Ž .Journal of Non-Crystalline Solids 213&214 1997 404–408

Crystallization kinetics of AlF -based glasses doped with rare3

earth ions

Tariq Iqbal a,), A.N. Kayani a, M.R. Shariari b, G.H. Sigel Jr. b

a Faculty of Mathematics and Applied Physics, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, District Swabi,N.W.F.P., Pakistan

b Fiber Optic Materials Research Program, Rutgers UniÕersity, P.O. Box 909, Piscataway, NJ 08855-0909, USA

Abstract

Different concentrations of NdF , PrF , and PrF , codoped with YbF , ranging from 0.25 to 2.0 wt%, have been added to3 3 3 3Ž .an AlF -base glass with composition mol% 30.2AlF –10.6BaF –20.2CaF –8.3YF –3.5MgF –3.8NaF–13.2SrF –10.2ZrF .3 3 2 2 3 2 2 4

The Avrami exponent, n, and activation energy, E, for the base glass, as well as the doped glasses, have been calculated byanalyzing the differential scanning calorimetry curves, according to the amended Ozawa–Chen and Augis and Benettequations. In light of these results, we observe that the introduction of suitable amounts of rare-earth ions improved thethermal stability of the base glass.

1. Introduction

Many studies of the crystallization kinetics ofvarious compositions of fluoride glasses have been

w xcarried out 1–3 . These glass systems are multicom-ponent, so crystallization behavior is typically com-plex and is influenced by the addition of new com-ponents. Rare-earth ions can easily be incorporatedinto many fluoride glasses in various concentrationsw x4 . These glasses have been successfully employed

w x w xfor optical amplifiers 5 upconverters and lasers 6 .An understanding of their crystallization behavior isessential in order to prevent devitrification duringfiber drawing. The characteristic temperatures of flu-

) Corresponding author. Tel.: q92-5372 71 861; fax: q92-537271 865.

oroaluminate glasses are 100 to 2008C higher thanthose of fluorozirconate glasses. AlF glasses also3

exhibit excellent chemical durability and superiormechanical strength compared with ZrF -based com-4

w xpositions 7 .In this work we investigated the effect of the

introduction of rare-earth fluorides including NdF ,3

PrF , and PrF , codoped with YbF on the thermal3 3 3

stability of an AlF -base glass whose properties are3

well known. Both Nd3q and Pr 3q are potentialcandidate ions for active device applications at 1.3mm. The values of activation energy, E, and Avramiexponent, n, for the base glass, as well as therare-earth-doped glasses, have been calculated by

Ž .analyzing the differential scanning calorimetry DSCw x w xdata according to the amended Chen 8 , Ozawa 9

w xand Augis and Benett equations 10 . We observedthat the introduction of suitable amounts of rare earthions improved the thermal stability of the base glass.

0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0022-3093 97 00044-6

( )T. Iqbal et al.rJournal of Non-Crystalline Solids 213&214 1997 404–408 405

2. Theoretical relations

The parameters of the crystallization kinetics ofglasses can be investigated either by isothermal,

w xpseudo-isothermal, or non-isothermal methods 11 .We have employed the non-isothermal approach forobtaining these parameters. Differential scanning

Ž .calorimetry data DSC were analyzed according tow xthe following amended equations of Chen 8 , Ozawa

w x w x9 and Augis and Benett 10 to obtain the values ofthe activation energy, E, and the Avrami exponent,n:

d lnT 2ra EŽ .pCHEN: s , 1Ž .½ 5d 1rT RŽ .p x

d lna EŽ .OZAWA: sy , 2Ž .½ 5d 1rT RŽ .p x

d ln yln 1yx� 4Ž .Ž .y sn , 3Ž .½ 5d ln aŽ . T

where T is the temperature at the apex of thep

crystallization peak, x is the volume fraction ofcrystals at a particular temperature T , a is a heatingrate, E is activation energy of crystallization, R isthe gas constant, T is temperature in degrees Kelvinand n is the Avrami exponent.

The modified forms of Augis and Benett andChen equations are similar. An alternate model, basedon a diffusion controlled process, was subsequently

w xproposed by MacFarlane 12 for fluoride glasses,leading to a relation from which ErR is deduced asin the Ozawa relation.

