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Crystals, Volume 9, Issue 6 (June 2019)

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Cover Story (view full-size image) The recent advances in liquid crystal (LC) beam steering devices are reviewed. We first describe [...] Read more.
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Open AccessArticle
Supramolecular Assemblies in Pb(II) Complexes with Hydrazido-Based Ligands
Crystals 2019, 9(6), 323; https://doi.org/10.3390/cryst9060323
Received: 29 May 2019 / Revised: 13 June 2019 / Accepted: 24 June 2019 / Published: 25 June 2019
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Abstract
Herein, we describe the synthesis and single crystal X-ray diffraction characterization of several Pb(II) complexes using Schiff base hydrazido-based ligands and different counterions (NO3, I and ClO4). In the three complexes reported in this work, the lead(II) [...] Read more.
Herein, we describe the synthesis and single crystal X-ray diffraction characterization of several Pb(II) complexes using Schiff base hydrazido-based ligands and different counterions (NO3, I and ClO4). In the three complexes reported in this work, the lead(II) metal exhibits a high coordination number (n > 8) and thus it is apparently not involved in tetrel bonding interactions. Moreover, the aromatic ligands participate in noncovalent interactions that play an important role in the formation of several supramolecular assemblies in the solid state of the three Pb(II) complexes. These assemblies have been analyzed by means of Hirshfeld surface analysis and DFT calculations. Full article
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Open AccessArticle
Field-Independent Features in the Magnetization and Specific Heat of Sm3Co4Ge13
Crystals 2019, 9(6), 322; https://doi.org/10.3390/cryst9060322
Received: 2 May 2019 / Revised: 7 June 2019 / Accepted: 10 June 2019 / Published: 25 June 2019
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Abstract
The cubic intermetallic compound Sm3Co4Ge13 (space group Pm3¯n) possesses a cage-like structure composed of Ge and displays an antiferromagnetic transition at TN 6 K in magnetization, M(T), [...] Read more.
The cubic intermetallic compound Sm 3Co 4Ge 13 (space group P m 3 ¯ n ) possesses a cage-like structure composed of Ge and displays an antiferromagnetic transition at T N 6 K in magnetization, M ( T ) , specific heat, C p ( T ) and in thermal conductivity, κ ( T ). The magnetic transition at T N is observed to be robust against applied magnetic fields up to 9 T. From the analysis of specific heat, a Sommerfeld coefficient γ = 80(2) mJ/mol-Sm K 2 is estimated. The magnetic entropy released at T N is estimated as lower than that of a doublet, R ln(2). A positive Seebeck coefficient is observed for the thermopower, S ( T ) . Photoemission spectroscopy reveals distinct electronic character of the near-E F valence band states arising out of Co( 3 d)-Sm( 4 f) hybridization and Sm( 4 f) electron correlation. The unusual field-independent features in magnetization, specific heat and electrical transport is an indication of the significant correlation between f and d wave functions. Full article
(This article belongs to the Special Issue Magnetic Field-induced Phase Transition)
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Open AccessArticle
Ice Crystal Coarsening in Ice Cream during Cooling: A Comparison of Theory and Experiment
Crystals 2019, 9(6), 321; https://doi.org/10.3390/cryst9060321
Received: 19 April 2019 / Revised: 20 June 2019 / Accepted: 21 June 2019 / Published: 25 June 2019
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Abstract
Ice cream is a complex multi-phase structure and its perceived quality is closely related to the small size of ice crystals in the product. Understanding the quantitative coarsening behaviour of ice crystals will help manufacturers optimise ice cream formulations and processing. Using synchrotron [...] Read more.
Ice cream is a complex multi-phase structure and its perceived quality is closely related to the small size of ice crystals in the product. Understanding the quantitative coarsening behaviour of ice crystals will help manufacturers optimise ice cream formulations and processing. Using synchrotron X-ray tomography, we measured the time-dependent coarsening (Ostwald ripening) of ice crystals in ice cream during cooling at 0.05 °C/min. The results show ice crystal coarsening is highly temperature dependent, being rapid from ca. −6 to −12 °C but significantly slower at lower temperatures. We developed a numerical model, based on established coarsening theory, to calculate the relationship between crystal diameter, cooling rate and the weight fraction of sucrose in solution. The ice crystal diameters predicted by the model are found to agree well with the measured values if matrix diffusion is assumed to be slowed by a factor of 1.2 due to the presence of stabilizers or high molecular weight sugars in the ice cream formulation. Full article
(This article belongs to the Special Issue Ice Crystals)
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Open AccessArticle
Simulation-Based Development of a New Cylindrical-Cavity Microwave-Plasma Reactor for Diamond-Film Synthesis
Crystals 2019, 9(6), 320; https://doi.org/10.3390/cryst9060320
Received: 17 May 2019 / Revised: 16 June 2019 / Accepted: 20 June 2019 / Published: 24 June 2019
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Abstract
A 2.45 GHz microwave-plasma chemical-vapor deposition (MPCVD) reactor was designed and built in-house by collaborating with Guangdong TrueOne Semiconductor Technology Co., Ltd. A cylindrical cavity was designed as the deposition chamber and a circumferential coaxial-mode transformer located at the top of the cavity [...] Read more.
