Cellular Ceramics / p2

2.2 Honeycombs 67

reactions required for its own fabrication: drying, debinding, and firing of large honeycombs.

About 50 vol % of the wet-green honeycomb web is binder (water, polymer, etc.), but on the basis of the entire volume of the substrate, it is only 10 vol % binder (for a honeycomb with 80 % open frontal area), and less than 2 vol % polymer for the entire object. Such a content of polymer is very low for such a large ceramic object; it has almost the same polymer volume fraction and diffusion distance as is found in the famous “Norton Sand Filled Balloon” demonstration [87].

2.2.4.2

Melt Manipulation

Cordierite (2 Al2O3·2 MgO·5 SiO2) honeycombs are used as substrates for ceramic automotive catalyst supports and diesel particulate filters (DPF). Most cordierite honeycombs are extruded from precursor powders that react on firing to become cordierite. There are many precursors and resulting microstructures; however, the “talc path” to cordierite is a good demonstration of the use of melt manipulation to make a unique porous ceramic [88].

For the talc path to cordierite, the cordierite precursors are talc, quartz, kaolin, calcined kaolin and alumina. During the reaction path to cordierite (1400–1430 C), at about 1355–1370 C the so-called talc melt event occurs [89]. It is a misnomer for a series of low-melt eutectics, but it picturesquely describes how the whole talc particle becomes part of a liquid phase that undergoes capillarily drainage into the surrounding, rigid, porous microstructure. The talc particle melts and drains away into its porous surroundings where it noninstantaneously reacts to form cordierite, and a relic pore of the same size and shape as the talc particle is left behind.

To demonstrate the power of the talc melt event, the precursor paste was divided into two pastes: a talc paste and a non-talc paste (quartz, kaolin, calcined kaolin, and alumina). Core-clad spaghetti was coextruded with a talc paste core and non-talc paste cladding; their proportions were such that together they would react to become cordierite. The wet-green core-clad spaghetti was bundled and isopressed to make a honeycomb precursor: an array of talc filaments in a matrix of non-talc. This composite billet was reduction extruded. The resulting extrudates were rebundled, isopressed, and reduction-extruded again. On firing, the array of talc filaments melted and were capillarily drained into the non-talc porous matrix where they reacted to form cordierite, and a cordierite honeycomb resulted (Fig. 5). Had the distribution been reversed (an array of non-talc filaments in a talc matrix), the honeycomb structure would have collapsed during the talc melt event. This demonstrates the need to engineer the non-talc distribution to percolate and sinter throughout the honeycomb in such a way as to form a strong enough skeleton to absorb and to hold/support the talc melt (which is a liquid and not load-bearing) without losing the honeycomb shape while reacting to form load-bearing cordierite. This same technique can be used to make silicon carbide honeycombs [43].

In the standard random mix of cordierite precursors, during the talc melt event, the honeycomb web is a porous non-talc ceramic, and its pore size distribution

2.2 Honeycombs 69

Talc and graphite are both platy and form platy relic porosity, and hence the way in which these two particle size distributions percolate together through the paste results in a pore network of high permeability and strength.

During honeycomb extrusion, under shear, the platy particles align their faces with the side faces of the pins of the honeycomb die, which results in a degree of coparallelness between the webs and the platy particles [88]. This can result in beneficial anisotropic cordierite crystal growth for microcracking to achieve a low coefficient of thermal expansion (CTE; e.g., 0.3 0 10–6 K–1), and this alignment can result in beneficial anisotropic drying and firing shrinkage for increasing OFA [90].

Ceramic processing is about the reduction of surface area without loss of shape. In special cases of “enhanced surface diffusion”, sintering can occur without loss of porosity [91–93], but in most cases some porosity must be traded in sintering for strength.

High porosity can be obtained by addition of pore formers, open packing particles, and shrinkage inhibitors [49], but some of the porosity is lost to shrinkage during firing. This shrinkage can decrease web thickness, width, and length, and the shrinkage distribution depends on particle arrangements, orientations, and constraints [93–95]. In a special case, a decrease in porosity can result in an increase in open frontal area and an increase in cell density during firing [90].

There is a practical limit to the maximum cell density that can be directly extruded. Everything in honeycomb extrusion process works against the direct extrusion of a honeycomb with higher cell density:

. The higher the cell density, the larger the drag area between the extrudate and the die.

. The higher the cell density, the thinner the webs, and the higher the flow intolerance to inhomogeneities in the batch and the die.

. The higher the cell density, the thinner the webs, and the faster the die wears out of tolerance.

