Cellular Ceramics / p2

57

2.2

Honeycombs

John Wight

2.2.1

Introduction

A paste is a viscoplastic formable body of dispersed particles in a polymer solution. This chapter describes the paste process for manufacturing ceramic and glass honeycombs with an emphasis on direct extrusion of the honeycomb shape. This paste process creates a porous, particulate honeycomb, the porous microstructure of which can be processed to change its composition, porosity, and connectivity. The honeycomb shape can also be changed by reduction extrusion and hot draw, enabling a wide range of honeycomb channel diameters from bigger than 1 cm to smaller than 1 mm. Glass honeycombs can be hot drawn down to fibers with channels and specific surface area comparable to the pore structure of the assembled particles from which they were first made – except that the channels are linear instead of random. The honeycomb extrusion process is a mechanical way to create linear porosity.

This chapter starts with the highest volume product, automotive catalyst supports, and ends with the lowest volume, photonic crystal fibers. The progression from one to the other is also a progression from large channels to small channels, and it demonstrates something fundamental to the honeycomb paste extrusion process: the interaction of shape and microstructure.

2.2.2

Forming the Honeycomb Geometry

2.2.2.1

Background

A porous ceramic is a composite of nonsolid (gas and/or liquid) and ceramic. It can have the surface area of ceramic powder in the shape of an object. If the nonsolid phase is open and continuous, the entire volume of the object is readily accessible for reaction. Ranges of specific strength (Chapter 4.1), thermal conductivity (Chapter 4.3), and permeability (Chapter 4.2) are possible, and depend upon the microstructure, that is, the connectivity of the solid and nonsolid phases. A disconnected

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

58 Part 2 Manufacturing

solid phase (e.g., packed beads) is immune to thermal shock since it is already “broken”, but is prone to self attrition [1]. A reticulated foam gives beneficial flow bifurcation for mixing, but at the cost of backpressure. Each arrangement has its own advantages and disadvantages.

There are many ways to distribute a nonsolid in a solid: random (dispersed bubbles and percolating paths) and nonrandom (honeycombs and woven structures). For some objects that are much more nonsolid than solid, the nonsolid and the solid percolate in such a way as to achieve optimum strength and permeability for a given porosity for a given application. The honeycomb (linear cellular) geometry is a popular design solution in nature and in industry (Fig. 1) [2].

1 cm

Fig. 1 Popular design solutions: Wasp nest (left) and ceramic honeycombs (right).

The largest volume honeycomb application is the automotive catalytic support: A typical automobile has about 800 m of honeycomb channels in its catalytic converter, which has 4 m2 of geometric surface in 1.2 L of substrate. It has two types of porosity: geometric (channels) and microstructural (pores and microcracks). These honeycombs have about 20 000 km of pore paths in the microstructure of its webs. The honeycomb market is about one hundred million substrates (two per car) per year for automotive catalytic converter substrates alone, and this is divided between Corning, NGK, Denso, Emitec, and others. Other markets include diesel particulate filter (Chapter 5.2), molten-metal filters (Chapter 5.1), membrane supports, and composite core material.

The cross-sectional geometry of the honeycomb is a tessellating pattern; the channels can be triangular, square, hexagonal, round, or of other shapes [3]. There can be gradients in pitch and web thickness in the radial [4, 5] and axial [6] directions, if this offers an advantage. Industrial honeycombs come in a variety of sizes: anodized alumina membranes (Anapore : 0.02–0.2 mm diameter channels [7]), photonic crystal fibers (0.5 mm [8]), glass capillary arrays (Burle: 2–50 mm [9]), multicapillary bundles (Alltech: 40 mm), “microcombs” (Philips: 100 mm [10]), automotive catalytic converter substrates (Corning: 0.8–1.3 mm [11]), lightweight mirror cores (10–150 mm [12], Fig. 2). And they are available in a variety of compositions: alumina, carbon, cordierite, fused silica, mullite, potassium borosilicate glass, silicon carbide, and so on. The combination of cell geometry and composition is designed to meet the application.

2.2 Honeycombs 61

exiting the die. The array of metal filaments (pins) in a matrix of paste becomes an array of air filaments in a matrix of paste as the honeycomb exits the die (it actually inhales). Buried in this matrix of paste is an array of knit lines whose properties are not detrimental.

This process creates a huge amount of well-defined porosity by purely mechanical means on the order of a 1600 km of channel per hour. A solid billet of paste or melt is pressurized to flow through a die to be plastically formed into a honeycomb with 90 % open frontal area (OFA, the channel face area divided by the total face area multiplied by 100) which has one tenth the bulk density of the billet (paste). Push 1 m of billet into the die and 10 m of honeycomb is extruded with the same diameter (Fig. 4). No chemistry is necessary: no pore formers [18], no etchants [12, 19–22], no burnout and intrude procedures [23, 24], and no washcoat and burnout procedures [25, 26] (Chapter 2.1).

