Dendrites, thermal

In our treatment of directional solidification above, only one diffusion field was treated explicitly, namely the compositional diffusion. If a simple material grows dendritically (thermal diffusion) one may worry about small amounts of impurities. This was reconsidered [132], confirming a qualitative previous result [133] that impurities may increase the dendritic growth rate. Recently some direct simulation results have been obtained with two coupled diffusion fields, one for heat and one for matter, but due to long computing times they are not yet in the state of standard applications [120,134]. 

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. 

In 1996, Hawker and Frechet83 discussed a comparison between linear hyperbranched and dendritic macromolecules (Fig. 5.17) obtained with the same monomeric structure, 3,5-dihydroxybenzoic. The thermal properties (glass transition and thermal decomposition) were not affected by the architecture. 

Among the many unusual properties that the arborescent architecture leads to, most notable is the discovery that block copolymers with a high MW dendritic (arborescent) polyisobutylene core and poly(para-methylstyrene) end blocks can manifest themselves either as a rubber, or as a plastic, depending on their environment (Figures 7.16 and 7.17). The behavior is thermally irreversible. 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

JH NMR) studies confirmed the presence of the AB diblocks in the product. This determination was facilitated by the fact that the dendritic nitroxide could be differentiated from the nonnitroxide-bearing dendron by NMR spectroscopy. Careful analytical studies confirmed that the pure ABA copolymers could be separated by column chromatography and that the undesired diblock impurity resulted mainly from the loss of the dendritic nitroxide during the course of the reaction. Obviously, this approach to ABA triblocks has rather limited practical value since the thermal stability of the final product is quite low. 

The separator should also not allow any dendrite to penetrate through the separator to avoid internal shorts. During an internal short, the separator is the only safety device that can stop the thermal runaway, if the failure is not instant. If the heating rate is too high, then instant failure will occur which cannot be stopped by separator shutdown. If the heating rate... 

While polymers most commonly consist of carbon, nitrogen and oxygen, hybrid materials containing metals or other elements represent an important class of materials [76-78]. Hybrid materials too have been used as hyperbranched grafts. Two examples of such materials are described here. The first is a polysiloxane graft. The second is a dendritic coordination polymer based on a thermally and oxygen stable Pd(II) pincer complex [79]. 

Our choice was the two series of dendritic polymers 5 and 6, depicted in Figure 4, which have all their open-shell centers (or trivalent carbon atoms) sterically shielded by an encapsulation with six bulky chlorine atoms in order to increase their life expectancies and thermal and chemical stabilities. Indeed, it is very well known that the monoradical counterpart of both series of polyradicals, the perchlo-rotriphenyl methyl radical, shows an astonishing thermal and chemical stability for which the term of inert free radical was coined. The series of dendrimer polymers 5 and 6 differ in the nature and multiplicity (or branching) of their central core unit, N, as well as in their branch-juncture multiplicities, N Thus, series 5 has a hyperbranched topology with = 3 and = 4, while dendrimer series 6 has a lower level of branching with = 3 and = 2, and the topology of a three-coordinated Cayley tree. 

Tomalia et al. (5) develop a synthetic method for preparing functionalized dendritic compositions having high thermal stability that did not undergo a reversible Michael addition reaction. 

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