The kinetics of crystal growth in an aluminosilicate glass containing small amounts of transition-metal ions

Transcription

1 MNERALOGCAL MAGAZNE, SEPTEMBER 970, VOL. 37, NO. 29 The kinetics f crystal grwth in an aluminsilicate glass cntaining small amunts f transitin-metal ins J. WLLAMSON Nuffield Research Grup in Extractin Metallurgy, mperial Cllege, Lndn, S.W.7 SUMMARY. Quantitative data are presented fr the kinetics f crystal grwth in an aluminsilicate glass t which a few wt % f a transitin-metal xide had been added. The cmpsitin f the base glass was CaO 3, MgO 2, A1208 5, SiO~ 53 wt %, t which the xides f irn, vanadium, chrmium, r zinc were added. Glasses were prepared with a range f high t lw valency ratis in cases where the transitin metal xide culd exist in tw r mre xidatin states. Small quantities f Fe ~, Fe 3+, Zn ~ V a+, V 4+, and V 5 were all fund t increase the kinetics f crystal grwth frm the glass phase. The magnitude f the effect is cmpared with additins f Mg 2+ t the glass. Cr 2 Cr 3 and Cr 6 were fund t decrease the rates f crystal grwth. Values fr the apparent activatin energy fr crystal grwth ranged frm ~50 kcalmle t ~5O kcalmle. The high values were fund t be f the same rder f magnitude as that fr the activatin energy fr viscus flw in the glass at the crystallizatin temperature. The results are interpreted in terms f the structure f the glasses and current theries n the rate f crystallizatin f glasses. KNETC studies n the crystallizatin f glasses prvide infrmatin n bth the structure and stability f the glass. The crystallizatin f glasses in the system CaO- MgO-A12Oa-SiQ is f interest fr the prductin f glass-ceramic materials. n rder t achieve a fine grained glass-ceramic, it is necessary t start with a hmgeneus glass, t which a small quantity f a 'nucleating agent' is frequently added. The rates f nucleatin and crystal grwth can then be cntrlled during the heat-treatment prcess. Transitin metal xides have been fund t be effective additives fr cntrlling the rate f nucleatin in many silicate glasses. Klemantaski and Kerrisn 0966) have recently described the use f irn blast-furnace slags, which cnsist essentially f lime, magnesia, alumina, and silica, as a starting material fr the prductin f a glass-ceramic material. The additin f a few wt % f irn xide, chrmium xide, r zinc xide t the slag glasses was fund t assist in bth the nucleatin and crystallizatin f the glass. n rder t understand the cmplex prcesses ccurring in the glasses during crystallizatin, a study has been made f the effect f additins f several transitinmetal xides t synthetic xide glasses in the system CaO-MgO-AlzOa-SiQ. The effect f the xidatin state f the transitin-metal in n bth the nucleatin and the crystallizatin prcess has been bserved. This paper reviews the results btained fr glasses cntaining irn and vanadium xides (Williamsn et al., 968, 969), and als the wrk f Keywrth (i969) n glasses cntaining chrmium xides. 9 Cpyright the Mineralgical Sciety.

