SiC颗粒增强镁基复合材料界面稳定性的第一性原理计算

针对SiC颗粒增强镁基复合材料界面的理论研究较少,大多研究仅停留在表征层面等问题.本文采用第一性原理方法,计算了四种不同SiC (0001)/Mg (0001)界面模型的电荷密度、布局分析和界面分离功.结果 表明:对于同种终端的SiC(0001)/Mg (0001)界面模型中,顶位型结构比心位型结构的稳定性好;不同终端...     

超声波振动下SiC陶瓷颗粒与Zn-Al液态合金的相互作用机制EI北大核心CSCD

采用半固态机械搅拌结合超声波振动过程制备了SiC颗粒增强的Zn-Al合金复合材料,研究了超声波作用下SiC陶瓷颗粒与液态Zn-Al合金的相互作用机制。结果表明,超声波作用在Zn基复合材料上可以在液态Zn-Al合金内产生强烈的空化作用,空化作用可以破坏Zn-Al合金表面的氧化物及陶瓷颗粒表面吸附的气膜,使Zn-Al合金润湿裸露的SiC陶瓷颗粒表面并形成冶金结合。在超声振动下,SiC颗粒与Zn-Al合金界面非常平直且洁净,没有孔洞存在,是一种润湿型结合界面。     

基于第一性原理的α-Fe(001)/Mo2FeB2(001)界面性能的研究

采用第一性原理方法,研究了α-Fe(001)/Mo_2FeB_2(001)界面性能。建立了4种不同的原子堆垛方式界面模型,计算了其界面粘附功、界面结合能和断裂功。结果表明,以空心位置堆垛的Fe终端界面性能最稳定,而顶部位置的Fe+B终端界面性能最不稳定,两者的裂纹断裂均趋向于发生在基体相或硬质相内。在此基础上,进一步分析了空心位置Fe终端界面模型和顶部位置Fe+B终端界面模型的电子结构,电荷差分密度图显示在空心位置的Fe终端界面系统中,界面处Fe原子与Fe原子间形成金属键,界面处Fe原子与Mo原子间形成金属键。在顶部位置的Fe+B终端界面系统中,界面处的Fe原子与B原子间形成共价键,但界面强度比空心位置的Fe终端界面系统低。分态密度显示界面处原子间重新排列,发生杂化,形成共价键,揭示了界面成键特性。     

增强体含量对SiC_W/SiC层状陶瓷复合材料强韧性的影响

采用流延-化学气相渗透(TC-CVI)工艺制备SiC晶须(SiC_W)/SiC层状陶瓷复合材料,研究了SiC_W含量对层状陶瓷复合材料力学性能和微观结构的影响,探讨了SiC_W/SiC层状陶瓷复合材料的强韧化机制。结果表明:TC-CVI工艺能够有效提高复合材料中晶须含量(40vol%),减少制备过程对晶须损伤,所制备的SiC_W/SiC层状陶瓷复合材料具有合适的层内及层间界面结合强度。随着SiC_W含量增加,层状陶瓷复合材料的密度和力学性能均有明显提高。含40vol%晶须的SiC_W/SiC层状陶瓷复合材料的密度、弯曲强度和断裂韧性均比含25vol%晶须的分别提高了8.4%、30.8%和26.7%。断口形貌中能够观察到层间及层内的裂纹偏转,层内的裂纹桥接和晶须拔出等,这些为主要的增韧机制。高含量SiC_W及合适的层间和层内界面结合强度,对提高SiC_W/SiC层状陶瓷复合材料强韧性有明显作用。     

