Classification of Mechanoluminescence

On the basis of the processes involved in inducing luminescence, mechanoluminescence (ML) may be classified into different types, such as: (i) piezo-electrification-induced fracto ML, (ii) defective-phase piezo-electrification-induced fracto ML, (iii) charged dislocation electrification-induced fracto ML, (iv) baro diffusion electrification-induced fracto ML, (v) chemically-induced fracto ML, (vi) thermally stimulated fracto ML, (vii) incandescent fracto ML, (viii) stress perturbation-induced fracto ML, (ix) thermal excitation-induced fracto ML, (x) mechanically excited ML, (xi) deep trap-induced fracto ML, (xii) dislocation mechanical interaction-induced plastico ML, (xiii) dislocation electrostatic interaction-induced plastico ML, (xiv) charged dislocation electrification-induced plastico ML, (xv) thermal excitation-induced plastico ML, (xvi) dislocation mechanical interaction-induced elastico ML, (xvii) dislocation electrostatic interaction-induced elastico ML, (xviii) thermal excitation-induced elastico ML, (xix) electrically induced tribo ML, (xx) chemically-induced tribo ML, (xix) electrically induced tribo ML, (xx) chemically-induced tribo ML, and (xxi) thermally-induced tribo ML. It is expected that the processes involving dislocation electrostatic interaction-induced elastico ML and thermal excitation-induced elastico ML may provide suitable ML materials in future.     

Cleavage Mechanoluminescence in Crystals

When a crystal is cleaved, initially the mechanoluminescence (ML) intensity increases linearly with time, attains an optimum-value I_m at a particular value of timet_m, and then decays exponentially with time. Cleavage ML provides a new tool to determine the velocity, v of cracks in crystals, and it may be given by v = H/t_m, where H is the thickness of the crystal. Both, the peak ML intensity I_m and total ML intensity I_T increase linearly with the area of newly created surfaces A as well as with the surface charge density γ. The ML intensity decreases with temperature primarily due to the decrease in the surface charge density. Beyond a particular temperature, the surface charge density may decrease to such a value where the breakdown of gases and solids may not be possible and thereby the ML may not appear. Depending on the prevailing conditions either the ML emission resembling gas discharge or other types of the luminescence of solids, or that having these two characters may be obtained. There exists a good correlation between the theoretical and experimental results obtained for cleavage ML in crystals.     

Mechanoluminescence in Centrosymmetric Crystals

The paper reports that the mechanoluminescence (ML) is not an inherent property of only the non-centrosymmetric crystals. The ML may appear in number of centrosymmetric crystals due to variety of processes. The ML of 82 centrosymmetric crystals are reported and different models are proposed for the ML excitation. The models proposed are: space charge electrification model, triboelectrification model, phase transformation model, gas adsorption model, chemical reaction model, thermal population model, molecular deformation model, cleavage electrification model, defective piezoelectric phase model, dislocation defect stripping model, dislocation unpinning model, dislocation annihilation model, charged dislocation model and incandecent light emission model. It is shown that on the basis of the proposed model, intense mechanoluminescent materials with desired nature and characteristics may be prepared.     

Mechanoluminescence and acoustical emission

It is reported on the measuring of the synchronic mechanoluminescent (ML) and acoustical emission (AE). The source of both ML and AE was the mechanical deformation of organic glass, of steel and copper plates. The results confirmed the connection between MA and AE stimulated by the same deformation source.     

Satatistical Model of Mechanoluminescence in Crtstals

The present paper reports on a statistical model of the mechanoluminescence (ML) excitation in crystals where attempts have been made to explain stress, temperature-, strain rate-, activator concentration dependence and several other aspects of ML. It is found that ML emission should take place only during the period at which stress will change with time. The total ML intensity should be directly proportional to the square of the stress and there should be phase difference between the ML pulse and the applied stress pulse. It is found that the ML intensity should decrease faster with temperature as compared to the corresponding photoluminescence intensity. within the limit of concentration quenching the ML intensity should increase directly with the defect centre concentration. The ML intensity should increase with the stress rate or strain rate of the solids. Furthermore, the concept of mechanoluminescent and non-mechanoluminescent materials is explored.     

Combinatorial analysis of crystals

The alphabetical, specific, partial, and complete codes of vertices are determined for diamond crystals and a number of Schlegel projections. The codes obtained are compared with the codes of the types of atomic environment.     

Nucleation of protein crystals

Protein crystal nucleation is a central problem in biological crystallography and other areas of science, technology, and medicine. Recent studies have demonstrated that protein crystal nuclei form within crucial precursors. Data for several proteins provided by these methods have demonstrated that the nucleation precursors are clusters consisting of protein dense liquid, which are metastable with respect to the host protein solution. The clusters are several hundred nanometers in size, they occupy from 10⁻⁷ to 10⁻³ of the solution volume, and their properties in solutions supersaturated with respect to crystals are similar to those in homogeneous, i.e., undersaturated, solutions. The clusters exist due to the conformation flexibility of the protein molecules, leading to the exposure of hydrophobic surfaces and enhanced intermolecular binding. These results indicate that protein conformational flexibility might be the mechanism behind the metastable mesoscopic clusters and crystal nucleation. The investigations of the cluster properties are still in their infancy. Results on direct imaging of cluster behaviors and characterization of cluster mechanisms with a variety of proteins will soon lead to major breakthroughs in protein biophysics.     