A 2.45 GHz microwave-plasma chemical-vapor deposition (MPCVD) reactor was designed and built in-house by collaborating with Guangdong TrueOne Semiconductor Technology Co., Ltd. A cylindrical cavity was designed as the deposition chamber and a circumferential coaxial-mode transformer located at the top of the cavity was adopted as the antenna. Two quartz-ring windows that were placed far away from the plasma and cooled by water-cooling cavity walls were used to affix the antenna to the cavity and act as a vacuum seal for the reactor, respectively. This design improved the sealing and protected the quartz windows. In addition, a numerical simulation was proposed to predict the electric-field and plasma-density distributions in the cavity. Based on the simulation results, a microwave-plasma reactor with TM021 mode was built. The leak rate of this new reactor was tested to be as low as 1 × 10−8 Pa·m3·s−1, and the maximal microwave power was as high as 10 kW. Then, single-crystal diamond films were grown with the morphology and crystalline quality characterized by an optical microscope, atomic force microscope (AFM), Raman spectrometer, photoluminescence (PL) spectrometer, and high-resolution X-ray diffractometer. It was shown that the newly developed MPCVD reactor can produce diamond films with high quality and purity. Full article
(This article belongs to the Special Issue Synthesis and Characterization of Diamond Crystals)
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Open AccessArticle
Extended Assemblies of Ru(bpy)(CO)2X2 (X = Cl, Br, I) Molecules Linked by 1,4-Diiodotetrafluoro-Benzene (DITFB) Halogen Bond Donors
Crystals 2019, 9(6), 319; https://doi.org/10.3390/cryst9060319
Received: 20 May 2019 / Revised: 12 June 2019 / Accepted: 19 June 2019 / Published: 24 June 2019
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Abstract
The ruthenium carbonyl compounds, Ru(bpy)(CO)2X2 (X = Cl, Br or I) act as neutral halogen bond (XB) acceptors when co-crystallized with 1,4-diiodotetrafluoro-benzene (DITFB). The halogen bonding strength of the Ru-X⋅⋅⋅I halogen bonds follow the nucleophilic character of the halido ligand. [...] Read more.
The ruthenium carbonyl compounds, Ru(bpy)(CO)2X2 (X = Cl, Br or I) act as neutral halogen bond (XB) acceptors when co-crystallized with 1,4-diiodotetrafluoro-benzene (DITFB). The halogen bonding strength of the Ru-X⋅⋅⋅I halogen bonds follow the nucleophilic character of the halido ligand. The strongest halogen bond occurs between the chlorido ligand and the iodide atoms of the DITFB. All three halogen bonded complexes form polymeric assemblies in the solid state. In Ru(bpy)(CO)2Cl2⋅DITFB (1) and in Ru(bpy)(CO)2Br2⋅DITFB (2) both halido ligands are halogen bonded to only one DITFB donor. In Ru(bpy)(CO)2I2⋅DITFB (3) only one of the halido ligands is involved in halogen bonding acting as ditopic center for two DITFB donors. The polymeric structures of 1 and 2 are isomorphic wave-like single chain systems, while the iodine complexes form pairs of linear chains attached together with weak F⋅⋅⋅O≡C interactions between the closest neighbors. The stronger polarization of the iodide ligand compared to the Cl or Br ligands favors nearly linear C-I⋅⋅⋅I angles between the XB donor and the metal complex supporting the linear arrangement of the halogen bonded chain. Full article
(This article belongs to the Special Issue Halogen-Bonded Cocrystals)
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Open AccessArticle
Effects of Sm2O3 and V2O5 Film Stacking on Switching Behaviors of Resistive Random Access Memories
Crystals 2019, 9(6), 318; https://doi.org/10.3390/cryst9060318
Received: 18 April 2019 / Revised: 3 June 2019 / Accepted: 10 June 2019 / Published: 19 June 2019
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Abstract
In this work, the resistive switching characteristics of resistive random access memories (RRAMs) containing Sm2O3 and V2O5 films were investigated. All the RRAM structures made in this work showed stable resistive switching behavior. The High-Resistance State and [...] Read more.
In this work, the resistive switching characteristics of resistive random access memories (RRAMs) containing Sm2O3 and V2O5 films were investigated. All the RRAM structures made in this work showed stable resistive switching behavior. The High-Resistance State and Low-Resistance State of Resistive memory (RHRS/RLRS) ratio of the RRAM device containing a V2O5/Sm2O3 bilayer is one order of magnitude higher than that of the devices containing a single layer of V2O5 or Sm2O3. We also found that the stacking sequence of the Sm2O3 and V2O5 films in the bilayer structure can affect the switching features of the RRAM, causing them to exhibit both bipolar resistive switching (BRS) behavior and self-compliance behavior. The current conduction mechanisms of RRAM devices with different film structures were also discussed. Full article
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Open AccessArticle
High-Density Well-Aligned Dislocations Introduced by Plastic Deformation in Bi1−xSbx Topological Insulator Single Crystals
Crystals 2019, 9(6), 317; https://doi.org/10.3390/cryst9060317
Received: 24 May 2019 / Revised: 13 June 2019 / Accepted: 16 June 2019 / Published: 19 June 2019
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Abstract
Topological insulators (TIs) have a bulk bandgap and gapless edge or surface states that host helically spin-polarized Dirac fermions. Theoretically, it has been predicted that gapless states could also be formed along dislocations in TIs. Recently, conductivity measurements on plastically deformed bismuth antimony [...] Read more.
Topological insulators (TIs) have a bulk bandgap and gapless edge or surface states that host helically spin-polarized Dirac fermions. Theoretically, it has been predicted that gapless states could also be formed along dislocations in TIs. Recently, conductivity measurements on plastically deformed bismuth antimony (Bi1−xSbx) TIs have revealed excess conductivity owing to dislocation conduction. For further application of them, fundamental study on dislocations in TIs is indispensable. Dislocations controlled based on fundamental studies could potentially be useful not only for experimental investigations of the dislocation properties but also for diverse device applications. In the present study, Bi1−xSbx TI single crystals were fabricated by a zone-melting method. The crystals were plastically deformed at room temperature. The resultant dislocations were observed by transmission electron microscopy (TEM). It was found that high-density dislocations with the Burgers vector satisfying the condition for the formation of gapless states were successfully introduced. The dislocations were mostly of edge type with lengths on the order of more than a few micrometers. Full article
(This article belongs to the Section Crystal Engineering)
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Open AccessArticle
Structural Insights into the Two-Step Spin-Crossover Compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2
Crystals 2019, 9(6), 316; https://doi.org/10.3390/cryst9060316
Received: 24 May 2019 / Revised: 6 June 2019 / Accepted: 16 June 2019 / Published: 19 June 2019
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Abstract
The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred [...] Read more.
