MnZn铁氧体/SrTiO3/PTFE复合材料的制备与表征

采用SrTiO3溶胶对MnZn铁氧体进行表面改性，得到MnZn铁氧体／SrTiO3复合粉体，以PTFE为基体，制备了的MnZn铁氧体／SrTiO3／PTFE复合材料。采用HP4292B和HP4294对复合材料电磁性能进行了表征。结果表明：复合材料在低频阶段具有较高磁导率和介电常数以及较低磁损耗和介电损耗，复合材料的介电频率响应符合德拜弛豫理论。     

Enhanced electromagnetic wave absorbing properties of Si-O-C ceramics with in-situ formed 1D nanostructures

The Si-O-C ceramics were prepared by polymer-derived ceramic method using polysiloxane/FeCl₃ as precursor with the FeCl₃ content of 1.0 wt%. The microstructure, dielectric properties, and electromagnetic wave (EMW) absorbing properties in X band of the Si-O-C ceramic were investigated. It was found that the pyrolysis temperature has a great influence on the amount of in-situ formed CNTs and the transformation from CNTs to 1D SiC nanostructures. With the temperature rising from 1000 to 1500°C, the SiC formed with various morphologies including SiC microspheres, needle-like SiC, and SiC nanowires which were transformed from CNTs. The EMW absorbing properties were dramatically improved when the pyrolysis temperature raised to 1500°C; the minimum reflection loss (RL) was −58.37 dB of sample with a thickness of 2.95 mm at 10.11 GHz, and the absorbing band (RL ≤−20 dB) of sample at a thickness of 3.0 mm covers 3.8 GHz (8.2-12.0 GHz), which means more than 99% of the EMW were absorbed. The enhancement of EMW absorbing properties of bulk Si-O-C ceramics was attributed to the interfacial polarization induced by in-situ heterogeneous nanostructures with complex interfaces.     

微螺旋炭纤维的微波电磁参数及其环氧复合材料的吸波性能研究

采用化学气相沉积法制备了微螺旋炭纤维(CMCs),研究了其在频率为8.2￣12.4GHz范围内的微波介电特性;以环氧树脂为胶粘剂制备了CMCs/环氧复合吸波涂层,并对其吸波性能进行了研究。结果表明,随着CMCs含量的增加,复介电参数实部ε′、虚部ε″和介电损耗tanδ均有所增加,当CMCs含量为1%￣5%时,ε″和tanδ增幅很小;当CMCs含量为10%时,两者增幅显著增大。ε′表现出高频减小的趋势,同时tanδ表现出高频增大的趋势,这些均有利于实现高频吸波。磁损耗tanξ较小且CMCs含量对其影响不明显。CMCs/环氧复合吸波涂层的反射率随着CMCs含量的增加而明显减小,表现出高频衰减略有增加的趋势,同时出现多吸收峰,这有利于实现宽频吸波。     

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.     

Electromagnetic wave absorbing properties of Cr2AlB2 powders and the effect of high-temperature oxidation

The electromagnetic (EM) wave absorbing properties of Cr₂AlB₂ powders and those after high-temperature oxidation were investigated. Coupling of magnetic and dielectric loss enables Cr₂AlB₂ with good absorption properties. The minimum reflection loss (RL) value is −44.9 dB at 8.5 GHz with a thickness of 2.7 mm, and the optimized effective absorption bandwidth (E_AB) is 4.4 GHz (13.0-17.4 GHz) with a thickness of 1.6 mm. After oxidation at 750, 900, and 1000°C for 2 h, the minimum RL values, respectively, are −23.9 dB (17.5 GHz, 1.5 mm), −41.4 dB (16.5 GHz, 1.5 mm), and −39.5 dB (8.0 GHz, 3.0 mm); and the corresponding E_AB values, respectively, are 3.8 GHz (13.6-17.4 GHz, 1.7 mm), 4.1 GHz (13.5-17.6 GHz, 1.6 mm), and 4.4 GHz (13.0-17.4 GHz, 1.7 mm). With an absorber thickness of 1.5-4.0 mm, the E_AB with a RL value of less than −10 dB can be tuned in a broad-frequency range 5.0-18.0 GHz, which basically covers C (4-8 GHz), X (8-12 GHz), and Ku (12-18 GHz) bands. These results demonstrate that Cr₂AlB₂, as a high-efficient and oxidation-resistant absorber, is a promising candidate for microwave absorption applications and can retain good EM wave absorbing properties after high-temperature oxidation.     

