Effect of tantalum pentoxide addition on the radiopacity performance of Bi₂ O₃ /Ta₂ O₅ composite powders prepared by mechanical milling

Among the various phases of bismuth oxide, the high temperature metastable face-centered cubic δ phase attracts great attention due to its unique properties. It can be used as an ionic conductor or an endodontic radiopacifying material. However, no reports concerning tantalum and bismuth binary oxide prepared by high energy ball milling and serving as a dental radiopacifier can be found. In the present study, Ta₂ O₅-added Bi₂ O₃ composite powders were mechanically milled to investigate the formation of these metastable phases. The as-milled powders were examined by X-ray diffraction and scanning electron microscopy to reveal the structural evolution. The as-milled composite powders then served as the radiopacifier within mineral trioxide aggregates (i.e., MTA). Radiopacity performance, diametral tensile strength, setting times, and biocompatibility of MTA-like cements solidified by deionized water, saline, or 10% calcium chloride solution were investigated. The experimental results showed that subsequent formation of high temperature metastable β-Bi_7.8 Ta_0.2 O_12.2, δ-Bi₂ O₃, and δ-Bi₃ TaO₇ phases can be observed after mechanical milling of (Bi₂ O₃)₉₅ (Ta₂ O₅)₅ or (Bi₂ O₃)₈₀ (Ta₂ O₅)₂₀ powder mixtures. Compared to its pristine Bi₂ O₃ counterpart with a radiopacity of 4.42 mmAl, long setting times (60 and 120 min for initial and final setting times) and 84% MG-63 cell viability, MTA-like cement prepared from (Bi₂ O₃)₉₅ (Ta₂ O₅)₅ powder exhibited superior performance with a radiopacity of 5.92 mmAl (the highest in the present work), accelerated setting times (the initial and final setting time can be shortened to 25 and 40 min, respectively), and biocompatibility (94% cell viability).