Microplasticity of CsI crystals

The study of the mechanisms of plastic deformation of CsI crystals has found the participation of not only the main {110} 〈100〉 slip system but also of the secondary one {110} 〈110〉. Besides that, production and motion of point defects (or small prismatic loops) take place. The gliding on secondary slip system and the deformation accounted for by the generation and motion of point defects is facilitated at low temperatures and high deformation rates. The character of the motion and multiplication of dislocations in the main slip system is investigated. From the temperature and stress dependence of the mobility of isolated dislocations quantitative data on the thermally activated motion of edge dislocations on the main slip system have been obtained. It is shown that, as in the case of other alkali halides, the thermally activated motion of edge dislocations in CsI crystals on {110} 〈100〉 system is limited by their interaction with local obstacles.     

Sublattices in crystals

A method of representation of a crystal structure as a set of its constituent Bravais sublattices is developed. The conditions for compatibility of sublattices related to the same or different systems are formulated and the connection matrices for primitive parallel-translation vectors of the crystal lattice and the sublattices are determined. A method of alignment of the first Brillouin zones of the sublattices with the first Brillouin zone of the crystal is described. It is shown that alignment may lead to quasi-degeneration of the energy levels in the case of weak hybridization of the sublattice states. A relationship between the sublattice method and the method of an extended unit cell is established.     

Ain single crystals

Single crystals of aluminum nitride up to 1 cm long and 0.3 in diameter have been grown by a sublimation-recondensation technique at about 2250°C. The starting material is prepared by the direct reaction of aluminum and nitrogen at 1850°C. The crystals are grown at a rate of 0.03 cm/hr. in sealed tungsten crucibes in an rf heated tungsten furnace. They are amber in color and have the wurtzite structure.     

Iso-epitaxial growth of n-octacosane crystals

Iso-epitaxial(epilayer)growth of n-octacosane crystals is reported. The morphology of the epilayers is in the form of triangular growth islands with or without truncature. Epilayer growth precedes spiral growth mechanism which occurs at medium and low supersaturations, respectively. The occurrence of triangular and truncated rhombic platelets is found to be a manifestation of growth at medium supersaturations.     

Growth of semiconductor silicon crystals

This paper focuses on the recent developments in Czochralski (CZ) crystal growth of silicon for large-scale integrated circuits (LSIs) and multi-crystalline silicon growth using a directional solidification method for solar cells. Growth of silicon crystals by the CZ method currently allows the growth of high-quality crystals that satisfy the device requirements of LSIs or power devices for electric cars. This paper covers how to obtain high-quality crystals with low impurity content and few point defects. It also covers the directional solidification method, which yields crystals with medium conversion efficiency for photovoltaic applications. We discuss the defects and impurities that degrade the efficiency and the steps to overcome these problems.     

Polymorphism of edible fat crystals

The course focuses on the polymorphism and polymorphic transformation of edible fat crystals, such as chocolate. The morphology, crystallization behavior and polymorphic transformation will be observed under optical microscopy, and melting point of each polymorph will be determined.     

Crystallization mechanisms of acicular crystals

In this contribution, we present an experimental investigation of the growth of four different organic molecules produced at industrial scale with a view to understand the crystallization mechanism of acicular or needle-like crystals. For all organic crystals studied in this article, layer-by-layer growth of the lateral faces is very slow and clear, as soon as the supersaturation is high enough, there is competition between growth and surface-activated secondary nucleation. This gives rise to pseudo-twinned crystals composed of several needle individuals aligned along a crystallographic axis; this is explained by regular over- and inter-growths as in the case of twinning. And when supersaturation is even higher, nucleation is fast and random.In an industrial continuous crystallization, the rapid growth of needle-like crystals is to be avoided as it leads to fragile crystals or needles, which can be partly broken or totally detached from the parent crystals especially along structural anisotropic axis corresponding to weaker chemical bonds, thus leading to slower growing faces. When an activated mechanism is involved such as a secondary surface nucleation, it is no longer possible to obtain a steady state. Therefore, the crystal number, size and habit vary significantly with time, leading to troubles in the downstream processing operations and to modifications of the final solid-specific properties.These results provide valuable information on the unique crystallization mechanisms of acicular crystals, and show that it is important to know these threshold and critical values when running a crystallizer in order to obtain easy-to-handle crystals.     

The cracking of Czochralski-grown crystals

Crystals of brittle materials are easily cracked during manufacture and processing. This paper examines the thermal strains existing in crystals during and after pulling and the strains which can be built-in by the facet effect. It is shown that during the growth of a crystal with a radius R, there is a maximum acceptable axial gradient roughly proportional to R^?1.5 and that to prevent cracking after growth there is a maximum rate of cooling proportional to R^?2. Results obtained with bismuth silicon oxide for which all the relevant parameters (breaking strain, expansion coefficient, thermal diffusivity and cooling constant) have been measured are in accord with these relations at least for 4 <R<23 mm. The analysis seems valid for other materials (e.g. lithium niobate) with small breaking strains and negligible plastic ranges. For bismuth silicon oxide, (110) and (100) facets have lattice constants larger than the bulk of the crystal by about 4 and 6 parts in 10⁵ respectively. The edges of these facets are thus regions which are strained by amounts corresponding to 20 or 30% of the breaking strain. Under normal growth conditions, this strain is not particularly significant, but it does affect post-growth handling and during rapid growth, interface instabilities develop first at the facet edges.     

Optical parameters of paratellurite crystals

Refractive indices and transmittances of paratellurite single crystals have been measured. The specific optical rotation is determined and the Verdet constants are calculated for a radiation wavelength of 355 nm.     

Dendritic growth of snow crystals

The hypothesis that allows the interpretation of dendritic growth of a snow crystal in terms of diffusion-limited aggregation is criticized. The results of simulation of growth of quasi-two-dimensional crystals in two-and three-dimensional media based on the classical two-parametric model of diffusion-limited aggregation are used as an argument in this criticism. It is established that the model dimensionality considerably influences morphology of the grown crystal. The mechanism of dendritic growth of a snow crystal in which the main part is played by the surface processes at the ice/water interface is suggested.