Intercrystalline Voids Research Articles

Study on LBE corrosion failure of FeAl/Al2O3 coatings after ion irradiation

FeAl/Al2O3 coatings, candidate coating materials for lead-cooled fast reactors, are first irradiated to produce pre-defects and then subjected to lead-bismuth eutectic corrosion to simulate the performance in the coupled field of irradiation and corrosion. Irradiation causes voids in the outer Al2O3 layer resulting in irradiation swelling and causes lattice distortion and dislocations in the inner FeAl layer resulting in compressive strain and intercrystalline void expansion. The weakened coatings crack during corrosion and suffer from Pb, Bi, and O penetration, causing internal corrosion and oxidation. Beyond the cracked areas, the coatings demonstrate commendable corrosion resistance with minimal penetration. The irradiation-induced structural evolution and the subsequent lead-bismuth eutectic corrosion failure of the coatings are elucidated in detail.

Capture of Volatile Iodine Gas and Identification of Adsorption Sites in the Pore Network of Unsubstituted Imidazole Linker-Incorporated Zeolitic Imidazolate Framework-8.

The incorporation of unsubstituted imidazole (Im) in zeolitic imidazolate framework-8 (ZIF-8) crystallized in sodalite topology is proposed to improve gas capture and gas separation performance drastically. However, the incorporation of unsubstituted Im in ZIF-8 has remained challenging due to the thermodynamic instability of zinc and Im bonding in sodalite topology. We have incorporated up to 24.4 (mol %) Im linker in highly crystalline ZIF-8 with similar morphology and sodalite topology using a delayed linker addition strategy at room temperature. Im incorporation brings significant tuning to the pore architecture of ZIF-8, as confirmed by positron annihilation lifetime spectroscopy. The modifications in the pore architecture are primarily due to linker defects produced in the frameworks during crystallization and the elimination of steric hindrance due to the absence of a methyl group on Im. The Im-incorporated ZIF-8 shows significant enhancement in iodine capture as well as higher crystal structure stability under iodine vapor exposure as compared to pristine ZIF-8. Through ortho-positronium interaction with the adsorbed molecular iodine in the pore network of the frameworks, it is confirmed that iodine is preferentially adsorbed at cavity and intercrystalline voids, whereas aperture sites remain unoccupied by iodine molecules.

Defect-Free Metal-Organic Framework Membrane for Precise Ion/Solvent Separation toward Highly Stable Magnesium Metal Anode.

Metallic magnesium batteries are promising candidates beyond lithium-ion batteries; however, a passive interfacial layer because of the electro-reduction of solvents on Mg surfaces usually leads to ultrahigh overpotential for the reversible Mg chemistry. Inspired by the excellent separation effect of permselective metal-organic framework (MOF) at angstrom scale, a large-area and defect-free MOF membrane directly on Mg surfaces is here constructed. In this process, the electrochemical deprotonation of ligand can be facilitated to afford the self-correcting of intercrystalline voids until a seamless membrane formed, which can eliminate nonselective intercrystalline diffusion of electrolyte and realize selective Mg2+ transport but precisely separate the solvent molecules from the MOF channels. Compared with the continuous solvent reduction on bare Mg anode, the as-constructed MOF membrane is demonstrated to significantly stabilize the Mg electrode via suppressing the permeation of solvents, hence contributing to a low-overpotential plating/stripping in conventional electrolytes. The concept is demonstrated that membrane separation can serve as solid-electrolyte interphase, which would be widely applicable to other energy-storage systems.

Rational Design of Pomegranate-like Base-Acid Bifunctional β Zeolite by Steam-Assisted Crystallization for the Tandem Deacetalization-Knoevenagel Condensation.

A highly crystalline pomegranate-like base-acid bifunctional beta zeolite was successfully synthesized by the steam-assisted crystallization method using a basic nitrided N-beta as the starting material. The secondary crystal growth of a beta zeolite generating acid functionality occurred over the outer surface and intercrystalline void spaces of the N-beta zeolite. The pomegranate-like N-beta@H-beta zeolite had a high surface area and base-acid dual functionality because of the well-connected framework topologies of the H-beta and N-beta crystallites. The N-beta@H-beta zeolite exhibited a superior yield of benzylidenemalononitrile during the tandem deacetalization-Knoevenagel condensation of benzaldehyde dimethyl acetal and malononitrile compared to H-beta, N-beta, and their physical mixture. This is likely due to the isolated and balanced activity of the base- and acid-catalyzed reactions.

