激光诱导气相合成Si_3N_4/SiC纳米粉试验工艺研究

选择廉价的六甲基乙硅胺烷((Me_3Si)_2NH)和氨气为原料,采用正交设计试验参数的方法,研究了激光功率密度、六甲基乙硅胺烷蒸汽流量、氨气流量对粉体化学组成的影响。制备出低游高碳含量,高Si_3N_4含量的Si_3N_4/SiC粉体。研究结果表明,在选定的试验参数范围内,形成的粉体中Si_3N_4、SiC的相对含量在较大的范围内变化。对Si_3N_4粉体中Si_3N_4的形成影响最大的是激光功率密度,六甲基乙硅胺烷流量对其影响最小。通过试验得到了高Si_3N_4含量,低游离碳含量的Si_3N_4/SiC纳米粉形成的最佳工艺参数。     

Si_3N_4对PTC瓷料性能的影响

研究了 Si_3N_4掺杂 BaTiO_3基 PTC 瓷料的显微结构和性能。掺 Si_3N_4的瓷料中出现了大量特征的棒状晶体。Si 富集在棒状晶体中。与掺 SiO_2的瓷料比较,Si_3N_4掺杂的 PTC 瓷料显示了更多的优越性。     

反应烧结Si_3N_4的热震断裂行为

研究了四种 Si_3N_4材料的热震断裂行为。分析了材料的断裂参数(K_(1c)、γ(?) 和 σ(?))与抗热震参数之间的关系。无掺杂的 Si_3N_4-B_0的临界抗热震温差ΔT_c 为400℃左右,其热震裂纹以准静态的方式扩展。含 BN 的 Si_3N_4-B_1,Si_3N_4-B_2和 Si_3N_4-B_2材料的临界抗热震温差较高(ΔT_c=500℃),并在该温度点发生动态的裂纹扩展。当热震温差进一步提高,BN 含量较高的 Si_3N_4-B_2以及含有 BN 和 C 的 Si_3N_4-B_3在一定的温差增量范围内(ΔT_c′-ΔT_c=200℃)保持了裂纹的稳定;然后随着温差的继续扩大,其裂纹又逐步扩展。BN 含量较低的Si_3N_4-B_1则不同,其热震裂纹在作了动态扩展之后紧接着再逐步扩展。在实验的基础上阐明了 Si_3N_4材料的热震断裂行为是由抗热震参数控制的,而抗热震参数是断裂参数的函数。指出 Si_3N_4-B_2是适用于力学和热学环境下的结构材料,因其具有较高的抗热震参数和较小的热震强度衰减率。     

纳米SiC纤维改性短切碳纤维增强Si_3N_4陶瓷介电响应及吸波性能

以甲基三氯硅烷为原料,采用催化化学气相沉积(CCVD)工艺在短切碳纤维(C_(fd))表面制备了纳米SiC纤维(nano SiC_f)改性层,并采用凝胶注模-无压烧结工艺制备了nano SiC_f-C_(fd)/Si_3N_4和C_(fd)/Si_3N_4复合材料。使用矢量网络分析仪研究了nano SiC_f-C_(fd)和C_(fd)对Si_3N_4陶瓷在X波段(8.2~12.4GHz)的介电响应和吸波性能的影响。结果表明:nano SiC_f-C_(fd)/Si_3N_4和C_(fd)/Si_3N_4复合材料的复介电常数和介电损耗角正切值(tanδ)均随纤维添加量增加而增大;相同纤维含量时,nano SiC_f-C_(fd)/Si_3N_4复合材料的介电常数实部比C_(fd)/Si_3N_4复合材料有所降低,但损耗角正切升高。反射损耗结果表明:nano SiC_f-C_(fd)/Si_3N_4复合材料拥有更优的电磁波吸收效果。nano SiC_f-C_(fd)含量为2wt%、d=2.5mm时,出现最大吸收峰-14.95dB,反射损耗优于-5dB,波段频宽达3.5GHz。nano SiC_f界面改性能有效提高C_(fd)/Si_3N_4复合材料的吸波性能。     

