Crystal Imperfections Research Articles

Measurement of the thulium ion spin Hamiltonian in an yttrium gallium garnet host crystal

We characterize the magnetic properties for thulium ion energy levels in the Y$_3$Ga$_5$O$_{12}$ (Tm:YGG) lattice with the goal to improve decoherence and reduce line-width broadening caused by local host spins and crystal imperfections. More precisely, we measure hyperfine tensors for the lowest level of the, $^3$H$_6$, and excited, $^3$H$_4$, states using a combination of spectral hole burning, absorption spectroscopy, and optically detected nuclear magnetic resonance. By rotating the sample through a series of angles with an applied external magnetic field, we measure and analyze the orientation dependence of the Tm$^{3+}$ ion's spin-Hamiltonian. Using this spin-Hamiltonian, we propose a set of orientations to improve material properties that are important for light-matter interaction and quantum information applications. Our results yield several important external field directions: some to extend optical coherence times, another to improve spin inhomogeneous broadening, and yet another that maximizes mixing of the spin states for specific sets of ions, which allows improving optical pumping and creation of lambda systems in this material.

In-Situ Isothermal Crystallization of Poly(l-lactide)

The isothermal crystallization of poly(l-lactide) (PLLA) has been investigated by in-situ wide angle X-ray diffraction (WAXD) and polarized optical microscopes (POM) equipped with a hot-stage accessory. Results showed that the spherulites of PLLA were formed at high temperature, whereas irregular morphology was observed under a low temperature. This can be attributed to the varying rates of crystallization of PLLA at different temperatures. At low temperatures, the nucleation rate is fast and hence the chains diffuse very slow, resulting in the formation of imperfect crystals. On the other hand, at high temperatures, the nucleation rate is slow and the chains diffuse fast, leading to the formation of perfect crystals. The change in the value of the Avrami exponent with temperature further verifies the varying trend in the morphological feature of the crystals.

Massive methane fluxing from magma\u2013sediment interaction in the end-Triassic Central Atlantic Magmatic Province

Exceptional magmatic events coincided with the largest mass extinctions throughout Earth’s history. Extensive degassing from organic-rich sediments intruded by magmas is a possible driver of the catastrophic environmental changes, which triggered the biotic crises. One of Earth’s largest magmatic events is represented by the Central Atlantic Magmatic Province, which was synchronous with the end-Triassic mass extinction. Here, we show direct evidence for the presence in basaltic magmas of methane, generated or remobilized from the host sedimentary sequence during the emplacement of this Large Igneous Province. Abundant methane-rich fluid inclusions were entrapped within quartz at the end of magmatic crystallization in voluminous (about 1.0 × 106 km3) intrusions in Brazilian Amazonia, indicating a massive (about 7.2 × 103 Gt) fluxing of methane. These micrometre-sized imperfections in quartz crystals attest an extensive release of methane from magma–sediment interaction, which likely contributed to the global climate changes responsible for the end-Triassic mass extinction.

The Effect of Dialkyl Peroxide Crosslinking on the Properties of LLDPE and UHMWPE.

Peroxide has been considered a chemical agent that can be used to tune the properties of polymeric materials. This research evaluated the influence of different concentrations of dialkyl peroxides on the mechanical, thermal, and morphological properties of linear low-density polyethylene (LLDPE) and ultra-high molecular weight polyethylene (UHMWPE). The neat polymer, as well as those with the addition of 1% and 2% by mass of dialkyl peroxides, were subjected to compression molding and immersion in water for 1 h, under controlled temperatures of 90 °C. The values of the gel content found in the samples indicated that the addition of peroxide to the LLDPE and to the UHMWPE promoted the formation of a reticulated network. The structure obtained by the crosslinking led to less reorganization of the chains during the crystallization process, resulting in the formation of imperfect crystals and, consequently, in the reduction in melting temperatures, crystallization and enthalpy. The mechanical properties were altered with the presence of the crosslinker. The polymers presented had predominant characteristics of a ductile material, with the occurrence of crazing with an increased peroxide content.

