Understanding behavior of machining interface and dielectric molecular medium in nanoscale electro-machining

Recently, a repeatable and scalable nanoscale electro-machining (nano-EM) process to produce sub-20 nm scale features has been demonstrated. In the presented research, the behavior of the liquid dielectric (n-decane) machining medium in nano-confinement (<3 nm) under physical boundary conditions is investigated using molecular dynamics (MD) simulation. Results show a four-fold increase in the density of n-decane indicating ‘quasi-solid’ behavior at the nano-EM interface, thereby acting as an effective charge transport medium between the nano-tool and the workpiece. The effect of such quasi-solid medium is demonstrated through the experimental observations of electrical breakdown (BD) at the sub-20 nm scale interface.     

How the crystallography and nanoscale chemistry of the metal/oxide interface develops during the aqueous oxidation of zirconium cladding alloys

Aqueous corrosion and hydrogenation have become major limiting factors to the use of zirconium alloys as fuel cladding and assembly components in water-cooled nuclear reactors. The metal–oxide interface has been a particular focus of previous research, but there is still no clear understanding of what is present at the interface at different stages of the complex oxidation process. We report here a systematic investigation using state-of-the-art instrumentation on the interfaces in several zirconium alloys corroded for different times. We have shown that thin intermediate oxide layers with compositions close to ZrO can be observed in almost all the pre-transition samples studied, and that this layer thickens during the pre-transition stage. Just before the kinetic transition, a large variation in the suboxide width was detected, suggesting that the kinetic transition is an extremely local process. After transition the suboxide was generally absent. In the suboxide locations different structures, including an unidentified phase, were found. The oxygen-saturated (～30 at.% O) metal regions found beneath the oxide are thickest in the (late) pre-transition samples and significantly thinner in the post-transition samples. We suggest that the suboxide cannot by itself act as a protective layer and conclude that it is the development of interlinked porosity down to the metal–oxide interface that is the reason for the transition in oxidation kinetics.     

In situ measurement of corrosion on the nanoscale

Measurement of the electrochemical properties phase-by-phase on the nanoscale in real (commercial) alloys is critical to understanding the microstructure-corrosion relationship and subsequently controlling it. This work presents a novel AFM based in situ corrosion probing methodology (for the first time) that is capable of resolving the electrochemical activity (impedance response) into the nanometer range; the method subsequently having major ramifications in the study of aluminium alloy corrosion, the interpretation of corrosion propagation, and the subsequent development of corrosion resistant aluminium alloys.     

Dislocation drag at the nanoscale

The mobility of dislocations is shown to be a size-dependent phenomenon. When dislocations intersect free surfaces, the mobility decreases as the dislocation length decreases, suggesting that dislocation motion in small structures may be more difficult. This increased drag may be related to surface forces acting where the dislocation intersects the free surface or from altered dislocation–phonon interactions. Mobility, however, is not as dependent on the film thickness and converges rapidly to bulk values.     

Mapping mechanical properties on the nanoscale using atomic-force acoustic microscopy

Tools are being developed that use the atomic-force microscope (AFM) to measure mechanical properties with nanoscale spatial resolution. Contact-resonance-spectroscopy techniques such as atomic-force acoustic microscopy involve the vibrational modes of the AFM cantilever when its tip is in contact with a material. These methods enable quantitative maps of local mechanical properties such as elastic modulus and thin-film adhesion. The information obtained furthers the understanding of patterned surfaces, thin films, and nanoscale structures.     

纳米薄膜X射线吸收光谱的鉴定

采用X射线吸收光谱研究了热丝化学气相沉积(CVD)合成的纳米金刚石薄膜和脉冲激光沉积的纳米SiC薄膜.结果表明:纳米金刚石薄膜的碳K边X射线吸收精细结构光谱显示的激发峰相当于微米金刚石薄膜的蓝移,是量子效应的显著特征,证明制备的是纳米金刚石薄膜,与高分辨透射电镜的结果完全吻合;纳米SiC薄膜的硅K边X射线吸收精细结构光谱和扩展X射线吸收精细结构光谱也显示了纳米薄膜短程有序的结构特征,表明获得的是纳米SiC薄膜.     

Parabolic growth of oxide solid solutions on binary alloys

The growth of oxide solid solutions on binary alloys has been described using the ternary diffusion model put forward by Wagner and subsequently modified by Dalvi and Coates and Bastow, Whittle, and Wood. A more refined defect model where the presence of all types of vacancy species within the scales is considered has been adapted to obtain the diffusivities of cations in the scales as a function of oxygen potential and cation composition. In addition, correlation effects on cation jump frequencies have been included to account for additional compositional dependence of the diffusivities. Results have been analyzed for oxidation of Ni-Co to (Ni, Co)O at 1273°K in 1 atm oxygen. Improved agreement between calculated and experimental cation concentration profiles indicates that this approach is better than to arbitrarily assign functional forms to the compositional and oxygen activity dependencies of the diffusivities, and demonstrates that an accurate as possible model for the point defect structure of the oxide is required for correct interpretation of the growth of these solid solution types of scales.Deceased July 23, 1982.     

