CN110330343B - Method for preparing nanocrystalline silicon carbide ceramic by utilizing core-shell structure nanoparticles - Google Patents
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Abstract
The invention discloses a method for preparing nanocrystalline silicon carbide ceramic by utilizing core-shell structure nano particles, which comprises the steps of preparing silicon carbide/carbon core-shell structure nano particles, uniformly mixing the silicon carbide/carbon core-shell structure nano particles with a sintering aid, carrying out pretreatment to obtain silicon carbide ceramic powder, and then carrying out high-temperature sintering to obtain the nanocrystalline silicon carbide ceramic. According to the invention, by preparing the silicon carbide/carbon core-shell structure nano-particles, the existence of the carbon layer can inhibit the grain growth in the sintering process, meanwhile, the carbon layer reacts with the sintering aid in situ to promote the sintering, the required sintering temperature and pressure are obviously reduced when the nanocrystalline silicon carbide ceramic is prepared after sintering, the prepared silicon carbide ceramic has high density and high mechanical strength, and the grain size is less than 200 nm; the silicon carbide/carbon core-shell structure nano-particles are prepared by an in-situ vapor deposition method, the dispersibility is good, the particle size is 10-30nm, and the thickness of the carbon shell layer is less than 2 nm; the process flow is simple, the cost is low, the continuous production can be realized, and the prepared silicon carbide ceramic has high purity, excellent performance and wide application.
Description
Technical Field
The invention belongs to the technical field of ceramic material production, and relates to a method for preparing nanocrystalline silicon carbide ceramic by utilizing core-shell structure nanoparticles.
Background
Silicon carbide ceramics are very important high-temperature structural materials due to excellent high-temperature mechanical properties, strong thermal shock resistance, large thermal conductivity and obvious oxidation resistance and chemical corrosion resistance, and are widely applied to the fields of aerospace, mechanical industry, electronic devices and nuclear reactors.
However, dense sintering of silicon carbide ceramics is difficult due to the extremely strong covalent bonding and extremely poor self-diffusion of silicon carbide itself. At present, sintering aids can only be added to promote sintering, and the sintering aids of silicon carbide are generally divided into a solid-phase sintering aid and a liquid-phase sintering aid, wherein the solid-phase sintering aid contains carbon, so that silicon carbide powder has no volatile components in the sintering process, the obtained silicon carbide ceramic has clean crystal boundary and high mechanical strength, but the temperature required by solid-phase sintering is generally more than 2000 ℃; although the liquid phase sintering aid can obviously reduce the sintering temperature of the silicon carbide, the volatilization of oxides in the sintering process makes the ceramic difficult to sinter and compact.
The nano powder has high surface energy and high sintering activity, but the huge specific surface area of the nano powder also greatly increases the content of a surface oxide layer of the nano powder, so that the nano powder is unfavorable for sintering silicon carbide ceramics. The silicon carbide/carbon core-shell structure nano-particles are used as sintering powder, so that the problem of surface oxidation of the nano-powder can be solved, and carbon can be introduced to prevent components from volatilizing in the sintering process. However, the coating of the carbon material on the surface of the nano silicon carbide particles in situ has a great technical challenge, and meanwhile, no relevant report is provided for a method for preparing the nano crystal silicon carbide ceramic by using the silicon carbide/carbon core-shell structure nano particles as sintering powder and matching the sintering powder with a certain sintering aid.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method for preparing nanocrystalline silicon carbide ceramic by utilizing core-shell structure nano particles.
The invention adopts the following technical scheme:
a method for preparing nanocrystalline silicon carbide ceramic by utilizing core-shell structure nano particles comprises the steps of preparing silicon carbide/carbon core-shell structure nano particles, uniformly mixing the prepared silicon carbide/carbon core-shell structure nano particles with a sintering aid, carrying out pretreatment to obtain silicon carbide ceramic powder, and then carrying out high-temperature sintering on the silicon carbide ceramic powder to obtain the nanocrystalline silicon carbide ceramic.
In the technical scheme, the particle size of the silicon carbide/carbon core-shell structure nano-particles is 10-30nm, and the thickness of a carbon shell layer coated on the surface of the silicon carbide is less than 2 nm.
