Cellular Ceramics / 5.11
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5.11
Other Developments and Special Applications
Paolo Colombo and Edwin P. Stankiewicz
5.11.1
Introduction
Cellular ceramics are versatile, controllable, and tailorable materials suited to varied applications. Advanced cellular ceramics must be properly engineered and integrated within the system to fulfil the requirements of a given application. Component design includes selecting the proper cell size, pore size, amount of porosity, material, and fabrication technique to ensure optimal performance. The fabrication technique heavily influences the properties because it affects the morphology, the compositional purity, and the flaw population of the ceramic material that constitutes the cellular component. For some applications, high-purity, stoichiometric materials are necessary, for their enhanced strength and corrosion resistance relative to lower purity materials.
Over the years there have been substantial developments – most of them discussed in the previous chapters – in the field of cellular ceramics. These developments relate to fabrication and applications.
This final chapter briefly describes some recent advancements, not covered elsewhere in this book, with the additional primary intent of giving the reader a better sense of the ever-growing range of applications that can be fulfilled by these fascinating materials. Practical examples are presented of how tailoring the material of construction, pore size, porosity, foam structure, and fabrication technique results in unique practical and functional components with the crucial strength, flow characteristics, and materials properties required for optimum performance in a given application.
5.11.2
Improving the Mechanical Properties of Reticulated Ceramics
Successful application of cellular ceramics typically demands high mechanical properties, preferably at low apparent densities. Mechanical properties models, such as those proposed by Gibson and Ashby for foams and honeycombs (see Chapter 4.1 and Ref. [1]), require use of empirically derived coefficients. These coefficients depend on the specific foam material and fabrication method. Selection of the fabrication method, in turn, depends upon a judicious balance of performance require-
Cellular Ceramics: Structure, Manufacturing, Properties and Applications.
Michael Scheffler, Paolo Colombo (Eds.)
Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-31320-6
5.11 Other Developments and Special Applications 597
ments, producer and user experience, and cost. Varied methods have been developed and used to improve mechanical properties, and each presents its own balance of benefits and limitations.
The production of ceramic foams by replication methods such as those introduced by Schwartzwalder and others (see Chapter 2.1) is still common in the industry, largely due to its simplicity and affordability. However, fabrication defects that significantly lower strength typically exist in the final product. Defects include longitudinal cracks that form during firing due to drying shrinkage and the gas evolution resulting from decomposition of the polymer foam substrate and to the mismatched thermal coefficients of polymer substrate and ceramic powder coating on heating. Increased strength would enhance structural efficiency and widen the applicability of these materials, particularly in advanced applications. Analysis of the fracture surface indicates that the origin of failure often develops in the strut corners near the apex of the triangular hole present in the hollow struts or is associated with macroscopic defects in the struts [1]. Furthermore, often due to cost constraints as well as for limiting shrinkage during firing, the ceramic material comprising the ligaments of the foam is often not fully sintered. This leads to porosity within the struts or cell walls. Thus, besides using a different fabrication procedure (e.g., direct blowing or gel casting, see Chapter 2.1), several approaches for improving the strength of ceramic foams have been proposed, including those discussed in the following.
5.11.2.1
Ceramic Foams by Reaction Bonding
Researchers at VITO (Mol, Belgium) are developing ceramic foams based on reac- tion-bonded aluminum oxide (RBAO) [2] that display bending strength far superior to that of conventional reticulated ceramics (see Fig. 1). The precursor Al/Al2O3
Flexural strength / MPa
8
6
4
2
0
5
RBAO Al2O3 foams
Reticulated Al2O3 foams
10 |
15 |
20 |
25 |
30 |
Relative density / %
Fig. 1 Comparison of three-point bending strength of Al2O3 ceramic foams produced by the conventional Schwartzwalder process (reticulated) and by reaction bonding (RBAO). Data courtesy of J. Luyten (VITO).
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mixture is intensively milled, and oxidation of the Al phase produces rather small grains with less glass phase at the grain boundaries [3]. Moreover, the precursor RBAO slurry wets the polyurethane sponge template better than conventional powder suspensions. This leads to better distribution of slurry on the template, with fewer cell blockings in comparison to poorly wetting slurries. Thus, the combination of a stronger coating and better wetting leads to fewer defects in the struts.
