Local Partial Pressure Research Articles

Reactivity of Laboratory and Industrial Limes

Quicklimes produced in an industrial kiln have a very low hydration activity compared to those produced in the laboratory. In this study, we seek to explain these differences in reactivities by providing quantitative data on the kinetics of the two mechanisms involved in the production of quicklime: the calcination of limestone to produce quicklime, and the undesirable sintering of quicklime. The first mechanism increases the specific surface area of the product from a few m 2g −1 to more than 80 m 2 g −1, making the quicklime highly reactive: the time to react with water is less than 10 s. The second mechanism reduces the specific surface area down to a few m 2 g −1—which is typical for an industrial quicklime—and severely reduces its reactivity: the time to react with water is around 15 min. The impact of the local temperature and of the local partial pressure of CO 2 is quantified for both mechanisms, by experimenting the two extreme cases of pure N 2 atmosphere and pure CO 2 atmosphere using specially designed laboratory apparatus. Some correlation between the quicklime reactivity and the specific surface area is found. The work demonstrates that industrial furnace calcining large limestone pellets, leads to low reactivity quicklimes both because the calcination temperature is too high and the CO 2 pressure is too elevated.

Redox competition mode of scanning electrochemical microscopy (RC-SECM) for visualisation of local catalytic activity

In order to locally analyse catalytic activity on modified surfaces a transient redox competition mode of scanning electrochemical microscopy (SECM) has been developed. In a bi-potentiostatic experiment the SECM tip competes with the sample for the very same analyte. This leads to a current decrease at the SECM tip, if it is positioned in close proximity to an active catalyst site on the surface. Specifically, local catalytic activity of a Pt-catalyst modified sample with respect to the catalytic reduction of molecular oxygen was investigated. At higher local catalytic activity the local 02 partial pressure within the gap between accurately positioned SECM tip and sample is depleted, leading to a noticeable tip current decrease over active sites. A flexible software module has been implemented into the SECM to adapt the competition conditions by proper definition of tip and sample potentials. A potential pulse profile enables the localised electrochemically induced generation of molecular oxygen prior to the competition detection. The current decay curves are recorded over the entire duration of the applied reduction pulse. Hence, a time resolved processing of the acquired current values provides movies of the local oxygen concentration against x,y-position. The SECM redox competition mode was verified with a macroscopic Pt-disk electrode as a test sample to demonstrate the feasibility of the approach. Moreover, highly dispersed electro-deposited spots of gold and platinum on glassy carbon were visualised using the redox competition mode of SECM. Catalyst spots of different nature as well as activity inhomogeneities within one spot caused by local variations in Pt-loading were visualised successfully.

A model for the oxidation of carbon silicon carbide composite structures

A mathematical theory and an accompanying numerical scheme have been developed for predicting the oxidation behavior of carbon silicon carbide (C/SiC) composite structures. The theory is derived from the mechanics of the flow of ideal gases through a porous solid. The result of the theoretical formulation is a set of two coupled non-linear differential equations written in terms of the oxidant and oxide partial pressures. The differential equations are solved simultaneously to obtain the partial vapor pressures of the oxidant and oxides as a function of the spatial location and time. The local rate of carbon oxidation is determined using the map of the local oxidant partial vapor pressure along with the Arrhenius rate equation. The non-linear differential equations are cast into matrix equations by applying the Bubnov–Galerkin weighted residual method, allowing for the solution of the differential equations numerically. The numerical method is demonstrated by utilizing the method to model the carbon oxidation and weight loss behavior of C/SiC specimens during thermogravimetric experiments. The numerical method is used to study the physics of carbon oxidation in carbon silicon carbide composites.

Water transport properties of Nafion membranes: Part I. Single-tube membrane module for air drying

A simple formula for water permeability of a Nafion hollow-fiber membrane has been determined. The result shows that water permeability of the membrane is proportional to the 1.5 power of the local partial pressure of water vapor in the gas phase. The water permeability was converted to a diffusion coefficient, which was found to attain a maximum value when the number of water molecules per sulfonate in the Nafion membrane is equal to 3.3. This agrees well with some reports in the literature. An effective mathematical model for a membrane process to remove water vapor from nitrogen gas was developed. Using the obtained permeability expression, the model simulations for the membrane module matched experimental data over a wide range of operating conditions. In most cases, the absolute errors between the predicted product relative humidity and the measured values are within ±2%, and the relative errors are less than 15%. Mass transfer resistances along the membrane module at typical operating conditions were also analyzed.

