Cellular Ceramics / p2
.1.pdf
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Part 2
Manufacturing
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
33
2.1
Ceramic Foams
Jon Binner
2.1.1
Introduction
Over the past few years there has been a significant increase in interest in the production and use of highly porous ceramic materials. This is associated mainly with the properties such materials offer, such as high surface area, high permeability, low density, low specific heat, and high thermal insulation. These characteristics are essential for technological applications such as catalyst supports, filters for molten metals, hot gases, and ion exchange, refractory linings for furnaces, thermal protection systems, heat exchangers, and as porous implants in the area of biomaterials. Cell size, morphology, and degree of interconnectedness are also important factors that influence potential applications for these materials. Predominantly closed-cell materials are needed for thermal insulation, while open-cell, interconnected materials are required for uses involving fluid transport such as filters and catalysts.
The different properties required of cellular ceramics mean that a range of processing routes is needed to manufacture them; no one route is sufficiently flexible to yield all the necessary structures [1]. This has led to a wide variety of routes being developed and patented in many countries around the world; however, they can be crudely divided into three categories with a series of variations on the basic themes. One of the oldest approaches is based on the replication of polymer foams by applying a ceramic slurry that is dried in place prior to the polymer template’s being burnt out and the ceramic sintered. While this leads to very open, reticulated foams, burning out of the polymer leaves hollow and damaged struts that can reduce the mechanical properties of the final foam significantly. Despite this, these foams are manufactured in large quantities and used extensively in industry, often as filters for molten metals. The second basic approach relies on foaming a ceramic slurry by mechanical agitation or in situ evolution of gases. These approaches probably yield the widest range of cellular structures and hence properties, but they are generally less open than the replicated foams. While a very wide range of applications are now being considered it is probably true to say that most of them are still in the development stage. The final approach relies on the incorporation of sacrificial additives in the form of beads or related materials. Depending on the quantity added, the foams can be predominantly open or closed in nature. In the sections below, each of the
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
34 Part 2 Manufacturing
processes outlined above is examined in more depth. An examination of the typical properties obtainable and the applications for which the ceramics are being used or considered for use can be found in other chapters of this book.
2.1.2
Replication Techniques
2.1.2.1
Slurry Coating and Combustion of Polymer Foams
The replication of polymer foams was one of the first manufacturing techniques developed for producing ceramics with controlled macroporosity, the first patent being taken out in 1963 [2]. However, despite its age it is still the most common and widely used technique in industry. The process involves coating a flexible, open-cell polymer foam with ceramic slurries [3–5]. After removal of the excess slip by squeezing and subsequent drying, the polymer is burned out and the ceramic sintered in a single step. A flowchart is shown in Fig. 1 [6].
Select foam 
Prepare ceramic slurry 
Additives
Immerse slurry in foam
Remove excess slurry
Dry
Burn out organics
Sinter
Fig. 1 Flowchart of the production of cellular ceramics by the replica process, adapted from Ref. [6].
Many different polymers can be used for the precursor foam; these include polyurethane (PU), poly(vinyl chloride) (PVC), polystyrene (PS), and cellulose. Reproducibility of the properties of the organic foam is extremely important. It must be able to spring back after being squeezed out and have controlled tolerances to ensure consistency in the ceramic product. Finally, the foam must burn out cleanly
2.1 Ceramic Foams 35
and completely during sintering without damaging the ceramic replica. As an example, Selee Corp. in the USA uses an interconnected, open-cell polyurethane foam with 97 % void volume as organic precursor, the structure of which consists of a complex pattern of dodecahedra repeated in three dimensions [4].
The production of a reticulated ceramic component begins with the machining of the original polymer foam to the desired shape. While theoretically there are relatively few limitations on the shape, in practice the brittle nature of the final ceramic replica means that failure can occur during burn out, sintering, or usage if the shape is too complex. Since the final ceramic foam is a direct replica of the original foam (Fig. 2), the polymer foam structure and pore size are critical in determining the properties of the final component, for example, its density and permeability.
Fig. 2 Reticulated ceramic foam produced by replication of a polyurethane foam.
Generally, any fine ceramic powder can be used that can be made into a suitable suspension; the rheological characteristics are similar to those required for other slurry-based production routes for ceramic components. The solids content usually lies in the range 50–70 wt%; a higher value can lead to an excessively high viscosity that can cause difficulties during infiltration. Typically, the suspension is thixotropic, that is, the viscosity decreases with time at a fixed rate of shear, and on removal of the stress the material regains its original structure with a consequent increase in viscosity. This type of slurry allows coating of the organic foam without excessive drainage; in some cases it is desirable for the slurry to bridge the polymer foam struts. This creates smaller window openings between the cells and thus increases the tortuosity of the flow path for fluids and therefore increases efficiency when the ceramic is used as a filter. Flocculating agents can also be added to improve the adherence of the slurry to the polymer material and thus reduce the chances of poor coating.
