Measured with vernier caliper, the average height of 12 sample is 42. 3 mm, with an average diameter of 18. 1 mm, and the average density is 1. 67 g/cm3. The difference between samples with different proportions is very small and almost negligible.
Measure and record the height and diameter of the molded samples before and after firing with vernier caliper, and calculate the longitudinal and transverse shrinkage of the fired samples, as shown in Table 6. 8.
Generally speaking, the shrinkage of sintered samples increases with the increase of temperature, and the longitudinal shrinkage is obviously higher than the transverse shrinkage. With the extension of constant temperature time, the shrinkage rate of the sample tends to increase. The shrinkage of A series samples is higher than that of B series and C series, and there is little difference between B series and C series, which shows that the shrinkage of synthetic samples is increased because of the high impurity content of fly ash without pickling and the high liquid phase content produced during sintering. The transverse shrinkage of B and C series is close to 6% of that of industrial cordierite.
The shrinkage of the sample is mainly caused by the melting of fly ash particles and hollow microspheres in fly ash at high temperature. After fine grinding for 5 h, a large number of microspheres can still be found in the sample, and their diameters are below several microns. Even if the fine grinding time is increased, it will not help, and the formation and recrystallization of cordierite can only partially offset this effect. Therefore, it is impractical to directly prepare sintered cordierite products with fly ash, because the shrinkage rate in the production process is so great that it is difficult to control the appearance size of sintered products to meet the design requirements. When using fly ash as raw material to prepare refractories or ceramic products, only the sintered fly ash can be used as their raw material, and the shrinkage will be greatly reduced after secondary sintering. Therefore, cordierite synthesized from high alumina fly ash is only used as the raw material of refractory or ceramic materials, and can not be directly made into refractory or ceramic products, which should be paid enough attention to.
(2) Physical properties of synthetic samples
The physical properties of synthetic cordierite samples were measured by the drainage displacement method (Archimedes method), as shown in Table 6. 9.
Table 6. 8 Shrinkage of sintered cordierite samples
Table 6. Physical properties of sintered cordierite samples
As can be seen from Table 6. The water absorption and apparent porosity of 9, A series samples are obviously lower than those of B and C series samples, and the apparent bulk density of A series samples is also slightly lower than that of B and C series samples. The water absorption of cordierite samples synthesized from A→B→C is 0. 32%→8.26% →6. The apparent porosity is 0. 55%. 66% →19.74 %→16.11%,and the average density changed. The water absorption and apparent porosity of series B samples are relatively high, which is related to the fact that the content of acid-washed fly ash in the sample ingredients is slightly higher than that in series C.
From the point of density change, C series samples are the highest. In the same series of samples, with the increase of temperature and the extension of constant temperature time, the water absorption and apparent porosity of the synthesized samples tend to decrease, with a large decline; Although the sample density has a downward trend, the decrease is small, which may be related to the increase of closed pores caused by the growth of cordierite crystals in sintered samples. Constant temperature time has little effect on sample density. The theoretical density of cordierite is 2. 48 g/cm3, the density of natural cordierite can reach 2. 53 ~ 2.78 g/cm3, while the density of industrial cordierite is generally 2. 35g/cm3. The density of cordierite samples obtained in this experiment is equivalent to that of cordierite samples synthesized from natural materials by Goren et al. (2006) (1350℃ × 1 h is 2. 32 g/cm3, and 1400 × 1 h is 2. 47 g/cm3).
(3) Mechanical properties of the synthesized samples
The two ends of the sintered cylindrical specimen were cut into planes, and the uniaxial compression failure test of the specimen was carried out with RMT- 150B multifunctional rock mechanics testing machine, and the stress-strain curve of the specimen was obtained, and the uniaxial compressive strength parameters of the sintered specimen were obtained. Figure 6. 13 uniaxial compressive strength test results of some samples.
Figure 6. Uniaxial compressive strength of cordierite samples synthesized by 13
The measured uniaxial compressive strength is very discrete, ranging from 60 to 284 MPa, and the average compressive strength is 139 MPa. The order of compressive strength from small to large is A 1→B 1→C2→B4→A2 (Table 6. 10), and their changes with sintering temperature and constant temperature time are not obvious. Individual samples are equivalent to cordierite synthesized by Kobayashi et al. (2000) by sintering ultrafine kaolinite and magnesium hydroxide at 1350℃ × 1 h, and the fracture pressure is 175 MPa.
Table 6. Uniaxial compressive strength of 10 sintered cordierite samples
(4) Phase analysis of synthetic samples.
