Crystals

2286 KiB

Open AccessFeature PaperReview

Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes

by Elena Spagnuolo, Oliver Plümper, Marie Violay, Andrea Cavallo and Giulio Di Toro

Crystals 2016, 6(7), 83; https://doi.org/10.3390/cryst6070083 - 22 Jul 2016

Cited by 7 | Viewed by 6286

Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s²), slip rates (~1 m/s), and normal stresses [...] Read more.

Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s²), slip rates (~1 m/s), and normal stresses (>>10 MPa) expected at the passage of the earthquake rupture along the front of fault patches, measured large fault dynamic weakening for slip rates larger than a critical velocity of 0.01–0.1 m/s. The dynamic weakening corresponds to a decrease of the friction coefficient (defined as the ratio of shear stress vs. normal stress) up to 40%–50% after few millimetres of slip (flash weakening), almost independently of rock type. The microstructural evolution of the sliding interfaces with slip may yield hints on the microphysical processes responsible for flash weakening. At the microscopic scale, the frictional strength results from the interaction of micro- to nano-scale surface irregularities (asperities) which deform during fault sliding. During flash weakening, the visco-plastic and brittle work on the asperities results in abrupt frictional heating (flash heating) and grain size reduction associated with mechano-chemical reactions (e.g., decarbonation in CO₂-bearing minerals such as calcite and dolomite; dehydration in water-bearing minerals such as clays, serpentine, etc.) and phase transitions (e.g., flash melting in silicate-bearing rocks). However, flash weakening is also associated with grain size reduction down to the nanoscale. Using focused ion beam scanning and transmission electron microscopy, we studied the micro-physical mechanisms associated with flash heating and nanograin formation in carbonate-bearing fault rocks. Experiments were conducted on pre-cut Carrara marble (99.9% calcite) cylinders using a rotary shear apparatus at conditions relevant to seismic rupture propagation. Flash heating and weakening in calcite-bearing rocks is associated with a shock-like stress release due to the migration of fast-moving dislocations and the conversion of their kinetic energy into heat. From a review of the current natural and experimental observations we speculate that this mechanism tested for calcite-bearing rocks, is a general mechanism operating during flash weakening (e.g., also precursory to flash melting in the case of silicate-bearing rocks) for all fault rock types undergoing fast slip acceleration due to the passage of the seismic rupture front. Full article

(This article belongs to the Special Issue Crystal Dislocations)

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13575 KiB

Open AccessFeature PaperArticle

Soda Cans Metamaterial: A Subwavelength-Scaled Phononic Crystal

by Fabrice Lemoult, Nadège Kaina, Mathias Fink and Geoffroy Lerosey

Crystals 2016, 6(7), 82; https://doi.org/10.3390/cryst6070082 - 21 Jul 2016

Cited by 46 | Viewed by 11740

Abstract

Photonic or phononic crystals and metamaterials, due to their very different typical spatial scales—wavelength and deep subwavelength—and underlying physical mechanisms—Bragg interferences or local resonances—, are often considered to be very different composite media. As such, while the former are commonly used to manipulate [...] Read more.

Photonic or phononic crystals and metamaterials, due to their very different typical spatial scales—wavelength and deep subwavelength—and underlying physical mechanisms—Bragg interferences or local resonances—, are often considered to be very different composite media. As such, while the former are commonly used to manipulate and control waves at the scale of the unit cell, i.e., wavelength, the latter are usually considered for their effective properties. Yet we have shown in the last few years that under some approximations, metamaterials can be used as photonic or phononic crystals, with the great advantage that they are much more compact. In this review, we will concentrate on metamaterials made out of soda cans, that is, Helmholtz resonators of deep subwavelength dimensions. We will first show that their properties can be understood, likewise phononic crystals, as resulting from interferences only, through multiple scattering effects and Fano interferences. Then, we will demonstrate that below the resonance frequency of its unit cell, a soda can metamaterial supports a band of subwavelength varying modes, which can be excited coherently using time reversal, in order to beat the diffraction limit from the far field. Above this frequency, the metamaterial supports a band gap, which we will use to demonstrate cavities and waveguides, very similar to those obtained in phononic crystals, albeit of deep subwavelength dimensions. We will finally show that multiple scattering can be taken advantage of in these metamaterials, by correctly structuring them. This allows to turn a metamaterial with a single negative effective property into a negative index metamaterial, which refracts waves negatively, hence acting as a superlens. Full article

(This article belongs to the Special Issue Phononic Crystals)

