Articles de revues : « Polytope de force »

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Wei, Baochen, et Feng Gao. « Output force capacity polytope approach for actuator forces selection of three degrees of freedom excavating manipulator ». Proceedings of the Institution of Mechanical Engineers, Part C : Journal of Mechanical Engineering Science 228, n^o 11 (2 décembre 2013) : 2007–17. http://dx.doi.org/10.1177/0954406213512629.

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Output force or velocity polytopes are usually used as an index of the manipulability of robot. This paper discusses the relationship between the actuator force and the variation of the output force capacity polytope and proposes the output force capacity polytope method for the selection of actuator forces of a three degrees of freedom excavating manipulator with the requirement that the output force on the end effector is a set of all possible forces rather than a single force. In this paper, the method to calculate capacity polytope is introduced with the consideration of gravity effect. With the concept that the required output force space should be within the output capacity polytope, the output force capacity polytope approach for selecting actuator forces is proposed.

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Rezzoug, Nasser, Vincent Hernandez et Philippe Gorce. « Upper-Limb Isometric Force Feasible Set : Evaluation of Joint Torque-Based Models ». Biomechanics 1, n^o 1 (18 avril 2021) : 102–17. http://dx.doi.org/10.3390/biomechanics1010008.

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A force capacity evaluation for a given posture may provide better understanding of human motor abilities for applications in sport sciences, rehabilitation and ergonomics. From data on posture and maximum isometric joint torques, the upper-limb force feasible set of the hand was predicted by four models called force ellipsoid, scaled force ellipsoid, force polytope and scaled force polytope, which were compared with a measured force polytope. The volume, shape and force prediction errors were assessed. The scaled ellipsoid underestimated the maximal mean force, and the scaled polytope overestimated it. The scaled force ellipsoid underestimated the volume of the measured force distribution, whereas that of the scaled polytope was not significantly different from the measured distribution but exhibited larger variability. All the models characterized well the elongated shape of the measured force distribution. The angles between the main axes of the modelled ellipsoids and polytopes and that of the measured polytope were compared. The values ranged from 7.3° to 14.3°. Over the entire surface of the force ellipsoid, 39.7% of the points had prediction errors less than 50 N; 33.6% had errors between 50 and 100 N; and 26.8% had errors greater than 100 N. For the force polytope, the percentages were 56.2%, 28.3% and 15.4%, respectively.

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Chiacchio, Pasquale, Yann Bouffard-Vercelli et Fran�ois Pierrot. « Force polytope and force ellipsoid for redundant manipulators ». Journal of Robotic Systems 14, n^o 8 (août 1997) : 613–20. http://dx.doi.org/10.1002/(sici)1097-4563(199708)14:8<613 ::aid-rob3>3.0.co;2-p.

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Peña Fernández, César A. « Control of Flexible Manipulator Robots Based on Dynamic Confined Space of Velocities : Dynamic Programming Approach ». Journal of Robotics and Control (JRC) 3, n^o 6 (4 décembre 2022) : 743–53. http://dx.doi.org/10.18196/jrc.v3i6.16454.

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Linear Parameter Varying models-based Model Predictive Control (LPV-MPC) has stood out in manipulator robots because it presents well-rejection to dynamic uncertainties in flexible joints. However, it has become too weak when the MPC's optimization problem does not include kinematic constraints-based conditions. This paper uses dynamic confined space of velocities (DCSV) to include these conditions as a recursive polytopic constraint, guaranteeing optimal dependency on a simplex scheduling parameter. To this end, the local frame's velocities and torque/force preload of joints (related to violation of kinematic constraints) are associated with different time scale dynamics such that DCSV correlates them as a polytope. So, a classical LPV-MPC will be updated using a dynamic programming approach according to the DCSV-based polytope. As a result, one lemma about DCSV-based recursive polytope and a five-step procedure for two decoupled close-loop schemes with different time scales compose the LPV-MPC proposed method. Numerical validation shows that even for relevant flexibility situations, trajectory tracking performance is improved by tuning finite horizons and optimization problem constraints regarding DCSV's behavior.

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Ferrolho, Henrique, Wolfgang Merkt, Carlo Tiseo et Sethu Vijayakumar. « Residual force polytope : Admissible task-space forces of dynamic trajectories ». Robotics and Autonomous Systems 142 (août 2021) : 103814. http://dx.doi.org/10.1016/j.robot.2021.103814.

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Firmani, Flavio, Alp Zibil, Scott B. Nokleby et Ron P. Podhorodeski. « Wrench capabilities of planar parallel manipulators. Part II : Redundancy and wrench workspace analysis ». Robotica 26, n^o 6 (novembre 2008) : 803–15. http://dx.doi.org/10.1017/s0263574708004396.

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SUMMARYThis part of the paper investigates the wrench capabilities of redundantly actuated planar parallel manipulators (PPMs). The wrench capabilities of PPMs are determined by mapping a hypercube from the torque space into a polytope in the wrench space. For redundant PPMs, one actuator output capability constrains the wrench space with a smaller polytope that is contained inside the overall polytope. Performance indices are derived from six study cases. These indices are employed to analyze the wrench workspace for constant orientation of the mobile platform of the non-redundant 3-RRR PPM, and actuation redundant 4-RRR and 3-RRR PPMs, where the underline indicates the actuated joints. A comparison of the results shows that both of the redundantly-actuated PPMs give better wrench capabilities than the non-redundant PPM. However, it is shown that scaled for the operational cost (wrench capabilities divided by total actuation output) the non-redundant 3-RRR PPM provides the highest maximum reachable force, the 3-RRR PPM produces the highest isotropic force, and the 4-RRR yields the highest reachable moment.

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Andrews, Lawrence C., et Herbert J. Bernstein. « The geometry of Niggli reduction:BGAOL–embedding Niggli reduction and analysis of boundaries ». Journal of Applied Crystallography 47, n^o 1 (30 janvier 2014) : 346–59. http://dx.doi.org/10.1107/s1600576713031002.

