Powders morphology

Experiment No. Powder morphology Peak pressure (GPa) Density (% solid)... 

Fig. 8.7. The influence of powder morphology (configuration) on shock modification controlling initiation of reaction is shown by the thermal response of mixed Ni-Al powders of different morphologies. The preinitiation event shown in Fig. 8.5 is observed to be strongly influenced by morphology at fixed shoek condition. The eoarse-medium mixture shows the largest preinitiation event [91D01]. The data show mueh larger preinitiation events for the flaky and fine morphologies.

The entire CRT industry is based on the quality and reproducibility of the phosphors used. As stated earlier the phosphors used for the different kinds of CRT displays are constantly being researched and improved. For high-definition television screens the phosphors must be prepared with great control over powder morphology and particle size (see Section 9.15.4.2.1). 

E Matijevic. Control of Powder Morphology. In LH Hench, JK West, eds. Chemical Processing of Advanced Materials. New York Wiley, 1992, pp 513-527. 

The nascent powder sample has quite a different morphology compared to the solution-crystallised or melt-crystallised samples. The nascent powder morphology mainly consists of particles connected by fibrils, which is called the "cobweb" structure.26 27 The nascent powder does not have any typical lamellar morphology but has a domain structure where crystalline domains distribute within the whole powder globule (Figure 3C). The domain size has a wide range of several tens of nanometres radius. 

Annealing of the nascent powder sample passed through Process 1, and above 90 °C, dynamic molecular motion started, as defined by Process 2. This critical temperature is slightly higher than that of the solution-crystallised sample. This difference indicates the restricted crystalline chain motion for the domain network structure crystallised during polymerisation. In Process 2, the crystallinity remained at a constant level for the nascent powder sample. This shows that the lamellar thickening is limited for the nascent powder morphology. 

Nascent powder morphologies of both the nascent and etched samples observed by SEM are shown in Figure 12 for the nascent and 4- and 12-month-etched UHMW-PE powders.24 Unique "cobweb" structures, composed of globules and fibrils, are often observed on the surfaces of nascent PE powders.2 26 27 43 44 In Figure 12A, very few fibrils are observed for the nascent powder. The paste-like morphologies cover the surface of this nascent powder. The major powder structure... 

Matijevic, E., Control of powder morphology, in Chemical Processing of Advanced Materials (L. H. Hench and X K. West, Eds.), p. 513. Wiley, New York (1992). 

The diameters of the particles, prepared from dispersion, are approximately 120 nm each. The powder morphology of spray-dried PMMA grafted siloxane particles is illustrated in Fig. 3a the spherical agglomerates are approximately 1-20 pm in size. The microstructure of the graft copolymer after processing is shown in Fig. 3b. 

Fig. 3a. Powder morphology (scanning electron micrograph) of spray-dried PMMA grafted siloxane particles (obtained from the dispersion of Fig. 2b)...

The powder morphology is observed with TEM (Japan H-600). Thermal analyses are carried out by using TG/DTA (Shimadzu DT-40). The powder x-ray diffraction patterns of BaTiOj is recorded with XRD (D/max-3B diffractometer), Surface area is measured with ST-08 surface area apparatus. 

Figure 3.32. High quality, nearly spherical powder prepared by high-pressure gas atomization from the melt and proper sample length, L. The x-ray powder diffraction data were collected from a continuously spinning sample (20 mm diameter and 1 mm deep) prepared as shown in Figure 3.22. Notations are the same as in Figure 3.29. The powder contains a small fraction of a second phase, which is identified by the series of vertical bars shifted downwards. The inset shows the scanning electron microscopy image of the powder morphology. (Powder courtesy of Dr. I.E. Anderson.)...

The initial characterization of the products was carried out by powder XRD measurements on thin films in transmission. Powder morphology was examined by SEM analysis. As an example in Fig. 11.6 the powder X-ray diffraction pattern of CoTi03/La displays the formation of single phase compound with CoTi03-structure (literature data ICSD 48107). The SEM image of the annealed material shows particles with a diameter of 30-100nm. All annealed materials appeared as open porous networks of interconnected particles, which should allow good interaction between gas and surface. 

Ruys, A.J., Sorrell, C.C., Brandwood, A., and Milthorpe, B.K. (1995) Hydroxyapatite sintering characteristics correlation with powder morphology by high resolution spectroscopy. J. Mater. Sci., 14, 744-747. 

Figure 12.2 Polypropylene TPO reactor powder morphology. AFM image of embedded and cryo-faced-off reactor powder particle.

Powder morphology was investigated using a transmission electron microscope (TEM, Model JEM-IOOCXII). Crystallite size of the powders and grain size of Nd YAG ceramics calcined at different temperatures were calculaied by X-ray diffraction (XRD, model D/maxrA, using nickel-filtered Cu-Ka radiation) patterns from the Scherrer s equation. Microstructures of the fractured and the thermal etched mirror-polished surfaces of Nd YAG specimens were observed by scanning electron microscopy (SEM, Model S-4800). Densities of the samples were measured by the Archimedes draining method. 

Eslamian and Ashgriz [11,12] systematically investigated the effect of pressure on powder morphology and other powder characteristics. Particle shape and morphology depends on the precursor properties and precipitation mechanism, as well as on the droplet evaporation rate. Droplet evaporation rate is a function of the reactor pressure and temperature. Evaporation rate controls the solute distribution profile within the droplet, and determines whether the particles are solid or hollow. Eslamian and Ashgriz [11] have shown that, when the ambient pressure is reduced to 60 Toir, the decrease of the evaporation rate due to the non-cmitinuum effects is about 60% of that of the continuum-based evaporatiOTi rate. 

The most notable finding from studies of powder morphology is that the Ticona resins are characterized by a fine network of submicron-sized fibrils that intercormect the microscopic spheroids. The fibrils are illustrated in the scanning electron microscopy (SEM) micrograph in Figure 2.5 (provided courtesy of Rizwan Gul [1997]). 

Reaction-bonded titanium nitride (RBTN) ceramics are like RBSN made from a porous green shape of titanium powder that is reacted with nitrogen to titanium nitride (TiN) at temperatures up to 1000°C. Here the titanium hardly increases in molar volume when nitrided and the initial porosity remains the same but the gas permeability of a pressed titanium tablet is increased after it has been converted to titanium nitride. If the titanium powder particles are too large, the reaction stops after passivation of the metal surfaces the TiN formed at the surface is a diffusion barrier that stops the reaction. A fractal powder morphology of the starting metal (such as can be obtained from gas-phase preparation) is a very suitable reactant for complete reaction at modest temperatures. 

It has been reported that polymerization temperature has a considerable effect on particle break-up and, therefore, on the final particle morphology [108, 109]. Maneshi et al. [69] observed that higher polymerization temperatures, up to a certain upper limit, enhanced clay exfoliation. Above this upper limit (which varies depending on the polymer and solvent type) active site and/or polymer chain start being extracted from the clay surface, resulting in poor exfoliation, inadequate powder morphology, and severe reactor fouling. 

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