10 research outputs found
Electrostatic Assembly Preparation of High-Toughness Zirconium Diboride-Based Ceramic Composites with Enhanced Thermal Shock Resistance Performance
The
central problem of using ceramic as a structural material is
its brittleness, which associated with rigid covalent or ionic bonds.
Whiskers or fibers of strong ceramics such as silicon carbide (SiC)
or silicon nitride (Si<sub>3</sub>N<sub>4</sub>) are widely embedded
in a ceramic matrix to improve the strength and toughness. The incorporation
of these insulating fillers can impede the thermal flow in ceramic
matrix, thus decrease its thermal shock resistance that is required
in some practical applications. Here we demonstrate that the toughness
and thermal shock resistance of zirconium diboride (ZrB<sub>2</sub>)/SiC composites can be improved simultaneously by introducing graphene
into composites via electrostatic assembly and subsequent sintering
treatment. The incorporated graphene creates weak interfaces of grain
boundaries (GBs) and optimal thermal conductance paths inside composites.
In comparison to pristine ZrB<sub>2</sub>–SiC composites, the
toughness of (2.0%) ZrB<sub>2</sub>–SiC/graphene composites
exhibited a 61% increasing (from 4.3 to 6.93 MPa·m<sup>1/2</sup>) after spark plasma sintering (SPS); the retained strength after
thermal shock increased as high as 74.8% at 400 °C and 304.4%
at 500 °C. Present work presents an important guideline for producing
high-toughness ceramic-based composites with enhanced thermal shock
properties
Sonochemical Transformation of Epoxy–Amine Thermoset into Soluble and Reusable Polymers
The
degradation and reuse of epoxy thermosets have significant impact
on the environments. We report that an epoxy–amine thermoset
embedded with Diels–Alder (DA) bonds was transformed into soluble
polymers via sonochemistry under mild temperature (ca. 20 °C)
for the first time. Sonication could effectively induce the position-oriented
cleavage of DA bonds (i.e., retro-DA) of the fully swelled epoxy thermoset
in dimethyl sulfoxide (DMSO), leading to the soluble polymers. Of
importance, such sonochemical process could be regulated on demand
via switching on-and-off of the sonication. The obtained soluble polymers
could be recured to form epoxy–amine thermosets via DA reaction.
This sonochemical method might provide an unprecedented and efficient
way to the controlled degradation and recycling of the epoxy thermosets
containing the dynamic covalent bonds likes DA groups
Thermosensitive Ionic Microgels via Surfactant-Free Emulsion Copolymerization and in Situ Quaternization Cross-Linking
A type of thermosensitive ionic microgel
was successfully prepared
via the simultaneous quaternized cross-linking reaction during the
surfactant-free emulsion copolymerization of <i>N</i>-isopropylacrylamide
(NIPAm) as the main monomer and 1-vinylimidazole or 4-vinylpyridine
as the comonomer. 1,4-Dibromobutane and 1,6-dibromohexane were used
as the halogenated compounds to quaternize the tertiary amine in the
comonomer, leading to the formation of a cross-linking network and
thermosensitive ionic microgels. The sizes, morphologies, and properties
of the obtained ionic microgels were systematically investigated by
using transmission electron microscopy (TEM), dynamic and static light
scattering (DLS and SLS), electrophoretic light scattering (ELS),
thermogravimetric analyses (TGA), and UV–visible spectroscopy.
The obtained ionic microgels were spherical in shape with narrow size
distribution. These ionic microgels exhibited thermosensitive behavior
and a unique feature of polyÂ(ionic liquid) in aqueous solutions, of
which the counteranions of the microgels could be changed by anion
exchange reaction with BF<sub>4</sub>K or lithium trifluoromethyl
sulfonate (PFM-Li). After the anion exchange reaction, the ionic microgels
were stable in aqueous solution and could be well dispersed in the
solvents with different polarities, depending on the type of counteranion.
