A study of a low copper dental amalgam by analytical transmission electron microscopy
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A study of a low copper dental amalgam by analytical transmission electron microscopy Tejpal Kaur Hooghan and Russell F. Pinizzotto Materials Science Department, University of North Texas, Denton, Texas 76203-0308
John H. Watkins and Toru Okabe Department of Biomaterials Science, Baylor College of Dentistry, Dallas, Texas 75246 (Received 14 December 1995; accepted 20 April 1996)
Analytical transmission electron microscopy was used to study specimens of a low-Cu dental amalgam (Velvalloy), prepared using the “wedge technique.” Analysis confirmed that the microstructure consists of a Ag2 Hg3 (g1 )yHgSn7–9 (g2 ) matrix surrounding unreacted Ag3 Sn(g) particles. In addition a hitherto uncharacterized reaction layer of fine grains between Ag3 Sn(g) and Ag2 Hg3 (g1 ) is a mixture of Ag3 Sn(g), Ag–Hg–Sn (b1 ), Ag2 Hg3 (g1 ), and occasionally Cu6 Sn5 (h 0 ). An Ag–Hg –Sn (b1 ) phase was clearly identified for the first time. Since Velvalloy is a simple commercial dental amalgam, it is a reasonable starting point for characterizing more complex dental amalgam microstructures.
I. INTRODUCTION
B. Amalgamation reactions
Dental amalgams are a major restorative material for the filling of tooth cavities. Amalgams are prepared by mixing alloy particles, which contain Ag, Sn, and Cu as the major constituent elements, with liquid Hg. The resultant plastic mass is compacted into the prepared tooth cavity where it solidifies and hardens by chemical reaction.
When liquid Hg reacts with the Ag –Sn (1Cu) alloy particles, both Ag and Sn and probably Cu dissolve into the Hg. In low Cu amalgams, Ag reaches supersaturation first, well before the Sn, which causes Ag2 Hg3 (g1 ) to precipitate before HgSn7-9 (g2 )3 (the solubilities of Cu, Ag, and Sn in Hg are 0.019 at. %, 0.66 at. %, and 1.68 at. %, respectively4 ). Since precipitated equiaxed Ag2 Hg3 (g1 ) grains occupy most of the matrix volume, and since Sn has the highest solubility in Hg, HgSn7–9 (g2 ) grains precipitate last, usually as elongated “discrete clumps”5 in open spaces between the Ag2 Hg3 (g1 ) grains. In high-Cu amalgams, there are two proposed mechanisms for Cu6 Sn5 (h 0 ) precipitation.6,7 Rod-shaped Cu6 Sn5 (h 0 ) may precipitate first because Cu has the lowest solubility in Hg. Alternatively, dissolved Sn may react with a Cu-containing phase forming Cu6 Sn5 (h 0 ). Later, the Ag2 Hg3 (g1 ) forms around the Cu6 Sn5 (h 0 ) precipitates. Since Sn is scavenged by the Cu, only a small amount of HgSn7–9 (g2 ) will form. During these reactions, the alloy powder and all precipitates coexist with liquid Hg, retaining the plastic consistency of the mixture for a short period of time, approximately 10 –15 min, depending on the alloy system. The material completely solidifies within 3–10 h.
A. Alloys for dental amalgam
The alloy powder for dental amalgam is produced either by water or gas atomization, or by lathe-cutting. In general, when the Cu concentration is less than 6 wt. % in the particles, the alloy is known as a low-Cu al
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