CN108203844B - Magnesium tantalate series crystal and its preparing process - Google Patents

Magnesium tantalate series crystal and its preparing process Download PDF

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CN108203844B
CN108203844B CN201810019067.8A CN201810019067A CN108203844B CN 108203844 B CN108203844 B CN 108203844B CN 201810019067 A CN201810019067 A CN 201810019067A CN 108203844 B CN108203844 B CN 108203844B
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CN108203844A (en
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马云峰
徐家跃
蒋毅坚
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Shanghai Institute of Technology
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    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/16—Oxides
    • C30B29/22—Complex oxides
    • C30B29/30—Niobates; Vanadates; Tantalates
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
    • C30B15/08—Downward pulling

Abstract

The invention discloses a magnesium tantalate series crystal, namely MgO-Ta2O5Pseudo-binary systems, each being Mg4Ta2O9、Mg3Ta2O8And MgTa2O6. Also provided are methods for preparing the magnesium tantalate crystals using excess MgO or Mg (OH)2Component compensation, synthesizing high-purity MgTa through solid-phase reaction2O6And Mg4Ta2O9The powder is grown into centimeter-level rod-shaped MgTa by adopting a micro-pulling-down method2O6And Mg4Ta2O9A single crystal; with Mg4Ta2O9And MgTa2O6As a melt initiator, using a spontaneous nucleation technique on MgTa2O6Centimeter-level rod-shaped Mg is grown on the ceramic seed crystal rod3Ta2O8A single crystal. The invention adopts a micro-pulling method, effectively utilizes the high temperature gradient near the growth interface, realizes rapid and high-uniformity crystallization, and effectively utilizes the post heater to avoid the cracking problem caused by overlarge thermal stress.

Description

Magnesium tantalate series crystal and its preparing process
Technical Field
The invention belongs to the field of chemical engineering, and relates to a microwave dielectric material, in particular to a magnesium tantalate series crystal and a preparation method thereof.
Background
With the rapid development of microwave communications, modern mobile phones and electronic communications, microwave dielectric materials playing an important role in the communications system have received great attention from researchers and manufacturers of the scientific and technological industries. Mg (magnesium)4Ta2O9、Mg3Ta2O8、MgTa2O6And the doped material has excellent microwave dielectric property and luminescent property, and is widely researched in the forms of ceramics and powder respectively. To deeply investigate MgO-Ta2O5The preparation of the crystal material is urgent due to the intrinsic properties of the microwave dielectric, the optical property and the like of the system material and the change rule of the system material along with the components, the structure and the crystal orientation.
Mg4Ta2O9The crystal material belongs to a hexagonal system and has an ilmenite structure, the space group is P3c1(165), the lattice constant is a 0.51611nm, c 1.40435nm and V0.32396 nm3。Mg4Ta2O9During the temperature rise to 1825 ℃, although no phase transformation occurs, MgO is volatile at high temperature, which causes Mg4Ta2O9Slightly decomposed to MgTa2O6Is not favorable for the synthesis of pure phase powder, the continuous growth of high-crystalline quality crystal and the component requirements of equal stoichiometric ratio, so the invention adopts MgO or Mg (OH)2Composition compensation for synthesizing high-purity Mg4Ta2O9Powders and crystals.
Mg3Ta2O8Is orthorhombic and belongs to the Ccm (63) space group, the unit cell parameters are a-1.0238 nm, b-1.1456 nm and c-1.0065 nm, and MgO-Ta2O5The phase diagram shows that the magnesium-magnesium alloy exists stably only at 1475-1675 ℃, and is decomposed into Mg at a temperature higher or lower than the temperature range4Ta2O9And MgTa2O6. Pure-phase powders or crystals, even when produced, produce more or less decomposition products during the course of heating, melting or cooling, so that pure-phase single-crystal Mg has hitherto been produced3Ta2O8There has been no report. The invention utilizes the high temperature gradient of the micro-pull-down method and the MgO compensation and spontaneous nucleation method of the initial powder to prepare pure-phase Mg3Ta2O8And (4) crystals.
