Hg Porosimetry Research Articles

Formation of zirconia foams using the thermostimulated sol–gel transition

In this paper we describe the production of zirconia-based foams by a novel thermostimulated sol–gel route, that employs the foaming of colloidal suspensions prior to the sol–gel transition promoted by small increase of temperature (≅3 °C). This method produces gelled bodies having porosity >70% in the wet stage, and can be used to produce complex-shaped components. The effect of a foaming agent (Freon11 or CCl 3F) and surfactant content on the formation and stability of the foams was analyzed. The rheologic measurements demonstrate that by increasing the surfactant concentration, the gelation time decreases increasing foam stability. As the surfactant concentration and quantity of foaming agent increase, the density decreases and the porosity increases. Hg porosimetry results show that the dry foam presents a bimodal pore size distribution. The family of sub-micrometer pores was attributed to the formation of a microemulsion between Freon11 and water. Scanning electron microscopy analysis shows that the foam structure consists of a three-dimensional network of spherical pores, which may be open and interconnected or closed, at larger or smaller porosities, respectively. Finally these results show that the thermostimulated sol–gel transition provides a potential route for ceramic foam manufacture.

Adsorption of zinc by activated carbons prepared from solvent extracted olive pulp

Activated carbons have been prepared by a two-step physical activation with steam at different burn-off levels to study the porosity development and its effect in zinc adsorption from aqueous solutions. The main material used was the residual from the extraction with solvent of the kernel-oil [solvent extracted olive pulp (SEOP)]. Olive, apricot and peach stone have been also used as different precursors. The products were characterized by N 2 at 77 K adsorption, Hg porosimetry and iodine number determination. The influence of surface complexes and pH has been investigated in an attempt to elucidate the adsorption phenomena. The effect of different treatments [demineralization with H 2SO 4 and oxidation with (NH 4) 2S 2O 8] was also evaluated for the adsorption of zinc species. Both basic and acidic carbons, originated from SEOP, show remarkable adsorption ability at solution pH=7. Their adsorption ability mainly depends on the content and nature of functional surface groups, the ash content of the precursors and the pH of the solution. These activated carbons were proved to be efficient adsorbents for the removal of water pollutants and contaminants.

Synthesis of porous hydroxyapatites by combination of gelcasting and foams burn out methods.

The biocompatibility and the osteoconductive behavior of hydroxyapatite (OHAp) ceramics are well established. Bioceramics made of OHAp are available in dense and porous form. Recently it has been proved that the volume of bone ingrowth at early times is primarily interconnectivity dependent. A new method for the obtention of porous OHAp ceramics that combine the in situ polymerization (gel casting method) and the foams burn out is proposed. Four polyurethane foams with different cells/cm were used. The foams were fully filled of an OHAp polymerizable suspension that after gelled produced very homogeneous and strong green bodies. After different thermal treatments the green bodies yield porous OHAp ceramics that were a replica of the foams used. Materials used in this work were studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), N(2) adsorption isotherm, particle size distribution, and Hg porosimetry. Porous pieces of OHAp obtained are constituted by polyhedral-like particles (0.45-1.0 microm) that are surrounded by an interconnected network of pores. A bimodal distribution of the pores size between 30.8-58.6 and 1.0-1.2 microm has been observed. The size of the interconnected pores (30.8-58.6 microm) was controlled as a function of the cells/cm of the foam while the volume of the small pores was modified as a function of the sintering time. The presence of pores could promote the bone ingrowth and also could be used to insert different drugs, which makes these porous pieces a potential candidate to be used as non-load-bearing bone implants and as drug delivery systems.

Investigations into provenance and properties of ancient building sandstones of the Zittau/Görlitz region (Upper Lusatia, Eastern Saxony, Germany)

Abstract Mineralogical and technical properties were investigated of building sandstones from ancient monuments of the Zittau/Görlitz region (Germany) as well as material from potential source quarries. The complex study included macroscopic rock description and detailed investigations by polarizing microscopy (phase composition, texture, grain size distribution), cathodoluminescence (quartz types, feldspar and kaolinite content), scanning electron microscopy (accessories, pore cement, diagenetic grain surface features) as well as the analysis of open macroporosity (total water uptake) and the pore size distribution function (Hg porosimetry). For the first time, mineralogical and technical data were obtained for building sandstones of the Zittau region. The results not only confirmed earlier conjectures concerning different source areas for the ancient building sandstones of the Zittau and Görlitz area but also allowed the unequivocal assignment of historically used material to specific sandstone occurrences. The data obtained provide a comprehensive basis for the interpretation of weathering damage suffered by the historical monuments and give useful hints for their successful conservation and reconstruction.

