Reactivity of Solids最新文献

The present paper is an extended abstract which should be considered as an introduction to the papers already published by us on this new topic. An extended synthesis of these papers was given in the lecture. The original information introduced here is the figure. It is a P_H2O vs. T diagram which gives the pre-reactional domain between the stability domains of calcium hydroxide and calcium oxide. Chemical reactions are proposed as an interpretation of the observed pre-reactional transformations.

In the introductory section the systematics of equilibrium and non-equilibrium interfaces which can be stationary or moving are pointed out. The basic thermodynamic features of interfaces, particularly with respect to its electrical structure, is briefly treated in section II. The main emphasis is laid on the kinetics of resting (stationary) and of moving interfaces in section III. The similarities with electrodes in electrochemistry are pointed out for ionic crystals. Special models are discussed which are constructed to explain the interface kinetics in heterogeneous solid-state reactions, e.g. cation reconstructive rearrangement in close-packed oxygen anion sublattices. Point defect fluxes to and from the interface and defect relaxation processes in the interface region may also influence the kinetics of the heterogeneous reactions decisively. Section IV is devoted to the discussion of the moving boundary in phase transformation, which in a sense is the most simple heterogeneous solid-state reaction. Silver chalcogenides serve as experimental examples. Finally, the morphological stability of moving interfaces is studied experimentally and theoretically on a (phenomenological) macroscopic and a submicroscopic (atomistic) scale in section V.

Solid-state amorphization reactions resulting from the severe mechanical deformation obtained by high-energy ball milling are reviewed. Two classes of such reactions are discussed: 1. “mechanical alloying” where material transfer occurs between elemental powders or alloys, and 2. “mechanical milling” where an equilibrium crystalline intermediate phase is transformed to the amorphous structure by milling. The thermodynamic and kinetic criteria for the crystalline-to-amorphous transition are outlined. Recent experimental results from the author's laboratory on the amorphization of intermediate phases by milling are presented. The implications of these results for mechanisms of the crystalline-to-amorphous transformation are discussed.

The usual procedure of evaluating kinetic parameters from thermogravimetric data is revised. It is shown that correct results can be obtained in the case of affinity of α(t) curves, because this type of kinetic energy of activation characterizes the process of reaction interface propagation (for heterogeneous reactions). A method of kinetic investigation based on the formation of a special reaction front with a fixed and constant-in-time reaction interface area is proposed. The connection of the observed kinetic parameters with the mechanism of interface propagation is considered within the scope of the model of a self-propagation reaction front.

The role of feed-back in solid-state reactions is discussed. Various types of feed-back loops are considered. Problems arising in studying feed-back in solid-state reactions and the means of their solution are illustrated by suitable examples.

A number of recent mechanistic studies of thermal decompositions of solids are discussed here with emphasis on the chemistry of the reactions involved. It is argued that greater insight into the sequence of steps participating can be achieved when kinetic observations are complemented with microscopic examinations and chemical analyses of the partially reacted salt as well as the final products. Comparisons with comparable reactions can also be valuable in providing insight into the mechanisms of solid state reactions.

A classification of the roles of nuclei is discussed with reference to a number of selected rate processes. Three types of nuclei are distinguished, these are: functional nuclei, the solid product is a catalyst for the changes proceeding at the reaction interface, fusion nuclei, chemical reaction proceeds preferentially in a molten zone which may be localized and temporary, and fluid-flux nuclei, reaction proceeds in a zone of fluid, condensed product temporarily retained within the nucleus.

It is argued that complementary measurements considered together, (kinetics, microscopy and analyses) provide greater insight into the chemistry of reactions of solids than is usually possible from studies using a less comprehensive experimental approach.

Current knowledge of topochemical reactions and their characteristic features is reviewed. Questions referring to the refinement of terminology and of the definition of topochemical processes are discussed.

Some problems connected with the variation of reactivity depending on the state of matter, the state of aggregation of a substance, the state of division of a solid, and the gradation of activity of crystal faces of a single crystal are discussed. On the basis of the results obtained, the possibility of synthesizing new triphosphate compounds is demonstrated.

Semiconductor particles composed of mixed CdS and CdS-ZnS were incorporated into an interlayer of hydrotalcite by chemical reaction between Cd(edta)²⁻ and S²⁻ in the interlayer. The incorporated particles seemed to be very small, less than 0.4 nm thick. The band gap energies of CdS and sequentially precipitated CdS followed by ZnS in the interlayer were slightly larger than that of normal-crystalline CdS. On the other hand, the band gap energy of CdS-ZnS mixture coprecipitated in the interlayer was almost equal to the average value of those of normal-crystalline CdS and ZnS. The CdS and CdS-ZnS mixture incorporated into hydrotalcite were capable of efficient hydrogen evolution following irradiation with visible light in the presence of Na₂S and/or Na₂SO₃ as a sacrificial donor. The hydrogen production activities of the catalyst incorporated in hydrotalcite were in the order of sequentially precipitated CdS followed by ZnS > simultaneously precipitated CdS-ZnS mixture ⪢ CdS. The difficulty of mass transfer of the hydrogen evolved in response to visible light through the interlayer restricted the efficiency of the semiconductor incorporated into hydrotalcite. Almost equal amounts of S₂O₆²⁻ and SO₄²⁻ were formed by the photochemical oxidation of SO₃²⁻ in aqueous solution catalysed by unsupported CdS/ZnS, but the amount of S₂O₆²⁻ produced in the same reaction with CdS/ZnS incorporated into hydrotalcite was significantly less.