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21740 KiB

Open AccessArticle

A Novel, Simple and Green Way to Fabricate BiVO₄ with Excellent Photocatalytic Activity and Its Methylene Blue Decomposition Mechanism

by Minna Guo, Qianglong He, Aiyang Wang, Weimin Wang and Zhengyi Fu

Crystals 2016, 6(7), 81; https://doi.org/10.3390/cryst6070081 - 20 Jul 2016

Cited by 24 | Viewed by 8301

Abstract

BiVO₄ photocatalysts were synthesized via a facile surfactant-free method with heat treatment. The heat treatment temperatures influenced the crystal structures and morphologies. The photocatalytic performance is associated with its crystallinity, Brunauer–Emmett–Teller (BET) specific surface area, and band gap energy. The BiVO₄ [...] Read more.

BiVO₄ photocatalysts were synthesized via a facile surfactant-free method with heat treatment. The heat treatment temperatures influenced the crystal structures and morphologies. The photocatalytic performance is associated with its crystallinity, Brunauer–Emmett–Teller (BET) specific surface area, and band gap energy. The BiVO₄ photocatalyst prepared by heat treatment at 700 °C showed the highest photocatalytic activity, promoting 100% degradation of methylene blue (MB) in 60 min under visible-light irradiation. Recycling experiments results indicated that the BiVO₄ photocatalysts have excellent photo-stability, and a possible mechanism for the photocatalytic process was proposed by examining the effects of the active species involved in MB degradation. This work could provide new insights into the fabrication of highly efficient and stable BiVO₄ photocatalysts for dye degradation. Full article

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4644 KiB

Open AccessArticle

Effect of HNO₃ Concentration on the Morphologies and Properties of Bi₂WO₆ Photocatalyst Synthesized by a Hydrothermal Method

by Wenjie Wang, Minna Guo, Dongliang Lu, Weimin Wang and Zhengyi Fu

Crystals 2016, 6(7), 75; https://doi.org/10.3390/cryst6070075 - 20 Jul 2016

Cited by 20 | Viewed by 6224

Abstract

Bi₂WO₆ samples were prepared by a hydrothermal method using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O as raw materials. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman and Brunauer–Emmett–Teller (BET) were [...] Read more.

Bi₂WO₆ samples were prepared by a hydrothermal method using Bi(NO₃)₃·5H₂O and Na₂WO₄·2H₂O as raw materials. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman and Brunauer–Emmett–Teller (BET) were employed for sample characterization. The photocatalytic activity of the samples was evaluated by the degradation of Rhodamine B under visible-light irradiation. The photocatalytic activity of Bi₂WO₆, as well as the effect of varying HNO₃ concentrations on the morphologies of Bi₂WO₆, was investigated. The HNO₃ concentration significantly affected the structure and morphology of the Bi₂WO₆. The photocatalytic performance varied with the structure, morphology, and surface area of the Bi₂WO₆ samples. The results indicated that the H10 sample exhibits uniform morphology and excellent photocatalytic performance; using this sample, the degradation of Rhodamine B reached 96% in 90 min under visible-light irradiation. Full article

(This article belongs to the Special Issue Mesocrystals and Hierarchical Structures)

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3067 KiB

Open AccessArticle

Adsorption, Desorption, Surface Diffusion, Lattice Defect Formation, and Kink Incorporation Processes of Particles on Growth Interfaces of Colloidal Crystals with Attractive Interactions

by Yoshihisa Suzuki, Yoshiaki Hattori, Jun Nozawa, Satoshi Uda, Akiko Toyotama and Junpei Yamanaka

Crystals 2016, 6(7), 80; https://doi.org/10.3390/cryst6070080 - 18 Jul 2016

Cited by 8 | Viewed by 6849

Abstract

Good model systems are required in order to understand crystal growth processes because, in many cases, precise incorporation processes of atoms or molecules cannot be visualized easily at the atomic or molecular level. Using a transmission-type optical microscope, we have successfully observed in [...] Read more.

