In this study, we investigate the oxidation behavior of copper at temperatures below 300 °C and its mechanism. A methodology to slow down the oxidation rate is then proposed based on the observed mechanism. The oxides formed after oxidation at low temperatures have fine crystal sizes; the rate constants reach 2×10 -15 m 2 /s and 6×10 -14 m 2 /s at 200 °C and 300 °C, respectively. A passivation treatment at 600 °C in nitrogen produces a thin oxide layer composed of relatively large Cu 2 O crystals. The presence of such a layer slows down the oxidation rate constants by an order of magnitude. This study demonstrates that the oxidation of copper at low temperatures is controlled by the grain boundary diffusion. Increasing the crystal size in the surface oxide reduces the oxidation rate significantly.

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An essential criterion for the selection of resorbable bioceramics is their ability to degrade inside human body within a reasonable time frame. Furthermore, if the bioceramic can release beneficial ions, such as strontium, as it degrades, recovery time might be shortened. The present study demonstrates that strontium-containing calcium sulfate (Sr,Ca)SO4 can fulfill these criteria. A long-term in vitro degradation analysis for 12 weeks using sintered (Sr,Ca)SO4 discs in phosphate buffered solution (PBS) was conducted. The sintered (Sr,Ca)SO4 disc was then implanted into defects in the distal femur of rats. The degradation rate of (Sr,Ca)SO4 discs showed a strong dependence on the Sr content. Similar results were observed between the long-term in vitro degradation analysis and the in vivo evaluation. The sintered (3.8%Sr,Ca)SO4 disc lost more than 80% of its initial weight after soaking in PBS with shaking at 37 °C for 12 weeks. After 12 weeks in vivo, the remaining volume of the (3.8%Sr,Ca)SO4 disc within the bone defect was ~25%. Over the same time period, new bone was formed at a relative volume of 40%. This study demonstrates the potential of (Sr,Ca)SO4 bioceramic, and the benefits of using a long-term degradation test during the evaluation of resorbable bioceramics.