Sintering and crystal growth of magnesia in the presence of lime and forsterite

Transcription

1 MINERALOGICAL MAGAZINE, SEPTEMBER 197, VOL. 37, NO. 29I Sintering and crystal grwth f magnesia in the presence f lime and frsterite D. R. F. SPENCER AND D. S. COLEMAN Department f Materials Technlgy, University f Technlgy, Lughbrugh, Leicestershire SUMMARY. The densificatin f pure magnesia was examined alng with mixtures cntaining '5 and I.O ml T f calcium xide and frsterite. The sintering behaviur at temperatures between x4 and 18 ~ was studied and measurements made f changes in shrinkage, prsity, bulk density, and grain size fr sintering times f up t 8 h at sintering temperature. It was fund that bth the '5 and I.O ml % additins enhanced the sintering f pure magnesia in the temperature range I5-17 ~ Increased grain grwth was fund fr mixtures cntaining frsterite but little change ccurred fr mixtures cntaining lime. Examinatin f the grain grwth data shwed that the mixtures beyed a relatinship : Time at temperature Cnstant ~ (grain size) n, where n = 2 fr the grain grwth f pure magnesia and mixtures cntaining lime, but n = 3 fr mixtures f magnesia with additins f a silicate such as frsterite. A value f 78 kcal/mle was btained fr the grain grwth f pure magnesia, which agrees with ther researches. The extent f the slubility f lime in periclase fr each lime-magnesia mixture at each temperature was als examined using electrn prbe analysis and related t the sintering behaviur bserved fr these mixtures. INTEREST in magnesia stems frm the wide use f this xide as a refractry in the steel-making industries. Because f its high melting pint (28 ~ and resistance t the high-temperature crrsive and ersive cnditins in steel-making furnaces, large quantities f this material are used fr the hearth, which cntains the mlten metal and its slag. Cmmercially available magnesia refractries nw apprach a high level f MgO cntent, where the main impurities are the xides CaO and SiO~, which are present as silicates. Lesser impurities are F%O8, A12~, and traces f xides such as B~Oa and Mn23. In the CaO-MgO-SiO~ system as given by Levin 964) there are a range f silicates that can cexist with a magnesia phase at high temperature: frsterite, mnticellite, merwinite, dicalcium silicate, and tricalcium silicate. The silicates, mnticellite, merwinite, dicalcium silicate, and tricalcium silicate have been examined after sintering in the presence f a magnesia phase by Spencer and Cleman (I969) and it has been fund that these silicates d affect the sintering and crystal (r grain) grwth f magnesia in the temperature range I4OO-I8OO ~ The silicates are either mlten within this temperature range, r decmpse liberating free CaO, which disslves in the periclase phase leaving a silicate system having a lwer CaO/SiO2 rati. In this system, nly the phases frsterite and lime can be cnsidered as slid phases in the range f temperatures I4OO-I8OO ~ This paper presents infrmatin n the densificatin and crystal grwth f magnesia, magnesia-frsterite, and magnesia-lime mixtures during sintering at these temperatures. 9 Cpyright The Mineralgical Sciety.

