Water Vapor Environment Research Articles

Novel aspects of the degradation process of PLA based bulky samples under conditions of high partial pressure of water vapour

This work describes differences between the degradation of poly(l-lactic acid) bulky samples in high partial pressure of a water vapour environment versus a buffered liquid medium. Analytical techniques (optical and electron microscopy, size exclusion chromatography, differential scanning calorimetry) were used to evaluate the material's morphology, molecular weight, crystallinity and thermal properties. In particular, the extent of degradation was studied separately in the core and surface of the testing specimens. The results reveal significant differences in the bulk degradation profiles and morphological changes for both environments. Degradation in the liquid medium leads to multiphase core–cortex morphology formation, while the environment with high partial pressure of water vapour provides homogeneous-like bulk degradation of the samples due to significantly different diffusion conditions on the liquid-polymer and water vapour saturated air-polymer interface.

Polymer-derived yttrium silicate coatings on 2D C/SiC composites

Abstract Yttrium silicate environmental barrier coatings (EBCs) on C/SiC composites were fabricated by using polysiloxanes-derived ceramic process. In order to reduce the free silica in the resultant ceramic coatings, Y 2 O 3 was added as an active filler. The materials with different weight ratios of polysiloxanes to Y 2 O 3 were synthesized. Their coefficient of thermal expansion (CTE) and water-vapor resistance were tested. The results indicated that the composition of 50% Y 2 O 3 –50% polysiloxanes (Y50) was suitable to be the EBCs for C/SiC composites. The C/SiC composites coated with Y50 were tested in the water-vapor environments at 1400 °C for 200 h. The results revealed that such a coating could effectively protect the C/SiC composites.

Studies on Initial Stage of High Temperature Oxidation of Fe - 9 to 12%Cr Alloys in Water Vapour Environment

Fe - 9 to 12%Cr alloys are a material for the thick sections boiler components and steam lines of a power plant. The role Fe - 9 to 12%Cr alloys is becoming more prominent in the development of a new generation of Ultra-Supercritical (USC) Power Plant due to the target operating temperature is reaching 620 °C (893 K), in 100% steam condition as well as pressure in excess of 300 bar (30 × 106 Pa). In such condition, the integrity of Fe - 9 to 12%Cr alloys relies on the oxide scale formed during the time of exposure. However due to the high temperature and water vapor condition, it is a well known fact that, the formation of oxide scale is accelerated thus depleting the structural integrity of the Fe - 9 to 12%Cr alloys over the time. Studies show that not only the formation of protective oxide scale was suppressed but the formation of non-protective oxide scale was accelerated instead. Decades of studies done by various groups around the globe has yet to have consensual on the exact mechanism of this phenomenon. Initial stage oxidation of these alloys plays great roles in hope to understand the formation of oxide scale in water vapor condition at high temperature. This paper reviews previous research works to understand the initial stage oxidation of Fe - 9 to 12%Cr alloys at high temperature in water vapor condition.

The Research on Film-Forming Performance of a New Kind of Compound Inhibitor

The film-forming effect of a new kind of compound inhibitor at high temperature was researched, with taking pure octadecyamine and pure imidazoline as comparison tests. Coupon test has been taken with autoclave simulating the on-site water-vapor environment; the film-forming effect of the coupon has been evaluated by hydrophobic test, acid copper sulfate drip test ,AC impedance test and scanning electron microscope(SEM) test. The GC-MS test was taken to analysis decomposition products of the compound inhibitor. The results show that the compound inhibitor can film well, and decomposition product doesn’t contain low molecular weight organic acids and other harmful ingredients. The compound inhibitor can applied into practice safely and efficiently.

