CVD金刚石薄膜二次形核机制的研究

根据实验观察分析提出了金刚石二次形核机制，此机制认为二次形核很容易在（100）晶面上和晶界上形成，通过比较（100）面的二次形核和形成新生长台阶的系统自由能差，可知当气氛中的碳氢基团浓度较大时，粘附在基底的碳氢基团发生堆集，如果堆集碳氢基团高度尺寸较大时将形成二次晶核，也对晶界二次形核的系统自由能差进行了，结果表明晶界二次形核是自发的，将导致体系自由能的下降。     

脉中电流对2091铝锂合金动态再结晶动力学的影响

研究了脉冲电流对2091铝锂合金超塑变形中动态再结晶及动力学的影响结果表明,脉冲电流加速动态再结晶,减小形核时的平均晶粒直径.脉冲电流能加快位错墙的形成并使其角度增大,使再结晶形核率提高.脉冲电流加快位错在晶界上的攀移及消失、减小形核界面两边的能量差,降低形核界面的迁移速率及再结晶形核的长大速率分析了脉冲电流作用下的动态再结晶动力学行为     

脉冲电流对2091铝锂合金动态再结晶动力学的影响

研究了脉冲电流对2091铝锂合金超塑变形中动态再结晶及动力学的影响,结果表明,脉冲电流加速动态再结晶,减少形核时的平均晶粒,脉冲电流能加快位错墙的形成并使其角度增大,使再结晶形核率提高,脉冲电流加快位错在晶界上的攀移及消失,减少形核界面两边的能量差,降低形核界面的迁移速率及再结晶形核的长大速率,分析了脉冲电流作用下的动态再结晶动力学行为。     

微细硬质合金刀具沉积金刚石薄膜的形核研究

形核是HFCVD金刚石薄膜沉积的重要阶段,对金刚石薄膜质量至关重要。为实现金刚石薄膜在微细硬质合金刀具表面的高质量形核,选取了形核过程中的几个核心参数:热丝功率(P_f)、碳氢比(X)以及形核气压(p),设计了三因素三水平正交实验,采用SEM形貌分析、拉曼光谱分析以及EDS能谱分析等对形核尺寸、表面覆盖率和形核质量进行了研究。实验结果表明,热丝功率对形核尺寸和形核速率的影响最大,形核气压的影响最小,而对沉积质量影响较大的为热丝功率和形核气压,碳氢比的影响则最小。实验得到的最佳形核参数为:P_f=2 000 W,X=2.0%,p=1 400 Pa。     

《马氏体相变》讲座

第二讲马氏体相变形核第一讲讨论了马氏体相变的基本特征,得出其定义,这一讲将转向讨论马氏体形核机制。图2—1表示奥氏体与马氏体单位体积自由能随温度的变化。奥氏体自由能随温度降低而增加,马氏体自由能也是随着温度降低而增加,两条曲线在T_温度处相交,在T_点,单位体积马氏体与奥氏体具有相同的自由能。当温度降低到T_下时,马氏体的自由能低于奥氏体的自由能,因此比奥氏体更稳定。所以,T_0点以下,马氏体与奥氏体自由能之差△g(△g=g~M—g~A)变为负值。相变驱动力定义为-△g,所以在T_0以下驱动力就变为正值。冷     

无取向硅钢中第二相的析出行为

使用扫描电镜(SEM)和能谱仪(EDS)等手段并结合热力学和动力学计算,研究了无取向硅钢900～1000℃常化处理过程第二相的析出行为。结果表明,无取向硅钢中的第二相主要为AlN和少量MnS。AlN和MnS在不同基体相(α相、γ相及(α+γ)两相）中有三种析出形核机制(均匀形核、晶界形核和位错形核),其临界形核半径(d*)都随常化温度的提高而增大。在同一温度下,相对于其他基体相AlN在(α+γ)两相区中晶界形核的临界形核功最小,相对形核率最大,因此以晶界形核为主;而MnS在α相中位错线上临界形核半径最小,相对形核率大,开始析出温度低,因此以位错形核为主。     

X70管线钢的局部脆化

研究了X70管线钢模拟热影响区的局部脆化现象和临界再热粗晶区粒状贝氏体的形成及其对韧性的影响规律.结果表明,二次峰值温度为780℃时,M-A组元在原奥氏体晶界成链状分布;二次峰值温度为840℃时,原粗晶区晶界位置生成了细晶贝氏体带,及大颗粒的M-A组元;二次峰值温度为900℃时,M A组元弥散分布,原粗晶区的奥氏体晶界消失.在二次加热过程中奥氏体主要在原粗晶区晶界形核长大.X70钢的粗晶区出现再加热脆化现象,它的脆化温度区间是一个比Ac1-Ac3更窄的温度区间,主要在T2P=Ac1-840℃温度范围内.临界再热粗晶区脆化的主要原因是在晶界上有链状分布的M-A组元.     

