Fabrication of Net-Shape Functionally Graded Composites by Electrophoretic Deposition and Sintering: Modeling and Experimentation

It is shown that electrophoretic deposition (EPD) sintering is a technological sequence that is capable of producing net-shape bulk functionally graded materials (FGM). By controlling the shape of the deposition electrode, components of complex shapes can be obtained. To enable sintering net-shape capabilities, a novel optimization algorithm and procedure for the fabrication of net-shape functionally graded composites by EPD and sintering has been developed. The initial shape of the green specimen produced by EPD is designed in such a way that the required final shape is achieved after sintering-imposed distortions. The optimization is based on a special innovative iteration procedure that is derived from the solution of the inverse sintering problem: the sintering process is modeled in the “backward movie” regime using the continuum theory of sintering incorporated into a finite-element code. The experiments verifying the modeling approach include the synthesis by EPD of Al2O3/ZrO2 3-D (FGM) structures. In order to consolidate green parts shaped by EPD, post-EPD sintering is used. The fabricated deposits are characterized by optical and scanning electron microscopy. The experimentally observed shape change of the FGM specimen obtained by EPD and sintering is compared with theoretical predictions

Processing model for tungsten powders and extention to nanoscale size range

Nanoscale tungsten powders promise access to very hard, strong and wear resistant materials via the press–sinter route. A small particle size changes the response during sintering, requiring lower temperatures and shorter times to attain dense but small grain size structures. On the other hand, oxide reduction and impurity evaporation favour high sintering temperatures and long hold times. Accordingly, press–sinter processing encounters conflicting constraints when applied to small particles. Presented here is an analysis of press–sinter tungsten particle processing to isolate conditions that balance the temperature and size dependent effects. The calculations are pinned by existing data. Opportunities are identified for new consolidation approaches to deliver a small grain size in a full density structure

Fabrication of functionally graded 3A/5A zeolites by electrophoretic deposition

Functionally graded zeolites of molecular sieve type 3A and 5A are deposited by electrophoretic deposition (EPD) from acetone suspension with 8% volume concentration of n-butylamine as particle charging agent. The EPD characteristics of both 3A and 5A suspensions are studied. Functionally graded zeolite 3A/5A deposits are obtained at 200 V DC. Energy dispersive X-ray dispersion (EDX) analysis results confirm the graded structure. The deposited zeolites are also analysed by scanning electron microscopy (SEM). The factors influencing the deposition process are discussed

Synthesis of gold micro- and nano-wires by infiltration and thermolysis

An approach for synthesizing micro- and nano-sized gold wires by infiltration and thermolysis is investigated. A porous ZrO2 ceramic preform with aligned pores obtained by unidirectional freezing and freeze-drying is employed as an infiltration template. The sintered porous ZrO2 preform is then infiltrated by a brushing gold solution. The thermolysis is conducted at 600 °C in air. Micro- and nano-sized gold wires are developed within the walls of the pores after thermolysis. The diameter of the gold wires ranges from several hundred nanometers to several microns

SIMULATION OF SINTERING OF LAYERED STRUCTURES

Abstract -An integrated approach, combining the continuum theory of sintering and Potts model based mesostructure evolution analysis, is used to solve the problem of hi-layered structure sintering. Two types of hi-layered structures are considered: layers of the same material with different initial porosity, and layers of two different materials. The effective sintering stress for the hi-layer powder sintering is derived, both at the meso-and the macroscopic levels. Macroscopic shape distortions and spatial distributions of porosity are determined as functions of the dimensionless specific time of sintering. The effect of the thickness of the layers on shrinkage, warpage, and pore-grain structure is studied. Ceramic ZnO powders are employed as a model experimental system to assess the model predictions.

Equivalence problem for the orthogonal webs on the sphere

We solve the equivalence problem for the orthogonally separable webs on the three-sphere under the action of the isometry group. This continues a classical project initiated by Olevsky in which he solved the corresponding canonical forms problem. The solution to the equivalence problem together with the results by Olevsky forms a complete solution to the problem of orthogonal separation of variables to the Hamilton-Jacobi equation defined on the three-sphere via orthogonal separation of variables. It is based on invariant properties of the characteristic Killing two-tensors in addition to properties of the corresponding algebraic curvature tensor and the associated Ricci tensor. The result is illustrated by a non-trivial application to a natural Hamiltonian defined on the three-sphere.Comment: 32 page

Maxwell Equations in Complex Form of Majorana - Oppenheimer, Solutions with Cylindric Symmetry in Riemann S_{3} and Lobachevsky H_{3} Spaces

Complex formalism of Riemann - Silberstein - Majorana - Oppenheimer in Maxwell electrodynamics is extended to the case of arbitrary pseudo-Riemannian space - time in accordance with the tetrad recipe of Tetrode - Weyl - Fock - Ivanenko. In this approach, the Maxwell equations are solved exactly on the background of static cosmological Einstein model, parameterized by special cylindrical coordinates and realized as a Riemann space of constant positive curvature. A discrete frequency spectrum for electromagnetic modes depending on the curvature radius of space and three parameters is found, and corresponding basis electromagnetic solutions have been constructed explicitly. In the case of elliptical model a part of the constructed solutions should be rejected by continuity considerations. Similar treatment is given for Maxwell equations in hyperbolic Lobachevsky model, the complete basis of electromagnetic solutions in corresponding cylindrical coordinates has been constructed as well, no quantization of frequencies of electromagnetic modes arises.Comment: 39 page

Iron oxide-based nanostructured ceramics with tailored magnetic and mechanical properties: Development of mechanically robust, bulk superparamagnetic materials

Nanostructured iron-oxide based materials with tailored mechanical and magnetic behavior are produced in bulk form. By applying ultra-fast heating routines via spark plasma sintering (SPS) to supercrystalline pellets, materials with an enhanced combination of elastic modulus, hardness and saturation magnetization are achieved. Supercrystallinity-namely the arrangement of the constituent nanoparticles into periodic structures-is achieved through self-assembly of the organically-functionalized iron oxide nanoparticles. The optimization of the following SPS regime allows the control of organics' removal, necking, iron oxide phase transformations and nano-grain size retention, and thus the fine-tuning of both mechanical properties and magnetic response, up until the production of bulk mm-size superparamagnetic materials.Deusche Forschungsgemeinschaft (DFG

Strain in the mesoscale kinetic Monte Carlo model for sintering

Shrinkage strains measured from microstructural simulations using the mesoscale kinetic Monte Carlo (kMC) model for solid state sintering are discussed. This model represents the microstructure using digitized discrete sites that are either grain or pore sites. The algorithm used to simulate densification by vacancy annihilation removes an isolated pore site at a grain boundary and collapses a column of sites extending from the vacancy to the surface of sintering compact, through the center of mass of the nearest grain. Using this algorithm, the existing published kMC models are shown to produce anisotropic strains for homogeneous powder compacts with aspect ratios different from unity. It is shown that the line direction biases shrinkage strains in proportion the compact dimension aspect ratios. A new algorithm that corrects this bias in strains is proposed; the direction for collapsing the column is determined by choosing a random sample face and subsequently a random point on that face as the end point for an annihilation path with equal probabilities. This algorithm is mathematically and experimentally shown to result in isotropic strains for all samples regardless of their dimensions. Finally, the microstructural evolution is shown to be similar for the new and old annihilation algorithms.Comment: 6 pages, 6 figure