Crystal Mechanical Properties Research Articles

Studies on the physico-chemical properties of semi-organic nonlinear optical (NLO) material: 2-CPPC [2-CNPyH][ClO4]

Semi-organic NLO single crystals 2-cyanopyridinium perchlorate (2-CPPC) were successfully grown by the method of slow evaporation of solvent at room temperature (27 °C) by using acetone as a solvent and of dimension 15×12×3mm3. The powder X-ray diffraction (XRD) of the grown crystal has been investigated. The UV–vis study estimates the transmission range, refractive index, extinction coefficient, band gap, and reflectance of the grown 2-CPPC crystal. The fluorescence spectra of the grown crystal were also recorded. The presence of different functional groups and the vibrational modes were confirmed by using FTIR analysis. The crystal's mechanical properties were studied by a Vickers microhardness tester. Single beam Z-scan technique using He–Ne laser is used to study the third-order NLO properties.

Local symmetry predictors of mechanical stability in glasses.

The mechanical properties of crystals are controlled by the translational symmetry of their structures. But for glasses with a disordered structure, the link between the symmetry of local particle arrangements and stability is not well established. In this contribution, we provide experimental verification that the centrosymmetry of nearest-neighbor polyhedra in a glass strongly correlates with the local mechanical stability. We examine the distribution of local stability and local centrosymmetry in a glass during aging and deformation using microbeam x-ray scattering. These measurements reveal the underlying relationship between particle-level structure and larger-scale behavior and demonstrate that spatially connected, coordinated local transformations to lower symmetry structures are fundamental to these phenomena. While glassy structures lack obvious global symmetry breaking, local structural symmetry is a critical factor in predicting stability.

Selection of Nanostructured Zirconium Dioxide Crystals under Dry Friction against Molybdenum

The article considers the issues of contact interaction of nanostructured zirconium dioxide crystals with molybdenum coating on titanium-based alloy ВТ9. The following statement is substantiated: the best mechanical properties of crystals correspond to the total amount of 2.8% additives of stabilizing and doping rare earths elements. Antifriction properties of crystals under dry friction against molybdenum are investigated. Tribological testing was performed on a reciprocating friction gauge simulating the operation of a plunger in a cylinder. The high scoring resistance of the friction couple under study was experimentally proved; the friction coefficient did not exceed 0.07. On the basis of the study performed, the friction couple is defined as promising for increasing the lifetime and reliability of friction units for hydraulic equipment and fuel pumps.

Methods to evaluate the twin formation energy: comparative studies of the atomic simulations and in-situ TEM tensile tests

Deformation twinning, one of the major deformation modes in a crystalline material, has typically been analyzed using generalized planar fault energy (GPFE) curves. Despite the significance of these curves in understanding the twin nucleation and its effect on the mechanical properties of crystals, their experimental validity is lacking. In this comparative study based on the first-principles calculation, molecular dynamics simulation, and quantitative in-situ tensile testing of Al nanowires inside a transmission electron microscopy system, we present both a theoretical and an experimental approach that enable the measurement of a part of the twin formation energy of the perfect Al crystal. The proposed experimental method is also regarded as an indirect but quantitative means for validating the GPFE theory.

Density functional theory calculation of fracture surfaces of siderite and hematite

The relationship between the fracture surface and mechanical properties of siderite and hematite was studied by density functional theory (DFT). The mechanical properties of crystals with an O-vacancy defect were also investigated to take into account the ubiquitous defects present in both minerals. Colored contour surfaces were generated to illustrate the elastic anisotropy of siderite and hematite with or without a defect. The calculated elastic-constant matrixes showed that shear deformation is the most likely deformation type in the crystals with or without a defect. The O-vacancy defect in both mineral types alters their crystal. Specifically, for siderite, the B/G ratio decreased to 1.66 in the presence of this defect, indicating a significant change in the brittle/ductile behavior of the crystal. The contour surfaces indicated that the symmetry of the distribution of mechanical properties was unaffected by the defects. For both siderite and hematite, the diagonal surfaces were weakest, so the (110), (101), and (011) planes were predicted to be the most commonly fractured surfaces. This was consistent with the surface fracture behavior of trigonal systems according to previous literature. This study provides a novel understanding of the link between the fracture surface and the mechanical properties of crystals.

Упругость и неупругость объемных кристаллов нитрида галлия

The results of the elastic and microplastic study of GaN single crystals and polycrystalline slabs are presented. Both types of the crystals grown by gas-phase epitaxy in HVPE reactor at deposition on sapphire and ceramic substrates respectively have a specific of structure. It was shown that crystals in slabs are grown in [10-10] perpendicular to substrate but have not primary directions in plane of substrate. The single crystals contains deep V-shaped defects (pits). The experimental results have been analyzed on the basis of existing the theoretical aspects of the influence of dislocations and grain boundaries on the acoustic and mechanical properties of crystals.

