Computer simulations of particle packing

Computer code has been developed to rapidly simulate the random packing of disks and spheres in two and three dimensions. Any size distribution may be packed. The code simulates varying degrees of inter particle conditions ranging from sticky to free flowing. The code will also calculate the overall packing density, density distributions, and void size distributions (in two dimensions). An important aspect of the code is that it is written in C++ and incorporates a user-friendly graphical interface for standard Macintosh and Power PC platforms. Investigations as to how well the code simulates the realistic random packing have begun. The code has been developed in consideration of the problem of filling a container (or die) with spray-dried granules of ceramic powder (represented by spheres). Although not presented here, the futuristic goal of this work is to give users the ability to predict homogeneity of filled dies prior to dry pressing. Additionally, this software has educational utility for studying relationships between particle size distributions and macrostructures

Characterizing and modeling organic binder burnout from green ceramic compacts

New characterization and computational techniques have been developed to evaluate and simulate binder burnout from pressed powder compacts. Using engineering data and a control volume finite element method (CVFEM) thermal model, a nominally one dimensional (1-D) furnace has been designed to test, refine, and validate computer models that simulate binder burnout assuming a 1-D thermal gradient across the ceramic body during heating. Experimentally, 1-D radial heat flow was achieved using a rod-shaped heater that directly heats the inside surface of a stack of ceramic annuli surrounded by thermal insulation. The computational modeling effort focused on producing a macroscopic model for binder burnout based on continuum approaches to heat and mass conservation for porous media. Two increasingly complex models have been developed that predict the temperature and mass of a porous powder compact as a function of time during binder burnout. The more complex model also predicts the pressure within a powder compact during binder burnout. Model predictions are in reasonably good agreement with experimental data on binder burnout from a 57--65% relative density pressed powder compact of a 94 wt% alumina body containing {approximately}3 wt% binder. In conjunction with the detailed experimental data from the prototype binder burnout furnace, the models have also proven useful for conducting parametric studies to elucidate critical i-material property data required to support model development

Modeling and characterization of molecular structures in self assembled and Langmuir-Blodgett films for controlled fabrication

Self Assembled (SA) thin films and Langmuir-Blodgett (LB) thin films are emerging technologies for the development of chemical and bio-chemical sensors, electrooptic films, second harmonic generators (frequency doublers), templates for biomimetic growth etc. One of the goals of this project was to extend Sandia`s characterization techniques and molecular modeling capabilities for these complex two-dimensional geometries with the objective of improving the control of the fabrication of these structures for specific applications. Achieving this requires understanding both the structure throughout the thickness of the films and the in-plane lattice of the amphiphilic molecules. To meet these objectives they used atomic force microscopy (AFM), X-ray reflectivity, and molecular modeling. While developing these capabilities, three different materials systems were fabricated and characterized: (1) Self Assembled Monolayers (SAMs) of octadecyltrichlorosilane (OTS) and LB films of arachidic acid on silicon wafers; (2) SAMs on PZT substrates; and (3) electrochemical deposition of CdS on LB film templates

Oriented inorganic thin film channel structures with uni-directional monosize micropores

The goal of this project was to develop a novel technology that may be used to eventually manufacture a new generation of inorganic membranes and sensors with oriented, unidirectional monosized pores. The premise is that very thin membranes with oriented channels as pores will have a high flux in addition to being highly selective. Applications include: (1) gas separation membranes for oxygen enrichment, partial oxidation, dehydrogenation, and purification of natural gas; (2) refractory catalytic membrane reactors; and (3) molecular recognition sensors. The methodology for making such membranes was to combine Langmuir - Blodgett (LB) technology with sol-gel chemistry to engineer pore channels within the range 3 to 20 K The channel structure was fabricated of amorphous SiO{sub 2} because of its good thermal, chemical, and mechanical stability. Our approach was to use LB techniques to uniformly place organic molecular spacers throughout a thin silica precursor matrix and apply this film to a substrate. LB films of solid solutions of commercially available silane amphiphiles and organic amphiphiles were fabricated. The siloxane groups were then hydrolyzed to form silica and the organic portions of the amphiphiles removed by thermal decomposition. With the completely fugitive organic spacer amphiphiles removed, a thin silica film with micropores resulted. The pore size was in the range of 6 - 8 {angstrom} and in an ultra-thin configuration. With further development this technique may be useful for fabrication of inorganic membranes which satisfy all the criteria of the ideal membrane

Heterodefloculação no sistema Al2O3 - SiC Heterodeflocculation in the Al2O3 - SiC system

