Cellular Ceramics / 5.11
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Fig. 7 a) The ceramic foam test specimen (macrocellular) is the black disk in the center of the holder (before testing). The bottom part was coated with a standard NASA tile coating. b) Same specimen after arc-jet testing (coated part is on the left-hand side; coating is
damaged because it reached temperatures higher than those for which it was developed). c) The arc-jet nozzle exit and swing arm are visible to the left and right of the sample being tested (center). Images courtesy of
M. Stackpoole (Eloret, NASA).
Fig. 7). Post-test phase analysis by X-ray diffraction confirmed that a phase change did not occur in the foam substrates during the test duration. Both types of samples showed minimal degradation (ablation or oxidation) to have occurred as a result of these test exposures.
Initial results are encouraging, but longer test durations are needed to better evaluate the long-term stability of these foam materials in the severe environments experienced in the arc-jet facility. Current work demonstrates that processing of preceramic polymer is a viable method to form refractory ceramic foams with tailorable properties at relatively low manufacturing temperatures (1200 C), which display encouraging properties under simulated reentry conditions.
5.11.8
Ceramic Foams for Impact Applications
5.11.8.1
Hypervelocity Impact Shields for Spacecrafts and Satellites
Cellular materials are currently discussed as candidates for micrometeorite and debris protection systems (MDPS) for spacecraft, because of their unique combination of properties including low density, high stiffness, and energy absorption that can be tailored through the design of their microstructure. A typical MDPS for a manned mission is based on the principle of multiple shocks followed by radial expansion of the impacting projectile, induced by several thin layers (bumpers) placed between the external environment and the panel walls composing the spacecraft’s shell [30]. The large amount of cell walls that a projectile impacting a cellular material would encounter along its path would induce a high number of consecutive shocks on the impactor. The protection effectiveness depends on several factors, among which are: 1) the ratio between the density of the projectile and of the cellu-
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lar material; 2) the relative dimension of the projectile (fragments) with respect to the cell walls; 3) the number of walls (or cells) per unit length and 4) the relative dimension of the projectile (fragments) with respect to the cell volume. Factor 2 controls the intensity of a single shock; factor 3 the number of shocks to which the projectile is subjected along its path through the material; and factor 4 the expansion of the cloud of new fragments.
Researchers at CISAS Hypervelocity Impact Facility (Padova, Italy) and at University of Bologna tested macro- (cell size 200–800 mm) and microcellular (cell size 10–100 mm) ceramic foam cores (2 cm thick) sandwiched between two thin aluminum sheets (0.4 mm thick) by impacting them with Al projectiles (diameter 1–1.5 mm) launched at 4, 4.5, and 5 km s–1 using a two-stage light-gas gun [31]. The experimental data collected so far indicate that denser foams can withstand more energetic impacts with a given damage. Thicker struts or higher solid material density result in more effective shocks within the cellular structure. However, the total weight of the shield is a paramount concern for space applications, and a suitable compromise between performance and MDPS mass budget should be found. It is difficult to independently analyze the role of the cell size with respect to the penetration depth because cell size often influences the mechanical strength of the material, and compressive strength is a critical parameter in impact-related problems, especially during the final phase of penetration. The foams tested in these experiments were neither completely perforated nor broken into several macroscopic fragments when impacted [32].
Hypervelocity impact tests on ceramic foams showed that these materials, if well designed, can effectively arrest (in a restricted space) the debris cloud generated from the collision between the projectile and the external bumper (see Fig. 8). Their performance compares favorably with that of traditional Whipple shields (two aluminum sheets at a given distance, with no core material between them) when only very limited space is available for the MPDS. With respect to metal-foam-based cores, ceramic foams can be used in high temperature environments (e.g., near-sun
Fig. 8 Images of the impact crater caused by a 1 mm Al projectile at 4 km s–1. a) Frontal
view of a gel-cast Al2O3 foam (bulk density 0.731 – 0.062 g cm–3, compression strength 16.9 – 4.5 MPa, average cell size 220 – 34 mm
– sample courtesy of Hi-Por Ceramics Ltd).
b) Cross-section view of an SiOC microcellular foam (bulk density 0.359 – 0.016 g cm–3, compression strength 11.0 – 1.9 MPa, average cell size 7.8 – 1.5 mm). Images courtesy of
A. Francesconi, D. Pavarin (CISAS), A. Arcaro and P. Colombo (University of Bologna).
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orbiting probes) or as thermal protection shields for reentry of spacecraft, and also provid debris-protection capabilities (i.e., they serve as a multifunctional component). Recently, the need for additional Space Shuttle TPS has been recognized, and this could lead to unexpected opportunities for the use of cellular ceramics with intrinsic protection capabilities.