5.11.2.2
Overcoating of Conventional Reticulated Ceramics
By repeating the impregnating and drying steps several times, it is possible to increase the strut size and to increase the strength of a ceramic foam, while only moderately increasing its bulk density or reducing permeability [4]. Alternatively, it is possible to recoat the reticulated ceramic foam after the sintering step [5, 6]; by coating with a low-viscosity slurry, thicker struts are obtained, and the occurrence of large flaws is minimized by the recoating process. Indeed, collaborative research performed at EMPA (Duebendorf, Switzerland) and at the University of Bologna showed that recoating a presintered ceramic foam with a slurry of suitable rheological characteristics allows not only healing of the macroscopic flaws in the struts but also penetration into the structure and at least partial filling of the hollow struts (see Fig 2a, b). In this set of experiments, the recoating process led to an approximately 40 % increase in relative density with only about 3 % increase in strut thickness. This resulted in an order of magnitude increase in mechanical strength, from about 0.2 MPa to greater than 2 MPa for alumina foams with a relative density of 0.13 when sintered at 1550 C. An effect of the temperature reached in the presintering step (during which the polymer sponge substrate is decomposed) and of the rheology of the second slurry was also observed. A lower temperature for the first sintering step (i.e., 1200 C) and a lower viscosity slurry in the second infiltration resulted in higher strength after recoating, likely due to better access of the slurry into the sintered ceramic body. The diameter of the struts and their uniformity in thickness, both of which affect the mechanical properties, can be controlled in reticulated ceramics also by adopting recoating (in the green stage) and centrifuging steps, as demonstrated recently [7].
Another possibility is to coat low-cost, low-strength reticulated ceramic foams by chemical vapor deposition (CVD). Experiments performed at Ultramet (Pacoima, CA) demonstrated that this method can result in a very large increase in strength, as well as an improvement in corrosion resistance. For example, the crushing strength of a 10 ppi ZrO2 foam increased from about 1.6 to about 5.0 MPa, after depositing a thin SiC layer.
An approach newly developed by Erbicol (Balerna, Switzerland) consists of immersing an SiC reticulated foam (presintered at a low temperature of 500–700 C) into a carbon-containing precursor (e.g., phenolic resin or preceramic polymer), followed by pyrolysis at 1000 C in inert atmosphere and infiltration with molten Si [8]. With this processing strategy, Si does not fill completely the struts but reacts with the carbon residue giving a SiC deposit that rounds off the tips of the triangular
Fig. 2 Comparison between the fracture surface of a strut of a) a conventional reticulated foam, b) the same foam after recoating [images courtesy of C. Testa, U. Vogt (EMPA), and P. Colombo (University of Bologna)],
5.11 Other Developments and Special Applications 599
c) strut in which the tips of the triangular void have been rounded off by a SiC coating [image courtesy of A. Ortona (ICIMSI)], and d) strut infiltrated by the LigaFill process [image courtesy of J. Adler (IKTS)].
voids within the strut (see Fig. 2c). This greatly reduces the formation of fatigue cracks during service, limits strut swelling due to the formation of silica from Si (only a limited amount of excess Si is present), and maintains a low thermal mass (low thermal capacity) and good permeability. The resultant ceramic foams are well suited for high-temperature applications, for instance, as porous-burner components, in which the material is subjected to an oxidative environment and severe thermal shock (especially in the power-off state in which the foam goes from 1500 C to room temperature in a few seconds; see Chapter 5.5).
5.11.2.3
Infiltration of the Struts of Reticulated Ceramics
The possibility of filling the hollow struts of reticulated ceramics to increase their strength has been proposed in several patents; a sintered foam can be immersed in a suspension containing colloidal silica, alumina, or other refractory oxides [9], which because of their very small size can penetrate the voids of the structure and fill the microcracks. The colloidal silica sol is transformed at about 150 C into silica
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gel retaining the refractory powder, before sintering the foam again at high temperature. With this process it is possible to modify the affinity of the foam surface for slag-type micro-inclusions in molten metals and thus improve the filtering efficiency. Alternatively, a sintered foam can be immersed in an aqueous sol of aluminum hydroxide or zirconium hydroxide, then refired [10].
