Dielectric and microwave absorption properties of FeSiAl/Al2O3 composites containing FeSiAl particles of different sizes

Here in, the effects of FeSiAl particle size on the dielectric and microwave absorption properties of FeSiAl/Al₂O₃ composites were studied. FeSiAl/Al₂O₃ composites containing 18–25 μm, 25–48 μm, and 48–75 μm FeSiAl particles were prepared by hot-pressed sintering based on uniformly mixed FeSiAl and Al₂O₃ powders. Results show that the real permittivity and the imaginary permittivity are significantly promoted with increasing FeSiAl particle size, which is ascribed to the enhanced interfacial polarization and conductance loss. In addition, the favorable matching impedance and suitable attenuation coefficient enabled the composite containing 25–48 μm FeSiAl powder to show a minimum reflection loss of ?34.4 dB at 11.7 GHz and an effective absorption bandwidth (<-10 dB) of 1.4 GHz in 11.0–12.4 GHz, when the thickness is 1.1 mm. By adjusting the thickness to 1.4 mm, the effective absorption bandwidth of the composite reaches a maximum value of 2.0 GHz in the 8.3–10.3 GHz range, indicating tunable, strong, and highly efficient microwave absorption performance.     

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.     

Physical sensors for water-in-oil emulsions

For many applications, the detection of water content in an oily base liquid is of crucial importance. Typical examples are automotive oils, e.g. engine and transmission oils, where water content in the order of a few percent needs to be detected. In this contribution, we analyze the suitability of permittivity and viscosity sensors for this application. It turns out, that permittivity sensors yield a clear indication of the water content in the oil being moreover to first order independent of the exact permittivity of the contaminating water. On the other hand, the output of microacoustic viscosity sensors, in contrast to conventional rotational rheometers, is hardly influenced by the water content as long as the droplet size is larger than the penetration depth of the evanescent microacoustic wave. Thus, they can be used for continued monitoring of the base oil’s viscosity even in case of an apparent water contamination.     

New frequency-dependent parametric modeling of dielectric materials

The recent increase in the use of electromagnetic (EM) waves raises more and more concern about the impact of EM radiation. Dielectric properties of materials are important indications of the depth of EM propagation and dispersion. This led to substantial research on the parametric modeling of dielectric properties of materials. In this paper, such modeling of material is studied for frequencies up to 100 GHz. The new model proposed was developed to describe the variation of dielectric properties of dielectricity as a function of frequency using experimental results on dielectric dispersion. Approximate modeling of electrical parameters can be very useful in that it allows the engineer to avoid complex equations of conventional models. Specifically, this work analyzes the dielectric properties of human parts, water, soil and vegetation. The new model is proposed for those materials. The theoretical background on those materials was investigated and simulation results on complex permittivity, the attenuation and phase constants of wave, of each of those materials were performed to assess the accuracy of the new approximate model. This model has further enabled conclusions in terms of measurable biophysical parameters.     

人工模拟肌肉组织复介电系数的微波测定

本文报导用置于恒温槽内的同轴开槽线和网络分析仪在200～2450MHz频带内测定一种人工模拟组织的复介电系数及其偏差讨论。本法基于传输线法并用微机对测量数据作实时直线回归处理以提高测量精度。对糊状模拟肌肉组织,本测量方法测定的的复相对介电系数(ε_1=ε′-jσ/ωε_0)的最大相对误差△ε′/ε′和△σ/σ均小于5％。本方法的优点是物理概念清晰,计算简便,便于自检,能宽带工作,特别适于具有中、高复介电系数的液体或糊状材料。     

Dielectric properties of polymer nanoparticle composites

Well-dispersed high dielectric permittivity titanium dioxide (TiO₂) nanoparticles were synthesized utilizing a block copolymer as a template. The nanoparticles were confined within microphase separated domains of sulfonated styrene-b-(ethylene-ran-butylene)-b-styrene (S-SEBS) block copolymers. A crosslinker (vinyltrimethoxysilane) was incorporated into the block copolymer matrices in order to decrease the dielectric loss from the free sulfonic acid groups. Dynamic mechanical analysis experiments confirmed that nanoparticles and crosslinker were confined within the crosslinked sulfonated styrene blocks and had no effect on the chain relaxation behavior of [ethylene-ran-butylene] blocks. Dielectric experiments showed that higher permittivity composites can thus be obtained with a significant decrease in loss tan δ (<0.01) when crosslinked with vinyltrimethoxysilane.     

