CN111977657A - Method for preparing silicon carbide nanowires by treating fly ash of coal-fired boiler by microwaves - Google Patents

Abstract

The invention discloses a method for preparing silicon carbide nanowires by treating fly ash of a coal-fired boiler by microwaves. The technology uses fly ash of a coal-fired power plant and low-quality carbonaceous materials, such as activated carbon, coal pyrolysis semicoke, coal gasification semicoke, petroleum coke, biomass semicoke, coal-based coke and the like as raw materials, adopts microwave heating, controls the heating temperature to be 800-1600 ℃ under the protection of nitrogen, and stays at the final heating temperature for 15-100 min to prepare the silicon carbide nanowire with the diameter of 5-100 nm and the length of 100-1500 mu m. The method takes fly ash of a coal-fired power plant and low-quality carbonaceous materials as raw materials, the raw materials are easy to obtain, the cost is low, the resource utilization of the fly ash of the solid waste can be realized, and the national policy requirements are met. Meanwhile, the method has the advantages of simple process and equipment, high product yield, easiness in large-scale production and the like, and the prepared silicon carbide nanowire is suitable for being applied to the fields of reinforced composite materials and semiconductor materials of high-performance high-temperature ceramics.

Description

A method for preparing silicon carbide nanowires by microwave treatment of coal-fired boiler fly ash

technical field

The invention relates to the technical field of high-end silicon carbide material and microwave heating production.

Background technique

Silicon carbide (SiC) nanowires are single crystal fibers with a radial dimension below 100 nm and a length much larger than the radial dimension. The characteristics of high temperature, high frequency and high radiation resistance of silicon carbide nanowires enable them to be used in some extremely harsh environments, which is of great significance to the development of military and aerospace industries.

Silicon carbide nanowire production technology has always been the center and difficulty of global research. The production of silicon carbide nanowires is not high in the world, and it is generally produced at the laboratory level (about tens of micrograms each time). At present, the methods for synthesizing SiC nanowires can be divided into three categories according to the different forms of carbon sources: (1) gas-phase carbon source method, which mainly uses chemical vapor deposition (CVD) to synthesize high-purity SiC nanowires; (2) ) liquid carbon source method, mainly sol-gel method (Sol-Gel); (3) solid phase carbon source method, mainly including template method, carbothermic reduction method, pyrolysis organic precursor method, arc discharge method, etc. . These methods have one or more of the problems of high equipment requirements, complex process conditions, long reaction time, inability to remove catalysts, and difficulty in large-scale production.

In addition to the advantages of low equipment requirements, simple process, uniform heating, and fast heating speed, microwave irradiation technology can also minimize the activation energy required for the reaction and realize the synthesis of target products and new structural products in a very short time. At present, some researchers have studied the preparation of silicon carbide nanowires by microwave. The methods are as follows:

Document 1 Patent CN102373505A "Microwave Preparation Method of Silicon Carbide Nanowires" discloses a microwave preparation method of silicon carbide nanowires, which includes the following steps: providing a silicon-containing substrate and placing it in a microwave reaction chamber; The reaction chamber is evacuated and a protective gas is introduced; the reaction chamber is heated to 1000-1300 ℃ with microwaves, and the carbon source gas and the protective gas are introduced into the reaction chamber to make the carbon source gas react and form carbonization on the surface of the silicon-containing substrate Silicon nanowires.

Document 2 Patent CN106544642A "A method for preparing silicon carbide nanowire film by microwave method" discloses a method for preparing silicon carbide nanowire film, which includes the following steps: fully mixing elemental carbon, elemental silicon and silicon dioxide into the ceramic crucible; put the ceramic crucible and the ceramic substrate pretreated with nickel salt or iron salt solution into the resonance cavity of the microwave oven, extract part of the air, and use the microwave under the low pressure state (the pressure is 10-30kPa). The heating reaction is carried out rapidly to the reaction temperature; after holding for a period of time, the microwave is turned off, and it is naturally cooled to room temperature, and the silicon carbide nanowire film is formed on the surface of the ceramic substrate.

