Particles copper

Extmded or roUed lead—copper alloys contain a uniform dispersion of copper particles in a lead matrix. Because the soHd solubiUty of copper in lead is very low, copper particles in the matrix remain stable up to near the melting point of lead, maintaining uniform grain size even at elevated temperature. 

Copper-containing lead alloys undergo less corrosion in sulfuric acid or sulfate solutions than pure lead or other lead alloys. The uniformly dispersed copper particles give rise to local cells in which lead forms the anode and copper forms the cathode. Through this anodic corrosion of the lead, an insoluble film of lead sulfate forms on the surface of the lead, passivating it and preventing further corrosion. The film, if damaged, rapidly reforms. 

He studied the sintering of copper particles in the diameter range 15-100 microns and of silver particles of diameter 350 microns. The results for the larger volume fraction of copper and for silver were shown to fit the volume diffusion mechanism and yielded the results for volume self-diffusion... 

Ashby pointed out diat die sintering studies of copper particles of radius 3-15 microns showed clearly the effects of surface diffusion, and die activation energy for surface diffusion is close to the activation energy for volume diffusion, and hence it is not necessarily the volume diffusion process which predominates as a sintering mechanism at temperatures less than 800°C. 

An example where one metal melts before the densihcation process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to dre Kirkendall effect. The compact is therefore seen to swell before the hnal sintering temperature of 1080 K is reached. After a period of homogenization dictated by tire criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

Some metals are soluble as atomic species in molten silicates, the most quantitative studies having been made with Ca0-Si02-Al203(37, 26, 27 mole per cent respectively). The results at 1800 K gave solubilities of 0.055, 0.16, 0.001 and 0.101 for the pure metals Cu, Ag, Au and Pb. When these metal solubilities were compared for metal alloys which produced 1 mm Hg pressure of each of these elements at this temperature, it was found drat the solubility decreases as the atomic radius increases, i.e. when die difference in vapour pressure of die pure metals is removed by alloy formation. If the solution was subjected to a temperature cycle of about 20 K around the control temperamre, the copper solution precipitated copper particles which grew with time. Thus the liquid metal drops, once precipitated, remained stable thereafter. 

Figure 9.7. Melallographic cross-section through a group of 3 copper particles sintered at 1300 K for 8 h. The necks arc occupied by grain boundaries (after Exner and Arzt 1996).

OT-nitrobenzenesulphonic acid to metanilic acid has been run. A cathode of copper particles was used in a 1000 A unit and the overall performance of the electrode was satisfactory (Smith, 1969). 

Copper clusters containing two to four atoms have been formed (94) in argon and methane, whereas large, colloidal-copper particles resulted in dodecane matrices (94). The authors suggested that the "birth of the band structure of copper is clearly visible on passing from the dimer to the tetramer, with CU4 already possessing many of the features of the bulk metal (94). 

This study relates to a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase. The water-alcohol method provided more dispersed catalysts than the impregnation method. The Cu particles of about 20 nm showed enhanced stability and higher activity than the particles larger than 40 nm. This was correlated with the distribution of copper particle sizes shown by XRD and TEM. Compared with silver and zinc, copper is better active and stable metal. 

As can be seen in table 1, with different preparation methods and active metals, the average size of the copper particle for the catalysts A and D were 20.3 nm and 50.0 nm. While those of the catalysts B and C were 51.3 nm and 45.4 run, respectively. CuO, non-supported metal oxide, made by impregnation is sintered and cluster whose particle size was 30 pm. The water-alcohol method provided more dispersed catalysts than the impregnation method. 

This strongly suggests that the synthesis takes place on the copper particles in the catalyst and not on the ZnO, vhich serves merely as the support. 

The dynamics of particle morphology can be used to an advantage, to counteract the effect of sintering ofthe copper particles. As Fig. 8.13 shows, a Cu/ZnO catalyst slowly loses activity, which is attributed to sintering. Exposing the catalyst for a short time to a highly reducing mixture of C02-free synthesis gas restores the activity. 

Figure 8.13. Rate of methanol synthesis of a Cu/Zn0/Al203 catalyst in a plug flow reactor as a function of time on stream. The catalyst was operated at 494 K and 63 bar in a gas steam of 5 % CO, 5 % COj, 88% H2, and 2% N2. Note the steady decrease in reactivity, which is ascribed to sintering ofthe copper particles. The CO2 was removed from the reactants for 4 h after 168 h. After reintroduction the catalyst displays a restored...

ZnO is, apparently, a very suitable support for the copper particles. Evidence exists, however, that its role does not have to be limited to that of a support only. Nakamura et al. have studied the influence of Zn on methanol synthesis on copper crystals by depositing Zn on the surface [J. Nakamura, I. Nakamura, T. Uchijima, Y. Kanai, T. Watanabe, M. Saito, and T. Fujitani, J. Catal. 160 (1996) 65]. They found that the rate was enhanced by a factor of six (see Fig. 8.14), suggesting that Zn atoms also act as a chemical promoter. Whether some of the ZnO in the real catalyst is actually reduced to such a degree that it can alloy into the copper particles and segregate to the surface, as suggested by Nakamura, is still a controversial topic. 

