CN115196634A

CN115196634A - Method for preparing silicon carbide and silicon carbide

Info

Publication number: CN115196634A
Application number: CN202210698507.3A
Authority: CN
Inventors: 赵海峰; 谢丹丹; 刘君; 朱国本; 马爱珍; 张宝荣; 宋文文; 宋安康; 方昭庆; 韩立宁
Original assignee: Qingdao Zhengwang Material Co ltd
Current assignee: Qingdao Zhengwang Material Co ltd
Priority date: 2022-06-20
Filing date: 2022-06-20
Publication date: 2022-10-18

Abstract

The invention relates to a method for preparing silicon carbide, which comprises the following steps of mixing powdery organic polymer material with CxHyOz composition with elemental silicon powder, pressing into blocks, and obtaining the silicon carbide by a microwave heating method. The method for preparing the silicon carbide mixes the cured phenolic resin powder with the elemental silicon powder and then heats the mixture by microwaves, and can generate more silicon carbide under the condition of lower temperature, thereby saving a large amount of energy.

Description

Method for preparing silicon carbide and silicon carbide

Technical Field

The invention relates to the technical field of silicon carbide preparation, in particular to a method for preparing silicon carbide by microwave heating.

Background

beta-SiC is an important inorganic non-metallic material, has excellent thermal stability, high thermal conductivity, high mechanical strength and hardness, high wear resistance and corrosion resistance, and is widely applied to a plurality of traditional industrial fields such as abrasive grinding tools, refractory materials, metallurgy, high-temperature structural ceramics and the like. Due to its high electron mobility, wide band gap and high frequency absorption, it has great engineering and industrial application potential in the emerging fields of electronics, photocatalysis, medicine, aerospace, etc.

Currently, most of industrially prepared silicon carbide is produced by adopting an Acheson process through a carbothermic reduction method, the reaction temperature of the method is 2200-2400 ℃, the reaction time is long, the obtained silicon carbide mainly takes an alpha-SiC phase, crystal grains are coarse, and multiple mechanical crushing and screening are needed, so the energy consumption is high, the pollution is serious, and the added value of products is low. In recent years, methods such as a gel-gel method, a chemical vapor synthesis method (CVD), a self-propagating combustion synthesis method, a plasma method, and a mechanical alloying method have also been used for the synthesis of silicon carbide. However, most of the preparation methods have the defects of low yield, complex synthesis steps, low efficiency, high cost and the like, and are not suitable for large-scale industrial production.

The microwave synthesis method has attracted extensive attention due to its low synthesis temperature, high thermal efficiency, low energy consumption and other features. Microwave heating is to utilize the coupling effect of microwave electromagnetic field and material medium, absorb microwave electromagnetic energy through the dielectric loss of material itself in the microwave electromagnetic field, and convert it into heat. The heating mechanism does not depend on heat conduction, and has the advantages of rapid, uniform and selective heating, obviously shortened heating time and the like. In recent years, many researchers have also successfully synthesized silicon carbide by means of microwave heating. Moshtaghioun et al amorphous SiO ₂ The micro powder and the microlite ink powder are respectively used as a silicon source and a carbon source, and the silicon carbide nano-particles with the particle size of 10-40 nm are synthesized by microwave heating and heat preservation for 5 min at 1200 ℃. Li et al reportAccording to the research of microwave synthesis of beta-SiC with a nano structure by taking rice hulls as precursors under the argon atmosphere, heating is carried out for 60min at 1300 ℃ or 15 min at 1500 ℃, and silicon dioxide in the rice hulls is completely subjected to carbon thermal reduction to generate the beta-SiC. Wei et al have studied alpha-SiC, C and SiO in different mixing processes ₂ Different distributions among the particles lead to different microwave heating behavior and growth mechanisms. Qi et al prepared one-dimensional beta-SiC silicon carbide nanowires by microwave heating at the optimum synthesis temperature of 1500 ℃ for 40 min. The microwave heating method is all made of SiO ₂ And C is used as a silicon source and a carbon source, and the reaction process mainly adopts carbothermic reduction reaction, so that the preparation temperature of the beta-SiC is higher.

