Reinforcement uniformity into matrix phase
Non-uniform distribution of reinforcement objects is one of the main obstacles for enhanced mechanical properties.58 Several techniques have been explored to circumvent this difficulty. Ultrasound-assisted casting is effective to reach an homogeneous dispersion of the reinforcement objects in the matrix phase.14,59,60 Another method consists in enhancing the wettability of reinforcement objects by the matrix. Pourhosseini et al.61 successfully modified Al2O3 nanoparticles via electroless deposition of Cu, Ni and Co and subsequently applied these particles to reinforce A356 alloy by stir casting. The wettability of the coated nanoparticles was enhanced, resulting in homogeneous distribution in the aluminum matrix. The final mechanical properties (such as microhardness and compressive strength) were then significantly improved, especially for composites encompassing Ni-coated Al2O3. Another method to reach dispersion focuses on the ability of the matrix to carry reinforcement objects. Mousavian et al.62 managed to reinforce an A356 aluminum matrix with SiC ceramic nanoparticles by combining techniques of stir casting and semi-solid extrusion. The authors found that the addition of nickel acted provided a SiC carrier, leading to a uniform dispersion of particles into the aluminum matrix. In view of the mechanical property, the ultimate tensile strength and elongation of the final Al/SiC composite in presence of nickel could reach around 304 MPa and 5.8%, respectively, while the introduction of titanium decreased the elongation ratio.
Reinforcement particle size
The size of the reinforcement particles plays a critical role in the mechanical properties of MMCs. For example, a A356 alloy reinforced with nanoscaled TiB2 possessed higher elongation and tensile strength than with microscaled particles.63 Another example from Youssef El-Kady et al.64 deals with alumina-reinforced A356 alloy. Rheocasting and squeeze casting were used to incorporate 60 and 200 nm Al2O3 nanoparticles into A356 alloy. The nanocomposite with 60 nm Al2O3 particles showed higher ultimate tensile strength, yield 20 strength and elongation compared to the composite with 200 nm Al2O3, while microhardness showed the opposite trend, probably because of the agglomeration of nanoparticles.
Volume fraction of the reinforcement phase
The content of reinforcement phase in the nanocomposite is also an important parameter to control the mechanical properties.20,63–67 Reddy et al.67 fabricated aluminum metal matrix nanocomposites reinforced by different contents of SiC nanoparticles of 55 nm diameter (0, 0.3, 0.5, 1.0 and 1.5 vol%) via microwave sintering and hot extrusion techniques. Increasing the SiC content from 0.3 to 1.5 vol% enhanced the final yield strength from 66 to 114 MPa, and the ultimate compressive strength from 313 to 392 MPa, respectively. A365/Al2O3 metal matrix nanocomposites also showed improved ultimate tensile strength, yield strength and elongation with particle size of 60 nm. However, the excessive incorporation of nanoparticles could be detrimental to mechanical properties. 64 Indeed, Akbari et al.63 found that increasing the content of TiB2 nanoparticles into A356 nanocomposites from 0.5 to 1.5 wt% significantly enhanced the final yield stress and ultimate yield strength, while an opposite trend appeared when the nanoparticle content kept increasing to 3.0 wt%. This behavior could be ascribed to increasing particle aggregation.
A strong interface between the metal matrix and reinforcement objects is necessary to ensure effective load transfer.68 Wang et al.69 utilized reduced graphene oxide (RGO) to reinforce a Cu alloy (in presence of Ti) and found that a TiC layer was formed at the interface of RGO/CuTi composite. The TiC layer led to enhanced interfacial bonding enabling yield strength and tensile strength of 242 MPa and 307 MPa, 24 and 16% higher than those of 2 vol% RGO/Cu composite, respectively. Another example relates to the interface reaction between BN and Al that leads to the formation of AlN and AlB2. 44 Their formation in Al-based composites is not as deleterious as compared with carbides (Al4C3).70 AlN reinforcement improves the hardness and flexural strength of Al matrix. AlB2 is also less detrimental to the fracture toughness of Al.
Phase change materials for thermal management of Li-ion battery
As mentioned above, MMNCs are potential light-weight materials for thermal control, which is essential for vehicle electrification based on lithium batteries technology. Below, we are presenting major strategies for thermal control of batteries together with a brief overview of phase change materials.
State of the art on the growth of silica shell to encapsulate bismuth particles
The synthesis of silica coated metallic nanoparticles has been well studied for nanoparticles such as silver and gold.32 Among the numerous applications of metal@silica core-shell nanoparticles, most biological applications of nanoparticles require a good control of the surface state of nanoparticles. The implementation of a silica shell helps to graft specific reactive functions such as antibodies or fluorophores.32 Among all synthesis methods, we should cite two main processes: a) the Stöber process, a sol-gel process where the Si precursor (typically tetraethyl orthosilicate (TEOS)) is hydrolyzed and condensed in an alcohol-ammonia-water solution;33 b) the use of a reverse microemulsion before condensation.
