Zhanqiu Tan

Uniform dispersion of graphene oxide in aluminum powder by direct electrostatic adsorption for fabrication of graphene/aluminum composites.

The excellent properties of graphene promote it as an ideal reinforcement in composites. However, dispersing graphene homogenously into metals is a key challenge that limits the development of high-performance graphene-reinforced metal matrix composites. Here, via simple electrostatic interaction between graphene oxide (GO) and Al flakes, uniform distribution of reduced graphene oxide (RGO) in an Al matrix is achieved. The adsorption process of GO on Al flakes is efficient, as it can be completed in minutes and proceeds spontaneously without any chemical agents. GO can be partially reduced by the electron interchange during the adsorption process and could be thoroughly reduced after subsequent thermal annealing. A densified RGO/Al composite can be obtained by hot pressing the RGO/Al composite powders. By employing the preceding fabrication process, a composite reinforced with only 0.3 wt.% of RGO shows an 18 and 17% increase in elastic modulus and hardness, respectively, over unreinforced Al, demonstrating RGO is a better reinforcement than most other reinforcements.

Development of Flake Powder Metallurgy in Fabricating Metal Matrix Composites: A Review

Powder metallurgy (PM) is one of the most applied processes in the fabrication of metal matrix composites (MMCs). Recently, a novel PM strategy called flake PM was developed to fabricate MMCs with nano-laminated or hierarchical architectures. The name “flake PM” was derived from the use of flake metal powders, which could benefit the uniform dispersion of reinforcements in the metal matrices and thus result in balanced strength and ductility. Flake PM has been proved to be successful in the dispersion of nano aluminum oxides, carbon nanotubes, graphene nano-sheets, and microsized B4C particles in aluminum or copper matrix. This paper reviews the technique and mechanism developments of flake PM in previous studies, and foresees the future develop of this new fabricating method.

Two-dimensional distribution of carbon nanotubes in copper flake powders

We report an approach of flake powder metallurgy to the uniform, two-dimensional (2D) distribution of carbon nanotubes (CNTs) in Cu flake powders. It consists of the preparation of Cu flakes by ball milling in an imidazoline derivative (IMD) aqueous solution, surface modification of Cu flakes with polyvinyl alcohol (PVA) hydrosol and adsorption of CNTs from a CNT aqueous suspension. During ball milling, a hydrophobic monolayer of IMD is adsorbed on the surface of the Cu flakes, on top of which a hydrophilic PVA film is adsorbed subsequently. This PVA film could further interact with the carboxyl-group functionalized CNTs and act to lock the CNTs onto the surfaces of the Cu flakes. The CNT volume fraction is controlled easily by adjusting the concentration/volume of CNT aqueous suspension and Cu flake thickness. The as-prepared CNT/Cu composite flakes will serve as suitable building blocks for the self-assembly of CNT/Cu laminated composites that enable the full potential of 2D distributed CNTs to achieve high thermal conductivity.

High content reduced graphene oxide reinforced copper with a bioinspired nano-laminated structure and large recoverable deformation ability.

By using CuO/graphene-oxide/CuO sandwich-like nanosheets as the building blocks, bulk nacre-inspired copper matrix nano-laminated composite reinforced by molecular-level dispersed and ordered reduced graphene oxide (rGO) with content as high as ∼45 vol% was fabricated via a combined process of assembly, reduction and consolidation. Thanks to nanoconfinement effect, reinforcing effect, as well as architecture effect, the nanocomposite shows increased specific strength and at least one order of magnitude greater recoverable deformation ability as compared with monolithic Cu matrix.

A Versatile Method for Uniform Dispersion of Nanocarbons in Metal Matrix Based on Electrostatic Interactions

Realizing the uniform dispersion of nanocarbons such as carbon nanotube and graphene in metals, is an essential prerequisite to fully exhibit their enhancement effect in mechanical, thermal, and electrical properties of metal matrix composites (MMCs). In this work, we propose an effective method to achieve uniform distribution of nanocarbons in various metal flakes through a slurry-based method. It relies on the electrostatic interactions between the negatively charged nanocarbons and the positively charged metal flakes when mixed in slurry. For case study, flake metal powders (Al, Mg, Ti, Fe, and Cu) were positively charged in aqueous suspension by spontaneous ionization or cationic surface modification. While nanocarbons, given examples as carboxylic multi-walled carbon nanotubes, pristine single-walled carbon nanotube, and carbon nanotube–graphene oxide hybrid were negatively charged by the ionization of oxygen-containing functional groups or anionic surfactant. It was found that through the electrostatic interaction mechanism, all kinds of nanocarbons can be spontaneously and efficiently adsorbed onto the surface of various metal flakes. The development of such a versatile method would provide us great opportunities to fabricate advanced MMCs with appealing properties.

