Reconfirmation of the existence of δ2 in dental amalgams

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The ∼90°C endotherm peak in the thermogram of a conventional dental amalgam is due to two overlapping peritectic transitions involving SnHg-δ2 and HgAgSn-γ1, respectively. The aim of the present study is to separate these two events and thus confirm that the δ2 phase is indeed a part of an amalgam microstructure. The materials used in this study were: sample 1, γ1+1 wt% Sn; sample 2, γ1+2 wt% Sn; sample 3, γ1+3 wt% Sn; and sample 4, a commercial conventional amalgam with 48 wt% Hg. In powder form, they were exposed to 1 wt% NaCl solution at 37 °C for up to 45 days. At 15 day intervals, samples were withdrawn from the solution, washed, dried and then characterized by the differential scanning calorimetry (DSC) technique. Corrosion of Sn from ternary γ1 matrix and intergranular δ2 in respective materials during their exposure to the NaCl solution led to the following: (a) a progressive increase in γ1→β1 transition temperature in samples 1 and 2; (b) a gradual decrease in δ2 peak and its disappearance in sample 2; and (c) in samples 3 and 4, initial splitting of the ∼90°C peak into two distinct endotherms associated with δ2 and γ1, respectively. Continued corrosion of the last two materials produced further changes in δ2 and γ1 in a manner similar to that seen in samples 1 and 2. On the basis of these observations, we have concluded that the ∼90°C endotherm is a valid indicator of the existence of the δ2 phase in dental amalgams.

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