Nucleation Of Crystallites Research Articles

Nucleation and Growth of Electrodeposited Manganese Dioxide for Electrochemical Capacitors

In this work the anodic electrodeposition of manganese dioxide from a solution of 0.01M MnSO4 in 0.1M H2SO4 onto a platinum substrate has been examined in order to determine the mechanism of deposition. Electrochemical data indicates that the dilute acid solution favours the formation of MnO2 via a MnOOH intermediate (hydrolysis). The electrodeposited material was examined further using AFM and TEM which showed that the oxidation of Mn2+ occurs in a number of steps. When a potential is applied, Mn2+ is electrochemically oxidised to soluble Mn3+. When the concentration of Mn3+ at the electrode reaches saturation, it chemically precipitates onto the electrode as MnOOH, causing an increase in the electroactive surface area. This stage of the deposition is characterised by the nucleation of MnOOH crystallites on the platinum surface. As the deposition proceeds, MnOOH undergoes solid state oxidation to MnO2. This later stage of the deposition favours the growth of existing MnO2 crystallites, leading to a decrease in the surface area of the electrodeposited material.

Impact of excess phosphorus doping and Si crystalline defects on Ag crystallite nucleation and growth in silver screen‐printed Si solar cells

AbstractGood quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass‐free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface‐embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P‐doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect‐property dependent. Copyright © 2013 John Wiley & Sons, Ltd.

Nanocomposites of silica reinforced polypropylene: Correlation between morphology and properties

Polypropylene/fumed hydrophilic silica nanocomposites were prepared via melt mixing method using a single‐screw extruder. Comparative study with and without compatibilizing copolymer agent (maleic anhydride grafted polypropylene: PP‐g‐AM) was conducted. The obtained results were interpreted in terms of silica nanoparticle–silica nanoparticle and silica nanoparticle‐polymer interactions. These results have shown that the addition of nanofillers improves the properties of the nanocomposites. From transmission electron microscopy, it was found that agglomerations of silica particles into the PP matrix increased in average size with increasing silica contents, except in presence of the copolymer. Storage modulus values of the nanocomposites measured by dynamic mechanical thermal analysis were sensitive to the microstructure of the nanocomposites. Higher silica contents resulted in higher storage modulus, revealing that the material became stiffer. By adding the compatibilizer, a further increase of storage modulus was observed due to the finer dispersion of the filler in the matrix and the increased interfacial adhesion. Crystallization rates were found to increase with the increase of silica nanoparticles as well as PP‐g‐MA content. In addition, silica nanoparticles and the compatibilizing agent present centers of germination and nucleation of crystallites. Thus, the use of the coupling agent resulted in a further enhancement of mechanical properties of the nanocomposites due to the reduction of silica agglomeration. POLYM. ENG. SCI., 54:2187–2196, 2014. © 2013 Society of Plastics Engineers

Nucleation effect of montmorillonite with β-nucleating surface on polymorphous of melt-crystallized isotactic polypropylene nanocomposites

Abstract Neat montmorillonite (MMT) causes strong heterogeneous nucleation of α-phase crystallites in isotactic polypropylene (iPP). By surface modification of MMT with pimelic acid (PA) MMT with tailored β-nucleation capacity (β-MMT) has been prepared and nanocomposites with β-iPP have been made. The polymorphism of the iPP in the nanocomposites, the effect of β-MMT on the morphology and its crystallization and melting behavior of iPP has been investigated by POM, DSC and XRD. β-MMT raises the crystallization temperature to a higher value than neat MMT. The β-nucleation ability of β-MMT depends both on the MMT/PA ratio and on the β-MMT content in the composite. The fraction of β-crystals with respect to the total amount of crystals increases with increasing β-MMT content and decreasing MMT/PA ratio. It can be adjusted between 0% and 98%. The accomplished surface modification of the MMT provides a simple and efficient way to prepare β-iPP nanocomposites.

Nanocrystallization in spark plasma sintered Fe48Cr15Mo14Y2C15B6 bulk amorphous alloy

Spark plasma sintering (SPS) is evolving as an attractive process for the processing of multi-component Fe-based bulk amorphous alloys and their in-situ nanocomposites with controlled primary nanocrystallization. Extended Q-range small angle neutron scattering (EQ-SANS) analysis, complemented by x-ray diffraction and transmission electron microscopy, was performed to characterize nanocrystallization behavior of SPS sintered Fe-based bulk amorphous alloys. The SANS experiments show significant scattering for the samples sintered in the supercooled region indicating local structural/compositional changes associated with the profuse nucleation of nanoclusters (∼4 nm). For the samples spark plasma sintered near and above crystallization temperature (>653 °C), the SANS data show the formation of interference maximum indicating the formation and growth of (Fe,Cr)23C6 crystallites. The SANS data also indicate the evolution of bimodal crystallite distribution at higher sintering temperatures (above Tx1). The growth of primary nanocrystallites results in impingement of concentration gradient fields (soft impingement effect), leading to non-random nucleation of crystallites near the primary crystallization.

