Relative Humidity Cycling Research Articles

Impact of the determination of the sorption-desorption curves on the prediction of the hemp concrete hygrothermal behaviour

Hemp concrete is a sustainable material with low embodied energy. However, its development as a building material requires a better evaluation of its moisture-thermal buffering abilities behavior, which are known to strongly depend on the amount of water contained in wall pores and its evolution. In this context, the aim of this paper is to study the experimental determination of sorption-desorption curves of hemp concretes following two methods and at two temperatures (23°C and 40°C). The former considers the total sorption-desorption loop, from the dried state to a relative humidity of 85%, while the second one is based on partial relative humidity cycles between 23% and 85%. At first, the difference between the curves obtained through these two methods were analysed. They were then implemented in a simple hygrothermal model to simulate dynamic hygroscopic and hygrothermal loadings. The comparison between the theoretical calculations and the experimental results eventually allows to scan which of these relations is more representative of the real dynamic behaviour of the tested hemp concretes.

Developing a Framework for Improved Accelerated Testing of Aluminum-Lithium Alloys

Third generation Aluminum-lithium (Al-Li) alloys such as AA2060 are attractive for aerospace applications due to their high strength-to-weight ratios and their corrosion resistance during seacoast exposures.1 However characterizing these alloys in laboratory corrosion tests can be difficult as the results from conventional accelerated tests do not always agree with seacoast results. For example, ASTM G34 is a standard test for exfoliation corrosion, which is a form of localized attack that produces blisters when corrosion products due to intergranular attack produce a wedging pressure and lift un-attacked grains upward. This test was developed for alloys 7075 and 7178,2 but is considered applicable to 2xxx and 7xxx series aluminum alloys.3 However when Al-Li alloys 2099 and 2060 are tested in ASTM G34, the trend in exfoliation susceptibility over different aging conditions is opposite to the trend observed after seacoast exposures.4 This kind of disagreement between laboratory testing and seacoast exposure slows down the process of optimizing aging practices for new products. As new generation aluminum alloys become available, improved accelerated corrosion tests must be designed to accurately predict the corrosion response under atmospheric conditions in a short time. Many different approaches can be used to accelerate corrosion (high testing temperature, high chloride content in the testing solution, added oxidizing agents, very acidic or very alkaline testing solution etc.), and each of these parameters alters the corrosion response by changing electrochemical kinetics. Improved accelerated test protocol requires a better understanding of the relationship between testing parameters, electrochemical kinetics, and corrosion morphology. The purpose of the current study is to develop a framework for new accelerated corrosion test design by connecting attack morphology to electrochemical kinetics. Both the exfoliation susceptible under-aged (T36) temper and the exfoliation resistant near peak-aged (T86) temper of Al-Cu-Li alloy 2060 were considered in this work. ASTM G343, ANCIT2, and ASTM G85-A25 were used to study the impact of various testing parameters on the corrosion response of AA2060. Electrochemical measurements were used to determine corrosion potential (Ecorr) and polarization resistance (Rp) during standard and modified versions of each test in-situ. After each exposure, samples were also cross-sectioned and examined with optical microscopy. Results indicate that for immersion tests (ASTM G34 and ANCIT), testing temperature has a significant impact on anodic kinetics, while solution pH affects cathodic kinetics. The higher testing temperature of ANCIT led to faster anodic kinetics and faster formation of exfoliation in the T36 temper compared to ASTM G34. For salt spray testing (ASTM G85-A2), it was found that the lower bound of relative humidity (RH) during RH cycling had an impact on Rp as well as extent of attack. When the test was run with a lower bound of 60% RH, Rp remained relatively constant throughout the salt spray, dry air purge, and dwell portions of the cycle. However when the test was run with a lower bound of 20% RH, Rp increased (corresponds to a decrease in corrosion rate) during the dry air purge and dwell periods. This time at very low RH resulted in much slower development of exfoliation on the T36 temper compared to the test with a lower RH bound of 60%. Acknowledgements This work is supported by the Office of the Undersecretary of Defense Corrosion University Pilot Program under the direction of Dr. D. Dunmire, Rolls-Royce, and the Virginia Space Grant Consortium. References R.J. Rioja, J. Liu, Metal. Mater. Trans. A 43 (2012): p. 3325-3337S. Lee, B.W. Lifka, “Modification of the EXCO Test Method for Exfoliation Corrosion Susceptibility in 7XXX, 2XXX, and Aluminum-Lithium Alloys,” in New Methods for Corrosion Testing of Aluminum Alloys, eds. V.S. Agarwala, G.M. Ugiansky, STP 1134 (West Conshohocken, PA: ASTM International, 1992), p. 1-19.ASTM G34-1, “Standard Test Method for Exfoliation Corrosion Susceptibility in 2XXX and 7XXX Series Aluminum Alloys” (West Conshohocken, PA: ASTM International, 2001), p. 1-7.J.P. Moran, F.S. Bovard, J.D. Chrzan, P. Vandenburgh, “Corrosion Performance of New Generation Aluminum-Lithium Alloys for Aerospace Applications,” 13th Int. Conf. on Aluminum Alloys, held June 3-7, 2012 (Hoboken, NJ: John Wiley & Sons, Inc., 2012).ASTM G85-A2, “Standard Practice for Modified Salt Spray (Fog) Testing” (West Conshohocken, PA: ASTM International, 2002), p. 1-14.

