Heterogeneous doped strontium titanate single crystal flame melting growth method

Abstract

The invention relates to a heterogeneously-doped strontium titanate single crystal flame fusion growth method, which comprises the following steps: taking strontium titanate powder with doping elements as a raw material for producing heterogeneously-doped strontium titanate single crystals, and sequentially carrying out single crystal core formation, shoulder expansion growth and equal-diameter growth to obtain heterogeneously-doped strontium titanate crystals, in the shoulder expanding growth process, hydrogen and oxygen are introduced into the hearth according to the increment ratio of the hydrogen to the oxygen, and the increment ratio of the hydrogen to the oxygen is larger than 2.5: 1 and smaller than or equal to 3.5: 1; the doping element of the strontium titanate powder with the doping element is any one of La, Nd, Sm, Gd, Dy, Er and Y. The method has the beneficial effects that the increment ratio of hydrogen to oxygen is controlled to be more than 2.5: 1 and less than or equal to 3.5: 1 in the heterogeneous doped strontium titanate single crystal growth process, and the effect of reducing the temperature of the edge of a melting cap is achieved by excessively introducing hydrogen, so that overflow is prevented.

Description

Technical Field

[0001] The invention relates to the field of artificial crystals, and in particular to a method for growing heterogeneous doped strontium titanate single crystals by flame melting. Background Art

[0002] There are no natural SrTiO3 single crystals in nature, and SrTiO3 single crystals can only be prepared by artificial synthesis. In recent decades, researchers have also tried many preparation techniques, mainly flame melting, pulling, cold crucible, floating zone and hydrothermal methods. Since there is no need for a crucible to hold the oxide melt, the flame melting process of preparing single crystals avoids the mutual reaction between the high-temperature melt and the crucible, eliminates the contamination of the melt by the crucible, and gets rid of the limitation of the crucible material on the preparation of high-melting-point oxide crystals. Therefore, the flame melting method is particularly suitable for preparing various high-temperature oxide crystals, such as SrTiO3 single crystals.

[0003] SrTiO3 single crystal has good chemical stability, electrochromic and photochromic properties, and its lattice constant is very compatible with the discovered high-temperature superconducting materials, and has been widely used in optical lenses, decorative gems, semiconductors and superconductors. Researchers have also tried to replace SrTiO3 with Al2O3 and MgO to become superconducting films, but only the lattice constant of SrTiO3 matches the 123 type superconductor (chemical formula YBa2Cu3O7-δ, where Y represents the rare earth element yttrium, Ba represents barium, Cu represents copper, O represents oxygen, and δ represents a non-integer ratio of oxygen) best, and the mismatch rate of the

[100] crystal direction is only 0.2%.

[0004] In the field of thermoelectrics, SrTiO3 single crystals have good thermoelectric properties because of their low toxicity, low cost, good high temperature stability, and high Seebeck coefficient. However, SrTiO3 single crystals are insulators and need to be doped to form thermoelectric semiconductor single crystal materials with good performance. Although the preparation process of SrTiO3 single crystals is relatively mature, the preparation process of heterogeneously doped SrTiO3 single crystals has rarely been studied. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for growing a heterogeneous doped strontium titanate single crystal by flame melting, which solves the problem of melt cap overflow in the process of growing a heterogeneous doped SrTiO3 single crystal.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: a method for growing a heterogeneous doped strontium titanate single crystal by flame melting, comprising the following steps: taking strontium titanate powder with doping elements as a raw material for producing a heterogeneous doped strontium titanate single crystal, sequentially performing single crystal nucleus formation, shoulder expansion growth and equal diameter growth to obtain a heterogeneous doped strontium titanate crystal; during the shoulder expansion growth process, introducing hydrogen and oxygen into the furnace according to the incremental ratio of hydrogen and oxygen, 2.5:1<the incremental ratio of the hydrogen and oxygen≤3.5:1;

[0007] The doping element of the strontium titanate powder with doping elements is any one of La, Nd, Sm, Gd, Dy, Er and Y.

[0008] The beneficial effect of the present invention is that during the growth of heterogeneous doped strontium titanate single crystals, the incremental ratio of hydrogen and oxygen is controlled at 2.5:1 < the incremental ratio of hydrogen and oxygen ≤ 3.5:1, and the temperature of the edge of the melt cap is reduced by excessive introduction of hydrogen, thereby preventing overflow.

