Method for preparing size-controllable black phosphorus-carbon composite material

Abstract

The invention discloses a method for preparing a size-controllable black phosphorus-carbon composite material, and the method comprises the following steps: carrying out necking treatment on a quartz tube, and placing a spacer at the necking part; placing a phosphorus source, a metal simple substance and a mineralizing agent at the head end of a quartz tube, and limiting a carbon material at the tail end of the quartz tube; and carrying out vacuum melting sealing on the quartz tube, horizontally placing the quartz tube in a heating device, carrying out multi-stage programmed heating and cooling, and keeping the quartz tube horizontally rotating at a constant speed to obtain the black phosphorus-carbon composite material. According to the preparation method, black phosphorus growth is realized through a chemical vapor transport method, meanwhile, compounding of black phosphorus and the carbon material is completed, the composite material formed in the process has very stable P-C bonds, and the two-dimensional layered structure of the black phosphorus material can be reserved to a great extent. Due to the introduction of the carbon material, the growth degree of the black phosphorus in the chemical vapor transport process stays at a nanometer or micrometer scale instead of a macroscopic cluster, and the two-dimensional structural property of the black phosphorus material is amplified.

Description

Technical Field

[0001] The invention belongs to the technical field of nanomaterials, and in particular relates to a method for preparing a size-controllable black phosphorus-carbon composite material. Background Art

[0002] Black phosphorus has a typical two-dimensional layered structure, which provides a cheap space for the embedding and migration of lithium ions. It is a good negative electrode material for lithium-ion batteries, and its theoretical lithium storage capacity is as high as 2596mAh / g. However, pure black phosphorus negative electrodes have problems such as high volume expansion rate and fast capacity decay during the charge / discharge process, and usually need to be compounded with carbon materials in applications.

[0003] At present, the preparation methods of black phosphorus-carbon composite materials are mostly ball milling, milling, chemical crosslinking and drip coating. The above methods generally have complicated steps, and cannot form stable and controllable PC bonds, and the microscopic size of black phosphorus-carbon composite materials cannot be precisely controlled. Therefore, it is of great significance to develop a one-step preparation method for black phosphorus-carbon composite materials with controllable microscopic size and stable PC bond structure for the energy storage application of black phosphorus. Summary of the invention

[0004] In order to solve the above technical content, the purpose of the present invention is to provide a one-step preparation method of a size-controllable and stable black phosphorus-carbon composite material.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a size-controlled black phosphorus-carbon composite material comprises the following steps:

[0007] The quartz tube is subjected to necking treatment, and a spacer is placed at the necking position;

[0008] Placing a phosphorus source, a metal element and a mineralizer at the head end of a quartz tube, and confining a carbon material at the end end of the quartz tube;

[0009] After the quartz tube is vacuum sealed, it is horizontally placed in a heating device, and the temperature is raised and lowered in multiple stages, and the quartz tube is kept horizontally and rotated at a constant speed to obtain a black phosphorus-carbon composite material.

[0010] Furthermore, the phosphorus source is one of white phosphorus or red phosphorus.

[0011] Furthermore, the metal element is one or more of Cd, In, Sn, Sb, Te, Pb and Bi, and is in the form of powder with a mesh size of less than 200.

[0012] Furthermore, the mineralizer is CdI 2 ,InI 3 SnI 2 SnI4 , SbI 3 ,TeI 4 、PbI 2 With BiI 3 One or more of the above, in the form of powder or granules.

[0013] Furthermore, the mass ratio of the phosphorus source, the metal element and the mineralizer is 5:1:1 to 60:6:1.

[0014] Furthermore, the carbon material is graphite, graphene, graphene oxide, carbon black, fullerene, carbon nanotube or carbon fiber, and is in the form of powder with a mesh size below 200 or a sheet with a thickness below 100 μm.

[0015] Furthermore, the mass ratio of the phosphorus source to the carbon material is 0.1 to 10:1.

[0016] Furthermore, the quartz tube rotates at a speed of 45 to 500 rpm.

