This article is dedicated to a class of specialty materials that have come to be known as low-density microcellular materials, or LDMMs. LDMMs have been developed and studied over the last decade at several U.S. Department of Energy (DOE) National Laboratories. LDMMs are used in various high energy physics programs where they meet unusual and demanding specifications (see Table 1.1).

To set the stage for introducing LDMMs, it is instructive to contrast them with more familiar cellular materials. Examples of commercially available low-density materials are plastic “blown cell” foams, such as polyurethane cushions and polystyrene coffee cups. Asymmetric membranes and filters, on the other hand, are representative of commercially available microcellular materials. The processes used to make these products generally are not suitable for making LDMMs, however, because they are limited by a tradeoff between density and cell size. That is, they yield low densities only at the expense of increased cell size, or small cell size at the expense of increased density. The challenge to DOE materials scientists, therefore, has been to develop new methods for making materials that simultaneously have low density and microcellular morphology, and also meet other application requirements (e.g., composition, homogeneity, size, and strength).

LDMMs are used by the DOE laboratories because their low density and high homogeneity (implied from their microcellular morphology) make them ideal precursors or substrates for forming high-density gases or expanded plasmas.

This section will provide a background for understanding and visualizing the microstructures observed in LDMMs. The word microcellular means small cell size, or more generally (because most low-density materials are not composed of easily recognizable “cells”) some measure of the spatial scale of the foam structure. The concept, measurement, and significance of cell size is intrinsically connected to the morphology, as are certain properties (e.g., stiffness). These matters are covered in later sections. As a background, this article describes some of the structural morphologies of low-density foams, not all of them available as LDMMs (i.e., simultaneously low-density and microcellular.) Interested readers are encouraged to also examine References 1 and 2 because a detailed discussion would take us too far afield. Instead, we shall indicate how most low-density morphologies may be considered in terms of a few simple models of morphological types encountered and how they arise. Often these are seen most cleanly in large-celled foams. Note that these are simplifications—there is no standard nomenclature.

This section describes and gives examples of processes developed at the DOE laboratories to make LDMMs. In general, these processes fall under one of two schemes: (1) phase separation of polymer or polymer-like solutions, or (2) replication of sacrificial pore formers. Both schemes produce a liquid-filled precursor. The LDMM can be obtained only if this precursor can be “dried” without collapsing the underlying structure. This key synthetic step is rarely a trivial task, and because of its importance, this section starts with a description of LDMM drying technologies.

Most liquid-filled LDMM precursors will collapse into a high-density mass if subjected to evaporative drying. This occurs because large capillary forces are generated when the liquid meniscus moves through the small cells and pores of the precursor. Only very strong and relatively large-celled precursor structures (e.g., inverse emulsions) can withstand these forces.

Some LDMM precursors of moderate strength can be dried by evaporation if the precursor is exchanged into a liquid of low surface tension. Liquified carbon dioxide is an example — at room temperature, liquid CO2 has a surface tension of 1–2 dynes/cm, compared to ˜70 dynes/cm for water, and 20–40 dynes/cm for most organic solvents. CO2 evaporative drying should not be confused with C02 supercritical drying discussed later.

Aerogels are a special class of open-cell foams derived from the supercritical drying of highly cross-linked inorganic or organic gels. These materials have ultrafine cell/pore sizes (less than 1,000 Å), continuous porosity, high surface area (400–1000 m2/g), and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 Å. This microstructure is responsible for the unusual optical, acoustic, thermal, and mechanical properties of aerogels. For example, aerogels can be prepared as transparent, porous solids because their ultrafine cell/pore size minimizes light scattering in the visible spectrum. Figure 4.1 shows the different aerogels that will be discussed in this article.

The hydrolysis and condensation of metal alkoxides is the most common synthetic route for the formation of inorganic aerogels. Inorganic aerogels have been prepared from monomers such as tetraisopropoxy titanate, aluminum secbutylate, and zirconium isopropoxide. Nevertheless, the majority of scientific research has concentrated on the sol-gel polymerization of tetramethoxysilane (TMOS), or the less toxic tetraethoxysilane (TEOS). The resultant silica aerogels are being investigated for applications ranging from window insulation to the collection of hypervelocity partis cles in space.

The sol-gel polymerization of a multifunctional monomer in solution, leading to the formation of an aerogel, is not unique to metal alkoxides. Organic reactions that proceed through a sol-gel transition have been discovered recently.

The low atomic number, good chemical reistance, and low thermal expansion of carbon foams make them attractive for many specialty applications. In general, we are interested in carbon LDMMs that have a maximum cell size of 25 μm and densities less than 0.05 g/cm3. These requirements are beyond the state of the art for most commercial carbon foams; thus, the DOE National Laboratories have concentrated on producing materials that meet these demanding requirements using various phase-separation, sol-gel, and replication processes.

