带有尖角基体上异质形核能力的研究

目的 基体表面分别为平面、圆柱面和椭圆柱面时,研究带有尖角基体上的异质形核的难易程度。方法 基于经典形核理论,应用热力学理论方法,分析形核基体的几何特征和材料属性对临界形核功的影响规律,并对其形核能力进行比较。结论 当润湿角为锐角时,基体形核能力均随着尖角的增大而减小,在带尖角平面基体上最容易形核;当润湿角为钝角时,随着尖角的增大,基体形核能力都是先增加后减小,相互之间形核能力的关系与几何特征相关。特别地,在具有特殊几何特征的带尖角椭圆柱面基体上会发生二次形核。     

Some current challenges in clathrate hydrate science: Nucleation,decomposition and the memory effect

Among outstanding issues still to be understood regarding the clathrate hydrates are the mechanism of the processes involved in the formation and decomposition of clathrates: nucleation, decomposition, and the memory effect during reformation. The latter involves the shorter induction times required for solutions of decomposed hydrate to nucleate as compared to those for freshly prepared solutions. The formation of the clathrate hydrate phases of insoluble gases in water is accompanied by a ∼6000 fold concentration of the gas content in the solid phase compared to the aqueous phase from which it forms. The nucleation mechanism for the solid hydrate which allows the delivery of such high concentration of gas and water in one location has been the subject of much experimental and computational study. While these studies have improved our understanding of the nucleation process, many unknown aspects remain. These developments are described in this Opinion.     

Theory of Nucleation on Dislocations

Cahn's theory of nucleation on dislocationshas been analyzed thoroughly and the imperfectionof the theory in some cases has been discussed.Arevised theory has been proposed with applicationto the problem of precipitation of M(CN)phase inaustenite.     

Controlling Nucleation in Lithium Metal Anodes

Rechargeable batteries are regarded as the most promising candidates for practical applications in portable electronic devices and electric vehicles. In recent decades, lithium metal batteries (LMBs) have been extensively studied due to their ultrahigh energy densities. However, short lifespan and poor safety caused by uncontrollable dendrite growth hinder their commercial applications. Besides, a clear understanding of Li nucleation and growth has not yet been obtained. In this Review, the failure mechanisms of Li metal anodes are ascribed to high reactivity of lithium, virtually infinite volume changes, and notorious dendrite growth. The principles of Li deposition nucleation and early dendrite growth are discussed and summarized. Correspondingly, four rational strategies of controlling nucleation are proposed to guide Li nucleation and growth. Finally, perspectives for understanding the Li metal deposition process and realizing safe and high‐energy rechargeable LMBs are given.     

Atomistic-continuum simulation of nano-indentation in molybdenum

Simulations of nano-indentation in bcc molybdenum are performed using a coupled atomistic-continuum method and a multi-body interatomic potential which includes angular forces. The indenter is flat and rigid while the indented material is a single crystal having its ⦑100⦒ and ⦑111⦒ directions respectively parallel to the indentation direction, in separate simulations. Indentation is accommodated by elastic deformation of the surface, up to an indenter displacement of about 6 ?, and by nucleation of crystalline defects for deeper indents. When indented in the ⦑100⦒ direction, the crystal twins under the indenter, while indentation in the ⦑111⦒ direction produces dislocation nucleation from the stress concentration sites at the indenter edge. The critical loads for these events are computed and the nucleation mechanism is observed. The results are compared with available experimental data. This revised version was published online in June 2006 with corrections to the Cover Date.     

塑料发泡成核过程分析与模拟计算

本文从发泡塑料的成型机械和工艺要求及计算机应用的需要出发,依靠经典成核理论,根据实验现象,唯象地建立了描述成核过程的微分方程,并进行了计算机模拟。尤其注意了释压速率与成核数的关系,补充了经典成核理论在塑料发泡中应用的不足     

用热涨落理论对塑料发泡中经典成核理论应用的修正

分析了经典成核理论对塑料发泡成核的解释,并从微泡实验的实验数据出发分析了实验与理论的偏差;在应用热涨落理论,修正了经典成核理论,导出新的成核模型,并比较了修正后理论与中成核率与释压压差的函变关系的结果。     