5.11.8.2
Armour Systems
Interest in composite, lightweight armour with enhanced performance for vehicular or blast protection led researchers at Ceramic Protection Corporation Inc. (Calgary, AB, Canada) and the University of Bologna to undertake the development and ballistic testing of 3D-reinforced composites based on polymer-infiltrated ceramic foams (see Fig. 9a). The armour system configuration comprised a thin (6 mm) ballisticgrade alumina face tile bonded to a backing system based on infiltrated SiC foams [20 mm thick, 10 or 20 ppi, samples courtesy of Foseco (Cleveland, OH)]. The alumina face tile, with optimized microcrystalline structure, high hardness, and mechanical properties, was used to fragment the bullet and to dissipate its impact energy. The functions of the polymer-infiltrated ceramic foams were to absorb the impact energy and stop the bullet. Two types of polyurethane (PU) – rigid crosslinked and elastomeric – were selected and vacuum infiltration was applied. Possible advantages of this design include the presence of a continuous polymeric phase with damping capability, the absorption of impact energy by crushing of the ceramic foams, and the presence of a continuous cellular ceramic structure which interferes with the propagation of the shock wave. Some polymers could also increase the strength and damage tolerance of the ceramic foams. This composite structure may be bonded to a metallic or plastic board that can be easily installed in mobile or stationary systems.
These composites were ballistically tested using 5.56 0 45 mm SS109 (steel tip ball), 7.62 0 51 mm NATO Ball FMJ (with a lead core), and 7.62 0 63 mm AP
Fig. 9 a) Microstructure of the 3D composite (SiC-foam-reinforced cross-linked PU). b) Composite made from alumina ceramic tile bonded with SiC foam infiltrated by cross-linked PU
b)
after ballistic testing (7.62 0 51 mm NATO Ball FMJ). Images courtesy of P. Colombo (University of Bologna) and E. Medvedovski (Ceramic Protection Corporation).
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M2 (with a tungsten carbide core) ammunition. The projectile velocities were 990–1000, 840–890, and 845–870 m s–1, respectively. Strong influence of the design and mechanical properties of the composite components, especially the nature of the selected polymer, on the performance was observed. In the case of SiC foam infiltrated with cross-linked PU, the 5.56 0 45 mm SS109 and 7.62 0 51 mm NATO rounds were stopped in the backing (Fig. 9b), while the 7.62 0 63 mm AP round was stopped only in the trauma pack. Similar systems made with elastomeric PU could not defeat even the 7.62 0 51 NATO round. Preliminary testing of the system based on the SiC foam infiltrated by cross-linked PU but without face alumina tile resulted in unsatisfactory performance against the 7.62 0 51 NATO projectile.
Further experimental work dealing with optimization of the system design, assessment of the influence of cell size and ceramic foam strength, and selection of a preferred polymer for foam infiltration are currently underway.
5.11.9
Heat Exchangers
Open-cell ceramic foams fabricated by CVD have been proposed by Ultramet as cores of compact heat exchangers for actively cooled, high-power, SiC-based electronics. Due to its high thermal conductivity, excellent corrosion resistance, and the perfect match of its coefficient of thermal expansion (CTE) with that of the electronic components, SiC is used (see Fig. 10a). The foam structure functions as cooling fin of high thermal conductivity and high surface area, in which the struts act as extended surface for heat dissipation. By forcing cooling air or water through the foam, heat can be removed very rapidly. The turbulent mixing caused by the threedimensional interconnected porosity of the foam structure continuously breaks down boundary layers within the fluid, causing more effective heat transfer. The completely open cell foam with high porosity offers low pressure drop for the cooling fluid.
Fig. 10 a) Cross section of a SiC foam-core heat-exchanger tube. Image courtesy of E. Stankiewicz (Ultramet). b) Carbonfoam heat sink with a fin design, used to cool a computer chip. Image courtesy of J. Klett (ORNL).