Exploration of Gamma irradiation effects on the structural, spectral characteristics, thermomechanical behaviour and optical constants in oriented glycine-Di-Glycinium sulphate (TGS) single crystals

Single crystals of glycine-di-glycinium sulphate (TGS) have been synthesized and grown along <011> direction. The crystals were subjected to gamma irradiation with dosage ranging 10-50 kGy. While observing the low temperature Raman scattering modes, it can be understood that there exists possible bond breaking mechanisms upon gamma irradiation and as its consequence free radical ions appeared to have been formed. Thermo-mechanical parameters such as linear thermal strain, thermal expansion and linear thermal expansion coefficient have been computed by performing thermo-mechanical analysis on the irradiated samples. Gamma irradiation induced crystal imperfections were assessed through the high resolution XRD (HRXRD) measurement and it provides a clue for the development of non-uniform strain in the crystal. Modulation in optical constants, on irradiation was also noticed from the refractive index measurements. Piezoelectric and ac- conductivity measurements confirmed the rupture of bonds and in turn causing deterioration of ferroelectric and piezoelectric properties of the irradiated crystal.

High pressure phase evolution under hydrostatic pressure in a single imperfect crystal due to nanovoids

In this paper, the evolution of high pressure phase (HPP) in an imperfect single crystal due to the existence of nanovoids is investigated. The nanovoid misfit strain and its stress are properly linked to its radius. The Ginzburg-Landau coupled with elasticity equations are solved for the HPP evolution. The energy barrier between the HPP and the low pressure phase, entropy jump, transformation strain function and the relation between critical and equilibrium temperatures are determined based on the phase transformation (PT) equilibrium and instability pressures and the HPP transformation strain. The obtained PT pressure which shows an exponential reduction vs. the radius and a linear increase vs. temperature, is found mostly smaller than the theoretical solution. The HPP grows from the nanovoid surface and along the line of zero shear transformation strain. Above some critical radius, the HPP growth is promoted and continues along the boundaries. The transformation work distribution reveals a faster initial growth for larger radii. The HPP growth is evaluated based on the thermodynamic equilibrium and instability criteria. The growth rate and velocity of the HPP tip nonlinearly decrease with temperature. Interestingly, the average shear stress varies very similar to the HPP concentration. The HPP growth is also investigated for different applied pressures so that above some pressure, the HPP growth proceeds along the boundaries and the growth rate increases as the pressure increases. The current study can be used to better understand the PT nano mechanisms of various HPPs.

Dynamical effects in the integrated X-ray scattering intensity from imperfect crystals in Bragg diffraction geometry. II. Dynamical theory.

The analytical expressions for coherent and diffuse components of the integrated reflection coefficient are considered in the case of Bragg diffraction geometry for single crystals containing randomly distributed microdefects. These expressions are analyzed numerically for the cases when the instrumental integration of the diffracted X-ray intensity is performed on one, two or three dimensions in the reciprocal-lattice space. The influence of dynamical effects, i.e. primary extinction and anomalously weak and strong absorption, on the integrated intensities of X-ray scattering is investigated in relation to the crystal structure imperfections.

Understanding the Reinforcement of Graphene in Poly(Ether Ether Ketone)/Carbon Fibre Laminates.

PEEK appears as an excellent candidate to substitute epoxy resins in carbon fibre laminates for high-performance aeronautical applications. The optimization of the properties and, in particular, of the transition region between the fibres and the matrix appear as a major issue prior to serial production. Graphene, modified with two compatibilizers, has been incorporated in the polymer layer with the purpose of imparting additional functionalities and enhancing the matrix-fibre interaction. It is found that both carbon fibres and modified graphene significantly influence the crystallization behaviour and smaller, and/or more imperfect crystals appear while the degree of crystallinity decreases. Despite this, nanoindentation studies show that the PEEK layer exhibits significant modulus improvements (≈30%) for 5 wt.% of graphene. Most importantly, the study of the local mechanical properties by nanoindentation mapping allows the identification of remarkably high modulus values close to the carbon fibre front. Such a relevant mechanical enhancement can be associated with the accumulation of graphene platelets at the polymer–fibre boundary, as revealed by electron microscopy studies. The results offer a feasible route for interlaminar mechanical improvement based on the higher density of graphene platelets at the fibre front that should promote interfacial interactions. Concerning electrical conductivity, a large anisotropy was found for all laminates, and values in the range ~10−4 S/cm were found for the through-thickness arrangement as a consequence of the good consolidation of the laminates.

