A rare earth Eu 2+ Doped fluorine oxyborate glass-ceramics and preparation method thereof

Abstract

The present invention provides a method for preparing a fluoroxyborate glass-ceramic capable of generating f-f transitions of rare earth Eu 2+ ions. The composition of the precursor glass of the present invention includes a matrix glass and rare earth ions. The molar percentage composition of the matrix glass is: 15-55 mol% of BaF2, 10-45 mol% of B2O3, 10-40 mol% of Al2O3; the rare earth ions are doped in an external doping form other than Eu2O3, and the molar percentage is 0.01-5 mol% of the matrix glass components. The preparation method of the glass-ceramic of the present invention is to first prepare a Eu 2+ -doped precursor glass by high-temperature melting in a reducing atmosphere protected by carbon, and then heat-treat the glass to precipitate BaAlBO3F2 microcrystalline particles to obtain a rare earth Eu 2+ -doped fluoroxyborate glass-ceramic. The glass-ceramic of the present invention has high transparency and a narrow emission line width, and can be used in the fields of laser materials and phototherapy lamps.

Description

Technical Field

[0001] The present invention relates to the technical field of microcrystalline glass, and in particular to a solid laser material and rare earth Eu for phototherapy lamps. 2+ Doped fluorine oxyborate glass-ceramics and preparation method thereof. Technical Background

[0002] Eu 2+ There are two transition modes, one is the 5d-4f transition that produces broadband luminescence, and the other is the 4f-4f transition that produces linear emission. 2+ The 4f energy level is located above the 5d energy level, and the 4f 6 5d and ground state 4f 7 (8S 7 / 2 ) between the two (df transitions) to produce broadband emission spectra. In a few fluorides with weak crystal field environments, such as RbMgF 3 , KMgF 3 , LiBaF 3 Crystal, Eu 2+ 4f 7 Energy level at 4f 6 5d below, 4f 7 (6P 7 / 2 )→4f 7 (8S 7 / 2 ) produces a 361nm linear emission peak from the forbidden transition (ff transition) of the alkaline earth metal fluoroaluminate crystal BaAlF in 1971. 5 Eu was observed in 2+ 4f 7 ( 6 P 7 / 2 )→4f 7 ( 8 S 7 / 2 ) transition linear emission (see RA Hewes and M. V. Hoffman, J. Luminescence, 3 (1971), 261.).

[0003] In 2012, K. Shionozaki first precipitated Eu in fluorobarium borate. 3+ Doped nonlinear BaAlBO 3 F 2 Microcrystalline particles (see K. Shionozaki, T. Honma and T. Komatsu, J. Appl. Phys., 112 (2012) 7.). In 2016, RG Kunghatkar et al. prepared Eu 2+ Doped BaAlBO 3 F2 Phosphor, a linear luminescence transition of 361nm was observed (see RG Kunghatkar, SJ Dhoble and PS Hemne, Luminescence, 31 (2016) 1503-1512.). However, the opacity of phosphor greatly limits its application range. 2+ The ff luminescence transition of transparent microcrystalline glass has great significance.

[0004] Laser output was achieved in Nd-doped silicate glass in 1961, and then phosphate glass replaced silicate glass. Today, phosphate laser glass is an excellent laser gain material in high-energy solid lasers and is widely used in defense, medical treatment, and high-power laser nuclear fusion. 3+ The output wavelength of 1064nm tripled frequency produces 355nm ultraviolet light, which causes great energy loss. 2+ The 361nm linear emission of the ff transition may produce stimulated radiation transition under pumping light and can be used as an ultraviolet laser material in solid lasers without additional frequency doubling, which can reduce devices and improve energy utilization.

[0005] The photolithography machine is the core machine in the semiconductor industry. Deep ultraviolet DUV and extreme deep ultraviolet EUV photolithography technology has been widely used in the preparation of the most advanced chips. The DUV photolithography machine uses a 193nm deep ultraviolet light source to prepare 7nm semiconductor chips. 2+ The ff transition can produce 361nm linear laser emission, and the 180nm laser generated by double frequency can be used in DUV lithography machines.

