Frequency dependence on dielectric constant and dielectric loss
The variation in the dielectric constant and dielectric loss of sesame oilseed at 4.5% moisture content is shown in Fig 3(a) and 3(b) for the frequency range of 5 kHz to 5 MHz at the indicated temperature. The figures show that the dielectric constant decreased from 5.558 to 1.478 and the dielectric loss decreased from 2.501 to 0.046 when the frequency was increased from 5 kHz to 5 MHz. This finding was consistent with that reported by
(Auksornsri et al., 2018). The decreasing trend of the dielectric constant was more regular than that of the changes in the dielectric loss factor at each temperature. According to the frequency-dependent variations in the dielectric constant and corresponding variations in the dielectric loss, the irregularities observed in the dielectric loss may be due to the complex structure of dielectric relaxation and dispersion events.
Frequency dependence on penetration depth and electrical conductivity
Fig 3(c) shows the relationship between the frequency and penetration depth of sesame oilseeds at the indicated temperatures. The value of the penetration depth decreased constantly with increasing frequency. At higher frequencies, the waves tend to be absorbed more by the materials. This absorption translates to energy loss in the form of heat or other forms of energy. As a result, as the frequency increases, more of the wave energy might be absorbed by the material, allowing less of it to penetrate deeper. It has also been reported that higher frequencies correspond to shorter wavelengths
(Singh et al., 2021). Shorter wavelengths may interact more with microstructures or particles within the material, leading to greater scattering or absorption. This is because of the longer wavelengths. The graph also illustrates that at higher temperatures, the penetration depth at any given frequency is generally lower than that at lower temperatures. This could be due to the increased molecular or atomic motion at higher temperatures, leading to more interactions and wave scattering, thereby reducing the penetration depth.
Fig 3(d) depicts the frequency dependence of the electrical conductivity at the indicated temperature, which shows that the electrical conductivity at the indicated temperature increased continuously with an increase in frequency.
Temperature dependence on dielectric constant and dielectric loss
Fig 4(a) and 4(b) show the variations in the dielectric constant and dielectric loss of sesame oilseed with temperature at the respective frequencies. It is evident that the nonlinear dielectric constant and dielectric loss of the effective complex permittivity increase with the temperature. Diverse frequencies also exhibit similar temperature-dependent dielectric characteristics
(Wang et al., 2013). The molecular mobility and ionic conduction of the material increase with temperature and at lower frequencies, these phenomena become more significant in influencing the dielectric properties as the temperature increases, similarly temperature and light effects on germination behaviour of African Eggplant (
Solanum aethiopicum L.) seeds were previously reported
(Botey et al., 2022).
Temperature dependence on penetration depth and electrical conductivity
The temperature dependence of the penetration depth of the sesame oilseed at the indicated frequency is shown in Fig 4(c). The interaction between ionic and dielectric loss causes the penetration depth to decrease significantly as the temperature increases. With the inclusion of loss from ionic conduction, the Cole-Cole relation can theoretically explain this pattern
(Feng et al., 2022).
The temperature dependence of electrical conductivity at the relevant frequency is shown in Fig 4(d). The electrical conductivity increased significantly with an increase in temperature and the increase was relatively large for higher frequencies and nearly linear for lower frequencies.
Moisture dependence on dielectric constant and dielectric loss
Fig 5 and 6 depict the variation in the dielectric constant and dielectric loss factor for the sesame oilseed sample at various moisture contents and 30°C. It was found that the dielectric constant and dielectric loss both showed an increasing trend as the moisture content increased and when the percentage of moisture content exceeded 13.2%, both parameters increased steeply. The rising rates of the dielectric constant and dielectric loss with moisture content were noticeably high at low frequencies, particularly at 5 kHz and 10 kHz. More water dipoles contribute to polarization at high moisture levels because of the higher water mobility, which shows how easily water dipoles follow applied field variations
(Ozturk et al., 2016). Below a moisture content of 9.5%, the dielectric constant and dielectric loss of the complex permittivity are both moderate. This is a result of the strongly bound water state (monolayer), in which the cell walls and water molecules are strongly attracted to one another while being relatively close to one another. As a result, both the dielectric loss and constant are quite low.
When the moisture content exceeds 9.5%, the change from the first bound water state-monolayer) to the second type-multilayer) could be responsible for the increase in the dielectric constant and dielectric loss of complex permittivity. When the moisture content was greater than 15.2%, there was a definite increase in frequency. With a high moisture content, particularly at a moisture content of 20.5% and frequency of 10 kHz, oilseeds exhibit extraordinarily high dielectric constant and dielectric loss values. The transition from the second (multilayer) type of bound water to the third (osmotic tension) type, or from free-state water, is the cause of this behavior. Large moisture levels and low frequencies result in strong ionic conductivity; hence, the dielectric losses are notably large under these conditions.
Effect of moisture content (%) on electrical conductivity
At a temperature of 30°C, experimental data on electrical conductivity with changes in moisture content percentage were gathered over the frequency range of 5 kHz to 5 MHz. Fig 7 shows how the electrical conductivity of sesame oilseed varies with moisture content at the indicated frequency and demonstrates how the electrical conductivity of each oilseed sample increases with increasing moisture content and frequency. At the indicated minimum and maximum moisture contents of the seeds, at 30°C temperature and over the frequency range of 5 kHz to 5 MHz, the electrical conductivity values of the sesame oilseed samples were found to range between 1.526×10
-7 S/m and 2.601×10
-4 S/m. This demonstrates that the electrical conductivity increases with moisture content. The electrical conductivity steeply increased for moisture contents greater than 13.2%. This behavior is similar to the dependency of moisture content on the dielectric constant and dielectric loss factor, which means that all these factors are related to each other.
Effect of moisture on penetration depth
At a temperature of 30°C, experimental data on penetration depth with varying moisture content percentages were gathered over the frequency range of 5 kHz to 5 MHz. Sesame oilseed sample penetration depth values ranged from 0.02 mm to 24.18 mm. Fig 8 demonstrates that the value of the sesame oilseed penetration depth constantly declined as moisture content increased at the observed specified set of frequencies between 5 kHz and 5 MHz at a constant temperature of 30°C. These results are consistent with those of a previous study
(Tripathi et al., 2015). With an increase in the moisture content percentage, the decrease in the penetration depth was due to strong ionic interactions and dielectric loss. This decreasing trend could also be explained by the Cole-Cole relation with the addition of loss from ionic conduction.
Statistical analysis
In this study, a linear regression model was developed to investigate the impact of the dielectric constant and loss on frequency and temperature in Fig 9 (a) and 9(b). It was hypothesized that the dielectric constant would have a significant impact of the applied frequency and temperature. To test this hypothesis, a regression was executed for the dielectric constant on the predicted variables, frequency and temperature
(Zuo et al., 2023). The results indicate that the dielectric constant can be significantly predicted with varying frequency and temperature (F = 408.4, p<0.001) similar to the previous study
(Mishra et al., 2024a). This suggests that the applied frequency and temperature play significant role in shaping the dielectric constant (b = -0.0000003, p<.001). An R-squared value of 0.705 indicates that the model explained 27.6% of the variance in temperature and frequency.