Complex Permittivity and Microwave Absorbing Property of Si3N4-SiC Composite Ceramic

Si3N4-SiC composite ceramics were fabricated by chemical vapor infiltration using porous Si3N4 ceramic as preform. The average grain size of SiC was 30 nm. Relationship between SiC content and relative complex permittivity of Si3N4-SiC within the frequency range of 8.2-12.4 GHz (X-band) was investigated. The average real part of relative complex permittivity ε of Si3N4-SiC increased from 3.7 to 14.9 and the relative imaginary part ε increased from 0.017 to 13.4 when the content of SiC increased from 0 to 10 vol.%. The Si3N4-SiC ceramic with 3 vol.% SiC achieved a reflection loss below 10 dB (90% absorption) at 8.6-11.4 GHz, and the minimum value was 27.1 dB at 9.8 GHz when the sample thickness was 2.5 mm. The excellent microwave absorbing abilities of Si3N4-SiC ceramic were attributed to the interfacial polarization at interface between Si3N4 and SiC and at grain boundary between SiC nanocrystals.     

Mechanical properties and microstructure of SiC-reinforced Mg-(2,4)Al-1Si nanocomposites fabricated by ultrasonic cavitation based solidification processing

Nano-sized SiC enhanced magnesium matrix nanocomposites, Mg-2Al-1SiC with 2% SiC and Mg-4Al-1Si with 2% SiC, were successfully fabricated by ultrasonic cavitation based dispersion of SiC nanoparticles in Mg-(2,4)Al-1Si magnesium alloy melts. As compared to the magnesium alloy matrixes, the mechanical properties including tensile strength and yield strength of the Mg-2Al-1Si/2% SiC and Mg-4Al-1Si/2% SiC nanocomposites were improved significantly, while the ductility of magnesium alloy matrix castings was retained. While there were some SiC micro-clusters in the microstructure of nanocomposites, the SiC nanoparticles were dispersed well outside the areas of micro-clusters. Most micro-clusters were located along the grain boundaries while most separate SiC nanoparticles were embedded inside the grains. TEM study of the interface between SiC nanoparticles and Mg-(2,4)Al-1Si metal matrixes suggested that SiC bonds well with the metal matrixes without forming an intermediate phase.     

Heteroepitaxy of zinc-blende SiC nano-dots on Si substrate by organometallic ion beam

The self-assembled SiC nano-dots were fabricated on Si(111) substrate at low-temperatures using the organometallic ion beam deposition technique. The single precursor of methylsilicenium ions (SiCH₃⁺) with the energy of 100 eV was deposited on Si(111) substrate at 500, 550 and 600 °C. The characteristics of the self-assembled SiC nano-dots were analyzed by reflection high-energy electron diffraction (RHEED), Raman spectroscopy and atomic force microscope (AFM). The RHEED patterns showed that the crystal structure of the SiC nano-dots formed on Si(111) substrate was zinc-blende SiC (3C-SiC) and it was heteroepitaxy. The self-assembled SiC nano-dots were like a dome in shape, and their sizes were the length of 200-300 nm and the height of 10-15 nm. Despite the low-temperature of 500 °C as SiC crystallization the heteroepitaxial SiC nano-dots were fabricated on Si(111) substrate using the organometallic ion beam.     

SiCP/ZL 109 复合材料中SiC 的界面行为

以常规TEM 为工具, 研究了SiCP/ ZL 109 复合材料中数十个SiC 颗粒及其界面,Si 优先在SiC 表面上形核、长大, 形成界面Si, 并形成大量SiC/S i 界面。靠近SiC 界面的Al 基体中, 普遍存在一层厚度小于1Lm 的“亚晶铝带”, 其内有大量位错。SiC 与Al、SiC 与Si 之间虽然没有固定的晶体学位向关系, 但是存在下列优先关系: (1103) _SiC//(111)_Al, [1120]_SiC//[110]_Al; (1101) _SiC//(111) _Si; [1120]_SiC//[112]_Si。      

Structural and optical properties of high quality ZnO thin film on Si with SiC buffer layer

ZnO thin films are grown on Si substrates with SiC buffer layer using ion plasma high frequency magnetron sputtering. These substrates are fabricated using a technique of solid phase epitaxy. With this technique SiC layer of thickness 20-200 nm had been grown on Si substrates consisting pores of sizes 0.5-5 μm at SiC and Si interface. Due to mismatching in lattice constants as well as thermal expansion coefficients, elastic stresses have been developed in ZnO film. Pores at the interface of SiC and Si are acting as the elastic stress reliever of the ZnO films making them strain free epitaxial. ZnO film grown on this especially fabricated Si substrate with SiC buffer layer exhibits excellent crystalline quality as characterized using X-ray diffraction. Surface topography of the film has been characterized using Atomic Force Microscopy as well as Scanning Electron Microscopy. Chemical compositions of the films have been analyzed using Energy Dispersive X-ray Spectroscopy. Optical properties of the films are investigated using Photoluminescence Spectroscopy which also shows good optical quality.     

