Deposits powdery

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Limiting currents measured for a deposition reaction may be excessively high due to surface roughness formation near the limiting current. Rough deposits in the case of copper deposition have been mentioned several times in previous sections, since this reaction is one commonly used in limiting-current measurements. However, many other metals form dendritic or powdery deposits under limiting-current conditions, for example, zinc (N lb) and silver. Processes of electrolytic metal powder formation have been reviewed by Ibl (12). 

Figure 6.12. Overpotential characteristic of transition from compact to powdery deposit in electrodeposition of Cu from CUSO4 (0.1 M) + H2SO4 (0.5 M) solutions. (From Ref. 22, with permission from Wiley.)...

Farinose Exudates of the Polypodlaceae and Prlmulaceae. Members of the Polypodlaceae produce a yellow or white powdery deposit on the lower surface of their fronds. These deposits are usually referred to as farinose exudates. Farina from species of Pltyrogramma, Chellanthes, Adlantum, and Notholaena have been carefully studied. The farinose coating of these plants Is formed by the terminal cell of small hairs usually found on the lower surface of the frond. 

Poly(bithiophene) films from these two ionic liquids are morphologically similar (Figure 7.14), even though the redox behavior (Figure 7.9) is markedly different, suggesting that the dominant differences in the films produced are on an atomic or sub-micron rather than macroscopic level. The morphology ofthe poly (bithiophene) films appears to be similar to that described by Roncali et al. [74] who reported a thin film on the surface of the electrode, covered by a thick brittle powdery deposit, from the galvanostatic polymerization of bithiophene in acetonitrile. The nodular structures are smaller in the poly (bithiophene) films than in the poly (thiophene), which is consistent with the formation of shorter chain polymers [73], but this does not... 

Sarac et al. [98] for poly(terthiophene) grown in acetonitrile, but without the large amount of powdery deposit observed in the poly(bithiophene) films (Figure 7.14(a, b)). Again, the film from the pyrrolidinium ionic liquid appears to be slightly more compact. 

In spring infected buds open late, the new leaves are covered with a white powdery deposit. They grow upwards at a steep angle and curl up from the edge on the top of the leaf, staying too narrow and becoming hard and brittle. 

Previous work has indicated that the physical state of the deposit can have a significant effect on the radiative properties, specifically molten deposits show higher emissivities/absorptivities than sintered or powdery deposits (1). Although thin, molten deposits are less troublesome from a heat transfer aspect than thick, sintered deposits, molten deposits are usually more difficult to remove and cause frozen deposits to collect in the lower reaches of the furnace where physical removal then becomes a problem for the wall blowers. 

Bis(4-methylphenyl)bismuth A -(4-methylphenyl)sulfonylamide 4 is obtained in moderate yield by the reaction of tris(4-methylphenyl)bismuth [A -(4-methylphenyl)sulfonyl]imide with anise alcohol or cinnamyl alcohol [96JCR(S)24]. This amide is insoluble in benzene, but soluble in chloroform, and not so sensitive to oxygen md moisture in the solid state. However, it slowly decomposes to tris(4-methylphenyl)bismuthine and a white powdery deposit when the amide is stood in CDCI3. 

When NbCls, methane, and ammonia were premixed prior to injection into the CVD reactor, (i.e., approach (i)), a parasitic side reaction occured which led to the formation of soot [69]. This soot formation was explained as the spontaneous precipitation of a niobium amido complex, NbC1.3(NH2)2 as shown in Eq. 2.5. This snowing effect critically limits this approach, yielding only powdery deposits. 

Hagevap LP (a mixture of sodium tripolyphosphate and lignin sulfonic acid derivatives). The concentration ratio of the sea water was not allowed to exceed 2 and therefore only the alkaline scales could form. As long as the maximum brine temperature did not exceed about 200° F., no evidence of scale deposition was obtained. At 210° to 215° F. there was definite indication of scale from the increase in terminal temperature difference (TTD). At the conclusion of a series of tests the inside surface of the tubes was examined and found to have a very thin layer of loose, powdery deposit (after drying) which rubbed off easily and would not be classed as scale. 

Chemical vapour deposition (CVD) processes have become a very important group of film-formation methods. Basically CVD is a material synthesis in which constituents of the vapour phase react chemically to form thin solid films as a solid-phase reaction product which condenses on the substrate. The reaction should take place very near to or on the substrate surface (heterogeneous reactions) and not in the gas phase to avoid powdery deposits. Activation of the reaction can be performed by various means such as the application of heat, high-frequency electrical fields, light or X-ray radiation, electric arc, electron bombardment or catalytic action of the substrate surface. A marked influence of the process parameters such as sub-... 

Chocolate (Figure 2.20) consists of a continuous phase of cocoa butter, substantially crystalline at room temperature and comprising 30 to 35% of the whole, in which sugar and cocoa solids are suspended. Milk chocolate also contains milk fat and solids. Lipids in chocolate have great importance in bloom formation. Bloom is a defect in the surface appearance of chocolate that appears as a white, powdery deposit. It is known to be associated with changes in the polymorphic forms of cocoa butter. 

To produce polymer wood, wood is degassed and then loaded according to wood type with 35%-95% monomer. The monomer is then converted by polycondensation or addition polymerization to polymer. For polycondensation, monomers that do not eliminate volatile components during polyreaction are, of course, preferred. Ring-shaped monomers as well as monomers with carbon-carbon double bonds can be polymerized. In the latter case, polymerization can be induced by 7-rays, peroxides, redox systems, etc. Not all monomers, however, are suitable for the preparation of polymer wood. For example, acrylonitrile is not soluble in its own monomer. In wood, therefore, the precipitation polymerization leads to powdery deposits and not to a continuous phase. The same problem occurs with vinyl chloride, and in this case, the boiling point of the monomer is too low. Poly (vinyl acetate) has a glass transition temperature which is too low. In addition, monomers with G values (see Chapter 12) which are too low require high 7-ray doses to induce polymerization. Copolymers of styrene and acrylonitrile, poly (methyl methacrylate), and unsaturated polyesters are used commercially. 

Electropolymerization produced polymers 53a-d as dark blue-green to blue-black films. All films were completely insoluble, but showed relatively high stability under normal conditions. Although the longer side chains of polymers 53c and 53d did not produce soluble materials, their inclusion did lead to smoother polymer films than the powdery deposits of 53a and 53b [76]. Both the CV and optical spectra of 53a gave an Eg of 1.0 eV [75,76], considerably lower than the analogous isothianaphthene-hased polymer 48a (1.58-1.77 eV) [63-66]. The incorporation of 2,3-dialkylthieno[3,4-h]pyrazine units in polymers 53h-d led to a slight increase in of less than 0.3 eV. 

In Fig. 9.15 are shown SEM images of silver produced with an immersion of niobium substrates into highly alkaline Ag(I) solutions above 90 °C. Agglomerated silver particles are clearly visible from these images. Deposition of silver onto niobium surface via galvanic displacement shows that unavoidably present oxide film at the surface of the substrate can be successfully removed. A production of powdery deposits suggests that oxide film is unevenly removed from the niobium surface. 

What causes the surface roughening and formation of powdery deposits According to the observations from the practice or from the experiments, the appearance and surface morphology of metals and alloys produced diuing autocatalytic deposition is significantly influenced by the rate of the reduction of metal ions. These observations are briefly summarized as follows ... 

Nickel-base alloys (Inconel) Generally has good corrosion-resistant qualities. Sometimes susceptible to pitting Green powdery deposit... 

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