Investigations of atomic-level structure-property relationship of ternary magnetic metallic alloys and metal-pipe waveguides for sub-terahertz application
Student thesis: Doctoral Thesis
The atomic-level structures and structure-property relationship of ternary alloy materials have been revealed by using state-of-the-art first principle computational techniques in combination with synchrotron radiation X-ray measurement. Their excellent magnetic properties and glass-forming ability (GFA) were also characterized. Three types of ternary magnetic metallic alloys have been investigated. Particular attention is paid to their correlation between their atomic structures and magnetic properties. In the second part, micromachined metal-pipe rectangular waveguide filters based on microelectromechanical system (MEMS) microfabrication technique have been successfully developed for sub-terahertz (sub-THz) application. Several FeB-based glassy alloys (Fe80B8Si12, Fe71B17Si12 and Fe71B17 (Nb4Y4Zr4)) with good mechanical properties and magnetic properties have recently been well developed by addition of solute atoms to tune the magnetic properties and GFA. The atomic structures were revealed by state-of-the-art extended X-ray absorption fine structure spectroscopy (EXAFS) combining with ab initio molecular-dynamics (AIMD) computational techniques and the electropulsing treatment (EPT) effect on magnetic properties of the FeBSi metallic glasses were further correlated with their structures. The experimental spectra were in good agreement with the EXAFS results. Besides, the magnetic behaviors was discussed in view of the EXAFS results and atomic structures of the glassy alloys. Based on a modified cluster model, two CoSiB metallic glasses of low Co contents (Co63.1B27Si9.9 and Co65B24.4Si10.6) with good mechanical properties and different physical properties have recently been developed by addition of solute atoms within their solubility limit to enhance GFA. Despite the similar compositions, these two glassy alloys consist of different clusters, and therefore exhibit different magnetic behaviors. Co63.1B27Si9.9 metallic glass, based on the Co7B3 cluster, shows good soft magnetic properties with low coercivity, while Co65B24.4Si10.6 metallic glass, based on the Co8B3 cluster, exhibits paramagnetic behavior. The atomic structures and the origins of the magnetic properties of two ternary CoBSi glassy alloys by state-of-the-art EXAFS combining with AIMD computational techniques were elucidated. The experimental spectra were in good agreement with the calculations of ab initio full multiple scattering theory using the FEFF8.4 code. Besides, the origin of the different magnetic behaviors, which is dominated by short-range orders (SROs), was discussed in view of the EXAFS results and cluster structures of the metallic glasses. Fe2P-type Gd6FeBi2 alloy with a high spin polarisation was successfully synthesized, and the compound shows good soft magnetic behavior at room temperature with a Curie temperature ~350 K. The compound exhibits non Curie-Weiss behavior in a large temperature range above Curie temperature, and slightly enhanced Gd moment at low temperature. The magnetocaloric effect was also measured in terms of the maximum magnetic entropy change of -4.3 J•Kg-1K-1 at 50 kOe and -2.3 J•Kg-1K-1 at 20 kOe. These findings may be of general importance for the understanding of the physical properties of magnetocaloric materials and spin polarised materials towards optimised performance in spintronics devices. It is very challenging to fabricate micromachined rectangular waveguide filters in the sub-THz regime with the dimension from several ten micrometers to several millimeters by traditional machining technology. Micromachined metal-pipe waveguides in the sub-THz regime based on deep reactive ion etching (DRIE) and bonding technologies have been successfully developed for THz applications. The effects of metallized layer and iris thickness, roughness on main performances of waveguide filters are also investigated. The test results show that the insertion loss is less than 1.8dB with the isolation larger than 15dB, which meet the needs of practical application. A promising application perspective that such a micromachined sub-microscale and nanoscale 3D structure as a mold for imprinting of the metallic glass to create a high-aspect-ratio TH plasmonic device is also discussed. The developed MEMS microfabrication technique is believed to be good at making high-performance rectangular waveguides at 0.10-1THz.
- Metallic glasses, Magnetic alloys, Electric filters, Wave-guide