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Maximum entropy ecological niche modelling and spatial scan statistic were utilized to predict the geographical range and to investigate clusters of infections for Neospora caninum and Coxiella burnetii in dairy cattle farms in Catalonia, northeastern Spain, using the Maxent and SaTScan programs, respectively. The geographical distribution of Neospora and Coxiella with the highest level of probability (P > 0·60) covers central Catalonia and spreads towards northeastern Catalonia which contains a high concentration of dairy cattle farms. The most important environmental factor that contributed to the ecological niche modelling was precipitation of driest month followed by elevation. Significant clusters (P < 0·001) were detected for Neospora and Coxiella infections in the western and eastern regions of Catalonia, respectively.
The growth of undoped GaAs films from Ga(C2H5)3 (TEG) and AsH3 by Metalorganic Chemical Vapor Deposition (MOCVD) has been studied in a low-pressure reactor operating at 3 Torr. This precursor combination is known to produce GaAs films with very low carbon incorporation when compared to films grown from Ga(CH3)3 (TMG) and AsH3. A kinetic model of the growth process has been developed that includes both gas-phase and surface reactions based on reported decomposition mechanisms of TEG and AsH3. The kinetic model was coupled to a transport model describing flow, heat and mass transfer. Finite element simulations were performed to determine the rate constants of the growth reactions that provide the best fit between predicted and observed growth rates. Under typical operating conditions the surface reactions were found to dominate the growth process and a reduced surface kinetic model was identified by sensitivity analysis. The proposed reaction-transport models can successfully predict observed growth rates of GaAs films and they can be used for identifying optimal operating conditions in MOCVD reactors.
An approach for optimal design of vertical stagnation flow Metalorganic Vapor Phase Epitaxy (MOVPE) reactors that minimizes parasitic pre-reactions between the film precursors is presented. The use of axisymmetric multi-aperture inlets (e.g. tube-in-tube or concentric-ring inlets) enables the separation of incompatible precursors, while preserving the axial symmetry of the reactor. A careful selection of the inlet velocity of each stream and the distance between the inlet and the susceptor (reactor height) can lead to complete mixing just above the substrate, while keeping the contact time between the precursors in the gas phase low enough to suppress pre-reactions. This idea has been used by our group for growing high quality ZnSe films on GaAs substrates from (CH3)2Zn:N(C2H5)3 and H2Se diluted in H2 in a stagnation flow MOVPE reactor with an axisymmetric split inlet. A transport model describing the MOVPE of ZnSe, for conditions at which the growth rate is limited by the precursors' arrival rate at the surface, has been developed. A parametric study was performed aiming at identifying operating conditions in industrial-scale reactors that maximize film thickness uniformity while minimizing precursor contact time. Operation atGr/Re2<100 eliminated flow recirculations in the region above the substrate. Such recirculations may lead to formation of particulates by trapping reactants. Optimal conditions correspond to equal velocities of the inlet streams, satisfying the above criterion, and to the minimum possible reactor height leading to uniform film thickness across the substrate.
A simple gas-phase and surface kinetic model describing the Metalorganic Vapor Phase Epitaxy (MOVPE) of GaAs from trimethyl-gallium (TMG) and arsine has been extracted from reported reaction mechanisms through sensitivity analysis. This model was coupled with fundamental descriptions of the flow, heat and mass transfer in MOVPE reactors. All the uncertainties of the model were lumped into the two rate constants of the surface growth reaction. Finite element simulations of GaAs growth in a rotating-disk MOVPE reactor reported in the literature were used to fit the two unknown rate constants. Without any adjustment, the model could predict observed growth rates at higher susceptor rotational rates. A more serious robustness test was the fully coupled 2-D simulation of GaAs growth in a horizontal MOVPE reactor by using a state-of-the-art code (MPSalsa) developed at Sandia National Laboratories. The model predicted observed growth rates reasonably well in 2-D fully coupled simulations of flow, heat and mass transfer. Further development of the kinetic model by testing different kinetic scenaria in 2- and 3-d simulations and by using data obtained at kinetically-limited growth conditions is planned in order to develop a reactor-independent simulator of the MOVPE of GaAs. Such a model will be essential for the development of a conceptual virtual MOVPE reactor, which can be used for reactor design, optimization and model-based control.
Metalorganic Vapor Phase Epitaxy (MOVPE) has emerged as the technique of choice for growing thin films and structures of group III-nitrides. The objective of this work is to address the optimal design of vertical rotating disk and stagnation flow MOVPE reactors in order to achieve film thickness uniformity over large area substrates. Gas inlets that preserve the axial symmetry and enable alternating feeding of the precursors through coaxial rings were studied. The growth of GaN films from trimethyl-gallium and ammonia was used as a typical example. A fundamental reaction-transport model of the MOVPE process including gas-phase reactions and gas-surface interactions has been developed. The model was validated through comparison with growth rate data obtained from both research-scale and industrial-scale reactors. Performance diagrams for industrial-scale stagnation flow and rotating disk reactors were developed by varying the reactor geometry and operating conditions to identify regions of uniform film growth.
Most sylvatic species of Trichinella are known to have poor infectivity to rats, but in the present study oral administration of bile from other hosts appeared to modify this infectivity. A total of 75 rats were inoculated in groups of 25 with 3 species of Trichinella (T. spiralis, T. nativa, and T. nelsoni) and each group of rats was given per os daily doses of bile from pig, sheep, chicken and fox respectively (4×5 rats). As a control 1 group of 5 rats was given daily doses of water. Whereas, the addition of bile did not increase the establishment of T. spiralis, fox bile had a significant positive effect on the establishment of muscle larvae of T. nativa and T. nelsoni. Addition of bile to cultures of the same Trichinella species had an overall negative effect on the in vitro survival of larvae. The present observation that carnivore bile favours the establishment of sylvatic Trichinella may explain why carnivores are equally receptive to all Trichinella species.
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