CN109704781B - Silicon nitride nanobelt aerogel and preparation method thereof - Google Patents
Silicon nitride nanobelt aerogel and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a silicon nitride nanobelt aerogel and a preparation method thereof, 1) polysiloxane sol is prepared; 2) preparing slurry; 3) constructing a three-dimensional porous carbon fiber framework formed by mutually lapping short carbon fibers bonded by siloxane sol; 4) curing and cracking: heating the three-dimensional porous carbon fiber skeleton to the curing temperature of the polysiloxane sol, performing heat preservation treatment, then performing temperature rise and heat preservation treatment in a nitrogen atmosphere, and cooling to room temperature along with a furnace to obtain a carbon fiber/silicon nitride nanofiber composite block; 5) carbon removal: and heating the carbon fiber/silicon nitride nanofiber composite block in the air to 400-1000 ℃, and carrying out heat preservation treatment for 2-4 h to obtain the silicon nitride nanobelt aerogel. The silicon nitride aerogel prepared by the method has excellent high-temperature stability and heat insulation performance, overcomes the brittleness problem of the traditional ceramic aerogel, does not need expensive drying equipment and an inefficient drying process, has low cost and high efficiency, and is suitable for being applied to the fields of heat insulation, heat preservation, wave transmission and the like.
Description
Technical Field
The invention belongs to the technical field of aerogel preparation, and relates to a silicon nitride nanobelt aerogel and a preparation method thereof.
Background
Aerogel is the lightest solid known by human beings at present, has ultrahigh porosity, thereby having extremely excellent heat insulation performance, and is considered as the best new material for replacing the traditional heat insulation material. The service life of the aerogel is more than ten times of that of the traditional heat insulation material, the aerogel is used as the heat insulation material, the thickness of the required material is only one fifth to one half of that of the traditional heat insulation material, and the material is lighter in weight, so that the aerogel not only has great application value in the civil field, but also has the advantage of no ethical ratio in the military heat insulation field. At present, the application of the aerogel in civil use mainly belongs to the fields of heat insulation and preservation of industrial heat transmission pipelines, LNG storage, battery heat insulation protection plates of new energy buses and the like; military applications are mainly in the thermal insulation of aircraft. In general, the application scenario of the current aerogel has not yet reached the expectation of people, mainly because the aerogel has high preparation cost, limited maximum use temperature, and is generally brittle and cannot be directly used, and certain reliability can be ensured after the aerogel is compounded with other inorganic fibers, but the density of the aerogel can also be increased.
Currently, the most mature preparation technology in ceramic aerogels is silica aerogel, and the aerogels in the market almost default to silica aerogel. However, the silicon dioxide aerogel has the defects of large brittleness, poor temperature resistance, long-time use temperature not exceeding 650 ℃, easy shrinkage failure and the like in a 'pearl neck' structure formed by mutually contacting nano silicon dioxide particles in a high-temperature oxygen-containing environment. Although the use temperature of the traditional alumina aerogel can reach 1000 ℃, the traditional alumina aerogel also has the defects of brittleness and low reliability, and the application field and the depth of the ceramic aerogel are greatly limited. With the increasing requirements of the national defense field on the service performance of materials in extreme environments and the increasing demands of the industrial field on high-efficiency energy-saving materials, the improvement and research of new preparation technologies are urgently needed, and the ceramic aerogel materials with excellent high-temperature stability, good heat insulation performance and good mechanical properties are prepared at lower cost.
The silicon nitride has good mechanical properties at high temperature and normal temperature, and simultaneously has excellent thermal stability and high ablation resistance, and compared with the traditional oxide ceramic, the silicon nitride has more excellent high-temperature comprehensive properties. But silicon nitride aerogels have been rarely reported. In the previous research process, a patent (application number: 201610566429.6) about 'silicon nitride nanowire aerogel capable of being compressed and recovered and a preparation technology thereof' is applied, nanowire paper is prepared by adopting a quasi-CVD method, and then bulk aerogel is obtained after compounding.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide the silicon nitride nanobelt aerogel and the preparation method thereof, and the method has the advantages of simple operation, low requirement on equipment, safe and controllable process and high efficiency; the silicon nitride nanobelt aerogel prepared by the method has excellent high-temperature stability, heat insulation property and mechanical property, and is controllable in size.
