Fig.6. Plot of efficiency vs. Cathode temperature for a number of materials for thermionic generators under investigation at the Westinghouse research laboratories

Note that each material dictates an operating temperature and that many materials reach excessive evaporation rates before interest­ing efficiencies can be achieved.

Another important factor determines the current flow in a ther­mionic converter. This phenomenon is called space charge the mu­tual repulsion of electrons. An electron emerging from the cathode finds itself in the company of a swarm of other electrons, all simi­larly charged, from which it is repelled. This will drive most of the electrons back into the cathode before they have a chance to reach the anode. One practical way for eliminating space charge consists of introducing heavy positive ions in numbers sufficient to neutralize the charge electrons.

Application of thermionic converters for the commercial genera­tion of power appears most favorable when the thermionic element is used as a topping unit for a nuclear steam plant, thereby taking ad­vantage of the high temperatures available from the fuel.

Thermionic conversion offers one possible means of obtaining ef­ficient conversion of heat to electrical power. Whether it becomes competitive with other means will depend largely on the solution of problems concerning the properties of materials. In the past there has been no particular urge to find or produce materials having the pecu­liar properties demanded by the thermionic converter. The field is therefore largely unexplored and advances of considerable magnitude can be expected.

By John Coltman

Vocabulary Study

Put the following words in their proper forms into the blanks:

apply, application, supply, efficient, efficiency, rate, available

1. High internal resistivity of the material will reduce its …

2. Compressive forces must be … through spring-loading.

3. These materials are … today and are used for military… .

4. Small conventional power … have an … of 5%.

5. Present energy sources are unable to … our needs for electric power.

6. Today a thermoelectric generator … at 5,000 watts is available.

Speaking

Task 9

Speak on different ways of generating electricity