激光法合成参数对 Si_3N_4 超细粉成分及表征性能的影响

本文研究了合成参数对形成 Si_3N_4超细粉化学成分、比表面积、粒径尺寸分布、形貌及结构等方面的影响规律。得出了制备粒径小于50nm,Si_3N_4含量大于96wt-%的最佳合成参数范围。     

g-C_(3)N_(4)/CeO_(2)/BiOBr三相复合材料的制备及其可见光催化降解RhB性能EI北大核心

以多孔层状g-C_(3)N_(4)为基体,引入沉淀法所得的CeO_(2)/BiOBr复合材料,经超声搅拌制得具有异质结结构的g-C_(3)N_(4)/CeO_(2)/BiOBr三相复合材料。采用X射线衍射、扫描电子显微镜、透射电子显微镜、X射线光电子能谱、紫外-可见光漫反射光谱、光致发光光谱等方法对g-C_(3)N_(4)/CeO_(2)/BiOBr进行了成分、结构和光学性质表征。结果表明:g-C_(3)N_(4)/CeO_(2)/BiOBr三相复合材料呈三明治层状堆叠结构,界面结构构建良好,光响应性能优异,各相分布均匀且结晶程度较高。当Ce∶Bi摩尔比为1∶1,g-C_(3)N_(4)质量分数为15%时所得三相复合材料表现出最高的光催化活性,RhB降解率高达99%,降解速率是纯相CeO_(2)的86倍、纯相BiOBr的3倍。此外,经过4次循环后,复合材料的RhB降解效率依然保持在89%,表现出良好的稳定性。     

低温 Hg 敏化光化学气相淀积 Si_3N_4薄膜

用光-CVD 技术,在100—200℃的温度下,在 Si 片上淀积出了 Si_3N_4薄膜。本文研究了薄膜淀积速率与淀积参数的关系,讨论了淀积的 Si_3N_4薄膜的物理性质、化学性质和力学性质。     

Fe_2(MoO_4)_3/Si_3N_4复合粉末还原过程中的微观组织结构

观测Fe_2(MoO_4)_3和Fe_2(MoO_4)_3/Si_3N_4粉末H_2还原后的微结构特征,研究了其微观组织结构的演变。结果表明:Fe_2(MoO_4)_3还原后转变为20 nm厚的Fe薄层包覆Mo颗粒的微结构;Fe_2(MoO_4)_3/Si_3N_4粉末被还原后转变为两种结构形式颗粒粉末,一种为3-5nm的薄层Fe包覆在Mo颗粒表面粉末,一种为粘附有纳米Fe-Mo氮化物、Si、Mo等颗粒的Si_3N_4粉末。Fe_2(MoO_4)_3/Si_3N_4粉末还原后形成这种微结构的原因是,在还原过程中同时发生了两种反应:一种是Fe_2(MoO_4)_3自身发生分解还原反应,另一种是Fe_2(MoO_4)_3与Si_3N_4颗粒表面发生反应。     

