Prediction of sorption and diffusion of water vapour by nylon-6,6

Sufficient measurements were made of the equilibrium moisture contents and diffusion coefficients of nylon-6,6, as a function of relative humidity, to draw graphs and obtain regression equations. Further measurements were made to determine whether either the equilibrium moisture content or the diffusion coefficient was dependent on stress or thickness. The equilibrium moisture content was found to be a function of the relative humidity only, and can therefore be predicted from a regression equation. The diffusion coefficient was found to be mainly dependent on the moisture content but slightly dependent on the thickness and shape, and it can therefore be predicted from suitable regression equations and a standard shape-correction factor. The resulting regression equations were used to predict moisture changes during air humidity changes and were found to give good predictions both for one-and two-directional diffusion and for humidity cycling.     

Transport and sorption of water vapour in activated carbons

The adsorption of water vapour on microporous carbons derived from the carbonization of coconut shell has been studied. The adsorption and desorption characteristics of water vapour on the activated carbons were investigated over the pressure range p/p₀ 0–0.95 in a static water vapour system. In these experiments the process of water adsorption/desorption was investigated by both kinetic and equilibrium experimental data. Activated carbons differing by the degree of burn-off have shown the importance of the microstructure. A carbon enriched with nitrogen functions underlined the influence of the surface chemistry.     

A comparison of desorption process of Chinese and Australian lignites by dynamic vapour sorption

Modified BET model and Do and Do (D.D.) model were chosen to interpret the desorption process of water on hard and soft lignites. The organic and inorganic hydrophilic sites were determined by X-ray photoelectron spectroscopy and X-Ray fluorescence spectroscopy. From the modified BET model, only a small part of hydrophilic sites (HSs) acted as effective primary sites due to space restraint, intramolecular hydrogen bond and maybe more than one hydrophilic site connecting with one water molecule. From the D.D. model, the total adsorption sites (S_t) of Loy Yang lignite (LY) is 18.7 mmol/g, which is higher than that of Shengli (SL) 17.0 mmol/g in spite of lower primary sites, and the saturation concentration of water (q_us) in the micropore of LY is 33.7 mmol/g, which is much higher than that of SL (12.5 mmol/g). So the high moisture-holding capacity of LY is mainly determined by the high S_t and q_us, and not the primary sites. The size of water clusters entering the micropores is 7 for SL and 6 for LY, which is related to relative location of HS. When dewatered, the higher total sites density and smaller water cluster size of LY both implied higher dewatering energy.     

A sorption balance study of water vapour sorption on anhydrous cement minerals and cement constituents

The phenomenon of water vapour sorption by powdered cement constituents exposed to different relative humidities and temperatures was studied. The individual clinker phases C₃S, C₂S, C₃A, C₄AF, calcium sulfates and CaO were tested. Using a water sorption balance, the amount of chemically and physically sorbed water per unit of surface area of the powders and the relative humidity at which water sorption starts to occur on the phases were determined. Various cement clinker phases prehydrate very differently. CaO and C₃A were found to be most reactive towards water vapour whereas the silicates react less. CaO starts to sorb water at very low RHs and binds it chemically. Beginning at 55% RH, orthorhombic C₃A also sorbs significant amounts of water and binds it chemically and physically. Water sorption of C₃S and C₂S only begins at 74% RH, and the amount of water sorbed is minor. Calcium sulfates sorb water predominantly physically.     

Concurrent effect of microporosity and chemical structure on the equilibrium sorption properties of cellulose-based hydrogels

Polyelectrolite hydrogels are crosslinked polymers which display high sorption capacity in water and water solutions. They are widely used in the marked of personal hygiene products, as well as in other biomedical and industrial applications. In the most industrial application they are not biodegradable and prepared starting from acrylamide.In this work, the chemical-physical analysis of a novel class of natural polymers-based microporous superabsorbent hydrogels has been presented. The main focus is the definition of the relationship between material's chemical-physical structure and its equilibrium sorption properties.The effect of the variation of the degree of crosslinking, the polyelectrolyte nature of the backbone and the hydrogel microporosity on its swelling properties has been analysed, and a good agreement with the theoretical statements has been displayed.The effect of the ionic strength and the pH of the external solution in contact with the hydrogel on its equilibrium sorption properties has also been assessed, and an high sensitivity to these variations was detected for all the tested chemical compositions and physical structures.This work have to be considered part of the efforts towards the reduction of the environmental impact of the large scale consumption industrial polymer based products.     

