Experiments have shown that the raw materials and firing process can have a tremendous influence on the formation of cordierite in MgO-Al₂O₃-SiO₂ ceramic bodies.



Ceramic bodies with a high cordierite content have excellent thermal shock properties, making them suitable for a wide range of high-temperature applications. Cordierite also lends the low dielectric loss properties required in electronic applications, and can help improve the spalling resistance of ordinary stoneware bodies. While cordierite can be obtained from some suppliers as a ready-to-use additive (see Ceramic Industry’s Data Book & Buyers’ Guide for supplier information), most large manufacturers develop or synthesize their own based on the other raw materials used and the temperatures at which those materials are processed and fired. The extent to which cordierite is developed determines the properties of the final product, but achieving the optimum cordierite development has always been a rather elusive goal. The basic variables of cordierite development (raw materials, thermal processing and firing) have been known for some time, but it has been unclear to what extent each of those variables influences the development process. For many manufacturers, developing cordierite in a ceramic body has simply been a result of trial and error within their own facilities.

To provide a more scientific analysis of the development of cordierite, researchers performed a series of experiments on a wide range of ceramic bodies, each containing the basic magnesium oxide-alumina-silica fume (MgO-Al₂O₃-SiO₂) ingredients necessary to develop cordierite. Following are the results of these experiments.

Facts and Theories about Cordierite Formation

Cordierite with a composition of 2MgO-Al₂O₃-5SiO₂ is the most important ternary compound in a magnesium oxide-alumina-silica fume (MgO-Al₂O₃-SiO₂) body. It is situated in the primary crystallization field of mullite and has a chemical composition (weight %) of MgO = 13.8, Al₂O₃ = 34.8 and SiO₂ = 51.4. Three forms of cordierite are known to exist: the a form, also known as indialite; and beta- and micro-cordierite. Of these, indialite is the stable high-temperature form and the only one found in nature or achievable in ceramic bodies. Beta- and micro- cordierite can only be formed under special conditions. At 1460°C, the indialite form undergoes an incongruent fusion that converts it first to a mullite phase, then to a liquid phase, from which forsterite can be developed. Both phases are crystalline phases with much higher expansion coefficients.

Cordierite, with a crystallo-chemical formula of Mg₂^[6]Al₃^[6][Si₅Al^[4]O₁₈], contains coplanar tetrahedral groups bound in the shape of hexagonal rings with five SiO₄ groups and one AlO₄ group. Between these tetrahedral groups are the octahedral groups MgO₆ and AlO₆, which form toward the inside of the structural unit’s free cavities. (A structural unit is formed by three planes of tetrahedral rings, with two cavities between them.) The thermal behavior of cordierite, the anisotropy of its thermal expansion and the possibility to influence these characteristics are explained by its structure and they are very important from a theoretical and practical point of view.^1-5

Cordierite can be obtained directly from oxides, as well from other raw materials whose chemical composition equals that of cordierite. Such materials include:^6-9

• elementary compounds, such as pure oxides, hydroxides and carbonates;
• double compounds, such as kaolins, clays, talc, steatite and sepiolite; and
• triple compounds, such as chlorite.

Double or triple compounds, especially those that are hydrated, are preferable. The presence of two or three oxides and the availability of the bonds resulting from the rupture of the lattice when water is released will guarantee a complete reaction and a broader vitrification range.

Besides the nature of the raw materials used, the synthesis of cordierite is also influenced by the initial structural state of the respective reactants; by the presence or absence of impurities, which act as mineralizers; by the grain composition and the specific surface area of the grains; and by the size of the contact area between the grains.¹⁰

Experiments

Based on the above theoretical considerations, 17 experimental bodies were prepared using the raw materials listed in Table 1.¹¹ Thermal analysis was used to determine the thermal behavior of each of the raw materials,¹¹ and these results, along with available technical literature on the raw materials,^6-9,12-14 were used as reference materials for this study.