Comparative microstructural studies using different methods: Effect of Cd-addition on crystallography, microstructural properties, and crystal imperfections of annealed nano-structural thin CdxZn1-xSe films

Thermally evaporated zinc-cadmium selenide (CdxZn1-xSe) thin films with different Cd-concentration (0.0≤x≤1.0, at%) have been prepared. The deposition process of films was controlled where the vacuum was fixed at ≈10−5 Pa, the film thickness at 7500 Å, and the rate of deposition at ≈100 Å/s. These prepared films have been annealed under vacuum at 500⁰C for 2-hrs. X-ray diffraction, XRD study confirmed the crystallinity nature of annealed CdxZn1-xSe films with a face-centered cubic structure, which is transformed into a hexagonal structure with increasing the cadmium. Energy-dispersive X-ray analysis shows that there is good consistency between the detected and selected element ratios. Scherrer, Williamson-Hall, Size-Strain plot, and Halder-Wagner methods were used to examine the crystallite size and microstrain using XRD peak-broadening analysis. According to the Williamson-Hall method the crystallite size increased from 14.70 nm to 31.30 nm and microstrain decreased from 14.19×10−3 to 7.29×10−3. The Cd-addition leads to improving the film crystallinity and reducing the crystal imperfections.

Elastic neutron scattering models for NCrystal

The NCrystal library provides a range of models for simulation of both elastic and inelastic scattering of thermal neutrons in a range of material structures. This article presents the available models for elastic scattering, and includes detailed discussion of their theoretical background, their implementation, and in particular their validation. The lineup includes a model for Bragg diffraction in crystal powders as well as one for incoherent elastic scattering, but the main focus is given to models of Bragg diffraction in ideally imperfect single crystals: both for the most widely applicable model of isotropic Gaussian mosaicity, and for a more specific model of layered single crystals which is relevant for materials such as pyrolytic graphite. Although these single crystal models are utilising computationally efficient approximations where appropriate, attention is given to the provision of precise and trustworthy results also for the extreme cases of back-scattering, forward-scattering, and crystals with very large mosaic spreads. Together with NCrystal's other features for crystal structure initialisation and inelastic physics, the presented models enable realistic modelling of components at neutron scattering instruments in frameworks like Geant4 and McStas, including monochromators, analysers, filters, support materials, shielding, and many kinds of samples. As a byproduct of the work, an improved formula for approximating cross sections in isotropic single crystals with Gaussian mosaicity is provided. Program summaryProgram Title: NCrystalCPC Library link to program files:https://doi.org/10.17632/rzmmzgpg7g.1Licensing provisions: Apache License, Version 2.0.Programming language:C++, C and PythonNature of problem: Thermal neutron transport in structured materials is inadequately supported in popular Monte Carlo transport applications, preventing simulations of a range of otherwise interesting setups.Solution method: Provide models for thermal neutron transport in open source library NCrystal, to be used standalone or as backend in existing Monte Carlo packages. Specifically, the present publication details the implemented elastic scattering models in crystalline materials.

Influence of nanometric microstructural development on thermophysical properties of lanthanum-doped strontium titanate

The evolution of imperfect crystal from a highly amorphous structure to an ordered crystalline structure via sintering has demonstrated a significant development onto the thermophysical properties. Therefore, a series of La-doped strontium titanate with different morphological properties have been synthesized via high energy ball milling with subsequent sintering. Doping of lanthanum tends to inhibit the grain growth where a significant reduction of grain size (range from 34 nm to 0.47 μm) could be observed. A large amount of grain boundaries presence with fine grains, resulting from lanthanum doping, has enhanced phonon scattering, thus yielding a low heat propagation in La-doped SrTiO3. Besides acting as a grain growth inhibitor, lanthanum also acts as a scattering centre due to imperfection created by the doping. The scattering mechanisms significantly decrease the phonon mean free path. Consequently, the thermal diffusivity has been efficiently reduced to 1.1 mm2/s as compared to that of pure SrTiO3 which showed a value of about 3.8 mm2/s, both observed for the complete phase polycrystalline materials. The systematic development of thermophysical and morphological properties of La-doped SrTiO3 can be served as a preliminary guide to engineer thermoelectric materials with low thermal diffusivity.

Role of the heat treatment of partial melt recrystallization method on microstructure change and toughness of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(HB-co-HV)

The microstructure of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(HB-co-HV)] was regulated by the heat treatment of partial melt recrystallization method (PMRM) to improve its mechanical properties effectively. The structural changes and toughening mechanism of P(HB-co-HV) were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), scanning electron microscope (SEM) as well as dynamic mechanical analysis (DMA) experiments. The imperfect crystals of P(HB-co-HV) melted at 176 °C while the remaining perfect crystals recrystallized at the higher temperature (Tc = 120–150 °C). The results demonstrated that the HV units were easily to be excluded from the PHB crystal lattice in the P (HB-co-HV) recrystallization process, which promoted the crystal perfection of P(HB-co-HV) and improved the tensile strength and melting temperature. Furthermore, the heat treatment of PMRM reduced the molecular chain density and entanglement in the amorphous region and made the distribution of the tensile degree of the molecular chains more uniform, which was conducive to the sliding of lamellar crystals along the impact tensile direction, inducing plastic deformation. In addition, the heat treatment of P(HB-co-HV) via PMRM produced a large number of micropores, which was conducive to dispersing stress and improving its macroscopic toughness during the subsequent stretching process.