10 cm

Fig. 4 Paste extrusion of honeycomb. Different views of ram extrusion of a ULE paste: the initial push through the die (bottom left), the honeycomb extruding/hanging down (bottom center), and the extrudate horizontally supporting itself (bottomright). A schematic of a ram extruder is shown in the top figure.

High OFA is greatly valued in that the higher the OFA, the lower the backpressure in a flow-through application (Chapter 4.2). However, in extrusion die design, the higher the OFA, the higher the extrusion pressure, for a given cell density and paste. A substrate which is designed for low backpressure makes for a die that has

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high backpressure; in some ways the extrusion and the application are the flow complements of one another. Ceramic foam forming processes (Chapter 2.1) do not have this particular high-OFA challenge.

2.2.2.3

Nonextrusion Fabrication Processes

Honeycombs can be constructed from lower dimensional parts (points, lines, and planes), which are much lower force forming operations than direct honeycomb extrusion, because the cross-sectional area of these extruded components is smaller than that of their assembled honeycomb. Another advantage of rods and sheets is that they are easier to coextrude (there is more room/dimension to manifold a rod or tape coextrusion die) and to decorate than directly extruded honeycombs. Once assembled these “two-color” composite honeycombs can have increased functionality [27–29]. Here are some examples of lower dimensional builds of honeycombs:

. A “point” can draw a honeycomb by a variety of rapid prototyping schemes: MIT’s 3DP [30], Rutgers’ FDC [31], Sandia’s Robocast (Chapter 2.3) [32], and others. These methods offer the opportunity to fabricate high aspect ratio walls [33] and periodic structures in the axial direction of the honeycomb, too [34, 35]. Here axial periodicity can be used to create a mixing exchange between adjacent channels by having a rod stacking sequence that bifurcates the flow (like reticulated foam), so as to prevent boundary layer formation on the channel wall, which otherwise would reduce the reaction rate [36–38]. Axial periodicity can also be achieved in direct honeycomb extrusion by employing moving dielets [39, 40], but this is not practical for substrates with high cell density.

. “Lines” can be bundled to make a honeycomb, as has already been described in the bundling of tubes to make a honeycomb. This is the quickest path to making a honeycomb from a tooling point of view. The technique has ancient roots in millefiori [41] and has modern practitioners. Different colored rods and tubes can be assembled to form a “dot-matrix” cross-section pattern: a simple build is a single core/clad design, and a more complicated build is a complete array/matrix honeycomb. The resulting composite billet can be reduction-extruded (MFCX: microfabrication by coextrusion) [27, 42–44] or hot drawn down (as is commercially done with core/clad rods by Galilieo Electro-Optics, Burle Technologies [9], and Schott Fiber Optics [45]) to miniaturize the dot-matrix pattern. The process is iterative in that the reduced parts can be bundled and reduced again and again to achieve high cell densities and large cross-sectional areas. The resulting composites have a range of useful properties:

–A refractive index contrast between the matrix and the array [46] results in image transfer fibers and taper optics.

–A leachable matrix creates flexible fiber bundles for fiberscopes [45, 47].

–A crackable matrix creates tough composites [48, 49].

–A leachable array results in thin channel array plates for photomultipliers [9, 50, 51].

2.2 Honeycombs 63

Note that honeycomb plates can be made by etching out the array of filaments due to their relatively small aspect ratio and thickness, but hollow honeycomb fibers cannot be made by etching due to their extremely high channel aspect ratio and length and insufficient etching contrast between the array and matrix. Hot drawing a honeycomb with an array of glass filaments is less prone to distortion (radial pitch gradient and loss of channel volume) than an array of air filaments, which does not have a constant-volume condition. Air filaments can be inflated and deflated as required during sintering and hot drawing so as to oppose surface tension (loss of channel volume by viscous sintering) and to modify OFA [21, 22, 52].

. “Planes” can be stacked to make a honeycomb. Sheets can be made by a papermaking process (like the Cercor substrate [53], which was the predecessor of the more cost effective Celcor substrate [1]), by tape casting and by extrusion and/or calendering with or without ribs [54]. Sheets can be embossed and corrugated to create troughs that become channels upon stacking. These can be rolled up or stacked to assemble honeycomb structures. Sheets can be selectively laminated into a stack, and then the stack can be expanded into a honeycomb [10, 55]. The sheets can be decorated before assembly so as to have periodic chemistry, windows, and so forth on the sheet [56]. The advantage of sheets is that there is no limit to the face area of the honeycomb made by stacking or rolling, because it occurs outside of the die. The face area of a honeycomb made by direct extrusion is mechanically and practically limited by force (extrusion pressure times the area). However, larger diameter substrates can be assembled from smaller substrates outside the die in order to make large segmented substrates [57].

