Publications

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Efficient and environmentally sound methods of producing hydrogen are of great importance to the US as it progresses toward the H2 economy. Current studies are investigating the use of high temperature systems driven by nuclear and/or solar energy to drive thermochemical cycles for H2 production. These processes are advantageous since they do not produce greenhouse gas emissions that are a result of hydrogen production from electrolysis or hydrocarbon reformation. Double-substituted perovskites, A1-xSrxCo1-yBy O3-δ (A = Y, La; B = Fe, Ni, Cr, Mn) were synthesized for use as ceramic high-temperature oxygen separation membranes. The materials have promising oxygen sorption properties and were structurally robust under varying temperatures and atmospheres. Post-TGA powder diffraction patterns revealed no structural changes after the temperature and gas treatments, demonstrating the robustness of the material. The most promising material was the La0.1Sr0.9Co1-xMnx O3-δ perovskite. The oxygen sorption properties increased with increasing Mn doping.

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All ceramics and powder metals, including the ceramics components that Sandia uses in critical weapons components such as PZT voltage bars and current stacks, multi-layer ceramic MET's, ahmindmolybdenum & alumina cermets, and ZnO varistors, are manufactured by sintering. Sintering is a critical, possibly the most important, processing step during manufacturing of ceramics. The microstructural evolution, the macroscopic shrinkage, and shape distortions during sintering will control the engineering performance of the resulting ceramic component. Yet, modeling and prediction of sintering behavior is in its infancy, lagging far behind the other manufacturing models, such as powder synthesis and powder compaction models, and behind models that predict engineering properties and reliability. In this project, we developed a model that was capable of simulating microstructural evolution during sintering, providing constitutive equations for macroscale simulation of shrinkage and distortion during sintering. And we developed macroscale sintering simulation capability in JAS3D. The mesoscale model can simulate microstructural evolution in a complex powder compact of hundreds or even thousands of particles of arbitrary shape and size by 1. curvature-driven grain growth, 2. pore migration and coalescence by surface diffusion, 3. vacancy formation, grain boundary diffusion and annihilation. This model was validated by comparing predictions of the simulation to analytical predictions for simple geometries. The model was then used to simulate sintering in complex powder compacts. Sintering stress and materials viscous module were obtained from the simulations. These constitutive equations were then used by macroscopic simulations for simulating shrinkage and shape changes in FEM simulations. The continuum theory of sintering embodied in the constitutive description of Skorohod and Olevsky was combined with results from microstructure evolution simulations to model shrinkage and deformation during. The continuum portion is based on a finite element formulation that allows 3D components to be modeled using SNL's nonlinear large-deformation finite element code, JAS3D. This tool provides a capability to model sintering of complex three-dimensional components. The model was verified by comparing to simulations results published in the literature. The model was validated using experimental results from various laboratory experiments performed by Garino. In addition, the mesoscale simulations were used to study anisotropic shrinkage in aligned, elongated powder compacts. Anisotropic shrinkage occurred in all compacts with aligned, elongated particles. However, the direction of higher shrinkage was in some cases along the direction of elongation and in other cases in the perpendicular direction depending on the details of the powder compact. In compacts of simple-packed, mono-sized, elongated particles, shrinkage was higher in the direction of elongation. In compacts of close-packed, mono-sized, elongated particles and of elongated particles with a size and shape distribution, the shrinkage was lower in the direction of elongation. We also explored the concept of a sintering stress tensor rather than the traditional sintering stress scalar concept for the case of anisotropic shrinkage. A thermodynamic treatment of this is presented. A method to calculate the sintering stress tensor is also presented. A user-friendly code that can simulate microstructural evolution during sintering in 2D and in 3D was developed. This code can run on most UNIX platforms and has a motif-based GUI. The microstructural evolution is shown as the code is running and many of the microstructural features, such as grain size, pore size, the average grain boundary length (in 2D) and area (in 3D), etc. are measured and recorded as a function of time. The overall density as the function of time is also recorded.

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Sintering is one of the oldest processes used by man to manufacture materials dating as far back as 12,000 BC. While it is an ancient process, it is also necessary for many modern technologies such a multilayered ceramic packages, wireless communication devices, and many others. The process consists of thermally treating a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles. During sintering, the individual particles bond, the pore space between particles is eliminated, the resulting component can shrinks by as much as 30 to 50% by volume, and it can distort its shape tremendously. Being able to control and predict the shrinkage and shape distortions during sintering has been the goal of much research in material science. And it has been achieved to varying degrees by many. The object of this project was to develop models that could simulate sintering at the mesoscale and at the macroscale to more accurately predict the overall shrinkage and shape distortions in engineering components. The mesoscale model simulates microstructural evolution during sintering by modeling grain growth, pore migration and coarsening, and vacancy formation, diffusion and annihilation. In addition to studying microstructure, these simulation can be used to generate the constitutive equations describing shrinkage and deformation during sintering. These constitutive equations are used by continuum finite element simulations to predict the overall shrinkage and shape distortions of a sintering crystalline powder compact. Both models will be presented. Application of these models to study sintering will be demonstrated and discussed. Finally, the limitations of these models will be reviewed.