Good model systems are required in order to understand crystal growth processes because, in many cases, precise incorporation processes of atoms or molecules cannot be visualized easily at the atomic or molecular level. Using a transmission-type optical microscope, we have successfully observed in situ adsorption, desorption, surface diffusion, lattice defect formation, and kink incorporation of particles on growth interfaces of colloidal crystals of polystyrene particles in aqueous sodium polyacrylate solutions. Precise surface transportation and kink incorporation processes of the particles into the colloidal crystals with attractive interactions were observed in situ at the particle level. In particular, contrary to the conventional expectations, the diffusion of particles along steps around a two-dimensional island of the growth interface was not the main route for kink incorporation. This is probably due to the number of bonds between adsorbed particles and particles in a crystal; the number exceeds the limit at which a particle easily exchanges its position to the adjacent one along the step. We also found novel desorption processes of particles from steps to terraces, attributing them to the assistance of attractive forces from additionally adsorbing particles to the particles on the steps. Full article

(This article belongs to the Special Issue Colloidal Crystals)

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13458 KiB

Open AccessReview

Zirconate Pyrochlore Frustrated Magnets: Crystal Growth by the Floating Zone Technique

by Monica Ciomaga Hatnean, Claudia Decorse, Martin R. Lees, Oleg A. Petrenko and Geetha Balakrishnan

Crystals 2016, 6(7), 79; https://doi.org/10.3390/cryst6070079 - 11 Jul 2016

Cited by 28 | Viewed by 7766

Abstract

This article reviews recent achievements on the crystal growth of a new series of pyrochlore oxides—lanthanide zirconates, which are frustrated magnets with exotic magnetic properties. Oxides of the type

A_{2}

B_{2}

O

_{7}

(where

A =

Rare Earth,

B =

Ti, [...] Read more.

This article reviews recent achievements on the crystal growth of a new series of pyrochlore oxides—lanthanide zirconates, which are frustrated magnets with exotic magnetic properties. Oxides of the type

A_{2}

B_{2}

O

_{7}

(where

A =

Rare Earth,

B =

Ti, Mo) have been successfully synthesised in single crystal form using the floating zone method. The main difficulty of employing this technique for the growth of rare earth zirconium oxides

A_{2}

Zr

_{2}

O

_{7}

arises from the high melting point of these materials. This drawback has been recently overcome by the use of a high power Xenon arc lamp furnace for the growth of single crystals of Pr

_{2}

Zr

_{2}

O

_{7}

. Subsequently, large, high quality single crystals of several members of the zirconate family of pyrochlore oxides

A_{2}

Zr

_{2}

O

_{7}

(with

A =

La → Gd) have been grown by the floating zone technique. In this work, the authors give an overview of the crystal growth of lanthanide zirconates. The optimum conditions used for the floating zone growth of

A_{2}

Zr

_{2}

O

_{7}

crystals are reported. The characterisation of the crystal boules and their crystal quality is also presented. Full article

(This article belongs to the Special Issue Traveling Solvent Floating Zone (TSFZ) Method in Crystal Growth)

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3981 KiB

Open AccessArticle

Magnetic-Sphere-Based Phononic Crystals

by Damien Leduc, Bruno Morvan, Alain Tinel, Rebecca Sainidou and Pascal Rembert

Crystals 2016, 6(7), 78; https://doi.org/10.3390/cryst6070078 - 8 Jul 2016

Cited by 6 | Viewed by 5939

Abstract

Periodic arrays in one, two, and three dimensions, made of magnetic spheres embedded in a fluid matrix, are considered in this study and utilized as phononic structures. The propagation of acoustic waves through these structures is analyzed experimentally, in low- and high-frequency region, [...] Read more.

Periodic arrays in one, two, and three dimensions, made of magnetic spheres embedded in a fluid matrix, are considered in this study and utilized as phononic structures. The propagation of acoustic waves through these structures is analyzed experimentally, in low- and high-frequency region, via laser vibrometry, as well as standard underwater acoustic measurements. A first comparison to theoretical calculations obtained through multiple-scattering techniques and multipole models reveals a distinct behavior depending on the immersion fluid and/or frequency regime. Our results show that the elastodynamic response of these systems can be, under conditions, simply described by classical elastic theory without taking directly (ab initio) into account the magnetic character of the spherical particles. The structures considered above could offer several possibilities including facility of construction and use in filtering applications, but they are also of interest from a theoretical point of view, as a means to investigate the validity of several approximate theoretical descriptions. Full article

(This article belongs to the Special Issue Phononic Crystals)

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10592 KiB

Open AccessArticle

Dislocation Nucleation on Grain Boundaries: Low Angle Twist and Asymmetric Tilt Boundaries

by Erman Guleryuz and Sinisa Dj. Mesarovic

Crystals 2016, 6(7), 77; https://doi.org/10.3390/cryst6070077 - 5 Jul 2016

Cited by 15 | Viewed by 9687

Abstract

We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain [...] Read more.

