Dielectric properties and electromagnetic wave transmission performance of aluminium silicate fibreboard at 915 MHz and 2450 MHz

Refractory lining is an important part of microwave heating equipment, and its wave-transparent effect significantly influences microwave heating efficiency. The complex dielectric constant of the material is the main factor determining its wave-transparent properties. In this study, with aluminium silicate fibreboard (ASF) as the study object, the complex dielectric constant of this material was investigated at temperatures ranging from 25 °C to 1000 °C at 915 MHz and 2450 MHz, respectively. Based on the theory of transmission lines, the wave-transparent properties of the material during the entire microwave heating process were calculated and analysed. It has been found that the dielectric constant and loss factor at both frequencies increase significantly when the temperature exceeds 700 °C; the wave-transparent properties of this material decrease with increasing temperature and present a fluctuating trend as the material thickness increases, with maximum and minimum values; in TM polarization mode, there is an optimal angle of incidence for total transmission, which contributes to a better overall wave-transparent effect of the material in TM polarization mode than that in TE polarization mode. Finally, this study provides the preferred thickness values at different working frequencies and temperatures under vertical incidence of electromagnetic waves for reference. This study is of certain theoretical significance to the research on the change mechanism of the material's dielectric and wave-transparent properties and has practical guidance on the optimization of microwave heating equipment and the selection of the technological parameters of furnace linings.     

Effect of polycrystalline mullite fibers on the properties of vitrified bond and vitrified CBN composites

The effect of polycrystalline mullite fibers (PMFs) on the properties of vitreous bonds and vitrified CBN composites was investigated. The results show that the addition of PMFs can increase the porosity of composites and reduce the fluidity of binders. The vitrified composites incorporating 6.4 wt% PMFs display excellent mechanical strength, which is enhanced by 21.2% compared with that of composites without PMFs sintered at the optimal sintering temperature. Meanwhile the thermal expansion coefficient of vitrified bond reduces from 6.256×10⁻⁶ °C⁻¹ to 4.805×10⁻⁶ °C⁻¹ with increasing fraction of PMFs. The improvement of mechanical strength is associated with the change of cracking mechanisms of the composites with fibrous crystals and the existence of several observed mechanisms, including fiber pull-out, fiber bridging and rupture.     

Microwave sintering study of strontium-doped lanthanum manganite in a single-mode microwave with electric and magnetic field at 2.45 GHz

The objective of this work is to study the changes in the physical and mechanical properties of strontium-doped lanthanum manganite (LSM) material and LSM-YSZ (ZrO₂ doped with 8 mol% yttria) composite, obtained by colloidal processing and sintered by 2.45 GHz microwave sintering at 1200 and 1300 °C using two different single-mode cavities. One circular cavity with TE₁₁₁ mode that has maximum in the electric field (E-field) and one rectangular cavity with TE₁₀₂ mode that has maximum in the magnetic field (H-field). As compared to conventional sintering at 1300 and 1400 °C, the microwave-heated samples exhibited a denser structure for shorter sintering times. LSM-based materials showed higher heating behavior in H-field, which translates into higher energy absorption. This fact can be attributed to an electromagnetic pressure induced by the combined effect of current loops subjected to H-field. Therefore, the interaction between the material and the electromagnetic waves depends on the dominant field of them.     

Dielectric properties and electromagnetic wave absorbing performance of single-source-precursor-derived carbon-rich NbC-SiC-C nanocomposites

For the first time, the present work reports the dielectric properties and electromagnetic wave (EMW) absorbing performance of polymer-derived carbon-rich NbC-SiC-C nanocomposites. In our previous work, NbC-SiC-C nanocomposites with the ultra-high temperature ceramic phase NbC as the main phase were synthesized with the allylhydridopolycarbosilane (AHPCS) and niobium pentachloride (NbCl₅) as starting materials. On this basis, divinyl benzene was chosen as carbon-rich source and introduced into the AHPCS and NbCl₅ to form a single-source-precursor. Finally, carbon-rich NbC-SiC-C nanocomposites were successfully synthesized by polymer-derived ceramic approach. Compared with ceramic samples without Nb and with lower carbon content, the carbon-rich NbC-SiC-C nanocomposites show extremely enhanced EMW absorbing performance with minimum reflection coefficient of −51.1 dB at 6.88 GHz for the thickness of 2.27 mm. As a consequence, the resultant carbon-rich NbC-SiC-C nanocomposite has to be considered as structure&function integrated material with excellent EMW absorption performance, which can be applied in hostile environment.     