SiCp/Al-Si复合材料中SiC/Si的晶体学位向关系

用TEM研究了离心铸造和挤压铸造的SiCp/ZL109复合材料,发现Si优先在SiC表面上形核、长大,并形成大量"界面Si"及SiC/Si界面.SiC与Si之间不存在固定的晶体学位向关系,但存在(1101)sic//(111)si,[1120]sic∥[112]si优先出现的位向关系,而(0001)sic∥(111)si不是优先出现的位向关系.     

SiC微粉含量对先驱体转化制备碳纤维布增强碳化硅复合材料性能的影响

以聚碳硅烷(PCS)、二乙烯基苯(DVB)和SiC微粉为原料制备了碳纤维布增强碳化硅复合材料，考察了SiC微粉含量对材料结构与性能的影响。实验表明，SiC微粉含量过低，材料内部存在大的孔洞，容易造成应力集中，导致材料的力学性能较差；而当SiC微粉含量较高时，在制备过程中微粉对碳纤维机械损伤加大，同样导致材料力学性能下降。当SiC微粉含量为30％(质量分数)时，所制备的材料的力学性能较好，其弯曲强度和拉伸强度分别为246．4MPa和72．5MPa。     

常规铸造工艺条件下SiCp/Al-Si复合材料中的界面反应

采用搅拌复合法制备了10%SiC／Al—5Si—Mg，10%SiC／Al—7Si—Mg(体积分数)复合材料，研究了在常规铸造工艺条件下重熔后复合材料中的界面反应。通过透射电镜和能谱分析可知，SiC界面基本上都是单一的SiC／Al及SiC／Si界面，部分界面上有MgAl2O4颗粒相形成，由于基体合金中Si的存在，生成Al2C3的有害界面化学反应得到了抑制。对不同文献中抑制Al4C3产生所需临界Si含量实验测定结果的差异进行了分析。     

Single-crystal SiC thin-film produced by epitaxial growth and its application to micro-mechanical devices

This paper deals with the fabrication process of single-crystal silicon carbide (SiC) thin-films and its application to microdevice. SiC thin-film was synthesized using molecular beam epitaxy, where single-crystal SiC layer was grown on single-crystal silicon (Si) substrate. Using lithography and etching process, microscopic cantilevers were fabricated. Typical dimensions of the cantilevers were 10-60 μm in length, 10-30 μm in width, typically 180 nm in thickness. Young's modulus estimated from bending test was almost the same with that of bulk material. Finally, an application is demonstrated where nickel was deposited on the cantilever and biomorphic actuation was carried out. The displacement at the tip was about 2 μm when the temperature change was 40 K. The time constant of the step response was about 0.07 s.     

碳模板转化法制备管状SiC/Si复合陶瓷

利用纸作为原材料,通过卷曲、树脂浸渍、碳化预制备出具有层状结构的管状碳模板,之后在1550℃通过原位反应液相渗Si0.5-1h,在常压烧结条件下制备出具有层状结构特征的SiC/Si管状陶瓷复合材料.采用XRD、SEM对碳模板反应前后的物相变化和显微结构进行了研究.结果表明了该材料的最终产物为β-SiC和Si,且两者分布表现出明显的交替成层现象,呈现出层状陶瓷的结构特征.     

新型SiBCN先驱体的合成及其陶瓷性能的研究进展

综述了近年来采用有机先驱体制备SiBCN复合陶瓷的研究现况.详细介绍了SiBCN先驱体合成的两种主要方法,并对利用这两种方法获得的先驱体和裂解后的陶瓷的产率等方面进行了比较.阐述了SiBCN复合陶瓷与纯SiC和Si3N4陶瓷在力学,抗蠕变,抗氧化等性能的差别.     