Oxygen vacancy formation in the SrTiO3 Σ5 [001] twist grain boundary from first‐principles

The SrTiO₃ 5 [001] twist grain boundary (GB) is studied using first‐principles density functional theory calculations. Three types of GB structures, SrO/SrO (S/S), SrO/TiO₂ (S/T), and TiO₂/TiO₂ (T/T), are modeled and their relative thermodynamic stabilities are examined. Our calculations show that the S/S and S/T structures can be formed within appropriate synthesis conditions, with the S/S structure thermodynamically favored over the S/T structure within a wide range of chemical potentials, while the T/T structure is unlikely to form. The segregation behavior of oxygen vacancies is also investigated by calculating oxygen vacancy formation energies with respect to the distance from GB plane. In the S/S system, oxygen vacancies tend to segregate to the layer adjacent to the GB layer, while in the S/T system, oxygen vacancies tend to segregate to the GB layer itself. In both S/S and S/T systems, oxygen vacancy formation energy is lower than that in bulk SrTiO₃. To clearly show the experimental conditions necessary to promote oxygen vacancy formation in the 2 GB systems, we also generate grain boundary phase diagrams for oxygen vacancy with respect to synthesis temperature and oxygen partial pressure. Our calculations reveal different segregation behaviors and distributions of oxygen vacancies in the S/S and S/T systems, providing a possible avenue for GB engineering.     

Microstructure and electrical properties of Nb-doped SrTiO3-BiFeO3 based lead-free ceramics

In this work, Nb-doped 0.75SrTiO₃-0.25BiFeO₃ (ST-BF) lead-free ceramics are designed and synthesized using a conventional solid-state reaction method. The influence of Nb doping on the microstructure, dielectric, and electrical properties are systematically investigated. With the increase of Nb concentration, the crystal structure of ST-BF remains pseudo-cubic as exhibited in the X-ray diffraction patterns. The grain size is found to increase from 0.33 to 6.23 μm, and then decrease to 1.88 μm by Nb doping, along with a clear heterogeneous core–shell microstructure. A relatively low dielectric loss (∼0.1, at 1 kHz) and a stable dielectric constant (∼700, at 1 kHz) are obtained for the 0.03 Nb-doped ST-BF composition at room temperature. Impedance spectroscopy analysis shows that Nb doping in ST-BF increases the total resistivity, forming an electrically conductive core and a nonconductive shell, with enhanced activation energy. The results may provide a feasible approach to develop novel ST-based lead-free dielectric ceramics for capacitor applications.     

Electrical and electromagnetic shielding behavior of laminated epoxy–carbon fiber composite

This paper reports results on the electrical properties of laminated epoxy composite containing 25 layers of carbon fibers in the form of mats. The dependence of the activation energy (calculated from DC resistivity measurements) on temperature reveals two independent conduction processes. The AC impedance is independent of the applied frequency below 75°C, and the real componet of the dielectric constant is also independent of temperature at high frequencies. The determined shielding effectiveness is dominated by the insertion loss. The observed optimum shielding effectiveness occurs at 30 mm spacing and applied frequency 9 GHz.     

水泥基电磁屏蔽与吸波材料的研究进展

阐述了电磁波污染对人类和环境的危害,分析了电磁屏蔽材料的屏蔽机理和吸波材料的吸波机理,探讨了水泥基电磁屏蔽材料与吸波材料的设计原理,并论述了它的研究现状,提出了水泥基电磁屏蔽材料与吸波材料今后的研究方向.     

Radiofrequency and microwave (10 kHz–8 GHz) electrical properties of polypyrrole and polypyrrole–poly(methyl methacrylate) composites

The complex permittivity ? = ?′ ?j ?″ of polypyrrole (PPY) samples and polypyrrole–poly(methyl methacrylate) (PPY‐PMMA) composites is calculated from measurements in the radiofrequency and microwave range (F = 10³–8 × 10⁹ Hz) at room temperature. A relaxation phenomenon is observed in PPY‐PMMA composites with polypyrrole concentration p = 6–12% by weight. The frequency F_max corresponding to the maximum of ?″ appears in the radiofrequency domain and increases with the PPY concentration from 10⁴ to 2 × 10⁶ Hz. This relaxation is caused by space charge moving into the conductive clusters of PPY. At low frequency F ? F_max, the real part of the permittivity ?′ becomes very high. For F ? F_max, PPY‐PMMA composites have a percolative behaviour, the percolation threshold p_c being 3.85%. For F ? F_max, in the microwave domain, an ac component of the conductivity σ_ac appears. σ_ac varies as a power function of the frequency, σ_ac ∝ ω ^x; with x < 1, x independent of p. © 2001 Society of Chemical Industry     