The D8 ADVANCE X-ray diffractometer produced by BrukerAX Company in Germany was used to analyze the phase of sintered cordierite samples, and the mineral types and content of sintered samples and the number of glass phases in the samples were obtained, which was helpful to optimize the experimental parameters. Polycrystalline diffraction spectra of different phases are always different in the number of diffraction peaks, the position and intensity of 2θ, and have phase characteristics. The diffraction spectrum of multiphase mixture is the weight superposition of polycrystalline diffraction spectra of each phase, so the diffraction spectrum of mixture can match the standard diffraction spectrum of various single phases, and each phase of mixture can be distinguished.
From the XRD curve (fig. 6. 14), it can be seen that the phase composition of A series samples is mainly cordierite, but also contains a very small amount of anorthite and spinel. The XRD baseline is horizontal, indicating that there is almost no glass phase. In other words, the minerals in the sample are relatively simple, and almost all of them are composed of cordierite minerals.
Figure 6. XRD patterns of cordierite samples of 14a series at different sintering temperatures: α-anorthite; S- spinel
According to the sintering temperature and lattice spacing d value (JCPDS card:, cordierite is all α-cordierite, that is, Indian stone. Figure 6. 14 shows that the phase composition of cordierite samples sintered at 1350℃ and 1370℃ is the same, but the peak strength of cordierite sintered at 1370℃ is obviously enhanced, and the increase of constant temperature time from 2 h to 3 h has little effect on the synthesis of cordierite. In order to further illustrate the phase composition characteristics of the sample, the XRD curve of a single sample is shown in Figure 6. 15 to reveal the exact diffraction peak position of the phase.
The XRD curve of C series samples is shown in Figure 6. 16, which shows that the main crystalline phase is cordierite, and there are very few sub-crystalline mullite and spinel. Compared with series A, anorthite disappears and a few mullite crystals appear. The crystallization strength of spinel decreased obviously. In C series, the diffraction peak intensity of cordierite sintered at 1350℃ and 1370℃ seems to have no obvious change, and the constant temperature time has little effect on it. Figure 6. 17 shows the detailed XRD curve of a single sample.
Comparing the XRD analysis results of A series and C series, it can be seen that although the main crystal phase in the two series samples is cordierite and the glass phase content is almost zero (the XRD baseline is horizontal), there are differences in the secondary crystal phase. The appearance of A series anorthite is related to the high CaO content in the original fly ash (4. 22%). Although the addition of talcum powder makes the relative content of CaO in the ingredients from 4. 22% to 2. It is still higher than that of fly ash treated with 20% hydrochloric acid. After hydrochloric acid treatment, the content of calcium oxide in fly ash is 0. Its relative content is reduced to zero. Therefore, there is no anorthite in C series sintered samples. Sub-crystalline mullite in C series was not found in A series.
Figure 6. XRD patterns of A4 samples sintered at 15 1370 ℃× 3 h
Figure 6. XRD patterns of cordierite samples of 16c series at different sintering temperatures: m- mullite; S- spinel
Figure 6. XRD spectrum of C2 samples sintered at 17 1350 ℃× 3 h
Comparing A and C series samples, it can also be found that the diffraction peak intensity of C series spinel (MgO Al2O3) is obviously lower than that of A series samples, which shows that the purity of ingredients has an important influence on the purity of synthesized cordierite samples.
The existence of CaO is very important to the phase composition of cordierite raw materials synthesized by sintering. Although the research of Sundar et al. (1993) points out that calcium ions can replace magnesium ions to make the content of calcium oxide in cordierite reach 4. 73%, that is, x can reach 0. 5 in Mg2-XCaxal4Si5O 18 system. Sundar synthesized cordierite by sol-gel method, and obtained single crystal cordierite with X as high as 0. 5. It is proved that the substitution of calcium ions can greatly reduce the anisotropy of thermal expansion of cordierite, which is related to the method of synthesizing cordierite, because the raw materials synthesized by sol-gel method are purer and the particles are finer and more uniform. When Chen (2008) sintered cordierite glass-ceramics with 3% CaO instead of MgO, only cordierite phase appeared; When the substitution amount is 5%, the main crystalline phase cordierite and the secondary crystalline phase anorthite appear, and the cordierite ceramics prepared at this time have the best density. When 10% is substituted, the XRD intensity of cordierite decreases obviously, while that of anorthite increases obviously.
