Aluminum titanate based composite porous ceramics with both high porosity and mechanical strength prepared by a special two-step sintering method

https://doi.org/10.1016/j.jallcom.2020.157193Get rights and content

Highlights

  • This work develops a special two-step sintering technology for porous ceramics.

  • Excellent comprehensive properties achieved in aluminum titanate-strontium feldspar-mullite composite porous ceramics.

  • The sample sintered at T1 = 1400 °C & T2 = 1500 °C shows optimized porous and mechanical performances.

Abstract

In this work, we develop a two-step sintering technology special for porous ceramics to resolve the contradiction between porosity and mechanical strength. It contains a first long-time sintering at the relatively low temperature (T1) and a second fast sintering at higher temperature (T2). Porous ceramics of aluminum titanate-strontium feldspar-mullite (ASM) ternary composite were prepared by this method, and exhibit both high porosity and high mechanical strength. The effects of T1 and T2 on the phase composition, microstructure, porosity, pore size distribution, mechanical strength and thermal expansion coefficient (TEC) of ASM porous ceramics are systematically studied. The sample sintered at T1 = 1400 °C & T2 = 1500 °C shows optimum performance: the porosity, bulk density, and mean pore diameter (D50) are 63.48%, 1.20 g/cm3 and 15.94 μm, respectively; while the flexural strength reaches 9.22 MPa and the TEC is 2.9 × 10−6/K. The two-step sintering technology provides a powerful tool to develop porous ceramics with excellent comprehensive properties.

Introduction

Porous ceramics are widely used in aerospace, energy, metallurgy, military industry, chemical industry, medicine, environmental protection and other fields, because of their porous structure, large specific surface area, low density and adjustable pore size distribution [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Aluminum titanate (Al2TiO5, AT) with a crystal structure of pseudo-brookite is currently the only structural material that integrates low thermal expansion coefficient (TEC) and high temperature resistance [[10], [11], [12], [13], [14]]. AT-based porous ceramics have wide engineering applications in filter, catalyst carrier, pipeline lining and thermal insulation at high temperatures [[15], [16], [17], [18], [19], [20]]. However, the low mechanical strength and the phase instability severely limit its applications [21,22]. Al2TiO5 easily decomposes into Al2O3 and TiO2 in the temperature range between 750 °C and 1280 °C; while the remarkable anisotropy of thermal expansion in the a- and c-axis of lattice tends to induce large amount of microcracks.

Researchers have made great efforts to improve the physical properties and the phase stability of AT-based porous ceramics. Previously, the phase stability is improved by various additives, including MgO, Fe2O3 and TiO2, which are isomorphous with the mineral pseudo-brookite [14,23]. Alternatively, second phases are introduced as phase stabilizer, such as mullite, SiC, CaAl4O7, etc. Since mullite is a high-temperature stable phase with high deformation resistance, its addition in AT-based ceramics can inhibit the grain growth, reduce microcracks and improve the mechanical strength [20,[24], [25], [26], [27]]. Stournaras et al. [25] enhanced the strength of AT ceramics by adding mullite, but the TEC was elevated at the same time. Kim et al. [20] used the wet foam method to prepare AT-based composite porous ceramics, and improved both the porosity and the strength by adding mullite. Their further work [26] indicates that the presence of mullite inhibits the grain growth of AT-based ceramics, thereby increasing the strength and maximizing the TEC.

The porous structure, mechanical strength and thermal expansion of porous ceramics always have contradictory relations in practice. When the strength of porous ceramics is enhanced by additives, the porosity is weakened at the same time, and vice versa. In our previous work [12], porous aluminum titanate-strontium feldspar-mullite fiber composite ceramics was prepared by a conventional sintering process, which exhibits a good porosity (70%) and acceptable mechanical properties (2.27 MPa). Moreover, the low TEC of AT-based ceramics mainly originates from microcracks, but the microcracks weaken the mechanical strength obviously. Hence, it is a great challenge to balance high porosity, high strength and low thermal expansion for porous ceramics.

