Theoretical models of mercury dissolution from dental amalgams in neutral and acidic flows
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ALTHOUGH mercury (Hg) dissolution from dental amalgam has attracted intense research interest, particularly over the last 2 decades, a detailed, quantitative understanding of the process remains elusive. A number of complicating features have slowed progress in this direction: the microstructure of amalgam is complex, typically composed of eight or more distinct metastable phases; in oxygen-containing atmospheres, metal oxides form on the amalgam surface and in intergranular spaces between constituent grains, forming effective, though difficult-to-quantitate barriers to Hg transport; Hg release is sensitive to the external environment and to the amalgam’s chemical composition, exhibiting qualitatively distinct, composition-dependent temporal behaviors in neutral and acidic solutions; stress, strain, surface abrasion, microfracture, and corrosion effects on Hg dissolution and solid diffusion remain largely unknown; and the chemistry of solid-phase Hg dissolution and consumption has not been fully characterized. Traditional low copper amalgam is made by mixing RUSSELL G. KEANINI, Associate Professor, is with the Department of Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte, Charlotte, NC 28223. JACK L. FERRACANE, Professor, is with the Department of Biomaterials and Biomechanics, School of Dentistry, Oregon Health Sciences University, Portland, OR 97201-3007. TORU OKABE, Regents Professor and Chairman, is with the Department of Biomaterials Science, Baylor College of Dentistry, Dallas, TX 75246. Manuscript submitted August 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
approximately equal weights of Hg and silver-tin alloy powder (composed primarily of Ag3Sn (␥)) to produce a solid composed primarily of silver-mercury (Ag2Hg3 (␥1)) and tin-mercury (Sn8Hg (␥2)) phases.[1] In contrast, high copper amalgam incorporates silver-copper particles either within the silver-tin alloy admixture or within a ternary silvertin-copper alloy, both of which result in a solid composed primarily of ␥1 and copper-tin (Cu6Sn5 (⬘)) phases when mixed with mercury. It is important to note that the relatively unstable, corrosion-prone ␥2 phase found in low copper amalgam appears in only trace amounts in high copper amalgam; in acidic solutions, this compositional difference leads to qualitatively distinct Hg release characteristics. One of the primary objectives of the present work is to characterize these distinct behaviors experimentally and theoretically. Surface oxide films and intergranular oxides play a predominant role in both suppressing and determining mercury release mechanisms in dental amalgam.[2,3,4] With regard to intergranular oxides, it is known that relatively thick tin oxide films cover ␥2 grains in low copper amalgam. Thus, Hg release from both low and high copper amalgam in oxygen-containing and neutral aqueous environments appears to originate from ␥1 grains within the matrix. Important insight into the composition, structure, and thickness of surface oxide films was recent
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