Solid-state decompositions

Formal theories of isothermal solid state decompositions... 

The process of calculation becomes more complicated on adding further terms. Coats and Redfem [555] effectively put (U-2)/U equal to a constant value and the relationship is equivalent to that already given for In g i/T2 from the single term expansion. They assumed that f(q) = (1 — q)" and determined n by testing values which have significance in solid state decomposition reactions (i.e. n = 0, 0.5, 0.67 and 1.00). Sharp [75,556] has shown that the approach may be applied to other functions of g(q). If it is assumed that the zero-order equation applied at low a, as q -> 0, then g(q) == a. 

The catalytic activity of doped nickel oxide on the solid state decomposition of CsN3 decreased [714] in the sequence NiO(l% Li) > NiO > NiO(l% Cr) > uncatalyzed reaction. While these results are in qualitative accordance with the assumption that the additive provided electron traps, further observations, showing that ZnO (an rc-type semi-conductor) inhibited the reaction and that CdO (also an rc-type semi-conductor) catalyzed the reaction, were not consistent with this explanation. It was noted, however, that both NiO and CdO could be reduced by the product caesium metal, whereas ZnO is not, and that the reaction with NiO yielded caesium oxide, which is identified as the active catalyst. Detailed kinetic data for these rate processes are not available but the pattern of behaviour described clearly demonstrates that the interface reactions were more complicated than had been anticipated. 

The solid-state decomposition of OV[OSi(O Bu)3]3 occurs with a precipitous weight loss at ca. 200 °C (as observed by TGA) and a final ceramic yield that is 10% less than the expected ceramic yield [79]. This discrepancy results from volatihzation and loss of HOSi(O Bu)3. However, solution thermolyses of OV[OSi(O Bu)3]3 in n-octane produce xerogels with an approximate composition of V2O5 6Si02 (after drying) with a quantitative ceramic yield (i.e., with no loss of HOSi(0 Bu)3) that have a BET surface area of 320 m g ... 

The stability of suspensions, emulsions, creams, and ointments is dealt with in other chapters. The unique characteristics of solid-state decomposition processes have been described in reviews by D. C. Monkhouse [79,80] and in the monograph on drug stability by J. T. Carstensen [81]. Baitalow et al. have applied an unconventional approach to the kinetic analysis of solid-state reactions [82], The recently published monograph on solid-state chemistry of drugs also treats this topic in great detail [83],... 

Palladium acetate, [PdO —02CCH3)2l3, possesses a unique quality that makes it attractive for solid state decomposition studies as well as technological applications. It can be spin-coated from solution to form a homogeneous, apparently amorphous solid film. This provides large uniform areas over which we can study the effects of various irradiation sources on the chemical nature of the film. The bulky structure of palladium acetate, shown in Figure 1 (8), may offer a partial explanation of the molecule s ability to achieve an amorphous metastable phase upon rapid evaporation of solvent. 

OH- HB. One can assnme that these dihydrogen bonds play a principal role in the formation of these dimer structures. Finally, the authors demonstrated that these dihydrogen bonds provide preservation of crystallinity during O-H H-B conversion into O-B bonds with H2 elimination. In fact, solid-state decomposition of NaBHt THEC and other systems leads to a crystalline covalent product by a crystal-to-crystal process. 

A discussion of the thermodynamics used in this book, which is essential for understanding the meaning of the tables, as well as a discussion of the kinetics of solid-state decomposition, is included in Chapter 1, which also includes a discussion of the sources of the data used. 

An endothermic solid-state decomposition proceeds as the product-reactant interface advances into the interior of the sample. The interface can, however, advance only if the necessary heat of reaction is applied to it. Since the source of heat is outside the sample, the rate of heat transfer to the interface may become rate-determining if the inherent rate of reaction exceeds the rate of heat transport. The rate of heat transport depends not only on the properties of the product through which heat must be transported, but also on the general experimental arrangement. It is for this reason that one expects and finds the literature to be conflicting. 

