Phase Discrimination Using Scanning Electron
Microscopy Techniques
LEONARD N. JOHNSON
Department of Restorative Dentisty, Division of Dental Materials Science, Faculty of Dentistry, University of Western Ontario, London, Ontario, Canada
Polished, unetched amalgam surfaces were
studied metallographically with a scanning
electron microscope (SEM). By use of different modes of SEM operation, micrographs were obtained from variations in the
electric properties of the respective phases
present. This technique minimizes artifact
formation that may result from conventional
electric or chemical etching techniques.
To gain an understanding of the amalgam
reaction, much emphasis has been placed on
the technique of polishing the specimen so
that a surface truly representative of the
mass can be obtained.1'2 More emphasis has
been placed on the methods of etching,
which have been reviewed by Allen and
Asgar2,3 and Wing.1 Despite the valuable
information that has been obtained from
such metallurgical techniques, it still is not
known whether one interprets fact or artifact when a chemical etch is used. As a result, a complete understanding of dental
silver amalgam is lacking. The purpose of
this paper is to describe a means of examining polished amalgam surfaces or any
polished metal surface without the aid of
electric or chemical etching, by considering
the electric properties of the sample.
Background and Rationale
Electric conductivity generally is associated with the metallic elements and their
alloys, and by definition, substances that
have relatively high electric conductivity are
classified as metals. There is a regularity in
electric conductivity that corresponds to the
Tlhis investigation was supported by Province of Ontario Health Research Grant No. PR 105.
This paper was presented, in part, at the 49th general
session of the IADR in Chicago, Illinois, March, 1971.
K4tceived for publication April 15, 1971,
regularity of the periodic table. Divalent
metals have lower conductivity than monovalent metals, and transition metals have
even lower conductivity. In addition to the
valency, the property of conductivity is decreased greatly with alloying and the change
is significant for even very small additions.
Also, differences in atomic size, valence,
crystal structure, or electronegativity tend
to make dissolved atoms more effective scatterers of conduction electrons, and thereby
lower the conductivity.5
Dental silver amalgam is a heterogeneous,
multiphase metal that may include several
solid solutions, several intermetallic compounds, and several eutectics. Each particle
or phase has its own unique electric conductivity value. When an electron beam
such as that generated in a scanning electron
microscope (SEM) is focused on the heterogeneous surface, the influence that the electric property of each phase has on the
electron beam should be discernible. Knowledge of what constituent elements are in the
alloy and their various possible combinations, and that the conductivities of the respective phases are a function of the atomic
number, and use of suitable means to evaluate the relative conductivity value of the
respective phases should enable one to define clearly a polished, unetched surface.
This may be accomplished by use of the
"specimen absorbed" current mode of operating the SEM. This mode considers the
phenomena that take place when a primary
electron beam interacts with a specimen that
is conductive and free of pn junctions and
other electric barriers. For example, if an
amalgam or an amalgam alloy in which a
charge collection does not occur when subjected to an electron beam is connected to
earth by an operational amplifier, one can
789
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790
JOHNSON
examine the electric behavior of the specimen. If the specimen is subjected to a primary electron beam current of I. amperes
and if the instantaneous secondary electron
and reflected electron currents are Is and
Irp respectively, an instantaneous residual
current of {Ip - (I, + Irp) } will have to be
neutralized by current flow through the amplifier. This "neutrality" or "specimen absorbed" current, as it is usually called, can
be used to obtain a micrograph, the contrast
of which is determined mainly by variations
in secondary electron emission and in the
back-scattered electron coefficient across the
specimen surface.6
This information can be displayed as
either an intensity modulation, where the
signal is sent to the grid of the cathode ray
tube (CRT), or as a line scan where the signal is sent to the y-plates of the CRT. This
paper deals with another means of displaying the information, a technique similar to
that used in microanalysis and scanning
electron diffraction studies. In this method
the signal is sent to the y-plates of the CRT
and the frame time-base simultaneously and
synchronously. In this way a series of line
scans are superimposed one behind the other
across the micrographs, and therefore, useful qualitative and quantitative data can be
presented accurately and quickly.
Materials and Methods
To demonstrate this technique amalgams
were made from two different alloys, Dispersalloy* and Spheraloyt with the manufacturers' recommended mercury-alloy ratio.
