S. Jill Glass

Ceramic Powder Compaction

With the objective of developing a predictive model for ceramic powder compaction we have investigated methods for characterizing density gradients in ceramic powder compacts, reviewed and compared existing compaction models, conducted compaction experiments on a spray dried alumina powder, and conducted mechanical tests and compaction experiments on model granular materials. Die filling and particle packing, and the behavior of individual granules play an important role in determining compaction behavior and should be incorporated into realistic compaction models. These results support the use of discrete element modeling techniques and statistical mechanics principals to develop a comprehensive model for compaction, something that should be achievable with computers with parallel processing capabilities.

Fracture behavior of engineered stress profile soda lime silicate glass.

Multi-step ion-exchange processing can produce complex stress profiles in glass surfaces, which can result in increased fracture strength, reduced strength dispersion, flaw tolerance and multiple cracking behavior. Glass displaying this set of properties is termed engineered stress profile (ESP) glass. Treatments at 400–450 C in molten KNO3 and NaNO3 salt baths were used to create residual stresses in the surface of soda lime silicate float glass. Stress profiles were measured using optical stress birefringence, allowing derivation of apparent fracture toughness curves and prediction of crack stability over a range of flaw sizes. Specimens were tested in the four-point bend configuration to determine fracture strength and to study the multiple cracking which results from crack growth stabilization. The results were compared to predictions from the fracture toughness curves, in terms of the strength dispersion and crack stability criteria. Indented specimens were tested to determine the response of the glass to contact damage. 2003 Elsevier Science B.V. All rights reserved.

Microstructure and properties of Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites formed byin situ reaction of Al with aluminosilicate ceramics

Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites have been formed byin situ reaction of molten Al with aluminosilicate ceramics. This reactive metal penetration (RMP) process is driven by a strongly negative Gibbs energy for reaction. In the Al/mullite system, Al reduces mullite to produce α-Al2O3 and elemental Si. With excess Al (i.e., x > 0), a composite of α-Al2O3, Al(Si) alloy, and Si can be formed. Ceramic-metal composites containing up to 30 vol pct Al(Si) were prepared by reacting molten Al with dense, aluminosilicate ceramic preforms or by reactively hot pressing Al and mullite powder mixtures. Both reactive metal-forming techniques produce ceramic composite bodies consisting of a fine-grained alumina skeleton with an interpenetrating Al(Si) metal phase. The rigid alumina ceramic skeletal structure dominates composite physical properties such as the Young’s modulus, hardness, and the coefficient of thermal expansion, while the interpenetrating ductile Al(Si) metal phase contributes to composite fracture toughness. Microstructural analysis of composite fracture surfaces shows evidence of ductile metal failure of Al(Si) ligaments. Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites produced byin situ reaction of aluminum with mullite have improved mechanical properties and increased stiffness relative to dense mullite, and composite fracture toughness increases with increasing Al(Si) content.

Controlling the Fragmentation Behavior of Stressed Glass

Inducing compressive surface stress profiles in brittle materials is a well-known approach for strengthening. The compressive stress inhibits crack initiation and propagation. The effect has been observed for tempered and ion-exchanged glasses,1, 2, 3, 4 and for oxide ceramics.5,6 While it is generally accepted that the magnitude of the stress and its depth determine the strength response, it has recently been demonstrated that the shape of the compressive stress profile can radically alter the strength distribution.7 For tempered glasses, the role of the internal tensile stress in causing fragmentation is well known,8 although it is not possible to predict the extent of fragmentation.9

Planar LTCC Transformers for High-Voltage Flyback Converters

This paper discusses the design and use of low-temperature (850°C to 950°C) cofired ceramic (LTCC) planar magnetic flyback transformers for applications that require conversion of a low-voltage to high-voltage (> 100-V) with significant volumetric constraints. Measured performance and modeling results for multiple designs show that the LTCC flyback transformer design and construction imposes serious limitations on the achievable coupling, and significantly impacts the transformer performance and output voltage. This paper discusses the impact of various design factors that can provide improved performance by increasing transformer coupling and output voltage. The experiments performed on prototype units demonstrate LTCC transformer designs capable of greater than 2-kV output. Finally, the paper investigates the effect of the LTCC microstructure on transformer insulation. Although this paper focuses on generating voltages in the kV range, the experimental characterization and discussion presented in this paper applies to designs requiring lower voltage.