The Influence of Oxalate-Promoted Growth of Saponite and Talc Crystals on Rectorite: Testing the Intercalation-Synthesis Hypothesis of 2:1 Layer Silicates

AbstractThe intercalating growth of new silicate layers or metal hydroxide layers in the interlayer space of other clay minerals is known from various mixed-layer clay minerals such as illite-smectite (I-S), chlorite-vermiculite, and mica-vermiculite. In a recent study, the present authors proposed that smectite-group minerals can be synthesized from solution as new 2:1 silicate layers within the low-charge interlayers of rectorite. That study showed how oxalate catalyzes the crystallization of saponite from a silicate gel at low temperatures (60ºC) and ambient pressure. As an extension of this work the aim of the present study was to test the claim that new 2:1 silicate layers can be synthesized as new intercalating layers in the low-charge interlayers of rectorite and whether oxalate could promote such an intercalation synthesis. Two experiments were conducted at 60ºC and atmospheric pressure. First, disodium oxalate solution was added to a suspension of rectorite in order to investigate the effects that oxalate anions have on the structure of rectorite. In a second experiment, silicate gel of saponitic composition (calculated interlayer charge -0.33 eq/O10(OH)2) was mixed with a suspension of rectorite and incubated in disodium oxalate solution. The synthesis products were extracted after 3 months and analyzed by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The treatment of ultrathin sections with octadecylammonium (nC =18) cations revealed the presence of 2:1 layer silicates with different interlayer charges that grew from the silicate gel. The oxalate-promoted nucleation of saponite and talc crystallites on the rectorite led to the alteration and ultimately to the destruction of the rectorite structure. The change was documented in HRTEM lattice-fringe images. The crystallization of new 2:1 layer silicates also occurred within the expandable interlayers of rectorite but not as new 2:1 silicate layers parallel to the previous 2:1 silicate layers. Instead, they grew independently of any orientation predetermined by the rectorite crystal substrate and their crystallization was responsible for the destruction of the rectorite structure.

Influence of argon dilution on the growth of amorphous to ultra nanocrystalline silicon films using VHF PECVD process

• Silicon films were grown by variation in argon concentration ( f ar %). • Nucleation and growth of crystallites were observed at f ar = 82% and 87%, respectively. • Amorphous films showed photosensitivity of five orders of magnitude. • FTIR showed changes in the hydrogen bonding configuration with variation in f ar %. Hydrogenated Silicon (Si:H) thin films were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF PECVD) technique from silane (SiH 4 ) diluted in various argon (Ar) concentration ( f ar ). The effect of argon concentration (53–95%) on the growth and properties of Si:H films was investigated. Device quality amorphous silicon films were grown at f ar = 53% and 63% showed photosensitivity of five orders of magnitude. Nucleation of crystallites were started at f ar = 82% whereas mixed-phase of amorphous (a-Si:H) and ultra nanocrystalline silicon thin film (ultra nc-Si:H) was grown at 87% of f ar . The nucleation and growth of crystallites were clearly observed in the AFM images. Raman spectra showed the splitting and blue shift of TO mode for the films grown at f ar = 82% and 87%, respectively. The variation in the hydrogen bonding configuration of these Si:H films were observed by FTIR measurement. The difference in the temperature dependent dark conductivity during heating and cooling runs decreases as the fraction of clustered Si–H bond increases in the films.

Well-dispersed mesoporous Ta2O5 submicrospheres: Enhanced photocatalytic activity by tuning heating rate at calcination

Well-dispersed mesoporous Ta2O5 submicrospheres were obtained by tailoring the heating rates (R) of calcining Ta2O5 precursor colloidal spheres which were synthesized by the hydrolysis of tantalum glycolate in a mixture of acetone and water. With increasing R, the pore size and the dispersibility of the as-prepared submicrospheres decrease gradually whereas the specific surface area and the pore volume reach maximum values at R=5°C/min. These phenomena are reasonably explained by the dependence of the temperature gradient within the Ta2O5 precursor colloidal spheres on R, which modulates the nucleation and growth behaviors of the Ta2O5 crystallites during the calcination process. Because of the good dispersibility, unique mesoporosity and high crystallinity, the Ta2O5 submicrospheres obtained at R=5°C/min show a significantly enhanced photocatalytic activity not only for hydrogen evolution but also for photodegradation of organic pollutants such as methylene blue and rhodamine B. The results here suggest that the meticulous control over the structural characteristics is an effective alternative to obtain highly efficient photocatalytic materials. The as-prepared well-dispersed mesoporous Ta2O5 submicrospheres are promising for the applications in clean energy access and environment remediation.

