The System CaO–“CaCr₂O₄”–CaAl₂O₄ in Air and under Mildly Reducing Conditions

Abstract

The system CaO-chromium oxide in air is reinvestigated and the existence of intermediate phases with chromium in oxidation states >3+ (Ca5Cr3OtZ, Ca3(Cr04)z, and Ca5(Cr04)3) confirmed. Under reducing conditions these phases are unstable. A metastable, polymorphic form of calcium chromite, 8-CaCr204, is observed. In the CaO-rich section of the CaO-AI2O3 -Cr203 system a ternary intermediate phase, chrome-hauyne, Ca4[(A1,C13')6012](Cr6+04), coexists with calcium chromate and calcium aluminate phases. In air, low melting temperatures are preserved in all assemblages containing calcium chromate phases. Under reducing conditions a new ternary phase, Ca6A14CrzOls, coexists with CaO, CaCr204, chrome-hauyne, and calcium aluminate phases. The influence of chromium oxide additions on the solidus temperatures of the CaO-AI,03 system is insignificant. [

Formation and Local Structure Analysis of High‐Valence Chromium Ion in Dicalcium Silicate

“…It should be noted that γ‐C 2 S spontaneously forms as a stable phase at temperatures lower than 773 K upon phase transformation of β‐C 2 S metastable phase . In addition, previous phase equilibrium studies on the CaO–Cr 2 O 3 system prepared in air revealed that CaCrO 4 forms as a stable phase at temperatures lower than 1346 K . Therefore, it is required to examine the onset of phase transformation of Cr‐doped C 2 S from the β to the γ phase and CaCrO 4 precipitation upon reheating at temperatures lower than the sintered temperature, for understanding of the phase stability of Cr‐doped β‐C 2 S.…”

High Temperature Reaction of MCrAlY Coating Compositions with CaO Deposits

“…This is due to the fact that L CaCrO dissolves very little (\1 at.%) nickel or cobalt oxide: as the reaction front advances into the c-phase, Ni and Co are rejected from the liquid and precipitate as oxide particles. Similarly, the solubility of Al 2 O 3 in the chromate was measured by SEM-EDS to be quite low (1-2 at.%), as also reported by Kaiser et al [27]. This led to an Al:Cr elemental ratio of about 1:5, similar to that in the original c phase in Ni-30Co-33Cr-12Al-0.1Y and Ni-33Co-35Cr-7Al-0.1Y.…”

Effect of thermal history on high‐valence chromium ion dissolution in merwinite (3CaO·MgO·2SiO2)

“…As discussed in Sections 3.2 and 3.3, the XAFS spectroscopy of the Cr‐doped merwinite in air revealed that chromium ions existed as either Cr 3+ in CrO 6 octahedron or high‐valence states in CrO 4 tetrahedron at 1673 K. However, the Cr ions were mostly present as Cr 6+ in CrO 4 tetrahedron in the sample synthesized at 1123 K. The formation of hexavalent chromium ions might be partially related to the phase relation of the CaO‐Cr 2 O 3 system in air, which accompanies CaCrO 4 formation as a stable phase below 1346 K . Therefore, it is of great importance to examine the oxidation of Cr ions doped into merwinite and CaCrO 4 precipitation when reheating the Cr‐doped merwinite at temperatures lower than the synthesis temperature (1673 K) to understand the stability in the electro‐valence of the Cr ions in merwinite.…”

“…Samples in the group I were used to examine solubility of Cr ion in merwinite, while the comparison between the sample AH1 and IH1 in group II was used to investigate the effect of atmospheric condition in annealing, and the comparison between AH1 and AL1 in group III was used to investigate the effect of annealing temperature, on the Cr ion dissolution behavior, respectively. The range of annealing temperatures was selected to avoid decomposition of the merwinite at the highest temperature but to ensure precipitation of the Cr 6+ ‐bearing CaCrO 4 in air at the lowest temperature . We expected that when annealed in air, the Cr 3+ ion should mainly exist in merwinite at high temperature and Cr 6+ was expected to present at lower temperatures.…”

“…[17][18][19] Particularly, this paper focuses on the effects of thermal history on the dissolution behavior of Cr ions in merwinite. Because phase diagram of the CaO-Cr 2 O 3 system in air 20 shows that the chromate of higher oxidation state tends to be thermodynamically stable at lower temperatures, we expect that the Cr ion oxidation state and occupying local structures in merwinite may be varied on the annealing temperature.…”