Synthetic Multifunctional Metallic-Intermetallic Laminate Composites

Overview Integrating Functionality Synthetic Multifunctional Metallic-Intermetallic Laminate Composites

Kenneth S. Vecchio

The field of material microstructure ing properties exhibited by hierarchical to develop synthetic multifunctional design targeted for a specific set of multiphase complex natural composites, materials. Mollusk shells are known to structural and functional properties such as mollusk shells,3,4 to design and possess hierarchical structures highly is now a recognized field of focus in synthesize multifunctional composites optimized for toughness. The two mol- materials science and engineering. This to optimize specific structural properties lusks that have been studied most exten- paper describes a new class of structural while facilitating low-cost, designable, sively are Haliotis rufescens (abalone) materials called metallic-intermetallic and functional microstructures. and Pinctata (conch) shells. Considering laminate (MIL) composites, which can Biological systems often exhibit a the weak constituents the shells are have their micro-, meso- and macro- wide array of multifunctional materials made from—namely calcium carbonate structure designed to achieve a wide and offer great biomimetic motivation (CaCO3) and a series of organic binders, array of material properties and tailored the mechanical properties of these shells to achieve specific functionalities. The are outstanding. Their tensile strength superior specific properties of this class varies between 100 MPa and 300 MPa of composites makes them extremely and fracture toughness between 3 MPa 1/2 attractive for high-performance aero- and 7 MPa-m . CaCO3 has correspond- space applications, and the fabrication ing strength and toughness values of 30 method for creating MIL composites MPa and <1 MPa-m1/2, respectively.5–9 allows new embedded technologies to be These mollusks owe their extraordinary incorporated into the materials, enhanc- mechanical properties to a hierarchically ing their functionality and utility. a 2 µm organized structure starting with single crystals of CaCO , with dimensions of INTRODUCTION 3 4–5 nm (nanostructure) and proceeding The field of material microstructural with bricks with dimensions of 0.5–10 design to achieve a set of targeted µm (microstructure, Figure 1a), and mechanical and functional properties finishing with layers of ~0.2–0.5 mm has become a mainstay of new mate- (mesostructure, Figure 1b). rial development strategies. Structural Building on the laminate microstruc- materials, which by their very nature ture design found in shells, Ti-Al3Ti MIL are intended to carry mechanical loads composites have been produced to mimic in service, can be designed to provide these structures from elemental titanium additional performance-enhancing func- and aluminum foils by a novel one-step tions through tailoring of meso-, micro-, process utilizing a controlled reaction at or nano-structures. Structural materials elevated temperature and pressure.2 The with these performance-enhancing novelty of this fabrication process lies capabilities have been termed “synthetic in the fact that it is performed in open multifunctional materials.”1 Structural air and produces a fully dense laminate composites, by their multiphase nature, composite. Figure 2 shows an illustration offer many opportunities for the design b 300 µm of the processing setup for fabrication of of performance-enhancing multifunc- Figure 1. (a) An SEM micrograph of the MIL composites using a simple open- the fracture surface of an abalone shell tional materials. Recently, a new class of showing the individual hexagonal aragonite air heated platen press. The thickness of structural materials has been developed tiles comprising the microstructure. (b) the original titanium and aluminum foils at the University of California, San An optical micrograph of the mesoscale is chosen to ensure that the entire alu- laminate structure of the abalone shell in Diego, termed metallic-intermetallic a bend fracture sample showing the crack minum layer is consumed upon reaction 2 laminate (MIL) composites. The goal of deflection achieved in this multi-layered with the adjacent titanium layers. Such a this materials development effort was to material. layering scheme results in a composite extrapolate upon the positive engineer- with alternate layers of Al3Ti (to mimic

