JO URNAL OF RESEARCH of the National Bureau of Standards- A. Physics and Chemistry Vol. 67A, No.4, July- August 1963 Thermodynamic Properties of Magnesium Oxide and Beryllium Oxide from 298 to 1,200 oK 1 Andrew C. Victor2 and Thomas B. Douglas (February 26, 1963) As a step in developin g new standards of hi gh-temperature h eat capacity and in deter­ mi ning accurate thermodynamic data fOl" simple s ubstances, t he enthalpy (heat content) relative to 273 oK, of hi gh purity fu sed magnesium oxide, MgO, and of sintered beryllium oxide, BeO, was measured up to 1,173 ole A B unsen ice calorimeter and the d rop m ethod were used. The two samples of BeO measured ha d s urface-to-volume ratios differing by a factor of 15 or 20, yet agreed with each other closely enough to preclude appreciable error attributable to the considerable surface a rea. The enthalpies found for MgO arc several percent highe r than most previously reported values. The values arc represented within t heir uncertainty (estimated to average ± 0.25 % ) by t he foll owing empirical equations 3 (cal mole- I at l' OK ) MgO : T-f ~- T-f;73 15= 10.74091'+ 1.2177(1 0- 3) 7'2 - 2.3183(10- 7) 1'3 + 2.26151 (10 5) 1'-'-3847.94. BeO: Tl ~- II;73 1 5= 11.10841'+ 7.1245(10- 4) 1'2 + 8.40705(105) 7'-1 - 5.31245(107) T-2- 5453.21. Value'S of rnthalpy, IwaL capacit.v, rntropy, a nd G ibbs free-energy function a rc tabulated from 298. 15 to 1, 200 oK. ( 1. Introduction Canada. Spectrochemical and spectrographic analyses at the Nation al Bureau or " Standards A previous paper [1] 4 has described the need for indicated that the sample contil.in ed 99 .90 weight heat-capacity standards at temperatures above the per cent MgO if the detected metallic impurities are range o/" the presen t a-aluminum oxide (corundum) assumed to be present as their highest sLable oxides standard. Enthalpy measurements on thorium (table 1) . dioxide have been presen ted [1] as a first step in the in vestigation of new materials for this purpose. TABLE 1. I mpurities in the samples The chemical stability and high melting poin ts Element a BeO M gO of MgO and B eO (3 ,000 and 2,800 oK , respectively sample 1 [2]) recommend these materials for consid eration ----------1----------- as possible heat-capacity standards. Although both A g ____________________________ _ lVeight % Weight % compounds have lower meltin g points than thorium Al _____________________________ _ ('J < 0. 001 Be ____________________________ _ 0. 007 . 004 dioxide (3 ,300 oK [2]), the lower sensitivity of their Ca __ __________________________ _ <bJ ('J Cr ___________ _________________ _ <. 001 . 025 heat capacities to the influence of common impurities Cs ____________________________ _ ('J <. 001 (because of smaller differences in atomic weights) is Cu _____________ __________ _____ _ . 001 ('J F e _______________ _____________ _ <. 001 < .001 a decided advantage. Tlle accurate knowledge of K ______ _______________________ _ . 001 .02 LL ____________________________ _ . 002 ('J the thermodynamic properties of these substances <. 00005 ('J M g ___________________________ _ (bJ over a large temperature range has added value MIl ___________________________ _ <. 00005 N a __ __________ ___ _____________ _ ('J .008 because of the very frequent occurrence of these SL ________ __ __________________ _ . 002 <. 002 materials in high-temperature reactions and installa­ . 01 . 009 tions. The results of enthalpy measurements on • The samples were also examined for th e foll owing clemen ts which were not fu sed 11g0 and on sintered BeO specim ens of two detected : As, Au, B, B ~ Bi ~ CeI, Ce, Co, Ga, Ge, TIf, In,lf) La, Mo, N b, Ni, Os, P, Pb, Pel, Pt, Rh, It u , ::ib, Sc, Sn, Sr, Ta, To, 'rh, 'ri, u, V, ,,~ Y I.-,Zn , Zr. different bulk densities are presen ted in this paper. In addition, the (ol1owing elements were undetected in BcD: Dy, l!. r , J ~ lI , Gel , lIo, Lu, Nel , P r, Rn, Rb, Re, 8m, Tb, Tm, Yb. b M ajor constituent. 