Tensile,impact and dielectric properties of three dimensional orthogonal aramid/glass fiber hybrid composites

Aramid/glass hybrid composites with three different stacking sequences and their corresponding single fiber type composites have been fabricated and their tensile, impact and dielectric properties were investigated. The trend of tensile strength and modulus of the composites followed the rule of mixture (ROM) closely and a small but positive hybrid effect for tensile strength of the hybrid composites was observed. The hybrid composites in general had a higher impact resistance than the single fiber type composites and the hybrid composite in which fiber volume fractions for glass and aramid fiber were the most balanced showed the highest impact ductility. The aramid fiber composite showed a lower dielectric constant and a higher dielectric loss than the glass fiber composites. However, the dielectric constant of the hybrid composites decreased first and then increased as the volume fraction of aramid fiber increased, which did not follow the mixing rule for dielectric constants of compounds. The dielectric loss of the composites increased monotonically as the volume fraction of aramid fiber increased which agreed well with the mixing rule.     

Effect of borax additive on the dielectric response of polypyrrole

The main aim of this study is to produce added polypyrrole (PPy) borax composites with high dielectric properties for technological applications. For this purpose, PPy–borax composites with different borax concentrations varying from 10 to 50 wt% have been prepared. To reveal their structural and morphological attributes, the composites have been characterized by Fourier-transform infrared spectroscopy and scanning electron microscopy. The real and imaginary parts of complex dielectric function, the imaginary component of complex electrical modulus and ac conductivity have been investigated at room temperature as a function of frequency in the range 100 Hz–15 MHz. It has been found that addition of borax increases the dielectric constant of pure PPy. In this respect, PPy–borax composites with the highest dielectric constant at low frequency may be utilized in charge storing devices. On the other hand, the dielectric loss is also very high in low-frequency region for the composites with high borax content. Exploiting this property, the material may also be used in decoupling capacitor applications. The relaxation mechanisms of the samples have also been determined as non-Debye type. The Nyquist curves of the samples have been analysed for calculating the grain and grain boundary resistance and capacitance values. In conclusion, borax has a promising potential to be used as a cheap and effective filler for improving the dielectric properties of PPy polymer.     

Enhanced dielectric and ferroelectric properties induced by TiO2@MWCNTs nanoparticles in flexible poly(vinylidene fluoride) composites

Flexible polymer based composites containing multi-walled carbon nanotubes (MWCNTs) have been reported to present high dielectric constant. However, the composites generally exhibit high dielectric loss and low dielectric breakdown strength, which prohibits their practical use in electronic and electric industry. MWCNTs were coated with a continuous layer of TiO₂ nanoparticles (TiO₂@MWCNTs) by a simple hydrothermal process and TiO₂@MWCNTs/poly(vinylidene fluoride) (PVDF) composites were prepared by a solution casting method. Compared to the pristine MWCNTs/PVDF composites, the TiO₂@MWCNTs/PVDF composites presented enhanced dielectric constant and lower dielectric loss. Additionally, the breakdown strength of the TiO₂@MWCNTs/PVDF composites was also improved, which is favorable for enhanced ferroelectric properties in the composites.     

The dielectric properties and dielectric mechanism of perovskite ceramic CLST/PTFE composites

Polytetrafluorethylene (PTFE) composites filled with perovskite (Ca,Li,Sm)TiO₃ (CLST) dielectric ceramic of various volume fractions filler up to 60% were prepared. The effects of volume fraction of ceramic filler on the microstructure and dielectric properties of the composites have been investigated. A comparative study of dielectric properties of experiment and modeling analysis has been carried out at high frequencies for the CLST/PTFE composites. The results indicate that both the dielectric constant and the dielectric loss increase with the filler. The CLST/PTFE composite with 40% ceramic has exhibited good dielectric properties: ε _r?=?7.92 (~10 GHz), tan δ?=?1.2?×?10^?3 (~10 GHz), and τ _f?=??45 ppm/°C. The dielectric properties are obviously better than most composites reported previously at high frequencies in the aspects of dielectric loss and thermal stability. The dielectric constant and dielectric loss of composites predicted by the Rother–Lichtenecker equation and the general mixing model are in good agreement with the experiment data when the volume fraction of ceramic is less than 40%. When the volume fraction of the ceramic is more than 40%, the deviation occurs. By introducing the correction factor, the theoretical values of the dielectric constant agrees well with the experimental values.     

