Dislocation controlled wear in single crystal silicon carbide

For better design and durability of nanoscale devices, it is important to understand deformation in small volumes and in particular how deformation mechanisms can be related to frictional response of an interface in the regime where plasticity is fully developed. Here, we show that when the size of the cutting tool is decreased to the nanometer dimensions, silicon carbide wears in a ductile manner by means of dislocation plasticity. We present different categories of dislocation activity observed for single asperity sliding on SiC as a function of depth of cut and for different sliding directions. For low dislocation density, plastic contribution to frictional energy dissipation is shown to be due to glide of individual dislocations. For high dislocation densities, we present an analytical model to relate shear strength of the sliding interface to subsurface dislocation density. Furthermore, it is shown that a transition from plowing to cutting occurs as function of depth of cut and this transition can be well described by a macroscopic geometry-based model for wear transition.     

Energy considerations in crack deflection phenomenon in single crystal silicon

Crack deflection in single-crystal brittle occurs when a crack, propagating on one cleavage plane, ‘chooses’, from energy considerations, to continue propagating on another cleavage plane. This phenomenon was identified during dynamic crack propagation experiments of thin, rectangular [0 0 1] single-crystal (SC) silicon specimens subjected to three-point bending (3PB). Specimens with long pre-cracks (hence propagating at a ‘low’ energy and velocity) cleave along the vertical (1 1 0) plane, while the same specimens but with short pre-cracks (and therefore with higher propagation energy and velocity) cleave along the inclined (1 1 1) plane. The same specimens with intermediate pre-crack length show that the crack first propagates on the (1 1 0) plane and then deflects to the (1 1 1) plane. We show that the deflection is due to variations of the material property that resists cracking, Γ, the dynamic cleavage energy, with velocity and crystallographic orientation. We propose selection criteria to explain the deflection: The crack will deflect to the plane with the lowest dynamic cleavage energy. We further suggest that crack deflection is the basic mechanism controlling the way the crack consumes energy while propagating and is the main cause of surface perturbations. The spatial temporal fracture energy along the (1 1 0) cleavage plane is evaluated.     

Mosaic structure and reflectivity of an as grown copper single crystal using neutron diffraction

A single crystal of copper of size 80 mm dia. and 50 mm length was grown by the Bridgman method. Neutron diffraction investigations were carried out to determine the mosaic spread and reflectivity of this as grown crystal. The crystal was found to be of good quality having structure free and symmetric rocking curves with mosaic spread of 14 min.arc and reflectivity of ∼ 50%.     

The preparation and properties of single crystal copper phosphide

Journal of Materials Science -     

硅晶片切割损伤层微观应力的研究

作者利用双晶Ｘ射线衍射技术，研究了不同切割速率下硅晶片的切割损伤。实验得出切割速率不影响损伤层的厚度，但影响损伤层内的微观应力，在一定范围内，切割速率越大，损伤层的微观应力越小，并运用Ｖｏｉｇｔ函数分析法，分析了硅晶片切割层的微观应力。     

Mechanical characterization of single crystal silicon and UV-LIGA nickel thin films using tensile tester operated in AFM

This paper describes the mechanical characteristics of microscale single crystal silicon (SCS) and UV‐LIGA nickel (Ni) films used for microelectromechanical systems (MEMS). A compact tensile tester, operated in an atomic force microscope (AFM), was developed for accurate evaluation of Young's modulus, tensile strain and tensile strength of microscale SCS and UV‐LIGA Ni specimens. SCS specimens with nominal dimensions of 20 μm in thickness, 50 μm in width and 600 μm in length were prepared by a conventional photolithography and etching process. UV‐LIGA Ni specimens, with a thickness of 15 μm, a width of 50 μm and a length of 600 μm in nominal dimensions, were also fabricated by electroplating using a UV thick photoresist mould. All specimens have line patterns on their specimen gauge section to measure axial elongation under tensile loading. The SCS specimens showed a linear stress–strain response and fractured in a brittle manner, whereas the UV‐LIGA Ni specimens showed elastic–inelastic deformation behaviour. Young's modulus of SCS and UV‐LIGA Ni specimens obtained from tensile tests averaged 169.2 GPa and 183.6 GPa, respectively, close to those of bulk materials. However, the tensile strength of both materials showed a larger value than the bulk materials: 1.47 GPa for the SCS and 0.98 GPa for the Ni specimens. Yield stress and breaking elongation of UV‐LIGA Ni specimens were also quite different from those of the bulk Ni because of the specimen size effect on inelastic properties.     

Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.     

Hexagonal single crystal domains of few-layer graphene on copper foils

Hexagonal-shaped single crystal domains of few layer graphene (FLG) are synthesized on copper foils using atmospheric pressure chemical vapor deposition with a high methane flow. Scanning electron microscopy reveals that the graphene domains have a hexagonal shape and are randomly orientated on the copper foil. However, the sites of graphene nucleation exhibit some correlation by forming linear rows. Transmission electron microscopy is used to examine the folded edges of individual domains and reveals they are few-layer graphene consisting of approximately 5-10 layers in the central region and thinning out toward the edges of the domain. Selected area electron diffraction of individual isolated domains reveals they are single crystals with AB Bernal stacking and free from the intrinsic rotational stacking faults that are associated with turbostratic graphite. We study the time-dependent growth dynamics of the domains and show that the final continuous FLG film is polycrystalline, consisting of randomly connected single crystal domains.     