In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:
the invention discloses a preparation method of silicon nitride nanobelt aerogel, which comprises the following steps of:
1) preparation of a polysiloxane sol: mixing siloxane sol, water and absolute ethyl alcohol to prepare polysiloxane sol;
2) preparing slurry: uniformly dispersing the chopped carbon fibers in the polysiloxane sol prepared in the step 1) to coat a layer of polysiloxane sol on the surfaces of the carbon fibers;
3) molding: constructing a three-dimensional porous carbon fiber framework formed by mutually lapping short carbon fibers bonded by polysiloxane sol;
4) curing and cracking: heating the three-dimensional porous carbon fiber skeleton to the curing temperature of the polysiloxane sol, carrying out heat preservation treatment for 4-8 h, then heating to 1200-1700 ℃ in a nitrogen atmosphere, carrying out heat preservation treatment for 1-3 h, and cooling to room temperature along with a furnace to obtain a carbon fiber/silicon nitride nanofiber composite block;
5) carbon removal: and heating the carbon fiber/silicon nitride nanofiber composite block to 400-1000 ℃ in air, and carrying out heat preservation treatment for 1-8 hours to obtain the silicon nitride nanobelt aerogel.
Preferably, step 1) comprises the following steps in percentage by mass: 10% -70% of siloxane sol, 10% -50% of water and 10% -80% of absolute ethyl alcohol; and the siloxane sol adopts methyltrimethoxysilane and/or dimethyldimethoxysilane.
Preferably, in the step 2), the average length of the chopped carbon fibers is 0.5 mm-2 mm.
Preferably, step 3) adopts a vacuum filtration method or a filter pressing method to remove unreacted siloxane sol, so that the chopped carbon fibers form a three-dimensional porous carbon fiber skeleton bonded by polysiloxane sol.
Preferably, in the step 4), the curing temperature of the polysiloxane sol is 80-120 ℃, the nitrogen pressure is 0.1-2 MPa, and the heating rate is 2-10 ℃/min.
Preferably, in the step 5), the temperature is raised to 400-1000 ℃ in the air at a temperature rise rate of 1-10 ℃/min.
Preferably, the strength and density of the silicon nitride nanobelt aerogel is adjustable, and is represented by:
the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by changing the solubility or the crosslinking degree of the siloxane sol;
or the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by changing the average length of the chopped carbon fibers;
or the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by changing the retention amount of polysiloxane sol in the three-dimensional porous carbon fiber skeleton;
or, the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by controlling the pressure of the nitrogen.
The invention also discloses the silicon nitride nanobelt aerogel prepared by the preparation method, and the density of the silicon nitride nanobelt aerogel is 5mg/cm3~50mg/cm3；
The thermal conductivity is 0.030W/mK-0.051W/mK; the silicon nitride nanobelt has a length of 100 to 500 μm and a width of 0.2 to 3 μm.
