Constant Water Vapor Pressure Research Articles

Operando 3D-Monitoring of the Oxygen Partial Pressure (p(O2)) within a Polymer Electrolyte Membrane Fuel Cell (PEMFC) during Cell Operations at Higher Temperatures

Introduction:Hydrogen technologies, especially the Proton Exchange Membrane Fuel Cell (PEMFC), are promising alternative power sources, due to their low to non-carbon emission, high power density, high efficiency, and fast start-up. Despite their high potential and expected major impact on the economy of the future, there are still challenges to overcome, before PEMFCs can be globally commercialized. Soon, PEMFCs operated above 100°C are expected to be used for automobiles, due to their use of smaller and lighter radiators, higher reaction rates and lower susceptibility to catalyst poisoning. To achieve this goal however, high-temperature PEMFCs need a higher performance, stability, and durability, while reducing the cost at the same time. The performance, durability and stability of a fuel cell are related to the distribution of physical and chemical parameters, like e.g., oxygen partial pressure (p(O2)). During the operation of the fuel cell, the distribution of those parameters is inhomogeneous.[1] Therefore, we used an in-house developed 3-dimensional non-destructive real-time/space visualization system to achieve an understanding of the p(O2) inside the fuel cell during operation at higher temperatures.Experimental:The PEMFC used for this experiment has an active area of 4 cm2 and uses 10 straight gas flow channels. Prior to the assembling, 5 pinholes with a diameter of 90 µm were manufactured into the GDL on the cathode side both underneath the rib and the gas flow channel (Fig. 1). Those pinholes are also manufactured into the current collector and insulator of the PEMFC on cathode side. The holes are used to insert optical fibers from outside the cell to approximately 20 µm (adjustable with an accuracy of 1 µm) above the catalyst layer of the CCM. Prior to the use, the fibers with a core diameter of 10 µm are etched in HF solution to reduce their clad diameter from 125 to 50 µm. Furthermore, the apex of the fiber is cleaved perpendicular to the axis of the fiber. Before insertion an oxygen sensitive dye (PtTFPP, (Fig. 2)) is applied to the apex of the fibers. PtTFPP has absorption peaks at 407, 530 and 540 nm and an emission peak at 650 nm. The emission peak is quenched by oxygen partial pressure, and the emission intensity decreases monotonically with increasing oxygen partial pressure. For the excitation, a laser with a wavelength of 532 nm is used and the emitted light is captured by a CCD camera.[2] Visualizations were carried out at 80, 90, 100, 110 and 120 oC at a constant water vapor pressure of 37.97 kPa, which equals 80, 53.6, 36.8, 25,9 and 18.5% RH respectively, at increasing current densities. Prior to the visualization, calibrations, and performance tests in form of IV and CV were carried out for each temperature. The gas flow rate during the experiments was set to 100 mL/min Air/H2 (parallel flow) at cathode and anode respectively. Results and Discussion:The IV performance tests showed decreasing cell performance with increasing temperatures, which was expected because of the increasing resistance due to drying out of the membrane. Furthermore, a decrease of the ECSA was observed with increasing temperatures, which could also be due to the drying out of the membrane as well as the binding material around the catalyst nanoparticles which reduces gas diffusion and the effective catalyst surface area. In this first try, the monitoring was done with 3 fibers under one rib located near the inlet, the middle of the cell and near the outlet of the cell. With increasing current density, the oxygen partial pressure was decreasing at every temperature. Interestingly, the oxygen partial pressure distribution changed with increasing temperature. The reason for the changing oxygen partial pressure distribution could be due to accumulation of heat (Fig. 3) and increasing water vapor pressure at higher temperatures. Further experiments with 5 fibers including measurements in the gas flow channel as well as measurements with back pressure to increase the cell performance will be done shortly to confirm this finding.[1] Y. Kakizawa, C. L. Schreiber, S. Takamuku, M. Uchida, A. Iiyama, J. Inukai, Visualization of the oxygen partial pressure in a proton exchange membrane fuel cell during cell operation with low oxygen concentrations, J. Power Sources, 483, 229193 (2021).[2] Y. Kakizawa, T. Kobayashi, M. Uchida, T. Ohno, T. Suga, M. Teranishi, M. Yoneda, T. Saiki, H. Nishide, M. Watanabe, A. Iiyama, J. Inukai, Oscillation mechanism in polymer electrolyte membrane fuel cell studied by operando monitoring of oxygen partial pressure using optical probes, J. Surf. Finish. Soc. Jpn, 72, 230 (2021). Figure 1