The sizes and thermosensitive behavior of the ionic microgels could
be well tuned by controlling the quaternization extent, the type of
comonomer, halogenated compounds, and counteranions. The ionic microgels
showed superior swelling properties in aqueous solution. Furthermore,
these ionic microgels also showed capabilities to encapsulate and
release the anionic dyes, like methyl orange, in aqueous solutions
In Situ Growth of Core–Sheath Heterostructural SiC Nanowire Arrays on Carbon Fibers and Enhanced Electromagnetic Wave Absorption Performance
Large-scale
core–sheath heterostructural SiC nanowires were facilely grown
on the surface of carbon fibers using a one-step chemical vapor infiltration
process. The as-synthesized SiC nanowires consist of single crystalline
SiC cores with a diameter of ∼30 nm and polycrystalline SiC
sheaths with an average thickness of ∼60 nm. The formation
mechanisms of core–sheath heterostructural SiC nanowires (SiC<sub>nws</sub>) were discussed in detail. The SiC<sub>nws</sub>-CF shows
strong electromagnetic (EM) wave absorption performance with a maximum
reflection loss value of −45.98 dB at 4.4 GHz. Moreover, being
coated with conductive polymer polypyrrole (PPy) by a simple chemical
polymerization method, the SiC<sub>nws</sub>-CF/PPy nanocomposites
exhibited superior EM absorption abilities with maximum RL value of
−50.19 dB at 14.2 GHz and the effective bandwidth of 6.2 GHz.
The SiC<sub>nws</sub>-CF/PPy nanocomposites in this study are very
promising as absorber materials with strong electromagnetic wave absorption
performance
S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Electrocatalysts
Replacement
of rare and precious metal catalysts with low-cost
and earth-abundant ones is currently among the major goals of sustainable
chemistry. Herein, we report the synthesis of S, N dual-doped graphene-like
carbon nanosheets via a simple pyrolysis of a mixture of melamine
and dibenzyl sulfide as efficient metal-free electrocatalysts for
oxygen reduction reaction (ORR). The S, N dual-doped graphene-like
carbon nanosheets show enhanced activity toward ORR as compared with
mono-doped counterparts, and excellent durability in contrast to the
conventional Pt/C electrocatalyst in both alkaline and acidic media.
A high content of graphitic-N and pyridinic-N is necessary for ORR
electrocatalysis in the graphene-like carbon nanosheets, but an appropriate
amount of S atoms further contributes to the improvement of ORR activity.
Superior ORR performance from the as-prepared S, N dual-doped graphene-like
carbon nanosheets implies great promises in practical applications
in energy devices
Improving Electrocatalysts for Oxygen Evolution Using Ni<sub><i>x</i></sub>Fe<sub>3–<i>x</i></sub>O<sub>4</sub>/Ni Hybrid Nanostructures Formed by Solvothermal Synthesis
Spinel-type
oxides have been found to be active electrocatalysts
for OER. However, their semiconductor character severely limits their
catalytic performance. Herein, we report a facile solvothermal pathway
for the synthesis of spinel-type Ni<sub><i>x</i></sub>Fe<sub>3–<i>x</i></sub>O<sub>4</sub> oxides/Ni metal nanocomposites.
The good electrical contact between the metal and semiconductor oxide
interface and well-tuned compositions of Ni<sub><i>x</i></sub>Fe<sub>3–<i>x</i></sub>O<sub>4</sub> spinel
oxides are crucial to achieve better OER performance. Specifically,
the Ni<sub><i>x</i></sub>Fe<sub>3–<i>x</i></sub>O<sub>4</sub>/Ni nanocomposite sample prepared from a metal
precursor ratio of <i>y</i> = 0.15 [<i>y</i> =
Fe/(Fe + Ni)] that results in an <i>x</i> value of about
0.36 exhibits catalytic activity with an overpotential of 225 mV to
achieve an electrocatalytic current density of <i>j</i> =
10 mA cm<sup>–2</sup> and a Tafel slope of 44 mV dec<sup>–1</sup> in alkaline electrolyte. This study not only provides new perspectives
to designing nanocomposite catalysts for OER but also opens a promising
avenue for further enhancing electrocatalytic performance via interface
and composition engineering
Carbon Nanofiber Arrays Grown on Three-Dimensional Carbon Fiber Architecture Substrate and Enhanced Interface Performance of Carbon Fiber and Zirconium Carbide Coating
Carbon
nanofibers (CNFs) were grown around the carbon fiber architecture
through a plasma enhanced chemical vapor deposition method to enhance
the interface performance between CF architecture substrate and ZrC
preceramic matrix. The synthesized 3D CF hierarchical architectures
(CNFs-CF) are coated with zirconium carbide (ZrC) ceramic to enhance
their antioxidant property and high temperature resistance. The composition
and the crystalline phase structure of the composite were detected
with the X-ray photoelectron spectroscopy and X-ray diffraction. The
results of scanning electron microscopy show that, the as-prepared
CNFs and consistent ZrC ceramic coating are uniformly covered on the
surface of carbon fiber architecture substrate. The ZrC ceramic products
with excellent crystallinity were got from the pyrolysis of preceramic
polymer at 1600 °C in inert atmosphere. Comparing with the untreated
CF, the loading of ZrC ceramics around the CNFs-CF architecture surface
are significantly increased. The thermal stability and mechanical
property of CNFs-CF/ZrC nanocomposites have been promoted obviously
compared with the CF/ZrC ceramic nanocomposite. The prepared CNFs-CF/ZrC
ceramic nanocomposite is one of the potential candidate materials
for the thermal protection application
Ordered Silica Nanoparticles Grown on a Three-Dimensional Carbon Fiber Architecture Substrate with Siliconborocarbonitride Ceramic as a Thermal Barrier Coating
Hierarchical structure consisting
of ordered silica nanoparticles grown onto carbon fiber (CF) has been
fabricated to improve the interfacial properties between the CFs and
polymer matrix. To improve the reactivity of CFs, their surface was
modified using polyÂ(1,4-phenylene diisocyanate) (PPDI) via in situ
polymerization, which also resulted in the distribution of numerous
isocyanate groups on the surface of CFs. Silica nanoparticles were
modified on the interface of CF-PPDI by chemical grafting method.