[0006] Phototherapy is the use of natural light or artificial light to treat skin diseases. However, excessive exposure to natural light can cause skin cancer, so artificial ultraviolet light has become a better physical means of treating various skin diseases such as psoriasis, vitiligo, and eczema. The efficacy of phototherapy lamps depends on the luminescent properties of the luminescent materials used. According to the different wavelengths of the ultraviolet spectrum, they are usually divided into three types: UVC (200-280nm), UVB (280-320nm), and UVA (320-400nm). The 361nm linear emission generated by the transparent microcrystalline glass of the present invention is within the UVA band and can be used to prepare phototherapy lamp light sources for the treatment of special skin diseases. Summary of the invention

[0007] The purpose of the present invention is to provide a solid laser material and a rare earth Eu for phototherapy lamp 2+The invention discloses fluorine oxyborate doped microcrystalline glass and a preparation method thereof. The microcrystalline glass has the characteristics of high transparency and narrow luminescence spectrum.

[0008] The technical solution adopted by the present invention is: a laser material and a rare earth Eu for phototherapy lamp 2+ The precursor glass of the doped fluorine oxyborate microcrystalline glass comprises a matrix glass and rare earth ions; wherein the matrix glass has the following molar percentages: 15-55 mol% of BaF 2 , 10~45mol% B 2 O 3 , 10~40mol%Al 2 O 3 ; Rare earth ions are Eu 2 O 3 It is added in the form of external doping, and the molar percentage is 0.01 to 5 mol% of the matrix glass component.

[0009] The present invention also provides the above rare earth Eu 2+ The preparation method of fluorine oxyborate doped glass-ceramics comprises the following steps:

[0010] (1) Precursor glass melting: According to the above-mentioned fluorine oxyborate glass composition and molar percentage, the mass of each glass component is calculated, and each raw material is accurately weighed. 2 O 3 By H 3 BO 3 Introduced, Al 2 O 3 From Al(OH) 3 or Al 2 O 3 Introduce; grind the weighed raw materials in an agate mortar and mix them evenly to form a mixture; put the mixture into a high-temperature furnace at 1000°C to 1500°C and melt it in a carbon-protected reducing atmosphere for 10-60 minutes; quickly transfer the glass to a muffle furnace at 470°C to 500°C for annealing, keep it warm for 2 to 5 hours, and then naturally cool it to room temperature with the furnace, and take out the glass after it is completely cooled;

[0011] (2) Preparation of glass-ceramics: The glass sample obtained in step (1) is cut and polished into blocks, and then heat-treated. The blocks are placed in a muffle furnace at 550-590° C. and kept warm for 0.5-5 hours. After that, the blocks are taken out to obtain glass-ceramics.

[0012] The carbon-protected reducing atmosphere is achieved by:

[0013] Use the shell crucible method, which is to place the raw materials in a corundum or quartz crucible, and then cover it with a corundum crucible with carbon powder.

[0014] Use the shell crucible method, which is to place the raw materials in a corundum or quartz crucible, and then cover it with a layer of corundum crucible with graphite powder on the outside;

[0015] Use the shell crucible method, which is to place the raw material in a graphite crucible and cover it with a corundum crucible.

[0016] Place the raw materials directly in a covered graphite crucible.

[0017] Technical effects of the present invention:

[0018] Solid laser material and rare earth Eu for phototherapy lamp disclosed in the present invention 2+ Doped fluorine oxyborate glass-ceramics, by adjusting the glass composition and rare earth ion Eu 2+ The doping concentration of Eu 2+ The doped precursor glass is then heat treated to produce BaAlBO in the glass matrix. 3 F 2 Tiny grains, Eu 2+ When entering the crystal phase from the glass phase, the luminescence transition mode is converted from the df transition that produces broadband luminescence to the ff transition that produces narrow-band luminescence, thereby obtaining a transparent microcrystalline glass with 361nm linear fluorescence emission.

