Intercrystalline Region Research Articles

Diffusion processes in a poly-crystalline zeolitic material: A molecular dynamics study.

Extensive molecular dynamics simulations of xenon in two classes of zeolite crystal systems, one consisting of purely intra-crystalline space and the other with both intra- and inter-crystalline space are reported. The latter mimics a typical poly-crystalline sample of zeolite. Comparison of results from these two systems provides insights into the structure and dynamics in the presence of inter-crystalline space. The temperature, as well as the distance between the crystallites, has been varied. The density distribution and diffusivities calculated inside the poly-crystalline system show that the interfacial region between the crystal and the inter-crystalline region acts as a bottleneck for diffusion through the system. At lower temperatures, the particles are trapped at the interface due to the pronounced energy minima present in that region. With the increase in temperature, the particles are able to overcome this barrier frequently, and the transport across the inter-crystalline region is increased. A ballistic or superdiffusive motion is seen in the inter-crystalline region along all the axes except along the axis which has the inter-crystalline space. The transition time for ballistic to diffusive motion increases with the increase in the length of the inter-crystalline space. Velocity auto- and cross correlation functions exhibit strong oscillations and exchange of kinetic energy along directions perpendicular to the direction of the inter-crystalline space. These results explain why uptake and PFG-NMR measurements exhibit lower values for diffusivity for the same system when compared to Quasi-Elastic Neutron Scattering. Thus, using molecular dynamics simulations, we were able to correlate the difference of diffusivity values measured using various experimental methods where these inter-crystalline regions are common.

Effect of Level of Crystallinity on Melt Memory Above the Equilibrium Melting Temperature in a Random Ethylene 1-Butene Copolymer

A strong memory effect of crystallization has been observed in melts of random ethylene copolymers even above the equilibrium melting temperature. Melt memory is correlated with self-seeds that increase the crystallization rate of ethylene copolymers. The seeds are associated with molten ethylene sequences from the initial crystals that remain in close proximity and are unable to diffuse quickly to the randomized melt state. Fast diffusion is restricted by topological chain constraints (loops, knots, and other entanglements) that build in the intercrystalline region during crystallization. The effect of topological constraints on melt memory, or on number of remaining self-seeds in the melt, is analyzed studying the melting and subsequent crystallization of a model ethylene 1-butene copolymer with 2.2 mol% ethyl branches prepared with different levels of crystallinity. There is a critical threshold level of crystallinity of 6–12% to observe the effect of melt memory on the subsequent crystallization rate of this copolymer. A faster development of the initial crystallinity may more efficiently trap knots and loops around the crystallites, leading to a lower crystallinity threshold than for slow or isothermally crystallized copolymers.

Evaluation of the segregation of paramagnetic impurities at grain boundaries in nanostructured ZnO films.

Magnetic and electrical properties of the nanostructured ZnO films are affected by the nonrandom distribution of impurities in the film due to segregation at grain boundaries (GBs) or extended defects. However, mapping the nature and distribution of the impurities in the film is not trivial. Here we demonstrate a simple, statistically relevant, and nondestructive procedure of quantitative determination of the paramagnetic impurities segregated at the GBs in nanostructured semiconducting and insulating films. From correlated electron paramagnetic resonance and transmission electron microscopy investigations, we determined the localization of trace amounts of Mn(2+) ions, present as native impurities in a ZnO film deposited by magnetron sputtering at room temperature. In the as-deposited ZnO film, the Mn(2+) ions were all localized in nanosized pockets of highly disordered ZnO dispersed between nanocrystalline columns. After the samples had been annealed in air at >400 °C, the size of the intercrystalline region decreased and the diffusion in GBs was activated, resulting in the localization of a portion of the Mn(2+) ions in the peripheral atomic layers of the ZnO columns neighboring the GBs. The proportion of Mn(2+) ions still localized at the GBs after annealing at 600 °C was 37%. The proposed method for the assessment of the presence and nature of impurities and the quantitative evaluation of their distribution in semiconducting and insulating nanostructures is expected to find direct applications in nanotechnology, in the synthesis and quality assurance of thin films for spintronics and opto- and nanoelectronics.