Sintering is one of the most important processes to optimize the microstructure of ceramics. For this purpose, some researchers prepare porous ceramics by special sintering technologies, such as spark plasma sintering [28], selective laser sintering [[29], [30], [31]], etc; while some researchers devote into modifying sintering schedule and improving parameters of the conventional sintering method for desired microstructure. Recently, the two-step sintering (TSS) is proposed as a more effective method for tailoring microstructure. Compared with the conventional sintering process, TSS is a more effective method for tailoring microstructure. The TSS controls the densification and the grain growth by different processes, so it has more freedom to achieve a desired microstructure. Chu et al. [32] first proposed the TSS technique, and obtained the microstructure with finer grains and homogeneous size distribution. It suggests that the TSS is a simple and convenient method to make ceramic body more homogeneous without resorting to specialized powders or complicated heat schedules, and the process provides a refined microstructure to improve material properties. Chen and Wang [33] prepared the fully dense nanocrystalline Y2O3 ceramics at a low temperature of ∼1000 °C and a normal pressure by the TSS method since the final-stage grain growth is suppressed by the difference in kinetics between the grain-boundary diffusion and the grain-boundary migration.

Up to now, most previous works on the TSS tend to prepare dense ceramics with fine grains [[34], [35], [36], [37], [38], [39], [40], [41], [42]]. They always adopt a fast sintering at high temperature with a very short dwell time as the first step, and a long-time sintering at low temperature as the second step. The first step achieves an intermediate density and restrains pores against shrinkage; while the second step freezes the particle network and promotes the densification mainly by grain-boundary diffusion, where the grain growth is suppressed due to slow kinetics [33]. Then many researchers use the TTS to prepare dense ceramics and control grain size, while someone also prepare porous ceramics with fine grains by this method [[40], [43], [44], [45]]. Our work proposes a two-step sintering process special for porous ceramics. It consists of a first long-time sintering at a relatively low temperature and a second fast sintering at higher temperature. In the first step, the grains begin to fuse to form a skeleton, which is stabilized after a long-time sintering, but the grain growth is suppressed by slow kinetics at low temperature. In the second step, the grain growth is promoted remarkably by the high sintering temperature and microcracks are formed in some large grains; while the porous ceramic skeleton is almost preserved due to the rapid heating rate and zero dwell time at high temperature.

In this work, we prepared the composite porous ceramics of aluminum titanate-strontium feldspar-mullite (ASM) by a novel TSS process, which efficiently improves both porous and mechanical properties. In the ternary composite, AT serves as the main phase while strontium feldspar and mullite fiber are additives to improve mechanical property. The effects of two-step sintering temperatures on the phase composition, micromorphology, porosity, pore size distribution, mechanical properties, and TEC of ASM porous ceramics are systematically studied.

Section snippets

Preparation procedure of ASM porous ceramics

AT ceramic powders were prepared by a conventional solid-state reaction method. α-Al2O3 (the average size ∼5.91 μm, purity > 99%), TiO2 (the average size ∼0.78 μm, purity > 99%), potato starch (the average size ∼36.5 μm, > 99%), SrCO3 (purity > 99%), SiO2 (99%) and mullite fibers were used as raw materials. For the preparation of Al2TiO5 clinker, the raw powders of Al2O3 and TiO2 were weighed according to the stoichiometric ratio and ball-milled for 4 h at 300 rpm in ethyl alcohol medium, where

Results and discussions

Fig. 2 shows the XRD patterns for the ASM porous ceramics sintered at different T1 and T2 temperatures. All the ASM porous ceramics exhibit Al2TiO5 phase and strontium feldspar phase mainly, as well as a small amount of Al2O3 impurity phase. It indicates that aluminum titanate and strontium feldspar are formed after the solid state reaction at high temperature, and some residual Al2O3 still exists. Our previous work indicates that the crystallinity is better if the ASM porous ceramics are

Conclusions

ASM porous ceramics were prepared by a novel two-step sintering method, which contains a long time sintering at the relatively low temperature firstly and a fast sintering at higher temperature secondly. Both excellent porous and mechanical properties are achieved because the forming of ceramic skeleton, sintering densification, and the grain growth processes are separately controlled by different sintering parameters of TSS process. The samples sintered at T1 = 1400 °C & T2 = 1500 °C show an

CRediT authorship contribution statement

Yirong Wang: Investigation, Formal analysis, Writing - original draft. Xueqian Wang: Validation, Formal analysis, Writing - review & editing. Chuanbao Liu: Visualization, Writing - review & editing. Xiaopo Su: Formal analysis. Chengye Yu: Formal analysis. Yanjing Su: Supervision. Lijie Qiao: Supervision. Yang Bai: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by grants from the Beijing Municipal Science and Technology Project (Z191100004819002), National Natural Science Foundation of China (91963114) and Fundamental Research Funds for the Central Universities (FRF-GF-19-012AZ).

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