Monte Carlo simulation of the electron beam scattering under water vapor environment at low energy

This paper reports on Monte Carlo simulations of electrons propagation in water vapor using a set of electron collision cross sections created with data published recently in order to follow the beam profile at low energy in environmental scanning electron microscopy (ESEM). Results show that lowering electrons beam energy or pressure has little effect on the skirt extend which is mainly function of working distance. The ionization and excitation events per track history are independent of the initial electron energy in the 1–5 keV range. Ionization occurs with low energy electrons, frequently above 50 eV, whereas the excitation processes are dominated by the electrons below 50 eV.

Recent advances in measurement of the water vapour continuum in the far-infrared spectral region

We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24 μm and 120 μm (85-420 cm(-1)) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer-Tobin-Clough-Kneizys-Davies (MT-CKD) 2.5 model parametrization above 300 cm(-1), but indicates the need to strengthen the parametrization below 300 cm(-1), by up to 50 per cent at 100 cm(-1). Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength.

Electron scattering cross section measurements in a variable pressure scanning electron microscope

Scattering of the incident electron beam in the variable pressure scanning electron microscope (VPSEM) affects the ability to perform quantitative chemical measurements. However, the manner in which the sum of the elastic and inelastic scattering cross sections varies as a function of gas type and accelerating voltage in the VPSEM is not well understood. A dual Faraday cup was constructed to measure the scattered fraction of the primary beam as a function of gas pressure, working distance, and accelerating voltage in air, water vapor, and argon environments. Experimentally measured values of the scattering cross section agree with previous experimental work, and agree within a factor of two with those values calculated carefully from theory.

SiC-based MIS gas sensor for high water vapor environments

Abstract In this work we will prove that SiC-based MIS capacitors can work in environments with extremely high concentrations of water vapor and still be sensitive to hydrogen, CO and hydrocarbons, making these devices suitable for monitoring the exhaust gases of hydrogen or hydrocarbons based fuel cells. Under the harshest conditions (45% of water vapor by volume ratio to nitrogen), Pt/TaOx/SiO2/SiC MIS capacitors are able to detect the presence of 1 ppm of hydrogen, 2 ppm of CO, 100 ppm of ethane or 20 ppm of ethene, concentrations that are far below the legal permissible exposure limits.

Inter-sheet-effect-inspired graphene sensors: design, fabrication and characterization **A part of this work has been published in the Proceedings of the 16th International Solid-State Sensors, Actuators and Microsystems Conference (Transducers ’11) [

With their sub-nanometer inter-sheet spacing, few-layer graphenes (FLGs) are alignment-free building blocks for nanosensors based on the inter-sheet effects. In this paper, we have tackled the challenges towards batch fabrication of inter-sheet graphene sensors through controlled layer engineering, edge tailoring and selective electrode fabrication on different atomic layers. An oxygen plasma etching (OPE) technique is developed to remove graphene layer by layer, enabling the batch fabrication of FLGs in a controllable fashion because of the faster speed and readiness of patterning of this process as compared to the conventional mechanical exfoliation. Vapor sensing experiments have shown that ‘inter-sheet’ sensors possess a higher sensitivity than conventional ‘intra-sheet’ ones. Vapor sensitivity is improved more than two times in normalized resistance changes by taking the ‘inter-sheet’ design upon exposure to 0.5% ethanol–nitrogen mixture and 500 Pa water vapor environments, respectively. These remarkable improvements can mainly be attributed to the inter-sheet effects such as electron tunneling, chemical doping, physical insertion and enhanced edge effects. Such effects may result from molecule adsorption/desorption, force/displacement, pressure, surface tension or thermal energy, and can potentially remarkably enrich the applicable transduction mechanisms.

