ALBERT

Notes: ZnSxSe1−x/ZnSe quantum wells (QW) with and without growth interruption at the interface were grown using atmospheric pressure metalorganic vapor-phase epitaxy. It has been shown that growth interruptions have a major influence on the optical properties of the QW. An interruption of growth in ternary ZnSSe layers causes a quasi-QW photoluminescence (PL), explained by sulfur depletion of the layer. A stabilization is necessary to avoid a quasi-QW. High-quality ZnSSe/ZnSe QWs were achieved by suitable stabilization with H2S and DESe during the growth interruption at the interface, so a sulfur diffusion out of the barrier can be avoided. The QW thicknesses grown under these conditions were in agreement with the nominal well thicknesses, and for stabilized 1-nm QWs we obtain a PL blueshift of 158 meV for x=0.68 and 60 meV for x=0.4 compared to the ZnSe band-edge emission. Characteristic QW-PL transitions at room temperature were observed for unstabilized QW grown with a long interruption time, tp=90 s. The thermal activation energies of the QW excitons investigated by temperature-dependent PL show that with decreasing interruption time an increased binding energy is obtained. The binding energy shows no dependence on the stabilization so that other effects (e.g., interface roughness and sulfur diffusion) may play a role. For QW with a low sulfur content (x=0.4) in the barrier material, we find binding energies which fit to theoretical models. This was not achieved for QWs with x=0.68.

Notes: A continuous bubble-rise-velocity (BRV) viscometry method was developed for Newtonian fluids which is based upon the remote determination of the rise velocity of a helium bubble. The bubble rise velocity was remotely determined by using a helium-leak detector to measure the rise time of a bubble passing through a constant distance. The density of the fluid was simultaneously obtained from differential pressure measurements in the bubble launch tubes. Empirical equations were developed which allow for calculation of the absolute viscosity and density from several remotely measurable variables: helium gas pressures in the bubble generation and launching system, liquid temperature, and helium concentrations at the liquid/air interface as a function of time. BRV viscometry results for highly viscous liquids (1 Pa s〈viscosity 〈40 Pa s) correlated within ±10% with data obtained with capillary or rotational viscometers. An integrated probe was conceived for the on-line monitoring of viscosity, density, temperature, and liquid level, especially in a hostile environment.

Notes: The rate and extent of soil–solution transfer of orthophosphate ions (Pi) depend both on soil solution Pi concentration and on time. This is so both in experiments on sorption–desorption and in those on isotopic exchange. Because the two methods are based on different principles, it is questionable whether they give a similar quantification of Pi transfer.The amount of isotopically exchangeable Pi, E, was determined over periods of 100 minutes and described as a function of both time and Pi concentration in solution for three soil samples taken from field plots having different P fertilizer histories over 26 years of annual application. In separate experiments, amounts of Pi, Qd, were extracted from soil suspensions using three levels of anion exchange resin strips for periods ranging from 10 minutes to 7 days. Both initial and final solution Pi concentrations were measured. These concentrations and periods of resin-contact were used to predict the difference in E, ΔE, between the initial and final states of the suspension–resin system using extrapolations of the equations fitted to the exchange data. Under conditions in which the solution concentration of Pi decreased during extraction the ΔE values and the resin-extracted Pi values, Qd, were equal. It is thus possible, using the description of E, to predict the amount of Pi released from soil suspension knowing the initial and final solution Pi concentrations and the time it took for the system to pass from the initial to the final state. For the soil studied, identical amounts of mobile Pi, i.e. the Pi which participates in the soil–solution dynamics, may therefore be assessed by either isotopic exchange or desorption.

Notes: Non-exchangeable K always contributes to some degree to plant nutrition. To understand this contribution and to deduce a predictive buffer power model the release and fixation of K was studied on a range of 44 calcareous clay soils. Short-term K sorption and desorption experiments (16 h) were followed by the measurement of soil exchangeable K (ammonium acetate extraction). Soil K-Ca exchange properties and the contributions of exchangeable K and non-exchangeable K to K dynamics of the soil-solution system were estimated. The change in the amount of non-exchangeable K during the experiment was generally proportional to the initial constraint imposed to the soil-solution system (ø), i.e. the solution: soil ratio multiplied by the difference between the solution K concentration imposed at the beginning of the experiment and the solution K concentration for which neither sorption nor desorption of K would occur. The proportionality coefficient (β) called the 'soil ability for K release and fixation’ was identical for release and fixation for 36 soils, whereas eight soils showed some difference between their ability for fixation and their ability for release of K. When β was considered identical in the release range and in the fixation range for all soils it was inversely proportional to the initial K saturation ratio of the CEC corrected for the amount of ammonium extractable K which was not in exchange equilibrium with Ca. This quantity of ammonium extractable K which is not in equilibrium with Ca probably contributes only partially to plant nutrition. For the studied soils this quantity contributed a large proportion of exchangeable K (26–65%) especially in soils with a small K content. Due to the contribution of non-exchangeable K to soil-solution K dynamics the buffer power of the system does not only depend on exchange properties but also on soil release and fixation properties. When β was taken into account buffer power was better estimated than when it was deduced from ion exchange alone.

Notes: The Ca concentration of the soil solution influences K plant nutrition by its influence on K concentration of the soil solution and on soil buffer power through ion exchange and K release or fixation. The effects of the imposed solution Ca concentration on the estimates of these parameters and on these two phenomena were studied on a loamy soil. Potassium sorption and desorption experiments were conducted for 16 h at five initial Ca concentrations (from 0 to 10−1 M) and followed by the measurement of soil exchangeable K (ammonium acetate extraction). Soil K-Ca exchange properties and the contributions of exchangeable K and non-exchangeable K to K dynamics of the soil-solution system were estimated. The‘Ratio Law’ applied for the medium range of Ca concentrations, i.e. 10−1 M to 10−3 M. But, it failed for some experiments at small initial Ca concentrations (0 M and 10−4 M). This failure went with a decrease of the number of sites of great affinity for K in K-Ca ion exchange and/or a decrease of the amount of K not in exchange equilibrium with Ca but extracted by M ammonium acetate. Release of K increased and fixation of K decreased when Ca concentration increased. The relation between the change in the amount of non-exchangeable K during the experiment and the initial constraint (ø) was curvilinear on the large range of ø investigated. But, this relation was independent of Ca concentration. The K concentration of the solution for which neither sorption of K by the soil nor desorption of K from the soil occurred decreased and the slope of the sorption-desorption curve at this K concentration increased when the solution Ca concentration decreased. These two parameters can be considered the K concentration of the soil solution of the soil and the buffer power of the soil, respectively, only if the initial Ca concentration imposed during the sorption-desorption experiments is close to the Ca concentration of the soil solution of the soil. A predictive model of the soil buffer power based on ion exchange and release-fixation properties is proposed. Despite some discrepancies at very low Ca concentrations (〈0·5 mM Ca) when‘Ratio Law’did not apply the agreement between calculated and observed values was good. The model permits the correction of the experimentally obtained buffer power for the bias related to the great solution volume: soil weight ratio commonly used during the sorption-desorption experiments.