BN/SiC复合涂层改性炭纤维的吸波性能研究

以尿素、硼酸为原料, 采用浸涂工艺先在炭纤维表面制备BN涂层, 再以三氯甲基硅烷为前驱体, 采用化学气相沉积工艺在纤维表面沉积SiC涂层, 制得了BN/SiC复合涂层改性炭纤维。对BN/SiC复合涂层改性炭纤维的微观结构、抗氧化性能、介电性能及吸波性能进行了研究。结果表明: 炭纤维表面BN涂层的厚度约为0.1 μm, SiC涂层的厚度约为0.7 μm。炭纤维经表面BN/SiC复合涂层改性后, 抗氧化性能明显提高, 开始明显氧化失重温度从560℃提高到790℃, 最终氧化温度从780℃提高到1200℃以上; 且介电性能得到有效改善, 吸波性能显著提高。相比于未改性炭纤维, 厚度为2 mm的BN/SiC复合涂层改性炭纤维的最小反射率减小到-13.3 dB, 小于-10 dB的带宽增加至2.5 GHz。     

PMN-PT陶瓷的制备及介电性能

采用NaCl-KCl熔盐法制备了钙钛矿相结构的0.8PMN-0.2PT陶瓷,研究了预烧温度、烧结温度和时间等工艺参数对样品的相结构、微观结构以及介电性能的影响,与传统陶瓷工艺相比,用熔盐法制备的样品介电性能好.     

石英纤维增强磷酸铬铝/有机硅杂化树脂基复合材料性能

通过甲基三乙氧基硅烷(MTEOS)在磷酸铬铝中水解制备了磷酸铬铝/有机硅的有机-无机杂化树脂,以石英纤维为增强材料,氧化铝为固化剂,采用模压成型工艺制备纤维增强杂化树脂基复合材料.考察了杂化树脂基体以及复合材料的力学、介电性能等,结果表明复合材料的弯曲强度大大提高,介电性能更加优越.     

PMN—PT陶瓷的制备及介电性能

采用NaCl-KCl熔盐法制备了钙钛矿相结构的0.8PMN-0.2PT陶瓷,研究了预烧温度,烧结温度和时间等工艺参数对样品的相结构,微观结构以及介电性能的影响,与传统陶瓷工艺相比,用熔盐法制备的样品介电性能好。     

制备工艺对BaO-TiO2-B2O3-SiO2体系LTCC性能和微观结构的影响

研究了溶胶-凝胶法、高温熔融法、预煅烧法分别制备的BaO-TiO2-B2O3-SiO2体系LTCC的介电性能、烧结性能以及微观结构.研究发现,溶胶-凝胶法制备的LTCC材料烧结温度较低,坯体在880℃烧结即可达到最大收缩,其烧结收缩率大于高温固相法制备的LTCC;高温熔融法制备的LTCC介电性能稳定,但烧结温度偏高,大约950℃才能达到最大收缩;预煅烧法制备粉体简单、有效,烧结温度在900℃左右,但LTCC中活性成分较多,容易造成气孔率偏高,致密化难度增大且介电常数稳定性不好等缺点;在总结这3种制备工艺对LTCC玻璃陶瓷材料性能和微观结构的基础上,利用适量掺杂Al2O3等添加剂的方法,在保证介电性能满足应用的前提下,大大提高了预煅烧法制备LTCC的烧结体致密度.     

聚四氟乙烯中空纤维膜的制备

通过聚四氟乙烯(PTFE)树脂糊料挤出和拉伸烧结成型方法,制备PTFE中空纤维膜.考察了拉伸倍数、温度和速度等拉伸工艺和烧结工艺对PTFE中空纤维膜的结构和性能的影响.实验结果表明,制膜参数中,随着拉伸倍数和温度的增加,平均孔径和孔隙率增加,泡点降低;随拉伸速度增大,膜平均孔径变小,泡点增加,拉伸速度对孔隙率的影响则呈现无规律变化;烧结可提高PTFE中空纤维膜的强度,孔径和孔隙率增加.PTFE中空纤维膜具有明显的非对称结构.     

