2.2.2 SINGLE-MODE APPARATUS
The single-mode cavity allows only a single-mode to enter the cavity by waveguide.
A properly designed single-mode cavity or reactor can prevent the formation of
“hot and cold” spots. This advantage is very important in organic synthesis since
the actual heating pattern can be controlled. Therefore, higher reproducibility and
predictability are achieved.
The basic difference of a single-mode apparatus is its ability to create a new
standing wave pattern. This pattern is generated by the interference of fields having
the same amplitude but different oscillating directions. This interface generates
an array of nodes (zero microwave intensity) and antinodes (highest magnitude of
microwave energy) (Fig. 2.1).
FIGURE 2.1 Nodes and antinodes standing wave pattern.
The design of a single-mode apparatus is governed by the distance of the sample
from the magnetron. It should be appropriate to ensure that the sample is placed at
the antinodes of the standing electromagnetic wave pattern (Fig. 2.2).
Several consequences of single-mode cavity design must be appreciated. First,
microwave-absorbing materials placed inside such a cavity will absorb microwaves
and as a consequence, the material is heated. The second consequence is more important.
There are some specific positions inside the single-mode cavity, where
items are placed so that the sample is heated. On the other hand, there are certain
positions in the single-mode cavity, where no heating will occur.
One of the limitations of this apparatus is that anything placed inside this singlemode
cavity can disrupt the standing wave pattern. Therefore, it is quite uncommon
for single-mode cavities to be designed to accept more than one target object (such
as a reaction vessel) to be heated at a time.