532 research outputs found
Attractive forces in microporous carbon electrodes for capacitive deionization
The recently developed modified Donnan (mD) model provides a simple and
useful description of the electrical double layer in microporous carbon
electrodes, suitable for incorporation in porous electrode theory. By
postulating an attractive excess chemical potential for each ion in the
micropores that is inversely proportional to the total ion concentration, we
show that experimental data for capacitive deionization (CDI) can be accurately
predicted over a wide range of applied voltages and salt concentrations. Since
the ion spacing and Bjerrum length are each comparable to the micropore size
(few nm), we postulate that the attraction results from fluctuating bare
Coulomb interactions between individual ions and the metallic pore surfaces
(image forces) that are not captured by meanfield theories, such as the
Poisson-Boltzmann-Stern model or its mathematical limit for overlapping double
layers, the Donnan model. Using reasonable estimates of the micropore
permittivity and mean size (and no other fitting parameters), we propose a
simple theory that predicts the attractive chemical potential inferred from
experiments. As additional evidence for attractive forces, we present data for
salt adsorption in uncharged microporous carbons, also predicted by the theory.Comment: 19 page
Analysis of ionic conductance of carbon nanotubes
We use space-charge (SC) theory (also called the capillary pore model) to describe the ionic conductance, G, of charged carbon nanotubes (CNTs). Based on the reversible adsorption of hydroxyl ions to CNT pore walls, we use a Langmuir isotherm for surface ionization and make calculations as a function of pore size, salt concentration c, and pH. Using realistic values for surface site density and pK, SC theory well describes published experimental data on the conductance of CNTs. At extremely low salt concentration, when the electric potential becomes uniform across the pore, and surface ionization is low, we derive the scaling G∝sqrt[c], while for realistic salt concentrations, SC theory does not lead to a simple power law for G(c)
Size scaling of strength in thin film delamination
We investigate by numerical simulation the system size dependence of the
shear delamination strength of thin elastic films. The films are connected to a
rigid substrate by a disordered interface containing a pre-existing crack. The
size dependence of the strength of this system is found to depend crucially on
the crack shape. For circular cracks, we observe a crossover between a
size-independent regime at large crack radii which is controlled by propagation
of the pre-existing crack, and a size-dependent regime at small radii which is
dominated by nucleation of new cracks in other locations. For cracks of finite
width that span the system transversally, we observe for all values of the
crack length a logarithmic system size dependence of the failure stress. The
results are interpreted in terms of extreme value statistics.Comment: 10 pages, 4 figure
Crack stability in the fracture of cementitious materials
The aim of the present study is to investigate the stability of crack propagation in cementitious materials. Tests were conducted on bend specimens in three-point and four-point loading conditions. Three-point bend specimens showed stable crack growth for mortar, normal strength and high strength concrete specimens. Alternatively, four-point bend specimens showed catastrophic failure for mortar and quasi-catastrophic failure for normal strength and high strength concrete specimens. Results will be discussed in relation to brittleness number model and specific microstructural features including the interfacial transition zone between the cement paste and the aggregate and the attendant toughening mechanisms
Crackling noise in three-point bending of heterogeneous materials
We study the crackling noise emerging during single crack propagation in a
specimen under three-point bending conditions. Computer simulations are carried
out in the framework of a discrete element model where the specimen is
discretized in terms of convex polygons and cohesive elements are represented
by beams. Computer simulations revealed that fracture proceeds in bursts whose
size and waiting time distributions have a power law functional form with an
exponential cutoff. Controlling the degree of brittleness of the sample by the
amount of disorder, we obtain a scaling form for the characteristic quantities
of crackling noise of quasi-brittle materials. Analyzing the spatial structure
of damage we show that ahead of the crack tip a process zone is formed as a
random sequence of broken and intact mesoscopic elements. We characterize the
statistics of the shrinking and expanding steps of the process zone and
determine the damage profile in the vicinity of the crack tip.Comment: 11 pages, 15 figure
Current-induced membrane discharge
Possible mechanisms for over-limiting current (OLC) through aqueous
ion-exchange membranes (exceeding diffusion limitation) have been debated for
half a century. Flows consistent with electro-osmotic instability (EOI) have
recently been observed in microfluidic experiments, but the existing theory
neglects chemical effects and remains to be quantitatively tested. Here, we
show that charge regulation and water self-ionization can lead to OLC by
"current-induced membrane discharge" (CIMD), even in the absence of fluid flow.
Salt depletion leads to a large electric field which expels water co-ions,
causing the membrane to discharge and lose its selectivity. Since salt co-ions
and water ions contribute to OLC, CIMD interferes with electrodialysis (salt
counter-ion removal) but could be exploited for current-assisted ion exchange
and pH control. CIMD also suppresses the extended space charge that leads to
EOI, so it should be reconsidered in both models and experiments on OLC.Comment: 4.5 page
Nonlinear viscoelasticity of freestanding and polymer-anchored vertically aligned carbon nanotube foams
Vertical arrays of carbon nanotubes (VACNTs) show unique mechanical behavior in compression, with a highly nonlinear response similar to that of open cell foams and the ability to recover large deformations. Here, we study the viscoelastic response of both freestanding VACNT arrays and sandwich structures composed of a VACNT array partially embedded between two layers of poly(dimethylsiloxane) (PDMS) and bucky paper. The VACNTs tested are ∼2 mm thick foams grown via an injection chemical vapor deposition method. Both freestanding and sandwich structures exhibit a time-dependent behavior under compression. A power-law function of time is used to describe the main features observed in creep and stress-relaxation tests. The power-law exponents show nonlinear viscoelastic behavior in which the rate of creep is dependent upon the stress level and the rate of stress relaxation is dependent upon the strain level. The results show a marginal effect of the thin PDMS/bucky paper layers on the viscoelastic responses. At high strain levels (ɛ = 0.8), the peak stress for the anchored CNTs reaches ∼45 MPa, whereas it is only ∼15 MPa for freestanding CNTs, suggesting a large effect of PDMS on the structural response of the sandwich structures
- …