Comparison of mechanical properties of zirconia crystals partially stabilized with yttria and gadolinia

(ZrO2) 0.97(Y2O3)0.03 and (ZrO 2)0.97(Gd2O3)0.03 solid solution crystals have been grown using directional melt crystallization in cold container. The structure and phase composition of the crystals have been studied using X-ray diffraction and transmission electron microscopy. The mechanical properties of the crystals e.g. microhardness and fracture toughness have been studied with microindentation. We show that the main hardening mechanism in the 3GdSZ and 3YSZ crystals is transformation hardening. No microhardness anisotropy has been observed in the crystals. The highest fracture toughness among the crystals has been obtained for the 3GdSZ crystals in the {100} plane.

On micro-level vulnerability of concrete surface layer in process of crystallization of hydrating Portland cement and upon mechanical load

The target of this research is to make a system of data on discontinuity of the cement stone microstructure in concrete. To achieve this, the following issues have to be addressed: a) a justification for the presence of dislocations in cement stone and definition of their type needs to be provided; b) the source of the considered material weakness has to be identified; c) an explanation of formation and propagation of micro cracks starting at the surface and going deeper into the concrete needs to be given; d) an educated guess has to be made for the opening/depth ratio of micro cracks; e) information about the distances between micro cracks in the capillary walls of the cement stone should be obtained; e) binding energy in the calcium silicate crystalline hydrate has to be determined. The research results show that the mechanical properties of crystals and polycrystals in the cement stone (concrete) depend on the ratio of small-angle (θ≤10…150) and large-angle boundaries. Due to a relatively small number of large-angle boundaries, intergranular and plastic flow deformations are dampened under straining, resulting in cleavage in the areas where stress-raisers are found (pores, capillaries, submicrocracks, etc.).

Nanostructured crystals of zirconium dioxide for mechanical modules of instruments

The work is devoted to the study of nanostructured partially stabilized crystals of zirconium dioxide with respect to the problem of improving the reliability of the jeweled bearings of precision instruments. The effect of yttrium oxide stabilizing additive on the crack resistance and mechanical properties of crystals has been investigated. A correlation has been established between the concentration of the stabilizing additive Y2O3 and the crack resistance. The tribological properties of crystals were determined using a friction test machine under sliding conditions. The crack resistance and mechanical characteristics of the crystals were determined by the kinetic microindentation method. Results of the comparative analysis of mechanical properties with the used crystals and ceramics are provided. Bench tests of full–scale thrust bearings confirmed a higher load–bearing capacity of the friction bearing of the instrument with a mock–up thrust bearing made of a zirconium dioxide crystal in comparison with a thrust bearing made of sapphire in the absence of significant damage to the friction surface.

High-power-load DCLM monochromator for a computed tomography program at BMIT at energies of 25–150 keV

The research program at the biomedical imaging facility requires a high-flux hard-X-ray monochromator that can also provide a wide beam. A wide energy range is needed for standard radiography, phase-contrast imaging, K-edge subtraction imaging and monochromatic beam therapy modalities. The double-crystal Laue monochromator, developed for the BioMedical Imaging and Therapy facility, is optimized for the imaging of medium- and large-scale samples at high energies with the resolution reaching 4 µm. A pair of 2 mm-thick Si(111) bent Laue-type crystals were used in fixed-exit beam mode with a 16 mm vertical beam offset and the first crystal water-cooled. The monochromator operates at energies from 25 to 150 keV, and the measured size of the beam is 189 mm (H) × 8.6 mm (V) at 55 m from the source. This paper presents our approach in developing a complete focusing model of the monochromator. The model uses mechanical properties of crystals and benders to obtain a finite-element analysis of the complete assembly. The modeling results are compared and calibrated with experimental measurements. Using the developed analysis, a rough estimate of the bending radius and virtual focus (image) position of the first crystal can be made, which is also the real source for the second crystal. On the other hand, by measuring the beam height in several points in the SOE-1 hutch, the virtual focus of the second crystal can be estimated. The focusing model was then calibrated with measured mechanical properties, the values for the force and torque applied to the crystals were corrected, and the actual operating parameters of the monochromator for fine-tuning were provided.

Representations in neural network based empirical potentials.

