I - Miscibility Gap (MG) Between Two Liquid Metals

Abstract

Two metals in their liquid state do not form a homogeneous solution but rather separate out in two distinct regions (phases)—much like water and oil. The miscibility gap (MG) between oil and water is understood readily in terms of their electronic bonding differences; water being ionic whereas oil being exclusively covalent bond. However, in the intermetallic system based on the free electron theory, the miscibility gap is totally unexpected and defies a conventional theoretical explanation. This is because the free electron theory is based on a sea of free-electrons that are roaming about among the positively charged nuclei. Thus, there is no reason for two metals in their liquid state to exist in MG state. It is even more perturbing when chemists find lithium (Li) and sodium (Na), both of which are I-A elements and therefore chemically identical, to form a MG. In fact, a MG also exists between potassium (K) and sodium (Na). The theory being proposed states that metals and alloys are consisted of covalent bond as well as metallic band. Also, it has been pointed out that covalent bond has many faces. Therefore, it can be said that while the free-electron part from both lithium metal and sodium metal are indistinguishable and may mix freely, the covalent bond part may be distinct from one another to the extent that they are prevented from mixing and forming a homogeneous solution. As stated in the proposed theory, the covalent bond is far more susceptible to the temperature change than the free- electrons. Therefore at higher temperature, the covalent bond may lose its covalency completely and no longer prevent the intermixing of two metals.