The microstructure, chemical composition, and interfacial properties
of CFs with ordered silica nanoparticles were comprehensively investigated
by scanning electron microscopy, X-ray photoelectron spectroscopy,
and Fourier transform infrared spectroscopy. Results indicated an
obvious increase in the interfacial shear strength, compared to that
of CF precursor, which was attributed to silica nanoparticles interacting
with the epoxy resin. Furthermore, siliconborocarbonitride (SiBCN)
ceramic was used as thermal barrier coating to enhance 3D CF architecture
substrate antioxidant and ablation properties. Thermogravimetric results
show that the thermal stability of the CF with SiBCN ceramic layer
has a marked increase at high temperature
Direct Transformation from Graphitic C<sub>3</sub>N<sub>4</sub> to Nitrogen-Doped Graphene: An Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction
Carbon-based nanomaterials provide
an attractive perspective to replace precious Pt-based electrocatalysts
for oxygen reduction reaction (ORR) to enhance the practical applications
of fuel cells. Herein, we demonstrate a one-pot direct transformation
from graphitic-phase C<sub>3</sub>N<sub>4</sub> (g-C<sub>3</sub>N<sub>4</sub>) to nitrogen-doped graphene. g-C<sub>3</sub>N<sub>4</sub>, containing only C and N elements, acts as a self-sacrificing template
to construct the framework of nitrogen-doped graphene. The relative
contents of graphitic and pyridinic-N can be well-tuned by the controlled
annealing process. The resulting nitrogen-doped graphene materials
show excellent electrocatalytic activity toward ORR, and much enhanced
durability and tolerance to methanol in contrast to the conventional
Pt/C electrocatalyst in alkaline medium. It is determined that a higher
content of N does not necessarily lead to enhanced electrocatalytic
activity; rather, at a relatively low N content and a high ratio of
graphitic-N/pyridinic-N, the nitrogen-doped graphene obtained by annealing
at 900 °C (NGA900) provides the most promising activity for ORR.
This study may provide further useful insights on the nature of ORR
catalysis of carbon-based materials
Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets
Molybdenum disulfide
(MoS<sub>2</sub>) is a promising nonprecious
catalyst for the hydrogen evolution reaction (HER) that has been extensively
studied due to its excellent performance, but the lack of understanding
of the factors that impact its catalytic activity hinders further
design and enhancement of MoS<sub>2</sub>-based electrocatalysts.
Here, by using novel porous (holey) metallic 1T phase MoS<sub>2</sub> nanosheets synthesized by a liquid-ammonia-assisted lithiation route,
we systematically investigated the contributions of crystal structure
(phase), edges, and sulfur vacancies (S-vacancies) to the catalytic
activity toward HER from five representative MoS<sub>2</sub> nanosheet
samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated
porous 2H phase. Superior HER catalytic activity was achieved in the
porous 1T phase MoS<sub>2</sub> nanosheets that have even more edges
and S-vacancies than conventional 1T phase MoS<sub>2</sub>. A comparative
study revealed that the phase serves as the key role in determining
the HER performance, as 1T phase MoS<sub>2</sub> always outperforms
the corresponding 2H phase MoS<sub>2</sub> samples, and that both
edges and S-vacancies also contribute significantly to the catalytic
activity in porous MoS<sub>2</sub> samples. Then, using combined defect
characterization techniques of electron spin resonance spectroscopy
and positron annihilation lifetime spectroscopy to quantify the S-vacancies,
the contributions of each factor were individually elucidated. This
study presents new insights and opens up new avenues for designing
electrocatalysts based on MoS<sub>2</sub> or other layered materials
with enhanced HER performance