The amalgam specimens were "potted" in a
polyester resin, one inch in diameter by a
half inch in thickness, and polished according to the method described by Allen, Asgar,
and Peyton.2,3 Micrographs of the unetched
amalgam specimens were obtained on a
metallography (Leitz MM5) and compared
with micrographs from the SEM§ generated
by secondary electron emission, "specimen
absorbed" current, and y-deflection modulation. X-ray analyses were made on an electron microprobel for compositional comparison.
Results and Discussion
The value of the technique just described
may best be assessed by viewing various
* Unitek Corp., Monrovia, Calif.
Kerr Manufacturing Co., Romulus. Mich.
Leitz MM5 metalograph.
§Cambridge Mark Ila Stereoscan.
1I Cambridge Mark V x-ray microanalysis system.
J Dent Res May-June 1972
micrographs. The upper left frame of Figure
1, top is an optical micrograph of a polished
unetched amalgam surface made from Dispersalloy. The three particular sites of interest are A, C, and D. The upper right frame
is a SEM micrograph of the same area as
left made from reflected secondary electrons. The contrast in this instance is improved greatly. A particle at site A is now
visible, whereas there was no indication of
a particle at site A in the polished, unetched
optical micrograph. The particle at site D
is visible clearly with corrosion products
forming on the surface; the same area appears only as a faint ghost in the optical
micrograph.
The contrast is improved further when
the SEM micrograph is generated by the
specimen absorbed current, as seen in the
lower left frame. Sites B and D that represent the spherical silver-copper eutectic
phase of this amalgam system display an
outer ring or halo that was not discernible
in either of the two micrographs described
previously. This micrograph records the
high conducting phases as white and the low
conducting phases as decreasing shades of
gray; black represents the least conducting
phase. Therefore, one can assume that all
areas with the same shade of gray will have
the same conductivity, and thus, the same
elemental composition.
Sites A, B, C, and D are located readily
with the use of the secondary electron micrograph as reference. The lower right
frame of Figure 1, top and Figure 1, center
are X-ray displays obtained from the scanning electron microprobe. The distribution
of the respective elements (zinc, silver, tin,
copper, and mercury) show the complexity
of this system, because the elements appear
in various combinations.
Figure 1, bottom is the y-deflection modulation display that reveals the relative conductivities of the various phases on the
surface. Even at low magnifications this
technique may be useful in scanning large
areas. The interpretation of the y-deflection
modulation micrograph is made by tracing
any one of the lines from left to right. As
the beam enters a phase of higher conductivity, a greater signal is produced, which
creates a sharp upward deflection of the line
which is proportional to the signal. The
converse is true when an area of low conductivity is traversed. Higher magnifications
show this in greater detail and expose the
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Vol 51 No. 3
NPHASE DISCRIMINATION WJIH SAEM
791
subtleties not otherwise seen by other metallographic techniques.
Figure 2, top shows the magnified phase
of the selected site A in Figure 1, top. The
optical micrograph (upper left frame) of
site A does not show the presence of the
particle in question. The micrographs obtained from secondary electrons (upper
right frame) and specimen absorbed current
(lower right frame) show the particle distinctly, in spite of the fact that all micrographs were made of the same polished
unetched surface. The specimen that absorbed current (lower right frame) shows a
superimposed line scan by double exposure.
A triple exposure was made by turning off
the beam current and allowing the CRT spot
to trace the exact path of the line scan. This
assists in the interpretation of the micrograph. One must not attach significance to
the distance between the two scan lines, because this is chosen arbitrarily. As the scanning spot approaches the phase in question
from the left (Fig 2, top, lower right
frame), a sharp rise in conductivity is noticed. This may be caused by a phase that
exhibits high conductivity or a void that
may act as an electron trap. The micrograph
generated by the reflected secondary electrons, which are most sensitive to changes
in topography, shows the latter to be true,
and thus the information is discounted. The
beam then enters a narrow segment of gray.
traverses a white-appearing "inclusion,"
passes into the gray area, and finally ends in
the black-appearing background. The Xray displays of the elemental distribution
(Fig 2, center) show that the white area
is primarily copper-tin, possibly similar to
that described by Allen ': and Johnson.78
The gray phase is silver-tin and the black
background matrix is primarily silver and
mercury. The y-modulated scan shows these
phases in order of decreasing conductivity,
and this is what one would expect. The ydeflection modulated scan summarizes this
information in the form of a relief or typographical map of the selected site (Fig 2,
bottom).
