Method for preparing boron nitride foamed ceramic from precursor
By preparing a boron nitride precursor, dissolving it in a xylene solution and then performing emulsification and foaming, vacuum freeze drying and nitrogen and ammonia calcination, the problem of difficult preparation of boron nitride foam ceramics was solved, and a boron nitride foam ceramic material with high temperature stability and chemical stability was obtained.
Patent Information
- Application Number
- CN202410350341.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-03-26
- Publication Date
- 2025-09-26
AI Technical Summary
Existing foam ceramic preparation technology is not suitable for boron nitride, which makes boron nitride foam ceramic difficult to prepare. The synthesis process is complex and the cycle is long, and the foam structure is easily broken, which limits its application in extreme environments.
Boron nitride precursor is dissolved in xylene solution and emulsified and foamed, and then vacuum freeze-drying, aging treatment and nitrogen and ammonia calcination are combined to form structurally stable boron nitride foamed ceramics.
The stable preparation of boron nitride foam ceramics was achieved, the collapse of the foam structure was avoided, and a boron nitride foam ceramic material with high temperature stability and chemical stability was obtained.
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of boron nitride foamed ceramics, and in particular relates to a method for preparing boron nitride foamed ceramics from a precursor. Background Art
[0002] Foam ceramics have a three-dimensional network skeleton structure and interconnected pores. It is an important multifunctional ceramic material with characteristics such as high permeability, high specific surface area and complex pore structure. These characteristics enable it to be used as a high-temperature filtration, purification, absorption, separation, mixing, insulation and heat exchange device in metallurgy, chemical industry, machinery, environmental protection, energy and other industries, and is widely used.
[0003] Currently, traditional foam ceramics are mostly made of silicon carbide, alumina, mullite, kaolin, etc. Due to the low thermal conductivity, poor high-temperature stability, and low resistance to thermal shock of these ceramic materials, the application of such materials in extreme environments is limited.
[0004] Hexagonal boron nitride is a wide bandgap insulator with a direct bandgap of 5.97 eV and excellent mechanical and optical properties. Most importantly, it has excellent thermal and chemical stability, with a maximum operating temperature of 900°C in an oxygen environment and even stable up to 2800°C in an inert environment.
[0005] Currently, there is no application of boron nitride foam ceramics. The main reason is that the existing preparation technology of foam ceramics is not suitable for boron nitride. In the early stage, some researchers used chemical vapor deposition to prepare boron nitride thin film network structure by high-temperature pyrolysis, but its synthesis process is complex, the cycle is long and it is impossible to prepare large-scale foam ceramics. The foam structure of boron nitride foam ceramics is easily broken during the preparation process, which limits the application of boron nitride in the field of foam ceramics. Summary of the Invention
[0006] The present invention provides a method for preparing boron nitride foam ceramics from a precursor, comprising the following steps: S1, preparing boron nitride precursor foam, comprising: S11, adding boron nitride precursor powder to a xylene solvent to obtain a boron nitride precursor solution; S12, emulsifying and foaming the boron nitride precursor solution to obtain boron nitride precursor foam;
[0007] S2, preparing a boron nitride precursor foam preform, comprising: S21, pouring the boron nitride precursor foam into a mold, and freeze-drying under vacuum to obtain a first boron nitride precursor foam preform;
[0008] S3, aging treatment: comprising: S31, aging the boron nitride precursor foam preform under constant temperature and humidity to obtain a second boron nitride precursor foam preform;
[0009] S4, preparing boron nitride foam ceramics: comprising: S41, calcining the second boron nitride precursor foam preform under nitrogen and ammonia to obtain boron nitride foam ceramics.
[0010] Compared with the prior art, the beneficial effects of the present invention are as follows: by dissolving a boron nitride precursor powder in a xylene solution, and by high-speed emulsification, stirring part of the gas to enter for emulsification and foaming, a foam with controllable structure and good morphology is formed on the boron nitride precursor solution; freeze-drying is performed under vacuum freezing, and the foam maintains its original shape during the freezing process. At the same time, the xylene solution in the boron nitride precursor solution is removed by vacuum drying, so that small micropores are formed on the bubble wall of the foam body, which is conducive to the discharge of internal gas and avoids destroying the structure of the foam; by aging treatment, the organic matter in the first boron nitride precursor foam preform is passivated with the reaction environment, so as to avoid violent reaction, the generation of a large amount of gas and the collapse of the foam structure, and the xylene is further removed; by calcination treatment under ammonia and nitrogen, the organic matter is decomposed to avoid oxidation, so that the foam ceramic precursor fully reacts with ammonia and is converted into the desired boron nitride foam ceramic.
