Screw Component Research Articles

Magnetic ordering and ferroelectricity in multiferroic2H−AgFeO2: Comparison between hexagonal and rhombohedral polytypes

Magnetic and dielectric properties of the hexagonal triangular lattice antiferromagnet 2H-AgFeO2 have been studied by neutron diffraction, magnetic susceptibility, specific heat, pyroelectric current, and dielectric constant measurements. The ferroelectric polarization, P ~ 5 {\mu}C/m2, has been found to appear below 11 K due to a polar nature of the magnetic ground state of the system. In the temperature range of 11 K < T < 18 K, an incommensurate spin density wave (ICM1) with the nonpolar magnetic point group mmm1' and the k1 = (0,q1_b,0; q1_b = 0.390-0.405)propagation vector takes place. Below 14 K, a proper screw ordering (ICM2) and k2 = (0,q2_b,0; q2_b = 0.385-0.396) appears as a minor phase which coexists with ICM1 and the ground state down to the lowestmeasured temperature 5.5 K. No ferroelectric polarization associated with the ICM2 phase was observed in agreement with its nonpolar point group 2221'. Finally, a spiral order with cycloid and proper screw components (ICM3), and k3 = (q3_a,q3_b,0; q3_a = 0.0467, q3_b = 0.349) emerges below 11 K as the ground state of the system. Based on the deduced magnetic point group 21', we conclude that the ferroelectric polarization in ICM3 is parallel to the c axis and is caused by the inverse Dzyloshinskii-Moriya effect with p1 ~ rij x (Si x Sj ). Unlike the rhombohedral 3R-AgFeO2 polytype, the additional contribution to the macroscopic polarization p2 ~ Si x Sj is not allowed in the present case due to the symmetry constraints imposed by the hexagonal lattice of 2H-AgFeO2.

Deformation mechanism study of a hot rolled Zr-2.5Nb alloy by transmission electron microscopy. I. Dislocation microstructures in as-received state and at different plastic strains

Thin foil dog bone samples prepared from a hot rolled Zr-2.5Nb alloy have been deformed by tensile deformation to different plastic strains. The development of slip traces during loading was observed in situ through SEM, revealing that deformation starts preferentially in certain sets of grains during the elastic-plastic transition region. TEM characterization showed that sub-grain boundaries formed during hot rolling consisted of screw ⟨a⟩ dislocations or screw ⟨c⟩ and ⟨a⟩ dislocations. Prismatic ⟨a⟩ dislocations with large screw or edge components have been identified from the sample with 0.5% plastic strain. Basal ⟨a⟩ and pyramidal ⟨c + a⟩ dislocations were found in the sample that had been deformed with 1.5% plastic strain, implying that these dislocations require larger stresses to be activated.

Mono-like silicon ingots grown on low angle misoriented seeds: Defect characterization by synchrotron X-ray diffraction imaging

The present work studies the generation and propagation of sub-grain boundaries and dislocations in mono-like silicon ingots grown on monocrystalline seeds with a very small relative misorientation between them (<0.06° around the x, y and z axes). Special emphasis is put on the region close to the area between the seeds at the bottom of the crucible, which appears to be crucial in determining the crystalline quality of the final ingot. For this investigation, X-ray rocking curve imaging (RCI) in transmission geometry, a directly quantitative version of monochromatic beam Bragg diffraction imaging (“topography”) has been used. This technique has been developed at the European Synchrotron Radiation Facility (ESRF), beamline BM05 and allows us to visualize the spatial distribution of the lattice distortion of a single crystal. It was found that the solidified ingot takes the crystallographic orientation of the seeds without creating any distorted area at the interface. However, dislocation bunches having a strong screw component are generated between the seeds and propagate along the growth direction. Sub-grain boundaries above the top of the seeds, mainly composed of edge dislocations, were also observed. These have a detrimental influence on the minority carrier lifetime of the ingot, since, as is known, sub-grain boundaries show active recombination. In this work taking advantage of the high angular resolution and the sensitivity of the technique it was shown that a relative misorientation between the seeds (δθx, δθy, δθz) as small as 0.02° can produce cascades of dislocations that propagate and multiply higher up in the ingot becoming electrically active.

