阅读说明：本技术 一种钙铕氮化物固溶体粉末及其制备方法 (Calcium-europium nitride solid solution powder and preparation method thereof ) 是由 申晋陇 周宏亮 张尚虎 张轩荣 于 2020-11-24 设计创作，主要内容包括：本发明公开了一种钙铕氮化物固溶体粉末及其制备方法,所述钙铕氮化物固溶体粉末由钙铕氮化物固溶体粉碎制成,所述钙铕氮化物固溶体的化学通式为(Ca_(1-x)Eu_x)_3N_2；其中,0＜x≤0.025。其制备方法包括以下步骤：S1、称取金属钙Ca、金属铕Eu,将其真空熔炼炉内,抽真空洗炉后通入氩气作为保护气体,熔融后浇筑于铸锭模具中,获得合金铸锭；S2、将合金铸锭进行表面清理后,置于氢破碎炉中,通入氢气并加热,使合金铸锭破碎,然后加热脱氢,获得合金粗粉末；S3、将氢破碎炉的温度降低至300℃以下,在氢破碎炉中通入氮气并逐步升温至460℃保温180min,然后升温至610℃保温300min,获得钙铕氮化物固溶体粉末。(The invention discloses calcium-europium nitride solid solution powder and a preparation method thereof, wherein the calcium-europium nitride solid solution powder is prepared by crushing calcium-europium nitride solid solution, and the chemical general formula of the calcium-europium nitride solid solution is (Ca) 1‑x Eu x ) 3 N 2 (ii) a Wherein x is more than 0 and less than or equal to 0.025. The preparation method comprises the following steps: s1, weighing calcium metal Ca and europium metal Eu, placing the calcium metal Ca and the europium metal Eu in a vacuum smelting furnace, vacuumizing and washing the furnace, introducing argon as protective gas, and pouring the argon into a casting mold after melting to obtain an alloy ingot; s2, cleaning the surface of the alloy ingot, placing the alloy ingot in a hydrogen crushing furnace, introducing hydrogen and heating to crush the alloy ingot, and then heating and dehydrogenating to obtain alloy coarse powder; s3, reducing the temperature of the hydrogen crushing furnace to be below 300 ℃, introducing nitrogen into the hydrogen crushing furnace and gradually increasing the temperatureAnd (3) heating to 460 ℃ and preserving the heat for 180min, and then heating to 610 ℃ and preserving the heat for 300min to obtain the calcium-europium nitride solid solution powder.)

1. A calcium-europium nitride solid solution powder, characterized in that: the calcium-europium nitride solid solution powder is prepared by crushing calcium-europium nitride solid solution, and the chemical general formula of the calcium-europium nitride solid solution is (Ca)_1-xEu_x)₃N₂(ii) a Wherein x is more than 0 and less than or equal to 0.025.

2. A method for producing a calcium europium nitride solid solution powder according to claim 1, characterized in that: the method comprises the following steps:

s1, weighing calcium metal Ca with the purity of more than or equal to 99.99% and europium metal Eu with the purity of more than or equal to 99.99%, placing the calcium metal Ca and the europium metal Eu into a vacuum smelting furnace of an alumina crucible, vacuumizing, washing the furnace, introducing argon as protective gas, melting at the temperature of 750-820 ℃, and pouring into a casting mold to obtain an alloy cast ingot;

s2, cleaning the surface of the alloy ingot obtained in the step S1, placing the alloy ingot in a hydrogen crushing furnace, washing the furnace with argon, introducing hydrogen with the purity of more than or equal to 99.99%, heating to 220-360 ℃, crushing the alloy ingot, vacuumizing, heating to 580 ℃ and dehydrogenating to obtain alloy coarse powder;

s3, reducing the temperature of the hydrogen crushing furnace to below 300 ℃, introducing nitrogen obtained by vaporizing liquid nitrogen with the purity of more than or equal to 99.999% into the hydrogen crushing furnace, gradually heating to 460 ℃, preserving the temperature for 180min, then heating to 610 ℃, preserving the temperature for 300min, and obtaining the calcium-europium nitride solid solution powder.

3. The method for producing a calcium-europium nitride solid solution powder according to claim 2, wherein: in the step S1, the furnace is washed three times during the vacuuming and furnace washing.