[0011] Furthermore, the mass ratio of the boron nitride precursor powder to the xylene solvent in S11 is 4:1.
[0012] Furthermore, the boron nitride precursor powder in S11 is polymerized boron nitride.
[0013] The beneficial effect of adopting the above further technical solution is that by dissolving the boron nitride precursor powder in the xylene solution, a colloidal substance is formed, which provides stability for maintaining the structure after foaming.
[0014] Furthermore, the freeze-drying temperature in S21 is -8-12°C, the vacuum pressure is -0.07-0.11 MPa, and the freeze-drying time is 18h-24h.
[0015] The beneficial effect of adopting the above-mentioned further technical solution is that: by freezing under vacuum freezing, the foam body maintains its original shape during the freezing process, and at the same time, the xylene solution in the boron nitride precursor solution is removed under vacuum drying. Drying under vacuum pressure causes the xylene solvent to sublime, turning from solid to gas, so that small micropores are formed on the bubble wall of the foam body, which is conducive to the discharge of internal gas while avoiding damage to the structure of the foam body.
[0016] Furthermore, the aging treatment time in S31 is 28-32 minutes, the constant temperature and humidity are 20-30° C., and the humidity is 28-32 RH.
[0017] The beneficial effect of adopting the above-mentioned further technical solution is that: through aging treatment, the organic matter in the first boron nitride precursor foam preform is passivated by the reaction with the environment, the activity of the organic matter in the boron nitride precursor foam preform is reduced, and the violent reaction to produce a large amount of gas leading to the collapse of the foam structure is avoided, while further removing the xylene solvent.
[0018] Furthermore, the calcination treatment in S41 includes a first stage sintering, a second stage sintering, a third stage sintering and a fourth stage sintering.
[0019] Furthermore, the temperature of the first stage sintering is 395-405° C., the heating rate is 8-12° C. / min, and the time is 0.45-0.55 h.
[0020] The beneficial effect of adopting the above further technical solution is that the foam ceramic precursor reaches the solidification temperature of the organic precursor at a faster rate, thereby preventing the organic precursor from melting.
[0021] Furthermore, the second stage sintering temperature is 595-605° C., the heating rate is 3-7° C. / min, and the heat preservation time is 0.8-1.2 h.
[0022] The beneficial effect of adopting the above further technical solution is that the foam ceramic precursor is decarbonized at 595-605° C. and undergoes a reduction reaction with ammonia, thereby avoiding further foaming and causing the foam structure to collapse.
[0023] Furthermore, the temperature of the third sintering stage is 995-1005° C., the heating rate is 1-4° C. / min, and the heat preservation time is 0.8-1.2 h.
[0024] The beneficial effect of adopting the above further technical solution is that the foam ceramic precursor is reacted with ammonia to be initially ceramicized at high temperature.
[0025] Furthermore, the temperature of the fourth sintering stage is 1595-1605° C., the heating rate is 3-7° C. / min, and the heat preservation time is 0.8-1.2 h.
[0026] The beneficial effect of adopting the above further technical solution is that the foam ceramic precursor is fully reacted with ammonia and finally completely ceramicized at high temperature to be transformed into the desired boron nitride foam ceramic. DETAILED DESCRIPTION
[0027] In order to make the objectives, technical solutions and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.
