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Crystals, Volume 8, Issue 9 (September 2018)

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Cover Story (view full-size image) The last nitronyl nitroxide bearing azolyl-substituent, namely [...] Read more.
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Open AccessArticle Selective Area Growth and Structural Characterization of GaN Nanostructures on Si(111) Substrates
Crystals 2018, 8(9), 366; https://doi.org/10.3390/cryst8090366
Received: 23 July 2018 / Revised: 27 August 2018 / Accepted: 13 September 2018 / Published: 16 September 2018
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Abstract
Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AlN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer.
[...] Read more.
Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AlN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer. Defects related to Al–Si eutectic formation were observed in all samples, irrespective of lattice mismatch and buffer layer polarity. Eutectic related defects in the Si surface caused voids in N-polar samples, but not in metal-polar samples. Likewise, inversion domains were present in N-polar, but not metal-polar samples. The morphology of Ga-polar GaN SAG on nitride buffered Si(111) was similar to that of homoepitaxial GaN SAG. Full article
(This article belongs to the Special Issue Growth and Structural Characterization of Self-Nucleated Nanowires)
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Open AccessReview Thermal and Chemical Expansion in Proton Ceramic Electrolytes and Compatible Electrodes
Crystals 2018, 8(9), 365; https://doi.org/10.3390/cryst8090365
Received: 18 August 2018 / Revised: 5 September 2018 / Accepted: 6 September 2018 / Published: 14 September 2018
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Abstract
This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while
[...] Read more.
This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while the chemical expansion is explained based on crystallography and defect chemistry. Modelling methods are used to predict the thermochemical expansion of PCCs and MIECs with two examples: hydration of barium zirconate (BaZr1−xYxO3−δ) and oxidation/reduction of La1−xSrxCo0.2Fe0.8O3−δ. While it is unusual for a review paper, we conducted experiments to evaluate the influence of the heating rate in determining expansion coefficients experimentally. This was motivated by the discrepancy of some values in literature. The conclusions are that the heating rate has little to no effect on the obtained values. Models for the expansion coefficients of a composite material are presented and include the effect of porosity. A set of data comprising thermal and chemical expansion coefficients has been gathered from the literature and presented here divided into two groups: protonic electrolytes and mixed ionic-electronic conductors. Finally, the methods of mitigation of the thermal mismatch problem are discussed. Full article
(This article belongs to the Special Issue Ceramic Conductors)
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Open AccessArticle Temperature Effects on the Elastic Constants, Stacking Fault Energy and Twinnability of Ni3Si and Ni3Ge: A First-Principles Study
Crystals 2018, 8(9), 364; https://doi.org/10.3390/cryst8090364
Received: 3 August 2018 / Revised: 5 September 2018 / Accepted: 9 September 2018 / Published: 14 September 2018
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Abstract
The volume versus temperature relations for Ni3Si and Ni3Ge are obtained by using the first principles calculations combined with the quasiharmonic approach. Based on the equilibrium volumes at temperature T, the temperature dependence of the elastic constants, generalized
[...] Read more.
The volume versus temperature relations for Ni 3 Si and Ni 3 Ge are obtained by using the first principles calculations combined with the quasiharmonic approach. Based on the equilibrium volumes at temperature T, the temperature dependence of the elastic constants, generalized stacking fault energies and generalized planar fault energies of Ni 3 Si and Ni 3 Ge are investigated by first principles calculations. The elastic constants, antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies and twinning energy decrease with increasing temperature. The twinnability of Ni 3 Si and Ni 3 Ge are examined using the twinnability criteria. It is found that their twinnability decrease with increasing temperature. Furthermore, Ni 3 Si has better twinnability than Ni 3 Ge at different temperatures. Full article
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Open AccessArticle Mosaicity of Spin-Crossover Crystals
Crystals 2018, 8(9), 363; https://doi.org/10.3390/cryst8090363
Received: 28 August 2018 / Revised: 11 September 2018 / Accepted: 11 September 2018 / Published: 13 September 2018
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Abstract
Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state
[...] Read more.
Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state and others in the low spin (LS) state. Since the volume of HS and LS crystal packings are known to be very different, the assembly of domains within the crystal, i.e., its mosaicity, may be modified at the spin crossover. With little data available in the literature we propose an investigation into the temperature dependence of mosaicity in certain spin-crossover crystals. The study was preceded by the examination of instrumental factors, in order to establish a protocol for the measurement of mosaicity. The results show that crystal mosaicity appears to be strongly modified by thermal spin-crossover; however, the nature of the changes are probably sample dependent and driven, or masked, in most cases by the characteristics of the crystal (disorder, morphology …). No general relationship could be established between mosaicity and crystal properties. If, however, mosaicity studies in spin-crossover crystals are conducted and interpreted with great care, they could help to elucidate crucial crystal characteristics such as mechanical fatigability, and more generally to investigate systems where phase transition is associated with large volume changes. Full article
(This article belongs to the Special Issue Synthesis and Applications of New Spin Crossover Compounds)
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Open AccessArticle Inducing Crystallinity of Metal Thin Films with Weak Magnetic Fields without Thermal Annealing
Crystals 2018, 8(9), 362; https://doi.org/10.3390/cryst8090362
Received: 31 May 2018 / Revised: 1 September 2018 / Accepted: 6 September 2018 / Published: 13 September 2018
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Abstract
Since the discovery of thin films, it has been known that higher crystallinity demands higher temperatures, making the process inadequate for energy-efficient and environmentally friendly methods of thin film fabrication. We resolved this problem by sparking metal wires in a 0.4 Tesla magnetic
[...] Read more.