Study of thermal deactivation of a de-NOx commercial catalyst

In the present paper, the thermal deactivation of a commercial de-NOx V2O5–WO3/TiO2 catalyst is investigated. In order to simulate long-term operation in gas firing samples of the catalyst are calcined at different temperatures from 773 to 1173K. The structural, morphological and chemico-physical changes caused by calcination at high temperatures are investigated by XRD, BET and Hg porosimetry measurements, FT-IR, FT-Raman and EPR spectroscopies, and the activity in the reduction of NO with ammonia, the direct oxidation of ammonia and the oxidation of SO2 is measured over the different catalyst samples. It is shown that sintering of the TiO2 support leads to aggregation of isolated vanadium ions; this favours the selective catalytic reduction (SCR) reaction at low temperatures, the oxidation of ammonia at high temperatures and the undesired oxidation of SO2 as well. Thus, it is expected that catalysts operated for long-term in gas firing at high temperatures are no longer appropriate for commercial use in the SCR process, because in spite of the greater activity in the de-NOx reaction, the temperature window is too narrow and the activity in the oxidation of SO2 is too high.

Effect of Exogenous Carbon on Movement of Simazine and 2,4‐D in Soils

Organic amendment (OA) addition is an agricultural practice that can greatly affect pesticide behavior in soil. The influence of three OAs, consisting of a liquid amendment (LOA), solid (SOA) humic amendment, and a solid urban waste (SUW), on sorption, persistence, and leaching potential of simazine (2‐chloro‐4,6‐bis (ethylamino)‐s‐triazine) and 2,4‐D (2,4‐dichlorophenoxyacetic acid) in a sandy soil (P2) has been investigated. The OAs were added to P2 at a rate of 5% (w/w). Sorption was studied using the batch equilibration technique, degradation by incubations at 20°C and −0.33 kPa moisture content, and leaching using handpacked soil columns. Simazine sorption increased in the order of P2 ≤ P2 + LOA < P2 + SUW < P2 +S OA. The herbicide, 2,4‐D, did not sorb on P2 soil, but in amended soils sorption increased in the same order as simazine. The lower sorption of both herbicides in P2 + LOA as compared with P2 + SOA and P2 + SUW is attributed to higher dissolved organic matter (DOM) content of LOA. Simazine's half‐life was reduced upon amendment. Degradation of 2,4‐D followed a sigmoidal rate in P2, P2 + SOA, and P2 + SUW, but not in P2 + LOA, in which 2,4‐D persistence was much higher. The higher stability of 2,4‐D in P2 + LOA has been attributed to 2,4‐D‐dissoved organic C interactions. Leaching studies indicate that degradation affects movement to a higher extent than sorption, specially in the case of 2,4‐D in P2 + LOA soil, in which the amounts leached are much higher than those in unamended soil. In the case of simazine and P2 + SUW soil, greater reduction in large‐size conducting pores upon amendment, as revealed in Hg porosimetry studies, resulted in a greater reduction of leaching than that suggested from the small differences in sorption and degradation between P2 and P2 + SUW soil.

Improvement in the strength of reticulated porous ceramics by vacuum degassing

SiC reticulated porous ceramics (RPCs) were fabricated from ceramic slurries by the replication process with polyurethane sponges as the templates. Vacuum degassing was adopted to remove air bubbles in the slurry. Effect of degassing on the rheological behavior of the slurry and the properties of RPCs was investigated. The slurry showed a slight increase in viscosity after degassing, which improved the loading of the slurry on the sponge. The large defects in the struts were significantly reduced by degassing, which was confirmed by scanning electron microscopy (SEM) observation and Hg porosimetry. Flexural strength of RPCs increases from 2.34 to 3.18 MPa after degassing. The results were discussed in terms of an available theoretical model on the mechanical behavior of open cell ceramics.

