56
3 Modification of the Work Function
Fig. 3.20 Schematic phase
diagram and relationship
between work function and
bulk content for binary
systems forming
intermetallic compounds
3.1.4 Two Elements with Miscibility Gap
There are combinations where two metals do not mix but separate, forming a miscibility gap in the phase diagram. The composition dependence of the work function
exhibits specific features in accordance with the miscibility gap. The characteristic
feature of this type of binary system is that the work function is insensitive to the
composition in the range of the gap. The reason for this feature is explained as
follows. When a small amount of element B is added to element A, B dissolves in A
until the concentration of B reaches the miscibility gap. Up to this concentration, the
composition dependence of the work function is the same as that of alloys because
the mixture is actually an alloy. When the concentration of B exceeds the solubility limit, the mixture separates into two phases, B-dissolved A and A-dissolved
B. Phase separation occurs until the concentration of B reaches the opposite side
of the limit, where A is dissolved in B. For the entire composition range where the
mixture separates into the two phases, the surface is covered by either B-dissolved A
or A-dissolved B, because the one with the lower surface energy preferentially exists
at the surface regardless of the mixing ratio. In this way, the work function of the
mixture remains as that of one of the two phases at the solubility limit in the phase
diagram with the lower surface energy. Beyond the gap, A is dissolved into B, so
the composition dependence of the work function is that of an alloy. Therefore, the
composition dependence of the work function is similar to that schematically shown
in Fig. 3.21, which also shows the phase diagram.
Fig. 3.21 Schematic phase diagram and relationship between work function and bulk composition
for binary systems having miscibility gap
3 Modification of the Work Function
Fig. 3.20 Schematic phase
diagram and relationship
between work function and
bulk content for binary
systems forming
intermetallic compounds
3.1.4 Two Elements with Miscibility Gap
There are combinations where two metals do not mix but separate, forming a miscibility gap in the phase diagram. The composition dependence of the work function
exhibits specific features in accordance with the miscibility gap. The characteristic
feature of this type of binary system is that the work function is insensitive to the
composition in the range of the gap. The reason for this feature is explained as
follows. When a small amount of element B is added to element A, B dissolves in A
until the concentration of B reaches the miscibility gap. Up to this concentration, the
composition dependence of the work function is the same as that of alloys because
the mixture is actually an alloy. When the concentration of B exceeds the solubility limit, the mixture separates into two phases, B-dissolved A and A-dissolved
B. Phase separation occurs until the concentration of B reaches the opposite side
of the limit, where A is dissolved in B. For the entire composition range where the
mixture separates into the two phases, the surface is covered by either B-dissolved A
or A-dissolved B, because the one with the lower surface energy preferentially exists
at the surface regardless of the mixing ratio. In this way, the work function of the
mixture remains as that of one of the two phases at the solubility limit in the phase
diagram with the lower surface energy. Beyond the gap, A is dissolved into B, so
the composition dependence of the work function is that of an alloy. Therefore, the
composition dependence of the work function is similar to that schematically shown
in Fig. 3.21, which also shows the phase diagram.
Fig. 3.21 Schematic phase diagram and relationship between work function and bulk composition
for binary systems having miscibility gap