Self-Healing Composite Membrane for Proton Exchange Membrane Fuel Cell Applications

We have developed a self-healing membrane based on microcapsules prefilled with Nafion solution. The microcapsules are designed to rupture when they encounter defects formed in the membrane such as cracks and pinholes, and then release the prefilled Nafion solution to heal the defects in-situ. A procedure was developed to prepare microcapsules with a urea-formaldehyde shell prefilled with a Nafion/tributyl phosphate self-healing solution. The prepared microcapsules were characterized by SEM with focused ion beam (FIB) and optical microscopy. Proton conductivity, mechanical properties, swelling and water uptake of the composite membranes based on microcapsules were measured and compared with that of pure recast Nafion membranes. Fuel cell performance with 6 and 10 wt% microcapsules/Nafion membranes was compared with that of a pure recast Nafion membrane. Durability testing comprising 220 hours of OCV-hold with relative humidity cycling confirmed that the self-healing functionality could greatly extend the life span of the fuel cell membrane.

Self-Healing Composite Membrane for Proton Exchange Membrane Fuel Cell Applications

We have developed a self-healing membrane based on microcapsules prefilled with Nafion solution. The microcapsules are designed to rupture when they encounter defects formed in the membrane such as cracks and pinholes, and then release the prefilled Nafion solution to heal the defects in-situ. A procedure was developed to prepare microcapsules with a urea-formaldehyde shell prefilled with a Nafion/tributyl phosphate self-healing solution. The prepared microcapsules were characterized by SEM with focused ion beam (FIB) and optical microscopy. Proton conductivity, mechanical properties, swelling and water uptake of the composite membranes based on microcapsules were measured and compared with that of pure recast Nafion membranes. Fuel cell performance with 6 and 10 wt% microcapsules/Nafion membranes was compared with that of a pure recast Nafion membrane. Durability testing comprising 220 hours of OCV-hold with relative humidity cycling confirmed that the self-healing functionality could greatly extend the life span of the fuel cell membrane.

Relative Humidity Cycling Effect on Ag and Cu Corrosion

In this work, corrosion behavior of Ag and Cu coupons was evaluated during relative humidity (RH) cycling in a mixed flowing gas (MFG) chamber. Three cycling conditions were performed: 52%-35%, 60%-38% and 70%-40%. One cycling period was set at 20 mins and total exposure time in MFG chamber was 2 days, which allowed for 144 cycles in each experiment. RH change rate as high as 180% per hour was achieved in the third condition. Corrosive gas concentrations were set close to field conditions in challenging environments: H2S-50ppb, NO2-200ppb, Cl2-20ppb, SO2-100ppb. Constant RH (maximum and minimum RH within cycling) exposure experiments were also conducted for comparison with RH fluctuation conditions. For all experiments, the corrosion rates of Ag and Cu coupons were found to be determined by the level of the RH, rather than the cycling of the RH. Cathodic reduction and SEM/EDX were also used to understand corrosion products and morphology of Ag and Cu under different RH conditions.