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Heat exchangers have also been built from metal foams (with the advantage of a more readily available joining technology, but a generally lower corrosion resistance in aggressive environments) and carbon foams. Researchers at Oak Ridge National Laboratories (Oak Ridge, TN) developed heat sinks based on special high-thermal- conductivity carbon foams (thermal conductivity of 150 W m–1 K–1 for a density of 0.54 g cm–3, see Chapter 2.7). Several applications have been proposed and tested. They range from computer chip cooling (see Fig. 10b) to power electronics cooling (a power density of 150 W cm–2 was attained at die temperatures of only 70 C; higher power densities allow faster processing speeds), transpiration/evaporative cooling (for electronics and leading edges), and personal cooling devices [33]. The use of these graphite foams significantly reduces temperatures compared to traditional heat sinks, such as Al foam or Al finned plate. However, in some applications the pressure drop can be exceedingly large when graphite foam block is used, so that pumping power rises. Thus, special engineering designs to reduce pressure penalties while maintaining heat transfer are being developed; they allow a reduction of the overall pressure drop of more than an order of magnitude.
In heavy vehicles, the radiator, located in the front, creates significant drag accounting for up to 12 % of fuel consumption at highway speeds. It has been shown that by utilizing a graphite foam as the fins of a radiator, the same heat can be dissipated in a significantly smaller package. For example, a radiator was designed for a racing car that can dissipate up to 33 kW in 60 % less volume than a typical radiator (see Fig. 11). However, the pressure drop across the radiator was higher than allowable due to fan considerations. Research is underway to minimize pressure drop while maintaining heat transfer. A smaller, lighter radiator will allow the redesign of the front cab and dramatically improve efficiency by allowing more heat to be dissipated in the same size and by improving aerodynamics, fuel efficiency, and load-carrying capacity of heavy vehicles.
Fig. 11 a) Vehicle modular radiator based on a high-thermal-conductivity graphite foam. b) Closeup of one element. Blocks of graphite foam were machined and soldered to the
cooling tubes on one side; the assembled foam and cooling tubes were then stacked together and soldered to the manifold. Images courtesy of J. Klett (ORNL).
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5.11.10
Ceramic Foams for Semiconductor Applications
Carbon (C) or silicon carbide (SiC) foams developed at ERG Materials and Aerospace Corporation (Oakland, CA) are being used to perform a number of critical functions in the manufacturing of integrated circuit chips. The processes of dry etching and chemical vapor deposition (CVD) require precise control of gas flows and the ionized plasma field to enable the uniform deposition and removal of materials. Through such precise control, the yield of chips with submicron path widths can be greatly improved, and this can be achieved by using a specifically designed inputgas foam diffuser disk. This component consists of a 40 ppi ceramic foam that performs several functions. It allows uniform gas distribution, because the disk provides uniform pressure drop and distribution of gases to all areas of the shower head regardless of turbulence or flow asymmetry in the diffusion chamber. The foam also operates as a fluid flow straightener, allowing gasses to enter all shower head ports uniaxially with uniform velocity minimizing any chugging in the orifices. Moreover, the diffuser disk acts as a grounding plate and flame arrestor that prevents backfiring or arcing of the plasma back through the shower head and into the diffusion chamber and thus stabilizes the plasma. Furthermore, the foam can collect and remove particulate contamination that might otherwise enter the diffusion chamber. Finally, their open-cell structure allows compact condensation of by-prod- ucts and process chemicals for disposal or recycling. High surface area foams doped with catalytic materials are also being examined as a means of artificially biasing the chemical population distribution in etching and CVD process chambers. Diffusers are commonly made of special-grade Al alloy, but C and SiC ceramic foams can be used in applications which involve higher temperatures or chemicals incompatible with Al.
5.11.11
Duplex filters
A Duplex filter combines two different porosities in a single filter, usually with a combination of porosities of 30/50 ppi or 40/60 ppi, but no particular restrictions exist. The alumina filters developed by Drache Umwelttechnik (Diez, Germany) allow a larger amount of particles to be filtered from a liquid-metal flow than conventional ceramic foam filters, are less expensive than any stacked filtration system, for which a special filterbox is needed, and have a lower release of particles, especially during longer casts, because any coarse particles released from the upper layer are caught by the finer bottom layer.