Nondestructive characterization of nanopore microstructure: Spatially resolved Brunauer–Emmett–Teller isotherms using nuclear magnetic resonance imaging

This article presents the results of nuclear magnetic resonance imaging (MRI) studies of gas adsorption/desorption in nanoporous solids. MR images obtained as a function of the equilibrium pressure, at constant temperature, form a pixel-by-pixel map of adsorption isotherms. Analysis of these isotherms using Brunauer–Emmett–Teller (BET) theory results in spatial maps of the specific surface area, the net energy of adsorption, and the pore morphology. Results obtained using MRI for γ-Al2O3 and ZnO powders and partially sintered ceramics of these materials, as well as Vycor® porous glass, compare well to results for bulk samples obtained using conventional N2 BET adsorption. MRI studies of gas adsorption are shown to provide statistical averages of the pore microstructure parameters, resolved on a macroscopic scale.

Joining SI3N4 for Advanced Turbomachinery Applications

The main objective of this project was to develop reliable, low-cost techniques for joining silicon nitride (Si{sub 3}N{sub 4}) to itself and to metals. For Si{sub 3}N{sub 4} to be widely used in advanced turbomachinery applications, joining techniques must be developed that are reliable, cost-effective, and manufacturable. This project addressed those needs by developing and testing two Si{sub 3}N{sub 4} joining systems; oxynitride glass joining materials and high temperature braze alloys. Extensive measurements were also made of the mechanical properties and oxidation resistance of the braze materials. Finite element models were used to predict the magnitudes and positions of the stresses in the ceramic regions of ceramic-to-metal joints sleeve and butt joints, similar to the geometries used for stator assemblies.

Ceramic-Metal Brazing, From Fundamentals to Applications: A Review of Sandia National Laboratories Brazing Capabilities, Needs and Opportunities

The purpose of the report is to summarize discussions from a Ceramic/Metal Brazing: From Fundamentals to Applications Workshop that was held at Sandia National Laboratories in Albuquerque, NM on April 4, 2001. Brazing experts and users who bridge common areas of research, design, and manufacturing participated in the exercise. External perspectives on the general state of the science and technology for ceramics and metal brazing were given. Other discussions highlighted and critiqued Sandias brazing research and engineering programs, including the latest advances in braze modeling and materials characterization. The workshop concluded with a facilitated dialogue that identified critical brazing research needs and opportunities.

Computational methods for coupling microstructural and micromechanical materials response simulations

Computational materials simulations have traditionally focused on individual phenomena: grain growth, crack propagation, plastic flow, etc. However, real materials behavior results from a complex interplay between phenomena. In this project, the authors explored methods for coupling mesoscale simulations of microstructural evolution and micromechanical response. In one case, massively parallel (MP) simulations for grain evolution and microcracking in alumina stronglink materials were dynamically coupled. In the other, codes for domain coarsening and plastic deformation in CuSi braze alloys were iteratively linked. this program provided the first comparison of two promising ways to integrate mesoscale computer codes. Coupled microstructural/micromechanical codes were applied to experimentally observed microstructures for the first time. In addition to the coupled codes, this project developed a suite of new computational capabilities (PARGRAIN, GLAD, OOF, MPM, polycrystal plasticity, front tracking). The problem of plasticity length scale in continuum calculations was recognized and a solution strategy was developed. The simulations were experimentally validated on stockpile materials.

Interfaces in High-Strength, High-Temperature Ceramic Joints

Development of stronger, more refractory joints in non-oxide ceramics has been accompanied by a better understanding of diffusion and reactivity at ceramic interfaces. Control of the chemistry and interfacial reactivity of refractory oxide joining compositions has produced silicon nitride joints with four-point bend strengths of 550 MPa at 1000°C. The oxide compositions were chosen to be compatible with the silicon nitride grain boundary phase, which enhances bond strength. Au-Pd braze alloys with small additions of V have been used to join silicon nitride between 1200 and 1300°C, giving strengths of 520 MPa at room temperature. Microprobe and SEM analysis suggests the interfacial reaction is Si3N4 + 6Pd + 4V = 4VN + 3Pd2Si. Initial tests of the suitability of both the refractory oxide and Au-Pd-V compositions for joining SiC have been conducted.