2005 March • JOM 25 the hard CaCO3 layers in shells) and residual titanium (to mimic the tough protein layers in shells). The thickness of the final layers is dependent upon the thickness of the original aluminum and titanium foils. This process is highly flexible since metal/alloy foils other than titanium can be used individually, or in combination, within the same composite, to produce their respective metal/metal- aluminide combinations. For example, MIL composites using iron-based, nickel-based, and cobalt-based alloys as the starting metal layer (instead of titanium) have been successfully fabricated using the described technique. The MIL composites produced by this method are Figure 2. An illustration of metallic- Figure 3. The microstructures typical of not hierarchical structures in the manner intermetallic laminate composite heated Ti-Al Ti MIL composites. using platen press apparatus for fabrication 3 that natural shells are, but are rather of planar laminates. Complex platen two-dimensional laminate structures. In designs can be used to fabricate 3-D, near-net-shape MIL composites in the these structures, the scale of the layering same one-step, open-air operation. can be controlled, tailored, subdivided, and compositionally altered to achieve desired properties and functions. The composition, physical, and mechanical properties of the MIL composites can be varied and tailored within the thickness of the composite by simply 0.1 varying the individual foil compositions, thicknesses, and layering sequence. The fabrication of MIL composites using this approach has several key advantages that make it ideally suited for the production of commercially scalable structural Specific Compressive Strength Specific Compressive materials as well as microstructures 0.01 designed for specific functionalities. First, since the initial materials utilized are in the form of commercially available 0.01 Specific Modulus 0.1 metallic foils, the initial material cost is Figure 4. A materials property map comparing specific compressive strength versus specific material stiffness. reasonably low compared to many of the exotic material processing routes that 10−1 are commonly pursued in small-scale research environments. This also means 10−2 that a wide array of compositions can be ready produced, although this paper will focus on the Ti-Al system because of its 10−3 high specific properties. Second, the use of initially ductile 10−4 metallic foils enables the layers to be formed into complex shapes. This opens

da/dN mm/cyc the door for non-planar structures, such 10−5 as rods, tubes, shafts, and cones, as well as simple machining of individual foils a 10−6 for complex, three-dimensional (3-D) Figure 5. (a) Quasi-static three-point structures and near-net shape forming bend samples for fracture-toughness of parts. The use of initial metallic 10−7 measurements, and (b) fatigue foils also allows the individual foils crack growth curves for various MIL 1 2 4 6 8 10 20 40 60 100 composites. b ∆K MPa√m to be machined to contain cavities and pathways facilitating the incorporation

26 2005 March • JOM of embedded functionalities, such as 5 passive damping or sensors, prior to ) 3 processing. /kg/m

Third, the processing conditions, 1/2 0.01 in terms of temperature, pressure, and atmosphere are very modest. Processing temperatures, in the case of aluminum-foil-containing samples, are below 700 C and the processing ° 0.03 pressures are below 4 MPa.2 Perhaps the most remarkable feature of the processing of these MIL composites is that the processing is carried out in open (MPam Toughness/Density Fracture air, and no special inert gas or vacuum chamber facilities are necessary. 0.01 0.1 The combination of these processing Young’s Modulus/Density (GPa/(kg/m3)) features makes the processing method Figure 6. A specific-fracture-toughness versus specific-modulus property map for an array of structural materials. Plot includes MIL (Ti-Al3Ti) composites (dark gray colored) itself low cost, allows for complex and other laminate systems (identified by (L) and outlined in red), metals, alloys, and shape fabrication, and is amenable to composites. computer control. Finally, the microstructure of the MIL composites is determined by the foil thickness and composition and the processing condition. Since the material make-up is based on the selection of the metal foils, it is possible to completely tailor the microstructure from one surface to the other. In addition, the physical and mechanical properties of a the MIL composites can be tailored by selection of the foil composition and Figure 7. (a) A cross section thickness, making the MIL composite of the impact location from material system ideally suited for a ballistic test on an MIL composite (the plate thickness is engineering the microstructure to 2 cm). (b) An optical micrograph achieve specific performance goals. of a ceramic-fiber-reinforced Of the various possible aluminides in MIL composite for enhanced ballistic performance. the Ti-Al system, the formation of the intermetallic Al3Ti is thermodynami- b 500 mm cally and kinetically favored over other aluminides when reacting aluminum directly with titanium. This preferential 100,000 formation of Al3Ti is fortuitous as its Young’s modulus (216 GPa) and oxida- tion resistance are higher, and the density · Specific Heat (3.3 g/cm3) lower than that of the other titanium aluminides such as Ti3Al and TiAl.10 The high compressive strength 10,000 and stiffness of Al3Ti (and intermetallics, in general) result from their high bond strength. However, intermetallics Thermal Conductivity are brittle at low temperatures due to the limited mobility of dislocations (and 1,000 paired superdislocations with anti-phase 0.1 1 (Modulus · Fracture Toughness) Density boundaries), insufficient number of slip Figure 8. A property map of specific heat capacity and thermal conductivity versus or twinning systems, and/or very low specific modulus and fracture toughness for an array of structural materials. Plot includes surface energy resulting in little or no MIL (Ti-Al3Ti) composites (red ellipse), which overlap diamond and aluminum at the plastic deformation at crack tips. For lower end of the structural performance range for the MIL composites and lie below beryllium alloys in terms of the thermal properties for a similar specific stiffness/fracture example, Al3Ti is extremely brittle at toughness value. room temperature and has a very low