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Niggli reduction can be viewed as a series of operations in a six-dimensional space derived from the metric tensor. An implicit embedding of the space of Niggli-reduced cells in a higher-dimensional space to facilitate calculation of distances between cells is described. This distance metric is used to create a program,BGAOL, for Bravais lattice determination. Results fromBGAOLare compared with results from other metric based Bravais lattice determination algorithms. This embedding depends on understanding the boundary polytopes of the Niggli-reduced coneNin the six-dimensional spaceG6. This article describes an investigation of the boundary polytopes of the Niggli-reduced coneNin the six-dimensional spaceG6by algebraic analysis and organized random probing of regions near one-, two-, three-, four-, five-, six-, seven- and eightfold boundary polytope intersections. The discussion of valid boundary polytopes is limited to those avoiding the mathematically interesting but crystallographically impossible cases of zero-length cell edges. Combinations of boundary polytopes without a valid intersection in the closure of the Niggli cone or with an intersection that would force a cell edge to zero or without neighboring probe points are eliminated. In all, 216 boundary polytopes are found. There are 15 five-dimensional boundary polytopes of the fullG6Niggli coneN.

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Boudreau, Roger, Scott Nokleby et Marise Gallant. « Wrench Capabilities of a Kinematically Redundant Planar Parallel Manipulator ». Robotica 39, n^o 9 (26 janvier 2021) : 1601–16. http://dx.doi.org/10.1017/s0263574720001381.

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SUMMARYThis paper presents a methodology to obtain the wrench capabilities of a kinematically redundant planar parallel manipulator using a wrench polytope approach. A methodology proposed by others for non-redundant and actuation-redundant manipulators is adapted to a kinematically redundant manipulator. Four wrench capabilities are examined: a pure force analysis, the maximum force for a prescribed moment, the maximum reachable force, and the maximum moment with a prescribed force. The proposed methodology, which finds the exact explicit solution for three of the four wrench capabilities, does not use optimization and is very efficient.

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Okabe, Kousuke. « Translating Manipulating Force Polytope by Dynamics on Kinematical Redundant Manipulators ». Journal of the Robotics Society of Japan 41, n^o 3 (2023) : 303–8. http://dx.doi.org/10.7210/jrsj.41.303.

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Zheng, Yu. « Real-time contact force distribution using a polytope hierarchy in the grasp wrench set ». Robotics and Autonomous Systems 99 (janvier 2018) : 97–109. http://dx.doi.org/10.1016/j.robot.2017.10.014.

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Jeong, Seonghee, et Takayuki Takahashi. « Impact Force Reduction of Manipulators Using a Dynamic Acceleration Polytope and Flexible Collision Detection Sensor ». Advanced Robotics 23, n^o 3 (janvier 2009) : 367–83. http://dx.doi.org/10.1163/156855308x397578.

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Firmani, Flavio, Alp Zibil, Scott B. Nokleby et Ron P. Podhorodeski. « Wrench capabilities of planar parallel manipulators. Part I : Wrench polytopes and performance indices ». Robotica 26, n^o 6 (novembre 2008) : 791–802. http://dx.doi.org/10.1017/s0263574708004384.

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SUMMARYThis paper is organized in two parts. In Part I, the wrench polytope concept is presented and wrench performance indices are introduced for planar parallel manipulators (PPMs). In Part II, the concept of wrench capabilities is extended to redundant manipulators and the wrench workspace of different PPMs is analyzed. The end-effector of a PPM is subject to the interaction of forces and moments. Wrench capabilities represent the maximum forces and moments that can be applied or sustained by the manipulator. The wrench capabilities of PPMs are determined by a linear mapping of the actuator output capabilities from the joint space to the task space. The analysis is based upon properly adjusting the actuator outputs to their extreme capabilities. The linear mapping results in a wrench polytope. It is shown that for non-redundant PPMs, one actuator output capability constrains the maximum wrench that can be applied (or sustained) with a plane in the wrench space yielding a facet of the polytope. Herein, the determination of wrench performance indices is presented without the expensive task of generating polytopes. Six study cases are presented and performance indices are derived for each study case.

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Kajio, Yusuke, Hiroyasu Iwata, Yoshihiro Tokumasu et Shigeki Sugano. « 2A1-S-047 Body Posture Control Coordinating Stability and Directional Force Tolerability Using Force Polytope(Humanoid 3,Mega-Integration in Robotics and Mechatronics to Assist Our Daily Lives) ». Proceedings of JSME annual Conference on Robotics and Mechatronics (Robomec) 2005 (2005) : 173. http://dx.doi.org/10.1299/jsmermd.2005.173_4.

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Frewin, Christopher L., Camilla Coletti, Christian Riedl, Ulrich Starke et Stephen E. Saddow. « A Comprehensive Study of Hydrogen Etching on the Major SiC Polytypes and Crystal Orientations ». Materials Science Forum 615-617 (mars 2009) : 589–92. http://dx.doi.org/10.4028/www.scientific.net/msf.615-617.589.

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A comprehensive study on the hydrogen etching of numerous SiC polytype surfaces and orientations has been performed in a hot wall CVD reactor under both atmospheric and low pressure conditions. The polytypes studied were 4H and 6H-SiC as well as 3C-SiC grown on Si substrates. For the hexagonal polytypes the wafer surface orientation was both on- and off-axis, i.e. C and Si face. The investigation includes the influence of the prior surface polishing method on the required etching process parameters. 3C-SiC was also studied grown in both the (100) and (111) orientations. After etching, the samples were analyzed via atomic force microscopy (AFM) to determine the surface morphology and the height of the steps formed. For all cases the process conditions necessary to realize a well-ordered surface consisting of unit cell and sub-unit cell height steps were determined. The results of these experiments are summarized and samples of the corresponding AFM analysis presented.

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Kim, Hong-Ki, Soo In Kim, Seongjun Kim, Nam-Suk Lee, Hoon-Kyu Shin et Chang Woo Lee. « Relation between work function and structural properties of triangular defects in 4H-SiC epitaxial layer : Kelvin probe force microscopic and spectroscopic analyses ». Nanoscale 12, n^o 15 (2020) : 8216–29. http://dx.doi.org/10.1039/c9nr10126h.