In Duplex filters constructed at Ultramet for gas-filtration applications, an oxide ceramic porous membrane filter is integrated into the first (surface) cells of a SiC foam on the inlet side of the foam filter structure (see Fig. 12a). The foam ligaments support and reinforce the submicrometer porous filtration membrane. The opencell foam component of the duplex filter fulfils several functions, as a coarse filter,
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Fig. 12 a) Cross section of a hot-gas duplex filter (SiC foam coated with a ceramic membrane). b) Hot-gas duplex filters (1 m long, based on SiC foam), covered with captured particu-
late, after high-temperature, high-pressure testing. Images courtesy of E. Stankiewicz (Ultramet).
as a structural element, and as a flow distributor/mixer for the process gas. Catalysts can also be coated onto the foam ligaments to add the function of catalytic converter to the duplex filter. These foams have been used in coal-combustion applications (see Fig. 12b). Other companies have also developed similar systems.
5.11.12
Lightweight Structures
Sandwich panels are valuable in applications where low weight and high stiffness are crucial [34]; consequently, lightweight structures have been fabricated with ceramic foam cores, in which the foam acts both as structural element and functional porous medium. Face sheets can be bonded to one or more sides of a ceramic foam to produce lightweight structures. Development of a sandwich panel made of a ce- ramic-fiber-reinforced SiC face shield and a CVI-SiC (CVI = chemical vapor infiltration) foam core for application in thermal protection systems (TPS) was reported as early as 1985 [35]. An example of a lightweight, high-strength, high-stiffness foam structure, developed at Ultramet, is an optical mirror for use in space applications (Fig. 13a). The open-cell foam serves as a stiff, light platform for the high-density CVD SiC mirror face sheet. The foam material can be chosen to match the CTE of the face-sheet mirror to minimize distortion.
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Fig. 13 a) Lightweight foam mirrors (foam core with face sheet). Image courtesy of E. Stankiewicz (Ultramet). b) Pyrolyzed ceramic sandwich structure. Image courtesy of T. Hoefner, J. Zeschky, and P. Greil (Erlangen University).
Utilizing a novel process with a single processing step, researchers at Erlangen University fabricated ceramic sandwich panels from Si and SiC filler-loaded polymethylphenylsilsesquioxane [36]. Green tapes are bonded to a preceramic polymerderived foam while it grows by self-blowing (see Chapter 2.1). Since the green tapes are made of filler-loaded preceramic polymer [37], they have the same composition as the foam material, and no interfacial adhesive is necessary to attach the face sheets to the foam core. Because of the fillers, shrinkage on pyrolysis at 1600 C is low (ca. 3.5 % linear shrinkage). The resulting panel is shown in Fig. 13b. These panels are proposed as carrier substrates for silicon-based photovoltaic cells, but their impact behavior is also currently being investigated.
SiSiC sandwiched foams could also be produced by conventional powder technology (Schwartzwalder + tape casting) without any shrinkage and with the advantage of achieving a more uniform porosity in the core material [38].
5.11.13
Ceramic Foams as Substrates for Carbon Nanotube Growth
Catalytic chemical vapor deposition (CCVD) is the most promising route for the synthesis of carbon nanotubes (CNTs), which have very high specific surface area and exceptional mechanical, electrical, and thermal characteristics that make them of particular interest for a variety of applications [39]. CCVD consists of the catalytic decomposition or dismutation of a carbonaceous gas on nanometric metal particles at high temperature (ca. 700–1100 C) [40]. Researchers at CIRIMAT (CNRS-Univer- sit Paul-Sabatier) recently proposed a method in which metal nanoparticles (< 5 nm) are formed in situ at the appropriate temperature by the reduction of a FeO/CoO-doped ceramic powder support (Al–Mg–O system) and are thus immediately active for the catalytic decomposition of CH4 [41]. Besides its composition, many parameters of this solid solution, such as specific surface area and granulometry, are important to ultimately determine the formation of the desired kind and
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proportion of CNTs. On the one hand, because the active metal particles are those formed at the surface of the oxide grains, it is of importance that the solid solution offers a large surface area. On the other hand, because the formation of each carbon nanotube needs a great amount of carbon, easy access of a large supply of carbonaceous gas (CH4) to the metal nanoparticles is of utmost importance. For these two reasons, the use of catalytic materials in the form of porous ceramic foam supports instead of powders becomes extremely advantageous. It was shown that an Mg0.9Co0.1Al2O4 solid solution foam prepared by gel casting (total porosity 98 %, cell window size < 300 mm), as compared to the corresponding powder, gave a fourfold increase in production of CNTs (> 95 % with only 1 or 2 walls, and ca. 70% single-wall nanotubes, SWNT, [42]), because it allowed for a higher quantity of surface metal nanoparticles, better dispersion of the metal particles (which hampers growth and therefore favors selectivity for CNT formation), and more space for the CNT to grow. In addition, a CNT–ceramic composite foam, with CNT in the open porosity of the matrix, would be of interest for many applications using the CNT as a catalyst substrate.