Compared with the prior art, the invention has the following beneficial effects:
the invention discloses a preparation method of silicon nitride nanobelt aerogel, which comprises the steps of mixing siloxane sol, water and absolute ethyl alcohol according to a certain proportion to prepare polysiloxane sol, then uniformly dispersing chopped carbon fibers in the prepared polysiloxane sol to construct a three-dimensional porous skeleton formed by mutually lapping the chopped carbon fibers bonded by the siloxane sol, and then carrying out curing and cracking treatment to obtain the silicon nitride nanobelt aerogel. In the method, the silica gel exists only on the surfaces and nodes of the chopped carbon fiber templates, the gaps formed among the chopped carbon fibers are not contained or are very little contained, and the whole chopped carbon fiber framework still has high porosity, so that the maximized space is provided for the growth of the silicon nitride nanobelt, and the high porosity of the aerogel is guaranteed. Meanwhile, the silica gel in the chopped carbon fiber framework is correspondingly in a porous framework structure, so that after pyrolysis at high temperature, the silica gel with high porosity (high specific surface area) can be cracked into a gas phase at the maximum conversion rate and used as a reactant to generate a silicon nitride nanobelt, and after the chopped carbon fiber template is finally removed by oxidation, the obtained aerogel is pure-phase silicon nitride aerogel, and has high purity and very little or no other impurities. According to the invention, the carbon fiber framework is introduced as the growth template of the silicon nitride nano fiber, so that the growth space is enlarged, the size of the final aerogel block can be controlled by controlling the size of the template, low-cost macro preparation can be realized, and a further technical basis is provided for industrialization of the silicon nitride ceramic aerogel.
The silicon nitride aerogel prepared by the method has excellent high-temperature stability and heat insulation performance, overcomes the brittleness problem of the traditional ceramic aerogel, does not need expensive drying equipment and an inefficient drying process, has low cost and high efficiency, and is suitable for being applied to the fields of heat insulation, heat preservation, wave transmission and the like.
Drawings
FIG. 1 is a process diagram of a process for preparing a silicon nitride aerogel according to the present invention;
FIG. 2 is a photograph of the macro-morphology of the silicon nitride aerogel prepared in example 1;
FIG. 3 is a photomicrograph of a silicon nitride aerogel from the silicon nitride aerogel prepared in example 1;
FIG. 4 is an XRD pattern of the silicon nitride aerogel prepared in example 2;
FIG. 5 is a thermogravimetric plot in an air atmosphere of the silicon nitride aerogel prepared in example 2;
FIG. 6 is a stress-strain curve of the silicon nitride aerogel prepared in example 2.
Detailed Description
In order to make the technical solutions of the present invention better understood, 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 by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. It should be noted that the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover non-exclusive inclusions, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
The invention is described in further detail below with reference to the accompanying drawings:
referring to fig. 1, a process flow diagram of a preparation method of the silicon nitride nanofiber aerogel of the present invention includes the following steps:
1) preparing polysiloxane sol by taking siloxane precursors such as methyltrimethoxysilane and dimethyldimethoxysilane as raw materials, water as a cross-linking agent and absolute ethyl alcohol as a solvent according to a certain proportion;
2) uniformly dispersing a certain amount of chopped carbon fibers in polysiloxane sol to enable the surface of the carbon fibers to be provided with a layer of siloxane sol;
3) molding: removing most of the silica sol, and forming a three-dimensional porous carbon fiber skeleton bonded by the polysiloxane sol from the chopped carbon fibers by adopting a vacuum filtration or filter pressing method;
4) and (3) curing: placing the three-dimensional porous carbon fiber framework in an environment with the temperature of 80-120 ℃ for heat preservation treatment for 4-8 h;
5) cracking: placing the cured carbon fiber framework under a certain nitrogen pressure, heating to 1200-1700 ℃, preserving heat for 1-3 hours, and then cooling to room temperature along with a furnace to obtain a carbon fiber/silicon nitride nanofiber composite block;
6) raising the temperature to 400-1000 ℃ in the air at a certain heating rate, and carrying out heat preservation treatment for 1-8 h to obtain the silicon nitride aerogel.