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In the defective SiC epitaxial layer, the work function variation was observed by Kelvin probe force microscopy (KPFM), and the work function difference came from the variation of polytype and the disordered surface.

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Rezzoug, N., V. Hernandez, J. Jacquier-Bret et P. Gorce. « Comparison between model-based and measured force polytopes : towards isometric force capacity evaluation ». Computer Methods in Biomechanics and Biomedical Engineering 16, sup1 (juillet 2013) : 172–74. http://dx.doi.org/10.1080/10255842.2013.815972.

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Corves, Burkhard, Jan Brinker, Michael Lorenz et Martin Wahle. « Design methodology for translational parallel manipulators exhibiting actuation redundancy ». Proceedings of the Institution of Mechanical Engineers, Part C : Journal of Mechanical Engineering Science 230, n^o 3 (18 juin 2015) : 425–36. http://dx.doi.org/10.1177/0954406215591048.

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In general, spatial manipulation of objects can be accomplished by parallel manipulators, whose number of actuators is equal to the demanded number of degrees of freedom. In order to improve, for example, positioning accuracy, stiffness characteristics, and transmission behavior, redundant drives can be added to the manipulator. Accordingly, this paper presents a methodology for the design of a translational parallel manipulator with redundant actuation. Based on the results of systematic structural syntheses and developed selection criteria, two valid configurations (i.e. 3-PŘŘŘ and 3-PUU) are analyzed. Since feasibility and performance of these configurations are dependent on the base geometry, five types of base geometries are introduced. First, the geometric parameters of each of the resulting 10 combinations of nonredundant configurations and base geometries are optimized by minimizing the maximal actuation force within a prescribed workspace. Second, the best combinations are used to generate redundant configurations with six legs. These redundant configurations are then analyzed with respect to the potential of improvement concerning homogenization of end-effector forces using force polytopes. It is shown that redundant actuation significantly improves the distribution of end-effector forces. This improvement has a positive influence on positioning accuracy and acceleration capabilities. In addition to these aspects, for further analysis it is planned to investigate the influence of homogenized end-effector forces on the dimensioning of actuators and finally on the energy efficiency of the entire configuration.

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Camarda, Massimo, Antonino La Magna, Andrea Severino et Francesco La Via. « Extended Study of the Step-Bunching Mechanism during the Homoepitaxial Growth of SiC ». Materials Science Forum 615-617 (mars 2009) : 117–20. http://dx.doi.org/10.4028/www.scientific.net/msf.615-617.117.

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We discuss the possible source of surface instabilities (with specific reference to the step bunching phenomena) during the growth of cubic and hexagonal Silicon Carbide polytypes. For this analysis we use: results from super-lattice Kinetic Monte Carlo simulations, atomic force microscope surface analysis and literature data. We show that only hexagonal polytypes with misorientation cut toward the <11-20> direction suffer “intrinsically” the step bunching phenomena (i.e. it are present, independently on the growth conditions) whereas cubic polytypes and hexagonal ones with misorientation cut toward the <10-10> direction do not.

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Kaur, Harjeet. « Points to Ponder in the Study of Cadmium Iodide ». International Letters of Chemistry, Physics and Astronomy 27 (février 2014) : 1–5. http://dx.doi.org/10.18052/www.scipress.com/ilcpa.27.1.

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During the growth of cadmium iodide crystals from solution, it has been observed that sometimes the crystal suddenly rotates. Probably, the convection currents are providing the force for the rotation. Mathematically, this force can be estimated and it must be greater than the van-der Walls force acting between adjacent sandwiches of cadmium iodide and can give rise to polytypic phase transformations. Further, studies of vacancies and the density measurements of various polytypes can help in understanding the phenomenon of polytypism better.

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Kaur, Harjeet. « Points to Ponder in the Study of Cadmium Iodide ». International Letters of Chemistry, Physics and Astronomy 27 (6 février 2014) : 1–5. http://dx.doi.org/10.56431/p-03c14b.

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During the growth of cadmium iodide crystals from solution, it has been observed that sometimes the crystal suddenly rotates. Probably, the convection currents are providing the force for the rotation. Mathematically, this force can be estimated and it must be greater than the van-der Walls force acting between adjacent sandwiches of cadmium iodide and can give rise to polytypic phase transformations. Further, studies of vacancies and the density measurements of various polytypes can help in understanding the phenomenon of polytypism better.

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Pezoldt, Jörg, et Andrei Alexandrovich Kalnin. « Defect Interactions and Polytype Transitions ». Advanced Materials Research 324 (août 2011) : 217–20. http://dx.doi.org/10.4028/www.scientific.net/amr.324.217.

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The polytype transitions are caused by disorder generation in the initial structure due to energy dissipation. The disorder is strongly related to the formation and propagation of stacking faults and partial dislocations. Collective and selective interactions between these defects result in a stability loss of the original structure leading to nonequilibrium phase transition occur if the critical point is reached. The stability of the defect subsystem was investigated in the stationary state for three types of stacking faults. The combination of the stability analysis with the defect generation processes during the different technological and devices operation processes allows predicting the critical values for the external forces and fluxes leading to phase transitions.

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Ben Haj Amara, A., J. Ben Brahim, N. Ben Ayed et H. Ben Rhaiem. « Presence de nacrite sur d'anciens gisements de Pb-Zn du Nord Tunisien ». Clay Minerals 31, n^o 1 (mars 1996) : 127–30. http://dx.doi.org/10.1180/claymin.1996.031.1.11.

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La nacrite constitue le plus rare polytype des kaolinites. Traditionnellement, on attribue á ce minéral une genèse hydrothermale (Ross & Kerr, 1931; Hanson et al., 1981 ), cependant Btihmann (1988) décrit sa formation en condition normale de température et de pression dans des milieux de solution et de force ionique élevées. Dans ce travail, la nacrite est mise en évidence dans deux anciens gisements de Pb-Zn situés dans les massifs de Jbel Slata et Garn Halfaya au nord de la Tunisie (Fig.l).

