Microwave susceptor

Microwave spectroscopy, 23 129, 135-136 Microwave susceptor, 16 528 Microwave techniques, applications in chemical technology, 16 539... 

The sample of desorbed tritide is placed inside a quartz tube that is connected to a gas-handling manifold by a TorrSeal . A quartz sleeve with Silicon Carbide (SiC) in the annular space is placed around the end of the quartz tube, surrounding the sample with microwave susceptor. The quartz tube and susceptor sleeve are thermally insulated from the rest of the microwave cavity. An internal thermocouple measures the temperature of the sample and provides the temperature signal for process control of the desired temperature. A shine block (alumina foam), attached to the thermocouple, blocks radiant heating of the TorrSeal and the upper area of the quartz tube and manifold. An IR pyrometer is used as a secondary measure of the temperature of the susceptor, and therefore of the sample. A stainless steel shield reflects microwaves from the quartz tube not in the susceptor sleeve, eliminating the production of a plasma at low pressure in the quartz tube. 

Begley, T.H., Dennison, J.L. and Hollifield, H.C. (1990). Migration into food of polyethylene terephthalate (PET) cyclic oligomers from PET microwave susceptor packaging. Food Addit. Contain., 7, 6, 797-803. 

Begley, T.H., Biles, J.E., and Hollifield. H.C., 1991, Migration of an Epoxy Adhesive Compound Into a Food Simulating Liquid and Food From Microwave Susceptor Packaging, J. Agricultural and Food Chem., November, 1944-1945. 

Sharman. M., Honeybone, C.A., Jickells, S.M., and Castle, L., 1995, Detection of residues of the epoxy adhesive component bisphenol A diglycidyl ether (BADGE) in microwave susceptors and its migration into food.. Food Add. Contam. 12,6,779-787. 

Microwave susceptors are used for crisping and browning food in a microwave oven. The susceptor is a multi-layer material usually made of a paper layer, a thin aluminium layer and finally a polyethylene terephthalate layer which is in contact with the food. The susceptor absorbs the energy of the microwaves and this heat is transferred to the food resulting in a crispy surface at the part in contact with the food. 

Begley, T.H. HoUifield, H.C. Migration of dibenzoate plasticizers and polyethylene terephthalate cyclic oligomers from microwave susceptor packaging into food-simulating liquids and foods. J. Food Prot. 1990, 53, 1062-1066. 

For high-temperature microwave ceramic processing, the refractory specimen enclosure (the casket) serves as both i) a thermal insulator and ii) a microwave susceptor. The susceptor allows the microwave energy to couple with the material at high temperature. The thermal insulation diminishes the dissipation of thermal energy. On occasion, the caskets are referred to as susceptors, which does not fully depict the dual role of the specimen enclosure or casket as both a thermal insulator and a microwave susceptor (absorber). 

Fig. 3.20. Schematic furnace for secondary microwave sintering MSCC = microwave susceptor ceramic composite (Pope, 1991).

Susceptor types. Currently there are three microwave susceptor types. 

For particulate heating, the dipolar heating is aspired in the majority of the cases with a minimizing of the resistive effects. Nevertheless, the dominant heating effect depends on the chemical natme of the microwave susceptors [23],... 

Recent research work has revealed carbon structures, silicon carbide, titanium dioxide, and metal flakes as suitable microwave susceptor additives [29]. 

Some work has been reported about the effect of different microwave susceptors mixed in a polymeric matrix. Most of the literature is related to carbon derivates, like carbon black and carbon microfibers [29,31]. Regarding the effectiveness, they are suitable susceptors for most applications. 

The microwave susceptors in this initial study have been carbon black, magnetite, lead zirconate titanate, and silicon carbide. The polymeric matrix for these trials has been mainly high density polyethylene, which is a nonpolar polymer without any absorption of microwave radiation. Other thermoplastic matrixes like polyamide 6 (PA6), polybutylene terephthalate (PBT) and metallocene polypropylene (m-PP) have been used as reference material. 

Table 2.3. Heating efficiency calculated for different microwave susceptors in HDPE...

The results presented by Ling et al. [31], which are focused on carbon fibers as susceptors, describe a similar heating behavior like the one being observed with carbon black filled HDPE material. The work of John Harper [29] is related to the heating behavior of carbon black as a microwave susceptor in high density polyethylene by the modification of the surface area. The specific surface was varied from 7.5 to 137 m /g (Figure 2.14). 

By the use of the single-mode microwave technology, the heating efficiency of carbon nanotubes as microwave susceptors could be further improved. The microwave power was reduced to 100 W. With this parameter setup, temperatures beyond 100 °C were reached in only 20 seconds, which previously have needed 400 W using multimode frequency setups. 

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