[0009] Based on the above technical solution, the present invention can also be improved as follows.

[0010] Furthermore, the specific steps of forming the single crystal nucleus include:

[0011] Introduce oxygen and hydrogen into the furnace, ignite the mixed gas of oxygen and hydrogen to heat the furnace to the set temperature, and raise the candlestick to the position of the observation window;

[0012] Increasing the oxygen flow rate, placing the seed crystal on the candlestick to bake, heating and melting the seed crystal, and after the seed crystal is melted, forming a solid-liquid interface between the seed crystal and the liquid melted seed crystal, and then continuously adding the strontium titanate powder with doping elements into the furnace;

[0013] The seed crystal is lowered, and a neck is gradually formed while maintaining the solid-liquid interface, until a clear solid-liquid interface appears, indicating that a single crystal nucleus is formed.

[0014] The beneficial effect of adopting the above further scheme is: by introducing oxygen and hydrogen into the furnace to heat the furnace, a growth environment is provided for the growth of single crystals, and increasing the amount of oxygen introduced can further enhance the heating effect and ensure that the seed crystal can be melted. At the same time, the solid-liquid interface is maintained on the basis of ensuring the descending rate of the seed crystal and the amount of powder added. A clear solid-liquid interface helps to ensure the orderliness and stability of crystal growth, reduce crystal defects, and improve crystal quality.

[0015] Furthermore, the strontium titanate powder with doping elements is Nb-doped strontium titanate powder, and the molar ratio of the doping amount of Nb in the Nb-doped strontium titanate powder is 2-8%.

[0016] Furthermore, before the oxygen and the hydrogen are introduced into the furnace, the initial introduction amounts of the oxygen and the hydrogen are set, the initial introduction amount of the oxygen is 4-6 L / min, the initial introduction amount of the hydrogen is 9-15 L / min, and the set temperature of the furnace is 500-600°C.

[0017] The beneficial effect of adopting the above further solution is: ensuring that the initial state is a hydrogen-rich state, providing a growth environment for the growth of heterogeneous doped strontium titanate single crystals.

[0018] Furthermore, after the furnace is heated to the set temperature, the oxygen is increased by 0.1L / min every 10-20 minutes; when the furnace temperature is 1000-1200°C, the seed crystal is placed on the candlestick for baking; during the baking process, the furnace temperature is increased to 2050°C, and the baking is continued for 10-20 minutes. When the upper surface of the seed crystal is melted, the barrel containing the Nb-doped strontium titanate powder is continuously knocked, and the Nb-doped strontium titanate powder drips into the molten pool at the top of the seed crystal along with the oxyhydrogen flame; the knocking amplitude of the barrel containing the Nb-doped strontium titanate powder is 20-35mm.

[0019] The beneficial effects of adopting the above further scheme are: gradually increasing the oxygen helps to increase the furnace temperature and maintain a stable growth environment, and by increasing the furnace temperature to 2050° C. during the baking process, it helps to quickly melt the seed crystal.

[0020] Furthermore, the seed crystal is lowered, while maintaining the solid-liquid interface, to gradually form a neck until a clear solid-liquid interface appears, representing the step of single crystal nucleus formation, and the descending rate of the seed crystal is 16-18 mm / h.

[0021] The beneficial effect of adopting the above further scheme is that the descending rate of the seed crystal is related to the knocking amplitude of the powder feeding barrel. The current descending rate of the seed crystal and the knocking amplitude of the powder feeding barrel match the temperature zone generated by the oxyhydrogen flame combustion, which can ensure that the position of the solid-liquid interface remains unchanged; at the same time, it prevents the excessively fast descending rate of the seed crystal from causing the liquid seed crystal at the top of the seed crystal to solidify.

[0022] Further, the step of shoulder expansion and growth comprises:

[0023] After the single crystal nucleus is formed, the tapping amplitude of the barrel containing the Nb-doped strontium titanate powder, the descending rate of the seed crystal, and the amount of hydrogen and oxygen introduced are adjusted to allow the Nb-doped strontium titanate crystal to expand and grow.

