A comparison study of tialite ceramics doped with various oxide materials and tialite–mullite composites: microstructural, thermal and mechanical properties

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Abstract

Tialite (Al2TiO5) is a material of very low thermal expansion coefficient, high thermal shock resistance, high refractoriness and good corrosion resistance. However, its applications are very limited due to its low mechanical strength and to its thermal instability in the temperature range 750–1350 °C, which leads to the decomposition of the material to its parent oxides alumina and rutile. To overcome both problems, stabilization of the structure is tried through doping with various oxides; in the present work, a comparison study of the properties that can be achieved and of the decomposition behavior of tialite ceramics stabilized by adding MgO, talc or feldspar and of tialite–mullite composites made by the addition of kaolin is carried out. The processing conditions are also investigated for preparing porous ceramics for applications in the area of soot traps and hot gas clean-up. It was found that talc addition has an excellent stabilizing behavior, whereas tialite–mullite composites exhibit increased strength. Such composites with 10–20 wt.% mullite present the appropriate properties for the applications under consideration. Mullite presence also brings a stabilizing effect, thus in combination with talc additions it could lead to a very stable product.

Introduction

Aluminum titanate (tialite) is a refractory material with very low thermal expansion coefficient (lower than that of fused silica), exhibiting as a consequence excellent thermal shock resistance. Additionally this material has very low thermal conductivity (of the order of 1.5 W m−1 K−1) and high melting point (about 1860 °C).1., 2., 3. These properties make it an excellent candidate material for refractory applications in the non-ferrous metallurgical industries, in the automotive industries for applications as exhaust port liners in automotive engines or as insulating components to increase heat efficiency4 such as exhaust manifold inserts, piston crowns and turbocharger liners where both thermal insulation and thermal shock resistance are required1., 5., 6. or even as thermal barriers.7 Furthermore, its high thermal shock resistance, high refractoriness and good corrosion resistance are potentially advantageous for soot traps for diesel engines and filters for hot gas clean-up applications.8., 9., 10.

However, these potential applications of this material have been severely restricted because it decomposes in the temperature range 800–1280 °C1., 4., 5., 11., 12., 13., 14. and it has very low fracture strength. Indeed tialite is stable from room temperature up to 750 °C1., 4. and from 1280 up to its melting temperature, being subject to a eutectoid decomposition to its parent oxides α-alumina and rutile between 750 and 1280 °C.1 Its low fracture strength is due to the extensive microcracking occurring during cooling from the firing temperature.1

The material is isomorphous with pseudobrookite, crystallizing in the orthorhombic space group Cmcm1., 12. and is characterized by a pronounced anisotropy in thermal expansion coefficient resulting in a distinct hysteresis.1., 6., 14. This pronounced anisotropy is the reason for the severe microcracking during cooling which leads to the poor mechanical properties of the sintered material.14., 15., 16., 17. The microcracking phenomenon is closely related to the material microstrucure.9., 15., 16., 17. Below a critical grain size, the elastic energy of the system is insufficient for microcrack formation during cooling and thus the mechanical properties are considerably enhanced.1 This critical grain size depends on the thermal history of the sample15., 16., 18. and is in the range of 1–2 μm.14., 15., 17., 18., 19., 20., 21., 22. The density of microcracks increases drastically with grain size increase once above the critical size.1., 14., 15., 16., 17., 18., 19., 20. The microcracking phenomenon determines finally the thermal expansion behavior of the material as well as the thermal diffusivity, the strength and the elastic modulus.1., 2., 7., 8., 11., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23.

