Lithium molybdate-chromate single crystal, preparation method thereof and optical filter

Abstract

The application discloses a lithium molybdate-chromate monocrystal, a preparation method thereof and an optical filter, and belongs to the technical field of optical materials. x Mo 1‑x O4, wherein 0.005<=x<=0.5, the lithium molybdate-chromate monocrystal is of a trigonal system, a space group is R, is of a cut-off absorption below a wavelength of 500 nm, has a high transmittance from above 500 nm to an infrared wave band, and has a room-temperature thermal conductivity of 4 W.m ‑1 ·K ‑1 -1.K-1, and is suitable for manufacturing a 500 nm long-wave-pass optical filter with high stability and high reliability. The application introduces appropriate hexavalent chromium (Cr 6+ ) into a lithium molybdate matrix, and can move an absorption edge and form a required long-wave-pass characteristic by adjusting the content of hexavalent chromium; meanwhile, the high thermal conductivity and mechanical stability of the oxide crystal make it more suitable for high-power transmission and optical application in a harsh environment, and solve the problems of the existing thin film and polymer optical filter in thermal stability and high-power bearing.

Description

Technical Field

[0001] This invention relates to the field of optical materials technology, and in particular to a lithium molybdenum chromate single crystal, its preparation method, and an optical filter. Background Technology

[0002] Optical filters are indispensable basic components in optical systems, widely used in imaging, spectral analysis, laser systems, and infrared detection. Existing long-pass filters typically employ multilayer dielectric film deposition, doped glass, or organic polymer substrates to achieve specific cutoff wavelengths and transmission characteristics. While multilayer film technology offers flexibility in narrowband and tunable cutoff, it exhibits limitations under conditions of high transmittance, thermal stability, and high power transmission over a wide spectral range: thermal expansion mismatch at film interfaces, film peeling or stress cracking, and layer-by-layer loss accumulation in the near-infrared band all limit its reliability in high-temperature or high-power applications. Furthermore, while polymer substrates are easy to process, they are weaker in terms of temperature resistance, radiation resistance, and long-term optical stability, making them unsuitable for long-term use under harsh conditions.

[0003] To meet the demand for long-wavelength optical components with broad-spectrum high transmittance, low scattering, and excellent thermal properties, single-crystal materials have become ideal candidates due to their regular atomic arrangement, good optical homogeneity, and superior mechanical strength and thermal conductivity. Common optical single crystals such as silicon and germanium have good transmittance in the infrared band, but they have limitations in terms of cutoff control in the visible-near-infrared transition region and material machinability. On the other hand, oxide crystal systems can finely adjust the absorption edge position through doping and solid solution design to achieve the required cutoff wavelength. In the preparation of large-size, low-defect-density single crystals, melt methods (such as Czochralski and crucible lowering methods) are the mainstream industrial preparation route, but they have strict requirements on the purity of raw materials, melt oxygen partial pressure, temperature gradient, and cooling profile. Otherwise, dislocations, cracks, or compositional segregation can be easily introduced, affecting optical performance and yield, and resulting in high manufacturing costs. Summary of the Invention

[0004] The technical problem that the invention aims to solve

[0005] To address the shortcomings of existing optical long-pass filters in terms of thermal stability and high power handling, this invention provides a lithium molybdenum chromate single crystal, its preparation method, and a filter. This invention achieves both large size and high optical uniformity, as well as good thermal properties, thus enabling the fabrication of a highly stable and reliable 500 nm long-pass filter.

[0006] Technical solution

[0007] To achieve the above objectives, the technical solution provided by this invention is as follows:

[0008] This invention relates to a single crystal of lithium molybdenum chromate, with the general chemical formula Li₂Cr₂.x Mo 1-x O4, where 0.005 ≤ x ≤ 0.5, is a trigonal crystal system with space group R. It exhibits cutoff absorption below 500 nm, high transmittance above 500 nm into the infrared band, and a room temperature thermal conductivity of 4 W·m. -1 ·K -1 This technology is suitable for fabricating highly stable and reliable 500 nm long-pass filters. It is achieved by introducing an appropriate amount of hexavalent chromium (Cr) into the lithium molybdate matrix. 6+ Furthermore, the absorption edge can be shifted and the desired long-wavelength pass characteristics can be formed by adjusting the hexavalent chromium content. At the same time, the high thermal conductivity and mechanical stability of oxide crystals make them more suitable for optical applications in high-power transmission and harsh environments, effectively solving the shortcomings of existing thin film and polymer filters in terms of thermal stability and high power handling.

[0009] A filter is disclosed, comprising a crystal sheet made of lithium molybdenum chromate single crystal as the filter medium, having a cutoff wavelength of 500 nm. It exhibits high transmittance above 500 nm and extends into the infrared band, while cutting off transmission below 500 nm. It is applicable to high-power laser transmission, infrared imaging systems, or broadband spectroscopy, providing superior performance and higher reliability long-pass optical elements for infrared imaging, laser transmission, and spectroscopy.