复合界面层对SiCf/SiC复合材料力学损伤行为的影响

SiC_f/SiC陶瓷基复合材料在航空航天领域具有广阔的应用前景,其界面层设计是研究重点。研究表明,复合界面层可以有效提升陶瓷基复合材料的抗氧化性能,但其对材料力学性能及损伤机制的影响尚不明确。本研究利用化学气相渗透法(CVI)制备得到具有BN及(BN/SiC)₃复合界面层的小复合材料,探究了复合界面层对SiC_f/SiC复合材料失效机制的影响。基于两种力学加载实验结合声发射探测分析了两种界面层小复合材料的损伤过程。实验结果表明,利用CVI制备的小复合材料界面结构清晰,基体致密。两类小复合材料均具有SiC_f/SiC陶瓷基复合材料的典型力–位移曲线,不同界面层小复合材料损伤过程具有不同的力声特征。通过两类力学加载试验的声发射特征能够有效分析小复合材料各阶段损伤发展情况。本实验中BN及(BN/SiC)₃复合界面层SiC_f/SiC小复合材料最大承受载荷分别为139和160 N,复合界面层小复合材料中的多层界面具有更强的偏转裂纹能力,降低裂纹延伸至纤维的速度,进而提高...     

ZnO压敏陶瓷与有机绝缘材料间界面电荷及其作用

用热刺激电流和电声脉冲法研究了ＺｎＯ压敏陶瓷与绝缘材料间界面电荷分布。发现绝缘材料显著地影响界面电荷性质和密度，压敏陶瓷酯型聚氨酯界面存在大是电荷，而压敏陶瓷与聚酯改性有机硅界而没有电荷。     

基体合金元素对SiC/Al界面结合影响的第一性原理及实验研究

采用基于密度泛函理论的第一性原理及实验相结合的方法, 探讨了Al基体中分别掺杂Mg、Si、Cu合金元素对SiC/Al界面结合的影响, 重点考察了合金元素在界面偏聚时的电子结构和成键情况。研究表明: 在未掺杂Al/SiC体系界面结构优化时, 以Si原子为终止面的Brigde结构是最稳定的结合方式; 当合金元素分别替换界面处的Al原子后, 界面处原子的分波态密度、Muliken电荷及成键原子集居数等电子结构参数均有不同程度的变化, 这不仅增加了界面处 Si与Al原子结合, 同时也增强了界面处和亚界面处的Al基体和SiC增强相原子之间的相互作用, 使体系更加稳定, 界面黏着功均有不同提升; 其中掺Mg提升效果最明显, 其次为掺Cu和掺Si; 利用第一性原理计算的掺杂Al/SiC体系黏着功和实验值较为接近且变化规律相同。     

活性填料在先驱体转化陶瓷基复合材料及其构件中有应用

有机聚合物先驱体由于其分子可设计性好、成型方便、低温裂解转化为陶瓷的特点,在陶瓷纤维和纤维增韧陶瓷基复合材料的制备中表现出极大的优势。针对先驱体转化法体积收缩大、孔隙率高的不足,在先驱体中引入适当的活性填料是解决其不足的有效方法。以聚碳硅烷(PCS)为 SiC 陶瓷的先驱体,以金属(Al、Cr、Mo、Ta、Ti、W、Zr)、非金属(B、Si)及其化合物(CrSi_2、TiH_2、TiB_2)微粉为活性填料,对活性填料控制的 PCS 裂解陶瓷的陶瓷产率、线收缩率、密度、力学性能、产物结构等进行了系统的研究。对活性填料控制的 PCS 先驱体裂解陶瓷的尺寸变化进行了模型分析。从理论上揭示了活性填料的临界体积分数与活性填料反应的产率、反应前后的密度比之间的相关性。通过对 PCS/Al/SiC/N_2裂解体系反应热力学参数的计算,从理论上预测可能发生的反应与不可能发生反应。将热分析反应动力学方法应用到活性填料 Al 控制的 PCS 先驱体的裂解中。分别研究了 N_2中纯 PCS、纯 Al、Al/SiC、PCS/Al/SiC 体系各阶段的表观活化能及其裂解-反应机理函数以 X 射线衍射,电子显微镜元素线扫描等方法对 PCS/Al/SiCN_2体系的裂解-反应机理进行研究。在含活性填料的先驱体转化法制备纤维复合材料的致密化-密度增长模型中,引入体积收缩率的参数,对先驱体转化法纤维复合材料的密度增长模型进行了修正。将活性填料 Al 应用到单向 M40JB 纤维和吉林碳纤维增强 SiC 陶瓷基复合材料及反坦克导弹陶瓷喷管的制备工艺中,并对裂解产物进行了表征,获得很好的效果。这一结果对促进先驱体转化法陶瓷构件在武器装备中的应用具有重要意义。     