[0017] Furthermore, the temperature difference between the head end and the tail end is 10-70°C.

[0018] Furthermore, the multi-stage heating and cooling program is as follows: at room temperature, the temperature is increased for 1 to 5 hours, the temperature is increased to 600 to 800°C at a heating rate of 120 to 600°C / h, and the temperature is kept for 1 to 5 hours; then the temperature is cooled for 2 to 10 hours, the temperature is cooled at a rate of 10 to 50°C / h, the temperature is cooled to 300 to 600°C and the temperature is kept for 2 to 10 hours; and the temperature is cooled for a second time for 2 to 10 hours, the temperature is cooled at a rate of 30 to 70°C / h, and the temperature is cooled to 180 to 270°C.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention discloses a method for preparing a size-controllable black phosphorus-carbon composite material, which uses a phosphorus source, a metal element and a mineralizer as raw materials, realizes the growth of black phosphorus through a chemical vapor transport method, and also completes the composite of black phosphorus and carbon materials. The composite material formed in the process has a very stable PC bond and can retain the two-dimensional layered structure of the black phosphorus material to the greatest extent. The introduction of carbon materials allows the growth of black phosphorus in the chemical vapor transport process to remain at the nanometer or micrometer scale, rather than a macroscopic cluster, which amplifies the two-dimensional structural properties of the black phosphorus material.

[0021] Furthermore, in the present invention, by changing the rotation speed of the quartz tube and controlling the coverage area and deposition thickness of black phosphorus on the surface of the carbon material, the microscopic size of the black phosphorus-carbon composite material can be regulated; by increasing or decreasing the addition ratio of the phosphorus source and the carbon material, auxiliary regulation of the thickness of the phosphorus layer in the composite material can also be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are used to provide a further understanding of the invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the invention but do not constitute a limitation of the invention. In the accompanying drawings:

[0023] Figure 1 This is a schematic diagram of the quartz tube after loading and vacuum sealing;

[0024] Figure 2 The schematic diagram of the preparation of the black phosphorus-graphene composite material in Example 1;

[0025] Figure 3 is a SEM image of the black phosphorus-graphene composite material in Example 1;

[0026] Figure 4 This is the SEM image of the black phosphorus-carbon black composite material in Example 2. DETAILED DESCRIPTION

[0027] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are given in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thoroughly and comprehensively understood.

[0028] A method for preparing a size-controlled black phosphorus-carbon composite material of the present invention comprises the following steps:

[0029] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0030] 2) Weigh a certain amount of phosphorus source, metal element and mineralizer and place them at one end of the quartz tube (raw material end), and confine the carbon material at the other end of the quartz tube (end).

[0031] 3) See Figure 1 After the above quartz tube is vacuum sealed, it is placed horizontally in a heating device, with the raw material end located at the hot end of the heating device and the terminal end located at the cold end of the heating device.

[0032] 4) Black phosphorus is prepared by multi-stage programmed heating and cooling, and a stable temperature difference is maintained between the hot end and the cold end. The multi-stage heating and cooling program is as follows: programmed heating - heat preservation - programmed cooling - heat preservation - secondary programmed cooling - natural cooling.

[0033] 5) During the multi-stage programmed heating and cooling process for preparing black phosphorus, the quartz tube is kept horizontal and rotated at a constant speed.

[0034] 6) Under vacuum conditions, the composite material prepared initially is washed, filtered and dried to obtain a black phosphorus-carbon composite material.

[0035] Furthermore, the phosphorus source is one of white phosphorus or red phosphorus.

[0036] Furthermore, the metal element is one or more of Cd, In, Sn, Sb, Te, Pb and Bi, and is preferably in the form of powder with a size of less than 200 mesh.

[0037] Furthermore, the mineralizer is CdI 2 ,InI 3 SnI 2 SnI 4 , SbI 3 ,TeI 4 、PbI 2 With BiI 3 One or more of the above, preferably in the form of powder or granules.