The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred at ca. 170 K and 145 K and a plateau at around 155 K. The origin of the two-step transition is discussed in light of these structural studies. The observations are compatible with a mostly disordered state between the two steps, consisting of mixing of high-spin and low-spin species, while weak substructure reflections in the mixed phase could indicate some degree of long-range order of the high-spin and low-spin sites. Full article
(This article belongs to the Special Issue Synthesis and Applications of New Spin Crossover Compounds)
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Open AccessArticle
High-Responsivity Photovoltaic Photodetectors Based on MoTe2/MoSe2 van der Waals Heterojunctions
Crystals 2019, 9(6), 315; https://doi.org/10.3390/cryst9060315
Received: 4 June 2019 / Revised: 16 June 2019 / Accepted: 17 June 2019 / Published: 19 June 2019
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Abstract
Van der Waals heterojunctions based on transition metal dichalcogenides (TMDs) show promising potential in optoelectronic devices, due to the ultrafast separation of photoexcited carriers and efficient generation of the photocurrent. Herein, this study demonstrated a high-responsivity photovoltaic photodetector based on a MoTe2 [...] Read more.
Van der Waals heterojunctions based on transition metal dichalcogenides (TMDs) show promising potential in optoelectronic devices, due to the ultrafast separation of photoexcited carriers and efficient generation of the photocurrent. Herein, this study demonstrated a high-responsivity photovoltaic photodetector based on a MoTe2/MoSe2 type-II heterojunction. Due to the interlayer built-in potential, the MoTe2/MoSe2 heterojunction shows obvious photovoltaic behavior and its photoresponse can be tuned by the gate voltage due to the ultrathin thickness of the heterojunction. This self-powered photovoltaic photodetector exhibits an excellent responsivity of 1.5 A W−1, larger than previously reported TMDs-based photovoltaic photodetectors. Due to the high-efficiency separation of electron-hole pairs and ultrafast charge transfer, the light-induced on/off ratio of current switching is larger than 104 at zero bias, and the dark current is extremely low (~10−13 A). These MoTe2/MoSe2 type-II heterojunctions are expected to provide more opportunities for future nanoscale optoelectronic devices. Full article
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Open AccessArticle
Electro-Optical Switching of Dual-Frequency Nematic Liquid Crystals: Regimes of Thin and Thick Cells
Crystals 2019, 9(6), 314; https://doi.org/10.3390/cryst9060314
Received: 29 May 2019 / Revised: 8 June 2019 / Accepted: 14 June 2019 / Published: 18 June 2019
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Abstract
Conventional display applications of liquid crystals utilize thin layers of mesogenic materials, typically less than 10 µm. However, emerging non-display applications will require thicker, i.e., greater than 100 µm, layers of liquid crystals. Although electro-optical performance of relatively thin liquid crystal cells is [...] Read more.
Conventional display applications of liquid crystals utilize thin layers of mesogenic materials, typically less than 10 µm. However, emerging non-display applications will require thicker, i.e., greater than 100 µm, layers of liquid crystals. Although electro-optical performance of relatively thin liquid crystal cells is well-documented, little is known about the properties of thicker liquid crystal layers. In this paper, the electro-optical response of dual-frequency nematic liquid crystals is studied using a broad range (2–200 µm) of the cell thickness. Two regimes of electro-optical switching of dual-frequency nematics are observed and analyzed. Full article
(This article belongs to the Special Issue Advances in Nematic Liquid Crystals)
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Open AccessArticle
Topological Phase Transition in a One-Dimensional Elastic String System
Crystals 2019, 9(6), 313; https://doi.org/10.3390/cryst9060313
Received: 20 May 2019 / Revised: 13 June 2019 / Accepted: 15 June 2019 / Published: 18 June 2019
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Abstract
We show that topological interface mode can emerge in a one-dimensional elastic string system which consists of two periodic strings with different band topologies. To verify their topological features, Zak-phase of each band is calculated and reveals the condition of topological phase transition [...] Read more.
We show that topological interface mode can emerge in a one-dimensional elastic string system which consists of two periodic strings with different band topologies. To verify their topological features, Zak-phase of each band is calculated and reveals the condition of topological phase transition accordingly. Apart from that, the transmittance spectrum illustrates that topological interface mode arises when two topologically distinct structures are connected. The vibration profile further exhibits the non-trivial interface mode in the domain wall between two periodic string composites. Full article
(This article belongs to the Special Issue Recent Advances in Novel Topological Materials)
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Open AccessArticle
Effect of Local Recrystallized Grains on the Low Cycle Fatigue Behavior of a Nickel-Based Single Crystal Superalloy
Crystals 2019, 9(6), 312; https://doi.org/10.3390/cryst9060312
Received: 29 April 2019 / Revised: 11 June 2019 / Accepted: 11 June 2019 / Published: 18 June 2019
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Abstract
This paper aims to understand the effect of local recrystallization (RX) on the low cycle fatigue fracture of a turbine-blade single crystal nickel-based superalloy. The fatigue life of the single crystal superalloy was evidently decreased by local recrystallization. In single crystal specimens, casting [...] Read more.
This paper aims to understand the effect of local recrystallization (RX) on the low cycle fatigue fracture of a turbine-blade single crystal nickel-based superalloy. The fatigue life of the single crystal superalloy was evidently decreased by local recrystallization. In single crystal specimens, casting porosity is the preferential fatigue crack initiation site, which is followed by crystallographic crack propagation along one or several octahedral slip planes. For all RX specimens, fatigue cracks preferred to initiate from local recrystallized grains and propagated through the recrystallized grains in a transgranular manner, followed by crystallographic crack propagation in the substrate single crystal superalloy. Moreover, fatigue tests indicated that locally recrystallized specimens exhibited temperature dependent fracture modes, i.e., transgranular cracking dominated at 550 °C, whereas intergranular cracking was preferred at 850 °C. Evident oxidation of fracture surfaces and strength degradation of grain boundaries at 850 °C was evidenced by scanning electronic microscopic observations. The present study emphasized the need to evaluate the effect of recrystallization according to the working conditions of turbine components, i.e., the local temperature. Full article
(This article belongs to the Special Issue Fundamentals of Superalloys)
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Open AccessArticle
Liquid Crystal Based Head-Up Display with Electrically Controlled Contrast Ratio
Crystals 2019, 9(6), 311; https://doi.org/10.3390/cryst9060311
Received: 9 May 2019 / Revised: 7 June 2019 / Accepted: 14 June 2019 / Published: 18 June 2019
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Abstract
With the growing demand for driving safety and convenience, Head-Up Displays (HUDs) have gained more and more interest in recent years. In this paper, we propose a HUD system with the ability to adjust the relative brightness of ambient light and virtual information [...] Read more.
