Mechanical property improvements in Nicalon SiC fibre reinforced silicon nitride ceramics by oxide coating of Si3N4 starting powders

Ceramic matrix composites are attractive as candidate materials for high-temperature applications offering some advantages compared to monolithic ceramics and high-temperature metal alloys. SiC fibre reinforced silicon nitride is one such composite system. However, the processing route is critical to the production of a reliable composite. In this study, silicon nitride matrix densification was improved and sintering temperature was lowered by coating of Si₃N₄ particles with oxides deposited from hydrolysed metal alkoxides. The solution containing oxide coated Si₃N₄ powders was used as a slurry to infiltrate Nicalon SiC fibre tows. Following previous studies, the fibres were heat-treated in carbon monoxide to improve mechanical and surface properties. Infiltrated green bodies were hot-pressed at elevated temperatures to produce dense composites. The results showed that particle coating accelerated densification kinetics, eliminated pores and reduced the required hot-pressing temperature. There was also less fibre degradation as a result of the lower temperature of densification. Bending strength and fracture toughness of the composites were measured and fractography was conducted using scanning electron microscope. Composites manufactured using coated Si₃N₄ powders showed improved properties, specifically matrix stiffening and delayed crack initiation under load.     

Preparation,characterization and microwave absorption properties of barium-ferrite-coated fly-ash cenospheres

Magnetic composites of barium ferrite coated on fly-ash cenospheres (BFACs) were prepared by sol–gel auto-combustion method. To promote surface activity, we modified fly-ash cenospheres (FACs) surfaces using γ-aminopropyltriethoxysilane (APS) as coupling agent and silver nitrate as activating agent before coating barium ferrite films on FACs. The morphology, composition, crystal structure, magnetic and microwave absorption properties of these composite powders were characterized by scanning electron microscope, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometer, and vector network analyzer. Continuous and uniform coatings of barium ferrite were found on the surfaces of the FACs. The BFACs powders-epoxy composite possesses excellent microwave absorption properties in the 2–18 GHz frequency range. The maximum microwave reflection loss reaches ?15.4 dB at 8.4 GHz with a thickness of 3.0 mm, and the widest bandwidth less than ?12 dB is 6.2 GHz with a sample thickness of 2.0 mm. The intrinsic reasons for microwave absorption were also investigated. Applications of this composite material in magnetic recording, electromagnetic wave shielding, and lightweight microwave-absorbing fields are promising.     

金属介质对超高频RFID被动标签读取效能的 影响及可用于金属表面标签的设计

 金属介质对射频信号的干扰一直以来影响着射频识别技术（RFID）的有效应用。为了研究导致干扰发生的机理以及解决办法，以超高频段RFID系统为对象，依据电磁场相关理论，进行了一系列实验。通过对实验所得数据的分析，验证了金属介质对RFID被动标签读取率的影响，并进一步发现标签和金属介质表面间隔距离与读取率之间的变化关系。另外，发现了附加金属层产生的同相位电场对标签读取性能的放大作用。最后，根据实验研究结果，提出一种可用于金属表面的RFID被动标签的设计方案，该设计方案不但可以有效消除金属干扰的负面影响，还可在原有读取性能的基础上进一步提高读取率，同时兼顾使用方便性和制造成本两方面。     

镀银羰基铁粉的制备及其性能的研究

用化学镀法,甲醛为还原剂,制备镀银羰基铁粉.用XRD,SEM和EDX对粉体进行表征.用重法测定粉体的抗氧化性能,并以镀银羰基铁粉制备电磁屏蔽材料,检测其在100kHz～1.5GHz的屏蔽效能.结果表明:用该法制备的镀银羰基铁粉能够实现表面银层包覆完整、致密;羰基铁粉镀银后的抗氧化性能得到明显提高;电磁屏蔽材料在100kHz～1.5GHz频率范围内获得优于-32dB的屏蔽效能.     