Si_2N_2O-Al_2O_3-La_2O_3和Si_2N_2O-Al_2O_3-CaO系统的亚固相关系

本文给出了 Si_2N_2O-Al_2O_3-La_2O_3和 Si_2N_2O-Al_2O_3-CaO 系统的亚固相图。实验结果表明:在 Si_2N_2O-Al_2O_3-CaO 系统中有一个未知结构的新化合物 CaO·Si_2N_2O,在3CaO·Si_2N_2O 和3CaO·Al_2O_3两化合物之间形成连续立方固溶体。而 Si_2N_2O-Al_2O_3-La_2O_3系统中则没有发现新化合物。在两个系统的富 Si_2N_2O区,过量的 Si_2N_2O 与 La_2O_3和 CaO 分别反应形成 Si_3N_4与 La_(10)[SiO_4]_(?)N_2(H-相)(和 CaSiO_3。所研究的这两个三元系统中,分别形成了如下几个四元相容性区。在 Si_2N_2O-Al_2O_3-La_2O_3系统内有:H-Si_3N_4-La_2O_3·Si_2N_2O-La_2O_3·Al_2O_3;H-Si_3N_4-La_2O_3·Al_2O_3-La_2O_3·11 Al_2O_3;H-Si_3N_4-La_2O_3·11 Al_2O_3-Al_2O_3;H-Si_3N_4-Al_2O_3-O′s.s;H-Si_3N_4-O′s.s-Si_2N_2O在 Si_2N_2O-Al_2O_3-CaO 系统内有:Si_3N_4-CaSiO_3-CaO·Si_2N_2O-3CaO·Al_2O_3;Si_3N_3-CaSiO_3-3CaO·Al_2O_3-2CaO·Al_2O_3·SiO_(?);Si_(?)N_(?)-CaSiO_3-2CaO·Al_2O_3·SiO_2-Al_2O_3;Si_3N_4-CaSiO_3-Al_2O_(?)-O′s.s;Si_3N_4-CaSiO_3-O′s.s-Si_(?)N_(?)O     

晶界相预合成对 Si_3O_4 陶瓷显微结构和高温力学性能的影响

将 Si_3N_4原料预热处理以及预合成 Si_3N_4-Al_2O_3-Y_2O_3系的晶界相,两者按适当比例制得的坯体,经无压烧结得到相对密度>97%的 Si_3N_4烧结体。其高温抗弯强度达400MPa(1300℃下),比通常工艺(未经处理的)制得的强度相应提高约25%。用 SEM、TEM 对这种 Si_3N_4试样的显微结构进行了研究分析,解释了强度提高的原因。     

Improved sinterability of SiC(w)/Si3N4 composites by whisker-oriented alignment

Effects of whisker-oriented alignment on sintering behavior of SiC(w)/Si₃N₄ composites were investigated by sintering curves of SiC(w)/Si₃N₄ compacts with different whisker alignment (random and unidirectionally oriented). It was found that whisker-oriented alignment led to considerable anisotropy of linear shrinkage and sintering rate of SiC(w)/Si₃N₄ compacts during sintering. Whisker-oriented alignment could improve the sinterability of the SiC(w)/Si₃N₄ composites and consequently improve the maximum fraction of whisker addition and reduce the content of sintering additives without obviously lowering the density, which are more beneficial to exert the best reinforcing effects of whiskers and mechanical properties of SiC(w)/Si₃N₄ composites at elevated temperature.     

Investigation of the physical and mechanical properties of hot-pressed machinable Si3N4/h-BN composites and FGM

A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si₃N₄ ceramics. Silicon nitride/hexagonal boron nitride (h-BN) ceramic composites and functionally graded materials were fabricated by hot pressing at 1750 °C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si₃N₄/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. The texture of h-BN and β-Si₃N₄ was observed during hot pressing sintering. Physical and mechanical properties of Si₃N₄/h-BN with different content of h-BN were investigated, such as bulk density, Vickers's hardness, flexural strength, and elastic modulus. All of these properties are important for the design of the machinable Si₃N₄/h-BN FGM (Functionally Graded Materials).     

The effect of Si3N4 on the thermal expansion behavior of MoSi2

The effect of Si₃N₄ particulates on the thermal expansion coefficient (CTE) of MoSi₂ was investigated. It was observed that as the volume percent of Si₃N₄ increases, the CTE of the MoSi₂-Si₃N₄ composites decreases. In the temperature range 1000–1500 °C, typical of that required for glass melting, about 30–35 vol% Si₃N₄ particulates are needed in the MoSi₂-Si₃N₄ composites such that the CTE of the composite matches the CTE of Mo.     