Dielectric response and electromagnetic wave absorption of novel macroporous short carbon fibers/mullite composites

For wider-band and stronger electromagnetic (EM) wave absorption, macroporous short carbon fibers/mullite matrix (C_f/Mu) composites were prepared via introducing short carbon fibers (0, 0.7, and 1.3 vol%) with length of 2-3 mm into macroporous mullite ceramic by gel-casting. The density of as-prepared C_f/Mu composites decreases from 2.93 g/cm³ to 2.74 g/cm³, while the porosity increases from 3.32% to 10.76% with the rise in carbon fibers content. The diameter of macropores in C_f/Mu composites is ranging from several microns to tens of microns. Complex permittivity and dielectric loss of the prepared composites in X-band (8.2-12.4 GHz) are significantly enhanced with increased carbon fibers content. The best EM wave absorption performance is obtained in the macroporous C_f/Mu composites containing only 0.7 vol% carbon fibers (C_f/Mu-0.7). The maximum absorption loss of C_f/Mu-0.7 is −38.3 dB at 12.08 GHz at the thickness of 2.1 mm, and effective absorption bandwidth below −10 dB (over 90% of EM wave absorption) covers the whole X band with the thickness of 2.35 mm. The results suggest that the C_f/Mu composites can be promising high-performance EM wave absorbing materials.     

Dielectric properties of alumina doped with TiO2 from 13 to 73 GHz

Recent studies show that alumina doped with TiO₂ exhibits promising dielectric properties, corresponding to low loss tangents and low temperature coefficients (or close to 0 ppmC⁻¹). This paper aims to confirm these trends and study the dielectric properties of alumina doped with TiO₂ from 0.5% to 12% wt. at high frequency, 13–73 GHz. This work demonstrates that alumina doped with TiO₂ corresponds to potential materials for frequency converter devices working at a high frequency (up to 50 GHz).     

Dielectric and thermal properties of dicyclopentadiene containing bismaleimide and cyanate ester. Part IV

A novel bismaleimide (BMI), bis(4-maleimidophenoxy-3,5-dimethylphenyl)dicyclopentadiene (DCPDBMI), containing a large dicyclopentadiene (DCPD) and aryl ether linkage, was synthesized from bis(4-aminophenoxy-3,5-dimethylphenyl)dicyclopentadiene and maleic anhydride by the usual two-step procedure that included ring-opening addition to give bismaleamic acid, followed by cyclodehydration to bismaleimide. The monomers were characterized by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (NMR), elemental analyses (EA), and mass spectra (MS). A series of bismaleimide-triazine (BT) resins were prepared from synthesized bismaleimide (DCPDBMI) and then cured with 2,6-dimethyl phenol-dicyclopentadiene dicyanate ester (DCPDCY) at various molar ratios. Thermal properties of cured BT resins (DCPDBMI/DCPDCY) were studied using dielectric analyzer (DEA), dynamic mechanical analyzer (DMA) and thermalgravimetric analyzer (TGA). These data were compared with that of commercial bismaleimide (DDMBMI) cured with bisphenol A dicyanate ester (BADCY). The cured DCPDBMI/DCPDCY exhibits lower dielectric constant, dissipation factor and moisture absorption than those of DDMBMI/BADCY. The effects of blend composition on the glass transition temperatures and thermal stability are discussed.     

In-situ measurement of microwave absorption properties at 2.45 GHz for the polycondensation of lactic acid

For the construction of a microwave-assisted organic synthesis plant, it is necessary to know the dielectric properties of the reaction system. Measurements of the dielectric properties of lactic acid aqueous solution, anhydrated lactic acid, oligo(lactic acid) and water, which are constituent materials in the polycondensation of lactic acid, confirm that dielectric properties decrease as reaction progresses. Calculated microwave penetration depths, obtained from the dielectric properties, show that microwaves penetrate deeply into the reaction system. This work should be useful for the development of microwave-assisted organic syntheses in the chemical industry.     

Evolution of the composition,microstructure and electromagnetic properties of HfOC ceramics with pyrolysis temperature

The evolution of phase composition, microstructure, and dielectric characteristics of HfOC ceramics pyrolyzed at various temperatures was studied in this work. When the pyrolysis temperature increased from 900 to 1500 °C, the composition of HfOC ceramics varies from HfO₂ and amorphous carbon (C_amp) at 900 °C to coexistence of HfO₂, C_amp, and HfC at 1100–1300 °C, and HfC and C_amp at 1500 °C. With the continuous consummation of C_amp, its distribution is transformed from a slice-like structure accumulating around the particles to a shell-like structure wrapping around the particles. The atomic ratios of as-obtained HfOC ceramics are HfO_2.0C_2.8, HfO_1.9C_2.7, HfO_1.0C_1.8, and HfO_0.1C_1.1, respectively, after being pyrolyzed at 900, 1100, 1300, and 1500 °C. As the pyrolysis temperature increases, the average value of the real part increases from 13.5 to 16.5, and the imaginary part rises from 12 to 14. The microwave absorption properties of HfOC ceramics need to be enhanced further in the future work.     

