CN108218467A - A kind of preparation method of high porosity and lower thermal conductivity porous nano silicon carbide ceramics - Google Patents

Abstract

The invention relates to the field of porous ceramic materials, in particular to a method for preparing porous nano-silicon carbide ceramic materials with high porosity and low thermal conductivity through a pore-forming agent method. The method uses β‑SiC nanoparticles and micron flake graphite as raw materials, and uses ethanol as the medium to perform ball milling and mixing to form a slurry; the slurry is dried, sieved into powder, pressed into a green body with a mold, and then cold isostatic pressure The green body is subjected to high-temperature pressureless sintering in an argon atmosphere, and then heat-treated in air to remove the pore-forming agent graphite to obtain a porous nano-silicon carbide ceramic material with both micropores and nanopores. The invention can prepare porous nano-SiC ceramic material with multi-level pore structure and controllable porosity, high porosity (54-76%) and low thermal conductivity (0.74-0.14W m ^-1 K ^-1 ).

Description

A preparation method of porous nano silicon carbide ceramics with high porosity and low thermal conductivity

technical field

The invention relates to the field of porous ceramic materials, in particular to a method for preparing porous nano-silicon carbide ceramic materials with high porosity and low thermal conductivity through a pore-forming agent method.

Background technique

Frontier technology applications in thermal insulation and energy conversion at high temperatures require new materials with low thermal conductivity and high reliability. Nanostructured materials have become a research hotspot in academic and technical fields due to their good thermal insulation properties. The basic mechanism of their thermal resistance originates from the huge interfacial thermal resistance in the nanostructures. However, as the particle size decreases, the specific surface area of the material increases, resulting in a sharp decline in the high-temperature thermal stability of the nanostructure. The oxide nanomaterials mainly used at present, such as silicon dioxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) aerogels and their composite materials, have thermal stability lower than 1200°C. Therefore, the bottleneck problem that urgently needs to be solved at present is how to simultaneously achieve extremely low thermal conductivity and high thermal stability in new nanomaterials.

Silicon carbide ceramics can exhibit excellent mechanical properties and corrosion resistance in harsh high-temperature environments, so they are widely used in industrial fields. More importantly, due to the strong Si-C covalent bond and low self-diffusion coefficient, it is reported that the cold-pressed SiC powder bulk can maintain its initial sample size at room temperature at a high temperature of 1800 °C without drastic shrinkage ( M. Fukushima et al. Mater. Sci. Eng. B 2008(148):211–214.). And β-SiC has ultra-high phase stability, and there is no phase transition within 2000°C (B.K.Jang et al.Adv.Mater.2007(8):655–659.). Although SiC is a ceramic with high thermal conductivity, when the particle size is reduced to the nanometer level, nano-SiC exhibits amazing thermal insulation properties (P.Wan et al.Scripta Mater.2017(128):1-5) . Therefore, porous nano-silicon carbide ceramics is a potential high-temperature-resistant thermal insulation and thermal protection material used in aviation and aerospace fields.

Contents of the invention

The purpose of the present invention is to provide a method for preparing porous nano-silicon carbide ceramics with high porosity and low thermal conductivity, so that the SiC porous ceramics have brand-new performance characteristics of low thermal conductivity.

Technical scheme of the present invention is as follows:

A method for preparing porous nano silicon carbide ceramics with high porosity and low thermal conductivity. The silicon carbide material is porous nano SiC ceramics. The specific steps are as follows:

1) Preparation of slurry: Prepare raw materials with a mass ratio of β-SiC nanoparticles and graphite powder of 9:1 to 5:5, use ethanol as a medium, and perform ball milling and mixing for 8 to 16 hours to form a slurry;

2) After the slurry is dried, it is sieved into powder, pressed into a green body with a mold, and then further densified under a cold isostatic pressure of 200-240 MPa for 10-30 minutes;

3) Carry out high-temperature pressureless sintering of the green body in an argon atmosphere at 1400-1600°C for 1-3 hours, and then heat-treat in the air at 600-800°C for 3-5 hours to remove the pore-forming agent graphite, and obtain micropores and nanopores. Porous nano-porous silicon carbide ceramic material.