Defect and Structure Engineering – Submicron Technology and Inverse Nanotechnology for Photo- and Ionizing Radiation Detectors

A review is given of what kind technological aspects were used for realizing the defect engineering in semiconductor layers or crystals. The possibilities to change the free carrier capture are presented. The effect of Fermi level pinning at the surface levels allow to avoid the influence of barriers on the photoconductivity as well as to increase a role of recombination in the inter-crystalline region. The isovalent doping or the creation of the clusters allows transforming the defect distribution in the crystal bulk. The detector structure using the high electric field can introduce the recombination in at the contacts therefore allow diminishing a role of carrier capture in the bulk of structure. The cluster generation allows to increase the capture rate in the definite volume by a proton irradiation. The experience of different technologies for Si, GaAs, PbS, CdSe are presented.

Modeling of methane permeation through a defective region in MFI-type zeolite membranes

Abstract Molecular modeling of methane permeation though silicalite membranes, including the intercrystalline region, was carried out. We focused on the effect of the intercrystalline region on sorption and diffusion properties. Molecular dynamics (MD) and grand canonical ensemble Monte Carlo (GCMC) simulations were performed to estimate the diffusion coefficient and adsorption parameters. The result of the MD calculation indicates that the intrinsic diffusivity of methane decreases by an order of magnitude due to the presence of the intercrystalline region, although the adsorption properties do not change except at higher pressure (> 2000 kPa) where the adsorption increased slightly. The estimated permeability using these parameters based on the combined method of molecular simulation and permeation theory agree well with experiments. These results indicate the importance of considering the effect of an intercrystalline region on the permeability in zeolite membranes.

Structure and thermal stability of nanocrystalline materials

Nanocrystalline materials, which are expected to play a key role in the next generation of human civilization, are assembled with nanometre-sized “building blocks” consisting of the crystalline and large volume fractions of intercrystalline components. In order to predict the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to understand precisely how the structures of crystalline and intercrystalline regions vary with decrease in crystallite size. In addition, study of the thermal stability of nanocrystalline materials against significant grain growth is both scientific and technological interest. A sharp increase in grain size (to micron levels) during consolidation of nanocrystalline powders to obtain fully dense materials may consequently result in the loss of some unique properties of nanocrystalline materials. Therefore, extensive interest has been generated in exploring the size effects on the structure of crystalline and intercrystalline region of nanocrystalline materials, and the thermal stability of nanocrystalline materials against significant grain growth. The present article is aimed at understanding the structure and stability of nanocrystalline materials.

Polymorphic transformation and lattice expansion in nanocrystalline niobium revealed by positron annihilation at grain boundaries

Positron lifetimes in nanocrystalline niobium were measured over the grain size range from 35 nm down to 5 nm. Two lifetimes were obtained indicating distinct annihilation sites at the grain surfaces and in the intercrystalline region. X-ray diffraction studies had indicated a bcc to fcc transformation and a concomitant lattice expansion. Positron lifetimes drastically varied below a grain size of 10 nm, coinciding with a sharp increase in the excess free volume of atoms at the grain boundaries as revealed in a model analysis. Doppler broadening measurements also indicated a redistribution of electron momentum, implying a restructuring of the grain surfaces. The occurrence of bcc to fcc transformation is also observed in binary Nb–Al and ternary Al–Nb–Cu alloys.

Transport phenomena through intercrystalline and intracrystalline pathways of silicalite zeolite membranes

Zeolite membranes are, generally, polycrystalline structures. Penetrant molecules pass through zeolite crystals (intracrystalline pathways) and intercrystalline pathways. The transport phenomena of ethanol through intracrystalline and intercrystalline pathways of silicalite membranes were separately examined. An intercrystalline–intracrystalline model (i–i model) is proposed for quantitative discussion of fluxes through intercrystalline and intracrystalline pathways. The concentration in the interface of the intracrystalline region is of the Langmuir type, and the concentration in the intercrystalline region is proportional to the activity on the outside of the membrane. This i–i model was investigated using silicalite membranes and silicalite membranes modified by chemical vapor deposition (CVD). CVD modification was carried out using a tetramethylorthosilicate (TMOS)/O 3 counter-diffusion CVD technique. The i–i model can quantitatively explain the permeate properties of ethanol transport both through the silicalite membrane and the CVD modified membrane. According to this model, ethanol is permselective from ethanol/water mixtures through intercrystalline pathways.

