CN109748282B - Method for preparing nano silicon carbide at low temperature - Google Patents
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Abstract
The invention provides a method for preparing nano silicon carbide at low temperature, which adopts a double-limited-domain process, firstly prepares a silicon dioxide/carbon compound by pyrolyzing a silicon dioxide/polymer compound, then mechanically mixes the obtained compound with metal magnesium or calcium, carries out heat treatment in a closed reactor, and finally uses hydrochloric acid and hydrofluoric acid to sequentially clean the mixture to obtain nano-structure SiC. In the synthesis route, a carbon skeleton in the silicon dioxide/carbon composite provides a first confinement effect to limit the growth of the nano SiC, and a closed reactor provides a second confinement effect to reduce the temperature of carbothermic reduction. The nano silicon carbide prepared by the method has large specific surface area and abundant pores, can be used as a carrier to load a metal silver catalyst, and can be used as a lithium ion battery cathode material. The preparation method of the nano silicon carbide provided by the invention has the advantages of simple process and convenience for realizing large-scale production.
Description
Technical Field
The invention belongs to the field of material preparation, and relates to a method for preparing nano silicon carbide at low temperature.
Background
Silicon carbide (SiC), commonly known as carborundum, is a new semiconductor material developed in succession to the first-generation semiconductor materials and the second-generation compound semiconductor materials, and has the advantages of wide band gap (2.3eV-3.3eV), high thermal conductivity, fast electron mobility and the like, so that the silicon carbide (SiC) is very suitable for manufacturing high-temperature, high-power and high-density integrated electronic devices. In addition, SiC has excellent chemical stability, good mechanical properties and thermal shock resistance, so that SiC is an ideal candidate material for a filter material and a catalyst carrier under severe conditions. The nano-structure SiC powder has better performance than the traditional SiC powder, can meet the strict requirements of higher technical fields, and has wider application in the fields of energy storage, catalytic conversion and the like. The most mature synthesis method for preparing SiC by carbothermic reduction method, namely, the method for preparing SiC by reducing silicon dioxide in quartz sand by carbon under high temperature condition, has high reaction temperature and is difficult to obtain nano-structure SiC. The carbothermic process is improved, and researchers use microwaves as a heat source and use phenolic resin and high-purity SiO2Nanoparticles as carbon and silicon sourcesThe nanometer SiC with the grain diameter of 5-80nm and the purity of 98 percent is obtained at lower temperature in a short time. The nano SiC material synthesized by the method mainly exists in the forms of nano whiskers, nano particles and the like, and is easy to agglomerate in the using process, so that the specific surface area is reduced. Chemical vapor deposition is also frequently used for the synthesis of nano SiC by forming a higher supersaturated vapor pressure of the reaction product vapor at a temperature higher than the theoretical reaction temperature, causing it to self-agglomerate into nuclei and then agglomerate into particles. Silane and hydrocarbon are used as raw materials, and particles with high purity and particle size of 10-100nm can be synthesized. However, the method has extremely high requirements on the purity of the raw materials, the obtained product has wide particle size distribution range, easy agglomeration and low yield. Therefore, it is still a challenge to find a method for preparing nano SiC with simple process, high yield, high purity, controllable particle size and controllable structure.
Disclosure of Invention
The invention aims to provide a preparation method for preparing nano SiC at low temperature, which realizes the nanocrystallization of a SiC structure by adopting a double-limited-domain (double definitions) process. Firstly, preparing a silicon dioxide/carbon composite by pyrolyzing a silicon dioxide/polymer composite, then mixing the obtained composite with a metal reducing agent, carrying out heat treatment in a closed reactor, and finally, sequentially cleaning with mixed acid and deionized water to obtain the nano-structure SiC. In the synthesis route, a carbon skeleton in the silicon dioxide/carbon composite provides a first confinement effect to limit the growth of the nano SiC, and a closed reactor provides a second confinement effect to reduce the temperature of carbothermic reduction.