An unique way to completely fill the struts of reticulated foams was developed at Fraunhofer IKTS (Dresden, Germany) using metal or glass melts [11]. The melts infiltrate readily into the struts if the melt has a sufficiently low viscosity and exhibits good wetting characteristics. More importantly, using a silicon melt allows complete infiltration in a single thermal process starting from a polyurethane foam coated with unsintered SiC powder and results in a high-strength SiC ceramic foam (see Fig. 2d). The process (trade name LigaFill) can be used for cell sizes ranging from 10 to 80 ppi and, because it consists of a single step, it is economically advantageous in comparison to competing technologies that require refiring. A compressive strength enhancement of more than 600 % compared to an unfilled sintered SiC foam has been measured, with maximum values reaching 24 MPa for relative densities of less than 30 %.
5.11.3
Microcellular Ceramic Foams
Conventional processing methods do not easily allow the fabrication of macroporous bodies in which the average cell size is less than 100 mm, because of the lack of convenient polyurethane foams with cell size above 100 ppi. Novel methodologies for the production of microcellular foams, in which the average cell size ranges from about 1 to less than 100 mm, are under development. A possible strategy is to infiltrate porous salt preforms with a molten preceramic polymer [12], while other approaches suggest using sacrificial fillers [mainly commercially available poly (methyl methacrylate), PMMA, microbeads or latex] either in combination with chemical precursors [13] or with preceramic polymers [14]. Another possibility is to dissolve CO2 gas under pressure (5.5 MPa for 24 h at room temperature) in preceramic polymers and introduce a thermodynamic instability by rapidly dropping the pressure (at a rate of 2.9 MPa s–1) [15]. A large shrinkage (ca. 30 % linear) occurs on firing, but it appears to be isotropic and thus limits the formation of cracks, and overall cost could become an issue for large components. Microcellular ceramic foams manufactured by the above processes have cell densities greater than 109 cells/cm3 and cells smaller than 50 mm (Fig. 3). Because of the more uniform distribution of cell size (especially when sacrificial fillers are used), thinner struts, and the possibility of being fabricated with either open or closed cells, they have different properties than macrocellular materials and extend the range of applications available to ceramic foams.
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Fig. 3 Typical microstructure of a microcellular SiOC foam: a) closed-cell, from CO2 processing [image courtesy of Y.-W. Kim (University of Seoul) and C. Wang, C. B. Park (University of Toronto)] and b) open-cell by burnout of PMMA microbeads [image courtesy of P. Colombo (University of Bologna)].
5.11.4
Porous Ceramics with Aligned Pores
Porous ceramics with aligned pores can be fabricated by several techniques (see Chapters 2.2 and 2.5) affording various degrees of porosity. With freeze-drying, a component (such as water or a solvent) is first allowed to solidify and grow unidirectionally, and then is eliminated by melting or directly by sublimation from the solid to the gas state under reduced pressure. The resulting structure is actually very complex and is comprised of continuously open, strongly textured channels with a flat shape and a size of about 10–30 mm, which afford high porosity (up to 70 vol %) [16–18].
Gas generation during an aqueous electrophoretic deposition (EPD) process, coupled with freeze-drying, was also used for producing thick (ca. 2 mm) porous ceramic layers containing many aligned continuous pores with a diameter of about 100 mm [19]. However, this process seems more suitable for the fabrication of membranes, rather than monolithic porous components.
Oriented continuous pores can also be fabricated by slurry coating of fugitive fibers, producing ceramic materials having pores with a diameter of 165 mm and 35 % open porosity. The pore size and the amount of porosity can be adjusted by varying the diameter of the sacrificial filaments and the solids concentration of the slurry, respectively [20].
Honeycomblike structures with oriented, tubular pores can be fabricated by using a gel-formation approach, in which ceramic particles are dispersed in a solution containing an inorganic polymer (alginate) that can be gelled [21]. Capillaries form in the direction of diffusion of the cross-linking agent (multivalent metal ions) through the slurry. The thickness of the components is limited to a few centimeters, and they have up to 80 vol % porosity and pore diameters ranging from 10 to 30 mm.
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Drying should be carefully controlled to avoid crack formation in the ceramic components, and pronounced shrinkage occurs in the transition from wet gel to sintered body. Well-structured samples were obtained by optimizing the amount of powder in the slurry.