Preparing the Experimental Bodies. The stoichiometric cordierite composition (2MgO-2Al₂O₃-5SiO₂) was used to calculate the composition of the raw materials mixtures for the experimental bodies. Only the three main oxides (MgO, Al₂O₃ and SiO₂) were considered in these calculations; the presence of all other oxides was ignored. This calculation method was justified by the following arguments:

• the raw materials used were very pure and contained only relatively small quantities of the other oxides; and
• the other oxides participate in the interactions within the real poly-compound system, especially in the formation of eutectic melts, and are incorporated in the developing liquid phase.

The chemical composition of the experimental bodies is presented in Table 2. For each of the experimental bodies, the raw material mixture was prepared by wet grinding. The materials were then dried and pressed into 17 sets of test specimens, some as 25-mm-diameter cylinders and others as 65 x 10 x 10 mm pieces.

The test specimens were pre-calcinated at 1100°C and were then subjected to three different thermal treatments. For two of the firing schedules, the firing was conducted in an electric laboratory kiln at 1300°C and 1350°C, respectively, with a one-hour holding time at peak temperature. The third firing (named “pen”) was conducted in an industrial gas kiln (used for in-glaze firing) at 1300°C, with about 20 minutes of holding time at the peak temperature and a firing cycle of about 17 hours (cold to cold).¹¹

Investigating the Unfired Bodies. Samples from each composition were analyzed using a sedimentation method (SediGraph 5100) to determine their grain size distribution.¹¹ The various grinding aptitudes of the raw materials were taken into account to eliminate possible significant differences between the grain size of the experimental mixtures, which could essentially influence the reactivity of the respective bodies. Some of the data resulting from these particle-size analyses are shown in Table 3.

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out on each unfired body using a-Al₂O₃ as the reference material.¹¹ The thermal diagrams indicated the evolution of the reacting systems on heating, showing the physical and chemical transformations that are characteristic for the raw materials used, as well as the formation of the new compounds by solid or solid-liquid phase reactions. The diagrams also indicated the formation of the liquid phases by eutectic interactions between initial phases, or between them and a reaction product. Thus, the exothermal peak—whose maximum was recorded between 1250-1300°C for each body tested—can be attributed to the formation of cordierite.^9,13

The DTA curves also showed two endothermal effects, one of them (I) at a lower temperature and the other one (II) at a higher temperature (see Figure 1). This compared to the temperature of the previously mentioned exothermal effect, which can be attributed to the formation of the liquid phases in the respective reacting systems. The thermal data obtained from these thermal analyses were used to determine the heat treatment schedule applied to the experimental bodies.

Investigating the Fired Bodies. The fired specimens of each experimental body were analyzed to determine their mineralogical composition, as well their degree of vitrification and thermal expansion.

The mineralogical composition was determined by X-ray diffraction (XRD) using a Siemens Diffrac 500 X-ray diffractometer with Ni filter CuK-alpha radiation. For the quantitative phase analysis, a method using external standards, in a version involving measurements of attenuation coefficients, was used.^11,13,15,16

To estimate the degree of vitrification of the fired samples, their water absorption capacities, apparent porosities and apparent densities were determined. The water absorption was accomplished under vacuum, and the necessary weightings were performed using a hydrostatic balance.¹¹

The thermal expansion of the fired samples was determined by measuring the lineal thermal expansion coefficients using a “differential” dilatometer with thermal expansion transmission accessories.¹¹

Results and Discussion

The XRD patterns¹¹ of the fired samples, along with the data provided in the material literature, enabled the researchers to identify the available crystalline phases in the fired samples.

Tables 4-6 present the results of these XRD analyses; c means the content of the crystalline phase in a sample.

However, a quantitative evaluation for sapphirine, spinel and protoenstatite could not be obtained because no pure minerals were available that could serve as reference materials.

