NaCl Single Crystals Research Articles

Microstructural evolution and mechanical behaviors of rock salt in energy storage: A molecular dynamics approach

The microstructure of rock salt significantly influences its macroscopic mechanical behaviors and deformation phenomena. Understanding the deformation and failure characteristics of rock salt at multiple scales is crucial for the secure and efficient functioning of energy storage in salt caverns. Although the macroscopic behaviors of rock salt are well understood, the microstructural changes occurring under uniaxial compression have not been thoroughly investigated. This study aims to investigate the evolving laws of rock salt microstructure and mechanical properties during the damage process, thereby providing a fundamental understanding of the underlying mechanism. To achieve this, a series of characterization tests on rock salt were conducted to study its microstructure and defects. Building on this, molecular dynamics simulations were utilized in rock mechanics to elucidate the mechanical behaviors, structural evolutions, and dislocation motions of rock salt during the damage process. The rock salt obtained from Qianjiang, China, is a polycrystalline material with an average subgrain size of 46 nm and an average dislocation density of 4.76 × 10−6 1/Å2. Uniaxial compression of single crystal NaCl exhibits three stages: elastic compression, rapid dislocation multiplication, and forest hardening. Scattered dislocations and isolated dislocation tangles trigger further plastic slippage, leading to decreased strength. Later, the formation of a dislocation cell network hinders further slip and reinforces the strength of rock salt. Plastic deformation is found to be a dominant factor in grain refinement and the formation of polycrystalline structures. Intergranular cracking results from cross-patterned dislocation lines on grain boundaries, which become vulnerable areas of the structure. This study describes the evolutionary process of rock salt microstructures and mechanical properties, identifies the micro-destruction mechanisms during the damage process at the ionic level, and provides insights into the micro-mechanical behavior of rock salt and for the design and stability assessment of underground energy storage facilities.

Macroscopic experimental study and microscopic phenomenon analysis of damage self-healing in salt rock

Salt rock is characterized by a low porosity, low permeability and self-healing ability, and it is one of the most common materials used for underground energy storage and construction in underground nuclear waste repositories. Studying the healing of macroscopic cracks in salt rock under different conditions is important for ensuring the stability of the rock surrounding a salt cavern. To evaluate the healing ability and effect of cracks in salt rock at the macroscale, two tests were designed to directly assess the healing ability of salt rock by analyzing the tensile strength and permeability recovery of healed crack surface. Furthermore, the microstructure of the healed macroscopic fractures in salt rock was observed using scanning electron microscopy (SEM), and the microscopic mechanisms of salt rock damage healing were analyzed. (1) The results show that the presence of water is an important condition for the healing of cracks in salt rocks. Fully fractured salt rock cracks can heal at a low pressure of 160 kPa, and the uniaxial tensile strength of the healed specimens can reach a maximum of 121.1 kPa. It was found via computed tomography (CT) that a healed crack was still a weak surface in the whole specimen. (2) Increases in temperature, environmental humidity, and normal stress all promote the self-healing of salt rock fractures. Based on the experimental data, a model describing the evolution of salt rock damage healing variables with respect to temperature, humidity, and stress was established. It was also observed that the change in salt rock damage healing variables over time during the healing process can be represented by a first-order exponential decay function. (3) Using SEM, detailed micro healing experiments were conducted on NaCl single crystal and impurity-containing salt rock specimens. The healing characteristics of intracrystalline cracks and intercrystalline cracks were studied. It was observed that salt rock damage healing manifests at the microscopic level as the filling of microcracks and crack segmentation caused by grain growth. Additionally, the presence of a certain amount of impurities was found to promote the growth of internal structures that heal cracks within salt rock. These findings are of great significance for evaluating the stability and tightness of the surrounding rock of salt cavern reservoirs.

Effect of preparation conditions on the surface morphology and edge absorption of ZnGa2O4:Cr thin films

Thin ZnGa2O4:Cr films were obtained by RF ion-plasma sputtering in argon and argon-oxygen atmospheres mixtures on single-crystal NaCl and amorphous υ-SiO2 substrates. The surface morphology of the films was studied by atomic force microscopy. It was found that during the annealing of ZnGa2O4:Cr films on υ-SiO2 substrates in oxygen, the grains forming the film grow along the film surface, and during annealing in an argon atmosphere, the growth of grains is observed normal to the film surface. The region of the fundamental absorption edge was studied by optical spectroscopy. The variation of the optical band gap of Eg depending on the deposition and annealing atmosphere was determined. The total effective mass of free charge carriers and the concentration of free charge carriers are estimated. It is shown that the shift of the fundamental absorption edge in ZnGa2O4:Cr thin films after annealing in an argon atmosphere and when oxygen is added to the sputtering atmosphere is due to the Burstein-Moss effect.

