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Development of a Novel Electroless Plating Approach for Coating Aluminum (Al) on Multi-Walled Carbon Nanotubes (MWCNTs) and Investigating their Use in Reinforcing Cast and Powder Compacted Aluminum Composites
Due to their superior mechanical properties, Aluminum-Carbon Nanotube (Al-CNT) composites have been widely investigated by various researchers using several processing techniques. One of the most successful techniques in creating Al-CNT composites is the ball milling technique followed by hot compac...
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AUC Knowledge Fountain
2019
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| Summary: | Due to their superior mechanical properties, Aluminum-Carbon Nanotube (Al-CNT) composites have been widely investigated by various researchers using several processing techniques. One of the most successful techniques in creating Al-CNT composites is the ball milling technique followed by hot compaction and hot extrusion for powder consolidation. However, this technique has some drawbacks including the limited design flexibility of the produced parts, the high initial cost of milling equipment, and the damage of CNTs during milling that has been reported to take place due to the harsh impact of milling balls on CNTs leading to the formation of brittle Al4C3 phases in the aluminum matrix with the increase of milling time. Therefore, researchers have investigated simple, cheap and flexible casting techniques in producing Al-CNT composites. However, it was found that CNTs are not wettable in molten aluminum due to the big difference in surface tension between them. Besides, CNTs have a lower condensed phase density than aluminum. Therefore, they tend to form segregations in the aluminum matrix that hinder the mechanical properties of the entire composite. Therefore, some researchers have experimented coating CNTs with metals by electroless plating to modify their surface characteristics and make them wettable with molten aluminum. However, these metallic coats were found to have a misleading alloying effect on the aluminum matrix that made the investigation of CNTs addition on aluminum not viable. In the current study, a novel aluminum electroless plating approach was developed and made it possible to cover CNTs with a nano-crystalline layer of pure aluminum at room temperature. The Al-coated CNTs are used as a reinforcement phase for pure aluminum without introducing any alloying elements. Previously, aluminum electroless plating in aqueous solutions represented an impossible challenge due to the high electrode potential of aluminum (-1.66 E(V)) that lies outside the electrochemical window (EW) of water. Therefore, limited number of trials have been made towards developing room temperature ionic liquids (RTILs) of a wide EW suitable for aluminum electroless plating. Researchers have focused on expensive RTILs such as AlCl3-EMIC that has a wide electrochemical window allowing aluminum to be deposited without decomposing the ionic compound. However, due to the high surface area to volume ratio of CNTs, they require a larger volume of the RTIL to be electroless plated. This could put Al-coated CNTs at a tremendously high cost of production. Therefore, the current study focused on finding an alternative cost effective RTIL for aluminum electroless plating by modifying AlCl3-Urea battery electrolyte. This modification was done by adjusting the molar ratio of AlCl3-Urea from 1.3:1 to 2:1 in addition to adding lithium aluminum hydride (LAH) as a reducing agent. The electrochemical window of the new electrolyte was measured against a standard Ag/Ag+ electrode and reported to be wider than the requirement of aluminum plating. This modified RTIL was used to coat MWCNTs (10 nm diameter) by aluminum in 3 steps starting by catalyzing CNTs using colloidal palladium-stannous (Pd-Sn) nanoparticles in a single step followed by an acceleration step in a group of acids to remove excess stannous hydroxide Sn(OH)2 from the surface of Pd-Sn particles. Finally, electroless plating of Al on CNTs was done in the developed RTIL. Electron microscopy imaging revealed that CNTs had up to 160 nm increase in their thickness after deposition indicating the formation of Al-coat on their surfaces. Energy dispersive X-ray (EDX) analysis along with the X-ray diffraction (XRD) pattern confirmed the presence of crystalline Al on the surface of CNTs. The Raman analysis revealed that CNTs did not undergo any damage during the chemical coating process as the D-band to the G-band intensity ratio of Al-coated CNTs did not differ from the one reported for the as-received CNTs. The prepared Al-CNT powders were used to reinforce pure aluminum using both the casting and hot compaction techniques such that the overall percent of CNTs in the final composite were 2% by weight in each sample. The XRD of bulk samples detected the presence of all the crystallographic planes of aluminum in addition to small spikes of Al4C3 in both cast and powder compacted samples. Raman analysis showed that a small increase in the D-band took place in both the cast and the hot compacted samples. However, the D-band was higher in the hot compacted sample than in the cast sample. Mechanical properties of the obtained Al-CNT composites were investigated for both the hot compacted-hot extruded samples and the cast samples. The coated layer of aluminum on CNTs resulted in excellent dispersion and wettability of CNTs in the cast sample reflected on the homogeneous mechanical properties obtained with a minimal standard error (SE) and on the SEM fractography imaging obtained for each sample. The mechanical testing showed an increase of 353.4 %, 338.7 %, and 266.1 % in the tensile strength, Vickers hardness, and nanoindentation hardness of the reinforced cast sample in addition to an increase of 164.8 %, 147.2 %, and 165.8 % of the same properties for the hot compacted-hot extruded reinforced samples respectively. These promising results will open the road for producing low cost, design flexible, and fast to produce Al-CNT composites with enhanced mechanical properties. |
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