Since the discovery of thin films, it has been known that higher crystallinity demands higher temperatures, making the process inadequate for energy-efficient and environmentally friendly methods of thin film fabrication. We resolved this problem by sparking metal wires in a 0.4 Tesla magnetic field at ambient conditions under ultra-pure nitrogen flow to replace the annealing of thin films, and thus designed an environmentally friendly and energy-efficient thin film fabrication method. We employed grazing incidence X-Ray Diffraction spectroscopy to characterize crystallinity of Iron, Nickel, Copper and Tungsten thin films prepared by a sparking discharge process in the presence of 0.4 T magnetic field at an ambient temperature of 25 °C. Control experiment was conducted by sparking without a magnetic field present and using ultra-pure nitrogen flow and ambient air containing oxygen. The Iron thin film prepared in ultra-pure nitrogen flow preserved crystallinity even after one year of ageing. Nickel exhibited higher crystallinity when sparked in nitrogen gas flow than when sparked in atmospheric air and was the only element to crystalize under atmospheric air. Tungsten successfully crystalized after just 40 min of sparking and aluminium failed to crystalize at all, even after 12 h of sparking under nitrogen flow. Full article
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Open AccessArticle Molecular Dynamics Investigation of Graphene Nanoplate Diffusion Behavior in Poly-α-Olefin Lubricating Oil
Crystals 2018, 8(9), 361; https://doi.org/10.3390/cryst8090361
Received: 27 June 2018 / Revised: 27 August 2018 / Accepted: 9 September 2018 / Published: 13 September 2018
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Abstract
Graphene as a type of novel additive significantly enhanced the tribological performance of blended lubricating oil. However, the dispersibility of graphene with long-term stability in lubricating oil is still a challenge. Chemical modification for graphene, rather than using surfactants, provided a better method
[...] Read more.
Graphene as a type of novel additive significantly enhanced the tribological performance of blended lubricating oil. However, the dispersibility of graphene with long-term stability in lubricating oil is still a challenge. Chemical modification for graphene, rather than using surfactants, provided a better method to improve the dispersibility of graphene in lubricants. In this study, the equilibrium molecular dynamics (EMD) simulations were carried out to investigate the diffusion behavior of graphene nanoplates in poly-α-olefin (PAO) lubricating oil. The effects of graphene-size, edge-functionalization, temperature, and pressure on the diffusion coefficient were studied. In order to understand the influence of edge-functionalization, three different functional groups were grafted to the edge of graphene nanoplates: COOH, COON(CH3)2, CONH(CH2)8CH3 (termed GO, MG, and AG, respectively). The EMD simulations results demonstrated that the relationships between diffusion coefficient and graphene-size and number of functional groups were linear while the temperature and pressure had a nonlinear influence on the diffusion coefficient. It was found that the larger dimension and more functional groups provided the lower diffusion coefficient. AG with eight CONH(CH2)8CH3 groups exhibited the lowest diffusion coefficient. Furthermore, the experimental results and radial distribution function for graphene-PAO illustrated that the diffusion coefficient reflected the dispersibility of nanoparticles in nanofluids to some degree. To our best knowledge, this study is the first time the diffusion behavior of graphene in PAO lubricating oil was investigated using EMD simulations. Full article
(This article belongs to the Section Crystal Engineering)
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Open AccessArticle Tracing Phase Transformation and Lattice Evolution in a TRIP Sheet Steel under High-Temperature Annealing by Real-Time In Situ Neutron Diffraction
Crystals 2018, 8(9), 360; https://doi.org/10.3390/cryst8090360
Received: 31 July 2018 / Revised: 22 August 2018 / Accepted: 6 September 2018 / Published: 11 September 2018
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Abstract
Real-time in situ neutron diffraction was used to characterize the crystal structure evolution in a transformation-induced plasticity (TRIP) sheet steel during annealing up to 1000 °C and then cooling to 60 °C. Based on the results of full-pattern Rietveld refinement, critical temperature regions
[...] Read more.
Real-time in situ neutron diffraction was used to characterize the crystal structure evolution in a transformation-induced plasticity (TRIP) sheet steel during annealing up to 1000 °C and then cooling to 60 °C. Based on the results of full-pattern Rietveld refinement, critical temperature regions were determined in which the transformations of retained austenite to ferrite and ferrite to high-temperature austenite during heating and the transformation of austenite to ferrite during cooling occurred, respectively. The phase-specific lattice variation with temperature was further analyzed to comprehensively understand the role of carbon diffusion in accordance with phase transformation, which also shed light on the determination of internal stress in retained austenite. These results prove the technique of real-time in situ neutron diffraction as a powerful tool for heat treatment design of novel metallic materials. Full article
(This article belongs to the Special Issue Neutron Diffractometers for Single Crystals and Powders)
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Open AccessArticle DFT Calculations and Mesophase Study of Coumarin Esters and Its Azoesters
Crystals 2018, 8(9), 359; https://doi.org/10.3390/cryst8090359
Received: 16 July 2018 / Revised: 30 August 2018 / Accepted: 5 September 2018 / Published: 8 September 2018
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Abstract
Two groups of coumarin derivatives, 4-methyl-2-oxo-2H-chromen-7-yl 4-alkoxybenzoates (coumarin esters), In, and 4-methyl-2-oxo-2H-chromen-7-yl 4-(2-(4-alkoxyphenyl)diazenyl) benzoates (coumarin azoesters), IIn, were synthesized and investigated for their mesophase behavior and stability. Each group constitutes five series that differ from each other by length
[...] Read more.