金属中裂纹形核的一个理论模型

建立了金属中裂纹形核的一个理论模型。通过理论分析，获得金属中裂纹的形核准则，定义了与材料晶粒和脆性沉淀物尺寸有关的材料影响参数，并与其他研究者的结果进行了对比分析结果表明，本文提出的模型有更明确的物理意义，形核准则较好地反映了形核反应力与材料组织的对应关系。     

甲基丙烯酸-2-羟丙酯/苯乙烯分散共聚合的研究

采用梳型结构分散剂,对亲水性单体甲基丙烯酸－2－羟丙酯（HPMA）的分散聚合进行了研究。通过反应在不同结构分散剂存在下表现的不同的动力学特征,提出了反相微悬浮成核机理。并通过HPMA与共聚单体苯乙烯（St)分散共聚合动力学规律的研究,证实了这种成核机理的存在。认为亲水性单体HPMA的分散聚合存在两种机理,均相成核机理和反相微悬浮机理,当采用F－1分散剂时倾向于遵循反相微悬浮成核机理。而且共聚单体苯乙烯,反应比例越小越倾向于反相微悬乳机理,当采用F－2分散剂时反应倾向于均相成核机理,而且苯乙烯单体共聚的引入,使聚合速率变快,有利于提高体系的稳定性。     

气体水合物超声结晶实验

实验中将15kHz超声波引入R141b和水体系,反应过程在恒温水浴中进行,对超声探头处于不同位置、不同过冷度情况下分别研究.结果表明在超声启动后5秒内就出现大量水合物浆,成核对过冷度的依赖程度降低,但结晶完成时间随过冷度的增加而减少.在同样条件下,探头位于两相界面处,消耗的能量较低.分析结果认为这可能与超声空化产生的一系列效应有关.     

细胞低温保存过程中冰晶成核的研究进展

细胞低温保存过程中,胞内冰损伤是其主要损伤因素之一,这可以通过控制冰晶成核来降低。本文总结了物理、化学及生物控制成核方法在低温保存领域的应用,归纳了各类控制成核方法的机理及对细胞产生的影响,对比得出生物及化学试剂控制成核方法较为优异。并提出最佳成核温度的概念,在该温度下,胞外冰产生且引导细胞适当脱水,减少胞内自由水,降低胞内冰的产生几率,为控制成核在细胞低温保存领域的应用指明了方向。     

奥氏体形变对铁素体形核动力学及生长动力学的影响~*

运用金相法测定了 Fe-Mn-C 合金铁素体相在形变的奥氏体界面的形核动力学及生长动力学。在经典非均匀形核理论的基础上给出了形核动力学方程,并对影响形核动力学的诸因素进行了讨论。形核后铁素体在形变的奥氏体界面上的生长具有一定方向性,对产生这一现象的原因进行了初步探讨。     

Self-preservation strategies during bacterial biomineralization with reference to hydrozincite and implications for fossilization of bacteria

The induction of mineralization by microbes has been widely demonstrated but whether induced biomineralization leads to distinct morphologies indicative of microbial involvement remains an open question. For calcium carbonate, evidence suggests that microbial induction enhances sphere formation, but the mechanisms involved and the role of microbial surfaces are unknown. Here, we describe hydrozincite biominerals from Sardinia, Italy, which apparently start life as smooth globules on cyanobacterial filaments, and evolve to spheroidal aggregates consisting of nanoplates. Complementary laboratory experiments suggest that organic compounds are critical to produce this morphology, possibly by inducing aggregation of nanoscopic crystals or nucleation within organic globules produced by metabolizing cells. These observations suggest that production of extracellular polymeric substances by microbes may constitute an effective mechanism to enhance formation of porous spheroids that minimize cell entombment while also maintaining metabolite exchange. However, the high porosity arising from aggregation-based crystal growth probably facilitates rapid oxidation of entombed cells, reducing their potential to be fossilized.     