4. The method for producing a calcium-europium nitride solid solution powder according to claim 2, wherein: in the step S2, the surface cleaning of the alloy ingot is mainly used to remove an oxide layer on the surface of the alloy ingot and slag-making impurities generated in the melting process, and then the surface of the alloy ingot is polished to completely remove the oxide layer remaining on the surface.

5. The method for producing a calcium-europium nitride solid solution powder according to claim 2, wherein: in the step S2, the furnace is washed three times with argon gas.

6. The method for producing a calcium-europium nitride solid solution powder according to claim 2, wherein: in the step S3, the pressure in the hydrogen fracturing furnace is maintained at 0.5 to 0.6 MPa.

7. The method for producing a calcium-europium nitride solid solution powder according to claim 2, wherein: the calcium-europium nitride solid solution powder obtained in the step S3 is coarse-particle powder, and the coarse-particle powder is transferred into a glove box with water and an oxygen content of less than 1ppm, and is pulverized by a pulverizer to obtain fine-particle powder with different mesh numbers.

Technical Field

The invention relates to the technical field of chemical industry, in particular to calcium-europium nitride solid solution powder and a preparation method thereof.

Background

At present, with the development of new material technology, calcium europium nitride solid solution powder with controllable component structure is flourishing in the industry gradually, and shows good market prospect, and the calcium europium nitride solid solution can be used as a raw material in the fields of LED fluorescent powder, ceramic pigment and the like.

The reaction of calcium metal with nitrogen started at 360 ℃ but the weight increase was small and stopped at the end of the reaction because the reaction was difficult to proceed due to the formation of an N-Ca film, and even at 650 ℃ for more than 50 hours, the reaction was difficult to proceed and a large flow of nitrogen was passed. At this time, if a small amount of impurity gas such as: moisture, carbon dioxide, oxygen and the like can be completely absorbed by metal at high temperature, the oxygen content of the finally obtained product is also very high, and the reaction time needs to last for more than 100 hours when the reaction is complete; therefore, the product obtained by the preparation method in the prior art has low purity and long time consumption, and simultaneously, a large amount of nitrogen is consumed, so that the production requirement cannot be met.

Disclosure of Invention

Aiming at the problems, the invention provides calcium-europium nitride solid solution powder which does not need to add any additive, is simple and convenient to operate, has higher purity, higher synthesis efficiency, less hydrogen and nitrogen consumption, higher production efficiency, simpler process control, short hydrogenation and nitridation time, less energy consumption and short production period, and a preparation method thereof.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a calcium europium nitride solid solution powder is prepared by crushing calcium europium nitride solid solution, and the chemical general formula of the calcium europium nitride solid solution is (Ca)_1-xEu_x)₃N₂(ii) a Wherein x is more than 0 and less than or equal to 0.025.

A preparation method of calcium europium nitride solid solution powder comprises the following steps:

s1, weighing calcium metal Ca with the purity of more than or equal to 99.99% and europium metal Eu with the purity of more than or equal to 99.99%, placing the calcium metal Ca and the europium metal Eu into a vacuum smelting furnace of an alumina crucible, vacuumizing, washing the furnace, introducing argon as protective gas, melting at the temperature of 750-820 ℃, and pouring into a casting mold to obtain an alloy cast ingot;

s2, cleaning the surface of the alloy ingot obtained in the step S1, placing the alloy ingot in a hydrogen crushing furnace, washing the furnace with argon, introducing hydrogen with the purity of more than or equal to 99.99%, heating to 220-360 ℃, crushing the alloy ingot, vacuumizing, heating to 580 ℃ and dehydrogenating to obtain alloy coarse powder;

s3, reducing the temperature of the hydrogen crushing furnace to below 300 ℃, introducing nitrogen obtained by vaporizing liquid nitrogen with the purity of more than or equal to 99.999% into the hydrogen crushing furnace, gradually heating to 460 ℃, preserving the temperature for 180min, then heating to 610 ℃, preserving the temperature for 300min, and obtaining the calcium-europium nitride solid solution powder.

Further, in the step S1, the furnace is washed three times during the vacuuming and furnace washing.