Once the polymer foam and ceramic slurry are ready the coating process is carried out. This involves immersing the foam in the slurry and compressing it to
36 Part 2 Manufacturing
remove air. While still in the slurry the foam is allowed to expand again, causing the slurry to be sucked into the open cells of the foam. This step can be repeated several times to achieve the desired coating density, after which pores that have become blocked with excess slip are cleared, typically by a defined compression of the foam.
When the foam has been coated appropriately, it is dried in an oven to solidify the ceramic structure, after which it is exposed to an initial thermal treatment that is designed to burn out the polymer from inside the ceramic struts as well as remove any organic additives from the ceramic slurry. Typically this involves heating the dried foam to a temperature in the range of 350–800 C, depending on the composition of the precursor polymer. The process can be performed in air or any other type of atmosphere as appropriate, and the heating rate must be carefully controlled, and generally it is very slow to prevent formation of residual stresses or cracking of the ceramic network due to volatilization of the polymer. This results in the formation of a green ceramic replica foam in which the struts are hollow and which requires a normal sintering schedule to achieve the required density and strength of the ceramic in the struts. The conditions required depend on the composition of the ceramic material used.
While this approach generally yields a very open reticulated structure with high permeability, the main disadvantage is associated with the hollow struts and large number of flaws that result from burning out the polymer foam substrate (Fig. 3); low mechanical properties are therefore a typical characteristic of this route. Another complication is related to the quantity and toxicity of the gases that are released during polymer burn out, for example, hydrogen cyanide in the case of polyurethane, which can make expensive scrubbing of the waste gases necessary.
Fig. 3 Ceramic struts resulting from the polymer foam reticulation process. Note how they are hollow and have triangular holes caused by escape of the polymer during combustion.
In an attempt to overcome the low strengths that arise from the replication technique, Luyten et al. [7] replaced the conventional (alumina) ceramic slurry with one that contains a passivated mixture of aluminum and alumina. During the sintering
2.1 Ceramic Foams 37
stage, the aluminum is oxidized to yield a reaction-bonded alumina foam. While the grain structure was finer and thus higher strength foams were achieved compared to conventionally produced foams by this route, the approach could not overcome the intrinsic disadvantage of hollow and cracked struts arising from the burnout of the polymer.
However, this has been achieved by Edirisinghe et al. [8], who electrosprayed alumina slurry onto a polyurethane foam prior to removing the polymer by pyrolysis after drying. The electrospray technique involved making the slurry flow through a nozzle kept at a high voltage relative to a ground electrode. This electrostatically atomized the suspension, creating very fine droplets that were used to coat a commercial polymer packaging foam (Fig. 4). To produce about 30 mm cubic samples capable of withstanding significant handling required 5 h of electrospraying and sintering at 1200 C. The most interesting feature of the process is that the strut cross
(a)
(b)
Fig. 4 Alumina foam produced by pyrolyzing electrosprayed polyurethane foam showing a) the open cell structure and b) the struts; after Ref. [8]. (S.N. Jayasinghe and M.J. Edrisinghe, J. Por. Mat. 2002 9 265–273). Reprinted with kind permission of Springer Science and Business Media.
38 Part 2 Manufacturing
(a)
(b)
Fig. 5 Struts of the alumina foam prepared by pyrolyzing electrosprayed polyurethane foam showing a) a solid cross section and b) a homogeneous crack-free microstructure incorporating fine details of the template; after Ref [8]. (S.N. Jayasinghe and M.J. Edrisinghe, J. Por. Mat. 2002 9 265–273). Reprinted with kind permission of Springer Science and Business Media.
sections show an homogeneous, crack-free microstructure, intriguingly without the central void typical of ceramic foams produced by immersion in a slurry (Fig. 5). Although the authors claim that the latter can be explained by the very fine size of the droplets, this argument is not convincing.
