Hollow-free high-quality bismuth vanadate single crystal and preparation method thereof

Abstract

The invention belongs to the technical field of artificial crystals, and particularly discloses a hollowing-free high-quality bismuth vanadate single crystal and a preparation method thereof. The preparation method comprises the following steps: mixing bismuth oxide and vanadium pentoxide to obtain a mixed raw material; then putting the mixed raw material into a platinum crucible, and heating for reaction to obtain bismuth vanadate; placing the obtained platinum crucible with bismuth vanadate in a pulling furnace, and preparing the hollowness-free high-quality bismuth vanadate single crystal by adopting a pulling method; the heating mode of the Czochralski method is medium-frequency induction heating. According to the invention, the heating mode is changed, so that the heat in the bismuth vanadate melt in the crucible can be quickly transferred and uniformly distributed, thereby keeping stable temperature distribution, realizing better thermal field stability, laying a foundation for preparing high-quality bismuth vanadate single crystals, and effectively preventing the generation of crystal hollowing. In addition, an iridium crucible is replaced by a platinum crucible, the pulling atmosphere does not need to be controlled, and low-cost preparation of the bismuth vanadate single crystal is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of artificial crystals, and particularly to a non-hollow high-quality bismuth vanadate single crystal and a preparation method thereof. Background Art

[0002] Bismuth vanadate (BiVO4) is a multifunctional crystal material with visible light response, having unique ferroelasticity, microwave dielectric response, ionic conductivity, acousto-optic effect, photochromic effect, photoelectric effect, and photo / photoelectrocatalytic activity. It can be applied to many fields such as microwave components, direct detection of high-energy rays, solar energy conversion and utilization, and degradation of organic pollutants, showing broad practical application prospects and huge potential market value.

[0003] Common bismuth vanadate materials are in polycrystalline (nanocrystalline, microcrystalline) and single crystal forms. According to the literature, only a few previous research works reported the successful preparation of BiVO4 single crystals by the Czochralski method. In 2013, Rettie et al. grew BiVO4, W:BiVO4, and Mo:BiVO4 single crystals using the floating zone method, but the obtained crystal diameter was only 6 mm. Chinese invention patent ZL201210188943.2 (Li Guoling et al., A preparation method of large-size bismuth vanadate single crystals) discloses a preparation method of large-size bismuth vanadate single crystals. In this method, the bismuth vanadate polycrystalline raw material obtained by high-temperature pre-sintering of Bi2O3 and V2O5 is put into an iridium crucible, and then the iridium crucible is put into a single crystal pulling furnace. N2 atmosphere is filled in the pulling furnace, and the oxygen partial pressure in the furnace is reduced to 1%. Using a YVO4 crystal as a seed crystal, controlling the temperature at the bottom of the iridium crucible to be 960 - 980 °C, the pulling speed of crystal growth to be 1 mm / h, and the rotation speed to be 8 rpm. After crystal growth stops, it is placed for 24 h to cool to room temperature, and then taken out to obtain BiVO4 single crystals. However, this method has the following problems: First, the obtained large-size bismuth vanadate single crystals have a relatively serious hollowing phenomenon, which results in a low effective utilization rate of the crystals. Second, the crucible material used in this method is the noble metal iridium. In recent years, due to market price fluctuations, the price of iridium has risen from about 0.5 times that of the noble metal platinum before to about 4 times at present. Therefore, the use of iridium crucibles not only increases the preparation cost but also limits the wide application of this method. In addition, only the German company SurfaceNet sells commercial bismuth vanadate single crystals in the world, but the crystal size is small and the shape is irregular, and the size of the single crystal slices that can be provided does not exceed 10 mm × 10 mm.

[0004] In view of the above problems, it is urgent to develop a new preparation process for large-size bismuth vanadate single crystals to avoid the hollowing phenomenon or irregular shape of bismuth vanadate single crystals, improve the quality of bismuth vanadate single crystals; at the same time, avoid the use of iridium crucibles, reduce the preparation cost, so as to achieve the low-cost and high-quality preparation of large-size bismuth vanadate single crystals. Summary of the Invention

[0005] In view of this, the present invention provides a non-hollow high-quality bismuth vanadate single crystal and a preparation method thereof, so as to solve the problem that the size of the bismuth vanadate single crystal prepared by the existing method is too small, as well as the problems of hollowness, irregularity and low quality existing in the large-size bismuth vanadate single crystal.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A preparation method of a non-hollow high-quality bismuth vanadate single crystal, comprising the following steps:

[0008] 1) Mix bismuth oxide and vanadium pentoxide to obtain a mixed raw material;

[0009] 2) Place the mixed raw material in a platinum crucible, heat it to react to obtain bismuth vanadate;

[0010] 3) Place the platinum crucible with bismuth vanadate obtained in step 2) in a pulling furnace, and use the pulling method to prepare a non-hollow high-quality bismuth vanadate single crystal;

[0011] The heating method of the pulling method is intermediate frequency induction heating.