With the growing demand for driving safety and convenience, Head-Up Displays (HUDs) have gained more and more interest in recent years. In this paper, we propose a HUD system with the ability to adjust the relative brightness of ambient light and virtual information light. The key components of the system include a cholesteric liquid crystal (CLC) film, a geometric phase (GP) liquid crystal lens, and a circular polarizer. By controlling the voltage applied to the GP lens, the contrast ratio of the virtual information light to ambient light could be continuously tuned, so that good visibility could always be obtained under different driving conditions. Full article
(This article belongs to the Special Issue Advances in Cholesteric Liquid Crystals)
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Open AccessArticle
Effect of Nitrogen Flow in Hydrogen/Nitrogen Plasma Annealing on Aluminum-Doped Zinc Oxide/Tin-Doped Indium Oxide Bilayer Films Applied in Low Emissivity Glass
Crystals 2019, 9(6), 310; https://doi.org/10.3390/cryst9060310
Received: 28 May 2019 / Revised: 11 June 2019 / Accepted: 13 June 2019 / Published: 17 June 2019
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Abstract
Low emissivity glass (low-e glass), which is often used in energy-saving buildings, has high thermal resistance and visible light transmission. Heavily doped wide band gap semiconductors like aluminum-doped zinc oxide (AZO) and tin-doped indium oxide (ITO) have these properties, especially after certain treatment. [...] Read more.
Low emissivity glass (low-e glass), which is often used in energy-saving buildings, has high thermal resistance and visible light transmission. Heavily doped wide band gap semiconductors like aluminum-doped zinc oxide (AZO) and tin-doped indium oxide (ITO) have these properties, especially after certain treatment. In our experiments, in-line sputtered AZO and ITO bilayer (AZO/ITO) films on glass substrates were prepared first. The deposition of AZO/ITO films was following by annealing in hydrogen/nitrogen (H2/N2) plasma with different N2 flows. The structure and optical and electrical properties of AZO/ITO films were surveyed. Experiment results indicated that N2 flow in H2/N2 plasma annealing of AZO/ITO films slightly modified the structure and electrical properties of AZO/ITO films. The X-ray diffraction peak corresponding to zinc oxide (002) crystal plane slightly shifted to a higher angle and its full width at half maximum decreased as the N2 flow increased. The electrical resistivity and the emissivity reduced for the plasma annealed AZO/ITO films when the N2 flow was raised. The optimum H2/N2 gas flow was 100/100 for plasma annealed AZO/ITO films in this work for low emissivity application. The emissivity and average visible transmittance for H2/N2 = 100/100 plasma annealed AZO/ITO were 0.07 and 80%, respectively, lying in the range of commercially used low emissivity glass. Full article
(This article belongs to the Special Issue Functional Oxide Based Thin-Film Materials)
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Open AccessCommunication
Design of Ionic Liquid Crystals Forming Normal-Type Bicontinuous Cubic Phases with a 3D Continuous Ion Conductive Pathway
Crystals 2019, 9(6), 309; https://doi.org/10.3390/cryst9060309
Received: 17 May 2019 / Revised: 12 June 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
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Abstract
We have prepared a series of pyridinium-based gemini amphiphiles. They exhibit thermotropic liquid–crystalline behavior depending on their alkyl chain lengths and anion species. By adjusting the alkyl chain lengths and selecting suitable anions, we have obtained an ionic amphiphile that exhibits a normal-type [...] Read more.
We have prepared a series of pyridinium-based gemini amphiphiles. They exhibit thermotropic liquid–crystalline behavior depending on their alkyl chain lengths and anion species. By adjusting the alkyl chain lengths and selecting suitable anions, we have obtained an ionic amphiphile that exhibits a normal-type bicontinuous cubic phase from 38 °C to 12 °C on cooling from an isotropic phase. In the bicontinuous cubic liquid–crystalline assembly, the pyridinium-based ionic parts align along a gyroid minimal surface forming a 3D continuous ionic domain while their ionophobic alkyl chains form 3D branched nanochannel networks. This ionic compound can form homogeneous mixtures with a lithium salt and the resultant mixtures keep the ability to form normal-type bicontinuous cubic phases. Ion conduction measurements have been performed for the mixtures on cooling. It has been revealed that the formation of the 3D branched ionophobic nanochannels does not disturb the ion conduction behavior in the ionic domain while it results in the conversion of the state of the mixtures from fluidic liquids to quasi-solids, namely highly viscous liquid crystals. Although the ionic conductivity of the mixtures is in the order of 10–7 S cm–1 at 40 °C, which is far lower than the values for practical use, the present material design has a potential to pave the way for developing advanced solid electrolytes consisting of two task-specific nanosegregated domains: One is an ionic liquid nano-domain with a 3D continuity for high ionic conductivity and the other is ionophobic nanochannel network domains for high mechanical strength. Full article
(This article belongs to the Special Issue Liquid-Crystalline Ion Conductors)
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Open AccessArticle
Indium-Incorporation with InxGa1-xN Layers on GaN-Microdisks by Plasma-Assisted Molecular Beam Epitaxy
Crystals 2019, 9(6), 308; https://doi.org/10.3390/cryst9060308
Received: 9 May 2019 / Revised: 6 June 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
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Abstract
Indium-incorporation with InxGa1-xN layers on GaN-microdisks has been systematically studied against growth parameters by plasma-assisted molecular beam epitaxy. The indium content (x) of InxGa1-xN layer increased to 44.2% with an In/(In + Ga) flux ratio [...] Read more.
