Effect of Binder Content on Properties of Cordierite-Mullite Porous Ceramics

In this paper, 70-80 mesh SiC particles were used as the base, and 2.5%MgO was added on the basis of quartz, kaolin and potassium feldspar as the binder. Meanwhile, graphite and activated carbon were added as pore-making agents for SiC porous ceramic supports. At the same time, the amount of binder added was changed under the condition of keeping SiC: pore-making agent 8:1 unchanged. The effects of binder content on flexural strength, filtration pressure drop and porosity of porous SiC ceramics were investigated. The results indicated that the comprehensive properties of SiC porous ceramics can be improved by adding proper amount of MgO. When the content of MgO in the binder was controlled at 2.5% and the amount of binder was 6%, the comprehensive performance of the sample was the best under the conditions of 1300 °C, holding time for 90 minutes and forming pressure of 4/10MPa, respectively. In this case, the flexural strength was 48mpa, the porosity was 25% and the filtration pressure drop was good.


Introduction
SiC porous ceramics have the characteristics of high hardness, high strength, corrosion resistance, good high temperature thermal stability, uniform pore distribution and low volume density.With these characteristics, it has become one of the best materials for high-temperature filter materials, and has great application prospects in high-temperature gas purification and catalyst carrier.However, due to the existence of covalent bonds in SiC itself, So the sintering of SiC ceramics requires a temperature of about 2000°C [1][2][3][4][5][6].Therefore, it is often used to add oxides with good performance and low sintering temperature to form a new phase with the SiO2 generated by the oxidation of SiC, so as to achieve the preparation of porous SiC ceramics at lower temperature [7].
Aluminosilicate-based ceramics had become the research focus of refractory materials because of their high temperature properties [7][8].In the process of sintering at high temperature, the addition of MgO increased the oxygen vacancy in Al2O3 and promoted the formation of liquid phase, which was conducive to physical transfer, thus promoting the formation of mullitite phase and cordierite phase, promoting the discharge of gas inside the ceramic, and improving the overall densification degree of the sample [9][10][11][12][13].Cordierite has poor mechanical properties and was not easy to prepare by solid phase sintering, mullite had good mechanical properties, and cordierite and mullite had good thermal expansion matching and good chemical compatibility with silicon carbide [14].Therefore, the preparation of mullite-cordierite composites may improve their mechanical and thermal properties [15].
In this paper, 70-80 mesh SiC particles were used as the base, sodium carboxymethyl cellulose(CMC) was used as the temporary binder, and 2.5%MgO was added into the binder composed of potassium feldspar, kaolin and quartz.At the same time, graphite and activated carbon were added as pore-making agents for SiC porous ceramic supports.The effects of binder ratio on flexural strength, porosity and filtration pressure drop of SiC porous ceramics were studied by XRD and SEM.

Effects of different binder contents on Cordierite-Mullite porous ceramics
The new binder was prepared by adding 2.5wt%MgO on the premise that 70-80 mesh SiC particles were used as the base and the binder components (kali-feldspar 64.53wt%, quartz 23.27wt%, kaolin 12.2wt%) remained unchanged (as shown in Table 1).Using layer-by-layer coating method.70-80mesh SiC particles, sodium carboxy methyl cellulose, binder and pore-making agent were mixed and stirred together successively (the dosage of each substance is shown in Table 2).Samples were prepared under pressure of 4/10MPa, respectively.Finally, the sintering furnace was heated to 1300°C at the rate of 2°C/min and held for 90min (as shown in Figure 1).

