Effect of neutron irradiation on stratigraphy in calcium fluoride single crystals

Natural single crystals of calcium fluoride have been cleaved along {111} planes and a number of matched pairs have been obtained. One piece from etch pairs have been irradiated with thermal and pile neutrons, the flux varying from 1.3 × 10⁴ to 3.0 × 10¹³ neutrons cm⁻². After irradiation, the irradiated and their non-irradiated counterparts have been matched. They were then etched simultaneously in 0.2 N hydrochlorid acid for the same period. The stratigraphical etch patterns produced have been studied optically. There is no exact correspondence of the stratigraphical etch patterns have been observed on the irradiated and non-irradiated matched pairs. The disappearing in the stratigraphical pattern of the irradiated crystals increases with increase in the dose of irradiation. The implications are dicussed.     

Oriented growth of large size calcium fluoride single crystals for optical lithography

We report (1 1 1), (1 1 0) and (1 0 0) growth of CaF₂ by the vertical Bridgman method. Crystals up to 250 mm diameter were grown and various growth parameters such as growth rate, temperature gradient and post-growth cooling rate were studied. It was found that the growth rate and the cooling rate are slower for the larger diameter crystals with a fixed temperature gradient. These growth parameters were optimized for growing the crystals along specific orientation after realizing that CaF₂ has a tendency to grow along an orientation close to 1 1 0. Degradation in optical transmittance was evaluated by irradiating the crystal to γ-rays up to a dose of 10⁵ rad. Optimized scavenger addition resulted in crystals with better radiation resistance and excellent VUV transmittance.     

Control of oxygen contamination during the growth of optical calcium fluoride and calcium strontium fluoride crystals

The progress, current state, and prognosis for CaF₂ as an optical material with wide applicability are reviewed by outlining the basic issues: technology, demand, supply, and metrology. To achieve perfect quality for all application grades of CaF₂ grown using a natural fluorite, investigations of the effect of PbF₂ and ZnF₂ scavengers for removal of oxygen contaminants from the starting material, representing chemically purified fluorspar, either granular or as dense precursors is reported. The chemical and crystal-chemical reactions occurring during the crystallization due to the presence of oxygen contaminants, scavengers, and rare-earth impurities are discussed based on data for the composition of the residual atmosphere in the reactor chamber. The water vapors have been shown to have a major impact in deteriorating the optical properties of the grown crystals. The use of precursors, together with 0.1n% ZnF₂ additive, is shown to be highly recommendable for the production of ultraviolet and vacuum ultraviolet grades of CaF₂ and Ca_1−xSr_xF₂ crystals. For visible and infrared grades PbF₂ appears to be a better additive than ZnF₂. The technique for growth of Ca_1−xSr_xF₂ in an argon flow with a 5 nines inlet purity has proved to be suitable for producing high-quality crystals, whose optical properties satisfy the demands of micro-photolithography provided that precise control over the total pressure inside the crucible is implemented.     

Etching behaviour of calcium fluoride crystals in hydrochloric acid vapour

Etching of octahedral cleavage faces of calcium fluoride crystals in hydrochloric acid vapour has been investigated. It is observed that dislocation etch pits and non-dislocation elevations are formed. The morphology of etch pits obtained by vapour etching is observed to be different from that obtained by weak acid solutions. The results are discussed in terms of the prevailing etching conditions.     

X-ray topographic investigation of growth defects in natural calcium fluoride crystals

Calcium fluoride crystals have been cleaved along {111} faces and the freshly cleaved faces have been chemically etched in 2.6 N nitric acid solution. The etched faces have been optically studied. One-to-one correspondence of dislocation etch pits have been established on the matched cleavage faces and on the opposite sides of thin flakes. The same crystal has been studied using X-ray topographic technique. By using 022 - and 02 2 reflections, stereopair projection topographs were studied and compared with the optical micrographs. The close resemblance between dislocation etch pits and dislocation out-crop images has been established. The orientation of the Burgers vectors of the dislocation lines has been identified using AgKα radiation with 111 -, 11 1- and 1 11 reflections. It has been confirmed that Burgers vectors of dislocation lines lie parallel to 〈110〉 directions. The implications are discussed.     

Macroscopic localization of plastic flow in zinc single crystals oriented for basal slip

The patterns of plastic-flow localization in hcp Zn single crystals oriented for slip in the {0001} 〈2110〉 systems are investigated. The main spatial and temporal regularities of the flow localization are established for all portions of the three-stage curve of such crystals. The relationship between the type of localization patterns and the regularities of strain-hardening of single crystals is traced for each stage. The role of the kinking in the formation of the observed macroscopic-flow distributions is estimated.     

Birefringence simulation of annealed ingot of calcium fluoride single crystal

We developed a method for simulating birefringence of an annealed ingot of calcium fluoride single crystal caused by the residual stress after annealing process. The method comprises the heat conduction analysis that provides the temperature distribution during the ingot annealing, the elastic thermal stress analysis using the assumption of the stress-free temperature that provides the residual stress after annealing, and the birefringence analysis of an annealed ingot induced by the residual stress. The finite element method was applied to the heat conduction analysis and the elastic thermal stress analysis. In these analyses, the temperature dependence of material properties and the crystal anisotropy were taken into account. In the birefringence analysis, the photoelastic effect gives the change of refractive indices, from which the optical path difference in the annealed ingot is calculated by the Jones calculus. The relation between the Jones calculus and the approximate method using the stress components averaged along the optical path is discussed theoretically. It is found that the result of the approximate method agrees very well with that of the Jones calculus in birefringence analysis. The distribution pattern of the optical path difference in the annealed ingot obtained from the present birefringence calculation methods agrees reasonably well with that of the experiment. The calculated values also agree reasonably well with those of the experiment, when a stress-free temperature is adequately selected.     

