Anomalous eutectic

The freezing point diagram for the hydrazine—water system (Eig. 1) shows two low melting eutectics and a compound at 64 wt % hydrazine having a melting point of —51.6°C. The latter corresponds to hydrazine hydrate [7803-57-8] which has a 1 1 molar ratio of hydrazine to water. The anomalous behavior of certain physical properties such as viscosity and density at the hydrate composition indicates that the hydrate exists both in the Hquid as well as in the soHd phase. In the vapor phase, hydrazine hydrate partially dissociates. 

K. Kassner, A. Valance, C. Misbah, D. Temkin. New broken-parity state and a transition to anomalous lamellae in eutectic growth. Phys Rev E 4S 1091, 1993. 

The extensive morphology studies conducted by Smith and his co-workers [23] enabled him to classify eutectic microstructures in terms of the parameters that influence the growth process. Some of the main features of this classification scheme are illustrated in Fig. 41, which shows the influence of solution entropy and volume fraction on the microstructure at a growth velocity of 5 X 10-4 cm/sec. The vertical line at ASa — 23 J/mol K 1 divides normal and anomalous structures. Whisker-like... 

Fig. 41. Classification of eutectic microstructures in terms of volume fractions and entropy of solution for an anomalous growth velocity of 5 X 10 4 cm/sec a, regular lamellar b, regular rods c, broken lamellar d, irregular e, complex regular f, quasiregular and g, irregular fibrous. After Elliot [23], Reproduced with permission of the ASM International, Metals Park.

The latter result is clearly quite acceptable, the sole exception being the point at x = 0.5 (cross in brackets). This datum is consistent with 1 1 compound formation of the solvent components, which is reflected also by the anomalous solution expansion at 30°C reported for this composition (44). Thus, since the partition-coefficient data were taken at 34 C, i.e., at only 3 above the congruent melting point of the aniline phenol complex, the deviation mi t well be due to the onset of stoichiometric solvent-solvent sociation. Moreover, we have observed similar anomalies in plots of partition coefficients against volume fraction at sis much sis 20° above eutectic points of mixed liquid-crystsdline phsises (45). 

The eutectic solvent HCIO4-H2O is especially suitable for Salzkryoskopie of cations which are stable only in acid solutions and of acids. With the latter the depression of freezing point is not affected by the degree of acid dissociation. On the other hand, polymerization of the acid molecule (or its dissociated anion) will lead to a decreased freezing-point depression. Solutions of nitric acid in this medium have been reported to produce anomalously low depressions, indicative of dimer formation. The available data are not adequate to distinguish between such species as HaNgOg, HNaO", NaOg", or... 

Growth. The growth mechanisms of eutectic Sn-Pb and eutectic Sn-Ag-Cu are also different. In the eutectic solidification process (Ref 5), the eutectic alloys are divided into two types normal and anomalous 63Sn-37Pb is a normal eutectic, while the Pb-free eutectic alloys (eutectic Sn-Ag, Sn-Cu, and Sn-Ag-Cu) are anomalous. 

The discussion in the previous section makes it clear that there are significant differences between lead-free, tin-based solder systems and other solders successfully used in the past. The anomalous crystal structure and allotropic transformations in tin-based alloys can affect the material properties that control the ultimate performance and reliability of these solders in microelectronic assemblies. Table 5 provides a comparison of several solder systems including high-lead and eutectic tin-lead alloys, white and gray tin, a lead-indium solder, gold-tin eutectic, and a fictitious ideal alloy. 

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