Tracer diffusion in proton-exchanged congruent LiNbO3 crystals as a function of hydrogen content.

The proton-exchange process is an effective method of fabricating low-loss waveguides based on LiNbO3 crystals. During proton-exchange, lithium is replaced by hydrogen and Li1-xHxNbO3 is formed. Currently, mechanisms and kinetics of the proton-exchange process are unclear, primarily due to a lack in reliable tracer diffusion data. We studied lithium and hydrogen tracer diffusion in proton-exchanged congruent LiNbO3 single crystals in the temperature range between 130-230 °C. Proton-exchange was done in benzoic acid with 0, 1, 2, or 3.6 mol% lithium benzoate added, resulting in micrometre thick surface layers where Li is substituted by H with relative fractions between x = 0.45 and 0.85 as determined by Nuclear Reaction Analysis. For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a Li tracer source and deuterated benzoic acid as a H tracer source. Isotope depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange as well as individual hydrogen and lithium tracer diffusivities are extracted. All three types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.0-1.2 eV and increase as a function of hydrogen content nearly independent of temperature. The effective diffusivities and the lithium tracer diffusivities are identical within a factor of two to five, while the hydrogen diffusivities are higher by three orders of magnitude. The results show that the diffusion of Li is the rate determining step governing the proton-exchange process. Exponential dependencies between diffusivities and hydrogen concentrations are determined. The observed increase of Li tracer diffusivities and effective diffusivities as a function of hydrogen concentration is attributed to a continuous reduction of the migration enthalpy of diffusion by a maximum factor of about 0.2 eV. Simulations based on the determined diffusivities can reproduce the step-like profile of hydrogen penetration during proton-exchange.

Lithium tracer diffusion in LiNi0.33Mn0.33Co0.33O2 cathode material for lithium-ion batteries.

The LiNi0.33Mn0.33Co0.33O2 compound is one of the most interesting cathode materials for Li-ion batteries. Li diffusion in this material directly influences charging/discharging times (and consequently power densities), maximum capacities, stress formation and possible side reactions. In the present study Li tracer self-diffusion is investigated in polycrystalline sintered bulk samples with an average grain size of about 50 nm in the temperature range between 110 and 350 °C. For analysis, stable 6Li tracers are used in combination with Secondary Ion Mass Spectrometry (SIMS). The diffusivities can be described by the Arrhenius law with an activation enthalpy of (0.85 ± 0.03) eV, which is interpreted as the migration energy of a single Li vacancy. Lithium diffuses via structural vacancies whose concentration is fixed by a Li deficiency of about 10%. An extrapolation of the diffusivities to room temperature gives significantly lower values than the diffusivities obtained by electrochemical measurements in literature.

Lithium permeation within lithium niobate multilayers with ultrathin chromium, silicon and carbon spacer layers.

Li permeation through ultrathin Cr, Si and C layers and interfaces is of interest in the optimization of lithium ion batteries with respect to the control of Li flux. Twenty-one LiNbO3 layers (9 nm), which serve as solid state Li reservoirs, were sputter deposited in an alternating sequence of enriched 6Li or 7Li isotope fractions spaced with (8 nm) thin Cr, Si and C layers. The Li isotope contrast was used to measure Li permeation using depth profiling by secondary ion mass spectrometry and neutron reflectometry on a nanometer scale. Extremely low Li permeation for Cr and Si at room temperature exemplifies the effective blocking of Li movement at least for five years. However, Li permeation through C layers was found to be faster than through Cr and Si layers. With temperature, the Li permeation is enhanced through Cr as compared to that through Si layers. Furthermore, material characterisation shows amorphous LiNbO3, C and Si layers and polycrystalline Cr layers (with 80% elemental bcc chromium and 20% chromium-oxide situated at Cr/LiNbO3 interfaces). Annealing in air at 100 °C (373 K) does not oxidize the Cr layers any further. A stress of 12 GPa, which was measured in Cr spacer layers at room temperature, remains unchanged upon annealing. The origin of a weak ferromagnetic order measured at room temperature (300 K) was attributed to some traces of Cr and Si inside LiNbO3.

Self-Diffusion in Amorphous Silicon.

The present Letter reports on self-diffusion in amorphous silicon. Experiments were done on ^{29}Si/^{nat}Si heterostructures using neutron reflectometry and secondary ion mass spectrometry. The diffusivities follow the Arrhenius law in the temperature range between 550 and 700 °C with an activation energy of (4.4±0.3) eV. In comparison with single crystalline silicon the diffusivities are tremendously higher by 5 orders of magnitude at about 700 °C, which can be interpreted as the consequence of a high diffusion entropy.

Surface controlled reduction kinetics of nominally undoped polycrystalline CeO2.

Ceria is an interesting material for high temperature redox applications like solar-thermal splitting of CO2 and H2O. Technical implementation and reactor design for solar-thermal redox-based fuel generation requires reliable data for the chemical surface exchange coefficient and the chemical diffusivity of oxygen. The results of thermogravimetric relaxation experiments and equilibrium oxygen isotope exchange experiments with subsequent depth profiling analysis suggest that the reduction reaction of even dense samples of pure ceria (1 mm thickness, 93% of theoretical density) with a grain size of about 20 µm is surface reaction controlled. The chemical surface exchange coefficient exhibits a negative apparent activation energy (-64 kJ mol(-1)). This finding is corroborated by similar data from literature for the tracer surface exchange coefficient. The structure of the derived expression for the apparent activation energy further suggests that the chemical surface exchange coefficient should show only a very weak dependence on temperature for ceria doped with lower valence cations.

Lithium transport through nanosized amorphous silicon layers.

Lithium migration in nanostructured electrode materials is important for an understanding and improvement of high energy density lithium batteries. An approach to measure lithium transport through nanometer thin layers of relevant electrochemical materials is presented using amorphous silicon as a model system. A multilayer consisting of a repetition of five [(6)LiNbO3(15 nm)/Si (10 nm)/(nat)LiNbO3 (15 nm)/Si (10 nm)] units is used for analysis, where LiNbO3 is a Li tracer reservoir. It is shown that the change of the relative (6)Li/(7)Li isotope fraction in the LiNbO3 layers by lithium diffusion through the nanosized silicon layers can be monitored nondestructively by neutron reflectometry. The results can be used to calculate transport parameters.