Ti-Si共晶钎料的制备及其对SiC陶瓷可焊性

采用海绵钛和单晶硅为原料,通过非自耗电弧熔融技术制备91.5Ti-8.5Si(wt%)和22Ti-78Si(wt%)共晶钎料.在真空中1400℃×10min条件下,研究了共晶钎料对SiC陶瓷的润湿性能和界面反应.结果表明:两种共晶钎料对SiC陶瓷均具有良好的润湿性能,润湿角分别约为10°和25°.在润湿实验后,91.5Ti-8.5Si(wt%)钎料和SiC陶瓷分离,22Ti-78Si (wt%)钎料和SiC陶瓷之间结合紧密.在润湿实验的温度制度下,用22Ti-78Si (wt%)钎料(厚度为0.2mm)对SiC陶瓷进行了初步连接,接头的抗弯强度为72MPa.采用SEM、EDS、XRD等检测手段对钎料的形貌和物相组成、钎料和SiC陶瓷的界面结合情况进行了表征,揭示了钎料对SiC陶瓷润湿性和界面反应的机理.     

烧结助剂种类对Si_3N_4/SiC陶瓷力学与摩擦性能的影响（英文）

分别以Y2O3-Al2O3(YA)和Y2O3-MgO(YM)为烧结助剂,采用气压烧结工艺制备了Si3N4/SiC陶瓷,研究了两种不同的烧结助剂对陶瓷的力学和摩擦性能的影响。研究结果表明:添加不同种类的烧结助剂对制备陶瓷的相对密度、抗弯强度、断裂韧性、硬度、摩擦系数和磨损率影响很大;与添加烧结助剂YM相比较,添加烧结助剂YA的Si3N4/SiC陶瓷在烧结过程中表现出了更好的烧结性能,得到的陶瓷样品最终显示了更好的力学和摩擦性能,尤其是SiC添加量为20wt%的Si3N4/SiC陶瓷。这主要归因于烧结助剂YA的添加使Si3N4/SiC陶瓷呈现出了更高的相对密度,获得的晶粒长径比更小。     

新型复式连通SiC/390Al复合材料的制备和性能

以空心多孔SiC泡沫陶瓷为增强体,用挤压铸造法制备了新型复式连通双连续相SiC/390Al复合材料,研究了泡沫陶瓷骨架筋的结构对复合材料的影响,以及复合材料中的界面对力学性能的影响.结果表明,SiC空心多孔泡沫陶瓷与390Al复合后形成了复式连通双连续相复合材料,具有独特的互穿式界面结构,材料界面的结合优异.随着复合材料界面结合的加强和泡沫增强体的复合韧化,复合材料的屈服强度、压缩强度和弯曲强度明显提高,韧性显著增强.     

重力分离SHS陶瓷内衬复合管界面现象

利用自蔓延高温合成-重力分离法制备了陶瓷内衬20碳钢、Cr25Ni20耐热钢和1Cr18Ni9Ti不锈钢高炉煤粉喷吹复合管。对其界面现象的研究表明,金属/陶瓷间的结合主要表现为机械结合。其中,在不锈钢复合管中除有更强的机械锚固效应外,还兼以由Ti元素扩散而形成的溶解和浸润结合迹象。各复合管经使用后,碳钢、耐热钢复合管界面仍表现为机械结合,而在不锈钢复合管中,钢基中Ti元素和煤粉中C元素在界面区富集,陶瓷/金属间过渡区加宽,且陶瓷一侧组织中出现TiC新相,使其结合方式逐渐向扩散结合过渡,并呈现部分反应结合迹象。      

熔盐辅助合成Dy3Si2C2包裹SiC粉体及其陶瓷的烧结行为

碳化硅陶瓷因自身优良的物理化学性能而具有广泛的应用前景.碳化硅的化学键结合特性决定了其难以烧结成型,因此如何制备高质量碳化硅陶瓷是领域内的难点之一.本研究以三元稀土碳化物Dy3Si2C2作为新型SiC陶瓷的烧结助剂,依据Dy-Si-C体系的高温相转变原位促进碳化硅的烧结致密化.采用放电等离子烧结技术,利用金属Dy与Si...     

