Polymorphic inversion

The sequence of the polymorphic inversions can be represented by the following simplified chart ... 

At room temperature, pure Zr02 is monoclinic and has a density of 5,7 gcm". When heated it undergoes a quite complex sequence of polymorphic inversions which have not yet been explained to general satisfaction. Ruh and Rockett (1970) attempted to summarize the available data in the phase diagram shown in Fig. 9. Some properties of the modifications are listed in Table 3. The monoclinic modification (baddeleyite) is stable at room temperature and under normal pressures, and is con-... 

In this way, it is possible to explain the low thermal expansion of silica refractories in the medium temperature range and a considerable expansion at low temperatures. This i.s also why silica differs considerably in thermal shock resistance in these temperature ranges. For this type of refractory, it is thus inadequate to specify the usual value of mean thermal expansion coefficient for a certain temperature range (e.g. 20—1000 C). The polymorphic inversion also results in an expansion anomaly of stabilized Z1O2 and of fusion-cast corundum-baddeleyite refractories around 1000 C (cf also Fig. 201). 

Efforts aimed at lowering the firing temperatures are motivated not only by economy but also by the quality of products (a reduced tendency to metal deformation and enamel peeling). The deformation of metal and creation of stresses in enamel is related to the polymorphic inversion of a-Fe (ferrite) to y-Fe (austenite) which takes place at 900 °C in pure iron the inversion temperature decreases with increasing carbon content down to 720 °C at 0.8% C. The aim is to fire the products below the temperature of this inversion. One of the ways is provided by use of low-carbon steels, the other by employing enamels with a reduced firing temperature. 

The initial mix for the manufacture of rutile ceramics usually contains about 90 % T1O2 in the form of anatase or rutile. Anatase is converted above 900 °C to rutile, which is then stable over the entire temperature range. Only rutile is therefore present in a sintered product. The polymorphic inversion involves a considerable volume contraction, since the density of anatase is 3.9 while that of rutile is 4.25 g/cm. This is why cracks occur on firing in anatase-containing bodies. This drawback can be eliminated by pre-calcining at least some of the Ti02 at temperatures exceeding 1000 C, to convert it to rutile. 

The principal properties of silica refractories are listed in Table 33. The porosity is relatively high, up to 30% with coke-oven silica. Attainment of values lower than those given in the table is rendered difficult by the volume changes involved in polymorphic inversions. Silica is typical by its very low content of closed pores, only 0.1 to 0.5%, whereas with fireclay their content amounts to 2 — 6%. There is no significant difference between the total porosities of standard fireclay and silica refractories however, substantially lower porosities can be attained with special grades of fireclay (less than 10%). 

Three polymorphic inversions (3m, 4k and 4t) are common to all 17 demes sampled, that is, they occur within both of the leg-bristle regions and may thus be judged as the oldest of the inversions. Two of these (3m and 4k ) show very high negative correlations with altitude, forming striking altitudinal dines in a number of places on the island the frequencies of these two variants are closely correlated. 

Knibb WR, East PD, Barker JSF (1987) Polymorphic Inversion and Esterase loci complex on chromosome 2 of Drosophila huzzatii I. Linkage disequilibria. Aust J Biol Sci 40 257-269... 

Of course, freezing of a liquid - or its inverse - are themselves phase transformations, but the scientific study of freezing and melting was not developed until well into the 20th century (Section 9.1.1). Polymorphism also links with metastability thus aragonite, one polymorphic form of calcium carbonate, is under most circumstances metastable to the more familiar form, calcite. 

N,A -bis(2-phyidylmethyl)pyrazine-2,3-dicarboxamide was crystallized in two orthorhombic forms, which differ from each other in the conformational arrangements of the compound [44], A new polymorph of 2-bromo-5-hydroxybenzaldehyde has been reported where a pair of hydrogen bonds linked molecules related by a center of inversion [45], One of the intermediates important in the synthesis of an antituberculosis drug, methyl p-aminobenzoate, was found to crystallize in a new monoclinic polymorph where molecules were arranged in head-to-tail linear ribbon arrays [46]. 

Fig. 13 Determination of 31P Tx values for fosinopril sodium using the inversion-recovery method. The spectra represent approximately a 50/50% w/w mixture of polymorphic forms A (8 = 52 ppm) and B (8 = 55 ppm). Spinning sidebands are represented by asterisks. 

Modified from Van Cappellen, 1991). The particle size, j, is critical for the inversion from one polymorphous form to another. 

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