Lawrence Berkeley National Laboratory Recent Work Title THE RATE OF SUBLIMATION OF MAGNESIUM NITRIDE FROM EFFUSION CELLS AND FROM FREE SURFACES IN VACUO AND IN ARGON AND NITROGEN GASES Permalink https://escholarship.org/uc/item/6jz7w3gt Authors Blank, Bette A.H. Searcy, Alan W. Publication Date 1967-07-01 eScholarship.org Powered by the California Digital Library University of California UCRL-16018-Rev. University of California Ernest 0. Lawrence Radiation Laboratory THE RATE OF SUBLIMATION OF MAGNESIUM NITRIDE FROM EFFUSION CELLS AND FROM FREE SURFACES IN VACUO AND IN ARGON AND NITROGEN GASES Bette A. H. Blank and Alan W. Searcy July 1967 TWO-WEEK LOAN COPY This is a library Circulating Copy which may be borrowed for two weeks. For a personal retention copy. call 1 ; . i ·. Tech. Info. Dioision, Ext. 5545 t L:JR.Il.R DOCUi\,1Ef·-JTS~~)~£~C-TI_O_N___________ ___.......,... 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The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. Submitted to Journal of Physical Chemistry UCRL-16018-Rev. Pre print .. ,... , \ ). UNIVERSITY OF CALIFORNIA Lawrence Radiation Laboratory Berkeley, California AEC Contract No. W-7405-eng-48 TFill RATE OF SUBL:LMATI:ON OF MA,GNES;f.:UM NITRIDE :Fl)0!1 EFFUSI~N CELLS AND FROM FREE SURFACE$ IN VACUO AND IN ARGON AND NITROGEN GASES Bette A. H. Blank and Alan W. Searcy July, 1967 .. -iii- UCRL-16018-Rev. THE RATE OF SUBLIMATION OF MAGNESIUM NITRIDE JROM EFFUSION · CELLS AND FROM FREE SURFACES IN VACUO AND IN ARGON AND NITBOGEN GASES .. .:~Bett.e A, H. Blank and Alan W, Searcy Inorganic Materials Research Division, Lawrence Radiation Laboratory, and Department of Mineral Technology, College of Engineering, University of California, Berkeley, California July, 1967 ABSTRACT Torsion effus;ton studies confirm earlier reports that effusion cell pressures for sublimation of magnesium nitride are significantly ., loyer than those calculated from thermochemical data for the expected sublimation reaction, Mg3N2(s) = 3Mg(g) + N2(g} eyen though usual tests .-' imply that eq_uilibrium is obtained in the cell, 'Free sur;t'ace sublimation rates range from 2 x 10-2 times the measured effusion pressures at 1000°K to 3 x 10-3 times the .measured effusion pressures at 1250°K. Free surface sublimation rates are lowered still further by addition of nitrogen or argon at about' 5 x 10-7 atm pressure. The rate of free surface sublimation is apparently limited by adsorption of gases on the sublimation sites, The rate limiting process in effusion is probably different from that for free surface sublimation, ... -1- UCRL-16018-Rev. I. ·INTRODUCTION Metal nitr;i.;dep when heated ;tn vacuo decoml?ose to y-;t:eld as ;reaction products, dia.tom~c nitrogen gap, and ga,peous metal atoms or solids depleted of nitrogen, The measured rates of several of these :reactions, 1 notably decomposition of boron nitride and aluminum ni:t:ride, gallium 2 nitride and beryllium nitride3 have been proved to be orders o;f magnitude lower than the maximum possible rates, which can be calculated ;from thermodynamic. data. by use of the Hertz-Langmuir e'luation, For the reaction Mg 3N2 (s) = 3Mg(g) + N2_(g)_, Soulen, Stha.pitononda. 4 . and Margrave have reported Hertz~Knudsen effusion pressures which lie well below pressures that they calculated from thermochemical data., This fact plus an increase in measured pressures when the effusion orifice was decreased suggested that magnesium nitride has a. kinetic barrier to decomposition .. Soulen et al, considered that they had too few data to permit calculation of the equilibrium pressures or evaporation coeffic.ient . A company report by Hildenbrand and Theard5 includes a study of the dependence of measured magnesium nitride dissociation pressures on . 6-9 .. • temperature and on effusion orifice area, By application of the equa,t~on p = p (1 + ~~ (1) e x aAl where P e is the equilibrium pressure, ;px is the mea.su.red pressure, W ;!.s 10 11 the orifice correction factor, ' a. ±:s the crops sect:);ona.l o.rif:);ce area, a is the evaporation coefficient, and A is the effective sa,mple surface area, Hildenbrand and Theard calcula.ted apparent e~uilibrium -2- . UCRL-16018-Rev. press~res .and :an :,,upper l~it to :the eya,;pora.t;ion c0e;t';ficient o;f 5 x 10-3 over the experbr1ental tempera.tu;re xa,nge, The equ;U~briUI!I pressures ' ....... "j ......... calculated iri the stud:yn are lower b:yn more than a factor o;f ten than ·.. pressures calculated for the dtssociation :reaction f:r0m da,ta. accepted ·:.