Lithium Fluoride Single Crystals Research Articles

Ultrafast and nanosecond laser-induced desorption of positive ions from lithium fluoride single crystals

Ultrafast and nanosecond laser-induced desorption of positive ions from lithium fluoride single crystals

Precursor decay anomaly in single-crystal lithium fluoride

The purpose of this study is to reduce the precursor decay anomaly in single-crystal lithium fluoride (LiF) using a macroscopic approach. To this end, a method of predicting the evolving unsteady plane wave fronts created in the crystal upon impact is developed. Parameters included in modeled strain waves in the fronts are determined such that the predicted particle velocity-time history at the impact surface fits the detector current at the LiF-quartz interface measured by Asay et al. [J. Appl. Phys. 43, 2132 (1972)]. Another condition used is that the particle velocity-time histories at and near the surface are initially parallel. It is assumed that when the amplitude of a near-steady precursor in the predicted unsteady wave front, which increases from a static yield stress, becomes a maximum, a kink occurs at the rear of the precursor and then it begins to decay. The precursor decay curves estimated, based on this assumption, are much lower than Asay’s decay curve. These lower curves are expected to reduce significantly the precursor decay anomaly in this crystal.

Shock jump equations for unsteady wave fronts of finite rise time

First, generalized Rankine–Hugoniot equations for unsteady wave fronts of finite width are derived. It is clarified from these jump equations for particle velocity, stress, and specific internal energy that shock jump conditions involve the effects of rise time of the front or the change in its velocity with time, in addition to that of strain rate and acceleration, and that the treatment in the previous study [Sano, J. Appl. Phys. 82, 5382 (1997)] where the rise time is reduced to an infinitesimal eliminates the terms of this change in the jump equations. Furthermore, the jump equations of the general form derived here support this elimination. Next, the influence of the rise time on the three jumps at the impacted surface of a lithium fluoride single crystal is calculated based on its experimental data and shown to be negligibly small. However, its influence, calculated for sandstone in a similar manner, is great. Finally, a parametric investigation of the influence is carried out for specific strain waves.

Radiation defects in lithium fluoride induced by heavy ions

Single crystals of lithium fluoride were irradiated with various species of heavy ions in the energy regime between 1 and 30 MeV/u. The induced radiation damage was studied with techniques such as optical absorption spectroscopy, small-angle X-ray scattering, chemical etching and profilometry, complemented by annealing experiments. Clear evidence is given for a complex track structure and defect morphology. Single defects such as F-centers are produced in a large halo of several tens of nanometers around the ion trajectory. The defect creation in this zone is similar to that under conventional radiation. For heavy ions above a critical energy loss of 10 keV/nm, new effects occur within a very small core region of 2–4 nm in diameter. The damage in this zone is responsible for chemical etching and for a characteristic anisotropic X-ray scattering. It is assumed that in this core, complex defect aggregates (e.g., cluster of color centers, molecular anions and vacancies) are created. Their formation is only slightly influenced by the irradiation temperature and takes place even at 15K where diffusion processes of primary defects are frozen. Furthermore, irradiation with heavy ions leads to pronounced swelling effects which can be related to an intermediate zone of around 10 nm around the ion path.

The Creation of Radiation Defects in Lithium Fluoride Single Crystals

The Creation of Radiation Defects in Lithium Fluoride Single Crystals

Mechanical healing of lithium fluoride single crystals at elevated temperatures

The mechanical healing of lithium fluoride (LiF) single crystals at elevated temperatures is studied. The LiF single crystal is cleaved into two halves and healed by compression at temperatures 25, 150, 300, 400, and 500 °C, respectively. The curves of compressive stress versus compressive strain are found to be separable into three regions for both healed and uncracked (virgin material) samples. The curves of fracture stress versus compressive stress consist of a) two regions if the temperature is equal to or larger than 300 °C and b) one linear region for a temperature equal to or less than 150 °C for healed samples; in addition, they are divided into two regions for temperatures 25 and 500 °C for uncracked ones. The compressive stress required to recover the same fracture stress decreases with increasing temperature. For the same compressive stress and temperature, the fracture stress of the healed sample is smaller than that of the uncracked one. The micrographs reveal the presence of many long and narrow stripes in the healed samples subjected to a small compression and many laminales in the healed ones subjected to a large compression. The two regions of the curves of fracture stress versus compressive stress for healed samples can possibly be accounted for by the morphology of the fracture surface.