Two groups of coumarin derivatives, 4-methyl-2-oxo-2H-chromen-7-yl 4-alkoxybenzoates (coumarin esters), In, and 4-methyl-2-oxo-2H-chromen-7-yl 4-(2-(4-alkoxyphenyl)diazenyl) benzoates (coumarin azoesters), IIn, were synthesized and investigated for their mesophase behavior and stability. Each group constitutes five series that differ from each other by length of the mesogenic part. Within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10, 12 and 16 carbons. Mesophase behavior was investigated by differential scanning calorimetry (DSC) and identified by polarized light microscopy (PLM). Density functional theory (DFT) calculations for coumarin derivatives were discussed. The results revealed that the incorporation of azo group incorporated in the mesogenic core decreases the energy differences, increases the dipole moments and stabilities of coumarin azoesters series more than coumarin esters. Full article
(This article belongs to the Section Liquid Crystals)
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Open AccessArticle Investigation of Inverted Perovskite Solar Cells for Viscosity of PEDOT:PSS Solution
Crystals 2018, 8(9), 358; https://doi.org/10.3390/cryst8090358
Received: 31 July 2018 / Revised: 16 August 2018 / Accepted: 28 August 2018 / Published: 6 September 2018
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Abstract
In this paper, we demonstrate that the inverted CH3NH3PbI3 (perovskite) solar cells (PSCs) based on fullerene (C60) as an acceptor is fabricated by applying an improved poly(3,4-ethlyenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution as a hole transport layer (HTL). The
[...] Read more.
In this paper, we demonstrate that the inverted CH3NH3PbI3 (perovskite) solar cells (PSCs) based on fullerene (C60) as an acceptor is fabricated by applying an improved poly(3,4-ethlyenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution as a hole transport layer (HTL). The power conversion efficiency (PCE) of inverted PSCs is increased by 37.5% with stable values of open-circuit voltage (VOC) and fill factor (FF) because we enhance the viscosity of the PEDOT:PSS solution, indicating the perfect effect on both external quantum efficiency (EQE) and surface grain size. The characteristics of the PEDOT:PSS solution, which is being improved through facile methods of obtaining excellent growth of PEDOT:PSS thin film, have a considerable impact on carrier transport. A series of further processing fabrications, including reliable and feasible heating and stirring techniques before the formation of the PEDOT:PSS thin film via spin-coating, not only evaporate the excess moisture but also obviously increase the conductivity. The raised collection of holes become the reason for the enhanced PCE of 3.0%—therefore, the stable performance of FF and VOC are attributed to lower series resistance of devices and the high-quality film crystallization of perovskite and organic acceptors, respectively. Full article
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Open AccessReview A Review of Current Development of Graphene Mechanics
Crystals 2018, 8(9), 357; https://doi.org/10.3390/cryst8090357
Received: 20 August 2018 / Revised: 28 August 2018 / Accepted: 30 August 2018 / Published: 6 September 2018
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Abstract
Graphene, a two-dimensional carbon in honeycomb crystal with single-atom thickness, possesses extraordinary properties and fascinating applications. Graphene mechanics is very important, as it relates to the integrity and various nanomechanical behaviors including flexing, moving, rotating, vibrating, and even twisting of graphene. The relationship
[...] Read more.
Graphene, a two-dimensional carbon in honeycomb crystal with single-atom thickness, possesses extraordinary properties and fascinating applications. Graphene mechanics is very important, as it relates to the integrity and various nanomechanical behaviors including flexing, moving, rotating, vibrating, and even twisting of graphene. The relationship between the strain and stress plays an essential role in graphene mechanics. Strain can dramatically influence the electronic and optical properties, and could be utilized to engineering those properties. Furthermore, graphene with specific kinds of defects exhibit mechanical enhancements and thus the electronic enhancements. In this short review, we focus on the current development of graphene mechanics, including tension and compression, fracture, shearing, bending, friction, and dynamics properties of graphene from both experiments and numerical simulations. We also touch graphene derivatives, including graphane, graphone, graphyne, fluorographene, and graphene oxide, which carve some fancy mechanical properties out from graphene. Our review summarizes the current achievements of graphene mechanics, and then shows the future prospects. Full article
(This article belongs to the Special Issue Graphene Mechanics)
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Open AccessArticle Microstructure and Mechanical Properties of Mg–6Al–1Sn–0.3Mn Alloy Sheet Fabricated through Extrusion Combined with Rolling
Crystals 2018, 8(9), 356; https://doi.org/10.3390/cryst8090356
Received: 30 July 2018 / Revised: 29 August 2018 / Accepted: 2 September 2018 / Published: 5 September 2018
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Abstract
Hot rolling was carried out in this study to modify the microstructures of an extruded Mg–6Al–1Sn–0.3Mn alloy sheet and investigate its effects on mechanical properties. After hot rolling, the grains and second phase of the extruded alloy sheet were remarkably refined, and the
[...] Read more.
Hot rolling was carried out in this study to modify the microstructures of an extruded Mg–6Al–1Sn–0.3Mn alloy sheet and investigate its effects on mechanical properties. After hot rolling, the grains and second phase of the extruded alloy sheet were remarkably refined, and the c-axis of a few grains was parallel to the transverse direction. The strength improvement was mainly attributed to the grain and Mg17Al12 particle refinement due to the Hall–Petch effect and the Orowan mechanism. The random orientation of the fine grains resulted in improving ductility and anisotropy. Full article
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Open AccessReview Microwave Liquid Crystal Technology
Crystals 2018, 8(9), 355; https://doi.org/10.3390/cryst8090355
Received: 2 August 2018 / Revised: 24 August 2018 / Accepted: 28 August 2018 / Published: 5 September 2018
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Abstract
Tunable Liquid Crystal (LC)-based microwave components are of increasing interest in academia and industry. Based on these components, numerous applications can be targeted such as tunable microwave filters and beam-steering antenna systems. With the commercialization of first LC-steered antennas for Ku-band
[...] Read more.
