Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 101, No. 6, December 1989, pp. 539-546. �9 Printed in India.

Morphological and solidification studies of the naphthalene-thymoi eutectic

P S BASSI 1 and G P S A C H D E V 2. t Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India 2 Department of Chemistry, Government G M Science College, Jammu 180001, India

MS received 11 May 1989; revised 28 September 1989

Mr, tract. The mechanism of eutectic solidification of the naphthalene-thymol system has been investigated by spontaneous crystallization, linear velocity of crystallization and microscopic studies. The kinetic data for all the compositions of the system sugg~'sts thai the growth is governed by a screw dislocation mechanism. The diffusion process has been found to be a dominant factor in explaining the decrease in the velocity of crystallization when one component is added to the other and also explains the higher growth rate for the eutectic mixture. Both the kinetic and microscopic Studies confirm the interdependent nature of the growth of the two phases of the eutectic. The two phases grow simultaneously side by side and are controlled by the cross-diffusion ahead of the solid-liquid interface.

Keywords. Heterogeneous nucleation; crystal growth; eutectic solidification; eutectic morphology; growth rate anisotropy.

I. Introduction

Although the subject of eutectic solidification (Hillig and Turnbul l 1956; Tiller 1958; Chadwick 1963; Chaimers 1964; Rastogi and Bassi 1964; Hunt and Jackson 1966; Jackson and Hun t 1966; Bassi and Sachdev 1974; Rastogi et al 1977) has been of absorbing interest for many years, much of the recent development in unders tanding crystal growth has been stimulated by its increasing commercial importance. Theories of crystal growth from the melts of one -componen t systems have been formulated and improved upon from time to time. Most of the work done on two-componen t systems relate to the systems forming regular eutectic morphologies. Jackson and Hunt ' s characterizat ion of eutectic morphologies is based on the ent ropy of fusion (Jackson and Hunt 1966; Hunt and Jackson 1966), while that of Croker et al (1973) is based on kinetics, en t ropy of solution and the volume fraction of each phase of the system. The kinetics and morphologies of irregular eutectic structures, however, remain little understood. The present investigation was under taken to study the solidification of the binary organic system made up of naphthalene and thymol which is expected to give faceted-faceted (irregular) crystal morphology.

* For correspondence

539

540 P S Bassi and G P Sachdev

2. Experimental

Naphthalene (BDH) was purified by ordinary distillation followed by slow sublimation/ in a china dish at 333K. Thymol (Schering, AG) was purified by repeated crystallization with absolute ethanol and kept in a vacuum desiccator for about a week. The melting points of purified material, i.e. naphthalene (353.3 K) and thymol (322.8K), are in good agreement with the literature values. The experimental techniques for the studying of the phase diagram (by the thaw melt method), temperature of spontaneous crystallization (using a sealed glass tube containing a definite amount of the melt, in a liquid bath stirred by a magnetic stirrer), linear velocity of crystallization (using a glass U-tube, 30cm long and of ID 0.6cm) and microscopic structures have been described earlier (Bassi and Sachdev 1974).

3. Results and discussion

3.! Heterogeneous nucleation

The phase diagram of the naphthalene-thymol system is given in figure 1, while the heterogeneous nucleation data is given in table I.

It is important to note that although the ASm values (entropy of fusion) of

: 3 5 0 -

A

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1

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M o l e - f roct ion of thymol

Figure I. Phase diagram for naphthalene-thymol system; (I) melting points, (I |) thaw points.

Study of naphthalene-thymol eutectic system 541

Table I. Heterogeneous nucleation data for different binary melts.

ASm x 103 of nucleating T,. AT

Mole fraction phase ~ = - - - of thymol (KJ tool- 1 K-- t ) T= Tm

0.0000 53"17 0"95 0.047 0.1004 53-17 0"96 0.037 0"2002 53-17 0-97 0"028 0-3204 53"17 0"95 0-045 0-3997 53' 17 0"94 0.054 0.5002 53"17 0"95 0"046 0.6008 53"17 0"96 0-035 0.6640 53"17 0"95 0"051 0.7008 53-44 0-94 0.060 0"799l 53"44 0'89 0" 110 0.9003 53"44 0"85 0" 150 1.0000 53"44 0'82 0.170

naphthalene and thymol are almost the same and that for the mixtures these values should only be higher by an amount contributed by the entropy of mixing, the ratio AT/T,, differs greatly. However, the ratio To~Tin (crystallographic thctor, ~) has a constant value for almost all the compositions except the thymol-rich compositions. AT is the undercooling (T in - To), Tm the melting point and Tc the temperature of spontaneous crystallization. A large difference in the AT/TIn values may be ascribed to the presence of very strong hydrogen bonding in thymol. The thymol melt is highly viscous and its nucleation is hindered to such an extent that our investigations reveal its undercooling to be of the order of 56 K. It therefore follows that a definite relationship exists between nucleation and the structure of the melt.