CVD BN界面层对Si3N4/SiBN复合材料弯曲性能的影响

连续Si3 N4纤维以其优异的热稳定性、高温力学性能和介电性能,被认为是耐高温陶瓷基透波复合材料的候选材料之一.采用连续Si3 N4纤维为增强体,以BCl3+NH3+H2+Ar反应体系,利用化学气相沉积工艺在Si3 N4纤维表面制备了BN界面层,并以聚硅硼氮烷为陶瓷先驱体,通过先驱体浸渍裂解工艺制备了Si3 N4/SiBN复合材料.研究了CVD BN界面层的合成及其对Si3 N4/SiBN复合材料弯曲性能的影响.结果表明:在Si3 N4纤维表面获得了均匀致密的BN界面层,该界面层有效改善了复合材料中纤维/基体的界面结合力,复合材料显示出典型的韧性断裂特征.当界面层的厚度为200 nm时,Si3 N4/SiBN的弯曲强度和断裂韧性分别为182.3 MPa和17.3 MPa·m1/2,比无涂层的复合材料分别提高了59.6％和94.4％.     

交替中间热处理BST薄膜介电性能研究

用溶胶凝胶(Sol-Gel)法制备了三种钛酸锶钡(Ba_0.6Sr_0.4TiO₃, BST)薄膜：常规的四层薄膜, 在逐层制备过程中,对首层薄膜进行中间热处理(Preheat-treatment, PT)的四层薄膜及间隔或交替地对奇数层薄膜进行中间热处理（称为交替中间热处理(Alternate-preheat-treatment, APT)）的八层薄膜. 用XPS研究薄膜表面成分化学态, 用SEM和AFM观察表面形貌及晶化, 并进行了介电性能测试. 结果表明, 常规薄膜介电性能差; 经PT, 薄膜裂纹和缩孔显著减少,形貌明显改善,表面非钙钛矿结构显著减少, 介电性能明显提高; 经APT, 薄膜形貌进一步改善,平均晶粒大小约30nm,非钙钛矿结构进一步减少, 介电损耗明显降低,介电稳定性和介电强度大幅度提高. APT为制备退火温度低及结构均匀致密的纳米晶BST薄膜提供了新方法, 可满足低频实用要求. 退火温度对薄膜厚度的影响也进行了讨论.     

BN 纤维氮化中晶体结构演化过程的研究

采用化学滴定、X 射线衍射(XRD) 及径向分布函数(RDF) 等方法, 测定了BN 纤维中残余B₂O₃ 含量与氮化温度的关系, 研究了氮化过程中BN 纤维的结构演变规律。分析表明: 经1100℃氮化后,BN 纤维以乱层结构为主, 同时存在少量类六方相(Hexagonal-BN ) 和菱方相(O rthorhombic-BN ) ; 随高温处理温度的升高, 纤维结构逐渐向菱方相结构转变, 经1900℃高温处理后,BN 纤维的结构主要为菱方相。     

溶胶-凝胶法制备Na0.5Bi0.5TiO3基无铅压电陶瓷的研究进展

钛酸铋钠(Na0.5Bi0.5TiO3,简写为NBT)基无铅压电陶瓷由于具有良好的压电性、高居里温度和烧结过程中无毒、易控制性等优点而备受关注.综述了溶胶-凝胶法制备Na0.5Bi0.5TiO3粉体及制备出的陶瓷的结构和性能特点.总结了用溶胶-凝胶法制备压电和介电性能显著提高的钛酸铋钠基无铅压电陶瓷的研究进展.研究表明,溶胶-凝胶法在制备压电陶瓷方面具有均匀性好、纯度高,烧结温度低等优点.展望了该工艺的发展方向.     

Si-B-N透波纤维的耐超高温性能

以聚硼硅氮烷（PBSZ）为先驱体,经熔融纺丝、不熔化以及在氨气气氛中高温裂解制备了Si-B-N 纤维,然后在高纯氮气保护下加热至超高温。利用元素分析、FTIR、XRD、SEM、力学性能分析和波导法等研究了纤维的耐超高温性能。结果表明:Si-B-N 纤维1 500 ℃退火几乎不失重,在惰性气体中非晶状态可以保持至1 700 ℃,加热到1 850 ℃才形成 Si₃N₄和BN等结晶相;Si-B-N 纤维的拉伸强度为1.72 GPa,弹性模量为196 GPa,1 500 ℃ 退火Si-B-N纤维的拉伸强度为1.86 GPa,弹性模量为205 GPa,Si-B-N纤维具有很好的耐超高温性能;此外,采用波导法测量,Si-B-N纤维表现出优良的介电性能,测试频率为 8~12 GHz,1 400 ℃退火的Si-B-N纤维平均介电常数和介电损耗角正切值分别为约3.68和0.001 1。      