[0028] Example 1:
[0029] The present embodiment provides a method for preparing boron nitride foam ceramics from a precursor, comprising the following steps: S1, preparing boron nitride precursor foam: comprising: S11, adding a boron nitride precursor to a xylene solvent to obtain a boron nitride precursor solution; S12, emulsifying and foaming the boron nitride precursor solution to obtain a boron nitride precursor foam; S2, preparing a boron nitride precursor foam preform: comprising: S21, pouring the boron nitride precursor foam into a mold, and freeze-drying it under vacuum to obtain a first boron nitride precursor foam preform; S3, aging treatment: comprising: S31, aging the boron nitride precursor foam preform at a constant temperature and humidity to obtain a second boron nitride precursor foam preform; S4, preparing a boron nitride foam ceramic: comprising: S41, calcining the second boron nitride precursor foam preform under nitrogen and ammonia to obtain a boron nitride foam ceramic. The method comprises dissolving a boron nitride precursor in a xylene solution, performing high-speed emulsification, stirring part of the gas to enter for emulsification and foaming, and forming a foam with a good structural morphology on the boron nitride precursor solution; freeze-drying is performed under vacuum freezing, and the foam maintains its original shape during the freezing process. At the same time, the xylene solution in the boron nitride precursor solution is removed by vacuum drying, so that small micropores are formed on the bubble wall of the foam body, which is conducive to the discharge of internal gas and avoids the destruction of the foam structure; passivating the organic matter in the first boron nitride precursor foam preform with the reaction environment through aging treatment, avoiding violent reaction, generating a large amount of gas and causing the collapse of the foam structure, and further removing the xylene; and calcining treatment under ammonia and nitrogen to decompose the organic matter and avoid oxidation, so that the foam ceramic precursor fully reacts with ammonia and is converted into the desired boron nitride foam ceramic.
[0030] The mass ratio of the boron nitride precursor powder to the xylene solvent in S11 is 4:1. The boron nitride precursor powder in S11 is polymerized boron nitride. Dissolving the boron nitride precursor in the xylene solution forms a colloid, which provides stability for the post-foaming structure.
[0031] In S21, the freeze-drying temperature is -10°C, the vacuum pressure is -0.09 MPa, and the freeze-drying time is 21 hours. Freezing under vacuum allows the foam to maintain its original shape during the freezing process. Simultaneously, the xylene solution in the boron nitride precursor solution is removed under vacuum drying. Drying under vacuum pressure causes the xylene to sublime, turning from a solid into a gas. This forms small micropores on the foamed cell walls, facilitating the discharge of internal gas while preventing damage to the foam structure.
[0032] The aging treatment in S31 is performed for 30 minutes at a constant temperature and humidity of 25°C and 30 RH. The aging treatment deactivates the organic matter in the first boron nitride precursor foam preform by reacting with the environment, reduces the activity of the organic matter in the boron nitride precursor foam preform, and prevents violent reactions that could generate a large amount of gas and cause the foam structure to collapse. Furthermore, the xylene solvent is further removed.
[0033] The calcination treatment in S41 includes the first sintering stage, the second sintering stage, the third sintering stage and the fourth sintering stage. The temperature of the first sintering stage is 400°C, the heating rate is 10°C / min, and the time is 0.5h. The foam ceramic precursor reaches the solidification temperature of the organic precursor at a faster rate to avoid melting of the organic precursor. The temperature of the second sintering stage is 600°C, the heating rate is 5°C / min, and the heat preservation is 1h. The foam ceramic precursor is decarbonized at 600°C and undergoes a reduction reaction with ammonia to avoid further foaming, which causes the foam structure to collapse. The temperature of the third sintering stage is 1000°C, the heating rate is 3°C / min, and the heat preservation is 1h. The foam ceramic precursor reacts with ammonia and is initially ceramicized at high temperature. The temperature of the fourth sintering stage is 1600°C, the heating rate is 5°C / min, and the heat preservation is 1h. The foam ceramic precursor is made to fully react with ammonia and finally be completely ceramicized at high temperature to be transformed into the desired boron nitride foam ceramic.
[0034] Example 2:
[0035] The same contents as those in the first embodiment will not be repeated here; the differences between the first embodiment and the present embodiment are as follows:
[0036] This embodiment provides a method for preparing boron nitride foam ceramics from a precursor
[0037] The freeze-drying temperature in S21 is -11°C, the vacuum pressure is -0.10 MPa, and the freeze-drying time is 23 hours.
[0038] The aging treatment time in S31 is 31 minutes, the constant temperature and humidity are 29° C., and the humidity is 31 RH.
[0039] The calcination process in S41 includes a first sintering stage, a second sintering stage, a third sintering stage and a fourth sintering stage. The temperature of the first sintering stage is 404° C., the heating rate is 11° C. / min, and the time is 0.54 h.
[0040] The second stage sintering temperature was 604° C., the heating rate was 6° C. / min, and the temperature was kept at this temperature for 1.1 h.
[0041] The temperature of the third sintering stage is 1004° C., the heating rate is 3° C. / min, and the temperature is kept at this temperature for 1.1 h.