[0024] Furthermore, during the shoulder expansion growth process, the melt volume at the top of the Nb-doped strontium titanate crystal gradually increases and forms a melt cap;

[0025] After the melt cap is formed, the descending rate of the seed crystal is adjusted to 16-18 mm / h, and the tapping amplitude of the barrel containing the Nb-doped strontium titanate powder is increased by 10-15 mm every 30 minutes;

[0026] The oxygen gas increases by 0.1 L / min-0.3 L / min every 5-10 min, and the hydrogen gas increases by 0.3 L / min-0.9 L / min every 5-10 min.

[0027] The beneficial effect of adopting the above further scheme is: controlling the incremental ratio of hydrogen and oxygen to 2.5:1<the incremental ratio of hydrogen and oxygen ≤3.5:1, preferably, controlling the incremental ratio of hydrogen and oxygen to about 3:1, and reducing the furnace temperature by controlling the amount of hydrogen introduced to prevent overflow.

[0028] Furthermore, when the oxygen intake reaches 9-10 L / min, the descent rate of the candlestick is reduced to 15-16 mm / h at a rate of 0.2 mm / hmin, and the hydrogen is increased by 0.2-0.5 L / min every 20-30 min.

[0029] The beneficial effect of adopting the above further solution is: by controlling the amount of hydrogen introduced, the furnace temperature is reduced and overflow is prevented.

[0030] Further, when the diameter of the Nb-doped strontium titanate crystal reaches 10-15 mm, the isodiameter growth stage is entered, and the tapping amplitude of the barrel containing the Nb-doped strontium titanate powder, the amount of hydrogen and oxygen introduced, and the descending rate of the seed crystal are maintained until the growth is completed;

[0031] When the total length of the Nb-doped strontium titanate crystal reaches 70-80 mm, the growth of the Nb-doped strontium titanate crystal is stopped, and the furnace is wrapped with heat-insulating cotton to cool the Nb-doped strontium titanate crystal to room temperature.

[0032] The beneficial effect of adopting the above further scheme is that in the isodiameter growth stage, it is necessary to stably control the powder feeding amount, gas supply amount and seed crystal descent rate to maintain the isodiameter growth of the crystal. The stable control of these parameters is essential to ensure the continuity and quality of single crystal growth. When the single crystal growth has reached the preset target, in order to evenly cool the crystal and avoid cracks or defects caused by thermal stress, the furnace is wrapped with insulation cotton to gradually cool the crystal to room temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic diagram of the single crystal growth principle of the flame melting method according to an embodiment of the present invention;

[0034] Figure 2 The XRD diffraction pattern of the Nb-doped strontium titanate single crystal grown in the embodiment of the present invention;

[0035] Figure 3 Transmission electron microscopy (TEM) of Nb-doped strontium titanate single crystal grown according to the embodiment of the present invention Figure 1 ;

[0036] Figure 4 Transmission electron microscopy (TEM) of Nb-doped strontium titanate single crystal grown according to the embodiment of the present invention Figure 2 . DETAILED DESCRIPTION

[0037] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0038] Example

[0039] Figure 1 The flame melting method single crystal growth principle diagram of an embodiment of the present invention is shown, specifically showing the candlestick descending direction, the crystal growth direction, the position of the observation window and the observation direction through the observation window. Figure 1 The relative position relationship between the observation window, seed crystal and candlestick at different growth stages can be intuitively observed.

[0040] The present invention provides a method for growing a heterogeneous doped strontium titanate single crystal by flame melting. The raw material is strontium titanate powder with a doping element. The doping element of the strontium titanate powder is any one of La, Nd, Sm, Gd, Dy, Er and Y. The molar ratio of the doping amount of the doping element in the strontium titanate powder is 2-8%. In this embodiment, Nb-doped strontium titanate powder is used as the raw material for producing the Nb-doped strontium titanate single crystal. In this embodiment, Nb-doped strontium titanate powder is used as an example for description. Figure 2 shows the XRD diffraction pattern of Nb-doped strontium titanate single crystal, Figure 3 , Figure 4 The transmission electron microscope (TEM) image of Nb-doped strontium titanate single crystal is shown. The steps for preparing crystals using other heterogeneous doped strontium titanate powders as raw materials are the same. As can be imagined, the specific powder feeding amount, seed crystal descent rate and gas supply amount can be adaptively adjusted according to actual conditions.