Rehydration of dehydrated-dehydroxylated smectite in a low water vapor environment

Thermal analysis experiments in the environment of an extremely low water vapor concentration provide insight into the first steps of the rehydration mechanism in smectite when completely dehydrated and the interlayer region is collapsed. The relative structural and compositional controls on dehydration and rehydration reactions are compared from a well-characterized suite of samples that vary with respect to chemical composition, octahedral and tetrahedral substitution, octahedral cation site vacancy, and degree of dehydroxylation. Techniques including multi-cycle heating-cooling thermogravimetric analysis and nitrogen gas adsorption on various smectite samples preheated at different temperatures followed by rehydration at ambient conditions were used to characterize the interaction of water molecules with completely dehydrated montmorillonite, beidellite, and nontronite smectite types. Beidellite with high-Al 3+ tetrahedral substitution results in electrostatically undersaturated basal oxygen atoms that exert strong repulsion between the tetrahedral sheets of adjacent 2:1 layers. The interlayer region of dehydrated or dehydroxylated beidellite is capable of being spontaneously rehydrated even in low water vapor environments. In completely dehydrated montmorillonite and nontronite, the external surface area of the crystallites is a primary control on water adsorption at low humidity when the molecules form a shell around the exchangeable cations present on external surfaces. The potential of montmorillonite and nontronite to reopen a collapsed interlayer is significantly lower than beidellite because of their crystal-chemical features that result in 2:1 layer and interlayer cation attraction. With increasing water vapor partial pressure, the hydration potential of interlayer cations provokes a reopening of the interlayer. In a dehydroxylated nontronite, the undersaturated residual oxygen atom strongly bonds the interlayer cation within the ditrigonal ring of the tetrahedral sheet, resulting in a permanent interlayer collapse. The specific surface area calculated from a conventional N 2 gas adsorption measurement using the BET model represents the external surface area of a dehydrated smectite crystallite and can be converted into the mean crystallite thickness. The mean crystallite thickness of a completely dehydrated smectite increases with an increase in preheating temperature.

Kinetics and the role of off-stoichiometry in the environmentally driven phase transformation of commercially available zirconia femoral heads

The low-temperature polymorphic transformation behavior of two types of commercially available femoral head, both made of 3mol.% Y2O3-stabilized tetragonal ZrO2 polycrystals (3Y-TZP), was examined by in vitro experiments. Both materials contained a small amount (0.25wt.%) of Al2O3, but they differed slightly in their SiO2 impurity content, in the morphology and crystallinity of the dispersed Al2O3 phase, and in grain size. In vitro experiments were conducted in a water-vapor environment at temperatures in the range 90–134°C and for periods of time up to 500h. Despite the materials having the same nominal composition, quite different behaviors were found in the hydrothermal environment for the two types of femoral head investigated. A phenomenological description of the kinetics of monoclinic nuclei formation/growth led to the experimental determination of activation energy values for the environmentally driven polymorphic transformation. From the material physics viewpoint, cathodoluminescence spectroscopy enabled us to rationalize the role of surface stoichiometry on the mechanisms leading to polymorphic transformation. Spectroscopic experiments unveiled some new relevant aspects of surface off-stoichiometry, which lie behind the different phase transformation kinetics experienced by the investigated femoral heads.

Increasing the Upper Temperature Oxidation Limit of Alumina Forming Austenitic Stainless Steels in Air with Water Vapor

A family of alumina-forming austenitic (AFA) stainless steels is under development for use in aggressive oxidizing conditions from ~600–900 °C. These alloys exhibit promising mechanical properties but oxidation resistance in air with water vapor environments is currently limited to ~800 °C due to a transition from external protective alumina scale formation to internal oxidation of aluminum with increasing temperature. The oxidation behavior of a series of AFA alloys was systematically studied as a function of Cr, Si, Al, C, and B additions in an effort to provide a basis to increase the upper-temperature oxidation limit. Oxidation exposures were conducted in air with 10% water vapor environments from 800–1000 °C, with post oxidation characterization of the 900 °C exposed samples by electron probe microanalysis (EPMA), scanning and transmission electron microscopy, and photo-stimulated luminescence spectroscopy (PSLS). Increased levels of Al, C, and B additions were found to increase the upper-temperature oxidation limit in air with water vapor to between 950 and 1000 °C. These findings are discussed in terms of alloy microstructure and possible gettering of hydrogen from water vapor at second phase carbide and boride precipitates.

