Open AccessThe system SrO-"chromium oxide" in air and oxygen
Author(s) -
T. Negas,
Robert Roth
Publication year - 1969
Publication title -
journal of research of the national bureau of standards. section a. physics and chemistry
Language(s) - English
Resource type - Journals
eISSN - 2376-5704
pISSN - 0022-4332
DOI - 10.6028/jres.073a.034
Subject(s) - liquidus , eutectic system , oxygen , ternary operation , solidus , chemistry , chromium , oxide , water vapor , phase (matter) , inorganic chemistry , analytical chemistry (journal) , thermodynamics , crystallography , environmental chemistry , alloy , microstructure , physics , organic chemistry , computer science , programming language
Phase relations in the system SrO-"chromium oxide" were determined in air and 1 atm O 2 (1 atm = 1.013 × 10 5 N/m 2 ) and are shown as isobaric projections on the SrO-Cr 2 O 3 pseudobinary. At both oxygen pressures, the system consists of three joins in the SrO-Cr 2 O 3 -oxygen ternary, (1) SrO-Sr 3 Cr 2 O 8 , (2) Sr 3 Cr 2 O 8 -SrCrO 4 , and (3) SrCrO 4 -Cr 2 O 3 . The former is binary from 1065 °C to liquidus temperatures in air and includes a eutectic near 79 mol percent SrO (in terms of SrO-Cr 2 O 3 starting materials). Sr 3 Cr 2 O 8 melts congruently at 1453 °C. Below 1065 °C in air, phase relations are complicated by reactions with atmospheric water vapor resulting in the formation of Sr 10 Cr 6 O 24 (OH) 2 which decomposes to SrO and Sr 3 Cr 2 O 8 above 1065 °C. In air, below 775 °C, Sr 3 Cr 2 O 8 reacts with water vapor and oxygen to form Sr 10 Cr 6 O 24 (OH) 2 and SrCrO 4 . Water vapor reactions are restricted in 1 atm O 2 . The Sr 3 Cr 2 O 8 -SrCrO 4 join contains a binary eutectic between 69-70 mol percent SrO but liquidus relations are ternary below 69 (air) and 68 (oxygen) mol percent SrO, as reduction of Cr 6+ occurs. Likewise, the SrCrO 4 -Cr 2 O 3 join is not binary at solidus and liquidus temperatures. In air, SrCrO 4 melts at 1251 °C to Cr 2 O 3 plus liquid with release of oxygen. In oxygen, the compound melts at 1283 °C with evolution of oxygen.