5.11.14
Metal Oxide Foams as Precursors for Metallic Foams
Researchers at Lehigh University developed a process for producing closed-cell metallic foams using a ceramic foam precursor constituted of Fe2O3 [43]. The oxide foam was obtained by reaction of an acidic solution with a metal blowing agent to generate hydrogen while cementitious materials simultaneously reacted to form a fast-setting hydrate. The mixture thus foams and sets simultaneously, and the evolved gas is enclosed to produce a ceramic foam with a predominantly closed cell structure [44]. The addition of carbon black increases the viscosity of the mixture, which in turn hinders the drainage of the foam and reduces cell coalescence. This ensures that a predominantly closed cell structure is developed. The ceramic precursor foam is then reduced by annealing at 1240 C in a nonflammable hydrogen/ inert gas mixture to obtain a metallic foam with variable density and cell size (typical values obtained so far are ca. 0.23 for relative density and 1.32 – 0.32 mm for the average cell diameter). Linear shrinkage on conversion from ceramic to metallic foam is about 25 %. The normalized strengths of the metal foams obtained with this method compare favorably with those of steel foams produced by other techniques. Closed-cell foams are more advantageous for structural applications.
This process employs cheap raw materials, the reduction process requires only standard equipment which is already widely used in industry, and it allows easy scale-up and fabrication of complex shapes with inexpensive mold materials. Moreover, this novel approach circumvents the typical problem of the production of steel foams, in which foaming normally can only be achieved at temperatures close to the melting point, which makes control of the metal foam morphology very challenging. Experimentation is currently continuing towards the adaptation of this ceramic foam precursor process to a wide range of steel compositions (e.g., to obtain stainless steel foams by adding other oxides such as Cr2O3 and /or NiO).
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5.11.15
Zeolite Cellular Structures
Zeolites are widely used in industry as active components in heterogeneous catalysts, adsorbents, and ion exchangers, and novel applications such as sensing/ molecular recognition, biomolecule separation, and chromatography are being explored. They are usually produced in the form of micrometer-sized powders, and, if inserted as such in a reactor, they would give rise to very high pressure drop. Conventional processing for making shaped bodies in the millimeter range includes dispersing these components into an inorganic binder matrix and shaping by pressing, extrusion, or droplet formation by peptization, as well as depositing zeolites on a substrate (see also Chapter 5.4 and references therein). Both approaches have advantages, such as good mechanical strength, and disadvantages, such as pore blocking by inorganic binders, diffusion limitations, single-sided mass transport or easy detachment of the microporous layer from the substrate due to thermal stresses. There is widespread interest in developing macroporous bodies constituted of microporous zeolites. Researchers from Sogang University produced self-support- ing pure zeolite open-cell foams with high thermal stability of the monolith (no dimensional variation on heating at 550 C for 1 d), limited mechanical strength (but sufficient for some applications), and high specific surface area (445 m2 g–1) [45]. This was achieved by dipping a polyurethane sponge in a sol–gel-based solution and placing the system in an autoclave. During zeolite synthesis the polyurethane template slowly decomposed, and the residues were eliminated by calcination at 550 C in air. With this process very large monoliths (13 cm diameter, 27 cm height) can be fabricated.
An alternative approach, exploited by researchers at the University of Erlangen, is that of using biotemplates for the fabrication of zeolite-based micro/macroporous components. A natural sponge (loofah gourd) was used as a macrostructural template for the development of self-supporting biomimetic zeolite macrostructures with hierarchical micro-/meso-/macroporosity. This was accomplished by two-step hydrothermal synthesis (in situ zeolite seeding and secondary crystal growth) utilizing a dual-template technique (quaternary ammonium salts as template molecules to tailor the micropore network of the zeolite crystals and the loofah sponge for the
Fig. 14 Silicalite-1 (MFI-type) zeolite replica of the loofah sponge. Images courtesy of A. Zampieri and W. Schwieger (Erlangen University).