Showing papers on "Grain size published in 2015"

Non-wetting surface-driven high-aspect-ratio crystalline grain growth for efficient hybrid perovskite solar cells

Abstract: Large-aspect-ratio grains are needed in polycrystalline thin-film solar cells for reduced charge recombination at grain boundaries; however, the grain size in organolead trihalide perovskite (OTP) films is generally limited by the film thickness. Here we report the growth of OTP grains with high average aspect ratio of 2.3-7.9 on a wide range of non-wetting hole transport layers (HTLs), which increase nucleus spacing by suppressing heterogeneous nucleation and facilitate grain boundary migration in grain growth by imposing less drag force. The reduced grain boundary area and improved crystallinity dramatically reduce the charge recombination in OTP thin films to the level in OTP single crystals. Combining the high work function of several HTLs, a high stabilized device efficiency of 18.3% in low-temperature-processed planar-heterojunction OTP devices under 1 sun illumination is achieved. This simple method in enhancing OTP morphology paves the way for its application in other optoelectronic devices for enhanced performance.

Low surface recombination velocity in solution-grown CH3NH3PbBr3 perovskite single crystal

Abstract: Surface recombination velocity can have a major impact on solar cell performance. Here, Yang et al. measure surface recombination dynamics in perovskite single crystals using broadband transient reflectance spectroscopy. Grain size is crucial to avoid the effects of surface recombination on carrier lifetime.

Synthesis of large single-crystal hexagonal boron nitride grains on Cu-Ni alloy.

Abstract: High nucleation density has thus far limited the quality and grain size of CVD-grown hexagonal boron nitride. Here, by optimizing the Ni ratio in Cu–Ni substrates, the authors successfully reduce nucleation density and report single-crystal hexagonal boron nitride grains up to 7500 μm2.

Millimeter-wave polarization of protoplanetary disks due to dust scattering

Abstract: We present a new method to constrain the grain size in protoplanetary disks with polarization observations at millimeter wavelengths. If dust grains are grown to the size comparable to the wavelengths, the dust grains are expected to have a large scattering opacity, and thus the continuum emission is expected to be polarized due to self-scattering. We perform 3D radiative transfer calculations to estimate the polarization degree for the protoplanetary disks having radial Gaussian-like dust surface density distributions, which have been recently discovered. The maximum grain size is set to be and the observing wavelength to be 870 . We find that the polarization degree is as high as 2.5% with a subarcsec spatial resolution, which is likely to be detected with near-future ALMA observations. The emission is polarized due to scattering of anisotropic continuum emission. The map of the polarization degree shows a double-peaked distribution, and the polarization vectors are in the radial direction in the inner ring and in the azimuthal direction in the outer ring. We also find the wavelength dependence of the polarization degree: the polarization degree is the highest if dust grains have a maximum size of , where λ is the observing wavelength. Hence, multi-wave and spatially resolved polarization observations toward protoplanetary disks enable us to put a constraint on the grain size. The constraint on the grain size from polarization observations is independent of or may be even stronger than that from the opacity index.

Unfolding grain size effects in barium titanate ferroelectric ceramics

Abstract: Grain size effects on the physical properties of polycrystalline ferroelectrics have been extensively studied for decades; however there are still major controversies regarding the dependence of the piezoelectric and ferroelectric properties on the grain size. Dense BaTiO3 ceramics with different grain sizes were fabricated by either conventional sintering or spark plasma sintering using micro- and nano-sized powders. The results show that the grain size effect on the dielectric permittivity is nearly independent of the sintering method and starting powder used. A peak in the permittivity is observed in all the ceramics with a grain size near 1 μm and can be attributed to a maximum domain wall density and mobility. The piezoelectric coefficient d33 and remnant polarization Pr show diverse grain size effects depending on the particle size of the starting powder and sintering temperature. This suggests that besides domain wall density, other factors such as back fields and point defects, which influence the domain wall mobility, could be responsible for the different grain size dependence observed in the dielectric and piezoelectric/ferroelectric properties. In cases where point defects are not the dominant contributor, the piezoelectric constant d33 and the remnant polarization Pr increase with increasing grain size.

Abnormal crystal growth in CH3NH3PbI3−xClx using a multi-cycle solution coating process

Abstract: Recently, the efficiency of organolead trihalide perovskite solar cells has improved greatly because of improved material qualities with longer carrier diffusion lengths. Mixing chlorine in the precursor for mixed halide films has been reported to dramatically enhance the diffusion lengths of mixed halide perovskite films, mainly as a result of a much longer carrier recombination lifetime. Here we report that adding Cl containing precursor for mixed halide perovskite formation can induce the abnormal grain growth behavior that yields well-oriented grains accompanied by the appearance of some very large size grains. The abnormal grain growth becomes prominent only after multi-cycle coating of MAI : MACl blend precursor. The large grain size is found mainly to contribute to a longer carrier charge recombination lifetime, and thus increases the device efficiency to 18.9%, but without significantly impacting the carrier transport property. The strong correlation identified between material process and morphology provides guidelines for future material optimization and device efficiency enhancement.