Although the reaction temperature of the microwave heating method is relatively low, the temperature of microwave sintering in the prior art also reaches 900-1600 ℃. For example, chinese patent publication No. CN 111762785A discloses a method for preparing granular silicon carbide by dual-frequency microwave, which comprises obtaining a precursor of silicon dioxide coated carbon by a sol-gel method using carbon and tetraethoxysilane, and pressing the precursor to form a green body; embedding the obtained blank in quartz sand, and simultaneously performing microwave sintering by using double-frequency microwaves to obtain granular silicon carbide; the temperature of the microwave sintering is 900-1600 ℃, and the time of the microwave sintering is 10-30 min.

Disclosure of Invention

The invention aims to provide a method for preparing silicon carbide by microwave heating and the silicon carbide prepared by the method.

In order to solve the technical problems, the method for preparing the silicon carbide comprises the following steps of mixing a powdery organic polymer material with CxHyOz composition with a simple substance silicon powder material, pressing the mixture into blocks, and heating the blocks by microwaves to obtain the silicon carbide.

Preferably, the organic polymer material is an organic material that can be pyrolyzed at 600 ℃.

Preferably, the organic polymer material is an organic polymer synthesized from one or two or more monomers.

Preferably, the organic polymer material is phenolic resin powder.

Preferably, the microwave heating temperature is 600-1000 ℃.

The microwave heating temperature is 800 ℃.

The mass part ratio of the phenolic resin powder to the elemental silicon powder is 90-110: 90-110.

The mass ratio of the phenolic resin powder to the elemental silicon powder is 1.

Preferably, the phenolic resin is ground to a powder particle size of less than 150 microns after curing.

Preferably, the porosity of the block pressed by the phenolic resin powder and the elemental silicon powder is 15-45%.

Preferably, after mixing the phenolic resin powder and the elemental silicon powder, putting the mixture into a die, and maintaining the pressure for 1min by using a hydraulic machine at the pressure of 35 MPa.

As a further improvement, a catalyst is added into the mixture of the phenolic resin powder and the elemental silicon powder, the catalyst is transition metal, and the content of the catalyst is 0.1-1%.

The invention also relates to silicon carbide prepared by the method for preparing the silicon carbide.

According to the method for preparing silicon carbide, the solidified phenolic resin powder and the elemental silicon powder are mixed and then heated by microwaves, so that more silicon carbide can be generated at a lower temperature, and a large amount of energy can be saved.

The foregoing description is only an overview of the technical solutions of the present application, so that the technical means of the present application can be more clearly understood and the present application can be implemented according to the content of the description, and in order to make the above and other objects, features and advantages of the present application more clearly understood, the following detailed description is made with reference to the preferred embodiments of the present application and the accompanying drawings.

The above and other objects, advantages and features of the present application will become more apparent to those skilled in the art from the following detailed description of specific embodiments thereof, taken in conjunction with the accompanying drawings.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following descriptions are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative efforts. Throughout the drawings, like elements or portions are generally identified by like reference numerals.

FIG. 1 a) is the XRD pattern of the product incubated for 30min at different reaction temperatures.

FIG. 1 b) is a graph of the object image and content of the product of 30min incubation at different reaction temperatures

FIG. 2 is an SEM image of silicon carbide particles after 30min microwave incubation at different reaction temperatures: a) 600 deg.C, b) 800 deg.C, c) 1000 deg.C.

FIG. 3 is an SEM image of silicon carbide fibers incubated for 30min at different reaction temperatures: a) 600 deg.C, b) 800 deg.C, c) 1000 deg.C.

Figure 4 is an XRD pattern of the product after the sample has "blasted".

FIG. 5 is an SEM image of the product after the sample "pops up".

FIG. 6 is a microwave heating curve of a sample of elemental silicon powder.

Fig. 7 is a schematic illustration of nucleation and growth of microwave heated beta-SiC particles and fibers.

FIG. 8 a) is XRD pattern of product at different incubation times at 800 ℃.

FIG. 8 b) is a graph of the physical image and the content of the product at different incubation times.

FIG. 9 is an SEM picture of silicon carbide particles heated at 800 ℃ for different holding times; a) 5 min, b) 10 min, c) 15 min, d) 30min and e) 60 min.

