Impregnation Cycles Research Articles

Ordered mesoporous silicoboron carbonitride ceramics from boron-modified polysilazanes: Polymer synthesis, processing and properties

Ordered two-dimensional (2D) mesoporous silicoboron carbonitride (SiBCN) ceramics were prepared by a nanocasting approach of a boron-modified polysilazane of the type [B(C 2H 4SiCH 3NH) 3] n (C 2H 4 = CHCH 3, CH 2CH 2) ([Si 3B 1.1C 10.5N 3.0H 25.5] n ) using mesoporous CMK-3 carbon as hard template. The polymer was synthesized according to a monomer route by hydroboration of CH 2 = CHSiCH 3Cl 2 followed by reaction of the as-made tris(dichloromethylsilylethyl)borane (B(C 2H 4SiCH 3Cl 2) 3 (TDSB, C 2H 4 = CHCH 3, CH 2CH 2) with lithium amide (LiNH 2). It was generated as a highly soluble compound which could easily impregnate mesoporous CMK-3 carbon. The derived [B(C 2H 4SiCH 3NCH 3) 3] n -carbon composite was directly pyrolyzed in flowing nitrogen at 1000 °C to generate a SiBCN-carbon composite. The carbon template was subsequently removed through thermal treatment at 1000 °C in a mixture of ammonia and nitrogen to generate ordered mesoporous Si 3.0B 1.0C 4.2N 2.4 structures. XRD and TEM analyses revealed that the obtained amorphous mesoporous ceramic exhibits open, continuous, and ordered 2D hexagonal frameworks which are strongly dependent on the number of impregnation cycles and the carbon removal step. Using a double impregnation cycle combined with a pyrolysis process up to 1000 °C in flowing nitrogen and a carbon removal step at 1000 °C for 3 h in a volumetric flow ratio between ammonia and nitrogen of 1, the ordered mesoporous SiBCN ceramic displays high surface area (630 m 2 g −1), high pore volume (0.91 cm 3 g −1), and narrow pore-size distribution (around 4.6 nm) with a thermal stability which extends up to 1180 °C under nitrogen.

General trends of deposit accumulation in porous hosts during cyclic impregnation with rare-earth and actinide waste solutions

A theoretical model is constructed to describe the dynamics of the increase in the weight of porous matrices upon introduction of rare-earth and actinide radioactive waste from solutions via repeated cycles of impregnation, drying, and high-temperature annealing. The modeling results for waste immobilization in porous Al2O3 and ZrO2 agree well with experimental data.

Fabrication of carbon fiber reinforced ceramic matrix composites with improved oxidation resistance using boron as active filler

Boron was introduced into C f/SiC composites as active filler to shorten the processing time of PIP process and improve the oxidation resistance of composites. When heat-treated at 1800 °C in N 2 for 1 h, the density of composites with boron (C f/SiC-BN) increased from 1.71 to 1.78 g/cm 3, while that of composites without boron (C f/SiC) decreased from 1.92 to 1.77 g/cm 3. So when boron was used, two cycles of polymer impregnation and pyrolysis (PIP) could be reduced. Meanwhile, the oxidation resistance of composites was greatly improved with the incorporation of boron-bearing species. Most carbon fiber reinforcements in C f/SiC composite were burnt off when they were oxidized at 800 °C for 10 h. By contrast, only a small amount of carbon fibers in C f/SiC-BN composite were burnt off. Weight losses for C f/SiC composite and C f/SiC-BN composite were about 36 and 16 wt%, respectively.

Detailed study of the pore-filling processes during nanocasting of mesoporous films using SnO 2/SiO 2 as a model system

Mesoporous SnO 2 films synthesized using mesoporous silica thin films as molds in a nanocasting process have been used as a model system in order to study the mechanism of the exotemplating process. A systematic study of the infiltration capability of the porous mold is presented, which also is suggested to shed some light on the general process of nanocasting from solution. The silica films were repeatedly dip-coated in an ethanolic SnCl 4 solution and the films were calcined between each impregnation cycle. The structural evolution during the process was followed by Environmental Ellipsometry Porosity (EEP), and the SnO 2 films were additionally studied by SEM–FEG, TEM, GI-SAXS and XRD. The SnO 2 films have a high porosity (over 60%) after leaching of SiO 2, and are of optical quality. Furthermore, the SnO 2 is present in the form of nanocrystallites about 5 nm in diameter. Despite the fact that the films were very homogeneous and showed a high mechanical stability, the results show that not all the mesopores in the SiO 2 films can be filled by SnO 2 before formation of a layer of nonporous SnO 2 on top of the SiO 2 films occur. A tentative model for the pore filling events is presented based on the experimental findings.