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Shin, Yun Ji, Soo In Kim, Hyeon Jin Jung, Chang Woo Lee et Wook Bahng. « Nanomechanical Analysis of Triangular Defect in 4H-SiC Epilayer ». Materials Science Forum 778-780 (février 2014) : 394–97. http://dx.doi.org/10.4028/www.scientific.net/msf.778-780.394.

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We report an investigation of the formation of triangular defects (TDs) in 4H–SiC expitaxial layers using Kelvin probe force microscopy (KPFM) and a nano-indenter. The results provide valuable information on the crystallographic structure, including the polytype nature of the TDs and surface potential profile. The TDs were also characterized using micro-Raman spectroscopy and high-resolution transmission electron microscopy. We found that the TDs were composed of a thick 3C-SiC band, as well as stacking faults (SFs) in the 4H-SiC epilayer.

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Zhong, Wei, et Cong Yu. « The Critical Core Mass of Rotating Planets ». Astrophysical Journal 926, n^o 1 (1 février 2022) : 43. http://dx.doi.org/10.3847/1538-4357/ac4261.

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Abstract The gravitational harmonics measured from the Juno and Cassini spacecraft help us specify the internal structure and chemical elements of Jupiter and Saturn, respectively. However, we still do not know much about the impact of rotation on the planetary internal structure as well as on their formation. The centrifugal force induced by the rotation deforms the planetary shape and partially counteracts the gravitational force. Thus, rotation will affect the critical core mass of the exoplanet. Once the atmospheric mass becomes comparable to the critical core mass, the planet will enter the runaway accretion phase and become a gas giant. We have confirmed that the critical core masses of rotating planets depend on the stiffness of the polytrope, the outer boundary conditions, and the thickness of the isothermal layer. The critical core mass with the Bondi boundary condition is determined by the surface properties. The critical core mass of a rotating planet will increase with the core gravity (i.e., the innermost density). For the Hill boundary condition, the soft polytrope shares the same properties as planets with the Bondi boundary condition. Because the total mass for planets with the Hill boundary condition increases with the decrease of the polytropic index, a higher core gravity is required for rotating planets. As a result, the critical core mass in the stiff Hill model sharply increases. The rotational effects become more important when the radiative and convective regions coexist. Further, the critical core mass of planets with the Hill (Bondi) boundary increases noticeably as the radiative layer becomes thinner (thicker).

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Pesnell, W. Dean. « Pulsations of Eötvös Spheres ». International Astronomical Union Colloquium 111 (1989) : 282. http://dx.doi.org/10.1017/s0252921100011908.

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AbstractA derivation of Eötvös spheres and their pulsations is presented. These spheres incorporate a short range force in their gravitational potential. We used a Yukawa formulation to agree with other workers in this field, although a slightly different form would guarantee a gravitational force that is always attractive. Various relationships are obeyed by these obejcts, much like the polytropes to which they are similar. An attempt is made to test whether a short range force can resolve two outstanding problems in astrophysics. These are the Cepheid mass discrepancy and the frequencies of high-order solar p-modes. The disagreement in predicted and observed Cepheid masses cannot be explained by a repulsive force, but the solar p-modes have their asympotoic frequency decreased. Linear fits to sequences of models are presented for the radial eigenfrequencies and asymptotic frequencies for nonradial pulsations.

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Nuzhdin, Sergey V. « The distribution of transposable elements on X chromosomes from a natural population of Drosophila simulans ». Genetical Research 66, n^o 2 (octobre 1995) : 159–66. http://dx.doi.org/10.1017/s0016672300034509.

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SummaryThe distribution of 13 transposable element families along 15 X chromosomes from an African natural population of Drosophila simulans was determined by in situ hybridization to polytene chromosomes. The transposable elements cloned from Drosophila melanogaster all hybridized with Drosophila simulans chromosomes. The number of copies per family was 3·5 times lower in the latter species and correlated with the copy number per family in Drosophila melanogaster. With the exception of 297, the copy number per chromosome followed a Poisson distribution. Element frequencies per chromosome band were generally low. However, several sites of the distal region and the base of the X chromosome had high frequencies of occupation. Elements had higher abundance at the base of the chromosome compared to distal regions. Overall, the distribution of transposable elements in Drosophila simulans is similar to that found in Drosophila melanogaster. These data provide evidence for the operation of a force (or forces) opposing transpositional increase in copy number, and that this force is weaker at the bases of chromosomes, consistent with the idea that recombination between elements at non-homologous sites contains TE copy number. The reduction in copy number of all TE families in Drosophila simulans compared to Drosophila melanogaster can be explained by stronger selection against transposable element multiplication and/or lower rates of transposition in Drosophila simulans.

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Eriksson, Jens, Donatella Puglisi, Remigijus Vasiliauskas, Anita Lloyd Spetz et Rositza Yakimova. « Thickness Uniformity and Electron Doping in Epitaxial Graphene on SiC ». Materials Science Forum 740-742 (janvier 2013) : 153–56. http://dx.doi.org/10.4028/www.scientific.net/msf.740-742.153.

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Large variations have been observed in the uniformity and carrier concentration of epitaxial graphene grown on SiC by sublimation for samples grown under identical conditions and on nominally on-axis hexagonal SiC (0001) substrates. We have previously shown that these issues are both related to the morphology of the graphene-SiC surface after sublimation growth. Here we present a study on how the substrate polytype, substrate surface morphology and surface restructuring during sublimation growth affect the uniformity and carrier concentration in epitaxial graphene on SiC. These issues were investigated employing surface morphology mapping by atomic force microscopy coupled with local surface potential mapping using Scanning Kelvin probe microscopy.

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Wasyluk, Joanna, Tatiana S. Perova, Sergey A. Kukushkin, Andrey V. Osipov, Nikolay A. Feoktistov et Sergey A. Grudinkin. « Raman Investigation of Different Polytypes in SiC Thin Films Grown by Solid-Gas Phase Epitaxy on Si (111) and 6H-SiC Substrates ». Materials Science Forum 645-648 (avril 2010) : 359–62. http://dx.doi.org/10.4028/www.scientific.net/msf.645-648.359.