Further, in the above technical solution, the sintering aid is one or more of boric acid, metal nitrate and metal chloride, and the metal is one of aluminum, yttrium, calcium and magnesium.
Specifically, the adding amount of the boric acid is 0.5-2 wt% of the silicon carbide/carbon core-shell structure nano-particles calculated by boron element; the addition amount of the metal nitrate and the metal chloride is 0.5-15 wt% of the silicon carbide/carbon core-shell structure nano-particles calculated by metal elements.
Still further, in the above technical solution, the pretreatment is a high temperature treatment under an inert gas.
Preferably, in the above technical solution, the pretreatment is carried out at 800-1000 ℃ for 1-4h under argon atmosphere.
Still further, in the above technical solution, the high temperature sintering is hot-pressing sintering or spark plasma sintering, the sintering temperature is 1450-.
In the technical scheme, the process of mixing the silicon carbide/carbon core-shell structure nano particles and the sintering aid specifically comprises the steps of weighing the silicon carbide/carbon core-shell structure nano particles and the sintering aid in proportion, adding the silicon carbide/carbon core-shell structure nano particles and the sintering aid into a solvent, performing ultrasonic dispersion, and drying for 2-12 hours at 50-90 ℃ in an air atmosphere.
In the above technical solution, the preparation method of the silicon carbide/carbon core-shell structured nanoparticle is an in-situ vapor deposition method, and specifically includes the following steps:
s1, heating the fluidized bed reactor, and introducing fluidizing gas;
and S2, introducing precursor steam from the bottom of the fluidized bed reactor through carrier gas, carrying out pyrolysis reaction in the fluidized bed reactor to form silicon carbide/carbon core-shell structure nanoparticles, conveying the silicon carbide/carbon core-shell structure nanoparticles to the top of the fluidized bed reactor under the action of the fluidized gas, sucking the silicon carbide/carbon core-shell structure nanoparticles through a negative pressure device, and collecting the silicon carbide/carbon core-shell structure nanoparticles.
Preferably, in the above technical solution, the precursor is hexamethyldisilane, the carrier gas is hydrogen, the fluidizing gas is hydrogen and/or argon, and the temperature of the precursor vapor pyrolysis reaction temperature range is 1000-1450 ℃.
The invention also provides the nanocrystalline silicon carbide ceramic prepared by the method.
Specifically, the nanocrystalline silicon carbide ceramic has a grain size of less than 200 nm.
The invention also provides the application of the nanocrystalline silicon carbide ceramic in the fields of nuclear fuel element matrixes and cladding, nuclear structural members, high-temperature structural members and electronic components.
Compared with the prior art, the invention has the following advantages:
(1) the invention constructs an in-situ coated nano powder structure by preparing silicon carbide/carbon core-shell structure nano particles, the coating of a carbon layer in the preparation process can effectively prevent the subsequent oxidation of the surface of nano silicon carbide particles, and simultaneously can be used as a rigid restraint layer to prevent the growth of the silicon carbide particles in the sintering process, on the other hand, the designed single-layer or multi-layer carbon coating structure can be uniformly mixed with a sintering aid added by a liquid phase method to carry out in-situ reaction, promote the sintering process and realize the regulation and control of a crystal boundary, the sintering aid can be solid phase sintering aid boric acid or liquid phase sintering aid metal chloride or nitrate, the nano crystal silicon carbide ceramic is prepared by adopting hot pressing sintering or discharge plasma sintering, the required sintering temperature and pressure are obviously reduced, the silicon carbide ceramic prepared by sintering has high density and high mechanical strength, the grain size is below 200 nm;
(2) in the invention, silicon carbide/carbon core-shell structure nano particles are prepared in a fluidized bed reactor by adopting an in-situ vapor deposition method, the nano particles with the core-shell structure are not contacted with an external medium in the preparation process, the prepared nano particles with the core-shell structure are in a monodisperse sphere shape, the particle size distribution range is very small, the particle size is 10-30nm, and the thickness of the carbon shell layer is less than 2 nm;
(3) the preparation method provided by the invention has the advantages of simple process flow, simple and convenient process operation, low cost and continuous production, and the prepared silicon carbide ceramic has high purity, less internal pores, high density and excellent performance, and can be widely applied to nuclear fuel element substrates and cladding, nuclear structural members, high-temperature structural parts and electronic components.