Feldspar is an end-member component of plagioclase, belonging to triclinic system, which can be subdivided into high-temperature body-centered feldspar (I- feldspar, 70% ~ 90% An component) and low-temperature original feldspar (P- feldspar, 90% ~ 100% An component), and the transition temperature between them is 200 ~ 3000. According to the composition-temperature diagram of AB-AN series (Figure 6. 18), it can be judged that the anorthite in sintered cordierite belongs to the body-centered anorthite and is a secondary mineral formed in the sintering process of ingredients. In figure 6. 18, Pe, вФ and Hu respectively represent biotite assemblage zone, Wokilde assemblage zone and Hutternrochelle assemblage zone.
Figure 6. Composition-temperature diagram of 18ab-an series (according to wang pu et al., 1984).
Figure 6. 19 shows Another phase diagram of albite (AB)- anorthite (an) at the temperature of11600℃, showing the phase states of different percentage combinations of AB-An plagioclase at different temperatures. The Na2O content in the batch is only 0. The content of CaO is 2. 07%. 84%. After sintering at 1350 ~ 1370℃, almost all of them are crystalline phase, which is consistent with the all-solid phase region shown in AB-An system high temperature phase diagram.
In the CaO-SiO _ 2-Al _ 2O _ 3 phase diagram (Figure 6. 20), anorthite is basically in the center of ternary composition diagram, and with the increase of Cao content, anorthite may appear. Although the research of Sundar et al. (1993) points out that calcium ions can replace magnesium ions to make the content of calcium oxide in cordierite reach 4. 73%, which may be the limit value of calcium ion replacing magnesium ion, and the corresponding conversion conditions are needed. The content of calcium oxide in this experiment is 2. The formation of anorthite in sintered samples shows that the number of calcium ions replacing magnesium ions is limited.
Mullite is an orthorhombic crystal system, and its crystals are in the form of needles extending parallel to the C axis or columns with quadrangular cross sections. The original content of mullite in high alumina fly ash is as high as 35. However, mullite crystal phase was not found in a series of sintered samples, indicating that the addition of MgO destroyed the existing mullite in the batch.
Figure 6. High temperature phase diagram of 19 AB-An system.
Figure 6. 20 CaO-SiO2-Al2O3 phase diagram (quoted from Mollah et al., 1999)
According to the research results of Lin et al. (1989), the sample contains 1. 5% MgO holding at 1500℃ for 2 ~ 10 h will not affect the structure of mullite, but when MgO is increased to 2% and holding time is prolonged, the amount of mullite will decrease. When 18. Adding 6% MgO, mullite is completely decomposed. The existence of CaO is also a factor to reduce the amount of mullite. Add 1. CaO in 12% sample can decompose 10%, but when 1 1 is added. 5% CaO and mullite are completely decomposed. It can be seen that these two factors promote the decomposition of mullite phase in the batch at high temperature, and then the chemical components in the batch are gradually transformed into cordierite crystal phase under the action of MgO.
There is a small amount of mullite phase in C series, which may come from two aspects: one is the residue of mullite phase in the original fly ash in the ingredients; The second is mullite related to the formation of cordierite. In order to distinguish the sources of these two kinds of mullite in detail, it is necessary to determine the lattice constants of mullite, that is, the values of A, B and C. The lattice constants of mullite change with the change of Al2O3 content in mullite, that is, with the increase of Al2O3, the value of A increases linearly, the value of C increases slightly, and the value of B decreases (Figure 6. 2 1).
Figure 6. Variation of lattice constant of 2 1 mullite with Al2O3 content (according to Fischer et al., 2005)
Characteristics of high alumina fly ash and its application in the synthesis of mullite and cordierite
Gomse et al. (2000) studied the fly ash of a thermal power plant in eastern France by XRD and NMR, and obtained that the chemical formula of mullite in fly ash is Al4. 70Si 1。 30O9。 65 (corresponding to x = 0. 35, Al2O3 content is 75. 5%), in which the Al2O3 content is slightly higher than the classical mullite chemical formula Al4. 5Si 1。 5O9。 75 (corresponding to x = 0. 25, Al2O3 content is 7 1. It is between 3∶2 sintered mullite and 2∶ 1. 5 fused mullite. The instantaneous cooling during the formation of fly ash makes mullite not fully crystallized and homogenized, which leads to the different structure and composition of mullite. If the content and lattice constant of Al2O3 in mullite are determined, the source of mullite in synthesized cordierite samples can be distinguished. In this experiment, the content of mullite in C series samples is very small, so it cannot be further studied.
