Observed Diffusion Profiles Research Articles

A coherent approach for cation surface diffusion in clay minerals and cation sorption models: Diffusion of Cs+ and Eu3+ in compacted illite as case examples

The conceptual setup and the parametrisation of surface diffusion models for different types of cations in negatively charged clay minerals has still not been fully developed. In particular, the contribution of different types of surface-associated cationic species to the overall mass transfer rates is an open question. Further, the transferability of sorption data gained on dispersed clay suspensions to the compacted clay minerals or consolidated clay rocks remains also unanswered. This contribution presents experimental results for the diffusion of 134Cs+ and 152Eu3+ in and sorption on compacted illite (bulk-dry densities of 1900 and 1700 kg m−3, respectively) and their interpretation using thermodynamic and transport modelling. Different solution parameters, such as pH, ionic strength and the concentration of stable isotope background were varied. The results give information on both diffusional processes of the two radionuclides and their sorption behaviour at relevant solid-to-liquid ratios. The existing two site protolysis non electrostatic surface complexation and cation exchange (2SPNE-SC/CE) model turned out to be fully valid for the description of the equilibrium distribution of the test cations between the solution and the clay phase in the compacted state. The model had to be extended by an electrical double layer description of cationic species bound to the planar surfaces, involving mobile species in the diffuse layer, in order to successfully model the observed diffusion profiles. Despite the notable differences in sorption behaviour of Cs+ and Eu3+ on illite, their diffusional behaviour could thus be satisfyingly described using the same model approach.

Conductivity and Degradation in Garnet Electrolytes

Lithium-stuffed garnets such as Li7La3Zr2O12 (LLZO) show promise as solid state lithium ion conductors for use in high energy density storage applications due to their chemical and thermal stability and electrochemical performance versus competing electrolytes. Much work has focused on improving the conductivity of these systems by optimising the lithium content through the introduction of lattice vacancies created by the addition of cation dopants (such as Al3+ and Ga3+)1 in the Li sites. The effect of adding cations on the observed Li conductivity depends strongly on the identity and concentration of cations; this process must be tuned such that an optimum number of Li ions and vacancies are present and can readily form an interconnected network of tetrahedral and distorted octahedral sites in the lattice, allowing long-range diffusion. The importance of obtaining the cubic garnet phase which is known to exhibit higher conductivities as a result of increased lithium disorder versus the tetragonal polymorph has been acknowledged and pursued by many.2 In addition, recent observations on the reactivity of the garnets with water have shown that lithium undergoes ion exchange with H+, resulting in a change in conductivity and degradation of the material, accompanied by the formation of passivating Li hydroxide and carbonates at the surface3–5. These phenomena occur even upon ambient air exposure of the garnet samples. Despite this knowledge, little is known about the pathway for lithium and proton diffusion during exchange, and many studies on LLZO garnet materials do not account for the role of this mechanism on the properties reported, with samples freely exposed to an air environment during synthesis and measurement. In this work, we use a Ge(IV) cation substituted at the lithium position to follow the role of aliovalent dopants on the lithium diffusion behaviour, building on previous work using Zn2+ and Ga3+. The high charge Ge4+ ion is chosen to achieve a greater theoretical Li vacancy concentration for the lowest amount of added dopant as a result of charge compensation effects. We have found the critical Li ion to vacancy ratio for optimum ionic conduction in the Ge-doped garnet, using ICP-OES to verify the composition and have measured ionic conductivities of 6-8 x10-4 S cm-1 using AC impedance for the material of nominal composition Li6.6La3Ge0.1Zr2O12. XRD confirms that the cubic garnet structure is stabilised by the addition of Ge. Moreover, the role of moisture in the degradation of LLZO is systematically investigated, through the use of controlled-atmosphere processing (glove box coupled to a high temperature furnace) and deliberate forcing of D/Li exchange in the lattice by exposing the otherwise ‘dry’ samples (synthesised and stored in a protective argon atmosphere) to deuterated water. We use secondary ion mass spectrometry (SIMS) to follow the depth profiles of ion species, specifically Li+, D+ and H+in the undoped and Ge-doped LLZO and have observed diffusion profiles indicative of the exchange process (Figure 1). Preliminary novel work on isotopic Li exchange is underway, following a similar protocol, to calculate the diffusivity in our samples, and the corresponding effect of dopants, stoichiometry and degradation on the observed diffusion profile. Altogether, this work is enabling the fundamental properties of this solid state garnet electrolyte to be better understood, identifying and optimising the factors governing conduction, with a view to its use in high energy density Li-metal batteries. A knowledge of the degradation processes occurring on Li exchange with protons could reveal important conductivity information about these two charge carriers, enabling us to harness this seemingly detrimental behaviour to our advantage.

