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The capacity of a substance to absorb microwave energy is expressed by its dielectric loss factor tan δ, which is represented by the dielectric loss (ε´´) and the dielectric constant (ε´).
tan δ = ε´´ / ε´
The lower the loss factor, the lower the absorption capacity of the substance. Materials with a low absorption capacity can be used as neutral components, e.g. as vessel material.
Some substances, e.g. water, have a relatively high loss factor at room temperature but this decreases once a certain temperature has been reached.
The dielectric loss factor is dependent on:
- Substance
- Frequency
- Temperature
| Materials |
tan δ * 10^-4 [2,5 GHz] |
| Quartz |
2,6 at 25 °C |
| Borosilicate |
12,3 at 25 °C |
| PTFE, PFA |
< 1,5 at 25 °C |
| PE |
3,1 at 25 °C |
| PVC |
56,1 at 20 °C |
| Ceramic (Al2O3) |
10,1 at 25 °C |
| Water, dist. |
1688,3 at 25 °C |
| Water, dist. |
535,7 at 85 °C |
Reference: Roger Meredith: Engineers´ Handbook of Industrial Microwave Heating. IEE Power Series 25, 1998, page 28ff.
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