Thus, when the cell density of the honeycomb can be increased without increasing the cell density of the honeycomb die, it is commercially significant. However, converting porosity into higher cell density is problematic because pore formers can be expensive and this transient high porosity is fragile and prone to damage and distortion during pronounced shrinkage during drying, debinding, and sintering.

2.2.5

Post-Extrusion Forming

Post-extrusion forming denotes shape-changing steps that can be performed on the honeycomb after it has been extruded. The most familiar one is the checkerboard plug pattern on the faces of a honeycomb that makes it into a “flow through the wall” filter, such as a diesel particulate filter (Section 5.2) [96, 97]. Less familiar postextrusion forming operations are reduction extrusion and hot draw for making honeycomb funnels (convergent channels) and ultrahigh cell density honeycombs.

70 Part 2 Manufacturing

These processes allow one to get around some of the cell density limits imposed by the paste and die technology. The following are some examples of how honeycombs can be viscoplastically altered by reduction extrusion [5] and by hot draw down [50, 93] to increase cell density.

2.2.5.1

Reduction Extrusion

Reduction extrusion entails ram extruding (Fig. 4) a billet of paste through a reduction die (frustum-shaped die) to plastically to reduce the diameter and to increase the length of the billet while maintaining its volume. When the billet is a composite of more than one paste of equivalent plasticity, the cross-sectional pattern of this billet can be reduced in a self-similar manner by reduction extrusion, and this process is popularly known as MFCX (microfabrication by coextrusion) [27, 48].

If a wet-green honeycomb extrudate is reloaded into an extruder barrel and ramextruded through a reduction die, it plastically deforms in an undesirable manner: the webs buckle and the channels collapse and it ultimately passes through the die as a solid plug. However, if the channels are backfilled with an incompressible, nonseeping material of about the same rheology as the webs, then the honeycomb plastically reduces through the reduction die without its channels collapsing. There is creep flow through the reduction die, so the flow lines (channels) do not cross. The reduction occurs in almost a self-similar manner. The channel filaments in the die show the flow path in the same detail as a FLUENT flow model trace of a homogeneous paste (Fig. 6). The volume and number of channels are conserved from the entrance to the exit of the reduction die. Channel continuity is maintained through the billet, die, and extrudate. The resulting novel products are honeycomb funnels and honeycombs with higher cell density than can be made by direct honeycomb extrusion.

Reduction extrusion of the wet-green honeycomb can be described from another perspective: the wet-green honeycomb is used as an extrudable mold for plast-cast- ing. Plast-casting is like gel-casting, but in plast-casting the viscous slurry sets to become an extrudable plastic paste [34, 99, 100] rather than an elastic body like in gel-casting [101, 102]. Here, the wet-green honeycomb is used as an extrudable mold to cast, in its matrix of plastic webs, an array of extrudable filaments, the goal being a homogeneous plasticity of a heterogeneous (composite) structure.

The following is an example of the reduction extrusion of a paste-extruded honeycomb [6, 103]: The extruded honeycomb is dried (it now has an open porosity from the loss of more water volume than shrinkage volume during drying) and then trimmed to fit a mold in which it is reconstituted by submersion in hot water (90 C). During reconstitution the web porosity is saturated with hot water, which is a nonsolvent for the methylcellulose binder (Section 2.2.3.3). While hot, saturated, and still in the mold, the hot channel water, but not the pore water, is drained and replaced with a hot liquid microcrystalline wax (90 C), which is very fluid. The wax easily fills the channels under only gravitational pressure and is immiscible with the pore water. The backfilled honeycomb is cooled, and the water in the pores redissolves the methylcellulose binder as the liquid wax in the channels crystallizes/soli-

2.2 Honeycombs 73

For direct paste extrusion of a honeycomb, a rule of thumb is that an extruded web should have at least ten particles spanning its width, otherwise flow disruptions (log jams) will occur in the die. Reduction extrusion does not have this problem, since the smallest features are defined by soft boundary conditions (wax filament array) and not by hard boundary conditions (metal honeycomb die). But a soft boundary condition has its own problems: reduction extrusion decreases the channel diameter and web thickness, but does not correspondingly decrease the size of the microstructure: the particles in the paste remain the same size before and after the reduction die.

Reduction extrusion of the honeycomb increases the cell density but loses some of the open frontal area, either due to shrinkage of the wax during solidification (ca. 15 vol %), and/or due to slip planes [105, 106], and/or due to intrusion of the wax during dilation or during roughening of the web by reorientation of platy particles. For reduction extrusion, the web roughness seems to scale to the largest particle in the paste and the wax backfill, the talc particle (ca. 8 mm) and microcrystalline wax crystal (ca. 18 mm), respectively. So, as the web thickness reduces to become comparable to the size of the largest precursor particle (and the fired mean pore size of ca. 2 mm), considerable roughening of the web occurs. This is expected since the paste can no longer be approximated as a continuum at this level of scrutiny. Its graininess/microstructure and degree of mixedness do not reduce as its honeycomb geometry does during reduction extrusion (Fig. 9) [107].

