Coated particle morphology

Figure 1.1.20 shows the differential thermal analysis (DTA) data for the cores, of chromium hydrous oxides particles prepared in the absence of hematite, and of coated particles. It is obvious that the latter behave as the coating material, when alone. This example clearly indicates the possibility of having the surface site characteristics of chromium hydrous oxide induced onto ellipsoidal iron oxide particles. The latter morphology cannot be achieved by diiecl precipitation of the same chromium compound. 

Several essential properties of cristobalite have influence on its applications. They include lower density than quartz (higher volume at the same mass), purity (low catalytic effect on many polymeric systems, excellent properties in exterior coatings due to low level of iron oxide), very low moisture (no need for drying in moisture sensitive systems), pure white color, less abrasive due to filler particle morphology. 

The most readily observed structure of PPy samples is the peculiar surface morphology common to all electropolymerized PPy films and coatings. The morphology consists of nodules ranging in size to hundreds of microns and that themselves consist of aggregations of smaller particles. The structure has been referred to as a cauliflower - or fractal -like surface. 

Surprisingly, all the quantitative spectra of the alcosol particles looked similar. As is also demonstrated in Table III, the siloxane structure of all the Stober silica spheres is more or less constant and independent of the particle size and thus the reaction conditions. Even the coated stearylsilica particle A3S is no exception, despite the 3 h at 200 °C necessary for the coating. Thus, the differences mentioned in the previous section in the particle morphology are not correlated with the siloxane structure. 

The selection of the proper intrinsic (plasma power, argon gas flow rate, auxiliary gas flow rate, powder carrier gas flow rate, etc.) and extrinsic (spray distance, powder feed rate, powder grain size, particle morphology, surface roughness, etc.) plasma parameters is crucial for sufficient powder particle heating, flow and surface wetting on impact and hence development of the desired coating porosity and... 

Over the decades that have passed since La Mer s work numerous examples of monodispersed particles of various composition, morphologies and properties, as well as methods for their preparation (not limited to condensational formation), were described in the literature. Extensive studies in this area were carried out by E. Matijevic and T. Sugimoto. Examples of monodisperse systems formed by precipitation from homogeneous solutions include dispersions of uniform particles of simple composition having different morphologies, such as metal halides, sulfides, phosphates, (hydrous) oxides, etc, various composite particles, including particles of internally mixed composition and coated particles. Both crystalline and amorphous materials can be obtained. Electron micrographs of some characteristic examples of monodispersed colloids are shown in Fig. IV-14. 

Fendler et al. [47] carried out experiments on vesicle-stabilized mixed crystals of Zn Cd. S and on CdS particles coated with ZnS. Besides absorption and fluorescence spectroscopy, x-ray diffractrometry was used for structural characterization of the various pure and mixed sulfide particles. In this article, the authors discuss thoroughly the fine interplay between kinetics and thermodynamics governing the rather complicated reaction scheme, finally yielding either separate particles of CdS and ZnS, mixed crystals Zn Cdj. S, or ZnS-coated particles of CdS. The latter may appear either as ZnS islands on the CdS particles or as a closed shell of ZnS surrounding a CdS core. In their experiments on capped particles, the authors of Ref. 47 do not attempt to decide which of the two morphologies occurs. 

The particle morphology is normally assessed by scanning electron microscopy (SEM). The sample preparation depends, in the first instance, on the type of equipment and normally requires the use of aluminum stubs with a carbon conductive tape and coating with gold-palladium layer. Sometimes the particles are placed on a double-sided carbon tape that is attached to aluminum stubs, without coating requirement. The analyses are carried out by applying to sample a difference of potential between 2 and 20 kV. 

In actual practice, there are often various levels of attrition, agglomeration, and accretion that will affect particle morphology and coat thickness. In addition, there is variation in the circulation of small particles compared to larger particles of the particle size distribution and the surface-to-volume ratio varies with particle size both of these concerns result in a theoretical... 

More recently, Lee et al. [119, 120] described the synthesis of the same kind of particles (without, however, referring to the previous work of Kondo). The difference lies in the coverage of FesOa nanoparticles, which were in this case coated with either a bilayer of lauric acid or with PAA oligomers. For each surface treatment, the influence of the initiator (either potassium persulfate (KPS) [119] or AIBA [120]) on the mechanism of particle formation, PSD, and particle morphologies was discussed. PSD was generally quite broad and the iron oxide nanoparticles were either located in the PS core or adsorbed at the surface. Further encapsulation with poly(NIPAM-co-MAA) provided core-shell particles. 

Coated particle technique has been well employed to synthesize powders for transparent ceramics, especially for YAG. YAG powder retaining the morphology of AI2O3 powder has been synthesized by using a partial wet-chemical process, in order to form yttrium precipitate coated AI2O3 particles [208]. The formation of the so-called core-shell structure had two steps, including (i) direct precipitation of yttrium component at the surface of the AI2O3 particles and (ii) assembly of the yttrium precipitate. A spherical surface reaction process was illustrated. YAG phase... 

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