[0012] Preferably, in step 1), the molar ratio of Bi to V elements in the bismuth oxide and vanadium pentoxide is 50 - X:50 + X, and the value of X is 0.5 to 3.5.

[0013] Preferably, in step 2), the reaction temperature is 700 to 750 °C, and the reaction time is 8 to 12 h.

[0014] Preferably, the purity of the platinum crucible in step 2) is ≥99.95%;

[0015] The shape of the platinum crucible is frustum-shaped, the upper mouth diameter of the platinum crucible is ≥70 mm, the lower bottom diameter is ≥60 mm, 1.50 ≥ upper mouth diameter / lower bottom diameter ≥ 1.05; the depth is ≥60 mm; the side wall thickness is ≥0.5 mm, and the bottom wall thickness is ≥1.0 mm; the volume is ≥350 mL.

[0016] Preferably, the frequency of the intermediate frequency induction heating is 5 to 50 kHz.

[0017] Preferably, at the initial stage of pulling in step 3), the temperature at the bottom of the crucible is 940 to 990 °C.

[0018] Preferably, there is also a heat insulation material outside the platinum crucible in step 3).

[0019] Preferably, the heat insulation material is zirconia sand;

[0020] The particle size of the zirconia sand is 0.1 to 4 mm.

[0021] Preferably, during the pulling process in step 3), the rotation speed of the seed rod is 2 - 20 rpm, the pulling speed is 0.2 - 3 mm / h, the crystal growth time is 15 - 240 h, and the cooling time is 20 - 80 h.

[0022] Another object of the present invention is to provide a high-quality bismuth vanadate single crystal without hollowing prepared by the preparation method.

[0023] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The present invention first introduces the method of intermediate frequency induction heating in the preparation process of large-size bismuth vanadate single crystals. By changing the heating method, the present invention can induce eddy currents in the crucible wall to achieve uniform heating. The intermediate frequency induction heating disclosed in the present invention can not only avoid surface overheating caused by high frequency but also avoid insufficient heating caused by low frequency, thus making the overall heating of the crucible wall more uniform; the bismuth vanadate melt in the crucible can also form a directional flow under the condition of intermediate frequency induction heating, strengthening heat transfer and reducing local temperature differences. This lays a foundation for the preparation of high-quality bismuth vanadate single crystals and at the same time prevents the generation of hollowing; in addition, the heating frequency can be flexibly adjusted according to actual needs to achieve precise control of heating power and temperature.

[0025] 2. The present invention also uses a platinum crucible for the preparation of bismuth vanadate single crystals. Compared with an iridium crucible, the platinum crucible has a lower cost, and there is no need to control the nitrogen / oxygen mixed atmosphere during the pulling process, avoiding the influence of the atmosphere on the product, reducing experimental control conditions, lowering the experimental difficulty, and being conducive to the stable and high-quality preparation of bismuth vanadate single crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0027] Figure 1 It is a physical diagram of the bismuth vanadate single crystal prepared in Example 1 of the present invention;

[0028] Figure 2 It is a physical diagram of the bismuth vanadate single crystal prepared in Example 2 of the present invention;

[0029] Figure 3 It is a physical diagram of the bismuth vanadate single crystal prepared in Example 3 of the present invention;

[0030] Figure 4Polarization pattern of the (004) crystal plane of the bismuth vanadate single crystal prepared in Example 2 of the present invention. Detailed implementation mode

[0031] The present invention provides a method for preparing a high-quality bismuth vanadate single crystal without hollowing, comprising the following steps:

[0032] 1) Mix bismuth oxide and vanadium pentoxide to obtain a mixed raw material;

[0033] 2) Place the mixed raw material in a platinum crucible and heat it in a muffle furnace for reaction to obtain bismuth vanadate;

[0034] 3) Place the platinum crucible with bismuth vanadate obtained in step 2) in a pulling furnace and use the pulling method to prepare a high-quality bismuth vanadate single crystal without hollowing.

[0035] In the present invention, the heating method of the pulling method is intermediate frequency induction heating.