木材陶瓷化反应机理的研究

研究了木材制备ＳｉＣ陶瓷的反应过程及熔融硅与多孔木炭反应的机理．结果表明,木材制得的ＳｉＣ陶瓷的最终组织取决于渗硅处理温度．较低温度下形成碳化硅多孔材料,较高温度下形成 Ｓｉ／ＳｉＣ复相致密材料．分析指出,木材制备 ＳｉＣ陶瓷中 Ｓｉ／Ｃ反应的大致过程为：熔融硅沿木炭毛细管壁上升,同时与接触的碳反应形成碳化硅,碳化硅层不断向碳层推进直至多孔碳骨架完全转化为碳化硅．生成的碳化硅在反应后期会发生再结晶,最终组织形态表现为多边形大颗粒碳化硅分布在自由硅基体上．     

在泡沫碳化硅载体上原位生长silicalite-1型沸石晶体

以多晶硅颗粒为硅源, 在泡沫碳化硅载体上原位水热合成silicalite--1型沸石晶体。研究了硅颗粒加入量、NaOH浓度以及合成时间等因素对沸石晶体的负载量、晶体尺寸和沸石晶体/泡沫碳化硅复合材料比表面积的影响。结果表明,
以多晶硅颗粒为硅源控制硅酸根的释放速度, 使沸石晶体在碳化硅载体表面异质界面形核, 从而实现沸石晶体在泡沫碳化硅载体表面的连续生长; 当多晶硅量过少时, 溶液中的硅酸根浓度过低, 不能在载体表面形成连续生长的沸石层;
而当多晶硅量过大时, 溶液中硅的浓度过高, 部分沸石晶体在溶液当中形核, 使沸石晶体在载体表面的负载量下降; 提高溶液中NaOH的浓度, 加快硅的溶解, 使溶液中硅的饱和浓度升高, 沸石晶体的形核率也随之升高, 使沸石晶体的负载量增加。在最优条件下制备的silicalite--1/泡沫碳化硅复合材料其沸石晶体的比表面积为81.28 m²g^-1。     

成形压力对SiC/TiB2复合材料组织与性能的影响

基于熔融Si浸渗法制备出较致密的SiC/TiB₂复合材料, 并研究了坯体成形压力对SiC/TiB₂复合材料致密度、相组成、显微组织和力学性能的影响。实验结果表明, 复合材料由TiB₂、SiC和Si相组成。SiC/TiB₂复合材料的显微组织特征为: TiB₂相和SiC相均匀分布, 游离Si填充在TiB₂相和SiC相的空隙处, 且形成了连续相。随成形压力的增大, 复合材料中游离Si含量降低, TiB₂颗粒尺寸减小, 复合材料的力学性能先增加后降低。坯体最佳成形压力为200 MPa, 对应SiC/TiB₂复合材料的体积密度、开口气孔率、抗弯强度、断裂韧性和维氏硬度分别为3.63 g/cm³、0.90%、(354±16) MPa、(6.8±0.2) MPa·m^1/2和(21.0±1.1) GPa。     

SiC thin films obtained by Si carbonization

This work reports the fabrication and characterization of SiC thin films obtained by carbonization of Si substrates with a recently designed and fabricated hot-wall reactor by rapid thermal chemical vapor deposition. The reactor design is less complex than other ones previously reported, but it permits to reach a similar SiC material crystalline quality. The composition and structure of the fabricated thin films have been assessed by scanning electron microscopy and transmission electron microscopy. The structural quality of the obtained films has been optimized as a function of the temperature ramp rate, flowing gases, Si surface pre-treatment and sample holder design. SiC obtained layers on Si exhibit a flat free surface, they mainly consist of highly oriented 3C-SiC and some hexagonal SiC inclusions have been detected.     

金刚石表面形成Ti3SiC2的反应机理

采用Ti、Si、TiC、金刚石磨料为原料,通过放电等离子烧结(SPS),制备了Ti3SiC2陶瓷结合剂金刚石材料.研究结果表明,Ti-Si-2TiC试样经SPS加热的过程中位移、位移率和真空度在1200℃时发生明显变化,表明试样发生了物理化学变化.XRD分析结果表明1200℃时试样发生化学反应生成了Ti3SiC2.随着温度升高,试样中Ti3SiC2含量逐渐增加.当烧结温度为1200℃、1300℃、1400℃和1500℃时,产物中Ti3SiC2含量分别为65.9%、79.97%、87.5%和90.1%.在Ti/Si/2TiC粉料中添加适量的金刚石5%和10%进行烧结,并未抑制Ti3SiC2的反应合成.SEM观察表明,金刚石与基体结合紧密,同时其表面生长着发育良好的Ti3SiC2板条状晶粒.提出了一种金刚石表面形成Ti3SiC2的机制,即金刚石表面的碳原子首先与周围的Ti反应生成TiC,然后TiC再与Ti-Si相发生化学反应,生成Ti3SiC2.     