Many structural and mechanical properties of crystals, glasses, and biological macromolecules can be modeled from the local interactions between atoms. These interactions ultimately derive from the quantum nature of electrons, which can be prohibitively expensive to simulate. Machine learning has the potential to revolutionize materials modeling due to its ability to efficiently approximate complex functions. For example, neural networks can be trained to reproduce results of density functional theory calculations at a much lower cost. However, how neural networks reach their predictions is not well understood, which has led to them being used as a "black box" tool. This lack of understanding is not desirable especially for applications of neural networks in scientific inquiry. We argue that machine learning models trained on physical systems can be used as more than just approximations since they had to "learn" physical concepts in order to reproduce the labels they were trained on. We use dimensionality reduction techniques to study in detail the representation of silicon atoms at different stages in a neural network, which provides insight into how a neural network learns to model atomic interactions.

Stratified plastic flow in metals

Experimentally determined possibility to control the localization of plastic flow (shear bands) and the mechanical properties of metals (viscosity) during a plastic deformation process that is relevantly changed, validates questioning the nature of the shear bands, the mechanisms behind their existence and their morphological form.In this paper, indications for a hypothesis, that there are possibilities and certain conditions in which a stratified plastic flow similar to a rigid (dislocation less) shear strain in metallic crystals can appear, are presented. The analysis conducted for the paper was based on experiments indicating the possible dominant role of point defects in structural processes (diffusion) and their influence on the mechanical properties of crystals (viscosity). The dependency between the diffusion coefficient and the point defects concentration and their migration enthalpy as well as the physical dependence of the viscosity coefficient on the diffusion coefficient might explain the low viscosity of the metal with high (in relation to thermodynamic equilibrium) concentration of self-interstitial atoms. It was proved that a high supersaturation of metal with such defects can be obtained either through a mechanically activated reverse dislocation movement or through changing the dominant slip system. The defects, generated as a result of collapse of dislocation dipoles, and their spatial distribution, change the properties of the plane in which they were generated.In other words, a planar zone along the active slip plane supersaturated by self-interstitial atoms with very low migration energy levels (which means their bonds within the lattice are weak) is treated as a layer (stratum), where the inter-planar bond in a crystal is weakened. A high saturation of vacancies in a given layer, which lowers the number of bonds within the first coordination sphere, gives similar results. High local saturation of metal with point defects results in the lattice's lower resistance to shear strain, and therefore, lower viscosity.

Influence of Alternating Magnetic Field on Physical and Mechanical Properties of Crystals

Influence of Alternating Magnetic Field on Physical and Mechanical Properties of Crystals

The effect of LaBr3:Ce single crystal aliovalent co-doping on its mechanical strength

Lanthanum bromide cerium doped single crystals are a transparent scintillator material that offers outstanding scintillation properties with high light yield, excellent energy resolution, fast emission and excellent proportionality. Although properly packaged detectors are robust enough to withstand operating conditions during geophysical oil logging operations, the material itself is brittle and exhibits low fracture toughness. We attempted to modify the composition of the crystals through aliovalent co-doping by Ba, Ca, Hf, Sr, Zn and Zr. These elements have been added in concentrations from 100ppm to 5000ppm to the growth bath. Ratio of co-doping in the crystalline matrices ranged from no incorporation (for Hf, Zn, and Zr) up to 200 ppm for Sr and 10–20ppm for Ca. The effect of the aliovalent co-doping on the mechanical properties of the crystals and in particular on their mechanical strength, hardness and toughness has been measured. As the crystals are extremely hygroscopic this demanded designing customized experiments in anhydrous environment in order to obtain reliable and accurate results. The ultimate strength has been measured by four points bending. Hardness has been measured by indentation and by the same technique we attempted to have some information on the fracture toughness. No improvement of the mechanical properties of the co-doped crystals with respect to the reference standard lanthanum bromide cerium doped could be found.

Dependence of dislocation damping on helium-3 concentration in helium-4 crystals

The mechanical properties of crystals are strongly affected by dislocation mobility. Impurities can bind to dislocations and interfere with their motion, causing changes in the crystal's shear modulus and mechanical dissipation. In 4He crystals, the only impurities are 3He atoms, and they can move through the crystal at arbitrarily low temperatures by quantum tunneling. When dislocations in 4He vibrate at speeds v < 45 μm/sec, bound 3He impurities move with the dislocations and exert a damping force B3v on them. In order to characterize 4He dislocation networks and determine the 3He binding energy, it is usually assumed that B3 is proportional to the concentration of 3He bound to the dislocations. In this preliminary report, we determine B3 in a crystal with 2.32 ppm 3He and compare with our previous measurements of B3 in natural purity 4He crystals to verify the assumption of proportionality.

Modification of structural, optical, thermal and mechanical properties of KDP crystals on the addition of Ni2+

Nonlinear optical materials are acquiring a new significance day by day with the advent of a large number of devices utilizing solid state laser sources. Owing to this technological application KDP having superior nonlinear optical property has been exploited for various applications. KDP crystals were grown from aqueous solutions added with Ni2+. SHG studies confirm that the title exhibits NLO property. The influence of doping Ni2+ on the structural, optical, thermal, NLO and mechanical properties has been studied in this present investigation. It is observed that addition of Ni2+ improves the optical, thermal and mechanical properties of the crystal.

