The reported thermal conductivity (kappa) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that kappa of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to kappa in suspended graphene according to a theoretical calculation.

Graphene monolayers supported on oxide substrates have been demonstrated with superior charge mobility and thermal transport for potential device applications. Morphological corrugation can strongly influence the transport properties of the supported graphene. In this paper, we theoretically analyze the morphological stability of a graphene monolayer on an oxide substrate, subject to van der Waals interactions and in-plane mismatch strains. First, we define the equilibrium separation and the interfacial adhesion energy as the two key parameters that characterize the van der Waals interaction between a flat monolayer and a flat substrate surface. By a perturbation analysis, a critical compressive mismatch strain is predicted, beyond which the graphene monolayer undergoes strain-induced instability, forming corrugations with increasing amplitude and decreasing wavelength on a perfectly flat surface. When the substrate surface is not perfectly flat, the morphology of graphene depends on both the amplitude and the wavelength of the surface corrugation. A transition from conformal (corrugated) to non-conformal (flat) morphology is predicted. The effects of substrate surface corrugation on the equilibrium mean thickness of the supported graphene and the interfacial adhesion energy are analyzed. Furthermore, by considering both the substrate surface corrugation and the mismatch strain, it is found that, while a tensile mismatch strain reduces the corrugation amplitude of graphene, a corrugated substrate surface promotes straininduced instability under a compressive strain. These theoretical results suggest possible means to control the morphology of graphene monolayer on oxide substrates by surface patterning and strain engineering.

We conducted in situ three-point bending experiments on beams with roughly square cross-sections, which we fabricated from the frustule of Coscinodiscus sp. We observe failure by brittle fracture at an average stress of 1.1 GPa. Analysis of crack propagation and shell morphology reveals a differentiation in the function of the frustule layers with the basal layer pores, which deflect crack propagation. We calculated the relative density of the frustule to be ∼30% and show that at this density the frustule has the highest strength-to-density ratio of 1,702 kN·m/kg, a significant departure from all reported biologic materials. We also performed nanoindentation on both the single basal layer of the frustule as well as the girdle band and show that these components display similar mechanical properties that also agree well with bending tests. Transmission electron microscopy analysis reveals that the frustule is made almost entirely of amorphous silica with a nanocrystalline proximal layer. No flaws are observed within the frustule material down to 2 nm. Finite element simulations of the threepoint bending experiments show that the basal layer carries most of the applied load whereas stresses within the cribrum and areolae layer are an order of magnitude lower. These results demonstrate the natural development of architecture in live organisms to simultaneously achieve light weight, strength, and exceptional structural integrity and may provide insight into evolutionary design.iatoms are single-cell algae that form a hard cell wall made of a silica/organic composite. The ability to produce a functional biosilica shell presents several natural precedents that fascinate and inspire scientists and engineers. One fascinating aspect of such silica glass shells is their intricate, varied, and detailed architecture. Diatoms are generally classified based on the symmetry of their shells: Centric diatoms display radial symmetry whereas pennate diatoms have bilateral symmetry. Fig. 1A shows a schematic of a typical centric diatom and reveals that the shells are composed of two halves, called frustules, that fit together in a Petri-dish configuration. The frustules are attached to each other around the perimeter of the shell by one or several girdle bands. The frustules are usually porous with pore size and density varying between species. The frustule shell can also be composed of multiple layers with a cellular structure within the shell.The proposed evolutionary functions for these intricate shell designs include nutrient acquisition, control of diatom sinking rate, control of turbulent flow around the cell, and protection from grazing and viral attack (1). Evidence in favor of a protective function is that the degree of shell silification depends on the environment, with greater amounts of silica found in shells grown in a predatory environment (2). As a deterrent to predation, the frustule makes use of an inherently brittle glass as a structurally protective material while balancing other evolutionary pressures. A denser ...

Simultaneously enhancing strength and ductility of metals and alloys has been a tremendous challenge. Here, we investigate a CoCuFeNiPd high-entropy alloy (HEA), using a combination of Monte Carlo method, molecular dynamic simulation, and density-functional theory calculation. Our results show that this HEA is energetically favorable to undergo short-range ordering (SRO), and the SRO leads to a pseudo-composite microstructure, which surprisingly enhances both the ultimate strength and ductility. The SRO-induced composite microstructure consists of three categories of clusters: face-center-cubic-preferred (FCCP) clusters, indifferent clusters, and body-center-cubic-preferred (BCCP) clusters, with the indifferent clusters playing the role of the matrix, the FCCP clusters serving as hard fillers to enhance the strength, while the BCCP clusters acting as soft fillers to increase the ductility. Our work highlights the importance of SRO in influencing the mechanical properties of HEAs and presents a fascinating route for designing HEAs to achieve superior mechanical properties.