Study of the crystalline quality of Czochralski grown barium lithium fluoride single crystals

The influence of crystallographic orientation and rotation rate on the crystalline quality of Czochralski grown BaLiF 3 crystals have been studied by neutron diffraction techniques. Analysis of the mosaicity of crystals showed that growth in the 〈111〉 direction with a flat interface resulted in crystals with good optical and crystalline quality. However, growth in the 〈100〉 direction with flat interface produced a low degree of optical quality but still with good crystallinity.

Photoionization processes in F aggregate centres of LiF crystal

Extensive studies have been carried out on the optical conversion of F and F-aggregate colour centres produced in lithium fluoride single crystal on gamma irradiation. Using 308 nm XeCl laser it has been shown that significantly large population build-up of F3+ centre and reduction in the population of undesirable F2 centres can be achieved in gamma irradiated crystal at room temperature due to multistep photoionization processes. These and other investigations have provided a scheme for possible laser action based on F3+ colour centres in LiF crystal at room temperature.

Peculiarities of LiF single-crystal deformation in vaseline oil at elevated temperatures

The influence of Vaseline oil on dislocation mobility and the microhardness under friction and indentation of lithium fluoride (LiF) single crystals was investigated. The experiments were carried out under a load of 0.1 N, at a temperature from 297 to 473 K, with an indentation time from 5 to 900 s and for a friction load of 0.45 N. The lengths of the screw and edge dislocation arms in air and Vaseline oil, as well as the width of the dislocation region (along the friction track), were estimated. It was established that at room temperature in oil, neither the dislocation arm lengths and dislocation region nor the microhardness changed in comparison with tests in air. The dual effect of Vaseline oil at elevated temperatures was revealed: the work-hardening in the impression region and the increase in screw dislocation mobility at the periphery of the dislocation rosette. Under friction, the essential increase in dislocation motion in {110} 45 sliding planes is observed. It was supposed that the increases in dislocation mobility and microhardness in Vaseline oil could be connected with the breaking of carbon-hydrogen bonds and hydrogen penetration and interaction with surface defects. It was suggested that the breaking of carbon-hydrogen bonds in mineral oil (as a basis of lubrication) may lead to a change in defect mobility in the surface layers of metals and alloys.

Environmental effect on sclerometric brittleness of ionic crystals

The conditions of plastic-to-brittle transition during microscratching with a Vickers diamond indentor of natural cleavage surfaces of lithium fluoride (LiF) single crystals (pure samples and those covered with octadecylamine films) in heptane, distilled water, and their saturated aqueous solution, were studied. Indentor loads varied in the range 3.2×10−3-1.9×10−1 N; indent or sliding velocity was 30 μm s−1. Quantitative characteristics of microplasticity and microbrittleness of near-surface layers in single crystals under appropriate test conditions were studied. The basic mechanisms of crack formation in non-uniform indentor-induced fields of stresses and deformations were revealed and the active media effect on the relative intensity of the functioning of these mechanisms was analysed.

Growth and properties of shaped single crystals of lithium fluoride

Shaped single crystals of lithium fluoride have been produced by the Stepanov method using shapers which are wetted and unwetted by the melt. Dislocation structure, porosity, optical properties, ion conductivity, and distribution of magnesium impurities have been studied.

Miniature active elements for color-center lasers with an extremely low lasing threshold

An active medium was formed from lithium fluoride single crystals with the maximum concentration of the active centers (F2, F3+), reaching the range of concentration quenching. The coloration method made it possible to reduce greatly the concentration of parasitic aggregate centers and, consequently, the optical losses in the medium. The lasing threshold for the F2 and F3+ centers was 4 and 60 kW/cm2, respectively.

A Comparative Study of Different (n,alpha) Converter Materials Placed in Contact with Cellulose Nitrate Track Etch Detectors for Neutron Detection

Abstract Different (n,a) converter materials placed in contact with cellulose nitrate detectors were tested as thermal neutron detectors. A plastic sheet doped with 1% boron, a solidified boron oxide plate, a single crystal plate of boron oxide and a single crystal plate of lithium fluoride were used as converter materials. After thermal neutron irradiation in the low power research reactor of the Institute, the cellulose nitrate detectors were etched in alkaline solution. The etched holes on the films were counted with a spark counter. The single crystal of lithium fluoride was found to be the most sensitive converter for the detection of thermal neutrons. A linear relation was observed between thermal neutron fluence and spark counts in the range 106 to 107 n.cm-2. A method of producing the single crystal converter is also described.

