Core and shell materials

Spray Drying. Spray-dry encapsulation processes (Fig. 7) consist of spraying an intimate mixture of core and shell material into a heated chamber where rapid desolvation occurs to thereby produce microcapsules (24,25). The first step in such processes is to form a concentrated solution of the carrier or shell material in the solvent from which spray drying is to be done. Any water- or solvent-soluble film-forming shell material can, in principle, be used. Water-soluble polymers such as gum arable, modified starch, and hydrolyzed gelatin are used most often. Solutions of these shell materials at 50 wt % soHds have sufficiently low viscosities that they stiU can be atomized without difficulty. It is not unusual to blend gum arable and modified starch with maltodextrins, sucrose, or sorbitol. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Fig. 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm. The emission band of CdS-ZnS core-shell nanoparticles is higher dian that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticl is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9], Althou the sur ce treatment conditions are different, the ranission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchan d by different surfaKs treatment. 

CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method. Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of CM and S precursor. However it was changed easily to cubic structure under the condition of sonication or higher concentration of Cd than S precursor. The interfacial state betwran CdS core and shell material was unchanged by different surface treatment. 

Silica has been used both as a core and shell material. For example, monodis-persed silica spheres were coated with titania by decomposing titanyl sulfate, TiOS04, in acidic solutions at 90°C (146). The particles so produced showed good hiding power, to be useful as paper whiteners (147). Due to the uniformity of the cores and shells, the optical properties of such dispersions were predictable and reproducible, as shown in Figure 1.1.22, which compares the scattering coefficient,... 

A general requirement for the synthesis of CS NCs with satisfactory optical properties is epitaxial type shell growth. Therefore an appropriate band alignment is not the sole criterion for choice of materials but, in addition, the core and shell materials should crystallize in the same structure and exhibit a small lattice mismatch. In the opposite case, the growth of the shell results in strain and the formation of defect states at the core-shell interface or within the shell. These can act as trap states for photogenerated charge carriers and diminish the fluorescence QY.95 The structural parameters of selected semiconductor materials are summarized in Table 5.1. 

Co-extrusion processes. Liquid core and shell materials are pumped through concentric orifices, with the core material flowing in the central orifice, and the shell material flowing through the outer annulus. A compound drop forms that is composed of a droplet of core fluid encased in a layer of shell fluid. 

Equqtion (51.15) provides the extinction cross section for spherical particles in a dielectric medium. When the particles are coated by a surface layer, the optical properties of both the core and shell materials must be considered. The extinction cross section of a concentric core-shell sphere is given by [144],... 

The selection of materials is a separate exercise that can often have more variables than process selection. Materials can be selected and used to eliminate potential processes, or materials can be selected and evaluated based on the preceding identification of a process. To further complicate interdependency of processes and materials, processing aids are often required and should be considered along with the basic core and shell materials. Table 2.1 lists limitations for both core and shell materials. 

Initially, dual fluid stream of liquid core and shell materials is pumped through concentric tubes. As the jet moves through the air, due to vibration, it breaks into droplets of core, each coated with the wall solution, as given in Figure 47.9. While the droplets are in flight, a molten wall may be hardened or a solvent may be evaporated from the wall solution. 

The co-extrusion process was developed by Southwest Research Institute in the United States, and has found a number of commercial applications. A dual fluid stream of liquid core and shell materials is pumped through concentric tubes and forms droplets under the influence of vibration (Fig. 1.14). The shell is then hardened by chemical crosslinkings, cooling, or solvent evaporation. Different types of extrusion nozzles have been developed in order to optimize the process [103]. 

Always in an attempt to compatibilize the core and shell materials, Yoshinaga and co-workers described the synthesis of a series of oHgomeric silane molecules and their use in encapsulation reactions [95]. However, as the polymerizations were performed in the absence of surfactant, the resulting composite particles were not colloidally stable. 

An IR-sensitive dye which had the peak of absorbance at 806 nm (IR-806) was used in work. Multiple types of capsules were fabricated using different core and shell materials. Capsules of the first type with (PAH/IR-806)4/PAH shell structure were fabricated on calcium carbonate cores. Capsules of the second type with IR-806 last layer adsorbed from the solution were fabricated on the surface of 4 fim manganese carbonate particles. 

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