Microwave sintering of nano-ceramics – FURNACE

The microwave and conventional sintering is implemented on two types of ceramic materials: zinc oxide (ZnO) and alumina (Al2O3). this experimental protocol is carried out by studying the influence of different parameters onto the microwave heating behavior but also on the final microstructure of the sintered samples. Several parameters have been studied, such as the powder granulometry, the eventual dopants (ex: MgO added dopant onto alumina), the microwave heating conditions (type of the cavity used, assembly of the process) etc. Morevover, a specific assembly has been developped in order to be able to measure both the temperature and the shrinkage of the heated sample. This systematic and comparative study should allow to clearly figure out the heating nature related to the experimental conditions used, and to the materials. In addition, the activation energy of the microwave sintering process could be determined and compared to the ones obtained by conventional processing. This latter is crucial to establish wether or not there are specific microwave effects during the sintering of ceramics. At last, a special attention will be focused on the research of the optimal conditions for the sintering of composites and large parts, using a large and muti-mode cavity.

The comparative study between microwave and conventional sintering allowed us to identify the own features of the microwave sintering:
- Lowering of the sintering temperature of alumina, which effect is more pronounced on MgO doped alumina ceramics. The point defects generated by this doping would promote the effect of microwaves onto the ceramic material.
- Lowering of the activation energies related to the densification process by microwave heating of alumina and zinc oxide, compared to the ones obtained during conventionnal sintering. This result is very important and proves the existence of a microwave effect.
- In the study of the sintering of ZnO by microwave heating , it was shown that the action of the microwave magnetic field (H) with respect to the electric field (E), improves the densification of ZnO but also leads to a more pronounced grain growth. The effect of the H field was interpreted as being analogous to the action of a pressure which has been called electromagnetic pressure.

Furthermore, the experimental study of the densification behaviour in single mode cavity allowed us to understand the key role of the susceptors dielectric properties on the sample-microwave interactions, and thereafter on the sintering behaviour of the material used.

From a practical point of view, this technological breakthrough make it possible to consider the microwave heating technique as an environmentally friendly alternative solution to conventional furnaces used in the industrial heat treatment of materials

The prospects of this study are to adapt the experimental conditions to the development of complex parts such as composites, or parts of large size in order to make the technique versatile and therefore enlarge the fields of applications. Moreover, due to the original microstructures obtained, it can be considered the development by microwave heating of ceramics with improved mechanical and functional properties. For example, increase in hardness and stiffness (modulus) can be foreseen on fine-grained ceramics prepared by microwave heating.

A comon paper to the three laboratories (LCG SIMAP CRISMAT) shows the comparison microwave sintering - conventional sintering of alumina doped MgO (Zuo Fei, Claude Carry Sebastien Saunier, Sylvain Marinel Dominique Goeuriot «Comparison of the Microwave and Conventional Sintering of Alumina: Effect of MgO Doping and Particle Size «In printing (Journal of the American Ceramic Society). CRISMAT Laboratory published an article on the electromagnetic pressure that could be generated by the action of the magnetic field during microwave sintering of ZnO (Badev A., R. Heuguet, S. Marinel. «Induced electromagnetic pressure during microwave sintering of ZnO in magnetic field. «Journal of the European Ceramic Society, vol.33, issue 6 (2013). Another article of the LCG partner on the non-thermal effects observed during the microwave sintering of alumina, is accepted in the journal Scripta Materialia. Further works are being published, including the activation energy and the influence of the nature of the susceptor on the heating behaviour.

The Powder Metallurgy route for the fabrication of metallic materials, ceramics and composites is fully consistent with the new economic and environmental constraints of energy and raw material saving. This route often provides near net shape parts and thus postprocessing operations are limited. Moreover, the sintering process is always performed at lower temperatures than classical processes that involve material melting. However, the sintering cycle is usually done in conventional furnaces, which are time and energy consuming and are inappropriate to the processing of novel materials as nano-structured ceramics, due to excessive grain growth and reactivity phenomena. In that context, the MicroWave Sintering (MWS) attracts more and more interest. Although there are not fully understood and tailored, they are well known to exhibit peculiar interests:
- energy saving (direct heating of the sample and processing time is lowered);
- additional driving forces for mass transport induced by electromagnetic fields (for instance, the ponderomotive force);
- the ability to consolidate new materials, such as Functional Graded Materials (FGM) in order to enhance existing properties or to create new ones (e.g. multiferroic composites);
- the ability to process ultrafine powders. These powders are more and more used and are currently available as industrial or laboratory products, but classic sintering ways may lead to uncontrolled grain growth. These nano-ceramics may exhibit improved properties or novel ones.
Although MWS technology is not really new - the first experiments were performed more than thirty years ago - the microwave furnaces available for sintering are most often laboratory devices, designed and constructed by the researchers themselves. As a consequence, there are a lot of possible configurations (multi-mode or single mode cavities) and many ways of using microwaves for heating the materials (the use or not of a susceptor, the sample position with respect to magnetic and electric field patterns etc.) Therefore it is really difficult to compare and discuss the results obtained in different laboratories. Most of the time, the success in using MWS involves a part of randomness and any specific situation has to be treated as a new case. Consequently, the goal of the FµRNACE project is to gather scientists in complementary fields (solid state chemistry, sintering, modelling, microscopy, crystallography) to focus on the MWS technique, with the aim of understanding its own features at various scales (nm to µm), improving our control of it and clearly identifying how to use it to get materials with original micro/nano structures. For this purpose, the microwave sintering of two reference materials, zinc oxide and alumina (respectively a semiconductor and an insulator), with two different particles sizes (micronic and nano-sized), will be carried out using different configurations. The microstructures and nano-structures after MWS will be deeply examined and compared with those conventionally obtained in relation with the microwave heating conditions. It will be developed as well an original numerical model providing a prediction of the electromagnetic field in the furnace and in the sintering part and determining its consequences in terms of heating, densification and grain growth. This coupling between electromagnetic, thermal and mechanical phenomena has never been done before in the case of MW sintering. It will allow an enlightened analysis of the experiments. Efforts will be put to improve our control of the overall process in designing a specific instrumentation (temperature and shrinkage measurements) and to increase microwave technology attractiveness to the industry. This will be done by exploring the microwave sintering of composites and large-size part in collaboration with a small company named Synerwave, expert in microwave technology.