Undercooling studies on dispersed bismuth droplets

In the zinc-bismuth system, a monotectic reaction occurs at 689 K and 0·6 at.% Bi composition. Rapid solidification of the as-cast monotectic alloy led to a micromorphology in which bismuth was uniformly and bimodally distributed as small droplets in the zinc matrix. Statistical analysis of the electron micrographs obtained from different transparent regions of the foils revealed that the size of most of the droplets was about 6 nm. These droplets undercooled by 132 K. An analysis of the nucleation rate measurements shows that the activation energy barrier to nucleation is of the order of 39·8 kcal/mol at the maximum undercooling.     

CVD金刚石薄膜的成核机理研究

利用热丝化学气相沉积 ,在预沉积无定形碳的硅镜面基底及表面研磨预处理的铜基底上 ,实现了金刚石薄膜的沉积 ,并由此讨论了金刚石的成核机理。研究表明 ,无定形碳是金刚石成核的前驱态 ;成核密度不仅与基底材料有关 ,更主要由基底的表面状态决定 ,基底表面状态的设计是改善成核密度的最有效的方法。     

Fretting fatigue test analysis of contact

Fretting fatigue tests conducted by Nowell and Szolwinski are analysed accurately by introducing several modifications needed to the classical formulation. With the total state of stress established, the crack‐tip stress intensity factor for a crack growing inward from the trailing edge of the contact is determined by the distributed dislocation technique. Finally, the results are correlated with local solutions for the contact stress field that enable an estimate of the crack nucleation life, and hence a characteristic material property quantifying initiation, to be found.     

聚对苯二甲酸乙二醇酯的结晶成核改性研究进展

聚对苯二甲酸乙二醇酯(PET)因其重复结构单元中含有刚性苯环而成核慢、结晶度低,从而导致综合力学性能与耐热性较差,限制了其工程化应用,故对PET进行成核改性以提高结晶速率与结晶度成为亟待解决的问题。综述了PET的4大类成核剂:无机填料、有机小分子、有机高分子及复合型成核剂;在此基础上,提出机理上不同于异相成核的"离子簇集诱导成核"的概念,即与离子共价相连的聚合物链段因离子簇集而在离子簇近围紧密堆砌,从而诱导"拥挤"链段结晶成核。无机填料类成核剂包括粘土、氧化物与氢氧化物、无机盐、Si_3N_4及碳纳米管/石墨等,其成核机理均为异相成核。有机小分子类成核剂涉及羧酸盐、二胺、双酰胺及改性山梨醇等,其中羧酸盐成核机理为离子簇集诱导成核,而其它均属于异相成核。有机高分子类成核剂分为结晶性聚合物、液晶高分子、嵌段共聚物及离子交联型聚合物等;其中前三者属异相成核,后者为离子簇集诱导成核。复合型成核剂为两种以上成核剂(或成核机理)配合使用协同促进PET结晶成核。对比发现,有机高分子与复合型成核剂效果较好,且不会引起PET的降解,为PET优良成核剂。异相与离子簇集诱导耦合高分子成核剂为PET结晶成核改性未来的重点发展方向之一。     

Al2O3陶瓷上沉积金刚石薄膜的研究

以酒精为碳源，用热丝ＣＶＤ法对不同表面状态的Ａｌ２Ｏ３衬底进行了金刚石薄膜沉积的比较，用扫描电镜，喇曼光谱和Ｘ射线衍射等方法检测了沉积出的金刚石膜的质量，并讨论了它们对成核和生长的影响。     

SiO2-MgO-Al2 O3-F系玻璃陶瓷的析晶

本文应用TEM,XRD和EDAX等技术,系统研究了SiO2-MgO-Al2O3-F系玻璃陶瓷的析晶过程.结果表明:在玻璃熔体的冷却过程中,由于弛豫作用而形成晶核,在随后的热处理过程中,中间相Mg2FB3和KAlSiO4以此核为中心析出;氟的存在促进了晶体的整体析出,而主晶相氟金云母在中间相的晶界处形核长大.     

热丝碳化促进CVD金刚石成核增强的物理机制

采用扫描电镜、X射线衍射及喇曼光谱等手段研究了HFCVD过程中热丝碳化对金刚石成核特性的影响。热丝经充分碳化后能发射出更多热电子，进而从反应气体中激发出更多原子H和CH3^ ，使金刚石晶核的孕育期大为缩短，3min内即可在未经表面预处理Si衬底上获得10^10cm^-2以上的高晶核密度。