直流热阴极PCVD法间歇生长模式间歇周期的研究

采用直流热阴极PCVD方法间歇生长模式,在CH4-H2气氛常规制备微米晶金刚石膜的参数条件下,利用人工干预二次形核工艺,研究了间歇周期变化对制备纳米晶金刚石膜的影响.人工干预二次形核是指通过生长温度的周期性改变而诱发二次形核行为,从而实现金刚石膜的纳米晶生长.金刚石膜周期性生长过程分为沉积阶段和干预阶段,沉积阶段主要完成金刚石膜的生长,干预阶段将沉积温度降低到600℃,然后恢复到生长温度,即完成一个生长周期.间歇周期研究主要是考察在不同间歇时间里人工干预诱导二次形核的效果,间歇时间设定为1 min、5 min、10 min、15min、20 min,生长时间设为20 min,总的沉积时间为6 h.采用拉曼光谱仪、SEM和XRD对样品进行了分析,结果表明直流热阴极PCVD方法间歇生长模式,间歇周期的变化,对二次形核的发生有诱导作用,适当选择间歇周期,有利于二次形核基团的生成.     

3004铝合金退火过程中的形核行为

研究了３００４铝合金经大变形量冷轧后进行退火时的形核行为。结果表明，在３００４铝合金再结晶退火时，主要有亚晶多边化和合并／长大形核以后粗大第二相变形区与晶界形核等形核机制。通过显微组织分析和数学计算，发现再结晶立方取向晶核的形成主要在变形基体中的三角晶界，粗大的Ａｌ（Ｆｅ，Ｍｎ）Ｓｉ和（Ｆ，Ｍｎ）Ａｌ第二相（尺寸大于２．５μｍ）周围强烈的晶格畸变区和取向紊乱区以及变形带趴有严格或近似立方取向的亚晶     

ZK60镁合金高温动态再结晶行为的研究

采用Gleeble-1500热模拟试验机进行压缩实验,研究了ZK60镁合金在变形温度为473～723K、应变速率为0.001～1s~(-1)范围内变形过程中的组织演变.分析了变形程度、变形温度、变形速率对其动态再结晶行为的影响,探讨了其动态再结晶的形核机制.结果表明:ZK60合金高温塑性变形时的主要软化机制为动态再结晶,变形温度623K,应变量超过0.24时,在原晶界处出现大量的动态再结晶晶粒,并形成易延展的剪切区.变形温度是影响ZK60合金动态再结晶晶粒尺寸的主要因素,变形温度高于623K时,动态再结晶晶粒超过25μm.ZK60合金动态再结晶晶核在晶界弓弯处形成,随着应变量增加,出现亚晶界合并长大,长条状亚晶快速长大以及在剪切带变形区形核等.     

Nucleation of Widmanstätten ferrite

Abstract

Recently published experimental data on the variation of the highest temperature at which Widmanstiittenferrite can be seen to form at a detectable rate, as a function of steel chemistry, are analysed theoretically. It is found that the data can be predicted to a fair accuracy if it is assumed that the nucleation of Widmanstiitten ferrite occurs by a mechanism similar to that of martensitic nucleation, but with the diffusion of carbon during nucleation, and if it is additionally assumed that the growth of Widmanstiitten ferrite can only be sustained when the chemical driving force exceeds a specific stored energy term.

MST/1210     

Nucleation and growth of single-walled nanotubes: the role of metallic catalysts

We present a review of experimental and theoretical results on the nucleation and growth of single-walled nanotubes, with particular emphasis on the growth of nanotube bundles emerging from catalyst particles obtained from evaporation-based elaboration techniques. General results are first discussed. Experiments strongly suggest a root-growth process in which carbon, dissolved at high temperatures in catalytic particles, segregates at the surface at lower temperatures to form tube embryos and finally nanotubes through a nucleation and growth process. A theoretical analysis of the reasons carbon does not always form graphene sheets to wrap the particles suggests analogies with other surface or interface instabilities, in particular, with those found in epitaxial growth. In the second part, detailed experimental results for nickel-rare earth metal catalysts are presented. By using various electron microscopy techniques, it is shown that carbon and the rare earth metal co-segregate at the surface of the particle and form carbide platelets, providing nucleation sites for nanotubes growing in directions perpendicular to the surface. A simple theoretical model is then presented in which the role of the rare earth metal is just to transfer electrons from metal to carbon. The graphene sheet is shown to become unstable; pentagons and heptagons are favored, which can explain the occurrence of local curvatures and of tube embryos. Finally, a brief discussion of some recent atomistic models is given.     

Evolution of 3C-SiC islands nucleated from a liquid phase on Si face α-SiC substrates

The contact between α-SiC crystals and Si-Ge based melts provokes the nucleation of 3C-SiC islands on the crystal surface. Evolution of these islands as a function of various parameters was studied. On both 4H and 6H substrates, it was found that, after nucleation, 3C-SiC islands first enlarge and may form a complete 3C layer under certain conditions. The 3C deposit can then be dissolved by the liquid phase at high temperature or for prolonged contact at relatively moderate temperature. The graphite crucible is proposed to play a central role in these enlargement and dissolution mechanisms by providing extra carbon atoms on the seed surface (enlargement) or provoking thermal induced carbon transport toward the sidewall (dissolution). Several differences between the use of 4H and 6H substrates were also observed.     