Mechanisms of the hydrogen reduction of molybdenum oxides

The two stages of the hydrogen reduction of MoO 3 to Mo were investigated in a thermal balance under well defined reaction conditions. Starting with different grain and agglomerate sizes for both stages, the influence of a set of parameters (temperature, local partial pressure of H 2O, gas flow, etc.) on the reaction progress and the final result were studied in detail. Depending on the set of parameters used, different reaction mechanisms like pseudomorphic transformation or chemical vapour transport (CVT) were observed. Taking into account that grains and agglomerates deviate from a spherical shape and a definite grain size, the extent of reaction is well described by standard theoretical gas–solid reaction models such as the shrinking core model or the crackling core model (CCM). Thermo-gravimetric analysis, X-ray diffraction, scanning electron microscopy, surface area measurements (BET method) and laser diffraction were used for these studies. Under all conditions, the first stage shows a reaction path MoO 3→Mo 4O 11→MoO 2 via CVT. The reaction extent follows the CCM. Depending on the local partial pressure of H 2O during reduction, the formed Mo 4O 11 and MoO 2 exhibit different size distributions and shapes of the grains. The extent of reaction of the second stage develops according to the shrinking core model. Depending on the local dew point, two different reaction paths can occur: Pseudomorphic transformation at low dew points and transformation via CVT at high dew points. This paper is an extract from the Ph.D. thesis of W.V. Schulmeyer “Mechanismen der Wasserstoffreduktion von Molybdänoxiden”, 1998, Darmstadt University of Technology, Institute of Material Science, Department of Chemical Analytics, FRG. It focuses on a phenomenological description of the most important results.

Studies on the interaction between thin film materials and Mo field emitter arrays

A simple method for the evaluation of materials suitable for the fabrication of field emission vacuum microelectronic devices is presented. Since there can be a wide range of electron and ion interactions with the device, it is important to be able to quickly assess if a material may have a particular adverse effect on emission performance under operational conditions. The technique is based on the sensitivity of a large field emitter array to the outgassing or desorption of gas species from thin films under electron beam excitation. We found that Mo field emitter arrays degraded rapidly with stainless steel anodes coated with various oxide materials. The extent of degradation was found to be the most rapid with SiO2, Si3N4, and MoO3 thin films. Stainless steel anodes with Mo and Nb thin films show a faster degradation rate than stainless steel anodes, most likely because of native oxides grown during processing and handling. The emission behavior in the presence of Ir, Pd, Al, Zn, and Ti metal films and barrier materials like C and TaN is similar to stainless steel reference data. We find that once the oxide films are covered with barrier layers like C and TaN, emission decay rates approach the values obtained with stainless steel reference anodes. The observed emission current degradation is consistent with a model based on the liberation of oxygen from the surface of electron beam bombarded materials. Using controlled oxygen exposure experiments, we have determined the equivalent local oxygen pressures in the presence of various thin films. We found that with thin films of Nb, ZrO2, Ta2O5, MgO, Nb2O5, and Al2O3, the emission degradation is akin to having a local O2 partial pressure in the 1×10−7–1×10−6 Torr range and with Mo, MoO3, Si3N4, and SiO2, this is equivalent to having local O2 pressures of 1×10−5 Torr.

Hypoxia and human placental development.