2. Samples and Containers 'Not detected . The magnesium oxide sample had been fused and Two samples of beryllium oxide were used in the was transparent, clear, and colorless; it was su-pplied present study. BeO powder was pressed, fired, and by the Norton Company, of Niagara Falls, Ontario, sintered to obtain bulk densities of 2.3 g cm- 3 and 1.6 g cm - 3 (firing temperatmes of 1,800 and 1,100 °C 1 The measurements on M gO were su pporteci by th e Wright Air Development respectively). These two samples, whose densities Divisioll..J... Air Research and Dc velopl11clli ComJl1 and, United States Air Force, Wright-L'atterson Air Force J3ase, Ohio. were about 72 and 50 percent of the single-crystal 2 Present acidress: U .S. Naval Orci nance T est Station, China L ake, Calif. (X-ray) value, will hereafter be referred to il.S BeO a US ing the defin ed t hermochemi cal calorie = 4.1840 joules . • F igures in brackets indicate the literature referell ees at tbe end of tbis p aper. samples 1 and 2, respectively. Spectrochem ical 325 analyses of both samples at the BUTeau indicated T ABLE 2. R elative enthalpy of magnesium oxde " t hat they contained 99 .96 percent BeO by weigh t (table 1). In a petrographic examination, sample 1 was found to consist of approximately isometric F urn a.ce I n dividual Mean Mean tempera- enth alpy observed Calc. eq observecl- particles 25f.J. on an edge. BeO sample 2 was ob­ turc, T ]11CaSllrc- enthalpy ( I) ca.lc. served to be composed of needlelike particles esti­ lncnts b ---- ",J mated to average 10}l in length and 1f.J.2 in cross \ oJ{ cal mole- 1 cal mole-I cal molrl ca l mole-1 section. The surface-to-volume ratio of such a 373. 15_. ____ { 923.5 922.7 } 023. 1 923.6 - 0.5 particle is the same as that for a cube with an edge ISS0.3 473.1 5_. ___ ~1 { 1960.7 ] 960. 1 of 1. 5f.J.. The sintered samples of BeO used in the 1961. 1 } + 0.6 3060. 4 573.15 __ __._ { 3050.2 3059.2 enthalpy measurem ents each consisted of two cylin­ 3058. 0 } + 0.0 4194.8 ders 2 cm long and 1 cm in diameter. 4203.6 4197.5 4109. 3 - 1.8 The samples were sealed in containers of annealed 4194.0 } "'I 5375.5 pme silver preparatory to making enthalpy measm e­ "WI773 .1 5_. ____ 5371. a } 5374. 4 5369.6 + 4.8 ments [1] . BeO sample 2 lost weight during the 5376.7 873.15 __ ___ ~ { 6560.6 - 4. 1 first attempts at sealing it in its container. Further 6558.2 } 6550. 4 6563.5 973.15 ___ __ ~ { 7777. 6 - 0.9 study of the weigh t following successive h eat treat­ 7773.6 } 77 75.6 7776. 5 lO73.15 ._. __ { 9007. 5 ment and exposure to the room atmosphere showed 9000.1 } 9008.3 9005.3 + 3.0 ) t hat at least 0.3 per cent of the original sample mass 1173.l5 .. __ ~ { lO245.4 10246.0 10247. 1 - 1.1 was lost on heating to about 1,100 oK, but was lO246.6 J regain ed by the sample after cooling in a desiccator a Mol wt= 40.311 g. an d then standing in room ail' for 30 min. T ests on b Sample lnass=JO.7578 g. six specimens of thi s lower-density sample or BeO all showed the same hygroscopic behavior. Simil ar tests mado with BeO sample 1 showed no detectablo mass change. When BeO sample 2 was fina ll)~ TABLE 3. Relative enthalpy of beryllium oxide a sealed in its container its mass was the lowest attain­ able by the heat treatmen t mentioned above. It is I ndividual enthalpy M ean possible, however, that the sample was still co n­ F urnace tOlll- lllcasurCHl.cn ts observed Mean taminated by a small amount of water. perature, l' enthalpy Calc. eq (2) obser ve cl - (sa lilple I) calc. Sampl e 1 h Sampl e 2 c 3. Enthalpy Measurements 0J< cal mole- 1 cal mole-1 cal mole- 1 cal mole I caZmole- 1 323. ] 5 ~~ ___ ~ __ ~ __ { 303.6 305.6 T 11 e "chop" method and cal orimoter employed 303.9 305. 5 } 303.7 303.7 +0.0 665.5 in the enthalpy measuremen ts have been described 373.15 ~ ~ _~ __ ~_ ~ __ 665.8 } 662.2 662.6 -0.4 in detail ill a previous publication [3]. In brief, t11 e L ~~ ~~~ ~ ~ 666.6 473. 1 5~~ ___ ~ ___ __ { 1503. 1 1490.3 -0.0 method used was as follows . The sample, sealed 1500.4 1497.8 } 1501. 8 1501. 8 2457.4 in it silver container, was suspended in a silver-core 573.15 __ ___ ______ 2452.6 2453.7 2452. 7 +1.0 furnace until it Imd time to come to a constant L ~ ~~~~ ~ ~ - 2453. 3 } 673.l5 __ ~ __ ~ __ ~ __ { 3481. 6 34.79.8 .! known temperatme. It was then dropped (wilh 3477. 0 3481. 0 } 3479.3 3478.0 + 0.4 I 4558. 0 4562.2 } almost free fall) into the Bunsen ice calorimeter, 773 .l 5 _~~~~ ____ ~~ { 4555 . 1 4557.8 4559.6 -2.