Structural and mechanical characterisation of MgO-, CaO- and BaO-doped aluminosilicate ceramics
Introduction
Ceramics based on alumina, Al2O3, and silica, SiO2, are used in a large number of engineering applications. Particularly in the aluminium foundry industry, relative high-Al2O3 ceramics, which secure remarkable chemical resistance, have been traditionally selected because of their low SiO2 contents [1], [2], [3]. SiO2 can be reduced by the aluminium to form silicon, Si, and Al2O3. When this happens, the ceramic is weakened and the metal can become contaminated with Si [4], [5]. Therefore, materials with low SiO2 contents are preferred. However, the presence of SiO2 in oxide ceramics made of Al2O3 is advantageous since the sintering temperature is lowered, as well as their production costs [2], [6]. Moreover, the combination of these two oxides with other oxides of calcium, magnesium, chromium and/or barium is commonly employed, donating the ceramic with specific properties related to phase formation, sintering temperature, porosity, resistance to chemical attack or thermal shock, wettability, etc. [3], [7]. Due to their relative low cost, these ceramics have been widely used in most of the ceramic applications in the aluminium foundry industry, such as refractories, transfer ladles, crucibles, etc.
Aluminosilicate compositions containing less than 72 wt.% Al2O3 lie between the crystobalite (α-SiO2) and mullite (3Al2O3·2SiO2) compounds, developing a silicate liquid phase above 1595 °C [2]. Basic oxides added to these compositions can act as fluxes, resulting in the displacement of the liquidus curve towards lower temperatures. Moreover, displacement can also occur towards lower SiO2 contents due to the formation of immiscible liquid phases so that the solubility of SiO2 in the melt decreases. In the case of oxides of divalent cations, liquid immiscibility is known to occur, except with Ba2+, which is the largest one. In fact, as the size of the cation increases miscibility gap is known to decrease. It is also known that, for these oxides, immiscibility occurs over the primary crystallisation field of silica, leading to the formation of silicates [7]. This can be correlated with the ability of the cations to fit into the interstices of the silicate structure [7], [8]. The stability of the silicates formed has important consequences on the evolution of the aluminosilicate structure, since the effect of Al2O3 is to reduce the liquid immiscibility. Thus, the less stable the silicate formed the more reactive with Al2O3. The stability of this type of silicates is known to increase as the ratio between the cation electronic charge, Z, and radius, r, (Z/r) decreases according to the following order: Ba<Ca<Mg [7], [9]. As low melting point liquid phases can be formed in the presence of such additives, the type, amount and homogeneity of the liquid, and its reactivity with the primary crystalline phases, determine the final microstructure formed at a given temperature and, consequently, the ceramic's properties [3], [7], [10].
On the other hand, since these aluminosilicates have complex microstructures, the combination of several crystalline phases with glassy phase and porosity is known to determine the structural performance of the ceramic component under mechanical or thermal loading. In the later case, factors controlling the high-temperature resistance of the ceramic would include: (a) the porosity volume fraction, (b) the size of pores and grains, (c) the elastic properties and (d) the thermal expansion and conductivity properties [11], [12], [13], [14], [15], [16], [17], [18]. Moreover, temperature, time and/or the number of cycles of exposure at high-temperature and cooling rate also interfere on the response of the material [19], [20], [21], [22].
The present work studies the influence of MgO, CaO and BaO additives on the sintering behaviour and on the properties (crystallinity, open porosity, microstructure, thermal expansion and mechanical strength) of ceramic bodies sintered at 1400 °C. Thermal and mechanical properties were seen to depend on the structural characteristics of the ceramics. Their response to cyclic thermal loading at 800 °C up to 10 cycles is also analysed. The cyclic loading effect over the stress-state of the samples was evaluated by measuring the mechanical resistance and associated to the observed crystallinity- and porosity-related features.