From these examples we may conclude that, as was indicated above, the question of having or not having a quantum confinement in a distinct particle allows different answers. All we may notice in this case is that gold, silver or copper particles of a distinct size must possess confined electron gases, but nanoparticles being too small to show a plasmon resonance cannot be excluded as having no confined electrons. On the contrary, as will be shown later by means of the Auss cluster. 

Pileni MP, Lisiecki I (1993) Nanometer metallic copper particle synthesis in reverse micelles. Colloids Surf A 80 63-68... 

The behavior of Cu(II)(aq) is relatively more understood than other metal ions. Haas and Gedanken [74] found only a partial reduction of Cu2+ ions to Cu+ (95%) instead of metallic copper (5.1%) in the presence of cetyltrimethylammonium bromide in an ultrasonic field and thus obtained CuBr particles instead of Cu. Nevertheless, when polymers such as poly(N-vinyl 2-pyrrolidone) or poly(vinyl alcohol) were used, the end product was metallic copper particles, as expected. They have proposed the reduction of Cu2+ ions to copper as the first stage, however, in the second stage Cu reacted with OH radicals or H2O2, formed by sonolysis of water to produce Cu+ and OH- ions as under ... 

Data of the chemisorption of hydrogen on small copper particles were obtained by Ward (Proc Roy Soc London A133 506, 1031) in the units P, Torr, and V in cc/g STP. The plots show the Langmuir to be a better fit than the Freundlich. 

Figure 1.7. Shapes of solidified droplets (particles) generated in powder production and spray forming processes, (a) Spherical shape gas-atomized gold alloy particles (b) near-spherical and dendritic shapes water-atomized bronze particles (c) irregular and porous (spongiform) shapes water-atomized zinc particles (d) irregular aggregates water-atomized copper particles (Cour. tesy of Atomizing Systems Ltd., UK.)...

Homogeneous, nanosized, copper-loaded anatase titania was synthesized by an improved sol-gel method [197], These titania composite photocatalysts were applied to the photoreduction of carbon dioxide to evaluate their photocatalytic performance. Methanol was found to be the primary hydrocarbon product [198], Under calcination conditions, small copper particles are well dispersed on the surface of anatase titania. According to XAS and XPS analysis, the oxidation state of Cu(I) was suggested to be the active species for C02 photoreduction [199], Higher copper dispersion and smaller copper particles on the titania surface are responsible for a great improvement in the performance of C02 photoreduction. 

This cycle uses solid reactants. Small dendritic copper particles are used to carry out the last reaction to make the transformation of all the solid copper to CuCl, thereby maximizing hydrogen yield. The reported efficiency of this cycle is 49% [66]. This low temperature cycle is believed to eliminate many of the engineering and materials issues associated with the other two previously discussed cycles, however this cycle is also in the initial stages of development [111]. The temperature ranges are such that lower temperature nuclear reactors, e.g. sodium-cooled fast reactors, could be used with this cycle [69]. A hybrid version of this cycle is under investigation in Argonne National Laboratory [66,112]. 

The above redox behaivor of copper ions in zeolites is very distinct from those on other supports or in aqueous solution and is a specific phenomenon observed only on the zeolite. Die copper (I) ion in zeolites is fairly stable, as mentioned above, whereas the copper(II) ion supported on silica gel is readily and directly reduced to copperfO). The difficulty of generating Cu in zeolites may prevent the formation of copper particles. This may be the reason why we need a zeolite framework structure and why silica gel is a poor support for reacti(Mi.26... 

The difference in the shape modification characteristics owing to the particle diameter was not recognized, as is the case with copper particles. 

Figure 13.3.10 shows the typical spheroidal deformation of stainless steel particles with treatment time. As compared with the result of copper particles, the rate of spheroidal deformation was lower. Thus, the stainless steel particles were easier for stepwise adjustment of the shape index with treatment time. 

Highly ordered, perfectly aligned copper particles... 

The microscopy results characterizing the Cu/ZnO catalyst are in accord with EXAFS data representing the dynamic morphology changes (39—41), and they also provide an important additional insight On the basis of the lattice-resolved images, the nature of the exposed facets of the projected copper nanoclusters and the epitaxial relationship between the copper and ZnO can be identified. The majority of the copper nanocrystals appear to be in contact with the ZnO support with their (111) facets, as was also observed for copper particles prepared by vapor... 

Fig. 11. Relationship between the apparent first-shell coordination number determined from standard EXAFS analysis and the true coordination number derived from the structures obtained from molecular dynamics simulations for copper particles [adapted from Clausen et al. (34)].

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