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Welcome to this short presentation of the double beam optical dilatometer
Misura® ODHT.
The double beam optical dilatometer was first developed by Expert System Solutions
in the year
1999
and it is protected by two international patents.
The highly innovative design
is based on two microscope optics with high magnification
which are framing both the ends
of the sample.
The optical dilatometer Misura® ODHT
is the only instrument available on the market
which is able to follow the process of expansion, sintering or bloating
with high resolution
and making no contact with the sample.
It is currently used by many companies and research laboratories
in several industrial fields like traditional and advanced ceramics,
glass,
cellular materials,
metallic
powders,
hard ceramic coating on metals
and in the management of industrial wastes.
The optical set up is based on two digital cameras and kept with a long
focal microscope
which are able to achieve an actual magnification of 0.6 microns
per pixel.
The two microscopes are mounted on micrometric slides
which allow the precise positioning of the microscope
in order to frame the top border of the sample and the sample holding plate.
The upper optical path is kept with a step motor which keeps always aligned
the microscope with the top of the sample,
even if the linear shrinkage overcomes the 50%.
The sample is 15 mm high with the base of 5 x 5 mm.
The sample can be cut from a piece of raw or fired material
using a diamond blade which can be supplied as an accessory.
As an alternative
the sample could be pressed from powders
using a small manual press,
also supplied as an accessory.
Since the sample is 15 mm high and the resolution of the measurement is
0.6 microns,
the resolution of the measurement is one part over 20.000.
The sample is introduced into a tubural kiln
which can be supplied in different versions up to 80 °C/min
and up to
1.600 °C.
The measurement is carried out with no contact on both sides of the sample
and for this reason
it is an absolute measurement.
In the traditional dilatometers the sample is contained inside the measuring
system
which itself
expands with temperature.
When the measurement system is made with Alumina,
the expansion of the measuring head may be much bigger than the expansion
of the specimen itself.
For this reason it is absolutely necessary
to apply a correction curve
which accounts for the expansion of the measuring system.
In the optical dilatometer the sample is completely free
and it is not necessary to record
and use the procedure of the calibration curve.
This graph is showing the result of the measurement
of the standard reference material 738
(blue line)
supplied
by the National Institute for Standards and Technology
(NIST)
in comparison with the data certified by NIST,
the red dot line.
Since
it is not necessary to correct the curve with the correction curve,
it becomes possible to use complex heating and cooling rates during
the test.
This makes it possible to reproduce industrial firing cycles.
This is the main window of the software during this measurement.
Here we can follow a sintering test on a ceramic body.
The two vertical windows on the left
show the images of the top end of the sample and of the sample holding
plate.
On the right we have the expansion/ contraction curve,
coloured in blue,
measured on the sample as a function of time
and temperature curve coloured in red.
The thermal treatment set for this test is made of three steps:
heating up to 1.200 °C
at 50 °C/ min
holding the temperature of 1.200 °C
for a period of 10 minutes
and then cooling.
The material is sintering very fast and the step-motor is working quite often
to bring back the image in the centre of the window.
The interval at constant temperature
starts when the temperature reaches 1.200 °C.
Note that the temperature of the kiln overcomes of only 1 °C
the setpoint.
The sintering process
continues even during the permanence at constant temperature
but the speed of sintering is progressively reduced.
At the end of the permanence at 1.200 °C the
cooling starts
and the temperature drops very quickly.
The height of the sample keeps reducing because of the thermal contraction
caused by
cooling.
This is an example of the graph of a sintering test.
The red curve is the temperature,
the blue curve is the expansion/ contraction,
the green
is the derivative of the sintering vs. the time and/ or the sintering speed,
the liliac curve
is the second derivative of the sintering.
One of the favourite field of application of the optical dilatometer
Misura® ODHT
is the control of the thermal behaviour of ceramic raw materials.
This is a comparison between several caolinitic clays
carried out with the heating rate of 50 °C/ min up to
1.400 °C.
Zooming the upper part of the graph
it is possible to have a better evaluation
of the differences between the materials.
Zooming only the part of the curve with the little fraction of the shrinkage
we can get results very similar to the ones of the traditional electronic
dilatometer.
This is a curve recorded on a sample
of pressed powders of glass.
The test continued up to the melting of the sample.
We can clearly see that the material expanded up to 640 °C
and then shrank up to 4%.
After that
it stays fairly stable up to 900 °C
and above this temperature it starts collapsing.
The test was ended when the temperature reached 1.000 °C
and the shrinkage overcome 12%.
Zooming on the upper part of the curve
we get the curve similar to the one obtained by an electronic dilatometer.
The transition temperature is
540 °C and the thermal expansion coefficient is
8.4 x
10 (-6).
The differences with the curves from traditional dilatometers are quite
obvious:
the part of the curve of the glass transition temperature,
which keeps going upwards,
is much wider
and no traditional dilatometers can follow the behaviour of the glass
up to complete melting.
Another quite interesting point
for technicians
who make daily use of the dilatometer
is that there is no danger of spoiling the measuring system
when the sample melts on the holding plate.
The fact
that the measurement is carried out with lower resolution
compared with the traditional dilatometer
is largely compensated by the fact
that the measurement is absolute
and it does not depend on the calibration curve.
One of the most interesting applications of the sintering curve
is the possibility to identify the optimal temperature to reach the
complete sintering in the shortest time
avoiding the risk of overfiring and bloating the material.
Running a test of constant heating rate up to bloating
it is possible to identify the temperature of maximum sintering rate.
This temperature is identified by the negative peak
of the first derivative of the sintering or
the sintering speed
which is the green curve.
In this case it is
1.230 °C.
Now, carrying out the second test with the same heating rate up to the temperature
of maximum sintering speed
followed by a permanence at that constant temperature,
it is possible to know how much time is necessary to hold the temperature
in order to reduce the sintering speed to 0.
Managing in this way it is possible to design a heat treatment with the optimal temperature
and the minimal holding time for the sintering process.
The result of the test can be confirmed by running several trials at slightly
different temperatures.
In the shown example it is quite obvoius that the best sintering conditions
are achieved at 1.210 °C,
since we get the maximum shrinkage and the sintering curve becomes perfectly flat.
Arising the temperature up to 1.220 or
1.230 °C
the material starts swelling,
the shrinkage
becomes lower
and the sintering curve
inverts the slope.
The curve of the heat treatment at 1.200 °C
clearly shows
that even after 10 minutes of constant temperature the sintering is not yet
completed
because the slope of the curve is still quite relevant.
The optical dilatometer Misura® ODHT
allows the study of the kinetics of sintering as a function of temperature
or as a function of time.
This graph
shows three sintering tests
at different temperatures
with the first derivative of sintering
or sintering speed
and the heat treatment.
A different colour is assigned to each test
for all the curves:
the sintering curve,
the speed of sintering and the temperature.
These tests show that the shrinkage increases
as the temperature increases
and it grows with the time at constant temperature.
The derivative of sintering vs. time or the sintering speed
is showing that the sintering speed is growing exponentially with
temperature,
but it decreases exponentially
at constant temperature.
This last example is an industrial waste slag
which shows bloating behaviour.
This material is used for the manufacturing of lightweight aggregates.
The analysis of this sintering test
clearly suggests the temperature and the time needed to obtain the maximum
bloating
before the material
starts collapsing.