Diffusion of volatile organics through porous snow: impact of surface adsorption and grain boundaries

Abstract. Release of trace gases from surface snow on earth drives atmospheric chemistry, especially in the polar regions. The gas-phase diffusion of methanol and of acetone through the interstitial air of snow was investigated in a well-controlled laboratory study in the temperature range of 223 to 263 K. The aim of this study was to evaluate how the structure of the snowpack, the interaction of the trace gases with the snow surface, and the grain boundaries influence the diffusion on timescales up to 1 h. The diffusive loss of these two volatile organics into packed snow samples was measured using a chemical ionization mass spectrometer. The structure of the snow was analysed by means of X-ray-computed micro-tomography. The observed diffusion profiles could be well described based on gas-phase diffusion and the known structure of the snow sample at temperatures ≥ 253 K. At colder temperatures, surface interactions start to dominate the diffusive transport. Parameterizing these interactions in terms of adsorption to the solid ice surface, i.e. using temperature-dependent air–ice partitioning coefficients, better described the observed diffusion profiles than the use of air–liquid partitioning coefficients. No changes in the diffusive fluxes were observed by increasing the number of grain boundaries in the snow sample by a factor of 7, indicating that for these volatile organic trace gases, uptake into grain boundaries does not play a role on the timescale of diffusion through porous surface snow. For this, a snow sample with an artificially high amount of ice grains was produced and the grain boundary surface measured using thin sections. In conclusion, we have shown that the diffusivity can be predicted when the structure of the snowpack and the partitioning of the trace gas to solid ice is known.

Accurate modelling of average phosphorus diffusivities in germanium after long thermal anneals: evidence of implant damage enhanced diffusivities

Average phosphorous diffusivities after implantation in ⟨1 0 0⟩ germanium have been measured for long anneals (i.e., 3–10 h) at temperatures from 600 to 800 °C. Considerable dose loss after annealing is also observed and quantified for temperatures below 800 °C. A diffusion model using an extrinsic diffusivity coefficient combined with a segregation component between the germanium and the oxide, to account for dopant loss, is found to be sufficient to completely explain the observed diffusion profiles. The best-fit diffusivity and segregation coefficients are reported for this model and the diffusivities are found to be over an order of magnitude slower than those measured after rapid thermal annealing (i.e., short anneals of only a few seconds). It is proposed that this disagreement of diffusivities between short and long anneals is due to implant damage perhaps similar to well-known transient enhanced diffusion effects observed in silicon.

Spectral Imaging and Electrochemical Study on the Response Mechanism of Ionophore‐Based Polymeric Membrane Amperometric pH Sensors

AbstractThe response features of amperometric ionophore‐based chemical sensors were evaluated in regular ion‐selective membranes, poly(vinyl chloride) plasticized with o‐nitrophenyl octyl ether, with a H+‐selective chromoionophore as model ionophore. Direct imaging experiments upon discrete applied voltage pulses revealed that the diffusion coefficients in such membranes are very similar to that found in membranes under zero current conditions, at about 10−8 cm2 s−1. In the potential range that defines the limiting current, it was found by these imaging experiments that the ionophore is saturated at the membrane surface, as expected. The observed concentration profiles could be well described with established diffusion theory. At more extreme potentials, the observed diffusion profiles were different, and could be explained by the extraction of additional, uncomplexed hydrogen ions into the membrane phase. The results were correlated to the amperometric response behavior of the same electrodes. At the limiting current, Cottrell fits were performed, and similar diffusion coefficients were found as in the imaging experiments. The pH‐dependent normal pulse voltammetric responses were analyzed in terms of their half wave potential, and found to show a Nernstian pH response, suggesting that they function in close analogy to their potentiometric counterparts. The breakdown of Nernstian pH response at low pH is explained by extraction of electrolyte into the membrane under these conditions, which renders the interface non‐polarizable and leads to loss of voltammetric features. The results adequately support the previously published response mechanism of such ionophore‐based amperometric sensors, including the origin of the limiting current, the capability of such membranes to be responsive to ion activities, the requirement of applying baseline potentials between pulses, and the reason for the altered selectivity at extreme potentials.

