Kevin G. Ewsuk

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.

Mechanical properties and shear failure surfaces for two alumina powders in triaxial compression

In the manufacture of ceramic components, near-net-shape parts are commonly formed by uniaxially pressing granulated powders in rigid dies. Density gradients that are introduced into a powder compact during press-forming often increase the cost of manufacturing, and can degrade the performance and reliability of the finished part. Finite element method (FEM) modeling can be used to predict powder compaction response, and can provide insight into the causes of density gradients in green powder compacts; however, accurate numerical simulations require accurate material properties and realistic constitutive laws. To support an effort to implement an advanced cap plasticity model within the finite element framework to realistically simulate powder compaction, we have undertaken a project to directly measure as many of the requisite powder properties for modeling as possible. A soil mechanics approach has been refined and used to measure the pressure dependent properties of ceramic powders up to 68.9 MPa (10,000 psi). Due to the large strains associated with compacting low bulk density ceramic powders, a two-stage process was developed to accurately determine the pressure-density relationship of a ceramic powder in hydrostatic compression, and the properties of that same powder compact under deviatoric loading at the same specific pressures. Using this approach, the seven parameters that are required for application of a modified Drucker-Prager cap plasticity model were determined directly. The details of the experimental techniques used to obtain the modeling parameters and the results for two different granulated alumina powders are presented.

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.

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.

Formation of structural intermetallics by reactive metal penetration of Ti and Ni oxides and aluminates

Alumina-aluminum composites can be prepared by reactive metal penetration (RMP) of mullite by aluminum. The process is driven by a strong negative free energy for the reaction (8 +x)Al + 3Al6Si2013 → 13Al2O3 + 6Si + xAl. Thermodynamic calculations reveal that titanium oxide, aluminum titanate, nickel oxide, and nickel aluminate all have a negative free energy of reaction with aluminum from 298 to 1800 K, indicating that it may be possible to form alumina-intermetallic composites by reactions of the type (2 +x)Al + (3/y) MOy → Al2O3 + AlxM3/y. Experiments revealed that aluminum reacts with titanium oxide, nickel oxide, and nickel aluminate, but not aluminum titanate, at 1673 K. Reaction with the stoichiometric amount of aluminum (x = 0) leads to the formation of alumina and either titanium or nickel. In some cases, reactions with excess aluminum (x > 0) produce intermetallic compounds such as TiAl3 and NiAl.

Effects of composition and atmosphere on reactive metal penetration of aluminium in mullite

Abstract Ceramic-metal composites can be made by reactive penetration of dense ceramic preforms by molten Al. Molten Al will reduce mullite to produce a composite of Al 2 O 3 , Si and Al. The reaction can be written as 3Al 6 Si 2 O 13 + (8 + x )Al → J3Al 2 O 3 + Al x Si y + (6 − y )Si. The penetration is driven by the strongly negative Gibbs free energy for reaction. In order to assess the influence of atmosphere and temperature in the penetration process, reaction couples of molten Al on mullite were heated at temperatures from 950 to 1150 °C, and at p ( O 2 ) from ~10 −10 to ~10 −20 atm. In this range p ( O 2 ) has little influence on reaction kinetics; the reaction rate is controlled by the rate of Si diffusion out of the preform to the external Al source.

Transmission electron microscopy study of interfacial microstructure formed by reacting Al-Mg alloy with mullite at high temperature

Abstract Transmission electron microscopy (TEM) has been used to study the interfacial microstructure formed by reacting Al–Mg alloy with mullite (Al 6 Si 2 O 13 ) at high temperature (>900°C). The TEM study was used in order to understand the strong effect of Mg addition on the nature of the reaction between Al and mullite used to form Al/Al 2 O 3 composite. After reaction at 1050°C, the formation of a layered structure between the Al–1% Mg alloy and mullite was observed. An alloy layer with a much higher concentration of Mg than the starting alloy was found present next to the initial mullite surface. Between the alloy layer and mullite, a dense and continuous layer made of small MgAl 2 O 4 (spinel) and Si particles was present. The layer apparently stopped further reaction between Al–Mg alloy and mullite by preventing transport of the metals to the reaction front and the Si reaction product away from the reaction front. The microstructure resulting from the initial reaction indicated the reaction proceeded by replacing Si atoms with Al and Mg atoms on mullite {210} lattice planes and forming MgAl 2 O 4 {311} lattice planes simultaneously.