异型纳米AlOOH在水热条件下的界面模型及生长机制

根据古典和现代结晶理论,结合AlOOH晶体结构特点,构建了晶体生长界面模型,预测了晶体生长速率,揭示了异型纳米AlOOH在水热过程中的生长机制.采用高分辩透射电镜、X射线等检测手段,分析了不同水热条件下的实验样品.结果表明;AlOOH在（010）、（001）面上显露的氧基和羟基多于（100）、（101）面,酸性环境促进...     

原位光发射谱研究横向偏压金刚石薄膜生长过程

利用热丝ＣＶＤ方法研究了横向偏压对金刚石薄膜成核和生长的影响．实验表明,随着偏流的增加,金刚石在光滑硅衬底上的成核密度得到显著提高,最高可达 １．１×１０^８ｃｍ^－２,但是横向偏压不利于金刚石薄膜的生长．原位光发射谱研究发现,横向偏流的增加提高了原子氢和ＣＨ基团的浓度,导致衬底表面非晶碳层的形成,这可能是造成横向偏压促进金刚石成核却不利于金刚石薄膜生长的主要原因．     

Spherulitic morphology of the matrix of thermoplastic PEEK/carbon fibre aromatic polymer composites

PEEK/carbon fibre composites (derived from APC-2) have been examined with a permanganic etching technique in order to reveal the crystalline, spherulitic morphology of the thermoplastic PEEK polymer matrix. The locations of the nucleation sites for spherulite growth have been categorized. Nucleation can occur both within the matrix and from the carbon fibres. Crystallization at lower temperatures favours matrix nucleation. Nucleation from fibres is dominated by sites associated with fibre-fibre contact. There is no evidence of “transcrystalline” growth. The study also identifies two types of crystal orientation effect in the polymer matrix. The first is a slight orientation that can occur in standard mouldings and is the result of the fibres constraining the shape of the volume into which spherulite growth can occur. The second effect produces abnormally high crystal orientation and is the result of improper processing at too low a melt temperature. Such conditions cause self-seeding during consolidation of laminates which, coupled with flow-induced orientation, can lead to directionally arranged spherulite precursors in different stages of morphological development.     

Kinetic study on the hexacelsian-celsian phase transformation

The hexacelsian-celsian transformation from grains of 1/4 in. (0.635 cm) size is slow and erratic. This is because the rate of heterogeneous nucleation in such grains is low and is influenced by contamination present in the furnace. When the grain size is reduced to a –200 mesh powder, heterogeneous nucleation becomes a dominant factor and the transformation is accelerated. The transformation has three stages: the first is controlled by the rate of crystal growth, the second is controlled by the rates of nucleation and crystal growth, and the third is controlled by the rate of nucleation.The activation energy of crystal growth in the first stage of transformation is 20.1 kcal mole^–1 ±20%. This suggests that the hexacelsian-celsian transformation does not involve Si-0 and Al-O bond openings, since this would be expected to involve an activation energy of at least 60 kcal mole^–1.     

Nucleation and growth of domains near crack tips in single crystal ferroelectrics

The nucleation and growth of domains is investigated near a stationary crack tip in a single crystal of ferroelectric material. The phase-field approach, applying the material polarization as the order parameter, is used as the theoretical modeling framework and the finite element method is used for the numerical solution technique. The electromechanical form of the J-integral is appropriately modified to account for the polarization gradient energy terms, and analyzed to illustrate the amount of shielding, or lack thereof, due to domain switching at the crack tip. It is shown that the nucleation of domains near the crack tip due to applied electric field is affected by applied stress. However, the crack-tip energy release rate can change significantly between the instant of domain nucleation and the final equilibrium domain configuration. Implications of these results for ferroelectric single crystal fracture criteria are discussed.     

Study on the initial electrodeposition behavior of Ni–P alloys

The initial electrodeposition behavior of Ni–P alloys on carbon steel has been studied in this paper and was found to be greatly influenced by the morphology of the substrate. Preferred nucleation of Ni–P alloy coatings takes place at the crystal boundaries and the surface of cementite. Preferred nucleation and growth of Ni–P coatings also takes place at pearlite because pearlite contains cementite phase and many crystal boundaries. The coatings exhibit a polycrystalline structure composed of gathering nano-sized grains. With a continued electroplating time prolonged, the grains not only present two-dimensional sideward growth, but also display three-dimensional growth on them to form a new polycrystalline layer.     

The effects of a glow discharge on the nucleation characteristics of gold on polymer substrates

Small amounts of gold were deposited onto organic substrates of stearic acid and polyethylene. Some nucleation characteristics of the gold were studied using transmission electron microscopy and were compared with the nucleation characteristics observed on the more conventional carbon substrates. It was shown that, by exposing the organic substrates to a glow discharge in air, their surface structure was changed by the removal of hydrogen by plasma etching. This produced a surface layer composed mainly of carbon or of carbon and oxygen and it was demonstrated that the subsequent nucleation behaviour of gold on these exposed surfaces was very similar to that of gold on pure carbon films.