纳米Fe3O4／石墨复合材料的制备及吸波性能研究

利用氧化石墨的强吸附性分别将两种铁盐吸附到氧化石墨层间,再通过热处理制备出纳米Fe3O4石墨复合材料,采用XRD和SEM对其晶体结构和微观形貌进行表征,并对其吸波性能进行了分析。研究结果表明,采用柠檬酸铁制备的纳米Fe3O4墨复合材料主要为介电损耗型微波吸收材料,而采用硝酸铁制备的纳米Fe3O4石墨复合材料在不同频段范围分别以介电损耗和磁损耗为主。两者相比,柠檬酸铁制备的纳米Fe3O4墨复合材料的微波吸收性能更能符合现代隐身技术所要求的薄、宽、轻、强的综合要求。     

铁氧体复合材料吸波性能研究进展

介绍了铁氧体吸波材料的概况。重点总结了近年来铁氧体复合材料的研究现状和吸波性能。详细介绍了聚苯胺/铁氧体复合材料、环氧树脂/铁氧体复合材料以及铁氧体与其它导电聚合物的复合材料的吸波性能,这些复合材料将是今后吸波材料研究和发展的重要方向。针对未来发展提出了建议。     

竹材制备SiC多孔陶瓷及吸波性能研究

以印度竹和竹炭粉为原料,采用溶胶凝胶法和液态渗硅法制备生物基SiC陶瓷块和陶瓷粉,并通过磁性金属担载制备了吸波材料。借助XRD、SEM、RAM反射率测试系统对材料的物相构成、微观构造、吸波反射率进行了分析。结果表明:竹材炭化及陶瓷化后均保持了多孔的骨架结构特征。溶胶凝胶法和液态渗硅法的陶瓷化反应都发生在竹炭孔道侧壁上,且溶胶凝胶法在竹炭孔道内部有硅基陶瓷晶须生成。要提高液态渗硅法竹炭向SiC的转化率和SiC的晶化程度,可以通过提高陶瓷化温度和延长保温时间的方法来实现。无论何种方法,竹炭粉比竹炭块的陶瓷转化率高。另外通过溶胶凝胶法制备的担载磁性金属的竹基陶瓷材料在低频波段有一定的电磁波吸收性能。草刺     

MoSi2对Cf/SiC/Ni复合材料微观结构及吸波性能的影响EI北大核心CSCD

以葡萄糖、Si粉、碳纤维为原料,镍为催化剂,采用水热反应-烧结法制备了Cf/SiC/Ni和Cf/MoSi2/SiC/Ni复合吸波材料。通过X射线衍射、扫描电子显微镜、波导法分别表征了Cf/SiC/Ni和Cf/MoSi2/SiC/Ni复合材料的相组成、微观结构和吸波特性。结果表明:Cf/SiC/Ni复合材料上生长的Si C纳米线稀疏且分布不均匀;厚度为1.5 mm时,在8.20 GHz处最小反射损耗为–14.61 dB,有效吸收带宽为0.23 GHz。Cf/MoSi2/SiC/Ni复合材料的碳纤维表面生长大量SiC纳米线,分布致密且均匀;厚度为2.0 mm时,在9.10 GHz时最小反射损耗为–34.14 dB,有效吸收带宽达2.18 GHz。与Cf/SiC/Ni复合材料相比,添加MoSi2的Cf/MoSi2/SiC/Ni复合材料吸波性能更好,说明MoSi2可有效改善Cf/SiC/Ni复合材料的微观结构及吸波性能。     

Al2O3/Fe3Al复合材料的制备及性能

利用热压烧结制备了致密的Al_2O_3/Fe_3Al复合材料。测试表明,该复合材料具有良好的力学性能,其抗弯强度和断裂韧性的最大值分别为832 MPa和7.96 MPa·m~1/2。对试样进行压痕实验,采用SEM对裂纹扩展方式进行观察,结果表明,在复合材料中存在着多种增韧机制,裂纹表现出复杂的扩展方式,这将在较大程度上吸收裂纹扩展能,从而使复合材料的断裂韧性得以提高。     