2005 March • JOM 27 11 fracture toughness of ~2 MPa√m. for Ti-Al3Ti MIL composites having dif- The unique properties of MIL com- ferent volume fractions of intermetallic posites arise from the combination of the phase. The crack growth curves for these high hardness and stiffness of the inter- MIL composites closely overlap similar metallic-aluminide phase alternatively data for monolithic titanium alloys such layered with the high strength, toughness, as Ti-6-4. Since the MIL composites and ductility of metal alloys. Figure 3 have higher stiffness and lower density shows two examples of the microstruc- than the monolithic titanium alloys ture of Ti-Al3Ti MIL composites with with similar crack growth resistance, significantly different layer thickness. enhanced performance can be obtained for demanding aerospace applications. STRUCTURAL Figure 6 plots the specific fracture PERFORMANCE toughness vs. the specific modulus of ATTRIBUTES Figure 10. An x-ray fluoroscope image of various engineering materials, including In the case of Ti-Al Ti MIL com- the through-thickness of a large (10 cm the MIL composites (shown by the dark 3 diameter) cavity filled with steel beads posites, the specific stiffness (modulus/ created within the intermetallic layer of a grey ellipse). Several other laminates are density) is nearly twice that of steel, the Ti-Al3Ti MIL composite. outlined in red in Figure 6. It can be seen specific toughness and specific strength are similar or better than nearly all metallic alloys, and specific hardness be obtained using a material property is on par with many ceramic materials. map. Figure 4 shows a plot having as An interesting comparison of material the x-axis the specific modulus of a properties for the MIL composites can material on a log scale and the y-axis the specific compressive strength on a log scale. In this plot, numerous material locations are shown, and in terms of optimizing these properties (combined compressive strength and stiffness), the upper right-hand corner represents the a goal. The location of the MIL composites (red ellipse) is shown to the right (higher specific modulus) of the typical structural metals such as steels, titanium alloys, nickel superalloys, aluminum alloys, and titanium-based intermetallics. The only metallic materials of higher specific stiffness are beryllium alloys. Several ceramic materials are shown, which have higher specific stiffness than the a MIL composites, including SiC, B4C,

Al2O3, and diamonds. Clearly, the MIL composites possess tremendous potential for structural applications, particularly b for demanding, high-specific-stiffness Figure 11. Schematic diagrams of MIL applications. composites containing embedded tubes within the intermetallic layers. (a) Initial The good fracture toughness of the microstructure, and (b) microstructure with MIL composites is derived from the collapsed tubes following blast. combination of the highly anisotropic layered structure of the material and the need for crack re-initiation at each suc- cessive metal layer. Figure 5a shows an example of the fracture behavior of three b different volume fraction Ti-Al3Ti MIL Figure 9. X-ray fluoroscope images of composites. In samples with as little as the through-thickness of cavities created 20% remnant titanium, the crack cannot within the intermetallic layer of a Ti-Al3Ti MIL composite. The grey circular regions propagate through the samples without are approximately 13 cm in diameter. being diverted and bifurcated at each Figure 12. A micrograph of a Ti-Al3Ti MIL titanium metal layer. Figure 5b shows a composite containing a ceramic tube filled with two metal wires. plot of the fatigue crack growth curves

