The Basics of Duocel® Foam
What is Duocel® Foam?
Duocel® is a porous structure or open-celled foam consisting of an interconnected network of solid struts. Like soap suds or beer foam, the original bubbles that formed the foam are a three-dimensional, perfectly packed array of similar sized bubbles where each bubble has the maximum volume for the minimal surface area and surface energy.
Given these common physical constraints, each bubble in the array is typically a 14-faceted polyhedral or solid shape called a tetrakaidecahedron. Once the foam is solidified, the thin membrane in each of the 14 facets or windows is removed by a reticulation process, creating an "open cell", and leaving only the thick outer perimeters of the window frames behind as a series of interconnected struts. The resulting bubble structure resembles a linked geodesic dome or "buckyball" structure where each link or strut is shared between three adjacent bubbles, thus creating the characteristic triangular cross-section.
Unlike honeycomb, this open-celled structure is identical in all three directions, and is therefore considered an "isotropic" foam. Just as all the structural ligaments or struts are interconnected, the open cell porosity is also interconnected, enabling fluids to pass freely into and out of the foam structure. While technically designated as an open-celled foam, these materials are also occasionally called porous metals or metal sponge.
How is it specified?
Duocel® foams can be specified by defining three independent characteristics. These independent characteristics provide a 3-dimensional design space from which a wide range of material properties can be attained.
Each bubble structure in the open-celled foam generally consists of 14 reticulated windows or facets. The polygonal opening through each open window is referred to as a "pore". In any given bubble, the polygonal pores actually are of two or three different characteristic sizes and shapes, but for material designation purposes, they are simplified to an average size and circular shape. The number of these pores that would subtend one inch then designates the foam "pore size". Duocel® metal foams are generally manufactured from 5 to 40 pores per inch, while Duocel® carbon and ceramic foams are manufactured from 5 to 100 pores per inch. An average pore diameter is about 50% to 70% the diameter of its parent bubble, thus a 10 pore per inch (PPI) foam would have roughly 5 to 7 bubbles per inch.
Effects of Pore Size (PPI) - Foam pore size defines how finely the raw material of a foam is divided. The bubble and strut structural shape is always constant, but a 5 pore per inch (PPI) will visually appear more open and course that a 40 or 100 PPI foam. Accordingly, the foam pore size directly affects nominal ligament length & cross section size, and pore diameter. In turn, these micro-structural features influence specific surface area, fluid flow resistance, and electromagnetic transmission or absorption. These are critical parameters when designing light diffusers, EMI shields, fluid flow diffusers, heat exchangers, stray light absorbers, laser dumps, filters, and porous electrodes.
Relative density is the density of a foam divided by the density of the solid parent material of the struts. In other words, it is the mass of real material in a block of foam compared to what it would be if it were a solid block of the same material. Typical relative densities for Duocel® foams run from about 2% to 15% depending on the material being foamed and the application. Due to the physics of small-scale structures, the majority of Duocel® foams are manufactured in the 3-10% density range. It should be noted that this characteristic has also been historically defined as "mass density", "void volume", "porosity", "solid fraction", and a number of other terms depending on the era, author, and industry. "Relative density" is currently the standard designation for this dimensionless characteristic as it is a more accurate and unambiguous description, and it correlates directly with the affected material properties to be discussed below.
Ligament Cross Sections
Effects of Relative Density - While pore size controls the number and nominal size of the foam ligaments, the relative density controls the ligament cross-section shape and actual size. Since foams can be compared to miniature three-dimensional truss structures, it is apparent that the cross section and moment of inertia of the struts or ligaments is a primary driver of foam mechanical properties like stiffness, rush strength, electrical conductivity, and thermal conductivity. Accordingly, relative density is the one foam characteristic that determines the structural behavior of monolithic components, composite panels, and energy absorbers. Similarly, it controls the electrical conductivity of porous electrodes and the thermal conductivity of heat exchangers.
Whether they are metal foams, carbon foams, or ceramic foams, all Duocel® open-celled foams can be categorized as either Primary foams or Secondary foams
Primary foams - Standard Duocel® foams are referred to as "primary" foams. In this case, the base material to be foamed is simply resolved to a liquid state, foamed directly, and then reticulated. The resulting foam strut or ligament then consists of a solid beam of roughly triangular section that is made of the solid, homogeneous base material chosen. While there is porosity in the bubble structure due to the foaming process, there are no porosities or discontinuities within the individual ligaments.
Ligament Cross Sections
Secondary foams - "Secondary" foams are generally produced on custom order for special application. These foams are made by post-processing a primary foam to upgrade it for a specific function that cannot be technically or economically provided by a primary foam. A typical secondary foam might be an aluminum primary foam that has been uniformly coated with platinum for use as a high surface area catalytic reactor element. While it is technically possible to make platinum primary foam, there would be a significant added cost of establishing new foaming equipment specifically for platinum.
In addition, there would be a major cost in the platinum metal itself. Since only the surface of the platinum is functional as a catalyst, the bulk of the material in a primary platinum foam would be wasted. By using a readily available primary foam as the skeletal substrate, and applying a thin, catalytically active platinum surface coating, a high specific surface area catalyst component can be produced much more rapidly, and at a much lower development and production cost. While some secondary foams as discussed above are produced purely for economic reasons, others are produced, like composites, for technical purposes to combine the best characteristics of two or more materials.
Effects of Base Material - The properties of the base material is what determines all of the physical properties of the resulting foam such as melt temperature, specific heat, coefficient of thermal expansion, strut hardness, galvanic behavior, oxidation limits and chemical reactivity. In combination with the Relative Density parameter, the base material selected to be foamed affects mechanical properties such as modulus, crush strength, electrical conductivity, and thermal conductivity. Of the two parameters (relative density and base material), relative density is often the dominant parameter in determining mechanical characteristics.