2 760 J. WLLAMSON ON New data are presented fr the effect f zinc xide n the kinetics f crystal grwth frm the glass. Sme aspects f the nucleatin prcesses invlved are described by Rgers 0970) in a separate paper. Results The glass cmpsitin chsen fr the study cntained CaO 3, MgO 2, A1203 5, and SiO2 53 wt ~, t which a few wt % f the transitin metal xide was added. The base glass was prepared by fusing tgether AnalaR xides and a pure frm f silica at 4OO ~ in platinum crucibles. Full preparative details have been given in an earlier paper (Williamsn, Tipple, and Rgers, ~968), and this prcedure was fllwed fr the glasses cntaining irn, vanadium, and zinc xides. Glasses cntaining chrmium xide were prepared at ~5 ~ by Keywrth 0969) t increase the limited slubility f chrmium xides in the base glass. Small pieces f glass apprximately mm square were crystallized in electric muffle furnaces. Chrmelalumel thermcuples were placed clse t the specimens t measure the crystallizatin temperature. After heat treatment the specimens were sectined and a thin sectin 2 t 3 ~m thick made frm ne half f each specimen. The ther half was retained fr pssible X-ray study r electrn-prbe investigatin. All the glasses studied nucleated and grew FG.. Typicalthin sectin f the crystalline surface crystals frm the surface f the glass. layer f a heat-treated glass cntaining a small Fig. shws a phtmicrgraph f a amunt f chrmium xide. The crystals are an prtin f such a thin sectin. The intimate mixture f wllastnite and anrthite, crystals can be seen t grw as thin 3. needles frm the surface f the glass twards the centre f the specimen. The crystal-grwth rates can therefre be derived frm the depth f the surface crystallizatin. The layer f surface crystallizatin shwed remarkable unifrmity in thickness ver all parts f the thin sectins fr mst f the systems studied. n all cases, X-ray pwder diffractin f the crystalline surface shwed that the phases were a mixture f anrthite and wllastnite. Frm the cmpsitin f the base glass, anrthite and wllastnite wuld be expected t crystallize in apprximately equal prprtins. t was nt pssible t distinguish between the crystalline phases in the thin sectin as individual needles were cnsiderably < ~ txm thick. n sme thin sectins, very small dendrites apprximately xm in size were visible at the surface f

3 CRYSTAL GROWTH N AN ALUMNOSLCATE GLASS 761 the crystalline layer. Similar dendrites were frequently encuntered in the bulk f the material and these were later shwn by X-ray diffractin t be dipside. Glasses cntaining irn xides. The results fr the glasses cntaining irn xides have already been published (Williamsn et al., ~968) and can be summarized as fllws: The rate f crystallizatin f the glass was fund t be dependent n the amunt f irn xide added and als n the state f xidatin f the irn in the glass. Furthermre, it was shwn that the rate f crystal grwth was prprtinal t the square f the ferrus in cncentratin, as shwn in tig. 2. The crystalgrwth rates were btained frm five glasses, all cntaining the equivalent f 5 wt % FeO, but with ferrusttal-irn ratis ranging frm 26 t 7 %. The istherms have been extraplated in bth directins t levels at which the irn wuld have been present either ttally as Fe 3+ (base line) r as % Fe 2+. Additins f Fe 3+ t the glass are then seen t have nly a marginal effect n the crystal grwth rates cmpared t equivalent amunts f Fe 2+. Glasses cntaining irn xides were fund t have crystal-grwth rates that 14 l ~ g S 11 ~" 11 j it s (~ 12 i i l ii m el l 900 ~ u g L2~ r. '" ; '.' ',, ' CRYSTAL GROWTH RAT[ (CM.JdTSEC -') FG. 2. Rate f crystal grwth versus the square f the ferrus in cncentratin fr glasses cntaining the equivalent f 5 g FeO per l g base glass. The results fr glasses with five different ferrusferric ratis arc shwn. remained linear with time nly when crystallized at the lwer temperatures. Fig. 3 shws the depth f surface crystallizatin versus time fr a glass cntaining 5 wt % FeO in which 26 % f the irn is present as Fe 2~. At the higher crystallizatin temperatures the rate f crystal grwth increased cnsiderably ver a perid f several hurs. t has been fund by using an electrn-prbe analyser that there is an increase in the cncentratin f the irn in the glass at the crystalglass interface, and a crrespnding decrease in the cncentratin f the irn in the crystalline layer cmpared with that in the bulk glass. t isnw thught that the increase in the cncentratin f the irn at the crystal interface is sufficient t lwer the viscsity f the glass in the regin f the advancing crystal frnt and that this leads t the increase in the measured crystalgrwth rates. This aspect f the wrk is t be dealt with in a paper by Schles. Glasses cntaining vanadium xides. Vanadium can be present in silicate glasses as either V 3-, V 4+, r V 5+ ins. Glasses were prepared in which the vanadium was present as mainly V 3 ~ r V 5~-, with small amunts f V 4+ in bth cases. The effect n the crystal-grwth rates f varying the ttal vanadium cntent and the rati f xidized t reduced vanadium has been bserved (Williamsn, Tipple, and Rgers, 969). Glasses cntaining mainly V '~+ ins were prepared by adding V205 t the glass befre the final melting. The glasses were analysed fr bth ttal and reduced