Human developmental biology has been stirred by the emergence of evidence that the early placental environment is hypoxic (1). Plugs of cytotrophoblasts block maternal spiral arteries that supply the placental site (2), and when the intervillous space is observed through a hysteroscope, it appears devoid of maternal erythrocytes. At about the 11th week of gestation, however, the plugs are displaced and blood flow begins. As a result, the local partial pressure of oxygen increases from about 18 to 60 mm (3), whereupon the placenta develops enhanced mechanisms for protection against oxidative damage (4). Pathways of regulation of trophoblast function by oxygen are not well known, but in this issue of the JCI, Caniggia and colleagues (5) investigate a transcription factor that appears to participate in physiological changes during this transition. This transcription factor, hypoxia-inducible factor 1α (HIF -1α), belongs to the basic helix-loop-helix family and functions as a heterodimer with its constitutively expressed partner HIF-1β, or ARNT, to bind gene regulatory regions. As Caniggia et al. now show, HIF-1α is present in the placenta before 10 weeks, but not at 11–14 weeks of gestation. Most placental villi float freely in the intervillous space, but specialized anchoring villi at the periphery attach the placenta to the decidual surface (6). These anchoring villi also act as feeder sites for the many migratory extravillous cytotrophoblasts (EVT) that infiltrate the decidual interstitium and spiral arteries (6, 7). These cells transform the arterial walls, allowing expansion, so that the blood supply can increase with fetal demands. Transformation proceeds from very early stages until about the 18th week, suggesting that trophoblast must be recruited into this lineage in both the first and second trimester. Similarly, to sustain placental growth, differentiation along another major pathway, from stem cytotrophoblast into villous syncytiotrophoblast, must occur efficiently both before and after the oxygen transition. The mechanism of anchoring villus formation has been illuminated by several groups using placental explants cultured ex vivo under varying concentrations of oxygen. First trimester villi, explanted onto a 3-dimensional extracellular matrix in 20% oxygen, form new sites spontaneously at their tips (8–10), whereas explants cultured in 2 or 3% oxygen show increased EVT production (5, 10, 11). Cytotrophoblast proliferation has long been known to be higher in hypoxia (12, 13), although the placental oxygen sensor still remains to be identified. Furthermore, as predicted from observations in vivo, HIF-1α mRNA levels are several-fold higher after culture in low oxygen. Now, following up on their earlier finding that TGF-β3 mRNA levels decline after the oxygen transition (14), the Caniggia group shows that TGF-β3 levels decline when HIF-1α mRNA is reduced by antisense treatment. Because reduction of TGF-β3 has no effect on HIF-1α levels (5), they conclude that TGF-β3 lies downstream of HIF-1α. Beyond its effects on TGF-β3, HIF-1α inhibits the expression of integrin α1β1 and MMP9, which are required for migration (5, 11). Caniggia and colleagues argue that TGF-β3 acts in an autocrine fashion to regulate these events. Certainly, TGF-β inhibits trophoblast migration in culture (15), but for several reasons interpretation of the new data is not straightforward. First, cytotrophoblast proliferation, migration, and colonization of decidua occur in vivo both before and after the oxygen transition (7, 16). Second, de novo column formation occurs in cultures in 20% oxygen (9, 10). Third, TGF-β is abundantly expressed in decidua (17), the very environment through which trophoblast migration occurs. Thus, observations made both in vivo and in vitro suggest that the placenta adapts to a changing oxygen environment by maintaining its core functions — anchorage, EVT migration, hormone production, and nutrient transfer — but that it adjusts the rate of each process to meet fetal requirements at specific developmental stages. Placental hypoxia, prolonged beyond the first trimester, is now recognized as a probable cause of pregnancy pathology (12, 13). Thus, for example, preeclampsia is associated with failed transformation of maternal spiral arteries by EVT (18). TGF-β3 overexpression has been reported in preeclamptic placenta (14). Some authors have linked preeclampsia to a failure of EVT migration, and Caniggia et al. (14) suggest that elevated TGF-β3 and a consequent inhibition of cell differentiation may be responsible for this failure. However, interstitially migrating EVT are abundant in the preeclamptic placental bed (18). Hence, the inability of these cells to enter and transform the arteries remains unexplained. The TGFβ family is controlled by proteases, binding proteins, and receptors, and more information is needed about the availability and activity of these molecules at the maternal-fetal interface. The explant model of EVT differentiation represents a significant technical advance. Assays of this kind that reflect, as faithfully as possible, the gravid uterine environment will play a vital role in elucidating developmental mechanisms and should lead to earlier and more effective treatments for pregnancy pathology.

Growth of SiC nanorods prepared by carbon nanotubes-confined reaction

SiC nanorods have been synthesized by means of the carbon nanotubes-confined reaction between SiO gas and carbon nanotubes. The diameters of SiC nanorods can be controlled by the local partial pressure of the CO gas produced during reaction. The morphology of SiC nanorods depends on the shape of the carbon nanotubes at high reaction temperatures. The growth mechanism of SiC nanorods is basically a shape memory synthesis.

