Water Vapor Diffusion Coefficient Research Articles

Mechanical, fracture-mechanical, hydric, thermal, and durability properties of lime–metakaolin plasters for renovation of historical buildings

An extensive set of parameters of several lime–metakaolin plasters, including basic material characteristics, mechanical and fracture-mechanical properties, durability characteristics, hydric parameters and thermal properties, is presented. In a comparison with the reference lime plaster without any additives, the experimental results show three- to five-fold increase in strength, about 25% decrease of water vapor diffusion coefficient, 20% decrease of water absorption coefficient and a very high increase of freeze/thaw resistance. Using the lime of very high purity in lime–metakaolin plasters (98.0% of CaO+MgO) is found preferable to lower-purity limes; the main differences are observed in the freeze/thaw resistance, mechanical and fracture-mechanical parameters. A comparison with two commercial renovation plasters shows that the only notable advantage of the best commercial plaster studied over the best lime–metakaolin plaster is in its lower moisture diffusivity but it can be matched relatively easily by using hydrophobization admixtures. Therefore, it can be concluded that the lime–metakaolin plasters can find a wider application range in the renovation of historical buildings in the future and may at least partially replace the current renovation plasters as a cheaper and equally effective solution.

Properties of lime composites containing a new type of pozzolana for the improvement of strength and durability

The basic material characteristics, mechanical and fracture-mechanical properties, durability characteristics, hydric parameters, and thermal properties of several lime–pozzolana composites with different dosage of the burnt Czech clay shale are investigated. Within the studied range of the pozzolana:lime (P:L) ratio of 0.2:2.3–1.7:0.8, the mix with P:L=0.9:1.6 is found to be the most effective solution. Its compressive strength, bending strength, effective fracture toughness, fracture toughness, and fracture energy are up to ten times higher, as compared with the reference lime composite without pozzolana addition, the water transport parameters are several times lower, the water vapor transport parameters are only about two times higher, the freeze/thaw resistance is significantly better, and the thermal properties are comparable. In a comparison with the reference lime–metakaolin composite, the corresponding material with burnt Czech clay shale is found to achieve similar mechanical and fracture-mechanical properties, its freeze/thaw resistance is somewhat worse, the water absorption coefficient is slightly lower, the water vapor diffusion coefficient is slightly higher, and the thermal properties are comparable. Therefore, the new lime–pozzolana composites have a good potential to be used as surface layers of building structures instead of lime composites without any addition, as an alternative solution to lime–metakaolin composites.

Innovative Lime-Pozzolana Renders for Reconstruction of Historical Buildings

Bulk density, matrix density, open porosity, compressive strength, bending strength, water sorptivity, moisture diffusivity, water vapor diffusion coefficient, thermal conductivity, specific heat capacity and thermal diffusivity of two innovative renovation renders on limepozzolana basis are analyzed. The obtained results are compared with reference lime plaster and two commercial renovation renders, and conclusions on the applicability of the particular renders in practical reconstruction works are drawn.

Determination of effective water vapor diffusion coefficient in pemfc gas diffusion layers

The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.

Tuning of Lower Critical Solution Temperature (LCST) of Poly(N-Isopropylacrylamide-co-Acrylic acid) Hydrogels

Temperature-responsive hydrogel with a lower critical solution temperature (LCST) close to human body temperature was prepared. Crosslinked N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc) copolymer networks were synthesized at various monomer ratios in the presence of ammonium persulfate (APS), N,N′-methylenebisacrylamide (NMBA) and N,N,N′,N′-tetramethylethylenediamine (TEMED) via a redox polymerization method. The resulting hydrogels possessed thermo- and pH-responsive characteristics. They were characterized in terms of swelling ratio, volume change, water uptake and diffusivity, water vapor uptake and diffusivity, and phase transition temperature. The water liquid and vapor diffusion coefficients for all the synthesized hydrogels were higher than the literature data, implying higher rates for drug release. The LCST of the hydrogel increased with higher AAc content in the copolymer. The gel containing 1.8% AAc exhibited an LCST similar to human body temperature, demonstrating a potential use in drug controlled release and biomedical applications.