FIG. 10 is an SEM image of silicon carbide fibers heated at 800 ℃ for various holding times; a) 5 min, b) 10 min, c) 15 min, d) 30min and e) 60 min.

FIG. 11 is the XRD pattern of the product after 30min reaction at 800 ℃.

FIG. 12 is an SEM picture of the product of the reaction at 800 ℃ for 30min, silicon carbide particles and fibers.

FIG. 13 is an XRD pattern and SEM picture of the product of a 30min reaction at 800 deg.C with catalyst added.

FIG. 14 is the XRD pattern of the product incubated at 600 ℃ for 30min.

FIG. 15 is the XRD pattern of the product incubated at 800 ℃ for 10 min.

FIG. 16 is the XRD pattern of the product of elemental silicon powder/uncured phenolic resin powder samples incubated at 800 deg.C for 30min.

FIG. 17 is an XRD pattern of a product of elemental silicon powder/polymethyl methacrylate powder sample incubated at 800 deg.C for 5 min.

FIG. 18 is the XRD pattern of the product of the 30min incubation at 800 deg.C with the addition of a portion of the silica sample.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. In the following description, specific details such as specific configurations and components are provided only to help the embodiments of the present application be fully understood. Accordingly, it will be apparent to those skilled in the art that various changes and modifications may be made to the embodiments described herein without departing from the scope and spirit of the present application. In addition, descriptions of well-known functions and constructions are omitted in the embodiments for clarity and conciseness.

It should be appreciated that reference throughout this specification to "one embodiment" or "the embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present application. Thus, the appearances of the phrase "one embodiment" or "the present embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Further, the present application may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

The term "and/or" herein is merely an association describing an associated object, meaning that three relationships may exist, e.g., a and/or B, may mean: the three cases of A alone, B alone and A and B together exist, and the term "/and" in this document describes another associated object relationship, which means that two relationships may exist, for example, A/and B, which may mean: a alone, and both a and B alone, and further, the character "/" in this document generally means that the former and latter associated objects are in an "or" relationship.

The term "at least one" herein is merely an association relationship describing an associated object, and means that there may be three relationships, for example, at least one of a and B, may mean: a exists alone, A and B exist simultaneously, and B exists alone.

It is further noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion.

The method for preparing the silicon carbide comprises the following steps of solidifying and grinding an organic polymer material consisting of CxHyOz, mixing the solidified powder with elemental silicon powder, pressing the mixture into blocks, and heating the blocks by microwaves to obtain the silicon carbide.

Preferably, the organic polymer material is an organic material that can be pyrolyzed at 600 ℃ or lower.

Preferably, the organic polymer material is a polymer of a phenol and an aldehyde.

The organic polymer material composed of CxHyOz may be a polymer synthesized from one monomer: polyethylene, polypropylene, polystyrene, polybutadiene, polyisoprene, etc.; it may be a polymer synthesized from two or more monomers: phenolic resin, epoxy resin, polymethacrylic resin, polymethyl acrylate, polyvinyl acetate, SBS resin, ABS resin and the like.

Preferably, phenolic resins are used, which have the advantage over other organic polymers: 1) The starting pyrolysis temperature of the phenolic resin is 350 ℃, a large amount of carbon-containing gas can be released by pyrolysis to fill sample pores, and then microwave plasma is excited, so that the reaction interface is rapidly heated, and the nucleation and growth of initial beta-SiC are promoted; 2) The phenolic resin pyrolytic carbon is amorphous glassy carbon, so that the activation energy is low, the reaction barrier can be reduced, and the formation and growth of beta-SiC are promoted; 3) Compared with other organic polymers, the pyrolytic carbon content is higher, so that sufficient carbon sources can be provided for the continuous growth of the beta-SiC in the later period, and the generation rate of the beta-SiC is higher.

Preferably, the microwave heating temperature is 600-1000 ℃.

The microwave heating temperature is 800 ℃.

The mass ratio of the phenolic resin powder to the elemental silicon powder is 1. A large number of experimental verifications and theoretical analyses prove that when the mass ratio of the phenolic resin powder to the elemental silicon powder is about 1. A slight deviation of about 10% may also be effective.

