CN102503430A - Method for preparing reaction-sintered silicon carbide ceramic by improved tape casting process - Google Patents

Abstract

The invention discloses a method for preparing reaction sintered silicon carbide ceramics by improving the cast molding process, which involves vacuum impregnating a phenolic resin solution in a porous biscuit after being debonded from a cast film, drying, carbonizing, and then reacting and siliconizing to obtain a dense reaction Sintered silicon carbide ceramics. The present invention comprises the following steps: first prepare a porous carbon-containing green body by tape casting method; then vacuum impregnate the porous green body into a phenolic resin solution, dry and carbonize the green body after complete impregnation, and finally carbonize the The green body is reacted siliconized in a vacuum furnace to obtain a dense reaction sintered silicon carbide ceramic. The present invention is based on the tape-casting process for the preparation of reaction sintered silicon carbide biscuits, and improves the problem of high free silicon content in the process, not only maintaining the advantages of the tape-casting process's stackable design, but also effectively The content of residual Si in the reaction sintered ceramics is reduced and the bending strength of the sintered body is significantly improved.

Description

Method for preparing reaction sintered silicon carbide ceramics by adopting improved tape casting process

technical field

The invention relates to an improved method for preparing reaction sintered silicon carbide ceramics by adopting an improved convective casting method, which belongs to the field of ceramic preparation.

Background technique

Tape casting is a method of colloidal molding. The ceramic material prepared by this method has the characteristics of uniform structure, designability, and high reliability. It has been widely used in the preparation of thin flat ceramic materials, especially in multilayer composite materials. The advantages are more obvious. In the previous experimental work, carbon-containing porous green bodies with uniform microstructure and narrow pore size distribution have been prepared by tape casting method, and dense reaction-sintered silicon carbide ceramics have been obtained by high-temperature liquid phase siliconizing. When the ratio of C/SiC in the green body is 3:10, the three-point bending strength of the ceramic after reaction sintering is as high as 410±14MPa.

However, the residual free silicon content in the ceramic sintered body obtained by this method is relatively high. When the C/SiC ratio in the green body is 3:10, the free silicon content after reaction sintering is 37vol%. The existence of excess free Si not only limits the service temperature of reaction sintered silicon carbide ceramics, but also is not conducive to the improvement of the mechanical properties of ceramics. Therefore, it is necessary to seek a method that can effectively reduce the content of free silicon in the reaction sintered body and further improve the flexural strength of the sintered body.

According to the existing literature reports, reducing the free silicon content in the reaction sintered silicon carbide ceramic sintered body can be considered from two aspects, one is to reduce the open porosity of the green body; the other is to increase the carbon content in the green body. But no matter which method is chosen, it must be considered that liquid phase siliconizing is a process of exothermic and volume expansion, and the increase of carbon content is also accompanied by greater thermal stress and volume expansion. If the green body does not have a certain strength, it is easy to form cracks in the sintered body after liquid-phase siliconizing, thereby reducing the strength of the ceramic body. Previous work has shown that when the C: SiC in the green body is increased to 5:10, crack-like defects appear in the sintered body, and the bending strength also decreases to 225±79MPa.

In terms of strengthening porous materials, many researchers have adopted the method of impregnating resins, for example, Furuno et al. (Furuno, T., Imamura, Y., Kajita, min., 2004.Wood Sci. 361.) impregnated phenolic resin into the cell walls and found that this method significantly increased the mechanical strength of wood.

Phenolic resin is a multifunctional substance compatible with various organic or inorganic fillers. It is often used as a binder and can provide certain mechanical strength after crosslinking. In addition, phenolic resin will decompose and leave about 55wt% carbon under vacuum or inert gas conditions at about 1000°C, which is also beneficial to reaction sintering.

In the previous work, the inventors of the present application prepared a carbon-containing porous green compact with uniform structure and obtained reaction-sintered silicon carbide ceramics with good mechanical properties by tape casting method. On this basis, the present invention intends to further vacuum-impregnate the porous green body with phenolic resin solution, which not only effectively improves the strength and carbon content of the green body, but also reduces the open porosity of the green body, and finally reduces the free Silicon content and enhance the mechanical strength of the sintered body. At present, there are few reports at home and abroad on the process of vacuum impregnating phenolic resin solution into carbon-containing porous green body and then performing reaction siliconizing.