Example 1
This example preparationHas a density of 5mg/cm3The silicon nitride aerogel with adjustable size and density comprises the following specific steps:
1) preparing silica sol by using methyltrimethoxysilane (the mass fraction is 10 wt.%) and silica sol raw materials, and using water as a cross-linking agent (the mass fraction is 30 wt.%) and absolute ethyl alcohol as a solvent (the mass fraction is 60 wt.%);
2) 2g of chopped carbon fibers (about 2mm in length) are dispersed in 100ml of silica sol and mechanically stirred for 10 min;
3) adopting a vacuum filtration method to enable carbon fibers dispersed in the sol to be mutually overlapped to form a three-dimensional porous carbon fiber skeleton;
4) placing the porous carbon fiber framework in an environment with the temperature of 100 ℃, and carrying out heat preservation treatment for 4 hours;
5) heating to 1400 deg.C in nitrogen gas with pressure of 0.2 Mpa, maintaining for 2 hr to generate a large amount of silicon nitride nanofibers in the carbon fiber skeleton, and furnace cooling;
6) heating to 400 ℃ in the air at the heating rate of 10 ℃/min, carrying out heat preservation treatment for 8h, and oxidizing to remove carbon fibers to obtain the silicon nitride aerogel.
Referring to fig. 2, in order to obtain a macroscopic photograph of the silicon nitride aerogel, it can be seen from fig. 2 that the silicon nitride aerogel obtained by the method of the present invention is macroscopically white, and the surface of the aerogel contains millimeter-sized ultra-long silicon nitride nanobelts.
Referring to fig. 3, a microscopic scanning photograph of the prepared silicon nitride aerogel is shown. As can be seen from the figure, the silicon nitride aerogel is a network-like structure formed by numerous silicon nitride nanobelts intertwined with each other on a microscopic scale. The network structure with high porosity ensures that the silicon nitride aerogel has excellent heat insulation performance.
Example 2
This example produced a density of 15mg/cm3The silicon nitride aerogel with adjustable size and density comprises the following specific steps:
1) taking dimethyl dimethoxy silane (with the mass fraction of 40 wt.%) as a raw material, taking water as a cross-linking agent (with the mass fraction of 10 wt.%) and absolute ethyl alcohol as a solvent (with the mass fraction of 50 wt.%), and preparing polysiloxane sol;
2) dispersing chopped carbon fibers (with the length of about 1 mm) in silica sol, and mechanically stirring for 10 min;
3) adopting a vacuum filtration method to enable carbon fibers dispersed in the sol to be mutually overlapped to form a three-dimensional porous carbon fiber skeleton;
4) placing the porous carbon fiber framework in a 70 ℃ oven, and carrying out heat preservation treatment for 8 hours;
5) raising the pressure to 1550 ℃ in nitrogen with the pressure of 0.5Mpa, carrying out heat preservation treatment for 2 hours, generating a large amount of silicon nitride nanofibers in the carbon fiber framework, and cooling along with a furnace;
6) raising the temperature to 1000 ℃ in the air at the heating rate of 2 ℃/min, carrying out heat preservation treatment for 1h, and removing carbon fibers by oxidation to obtain the silicon nitride aerogel.
Referring to fig. 4, the XRD spectrum of the silicon nitride aerogel prepared in this example is shown. As can be seen from the figure, the characteristic peak of the silicon nitride aerogel belongs to the typical alpha-Si3N4And no other impurity peaks exist, which indicates that the silicon nitride aerogel prepared by the method has high purity.
Referring to fig. 5, the thermogravimetric plot in air of the silicon nitride aerogel prepared in this example is shown. The heating rate is 10 ℃/min, and as can be seen from the figure, the quality technology of the silicon nitride aerogel is kept unchanged in the air environment below 1000 ℃, and the silicon nitride aerogel has extremely excellent high-temperature oxidation resistance and high-temperature stability; when the temperature is higher than 1000 ℃, Si3N4The aerogel mass began to slowly increase, mainly due to the formation of a silicon oxide layer on the surface of the silicon nitride nanowires in the silicon nitride aerogel. When the temperature is increased to 1200 ℃, the weight gain of the silicon nitride aerogel is less than only 103wt.%, which shows that the silicon nitride aerogel has excellent oxidation resistance and thermal stability in a high-temperature air environment and is suitable for being used as a material for high-temperature heat insulation, filtration and the like.