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The use of oxidized metal powders in mechanical shock or crush safety enhancers in nuclear weapons has been investigated. The functioning of these devices is based on the remarkable electrical behavior of compacts of certain oxidized metal powders when subjected to compressive stress. For example, the low voltage resistivity of a compact of oxidized tantalum powder was found to decrease by over six orders of magnitude during compaction between 1 MPa, where the thin, insulating oxide coatings on the particles are intact, to 10 MPa, where the oxide coatings have broken down along a chain of particles spanning the electrodes. In this work, the behavior of tantalum and aluminum powders was investigated. The low voltage resistivity during compaction of powders oxidized under various conditions was measured and compared. In addition, the resistivity at higher voltages and the dielectric breakdown strength during compaction were also measured. A key finding was that significant changes in the electrical properties persist after the removal of the stress so that a mechanical shock enhancer is feasible. This was verified by preliminary shock experiments. Finally, conceptual designs for both types of enhancers are presented.

An integrated approach, combining the continuum theory of sintering and Potts model based mesostructure evolution analysis, is used to solve the problem of bi-layered structure sintering. Two types of bi-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 bi-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.

A microfabrication process is described that provides for the batch realization of miniature rare earth based permanent magnets. Prismatic geometry with features as small as 5 microns, thicknesses up through several hundred microns and with submicron tolerances may be accommodated. The processing is based on a molding technique using deep x-ray lithography as a means to generate high aspect-ratio precision molds from PMMA (poly methyl methacrylate) used as an x-ray photoresist. Subsequent molding of rare-earth permanent magnet (REPM) powder combined with a thermosetting plastic binder may take place directly in the PMMA mold. Further approaches generate an alumina form replicated from the PMMA mold that becomes an intermediate mold for pressing higher density REPM material and allows for higher process temperatures. Maximum energy products of 3--8 MGOe (Mega Gauss Oersted, 1 MGOe = 100/4{pi} kJ/m{sup 3}) are obtained for bonded isotropic forms of REPM with dimensions on the scale of 100 microns and up to 23 MGOe for more dense anisotropic REPM material using higher temperature processing. The utility of miniature precision REPMs is revealed by the demonstration of a miniature multipole brushless DC motor that possesses a pole-anisotropic rotor with dimensions that would otherwise prohibit multipole magnetization using a multipole magnetizing fixture at this scale. Subsequent multipole assembly also leads to miniaturized Halbach arrays, efficient magnetic microactuators, and mechanical spring-like elements which can offset miniaturized mechanical scaling behavior.

Conductor inks containing silver and palladium, used in ceramic co-fired circuits, sometimes undergo an anomalously large expansion during heating in the temperature range where interdiffusion occurs. Therefore, the interdiffusion of silver and palladium was studied during heating in both air and argon using both powder and foil samples. Measurements on a powder compact made of a mixture of Ag and Pd (80% Ag) particles indicated that a very rapid expansion occurred between 375 and 400 C when heated in air but only a slight expansion occurred in Ar. A pre-alloyed powder with the same composition did not expand during heating. In situ high temperature x-ray diffraction studies indicated that both powders oxidized during heating in air, with the mixture oxidizing more and that interdiffusion occurred between 300 and 500 C. Microstructural examination indicated that larger particles with internal pores had formed in the mixture heated in air to 375 C due to rearrangement during interdiffusion. A porous region much thicker than the original silver film formed on a palladium foil sample when it was heated in air, whereas in inert atmosphere pores formed only in the silver film, indicating a Kirkendall effect occurs in both cases. Based on these results, it was concluded that the expansion of the Ag-Pd powder mixture was due to interdiffusion in the presence of oxygen, not solely to the oxidation of the Pd.

A new mold material has been developed for use in making rare-earth permanent magnet components with precise dimensions in the 10 to 1000 µm range by hot-forging. These molds are made from molds poly(methyl)methacrylate (PMMA) made by deep x-ray lithography (DXRL). An alumina bonded with colloidal silica has been developed for use in these molds. This material can be heated to 950°C without changing dimensions where it develops the strength needed to withstand the hot-fmging conditions (750°C, 100 MPa). In addition, it disintegrates in HF so that parts can be easily removed after forging.