Spatial and Temporal Dependence of Diffusion in Polystyrene Thin Films on Silicon and Carbon Surfaces

The strong inhibition of chain diffusion in thin polystyrene (PS) films has been observed near an attractive silicon surface, and diffusion remains inhibited out to distances of several radii of gyration from the surface. The present study seeks to determine the time dependence of the diffusion coefficient, and to examine the effect of a carbon surface on this diffusion. The sputter-deposited carbon surface may serve as a model for carbon-black particles employed in nanocomposites, which have recently been observed to reduce diffusion throughout a nanocomposite layer. The experiments employed a thin (∼15 nm) deuterated polystyrene (dPS) marker layer sandwiched between two normal PS layers. Deuterium profiles were monitored in the annealed samples by secondary ion mass spectrometry. Strong segregation was observed at the silicon surface, but was inhibited at the carbon surface, allowing the diffusion behaviour to be studied in the latter case over longer annealing times. A finite-element computer program was developed to fit the observed diffusion profiles. The variation of the diffusion coefficient with depth is shown to be consistent with previous results, and diffusion is comparable at both the carbon and silicon surfaces. The diffusion coefficient decreases roughly in proportion to t−1/2, and is discussed in the context of reptation theory.

Diffusion of heterocyclic compounds from a complex mixture of coal–tar compounds in natural clayey till

The diffusion of coal–tar compounds in natural clayey till was studied experimentally. Cores were exposed to a solution with near constant concentration of coal–tar compounds in a multi-component mixture for 5 months, the cores were subsequently sub-sampled and analyzed. Diffusion profile data strongly indicates highly non-linear sorption isotherms. For dibenzofuran, dibenzothiophene, 2-methylquinoline, carbazole, phenanthrene, and fluorene, the profiles indicated significantly stronger sorption at high solute concentrations than at low solute concentrations, and than expected based on linear sorption isotherms. This corresponds with observed dramatic increase in sorption of these compounds at high surface density. For benzofuran, benzothiophene, quinoline, phenols, naphthalenes, and BTEXs the profiles indicated significantly weaker sorption at high solute concentrations than at low solute concentrations and than expected based on linear or Freundlich isotherms. The observed diffusion profiles have important implications with respect to transport of dissolved coal–tar compounds in multi-component mixtures of high concentrations as expected near immiscible phase coal–tar sources. Breakthrough times and concentration levels are affected significantly.

Zn diffusion in InP: Effect of substrate dopant concentration

We have performed open-tube diffusion of Zn into n-type InP over the temperature range 525–675 °C. For heavily doped InP, the observed diffusion profiles are consistent with an interstitial-substitutional model where the interstitial Zn is doubly ionized. However, for undoped InP the profiles have a significantly different shape and the diffusion is deeper. Assumption of neutral interstitial Zn is able to account for these two changes. A tentative model is proposed to explain the dependence of the interstitial Zn ionization state on the starting substrate dopant concentration.

Elimination of ionized impurity scattering in heavily doped GaxIn1−xAsyP1−y

Unexpectedly high hole mobilities occur in p-type layers of Ga0.03In0.97As0.07P0.93 grown by liquid phase epitaxy and concurrently doped with high concentrations (1017–1019 cm−3) of Mn and Ge. The temperature dependence of the hole mobility indicates that ionized impurity scattering is insignificant. The effect is attributed to the diffusion of neutral Mn to form donor-acceptor complexes instead of isolated ionized donors and acceptors. Such a model is consistent with observed diffusion profiles into a variety of substrates from Mn-doped epilayers and the presence of ionized impurity scattering where either the donor concentration or the depth of the Mn acceptor level is such that little or no neutral Mn is present.

Gleichzeitige diffusion von Ga und As in silizium aus einer GaAs-quelle

The observed diffusion profiles obtained by the simultaneous diffusion of Ga and As from GaAs sources with temperatures of 1040, 1100 and 1140°C into n-type Si at 1250°C are reported. The distribution of the As concentration will be explained by a diffusion coefficient depending on concentration and for the source temperatures 1100 and 1140°C by the formation of As precipitations at As concentrations higher than 10 20 cm -3. The electric field between the Ga acceptors and the more slowly diffusing As donators causes the effective Ga diffusion coefficient to be smaller than that with undisturbed Ga diffusion.