In the preparation method of the porous nano-SiC ceramic with high porosity and low thermal conductivity, the porosity of the porous nano-SiC ceramic material ranges from 54% to 76%, the micro-pores are in the form of thin slices and are uniformly distributed, and the pore size is 2 μm in length and width. ～6.5 μm, thickness: 0.5～1.5 μm; nanopores are distributed on the skeleton of the ceramic material, and the pore diameter range is less than 50nm.

In the preparation method of porous nano-silicon carbide ceramics with high porosity and low thermal conductivity, the average particle size of β-SiC nanoparticles is 20-50 nm, and the particle size distribution of graphite powder is D ₉₀ =6.5 μm.

In the preparation method of porous nano-silicon carbide ceramics with high porosity and low thermal conductivity, the heating rate of high-temperature sintering is 8-12°C/min below 1000°C, and 4-6°C/min above 1000°C.

Advantage of the present invention and beneficial effect are:

1. The present invention proposes a method for preparing a porous nano-silicon carbide ceramic material with high porosity and low thermal conductivity by a pore-forming agent method. SiC porous ceramics with low thermal conductivity.

2. The operation of the present invention is convenient, and only needs to go through processes such as slurry preparation, press molding, high-temperature sintering, and carbon removal treatment.

3. The present invention does not require special equipment and is easy to produce on a large scale.

4. The present invention can prepare porous nano-SiC ceramic material with higher porosity. The porous ceramic has a multi-level nanostructure heat resistance mechanism and a multi-level pore structure, which can greatly reduce thermal conductivity, and is a lightweight material with excellent performance. High temperature resistant insulation material.

In a word, this method uses β-SiC nanoparticles and micron flake graphite as raw materials, and after 12 hours of ball milling, a uniformly mixed powder is obtained. After the powder was cold isostatically pressed at 220MPa, it was sintered in an argon atmosphere at 1500°C for 2 hours without pressure, and then the sample was heat-treated at 700°C in air for 4 hours to remove the graphite pore-forming agent, and finally a porous nano-SiC ceramic was prepared. The method of the present invention has simple process and strong operability, and can prepare high porosity (54-76%) and low thermal conductivity (0.74-0.14W m ^-1 K ^-1 ) with multi-level pore structure and controllable porosity. ) of porous nano-SiC ceramic materials.

Description of drawings

FIG. 1 is a transmission electron micrograph of SiC nano powder in Example 1. In the figure, the upper inset is the morphology of a single SiC nanoparticle under high magnification; the lower inset is the overall morphology of SiC nanopowder under low magnification.

Fig. 2(a) is an X-ray diffraction three-dimensional imaging (XRT) image of the porous nano-SiC ceramic material in Example 2.

FIG. 2( b ) is a scanning electron microscope (SEM) photo of the porous nano-SiC ceramic material in Example 2.

Detailed ways

In the specific implementation process, the preparation method of the porous nano silicon carbide (SiC) ceramics with high porosity and low thermal conductivity of the present invention, the specific steps are as follows:

1) Preparation of slurry: The mass ratios of β-SiC nanoparticles and graphite powder are: 9:1, 8:2, 7:3, 6:4 and 5:5 to prepare raw materials, and use ethanol as the medium for ball milling Mix for 12 hours to form a slurry; wherein, the average particle size of the β-SiC raw powder is 35 nm, and the particle size distribution of the graphite powder is D ₉₀ =6.5 μm.

2) The slurry is dried and sieved into powder, pressed into a green body with a mold of the required shape, and then further densified under a cold isostatic pressure of 220 MPa;

3) The green body is subjected to high-temperature pressureless sintering in an argon atmosphere at 1500°C, and then heat-treated at 700°C in air to remove the pore-forming agent graphite, so that a SiC porous ceramic material with both micropores and nanopores can be obtained. The porosity of the porous nano-SiC ceramic material ranges from 54 to 76%. The micro-pores are thin and uniformly distributed. 50nm.

