RU2358045C2 - Ceramic laser microstructured material with twinned nanostructure and method of making it - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: proposed laser material is a ceramic polycrystalline microstructure substance with particle size of 3-100 mcm, containing a twinned nanostructure inside the particles with size of 50-300 nm, made from halides of alkali, alkali-earth and rare-earth metals or their solid solutions, with vacancy or impurity laser-active centres with concentration of 10¹⁵-10²¹ cm^-3. The method involves thermomechanical processing a monocrystal, made from halides of metals, and cooling. Thermomechanical processing is done until attaining 55-90% degree of deformation of the monocrystal at flow temperature of the chosen monocrystal, obtaining a ceramic polycrystalline microstructure substance, characterised by particle size of 3-100 mcm and containing a twinned nanostructure inside the particles with size of 50-300 nm.

EFFECT: improved mechanical properties, increased microhardness and failure viscosity.

5 cl, 1 tbl, 4 ex, 1 dwg

Description

The invention relates to the field of technology of optical materials, and in particular to a technology for producing laser materials used as an optical medium for generating or (and) converting laser radiation.

Laser materials with color centers are widely known. These materials make it possible to obtain powerful narrowly focused spectrally narrow light beams with a tunable wavelength of the generated radiation over a wide range (0.5-4 microns). Appropriate materials and devices based on them are used in high resolution spectroscopy, nonlinear optics, spectral-selective photochemistry, intracavity spectroscopy, holography, as well as in biology and medicine. These materials are used as active and passive laser elements, including passive laser shutters, aposing diaphragms and other optical elements.

For the preparation of active centers operating at room temperature, the most promising were fluorides of alkali and alkaline earth metals, especially lithium fluoride (LiF).

Additive or electrolytic dyeing technologies or irradiation with ionizing radiation (ultraviolet, x-ray or gamma rays, neutrons or high-energy electrons) are used to create color centers.

Known laser material, which is a single crystal of lithium fluoride with F ₂ color centers, which is obtained by irradiating a single crystal with electrons with an energy of 0.08-0.7 MeV to an absorbed dose of (0.7-5.0) · 10 ⁷ Gy (SU 1322948, 02.27.1995).

A disadvantage of the known material is its low hardness and strength.

Known laser materials based on oxide ceramics with a garnet structure. The methods for obtaining these materials are reduced to the synthesis of powder from solutions of the corresponding composition and its compaction (JP 05-286762, 02/11/1993, JP 10-101333, 04/21/1998).

Known polycrystalline laser material based on alkaline earth metal fluorides doped with rare earth ions, which is produced by hot pressing of previously obtained powders of the required composition (US 3453215, 07/01/1969). The disadvantage of this method is the high adsorption ability of the developed surface of the powders, including with respect to water vapor, and the possibility of pyrohydrolysis in the preparation of optically transparent ceramics.

A known laser material containing optical fluoride ceramics with a large number of working laser centers (F2). The material was obtained by hot pressing of polycrystalline fluorides followed by irradiation (SU 1538846, 02.27.1995). The disadvantages of this laser material are the low light transmittance in the region of 0.2-7.0 μm, the high concentration of impurity oxides in the composition of the main substance, which causes large losses of the light beam due to scattering, and, accordingly, high thresholds and low generation efficiency.

The material described above is the closest analogue of the proposed material.

However, the method by which the material described above is obtained does not allow to obtain pure high-quality material.

For the prototype of the proposed method, a method of manufacturing an optical material made in the form of a polycrystalline plate, including thermomechanical processing of a single crystal based on a metal halide by heating a single-crystal billet in a temperature range of 0.5 T _pl <T <T _pl . Thermomechanical deformation is carried out first by uniaxial compression with a profiled surface, and then with a plane-parallel surface and cooled to room temperature (RU 2083733, 07.10.1997).

However, it is not possible to obtain a nanomicrostructured polycrystalline substance with increased hardness and strength in a known manner.

The objective of the invention is the creation of a laser material from highly transparent halide ceramics suitable for the transmission, generation and conversion of optical radiation with different frequencies and power of optical signals with increased hardness and strength, and the development of a method for producing the material.

The problem is solved by the described method of manufacturing a laser material, which includes thermomechanical processing of a single crystal made of metal halides, carried out to a degree of deformation of a single crystal of 55-90% at a pour point of the selected single crystal to obtain a ceramic polycrystalline microstructured substance characterized by a crystal grain size of 3-100 μm.

Single crystals of alkali, alkaline earth and rare earth fluorides or their solid solutions are fed for processing.

The deformation is carried out at the pour point of the corresponding single crystal, in the temperature range above 2/3 of the absolute melting temperature of the single crystal to its melting temperature.

After cooling, the creation of crystals of laser-active color centers in the crystals by the method of additive or electrolytic staining or by ionizing radiation.

The problem is also solved by the described laser material, which is a ceramic microstructured polycrystalline substance with a grain size of 3-100 μm, made of halides of alkali, alkaline earth and rare earth metals or their solid solutions, having vacancy or impurity laser-active centers with a concentration of 10 ¹⁵ -10 ²¹ cm ^-3 , which is made by the method described above.