[0036] In the present invention, in step 1), the molar ratio of Bi to V elements in the bismuth oxide and vanadium pentoxide is 50 - X:50 + X, and the value of X is 0.5 to 3.5.

[0037] In the present invention, the chemical reaction formula is: V2O5 + Bi2O3 → 2BiVO4, and the added excessive V2O5 is to supplement the vanadium volatilization loss occurring during crystal growth.

[0038] In the present invention, in step 1), the mixing is preferably carried out by ball milling, and the ball milling time is preferably 8 to 12 h, specifically it can be 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h, 11.5 h.

[0039] In the present invention, in step 2), the reaction temperature is 700 to 750 °C, specifically it can be 710 °C, 720 °C, 730 °C, 740 °C; the reaction time is 8 to 12 h, specifically it can be 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h, 11.5 h.

[0040] In the present invention, in step 2), the purity of the platinum crucible is ≥ 99.95%, specifically it can be 99.95%, 99.99%.

[0041] In the present invention, the shape of the platinum crucible is frustum-shaped, and its side view is an inverted trapezoid. The upper mouth diameter of the platinum crucible is ≥70 mm, specifically it can be 72 mm, 75 mm, 78 mm, 80 mm, 85 mm, 90 mm; the lower bottom diameter is ≥60 mm, specifically it can be 62 mm, 65 mm, 68 mm, 70 mm; 1.50 ≥ upper mouth diameter / lower bottom diameter ≥ 1.05, and the upper mouth diameter / lower bottom diameter can specifically be 1.1, 1.15, 1.2, 1.25, 1.28, 1.3, 1.35, 1.4, 1.45; the depth of the platinum crucible is ≥60 mm, specifically it can be 62 mm, 65 mm, 68 mm, 70 mm, 75 mm, 80 mm; the side wall thickness is ≥0.5 mm, specifically it can be 0.55 mm, 0.6 mm, 0.65 mm, 0.7 mm, 0.75 mm, 0.8 mm, 0.9 mm, 1 mm; the bottom wall thickness is ≥1.0 mm, specifically it can be 1.1 mm, 1.2 mm, 1.4 mm, 1.5 mm, 1.6 mm, 1.8 mm, 2 mm; the volume is ≥350 mL, specifically it can be 360 mL, 400 mL, 500 mL, 600 mL, 700 mL, 800 mL, 1000 mL.

[0042] In the present invention, the shape of the platinum crucible is conducive to the directional flow of the bismuth vanadate melt, and can promote more uniform temperature inside the crucible; in addition, bismuth will corrode and penetrate the platinum crucible by forming a platinum-bismuth alloy at high temperatures. The shape of the platinum crucible disclosed in the present invention is conducive to slowing down bismuth corrosion, and has a longer service life compared to a cylindrical crucible. The high-purity platinum crucible helps to reduce the influence of impurities on crystal growth, ensures the purity and quality of the crystal, and can effectively extend the service life of the crucible.

[0043] In the present invention, the frequency of the intermediate frequency induction heating is 5 - 50 kHz, specifically it can be 10 kHz, 15 kHz, 20 kHz, 30 kHz, 40 kHz.

[0044] In the present invention, at the initial stage of pulling in step 3), the temperature at the bottom of the crucible is 940 - 990 °C, specifically it can be 950 °C, 960 °C, 970 °C, 980 °C.

[0045] In the present invention, outside the platinum crucible in step 3), there is also a heat-insulating material.

[0046] In the present invention, the heat-insulating material is zirconia sand; it has good heat-insulating performance and can effectively maintain the temperature of the crucible. The particle size of the zirconia sand is 0.1 - 4 mm, specifically it can be 0.2 mm, 0.5 mm, 0.8 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm.

[0047] In the present invention, the seed crystal for pulling is preferably a bismuth vanadate seed crystal obtained by cutting a bismuth vanadate crystal prepared by the applicant in the early stage (adopting the solution of Chinese invention patent ZL201210188943.2). Its cross-section is preferably 5 mm × 5 mm, and the length is preferably 12 - 30 mm, specifically it can be 14 mm, 15 mm, 18 mm, 20 mm, 22 mm, 25 mm, 28 mm; the orientations are <100>, <001>, <110>, <101>, <111>, etc. Seed crystals with different orientations can provide different growth directions for crystal growth, thereby preparing single crystals with specific orientations.

[0048] In the present invention, during the pulling in step 3) by the pulling method, the rotation speed of the seed crystal rod is 2 - 20 rpm, the pulling speed is 0.2 - 3 mm / h, the crystal growth time is 15 - 240 h, and the cooling time is 20 - 80 h.