Acoustic shock wave induced chemical reactions–A case study of NaCl single crystal

Herein, we report the catalytic-free acoustic shock wave-induced synthesis of the non-stoichiometry compound of sodium chloride (Na3Cl2-P4/m) from the stoichiometry NaCl (B1). The shocked NaCl crystal structure is formed as a mixed phase together with the NaCl (B1) and (Na3Cl2-P4/m) thereby the formation of non-stoichiometry compound of Na3Cl2 is confirmed by the diffraction, vibrational, spectroscopic and microscopic techniques. X-ray photoelectron spectroscopic study reveals the positive energy shift of Na 1 s peak which confirms the formation of higher coordination number Na3Cl2 (six to eight) such that it can be considered as Na rich structure. The typical cubic morphology has changed into butterfly wing structure under shocked conditions which is found to be novel, to date. Due to the formation of Na-rich crystal structure, the electrical properties of the shocked crystal are greatly altered. The maximum values of the real part of the impedance of the control and shocked crystals are 1.9 × 107 ohm and 1.0 × 107 ohm, respectively at the lower frequency regions. The present study offers a new platform to synthesize the stable non-stoichiometry NaCl structures under ambient conditions by low pressure acoustic shock waves which could bring about new understanding and implications on NaCl crystals and their applications.

Development of slurry targets for high repetition-rate x-ray free electron laser experiments

Combining an x-ray free electron laser with a high-power laser driver enables the study of equations-of-state, high strain-rate deformation processes, structural phase transitions, and transformation pathways as a function of pressure to hundreds of GPa along different thermodynamic compression paths. Future high repetition-rate laser operation will enable data to be accumulated at >1 Hz, which poses a number of experimental challenges, including the need to rapidly replenish the target. Here, we present a combined shock compression and an x-ray diffraction study on epoxy (50% vol.)-crystalline grains (50% vol.) slurry targets, which can be fashioned into extruded ribbons for high repetition-rate operation. For shock-loaded NaCl-slurry samples, we observe pressure, density, and temperature states within the embedded NaCl grains consistent with observations from shock-compressed single-crystal NaCl.

CONTROL OF THE OPTICAL SURFACE PURITY OF THE ELEMENTS BY THE ELLIPSOMETRIC METHOD

The possibility of controlling the chemical purity of the surface of optical elements by the ellipsometric method has been analyzed. The rationale of the possibility of measuring the parameters of contaminating films on the optical surface of elements by the ellipsometric method has been given simplification has been
 shown of the process of determining the thickness of the contaminating film while expanding the possibility of its measurement on an optical element made of different materials. Ellipsometric studies of freshly polished and used metal mirrors made of copper and copper alloy (zirconium bronze), aluminum and its alloys AMG-6, AL-9, AL-24 have been carried out. Research has also been conducted on elements made of K-8 and
 K-108 (State Standard 3514-94) optical glasses, which are the most typical materials used for manufacture of optical parts for laser technique of visible and near IR-range, from single crystals of NaCl, BaF2 and sapphire (Al2O3). Parameters of contaminating films on the surface of these elements have been measured.
 It has been concluded that it is advisable to use the ellipsometry method during the input (before carrying out physicochemical cleaning) and during the output (after cleaning) control of the optical element to assess the contamination of the optical surface and also for the quantitative analysis of the concentration of contaminants on the optical surface of the elements while working off the technology of their physicochemical cleaning.

Experimental Determination of Polycrystalline Salt Rock Thermal Conductivity, Diffusivity and Specific Heat From 20 to 240°C