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Raman spectroscopy was applied to investigate a series of SiC films grown on Si and 6H-SiC substrates by a new method of solid gas phase epitaxy. During the growth characteristic voids are formed in Si at the SiC/Si interface. Raman peak position, intensity and linewidth were used to characterize the quality and the polytype structure of the SiC layers. A large enhancement in the peak intensity of the transverse optical and longitudinal optical phonon modes of SiC is observed for the Raman signal measured at the voids. In addition, scanning electron microscopy and atomic force microscopy were used to investigate the surface morphology of SiC layers.

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Miao, M. S., et Walter R. L. Lambrecht. « Stacking Faults and 3C Quantum Wells in Hexagonal SiC Polytypes ». Materials Science Forum 527-529 (octobre 2006) : 351–54. http://dx.doi.org/10.4028/www.scientific.net/msf.527-529.351.

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The electronic driving force for growth of stacking faults (SF) in n-type 4H SiC under annealing and in operating devices is discussed. This involves two separate aspects: an overall thermodynamic driving force due to the capture of electrons in interface states and the barriers that need to be overcome to create dislocation kinks which advance the motion of partial dislocations and hence expansion of SF. The second problem studied in this paper is whether 3C SiC quantum wells in 4H SiC can have band gaps lower than 3C SiC. First-principles band structure calculations show that this is not the case due to the intrinsic screening of the spontaneous polarization fields.

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Tymicki, Emil, Krzysztof Grasza, Katarzyna Racka, Marcin Raczkiewicz, Tadeusz Łukasiewicz, Maciej Gała, Kinga Kościewicz, Ryszard Diduszko et Rafał Bożek. « Growth of 4H-SiC Single Crystals on 6H-SiC Seeds with an Open Backside by PVT Method ». Materials Science Forum 615-617 (mars 2009) : 15–18. http://dx.doi.org/10.4028/www.scientific.net/msf.615-617.15.

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4H-SiC single crystals grown by the seeded physical vapour transport method have been investigated. These crystals were grown on 6H-SiC seeds. The influence of the seed temperature, form and granulation of SiC source materials on the stability and efficiency of the 4H polytype growth have been investigated. A new way of the seed mounting - with an open backside - has been used. Crystals obtained were free of structural defects in the form of hexagonal voids. The crystalline structure of SiC crystals was investigated by EBSD (Electron Backscatter Diffraction) and X-Ray diffraction methods. Moreover, defects in crystals and wafers cut from these crystals were examined by optical, scanning electron and atomic force microscopy combined with KOH etching.

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Patton, Ron, Lejun Chen et Supat Klinkhieo. « An LPV pole-placement approach to friction compensation as an FTC problem ». International Journal of Applied Mathematics and Computer Science 22, n^o 1 (1 mars 2012) : 149–60. http://dx.doi.org/10.2478/v10006-012-0011-z.

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An LPV pole-placement approach to friction compensation as an FTC problemThe concept of combining robust fault estimation within a controller system to achieve active Fault Tolerant Control (FTC) has been the subject of considerable interest in the recent literature. The current study is motivated by the need to develop model-based FTC schemes for systems that have no unique equilibria and are therefore difficult to linearise. Linear Parameter Varying (LPV) strategies are well suited to model-based control and fault estimation for such systems. This contribution involves pole-placement within suitable LMI regions, guaranteeing both stability and performance of a multi-fault LPV estimator employed within an FTC structure. The proposed design strategy is illustrated using a nonlinear two-link manipulator system with friction forces acting simultaneously at each joint. The friction forces, regarded as a special case of actuator faults, are estimated and their effect is compensated within a polytope controller system, yielding a robust form of active FTC that is easy to apply to real robot systems.

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Lejda, Katarzyna, Janusz Partyka et Jerzy F. Janik. « Thermogravimetric/Thermal–Mass Spectroscopy Insight into Oxidation Propensity of Various Mechanochemically Made Kesterite Cu2ZnSnS4 Nanopowders ». Materials 17, n^o 6 (7 mars 2024) : 1232. http://dx.doi.org/10.3390/ma17061232.

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Thermogravimetry coupled with thermal analysis and quadrupole mass spectroscopy TGA/DTA-QMS were primarily used to assess the oxidation susceptibility of a pool of nanocrystalline powders of the semiconductor kesterite Cu2ZnSnS4 for prospective photovoltaic applications, which were prepared via the mechanochemically assisted synthesis route from two different precursor systems. Each system, as confirmed by XRD patterns, yielded first the cubic polytype of kesterite with defunct semiconductor properties, which, after thermal annealing at 500 °C under neutral gas atmosphere, was converted to the tetragonal semiconductor polytype. The TGA/DTA-QMS determinations up to 1000 °C were carried out under a neutral argon Ar atmosphere and under a dry, oxygen-containing gas mixture of O2:Ar = 1:4 (vol.). The mass spectroscopy data confirmed that under each of the gas atmospheres, a distinctly different, multistep evolution of such oxygen-bearing gaseous compounds as sulfur oxides SO2/SO3, carbon dioxide CO2, and water vapor H2O was taking place. The TGA/DTA changes in correlation with the nature of evolving gases helped in the elucidation of the plausible chemistry linked to kesterite oxidation, both in the stage of nanopowder synthesis/storage at ambient air conditions and during forced oxidation up to 1000 °C in the dry, oxygen-containing gas mixture.

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Charlesworth, Brian, et Angela Lapid. « A study of ten families of transposable elements on X chromosomes from a population of Drosophila melanogaster ». Genetical Research 54, n^o 2 (octobre 1989) : 113–25. http://dx.doi.org/10.1017/s0016672300028482.

