Fragile nanoparticles, such as carbon nanotubes, typically undergo chemical dispersion such as with the use of surfactants. Methods of nanoparticle dispersion include melt-state sheer, ultrasonication, and solid-state pulverization. Uniform particle dispersion is therefore essential in obtaining the desired composite properties. Smaller particle diameters exhibit an increasing probability towards particle agglomeration due to electrostatic, steric, and Van der Waals forces. Particle size has a direct influence on the surface and interfacial properties of nanoparticles. This was because of pseudo-tetragonal distortion of the crystal lattice. Below a particle size of 70 nm, the paraelectric phase was stable whilst above 100 nm, the tetragonal ferroelectric phase was stable. A decrease in ferroelectric to paraelectric phase transition in PbZrO 3 at particle sizes below 100 nm was observed, as well as a decrease in dielectric constant. A decrease in particle size for TiO 2 resulted in an increase in electrical storage capacity with similar effects observed for rutile particles. For a ZrO 2 nanoparticle embedded matrix a decrease in particle size resulted in a decrease in transition temperatures which is a typical effect experienced when incorporating ceramics into a polymer matrix. For SnO 2 nanoparticles, a significant increase in energy band gap was observed with a decrease in particle size. Hanemann and Szabó noted that the refractive index of PbS nanoparticles with sizes below 25 nm was significantly less compared to larger particles. Embedding ZnO nanoparticles into a PANI (polyaniline) polymer matrix resulted in a spectral red shift whilst reducing the optical band gap energy by 1.9%. This is due to a high attractive force between silver and nickel relative to the low surface energy of silver nanoparticles. As the size of nanoparticles decrease, the effect of cohesive forces attracting the particles decrease hyperbolically except for Ag nanoparticles embedded in a Ni containing bulk material when the inverse effect occurs. Theory states that with a decrease in particle size the ratio of particle surface area respective to its volume increases hyperbolically. This definition restricts true nanoparticles to particles with dimensions in the range of 10–20 nm. Nanoparticle size has a direct impact on the electrical and thermal conductivity, polymer phase behavior and stability, mechanical properties, flame retardancy, density, magnetic, optic, or dielectric properties of the polymer matrix. Most of the properties associated with nanoparticles can be attributed to particle size. This is followed by a subsequent coating step in which organic compounds are grafted onto the formed nanoparticle surfaces for coating, encapsulation, or surface functionalization. Energy is applied to a precursor gas of chemical compounds which then forms inorganic nanoparticles. Physical in situ methods are mainly gas-phase methods used to produced encapsulated nanoparticles which appears as hybrid nanoparticles. This method typically ensures a more homogeneous dispersion of the nanoparticles compared to the ex situ process. Chemical in situ methods use a liquid environment to generate the nanoparticles as either hybrid particles or directly into the polymer matrix. This process has a typical drawback of nanoparticle agglomeration which can be difficult to overcome and resolve. Ex situ processes are where nanoparticles are synthesized and introduced into the polymer matrix in solution. Composite formation techniques include ex situ, chemical, and physical in situ methods which lead to the final composite or the formation of hybrid nanoparticles. Composites can be classified as either ceramic matrix nanocomposites (CMNC), metal matrix nanocomposites (MMNC), or polymer matrix nanocomposites (PMNC) to which nanoparticles can be added to either enhance, change, or add to the properties of the matrix material. Composites combine the properties of a bulk material with that of a size-dependent particle. Nanostructured materials for this purpose are typically used in the form of nanoparticles, nanowires, nanotubes, nanofibers, etc. Nanostructured materials are receiving increasing attention for their advantageous properties such as extremely huge relative surface area and surface reactivity, no contact heat transfer problems, and availability for natural convection to enhance heat transfer during melting. Nanoparticles could be either organic or inorganic in nature with dimensions below 100 nm.
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