Zn-dopant dependent defect evolution in GaN nanowires.

Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101¯3), (101¯1) and (202¯1), as well as Type I stacking faults (…ABABCBCB…), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (…ABABACBA…) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.

Microscopic crystalline structure of a thick AlN film grown on a trench-patterned AlN/α-Al2O3 template

The microscopic crystalline structure (MCS) such as domain texturing, lattice tilting fluctuation and strain fluctuation in thick AlN films grown on trench-patterned AlN/α-Al2O3 templates was clarified by utilizing position-dependent X-ray microdiffraction measurements in combination with transmission electron microscopy observations. The results clearly demonstrate that the trench-patterned templates have a strong influence on the MCS in the thick AlN films. The MCS is anisotropic between the [112¯0] and [11¯00] directions. Periodic domain texturing was observed along the [112¯0] direction, corresponding to the periodicity of the patterning pitch of the template. Submicron crystal domains are tilted at different angles to the [112¯0] direction, and the crystal domain tilting becomes larger in the void-containing trench regions than in the terrace regions. This generates the periodicity in lattice tilting fluctuation and strain fluctuation along the [112¯0] direction. These were found to be caused primarily by two factors; one of which being the elastic strain relaxation that depends on the growth direction in the terrace regions, and the other being the inhomogeneity in distribution and morphology of dislocations that have Burgers vectors with a screw component in the void-containing trench regions.

Evaluation of stacking faults and associated partial dislocations in AlSb/GaAs (001) interface by aberration-corrected high-resolution transmission electron microscopy

The stacking faults (SFs) in an AlSb/GaAs (001) interface were investigated using a 300 kV spherical aberration-corrected high-resolution transmission electron microscope (HRTEM). The structure and strain distribution of the single and intersecting (V-shaped) SFs associated with partial dislocations (PDs) were characterized by the [110] HRTEM images and geometric phase analysis, respectively. In the biaxial strain maps εxx and εyy, a SF can be divided into several sections under different strain states (positive or negative strain values). Furthermore, the strain state for the same section of a SF is in contrast to each other in εxx and εyy strain maps. The modification in the strain states was attributed to the variation in the local atomic displacements for the SF in the AlSb film on the GaAs substrate recorded in the lattice image. Finally, the single SF was found to be bounded by two 30° PDs. A pair of 30° PDs near the heteroepitaxial interface reacted to form a Lomer-Cottrell sessile dislocation located at the vertices of V-shaped SFs with opposite screw components. The roles of misfit dislocations, such as the PDs, in strain relaxation were also discussed.

Direct observation of depth-dependent atomic displacements associated with dislocations in gallium nitride.

We demonstrate that the aberration-corrected scanning transmission electron microscope has a sufficiently small depth of field to observe depth-dependent atomic displacements in a crystal. The depth-dependent displacements associated with the Eshelby twist of dislocations in GaN normal to the foil with a screw component of the Burgers vector are directly imaged. We show that these displacements are observed as a rotation of the lattice between images taken in a focal series. From the sense of the rotation, the sign of the screw component can be determined.

