Towards solving the crystal structure of polytype 3R2 Mg–Al layered double hydroxides

An analysis of the crystal structure of polytype 3R₂ layered double hydroxides has been carried out based on XRPD diffractograms. Measurement of the transmission-XRPD pattern carried out in a spinning and tilting capillary using synchrotron radiation has been shown to diminish the preferred orientation effect of the sample. In order to approach the structure of polytype 3R₂, structural models, based on possible interlayer anion arrangement as revealed by FT-IR, 27-Al solid state NMR and XRPD, were constructed. A structural model is proposed that is consistent with the by XRPD measured interlayer distance, in which tetrahedral aluminate ions are grafted with their apical oxygen ions onto the octahedral sheet. The presence of a measured extra reflection at 2θ value of 6.3, which is not predicted by this structure, can be obtained however by enlargement of the unit cell to a′ = a√3 = 0.5279 nm and c′ = c = 2.1985 nm, which simulates the aluminate anions ordering in the interlayer.     

Dissociation pressure and Gibbs energy of formation of Y3Fe5O12 and YFeO3

Compounds in the ternary system Y  Fe  O are of significant interest due to their magnetic properties. Electrochemical solid state galvanic cell techniques have been employed to study their thermodynamic properties in the temperature range from 900 to 1250 °C. The following oxygen dissociation pressures were obtained: log P_O2$\text{[}\text{Pa}\text{] = ?}\text{26365}\text{T}\text{+ 17.13}$ for yttrium iron garnet, Y₃Fe₅O₁₂, in the entire temperature range, log P_O2$\text{[}\text{Pa}\text{] = ?}\text{29345}\text{T}\text{+ 13.00}$ for perovskite, YFeO₃, below 1080 °C and log P_O2$\text{[}\text{Pa}\text{] = ?}\text{39375}\text{T}\text{+ 20.43}$ for YFeO₃ above 1080 °C. The standard Gibbs energies of formation from metallic iron, yttria (Y₂O₃) and oxygen were determined to be $\text{2}\text{G}{}^{\mathrm{o}}\text{= ? 2078.2 + 0.5850·}\text{T kJ}\text{/}\text{mol}$ for the formation of Y₃Fe₅O₁₂ and $\text{2}\text{G}{}^{\mathrm{o}}\text{= ? 421.3 + 0.1148·}\text{T kJ}\text{/}\text{mol}$ for the formation of YFeO₃. The standard enthalpies and entropies of formation are ? 2078.2 kJ/mol and ? 585.0 J/mol·K for Y₃Fe₅O₁₂ and ? 421.3 kJ/mol and ? 114.8 J/mol·K for YFeO₃, respectively.     

Fatigue crack growth in polyurethane foam

The growth of fatigue cracks in compact tension specimens of rigid polyurethane foam has been studied at room temperature under conditions of constant load-amplitude cycling. The growth of the cracks at the frequencies employed (0.2 Hz) is found to be reasonably reproducible and the growth rate can be related to the cyclic stress intensity range in the conventional way. The rate of growth of the cracks is also found to depend on the mean stress level and an attempt has been made to separate out the effects of stress-intensity range, K, and the maximum stress intensity in each cycle, K _max, by combining the data obtained under a variety of loading conditions.     

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