Dispersion characteristics and effect of heating rates on the mechanical properties of spark plasma sintered multi-walled carbon nanotubes reinforced nickel aluminide composites
- Authors: Ayodele, Olusoji Oluremi
- Date: 2020
- Subjects: Carbon nanotubes , Nickel-aluminum alloys , Carbon composites
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/481841 , uj:43673
- Description: D.Phil. , Abstract: The dispersion of multi-walled carbon nanotubes (MWCNTs) in NiAl matrix was successfully carried out through the combination of two steps planetary ball millings. Low energy ball milling (LEBM) was employed for preliminary milling of the starting powders at 150 rpm. Subsequently, the milled powders were transferred to the high energy ball milling (HEBM) for auxiliary milling, and to ensure proper dispersion. This was performed at different milling hours (1 h, 2 h and 3 h) to further de-bundle the MWCNTs. The milled powders were consolidated via spark plasma sintering (SPS) using the sintering temperature of 1000 oC, sintering pressure of 50 MPa, holding time of 10 min and heating rates from 50-150 oC/min. The morphology of the milled powders and the consolidated compacts were observed using the scanning electron microscopy (SEM) and the structural and phase identifications were examined through the X-ray diffractometer (XRD). Raman spectroscopy was used to determine the structural integrity of MWCNTs in the metal matrix and Transmission electron microscopy (TEM) revealed the level of damages or defects done to the MWCNTs. The SEM micrograph of the consolidated NiAl-1wt% MWCNTs composite (HEBM, 1 hour) revealed a relative dispersion of MWCNTs with a few agglomerations and this was corroborated by the TEM images. Furthermore, the Vickers microhardness and relative density of the consolidated samples was observed to decrease as the heating rate was increased and further increase in heating rate led to the increase in the density of the consolidated NiAl. Conversely, the Vickers microhardness and relative density of the consolidated NiAl-1wt% MWCNTs composites decreased as the heating rate was increased from 50-150 oC/min. The nanoindentation plot indicates that the sintered samples with higher penetration depths at the indentation load of 75 mN had lower nanohardness values. MWCNTs was observed to have decreased the coefficient of friction due to the formation of lubricating tribolayer at the contact...
- Full Text:
- Authors: Ayodele, Olusoji Oluremi
- Date: 2020
- Subjects: Carbon nanotubes , Nickel-aluminum alloys , Carbon composites
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/481841 , uj:43673
- Description: D.Phil. , Abstract: The dispersion of multi-walled carbon nanotubes (MWCNTs) in NiAl matrix was successfully carried out through the combination of two steps planetary ball millings. Low energy ball milling (LEBM) was employed for preliminary milling of the starting powders at 150 rpm. Subsequently, the milled powders were transferred to the high energy ball milling (HEBM) for auxiliary milling, and to ensure proper dispersion. This was performed at different milling hours (1 h, 2 h and 3 h) to further de-bundle the MWCNTs. The milled powders were consolidated via spark plasma sintering (SPS) using the sintering temperature of 1000 oC, sintering pressure of 50 MPa, holding time of 10 min and heating rates from 50-150 oC/min. The morphology of the milled powders and the consolidated compacts were observed using the scanning electron microscopy (SEM) and the structural and phase identifications were examined through the X-ray diffractometer (XRD). Raman spectroscopy was used to determine the structural integrity of MWCNTs in the metal matrix and Transmission electron microscopy (TEM) revealed the level of damages or defects done to the MWCNTs. The SEM micrograph of the consolidated NiAl-1wt% MWCNTs composite (HEBM, 1 hour) revealed a relative dispersion of MWCNTs with a few agglomerations and this was corroborated by the TEM images. Furthermore, the Vickers microhardness and relative density of the consolidated samples was observed to decrease as the heating rate was increased and further increase in heating rate led to the increase in the density of the consolidated NiAl. Conversely, the Vickers microhardness and relative density of the consolidated NiAl-1wt% MWCNTs composites decreased as the heating rate was increased from 50-150 oC/min. The nanoindentation plot indicates that the sintered samples with higher penetration depths at the indentation load of 75 mN had lower nanohardness values. MWCNTs was observed to have decreased the coefficient of friction due to the formation of lubricating tribolayer at the contact...
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Microstructural characterization and mechanical properties of carbon nanotube reinforced nickel aluminide composites
- Authors: Awotunde, Mary Ajimegoh
- Date: 2020
- Subjects: Nickel - Aluminum alloys , Diffusion - Defects , Nanotubes - Carbon content , Fracture mechanics , Carbon composites
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/413118 , uj:34791
- Description: Ph.D. (Metallurgy) , Abstract: Toughened nickel aluminides were successfully synthesized by incorporating multi-walled carbon nanotubes (MWCNTs) into nickel aluminide (NiAl) intermetallic matrix. The drive to retain the relative lightweight of NiAl motivated the choice of MWCNTs as the reinforcing agent in this study. Moreover, enhanced mechanical properties were anticipated in the reinforced composites owing to the exceptional properties of the MWCNTs. Elemental powders of nickel and aluminium were blended together with MWCNTs in a novel two stage ball milling for optimum dispersion and preservation of the structural integrity of the MWCNTs. The milled powders were consolidated by Spark Plasma Sintering (model HHPD- 25, FCT GmbH, Germany). The milled powders and sintered samples were characterized using Scanning Electron Microscopy, Transmission Electron Microscopy and X-Ray Diffraction. The nano-structural evolution of the MWCNTs during their dispersion via dry ball milling was further evaluated using Raman Spectroscopy. The mechanical properties and fracture behaviours of the reinforced sintered samples were critically evaluated using nanoindentation techniques. Results show that the integration of MWCNTs into the NiAl matrix led to an enhancement of the fracture toughness. An inverse relationship between the hardness and fracture toughness of the NiAl composites was established. The intergranular fracture morphology of the unreinforced NiAl transited to a dominantly dimpled fracture morphology in the NiAl-1.0 wt% CNTs composites indicating enhanced ductility and fracture toughness. The improvement of the fracture toughness of the reinforced NiAl is attributed to the uniform dispersion of theMWCNTs within the NiAl matrix, the preservation ofMWCNTs aspect ratios and the disordering of the B2 ordered NiAl intermetallic structure.
