J. BIOMED. MATER. RES.
VOL. Y, PP. 253-254 (1975)
NOTES Diflusion of H g in Dental Amalgams I n a recent note' to this journal, we reported on a method of estimating diffusion coefficients D of Hg in the pure y1 phase (Ag2Hg3)and the AgSn phase of dental amalgams. The method involved the measurement of ,weight losses vs. time at a given temperature. The most significant fact we were attempting to make, namely t h a t D (Hg:rl) was much larger than D (Rg: Ag,Sn-amalgam), was clear enough from the observations but the data analyzed by the method were actually taken for another purpose and, as suggested in the note, the different dependences on weight loss vs. time for the two systems required some rationalization in terms of differences in their morphological structures. Also the reported diffusion coefficients were a t 70°C, which is somewhat high for many applications of interest t o dentists. For these reasons and because the diffusion of mercury in the various phases of dental amalgams is a matter of considerable importance with regard to mechanical strength and corrosive properties, a more direct and accurate way to determine D in small particleswas developed. The details of the method will be described in another publication2 but it seems appropriate here to state the more accurate values of D of Hg into Ag3Sn and to mention the effect of temperature T. As might be expected, D as a function of T is described rather well by an Arrhenius expression D = DO exp ( - E o / k T ) where Lto is constant and E D is the activation energy for diffusion. The new experiments give DO = 4.2 cm2/sec and E D = 78 f 6 kJ/mol (18.6 f 1.5 kcal/mol). Relevant values of the Hg diffusion coefficient D a t various temperatures are listed in Table I. In very brief terms, the new method utilizes a Cambridge scanning electron microscope with an attached Ortec Si(Li) X-ray energy dispersive spectrometer to TABLE I Diffusion of Hg in Ag,Sn - Particles in Amalgam* T ["C] 24 37
55 72 98
D [cmz/sec] 9 X TO-'* 2 . 7 x 10-13c 2 X 3.7 X 6 . 5 X lo-"
thlb
_.
590 241 257 8
a The Ag,Sn particles were about 50pm in diameter and embedded in the amalgam matrix. b Time maintained at experimental temperature. Calculated from Arrhenius expression with our values for D O and E D .
253
@ 1975 by John Wiley & Sons, Inc.
254
NOTES
measure the relative Hg concentration profile C/Csurfaoe vs. distance z from the particle boundary. At a sufficiently early stage 01 the diffusion of Hg into the alloy, the very small particles can be treated aa semi-infinite bodies. I n the early stages theory predicts that a plot of zvs.erf-l[l- (C/Cs)] should be a straight line through the origin and that the initial slope should be (4Dt)1/P. Both mathematical and experimental details will be discussed in another paper.2 This research was supported in part by a National Institute of Dental Research Training Grant and Program/Project DE-02111.
References 1. C. L. Reynolds, Jr. and R. E. Barker, Jr., J . Biorned. Mater. Res., 7,489 (1973). 2. M. L. Malhotra, C. L. Reynolds, Jr., and R. E. Barker, Jr., J . Chem. Phys., 60, 3831 (1974).
C. L. REYNOLDS, JR. R. E. BARKER, JR. Department of Materials Science School of Engineering and Applied Science University of Virginia Charlottesville, Virginia Received February 10, 1974
VOL. Y, PP. 253-254 (1975)
NOTES Diflusion of H g in Dental Amalgams I n a recent note' to this journal, we reported on a method of estimating diffusion coefficients D of Hg in the pure y1 phase (Ag2Hg3)and the AgSn phase of dental amalgams. The method involved the measurement of ,weight losses vs. time at a given temperature. The most significant fact we were attempting to make, namely t h a t D (Hg:rl) was much larger than D (Rg: Ag,Sn-amalgam), was clear enough from the observations but the data analyzed by the method were actually taken for another purpose and, as suggested in the note, the different dependences on weight loss vs. time for the two systems required some rationalization in terms of differences in their morphological structures. Also the reported diffusion coefficients were a t 70°C, which is somewhat high for many applications of interest t o dentists. For these reasons and because the diffusion of mercury in the various phases of dental amalgams is a matter of considerable importance with regard to mechanical strength and corrosive properties, a more direct and accurate way to determine D in small particleswas developed. The details of the method will be described in another publication2 but it seems appropriate here to state the more accurate values of D of Hg into Ag3Sn and to mention the effect of temperature T. As might be expected, D as a function of T is described rather well by an Arrhenius expression D = DO exp ( - E o / k T ) where Lto is constant and E D is the activation energy for diffusion. The new experiments give DO = 4.2 cm2/sec and E D = 78 f 6 kJ/mol (18.6 f 1.5 kcal/mol). Relevant values of the Hg diffusion coefficient D a t various temperatures are listed in Table I. In very brief terms, the new method utilizes a Cambridge scanning electron microscope with an attached Ortec Si(Li) X-ray energy dispersive spectrometer to TABLE I Diffusion of Hg in Ag,Sn - Particles in Amalgam* T ["C] 24 37
55 72 98
D [cmz/sec] 9 X TO-'* 2 . 7 x 10-13c 2 X 3.7 X 6 . 5 X lo-"
thlb
_.
590 241 257 8
a The Ag,Sn particles were about 50pm in diameter and embedded in the amalgam matrix. b Time maintained at experimental temperature. Calculated from Arrhenius expression with our values for D O and E D .
253
@ 1975 by John Wiley & Sons, Inc.
254
NOTES
measure the relative Hg concentration profile C/Csurfaoe vs. distance z from the particle boundary. At a sufficiently early stage 01 the diffusion of Hg into the alloy, the very small particles can be treated aa semi-infinite bodies. I n the early stages theory predicts that a plot of zvs.erf-l[l- (C/Cs)] should be a straight line through the origin and that the initial slope should be (4Dt)1/P. Both mathematical and experimental details will be discussed in another paper.2 This research was supported in part by a National Institute of Dental Research Training Grant and Program/Project DE-02111.
References 1. C. L. Reynolds, Jr. and R. E. Barker, Jr., J . Biorned. Mater. Res., 7,489 (1973). 2. M. L. Malhotra, C. L. Reynolds, Jr., and R. E. Barker, Jr., J . Chem. Phys., 60, 3831 (1974).
C. L. REYNOLDS, JR. R. E. BARKER, JR. Department of Materials Science School of Engineering and Applied Science University of Virginia Charlottesville, Virginia Received February 10, 1974