原位生成TiC/Ti5Si3纳米复合材料的显微结构研究

研究了原位生成ＴｉＣ／Ｔｉ_５Ｓｉ_３纳米复合材料的显微结构．实验结果表明,以ＳｉＣ和Ｔｉ 为原料,通过反应热压工艺可以原位合成ＴｉＣ／Ｔｉ_５Ｓｉ_３复合材料,其中的大部分ＴｉＣ粒子为纳 米粒子．ＴｉＣ晶粒与Ｔｉ_５Ｓｉ_３晶粒的晶界上存在原子台阶．复合材料还含有少量Ｔｉ_３ＳｉＣ_２相．这 些Ｔｉ_３ＳｉＣ_２相主要呈棒状分布在Ｔｉ_５Ｓｉ_３基体中,另有少量Ｔｉ_３ＳｉＣ_２相位于大的ＴｉＣ晶粒内．     

添加Al对MSI制备C/C-SiC复合材料组织和力学性能的影响

以聚丙烯腈( PAN) 基炭纤维(Cf ) 针刺整体毡为预制体, 用化学气相渗透(CVI) 法制备炭纤维增强炭基体(C/ C) 的多孔坯体, 采用熔融渗硅(MSI) 法制备C/ C-SiC 复合材料, 研究了渗剂中添加Al 对复合材料组织结构和力学性能的影响。结果表明: C/ C 坯体反应溶渗硅后复合材料的物相组成为SiC 相、C 相及残留Si 相。随着渗剂中Al 量的增加, 材料组成相中的Al 相也增加而其它相减少; SiC 主要分布在炭纤维周围, 残留Si 相分布在远离炭纤维处, 而此处几乎不含Al ; 当渗剂中Al 量由0 增加到10 %时, 复合材料的抗弯强度由116. 7 MPa 增加到175. 4 MPa , 提高了50. 3 % , 断裂韧性由5. 8 MPa·m1/2增加到8. 6 MPa·m1/2 , 提高了48. 2 %。Al 相的存在使复合材料基体出现韧性断裂的特征。      

熔融渗透工艺制备SiC-TiSi2复相陶瓷的反应机理

熔融Si渗透过程伴随着复杂的化学反应及多组分扩散,对该过程进行研究有助于更好地理解熔渗反应机理。本工作采用熔融渗透工艺制备SiC-TiSi2复相陶瓷,在生成SiC基体的同时原位生成TiSi2。通过扫描电子显微镜(SEM)、X射线能谱分析(EDS)和微区X射线衍射(micro-beam XRD)分别对熔融硅区域、Si/SiC界面以及SiC基体的微观结构和相组成进行表征和分析,研究了熔渗工艺制备SiC-TiSi2的反应机理。结果表明:高温下液Si渗入C-TiC预制体,发生化学反应生成SiC、TiSi2以及少量副产物Ti5Si3,其中Ti5Si3主要集中于Si/SiC界面处。随着反应进行,液Si与TiSi2形成液态Ti-Si共晶。该液态共晶通过流动扩散在Si区域中析出TiSi2。而预制体中的少量固态C在液Si中溶解、扩散,并在Si区域生成均匀分布的孤立SiC颗粒。     

SiC/SiC and SiC/Kovar joining by Ni–Si and Mo interlayers

A composite joining technique, using a Ni–56Si filler alloy and Mo as interlayers, was used to join SiC to SiC and to Kovar. The wetting of the Ni–Si alloy on SiC ceramic was studied in a vacuum at 1,350 °C by the sessile drop technique as a function of time; the non-reactive wetting characteristics in the Ni–Si/SiC system were confirmed, with an equilibrium contact angle of about 23°. SiC/SiC joints were fabricated by two processes using a Ni–Si/Mo/Ni–Si structure as the interlayer. SiC/Kovar joints were produced by means of a multilayer structure: molybdenum, which is used as the interlayer, was joined to Kovar on one side by means of transient liquid phase bonding and to SiC on the other side, using a Ni–Si coating as a filler alloy. The resulting joints were analyzed and discussed in terms of joint morphology and microstructure, joint strength, and fracture behavior. Two interfacial layers form at the Kovar/Mo and the Mo/Ni–Si interfaces due to dissolution and interdiffusion phenomena between the metallic elements, without there being any observable reactions with the SiC component. The type of joining process and the experimental conditions used play a key role in determining the joint microstructure and composition, the joint strength and its fracture behavior.