Closing of cracks under impact loading

The healing of cracks has been studied in crystalline materials such as diamond, sodium chloride, tungsten, molybdenum and quartz, and the possibility has also been studied of restoring material continuity to sodium chloride and lithium fluoride single crystals under conditions of relatively prolonged compression over a time range of from one to tens of seconds. Potential restoration of interatomic bonds between surfaces of failed material (reanimation) precedes collapse of a crack as a process of approach of its edges before mechanical contact. The goal of this work is to study crack closing with short-term impact.

Atomic mixing in insulating crystals containing small metallic aggregates

Magnesium oxide and lithium fluoride single crystals are implanted with metallic ions (Na +, Rb +) and subsequently annealed to form small metallic precipitates of sodium or rubidium embedded in the insulating matrix. They are reimplanted at low dose rate with the same particles and analyzed using optical absorption measurements and transmission electron microscopy. In MgO samples, the metallic precipitates are dissolved in the lattice and they all disappear for doses of the order of 10 15 ions cm −2, whereas, to the contrary, in LiF the precipitation increases. This result seems to indicate the strong influence of intrinsic defects on the atomic mixing process.

Single Crystal LiF Thermoluminescence Dosemeters

Abstract Commercial Thermoluminescence dosemeters (TLDs) currently used are finely powdered lithium fluoride (LiF), calcium sulphate or magnesium silicate etc. sealed in glass tubes or those melted with special resins. These are opaque or semi-transparent and therefore thermoluminescence light emitted inside does not come out efficiently due to the self-absorption. To avoid the self-absorption LiF single crystals have been examined. A cylindrical graphite crucible including LiF mixed with trace amounts of magnesium, copper and phosphorus was heated in a vacuum stainless tube with a slowly moving heater coil along the axis. The LiF single crystal grew gradually from the bottom tip of the crucible. Thermoluminescence emitted from a LiF single crystal after irradiation with ? rays increased as phosphorus in the LiF crystal increased. Thermoluminescence emitted from a single crystal with 0.25 mole percent phosphorus was one order higher than that emitted from powdered LiF with the same mole percent phosphorus and the same weight. A single crystal could not grow with phosphorus of more than 0.25 mole percent. However, lithium phosphate appears to be a better activator material than the ammonium phosphate used in our experiments to overcome this 0.25 mole percent barrier.

Thermal annealing effects in LiF crystals implanted with57Fe ions

Thermal annealing effects in lithium fluoride single crystals implanted with57Fe ions were studied by conversion electron Mossbauer spectroscopy. It was found that annealing in vacuum leads mostly to the precipitation of metallic aggregates in contrast to the annealing in He and H2 when the formation of ferric compounds, having high magnetic transition temperatures, takes place.

Strain-rate history effects and microstructural aspects in lithium fluoride and aluminium single crystals after dynamic loading

Strain-rate history effects and microstructural aspects in lithium fluoride and aluminium single crystals after dynamic loading

Thermal Conductivity, Thermal Diffusivity and Specific Heat of Lithium Fluoride Crystals Containing Dislocations

The thermal conductivity and thermal diffusivity of lithium fluoride single crystals have been measured simultaneously by the temperature wave method at 1.5–20 K. The specimens were successively deformed by compression and dislocations with densities of 10 6 –10 8 cm -2 were introduced. The diffusivity data were used to analyze the scattering of phonons by dislocations. The analysis was made with the Callaway's method, and the empirical representation of the relaxation rate for the dislocation scattering was found to be a sum of two rates with different dependences on phonon frequency: τ F -1 ∝ω -1 and τ A -1 ∝ω 2 . The former corresponds to the phonon scattering by the fluttering of dislocations, while the latter represents other unknown scattering mechanism effective at high temperatures. Specific heat of the crystals was also determined from the conductivity and diffusivity data. The Debye temperature was found to decrease or the specific heat to increase by fairly large amount when dislocations were i...

Disappearance of voids in lithium fluoride single crystals

The disappearance of radiation-induced voids in LiF single crystals is investigated. The activation energy of the process is determined by studying the temperature dependence of the rate of disappearance of individual needle-shaped voids. The value obtained is in good agreement with the figures reported for F− ion diffusion. On the basis of kinetic studies it is shown that radiation-induced voids contain gas under low pressure.