·.:_.·.. :, . 12 in the J'J:iliAJ' Tables, · .\. ' • '<":·: .·' Hildenbrand and Th.ea,rd concluded that e:tther the .hea,t of ;format;!:on ·· i~. • '•.. for Mg3N2 ( s) is in erre.r, or the ·rate of ·vaporiza.t;ion ·i~· l;tJnited by a ',•, secondary process "fh±ch leads to :false apparent ee];u:Ui~brium pressures when Eq. (1) is applied, They favored the second expla,nat;ton, la.rgely •. i .. ', ~ bec;:ause thermodynamic quantities calculated f:rom their data were inconsistent with apparently reliable values ca.lculated from independent-· '. I ·~. '· ': •. • ':.· . 13 ' ' · thermodynamic data, They noted that Kay and Gregory- ha.ye :reported ' . .-:·. t ··'' that use of Eq, (1) for effusion data for the :reaction J11g(OHb (s )_ = ·MgO(s) + H20(g) yields apparent equilibrium pressures far lower than the • ~ t :. ' ~' ., . known equilibrium pressures. Since Eq, (1} has- come into increasing use for ca.lcula.ti<;m of equilibrium pressures for substances w;i.th low evaporation coefficients; . ~.... :. ,::. '.\ ...... .. ~ ,. : . ;. · · the· indications reported by Kay and Gregory and by H;i;ldenbre,nd and Theard that its use may sometimes be invalid are o;f grea.t concern, and :, ::, . : ~ · .. .j .• ,. '· . further studies of· substance::~ for which the eq,ua.t;lon m,a,y. not be yal;ld i'· . ·, are of particular importance, '' .i .' ' . ·~ . ,- -~ This paper reports a torsion~e;f;fusi.on and tor:;>;t:on't'La.ng:g!uir study ~- ...... · .. of magnesium nitride, The torsion.... la.ngmu±T study was made. both in f',· • I ........... ·. :· : .·· . ~ ·. ~ i vacuo and in controlled pressures of nitrogen and argon gases, The .) .. .... ~ .. · :-· .. ,: ,_ . " ..~·· torsion-effusion stud;r provides a direct check of the· exper:l:rnental .: .•· -3- UCRL-16018-Rev. findings of Hildenbrand and Theard and the torsion-Langmuir study provides an independent means for calculating the evaporation coefficient and temperature dependence and provides evidence on the effects of adsorbed gases on the rate of a sublimation reaction, II, EXPERIMENTAL The torsion-effusion and torsion-Langmuir methods as used in 14 this laboratory have been previously describea •15 and only, special features of the present study need be described here, That leakage from the effusion cells of National Carbon ZTlOl. grade graphite, caused no deflection was demonstrated by heating magnesium nitride in the ·cell before effusion holes were drilled, Orifice dimensions for the effusion cells and for the gra:r?hite washers behind which wafers were mounted in the torsion-Langmuir experiments and the orifice·correction factors used are given in Table I. Temperature measurements were made by means of a 20 mil Pt - Pt 10% Rh thermocouple which was located in a dummy cell that was :Placed just below the torsion cell; To calibrate the torsion cell temperatures against temperatures of the dummy cell, another thermocou:r?le, which had been calibrated by measurement of the melting po:!;nts o:t' tin and silver was placed inside a second cell, This cell was suspended in the • position where the torsion cell is usually located, and the corresponding temperatures of the two thermocouples were measured in the ex~erimental range, The vapor pressure of tin was measured as a test of our experimental methods, A value of 72,0 kcal/mole for the heat of sublimation at -4- UeRL-16018-Rev; . 298°K was obtained by the third law method which agrees well with the . •. 16 . ·-·­ value. of 71.8 kcal/mole measured by Schulz by the tors;t<on effus.:ton .. ·. ··:,,···---~ 17 method and w~th72.2 ± 0,5 calculated by Hultgxen et al, from results of earlier studies.· :For experiments on the e;f;fect of gas-es on the ·rate of sublin;ta,t;i:on, ·.·.- '· '·.·· nitrogen or argon was admitted into the furnace cha.TI}ber bymea,ns of a ; . _,·· . ' t :....... :: ... =.·,t. .. needle valve. If nitrogen was used dixectly frOI!l the ta.nk, oxygen and· ,· I• , . water vapor· impur;tties in the gas completely ccmyerted the ma,gnesiUill . '• :• .. nitride to magnesium oxide, Accordingly the gases were passed through . '·.· " ~ . '' ··.. _ ·' a ;Liquid nitrogen cold trap and subsequently through an oxygen getter of . - .-~~ ..·... · ' '~-: ~ :•. .L '.copper shavings, maintained at 680°e •. Upon completion of each sublimation ·· .. -~ ' ·" .· .. ... · ~. experiment in a foreign gas, the La,ngmuir pressure inyacuq Wa$·mea,sured 'I.