Further, in the step S2, the cleaning of the surface of the alloy ingot is mainly used to remove an oxide layer on the surface of the alloy ingot and slag-forming impurities generated in the melting process, and then the surface of the alloy ingot is polished to completely remove the oxide layer remaining on the surface.

Further, in the step S2, the furnace is washed three times with argon gas.

Further, in the step S3, the pressure in the hydrogen decrepitation furnace is maintained at 0.5 to 0.6 Mpa.

Further, the calcium-europium nitride solid solution powder obtained in step S3 is coarse-particle powder, and the coarse-particle powder is transferred into a glove box with water and an oxygen content of less than 1ppm, and is pulverized by a pulverizer to obtain fine-particle powder with different mesh numbers.

Compared with the prior art, the invention has the advantages and positive effects that:

1. the purity of the calcium europium nitride solid solution powder prepared by the invention is higher, and the main reasons are as follows:

(1) the purity of the raw materials is high, in the raw materials used by the invention, the purity of metal calcium is 99.99 percent, the purity of metal europium is 99.99 percent, argon is vaporized by liquid argon to be used as shielding gas, the purity reaches more than 99.999 percent, nitrogen is vaporized by liquid nitrogen, the purity is more than 99.999 percent, and the purity of hydrogen reaches 99.99 percent;

(2) the process is strictly controlled, the invention firstly melts metal calcium and metal europium into an ingot to form calcium-europium alloy, removes an oxide layer on the metal surface of the ingot and slagging impurities generated in the smelting process, and then continues to polish the metal surface before entering a hydrogen furnace to completely remove the oxide layer remained on the surface;

(3) the introduction of a hydrogen crushing process can avoid the contact of large products with gases such as oxygen, moisture and the like in the later process, and the hydrogen can reduce part of incompletely oxidized metals to further reduce the oxygen content;

(4) the coarse powder is treated in a glove box with water and oxygen content less than 1ppm in the post-treatment process, and is crushed by a crusher coated with a high-hardness wear-resistant tungsten carbide coating, so that the introduction of mechanical impurities is further avoided in the process.

2. After various metals are smelted, various raw materials are uniformly distributed in a molten state, namely, the raw materials are mixed from the atom level, and then the molten raw materials are subjected to hydrogen cracking, nitriding and other processes, so that the uniformity and consistency of the products obtained in the whole process are high.

3. The invention uses a static high-pressure method to prepare the nitride solid solution powder, does not need a large amount of nitrogen gas flow, and can avoid the situation that impurities in the product are uncontrollable due to accumulation of a small amount of impurity gas mixed in the nitrogen.

4. The calcium europium nitride solid solution powder is subjected to a hydrogen crushing link, large and small or more or less cracks are arranged on the surface and inside of the metal, and the cracks form a channel for nitrogen to enter the metal when the nitride solid solution is synthesized in the later period, so that the reaction is easier to carry out, and the small cracks provide assistance for obtaining powder with different meshes by mechanical crushing in the later period.

5. (Ca) produced from calcium europium nitride solid solution powder in the present invention_0.98Eu_0.02)AlSiN₃Eu red phosphor, whose emission intensity is higher than that obtained directly with calcium nitride, europium nitride, silicon nitride, aluminum nitride, for reasons derived from three aspects:

(1) the raw materials for synthesizing the fluorescent powder comprise calcium,The europium is already distributed homogeneously at the atomic level, since in CaAlSiN₃Eu in the fluorescent powder occupies the position of Ca, compared with a preparation scheme of calcium nitride and europium nitride that only mixing at a molecular level, the method improves the mixing of Ca and Eu from a small particle molecular level to the mixing at an atomic level, greatly shortens the diffusion distance of Eu replacing Ca atoms during sintering, can be uniformly distributed in the whole crystal, can reduce the reaction time, and has higher brightness of the product;

(2) europium nitride cannot be mechanically ground to a very fine size, which causes Eu ions to segregate more or less at different parts of a single particle of the phosphor, or on different particles, resulting in failure to exert optimum intensity of light emission.

(3) In the calcium europium nitride solid solution powder of the present invention, Eu ions are first solid-dissolved in a divalent form at Ca sites, CaAlSiN₃The luminescence of the phosphor is Eu²⁺Can eliminate the existence of Eu due to incomplete reduction in the preparation process of the fluorescent powder³⁺Leading to a case where the light emission luminance is weakened.

