## 4 Composite Formation Techniques
### Chemical in situ _methods_  
This approach uses chemical reactions in a liquid environment to generate nanocomposites. The result may be either nanocomposite particles, or compact nanocomposite material. Very comprehensive reviews on the variety of chemical synthesis methods are given by [PERSON] [34] and [PERSON] _et al._[74].  
Already in the early 1990 ies [PERSON] _et al._[75, 76] elaborated a one-step chemical method to synthesize fine dispersed Fe\({}_{2}\)O\({}_{3}\) nanoparticles in a cross-linked polystyrene resin. They used a synthetic ion-exchange resin and aqueous solutions of Fe(II) or Fe(III)-chloride, respectively, to exchange the ions. [PERSON] synthesized Fe\({}_{3}\)O\({}_{4}\)/PMMA composite particles by a one-pot hydrothermal method [77]. [PERSON] _et al._[78] report about the synthesis of transparent polymer nanocomposites containing ZnS using a one-pot route via _in situ_ bulk polymerization.  
The common feature of most materials described below, in contrast to the one described before, is, that (functionalized) nanoparticles are synthesized in a first step, mostly as a sol or dispersed in a solution, followed by a second step where a monomer or resin is added and brought to polymerization.  
[PERSON] _et al._[79] synthesized AlN nanoparticles with a sol-gel method, and then applied an effective solution mixing method to generate a homogeneous dispersion of AlN nanoparticles in polyimide. GaN/polymer nanocomposites were synthesized by _in situ_ thermal decomposition of a precursor incorporated into a copolymer [80]. [PERSON] and [PERSON] [81] prepared colloidal solutions of Fe\({}_{2}\)O\({}_{3}\) nanoparticles, which then were added to the conducting polymer PPy. The whole mixture finally was polymerized to obtain a nanocomposite. [PERSON] _et al._[82] prepared TiO\({}_{2}\)/polymer nanocomposites by mixing (3-methacryloxymethyl)trimethoxysilane (MPMS)-capped acrylic resins with sol-gel synthesized TiO\({}_{2}\). Quantum dot/polymer nanocomposites were synthesized by polymerization in microemulsion after synthesis of the nanoparticles by thermal decomposition of a precursor [83, 84]. [PERSON] _et al._[85] applied a two-step process to synthesize ZnO in a colloidal suspension, which finally was photopolymerized. [PERSON] [86] prepared magnetic nanocomposites containing Ni\({}_{0.5}\)Zn\({}_{0.5}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles via a wet-chemical method leading to a colloidal suspension, followed by _in situ_ polymerization of a monomer. A similar method is applied by [PERSON] _et al._[87] for the synthesis of ZnS containing nanocomposites. The _in situ_ generation of SiO\({}_{2}\) nanoparticles via sol  
Figure 6: Sketch of dispersing nanoparticles in a monomer, polymer or resin (organic solution) by use of external shear forces, e.g., by a stirrer (left) or by sonification prior to polymerization. In an advanced set-up, the nanoparticles may be coated with a coupling agent/surfactant before mixing (right) with the monomer/polymer/resin.  
gel techniques in an organic solvent, which contains dissolved PMMA, lead to PMMA-nanosilicate composites after solvent evaporation and drying [88].  
Chemical routes based on sol-gel processes and subsequent _in situ_ polymerization are commonly used for the synthesis of hybrid nanocomposite particles and nanocomposites.