凝胶注模成形氧化锆陶瓷性能研究

笔者以氧化锆陶瓷浆料,采用凝胶注模法制备氧化锆坯体,并对其坯体和烧结体进行性能分析。     

Combined zirconia toughened alumina (ZTA) stacks obtained by electron beam physical vapour deposition

Ion assisted electron beam physical vapour deposition (EB-PVD) technique was used for three zirconia toughened alumina (ZTA) batches containing 15, 25, 35, mole% ZrO₂ and c-zirconia ceramic multilayer deposition on 617-Ni based alloy substrate. The ceramic batches were prepared via sol-gel technique. The thickness of the coated layers was found to be 0.66 μm for ZTA layers and 0.65 μm for c-zirconia layer. Evaluation of the microstructure of the samples coated with c-zirconia on the top of different ZTA coats reveals three different regions. First region indicates c-ZrO₂ coat, second region exhibits ZTA layers, while third is the metallic substrate. The top coat microstructure can be divided into two zones. The inner zone (ZTA zone), which is the early part of multiple nucleation and subsequent growth of the columnar microstructure. The outer zone (c-ZrO₂) which is crystallographically perfect columnar YSZ coatings produced by EB-PVD.     

Prediction of water vapour sorption isotherms and microstructure of hardened Portland cement pastes

Water vapour sorption isotherms of cementitious materials reflect the multi-scale physical microstructure through its interaction with moisture. Our ability to understand and predict adsorption and desorption behaviour is essential in the application of modern performance-based approaches to durability analysis, along with many other areas of hygro-mechanical and hygro-chemo-mechanical behaviour. In this paper, a new physically based model for predicting water vapour sorption isotherms of arbitrary hardened Portland cement pastes is presented. Established thermodynamic principles, applied to a microstructure model that develops with hydration, provide a rational basis for predictions. Closed-form differentiable equations, along with a rational consideration of hysteresis and scanning phenomena, makes the model suitable for use in numerical moisture simulations. The microstructure model is reconciled with recently published ¹H NMR and mercury intrusion porosimetry results.     

Fabrication of bulk macroporous zirconia by combining sol–gel with calcination processes

Macroporous yttria-stablized tetragonal zirconia has been synthesized combining the sol–gel with Pechini method. Polyacetylacetonatozirconium (PAZ) served as the zirconium precursor while polyvinyl alcohol (PVA) was used as pore template. The thermal behaviors of xerogel and porous zirconia were determined by thermogravimetric/differential thermal analysis (TG/DTA). FT-IR, FE-SEM, and XRD analysis indicated that the organic components of xerogel have been completely removed and porous zirconia was formed after annealing at 600 °C for 2 h in air. The skeleton is composed of zirconia particles with diameter of about 60 nm and the pore diameter is about 68 nm. The surface area and pore volume of the bulk macroporous yttria-stablized tetragonal zirconia are 9.4 m²/g and 0.016 cm³/g, respectively, evaluated from nitrogen adsorption/desorption measurements.     

Preparation and stabilization of high specific area zirconia carriers

Zirconia samples have been prepared by precipitation from an aqueous solution of zirconyl nitrate followed by calcination in flowing air at temperatures up to 970 K. The textural properties (S_BET and pore size distribution) of these zirconia powders were very sensitive to the activation procedure. Calcination in carefully controlled conditions at 770 K yielded crystallized zirconia with surface area (S_BET ≈ 130 m² g⁻¹) and porosity suitable for use as a catalyst carrier.

Zirconia samples doped with yttrium, nickel or aluminium were obtained by impregnation of the amorphous hydrous oxide. For solids calcined at 770–970 K, the surface area and thermal stability were improved, but the porosity was lower.     

The role of sorption of water vapour in the spontaneous heating of coal

Laboratory experiments have been carried out to measure the rates of heat release from different coals by a calorimeter during sorption of water vapour in isothermal conditions. It has been observed that the rate of heat generation in a particular coal increases with the equilibrium humidity deficiency of the coal, i.e.with the difference of equilibrium humidity of air and coal. For a given coal, the rate of heat generation due to oxidation has been found to be negligible in comparison with that due to sorption of water vapour. A small ‘peak’ at the beginning of the rate curves has been observed during the tests with dry coals, with the exception of anthracite; explanations for this phenomenon have been attempted. The results also show that under a given test condition the characteristic rate of heat release is dependent on the type of coal, its particle size and its weathering.     

Thermal decomposition,pore structure and heats of immersion of zirconia gel

The surface properties of zirconia gel and its thermal dehydration products obtained in the temperature range 300–1000°C were investigated by N₂ and C₆H₁₂ adsorption. Structural and phase changes were studied by X-ray diffractometry and differential thermal analysis. Samples obtained up to 500°C were amorphous to X-rays whereas those obtained at higher temperatures proved to be a mixture of tetragonal and monoclinic modifications. The glow phenomenon is attributed to the conversion of the amorphous oxy-hydroxide of zirconium to the amorphous zirconium dioxide. Complete pore structure analysis showed the original parent material to be microporous. The pores widen with increase of temperature but at 500°C some narrowing occurs when the structure is changed to the oxide. Heats of immersion in cyclohexane were measured at 35°C. A correlation between the micropore fraction and the heats of immersion, normalised per unit area, could be traced for all samples.     

Changes in mechanical properties of coal due to sorption of carbon dioxide vapour

An apparatus for measuring the mechanical resistance of bituminous coal is described. The coal samples are placed in a pressure container, saturated with test gas, and then submitted to crushing, with simultaneous pressure. Coal resistance to crushing, when saturated with carbon dioxide under a 2.0 M Pa pressure, decreases markedly.     

