Calcium Fluoride Single Crystals Research Articles

Crack propagation in calcium fluoride single crystals

Crack propagation in calcium fluoride single crystals

Growth and characterization of thorium-doped calcium fluoride single crystals

We have grown ^{232}Th:CaF_2 and ^{229}Th:CaF_2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of ^{229}Th, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of ^{229}Th, we have scaled down the crystal volume by a factor 100 compared to established commercial or scientific growth processes. We use the vertical gradient freeze method on 3.2 mm diameter seed single crystals with a 2 mm drilled pocket, filled with a co-precipitated CaF_2:ThF_4:PbF_2 powder in order to grow single crystals. Concentrations of 4cdot 10^{19} cm^{-3} have been realized with ^{232}Th with good (> 10%) VUV transmission. However, the intrinsic radioactivity of ^{229}Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the ^{229}Th concentration to <5times 10^{17} cm^{-3}.

Predictive modelling for enhanced scratching of brittle ceramics with magneto-plasticity

The micro-cutting of brittle ceramics starts with elastic-plastic deformation followed by severe brittle fracture, which significantly restricts the machinability and application of these materials. Magnetic field-enhanced plasticity (i.e., magneto-plasticity) has shown a positive potential to augment the ductile-brittle transition and machinability but the material removal mechanism of brittle ceramics with magneto-plasticity and its dependence on strain rate remains unclear. In this study, a deformation energy-based model was developed to bring light to the deformation mechanisms affecting the ductile-brittle transition under magneto-plasticity and the corresponding dependency on strain rate. MD simulations assisted the analytical modeling by deriving the orientation-dependent dislocation movement and stress distribution that contribute to single-crystal anisotropy in the surface formation and ductile-brittle transitions during scratching. The proposed model accurately predicted the improvement in ductile-brittle transition under various magnetic field intensities and orientations as confirmed in the experimental magnetic field-enhanced micro-scratching tests on single-crystal calcium fluoride (CaF2). Scratched surface morphologies, acoustic emission (AE) signals, and critical loading conditions demonstrated the improvements in ductile-brittle transition with magneto-plasticity. Additionally, the progression of lateral slip trace and tangential surface pile-up under various magnetic field conditions confirm the direct influence of magneto-plasticity on dislocation mobility and the change in surface formation mechanism of brittle ceramics. This study not only deepens the understanding of the manifestation of magneto-plasticity during the deformation of brittle ceramics, but also opens new avenues for the integration of magnetic field-assisted technology in manufacturing processes.

Investigation of the temperature effect on cutting of calcium fluoride single crystal using molecular dynamics simulation

Thermal assisted machining has been demonstrated to enhance the machinability of single-crystal CaF2 but the underlying mechanisms of the thermal effect on the mechanical behavior of the material remain unclear. Therefore, molecular dynamics was employed in this research to study the deformation process during machining from the perspective of dislocation activity. The results illustrate the enhanced plasticity with longer dislocation line lengths and the reduction in cutting forces at an elevated temperature. Micro-cutting experiments confirm the simulated results by the delay in ductile–brittle transition by a magnitude of 1.34.

Growth and characterization of thorium doped calcium fluoride single crystals

Growth and characterization of thorium doped calcium fluoride single crystals

Effect of point defects on the STE luminescence of CaF2 single crystals

The study is devoted to the radiation defects created in the calcium fluoride single crystals with reactor irradiation to the fluence of 3.7 × 1017 thermal and 4.6 × 1017 fast (En > 1 MeV) neutrons per cm2, and their effect on the luminescence of the crystals. UV–VIS absorption spectroscopy revealed the formation of point defects, mainly F and F2 centers. From the comparison of the annealing behavior of the absorption and the ion-beam-induced luminescence (IBIL), we conclude that the color centers are responsible for a non-radiation decay of self-trapped excitons (STEs) and thus cause the luminescence quenching. In addition, the Raman analysis revealed a disorder of about 44% and a stress formation in the material after the neutron irradiation. Annealing at 100 °C reduces the disorder to 13% and eliminates the stress.