Preferably, the phenolic resin is ground to a powder particle size of less than 150 microns after curing. The phenolic resin is solidified and ground into powder with the grain size less than 150 microns, so that a sample is conveniently pressed and molded, and in addition, the smaller the grain size is, the larger the specific surface area is, the more reactive active sites are formed after the phenolic resin is carbonized, and the more silicon carbide is generated. Similarly, the smaller the particle size of the simple substance silicon micro powder is, the larger the specific surface area is, the larger the contact area between reactants is, the more sufficient the contact of reaction particles or reaction gas is, the shorter the reaction diffusion displacement is, and the easier the reaction is.

Preferably, the porosity of the block body formed by pressing the phenolic resin powder and the elemental silicon powder is 15-45%. The porosity of the pressed sample is reasonably between 15 and 45 percent through theoretical calculation. Neither too dense nor too loose a sample mass leads to excitation of the plasma effect.

In order to achieve the above porosity, it can be achieved by: mixing phenolic resin powder with elemental silicon powder, placing the mixture into a die, and maintaining the pressure for 1min at the pressure of 35MPa by using a hydraulic press.

As a further improvement, a catalyst is added into the mixture of the phenolic resin powder and the elemental silicon powder, the catalyst is transition metal, and the content of the catalyst is 0.1-1%. A small amount of catalyst (typically transition metals Fe, co, ni, etc.) may be added, which promotes the formation of α -SiC.

The phenolic resin powder and the elemental silicon powder are mixed and then heated by microwaves, so that more silicon carbide can be generated under the condition of low temperature, and a large amount of energy can be saved.

Other carbon sources, such as pitch, have fluidity after being heated, can close part of air holes, and cannot effectively excite the plasma effect, so that the silicon carbide generation rate is relatively low under the same experimental conditions.

Other silicon sources, such as silicon dioxide, have no wave-absorbing property and conductivity of elemental silicon, so that the silicon carbide formation rate is low under the same experimental conditions.

Experimental procedures

1. Raw material

The simple substance silicon powder (1-30 μm, purity > 99.9%) is from Yingkou Xianxingtai and Si industries, inc.; phenolic resins (solids > 98%) were obtained from san Dong Shengquan chemical Co., ltd.

2. Sample preparation

Firstly, curing phenolic resin at 120 ℃ for 24 hours, and then raising the temperature to 160 ℃ for curing for 12 hours to obtain massive cured phenolic resin; then grinding and sieving the cured phenolic resin, and taking the cured phenolic resin powder smaller than 150 mu m for later use; uniformly mixing the silicon powder and the cured phenolic resin powder according to the element molar ratio of 1.5, placing the mixture in a die, and then maintaining the pressure for 1min at 35MPa by using a hydraulic press to obtain a cylindrical sample with the diameter of 25 mm.

The sample was placed in a covered transparent quartz crucible, and the quartz crucible was placed in an ANKS-M12-A industrial microwave muffle furnace of AINIXX microwave Automation Equipment Co., ltd. In Qingdao, with a microwave emission frequency of 2.45 GHz. Starting microwave, heating the sample to a target temperature with 1500W power, then preserving heat for different time with 1000W power, and taking out a product after the sample is cooled to room temperature; finally, the product was heat treated at 600 ℃ for 30min to remove unreacted carbon.

3. Testing and characterization

The product phase was analyzed with an X-ray diffractometer (XRD, DX-2700BH, expensive instruments limited, dengtongeastern university) using a Cu ka radiation source with an incident wavelength dimension of λ =1.5418 a, a 2 θ test range of 20-80 °, a step size of 0.02 °, a tube pressure of 40 kV, a tube flow of 70 mA; the microscopic morphology of the product is characterized by adopting a scanning electron microscope (SEM, phenom ProX, holland Saimer fly group), a sample is directly adhered on an objective table by using a conductive adhesive tape, metal spraying treatment is carried out by adopting an SBC-12 type ion sputtering instrument, and then the sample is sent into a sample chamber for observation. The accelerating voltage of the scanning electron microscope is set to be 10 kV, the beam intensity is set to be standard current, and the probe mode is secondary electrons.