Evolution, Control, and Effects of Interface in CNT/Al Composites: a Review

This review summarizes the work carried out in the field of interface study in carbon nanotube reinforced aluminum (CNT/Al) composites. Much research work has been conducted to reveal the evolution of CNT/Al interface in producing the composite with the purpose of achieving uniform distribution of CNTs and tight interfacial bonding. The effect and principles of coating were reviewed along with the illustration of “intermetallic interphases” design. Different roles of CNT/Al interface in structural and functional application were elucidated, and the future work that needs attention was addressed.

Back stress in strain hardening of carbon nanotube/aluminum composites

ABSTRACT As demonstrated by the loading–unloading tests and the modeling of the grain size effect and the composite effect, mainly owing to the back stress induced by CNTs, carbon nanotube/aluminum (CNT/Al) composites exhibit higher strain hardening capability than the unreinforced ultrafine-grained Al matrix. The back stress induced by CNTs should arise from the interfacial image force and the long-range interaction between statically stored dislocations and geometrically necessary dislocations around the CNT/Al interface. Therefore, this CNT-induced interfacial back stress strengthening mechanism is supposed to provide a novel route to enhancing the strain hardening capability and ductility in CNT/Al composites. IMPACT STATEMENT The present work investigates the roles and origins of back stress in the strain hardening of the carbon nanotubes/aluminum composites for the first time. GRAPHICAL ABSTRACT

Size and Crystallographic Orientation Effects on the Mechanical Behavior of 4H-SiC Micro-/nano-pillars

Single crystalline 4H-SiC micro-/nano-pillars of various sizes and different crystallographic orientations were fabricated and tested by uniaxial compression. The pillars with zero shear stress resolved on the basal slip system were found to fracture in a brittle manner without showing significant size dependence, while the pillars with non-zero resolved shear stress showed a “smaller is stronger” behavior and a jerky plastic flow. These observations were interpreted by homogeneous dislocation nucleation and dislocation glide on the basal plane.

Orientation-Dependent Tensile Behavior of Nanolaminated Graphene-Al Composites: An In Situ Study

We conducted in situ microtension experiments in a scanning electron microscope (SEM) to study the orientation-dependent mechanical behavior of nanolaminated graphene-Al composite. We found a transition from a weak-and-brittle behavior in the isostress composite configuration to a strong-yet-ductile tensile response in the composite under isostrain condition. This is explained by the excellent load-bearing capacity of the graphene nanosheets and a crack deflection mechanism rendered by the laminate structure. These in situ measurements enabled direct observation of the deformation procedure and the exact failure mode, which highlight the importance of microstructural control in tailoring the mechanical properties of advanced metal matrix composites (MMCs).

Grain boundary-assisted deformation in graphene–Al nanolaminated composite micro-pillars

ABSTRACT Micro-pillars with diameters varying from 0.5 to 3.5 µm were fabricated from bulk nanolaminated graphene (in the form of reduced grapheme oxide, RGO)–Al composite. Upon uniaxial compression, the pillar strengths exhibited no obvious size effect, and the pillars of larger diameters possessed smoother stress–strain response, as opposed to the jerky deformation of their smaller counterparts. A corresponding transition in the deformation mode from Al layer extrusion to shear fracture over decreasing pillar diameter was observed. These observations were explained by the competing effect of dislocation accumulation and annihilation, and a grain boundary-assisted deformation mechanism. GRAPHICAL ABSTRACT IMPACT STATEMENT The transition in the deformation mode of graphene–Al composite micro-pillars from localized shear facture to Al layer extrusion over increasing pillar diameter is attributed to a grain boundary-assisted deformation mechanism.