28 2005 March • JOM that the Ti/Al3Ti laminate composites have higher specific toughness than other laminate systems and the Ti/Al3Ti specific modulus is surpassed only by the metal/Al2O3 system. Relative to γ- TiAl/TiNb, MIL composites have higher specific fracture toughness and a higher specific modulus for the same volume fraction of the ductile phase. Further, relative to the various metal/Al2O3 systems, the MIL composites have higher specific fracture toughness for the same Figure 14. An x-ray fluoroscope volume fraction of the ductile phase. image of the through-thickness Thus, owing to the ease of fabrication of cavities, interconnected by of Ti/Al Ti laminates, low fabrication ceramic insulators containing 3 wires created within the costs, and their attractive mechanical intermetallic layer of a Ti-Al3Ti MIL composite. properties, MIL (Ti/Al3Ti) composites are excellent candidates for engineering applications requiring a combination of sample is a 20% Ti-6-4 and 80% Al3Ti the ballistic performance makes them low density, high strength, high tough- laminate with an initial thickness of attractive for armor applications, creating ness, and high stiffness. approximately 2 cm. This composition a multifunctional (structural + ballistic) produces a sample with a density of 3.5 material. The ballistic performance of STRUCTURAL PLUS g/cm3; therefore, the specific target in these MIL composites can be further BALLISTIC ATTRIBUTES this figure would have an areal density enhanced by the incorporation of harder The specific physical and mechanical of 7 g/cm2. The penetrator used was a constituent phases, such as ceramics. properties of the MIL composites make tungsten heavy alloy rod (93W-7FeCo) Metallic-intermetallic laminate compos- them an attractive material for structural with a mass of approximately 10 g and ites have been successfully fabricated applications. However, the ballistic initial diameter of 6.15 mm. The penetra- using aluminum-based ceramic-particu- performance of these materials is also tor was fired at the target at a velocity of late-reinforced metal-matrix composites. quite impressive, when compared on the 900 m/s in a normal incidence depth-of- Also successful has been the direct basis of areal density for a given threat, penetration (DOP) orientation. The DOP incorporation of ceramic fibers within against other structural materials. Figure within the MIL target is less than 1 cm. the intermetallic layer by layering the 7a shows a photograph of a cross sec- The demonstrated mechanical proper- ceramic fiber tapes between aluminum tion through the impact location from a ties of the MIL composites make them sheets prior to reaction sintering of the ballistic test on an MIL composite. The suitable for structural applications, while MIL composites. Figure 7b shows an

optical micrograph of a Ti-Al3Ti-Al2O3

ﬁber-Al3Ti-Ti layered MIL composite.

The Al2O3 ceramic ﬁbers are stacked in two layers in a 0–90 orientation within

the center of each thicker Al3Ti layer. STRUCTURAL PLUS THERMAL-MANAGEMENT ATTRIBUTES For applications such as structural heat sinks, a high-performing material must possess the structural attributes described previously in addition to a high specific heat capacity to store the thermal energy and a high thermal conductivity to transport the heat throughout the structural heat sink. Figure 8 shows a plot of the product of thermal conductivity and specific heat capacity (on the y-axis) versus the product of specific modulus Figure 13. A micrograph showing an example of a and fracture toughness (on the x-axis). through-thickness titanium The product of specific modulus and wire embedded in a Ti-Al MIL fracture toughness was chosen because composite plate. it represents an optimum for many struc-

2005 March • JOM 29 within the MIL composite hierarchy can be selected in the material design process by the placement of the cavities within the individual aluminum layers and the placement of these layers within the foil stack. These cavities can be filled with granular materials to serve as particle dampers. Figure 10 shows an example of an MIL composite fabricated with a large cavity filled with steel beads to b demonstrate the concept of a particle- filled cavity. Initial damping results have Figure 15. (a) An illustration of a 11 an MIL composite having an been presented elsewhere. embedded piezoelectric sensor within its structure, and (b) a photo Enhanced Energy-Absorbing of embedded piezoelectric sensors or Fluid-Conduit-Modified MIL prior to reaction. Composites By embedding tubes between various layers of MIL composites, it is possible tural applications wherein high stiffness alloys, which can be critical for high- to incorporate energy-absorbing capacity and high fracture toughness are desired. performance aerospace applications. into these materials, specifically for blast This combination of properties is usually As such, Ti-Al MIL composites offer an mitigation. These tubes would deform difficult to achieve considering the high- attractive alternative to beryllium alloys during impact and absorb the incident est stiffness materials, such as ceram- for structural heat sink (multifunctional shock energy. Figure 11 illustrates ics, usually possess the lowest fracture structural + thermal) applications. the concept for this energy-absorbing toughness. In terms of thermal-manage- MIL composite system. In addition, MULTIFUNCTIONAL ment properties, the highest-stiffness the embedding of tubes within the MIL METALLIC-INTERMETALLIC materials, such as ceramics, are usually composites would facilitate the passage LAMINATE COMPOSITES poor thermal conductors, and the high- of fluids or gases within the structure, thermal-conductivity materials such Additional functionalities can be read- as aluminum alloys and copper alloys ily incorporated into MIL composites due have relatively low specific stiffness. to the layer-by-layer assembly nature of The exception to these trends is beryl- the materials. Since each layer of the MIL lium alloys, which possess high specific composite starts out as a metal foil, it is 4 stiffness, high thermal conductivity, and possible to create holes and slits in these high heat capacity. On the other hand, layers forming open space in individual 3 beryllium alloys have significant draw- layers or multiple layers. Within these backs to their widespread use, such as cavities and slits, additional functional- 2 the limited availability of beryllium, the ities can be embedded to further enhance high cost of beryllium alloys, and the the properties of the MIL composites. 1 serious health concerns with beryllium The approach follows, to some extent, a manufacturing. As such, alternatives to multi-layer electronic circuit board meth- 0 beryllium alloys for combined structural odology with interconnections occurring