The implementation of the method in the amount of the above set of features provides a material having a complex hierarchical nanostructure, including a dislocation ensemble, twin and grain boundaries. The grain size ranges from 3-100 microns, while a twin structure with a characteristic size of 50-300 nm takes place inside the grains.

In the process of manufacturing the material, the formation of grain boundaries occurs inside the volume of the single crystal, which eliminates chemical interactions with the surrounding gas medium during the formation of the polycrystalline phase and, thus, increases the purity of the material, reduces spurious losses and simplifies the process.

Deformation can be produced both under uniaxial pressure and comprehensive compression. As a result of deformation in the single crystal, point, linear, and two-dimensional defects accumulate. When the degree of deformation is above 55% up to 90%, the sample acquires a polycrystalline structure, which is detected by x-ray diffraction, electron diffraction, and optical microscopy.

As a result of the method, ceramics is obtained from a single crystal. This transformation is accompanied by an improvement in the mechanical characteristics of the samples, namely, microhardness increases by 15%, and fracture toughness by 2-6 times. In this case, as shown experimentally, the spectral-lasing characteristics do not change and laser generation on ceramic samples is possible.

After cooling the sample, laser-active color centers are created by any known method.

The degree of polycrystallinity of the samples obtained after thermomechanical treatment was determined by determining the intensities of various hkl reflections for a LiF single crystal, LiF powder, and LiF samples of various degrees of deformation, which are given in Table No. 1.

Table number 1 No. Sample Relative intensities of hkl reflections in% (111)
54.7 (200)
0.0 (220)
45.0 (311)
25.1 (222)
54.7 (400)
0.0 (331)
46.5 (420)
26.6 (422)
26.6 one Original single crystal one hundred 2 2 Powder 40 one hundred 29th eleven 5 6 2 6 four 3 Sample No. 1, rel. deformation 0.37 0 one hundred 0 0 0 6 0 0 0 four Sample No. 2, rel. deformation 0.52 2.4 one hundred 0.6 0.3 0.35 9 0.2 0.3 0.3 5 Sample No. 3, rel. deformation 0.69 3 one hundred one 0.7 0.6 3.5 0.1 0.5 0.2 6 Sample No. 4, rel. strain 0.84 8 one hundred 7 10 0.6 5 0.2 thirty 2 Note: in the table below the reflex indices are shown minimum angles between the (100) plane and the (hkl) plane.

An analysis of the data showed that prior to deformation of less than 50% of the initial sample thickness, misorientation of single crystal blocks to large angles was not observed. With large deformations, rotations of the crystallographic planes at any angles are observed. With increasing strain, the volume of sample blocks rotated at large angles increases. The drawing shows x-ray: a single crystal LiF (see drawing, a), powder LiF (see drawing, b) and optical ceramics LiF - sample No. 4 (see drawing, c). The X-ray diffraction pattern of a single crystal contains two reflections (200) and (400), which are a reflection from one crystallographic plane (100). The X-ray powder pattern of lithium fluoride corresponds to the powder database card PDF No. 78-1217. The X-ray diffraction pattern of the ceramic sample contains a complete peak pattern, which is inherent in the powder spectrum, from which we can conclude that when strains are more than 80%, the diffraction patterns of the powder and the deformed sample do not differ qualitatively, that is, the sample becomes a polycrystal.

The following are some specific examples of the invention, not limiting its scope.

Example 1

The CaF ₂ laser single crystal containing 5 mol% YbF ₃ grown by the Bridgman method was heated to a temperature of 1350 ° C and subjected to pressure deposition with a plastic strain of 80% in a vacuum of 10 ^-2 Torr. The sample was cooled to room temperature. While maintaining laser quality, the sample acquired a polycrystalline structure according to the data of X-ray diffraction and diffraction of back-reflected electrons. The microhardness was 600 GPa (for a single crystal 500 GPa), fracture toughness

K _1C 10.2 MPa · m ^–0.5 (for a single crystal 1.5 MPa · m ^–0.5 ). The grain size was 10-40 microns.

Example 2

The lithium fluoride single crystal grown by the Kyropoulos method was heated to a temperature of 600 ° C and, by applying pressure, was subjected to deposition with a plastic strain of 84% in a press with a force of up to 1 ton. According to X-ray diffraction and diffraction of back-reflected electrons, the sample acquired a polycrystalline structure. The grain size according to optical microscopy was 3-7 microns. The microhardness measured by indentation by the Vickers diamond pyramid has a value of 150 kg / mm ² (for a single crystal 137 kg / mm ² ). Optical microscopy of the samples after chemical etching revealed the structure of the sample with a characteristic size of 100-150 nm. Then, a polycrystalline material was irradiated with ionizing radiation with a dose of 10 ⁸ Rad, as a result of which F ₂ ^- laser color centers were induced. The absorption coefficient at a wavelength of 965 nm was 3.1 cm ^-1, which corresponds to the concentration of F ₂ ^- centers 10 ¹⁷ cm ^-3. The microhardness of the irradiated polycrystalline sample was 186 kg / mm ² , which indicates an improvement in mechanical characteristics. On the obtained sample, laser generation was obtained at a wavelength of 1112 nm when pumped with 975 nm radiation from an IPG laser diode with a 12 W fiber optic output. The generation threshold was 2.2 W, which is lower than that of a single-crystal sample (4.6 W) irradiated under the same conditions, the differential generation efficiency was 23% compared to 18% for a single-crystal sample.