Investigation on the Formation and Propagation of Defects in GeSn Thin Films

Silicon photonics is rapidly growing because of the recent introduction of GeSn alloys as a group IV direct bandgap material. Viability of GeSn alloys for complementary metal-oxide-semiconductor technology, as demonstrated recently, is another step towards commercializing this material for integrated circuits applications [1]. However, ability to grow high quality materials is imperative for device applications. In order to compensate the high lattice mismatch between the Si substrate and the GeSn film different strain relaxed buffer layers, such as Ge, are used. Relaxation of Ge buffer layer introduces a network of misfit dislocations. This can result in high threading dislocation density (TDD) in GeSn degrading the performance of the final device. In this paper, we report on defect formation and propagation mechanism in Ge1-xSnx films.Plastic relaxation of higher lattice constant materials (SiGe, Ge, GeSn) on top of Si forces the layers to form 60o misfit dislocations. These dislocations are caused by slip of (110) and (111) planes in the diamond lattice structure, which have a screw component that threads through the grown film. Fig. 1 (a) schematically shows how defects form and propagate through the film. A network of misfit dislocations results in a cross-hatch pattern on the surface of the film. Fig. 1 (b) shows the plan view transmission electron microscopy (TEM) image of Ge.93Sn.07 on top of Ge buffer layer. The cross-hatch patterns formed at GeSn/Ge interface have overlapped with those of Ge/Si. The denser pattern is attributed to the Ge/Si interface due to higher lattice mismatch. Fig. 1 (c) delineates scanning electron microscopy image of decorated threading dislocations at the surface by using cold Schimmel solution (CrO3+HF) for 10 minutes. Density of pits (1.1×107 cm-2) is in good agreement with TDD measured from TEM micrographs (3×107 cm2)[1]. It also confirms that the quality of GeSn material is higher than Ge buffer layer (TDD: 9.71×108 cm-2) by an order of magnitude.Formation of Lomer misfit dislocaltion prevents propagation of threadig dislocation in the top layer.The main reasons behind this phenomenon is that the Burgers vector in Lomer misfit dislocation, for a (001) substrate, lies parallel to the interface [2]. Therefore, the substrate and film lattices can match while no prependicular component exists. In this case the film is grown relaxed without introducing structural imperfection such as threading dislocations, which degrades the material quality. Different models of Lomer misfit dislocation are proposed including a five-atom ring and seven-atom ring without dangling bonds [3]. Available models show that Lomer dislocations can combine two lattices through symmetric and asymmetric loops [4]. Fig. 1 (d-g) shows formation of the symmetric and asymmetric loops at Ge/GeSn interface films using high resolution TEM images.Our investigations show that formation of symmetric and asymmetric Lomer dislocation at the interface of Ge/GeSn film has reduced the TDD in GeSn film. Results from TEM images and etch pit density measurement verifies this conclusion by showing that 80% of the threading dislocations has not passed through the GeSn film. The current results show achievement of high quality GeSn film as well as indicating that GeSn is a unique candidate to be used as a universal compliant layer.

Plastic deformation in nanoindentation of tantalum: A new mechanism for prismatic loop formation

The mechanisms of deformation under a nanoindentation in tantalum, chosen as a model body-centered cubic (bcc) metal, are identified and quantified. Molecular dynamics (MD) simulations and indentation experiments are conducted for [100], [110] and [111] normals to surface orientations. The simulated plastic deformation proceeds by the formation of nanotwins, which rapidly evolve into shear dislocation loops. It is shown through a dislocation analysis that an elementary twin (three layers) is energetically favorable for a diameter below ∼7nm, at which point a shear loop comprising a perfect dislocation is formed. MD simulations show that shear loops expand into the material by the advancement of their edge components. Simultaneously with this advancement, screw components of the loop cross-slip and generate a cylindrical surface. When opposite segments approach, they eventually cancel by virtue of the attraction between them, forming a quasi-circular prismatic loop composed of edge dislocation segments. This “lasso”-like mechanism by which a shear loop transitions to a prismatic loop is identified for both [001] and [111] indentations. The prismatic loops advance into the material along 〈111〉 directions, transporting material away from the nucleation site. Analytical calculations supplement MD and experimental observations, and provide a framework for the improved understanding of the evolution of plastic deformation under a nanoindenter. Dislocation densities under the indenter are estimated experimentally (∼1.2×1015m−2), by MD (∼7×1015m−2) and through an analytical calculation (2.6–19×1015m−2). Considering the assumptions and simplifications, this agreement is considered satisfactory. MD simulations also show expected changes in pile-up symmetry after unloading, compatible with crystal plasticity.