Um estudo do comportamento reológico de suspensões aquosas de alumina e carbeto de silício foi desenvolvido, afim de se encontrar condições de processamento que otimizem a dispersão simultânea dessas matérias-primas. Os agentes dispersantes utilizados foram os polieletrólitos: sal de ácido polimetacrílico e polietilenoimina. Os pontos isoelétricos (pie) dos pós em suspensão aquosa e seus deslocamentos na presença dos defloculantes utilizados foram determinados. Obteve-se através de medidas de viscosidade e potencial zeta a concentração de defloculante que proporciona melhor dispersão de ambas as suspensões, bem como a influência do pH e da fração volumétrica de sólidos na estabilização. Neste trabalho, verificou-se que a defloculação simultânea de alumina e carbeto de silício pode ser conseguida em pHs nos quais os pós apresentem maior densidade de carga de mesmo sinal na superfície das partículas, preferencialmente utilizando-se o mesmo tipo de dispersante.<br>A study of the rheological behaviour of alumina and of silicon carbide in aqueous suspensions was carried out with the purpose of evaluating processing conditions that enable simultaneous dispersion of both raw materials. The dispersing agents used in this work were polyelectrolytes, including a salt of polymethacrylic acid and polyethyleneimine. The isoelectric points (iep) of powders were determined in aqueous suspensions either in the presence or not of deflocculants. The deflocculant concentration that promotes best dispersion, as well as the influence of pH and of solids content on the stabilisation were evaluated through viscosity and zeta potential measurements. It was verified that alumina and silicon carbide can be simultaneously dispersed at pHs where high surface charge of same sign is obtained for both powders, preferably using the same type of dispersant

Robocast Pb(Zr{sub 0.95}Ti{sub 0.05})O{sub 3} Ceramic Monoliths and Composites

Robocasting, a computer controlled slurry deposition technique, was used to fabricate ceramic monoliths and composites of chemically prepared Pb(Zr{sub 0.95}Ti{sub 0.05})O{sub 3} (PZT 95/5) ceramics. Densities and electrical properties of the robocast samples were equivalent to those obtained for cold isostatically pressed (CIP) parts formed at 200 MPa. Robocast composites consisting of alternate layers of the following sintered densities: (93.9%--96.1%--93.9%), were fabricated using different levels of organic pore former additions. Modification from a single to a multiple material deposition robocaster was essential to the fabrication of composites that could withstand repeated cycles of saturated polarization switching under 30 kV/cm fields. Further, these composites withstood 500 MPa hydrostatic pressure induced poled ferroelectric (FE) to antiferroelectric (AFE) phase transformation during which strain differences on the order of 0.8% occurred between composite elements

Filling Source Feedthrus with Alumina/Molybdenum CND50 Cermet: Experimental, Theoretical, and Computational Approaches

This report is a summary of the work completed in FY00 for science-based characterization of the processes used to fabricate cermet vias in source feedthrus. In particular, studies were completed to characterize the CND50 cermet slurry, characterize solvent imbibition, and identify critical via filling variables. These three areas of interest are important to several processes pertaining to the production of neutron generator tubes. Rheological characterization of CND50 slurry prepared with 94ND2 and Sandi94 primary powders were also compared. The 94ND2 powder was formerly produced at the GE Pinellas Plant and the Sandi94 is the new replacement powder produced at CeramTec. Processing variables that may effect the via-filling process were also studied and include: the effect of solids loading in the CND50 slurry; the effect of milling time; and the effect of Nuosperse (a slurry ''conditioner''). Imbibition characterization included a combination of experimental, theoretical, and computational strategies to determine solvent migration though complex shapes, specifically vias in the source feedthru component. Critical factors were determined using a controlled set of experiments designed to identify those variables that influence the occurrence of defects within the cermet filled via. These efforts were pursued to increase part production reliability, understand selected fundamental issues that impact the production of slurry-filled parts, and validate the ability of the computational fluid dynamics code, GOMA, to simulate these processes. Suggestions are made for improving the slurry filling of source feedthru vias

Influência de aditivos sobre a trabalhabilidade de concretos refratários de ultra-baixo teor de cimento Influence of additives on the working time of ultra-low cement refractory castables

A trabalhabilidade de concretos refratários é determinada principalmente pela cinética de hidratação do ligante hidráulico. Têm-se observado que o processo de gelificação/hidratação desse material em meio aquoso é sensivelmente influenciado pela presença de aditivos dispersantes. Em virtude disso, este trabalho investigou a correlação entre a trabalhabilidade de concretos refratários de ultra-baixo teor de cimento no sistema Al2O3-SiC-SiO2-C e as características apresentadas por suspensões aquosas de cimento na presença de diferentes aditivos. A influência desses aditivos no processo de hidratação do cimento foi estudada através de ensaios de microscopia eletrônica de varredura, difração de raios X, sedimentação, condutividade elétrica e ensaios reológicos. Os resultados obtidos indicam que a "pega" inicial do concreto ocorre devido à aglomeração das partículas induzida pela formação de um produto coloidal amorfo ("gel") entre elas. Observou-se que o citrato de sódio favorece a formação de uma quantidade significativa de "gel" entre as partículas, reduzindo a trabalhabilidade do concreto.<br>The working time of refractory castables is mainly determined by the hydration kinetic of the hydraulic binder. It has been observed that the gelation/hydration process of this material in aqueous solutions is significantly influenced by the addition of dispersing additives. Hence, this work investigated the correlation between the working time of ULC refractory castables in the Al2O3-SiC-SiO2-C system and the physic-chemical characteristics of cement aqueous suspensions in the presence of different sort of additives. The influence of these additives in the cement hydration process was studied by scanning electron microscopy, X-ray diffraction, sedimentation experiments, electrical conductivity and rheological tests. The initial setting of castables was found to be associated to particle coagulation and the development of an amorphous colloidal product (gel), that gives rise to a stiff gel-like structure. Sodium citrate favored the development of a significant content of such "gel" among particles, decreasing the castable working time