Special microstructures by microwave sintering

29.08.2008


Figure 1: SEM image of polished furnace sintered samples made of nano- scaled barium titanate powders. Micro structural coarsening due to formation of agglomerates in the intermediate sintering stage.
Figure 1: SEM image of polished furnace sintered samples made of nano- scaled barium titanate powders. Micro structural coarsening due to formation of agglomerates in the intermediate sintering stage.

Figure 2: SEM image of polished microwave sintered samples made of nano-scaled barium titanate powders
Figure 2: SEM image of polished microwave sintered samples made of nano-scaled barium titanate powders

Figure 3: Comparison of sintering temperatures of furnace and microwave sintered samples
Figure 3: Comparison of sintering temperatures of furnace and microwave sintered samples

Figure 4: Comparison of thermal diffusivities of furnace and microwave sintered samples
Figure 4: Comparison of thermal diffusivities of furnace and microwave sintered samples

Initial situation
For about two decades microwave sintering has been extensively investigated. Although there are just a few applications in industry yet, the advantages of microwave sintering are quite promising. Results of the latest research show that microwave sintering leads to more homogeneous microstructures and to improved material properties in comparison to conventional furnace sintering. It is assumed that microwave plasmas are generated which enhance surface diffusion, thus leading to enhanced growth of sintering necks in the initial sintering stage.

In the conventional sintering process agglomerates are often formed in the intermediate stage of sintering. Thereby a few particles sinter locally together and density is locally increased. Different agglomerates are loosely bound together. Because of the formation of these agglomerates, the microstructure is becoming more inhomogeneous (Figures 1 and 2). Especially many advantages of fine, nano-scaled powders are therefore lost in conventional sintering.

Our solution
Microwave sintering is systematically analyzed to improve microstructures by a better understanding of the special diffusion processes caused by the microwave field. For example, the sintering behavior of furnace sintered and microwave sintered barium titanate was investigated. To achieve a homogeneous distribution of microwave energy, a special multi-mode microwave furnace was used which has four magnetrons and two microwave frequencies: 2.45 and 5.8 GHZ (Figure 5). Micro- and nano-scaled powders were sintered to 80 per cent of theoretical density to examine the intermediate sintering stage. For both heating methods, identical heating rates and hold times were used. The sintered samples were characterized by thermal diffusivity measurements and scanning electron microscopy.

The thermal diffusivity of partially sintered ceramics increases strongly with the sintering neck area and can therefore show whether or not enhanced surface diffusion occurs in microwave sintering. The sintering temperatures necessary to reach 80 per cent of theoretical density are significantly reduced in microwave sintering in comparison to the conventionally furnace sintered samples (Figure 3). Moreover, a distinct increase in thermal diffusivity for the microwave sintered samples was observed (Figure 4). This indicates that the sintering neck areas of the microwave sintered samples are larger than those of the furnace sintered samples. These results show that enhanced surface diffusion occurs during microwave processing. It was also shown that the measurement of thermal diffusivity can be more sensitive to microstructural changes than customary image analysis.

Customer benefit
The investigations of microwave sintering showed the differences in diffusion processes compared to conventional furnace sintering. The enhancement of surface diffusion in the initial stage of microwave sintering results in a larger sintering neck area between particles. This could give the possibility to prevent the formation of agglomerates during the intermediate stage of sintering. Therefore, more homogeneous microstructures may be realized.

Additional advantages of microwave sintering include:

  • Very high heating rates
  • Reduced temperature gradients within sintered parts due to volumetric heating
  • Lower sintering temperatures
  • Reduced processing times
  • More homogeneous microstructures

Your contact
Dr. Friedrich Raether
Phone +49 (0)931 4100-200
Fax +49 (0)931 4100-299
E-Mail: raether@isc.fraunhofer.de

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