The technical scheme of the invention is as follows:
a method for preparing nano silicon carbide at low temperature comprises the following steps:
(1) adopting polymer and nano-scale silicon dioxide as raw materials, and mechanically mixing the raw materials according to the mass ratio of the polymer to the silicon dioxide of 1: 5-0.5;
(2) pyrolyzing the mixture prepared in the step (1) at the temperature of 500 ℃ and 900 ℃ for 1-4h to generate a silicon dioxide/carbon composite; the pyrolysis atmosphere is argon or nitrogen, the gas flow is 0.2-3L/min, and the heating rate is 1-10 ℃/min;
(3) mechanically mixing the silicon dioxide/carbon compound obtained in the step (2) with a metal reducing agent, transferring the mixture into a closed reactor, and then carrying out heat treatment; the heat treatment temperature is 400-750 ℃; obtaining a carbothermic product;
(4) placing the carbothermic reduction product obtained in the step (3) in mixed acid, standing for 3-48h, washing with deionized water, and finally placing in an oven for drying to obtain nano silicon carbide; the mixed acid is hydrofluoric acid and concentrated hydrochloric acid mixed according to the volume ratio of 1:1-5, and the concentration of hydrogen ions in the mixed acid is 0.1-1.0M/L.
Further, the heat treatment temperature in (3) is preferably 400-650 ℃.
Further, the polymer described in the above (1) is a polymer having a carbon content of 30wt% or more.
Further, the polymer in the above (1) is one or a mixture of two or more of phenolic resin, starch, cellulose, chitin and urea resin.
Furthermore, the shape of the nano-scale silicon dioxide in the step (1) is spherical and rod-shaped, and the particle size is in the range of 5-80 nm.
Further, the metal reducing agent in (3) above is one of sodium, magnesium or calcium; the material of the closed reactor is stainless steel or copper.
Further, the drying temperature in the step (4) is 80 ℃, and the drying time is 4-10 h.
Further, the mechanical mixing method in the above (1) and (3) is ball milling or mechanical stirring.
The invention also provides application of the nano silicon carbide prepared by the method in battery cathode materials and catalyst carriers. The material can be further doped with a large amount of heterogeneous elements for application in super capacitors, hydrogen storage materials, gas sensing and the like due to large specific surface area; the high porosity is beneficial to gas adsorption, storage and electrolyte transfer, and effectively improves gas sensing, storage and electrical properties.
The invention has the technical effects that: the low-temperature synthesis method of the silicon carbide with the structural structure is provided, SiC is generated by the silicon dioxide/polymer pyrolysis and the double-confinement effect reduction method, and the nano SiC with various structures such as mesopores, spheres and the like can be obtained. The morphology and the pore structure of the obtained nano SiC are consistent with those of the raw materials. The double-confinement effect provided by the invention can limit the agglomeration of SiC nano particles, and the nano SiC can be prepared at a lower temperature, so that the nano structure can be easily maintained. The preparation method of the nano silicon carbide provided by the invention has the advantages of simple process and convenience for realizing large-scale production.
Drawings
FIG. 1(a, b) is a Scanning Electron Microscope (SEM) image of a silica/carbon composite prepared by using silica with a particle size of 16nm as a silicon source and cellulose as a carbon source under different magnifications, (C, d) and an energy spectrum (EDX) plane scanning Si and C distribution image of FIG. b.
FIG. 2 is XRD diffraction patterns of nano SiC at different reduction temperatures.
FIG. 3 is a transmission electron microscope photograph of (a) low power and (b) high power nano SiC at a reduction temperature of 650 ℃.
FIG. 4 is a Scanning Electron Microscope (SEM) image of nano SiC at a reduction temperature of 650 ℃.
FIG. 5 shows (a) nitrogen adsorption isotherms and (b) pore size distributions of nano SiC at a reduction temperature of 650 ℃.