溶解乳化法制备SBS包覆FeSiAl片状吸收剂的微波电磁性能

片状FeSiAl吸收剂低频磁性能优异,但复介电常数大、阻抗失配严重,导致难以兼顾高频吸波性能。通过将有机高分子溶解、混合、乳化及吸附析出的多步骤过程,直接将线性苯乙烯-丁二烯-苯乙烯嵌段共聚物（SBS）包覆到微米级片状FeSiAl合金粒子表面,并研究其电磁性质。结果表明,SBS在片状FeSiAl合金粒子表面形成了均匀致密的包覆层,显著提高了合金粒子的电阻率,其与石蜡2：1混合样品复介电常数的实部由72降低到30左右。包覆后片状FeSiAl合金粒子的低频磁性能没有受到负面影响,而其高频性能由于阻抗匹配性的提升得到了明显的改善,因而片状FeSiAl@SBS复合粒子具有兼顾低频与宽带的吸波特性。     

环氧树脂基复合材料表面金属化处理及其性能探讨

采用特殊的表面活化处理方法对环氧树脂基复合材料进行处理,然后采用电镀工艺在复合材料表面沉积铜-镍复合镀层,并对复合材料金属镀层附着力、环境适应性能和电磁屏蔽性能进行了测试分析。结果表明,金属层与复合材料基材结合良好;经过温度冲击试验,金属镀层无起泡、开裂、脱落等缺陷;通过雷达波反射特性测试,表面金属化处理的复合材料与金属具有相同的雷达波反射特性,可用于防静电性、电磁屏蔽性要求较高的包装材料。     

Titanate coupling agent effects on nonaqueous Co2Z ferrite suspensions dispersion

Co₂Z ferrite powders with the chemical composition 3Ba_0.5Sr_0.5O·2CoO·12Fe₂O₃ have superior high frequency magnetic properties. However, Co₂Z ferrite powders are difficult to apply to practical processes because of agglomeration induced by the strong magnetic attraction between particles. In this study, Co₂Z ferrite powder pretreatment using a titanate coupling agent—Neopentyl (dially)oxy tri(dioctyl)pyrophosphate titanate (Lica 38) on the sedimentation and rheological behavior is investigated. The bonding mechanisms between ferrite powder, Lica 38, and dispersant (KD1) are studied using diffuse reflectance Fourier transform infrared spectroscopy is used to explain the difference in the rheological and sedimentation behaviors of untreated and titanate coupling agent-modified ferrite powders. The affinity of Co₂Z ferrite and dispersants could be substantially enhanced by coating a titanate- coupling agent onto the ferrite surface. The coated layer could prevent particles from agglomerating from magnetic interaction.     

碳纤维粉化学镀镍及其电磁参数研究

用联氨作为还原剂,通过化学镀的方法在碳纤维粉基体上包覆纯金属镍层,获得一种新的功能复合材料.SEM电镜和EDX能谱分析证明了碳纤维粉基体上包覆了1层致密均匀而且连续的铁磁性金属镍层,并且没有引入其它杂质元素.通过对包覆前后样品在2～18GHz范围内的复介电常数的实部ε′、虚部系数ε″、复磁导率的实部μ′和虚部系数μ″等电磁参数进行分析比较,可以看出在低频处复合材料的电磁损耗都较纯碳纤维粉有较大提高,可应用于微波吸收材料.     

铁氧体/碳化硅/石墨三层涂层复合材料介电性能

选用柔性涤纶针织物为基材,以铁氧体、碳化硅和石墨分别作为底层、中层和表层吸波剂,在基材上进行三层复合涂层整理,制备不同厚度的复合材料。探讨了底层、中层、表层厚度对介电常数实部、虚部和损耗角正切的影响。结果表明:该复合材料在低频段具备良好的介电性能,涂层厚度对介电常数实部、虚部和损耗角正切影响较大;底层铁氧体涂层厚度为0.5mm,中层碳化硅涂层厚度为0.3mm,表层石墨涂层厚度为0.3mm时,铁氧体/碳化硅/石墨三层涂层复合材料的介电常数实部、虚部、损耗角正切最大。     

Ag包覆Fe3O4复合粉体的制备及其性能研究

用化学镀法,甲醛为还原剂,制备Fe3O4/Ag包覆复合粉体.用XRD、SEM和EDX对粉体进行表征.用重法测定粉体的抗氧化性能,并研究了AgNO3用量对Fe3O4/Ag包覆复合粉体的导电性能的影响.结果表明,用该法制备的Fe3O4/Ag包覆复合粉体能够实现表面银层包覆完整;Fe3O4粉镀银后的抗氧化性能得到一定程度的提高;AgNO3用量越多,Fe3O4/Ag包覆复合粉体的导电性能越好.     