MgTa2O6Is a tantalate with a trirutile structure, belongs to a P42/mnm space group and has a lattice constant
Figure GDA0002781938390000011
And
Figure GDA0002781938390000012
in the synthesis of MgTa2O6Powder and growing MgTa2O6In the process of crystallization, Ta is caused by volatilization of MgO at high temperature2O5Impurity phase, so that MgO component compensation is required to prepare MgTa with high crystal quality and the like in stoichiometric ratio2O6And (4) crystals.
The micro-pulling-down method is a single crystal growth technology capable of realizing the preparation of high-quality single crystal optical fibers, has the advantages of raw material saving, cost reduction, simple crucible post-treatment, high single crystal growth rate, very large crystal aspect ratio, controllable crystal section shape, capability of growing crystals with high segregation coefficient and the like, and has great development value in the aspects of new material exploration and single crystal performance optimization.
Disclosure of Invention
Aiming at the technical problems in the prior art, the invention provides a magnesium tantalate series crystal and a preparation method thereof, and aims to solve the technical problem that the quality and the performance of the magnesium tantalate series crystal prepared in the prior art are poor.
The invention provides a magnesium tantalate crystal with a molecular formula of Mg4Ta2O9The crystal is a hexagonal crystal system, the space group is P-3c1(165), the unit cell parameters are a-0.51625 (10) nm and c-1.4062 (4) nm, and the density is 6.171g/cm3The refined structure constant was 0.0356.
The invention also provides a magnesium tantalate crystal with the molecular formula of Mg3Ta2O8Belongs to an orthorhombic system, belongs to a space group of Ccm (63), has a lattice constant of 0.38087(12) nm, b of 1.0034(2) nm, c of 1.0206(3) nm and a density of 5.965g/cm3The refined structure constant is 0.0207.
The invention also provides a magnesium tantalate crystal with the molecular formula of MgTa2O6Belonging to the tetragonal system, theThe space group is P42/mnm (136), the lattice constant is a-0.46821 (3) nm, c-0.91382 (7) nm, and the crystal density is 8.03265g/cm3The refinement parameter RAII was 0.026.
The invention also provides the Mg4Ta2O9The preparation method of the crystal comprises the following steps:
1) weighing pure phase Mg (OH) according to a molar ratio of 5.2:1 or 5.6:12And Ta2O5Grinding, mixing, sieving, ball milling, pressing into cylindrical tablet, presintering at 1300 deg.C or 1350 deg.C for 16 hr, and heating and cooling at 100 deg.C/hr; or weighing pure-phase MgO and Ta according to the molar ratio of 4.04:12O5Grinding raw materials, mixing, sieving, ball milling, pressing into cylindrical tablet, presintering at 1400 deg.C for 2 hr, and heating and cooling at 1400 deg.C/h to obtain pure-phase Mg4Ta2O9Grinding the ceramic cylindrical sheet into powder;
2) mixing Mg4Ta2O9And Mg (OH)2Or MgO powder is weighed according to the molar ratio of 10:1, ball-milled and uniformly mixed, put into an Ir crucible and added with Mg4Ta2O9Taking a single crystal rod as seed crystal, vacuumizing for 12h, introducing protective atmosphere Ar, regulating the flow of gas inlet and outlet to 20mL/min to ensure that the gas pressure is stabilized at 1.5atm, setting the micro-pulling-down speed to be 0.05mm/min for growth, and cooling for 3h after the growth is finished to obtain colorless transparent single crystal Mg4Ta2O9. The invention also provides the Mg3Ta2O8A method of crystallizing comprising the steps of:
1) weighing pure phase Mg (OH) according to a molar ratio of 4:12And Ta2O5Grinding, mixing, sieving, ball milling, pressing into cylindrical tablet, pre-sintering at 1300 deg.C for 16 hr, and heating and cooling at 100 deg.C/hr;
2) placing the synthetic ceramic cylindrical sheet in Ir crucible, and adding MgTa2O6Taking a polycrystalline rod as a seed crystal, vacuumizing for 12h, introducing Ar gas, controlling the pressure to be 1.5atm, controlling the flow to be 20mL/min, setting the pull-down speed to be 0.2mm/min, performing spontaneous nucleation, adjusting the power to perform multiple necking and shoulder expanding processes, optimizing the crystallization quality, setting the cooling time to be 10h, cooling to room temperature, and growing until the temperature reaches the room temperatureFinally, obtaining light yellow transparent single crystal Mg3Ta2O8。
The invention also provides the MgTa2O6A method of crystallizing comprising the steps of:
