Solid solid-state reactions

The amazing evolution of solid state physics and chemistry over the last 30 years induced an intensive study of various solid state processes, particularly in the context of materials science. Materials have always been an important feature of civilization and are the basis of our modern technical society. Their preparation is often due solely to reactions between solids. Solid state reactions are also often responsible for the materials adaptation to a specific technical purpose, or for the degradation of a material. 

Many important phenomena in solid state ionics, such as ionic conduction, gas permeation through dense solids, solid state reactions, high temperature corrosion, or sintering of polycrystals, involve mobile ionic charge carriers. In most crystalline... 

This section gives an overview of the sort of chemistry that is observed in or on solids. Solid state reactions are those chemical reactions in which at least one solid product is formed from at least one solid reactant. This definition excludes processes for synthesis of solids from nonsolid reactants. If solids are made from gases and liquids as in chemical vapor deposition or in sol-gel reactions, it is not strictly speaking solid state chemistry. Defect chemistry is a central subject in solid state chemistry, and the physical chemistry of defects is discussed in Chapter 10. 

A deeper insight into electrochemical reaction mechanisms is possible by electrochemical studies employing solid electrolyte instead of liquid electrolyte With a solid electrolyte having preponderantly only one mobile ionic species electrode polarization can be studied under thermodynamically well-defined conditions without superimposed side effects by solvents and without the complications created by the presence of hydrated films or hydrolytic layers. Such measurements can be used, for instance, for the study of electrodeposition, formation of monolayers or of dendrites due to nucleation, for the study of polarization phenomena in ionic solids, solid-state reaction kinetics, transport phenomena, thermodynamics or constitutional diagrams, and for the development of practical devices. 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

Copper(II) oxide, CuO. Black solid formed by heating Cu(OH)2, Cu(N03)2, etc. Dissolves in acid to Cu(II) salts, decomposes to CU2O at 800 C. Forms cuprates in solid state reactions. A cuprate(III), KCUO2, is also known. 

Plumbaies IV), e.g. ICjPbOj, are formed by solid state reactions and plunibates containing [Pb(OH)6] " ions are formed from aqueous solution. 

Manganaies IV), manganites. Mixed-metal oxides containing Mn(IV). Prepared by solid state reactions. 

The essentially non-destmetive nature of Rutherford backscattering spectrometry, combmed with the its ability to provide botli compositional and depth mfomiation, makes it an ideal analysis tool to study thm-film, solid-state reactions. In particular, the non-destmetive nature allows one to perfomi in situ RBS, thereby characterizing both the composition and thickness of fomied layers, without damaging the sample. Since only about two minutes of irradiation is needed to acquire a Rutherford backscattering spectmm, this may be done continuously to provide a real-time analysis of the reaction [6]. 

The application of RBS is mostly limited to materials applications, where concentrations of elements are fairly high. RBS is specifically well suited to the study of thin film stmctures. The NMP is usefiil in studying lateral inliomogeneities in these layers [30] as, for example, in cases where the solid state reaction of elements in the surface layers occur at specific locations on the surfaces. Other aspects, such as lateral diffusion, can also be studied in tluee-dimensions. 

Theron C C 1997 In situ, real-time characterization of solid-state reaction in thin films PhD Thesis University of Stellenbosch... 

Karge FI G 1997 Post-synthesis modification of microporous materials by solid-state reactions Stud. Surf. Sol. Catal. 105 1901-48... 

It is noteworthy that it is the lower cross-over temperature T 2 that is usually measured. The above simple analysis shows that this temperature is determined by the intermolecular vibration frequencies rather than by the properties of the gas-phase reaction complex or by the static barrier. It is not surprising then, that in most solid state reactions the observed value of T 2 is of order of the Debye temperature of the crystal. Although the result (2.77a) has been obtained in the approximation < ojo, the leading exponential term turns out to be exact for arbitrary cu [Benderskii et al. 1990, 1991a]. It is instructive to compare (2.77a) with (2.27) and see that friction slows tunneling down, while the q mode promotes it. 

Since only the vibrational degrees of freedom take part in a solid-state reaction, the sole reason for this change may be the increase in their frequencies in the transition state... 

H. Schmalzried, Solid State Reactions, (Verlag Chemie, Basel 1981). 

The corrosion rate is greatest close to the reversible Pb02/PbS04 potential as a result of a solid state reaction between PbOj and the underlying lead surface". This corresponds to the rest or open circuit condition. 

Fej04 . A similar correspondence between theory and practice has been found for growth of Fej04 by the solid state reactions from FeO and Fe, , between 600 and 1 200°C. The growth rate of FeO is within 10% of the theoretical rate expected from Fe lattice diffusion, calculated according to the Wagner theory . 

The Pb02/PbOx border slowly penetrates into the metal, but only at a very slow rate as a solid-state reaction. Cracks are formed when the oxide layer exceeds a given thickness, on account of the growth in volume when lead becomes converted into lead dioxide (Table 7). Underneath the cracks the corrosion process starts again and again. As a whole, the corrosion proceeds at a fairly constant rate. It never comes to a standstill, and a continually flowing anodic current, the corrosion current is required to re-establish the corrosion layer. 

The third aspect, the stability range of solid electrolytes, is of special concern for alkaline-ion conductors since only a few compounds show thermodynamic stability with, e.g., elemental lithium. Designing solid electrolytes by considering thermodynamic stability did lead to very interesting compounds and the discovery of promising new solid electrolytes such as the lithium nitride halides [27]. However, since solid-state reactions may proceed very slowly at low temperature, metasta-... 

Both of these mechanistic representations have been widely applied in interpretations of observations on solid state reactions and there is ample experimental evidence for their existence in most, but not necessarily all, systems. 

Classification of solid state reactions according to Boldyrev [100] Initial step Reaction mechanism Examples... 

Magnetic resonance techniques, EPR (ESR) and NMR, can be used [341,342] to obtain information about atomic, ionic, molecular and crystallographic states before, during and after solid state reactions. Only a very restricted use has been made of the NMR of solids [342—345]. 

In solid state reactions, the rate of nucleation may be given by either of the expressions dN/dt = const, or dN/dt = t° const. For both expressions, the probability (pdf) is proportional to the total volume of the spherical layers at the instant t at the peripheries of nuclei which originated at time r. The radii of the spheres at the inner and outer boundaries of these layers are... 

It is appropriate to terminate this section, concerned with the formal theories of solid state reactions, with a table summarizing those a—time... 

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