Durable and self-hydrating tungsten carbide-based composite polymer electrolyte membrane fuel cells.

Proton conductivity of the polymer electrolyte membranes in fuel cells dictates their performance and requires sufficient water management. Here, we report a simple, scalable method to produce well-dispersed transition metal carbide nanoparticles. We demonstrate that these, when added as an additive to the proton exchange Nafion membrane, provide significant enhancement in power density and durability over 100 hours, surpassing both the baseline Nafion and platinum-containing recast Nafion membranes. Focused ion beam/scanning electron microscope tomography reveals the key membrane degradation mechanism. Density functional theory exposes that OH• and H• radicals adsorb more strongly from solution and reactions producing OH• are significantly more endergonic on tungsten carbide than on platinum. Consequently, tungsten carbide may be a promising catalyst in self-hydrating crossover gases while retarding desorption of and capturing free radicals formed at the cathode, resulting in enhanced membrane durability.The proton conductivity of polymer electrolyte membranes in fuel cells dictates their performance, but requires sufficient water management. Here, the authors report a simple method to produce well-dispersed transition metal carbide nanoparticles as additives to enhance the performance of Nafion membranes in fuel cells.

Effects of interaction volume on X-ray line-scans across an ultrasonically consolidated aluminum/copper interface.

Diffusion as a bonding mechanism for ultrasonic consolidation of metals is widely debated due to the short weld times and low processing temperatures. To quantify interdiffusion coefficients, X-ray energy dispersive spectroscopy (XEDS) line-scans were performed across an Al-Cu interface using the Scanning Electron Microscope (SEM) with accelerating voltages ranging from 6 to 22 KeV in increments of 2 KeV and a step size of 0.05 microns. Higher accelerating voltages resulted in broader concentration profiles, indicating higher apparent interdiffusion coefficients when scanned at the same location of the same sample. This error due to the interaction volume interference was quantified using Monte Carlo simulations. It was found that an accelerating voltage of 22 KeV and diffusion distance less than 5 microns resulted in at least 50% error. Even at a smaller accelerating voltage of 6 KeV, the percent error in calculation of the interdiffusion coefficient for a diffusion distance of 0.5 microns is expected to be 15-20%. An approximate diffusion distance and apparent interdiffusion coefficient for ultrasonically consolidated Al-Cu were 0.503 microns and 0.013 um(2) /s, respectively. In this study, a methodology is presented that allows one to estimate the error in the calculation of an interdiffusion coefficient from the accelerating voltage used and the diffusion distance measured by the SEM XEDS at that accelerating voltage.

WEAR BEHAVIOR OF CARBON NANOTUBE/HIGH DENSITY POLYETHYLENE COMPOSITES.

Carbon Nanotube/High Density Polyethylene (CNT/HDPE) composites were manufactured and tested to determine their wear behavior. The nanocomposites were made from untreated multi-walled carbon nanotubes and HDPE pellets. Thin films of the precursor materials were created with varying weight percentages of nanotubes (1%, 3%, and 5%), through a process of mixing and extruding. The precursor composites were then molded and machined to create test specimens for mechanical and wear tests. These included small punch testing to compare stiffness, maximum load and work-to-failure and block-on-ring testing to determine wear behavior. Each of the tests was conducted for the different weight percentages of composite as well as pure HDPE as the baseline. The measured mechanical properties and wear resistance of the composite materials increased with increasing nanotube content in the range studied.

Capillary effect of multi-walled carbon nanotubes suspension in composite processing.

Carbon nanotubes (CNTs) do have the potential to improve the interlaminar shear strength (ILSS) of composites if they can be successfully integrated into the matrix as it infuses into the fiber preform. The infusion under capillary action of Multi-Walled Carbon Nanotubes (MWNT)/Epoxy suspension with tubes of length 0.3 approximately 1 microm in glass fiber bundles containing pores of the order of 5 nm approximately100 microm was investigated. The influence of parameters such as suspension concentration, viscosity, porous media architecture, surface tension and contact angle were explored. It was found that filtering of the suspension is a major challenge for uniform infusion for concentrations beyond 0.5% MWNT by weight. This is even truer for fiber bundles that are compacted. Hence for successful manufacturing, new infusion techniques that rely on fabrics of high permeability will have to be developed to fabricate such nanocomposites.

Nonequilibrium molecular dynamics simulation to describe the rotation of rigid, low aspect ratio carbon nanotubes in simple shear flow.

