Difference In Dielectric Loss Research Articles

Caffeic acid assists microwave heating to inhibit the formation of mutagenic and carcinogenic PhIP

The inhibitory effect of caffeic acid on the formation of 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP) was investigated in chemical model systems under microwave heating (MW). A mechanistic study was subsequently carried out to identify the inhibitory mechanism. The results showed that both for conductive heating (CV) and MW, the inhibition of PhIP increased with the concentration of caffeic acid but decreased with the prolongation of heating time. The inhibition on PhIP under MW was always higher than under CV, which were dominated by the difference in dielectric loss (ε″). UPLC-MS analysis showed that caffeic acid releases a CO2 molecule to produce 4-vinylcatechol which can form adducts with phenylacetaldehyde, thus reducing its availability for PhIP formation. The structure of adduct was characterized as 3-(3,4-dihydroxyphenyl)-2-phenylbutanal with a molecular weight of 256. The findings indicate that trapping of phenylacetaldehyde by 4-vinylcatechol is a key mechanism of caffeic acid in inhibiting PhIP formation.

Dielectric properties and electromagnetic interference shielding effectiveness of graphene-based biodegradable nanocomposites

Graphene nanoplatelets (GNPs) were dispersed in poly lactide (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) via melt-mixing. Effect of GNP incorporation on electromagnetic properties and electromagnetic interference shielding effectiveness (SE) of PLA and PBAT was investigated and the two systems were systematically compared. Furthermore, applicability of Sihvola's mixing rule of complex electrical permittivity to these nanocomposites was studied. GNP addition significantly enhanced permittivity of both polymers. Dielectric constants of PLA and PBAT nanocomposites had comparable values. However, above 6wt% GNPs, PLA nanocomposites showed significantly higher dielectric loss than PBAT nanocomposites, even though pure PLA had lower dielectric loss than pure PBAT. This was attributed to the dispersion state of GNPs in the two matrices, detected in morphological studies. SE of both polymers increased with GNP addition due to enhancement of their dielectric properties. The difference in dielectric loss of the two systems was revealed in their ability to attenuate the radiation by absorption. At 15wt% GNPs, 1mm-thick PLA/GNP nanocomposite had an effective absorbance of 70%. This value was only 43% for PBAT/GNP nanocomposite. Variations of polymers' permittivities with GNPs were successfully modelled by Sihvola's rule. While both systems returned close values for model's fitting parameters, it better fitted PBAT/GNP nanocomposites.

Huge low-frequency dielectric response of (Nb,In)-doped TiO2 ceramics

The (Nb,In)-doped TiO2 ceramics have drawn considerable attention as a type of promising giant-permittivity dielectric materials in recent years. However, a significant controversy concerning the giant dielectric mechanism currently exists, and clarifying it is vitally important from both scientific and technological viewpoints. This letter reports the results of a systematical comparison study, where two kinds of (Nb,In)-doped TiO2 ceramics with a substantial difference in dielectric loss are used. Dielectric properties and complex impedance are investigated over a broad frequency band of 3 mHz–110 MHz. A huge low-frequency dielectric response in addition to the giant dielectric relaxation appearing above 1 MHz is observed for both kinds of (Nb,In)-doped TiO2 ceramics in dielectric dispersion. The huge dielectric response observed in the low frequency range can be ascribed to a non-ohmic electrode-contact, and the dielectric relaxation appearing above 1 MHz can be attributed to an internal barrier layer capacitance effect. An electrical equivalent circuit model suggested can well describe the observed dielectric properties and electrical behaviors.

Effect of moisture on temperature rise of composite insulators operating in power system

The local heating phenomenon of composite insulator has drawn more and more attention gradually. From our analysis, the temperature rise of composite insulator is closely related to the moisture in atmospheric environment according to research results. Effect of moisture on the temperature rise of composite insulators is studied in this paper. The composite insulators, which have operated in power system for a long time and have been detected for local heating, and new insulator are taken as test samples. The temperature rise of operated composite insulators, which have been detected for local heating, increases with the rise of the relative humidity, while new insulator shows no temperature rise can be detected. The temperature rise of sample is not relevant to the core and the interface between core and housing material practically, but the high voltage end housing. The moisture content of the housing material is different at various relative humidity, and this leads to difference in dielectric loss, which is a significant source of local heating phenomenon. The difference of dielectric loss at different relative humidity leads to the temperature rise variation with the relative humidity.

Dielectric loss determination of fine residual waste electrical and electronic equipment for understanding of heat development during microwave pyrolysis

Waste electrical and electronic equipment (WEEE) contains rare and valuable metals which are important to recycle, but it also has high organic content. The conventional methods used today for treatment of WEEE cannot recycle 100%; they generate valuable fine residual fractions. For further processing of these residues, microwave pyrolysis has shown to be promising since it reduces the organic content and liberate enclosed metal pieces. However, to fully control the process, the dielectric properties of the materials need to be known but are difficult to determine due to the complex composition and structure of WEEE. This paper aims to describe a suitable procedure for determination of dielectric loss in six WEEE residual fine fractions and to correlate the results to heating behaviour during microwave pyrolysis. Three fractions are “dusts” (light, medium and heavy dust) and three fractions are called “particulate materials” and contain smaller particles (0–7mm, 0–20mm and 7–12mm). The method chosen was Vector Network Analyzer (VNA) using waveguide WR430 equipped with polymer foam to contain the samples which consisted of 1dl of WEEE fine fraction. The scattering parameters were measured and dielectric loss in the materials was calculated. The measurements were performed in frequency region 1.3–2.7GHz to include the industrial frequency 2.45GHz. Differences between dielectric losses for the materials were determined by ANOVA. It was shown that the scattering parameter measurements were rather stable for the dust materials and the losses could be determined. For the larger sized particulate materials a lot of reflection was observed during the measurements, due to fine metal wires in the material. The losses could be determined after removing these wires. The previously observed higher heating rate and shorter time for mass reduction of dusts could not be explained by differences in dielectric loss. However, since VNA measurements indicate that metal wires in the material disturb the field; presence of wires in the particulate materials might be one cause for slower heating rate and observed temperature drops during pyrolysis.

Microwave dielectric properties of lanthanum aluminate ceramics and single crystal

Single crystal and ceramics of LaAlO 3 were prepared by a Czochralski method and conventional solid phase reaction, respectively, using high purity reagents. Far infrared reflectivity spectra for single crystals and ceramics of LaAlO 3 were measured and eigenfrequencies and damping constants of transverse and longitudinal optical modes were evaluated in order to discuss variations in the dielectric properties. The observed reflectivity spectra were fitted by four IR active modes (predicted by factor group analysis) in order to calculate the vibration eigenfrequencies and damping constants. Differences in dielectric loss were found between the two kinds of single crystals, having different crystal orientations. It was inferred that the difference in dielectric loss was due to difference in cut off frequencies of these single crystals. The loss obtained from the IR reflectivity of the ceramics was used to evaluate the extrinsic loss in the ceramics.