Tunable Liquid Crystal (LC)-based microwave components are of increasing interest in academia and industry. Based on these components, numerous applications can be targeted such as tunable microwave filters and beam-steering antenna systems. With the commercialization of first LC-steered antennas for Ku-band e.g., by Kymeta and Alcan Systems, LC-based microwave components left early research stages behind. With the introduction of terrestrial 5G communications systems, moving to millimeter-wave communication, these systems can benefit from the unique properties of LC in terms of material quality. In this paper, we show recent developments in millimeter wave phase shifters for antenna arrays. The limits of classical high-performance metallic rectangular waveguides are clearly identified. A new implementation with dielectric waveguides is presented and compared to classic approaches. Full article
(This article belongs to the Section Liquid Crystals)
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Open AccessArticle Highly Efficient Nano-Porous Polysilicon Solar Absorption Films Prepared by Silver-Induced Etching
Crystals 2018, 8(9), 354; https://doi.org/10.3390/cryst8090354
Received: 1 August 2018 / Revised: 25 August 2018 / Accepted: 2 September 2018 / Published: 4 September 2018
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Abstract
Nano-porous polysilicon high-temperature resistant solar absorption films were prepared by a thin layer of silver nanoparticles catalytic chemical etching. The polysilicon films with average tiny grain size of approximately 30 nm were obtained by high-temperature 800 °C furnace annealing of hydrogenated amorphous silicon
[...] Read more.
Nano-porous polysilicon high-temperature resistant solar absorption films were prepared by a thin layer of silver nanoparticles catalytic chemical etching. The polysilicon films with average tiny grain size of approximately 30 nm were obtained by high-temperature 800 °C furnace annealing of hydrogenated amorphous silicon films that were deposited on stainless substrate by plasma-enhanced chemical vapor deposition. The uniformly distributed 19 nm sized silver nanoparticles with 8 nm interspacing deposited on poly-Si film, were controlled by thin 4 nm thickness and very slow deposition rate 0.4 nm/min of thermal evaporation. Small silver nanoparticles with short spacing catalyzes the detouring etching process inducing the nano-porous textured surface with deep threaded pores. The etching follows the trail of the grain boundaries, and takes a highly curved thread like structure. The etching stops after reaching a depth of around 1100 nm, and the rest of the bulk thickness of the film remains mostly unaffected. The structure consists of three crystal orientations (111), (220), and (331) close to the surface. This crystalline nature diminishes gradually in the bulk of the film. High absorbance of 95% was obtained due to efficient light-trapping. Hence, preparation of nano-porous polysilicon films by this simple method can effectively increase solar absorption for the receiver of the solar thermal electricity Stirling Engine. Full article
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Open AccessArticle New Topologically Unique Metal-Organic Architectures Driven by a Pyridine-Tricarboxylate Building Block
Crystals 2018, 8(9), 353; https://doi.org/10.3390/cryst8090353
Received: 29 July 2018 / Revised: 21 August 2018 / Accepted: 23 August 2018 / Published: 3 September 2018
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Abstract
Two new three-dimensional (3D) coordination compounds, namely a lead(II) coordination polymer (CP) {[Pb35-cpta)(µ6-cpta)(phen)2]·2H2O}n (1) and a zinc(II) metal-organic framework (MOF) {[Zn24-cpta)(µ-OH)(µ-4,4′-bipy)]·6H2O}n (2
[...] Read more.
Two new three-dimensional (3D) coordination compounds, namely a lead(II) coordination polymer (CP) {[Pb35-cpta)(µ6-cpta)(phen)2]·2H2O}n (1) and a zinc(II) metal-organic framework (MOF) {[Zn24-cpta)(µ-OH)(µ-4,4′-bipy)]·6H2O}n (2), were hydrothermally assembled from 2-(5-carboxypyridin-2-yl)terephthalic acid (H3cpta) as an unexplored principal building block and aromatic N,N-donors as crystallization mediators. Both products were isolated as air-stable microcrystalline solids and were fully characterized by IR spectroscopy, elemental and thermogravimetric analysis, and powder and single-crystal X-ray diffraction. Structural and topological features of CP 1 and MOF 2 were analyzed in detail, allowing to identify a topologically unique 4,5,5,6-connected net in 1 or a very rare 4,4-connected net with the isx topology in 2. Thermal stability and solid-state luminescent behavior of 1 and 2 were also investigated. Apart from revealing a notable topological novelty, both compounds also represent the first structurally characterized Pb(II) and Zn(II) derivatives assembled from H3cpta, thus opening up the application of this unexplored pyridine-tricarboxylate block in the design of new metal-organic architectures. Full article
(This article belongs to the Section Crystal Engineering)
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Open AccessArticle Improved Solubility of Vortioxetine Using C2-C4 Straight-Chain Dicarboxylic Acid Salt Hydrates
Crystals 2018, 8(9), 352; https://doi.org/10.3390/cryst8090352
Received: 5 August 2018 / Revised: 25 August 2018 / Accepted: 31 August 2018 / Published: 2 September 2018
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Abstract
The purpose of this study was to improve the solubility of vortioxetine by crystal engineering principles. In this paper, three C2-C4 straight-chain dicarboxylic acid salt hydrates of vortioxetine (VOT-OA, VOT-MA-H2O, and VOT-SUA-H2O, VOT = vortioxetine, OA = Oxalic acid,
[...] Read more.
The purpose of this study was to improve the solubility of vortioxetine by crystal engineering principles. In this paper, three C2-C4 straight-chain dicarboxylic acid salt hydrates of vortioxetine (VOT-OA, VOT-MA-H2O, and VOT-SUA-H2O, VOT = vortioxetine, OA = Oxalic acid, MA = malonic acid, SUA = succinic acid) were synthesized and characterized by single X-ray diffraction, powder X-ray diffraction, and differential scanning calorimetry. The single crystal structure of three salts reveals that vortioxetine has torsional flexibility, which can encourage VOT to allow combination with aliphatic dicarboxylic acids through N+-H···O hydrogen bonds. The solubility of all salts exhibits a dramatic increase in distilled water, especially for VOT-MA-H2O salt, where it shows the highest solubility, by 96-fold higher compared with pure vortioxetine. Full article
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Open AccessArticle Band Structures Analysis of Elastic Waves Propagating along Thickness Direction in Periodically Laminated Piezoelectric Composites
Crystals 2018, 8(9), 351; https://doi.org/10.3390/cryst8090351
Received: 18 July 2018 / Revised: 12 August 2018 / Accepted: 20 August 2018 / Published: 1 September 2018
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Abstract
Existing studies on elastic waves in periodically laminated piezoelectric structures mainly concerned the passive band properties, since the electrical boundaries in the considered structures cannot vary. This paper investigates the tuning of band properties of uncoupled primary and shear (P- and S-) waves
[...] Read more.
