Sang-Pil Kim

Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations.

We have studied the thermal conductance of tilt grain boundaries in graphene using nonequilibrium molecular dynamics simulations. When a constant heat flux is allowed to flow, we observe sharp jumps in temperature at the boundaries, characteristic of interfaces between materials of differing thermal properties. On the basis of the magnitude of these jumps, we have computed the boundary conductance of twin grain boundaries as a function of their misorientation angles. We find the boundary conductance to be in the range 1.5 × 10(10) to 4.5 × 10(10) W/(m(2) K), which is significantly higher than that of any other thermoelectric interfaces reported in the literature. Using the computed values of boundary conductances, we have identified a critical grain size of 0.1 μm below which the contribution of the tilt boundaries to the conductivity becomes comparable to that of the contribution from the grains themselves. Experiments to test the predictions of our simulations are proposed.

Aerosol synthesis of cargo-filled graphene nanosacks

Water microdroplets containing graphene oxide and a second solute are shown to spontaneously segregate into sack-cargo nanostructures upon drying. Analytical modeling and molecular dynamics suggest the sacks form when slow-diffusing graphene oxide preferentially accumulates and adsorbs at the receding air-water interface, followed by capillary collapse. Cargo-filled graphene nanosacks can be nanomanufactured by a simple, continuous, scalable process and are promising for many applications where nanoscale materials should be isolated from the environment or biological tissue.

Graphene-Based Environmental Barriers

Many environmental technologies rely on containment by engineered barriers that inhibit the release or transport of toxicants. Graphene is a new, atomically thin, two-dimensional sheet material, whose aspect ratio, chemical resistance, flexibility, and impermeability make it a promising candidate for inclusion in a next generation of engineered barriers. Here we show that ultrathin graphene oxide (GO) films can serve as effective barriers for both liquid and vapor permeants. First, GO deposition on porous substrates is shown to block convective flow at much lower mass loadings than other carbon nanomaterials, and can achieve hydraulic conductivities of 5 × 10(-12) cm/s or lower. Second we show that ultrathin GO films of only 20-nm thickness coated on polyethylene films reduce their vapor permeability by 90% using elemental mercury as a model vapor toxicant. The barrier performance of GO in this thin-film configuration is much better than the Nielsen model limit, which describes ideal behavior of flake-like fillers uniformly imbedded in a polymer. The Hg barrier performance of GO films is found to be sensitive to residual water in the films, which is consistent with molecular dynamics (MD) simulations that show lateral diffusion of Hg atoms in graphene interlayer spaces that have been expanded by hydration.

Fluorination of Graphene Enhances Friction Due to Increased Corrugation

The addition of a single sheet of carbon atoms in the form of graphene can drastically alter friction between a nanoscale probe tip and a surface. Here, for the first time we show that friction can be altered over a wide range by fluorination. Specifically, the friction force between silicon atomic force microscopy tips and monolayer fluorinated graphene can range from 5-9 times higher than for graphene. While consistent with previous reports, the combined interpretation from our experiments and molecular dynamics simulations allows us to propose a novel mechanism: that the dramatic friction enhancement results from increased corrugation of the interfacial potential due to the strong local charge concentrated at fluorine sites, consistent with the Prandtl-Tomlinson model. The monotonic increase of friction with fluorination in experiments also demonstrates that friction force measurements provide a sensitive local probe of the degree of fluorination. Additionally, we found a transition from ordered to disordered atomic stick-slip upon fluorination, suggesting that fluorination proceeds in a spatially random manner.

Electrochemical performances of gel polymer electrolytes using tetra(ethylene glycol) diacrylate

Abstract A gel polymer electrolyte (GPE) was prepared using tetra(ethylene glycol) diacrylate monomer, benzoyl peroxide, and 1.0 M LiPF 6 / EC-DEC ( 1 : 1 vol % ). The LiCoO 2 /GPE/graphite cells were prepared and their electrochemical properties were evaluated at various current densities and temperatures. The viscosity of the precursor containing the 5 vol % tetra(ethylene glycol) diacrylate monomer was around 4.6 mPa s . The ionic conductivity of the gel polymer electrolyte at 20°C was around 5.9×10 −3 S cm −1 . The gel polymer electrolyte had good electrochemical stability up to 4.3 V vs. Li/Li + . The capacity of the LiCoO 2 /GPE/graphite cell at 2.0 C rate was 63% of the discharge capacity at 0.2 C rate. The capacity of the cell at −10°C was 81% of the discharge capacity at 20°C. Discharge capacity of the cell with gel polymer electrolyte was stable with charge–discharge cycling.

Synthesis of Cross-Linked Polyurethane-Based Gel Polymer Electrolyte and Its Electrochemical Properties

Urethane acrylate oligomer was synthesized and used in a gel polymer electrolyte (GPE) and then its electrochemical performances were evaluated. cells were prepared and their performances depending on discharge currents and temperatures were evaluated. The precursor containing curable mixture had a low viscosity relatively. ionic conductivity of the gel polymer electrolyte at room temperature and was ca. , respectively. GPE showed good electrochemical stability up to potential of 4.5V vs. cell showed a good high-rate and low-temperature performance.

Optimization Study on Polymerization of Crosslink-type Gel Polymer Electrolyte for Lithium-ion Polymer Battery

In this work, polymerization conditions of the gel polymer electrolyte (GPE) were studied to obtain better electrochemical performances in a lithium-ion polymer battery. When the polymerization temperature and time of the GPE were 70 and 70 min, respectively, the lithium polymer battery showed excellent a rate capability and cycleability. The TMPETA (trimethylolpropane ethoxylate triacrylate)/TEGDMA (triethylene glycol dimethacrylate)-based cells prepared under optimized polymerization conditions showed excellent rate capability and low-temperature performances: The discharge capacity of cells at 2 Crate showed 92.1 % against 0.2C rate. The cell at -20 also delivered 82.4 % of the discharge capacity at room temperature.