What is a material?
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What is a material?
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f you are reading this article, you are probably a member of the Materials Research Society or at least are very interested in materials research. Consequently, I pose to you this burning question. What is a material? My belief is that many people, if asked this question, would focus first on solids as materials. But what about liquids, gases, and plasmas? Dictionaries provide various definitions of “material” as a noun.1 The definitions fall into three main categories: (1) matter, (2) textiles/cloth/fabric, and (3) information/data/ideas. The first definition seems circular to me, but has some aspects worth further consideration. The second is certainly relevant to our colleagues who study textiles, but is hardly complete. (Although, when I worked in the high fashion industry, many of the folks in that industry assumed that textiles, jewelry, and fragrances were the only materials worth considering.) The third group of definitions is not relevant to this discussion. In some definitions, the elements of the periodic table are invoked as materials. The definition does not discuss the thermodynamic state of the element and so that leaves room for the liquids, gases, and plasmas as well as solids of elements to be considered materials. In fact, phase diagrams generally show that elements have some region of the pressure/temperature/volume phase diagram in which they manifest as solid, liquid, or gas. Consequently, it makes sense to consider all elements as materials without regard to their phase. I also believe that some people would likely invoke issues of homogeneity (e.g., highly pure, crystalline silicon) in regard to the idea of what is a material. However, this limited perspective certainly leaves out mixtures, alloys, compounds, polymers, and other forms of heterogeneous materials. It also does not encompass amorphous or polycrystalline materials. It also seems to me the idea of what constitutes a material must have some element of reasonableness to it. Is the gunk in my shower drain a material? If I collect random articles from my trash bin and mash them together into a solid or even a gelatinous mélange, do those objects constitute a material? Something deep inside me rejects that, although it might fit into our discussion of Strange Matter (the MRS traveling science exhibition).2 I know that there are mixtures that make good materials. Otherwise, metallurgy would be a much different discipline than we now know and alloy semiconductors would not exist. However, randomly mixing various things together does not make them a material any more than blending raw fish heads, cow brains, tripe, strawberries, and milk makes an enticing milkshake. A collection of like atoms certainly seems to form an item that we would call a material. If I put enough gold atoms together to make a bar of gold, most of us would agree that this is a material. Nanoscale clusters of gold atoms would also seem to fit what we would call materials. However, what about two atoms? Is a single atom of gold a material? Is
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