Effects of diaspirin-cross-linked hemoglobin (dclhb tm) on local tissue oxygen tension in striated skin muscle: An efficacy study in the hamster

Using the dorsal skin fold chamber model in the hamster, we analyzed local tissue partial oxygen pressure (PO 2) in the striated skin muscle under nonischemic and postischemic conditions with a Clark-type multiwire oxygen surface electrode. Hypervolemic infusion (500 mg · kg −1 IV) or isovolemic exchange transfusion (3.3 gm · kg −1 IV; hematocrit 30%) with diaspirin-cross-linked hemoglobin (DCLHb) resulted in a slight decrease of the mean value of the local tissue PO 2 (mm Hg) 1 hour after administration. Concomitantly, the frequency distribution curves of local tissue PO 2 values were found to be more narrow (fewer values > 25 mm Hg and < 10 mm Hg). Resuscitation from severe hemorrhagic shock (bleeding of 33 ml · kg −1 at 0.4 ml · min −1) with autologous blood (AuB), Dx-60, or DCLHb TM led to an increase of mean tissue PO 2 values by 4.2-fold ( p < 0.05 versus Dx-60), 1.9-fold, and 3.7-fold ( p < 0.05 versus Dx-60), respectively, 2 hours after resuscitation. The reduction of tissue hypoxia (0-5 mm Hg) was significant only in the AuB- and DCLHb TM-treated animals. This study indicates that DCLHb TM effectively reverses tissue hypoxia after resuscitation from severe hemorrhagic shock by inducing a more homogeneous distribution of the local tissue PO 2 levels.

Effect of atmospheric CO 2 enrichment on rubisco content in herbaceous species from high and low altitude

Atmospheric CO 2 enrichment reduces Rubisco content in many species grown in controlled environments; however, relatively few studies have examined CO 2 effects on Rubisco content of plants grown in their natural habitat. We examined the response of Rubisco content to atmospheric CO 2 enrichment (600–680 μmol mol −1 in place of ppm) in 5 herbaceous species growing in a low altitude grassland (550 m) near Basel, Switzerland, and 3 herbaceous species from Swiss alpine grassland at 2470 m. At low elevation, the dominant grass Bromus erectus and the subdominant dicot Sanquisorba minor exhibited 20% to 25% reduction of Rubisco content following high CO 2 exposure; no CO 2 effect was observed in the subdominants Carex flacca, Lotus corniculatus and Trifolium repens. At the Alpine site, the subdominant grass Poa alpina maintained 27% less Rubisco content when grown at high CO 2 while the co-dominant forb Leontodon helveticus had 19% less Rubisco in high CO 2. Rubisco content was unaffected in the tundra dominant Carex curvula. Because the degree of Rubisco modulation was similar between high and low elevation sites, it does not appear that differences in local partial pressure of CO 2 (altitude) or differences in stress in general induce different patterns of modulation of photosynthetic capacity in response to high CO 2. In addition, the degree of Rubisco reduction (<30%) was less than might be indicated by the low biomass response to CO 2 enrichment previously observed at these sites. Thus, plants in Swiss lowland and alpine grassland appear to maintain greater Rubisco concentration and photosynthetic capacity than whole plants can effectively exploit in terms of harvestable biomass.

Bedeutung der Messung des Sauerstoffpartialdruckes bei experimenteller Lebertransplantation

Disturbances of microcirculation followed by an impaired tissue oxygenation contribute significantly to a possible organ dysfunction after liver implantation. This study investigated the influence of the explantation technique on and the possible predictive value of the liver transplant oxygenation. Two different explantation techniques were used. The livers were either explanted as rapidly as possible followed by the careful dissection of the vessels and the bile duct ex situ-rapid perfusion or were completely prepared--in situ perfusion in both groups, the livers were preserved by arterial and porto-venous infusion of ice-cold "UW"--solution. Local partial pressure values were obtained polarographically with miniaturized needle electrodes. The liver oxygenation directly after laparotomy was comparable in both groups. We systematically investigated the potential of liver oxygenation as a predictor of transplant quality.