[0038] Furthermore, the mass ratio of the phosphorus source, the metal element and the mineralizer is 5:1:1 to 60:6:1.

[0039] Furthermore, the carbon material includes but is not limited to graphite, graphene, graphene oxide, carbon black, fullerene, carbon nanotube or carbon fiber, and is preferably in the form of powder with a mesh size below 200 or a sheet with a thickness below 100 μm.

[0040] Furthermore, the mass ratio of the phosphorus source to the carbon material is 0.1 to 10:1.

[0041] Furthermore, the phosphorus source, metal element and mineralizer are placed at one end (raw material end) of the quartz tube, and the carbon material is placed at the other end (terminal end) of the quartz tube.

[0042] Furthermore, the raw material end is located at the hot end of the heating device, and the terminal end is located at the cold end of the heating device.

[0043] Furthermore, the heating device has two or more heating zones, including but not limited to a tube furnace, a crystal furnace or an electromagnetic heater.

[0044] Furthermore, the quartz tube rotates at a speed of 45 to 500 rpm.

[0045] Furthermore, the temperature difference maintained between the hot end and the cold end of the heating device is 10-70°C.

[0046] Furthermore, the multi-stage heating and cooling program is specifically as follows: under room temperature conditions, heating for 1 to 5 hours at a heating rate of 120 to 600°C / h, and keeping the temperature at the current temperature for 1 to 5 hours after heating; cooling for 2 to 10 hours at a cooling rate of 10 to 50°C / h, and keeping the temperature at the current temperature for 2 to 10 hours after cooling; and cooling for a second time for 2 to 10 hours at a cooling rate of 30 to 70°C / h.

[0047] The following are specific embodiments.

[0048] Example 1

[0049] See also Figure 2 , a method for preparing a size-controlled black phosphorus-carbon composite material, the specific preparation method is as follows:

[0050] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0051] 2) Weigh 250 mg white phosphorus, 60 mg Sb powder and 50 mg SbI 3 was placed at one end of the quartz tube (the feedstock end), while 250 mg of graphene powder was confined at the other end (the terminal end) of the quartz tube.

[0052] 3) After the above quartz tube is vacuum sealed, it is placed horizontally in a double-temperature zone tubular furnace, with the raw material end located at the hot end of the tubular furnace and the terminal end located at the cold end of the tubular furnace.

[0053] 4) Black phosphorus was prepared by multi-stage programmed heating and cooling, and the temperature difference between the hot end and the cold end was controlled to be stable at 50°C.

[0054] Hot end temperature control program: heating from 20℃ for 5h, heating rate of 132℃ / h, keeping at 680℃ for 2.5h; cooling for 4h, cooling rate of 30℃ / h, keeping at 560℃ for 6h; cooling for a second time to 250℃ for 5h, cooling rate of 62℃ / h, and then naturally cooling to room temperature.

[0055] Cold-end temperature control program: heating from 20℃ for 5h at a heating rate of 122℃ / h, keeping at 630℃ for 2.5h after heating; cooling for 4h at a cooling rate of 30℃ / h, keeping at 510℃ for 6h after cooling; cooling for a second time to 200℃ for 5h, and then naturally cooling to room temperature.

[0056] 5) During the multi-stage programmed temperature rise and fall process for preparing black phosphorus, the quartz tube was kept horizontal and rotated at a constant speed of 200 rpm.

[0057] 6) Under vacuum conditions, the preliminarily prepared composite material is washed to remove impurities, and a black phosphorus-graphene composite material is obtained after filtering and drying.

[0058] Figure 3 is a SEM image of the black phosphorus-graphene composite material in Example 1; Figure 3 It can be seen that black phosphorus and graphene form a composite material with an obvious layered structure and a length of about 500nm.

[0059] Table 1 shows the element distribution data of the black phosphorus-graphene composite material in Example 1. As can be seen from Table 1, the mass ratio (wt%) of black phosphorus to graphene meets the 1:1 ratio when the raw materials are added.