1) weighing pure phase Mg (OH) according to the molar ratio of 1.15:12And Ta2O5Grinding, mixing, sieving, ball milling, pressing into cylindrical tablet, presintering at 1400 deg.C for 8 hr, and heating and cooling at 100 deg.C/hr;
2)MgTa2O6and Mg (OH)2Weighing powder according to a molar ratio of 10:1, ball-milling and uniformly mixing, putting the powder into an Ir crucible, and adding Y3Al5O12Taking a single crystal rod as a seed crystal, vacuumizing for 12h, introducing protective atmosphere Ar, regulating the flow rate of gas inlet and gas outlet to 20mL/min to ensure that the gas pressure is stabilized at 1.5atm, setting the micro-pulling-down speed to be 0.2mm/min for growth, cooling for 8h after the crystal growth is finished, taking out the crystal, annealing at 1000 ℃ in the air for 12h to obtain a transparent single crystal MgTa, and obtaining the transparent single crystal MgTa2O6。
The invention adopts MgO or Mg (OH)2Component compensation, synthesizing high-purity MgTa through solid-phase reaction2O6And Mg4Ta2O9The powder is grown into centimeter-level rod-shaped MgTa by adopting a micro-pulling-down method2O6And Mg4Ta2O9Monocrystals with the size of phi (2-3) mm multiplied by 60mm and phi (1.2-1.6) mm multiplied by 97mm respectively; with Mg4Ta2O9And MgTa2O6As a melt initiator, using a spontaneous nucleation technique on MgTa2O6Centimeter-level rod-shaped Mg is grown on the ceramic seed crystal rod3Ta2O8The size of the single crystal is phi 1.2mm multiplied by 19mm, the grown crystal has high crystal integrity, is transparent and has no macroscopic defects, and completely meets the characterization and test of various intrinsic properties of the crystal.
Compared with the prior art, the invention has remarkable technical progress. The invention adopts excessive MgO or Mg (OH)2 to carry out component compensation, thereby effectively inhibiting the volatilization of MgO and the decomposition and impurity phase generation of compounds in the preparation process of pure-phase powder, ceramics and crystals, and obtaining an isochemical meterThree crystals in quantitative ratio. The micro-pulling-down method used by the invention has high temperature gradient near a solid-liquid interface, and is easier to grow MgO-Ta which is easy to decompose and easy to change phase compared with other crystal growth methods2O5The system crystal material can effectively solve the problem of MgO-Ta because the method is provided with the rear heater near the solid-liquid interface2O5The problem of easy cracking of the crystalline material of the system.
Description of the drawings:
FIG. 1 is a schematic structural view of a radio frequency induction heating type micro pull-down crystal growth furnace for growing micro pull-down crystals;
FIG. 2(a) is 5.2Mg (OH)2:Ta2O5Proportioning raw material synthesized Mg4Ta2O9A powder XRD pattern;
FIG. 2(b) is 5.6Mg (OH)2:Ta2O5Proportioning raw material synthesized Mg4Ta2O9A powder XRD pattern;
FIG. 2(c) shows 4.04MgO: Ta2O5Proportioning raw material synthesized Mg4Ta2O9A powder XRD pattern;
FIG. 3 is a graph showing the results of the reaction with an excess of 10% Mg (OH)2Mg of (2)4Ta2O9Mg grown in powder form4Ta2O9A rod-shaped single crystal;
FIG. 4Mg with 10% excess MgO4Ta2O91.6mm diameter Mg for powder growth4Ta2O9A rod-shaped single crystal;
FIG. 5 shows Mg with 10% excess MgO4Ta2O92mm diameter Mg for powder growth4Ta2O9A rod-shaped single crystal;
FIG. 6(a) with a 10% excess of Mg (OH)2Mg of (2)4Ta2O9Mg grown in powder form4Ta2O9XRD pattern of rod-shaped single crystal powder;
FIG. 6(b) Mg with 10% excess MgO4Ta2O91.6mm diameter Mg for powder growth4Ta2O9Rod-shaped single crystal powder XRA D map;
FIG. 6(c) Mg with 10% excess MgO4Ta2O92mm diameter Mg for powder growth4Ta2O9XRD pattern of rod-shaped single crystal powder;
FIG. 7(a) is 4Mg (OH)2:Ta2O5Powder XRD pattern of ceramic synthesized by the raw materials in proportion;
FIG. 7(b) shows Mg grown by micro-pull-down method3Ta2O8A powder XRD pattern of the crystals;
FIG. 8 shows Mg grown by the micro-pull-down method3Ta2O8A crystal;
FIG. 9 is Mg3Ta2O8A structure of a crystal;
FIG. 10(a)1.15Mg (OH)2:Ta2O5MgTa synthesized from proportioned raw materials2O6A powder XRD pattern of the ceramic;
FIG. 10(b) MgTa grown by the micro-pull-down method2O6A powder XRD pattern of the crystals;
FIG. 11 shows a view of a solution of MgTa2O6The powder was supplemented with 1% Mg (OH)2MgTa grown by micro-pull-down method2O6A crystal;
FIG. 12 is MgTa2O6Structure of crystal.