Exploring the reactivity and nanoscale morphology of de-alloyed layers

De-alloying of a 70Cu-30Pt alloy has been studied with a view to testing the thermodynamic prediction that a novel kind of underpotential or sub-potential electrodeposition may be possible in such highly curved nanoporous solids. After de-alloying in H₂SO₄ solution containing CuSO₄, successive cathodic processes of PtO monolayer reduction, UPD of Cu, and a Cu plating process were identified, all occurring at potentials above the Cu equilibrium potential. The maximum amount of charge in the third plating process was determined to be about three monolayers, averaged over the whole porous surface. This seems to rule out the possibility that this process is some kind of second UPD layer or other non-bulk form of Cu plating. A probable explanation is that small amounts of bulk Cu are plating into regions of high negative curvature within the de-alloyed material. However X-ray diffraction did not show any evidence of bulk Cu.     

Effects of anisotropy on the equilibrium shape of nanoscale pores at grain boundaries

Molecular dynamics simulations have been performed to study the interaction between a faceted pore and an anisotropic grain boundary (GB). Nickel was chosen as a convenient model system. In order to establish the equilibrium crystal shape (ECS) of the pore, studies were also conducted on isolated pores. Isolated pores were found to be subject to the nucleation inhibition of equilibration that has been predicted by Rohrer et al. (J Am Ceram Soc 2000;83:214, 2001;84: 2099). This work shows that configurations close to the ECS can be obtained if supersaturation within a pore is artificially increased by adding mobile adatoms to the internal surfaces of the pores. In the case of pores located at GBs, the nucleation energy barriers to facet displacement are not present for facets in contact with the GB at the triple line, but may still persist for facets that have no contact with the GB. This problem can be overcome by approaching the equilibrium shape from different initial configurations. The configuration of the GB in the vicinity of the pore has been found to be essentially planar, indicating that GB puckering in the vicinity of anisotropic pores is not generally necessary. The present calculations show that incompatibilities between misoriented pore facets that meet at the triple line with the GB are easily accommodated by local atomic rearrangements at the disordered region of intersection with the GB.     

Roughness effect on the measurement of interface stress

Stimulated by a recent paper by Spaepen (Acta mater. 48 (2000) 31) we concentrate on the effect of roughness parameters on stress measurements in thin films for self-affine and mound rough interfaces. A self-affine interface is characterized by a lateral correlation length ξ, an rms roughness amplitude σ, and a roughness exponent H (0<H<1). With increasing long wavelength roughness ratio σ/ξ, the ratio between the measured and the actual interface stress decreases. It decreases with a decreasing roughness exponent H that leads to rougher interfaces at short roughness wavelengths (<ξ). For mound roughness which is characterised besides σ by an average mound separation λ and a system correlation length ζ, the force ratio decays in an oscillatory manner as a function of σ/λ as long as λ<ζ. It is concluded that for both cases a more precise knowledge of roughness morphology is required in order to address the influence of interface roughness on the interface stress in thin films.     

Investigation on sintering mechanism of nanoscale tungsten powder based on atomistic simulation

Atomistic simulations focusing on sintering of crystalline tungsten powders at the submicroscopic level are performed to shed light on the processing on the nanoscale powders. The neck growth and shrinkage were calculated during these sintering simulations, making it possible to extend these results to the evolution of global physical properties that occurs during sintering. The densification and grain growth during sintering were calculated with variations in temperature, pressure, particle configuration and crystalline misalignment between particles. These findings lay the foundation for a virtual approach to setting the processing cycles and materials design applicable to nanoscale powders.     

Investigation on the displacement fluctuation of the linear motor feed system considering the linear encoder vibration

Optical linear encoder is generally adopted to realize the full-closed loop control for the linear motor feed system. Its reading head installed on the worktable may vibrate as a result of the excitation of disturbance harmonics, which leads to the encoder's error. This study aims to investigate the mechanism of displacement fluctuation due to the encoder's error caused by the worktable vibration in the linear motor feed system. The vibrating modes of feed system are analyzed firstly, and then the influences of three torsional vibrations on encoder's errors are investigated. The transfer function between disturbance and output response is proposed to analyze the influence of encoder's error on the displacement fluctuation. The relationships among vibration, encoder's error, thrust harmonic and displacement fluctuation are discussed in detail. The results show that the encoder's error can produce obvious displacement fluctuation for the linear motor feed system, which is actually an electromechanical coupling process due to the direct drive and full-closed loop control. Finally, three effective measures are put forward to diminish the system displacement fluctuation.     

Mechanisms of the gold-catalysed water-gas shift

Mechanisms proposed for the gold-catalysed Water-Gas Shift (WGS) are critically reviewed, and a complete reaction set and cycle involving activation of water on reduced support sites is considered; however, a modified route to CO2 formation by thermal decomposition of a carboxyl species results in a simpler and therefore more probable mechanism.     

试样厚度对韧脆转变温度区间的影响

根据BSI 7448 Part I标准,测定了海洋平台用钢E36以裂纹尖端张开位移(CTOD)为表征的韧脆转变温度曲线. 通过改变试样厚度,研究面外拘束对韧脆转变温度区间的影响并分析了其变化规律. 结果表明,E36钢具有典型的韧脆转变特征,其韧脆转变温度曲线可以用Boltzmann函数进行拟合,具有良好的相关性;试样厚度越大,韧脆转变温度越高,断裂性能下降. 通过有限元模拟分析三维裂纹尖端应力状态,选择了面外拘束参数Tz,用来阐述厚度效应对韧脆转变温度的影响,具有良好的相关性.