Spinel (MgO·al2o 3) is also an associated phase in the process of cordierite synthesis, with a small overall content, and the content in C series is slightly lower than that in A series. Spinel belongs to the equiaxed crystal system, usually in octahedral crystal form, and sometimes forms clusters with rhombic dodecahedron and cube, and often forms twins with (11) as the double crystal plane and joint plane. This twin law is called spinel law. Spinels exist in many forms, such as magnesium spinel, iron spinel and zinc spinel. This is because the isomorphism of spinel is very common and the isomorphism of divalent cation Mg2+ is often replaced by Fe2+ and Zn2+. The so-called spinel usually refers to magnesium spinel (MgAl2O4), and its theoretical chemical composition is 28. 2% MgO and 7 1. 8% alumina.
The solid-state reaction between MgO and Al2O3 can be carried out at a relatively low temperature. Hlaivac (196 1) studied the reaction kinetics of Al2O3+ MgO at 950 ~ 1300℃, and explained that γ-Al2O3 has great chemical activity (activation energy: α-Al2O3 is 107 kJ). γ-Al2O3 is 342. 76 kJ/mol) to promote the synthesis reaction. The process of cation interdiffusion given by Wagner is shown in Figure 6. 22. The reaction model can be verified by experiments, but it can't completely calculate the actual reaction rate constant.
8. No corundum phase of 4% in the fly ash was found in the sintered cordierite samples, indicating that the addition of MgO made corundum (α-Al2O3) disappear, then magnesium and aluminum ions diffused to form spinel, and cordierite could also be formed with the participation of silicon.
According to the research (Winnie et al., 1995, 1996, 1997), there are two different tetrahedrons in the structure of high-temperature α-cordierite, namely, the tetrahedron located in six rings and the tetrahedron playing a connecting role. Meger et al. (1977) think that the tetrahedron that plays a connecting role is obviously larger than the tetrahedron in the six rings, so the larger aluminum atoms will have a greater chance to enter these larger tetrahedrons (Figure 6. Article 23 (a)).
Fig. 6.22 Wagner model of ion diffusion and phase boundary reaction in MgO-Al2O3 system
Figure 6. 23 structural comparison between typical α-cordierite (a) and typical β-cordierite (b) (according to Winnie et al., 1995).
For the low temperature variant β-cordierite, Gibbs (1966) thinks that there are two large tetrahedrons in the six rings which are easily filled with aluminum. Therefore, in an ideal cordierite structure, there are two Al-O tetrahedrons in the six rings, and 1/3 of the tetrahedron which plays a connecting role is occupied by silicon. In the whole three-dimensional space skeleton, except two pairs of silicon-rich tetrahedrons in the six rings share one oxygen atom, other aluminum-rich tetrahedrons and silicon-rich tetrahedrons are alternately arranged in strict order (Figure 6. Article 23 (b)).
μ-cordierite is a low-temperature cordierite glass (
The basic unit of cordierite structure is six rings formed by connecting six (Si, Al) O4 tetrahedrons. These six rings are arranged in parallel along the C-axis, forming a channel of the C-axis. Due to the large space in the channel, some smaller molecules, such as H2O and CO2, and electricity price compensation ions, such as K+, Na+, Li+, Cs+, Ca2+ and Ba2+, can enter the channel without affecting the basic structure of cordierite. These molecules or ions are collectively called channel particles. Although most pore particles do not affect the basic structure of cordierite, some larger particles will affect the lattice distortion of cordierite, thus affecting the stability of cordierite polymorphs.
Cordierite has a complex uniform image. In cordierite crystals, there are still structural distortions that reduce its symmetry. Capital Qiu Sui (1957) thinks that the degree of cordierite distortion can be changed from (5 1 1), (42 1) and (13 1) in the X-ray powder diagram to 3. They recombine into a single peak in unreflected hexagonal Indian stone.
Japanese scholar Akisui put forward the concept of distortion index (δ) when studying the crystallization of cordierite:
Characteristics of high alumina fly ash and its application in the synthesis of mullite and cordierite
He found that the highest distortion index of cordierite did not exceed 0. 3 1, he called the cordierite with the highest distortion index (0.29 ~ 0.3 1) as over-distorted cordierite. 0 & ltδ& lt； 0.29 cordierite is called sub-twisted cordierite; Cordierite with δ = 0 is called Indian stone. The distortion index has nothing to do with the composition of cordierite, but with the formation temperature of cordierite. Cordierite with δ = 0 is stable at very high temperature, and cordierite with δ = 0 is excessively twisted. 29 ~ 0.3 1 is stable at medium temperature, and sub-distorted cordierite is between the two. It can be divided into high-order distorted cordierite and low-order distorted cordierite. The former occurs in andesite, while the latter is widely distributed in metamorphic rocks, pegmatites and timely veins. Therefore, the distortion index of cordierite can be used as a geological thermometer (Ye Danian et al., 1984). In fact, the distortion index of cordierite synthesized by artificial sintering can be used to represent the thermal state of cordierite during crystallization.
