GUO Huiming, LI Xiuying, LAWRENCE Cooper

(School of Material Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)

Extended abstract:[Background and purposes] Bismuth borate glasses reported in literature mainly include Bi₂O₃-B₂O₃, Bi₂O₃-ZnO/BaO/PbO-B₂O₃, Bi₂O₃-ZnO-B₂O₃-SiO₂ systems. In transition metal doped bismuth borate glasses, CuO-doped Bi₂O₃-B₂O₃ glasses have been rarely studied, while the doping concentration of CuO is usually less than 10 mol%. It has been indicated that the doping of CuO in bismuth borate glass has several advantages. Firstly, it helps to increase the density of the glass, thus making the liquid phase to be at a lower temperature. Secondly, it improves the alkaline resistance of glass. Thirdly, the thermal expansion coefficient of the glass has a very weak change, while slightly reducing the characteristic temperature of the glass, which benefits its application at low temperatures. The current authors found that high concentrations of CuO [specifically in (40−x)ZnO-xCuO-40Bi₂O₃-20B₂O₃(x>20 mol%) system] lead to the precipitation of crystalline phases during the sintering process, adversely impacting the fluidity and thermal expansion coefficient of the solder and hence complicating the sealing processes. To systematically study the influence of CuO concentration on the composition, structure, and properties of bismuth borate glass, it is imperative to first study the glass formation range in the CuO-Bi₂O₃-B₂O₃ ternary system and identify glass compositions from the view of practical applications. In oxide-doped bismuth borate glasses, the glass formation region, of PbO/ZnO/BaO-Bi₂O₃-B₂O₃, CaO/SrO-Bi₂O₃-B₂O₃ and TeO₂-Bi₂O₃-B₂O₃, has been reported, while the glass formation region of CuO-Bi₂O₃-B₂O₃ ternary system has not been presented.[Methods] Analytical grade raw materials, including Bi₂O₃, CuO and H₃BO₃, were weighed according to ratio of each composition and ball milled to obtain glass batch. The batch is then placed in a corundum crucible and heated in an electric furnace to 1000 ℃ for 1 h. After that, the molten glass is poured into molds, annealed and subsequently cooled within the furnace to obtain solid glass blocks. X-ray diffraction (XRD) is utilized for characterization of the samples exhibiting incomplete melting, poor glass luster, flocculation, stratification or other anomalies visible to the naked eye. The glass forming ability is evaluated based on sample appearance and XRD results, thereby establishing the glass formation region of the CuO-Bi₂O₃-B₂O₃ ternary system.[Results] The content of Bi₂O₃ in the Bi₂O₃-B₂O₃ binary glass is found to be in the range from 30 mol% to 70 mol%. Samples with Bi₂O₃ content <30 mol% exhibit tendency of phase separation, whereas the samples with >70 mol% Bi₂O₃ results in the precipitation of Bi₂O₃. In the CuO-B₂O₃ binary system, samples with high CuO concentrations demonstrate strong crystallization, leading to the precipitation of Cu₂O and aluminium-containing crystalline phases. Conversely, samples with lower CuO contents are amorphous but lack glass lustre, rendering them unsuitable for practical applications, while also exhibiting severe corrosion of the alumina crucible. In the CuO-Bi₂O₃-B₂O₃ ternary glass, maintaining Bi₂O₃ content at 5 mol%, while controlling CuO or B₂O₃ content to below 55 mol%, facilitates the production of uniform glass, with CuO concentrations up to 50 mol%. An increase in the content of Bi₂O₃ significantly enhances the glass formation ability and thermal stability, with the maximum CuO concentration in glass compositions containing 10–20 mol% Bi₂O₃ reaching 40 mol%. For samples with Bi₂O₃ content exceeding 20 mol%, the maximum permissible CuO concentration is limited to 30 mol%.[Conclusions] Homogeneous glasses cannot be prepared from compositions with the content of B₂O₃ below 15 mol% in the CuO-Bi₂O₃ ternary system, because of the precipitation of Bi₂O₃ and CuBi₂O₄ crystalline phases during the cooling process of the melts. Compositions with >70 mol% B₂O₃ also fail to produce glass with practical utility, as they are prone to phase separation and cracking. Compositions with B₂O₃ content in the range of 50–70 mol% can favorably produce glass, when the content of Bi₂O₃ is no less than 15 mol%. Additionally, glass can be formed provided Bi₂O₃ content is not less than 5 mol%, when the content of B₂O₃ ranges from 45 mol% to 50 mol%. When the B₂O₃ content is reduced from 45 mol% to 15 mol%, glass can be prepared by increasing the ratio of Bi₂O₃/CuO. Otherwise, Cu₂O crystals tend to crystallize from the melt during the cooling process. The CuO-Bi₂O₃-B₂O₃ ternary system has a wide glass formation region, where the maximum contents of CuO and Bi₂O₃ in the glass are 50 mol% and 75 mol%, respectively.

Key words: bismuth borate glass; phase identification; glass forming region