Salt rock is recognized as one of the most suitable parent rocks for geological disposal of high level radioactive waste due to its low permeability, good ductility, good thermal conductivity and damage self-healing properties. The thermal conductivity of salt rock directly affects the temperature of disposal storage and surrounding rock, high temperature will lead to a series of problems such as nuclear waste storage tank rupture, mechanical and permeability reduction of surrounding rock. In this paper, the thermal conductivity, specific heat and thermal diffusion coefficient of NaCl single crystal made by crystallization method in the laboratory and polycrystalline salt rock from Khewra salt mine were measured in the range of 22–240°C by the transient plane source method. The results showed that the thermal conductivity and thermal diffusivity of salt rock decrease gradually with the increase of temperature, while the specific heat capacity increases with the increase of temperature. The thermal conductivity of salt rock is slightly lower than that of single-crystal NaCl. The reason for this phenomenon may be that there are a few pores in salt rock. Based on the experimental data, the models of thermal conductivity, thermal diffusivity and specific heat of salt rock with temperature were established. The results can provide a reference for the construction of underground salt rock high-level radioactive waste disposal repository. Based on the thermal conductivity model of polycrystalline salt rock established in this paper, the temperature field evolution during the operation of the underground salt rock high-level nuclear waste repository in 1,000 years was studied. It was found that the temperature of the glass solidified of high-level radioactive waste reached the highest (177.6°C) and then dropped rapidly. The decay heat radiation influence radius of the nuclear waste reaches its maximum in about 50 years of operation of the repository and then gradually decreases.

Dielectric response of rock-salt crystals at finite temperatures from first principles

We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature-dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasiharmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering. Our model fully captures the linear dielectric response of a crystal at finite temperatures and may therefore be used to calculate the temperature dependence of other material properties governed by it.

Electron Diffraction Study of Phase Formation in Nano Layers of Cu-=SUB=-2-=/SUB=-Te-In-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=-, Cu-In-Te Systems and Short-Range Atomic Order in Amorphous CuIn-=SUB=-5-=/SUB=-Te-=SUB=-8-=/SUB=- Films

It is shown that with the simultaneous and sequential deposition of films of the Cu2Te-In2Te3 system, as well as copper, indium, and tellurium of the highest purity used as starting materials, ~99.999%, taken in the ratio Cu : In : Te = 1 : 5 : 8, regardless of the deposition order ternary compounds of the compositions CuInTe2, CuIn3Te5, and CuIn5Te8 in the crystalline state are distinguished. During vacuum condensation of films on single-crystal substrates NaCl, KCl and amorphous celluloid cooled by liquid nitrogen to 203 K, the resulting films obtained both by co-evaporation of binary compounds of the Cu2Te-In2Te3 system and by the synthesis of thin layers applied by Cu, In, Te are amorphous. For the first time in nanosized amorphous films of the composition CuIn5Te8, crystallizing in the tetragonal system with the periods of unit cells a=6.162 Angstrem, c=12.291 Angstrem, obtained both under normal conditions and under the influence of an external electric field with a strength of 500 V · cm-1, the structure of the short-range atomic order is established --- the number of nearest neighbors --- coordination numbers and radius of coordination spheres. It was revealed that in amorphous CuIn5Te8 films obtained under the influence of an external electric field, in which the matrices consist of tetrahedral and octahedral environments of atoms, in opposite to films that are formed outside the field, the number of nearest neighbors remains unchanged, the radius of coordination spheres and interatomic distances are somewhat are shortened. Keywords: phase formation, elektronogram, radial distribution function of atoms (RDFA), coherent electron scattering.

Application of Particular Solutions of the Burgers Equation to Describe the Evolution of Shock Waves of Density of Elementary Steps

Particular solutions of the Burgers equations (BE) with zero boundary conditions are investigated in an analytical form. For values of the shape parameter greater than 1, but approximately equal to 1, the amplitude of the initial periodic perturbations depends nonmonotonically on the spatial coordinate, i.e. the initial perturbation can be considered as a shock wave. Particular BE solutions with zero boundary conditions describe a time decrease of the amplitude of initial nonmonotonic perturbations, which indicates the decay of the initial shock wave. At large values of the shape parameter , the amplitude of the initial periodic perturbations depends harmoniously on the spatial coordinate. It is shown that over time, the amplitude and the spatial derivative of the profile of such a perturbation decrease and tend to zero. Emphasis was put on the fact that particular BE solutions can be used to control numerical calculations related to the BE-based description of shock waves in the region of large spatial gradients, that is, under conditions of a manifold increase in spatial derivatives. These solutions are employed to describe the profile of a one-dimensional train of elementary steps with an orientation near <100>, formed during the growth of a NaCl single crystal from the vapor phase at the base of a macroscopic cleavage step. It is shown that the distribution of the step concentration with distance from the initial position of the macrostep adequately reflects the shock wave profile at the decay stage. The dimensionless parameters of the wave are determined, on the basis of which the estimates of the characteristic time of the shock wave decay are made.