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SummaryData were collected on the distribution of ten families of transposable elements among fourteen X chromosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization to polytene chromosomes. It was found that, with the exception of roo, the copy number per chromosome followed a Poisson distribution. There was no evidence for linkage disequilibrium, either within or between families. Some pairs of families of elements were correlated with respect to the identity of the sites that were occupied in the sample, although there was no evidence for a correlation with respect to the sites at which elements attained relatively high frequencies. Elements appeared to be distributed randomly along the distal part of the X chromosome. There was, however, a strong tendency for elements to accumulate at the base of the chromosome. Element frequencies per chromosome band were generally low, except at the base of the chromosome where bands in subdivisions 19E and 20A sometimes had high frequencies of occupation. These results are discussed in the light of models of the population dynamics of transposable elements. It is concluded that they provide strong evidence for the operation of a force or forces opposing transpositional increase in copy number. The accumulation of elements at the base of the chromosome is consistent with the idea that unequal exchange between elements at non-homologous sites is such a force, although other possibilities cannot be excluded at present. The data suggest that the rate of transposition per element per generation is of the order of 10−4, for the elements included in this study.

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Baghestan, K., S. M. Rezaei, H. A. Talebi et M. Zareinejad. « Robust force control in a novel electro-hydraulic structure using polytopic uncertainty representation ». ISA Transactions 53, n^o 6 (novembre 2014) : 1873–80. http://dx.doi.org/10.1016/j.isatra.2014.08.002.

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Davydov, Sergey Y. « Pressure Effect on the Elastic Properties of SiC Polytypes ». Materials Science Forum 483-485 (mai 2005) : 303–6. http://dx.doi.org/10.4028/www.scientific.net/msf.483-485.303.

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The pressure dependences of the second-order elastic constants ij C and the velocity of sound in 3C-SiC and 2H-SiC crystals are calculated in the framework of the Keating model with the additional assumption that the central α and the noncentral β force constants are linear functions of external hydrostatic pressure. Grüneisen parameters for the different acoustic modes of 3C-SiC have been also calculated. The third –order elastic constants ijk C for 3C-SiC are determined from the dependences of ij C on the pressure.

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Ashida, Koji, Toru Aiso, Manabu Okamoto, Hirokazu Seki, Makoto Kitabatake et Tadaaki Kaneko. « Low Energy Electron Channeling Contrast Imaging from 4H-SiC Surface by SEM and its Comparison with CDIC-OM and PL Imaging ». Materials Science Forum 897 (mai 2017) : 193–96. http://dx.doi.org/10.4028/www.scientific.net/msf.897.193.

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Low energy electron channeling contrast imaging (LE-ECCI) by scanning electron microscopy (SEM) was adopted to evaluate both the direction and length of the topmost hexagonal stacking sequence of the Si–C bilayers on 4H-SiC (0001). LE-ECCI revealed the change in the dangling bond configuration at step edges depending on SiC thermal etching rate, which was difficult to be detected by optical microscope (OM) and even by atomic force microscopy (AFM). Furthermore, LE-ECCI was applied to evaluate the atomic structure of polytype inclusions in commercially available 3-inch diameter 4o off-axis 4H-SiC (0001) epitaxial wafer. The validity of LE-ECCI was discussed by comparing the one with two kinds of widely used wafer inspection methods: confocal OM with differential interference contrast (CDIC-OM) and photoluminescence (PL) imaging.

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Eid, Jessica, Jean Louis Santailler, Bernard Ferrand, Pierre Ferret, J. Pesenti, Alain Basset, Antoine Passero et al. « Growth of Cubic Silicon Carbide Crystals from Solution ». Materials Science Forum 527-529 (octobre 2006) : 123–26. http://dx.doi.org/10.4028/www.scientific.net/msf.527-529.123.

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Cubic-silicon carbide crystals have been grown from solution by using the traveling-zone method. In this technique a molten silicon zone heated by induction coils is held between two rods of polycrystalline silicon carbide. Due to the growth set-up and boundary conditions, different mass transfer mechanisms are operative : diffusion, buoyancy, Marangoni convection and forced convection. The growth experiments have been performed on various seed crystals. Cubic SiC crystals were grown with a [111] habit on the [0001] silicon faces of 4H SiC seeds. The polytype 3C-SiC was identified by Transmission Electron Microscopy. Micro Raman spectroscopy and photoluminescence analyses showed good crystalline quality with few 6H inclusions.

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Lignières, F., et M. Rieutord. « Oscillations of Fast Rotating Stars : p-Modes in Centrifugally Flattened Polytropes ». Symposium - International Astronomical Union 215 (2004) : 414–15. http://dx.doi.org/10.1017/s0074180900195956.

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Oscillation modes of rapidly rotating stars have not yet been calculated with precision, rotational effects being generally approximated by perturbation methods. We developed a mathematical formalism and a numerical method which fully account for the deformation of the star by the centrifugal force. The method has been first tested in the case of Maclaurin spheroids and then applied to uniformly rotating polytropic stars.

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Charlesworth, Brian, Angela Lapid et Darlene Canada. « The distribution of transposable elements within and between chromosomes in a population of Drosophila melanogaster. I. Element frequencies and distribution ». Genetical Research 60, n^o 2 (octobre 1992) : 103–14. http://dx.doi.org/10.1017/s0016672300030792.

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SummaryData were collected on the distribution of nine families of transposable elements among second and third chromosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization of element probes to polytene chromosomes. It was found that the copy numbers per chromosome in the distal sections of the chromosome arms followed a Poisson distribution. Elements appeared to be distributed randomly along the distal sections of the chromosome arms. There was no evidence for linkage disequilibrium in the distal sections of the chromosomes, but some significant disequilibrium was detected in proximal regions. There were many significant correlations between different element families with respect to the identity of the sites that were occupied in the sample. There were also significant correlations between families with respect to sites at which elements achieved relatively high frequencies. Element frequencies per chromosome band were generally low in the distal sections, but were higher proximally. These results are discussed in the light of models of the population dynamics of transposable elements. It is concluded that they provide strong evidence for the operation of a force or forces opposing transpositional increase in copy number. The data suggest that the rate of transposition perelement per generation is of the order of 10−4, for the elements included in this study.

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Lozman, Owen R., Richard J. Bushby et Jeremy G. Vinter†. « Complementary polytopic interactions (CPI) as revealed by molecular modelling using the XED force field ». Journal of the Chemical Society, Perkin Transactions 2, n^o 9 (2001) : 1446–52. http://dx.doi.org/10.1039/b103390p.