Detailed Description
Micro-pull-down crystal growth of MgO-Ta2O5The crystal growth furnace is a radio frequency induction heating type micro pull-down crystal growth furnace produced by French Cyberstar company, as shown in figure 1, the front end of an Ir crucible 1 used is conical, the size of the Ir crucible is phi 16mm multiplied by L10mm, the size of a nozzle of the Ir crucible is phi 3mm multiplied by L3mm, the diameter of a nozzle through hole is 1mm, the rear end of the Ir crucible is cylindrical, the size of the Ir crucible is phi 16mm multiplied by L30mm, a meniscus 13 is formed in the falling process of a melting material 12, an Ir rear heater 2 is placed at the bottom of the Ir crucible, the size of the Ir rear heater is phi 16mm multiplied by L15mm, the Ir rear heater is used for adjusting the temperature field distribution near a solid-liquid interface at the nozzle, and the crystal 11 which is installed on a pull rod 15. Two Al with different inner and outer diameters are placed below the rear heater 22O3Made of ringsThe column insulating supports 3 and 14 are used to further adjust the temperature field distribution in the growth region of the crystal 11 to provide a suitable crystallization temperature gradient. Below the heat-insulating support 14 is placed a transparent quartz glass tube 4 serving to adjust the vertical position of the crucible 1 at the radio-frequency heating coil 5 and to visualize the macroscopic state of crystal 11 growth. An inner heat-insulating wall 6, an outer heat-insulating wall 7 and an outer heat-insulating cover 8 are arranged outside the crucible 1, so that the temperature field distribution is optimized. And a glass shield 9 and a top protective cover 10 are additionally arranged outside the glass shield, so that gas leakage and pollution of the radio frequency heating coil 5 outside the shield when the melting material 12 is at high temperature are prevented, and pollution of uncontrollable impurities outside to the growing environment in the glass shield is also prevented.
The invention is further illustrated by the following examples, without restricting the invention thereto.
EXAMPLE 1 micro-Pull-Down growth of Mg4Ta2O9Crystal
(1) Solid phase reaction synthesis of high purity Mg4Ta2O9Powder material
1)5.2Mg(OH)2:1Ta2O5Synthesis of pure phase Mg4Ta2O9Powder material
Mg (OH) with the purity of 99.999 percent and 99.99 percent is weighed according to the molar ratio of 5.2:12And Ta2O5The raw materials (a) are put into a mortar for grinding and uniformly mixing, sieved by a 220-mesh sieve and put into a container filled with ZrO2In the ball milling pot, the ball was milled for 1 hour in a ball mill. Preparing a cylindrical sheet with the size of phi 13mm multiplied by 5mm under the action of 350MPa pressure, putting the cylindrical sheet into a muffle furnace for presintering at 1300 ℃ for 16h, and the temperature rising and falling speed is 100 ℃/h. The phase purity of the resulting ceramic cylindrical sheet was characterized by powder XRD (fig. 2 (a)).
2)5.6Mg(OH)2:1Ta2O5Synthesis of pure phase Mg4Ta2O9Powder material
The raw material ratio is increased to 5.6:1, the pre-sintering temperature is increased to 1350 ℃, other conditions are unchanged, and the phase purity of the obtained ceramic cylindrical chip is characterized by powder XRD (figure 2 (b)).