Preparation of low cost NaCl single crystal for IR optical window applications

Preparation of low cost NaCl single crystal for IR optical window applications

Effect of Pr ion concentration on the luminescence properties of NaCl:0.01Ce3+,Pr3+ crystals grown in large size

Large size single crystals of NaCl:Ce3+,XPr3+ (with X = 0.0, 0.002, 0.005, 0.008, 0.02, 0.05 and 0.08) were successfully grown by the melt (Czochralski) method and characterized structurally and optically. X-ray diffraction analysis has revealed the incorporation of Ce3+ and Pr3+ ions into the NaCl matrix and the creation of vacancy centers (VʺNa) and positive defects (CeNa●● and PrNa●●) to keep the charge balance. Optical absorption spectral analysis has indicated the presence of absorption peaks at 214, 225, 305, 342, 446, 469, 483 and 592 nm. Photoluminescence (PL) spectral analysis (excited with 259 nm) has revealed dominant emission peaks at 386 and 390 nm due to 5d→4f transition of Ce3+ ions and sharp lines at 479, 491, 532, 600 and 621 nm due to 3P1→3H4, 3P0→3H4, 3P1→3H5, 3P0→3H6 and 3P0→3F2 transitions of Pr3+ ions. The energy transfer mechanism from Ce3+ to Pr3+ in the NaCl crystal has been described and the color coordinates of the luminescence of NaCl:Ce3+,XPr3+ crystals have been calculated by using the emission spectral data. The thermo-luminescence (TL) glow curves of NaCl:Ce3+,XPr3+ crystals obtained show three peaks at about 83, 150 and 239 °C, suitable trap depths induced by Ce3+ and Pr3+ and resulted in the characteristic emission. The TL mechanism of NaCl:Ce3+,XPr3+ crystals has been discussed. The photoluminescence and thermo-luminescence performances have been found to depend strongly on the Pr ion concentration and the maximum intensity has been obtained for the crystal with X = 0.008. Also, the increase in Pr ion concentration has led to gradual decrease of luminescence decay time.

Characterization of optically-stimulated luminescence properties by NaCl:Eu2+ crystal and the thermal response

In this paper, we report optically-stimulated luminescence (OSL) properties of Eu2+-doped NaCl single crystal as a candidate phosphor for use in near real-time radiation measurement and dosimetry. The crystal was grown by the vertical Bridgman–Stockbarger method. The obtained crystal shows a strong photoluminescence (PL) band peaking at 430 nm due to the parity-allowed 5d-4f transitions of Eu2+. After irradiating with X-rays, it demonstrates OSL also at 430 nm due to the Eu2+ ion acting as an emission center while stimulating by light (500 ± 30 nm). Compared with a commercial BaFBr:Eu2+ OSL detector, the sensitivity to X-ray dose is lower by about one order of magnitude. In addition, while the crystal is reused multiple times, the OSL response signal increases despite the same given radiation dose. The OSL decay curve can be decomposed into three exponential decay functions with the lifetimes of 0.6, 6.4, and 120.5 s, and the decomposition analyses revealed that only the intensity of intermediate component increases with the number of reused cycles. Further analyses with thermally-stimulated luminescence (TSL) revealed that the intermediate OSL component is related to a TSL glow signal originated from trapping centers having the activation energies of ~0.93 eV and ~1.27 eV and the frequency factors of ~7.52 × 108 s−1 and ~6.29 × 1011 s−1, respectively.

The orientation relationships and interfaces of θ-Al2Cu/Cu and γ2-Al4Cu9/Cu and the formation sequence of θ-Al2Cu and γ2-Al4Cu9 at the Cu/Al interface

The orientation relationships and interfacial planes of the theta (θ)-Al2Cu/Cu and gamma2 (γ2)-Al4Cu9/Cu interfaces formed on the Cu (001) and (111) surfaces have been studied by transmission electron microscopy (TEM). Epitaxial thin Cu films are grown on the single-crystal NaCl surfaces and then Al is evaporated to form θ-Al2Cu and γ2-Al4Cu9. Results showed that the θ-Al2Cu first formed and then the γ2-Al4Cu9 formed. The orientation relationships at the Al/Cu interface were found: (1) [110]θ//[111]Cu (zone axis) and (1¯10)θ //(1¯1¯2)Cu on the (111)Cu surface, (2) the (110)γ2 surface is the interface with both the Cu (001) and (111) surfaces with a common orientation relationship of (1¯11)γ2//(11¯0)Cu, and (3) [110]θ//[110]γ2 (zone axis) and(1¯10)θ//(1¯12¯)γ2 on the (111)Cu surface. The θ/Cu, γ2/Cu, and γ2/θ interfaces were further analyzed. The γ2-Al4Cu9 phase has good orientation relationships to Cu and therefore low interfacial energies. In addition, the energy of formation of γ2-Al4Cu9 is about 1.2 times than that of eta (η)-AlCu. Therefore γ2-Al4Cu9 nucleates first after θ-Al2Cu.