Existing studies on elastic waves in periodically laminated piezoelectric structures mainly concerned the passive band properties, since the electrical boundaries in the considered structures cannot vary. This paper investigates the tuning of band properties of uncoupled primary and shear (P- and S-) waves along the thickness direction by actively varying the electrical field in periodically multilayered piezoelectric structures consisting of orthotropic materials. The alteration of the electrical field is realized in the multilayered unit cell here by either applying or switching four kinds of electrical boundary conditions, including the electric-open, applied electric capacitance, electric-short, and applied feedback voltage, to the constituent piezoelectric layer via the constituent electrode layers covering both its surfaces. First, the state space formalism is introduced to obtain the partial wave solution of any constituent orthotropic layer in the unit cell. Second, the traditional transfer matrix method is adopted to derive the dispersion equation of general, periodically laminated piezoelectric composites with unit cells consisting of an arbitrary number of piezoelectric layers with various boundaries and of elastic layers. Third, numerical examples are provided to verify the proposed analysis method, and to study the influences of electrode thickness as well as four electrical boundaries on the band structures. All the frequency-related dispersion curves are also illustrated by numerical examples to summarize the general dispersion characteristics of uncoupled P- and S-waves in periodically laminated piezoelectric composites. The main finding is that the innovative dispersion characteristic resulting from the negative capacitance may also be achieved via feedback control. Full article
(This article belongs to the Special Issue Phononics)
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Open AccessArticle A Screen Method for the Mass-Production of the Small-Size and Side-View LED Package
Crystals 2018, 8(9), 350; https://doi.org/10.3390/cryst8090350
Received: 14 July 2018 / Revised: 9 August 2018 / Accepted: 28 August 2018 / Published: 31 August 2018
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Abstract
In this study, the small-size and side-view LED packages used in the backlight modules were examined by performing the electrostatic discharge (ESD) process. The high voltages used for the ESD process were 2 kV, 3 kV, and 4 kV. After ESD, the current–voltage
[...] Read more.
In this study, the small-size and side-view LED packages used in the backlight modules were examined by performing the electrostatic discharge (ESD) process. The high voltages used for the ESD process were 2 kV, 3 kV, and 4 kV. After ESD, the current–voltage curves were scanned from −7 V to 3 V. It was found that the significant leakage currents were in the reverse bias of 4 V~7 V and also in the forward bias of 1 V~2.5 V for ESD-damaged LED chips. However, the phenomenon of a slight increase in current was not found for the non-damaged samples. In our study case, the screening conditions could be set at a bias of −7 V or 2.3 V and the current of 0.1 µA. Full article
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Open AccessArticle A Revisit to the Notation of Martensitic Crystallography
Crystals 2018, 8(9), 349; https://doi.org/10.3390/cryst8090349
Received: 21 July 2018 / Revised: 23 August 2018 / Accepted: 29 August 2018 / Published: 30 August 2018
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Abstract
As one of the most successful crystallographic theories for phase transformations, martensitic crystallography has been widely applied in understanding and predicting the microstructural features associated with structural phase transformations. In a narrow sense, it was initially developed based on the concepts of lattice
[...] Read more.
As one of the most successful crystallographic theories for phase transformations, martensitic crystallography has been widely applied in understanding and predicting the microstructural features associated with structural phase transformations. In a narrow sense, it was initially developed based on the concepts of lattice correspondence and invariant plane strain condition, which is formulated in a continuum form through linear algebra. However, the scope of martensitic crystallography has since been extended; for example, group theory and graph theory have been introduced to capture the crystallographic phenomena originating from lattice discreteness. In order to establish a general and rigorous theoretical framework, we suggest a new notation system for martensitic crystallography. The new notation system combines the original formulation of martensitic crystallography and Dirac notation, which provides a concise and flexible way to understand the crystallographic nature of martensitic transformations with a potential extensionality. A number of key results in martensitic crystallography are reexamined and generalized through the new notation. Full article
(This article belongs to the Special Issue Microstructures and Properties of Martensitic Materials)
Open AccessArticle Bending Behavior of a Wrought Magnesium Alloy Investigated by the In Situ Pinhole Neutron Diffraction Method
Crystals 2018, 8(9), 348; https://doi.org/10.3390/cryst8090348
Received: 31 July 2018 / Revised: 26 August 2018 / Accepted: 27 August 2018 / Published: 30 August 2018
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Abstract
The tensile twinning and detwinning behaviors of a wrought magnesium alloy have been investigated during in situ four-point bending using the state-of-the-art high spatial resolution pinhole neutron diffraction (PIND) method. The PIND method allowed us to resolve the tensile twinning/detwinning and lattice strain
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The tensile twinning and detwinning behaviors of a wrought magnesium alloy have been investigated during in situ four-point bending using the state-of-the-art high spatial resolution pinhole neutron diffraction (PIND) method. The PIND method allowed us to resolve the tensile twinning/detwinning and lattice strain distributions across the bending sample during a loading-unloading sequence with a 0.5 mm step size. It was found that the extensive tensile twinning and detwinning occurred near the compression surface, while no tensile twinning behavior was observed in the middle layer and tension side of the bending sample. During the bending, the neutral plane shifted from the compression side to the tension side. Compared with the traditional neutron diffraction mapping method, the PIND method provides more detailed information inside the bending sample due to a higher spatial resolution. Full article
(This article belongs to the Special Issue Neutron Diffractometers for Single Crystals and Powders)
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Open AccessReview GaAs Nanowires Grown by Catalyst Epitaxy for High Performance Photovoltaics