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de Grauw, C. J., A. Avogadro, D. J. van den Heuvel, K. O. vd Werf, C. Otto, Y. Kraan, N. F. van Hulst et J. Greve. « Chromatin Structure in Bands and Interbands of Polytene Chromosomes Imaged by Atomic Force Microscopy ». Journal of Structural Biology 121, n^o 1 (1998) : 2–8. http://dx.doi.org/10.1006/jsbi.1997.3929.

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Hindman, Bradley W., et Rekha Jain. « Overstable Convective Modes in a Polytropic Stellar Atmosphere ». Astrophysical Journal 943, n^o 2 (1 février 2023) : 127. http://dx.doi.org/10.3847/1538-4357/acaec4.

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Abstract Within the convection zone of a rotating star, the presence of the Coriolis force stabilizes long-wavelength convective modes. These modes, which would have been unstable if the star lacked rotation, are called overstable convective modes or thermal Rossby waves. We demonstrate that the Sun’s rotation rate is sufficiently rapid that the lower half of its convection zone could possess overstable modes. Further, we present an analytic solution for atmospheric waves that reside within a polytropic stratification. We explore in detail the properties of the overstable and unstable wave modes that exist when the polytrope is weakly unstable to convective overturning. Finally, we discuss how the thermal Rossby waves that reside within the convection zone of a star might couple with the prograde branch of the g modes that are trapped within the star’s radiative zone. We suggest that such coupling might enhance the photospheric visibility of a subset of the Sun’s g modes.

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Powell, J. Anthony, Philip G. Neudeck, Andrew J. Trunek et Phillip B. Abel. « Step Structures Produced by Hydrogen Etching of Initially Step-Free (0001) 4H-SiC Mesas ». Materials Science Forum 483-485 (mai 2005) : 753–56. http://dx.doi.org/10.4028/www.scientific.net/msf.483-485.753.

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This paper presents new observations resulting from in-situ high temperature hydrogen etching of 4H-SiC mesas that were step-free prior to initiation of etching. In particular, it was found that well-ordered pyramidal-shaped step train structures could be produced on mesa top surfaces via stepflow etching proceeding inward from the sides of mesas. In many cases, the height of steps etched inward from {112 0} mesa sides is 0.5 nm (2 Si-C bilayers), while the height of steps etched inward from {11 00} sides of the same mesa is 1.0 nm (4 Si-C bilayers, the repeat distance of the 4H-SiC polytype). We propose that stepflow etching starting from the mesa sidewall and involving step-to-step repulsive forces produces the observed step train structures.

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Damiani, C., R. H. Cameron, A. C. Birch et L. Gizon. « Rossby modes in slowly rotating stars : depth dependence in distorted polytropes with uniform rotation ». Astronomy & ; Astrophysics 637 (mai 2020) : A65. http://dx.doi.org/10.1051/0004-6361/201936251.

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Context. Large-scale Rossby waves have recently been discovered based on measurements of horizontal surface and near-surface solar flows. Aims. We are interested in understanding why it is only equatorial modes that are observed and in modelling the radial structure of the observed modes. To this aim, we have characterised the radial eigenfunctions of r modes for slowly rotating polytropes in uniform rotation. Methods. We followed Provost et al. (1981, A&A, 94, 126) and considered a linear perturbation theory to describe quasi-toroidal stellar adiabatic oscillations in the inviscid case. We used perturbation theory to write the solutions to the fourth order in the rotational frequency of the star. We numerically solved the eigenvalue problem, concentrating on the type of behaviour exhibited where the stratification is nearly adiabatic. Results. We find that for free-surface boundary conditions on a spheroid of non-vanishing surface density, r modes can only exist for ℓ = m spherical harmonics in the inviscid case and we compute their depth dependence and frequencies to leading order. For quasi-adiabatic stratification, the sectoral modes with no radial nodes are the only modes which are almost toroidal and the depth dependence of the corresponding horizontal motion scales as rm. For all r modes, except the zero radial order sectoral ones, non-adiabatic stratification plays a crucial role in the radial force balance. Conclusions. The lack of quasi-toroidal solutions when stratification is close to neutral, except for the sectoral modes without nodes in radius, follows from the need for both horizontal and radial force balance. In the absence of super- or sub-adiabatic stratification and viscosity, both the horizontal and radial parts of the force balance independently determine the pressure perturbation. The only quasi-toroidal cases in which these constraints on the pressure perturbation are consistent are the special cases where ℓ = m and the horizontal displacement scales with rm.

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Galambos, Péter, Péter Baranyi et Gusztáv Arz. « Tensor product model transformation-based control design for force reflecting tele-grasping under time delay ». Proceedings of the Institution of Mechanical Engineers, Part C : Journal of Mechanical Engineering Science 228, n^o 4 (23 mai 2013) : 765–77. http://dx.doi.org/10.1177/0954406213490375.

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The improvement of direct human–robot physical interaction has recently become one of the strongest motivation factors in robotics research. Impedance/admittance control methods are key technologies in several directions of advanced robotics such as dexterous manipulation, haptics and telemanipulation. In this paper, we propose a control scheme and a design technique for stabilising shared impedance/admittance-based bilateral telemanipulation under varying time delay. The proposed scheme introduces delay-adaptive non-linear damping to stabilise the impedance model. A modified version of the tensor product model transformation is applied to determine the tensor product type polytopic linear parameter varying (LPV) representation of the impedance controlled interaction model, such that the value of the actual time delay becomes an external parameter rather than an inherent property of the system. The main benefit of the proposed approach is that the model form it produces is amenable to the immediate application of modern, linear matrix inequality (LMI)-based multi-objective synthesis methods. The viability of the proposed methodology is demonstrated through a single DoF force reflecting tele-grasping application. The results are also confirmed through laboratory experiments which help to further highlight the perspectives of this novel approach.

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Zehe, A., A. Ramírez et J. Vázquez. « Dimorphism and perturbed growth kinetics of cadmium sulfide thin films in a chemical working solution ». Canadian Journal of Physics 80, n^o 3 (1 mars 2002) : 299–307. http://dx.doi.org/10.1139/p01-135.