3. Experimental

Ž .Different concentrations 0.25 to 2.0 wt% ofNdF , PrF and PrF , mixed with YbF , were added3 3 3 3

Žto AlF -based glasses of nominal composition in3.mol% 30.2AlF –10.6BaF –20.2CaF –8.3YF –3 2 2 3

3.5MgF –3.8NaF–13.2SrF 10.2ZrF . Typically, 20 g2 2 4

batches were melted in platinum crucibles in a glovebox environment in which moisture and oxygen lev-els were below 1 ppm. The glass melts were cast intopreviously heated, gold plated, brass molds andcooled to room temperature. Rods, 10 mm in diame-

ter and 75 mm long were obtained using theseprocedures.

DSC measurements were carried out at differentŽ .heating rates Perkin–Elmer model DSC-7 . The ac-

tivation energy was calculated by obtaining the tem-peratures of an exothermic peak, T , at differentp

Ž .heating rates and then plotting ln a versus 1rTpŽ 2 .and ln T ra versus 1rT . From these plots, thep p

value of the crystalline fraction, x, at Ts5308C wasw Ž .xcalculated. Finally the plot of ln yln 1yx against

Ž .ln a provided a value of Avrami exponent, n.

4. Results

Fig. 1 shows the DSC curves measured at differ-ent heating rates for an AlF -based glass, codoped3

with 0.5 wt% PrF and 1 wt% YbF . A plot of3 3Ž 2 .ln T ra versus 1rT for the AlF -base glass,p p 3

codoped with 0.5 wt% PrF and 2.0 wt% YbF , is3 3Ž .given in Fig. 2. Fig. 3 shows a typical graph of ln a

against 1rT for the same composition. The varia-pw Ž .x Ž .tion of ln yln 1yx as a function of ln a for the

base glass, codoped with 0.5 wt% PrF and 0.5 wt%3

YbF , is shown in Fig. 4. Table 1 shows the values3

of activation energy, E, and Avrami exponent, n, at5308C for the different concentrations of PrF in the3

base glass. Table 2 shows the effect of NdF on the3

Fig. 1. DSC curves at different heating rates for AlF -based glass3Ž . Ž .codoped with 0.5 wt% PrF and 1.0 wt% YbF . 1 68Crmin, 23 3

Ž . Ž .58Crmin, 3 48Crmin The curves are offset for clarity .

( )T. Iqbal et al.rJournal of Non-Crystalline Solids 213&214 1997 404–408406

Ž 2 .Fig. 2. Plot of ln T ra versus 1rT for AlF -based glassp p 3Žcodoped with 0.5 wt% PrF and 2.0 wt% YbF Chen’s method3 3

.for the determination of activation energy .

Ž .Fig. 3. Plot of ln a versus 1rT for AlF -based glass codopedp 3Žwith 0.5 wt% PrF and 2.0 wt% YbF Ozawa’s method for the3 3

.determination of activation energy .

Table 1Activation energy and Avrami’s exponent values for base glassdoped with PrF3

Ž . Ž . Ž . Ž .Sr No. Composition Activation energy kcalrmol by Eq. 2 Activation energy kcalrmol by Eq. 1 Avrami exponent ‘n’ at 5308C

1 base glass 45.4"0.3 42.2"0.3 2.6"0.12 0.25 PrF 20.6"0.1 17.3"0.1 2.5"0.13

3 0.5 PrF 42.1"0.3 38.9"0.2 1.9"0.13

4 1.0 PrF 24.3"0.2 21.0"0.2 2.5"0.13

5 1.5 PrF 22.7"0.1 19.4"0.1 1.7"0.13

6 2.0 PrF 20.3"0.1 17.0"0.1 1.5"0.13

Table 2Activation energy and Avrami’s exponent values for base glassdoped with NdF3

Ž . Ž . Ž . Ž .Sr No. Composition Activation energy kcalrmol by Eq. 2 Activation energy kcalrmol by Eq. 1 Avrami exponent ‘n’ at 5308C