水泥基复合材料的吸波性能

对普通硅酸盐水泥与吸波材料制成的复合材料的吸波性能进行了研究。为军事掩体、军用机场及其它固定军用目标干扰雷达探测找到了一种合适的方法,也为大型建筑物的电磁波防护提供了一种新途径。研究了羰基铁粉、氧化镍和纳米氧化钛3种吸波材料与水泥制成的复合材料的吸波性能,并分析了纳米氧化钛的用量、分散方式及试样厚度对电磁波反射衰减的关系。结果表明：在8～18GHz频率范围内。纳米氧化钛与水泥制成的复合材料的反射率均小于-7dB,在16．24GHz时其反射率达-16．34dB,反射率小于-10dB的带宽达4．5GHz。     

Nb2C/MnO2复合材料的制备及吸波性能研究

二维过渡金属碳化物（MXenes）由于其多层结构、优异的导电性和较大的层间距,在电磁波吸收领域具有广阔的发展前景。本采用静电自组装法制备了一种多层结构的Nb2C纳米片和一维纳米棒形貌的MnO2组成的复合体系。通过研究发现,Nb2C/MnO2复合材料质量比为1∶1时,最小反射损耗值在16.98GHz处可达到-29.22d...     

聚氨酯–羰基铁泡沫复合材料的制备及吸波性能

将适量羰基铁吸收剂填充到发泡体系中,采用一步法制备了形貌结构良好的硬质聚氨酯基羰基铁泡沫吸波材料(CIPRPU)。用扫描电子显微镜和矢量网络分析仪对样品的结构及电磁性能进行了表征。结果表明,吸收剂的填充量对电磁参数影响较大,整体上呈现出随吸收剂的填充量升高,电磁参数变大的趋势。CIPRPU的介电常数ε′和ε′′分别在3.0和0.5以下,磁导率μ′和μ′′则分别在1.5和0.5以下。利用测得的电磁参数对CIPRPU的反射率进行了模拟。当厚度为3 mm时,在X波段内,CIPRPU(m(聚氨酯基体):m(吸收剂)=1:5)的峰值吸收达到了–22 d B,表明制备的泡沫型吸波材料能够充分利用其独特的孔结构,使材料由单纯的吸收作用转变为透波–吸波作用,取得了较好吸波效果。     

Ti3SiC2/Al2O3复合材料的制备与性能研究

以TiC／TiO2/Si／Al／Ti等为主要原料，采用热压法原位合成Ti2SiC2／Al2O3复合材料，分别探讨了Al掺入量和工艺制度对Ti3SiC2/Al2O3复合材料物相、显微结构以及性能的影响。结果表明：原位合成制备的Ti3SiC2/Al2O3复合材料与传统方法合成制备的纯Ti3SiC2材料相比，材料的硬度和致密度均有很大的提高。     

Al2O3/SiC复合陶瓷的反应烧结

在空气中于1600℃对Al2O3/0.78%SiC纳米复合体进行2h的反应烧结,制得Al2O3/5%莫来石复合陶瓷,其中的莫来石分为两类,即3Al2O3·2SiC和Al5.65Si0.35O9.175。采用SEM和TEM研究Al2O3/莫来石复全陶瓷的微观结构。对Al2O3/莫来石复合陶瓷的密度和力学性能如杨氏模量、泊松比、硬度、韧性和强度进行了研究。     

Preparation of Al2O3-mullite thermal insulation materials with AlF3 and SiC as aids by microwave sintering

Al₂O₃-mullite composites were prepared under the synergy effect of AlF₃ and SiC aids by microwave heating. The phase composition, microstructure, porosity, flexural strength, thermal shock resistance, and thermal conductivity were investigated. The XRD results revealed that the content of mullite phase steadily increased with the increasing of AlF₃ content. The microstructure showed that the lower content (≤1 wt%) of AlF₃ led to the formation of granular mullite and the higher content (≥3 wt%) of AlF₃ led to the formation of mullite whiskers, which could form an interlocking structure. In addition, the SiC hot spots can also promote the generation of mullite whiskers by microwave sintering. The thermal shock resistance was significantly improved by the interlocking structure of mullite whiskers. The residual rate of flexural strength of the composite with 3 wt% AlF₃ was 86%. The composite with 3 wt% AlF₃ additives got its optimized thermal conductivity from 30°C to 950°C, the value was between 0.819 and 1.021 W/(mK), which possess excellent thermal insulation performance.     