plus thermal management applications either within a given layer or between Sensor Signal –1 are in great demand. Figure 8 shows layers, creating a 3-D architecture to the that MIL composites are second only structure of embedded functionalities. to beryllium alloys, in terms of thermal –2 MIL Composites with management capacity, for an equiva- Meso-Scale Cavities to lent structural property level. In terms –3 Incorporate Vibration Damping of thermal management capacity, the MIL composites are only surpassed By designing cavities within the alu- –4 0 0.5 1 1.5 2 2.5 by beryllium alloys, some aluminum minum layers and ﬁlling these cavities Time (s) x 10–3 alloys, and diamond. Given the high with granular material, it is possible to Figure 16. Voltage signals recorded cost of diamonds and the inability to produce MIL composites with tailored simultaneously from four piezoelectric produce structural components from vibration damping within the intermetal- sensors embedded within the MIL 11 composites following reacting of the them, diamonds can be eliminated as a lic layers. Figure 9 shows an example plate. choice. Furthermore, the speciﬁc struc- of the presence of these cavities within tural performance of MIL composites is a Ti-Al3Ti MIL composite. The size, nearly three times greater than aluminum distribution, and location of the cavities

30 2005 March • JOM which might be used for heat exchange, ded electrical pathways. Figure 14 shows rial, with experimental verification of the fluid transport, or embedded reactions. an x-ray fluoroscope image of an MIL models, will lead to greater implemen- composite containing a series of cavities tation of these materials in demanding MIL Composites with Embedded with interconnected electrical insulators multifunctional structural applications. Sensing Capability and a pair of wires running through the ACKNOWLEDGEMENTS Synthesis of in-plane embedded wires cavities. These cavities can also be filled and tubes as electrical and/or optical with high-temperature-capable devices The initial development of the MIL pathways for damage detection and life such as lithium niobate piezoelectric composites was conducted as part of a monitoring can make these MIL com- crystals that can be used for detection Multidisciplinary University Research posites truly multifunctional. By creating of mechanical impulses, or conversely Initiative program from the Army slots in the aluminum foils prior to MIL excited electrically to produce mechani- Research Office to the University of synthesis, it is possible to introduce metal cal vibration of the material. Figure 15 California, San Diego (Contract No. wires, metal or ceramic tubes, ceramic shows an illustration of an MIL com- DAAH04-96-10376). The follow-on work tubes containing metal wires, optical posite having an embedded piezoelectric to develop the multifunctional aspects of fibers, etc. By monitoring the wires sensor within its structure and a photo MIL composites has been funded by the or fibers, it is possible to monitor and of an actual embedded sensor prior to Defense Advanced Research Projects detect damage within the intermetallic reaction of the MIL composite. Figure Agency under Contract No. DAAD19- layers. The embedded wires also serve 16 shows the voltage output from four 00-1-0511, with Leo Christodoulou as as micro-rebars within the intermetallic piezoelectric sensors embedded within program manager. layers to further toughen these layers. an MIL composite following reacting of Figure 12 shows an example of embed- the plate. The responses of the sensors References ded ceramic tubes in the intermetallic are identical to their responses before layers. The embedded ceramic tube has embedding in the plate. The signals from 1. L. Christodoulou and J.D. Venables, “Multifunctional this array of sensors have been used to Material Systems: The First Generation,” JOM, 55 (12) two wires within it that are electrically (2003), pp. 39–45. isolated from the sample itself. perform impact location determination 2. David J. Harach and Kenneth S. Vecchio, and through modal analysis of the signals “Microstructure Evolution in Metal-Intermetallic MIL Composites with Through- determine the magnitude of the impact