[0060] Table 1 Element distribution data of black phosphorus-graphene composite material in Example 1

[0061]

[0062] Figure 4

[0063] Example 2

[0064] A method for preparing a size-controllable black phosphorus-carbon composite material, the specific preparation method is as follows:

[0065] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0066] 2) Weigh 500 mg red phosphorus, 60 mg Pb powder, 36 mg I 2 placed at one end of the quartz tube (the feedstock end), while 80 mg of carbon black powder was confined at the other end (the end) of the quartz tube.

[0067] 3) After the above quartz tube is vacuum sealed, it is placed horizontally in a double-temperature zone tubular furnace, with the raw material end located at the hot end of the tubular furnace and the terminal end located at the cold end of the tubular furnace.

[0068] 4) Black phosphorus was prepared by multi-stage programmed heating and cooling, and the temperature difference between the hot end and the cold end was controlled to be stable at 20°C.

[0069] Hot end temperature control program: heating from 30℃ for 2.5h at a heating rate of 288℃ / h, and keeping at 750℃ for 4.5h after heating; cooling for 10h at a cooling rate of 20℃ / h, and keeping at 550℃ for 6.5h after cooling; cooling for a second time to 250℃ for another 10h at a cooling rate of 30℃ / h, and then naturally cooling to room temperature.

[0070] Cold end temperature control program: heating from 30℃ for 2.5h at a heating rate of 280℃ / h, keeping at 730℃ for 4.5h; cooling for 10h at a cooling rate of 20℃ / h, keeping at 530℃ for 6.5h; cooling for a second time to 230℃ for another 10h at a cooling rate of 30℃ / h, and then naturally cooling to room temperature.

[0071] 5) During the multi-stage programmed heating and cooling process for preparing black phosphorus, the quartz tube was kept horizontal and rotated at a constant speed of 60 rpm.

[0072] 6) Under vacuum conditions, the composite material prepared initially is washed to remove impurities, and the black phosphorus-acetylene black composite material is obtained after filtering and drying.

[0073] Figure 4 is a SEM image of the black phosphorus-carbon black composite material in Example 2. Figure 4 It can be seen that due to the slower rotation speed and higher P / C mass ratio, the lateral size (>10μm) of the composite material is larger and its structure is dense, but the layered structure is still obvious.

[0074] Example 3

[0075] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0076] 2) Weigh 80 mg red phosphorus, 37 mg Sn powder, 25 mg SnI 2 placed at one end of a quartz tube (the feedstock end), while 40 mg of porous carbon fiber was confined at the other end (the end) of the quartz tube.

[0077] 3) After the above quartz tube is vacuum sealed, it is placed horizontally in a double-temperature zone tubular furnace, with the raw material end located at the hot end of the tubular furnace and the terminal end located at the cold end of the tubular furnace.

[0078] 4) Black phosphorus was prepared by multi-stage programmed heating and cooling, and the temperature difference between the hot end and the cold end was controlled to be stable at 60°C.

[0079] Hot end temperature control program: heating from 15℃ for 3h, heating rate of 215℃ / h, keeping at 660℃ for 3.5h; cooling for 10h, cooling rate of 10℃ / h, keeping at 560℃ for 10h; cooling for the second time to 260℃ for 6h, cooling rate of 50℃ / h, and then naturally cooling to room temperature.

[0080] Cold end temperature control program: heating from 15℃ for 3h, heating rate of 195℃ / h, keeping at 600℃ for 3.5h; cooling for 10h, cooling rate of 10℃ / h, keeping at 600℃ for 10h; cooling for a second time to 200℃ for 8h, cooling rate of 50℃ / h, and then naturally cooling to room temperature.

[0081] 5) During the multi-stage programmed temperature rise and fall process for preparing black phosphorus, the quartz tube was kept horizontal and rotated at a constant speed of 45 rpm.

[0082] 6) Under vacuum conditions, the composite material prepared initially is washed to remove impurities, and a black phosphorus-porous carbon fiber composite material is obtained after filtering and drying.