Section snippets
Materials and experimental procedure
Powder of α-Al2O3 (Grade A16SG, Alcoa Chemicals, USA) was used. SiO2 (Soc. Portuguesa de Diatomite, P) was introduced as natural diatomite. MgO, CaO and BaO additives were introduced as magnesium hydroxide carbonate, 4MgCO3·Mg(OH)2·5H2O (Merck, Germany), calcium carbonate, CaCO3 (M1, Mineraria Salcilese, I) and barium carbonate, BaCO3 (Grade 99+%, Aldrich, Austria), respectively. Aqueous suspensions containing 1:1 volume ratio of Al2O3 and SiO2 powders and the proper amount of each precursor,
Phase formation and microstructure
The XRD spectra of the sintered non-doped and BaO-doped ceramics as well as the MgO- and CaO-doped ones are presented in Fig. 1.
In the pure Al2O3–SiO2 system, Fig. 1(a), α-Al2O3 and crystobalite, α-SiO2, are the main crystalline phases formed while 3Al2O3·2SiO2 appears as a minor phase. The same phases were found for 5% BaO, although α-SiO2 peaks appear less intense and a more abundant glassy phase is apparent. Since no BaO-containing crystalline phase was formed, it suggests that the glassy
Conclusions
The different alkaline-earth oxides added to aluminosilicate compositions produced different effects on final microstructures, mechanical and thermal properties:
(1) High-density structures and good mechanical properties were obtained for 5% CaO-, 5% BaO- and 10% BaO-doped samples. 5% MgO-doped ceramics presented full-crystalline porous structures with low mechanical strength.
(2) The elastic, thermal and mechanical properties show a complex dependence on pore volume fraction, pore size and
Acknowledgements
The first author wishes to thank Fundação para a Ciência e a Tecnologia (FCT) of Portugal for the financial support under the grant PRAXIS XXI/BD/18605/98. The authors are also in debt with Agência de Inovação, S.A., in Portugal for the financial support in the frame of the Project P051-P31B-05/97-DISASVAL.
References (30)
- et al.
Cement Concrete Res.
(1997) - et al.
Ceramics International
(1999) - et al.
Acta Mater.
(2002) Br. Ceram. Trans. J.
(1992)- et al.
J. Am. Ceram. Soc.
(1998) - et al.
J. Am. Ceram. Soc.
(1998) - et al.
J. Phys. III France
(1994) - et al.
JOM
(1998)
Brit. Ceram. T.
CFI/Ber. DKG
Cited by (13)
Study of the hexagonal to monoclinic celsian phase transition induced by magnetic phase nucleation in barium aluminosilicate glass-ceramics
2022, Journal of the European Ceramic SocietyCitation Excerpt :Adding mineralizers is another useful method. In a lot of previous literature, Li2O [23–27], LiF, Na2O [28], CaO [29,30], etc. have been applied to facilitate the formation of celsian phase at slightly low temperature through decreasing the viscosity of BAS glass system [31,32]. Sometimes, these additions tend to replace the Ba2+ sites leading to the difference of aggregation degree of glass network due to the difference of ionic field strength.
The effect of aqueous media on the mechanical properties of fluorapatite-mullite glass-ceramics
2015, Dental MaterialsCitation Excerpt :The reduction of the water stability and hardness values was also observed in G and GB glass–ceramic compositions. These could be explained by less cross-linked glass network and easier diffusion of ions [31,32]. The MTT results indicate that all of the glass–ceramic samples are biocompatible and the additive oxides and extra amount of SiO2 don not led to the toxicity after 28 days cell proliferation.
Comparison of sintering and compressive strength tendencies of a model coal mineral mixture heat-treated in inert and oxidizing atmospheres
2011, Fuel Processing TechnologyCitation Excerpt :An increase in porosity reduces mechanical resistance and enables easier crack propagation when a force is applied [57,64]. Changes in porosity can also affect elastic constants [64]. A decrease in the pellet's ability to deform ensures easy crack initiation [62].
Preparation, characterisation and industrial evaluation of foamed fireclay
2020, Journal of the Australian Ceramic Society
Al2O3