6. The nitride formed by metallic calcium and calcium in the invention is a divalent ion compound, and Ca is used²⁺And europium nitride forms a trivalent ionic compound, in the form of Eu³⁺In the calcium europium nitride solid solution powder of the present invention, since Eu ions enter Ca sites, the powder becomes a calcium-based solid solution, and the stability is better.

7. The temperature required by the final synthesis is lower, the highest temperature only needs 610 ℃, and compared with other preparation methods, the preparation method is simpler and faster in preparation operation.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

FIG. 1 is an XRD pattern of calcium europium nitride powder obtained in examples 1-4;

FIG. 2 is a graph showing emission spectra of two phosphor samples in comparative examples 1-2.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived from the embodiments of the present invention by a person skilled in the art without any creative effort, should be included in the protection scope of the present invention.

The calcium metal used in the examples and the comparative examples of the invention has a purity of 99.99%, the europium metal has a purity of 99.99%, the silicon nitride is UBE, the E10 product, the aluminum nitride, the calcium nitride and the calcium nitride are sigma raw materials, the nitrogen is vaporized by liquid nitrogen with a purity of 99.999%, the argon is vaporized by liquid argon with a purity of 99.999%, and the hydrogen is hydrogen with a purity of 99.99%.

Example 1: preparation of (Ca)_0.995Eu_0.005)₃N₂；

981.3g of calcium metal and 1.87g of europium metal are weighed, the furnace is washed three times by argon in a vacuum melting furnace, argon is flushed to 80KPa, the mixture is heated to 780 ℃ and melted into a slurry state, and an alloy ingot is obtained by pouring and cooling. Cleaning the surface of an alloy ingot, putting the alloy ingot into a hydrogen breaking furnace, vacuumizing for three times, heating to 220 ℃ for activation, introducing hydrogen, heating to 360 ℃, keeping the temperature for 4 hours, heating to 580 ℃, vacuumizing to remove the hydrogen, cooling to below 300 ℃ after the vacuum degree is less than 40pa (shown by a meter), introducing nitrogen to 0.5MPa, gradually heating to 460 ℃, maintaining the gas pressure at 0.5MPa, keeping the temperature for 3 hours, then heating to 610 ℃, keeping the temperature for 5 hours, keeping the pressure at 5 minutes until the pressure is not reduced, and finishing the reaction. And then cooling to a room temperature state, charging the materials by using a stainless steel tank, moving the materials into a glove box with water and oxygen content less than 1ppm, crushing by using a crusher, sieving to obtain a powder sample with the granularity of 1-150 meshes, and performing XRD detection by using a serial number S-1.

Example 2: preparation of (Ca)_0.985Eu_0.015)₃N₂；

945.3g of calcium metal and 54.7g of europium metal are weighed, the furnace is washed three times by argon in a vacuum melting furnace, argon is flushed to 80KPa, the mixture is heated to 800 ℃ to be melted into a slurry state, and an alloy ingot is obtained by pouring and cooling. Cleaning the surface of an alloy ingot, putting the alloy ingot into a hydrogen breaking furnace, vacuumizing for three times, heating to 220 ℃ for activation, introducing hydrogen, heating to 360 ℃, keeping the temperature for 4 hours, heating to 580 ℃, vacuumizing to remove the hydrogen, cooling to below 300 ℃ after the vacuum degree is less than 40pa (shown by a meter), introducing nitrogen to 0.55MPa, gradually heating to 460 ℃, maintaining the gas pressure at 0.5MPa, keeping the temperature for 4 hours, then heating to 610 ℃, keeping the temperature for 5 hours, keeping the pressure in 5 minutes and showing that the pressure does not drop, and finishing the reaction. And then cooling to a room temperature state, charging by using a stainless steel tank, moving into a glove box with water and oxygen content less than 1ppm, crushing by using a crusher, sieving to obtain a powder sample with the granularity of 1-150 meshes, and performing XRD detection by using a serial number S-2.