[0010] A method for preparing the above-mentioned lithium molybdenum chromate single crystal by melt growth includes the following steps:

[0011] Li₂CO₃, MoO₃, and Cr₂O₃ were prepared according to stoichiometric ratios to obtain a final composition of Li₂Cr. x Mo 1-x A mixture of O4, 0.005 ≤ x ≤ 0.5;

[0012] The mixture is pre-sintered and melted to obtain a homogeneous melt;

[0013] Single crystal growth of the melt is carried out by the Czochralski method or the crucible lowering method, with a Czochralski rate of 0.5-2.0 mm / h, and the growth is carried out in an inert gas (such as pure argon) or a weak oxidizing atmosphere.

[0014] After growth, the single crystal is annealed, cut, ground and polished.

[0015] A further preparation method involves a single crystal growth temperature of 680-750℃, during which no phase transformation occurs during the cooling and crystallization process of the melt.

[0016] Further preparation methods include a crystal rotation speed of 5-15 rpm in the Czochralski method to adapt to the melt environment and ensure crystal uniformity; and a crystal pulling rate of 0.5-2.0 mm / h to control the growth rhythm of lithium molybdenum chromate single crystals, control the matching of crystallization rate and atomic diffusion, and avoid compositional undercooling and defect surge.

[0017] Further preparation methods, temperature gradient and cooling: control the temperature gradient at the crystal-melt interface to reduce lattice defects; after growth, slowly cool to room temperature at a rate of 10-50℃ / h to reduce thermal stress.

[0018] Further preparation methods utilize platinum crucibles in melt growth. Within the crystal growth temperature range of 680-750℃, high-purity platinum exhibits strong chemical inertness to most molten materials (including oxides, halides, semiconductor melts, and noble metal alloys), exhibiting no chemical reaction, dissolving in the melt, or forming compounds that contaminate the crystal. When the purity is ≥99.99%, the content of impurities (such as Fe, Cu, Ni, Si, etc.) is extremely low, and the impurity diffusion coefficient is small at high temperatures, preventing migration into the melt and avoiding impact on the electrical and optical properties of the crystal (such as semiconductor carrier concentration and laser crystal luminescence efficiency). The uniform heat distribution of the platinum crucible prevents localized overheating or undercooling in the melt, reducing stress, dislocations, inclusions, and other defects during crystal growth, thus improving the integrity of the single crystal.

[0019] Beneficial effects

[0020] Compared with existing known technologies, the technical solution provided by this invention has the following significant advantages:

[0021] This invention provides a melt growth method for preparing large-size, high-quality lithium molybdenum chromate single crystals and its application in 500 nm long-pass filters. The method described in this invention can yield large-size Li₂Cr crystals with diameters reaching tens of millimeters or even larger, low dislocation density, and high optical uniformity. x Mo 1-x O4 single crystals exhibit cutoff transmission (significant absorption) below 500 nm and high transmittance above 500 nm, extending into the near-infrared or even far-infrared range; furthermore, the crystal's room-temperature thermal conductivity is approximately 4 W·m. -1 ·K -1 It has excellent thermal properties, making it suitable for manufacturing long-pass filters that operate under high power and wide spectral conditions; the mechanical and chemical stability of the crystal also facilitates subsequent processing and packaging. Attached Figure Description

[0022] Figure 1 The Li2Cr grown using the Czochralski method in this specific embodiment 0.005 Mo 0.995Physical image of O4 single crystal;

[0023] Figure 2 For a specific embodiment, Li2Cr x Mo 1-x X-ray diffraction (XRD) pattern of O4 single crystal;

[0024] Figure 3 For a specific embodiment, Li2Cr 0.005 Mo 0.995 Ultraviolet-visible-infrared transmittance spectrum of O4 single crystal wafer;

[0025] Figure 4 For a specific embodiment, Li2Cr 0.005 Mo 0.995 O4 single crystal thermal conductivity test diagram;

[0026] Figure 5 The Li2Cr grown using the Czochralski method in this specific embodiment 0.1 Mo 0.9 Physical image of O4 single crystal;

[0027] Figure 6 For a specific embodiment, Li2Cr 0.1 Mo 0.9 Differential scanning calorimetry (DSC) test pattern of O4 single crystal;

[0028] Figure 7 For a specific embodiment, Li2Cr 0.1 Mo 0.9 Spectrum of ultraviolet-visible-infrared transmittance of O4 single crystal. Detailed Implementation

[0029] To further understand the content of this invention, a detailed description of the invention will be provided in conjunction with the accompanying drawings and embodiments.