2 84 D. R. F. SPENCER AND D. S. COLEMAN ON Raw materials Magnesia. The surce material fr this xide was the xalate prepared via the nitrate btained frm magnesium metal f stated purity 99"97 %. Upn calcinatin f the magnesium xalate in platinum crucibles at 8 ~ fr 5 h a pure magnesium xide was btained having the fllwing impurities, when analysed by spectrscpic techniques: A1 3, Ca 4, Cu 3, Fe 4, K 3, Mn 4, Na 2, Ni I, Pb I, Si 2 ppm (ttal 27 ppm). Other metals were sught but nt detected. Frsterite. Pure precipitated silica (~ 99"9 ~ SiO2; X-ray amrphus) was dried at 25 ~ and used t make the frsterite additin. The crrect mlar amunts f the dried magnesia and silica were fired (as pressed pellets) fr 3 h at I65 ~ and then quenched t rm temperature. The fired pellets were grund with an agate pestle and mrtar t pass thrugh a 3 B.S. sieve. This pwder was examined by X-ray diffractin techniques t ensure that the reactin f the raw materials was cmplete and that frsterite was frmed. This prcedure has been previusly used successfully by Spencer and Cleman 969) t prepare the ther silicates in the CaO-MgO-SiO2 system fr the determinatin f their densities. Lime. The additin f lime t magnesia was made using Analar grade calcium carbnate that had been dried at 25 ~ Mixture preparatin. Mixtures f magnesia with '5 and I. ml % f frsterite r lime were prepared by repeatedly passing the material thrugh a 3 B.S. sieve, and frmed int pellets by pressing at i tn/in 2, using a few drps f carbn tetrachlride as a lubricant t prevent laminatin. These levels f additin, which crrespnd t '73 and 1-45 ~ SiQ r '69 and I'38 % CaO, were chsen t give cmpsitins appraching thse f cmmercially available magnesia refractry materials. Experimental details Sintering prcedure. The pellets were sintered in air in a hrizntal alumina tube furnace fr q, I, 2, 4, and 8 h at each f the fllwing temperatures: I4, I5, I6, I7, and I8OO ~ Five pellets f each mixture were sintered at a cnstant heating rate f 3 ~ per h. A thermcuple was placed n each side f the pellets within the furnace and ne thermcuple in their centre. Up t and including i7 ~ the cntrl f the sintering temperature was within -L2 ~ difference between the three thermcuples. Fr sinterings at I8 ~ the cntrl f the sintering temperature was cnsidered t be better than 4-5 ~ Determinatin f densities and vlumes: The pressed pellets were weighed t 4-.z g. Their vlumes were determined by weighing t within 4-.2 g immersed in mercury, using the 'mercury balance' methd; this crrespnds t g/cm 3 in the bulk densities and cma/g in the bulk vlumes calculated frm these determinatins. After sintering, the pellets were reweighed as accurately as befre, and then impregnated with xylene under vacuum. The sintered bulk vlume and apparent slid vlume f the pellets were determined by weighing t within 4-.i g immersed in

3 CRYSTAL GROWTH OF MAGNESIA 841 xylene, and m air still impregnated with xylene. This enabled the apparent prsities f the pellets t be calculated. Knwing the true densities (Spencer and Cleman, ~969 b) f the silicates, the true prsities f the sintered mixtures can als be calculated. The values presented in this paper are the average f at least IO pellets (i.e. 2 sinterings f 5) and in sme cases ~5 pellets (i.e. 3 sinterings). Results f pellets that were severely cracked were ignred, and results f sinterings that gave dubtful data were repeated. Strict cntrl f the pressed bulk densities f the pellets was bserved. Only thse pellets that had bulk densities that came within the range 1.58~.2 g/cm a were sintered, all pellets with different densities t this were rejected. Preparatin Jbr micrscpic examinatin. The sintered pellets were impregnated under vacuum with a plystyrene resin and allwed t set slwly in air. They were grund n carbrundum papers and with diamnd paste t a x /~m finish. Measurement f grain size. The 'randm intercept' methd f Fullman (1953) was used t determine the mean grain diameter D. Fr randm straight lines drawn acrss a sectin f a micrstructure cmpsed f equal spherical grains, he shwed that D = p5l, where I is the average length f the intercepts f individual grains n the lines. This methd is cnsidered valid if the range f grain size is nt t wide as in this research. The plished surfaces, after etching, f three pellets frm each sintering treatment were phtgraphed under the micrscpe. Phtgraphs were develped at a magnificatin that gave an average intercept length f between 5 and IO mm. An average f 5 measurements f intercepts were taken frm each phtgraph. Electrn-prbe examinatin. Pellets that had been sintered fr 8 h at each temperature were munted tgether with a reference sample f mnticellite, since this cntains equal mlar prprtins f Ca, Mg, and Si. The munt was plished and given a thin carbn cating. Pint cunts in the centre f the periclase grains were made f Mg, Ca, and Si, and cmpared with the cunts taken frm the reference sample t give a quantitative estimate f Ca ~+ and Si 4+ present. Scans acrss the periclase grains shwed that n change in the pint cunts ccurred, inferring that the phases present had reached equilibrium. Results Sintered densities and vlumes. The shrinkage f the sintered mixtures is presented here as a vlume shrinkage parameter F v fr a given firing temperature: F v = IOO (vl. change f mixture)/(max, vl. change pssible) = IOO (pressed bulk vl. -- fired bulk vl.)/(pressed bulk vl.--specific vl.). The values f F v against time at sintering temperature fr pure magnesia are shwn graphically in fig. ~. Similar values fr the mixture f magnesia and '5 mle % frsterite are pltted in fig. 2, and a crrespnding set f values fr the mixture f magnesia and "5 mle % lime are pltted in fig. 3. A cmparisn between the effects f bth levels f '5 and I. mle % f additin n the sintering f the magnesia mixtures at each temperature with that f pure

4 842 D.R.F. SPENCER AND D. S. COLEMAN ON magnesia is given in table I, where F v values after 4 h f sintering have been chsen as a time cmparable with cmmercial sintering practice. Prsities. The cmparative effects f the additins n the densificatin f the mixtures, when cmpared with the pure magnesia, can be demnstrated by cnsidering the I teooa'r s *c iic F~ is~ 7c 6~ I 14 ~ 5(: 4c FIRING TIME 1: ~HOURS) FIRING TIME 1: (HOURS) FIGS. I and 2 : Fig. I (left). Shrinkage f pure magnesia : plts f shrinkage parameter F;,- % against time at tenapcrature. Fig. 2 (right). Shrinkage f mixtures f magnesia cntaining '5 mle i~ f frsteritc: plts f shrinkage parameter t:v % against time at temperature. prsities f the sintered peiiets after 4 h. Fig. 4 cmpares the effect f - 5 and z. mle % frsterite additins t pure magnesia n the true and apparent prsities when pltted against sintering temperature. A similar series f graphs cmpares the effects f the same levels f additin f lime t pure magnesia in fig. 5. Grain sizes. The grain sizes f purc magnesia cmpared with thse f mixtures cntaining "5 tl O// additins are pltted against time at the sintering temperature in

5 CRYSTAL GROWTH OF MAGNESIA 843 fig. 6. The effects f '5 and t. mle % additins n the grain size f the mixtures are shwn in cmparisn with pure magnesia in fig. 7, where the grain sizes after 4 h are pltted against sintering temperature. Grain grwth kinetics. At each temperature T ~ the grain grwth f pure magnesia and its mixtures was assumed t be a simple functin f time t, as given by Turnbull IOC ~ i, i i 9C 16~ ISO "IC?r i ~! i i ~17.4 L._ t \ i O.S.OLE "/ \ i... \ \... 6C SO PORO$1 Z,.,\ \ z., I "~ i I APPA~IN " i ~.'x>.. 4C I I I i i l ,i,~,,,~ "r,b,, t (~u,.s) O[ t I I 9 14OO 15OO 16OO 17OO 18 FIRING TEMPERATURE (*C) F]Gs. 3 and 4: Fig. 3 (left). Shrinkage f mixtures f magnesia cntaining '5 mle % f lime: plts f shrinkage parameter F v % against time at temperature. Fig. 4 (right). Cmparisn f prsities (after 4 h at temperature) f pure magnesia with mixtures cntaining '5 and I. mle % frsterite additins. 