SiC-Based MIS Gas Sensor for High Water Vapor Environments

In this work we will prove that SiC-based MIS capacitors can work as efficient gas sensors in environments with extremely high concentrations of water vapour, still being sensitive to hydrogen, CO and hydrocarbons. This makes these devices suitable for long term and efficient operation in harsh environments, such as the exhaust gas atmosphere of hydrogen or hydrocarbon-based fuel cells.

Elastic Modulus Evolution and Behavior of Si/Mullite/BSAS-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapor Environment

Si-based ceramics (e.g., SiC and Si3N4) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the surface recession that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H2O vapor), appropriate refractory oxides are engineered to serve as environmental barrier coatings (EBCs). The current state-of-the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al2O3·2SiO2) interlayer and (1 − x)BaO·xSrO·Al2O3·2SiO2, 0 ≤ x ≤ 1 (BSAS) top coat. In this article, the role of high-temperature exposure (1300 °C) performed in H2O vapor environment (for time intervals up to 500 h) on the elastic moduli of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates was investigated via depth-sensing indentation. Laser-ultrasonics was employed to evaluate the E values of as-sprayed BSAS coatings as an attempt to validate the indentation results. Fully crystalline, crack-free, and near-crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The absence of phase transformation and stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers after harsh environmental exposure provides a plausible explanation for the almost crack-free coatings observed. The relationships between the measured elastic moduli of the EBCs and their microstructural behavior during the high-temperature exposure are discussed.

Phase Composition and Microstructural Responses of Graded Mullite/YSZ Coatings Under Water Vapor Environments

Mullite-based systems have been considered as environmental barrier coatings (EBCs) for high temperature protection of Si-based ceramic (Si3N4, SiC) substrates against water vapor corrosion, for application in forthcoming turbine engines. Graded mullite/Y-ZrO2 composites plasma sprayed over Hexoloy SiC substrates were analyzed as EBCs. All feedstock materials were purposely prepared and singular spraying conditions were used to assure superior crystallization. The different coated specimens were subjected to temperatures of 1300 °C for 100-500 h under water vapor environment. The effect of water corrosion on the exposed coatings was investigated by focusing on their phase and microstructure changes.

Mechanical Behavior of Air Plasma-Sprayed YSZ Functionally Graded Mullite Coatings Investigated via Instrumented Indentation

Yttria-stabilized zirconia (YSZ)-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat are envisioned as solutions to ease their coefficient of thermal expansion mismatch induced stress. In this work, two different types of mullite powder (spray-dried and freeze-granulated) and a mullite-YSZ 75/25 vol.% mixture spray-dried powder were employed. Using instrumented indentation with loads between 10 and 500 mN, the role of the powder characteristics on the mechanical behavior of air plasma-sprayed mullite bond coats deposited on SiC substrates was investigated. Hardness (H) and elastic modulus (E) were measured for the as-sprayed coatings and for coatings heat-treated at 1300 °C, in water vapor environment, for periods up to 500 h. Both H and E values of the coatings are found to be highly dependent on the size distribution of the starting powders. It is aimed the fabrication of an efficient and cost-effective EBC prototype based on YSZ compositionally graded mullite.

Antibacterial coatings of fluoridated hydroxyapatite for percutaneous implants

Percutaneous orthopedic and dental implants require not only good adhesion with bone but also the ability to attach and form seals with connective tissues and the skin. To solve the skin-seal problem of such implants, an electrochemical deposition method was used to modify the surfaces of metallic implants to improve their antibacterial ability and skin seals around them. A dense and uniform fluoridated calcium phosphate coating with a thickness of about 200 nm was deposited on an acid-etched pure titanium substrate by controlling the current density and reaction duration of the electrochemical process. The as-deposited amorphous fluoridated calcium phosphate transformed to fluoridated hydroxyapatite (FHA) after heat treatment at 600°C in a water vapor environment for 3 h. Both single crystal diffraction patterns and high-resolution transmission electron microscope (HRTEM) images confirmed the phase of the fluoridated calcium phosphate after the heat treatment. The antibacterial activities of FHA coatings were tested against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Porphyromonas gingivalis (P. gingivalis) with the film attachment method. The antibacterial activity of FHA coating is much higher than that of pure hydroxyapatite (HA) coating and acid-etched pure titanium surface. The promising features of FHA coating make it suitable for orthopedic and dental applications.