Contents of the invention

The purpose of the present invention is to provide a method for preparing reaction sintered silicon carbide ceramics by adopting an improved tape casting process. The invention aims at the disadvantages of high content of free silicon and relatively low bending strength in the process of preparing reaction sintered silicon carbide ceramics by tape casting process, and provides a simple and effective method to improve it. It is characterized in that: the method of vacuum impregnation is used to impregnate the phenolic resin solution into the porous biscuit obtained after lamination and debonding of the cast film, and the phenolic resin remains on the hole wall after solidification; and then debonded at medium temperature, The amorphous carbon formed after the cracking of phenolic resin remains in the green body, which not only reduces the open porosity of the green body but also increases the carbon content in the green body. Compared with the sample not impregnated with phenolic resin, the free silicon content in the sintered body of the reaction sintered silicon carbide ceramics obtained after liquid-phase siliconizing the green body obtained is greatly reduced, and the bending strength is significantly increased.

The concrete technology that the present invention adopts comprises the steps:

1) Prepare porous carbon-containing blanks with different C/SiC ratios and uniform pore size distribution by tape casting;

2) Vacuum impregnating the porous carbon-containing blank obtained above in different concentrations of phenolic resin solutions;

3) After the impregnation is complete, the excess phenolic solution on the surface of the carbon-containing porous biscuit impregnated with the phenolic resin solution is blotted dry with filter paper;

4) after drying the green body with excess phenolic solution on the surface, place it in a drying oven to dry to make the phenolic resin solidify;

5) heating up the dried green body under vacuum conditions to crack the phenolic resin into amorphous carbon;

6) Place the obtained carbon-containing green body on a silicon wafer, and react siliconizing under vacuum conditions, and the reaction temperature is between 1420-1550°C.

The C/SiC mass ratio in the above step 1) is between 1:10-10:1;

Preferably, the C/SiC mass ratio in step 1) is between 1.4-7:1;

The phenolic resin in the step 2) is phenol formaldehyde resin, polyvinyl acetal modified phenolic resin, polyamide modified phenolic resin, epoxy modified phenolic resin, organosilicon modified resin, and other modified resins wait;

Described phenolic resin organic solution is any one in ethanol, isopropanol, n-butanol, n-octanol, acetone, methyl ethyl ketone, toluene, xylene, normal hexane, hexanaphthene organic solvent;

Preferably, the concentration of the organic solution of the phenolic resin is 5-70wt%, preferably 10wt%-60wt%;

Preferably, the vacuum impregnation in step 2) is to vacuum the porous green body first, and then add the phenolic resin solution to the porous green body; vacuum impregnate the porous green body in the phenolic resin solution.

Preferably, the vacuum degree in the step 2) is lower than 0.1MPa;

Preferably, the vacuum pumping time in the step 2) is 2-30 minutes;

Preferably, the drying oven temperature in step 4) is between 50-170°C, preferably 70-160°C;

Preferably, the drying time in step 4) is 2-72 hours, preferably 2-24 hours;

Preferably, the heating rate in step 5) is lower than 10°C/min, preferably 0.3-5°C/min;

Preferably, the cracking temperature in step 5) is 700°C-1200°C, preferably 800-1000°C; the heating rate is less than 10°C/min;

Preferably, the heat preservation time in the step 5) is 10-300 minutes, preferably 10-120 minutes;

Preferably, the reactive siliconizing in step 6) is completed in a carbon tube furnace with a vacuum of 1-20Pa.

The process described in the above step 2) to step 5) is a cycle, which can be performed only once or repeated many times.

The present invention is based on the tape-casting process for the preparation of reaction sintered silicon carbide biscuits, and improves the problem of high free silicon content in the process, not only maintaining the advantages of the tape-casting process's stackable design, but also effectively The content of residual Si in the reaction sintered ceramics is reduced and the bending strength of the sintered body is significantly improved.

Description of drawings

Figure 1 is a comparison of the microstructure of the green body mentioned in Example 2 before and after impregnation with phenolic resin. Wherein (a) is the microstructure and pore size distribution before the green body is impregnated with phenolic resin, and (b) is the microscopic structure and pore size distribution after the green body is impregnated with phenolic resin.

Detailed ways

Below by concrete example, the present invention is further described:

Comparative Example 1: The density and carbon content of the prepared porous green body were 1.43g/cm ³ and 24wt%, respectively, by laminating cast films with a C/SiC ratio of 3:10 and debonding at 500°C. After siliconizing the green compact at 1450°C/30 minutes, the obtained sintered body has a density of 2.89g/cm ³ , a content of free silicon of 37vol%, and a three-point bending strength of 410±14MPa.

Example 1: A porous green body with a C/SiC ratio of 3:10 was impregnated with a phenolic resin solution with a concentration of 30 wt% at a vacuum of 0.1 MPa, dried at 80°C for 12 hours, and then heated at a rate of 5°C/min After carbonization at 800°C/1 hour, the density and carbon content of the green body were 1.57g/cm ³ and 25wt%, respectively, which were 10% and 4% higher than those before the green body was impregnated with phenolic resin.