Verification of hydrothermal stability of adsorbent materials for thermal energy storage

This paper presents an experimental protocol for the cycling stability of adsorbent materials for thermal energy storage (TES) applications under hydrothermal conditions. Two different aging conditions were identified, namely, cycle and shelf test. The former one mimicking the cycling between desorption and adsorption conditions, while the latter one keeping a constant temperature for long time under constant water vapor pressure. A flexible experimental setup was then designed and realized to contemporarily perform both aging condition under selectable operating conditions. The protocol defines different characterization methods to compare the fresh and the aged samples. The measurement of the water vapor adsorption equilibrium isobars represents the main parameter to directly highlight possible degradation phenomena. Subsequently, X-ray diffraction patterns (XRD), nitrogen physisorption, and scanning electron microscopy coupled to energy dispersive x-ray (SEM–EDX), are used to evaluate structural, textural, morphological, and elemental composition variation that can help in identifying the causes of possible degradation. The proposed protocol was employed to validate the stability of a commercial adsorbent, AQSOA Z02, that proved a quite stable behavior both under cycle and shelf investigated conditions.

Effect of Humidity on Wear of TiN Coatings: Role of Capillary Condensation

Coated ball-on-disk wear configuration was used to study the effect of relative humidity, water vapor pressure, and water on wear of TiN coatings in the temperature range of room temperature to 100 °C. Two kinds of experiments were designed: one at constant temperature with varying humidity and the second at constant water vapor pressure with varying temperature. Temperature variation experiments were also conducted in water. The trends in wear volume after a fixed sliding distance were analyzed. At constant temperature, the wear volumes increased with humidity/water vapor pressure. However, at a constant vapor pressure, wear volumes remained roughly invariant with temperature until a critical temperature below which they rose sharply. In contrast, the wear rate increased with temperature for tests in liquid water. Considering calculations based on Kelvin’s equations and further characterization of the wear surfaces using profilometry, XPS and FIB techniques, it has been strongly suggested that the anomalous temperature dependence of wear in humid air might be due to capillary condensation occurring at the contacting asperities. The wear of TiN showed two regimes in the influence of humidity. Where the humidity was below a threshold value of ~ 50 pct relative humidity (RH), there was negligible condensation and wear was low. Above 50 pct RH, the wear rate increased due to the availability of liquid water at asperities. The reason for the difference in the wear rate in vapor and liquid was hypothesized, based on XPS data, to depend on the formation of soft hydroxides in the presence of water, in contrast to the harder barrier oxide formed in the presence of oxygen, the dominant gas species in the vapor phase.

Modeling of the lithium hydride hydrolysis under low relative humidity

A reactional mechanism describing the hydrolysis of lithium hydride (LiH) under moist atmosphere is proposed and modeled. It involves the formation of lithium oxide (Li2O) at low vapor pressure and both lithium oxide and lithium hydroxide (LiOH) at higher vapor pressures, with diffusion of water in these layers. A numerical model based on this mechanism is implemented with COMSOL Multiphysics to simulate the hydrolysis of LiH particles in open system (constant water vapor pressure). Kinetic parameters such as rate constant of reactions and diffusion coefficient of water in Li2O and LiOH are first fitted against experimental data. The best agreements are obtained when the diffusion coefficient of water is 10 times higher in LiOH than in Li2O. The resulting model accurately predicts the hydrolysis rates experimentally measured for a wide range of water vapor pressures (0.3–17 Pa). The hydrolysis mechanisms are compatible with a Li2O production limited by water diffusion and a LiOH production governed by the kinetics. Finally, simulation suggest that the thickness of the Li2O layer does not depends much on the water vapor pressure whereas that of LiOH increases drastically at high water vapor pressures.

