Effect of La2O3 adding on Properties of SiC Porous Ceramic Support

In this paper, 80-90 grains of SiC as the main raw material, doped La2O3 into the binder composed of potassium feldspar, kaolinite and quartz, and then graphite and activated carbon as pore-making agents to prepare silicon carbide porous ceramic carrier. The effects of doped La2O3 content on the bending strength, porosity and filtration pressure drop of porous SiC ceramics were studied. The results show that the formation of mullite phase and the bending strength of SiC porous ceramics can be improved by adding proper amount of La2O3. Under the sintering temperature of 1300°C for 90 minutes, when the content of La2O3 in the binder is 2.5wt%, the comprehensive performance is the best, the porosity is 21.5%, the bending strength is 53.5MPa, and the filtration pressure drop is low.


Introduction
SiC porous ceramics have low thermal expansion coefficient, low volume density, high bending strength, uniform distribution of micropores and pores, high porosity, and good thermal stability and chemical stability.3][4][5] Mullite has high hardness, low thermal conductivity, good impact resistance, good temperature stability, good creep resistance and other excellent properties, and is often selected as a preparation material for precision ceramics. [6]The reason why mullite can be used to prepare porous silicon carbide ceramics is that its thermal expansion is similar to silicon carbide and its chemical compatibility is good.These characteristics ensure that mullite can combine with silicon carbide to produce porous ceramics with excellent high temperature strength and shock resistance [7][8] .The formation of mullite is related not only to material processing but also to the addition of other compounds.It has been confirmed that self-reinforcing bubble mullite has been obtained by adding AlF 3 , V 2 O 5 , WO 3 and some rare earth oxides.Rare earth oxides have a positive effect on mullite formation [9][10][11][12] .Relevant literature shows that the addition of Y 2 O 3 enhances the formation of neck (the binding part between SiC particles) and achieves a high strength of 27.5MPa, the open porosity is 44.4% [13] .The density of porous silicon carbide ceramics can be effectively improved by adding high energy grinding fine additives, mixing Sc 2 O 3 and Ho 2 O 3 [14]   .With the addition of 2.0wt% Ce 2 O 3 , the porosity reaches 42.1% and the bending strength is 35.3MPa [15] .However, there are few reports on the effects of La 2 O 3 on on SiC porous ceramics.In this study, the effect of La 2 O 3 on the formation performance of SiC porous ceramics was studied.It is found that the addition of La 2 O 3 can promote the formation of mullite, and there will be a long columnar interlocking network structure, which greatly improves the densification performance and mechanical strength of porous ceramics.Therefore, the mechanical properties of the prepared SiC porous ceramics are improved accordingly.
In this study, 80-90 mesh silicon carbide are used as the main raw material.Sodium carboxymethyl cellulose as the temporary binder, potassium feldspar, kaolin, quartz and doped lanthanum oxide as the binder, and graphite and activated carbon as pore-making agents.The effects of La 2 O 3 doping on the microstructure, phase composition, bending strength and porosity of porous silicon carbide ceramics were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), bending strength, filtration pressure drop and porosity.

Preparation of raw materials and samples
Using 80-90 mesh SiC particles as the main material, CMC solvent as the temporary binder, activated carbon and graphite as pore-making agents, doping different content of La 2 O 3 into the binder prepared by kaolin, potassium feldspar and quartz (Doping content is shown in Table 1) to prepare silicon carbide porous ceramics samples.(The content of each substance is shown in Table 2) Table 1.Ratio series of doped lanthanum oxide (%wt)

Table 2. The content of various substances(g)
The samples were prepared by layer-by-layer coating method.SiC particles, CMC temporary binder, binder and pore-making agent (composed of graphite and activated carbon with mass ratio of 1:1) were successively added into beakers in small amounts and multiple times according to the mass ratio of 40:4:5:5, and stirred as they were added.Weigh 15g spare material and place it in the tablet die of the tablet press, and press it under 10 MPa to make a sheet model.Weigh 3 g spare material and put it in the die of the tablet press, and press it under 4MPa to make a strip model.Finally, the pressed support body was put into Muffle furnace for sintering at a heating rate of 2 ℃/min.First, it was heated to 900℃ and then kept for 30min.Then, it was heated to 1300℃ and then kept for 1.5 h.In the experiment, sheet samples were used to test the filtration pressure drop and porosity, and strip samples were used to characterize the bending strength, SEM morphology and XRD phase.