Highly porous cellular ceramic components with a thickness of about 1 cm and oriented cylindrical pores with a diameter up to about 2 mm were fabricated by forming hydrogen gas in an alumina sol–gel system by reaction of dispersed Al particles with H+ ions. The large shrinkage and the difficulty of eliminating the solvent during drying limit the size of the components achievable with this method [22].
5.11.5
Porous Superconducting Ceramics
High-temperature superconducting ceramics have a host of possible applications, for which various shapes and sizes of the ceramic material are required. The simple extension of conventional ceramic shaping techniques to these materials is not possible because of the stringent requirement of achieving a specific microstructural texture affording superconducting properties suitable for the applications. Foams and interconnected porous structures of superconducting ceramics could provide solutions for some of the problems encountered in applications of bulk or film-type superconductors. For instance, porous components may find applications in resistive superconducting fault-current limiters (requiring efficient heat extraction from the superconducting components) or could be reinforced by infiltration with suitable phases. Ceramic superconductors with enhanced mechanical strength are useful for applications like flywheel energy storage and levitation devices or quasipermanent magnets for magnetic fields exceeding 17 T at 25 K.
A biaxial grain texture, the entire superconductor ideally being a large single grain, with fine and homogeneously distributed normal conducting particles, are the two essential microstructural features that make the superconducting oxide YBa2Cu3O7–x (123) suitable for practical applications. Such a microstructure has been successfully achieved by various melt-processing techniques [23]. However, intrinsic drawbacks of these techniques, such as large amounts of low-viscosity liquids and distortions due to large and anisotropic shrinkage, limit their straightforward extension to complex shaped bodies such as foams.
The manufacture of superconducting 123 foams and porous bodies with high critical current densities has become possible by an infiltration and growth process developed by Reddy and Schmitz [24, 25] at ACCESS Materials & Processes (Aachen, Germany); see Fig. 4. In this process, standard ceramic shape-processing techniques are coupled with methods to avoid the shrinkage and distortion that typically occur in melt processing during the peritectic conversion of the green body to a single superconducting grain. Reticulated foams produced by this method typically have strut thickness of a few hundred micrometers and pore sizes ranging from 10 to 100 ppi. 3D interconnected porous bulk structures with a relative density of about 0.35, a pore size of about 1 to 2 mm, and strut thickness in the millimeter range
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Fig. 4 Schematic diagrams detailing the fabrication by infiltration process of a) a singlegrain superconducting YBa2Cu3O7–x (123) foam and b) a porous single-grain bulk material. The processing method involves two stages: in the first step a Y2BaCuO5 (211) porous structure is processed either as a replica of a commercial polyurethane foam or by creating a hollow replica of a wax model in 211 castings by standard ceramic processing. Subsequently this porous 211 body is converted to a singlegrain 123 foam or porous bulk by infiltrating it
with liquid phases from a liquid-phase source containing barium cuprates and copper oxides. Slow solidification through the peritectic temperature in the presence of a higher melting NdBa2Cu3O7–x (“Nd123”) seed crystal having a similar crystal structure results in nucleation and growth of a single superconducting grain covering the entire skeleton of the 211 preform. Images courtesy of G.J. Schmitz (ACCESS, Materials & Processes) and E.S. Reddy (IRC in Superconductivity).
have also been produced by the approach described in Fig. 4b [25], or by using spherical wax balls of desired size [26].
The high surface area of the foams, which is controllable partly by selecting the pore size, also allows the investigation of fundamental aspects of superconductivity, such as the extent of surface pinning and hence the critical current densities.
5.11.6
Porous Yb2O3 Ceramic Emitter for Thermophotovoltaic Applications
Thermophotovoltaics (TPV) is a technique to convert radiation from a synthetic emitter to electricity by using commercial silicon photocells [27] (Fig. 5). Researchers at EMPA (Duebendorf, Switzerland) and PSI (Villigen, Switzerland) recently proposed the use of reticulated ceramics for TPV converters for residential heating systems. The main concept is to combine heat and power generation in one compact unit, providing electrically self-powered operation of the furnace. For gas-fired TPV systems, which would operate as a porous burner (see Chapter 5.5), Yb2O3 ceramic foams were chosen as the selective mantle emitter because of their favorable emission spectrum and because they allow a high mechanical and temperature stability
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Fig. 5 The thermophotovoltaic principle with a ceramic foam-based mantle emitter. Pth is the thermal power of the burner which is supplied as combustion heat (proportional to the thermal power of the fuel), Pem the energy released by radiation from the emitter
(ca. 10–20 % of the combustion heat), Prad the energy available to the Si photocell after the filter, and Pel is the generated electric power. Image courtesy of U. Vogt (EMPA) and
W. Durisch (PSI).
of the emitter structure to be obtained. Al2O3, Y2O3, CeO2, or Zr2O3 was added as sintering additives, and processing was performed at 1550–1650 C.