In-plane biaxial cyclic mechanical behavior of proton exchange membranes

The durability of a proton exchange membrane is affected by both mechanical degradation and chemical degradation. While fatigue and relative humidity cycling tests have been conducted to address mechanical degradation, the cyclic behavior that bridges the gap between the stress-strain response and fatigue behavior is not well established. The objective of this study is to understand the strain evolution during biaxial cyclic loading that resemble the actual stress state of the membrane. In particular, the effect of loading paths on strain evolution is examined to account for the stress state on strain accumulation. It is found that the constraint effect of stress in one direction on strain evolution in another direction strongly depends on the stress state of the membrane, and the equibiaxial stress state imposes the most significant constraint on strain evolution. Furthermore, the constraint effect induced by biaxial loading is more significant at higher relative humidity values. Moreover, high-stress amplitude cycle acts to retard strain accumulation in the subsequent low-stress amplitude cycle. The findings reported here will provide new evidence for an understanding of the fatigue behavior of a proton exchange membrane as well as durability modeling of proton exchange membrane fuel cells.

Relative Humidity Cycling Effect on Ag and Cu Corrosion

Introduction Corrosion-induced failures remain frequent in electronic equipment deployed in industrial and aggressive environments. ISA 71.04 standard [1] provides a classification system to evaluate the corrosive potential of an environment. Four levels of environmental severities (G1 Mild, G2 Moderate, G3 Harsh and GX Severe) are proposed based on Cu and Ag corrosion rates. Furthermore, ISA 71.04 standard posits that severity level can be increased by one level for a relative humidity (RH) fluctuation rate of change greater than 6% per hour. However, no experimental data is provided to validate this statement. Anecdotal evidence for the potential effect of RH cycling on the metal corrosion have been reported [2-3], where the corrosion rate in data centers with large humidity variations appears to be higher than data centers with small humidity variations. However, containment species and gas concentrations in these data centers were not monitored and controlled. Testing with well-controlled conditions is therefore needed to fully understand the role of RH fluctuation on metal corrosion rate. Experimental A mixed flowing gas (MFG) chamber was used to perform the RH cycling experiments. A three-way solenoid valve is added to the MFG system to produce RH fluctuations. Three cycling conditions were set: 52%-35%, 60%-38% and 70%-40%. The cycling time was set as 20 minutes and total exposure time in the chamber is 2 days for each experiment. The RH change rate as high as 180% per hour has been achieved in third condition. Corrosive gas concentrations are set close to field condition: H2S-50ppb, NO2-200ppb, Cl2-20ppb, SO2-100ppb. Constant RH (high limit and low limit RH) experiments were also conducted for comparison. The corrosion behavior of Ag and Cu are studied using different metrologies: weight gain, SEM/EDX, cathodic reduction (CR). Results For all three conditions, RH cycling did not increase the corrosion rate as measured by the weight gain of Ag and Cu coupons compared with the constant RH conditions. Based on cathodic reduction results, the main corrosion products for Cu and Ag are not changed under RH cycling. For Ag, the dominant product is Ag2S and for Cu, copper oxides and Cu2S are main corrosion products. SEM also shows no significant surface morphology change between the cycling condition and constant RH conditions. The corrosion on both Ag and Cu coupons is mostly determined by the level of the RH, rather than the cycling of the RH. The ISA 71.04 statement with respect to RH cycling therefore has been demonstrated to be invalid. [1] ISA-71.04-2013 “Environmental Conditions for Process Measurement and Control Systems: Airborne Contaminants” [2] Abbott, W. H., Natural and Laboratory Environmental Reactions on Materials and Components, 9th Progress Report, 1986. [3] Singh, P., Klein, L., Agonafer D., Shah, J.M., Pujara, K.D., „Effect of Relative Humidity, Temperature and Gaseous and Particulate Contaminations on Information Technology Equipment Reliability”, Proceedings of the ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems and ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels InterPACKICNMM2015, July 6-9, 2015, San Francisco, California, USA

The response of Opalinus Clay when exposed to cyclic relative humidity variations

Abstract. Clay shale specimens were exposed to cyclic relative humidity (RH) variations to investigate the response of the material to natural environmental changes. Opalinus Clay, a clay shale chosen as host rock for nuclear waste disposal in Switzerland, was utilized. The specimens were exposed to stepwise relative humidity cycles in which they were alternately allowed to equilibrate at 66 and 93 % relative humidity. Principal strains were monitored throughout the experiments using strain gauges. After each relative humidity cycle, Brazilian tensile strength tests were performed to identify possible changes in tensile strength due to environmental degradation.Results showed that Opalinus Clay follows a cyclic swelling–shrinkage behaviour with irreversible expansion limited to the direction normal to bedding, suggesting that internal damage is restricted along the bedding planes. The Brazilian tensile strength in direction parallel and normal to bedding as well as the water retention characteristic remained unaffected by the RH variations.