Figure 3, top is a composite of similar
micrographs of the selected site D of Figure
3175x
-*J37
1, top. The X-ray displays of Figure 3, cen- Y Mod.-Scan
FIG 1. Top, micrographs representative of
ter and the lower left frame of Figure 3,
including X-ray microanalysis
top show that this is the spherical silver- Dispersalloy,
for zinc. Cenjer, X-ray microanalysis displays
copper eutectic dispersed phase of the Dis- of constituent elements of area shown in Fig
persalloy system, and that this phase obvi- 1. top. Bottom, Y-deflection modulation microouLsly has anl afLility for tin, as indicated in ,i apl of ai-ca bhown in 1 iguIc 1, top.
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Y Mod, Scan
FIG 2.-Top, micrographs of selected area
of Dispersalloy amalgam indicated as site A
in Figure 1, top. Center, X-ray microanalysis
displays of constituent elements of area shown
in Figure 2, top. Bottolm, Y-deflection moduilation micrograph of area shown in Figure 2,
top.
FIG 3. Top, micrographs of selected area
of Dispersalloy amalgam indicated as site D
in Figure 1, top. Center, X-ray microanalysis
displays of constituent elements of area
shown in FigUre 3, lop. Bottoin, Y-deflection
modulation micrograph of alrea shown in
Figure 3, top.
792
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T moo. *can
T._ Mot. -can _.
FIG 4.-Top, micrographs of selected area
of Dispersalloy amalgam indicated as site C
in Figure 1, top. Center, X-ray microanalysis
displays of constituent elements of area shown
in Figure 4, lop. Boltnto, Y-deflection modUlation micrograph of area shown in Figure 4,
FIG 5.-Top, Micrographs that represent
amalgam made from spherical alloy particles.
Center, X-ray microanalysis displays of constituent elements of area shown in Figure 5,
top. Bottom, Y-deflection modulation micrograph of area shown in Figure 5, top.
lop.
793
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794
JOHNSON
J Dent Res May-June 1972
the tin analyses. It appears that tin replaces mental analyses of Figure 5, center show the
the silver as the tin diffuses into the silver- primary particle to be silver-tin clad with
copper eutectic( Fig 3, center). The effect copper. Apparently the tin from the amalof this diffused tin reduces the conduc- gam matrix diffuses into the copper clad to
tivity of the eutectic, as seen by the in- form a ring of copper enriched with tin.
termediate step beween the black matrix This latter phase seems to be the least noble
and the silver-copper eutectic on the surface of all phases, as indicated by the micrograph
of the silver-copper eutectic (Fig 3, top, generated by back reflected secondary eleclower right frame). Oxidation products rich trons (Fig 5, top). The y-deflection moduin mercury assume a ring shape at the tin- lation mode of scanning electron microscopy
copper, silver-copper interface. This is com- summarizes this information by recording
patible with the theories of Jorgenson with the electric behavior of the polished, unregard to the mechanism of amalgam cor- etched amalgam under the electron beam.
rosion.9 One would expect oxides to have
Conclusions
insulative properties, and thus be relatively
poor conductors. This is indeed confirmed
Two different dental amalgams were polin Figure 3, top (lower right frame) where ished and viewed in a metallograph and a
in the center of the spherical eutectic phase scanning electron microscope. By use of
the beam strikes a particle of relatively high various modes of operation, one may obtain
resistance, and a sharp drop in conductivity qualitative and quantitative information
is seen in the y-modulated line scan. Figure without the need of electric or chemical
3, bottom represents the same silver-copper etching. In addition, the discrimination of
eutectic particle as seen before generated by the respective phases present can be prethe y-deflection modulation scan. Here the sented in a manner free of artifacts.
heterogeneous nature of the surface is seen
References
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F.A.: Microstructure of Dental Amalgam,
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Figure 4, top is a composite of the vari- 3. ASGAR, K.; ALLEN, F.C.; and PEYTON, F.A.:
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1966 (microfilm of proceedings of dental
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F.A.: Microanalysis of Copper-Tin Phases
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in Dental Amalgam, J Dent Res 48:872The last set of micrographs is of amalgam
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permission.