MODIFICATION OF GRAPHITE BY A FLUORINE- CONTAINING OLIGOMER


 
 
 Impact of the process of modification on graphite structure at the application of fluorine-containing oligomer - chloroperfluorododecylfluoro sulfate has been studied. It was shown that during this process graphite structure doesn’t change. Only particles of chloroperfluorododecylfluoro sulfate are destructed into monolayers, which are spread in graphite intercrystalline voids. At the mechanical impact it protects graphite from destruction and correspondingly, improves some of its tribotechnical characteristics.
 
 

Sequential Template Decomposition to Adjust the Performance of Imperfect Zeolite BEA Membranes

AbstractZeolite membranes offer superior thermal, chemical, and mechanical stability compared to polymeric membranes. However, it is still a challenge to prepare completely defect‐free membranes without any intercrystalline voids, which is necessary for gas separation processes. In this study zeolite beta (*BEA) membranes on stainless‐steel supports were prepared by applying the multiple in situ crystallization technique. The membranes were used as a model system to systematically study the decomposition of the organic structure directing agent tetraethylammoniumhydroxide (TEA‐OH). It was evaluated if the organic decomposition products of TEA‐OH can be used for enhancing the membranes selectivity. Post‐treatment experiments have been carried out to adjust surface properties and pore size dimensions in the zeolitic membrane layer. The results show that membranes calcined at lower temperatures exhibit a higher gas selectivity.

Development and application of an exchange model for anisotropic water diffusion in the microporous MOF aluminum fumarate

Diffusion of water in aluminum fumarate was studied by means of pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Due to water molecules exchanging between the intracrystalline anisotropic pore space and the isotropic intercrystalline void space the model of intracrystalline anisotropic diffusion fails to describe the experimental PFG NMR data at high observation times. Therefore, the two-site exchange model developed by Kärger is extended to the case of exchange between an anisotropic and an isotropic site. This extended exchange model is solved by numerical integration. It describes the experimental data very well and yields values for the intracrystalline diffusion coefficient and the mean residence times of the respective sites. Further PFG NMR studies were performed with coatings consisting of small aluminum fumarate crystals, which are used in adsorptive heat transformation applications. The diffusion coefficients of water in the small crystal coating are compared to the values expected from the extended two-site exchange model and from the model of long-range diffusion.

Efficient and selective conversion of methanol to para-xylene over stable H[Zn,Al]ZSM-5/SiO2 composite catalyst

A highly shape-selective H[Zn,Al]ZSM-5/SiO2 composite catalyst was prepared by direct incorporating Zn species via isomorphous substitution into the zeolite framework and subsequent SiO2 deposition on zeolite surface. Its catalytic performance in methanol to aromatics (MTA) was compared with Zn/HZSM-5/SiO2 prepared by impregnation of Zn species and subsequent SiO2 deposition. The textural properties and acidic properties as well as the subsequent catalytic performances of the resultant catalysts in MTA reaction were obviously influenced by the introduction method of Zn species. H[Zn,Al]ZSM-5/SiO2 prepared by isomorphous substitution contained more inter-crystalline voids, which provided much space to resist deposition of coke and avoided quick blockage of micropore entrances. In contrast, Zn/HZSM-5/SiO2 prepared by impregnation contained sub-nanometric ZnO clusters at the pore entrances, which might restrict large molecule products diffusion and deteriorated the catalyst deactivation. Moreover, the sum of strong and medium acid sites in H[Zn,Al]ZSM-5/SiO2 was more than that in Zn/HZSM-5/SiO2, which meant there were more active sites for cyclization and aromatization steps in MTA reaction in the former. As a result, the lifetime of H[Zn,Al]ZSM-5/SiO2 was almost twice that of Zn/HZSM-5/SiO2. In addition, the enhancement in catalytic activity by Zn introduction and the improvement in para-selectivity in xylene by SiO2 deposition were also studied. Finally, nearly 100% methanol conversion, 95.6% para-selectivity in xylene and 18.2% para-xylene yield were simultaneously obtained over the stable H[Zn,Al]ZSM-5/SiO2 (with 24 h lifetime) in MTA reaction.

Effects of Sn Layer Orientation on the Evolution of Cu/Sn Interfaces

The effects of Sn layer orientation on the evolution of Cu/Sn joint interfaces were investigated. Three Sn layers possessing (112), (321) and (420) orientations were electroplated on polycrystalline Cu substrates respectively. The orientations of Sn layer preserved during reflowing at 250 °C for 10 s. After aging at 150 °C for different time, the interfacial microstructures were observed from the cross-section and top-view. The alignment between the c-axis of Sn and Cu diffusion direction significantly sped up the Cu diffusion, leading to the thickest intermetallic compound layer formed in (112) joint. Two types of voids, namely, intracrystalline voids and grain islanding caused intercrystalline voids generated at Cu/Cu3Sn interfaces due to the different interdiffusion coefficients of Cu and Sn (112) oriented Sn/Cu joint produced many more voids than (321) joint, and no voids were detected in (420) joint. Therefore, to enhance the reliability of solder joints, using (420) oriented Sn as solder layer could be an efficient way.