水热合成钛酸锶粉体增强压敏电阻器电性能的研究

以硝酸锶Sr(NO3)2为锶源、酞酸四丁酯Ti(OC4H9)4为钛源,加入矿化剂NaOH/KOH,在130℃下反应24h水热合成了SrTiO3粉体。将水热粉体应用到压敏电阻器中,分别采用XRD、电性能测试、SEM等手段对产物进行表征。电性能测试结果显示,与传统固相法制备的SrTiO3压敏电阻器相比,同配方下此类压敏电阻器电性能获得显著提高,压敏电压V1mA和非线性系数ɑ分别达到120V和8。     

Directional properties of ordered 3‐3 piezocomposites fabricated by sacrificial template

Fabrication of piezoelectric composites with 3‐3 connectivity and their properties have attracted attention due to the application of these composites in acoustic transducers, medical imaging, and nondestructive testing. In this research, directional piezoelectric‐polymer composites with 3‐3 connectivity are prepared by a relatively simple fabrication process of dipping an ordered polymeric template (mesh) into lead zirconate titanate suspension followed by drying, pyrolysis, and sintering. The resulting porous ordered structures of lead zirconate titanate‐5A ranging from 18 to 32 vol% ceramic were subjected to polymer injection to form composites to be cut and polarized in different directions. In the composites, the effects of (i) polarization direction and (ii) active piezoelectric phase content on the dielectric and piezoelectric properties were investigated. The results showed that the dielectric constant and the piezoelectric properties are significantly dependent on the direction in composites of directional structure. In these composites, dielectric constant values were found to be higher in the direction parallel to ceramic ligaments. Moreover, the highest values for piezoelectric charge and voltage coefficient and figure of merit were found in the direction 45° to ceramic ligaments. In all composites, both dielectric constant and piezoelectric property values were proportional to the active piezoelectric phase content.     

Preparation and properties of castor oil‐based polyurethane/α‐zirconium phosphate composite films

Novel castor oil‐based polyurethane/α‐zirconium phosphate (PU/α‐ZrP) composite films with different α‐ZrP loading (0–1.6 wt %) and different NCO/OH molar ratios were synthesized by a solution casting method. The characteristic properties of the PU/α‐ZrP composite films were examined by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing. The results from Fourier transform infrared spectroscopy indicated that strong intermolecular hydrogen bonding formed between α‐ZrP and PU, XRD and SEM results revealed that the α‐ZrP particles were uniformly distributed in the PU matrix at low loading, and obvious aggregation existed at high loading. Because of hydrogen bonding interactions, the maximum values of tensile strength were obtained with 0.6 wt % α‐ZrP loading and 1.5 of NCO/OH molar ratio in the matrix. Evidence proved that the induced α‐ZrP used as a new filler material can affect considerably the mechanical and thermal properties of the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Hydrothermal synthesis of Litchi-like SrTiO3 with the help of ethylene glycol

Templated by TiO₂ microspheres , litchi-like SrTiO₃ crystals with a narrow size distribution and monodispersity were synthesized through the combination of regulating the ethylene glycol concentration during the hydrothermal process and the post heat treatment. The results show that when the volume ratio of water and ethylene glycol reached 10:70, microsized SrCO₃ was firstly formed under the hydrothermal process, and then the litchi-like SrTiO₃ powder was obtained after the postheat treatment at 700°C for 4 hours, which shows a large specific surface area of 37.41 m²/g. It is found that the concentration of ethylene glycol aqueous solution plays an important role on the morphology of the SrTiO₃ crystals, and the slightly higher viscosity and proper amount of OH hydroxyl groups facilitate the formation of the micrometer spherical hierarchical surface.     

Effect of SiBCN content on the dielectric and EMW absorbing properties of SiBCN-Si3N4 composite ceramics

Siliconboron carbonitride ceramics (SiBCN) were introduced into porous Si₃N₄ substrates via low pressure chemical vapor deposition and infiltration from SiCl₃CH₃-NH₃-BCl₃-H₂-Ar system. To improve the electromagnetic wave(EMW) absorbing properties, the molar ratio, n_CH3SiCl3/(n_NH3 + n_BCl3), was increased based on thermodynamics analysis. The results show that nanosized silicon carbide crystals and free carbon dispersed uniformly in the amorphous SiBCN phase, resulting in suitable dielectric properties and improved absorption capabilities of SiBCN-Si₃N₄ ceramics. Additionally, with increasing SiBCN ceramics loading, the amount of nanocrystals and interface between nanocrystals and amorphous SiBCN phase increased, leading to enhanced polarization and dielectric loss of the composite ceramics. When SiBCN content was up to 3.64 wt%, the electromagnetic reflection coefficient (RC) of SiBCN-Si₃N₄ composite ceramics reached ?40 dB (>99.97% absorbing) with the effective electromagnetic absorbing bandwidth of 3.64 GHz in the X-band. This study makes it possible to fabricate SiBCN-based composite materials with excellent EMW absorbing properties at a low temperature.     