Laminate (MIL) Composites Synthesized by Reactive Thickness Wires or Tubes Foil Sintering in Air,” Metallurgical and Materials force. Transactions, 32A (2001), pp. 1493–1505. Since metal foils are used as the start- 3. R. Menig et al., “Quasi-static and Dynamic CONCLUSION Mechanical Response of Haliotis Rufescens (Abalone) ing material, it is possible to machine Shells,” Acta Materialia, 48 (2000), pp. 2383–2398. the foils individually or in a group to Metallic-intermetallic laminate com- 4. R. Menig et al., “Quasi-Static and Dynamic facilitate fabrication steps. By drilling posites embody and exploit the concept Mechanical Response of Strombus Gigas (Conch) Shells,” Mater. Sci. & Eng., A297 (2001), pp. 203–211. a hole through the entire foil stack and of synthetic multifunctional materials. 5. M. Sarikaya, “An Introduction to Biomimetics: A placing either a wire or tube in the hole, They have the potential to perform Structural Viewpoint,” Microsc. Res. Tech., 27 (1994), it is possible to create a through-thick- various other functions, such as thermal pp. 360–375. 6. M. Sarikaya and I.A. Aksay, Results and Problems ness-strengthening feature. Figure 13 management, ballistic protection, blast in Cell Differentiation in Biopolymers, ed. S. Case shows an example of a through-thickness mitigation, heat exchange, vibration (Amsterdam: Springer Verlag, 1992), pp. 1–25. titanium wire embedded in a Ti-Al MIL damping, and sensing of various types 7. M. Sarikaya et al., “Mechanical Property- Microstructural Relationships in Abalone Shell,” MRS composite plate. The location and dis- through embedded devices. The materi- Symp. Proc. Vol. 174 (Warrendale, PA: Materials tribution of these wires can be designed als are assembled layer by layer, with the Research Society, 1990), pp. 109–116. to regulate the balance between through functional features incorporated primar- 8. B.J.J. Zelinsky et al., eds., Better Ceramics through Chemistry IV (Warrendale, PA: MRS, 1990), p. 625. thickness and transverse strength. These ily within the intermetallic layers, and 9. M. Sarikaya, Microsc. Res. Techn., 27 (1994), p. wires can also serve as embedded electri- interconnections are completed within 369. cal resistors for strain sensors or damage a given layer and between layers using 10. G. Sauthoff, Intermetallics (Weinheim, Germany: VCH Publishers, 1995), p. 14. detection. Replacing the wires with tubes electrically insulated wires. Strategies 11. R.A. Varin et al., Materials Science and Engineering provides a method to introduce rivet-type need to be developed that would allow the A, A300 (2001), p. 1. attachment holes and through-thickness optimal integration of these interconnec- 12. A. Rohatgi et al., “Development of Multifunctional Metallic-Intermetallic Laminate (MIL) Composites with fiber optics for imaging or environmental tions while not significantly degrading Particulate-based Damping” Proceedings of the 16th sensing. material properties and performance. Annual Technical Conference of the American Society In addition, constitutive and damage for Composites (Dayton, OH: University of Dayton, Fully Functional MIL Composites 2001), pp. 203-211. evolution models need to be developed The next step to multifunctional MIL that could be integrated into large-scale Kenneth S. Vecchio is a professor in the Department composites having meso-scale cavities computer codes to predict the accuracy of Mechanical and Aerospace Engineering at and electrical conduits to incorporate and effectiveness of performance indices the University of California at San Diego, La Jolla, CA. sensing devices, such as piezoelectric for both properties and functionality. devices, accelerometers, gyroscopes, and The development of “rules and tools” For more information, contact Kenneth S. Vecchio, microelectromechanical system devices for designers to utilize these inherent University of California at San Diego, Department of Mechanical and Aerospace Engineering, La is to combine the technology of embed- and embedded functionalities, and their Jolla, CA 92093-0411; (858) 534-6076; fax (858) ded cavities with the concept of embed- distribution and density within the mate- 534-5698; e-mail [email protected].

2005 March • JOM 31