FIG. 6 shows the performance test of the lithium ion battery with nano SiC at the reduction temperature of 650 deg.C: (a) a charge-discharge curve; (b) at 1A g-1Cycling performance at current density.
FIG. 7 is a transmission electron microscope picture of Ag loaded with nano SiC as a catalyst carrier at a reduction temperature of 650 ℃.
Detailed Description
Example 1
First confinement effect: weighing a certain amount of cellulose and nano silicon dioxide (the mass ratio is 3:1), placing the mixture into a 100ml ball milling tank, carrying out ball milling at 450rpm for 2h, and then placing the mixture into a tube furnace for pyrolysis treatment. The treatment temperature is 600 ℃, the treatment time is 4 hours, and the heating rate is 5 ℃ for min-1. The obtained sample was naturally cooled to room temperature to obtain a silica/carbon composite. FIGS. 1(a) and (b) are scanning electron micrographs of the silica/carbon composite prepared in this example, and it can be seen from the energy spectrum plane scanning images of FIGS. 1(c) and (d) that silicon is uniformly distributed on the carbon skeleton and carbon is uniformly distributed on the carbon skeletonThe skeleton forms a good composite structure.
Second confinement effect: mechanically mixing the obtained silicon dioxide/carbon compound with metal magnesium according to a certain mass ratio (1:1), transferring the mixture to a stainless steel reactor, and finally placing the reactor and a tubular furnace to heat to 550 ℃ and stay for 4 hours. Naturally cooling to room temperature, taking out the compound in the reactor, and placing the compound in mixed acid of hydrochloric acid and hydrofluoric acid, wherein the volume ratio of the mixed acid is 1:1, and the concentration of hydrogen ions is 0.5M/L. And (3) standing the compound in mixed acid for 5h, washing the compound to be neutral by using deionized water, and drying the compound in an oven at the temperature of 80 ℃ for 10 h. Fig. 2 is an XRD diffractogram of the nano silicon carbide prepared in this example, which is compared with the standard card JCPDS29-1129, and has 2 θ of 35.6 °,60 ° and 71.7 ° corresponding to (111), (220) and (311) crystal planes of SiC, respectively.
Example 2
First confinement effect: same as in example 1.
Second confinement effect: the reduction temperature was 650 ℃, the other conditions were the same as in example 1, and the XRD diffraction pattern of the obtained SiC nanoparticles is shown in fig. 2, and in contrast to JCPDS29-1129, the 2 θ of the SiC nanoparticles was 35.6 °,60 ° and 71.7 ° respectively corresponding to the (111), (220) and (311) crystal planes of SiC. In FIG. 3, TEM images of the sample are shown (a) and (b), respectively, and the sample can be seen to have a certain pore structure at low resolution, which is mainly due to the reduction of SiO2Deoxygenation leaves pores behind after reaction with the carbon skeleton. The (111) interplanar spacing of SiC was found to be 0.252nm in high resolution transmission electron microscopy pictures. Fig. 4 is a scanning electron microscope picture of the sample, which shows that the sample has an obvious mesoporous structure. Fig. 5(a) is a nitrogen adsorption-desorption isotherm of the sample, which has a typical type IV isotherm structure with a significant hysteresis loop and is a mesoporous structure. Specific surface area SBET=267.3m2G, total pore volume Vtotal=0.41cm3(ii)/g, (b) is the corresponding pore size distribution with a more concentrated pore size of about 10 nm.
Example 3
First confinement effect: same as in example 1.
Second confinement effect: the reduction temperature was 400 ℃ and other conditions were the same as in example 1, and the obtained SiC nanoparticles had an XRD diffraction pattern as shown in fig. 2, which corresponds to the (111), (220) and (311) crystal planes of SiC, respectively, at 2 θ of 35.6 °,60 ° and 71.7 ° in comparison with JCPDS29-1129, which is standard card.