And analyzing the content of each phase in the product by using an XRD diffraction pattern according to the principle of an adiabatic method. Because the unreacted carbon in the sample is removed by oxidation, phases such as silicon, beta-SiC, alpha-SiC and the like which are not completely reacted may exist in the product at the same time, the beta-SiC phase is selected as a standard sample, the RIR value (relative to the K value of corundum) of each phase is found out through a PDF card, and then the K value of each phase in the sample relative to the selected beta-SiC phase, namely the K value of each phase in the sample relative to the selected beta-SiC phase can be calculated, namely

According to the adiabatic method, if N phases exist in a system, the mass fraction of x phases is:

wherein, I _i Is the integrated intensity of the strongest peak.

4. Results and analysis of the experiments

The XRD patterns of the product and the contents of the phases in the product are shown in figure 1. As can be seen from the figure, after the sample is kept at 600 ℃ for 30min, the 2 theta diffraction angle of the product has obvious characteristic diffraction peaks at 35.65 degrees, 41.40 degrees, 59.98 degrees, 71.77 degrees and 75.50 degrees, and the characteristic diffraction peaks respectively correspond to the (111), (200), (220), (311) and (222) crystal faces of a beta-SiC phase (JCPDS 74-2307) with a cubic structure, which indicates that the beta-SiC can be generated at 600 ℃ of microwave and is far lower than the temperature for synthesizing the beta-SiC by other microwaves. The unreacted silicon and the produced beta-SiC content in the product were 66.05% and 33.95%, respectively. Along with the increase of the reaction temperature, the intensity of the characteristic diffraction peak of silicon in the product is gradually reduced, the intensity of the characteristic diffraction peak of beta-SiC is gradually enhanced, and the content of the beta-SiC in the product at 800 ℃ and 1000 ℃ is 95.31 percent and 97.22 percent respectively. Meanwhile, about 1.24% of α -SiC was formed in the product at 1000 ℃ indicating that an increase in the reaction temperature promotes the conversion of a part of β -SiC to α -SiC to some extent, and thus 800 ℃ can be considered as the optimum reaction temperature for synthesizing β -SiC. In addition, the product shows a remarkable Stacking Faults (SF) near 33.6 degrees 2 theta, which is caused by defects such as twins and Stacking Faults in the beta-SiC.

Fig. 2 and 3 show the shapes of the products of microwave heating at 600 deg.c, 800 deg.c and 1000 deg.c for 30min, and it can be seen that the produced beta-SiC product is a mixture of particles and whiskers. As can be seen from FIG. 2, the beta-SiC particles have a regular cubic structure, with the beta-SiC particles being about 150-300 nm at 600 deg.C (FIG. 2 a); as the microwave heating temperature is increased to 800 ℃ and 1000 ℃, the maximum grain size of the beta-SiC particles is respectively increased to 2.5 μm and 3 μm, and the morphology of the beta-SiC particles is not obviously changed. The beta-SiC fiber is mostly filled in the pores of the sample, the diameter of the generated beta-SiC fiber is 40-50 nm, and the length of the beta-SiC fiber is increased from several microns to tens of microns along with the increase of the reaction temperature from 600 ℃ to 1000 ℃. Different nucleation and growth mechanisms may lead to differences in the morphology of the beta-SiC. At the contact interface of the elemental silicon powder and the cured phenolic resin powder, the generation of the beta-SiC particles is mainly a solid-solid (SS) or solid-liquid-solid (SLS) reaction mechanism. While in the pores, the formation of beta-SiC whiskers is mainly based on a gas-solid reaction mechanism.

According to the experimental data, the beta-SiC can be generated in the sample by heating the sample to 600 ℃ with microwave and preserving the heat for 30min. During the microwave heating experiment, when the temperature rose to 600 ℃, an obvious "pop" sound could be heard and sparks were observed. When the above phenomenon occurs, the microwave heating is immediately stopped, and the XRD result and SEM image of the sample are shown in fig. 4 and 5. The product produced 18.71% yield of β -SiC (figure 4) with β -SiC particles and whiskers (figure 5) grown on the surface of the reaction particles (β -SiC particle size about 40-50 nm, whisker diameter about 40-50 nm, whisker length about 1 μm), indicating that with the occurrence of "pop" sounds and sparks, initial β -SiC can be formed. In addition, it can be seen from the red region of FIG. 5a that discrete liquid regions are formed on the surface of the silicon powder.

