Bulk materials synthesis

Whichever method is followed, a protective agent able to induce a repulsive force opposed to the van der Waals forces is generally necessary to prevent agglomeration of the formed particles and their coalescence into bulk material. Since aggregation leads to the loss of the properties associated with the colloidal state, stabilization of metallic colloids - and therefore the means to preserve their finely dispersed state - is a cmcial aspect for consideration during their synthesis. 

The excellent prospects of PEFCs as well as the undesirable dependence of current PEMs on bulk-like water for proton conduction motivate the vast research in materials synthesis and experimental characterization of novel PEMs. A major incentive in this realm is the development of membranes that are suitable for operation at intermediate temperatures (120-200°C). Inevitably, aqueous-based PEMs for operation at higher temperatures (T > 90°C) and low relative humidity have to attain high rates of proton transport with a minimal amount of water that is tightly bound to a stable host polymer.33 37,40,42,43 yj-jg development of new PEMs thus warrants efforts in understanding of proton and water transport phenomena under such conditions. We will address this in Section 6.7.3. 

The preparation and study of nanoparticles has attracted a remarkable academic and industrial research effort because of their potential applications, ranging from fundamental studies in quantum physics, fabrication of composite materials, information storage/optoelectronics, immunoassays, to catalysts. The precise control of size and chemical behavior (stabihty and reactivity) by means of the synthesis itself is still one of the main targets because the direct correlation of the new intriguing properties with the particle size which is just between a molecule and a bulk material [140]. 

Whiskers are synthetic crystalline fibers of variable size, but with diameters of usually less than 25 microns. An upper limit in diameter exists because the physical and chemical properties approach those of the bulk material as the diameter of fibrous sample increases. Since the purpose of synthesis is to take advantage of some characteristic property of the material in fibrous form, such as enhanced strength for small volume, the optimum material has a small diameter. For similar reasons the aspect ratio of useful whiskers is often well over 100. Whiskers can be single crystals, but many are polycrystalline aggregates of fibrils with preferred orientations. The compositions and crystal structures of the compounds synthesized as crystalline fibers also have the broadest possible variety (Brenner, 1958). 

Nature has provided a wide variety of chiral materials, some in great abundance. The functionality ranges from amino acids to carbohydrates to terpenes (Chapters 2-5). All of these classes of compounds are discussed in this book. Despite the breadth of functionality available from natural sources, very few compounds are available in optically pure form on large scale. Thus, incorporation of a chiral pool material into a synthesis can result in a multistep sequence. However, with the advent of synthetic methods that can be used at scale, new compounds are being added to the chiral pool, although they are only available in bulk by synthesis. When a chiral pool material is available at large scale, it is usually inexpensive. An example is provided by L-aspartic acid, where the chiral material can be cheaper than the racemate. 

Usually there is a lot of effort required to make nanomaterials by electrochemical means. In aqueous solutions the electrodeposition of nanocrystalline metals requires pulsed electrodeposition and the addition of additives whose reaction mechanism hitherto has only been partly understood (see Chapter 8). A further shortcoming is that usually a compact bulk material is obtained instead of isolated particles. The chemical synthesis of metal or metal oxide nanoparticles in aqueous or organic solutions by colloidal chemistry, for example, also requires additives and often the desired product is only obtained under quite limited chemical conditions. Changing one parameter can lead to a different product. 

The top-down approach starts with a bulk material and attempts to break it down into nanoscaled materials through physical methods. Hence, most of these techniques are really forms of fabrication rather than synthesis. For nanostructured bulk phases, including powders, the common methods are milling, devitrification of metallic glass, and severe plastic deformation. For nanocrystalline thin films (films with nanosized crystallites), methods include thermal vaporization (under high vacuum), laser ablation, and sputtering (thermal plasma), all of which were... 

Caveats. Those familiar with cluster chemistry will mark the absence of cluster synthesis, framework dynamics and reactivity. Considerable information exists and these topics for selected cluster types are well developed in cluster reviews and edited volumes. However, our focus on electronic structure is deliberate. We wished to compare and contrast geometric and electronic structure across the large sweep of element composition and cluster size up to and including bulk materials. To keep the book of manageable size relative to a typical one-semester advanced course yet... 

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