The effect of surfactant Tween 80 on the hydrophilicity, water vapor permeation, and the mechanical properties of cassava starch and poly(butylene adipate-co-terephthalate) (PBAT) blend films

In this work, the affinity of water to cassava thermoplastic starch (TPS) and PBAT blend films with the addition of surfactant Tween 80 was investigated. The TPS and PBAT blend films were produced by extrusion with different starch concentrations added with 2% surfactant Tween 80, without surfactant Tween 80 or with 100% starch films (F100) as controls. The sorption isotherms, water vapor permeability (WVP), solubility coefficients, and diffusion coefficients of the films were evaluated. F100 films demonstrated the highest WVP and solubility coefficient values. Addition of the surfactant increased the diffusion coefficient and WVP. Films with the addition of surfactants had lower structural integrity and mechanical resistance than control films. The results indicated that the surfactant increased the free volume between the starch chains, thus, favoring the diffusion of water vapor. The combined effects of solubility coefficients and diffusion coefficients controlled the WVP of the films.

Distribution of morphology and transport properties of water vapor in injection‐molded biodegradable aliphatic polyester

AbstractA biodegradable polyester, poly(tetramethylene dodecanedioate), was injection molded in a rectangular cavity. Thin sections were sliced from molded samples starting from the skin. The morphology distribution inside the molded samples was studied by X‐ray analysis. The diffusion and solubility coefficient of water vapor were then measured using the microgravimetric method at the temperature of 30°C. Results show that morphology developed during the process influences the diffusion of water molecules through the polymer matrix. In particular, a direct influence of crystalline degree on the sorption and diffusion parameters was identified. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Experimental Measurement and Numerical Simulation of Water Vapor Diffusion through Asphalt Pavement Materials

Moisture damage in asphalt mixtures is defined as the gradual loss of structural integrity caused by the presence of moisture. A simple experimental procedure was developed in this study to measure water vapor diffusion coefficients in coarse aggregates, fine aggregate mixture (blend of the fine portion of the aggregates with the asphalt binder), and hot-mix asphalt. The procedure is based on periodic weight measurements of specimen-container ensembles subjected to a controlled temperature and relative humidity environment. Fick’s first law was used to estimate the diffusion coefficients of the materials. The results show that the proposed experimental method is an economic and efficient tool to quantify water vapor diffusion coefficients. Determining these material properties is fundamental to develop numerical models to study the deleterious effects of moisture vapor on the mechanical performance of asphalt mixtures. In order to exemplify the significance of the experimental measurements, a numerical simulation of transient moisture diffusion within the cross section microstructure of an asphalt mixture was conducted. The results of the simulations suggest that diffusion coefficients are fundamental for tracking the potential of a mixture to develop moisture-related degradation processes. Furthermore, it was observed that under the same environmental conditions, moisture damage in an asphalt mixture is highly influenced by the moisture diffusion coefficient of its constitutive phases.

Thermophysical characterization of a laminated solid-wood pine wall

The experimental results obtained by the thermophysical characterization of a wall made of stacked laminated solid-wood planks are reported in this article. The interface between two logs is filled with a felt packing that is compressed under the weight of the upper plank. The characteristics which affect heat, air and water vapour transfers through the wall have been determined, i.e. the sorption isotherm, density, thermal conductivity and diffusivity, water vapour diffusion coefficient, and air permeability of the wall. The results achieved demonstrate the influence of the presence of the laminating adhesive on water vapour and air diffusion, and the influence of the presence of the felt packing on air permeability. The result is that the hygrothermal behaviour of the wall studied cannot be described properly by simply taking into account the properties of the solid wood, and it is thus necessary to determine the global properties of the composite wall.

On the effect of the atmosphere on the evaporation of sessile droplets of water

An experimental and theoretical study of the effect of the atmosphere on the evaporation of pinned sessile droplets of water is described. The experimental work investigated the evaporation rates of sessile droplets in atmospheres of three different ambient gases (namely, helium, nitrogen, and carbon dioxide) at reduced pressure (from 40 to 1000 mbars) using four different substrates (namely, aluminum, titanium, Macor, and polytetrafluoroethylene) with a wide range of thermal conductivities. Reducing the atmospheric pressure increases the diffusion coefficient of water vapor in the atmosphere and hence increases the evaporation rate. Changing the ambient gas also alters the diffusion coefficient and hence also affects the evaporation rate. A mathematical model that takes into account the effect of the atmospheric pressure and the nature of the ambient gas on the diffusion of water vapor in the atmosphere and the thermal conductivity of the substrate is developed, and its predictions are found to be in encouraging agreement with the experimental results.