Fabrication and properties of SiNO continuous fiber reinforced BN wave-transparent composites

SiNO continuous fiber reinforced boron nitride (BN) wave-transparent composites (SiNO _f /BN) have been fabricated by a precursor infiltration pyrolysis (PIP) method using borazine as the precursor. The densification behavior, microstructures, mechanical properties, and dielectric properties of the composites have been investigated. After four PIP cycles, the density of the composites had increased from 1.1 g·cm^?3 to 1.81 g·cm^?3. A flexural strength of 128.9 MPa and an elastic modulus of 23.5 GPa were achieved. The obtained composites have relatively high density and the fracture faces show distinct fiber pull-out and interface de-bonding features. The dielectric properties of the SiNO _f /BN composites, including the dielectric constant of 3.61 and the dielectric loss angle tangent of 5.7×10^?3, are excellent for application as wave-transparent materials.     

石英纤维/KH308复合材料的介电性能

为研究石英纤维/聚酰亚胺（KH308）复合材料介电性能与纤维体积分数、频率、温度和吸水率之间的关系,通过热压成型法,制备了4种不同纤维体积含量的石英纤维/KH308复合材料,采用高Q谐腔法分别测试这4种复合材料在不同状态下的介电常数和介电损耗。结果表明：石英纤维/KH308复合材料的介电常数随着纤维体积分数增加而变大,介电损耗随纤维体积分数变化不大;7~18 GHz频率下,复合材料的介电常数和介电损耗基本不随频率变化;25~300℃下,复合材料的介电常数随温度增加变化比较平缓,而介电损耗随温度的增加而降低;复合材料吸水后,介电常数和介电损耗都会增加;复合材料介电常数ε<4,介电损耗tanδ<0.1,能满足导弹天线罩透波材料介电性能的要求。     

ZrO2/聚偏氟乙烯复合材料介电性能及松弛行为

通过熔融法制备了ZrO₂/聚偏氟乙烯（PVDF）复合材料,采用SEM、XRD和FTIR对其形貌和结构进行分析,结果表明,ZrO₂分布较为均匀,ZrO₂/PVDF复合材料主要含α相和少量γ相。采用宽频介电谱（BDS）测试,ZrO₂/PVDF复合材料介电常数ε'随ZrO₂含量的增加而增加,而介电损耗tanδ保持定值,表明ZrO₂的加入可以显著提高ZrO₂/PVDF复合材料的介电性能。经计算ZrO₂/PVDF复合材料介电模量M″和活化能,发现有玻璃化转变峰、缺陷峰和界面极化峰存在,而加入ZrO₂后,ZrO₂/PVDF复合材料活化能增加。      

Dielectric behaviour of barium titanate-polyvinylidene fluoride composites

The dielectric behaviour of composites of barium titanate (BaTiO₃) and polyvinylidene fluoride (PVDF) has been studied by changing the weight fraction of BaTiO₃. The dielectric behaviour of the composites has a significant influence of PVDF up to 50wt% BaTiO₃ in the composite, but this effect is insignificant at higher weight fractions of BaTiO₃. X-ray diffraction studies on composites are also reported to correlate the observed dielectric changes with the corresponding structural changes.     

三维正交机织复合材料的拉伸力学性能及介电性能研究

三维纺织复合材料的发展越来越广泛,其轻质,抗分层等优点是层合板式复合材料无法比拟的.本工作对自行设计并制作的五种玻璃纤维芳纶纤维混合增强的环氧树脂复合材料的拉伸力学性能和介电性能进行了研究.结果表明纯芳纶结构的三维复合材料有着最高的比强度和比模量;而在玻璃纤维含量较多的结构材料里介电性能呈现集中且稳定的趋势.在不同的应用中,可以将不同纤维混合作为增强体以发挥各种纤维的优势和特点,满足不同的设计和实际需要.     

