3 research outputs found
Fluorination of an Alumina Surface: Modeling Aluminum–Fluorine Reaction Mechanisms
Density functional
theory (DFT) calculations were performed to examine exothermic surface
chemistry between alumina and four fluorinated, fragmented molecules
representing species from decomposing fluoropolymers: F<sup>–</sup>, HF, CH<sub>3</sub>F, and CF<sub>4</sub>. The analysis has strong
implications for the reactivity of aluminum (Al) particles passivated
by an alumina shell. It was hypothesized that the alumina surface
structure could be transformed due to hydrogen bonding effects from
the environment that promote surface reactions with fluorinated species.
In this study, the alumina surface was analyzed using model clusters
as isolated systems embedded in a polar environment (i.e., acetone).
The conductor-like screening model (COSMO) was used to mimic environmental
effects on the alumina surface. Four defect models for specific active −OH
sites were investigated including two terminal hydroxyl groups and
two hydroxyl bridge groups. Reactions involving terminal bonds produce
more energy than bridge bonds. Also, surface exothermic reactions
between terminal −OH bonds and fluorinated species produce
energy in decreasing order with the following reactant species: CF<sub>4</sub> > HF > CH<sub>3</sub>F. Additionally, experiments were
performed on aluminum powders using thermal equilibrium analysis techniques
that complement the calculations. Consistently, the experimental results
show a linear relationship between surface exothermic reactions and
the main fluorination reaction for Al powders. These results connect
molecular level reaction kinetics to macroscopic measurements of surface
energy and show that optimizing energy available in surface reactions
linearly correlates to maximizing energy in the main reaction
Reaction Dynamics of Rocket Propellant with Magnesium Oxide Nanoparticles
The combustion behavior of rocket
propellant grade 2 (RP-2) was
investigated as a function of magnesium oxide (MgO) nanoparticles
(i.e., 20 nm diameter) added at varied concentrations. The MgO nanoparticles
were surface-treated with a long-chain carboxylic acid to aid their
dispersion in RP-2. The fuel droplet regression rate, surface tension,
and heat of combustion of RP-2 with MgO nanoparticle additives were
measured to characterize combustion behavior. Heat of combustion and
surface tension measurements varied negligibly among all samples indicating
that calorific output and surface tension are not controlling parameters
influencing fuel combustion behavior. However, fuel droplet regression
rates were considerably increased by adding 0.5 wt % MgO from 0.225
to 66.16 mm/s, which is an improvement by 2 orders of magnitude. Further
analysis showed that MgO particles enhance diffusive heat transfer,
which promotes nucleation and disruptive burning throughout the three
stages of regression, heating/evaporation (stage 1), combustion of
RP-2 (stage 2), and combustion of carboxylic acid dispersant (stage
3), and, thus, lead to improved fuel droplet combustion
Porphyrin Immobilized Nanographene Oxide for Enhanced and Targeted Photothermal Therapy of Brain Cancer
Brain
cancer is a fatal disease that is difficult to treat because
of poor targeting and low permeability of chemotherapeutic drugs through
the blood brain barrier. In a comparison to current treatments, such
as surgery followed by chemotherapy and/or radiotherapy, photothermal
therapy is a remarkable noninvasive therapy developed in recent years.
In this work, porphyrin immobilized nanographene oxide (PNG) was synthesized
and bioconjugated with a peptide to achieve enhanced and targeted
photothermal therapy for brain cancer. PNG was dispersed into the
agar based artificial tissue model and demonstrated a photo-to-thermal
conversion efficiency of 19.93% at a PNG concentration of only 0.5
wt %, with a heating rate of 0.6 °C/s at the beginning of irradiation.
In comparison, 0.5 wt % graphene oxide (GO) indicated a photo-to-thermal
conversion efficiency of 12.20% and a heating rate of 0.3 °C/s.
To actively target brain tumor cells without harming healthy cells
and tissues surrounding the laser path, a tripeptide l-arginyl-glycyl-l-aspartic (RGD) was further grafted to PNG. The photothermal
therapy effects of PNG-RGD completely eliminated the tumor <i>in vivo</i>, indicating its excellent therapeutic effect for
the treatment of brain cancer