Frustration of the dislocation density in NaCl and its implication on “Harper-Dorn” creep

Static annealing of (100)-oriented NaCl single crystals at a homologous temperature of ~0.92 showed a significant reduction in the etch-pit dislocation density during the first 100 h, which remained constant during subsequent annealing up to 225 h. The existence of the frustration of the dislocation density, at a value of ~109 m-2, in NaCl is similar to the observations in the high purity Al and LiF single crystals. The implication of the frustration dislocation density on the observation of a creep stress exponent of >1 in the Harper-Dorn creep regime is discussed.

Elasticity of single-crystal NaCl under high-pressure: simultaneous measurement of x-ray inelastic scattering and diffraction

ABSTRACT Elastic stiffness constants, C ijs, of single crystal of NaCl B1 phase have been determined at pressure up to 20 GPa using inelastic x-ray scattering to measure elastic wave velocities and x-ray diffraction to measure densities in the same experimental setup. The Voigt-Reuss-Hill average velocities calculated from the present Cijs are in good agreement with previous ultrasonic measurements of polycrystalline samples. Absolute pressures based on the present elastic constants and densities are proposed. In order to fit our results an equation of state with at least three parameters (an addition to a reference volume), such as the fourth-order form of Birch–Murnaghan equation or the higher order form of Vinet equation, is required, as fewer parameters do not reproduce the observed pressure dependence of the volume and bulk modulus.

Control of Radiation Defects and Na Clusters in the Process of Radiation Degradation of Natural Stone Salt Single Crystals

Using the methods of positron annihilation and optical spectroscopy, the effect of gamma (Co60) and electron irradiation (Cockcroft–Walton accelerator) on NaCl single crystals, followed by annealing at various temperatures. It was found that electron color centers are effective traps of positrons diffusing in the lattice. At the same time, hole centers do not capture positrons. It was shown that positrons are captured by sodium clusters formed in the bulk of NaCl crystals upon annealing (443 K) of irradiated (2650 Mrad) samples. The annihilation characteristics of captured positrons allow us to estimate size of sodium clusters R = 23.0 nm and their concentration Nx = 5.3 × 1017 cm–3. Taking into account the fact that modern optical and positron spectrometers are quite compact and sensitive instruments, the monitoring of the processes of radiation degradation of the geological rock of rock salt can be carried out directly at the disposal sites of radioactive waste rather quickly and with the necessary degree of sensitivity to the accumulation of radiolytic products.

Influences of quantum fluctuation on superfluorescent spectra observed by single-shot measurement for semiconductor quantum dots

Fluctuations in superfluorescence were evaluated for a CuCl quantum dot assembly in a NaCl single crystal by measuring single superfluorescent spectra. The distributions of the spectral widths and peak intensities of the individual spectra were obtained as a function of the excitation density. The result showed that fluctuations decrease with an increase in excitation density due to the rapid establishment of coherent coupling between the quantum dots, which suppresses the fluctuation in superfluorescence. Based on the results of our study, influences of quantum fluctuations on the generation of macroscopic coherence can be evaluated.

Analysis of the Dynamic and Structural Characteristics’ Behavior in NaCl Single Crystals Pre-deformed and Х-ray Irradiated

Analysis of the Dynamic and Structural Characteristics’ Behavior in NaCl Single Crystals Pre-deformed and Х-ray Irradiated

Synthesis of Lithium Niobate during Crystallization of Amorphous Li–Nb–O Film

The structural and phase changes induced by heat treatment (HT) and brief lamp processing (LP) in 70- and 1500-nm-thick Li–Nb–O films similar in elemental composition to LiNbO3, grown on the surface of unheated substrates (monocrystalline Si wafer and NaCl single crystal) by rf magnetron sputtering and ion-beam sputtering of a LiNbO3 target have been studied by reflection high-energy electron diffraction and X‑ray diffraction. The as-grown films were extremely nanostructured and consisted of crystalline lithium oxides, niobium oxides, and LiNbO3 as the product of reaction between them. We have demonstrated the conceptual feasibility of driving the synthesis of polycrystalline LiNbO3 films to completion using HT or brief LP. In the case of LP, LiNbO3 synthesis is three orders of magnitude faster than in the case of HT.