[0041] The growth method of niobium-doped strontium titanate single crystal by flame melting method comprises the following steps:

[0042] S1: Forming a single crystal nucleus. Specifically including:

[0043] S11: introduce oxygen and hydrogen into the furnace, ignite the mixed gas of oxygen and hydrogen to heat the furnace to the set temperature, and raise the candlestick to the position of the observation window; Figure 1As shown in (1), specifically, the growth of niobium-doped strontium titanate single crystal by flame melting is carried out in a special crystal growth device, which includes a furnace body, an observation window is provided on the furnace body, a growth chamber for growing heterogeneous doped strontium titanate crystals is provided in the furnace body, and a candlestick for placing seed crystals is provided at the lower end of the growth chamber; because the crystal growth device is an existing device, it will not be described in detail here. In this step, the temperature is set to 500-600°C, and the heat generated by the combustion of hydrogen and oxygen is used to heat the furnace of the furnace body to 500-600°C.

[0044] Among them, before oxygen and hydrogen are introduced into the furnace, it can be imagined that the initial introduction amount of oxygen and hydrogen needs to be set, wherein the initial introduction amount of oxygen is 4-6L / min, and the initial introduction amount of hydrogen is 9-15L / min. In this embodiment, the initial introduction amount of oxygen is set to 5L / min, and the initial introduction amount of hydrogen is set to 12L / min, because the reaction ratio of hydrogen and oxygen is 2:1, and after hydrogen and oxygen are introduced into the furnace according to the setting, the initial state in the furnace is a hydrogen-rich state, which provides a growth environment for the growth of Nb-doped strontium titanate single crystals.

[0045] S12: increasing the oxygen flow rate, placing the seed crystal on the candlestick to bake, heating and melting the seed crystal, and after the seed crystal is melted, forming a solid-liquid interface between the seed crystal and the liquid melted seed crystal, and then continuously adding strontium titanate powder with doping elements into the furnace;

[0046] In step S12, the amount of oxygen introduced is slowly increased to increase the temperature of the single crystal furnace. After the furnace is heated to the set temperature, the oxygen is increased by 0.1L / min every 10-20 minutes. By increasing the oxygen to increase the heating power of the furnace body, the temperature zone is lowered to the top of the seed crystal while increasing the temperature. The temperature in the furnace gradually decreases along the height difference, that is, the temperature in each area is inconsistent. By controlling the amount of hydrogen and oxygen introduced, the temperature zone of the corresponding temperature is controlled at the top of the seed crystal, and the seed crystal is heated and melted. The seed crystal is a small crystal with the same crystal orientation as the desired crystal, which is the seed for growing a single crystal, also called a seed crystal. In this embodiment, SrTiO3 seed crystal is used. Specifically, when the furnace temperature is 1000-1200°C, the seed crystal is placed on the candlestick of the furnace for baking. It is usually necessary to heat the furnace for 20-40 minutes, and gradually increase the furnace temperature to 2050°C. After continuing to bake for 10-20 minutes, the seed crystal is raised to the crystal growth zone. In this embodiment, the position of the observation window is defined as the crystal growth zone. When the upper surface of the seed crystal is melted, powder feeding begins. The strontium titanate powder with doping elements is loaded in the barrel. By knocking the barrel, the Nb-doped strontium titanate powder drips into the molten pool at the top of the seed crystal with the hydrogen-oxygen flame. In this step, the molar ratio of the doping amount of Nb in the Nb-doped strontium titanate powder is 2-8%, and the knocking amplitude of the powder feeding barrel is 20-35mm.

[0047] S13: The seed crystal is lowered, and a neck is gradually formed while maintaining the solid-liquid interface until a clear solid-liquid interface appears, indicating the formation of a single crystal nucleus.