Jet-Flow Scavenging of a Curing Oven—Part II: Numerical Simulation

In this part the heat, mass, and momentum transfer within a curing over that is commonly used in microelectronic manufacturing have been numerically analyzed. The local partial pressure of the vapor is a determining factor in the promotion of contamination. Based on the results of this study a new design is recommended. The significant features of the new design are: (i) ability to maintain the partial pressure of the vapor below the saturation pressure corresponding to the temperature at every location within the oven, and (ii) improved facility for scavenging of the vapor. Qualitative comparisons of results obtained here with flow visualizations (Part I) are excellent.

The influence of atmosphere on the formation behaviour of YBa2Cu3O7-x

The influence of reduced local partial pressure of CO2 due to virtually open atmosphere conditions on the sequence of reactions during the synthesis of YBa2Cu3O7-x from Y2O3, BaCO3 and CuO in air has been studied. Under these conditions, formation of (123) is observed to be complete within the first 30 min of calcination, through a reaction between intermediate phases Y2Cu2O5 and BaCuO2. Y2Cu2O5 also reacts with BaCO3, BaCuO2, and probably YBa2Cu3O7-x, individually to form Y2BaCuO5. The rapidly formed superconducting phase (123) is, however, unstable and decomposes into (211), BaO and CuO. BaO, thus released reacts with CO2 to form BaCO3. (211) reacts with other raw materials forming a stable (123) with (211) as the major impurity phase.

Monitoring of the lifetimes of the metastable states in Au in an air-acetylene flame as a probe for local stoichiometric conditions by the laser-enhanced ionization technique

The behaviour of the lifetimes of the metastable 5d9 6s22D3/2 and 5d9 6s22D5/2 states in Au in an air/acetylene flame has been studied using the step-wise delayed laser-enhanced ionization technique. Using a new, fast and reliable, automated experimental procedure, we have conducted a systematic investigation of the lifetimes for various flame characteristics, namely lateral and vertical position in the flame and flame composition. It was found that the lifetimes of the metastable states are affected by at least three different mechanisms: i) a rather small but almost constant quenching rate, unaffected by the variations in the flame environment; ii) quenching by oxygen, which is proportional to the local partial pressure of oxygen; and iii) quenching by unburned fuel components. We have shown that by monitoring the lifetime of a metastable state as a function of various flame parameters, the conditions for local stoichiometry in the flame can be determined by this technique.

Effect of Epinephrine on Local Partial Pressure of Oxygen in the Ciliary Processes

Local pO2 was measured close to the ciliary process of the rabbit eye using polarographic microelectrodes under microendoscopic observation. The experiments were performed in 5 anaesthetized New Zealand rabbits. Changes in local pO2, intraocular pressure (IOP) and blood flow velocity (BFV) were investigated after intra-arterial and topical application of epinephrine. After intra-arterial epinephrine, a simultaneous decrease in BFV, IOP and local pO2 was found. pO2 decrease was more pronounced in the anterior part of the ciliary processes than in the posterior part. Topical epinephrine lead to similar changes of all parameters mentioned above, the time constants being 10-20 times longer. In these topical experiments, no recovery of local pO2 was found over a period of at least 30 min despite an increase in BFV. Our results indicate that in the hyperemic phase after epinephrine the activity of the ciliary epithelium (and probably the total aqueous formation) is higher than normal. Thus, the decrease in IOP in this phase must be due to an increased outflow facility.

A method to estimate local deposition conditions using film profiles in features

A method to estimate the local partial pressures and temperature during low pressure chemical vapor deposition (LPCVD) proceses using film profile data is demonstrated using the chemistry of blanket tungsten deposition by the hydrogen reduction of tungsten hexafluoride. EVREST, the computer program which performs the estimates, uses EVOLVE, a physically based process simulator for ballistic transport and heterogeneous reactions in features on patterned wafers, to compute film profiles for a given set of deposition conditions. In order to validate the estimation method and to demonstrate EVREST, test calculations are performed using film profiles generated by EVOLVE instead of experimental scanning electron microscopy cross-sections. Calculated films profiles are compared with the generated film profiles (the data) and the simulated deposition conditions are adjusted using an algorithm based on Marquardt's method to minimize the sum of squared differences between points on the calculated and generated profiles. Test calculations confirm that film profiles in more than one feature can be used to improve the reliability of the partial pressure and temperature estimates.

