Cellular Ceramics: Intriguing Structures, Novel Properties, and Innovative Applications
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Cellular Ceramics:
Intriguing Structures, Novel Properties, and Innovative Applications David J. Green and P. Colombo
Abstract Cellular ceramics are a class of high-porosity materials that are used or are being considered for a wide range of technological applications. A critical aspect of this development is the materials science approach required to understand the relationships between the properties of these materials and their structure. Of particular interest are the parameters that control mechanical reliability, as ceramic materials are usually brittle. In addition, it is critical to understand the way in which processing methods can influence the cellular structure. This article emphasizes one particular group of cellular ceramics known as ceramic foams. Understanding these materials involves various interdisciplinary scientific challenges in characterizing structure, developing micromechanical models, experimentally measuring properties, developing new processing approaches, and optimizing performance. Keywords: cellular solids, ceramic materials, foams, mechanical properties.
Introduction The presence of porosity in a material is often viewed as problematic. However, there are many applications in which the use of porous materials can be advantageous or necessary—for example, filters, membranes, catalytic substrates, thermal insulation, gas-burner media, and refractory materials. In these applications, materials are chosen for their special functional properties, such as low thermal conductivity, high permeability, high-temperature stability, excellent thermal shock resistance, or low dielectric constant. Although the primary function of porous materials may not be structural, many of their applications require a high degree of mechanical reliability. This is of particular concern when the porous materials are brittle, as
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is usually the case with porous ceramics. These materials can be produced with a variety of structures, but of interest here is the group called cellular ceramics, in which the structure consists of an array of hollow cells. The porosity in these materials usually exceeds 70% of the total volume. Gibson and Ashby1 have written a comprehensive review of the literature on cellular materials, and Brezny and Green2 reviewed the mechanical behavior of cellular ceramics. There are other groups of high-porosity ceramic materials that can compete with cellular ceramics (e.g., bonded fibers, hollow spheres, honeycombs, aerogels, xerogels), but this article concentrates on cellular ceramics and, more specifically, on ceramic foams.
Foams are surprisingly intricate formations, and understanding these structures is of interest to a wide range of scientific disciplines such as astronomy, biology, chemistry, physics, mathematics, food science, and engineering. Foam structures permeate the cosmos, from subatomic quantum foams to the air-riddled magma that bubbles below planetary surfaces and the foamlike cancellous bones that bear weight in animals and humans. Ceramic foams are rather novel m
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