Large dielectric constant of the chemically purified carbon nanotube/polymer composites

The chemically purified multiwalled carbon nanotube/poly(vinylidene fluoride) (MWCNT/PVDF) composites were fabricated. Raman spectroscopy and transmission electron microscopy micrographs indicated that the catalysts metal particles and amorphous carbon had been removed from the purified MWCNTs. The percolation threshold of the composites is relatively large, about 3.8 vol.%. The most important result is that the dielectric constant of the composites is enhanced remarkably, and the dielectric constant of 3600 is obtained in the composite with 8 vol.% purified MWCNT at 1 kHz. The large dielectric constant can be attributed to the preparation procedure and the interface effect between the MWCNTs and the polymer.     

有限元法研究填料形貌与介电常数对无机/有机介电复合材料介电性能的影响

为了开发高储能密度的无机/有机介电复合材料,本文采用有限元法分别研究了直径为100 nm的球形填料与基体介电常数的比值（k）、球形填料在复合材料中的排列方式、球形填料尺寸（100~300 nm）、纤维状填料长径比（α）和片状填料的球形度（β）对复合材料介电性能的影响。计算结果表明,当k值大于20时,复合材料的介电常数变化不明显;球形填料沿电场方向成链式排列时,复合材料有较大的介电常数,且材料中球形填料附近处存在较大的电位移和较大的电场,说明这种填料排列方式有利于材料介电常数的提高,但会削弱材料的耐击穿能力;当球形填料随机分布时,颗粒尺寸变化对复合材料介电常数的影响不明显。对于纤维状填料,其长径比α越大且长轴沿电场方向分布时,填料自身及周边会产生较大的电位移,表明这种情况有利于复合材料介电常数的提高。对于片状填料,其球形度β越小,填料与基体界面处高电场区域越小,表明材料的耐击穿能力越高。本研究可为高介高储能材料的实验研究提供理论指导。      

Large dielectric constant and high thermal conductivity in poly(vinylidene fluoride)/barium titanate/silicon carbide three-phase nanocomposites

Dielectric polymer composites with high dielectric constants and high thermal conductivity have many potential applications in modern electronic and electrical industry. In this study, three-phase composites comprising poly(vinylidene fluoride) (PVDF), barium titanate (BT) nanoparticles, and β-silicon carbide (β-SiC) whiskers were prepared. The superiority of this method is that, when compared with the two-phase PVDF/BT composites, three-phase composites not only show significantly increased dielectric constants but also have higher thermal conductivity. Our results show that the addition of 17.5 vol % β-SiC whiskers increases the dielectric constants of PVDF/BT nanocomposites from 39 to 325 at 1000 Hz, while the addition of 20.0 vol % β-SiC whiskers increases the thermal conductivity of PVDF/BT nanocomposites from 1.05 to 1.68 W m(-1) K(-1) at 25 °C. PVDF/β-SiC composites were also prepared for comparative research. It was found that PVDF/BT/β-SiC composites show much higher dielectric constants in comparison with the PVDF/β-SiC composites within 17.5 vol % β-SiC. The PVDF/β-SiC composites show dielectric constants comparable to those of the three-phase composites only when the β-SiC volume fraction is 20.0%, whereas the dielectric loss of the PVDF/β-SiC composites was much higher than that of the three-phase composites. The frequency dependence of the dielectric property for the composites was investigated by using broad-band (10(-2)-10(6) Hz) dielectric spectroscopy.     

Tailoring of thermal and dielectric properties of LDPE-matrix composites by the volume fraction,density, and surface modification of hollow glass microsphere filler

