Quantitative
Analysis
X-Ray Diffraction:
Dry grinding can be used to induce microstrains in the
crystal lattice for the determination of ultrastructural damage by
XRD line broadening measurements.
Principle of Operation
A 125 ml polypropylene jar containing 48 grinding pellets
in ordered array is gyrated around a horizontal axis.
Grinding may be carried out wet or dry.
Full description
Grinding Methods
Both methods are suitable but wet grinding
(as a slurry) is the preferred method for sample preparation in the
McCrone Micronising Mill in most laboratories.
WET GRINDING allows total recovery of the
ground sample and cleaning of the jar to be combined in one set of
operations.
When grinding has finished remove the lid from the jar and
replace it with the pouring lid that has two 6 mm holes. The ground
slurry can then be poured out through one of the holes and the jar,
with the elements still in place, is washed a further to or three
times with intermediate shakings. This procedure yields the
combined pourings and washings together with a clean jar and
pellets without having to remove the pellets from the jar.
To obtain a dry sample clear supernatant fluid can be safely
decanted and the rest of the liquid is evaporated.
In the case of water, the remaining liquid can be replaced by
acetone which is lighter and less viscous than water thus allowing
the ground powder to settle out more rapidly. After decanting off
the clear acetone layer the remainder can be evaporated under an
infra red lamp.
The densities of practically all liquids used in wet grinding are
greater than that of the polypropylene jar material. Any jar
material transferred to the sample during grinding will therefore
float on the surface of the carrier liquid and can be removed when
the supernatant liquid is decanted off the ground sample.
Theoretically, complete removal of any traces of corundum from
the sample can be achieved by the use of a heavy liquid suspension
centrifuging technique but this would only be used in the
preparation of micronised products where the highest purity is
required.
In practice the material transfer form jar and grinding pellets
to the sample is minimal.
DRY GRINDING may be preferable when the
relationship between amount of mechanical work put into a sample
and the amount of ultrastructural damage produced needs to be
studied.
The work of Burton on the production of gram quantities of
materials with unusual properties for research and development
purposes is a good example.
Increasing attention is also being given to the examination of
various crystallographic transformations and tribochemical
reactions induced by prolonged dry grinding
Lewis and his colleagues have made use of the x-ray line
broadening effects observed when powders are dry ground for
different times. They were able to measure the amount of lattice
microstrain produced in brittle substances as diverse as calcite,
lithium fluoride, corundum and tungsten carbide. Such measurements
have been shown to be of great value in fundamental studies of the
sintering of metal powder compacts. The McCrone mill has the virtue
that close control can be maintained over every aspect of the
strain-inducing milling operation.
With the quantity of sample and the grinding time as the only
variables, the McCrone Micronising Mill is the appropriate
quantitative tool for such studies.
Full description
Experimental Results