965): Kt = D'~,, where K is the rate cnstant fr grain grwth at the given temperature. This becmes K(t--t) = Db--D'd = (kyv)(t--t), where t is sme arbitrary zer time and D the grain size at that mment, t a later time when the grain size is DT, k is related t K, y is the interracial energy and V is the gram-atmic vlume f the material. We tk t =.I h and the value f D at that time and applied the abve equatin t the grain size measurements btained. Examinatin f plts f lg(t--t) versus lg(d~--d~) shwed that when 17 = 2

6 844 TABLE D. R. F. SPENCER AND D. S. COLEMAN ON I. Shrinkage parameter." F v % values. Cmparisn f the effect f "5 and I' mle % additins //brsterite and lime Material Additin Sintering fr 4 h at temperatures f: mle % I4OO ~ I5OO ~ I6OO ~ ]7 ~ ]8 ~ MgO -- 58"] "6 96"9 97'5 MgO-t-M~S. 5 55'] 9'2 93"9 96"2 96"5 MgO+M~S I. 89"9 94"3 94"8 95"6 96"I MgO+CaO. 5 76" "4 97"[ MgO Jr- CaO ] q 94"9 94"9 95"] aa ~., ' ~- ' ' ' 9 18 e i 2- I "5 MOLE ~O " I O \\ : I'O MOLE /... -O," : M~O O.5 MOLE ~ ~ 1. 9 "--- PURE M~O,5 - ', "~ 6,',.~... I 8OO ~ i \ I O ',TRUE true $ O. I 7 ~ ~ POROSITY 7. tt ', ~ 9, ~ ~ ~,, ~" II "" ~ "" * ~ " 6., -. ;,.4.--+','7 ~, ",--,.-".._..,;5 ~ APPARENT '~ APPtA REN T... ~1...~ 6 e,~ '~Q, I 5 ~... ~--'~= -" --'8"TM ~ i,4,s,6,7,. - = FIRING TEMPERATURE (It) FIRING T'ME ~ ~HOURS). FIGS. 5 and 6: Fig. 5 (left) Cmparisn f prsities (after 4 h at temperature) f pure magnesia with mixtures cntaining '5 and z. mle % lime additins. Fig. 6 (right). Grain grwth f pure magnesia, cmpared with the grain grwth f mixtures f magnesia cntaining "5 mle % frsterite and lime additins: plts f average grain size D T (micrmetres) against time at temperature.

7 CRYSTAL GROWTH OF MAGNESIA 845 the data fr pure magnesia and mixtures f magnesia with lime additin were linear plts with slpes appraching unity, whereas a linear plt with a slpe appraching unity nly ccurred fr n = 3 with data fr the mixtures f magnesia cntaining additins f frsterite. go ~r I S-4 9! f!! 1! I I I 4"6 8! ~, 35 5 LOG I K, 5 3" 2"2, 3 2 I "4 ' I " M~U~'b~U AUUII IUN~,,, 1,,,, 1 1, : ! 8 4"5 5' "b 5'5 6. I FIRING TEMPERATURE ~ ~'~A x IO '1" FIGS. 7 and 8 : Fig. 7 (left). Cmparisn f average grain size D T (after 4 h at temperature) f pure magnesia with mixtures cntaining "5 and I.O mle % frsterite and lime additins. Fig. 8 (right). Plts f lg1, K against I/T ~ t determine activatin energies f grain grwth. Systems beying the relatinship: K(t--t) = Dr--D.~ ~" A: Pure magnesia, n = z. B: Magnesia with -5 mle % lime, n = 2. C: Magnesia with "5 mle % frsterite, n = 3. Table II gives the values f n/2 r n/3, which shuld be I-OO if the equatins are true quadratic r cubic respectively. These values indicate hw clsely the equatin is fllwed. This table als presents the values f K determined frm the lgarithm plts, and Fig. 8 shws the lg K versus I/T plts fr pure magnesia and mixtures cntaining "5 mle % additins. Grain grwth and sintering temperature. Using the data fr pure magnesia, a plt f lg K versus I/T is linear. This indicates that an Arrhenius relatinship hlds "6

8 846 D. R. F. SPENCER AND D. S. COLEMAN ON.~ r~ ~ e~ ~ ~.~? ~ ~ ~ 2 X r~ O ~ c~ [I? ~D r~?. ~ 2? 7.~.~ ~ b r~ ~ ~ 66 6 % et ~ e,l 2.~ ~.~ b~ b~

9 CRYSTAL GROWTH OF MAGNESIA 847 K = cnstant, e -~mt, where E cal/mle is a phenmenlgical 'activatin energy' fr the grain grwth prcess. A similar analysis was carried ut fr the mixtures cntaining the '5 tl % additins, and it was fund that a break in the Arrhenius plts ccurred in the temperature range I7-I8 ~ fr the magnesia-frsterite mixtures, and within the range ~6-I7 ~ fr the magnesia-lime mixtures. The 'activatin energies' fr the temperature ranges n each side f these breaks are given in table II. Electrn-prbe infrmatin. The percentages f CaO fund in slid slutin in the periclase f