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.
|Instrument||NETZSCH DIL 402 C|
|Sample length/mm||18 ... 25|
|Temperature range/¡C||RT ... 1700|
|Heating rates/(K/min)||5, 10 and 20|
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).