4 76z J. WLLAMSON ON 01 l 'l' 8-7 Crystal ~ (pxlo 2) _...-~908*C g TME ( hurs ) F[6. 3. Thickness f surface crystallizatin versus time fr a glass cntaining 5 g FeO per OO g base glass with 26 % f the irn as Fe ~+. lc 9G 8c 7c 6G 5G - 30 l Added wt.% V205 F6. 4. Prprtin f vanadium present as V 5+ fr glasses melted in air fr ne hur at 4 ~ vanadium. The spectral evidence f Kakabadse and Vassiliu 0965) shws that the reduced vanadium wuld be essentially V 4+. Fig. 4 shws the results f the chemical analysis. The prprtin f V 5+ increased apprximately linearly with increasing cncentratins f V205 in the glass fr glasses prepared under identical cnditins. Fig. 5 shws the effect f additins f V205 t the glass n the rates f crystal grwth. Fr crystallizatin temperatures between 9 and 980 ~ additins f V205 t the 7 base glass initially increased the crystalgrwth rates. The rates reached a maximum at between 2 and 4 wt % V2Os depending n the temperature f crystal-

5 CRYSTAL GROWTH N AN ALUMNOSLCATE GLASS 763 lizatin. Glasses cntaining mainly V 3+ ins were prepared by the additin f V203 t the melt. The results fr tw glasses cntaining 1"3 and 4.r wt Y V203, f which apprximately 8 % f the vanadium was in a reduced frm are shwn in fig. 6. The slubility limit f V203 in the silicate melt at 4 ~ appeared t be clse t 4.r wt %, G i 7. s X ~ 40 ;. 980"C g60"c V i l, Vandium cntent (expressed as wt.~ Fi. 5. Rate f crystal grwth in glasses cntaining V205. as the last traces f the V20 a were slw t disslve. Cmparisn f figs. 5 and 6 shws that fr small additins f vanadium xide t the glass, V205 is mre effective at increasing the grwth rates than similar quantities f V203. Little can be deduced abut the effect f V 4~ ins. Glasses cntaining chrmium xides. A study f the effect f additins f chrmium xides t the same base glass n bth the nucleatin and crystal grwth rates has been made by Keywrth (969). Cr203 is nly slightly sluble in these glasses. At r5 ~

6 764 J. WLLAMSON ON apprximately 0.6 wt % Cr,,Oa disslved in the melt. The rati Cr 6 ~C1 a~-) fr glasses melted under an air atmsphere was fund t be. 7. This rati culd be dubled by melting the glass under an atmsphere f pure xygen. Glasses were als prepared by melting under an atmsphere f frming gas (t % H2, 90 % N2) and the rati Cr 2 ~-ECr was fund t be '85. Chrmium xides, unlike the ther transitin metal b- ' x u 40-- V203 cntent f glss (wt~ ~9 30 i_ u 20 2 : GO TemperQture ~ FG. 6. Rate f crystal grwth in glasses cntaining VzOa. ins studied, were fund t decrease the crystal-grwth rates. The results fr the glasses melted in air and under reducing cnditins are summarized in fig. 7. Glasses melted under xidizing cnditins were fund t have almst identical crystal-grwth rates t the air-melted glasses. An increase in the Cr 6~ cntent f the glass has therefre negligible effect n the crystal-grwth rates f the glass. The results btained frm the xidized glasses have theretbre been mitted frm the figure fr the sake f clarity. The magnitude f the reductin in the crystal-grwth rates fr the air-melted glasses increased almst linearly with increasing Cr 3 ~- cntent fr crystallizatin temperatures in the range 94- ~ Fig. 8 shws the percentage reductin in the grwth rate