a sintered mixture cntaining '5 mle % ('69 wt %) CaO after 8 h sintering were: sintered at I4 ~ nil; at I5 ~ '5z %, at I6, ~7, and at 18 ~ '69 %;fr a mixture cntaining I. mle % (1.38 wt %) CaO, the crrespnding figures were: I5 ~ '7 z %, I6 ~ '9I%, I8 ~ r'38 %. Allwing fr crrectins fr backgrund cunts, these results are cnsidered t be accurate t wt %. N appreciable amunts f SP + ins were fund in the periclase grains. Discussin f results The cntrlling prperty that can explain the results btained appears t be the average grain size D~, f the materials. This can be related t prsity (and therefre shrinkage) and accrding t Turnbull (I965) it is related t time by the abve equatin. Further, Burke 957) has suggested that D~, is a functin f the average pre size d and the ttal prsity P. Pres can be cnsidered as a secnd phase that inhibits cntinuus grain grwth and Smith (I948) says that this grwth will stp when D~, = dip. Cnsidering the equatin fr grain grwth, it is pssible t suggest suitable mechanisms that will explain the grain grwth behaviur f the magnesia with additins f frsterite and lime. These additins can be shwn t have different effects n the interracial energy f the periclase phase f the mixtures, which alters the D~, values. These in turn affect the values f prsity, and hence density and shrinkage values, which are related as Burke suggests. Magnesia Grain grwth. The D T ~ t 89 relatinship is expected fr the grain grwth f nrmal crystalline slids where the grain bundaries mve twards their centre f curvature and the pres, if present, d nt drastically affect the grwth rate. The grain grwth behaviur described here fr ur pure magnesia has been fund by ther researchers. Spriggs, Brissette, and Vasils (I963) examined the grain grwth f fully dense magnesia having an impurity cntent f 2 ppm. They fund that the D T ~-~ t 89 relatinship held fr their material and determined an activatin energy f 8I kcal/mle. Our value f 78:~:2 kcal/mle agrees favurably with this cnsidering that ur level f impurity is different. Hwever, Daniels, Lwrie, Gibby, and Cutler 96z) examined a very impure magnesia and fund the same grain grwth behaviur. They determined an activatin energy f 6 kcal/mle, which is nearer t the value determined fr ur magnesia and lime mixture with '69 wt % CaO, 62~4 kcal/mle. Mixtures f magnesia and Jbrsterite Grain grwth. Fig. 6 shws that the presence f. 5 mle % frsterite in the magnesia mixtures gives a greatly increased grain size when cmpared with pure magnesia, fr

10 848 D. R. F. S P E N C E R A N D D. S. C O L E M A N ON sintering temperatures up t r7 ~ This effect increases with the cncentratin f frsterite f r m '5 t ~. mle % (fig. 7)- U p t I7 ~ the grain grwth f the mixtures beys the grwth equatin fr a value f n = 3. This is unusual, since we cnsider that the M g O and 2 M g O. S i O 2 phases are slid in this temperature range. The MgO-SiO2 phase diagram (Levin et al, I964) shws that a liquid phase nly ccurs fr temperatures abve ~85 ~ The equatin has been fund by Spencer and Cleman (~969a) t be beyed fr silicates in the C a O - M g O SiO2 system that are liquid at the sintering temperature and in cntact with a periclase phase. Hwever, Kingery and Francis (I965) present evidence fr UO2 shwing that a single slid phase can sinter in such a way that the grain size is prprtinal t t ~ prvided that the pres migrate alng with grain bundaries during nrmal grain grwth. Examinatin f the micrstructure f these sintered mixtures shws that pres d cncentrate n grain bundaries, particularly at grain bundary junctins, as shwn in the typical micrstructure given in fig. 9b. Electrn-prbe analyses searching fr the presence f silicn in the periclase phase f mixtures sintered frm ~4 t I8OO ~ shwed that there was n measurable slutin f Sin+ ins retained in the periclase lattice. This has als been shwn by Henney and FIG. 9. Examples f micrstructures f materials after sintering