Effect of a weak magnetic field on ductile–brittle transition in micro-cutting of single-crystal calcium fluoride

• A weak magnetic field can enhance ductile-mode cutting of CaF 2 single crystal. • Magnetic field direction shows anisotropy in affecting ductile–brittle transition. • A coupled analysis of magneto-plasticity and crystal plasticity is developed. • Dislocation behaviour and fracture toughness are revealed under magnetic field. • DFT computation corroborates the magnetic-induced dislocation depinning theory. Magneto-plasticity occurs when a weak magnetic field alters material plasticity and offers a viable solution to enhance ductile-mode cutting of brittle materials. This study demonstrates the susceptibility of non-magnetic single-crystal calcium fluoride (CaF 2 ) to the magneto-plastic effect. The influence of magneto-plasticity on CaF 2 was confirmed in micro-deformation tests under a weak magnetic field of 20 mT. The surface pile-up effect was weakened by 10–15 nm along with an enlarged plastic zone and suppressed crack propagation under the influence of the magnetic field. Micro-cutting tests along different crystal orientations on the ( 111 ) plane of CaF 2 revealed an increase in the ductile–brittle transition of the machined surface with the aid of magneto-plasticity where the largest increase in ductile–brittle transition occurred along the [ 11 2 ¯ ] orientation from 512 nm to a range of 664–806 nm. Meanwhile, the subsurface damage layer was concurrently thinner under magnetic influence. An anisotropic influence of the magnetic field relative to the single-crystal orientation and the cutting direction was also observed. An analytical model was derived to determine an orientation factor M that successfully describes the anisotropy while considering the single-crystal dislocation behaviour, material fracture toughness, and the orientation of the magnetic field. Previously suggested theoretical mechanism of magneto-plasticity via formation of non-singlet electronic states in defected configurations was confirmed with density functional theory calculations. The successful findings on the influence of a weak magnetic field on plasticity present an opportunity for the adoption of magnetic-assisted micro-cutting of non-magnetic materials.

Beneficial stress of a coating on ductile-mode cutting of single-crystal brittle material

Innovative techniques have been proposed to overcome the challenge of the strong tendency of brittle materials to crack during machining. One of them is the use of an epoxy coating to serve as a crack formation restraint. However, this only serves to achieve ductile-mode grinding along the uncut shoulders. Therefore, this study evaluates the effect of a layer of epoxy resin on the machined surface perpendicular to the micro-cutting direction of a brittle material, single-crystal calcium fluoride. An increase in the ductile–brittle transition was observed in micro-cutting experiments on the (111) plane of calcium fluoride with the use of a 4-μm thick epoxy resin coating, ranging from 167–347 nm to 213–476 nm for the 0° rake angled tool. A similar increase was also observed with the +5° rake angle, ranging from 91–233 nm to 187–310 nm. An energy-based ductile–brittle transition predictive model is introduced, which characterises the anisotropic behaviour of the single crystal and incorporates the additional stress induced during cutting with a coating as a fraction of the coating hardness. The analytical model accurately predicts a consistent range of improvement from 69–325 nm to 257–485 nm for the 0° rake angle and from 62–285 nm to 215–460 nm for the +5° rake angle. The ductile-mode cutting energy increases to preserve its dominance over brittle-mode cutting and delays the onset of brittle-mode activation. The validity of the model extends the understanding of a surface coating as a restraining technology to include the beneficial stress acting in the deformation zone during cutting.

Characterizing crack morphology toward improving ductile mode cutting of calcium fluoride

Delaying the onset of brittle mode cutting does not only involve the augmentation of slip deformation but could also be tackled from the perspective of suppressing the leading causes for crack formation. This work studies the micro-cutting of calcium fluoride single crystals with diamond turning and orthogonal plunge-cutting. An applied coating shows promise to enhance ductile mode cutting during diamond turning and also alters the crack morphology during cutting in the brittle regime. Conventionally, calcium fluoride exhibits anisotropic pyramidal cracks that were associated with the activation of the {111} cleavage plane while the adoption of the coating prior to micro-cutting alters the cracks to become laminar in nature associated with the (1‾11‾) cleavage plane. The resistance to deformation of the coating is proposed to create reaction stress along the shear plane of the workpiece to reduce the effective tensile stresses acting on cleavage planes. Crystal plasticity finite element method modelling demonstrates the relationship between the coating stress distribution and the intensified concentration of shear stresses in the workpiece, which confirms the notion of the reaction stress acting along the shear plane. Moreover, the simulations with the coating present the affected tensile stresses acting on a majority of the {111} cleavage planes with the exception of the (1‾11‾) cleavage plane, which supports the crack orientations in brittle mode cutting.