Synthesis and characterisation of a vanadium-based ‘chemical valve’ membrane

In this work, the reversible red/ox properties and related textural variations of vanadium oxide (V2O5/V2O3) have been exploited to prepare an original composite inorganic porous membrane acting as a 'chemical valve'. A mixture of vanadium and phosphorous molecular precursors were infiltrated into a commercial αAl2O3 tubular support. After a thermal treatment in air at 650°C, the pores of the internal and intermediate layers the support were entirely filled with a ceramic material containing V2O5 and AlPO4 crystallites. A number of techniques (FESEM, N2 ads-desorption, Hg porosimetry, XRD, LRS, NMR and TGA) were used to study the morphology, the porous texture and the structural characteristics of the membrane. The red/ox properties of V2O5 crystallites, which transform reversibly to V2O3 under reducing atmosphere, were found to control the porous characteristics of the membrane (grains morphology, pore sizes distribution, and pore volume) and consequently its permeability. The original and reversible membrane red/ox properties were clearly evidenced by single gas permeance and gravimetric studies. The membrane permeance is higher when it is reduced and lower when it is oxidised: the ratio between the permeances of pure n-C4H10 and pure O2 is about 70 at 500°C. The red/ox process was reversible and the membrane permeance behaviour was not altered after 10 red/ox cycles, even under drastic conditions (heating–cooling red/ox cycles with pure gases up to 500°C). This type of infiltrate composite membrane is reproducible, and thermally and chemically stable.

Experimental Investigation of the Porous Nickel Anode in the Molten Carbonate Fuel Cell

The electrochemical performance of the porous nickel anode in a molten carbonate fuel cell was experimentally investigated in this study. The electrode structure was studied by scanning electron microscope and Hg porosimetry. The effect of electrolyte filling, on overpotential at a constant current density, was also examined. Kinetics for the hydrogen reaction was studied by obtaining stationary polarization curves, for porous nickel anodes at varying temperatures and anode gas compositions. The slopes of these polarization curves were analyzed at low overpotentials, with the assumption that the porous anode was under kinetic control, and the corresponding exchange current densities were determined using a simplified porous electrode model. The obtained partial pressure dependencies of the exchange current density were high, and therefore, difficult to explain by the generally assumed mechanisms. © 2001 The Electrochemical Society. All rights reserved.

Pore sizes and pore connectivity in rocks using the effect of internal field

A novel nuclear magnetic resonance method has been applied to several sandstone rocks to measure the pore size distribution using the magnetization decay due to diffusion in the internal magnetic field (DDIF). By comparing the results of the DDIF and Hg porosimetry experiments, a clear picture of pore connectivity emerges. The pore body diameter can be defined using the DDIF data and is found to have a clear trend as a function of porosity.

Sintering of zirconia composites obtained by slip casting

Composites containing a matrix of nanometric Ce-stabilized zirconia with an addition of micrometric monoclinic zirconia were processed by slip casting and sintered at a relatively low temperature. The ratio between nanometric and micrometric particles was determined according to the viscosity of the suspensions and the final density of the pellets. An optimum amount of micrometric particles was necessary to achieve improved suspension dispersion and higher pellet density. The amount of deflocculant in the suspensions containing the mixture of micrometric and nanometric particles was optimized by viscosity measurements. The pellets were characterized by dilatometry, Hg porosimetry, density measurement (the Archimedes method) and scanning electron microscopy. Despite the low green density obtained (35–38% of the theoretical density), densities as high as 97.5% were achieved after sintering.

Sintering of tin oxide processed by slip casting

Sintering of SnO 2 compacts, obtained through slip casting, was studied by means of dilatometry, Hg porosimetry, scanning electron microscopy, and density measurement (Archimedes method). Sintering is strongly influenced by the green microstructure. Moreover, the sintering mechanisms are not dependent on the slurries’ solid content up to 50% of solids in volume. Above this value, agglomerates are formed, leading to differential sintering inside and among the agglomerates. Another important point is the reduction of the temperature of maximum shrinkage rate when compared to tin oxide processed by isostatic pressing. This reduction is more accentuated when ammonium hydroxide is added to the suspension.

Surface and Chemical Study of SiO2·P2O5·CaO·(MgO) Bioactive Glasses

Glasses in SiO2·CaO·P2O5 and SiO2·CaO·P2O5·MgO systems have been prepared by a sol−gel synthesis procedure. The calcined glasses have been characterized by XRD, N2 adsorption, Hg porosimetry, XPS and TEM and have been also subjected to in vitro tests (immersion in a simulated body fluid) to evaluate their bioactivity. The presence of magnesium in the glasses increases the surface area and porosity, but it retards the formation of an apatite layer on the surface of glasses in the in vitro test. The XPS reveals that the surfaces of the glasses are richer in phosphorus and poorer in calcium than the bulk, whereas the magnesium, if present, associates preferentially to phosphorus at the glass surface. The TEM shows the presence of apatite-like calcium phosphate domains in the magnesium-free glasses, which are barely detected in the glasses, which contain this element. These apatitic domains are proposed to be the nucleation centers for the crystallization of apatite in the in vitro tests.

