Dilatometer System with Hybrid Heating Technique for Detailed Investigations on Sintering of Ceramics
*G. Link, M. Thumm#
Forschungszentrum Karlsruhe GmbH, Institut fuer Hochleistungsimpuls- und Mikrowellentechnik, Germany
#also University of Karlsruhe, IHE, Germany
In materials science and development, dilatometry is a widely used technique to measure thermal expansion coefficients, phase transformations or the linear shrinkage during a sintering processes. The basic information such a dilatometer gives is the variation in sample length during heating or cooling through a programmed cycle. This information can be very effectively used to optimize thermal processing.
One of the major drawbacks for exact measurements is the fact that one has to make sure that the sample is in thermal equilibrium with its environment. This gives strong restrictions to the programming of heating cycles with respect to the heating rates. With conventional heating, that means infrared heating, we can observe a sample surface with temperatures higher than in the volume. This results in heat flow from the sample surface into the volume until sample is in thermal equilibrium. But this takes some time, depending on thermal conductivity, which as for example for powder compacts can be rather poor. With pure mm-wave heating the problem appears the other way round. If the sample shows low dielectric losses, resulting in volumetric heating, then there is permanent heat flow from the inside out where heat is lost from the sample surface by radiation and convection. But in contrast to conventional heating this cannot be avoided by slow heating. Here a real thermal equilibrium can never be achieved.
Figure 1: Dilatometer system viewgraphin thesystem installed in the mm-wave applicator with conventional, resistant heating.
There are different ways to overcome this problem. One is placing the dilatometer sample holder in a box of ceramic fiber boards, as it has been performed up to now . Another solution is the combination of conventional and mm-wave heating technique. This allows to control temperature gradients during the sintering process. At the same time it is a versatile tool to investigate the influence of different heating methods on the sintering process of ceramic compacts in a single system. The compact 30 GHz, 15 kW gyrotron installation of the Forschungszentrum Karlsruhe, Germany has been equipped with such a dilatometer system (see Figure 1) for detailed investigations in sintering of various functional and structural ceramics by means of millimeter wave (mm-wave) radiation.
Large efforts have been undertaken to study the intrinsic behavior of the dilatometer under various heating conditions for calibration purposes. This behavior is determined by the temperature evolution in the sensor head. If residual temperature gradients are present, the sensor rod may show a thermal expansion different to the sample holder. But this will result in an intrinsic dilatometer signal superimposed to the signal coming from the sample under test. It was found that with pure mm-wave heating temperature profiles strongly depend on the heating rates and the dielectric loss behavior of sample itself, so that a calibration of the dilatometer system is unsatisfactory. However with hybrid heating, where temperature profiles can be actively controlled, a proper calibration is feasible.
The sintering behavior of ceramic compacts has been investigated in the prescribed dilatometer setup. It allows a direct comparison of conventional, mm-wave and hybrid heating without changing the system, that means without additional systematic errors in temperature measurement. Even the effect of changing heating conditions during the sintering process can be investigated and gives interesting additional information on the effect of mm-waves on the sintering process (see Figure 2).
While the temperature of maximum shrinkage rate is shifted to lower values with increasing heating rates during mm-wave processing, this point is shifted in the opposite direction with conventional heating. This gives the impression of enhanced sintering under the influence of mm-waves. In case of conventional sintering, effects can be explained by temperature gradients and reduced annealing time at equal temperatures if heating rates are increased. Careful analysis of these measurements show that the temperature gradients have to be taken into account carefully in order to avoid misinterpretations. Hybrid heating, where temperature profiles can be controlled is a very helpful tool to understand these effects.
Figure 2: Linear shrinkage of nano-crystalline YSZ under various heating conditions.