For greater reduction to higher cell densities, finer web-paste and channel-fill microstructures are required, and this is an incentive to use nanoparticles [108]. The ultimate goal is to have no microstructure.

2.2.5.2

Hot Draw Reduction

A paste of glass particles can be honeycomb extruded, and its particle size distribution of the glass can be eliminated by viscous sintering so that, ideally, there is no microstructure left to scale to the original glass particle size distribution. This glass honeycomb can be hot drawn down (Figs. 9 and 10) to make a honeycomb fiber. Ideally, this fiber can be drawn down to a submicrometer web thickness (see Fig. 2) smaller than the original particles from which it was made [98]. However, glass has its own problems. Glass powder, made by comminution (ball mill, attritor, jet mill, etc.) has contaminants that scale to the glass particle size distribution. This wear debris is often a nonglass which does not viscously hot draw; hence, the debris particle stays the same size as the web shrinks around it. Ultimately, these stones limit the diameter down to which the fiber can be drawn. Superfine glass powders which are built up (e.g., pyrolysis sootlike Cab-o-sil fumed silica, and sol colloids like Ludox colloidal silica) rather than broken down do not have grinding debris.

Other sources of microstructure are leaching (alkali depletion of the glass powder or web surface), devitrification, and coarse porosity resulting from unintended pore formers. The pore size distribution in the debinded particle network can be bimodal as the result of a pore-former size distribution in addition to the normal interstices

76 Part 2 Manufacturing

have a nondiffusing atmosphere in the bubble, because a nondiffusing atmosphere would resist the viscous collapse of the bubble [113–115]. Some large bubbles can persist, because the larger their diameter, the longer the time and/or the higher the temperature needed to collapse even a vacuum bubble. It is better to prevent or eliminate unintended pore formers during batching and mixing rather than to eliminate their conesquences during sintering. However, if a “fix” is still needed, hot isopressing of the closed porosity with argon gas (nondiffusing) can collapse the bubbles at a lower temperature and in a shorter time.

The sintering time and temperature schedule must be such as to avoid phase separation and devitrification in the glass, which affect the viscosity. Devitrification/precipitation can grow crystallites sufficiently large to prevent useful draw-down. Potassium borosilicate glass is particularly resistant to devitrification, and fused silica is not. The fused-silica strategy is to sinter for a shorter time at a lower temperature by starting with cleaner and smaller particles and pores.

Viscous sintering of a glass powder is a careful balancing act for processing of a honeycomb of glass particles. Sintering consumes fastest the surfaces with the smallest radii of curvature. The art and science of ceramic processing is to reduce surface area without losing the shape of the object, without turning the mass into a sphere or a puddle. The goal is for the pore size distribution of the packed glass particles to be much smaller than the web thickness to minimize sintering away of the channel surface area (which would result in a loss of open frontal area) and of the honeycomb shape (rounding of the channels and gravitational slump) during viscous sintering to zero web porosity.

In the forming of a glass honeycomb by paste extrusion, it is interesting to track the surface area throughout the process: surface area is created by pulverization/- comminution or precipitation to create a glass powder, some surface area is lost in sintering to create a nonporous honeycomb preform, and then surface area is regained in hot draw to create a honeycomb fiber. Here, the random surface area of the powder is eventually recreated as the nonrandom surface area of the honeycomb fiber which defines the microfilament array of air.

An example is the fabrication of a glass honeycomb funnel from potassium borosilicate glass (7761 Pyrex glass, see Fig. 11 for viscosity) [52, 107].

Glass tubes were hot melt extruded from a glass melt tank. The tubes were crushed, dry ball-milled, and dry sieved to make a particle size distribution with an average particle size of about 12 mm (Fig. 12). The glass powder was dry blended with Methocel powder, and liquids were added during the mulling process, whereby the powder was consolidated into paste chunks. The stiff paste was then evacuated and ram-extruded three times through a spaghetti die (an array of 3 mm diameter holes) for high-shear mixing, and then ram-extruded through a honeycomb die. The honeycomb extrudate was dielectrically dried enough to fix its shape and then con- vection-dried (70 C). It was then trimmed (30-inch length) and vertically suspended in an electric furnace (700 C max.) in a controlled atmosphere to debind and then to sinter. The length of the honeycomb shrank due to sintering while it lengthened owing to gravitational slump. This vertically suspended fired honeycomb ended with a slight axial taper due to the gravitational stress gradient along its length. This