3.2 Linear velocity of crystallization

The linear velocity of crystallization (v) for the naphthalene- thymol system has been plotted for various degrees of undercooling (/ST) in figure 2. The following equation explains the dependence of the velocity of crystallization on the undercooling reckoned from the corresponding liquidus temperature,

V = K(AT)", (1)

where K and n are constants. The values of these constants and the velocities of crystallization of various compositions of the system for an undercooling of 5 K are given in table 2.

It is evident that the value of n is nearly 2 for most of the compositions. The lower value of n for thymol may be explained on the basis of strong hydrogen bonding. With the increase in AT, the velocities of crystallization do not increase much because of the unusual increase in viscosities. This is in agreement with our earlier investigations that the activation energy for viscous flow is maximum for thymol

542 P S Bassi and G P Sachdev

2.0"

O

~..o-

~.o

,q\

0 0:4 0.8 I"2 1:6 log AT

Figure 2. Linear velocity of crystallization at various degrees of undercooling. (I) Pure naphthalene, (11) pure thymol and (I!I) to (VIII) mixtures with 0-7898, 0.6990 (eutectic), 0-6805, 0-6002, 0.4000 and 0-2003 mole fractions of thymol respectively.

Table 2. Linear crystallization growth parameters for various compositions.

Vx 103 Mole fraction K x 103 for AT = 5 K of thymol (cm/s K") n (cm/s)

0.0000 89.9500 2"33 3548.00 0-2003 19.0500 1"35 166-00 0-4000 1.4130 I "83 25-70 0-6002 0-0933 2" 17 3"02 0-6805 0-0024 2.47 0.12 0-6990 0-0093 2'61 0-60 leutectic) 0-7898 0.0007 2.40 0"03 1.0000 0-3162 1'15 1.99

(Sachdev et ai 1984). The dev i a t i ons (Singh et al 1985) in va lues o f n f rom 2 m a y also

be due to differences in the ba th t e m p e r a t u r e s a n d the t e m p e r a t u r e s of the g r o w i n g interface.

The mos t p laus ib le theo ry (Hil l ig a~d T u r n b u l l 1956) which predic ts n = .2, a s sumes

Study of naphthalene-thymol eutectic system 543

the growth to be due to the presence of repeatable steps caused by screw dislocations on the planar growing face. The rate of growth is given by

3(AS) 2 D(AT) 2 v = (2)

4rr V,,,R Ta '

where AS is the molar entropy of fusion, D is the diffusion coefficient for transport across the interface, Vm is the molar volume, R is the gas constant, T is the equilibrium temperature and ~ is the interfaciai tension.

Table 2 reveals that for the same undercooling (AT = 5 K), there is a large decrease in the velocity of crystallization when one component is gradually added to the other. But for the eutectic composition an increase in the value of v has been observed. Similar results were observed by Parkhutik et al (1971) as well. These workers have observed higher growth rates for the pure components and the eutectic mixtures of AI-Mg, Mg-Zn, Bi-Sn, Sb-Zn binary systems.

The reason for the fall in velocities of crystallization for binary mixtures has to be sought either from the growth mechanism or the change in the parameters of (2). The present study rules out the change in growth mechanism on the considerations of kinetics and morphologies of the growing front. Therefore, the observed results can be explained on the basis of change in the parameters of (2). Since Vm, R, T and are constants or have nearly the same values for pure melts and their mixtures, AS for the mixtures is larger than for the pure components. Therefore, the fall of growth rate for the mixtures must be dependent on D, i.e. diffusion. Diffusion is much hindered in viscous melts but the decrease in the viscosities of the mixture of the system as reported elsewhere (Bassi and Sachdev 1974; Sachdev et al 1984), does not explain the much greater fall in the velocities of crystallization. The diffusion mechanism required to explain the kinetics of growth is discussed below.

in the crystallization from melts of non-eutectic compositions at a given under- cooling, fluctuations lead to the formation of a nucleus of a component (say component I) having a greater concentration than the eutectic mixture. As this nucleus grows, the surrounding liquid becomes richer in component 2. The solid/liquid interface is surrounded by a layer called the diffusion layer which contains more of component 2. The growth of component 1 is guided by the diffusion of this component from the bulk liquid to the solid/liquid interface through the diffusion layer. For different melts, the thickness of this diffusion layer goes on increasing with the increase in concentration of component 2 till the eutectic composition is reached. Thus the progressive fall in the velocities of crystallization when component 2 is added to component 1 (and vice versa) is explained. However, the situation in the melt of a eutectic composition is different. Here, the two components grow simultaneously, although one of the components is always ahead of the other. The diffusion of each component is facilitated by cross-diffusion and therefore a higher growth rate for the eutectic composition has been observed.