Sample Characterization
The microstructure of the sample was observed by scanning electron microscope and the diffraction pattern of the sample was obtained by X-ray diffractometer.YLN-electronic press was used to test the flexural strength of the sample.The porosity of the sample was obtained by electronic balance and Archimedean drainage method.The filtration pressure drop of the samples was measured by the multi-purpose circulating water vacuum pump, the testo-510 differential pressure meter and the glass flow meter. 2 can be seen that a small amount of cordierite phase and mullite phase appeared in the process of changing the amount of binder added, among which 6% binder showed obvious cordierite peak and mullite peak.With the increase of binder content, the peak strength of mullite phase increased slightly, while the cordierite phase gradually disappeared, and the reduction of cordierite promoted the formation of mullite.However, the MgO promoted more low-viscosity liquid phase to participate in the reaction, and a small amount of magnesium-aluminum spinel phase appeared [10], leading to the gradual reduction of mullite phase.3 can be seen that as the binder content increases, the pore-forming agent content decreases [16], part of the pore space was occupied, and the number of small pores also increases gradually.With the increase of binder content, the molten liquid phase increased, the particle surface gradually became smooth, and the whole SiC ceramics gradually became dense.With the further reduction of pore-making agent, the liquid phase gradually occupied most of the pore space [17], and the density was further improved.Meanwhile, a large number of tiny pore appear at the junction of the neck., which further reduced the overall strength of SiC ceramics.4 can be seen that with the increase of binder content, the overall porosity showed a trend of gradual decline, gradually decreasing from 31.49% to 9.85%.Among them, the change of binder content in the stage of 4%-8% was drastic, which may be due to the increase of liquid phase, occupying part of the pore space, leading to the decline of porosity.At the same time, it can be seen that the flexural strength changed with the increased of binder content.The addition of MgO promoted the presence of more liquid phase in the sample, and the diffusion of liquid phase made the particles rearrange, crystallize and grow.When the binder was 6%, the peak strength of cordierite reached 48.52MPa, and a small amount of mullite peak appeared, showing a good flexural strength [18].Figure 5 can be seen that adding 4% binder had the lowest filtration pressure drop.With the increase of binder content, the filtration pressure drop gradually increased, which was consistent with the previous trend of porosity.There were more pore-making agents in 4% binder, and more pores were formed.When a small amount of binder was added, the magnesium-aluminum spinel formed will bring partial volume expansion due to the addition of too much MgO, thus occupying part of the pore position, resulting in a large increase in filtration pressure drop.

Conclusion
When the content of binder was 6% and the proportion of MgO in binder was 2.5%, the samples with the best comprehensive performance can be obtained by holding them at 1300°C for 90minutes.At this time, the flexural strength was 48.52MPa, the porosity was 25.08%, and the filtration pressure drop was good.

Figure 2 .
Figure 2. Doped with different binder content of SiC porous ceramics samples by XRD Figure.2 can be seen that a small amount of cordierite phase and mullite phase appeared in the process of changing the amount of binder added, among which 6% binder showed obvious cordierite peak and mullite peak.With the increase of binder content, the peak strength of mullite phase increased slightly, while the cordierite phase gradually disappeared, and the reduction of cordierite promoted the formation of mullite.However, the MgO promoted more low-viscosity liquid phase to participate in the reaction, and a small amount of magnesium-aluminum spinel phase appeared[10], leading to the gradual reduction of mullite phase.

Figure 3 .
Figure 3. Doped with different binder contents of porous SiC ceramic samples by SEM

Figure 4 .
Figure 4. Porosity and flexural strength of SiC porous ceramics doped with different contentsof binder.Figure4can be seen that with the increase of binder content, the overall porosity showed a trend of gradual decline, gradually decreasing from 31.49% to 9.85%.Among them, the change of binder content in the stage of 4%-8% was drastic, which may be due to the increase of liquid phase, occupying part of the pore space, leading to the decline of porosity.At the same time, it can be seen that the flexural strength changed with the increased of binder content.The addition of MgO promoted the presence of more liquid phase in the sample, and the diffusion of liquid phase made the particles rearrange, crystallize and grow.When the binder was 6%, the peak strength of cordierite reached 48.52MPa, and a small amount of mullite peak appeared, showing a good flexural strength[18].

Figure
Figure 4. Porosity and flexural strength of SiC porous ceramics doped with different contentsof binder.Figure4can be seen that with the increase of binder content, the overall porosity showed a trend of gradual decline, gradually decreasing from 31.49% to 9.85%.Among them, the change of binder content in the stage of 4%-8% was drastic, which may be due to the increase of liquid phase, occupying part of the pore space, leading to the decline of porosity.At the same time, it can be seen that the flexural strength changed with the increased of binder content.The addition of MgO promoted the presence of more liquid phase in the sample, and the diffusion of liquid phase made the particles rearrange, crystallize and grow.When the binder was 6%, the peak strength of cordierite reached 48.52MPa, and a small amount of mullite peak appeared, showing a good flexural strength[18].

Figure 5 .
Figure 5.The filtration pressure drop of samples doped with different binder contents.
Table1.Binder and Magnesium Oxide Content in sample (wt%) Table2.Experimental Dosage of each substance (g)