Example 3: preparation of (Ca)_0.98Eu_0.02)₃N₂；

728g of calcium metal and 72g of europium metal are weighed, the furnace is washed by argon gas for three times in a vacuum smelting furnace, argon gas is filled until 80KPa, the temperature is increased to 810 ℃, the materials are melted into a slurry state, and an alloy ingot is obtained by pouring and cooling. Cleaning the surface of an alloy ingot, putting the alloy ingot into a hydrogen breaking furnace, vacuumizing for three times, heating to 220 ℃ for activation, introducing hydrogen, heating to 360 ℃, keeping the temperature for 4 hours, heating to 580 ℃, vacuumizing to remove the hydrogen, cooling to below 300 ℃ after the vacuum degree is less than 40pa (shown by a meter), introducing nitrogen to 0.6MPa, gradually heating to 460 ℃, maintaining the gas pressure at 0.6MPa, keeping the temperature for 4 hours, then heating to 610 ℃, keeping the temperature for 5 hours, keeping the pressure in 5 minutes and showing that the pressure does not drop, and finishing the reaction. And then cooling to a room temperature state, charging by using a stainless steel tank, transferring into a glove box with water and oxygen content less than 1ppm, crushing by using a crusher, sieving to obtain a powder sample with the granularity of 1-150 meshes, and performing XRD detection by using a serial number S-3.

Example 4: preparation of (Ca)_0.975Eu_0.035)₃N₂；

911.2g of calcium metal and 88.8g of europium metal are weighed, the furnace is washed three times by argon in a vacuum melting furnace, argon is filled until 80KPa, the temperature is heated to 820 ℃, the materials are melted into a slurry state, and an alloy ingot is obtained by pouring and cooling. Cleaning the surface of an alloy ingot, putting the alloy ingot into a hydrogen breaking furnace, vacuumizing for three times, heating to 220 ℃ for activation, introducing hydrogen, heating to 360 ℃, keeping the temperature for 4 hours, heating to 580 ℃, vacuumizing to remove the hydrogen, cooling to below 300 ℃ after the vacuum degree is less than 40pa (shown by a meter), introducing nitrogen to 0.6MPa, gradually heating to 460 ℃, maintaining the gas pressure at 0.6MPa, keeping the temperature for 4 hours, then heating to 610 ℃, keeping the temperature for 5 hours, keeping the pressure in 5 minutes and showing that the pressure does not drop, and finishing the reaction. And then cooling to a room temperature state, charging by using a stainless steel tank, moving into a glove box with water and oxygen content less than 1ppm, crushing by using a crusher, sieving to obtain a powder sample with the granularity of 1-150 meshes, and performing XRD detection by using a serial number S-4.

The high-purity calcium europium nitride synthesized by the invention is a pure solid solution, is not two mechanical mixtures of calcium nitride and europium nitride, and as seen from figure 1, the diffraction peak of the solid solution of the invention is similar to that of standard card pure calcium nitride and moves to a small angle along with the increase of Eu amount, because Eu²⁺The ion radius of (2) is large.

Comparative example 1:

153.04g of the high-purity calcium-europium nitride solid solution powder obtained in example 2 was weighed in a glove box, and 140.28gSi was added₃N₄122.96gAlN, mixed evenly by special mixing bags, put into a molybdenum crucible in a carbon tube furnace, and keep warm for 4 hours under nitrogen atmosphere, 1760 ℃ and 0.6MPa to obtain a fluorescent powder sample, numbered D-1, and tested for spectral performance.

Comparative example 2:

146.2g of calcium nitride and 7.47g of europium nitride are weighed in a glove box, and 140.28gSi is added₃N₄122.96gAlN, mixing uniformly, putting in carbon tube furnace, keeping temperature for 4 hours at 1760 deg.C and 0.6MPa in nitrogen atmosphere, obtaining fluorescent powder sample, numbering D-2, testing spectrum performance.

It can be seen from fig. 2 that the emission intensity of the phosphor powder synthesized by using the calcium-europium solid solution powder as the raw material is higher than that of the phosphor powder synthesized by directly using a single raw material, which fully illustrates that the calcium-europium solid solution powder prepared by the present invention has better mixing uniformity.