A plasticity model for predicting the rheological behavior of paperboard

The sorption of water into the paperboard exposes a container to reversible and irreversible deformations under relative humidity variations. In this study, an elasto-plastic material model is used to demonstrate how through-thickness dry solids content gradients can generate permanent in-plane strains in paperboard. The measurements presented in this paper indicate that in consecutive loading-unloading cycles, the yield stress either remains roughly constant or decreases, and an additional permanent set of strain is obtained even when the maximum tension of repetitions stays constant. Two modified approaches concerning elasto-plastic hardening behavior based on the measurements of this work and the observations of previous studies are introduced. The simulated results exhibit some shared features of the frequently observed shrinkage behavior of paperboard exposed to cyclic relative humidity changes. The results suggest that with the use of a suitable hardening approach, the plastic deformations arising from through-thickness dry solids content gradients may be considered as a time-independent component for simulations of phenomena such as moisture-accelerated creep and release of dried-in stresses.

Strength and Water Interactions of Cellulose I Filaments Wet-Spun from Cellulose Nanofibril Hydrogels

Hydrogels comprising cellulose nanofibrils (CNF) were used in the synthesis of continuous filaments via wet-spinning. Hydrogel viscosity and spinnability, as well as orientation and strength of the spun filaments, were found to be strongly affected by the osmotic pressure as determined by CNF surface charge and solid fraction in the spinning dope. The tensile strength, Young’s modulus and degree of orientation (wide-angle X-ray scattering, WAXS) of filaments produced without drawing were 297 MPa, 21 GPa and 83%, respectively, which are remarkable values. A thorough investigation of the interactions with water using dynamic vapour sorption (DVS) experiments revealed the role of sorption sites in the stability of the filaments in wet conditions. DVS analysis during cycles of relative humidity (RH) between 0 and 95% revealed major differences in water uptake by the filaments spun from hydrogels of different charge density (CNF and TEMPO-oxidised CNF). It is concluded that the mechanical performance of filaments in the presence of water deteriorates drastically by the same factors that facilitate fibril alignment and, consequently, enhance dry strength. For the most oriented filaments, the maximum water vapour sorption at 95% RH was 39% based on dry weight.

Mechanism of craquelure pattern formation on panel paintings

The drying shrinkage accumulation from exposure of freshly prepared gesso layers to relative humidity (RH) cycles was determined to elucidate the mechanism of craquelure pattern formation on panel paintings. The progresive drying shrinkage of the gesso is observed only under the cycles going to high RH levels which bring about transitions from brittle to ductile state of the material. The first incidence of fracture on the gesso layers occurred after a limited number of cycles ranging between a few and 100 for a range of layer thickness between 0.5 and 1 mm. The craquelure patterns stabilised also after a limited number of cycles (30 for the 1-mm thick layer). Upon increase in the gesso layer thickness, the strength of the layer is reduced and the spacing of shrinkage fractures increases. The study demonstrated that craquelure patterns, mimicking historical ones, can be realistically produced in laboratory conditions. Such studies would provide useful information for preparing specimens simulating historic panel paintings and would inform the current efforts on automatic, computer-aided classifications of crack formations on paintings.

Numerical evaluation of crack growth in polymer electrolyte fuel cell membranes based on plastically dissipated energy

Understanding the mechanisms of growth of defects in polymer electrolyte membrane (PEM) fuel cells is essential for improving cell longevity. Characterizing the crack growth in PEM fuel cell membrane under relative humidity (RH) cycling is an important step towards establishing strategies essential for developing more durable membrane electrode assemblies (MEA). In this study, a crack propagation criterion based on plastically dissipated energy is investigated numerically. The accumulation of plastically dissipated energy under cyclical RH loading ahead of the crack tip is calculated and compared to a critical value, presumed to be a material parameter. Once the accumulation reaches the critical value, the crack propagates via a node release algorithm. From the literature, it is well established experimentally that membranes reinforced with expanded polytetrafluoroethylene (ePTFE) reinforced perfluorosulfonic acid (PFSA) have better durability than unreinforced membranes, and through-thickness cracks are generally found under the flow channel regions but not land regions in unreinforced PFSA membranes. We show that the proposed plastically dissipated energy criterion captures these experimental observations and provides a framework for investigating failure mechanisms in ionomer membranes subjected to similar environmental loads.