In order to study the microwave heating behavior of the sample in the microwave heating process, under the same experimental conditions, the silicon powder and the cured phenolic resin powder were pressed into cylindrical samples, and then heated in a microwave electromagnetic field, with the silicon powder heating curve as shown in fig. 6. It can be seen that the silicon powder sample rapidly rises to 660 ℃ within 2.5 min, indicating that the silicon powder has good microwave absorption capacity and can be rapidly heated by microwaves. The heating rate then slowed after 660 ℃. However, the temperature of the cured phenolic resin powder sample is still lower than 300 ℃ after microwave heating for 30min, which shows that the microwave absorption capacity of the cured phenolic resin powder sample is poor. It can be seen that the silicon powder in the sample acts as a heat source to heat the cured phenolic resin powder around it during the initial microwave heating stage.

Figure 7 is a schematic representation of microwave heating of nucleation and growth of beta-SiC particles and whiskers. When the sample temperature reaches 350 ℃, the cured phenolic resin powder releases carbon-containing gases (CxHyOz) such as methane, ethylene, phenol, methanol, carbon dioxide, methyl phenol, and phenol alkyl derivatives, and then is converted into amorphous pyrolytic carbon. When the temperature rises to 600 ℃, the carbon-containing gas and the air filled in the pores can be activated by microwaves, creating a local microwave plasma, thus creating a "pop" sound and a spark (fig. 7 c). This process generates a large amount of energy, resulting in a transient high temperature at the reaction interface (discrete liquid areas on the surface of the local silicon particles). At the contact interface of the elemental silicon powder and the pyrolytic carbon, carbon atoms diffuse into silicon, and the formation of beta-SiC particle crystal nuclei is promoted. Meanwhile, elemental silicon and pyrolytic carbon can also react with oxygen to generate SiO and CO gases, and then SiO reacts with CO and carbon-containing gas (CxHyOz) filled in the sample pores to grow beta-SiC whisker crystal nuclei from the surface of the silicon powder (FIG. 7 d). The above process can be described in the system by the following reaction:

PF(s)→CxHyOz(g)+C(s) （5）

2Si(l)+O ₂ (g)→2SiO(g) （6）

2C(s)+O ₂ (g)→2CO(g) （7）

Si(l)+C(s)→SiC(s) （8）

Si(s)+C(s)→SiC(s) （9）

SiO(g)+CO(g)→SiC(s)+O ₂ (g) （10）

SiO(g)+CxHyOz(g)→SiC(s)+H ₂ O(g) （11）

the nucleation and growth process of the beta-SiC particles is mainly dominated by the reaction (8) and the reaction (9), and the reaction (10) and the reaction (11) mainly act on the nucleation and growth process of the beta-SiC whiskers.

With the continuous microwave heating, the unreacted silicon powder, the generated beta-SiC crystal nucleus and the pyrolytic carbon can absorb the microwave, so that the temperature of the sample is increased. With the increase of the microwave temperature and the holding time, the diffusion rate of silicon atoms and carbon atoms at the contact interface of silicon and pyrolytic carbon is gradually increased, and further growth of beta-SiC particles is promoted. Because the sample contains a large amount of oxygen, siO and CO gases are continuously generated and filled in the pores of the sample. The sufficient gas source promotes the rapid and continuous reaction of the growth of the beta-SiC crystal whisker. The main reactions of the β -SiC particles and whisker growth are formula (8-11).

The XRD patterns and contents of the phases in the product at 800 deg.C with different holding times are shown in FIG. 8. And gradually reducing the characteristic diffraction peak intensity of the silicon and gradually increasing the characteristic diffraction peak intensity of the beta-SiC along with the heat preservation time from 5 min to 60 min. When the heat preservation time is 15 min, the content of the beta-SiC in the product is 86.27 percent. When the heat preservation time is 30min and 60min, the content of the beta-SiC is respectively improved to 95.31 percent and 97.49 percent. This is probably due to the fact that the silicon source and the carbon source are sufficient in the sample at the initial stage of the reaction, and the formation process of the beta-SiC is continuously carried out. When the heat preservation time exceeds 30min, the yield of the beta-SiC is not greatly influenced by the extension of the heat preservation time, and the reaction cost is increased. It is considered that the holding time is preferably 30 minutes.