Crystals 2018, 8(9), 347; https://doi.org/10.3390/cryst8090347
Received: 13 August 2018 / Revised: 26 August 2018 / Accepted: 27 August 2018 / Published: 29 August 2018
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Abstract
Photovoltaics (PVs) based on nanostructured III/V semiconductors can potentially reduce the material usage and increase the light-to-electricity conversion efficiency, which are anticipated to make a significant impact on the next-generation solar cells. In particular, GaAs nanowire (NW) is one of the most promising
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Photovoltaics (PVs) based on nanostructured III/V semiconductors can potentially reduce the material usage and increase the light-to-electricity conversion efficiency, which are anticipated to make a significant impact on the next-generation solar cells. In particular, GaAs nanowire (NW) is one of the most promising III/V nanomaterials for PVs due to its ideal bandgap and excellent light absorption efficiency. In order to achieve large-scale practical PV applications, further controllability in the NW growth and device fabrication is still needed for the efficiency improvement. This article reviews the recent development in GaAs NW-based PVs with an emphasis on cost-effectively synthesis of GaAs NWs, device design and corresponding performance measurement. We first discuss the available manipulated growth methods of GaAs NWs, such as the catalytic vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) epitaxial growth, followed by the catalyst-controlled engineering process, and typical crystal structure and orientation of resulted NWs. The structure-property relationships are also discussed for achieving the optimal PV performance. At the same time, important device issues are as well summarized, including the light absorption, tunnel junctions and contact configuration. Towards the end, we survey the reported performance data and make some remarks on the challenges for current nanostructured PVs. These results not only lay the ground to considerably achieve the higher efficiencies in GaAs NW-based PVs but also open up great opportunities for the future low-cost smart solar energy harvesting devices. Full article
(This article belongs to the Special Issue III-V Heteroepitaxy for Solar Energy Conversion)
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Open AccessArticle Formation of Dislocations in the Growth of Silicon along Different Crystallographic Directions—A Molecular Dynamics Study
Crystals 2018, 8(9), 346; https://doi.org/10.3390/cryst8090346
Received: 15 July 2018 / Revised: 19 August 2018 / Accepted: 21 August 2018 / Published: 29 August 2018
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Abstract
Molecular dynamics simulations of the seeded solidification of silicon along <100>, <110>, <111> and <112> directions have been carried out. The Tersoff potential is adopted for computing atomic interaction. The control of uniaxial strains in the seed crystals is enabled in the simulations.
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Molecular dynamics simulations of the seeded solidification of silicon along <100>, <110>, <111> and <112> directions have been carried out. The Tersoff potential is adopted for computing atomic interaction. The control of uniaxial strains in the seed crystals is enabled in the simulations. The results show that the dislocation forms stochastically at the crystal/melt interface, with the highest probability of the formation in <111> growth, which agrees with the prediction from a previously proposed twinning-associated dislocation formation mechanism. Applications of the strains within a certain range are found to inhibit the {111}-twinning-associated dislocation formation, while beyond this range they are found to induce dislocation formation by different mechanisms. Full article
(This article belongs to the Special Issue Growth and Evaluation of Multicrystalline Silicon)
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Open AccessArticle Microstructure and Phase Transformation Analysis of Ni50−xTi50Lax Shape Memory Alloys
Crystals 2018, 8(9), 345; https://doi.org/10.3390/cryst8090345
Received: 20 July 2018 / Revised: 16 August 2018 / Accepted: 24 August 2018 / Published: 29 August 2018
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Abstract
The microstructure and martensitic transformation behavior of Ni50−xTi50Lax (x = 0.1, 0.3, 0.5, 0.7) shape memory alloys were investigated experimentally. Results show that the microstructure of Ni50−xTi50Lax alloys consists of
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The microstructure and martensitic transformation behavior of Ni50−xTi50Lax (x = 0.1, 0.3, 0.5, 0.7) shape memory alloys were investigated experimentally. Results show that the microstructure of Ni50−xTi50Lax alloys consists of a near-equiatomic TiNi matrix, LaNi precipitates, and Ti2Ni precipitates. With increasing La content, the amounts of LaNi and Ti2Ni precipitates demonstrate an increasing tendency. The martensitic transformation start temperature increases gradually with increasing La content. The Ni content is mainly responsible for the change in martensite transformation behavior in Ni50−xTi50Lax alloys. Full article
(This article belongs to the Special Issue Microstructures and Properties of Martensitic Materials)
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Open AccessArticle Temperature Dependence of the Vacancy Formation Energy in Solid 4He
Crystals 2018, 8(9), 344; https://doi.org/10.3390/cryst8090344
Received: 14 August 2018 / Accepted: 23 August 2018 / Published: 28 August 2018
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Abstract
We studied the thermal effects on the behavior of incommensurate solid 4He at low temperatures using the path integral Monte Carlo method. Below a certain temperature, depending on the density and the structure of the crystal, the vacancies delocalize and a finite
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We studied the thermal effects on the behavior of incommensurate solid 4He at low temperatures using the path integral Monte Carlo method. Below a certain temperature, depending on the density and the structure of the crystal, the vacancies delocalize and a finite condensate fraction appears. We calculated the vacancy formation energy as a function of the temperature and observed a behavior compatible with a two-step structure, with a gap of few K appearing at the onset temperature of off-diagonal long-range order. Estimation of the energy cost of creating two vacancies seems to indicate an effective attractive interaction among the vacancies but the large error inherent to its numerical estimation precludes a definitive statement. Full article
(This article belongs to the Special Issue Quantum Crystals)