Self-Healing Composite Membrane for Proton Electrolyte Membrane Fuel Cell Applications

We have developed a self-healing membrane based on microcapsules prefilled with Nafion solution. The microcapsules are designed to rupture when they encounter defects formed in the membrane such as cracks and pinholes, and then release the prefilled Nafion solution to heal the defects in-situ. A procedure was developed to prepare microcapsules with a urea-formaldehyde shell prefilled with a Nafion/tributyl phosphate self-healing solution. The prepared microcapsules were characterized by SEM with focused ion beam (FIB) and optical microscopy. Proton conductivity, mechanical properties, swelling and water uptake of the composite membranes based on microcapsules were measured and compared with that of pure recast Nafion membranes. Fuel cell performance with 6 and 10 wt% microcapsules/Nafion membranes was compared with that of a pure recast Nafion membrane. Durability testing comprising 220 hours of OCV-hold with relative humidity cycling confirmed that the self-healing functionality could greatly extend the life span of the fuel cell membrane.

Investigation on creep behavior of geo-materials with suction control technique

The compacted bentonite which has typical couple problem associated to thermal - hydration - mechanical – chemical (THMC) consist of one component of engineered barrier. Recently, the couple THMC formulation modelling suggested by some researchers can be predicted basically phenomena for engineered barrier that approach to correct evaluate satisfied facilities. The compacted bentonite is essentially unsaturated condition, some behaviors for bentonite has similar or close with generally expansive unsaturated soils. Therefore, hydrations have given significant influence on deformation of compacted bentonite such as swelling. There are many researches for swelling behavior of compacted bentonite within soaking. Extended theoretical or experimental investigations for unsaturated soil mechanics are possible to describe the strength-deformation behavior of compacted bentonite with suction controlling principle. A new method of determining the failure phase such as great axis deformation and destructions like strip of surface in the laboratory is described and the creep behavior of compacted bentonite is considered under maintain of high relative humidity environment. The creep deformation measured using improved cyclic relative humidity control apparatus in terms of specific suction control technique.

Impacts of Sulfate Seed Acidity and Water Content on Isoprene Secondary Organic Aerosol Formation.

The effects of particle-phase water and the acidity of pre-existing sulfate seed particles on the formation of isoprene secondary organic aerosol (SOA) was investigated. SOA was generated from the photo-oxidation of isoprene in a flow tube reactor at 70% relative humidity (RH) and room temperature in the presence of three different sulfate seeds (effloresced and deliquesced ammonium sulfate and ammonium bisulfate) under low NOx conditions. High OH exposure conditions lead to little isoprene epoxydiol (IEPOX) SOA being generated. The primary result is that particle-phase water had the largest effect on the amount of SOA formed, with 60% more SOA formation occurring with deliquesced ammonium sulfate seeds as compared to that on effloresced ones. The additional organic material was highly oxidized. Although the amount of SOA formed did not exhibit a dependence on the range of seed particle acidity examined, perhaps because of the low amount of IEPOX SOA, the levels of high-molecular-weight material increased with acidity. While the uptake of organics was partially reversible under drying, the results nevertheless indicate that particle-phase water enhanced the amount of organic aerosol material formed and that the RH cycling of sulfate particles may mediate the extent of isoprene SOA formation in the atmosphere.