碳纳米管材料低频电磁参数及吸波产热特性

微波的热利用技术促进了吸波材料的应用研究。碳纳米管（CNTs）是近年来新兴的强吸波材料，具有密度小、比表面积大、量子尺寸效应的特点。对碳纳米管吸波材料的复介电常数和复磁导率随碳纳米管含量的变化进行探究。在此基础上，以石蜡油为蓄热介质探究了碳纳米管材料在微波辐照下吸波产热特性。同轴传输法适用于小型样品的测量，具有误差小的优点，故采用此种方法作为测量电磁参数手段。对碳纳米管电磁参数测量实验结果表明，碳纳米管吸波材料在低频下对于微波能的损耗兼具电损耗和磁损耗。对碳纳米管吸波产热特性实验结果表明，碳纳米管是一种强吸波材料。     

细菌纤维素@Fe3O4/AgNWs复合薄膜的制备与电磁屏蔽性能

采用原位共沉淀法在高性能细菌纤维素（BC）表面负载磁性四氧化三铁（Fe3O4）纳米粒子得到BC@Fe3O4，进而采用两步真空辅助抽滤法制得具有磁性导电层级结构的BC@Fe3O4/AgNWs复合薄膜。通过扫描电镜（SEM）、透射电镜（TEM）、傅里叶红外光谱（FTIR）、X射线衍射（XRD）和矢量网络分析仪等对纳米材料和复合薄膜的微观结构与性能进行分析。结果表明：当AgNWs面积含量为1.8 g/m2时，复合薄膜的电磁屏蔽效能（EMI SE）可达56 dB。AgNWs与BC@Fe3O4基体之间具有良好的界面相互作用，使BC@ Fe3O4/AgNWs复合薄膜具有优异的力学性能，拉伸强度和断裂伸长率最高达到84.6 MPa和4.05%。所得柔性、高强且高电磁屏蔽效能细菌纤维素基电磁屏蔽复合薄膜在柔性可穿戴电子设备等领域具有良好的应用前景。     

Separation of bovine serum albumin (BSA) using γ‐Al2O3–clay composite ultrafiltration membrane

BACKGROUND: Ceramic membranes have received more attention than polymeric membranes for the separation and purification of bio‐products owing to their superior chemical, mechanical and thermal properties. Commercially available ceramic membranes are too expensive. This could be overcome by fabricating membranes using low‐cost raw materials. The aim of this work is to fabricate a low‐cost γ‐Al₂O₃–clay composite membrane and evaluate its potential for the separation of bovine serum albumin (BSA) as a function of pH, feed concentration and applied pressure. To achieve this, the membrane support is prepared using low‐cost clay mixtures instead of very expensive alumina, zirconia and titania materials. The cost of the membrane can be further reduced by preparing a γ‐alumina surface layer on the clay support using boehmite sol synthesized from inexpensive aluminium chloride instead of expensive aluminium alkoxide using a dip‐coating technique. RESULTS: The pore size distribution of the γ‐Al₂O₃‐clay composite membrane varied from 5.4–13.6 nm. The membrane was prepared using stable boehmite sol of narrow particle size distribution and mean particle size 30.9 nm. Scanning electron microscopy confirmed that the surface of the γ‐Al₂O₃–clay composite membrane is defect‐free. The pure water permeability of the support and the composite membrane were found to be 4.838 × 10^?6 and 2.357 × 10^?7 m³ m^?2 s^?1 kPa^?1, respectively. The maximum rejection of BSA protein was found to be 95%. It was observed that the separation performance of the membrane in terms of flux and rejection strongly depends on the electrostatic interaction between the protein and charged membrane. CONCLUSION: The successively prepared γ‐Al₂O₃‐clay composite membrane proved to possess good potential for the separation of BSA with high yield and could be employed as a low cost alternate to expensive ceramic membranes. Copyright © 2009 Society of Chemical Industry     

pH值对化学共沉淀法制备SrTiO3粉料性能的影响

借助ＸＲＤ、ＳＥＭ、化学分析、ＴＧ－ＤＴＡ、原子发射光谱等近代仪器及测试手段分析研究了反应溶液ｐＨ值对化学共沉淀法制备ＳｒＴｉＯ３超细粉料的影响。结果表明：ｐＨ值不同,所制备粉料的Ｓｒ２＋／Ｔｉ４＋摩尔配比不同,颗粒形貌及粉料前驱体的热分解机理不同。当ｐＨ值在２～４范围内,反应溶液中Ｓｒ２＋／Ｔｉ４＋摩尔配比为１．１时,可获得精确符合化学计量比（１．００１）、纯度＞９９．８５％、且大小均匀、颗粒细小的ＳｒＴｉＯ３粉料。