In the summer of 2012, during a Pulsar Search Collaboratory workshop, two high-school students discovered J1930−1852, a pulsar in a double neutron star (DNS) system. Most DNS systems are characterized by short orbital periods, rapid spin periods and eccentric orbits. However, J1930−1852 has the longest spin period (P spin ∼185 ms) and orbital period (P b ∼45 days) yet measured among known, recycled pulsars in DNS systems, implying a shorter than average and/or inefficient recycling period before its companion went supernova. We measure the relativistic advance of periastron for J1930−1852,ω = 0.00078(4) deg/yr, which implies a total mass (M tot = 2.59(4) M ) consistent with other DNS systems. The 2σ constraints on M tot place limits on the pulsar and companion masses (m p < 1.32 M and m c > 1.30 M respectively). J1930−1852's spin and orbital parameters challenge current DNS population models and make J1930−1852 an important system for further investigation.
The advantaged intrinsic and scale-dependent properties of aligned nanofibers (NFs) and their assembly into 3D architectures motivates their use as dry adhesives and shape-engineerable materials. While controlling NF-substrate adhesion is...
The excellent intrinsic properties of aligned nanofibers,
such
as carbon nanotubes (CNTs), and their ability to be easily formed
into multifunctional 3D architectures motivate their use for a variety
of commercial applications, such as batteries, chemical sensors for
environmental monitoring, and energy harvesting devices. While controlling
nanofiber adhesion to the growth substrate is essential for bulk-scale
manufacturing and device performance, experimental approaches and
models to date have not addressed tuning the CNT array–substrate
adhesion strength with thermal processing conditions. In this work,
facile “one-pot” thermal postgrowth processing (at temperatures T
p = 700–950 °C) is used to study
CNT–substrate pull-off strength for millimeter-tall aligned
CNT arrays. CNT array pull-off from the flat growth substrate (Fe/Al2O3/SiO2/Si wafers) via tensile testing
shows that the array fails progressively, similar to the response
of brittle microfiber bundles in tension. The pull-off strength evolves
nonmonotonically with T
p in three regimes,
first increasing by 10 times through T
p = 800 °C due to graphitization of disordered carbon at the
CNT–catalyst interface, and then decreasing back to a weak
interface through T
p = 950 °C due
to diffusion of the Fe catalyst into the substrate, Al2O3 crystallization, and substrate cracking. Failure is
observed to occur at the CNT–catalyst interface below 750 °C,
and the CNTs themselves break during pull-off after higher T
p processing, leaving residual CNTs on the substrate.
Morphological and chemical analyses indicate that the Fe catalyst
remains on the substrate after pull-off in all regimes. This work
provides new insights into the interfacial interactions responsible
for nanofiber–substrate adhesion and allows tuning to increase
or decrease array strength for applications such as advanced sensors,
energy devices, and nanoelectromechanical systems (NEMS).
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