The invention has the following beneficial effects:

1. the purity of the calcium europium nitride solid solution powder prepared by the invention is higher, and the main reasons are as follows:

(1) the purity of the raw materials is high, in the raw materials used by the invention, the purity of metal calcium is 99.99 percent, the purity of metal europium is 99.99 percent, argon is vaporized by liquid argon to be used as shielding gas, the purity reaches more than 99.999 percent, nitrogen is vaporized by liquid nitrogen, the purity is more than 99.999 percent, and the purity of hydrogen reaches 99.99 percent;

(2) the process is strictly controlled, the invention firstly melts metal calcium and metal europium into an ingot to form calcium-europium alloy, removes an oxide layer on the metal surface of the ingot and slagging impurities generated in the smelting process, and then continues to polish the metal surface before entering a hydrogen furnace to completely remove the oxide layer remained on the surface;

(3) the introduction of a hydrogen crushing process can avoid the contact of large products with gases such as oxygen, moisture and the like in the later process, and the hydrogen can reduce part of incompletely oxidized metals to further reduce the oxygen content;

(4) the coarse powder is treated in a glove box with water and oxygen content less than 1ppm in the post-treatment process, and is crushed by a crusher coated with a high-hardness wear-resistant tungsten carbide coating, so that the introduction of mechanical impurities is further avoided in the process.

2. After various metals are smelted, various raw materials are uniformly distributed in a molten state, namely, the raw materials are mixed from the atom level, and then the molten raw materials are subjected to hydrogen cracking, nitriding and other processes, so that the uniformity and consistency of the products obtained in the whole process are high.

3. The invention uses a static high-pressure method to prepare the nitride solid solution powder, does not need a large amount of nitrogen gas flow, and can avoid the situation that impurities in the product are uncontrollable due to accumulation of a small amount of impurity gas mixed in the nitrogen.

4. The calcium europium nitride solid solution powder is subjected to a hydrogen crushing link, large and small or more or less cracks are arranged on the surface and inside of the metal, and the cracks form a channel for nitrogen to enter the metal when the nitride solid solution is synthesized in the later period, so that the reaction is easier to carry out, and the small cracks provide assistance for obtaining powder with different meshes by mechanical crushing in the later period.

5. (Ca) produced from calcium europium nitride solid solution powder in the present invention_0.98Eu_0.02)AlSiN₃Eu red phosphor, whose emission intensity is higher than that obtained directly with calcium nitride, europium nitride, silicon nitride, aluminum nitride, for reasons derived from three aspects:

(1) in the raw materials for synthesizing the fluorescent powder, calcium and europium are uniformly distributed on the atomic layer surface because of CaAlSiN₃Eu in the fluorescent powder occupies the position of Ca, compared with a preparation scheme of calcium nitride and europium nitride that only mixing at a molecular level, the method improves the mixing of Ca and Eu from a small particle molecular level to the mixing at an atomic level, greatly shortens the diffusion distance of Eu replacing Ca atoms during sintering, can be uniformly distributed in the whole crystal, can reduce the reaction time, and has higher brightness of the product;

(2) europium nitride cannot be mechanically ground to a very fine size, which causes Eu ions to segregate more or less at different parts of a single particle of the phosphor, or on different particles, resulting in failure to exert optimum intensity of light emission.

(3) In the calcium europium nitride solid solution powder of the present invention, Eu ions are first solid-dissolved in a divalent form at Ca sites, CaAlSiN₃The luminescence of the phosphor is Eu²⁺Can eliminate the existence of Eu due to incomplete reduction in the preparation process of the fluorescent powder³⁺Leading to a case where the light emission luminance is weakened.

6. The nitride formed by metallic calcium and calcium in the invention is a divalent ion compound, and Ca is used²⁺And europium nitride forms a trivalent ionic compound, in the form of Eu³⁺In the calcium-europium nitride solid solution powder of the present invention, since Eu ions enter the Ca site, the Eu ions become calcium-based solidsThe stability of the solution is better.

7. The temperature required by the final synthesis is lower, the highest temperature only needs 610 ℃, but the reaction pressure is higher, the product can have incomplete reaction when the pressure is lower than 0.5MPa, and when the pressure is higher than 0.6MPa, the local temperature is too high to generate melting due to too high reaction speed, so that the reaction is not facilitated, and the preferable synthesis pressure is 0.5-0.6 MPa.