The present invention is described in detail below by way of examples.

Example 1

Put a small amount of SiC nanometer powder into ethanol and ultrasonically disperse it for 30 minutes, absorb the mixture of powder and ethanol with a glass capillary, then drop 2 to 3 drops onto the 200-mesh microgrid support film, and observe it under a transmission electron microscope after fully drying. Figure 1. The results in this example show that the average particle diameter of β-SiC nanoparticles is about 35nm and the particles contain a lot of stacking faults. The fine particle size produces large interface and grain boundary thermal resistance, while lattice defects (such as stacking faults) further hinder the transmission of heat transfer phonons, thus effectively reducing thermal conductivity.

Example 2

The mass ratio of β-SiC nanoparticles and graphite powder is 7:3 to prepare raw materials, using ethanol as the medium, ball milling and mixing in a silicon nitride ball mill tank for 12 hours to form a slurry; then dry the slurry and sieve it into powder , pressed into a green body with a mold of the required shape, and then further densified under a cold isostatic pressure of 220MPa for 10 minutes;

The green body was sintered at 1500°C in an argon atmosphere at high temperature for 2 hours without pressure, and then heat-treated at 700°C in air for 4 hours to remove the pore-forming agent graphite to obtain a porous nano-SiC ceramic material. In this embodiment, the porosity of the sample is 66.8%, and the thermal conductivity is 0.42 W m ^-1 K ^-1 . The microscopic morphology of the porous ceramics is shown in the XRT photo of Figure 2(a). The micro-pores are thin sheets and evenly distributed. The nanopores on the porous ceramic skeleton are shown by the SEM photo of Figure 2(b), there are nanopores between the particles, and the pore diameter range is <50nm.

The results of the examples show that the present invention combines the nanopowder with the simple pore-forming agent method, and the porosity can be controlled by adjusting the amount of the pore-forming agent added. The porous nano-SiC ceramics prepared by this method have higher porosity and Low thermal conductivity, and porous ceramics with both micropores and nanopores can be prepared.

Claims (4)

1. a preparation method of high porosity and low thermal conductivity porous nano silicon carbide ceramics, characterized in that the silicon carbide material is porous nano SiC ceramics, and the concrete steps are as follows: 1) Preparation of slurry: Prepare raw materials with a mass ratio of β-SiC nanoparticles and graphite powder of 9:1 to 5:5, use ethanol as a medium, and perform ball milling and mixing for 8 to 16 hours to form a slurry; 2) After the slurry is dried, it is sieved into powder, pressed into a green body with a mold, and then further densified under a cold isostatic pressure of 200-240 MPa for 10-30 minutes; 3) Carry out high-temperature pressureless sintering of the green body in an argon atmosphere at 1400-1600°C for 1-3 hours, and then heat-treat in the air at 600-800°C for 3-5 hours to remove the pore-forming agent graphite, and obtain micropores and nanopores. Porous nano-porous silicon carbide ceramic material. 2. according to the preparation method of high porosity and low thermal conductivity porous nano-silicon carbide ceramics according to claim 1, it is characterized in that the porosity range of porous nano-SiC ceramic material is 54～76%, and the micron hole is sheet and shape Uniform distribution, pore size: length and width: 2 μm to 6.5 μm, thickness: 0.5 to 1.5 μm; nanopores are distributed on the skeleton of the ceramic material, and the pore diameter range is less than 50 nm. 3. according to the preparation method of high porosity and low thermal conductivity porous nano-silicon carbide ceramics according to claim 1, it is characterized in that the average particle size of β-SiC nanoparticles is 20～50nm, and the particle size distribution of graphite powder is D ₉₀ =6.5 μm. 4. According to the preparation method of high porosity and low thermal conductivity porous nano-silicon carbide ceramics according to claim 1, it is characterized in that the heating rate of high-temperature sintering: below 1000°C is 8～12°C/min, above 1000°C is 4～6℃/min.