Physical properties of nanomaterials – mechanical and optical properties

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Physical properties of nanomaterials – mechanical and optical properties

·         A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case

·         Copper –an opaque substance become transparent

·         Platinum inert material become catalyst

·         Aluminum –a stable material turns combustible

·         Silicon insulator become conductors

·         Gold- solid, inert, yellow at room temperature become liquid and red in colour

·         Due to their small dimensions, nanomaterials have extremely large surface area to volume ratio, which makes a large fraction of atoms of the materials to be the surface or interfacial atoms, resulting in more “surface” dependent material properties

·         Nanosturctures and nanomaterials favors of a self-purification process in that the impurities and intrinsic material defects will move to near the surface upon thermal annealing. This increased materials perfection affects the properties of nanomaterials. For example, the chemical stability for certain nanomaterials may be enhanced, the mechanical properties of nanomaterials will be better than the bulk materials. The superior mechanical properties of carbon nanotubes are well known

·         The excellent mechanical properties of nanomaterials could lead to many potential applications in all the nano, micro and macro scales. High frequency electro-mechanical resonators have been made from carbon nanotubes and nanowires

·         Polymer nanocomposites from nano size fillers could result in unique mechanical properties at very low filler weight fractions

·         Nanocomposite materials from carbon nanotubes are expected to exhibit outstanding mechanical properties, such as high Young’s modulus, stiffness and flexibility.

·         Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials.

·         Nanoparticles exhibit a number of special properties relative to bulk material. For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper

·         Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage.

·         Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid.

·         Nanoparticles often have unexpected visual properties because they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep red to black in solution.

·         The often very high surface area to volume ratio of nanoparticles provides a tremendous driving force for diffusion, especially at elevated temperatures.

·         Sintering is possible at lower temperatures and over shorter durations than for larger particles. This theoretically does not affect the density of the final product, though flow difficulties and the tendency of nanoparticles to agglomerate do complicate matters. The surface effects of nanoparticles also reduces the incipient melting temperature.

Thermal properties

·         In non-metallic material system, the thermal energy is mainly carried by phonos, which have a wide variation in frequency and the mean free paths (mfp). The heat carrying photons often have large wave vectors and mfp in the order of nanometer range at room temperature, so that the dimensions of the nanostructures are comparable to the mfp and wavelengths of photons.

·         Small size, the special shape, the large interfaces modified the thermal properties of the nanomaterials, rendering them the quite different behavior as compared to the macroscopic materials.

·         As the dimension goes down to nano scales, the size of the nanomaterials is comparable to the wavelength and the mean free path of the photons, so that the photon transport within the materials will be changed significantly due the photon confinement and quantization of photon transport, resulting in modified thermal propeties. For example, nanowires from silicon have a much smaller thermal conductivities compared to bulk silicon

·         The special structure of nanomaterials also affects the thermal properties. For example, because of it tubular structures of carbon nanotubes, they have extreme high thermal conductivity in axial directions, leaving high anisotropy in the heat transport in the materials

·         The in interfaces are also very important factor for determine the thermal properties of nanomaterials. Generally, the internal interfaces impede the flow of heat due to photon scattering. At interface or grain boundary between similar materials, the interface disorder scatters phonons, while as the differences in elastic properties and densities of vibrational states affect the transfer of vibrational energy across interfaces between dissimilar materials. As a result, the nanomaterials structures with high interfaces densities would reduce the thermal conductivity of the materials

·         Due to their high thermal conductivities, carbon nanotubes or nanotubes based nanocomposite could be promising candidates for heat transport management in many applications such as in the integrated circuits, optoelectronic devices and MEMS structures.

·         Although the nanowires had well-defined crystalline order as in bulk materials, the observed thermal conductivity was more than two orders of magnitude smaller than that of bulk silicon, which also showed a strong dependence on the nanowires size.

·         Thermal conductivity on Si/SiGe superlattice nanowires, with even smaller thermal conductivity reported on the superlattice nanowires compared to pure silicon nanowires

·         The low thermal conductivity in these one dimensional semiconducting nanowires will find applications in thermoelectric power generation and the thermoelectric refrigeration. However, they are not desired in for thermal management in electronics, which generally need much higher thermal transport capabilities.

·         Nanofluids are generally referred to the solid-liquid composite materials, which consist of nanomaterials of size in the range 1-100nm suspended in a liquid. Nanofluids hold increasing attentions in both research and practical applications due to their greatly enhanced thermal properties compared to their base fluids. Many type of nanomaterials can be used in nanofluids including nanoparticles of oxides, nitrides, metals, metal carbides, and nanofibers such as single wall and multi wall carbon nanotubes, which can be dispersed in to a variety of base liquid depending on the possible applications, such as water, ethylene glycol, and oils . The most important features of nanofluids are the significant increase of thermal conductivity compared with liquids without nanomaterials, which have been proved by many experimental works. 30% increase in the thermal conductivity of water when 4.3 vol.% Al2O3 nanoparticles was added into the nanofluid. An addition of 0.3 vol% Cu nanoparticles of mean diameter~10 nm in ethylene glycol resulted in an increase of 40% in the effective thermal conductivity of the nanofluid . A thermal conductivity enhancement of 5%–21% in the temperature range of 30–60 °C at an Au nanoparticle loading of 0.000 26 (by volume)

Optical properties

Nanomaterials with Photo Luminescence (PL) effects, in particular nanocrystals, can reveal many interesting and improved optical properties. These include brighter emission, narrower emission band, and broad UV absorption. For example, semiconductor nanocrystals produce narrower emission peaks than luminescent organic molecules, with bandwidth of around 30-40 nm. Having a smaller bandwidth, it is much easier to discriminate individual wavelengths emanating from multiple sources, such as in an array of nanocrystals. The PL emission intensities and wavelengths are dependent on particle size. Hence, PL spectroscopy directly enables particle size effects, in particular those in the nanoscale, to be observed and quantified.