Design and growth of InAsP metamorphic buffers for InGaAs thermophotovoltaic cells

The structural and optical properties of InAsxP1-x metamorphic buffers grown by metal–organic chemical vapor deposition on InP (100) substrates have been investigated. High-resolution X-ray reciprocal space mapping around the (115) InP lattice point reveals that the strain relaxations of the InAsxP1-x with x = 0.5, 0.55, and 0.7 are 98%, 92%, and 96%, while the lateral correlation lengths are 17, 62, and 28 nm, respectively. The optical bandgap energy of the InAsP derived from photoreflectance (PR) measurements decreases from 0.819 to 0.621 eV at 300 K when increasing As composition from x = 0.5 to 0.7. The bowing parameter for the optical bandgap of the InAsP is increased with increasing As composition, which is attributable to the increased spontaneous CuPt-type ordering in InAsP. It is found from the excitation power-dependent PR measurement that the InAsxP1-x layers have different degrees of the bandgap redshift due to the reduced thermal conductivity caused by crystal imperfections generated during the strain relaxation process.

Modelling Electron Channeling Contrast Intensity of Stacking Fault and Twin Boundary Using Crystal Thickness Effect.

In a scanning electron microscope, the backscattered electron intensity modulations are at the origin of the contrast of like-Kikuchi bands and crystalline defects. The Electron Channeling Contrast Imaging (ECCI) technique is suited for defects characterization at a mesoscale with transmission electron microscopy-like resolution. In order to achieve a better comprehension of ECCI contrasts of twin-boundary and stacking fault, an original theoretical approach based on the dynamical diffraction theory is used. The calculated backscattered electron intensity is explicitly expressed as function of physical and practical parameters controlling the ECCI experiment. Our model allows, first, the study of the specimen thickness effect on the channeling contrast on a perfect crystal, and thus its effect on the formation of like-Kikuchi bands. Then, our theoretical approach is extended to an imperfect crystal containing a planar defect such as twin-boundary and stacking fault, clarifying the intensity oscillations observed in ECC micrographs.

Preparation of novel poly(vinyl alcohol)/chitosan lactate-based phase-separated composite films for UV-shielding and drug delivery applications

In this study, novel phase-separated composite films based on poly(vinyl alcohol) (PVA) and chitosan lactate (CL) were prepared by the solvent casting method. The influence of varying concentrations of CL (2.5–10% w/w) on the physicochemical properties of the films was assessed. Bright-field micrographs showed the formation of phase-separated polymeric matrices of different architectures in the CL concentration range of 2.5 and 7.5 wt%. At 10 wt% of CL, a homogenous matrix of polymer blend was observed. The developed films demonstrated excellent shielding properties against UV and visible radiation. FTIR studies confirmed the presence of hydrogen bonding between the parent polymers, i.e., PVA and CL, of the films. The film containing 7.5 wt% CL exhibited the highest crystalline nature with little crystal imperfections as compared to the remaining films. Impedance spectroscopy confirmed the conductive nature of the prepared films. The thermograms revealed that the addition of CL strongly influenced the hydrophilic interactions between the functional groups of the polymer components and the water molecules. PVA/CL composite films showed a higher % release of ciprofloxacin HCl (CPH) as compared to the pristine PVA film. CPH-loaded films showed excellent antimicrobial efficacy against model organisms. Therefore, the novel PVA/CL phase-separated composite films can potentially be used as matrices for UV-shielding and drug delivery applications.

Mechanochemical Synthesis of a Hydrogenation Catalyst from Nickel Oxide and Silica Gel

For the first time, silica gel-supported nickel hydrogenation catalyst has been synthesized via the mechanochemical method from oxides in a single stage. A set of data on the effect of the catalyst mechanochemical activation conditions on specific surface area, pore size distribution, crystallites size, and crystal lattice imperfection has been obtained. Activity of the obtained catalyst towards mild-conditions reduction of multiple carbon bonds in styrene, prop-2-en-1-ol, and diethyl maleate has been studied. Recommendations for establishing the optimal parameters for obtaining a nickel catalyst using mechanochemical synthesis have been elaborated.