The distortion of cordierite structure may be related to the ordered and disordered distribution of silicon and aluminum in Si5AlO 18 ring, so the distortion index can be used as the scale of cordierite order-disorder.
In this experiment of synthesizing cordierite, referring to JCPDS card, the positions of three peaks (5 1 1), (42 1) and (13 1) on the XRD pattern of cordierite are between 2θ = 28 ~ 30, corresponding to each other. (fig. 6. 24), as can be seen from the XRD pattern of this sample, the three peaks completely overlap (see Figure 6. 14 to figure 6. 17), indicating that the cordierite in the sample is Indian stone, that is, high temperature α-cordierite.
Figure 6. 24 diffraction line (Cu, Kα) characteristics of various cordierite at 2θ = 28 ~ 30 (according to Ye Danian et al., 1984).
(5) SEM observation of synthetic samples
The fresh fracture of sintered cordierite sample was put into a vacuum coating device, plated with platinum for 30s, and observed by scanning electron microscope. At low magnification, it is found that sintered samples usually have unequal number of pore structures, and most of them are irregular (Figure 6. 25a)。 Only in A4 sample (softening collapse), a large number of bubble-like pores with different sizes were found (Figure 6. 25b)。
At high magnification, cordierite crystals in the sample develop well, especially in pores, because the existence of pores provides a good development space for crystal growth (Figure 6. 26). The crystal form of cordierite is generally short columnar, and the aspect ratio is mostly between 1.5 ~ 2.0. The cross section is hexagonal or nearly round, and complete hexagonal columnar crystals can be seen. Mullite crystals are generally needle-like or long-columnar, and the cross section is quadrilateral, which can be distinguished from cordierite crystals. Feldspar is a parallel double-sided crystal, which is generally in the form of a pseudo-hexagonal plate along (0 10), and sometimes it can be regarded as a polycrystalline twin. Spinel is basically octahedral crystal, but it is also possible to find a polygon formed by composite spinel, which is easy to identify. Figure 6. 26 shows the crystal morphology of each sample under SEM. Unless otherwise specified, all samples are cordierite crystals.
Figure 6. Microstructure of 25 sintered cordierite samples
According to the research of Gao Zhenxin et al. (2002), there may be a very small amount of hexagonal calcium hexaaluminate (CA6) crystal in the synthesized mullite-cordierite sample, which belongs to hexagonal crystal system. CA6 usually exists in CaO-Al2O3 or CaO-Al2O3-SiO2 system. Some people think that CA6 is crystallized from bauxite fused corundum abrasive containing 1% ~ 2% Cao. Gao Zhenxin (1982) found authigenic CA6 with good crystallization in calcareous caves of calcined bauxite, and observed it by chemical analysis, microscope observation, XRD and SEM. It is pointed out that calcite in bauxite reacts with diaspore (corundum) at high temperature to form CA6, and it is considered that its crystallization environment is mostly liquid phase.
Although there is no diffraction peak on XRD in this experiment, there are a few hexagonal flake crystals with perfect crystallization in individual samples, which are difficult to distinguish in detail because of their small content and similar crystal form to albite. In fact, if we want to distinguish them, we can judge them by crystal morphology and chemical composition, and attach the results of EDX analysis. Trace minerals in samples can be determined by scanning electron microscope -EDX analysis to make up for the deficiency of XRD analysis.
A small amount of round particles and irregular particles in the sample generally belong to RO phase, which is caused by impurity oxide components contained in the composition.
By scanning electron microscope observation, the structure of B series samples is loose and irregular pores are common. Cordierite crystal phase still exists, and the degree of crystal development is not as good as that of C series, so coarse mullite can be seen (Figure 6. 27). In addition, spinel phase was found in B series. Because the crystal phase development is not as good as that of A and C series samples, XRD study is not carried out, but SEM observation is used. However, from the physical properties and compressive strength indexes of the samples, the phase composition is not much different from that of A and C series samples.
