The anti water vapor oxidation performance of SiO2 combined with SiC bricks

SiC material has high thermal conductivity and is an ideal material for hanging bricks on the inner wall of garbage incinerators [1-4]. However, during use, the material may expand and peel off due to water vapor oxidation. Si3N4 combined with SiC hanging bricks has good volume stability against water vapor oxidation [5], but its manufacturing process is complex and costly. Calcium aluminate cement combined with SiC castable has a low price and easy construction, but the presence of Ca and Al elements makes its volume stability poor after water vapor oxidation [6].

The use of SiO2 combined with SiC bricks should be a compromise choice. In this work, SiO2 bonded SiC bricks were first self-made, and their resistance to water vapor oxidation was tested at 1000 ℃ according to ASTM C863-2000.

1 .  2 antioxidant test

Referring to ASTMC863-2000, cut the fired brick sample into strip shaped specimens of 40mm × 40mm × 75mm, and place them in a controlled atmosphere furnace. The steam flow rate is 32kg · m-3 · h-1, and the temperature is raised to 1000 ℃ at a rate of 200 ℃ · h-1. The specimens are kept at room temperature for 50, 100, 150, 200, 250, and 300 hours, respectively.

Observe the appearance changes of the oxidized sample. Detect the apparent porosity of the oxidized sample. Measure the mass and volume of the sample before and after oxidation, and calculate its mass change rate and volume change rate. Analyze the phase composition and microstructure of the samples before and after oxidation using XRD and SEM. Perform chemical analysis on the samples before oxidation and after 300 hours of oxidation.

2 .  Volume change rate and mass change rate

The volume change rate and mass change rate of the sample after different oxidation times are shown in Figure 2. It can be seen that with the extension of oxidation time, the volume change rate and mass change rate of the sample show a trend of first increasing and then decreasing, with values at 200 and 250 hours, respectively. The molar masses of SiC and SiO2 are 40.1 and 60.08 g, respectively, and the molar volumes are approximately 12.45 and 27.31 cm3, respectively. Therefore, after 1mol of SiC is oxidized to 1mol of SiO2, its mass change rate reaches 49.83%, and its volume change rate reaches 119.36%, with significant increases in both mass and volume. After more than 200 hours, the volume begins to decrease, possibly due to a decrease in oxidation rate. The volume shrinkage caused by sintering densification of the sample is greater than the volume expansion caused by SiC oxidation.

2 .  4 pore size distribution

The pore size distribution of the sample before and after 300 hours of oxidation is shown in Figure 4. It can be seen that the percentage of pores with a pore size<1000nm in the samples before and after oxidation is about 36%, and the d50 is about 2000nm; After 300 hours of oxidation, there were more pores with a pore size<200nm in the sample than before oxidation, while there were fewer pores with a pore size of 200-10000nm in the sample than before oxidation, indicating that the pore size of the sample decreased after oxidation. This is because the volume expansion and compression generated by SiC oxidation fill the pores of the sample.

2 .  5 Phase Composition

The XRD patterns of the samples before and after oxidation at different times are shown in Figure 5, and the semi quantitative analysis results of different crystal forms of SiO2 are shown in Table 1. It can be seen that with the extension of oxidation time, the crystalline SiO2 shows an increasing trend. This is because SiC oxidizes to form SiO2.

According to chemical analysis, the chemical composition (w) of the sample before oxidation is: SiC83.45%, SiO215.03%. After 300 hours of oxidation, the chemical composition (w) of the sample is: SiC78.61%, SiO217.60%. The decrease in SiC and the increase in SiO2 after oxidation do not correspond significantly, which may be related to the loss of some SiO2 converted to SiO (g) in the later stage of oxidation.

2 .  6 Microstructure

The microstructure photos of the sample cross-section after different oxidation times are shown in Figure 6. It can be seen that after 50 hours of oxidation, the particles in the sample are tightly bound to the matrix, and the sample structure is basically intact without obvious cracks. After 200 hours of oxidation, there were obvious gaps between the particles and the matrix in the sample, and even separation occurred. After 300 hours of oxidation, the surface of SiC particles in the sample was covered by an oxide film, and there were cracks on the oxide film. The mismatch of thermal expansion coefficients between crystalline SiO2 and SiC, as well as the phase transition between crystalline SiO2, can lead to the rupture and detachment of the SiO2 oxide film on the surface of SiC. The key to improving the water vapor oxidation resistance of SiO2 bonded SiC materials is to allow more SiO2 to enter the glass phase.

3    结论

(1) The bulk density, apparent porosity, and SiC content of self-made SiO2 bonded SiC bricks are comparable to existing SiC castables, while the flexural strength and compressive strength are significantly higher than those of SiC castables.

(2) After 100 hours of oxidation, cracks began to appear on the surface of the sample; After 200 hours of oxidation, cracks increased and widened.

(3) As the oxidation time increases to 200 hours, the rate of volume increase of the sample gradually increases; But after oxidation for 250 and 300 hours, it gradually decreased again, which is caused by sintering shrinkage exceeding oxidation expansion.

(4) As the oxidation time increases to 250 hours, the rate of mass increase of the sample gradually increases; But after 300 hours of oxidation, it decreased again, which may be related to the loss of some SiO2 converted to SiO (g) in the later stage of oxidation.

(5) The variation trend of the apparent porosity of the sample with oxidation time is opposite to that of the mass change rate.

(6) As the oxidation time prolongs, the amount of crystalline SiO2 in the oxidized sample increases; The mismatch of thermal expansion coefficients between crystalline SiO2 and SiC, as well as the phase transition between crystalline SiO2, can lead to the rupture and detachment of the SiO2 oxide film on the surface of SiC. The key to improving the water vapor oxidation resistance of SiO2 bonded SiC materials is to allow more SiO2 to enter the glass phase.