Hollow glass microsphere (HGM) filled low-density polyethylene (LDPE) composites were prepared, and the effects of density, content, and surface modification of HGM on the thermal and dielectric properties of the composites were investigated. It is found that the thermal conductivity of the composites decreases with increasing HGM content or decreasing HGM density. At the same HGM content and density, the composites filled with suitable amount of silane coupling agent (KH570) modified HGM exhibit higher thermal conductivity than unmodified-HGM filled composites. The dielectric constant at 1 MHz of the composites also decreases with increasing HGM content or decreasing HGM density, but their dielectric loss increases with increasing HGM content or increasing HGM density. By modifying the surface of HGM with suitable amount of KH570, the dielectric constant and loss at 1 MHz of the composites can be decreased at the same time. The results of microwave dielectric properties of the composites indicate that the dielectric constant decreases with increasing HGM content or decreasing HGM density, the quality factor (Q × f) decreases with increasing HGM content or increasing HGM density, but both dielectric constant and quality factor are slightly affected by the surface modification of HGM. Due to lower intrinsic thermal conductivity and dielectric constant but higher dielectric loss of HGM than LDPE, the thermal conductivity and dielectric properties of the composites can be controlled with adding HGM and varying its volume fraction. The surface modification of HGM improves the interface contact between HGM and LDPE in the composites, which is confirmed by the SEM observation, and thus the heat conduction and dielectric properties at low frequency are improved. Based on calculated thermal conductivity and dielectric constant of HGM, the experimental trends of thermal conductivity and dielectric constant at 1 MHz of the composites are analyzed by using different models, including typical models for particles-filled composites and special models developed for hollow microsphere filled composites. The results from suitable models show close correlation with the experimental values.     

Dielectric,thermal and mechanical properties of Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> based polymer/ceramic composites

Polymer/Sr₂ZnSi₂O₇ (SZS) ceramic composites suitable for substrate applications have been developed using the polymers polystyrene (PS), high density polyethylene (HDPE) and Di-Glycidyl Ether of Bisphenol A (DGEBA). The dielectric, thermal and mechanical properties of the composites are investigated as a function of various concentrations of the ceramic filler. The obtained values of relative permittivity, dielectric loss tangent, thermal conductivity and coefficient of thermal expansion of the composites are compared with the corresponding theoretical predictions. The relative permittivity of the polymer/ceramic composites increases with filler loading. The dielectric loss tangent also shows the same trend except for DGEBA/SZS composites. The major advantages of the ceramic loading are improvement in thermal conductivity and a decrease in the coefficient of thermal expansion. The tensile strength of the composites decreases with increase in filler content, whereas an improvement is observed in microhardness. The variation of relative permittivity (at 1 MHz) of the composites is also studied as a function of temperature.     

氧化石墨/钛酸钡/环氧树脂三相复合材料的制备及其介电性能研究

利用浓硫酸、高锰酸钾等强氧化剂制备了氧化石墨, 将其与钛酸钡和环氧树脂复合, 制备了三相复合材料。研究了氧化石墨的添加量对于复合材料介电性能的影响。结果发现在氧化石墨的添加量很少时, 三相复合材料的介电常数显著地高于钛酸钡/环氧树脂两相复合材料, 同时介电损耗仍然维持在较低的水平。钛酸钡/环氧树脂的介电常数为17.7 (20℃, 1 kHz), 当加入3wt%氧化石墨, 介电常数增加到42.6, 介电损耗为0.043。因此该三相复合材料适合用于埋入式电容器的介质材料。最后初步探讨了氧化石墨对复合材料介电性能的影响机理。     

SrTiO_3/CaCu_3Ti_4O_(12)复合陶瓷材料的介电性能

本文采用固相反应法制备了xSrTiO_3/(1-x)CaCu_3Ti_4O_(12)(x=0,0.2,0.4,0.6,0.8,1)复合陶瓷,研究了复合材料的物相、微观结构和宽温度宽频率范围内的介电性能。结果表明:在1348~1600K的温度范围内烧结能够得到致密性良好的xSrTiO_3/(1-x)CaCu_3Ti_4O_(12)(x=0,0.2,0.4,0.6,0.8,1)复合陶瓷。频率为100kHz时,样品的室温介电常数随SrTiO_3含量的增加而减少,从71358(x=0)单调减少至270(x=1),其变化规律遵循Lichtenecker法则。介电损耗随SrTiO_3含量的增加先增大后减少。当x=0.2时,样品与CaCu_3Ti_4O_(12)陶瓷的介电性能相似,存在低温的介电弛豫和巨介电常数平台。随着SrTiO_3含量的增加,复合陶瓷的低温介电弛豫激活能增大,介电响应被抑制,而高温介电响应由于高温电导的影响而增强,使得CaCu_3Ti_4O_(12)特有的巨介电常数平台随着SrTiO_3的增加逐渐消失,xSrTiO_3/(1-x)CaCu_3Ti_4O_(12)复合材料的温度依赖性增强。     