Biomechanical evaluation of screw- and cement-retained implant-supported prostheses: A nonlinear finite element analysis

The mechanical stability of the prosthetic components in the implant-prosthesis complex is essential to the long-term success of the restorations. However, little is known about the differences in the biomechanical behavior of screw- and cement-retained prostheses. The purpose of this study was to compare the preload maintenance, stresses, and displacements of prosthetic components of screw- and cement-retained implant-supported prostheses by using the finite element method in a nonlinear analysis. Two 3-dimensional models were constructed: implant-supported fixed partial prostheses with 3 elements retained either by screws (SFP) or cement (CFP). After the simulation of screw tightening, the preload was calculated for both prostheses. Then vertical and oblique loads (100 N) were applied on the models. The preload was identified, the maximum von Mises equivalent stresses (SEQV) were obtained on the screws, and the displacement among the abutment, the implant, and screw was identified by observing the penetration and gap in the contact interfaces. Under vertical load, there was a higher decrease in the preload and in the SEQV on the screw in the SFP. Under oblique load, the SEQV was 24% higher on the screw of the SFP. In the displacement analysis under vertical load, penetration was concentrated in the threads of the screw in the SFP and between the abutment and implant in the CFP. The gap was 118% greater for the SFP and was concentrated on the abutment extension. Under oblique load, the displacement pattern was similar for both prostheses, but with values 66% higher for penetration and 96% higher for gap for the SFP. The SFP showed a higher biomechanical risk of failure than the CFP.

Contrast analysis of Shockley partial dislocations in 4H-SiC observed by synchrotron Berg–Barrett X-ray topography

Shockley partial dislocations in 4H-SiC were observed using monochromatic synchrotron X-ray topography with a grazing-incidence Bragg-case geometry, that is, Berg–Barrett topography. The contrast of partial dislocations at the edges of Shockley-type stacking faults is discussed in terms of whether they have C- or Si-core edge components, or screw components. The dissociated state of basal-plane dislocation is discussed on a basis of the stacking sequence for basal-planes in the 4H-SiC crystal structure. It is expected that the results obtained in this study will be useful for characterizing Shockley-type stacking faults in Berg–Barrett topography.

Theoretical modeling and error analysis for CNC whirling of the helical surfaces of custom screws using common inserts

With the wide use of complex helical surfaces in screw components, it becomes more and more urgent to investigate efficient ways for the design and manufacture of components of this kind. This study establishes the mathematical model of helical surface based on discrete points on its axial section profile using cubic spline curve fitting method, and presents a theoretical approach to calculate the tool path for the whirling process which aims to produce the helical surface through enwrapping movements of standard cutters inserted in the tool ring. The approach is implemented in MATLAB environment focusing on the calculation of the cutter location points. After that, the theoretical machining error is analyzed focusing on the scallop height, which can be divided into axial and cross-sectional errors. Finally, a case study is provided, together with numerical simulation demonstrating the validity of the proposed approach.

Coincident Electron Channeling and Cathodoluminescence Studies of Threading Dislocations in GaN

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black-white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

Reduction of threading dislocations in N-polar GaN using a pseudomorphicaly grown graded-Al-fraction AlGaN interlayer

We report on an improvement for the crystal quality of N-polar GaN with a pseudomorphicaly grown graded-Al-fraction AlGaN interlayer which introduces strain management in heterostructure and brings in the inclination and annihilation of threading dislocations (TDs). Significant blocking of screw and edge component TDs are observed by transmission electron microscopy, and the reduction of TDs densities are estimated by high resolution X-ray diffraction and plan-view cathodoluminescence measurement. Photoluminescence measurement shows a good optical property associated with the significant reduction of edge TDs. All the results show that insertion of graded AlGaN interlayer is a convenient method to achieve excellent crystal quality in N-polar GaN growth.

Mathematical Model and Tool Path Calculation for Helical Groove Whirling

The helical groove shape plays a key role in ensuring the adequate flute space of many screw components. In many situations, the helical groove is machined through profiled grooving cutter, which brings a huge cost. This study establishes the mathematical model of helical groove based on cross-section and presents an approach to calculate tool path using the whirling process which machines helical groove through enwrapping movements with standard cutters. Finally, a case study and the error analysis are provided to illustrate the validity of the developed models and algorithms, which offers an alternative method for further computer aided manufacturing.