Microstructural effects on the electrical properties of grain boundary Fe-doped LSGM

Mixed conductors based on grain boundary Fe-doped La 0.95Sr 0.05Ga 0.90Mg 0.10O 3 − δ (LSGM) ceramics were obtained by selectively doping the grain boundaries with Fe. This was achieved using LaFeO 3 − δ layers screen-printed onto LSGM, after annealing at high temperature in air for several hours to promote Fe diffusion into LSGM. The influence of the number of impregnation cycles, temperature of impregnation and microstructure of the host LSGM was evaluated by impedance spectroscopy and oxygen permeability. The impedance spectra consist of high and low frequency semicircles, ascribed to bulk and grain boundaries. The amplitude of both contributions decreases with increasing impregnation temperature and time, suggesting the onset of electronic conduction along grain boundaries. The effect is stronger for ceramics with larger grain size. The observed trends are fully consistent with estimates of p-type electronic conductivity obtained from oxygen permeability measurements using a simplified electrochemical cell.

Synthesis of mesoporous metal oxides by structure replication: Strategies of impregnating porous matrices with metal salts

Various measures to optimise the impregnation of mesoporous CMK-3 carbon and SBA-15 silica matrices with metal nitrates for the synthesis of mesoporous metal oxides by structure replication are investigated. The effect of surface modification of the matrix pores, the choice of a solvent with suitable polarity, and the concentration of the metal nitrate solution are studied in detail. The efficiency of pore loading is monitored by nitrogen physisorption measurements. The creation of polar functions at the pore surface of CMK-3 carbon is shown to increase the impregnation efficiency substantially while maximizing the pore wall polarity of SBA-15 silica by increasing the amount of free silanol groups does not have any significant impact. The choice of a less polar solvent (THF instead of water) has a positive effect on the wettability of CMK-3 carbon in the first impregnation cycle but turns out to be disadvantageous in the second cycle; a similar trend is observed for variation of the metal salt concentration.

Improvement in the quality of self-firing anodes of aluminum electrolyzers and other carbon articles

An effective method is demonstrated for improving the quality of carbon materials and graphite, whose articles are used extensively in different branches of the national economy. This method with use of the “nudel process” includes preparation of a strong and compact filler. The low temperature for filler preparation (550°C) provides a synchronous nature for its shrinkage with the binder during subsequent heat treatment, which reduces to a minimum structural defects for the final material, and as a consequence, it considerably increases the physicomechanical and operating properties similar to the properties of normal high quality materials, and the level of quality for the latter is achieved by repeated production cycles of impregnation of coal tar medium-temperature pitch, i.e. firing. The operational properties of articles made from the material exceeds the similar properties of articles made from traditional materials. The marked production advantages of the suggested version for carbon material using the nudel process are a significant reduction in the time of graphite preparation, a reduction of discharge into the atmosphere of cancerous substances (pitch vapors) and carbon dust; reduced gas flow rate, electric power, spilled materials, and other material expenditures.

Effect of SDC-impregnated LSM cathodes on the performance of anode-supported YSZ films for SOFCs

Sm 0.2Ce 0.8O 1.9 (SDC)-impregnated La 0.7Sr 0.3MnO 3 (LSM) composite cathodes were fabricated on anode-supported yttria-stabilized zirconia (YSZ) thin films. Electrochemical performances of the solid oxide fuel cells (SOFCs) were investigated in the present study. Four single cells, i.e., Cell-1, Cell-2, Cell-3 and Cell-4 were obtained after the fabrication of four different cathodes, i.e., pure LSM and SDC/LSM composites in the weight ratios of 25/75, 36/64 and 42/58, respectively. Impedance spectra under open-circuit conditions showed that the cathode performance was gradually improved with the increasing SDC loading. Similarly, the maximum power densities (MPD) of the four cells were increased with the SDC amount below 700 °C. Whereas, the cell performance of Cell-4 was lower than that of Cell-3 at 800 °C, arising from the increased concentration polarization at high current densities. This was caused by the lowered porosity with the impregnation cycle. This disadvantage could be suppressed by lowering the operating temperature or by increasing the oxygen concentration at the cathode side. The ratio of electrode polarization loss in the total voltage drop versus current density showed that the cell performance was primarily determined by the electrode polarization. The contribution of the ohmic resistance was increased when the operating temperature was lowered. When a 100 ml min −1 oxygen flow was introduced to the cathode side, Cell-3 produced MPDs of 1905, 1587 and 1179 mW cm −2 at 800, 750 and 700 °C, respectively. The high cell outputs demonstrated the merits of the novel and effective SDC-impregnated LSM cathodes.