Characterization of samples
In this experiment, S4800/TM3000 scanning electron microscope was used to observe the microstructure of the sample.The diffraction pattern was obtained by X-ray diffractometer.The flexural strength of the samples was determined by YLN electronic press and three-point method.The porosity was determined by ultrasonic cleaning tank method, Archimedes drainage method and electronic balance method.The filter pressure drop of the sample was measured by differential pressure meter, glass flowmeter and circulating water multipurpose vacuum pump.In conclusion, the presence of La 2 O 3 promotes the combination of alumina and silica to promote the secondary formation of mullite, and La 2 O 3 has a good effect on the densification behavior of porous ceramics, improving the densification degree and strength.As can be seen from figure 2, when La 2 O 3 is not doped, the pore size formed by the accumulation of SiC particles is larger and the surface of particles is uneven.With the addition of La 2 O 3 in the support body, the gap becomes uniform, which is mainly because the mullite phase increases after La 2 O 3 doping.At the same time, the generated LAS liquid liquid fluidity enhancement covers the surface of SiC particles, fills the pores, makes the particles bond more closely, and the surface of silicon carbide particles is uniformly covered by the binder.However, with the increase of lanthanum oxide doping content and the decrease of mullite content, stable lanthanum silicate was generated and blocked the pore.Filtration pressure drop is a physical quantity of the flow rate of the reaction gas.The higher the filtration pressure drop is, the stronger the obstruction effect on the air is.As can be seen from Figure 3, after La 2 O 3 is added to the support body, the filtration pressure drop of the support body decreases and then increases.This situation is mainly due to the increase of mullite phase proportion and glass phase proportion in the sample after La 2 O 3 is added.The glass phase helps the binder to be tightly coated on the surface of silicon carbide particles after melting, and does not plug the pores between silicon carbide particles and poremaking agent after sintering, so as to reduce the filtration pressure drop of the support body.When the content of La 2 O 3 is more than a certain amount, the content of mullite phase and glass phase is reduced, and the formation of new lanthanum silicate material to plug the pores to increase the filtration pressure drop.

The bending strength and porosity of ceramic samples mixed with different La 2 O 3 content in binder
Figure 4.The bending strength and porosity of samples mixed with different La 2 O 3 content in binder Porosity is a measure of the porosity and compactness of porous ceramics and a characterization of the compactness of porous ceramic materials.It is expressed as a percentage of pore volume per unit volume of ceramic material.As can be seen from Figure 4, the porosity of the support doped with 2.5wt%La 2 O 3 is higher than that of the support without La 2 O 3 , which increases from 17.8% to 21.5%.This is because the increase in the number of glass phases leads to the melting of the binder after sintering at high temperature, which can completely cover and wrap the surface of SiC particles, and the porosity increases.In addition, the bending strength of La 2 O 3 doped support body is obviously improved.The bending strength can reach 51.5MPa which is mainly due to the increase of mullite phase in the sintering process of the support body, forming a long and columnar interlocking network structure, which enhances the densification performance of porous ceramics, and thus improves the bending strength.When the doping content is too high, the bending strength will be reduced because of the reduction of mullite phase, and the porosity will be reduced because of the reduction of glass phase.

Conclusion
The doping of La 2 O 3 significantly improved the formation, microstructure and bending strength of mullite phase.When the content of the doped La 2 O 3 is 2.5wt%, the flexural strength of the SiC porous ceramic support body can reach 53.5MPa, the porosity is 21.5%, the filter pressure is low, and the comprehensive performance is the best.

Figure 1 .
Figure 1.XRD analysis of ceramic samples mixed with different La 2 O 3 content in binder As can be seen from Figure 1, the initial crystal phase of the samples without La 2 O 3 doping is silicon carbide.Quartz peak and mullite peak are found in porous SiC ceramic samples doped with La 2 O 3 .The diffraction of mullite phase increases significantly, which means that the addition of La 2 O 3 also accelerates the polycrystallization of alumina and silica mixtures.Moreover, the addition of La 2 O 3 will increase the glass phase, showing a strong tendency of glass transition.It has been reported by Kong et al that La 2 O 3 can promote the formation of cristobalite marble phase and can promote secondary mullite fossilization .It should be noted that even when 2.5wt%La 2 O 3 is incorporated into the sample, no La 2 O 3 or compounds associated with La 2 O 3 were observed in the XRD pattern.Here, La 2 O 3 is concentrated in the silica-rich glass phase of the La 2 O 3 -Al 2 O 3 -SiO 2 ternary system.The formation of the glass phase consumes silica, leaving residual alumina in the sample.It reduces the viscosity of the glass and forms a new liquid phase, known as (La, Al, Si) x O y (LAS liquid).The LAS liquid encourages the secondary formation of mullite.As the La 2 O 3 doping content increases, it consumes the silicon phase to form lanthanum silicate, leaving unreacted alumina, prompting the reduction of the mullite phase.Other stable compounds such as La 14 Si 9 O 39 and LaAl 11 O 8 have also been reported to be stable in the La 2 O 3 -Al 2 O 3 -SiO 2 ternary system.When La 2 O 3 is doped, the secondary mullite formation equation can be expressed as: La 2 O 3 +Al 2 O 3 +SiO 2 →LAS eutectic melt Al 2 O 3 +SiO 2 +LAS eutectic melt→3Al 2 O 3• 2SiO 2 grain boundary liquid phase

Figure 2 .
Figure 2. SEM of ceramic samples mixed with different La 2 O 3 content in binderAs can be seen from figure2, when La 2 O 3 is not doped, the pore size formed by the accumulation of SiC particles is larger and the surface of particles is uneven.With the addition of La 2 O 3 in the support body, the gap becomes uniform, which is mainly because the mullite phase increases after La 2 O 3 doping.At the same time, the generated LAS liquid liquid fluidity enhancement covers the surface of SiC particles, fills the pores, makes the particles bond more closely, and the surface of silicon carbide particles is uniformly covered by the binder.However, with the increase of lanthanum oxide doping content and the decrease of mullite content, stable lanthanum silicate was generated and blocked the pore.

Figure 3 .(
Figure 3. Filtration pressure drop of samples with different La 2 O 3 content in binder