Theoretical infrared spectra of OH defects in corundum (<i>α</i>-Al₂O₃)

The atomic-scale structure, relative stability and infrared spectroscopic properties of OH defects in corundum (α-Al2O3) are theoretically investigated at the density functional theory level. Comparison with experimental data makes it possible to assign most of the narrow bands observed between 3150 and 3400 cm−1 in natural and Ti- or V-doped synthetic corundum to specific defects. These defects correspond to the association of one OH group with an Al vacancy and M4+ for Al3+ substitutions in neighboring sites. The OH group is located in the large oxygen triangle forming the base of the vacant Al site. Models of interstitial proton associated with a nearby Mg2+ for Al3+ substitution are consistent with the broad band observed at 3010 cm−1 in Mg-doped corundum. Its is also suggested that two weaker OH-stretching bands observed in nominally pure synthetic corundum at 3163 and 3209 cm−1 could be associated with intrinsic defects combining an Al and an O vacancy. These results highlight the importance of defect clustering in the high-temperature incorporation of hydrogen in nominally anhydrous minerals.

Selection of Synthesis Corundum Grain in Grinding Flat Parts from Hardened Steel 30ChGSA by the Macrogeometry Criterion

The stochastic nature of grinding predetermines the necessity of attracting statistical methods to predict output parameters. In this research the form accuracy indices of flat workpieces EFEmax (the main parameter) and two additional indices EFEa and EFEq called the arithmetic average and the quadratic average, respectively are used (GOST 24642-81). In the conditions of violating normality and homoscedasticity of distributions the selection of the grain in the Norton high porous wheels (HPW) made of synthetic corundum 5SG (46, 60) K12VXP is made with an emphasis on a non-parametric method, wherein medians and quartile latitudes are used as one-dimensional frequency distributions. This paper states that grinding by the coarse-grained HPW 5SG46 in comparison with the small-grained 5SG60 made it possible to increase the form accuracy by the predicted medians within the same accuracy quality class: for EFEmax – TFE7; for EFEa and EFEq – TFE6. The precision of the form accuracy of the parts 30ChGSA increased most significantly: for EFEmax it increased by 5 times, for EFEa by 3.1 times, and for EFEq by 4.2 times. The fact that the heights of microasperity remained practically at the same level , μm; , μm was due to the features of the synthetic corundum grain structure. In this study the EFEmax parameter is used for selecting the wheel grain. The auxiliary parameters of form accuracy are used in calculating the correction indexes of basic models in the multivariate dispersion analysis because they have more information about the status of the part surface.