7 CRYSTAL GROWTH N AN A.UMNOSLCATE GLASS p. (. O x 20 Glasses melted in Air. Analysed Cr203 cntent wt.% X 0.16,,. ( 0.35,,. Glass melted under reducing. cnditins \ u v 15 & 0'50 " 9 td O.G1,, 9 u G wt; Cr203 present mstly as Cr 2.. ix 920 g40 ~i g Crystallisatin Temperature =C FG. 7. Crystal-grwth rates in glasses cntaining increasing amunts f chrmium xide, melted in air and melted under reducing cnditins. as a functin f analysed Cr 3+ cntent f the glass, expressed as wt % Cr2Oa. Reductin in grwth rate = (U- U1)U, where U0 =: grwth rate f base glass at tern- perature T, and 1 = grwth rate at temperature T fr given Cr 3+ cntent. The results fr the glasses prepared under reducing cnditins and cntaining appreciable quantities f Cr 2+ indicated that the reductin in the grwth rate was nt as marked as with the crrespnding air-melted glasses. Evaluatin f the results is difficult, due partially t the nature f the effect f the presence f small quantities f Cr 6+ in the air-melted glasses. On a qualitative basis, Cr ~+ des nt appear t reduce the grwth rates as much as Cr ~+ and Cr e ~. ir 100 ~0 ; "~.E 4.~_ " " 20 Ar ct!500"c. 0! G Cr 3" cnte~t (expressed as wt.,cr?o 3 ) Fla. 8. Reductin in the rate f crystal grwth as a functin f Cr 3+ cntent f the glass. ~ Glas,es melted in 08

8 766 J. WLLAMSON ON Glasses cntaining zinc xide. Glasses were prepared cntaining up t 8 wt % ZnO and the crystal-grwth rates determined between 900 and i ~ The relatinship between the crystal-grwth rate and the temperature is shwn fr the base glass and fur different cncentratins f ZnO in fig. 9. The results btained were similar t thse btained fr the additin f irn xide t the glass, althugh Fe 2+ had a greater effect 'd 7- s X E U Acldltin~ f ZnO t Gl wt.%. A 3.5,, : b Crystallistin Temperature *C FG. 9. Rate f crystal grwth in glasses cntaining ZnO. n the grwth rates than Zn 2+. Fig. O shws that there was a linear relatinship between the square f the zinc in cncentratin and the crystal-grwth rates ver a greater temperature range than that bserved fr the irn-cntaining glasses. The apparent activatin energies fr crystal grwth varied frm O kcalmle fr a glass cntaining 3"5 wt % ZnO t 6 5 kcalmle fr a glass with 8 wt % ZnO. Discussin Crystal-grwth rates in bth rganic and inrganic glasses initially increase with the degree f supercling. The grwth rates pass thrugh a maximum and eventually reach zer fr extreme degrees f undercling. The general equatin fr the rate f crystal grwth, u, can be expressed as u = a v exp[-- AG"RT][--exp AG'RT], ()

9 CRYSTAL GROWTH N AN ALUMNOSLCATE GLASS 767 where a0 represents the jump distance, v is the vibratinal frequency, AG" is the free energy f activatin fr an atm t mve frm the amrphus t the crystalline slid and zxg' is the bulk free energy f crystallizatin. The first expnential term is knwn as the kinetic barrier t crystallizatin and the secnd expnential term the thermdynamic barrier. Fr small degrees f undercling the secnd expnential term is.l,, ' f ~n.c} O --..._ t- N 30 (]l y f 9 _ 20 m i.-) i Crystl Grwth Rate (r ser -1) F. [. Rate f crystal grwth versus the square f the zinc in cncentratin. dminant and AG', and hence the grwth rate, u, increases with increasing degrees f undercling (AT). Eventually the kinetic term becmes the rate-cntrlling factr and the grwth rates then decrease with increasing ~,T. Turnbull and Chen (96) suggested that fr many substances the activatin energy cntrlling the kinetics f crystal grwth was equal t that fr viscus flw in the same substance. The kinetic term in the abve equatin was therefre replaced by ne t include the viscsity, ~, s that u --JRT[--exp ~GRT]3Na~, where f is the fractin f sites t which a mlecule can be attached. Fr small departures frm the equilibrium temperature AG ~ L v ATTz, where L~ is the latent heat f fusin per unit vlume and T~ is the equilibrium temperature. Jacksn (1958) has characterized materials with small entrpies f fusin (LRT E < z)