The origin of anomalous water diffusion in silica glasses at low temperatures

AbstractAnomalous water diffusion into SiO2 glass was observed in a low temperature range, below ~850°C, under a constant water vapor pressure of 355 Torr (47.3 kPa). Both the effective water diffusion coefficient and water solubility exhibited an anomalous time dependence. For example, water solubility in the low temperature range increased initially, achieving much higher values than expected based on extrapolation from higher temperature data, and then decreased with time toward an equilibrium value. This phenomenon was reported earlier, but a complete explanation was not possible; a new model is presented based upon glass surface compressive stress generation and subsequent surface stress relaxation. Water diffusion can promote stress generation and stress relaxation, both of which affect the reaction between diffused molecular water and the glass structure. By considering these stress effects, the anomalous water diffusion behavior in silica glass is explained. Furthermore, the same model can account for the reversal of external tensile and compressive stress effects on water solubility and diffusivity in silica glass observed after a few hours of heat treatment at 650°C in 355 Torr water vapor pressure.

Hydrolysis of lithium hydride under low relative humidity

Hydrolysis of LiH at room temperature and under low relative humidity (RH < 2%) is studied by manometry working with heavy water either in closed (variable water vapor pressure) or open (constant water vapor pressure) system. Products of the reaction are characterized by XRD and FTIR. It is shown for the first time that the hydrolysis reaction occurs in two steps: first water is adsorbed on the LiH surface and then the hydrolysis reaction starts. In open system, for relative humidity lower than 0.04% only the formation of Li2O is observed while for higher moisture the reaction continues with the production of LiOD. The reaction rate is however extremely low and only a very small amount of LiH is transformed. The kinetic can be well predicted by the model of core shrinking limited by the diffusion through the layer of Li2O and/or LiOD surrounded LiH particles. For practical application, it is concluded that if LiH is stored under controlled humidity lower than 0.04% (40 ppm) there is no major risk to form LiOH in significant amount.

Characterizing Penetration of Aminoethoxyvinylglycine (AVG) through Isolated Tomato Fruit Cuticles

Aminoethoxyvinylglycine (AVG) is an ethylene biosynthesis inhibitor that is commonly applied to apple trees prior to harvest to delay ripening and reduce fruit drop. To help understand how selected environmental factors and spray adjuvants affect AVG uptake, penetration of 14C-AVG through enzymatically isolated tomato (Solanum lycopersicon L.) fruit cuticular membranes (CM) was studied using a finite-dose diffusion system in which penetration is monitored from a drying spray droplet/deposit through an interfacing CM into a receiver solution. Penetration of AVG was initially rapid (4.1 % at 1 h after application), slow after droplet drying (12.5 % by 120 h after application), and averaged 20.7 % of the amount applied at 37 days after application. Rate and amount of AVG penetration were positively related to AVG concentration. Rewetting the dried droplet deposit with deionized water caused a transient increase in penetration that ceased when the droplet dried again. Increasing relative humidity from 50 to 100 % above the dried droplet deposit markedly increased penetration. Increasing temperature from 10 to 30 °C at constant water vapor pressure deficit (0.35 kPa) increased AVG penetration between 0 and 6 h after application but had little effect on penetration thereafter. LiCl, CaCl2, and MgCl2 at 100 mM increased AVG penetration at 120 h after application; lower concentrations had no effect. Our results indicate that AVG penetration was enhanced by increasing humidity above the droplet deposit or by the addition of hygroscopic salts to the spray solution, thereby maintaining the AVG mobility in the droplet deposit.

Application of sorption tests to estimate selected properties of alluvial soils in Żuławy Elbląskie

Application of sorption tests to estimate selected properties of alluvial soils in Żuławy Elbląskie The specific surface area and hence the sorption or desorption moisture at constant water vapour pressure will increase with increasing content of humus and clay fraction in soils. Adopting such an assumption, preliminary experimental studies were performed to assess the possibility of using sorption/desorption tests to determine some features of alluvial soils from Żuławy such as cation exchange capacity CEC and the specific external surface area Se. Results of the sorption tests were compared with the analyses of the same soil properties determined with standard methods to evaluate the usefulness of the former. Preliminary studies showed a high similarity in the determination of CEC and Se with both methods. Confirmed usefulness of sorption/desorption methods for determination of the specific surface area of mineral soil particles and particularly for analyses of cation exchange capacity would bring a significant simplification of studies and a possibility of their dissemination due to the ease of such analyses. Determination of the specific surface area with the sorption and desorption method does not require sophisticated equipment and laborious and costly preliminary procedures. Dissemination of this method and recognition it as a reference one would, however, need further studies on various soil types.