Instead, the intensity (Ip) values of these phases were used to describe their content in semi-quantitative terms only. Figure 2 compares the cordierite content in the tested samples using the XRD analyses data.

In addition to the crystalline phases detected by X-ray diffraction, the experimental fired bodies also contained some amorphous and cryptocrystalline phases. However, because not all of the crystalline phases could be determined quantitatively, the amorphous and cryptocrystalline phases could not be estimated. The values obtained for the water absorption capacity, apparent porosity and apparent density—microstructural characteristics that define the compactness of the ceramic material—(presented in Table 7) provided additional proof of the reactivity of the experimental bodies.

Two of the experimental bodies fired at 1300°C (Samples 4.1 and 4.2) were selected to study their thermal expansion. Table 8 shows the values of the lineal thermal expansion coefficients, which were determined between 20-600ºC. The small values of the determined lineal thermal expansion coefficients (alpha _20-600°C=2.54-10^-6 °C^-1) for both samples can be explained by 1) the presence of cordierite, which has a very low thermal expansion (alpha _20-1000°C=2-10^-6 °C^-1), as a prevailing crystalline phase; and 2) to a lesser extent, the porosity of the samples (see Table 7).

Conclusions

Several conclusions can be drawn from these experiments and their results.

Cordierite was formed in all experimental bodies and was the prevailing phase constituent in the fired samples. However, the ceramic bodies containing double hydrated compounds, such as kaolin and steatite, achieved a much higher reactivity than the bodies containing elementary compounds—especially oxides, such as aluminum oxide (alpha-alumina, gamma-alumina), silicon dioxide (beta-quartz, amorphous silica) and magnesium oxide; hydroxides, such as aluminium hydroxide; and carbonates, such as magnesium bicarbonate. This fact was determined both from the lower temperature interval of the cordierite and liquid phase formation in the unfired bodies, and from the higher content of cordierite (ternary compound), as well as the reduced (or even absent) proportion (based on a semi-quantitative estimation) of spinel (binary compound) in the fired bodies. This confirms the theoretical assumption that the nature and initial structural state of the reactants influences the reactivity of cordierite bodies.

Increasing the thermal treatment temperature had a positive influence on the physical and chemical interactions within the reacting systems. This was illustrated by the fact that the bodies fired at 1350°C contained a higher content of cordierite, as well as a reduced (and in some cases nonexistent under the detection limit of XRD) mullite content; and reduced corundum, beta-quartz and protoenstatite (unreacted compounds available in the raw materials or compounds derived from these by their transformation on heating) contents compared to the bodies fired at 1300°C.

Increasing the duration of the thermal treatment also positively affects the interactions leading to the formation of cordierite. The bodies fired at 1300ºC with a one-hour holding time had a higher cordierite content than the bodies fired at the same temperature with only 20 minutes of holding time.

Firing in a gas kiln appears to produce better results than firing in an electric kiln. The bodies fired in the gas kiln were more compact than the bodies fired at the same temperature in the electric kiln, perhaps due to the circulation and composition of the combustion gases.

Additionally, the variation in reactivity of the bodies under the interactions that led to the formation of liquid phases (illustrated by the determined values of the compactness of the fired bodies) was similar to that indicated by the results of the thermal and X-ray diffraction analyses. The degree of vitrification of the fired bodies was further proof of their reactivity.

Finally, the decreased thermal expansion of the fired bodies due to the presence of cordierite, as well as the porosity of the materials, proves that a correlation exists between the structure and properties of a ceramic material.

Based on the structure of the fired bodies and on the properties that define the applications of a product, some of the experimental bodies can be recommended for use in various technical applications, including refractories for firing other ceramics (saggers, kiln furniture, gas burners, etc.), electro-ceramics (resistors, fusibles, flame guards, etc.), flame-proof applications (cooking ware, laboratory items, etc.), catalyst carriers, and a variety of other areas where increased thermal shock resistance would be an advantage.