2. Synthesis and Processing

How will the materials of the future be discovered and made? We are faced with the curious situation that two current trends in materials synthesis are taking the process of innovation in opposite directions. On the one hand, there is a greater element of design than ever before-materials are planned as if at the drawing board. For example, one can now rationally design and then fabricate organic or inorganic materials perforated with microscopic pores of a well-defined shape and size. This design process may sometimes permit of a modular approach, in the same way that an electronic circuit consists of modules such as amplifiers or pulse generators. A polymer for use in optoelectronic technology might be given some side chains that absorb light at a certain frequency, others that release charged particles when stimulated by the absorbed energy, still others that transport or trap these charges, and so forth.

On the other hand, one of the major innovations of the 1990s in the arena of materials synthesis and discovery was the development of combinatorial methods. These entail the creation of a huge library of materials whose compositions are blends of several different components or substances, mixed at random or in gradually modulated steps. These libraries are analogous to (and sometimes visibly resemble) a color chart for commercial paints, in which each unique hue is composed of a mixture of several different pigments.

Figure 1. A Combinatorial Array (Library) of Ceramic Materials Prepared
by Mixing Four Elements in Different Ratios
These are all potential superconductors, and each block will be tested
individually to discover its properties

By blending chemical elements in different ratios, or assembling molecular units at random into new molecules, one samples different materials over a whole region of "composition space." The challenge is then to find an effective, rapid and sensitive way of screening these candidate materials in the hope of discovering one that performs in the manner desired-a superior phosphor, say, or a catalyst, dielectric or superconductor. Combinatorial synthesis may be intellectually less satisfying than rational design-it amounts to little more than trial and error writ large by automated technology-but the accurate, quantitative prediction of materials properties from theory alone remains severely challenging, and so the combinatorial approach may be the most pragmatic for many situations.

It is also important to appreciate that serendipity is as important to the materials scientist as ever it was. One of the most important new materials of the 1990s is the mesoporous form of silica known as MCM-41, created by researchers at Mobil's research laboratories in the early part of the decade. "Mesoporous" denotes the fact that the silica is laced with long cylindrical channels ~10-100 nm across; and the critical feature is that these are uniform in size, and arranged in an orderly fashion, packed hexagonally as if in a honeycomb.

Figure 2. MCM-41, A Porous Form of Silica Permated
by Cylindrical Channels of Uniform Size
These can range from ~10-100 nm in width, depending
on the method of preparation

This makes MCM-41 a scaled-up version of the aluminosilicate zeolites that are widely used in the chemical industry as highly selective catalysts and "molecular sieves"-their smaller pores are the width of small molecules. MCM-41 is now simply a member of a much broader family of ordered mesoporous inorganic materials: others have slit-like pores or three-dimensional connectivity between pores, and their walls can be made of oxides other than silica. An understanding of the formation process, which involves templating by spontaneously assembling clusters of surfactant molecules, has now led to a strong element of rational design in the creation of these materials. But the initial discovery was unexpected, and stemmed from studies directed at the creation of new types of (small-pore) zeolites.

Rational design of materials has emphasized the importance of the interface with chemistry, in particular because developments in the field of supramolecular chemistry have a great deal to offer the materials scientist seeking control of structure on the scale of nanometers or less. Supramolecular chemistry is the "chemistry beyond the molecule." Whereas the traditional synthetic chemist strives to assemble atoms into a specific molecular geometry, the supramolecular chemist typically uses whole molecules as the fundamental building blocks, and devises ways of assembling them into organized structures and arrays, generally using non-covalent interactions, such as hydrogen bonding or metal-ion coordination chemistry. This makes synthesis possible at a scale extending upwards to meet that at which the engineer can carve structures from monolithic materials. Thus, to make the kinds of microstructures required for, say, electronic circuitry or micromechanical engineering, one now has the choice of adopting either a top-down or a bottom-up approach. Examples later in this section should serve to illustrate the possibilities this presents. One consequence is that the very meaning of the word "material" becomes ill-defined. Supramolecular and colloid chemistry can afford assemblies of diverse components, some of which are single molecules. The term "integrated chemical systems" has been proposed for heterogeneous molecular assemblies of this type that are designed for a particular function. Perhaps the central point is that "materials" are no longer to "chemistry" as "bulk" is to "molecular": much of the action is taking place somewhere in between, at the mesoscopic scale of nanometers to micrometers-the dimensions typical of many of the structures in living cells. Many of the advanced materials of the future will surely be engineered at this size scale.

How will they be put together? Several considerations are leading to a shift away from energy-intensive, "harsh" conditions of synthesis and towards what one might call "soft processing" technologies: solution-phase chemistry (often with water as the solvent), low temperatures, ambient pressure. In part, this trend is driven by environmental considerations: a reduction in energy consumption and in the use of hazardous solvents. In part, it is dictated by the kinds of synthetic procedures pertinent to supramolecular chemistry, which employs gentle inter-particle forces, rather than the strong (and therefore somewhat intransigent) bonds that maintain the integrity of individual molecules.

It is certainly true that toxic organic solvents, high temperatures and ultrahigh vacuums have been widely used in the synthesis of advanced materials-in semiconductor processing, for example. But the recognition that the solvation properties of benign fluids, such as water and carbon dioxide are markedly different in the supercritical state, has led to their introduction as solvents for several industrial chemical processes that might otherwise utilize organics. Water's critical point, where the distinction between gas and liquid disappears, is at 374 ĄC and 218 atmospheres. Substances that are sparingly soluble in liquid water may become appreciably so in the supercritical fluid, and vice versa. Reaction rates also differ significantly: many organic compounds, for example, can be efficiently oxidized in supercritical water. Carbon dioxide, with its lower critical temperature of 31 ĄC, is a more amenable supercritical solvent, and has been used, e.g., in the fabrication of nanoscale particles of various materials, including drugs.

Liquid water too, is finding increasing use as a solvent. Electrochemical processing from aqueous solution can deliver thin films of technologically useful oxide ceramics such as barium, strontium and lead titanate, and lithium niobate. Electrodeposition has been used to make complex many-layered "superlattices" of metals and ceramic materials, which are more conventionally fabricated using high-temperature vapor deposition methods. And hydrothermal methods, which employ moderate temperatures and aqueous solutions comparable to the conditions of some geological processes are used in zeolite synthesis amongst other things, and have been proposed for the fabrication of diamond films.

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