Matrix Diffusion of Simple Cations, Anions, and Neutral Species in Fractured Crystalline Rocks

The diffusion of radionuclides into the pore spaces of a rock matrix and the pore properties in fractured crystalline rocks were studied. The work concentrated on the predominant water-conducting fracture system in the host granodiorite of the Kamaishi In Situ Test Site, which consists of fracture fillings and altered granodiorite. Through-diffusion experiments to obtain effective and apparent diffusion coefficients (De and Da, respectively) for Na+, Cs+, HTO, Cl-, and as a function of ionic charge were conducted through the fracture fillings and altered and intact granodiorite. The total porosity ϕ, density, pore-size distribution, and specific surface area of the pores of the rocks were also determined by a water saturation method and Hg porosimetry. The average ϕ is, in the order from highest to lowest, as follows: fracture fillings (5.6%) greater than altered granodiorite (3.2%) greater than intact granodiorite (2.3%), and gradually it decreases into the matrix. The pore sizes of the intact and altered granodiorite range from 10 nm to 200 μm, and the fracture fillings from 50 nm to 200 μm, but almost all pores are found around 0.1 and 200 μm in the fracture fillings. The De values for all species are in the following order: fracture fillings greater than altered granodiorite greater than intact granodiorite, as with the rock porosity. In addition, no effect of ionic charge on De is found. No significant dependence for Da values on the rock porosity is found. The formation factors FF and geometric factors G of the rocks were evaluated by normalizing the free water diffusion coefficient Do for each species. The FF decreased with decreasing rock porosity, and an empirical equation for the rock porosity was derived to be FF = ϕ1.57±0.02. The G values showed a tendency to slightly decrease with decreasing rock porosity, but they were approximately constant (0.12 to 0.19) in this porosity range. This indicates that accessible pores decrease into the rock matrix from the fractures, and diffusion is also reduced into the rock matrix. Furthermore, it was concluded that a simplified model based on FF and Do values is approximately reasonable to predict De.

Porosity measurements by a gas penetration method and other techniques applied to membrane characterization

In this paper several methods that can be used to determine membrane porosities are studied. Apparent densities, water pycnometry, Hg porosimetry and the gas penetration technique can be considered direct methods since no specific pore geometry needs to be assumed, whereas computerized analysis of scanning electron microscopy images and air–liquid displacement techniques are indirect methods since pores are usually assumed to have a specific shape. Both direct and indirect methods for polymeric and very regular inorganic membranes, Cyclopore and Anopore filters respectively, are compared. Other porous films with more complex structures such as asymmetric or composite membranes are also analysed, namely polymeric ultrafiltration membranes made by Gelman and Desalination, and inorganic filters made by Tami industries. As a consequence of a detailed comparison of the porosities obtained, we conclude that the gas penetration volumetry is preferable when we aim to obtain the actual void volume fraction. This method is highly reproducible and gives precise results which allow us to detect very small pores without any interaction with the structure due to the inertness of He and the low pressures applied.

A comparative study of NiZn ferrites modified by the addition of cobalt

Off-stoichiometric NiZn ferrite was obtained by hydrothermal process and compacted in torus form under different pressures. Two samples A1 and A2 - cobalt doped (0.5 %) were sintered at 1573 K in air atmosphere during 3 h. The magnetic properties were studied by vibrating sample magnetometry, Mössbauer spectroscopy and complex impedanciometry. X-ray diffraction and Hg porosimetry were used in order to determine the average grain size and the type of packing in the samples. Both samples exhibited superparamagnetic behavior in the hysteresis loop. This effect does not agree with Mössbauer results, which were fitted using Normos, a commercial computer program. All samples parameters were compared.

Influence of particle size and particle size distribution on drying-shrinkage behaviour of alumina slip cast bodies