[0048] In this embodiment, the growth of Nb-doped strontium titanate single crystals is carried out in a special crystal growth device. The crystal growth device is composed of a rotating mechanism, a lifting mechanism, a knocking mechanism, a furnace body, a burner, a raw material bin and other parts. The crystal growth device adopts existing equipment. The existing technology is relatively complete and is not the focus of protection of this embodiment, so it is not limited here. Therefore, in step S13, the lifting system is connected to the candlestick, and the movement of the candlestick is controlled by the lifting system, which in turn drives the movement of the seed crystal. The lifting system is started, and the seed crystal is lowered by the lifting system on the crystal growth equipment. The seed crystal descends at a rate of 16-18mm / h to prevent the liquid seed crystal at the top of the seed crystal from solidifying due to excessive deceleration; the solid-liquid interface is the phase interface between the crystal and the melt, that is, the phase interface between the seed crystal and the liquid molten seed crystal. On the basis of maintaining the solid-liquid interface, a neck is gradually formed. After a clear solid-liquid interface, that is, a growth interface, appears, it indicates the formation of a single crystal nucleus. As Figure 1 (2)-(3) shows the process of melting the seed crystal to form a single crystal.

[0049] Due to the inhomogeneity of the melting points of Nb-doped strontium titanate single crystals and pure phase strontium titanate single crystals, it is necessary to slow down the powder feeding and seed crystal descent rates. In this embodiment, the seed crystal descent rate and the powder feeding tapping amplitude are both smaller than the corresponding parameters for the growth of pure phase strontium titanate single crystals, and the seed crystal descent rate is related to the tapping amplitude of the powder feeding barrel. In this embodiment, the current seed crystal descent rate and the tapping amplitude of the powder feeding barrel match the temperature zone generated by the oxyhydrogen flame combustion to ensure that the position of the solid-liquid interface remains unchanged.

[0050] S2: Shoulder expansion growth. Specifically, after the single crystal nucleus is formed, the tapping amplitude of the barrel containing Nb-doped strontium titanate powder, the descending rate of the seed crystal, and the amount of hydrogen and oxygen introduced are adjusted to make the Nb-doped strontium titanate crystal grow with shoulder expansion, such as Figure 1As shown in (3)-(4). And as the crystal growth process proceeds, the temperature in the furnace near the observation window gradually increases. In the actual shoulder expansion process, the growth interface must be kept slowly descending to prevent overflow. In the shoulder expansion growth process, the volume of the melt on the top of the Nb-doped strontium titanate crystal gradually increases, that is, a "hat"-shaped melt is formed on the top of the grown Nb-doped strontium titanate crystal, which is the melt cap. After the melt cap is formed, the shoulder expansion growth begins, and the descent rate of the seed crystal is kept unchanged, that is, the descent rate of the seed crystal is kept at 16-18mm / h. The tapping amplitude of the powder feeding barrel is adjusted in increments of 10-15mm every 30min. At the same time, the amount of oxygen and hydrogen introduced is slowly increased. Specifically, the oxygen is increased by 0.1L / min-0.3L / min every 5-10min, and the hydrogen is increased by 0.3L / min-0.9L / min every 5-10min. In this embodiment, the hydrogen to oxygen increment ratio is hydrogen every 5-10min. The ratio of the amount of increase to the amount of oxygen increase every 5-10 minutes, wherein in the calculation process, the amount of oxygen increase is in the range of 0.1L / min-0.3L / min, and the amount of hydrogen increase is in the range of 0.3L / min-0.9L / min. After ANSYS analysis, when the incremental ratio of hydrogen to oxygen is 2.5:1-3:1, the combustion heat is the highest. In a preferred embodiment of the growth process of niobium-doped strontium titanate single crystal flame melting method, the incremental ratio of hydrogen to oxygen is 3:1; wherein the incremental ratio is the ratio of the incremental hydrogen introduction to the incremental oxygen introduction after entering the shoulder expansion growth stage, and the amount of hydrogen and oxygen introduced before entering the shoulder expansion growth and the existing hydrogen and oxygen content in the furnace have no effect on the incremental ratio. By expanding the area of ​​the isothermal zone, the shoulder expansion growth of the crystal is ensured.

[0051] In this embodiment, in order to prevent overflow, during the growth of Nb-doped strontium titanate single crystal, the incremental ratio of hydrogen and oxygen is controlled at 2.5:1 < the incremental ratio of hydrogen and oxygen ≤ 3.5:1. Preferably, the incremental ratio of hydrogen and oxygen is controlled at about 3:1. By introducing excessive hydrogen, the temperature of the edge of the melt cap is reduced, thereby preventing overflow; however, further increasing the incremental amount of hydrogen will cause the melt cap to solidify. In the specific growth process, when the ratio of the amount of hydrogen and oxygen introduced is 2.5:1, the crystal furnace can obtain a higher temperature, and overflow is likely to occur if the furnace temperature is too high. With the further increase of hydrogen, the furnace temperature decreases accordingly. If the amount of hydrogen and oxygen introduced is 4:1, this will cause the melt cap to solidify. Therefore, it is necessary to control the incremental ratio of hydrogen and oxygen to 2.5:1 < the incremental ratio of hydrogen and oxygen ≤ 3.5:1. Preferably, the incremental ratio of hydrogen and oxygen is controlled at about 3:1 to prevent overflow.