[0030] Example 1

[0031] In this embodiment, the lithium molybdenum chromate single crystal was grown using the Czochralski method. 0.005 Mo 0.995 The steps for preparing O4 single crystal by melt growth are as follows:

[0032] Ingredients: Li₂CO₃, MoO₃, and Cr₂O₃, all with a purity of 99.99%, are weighed out according to the stoichiometric ratio as raw materials, and the target stoichiometric Li₂Cr₃ is prepared according to the ratio. 0.005 Mo 0.995 80g of mixed O4 powder, of which Li2CO3 weighs 27.13g, MoO3 weighs 52.73g, and Cr2O3 weighs 0.14g;

[0033] Pretreatment: The mixed powder is ground evenly and pressed into a cake shape and placed in a high-purity platinum crucible with a diameter of 40 mm. Then, the crucible containing the raw materials is placed in a muffle furnace and pre-fired at 500°C in air for 6 hours to decompose the carbonates and remove volatiles. The pre-synthesized mixture is melted to obtain a uniform parent melt.

[0034] Melt growth: Take out the crucible containing the raw material and put it into the Czochralski single crystal growth furnace to melt the raw material at 700℃ and hold it at that temperature for 2 hours to ensure uniformity.

[0035] Growth atmosphere: directional single crystal seed crystals are used, the rotation speed is 8 rpm, the pulling rate is 1.0 mm / h, and crystal growth is carried out in an air atmosphere; air atmosphere: it is carried out under the protection of argon or argon / oxygen mixture with controllable oxygen content, or it can be carried out in an inert gas (such as pure argon) or a weak oxidizing atmosphere to control the chromium valence state and redox conditions.

[0036] Post-processing: After growth, the temperature is reduced to room temperature at a rate of 30℃ / h to eliminate thermal stress and stabilize the crystal structure. Then, the crystal is cut, ground and polished to prepare optical sheets.

[0037] The grown crystals are shown in the attached image. Figure 1 As shown, the crystals are light yellow, transparent, and of high quality, with a complete shape and no visible scattering or inclusion impurities. The crystal diameter reaches 20 mm, and the uniform length is 35 mm. The XRD characterization results of the obtained crystals are attached. Figure 2 As shown, the crystal structure is consistent with that of Li₂MoO₄, belonging to the trigonal crystal system with space group R. The transmittance spectrum of the obtained crystal in the ultraviolet-visible-infrared range is shown in the attached figure. Figure 3 As shown, the crystal exhibits significantly low transmittance (cutoff) below 460 nm, but high transmittance in the 470-2500 nm region. The thermal conductivity test results of the crystal are attached. Figure 4 As shown, the thermal conductivity of the crystal at room temperature is 4.1 W·m. -1 ·K -1 Therefore, this crystal wafer is suitable for manufacturing 500 nm long-pass filters with high stability and high reliability.

[0038] Example 2

[0039] In this embodiment, the lithium molybdenum chromate single crystal was grown using the Czochralski method. 0.1 Mo 0.9 O4 single crystal, the basic steps are the same as in Example 1, the specific steps are as follows:

[0040] Ingredients: Weigh out Li2CO3, MoO3 and Cr2O3, all of high purity (99.99%), as raw materials according to stoichiometric ratio, and prepare 80g of mixed powder of the target stoichiometric Li2Cr0.1Mo0.9O4, of which Li2CO3 weighs 21.01g, MoO3 weighs 36.83g and Cr2O3 weighs 2.16g.

[0041] Pretreatment: The mixed powder is ground evenly and pressed into a cake shape and placed in a high-purity platinum crucible with a diameter of 40 mm. Then, the crucible containing the raw materials is placed in a muffle furnace and pre-fired at 500°C in air for 6 hours to decompose the carbonates and remove volatiles. The pre-synthesized mixture is melted to obtain a uniform parent melt.

[0042] Melt growth: Take out the crucible containing the raw material and put it into the Czochralski single crystal growth furnace to melt the raw material at 700℃ and hold it at that temperature for 2 hours to ensure uniformity.

[0043] Growth atmosphere: directional single crystal seed crystals are used, the rotation speed is 5 rpm, the pulling rate is 1.5 mm / h, and crystal growth is carried out in an air atmosphere; air atmosphere: it is carried out under the protection of argon or argon / oxygen mixture with controllable oxygen content, or it can be carried out in an inert gas (such as pure argon) or a weak oxidizing atmosphere to control the chromium valence state and redox conditions.

[0044] Post-processing: After growth, the temperature is reduced to room temperature at a rate of 50℃ / h to eliminate thermal stress and stabilize the crystal structure. Then, the crystal is cut, ground and polished to prepare optical sheets.

