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Welcome to this short presentation of the hot stage microscope Misura® HSM.
This instrument was
first developed by Expert System Solutions
in the year 1990.
In the following years
it was continuously updated
and developed
and now
it is currently used by many companies
which work in several industrial fields liked glass, ceramics, enamels, steel,
power generation and waste management.
The hot stage microscope is the ideal instrument
to study phenomena like softening, melting, swelling, burning at high
temperatures
on materials like glass, ceramics, metal,
casting lubricants
and flying ashes from the combustion
of fossil fuels
or from the waste incineration.
Thanks to the restless research efforts,
Expert System Solutions has now implemented the measurement of the
surface tension,
using the method of the analysis of the profile of the sessile drop.
The hot stage microscope Misura® HSM is now the only instrument
available on the market
which can
measure the surface tension on molten materials up to
1.600 °C.
In order to carry out the softening and melting tests,
the hot stage microscope Misura® HSM
uses a cylindrical specimen
with 2 mm of diameter
and 3 mm of height.
This specimen is prepared with a small manual press which is supplied with
the instrument.
It is also possible to follow the behaviour
of specimens with an irregular shape
or analyse two specimens at the time.
The specimen is positioned on a sample holder rod
and it is inserted in a tubural kiln.
Misura® HSM is available in several versions
with the heating rate up to 80 °C/ min
and up to a maximum temperature of 1.600 °C.
The image of the sample is captured by a digital camera
equipped with the microscope with a very long focal length.
The image is transferred to the computer
and since the picture includes the sample holding plate,
the system automatically corrects for displacement of the sample
holder. For this reason it is not necessary to run a calibration
curve.
This is an example of analysis of the softening of a ceramic frit.
The heating rate is 50 °C/ min.
The image acquisition starts from the temperature set by the user
and in this case it is 600 °C.
The size and shape of the specimen are automatically analysed.
Let's follow the analysis when the material is already above
1.000 °C.
The shape of the sample
is changing
because the material is becoming liquid
and the surface tension is trying to reduce the surface of the sample to the
minimum.
At 1.100 °C
the material is completely liquid and the shape of the sample becomes spherical.
In order to get the
sphere shape,
the sample must be very small.
In this way the hydrostatic pressure,
which would flopped the drop of liquid,
is negligible
compared to the surface tension.
In order to carry out surface tension measurements,
the sample must be much bigger
and so the sessile drop
will be more flat.
The heating
of the sample may go on up to the complete flattening of the sample
which corresponds to the melting.
The control software
will automatically stop the test when the sample reaches the flattening.
This function can be configurated by the user
and it is very useful for the tests of unknown material.
Some materials may react with the sample holder and damage the
thermocouple.
The images taken during the test can be seen as a movie
and exported in .avi format.
Looking at the movie at higher speed we can enhance the transformation of the
sample.
In the initial part of the test the material undergoes a simple thermal expansion,
then the sintering process starts and the shape is maintained, while
the dimension is getting smaller.
At a certain temperature the liquid phases are emerging on the surface
provoking the softening of the sample.
From this point on,
the surface tension is taking control of the shape which is continuously changing
to sphere,
half sphere and finally complete flattening.
The data gathered during the test can be seen as a plot.
This is an example of the softening plot of the glaze.
The lilac lines on the plot
are the temperatures of sintering, softening, sphere, half sphere and melting
which are identified automatically.
It is also possible to extrapolate the value of the viscosity of the molten
glass
using the information from the hot stage microscope and from the optical dilatometer
Misura® ODHT or ODLT.
This calculation uses the the V.F.T. equation
with known values of viscosity given to the temperature of glass
transition, softening and half sphere.
This is an example of animation of two samples of different minerals.
The images are synchronized by the temperature.
In this case we can see a comparison between the incoherent
melting of Sodium feldspar
which becomes progressively liquid forming a glassy phase and the melting
of the diopside which is becoming suddenly liquid when it reaches the
melting temperature.
Another very interesting comparison is between the swelling
behaviour
of different flying ashes.
We can clearly see
the swelling after the softening and the incoherent melting which gives rise
to high viscosity liquid state.
The lubricants for a continuous casting are becoming a very important
technological auxiliary
in the continuous casting of steel
because they enable higher
casting speed.
The two materials shown in this example are spray dry powders use as
lubricants for steel casting.
The images can be printed out as a complete sequence or
as a selection of the characteristic shapes
which are automatically identified.
The latest development on the hot stage microscope
makes it possible to measure the surface tension using the method
of the analysis of the profile of the sessile drop.
This is the image of the molten drop of soda lime glass on a Platinum foil.
The calculation of the surface tension at 1.100 °C
gave the value of
346
x 10(-3)
N/ m
which is in a good agreement with the bibliography.
This is a sessile drop of molten solder
over an Alumina plate at 200 °C.
The measured value
is 482
x 10(-3)
N/ m.
The calculation
of the surface tension is made possible thanks to the development of a new
mathematical algorithm.
This function is based on the assumption that the curvature of the
surface of a sessile drop is given by the differential pressure between the
internal
and the external surface, given by the surface tension.
Each point of the surface of the drop can be described by two curvature radii.
The two mathematicians Young and Laplace
wrote the equation which describes the curvature of the sessile drop as a
function of the surface tension
almost 200 years ago
but this function
has no known analytical solution.
The solution for this equation can be found using
an algorithm for numerical integration
which solves the equation by iteration
until the calculated drop profile fits with image profile.
The curvature of the drop profile
is also dependent
on the density of the molten material.
Since it is nearly impossible to get a direct measurement of the density of
the molten glass,
we use an extrapolation method.
The density of the glass at
temperatures higher than the transition temperature
is extrapolated extending the thermal expansion curve
obtained with the optical dilatometer Misura® ODLT.
This instrument is carrying out the measurement with no contact and it is
not deforming the sample.
The part of the curve above the transition temperature (Tg)
and before the softening can be several hundred degrees wide.
This is the main window for the surface tension measurements.
The shown example is the measurement of the surface tension
on a drop of distilled water over a Teflon plate
at room temperature.
The measured surface tension is
70,3 x 10(-3)
N/ m
and it is in perfect agreement with the values obtained on water with all the
other methods.