Behavior of GFRP bridge deck panels infilled with polyurethane foam under various environmental exposure

This paper investigates the performance of polyurethane foam-infill bridge deck panels (PU sandwich panels) after being exposed to various environmental conditions. These panels were constructed with woven E-glass fiber/polyurethane facesheets that were separated by a trapezoidal-shaped, low-density polyurethane foam. Corrugated web layers were introduced into the core to enhance the panel's structural characteristics. The PU panels were manufactured through a one-step vacuum assisted resin transfer molding (VARTM) process. An experimental program was designed to simulate their in situ environments. The environmental conditions used included different conditioning regimens to examine the behavior of both GFRP laminates and PU sandwich panels. The GFRP laminates, which were made from the same materials as the PU sandwich panels, were exposed to ultraviolet radiation, a deicing solution at both room temperature and elevated temperature, and thermal cycling. The PU sandwich panels were exposed to thermal cycling (a series of freeze–thaw, mid–high temperatures, and mid–high relative humidity cycles). The thermal cycling exposure was conducted in a computer-controlled environmental chamber to duplicate seasonal effects in Midwestern states. Following the exposure regimens, tensile strength tests and four-point loading tests were performed on the GFRP laminates and the PU sandwich panels, respectively. The evaluation was based on visual inspection, strength, stiffness, and failure modes, as compared to those that were not conditioned (the control). The results of this study revealed that degradation did exist due to the effects of thermal cycling and the deicing solution. The ultraviolet radiation, however, did not cause any degradation. These results were within the recommended environmental durability design factors of the FHWA guidelines.

The effect of wetting cycles on moisture behaviour of thermally modified Scots pine (Pinus sylvestris L.) wood

The moisture behaviour of thermally modified Scots pine (Pinus sylvestris L.) exposed to cyclic conditions was analysed. Specimens of dimensions 15 × 15 × 5 mm3 were thermally modified at 180 °C (TM1) and 220 °C (TM2) using atmospheric pressure and superheated steam. Radial, tangential, volumetric swelling and anti-swelling efficiency (ASE) were calculated during six consecutive drying–soaking cycles. Afterwards, additional specimens were exposed to ten relative humidity cycles (0 and 95 %) at temperature 25 and 40 °C in order to analyse its influence on sorption behaviour. Application of thermal modification led to significant reduction of swelling from original 18.4–13.3 % for TM1 and to 10.5 % for TM2. However, after exposure to six consecutive soaking–drying cycles, the swelling of control specimens slightly decreased, whereas the swelling of thermally modified specimens increased. Due to the increased swelling after repeated cycles, the original ASE (28.6 and 42.7 %) decreased to 22.5 % for TM1 and to 36.88 % for TM2. The presence of leachable compounds and release of internal stresses are mainly attributed to that phenomenon. The EMC of the reference specimens decreases over the repeated humidity cycles for approximately 1 %–units. Same trend was found for the mild thermal modification TM1, but decreasing only in the range of 0.5 %–units. However, the EMC of the TM2 specimens during humidity cycles behaved differently. The results provide a better insight into details of thermal modification of wood and its behaviour under cyclic conditions.

Preventive deformation measurements on cultural heritage materials based on non-contact surface response of model samples

Due to climate change a slowly increasing annual temperature may be experienced by structures. Relative humidity (RH) fluctuations affect the equiibrium moisture content of materials. Repeated RH cycling leads to mechanical failure and may endanger an object's structural integrity. Preventive conservation is based on adopting measures that will prevent fracture. Real-time interferometry allows the acquisition of sequential high-resolution full-field surface images from hygroscopic materials used in cultural heritage by recording during cycles of changing RH. The differential images allow the development of a preventive methodology directly through surface responses. Indications of the natural onset of degradation can be followed and traced before visible damage occurs, allowing preventive measures to be taken in advance. An ongoing study (Climate for Culture European project (FP7-ENV-2008-1 CfC no. 226973)) aims to experimentally classify structural deterioration as a function of acclimatization and confirm the hypothesis that surface responses before deformation can indicate deformation threshold values as reference points for the onset of RH-induced deterioration.

A Mechanistic Study of Perfluorosulfonic Acid Membrane Water Permeance Degradation in Air

