MELTING TEMPERATURE OF THE CONTACT LAYER OF CRYSTALS

P. A. SAVINTSEV, V. E. AVERICHEVA

(Presented by Professor Doctor A. A. Vorob’ev)

Introduction

Eutectic alloys differ from other heterogeneous and homogeneous alloys in a number of properties, one of which is, for example, their fusibility. Eutectics, unlike other alloys, crystallize at a constant temperature corresponding to the lowest point of the liquidus line. It is assumed that, in order to obtain a eutectic, it is necessary either to dissolve a high-melting component in a liquid low-melting one, or to mix liquid components taken in amounts corresponding to the eutectic composition. Upon solidification of such a solution, an alloy is formed with a definite eutectic structure characteristic of it. In many cases, finding the eutectic temperature and the eutectic composition of the given components is an important practical problem.

Many attempts have been made to calculate the composition and melting temperature of eutectics as functions of the heat of fusion and the melting temperature of the components forming the alloy, and also as functions of the lattice energy and heat capacity \[1, 2\]. However, a reliable solution of the problem has not yet been found.

The most correct method remains the laborious one of constructing fusibility diagrams, which consists in taking cooling curves for a series of alloys of the given system at different concentrations of the components. In some cases even this method of finding the eutectic point of a system proves unacceptable, for example, in cases where, during melting of the components, transformations occur and complex systems are formed containing both the pure components and the products of their transformations \[3\].

A number of authors \[4, 5, 6\] have shown that, at the contact point of two crystals constituting a eutectic pair, a liquid phase appears.

N. S. Kurnakov and N. N. Efremov \[4\], carrying out experiments at room temperature (15°C), associated the appearance of liquid in an arbitrary mixture of crystals of methyl mustard oil ($t_{\text{m}} = 34^\circ$) and camphene ($t_{\text{m}} = 49.3^\circ$), which form a eutectic at $t = -8^\circ$, with the form of the fusibility diagrams of eutectic systems and with the considerable excess of the experimental temperature over the eutectic temperature.

It is seen from Fig. 1 that the region of existence of the liquid phase at the experimental temperature covers a certain interval of concentrations. With approxi-

as the temperature of the experiment approaches the eutectic temperature, this concentration interval becomes narrower. On this basis N. S. Kurnakov and N. N. Efremov concluded that the appearance of a liquid in a mixture of crystals of noneutectic composition is possible only when the temperature of the experiment considerably exceeds the eutectic temperature. For eutectic melting, however, a eutectic composition of the components or an already prepared eutectic structure is necessary.

The erroneousness of the above ideas was shown by the work of D. D. Saratovkin and P. A. Savintsev \[5, 7, 8\], who investigated eutectic pairs of organic substances and metals and showed that, for eutectic melting, simple contact of solid heterogeneous substances at a temperature only slightly exceeding the eutectic temperature is sufficient. The appearance of a liquid on the contact surface of the crystals was called contact melting \[7\]. The eutectic structure of an alloy always ensures contact of the components and, consequently, the conditions for contact melting. Thus, contact melting is also the cause of the low melting point of eutectics \[7\].

D. D. Saratovkin and P. A. Savintsev showed that the alloy formed during contact melting of metals is a eutectic alloy. Thus arose the “contact method” of obtaining a eutectic, a very simple and, it seemed, reliable one.

The contact method has found practical application as a method for detecting eutectic points of unstudied systems, chiefly systems formed by substances that decompose on heating to high temperatures \[8, 9, 10\], and also for detecting metastable eutectics \[3\]. It is considered \[11\] that, in speed of determination and ease of execution, the method of contact melting is the simplest of all existing methods for determining the melting temperature of eutectics, both in binary and in multicomponent systems.

The method of contact melting was used \[12\] to determine the eutectic points of systems whose components are inorganic salts and ice.

Thus:

1. The phenomenon of contact melting was first discovered in substances forming systems with a eutectic.
2. The phenomenon of contact melting has been studied in organic substances and metals that constitute eutectic systems.
3. The phenomenon of contact melting has been used to detect eutectic points of unstudied systems formed by organic substances, certain inorganic salts, and metals.

It became widely accepted that contact melting is a feature of pairs of substances forming eutectic systems, i.e., mechanical mixtures.

Contact Melting of Alkali-Halide Crystals

Many physical properties of alloys of alkali-halide crystals are connected with the molecular interaction of the components. Molecular interaction is also manifested in the phenomenon of contact melting. Contact melting of alkali-halide compounds had not previously been observed; however, its study is of both theoretical and practical interest.

In the work of D. D. Saratovkin and P. A. Savintsev [13], eutectic (contact) melting is considered as a consequence of distortion of the lattice of one crystal at the point of its contact with the lattice of another crystal. Obviously, this factor should also manifest itself upon contact of certain substances possessing unlimited mutual solubility. In this case, contact melting should be expected for substances that form a series of solid solutions.

It is evidently necessary to determine whether contact melting is a general property of substances, and also to evaluate the method of contact melting as a method for detecting eutectic points.

We have carried out a study with alkali-halide salts of chemically pure grade.