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Open AccessArticle Investigation of Approaches to Control the Compositions of Zn(Se,OH) Buffers Prepared by Chemical Bath Deposition Process for Cu(In,Ga)Se2 (CIGS) Solar Cells
Crystals 2018, 8(9), 343; https://doi.org/10.3390/cryst8090343
Received: 31 July 2018 / Revised: 20 August 2018 / Accepted: 23 August 2018 / Published: 26 August 2018
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Abstract
We deposited zinc-based films with various ammonia (ammonium hydroxide; NH4OH) and selenourea concentrations, at the bath temperature of 80 °C, on soda-lime glass substrates using the chemical bath deposition (CBD) process. We analyzed the results using X-ray photoelectron spectroscopy (XPS), which
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We deposited zinc-based films with various ammonia (ammonium hydroxide; NH4OH) and selenourea concentrations, at the bath temperature of 80 °C, on soda-lime glass substrates using the chemical bath deposition (CBD) process. We analyzed the results using X-ray photoelectron spectroscopy (XPS), which showed binding energies of zinc, selenium, and oxygen. The as-deposited films, containing zinc selenide, zinc oxide, and zinc hydroxide, were also verified. The films prepared in this investigation can be referred to a zinc compound, characterized as Zn(Se,OH). A conformal coverage of the Zn(Se,OH) films, with the smooth surface morphologies, was obtained by optimizing the ammonia or selenourea concentrations in the deposition solutions. The Zn(Se,OH) films had a preferred (111) orientation, corresponding to a cubic crystal structure. The bandgap energies of the as-deposited Zn(Se,OH) films were determined from the optical absorption data, suggesting a dependence of the bandgap energies on the atomic percentages of ZnSe, Zn(OH)2 and ZnO in the films. The same variation tendency of the compositions and the bandgap energies for the films, deposited with an increment in the ammonia or selenourea concentrations was achieved, attributing to the facilitation of ZnSe formation. These results show that the compositions, and therefore the bandgap energies, can be controlled by the ammonia concentrations, or selenourea concentrations. Full article
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Open AccessArticle Simulation and Analysis of Single-Mode Microring Resonators in Lithium Niobate Thin Films
Crystals 2018, 8(9), 342; https://doi.org/10.3390/cryst8090342
Received: 4 August 2018 / Revised: 21 August 2018 / Accepted: 22 August 2018 / Published: 24 August 2018
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Abstract
The single-mode microring resonators on lithium niobate thin films were designed and simulated using 2.5-D variational finite difference time domain mode simulations from Lumerical mode Solutions. The single-mode conditions and the propagation losses of lithium niobate planar waveguide with different SiO2 cladding
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The single-mode microring resonators on lithium niobate thin films were designed and simulated using 2.5-D variational finite difference time domain mode simulations from Lumerical mode Solutions. The single-mode conditions and the propagation losses of lithium niobate planar waveguide with different SiO2 cladding layer thicknesses were studied and compared systematically. The optimization of design parameters such as radii of microrings and gap sizes between channel and ring waveguides were determined. The key issues affecting the resonator design such as free spectral range and Quality Factor were discussed. The microring resonators had radius R = 20 μm, and their transmission spectrum had been tuned using the electro-optical effect. Full article
(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials)
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Open AccessArticle Investigation of the Grain Boundary Character and Dislocation Density of Different Types of High Performance Multicrystalline Silicon
Crystals 2018, 8(9), 341; https://doi.org/10.3390/cryst8090341
Received: 18 July 2018 / Revised: 20 August 2018 / Accepted: 21 August 2018 / Published: 24 August 2018
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Abstract
Wafers from three heights and two different lateral positions (corner and centre) of four industrial multicrystalline silicon ingots were analysed with respect to their grain structure and dislocation density. Three of the ingots were non-seeded and one ingot was seeded. It was found
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Wafers from three heights and two different lateral positions (corner and centre) of four industrial multicrystalline silicon ingots were analysed with respect to their grain structure and dislocation density. Three of the ingots were non-seeded and one ingot was seeded. It was found that there is a strong correlation between the ratio of the densities of (coincidence site lattice) CSL grain boundaries and high angle grain boundaries in the bottom of a block and the dislocation cluster density higher in the block. In general, the seeded blocks, both the corner and centre block, have a lower dislocation cluster density than in the non-seeded blocks, which displayed a large variation. The density of the random angle boundaries in the corner blocks of the non-seeded ingots was similar to the density in the seeded ingots, while the density in the centre blocks was lower. However, the density of CSL boundaries was higher in all the non-seeded than in the seeded ingots. It appears that both of these grain boundary densities influence the presence of dislocation clusters, and we propose they act as dislocation sinks and sources, respectively. The ability to generate small grain size material without seeding appears to be correlated to the morphology of the coating, which is generally rougher in the corner positions than in the middle. Furthermore, the density of twins and CSL boundaries depends on the growth mode during initial growth and thus on the degree of supercooling. Controlling both these properties is important in order to be able to successfully produce uniform quality high-performance multicrystalline silicon by the advantageous non-seeding method. Full article
(This article belongs to the Special Issue Growth and Evaluation of Multicrystalline Silicon)
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Open AccessArticle Novel Devices for Transporting Protein Crystals to the Synchrotron Facilities and Thermal Protection of Protein Crystals
Crystals 2018, 8(9), 340; https://doi.org/10.3390/cryst8090340
Received: 10 August 2018 / Revised: 18 August 2018 / Accepted: 20 August 2018 / Published: 23 August 2018
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Abstract
In this article, we use novel and non-conventional devices, based on polyolefins that help to increase the thermal protection of protein crystals in their crystallization conditions for crystallographic applications. The present contribution deals with the application of some ad hoc devices designed for