Suppression of Eu2+ luminescence and enhancement of Eu3+ emission in Eu: CaF2 single crystal via Gd3+ co-doping

Eu-doped calcium fluoride (CaF2) single crystals co-doped with various concentrations of Gd3+ were grown by the Bridgman-Stockbarge method. A broad excitation band in the ultraviolet (UV) region were observed in the crystal, which is attributed to the existence of Eu2+ ions detected with the help of XPS measurements. The Gd3+ doping was found to have a significant influence on the luminescence properties of the Eu: CaF2, resulting in a suppression of Eu2+ luminescence and enhancement of Eu3+ emission at high Gd3+ co-doping level. The photoluminescence excitation and emission spectra of Eu2+ and Eu3+ ions were measured using luminescence spectrometer. The local site symmetry in the crystals was investigated to explain the influence of Gd3+ on suppression of Eu2+ luminescence and emission of 5D3→7FJ (J = 1, 2, 3) in Eu3+ transition. The fluorescence emissions of the 3%Eu, 6%Eu: CaF2 and 3%Eu, 12%Gd: CaF2 crystals excited at 398 nm relate to the Commission internationale de I’éclairage (CIE) color coordinates of (0.6012, 0.3870) and (0.5930, 0.3953), respectively. A highest quantum yield of 40.53% and 5D0→7F1 transition decay time of 9.54 ms were obtained on the 3%Eu, 6%Gd: CaF2 crystal. This investigation suggests that Eu, Gd: CaF2 has great potentials for optimizing solid state UV–visible converter performances through appropriate concentration tuning of dopants.

Machinability of single crystal calcium fluoride by applying elliptical vibration diamond cutting

Functional micro/nano structures are promising for enhancing the performance of CaF2-based devices. However, it is still a challenge to precisely manufacture CaF2 micro/nano structures due to their brittleness. In the present work, we demonstrated the machinability of ductile-mode diamond cutting of CaF2 and the ability to sculpture sophisticated micro/nano structures on CaF2 by applying elliptical vibration cutting. Firstly, the nanoindentation of CaF2 reveals the crystal orientation-dependent interaction between dislocation slip and crack propagation, thus obtaining the optimal crystal orientation for plasticity. Subsequently, the grooving tests were conducted along the crystal orientation of (111)[1‾21‾]. With elliptical vibration cutting, the critical depth of cut from ductile-to-brittle transition is increased by 42 times as compared with ordinary cutting. Furthermore, considering the instantaneous uncut chip thickness in each vibration cycle, the influence of vibration amplitude on the ductile machinability of CaF2 is discussed in detail. Finally, based on these fundamental results, ultra-precision hexagonal microlenses were successfully sculptured on CaF2 by applying the amplitude-controlled sculpturing method in elliptical vibration cutting.

Laser-induced damage and periodic stripe structures of a CaF2 single crystal by an ArF excimer laser

The laser-induced damage threshold of a calcium fluoride (CaF2) single crystal was obtained by a 193 nm ArF excimer laser. The damage morphology of the crystal was analyzed. The results showed that the surface of CaF2 single crystal broke along the natural cleavage plane under ArF excimer laser irradiation, some fragments fell off, and Newton’s rings were observed on the curved fragments. Laser-induced periodic stripe structures (LIPSS) appeared on the surface layer beneath the fragments that peeled off. The spacing of LIPSS was measured, and the formation mechanism of LIPSS was analyzed based on the interference model.

