputter source for the preparation of the clusters; (b) lens system for formation of cluster ion beam; (c) first deposition stage for non-size selected clusters; (d) time-of-flight mass spectrometer; (e) lens system and Faraday cup; (f) 90° bending magnet; (g) second deposition system for size selected clusters.
Furthermore, nanoglass structures can be prepared by sputtering thin and thick films of the respective alloys under certain deposition parameters, such as gas pressure, angle of deposition, etc. With the correct parameters, columnar structures can be obtained, which resemble the structures observed using the compaction route described above.
Fig. NG3: Photo of the sputter deposition system for the preparation of thin and thick films with a columnar nanoglass structure (left) and SEM image of the columnar structures observed in a sputtered film (right).
Using the above synthesis routes, our group has synthesized many different amorphous alloy systems in the nanoglass structure, including PdSi, FeSc, CuZr, CuSc using the compaction route starting from nanoparticles, and NiTiCu, PdSi and TiZrCuPd films using the film sputtering technique.
We have employed many different techniques to analyze the structure of nanoglasses and always compared the results with the structure of reference glasses, prepared by rapid quenching from the melt. The characterization techniques include conventional X-ray diffraction (XRD), determination of the radial distribution function (RDF) using synchrotron radiation and transmission electron microscopy , EXAFS, SAXS, Mössbauer spectroscopy (MS), positron annihilation spectroscopy (PAS), scanning (SEM) and transmission (TEM) electron microscopy, atom probe tomography (APT), atomic force (AFM) and scanning tunneling (STM) microscopy, and others.
The current understanding based on the results of the above mentioned techniques of the structure of nanoglasses is as follows:
- In the nanoglass structure two distinct atomic environments are identified, while in rapidly quenched amorphous alloys of identical composition only one atomic environment is observed (concluded from results of TEM, AFM, STM, SAXS, PAS, MS, APT).
- One component corresponds in respect of its structure and atomic environment to that of the rapidly quenched material, while the other component, called interfacial component, is characterized by a different chemical composition and by an increased free volume.
- The interfacial component only appears after compaction of the amorphous nanoparticles.
- The interfacial structure is well defined (as seen from the line width of MS) and seems to exhibit a higher degree of short- and medium range order (as concluded from the RDF), compared to the well defined rapidly quenched structure.
- The free volume of the interfacial component is substantially larger than that of the rapidly quenched material.
As a consequence of the structural differences between the nanoglasses and the rapidly quenched counterparts, also the mechanical, magnetic and thermal properties are drastically different.
Fig. NG4: Nanoindentation of different amorphous materials of Cu50Zr50 (left) and SEM images of samples of micropillars made of a nanoglass (middle) and a rapidly quenched glass (right). No shear bands are observed in the nanoglass, while the rapidly quenched glass fails catastrophically.
Fig. NG5: Magnetization curves of Fe90Sc10 nanoglass (red) and rapidly quenched glass (black) measured at room temperature. Clearly the ferromagnetic behavior of the nanoglass can be seen, while the rapidly quenched glass is paramagnetic.
Fig. NG6: Differential scanning calorimetry of Cu50Zr50 nanoglass, rapidly quenched glass and a nanoglass after annealing at 350 °C. It is obvious that the crystallization temperature is higher for the nanoglass than for the rapidly quenched glass.