Nanosized Mesoporous Silica Coatings on Ceramic Foams: New Hierarchical Rigid Monoliths

Silica-based rigid monoliths exhibiting a trimodal hierarchical pore system have been successfully prepared through coating of a ceramic foam (CF) with sub-micro-/nanometric mesoporous particles (as building blocks). We have selected a bimodal porous silica, denoted as UVM-7 (a nanometric version of the well-known MCM-41 materials), consisting of small aggregates of nanometric surfactant-assisted mesoporous particles. A colloidal suspension of this material in water is used to coat through successive impregnation cycles the CF surface. The small intraparticle mesopore system (with pore diameters around 2−3 nm) is due to the supramolecular templating effect of the surfactant. Textural large-mesopores/macropores (in the 20−70 nm range) have their origin in the interparticle UVM-7 voids. The large macrocellular macropores are due to the CF support. The resulting monoliths present a good and homogeneous coverage level. Moreover, these composites display better mechanic properties than those of related silica self-supported monoliths.

New processing methods to produce silicon carbide and beryllium oxide inert matrix and enhanced thermal conductivity oxide fuels

For inert matrix fuels, SiC and BeO represent two possible matrix phase compounds that exhibit very high thermal conductivity, high melting points, low neutron absorption, and reasonably high radiation stability. BeO is chemically compatible with UO2, PuO2 and Zircaloy to very high temperatures, but SiC reacts with all three at somewhat lower temperatures. We have developed the Polymer Impregnation and Pyrolysis or PIP method, making use of a commercial SiC polymeric precursor, to consolidate both particulate fuels like ‘TRISO’ microsphere fuels, and to impregnate UO2 fuels with pure stoichiometric SiC to improve their thermal conductivity. This method was employed to fabricate Enhanced Conductivity Oxide fuels, or ECO fuels with 5–10vol.% of the high conductivity phase, and with 50vol.% for TRISO dispersion fuels. For ECO fuels, a new ‘slug/bisque’ method of fabricating the UO2 fuel granules was necessary to produce sintered fuel with open pore structures, allowing almost complete impregnation of the continuous SiC phase. The advantages of the PIP process are that it is a non-damaging consolidation process for particulates (TRU, UC or TRISO microspheres), forms a continuous, pure β-SiC phase at temperatures as low as 1573K, and allows the maximum in fissile atom density. However, several PIP impregnation cycles and high crystallization temperatures are necessary to obtain high thermal conductivity SiC. For producing IMF fuels using the PIP process, the fissile PuC and/or TRU actinides can be added in small concentrations along with SiC ‘filler particles’ and consolidated with the SiC precursor for either open or closed fuel cycles. For BeO, a second approach was developed for ECO fuels that involves a ‘co-sintering’ route to produce high density fuels with a continuous BeO phase of 5–10vol.%. Special granulation and mixing techniques were developed, but only one normal sintering cycle is required. For BeO matrix IMF fuels, PuO2 granules and TRU actinides or YSZ granules containing actinides could be simply dry-mixed with BeO powder and co-sintered to produce a dispersed fuel form. Alternatively, the BeO could be granulated, and the PuO2 and/or TRU oxides would fill the interstices forming a continuous minor phase that could be recycled in closed fuel cycles. Some advantages and disadvantages of these matrices are discussed.

Development of Magnetic Nanostructured Silica-Based Materials as Potential Vectors for Drug-Delivery Applications

Metallic iron nanoparticles were synthesized within micron-sized mesoporous molecular sieves (with 2.9-nm pores) and hollow silica microcapsules (pores of 2.7 and 15 nm) using several cycles of wet impregnation under vacuum, followed by drying, oxidation, and reduction steps. For iron-loaded MCM-48, SQUID measurements revealed ferromagnetic behavior at room temperature with a magnetic moment as high as 3.40 emu/g (measured at 2 T) after four deposition cycles. Iron-loaded hollow silica microcapsules (250-nm wall thickness) showed a magnetic moment of 2.40 emu/g (at 2 T) after three deposition cycles and a coercivity as low as 12.9 Oe.