Example 2: A porous green body with a C/SiC ratio of 3:10 was impregnated with a 50 wt% phenolic resin solution at a vacuum of 0.1 MPa, dried at 80°C for 12 hours and then heated at a rate of 5°C/min After carbonization at 800°C/1 hour, the density and carbon content of the green body are 1.60g/cm ³ and 26wt%, respectively, and the increase of the green body before impregnating phenolic resin is 11% and 8%, respectively. The structure is shown in Figure 1. It can be seen that the green body after impregnated with phenolic resin can still maintain a connected pore structure, and still maintain a narrow pore size distribution.

Example 3: A porous green body with a C/SiC ratio of 1.4:10 is impregnated with a phenolic resin solution with a concentration of 50 wt% under a vacuum of 0.1 MPa, then dried at 80°C for 12 hours and then heated at a rate of 5°C/ minutes to 800°C/1 hour for carbonization. Afterwards, the carbonized porous green body was reacted and siliconized in a carbon tube furnace with a vacuum degree of 6Pa at 1450°C/30 minutes, and the three-point bending strength of the obtained sintered body was 430±45MPa, and the silicon content was 24vol%. Compared with the sintered green compact not impregnated with phenolic resin (comparative example), the strength value after sintering increased by 28%, and the content of free silicon decreased by 44%.

Example 4: A porous green body with a C/SiC ratio of 2:10 is impregnated with a phenolic resin solution with a concentration of 50 wt%, and then dried, carbonized, and siliconized in the same manner as in Example 4, and the obtained sintered body is three-point resistant. The bending strength is 549±86MPa, and the silicon content is 20vol%. Compared with the sintered green compact not impregnated with phenolic resin (comparative example), the strength value after sintering increases by 50%, and the content of free silicon decreases by 49%.

Example 5: The porous green body with a C/SiC ratio of 3:10 is impregnated with a phenolic resin solution with a concentration of 50 wt%, and then dried, carbonized, and siliconized in the same manner as in Example 4, and the obtained sintered body is three-point resistant. The bending strength is 598±112MPa, and the silicon content is 17vol%. Compared with the sintered green compact not impregnated with phenolic resin (comparative example), the strength value after sintering increased by 46%, and the content of free silicon decreased by 54%.

Claims (4)

1. A method for adopting an improved tape-cast molding process molding reaction sintered silicon carbide ceramics, characterized in that it comprises the steps: 1) Prepare porous carbon-containing blanks with different C/SiC ratios and uniform pore size distribution by tape casting; 2) vacuum impregnating the porous carbon-containing blank obtained above in an organic solution of phenolic resin after debonding; 3) After the impregnation is complete, the excess phenolic solution on the surface of the carbon-containing porous biscuit containing the phenolic resin solution is blotted dry with filter paper; 4) After the excess phenolic solution on the surface of the green body is blotted dry, it is placed in a drying oven to dry to cure the phenolic resin; 5) heating up the dried green body under vacuum conditions to crack the phenolic formaldehyde into amorphous carbon; 6) The obtained carbon-containing billet is placed on a silicon wafer, and siliconized under vacuum conditions, and the reaction temperature is between 1420-1550°C. 2. by the described method of claim 1, it is characterized in that: a) The C/SiC ratio in step 1) is 1:10-10:1; b) The phenolic resin described in step 2) includes one of phenol formaldehyde resin, polyvinyl acetal modified phenolic resin, polyamide modified phenolic resin, epoxy modified phenolic resin and silicone modified resin; c) step 2) in the phenolic resin solution, the solvent is any one of ethanol, Virahol, n-butanol, n-octanol, acetone, methyl ethyl ketone, toluene, xylene, n-hexane, hexanaphthene organic solvent ; d) The concentration of the organic solution of the phenolic resin in step 2) is 5wt%-70wt%; e) The vacuum degree of vacuum impregnation in step 2) is lower than 0.1MPa; the vacuum pumping time is 2-30 minutes; f) The drying temperature in step 4) is between 50°C and 170°C, and the drying time is 2-72 hours; g) step 5) the phenolic pyrolysis temperature is 700°C-1200°C, the heating rate is less than 10°C/min; the pyrolysis holding time is 10-300 minutes; h) The reactive siliconizing described in step 6) is completed in a carbon tube furnace with a vacuum degree of 1-20Pa. 3. by the described method of claim 1 or 2, it is characterized in that: a) The mass ratio of C/SiC in step 1) is between (1.4-7): 1; b) the concentration of the organic solution of phenolic resin in step 2) is 10wt%-60wt%; c) The drying temperature in step 4) is 70-160°C; the drying time is 2-24 hours; d) The cracking temperature in step 5) is 800-1000°C; e) The heat preservation time for cracking in step 5) is 10-120 minutes. 4. The method according to claim 1, characterized in that the process described in step 2) to step 5) is a cycle, which can be carried out once or repeatedly.

Images