Mesoporous EU-1 zeolite with enhanced accessibility and diffusion for bulky molecular reaction

Mesoporous EU-1 zeolites with adjustable mesoporosity have been prepared by employing silanized silica into the starting synthesis gel, based on the hinder effect of organic moiety to crystal growth. The synthesized mesoporous EU-1 zeolites were characterized by XRD, NMR, SEM/TEM and N2 adsorption/desorption at 77K, showing the persistence of inherent microporous framework structure of EU-1 zeolite and the existence of additional mesopores within the range of 3–10nm. The generation of mesoporosities is from the intercrystalline voids of nanocrystal aggregates. The introduction of mesopores in EU-1 zeolite increased the adsorption of mesitylene with the kinetics diameter larger than the micropore size, and made it possible for the diffusion pattern of n-heptane from one-dimensional in microporous EU-1 to three-dimensional in mesoporous EU-1. This result together with the enhanced accessibility of acidic sites of mesoporous EU-1 improved greatly the catalytic activity and product yields of cumene cracking reaction involving bulky molecules.

The assessment of pore connectivity in hierarchical zeolites using positron annihilation lifetime spectroscopy: instrumental and morphological aspects

Recent studies demonstrated the power of positron annihilation lifetime spectroscopy (PALS) to characterise the connectivity and corresponding effectiveness of hierarchical pore networks in zeolites. This was based on the fractional escape of ortho-positronium (Ps), formed within the micropore framework, to vacuum. To further develop this technique, here we assess the impact of the positron implantation energy and of the zeolite crystal size and the particle morphology. Conventional measurements using fast positrons and beam measurements applying moderated positrons both readily distinguish purely microporous ZSM-5 zeolites comprised of single crystals or crystal aggregates. Unlike beam measurements, however, conventional measurements fail to discriminate model hierarchical zeolites with open or constricted mesopore architectures. Several steps are taken to rationalise these observations. The dominant contribution of Ps diffusion to the PALS response is confirmed by capping the external surface of the zeolite crystals with tetraethylorthosilicate, which greatly enhances the sensitivity to the micropore network. A one-dimensional model is constructed to predict the out-diffusion of Ps from a zeolite crystal, which is validated experimentally by comparing coffin-shaped single crystals of varying size. Calculation of the trends expected on the application of fast or moderated positrons indicates that the distinctions in the initial distribution of Ps at the crystal level cannot explain the limited sensitivity of the former to the mesopore architecture. Instead, we propose that the greater penetration of fast positrons within the sample increases the probability of Ps re-entry from intercrystalline voids into mesopores connected with the external surface of zeolite crystals, thereby reducing their fractional escape.

Impact of Modification Process on Graphite Structure at Application of Chloroperfluorododecylfluoro Sulfate

Impact of the process of modification on graphite structure at the application of fluorine-containing oligomer-FDS (chloroperfluorododecylfluoro sulfate) has been studied. It was shown that during this process graphite structure does not change. Only particles of FDS are destructed into monolayers, which are spread in graphite intercrystalline voids. At the mechanical impact it protects graphite from destruction and correspondingly, improveing some of its tribotechnical characteristics.

Microstructure of porous carbons derived from phenolic resin – Impact of annealing at temperatures up to 2000 °C analyzed by complementary characterization methods

To investigate the impact of thermal annealing on the micro- and macrostructure of amorphous carbons two series of synthetic disordered porous carbons (carbon xerogels) were analyzed with respect to their structural properties after exposure to different annealing temperatures under argon between 800 and 2000°C. One series has micro- and mesopores, while the second set of samples is micro- and macroporous only.Gas sorption measurements and small angle X-ray scattering (SAXS) were applied to determine characteristic parameters of the micro- and mesoporosity present and in particular to distinguish between accessible and inaccessible porosity. XRD-spectra were taken to analyze the size of the graphitic domains (microcrystallites) defining the micropores as intercrystalline voids. The macroscopic properties of the carbons were quantified in terms of their apparent density and their elastic modulus.The combination of these characterization methods yields a detailed picture of the inner structure of the porous carbons and the changes upon annealing.The experimental data reveal that the graphitic domains as well as the size of the micropores grow with the annealing temperature. However, for T>1000°C micropore accessibility is continuously decreasing and for T>1800°C practically no microporosity is detected with N2 sorption analysis, while SAXS still shows significant micropore volume. At the same time the structure on the meso- and macroscale is almost unaffected.