Application 1:
the sample prepared in example 2 was used as an active material, and a half cell was assembled with a lithium plate as a counter electrode to perform electrochemical performance tests. Firstly, 6g of a sample prepared at 650 ℃, 3g of conductive carbon black and 1g of vinylidene fluoride serving as a binder are weighed, mixed and ground, 80mL of N-methylpyrrolidone is added, and the mixture is magnetically stirred for 10 hours. Coating the prepared slurry on copper foil to prepare an electrode plate, slicing and tabletting the electrode plate and a lithium sheet serving as a counter electrode, and assembling the CR2025 button cell in a glove box. The electrolyte of the button cell is 1.0M lithium hexafluorophosphate (LiPF)6) Dissolved in Ethylene Carbonate (EC), dimethyl carbonate (DMC), and methylethyl carbonate (EMC) in a mass ratio of 1:1: 1. Electrochemical performance tests were performed on a constant current charge-discharge system at a rate of 1A/g. The cell assembly operation was carried out in a glove box with water oxygen content below 0.1 ppm. The voltage plateau around 0.5V in the first discharge curve of fig. 6 corresponds to the formation of a Solid Electrolyte Interface (SEI) film, and the first coulombic efficiency is 79%. The capacity retention rate is good in the subsequent 100 cycles, which shows that the battery has good cycle performance, and the specific capacity is about 320 mAh/g.
Application 2:
the silicon carbide prepared in example 2 was used as a catalyst carrier, 10 wt% of elemental silver was impregnated by a wet impregnation method, and it was tested that the silicon carbide had a good catalytic activity when applied to a p-nitrophenol reduction reaction. The transmission electron microscope of the Ag/SiC catalyst is shown in FIG. 7, and the particle size of Ag particles is about 10 nm.
Claims (5)
1. A method for preparing nano silicon carbide at low temperature is characterized by comprising the following steps:
(1) adopting polymer and nano-scale silicon dioxide as raw materials, and mechanically mixing the raw materials according to the mass ratio of the polymer to the silicon dioxide of 1: 5-0.5; the polymer is a polymer with carbon content of more than 30 wt%; the nanometer silicon dioxide is spherical and rod-shaped, and the particle size is in the range of 5-80 nm;
(2) pyrolyzing the mixture prepared in the step (1) at the temperature of 500 ℃ and 900 ℃ for 1-4h to generate a silicon dioxide/carbon composite; the pyrolysis atmosphere is argon or nitrogen, the gas flow is 0.2-3L/min, and the heating rate is 1-10 ℃/min;
(3) mechanically mixing the silicon dioxide/carbon compound obtained in the step (2) with a metal reducing agent, transferring the mixture into a closed reactor, and then carrying out heat treatment; the heat treatment temperature is 400-oC; obtaining a carbothermic product; the metal reducing agent is magnesium; the material of the closed reactor is stainless steel or copper;
(4) placing the carbothermic reduction product obtained in the step (3) in mixed acid, standing for 3-48h, washing with deionized water, and finally placing in an oven for drying to obtain nano silicon carbide; the mixed acid is hydrofluoric acid and concentrated hydrochloric acid mixed according to the volume ratio of 1:1-5, and the concentration of hydrogen ions in the mixed acid is 0.1-1.0 mol/L.
2. The method for preparing nano silicon carbide at low temperature according to claim 1, wherein the polymer in (1) is one or a mixture of more than two of phenolic resin, starch, cellulose, chitin and urea resin.
3. The method for preparing nano silicon carbide at low temperature according to claim 1 or 2, wherein the drying temperature in (4) is 80 DEGoAnd C, drying for 4-10 h.
4. The method for preparing nano silicon carbide at low temperature according to claim 1 or 2, wherein the mechanical mixing manner in (1) and (3) is ball milling or mechanical stirring.
5. The use of the nano-silicon carbide prepared by the method of any one of claims 1 to 4, wherein the nano-silicon carbide with a mesoporous structure is applied to electrode materials of lithium ion batteries and catalyst carriers.
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