Drawings
FIG. 1 is a schematic structural diagram of a fluidized bed reactor used in an embodiment of the present invention for preparing silicon carbide/carbon core-shell structured nanoparticles;
FIG. 2 is a TEM image of the SiC/C core-shell structured nanoparticle prepared according to example 1 of the present invention;
FIG. 3 is an EDS scan of silicon carbide/carbon core-shell structured nanoparticles prepared in example 3 of the present invention;
FIG. 4 is a sectional SEM photograph of the silicon carbide nano-crystalline ceramic prepared in example 3 of the present invention;
in the figure:
1. fluidizing gas hydrogen, 2, fluidizing gas argon, 3, carrier gas hydrogen, 4, a precursor hexamethyldisilane, 5, a precursor inlet, 6, an infrared pyrometer, 7, a water cooling system, 8, a heating furnace, 9, a conical spouted bed, 10 and a particle absorption device; 11. silicon carbide/carbon core-shell structured nanoparticles.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples.
The following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention.
The experimental procedures used in the following examples are conventional unless otherwise specified.
Materials, reagents and the like used in the following examples are commercially available unless otherwise specified.
Example 1
Heating the nanoparticle precursor hexamethyldisilane to 80 ℃ by adopting a water bath heating mode, and referring to fig. 1, adopting hydrogen as a carrier gas, wherein the flow rate of the carrier gas is 1L/min. Hydrogen is used as fluidizing gas, and the flow rate of the hydrogen is 4L/min. Heating the fluidized bed reactor to 1200 ℃ to start reaction, and collecting powder through a powder collecting system at the top end of the reactor. And uniformly mixing the obtained powder with a boric acid aqueous solution, and ultrasonically dispersing for 2 hours, wherein the content of boron in boric acid is 0.5 percent of the mass of the silicon carbide nano particles. And drying the mixture in an oven at 80 ℃, and carrying out heat treatment for 1h at 1000 ℃ under an argon atmosphere to obtain sintered powder. And carrying out hot-pressing sintering on the sintered powder, wherein the sintering temperature is 1800 ℃, the sintering pressure is 30MPa, the sintering time is 1h, and the heating rate is 10 ℃/min, so as to obtain the silicon carbide ceramic.
Fig. 2 is a TEM image of the silicon carbide/carbon core-shell structured nanoparticle prepared in example 1 of the present invention, and it can be seen from the TEM image that the obtained core-shell structured particle is a monodisperse spherical particle, the particle size distribution of the particle is narrow, the average particle size of the product is 10nm, the thickness of the carbon shell structure is 2nm, and the core-shell structure is uniformly coated on the surface of the core structure; the obtained silicon carbide ceramic has high density, the relative density is 95 percent, and the grain size is below 200 nm.
Example 2
Heating a nanoparticle precursor hexamethyldisilane to 80 ℃ by adopting a water bath heating mode, and adopting hydrogen as a carrier gas, wherein the flow rate of the carrier gas is 1L/min. The mixed gas of hydrogen and argon is adopted as fluidizing gas, the flow of hydrogen is 2L/min, and the flow of argon is 2L/min. Heating the fluidized bed reactor to 1300 ℃ to start reaction, and collecting powder through a powder collecting system at the top end of the reactor. And uniformly mixing the obtained powder with a boric acid aqueous solution, and ultrasonically dispersing for 2h, wherein the content of boron in boric acid is 1% of the mass of the silicon carbide nano particles. And drying the mixture in an oven at 80 ℃, and carrying out heat treatment for 1h at 1000 ℃ under an argon atmosphere to obtain sintered powder. And carrying out hot-pressing sintering on the sintered powder, wherein the sintering temperature is 1700 ℃, the sintering pressure is 30MPa, the sintering time is 1h, and the heating rate is 10 ℃/min, so as to obtain the silicon carbide ceramic.