1 base glass 45.4"0.3 42.2"0.3 2.6"0.12 0.25 NdF 23.3"0.1 19.7"0.1 2.2"0.13

3 0.5 NdF 16.5"0.1 13.2"0.1 1.6"0.13

4 1.0 NdF 19.4"0.1 16.1"0.1 –3

5 1.5 NdF 20.4"0.1 17.1"0.1 2.5"0.13

6 2.0 NdF 19.1"0.11 16.4"0.1 2.4"0.13

Table 3Activation energy and Avrami’s exponent values for base glass codoped with PrF and YbF3 3

Ž . Ž .Sr No. Composition Activation energy kcalrmol by Activation energy kcalrmol Avrami exponent ‘n’ at 5308CŽ . Ž .Eq. 2 by Eq. 1

1 base glass 45.4"0.3 42.2"0.3 2.6"0.12 0.5 PrF q0.5 YbF 23.0"0.1 19.7"0.1 2.4"0.13 3

3 0.5 PrF q1.0 YbF 22.1"0.1 18.8"0.1 1.5"0.13 3

4 0.5 PrF q1.5 YbF 21.7"0.1 18.4"0.1 2.5"0.13 3

5 0.5 PrF q2.0 YbF 35.3"0.2 32.1"0.2 2.0"0.13 3

( )T. Iqbal et al.rJournal of Non-Crystalline Solids 213&214 1997 404–408 407

w Ž .x Ž .Fig. 4. Plot of ln yln 1y x versus ln a for AlF -based glass3Žcodoped with 0.5 wt% PrF and 0.5 wt% YbF at 5308C Ozawa’s3 3

.method for the determination of Avrami’s exponent n .

activation energy and Avrami exponent at 5308C ofŽthe base glass. Similarly, the value of E and n at

.5308C for AlF -based glasses, codoped with 0.53

wt% PrF and various concentrations of YbF , are3 3

listed in Table 3.

5. Discussion

lt is generally assumed that the activation energy,E, for crystallization is correlated with the glassstability such that smaller E corresponds to morestable glasses. The E, for the base glass is ;45 Kcalrmol and decreases upon the addition of PrF3Ž .Table 1 . The smallest E is for the 2.0 wt% dopedPrF glass. No systematic trend is observed. The data3

show that by adding PrF to the base glass in concen-3

trations ranging from 0.25 to 2.0 wt%, the crystal-lization rate decreased and the thermal stability ofthe base glass increased. The activation energies fordifferent concentrations of NdF in the base glass are3

listed in Table 2. These results show that the thermalstability of the base glass increases by the addition ofNdF , up to 0.5 wt%, and then begins to decrease.3

Similarly, the activation energy, E, also decreasesfor the glasses codoped with PrF and YbF . When3 3

Ž .the concentration of PrF is fixed 0.5 wt% , varying3

the concentration of YbF also changes the activa-3

tion energy. Values of E decrease up to 1.5 wt%YbF , and any attempt to add further amounts of3

YbF , resulted in an increase in the activation energy3

or a corresponding decrease in the thermal stability.The values of the Avrami exponent, n, directly

w xdepends on the mechanism of crystal growth 13 .The n for the base glass is 2.6 and corresponds todiffusion controlled growth and increasing nucle-

w xation rate 13 , while the n for the doped glasses liesbetween 1.5 to 2.5 and are linked with decreasing

w xnucleation rate, indicating more stable glasses 13 .

6. Conclusions

A kinetic analysis based on Chen and Ozawa’samended equations applied to DSC curves of AlF -3

based glasses, doped with different concentration ofNdF , PrF and PrF , codoped with YbF , ranging3 3 3 3

from 0.25 to 2.0 wt% showed that the thermalstability of the base glass increases by the addition ofthese rare-earth ions. Each rare-earth ion has a uniqueeffect on the stability of the base glass. The additionof 0.25 to 2.0 wt% of PrF , 0.25 to 0.5 wt% NdF ,3 3

and 0.5 to 1.5 wt% of YbF , codoped with 0.5 wt%3

of PrF , increases the thermal stability of the base3

glass. Any further addition of these rare-earth fluo-rides decreases the stability. This enhanced thermalstability in the presence of a rare-earth ion coupledwith the superior chemical durability and mechanicalproperties of AlF -based glasses, makes these materi-3

als candidates for fiber applications.

Acknowledgements

T.I. and A.N.K. are grateful to Ghulam IshaqKhan Institute of Engineering Sciences and Technol-ogy for providing facilities to carry out this researchwork.

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