For this application, a specific pore size is required, since pore size and the porosity of the ceramic foam determine the gas flow resistance and the emission behavior. Experimentally, 20 ppi foams were found to allow easy ignition of the flame and to stabilize the flame in the ceramic emitter structure (see Chapter 5.5). To improve the mechanical stability of the foams, the presintered Yb2O3 foams can be impregnated with a ceramic slurry (recoating), followed by an additional sintering step. Alternatively, presintered Al2O3 foams can be recoated with a Yb2O3 slurry to give a defect-free emitting coating.
TPV systems with commercial silicon solar cells (Type SH2 from ASE GmbH, Alzenau, Germany) and the developed selective Yb2O3 foam emitter have been demonstrated to be suitable for application in residential heating systems. For self-pow- ered operation of the heater, an electrical power of 120 W is sufficient, and safety requirements as well as cost effectiveness could be met. The Yb2O3 foam ceramic emitter has acceptable mechanical and thermal stability at 1400 C. Thermal-shock tests up to 200 ignition cycles showed that the emitters have satisfactory thermalcycling resistance, as required for TPV systems. The selectivity of the solar cell with respect to the emitter radiation is currently 10 %, and a successfully tested TPV prototype system had an overall electrical efficiency of gsys » 1 % [28].
5.11.7
Ceramic Foams for Advanced Thermal Management Applications
Open-cell vitreous carbon foams with porosity ranging from 75 to 97 % can be used as lightweight, low thermal conductivity insulation. Heat is conducted through the
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foam by conduction through the solid ligaments, convection through any gas in the pores of the foam, and radiation (see Chapter 4.3). To minimize thermal conduction through the solid, the lowest mass or highest porosity foam is used, and the solid should have low intrinsic thermal conductivity. Small pore diameters reduce heat transport by convection. Radiative heat transfer can be reduced by filling the pores of the open-cell foam with an aerogel of low thermal conductivity. Figure 6 shows the thermal conductivity of a vitreous carbon foam, a carbon aerogel, and an aero- gel-filled foam, as developed by Ultramet (Pacoima, CA). The data indicate that the filled reticulated vitreous carbon retains the excellent thermal insulation characteristics of low-density aerogel. Combining the carbon foam skeleton with the aerogel provides adequate structural performance. This material is being developed for lightweight insulation panels for aerospace applications.
Fig. 6 Thermal conductivity of aerogel, foam, and aerogel-filled foam. Data courtesy of E. Stankiewicz (Ultramet).
Realization of a viable reusable launch vehicle (RLV) is a primary goal for NASA. Compared with the existing Space Shuttle orbiter, the next-generation vehicle needs lower operating costs, increased reliability and safety, and faster turnaround between flights. The options currently under consideration range from major upgrade programs to various new vehicle concepts. For the thermal protection system (TPS), the key goals are improvements in both durability and temperature capability and a reduction in maintenance cost. A research effort is underway at NASA Ames Research Center (Moffett Field, CA) to develop durable, oxidation-resistant, and reusable foams for both acreage and leading-edge TPS applications. These foam systems need low to moderate density and temperature capability comparable to those of carbon TPS systems (reusable at 1650 C) with application of a suitable coating.
In a collaborative effort between researchers at The Pennsylvania State University and the University of Bologna, preliminary arc-jet testing was conducted on SiOC open-cell macrocellular (density ca. 0.73 g cm–3, cell size ca. 700 mm, crushing strength ca. 5 MPa) and microcellular (density ca. 0.35 g cm–3, cell size ca. 8 mm, crushing strength ca. 10 MPa) foams processed from preceramic polymers [29]. The tests were conducted in the NASA Ames 60 MW Interaction Heating Facility (IHF). Small diameter (2 cm) cylinders were tested for 120 s exposure to 1690 C (see