[0083] Example 4

[0084] A method for preparing a size-controllable black phosphorus-carbon composite material, the specific preparation method is as follows:

[0085] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0086] 2) Weigh 6000 mg white phosphorus, 600 mg Cd powder and 10 mg CdI 2 was placed at one end of a quartz tube (the feedstock end), while 600 mg of fullerene powder was confined at the other end of the quartz tube (the terminal end).

[0087] 3) After the above quartz tube is vacuum sealed, it is placed horizontally in a double-temperature zone tubular furnace, with the raw material end located at the hot end of the tubular furnace and the terminal end located at the cold end of the tubular furnace.

[0088] 4) Black phosphorus was prepared by multi-stage programmed heating and cooling, and the temperature difference between the hot end and the cold end was controlled to be stable at 10°C.

[0089] Hot end temperature control program: heating from 20℃ for 1h, heating rate of 590℃ / h, keeping at 610℃ for 1h after heating; cooling for 6h, cooling rate of 50℃ / h, keeping at 310℃ for 2h after cooling; cooling for a second time to 190℃ for 2h, cooling rate of 60℃ / h, and then naturally cooling to room temperature.

[0090] Cold end temperature control program: heating from 20℃ for 1h, heating rate of 580℃ / h, keeping at 600℃ for 1h; cooling for 6h, cooling rate of 50℃ / h, keeping at 300℃ for 2h; cooling for a second time to 180℃ for 2h, cooling rate of 60℃ / h, and then naturally cooling to room temperature.

[0091] 5) During the multi-stage programmed temperature rise and fall process for preparing black phosphorus, the quartz tube was kept horizontal and rotated at a constant speed of 45 rpm.

[0092] 6) Under vacuum conditions, the preliminarily prepared composite material is washed to remove impurities, and a black phosphorus-graphene composite material is obtained after filtering and drying.

[0093] Example 5

[0094] A method for preparing a size-controllable black phosphorus-carbon composite material, the specific preparation method is as follows:

[0095] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0096] 2) Weigh 50 mg white phosphorus, 10 mg Sb powder and 10 mg SbI 3was placed at one end of the quartz tube (the feedstock end), while 500 mg of carbon nanotube powder was confined at the other end of the quartz tube (the terminal end).

[0097] 3) After the above quartz tube is vacuum sealed, it is placed horizontally in a double-temperature zone tubular furnace, with the raw material end located at the hot end of the tubular furnace and the terminal end located at the cold end of the tubular furnace.

[0098] 4) Black phosphorus was prepared by multi-stage programmed heating and cooling, and the temperature difference between the hot end and the cold end was controlled to be stable at 70°C.

[0099] Hot end temperature control program: heating from 20℃ for 3h, heating rate of 260℃ / h, keeping at 800℃ for 5h; cooling for 4h, cooling rate of 50℃ / h, keeping at 600℃ for 2h; cooling for a second time to 270℃ for 10h, cooling rate of 33℃ / h, and then naturally cooling to room temperature.

[0100] Cold end temperature control program: heating from 20℃ for 3h, heating rate of 237℃ / h, keeping at 730℃ for 5h; cooling for 4h, cooling rate of 50℃ / h, keeping at 530℃ for 2h; cooling for the second time to 180℃ for 10h, cooling rate of 35℃ / h, and then naturally cooling to room temperature.

[0101] 5) During the multi-stage programmed temperature rise and fall process for preparing black phosphorus, the quartz tube was kept horizontal and rotated at a constant speed of 500 rpm.

[0102] 6) Under vacuum conditions, the preliminarily prepared composite material is washed to remove impurities, and a black phosphorus-graphene composite material is obtained after filtering and drying.

[0103] Example 6

[0104] A method for preparing a size-controllable black phosphorus-carbon composite material, the specific preparation method is as follows:

[0105] 1) Use high temperature flame to shrink the quartz tube and place a spacer at the neck.