10 768 J. WLLAMSON ON as having rugh interfaces and has als shwn that materials with large entrpies f fusin (LRT~ > 4) have smth interfaces and anistrpic grwth. Hillig and Turnbull (1956) fund that the use f equatin () predicted values fr the grwth rates f cristbalite frm pure fused silica that were in clse agreement with the bserved values. t wuld be unreasnable t expect the equatin t satisfy mre cmplex glasses. Multicmpnent xide glasses generally exhibit grwth rates that vary less rapidly than equatin () wuld predict. n the present case the bulk cmpsitin f the glass remains fairly cnstant, as d the phases crystallizing frm the glass. Hwever, a change in the viscsity f the glass with substitutin f a different transitin metal in r with a change in the xidatin state f the in, shuld have a marked effect n the crystal grwth rates. The viscsity f sme f the glasses has been measured in the temperature range 9-98 ~ the temperature at which the crystallizatin had been studied (Williamsn et al., 1968 ). t was fund that the presence f a few wt % irn xide mainly in the Fe 3+ state slightly lwered the viscsity ver the whle temperature range. The same amunt f irn xide present in the Fe 2+ state lwered the viscsity even further, the effect being mst nticeable at the higher temperatures. t was nt surprising therefre that glasses cntaining Fe z+ ins had crystal-grwth rates greater than glasses cntaining an equivalent amunt f Fe 3+. The apparent activatin energy fr the crystallizatin f the glasses was calculated frm the rates at zer time using the Arrhenius equatin k = A exp(--ejrt). The activatin energy fr crystallizatin f the glasses cntaining irn xides was fund t vary frm ~3 kcalmle fr a glass cntaining 5 wt % irn xide in the Fe 8+ state, t -~5O kcalmle fr a glass cntaining the same amunt f irn xide in the Fe 2+ state. The activatin energies increased linearly with increasing prprtin f Fe 2+ in up t a rati f Fe 2+ : Fe 3+ f 9 1, at which pint there was a sharp increase in the slpe; thereafter the activatin energies rse steeply with increasing Fe 2 : Fe 3+ rati. t appeared that there were tw mechanisms fr crystal grwth in the irn-cntaining glasses. The activatin energy fr viscus flw in the glasses cntaining ferrus in was fund t be ~5 kcalmle. Glasses cntaining irn xide with the irn present essentially as Fe 2+ are therefre thught t crystallize by the mvement f large aninic units t the advancing crystal interface, whereas glasses cntaining irn xide with the irn as Fe 3+ appear t crystallize by the diffusin f single catins t the crystal frnt. Small additins f Mg 2+ t glasses and melts in the system CaO-A1203-SiO 2 are knwn t reduce the viscsity (Machin, Yee, and Hanna, 952) and Zn 2+ is likely t have a similar effect. On the ther hand Cr203 is knwn t increase the viscsity f a slag (Higgins and Jnes, 963), and this n dubt accunts fr the reductin f the crystal-grwth rates fr additins f Cr203 t the glass. There appears t be n infrmatin available in the literature fr the effect f additins f V205 n the viscsity f silicate melts r glasses. V205 itself frms a very fluid melt (Pantny and Vasu, 968 ) and s it appears likely that small additins f V205 t the silicate glass will als reduce the viscsity. A direct cmparisn f the effects f the different transitin-metal xides n the rates f crystal grwth presents sme difficulty. Cr 2+, Cr 3+, and Cr 6+ all reduced the

