Thermionic devices have held some stead into present day despite competing technologies often taking their place, most noticeably the development of the silicon transistor. However, while its applications in computing have for the most part ceased to exist, thermionic valves still have distinct advantages over newer technologies in certain applications. The majority of current uses are continuations of its uses in the past which, generally speaking owes to the reliability and high power usability not efficiently reproducible by transistor based counterparts.
Thermionic devices have practical value in the present day, for example, the x-ray tube is used in a host of applications, from airport scanners to CT-Scanners. They have also long played important roles in scientific research such as particle acceleration, integral to the furthering of modern science. Almost none of these are new inventions, the basis of their operation has always remained fairly elegant with few changes. Present day manufacturing processes and complementary technologies have improved their operation to the often more sophisticated tasks of modern day science.
In light of these observations it seems more appropriate to see some of the main thermionic devices utilised in modern day applications:
Sound and Transmission
The Magnetron
The Cathode Ray Tube
Travelling Wave Tubes
The Klystron