Dependence of dielectric properties on the crystallinity of ceramic/polymer composites

Effects of ZnNb(2)O(6) content and crystallinity of polymers on the dielectric properties of ZnNb(2)O(6)/polytetrafluoroethylene (PTFE), polypropylene (PP), and polystyrene (PS) composites were investigated at microwave frequencies. With increasing ZnNb(2)O(6) content, the dielectric constant (K) of the composites increased, whereas the dielectric loss (tanδ) and temperature coefficient of resonant frequency (TCF) decreased. The tanδ of the composites with amorphous PS was lower than those of the composites with semi-crystalline PP and PTFE. For the composites with semi-crystalline PTFE and PP, the tanδ was strongly dependent on the degree of crystallinity of composites. Several types of theoretical models were applied to predict the effective dielectric properties of the composites. Typically, K of 5.73, tan δ of 1.45 x 10(-3), and TCF of 2.66 ppm/°C were obtained for the PP composites with 0.5 volume fraction V(f) of ZnNb(2)O(6).     

Studies on conductivity and dielectric properties of polyaniline-zinc sulphide composites

In the present paper, we report electrical conductivity and dielectric studies on the composites of conducting polyaniline (PANI) with crystalline semiconducting ZnS powder, wherein PANI has been taken as inclusion and ZnS crystallites as the host matrix. From the studies, it has been observed that the value of room temperature d.c. conductivity of the composites with volume fraction of PANI > 0.65 shows an unusual behaviour wherein, conductivity values of the composites exceed that of PANI itself with maximum value as high as 6 times that of PANI at the volume fraction of 0.85. A similar trend has also been observed for the real and imaginary parts of complex dielectric constant values of the composites. This unusual behaviour in the d.c. conductivity and dielectric properties has been attributed to the enhancement in the degree of crystallinity of PANI as a consequence of its interfacial interaction with ZnS matrix. The results of optical microscopy show coating of PANI all around the ZnS particles. The temperature dependent conductivity studies suggest the quasi one-dimensional VRH conduction in PANI as well as its composites with ZnS. FTIR and XRD studies have also been reported.     

Styrene butadiene rubber/aluminum powder composites—mechanical,morphological and electrical behaviors

Different weight fractions of aluminum (Al) powder viz., 10, 20, 30, 40, 50, 60 and 70 phr were incorporated into styrene butadiene rubber (SBR) matrix. The Al powder filled and vulcanized SBR composites have been characterized for mechanical properties such as tensile strength, tensile modulus and surface hardness. A drastical improvement in tensile strength and tensile modulus with increase in filler content of the composites was noticed. The electrical properties such as dielectric constant, tan delta and dielectric loss were measured for all the four compositions. The effect of volume fraction (0–70 phr) of conducting filler, frequency (100 kHz–30 MHz), temperature (25–75°C) and relative humidity on dielectric constant, dielectric loss and tan delta values of the composites were studied.     

BaTiO3纳米线的长径比对聚间苯二甲酰间苯二胺复合材料介电性能的影响

用水热法合成了不同长径比的钛酸钡纳米线(BaTiO₃ nanowires (BTN)),用聚乙烯吡咯烷酮(PVP)调节其表面化学能和静电力(标记为P-BTN)。将P-BTN加入聚间苯二甲酰间苯二胺(PMIA)基体中制备出P-BTN含量(质量分数)为10%的介电复合材料P-BTN/PMIA。研究了合成温度对BTN长径比的影响、P-BTN对P-BTN/PMIA复合材料介电性能和电学性能的影响以及P-BTN/PMIA复合材料在不同温度下的介电性能和电学性能。结果表明：随着BTN合成温度的提高其长径比明显增大,从130℃时的7.2增大到250℃时的46;随着PMIA复合材料中P-BTN长径比的增大其介电常数从6.6增大到9.8,其介电损耗在整个频率范围内小于0.025并保持了良好的绝缘性能;在-20℃-200℃复合材料P-BTN-250-10介电常数和介电损耗保持稳定。高长径比的BTN更利于提高耐高温聚合物基复合材料的介电常数,进而提高其储能密度。