Synthesis and hydration behavior of calcium zirconium aluminate (Ca7ZrAl6O18) cement
Introduction
In dentistry, calcium silicate-based cements are used for dental root canal filling and repair materials, but their radiopacity is not sufficient to be visualized radiographically [1], [2], [3], [4]. To distinguish the filling and repair materials from the surrounding anatomical structures (dentine), radiopacifying materials such as bismuth oxide, zinc oxide, and zirconium oxide has to be added to the calcium silicate-based cements [1], [5], [6]. However, the addition of a radiopacifier in a minimal amount can change the setting chemistry, biocompatibility, and physical properties of the cements. In this aspect, it is beneficial to develop the cement with the radiopacifier as a component instead of as an additive, and ZrO2-containing calcium aluminate (Ca7ZrAl6O18) cement is a promising candidate.
Ca7ZrAl6O18 is only a compound with hydration reactivity in CaO–Al2O3–ZrO2 system [7], [8], [9]. Its crystal structure is orthorhombic with a space group of Pmn21, in which five types of Ca atoms and four types of AlO4 tetrahedra are positionally and orientationally disordered, respectively [9]. The hydration behavior of Ca7ZrAl6O18 is comparable to that of Ca3Al2O6 (C3A), and Ca7ZrAl6O18 is free from premature setting. Ca7ZrAl6O18 is expected to take the place of Ca3Al2O6 in normal Portland cement and have a potential application as dental root canal filling and repair materials, but the studies were very limited and very few reports have been published [7], [8], [9], [10].
Ca7ZrAl6O18 has been synthesized by a conventional solid state reaction [7], [9], which requires a high temperature and a prolonged sintering time. Polymeric precursor (or Pechini) method is a low temperature sol–gel route based on the polybasic acids (e.g., citric acid) to form chelates with metallic ions. The chelates undergo polyesterification upon heating with the polyhydroxyl alcohols (e.g., ethylene glycol) to form the large metal–organic complexes [11], [12]. This method offers a molecular level mixing resulting in the powders with a high purity, an ultrahigh homogeneity, a wide range of particle size, and a lower calcination temperature. The polymeric precursor method has been employed to prepare the calcium aluminate and calcium silicate cements [13], [14], [15], but it has not been applied to synthesize the Ca7ZrAl6O18 cement.
In this study, calcium zirconium aluminate (Ca7ZrAl6O18) was synthesized by polymeric precursor and solid-state reaction methods, and their phase evolution and morphology were investigated. The synthesized Ca7ZrAl6O18 powder was treated by planetary ball milling to control the particle size and morphology. In addition, the hydration behavior and radiopacity of Ca7ZrAl6O18 were examined and compared with that of Ca3Al2O6 (C3A), which was prepared in the similar manner.
Section snippets
Materials and methods
For solid state reaction synthesis, the stoichiometric mixture of CaCO3 (Acros, Geel, Belgium), ZrO2 (Fine Materials, Korea), and Al2O3 (Aldrich, Milwaukee, WI) was mixed in acetone, ball milled for 24 h, and then dried in an oven for 24 h. The dried mixtures were pre-calcined at 900 °C for 3 h in air and then annealed at 1000–1400 °C for 3 h. In polymeric precursor process, Ca(NO3)2·4H2O (Aldrich, Milwaukee, WI), ZrO(NO3)2 (Aldrich, Milwaukee, WI), and Al(NO3)3·9H2O (Aldrich, Milwaukee, WI) were
Phase and morphology
The XRD patterns for Ca7ZrAl6O18 powders prepared by solid-state reaction are shown in Fig. 1 as a function of annealing temperature. At 1000 °C, CaZrO3 and Ca3Al2O6 (C3A) were formed, but unreacted starting materials (CaO, ZrO2, and Al2O3) were still observed (Fig. 1(A)). With increasing the annealing temperature, the peak intensity for CaZrO3 and C3A increased with the expense of starting materials (Fig. 1(B), (C)). At 1300 °C, these two phases were transformed into Ca7ZrAl6O18 with a small
Conclusions
A nearly single phase Ca7ZrAl6O18 was synthesized at relatively low temperature of 1200 °C through polymeric precursor process. The hydration behavior of Ca7ZrAl6O18 was similar to that of C3A, but the hydration products were different. The intermediate hydration products of C3A persistently exist in the hydrates of Ca7ZrAl6O18, which appears to be related to the degree of heat evolution, but further studies are required to find out the reason. The Ca7ZrAl6O18 shows a better radiopacity than C3A
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012-008226).
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