Predicting the diffusion coefficient of water vapor through glassy HPMC films at different environmental conditions using the free volume additivity approach

Prediction of diffusion coefficient of polymer materials is important in the pharmaceutical research and becomes the aim of this paper. This paper bases the prediction method on the estimation of the polymer fractional free volume at different environmental conditions. Focussing on glassy polymers, the free volumes of polymer films were estimated using the model of Vrentas et al. [J.S. Vrentas, J.L. Duda, H.-C. Ling, Antiplasticization and volumetric behavior in glassy polymers, Macromolecules 21 (1988) 1470–1475]. The required data are the moisture sorption and glass transition temperature data, which were measured on various hydroxypropyl methylcellulose (used as a model material) free films at different water activities. The temperature and molecular weight particularly determine the free volume of the polymer, while the sorbed water can either decease or increase the specific free volume of the polymer. At high water activity, the amount of water sorbed in the film increases to such level that the direct free volume addition by water becomes proportional to the contribution of the polymer itself. This confirms the importance of considering the environmental effect on the diffusivity of polymer during coating material selection. The presented approach enables the prediction of the diffusivity at any given relevant material variable and therefore has the potency to be used as a formulation development tool.

The impact of thermal conductivity and diffusion rates on water vapor transport through gas diffusion layers

Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively.

Effect of cellulose fibers addition on the mechanical properties and water vapor barrier of starch-based films

Starch-based films have promising application on food packaging, because of their environmental appeal, low cost, flexibility and transparency. Nevertheless, their mechanical and moisture barrier properties should be improved. The aim of this work was to enhance these properties by reinforcing the films with cellulose fibers. Besides, the influences of both the solubility coefficient of water in the films ( β) and the diffusion coefficient of water vapor through the films ( D w) on the films' water vapor permeability ( K w) were investigated. Films were prepared by the so-called casting technique, from film-forming suspensions of cassava starch, cellulose fibers (1.2 mm long and 0.1 mm of diameter), glycerol and water. The influence of fibers addition on K w was determined at three relative humidity gradient ranges, ΔRH (2–33%, 33–64% and 64–90%). Films reinforced with cellulose fibers showed higher tensile strength and lower deformation capacity, and presented lower K w than films without fibers. K w showed strong dependency of β and D w, presenting values up to 2–3 times greater at ΔRH = 64–90% than at ΔRH = 33–64%, depending on the film formulation. Therefore, adding cellulose fibers to starch-based films is a viable alternative to improve their mechanical and water barrier properties. Besides, this work showed the importance of determining film's water vapor permeability simulating the real environmental conditions the film will be used.

Diffusivity enhancement of water vapor in poly(vinyl alcohol)–fumed silica nano-composite membranes: Correlation with polymer crystallinity and free-volume properties

The diffusion coefficients of water vapor in poly(vinyl alcohol)–fumed silica (PVA–FS) nano-composite membranes were determined using the gravimetric method. Water vapor was observed to diffuse more rapidly in membranes with increased FS content. The vapor diffusion coefficient was determined as 1.2 × 10 −13 m 2/s in pure PVA and was observed to increase to 3.0 × 10 −13 m 2/s in PVA composites containing 30% FS nano-particles. The free-volumes of PVA–FS membranes were characterized using positron annihilation lifetime (PAL) spectroscopy. PAL results showed that both the ortho-positronium (o-Ps) lifetime and intensity increased with the addition of FS. The intensity ( I 3) was found to be higher than the estimated value determined from the linear combination of the data from pristine PVA and FS, and correlated excellently with the polymer amorphous content. The PAL results indicate that a higher FS content in PVA increases the free-volume hole size (a volume increase from 40 to 55 Å 3) and free-volume hole density (an I 3 increase from 23 to 28%), resulting in a higher fractional free-volume in the nano-composites. The increase in the relative polymer free-volume with higher FS content was associated with a decrease in the PVA crystallinity, as determined from differential scanning calorimetry measurements. It is postulated that the incorporated FS nano-particles interrupt polymeric chain packing and retard crystallization during membrane formation. More crystalline segments were transformed into amorphous regimes in the nano-composites containing more FS. A correlation between water diffusivity and the fractional free-volume was obtained, and the water diffusivity was successfully expressed by the free-volume theory.