Indium-incorporation with InxGa1-xN layers on GaN-microdisks has been systematically studied against growth parameters by plasma-assisted molecular beam epitaxy. The indium content (x) of InxGa1-xN layer increased to 44.2% with an In/(In + Ga) flux ratio of up to 0.6 for a growth temperature of 620 °C, and quickly dropped with a flux ratio of 0.8. At a fixed In/(In + Ga) flux ratio of 0.6, we found that the indium content decreased as the growth temperature increased from 600 °C to 720 °C and dropped to zero at 780 °C. By adjusting the growth parameters, we demonstrated an appropriate InxGa1-xN layer as a buffer to grow high-indium-content InxGa1-xN/GaN microdisk quantum wells for micro-LED applications. Full article
(This article belongs to the Special Issue Thin Film Growth by Molecular Beam Epitaxy)
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Open AccessArticle
Morphology Investigation on Cyclopentane Hydrate Formation/Dissociation in a Sub-Millimeter-Sized Capillary
Crystals 2019, 9(6), 307; https://doi.org/10.3390/cryst9060307
Received: 24 April 2019 / Revised: 4 June 2019 / Accepted: 6 June 2019 / Published: 14 June 2019
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Abstract
The formation, dissociation, and reformation of cyclopentane (CP) hydrate in a sub-millimeter-sized capillary were conducted in this work, and the morphology of CP hydrate was obtained during above processes, respectively. The influences of the supercooling degree, i.e., the hydrate formation driving force, on [...] Read more.
The formation, dissociation, and reformation of cyclopentane (CP) hydrate in a sub-millimeter-sized capillary were conducted in this work, and the morphology of CP hydrate was obtained during above processes, respectively. The influences of the supercooling degree, i.e., the hydrate formation driving force, on CP hydrate crystals’ aspect and growth rate were also investigated. The results demonstrate that CP forms hydrate with the water melting from ice at the interface between the CP and melting water at a temperature slightly above 273.15 K. With the action of hydrate memory effect, the CP hydrate in the capillary starts forming at the CP-water interface or CP–water–capillary three-phase junction and grows around the CP–water interface. The appearance and growth rate of CP hydrate are greatly influenced by the supercooling degree. It indicates that CP hydrate has a high aggregation degree and good regularity at a high supercooling degree (or a low formation temperature). The growth rate of CP hydrate crystals greatly increases with the supercooling degree. Consequently, the temperature has a significant influence on the formation of CP hydrate in the capillary. That means the features of CP hydrate crystals in a quiescent system could be determined and controlled by the temperature setting. Full article
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Open AccessArticle
Research on Optical Properties of Tapered Optical Fibers with Liquid Crystal Cladding Doped with Gold Nanoparticles
Crystals 2019, 9(6), 306; https://doi.org/10.3390/cryst9060306
Received: 15 May 2019 / Revised: 11 June 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
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Abstract
This paper presents results obtained for biconical tapered fibers surrounded/immersed in liquid crystal mixtures. The phenomenon of light propagating in the whole structure of a tapered fiber allows the creation of a sensor where the tapered region represents a core whereas the surrounding [...] Read more.
This paper presents results obtained for biconical tapered fibers surrounded/immersed in liquid crystal mixtures. The phenomenon of light propagating in the whole structure of a tapered fiber allows the creation of a sensor where the tapered region represents a core whereas the surrounding medium becomes a cladding. Created devices are very sensitive to changing refractive index value in a surrounding medium caused by modifying external environmental parameters like temperature, electric or magnetic field. For this reason, the properties of materials used as cladding should be easily modified. In this investigation, cells have been filled with two different nematic liquid crystals given as 1550* and 6CHBT (4-(trans-4-n-hexylcyclohexyl) isothiocyanatobenzoate), as well as with the same mixtures doped with 0.1 wt% gold nanoparticles (AuNPs). Optical spectrum analysis for the wavelength range of 550–1150 nm and time-courses performed for a wavelength of 846 nm at the temperature range of T = 25–40 °C were provided. For all investigations, a steering voltage in the range of 0–200 V which allows establishing the dopes’ influence on transmitted power and time response at different temperatures was applied. Full article
(This article belongs to the Special Issue Liquid Crystal Optical Device)
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Open AccessArticle
Advanced AlGaAs/GaAs Heterostructures Grown by MOVPE
Crystals 2019, 9(6), 305; https://doi.org/10.3390/cryst9060305
Received: 21 April 2019 / Revised: 24 May 2019 / Accepted: 29 May 2019 / Published: 14 June 2019
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Abstract
AlGaAs/GaAs heterostructures are the base of many semiconductor devices. The fabrication of new types of devices demands heterostructures with special features, such as large total thickness (~20 μm), ultrathin layers (~1 nm), high repeatability (up to 1000 periods) and uniformity, for which a [...] Read more.
AlGaAs/GaAs heterostructures are the base of many semiconductor devices. The fabrication of new types of devices demands heterostructures with special features, such as large total thickness (~20 μm), ultrathin layers (~1 nm), high repeatability (up to 1000 periods) and uniformity, for which a conventional approach of growing such heterostructures is insufficient and the development of new growth procedures is needed. This article summarizes our work on the metalorganic vapour-phase epitaxy (MOVPE) growth of AlGaAs/GaAs heterostructures for modern infrared devices. The growth approaches presented allow for the improved output characteristics of different emitting devices such as multi active region lasers, epitaxially integrated via highly doped tunnel junctions (emission wavelength λ ~ 1 μm), quantum cascade lasers (λ ~10 μm) and THz laser (λ ~100 μm), based on short-period superlattice with 500–2000 layers. Full article
(This article belongs to the Special Issue MOVPE Growth of Crystalline Film)
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Open AccessArticle
Effects of Anion Impurities on the Solubility and Nucleation of Borax Decahydrate Crystals in Carbonate-Type Brine
Crystals 2019, 9(6), 304; https://doi.org/10.3390/cryst9060304
Received: 13 May 2019 / Revised: 5 June 2019 / Accepted: 11 June 2019 / Published: 13 June 2019
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Abstract
Measurements of the solubility and metastable zone width (MSZW) of borax decahydrate (disodium tetraborate decahydrate) within chloride, sulfate and carbonate solutions were carried out. The MSZW was measured by the conventional polythermal method with turbidity technology. The results showed that the addition of [...] Read more.