Chemical degradation of perfluorosulfonic acid (PFSA) membranes of proton exchange membrane (PEM) fuel cells is a well-known process which can limit PEM fuel cell lifetimes (1, 2). Chemical degradation can weaken membranes to the point where they no longer provide an effective barrier to reactant gas permeation or electrical shorting. PEM chemical degradation is the result of a complex interplay between mechanical and chemical degradation processes, both of which are strong functions of operating conditions. In particular, hot and dry operating conditions induce high rates of chemical degradation whereas relative humidity cycling between wet and dry states challenges the mechanical integrity of the membranes. Because chemical degradation of PEMs is accelerated under low humidity operating conditions, membrane lifetimes are significantly enhanced by providing humidified inlet gas streams to the fuel cell stack. In addition, fuel cell performance is improved with the use of humidified input streams. An efficient method of humidifying the cathode inlet stream employs a passive water vapor transfer (WVT) device that delivers water vapor from the wet cathode exhaust to the cathode inlet stream (Figure 1). The key component of a WVT device is a highly water permeable, but gas impermeable membrane. Not surprisingly, ionomeric membranes such as PFSAs function as excellent WVT materials. Ideally, WVT membrane materials will maintain adequate durability and performance characteristics to provide necessary stack humidification throughout the target operational life of an automotive fuel cell system (>10years />8000 operational hours). Several general pathways known to reduce water permeance values include cation contamination, mechanical degradation and chemical degradation. We have observed during in-situ and ex-situ testing of WVT materials in air that the water permeance values of PFSA-based WVT membranes can degrade by 75% over time. This degradation of water permeance is associated with a physical and chemical change of the membrane. The extent of ionomer transformation is readily monitored using transmission FTIR spectroscopy by the appearance of new absorption bands centered at 3200 and 1434 cm-1 and also by the decreases in water stretching and bending bands centered near 2800 and 1700 cm-1, respectively (Figure 2). In addition to water permeance, other membrane properties including proton conductivity and ion exchange capacity decrease as a function of this degradation reaction. The fully reversible, deuterium exchanged FTIR spectrum proves that the degradation product is not the cross-linked sulfonic acid anhydride as previously suggested (3). Through an extensive series of kinetic studies and analytical characterization, we have determined that the membrane degradation product results from a reaction between the ionomer and a trace atmospheric gas. We will demonstrate that the definitive structural assignment is fully consistent with all known properties and behaviors of the degraded membrane. The revelation of the PFSA degradation product and its formation mechanism will aid in development of highly durable fuel cell systems. References R. Borup, J. Meyers, B. Pivovar, Y. Kim, R. Mukundan, N. Garland, D. Meyers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K Yasuda, K. Kimijima, N. Iwashita, Chem. Rev. 107, 3904, (2007)C. S. Gittleman, F. D. Coms, Y-H. Lai, Membrane Durability: Physical and Chemical Degradation, in Polymer Electrolyte Fuel Cell Degradation, pp 15-88, M. M. Mench, E. C. Kumbur, T. N. Veziroglu, eds, Academic Press, 2012F. M. Collette, C. Lorentz, G. Gebel, F. Thominette, J. Mem. Sci. 330, 21-29, (2009) Figure 1

Design and construction of an inexpensive homemade plant growth chamber.

Plant growth chambers produce controlled environments, which are crucial in making reproducible observations in experimental plant biology research. Commercial plant growth chambers can provide precise controls of environmental parameters, such as temperature, humidity, and light cycle, and the capability via complex programming to regulate these environmental parameters. But they are expensive. The high cost of maintaining a controlled growth environment is often a limiting factor when determining experiment size and feasibility. To overcome the limitation of commercial growth chambers, we designed and constructed an inexpensive plant growth chamber with consumer products for a material cost of $2,300. For a comparable growth space, a commercial plant growth chamber could cost $40,000 or more. Our plant growth chamber had outside dimensions of 1.5 m (W) x 1.8 m (D) x 2 m (H), providing a total growth area of 4.5 m2 with 40-cm high clearance. The dimensions of the growth area and height can be flexibly changed. Fluorescent lights with large reflectors provided a relatively spatially uniform photosynthetically active radiation intensity of 140–250 μmoles/m2/sec. A portable air conditioner provided an ample cooling capacity, and a cooling water mister acted as a powerful humidifier. Temperature, relative humidity, and light cycle inside the chamber were controlled via a z-wave home automation system, which allowed the environmental parameters to be monitored and programmed through the internet. In our setting, the temperature was tightly controlled: 22.2°C±0.8°C. The one-hour average relative humidity was maintained at 75%±7% with short spikes up to ±15%. Using the interaction between Arabidopsis and one of its bacterial pathogens as a test experimental system, we demonstrate that experimental results produced in our chamber were highly comparable to those obtained in a commercial growth chamber. In summary, our design of an inexpensive plant growth chamber will tremendously increase research opportunities in experimental plant biology.