Characterization and non-isothermal crystallization behavior of biodegradable poly(ethylene sebacate)/SiO2 nanocomposites

Poly(ethylene sebacate) (PESeb) and PESeb/silica nanocomposites (PESeb/SiO_2) were prepared by in situ polymerization from the direct esterification of ethylene glycol with sebacic acid in the presence of proper amounts of silica nanoparticles. The non-isothermal crystallization behavior of PESeb/SiO_2 nanocomposites has been studied using different theoretical equations such as Avrami, Ozawa and combined Avrami and Ozawa equations. It is found that the addition of nanoparticles of SiO_2 influenced the mechanism of nucleation and the growth of PESeb crystallites. Also, the nanocomposites show a higher Avrami value than the neat PESeb, implying a more complex crystallization configuration. Moreover, the combined Avrami and Ozawa equation can successfully describe the crystallization model under the non-isothermal crystallization. The crystallization activation energies, E_a, calculated from ‘‘Kissinger’s equation’’ have shown that the synthesized PESeb/SiO_2 nanocomposites have lower energy than the neat PESeb, reflecting the much lower energy barrier for the rapid heterogeneous nucleation.

Nucleation rate reduction through stress relief of thermally annealed hydrogenated amorphous silicon films

The effect of film stress on crystallite nucleation is investigated in 0.11 μm thick, thermally annealed hydrogenated amorphous silicon films. Using a recently developed optical method, the crystallite density is measured as the films are isochronally annealed at 600 °C, which enables the determination of the crystallite nucleation rate. This rate is significantly suppressed around scratches, cleaved film edges, and laser ablated areas, extending laterally as much as 100–150 μm from these regions where the film connectivity is disrupted. μ-Raman measurements of the transverse optical mode of Si demonstrate an accompanying reduction in tensile stress in the regions where nucleation is suppressed. The first measurements of nucleation rate in stress and in stress relieved areas in the same film are presented.

Electrochemically active surface area effects on the performance of manganese dioxide for electrochemical capacitor applications

The specific surface area, morphology and electrochemical performance of thin films of electrodeposited manganese dioxide have been examined. Electrodeposition of these films was carried out using chronoamperometry, using times ranging from 10 to 120s in order to obtain deposits with different masses. Using a novel approach to analysing the chronoamperometric i–t data, the specific surface area of the electrodeposited material was found to range from 13 to 67m2/g across the range of deposition times, with short deposition times leading to higher specific surface areas. This has implications on the electrodeposition mechanism of manganese dioxide, which favours crystallite nucleation initially, leading to a high surface area material, followed by growth of these crystallites leading to a denser, lower surface area electrode material. This is the first time that the electrochemically active surface area of porous electrode materials has been determined. This decrease in surface area with deposition time was also reflected in the specific capacitance values of the material, which decreased slightly with increased deposition time, and hence lower surface area.

TILT-A-WHIRL: a texture analysis package for 3D rendering of pole figures using Matlab

A new Matlab-based software suite called Tilt-A-Whirl has been applied to XRD data from textured gold films electro-deposited onto nickel substrates. The software routines facilitate phase identification, texture analysis via pole figure visualization, and macrostrain determination. The use of principal component analysis with multivariate curve resolution (PCA/MCR) revealed the extraction of texture components. The unusual hardness properties of one Au film (deposited from a 30% gold depleted BDT-200 bath) were found to be dependent on the (210) out-of-plane preferred orientation of the polycrystalline gold film. The progressive nucleation of Au crystallites during electro-plating has been tied to improved hardness properties of this film.

