3 research outputs found
Coating of Open Cell Foams
The interior surfaces of three-dimensional open cell
foams were coated by a combination of dip coating and spin coating.
Glycerol/water solutions were used as model Newtonian liquids, and
the coating processes were studied on open cell carbon foams with
10 or 30 pores per inch (PPI). The amount of liquid retained in the
foam structures after dip coating increased with withdrawal speed
and coating viscosity, as expected from the conventional understanding
of dip coating onto nonporous substrates such as flat plates and rods.
However, the liquid retention and hence average coating thickness
increased with surface tension, a result counter to the observation
with coating onto nonporous substrates. Pockets of liquid were observed
after dip coating and results with coatings of alumina suspension
showed that after drying, the trapped liquid can block pore windows.
Spinning the foams after dip coating resulted in uniform liquid distribution
and uniform coatings. Foams were placed in a special apparatus and
rotated using a commercial spin coater. The liquid layer thickness
decreased with spinning time and rotational speed, and increased with
the liquid viscosity, results consistent with spin coating theory.
The coating thickness after spinning was not affected by the initial
dip coating procedure. The dip and spin process was also used to create
γ-alumina
and zeolite coatings, which are of interest for catalysis applications
Understanding and Analyzing Freezing-Point Transitions of Confined Fluids within Nanopores
Understanding phase transitions of
fluids confined within nanopores
is important for a wide variety of technological applications. It
is well known that fluids confined in nanopores typically demonstrate
freezing-point depressions, Δ<i><i>T</i></i><sub>f</sub>, described by the Gibbs–Thomson (GT) equation.
Herein, we highlight and correct several thermodynamic inconsistencies
in the conventional use of the GT equation, including the fact that
the enthalpy of melting, Δ<i><i>H</i></i><sub>m</sub>, and the solid–liquid surface energy, γ<sub>SL</sub>, are functions of pore diameter, complicating their prediction.
We propose a theoretical analysis that employs the Turnbull coefficient,
originally derived from metal nucleation theory, and show its consistency
as a more reliable quantity for the prediction of Δ<i><i>T</i></i><sub>f</sub>. This analysis provides a straightforward
method to estimate Δ<i><i>T</i></i><sub>f</sub> of nanoconfined organic fluids. As an example, we apply this
technique to ibuprofen, an active pharmaceutical ingredient (API),
and show that this theory fits well to the experimental Δ<i><i>T</i></i><sub>f</sub> of nanoconfined ibuprofen
Mechanism and Prediction of Gas Permeation through Sub-Nanometer Graphene Pores: Comparison of Theory and Simulation
Due to its atomic
thickness, porous graphene with sub-nanometer
pore sizes constitutes a promising candidate for gas separation membranes
that exhibit ultrahigh permeances. While graphene pores can greatly
facilitate gas mixture separation, there is currently no validated
analytical framework with which one can predict gas permeation through
a given graphene pore. In this work, we simulate the permeation of
adsorptive gases, such as CO<sub>2</sub> and CH<sub>4</sub>, through
sub-nanometer graphene pores using molecular dynamics simulations.
We show that gas permeation can typically be decoupled into two steps:
(1) adsorption of gas molecules to the pore mouth and (2) translocation
of gas molecules from the pore mouth on one side of the graphene membrane
to the pore mouth on the other side. We find that the translocation
rate coefficient can be expressed using an Arrhenius-type equation,
where the energy barrier and the pre-exponential factor can be theoretically
predicted using the transition state theory for classical barrier
crossing events. We propose a relation between the pre-exponential
factor and the entropy penalty of a gas molecule crossing the pore.
Furthermore, on the basis of the theory, we propose an efficient algorithm
to calculate CO<sub>2</sub> and CH<sub>4</sub> permeances per pore
for sub-nanometer graphene pores of any shape. For the CO<sub>2</sub>/CH<sub>4</sub> mixture, the graphene nanopores exhibit a trade-off
between the CO<sub>2</sub> permeance and the CO<sub>2</sub>/CH<sub>4</sub> separation factor. This upper bound on a Robeson plot of
selectivity <i>versus</i> permeance for a given pore density
is predicted and described by the theory. Pores with CO<sub>2</sub>/CH<sub>4</sub> separation factors higher than 10<sup>2</sup> have
CO<sub>2</sub> permeances per pore lower than 10<sup>–22</sup> mol s<sup>–1</sup> Pa<sup>–1</sup>, and pores with
separation factors of ∼10 have CO<sub>2</sub> permeances per
pore between 10<sup>–22</sup> and 10<sup>–21</sup> mol
s<sup>–1</sup> Pa<sup>–1</sup>. Finally, we show that
a pore density of 10<sup>14</sup> m<sup>–2</sup> is required
for a porous graphene membrane to exceed the permeance-selectivity
upper bound of polymeric materials. Moreover, we show that a higher
pore density can potentially further boost the permeation performance
of a porous graphene membrane above all existing membranes. Our findings
provide insights into the potential and the limitations of porous
graphene membranes for gas separation and provide an efficient methodology
for screening nanopore configurations and sizes for the efficient
separation of desired gas mixtures