Influence of additives on the permeability of impregnating coal-tar pitch

Coal-tar pitch is widely used to prepare composite materials by the several cycles of impregnation and carbonization. Impregnation becomes the control step for cost when the high-density carbon preform is densified. Thus, effects of additives on the permeability of the pitch are investigated by experiments and their mechanisms are discussed according to theoretical analysis of fluid kinetics. Direactive glyceride as well as aluminum sulfate and polyethylene oxides evidently increase permeability by changing the physical structure of pitch. However, divinylbenzene has chemical modification to the pitch in the presence of p-toluene sulfonic acid, which leads to a marked decrease in permeability. In this regard, the former two types of additives are more suitable than the latter one to modify the coal-tar pitch as a precursor to densify the carbon preform with high density. In addition, the permeability drastically depends on the concentrations of additives.

Optimization of anodic oxidation and Cu–Cr oxide catalyst preparation on structured aluminum plates processed by electro discharge machining

This paper describes the optimization of three processes applied in fabrication of a microstructured reactor for complete oxidation of volatile organic compounds. The first process involves the optimization of the electro discharge machining (EDM) method to produce a set of microchannels with a high length to diameter ratio of 100, with a standard deviation from the average diameter below 0.2%, and with a surface roughness not higher than 2.0 μm. To satisfy these criteria, fabrication of microchannels must be carried out with two machining passes in the Al51st alloy. Then, the effect of several parameters on the anodization current efficiency with respect to oxide formation was studied. The best process conditions to get a 30 μm porous alumina layer in a 0.4 M oxalic acid electrolyte, were found to be a temperature of 1 °C, an anodic current density of 5 mA/cm 2, and 23 h oxidation time. At last, the resulting coatings were impregnated with an aqueous solution of copper dichromate followed by drying and calcination at 450 °C to produce active catalysts. The effect of a copper dichromate concentration, number of impregnation cycles (1 or 2), and different after-treatments on catalytic activity and stability in complete oxidation of n-butane were studied. The catalytic activity of the obtained coatings is superior to that of alumina supported pelletized catalysts even at much lower loadings of active metals.

Study of the degradation causes affecting stucco sculptures from the Valentino Castle in Turin

17th century stuccos located in The Valentino Castle in Turin (Northern Italy) have been studied by means of XRD, TG-DTA, Hg porosimetry and SEM coupled with EDX chemical analysis in order to understand their composition and the degradation causes. The investigated un-degraded samples were made of gypsum, hydrated lime and other carbonates phases on the external surface and up to ca. 20 mm in depth but the compositions were highly heterogeneous. Repeated cycles of water impregnation, coming from roofing infiltration, and evaporation have reached the investigated area and led to severe damages to stuccos due to crystallization of hydrated magnesium sulphate salts.

Treatment of paper with basic agents in alcohols and supercritical carbon dioxide to neutralize acid and prolong storage time

The efficiency of treatment of paper prepared from 100% sulfite cellulose with basic agents (solutions of magnesium alkoxides or methoxycarbonate in alcohols or in supercritical carbon dioxide) to neutralize acid was studied, as influenced by the following factors: concentration of a basic agent in the neutralizing solutions, repetition of impregnation of paper with a neutralizing solution and hydrolysis of the neutralizing agent after each impregnation cycle, temperature and pressure of supercritical carbon dioxide, mixing of a neutralizing agent with supercritical CO2, and conditions (dynamic or static) the paper treatment with a neutralizing solution.

The electric conductivity of nanodimensional cobalt oxide in a porous glass

Cobalt(II) oxide was introduced in an amount of 3.5–56.4 mass % into a porous glass by repeated cycles of impregnation with an aqueous solution of cobalt(II) nitrate followed by dehydration and thermal decomposition of the salt. The results of conductivity measurements, in combination with the data on the cobalt oxide volume and the specific area of the porous carrier, reflect the formation of sequential cobalt oxide monolayers on the surface of pore walls.

