ANOMALIES AT THE Sn MELTING POINT

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ANOMALIES AT THE Sn MELTING POINT

W. Wildner, H.-D. Pfannes, U. Gonser

To cite this version:

W. Wildner, H.-D. Pfannes, U. Gonser. ANOMALIES AT THE Sn MELTING POINT. Journal de

Physique Colloques, 1974, 35 (C6), pp.C6-381-C6-382. �10.1051/jphyscol:1974670�. �jpa-00215827�

JOURNAL DE PHYSIQUE

Colloque C6, supplément au

12, Tome 35, Décembre 1974, page C6-381

ANOMALIES AT THE Sn MELTING POINT

W. WILDNER, H.-D. PFANNES and U. GONSER

Fachbereich Angewandte Physik, Werkstoffphysik und Werkstofftechnologie Universitat des Saarlandes, Saarbrücken, Germany

Résumé. -

De petites particules d'étain métallique ont été incorporées dans une matrice de BN.

L'effet Mossbauer

mesuré autour du point de fusion. Les résultats essentiels sont

1) Au point de fusion, le signal de résonance chute

une valeur incommensurablement faible et ceci dans un intervalle de température très limité. Un élargissement de raie par diffusion n'a pas pu être détecté, confirmant Packwood et Longworth. L'observation d'un élargissement de raie par Boyle

2) La probabilité de résonance montre un phénomène prononcé d'hystérésis dû

la surfusion.

L'analyse de ce phénomène, en association avec la taille des particules d'étain, a permis de déduire une énergie d'interphase liquide-solide. Cette énergie (48 ergs cm-2) est en accord raisonnable avec les résultats d'autres techniques.

Abstract. -

Small particles of metallic Sn were embedded in a BN matrix. The Mossbauer effect was measured around the melting point. The main results are

1) At the melting point the resonance signal drops to an unmeasurable small value in a very limited temperature interval. Line broadening by diffusion could not be detected, confirming Packwood and Longworth. The observation of line broadening by Boyle

must have been due to impurities.

2)

The resonance effect shows a pronounced hysteresis behavior due to undercooling. From the analysis of this phenornenon in conjunction with the Sn particle size an interphase energy between liquid and solid phase could be deduced. The energy (48 ergs cm-2) is in reasonable agreement with results by other techniques.

Mossbauer spectroscopy has been used previously t o investigate the behavior of tin a t the melting point [l-31. However, the results were contradictory.

Deviating from earlier experimental arrangement we have prepared a sample where the tin was embedded as small particles. Specifically, Sn particles with a diameter of (5 2 1)

were mixed with boronnitride (BN) powder (2 p in diameter) and pressed. A volume ratio of Sn

BN equal to 1

6 was obtained. Optical microscopy of the sample indicates an homogeneous mixture of Sn and BN particles and furthermore, the Sn particles were to a great extent isolated from each other by the BN matrix as shown schematically in figure 1. Such an absorber is easy to handle in the temperature region of the melting point of Sn (505 K), particularly repeated variation in temperature above and below the melting point of Sn does not change the consistence of the sample in any way. For the Moss- bauer measurements the absorber was placed into a furnace. The temperature deviation never exceeded 0.2 K over a period of many hours and over the entire sample a t any measuring temperature.

Two main results were obtained :

1) At the melting point the resonance signal d r o ~ s Fm.

Consistence of the BN-Sn absorber. Hatched circIes to an unmeasurable small value in a very small tem- indicate Sn particles, between them

crystallites.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974670

C6-382 W. WILDNER, H.-D. PFANNES AND

GONSER

2) The resonance effect shows a pronounced hyste- resis resulting from an undercooling behavior of the specimen as shown in figure 2. The intensity of the resonance line makes an hysteresis loop which is completely reversible and extends over a wide tempe- rature range of 80 K. This undercooling behavior is mainly due to the consistence of Our specimen. Because the Sn particles are isolated from each other, the existence of a nucleus for solidification in one or a few Sn particles does not affect an instantaneous solidi- fication of the whole Sn present in Our sample. A comparison of the intensities in the heating-up and cooling-down branches gives an estimate of the frac- tional parts of liquid and solid phase present. Accord- ing to the theory of homogeneous nucleation [4,

the number of nuclei per unit volume and per unit time is given by

perature interval whose width corresponds to the

I(AT)

nkT exp

exp -

h

i ? 3 A ~ ( A T ) ~ kT limitation in Our temperature resolution. Line broaden- ing by diffusion could not be detected, confirming Packwood and Langworth [2]. The observation of

line broadening by Boyle

al. [Il must have been due to impurities.

where Tis the temperature, To the melting temperature, AT

To - T the actual undercooling temperature, k Boltzmann's constant, h Planck's constant, n the number of metal atoms per cm3, AGA the free energy of activation for transport of atoms across the liquid solid interface,

the interfacial energy per cm2 bet- ween solid and liquid, and

the latent heat of fusion of the metal per cm3. This equation indicates that I(AT) remains very small until AT reaches a critical value, then I(AT) increases very fast, in fact in metallic sys- tems the increase is so fast that the actual nucleation rate can not be measured. For instance as can be seen from eq. (1) Z(AT) increases from about 10' to 106 nuclei per second and per cm3 when the temperature lowers from 1 K above the maximum undercooling temperature to 1 K below. This narrow range is often called nucleation temperature [4]. According to Turnbull

the factor exp(- AGA/kT) is approxima- tely IO-' over the whole temperature region

interest and the rate of nucleation at the maximum of

- O

O

O heating up a cooling d o w n

Temperature [ K I

FIG. 2. -

Hysteresis loop of

resonance absorption.

the undercooling temperature is about 1O6''. With these approximations and the measured .undercooling temperature for tin AT,,,,,

80 K one obtains an interfacial energy

(tin)

48 ergs/cm2. Turnbull obtained dilatometrically from small tin particles dispersed in methyl cyclopentane AT,,,,, (tin)

105 K and a resulting a,, (tin)

54.5 ergs/cm2. The diffe- rence in the results can be attributed to the higher impu- rity content of the tin used in Our experiment (99.9 %) whereas Turnbull made his investigation with 99.999 % tin specimens. The influence of the heterogeneous nucleation which is not taken into account by eq. (1) therefore may play a role in Our investigation.

The common methods to determine undercooling temperatures are macroscopic in nature. The Moss- bauer effect as a microscopic probe offers, under certain conditions, a possibility to determine and to differentiate the solid and liquid phases. The statistical nature of the Mossbauer effect makes relatively long measuring time

30 min) necessary and therefore it can not be applied to rather dynamic systems. However, in cases as described here Mossbauer spectroscopy is a convenient tool to determine the « nucleation tempe- rature » without interferring into the system.

References

111

BUNBURY, D. S. P., EDWARDS, C., HALL, [41 CHALMERS,B.,

&Sons, H. E.,

(1961) 129. Inc.,

York) 1964.

[2] LANGWORTH,

G., PACKWOOD, R.

(1965) 75. [5] TURNBULL, D.,

(1950) 769.

[3] PACKWOOD,

H., BLACK,

(1965)

653.