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Ceramic nanocomposites

Ceramics are used as structural materials under conditions of high loading rates, high temperature, wear, and chemical attack that are too severe for metals. However, inherent brittleness of the ceramics has prevented their wide use in different applications. Improvements have been made in the material by incorporating of fibers or whiskers to bridge the crack faces just behind the crack tip. Second phase particles are incorporated to deflect the crack making it travel a more tortuous path, a secondary phases is also incorporated to make stress induced volume expansion so that the crack faces can be forced together.
The definition of nanocomposite material has broadened significantly to encompass a large variety of systems such as one-dimensional, two-dimensional, three-dimensional and amorphous materials, made of distinctly dissimilar components and mixed at the nanometer scale. Presently multiple phases are distributed in a ceramic composite at the nanoscopic length scale to make ceramic nanocomposites.
Ceramic nanocomposites
In recent years an increasing interest has been devoted to nanostructured composites because of exciting possible applications ranging from new catalysts to the preparation of nanocomposites ceramics. Nanopowders have been synthesis and of nanostructures silicon based composites have been produced by liquid-phase sintering using nano-sized powders where interfaces play a decisive role for the evolution of properties.
Ceramic-based nanocomposites have high permittivity, breakdown strength (BDS), and reduced strain, which can increase the energy density of capacitors and increase their shot life. Ceramic nanoparticles upto 30% loadings are incorporated in polymer-based composite capacitors; they have high energy densities but not extended performance in a robust device. This new class of ceramic microstructure consists of a dispersion of small inclusions (<300nm) in a matrix with a much larger grain size (1-5µm).
Alumina matrix nanocomposites
Alumina matrix nanocomposites containing reinforcements of SiC or TiN (20-200nm) have been produced. Improvements in strength of up to 30% over comparable monolithic alumina have been achieved. The addition of as little as 5vol% SiC to alumina changes the fracture mode from intergranular to transgranular, increasing the fracture toughness of the material. This reduction in the propensity for grain boundary fracture also eliminates the grain pullout which often frustrates attempts to polish monolithic alumina to a flawless mirror finish, and reduces the wet erosive wear rate by 65%.
Non-Oxide Ceramic nanocomposites
Two types of Aluminum nitride (AlN) based ceramic nanocomposite with multifunctionality improve machinability or electrical conductivity of AlN ceramics with high thermal conductivity. The AlN/BN nanocomposite is fabricated by hot-pressing AlN-BN composite powder, which is prepared by reducing and heating AlN particles containing a mixture of boric acid, urea and carbon. The nanocomposite containing 20 vol. % BN show high strength, good machinability and relatively high thermal conductivity. On the other hand, the sintered AlN ceramics with CeO2 as an additive indicate high thermal conductivity and electric conductivity which is possible for electric discharge machining.
Ceramic nanoparticles for entrapping drug
The composition for photodynamic therapy comprises ceramic nanoparticles in which a photosensitive drug/dye is entrapped. The ceramic nanoparticles are made by formation of a micellar composition of the dye. The ceramic material is added to the micellar composition and the ceramic nanoparticles are precipitated by alkaline hydrolysis. The precipitated nanoparticles in which the photosensitive dye/drug is entrapped can be isolated by dialysis. The resulting drug doped nanoparticles are spherical, highly monodispersed, and stable in aqueous system. Irradiation with light of suitable wavelength of the photosensitizing drug entrapped inside nanoparticles results in generation of singlet oxygen, which was able to diffuse out through the pores of the ceramic matrix. The drug loaded ceramic nanoparticles can be used as drug carriers for photodynamic therapy.
Properties of nanocomposites
The properties of nanocomposite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics. The nanocomposites find their use in various applications because of the improvements in the properties over the simpler structures. Few of such advantages can be summarized as:
• Improved Mechanical properties e.g. strength, hardness, toughness, modulus and dimensional stability
• High strength even at elevated temperature
• Better creep resistance
• Decreased permeability to gases, water and hydrocarbons
• Higher thermal stability and heat distortion temperature
• Higher flame retardancy and reduced smoke emissions
• Higher chemical resistance
• Smoother surface appearance
• Higher electrical conductivity
• lifetime upto 10000 h for components used in a gas turbine engine
• high resistance to thermal shock, oxidation, and sub critical crack growth
• can withstand high temperatures (>1500 °C) without degradation or oxidation
• A 200% improvement in both strength and fracture toughness, better retention of strength at high temperatures.
Advanced bulk ceramic composite materials can be used for applications such as structural parts of motor engines, catalytic heat exchangers, nuclear power plants, and combustion systems, besides their use in fossil energy conversion power plants. These hard, high-temperature stable, oxidation-resistant ceramic composites and coatings are also used for aircraft and spacecraft applications.
Examples of nanocomposites
One such nanocomposite is Silicon Carbide/Silicon Nitride (SiC/Si3N4) composites, which perform very well under high temperature oxidizing conditions. Large improvements have been achieved in both the fracture toughness and the strength of materials by embedding nanometer range (20-300 nm) particles within a matrix of larger grains and at the grain boundaries. This advanced nanocomposite microstructure such as that of polycrystalline Silicon Carbide (SiC)-Silicon Nitride (Si3N4) nanocomposites, contains multiple length scales with grain boundary thickness of the order of 50 nm, SiC particle sizes of the order of 200-300 nm and Si3N4 grain sizes of the order of 0.8 to 1.5 µm.

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