This paper focuses on the possibility of characterising slip cast bodies microstructures through their behaviour during drying, by using the Bigot’s curves. The Bigot’s curves are normally used as routine control in traditional clay-based ceramic production (tiles, sanitary wares, earthenware, etc.) for testing the sensitivity of clays and pastes to drying. In order to test the validity and usefulness of the Bigot’s curves to characterise wet porous structures as that obtained by slip casting, “tailored” alumina slip cast bodies, prepared from suspensions having different mean particle sizes (PSs) and particle size distributions (PSDs), were used. The results showed that a decrease of the mean PS leads to an increase of the shear-thinning behaviour in the low shear rate range due to the predominance of the surface forces, relative to the hydrodynamic forces. In the slip cast bodies, a diminution of the mean PS leads to an overall decrease of green density and to an increase of both Critical Moisture Content (CMC) and total shrinkage in the first stage of the drying process. Regarding the effect of the PSD shape, alumina CT530 characterised by a bimodal PSD, gave slips with lower viscosity and denser slip cast bodies, compared with the other aluminas characterised by a continuous PSD. Accordingly, the Bigot’s curves showed a decreasing trend of CMC and total shrinkage. Overall, this work demonstrates that the use of Bigot’s curves can be extended to the advanced ceramic field, as an important tool for characterisation of slip cast bodies, including the wet state, giving complementary information in respect of Hg porosimetry or density.

Durability of ceramic filters in the presence of some diesel soot oxidation additives

The chemical interactions between sodium (as sulphite), iron and cerium (as oxides), coming from fuel additives, and two ceramic filters, in the working temperature range of a Diesel soot filter, were considered. The filters were a cordierite monolith filter and a wound-fiber filter, made by continuous Al 2O 3–B 2O 3–SiO 2 fibers, respectively. Using X-ray diffraction (XRD) and Hg porosimetry analyses, scanning electron microscopy (SEM) observations and microprobe investigations, the extent of chemical etching, the involved reaction mechanisms and porosity changes were investigated. From the experimental results it was concluded that cerium and iron oxides do not react significantly with these filters, whereas sodium leads to a diffuse etching starting from low working temperatures, by many different mechanisms (solid state diffusion, liquid phase diffusion, evaporation–condensation) as a function of the temperature. However the porosity characteristics of the filters, in terms of total porosity and pore size distribution, were not significantly affected by the described chemical interactions.

Interaction between clay microstructure, decomposition of plant residues and humification

SummaryWe have studied microstructural changes in montmorillonite and kaolinite, which were amended with organic debris (leaves of chestnut or beech), microbial inoculum and nutrients and subjected, for 30 days at 25°C, to alternate drying and wetting (D–W) or kept continuously moist at water‐holding capacity (WHC). The objective was to evalute the interactions between the decomposition of plant residues and clay microstructural organization in different pore–water regimes. The microstructure was studied by Hg porosimetry, N adsorption at 77 K, and scanning electron microscopy. Decomposition was assessed by measuring residual C and N, the amount of humified material, the relative contents of humic (HA) and fulvic (FA) acids, and their molecular weight distribution. The structural organization at the end of the experiments was different for the two clays, and new classes of pores were found as the result of the microbial metabolism. The decomposition was also significantly affected by clay type. Decomposition of the organic matter (OM) was rapid in the montmorillonite. The humified material occured mainly as low molecular weight FAs. By contrast, transformation was slow in kaolinite, leaving much residual C, and more of the humified material consisted of HAs. Alternate D–W had little effect on microstructure and decomposition. Samples that were kept moist and amended with chestnut leaves produced the greatest amount of humified substances. The dynamics of the decomposition was significantly affected by the nature of the leaves as indicated by the differences in residual C, humified material and C/N ratio. By contrast, changes in pore size distribution (PSD) were litle influenced by the type of organic debris.

Influence of the stabilising mechanism and solid loading on slip casting of alumina

Two commercial surface active agents that act through different stabilisation mechanisms, were used to disperse aqueous alumina suspensions. Electrophoretic measurements were performed to determine the zeta potential curves of the bare and surface charge modified alumina particles and, hence, to determine the stabilising performance of the dispersants. Drying–shrinkage behaviour (Bigot’s curves) and Hg porosimetry were used to characterise the slip cast bodies prepared from the suspensions at different solid loading. The rheological behaviour of the starting alumina suspensions was also determined in order to relate the flow properties with the superficial nature of the suspended particles and the wet body microstructure. Rheological measurements have showed that the slips containing the dispersant that confers the highest zeta potential values to the alumina particles were more viscous than when using the less capable dispersant in terms of surface charge modifier. These findings were explained by a larger interaction size of the particles dispersed through a pure electrostatic mechanism which resulted in a higher effective volume solid fraction of these suspensions. For both dispersants used, the Bigot’s curves indicated that the shrinkage and the slopes of the straight lines corresponding to the first stage of the drying decrease with increasing solids concentration. The variations in the slope derive from a broadening pore size distribution as a result of the increasing difficulties in stabilising the concentrated alumina suspensions, in agreement with the rheological measurements. Good correlations were found between the green microstructure, the rheology of the suspensions and the particle interaction forces.