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This thesis focuses on a new method called Bombardment Induced Iontransport (BIIT). The method is being improved and its potential and error margins are analyzed.
The method uses a thermionic emitter to generate an alkali ion beam of controlled, low kinetic energy. This beam charges the surface of a solid state electrolyte such as an ion-conducting glass in high vacuum. Depending on the acceleration voltage, a neutralization current will flow. This results in a linear current-voltage curve, which yields the temperature dependent dc-conductivity of the material. Typically, this curve does not cross the voltage axis at 0 V, but at values between +1 V and +4 V. This phenomenon is referred to as voltage offset.
This thesis deals with three key aspects of BIIT: First of all, the measurement of conductivities was optimized. A high temperature sample holder was developed to increase the accessible measurement window to temperatures up to 260 °C (533 K). By reducing the voltage difference between sample surface and backside to values of down to 100 mV, conductivities of up to 2 · 10^-10 S/cm were measured. The reproducibility of conductivity measurements was verified and the experimental deviations between BIIT and impedance spectroscopy were discussed. In order to examine the charging process in BIIT, an experimental model system was developed, which consists of a metal electrode in series with an ohmic resistor.
As second key aspect, the voltage offset was surveyed. Different metal electrodes (Pt, Au, Cu, Ag) were bombarded with K+, Rb+ and Cs+. The voltage offset was analyzed as a function of electrode material and of time or deposited charge. Independently of the material, the voltage offset rapidly increased with time and became constant after a shift of roughly 1.7 V. By using a reference electrode, it was shown that this shift occurs due to the bombardment. The observations for the voltage offset at the beginning of the experiment were ambiguous: Either it does not depend on the electrode material, or it decreases as the electronic work function of the material increases. Based on electron emission, a theoretical model for BIIT was developed, in order to explain phenomena such as the voltage offset. The voltage offset might be the Volta potential difference (or contact potential) or an electromotive force.
As third key aspect, the BIIT method was used to create concentration profiles in glasses. By bombarding the glass with a foreign alkali ion, this ion will replace the native ion, which was present in the glass at the beginning of the experiment. This typically results in a roughly 150 nm deep depletion layer, in which up to 80% of the native ion were substituted. The created concentration profiles were measured using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and numerically modeled using the Nernst-Planck- and Poisson equation. The resulting NPP diffusion coefficient of one of the alkali ions systematically appeared to be constant, whereas the other strongly depends on the local composition.
The calcium phosphate glasses Ca30-K and Ca30-Rb (0.25 M2O · 0.30 CaO · 0.45 P2O5 with M = K, Rb) and the mixed alkali borate glass 16Na04Rb80B (0.16 Na2O · 0.04Rb2O · 0.8B2O3) were studied. In a comparative experiment Rb@Ca30-K vs. K@Ca30-Rb, it has been shown that in both cases the rubidium diffusion coefficient appears constant. Therefore, the constancy does not depend on whether the ion is the bombarder or native. The physical nature of the NPP diffusion coefficient was thus analyzed and discussed. By analyzing the NPP model, it became clear that the model is very sensitive to changes in the ratio of both diffusion coefficients. However, it’s not very sensitive to their absolute values. In principle, the concentration dependence of the second diffusion coefficient could be overlooked this way.
The alkali borate glasses were bombarded with Rb+, yielding for the first time temperature dependent NPP diffusion coefficients. These were compared to literature values from impedance spectroscopy and radio tracer diffusion (RTD), since those diffusion coefficients are known from studies of the mixed alkali effect in this glass system. The uncertainty that arises from converting SIMS data into concentration profiles was quantified. Furthermore the results of the BIIT-NPP-method were at length checked for self-consistency, for example whether the diffusion coefficients show Arrhenius behavior or whether the deposited charge resulting from the neutralization current agrees with the charge found in the SIMS profiles.