UCLA

The objective of this study is to significantly advance the fundamental knowledge to enable scalable nano-manufacturing of metal-based nanocomposites by overcoming the grand challenges that exist in both fundamental and manufacturing levels. It especially seeks to manufacture bulk aluminum nanocomposite electrical conductors (ANECs) with uniform dispersion and distribution of nanoparticles that offer excellent mechanical and electrical properties.

Polymer-metal nanocomposite is an emerging class of hybrid materials which can offer significantly improved functional properties (e.g. electrical conductivity). Incorporating proper nanoscale metallic elements into polymer matrices can enhance the electrical conductivity of the polymers. To achieve such polymer nanocomposites, the longstanding challenge of uniform dispersion of metal nanoparticles in polymers must be addressed. Conventional scale-down techniques often are only able to shrink larger elements (e.g. microparticles and microfibers) into micro/nano-elements (i.e. nanoparticles and nanofibers) without significant modification in their relative spatial and size distributions. This study uncovers an unusual phenomenon that tin (Sn) microparticles with both poor size distribution and spatial dispersion were stretched into uniformly dispersed and sized nanoparticles in polyethersulfone (PES) using thermal drawing method. It is believed that the capillary instability plays a crucial role during thermal drawing. This novel, inexpensive, and scalable method overcomes the longstanding challenge to produce bulk polymer-metal nanocomposites (PMNCs) with a uniform dispersion of metallic nano-elements (Chapter 3).

Nano-elements (e.g. nanoparticles) are one of the most important constituent of the nanocomposite materials. Since titanium diboride (TiB2) nanoparticles is of a crucial factor in this study, and more importantly is not commercially available, we synthesized these reinforcements to ensure high purity and size uniformity. Our preliminary results show that TiB2 nanoparticles with a uniform size can be produced. Further characterization confirmed the presence of crystalline TiB2 nanoparticles with average size of 8.1�0.4 nm. The in-house synthesized TiB2 nanoparticles were used to reinforce both aluminum and magnesium matrices. Successful incorporation of TiB2 nanoparticles in the aforementioned matrices was another indirection indication of high purity and surface-clean TiB2 nanoparticles (Chapter 4).

Lightweight metallic systems (e.g. Al) have promising potentials for applications in metal-based laser additive manufacturing. Lightweight metals exhibit moderate mechanical properties compare to high density metals (e.g. steel). However, lightweight metal matrix nanocomposites (LMMNCs) offer excellent mechanical properties desirable to improve energy efficiency and system performance for widespread applications including, but not limited to, aerospace, transportation, electronics, automotive, and defense. It has been a longstanding challenge to realize a scalable manufacturing method to produce metal nanocomposite microparticles. This study demonstrates high volume manufacturing of Al and magnesiuim (Mg) nanocomposite microparticles. In-house synthesized TiB2 and commercial titanium carbide (TiC) nanoparticles were chosen as nano-scale reinforcements. Using a flux-assisted solidification processing method, up to 30% volume fraction nanoparticles were efficiently incorporated and dispersed into Al and Mg microparticles. Theoretical study on nanoparticle interactions in molten metals revealed that TiC and TiB2 nanoparticles can be self-dispersed and self-stabilized in molten Al and Mg matrices. Metal-based additive manufacturing and thermal spraying coating can significantly benefit from these novel Al and Mg nanocomposite microparticles. This simple yet scalable approach can broaden the applications of such nanocomposite in additive manufacturing of the functional parts. Moreover, the metal nanocomposite microparticles can be applied in conventional manufacturing processing. For example, bulk Al-30 volume percent (vol. %) nanocomposites were produced by cold compaction of Al-30 vol. % TiB2 nanocomposite microparticles followed by melting. Al-30 vol. % TiB2 nanocomposites with average Vickers hardness of 458 HV was successfully produced (Chapter 5).

Magnesium is the lightest structure metal applied in broad range of applications in various industries such as biomedical, transportation, construction, naval and electronic. Strengthening Mg is of significance for energy efficiency of numerous transportation systems. Traditional metal strengthening approaches such as elemental alloying have reached their fundamental limits in offering high strength metals functioning at elevated temperature. Adding nanoparticle reinforcements can effectively promote the mechanical properties of Mg nanocomposites. However, manufacturing of bulk magnesium nanocomposites with populous and dispersed nanoparticles remains as a great challenge. Here we report a novel flux-assisted liquid state processing of bulk Mg nanocomposites with TiC as the nanoscale reinforcements. TiC nanoparticles with high hardness and high elastic modulus is well-distributed and uniformly dispersed in the Mg matrix, resulting in a significantly improved Vickers hardness of 143.5�11.5 HV (pure Mg Vickers hardness is about 35 HV). Further theoretical study suggested that TiC nanoparticles can be self-dispersed and self-stabilized in Mg matrix (Chapter 6).

Aluminum is one of the most abundant lightweight metal on Earth with a wide range of practical applications such as electrical wire. However, traditional aluminum manufacturing processing approaches such as elemental alloying, deformation and thermomechanical cannot offer further property improvement due to fundamental limitations. Successful incorporation of ceramic nanoparticles into aluminum have shown unusual property improvements. Adding metal-like ceramic nanoparticles into aluminum matrix can be a promising alternative to produce high performance aluminum electrical wires. Here we show a new class of aluminum nanocomposite electrical conductors (ANECs), with significantly improved average Vickers hardness (130 HV) and good electrical conductivity (41% IACS). The as-cast Al-3 vol. % TiB2 nanocomposites exhibit yield strength of 206.6 MPa, UTS of 219.6 MPa, tensile strain of 4.3% and electrical conductivity of 57.5% IACS (pure Al has yield strength of 35 MPa, UTS of 90 MPa, tensile strain of 12% and electrical conductivity of 62.5% IACS). We also observed an unusual ultra-fine grain (UFG) size, as small as 300 nm, in the ANEC samples under slow cooling. We believe that the significant mechanical property enhancements can be partially attributed to the existence of the UFG. Further investigations demonstrated that UFG can be achieved when nanoparticles are uniformly dispersed and distributed in the aluminum matrix (Chapter 7).

In summary, analytical, numerical and experimental approaches have been established to significantly advance fundamental understanding of polymeric and metallic matrix nanocomposites, in particular the effect of metal-like ceramics on mechanical and electrical properties of lightweight metals. This study has demonstrated scalable production of multi-functional metal and polymer matrix nanocomposites. Metal-like ceramic nanoparticles can significantly enhance the mechanical properties of metal matrix while retaining good electrical properties.