3)4.04MgO:1Ta2O5Synthesis of pure phase Mg4Ta2O9Powder material
MgO and Ta with the purity of 99.9995 percent and 99.99 percent are weighed according to the molar ratio of 4.04:12O5The raw materials (a) are put into a mortar for grinding and uniformly mixing, sieved by a 220-mesh sieve and put into a container filled with ZrO2In the ball milling pot, the ball was milled for 1 hour in a ball mill. Preparing a cylindrical sheet with the size of phi 13mm multiplied by 5mm under the action of 350MPa pressure, putting the cylindrical sheet into a muffle furnace for presintering at 1400 ℃ for 2h, and the temperature rising and falling speed is 1400 ℃/h. The phase purity of the resulting ceramic cylindrical sheet was characterized by powder XRD (fig. 2 (c)).
(2) Growing Mg by micro-pulling-down method4Ta2O9Single crystal rod
1)Mg4Ta2O9+0.1Mg(OH)2Growing optical fiber crystal by micro-pulling down method
With Mg (OH)2And Ta2O5Pure phase Mg synthesized as raw material4Ta2O9Powder and Mg (OH)2Ball-milling and uniformly mixing the powder material according to the molar ratio of 10:1, putting 5g of the powder material into an Ir crucible shown in figure 1, and optimizing the Mg by spontaneous nucleation in the early period4Ta2O9Taking the single crystal rod as seed crystal, cutting and polishing the upper end of the seed crystal, keeping clean and dry, and fixing in a hollow to obtain Al2O3On the seed rod. And adjusting various components such as the crucible and the like and the axis of the seed crystal to be positioned on the vertical line, and enabling the upper surface of the seed crystal to be close to the lower surface of the nozzle of the crucible. And (3) sealing the spherical growth chamber, vacuumizing for 12h, introducing protective atmosphere Ar, and regulating the flow of gas to be inlet and outlet to 20mL/min to ensure that the pressure of the gas is stabilized at 1.5 atm. After the atmosphere flow and the pressure are stabilized for 1h, setting the heating rate to 1h, heating to 35% of the rated power, keeping the temperature for 1h, after the molten liquid flows out of a nozzle through hole of a crucible, slowly expanding and spreading to form a meniscus, moving up the seed crystal to be in butt joint with the meniscus, keeping the temperature for 1h, setting the micro-pulling-down speed to be 0.05mm/min for growth when the solid-liquid interface is not changed, and finely adjusting the temperature in a range of 35-36% of the rated power according to the variation condition of the growth interface. After the crystal growth is finished, the temperature is reduced for 3h and then the crystal is taken out, and the colorless transparent single crystal with the size of phi 1.2mm multiplied by 97mm is obtained as shown in figure 3.
2)Mg4Ta2O9+0.1MgO micro-pull-down method for Mg growth4Ta2O9Crystal
With MgO and Ta2O5Pure phase Mg synthesized as raw material4Ta2O9The powder and MgO powder are uniformly mixed by ball milling in a molar ratio of 10:1, the mixture is put into an Ir crucible for growth, the growth power is 36 percent of the rated power of the equipment, the growth speed is 0.05mm/min, the size of the grown crystal is phi 1.6mm multiplied by 76mm, the crystal is subjected to powder XRD characterization as shown in figure 4, and the crystal is a pure phase as shown in figure 6 (b). Under the conditions that the rated power is 37.5 percent and the growth speed is 0.05mm/min, the same powder is used for growing crystals with the size of phi 2.0mm multiplied by 16mm, the crystals are more transparent, as shown in figure 5, and the powder XRD pattern is shown in figure 6(c), which shows that the crystals are pure phases.
(3)Mg4Ta2O9Crystal characterization
As shown in FIGS. 6(a), (b) and (c), 10% excess of Mg (OH) is used2Or Mg of MgO4Ta2O9Mg grown in powder form4Ta2O9XRD pattern of the rod-shaped single crystal powder shows that the grown crystal is pure-phase Mg4Ta2O9。
Single crystal X-ray diffraction data analysis, Mg4Ta2O9The crystal is a hexagonal system, the space group is P-3c1(165), the unit cell parameters are a-0.51625 (10) nm and c-1.4062 (4) nm, and the density is 6.171g/cm3The refined structure constant is 0.0356, which proves that the crystal quality is better.