[0045] The grown crystals are shown in the attached image. Figure 5 As shown, the crystal exhibits a deep yellow, transparent, and high-quality appearance, with a color deeper than the crystal in Example 1, primarily due to a higher Cr6+ content. The crystal has a complete shape, free from visible scattering and inclusion impurities. The crystal diameter is approximately 20 mm, with a constant diameter length of 35 mm. The XRD characterization results of the obtained crystal are attached. Figure 2 The crystal structure is consistent with that of Li₂MoO₄, belonging to the trigonal crystal system with space group R. The DSC characterization results of the obtained crystals are attached. Figure 6 As shown, the crystal has a melting point of approximately 698℃, and there is no phase transition during the melt-to-crystallization process, which is beneficial for large-size, high-quality growth. The UV-Vis-IR transmittance test results of the obtained crystal are attached. Figure 7 As shown, the crystal has significantly low transmittance (cutoff) below 490 nm, but high transmittance in the 500-2500 nm region, making it suitable for fabricating highly stable and reliable 500 nm long-pass filters.

[0046] Example 3

[0047] In this embodiment, the lithium molybdenum chromate single crystal was grown using the Czochralski method.x Mo 1-x O4, where x is 0.3 and 0.5 respectively, that is, to prepare the target stoichiometry of Li2Cr according to the formula. 0.3 Mo 0.7 O4 and Li2Cr 0.5 Mo 0.5 The O4 mixed powder is prepared using the same basic steps as in Example 2, with different parameters shown in Table 1.

[0048] Table 1: Process parameters for melt growth preparation methods of different lithium molybdenum chromate single crystals

[0049]

[0050] The grown Li2Cr 0.3 Mo 0.7 O4 and Li2Cr 0.5 Mo 0.5 O4 single crystals all exhibit a deep yellow, transparent, and high-quality characteristic.

[0051] After testing, combined with the attached Figure 1-7 The Li2Cr prepared in Examples 1-3 0.005 Mo 0.995 O4, Li2Cr 0.1 Mo 0.9 O4, Li2Cr 0.3 Mo 0.7 O4 and Li2Cr 0.5 Mo 0.5 O4 single crystals are all trigonal crystals with space group R. Its core features are "rhombohedral lattice (R lattice) + triple inversion symmetry ( "Atomic packing is a variant of hexagonal close packing. The overall structure is a periodic repetition of rhombohedral atoms. After rotating the atoms 120° around the axis, and then inverting them through a point on the axis (equivalent to "rotation + mirroring"), the structures completely overlap, achieving symmetrical matching after the 120° rotation inversion. Due to its good optical homogeneity and absence of piezoelectric and thermoelectric interference, it is a superior material for use as an infrared optical window. It exhibits cutoff absorption below 500 nm and high transmittance above 500 nm into the infrared band, with a room temperature thermal conductivity of 4 W·m." -1 ·K -1 It is especially suitable for manufacturing 500 nm long-pass filters with high stability and high reliability.

[0052] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention. The actual structure and manufacturing steps are not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A single crystal of lithium molybdenum chromate, characterized in that: Li2Cr x Mo 1-x O4, wherein 0.005≤x≤0.5; It belongs to the trigonal crystal system and has the space group R. .

2. The lithium molybdenum chromate single crystal according to claim 1, characterized in that, It exhibits cutoff absorption below 500 nm, high transmittance above 500 nm into the infrared band, and a room temperature thermal conductivity of 4 W·m. -1 ·K -1 .

3. A filter, characterized in that, A crystal sheet is fabricated using the lithium molybdenum chromate single crystal as described in claim 2 as the filter medium. Its cutoff wavelength is 500 nm, it has high transmittance in the band above 500 nm and can extend to the infrared band, and it has cutoff transmittance below 500 nm.

4. A method for preparing lithium molybdenum chromate single crystals by melt growth according to any one of claims 1-2, characterized in that, Includes the following steps: Li₂CO₃, MoO₃, and Cr₂O₃ were prepared according to stoichiometric ratios to obtain a final composition of Li₂Cr. x Mo 1-x A mixture of O4, 0.005 ≤ x ≤ 0.5; The mixture is pre-sintered and melted to obtain a homogeneous melt; The melt is grown into a single crystal using the Czochralski method or the crucible lowering method, with a pulling rate of 0.5-2.0 mm / h, in an inert gas or weak oxidizing atmosphere. After growth, the single crystal is annealed, cut, ground and polished.

5. The preparation method according to claim 4, characterized in that: The single crystal growth temperature is 680-750℃, and there is no phase transformation during the cooling and crystallization process of the melt.

6. The preparation method according to claim 4, characterized in that: In the Czochralski method, the crystal rotation speed is 5-15 rpm and the crystal pulling rate is 0.5-2.0 mm / h.

7. The preparation method according to claim 4, characterized in that: The crucible used in the melt growth process is made of platinum.

8. The preparation method according to claim 4, characterized in that: After growth is complete, cool the temperature to room temperature at a rate of 10-50℃ / h.

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