Further Study of the Effect of Water Vapor on Slider Air Bearing

It has been proved that water vapor in moist air contributes to slider air-bearing pressure in a totally different way from dry air. It is a very good first order approximation to take a constant water vapor pressure inside and outside the interface. The simulated slider flying height and attitude in moist air are different from that in dry air and the higher the relative humidity the bigger the difference. This work is a further study of the effect of water vapor on slider air bearing. In this paper, the effect of slider air-bearing pressure on the saturated water vapor pressure is explained. The adsorbed water film thickness on slider and disk surfaces as a function of relative humidity at different temperatures is calculated. The effects of slider air-bearing pressure, temperature, slider-disk spacing, and relative humidity on the water vapor pressure in slider-disk interface are studied. An empirical equation is developed for the calculation of accurate water vapor pressure distribution in slider-disk interface.

Chemical preparation and thermal behavior of neodymium cyclotriphosphate pentahydrate NdP 3O 9·5H 2O: A study by Controlled Rate Thermal Analysis (CRTA)

A new neodymium cyclotriphosphate pentahydrate powder, NdP 3O 9·5H 2O, has been prepared using a classical chemistry method and characterized by X-ray diffraction and infrared spectroscopy techniques. The IR spectrum shows the characteristic bands of cyclotriphosphates as the triplet in the range 1055–1007–917 cm −1. The thermal behavior of the titled compound was also carried out using conventional thermal analysis techniques (TG and DSC) under air atmosphere and Controlled transformation Rate Thermal Analysis (CRTA) technique under constant water vapour pressure of 5 hPa. It was shown that thermal treatment in air, with linear heating rate of 10 K min −1, favourites the structure breakdown as soon as the release of water molecules starts. In this case an amorphous compound is obtained, which then crystallizes at 1073 K and gives the polyphosphate Nd(PO 3) 3. However, by CRTA technique at P H 2 O = 5 hPa , the water molecules are eliminated with a lower rate which permits to save the initial structure up to an advanced dehydration step. The breakdown of the initial structure is immediately followed by the crystallization of the anhydrous polyphosphate Nd(PO 3) 3 phase in the temperature range 455 K < T < 793 K.

Water Vapor Diffusion through Wheat Straw Residue

Accurate prediction of evaporative water loss from dryland agricultural soils requires knowledge of diffusive resistance. Our objective was to experimentally determine the effective vapor diffusion coefficients and the diffusive resistances for water vapor through wheat (Triticum aestivum L.) residue layers. A laboratory diffusion chamber was designed to investigate the effects of wheat residue type (straw vs. chaff), residue amount (2, 4, 8, and 12 Mg ha−1), and residue orientation (horizontal vs. vertical straw) on vapor diffusion. The diffusion chamber consisted of two well‐stirred chambers separated by a residue layer. In one chamber, a constant saturated water vapor pressure was maintained, while in the other chamber, vapor pressure increased with time due to the diffusion of vapor through the residue layer. Vapor pressure was measured with a humidity sensor. The diffusion equation was used to obtain the effective diffusion coefficient from the measured data by inverse modeling. Results showed that for both straw and chaff residue, increasing the quantity of residue did not necessarily decrease the effective diffusion coefficient but did increase the diffusive resistance. For the same quantity of residue, chaff had a lower effective diffusion coefficient than straw, but the diffusive resistances were similar. The diffusive resistances were in the order of 800 to 3700 s m−1 for 2 to 12 Mg ha−1 residue treatments. The factor affecting the diffusive resistance the most was the thickness of the residue layer: the thicker the residue layer, the larger the diffusive resistance.

The interaction of the theophylline metastable phase with water vapor

The conditions of hydration of the stable and metastable theophylline phases were determined. Two-phase metastable phase/monohydrate and stable phase/monohydrate equilibrium pressures were measured at 25, 30, and 35°C. The metastable phase began to react with water vapor at lower relative humidities than the stable phase. Processes that occurred with the metastable and stable theophylline phases over various water pressure ranges were considered. The metastable phase exhibited an unusual behavior at 25°C and relative humidity 47%. At constant water vapor pressure and temperature, theophylline was initially hydrated and then lost water and again became anhydrous. Two consecutive processes occurred in the system, the formation of theophylline monohydrate from the metastable phase and its decomposition to the stable phase. The ratio between the rates of these processes determined the content of the monohydrate at the given time moment.