EXAMPLE 2 micro-Pull-Down growth of Mg3Ta2O8Crystal
Mixing high-purity powder raw material Mg (OH)2(99.999%) and Ta2O5(99.99%) 5g of the mixture is put into a mortar to be ground and uniformly mixed according to the molar ratio of 4:1, and the mixture is sieved by a 220-mesh sieve and put into a mortar filled with ZrO2Ball-milling for 1h in a ball-milling tank of the ball, grinding the ball-milled materials in a mortar again, sieving by a 220-mesh sieve, pressing the materials under 350MPa into cylindrical pieces with the sizes of phi 13mm multiplied by 5mm, putting the cylindrical pieces into a high-temperature muffle furnace, setting the presintering temperature to 1300 ℃, the heat preservation time to 16h, and increasing and decreasing the temperature at the speed of 100 ℃/h. The obtained phase-pure powder of ceramic cylindrical sheetXRD was characterized and its pattern features are shown in FIG. 7 (a). Found to be free of Mg3Ta2O8Phase, 0.62Mg4Ta2O9+0.38MgTa2O6Mixed phase, the components of which are approximately in Mg by phase diagram analysis3Ta2O8Is near the component (a).
Placing the synthetic ceramic cylindrical sheet into Ir crucible as shown in FIG. 1, and adding MgTa2O6Taking a polycrystalline rod as a seed crystal, moving a seed crystal rod to the top end of the seed crystal to be approximately aligned with a conical bottom nozzle of an Ir crucible, properly adjusting all the steps, closing a growth chamber to enable the growth chamber to be in a closed state, vacuumizing for 12h, introducing Ar gas, controlling the flow rate of an atmosphere inlet and an atmosphere outlet to enable the pressure to be stabilized at 1.5atm, controlling the flow rate of the gas in the chamber to be 20mL/min, setting the temperature rise time to be 1h after the atmosphere flow rate and the pressure are stabilized for 1h, raising the temperature to 25% of rated power, preserving the temperature for a period of time, forming crescent wetting liquid drops when molten liquid flows out from a nozzle through hole, keeping the power at 1h, slowly moving the seed crystal upwards after the appearance is stabilized, connecting the seed crystal with the seed crystal, keeping the connection state for 1h, and slowly adjusting the growth power to enable the width of a melting zone to be. And after all the seed crystal rods are ready, setting the pull-down speed of the seed crystal rod to be 0.2 mm/h. And (4) carrying out multiple necking and shoulder expanding processes by adjusting the power so as to accelerate the growth rate of spontaneous nucleation crystals. The crystal to be grown is transparent and brighter than the previous crystal, and the growth parameters are kept unchanged. And (4) continuing the constant-diameter growth until no solution flows out from the nozzle of the crucible. Setting the temperature reduction time to be 10h, and slowly cooling to room temperature until the growth is finished. As shown in FIG. 8, it underwent a process of optimizing the crystalline quality by spontaneous nucleation of about 20cm long, and it was pale yellow and transparent without macroscopic defects, and Mg having a size of 1.2 mm. times.19 mm3Ta2O8The crystal is grown by spontaneous nucleation. The optimized growth power is 29.5% of the maximum power of the apparatus, and the growth speed is 0.2 mm/min.
The single crystals were cut from any portion and ground to a powder form for XRD pattern testing, as shown in FIG. 7(b), and characterized as pure Mg3Ta2O8And (4) phase(s).
Analysis of Single Crystal X-ray diffraction data, Mg3Ta2O8The crystal belongs to an orthorhombic system, the space group is Ccm (63), the lattice constant is a-0.38087 (12) nm, b-1.0034 (2) nm, c-1.0206 (3) nm, and the density is 5.965g/cm3The refined structure constant is 0.0207, which proves that the crystallization quality is good, and the test structure chart is shown in figure 9.