Referring to FIG. 6, the density obtained for this example was 16mg/cm3The stress-strain curve of the silicon nitride aerogel of (1). As can be seen from the figure, the density produced by the method disclosed in the present invention is 16mg/cm3The silicon nitride aerogel with the porosity of 99.5 percent overcomes the defects of the traditional potteryThe porcelain aerogel has the brittleness problem and certain elasticity and compression recovery. When the compression deformation amount reaches 90%, the integral structure of the silicon nitride aerogel keeps complete, no macrocracks are generated, and the compression stress is as high as 0.15MPa, so that the silicon nitride aerogel belongs to the class of other aerogel materials with extremely excellent performance, wherein the porosity of the aerogel materials is as high as 99.5%. The silicon nitride aerogel prepared by the invention can realize the application of various scenes such as heat transmission pipelines, new energy battery thermal insulation boards, LNG storage and transportation and the like due to the three mechanical characteristics of no brittleness, compressibility and high strength.
Example 3
This example produced a density of 30mg/cm3The silicon nitride aerogel with adjustable size and density comprises the following specific steps:
1) taking methyltrimethoxysilane (with the mass fraction of 60 wt.%) as a raw material, taking water as a cross-linking agent (with the mass fraction of 30 wt.%) and absolute ethyl alcohol as a solvent (with the mass fraction of 10 wt.%), and preparing silica sol;
2) dispersing 2g of chopped carbon fibers (about 0.5mm in length) in 100ml of silica sol, mechanically stirring and ultrasonically treating for 5 min;
3) adopting a filter pressing method to enable carbon fibers dispersed in the sol to be mutually overlapped to form a three-dimensional porous carbon fiber framework;
4) placing the porous carbon fiber framework in a 70 ℃ oven, and carrying out heat preservation treatment for 8 hours;
5) heating to 1700 ℃ in nitrogen with the pressure of 1Mpa, carrying out heat preservation treatment for 3h, generating a large amount of silicon nitride nanofibers in the carbon fiber framework, and cooling along with the furnace;
6) heating to 700 ℃ in the air at the heating rate of 1 ℃/min, carrying out heat preservation treatment for 4h, and oxidizing to remove carbon fibers to obtain the silicon nitride aerogel.
Example 4
This example produced a density of 50mg/cm3The silicon nitride aerogel with adjustable size and density comprises the following specific steps:
1) taking methyltrimethoxysilane (with the mass fraction of 70 wt.%) as a raw material, and taking water as a cross-linking agent (with the mass fraction of 20 wt.%) and absolute ethyl alcohol as a solvent (with the mass fraction of 10 wt.%), preparing silica sol;
2) dispersing 2g of chopped carbon fibers (about 1.5mm in length) in 100ml of silica sol, mechanically stirring and ultrasonically treating for 5 min;
3) adopting a filter pressing method to enable carbon fibers dispersed in the sol to be mutually overlapped to form a three-dimensional porous carbon fiber framework;
4) placing the porous carbon fiber framework in an oven at 80 ℃, and carrying out heat preservation treatment for 6 hours;
5) heating to 1600 deg.C in nitrogen gas with pressure of 1Mpa, maintaining the temperature for 3h to generate a large amount of silicon nitride nanofibers in the carbon fiber skeleton, and furnace cooling;
6) raising the temperature to 650 ℃ in the air at the heating rate of 1 ℃/min, carrying out heat preservation treatment for 5h, and removing carbon fibers by oxidation to obtain the silicon nitride aerogel.