A Coprecipitation Coating Synthesis of SiC/YAG Composites

The α-SiC in 0.5μm size powders were coated with Al_2O_3 and Y_2O_3 by a coprecipitation coating (CPC) method forfabrication of SiC/YAG composites. The same powder preparation was carried out by conventional mechanical mixing(MM) method for comparison. Two kinds of SiC/YAG composites were manufactured by pressureless sintering usingthe different powders, named CPC composite and MM composite thereafter respectively. It is shown that the CPCcomposite has the advantages of homogeneous distribution of YAG phase and of being sintered to high density ata low temperature, 100℃ lower than that of MM composite. The strength (573 MPa) and hardness (23.3 GPa) ofthe CPC composite are significantly higher than those (323 MPa and 13.5 GPa) of the MM composite, respectively.     

纳米陶瓷微米高温合金复合涂层组织与性能

采用溶胶-凝胶化学包覆法制备纳米陶瓷微米高温合金复合粉末,用HVOF喷涂技术制备了复合涂层,采用SEM观察和摩擦磨损实验分析了复合粉末和复合涂层的组织和性能.研究表明:复合粉末是以纳米陶瓷为外壳包覆微米级高温合金颗粒核心的核壳式结构;陶瓷壳在喷涂过程中形成液相与高温合金液相熔合,烧结成致密陶瓷相,部分陶瓷在冷却过程中析出结晶体;复合涂层与基体的结合强度为59.2 MPa,摩擦系数为0.766,磨损率比纯高温合金涂层降低了32%.     

堇青石-氧化锆红外辐射复合粉体的制备

为获得具有高红外辐射性能的粉体,以堇青石颗粒为基体,采用液相共沉淀法,用氧化锆前驱体包覆堇青石基体,煅烧后得到堇青石-氧化锆红外辐射复合粉体;对制备的复合粉体进行热分析、物相分析及形貌分析,并测试其红外辐射性能。结果表明,利用非均匀形核可将氧化锆前驱体包覆于堇青石颗粒表面,氧化锆结晶温度在800～900℃之间,900℃煅烧后得到的氧化锆晶粒尺寸约为8 nm;当在一定初始溶液浓度范围内改变反应物的用量时,可以控制氧化锆的包覆量并且避免其自发团聚;通过氧化锆纳米颗粒包覆的工艺可以提高堇青石粉体在3～5μm波段的红外辐射性能,当氧化锆包覆质量分数为21.11%时,复合粉体具有最佳的红外辐射性能。     

原位聚合法制备铁氧体/聚苯胺吸波复合材料的研究进展

电磁污染已成为继空气污染、水污染和噪声污染之后的第四大污染, 吸波材料因其吸收和衰减特性, 可以作为解决电磁污染的有效手段。聚苯胺(PANI)作为一种电阻损耗型吸波材料, 可以满足吸波材料"厚度薄"、"质量轻"的发展理念, 但由于阻抗匹配度差, 吸波性能并不理想。铁氧体作为一类传统的磁损耗型吸波材料, 因其密度较高使其适用范围受到限制。高密度的铁氧体与低密度的聚苯胺复合制备的吸波材料, 不仅可以调整复合材料的密度, 而且还能改善复合材料的阻抗匹配, 提高铁氧体/聚苯胺复合材料的吸波性能。本文首先探讨了聚苯胺以及铁氧体/聚苯胺复合材料的制备方法, 其次阐述了铁氧体/聚苯胺复合材料的吸波机理。然后分别归纳了尖晶石型、磁铅石型、石榴石型铁氧体与聚苯胺制备的复合材料在吸波领域的研究进展。最后指出铁氧体/聚苯胺复合材料应趋向于电磁仿真和多元复合化的方向发展。     

Sintering characteristics of nano-sized Ag–Pd–glass composite powders with high Pd content

Nano-sized Ag–Pd (50–50) alloy powders coated with Pb-based glass material with low and high glass transition temperature are directly prepared by high-temperature flame spray pyrolysis. Nano-sized Ag–Pd–glass composite powder is formed from the evaporated vapors by nucleation and growth process, and then glass material moves out to the outside of the powder by crystallization process of alloy. The thickness of the glass coating layer measured from the TEM image is 2.8 nm. The mass changes of the Ag–Pd alloy and Ag–Pd–glass composite powders in the TG analysis under 900 °C are 10.9 and 6.8%, respectively. Glass materials improve the uniformity and density of the Ag–Pd electrode layers by act as sintering agent and adhesion improvement. The Ag–Pd electrode formed from the composite powders with high glass transition temperature glass material has thin and uniform thickness. The specific resistances of the electrodes formed from the nano-sized Ag–Pd–glass composite powders are 0.27, 0.09, and 0.03 mΩ cm at firing temperatures of 700, 800, and 900 °C, respectively.     

