SPI Mica Sheets and Disks
Air inclusion content and mica microholes
This is a pretty arcane kind of topic, yet it can be of critical importance
for one having the need to have well understood mica, both from the
standpoint of surface smoothness as well as mica that is free of damage or
distortions.
We like to think of Grade V1 mica as being the perfect mica. It has zero
air content. It also has no color, meaning it has essentially no impurities.
Of course, as is the case with other things in life, and nature, that which
is that "perfect" is never found in abundant supply. Indeed mica deserving
of a V1 classification is becoming increasingly more rare and difficult
to find.
As a few overall comments, mica is reasonably clear, if thin enough, to the
point of being transparent, from V1 down to V-4 and possibly to V-5.
Color comes from impurities, such as Fe in the form of various oxides and
other minerals, as well as other species, all depending upon the precise
place where the mica was found. For analytical purposes, SPI Supplies
offers nothing further down the quality scale than V5. So if you have
obtained mica from other sources, and it exhibits color, then you can be
sure that you have mica that has more than the amount of impurities one
normally desired to have when doing careful analytical studies. We will
later say a few things about grades lower than V-5 but only for pedagogical
value.
Now focussing on grades V-1 to V-4, and going from an air content from zero
to 25%, it turns out that the air, which by the way is "ancient" air, air
that has been trapped for millions of years, exists not as actual "pores" or
"voids" but in layers parallel to the micaceous cleavage planes. So when
mica of these grades (except V-1) is cleaved, if the cleavage plane was
close to one of these trapped layers of ancient air, when the pressure is
relieved, you will see a "blistering" effect. One can not use the area of a
blister, if for no other reason that the mica itself has been damaged, and
it must itself be cleaved off to expose a presumably undamaged surface.
Interestingly, this cleaved blister material might be the thinnest possible
mica cleavings, perhaps not more than 5-10 µm in thickness.
Since V-1 is essentially air free, then V-1 when being cleaved, literally
does not exhibit blister formation. We hesitate to say "never", but its
frequency of occurrence should not be very often.
And since V-4 grade has undergone somewhat more stress in the release of the
ancient air, we would expect that there could be more of this natural
"damage" present on both surfaces as well as cleaved layers than of V-2 and
certainly, V-1.
Once the grade drops to V-5 and certainly V-6, color is developing and there
are indeed microinclusions, not of ancient air, but of iron in some of its
various compounds, generally as a small mineral inclusion in the mica. So
let us consider a typical inclusion that might be on the order of several µm.
And we are now cleaving the mica, and during the cleaving step, the
separation plane encounters this several µm iron containing mineral
inclusion. The inclusion says with the substrate, but the point is that in
the cleaved layer, there is either a "hole" or "ghost" from where the
particle inclusion was originally, or in the case of a very large inclusion,
common in some of the grades still further down the quality scale, there
could actually be a hole straight through the cleaved layer. Indeed one
can get at times something almost suggesting a "Swiss cheese" effect.
But these grades beyond V-5 are not offered by SPI Supplies to researchers
because of the variability of the material and the impact on freshly cleaved
surfaces, if not also on the defect structure of the crystalline lattice of
the mica itself.
We have tried to explain a complicated kind of story, but one which is of
intense interest to at least some doing scanning probe studies. While it
does cost more, our philosophy is simply that if the experiment is important,
pay the higher amount of money, it is not really all that much anyhow, and
remove from the variables the various features that can result from non-V-1
grade mica.
Let us know if you have still some unanswered questions or if you feel that
there are still some things that have to be better explained.
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Saturday March 20, 2010
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