- THE MAGAZINE
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 FormationCordierite 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 Mg2Al3[Si5AlO18], 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.
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
ExperimentsBased 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 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
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
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 DiscussionThe 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.
ConclusionsSeveral 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.