Mechanical Properties of Concrete with partial replacement of Coarse aggregates by Coconut Shells and reinforced with Coconut Fibre

This paper centers around the study of physical and mechanical properties of concrete reinforced with coconut fibre and the coarse aggregates of which are partially replaced with coconut shells. American Concrete Institute (ACI) method has been used to design M20 concrete wherein coconut shells (CS) replaced the coarse aggregates (CA) by 6%, 8%, 10%, 12% and 14% by volume. Under each replacement of CA by CS, coconut fibres were added by 3%, 4% and 5% of cement content. Compressive strength of concrete was found to comply with characteristic strength for certain mixes which avers that the replacements were justifiable for concrete production and thus, the optimum mix for the concrete prepared thereby, would have to be considered accentuating the tensile strength which was actually the one with 10% replacement of CA by CS in terms of volume and 3% addition of coconut fibre. The research vividly evinces a decrease in overall density of the concrete thus prepared. The authors suggest the use of coconut shell and fibre in the production of concrete not only because they impart themselves as viable materials, but their use would also assist to abate the amount of environmental waste

ハロゲン化鉛ペロブスカイトのマイクロ結晶、ナノ結晶および集合体の発光挙動に関する顕微分光学的相関研究 [全文の要約]

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Photon Recycling by Energy Transfer in Piezochemically Synthesized and Close-Packed Methylammonium Lead Halide Perovskites

Photon recycling by multiple reabsorption-emission is responsible for the long-range energy transport in large crystals and thick films of lead halide perovskites, resulting in red-shifted and delayed emission. Apart from such a radiative process, nonradiative energy transfer influences photon recycling in perovskites with close-packed donor-acceptor-type states. In this study, we report the role of nonradiative energy transfer on photon recycling in piezochemically synthesized and close-packed pure and mixed halide methylammonium lead perovskites. Here, the pressure applied to precursors of perovskites helps us to synthesize and close-pack perovskite crystallites into pellets. Nonetheless, interestingly, we find that the applied pressure redistributes the emission maxima or band- gap of these perovskites. The temporally and spectrally resolved photoluminescence from the mixed halide sample unveils nonradiative energy transfer from a higher (bromide) to a lower (iodide) band-gap domain, where the rate of relaxation of the bromide domain is higher than that of the pure bromide perovskite. These results help us to confirm the role of nonradiative energy transfer on photon recycling in perovskites

Synthesis, optoelectronic properties and applications of halide perovskites

Halide perovskites have emerged as a class of most promising and cost-effective semiconductor materials for next generation photoluminescent, electroluminescent and photovoltaic devices. These perovskites have high optical absorption coefficients and exhibit narrow-band bright photoluminescence, in addition to their halide-dependent tuneable bandgaps, low exciton binding energies, and long-range carrier diffusion. These properties make these perovskites superior to classical semiconductors such as silicon. Most importantly, the simple synthesis of perovskites in the form of high quality films, single crystals, nanocrystals and quantum dots has attracted newcomers to develop novel perovskites with unique optoelectronic properties for optical and photovoltaic applications. Here, we comprehensively review recent advances in the synthesis and optoelectronic properties of films, microcrystals, nanocrystals and quantum dots of lead halide and lead-free halide perovskites. Followed by the classification of synthesis, we address the ensemble and single particle properties of perovskites from the viewpoints of the confinement and transport of charge carriers or excitons. Further, we correlate the charge carrier properties of perovskite films, microcrystals, nanocrystals and quantum dots with the crystal structure and size, halide composition, temperature, and pressure. Finally, we illustrate the emerging applications of perovskites to solar cells, LEDs, and lasers, and discuss the ongoing challenges in the field

Mechano-optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots

Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-Assembled formamidinium lead bromide (FAPbBr3) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20-300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-Assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-Assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and a lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors. © 2022 American Chemical Society. All rights reserved

Nonradiative Energy Transfer through Distributed Bands in Piezochemically Synthesized Cesium and Formamidinium Lead Halide Perovskites

Repeated absorption of emitted photons, also called photon recycling, in large crystals and thick films of perovskites leads to delayed photoluminescence (PL) and decrease of PL intensity. The role of distinct band gaps, which act as donors and acceptors of energy, and nonradiative energy transfer on such delayed, low intensity emission is yet to be rationalized. Here we report delayed emission by nonradiative energy transfer across a distribution of energy states in close‐packed crystallites of cesium lead bromide CsPbBr3, formamidinium lead bromide FAPbBr3, or the mixed halide FAPb(BrI)3 perovskite synthesized in the form of thick pellets by the piezochemical method. The PL lifetime of the bromide‐rich domain in the mixed halide pellet is considerably decreased when compared with a pure FAPbBr3 pellet. Here the domains with higher bromide composition act as the energy donor, whereas the iodide‐rich domains are the acceptors. Time‐resolved PL measurements of CsPbBr3, FAPbBr3, and the mixed halide FAPb(BrI)3 perovskite pellets help us to clarify the role of nonradiative energy transfer on photon recycling