Diffusion barriers at Mars surface conditions: Salt crusts, particle size mixtures, and dust

The diffusion coefficient of water vapor through porous media at Mars‐like surface conditions is measured for a variety of complex particle size distributions and soil compositions. Micron‐sized dust simulants, mixtures of sand‐ and dust‐sized particles, and salt‐encrusted sand are examined. We find that while the value of the diffusion coefficient,D, can be reduced by up to a factor of 10 for heavily salt‐encrusted soils (minimum observedD= 0.4 ± 0.04 cm2s−1), moderate amounts of salt only produce minor reductions inD. Mechanical packing of pure dust can lowerDby a similar amount, while mixtures of dust with sand‐sized particles produce at most a factor of ∼4 reduction. We conclude that present‐day processes of aeolian redistribution, moderate levels of salt encrustation, and volatile loss from dirty ice would be inefficient at producing soil deposits and lags on Mars that pose significant barriers to diffusion. Therefore, subsurface ice deposits that are thermally unstable would not be protected against sublimative loss by such materials.

Concentration Profiles of Tritium Penetrated into Concrete

Concentration profiles of tritium in cement paste, mortar and concrete were measured after exposure to tritiated water vapor for a given time. Tritium penetrated a distance of about 5 cm from the exposed surface during an exposure of 6 months. The model of tritium behavior in concrete materials reported by the present authors was developed in this study with the consideration of the effects of sand and aggregate on both the diffusion coefficient of tritiated water vapor and the isotope exchange capacity. Predictive calculations based on the tritium transport model were also carried out in some situations of tritium leakage. The results of the calculations show that a large amount of tritium will be trapped in the concrete walls, and the trapped tritium will be gradually released back to the tritium handling room over the time of months to years even after the decontamination of the room is completed.

Experimental study of the sublimation of ice through an unconsolidated clay layer: Implications for the stability of ice on Mars and the possible diurnal variations in atmospheric water

We have studied the sublimation of ice and water vapor transport through various thicknesses of clay (<63 μm grain size). We experimentally demonstrate that both adsorption and diffusion strongly affect the transport of water, and that the processes of diffusion and adsorption can be separately quantified once the system comes to a steady state. At shallow depths of clay, water vapor transport is determined by diffusion through both the atmosphere and the clay layer, whereas at greater depth the rate of sublimation of the ice is governed only by diffusion through the clay. Using two different models, we determine the diffusion coefficient for water vapor through unconsolidated clay layer to be 1.08 ± 0.04 × 10 −4 and 1.29 ± 0.06 × 10 −4 m 2 s −1 . We also determined the adsorption isotherms for the clay layer, which follow the Langmuir theory at low water vapor pressure (<100 Pa, where a monolayer of water molecules forms on the surface of the clay) and the BET theory at higher pressure (where multiple water layers form). From our analysis of both types of isotherms we determined the adsorption constants to be α = 4.9 ± 1.0 × 10 −2 Pa −1 and c = 30 ± 10 , respectively, and specific surface areas of 1.10 ± 0.2 × 10 5 and 9.0 ± 0.7 × 10 4 m 2 kg −1 , respectively. Finally, we report a theoretical kinetic model for the simultaneous diffusion and adsorption from which we determine adsorption kinetic constants according to the Langmuir theory of k a = 2.5 ± 0.5 × 10 −4 s −1 and k d = 8.7 ± 3.6 × 10 −5 s −1 . If the martian regolith possesses diffusive properties similar to those of the unconsolidated montmorillonite soil we investigated here, it would not represent a significant barrier to the sublimation of subsurface ice. However, at the low subsurface temperatures of high latitude (180 K on average), ice could survive from the last glaciation period (about 300 to 400,000 years ago). Higher subsurface temperatures in the equatorial regions would prevent long-timescale survival of ice in the shallow subsurface. In agreement with previous work, we show that adsorption of water by a clay regolith could provide a significant reservoir of subsurface water and it might account for the purported diurnal cycle in the water content of the atmosphere.