Anatase-to-Rutile Phase Transition in TiO2 Nanoparticles Irradiated by Visible Light

The light-induced phase transition of TiO2 nanoparticles from anatase to rutile structure is reported depending on the surrounding environment, the transition being accomplished under oxygen-poor conditions. The transition mechanism is interpreted in the framework of oxygen adsorption and desorption phenomena with the involvement of surface oxygen vacancies and F centers. It is shown that the observed phase transition is not thermally driven because the local temperature of the nanoparticles during irradiation is about 370 K (estimated through the Stokes to anti-Stokes Raman peaks ratio). On the contrary, the phase transition is initiated by intragap irradiation (with the exception of the red light one) that acts as TiO2 surface sensitizer, promoting the activation of the surface and the nucleation of rutile crystallites starting from two activated anatase neighboring nanoparticles.

Facile and effective promotion of β crystalline phase in poly(vinylidene fluoride) via the incorporation of imidazolium ionic liquids

AbstractIn this study, a facile and efficient protocol to enhance the β‐phase content of poly(vinylidene fluoride) (PVDF) is developed, in which the effect of room temperature ionic liquids (RTILs), including [1‐butyl‐3‐methylimidazolium (bmim)][PF6], [bmim][BF4], [bmim][FeCl4] and [bmim][Cl], on the crystallization behavior of PVDF is investigated. The resulting PVDF/RTIL hybrids are characterized by Fourier transform infrared (FTIR) spectroscopy, XRD, polarized optical microscopy (POM) and DSC. The FTIR spectroscopy and XRD results show that the fraction of β‐phase, F(β), is significantly enhanced by the incorporation of RTILs, specifically from 49.2% for neat PVDF to 92.6% for hybrid filled with 15 wt% [bmim][PF6]. The analysis of the crystallization behavior based on the DSC tests reveals that the degree of crystallinity increases with incorporation of RTILs, implying that RTILs could act as directing agents to facilitate the crystallization process, which is further evidenced by the POM results. In addition, the non‐isothermal crystallization kinetics of PVDF and PVDF/RTIL composites are investigated by means of DSC and the results indicate that the addition of the RTILs significantly influences the mechanism of nucleation and growth of PVDF crystallites. © 2013 Society of Chemical Industry

Ni Layer Thickness Dependence of the Interface Structures for Ti/Ni/Ti Trilayer Studied by X-ray Standing Waves

X-ray standing waves generated by periodic multilayers have been used to characterize the interface microstructures of Ti/Ni/Ti trilayers based on the X-ray reflectivity (XRR) and grazing incidence X-ray fluorescence (GIXRF) methods. For the Ni layer having thickness of 1.7 nm, it is observed that the roughness of Ti-on-Ni interface is 0.64 nm and that of Ni-on-Ti interface is 0.40 nm, which can be explained by an additional roughness on the Ti-on-Ni interface induced by the nucleation of Ni crystallites when the Ni layer thickness is at the amorphous-to-crystalline transition region. For the Ni layer thickness of 3.3 nm beyond this transition region, the roughness of Ti-on-Ni interface is 0.42 nm and Ni-on-Ti interface is 0.46 nm, which is consistent with the expectation on wetting and dewetting conditions.

Isothermal and nonisothermal crystallization of isotactic polypropylene/graphene oxide nanosheet nanocomposites

Isothermal and nonisothermal crystallization of isotactic polypropylene (iPP)/graphene oxide nanosheet (GONS) nanocomposites were investigated by differential scanning calorimetry, polarizing optical microscopy and synchrotron wide-angle X-ray diffraction. Nonisothermal crystallization measurement revealed that GONSs significantly elevated crystallization peak temperature of iPP. Nucleating activity of the nanocomposites containing 0.05 wt% and 0.1 wt% GONSs was 0.61 and 0.56, respectively, indicating strong nucleation ability of GONSs. Induction period and half crystallization time of nanocomposites was greatly reduced during isothermal crystallization process. Moreover, only 0.05 wt% and 0.1 wt% caused less temperature dependence of crystallization kinetics in a wide temperature range of 132–140 °C. A very low GONS loading provided sufficient surface area for crystallite nucleation. Polarized optical microscopy showed that nucleation density of nanocomposites was much larger than that of neat iPP, accelerating overall spherulite growth rate. Crystal structure of iPP was not affected by GONSs, confirming the α-nucleating ability of GONSs.