[0019] The present invention Eu 2+ The doped microcrystalline glass has high transparency and narrow luminescence linewidth, and can be used in applications in the field of ultraviolet solid laser materials and phototherapy lamps. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The XRD diagrams of the precursor glass and Example 1# in the present invention are shown in FIG.

[0021] Figure 2 The EPR diagrams of the precursor glass and Example 1# in the present invention are shown in FIG.

[0022] Figure 3 The emission spectra of the precursor glass, comparative example 1# and example 1# in the present invention under 270nm blue light excitation. DETAILED DESCRIPTION

[0023] The specific technical solutions in the embodiments of the present invention are described below, and the following descriptions are not all examples of the present invention. Other examples without creativity made based on the present invention all belong to the protection scope of the present invention.

[0024] Laser material and rare earth Eu for phototherapy lamp of the present invention 2+ The specific glass composition of the doped fluorine oxyborate glass-ceramics is as follows:

[0025] Table 1: Glass composition of Comparative Example 1# and Examples 1-9#

[0026]

[0027] Comparative Example 1#:

[0028] The preparation process of Comparative Example 1 is as follows:

[0029] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (18.603g), H 3 BO 3 (13.121 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (8.276 g)(Al 2 O 3 From Al(OH) 3 Introduce); put all the raw materials in an agate mortar and mix them evenly; after mixing, put the raw materials into a graphite crucible and melt them in a silicon carbon rod electric furnace at 1300℃ for 20 minutes, and pour the clarified glass liquid on an aluminum plate; transfer the obtained glass to a 470℃ annealing furnace for annealing for 3 hours, cool it to room temperature with the furnace and take it out to obtain a colorless and transparent fluorine oxyborate matrix glass. Subsequently, the glass sample was kept at 580℃ for 2 hours for heat treatment to obtain a transparent microcrystalline glass. The photoluminescence spectrum of the sample was measured by the FP-6500 fluorescence spectrometer of Japan Spectroscopy Corporation. Figure 3 As shown, no Eu 2+ The doped comparative example 1# does not produce fluorescence emission under 270nm blue light excitation.

[0030] Embodiment 1#:

[0031] The preparation process of Example 1 is as follows:

[0032] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (18.603g), H 3 BO 3 (13.121 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (8.276 g)(Al 2 O 3 From Al(OH) 3 Then weigh 0.1mol% Eu of the matrix glass component 2 O 3(0.0931g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1300℃ for 20 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470℃ annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ The glass sample was then heat treated at 580°C for 2 hours to obtain transparent microcrystalline glass. A linear emission peak of 361 nm appeared under 270 nm ultraviolet light excitation. Figure 1 The XRD pattern measured by the Netherlands PANalytical Empyrean multifunctional X-ray diffractometer shows that the precursor glass exhibits an amorphous scattering peak, indicating that it is in a glassy state. 3 F 2 The crystal diffraction peaks indicate that BaAlBO 3 F 2 Crystalline glass. Figure 2 The samples were tested using the German Bruker Elexsys-E580 electron paramagnetic resonance tester. The signals near g = 6.0 and 4.5 in the precursor glass were Eu 2+ The paramagnetic signal of the reducing atmosphere successfully converted Eu 3+ Reduction to Eu 2+ In Example 1# glass-ceramic sample, Eu appeared near g=1.99 2+ The weak crystal field paramagnetic signal indicates that a part of Eu 2+ BaAlBO from glass phase to weak crystal field 3 F 2 The photoluminescence spectrum of the sample was measured by FP-6500 fluorescence spectrometer of Japan SCOP Corporation. Figure 3 As shown, Eu 2+ Broadband luminescence of df transition is generated in the precursor glass, while Eu 2+ The narrow band emission of 361 nm indicates that some Eu 2+ Enter BaAlBO 3 F 2 After the crystal is formed, its luminescence transition mode changes from df transition to ff transition.