Controllable dielectric properties and strong electromagnetic wave absorption properties of SiC/spherical graphite-AlN microwave-attenuating composite ceramics

Fully dense SiC/spherical graphite-AlN microwave-attenuating composite ceramics were manufactured via hot-pressing sintering, in which, apart from the primary SG (spherical graphite) attenuating agent, 5–30 wt% semiconductive α-SiC was employed as the second attenuating agent. The incorporation of SiC contributed to a slightly decreasing electrical conductivity and enhanced polarization relaxation. Controllable complex permittivities were obtained, namely, both the real and imaginary permittivities exhibit first a decrease and then an increase with the SiC addition, and which delivers an optimized impedance matching of the composites. RL_min values below ?10 dB (more than 90% absorption) were achieved by all the composites containing 5–20 wt% SiC with the sample thickness of 1–1.4 mm, and the absorption performance characteristics were significantly tunable by controlling the of SiC content at 8.2–12.4 GHz. Impressively, a superior reflection loss of ?46 dB (1.1 mm) and wide effective absorption bandwidth of 2.1 GHz were achieved at a 5 wt% SiC content, respectively, rendering SiC/SG–AlN composites a potential ultra-thin and highly efficient microwave-attenuating ceramic candidate.     

Synthesis and electrical properties of the (PVA)0.7(KI)0.3·xH2SO4 (0 ≤ x ≤ 5) polymer electrolytes and their performance in a primary Zn/MnO2 battery

Solid acid polymer electrolytes (SAPE) were synthesised using polyvinyl alcohol, potassium iodide and sulphuric acid in different molar ratios by solution cast technique. The temperature dependent nature of electrical conductivity and the impedance of the polymer electrolytes were determined along with the associated activation energy. The electrical conductivity at room temperature was found to be strongly depended on the amorphous nature of the polymers and H₂SO₄ concentration. The ac (100 Hz to 10 MHz) and dc conductivities of the polymer electrolytes with different H₂SO₄ concentrations were analyzed. A maximum dc conductivity of 1.05 × 10⁻³ S cm⁻¹ has been achieved at ambient temperature for electrolytes containing 5 M H₂SO₄. The frequency and temperature dependent dielectric and electrical modulus properties of the SAPE were studied. The charge transport in the present polymer electrolyte was obtained using Wagner's polarization technique, which demonstrated the charge transport to be mainly due to ions. Using these solid acid polymer electrolytes novel Zn/SAPE/MnO₂ solid state batteries were fabricated and their discharge capacity was calculated. An open circuit voltage of 1.758 V was obtained for 5 M H₂SO₄ based Zn/SAPE/MnO₂ battery.     

The role of particle size on the electrochemical properties at 25 and at 55 °C of the LiCr0.2Ni0.4Mn1.4O4 spinel as 5 V-cathode materials for lithium-ion batteries

The role of the particle size on the electrochemical properties at 25 and at 55 °C of the LiCr_0.2Ni_0.4Mn_1.4O₄ spinel synthesized by combustion method has been determined. Samples with different particle size were obtained by heating the raw spinel from 700 to 1100 °C, for 1 h in air. X-ray diffraction patterns revealed that all the prepared materials are single-phase spinels. The main effect of the thermal treatment is the remarkable increase of the particles size from 60 to 3000 nm as determined by transmission electron microscopy. The electrochemical properties were determined at high discharge currents (1C rate) in two-electrode Li-cells. At 25 and at 55 °C, in spite of the great differences in particle size, the discharge capacity drained by all samples is similar (Q_dch ≈ 135 mAh g⁻¹). Instead, the cycling performances strongly change with the particle size. The spinels with Φ > 500 nm show better cycling stability at 25 and at 55 °C than those with Φ < 500 nm. The samples heated at 1000 and 1100 °C, with high potential (E ≈ 4.7 V), elevate capacity (Q ≈ 135 mAh g⁻¹), and remarkable cycling performances (capacity retention after 250 cycles >96%) are very attractive materials as 5V-cathodes for high-energy Li-ion batteries.