Effect of environment upon the melting point of Al2O3

The me ltin g point of AI, O" has bee n d e termined in vac uum . a ir. a rgon. a nd he lium utilizin)': iridium or tun ),:s te n co ntain ers. A ll me ltin g po int expe rim e nt s we re conduc ted in indu c ti on furna ces capab le of hea ti ng s ma ll sa mpl es und er b lackbody co ndition s to 3000 C in vacuum. 2600 C in inert at mosp he res. a nd 2400 C in air. Temperatures we re co ntinu ous ly mo nit o red wit h a reco rding phutoe lec tri c pyrome ter whi c h has a se n sitivity of0.2 at 1063 C and 10 at 2000 0c. Meltin g points were determ in ed on s pec im ens having a purity in excess of 99.9 pe rce nt. T he data indi c at e d that th e me ltin g poin t of AI ,O" d e te rmin ed in vac uum (-6.5 X 10 -5 torr) is 2051 C OPTS). T he overa ll maximum un ce rt a int y was es tim ate d to be 6 0c. Th e mea s ure me nt s th e m se lves we re prec ise within 1.5 0c. Vacuum trea tm e nt had leas t de t rimenta l e ffec t upon th e AI,03 spec im e ns. Eac h of the gaseous a tmo s ph e res in c lu ding air yie lded lower me lti ng point s for AI,O", p res umably d ue to a premeltin g p he nom e no n.