[0049] By precisely controlling the rotation speed of the seed crystal rod and the pulling speed in the present invention, the growth rate and quality of the crystal can be effectively adjusted; from the growth temperature to room temperature, the slow cooling process helps to stabilize the internal structure of the crystal and reduce the defects inside the crystal.

[0050] The pulling method of the present invention includes steps such as melting the material, seeding, necking down, shoulder releasing, shoulder turning, equal diameter growth, ending, and cooling and furnace shutdown in sequence.

[0051] The present invention also provides a high-quality bismuth vanadate single crystal without hollowing prepared by the preparation method.

[0052] Next, the technical solutions in the embodiments of the present invention will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

[0053] In the following examples, bismuth vanadate single crystals are grown by the intermediate frequency induction pulling method under one atmospheric pressure and in an air atmosphere. The upper diameter of the frustum-shaped platinum crucible used is 90 mm, the lower bottom diameter is 70 mm, the upper diameter / lower bottom diameter = 1.28, the depth is 80 mm, the side wall thickness is 1 mm, the bottom wall thickness is 1.5 mm, and the volume is ~400 mL.

[0054] Example 1

[0055] Mix bismuth oxide and vanadium pentoxide according to Bi:V = 49:51, then ball-mill and mix for 10 h to obtain a mixed raw material. Place the mixed raw material in the above-mentioned platinum crucible and place the platinum crucible in a muffle furnace. Raise the furnace temperature to 700 °C at a heating rate of 200 °C / h and keep it constant at 700 °C for 10 hours to obtain a bismuth vanadate raw material with a total weight of 2 kg.

[0056] Place the above platinum crucible with the bismuth vanadate raw material in a Czochralski furnace for pulling (melting the material, seeding, necking, shoulder opening, shoulder turning, equal-diameter growth, ending, and cooling and stopping the furnace). Set a zirconia sand insulation layer with a particle size range of 0.1 - 4 mm outside the platinum crucible, and use an intermediate frequency induction furnace for heating with the working frequency set to 22 kHz. The temperature at the bottom of the crucible at the beginning of pulling is 968 °C; the orientation of the bismuth vanadate seed crystal is <001>, and its size is 5 mm × 5 mm × 15 mm. The process parameters for crystal growth are as follows: the rotation speed of the seed rod is 15 rpm, the pulling speed is 2 mm / h, the growth time is 20 h, and the cooling time is 24 h.

[0057] The prepared large-size bismuth vanadate single crystal is as Figure 1 shown. Through Figure 1 it can be seen that the diameter is ~20 mm, the effective length is ~25 mm, and the bottom photo shows no hollowing.

[0058] Example 2

[0059] Mix bismuth oxide and vanadium pentoxide according to Bi:V = 48.5:51.5, then ball-mill and mix for 10 h to obtain a mixed raw material. Place the mixed raw material in the above-mentioned platinum crucible and place the platinum crucible in a muffle furnace. Raise the furnace temperature to 700 °C at a heating rate of 200 °C / h and keep it constant at 700 °C for 10 hours to obtain a bismuth vanadate raw material with a total weight of 2.2 kg.

[0060] Place the above platinum crucible with the bismuth vanadate raw material in a Czochralski furnace for pulling (melting the material, seeding, necking, shoulder opening, shoulder turning, equal-diameter growth, ending, and cooling and stopping the furnace). Set a zirconia sand insulation layer with a particle size range of 0.1 - 4 mm outside the platinum crucible, and use an intermediate frequency induction furnace for heating with the working frequency set to 20 kHz. The temperature at the bottom of the crucible at the beginning of pulling is 961 °C; the orientation of the bismuth vanadate seed crystal is <100>, and its size is 5 mm × 5 mm × 15 mm. The process parameters for crystal growth are as follows: the rotation speed of the seed rod is 10 rpm, the pulling speed is 1.5 mm / h, the growth time is 28 h, and the cooling time is 36 h.

[0061] The prepared large-size bismuth vanadate single crystal is as Figure 2 shown. Through Figure 2 it can be seen that the diameter is ~30 mm, the effective length is ~30 mm, and the bottom photo shows no hollowing.

[0062] Example 3

[0063] Bismuth oxide and vanadium pentoxide were proportioned according to Bi:V = 48:52, and then ball-milled and mixed for 10 h to obtain a mixed raw material. The mixed raw material was placed in the above-mentioned platinum crucible, and the platinum crucible was placed in a muffle furnace. The furnace temperature was raised to 700 °C at a heating rate of 200 °C / h and held at 700 °C for 10 hours to obtain a bismuth vanadate raw material with a total weight of 2.4 kg.