We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain boundary (GB) dislocation network deforms under load until a dislocation segment compatible with glide on a lattice slip plane is created. The half loops are then emitted into the bulk of the crystal. Asymmetric twist boundaries considered here did not produce bulk dislocations under load. Instead, the boundary with a low excess volume nucleated a mobile GB dislocation and additional GB defects. The GB sliding proceeded by motion of the mobile GB dislocation. The boundary with a high excess volume sheared elastically, while bulk-nucleated dislocations produced plastic relaxation. Full article

(This article belongs to the Special Issue Crystal Dislocations)

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1318 KiB

Open AccessArticle

Preparation of Three-Dimensional Photonic Crystals of Zirconia by Electrodeposition in a Colloidal Crystals Template

by Lei Pan, Hongbo Xu, Yunyong Sun, Jiupeng Zhao and Yao Li

Crystals 2016, 6(7), 76; https://doi.org/10.3390/cryst6070076 - 2 Jul 2016

Cited by 10 | Viewed by 5102

Abstract

Three-dimensional photonic crystals of zirconia were prepared by electrodeposition in a colloidal crystals template following calcination at 500 °C. Scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and reflectance spectroscopy were employed to characterize the photonic crystals of zirconia. It was found that hydrated [...] Read more.

Three-dimensional photonic crystals of zirconia were prepared by electrodeposition in a colloidal crystals template following calcination at 500 °C. Scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and reflectance spectroscopy were employed to characterize the photonic crystals of zirconia. It was found that hydrated zirconium ions could penetrate the colloidal crystals template and reach the substrate easily by electrodeposition, which resulted in stronger bonding between the substrate and the as-deposited membrane. Moreover, the electrodeposited membrane had low water content, leading to a low amount of shrinkage during calcination. Both these properties could suppress detachment from the substrate upon removal of the colloidal crystals template. Therefore, the three-dimensional photonic crystals of zirconia synthesized in this study exhibited very good preservation of the ordered structures of the colloidal crystals template with a high density. A peak of reflection higher than 70% was formed in the reflectance spectrum because of the strong diffraction of the ordered structures. Full article

(This article belongs to the Special Issue Colloidal Crystals)

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8340 KiB

Open AccessArticle

Electronic and Optical Properties of Dislocations in Silicon

by Manfred Reiche and Martin Kittler

Crystals 2016, 6(7), 74; https://doi.org/10.3390/cryst6070074 - 30 Jun 2016

Cited by 13 | Viewed by 10965

Abstract

Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase of the drain current of metal-oxide-semiconductor field-effect transistors (MOSFETs) if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic [...] Read more.

Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase of the drain current of metal-oxide-semiconductor field-effect transistors (MOSFETs) if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed based on experimental measurements and tight binding simulations. It is shown that the high strain level on the dislocation core—exceeding 10% or more—causes locally dramatic changes of the band structure and results in the formation of a quantum well along the dislocation line. This explains experimental findings (two-dimensional electron gas and single-electron transitions). The energy quantization within the quantum well is most important for supermetallic conductivity. Full article

(This article belongs to the Special Issue Crystal Dislocations)

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7908 KiB

Open AccessArticle

Optical and X-Ray Topographic Studies of Dislocations, Growth-Sector Boundaries, and Stacking Faults in Synthetic Diamonds

by Moreton Moore, Stuart G. Nailer and Wojciech K. Wierzchowski

Crystals 2016, 6(7), 71; https://doi.org/10.3390/cryst6070071 - 24 Jun 2016

Cited by 16 | Viewed by 7123

Abstract

The characterization of growth features and defects in various high-pressure high-temperature (HPHT) synthetic diamonds has been achieved with optical and X-ray topographic techniques. For the X-ray studies, both characteristic and synchrotron radiation were used. The defects include dislocations, stacking faults, growth banding, growth [...] Read more.

The characterization of growth features and defects in various high-pressure high-temperature (HPHT) synthetic diamonds has been achieved with optical and X-ray topographic techniques. For the X-ray studies, both characteristic and synchrotron radiation were used. The defects include dislocations, stacking faults, growth banding, growth sector boundaries, and metal inclusions. The directions of the Burgers vectors of many dislocations (edge, screw, and mixed 30°, 60°, and 73.2°), and the fault vectors of stacking faults, were determined as <110> and 1/3 <111> respectively. Some dislocations were generated at metallic inclusions; and some dislocations split with the formation of stacking faults. Full article

(This article belongs to the Special Issue Crystal Dislocations)

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Crystals, Volume 6, Issue 7 (July 2016) – 11 articles

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