Effects of Using Different Polymers in Post-Synthesis Treatments of Zeolite a Coatings

Zeolite A coatings were prepared by the substrate heating and repeated conventional syntheses methods, targeting their use especially in adsorption heat pump applications. The coatings, grown on stainless steel, were then covered by polyvinyl acetate, polyvinyl alcohol, and cellulose acetate as post-synthesis treatments for improving the stabilities of the coatings. The water desorption properties of the metal-zeolite-polymer integrated systems were investigated by thermogravimetry (TG). Their stabilities were determined by the application of consecutive ultrasonic treatments and heating/cooling cycles. The zeolite coatings prepared by substrate heating contained void fractions of about 45%, significantly higher values than those obtained by repeated syntheses, which remained below 10%. When the former coatings were covered by polymer, they exhibited water desorption of about 0.13–0.15 g/g sample which was higher than the amount of water desorbed from both pure zeolite (0.12–0.14 g/g sample) and pure polymer (0.038–0.083 g/g sample) under the conditions investigated. Such behavior was absent for the more compact zeolite coatings prepared by repeated syntheses. This might signify that the zeolite coatings with larger intercrystalline void regions provided higher amount of surface and/or interactions between the polymer and zeolite phases, which mostly favored water desorption properties of composite coatings. Zeolite coatings prepared by both synthesis methods exhibited high stabilities after the post-synthesis treatments applied.

Preparation of mesoporous CeO2 and monodispersed NiO particles in CeO2, and enhanced selectivity of NiO/CeO2 for reverse water gas shift reaction

The preparation of mesoporous ceria (CeO2) and NiO/CeO2 was investigated using a simple calcination method. Though the oxidation of Ce3+ ions, the hydrolysis of Ce4+ ions, polymerization and precipitation in melted liquid, the nanosized mesoporous CeO2 and NiO/CeO2 could be obtained after calcination. The mesoporous CeO2 and NiO/CeO2 that were obtained had high surface areas, narrow pore size distributions and uniform mesopores. The mesoporous CeO2 had very high thermal stability and a surface area of 100m2g−1 even after calcination at 600°C. The mesoporous CeO2 and NiO/CeO2 had nanoarrays with uniform mesopores (intercrystalline voids). The reverse water gas shift reaction was studied using mesoporous NiO/CeO2 as a catalyst. The CO2 conversion increased with increasing temperature and NiO amount. With less than 3wt% NiO, NiO particles were monodispersed in mesoporous CeO2, and provided a CO selectivity of 100%, regardless of temperature. These monodispersed NiO particles in mesoporous CeO2 were observed for the first time. NiO particles that aggregated in the presence of more than 3.5wt% NiO, resulted in a CO selectivity of 100% at temperatures above 700°C, whereas the CO selectivity is less than 100% at temperatures below 650°C. Therefore, the mondispersion of NiO particles in mesoporous CeO2 enhanced CO selectivity.

Hierarchical ZSM-11 with intergrowth structures: Synthesis, characterization and catalytic properties

Hierarchical ZSM-11 microspheres with intercrystalline mesoporous properties and rod-like crystals intergrowth morphology have been synthesized using a spot of tetrabutylammonium as a single template. XRD, FTIR, SEM, TEM and N2 adsorption analysis revealed that each individual particle was composed of nanosized rod crystals inserting each other and the intercrystalline voids existing among rods gave a significant mesopore size distribution. Steam treatment result demonstrated the excellent hydrothermal stability of samples. Various crystallization modes including constant temperature crystallization (one-stage crystallization) and two-stage temperature-varying crystallization with different 1st stage durations were investigated. The results suggested that the crystallization modes were mainly responsible for the adjustable particle size and textural properties of samples while the small amount of tetrabutylammonium bromide was mainly used to direct the formation of both ZSM-11 framework and its intergrowth morphology. Furthermore, the performance of optimal ZSM-11 as an active component for the catalytic pyrolysis of heavy oil was also investigated. Compared with the commercial pyrolysis catalyst, the hierarchical ZSM-11 catalyst exhibited a high selectivity to desired products (LPG + gasoline + diesel), as well as a much lower dry gas and coke yield, plus a high selectivity and yield of light olefins ( C3=-C4=) and very poor selectivity to benzene. Therefore, fully open micropore-mesopore connectivity would make such hierarchically porous ZSM-11 zeolites very attractive for applications in clean petrochemical catalysis field.

