Thermal, solid–liquid equilibrium, crystallization, and microstructural studies of organic monotectic alloy: 4,4′-Dibromobiphenyl–succinonitrile
Introduction
The investigations on the temperature dependent solidification behaviour of monotectic alloy are of potential importance both from fundamental understanding of the development of self-lubricating alloys and for industrial applications [1], [2]. Although, metallic systems constitute an interesting area of investigations [3], [4], [5], they are not suitable for detail study due to high transformation temperature and wide density difference of the components involved. However, low transformation temperature, transparency, wider choice of materials and minimised convection effects are the special features that have prompted a number of research groups [6], [7] to work on organic eutectics, monotectics and olecular complexes. As such organic systems are used as model systems for detailed investigation of the parameters which control the mechanism of solidification which decides the properties of materials. In addition, these materials are being used for various physicochemical investigations for their use for non-linear optical effects and different electronic applications [8], [9], [10].
The monotectic alloys have been less studied due to several difficulties associated with the miscibility gap systems while some of the articles [2], [11], [12] explain various interesting phenomena of monotectic alloys. The main problem arises due to a wide freezing range and large density difference between two liquid phases. The role of wetting behaviour, interfacial energy, thermal conductivity and buoyancy in a phase separation process has been the subject of great discussion. 4,4′-Dibromobiphenyl (DBBP) is a material of high enthalpy of fusion (28.38 kJ/mole) and simulates the nonmetallic solidification (faceted morphology) where as succinonitrile (SCN) is a material of low enthalpy of fusion (3.70 kJ/mole) and corresponds the metallic solidification (nonfaceted morphology). Therefore, the present DBBP–SCN system is very good organic analog of metal–nonmetal systems like Al–Si and Zn–Bi. In the present paper, the details concerning phase diagram, thermochemistry, linear velocity of crystallization at different undercoolings, heat of fusion, Jackson's roughness parameter, interfacial energy and microstructure of DBBP–SCN system are reported.
Section snippets
Materials and purification
Succinonitrile, obtained from Aldrich, Germany, was purified by repeated distillation under reduced pressure while 4,4′-dibromobiphenyl (Aldrich, Germany) was used as received. The melting temperatures of DBBP and SCN were found to be 167.5 °C and 56.5 °C, respectively which are quite close to their respective values reported [13].
Phase diagram
The phase diagram of DBBP–SCN system was determined by the thaw–melt method in the form of temperature–composition curve. In this method [14], [15], mixtures of two
Phase diagram
The phase diagram, between different compositions of DBBP–SCN and their melting/miscibility temperature, shows the formation of a monotectic and a eutectic as depicted in Fig. 1. The numerical values of different composition and its melting/miscibility temperature are tabulated in Table 1. Melting point of DBBP is 167.5 °C and it decreased by the addition of SCN. When the mole fraction of SCN is 0.15 immiscibility appears and at certain temperature the two liquids are completely miscible. With
Enthalpy of fusion
The values of enthalpy of fusion of the pure components, the eutectic and the monotectic, determined by the DSC method, are reported in Table 3. For comparison, the value of enthalpy of fusion of eutectic calculated by the mixture law [15] is also included in the same table. The enthalpy of mixing which is the difference of experimentally determined and the calculated values of the enthalpy of fusion are found to be 0.63 kJ mol−1. As such, three types of structures are suggested [22];
Conclusions
The 4,4′-dibromobiphenyl–succinonitrile binary phase diagram was experimentally studied in detail which shows the formation of a monotectic and a eutectic with 0.15 and 0.9997 mole fractions of succinonitrile, respectively. The consolute temperature was found to be 67 °C above the monotectic horizontal. The growth behaviour of the pure components, the eutectic and the monotectic determined by measuring the rate of movement solid–liquid interface in a capillary suggest that growth data obey the
Acknowledgement
The authors would like to thank Board of Research in Nuclear Science, Department of Atomic Energy, Mumbai, India, for financial support.
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