Example 3

A CaF ₂ single crystal containing 0.01% NaF grown by the Bridgman method was heated to a temperature of 1100 ° C and, by applying pressure, it was deposited with a plastic strain of 55% in a vacuum of 10 ^-2 Torr. The sample acquired a polycrystalline structure. The microhardness was 255 GPa (for a single crystal 200 GPa), fracture toughness K _1C 4.7 MPa · m ^–0.5 (for a single crystal 1.5 MPa · m ^–0.5 ). Then, additive staining was carried out, as a result of which laser color centers with a concentration of 10 ¹⁵ cm ^{-3 appeared} . The grain size was 40-70 microns.

Example 4

The SrF ₂ single crystal containing 0.7% NdF ₃ grown by the Bridgman method was heated to a temperature of 1480 ° C in a fluorinating atmosphere (CF ₄ ), as a result of which it melted and was squeezed between graphite plates to a degree of deformation of 90%. The size of the obtained crystal grains was 80-100 microns. The concentration of laser-active centers was 10 ²¹ cm ^{-3, the} characteristics of the obtained laser material are similar to those specified in example 2.

The claimed methods have also been obtained ceramic laser materials based on CaF ₂ : Nd, SrF ₂ : Yb, CaF ₂ -SrF ₂ : Yb, BaF ₂ : Er, NaF, LiYF ₄ , which were devoid of cleavage.

The fracture toughness of the obtained samples is 2-6 times higher in comparison with the corresponding single-crystal materials, and the microhardness is 10-15% higher. The thermal conductivity and spectroscopic characteristics of the obtained fluoride nanoceramics practically do not differ from the characteristics of the corresponding single crystal.

Comparative characteristics of the lasing properties of single crystals and ceramic samples obtained by the claimed method are shown below on the example of a sample of lithium fluoride.

The measured spectra and luminescence lifetime of F ₂ ^- color centers in ceramic samples with excitation by nanosecond laser tunable LiF: F ₂ ⁺ with lasing wavelengths near 920 nm were not different from those measured under similar conditions for the single crystal sample of 7 mm in thickness. Laser generation of single crystals and LiF: F ₂ ceramics ^{- was} investigated when pumped with an IPG laser diode with a fiber (fiber diameter of 100 μm) radiation output with a power of 12 W and a generation wavelength of 967 nm. The laser diode operated in a quasi-continuous mode of generation with a pump duration of 1 ms and a repetition rate of 5 Hz. The laser diode radiation was focused into the sample with a single lens with a focal length of 5 mm. The laser cavity was formed by a dichroic plane mirror with a transmission of 95% at the pump wavelength and with a reflection of 100% at the laser generation wavelength (about 1.117 μm) and an output spherical mirror 40 mm from the plane mirror. The average output and pump power was measured with an EPM-100 power meter, and the generation and pump wavelengths were measured with a LRL005 wave meter.

Ceramic samples had a lower generation threshold. The maximum generation efficiency was obtained for a ceramic sample 4 mm thick and amounted to 26%, and its generation efficiency at a slightly higher maximum output power and about 1.5 times lower absorption of a ceramic sample 4 mm thick significantly exceeded the efficiency of a single-crystal sample (18%). The laser emission spectrum based on a ceramic sample was measured. The maximum wavelength of the generation was at a wavelength of 1.117 μm with a spectral width of about 5 nm.

The study of the lasing properties of the materials in accordance with the present invention showed the promise of the obtained fluoride ceramics for developing lasers based on it, including those providing a mode locking mode for pico- and femtosecond oscillators.

Claims (5)

1. A method of manufacturing a laser material, including thermomechanical processing of a single crystal made of metal halides, and cooling, characterized in that the thermomechanical processing is carried out to a degree of deformation of the single crystal of 55-90% at a pour point of the selected single crystal to obtain a ceramic polycrystalline microstructured substance characterized by grain size crystals of 3-100 microns and including a twin nanostructure inside grains with a size of 50-300 nm. 2. The method according to claim 1, characterized in that the metal halides use fluorides of alkali, alkaline earth and rare earth metals or their solid solutions. 3. The method according to claim 1, characterized in that the deformation is carried out at a pour point of the corresponding single crystal in the temperature range above 2/3 of the absolute melting temperature of the single crystal to its melting temperature. 4. The method according to claim 1, characterized in that after cooling in the crystals create laser-active color centers by the method of additive or electrolytic staining or ionizing radiation. 5. The laser material, which is a ceramic polycrystalline microstructured substance with a grain size of 3-100 microns, including a twin nanostructure inside the grains of 50-300 nm in size, made of halides of alkali, alkaline earth and rare earth metals or their solid solutions, having vacancy or impurity laser active centers with a concentration of 10 ¹⁵ -10 ²¹ cm ^-3 , and manufactured by the method described in claims 1-4.