A new method of grafting multi-walled carbon nanotubes on carbon fibers for improving the mechanical and thermal properties of woven fabric composites

A new method of grafting multi-walled carbon nanotubes (MWCNTs) onto carbon fiber surface to improve the thermo-mechanical properties of woven carbon fabric reinforced composites was proposed. In this method, both carbon woven fabrics and MWCNTs were oxidized by sulfuric acid to generate carboxyl groups on their surfaces, respectively. Then silane coupling agent was used to react with the carboxyl groups to graft MWCNTs onto the carbon fiber surfaces of the woven fabric. The untreated, acid treated and MWCNTs grafted carbon woven fabrics were separately combined with polypropylene films to form composite plates by thermal-stamping. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were conducted to estimate the changes of element contents and functional groups on surfaces of carbon fibers and MWCNTs. Atomic force microscope was used to estimate the roughness of carbon fiber surfaces. Scanning electron microscopy, differential scanning calorimeter, dynamic mechanical thermal analysis and tensile tests were carried out to analyze the surface morphology, thermal, and mechanical properties of carbon fabrics and their composites. Testing results showed that MWCNTs could be successfully grafted onto the carbon fibers by using silane as an intermediate bridge. Compared with the untreated and acid treated composites, the in-plane shearing stiffness and fracture strength of the composites were increased significantly by MWCNTs grafting. In terms of thermal properties, acid treatment and MWCNTs grafting have little effect on melting point of composites. MWCNTs can promote the recrystallization process of the PP and reduce the numbers of imperfect crystals. As for thermo-mechanical properties, acid treatment deteriorated the bending storage modulus of the composite, while MWCNTs grafting could compensate it.

Disordered exchange is biased

The magnetic properties of intercalated metal dichalcogenides are dramatically affected by small crystal imperfections, potentially providing design principles and materials for spintronic devices.

Prediction of heterogeneity and anisotropy of oxidised pyrite crystals using electrical measurements

ABSTRACT Some electrical, geochemical, and mineralogical investigations and analyses were made on some cubic oxidized Pyrite crystals (Gabal El-Sibai). Samples (chemically) are constituted from Fe2O3, SiO2, and Al2O3. Electrical features measured at frequency range (100 Hz– 5 MHz). Oxidized Pyrite crystals (cubes) are homogeneous, with no distortion or high variation in their composition. Also, samples have roughly the same structure. Samples were measured in 3-directions. It is supposed that any difference in a lattice structure, in any direction, will be reflected directly in the measurements, i.e. electrical features will attempt to detect variations of homogeneity and anisotropy of samples. The alterations in electrical functions are from mineral composition changes. Changes in texture, at specimens, do not exist (grain size and shape, mineral constituents, and pore throats between grains), and it is supposed that electrical features will be similar in three directions. There were some slight changes in electrical characteristics, due to some imperfections in natural crystals, with some impurities on crystal surfaces that we could not remove. Conductivity rises with frequency increase and accordingly, dielectric constant decreases. The paper aims to prove that the electrical properties, due to homogeneity, are the same at the three perpendicular directions.

Термодинамика роста металлической фазы при твердофазном восстановлении металлов в комплексных оксидах

To improve the technologies of solid-phase reduction of metals in ores, it is necessary to study in detail the regularities of the growth of nuclei of the metal phase, taking into account the conditions existing in the reduction aggregates. At the first stage of the study, the general regular-ities of the growth of a new phase in a multicomponent oxide system were studied. Using the methods of non-equilibrium thermodynamics, a complex physicochemical description of the solid-phase reduction of metals in a multicomponent oxide phase was obtained, taking into account the thermal and diffusion processes occurring in this case. Additionally, using the methods of chemistry of imperfect crystals, the regularities of the influence of the movement of anionic vacancies on the thermodynamic state of the system were formulated. For the considered system the physicochemical equations for the balance of vacancies and entropy were written. The developed new theory made it possible to consider thermal and diffusion processes, as well as the processes of vacancy movement in a multicomponent oxide system in their complexity. For a practical study of the reduction of metals in ores, the case of the growth of a metal particle from the initial phase of the ore was considered, taking into account the additional effect of the reducing agent on the process. On the basis of an integrated approach, a general expression for the production of entropy in the system of a growing metal nucleus, an initial ore phase and a phase of reducer inclusions was obtained. The carried out research made it possible to develop a mathematical model of the growth of a metal crystal in the volume of the oxide phase, depending on the heating mode and the composition of complex oxides. In particular, the developed mathematical model made it possible to calculate the growth rate of the metal nucleus in the initial multicomponent oxide phase. The obtained technique also made it possible to evaluate the degree of influence of the movement of anionic vacancies on the regularities of the reduction process.