At lower temperatures (330 to 480°C) a much lower activation energy, 192 kJ, was observed for the initial 3% of the solid-state decomposition.84 This, and the observed pre-exponential factor, were interpreted in terms of the abstraction of 02 from two neighboring C104 ions, (C103=0=0=(C103). The theory is discussed in more detail under KC104. 

The initial stages of the solid-state decomposition have been studied by Cordes and Smith84 between 330 and 440°C. An Arrhenius plot was linear to 390°C. Above this temperature, the slope of the plot increased. This result, as well as the low preexponential factor, was accounted for by a detailed molecular model (discussed under KC104) in which 02 is abstracted from two neighboring perchlorate ions ... 

The growth of the inner layers L,- (i = 2, 3,. . . , N — 1) are first considered. By hypothesis, the cations are immobile for the present case, so the solid-state decomposition of layer i — 1 denoted by (dL j/dt). yields cations for layer i formation equivalent to an effective cation current of (1/iZ f5) (dL,- j /df). The product of this quantity with the new oxide volume of layer i which results from each cation pro-... 

The superscript (Oxv) is an abbreviation for oxygen vacancy. Because the newly forming layer i is oxygen-rich relative to the decomposing layer — 1, an additional number of oxygen anions are required in order to utilize all of the cations released in the solid-state decomposition of layer i — 1 at xt = 0 and in the presently considered case, these are provided by a new component of the anion vacancy current generated in layer i by this phase boundary reaction. Since layer i must carry, in addition, the anion vacancy currents generated by the phase boundary reactions at Xi = 0, x2 = 0,. . . , j = 0, we have the identity... 

Consider, also, that layer i will likewise undergo solid-state decomposition at the interface xt = L, in the formation of layer i + 1, yielding a negative contribution (cLL,/df) to the growth of layer i. The net growth rate of layer j will be given by the difference in the formation rate of layer i at xt = 0 and the decomposition rate of layer i at xt = L,... 

A broad endothermic trend begins at 733°C and continues until 879°C. This was accompanied by an accelerating weight loss, as indicated by the DTG peak starting at the same temperature. This trend corresponds in part to the solid state decomposition of CaCO,3(s) to CaO(g) and C02(s) as indicated by the decreasing CaCC>3 and increasing CaO relative XRD peak intensities in Figure 5.15. 

A solid state reaction is said to be topochemically controlled when the reactivity is controlled by the crystal structure rather than by the chemical nature of the constituents. The products obtained in many solid state decompositions are determined by topochemical factors, especially when the reaction occurs within the solid without the separation of a new phase [27, 28], In topotactic solid state reactions, the atomic arrangement... 

No dimerization of acetic anhydride has been observed in either die liquid or solid state. Decomposition, accelerated by heat and catalysts such as mineral acids, leads slowly to acetic acid (2). Acetic anhydride is soluble in many common solvents, including cold water. As much as 10.7 wt % of anhydride will dissolve in water. The unbuffered hydrolysis rate constant k at 20°C is 0.107 min 1 and at 40°C is 0.248 min-1. The corresponding activation energy is about 31.8 kj/inol (7.6 kcal/mol) (3). Aldiougli aqueous solutions are initially neutral to litmus, they show acid properties once hydrolysis appreciably progresses. Acetic anhydride ionizes to acetylium, CH CO+, and acetate, CH - CO, ions in the presence of salts or acids (4). Acetate ions promote anhydride hydrolysis. A summary of acetic anhydride s physical properties is given in Table 1. 

Vibrations of the crystal components may destabilise a lattice structure above a particular temperature, but this does not necessarily result in melting if an alternative phase is stable. Normally the form stable at the higher temperature, generated by a reversible recrystallisation, is less ordered. An example is the orthorhombic to cubic transformation of the alkali metal perchlorates [7] which is associated with increased rotational freedom of the perchlorate anion. Studies of phase transformations are of value in advancing understanding of solid state decompositions. 

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