Synthesis of Dense Materials

When the temperature of the SHS process exceeds the melting points of all product components, the product can be highly dense without the use of external pressure. Such products are referred to as cast SHS materials and are primarily the result of thermite reactions [36]. However, in other than these extremely exothermic reactions the products tend to be highly porous. While this is probably the most important disadvantage of the SHS process, it can actually be an advantage in some special cases, as in the synthesis of electrode materials [37], A direct relationship between the initial and the final densities is observed, see Fig. 26 for the case of TiC [30]. However, attempts to use highly dense reactants, taken as a step towards ameliorating 

The sources of porosity in SHS products have been analyzed and classified as intrinsic and extrinsic [38], The extrinsic sources of porosity are the initial porosity of the reactants and the porosity generated by the expulsion of volatile impurities during synthesis. The contribution of the latter is difficult to calculate exactly, but an approximate relationship has been derived which shows the dependence of the level of porosity on the amount of impurity as 

The relationship between reactants and product molar volume for selected compounds 

2 Self-Propagating High-Temperature Synthesis of Hard Materials 

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Cubic BC2N. Hetero-diamond B C—N compounds have recently received a great interest because of their possible applications as mechanical and optical devices. The similar properties and structures of carbon and boron nitrides (graphite and hexagonal BN, diamond, and cubic BN) suggested the possible synthesis of dense compounds with all the three elements. Such new materials are expected to combine the best properties of diamond (hardness) and of c-BN (thermal stability and chemical inertness). Several low-density hexagonal phases of B,C, and N have been synthesized [534] while with respect to the high-density phases, different authors report contradictory data [535-538], but the final products are probably solid mixtures of c-BN and dispersed diamonds [539]. 

This is a very simple methodology for the synthesis of different materials [41], The used precursors for the procedure are acetates. This methodology gives an excellent molecular mixing of the reaction components, and provides a reactive milieu throughout the successive heating and decomposition processes required to obtain the end product [41], This methodology is especially adaptable for the synthesis of thin, dense films for the manufacture of membranes [50],... 

To expedite the synthesis of densely packed dendrons, Percec and co-workers110 have utilized the triply branched monomer 37, methyl 3,4,5-trihydroxy-benzoate.84-85 111-113 The materials were synthesized by coupling the phenolic groups of the monomer to 3 equiv of a benzylic chloride, followed by transformation of the focal ester functionality to a chloromethyl group, via LiAlFF reduction and SOCT chlorination. This procedure could be repeated up to the fourth generation. 

Zeolites are a subclass of microporous materials in which the crystalline inorganic framework is composed of four-coordinated species interconnected by two-coordinated species. Traditionally these materials are aluminosilicates however, many different compositions have been synthesized. The templates used in the synthesis of microporous materials are typically small ionic or neutral molecular species. The function of the template in the synthesis of microporous materials is little understood, and there are at least four different modes by which an additive can operate in a zeolite synthesis a) It may act as a space filler occupying the voids in the structure, thereby energetically stabilizing less dense inorganic framework b) the additive may control the equilibria in the synthesis mixture, such as solution pH or complexation equilibria c) it may preorganize the solution species to favor the nucleation of a specific structure d) it may act as a true template determining the size and the shape of the voids in the structure. 

A few varieties of nonsilicate oxide ceramic powders synthesized through alkoxide processing are presented here. These oxide powders were developed for the use of electronic, optical, and high-temperature structural applications. For each material, we start with a brief description of the synthesis, which is followed by powder characteristics (e.g., particle sizes, morphologies, and size distributions) and densification behavior and some properties of dense material. 

A different approach to the synthesis of dense hard materials is reported by Degnan and Wood [91]. They describe a process in which Fe-(W,Ti)C cermet powders, produced by the SHS method, are used as additives to molten iron for the production of a particulate-reinforced material with improved wear properties. The presence of the additive provided an enhancement of wetting between the metal matrix and the added particles. The product was a fairly uniform material which showed a considerably improved wear resistance over iron-based alloys. Sharivker et al. [92] followed a somewhat similar route by using SHS-produced TiC or Tio 97Moo.o3C as additives to produce metal-based hard materials. 

Reviews on the synthesis of sinter materials consisting of p-BN and/or y-BN and their use as machining tools are numerous and have been cited with the respective applications. Variable intergrain bonding additives are contained in polycrystalline hard boron nitride sinters, and it is difficult to decide if a material should be classified as a polycrystalline hard boron nitride sinter, as a sintered boron nitride base ceramic, or as hard metal-bonded dense boron nitride. 

Zhou, Y., Sun, Z., Chen, S., Zhang, Y. (1998). In-situ hot pressing/solid-Uquid reaction synthesis of dense titanium silicon carbide bulk eeramies. Materials Research Innovations, 2(3), 142-146. doi 10.1007/sl00190050076. 

Because the condensation between a diketene acetal and a diol proceeds without the evolution of volatile byproducts, this method allows the preparation of dense, crossUnked materials by using reagents having a functionality greater than 2 (15). Even though either or both the ketene acetal and alcohol could have functionalities greater than 2, only triols were investigated because the synthesis of trifunctional ketene acetals is extremely difficult. 

Very recently, highly regular, highly controlled, dense branching has been developed. The resulting dendrimers often have a spherical shape with special interior and surface properties. The synthesis and properties of dendrimers has been reviewed (see e.g. G.R. Newkome et al. Dendritic Molecules , VCH, 1996). In this series, a chapter deals with the molecular dimensions of dendrimers and with dendrimer-polymer hybrids. One possible development of such materials may be in the fields of biochemistry and biomaterials. The less perfect hyper-branched polymers synthesized from A2B-type monomers offer a real hope for large scale commercialization. A review of the present status of research on hyperbranched polymers is included. 

P-Amino carbonyl compounds containing an a-atkyUdene group are densely functionalized materials, which are widely applied in the synthesis of medicinal reagents and natural products [265]. These products are usually prepared through the classic aza-Morita-Baylis-Hillman reaction [176, 177] of activated imines and electron-deficient alkenes catalyzed by tertiary amines or phosphines. Chen and co-workers, in 2008, identified bis-thiourea 106 as a suitable catalyst for the... 

A dense polymer brush is obtained using the grafting from techniques. Surface-initiated polymerization in conjunction with a living polymerization technique is one of the most useful synthetic routes for the precise design and functionalization of the surfaces of various solid materials with well-defined polymers and copolymers. Above all, surface-initiated living radical polymerization (LRP) is particularly promising due to its simplicity and versatility and it has been applied for the synthesis of Au NPs. 

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