高性能SiBN(C)陶瓷纤维的热稳定性能及透波性能

SiBN(C)陶瓷纤维因其优异的性能(高温稳定性、高温抗蠕变和高温抗氧化性能等)被认为是高温高性能陶瓷基复合材料的理想增强体。研究了SiBN(C)陶瓷纤维的热稳定性能及微观结构, 探索了SiBN(C)陶瓷纤维在1 100~1 500 ℃的抗氧化过程, 并研究了C含量对SiBN(C)陶瓷纤维介电性能的影响。结果表明: SiBN(C) 陶瓷纤维在高温热处理至1 600 ℃的N₂气氛下仍然呈现无定形结构;HT-TGA结果表明该SiBN(C)陶瓷纤维具有良好的高温热稳定性, 该陶瓷纤维的热失重率(1 450 ℃, N₂气氛)仅为1.5wt%; 同时SiBN(C)纤维也表现出优良的高温抗氧化性能, SiBN(C)陶瓷纤维在1 400 ℃, 空气中处理5 h后, 纤维致密且无裂纹, XRD分析表明SiBN(C)陶瓷仍然呈现无定形结构, 1 500 ℃处理5 h后, SiBN(C)陶瓷纤维开始出现皮芯结构, 并且出现微晶现象; XRD、SEM和EDX等测试手段表明氧化后样品的表面主要以SiO₂微晶形式存在; 介电性能研究表明当C含量低至0.1wt%时, SiBN(C)陶瓷纤维的介电常数为2.1, 介电损耗为0.001 7(频率为10 GHz)。性能评价说明该SiBN(C)陶瓷纤维可满足高温透波材料对增强体的要求。      

Hollow fibers of yttria-stabilized zirconia (8YSZ) prepared by calcination of electrospun composite fibers

8YSZ fibers were synthesized by calcination of PVP/zirconium oxychloride/yttrium nitrate composite fibers (PVP-Precursor) obtained by electrospinning. Scanning electron microscopy (SEM) indicated that the 8YSZ fibers are hollow and the gas released during organic binder decomposition resulted in the formation of hollow center in fibers. Proper controlling heating rate is necessary to the formation of hollow structure. Both X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the 8YSZ fibers have crystal structure of tetragonal phase. The hollow structure should make the fibers to have a higher resistance to sintering than solid fibers at elevated temperature. This result has important applications in catalytic combustion.     

Dielectric modeling of multiphase composites

In this paper effective medium theory based on Clausius–Mossoti relation was used to predict dielectric properties of multiphase composite system. The composite consisted of E-glass fibers (plain weave S-glass, J.B. Martin) embedded in bisphenol A diglycidylether epoxy matrix (D.E.R. 324, D.O.W. chemicals) with hollow ceramic spherical inclusions (SF 14, P.Q. Corp.) at different volume fractions. In many engineering applications, materials with designed dielectric properties are often sought. An important question in engineering design of a composite material is how the overall properties of the composite depend on those of the individual constituents. Mixing the epoxy resin with hollow ceramic inclusions can effectively reduce dielectric constant of the resin, which is often desirable, rendering composites as good candidates for many applications, especially in telecommunications. Compensation for degradation of mechanical properties of matrix (due to inclusion insertion) is obtained by embedding fibers into the mixture. Measurements of dielectric constant and loss tangent for this multiphase composite system were conducted in the region between 0.1 and 100 kHz range using DEA 290 (T.A. Instruments) dielectric analyzer and experimental results are presented. Good correlation between analytical model and experimental results was observed throughout all frequency range of investigation.     

Micromechanics-based predictions of effective properties of a 1-3 piezocomposite reinforced with hollow piezoelectric fibers

Abstract

Presented is a development of models for predicting the effective properties of a piezocomposite reinforced with hollow piezoelectric fibers. The models are established based on micromechanics of representative volume element so-called modified concentric cylinders model. Predicted are five effective elastic constants, two effective piezoelectric coefficients, one effective dielectric permittivity, and two thermal expansion coefficients. Numerical results of a chosen material system are discussed. Comparisons with finite-element method show very good agreements. The predicted effective properties of the piezocomposite with hollow fibers provide the enhancement in several performance parameters up to three times as high as those with solid piezoelectric fibers.     