Methods for Determining the Marangoni Numbers in Studying the Absorption in the Operation of a Heat Pump

It is analyzed whether or not experimental data on the surface tension of an aqueous lithium bromide solution with or without surfactants can be used to study the heat and mass transfer in the operation of the absorber of a heat pump. Examples of the erroneous determination of the Marangoni numbers under these conditions are given. A method for calculating the interfacial tension gradients and the Marangoni numbers is proposed. It is argued that it is necessary to perform additional experimental investigations for determining the interfacial tension under the conditions of equilibrium in the surfactant-containing aqueous lithium bromide solution-water vapor system at constant water vapor pressure.

Metaettringite, a decomposition product of ettringite

Dehydroxylation of ettringite in an atmosphere of constant partial water vapour pressure (30–400 Torr) and controlled temperature (55–95 °C) yields an X-ray amorphous product containing 11–13 H2O per ettringite formula unit. The X-ray amorphous product does, however, give electron diffraction patterns similar to those of ettringite but with a considerably reduced from ∼1.123 nm (ettringite) to 0.85 nm in the partially dehydroxylated product, termed metaettringite. The structure of metaettringite is closely related to that of fleischerite, Pb3Ge[(OH)6](SO4)2·3H2O and the isostructural despujolsite, Ca3MnIV(SO4)2(OH)6·3H2O. These minerals contain columnar units. The columnar structure is like that of ettringite but with closer packing of columns in the a direction, resembling metaettringite.The mechanism of collapse of ettringite to metaettringite, involving loss of water and motion of columns, cannot be achieved without scissoring and possibly rotation of individual columns; this and other defect-producing mechanisms result in loss of crystallinity of the metaettringite product.

Heat Balance Limits in Football Uniforms

ABSTRACTBACKGROUND: Scant evidence documents the physiological and environmental stresses for football players wearing partial or full uniforms, but such information would be useful for determining the ambient temperatures and humidities associated with uncompensable heat stress during practice and games.OBJECTIVE: This laboratory study used a physiological approach to determine critical heat balance limits (various combinations of ambient temperature and relative humidity) for subjects exercising in typical American football uniforms.DESIGN: Eight nonheat-acclimatized men exercised at 35% Vo2max in a programmable environmental chamber. In multiple trials, either dry-bulb temperature (Tdb) was held constant and ambient water vapor pressure (Pa) was systematically increased, or Pa was held constant and Tdb was systematically increased. The critical heat balance limits were determined as the environments at which body core (esophageal) temperatures were forced out of equilibrium, reflecting uncompensated heat storage imposed by those environments.RESULTS: Critical environmental limits are presented that define the combinations of air temperature and relative humidity above which thermal balance cannot be maintained. These zones of uncompensable heat stress result in continuously rising core temperatures. Retrospective analysis reveals that documented heatstroke deaths in football players wearing full uniforms occurred at or above these critical environments.CONCLUSION: Heat balance limits can be used in decision making and are especially relevant for preventing heat-related illness or injuries early in the football practice season. The critical limits are expanded when shorts are substituted for football pants with pads.

Curvature of carrot (Daucus carota L.) sticks is related to number and distribution of xylem vessels