The obtained powder product is silicon carbide/carbon core-shell structure nano-particles, the obtained core-shell structure particles are monodisperse spherical particles, the particle size distribution of the particles is narrow, and the average particle size of the product is 15 nm. The thickness of the carbon shell structure is 2nm, and the carbon shell structure is uniformly coated on the surface of the core structure. The obtained silicon carbide ceramic has high density, the relative density is 98 percent, and the grain size is below 200 nm.
Example 3
Heating a nanoparticle precursor hexamethyldisilane to 80 ℃ by adopting a water bath heating mode, and adopting hydrogen as a carrier gas, wherein the flow rate of the carrier gas is 1L/min. Argon is used as fluidizing gas, and the flow rate of the argon is 5L/min. Heating the fluidized bed reactor to 1000 ℃ to start reaction, and collecting powder through a powder collecting system at the top end of the reactor. And uniformly mixing the obtained powder with an aqueous solution of aluminum nitrate and yttrium nitrate, and ultrasonically dispersing for 2 hours, wherein the total content of aluminum and yttrium elements in the aluminum nitrate and yttrium nitrate is 5% of the mass of the silicon carbide nano particles. And drying the mixture in an oven at 80 ℃, and carrying out heat treatment for 1h at 1000 ℃ under an argon atmosphere to obtain sintered powder. And carrying out hot-pressing sintering on the sintered powder, wherein the sintering temperature is 1600 ℃, the sintering pressure is 30MPa, the sintering time is 1h, and the heating rate is 10 ℃/min, so as to obtain the silicon carbide ceramic.
The obtained powder product is silicon carbide/carbon core-shell structure nano-particles, the obtained core-shell structure particles are monodisperse spherical particles, the particle size distribution of the particles is narrow, and the average particle size of the product is 30 nm. The thickness of the carbon shell structure is 2nm, and the carbon shell structure is uniformly coated on the surface of the core structure. The obtained silicon carbide ceramic has high density, the relative density is 98 percent, and the grain size is below 200 nm.
The obtained powder product is nano-particles with a silicon carbide/carbon core-shell structure, the obtained core-shell structure particles are monodisperse spherical particles, the particle size distribution of the particles is narrow, the average particle size of the product is 25nm, and fig. 3 is an EDS scanning diagram of the silicon carbide/carbon core-shell structure nano-particles prepared in embodiment 3 of the invention; fig. 4 is a sectional SEM image of the silicon carbide nanocrystalline ceramic prepared in example 3 of the present invention, and it can be seen from the image that the obtained ceramic has a nanocrystalline structure with a grain size of 200nm or less.
Example 4
Heating a nanoparticle precursor hexamethyldisilane to 80 ℃ by adopting a water bath heating mode, and adopting hydrogen as a carrier gas, wherein the flow rate of the carrier gas is 1L/min. Argon is used as fluidizing gas, and the flow rate of the argon is 10L/min. Heating the fluidized bed reactor to 1300 ℃ to start reaction, and collecting powder through a powder collecting system at the top end of the reactor. And uniformly mixing the obtained powder with an aqueous solution of calcium nitrate and magnesium nitrate, and ultrasonically dispersing for 2 hours, wherein the total content of calcium and magnesium elements in the calcium nitrate and the magnesium nitrate is 5 percent of the mass of the silicon carbide nano particles. And drying the mixture in an oven at 80 ℃, and carrying out heat treatment for 2h at 1000 ℃ under an argon atmosphere to obtain sintered powder. And (3) performing discharge plasma sintering on the sintered powder, wherein the sintering temperature is 1700 ℃, the sintering pressure is 40MPa, the sintering time is 10min, and the heating rate is 100 ℃/min, so as to obtain the silicon carbide ceramic.
The obtained powder product is nano-particles with a silicon carbide/carbon core-shell structure, the obtained core-shell structure particles are monodisperse spherical particles, the particle size distribution of the particles is narrow, and the average particle size of the product is 10 nm. The thickness of the carbon shell structure is 2nm, and the carbon shell structure is uniformly coated on the surface of the core structure. The obtained silicon carbide ceramic has high density, the relative density is 98 percent, and the grain size is below 100 nm.
