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Title: Synthesis of SmCo for 3D components by metal-organic compounds
Authors: Huang, Junfeng
Degree: Ph.D.
Issue Date: 2019
Abstract: Functional alloy materials are widely used for various applications. Alloys of rare-earth metals (REMs) and transition metals (TMs) are getting important due to their outstanding magnetic properties. Samarium-cobalt (SmCo) alloy is one of the most outstanding magnetic materials in this group due to its large magneto-crystalline anisotropy and high Curie temperature. In this PhD thesis, it presents my research studies on how to fabricate the three-dimensional (3D) structure of SmCo alloys. Three technical challenges are involved in the synthesis of SmCo alloys with 3D structure: The synthesis of SmCo alloys with 3D structure involves 3 challenges as follows: (i) The current practice of synthesizing SmCo powders uses acetylacetonate precursors in highly volatile organic solvents. The metal (Sm and Co) acetylacetonates are toxic and the synthesis process is not environmental-friendly. (ii) The demand for magnetic materials with high flexibility in shape has increased to cater for different applications. There exist several techniques which produce the materials in 3D structure, such as laser sintering. However, laser sintering is not applicable for 3D structure fabrication of SmCo alloys due to its geometric limit: the non-spherical shape of SmCo powder does not meet the requirement of raw material for laser sintering. (iii) Before sintering to obtain final products, polymer binders are added to adjust densification and improve structural stability of the shape of 3D structure. However, the content of binder affects physical properties of products. Also, crack formation during debinding easily occurs and deforms the shape of 3D structure. Solutions for addressing the challenges: This research investigates and develops pathways for synthesis of SmCo alloys using metal-organic compounds and appropriate fabrication methods to form 3D structure alloys. The research consists of three parts in addressing the challenges: (1) Development of an alternative precursor, named ascorbate, from the derivation of vitamin C with water as the solvent for the sake of sustainability. (2) Development of appropriate methods to fabricate 3D structure for SmCo alloys. Laser system and inkjet printing were developed using TMs to find out the advantages and disadvantages for fabricating SmCo alloys with 3D structure. (3) Development of the control mechanism in binder content and thermal profile to enhance the crystal structure and magnetic properties of SmCo alloys.
The first part of this work was to synthesize metal-organic compounds, cobalt ascorbate and samarium ascorbate. The ratio of element and weight ratio between cobalt and samarium for the mixture were 4.3:1 and 1.8:1 respectively, which corresponded to findings in available literature. The decomposition of samarium ascorbate and cobalt ascorbate was found to occur at 326 ℃ and 380 ℃ respectively before the alloying of SmCo began. This addresses challenge (i). The second part of this work focuses on the development of 3D structure fabrication for metals or metal-organic compounds using laser system and inkjet printing. The laser system was used to produce 3D structures of (a) copper by reducing copper oxide and copper carbonate hydroxide to form copper circuit lines with resistivity of 1.46×10⁻⁶ Ωm, which is slightly higher than that of the bulk copper (1.68×10⁻⁸ Ωm); and (b) cobalt embedded within the carbon matrix by reducing ZIF-67 (a type of cobalt-based organic compound) to perform as oil absorbents. Inkjet printing system was used to produce 3D cobalt-based material (Co(tpy)Cl₂ · MeCN) on a silicon wafer with a droplet spacing of 110 μm. This addresses challenge (ii). The third part of the work was to produce 3D SmCo alloys. The thermal sintering (by tube furnace) of metal-organic compounds, samarium ascorbate and cobalt ascorbate, using 6% PVA as the binder was able to produce SmCo alloys with 1:3 phase (SmCo₃). Laser sintering of SmCo powder with 1:5 phase (SmCo₅) using laser sintering parameters of 250 μm hatch space, 150 W laser power, 1270 mm/min scanning speed in one-time sintering, was able to produce 3D structure for SmCo5 alloys with coercivity and saturation magnetization of 1586.4 Oe and 83.49 emu/g respectively, similar to the magnetic properties of SmCo5 powders. This addresses challenge (iii). In conclusion, this study has successfully synthesized samarium ascorbate and cobalt ascorbate as the metal-organic precursors in chemical reaction in a non-hazardous, environmental-friendly manner. SmCo alloys were fabricated into a 3D structure using the proposed precursors and thermal sintering, which can overcome the geometric limitation of SmCo powders for laser sintering. Also, the control mechanisms in binder content and thermal profile in thermal sintering were developed to improve the crystallization of SmCo alloys and avoid the crack formation in the 3D structure.
Subjects: Hong Kong Polytechnic University -- Dissertations
Organic compounds -- Synthesis
Pages: xx, 247 pages : color illustrations
Appears in Collections:Thesis

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