Preparation of Zeolite A and Faujasite Membranes from a Clear Solution

Zeolite A and faujasite- (FAU-)type zeolite membranes were prepared under identical conditions from solutions of the same composition, with the only difference being the seed crystal used. The zeolite A membrane was formed on a substrate seeded with zeolite A crystals, while the FAU membrane was formed with zeolite Y seeds. The induction period of clear solution was used to prevent the influence of precipitated crystals that will eventually form in the clear solution and to keep the composition of the synthesis solution constant. Zeolite crystals increased in size with synthesis time and formed a continuous membrane on a porous substrate. Selectivity in a liquid mixture separation by pervaporation increased as crystals grew, indicating a decrease in intercrystalline region size.

Silicalite Membranes Modified by Counterdiffusion CVD Technique

A new technique to modify the zeolite molecular sieve membrane has been developed. Zeolite membranes have polycrystalline structure, and penetrant molecules can transport through zeolite crystals and among crystal faces. When the intercrystalline region are plugged, the zeolite crystal pores are the only pathway for membrane transport. That is a promising way to obtain a good molecular sieving effect. Counterdiffusion CVD technique was carried out to fill the intercrystalline region. Tetraethyl orthosilicate (TEOS) was employed as a silica source, and O3 was used as an oxidizing agent; these reactants form amorphous silica by CVD reaction. TEOS hardly penetrated into zeolitic pores because of its molecular size, and zeolitic pores may not have been changed by the reaction. Thus, only the intercrystalline region was filled due to the silica formation. The CVD-modified silicalite membrane produced a n-butane selectivity of 87.8 over isobutane at 288 K.

Room temperature creep behavior of nanocrystalline nickel produced by an electrodeposition technique

Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

Variation of trap state density and barrier height with Cu/In ratio in CuInSe 2 films

Optical properties of CuInSe 2 films, deposited by the three-source evaporation technique at a substrate temperature of 670 K, were studied critically to determine the grain boundary scattering effects from the tailing of the absorption edge. The experimental data were analysed in the light of the grain boundary trapping model in polycrystalline films. The crystallites were found to be partially depleted of carriers. The barrier height along with the carrier concentration in the films were determined from optical reflectance measurements. The effect of the variation of the Cu/In ratio (0.4–1.18) on the grain boundary scattering parameters has been critically discussed. The density of trap states in the intercrystalline region of the films was found to increase with increasing Cu/In ratio. The built-in electric field in the crystallites was evaluated for different sizes of the crystallites. The effect of illumination on the grain boundary barrier height was studied and this indicated that the grain boundary charges are depleted by subbandgap photons resulting in the reduction of the barrier height with increase of illumination level.

Grain boundary effect in polycrystalline ZnTe films

Electrical and optical properties of ZnTe films, deposited at different substrate temperatures (300–630 K) by hot-wall vacuum evaporation, were studied. The experimental data were analysed in the light of the grain boundary trapping model in polycrystalline films. The crystallites were found to be partially depleted of carriers. The barrier height along with the carrier concentration in the films were determined from the optical reflectance measurements. The effect of deposition temperature on the film properties has been critically discussed. The density of trap states in the intercrystalline region of the films was found to decrease with increasing deposition temperature. The built-in electric field in the crystallites was evaluated for different sizes of the crystallites.

Thermo-oxidative degradation of isotactic polypropylene film: Structural changes and its correlation with properties

Degradation of isotactic polypropylene has been studied by ageing the film samples at 70°, 80° and 90°C in air. Reactions involved in changing the molecular structure and formation of oxygenated and unsaturated groups, during thermo-oxidative degradation, are discussed. The increase in molecular weight ( M ̄ v ) suggests molecular enlargement mainly by recombination of alkyl and alkoxy radicals which produces non-radical products. Tensile strength has been found to increase initially and then decreases with ageing time. Increase in isotacticity and density of polypropylene may be due to the cleavage of the backbone chain in the amorphous intercrystalline region, facilitating poorly ordered molecular segments into a more ordered helical form.