Growth of metals

Historically, strontium metal was produced only in very small quantities. Rapid growth of metal production occurred during the late 1980s, however, owing to use as a eutectic modifier in aluminum—silicon casting alloys. The addition of strontium changes the microstmcture of the alloy so that the siUcon is present as a fibrous stmcture, rather than as hard acicular particles. This results in improved ductility and strength in cast aluminum automotive parts such as wheels, intake manifolds, and cylinder heads. 

We saw in Chapter 6 that diffusive transformations (like the growth of metal crystals from the liquid during solidification, or the growth of one solid phase at the expense of another during a polymorphic change) involve a mechanism in which atoms are attached to the surfaces of the growing crystals. This means that diffusive transformations can only take place if crystals of the new phase are already present. But how do these crystals - or nuclei - form in the first place ... 

In 1985 Car and Parrinello invented a method [111-113] in which molecular dynamics (MD) methods are combined with first-principles computations such that the interatomic forces due to the electronic degrees of freedom are computed by density functional theory [114-116] and the statistical properties by the MD method. This method and related ab initio simulations have been successfully applied to carbon [117], silicon [118-120], copper [121], surface reconstruction [122-128], atomic clusters [129-133], molecular crystals [134], the epitaxial growth of metals [135-140], and many other systems for a review see Ref. 113. 

The crystal growth of metal borides by gas-phase methods permits preparation of products at moderate T (1000-1500°C). This is an important advantage since most borides melt at high T (ca. 3000°C), which makes their crystal growth from melts difficult. In addition, the gas-phase methods lead to the formation of single crystals and solid films of incongruently melting borides. 

Important results and a detailed insight into aqueous chemical deposition processes have been reported and discussed elsewhere for CdSe [248, 249] and ZnS [250, 251] target products. We should note also the work of Davies et al. [252] who described an alternative method for the chemical growth of metal sulfides and selenides on the basis of polysulfide or polyselenide solutions (containing hexa- and tetra-chalcogen anions) formed by the dissolution of sulfur or selenium in hydrazine monohydrate. ... 

In summary, we found that Ugands indeed coordinate at the surface of nanoparticles and that they can be firmly or loosely attached to this surface according to their chemical nature. Furthermore, the hgands influence the reactivity of the metal nanoparticles. This is important in catalysis but, as we will see later in this paper, is also important for the control of the growth of metal nanoparticles of defined size and shape. 

The first one is the direct synthesis of metallic nanoclusters, not via formation of (hydro)oxides and their reduction in gas-phase, because the successive reduction for formed (hydro)oxides sometimes results in the size growth of metal particles due to the aggregation and/or sintering. The second one is the use of precisely designed metal complexes, which are well adsorbed on the support surfaces, as shown in Figure 1. 

The fabrication of such a system can be accomplished only by nanofabrication, and different routes can be imagined in this context. We will focus in the following section on the template-controlled growth of metal clusters on thin oxide films, which has proven to give excellent results in terms of low complexity. This approach has been successfully employed for metal-on-metal systems (for a comprehensive review see [6]) and has recently been extended to metal growth on oxide films. 

In order to prepare functional particles, it is important to control the size of metal particles accurately. To achieve such size controlled synthesis of metal particles, inorganic or organic templates are useful to suppress the growth of metal particles. 

The relationship between "conventional" and "high energy" processes in organometallic chemistry is defined. The growth of metal atom chemistry is surveyed as a model for the impact of a "high energy process" on the field of organometallic chemistry. 

Charged polysaccharides can also serve as templates for the growth of metallic, semiconductor and magnetic nanoparticles. For instance, chitosan has been reported as a catalyst and stabilizing agent in the production of gold nanoparticles by the reduction oftetrachloroauric (III) acid by acetic acid. The biopolymer controls the size and the distribution of the synthesized Au nanoparticles and allows the preparation... 

All the examples gathered here demonstrate the possibility to control the growth of metallic and oxide nanoparticles using biological templates. A wide variety of chemical composition, particle size and assemblage can be obtained via these approaches. Moreover, due to the biological nature of the template, applications in fields related to biotechnology and medicinal science can be envisioned. 

Fig. 12.15. Growth of metallicity with time in regions of differing overdensity, corresponding approximately to clusters of galaxies (5 = 103 thick curve), DLA and Lyman-limit systems (5 = 102 thinner curve), moderate-column-density Ly-a clouds (5 = 10 dotted curve) and low-column-density Ly-a clouds (8 = 0 dashed curve), The dot-dashed curve shows the global average. After Cen and Ostriker (1999). Courtesy Renyue Cen.

Ruckenstein E 1981 Growth of Metal Clusters ed J Bourdon (Amsterdam Elsevier) p 57 Ruckenstein E and Lee S 1984 J. Catal. 86 457... 

This section is concerned with the mechanism of formation and growth of metal nanoparticles in homogeneous solutions. As mentioned in the section on synthesis of metal nanoparticles, there are many kinds of synthetic methods. Each method has its own process and mechanism. However, there are four main processes ... 

In some cases, reduction, aggregation, and growth occur at once. Stabilizers such as surfactants, polymers, etc. can be added to the solution before or after the growth of metal nanoparticles. 

Enzymes functionalized with metallic NPs were used as biocatalytic hybrids for the growth of metallic nanowires. The catalytic enlargement of metal nanoparticles by products generated by different enzymes was used to develop different optical sensors that follow the activities of enzymes and analyze their substrates.57 For example, hydrogen peroxide generated by the biocatalyzed oxidation of glucose by O2 in the... 

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