1. The powder of the more refractory component was melted and crystallized in a porcelain crucible. The less refractory component was crystallized in the form of a cylindrical rod in a special nickel mold. Then the cylindrical specimen was placed with its base on the surface of the first component, and the resulting system was placed in a crucible furnace. In this way continuous contact of the crystal surfaces was ensured during their heating. The temperature of the contact surface was measured during heating by means of a chromel–alumel thermocouple. At the same time, visual observation was carried out. The moment of appearance of liquid was recorded from the slowing of the temperature rise and visually. The liquid appeared at a temperature considerably lower than the melting temperature of the components.

In some experiments the crystals were heated separately to the temperature of contact melting and were then brought into contact. Liquid appeared only when the crystals were brought into contact.

The results of the study are given in Table 1.

It is clear from Table 1 that contact melting is observed for all the alkali-halide crystals studied, regardless of whether they form eutectic systems or a continuous series of solid solutions. The order of magnitude of the lowering of the melting temperature of the contacting surfaces relative to the melting temperatures of the pure components is approximately the same both for substances of the first group and for substances of the second group. For some systems the lowering of the temperature reaches several tens and hundreds of degrees.

1. As was shown earlier, as a result of contact melting of substances constituting a system with a eutectic, a eutectic alloy is formed. To determine the character of the alloy obtained by the method of contact melting of substances forming solid solutions, we recorded Debye–Scherrer X-ray diffraction patterns of the alloys. Table 2 gives the diameters of the interference lines of the X-ray diffraction patterns of the alloy and of the components forming it. It is clear from Table 2 that the lines of the X-ray diffraction pattern are shifted both relative to the lines of KCl and relative to the lines of KBr. Consequently, the contact alloy KCl—KBr is a solid solution. Similar X-ray diffraction patterns were also obtained for the contact alloys KCl—NaBr, KBr—NaBr.

2. From the data presented the following follows: contact melting is observed not only in alkali-halide crystals forming eutectics, but also in crystals forming solid solutions; consequently, by the method of contact melting it is impossible to resolve the question of mutual solubility of alkali-halide salts. As a result of their contact melting, both mechanical mixtures and solid solutions are formed.

Table 1

Temperature of contact melting of alkali-halide crystals

Contact Melting of Organic Substances

In the following series of experiments we discovered contact melting of organic substances capable of forming solid solutions. Para-chlorobromobenzene with para-nitrochlorobenzene form a continuous series of solid solutions with descending solidus and liquidus lines. A similar melting diagram also characterizes the solid solutions of para-iodobromobenzene with para-dibromobenzene.

Table 2

Distance between symmetrical interference lines of X-ray diffraction patterns of KCl, KBr, and their contact alloy

We measured the melting temperatures of the indicated substances, as well as the melting temperatures of mixtures of their powders taken in arbitrary ratios.

The temperature values are given in Table 3.

Table 3

Contact melting temperature of organic substances

The lowering of the contact-melting temperature relative to the melting temperature of the pure components amounts approximately to from 5 to 27%, i.e., the effect of contact melting is manifested to a considerable degree.

Conclusions

1. Contact melting has been discovered in alkali-halide crystals that form both eutectic systems and solid solutions.

2. Contact melting has been discovered in organic substances forming a continuous series of solid solutions.

3. The method of contact melting cannot be used, without additional investigations, to detect eutectic points of alkali-halide crystals.

1. The use of the contact method for detecting eutectic points of organic substances is questionable, since on the basis of this method an erroneous conclusion may be drawn about the presence of a eutectic in a system that does not contain one.

2. Contact melting is evidently observed in all pairs of substances that form systems with a descending liquidus line.

REFERENCES

1. A. F. Kapustinskii, Tr. VNIIMS, vol. 109, 10, 1936.
2. M. Usanovich, DAN SSSR, 112, No. 4, 1957.
3. M. Kh. Gluzman, A. L. Gershuns, L. S. Palatnik, D. E. Plotkina, R. S. Milner. ZhFKh, 27, vol. 9, 1953.
4. N. S. Kurnakov, N. N. Efremov. Izvestiya Petrogradskogo politekhnicheskogo instituta, 18, 1912.
5. D. D. Saratovkin, P. A. Savintsev. DAN SSSR, 33, vol. 4, 1941; DAN SSSR, 80, vol. 4, 1951.
6. S. V. Avakyan and N. F. Lashko, DAN SSSR, 65, No. 1, 1949.
7. D. D. Saratovkin and P. A. Savintsev. DAN SSSR, 58, vol. 9, 1947.
8. M. Kh. Gluzman, A. L. Gershuns, Ya. E. Geruzin. ZhPKh, 26, 1953.
9. M. Kh. Gluzman, V. P. Rubtsova. ZhAKh, II, 1956.
10. M. Kh. Gluzman, V. P. Rubtsova. Aptechnoe delo, No. 1, 15, 1957.
11. M. Kh. Gluzman and V. P. Rubtsova. ZhOKh, 27, 3, 1957.
12. V. B. Chernogorenko. ZhNKh, 1, 2, 1956.
13. D. D. Saratovkin and P. A. Savintsev. Izvestiya TPI, 66, No. 3, 1948.

Errata

Publishing House of Technical and Theoretical Literature, vol. 95.