In summary, honeycombs made from points, lines, and planes are assembled outside of their die; they are alternatives to direct honeycomb extrusion, a process in which the flow streams assemble and knit inside the die. Ultimately, profitability, speed, flexibility, and process-dependent properties (quality of knit and total knit area per unit volume of honeycomb) are the criteria for comparing these different honeycomb fabrication processes.

2.2.3

Composition

For paste extrusion of a honeycomb, the particles, binder, and processing equipment are designed to work together as a system of compromises and synergies to obtain a successful honeycomb product.

2.2.3.1

Paste

The direct extrusion of a honeycomb requires that the material be viscoplastically formed into a honeycomb, and requires that this shape and composition neither slump nor fracture during handling, drying, debinding, firing, finishing, assem-

64 Part 2 Manufacturing

bling, and operating. There are two classes of materials for honeycomb extrusion: melts and pastes. Melts are densified and then shaped (like a glass melt). Pastes are shaped and then densified (like a ceramic paste). A melt is homogeneous. A paste is a composite in that it is a mixture of binder “liquid” and particles, a powder which is usually either too refractory or reactive to be economically melt-processed. Any powder (polymer, metal, glass, ceramic, etc.) can be extruded into a honeycomb provided it is compatible with its binder, a polymer solution.

Examples of paste extrusion are spaghetti [58] and pencil lead, the first high-tech ceramic [59]. For a paste, the shaping shear occurs in the binder phase, such that the particle assembly transforms from one shape to the next without fracture. The role of the binder is to saturate (no compressible gas phase) and to not seep/leak through the particulate pack. “Incompressibilility” and “no seepage” are the necessary conditions to create a hydrostatic condition for shear strain for extrusion for plastic shaping of the particle network (soil mechanics calls this the critical state) [60–64]. The other role of the binder is to build viscosity during mixing to create sufficient shear stress for deagglomeration of the particles (viscous processing [65, 66]) and then to create sufficient yield stress to maintain the shape of the wet-green extrudate upon exiting the die (Terminology: “wet-green” is as-extruded: undried and unfired, “dry-green” is dried and unfired, “brown” is debinded, and “fired” is the final sintered ceramic). The goal is to disperse the particles and then to agglomerate them into a single uniform agglomerate (the paste) and then to reshape/extrude it into a honeycomb.

2.2.3.2

Mixing

The formation of the paste is a transient event. Particles and liquid are brought together. The components are mixed and “sintered”: liquid–solid interfaces displace/ minimize the air–solid and air–liquid interfaces, expelling the air phase, coalescing/consolidating the free-flowing, air-dispersed ingredients into a saturated (no gas phase) plastic paste, a dough that sticks to itself. The transition from free-flow- ing powder to paste results in an increase in shear stress, and this works the paste to shear apart hard agglomerates and elongate liquid droplets. The paste is homogenized from being a poorly mixed composite of low and high viscosities and solids loadings to being a paste which is homogeneous in viscosity and composition. The mixing process breaks/shears the paste at its inhomogeneities and then reknits, so as to progressively remove the defect population, until the paste uniformly plastically deforms rather than fractures when its spaghetti extrudate is bent around a radius in an unconfined state (volume is not kept constant). The tighter the bend radius without fracture, the “longer” the paste is said to be. A paste that readily fractures is called “short”. Some “long” pastes dilate (increase in volume), imbibe air so as to uniformly open the particle network in a show of fracture toughness, rather than undergoing brittle opening of one large crack in the unconfined state. Extrusion is a confined deformation process (constant volume), in which cracks cannot open up within the die, and thereby the fracture response to stress is suppressed to favor the plastic response.

2.2 Honeycombs 65

In manufacturing, high-shear mixing and pressurization is accomplished with a twin-screw auger [67–69]. In the laboratory, the paste can be evaluated with a Brabender mixer/rheometer and/or calendered (pasta maker: fold and roll again and again) to observe how the properties evolve with shear stress and strain as the paste is homogenized. The paste can be calendered into thin sheets, as thin as the honeycomb web, to observe flexibility, drying, and the population of mixing defects [70, 71]. Calendering is a good surrogate test for evaluating the honeycomb extrudability of a paste. A paste will calender well if it extrudes well as a honeycomb. But, a paste that calenders well does not necessarily knit well during honeycomb extrusion.

