12,615 research outputs found
Dynamic response of elastic beam to a moving pulse: finite element analysis of critical velocity
Dynamic behaviour of a semi-infinite elastic beam subjected to a moving single sinusoidal pulse was investigated by using finite element method associated with dimensionless analysis. The typical features of the equivalent stress and beam deflection were presented. It is found that the average value of maximal equivalent stress in the beam reaches its maximum value when the velocity of moving pulse is closed to a critical velocity. The critical velocity decreases as the pulse duration increases. The material, structural and load parameters influencing the critical velocity were analysed. An empirical formula of the critical velocity with respect to the speed of elastic wave, the gyration radius of the cross-section and the pulse duration was obtained
Condensed Multiwalled Carbon Nanotubes as Super Fibers
The ultra-low intershell shear strength in carbon nanotubes (CNTs) has been
the primary obstacle to applications of CNTs as mechanical reinforcements. In
this paper we propose a new CNT-system composed of comprising of coaxial
cylindrical shells of sp2-bonded carbons with condensed intershell spacings.
Our atomistic calculations show that such condensed multiwalled carbon
nanotubes (CMWNTs) can greatly enhance intershell shear strengths by several
orders, and can simultaneously generate higher tensile strengths and moduli
respectively than those of ordinary CNTs. It has further shown that CMWNTs can
maintain thermally stable up to 2,000 K. By taking advantage of the primary
enhancement mechanism of CMWNTs, a method of producing CMWNTs is therefore
proposed tentatively. It is believed that CMWNTs featured with those properties
can be taken as excellent candidates of super fibers for creating space
elevators
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Dynamic deformability of individual PbSe nanocrystals during superlattice phase transitions
The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole-dipole interactions
Diffusion Language Models Can Perform Many Tasks with Scaling and Instruction-Finetuning
The recent surge of generative AI has been fueled by the generative power of
diffusion probabilistic models and the scalable capabilities of large language
models. Despite their potential, it remains elusive whether diffusion language
models can solve general language tasks comparable to their autoregressive
counterparts. This paper demonstrates that scaling diffusion models w.r.t.
data, sizes, and tasks can effectively make them strong language learners. We
build competent diffusion language models at scale by first acquiring knowledge
from massive data via masked language modeling pretraining thanks to their
intrinsic connections. We then reprogram pretrained masked language models into
diffusion language models via diffusive adaptation, wherein task-specific
finetuning and instruction finetuning are explored to unlock their versatility
in solving general language tasks. Experiments show that scaling diffusion
language models consistently improves performance across downstream language
tasks. We further discover that instruction finetuning can elicit zero-shot and
few-shot in-context learning abilities that help tackle many unseen tasks by
following natural language instructions, and show promise in advanced and
challenging abilities such as reasoning.Comment: added reference
Policy Optimization with Stochastic Mirror Descent
Improving sample efficiency has been a longstanding goal in reinforcement
learning. In this paper, we propose the : a sample efficient
policy gradient method with stochastic mirror descent. A novel variance reduced
policy gradient estimator is the key of to improve sample
efficiency. Our needs only sample
trajectories to achieve an -approximate first-order stationary point,
which matches the best-known sample complexity. We conduct extensive
experiments to show our algorithm outperforms state-of-the-art policy gradient
methods in various settings
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In Situ TEM Study of the Degradation of PbSe Nanocrystals in Air
PbSe
nanocrystals have attracted widespread attention due to a
variety of potential applications. However, the practical utility
of these nanocrystals has been hindered by their poor air stability,
which induces undesired changes in the optical and electronic properties.
An understanding of the degradation of PbSe nanocrystals when they
are exposed to air is critical for improving the stability and enhancing
their applications. Here, we use in situ transmission electron microscopy
(TEM) with an environmental cell connected to air to study PbSe nanocrystal
degradation triggered by air exposure. We have also conducted a series
of complementary studies, including in situ environmental TEM study
of PbSe nanocrystals exposed to pure oxygen and PbSe nanocrystals
in H2O using a liquid cell, and ex situ experiments, such
as O2 plasma treatment and thermal heating of PbSe nanocrystals
under different air exposure. Our in situ observations reveal that
when PbSe nanocrystals are exposed to air (or oxygen) under electron
beam irradiation, they experience a series of changes, including shape
evolution of individual nanocrystals with the cuboid intermediates,
coalescence between nanocrystals, and formation of PbSe thin films
through drastic solid-state fusion. Further studies show that the
PbSe thin films transform into an amorphous Pb rich phase or eventually
pure Pb, which suggest that Se reacts with oxygen and can be evaporated
under electron beam illumination. These various in situ and ex situ
experimental results indicate that PbSe nanocrystal degradation in
air is initiated by the dissociation and removal of ligands from the
PbSe nanocrystal surface
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