[0042] The temperature of the fourth stage sintering is 1604° C., the heating rate is 6° C. / min, and the temperature is kept at this temperature for 1.1 h.
[0043] Example 3:
[0044] The same contents as those in the second embodiment will not be repeated here; the differences between the second embodiment and the present embodiment are as follows:
[0045] This embodiment provides a method for preparing boron nitride foam ceramics from a precursor
[0046] The freeze-drying temperature in S21 is -9°C, the vacuum pressure is -0.08 MPa, and the freeze-drying time is 19 hours.
[0047] The aging treatment time in S31 is 29 minutes, the constant temperature and humidity are 21° C., and the humidity is 29 RH.
[0048] The calcination process in S41 includes a first sintering stage, a second sintering stage, a third sintering stage and a fourth sintering stage. The temperature of the first sintering stage is 396° C., the heating rate is 9° C. / min, and the time is 0.46 h.
[0049] The second stage sintering temperature is 596° C., the heating rate is 4° C. / min, and the temperature is kept at this temperature for 0.9 h.
[0050] The temperature of the third sintering stage is 996° C., the heating rate is 2° C. / min, and the temperature is kept at this temperature for 0.9 h.
[0051] The temperature of the fourth stage sintering is 1596° C., the heating rate is 4° C. / min, and the temperature is kept at this temperature for 0.9 h.
[0052] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in this field should understand that they can still modify the technical solutions described in the above embodiments, or replace some of the technical features therein with equivalents; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims
1. A method for preparing boron nitride foam ceramics from a precursor, characterized in that: The following steps are involved: S1, preparing boron nitride precursor foam: comprising: S11, adding boron nitride precursor powder into xylene solvent to obtain a boron nitride precursor solution; S12, emulsifying and foaming the boron nitride precursor solution to obtain boron nitride precursor foam; S2, preparing a boron nitride precursor foam preform, comprising: S21, pouring the boron nitride precursor foam into a mold, and freeze-drying under vacuum to obtain a first boron nitride precursor foam preform; S3, aging treatment: comprising: S31, aging the boron nitride precursor foam preform under constant temperature and humidity to obtain a second boron nitride precursor foam preform; S4, preparing boron nitride foam ceramics: comprising: S41, calcining the second boron nitride precursor foam preform under nitrogen and ammonia to obtain boron nitride foam ceramics.
2. The method for preparing boron nitride foam ceramics from a precursor according to claim 1, wherein: The mass ratio of the boron nitride precursor powder to the xylene solvent in S11 is 4:
1.
3. The method for preparing boron nitride foam ceramics from a precursor according to claim 2, wherein: The boron nitride precursor powder in S11 is polymerized boron nitride.
4. The method for preparing boron nitride foam ceramics from a precursor according to claim 1, wherein: The freeze drying temperature in S21 is -8-12°C, the vacuum pressure is -0.07-0.11 MPa, and the freeze drying time is 18h-24h.
5. The method for preparing boron nitride foam ceramics from a precursor according to claim 1, characterized in that: The aging treatment in S31 lasts for 28-32 minutes, the constant temperature and humidity are 20-30° C., and the humidity is 28-32 RH.
6. The method for preparing boron nitride foam ceramics from a precursor according to claim 1, characterized in that: The calcination process in S41 includes a first stage sintering, a second stage sintering, a third stage sintering and a fourth stage sintering.
7. The method for preparing boron nitride foam ceramics from a precursor according to claim 6, characterized in that: The temperature of the first stage sintering is 395-405° C., the heating rate is 8-12° C. / min, and the time is 0.45-0.55 h.
8. The method for preparing boron nitride foam ceramics from a precursor according to claim 7, characterized in that: The second stage sintering temperature is 595-605° C., the heating rate is 3-7° C. / min, and the heat preservation is 0.8-1.2 h.
9. The method for preparing boron nitride foam ceramics from a precursor according to claim 8, characterized in that: The temperature of the third stage sintering is 995-1005° C., the heating rate is 1-4° C. / min, and the heat preservation is 0.8-1.2 h.
10. The method for preparing boron nitride foam ceramics from a precursor according to claim 9, characterized in that: The temperature of the fourth stage sintering is 1595-1605° C., the heating rate is 3-7° C. / min, and the heat preservation is 0.8-1.2 h.