The preparation methods of the above documents inevitably use pure silicon substrates, pure silicon, etc. as substrates, and carbon-containing gas, elemental carbon, etc. as carbon sources, which require high quality of raw materials and high preparation costs. The present invention studies the composition of fly ash from coal-fired power plants and low-quality carbonaceous materials (activated carbon, coal pyrolysis semi-coke, coal gasification semi-coke, petroleum coke, biomass semi-coke and coal-based coke), and its application in microwave treatment. According to the change mechanism, a method for preparing silicon carbide nanowires by microwave treatment of coal-fired boiler fly ash is proposed. The method uses fly ash from coal-fired power plants and low-quality carbonaceous materials as raw materials, the raw materials are readily available, and the cost is low, and the resource utilization of solid waste fly ash can be realized. At the same time, the method has the advantages of simple process and equipment, high product yield, easy mass production, etc. The prepared silicon carbide nanowires are suitable for application in the fields of high performance high temperature ceramic reinforced composite materials and semiconductor materials.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for preparing silicon carbide nanowires by microwave treatment of coal-fired boiler fly ash, which solves the problem that the prior art has higher requirements for equipment, complicated process conditions, long reaction time, inability to remove catalysts, and difficulty in large-scale production. And other issues.

To achieve the above object, the present invention is achieved through the following technical solutions:

Firstly, the fly ash of the coal-fired boiler with particle size < 0.106 μm and the carbonaceous material with particle size < 0.5 mm are uniformly mixed according to the mass ratio of 1:1 to 5:1, then the mixture is put into the quartz reactor, and then the The quartz reactor of the mixture is placed in a microwave heating device, and the heating temperature is controlled at 800 ℃ ~ 1600 ℃ under nitrogen protection, the heating rate is controlled at 20 ℃/min ~ 50 ℃ /min, and the final temperature of heating is kept for 15min ~ 100min, preparation silicon carbide nanowires.

The content of silica in the fly ash is more than 60%, the content of residual carbon is less than 1%, and the content of volatile matter is less than 1.5%.

The carbonaceous material is activated carbon, coal pyrolysis semi-coke, coal gasification semi-coke, biomass semi-coke, petroleum coke, coal-based coke or a mixture of any of them.

The silicon carbide nanowires have a diameter of 5 nm to 100 nm and a length of 100 μm to 1500 μm.

For achieving the above object, the present invention realizes through the following technical steps:

(1) uniformly mix the particle size <0.106μm coal-fired boiler fly ash and the carbonaceous material with particle size <0.5mm according to the mass ratio of 1:1～5:1, and then load the mixture into the quartz reactor;

(2) put the quartz reactor equipped with the mixture on the rotary table of the microwave heating device, and insert a thermocouple in the raw material to monitor the temperature of the material bed;

(3) Pour nitrogen into the quartz reactor, turn on the microwave heating device, set the heating temperature to 800℃～1600℃, control the heating rate at 20℃/min～50℃/min, and stay at the final temperature for 15min～100min .

(4) cooling to obtain silicon carbide nanowires with a purity of 90%, a diameter of 5 nm to 100 nm, and a length of 100 μm to 1500 μm.

The preparation of silicon carbide nanowires adopts the setting and display of process parameters such as temperature, heating time and heating rate of microwave treatment through a computer. The processing capacity of each batch of the microwave device can be determined according to the production scale, ranging from several kilograms to tens of kilograms, and the daily production volume can be from tens of kilograms to hundreds of kilograms; it is also possible to increase the number of microwave devices to expand silicon carbide. Nanowire production capacity, enabling production to reach hundreds of tons per year. The silicon carbide nanowires are suitable for preparing reinforced composite materials of high-performance high-temperature ceramics, and high-purity silicon carbide nanowires can be obtained after purification, which can be used for preparing semiconductor materials.

