Design of microwave sintering furnace

1. Principle of microwave sintering of ceramic materials
The reaction of materials to microwave can be divided into four situations: microwave reflection microwave transmission; microwave absorption; partial absorption of microwave. Most metals fall into the first category, while all glass and ceramic materials fall into the latter three categories. When a ceramic body is placed in a microwave field, the absorbed power can be expressed by the following equation：
P = (2π fε ) ( E2/ 2) tan δ，When the microwave penetrates the material, its intensity decreases with the penetration depth. The distance from the material surface to the attenuation to 1/ e of microwave energy is defined as the penetration depth Dp of microwave:
3λ 0
DP = π tan δ (ε r/ε 0) 1/ 2
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Where, P is the microwave power absorbed by the ceramic body; F is the frequency; ε is the composite dielectric constant; λ 0 is the wavelength of microwave in vacuum; E is the intensity of electric field; tan δ is the loss tangent of dielectric ceramics; The loss tangent (the ratio of the loss factor to the dielectric constant) is usually used to express the coupling capability of the material to the microwave.The higher the loss tangent value is, the stronger the coupling ability between the material and microwave is.

2. Design of microwave high temperature heating furnace
Microwave heating furnace is mainly composed of monitoring system, control system, insulation box, microwave generator, heating box and gas storage tank. The heating box is made of stainless steel mirror plate. Cooling water can flow in the sandwich between two layers of stainless steel plate to achieve the cooling of heating box. The temperature sensor we use is RAYR3I1MSCL2U infrared thermometer of American leitai company. The control system can realize manual control and automatic control. In order to prevent in the microwave oven door closed after microwave heating furnace from the oven door and leak out of the gaps between the cavity, in addition to heating furnace door in processing and manufacturing to ensure the high dimensional precision and assembly precision, we in the microwave oven door installed around the choke groove structure, this structure can effectively reduce the leakage of microwave. The selection, field design and insulation of the microwave generator are the key factors in the sintering furnace design.

3. Microwave generator selection
Mag netron, Klystron and Gyrotron are generally selected in microwave heating devices. As the “heart” of microwave sintering equipment, its choice will directly affect the performance and cost of the whole equipment. The commonly used frequency in microwave sintering device is:
915 MHz, 2.45 GHz, 6 GHz, 28 GHz and 60 GHz, etc. Generally, magnetron can be selected as microwave generator for lower frequencies such as 915 MHz and 2.45 GHz,6 GHz can be selected as speed regulating tube, and higher frequencies such as 28 GHz and 60 GHz can be selected as magnetic coil.
As the frequency and power of various microwave generators increase, their price ratio ($/ Watt) will increase significantly. If high power microwave generator is needed in the design, it is recommended to use the same frequency and low power generator to obtain by power superposition.

4.Design of insulation structure
The most commonly used insulation structure in microwave sintering furnace is buried powder type and box type. Buried powder insulation structure has the advantages of good insulation effect. However, when samples are sintered at a higher temperature, adhesion between samples and buried powder is easy to occur. After the samples are sintered, direct contact will occur. But this structure is not as good as buried powder insulation structure. Combining the characteristics of two insulation structures, we designed a box-type insulation structure.