From the SEM analysis results of cordierite synthesized by solid-state reaction sintering of different components, it can be seen that the grain size seems to be not particularly different, mostly 5 ~ 10 μ m, sintered at 1350℃ and 1370℃, the phase composition is basically the same and the grain size is almost the same; The different constant temperature time has little effect on the crystallization of cordierite. The phase composition mainly depends on the ratio of raw materials, and the phase composition is different with different ratios. Compared with the C series samples, the B series samples are all acid-washed fly ash, so the crystals in the C series can also be seen in the B series, but the development degree is slightly inferior. B-series mullite crystals are relatively large, which may be related to the high Al2O3 content and low MgO content in the ingredients. Comparing A with C series, the crystals in C series are well developed, and many cordierite crystals can be seen not only in the pores, but also anywhere in its fracture, especially in C series, and the sample of C 1 is the most obvious.
Characteristics of high alumina fly ash and its application in the synthesis of mullite and cordierite
Characteristics of high alumina fly ash and its application in the synthesis of mullite and cordierite
Characteristics of high alumina fly ash and its application in the synthesis of mullite and cordierite
Characteristics of high alumina fly ash and its application in the synthesis of mullite and cordierite
Figure 6. Morphology of samples under scanning electron microscope
Figure 6. SEM images of 27b series partially sintered samples.
Whether cordierite synthesis depends entirely on the specific surface area of raw materials and firing temperature; The purity of synthesis mainly depends on the composition of components, which is technically difficult. Due to the different kinds and quantities of impurity oxides in raw materials, the synthesis temperature is also different. The particle size of synthetic raw materials also affects the synthesis temperature. In addition, in order to reduce the sintering temperature or improve some properties of products, many researchers use different additives to carry out experiments and draw different conclusions. For example, Torres et al. (2005) used 55% silica, 2 1.5% alumina, (16. 5-x)% MgO，x% CaO，3。 8% titanium dioxide and silicon dioxide. 9% B2 O3 as raw material, x = respectively. It is concluded that when x = 4. 6. α -cordierite ceramics with single crystal phase can be obtained at1160 ~1190℃, with the maximum microhardness and the most complete crystal.
Chen (2008) pointed out that in the MgO-Al2O3-SiO _ 2 system, when the substitution of CaO for MgO is less than 3%, when sintered at 900℃, the main crystal phase is α-cordierite and the secondary crystal phase is μ-cordierite; When 10% is substituted, the main crystal phase is anorthite and the secondary crystal phase is α-cordierite. The main crystalline phase is α -cordierite and the secondary crystalline phase is anorthite. At this time, the sample density is close to 98% of the theoretical value of cordierite, and it has low dielectric constant, low thermal expansion and high flexural strength (≥ 134 MPa).
Dai Gangbin et al. (2003) found that the microstructure and high-temperature properties of synthetic cordierite materials were obviously affected when the Al2O3 content in the ingredients changed within 5% of the theoretical composition. Among them, increasing the mass ratio of al2o 3/SiO 2 or al2o 3/MgO is beneficial to improve the microstructure and high temperature properties of cordierite materials. In cordierite materials synthesized from aluminum-rich components, the content of glass phase is relatively low, acicular mullite precipitates in the glass phase, and the particles connected by acicular mullite crystals are evenly distributed in the cordierite phase. This microstructure is very helpful to improve the high temperature properties of materials.
In the experiment, if the proportion of talcum powder is reduced, the symbiotic combination structure of mullite and cordierite can be formed. Taking this symbiotic structure as the combined matrix, adding sintered mullite particles or synthetic cordierite particles can produce products with different phase combinations to adapt to the changes of different temperature conditions. There have been industrial examples of producing mullite-cordierite products, and the methods adopted include primary firing and secondary firing based on the principle of in-situ reaction. We can find the symbiotic form of mullite and cordierite in the product matrix produced by Acme company. The former is a thick column, and the latter is a slender needle or fiber. The two are symbiotic and inseparable. This structural feature is a sign of the close combination of particles and matrix, and it is also a fundamental factor to ensure that the product has a series of advantages. Camerucci et al. (200 1) mixed 30% mullite with 70% cordierite to prepare composites with thermal expansion coefficient equivalent to that of silicon, and confirmed that the mullite content had little effect on the electrical properties of the materials. The purpose of such experiments is to combine the advantages of mullite (high melting point) and cordierite (low thermal expansion and low dielectric constant) to prepare high-performance composites.