Enhancing Ductile-mode Cutting of Calcium Fluoride Single Crystals with Solidified Coating

Positive improvements have been observed during machining brittle materials under high hydrostatic pressure, but the techniques to achieve such desirable effects often utilize complex and expensive equipment or tools. This work presents a cost-efficient method to achieve ductile-mode machining of brittle materials at higher uncut chip thicknesses, by the application of a solidified coating on workpiece surface before a machining process. Orthogonal microcutting experiments were conducted on calcium fluoride single crystals oriented with the (111) plane and an increase in critical uncut chip thickness was observed with the solidified coating. The primary cause has been resolved to be mechanical-related and results in a stabilized microcutting process. Transmission electron microscopy provided evidence of slip deformation occurring in the machined subsurface regions and a layer thickness of subsurface damages reduced by ~ 45% under the influence of the solidified coating. In addition, erratic fluctuations in direction of the resultant machining force were subdued with the applied coating, which is proved to be caused by the compressive stresses induced from the sandwiching of the CaF2 material between the tool and the solidified coating. The proposed technique successfully reduces the cost and pollution in the fabrication process of optical components from the use of coolant in an ultraprecision machining process to the time consumed by eliminating the subsurface damage with abrasive slurries in post-machining polishing.

Stress-enhanced dissolution and delamination wear of crystal CaF2 in water condition

The role of water in the material removal of single crystal calcium fluoride (CaF2) was studied using a scratch tester with a diamond tip. Water can facilitate the sliding wear of CaF2, and the degree strongly depended on the amount of water molecules involved in the wear process. CaF2 wear increases with the increase of relative humidity (RH). When the sliding wear tests were conducted in dry condition, the wear rate increases linearly with applied load, obeying the Archard wear equation. When water participated in the wear process, the Archard wear equation was failure and a exponential dependence of wear rate on contact stress was given under water condition. The transformation of the applied load (or contact stress) dependence indicates the change of wear mechanism in water condition. In dry condition, the surface damage of CaF2 is mainly attributed to mechanical wear where the stress-induced plastic flow under the activation of the slip systems results in a relatively smooth worn surface. In humid nitrogen or water condition, the plastic wear of CaF2 weakens due to the stress-associated dissolution though the wear volume increases compared to that in dry case. Both the water-associated dissolution itself and induced delamination promote the sliding wear of CaF2 in water. The results may help elucidate the water-associated wear mechanism of CaF2 and optimize the processing parameters in the ultra-precision machining to reduce the residual surface/subsurface defects.

Revisit of the anisotropic deformation behavior of single-crystal CaF2 in orthogonal cutting

A resolved stress model is presented to re-discuss the anisotropic deformation mechanism of single-crystal calcium fluoride in plunge-cut tests. The authors have investigated the machinability of CaF2 in ultra-precision cutting. However, the influence of crystal anisotropy on the brittle–ductile transition was discussed in a qualitative manner using optical microscopy and white light interferometer. Moreover, only the primary {100}<110> slip system and {111} cleavage were considered as deformation mechanisms. In this study, the ductility and brittleness of CaF2 is semi-quantitatively re-discussed on the basis of computation of the resolved stresses, by considering a secondary slip system and cleavage. The surface morphologies of the machined surface were characterized using field-emission scanning electron microscopy. The primary {100}<110> slip system is assumed to be dominant for the ductility. The brittle fracture initiated by the {111} cleavages was observed in a different manner in dependency rwith cutting directions.

On the improvement of subsurface quality of CaF2 single crystal machined by boron-doped nano-polycrystalline diamond tools

The subsurface quality of single-crystal calcium fluoride (CaF2) is examined by ultra-precision cutting experiments using a boron-doped nano-polycrystalline diamond (B-NPD) tool and compared with that of CaF2 machined with a single-crystalline diamond tool (SCD). The critical depth of cut, surface morphologies, and process force did not change significantly in the plunge-cut tests. In the ultra-precision cylindrical turning tests, the surface roughness was almost the same, whereas the subsurface damage depth (SSD) changed remarkably, from ten to several hundred nanometers. The SSD depth was reduced by the use of B-NPD. Additionally, shear deformation and rotational deformation of the subsurface layer were observed. The lowering of friction coefficient at the interface between the cutting tool and the machined surface is assumed to affect the subsurface layer.