3.3 Microscopic and microphotographic studies

Microscopic studies confirm that the solid immediately separating out from a eutectic melt has an entirely different structure as compared to the pure components. The crystal morphologies of pure components and the eutectic solid grown from their

544 P S Bassi and G P Sachdev

melts are shown in figures 3, 4 and 5. It is quite evident that the eutectic solid is heterogeneous and the two components can be seen side by side. It has a spherulitic growth form similar to the one observed in the crystallization of organic high polymers IBillimeyer 1962; Magill 1965) from melts and the crystallization of CrTC 3 rods IVan den Boomgaard and Wolff 1972) in an Fe matrix.

Eutectic morphology can be studied in the light of classification proposed by Jackson and Hunt (1966), based on thermodynamic considerations. According to these workers, when both the components have a > 2, where ~t = ~ AS,n/R, each phase grows with a faceted solid- liquid interface and an irregular morphology of the eutectic is obtained. Since the crystallographic factor, ~ is less than and nearly equal to 1, AS,,,/R values for naphthalene and thymol are 6-40 and 6.43 respectively (Weast 1976-77). The values are much greater than 2 and an irregular growth is to be expected. However, at moderate growth rates, the eutectic solid does not possess a completely irregular structure but has a quasi-regular structure. A eutectic micro- structure solidified directionally has been shown in figure 6. The two favourably oriented grains have been elongated in the growth direction. These grains grow side by side to give an aligned preferred crystallographic morphology.

In random solidification ofeutectic melts, freezing begins at a number of nucleation sites and each nucleus grows to a eutectic grain. Each grain continues growing into the melt until it impinges on a grain growing from some other nucleus. It was observed that the nucleation centres and hence the number of eutectic grains increase with increase in undercooling.

The formation of these micro-structures is due to the simultaneous growth of two phases. This imposes a restriction on nucleation and initial growth. The growth of the two phases is quite inter-dependent and cross-diffusion, ahead of the interface, is required is sustain the growth process. Because of large entropy changes in organic compounds, the influence of the diffusiveness of the growth interface is predominant. Figure 7 shows the microphotograph of the solid/liquid interface of the eutectic mixture. The faceting tendency of the two phases is self-evident. Neither phase can escape the influence of the other and the normal dendritic growth is prevented.

The formation of these microstructures can also be explained on the basis of the morphological stability theory discussed by Sekerka (1973). Both the compounds forming the eutectic have pronounced growth rate anisotropy. Since one of the directions of growth has much greater rate, splitting along that direction is obtained. In a super-cooled melt one of the components first nucleates and then grows. The adjacent liquid becomes rich in the second component, which also nucleates. Now the two phases grow simultaneously side by side to give a quasi-regular structure.

Therefore, it can be concluded that the process of diffusion plays a major role in the solidification of binary systems. Eutectic morphology is guided by the faceting tendency of each phase and the growth rate anisotropy. Simultaneous growth of the two phases in the eutectic, gives rise to a quasi-regular micro-structure.

Study o f naphthalene-thymol eutectic system 545

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546 P S Bass i and G P Sachdev

Figure 7. Microphotograph of solid/liquid interface of eutectic mixture showing faceting tendency (200 • ).

References

Bassi P S and Sachdev G P 1974 Indian J. Chem. 12 727 Billimeyer F W Jr 1962 Textbook of polymer science (New York: John Wiley) p, 148 Chadwick G A 1963 Prog. Mater. Sci. 12 97 Chalmers B 1964 Principles of solidification (New York: John Wiley) p. 194 Croker M N, Fidler R A and Smith R W 1973 Proc. R. Soc. London A335 15 Hillig W B and Turnbull D 1956 J. Chem. Phys. 24 914 Hunt J D and Jackson K 1966 Trans. Metall. Soc. AIME 236 843 Jackson K A and Hunt J D 1966 Trans. Metall. Soc. AIME 236 1129 Magill J H 1965 J. Polym. Sci. 3 1195 Parkhutik P A, Lubenskii M Z and Zagorskii G G 1971 Vestsi Akad. Navuk B. SSR, Ser. Fiz.-Mat. Navuk 4

40 (1972 Chem. Abstr. 76 89269j) Rastogi R P and Bassi P S 1964 J. Phys. Chem. 68 2398 Rastogi R P, Singh N B and Singh N B 1977 J. Cryst. Growth 37 339 Sachdev G P, Sharma B L, Sharma N K and Bassi P S 1984 J. Indian Chem. Soc. 61 673 Sekerka R F 1973 Crystal growth: An introduction L morphological stability (ed.) P Hartman (Amsterdam:

North Holland) Singh N B, Rai U S and Singh O P 1985 J. Cryst. Growth 71 353 Tiller W A 1958 Liquid metals and solidifications (Cleveland, OH: American Society of Metals) p, 276 Van den Boomgaard J and Wolff L R 1972 J. Cryst. Growth 15 11 Weast R C 1976 77 Handbook of chemistry and physics (57th edn) (CRC press)