In this paper, the rotation of short carbon nanotubes in simple shear liquid argon flow was investigated by nonequilibrium molecular dynamics (MD) simulation. In their simulations, nanotubes were described as rigid cylinders of carbon atoms. Lennard-Jones potential was employed to represent both argon-argon and argon-carbon interactions. Results show that time period of a nanotube as calculated from MD simulations is longer than what would be calculated from Jeffery's equation based on the aspect ratio of the cylinder. The difference is much higher at low shear rates and for small aspect ratios. Results also reveal that adding caps to an open-ended nanotube speeds up its rotation.

Drag on a nanotube in uniform liquid argon flow.

In this work, nonequilibrium molecular dynamics (MD) simulations were performed to investigate uniform liquid argon flow past a carbon nanotube. In the simulation, nanotubes were modeled as rigid cylinders of carbon atoms. Both argon-argon and argon-carbon interactions were calculated based on Lennard-Jones potential. Simulated drag coefficients were compared with (i) published empirical equation which was based on experiments conducted with macroscale cylinders and (ii) finite element (FE) analyses based on Navier-Stokes equation for flow past a circular cylinder using the same dimensionless parameters used in MD simulations. Results show that classical continuum mechanics cannot be used to calculate drag on a nanotube. In slow flows, the drag coefficients on a single-walled nanotube calculated from MD simulations were larger than those from the empirical equation or FE analysis. The difference increased as the flow velocity decreased. For higher velocity flows, slippage on the surface of the nanotube was identified which resulted in lower drag coefficient from MD simulation. This explains why the drag coefficient from MD dropped faster than those from the empirical equation or FE simulation as the flow velocity increased. It was also found that the drag forces are almost equal for single- and double-walled nanotubes with the same outer diameter, implying that inner tubes do not interact with fluid molecules.

Slow drag in polydisperse granular mixtures under high pressure.

The behavior of polydisperse granular materials, composed of mixtures of particles of different sizes, is studied under conditions of high pressure and confinement. Two types of experiments are performed. In the first type, granular mixtures are compressed, with the resulting force-displacement curve used to calculate density and volume modulus. In the second set of experiments, the drag force is measured by pulling a cylinder, horizontally, through a compressed granular mixture. The density, volume modulus, and drag forces for the mixtures are quantified in terms of the mixture composition. The results show that the behavior of these mixtures depends strongly on the mass fractions of the different sized particles, with density, volume modulus, and drag force all reaching values significantly higher than observed in the monodisperse granular materials. Furthermore, the trends for density and drag force show strong correlation, suggesting that drag resistance of confined granular media could be directly related to packing effects. These results should prove useful in understanding the physics of drag in granular materials under high pressure, such as ballistic penetration of soils or ceramic armors.

Slow drag in granular materials under high pressure.

The resistance offered by a cylindrical rod to creeping cross flow of granular materials under pressure is investigated. The experimental system consists of a confined bed of granular particles, which are compacted under high pressure to consolidate the granular medium. A cylindrical rod is pulled at a constant and slow rate through the granular medium, and the measured pulling resistance is characterized as a drag force. The influence of various parameters such as the velocity of the cylindrical rod, the rod diameter and length, the granular particle size, and the compaction pressure on the required drag force is investigated experimentally. Nondimensional analysis is performed to simplify the relationships between these variables. The results show that the drag force is independent of the drag velocity, is linearly proportional to compaction pressure and rod diameter, and increases with rod length and particle size. Additional compaction experiments show that the effective density of the granular bed increases linearly with pressure, and similar behavior is noted for all particle sizes. These results should prove useful in the development of constitutive equations that can describe the motion of solid objects through compacted granular media under high pressure, such as during ballistic penetration of soils or ceramic armors.

Analysis and characterization of relative permeability and capillary pressure for free surface flow of a viscous fluid across an array of aligned cylindrical fibers.

In LCM processes of fiber-reinforced composite manufacturing, resin is injected into a closed mold with a preplaced stationary fiber preform. If these preforms are created from fiber tows, resin progression at the microlevel during infiltration is often non-uniform. Consequently, macroscopic description of the filling phase requires a theory of flow through unsaturated porous media in which the transition (partly saturated) region must be taken into account. Unsaturated flows must consider surface tension effects; therefore, capillary pressure and relative permeability must be included in governing equations. This paper presents a methodology to determine relative permeability and macroscopic capillary pressure for simple flows. The results lead to important conclusions and the methodology can be generalized to other flow fields.