Preparation and microwave dielectric properties of B2O3 bulk

The transparent B₂O₃ bulks were prepared by quenching B₂O₃ melt or cooling it down to room temperature with the rate of 0.1-1°C/min. The quenched sample exhibited the dense microstructure, while the pores were observed for low cooling rates, and this is consistent with the slight decrease in density and permittivity (ε_r) with decreasing the cooling rate. Both the amorphous and cubic phases of B₂O₃ were indicated by XRD, and the content of the cubic phase increased with the decrease in cooling rate, which was responsible for the decreasing Qf value with the lower cooling rate. The temperature coefficient of resonant frequency (τ_f) was insensitive to the cooling rate, and the microwave dielectric properties with ε_r = 3.86-3.97, Qf = 3,200-5,300 GHz and τ_f = −48.9 to −42.8 ppm/°C were obtained in the B₂O₃ bulks. The reported microwave dielectric properties of B₂O₃ bulk are helpful to estimate the effects of the residual B₂O₃ on the properties of the microwave dielectric ceramics with B₂O₃ and H₃BO₃ as the sintering aid.     

Design and fabrication of Al2O3f/SiCN composite with excellent microwave absorbing and mechanical properties

A new kind of structural and functional integration ceramic matrix composite material was prepared from high-performance alumina (Al₂O₃) fibers and absorbing silicon carbonitride (SiCN) ceramics via a combination of polymer infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The Al₂O₃ fiber annealed at its cracked temperature had enhanced permittivity, because the sizing agent on the Al₂O₃ fiber surface was cracked into pyrolysis carbon. For PIP + CVI Al₂O_3f/SiCN composites, PIP SiCN matrix with low conductivity was used as the matching phase, while CVI SiCN matrix with medium permittivity and dielectric loss was regarded as the reinforcing phase distributed in porous PIP SiCN matrix and inter-bundles of Al₂O₃ fiber to improve their mechanical and microwave absorption properties. The fracture toughness and flexural strength of Al₂O_3f/SiCN composite were determined to be 9.4 ± 0.5 MPa m^1/2 and 279 ± 28 MPa, respectively. Based on the design principles for impedance matching, the Al₂O_3f/SiCN composites before and after oxidation were used as loss and impedance layers, respectively. It was found that the optimized composite had the lowest reflection coefficient (RC) of −70 dB and the effective absorption bandwidth covering the whole X-band. In conclusion, Al₂O_3f/SiCN composite can serve as a high-temperature structural material with excellent microwave absorption properties for aerospace applications.     

Investigation of preparation and properties of Ti3SiC2/Al2O3 multilayered composites

Ti₃SiC₂/Al₂O₃ multilayered composites were prepared by the combination of tape casting and hot pressing sintering. The slurry was produced by adjusting the amounts of each organic material, including triethyl phosphate (TEP) as a dispersion, polyvinyl butyrate (PVB) as a binder, dioctyl phthalate (DOP) as a plasticizer, and anhydrous ethanol as an organic solvent. When TEP content was 3 wt.%, PVB content was 4.5 wt.%, R-value (DOP/PVB) was 1.4, and solid content was 38 wt.%; the cast film with a smooth surface, good flexibility, and uniform thickness was obtained after defoaming, tape casting, and drying. Three samples were prepared, namely, S1–S3. The S1 was monolithic Ti₃SiC₂/Al₂O₃ (mass ratio is 1:1) composites. S2 and S3 were Ti₃SiC₂/Al₂O₃ multilayered composites, which matrix layers were Ti₃SiC₂/Al₂O₃ composites (mass ratio is 1:1) and Al₂O₃, respectively, and their interface layer was Ti₃SiC₂. S1–S3 were also sintered at 1550°C. The bending strength of multilayered materials were lower than that of monolithic material, but the fracture toughness of multilayered materials significantly increased. Due to the introduction of Ti₃SiC₂ interface layer, the friction coefficient and wear rate of Ti₃SiC₂/Al₂O₃ multilayered composites were reduced by 30.7% and 33.8%, respectively, compared with monolithic material.     

