|
During a sintering
process two critical stages exist:
- the burnout of the polymer binder and
- the sintering of the material itself.
During the
burnout the speed
of heating up must be controlled in such a way that the mass
loss rate is nearly constant in order to avoid the
production of micro-cracks.
The properties of the
final product are strongly influenced by the temperature
profile. In general the densification is higher if the
shrinkage rate is smaller. Sometimes the best temperature
profile exhibits a constant shrinkage rate.
The general procedure for
determination of a temperature profile resulting in a
constant shrinkage rate is the following:
measurement using at least 3 different heating rate in a
logarithmic order
kinetic analysis
estimation of the temperature profile for a certain
shrinkage rate.
The advantage of this
procedure in relation to the direct experimental
determination is that on the basis of these three
measurements we are able to predict the temperature profile
for each value of shrinkage rate. This handling reduced
costs for the evaluation of the optimum firing
process.
Measurements
|
Instrument
|
NETZSCH
DIL 402 C
|
|
Sample
length/mm
|
18 ...
25
|
|
Temperature
range/¡C
|
RT ...
1700
|
|
Heating
rates/(K/min)
|
5, 10 and
20
|
Model fit
Dilatometric
measurements of Al2O3-sintering and
their fit by multiple step reaction.
The total shrinkage increases with increasing heating rate.
This is an abnormal behavior. The branched reaction path is
necessary for the description of the reaction with a
heating-rate dependant total shrinkage.
Simulation of rate
controlled sintering
Determination of a
temperature profile, realizing a rate-controlled
sintering with a sintering rate of
0.1%/min.
The experimental proof of
the estimated temperature profiles shows that differences
between the experiment and the prediction are within the
experimental error (J. R. Opfermann, J. Blumm, W.-D.
Emmerich: Thermochimica Acta 318 (1998) 213-
220).
Comparison between
experiment and simulation shows the high level of confidence
of the predictions.
The general advantage of
this technique, consisting in a small number of measurements
and kinetic analysis, is the reduction of time, reduction of
costs and enhancement of the speed of developments. |