The growth of a silica shell on Bi nanoparticles has been less studied in the literature. Lee et al.35 developed Bi nanotube@SiO2 core-shell structures by atomic layer deposition. Chen et al.36 reported Bi@SiO2 nanospheres of 100 nm via in situ reduction of Bi2O3@SiO2 by NaBH4. Luo et al.37 synthesized around 80 nm Bi@SiO2 with 5 to 20 aggregates encapsulated in the same silica shell. The synthesis method was not described in the article. Li et al.38 synthesized 40 – 60 nm Bi nanoparticles in acidic solution at 80 °C, then the authors grew a 2 – 5 nm SiO2 layer on the Bi nanoparticles by the Stöber process. Li et al.39 succeeded to achieve monodisperse core-shell objects. Spherical Bi nanoparticles (70–75 nm) were synthesized by thermal reduction of Bi(NO3)3 in dodecanethiol at 178 °C. Then, a 40 nm-thick SiO2 shell was grown by employing the reverse-microemulsion method: Bi nanoparticles were dispersed in cyclohexane and treated with ultrasonication, then polyoxyethylene, nonylphenylether and ammonia solution were added and stirred for 24 h. In view of the facility of the synthesis and of the quantities of product achievable, the Stöber process has been applied in our work.
Synthesis and characterization of Bi nanoparticles
Firstly, we applied the protocol developed by Scheele et al.8 for the synthesis of Bi nanoparticles. This protocol consists in reducing bismuth acetate in oleylamine in the presence of 1-dodecanethiol as ligand at 60 °C following a hot injection protocol, the experimental details are described herein: Bismuth acetate (1 mmol) was mixed with 1- dodecanethiol (11.1 mL) and heated to 45 °C for 45 min under vacuum. The reaction flask was then purged with N2 and heated to 60 °C. Oleylamine (22.2 mL) was quickly added under stirring and the reaction mixture was maintained at 60 °C for 24 h. Finally, Bi nanoparticles were washed twice by an ethanol/chloroform mixture (20/1 vol./vol.) and then re-dispersed in ethanol.
The powder X-ray diffraction (XRD) patterns confirmed that Bi nanoparticles were obtained (Figure 2.1a). TEM images (Figure 2.1b) showed that the sample consists in bismuth nanoparticles with diameter around 25 nm with a relatively narrow size distribution. Contrary to the literature in which nanoparticles of 12-14 nm were obtained by this process,8 the nanoparticles we synthesized are larger and the sample is more polydisperse. We then evaluated the impact of a range of parameters to understand this difference. In brief, the particle size distribution did not exhibit significant dependence on the origin and purity of the reagents, on the synthesis temperature (60 ± 2 °C), neither on the reaction duration from 20 to 24 and 28 h. When the reaction time was decreased to 6 h, the nanoparticles (figure 2.1c) were more faceted. The only remaining synthesis parameter is the injection speed and/or the heating ramp after the injection. These parameters have not been described in the original article of Scheele et al.8 Then, we have attempted to preheat oleylamine at 55 °C before injection. The resulting nanoparticles shown in figure 2.1d exhibit smaller particle size ranging from 4 to 24 nm with a wide range of morphologies compared with the sample prepared by hot injection of room-temperature oleylamine (figure 2.1b). Because the final nanoparticles are dedicated to the design of phase change materials relying on the melting temperature decrease of nano-sized bismuth, nanoparticles smaller than 30 nm are sufficient for this study as this size range corresponds to a noticeable decrease of the melting temperature.40 Thus, we selected nanoparticles of 25 nm diameter (figure 2.1b) from room temperature oleylamine injection for the following steps.
Table of contents :
1.1.1 Metal matrix composites
1.1.2 How can aluminum matrix nanocomposites improve sustainability in the automotive industry?
1.2 Mechanical property improvement
1.2.1 Strengthening mechanisms
1.2.2 Strategies to improve mechanical properties
1.3 Phase change materials for thermal management of Li-ion battery
1.3.1 Importance of thermal management of the Li-ion battery
1.3.2 Cooling strategies
1.3.3 Metal matrix nanocomposite with phase change materials for temperature
1.4 MMCs fabrication techniques
1.4.1 Powder metallurgy
1.4.2 Liquid state process
1.4.3 Squeeze casting
1.4.4 Other techniques
1.5 Conclusion and motivation of this work
2.1.1 State of the art on the Bi synthesis
2.1.2 State of the art on the growth of silica shell to encapsulate bismuth particles
2.2 Synthesis and characterization of Bi nanoparticles
2.3 Synthesis and characterization of Bi@SiO2 nanoparticles
3.2 Materials and methods
3.3 Results and discussion
4.2 Experimental section
4.3 Results and discussion
4.3.1 Synthesis and characterization of Bi and Bi@SiO2 nanoparticles
4.3.2 Synthesis and characterization of Bi@SiO2-Al nanocomposite
4.3.3 Synthesis and characterization of Bi-Al nanocomposite
4.3.2 Heat absorption ability evaluated by DSC measurements