The acute effects of MK-801 on cerebral blood flow and tissue partial pressures of oxygen and carbon dioxide in conscious and alpha-chloralose anaesthetized rats

The dynamic effects of the non-competitive N-methyl- d-aspartate receptor antagonist, MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo{ a,d}cyclohepten-5,10-imine], on cerebral blood flow and tissue partial pressures of oxygen and carbon dioxide were investigated in the striatal and occipital regions of conscious and anaesthetized rats by mass spectrometry. MK-801 (0.5 and 5 mg/kg, i.p.) induced a large increase in the blood flow of both cerebral regions of conscious rats, without significant changes in local tissue partial pressures of oxygen and carbon dioxide. The increase in cerebral blood flow was maximal within 30 min after injection. Its amplitude was independent of the dose of MK-801, but cerebral blood flow remained elevated for up to 4 h after 5 mg/kg MK-801, while it progressively decreased towards its basal level in rats given 0.5 mg/kg MK-801. The amplitude and time-course of the vascular changes were similar in the two cerebral regions studied. The difference in the changes in tissue partial pressure of oxygen induced by MK-801 and by a 6% CO 2 inhalation suggests that the MK-801-induced rise in cerebral blood flow in conscious rats is, at least partly, due to an increase in oxidative metabolism. In contrast, MK-801 induced either no changes or decreases in cerebral blood flow in alpha-chloralose-anaesthetized rats. The present results should be taken into account not only to determine the mechanisms by which N-methyl- d-aspartate receptor antagonists may exert their neuroprotective effects but also to further elucidate the sites of action of MK-801 in the central nervous system.

Estimating Local Deposition Conditions and Kinetic Parameters Using Film Profiles

ABSTRACTEVREST, a program which uses film profile data in features obtained from low pressure chemical vapor deposition (LPCVD) processes to estimate the local partial pressures, temperature, and kinetic parameters is demonstrated for blanket tungsten deposition by the hydrogen reduction of tungsten hexafluoride. EVREST uses EVOLVE, a physically based process simulator for ballistic transport and heterogeneous reactions in features on patterned wafers, to compute film profiles for a given set of deposition conditions and kinetic parameters. Calculated film profiles are compared with the experimental film profile and the free parameters are adjusted to minimize the sum of squared differences between points on the calculated and experimental profiles. EVREST uses Marquardt's method to determine the set of deposition parameters which provide the best calculated profile. Simulated and experimental profiles are in very good agreement.

Increase in the PSC‐formation probability caused by high‐flying aircraft

The saturation temperature Tsat for the formation of polar stratospheric clouds (PSC) strongly depends on the local partial pressures of nitric acid and water vapour, and thus is sensitive to NOx and H2O‐injection due to exhaust from aircraft in the stratosphere. The present paper investigates this effect, using daily stratospheric temperature data from the northern hemisphere compiled over the last 25 years by the Free University of Berlin. For Type‐I PSC the data were examined for temperatures T &lt; Tsat ‐ 3 K, which would allow for a supercooling of about the 3 K measured in a previous arctic winter balloon mission, in effect corresponding to a large HNO3‐supersaturation. We show that this is required to overcome the heterogeneous nucleation energy barrier. We compare the analyses of both a background atmosphere and one perturbed by a fleet of 600 stratospheric aircraft flying at ∼22 km altitude. The result is that between December and March in the polar cap region there might be more than a doubling in the occurence of Type‐I PSCs and an even stronger increase of Type‐II PSCs, and accordingly a substantial enhancement in the potential ozone destruction by chlorine radicals.

Minimization of Transient Emissions from Rotary Kiln Incinerators

Abstract Transient emissions of organics can occur from rotary kiln incinerators when drums containing liquid wastes bound on sorbents are introduced in a batch-wise fashion. Physical processes controlling the release of waste from the sorbent material are greatly affected by the rotation speed of the kiln and the kiln temperature. Local partial pressure of oxygen influences the rate of oxidation of the puff formed inside the kiln. These physical and chemical phenomena can be exploited to effect control of transient emissions by oxygen enrichment, where this is accomplished in either a steady or a dynamic mode. Experimental results from a pilot-scale rotary kiln incinerator simulator were combined with a theoretical model in order to explore the potential of minimizing transient emissions through changes in kiln rotation speed, kiln temperature, steady oxygen enrichment, and oxygen enrichment in a dynamic mode. Results indicated that transient organic emissions could indeed be minimized by changes in thes...