Nanocomposites monoliths

Figures. Procedures to create the chemically converted GO/epoxy nanocomposite monoliths. Reprinted from reference [29] by permission of The Royal Society of Chemistry.

In this chapter we review the principles of monolith fabrication, routes for febrication of monoUths such as xerogel monoliths, nanocomposite monoliths, ORMOSIL monohths, hybrid monoliths, and aerogel monohths. In addition, we will present several characterization methods of monohths, encapsulation or doping in monohths, and examples of apphcations of monohths. 

The sol—gel process can be utilized to yield products within a wide range of appHcations. Some of these appHcations include production of nanocomposites, films, fibers, porous and dense monoliths, and biomaterials. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

The sol-gel method has been extensively used for the preparation of n-metal oxides and organic compounds. The important examples are n-NiO, n-Mn02, n-W03 and n-Fe203 etc. which have homogeneous particles, pore sizes and densities. This method affords easy control over the stoichiometry and homogeneity which is not possible with conventional methods. Further, the materials with special shapes monoliths, fibers, films and powders of uniform and very small particle sizes can also be prepared. The most important attribute of NMs prepared by this method is that they also contain pores of similar dimensions. These pores may be filled with another phase to form a nanocomposite which has proved to be of significant use to the HEMs community [98]. 

Another notable example of a reduction in creep rate through the addition of second-phase particles concerns nanocomposites . In alumina-SiC, systems, several investigations have reported significant reductions in creep rate compared with monolithic alumina [48, 49], Figure 4.9 shows the results of Ohji and co-workers [48], At 1200°C the creep rate of an AI203-17vol%SiC nanocomposite was less than that of alumina for a given stress by a factor of 250, and the time to rupture at 50 MPa was increased from 120 h to 1120 h. The SiC inhibits creep primarily because it is difficult to remove or deposit... 

All the above nanocomposites showed improvements of thermal resistance by h-BN addition. In particular, Si3N4/BN has aA /c (the temperature difference above which the residual strength decreases suddenly) as high as 1500°C, 50% higher than that of Si3N4 monolithic ceramics. 

Feng and coworkers prepared nanocrystalline tin oxide on monolithic mesoporous silica starting from Sn(acac)2Cl2 by simple immersing of the substrate in the precursor solution. Heat treatment (300-600 °C) leads to nanocomposites with a large specific surface area. The electrical properties of these nanocomposites were also investigated. The authors found an inverse correlation between the precursor concentration and the electrical resistivities of the samples. ... 

Tahir, M. Tahir, B. Amin, N. S. Photocatalytic C02 Reduction by CH4 over Montmorillonite Modified Ti02 Nanocomposites in a Continuous Monolith Photoreactor. Mater. Res. Bull., 2015, 63, 13-23. 

Classification of composites by the phase inclusion size bears a philosophical aspect how small should a component in the matrix be not to make the term composite material so universal as to include in fact all materials Interatomic distances in molecules and crystals are of 1.5 10 m dimensionality, distances between iterative elements of the crystalline structure are 10 —10 m, while the size of the smallest intermolecular voids in polymers is 10 m. Note that mean nanoparticle size (plastic pigments are 10-8-10 m in size, the diameter of monocrystalline fibers or whiskers is 10 —10 m, glass microspheres are 10 —10 m) is commensurate with parameters of monolithic simple materials. This means that in the totality of engineering materials, nanocomposites occupy a place at the boundary between composite and simple materials. 

Wang Z, Li F, Eigang NS, Stein A (2006) Effects of hierarchical architecture on electronic and mechanical properties of nanocast monolithic porous carbons and carbon—carbon nanocomposites. Chem Mater 18 5543-5553... 

Worsley, M.A., Satcher, J.H., and Baumann, T.E (2008) Synthesis and characterization of monolithic carbon aerogel nanocomposites containing double-walled carbon nanotubes. Langmuir, 24(17), 9763-9766. 

Wang, Zh. and Stein, A. 2008. Morphology control of carbon, silica, and carbon/silica nanocomposites From 3D ordered macro-Zmesoporous monoliths to shaped mesoporous particles. Chem. Mater. 20 1029-1040. 

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