[0064] The platinum crucible with the bismuth vanadate raw material was placed in a Czochralski furnace for pulling (melting, seeding, necking, shoulder opening, shoulder turning, isodiametric growth, ending, and cooling and furnace shutdown). An insulating layer of zirconia sand with a particle size range of 0.1 - 4 mm was set outside the platinum crucible, and it was heated by an intermediate frequency induction furnace with the working frequency set at 18 kHz. The temperature at the bottom of the crucible was 952 °C at the beginning of pulling; the orientation of the bismuth vanadate seed crystal was <110>, and its size was 5 mm × 5 mm × 15 mm. The process parameters for crystal growth were as follows: the rotation speed of the seed rod was 3 rpm, the pulling speed was 0.5 mm / h, the growth time was 96 h, and the cooling time was 48 h.

[0065] The obtained large-size bismuth vanadate single crystal is as Figure 3 shown. It can be seen from Figure 3 that the diameter is ~40 mm, the effective length is ~40 mm, and the bottom photo shows no hollowing.

[0066] The bismuth vanadate single crystal samples prepared in Examples 1 - 3 were all transparent yellowish-brown, with a complete appearance and no cracking, and no hollowing phenomenon occurred. Through XRD characterization, it was determined that the bismuth vanadate crystals prepared in Examples 1 - 3 were all monoclinic structures (space group I2 / b), and the unit cell parameters were: γ = 90.38°. Figure 4 Figure 20 is a polarized light image of the (004) crystal plane of the bismuth vanadate single crystal prepared in Example 2 taken by a polarized light microscope. It is circular and the brightness is relatively uniform, indicating that the internal structure of the single crystal is relatively uniform. Further, through the X-ray diffraction rocking curve test, the full width at half maximum of the (004) crystal plane of the bismuth vanadate single crystal was 40 arcseconds, indicating its high crystallization quality.

[0067] In this specification, each example is described in a progressive manner. The key point of each example is to illustrate the differences from other examples. For the same or similar parts among the examples, reference can be made to each other.

[0068] The foregoing description of the disclosed embodiments enables those skilled in the art to practice or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing high-quality bismuth vanadate single crystal without hollowing, characterized in that: The steps include: 1) mixing bismuth oxide and vanadium pentoxide to obtain a mixed raw material; 2) placing the mixed raw materials in a platinum crucible and heating them for reaction to obtain bismuth vanadate; 3) placing the platinum crucible with bismuth vanadate obtained in step 2) in a pulling furnace, and preparing high-quality bismuth vanadate single crystals without hollowing by a pulling method; The heating method of the Czochralski method is medium frequency induction heating.

2. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 1, characterized in that: In step 1), the molar ratio of Bi to V in the bismuth oxide and vanadium pentoxide is 50-X:50+X, and the value of X is 0.5-3.

5.

3. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 2, characterized in that: The reaction temperature in step 2) is 700-750° C., and the reaction time is 8-12 hours.

4. A method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to any one of claims 1 to 3, characterized in that: The purity of the platinum crucible in step 2) is ≥99.95%; The shape of the platinum crucible is a truncated cone, with an upper diameter of ≥70 mm, a lower bottom diameter of ≥60 mm, 1.50≥upper diameter / lower bottom diameter≥1.05; a depth of ≥60 mm; a side wall thickness of ≥0.5 mm, a bottom wall thickness of ≥1.0 mm; and a volume of ≥350 mL.

5. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 4, characterized in that: The frequency of the medium frequency induction heating is 5 to 50 kHz.

6. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 5, characterized in that: Step 3) In the initial stage of the pulling process, the temperature of the bottom of the crucible is 940-990°C.

7. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 5 or 6, characterized in that: Step 3) The platinum crucible also includes a heat-insulating material.

8. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 7, characterized in that: The thermal insulation material is zirconium oxide sand; The particle size of the zirconium oxide sand is 0.1-4 mm.

9. The method for preparing a non-hollowing high-quality bismuth vanadate single crystal according to claim 8, characterized in that: Step 3) During the pulling method, the rotation speed of the seed crystal rod is 2 to 20 rpm, the pulling speed is 0.2 to 3 mm / h, the crystal growth time is 15 to 240 h, and the cooling time is 20 to 80 h.

10. The high-quality bismuth vanadate single crystal without hollowing prepared by the preparation method according to any one of claims 1 to 9.

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