Measurements of the solubility and metastable zone width (MSZW) of borax decahydrate (disodium tetraborate decahydrate) within chloride, sulfate and carbonate solutions were carried out. The MSZW was measured by the conventional polythermal method with turbidity technology. The results showed that the addition of impurities decreased solubility. The main factor reducing solubility is the common ion effect, but the salt effect is also significant in the complex system. The addition of impurities broadened the MSZW. The MSZW was wider at first, then decreased with the addition of NaCl, yet the MSZW widened with the addition of Na2SO4 within the concentration range studied. This can be explained by adsorption and diffusion. The data of solubility and MSZW were analyzed by applying the Van’t Hoff equation and the classical 3D nucleation theory approach, respectively. By calculating the corresponding parameters, the experimental phenomena are further explained from the perspective of thermodynamics and dynamics. Full article
(This article belongs to the Section Crystal Engineering)
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Open AccessArticle
Novel Cerium Bisphosphinate Coordination Polymer and Unconventional Metal–Organic Framework
Crystals 2019, 9(6), 303; https://doi.org/10.3390/cryst9060303
Received: 30 April 2019 / Revised: 7 June 2019 / Accepted: 7 June 2019 / Published: 12 June 2019
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Abstract
The first Ce(III)-based coordination polymer ICR-9 (ICR stands for Inorganic Chemistry Řež), with the formula Ce2(C8H10P2O4)3, containing ditopic phenylene-1,4-bis(methylphosphinic acid) linker, was synthetized under solvothermal conditions. The crystal structure, solved using [...] Read more.
The first Ce(III)-based coordination polymer ICR-9 (ICR stands for Inorganic Chemistry Řež), with the formula Ce2(C8H10P2O4)3, containing ditopic phenylene-1,4-bis(methylphosphinic acid) linker, was synthetized under solvothermal conditions. The crystal structure, solved using electron diffraction tomography (EDT), revealed 2D layers of octahedrally coordinated cerium atoms attached together through O-P-O bridges. The structure is nonporous, however, the modification of synthetic conditions led to unconventional metal–organic framework (or defective amorphous phase) with a specific surface area up to approximately 400 m2 g-1. Full article
(This article belongs to the Special Issue Metal Phosphonates and Phosphinates)
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Open AccessArticle
A High-Resolution Terahertz Electric Field Sensor Using a Corrugated Liquid Crystal Waveguide
Crystals 2019, 9(6), 302; https://doi.org/10.3390/cryst9060302
Received: 28 May 2019 / Revised: 8 June 2019 / Accepted: 10 June 2019 / Published: 12 June 2019
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Abstract
Liquid crystals (LCs) can always reflect variable optical properties in a broad terahertz (THz) band under external electric or magnetic fields. Based on the measurements of these varying properties, we can realize electric and magnetic field sensing with very high sensitivity. Here, we [...] Read more.
Liquid crystals (LCs) can always reflect variable optical properties in a broad terahertz (THz) band under external electric or magnetic fields. Based on the measurements of these varying properties, we can realize electric and magnetic field sensing with very high sensitivity. Here, we theoretically and numerically demonstrate a type of electric field sensor in the THz frequency range based on the defect mode arising in a periodically corrugated waveguide with liquid crystals. The Bragg defect structure consisting of periodically corrugated metallic walls and a defect in the middle can provide a narrow transmitted peak with controllable bandwidth, which can be used for external field sensing when it is filled with LCs. The molecular orientation of nematic LCs (E7) is not only very sensitive to the applied DC electric field but also very crucial to the effective refractive index of E7. Changing the effective index can efficiently shift the frequency of the transmitted peak in the THz spectrum. The simulated results show that the sensitivity can reach as high as 9.164 MHz/(V/m) and the smallest resolution is 0.1115 V/m. The proposed sensor and its significant performance could benefit electric field sensing and extend the applications of THz technology. Full article
(This article belongs to the Special Issue Liquid Crystal Optics and Physics: Recent Advances and Prospects)
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Open AccessArticle
Platonic Relationships in Metal Phosphonate Chemistry: Ionic Metal Phosphonates
Crystals 2019, 9(6), 301; https://doi.org/10.3390/cryst9060301
Received: 15 May 2019 / Revised: 3 June 2019 / Accepted: 7 June 2019 / Published: 11 June 2019
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Abstract
Phosphonate ligands demonstrate strong affinity for metal ions. However, there are several cases where the phosphonate is found non-coordinated to the metal ion. Such compounds could be characterized as salts, since the interactions involved are ionic and hydrogen bonding. In this paper we [...] Read more.
Phosphonate ligands demonstrate strong affinity for metal ions. However, there are several cases where the phosphonate is found non-coordinated to the metal ion. Such compounds could be characterized as salts, since the interactions involved are ionic and hydrogen bonding. In this paper we explore a number of such examples, using divalent metal ions (Mg2+, Ca2+, Sr2+ and Ni2+) and the phosphonic acids: p-aminobenzylphosphonic acid (H2PABPA), tetramethylenediamine-tetrakis(methylenephosphonic acid) (H8TDTMP), and 1,2-ethylenediphosphonic acid (H4EDPA). The compounds isolated and structurally characterized are [Mg(H2O)6]·[HPABPA]2·6H2O, [Ca(H2O)8]·[HPABPA]2, [Sr(H2O)8]·[HPABPA]2, [Mg(H2O)6]·[H6TDTMP], and [Ni(H2O)6]·[H2EDPA]·H2O. Also, the coordination polymer {[Ni(4,4’-bpy)(H2O)4]·[H2EDPA]·H2O}n was synthesized and characterized, which contains a bridging 4,4’-bipyridine (4,4’-bpy) ligand forming an infinite chain with the Ni2+ cations. All these compounds contain the phosphonate anion as the counterion to charge balance the cationic charge originating from the metal cation. Full article
(This article belongs to the Special Issue Metal Phosphonates and Phosphinates)
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Open AccessArticle
Effect of Rare-Earth Element Oxides on Diamond Crystallization in Mg-Based Systems
Crystals 2019, 9(6), 300; https://doi.org/10.3390/cryst9060300
Received: 14 May 2019 / Revised: 4 June 2019 / Accepted: 10 June 2019 / Published: 11 June 2019
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Abstract
Diamond crystallization in Mg-R2O3-C systems (R = Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb) was studied at 7.8 GPa and 1800 °C. It was found that rare-earth oxide additives in an amount of 10 wt % [...] Read more.