fr 8 h at I8 ~ a (tp), pure magnesia; unetched as plished. Ntice the angular grains and the small pres. l. b (centre), magnesia with 1. mle T frsterite additin; etched with sulphuric acid. Ntice the runding f the grain bundaries at grain bundary junctins, als the large runded pres (black) in the centre f the grains. The grain bundaries appear t have swept up pres during the grain grwth, pres appear at grain bundary junctins. The dark grey phase is frsterite. l. c (bttm), magnesia with I. mle % lime additin: etched with sulphuric acid. Ntice the smaller angular grains and the small pres. 2.

11 CRYSTAL GROWTH OF MAGNESIA 849 Jnes (1969), wh examined by X-ray diffractin mixtures quenched frm 175~ and were unable t detect lattice parameter changes that wuld indicate the presence f SP + ins in the periclase phase. It is therefre cncluded that during sintering the frsterite diffuses arund the grains, alng grain bundaries, s that the interfacial energy f the grains is changed. Since this change allws increased grain grwth cmpared with pure magnesia, it can be assumed that the presence ef the frsterite causes a reductin in the interracial energy 7 with a resultant change in the grwth rate cnstant K (cf. table II). The grain grwth rate fr the mixtures has a greater temperature dependence than the grwth rate fr magnesia, as shwn by the higher activatin energy f 85 kcal/mle fr the magnesia-frsterite mixtures, cmpared with 78 kcal/ mle fr pure magnesia (table II). Abve 17 ~ a change ccurs in the slpe f the graph given in fig. 8. This suggests the presence f a liquid phase, presumably frmed fi'm the impurities present in the silica used t make the frsterite phase, since the magnesia is very pure. Traces f this liquid phase were seen in the micrstructures f mixtures sintered at 18 ~ where sme runding f grain bundaries ccurred. If this is s, a large change in the interracial energy at the grain bundary wuld be expected and wuld give rise t this break in the grain grwth behaviur. Shrinkage and prsity. Kingery and Francis (I965) cnsider that fr cases such as this, where the D T ~ t ~ relatinship hlds, the grain (and pre) grwth is mre rapid than ther changes, s the pre-grain gemetry remains abut the same. Thus the pre size f the material remains nearly prprtinal t the grain size. These effects have a bearing n the shrinkage fr these mixtures in the early stages f sintering, when the number f pres is high. Fig. a shws the marked difference in the values f F v fr the mixtures abve I4 ~ cmpared with thse values f F v fr pure magnesia after the same sintering time. The values at 14 ~ are abut the same in figs. I and 2; this is attributed t the lw diffusin rate f frsterite arund the grains at this temperature fr the lwer level f additin f frsterite. At the higher level f additin this des nt ccur. At temperatures f 17 and I8OO ~ the amunt f sealed prsity increases with frsterite cntent (cf. fig. 4), as wuld be expected and the ver-all prsities are slightly higher than the cmparable values fr pure magnesia. In the later stage f sintering at these high temperatures, althugh the grain size is increasing mre rapidly fr the mixtures than fr the pure magnesia (and the shrinkages crrespndingly increased) the grwing amunt f sealed prsity is nw cntrlling the final density and ultimate shrinkage. Then, fr the same level f prsity, a high value f D~, means a large value f d, and therefre large diameter pres wuld be expected in magnesia mixtures cntaining frsterite after sintering fr a lng time at high temperatures, when the grain grwth is slwing dwn. This is exactly what is seen n the examinatin f micrstructures, such as the example in fig. 9b, where pres at grain bundaries are as large as thse left in the centre f the grains by the receding grain bundary. Few pres are seen in the areas thrugh which a grain bundary has mved; this again infers that the pres mve with the grain bundary.