FIGS. 9 and 10 are SEM images of samples heated by microwave at 800 deg.C for different reaction times. As can be seen from fig. 9a, at the initial stage of the reaction, beta-SiC particles of nanometer scale are formed on the surface of the reactant particles. As the incubation time increased, the β -SiC particles grew gradually and the reactant particles were gradually consumed and decomposed (fig. 9 b-c). When the holding time was 30min and 60min, the maximum grain sizes of the β -SiC particles were 2.5 μm and 3 μm, respectively, indicating that extension of the holding time promoted the growth of SiC particles to some extent, which may be related to the kinetic reaction process. As can be seen from fig. 10, a large number of β -SiC whiskers were filled in the pores of the sample. With the prolonged holding time, the diameter of the whisker does not change much, and the length of the whisker increases from several micrometers to tens of micrometers. The length-diameter ratio of the beta-SiC crystal whisker increases along with the prolonging of the heat preservation time.

Conclusion

In conclusion, the successful synthesis of the beta-SiC powder can be realized by using the elemental silicon powder and the cured phenolic resin powder as a silicon source and a carbon source and adopting a microwave heating method. When the temperature is increased to 600 ℃, carbon-containing gas released by the cured phenolic resin and air filled in sample pores are excited by microwaves to generate microwave plasmas, so that the local reaction interface is instantaneously high in temperature, and the nucleation and growth of beta-SiC particles and whiskers are promoted. As the microwave continues to heat up, the size of the β -SiC particles and whiskers increases. Microwave heating is adopted, the temperature is kept at 800 ℃ for 30min, and the yield of the beta-SiC can reach more than 95%. Compared with other preparation methods, the method has lower synthesis temperature and better practical application value.

Example 1

The method for preparing the silicon carbide by microwave heating comprises the following steps:

1) Preparation of cured phenolic resin powder

Curing the phenolic resin: curing the phenolic resin at 120 ℃ for 24 h, and then raising the temperature to 160 ℃ for 12 h to obtain massive cured phenolic resin;

grinding the cured phenolic resin into powder in a sample grinder, and sieving to obtain < 100# phenolic resin powder for later use;

2) Sample block preparation

Uniformly mixing the phenolic resin powder prepared in the step (1) with elemental silicon powder according to the mass ratio of 1.

3) Preparation of silicon carbide

And (3) putting the sample obtained in the step (2) into a transparent quartz crucible with a cover, positioning an infrared temperature measuring point on the sample, and heating the crucible in an industrial microwave muffle furnace.

And (3) starting a microwave muffle furnace switch, setting 1500W microwave power to heat the sample to 800 ℃, then setting 1000W microwave power to preserve heat for 30min, and thus obtaining the silicon carbide. And taking out the sample when the sample is cooled to room temperature.

The phase of the product prepared in example 1 was analyzed by XRD, and the XRD pattern is shown in fig. 11. As is clear from the figure, the silicon carbide obtained in example 1 corresponded to the 3C-SiC (JCPDS 74-2307) standard diffraction peak, and the silicon carbide obtained was cubic phase β -SiC.

The product prepared in example 1 was analyzed for its morphology by SEM, and the picture is shown in fig. 12. As can be seen, the silicon carbide prepared in example 1 includes both particle and whisker morphologies. In the contact area of the simple substance silicon particles and the resin carbon particles, the shape of the granular crystalline silicon carbide is mainly used, and the shape of the silicon carbide whisker is mainly used in the pores of the sample.

Example 2

This example is different from example 1 only in that 1wt% of nickel nitrate catalyst is loaded in elemental silicon powder, and silicon carbide is prepared under the conditions of example 1. The XRD pattern and scanning electron microscope image obtained by characterizing the silicon carbide are shown in figure 13. Besides the generation of beta-SiC phase, alpha-SiC phase is generated in the product, and the addition of the catalyst can promote the conversion of beta-SiC to alpha-SiC and promote the generation of needle-shaped or "club-shaped" silicon carbide.