Diamond crystallization in Mg-R2O3-C systems (R = Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb) was studied at 7.8 GPa and 1800 °C. It was found that rare-earth oxide additives in an amount of 10 wt % did not significantly affect both the degree of graphite-to-diamond conversion and crystal morphology relative to the Mg-C system. The effect of higher amounts of rare-earth oxide additives on diamond crystallization was studied for a Mg-Sm2O3-C system with a Sm2O3 content varied from 0 to 50 wt %. It was established that with an increase in the Sm2O3 content in the growth system, the degree of graphite-to-diamond conversion decreased from 80% at 10% Sm2O3 to 0% at 40% Sm2O3. At high Sm2O3 contents (40 and 50 wt %), instead of diamond, mass crystallization of metastable graphite was established. The observed changes in the degree of the graphite-to-diamond conversion, the changeover of diamond crystallization to the crystallization of metastable graphite, and the changes in diamond crystal morphology with increasing the Sm2O3 content attested the inhibiting effect of rare-earth oxides on diamond crystallization processes in the Mg-Sm-O-C system. The crystallized diamonds were studied by a suite of optical spectroscopy techniques, and the major characteristics of their defect and impurity structures were revealed. For diamond crystals produced with 10 wt % and 20 wt % Sm2O3 additives, a specific photoluminescence signal comprising four groups of lines centered at approximately 580, 620, 670, and 725 nm was detected, which was tentatively assigned to emission characteristic of Sm3+ ions. Full article
(This article belongs to the Special Issue Diamond Crystals Volume II)
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Open AccessArticle
Elasticity of Phases in Fe-Al-Ti Superalloys: Impact of Atomic Order and Anti-Phase Boundaries
Crystals 2019, 9(6), 299; https://doi.org/10.3390/cryst9060299
Received: 26 April 2019 / Revised: 4 June 2019 / Accepted: 6 June 2019 / Published: 8 June 2019
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Abstract
We combine theoretical and experimental tools to study elastic properties of Fe-Al-Ti superalloys. Focusing on samples with chemical composition Fe71Al22Ti7, we use transmission electron microscopy (TEM) to detect their two-phase superalloy nano-structure (consisting of cuboids embedded into [...] Read more.
We combine theoretical and experimental tools to study elastic properties of Fe-Al-Ti superalloys. Focusing on samples with chemical composition Fe71Al22Ti7, we use transmission electron microscopy (TEM) to detect their two-phase superalloy nano-structure (consisting of cuboids embedded into a matrix). The chemical composition of both phases, Fe66.2Al23.3Ti10.5 for cuboids and Fe81Al19 (with about 1% or less of Ti) for the matrix, was determined from an Energy-Dispersive X-ray Spectroscopy (EDS) analysis. The phase of cuboids is found to be a rather strongly off-stoichiometric (Fe-rich and Ti-poor) variant of Heusler Fe2TiAl intermetallic compound with the L21 structure. The phase of the matrix is a solid solution of Al atoms in a ferromagnetic body-centered cubic (bcc) Fe. Quantum-mechanical calculations were employed to obtain an insight into elastic properties of the two phases. Three distributions of chemical species were simulated for the phase of cuboids (A2, B2 and L21) in order to determine a sublattice preference of the excess Fe atoms. The lowest formation energy was obtained when the excess Fe atoms form a solid solution with the Ti atoms at the Ti-sublattice within the Heusler L21 phase (L21 variant). Similarly, three configurations of Al atoms in the phase of the matrix with different level of order (A2, B2 and D03) were simulated. The computed formation energy is the lowest when all the 1st and 2nd nearest-neighbor Al-Al pairs are eliminated (the D03 variant). Next, the elastic tensors of all phases were calculated. The maximum Young’s modulus is found to increase with increasing chemical order. Further we simulated an anti-phase boundary (APB) in the L21 phase of cuboids and observed an elastic softening (as another effect of the APB, we also predict a significant increase of the total magnetic moment by 140% when compared with the APB-free material). Finally, to validate these predicted trends, a nano-scale dynamical mechanical analysis (nanoDMA) was used to probe elasticity of phases. Consistent with the prediction, the cuboids were found stiffer. Full article
(This article belongs to the Special Issue Fundamentals of Superalloys)
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Open AccessArticle
Stress Heterogeneity Leading to Void Nucleation within Spherulites for Semi-Crystalline Polymers
Crystals 2019, 9(6), 298; https://doi.org/10.3390/cryst9060298
Received: 26 April 2019 / Revised: 31 May 2019 / Accepted: 6 June 2019 / Published: 8 June 2019
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Abstract
Linking the microstructure to the mechanical properties is a key feature in the design and assessment of the durability of semi-crystalline polymers. This paper addresses the importance of a particular architecture inside spherulites. The use of the theoretical tools of continuum mechanics has [...] Read more.
Linking the microstructure to the mechanical properties is a key feature in the design and assessment of the durability of semi-crystalline polymers. This paper addresses the importance of a particular architecture inside spherulites. The use of the theoretical tools of continuum mechanics has been combined with experimental observations of the deformation of the microstructure. Microstructural heterogeneities at various scales induced critical regions in the spherulite where voiding has been reported. The local stress state has been investigated using a Finite Element code. A simple Hooke’s law was used for meshes accounting for the alternating crystalline and amorphous lamellae. This allowed a better understanding of the loci of void nucleation in the equatorial region, as well as in the “polar fans”, and were explained by using a criterion based on stress equi-triaxiality. Full article
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Open AccessArticle
X-ray Structure Refinement and Vibrational Spectroscopy of Metavauxite FeAl2(PO4)2(OH)2·8H2O
Crystals 2019, 9(6), 297; https://doi.org/10.3390/cryst9060297
Received: 11 May 2019 / Revised: 1 June 2019 / Accepted: 3 June 2019 / Published: 6 June 2019
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Abstract
In this paper, we provide a crystal-chemical investigation of metavauxite, ideally FeAl2(PO4)2(OH)2·8H2O, from Llallagua (Bolivia) by using a multi-methodological approach based on EDS microchemical analysis, single crystal X-ray diffraction, and Raman and Fourier [...] Read more.