Cutting performance of microwave-sintered sub-crystal Al2O3/SiC ceramic tool in dry cutting of hardened steel

Tool life and failure mechanisms of a microwave-sintered sub-crystal Al₂O₃/SiC ceramic tool (AS) in dry turning hardened steel were studied. The AS tool with plane face shows better cutting performance and wear resistance than the commercial ceramic tool SW500 and cemented carbide tool YG8 at both low and high cutting speeds. It's suitable for dry cutting at high speed (210–270 m/min), the cutting distance is 5–8 times longer than that of other two tools. The results indicate that the ceramic tool fabricated by this pressureless sintering technology can satisfy the requirements of high-speed machining. Wear forms of AS tool at low cutting speed are slight crater wear and groove wear, which were mainly caused by abrasion. At high cutting speed, tool failure forms are cater wear, groove wear and slight chipping caused by severe abrasion and adhesion.     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

Origin of dielectric loss in Ba(Co1/3Nb2/3)O3 microwave ceramics

The microwave dielectric loss of stoichiometric and non‐stoichiometric Ba(Co_1/3Nb_2/3)O₃ ceramics have been measured from 2 to 300 K in magnetic fields ranging from 0 to 5 T using a dielectric resonator (DR) technique. The microwave absorption from spin excitations of unpaired d‐electrons in exchange coupled Co²⁺ ions dominate the loss of the Ba(Co_1/3Nb_2/3)O₃ ceramics at cryogenic temperatures. Two peaks in the loss tangent (tan δ) vs temperature relation from a distinctly different origin occur at 25‐30 K and 90 K, which increase in magnitude with increasing Co content in the bulk dielectric samples. Evidence that these peaks result from polaron conduction from hopping between Co²⁺ and Co³⁺ ions includes (i) the peak's observed temperature range; (ii) the decrease in peak intensity of approximately a factor of two in a large applied magnetic fields (5 T); and (iii) a strong correlation between the peak's magnitude and both the fraction of the minority Co³⁺ in the dominant Co²⁺ matrix and D.C. conductivity at elevated temperatures. A magnetic‐field independent high temperature peak with a maximum at 250 K dominates the room temperature microwave loss whose magnitude correlates with those of the low temperature peaks. This suggests that the defects responsible for carrier conduction play an important role in establishing the loss tangent at room temperature.     

烧结助剂对CaO-B2O3-SiO2介电陶瓷结构与性能的影响

以CaO-B203-Si02(CBS)系微波介质陶瓷为基体材料,采用ZnO和Na2O作为烧结助剂,研究了其微观结构、相组成及介电特性.研究结果表明ZnO在烧结过程中与B2O3及SiO2生成低熔点玻璃相,促进以片状集合体形式存在的CaSiO3晶相合成,显著降低了材料的致密化温度;Na2O虽可促进烧结,但会破坏硅灰石晶体结构,导致微波介电性能显著降低2%(摩尔分数)ZnO取代CaO,在1000℃保温2h,具有较好的微波介电性能εr为5.4.Q·f为22000GHz(测试频率f0为8.5GHz).     

High temperature absorbing coatings with excellent performance combined Al2O3 and TiC material

A series of challenges are impeding the development of high temperature electromagnetic (EM) wave absorbing materials in x band. In this study, to deal with this problem, a coating, (1-x).wtAl₂O₃∼x.wtTiC, is designed and prepared using plasma spraying technology. Its permittivity increases with temperature and TiC content, which endows it with a good EM impedance at high temperature. The coating possesses excellent EM absorbing performance at 800 ℃. When the x value equals 0.2 and thickness 1.6 mm, the coating exhibits an EAB of 3.45 GHz at 800℃, and a reflection loss lower than -8 dB in whole x band. The XRD result shows that only two phases exist in the coating. The SEM images illustrate that TiC is unevenly dispersed in Al₂O₃, causing loss of conductivity and interface polarization. The finding not only broadens the application of TiC-based materials but also indicates the promising future of this material system.