We have developed a variety of processes for fabricating components for micro devices based on deep x-ray lithography (DXRL). Although the techniques are applicable to many materials, we have demonstrated them using hard (Nd{sub 2}Fe{sub 14}B) and soft (Ni-Zn ferrite) magnetic materials because of the importance of these materials in magnetic micro-actuators and other devices and because of the difficulty fabricating them by other means. The simplest technique involves pressing a mixture of magnetic powder and a binder into a DXRL-formed mold. In the second technique, powder is pressed into the mold and then sintered to densify. The other two processes involve pressing at high temperature either powder or a dense bulk material into a ceramic mold that was previously made using a DXRL mold. These techniques allow arbitrary 2-dimensional shapes to be made 10 to 1000 micrometers thick with in-plane dimensions as small as 50 micrometers and dimensional tolerances in the micron range. Bonded isotropic Nd{sub 2}Fe{sub 14}B micromagnets made by these processes had an energy product of 7 MGOe.

Motion detectors consisting of Pb(Zr{sub x}Ti{sub (1{minus}x)})O{sub 3} (PZT) thin films, between platinum electrodes, on micromachined silicon compound clamped-clamped or cantilever beam structures were fabricated using either hot KOH or High Aspect Ratio Silicon Etching (HARSE) to micromachine the silicon. The beams were designed such that a thicker region served as a test mass that produced stress at the top of the membrane springs that supported it when the object to which the detector was mounted moved. The PZT film devices were placed on these membranes to generate a charge or a voltage in response to the stress through the piezoelectric effect. Issues of integration of the PZT device fabrication process with the two etching processes are discussed. The effects of PZT composition and device geometry on the response of the detectors to motion is reported and discussed.

Techniques for characterizing density gradients generated during typical powder compaction processes are reviewed and several are evaluated. The techniques reviewed are ultrasonic velocity measurements, laser ultrasonic velocity measurements, x-ray radiography, autoradiography, computer tomography (CT), magnetic resonance imaging (MRI), and simple image analysis of polished cross-sections. Experimental results are reported for all of these techniques except autoradiography, CT and MRI. The test specimens examined were right circular cylinders of a high length/diameter ratio (to ensure significant density variation) pressed from commercial spray-dried alumina powders. Although the density gradients could be detected with all four techniques, ultrasonic velocity measurements gave the best contour map of gradients and is therefore most suitable for model validation. On the other hand, it was concluded that x-ray radiography is preferable in situations where cost and/or number of samples are more important that high resolution.

Thin film decoupling capacitors consisting of submicron thick, sol-gel Pb(Zr,Ti)O{sub 3} layers between Pt electrodes on a Si substrate have recently been developed. Because the capacitor structure needs to be only {approximately}3 {mu}m thick, these devices offer advantages such as decreased package volume and ability to integrate so that interconnect inductance is decreased, which allows faster IC processing rates. To fully utilize these devices, techniques of integrating them onto packages such as multi-chip modules and printed wiring boards or onto IC dies must be developed. The results of our efforts at developing integration processes for these capacitors are described here. Specifically, we have demonstrated a process for printing solder on the devices at the Si wafer level and reflowing it to form bumps and have developed a process for fabricating the devices on thin (25 to 75 {mu}m) substrates to facilitate integration onto ICs and printed wiring boards. Finally, we assessed the feasibility of fabricating the devices on rough surfaces to determine whether it would be possible to fabricate these capacitors directly on multi-layer ceramic substrates.

We studied the sintering behavior of high field chem-prep ZnO varistors using dilatometry and determined how sintering conditions influence the varistor's electrical properties. The dilatometric results indicated that 95% theoretical densities could be obtained with a soak time of only 0.5 hour above 752{degree}C, using a 3{degree}C/min ramp rate; greater shrinkage did not occur for soak temperatures above 752{degree}C. A variety of soak times and temperatures was found to be capable of producing varistors with acceptable electrical properties. The variation in the switching field, E{sub s} (at a current density of 10 A/cm{sup 2}), with soak temperature was found to be significantly less for short soak times than for the standard 16 hour soak time. Annealing sintered pellets at 700{degree}C for 4 hours raised the nonlinearity coefficients by {approximately}25% and decreased E{sub s} by {approximately}10%. This uniform decrease in E{sub s} led to a further reduction in the slope of E{sub s} vs soak temperature for values of E{sub s} in the specified range of interest. Further testing of more samples, of samples from different powder batches, and of samples annealed at various temperatures needs to be performed to verify these results. 19 refs., 10 figs., 1 tab.