Example 3 micro-pull-down growth of MgTa2O6Crystal
The raw material Mg (OH)2(Noah chemical, 99.999%) and Ta2O5(Aldrich, 99.99%) are weighed according to the molar ratio of 1.15:1, mixed evenly in a mortar of 5g, sieved by a 220-mesh sieve, ball-milled for 1h under 350MPa to prepare a cylindrical sheet with phi 13mm multiplied by 5mm, put into a muffle furnace for presintering at 1400 ℃ for 8h, and heated and cooled at 100 ℃/h. Powder XRD, as shown in FIG. 10(a), shows that the obtained ceramic sheet is pure phase MgTa2O6. The synthesized pure phase MgTa2O6Powder and Mg (OH)2Ball milling the powder at a molar ratio of 100:1, pressing into 13mm column shape with 5g dry, placing into Ir crucible shown in FIG. 1, and mixing with Y3Al5O12Using a (YAG for short) single crystal rod as a seed crystal, cutting and polishing the upper end of the seed crystal, keeping clean and dry, and fixing in a hollow to obtain Al2O3On the seed rod. And adjusting various components such as the crucible and the like and the axis of the seed crystal to be positioned on the vertical line, and enabling the upper surface of the seed crystal to be close to the lower surface of the nozzle of the crucible. And (3) sealing the spherical growth chamber, vacuumizing for 12h, introducing protective atmosphere Ar, and regulating the flow of gas to be inlet and outlet to 20mL/min to ensure that the pressure of the gas is stabilized at 1.5 atm. After the atmosphere flow and the pressure are stabilized for 1h, setting the heating rate for 1h to heat to 29.6% of the rated power, keeping the temperature for 1h, after the molten liquid flows out of the nozzle through hole of the crucible and slowly expands and spreads to form a meniscus, moving up the seed crystal to be in butt joint with the meniscus, keeping the temperature for 1h, setting the micro-pulling-down speed to be 0.2mm/min for growth when the solid-liquid interface is not changed, and finely adjusting the micro-pulling speed in a range of 29.6-34% of the rated power according to the variation condition of the growth interface. And after the crystal growth is finished, cooling for 8 hours and taking out, thus obtaining the crystal shown in figure 11, wherein the crystal is black in color, bright in color, 2-3 mm in diameter and 60mm in length, and has no macroscopic defect. The black color is caused by that the growth atmosphere is Ar gas in a reducing atmosphereAnnealing at 1000 deg.C for 12h in air turns into light yellow transparent.
Randomly cutting part of the single crystal, grinding into powder, and performing XRD pattern test to characterize as pure-phase MgTa2O6As shown in fig. 10 (b).
Analysis of Single Crystal X-ray diffraction data, MgTa2O6The crystal belongs to a tetragonal system, the space group is P42/mnm (136), the lattice constant is a-0.46821 (3) nm, c-0.91382 (7) nm, and the crystal density is 8.03265g/cm3And the fine modification parameter RAII is 0.026, which proves that the crystallization quality is better. The test structure is shown in fig. 12. The grown crystal is cut, oriented, processed and the like, and is used for testing various performances.
The above description is only a basic description of the present invention, and any equivalent changes made according to the technical solution of the present invention should fall within the protection scope of the present invention.

Claims (2)

1. Preparation of Mg3Ta2O8A method of crystallizing comprising the steps of:
1) weighing pure phase Mg (OH) according to a molar ratio of 4:12And Ta2O5Grinding, mixing, sieving, ball milling, pressing into cylindrical tablet, pre-sintering at 1300 deg.C for 16 hr, and heating and cooling at 100 deg.C/hr;
2) putting the synthetic ceramic cylindrical sheet into an Ir crucible and adding MgTa2O6Taking a polycrystalline rod as seed crystal, vacuumizing for 12h, introducing Ar gas, controlling the pressure to be 1.5atm, controlling the flow to be 20mL/min, setting the pull-down speed to be 0.2mm/min, performing spontaneous nucleation, adjusting the power to perform multiple necking and shoulder expanding processes, optimizing the crystallization quality, setting the cooling time to be 10h, cooling to room temperature, and obtaining the Mg after the growth is finished3Ta2O8And (4) crystals.
2. Preparation of Mg according to claim 13Ta2O8Method for the crystallization, characterized in that the Mg3Ta2O8The crystal belongs to the orthorhombic system, the space group belongs to Ccmm (63), the lattice is constantNumber isa=0.38087(12) nm，b=1.0034(2)nm，c=1.0206(3) nm, density 5.965g/cm3The refined structure constant is 0.0207.
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