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Thermionic emission

Thermionic emission (T.E.) is a very powerful technique for generating intense, continuous beams of charge carriers, in particular electrons and alkali ions. The technique itself has been developed at the beginning of the 20th century and was recognized by the Nobel prize awarded to Richardson in 1928. For references to reviews on thermionic emission, see our literature below.

The Weitzel group has developed intense ion beams of all natural alkali elements: Li, Na, K, Rb and Cs. The ion beams are chemically pure, the level of impurities being on the order of a percent.[1,2]. The original reason for developing alkali ion sources was the need of such ion beams in the CAIT approach

Fig. 1   Mass spectra obtained for alkali ion emission from MAlSi2O6

One of the physical advantages of T.E. is that it allows to determin the work functions (both for electrons and ions). As an example we recently reported an extensive study of T.E. of Li+ from spodumene (LiAlSi2O6) covering all relevant ranges from the Child-Langmuir regime to the Richardson-Dushman regime and the Schottky regime. The ionic work function has been determined with an accuracy on the order of 15 meV [2].

Figure 2  Ion currents observed in T.E. from spodumene as function of the emitter potential covering the Child-Langmui and Richardson-Dushmann regime and Arrhenius plot leading to a work function of (2.46 ± 0.01) eV [3]

Ultimately the thermionic emission studies provide access to a better understanding of electrode materials in lithium ion batteries. That work is described in more details at https://www.uni-marburg.de/en/fb15/researchgroups/ag-weitzel/research/energy-storage-material.

We are continuing our efforts in developing thermionic emission studies towards new frontiers. Recently we succeeded in mmeasuring electronic and ionic work function in lithium phosphates with a detection limit for emission currents on the order of atto-amperes. This is achieved by employing single ion and electron counting using micro channel plate detectors.

Literatur

[1]  T. Kolling, A. Schlemmer, C. Pietzonka, B. Harbrecht, K.-M. Weitzel,
Field effects in alkali ion emitters: Transition from Langmuir–Child to Schottky regime.
J. Appl. Phys., 107, 014105, (2010)

[2]     S. Schuld, B. Harbrecht, K.-M. Weitzel
Ionic work functions of alkali aluminosilicates - Correlations with structural and energetic landscapes.
Int. J. Mass Spectrom. 435, 291–297, (2019)

[3]  S. Schuld, M. Diekmann, M. Schäfer, and K.-M. Weitzel
The work function for Li+-ion emission from spodumene: A complete characterization of thermionic emission
Journal of Applied Physics, 120, 185102 (2016)

[4]     Johanna Schepp, Dominik Plamper, Jon Henrik Both, Karl-Michael Weitzel,
Combined measurement of electronic and ionic work functions, w(e-) and w(Li+), for Lithium Phosphate,
J. Appl. Phys. 128, 115108 (2020).