The method for preparing the silicon nitride aerogel does not relate to expensive, time-consuming and inefficient drying equipment required in the traditional aerogel preparation, can meet the preparation requirements only by using a common air pressure sintering furnace and an air furnace, has the advantages of simple raw materials, low cost, no need of consuming a large amount of solvent and simple preparation process, has the period of 1/8-1/5 of the traditional method, greatly reduces the preparation period and cost of the aerogel, and greatly improves the yield.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (6)
1. A preparation method of silicon nitride nanobelt aerogel is characterized by comprising the following steps of:
1) preparation of a polysiloxane sol: mixing 10-70% of siloxane sol, 10-50% of water and 10-80% of absolute ethyl alcohol by mass percent to prepare polysiloxane sol; and the siloxane sol adopts methyltrimethoxysilane and/or dimethyldimethoxysilane;
2) preparing slurry: uniformly dispersing the chopped carbon fibers in the polysiloxane sol prepared in the step 1) to coat a layer of polysiloxane sol on the surfaces of the carbon fibers; the average length of the short carbon fibers is 0.5 mm-2 mm;
3) molding: constructing a three-dimensional porous carbon fiber framework formed by mutually lapping short carbon fibers bonded by polysiloxane sol;
4) curing and cracking: heating the three-dimensional porous carbon fiber skeleton to the curing temperature of the polysiloxane sol, carrying out heat preservation treatment for 4-8 h, then heating to 1200-1700 ℃ in a nitrogen atmosphere, carrying out heat preservation treatment for 1-3 h, and cooling to room temperature along with a furnace to obtain a carbon fiber/silicon nitride nanofiber composite block;
5) carbon removal: and heating the carbon fiber/silicon nitride nanofiber composite block to 400-1000 ℃ in air, and carrying out heat preservation treatment for 1-8 hours to obtain the silicon nitride nanobelt aerogel.
2. The method for preparing the silicon nitride nanobelt aerogel according to claim 1, wherein the step 3) is to remove the unreacted polysiloxane sol by a vacuum filtration method or a filter pressing method, so that the chopped carbon fibers form a three-dimensional porous carbon fiber skeleton bonded by the polysiloxane sol.
3. The method for preparing the silicon nitride nanobelt aerogel according to claim 1, wherein in the step 4), the curing temperature of the polysiloxane sol is 80 to 120 ℃, the nitrogen pressure is 0.1 to 2MPa, and the temperature rise rate is 2 to 10 ℃/min.
4. The method for preparing the silicon nitride nanobelt aerogel according to claim 1, wherein in the step 5), the temperature is raised to 400-1000 ℃ in air at a temperature rise rate of 1-10 ℃/min.
5. The method for preparing the silicon nitride nanobelt aerogel according to any one of claims 1 to 4, wherein the strength and density of the silicon nitride nanobelt aerogel are adjustable, and are embodied in that:
the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by changing the solubility or the crosslinking degree of the siloxane sol;
or the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by changing the average length of the chopped carbon fibers;
or the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by changing the retention amount of polysiloxane sol in the three-dimensional porous carbon fiber skeleton;
or, the strength and the density of the silicon nitride nanobelt aerogel are regulated and controlled by controlling the pressure of the nitrogen.
6. The silicon nitride nanobelt aerogel prepared by the preparation method of any one of claims 1 to 5, wherein the density of the silicon nitride nanobelt aerogel is 5mg/cm3~50mg/cm3The thermal conductivity is 0.030W/mK to 0.051W/mK, the length of the silicon nitride nanobelt is 100 mu m to 500 mu m, and the width of the silicon nitride nanobelt is 0.2 mu m to 3 mu m.
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| WO2020167442A1 (en) * | 2019-02-01 | 2020-08-20 | The Regents Of The University Of California | Double-negative-index ceramic aerogels for thermal superinsulation |
| CN111205106B (en) * | 2020-01-11 | 2021-05-28 | 西安交通大学 | Silicon nitride @ carbon wave-absorbing foam and preparation method and application thereof |
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| CN112047742B (en) * | 2020-09-03 | 2022-03-15 | 中钢南京环境工程技术研究院有限公司 | Low-cost preparation method of large-size silicon nitride nanobelt aerogel |
| CN112811933B (en) * | 2021-01-18 | 2022-11-01 | 中国人民解放军海军工程大学 | Preparation method of nanowire-reinforced silicon nitride foamed ceramic composite silicon dioxide aerogel and product thereof |
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