2.1.2.2
Pyrolysis and CVD Coating of Polymer Foams
A variation of the replication process described above is the pyrolysis process, in which the polymer material is not burned out but pyrolyzed to yield a carbonaceous skeleton that can then be coated by the appropriate ceramic. The process begins with the pyrolysis of a resin-impregnated thermosetting foam to obtain a reticulated carbon skeleton. This can then be coated with a ceramic slurry as described above, but it is more usual to use chemical vapor deposition (CVD) [9, 10] to coat the indi-
2.1 Ceramic Foams 39
vidual ligaments. Any material that can be deposited by the CVD process can be used for the coating, including oxides, nitrides, carbides, borides, silicides, and metals.
The reticulated carbon network is heated to the required deposition temperature, and then a gaseous precursor compound is passed through the hot body. The gas is reduced or decomposed on the carbon surfaces throughout the internal structure of the foam according to any one of a number of different chemical reactions (Tab. 1), to form a uniform coating that is typically 10–1000 mm thick and has up to 50 % of the theoretical density. The process utilizes the high rates of deposition available with CVD, typically 100–400 mm h–1.
Table 1. Chemical vapor deposition; examples of precursors and reaction temperatures; after Ref. [65].
Coating |
Reaction |
Temperature/ C |
|
|
|
AlB2 |
AlCl3 + BCl3 |
1000 |
Al2O3 |
AlCl3 + CO2 + H2 |
800–1300 |
B4C |
BCl3 + CO + H2 |
1200–1800 |
|
B2H6 + CH4 |
1200 |
|
(CH3)3B |
560 |
BN |
BCl3 + NH3 |
1000–2000 |
|
B3N3H3Cl3 |
1000–1500 |
SiC |
CH3SiCl3 + H2 |
1000 |
|
SiCl4 + C6H5CH3 |
1500–1800 |
Si3N4 |
SiH4 + NH3 |
950–1050 |
|
SiCl4 + NH3 |
1000–1500 |
TiB2 |
TiCl4 + BBr3 |
300–850 |
TiC |
TiCl4 + H2 + CH4 |
980–1400 |
ZrB2 |
ZrCl4 + BBr3 |
1700–2500 |
To increase the flexural and tensile properties of the foam, a dense face sheet can be applied to one or more surfaces of the reticulated material by changing the gas flow patterns during the CVD process. Other unusual structures developed include a material that is thermally insulating at one end and thermally conducting at the other by deposition of different materials on each half of the preform and fabrication of insulators with a density gradient throughout the material. The latter method makes it possible to blend a high-density surface into an area of low thermal conductivity.
2.1.2.3
Structure of Reticulated Ceramics
An oft-quoted characteristic of reticulated ceramics is the number of pores per linear inch (ppi value), even in countries where SI units are used. Ceramic foams produced by the replication methods described above usually have pore sizes between 5 and
40 Part 2 Manufacturing
65 ppi (2–25 pores per cm) and densities ranging between 5 and 30 % of theoretical. The foams prepared are typically 10–100 cm wide and 1–10 cm thick.
Reticulated ceramics produced by replication of a polyurethane foam exhibit large cracks along the struts, which are also hollow (Fig. 3). Both these features occur as a result of the elimination of the polymeric precursor. Green et al. [11–14] have examined these materials in some depth and concluded that while the strength could be improved if the strut cracks and other large flaws were eliminated, the large triangular holes in the struts do not necessarily reduce the strut strength unless the apex is near the outside of the strut or is associated with a strut crack. In addition, they found that most of the theoretical relationships developed by Gibson and Ashby [15] for the mechanical behavior of cellular materials hold quite well for these reticulated ceramics. While some differences were apparent, these could be attributed to microstructural defects.
Gauckler et al. [16] used stereological methods to characterize the microstructure of reticulated ceramics produced by replication of a polyurethane foam to understand their flow characteristics since they were being used for metal infiltration. The basic stereological parameters used are shown in Fig. 6. The structure of the reticulated ceramic consisted of rounded polyhedra with a nominal diameter P, connected by openings or windows with diameter u. The total porosity of the reticulated body was designated f, the total internal cell surface area per unit volume Sv and the distance between two pore centers S. The structure revealed anisotropy; in the plane of the filter plate the pores were spherical, whereas in the perpendicular direction the pores were both larger and elongated. With respect to the permeability of the ceramics, as expected the most important features were the pore size P, and window size u. The relationship between pore and window sizes (Fig. 7a), was linear, and that between the internal surface area per unit volume and window size is shown in Fig. 7b.
Fig. 6 Stereological parameters of reticulated ceramics; after Ref. [16]. (L.J. Gauckler and M.M. Waeber, in Light Metals 1985, Proc. 114th Ann. Meet. Metal. Soc. AIME, 1985, pp. 1261–1283.) Reprinted with permission of The Minerals, Metals and Materials Society.