Abstract Carrot sticks, marketed as vegetable snacks, often develop significant curvature which is considered to be undesirable. Factors affecting curvature of sticks were investigated following incubation under defined conditions. Curvature was quantified by determining the angle between two tangents fitted to projections of a carrot stick. Curvature of commercially produced sticks following a 3 day-incubation period (5°C, 75% RH) was highly variable and weakly related to segment angle (r=0.23*) or the volume ratio of xylem to phloem tissue (r=−0.24*), but not affected by fresh mass of sticks (r=−0.08), loss of fresh mass during incubation (r=0.11) or volume of xylem (r=−0.02) or phloem (r=0.22). Since geometry and mass of commercially produced sticks were variable, standardized sticks (80 mm length, 45° segment angle, 5.5±0.5 g fresh mass and with a volume ratio of phloem to xylem of 1:0.33) were used in subsequent experiments. Curvature increased with increasing water vapor pressure deficit (range 20–566 Pa) at constant temperature (6°C), but temperature (range 6–20°C) at constant water vapor pressure deficit (560 Pa) had no effect on curvature. Variation in curvature was larger for sticks prepared from different roots than for sticks from the same root. Proximal xylem vessels were arranged at high density in radial rays adjacent to the cambium (0–0.6 mm inwards from cambium), but distal vessels (0.6–5 mm inwards from cambium) were more randomly distributed at markedly lower density. Curvature was related to the number of distal xylem vessels (r2=0.43**) or to the ratio of number of distal to proximal xylem vessels (r2=0.66***), but only weakly to mass loss (r2=0.22*). Our data suggest that number and distribution of xylem vessels were key factors determining curvature of carrot sticks.

Time dependent diffusion coefficient of water into silica glass at low temperatures

The diffusion of water into silica glass at low temperatures (<1000°C) exhibits a peculiar time dependence: the diffusion coefficient decreases at first and then increases with time at a constant temperature under a constant water vapor pressure. Water solubility exhibits the opposite trend: it initially increases with time and subsequently decreases with time. The initial decrease in diffusion coefficient and increase in solubility with time is attributed to the decrease of the self-stress around the diffusing water molecules by stress relaxation of the surrounding glass. The subsequent increase in diffusion coefficient and decrease in solubility with time is attributed to the structural relaxation of the glass and the corresponding change of the glass–water reaction equilibrium constant.

Effect of water content in phosphate glasses on slow crack growth rate

Slow crack growth in phosphate glasses with different water contents was measured at 225°C and room temperature under different water vapor pressures using double-cleavage-drilled compression (DCDC) samples. Higher water vapor pressure caused a faster crack growth velocity at a constant temperature for all samples. At 225°C under a constant water vapor pressure, glasses with higher water content exhibited greater crack velocity. However, at room temperature under a constant water vapor pressure, samples with lower water content exhibited a higher crack velocity. This room temperature behavior is probably caused by water uptake by the sample during the crack growth measurement. In support of this hypothesis, weight gain was observed when the glasses were exposed to humid atmosphere at room temperature, with the samples having lower water content exhibiting the greater weight gain.

Effects of water and combustibles on single crystal zeolite conductivity

The conductivity behavior of the natural single crystal zeolites ANA, BRE, CHA, HEU, PHI and STI was investigated in the temperature range between 60 °C and 110 °C. The effect of combustible gas components on the conductivity was also investigated using constant water vapor pressure conditions. In the temperature range studied, the conductivity is assumed to be mainly ionic and zeolitic water remains within the channel and cage system of the structures. All zeolites do have different frameworks, channel systems, channel sizes, channel cations and water contents.

Water diffusion, oxygen vacancy annihilation and structural relaxation in silica glasses

Water diffusion into silica glasses causes structural changes which can be readily detected by ultraviolet (UV) and infrared (IR) spectroscopy. One such change is the annihilation of the oxygen vacancies which exist in some silica glasses. This process can be monitored using the UV absorption at ∼ 240 nm (∼ 5.0 eV). Water can also accelerate local structural relaxation of silica glass. This relaxation can be monitored by measuring the IR absorption peak at 1873 cm −1. The diffusion coefficients for water penetration and local structural relaxation were measured for various silica glasses with low water concentrations in the high temperature range of 750–1200°C under a constant water vapor pressure of 355 mm Hg. The diffusion coefficient for oxygen vacancy annihilation was also measured for oxygen-deficient glasses under the same conditions. The water diffusion coefficient was nearly identical for all glasses investigated. The diffusion coefficients for oxygen vacancy annihilation in oxygen-deficient glasses and for local structural relaxation of stoichiometric silica glass were higher than the water diffusion coefficient even though both structural changes are accelerated by water. Water diffusion into silica at high temperatures has been explained in terms of the local equilibrium of the reaction between mobile molecular water and silica glass to form immobile hydroxyl water. The phenomena observed here can be explained by postulating the presence of a minute amount of molecular water species which are not restricted by the equilibrium condition for the reaction and can diffuse faster than the majority of the molecular water species.