[0106] 2) Weigh 200 mg red phosphorus, 30 mg Sb powder and 10 mg SbI 3 was placed at one end of a quartz tube (the feedstock end), while 40 mg of graphite powder was confined at the other end of the quartz tube (the terminal end).

[0107] 3) After the above quartz tube is vacuum sealed, it is placed horizontally in a double-temperature zone tubular furnace, with the raw material end located at the hot end of the tubular furnace and the terminal end located at the cold end of the tubular furnace.

[0108] 4) Black phosphorus was prepared by multi-stage programmed heating and cooling, and the temperature difference between the hot end and the cold end was controlled to be stable at 30°C.

[0109] Hot end temperature control program: heating from 20℃ for 5h, heating rate of 136℃ / h, keeping at 700℃ for 3h; cooling for 10h, cooling rate of 20℃ / h, keeping at 500℃ for 5h; cooling for a second time to 230℃ for 7h, cooling rate of 39℃ / h, and then naturally cooling to room temperature.

[0110] Cold end temperature control program: heating from 20℃ for 5h, heating rate of 130℃ / h, keeping at 670℃ for 3h; cooling for 10h, cooling rate of 20℃ / h, keeping at 470℃ for 5h; cooling for a second time to 270℃ for 5h, cooling rate of 40℃ / h, and then naturally cooling to room temperature.

[0111] 5) During the multi-stage programmed temperature rise and fall process for preparing black phosphorus, the quartz tube was kept horizontal and rotated at a constant speed of 300 rpm.

[0112] 6) Under vacuum conditions, the preliminarily prepared composite material is washed to remove impurities, and a black phosphorus-graphene composite material is obtained after filtering and drying.

[0113] The above description is only for the best embodiment of the present invention, but it should not be understood as limiting the claims. The present invention is not limited to the above embodiments, and its specific structure is allowed to be changed. However, all changes made within the protection scope of the independent claims of the present invention are within the protection scope of the present invention.

[0114] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

Claims

1. A method for preparing a size-controlled black phosphorus-carbon composite material, characterized in that: The following steps are involved: The quartz tube is subjected to necking treatment, and a spacer is placed at the necking position; Placing a phosphorus source, a metal element and a mineralizer at the head end of a quartz tube, and confining a carbon material at the end end of the quartz tube; After the quartz tube is vacuum sealed, it is horizontally placed in a heating device, and the temperature is raised and lowered in multiple stages, and the quartz tube is kept horizontally and rotated at a constant speed to obtain a black phosphorus-carbon composite material.

2. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The phosphorus source is one of white phosphorus or red phosphorus.

3. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The metal element is one or more of Cd, In, Sn, Sb, Te, Pb and Bi, and is in the form of powder with a mesh size of less than 200.

4. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The mineralizer is one or more of CdI2, InI3, SnI2, SnI4, SbI3, TeI4, PbI2 and BiI3, and is in the form of powder or granules.

5. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The mass ratio of the phosphorus source, the metal element and the mineralizer is 5:1:1 to 60:6:

1.

6. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The carbon material is graphite, graphene, graphene oxide, carbon black, fullerene, carbon nanotube or carbon fiber, and is in the form of powder with a mesh size of less than 200 or a sheet with a thickness of less than 100 μm.

7. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The mass ratio of the phosphorus source to the carbon material is 0.1 to 10:

1.

8. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The quartz tube rotation speed is 45-500 rpm.

9. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: The temperature difference between the head end and the tail end is 10-70°C.

10. The method for preparing a size-controlled black phosphorus-carbon composite material according to claim 1, characterized in that: Multi-stage The specific cooling procedure is: under room temperature, the temperature is raised to 600-800°C at a heating rate of 120-600°C / h after 1-5h, Keep the temperature for 1 to 5 hours; then cool down for 2 to 10 hours at a cooling rate of 10 to 50°C / h to 300 to 600°C and keep the temperature for 2 to 10 hours; The temperature is then lowered for a second time for 2 to 10 hours at a rate of 30 to 70°C / h to a temperature of 180 to 270°C.

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