11 CRYSTAL GROWTH N AN ALUMNOSLCATE GLASS 769 crystal-grwth rates, while the ther transitin ins increased the grwth. Less than wt % chrmium xide disslved in the glass at 5OO ~ whereas several wt % f the ther xides were sluble at 14 ~ The effect f a few wt % MgO n the crystal grwth rates has als been determined (Williamsn et al., 1968; Keywrth, 969), and these results can be cmpared with thse f the transitin-metal ins. Table lists the xides studied and the crystal-grwth rates at 950 ~ n rder t make a direct cmparisn between the different transitin-metal xides, the grwth rates crrespnding t a catin fractin f 2 % have been determined. The xides f chrmium, all f TABLE. Rates f crystal grwth at 950 ~ fr a catin fractin f 2 % Oxide g g base glass Rate f Catin Oxygen-in t give catin crystal grwth radius attractin fractin f 2 T ~ -7 cm sec -1 VOw ~ 35 0"59 2'5 FeO " ZnO 2.88 O 0"74 0"87 VO "74 '3 MgO "43 4 '65 '95 FeO ~ 4 0"64 "44 Base glass 3 CrO * < 3 ~.84.8 CRO15 * ~ 3 '69 '3 CrO~ * ~ 3 0"52 3"25 * Slubility limit f CrOt.5 ~ 0"46 catin %. which reduce the grwth rates, are nly slightly sluble in the glass, the slubility limit f Cr2Oa being apprximately.46 catin % at 5OO ~ The influence f the transitin-metal in n the silicate glass will depend largely n the charge and size f the in. Excluding the chrmium ins a rugh crrelatin exists between the inic ptential (Zr) and the effect n the grwth rate. Fr the divalent ins, as the inic ptential increased the in exerted less influence n the grwth rate. A similar trend was fund fr the trivalent ins V a+ and Fe 3+. Dietzel (1942) used the attractin between a catin and the xygen in in cntact with it as a means f classifying metal xides in glass-frming systems. The xygen-in attractin,, is defined as 2z12, where z is the charge n the catin and l is the sum f the radii f the catin and xygen in. Catins with > -6 were classified as being strngly crdinated t xygen atms (i.e. netwrk frmers). Values fr the xygen-in attractin,, fr the catins studied are included in table. The treatment makes n allwance fr the cvalent character f the ins. Despite this, the trend is fr the xygen-in attractin t increase as the rates f crystal grwth decreased. Glasses cntaining V 5+ ins shw a marked deviatin frm this trend. A similar deviatin is shwn by glasses cntaining V a+ ins. The grwth rates fr glasses cntaining V20~ exhibited a maximum in the curve f grwth rate versus V205 cntent. At a catin fractin f 2 %, the crystalgrwth rate was clse t the bserved maximum. A further increase in the V205 cntent

12 770 J. WlLLAMSON ON CRYSTAL GROWTH N A GLASS resulted in a lwering f the grwth rate. Hence at a catin fractin greater than 2 %, V205 wuld appear much lwer dwn in the table. N satisfactry explanatin exists at present t explain the b3erved maxima inthe grwth-rate curve f the glasses cntaining V205. The phenmena may well be related t the frmatin f vanadate cmplexes in the glass. The anmalus psitin f V2Oz in table is als difficult t explain. This may be related t the V a+ als present in the glasses cntaining V203. Little culd be deduced f the effect f V 4+ ins n the crystal-grwth rates f the glass. The evidence btained frm the presence f ther transitin-metal ins in the glass suggests that the stability f these glasses twards devitrificatin is cnditined largely by the size and charge n the transitin metal in added t the glass. REFERENCES DETZEL (A.), 942. Zeits. Elektrchem. 48, 9. H~GGNS (R.) and JONES (J. B.), 963. Bull. nst. Min. Met. 72, 8z5. HmLG (W. B.) and TURNBULL (D.), Jurn. Chem. Phys. 24, 914. JACKSON (K. A.), 958. Liquid Metals and Slidificatin, Amer. Sc. Metals (Cleveland). KAKABADSE (G. J.) and VASSmOU (E.), 965. Phys. Chem. Glasses, 6, 33. KEYWORTH (B.), 969. Ph.D. Thesis, Lndn University. KLEMANTASK (S.) and KERRSON (B.), Chem. nd MACrN (J. S.), YEE (T. B.), and HANNA (D. L.), 952. Jurn. Amer. Ceramic Sc. 35, 322. PANTONY (D. A.) and VASt; (K..), 968. Jurn. nrg. Nuc. Chem. 30, 433. ROGERS (P. S.), 97O. Min. Mag. 37, 74. SCHOLES (S.), private cmmunicatin. TURNaULL (D.) and COHEN (M. H.), 96O. Mdern Aspects f the Vitreus State, 1, Ed. J. D. Mackenzie, Butterwrths (Lndn). WLHAMSON (J.), TPPLE (A. J.), and ROGERS (P. S.), 968. Jurn. rn Steel nst. 206, Jurn. Mat. Sci. 4, O69. [Manuscript received 24 March r97]