Negative linear compressibility and complex high-Pstructure in Zn[Au(CN)2]2

The structure of the framework material zinc dicyanoaurate is built up of tetrahedrally-coordinated Zn cations linked by linear [NC-Au-CN]- complex anions, which form a six-fold interpenetrated -quartz-type network. This material exhibits strong negative linear thermal expansion along the c direction due to framework flexibility [1]. In situ, single-crystal and powder x-ray diffraction under high pressure in diamond anvil cells indicate that the c lattice parameter increases by 8% from ambient pressure up to 1.8 GPa, which is the highest negative linear compressibility (NLC) known for any material. Structural data obtained from single crystal x-ray diffraction indicate that the NLC mechanism is linked to the closing of N-Zn-N angles and the bending of Au-C-N-Zn linkages. A large number of superlattice reflections are observed above 2 GPa, indicating that the unit cell is doubled in all three directions. This phase transition is accompanied by a strong decrease in NLC. Systematic absences indicate that the space group changes from P6222 to P6422 or vice versa depending on the absolute configuration of the initial crystal corresponding to an instability at one of the L points in the Brillouin zone. The resulting unit cell has the following lattice parameters at 3.6 GPa: a=14.485(2)A, c=45.796(6)A with 528 atoms per unit cell. The ZnAu2 sublattice of the new, high-pressure form was solved by direct methods and followed by the use of difference Fourier maps to locate the C and N atoms. A significant change occurs in the Au sublattice with the Au-Au interatomic vector becoming canted with respect to c. This provides an additional compression mechanism, which reduces the need for the structure to expand along c. These results provide a detailed structural mechanism for the exceptional and unusual mechanical properties of this material. [1] Goodwin, A. L., Kennedy, B. J.. & Kepert, C. J. (2009). J. Am. Chem. Soc. 131, 6334-6335. Keywords: structural phase transitions; high-pressure structure determination; mechanical properties of crystals

Relationship between Tribology and Optics in Thin Films of Mechanically Oriented Nanocrystals

Many crystalline dyes, when rubbed unidirectionally with cotton on glass slides, can be organized as thin films of highly aligned nanocrystals. Commonly, the linear birefringence and linear dichroism of these films resemble the optical properties of single crystals, indicating precisely oriented particles. Of 186 colored compounds, 122 showed sharp extinction and 50 were distinctly linearly dichroic. Of the latter 50 compounds, 88% were more optically dense when linearly polarized light was aligned with the rubbing axis. The mechanical properties of crystals that underlie the nonstatistical correlation between tribological processes and the direction of electron oscillations in absorption bands are discussed. The features that give rise to the orientation of dye crystallites naturally extend to colorless molecular crystals.

The growth and mechanical properties of paratellurite single crystals at high temperatures

α-TeO2 single crystals have been grown by the Czochralski method on specially developed systems with automatic growth control. The mechanical properties of crystals under compression in the [100], [110], and [221] directions at a constant strain rate of about 10−4 s−1 in the temperature range T = 733−993 K have been investigated. The distribution of plastic shear strain in deformed samples is has been experimentally studied. Manifestations of localized strain in covalent paratellurite crystals at T > 900 K are revealed for the first time. The active slip systems along which localized shear band are oriented are determined. The temperature dependence of the critical stress of transition to localized flow is investigated.

Deformation Mechanisms, Microstructure and Mechanical Properties of Nanoscale Crystalline and Noncrystalline Materials in Different Temperature Ranges

ABSTRACTA review of the influence of nanoscale structural elements on the mechanical properties of crystals, quasicrystals, and metallic glasses (MG) is presented. Temperature ranges of cold, warm, and hot deformation are distinguished for crystalline materials, but a nanocrystalline (NC) structure may be formed by severe plastic deformation in the temperature ranges of warm and hot deformation. The plasticity characteristic obtained by indentation can be used for the characterization of low-ductile NC materials. The main features of the plastic deformation mechanisms of NC materials, including results obtained by molecular dynamic simulation, are considered.For MG, the following two problems are discussed: the comparison of the yield stresses for NC and MG and the possibility of strengthening of MG by disperse crystalline nanoscale particles.Quasicrystals with nanosize grains, which are also called nanoquasicrystals (NQC), form a separate class of materials. The mechanical properties of NQC and crystalline materials strengthened by NQC particles are analyzed. Dispersion hardening of metals by NC particles was the first application of nanoscale structures for structural materials. New possibilities of such strengthening are considered.