The thinness of a sheet which can be made by calendering is determined by the tensile cohesiveness of the paste and its peal adhesiveness to the roller, and for extrusion these properties relate to die-entrance yield stress and die-land yield stress [72], respectively. Honeycomb extrusion has the additional constraint that self-adhe- sion (knit) is also required. To simulate the honeycomb knit, sheets can be laminated in the calender. The goal is for the strength of lamination to be the same as the cohesiveness of the original tape. The quality of the knit is expected to be a function of thickness reduction (elongation) in the nip of the rollers, a function of particle rearrangement and accommodation at the lamination interface.

2.2.3.3

The Binder

C.F. Binns described the paste process as “making ropes out of sand” [73]. To do this, a binder is required. The binder is usually a solution of a high molecular weight polymer. The required molecular weight and concentration increase with particle size. A good starting point for 5 mm particles is a 15 % solution of a polymer with a molecular weight greater than 200 000. The polymer concentration can be lower if the particles have a natural plasticity, like clay as opposed to glass powder. A good starting point for solids loading (volume fraction of the paste occupied by nondeformable particles times 100) is 50 vol % for a typical powder [72, 74, 75]. The binder can also contain immiscible liquids [76], surface active agents like lubricants, and others. Composition adjustments can quickly be evaluated with the calender method described above.

The rheology of the paste can be described as Hershel–Bulkley (a shear-thinning paste with a yield stress), and its Benbow parameters [72] can be measured by capillary rheometry. The Benbow parameters can be used to describe the honeycomb extrusion flow, which can be used to quantify the rheological and tribological effect of each component of the paste [63, 64, 77].

The target yield stress of a paste depends on many factors, for example, workable extrusion pressure and necessary wet strength (the necessary yield stress of the wetgreen honeycomb extrudate to prevent deformation during handling). There are trade-offs: pastes with higher yield stresses result in higher extrusion pressures, and this limits the possible combinations of maximum cell density, open frontal area, diameter, and extrusion rate for the paste–extruder–die system. The combination of high cell density and high open frontal area is a challenging target. For the current

66 Part 2 Manufacturing

Corning product line, the maximum combination is the 900/2 Celcor substrate [11]: 900 channels per square inch (csi) with a 2 mil web (nominally, a 50 mm thick web with a 800 mm pitch) with 88 % OFA .

The binder must also enable sufficient lateral paste flow and knit between the pins in the honeycomb die. Webs or corner intersections (depending on the location of the feed hole) contain the knit lines that run the axial length of the honeycomb. The binder contributes to the quality of this knit. An overlubricated stiff paste may calender well, but does not knit, and thus would result in a loose bundle of ribbed spaghetti exiting the extrusion die instead of a honeycomb.

The patent literature covers a wide variety of binder systems for paste extrusion: aqueous, nonaqueous, and combinations [72, 74, 76, 78–81]. Of these, that based on Methocel polymer (methylcellulose and hypromellose, manufactured by Dow Chemical Co.) are the most fascinating in that their aqueous polymer solutions gel reversibly on heating [82, 84]. Methocel is insoluble in hot water and soluble in cold water; it is a unique commercial polymer. The consequence is that the wet-green extrudate stiffens upon heating (the batch can be cooled during mixing to prevent gelation from the heat from the work of mixing), even before it dries. And, it dries very quickly, because the water is no longer osmotically held by this polymer once the temperature is above its gel point. This is very attractive for manufacturing.

2.2.4

Thermal Processing

Once the particles are assembled into the shape of a honeycomb, a series of extraction processing steps are executed to transform the particle assembly into a sintered ceramic. Water is removed, then polymer, then contaminants and inorganic decomposition products, then surface area. The loss of surface area, the loss of porosity, is due to transition from porous to nonporous ceramic. Surface area is traded for strength.

2.2.4.1

Diffusion: Drying and Debinding

After the honeycomb shape is extruded, a series of subtraction processes are preformed that requires chemical and thermal diffusion and exchange. The honeycomb geometry is perfect to aid these processes. The great advantage of the honeycomb geometry is that its diffusion distances can be effectively short, because honeycomb channels are like diffusion superhighways/arteries connecting to the capillaries of web porosity. Thus, a honeycomb with a large volume dries and debinds like a thin self-supporting tape. Diffusion times that would normally scale to the square of half the thickness of an object [85] can be made to scale to the square of the radius of the channel or of half the web thickness because of high open frontal area and channels that are easily flowed through. The optimized “fast-light-off” design of the automotive catalytic substrate [86] is beneficial for the flow-through