In this paper, we provide a crystal-chemical investigation of metavauxite, ideally FeAl2(PO4)2(OH)2·8H2O, from Llallagua (Bolivia) by using a multi-methodological approach based on EDS microchemical analysis, single crystal X-ray diffraction, and Raman and Fourier transform infrared (FTIR) spectroscopy. Our new diffraction results allowed us to locate all hydrogen atoms from the structure refinements in the monoclinic P21/c space group. Metavauxite structure displays a complex framework consisting of a stacking of [Al(PO4)3(OH)(H2O)2]7− layers linked to isolated [Fe(H2O)6]2+ cationic octahedral complex solely by hydrogen bonding. The hydrogen-bonding scheme was inferred from bond-valence calculations and donor-acceptor distances. Accordingly, strong hydrogen bonds, due to four coordinated H2O molecules, bridge the [Fe(H2O)6]2+ units to the Al/P octahedral/tetrahedral layer. The hydroxyl group, coordinated by two Al atoms, contributes to the intra-layer linkage. FTIR and Raman spectra in the high-frequency region (3700–3200 cm−1) are very similar, and show a complex broad band consisting of several overlapping components due to the H2O molecules connecting the isolated Fe(H2O)6 and the adjacent Al/P octahedral/tetrahedral layers. A sharp peak at 3540 cm−1 is assigned to the stretching mode of the OH group. The patterns collected in the low-frequency region are dominated by the stretching and bending modes of the PO43− group and the metal-oxygen polyhedra. Full article
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Open AccessArticle
Luminescent and Scintillation Properties of CeAlO3 Crystals and Phase-Separated CeAlO3/CeAl11O18 Metamaterials
Crystals 2019, 9(6), 296; https://doi.org/10.3390/cryst9060296
Received: 29 April 2019 / Revised: 29 May 2019 / Accepted: 4 June 2019 / Published: 6 June 2019
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Abstract
This work is dedicated to the growth process and investigation of luminescent and scintillation properties of CeAlO3 single crystals and CeAlO3/CeAl11O18 metamaterials under e-beam and α-particles excitation. It has been shown that cathodoluminescence and radioluminescence spectra of [...] Read more.
This work is dedicated to the growth process and investigation of luminescent and scintillation properties of CeAlO3 single crystals and CeAlO3/CeAl11O18 metamaterials under e-beam and α-particles excitation. It has been shown that cathodoluminescence and radioluminescence spectra of CeAlO3 crystals contain two bands, peaking at 440 and 500 nm, and caused by the Ce3+ 5d–4f transitions into CeAl11O18 phase, which is present in these crystals as an admixture. Under 270 nm ultraviolet (UV) light excitation, a CeAlO3 crystal possesses complicated non-exponential luminescence decay, with the average decay time of 16 ns. The light yield of CeAlO3 crystals under α-particle excitation is about 16% and 12%, in respect to the standard Bi4Ge3O12 (BGO) crystal and Y3Al5O12:Ce (YAG:Ce) single crystalline film samples, respectively. The CeAlO3 scintillation decay is quite fast, with the decay time value t1/e in the 54–56 ns range. Full article
(This article belongs to the Special Issue Crystals, Films and Nanocomposite Scintillators)
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Open AccessArticle
Comparative Study of Different Crystallization Methods in the Case of Cilostazol Crystal Habit Optimization
Crystals 2019, 9(6), 295; https://doi.org/10.3390/cryst9060295
Received: 10 May 2019 / Revised: 3 June 2019 / Accepted: 4 June 2019 / Published: 5 June 2019
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Abstract
The therapeutic usage of cilostazol is limited owing to its poor aqueous solubility and oral bioavailability. Our aim was to produce cilostazol crystals with small average particle size; besides suitable roundness, narrow particle size distribution and stable polymorphic form to increase its dissolution [...] Read more.
The therapeutic usage of cilostazol is limited owing to its poor aqueous solubility and oral bioavailability. Our aim was to produce cilostazol crystals with small average particle size; besides suitable roundness, narrow particle size distribution and stable polymorphic form to increase its dissolution rate and improve processability. Different conventional crystallization methods with or without sonication were compared with impinging jet crystallization combined with cooling, and the optimization of the various parameters was also implemented. The effects of post-mixing time and temperature difference were studied by means of a full factorial design. The physical properties of powder particles were characterized by, i.a., XRPD, DSC and SEM. The dissolution rate and the contact angle of solid surfaces were also determined to elucidate the relationship between wettability and dissolution. It was observed that impinging jet crystallization combined with cooling is a very effective and reproducible method for reducing the particle size of cilostazol. This method resulted in significantly smaller particle size (d(0.5) = 3–5 μm) and more uniform crystals compared to the original ground material (d(0.5) = 24 μm) or the conventional methods (d(0.5) = 8–14 μm), and it also resulted in a stable polymorphic form and enhanced the dissolution rate. Full article
(This article belongs to the Special Issue Anti-Solvent Crystallization)
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Open AccessArticle
The Orientation of Strain-Induced Crystallites in Uniaxially-Strained, Thin and Wide Bands Made from Natural Rubber
Crystals 2019, 9(6), 294; https://doi.org/10.3390/cryst9060294
Received: 20 April 2019 / Revised: 14 May 2019 / Accepted: 2 June 2019 / Published: 5 June 2019
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Abstract
Vulcanized natural rubber (unfilled and filled with 20 phr carbon black) is strained. We suppress the macroscopic formation of fiber symmetry by choosing strip-shaped samples ("pure-shear geometry") and investigate the orientation of the resulting crystallites by two-dimensional wide-angle X-ray diffraction (WAXD), additionally rotating [...] Read more.
Vulcanized natural rubber (unfilled and filled with 20 phr carbon black) is strained. We suppress the macroscopic formation of fiber symmetry by choosing strip-shaped samples ("pure-shear geometry") and investigate the orientation of the resulting crystallites by two-dimensional wide-angle X-ray diffraction (WAXD), additionally rotating the sample tape about the straining direction. Indications of a directed reinforcing effect of the strain-induced crystallization (SIC) in the thin strip are found. In the filled material fewer crystallites are oriented and the orientation distribution of the oriented crystallites is less perfect. The results confirm, that it is important for the evaluation of crystallinity under deformation to check, whether fiber symmetry can be assumed. This has consequences in particular on the quantitative interpretation of space-resolved scanning experiments in the vicinity of crack tips. Furthermore it raises the question, whether there is an asymmetric reinforcing effect of the SIC in the vicinity of crack tips inside natural rubber. Full article
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