G. Link, S. Rhee*, M. Thumm^#

Forschungszentrum Karlsruhe GmbH, IHM, POBox 3640, 76021 Karlsruhe, Germany

* now with NIH Transducer Resource Center, Pennsylvania State University, USA

INTROduction

Sintering is one of the most crucial steps in the processing of modern high-performance ceramics. Due to the low penetration depth of IR-radiation in a conventional sintering furnace, temperature gradients are induced in the ceramic samples resulting in thermal stresses and inhomogeneous shrinkage, if not an optimized temperature time program is applied. The use of microwaves allows to reduce such problems since energy is directly transferred into the materials, where it is converted to heat. Furthermore, the densification process of ceramic bodies seems to be enhanced by sintering in a microwave field, which has been demonstrated by several authors [1]. This makes the use of microwave technology very attractive.

At the Forschungszentrum Karlsruhe, Germany, a compact gyrotron system has been established in order to investigate technological applications in the field of high temperature materials processing by means of millimeter wave (mm-wave) radiation. For more detailed investigations in the field of sintering of various functional and structural ceramics a specific dilatometer setup has been designed and build into the mm-wave oven of the gyrotron system. For this purpose a commercially available dilatometer has been used and modified in order to preventthat way that a leakage of mm-wave radiation out of the applicatoris avoided. ThereforTthe alumina ceramic rods used in the dilatometer sensor head were exchanged by metallic components in the cold part of the applicator system where it is fed through the applicator wall.

In materials science and development dilatometry is a widely used technique to measure thermal expansion coefficients, phase transformations or the shrinkage during a sintering processes. The basic information such a dilatometer gives is the variation in sample length during a temperature change. The temperature change is achieved by heating or cooling through a programmed cycle.

Figure 1: Photograph of the dilatometer system and schematicviewgraph of thesystem installed in the mm-wave applicator

Experimental REsults

Detailed investigations have been performed for calibration of the mm-wave heated dilatometer system which gives its intrinsic temperature behavior. Heating with mm-waves which is a selective heating means that the temperature distribution effecting the intrinsic dilatometer signals strongly depends on process conditions, such as dielectric properties of the material under test and heating rates. But this makes a reasonable calibration of the system much more difficult than for a conventional dilatometer.

References