Particle anisotropic shape

When a nanoporous Ti02 film consisting of Ti02 nanoparticles is used instead of the single crystal, the extinction band of silver nanoparticles deposited by UV-irradiation is much broader. This is probably because the nanopores in the Ti02 film mold the silver nanoparticles into various anisotropic shapes [9], although direct observation of the particles in the nanopores is difficult. 

Transmission electron microscopy ( ) analysis reveals that these materials crystallize as hexagonal planar particles with marked anisotropic shape,8,37 as shown in Figure 6. When appropriate preparation methods are used, plate-like crystals are obtained with small thickness of about 20-30 nm and an aspect ratio D/h=5-10. Selected area diffraction (SAD) patterns of incident beams perpendicular and parallel to the large hexagonal facet show that they correspond to the crystallographic planes perpendicular to the c axis. The anisotropic shape of the... 

The lengths of the elongated particles can be shortened by dissolution in hydrochloric acid or oxalic acid. It is interesting to note that only the length becomes shorter by dissolution. This makes it easy to obtain hematite particles having the desired aspect ratio. This technique may also be applied to other particles having anisotropic shapes. 

In the previous sections, we discussed the influence of the number of crystals in the sample. The orientations of the crystals were assumed to be random, and obviously, this factor comes into play. Theoretically, quantitative analyses by X-ray diffraction are conducted on samples comprised of a very large number of micrometric crystals without any preferential orientation. This latter condition is sometimes difficult to meet, since it can be sometimes complicated to give the crystals in the sample a random orientation. This effect often occurs when crystals have an anisotropic shape. Clays are an extreme example of this behavior [BRI 80]. Their layered stracture naturally causes a preferential orientation along the (001) planes. Some authors pLO 55, SMI 79, HIL 99] have used atomization methods to produce polycrystalhne particles in which the clay crystals have a random orientation. Another approach consists of quantifying the preferential orientation and to take it into accoimt when calculating the proportions of the phases in the sample. We will not be giving arty details on this method, since it requires considerable skill in the production of pattern and data analysis. It is always better not to have a preferential orientation. 

The polarizability of cylindrically symmetric particles (rod-shaped or ellipsoidal particles) can be characterized by isotropic (a) and anisotropic (P) parts of the polarizability tensors ... 

The particle sizing by field flow fractionation (FFF) is based on the different effect of a perpendicular applied field on particles in a laminar flow [63-66], The separation principle corresponds to the nature of the perpendicular field and may, for example, be based on different mass (sedimentation FFF), size (cross-flow FFF), or charge (electric-field FFF). Cross-flow FFF has been applied recently to investigate nanoemulsions, SLN, and nanostructured lipid carriers (NLC, particles composed of liquid and solid lipids) [58], Although all samples had comparable particle sizes in PCS, their retention in the FFF was very different. Compared to the spherical droplets of the nanoemulsion, SLN and NLC were pushed more efficiently to the bottom of the channel because of their anisotropic shape. Their very different shapes have been confirmed by electron microscopy. 

When the crystal size approaches a, the pair becomes confined and the effects are observable. This model is a simplification, since other correlation functions (with more complex solutions) are possible. However, several experimental works have demonstrated the validity of the expression for near-spherical nanoparticles [65-79]. More complex solutions may be necessary for anisotropic shapes, as experimentally demonstrated by Buhro and Covin [80] for InAs. In general, the third term of (18) is neglected and the second term is only significant when < Rp [79]. hi these particles, the mode is known as strong confinement, whereas for larger, albeit still small particles, the mode is known as weak confinement [64]. 

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

In addition, controlled aggregation gives anisotropic-shaped particles similar to those reported for ZnO nano-rods. The confined space growth process has a rather long history. This process makes particles in a confined space such as a microemulsion. In this case, the collision frequency, stabiUty, and shape of the emulsions affect the particle-size distribution and shapes. 

Simpler dripping methods have been reported where the solidification starts immediately after contact of the drops with the liquid. An example represents freezing of slurry droplets in liquid nitrogen and subsequent freeze-drying and calcination of the bodies [50]. However, such methods produce particles of anisotropic shape, and the size distributions are broad because many droplets break apart upon bouncing against the liquid surface. 

The concentration of Ba(OH)2 significantly affects the final morphology of barium titanate nanopowder. The BaTiOs particles acquire spherical shape at sufficiently high concentrations of Ba (OH)2 only. In the opposite case crystals take various anisotropic shapes. Addition of surfactants [90, 94, 100] improves the crystallization of perovskite phase. 

In the case of filler particles in shape of fibers or platelets (as, e.g., organoclays), oriented randomly, the distribution of voids created at their interfaces on loading can be not homogeneous enough to produce a uniform porous structure needed for successful toughening. Therefore, toughening with anisotropic rigid particles seems more difficult than with semi-equiaxed ones. 

Weinberg et al. (1997), showed that the JMAK theory is invalid for non-spherical particles, e.g., needlelike crystalline phases. Therefore, Weinberg and Birnie III (2000), proposed theoretical models describing the kinetics of the crystallization of highly anisotropic particles. These models took into account the influence of blocker particles and shaped a ressors on the precipitation of anisotropic crystals. 

Nisisako T, Hatsuzawa T A microfluidic cross-flowing emulsion generator for producing biphasic droplets and anisotropically shaped polymer particles, Micrqfluid Nanojluid 9 427-437, 2010. 

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