Tailored and deep porosification of LTCC substrates with phosphoric acid

Low temperature co-fired ceramics (LTCC) as an advanced technology for robust assembly of electronic components, has attracted significant attention in a wide application range such as in wireless communication or automotive radar systems. However, accurate designs of micromachined devices operated at high frequencies require substrates with regions of tailored permittivities. Introduction of controlled porosity into the substrate via wet-chemical etching procedure, is a promising approach for permittivity reduction which can be applied to commercially available LTCC without necessitating to alter their composition or sintering process. In the present study, by selective dissolution of celsian phase a very deep porosification (highest reported so far) could be realized while preserving the surface quality. Also, by a careful selection of the etching parameters, the depth of porosification and hence the permittivity reduction can be delicately tailored. Laser ablation inductively coupled plasma mass spectrometry was used for the investigation of chemical compositions of substrates.     

3D printing of PDC-SiOC@SiC twins with high permittivity and electromagnetic interference shielding effectiveness

Effective electromagnetic interference (EMI) shielding requires materials with high permittivity. The current study reports 3D printed polymer-derived SiOC ceramics (PDC) modified with SiC nanowires (SiC_nw) exhibiting both high real and imaginary parts of permittivity within X-band. SEM results indicated that a large number of pores and cracks exist in the SiOC, and twinned SiC_nw were uniformly grown among them along with the existence of graphite microcrystals when the sintering temperature was 1500 ℃. The real part of permittivity ranged from 16.6 to 28.9 while the imaginary part from 31.7 to 34.2 in X-band. The EMI total shielding effectiveness (SE_T) of the ceramics could reach 34.7 dB with absorption loss (SE_A) of 29.3 dB and reflection loss (SE_R) of 5.4 dB. Meanwhile, the 3D printed PDC-SiOC ceramics at 900 ℃ sintering temperature possess certain mechanical properties with the magnitude of compressive strength being 12.57 MPa.     

High temperature dielectric properties of cubic bismuth zinc tantalate

Electrical properties of the parent phase in the Bi₂O₃–ZnO–Ta₂O₅ ternary system, cubic Bi_1.5ZnTa_1.5O₇ (α-BZT), P, are investigated using impedance spectroscopy. P has permittivity (?′) of 58, dielectric loss (tan δ) of 0.0023 at 30 °C and 1 MHz; temperature coefficient of capacitance (TCC) of −156 ppm/°C in the range of 30–300 °C at 1 MHz. A high degree of dispersion in the permittivity at low frequencies (<1 kHz) and temperatures above 500 °C is apparent. Dielectric losses exhibit non-frequency dependence at low temperatures presenting an increase at temperatures above 500 °C. A decrease of the loss occurs with increasing frequency.     

Dielectric spectroscopy measurements for moisture prediction in Vidalia onions

Microwave sensing offers an opportunity to determine nondestructively the amount of moisture in materials by sensing the dielectric properties of the material. Dielectric properties of Vidalia onions grown in southeastern Georgia were measured with an open-ended coaxial-line probe and network analyzer in the range from 200 MHz to 20 GHz. Frequency dependence and moisture dependence of dielectric properties were analyzed for moisture contents between 8% and 91%. Moisture content was linearly correlated with the dielectric constant at higher frequencies for the entire moisture range. A density-independent function that incorporates both the dielectric constant and loss factor was tested across multiple frequencies and moisture ranges. Use of this function enabled prediction of moisture content with high accuracy (R² = 0.99) up to 40% moisture content.