Electrical conductivity of sol–gel derived yttria-stabilized zirconia

A 3 mol% Y₂O₃ zirconia stabilized powder has been synthesized by destabilization of an aqueous zirconia sol prepared by the alkoxide hydrolysis method. The powder calcined at 500°C is ultrafine with tetragonal crystallites of about 8 nm, slightly agglomerated and with a narrow pore size distribution having an average pore size of 5.2 nm. Zirconia ceramics with density higher than 92%TD and grain size on the order of 100 nm have been obtained by uniaxial pressing at 500 MPa and vacuum sintering at 1000°C. Electrical conductivity of sintered samples, evaluated by complex impedance spectroscopy measurements, indicated that the zirconia stabilized with 3 mol% Y₂O₃ can potentially be used as an oxygen semipermeable dense membrane, but only at a relatively high temperature.     

Diffusion of water vapour in cellulose acetate: 2. Permeation and integral sorption kinetics

A series of permeation and integral sorption kinetic experiments were performed under both absorption and desorption conditions on membranes of cellulose acetate prepared by the same methods and conditions as before¹. Alternative methods of data treatment are presented. The slow approach to steady state as well as analysis of time lag data suggest that the limiting value of the thermodynamic permeability coefficient is approached at a slow rate. In view of a comparable time-variability already observed in the solubility coefficient¹, these results indicate that the mobility of penetrant molecules does not decrease with time; hence they imply that the effect of partial immobilisation of the water molecules in clusters is offset by polymer plastisization caused by the penetrant. The diffusion coefficient D is shown to increase with concentration in all cases, thus confirming the conclusions based on differential sorption data. D measured in absorption is, at any concentration, lower than in desorption, but in desorption it shows satisfactory agreement between the two limbs of the equilibrium isotherm.     

Surface area and pore structure of stannic oxide gel from organic vapour adsorption

Surface area measurements were performed on stannic oxide gel and its thermal dehydration products obtained both in vacuo and in presence of air in the temperature range 100–600°C by benzene and carbon tetrachloride adsorption at 35°C. The presence or absence of air is found to impose changes in the surface area and consequently vary the pore structure of the solid material. Pore size analyses for both micro- and mesopores are performed. The low temperature samples are characterised by the presence of micropores only. At 350°C widening of pores commences and at higher temperatures the results indicate the existence of pores of “submicro” dimensions together with the mesopores. These changes in pore structure go in parallel with the gradual development of the crystalline cassiterite structure.     

Some effects of formaldehyde crosslinking on the kinetics of water vapour sorption in cellulose

New data are presented on the variation in the fraction sorbed with activity in the initial stage of the two stage sorption process for a crosslinked cellulose and compared to those for the original material and analysed in terms of the water—cellulose interaction. For both original and crosslinked material the fractions sorbed in the initial stage depend on the amount sorbed in the first sorption from dryness. The amounts sorbed in the initial stage starting at a given activity and varying the activity increment bear a similar relation to the activity increment as the amounts sorbed at equilibrium bear to activity.     

Thermal evolution and crystallisation of polydimethylsiloxane–zirconia nanocomposites prepared by the sol–gel method

Polydimethylsiloxane–zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in different molar ratios. Transparent, homogeneous and non-porous xerogels have been obtained up to 70 mol% ZrO₂ content. The starting xerogels have been pyrolyzed under argon atmosphere up to 1400°C and the structural evolution of samples treated at different temperatures has been followed by X-ray diffraction, transmission electron microscopy, infrared and ²⁹Si solid state nuclear magnetic resonance spectroscopies, thermal analyses and N₂ sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600°C of high surface area materials with pore sizes below 3 nm. Samples are amorphous up to 800°C. At 1000°C, the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallisation of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO₂ content. At 1400°C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO₂ phases are present with ZrO₂ average crystallite dimensions never exceeding 20 nm, for ZrO₂ content ≤50 mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. ©     

Diffusion of water vapour in cellulose acetate: 1. Differential transient sorption kinetics and equilibria

The concentration dependence of the diffusion coefficient D, of the cellulose acetate/water vapour system has been investigated: aspects of the non-Fickian behaviour of the system are reported. Kinetic and equilibrium data, were obtained for membranes of different thicknesses from a series of differential sorption experiments along both limbs of the hysteresis loop. The data are treated in alternative ways. It is concluded that we may distinguish two rate-determining molecular relaxation processes: a relatively fast one, prevailing during the initial stages of absorption, which is succeeded by a considerably slower and more drastic reorganization of the polymer structure. During the latter stage the solubility coefficient S is shown to possess a long-term time dependence, which is attributed to penetrant cluster formation. D obtained from desorption kinetics increases with concentration, whereas the apparently concentration-independent D on the absorption side is assumed to be due to partial immobilization of penetrant in regions where penetrant clusters are developed.