No abstract

Rapidly evolving fields of biomedical, energy, and (opto)electronic devices bring forward the need for deformable conductors with constantly rising benchmarks for mechanical properties and electronic conductivity. The search for conductors with improved strength and strain have inspired the multiple studies of nanocomposites and amorphous metals. However, finding conductors that defy the boundaries of classical materials and exhibit simultaneously high strength, toughness, and fast charge transport while enabling their scalable production, remains a difficult materials engineering challenge. Here, composites made from aramid nanofibers (ANFs) and gold nanoparticles (Au NPs) that offer a new toolset for engineering high strength flexible conductors are described. ANFs are derived from Kevlar macrofibers and retain their strong mechanical properties and temperature resilience. Au NPs are infiltrated into a porous, free-standing aramid matrix, becoming aligned on ANFs, which reduces the charge percolation threshold and facilitates charge transport. Further thermal annealing at 300 °C results in the Au-ANF composites with an electrical conductivity of 1.25 × 10 4 S cm −1 combined with a tensile strength of 96 MPa, a Young's modulus of 5.29 GPa, and a toughness of 1.3 MJ m −3 . These para meters exceed those of most of the composite materials, and are comparable to those of amorphous metals but have no volume limitations. The plasmonic optical frequencies characteristic for constituent NPs are present in the composites with ANFs enabling plasmon-based optoelectronic applications.

No abstract

Materials that are both stretchable and electrically conductive enable a broad spectrum of applications in sensing, actuating, electronics, optics and energy storage. The materials engineering concept of stretchable conductors is primarily based on combining nanowires, nanoribbons, nanoparticles, or nanocarbons with rubbery polymers to obtain composites with different abilities to transport charge and alter their nanoscale organization under strain. Although some of these composites reveal remarkably interesting multiscale reconfigurability and self-assembly phenomena, decreasing conductance with increased strain has restricted their widespread implementation. In a broader physical sense, the dependence of conductance on stress is undesirable because it requires a correlated change of electrical inputs. In this paper, we describe highly conductive and deformable sheets with a conductivity as high as 230 000 S cm−1, composed of silver nanoparticles, infiltrated within a porous aramid nanofiber (ANF) matrix. By forming a kirigami pattern, consisting of a regularized network of notches cut within the films, their ultimate tensile strain is improved from ∼2% to beyond 100%. The use of ANFs derived from well-known ultrastrong Kevlar™ fibers imparts high mechanical performance to the base composite. Importantly, the conductance of the films remains constant, even under large deformation resulting in a material with a zero conductance gradient. Unlike other nanocomposites for which strain and conductance are strongly coupled, the kirigami nanocomposite provides a pathway to demanding applications for flexible and stretchable electronics with power/voltage being unaffected by the deformation mode and temperature.

Using several-mJ energy pulses from a high-repetition rate ( 1 ⁄2 kHz), ultrashort (35 fs) pulsed laser interacting with a ∼ 10 µm diameter stream of free-flowing heavy water (D 2 O), we demonstrate a 2.45 MeV neutron flux of 10 5 /s. Operating at high intensity (of order 10 19 Wcm −2 ), laser pulse energy is efficiently absorbed in the pre-plasma, generating energetic deuterons. These collide with deuterium nuclei in both the bulk target and the large volume of low density D 2 O vapor surrounding the target to generate neutrons through d (d, n) 3 He reactions. The neutron flux, as measured by a calibrated neutron bubble detector, increased as the laser pulse energy was increased from 6 mJ to 12 mJ. A quantitative comparison between the measured flux and results derived from 2D particle-in-cell simulations show comparable neutron fluxes for similar laser characteristics to the experiment. The simulations reveal that there are two groups of deuterons; forward moving deuterons generate D−D fusion reactions in the D 2 O stream and act as a point source of neutrons, while backward moving deuterons propagate through the low-density D 2 O vapor filled chamber and yield a volumetric source of neutrons.Energetic neutrons have numerous applications in many fields, including medicine 1 , homeland security 2 , and material science 3 . Conventional fast neutron sources include deuterium−deuterium (D−D) and deuteriumtritium (D−T) fusion generators, as well as light-ion, photoneutron and spallation sources. Laser plasma interactions in the relativistic regime can also generate charged particles and subsequently accelerate them to energies high enough to trigger nuclear fusion reactions, resulting in neutron production [4][5][6][7][8][9][10][11][12][13][14][15][16] . Recent advances in ultra-high power laser technology now enable tabletop scale systems, which may be further reduced in size for use as drivers for portable neutron generators in the future. One of the methods for neutron production is through the acceleration of high-energy ions (keV-MeV) impinging upon an appropriate converter target, such as deuterated plastic. Typically, thin solid targets are used in these experiments to accelerate deuterons.Using solid targets in the form of a thin (1µm) foil has some drawbacks for high repetition-rate (>kHz) operation; for example, one has to replace the target after each shot. To resolve target life-time issues, fast target replacement schemes have been introduced by some groups 8,15,[17][18][19] . In particular, using ∼ 100 mJ of pulse energy at 10 Hz repetition-rate, Ditmire et. al. 8 used deuterium clusters, which were rapidly heated by the laser pulse (on a femtosecond time scale) and launched few a) Now at Physics Department, Lancaster University, UK keV deuterons to drive D−D fusion reactions.In this Letter, we report the production of neutrons using a high repetition-rate ( 1 ⁄2 kHz) femtosecond laser at high intensities (> 10 19 Wcm −2 for vacuum focus) but low pulse energies (several-mJ) interacting with a heavy...