Electric field-assisted and field-depressed segregation of reactive metals to the bonding interface in braze alloy joining

Segregation of reactive metals at the bonding interface has been observed in various ceramic and/or metal joints bonded with reactive metal-bearing braze alloys. When a d.c. of 20 mAcm⁻² is applied to the ceramic/braze/ceramic system at a brazing temperature of, say, 1373 K, the electric field assists the segregation at the braze-ceramic interface on the cathode side and suppresses the segregation at the interface on the anode side. This may imply that reactive metal atoms in the braze can migrate as a cation. E.m.f. measurement on the ceramic (AIN or ZrB₂)-metal foil systems with increasing temperature shows that a negative e.m.f. to the ceramic pole appears from about 900 K for AIN and from 500 K for ZrB₂, as does the thermally stimulated current in polymers. These temperatures coincide well with those where the electrical conductivity of AIN and ZrB₂, respectively, begins to increase with increasing temperature. Therefore, it is considered that the polarization of the ceramics may take place and assist the migration, and consequently segregation, of reactive metals in braze alloys to the braze-ceramic interface during brazing.     

First principles study of the adhesion asymmetry of a metal/oxide interface

The observed asymmetry in the interfacial adhesion between ZnO and Ag is studied using a first principles density functional approach. The interface formed when ZnO is deposited on Ag (111) is experimentally measured to be stronger than that formed when Ag is deposited on ZnO (0001) and this indicates a possible difference in bonding geometries. It is found that because the ZnO (0001) surface does not exhibit atoms which are onefold coordinated with the sub-surface layer this restricts the way Ag can bond to the surface whereas no such restriction exists, when ZnO is deposited on the Ag (111) surface. The study focuses on a particular Ag (111)/ZnO (0001) interface which has rotated epitaxy and by calculating the ideal work of separation it is found that an O-terminated interface which is onefold coordinated with an adjacent Zn layer is significantly stronger than the corresponding interface which is threefold coordinated with an adjacent Zn layer. This is consistent with the observations, since the onefold coordinated interface cannot form when Ag is deposited on to ZnO (0001). Additional calculations indicate that the stronger onefold coordinated interface can also separate leaving oxygen on the Ag surface provided that surface relaxation effects are suppressed.     

Sequential assembly of metal-free phthalocyanine on few-layer epitaxial graphene mediated by thickness-dependent surface potential

Due to strong interactions between epitaxial graphene and SiC(0001) substrates, the overlayer charge density induced by the interface charging effect is much more attenuated than that of exfoliated graphene on SiO₂. We report herein a quantitive detection of the charge properties of few-layer graphene by surface potential measurements using electrostatic force microscopy (EFM). A minor difference in surface potential is observed to mediate a sequential assembly of metal-free phthalocyanine (H₂Pc) on monolayer, bilayer and trilayer graphenes, as demonstrated by scanning tunneling microscopy (STM). In order to understand this, we further executed density functional theory (DFT) calculations which showed higher adsorption energies for Pc on thinner graphenes. In this case, we attribute the unique growth behavior of Pc to its variable adsorption energies on few-layer graphene, and in turn the layer charge variations from the viewpoint of energy minimizations. This work is expected to provide fundamental data useful for related nanodevice fabrications.      

Transient liquid phase bonding of 2124 aluminium metal matrix composite

Abstract

The transient liquid phase (TLP) bonding of particle reinforced aluminium metal matrix composites (MMCs) using copper interlayers often results in the segregation of SiC particles to the bond region, and this has the effect of producing bonds with poor mechanical strengths. In this preliminary study, the TLP bonding of a 2124 aluminium alloy MMC is investigated using nickel interlayers, and the initial results show that good bonds are produced with no effect on the SiC dispersion in the matrix. The absence of segregation is attributed to the high diffusivity of the nickel in the aluminium MMC, which produces rapid isothermal solidification at the bonding temperature. Bond shear tests show that near parent metal strengths are possible when thin nickel interlayers are used, and failure occurs at the MMC/bond interface. When thick interlayers are used, failure is predominately through the centre of the bondline.