[0052] Furthermore, during the crystal shoulder growth process, it is necessary to observe the growth interface in real time and adjust the hydrogen intake and candlestick descent rate in real time according to the clarity of the growth interface. If the growth interface is not clear, it is necessary to increase the hydrogen intake and candlestick descent rate in real time to reduce the crystal growth temperature.

[0053] During this process, when the oxygen intake reaches 9-10L / min, slowly reduce the candlestick's descent rate from 0.2mm / hmin to 15-16mm / h, and increase the hydrogen intake every 20-30min with an increment of 0.2-0.5L / min to control the furnace temperature. The furnace temperature can be lowered by controlling the hydrogen intake to prevent overflow.

[0054] During the growth of pure phase strontium titanate, after the melt cap is formed, the amplitude growth rate is 20-25mm in 30 minutes, and the incremental ratio of hydrogen and oxygen is about 2.5:1, which needs to be increased every 2-3 minutes; in the present application, the powder feeding and knocking amplitude is coordinated with the slow descent rate of the seed crystal during the growth of Nb-doped strontium titanate, and an excessive amount of hydrogen is introduced into the furnace, thereby solving the melt cap overflow problem in the growth process of heterogeneous doped SrTiO3 single crystal. The excess hydrogen has the effect of reducing the edge temperature of the melt cap, effectively reducing the furnace temperature, thereby preventing overflow.

[0055] S3: Equal diameter growth. Specifically includes:

[0056] When the diameter of Nb-doped strontium titanate crystal reaches 10-15 mm, it enters the isodiameter growth stage. Figure 1 As shown in (5), the tapping amplitude of the barrel containing Nb-doped strontium titanate powder, the amount of hydrogen and oxygen introduced, and the descending rate of the seed crystal are maintained until the growth is completed. In this growth stage, the tapping amplitude of the barrel containing Nb-doped strontium titanate powder, the amount of hydrogen and oxygen introduced, and the descending rate of the seed crystal are maintained constant when the shoulder expansion growth enters the equal diameter growth until the growth is completed.

[0057] S4: Finishing. After the equal-diameter growth is completed, the furnace is insulated. Specifically, when the overall length of the Nb-doped strontium titanate crystal reaches 70-80mm, the growth of the Nb-doped strontium titanate crystal is stopped. It can be imagined that after the Nb-doped strontium titanate crystal grows to the required size, it is necessary to reduce the powder supply and the gas supply until the shoulder is completed, and then stop the gas supply, material supply and crystal growth equipment movement and wrap the crystallization furnace with insulation cotton to allow the Nb-doped strontium titanate crystal to slowly cool to room temperature. Among them, the existing technology for the shoulder-retracting step is relatively complete, and no further elaboration is made here.

[0058] The X-ray diffraction pattern of Nb-doped SrTiO3 single crystal is shown in Figure 2As shown in the figure, it can be seen that there are only diffraction peaks of the (100) and (200) crystal planes, and no diffraction peaks of other crystal planes are observed, which indicates that Nb-doped SrTiO3 single crystals have been generated.

[0059] from Figure 3 , Figure 4 It can be seen from the TEM image that the electron diffraction pattern of Nb-doped SrTiO3 single crystal forms electron diffraction spots rather than electron diffraction rings, which also confirms the formation of single crystals.

[0060] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for growing heterogeneous doped strontium titanate single crystal by flame melting, characterized in that: The method comprises the following steps: taking strontium titanate powder with doping elements as a raw material for producing heterogeneous doped strontium titanate single crystals, sequentially performing single crystal nucleus formation, shoulder expansion growth and equal diameter growth to obtain heterogeneous doped strontium titanate crystals; during the shoulder expansion growth process, introducing hydrogen and oxygen into the furnace according to the incremental ratio of hydrogen and oxygen, 2.5:1<the incremental ratio of hydrogen and oxygen≤3.5:1; The doping element of the strontium titanate powder with doping elements is any one of La, Nd, Sm, Gd, Dy, Er and Y.

2. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 1, characterized in that: The specific steps of forming the single crystal nucleus include: Introduce oxygen and hydrogen into the furnace, ignite the mixed gas of oxygen and hydrogen to heat the furnace to the set temperature, and raise the candlestick to the position of the observation window; Increasing the oxygen flow rate, placing the seed crystal on the candlestick to bake, heating and melting the seed crystal, and after the seed crystal is melted, forming a solid-liquid interface between the seed crystal and the liquid melted seed crystal, and then continuously adding the strontium titanate powder with doping elements into the furnace; The seed crystal is lowered, and a neck is gradually formed while maintaining the solid-liquid interface, until a clear solid-liquid interface appears, indicating that a single crystal nucleus is formed.

3. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 2, characterized in that: The strontium titanate powder with doping elements is Nb-doped strontium titanate powder, and the molar ratio of the doping amount of Nb in the Nb-doped strontium titanate powder is 2-8%.

4. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 3, characterized in that: Before introducing the oxygen and the hydrogen into the furnace, the initial introduction amount of the oxygen and the hydrogen is set, the initial introduction amount of the oxygen is 4-6L / min, the initial introduction amount of the hydrogen is 9-15L / min, and the set temperature of the furnace is 500-600°C.

5. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 3, characterized in that: After the furnace is heated to the set temperature, the oxygen is increased by 0.1L / min every 10-20 minutes; when the furnace temperature is 1000-1200°C, the seed crystal is placed on the candlestick for baking; during the baking process, the furnace temperature is increased to 2050°C, and the baking is continued for 10-20 minutes. When the upper surface of the seed crystal is melted, the barrel containing the Nb-doped strontium titanate powder is continuously knocked, and the Nb-doped strontium titanate powder drips into the molten pool at the top of the seed crystal along with the oxyhydrogen flame; the knocking amplitude of the barrel containing the Nb-doped strontium titanate powder is 20-35mm.

6. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 5, characterized in that: The seed crystal is lowered, and a neck is gradually formed on the basis of maintaining the solid-liquid interface until a clear solid-liquid interface appears, representing the step of single crystal nucleus formation. The descending rate of the seed crystal is 16-18 mm / h.

7. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 3, characterized in that: The step of shoulder expansion and growth comprises: After the single crystal nucleus is formed, the tapping amplitude of the barrel containing the Nb-doped strontium titanate powder, the descending rate of the seed crystal, and the amount of hydrogen and oxygen introduced are adjusted to allow the Nb-doped strontium titanate crystal to expand and grow.

8. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 7, characterized in that: During the shoulder expansion growth process, the melt volume at the top of the Nb-doped strontium titanate crystal gradually increases and forms a melt cap; After the melt cap is formed, the descending rate of the seed crystal is adjusted to 16-18 mm / h, and the tapping amplitude of the barrel containing the Nb-doped strontium titanate powder is increased by 10-15 mm every 30 minutes; The oxygen gas increases by 0.1 L / min-0.3 L / min every 5-10 min, and the hydrogen gas increases by 0.3 L / min-0.9 L / min every 5-10 min.

9. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 8, characterized in that: When the oxygen intake reaches 9-10 L / min, the descent rate of the candlestick is reduced to 15-16 mm / h at a rate of 0.2 mm / hmin, and the hydrogen is increased by 0.2-0.5 L / min every 20-30 min.

10. The method for growing heterogeneous doped strontium titanate single crystal by flame melting method according to claim 9, characterized in that: When the diameter of the Nb-doped strontium titanate crystal reaches 10-15 mm, the isodiameter growth stage is entered, and the tapping amplitude of the barrel filled with the Nb-doped strontium titanate powder, the amount of hydrogen and oxygen introduced, and the descending rate of the seed crystal are maintained until the growth is completed; When the total length of the Nb-doped strontium titanate crystal reaches 70-80 mm, the growth of the Nb-doped strontium titanate crystal is stopped, and the furnace is wrapped with heat-insulating cotton to cool the Nb-doped strontium titanate crystal to room temperature.

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