Characteristics of Millimeter-wave Heating and Smart Materials Synthesis

Abstract

Characteristics and advantages of heating processing based on millimeter-wave are described from the standpoint of the interaction between electromagnetic energy and solid materials. High capabilities of the electromagnetic processing are indicated by exemplifying several successful results such as sintering of alumina and AlN and post-annealing of aerosol-deposited PZT films. In these examples, it is shown that well-characterized properties such as high thermal conductivity and high bending strength are obtained by the inherent effect in the millimeter-wave processing. In the millimeter-wave processing, high thermal conductive AlN with over 210 W/(m · K) can be synthesized by rapid sintering at lower temperature, compared with the conventional method. Suppression of the interfacial reaction between PZT film and substrate steel can also be attained using the millimeter-wave processing. It is explained that the high capabilities of applying millimeter-wave processing to smart materials synthesis originates from the non-thermal effects due to high frequency field of millimeter-wave.KEY WORDS: millimeter-wave; microwave effect; ponderomotive force; multi-phonon; enhanced mass transfer; rapid densification; selective heating. ISIJ International, Vol. 47 (2007), No. 4, pp. 539-544 Review(ii) easier injection of high powder, (iii) easier designing of compact applicator, (xi) suppression of local overheating (thermal runaway) due to the less temperature dependence of dielectric loss. Further, stable heating can be attained in the composite materials even if selective heating arises from the difference of dielectric losses in constitutional materials.In the heating of materials by microwave, dielectric loss, eЉ(w), is the important factor and can be divided into three factors due to ionic conduction, electric dipole and phonon in the following;where e c Љ(w), e d Љ(w) and e Љ ph (w) are the dielectric loss contributed by ionic conduction, electric dipole and phonon. In an ionic crystal having cation and anion vacancies eЉ(w) can be expressed by the following equation 2,11) ;....... (2) where where e(0), e(∞) and t i are the dielectric losses at zero and infinite frequencies and relaxation time, respectively.The term of eЉ ph (w) is generated from multi-phonon process, which is one of the non-thermal effects, and becomes important in the millimeter-wave and far-infrared regions. In an ionic crystal with low concentration of vacancies, however, the multi-phonon process is also the important mechanism for the absorption of microwave energy in the range from 2 to 20 GHz. According to Sparks et al., (4) where w f and G are the resonant frequency of fundamental reststrahrent transverse phonon mode and relaxation velocity coefficient, respectively.Influence of vacancy concentration on the dielectric loss eЉ(w) has been indicated by examining the dependence of eЉ(w) on the microwave frequency in NaCl single crystals with different vacancy concentration.11) In the temperature range below about 500 K, the difference in the v...

Microwave processing - A new dimension in synthesis of materials

“…Slip-casted pellets of 3 to 5 wt% Yb 2 O 3 in AlN were sintered under N 2 or N 2 + H 2 atmosphere by 28 GHz generator at 1650 °C up to 3 h. Samples with 3 wt% Yb 2 O 3 exhibited 98% TD (theoretical density) whereas with 5 wt% the sample was nearly 100% dense [31]. This rapidly processed sample at low temperature has exhibited very high thermal conductivity (> 200 W/(m.K)) due to the formation of ultra-thin (< 1nm) intergranular oxide layer [32].…”

Systematic study on densification of alumina fine powder during milliwave sintering at 28 GHz

“…The external electromagnetic radiation couples with dielectric materials at ionic level through a combination of dissipation (mainly polarization and conduction) mechanisms [6,[10][11][12]. From the basic theory of electromagnetic field [12] it is well-known that the absorption of electromagnetic energy in materials depends upon their complex permittivity ε.…”

“…Among ceramic materials alumina has several merits and can be used either as a benchmark material [5,6,17,19,[22][23][24][25], i.e., to compare different processing routes, or because of its outstanding properties which are of interest in various engineering applications (cutting tools, armor plates, dielectric resonators, patch antennas, laser sources, etc.). However, alumina is a low loss material and is hardly heated by CMW fields, especially from room temperature [11,13,15,26].…”

“…To date a number of millimeter-wave sintering (MMWS) devices have been commercialized worldwide. Most of them operate in the band of 28-83 GHz [4][5][6][7][8][9], specifically 28, 30, 60 and 83 GHz [10].…”

Microwave Penetration Depth in Materials with Non-zero Magnetic Susceptibility

“…[1][2][3][4][5][6][7][8][9][10][11] The main advantage of this technology, compared with other conventional methods (e.g. infrared radiation or convective transfer), is that microwaves penetrate into materials, heating volumetrically, and significantly improving the heat transfer to the interior of a sample.…”