Effect of oil films on moisture vapor absorption on human hair

In this paper sorption and desorption of water vapor on hair fibers treated with various oils is investigated, using a dynamic vapor sorption (DVS) apparatus. Results show lower "equilibrium" sorption of moisture for various oil-treated samples compared to the untreated sample. Coconut oil-treated hair had a higher regain than mineral oil-treated hair. Although treating the hair samples with oil reduced moisture pickup, a considerable amount of moisture vapor was still able to penetrate into hair fibers. Calculated hysteresis plots show that the samples treated with different oils have slightly higher moisture retention at low relative humidities compared to that of the untreated sample, which suggests a beneficial effect. The calculated moisture diffusion coefficients for oil-treated samples were much lower compared to the untreated hair fibers, suggesting that surface oil films and penetrated oil molecules form a diffusion barrier. A moisture diffusion model is discussed in terms of the possible role of fiber swelling on restrictive narrowing of the cell membrane complexes (CMCs), which form the diffusion pathways in the fiber. The effect of film thickness on moisture absorption and the reverting of the sorption isotherm to that of the untreated hair after removal of the oil film shows that oil film is the main resistance to moisture diffusion. The lowering of the diffusion coefficient of water vapor by oil films will slow the loss of moisture, an effect similar to "moisturization" of hair.

Mass Transfer Characteristics Inside PEMFC and its Unsteady Power Generation Analysis

Water management factors such as diffusion coefficient and electro-osmotic coefficient of water vapor through MEA have been measured in our previous report, since these factors have important effects on performance of PEMFC. For unsteady analysis, more factors such as water content of MEA and mass transfer coefficients at both side of GDL boundary must be known. Then we have measured the water content of MEA and unsteady mass transfer coefficient between MEA and GDL. Since the local water transfer inside the PEMFC is quite difficult to be measured, so that the three dimensional analysis of PEMFC has been performed to estimate the mass transfer coefficient between channel and GDL. Then, unsteady power generation analysis with those measured mass (water) transport factors has been conducted. The computed results such as cell voltage transient response, current distributions, and membrane resistivity distributions are compared and agreed well with the measured results.

On the importance of the diffusional uptake of water vapour for the development and radiative properties of high altitude clouds: a large eddy model sensitivity study

AbstractHigh altitude clouds such as Cirrus have a substantial impact on the global radiation budget and their climatic impact depends on their micro‐structure. Clouds composed of small crystals with effective radii less than 16 µm have a cooling effect, whilst clouds made up of larger crystals have a warming effect (Lynch et al., 2002; p 397). The success of cirrus microphysical modelling depends largely on the right choice of microphysical parameters and the diffusivity of water vapour (Dv) is one such crucial parameter. In this study we have explored the sensitivity of cirrus microphysics to the specification of Dv in large eddy model (LEM) simulations of cirrus case studies. We analysed important processes where vapour diffusion plays a role in the evolving microphysics. Cirrus clouds form at high altitudes where the molecular mean free path and hence Dv is significantly larger than the ground level value. To date some LEMs (e.g. the UK Met Office LEM) do not consider the height‐dependence of the diffusivity of water vapour. Although this may not pose much of a problem for warm clouds formed in the lower boundary layer, for high altitude cirrus clouds, using ground level values could affect the microphysical development. In this study we have shown that crystal growth rates, the ice water mixing ratios, crystal number concentrations, auto‐conversion rates of ice particles to form aggregates, the deposition rates of water vapour on aggregates, and the long‐wave radiative cooling rates depend sensitively on the choice of the diffusivity of water vapour. The most widely used empirical formulation on the height‐dependence of the diffusion coefficient is that described by Pruppacher and Klett (1997) and is valid over a temperature regime between − 40 °C and 40 °C. Since many cirrus clouds form at temperatures colder than − 40 °C, it becomes imperative to use sophisticated formulations for an accurate prescription of Dv for cirrus studies. In this first LEM study we have used a Lennard‐Jones (L‐J) model to estimate Dv and applied it to two cirrus case studies. This formulation is accurate and valid over cirrus forming altitudes and is effective even when temperatures are colder than − 40 °C. First, we have shown that the L‐J model can be easily adapted within LEMs to study cirrus clouds and thence we examined the resulting microphysics through simulations with and without the L‐J update. We observed that the overall microphysical development was sensitive to the choice of the diffusion coefficient of water vapour. We believe that this study will aid cirrus modellers worldwide who are often constrained by the availability of microphysical observational data. Copyright © 2007 Royal Meteorological Society