Finally, the above is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (3)
1. A method for preparing nanocrystalline silicon carbide ceramic by utilizing core-shell structure nano particles is characterized by comprising the steps of preparing silicon carbide/carbon core-shell structure nano particles, uniformly mixing the prepared silicon carbide/carbon core-shell structure nano particles with a sintering aid, pretreating to obtain silicon carbide ceramic powder, and sintering the silicon carbide ceramic powder at a high temperature to obtain nanocrystalline silicon carbide ceramic;
the preparation method of the silicon carbide/carbon core-shell structure nano-particles is an in-situ vapor deposition method, and specifically comprises the following steps:
s1, heating the fluidized bed reactor, and introducing fluidizing gas;
s2, introducing precursor steam from the bottom of the fluidized bed reactor through carrier gas, performing pyrolysis reaction in the fluidized bed reactor to form silicon carbide/carbon core-shell structure nanoparticles, conveying the silicon carbide/carbon core-shell structure nanoparticles to the top of the fluidized bed reactor under the action of the fluidized gas, sucking the silicon carbide/carbon core-shell structure nanoparticles through a negative pressure device, and collecting the silicon carbide/carbon core-shell structure nanoparticles;
the precursor is hexamethyldisilane, and the carrier gas is hydrogen;
the flow rate of the carrier gas is 1L/min, hydrogen is adopted as fluidizing gas, the flow rate of the hydrogen is 4L/min, and the temperature zone of the precursor steam for pyrolysis reaction is 1200 ℃;
or the flow rate of the carrier gas is 1L/min, the mixed gas of hydrogen and argon is adopted as the fluidizing gas, the flow rate of hydrogen is 2L/min, the flow rate of argon is 2L/min, and the temperature of the temperature zone of the precursor steam for pyrolysis reaction is 1300 ℃;
the process for mixing the silicon carbide/carbon core-shell structure nano particles with the sintering aid specifically comprises the steps of weighing the silicon carbide/carbon core-shell structure nano particles and the sintering aid respectively according to a proportion, adding the silicon carbide/carbon core-shell structure nano particles and the sintering aid into a solvent for ultrasonic dispersion, and drying the mixture for 2 to 12 hours at the temperature of 50 to 90 ℃ in an air atmosphere;
the particle size of the silicon carbide/carbon core-shell structure nano-particles is 10-30nm, and the thickness of a carbon shell layer coated on the surface of the silicon carbide is less than 2 nm;
the sintering aid is boric acid, and the addition amount of the boric acid is 0.5-2 wt% of the silicon carbide/carbon core-shell structure nano-particles by the element of boron;
the pretreatment is carried out for 1-4h at the temperature of 800-1000 ℃ under the atmosphere of argon;
the high-temperature sintering is hot-pressing sintering or discharge plasma sintering, the sintering temperature is 1450-1900 ℃, the sintering pressure is 10-50MP a, and the sintering time is 1-120 min.
2. Nanocrystalline silicon carbide ceramic produced by the method of claim 1, having a grain size of less than 200 nm.
3. The nanocrystalline silicon carbide ceramic of claim 2 for use in the field of nuclear fuel substrates and cladding, nuclear structural components, high temperature structural components, and electronic components.
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| CN101805201A (en) * | 2010-04-19 | 2010-08-18 | 清华大学 | Preparation method of porous silicon carbide ceramics with high thermal shock resistance |
| CN103011824A (en) * | 2011-09-28 | 2013-04-03 | 深圳光启高等理工研究院 | Metamaterial dielectric substrate material and preparation method thereof |
| CN102757237A (en) * | 2012-06-28 | 2012-10-31 | 航天材料及工艺研究所 | Method for adding ceramic material sintering aid |
| CN106082227A (en) * | 2016-06-07 | 2016-11-09 | 清华大学 | A kind of fluidized-bed chemical vapor deposition preparation method of nanometer silicon carbide granule |
| CN108675300A (en) * | 2018-06-15 | 2018-10-19 | 清华大学 | A kind of nuclear shell structure nano composite particles and preparation method thereof, its preparation facilities |
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