Extrusion of mechanically milled composite powders

Mechanical alloying has come to the fore in fields such as the production of intermetallic compounds, supersaturated solid solutions, amorphous materials and metal matrix composites. With this process, composite powders are obtained with characteristics that are impossible to achieve employing conventional powder metallurgy techniques. In this work, aluminium powder AA6061 is mixed with silicon and aluminium nitrides in a conventional mixer and in a high-energy ball mill to obtain composite powders that are subsequently uniaxially cold pressed and hot extruded. The necessary pressure to extrude the composite cold pressed powders varies with the powder condition, being lower for the composite powder conventionally mixed, higher for the composite powder after a short time of milling, and intermediate after a longer time of milling, due to the morphological and structural changes typical of the mechanical alloying process.     

Preparation of composite particles coated with two kinds of solid powders by semi-chemical recycle method

Polymer microspheres coated with two kinds of solid powders were prepared by the semi-chemical recycle method of waste plastics. Magnetite (Fe₃O₄) and silicon carbide (SiC) were added as solid powders so as to give thermal and electric conductivity and magnetization. Waste expanded polystyrene was used as raw materials of polymer matrix of composite particles. In the experiment, the oil phase dissolving expanded polystyrene was dispersed to form the O/W dispersion in the continuous water phase in which polyvinyl alcohol and magnetite powders were added. And then, silicon carbide powder was added into the O/W dispersion. Composite particles prepared had the structure that a polymer particle was covered with two kinds of solid powders. The amount of each solid powder adhered on the surface of polymer microsphere was strongly affected by an addition time of silicon carbide powder. Furthermore, the gradient adhesion layer due to two solid powders was found to be formed on the surface of polymer microsphere.     

涂层与镀层复合雷达波吸收性能研究

以纳米铁酸镍钴铁氧体复合Co粉、羰基铁粉等为吸收剂,并采用化学镀层和涂层方法,进行了单层、双层和三层沣涂层的吸波性能实验研究.结果表明:双层复合涂层的吸波性能较单层涂层在低频段有较大的提高;三层复合涂层的吸波性能优于双层复合涂层,三层复合涂层反射率小于-5dB的频宽为4.5～18GHz,较双层涂层提高5.4GHz.其中,镀镍层对提高吸波性能作用明显.     

三氧化二锑-硅灰石复合填料的制备及在聚丙烯中的填充性能

以硅灰石、三氧化二锑为主要原料,采用化学沉淀法在硅灰石表面包覆纳米三氧化二锑,制备一种无机复合型阻燃粉体材料,并用硅烷1113和硬脂酸对这种复合粉体进行表面改性后填充聚丙烯制备复合阻燃材料。采用扫描电镜、粒度仪、白度仪、X射线衍射等手段对改性粉体材料表面状况进行了表征,并检测了填充聚丙烯材料的力学性能、热学性能及垂直燃烧和水平燃烧阻燃性能。结果表明:硅灰石表面均匀的包覆了一层纳米三氧化二锑;其水平燃烧时间由90s降到30s,燃烧长度由75mm降到20mm,垂直燃烧时间由60s降到10s,由不具阻燃性升高到阻燃级别为V-2。     

Rod-shaped polyaniline-barium ferrite nanocomposite: preparation, characterization and properties

Rod-shaped polyaniline (PANI)-barium ferrite nanocomposite was synthesized by in situ polymerization of aniline in the presence of BaFe(12)O(19) nanoparticles with diameters of 60-80?nm. The structure, morphology and properties of the nanocomposite were measured using powder x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and vibrating sample magnetometry. Different ferrite/PANI ratios were selected in order to study magnetic and conductive properties. The results indicated that there were some interactions between PANI chains and ferrite particles. The saturation magnetization and the coercivity varied with the ferrite content. The conductivity at room temperature decreased from 43.35 to 6.9 × 10(-2)?S?cm(-1) when the ferrite content changed from 0 to 50?wt%. The composite has excellent electromagnetic parameters which indicates potential application in high performance adsorbing materials in broadband and high frequency ranges. The polymerization mechanism and interactions in the nanocomposites were also studied.