12 85 D.R.F. SPENCER AND D. S. COLEMAN ON Mixtures f magnesia and lime Grain grwth. Electrn prbe data shw that belw I6OO ~ the lime additin present at the grain bundaries is disslving in the magnesia grains. Als, cmplete slutin is achieved abve this temperature, but nly after lng sintering times. Belw I6OO ~ the grain size f the mixtures, as shwn in fig. 7, is dependent n the lime cntent. The mre rapid increase in grain size f the 1- mle % mixtures ver that f the pure magnesia culd be explained by this slutin f the Ca 2+ ins in the periclase. Since there is a mismatch f in radii given by Wells 962) as: Ca 2+ '99 A, Mg A, fr every Ca 2+ in replacing a Mg 2+ in a distrtin in the periclase is prduced. X-ray analysis f these mixtures has been carried ut by Mead (1969) and by Spencer, Beamnd, and Cleman (197), and the periclase lattice parameter was shwn t expand with the presence f Ca 2+ ins in prprtin t the lime cntent f the mixture. If the lattice is distrted sufficiently, vacancies can be generated at the grain bundaries, as well as being adsrbed there, as Burke suggests. The resultant flw f vacancies t the grain bundary 'sinks' is altered, resulting in larger grains fr the mixtures at temperatures belw 16 ~ This effect wuld be expected t be dependent mre n the lime cntent at the grain bundaries than the sintering temperature. This wuld accunt fr the lw temperature-dependence f the grain grwth at this stage, as shwn by the very lw activatin energy f 22 kcal/mle. Abve 16 ~ when the slutin f Ca ~+ ins in the periclase grains has been cmpleted, the disturbing effect f vacancy generatin at the grain bundary (which must affect the interfacial energy there) is remved. The grain grwth reverts t a rate very similar t that fr pure magnesia, as shwn by the increase in activatin energy frm 2z t 6z kcal/mle. This indicates that the grwth has becme mre temperaturedependent than belw 16 ~ which is mre cnsistent with a diffusin prcess by vacancy flw than with a diffusin prcess by a vacancy flw invlved als in slid slubility effect at the grain bundaries. Fr this reasn, the grain sizes are very similar fr pure magnesia and the mixtures at abut 17 ~ as shwn in fig. 7; but at 18 ~ the presence f lime in the periclase has an inhibiting effect n the grain grwth and the prsity develpment has been different. Shrinkage and prsity. The enhancement f sintering at lw temperatures caused by the additin f lime t magnesia has als been fund by Degtyareva, Kainarski, and Prkpenk (1967), wh fund that magnesia with the additin f '45 wt % CaO had a greater shrinkage than pure magnesia in the sintering temperature range 8-14 ~ The decrease in the grain grwth behaviur shwn in fig. 7, which ccurs abve 16 ~ fr the magnesia and lime mixtures, helps t explain the prsity changes shwn in fig. 5. The mixtures decrease in prsity mre rapidly as the temperature increases than des the pure magnesia, and develp a sealed prsity, which appears t increase with the greater lime cntent f the mixture. Hwever, the shrinkage values, althugh related t the prsities, tend t the same value at 18 ~ Belw 16 ~ the grain grwth f the lime mixtures increases with lime cntent as des the shrinkage, but abve this temperature, due t the inhibiting effect f lime nw disslved cmpletely in the periclase, the retarded grain grwth cunteracts the

