Research & Developments of Sintering of Ceramics by Microwave
Date of publication:
Microwave, Kilns, Ceramics , Isothermal Barrier, Rapid Sintering, Stress-free sintering, Low energy consumption
New technology is required in the field of industrial kilns reducing for green house effect and increasing productivity. Microwave sintering methods have been expecting to the merits of energy saving, inhibition of grain growth, higher density, less evaporation for existing material and innovations to new materials by their properties of internal, selective and rapid heating. However, it cannot be extending to the industrial scale from laboratory, as the problems of distortions and cracking did not solved in the systems pricing with compatible to the conventional heating. The key ward of "stress free sintering" is "the reduction of temperature gradient" in the work piece. The energy flow in the kiln is determined by conduction, convection and infrared radiation. Especially, the infrared radiation is the largest factor at high temperature such as ceramic sintering, as it increases in proportion to fourth power of temperature. The energy loss or input through the surface of work piece generates the temperature gradient. If the transfer is prevented, the temperature gradient between surface and body is zero. It is principally difficult to expect in the conventional kilns of external heating. On the contrary, microwave can transmit electromagnetic energy and converts to thermal energy in the work piece. The microwave heating can realize the uniform body heating. Why not? The scientist of microwaves considered that the non-uniformity of electromagnetic fields generate the local heating. The engineers of ceramics bereaved that the cracking was inevitable for the rapid temperature rise by their experiences in the traditional kilns. The right answer is to cover the work pieces by equal temperature wall. There will be two methods. The first is the hybrid heating. Heaters in reference to the temperature of work pieces control the wall temperature externally. The other is a self-consistent susceptor wall that has certain dielectric constant and loss-tangent to be heated by microwave in keeping the equal temperatures and equal rising rates with the work pieces. The thermal energy flux balances on the surface of work pieces, i.e. zero in total. Under such an ideal thermal insulation, work pieces get energy only by electric field of microwave and heat themselves uniformly. The stress free system realizes the rapid sintering. Microwave field distribution in the work piece is another key word for the "stress free sintering". More than two magnetrons are operated simultaneously with mode stirrers (rotating metallic fans). For heating, microwaves from two or more magnetrons are coupled through the ports into the chamber. The magnetrons generate 1.5 or 2.5kW at 2.45GHz. The frequencies of each magnetron are slightly different as a result of the manufacturing process. This allows increasing the number of excited modes. In addition, a microwave scattering fan rotates in the chamber. This prevents a static field and improves the uniformity of the field distribution in the chamber. Isolators do not apply to the magnetrons. The VSWR changes between 1.4 and 2.7 and the frequencies of magnetrons shift in a few MHz in accordance with the rotation of fans. The beating frequency of microwave is scattered and mode mixing becomes more complicated. The numerical simulations are now trying to, however, as it need very large number of memories in PC's, the calculation with work pieces has not been done yet. The microwave power is continuous waves (CW) and is controlled manually. The 2.45GHz low frequency microwave does not show any non-uniform heating in our experiment and industrial systems. The application of isothermal wall proved that uniform sintering is realized by magnetrons at low to medium power of a few kW. The prices reduced to the competitive to conventional kilns. The microwave kilns need only 1/2 ~1/5 energy for the unit weight of products.
Now, many industries pay attention and are planning to introduce the microwave kilns into their factories in Japan.