原位引入BN-SiC燃烧合成Si3N4-BN-SiC复合材料

Si₃N₄-BN-SiC复合材料以其良好的力学性能和抗氧化性能而具有良好的工程应用前景。本研究以Si、Si₃N₄稀释剂、B₄C和Y₂O₃为原料, 采用燃烧合成法成功制备了Si₃N₄-BN-SiC复合材料。通过Si、B₄C和N₂气之间的反应, 在Si₃N₄陶瓷中原位引入BN和SiC, 制备的Si₃N₄-BN-SiC复合材料由长棒状的β-Si₃N₄和空心球形复合材料组成。实验研究了空心球微结构的形成机理, 结果表明, 生成的SiC、BN颗粒及玻璃相覆盖在原料颗粒上, 当原料颗粒反应完全时, 形成空心球形微结构。并进一步研究了B₄C含量对Si₃N₄-BN-SiC复合材料力学性能的影响。原位引入SiC和BN在一定程度上可以提高复合材料的力学性能。当B₄C添加量为质量分数0~20%时, 获得了抗弯强度为28~144 MPa、断裂韧性为0.6~2.3 MPa·m ^1/2, 杨氏模量为17.4~54.5 GPa, 孔隙率为37.7%~51.8%的Si₃N₄-BN-SiC复合材料。     

Effect of the microstructure of Si3N4 on the adhesion strength of TiN film on Si3N4

The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si₃N₄ was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase ( or β) of starting Si₃N₄ powder, and sintering temperature. The microstructure of Si₃N₄ was characterized in terms of grain size, aspect ratio of the elongated grain, and β-to- phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si₃N₄ were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si₃N_4, such as critical loads for yield (P_y) and for ring cracking (P_c). The effect of the microstructure of Si₃N₄ on adhesion strength evaluated by scratch test is discussed. TiN films on Si₃N₄ showed high adhesion strengths in the range of 80–140 N. Hardness and the P_y of Si₃N₄ substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si₃N₄, substrates with finer grain sizes and higher phase ratios, which show high hardness and high P_y, were suitable for higher adhesion strength of TiN film.     

添加BaTiO3的热压烧结Si3N4陶瓷的力学和介电性能

本研究以Al₂O₃和Nd₂O₃为烧结助剂, 采用热压烧结法制备Si₃N₄陶瓷, 系统研究了添加BaTiO₃对Si₃N₄陶瓷力学和介电性能的影响。研究结果表明, 随着BaTiO₃含量的增加, 相对密度、抗弯强度和维氏硬度都随之降低, 而断裂韧性有所升高; 即使添加5wt%~20wt%的BaTiO₃, Si₃N₄陶瓷的抗弯强度依然可以保持在600 MPa以上。Si₃N₄陶瓷的介电常数可以提高到9.26~11.50, 而介电损耗保持在10^-3量级。在Si₃N₄陶瓷中未检测到BaTiO₃结晶相, 可以认为Si₃N₄陶瓷介电常数的提高主要来源于烧结过程中形成的TiN。这些结果有助于拓展Si₃N₄陶瓷的应用领域。     

Creep behavior of SiC fiber-reinforced hot-pressed Si3N4 composites

The creep response of SiC fiber-reinforced Si₃N₄ composites has been measured using four-point flexural loading at temperatures of 1200–1450°C and stress levels ranging from 250 to 350 MPa. Parameters characterizing the stress and temperature dependence of flexural creep strain rates were determined. A numerical analysis was also performed to estimate the power-law creep parameters for tensile and compressive creep from the bend test data. The incorpoporation of SiC fiber into Si₃N₄ resulted in substantial improvements in creep resistance even at very high stresses. The steady-state creep deformation mechanism, determined to be subcritical crack growth in the unreinforced matrix, changed to a mechanism in the composites of repeated matrix stress relaxation-fiber rupture-load dispersion by the matrix. Multiple fiber fracture rather than multiple matrix cracking resulted. The tertiary creep in the composite resulted from the rapid growth of the microcracks which initiated from the fiber rupture sites. Fiber strength, matrix cracking stress and interfacial shear strength have been identified as the key microstructural parameters controlling the creep behavior of the composite.     

