Main Content
Introduction to Charge Attachment Induced Transport (CAIT)
The ion conductivity of glassy material is the basis for several technical applications in the field of energy conversion and storage. The generally accepted model for ion transport in ion conducting glasses implies the thermally activated hopping of ions between different sites separated by an activation barrier. Established techniques for determining ion conductivities include impedance spectroscopy, the tracer diffusion technique and pulsed field gradient NMR.
Here, we describe an alternative approach based on measuring the macroscopic transport of ions through the material of interest induced by charge carrier attachment to the front side. Hence, the technique is termed Charge Attachment Induced Transport (CAIT) [1]. Originally, the approach was termed Bombardment Induced Ion Transport (BIIT) [2],[3]. The term CAIT is more general, since the charge carrier to be attached can be an ion (of either polarity) or an electron. The particle transported in the sample can also either be an ion or an electron, or both, in the case of mixed ionic electronic conduction. Finally, the term bombardment suggested the possibility of ion implantation, which can be safely excluded by choice of experimental parameters.
Figure 1 Sketch of the setup for the original version of ion attachment induced transport. Taken from ref. [1]
The measurement principle is based on shining a charge carrier beam on to the surface of the sample that is to be examined. The particles, ions of both polarities, electrons or plasma, are accelerated by the source potential, Usource. The sample is mounted on top of a block metal electrode, which is temperature controlled. Attachment of charge carriers to the surface induces a well-defined electrical surface potential, a surface charge carrier density and hence an electrochemical surface potential, c.f. Figure 1. Gradients of the respective quantities induces transport in the sample of interest.
There are two basic versions of CAIT, i. the native charge carrier CAIT and ii. the foreign charge carrier CAIT. The native charge carrier CAIT is particularly well suited to determining absolute conductivities and activation energies for ion hopping (see URL Native ion CAIT for details).
|
|
Figure 2 Data from the first BIIT manuscript. Current-voltage plot (left) and Arrhenius plot (right) [2].
The foreign CAIT inherently introduces foreign charge carriers into the sample, i.e. foreign ions. This causes concentration depth profiles to evolve. These profiles carry all the relevant information on charge carrier transport, see URL Foreign Ion CAIT.
Concentration depth profiles are quantitative determined by means of time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS) which is explained on a separate page. The concentration depth profiles in general exhibit pronounced depletion / replacement zones, unique characteristics of the CAIT technique. These concentration depletion profiles are quantitatively described by means of the Nernst-Planck-Poisson theory.
Figure 3 Illustration of the discretization employed in the numerical Nernst-Planck-Poisson calculations [2].
Possible applications of the CAIT method are very broad. They are covered in various specific pages as listed on the main research page
Literature
[1] K.-M. Weitzel, Charge attachment induced transport - Towards new paradigms in solid state electrochemistry, in Current opinions in electrochemistry, in press (2020)
[2] M. Schäfer and K.-M. Weitzel
Bombardment induced ion transport – Part I Numerical investigation of bombardment induced ion transport through glasses and membranes on the basis of the Nernst-Planck-Poisson equations
PCCP, 13, 20112-20122 (2011)
http://dx.doi.org/10.1039/c1cp21215j
[3] P.V. Menezes, J. Martin, M. Schäfer, H. Staesche, B. Roling and K.-M. Weitzel
Bombardment induced ion transport – Part II Experimental potassium ion conductivities in borosilicate glass
PCCP, 13, 20123-20128 (2011)
http://dx.doi.org/10.1039/c1cp21216h