Tribological Behavior of Friction Materials Affected by Boehmite Impregnation

Focus of this study has been placed on the effect of boehmite impregnation methods at densification on tribological and mechanical behavior of friction materials. Experimental results indicate that open porosity decreaseswith the numbers of impregnation cycle. Both bulk density and flexural strength increase with the numbers of impregnation cycle. The friction coefficient and the mass loss of the impregnated specimen is shown more stable and lower than that of the green specimen. Morphological observations show that the number of open pores for these materials decreases but they exhibit a denser and smoother morphology as the number of impregnation cycle increases. Furthermore, as the number of densification cycle increase, the friction behavior of impregnated specimens becomes more sensitive to the presentation of γ-Al 2 O 3 particles. Therefore, the friction coefficient of specimens becomes higher but more unstable; in the mean time, the mass loss becomes larger.

Uranium oxide nanoparticles dispersed inside the mesopores of MCM-48: synthesis and characterization

The nanocrystallites of uranium oxide were dispersed in the mesopores of siliceous MCM-48 by subjecting it to the repeated cycles of incipient wet impregnation followed each time by drying and calcination steps. A loading of up to ∼20 wt% of U could be achieved in this manner. The physico-chemical characteristics of the samples were examined using XRD, XPS, IR, DR UV–VIS spectroscopy, Nitrogen sorption and HRTEM techniques. In spite of progressive disordering in the host structure and accompanying decrease in the unit cell parameter after each cycle of impregnation/calcination, the mesoporosity of the host was always preserved to certain extent. Also, the BET area, pore size and pore volume measurements indicated the gradual filling of the pores of MCM-48 as a result of uranium loading. The spectroscopy results revealed that the uranium species were initially bonded to Si–OH sites of the host matrix in the form of UO 2 2+ groups which in turn converted to U 3O 8 upon calcination. TEM examination confirmed that the majority (∼85%) of U 3O 8 particles were of less than 3 nm size and these were located exclusively within the pore system of host matrix. On the other hand, larger size crystallites were dispersed at the external surfaces. Furthermore, the size of the U 3O 8 particles and the physical characteristics of the U/MCM samples depended considerably on synthesis conditions, the post-impregnation ambient temperature drying under vacuum facilitating the formation of smaller size and more uniformly distributed particles.

Iron (III) oxide nanoparticles within the pore system of mesoporous carbon CMK-1: intra-pore synthesis and characterization

Iron (III) oxide nanoparticles were synthesized inside the pore system of mesoporous carbon CMK-1. This intra-pore synthesis was carried out using several cycles of wet impregnation, drying and calcination procedures. The existence of iron (III) oxide nanoparticles within the pore system was proved by powder X-ray diffraction, nitrogen physisorption, preservation of the host structure by Raman spectroscopy and transmission electron microscopy. Finally, the local structure of the iron oxide was determined by X-ray absorption spectroscopy.

Nanoparticles of uranium oxide occluded in MCM-41 silica host: Influence of synthesis condition on the size and the chemisorption behavior

The paper presents a comparative study on the characteristics of uranium oxide crystallites deposited in the mesopores of MCM-41 by two different methods, one involving the repeated cycles of wet impregnation of template-free MCM-41 with uranyl acetate and the other by way of exchanging the template cations in an as-synthesized host matrix. XPS, DRUV visible and XRD studies revealed that both the preparation procedures resulted in initial binding of uranyl groups with the Si–OH sites of host matrix, a part of which converted to U3O8 particles upon calcination. The size of the U3O8 crystallites, however, depended upon the synthesis conditions. Thus, more uniform and smaller size (<3 nm) particles of uranium oxide were formed in the case of the impregnation-prepared samples, as compared to the crystallites of 3–15 nm size formed using a template-exchanged procedure. Furthermore, the post-impregnation drying of a sample under the conditions of vacuum and room temperature was vital for the uniformity and for the small size of these crystallites. XRD, TEM and N2 sorption results showed that the U-loading resulted in a decrease in the unit cell parameter and also in a decrease in the long range ordering of the host matrix while the mesoporosity was preserved in both kinds of samples. At the same time, whereas the crystallites of U3O8 were largely dispersed within the pore system of MCM-41 in the case of impregnation-prepared samples , a large fraction existed at the external surface of the samples synthesized through template exchange route. This is attributed to the restriction imposed by template cations to the transport of uranyl species into the pore system. The monitoring by in situ IR spectroscopy revealed that the methanol molecules undergo dehydrogenation as a result of room temperature adsorption over uranium oxide crystallites, giving rise to oxymethylene (–OCH2) species as primary products which transform subsequently over larger size crystallites to form polyoxymethylene, i.e. (–OCH2)n, species. Smaller size crystallites of U3O8, on the other hand, promoted the further oxidation of oxymethylene groups resulting thereby in the formation of formate-type complexes.