In all cases, the final carbon foam is derived from a polymer precursor. Most polymers (e.g., polymethyl methacrylate) simply depolymerize and revert to their monomeric form when heated to high temperatures in an inert atmosphere. Cross-linked phenol-formaldehyde and polyfurfuryl alcohol are polymers that leave substantial carbon chars (greater than 50 wt%) after pyrolysis near 1000°C. Polyacrylonitrile also leaves a large carbon char (˜40 wt%)) after partial oxidation at 200°C followed by heat treatment at 1000°C. The latter char is a partially ordered graphite while the former chars are totally amorphous.

The microstructure and properties of carbon LDMMs depend on synthetic procedure and heat treatment conditions. We have been able to make carbon LDMMs with cell sizes as small as 100 Å or with densities as low as 0.015 g/cm3. Furthermore, surface areas of 50–1000 m2/g have been obtained with microstructures ranging from ribbonlike webs to interconnected colloidal-like particles. The ability to control these various parameters presents new opportunities to design carbon LDMMs for high technology applications, examples of which include sensors, electrodes, and high temperature filters.

The previous sections of this article described the synthesis, morphologies, and properties of a variety of low-density microcellular materials. This section discusses several of the analytical methods used and developed at the DOE laboratories to characterize these state-of-the-art materials.

In some LDMM applications, quantitative measurements of the material's average cell size and cell size distribution are desired. Indeed, the term “microcellular” has little meaning without such information. As seen throughout this article, however, most LDMMs do not have a readily defined cellular character. The more general problem is to quantify the spatial scale(s) of the foam. For this purpose it is necessary to define one or more “measures” of the spatial scale. The possibilities are many and include not only single numbers (e.g., cell size and cell size standard deviation, where “cell size” is meaningful) but also functional descriptions (e.g., correlation functions).

SEM provides direct images and, therefore, is the most popular technique for examining LDMM morphology. SEM, however, suffers from at least three limitations: (1) SEM examines only a very small volume of material, and thus is impractical for obtaining average morphological properties; (2) SEM requires that nonconductive LDMMs be coated, a process step that can alter the structure and introduce artifacts (particularly with delicate structures); and (3) SEM images are only two-dimensional projections of real three-dimensional structures.

The development of LDMMs is a significant scientific accomplishment. This article provided an overview of the synthesis, structural characterization, and properties of these materials. Hopefully, the reader gained an appreciation for (1) the diverse morphologies of these materials, (2) their unique structure-property relationships, and (3) the difficulty in simultaneously achieving low densities with small cell sizes. This section will briefly discuss the future development of these materials and their potential commercial applications.

Although substantial progress has been made in the scientific understanding of the formation of LDMMs, many important questions remain. As mentioned earlier, LDMMs rarely display an ideal spinodal morphology even though many of these structures have been established by phase-separation of polymer/solvent systems. Such morphologies appear to coarsen after the initial structure is established. The question arises — on what length scales is phase-separation taking place and how does its temporal evolution affect LDMM structure? In aerogel processing, polymerlike gels are formed, but certain sol-gels do not behave in accordance with theories on the swelling of conventional polymer gels. Why is this, and what is the effect on the final aerogel morphology? These and other questions can only be answered with a better understanding of gel structures and the dynamics of their formation. This is an important area for future research because the information gained can be used to tailor LDMMs for specific applications.

Although intermetallics based on aluminum or silicon tend to have a very attractive combination of low density and excellent oxidation resistance, they suffer from lack of adequate creep strength and, in most cases, from inadequate ductility and toughness. It has been recognized for several years that an approach which could simultaneously solve both problems, without degrading other properties, is to utilize the intermetallics as matrices for composite materials. The consequence has been an explosion of interest in two-phase intermetallic-based alloys.

With the exception of some early work by Seybolt, intermetallic matrices have been utilized for composites only for the past five to six years; the first published reference to systematic studies of fibrous composites dates from the proceedings of an MRS meeting in December 1986. The published literature is much sparser than, for example, that on ceramic matrix composites, which have been under development for a much longer period. Nevertheless, an appreciable number of intermetallic matrices have been reinforced with fibers or particles.

Because of the relatively high melting points and extreme brittleness of most intermetallic compounds utilized as matrices, as well as other significant advantages, enormous effort has been devoted to powder metallurgical techniques. In this category we include reactive sintering of elemental or elemental plus prealloyed powders, reactive hot pressing, reactive HIPing, injection molding, the XD process, dynamic compaction of powders, mechanical alloying, the powder cloth method, and, of course, traditional hot pressing techniques.