Recently, fundamental (thermophysical property) research on ionic clathrate hydrates has experienced remarkable growth, particularly over the last ten years [21–30]. Previously, beginning with the first paper on unusual hydrates of tetrabutylammonium salts in 1940 [31], a number of studies could be found on ionic clathrate hydrates (hereafter, semiclathrate hydrates) [32–35] before the unified terminology semiclathrate hydrate was generally accepted. Semiclathrate hydrates have been attracting increased attention because of their promising applications as phase change materials for refrigeration systems and in gas capture and storage [36–41]. In addition, there is interesting speculation that semiclathrate hydrate may be regarded as a representative substance for the study of thermal conductivity in clathrate hydrate in general. This is because: (1) it can reduce characterization problems as a solid sample, since semiclathrate hydrate is formed around ambient temperature under atmospheric pressure and is easy to handle; (2) accurately measuring the thermal conductivity of semiclathrate hydrates, which have many similarities to clathrate hydrates, may make possible a deeper understanding of the unique (anomalous) behavior of the thermal conductivity of clathrate hydrates; and (3) currently, there are no experimental studies on the thermal conductivity of semiclathrate hydrates.