Theoretical model for CNC whirling of screw shafts using standard cutters

Widely increased use of screw components with complex helical surfaces in contemporary machinery makes a huge demand for high-efficiency fabrication of these components. In this paper, a typical whirling process is introduced and the helical surface of a screw shaft is mathematically modeled based on its axial section profile, which is depicted usually by a number of sampled points. Following this, a theoretical approach is proposed to whirling the screw by using standard cutters, aligned with a detailed investigation into related issues including calculations of the cross section profile and the cutting tool location points. In this approach, the helical surface can be generated according to the shaft cross section which is used as the envelope of the cutting-tool motions in the machining process. The machining error is also analyzed with focuses on the axial- and cross section scallop height. Finally a case study is presented, which demonstrates the whole process including CAD models and cutting tool in-process positioning geometry data. The error distribution of the machined component along the profiles is also simulated.

Study of $$ \{ 11\bar{2} 1\} $$ Twinning in α-Ti by EBSD and Laue Microdiffraction

Activity of the $$ \{ 11\bar{2} 1\} \langle \bar{1} \bar{1} 26 \rangle $$ extension twinning (T2) mode was analyzed in a commercial purity Ti sample after 2 pct tensile strain imposed by four-point bending. The sample had a moderate c-axis fiber texture parallel to the tensile axis. Compared with the many $$ \{ 10\bar{1} 2\} \langle \bar{1} 011 \rangle $$ extension (T1) twins that formed in 6 pct of the grains, T2 twins were identified in 0.25 pct of the grains by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) maps. Most of the T2 twins exhibited irregular twin boundaries (TBs) on one side of the twin. High-resolution EBSD revealed both intermediate orientations at some matrix/twin interfaces and substantial lattice rotation within some T2 twins. Interactions between matrix 〈c + a〉 dislocations $$ \frac{1}{3} \langle 1\bar{2} 13 \rangle $$ and a $$ \{ 11\bar{2} 1\} $$ T2 twin were investigated by combining SEM/EBSD slip trace characterization and Laue microdiffraction peak streak analysis. 〈c + a〉 dislocations that originally glided on a pyramidal plane in the matrix were found on other planes in both the matrix and the twin, which was attributed to extensive cross-slip of the screw component, whose Burgers vector was parallel to the twinning plane. On the other hand, thickening of the twin could engulf some pile-up edge components in front of the TB. During this process, these 〈c + a〉 dislocations transmuted from a pyramidal plane $$ (0\bar{1} 11) $$ in the matrix to a prismatic plane $$ (\bar{1} 010)_{\text{T}} $$ in the twin lattice. Finally, possible mechanisms for the nucleation and growth of T2 twins will be discussed.

Tool-Path Calculation for NC Machining the Helical Surface of Screw Shaft Based on the End Section

With the increasingly wide use of screw components in industry, technologies for efficiently fabricating the complex helical surfaces on these components play an important role in modern metal-working. In this paper, the helical surface of a screw shaft is mathematically modeled based on the profile of its end section, and the whirlwind milling process and its requirements for machining the screw using standard cutter blades are overviewed. Then a theoretical approach to calculating the tool path and the corresponding movements of the machine axes for producing such a helical surface are presented. Finally a case study is provided with an example screw whose end section is described using sampled points.

Safety Recession Characteristics of High Speed Milling Cutter

In order to research the influence property of pre-tightening force, centrifugal force and dynamic cutting force on cutter components, and reveal safety recession of the cutter components, this paper makes load analysis for high speed milling cutter, studies cutter deformation, joint surface crushing, ductile fracture and other damage characteristics, gets the safety recession order of cutter components by safety margin theory. By means of the grey correlation analysis for safety recession, acquires the safety recession model of cutter. The experiment results confirmed that screw component firstly occurs the safety recession, and the safety recession model has certain reliability.

Threading Dislocations in 4H-SiC Observed by Double-Crystal X-Ray Topography

Threading dislocations in 4H-SiC wafer were observed by using a double crystal X-ray topography technique. The incident beam was collimated by the first crystal so that the angular divergence was reduced to sufficiently smaller than the rocking curve of a diffraction of the specimen (the second crystal). At off-Bragg angles, cross-sectional projection of threading dislocations appeared on the topographic images. From these results, we found that outcrops of the screw dislocation at the wafer surfaces suffered from significant strain, giving strong contrast on the topograph even under the condition, $\vec{g}\cdot\vec{b}=0$. In addition, the edge and screw components in the mixed dislocations were clearly distinguished.