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In this article, we use novel and non-conventional devices, based on polyolefins that help to increase the thermal protection of protein crystals in their crystallization conditions for crystallographic applications. The present contribution deals with the application of some ad hoc devices designed for transporting protein crystals to the synchrotron facilities. These new devices help transporting proteins without cryo-cooling them, therefore replacing the conventional dry Dewars. We crystallized four model proteins, using the classic sitting-drop vapor diffusion crystallization setups. The model proteins lysozyme, glucose isomerase, xylanase, and ferritin were used to obtain suitable crystals for high-resolution X-ray crystallographic research. Additionally, we evaluated the crystallization of apo-transferrin, which is involved in neurodegenerative diseases. As apo-transferrin is extremely sensitive to the changes in the crystallization temperature, we used it as a thermal sensor to prove the efficiency of these thermal protection devices when transporting proteins to the synchrotron facilities. Full article
(This article belongs to the Section Biomolecular Crystals)
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Open AccessArticle Effects on the Surface and Luminescence Properties of GaAs by SF6 Plasma Passivation
Crystals 2018, 8(9), 339; https://doi.org/10.3390/cryst8090339
Received: 17 July 2018 / Revised: 18 August 2018 / Accepted: 19 August 2018 / Published: 23 August 2018
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Abstract
The passivation effects of the SF6 plasma on a GaAs surface has been investigated by using the radio frequency (RF) plasma method. The RF’s power, chamber pressure, and plasma treatment time are optimized by photoluminescence (PL), atomic force microscopy (AFM), and X-ray
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The passivation effects of the SF6 plasma on a GaAs surface has been investigated by using the radio frequency (RF) plasma method. The RF’s power, chamber pressure, and plasma treatment time are optimized by photoluminescence (PL), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The PL intensity of passivated GaAs samples is about 1.8 times higher than those which are untreated. The oxide traps and As-As dimers can be removed effectively by using SF6 plasma treatment, and Ga-F can form on the surface of GaAs. It has also been found that the stability of the passivated GaAs surface can be enhanced by depositing SiO2 films onto the GaAs surface. These indicate that the passivation of GaAs surfaces can be achieved by using SF6 plasma treatment. Full article
(This article belongs to the Section Interactions in Crystal Structures)
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Open AccessReview Luminescent Metal–Organic Framework Thin Films: From Preparation to Biomedical Sensing Applications
Crystals 2018, 8(9), 338; https://doi.org/10.3390/cryst8090338
Received: 4 August 2018 / Revised: 15 August 2018 / Accepted: 17 August 2018 / Published: 23 August 2018
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Abstract
Metal-organic framework (MOF) thin films are receiving increasing attention in a number of different application fields, such as optoelectronics, gas separation, catalysis electronic devices, and biomedicine. In particular, their tunable composition and structure, accessible metal sites and potential for post-synthetic modification for molecular
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Metal-organic framework (MOF) thin films are receiving increasing attention in a number of different application fields, such as optoelectronics, gas separation, catalysis electronic devices, and biomedicine. In particular, their tunable composition and structure, accessible metal sites and potential for post-synthetic modification for molecular recognition make MOF thin films promising candidates for biosensing applications. Compared with solution-based powdery probes, film-based probes have distinct advantages of good stability and portability, tunable shape and size, real-time detection, non-invasion, extensive suitability in gas/vapor sensing, and recycling. In this review, we summarize the recent advances in luminescent MOF thin films, including the fabrication methods and origins of luminescence. Specifically, luminescent MOF thin films as biosensors for temperature, ions, gases and biomolecules are highlighted. Full article
(This article belongs to the Special Issue MOF-Based Membranes)
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Open AccessArticle Aging Behavior of Aluminum Alloy 6082 Subjected to Friction Stir Processing
Crystals 2018, 8(9), 337; https://doi.org/10.3390/cryst8090337
Received: 27 July 2018 / Revised: 14 August 2018 / Accepted: 16 August 2018 / Published: 22 August 2018
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Abstract
The present work examined the effect of artificial aging on the microstructure, texture, and hardness homogeneity in aluminum alloy AA6082 subjected to friction stir processing (FSP). Aging was applied to FSP samples at three different temperatures (150 °C, 175 °C, and 200 °C)
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The present work examined the effect of artificial aging on the microstructure, texture, and hardness homogeneity in aluminum alloy AA6082 subjected to friction stir processing (FSP). Aging was applied to FSP samples at three different temperatures (150 °C, 175 °C, and 200 °C) for a period of 1 h, 6 h, and 12 h. Microstructure analysis using optical Microscopy (OM) and Electron Back-Scattered Diffraction (EBSD) indicated that FSP produced fine equiaxed grains, with an average grain size of 6.5 μm, in the stir zone (SZ) due to dynamic recrystallization. Aging was shown to result in additional grain refinement in the SZ due to the occurrence of recovery and recrystallization with either increasing aging temperature and/or aging time. An optimum average grain size of 3–4 μm was obtained in the SZ by applying aging at 175 °C. This was accompanied by an increase in the fraction of high-angle grain boundaries. FSP provided a simple shear texture with a major component of B fiber. Increasing aging temperature and/or time resulted in the formation of recrystallization texture of a Cube orientation. In addition, Vickers microhardness was evaluated for the FSP sample, indicating a softening in the SZ due to the dissolution of the hardening precipitates. Compared to other aging temperatures, aging at 175 °C resulted in maximum hardness recovery (90 Hv) to the initial value of base metal (92.5 Hv). The hardness recovery is most likely attributed to the uniform distribution of fine hardening precipitates in the SZ when increasing the aging time to 12 h. Full article
(This article belongs to the Special Issue Recrystallization of Metallic Materials)
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