频率、温度和含水率对玉米介电性能的影响

Frequency,moisture and temperature-dependent dielectric properties of maize

应用网络分析仪和同轴探头技术,研究含水率为13%~22%的玉米在频率10~3 000 MHz,温度25~85℃下的介电性能。结果表明：玉米的介电常数和介电损耗因数随频率升高而降低,随温度和含水率的增加而增大。高温低频条件下,在双对数图中高含水率玉米的介电损耗因数与频率呈现明显的负线性关系,表明离子导电引起介电损耗是主要的加热机理。玉米射频加热时容易引起＂热逸散＂现象,因此均匀的电磁场分布至关重要。微波、射频在玉米中的能量穿透深度随水分、温度和频率的升高而降低。射频频段的电磁能比微波频段有更深的穿透深度,因此玉米射频加热适合于大容器厚层物料,而微波加热时可采用小容器薄层物料。

Dielectric properties（the dielectric constant,loss factor and power penetration depth） of compressed maize flour samples were determined used by an open-ended coaxial-line probe with an impedance analyzer over the frequency ranging from 10 to 3 000 MHz,moisture contents from 13% to 22% w.b.,and temperature from 25 to 85 ℃.The results indicated that the dielectric constant and loss factor of the maize samples decreased with the increasing frequency but increased with the increasing temperature and moisture content.Under the conditions of low frequency and high temperature,negative linear correlations between the loss factor and the frequency on a log-log plot suggested that ionic conductance was the dominant heating mechanism.Compressed maize flour samples are likely to experience thermal runaway heating at radio frequencies,thus it is extremely critical to provide a uniform electric field for RF heating applicators.Power penetration depth decreased with the increasing frequency,temperature and moisture content.Maize samples could be treated in larger containers and thicker layers in RF heating due to deeper penetration.In microwave heating,however,small containers and thin layers of products could be used.