Tacky Dot™ Slide Arrays and Double Sided Adhesive Products
Procedures for the mounting of powders
The proper mounting of dry powders for SEM and LM examination is often times
a more complicated issue that might at first appear, in part because of the
need to get good representative sampling of the bulk sample and yet at the
same time, to have the individual features sufficiently dispersed that one
can see individual particles in isolation so that accurate diameter and
other measurements can be made.
We describe several different methods that have been developed, both in our
own laboratories at Structure Probe, Inc., and also other laboratories. Our
own experience dates back to the year of 1967, when we first started working
with one of the first SEMs ever manufactured and imported into the
United States.
Tacky Dot Slides™
The use of the Tacky Dot Slide product offered
the first real opportunity to mount a dry powder in some form of an
orthogonal array so that what is presented to either an SEM or LM is of the
type that with stage automation, large numbers of samples could be analyzed
automatically, and without human intervention.
There are basically three different ways to mount dry powders onto Tacky Dot
Slides:
- Recommended way: Here the correct dot size Tacky Dot Slide is inserted
into the slide holder, a representative amount of the powder sample is
poured into the cavity, a "backing" slide is put onto the other side of the
cavity, and then the entire assembly is shaken, vigorously, and in the end,
only one particle will be sticking per dot so long as as there are no
particles present less than one fourth the selected dot size. We believe
that for the typical powder, absent any reason to do it some other way,
this is by far the most representative sampling of any dry powder.
- Short cut way: Here the powder to be sampled is put into a plastic
zipper-lock sandwich bag, along with the Tacky Dot Slide, the top of the
bag then being closed, and the entire bag, powder and slide, vigorously
shaken. This shaking process usually generates considerable electronic
charge on the powder, and to reduce the charge inside the bag, we recommend
the use of the Zerostat®
Antistatic Gun.
After discharging, the slide is then removed, using
extra long tweezers. The
excess powder clinging to the slide is "blown off" using an
SPI "Two-in-One" Easy Duster.
- Double sided (dry adhesive) products: SPI Supplies offers
three different forms of
these conductive carbon filled products, so one can select between
sheets, cut discs, or rolls of tape. In this instance the powder is
"dusted" onto the tacky surface, but one has to be very careful in what is
done from this point. If a duster blast
is too vigorous, then the smaller particles that are not so tenaciously held
might get blown off preferentially, leaving behind only the large particles
and not the fines. Also it is hard to control particle agglomeration, and
therefore difficult, once in the microscope, to determine agglomerates of
several or more individual particles or aggregates that existed in the bulk
and therefore, from a rheology standpoint, would behave a one larger
particle and not a multitude of smaller particles.
Some times it is helpful to use special fine brushes, such as those made of
camel hair, for the spreading
around of dry or otherwise fragile powders on the tacky surface.
- "Quick and dirty" approaches: One can take conductive paint products,
such as SPI Silver Paint
or even SPI Carbon Paint
and on either an SEM mount or a glass cover slip, draw down a thin film of
the paint, and as it is evaporating to a dry film, sprinkling on or "dusting"
some of the powder to be examined. This approach has the advantage of being
certainly "quick" and it does not even require anything special in the way
of mounting supplies. However, one has to be very careful that the
evaporating liquids from the paint do not alter fragile surface structures
on the particles to be examined. This is of course much more of a concern
with critical point dried powders (we absolutely do not recommend this
approach even be considered for those kinds of powders) or any organic or
polymeric powder. And capillary action could also be responsible for
swelling of the particles which could lead to artifacts in the final images.
A variation on this method is the use of the
SPI "Five Minute" Epoxy. This approach is used when one does not want
to see in the back ground of the SEM micrograph, for example, the silver or
carbon solids from the paint. The back ground of the epoxy is structureless
and featureless, however it is non-conductive and therefore must be either
gold coater or carbon coated in a
sputter or carbon coater prior to examination.
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Tuesday May 13, 2008
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