Effet Jahn-Teller dans le système Ta2CrO6TaCrO4

“…Oxides with the trirutile structure of composition M>B> O exist for various combinations of divalent pentavalent ions, for example, M"V, Cr, V, Fe, Co, and Ni for B"Ta and M"Co, Ni, and Cu for B"Sb (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). All crystallize in P4 /mnm or P2 /n, when M is a Jahn-Teller species such as Cr> or Cu>.…”

Section: Introductionmentioning

confidence: 99%

Structure and Magnetism in CrTa2O6: A Trirutile Oxide Based on Cr2+

Saes

Raju

Greedan

1998

Journal of Solid State Chemistry

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“…It was thought earlier [21] that tantalum oxide which formed just under the chromia scale may threaten the adherence of chromia on substrate. This was found again here for the alloys containing between 30 and 50wt.%Cr which displayed both a very regular alloy/oxide scale interface and an almost continuous sub-oxide of CrTaO 4 .The deleterious effect of the CrTaO 4 sub-layer on the adherence of chromia may be explained by the mismatch of crystalline network between chromia (eskolaite) which possesses a corundum crystal structure (with Cr III replacing Al III at the same locations in the crystal) and the rutile structure of CrTaO 4 [24,25]. Classically encountered in case of high Ta contents (5 to 9 wt.%) in polycrystalline [26], directionally solidified [27] and single-crystalline [28] nickel-based superalloys, it was reported that tantalum often leads to the formation of more or less continuous sub-layers of CrTaO 4 during high temperature oxidation.…”

Section: Discussionmentioning

confidence: 99%

Dependence on the Chromium Content of the High-Temperature Oxidation Behavior of Ta-Rich Nickel-Based Cast Alloys

Berthod

Himeur

2017

Oxid Met

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The high-temperature stability of the primary tantalum carbides is a problem of importance for chromium-rich cast alloys, based on cobalt or nickel. The focus of this study was nickel-based alloys, as these alloys are particularly sensitive to a lack of TaC carbides in the ascast state and by dissolution due to high-temperature exposure. In this work, a possible way for promoting the formation of many TaC carbides by changing from the usual 30wt.% chromium content was investigated. Five alloys with Cr content varying from 10 to 50 wt.% were prepared and then subjected to microstructure characterization and to oxidation tests. In contrast with what was expected, decreasing the Cr content in comparison to the Ni-30Cr-0.4-6Ta reference alloy did not succeed in obtaining more TaC carbides, but instead had the opposite effect. Concerning the high-temperature oxidation behavior at 1127 and 1237°C, loss of resistance was observed only for a Cr content at the lower level of 10 wt.%. It was noticed that a subscale CrTaO 4 developed during oxidation and seemed to promote oxide spallation during cooling.

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“…Since no SiO 2 was observed to form in the alloys at 1100 C, this suggests either its formation is kinetically inhibited or that the amount of silicon in the alloys is not sufficient to permit its formation. Any Ta 2 O 5 that forms as a result of transient oxidation may then readily combine with chromia to form the TaCrO 4 and Ta 2 CrO 6 mixed oxides respectively according to the reactions [32]:…”

Section: Oxidation Reactionsmentioning

confidence: 99%