[0033] Example 2#:

[0034] The preparation process of Example 2 is as follows:

[0035] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (13.833 g), H 3 BO3 (9.757 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (16.411 g)(Al 2 O 3 From Al(OH) 3 Then weigh 0.25mol% Eu of the matrix glass component 2 O 3 (0.2301g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1300°C for 20 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470°C annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ Doped light yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 560°C for 2 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0036] Embodiment 3#:

[0037] The preparation process of Example 3 is as follows:

[0038] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (23.606g), H 3 BO 3 (3.027 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (13.366 g)(Al 2 O 3 From Al(OH) 3 Then weigh 1mol% Eu of the matrix glass component 2 O 3 (0.8434g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1500℃ for 30 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470℃ annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+Doped yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 550°C for 0.5 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0039] Embodiment 4#:

[0040] The preparation process of Example 4 is as follows:

[0041] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (24.835g), H 3 BO 3 (11.147 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (4.018 g)(Al 2 O 3 From Al(OH) 3 Then weigh 1.5mol% Eu of the matrix glass component 2 O 3 (1.3149g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1300℃ for 20 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470℃ annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ Doped yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 550°C for 0.5 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0042] Example 5#:

[0043] The preparation process of Example 5 is as follows:

[0044] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (16.561g), H 3 BO 3 (15.019 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (8.420 g)(Al 2 O 3 From Al(OH) 3Then weigh 2mol% Eu of the matrix glass component 2 O 3 (1.8134g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1200°C for 20 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470°C annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ The doped yellow transparent fluorine oxyborate glass was then heat treated at 570°C for 1 hour to obtain transparent microcrystalline glass. A linear emission peak of 361 nm appeared under 270 nm ultraviolet light excitation.

[0045] Embodiment 6#:

[0046] The preparation process of Example 6 is as follows:

[0047] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (9.546 g), H 3 BO 3 (13.467 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (16.988g)(Al 2 O 3 From Al(OH) 3 Then weigh 2mol% Eu of the matrix glass component 2 O 3 (1.8285g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1100°C for 20 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470°C annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ Doped yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 580°C for 2 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0048] Example 7#:

[0049] The preparation process of Example 7 is as follows:

[0050] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (11.726 g), H3 BO 3 (11.580 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (16.694g)(Al 2 O 3 From Al(OH) 3 Then weigh 3mol% Eu of the matrix glass component 2 O 3 (2.6383g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1300℃ for 30 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470℃ annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ Doped deep yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 590°C for 2 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0051] Example 8#:

[0052] The preparation process of Example 8 is as follows:

[0053] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (14.133 g), H 3 BO 3 (13.292g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (13.575 g)(Al 2 O 3 From Al(OH) 3 Then weigh 5mol% Eu of the matrix glass component 2 O 3 (4.2284g). All raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1200°C for 10 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470°C annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+Doped deep yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 590°C for 1.5 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0054] Example 9#:

[0055] The preparation process of Example 9 is as follows:

[0056] First, according to the matrix glass composition in Table 1, weigh 40g of the required raw materials, including BaF 2 (7.288 g), H 3 BO 3 (15.421 g)(B 2 O 3 By H 3 BO 3 Introduction), Al(OH) 3 (17.292g)(Al 2 O 3 From Al(OH) 3 Then weigh 0.01mol%Eu of the matrix glass component 2 O 3 (0.0097g). All the raw materials were placed in an agate mortar and mixed evenly; after mixing, the raw materials were placed in a graphite crucible and melted in a silicon carbon rod electric furnace at 1100°C for 20 minutes, and the clarified glass liquid was poured on an aluminum plate; the obtained glass was transferred to a 470°C annealing furnace for annealing for 3 hours, and then taken out after cooling to room temperature in the furnace to obtain Eu 2+ Doped light yellow transparent fluorine oxyborate glass. Subsequently, the glass sample was heat treated at 550°C for 5 hours to obtain transparent microcrystalline glass. A linear emission peak of 361nm appeared under 270nm ultraviolet light excitation.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A rare earth Eu 2+ Doped fluoride oxyborate glass-ceramics, characterized in that The precursor glass comprises a matrix glass and rare earth ions, wherein the matrix glass has the following molar percentages: 15-55 mol% of BaF2, 10-45 mol% of B2O3, and 10-40 mol% of Al2O3, and the rare earth ions are doped in the form of Eu2O3, and the molar percentage is 0.01-5 mol% of the matrix glass component; and through heat treatment, BaAlBO3F2 micro-grains are generated in the matrix glass, Eu2O3 and Al2O3 are added to the matrix glass. 2+ From the glass phase to the crystal phase, the luminescence transition mode is converted from the df transition that produces broadband luminescence to the ff transition that produces narrow-band luminescence.