Kinetics of dissolution and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

The effect of temperature (20–100°C) and concentration of H₂SO₄ etching solutions on the etching behaviour of {111} faces of calcium fluoride crystals is investigated. Whisker growth of calcium sulphate is observed. Temperature of the etchant also has an effect on their formation. The kinetics of dissolution at the sites of dislocations and general dissolution have been studied. Studies on induction period and its dependence on the concentration of the acid and temperature are also described. It is observed that: (i) at low and high acid concentrations, the dissolution is diffusion controlled while it is predominantly reaction-rate controlled in the intermediate concentration range and (ii) the growth rate of calcium sulphate whiskers decreases with time and their induction period decreases with increase in the temperature of the solution. The implications are discussed.     

Etch pit morphology and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

An investigation of the etching behaviour of cleaved octahedral faces of calcium fluoride crystals in sulphuric acid of different concentrations and at various temperatures is carried out. It is observed that the morphology of dislocation etch pits depends on the temperature and concentration of the acid. With an increase in the acid concentration, growth of whiskers, elongated platelets, hexagonal and dice-shaped elevations, spherulites and sheaves is observed. Temperature of the etchant also has an effect on their formation. By X-ray diffraction techniques, the compounds forming the whiskers and elongated platelets, and hexagonal and dice-shaped features are identified as CaSO₄ · 2 H₂O and CaSO₄, respectively. The mechanism of the growth of CaSO₄ · 2 H₂O and CaSO₄ and the change in the morphology of selective etch pit are discussed.     

A method for the determination of the cyclic stress-strain curve of presistent slip bands in fatigued single crystals

Single crystals of pure nickel oriented for single slip were fatigued at constant total strain amplitude (ε_at) at room temperature. The dependence of the saturation resolved shear stress amplitudes (τ_as) on the saturation plastic resolved shear strain amplitudes (γ_aps) (cyclic stress-strain curve — CSSC) was determined. In the plateau range of the CSSC the volume fraction f of persistent slip bands (PSBs) was found to be linearly related to γ_aps. The reduction of the total strain amplitude after saturation in the plateau range of the CSSC leads to the so-called secondary CSSC with a slope n > 0 in the log-log plot. From the secondary CSSC the ”︁true”︁ cyclic stress-strain curve of the PSB-volume (PSB-CSSC) can be determined. The method is based on the two assumptions that firstly the PSB-groups (or ”︁macrobands”︁) penetrate the whole cross-section of the specimen and secondly the volume fraction of PSBs in the secondary range of the CSSC remains constant.     

Defects in calcium tungstate crystals

The role of defects formed owing to the deviations from stoichiometry is analyzed for CaWO₄ single crystals grown by the Czochralski method. It is shown that the defects leading to the light scattering in CaWO₄ crystals affect the spectra of X-ray luminescence and the luminescence kinetics only slightly, but give rise to an additional peak in the thermoluminescence spectrum. Addition of WO₃ to the melt eliminates the light scattering and the additional thermoluminescence peak.     

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.     

Growth and characterization of calcium hydrogen phosphate dihydrate crystals from single diffusion gel technique

Calcium hydrogen phosphate dihydrate (CaHPO₄·2H₂O, CHPD) a dissolved mineral in urine is known to cause renal or bladder stones in both human and animals. Growth of CHPD or brushite using sodium metasilicate gel techniques followed by light and polarizing microscopic studies revealed its structural and morphological details. Crystal identity by powder x‐ray diffraction confirmed the FT‐IR and FT‐Raman spectroscopic techniques as alternate methods for fast analysis of brushite crystals which could form as one type of renal stones. P‐O‐P asymmetric stretchings in both FT‐IR (987.2, 874.1 and 792 cm^‐1) and FT‐Raman (986.3 cm^‐1, 1057.6 cm^‐1 and 875.2 cm^‐1) were found as characteristics of brushite crystals. Differential Scanning Calorimetry (DSC) analysis revealed brushite crystallization purity using gel method by studying their endothermic peaks. This study incorporated a multidisciplinary approach in characterizing CHPD crystals grown in vitro to help formulate prevention or dissolution strategy in controlling urinary stone growth. Initial studies with 0.2 M citric acid ions as controlling agent in the nucleation of brushite crystals further support the presented approach. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Point defects isomerism in lithium fluoride crystals and nanocrystals

It has been established that the intrinsic radiation defects F₃ in lithium fluoride crystals and nanocrystals exist in three different configurations (isomers). It is found that at influence on a sample by a radiation with a wavelength that falls in the absorption band of one of the isomers this isomer is transformed to another one. After treatment cessation the equilibrium ratio of concentrations for these isomers is restored. The value of the energy barrier separating equilibrium states of isomers and the contribution of entropy in this barrier are defined.     

Impact damage in MgO single crystals

Optical absorption has been used to study the impact damage in MgO and MgO:Li single crystals. The results are similar to that obtained in compressed crystals, except in MgO:Li with stable Li° defects, in which the impacts do not produce an increase of the 5.7 eV absorption band.     

Fluctuations of impurity composition and their role in dislocation cross slip in crystals

The influence of impurities on the kinetics of split-dislocation cross slip caused by a change in the stacking-fault energy is studied theoretically. It is shown that the fluctuations in the impurity composition of a crystal make a considerable contribution to the kinetics of dislocation cross slip. The activation-energy spectrum and the average frequency of the processes of dislocation cross slip are calculated for a model of random impurity distribution in a crystal. The calculation shows that the fluctuations in impurity concentration, reducing the stacking-fault energy, play an important role in the low-temperature region.