The compound Cr2TiO5 in the system Cr2O3TiO2

On the Microstructure and Isothermal Oxidation of Silica and Alumina Scale Forming Si-23Fe-15Cr-15Ti-1Nb and Si-25Nb-5Al-5Cr-5Ti (at.%) Silicide Alloys

An Nb-silicide based alloy will require some kind of coating system. Alumina and/or SiO₂ forming alloys that are chemically compatible with the substrate could be components of such systems. In this work, the microstructures, and isothermal oxidation at 800 °C and 1200 °C of the alloys (at.%) Si-23Fe-15Cr-15Ti-1Nb (OHC1) and Si-25Nb-5Al-5Cr-5Ti (OHC5) were studied. The cast microstructures consisted of the (TM)₆Si₅, FeSi₂Ti and (Fe,Cr)Si (OHC1), and the (Nb,Ti)(Si,Al)₂, (Nb,Cr,Ti)₆Si₅, (Cr,Ti,Nb)(Si,Al)₂ (Si)ss and (Al)ss (OHC5) phases. The same compounds were present in OHC1 at 1200 °C and the (Nb,Ti)(Si,Al)₂ and (Nb,Cr,Ti)₆Si₅ in OHC5 at 1400 °C. In OHC1 the (TM)₆Si₅ was the primary phase, and the FeSi and FeSi₂Ti formed a binary eutectic. In OHC5 the (Nb,Ti)(Si,Al)₂ was the primary phase. At 800 °C both alloys did not pest. The scale of OHC1 was composed of SiO₂, TiO₂ and (Cr,Fe)₂O₃. The OHC5 formed a very thin and adherent scale composed of Al₂O₃, SiO₂ and (Ti(1-x-y),Crx,Nby)O₂. The scale on (Cr,Ti,Nb)(Si,Al)₂ had an outer layer of SiO₂ and Al₂O₃ and an inner layer of Al₂O₃. The scale on the (Nb,Cr,Ti)₆Si₅ was thin, and consisted of (Ti(1-x-y),Crx,Nby)O₂ and SiO₂ and some Al₂O₃ near the edges. In (Nb,Ti)(Si,Al)₂ the critical Al concentration for the formation of Al₂O₃ scale was 3 at.%. For Al < 3 at.% there was internal oxidation. At 1200 °C the scale of OHC1 was composed of a SiO₂ inner layer and outer layers of Cr₂O₃ and TiO₂, and there was internal oxidation. It is most likely that a eutectic reaction had occurred in the scale. The scale of OHC5 was α-Al₂O₃. Both alloys exhibited good correlations with alumina forming Nb-Ti-Si-Al-Hf alloys and with non-pesting and oxidation resistant B containing Nb-silicide based alloys in maps of the parameters δ, Δχ and VEC.

Structure and thermal stability of toxic chromium(VI) species doped onto TiO(2) powders through heat treatment

Chromium(VI)-containing sorbents in the form of sludge or solid residue from treatment processes are often landfilled or used as fill materials, therefore the long-term stability of metal binding is important. The reduction of Cr(VI)-Cr(III) through heat treatment may be a useful detoxification method. After heating at 500, 900, 1000, and 1100 degrees C for 4h, the transformation of chemical states of chromium on 105 degrees C-dried, 7.9% Cr(VI)-doped TiO(2) powders was studied on the basis of surface area measurements, scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) spectra. It was shown that Cr(VI) was reduced to Cr(III) in the Cr(VI)-doped samples after heating within 500-900 degrees C. The present results also suggested that the chromium octahedral was bridged to the titanium tetrahedral and was incorporated in TiO(2) minerals formed after 1000 degrees C treatment.