The decomposition phenomenon has been addressed through various oxide additives that form solid solutions with the aluminum titanate. The oxides that are considered to have a good stability effect are Fe2O3, MgO, SiO2, ZrO2, ZrSiO4, La2O3.2., 4., 7., 10., 11., 13., 14., 24., 25., 26., 27., 28., 29., 30. Among them, Fe2O3 and MgO have been considered to be the most effective ones.4., 7., 13., 14., 24., 25., 26., 27., 28., 29., 30. However, a quite high percentage of MgO (of the order of 10–25 mol%) is required for an effective stabilization, and even this could not keep the material stable for more than a hundred of hours at the temperature of the maximum decomposition rate (1100 °C).7., 14., 27. Slightly better stability efficiency is reported with Fe2O3 additions,4., 13., 24. which in some cases led to material stable for about 300 h of annealing at 1000 °C in an oxidizing atmosphere.24 In all cases, the decomposition proceeds fast after an initial time period following a nucleation and grain growth mechanism and its rate varies with both the type of additive and the material particle size.2., 7., 23. Recently, it has been recognized that the combined effect of MgO and SiO2 enhance significantly the material stability.6., 31. In a recent work, very positive results in terms of stability were also obtained by the addition of feldspar.11 The additives, besides the stabilization effect, have a considerable influence on the microstructure thus affecting the mechanical and thermal properties of the material.

Besides the decomposition problem, another important concern is the low strength of the material due to the microcracking phenomenon. A solution that is proposed is the development of composite structures including as a second phase mainly mullite,3., 5., 6., 8., 12., 15., 31., 32. whereas zirconia and alumina have been also reported.18., 33., 34., 35., 36., 37., 38., 39., 40., 41. Mullite incorporation in the structure strengthens significantly the material, reducing also the grain growth rate and thus decreasing the microcrack density in the material; it also exhibits a stabilization effect in the structure.6., 12., 31. Various efforts have also been made to control the tialite structure and enhance the mechanical and thermal properties of the material by producing nanosized powders consisted of either a single phase doped tialite or a composite structure through chemical methods.2., 3., 5., 6., 31., 41., 42., 43., 44., 45.

The present work concerns an analytical and comparative study of the properties of porous ceramic tialite supports for applications mainly in the area of filters for soot traps or for hot gas clean-up. The work comprises the manufacturing of both doped tialite ceramics stabilized with various oxide additives and mullite–tialite composites. In the first case, the study concerned the effect of various additives based on magnesia and silica on the material properties (thermal expansion, mechanical strength, porosity) and thermal stability. The additives were pure magnesia, commercial talc (for achieving the combined effect of magnesia and silica) and a combination of talc and feldspar. Mullite composites varying in the mullite percentage were also manufactured using kaolinite as the mullite source; these were compared with the doped tialite ceramics.

Section snippets

Experimental

For tialite formation, calcined alumina (Nabaltec 115-25) of purity 99.5% and a mean particle size d50 of 4.6 μm and rutile powder (Aldrich Chemical Co. Inc., purity 99.9%) of a mean particle size of 1 μm were mixed in the proper stoichiometry. As stabilizing agents, various oxide raw materials were also added in the alumina–titania mixture. These included MgO (Aldrich Chemical Co. Inc., purity 99.9%) of either 8 or 3 wt.%, talc (Luzenac, Cyprus International Minerals Corp.) of 6 or 9 wt.%, or a

Dynamic sintering

Dilatometric analyses on the raw samples after their shaping and drying are shown in Fig. 1, whereas the various transformation phenomena occurring are described in Table 2, Table 3. Comparing the shrinkage curves of the MgO containing samples (Fig. 1a) with those of the talc or the talc–feldspar containing samples (Fig. 1b, Table 2) it is easily noticed that the addition of talc to the alumina–titania mixture leads to a higher shrinkage rate at the initial stages, whereas the maximum shrinkage

Conclusions

Tialite ceramics and tialite–mullite composites were prepared from commercial raw materials and were compared in terms of their properties for applications in the area of hot gas clean-up. Various oxide based materials such as MgO, talc, feldspar were investigated as stabilizing agents of tialite structure. The conclusions derived are summarized as follows:

  • Both the type and the percentage of the stabilizing agent affect the final ceramic properties by influencing the ceramic microstructure. The

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