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Thin films of numerous compound semiconductors, which crystallize in a (cubic) sphalerite structure, often display an alternative allotropic form as (hexagonal) wurtzite. The relative proportion of each depends in a characteristic manner on the growth conditions chosen. We applied an external magnetic flux during the growth of cadmium sulfide thin films from a chemical working solution, where the CdS molecules or small clusters are formed by a controlled reaction in an aqueous solution. The incorporation mechanism of such neutral particles into the polycrystalline CdS thin film is supposed to happen in a transition layer via a dissociation into charged constituents close to the growing interface. We found a clear effect of the external magnetic field on the film morphology, when the Lorentz force was acting parallel to the surface plane of the layer. Band-gap energy data from optical absorption measurements suggest dimorphism of the grown films, where the field application turns the commonly cubic crystal modification (Eg = 2.38 eV) into a hexagonal polytype with a 200 meV larger energy gap. PACS Nos.: 81.05Dz, 81.15Lm, 81.15Aa, 81.70Fy, 82.65My, 82.80Ch, 82.80Ej

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Song, Li, Guo Tian He, Zhi Zeng, Yan Ma, Ming Li, De Sheng Zhang, Song Wang et Ying Chun Ran. « The Model and Simulation of Magnetorheological Grease Characteristics ». Applied Mechanics and Materials 246-247 (décembre 2012) : 1214–19. http://dx.doi.org/10.4028/www.scientific.net/amm.246-247.1214.

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Magneto-rheological grease attracts the attention as a new kind of intelligent material. Under the external magnetic force, it is difficult to use traditional mathematics model to solve and analysis internal structure because of its complexity and polytropy. The modeling and simulation for complex system using computer becomes a good way to investigate complex system. Based on the electromagnetic theory and computer simulation, this paper constructed the model of shear stress to describe the characteristic of magneto-rheological grease that is to use body centered cubic pillars model to analysis the mechanism and characteristics of magneto-rheological grease. Using matlab simulink toolkit to compose the simulation program, simulate and analysis using theoretic model, and plot the shear stress curve of magneto-rheological grease with magnetic strength and particle size, and the shearing force under zero field. The simulation and experimental results indicate that the simulated shear stress curve is coincidence with experimental curve when the density of ferromagnetic particle is low, and it is lower than experimental curve when the density is high (the variation tendency is close), which verified the accuracy of theoretical model. It needs to improve the model by considering the still friction between ferromagnetic particles.

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McRobie, Allan. « The geometry of structural equilibrium ». Royal Society Open Science 4, n^o 3 (mars 2017) : 160759. http://dx.doi.org/10.1098/rsos.160759.

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Building on a long tradition from Maxwell, Rankine, Klein and others, this paper puts forward a geometrical description of structural equilibrium which contains a procedure for the graphic analysis of stress resultants within general three-dimensional frames. The method is a natural generalization of Rankine’s reciprocal diagrams for three-dimensional trusses. The vertices and edges of dual abstract 4-polytopes are embedded within dual four-dimensional vector spaces, wherein the oriented area of generalized polygons give all six components (axial and shear forces with torsion and bending moments) of the stress resultants. The relevant quantities may be readily calculated using four-dimensional Clifford algebra. As well as giving access to frame analysis and design, the description resolves a number of long-standing problems with the incompleteness of Rankine’s description of three-dimensional trusses. Examples are given of how the procedure may be applied to structures of engineering interest, including an outline of a two-stage procedure for addressing the equilibrium of loaded gridshell rooves.

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MIRZA, BABUR M. « THE EQUILIBRIUM STRUCTURE OF CHARGED ROTATING RELATIVISTIC STARS ». International Journal of Modern Physics D 17, n^o 12 (novembre 2008) : 2291–304. http://dx.doi.org/10.1142/s021827180801387x.

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General relativistic equilibrium conditions imply that an electrically charged compact star, in a spherically symmetric configuration, can sustain a huge amount of electric charge (up to 1020 C). The equilibrium, however, is reached under very critical conditions such that a perturbation to the stellar structure can cause these systems to collapse. We study the effects of rotation in charged compact stars and obtain conditions, the modified Tolman–Oppenheimer–Volkoff (TOV) equations, under which such stars form a stable gravitational system against Coulomb repulsion. We assume the star to be rotating slowly. We also assume that the charge density is proportional to the mass density everywhere inside the star. The modified TOV equations for hydrostatic equilibrium are integrated numerically for the general equation of state for a polytrope. The detailed numerical study shows that the centrifugal force adds to the Coulomb pressure in the star. In the stable equilibrium configurations, therefore, a loss in stellar mass (energy) density occurs for higher values of the angular frequency. The additional energy is radiated in the form of electrical energy. The stellar radius is also decreased so that the star does not necessarily becomes more compact.

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Korucuk, Selçuk, et Ahmet Aytekin. « A field study examining barriers to logistics 4.0 using polytopic fuzzy RANCOM ». Journal of Process Management and New Technologies 12, n^o 3-4 (2024) : 90–100. https://doi.org/10.5937/jpmnt12-54800.

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Logistics companies are increasingly focusing more on technological transformation and development to save costs and transition to a more flexible structure, and their business objectives are closely tied with this framework. Changing business processes and technological revolutions force logistics companies to become more adaptive and agile, demanding the creation of organizational procedures in this context. To do this, the technology revolution, also known as logistics 4.0, is gaining popularity in the logistics industry and playing an essential role in cost savings for businesses. Logistics 4.0 applications are one approach for implementing procedures in logistics organizations in an effective and efficient manner. However, logistics 4.0 applications encounter many administrative and operational barriers. In this regard, the study identified and weighted barriers to logistics 4.0 applications in logistics companies with corporate identities. The polytopic fuzzy RANCOM approach was employed for this aim. The study found that "costs of implementing logistics 4.0" and "necessity of implementing process-driven management approaches" are the most significant barriers to implementing logistics 4.0, while "existence of cyber-attack threats" is the least important criterion.

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