Compared with the prior art, the beneficial effects of the present invention are:

(1) Using fly ash from coal-fired power plants as raw materials, the cost of raw materials is low, and the resource utilization of solid waste fly ash can be realized, which meets the requirements of national policies;

(2) This technology can realize large-scale production of silicon carbide nanowires, with short production cycle, few by-products, and high yield;

(3) Low requirements for carbonaceous raw materials, coal-based semi-coke, biomass semi-coke, petroleum coke, coal-based coke, etc. can be used;

(4) The production equipment and process are simple, easy to operate, the heating conditions are not demanding, and the investment cost is low;

(5) The produced silicon carbide nanowire products can be used to produce high temperature ceramic reinforced composite materials, and can be used to prepare semiconductor materials after purification treatment.

Description of drawings

FIG. 1 is a flow chart of the fabrication process of silicon carbide nanowires.

Figure 2 is a microwave production device for silicon carbide nanowires; wherein 1) thermocouple; 2) microwave oven cavity; 3) quartz reactor; 4) reaction material; 5) refractory insulation brick; 6) microwave oven rotary table; 7) Computer; 8) Microwave oven display; 9) Microwave oven magnetron unit; 10) Microwave oven power supply control unit.

3 is an electron microscope image of silicon carbide nanowires; wherein (a) the morphology of the silicon carbide nanowires under a scanning electron microscope; (b) the structural morphology of the silicon carbide nanowires under a transmission electron microscope.

Detailed ways

A new method for preparing silicon carbide nanowires by microwave treatment of fly ash of coal-fired boilers, the following examples are used to illustrate the specific embodiments and implementation effects of the present invention.

Specific examples:

Mix 10g of fly ash from a coal-fired power plant with a particle size of less than 0.106 μm and 5g of pyrolysis semi-coke with a particle size of less than 0.5mm, then put the mixture into a quartz reactor, and then put the mixture into the quartz reactor. In the microwave oven (see Figure 2 for the schematic diagram of the structure), the heating temperature is controlled at 1200 °C under nitrogen protection, the heating rate is controlled at 20 °C/min, and the temperature is kept at 1200 °C for 60 minutes to prepare silicon carbide nanowires. The process flow chart is shown in Figure 1 .

The prepared silicon carbide nanowires were observed by electron microscope, and their morphology and structure are shown in Figure 3. It can be seen from Figure 3(a) that the silicon carbide nanowires have a diameter of 5-50 nm and a length of 1000 μm, with a high yield, which can be used to prepare reinforced composites for high-performance high-temperature ceramics. Impurities such as iron oxide and aluminum oxide (the impurities contained in fly ash and carbonaceous materials themselves) in the silicon carbide nanowires are removed to obtain high-purity silicon carbide nanowires, which can be used to prepare high-end silicon carbide functional materials.

Those skilled in the art can easily understand that the above are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, etc., All should be included within the protection scope of the present invention.

Claims (4)

1. A method for preparing silicon carbide nanowires by using microwave to treat fly ash of a coal-fired boiler is characterized by comprising the steps of firstly, uniformly mixing the fly ash of the coal-fired boiler with the particle size of less than 0.106 mu m with a carbonaceous material with the particle size of less than 0.5mm according to the mass ratio of 1: 1-5: 1; then the mixture was charged into a quartz reactor; then placing the quartz reactor filled with the mixture into a microwave heating device, controlling the heating temperature to be 800-1600 ℃ under the protection of nitrogen, controlling the temperature rise rate to be 20 ℃/min-50 m DEG C i/n, and staying for 15 min-100 min at the final heating temperature to prepare the silicon carbide nanowire.

2. The method for preparing silicon carbide nanowires by treating fly ash of a coal-fired boiler with microwaves as claimed in claim 1, wherein the fly ash contains more than 60% of silica, less than 1% of residual carbon and less than 1.5% of volatile components.

3. The method for preparing the silicon carbide nanowires by using the microwave to treat the fly ash of the coal-fired boiler according to claim 1, wherein the carbonaceous material is one or a mixture of any several of activated carbon, coal pyrolysis semicoke, coal gasification semicoke, biomass semicoke, petroleum coke and coal-based coke.

4. The method for preparing the silicon carbide nanowire by using the microwave to treat the fly ash of the coal-fired boiler according to claim 1, wherein the diameter of the silicon carbide nanowire is 5nm to 100nm, and the length of the silicon carbide nanowire is 100 μm to 1500 μm.

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