A study on thermal emission of charges at Si3N4—GaAs interfaces after annealing in N2 and N2 + H2 mixtures

Si₃N₄—GaAs interfaces subjected to annealing in N₂ + H₂ mixture or pure N₂ atmosphere were investigated by a small-signal charge deep-level transient spectroscopy (Q-DLTS) method. The method measures the physical parameters of selective populations of the interface traps continuum. A dependence of the capture cross-section on activation energy was constructed for the continuum of interface states at the Si₃N₄—GaAs interface. The dependence shows an exponential character in the part of the gap ranging from 0.3 to 1.0 eV below the conduction band minimum. It was found that annealing in the temperature interval 400–450 °C reduces the zero-bias band bending by about 0.1 eV. At temperatures of 500 °C and more, degradation of the interface started; compared with annealing in pure N₂ ambient, annealing in an N₂ + H₂ mixture degraded the interface slightly more.     

氮化硼纳米管增强氮化硅复合材料的裂纹扩展阻力行为

用氮化硼纳米管(BNNT)增强氮化硅(Si₃N₄)陶瓷制备了BNNT/Si₃N₄复合材料, 利用三点弯曲强度及单边切口梁(SENB)法测定了BNNT/Si₃N₄复合材料的弯曲强度和断裂韧性。通过SEM观察了BNNT/Si₃N₄复合材料微观形貌。基于BNNT增强Si₃N₄陶瓷复合材料的裂纹扩展阻力计算公式, 构建了BNNT对Si₃N₄陶瓷裂纹屏蔽区的裂纹扩展阻力的数学模型。用该模型的计算结果与Si₃N₄陶瓷的裂纹扩展阻力进行了对比。结果表明: BNNT/Si₃N₄复合材料的弯曲强度和断裂韧性明显高于Si₃N₄陶瓷, 说明BNNT对Si₃N₄陶瓷的裂纹扩展有阻力作用, 摩擦拔出是Si₃N₄陶瓷抗裂纹扩展能力提高的主要原因; BNNT对Si₃N₄陶瓷有明显的升值阻力曲线行为。通过有限元模拟裂纹尖端应力分布, 发现BNNT使Si₃N₄陶瓷裂纹尖端的最大应力转移到纳米管上, 而且BNNT降低了Si₃N₄陶瓷裂纹尖端的应力, 对Si₃N₄陶瓷尖端的裂纹有屏蔽作用, 从而提高了Si₃N₄陶瓷的裂纹扩展阻力。     

Si3N4/FePd/Si3N4薄膜的晶体结构和磁性能研究

采用磁控溅射的方法制备了Si₃N₄/FePd/Si₃N₄三层膜, 研究了非磁性材料Si₃N₄作为插入层对磁记录FePd薄膜结构与磁性能的影响。结果表明, 热处理后Si₃N₄分布在FePd纳米颗粒之间, 抑制了FePd晶粒的生长, 与纯FePd薄膜相比, Si₃N₄/FePd/Si₃N₄薄膜的颗粒明显得到细化; 通过添加Si₃N₄层, FePd薄膜的晶体学参数c/a从0.960减小到0.946, 表明Si₃N₄可以有效促进FePd薄膜的有序化进程, 同时提升了矫顽力和剩磁比, 分别提高到249 kA/m、0.86; 随着600℃退火时间的进一步延长, 添加Si₃N₄的薄膜磁性没有迅速下降, 在较宽的热处理时间范围内磁性能保持在比较高的水平, 提高了抗热影响的能力。Si₃N₄作为插入层对FePd薄膜的磁性能具有较大的提升作用, 这对磁记录薄膜的发展具有重要意义。