Example 3

The method for preparing silicon carbide by microwave heating in the embodiment is different from the method in embodiment 1 only in that the microwave heating temperature is different, and the microwave heating temperature in the embodiment is 600 ℃ and the heat preservation time is 30min. The remaining steps and parameters were the same as in example 1. The XRD characterization pattern of the product is shown in FIG. 14, and it can be seen that beta-SiC is generated at 600 ℃, but the yield of the generated silicon carbide is low.

Example 4

The method for preparing silicon carbide by microwave heating in this embodiment is different from that in embodiment 1 only in the difference of the microwave heating heat preservation time, and the microwave heating temperature in this embodiment is heat preservation for 10 min. The remaining steps and parameters were the same as in example 1. The XRD characterization pattern of the product is shown in FIG. 15, which shows that a large amount of beta-SiC is generated after the product is kept at the temperature for 10 min, but the yield of the generated silicon carbide is lower than that of the silicon carbide in example 1.

Example 5

The method for preparing silicon carbide by microwave heating in this example is different from example 1 only in that the carbon source used is uncured phenolic resin powder, and the rest of the steps and parameters are the same as those in example 1. The XRD pattern obtained by characterizing the product is shown in fig. 16. beta-SiC was also produced in this example, but the silicon carbide yield was low.

Example 6

The method for preparing silicon carbide by microwave heating in the embodiment is different from the method in the embodiment 1 only in that the carbon source used is powdered polymethyl methacrylate, and the temperature in the embodiment is 800 ℃ for heat preservation for 5 min. The XRD pattern obtained by characterizing the product is shown in fig. 17. beta-SiC can also be produced in this example, but the silicon carbide yield is low.

Example 7

The method for preparing silicon carbide by microwave heating in the embodiment is different from the embodiment 1 only in that a fused silica powder is used to partially replace elemental silicon powder as a silicon source, wherein the mass ratio of the fused silica powder to the elemental silicon powder is 1. The XRD pattern obtained by characterizing the product is shown in fig. 18. beta-SiC was also produced in this example, but the yield was lower than in example 1.

Claims

1. A method of preparing silicon carbide, comprising: the preparation method comprises the following steps of mixing powdery organic polymer material with CxHyOz composition with elemental silicon powder, pressing into blocks, and heating by microwave to obtain the silicon carbide.

2. The method for producing silicon carbide according to claim 1, wherein: the organic high molecular material is an organic matter which can be pyrolyzed below 600 ℃.

3. The method for producing silicon carbide according to claim 2, wherein: the organic high molecular material is an organic high molecular polymer synthesized by one or more than two monomers.

4. A method for producing silicon carbide according to claim 3 wherein: the organic high polymer material is phenolic resin.

5. The method for producing silicon carbide according to any one of claims 1 to 4, wherein: the microwave heating temperature is 600-1000 ℃.

6. The method for producing silicon carbide according to claim 5, wherein: the microwave heating temperature is 800 ℃.

7. The method for producing silicon carbide as set forth in claim 4, wherein: the mass part ratio of the phenolic resin powder to the elemental silicon powder is 90-110: 90-110.

8. The method for producing silicon carbide according to claim 7, wherein: the mass ratio of the phenolic resin powder to the elemental silicon powder is 1.

9. The method for producing silicon carbide according to claim 4, wherein: having one or more of the following features,

(1) The particle size of the phenolic resin is less than 150 microns after being solidified and ground into powder;

(2) The porosity of a block formed by pressing the cured phenolic resin powder and the elemental silicon powder is 15-45%;

(3) Mixing the cured phenolic resin powder with the elemental silicon powder, putting the mixture into a die, and maintaining the pressure for 1min at the pressure of 35MPa by using a hydraulic machine;

(4) And adding a catalyst into the mixture of the cured phenolic resin powder and the elemental silicon powder, wherein the catalyst is transition metal and the content of the catalyst is 0.1-1%.

10. A silicon carbide, characterized by: produced by the method for producing silicon carbide according to any one of claims 1 to 9.