On the role of oxygen vacancies and lattice strain in the tetragonal to monoclinic transformation in alumina/zirconia composites and improved environmental stability

The aim of this paper is to clarify at the nanometer scale the relevant factors influencing the hydrothermal resistance to polymorphic transformation of alumina/zirconia composites, primary candidates for artificial joint applications. The topographic distribution of oxygen vacancies and lattice strain on the composite surface were visualized by means of cathodoluminescence spectroscopy and mapped as a function of exposure time in a thermally activated water vapor environment (i.e., simulating the exposure in human body). Systematically monitoring the optical activity of oxygen vacancies in both alumina and zirconia phases also revealed the effect of surface lattice strain accumulation on the kinetics of polymorphic transformation. From the presented data, an explicit role is evinced for surface oxygen vacancy formation in the alumina matrix, an important step in the complex cascade of mechanochemical events determining the superior environmental resistance of the composite.

Nano‐scale topography of bearing surface in advanced alumina/zirconia hip joint before and after severe exposure in water vapor environment

The aim of this study was to perform a surface morphology assessment with nanometer scale resolution on femoral heads made of an advanced zirconia toughened alumina (ZTA) composite. Femoral heads were characterized to a degree of statistical accuracy in the as-received state and after exposures up to 100 h in severe vapor-moist environment. Surface screening was made using an atomic force microscope (AFM). Scanning was systematically repeated on portions of surface as large as several tens of micrometers, randomly selected on the head surface, to achieve sufficient statistical reliability without lowering the nanometer-scale spatial resolution of the roughness measurement. No significant difference was found in the recorded values of surface roughness after environmental exposure (at 134 degrees C, under 2 bar), which was always comparable to that of the as-received head. Surface roughness safely lay <10 nm after environmental exposures up to 100 h, which corresponded to an exposure time in vivo of several human lifetimes (i.e., according to an experimentally derived thermal activation energy). In addition, the roughness results were significantly (about one order of magnitude) lower as compared to those recorded on femoral heads made of monolithic zirconia tested under the same conditions.

Behavior of a barrier layer of corrosion films on zirconium alloys

Capacitances of oxide films obtained on a EZ-1 alloy during the corrosion tests in water-vapor environment at 300, 350, and 400°C are measured. In order to estimate the thickness of a barrier layer, the barrier potential at a constant anodic current (dielectric puncture potential) is measured. It is shown that the barrier layer cannot be treated as a homogeneous environment. At the sites of intermetallide inclusions in the oxide film, the thickness of the dielectrics is locally decreased. In the first approximation, the heterogeneity of the oxide film can be taken into account by inserting two parallel RC subcircuits in the equivalent scheme. One subcircuit (C1, R1) describes the electrophysical properties of the capacitance whose insulator thickness corresponds to the total thickness of the oxide film. The other subcircuit (C2, R2) describes the electrophysical properties of the nonporous part of the oxide film between the intermetallide particles and the outer surface. Then, the results of measurements can be written as follows: Cexp = θC2 + (1 − θ)C1, where θ is the surface part of the oxide film whose dielectric properties are changed due to the inclusion of intermetallide particles. Assuming that the mean spatial size of intermetallide particles falls in the range of 200–400 nm, one can estimate the mean concentration of the particles on the metal surface in agreement with the metallographically determined concentration of the second-phase particles (approximately 106–107 cm−2). The obtained results indicate the substantial heterogeneity of the barrier layer structure, which may cause local corrosion and premature failure of zirconium items.