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
8. 686
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.
