Dendritic metal

Complexation of gold ions, [Au(I)], with peripheral phosphine groups of a P-based dendrimer was reported by Majoral et al. [185]. Transmission electron microscopy (TEM) was used to analyze the large aggregates formed by the dendritic gold complexes and a direct correlation was observed between the size of the particles and the dendrimer generation number. In a recent report [186], Majoral et al. further demonstrated that up to 48 diphosphino groups could be anchored to the surface of dendrimers and various dendritic metal-complexes... 

Dendrimer synthesis, 26 787-788 breakthrough approaches in, 26 788 Dendritic box, 26 790 Dendritic metallic silver, 19 367—368 Dendritic nanoparticles, water-soluble, 26 796... 

Fig. 2.11 Lanthanide ion as core unit of a dendritic metal complex...

The cross sections of the copper and cadmium deposits obtained at //i < // < tjc, and // > //c are shown in Fig. 2.15a, b, respectively. It can be seen that there is no dendrite formation when t] < t, both compact and dendritic deposits are formed when rji This is in perfect agreement with findings of Calusaru [44] for the morphology of deposits of the same metals deposited at overpotentials corresponding to full diffusion control. 

A problem initially experienced with rechargeable Li batteries was that during a recharge cycle, the Li metal tended to form dendrites (metal whiskers) that shorted the electrodes and limited the number of charge/recharge cycles. It also had an incipient fire risk [53]. 

Metal surfaces in motion have also been characterized by STM, one of the clearest examples bemg tire surface diflfiision of gold atoms on Au(l 11) [29] (figure Bl.19.7). Surface diflfiision of adsorbates on metals can be followed [30] provided that appropriate cooling systems are available, and STM has been successfiilly employed to follow the 2D dendritic growtli of metals on metal surfaces [31]. 

Fig. 7. (a) Impurity elements are rejected into the Hquid between the dendritic solidification fronts, (b) Corresponding impurity concentration profiles. Cq, weld metal composition k, impurity partitioning coefficient in the Hquid maximum impurity soHd solubiHty eutectic composition at grain... 

Miscellaneous. Electron beams can be used to decompose a gas such as silver chloride and simultaneously deposit silver metal. An older technique is the thermal decomposition of volatile and extremely toxic gases such as nickel carbonyl [13463-39-3] Ni(CO)4, to form dense deposits or dendritic coatings by modification of coating parameters. 

Fig. 4. Shapes of metal powder particles (a) spherical (b) rounded (c) angular (d) acicular (e) dendritic (f) kregular (g) porous and (h) fragmented. Density. The density of a metal powder particle is not necessarily identical to the density of the material from which it is produced because of...

A number of attempts to produce tire refractory metals, such as titanium and zirconium, by molten chloride electrolysis have not met widr success with two exceptions. The electrolysis of caesium salts such as Cs2ZrCl6 and CsTaCle, and of the fluorides Na2ZrF6 and NaTaFg have produced satisfactoty products on the laboratory scale (Flengas and Pint, 1969) but other systems have produced merely metallic dusts aird dendritic deposits. These observations suggest tlrat, as in tire case of metal deposition from aqueous electrolytes, e.g. Ag from Ag(CN)/ instead of from AgNOj, tire formation of stable metal complexes in tire liquid electrolyte is the key to success. 

Fig. 6.8. Most metals solidify with a dendritic structure. It is hard to see dendrites growing in metals but they con be seen very easily in transparent organic compounds like camphene which - because they have spherical molecules - solidify just like metals.

Beryllium is extracted from the main source mineral, the alumino-silicate beryl, by conversion to the hydroxide and then through either the fluoride or the chloride to the final metal. If the fluoride is used, it is reduced to beryllium by magnesium by a Kroll-type reaction. The raw metal takes the form of pebble and contains much residual halides and magnesium. With the chloride on the other hand, the pure metal is extracted by electrolysis of a mixture of fused beryllium chloride and sodium chloride. The raw beryllium is now dendritic in character, but still contains residual chloride. 

In the second part of the 20th century, the tantalum capacitor industry became a major consumer of tantalum powder. Electrochemically produced tantalum powder, which is characterized by an inconsistent dendrite structure, does not meet the requirements of the tantalum capacitor industry and thus has never been used for this purpose. This is the reason that current production of tantalum powder is performed by sodium reduction of potassium fluorotantalate from molten systems that also contain alkali metal halides. The development of electronics that require smaller sizes and higher capacitances drove the tantalum powder industry to the production of purer and finer powder providing a higher specific charge — CV per gram. This trend initiated the vigorous and rapid development of a sodium reduction process. 

After the end of the interaction, the melt is cooled down to room/ambient temperature, and the metal and salts are crushed and leached using mineral acids and water to separate the metal. The metal precipitates mostly in the form of dendrites, which are pressed and sintered into bars to be converted into wire, sheet and powder. 

The addition of some metal ions, such as Mg2+,Zn2+, In3+,orGa3+, and some organic additives, such as 2-thiophene, 2-methylfuran, or benzene, to propylene carbonate-LiC104 improved the coulombic efficiency for lithium cycling [112]. Lithium deposition on a lithium surface covered with a chemically stable, thin and tight layer which was formed by the addition of HF to electrolyte can suppress the lithium dendrite formation in secondary lithium batteries [113]. 

Another complication had to be matched when the zinc electrode was made reversible in a battery with unstirred electrolyte or an electrolyte gel, dendritic growth of the electrolytically deposited metal takes place. The formation of dendrites cannot be fully suppressed by the use of current collectors with large surface areas (grids, wire fabrics). However, by using improved separators combined in multi layer arrangements, the danger of short-circuiting is reduced. 

However, even these small pores cannot prevent the formation of so-called microshorts , arising by metal deposition (e.g., dendrites) from the solution phase. The pores of modern separators have a diameter of about 0.1 /mi, equal to 100 nm, while metal ions have a diameter of few angstroms, equal to 0.5-1 nm. On an atomic scale even micropores are barn doors ... 

Occasionally the zinc electrode is wrapped in a polypropylene fleece filled with inorganic substances, such as potassium titanate, in order to reduce the solubility of zinc since the problem of dendrite growth is aggravated even by the metallization of the cellophane separator due to the aforesaid silver reduction and its promoting the generation of shorts. 

A third type of problem, that is often mistakenly confused with dendrite formation, is due to the presence of a reaction-product layer upon the growth interface if the electrode and electrolyte are not stable in the presence of each other. This leads to filamentary or hairy growth, and the deposit often appears to have a spongy character. During a subsequent discharge step the filaments often become disconnected from the underlying metal, so that they cannot participate in the electrochemical reaction, and the rechargeable capacity of the electrode is reduced. 

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