13 b. CRYSTAL GROWTH OF MAGNESIA shrinkage. Therefre, the pure magnesia and the mixtures arrive at similar prsity and shrinkage values (cf. table ]) by different prcesses f pre develpment and grain grwth. Cnchtsins Magnesia-frsterite mixtures. The additin f up t I. mle O// frsterite t magnesia increases its grain grwth within the temperature range I4-I8OO ~ This grwth fllws a relatinship f (grain size) 3 prprtinal t time at temperature, since the pres mve with the grain bunderies. The densificatin f these mixtures at temperatures belw 16 ~ is initially increased during the high grain grwth, but the final density f these mixtures at I8 ~ is lwer than that fr pure magnesia under the same cnditins, since the presence f the insluble frsterite phase causes an increase in the sealed prsity. Magnesia-lime mixtures. The additin f I. mle ~ lime t magnesia disslves cmpletely in the periclase abve a sintering temperature f I6 ~ Belw I6OO ~ during the slutin f lime, the grain grwth f the mixtures is slightly increased and the grain grwth prcess has a lw activatin energy. After cmplete slutin f Ca.'.+ ins in the periclase abve I6 ~ the grain grwth is inhibited and reduced belw cmparable values fr pure magnesia. The grain grwth fllws a relatinship f (grain size) 2 prprtinal t time at temperature; the same relatinship hlds fr pure magnesia. Althugh densificatin is initially increased while the grain grwth is increased at temperatures belw i6 ~ the final densities at I8OO ~ f these mixtures is als lwer than fr pure magnesia under the same cnditins due t the increase in sealed prsity. REFERENCES LEVIN (E. M.) et al., r964. Phase Diagrams fr Ceramists, Amer. Ceram. Sc., Clumbus, Ohi. SPENCER (D. R. F.) and COLEMAN (D. S.), I969a. Trans. Brit. Ceram. Sc. 68 (3), IZ5. Science f Ceramics, 5th edn, 23 April, Rnneby Brunn (Sweden). FULLMAN (R. L.), I953. Trans. A.LM.E. 197, 8I. TURNBULL (D.), I965. J. Metals, 3, Trans. A.LM.E. 191 (8), 661. BURKE (J. E.), J- Amer. Ceram. Sc. 4 (3), 8. SMITH (C. S.) Grains, phases and interfaces: an interpretatin f micrstructure, Metals Technl. 15, (4), Trans. A.I.M.M.E. 175, 15. SPRIGGS (R. M.), ]]RISSETTE(L. A.), and VASILOS (T.), J. Amer. Ceram. Sc. 46 (IO), 58. DANIELS (A. U.), LOWRIE (R. C.), GIBBY (R. L.), and CUTLER (L B.), Ibid. 45 (6), 282. KINGERY (W. O.) and FRANCOIS (B.), I965. Ibid. 48 (lo), 546. HENNEY (J. W.) and JONES (J. W. S.), Trans. Brit. Ceram. Sc. 68 (4), 2i. WELLS (A.), Structural Inrganic Chemistry, 3rd edn, Clarendn Press (Oxfrd). MEAD (G. F.), ~969. Unpublished research, dissertatin, Department f Materials Technlgy, University f Technlgy, Lughbrugh. SPENCER (D. R. F.), BEAMOND (T. W.), and COLEMAN (D. S.) x97, Trans. Brit. Ceram. Sc. (in press). DEGTYAREVA (E. V.), KAINARSKII (I. S.), and PROKOPENKO (M. I.), Ogneupry, (3), I