- Full Text:
- Authors: Awotunde, Mary Ajimegoh
- Date: 2020
- Subjects: Nickel - Aluminum alloys , Diffusion - Defects , Nanotubes - Carbon content , Fracture mechanics , Carbon composites
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/413118 , uj:34791
- Description: Ph.D. (Metallurgy) , Abstract: Toughened nickel aluminides were successfully synthesized by incorporating multi-walled carbon nanotubes (MWCNTs) into nickel aluminide (NiAl) intermetallic matrix. The drive to retain the relative lightweight of NiAl motivated the choice of MWCNTs as the reinforcing agent in this study. Moreover, enhanced mechanical properties were anticipated in the reinforced composites owing to the exceptional properties of the MWCNTs. Elemental powders of nickel and aluminium were blended together with MWCNTs in a novel two stage ball milling for optimum dispersion and preservation of the structural integrity of the MWCNTs. The milled powders were consolidated by Spark Plasma Sintering (model HHPD- 25, FCT GmbH, Germany). The milled powders and sintered samples were characterized using Scanning Electron Microscopy, Transmission Electron Microscopy and X-Ray Diffraction. The nano-structural evolution of the MWCNTs during their dispersion via dry ball milling was further evaluated using Raman Spectroscopy. The mechanical properties and fracture behaviours of the reinforced sintered samples were critically evaluated using nanoindentation techniques. Results show that the integration of MWCNTs into the NiAl matrix led to an enhancement of the fracture toughness. An inverse relationship between the hardness and fracture toughness of the NiAl composites was established. The intergranular fracture morphology of the unreinforced NiAl transited to a dominantly dimpled fracture morphology in the NiAl-1.0 wt% CNTs composites indicating enhanced ductility and fracture toughness. The improvement of the fracture toughness of the reinforced NiAl is attributed to the uniform dispersion of theMWCNTs within the NiAl matrix, the preservation ofMWCNTs aspect ratios and the disordering of the B2 ordered NiAl intermetallic structure.
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Synthesis and modifications of polymer-based carbon/metal organic framework composites for hydrogen storage applications
- Authors: Molefe, Lerato
- Date: 2019
- Subjects: Polymeric composites , Carbon nanotubes , Carbon composites , Nanocomposites (Materials)
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/417836 , uj:35402
- Description: Abstract: The South African energy landscape is dominated by carbon dioxide (CO2) emitting coal combustion which is currently plentiful and cheap however because of its finite nature, it will soon diminish. With the current growing population and increasing energy need, research scientists all over the world are continuously working towards shifting to cleaner renewable energy sources of which hydrogen (H2) is a promising candidate. Hydrogen has a high gravimetric energy density than any other fuel and its combustion in pure oxygen only produces water vapour and heat. However, since it is very light and as a result has a low volumetric energy density, its storage is very difficult. Generally, adsorbents materials such as metal-organic frameworks (MOFs) such as Materials Institute Lavoisier (MIL-101(Cr)) and Universitetet i Oslo (UiO-66(Zr)) as well as porous carbons such as zeolite templated carbon (ZTC) have been well investigated for H2 storage due to their unique properties such as extremely high surface areas and ultrahigh porosities. However, their great potential in H2 storage applications is limited by their lack of immediate processability, because they are obtained as fine powders in their as-synthesised form. For these materials to gain practical applications in H2 storage they must be shaped into mechanically stable and easy to handle bodies like monoliths. The process of shaping MOFs and carbon powders often includes the use of various non-porous polymers and inorganic materials as binders and that often results in pore-blocking effects and low H2 uptake... , Ph.D. (Chemistry)
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- Authors: Molefe, Lerato
- Date: 2019
- Subjects: Polymeric composites , Carbon nanotubes , Carbon composites , Nanocomposites (Materials)
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/417836 , uj:35402
- Description: Abstract: The South African energy landscape is dominated by carbon dioxide (CO2) emitting coal combustion which is currently plentiful and cheap however because of its finite nature, it will soon diminish. With the current growing population and increasing energy need, research scientists all over the world are continuously working towards shifting to cleaner renewable energy sources of which hydrogen (H2) is a promising candidate. Hydrogen has a high gravimetric energy density than any other fuel and its combustion in pure oxygen only produces water vapour and heat. However, since it is very light and as a result has a low volumetric energy density, its storage is very difficult. Generally, adsorbents materials such as metal-organic frameworks (MOFs) such as Materials Institute Lavoisier (MIL-101(Cr)) and Universitetet i Oslo (UiO-66(Zr)) as well as porous carbons such as zeolite templated carbon (ZTC) have been well investigated for H2 storage due to their unique properties such as extremely high surface areas and ultrahigh porosities. However, their great potential in H2 storage applications is limited by their lack of immediate processability, because they are obtained as fine powders in their as-synthesised form. For these materials to gain practical applications in H2 storage they must be shaped into mechanically stable and easy to handle bodies like monoliths. The process of shaping MOFs and carbon powders often includes the use of various non-porous polymers and inorganic materials as binders and that often results in pore-blocking effects and low H2 uptake... , Ph.D. (Chemistry)
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