2. A rare earth Eu as claimed in claim 1 2+ Doped fluoride oxyborate glass-ceramics, characterized in that In terms of molar percentage, the molar percentage of the matrix glass composition is as follows: 40 mol% BaF2, 40 mol% B2O3, 20 mol% Al2O3, and the molar percentage of rare earth ions is 0.1 mol% of the matrix glass component.

3. A rare earth Eu as claimed in claim 1 2+ Doped fluoride oxyborate glass-ceramics, characterized in that The B2O3 is introduced from H3BO3, and the Al2O3 is introduced from Al(OH)3 or Al2O3.

4. A rare earth Eu 2+ A method for preparing doped fluorine oxyborate glass-ceramics, characterized in that: The following steps are involved: (1) Precursor glass melting: According to the composition and molar percentage of the fluorine oxyborate glass of claim 1, the mass of each corresponding glass composition is calculated, and each raw material is accurately weighed, wherein the B2O3 is introduced by H3BO3, and the Al2O3 is introduced by Al(OH)3 or Al2O3; the weighed raw materials are ground in an agate mortar and mixed evenly to form a mixture; the mixture is put into a high-temperature furnace at 1000°C to 1500°C, and melted in a carbon-protected reducing atmosphere for 10-60 minutes; The glass is quickly transferred to a muffle furnace at 470°C to 500°C for annealing. After keeping the temperature for 2 to 5 hours, the glass is naturally cooled to room temperature in the furnace. After being completely cooled, the glass is taken out. (2) Preparation of glass-ceramics: The glass sample obtained in step (1) is cut and polished into blocks, and then heat-treated. The blocks are placed in a muffle furnace at 550-590° C. and kept warm for 0.5-5 hours. After that, the blocks are taken out to obtain glass-ceramics.

5. A rare earth Eu as claimed in claim 4 2+ A method for preparing doped fluorine oxyborate glass-ceramics, characterized in that: In step (1), the mixture is placed in a high-temperature furnace at 1300° C. and melted in a carbon-protected reducing atmosphere for 10-60 minutes; the glass is quickly transferred to a muffle furnace at 470° C. for annealing, and after being kept warm for 3 hours, it is naturally cooled to room temperature in the furnace, and the glass is taken out after being completely cooled; In step (2), the glass sample obtained in step (1) is cut and polished into blocks, and then heat-treated. The blocks are placed in a muffle furnace at 580° C. and kept warm for 2 hours, and then taken out to obtain microcrystalline glass.

6. A rare earth Eu according to claim 4 or 5 2+ A method for preparing doped fluorine oxyborate glass-ceramics, characterized in that: The carbon-protected reducing atmosphere can be achieved by one of the following methods: Use the shell crucible method, which is to place the raw materials in a corundum or quartz crucible, and then cover it with a corundum crucible with carbon powder. Use the shell crucible method, which is to place the raw materials in a corundum or quartz crucible, and then cover it with a corundum crucible with graphite powder. Use the shell crucible method to place the raw materials in a graphite crucible, and then cover it with a layer of covered corundum crucible; directly place the raw materials in the covered graphite crucible.

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