Aerogel
fibers, the simultaneous embodiment of aerogel 3D network
and fibrous geometry, have shown great advantages over natural and
synthetic fibers in thermal insulation. However, as a fast gelation
to ensure aerogel fiber spinning generally induces rapid local clustering
of precursor particles (i.e., phase separation) and
unavoidably results in nontransparency and nonuniformity in the gel
state, a severe challenge remains in remedying the spinning to make
transparent aerogel fibers come true. Herein, we report a reaction
spinning toward highly porous silica aerogel fibers, where the Brownian
motion (i.e., diffusion) of colloidal particles is
hampered during spinning to allow the maintaining of the fiber shape,
while a rapid gelation reaction is activated by concentrated ammonia
to solidify the fiber. The aggregation degree of the primary particles
can be precisely controlled by pH-dependent hydrolyzation, and thus,
the final aerogel fiber can be either transparent or opaque, as dominated
by Rayleigh or Mie scattering. The resulting transparent silica aerogel
fibers with low density, high specific surface area, and flexibility
can inherit advanced features including excellent thermal insulation,
wide temperature stability, and optional hydrophobic functionalization
and, thus, be suitable for wearable applications.
To obtain ideal sensing materials with nearly zero temperature coefficient resistance (TCR) for self-temperature-compensated pressure sensors, we proposed an Incipient Network Conformal Growth (INCG) technology to prepare hybrid and elastic porous materials: the nanoparticles (NPs) are first dispersed in solvent to form an incipient network, another component is then introduced to coat the incipient network conformally via wet chemical route. The conformal coatings not only endow NPs with high stability but also offer them additional structural elasticity, meeting requirements for future generations of portable, compressive and flexible devices. The resultant polypyrrole/silver coaxial nanowire hybrid aero-sponges prepared via INCG technology have been processed into a piezoresistive sensor with highly sensing stability (low TCR 0.86 × 10(-3)/°C), sensitivity (0.33 kPa(-1)), short response time (1 ms), minimum detectable pressure (4.93 Pa) after suffering repeated stimuli, temperature change and electric heating. Moreover, a stress-triggered Joule heater can be also fabricated mainly by the PPy-Ag NW hybrid aero-sponges with nearly zero temperature coefficient.
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