Microstructure and composition of Al-Al2O3 composites made by reactive metal penetration

The microstructure of Al-Al2O3 composites made by reactive penetration of Al (or Al alloy) into ceramic (mullite or kaolin) preforms has been investigated using transmission electron microscopy (TEM). The Al-Al2O3 composites were found to contain a mutuallyinterconnected network of Al and Al2O3. No crystallographic orientation was observed between the Al and Al2O3 phase. Impurities and pores in the ceramic preforms were found to have a strong effect on the microstructure of the composites. The impurities resulted in formation of small particles in the Al2O3 grains of Al-Al2O3 composites, whereas the porosity yielded a varied ratio of Al to Al2O3 in the composites. The growth rate of the Al-Al2O3 composites was found to depend on the microstructure and composition of the ceramic preforms as well as the composition of the reactive metals. Pure aluminium penetrated into a dense mullite faster than into a porous mullite at temperatures below 1200 °C. Addition of Mg to Al reduced the growth rate, whereas a continuous phase of amorphous SiO2 in the ceramic preforms increased the growth rate.

Controlled Ceramic Powder Compaction Through Science-Based Understanding

Reproducible manufacturing of ceramic components requires understanding and controlling materials and processing. Utilizing characterization and modeling to develop sciencebased understanding, significant advances have been made to better understand and control ceramic pressing powders, powder compaction, and sintering. This includes identifying some of the critical relationships between powder characteristics/properties, powder compaction behavior, and sintering. Another significant advance includes the development of computer simulation technology for compaction and sintering that provides guidance to improve process reproducibility and control. For powder compaction, a cap-plasticity constitutive model is implemented within a finite element (FE) framework. For sintering, a linear viscous sintering constitutive model is implemented within an FE framework. Both models have been tested and validated by comparing model predictions to experimental observations. The computer modeling technology developed can be used to improve and expand ceramic component designs, to help optimize powder pressing and sintering, and to anticipate and minimize defects during processing. The application of characterization and modeling technology to develop better powders and more robust processes will contribute to more reproducible, efficient, and cost effective manufacturing technology for ceramic components. Introduction Processing Particulate Ceramic Components. Ceramic component manufacturing is a multi-step process that involves beneficiating a ceramic powder (e.g., by granulation), shape-forming the powder into a green body (e.g., by dry pressing [1-2]), and sintering at elevated temperature to produce the desired size and shape ceramic body with the requisite properties for a given application [3]. Reproducible processing is critical in advanced ceramic component manufacturing, both to ensure the fabrication of a reliable product, and to achieve the process yields required for cost-effective manufacturing. Historically, traditional shape-forming processes such as dry powder pressing have been engineered empirically. This approach is typically time intensive, and often does not provide the understanding necessary design a new powder/component or to troubleshoot processing problems. Statistical process control (SPC) techniques have been used to help identify and eliminate the causes of non-random variability in processes such as dry pressing [4]; however, they also do not necessarily provide the insight required to fully optimize processing. Science-Based Processing. As a complement to empirical engineering and SPC techniques, scientific principals can be applied to better understand and control ceramic processing. Sciencebased understanding can be used to identify and control critical relationships between powder properties/characteristics, pressing, and sintering response that impact manufacturing, and finished component performance and reliability. A combination of characterization and modeling are required to develop the fundamental scientific understanding required to achieve this goal. We have characterized the physical properties and characteristics of granulated ceramic powders [57], and their behavior during dry pressing [8-9] and sintering [6-7,10-11]. Additionally, measured constitutive behavior has been implemented within predictive models for powder compaction and sintering to better understand and control ceramic powder processing. [1,12-17]. Ceramic Powder Processing and Modeling Dry Pressing. Because it is fast, simple, and well suited to high-volume production, powder pressing is commonly used to shape-form ceramic components [1-2]. Dry pressing also provides the Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 149-154 doi:10.4028/www.scientific.net/KEM.264-268.149

An Assembly-Based Structural Model for LTCC Package Design and Reliability

Packaging a high power radio frequency integrated circuit (RFIC) in low temperature cofired ceramic (LTCC) presents many challenges. Within the constraints of LTCC fabrication, the design must provide the usual electrical isolation and interconnections required to package the IC, with additional consideration given to RF isolation and the structural integrity issues. While iterative design and prototyping is an option for RFIC packaging development, it is a tedious and expensive process that would most likely be unsuccessful due to the complexity of the problem. To facilitate package design and optimization, thermo-mechanical assembly simulations were used to identify and manage the critical process parameters to control solder failures in the LTCC package assembly. The modeling results were confirmed through comparisons to prototype testing. This paper summarizes an assembly-based modeling approach to RFIC package design and solder failure analysis, and presents some results and key findings to date. San...