Reaction solid-state displacement

The solid state displacement reaction method and wet chemical precipitation method were employed for synthesizing the ceria powders, and thus the ceria properties showed different features in several experiments. Figure 15.10 shows the morphology of the ceria particles observed with high-resolution scanning electron microscopy (SEM S900, Hitachi, Japan) and transmission electron microscopy (TEM JEM-2010, JEOL, Japan). In the figure, the ceria particles have a polyhedral shape. Both of the powders have nearly the same size. The primary particle size is approximately 40 mn. However, the difference in crystal shape of the ceria particles was found on TEM analysis. 

Copolymer formation by displacement reaction Solid-state chlorination of PP may lead to low levels of chlorine incorporation along the PP chain with minimal PP molecular weight degradation. Coran and Patel [1983 b] have blended (internal mixer, T = 190°C) 50 parts chlorinated PP with 45 parts NBR containing 5 parts amine-terminated NBR. The blend was characterized by mechanical properties vs. the use of unfunctionalized PP. Compatibilization resulted from copolymer formation through displacement reaction of chloride by amine groups. The blend was prepared in the presence of a vulcanization agent (3.75% of dimethylol phenol + 0.5% of SnCl ) to cause concomitant dynamic vulcanization of the rubber phase. 

There are reports on the use of solid state displacement reactions, too. The joints were made from Tic and Si powder processed at 1400 C, and their shear strength at room temperature reached 50 MPa Nevertheless, these joints were weak compared to SiC/SiC composites, so strength improvement is required for their applications to gas/liquid cooled reactor systems. Additionally, irradiation behavior of these joints will also require to be studied... 

In the samples without excess silicon, Ti3SiC2 is assumed to form via a direct solid state displacement reaction. 

According to Bayern and Mocellin, a solid state displacement reaction takes place in the Zr02-A1N system at the temperature studied ... 

Electron donor groups (EDG) on the aromatic ring favour its displacement to the right while electron withdrawing group (EWG) favours its displacement to the left. Selenobenzophenone monomer in solution is isolated as a dimer in the solid state. Dimerization of the stable 12 to the 1,3-ditelluretane was observed in the solid state, in solution the dimer reforms 12.25 26 Contrary to the reaction in the solid state dimerization does not take place in solution. 

Figure 10. Kleitz s reaction pathway model for solid-state gas-diffusion electrodes. Traditionally, losses in reversible work at an electrochemical interface can be described as a series of contiguous drops in electrical state along a current pathway, for example. A—E—B. However, if charge transfer at point E is limited by the availability of a neutral electroactive intermediate (in this case ad (b) sorbed oxygen at the interface), a thermodynamic (Nernstian) step in electrical state [d/j) develops, related to the displacement in concentration of that intermediate from equilibrium. In this way it is possible for irreversibilities along a current-independent pathway (in this case formation and transport of electroactive oxygen) to manifest themselves as electrical resistance. This type of chemical valve , as Kleitz calls it, may also involve a significant reservoir of intermediates that appears as a capacitance in transient measurements such as impedance. Portions of this image are adapted from ref 46. (Adapted with permission from ref 46. Copyright 1993 Rise National Laboratory, Denmark.)...

In the crystal structure of the polymer phase (Fig. 17a), the polymer chains are aligned along the c-axis and the distance (3.71 A) between the centres of adjacent cyclobutane and pyrazine rings corresponds to half the c-axis repeat of the unit cell. For comparison between the monomer and polymer structures, an overlay plot of these structures is shown in Fig. 17b. It is clear that the solid-state reaction is associated with only very small atomic displacements at the site of the [2-1-2] photocyclization reaction (the displacement of the carbon atoms of the C=C double bonds of monomer molecules on forming the cyclobutane ring of the polymer is only ca. 0.8 A for one pair of carbon atoms and ca. 1.6 A for the other pair). Such small displacements are completely in accord with the assignment of this solid-state reaction as a topochemical transformation [124—127] (in which the crystal structure of the reactant monomer phase imposes geometric control on the pathway of the... 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is believed to occur by surface nucleation and ledge movement by face specific reactions (37). The solid-state transformation from one polytype to another is believed to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

Displacement reactions are quite common in solid state chemistry. The classical reactions of this type were already investigated many years ago by Tammann as powder reactions (e.g., PbS+CdO = PbO + CdS or ZnS+CdO = ZnO+CdS [G. Tammann (1925)]). Systematic kinetic studies in this field of research are scarce because the complex morphology adds to all the other complications of solid state... 

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