Frequency- and temperature-dependent dielectric properties of fruit juices associated with pasteurization by dielectric heating

Dielectric properties of apple, pear, orange, grape and pineapple juices were studied from 20 to 4500 MHz at 15-95 °C. The dielectric constants ε^′ of fruit juices decreased as frequency increased. The loss factors ε^″ decreased with increasing frequency to a minimum between about 1000 and 3000 MHz, depending on temperature, and then increased as frequency increased. The frequency of that turning point increased, whereas the minimum ε^″ decreased with increasing temperature. The ε^″ values increased with increasing temperature below about 1000 MHz, and decreased with increasing temperature above 3000 MHz. Dielectric constants decreased consistently and linearly with increasing temperature. Linear or polynomial correlations for the dependence of dielectric properties on temperature were developed. The power penetration depth of all fruit juices decreased with increasing frequency. The depth decreased as temperature increased at lower frequencies and increased with temperature above 3000 MHz. The dielectric properties obtained in the study provide important parameters for juice pasteurization by dielectric heating.     

吸水性对介电常数和介质损耗角影响因素研究

因为电子产品容易吸收水分,所以电子行业对介电常数、介质损耗角的关注程度一直居高不下。文章概述了在覆铜板行业中测试介电常数、介质损耗角的影响因素。     

Radiofrequency dielectric properties of amorphous semiconducting Y-Ba-Cu-O oxide thin films for bolometric detection

We report the study of dielectric properties of amorphous semiconducting YBCO thin films measured in a broad frequency range, from 40 Hz to 2 GHz and in the 210 to 430 K temperature range, using a coaxial discontinuity technique. At all temperatures, dielectric permittivity and conductivity spectra exhibit typical features of dielectric relaxation processes. A first relaxation at low frequency (300 Hz-2 kHz), observed only at high temperature, might be attributed to interfacial effects. Polarization in the grains of the YBCO thin films could lead to the second relaxation observed at higher frequency (800 Hz-660 kHz). The relaxation frequencies increase with temperature and follow a thermally activated behavior of the Arrhenius-type. Optical near-infrared response of YBCO bolometers exhibits a corner frequency around 40 kHz. Above this frequency, the thermal diffusion is localized inside the YBCO thin film. This frequency is in good agreement with the relaxation frequency attributed to YBCO grains in the thin film, i.e. around 40 kHz at 300 K.     

Field-assisted sintering of undoped BaTiO3: Microstructure evolution and dielectric permittivity

We report, for first time, how electric fields influence the sintering of undoped BaTiO₃, a ferroelectric material, and how this process affects the microstructure and the dielectric properties. Flash sintering is achieved at a furnace temperature of 688 °C under a field of 500 V cm⁻¹, producing specimens that are 94% dense. As a consequence, the grain size is much finer than in conventional sintering, which is shown to influence the Curie temperature and dielectric permittivity. Data obtained at different strengths of the electrical field, and current limits imposed on the specimen are presented in the form of a “processing map” that separates the safe region, where sintering is uniform, from the fail region, where the current flow in the sample becomes localized. The map illustrates that ceramics can respond by different mechanisms, with the dominant mechanism changing with the strength of the electrical parameters.     

Effect of fillers and vulcanizing systems on the physicomechanical and electrical properties of EPDM vulcanizates

The physicomechanical and dielectric properties were investigated for ethylene propylene diene terpolymer (EPDM) rubber loaded with different white fillers, namely, kaolin, quartz, polyvinylchloride PVC, and talc. The white fillers were added at a fixed level of 20 phr. Two vulcanizing systems were used, tetramethyl thiuram disulfide (TMTD) and sulfur/N-cyclohexyl-2-benzothiazyl sulfenamide (S/CBS). It was found that the curing systems affect the dielectric properties more than the filler type. The effect of the curing system and type of filler on the physicomechanical properties before and after thermal aging were also investigated, and the data obtained were interpreted.     

Synthesis, Characterization, and Microwave Absorption Property of the SnO2 Nanowire/Paraffin Composites

In this article, SnO₂ nanowires (NWs) have been prepared and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the SnO₂ NWs/paraffin composites have been measured in a frequency range of 0.1–18 GHz, and the measured results are compared with that calculated from effective medium theory. The value of maximum reflection loss for the composites with 20 vol.% SnO₂ NWs is approximately −32.5 dB at 14 GHz with a thickness of 5.0 mm.     

Nanocomposite ceramics based on La-doped BaTi2O5 and BaTiO3 with high temperature-independent permittivity and low dielectric loss

Nanocomposite ceramics containing a mixture of two ferroelectric phases, La-doped BaTi₂O₅ and BaTiO₃, with carefully-controlled phase amounts and ceramic microstructure have temperature-independent permittivity and low dielectric loss over very wide temperature ranges: ɛ = 95 ± 10 from 25 to 600 °C; tan δ = 0.02(2) from 25 to 400 °C, measured at 100 kHz. Further optimisation of properties should be possible.     

Temperature-independent permittivity of xBaTiO3–(1−x)(0.5Bi(Mg1/2Ti1/2)O3–0.5BiScO3) ceramics

xBaTiO₃–(1−x)(0.5Bi(Mg_1/2Ti_1/2)O₃-0.5BiScO₃) or xBT–(1−x)(0.5BMT–0.5BS) (x=0.45–0.60) ceramics were prepared by using the conventional mixed oxide method. Perovskite structure with pseudo-cubic symmetry was observed in all the compositions. Dielectric measurement results indicated that all the samples showed dielectric relaxation behavior. As the content BaTiO₃ was decreased from 0.60 to 0.45, temperature coefficient of permittivity (TCε) in the range of 200–400 °C was improved from −706 to −152 ppm/°C, while the permittivity at 400 °C was increased from 1208 to 1613. The temperature stability of permittivity was further improved by using 2 mol% Ba-deficiency. It was found that lattice parameter and grain size of the 2 mol% Ba-deficient ceramics were smaller than those of their corresponding stoichiometric (S) counterparts, with TCε in the range of 200–400 °C to be improved noticeably. For example, TCε of the Ba-deficiency sample with x=0.45 was −75 ppm/°C in the temperature range of 200–400 °C and the permittivity was 1567 at 400 °C. The results obtained in this work indicated that xBT–(1−x)(0.5BMT–0.5BS) ceramics are very promising candidates for high temperature capacitor applications.     

微波铁氧体吸收剂复介电常数的研究

对W型六角晶系铁氧体吸收剂复介电常数进行了研究，通过实验着重研究配方中Fe2O3的掺入量对介电常数的影响，研究发现，在分子式正分配方附近复介电常数ε的实部值ε′与虚部ε″值随Fe2O3的掺入量增加而升高，对材料中出现二价铁离子Fe^ 对介电常数的影响以及介电常数随频率的变化进行了分析。