Formation mechanism and catalytic application of hierarchical structured FeAlPO-5 molecular sieve by microwave-assisted ionothermal synthesis

At atmospheric pressure, FeAlPO-5 molecular sieve with hierarchical micro- and meso-porous structure has been ionothermally synthesized by microwave heating with eutectic mixture as the solvent. Among the synthesis parameters investigated, the ratio of P2O5/Al2O3, aluminum source, and heating methods significantly affected the mesoporosity of FeAlPO-5 molecular sieve. N2 Physisorption, SEM and TEM characterizations showed that the resultant material possessed inter- and intra-crystalline mesopores simultaneously. The larger intercrystalline mesopores in size resulted from the intercrystalline void space between bar crystals and nanoparticles. The calcination removed the organic species embedded in the crystals of FeAlPO-5 molecular sieve, leading to the formation of smaller intracrystalline mesopores. The catalytic properties of hierarchical FeAlPO-5 molecular sieve were investigated in the hydroxylation of phenol with microporous FeAlPO-5 as a reference. The results showed that the catalytic performance of the former catalyst was superior to the latter.

Synthesis of mesoporous ceria without template

The synthesis of mesoporous ceria (CeO2) without a template was studied in detail using NaOH and NH4OH solution. We first synthesized nanosized mesoporous CeO2 without a template by the direct hydrothermal synthesis method with NaOH. By using NaOH, mesoporous CeO2 could be obtained without a template through the oxidation of Ce3+ ions, the hydrolysis of Ce4+ ions, polymerization and precipitation. Under appropriate synthesis conditions, the obtained mesoporous CeO2 had a high surface area, a narrow pore size distribution and an ordered structure with uniform mesopores. The surface area of the mesoporous CeO2 increased with increasing synthesis temperature. The NaOH concentration induced different behavior on the mesoporous CeO2 surface with and without a template. The mesoporous CeO2 had nanoarrays with uniform mesopores (intercrystalline voids). However, when NH4OH solution was used, the CeO2 had nanoarrays with a hierarchical pore network. The mesoporous CeO2 obtained without template had relatively high thermal stability that increased with increasing synthesis temperature.

Direct hydrothermal synthesis of nanosized mesoporous ramsdellite manganese oxide with high surface area

We first synthesized nanosized mesoporous R-MnO2 using different block copolymers and PEG by direct hydrothermal synthesis method. The block copolymers seemed to play the important role. At appropriate synthesis conditions, the mesoporous R-MnO2 with high surface area, narrow pore size distributions and ordered structure with uniform mesopores could be obtained. The mesoporous R-MnO2 had nanoarrays with uniform mesopores (intercrystalline voids). Because the residual copolymer in the product could be easily removed by washing after synthesis, the calcination procedure of the final mesoporous R-MnO2 obtained was not necessary, suggesting that this method is economical and environmental. This direct hydrothermal synthesis method can potentially develop a new method for preparing other metal oxide. We think that the mesoporous R-MnO2 formed through a mechanism that combines PEO moieties in block polymers self-assembly with complication of Mn species. The mesoporous R-MnO2 had high thermal stability.

Mineral composition and structure of the stalagmite laminae from Chulerasim cave, Indian Himalaya, and the significance for palaeoclimatic reconstruction

Many noteworthy properties of climate recorded by stalagmites can result from their mineralogy and fabric as well as their mode of occurrence. In this study, X-ray diffraction (XRD) and Scanning electron microscope (SEM) investigations were carried out for a well laminated stalagmite from Chulerasim cave, north India, to identify the mineral composition and structure of the laminae. As some early reported stalagmite laminae from Thailand and Southwestern China, the laminae of this stalagmite are composed of alternating compact and porous sub-layers. The XRD results confirm that the stalagmite is composed mainly of primary aragonite, which corrects the previous interpretation. The SEM results show that the compact sub-layer is composed of elongated columnar aragonites with a general longitudinal orientation (parallel to the vertical growth axis) and the coalescence of the aragonite crystals is well developed, leaving few inter-crystalline voids. The compact sub-layer may have formed in quasi-equilibrium conditions and provides the main carrier of climate proxies. The porous sub-layer is made up of needles, drusy and fibrous aragonites intersecting each other. Accordingly, the coalescence is low, with many inter-crystalline voids, which suggests a short hiatus between two adjacent compact sub-layers. Therefore, the growth of alternation of compact/porous sub-layers may not be successive, and they may have formed in different seasons. The results suggest that, for stalagmite/palaeoclimate research, cave monitoring should be performed to reveal when and how the compact sub-layers were formed.