Structural defects caused by swift ions in fluorite single crystals

A comparative study of radiation damage caused by the irradiation of oxygen-free calcium fluoride single crystals with ∼GeV 132Xe or 209Bi heavy ions, 100-keV light hydrogen ions (protons) or X-rays at room temperature has been performed. Optical absorption in a wide spectral region from NIR to VUV (1.5–10.5 eV), its dependence on stepwise preheating of the irradiated CaF2 crystals to a certain temperature as well as thermally stimulated luminescence accompanying the main annealing stages have been analyzed. It is shown that in addition to different F-type aggregates, Ca colloids and trifluorine quasi-molecules, complex and temperature stable structural defects responsible for VUV absorption (in particular, the 9.8 eV band) are induced in CaF2 only after irradiation with swift heavy ions. The origin and tentative creation mechanisms of such defects as well as the features of the used irradiation types are considered.

Effect of crystallographic orientation on mechanical removal of CaF2

The anisotropic removal behavior of single crystal calcium fluoride (CaF2) severely restricts the improvement of surface quality in ultra-precision machining. In this study, the anisotropic mechanical removal of CaF2 material was investigated quantitatively along various crystal orientations on CaF2(111), (110), and (001) surfaces by a conical diamond tip. With the increase of load, the wear of CaF2 material has an elastic–ductile–brittle transition, and the corresponding critical load and wear depth strongly depend on the basis of crystal plane and orientation. Plastic deformation initially occurs in the directions of mechanical stress coinciding with the slip system of CaF2 material, and the crack grows preferentially on the cleavage plane. The contact stress range of 10.8GPa to 11.4GPa is defined as the security region in which the material removal of CaF2 can be controlled in the ductile wear under all crystal plane and orientation conditions. The results may help to optimize the processing parameters to achieve consistent and controllable material removal in the fabrication of CaF2 with complicated shapes.

Influence of crystal anisotropy on subsurface damage in ultra-precision cylindrical turning of CaF2

An optical microcavity, which stores light at a certain spot, is an essential component to realize all-optical signal processing. Single-crystal calcium fluoride (CaF2) theoretically shows a high Q-factor which is a desirable optical property. The CaF2 microcavity can only be manufactured by ultra-precision cylindrical turning (UPCT). The authors have studied UPCT of CaF2 and shown the influence of crystal anisotropy and tool geometry on surface roughness and subsurface damage. The study indicated that a smaller nose radius of the cutting tool led to shallower subsurface damage. Thus, it is inferred that a smaller nose radius compared to the previous nose radius (0.05mm) can further reduce subsurface damage. Nevertheless, the mechanism that causes a difference in subsurface damage due to crystal anisotropy is not sufficiently clear. The influence of subsurface damage on microcavity performance is still unclear. In this study, the UPCT of CaF2 was conducted using a tool with a nose radius of 0.01mm. The subsurface damage was investigated by transmission electron microscope (TEM) observation from the viewpoint of the change in crystal lattice arrangement. In our previous study, fast Fourier transfer (FFT) analysis was used for confirmation of change of crystal structure. In this study, FFT analysis was also used to quantitatively evaluate the depth of subsurface damage. In addition, inverse fast Fourier transfer (IFFT) was used to analyze change of crystal lattice arrangement clearly, which enables discussion of the influence of slip systems. Finally, optical microcavities are manufactured without any crack, and the influence of subsurface damage on microcavity performance is experimentally evaluated using a wavelength tunable laser and power meter.

Factors controlling the thermoluminescence spectra of rare earth doped calcium fluoride

Thermoluminescence spectra of rare earth doped calcium fluoride samples, both powder and single crystal, were recorded over the temperature range from 25K to 673K. Although some broad band features exist, the spectra are dominated by the rare earth line transitions. The glow peak temperatures are slightly sensitive both to the ionic size of the dopants and the dopant concentration. By contrast, very considerable differences are generated by heat treatments, such as annealing followed by either fast or slow cooling. Comments are included on the reasons for such sensitivity in terms of association of dopant and intrinsic defect sites and why the results of dosimetry powder differ from those from single crystals.