High-temperature properties and associated structure evolution of continuous SiNO fiber-reinforced BN composites for wave transparency

Continuous SiNO fiber-reinforced boron nitride (SiNO_f/BN) composites for high-temperature wave transparency have been fabricated by a precursor infiltration pyrolysis (PIP) process using borazine as the BN precursor. The evolution of the properties of the composites at elevated temperatures have been measured in terms of strength, dielectric constant, and loss tangent, and the associated structure evolution has been investigated by XRD, SEM, HRTEM, and FTIR. A flexural strength of 138 MPa was maintained up to 1200 °C. Deterioration of the mechanical properties at elevated temperature was due to crystallization of the amorphous SiNO fibers into Si₂N₂O and internal pore formation, coupled with interface reactions. The composites display a low dielectric constant of 3.38 and a low loss tangent of 0.0017, which increases slightly with temperature due in part to the crystallization into Si₂N₂O and high-temperature polarization. The high-temperature mechanical properties and good dielectric properties of these composites make them useful for wave transparency.     

热解炭(PyC)/BN复合粉的制备及其吸波性能

以尿素、硼酸和热解炭(PyC)粉为原料, 利用化学转化法, 在1750℃、N₂气氛下反应6 h, 制备了PyC/BN复合粉。采用XRD、XPS、SEM、DTA/TGA对PyC/BN复合粉的物相组成、化学成分、表面形貌及抗氧化性能进行了表征, 并用矢量网络分析仪测试了PyC/BN复合粉在2~18 GHz频段内的吸波性能。结果表明: 生成的h-BN颗粒在PyC粉表面形成包裹层, 其尺寸为几十至几百纳米。相比于PyC粉, PyC/BN复合粉的抗氧化性能明显提高, 在1200℃时其质量仍剩余50%以上, 而PyC粉在940℃时已被氧化完全。同时, 由于波阻抗增大, 电子位移极化、界面极化、多重散射和λ / 4干涉相消作用, 使得PyC/BN复合粉在相同厚度下(d=2 mm), 其反射率峰值达到-27.2 dB, 反射率小于-10 dB的频宽为2.6 GHz; 且随着厚度的增加, 其反射率峰值均小于-10 dB, 并出现多重吸收峰。PyC/BN复合粉可作为理想的轻质、耐高温、强吸收的吸波剂应用于微波领域。     

Hierarchical composites of conductivity controllable polyaniline layers on the exfoliated graphite for dielectric application

Hierarchical composites of polyaniline fibers on the surface of exfoliated graphite were synthesized by chemical oxidation method. The conductivity of polyaniline fibers was controlled by doping and dedoping procedures. The morphology, structure and chemical character of doped and dedoped hierarchical composites were systemically investigated by SEM, TEM, XRD, FT-IR, XPS and conductivity measurement. After dedoping procedure, polyaniline/exfoliated graphite composites were used to improve dielectric property of the electroactive polymer poly(vinylidene fluoride). The dielectric constant and loss tangent of composites were 17 and 0.06 (10³ Hz) when the polyaniline/exfoliated graphite loading was 3 wt%. Of great interest was that the dielectric loss tangent of composites showed inherent low loss of poly(vinylidene fluoride) because insulating polyaniline fibers on the surface of exfoliated graphite effectively prevented direct contact of exfoliated graphite flakes in poly(vinylidene fluoride) matrix.     

Facile synthesis of hexagonal boron nitride fibers and flowers

Hexagonal boron nitride (h-BN) fibers and flowers were synthesized with a facile synthesis route. The precursor was obtained as an intermediate material by the reaction of KBH₄ and NH₄Cl at 120 °C for 48 h. h-BN was obtained by heating the intermediate material at 1250 °C for 10 h. The sample obtained was characterized by X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetry (TG/DTA) and environment scanning electron microscopy (ESEM), which matched with h-BN. ESEM image indicated that the diameter of the BN fibers is mainly in the range of 1–2 μm.