Vertically bent single crystal silicon wafers as a neutron monochromator

A simple technique to construct monochromators for neutron beams is presented. The monochromator consists of a set of Si wafers stacked together and elastically bent to focus in the vertical geometry. The apparatus is described. It is shown that the peak reflectivity of the set is mainly governed by the number of wafers, while symmetry and smoothness of the rocking curve is achieved by the bending process. A prototype monochromator based on the (220) reflection of Si wafers has been tested and has successfully performed as a fixed monochromator selecting a flux of 3 × 10⁶ n/(s cm²) (at 45 meV) at the sample position (with 20′ collimation in the white beam and 10′ after the reflection from the monochromator). We conjecture that such a technique could be applied to any kind of material provided the wafers are cut from a single crystal with the surface parallel to the reflecting planes.     

Molecular dynamics study on the nano-void growth in face-centered cubic single crystal copper

Cylindrical nano-void growth in face-centered cubic single crystal copper is studied by mean of molecular dynamics with the Embedded Atom Method. The problem is modeled by a periodic unit cell containing a centered nano-sized cylindrical hole subject to uniaxial tension. The effects of the cell size, crystalline orientation, and initial void volume fraction on the macroscopic stress–strain curve, incipient yield strength, and macroscopic effective Young’s modulus are quantified. Defect evolution in terms of dislocation emission immediately after incipient yielding is also investigated. Obtained results show that, for a given void volume fraction, cell size has apparent effects on the incipient yield strength but negligible effects on the macroscopic effective Young’s modulus. Moreover, the macroscopic effective Young’s modulus and incipient yield strength of the [ 1 0]–[1 1 1]–[1 1 ] orientated system are found to be much more sensitive to the presence of void than those of the [1 0 0]–[0 1 0]–[0 0 1] system.     

Influence of sub-micrometer notches on the fracture of single crystal silicon thin films

The influence of notches on the fracture of single crystal silicon thin films was investigated. The tests were conducted on notched and smooth tensile specimens micromachined on a silicon wafer. The specimen geometry was 100 μm long, 50 μm wide and 5 μm thick. For the notched specimen, a V‐shaped sub‐micrometer notch was introduced on one edge of it by using a focused ion beam (FIB) process. The notch lengths ranged from 0.07 to 1.3 μm. Four types of specimens with different surfaces and tensile orientations were tested. The smooth specimens showed scattered fracture strengths and ‘collapsed’ fractures. For the restrictive‐shaped notches, the critical length was 0.5 μm. The short‐notched (<0.5 μm) specimens also showed ‘collapsed’ fractures, and the stress concentrations on notch tips decreased their fracture strengths. For the long‐notched (>0.5 μm) specimens, the notch was equivalent to a crack in the Griffith model and the crack mainly propagated on {111} cleaved planes.     

单晶硅表面溶胶-凝胶法制备磷灰石涂层的研究

采用溶胶-凝胶法在单晶硅表面制备了磷灰石(apatite,AP)涂层。借助SEM、EDS、XRD研究了单晶硅表面AP涂层的形貌结构与成分特征,通过模拟体液(simulated body fluid,SBF)浸泡实验研究了硅基AP涂层的体外诱导性能。研究结果表明,在500℃保温2h即可使sol-gel层转变为较致密的AP涂层,700与800℃分别保温2h得到的硅基AP涂层晶体长大且出现间隙,涂层的结晶性随热处理温度升高而增加,在800℃处理未出现AP的分解;SBF中浸泡1周后,AP涂层未出现剥离单晶硅的现象,且能够诱导骨状磷灰石新沉积层的形成,体现出较好的体外诱导性能。     

Residual stress measurement in filament-evaporated aluminium films on single crystal silicon wafers

Shadow Moire interferometry was used to determine the residual stresses in thin, filament-evaporated aluminium films deposited on (100) p-type, 10.16 cm diameter, 0.05 cm thick, circular single crystal silicon wafers. The aluminium film thicknesses ranged from 70 to 780 nm. Benchmark experiments on wafers without aluminium films showed that wafers possess in-plane residual stresses; the centre of the wafer is under tensile stresses of the order of 30 M Pa and these stresses decrease toward the edge of the wafers to approximately 10 M Pa. The deposition of aluminium films increases these tensile residual stresses by 3 to 15 M Pa depending on the film thickness. The increase in the stress is attributed to the stresses in the films.     

Dislocation structures on the plane of shear in compressed double-notch copper single crystal specimens

Dislocation structures in the zone of shear of double-notch copper single crystals, generated by a shear stress 20 MPa acting in the specimen axis ([110] direction) at 773 K were investigated by means of transmission electron microscopy. Dislocation configurations in different crystal planes of shear, and (001), were compared. The role of the 110{111} slip systems in the process of shear is assessed and possible indications of a direct glide of dislocations with Burgers vector b=a/2[110] in the mentioned compact and non-compact planes are presented.     

Influence of crystal orientation on the processing of copper single crystals by ECAP

Single crystals of high-purity copper, having two different orientations, were pressed through one pass in equal-channel angular pressing (ECAP) at room temperature and then examined using several different analytical techniques. For both orientations, it is shown that elongated arrays of cells or subgrains are formed in the first pass with their long axes aligned parallel to the primary slip system. The average width of these subgrains was measured as ∼0.2 μm which is similar to the equilibrium grain size reported in polycrystalline Cu after processing by ECAP. These results confirm earlier observations using an aluminum single crystal except only that the subgrain width in copper is significantly smaller. This difference is attributed to the lower stacking-fault energy in copper and the consequent low rate of recovery.