Effect of consolidation pressure up to 1.8 GPa on the sintering of nanocrystalline nc-Y2O3

Sintering of nanocrystalline (nc) monoclinic yttrium oxide (Y₂O₃) was studied in the homologue temperature range of 0.4–0.7T_M. Samples were isostatically consolidated at super high pressure (SHP), up to 1.8 GPa. The combined effects of consolidation pressure and sintering temperature on the properties and microstructure are explored. The physical properties of the samples, mainly elastic modulus follow in general the density changes, or the interparticle contact area, but are also affected by sintering temperature. The effect of compaction pressure and sample density on the phase transition is studied and discussed.     

Comparative study of steel alloys using STM and XRD

The STM (Scanning Tunneling Microscope) has been used for characterizing the microstructure of various steel samples by utilizing its surface imaging properties. The subgrain sizes of 15 different samples of steel (in the range 20 nm to 250 nm) were studied by the STM and compared with the results obtained using X-ray diffraction (XRD) by employing line profile analysis of the Bragg peaks. Good agreement was observed between the two sets of data of subgrain sizes. This work establishes STM as a useful characterization tool for studies in metallurgy and metal physics.     

Microbehavior of U3Si during deformation

Metallography and X-ray diffraction were used to investigate the texture and microstructure of the intermetallic compound U₃Si after bulk and surface loading. It was found that there is neither an order-disorder transition nor a phase transformation as a result of cold-working of UaSi. Changes in the X-ray and in the metallographic patterns in deformed martensitic U₃Si were interpreted as being due to detwinning of the {112} 〈111〉 systems. Detwinning was moderate in the elastic range and considerable in the plastic range.     

Performance of ad hoc networks with two-hop relay routing and limited packet lifetime (extended version)

We consider a mobile ad hoc network consisting of three types of nodes (source, destination and relay nodes) and using the two-hop relay routing. This type of routing takes advantage of the mobility and the storage capacity of the nodes, called the relay nodes, in order to route packets between a source and a destination. Packets at relay nodes are assumed to have a limited lifetime in the network. Nodes are moving inside a bounded region according to some random mobility model. Closed-form expressions and asymptotic results when the number of nodes is large are provided for the packet delivery delay and for the energy needed to transmit a packet from the source to its destination. We also introduce and evaluate a variant of the two-hop relay protocol that limits the number of generated copies in the network. Our model is validated through simulations for two mobility models (random waypoint and random direction mobility models), and the performance of the two-hop routing and of the epidemic routing protocols are compared.     

Optimal Algorithm for Shape from Shading and Path Planning

An optimal algorithm for the reconstruction of a surface from its shading image is presented. The algorithm solves the 3D reconstruction from a single shading image problem. The shading image is treated as a penalty function and the height of the reconstructed surface is a weighted distance. A consistent numerical scheme based on Sethian's fast marching method is used to compute the reconstructed surface. The surface is a viscosity solution of an Eikonal equation for the vertical light source case. For the oblique light source case, the reconstructed surface is the viscosity solution to a different partial differential equation. A modification of the fast marching method yields a numerically consistent, computationally optimal, and practically fast algorithm for the classical shape from shading problem. Next, the fast marching method coupled with a back tracking via gradient descent along the reconstructed surface is shown to solve the path planning problem in robot navigation.     

Patch‐Collaborative Spectral Point‐Cloud Denoising

We present a new framework for point cloud denoising by patch‐collaborative spectral analysis. A collaborative generalization of each surface patch is defined, combining similar patches from the denoised surface. The Laplace–Beltrami operator of the collaborative patch is then used to selectively smooth the surface in a robust manner that can gracefully handle high levels of noise, yet preserves sharp surface features. The resulting denoising algorithm competes favourably with state‐of‐the‐art approaches, and extends patch‐based algorithms from the image processing domain to point clouds of arbitrary sampling. We demonstrate the accuracy and noise‐robustness of the proposed algorithm on standard benchmark models as well as range scans, and compare it to existing methods for point cloud denoising.     

Scaling of the Shaped Charge Jet Break‐Up Time

Since the formula for the shaped charge jet break‐up time was published on 1979 many attempts were made to interpret it and to make its use more efficient. It is shown herein that the V_pl parameter depends on the ratio of the liner thickness to the charge explosive diameter by the formula: 1/V_pl=13.9−101⋅(T_L/CD), where T_L is the liner thickness, CD is the explosive charge diameter and the numbers are for a published set of measurements with an OFE copper liner driven by COMP‐B explosive. To find how the numbers used in this formula change with the liner material and its metallurgical state and with the type of explosive, measurements should be made as prescribed herein. An attempt to begin explaining this formula is made in the discussion.     

(Semi)alternating stack automata

New results concerning the space complexity of languages accepted by stack automata, alternating stack automata, and alternating pushdown automata are derived. Some of the results generalize previously known results.Work supported by NSF Grant MCS79-09967.Work supported by NSF Grant MCS78-01736.     

A Simple Discretization of the Vector Dirichlet Energy

We present a simple and concise discretization of the covariant derivative vector Dirichlet energy for triangle meshes in 3D using Crouzeix-Raviart finite elements. The discretization is based on linear discontinuous Galerkin elements, and is simple to implement, without compromising on quality: there are two degrees of freedom for each mesh edge, and the sparse Dirichlet energy matrix can be constructed in a single pass over all triangles using a short formula that only depends on the edge lengths, reminiscent of the scalar cotangent Laplacian. Our vector Dirichlet energy discretization can be used in a variety of applications, such as the calculation of Killing fields, parallel transport of vectors, and smooth vector field design. Experiments suggest convergence and suitability for applications similar to other discretizations of the vector Dirichlet energy.     

Active current balance between parallel thyristors in multi-bridgeAC-DC rectifiers

New technology in digital excitation control provides a method to balance thyristor currents in parallel power rectifier bridges that provide current to a synchronous generator field. Balancing current between parallel bridges feeding a common load has traditionally been accomplished by using current balancing transformers in each bridge or adding inductance in series with the thyristors or SCRs (silicon controlled rectifiers). This paper introduces Active Current Balance, a new technique which modifies the average current in parallel leg thyristors to facilitate current balance between the bridges. This is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are fired carry more current during this interval, increasing their average current. The thyristors that are not fired do not carry current during this interval and thus their average current is reduced. Using this technique (referred to in this paper as skip firing), the average current in parallel legs can be adjusted through feedback and control to equalize the average thyristor current loading. Additional feedback and control parameters are proposed, such as heatsink temperatures, which can be measured and utilized to program the skip firing, in order to balance thyristor heat-sink temperatures