Impact of crystalline defects and size on X-ray line broadening: A phenomenological approach for tetragonal SnO2 nanocrystals

Abstract: Nanocrystalline tin oxide (SnO2) powders with different grain size were prepared by chemical precipitation method. The reaction was carried out by varying the period of hydrolysis and the as-prepared samples were annealed at different temperatures. The samples were characterized using X-ray powder diffractometer and transmission electron microscopy. The microstrain and crystallite size were calculated for all the samples by using Williamson-Hall (W-H) models namely, isotropic strain model (ISM), anisotropic strain model (ASM) and uniform deformation energy density model (UDEDM). The morphology and particle size were determined using TEM micrographs. The directional dependant young’s modulus was modified as an equation relating elastic compliances (sij) and Miller indices of the lattice plane (hkl) for tetragonal crystal system and also the equation for elastic compliance in terms of stiffness constants was derived. The changes in crystallite size and microstrain due to lattice defects were observed while ...

A New Electrochemical Approach for the Synthesis of Copper-Graphene Nanocomposite Foils with High Hardness

Abstract: Graphene has proved its significant role as a reinforcement material in improving the strength of polymers as well as metal matrix composites due to its excellent mechanical properties. In addition, graphene is also shown to block dislocation motion in a nanolayered metal-graphene composites resulting in ultra high strength. In the present paper, we demonstrate the synthesis of very hard Cu-Graphene composite foils by a simple, scalable and economical pulse reverse electrodeposition method with a well designed pulse profile. Optimization of pulse parameters and current density resulted in composite foils with well dispersed graphene, exhibiting a high hardness of ~2.5 GPa and an increased elastic modulus of ~137 GPa while exhibiting an electrical conductivity comparable to that of pure Cu. The pulse parameters are designed in such a way to have finer grain size of Cu matrix as well as uniform dispersion of graphene throughout the matrix, contributing to high hardness and modulus. Annealing of these nanocomposite foils at 300°C, neither causes grain growth of the Cu matrix nor deteriorates the mechanical properties, indicating the role of graphene as an excellent reinforcement material as well as a grain growth inhibitor.

High temperature deformation behavior and dynamic recrystallization in CoCrFeNiMn high entropy alloy

Abstract: Microstructure evolution in high-entropy alloy CoCrFeNiMn during uniaxial compression to a height reduction of true strain of ≈1.4 in the temperature interval 600–1100 °C was studied. Although some differences was observed in the mechanical behavior of the alloy and the activation energy of deformation in warm (below 800 °C) and hot (above 800 °C) temperature intervals, microstructure evolution at all studied temperatures was found to be accompanied by discontinuous dynamic recrystallization (dDRX). During hot deformation recrystallization was primary associated with nucleation of new grains on the initial grain boundaries, while in the warm interval dDRX was mainly observed in shear bands. The volume fraction of the recrystallized structure was respectively 0.085 and 0.95 at 600 and 1000 °C and the recrystallized grain size was found to be 0.2 and 40.4 µm for 600 and 1100 °C, respectively.

Grain-size dependent mechanical behavior of nanocrystalline metals

Abstract: Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 106 s−1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d>1 μm, 1 μm

Relative importance of grain boundaries and size effects in thermal conductivity of nanocrystalline materials

Abstract: A theoretical model for describing effective thermal conductivity (ETC) of nanocrystalline materials has been proposed, so that the ETC can be easily obtained from its grain size, single crystal thermal conductivity, single crystal phonon mean free path (PMFP), and the Kaptiza thermal resistance In addition, the relative importance between grain boundaries (GBs) and size effects on the ETC of nanocrystalline diamond at 300 K has been studied It has been demonstrated that with increasing grain size, both GBs and size effects become weaker, while size effects become stronger on thermal conductivity than GBs effects

Effect of grain size on dielectric and ferroelectric properties of nanostructured Ba 0.8 Sr 0.2 TiO 3 ceramics

Abstract: Barium strontium titanate (Ba0.8Sr0.2TiO3, BST) nanocrystalline ceramics have been synthesized by high energy ball milling. As the sintering temperature increases from 1200 °C to 1350 °C, the average grain size of BST ceramics increases from 86 nm to 123 nm. The X-ray diffraction (XRD) studies show that these ceramics are tetragonal. The phase and grain size of the sintered pellets have been estimated from the XRD patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The effect of grain size on dielectric and ferroelectric properties is studied. The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size. The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm. The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases. These ceramics are promising materials for tunable capacitor device applications.