Framboidal and idiomorphic pyrite in the upper Maastrichtian sedimentary rocks at Gabal Oweina, Nile Valley, Egypt: Formation processes, oxidation products and genetic implications to the origin of framboidal pyrite

The upper Maastrichtian organic-rich sediments studied at Gabal Oweina, Egypt, are moderately enriched in syngenetic and diagenetic pyrite. Pyrite occurs mostly as layers or bands, group of lamina, lenses, diagenetic intercalated pockets, burrow fills and disseminated individual pyrite framboids and crystals within the host sediments. The pyritic thin bands and lamina consist mostly of unconsolidated to compact-oriented pyrite (oriented along the bedding planes) in gypsiferous–clayey matrix and less common as poorly oriented pyrite crystallites. In several cases, pyrite crystals of the latter type depict zoning, fracturing and micro-concretions. Pyritic burrow fills are composed mainly of pyrite, phosphatic ooids, microfossils, glauconitic grains, poorly graphitized carbon and native sulfur. Pyrite replaces minerals other than gypsum, sulfur or carbon. It also replaces microfossils thus turning some of the phosphatic ooids and microfossils to pyritized pseudomorphs. None of the studied phosphate ooids or framboids contains any mackinawite, pyrrhotite or greigite. Based on the microscopic and SEM observations of the micro-textures of disseminated pyrite found at Gabal Oweina section, four morphological forms of primary pyrite could be identified: (1) Grouped multiple-framboids; (2) Individual framboids; (3) Pyrite idiomorphic crystal overgrowths on framboids and (4) Single and aggregates of euhedral pyrite crystals. The multiple-framboid formation may have emerged from three successive processes: nucleation and growth of individual aggregates of the microcrystals to form combined micro-framboids (the growth of framboids); and followed by grouping of the several pyrite framboids. Direct pyrite nucleation (shell formation), crystallization, and aggregation processes might complete a single framboid. The disseminated single and aggregated euhedral pyrite crystals bear evidence indicating that their formation was via nucleation and growth of pyrite crystallites and their aggregation (to individual framboids), infilling (in the interstices by additional pyritic material), compaction and homogenization (of all these materials). Furthermore, we encounter for the first time in nature idiomorphic pyrite crystals that integrated numerous framboids, using them as their nucleation and growth sites without erasing or modifying their pristine morphology. Elemental sulfur contains minor concentration of Sb, Ni, Cd and Cu strongly suggesting their presence as submicron sulfide crystallite inclusions. SEM and microprobe investigations revealed that goethite is present as a weathering product in all morphological types of pyrite however; only an iron-sulfate (presumably melanterite) was encountered as oxidation product of the multi-framboids and the euhedral aggregate crystals. The upper Maastrichtian sediments not only contain a menagerie of pyrite morphologies but probably a complete record of the formation process and the geochemical conditions of the growth of framboids, single pyrite crystals, pyrite burrows, pyritized Mn–Fe-oxide framboids and finally their weathered products. The various pyrite forms strongly suggest a multistage process that led to their formation without any evidence for mackinawite, pyrrhotite or greigite, precursors. There is also no evidence in the Oweina sediments for post pyrite formation of mackinawite, pyrrhotite or greigite. The presence of elemental sulfur containing minor concentrations of Sb, Ni, Cd and Cu with pyrite framboids indicates that the pore solutions were geochemically supersaturated in sulfur thus inhibiting the crystallization of any iron sulfide other than pyrite. This cast considerable doubt on the assumed mackinawite or greigite precursors as prerequisite for formation of pyrite framboids.

The use of optical microscopy to examine crystallite nucleation and growth in thermally annealed plasma enhanced chemical vapor deposition and hot wire chemical vapor deposition a-Si:H films

We report a simple method to investigate crystallite nucleation and growth in stepwise, thermally annealed plasma enhanced chemical vapor deposition and hot wire chemical vapor deposition a-Si:H films. By confining film thicknesses to the range 500–4000 Å, optical microscopy in the reflection mode can be used to readily detect crystallites in the thermally annealed a-Si:H lattice. Measurements of the crystallite density versus annealing time for identically prepared films of different thickness show that the crystallite nucleation rate is smaller for thinner films, suggesting that crystallite nucleation is homogeneous, in agreement with previous results. A comparison of film nucleation rates with those obtained by other methods on identically prepared films shows excellent agreement, thus establishing the validity of the current technique. The potential effect of impurity (oxygen) incorporation during the stepwise annealing in air is shown not to affect crystallite nucleation and growth, in that SIMS oxygen profiles for stepwise versus continuous annealing show not only similar impurity profiles but also similar bulk impurity densities.

Time-Resolved Structural Evolution of Additive-Processed Bulk Heterojunction Solar Cells

Solution deposition using high-boiling-point additives such as octanedithiol (ODT) provides a simple and widely used fabrication option for improving the power conversion efficiencies of solar cells composed of narrow-band-gap conjugated polymer donor/fullerene acceptor blends. Previous examination of the resulting device active layers has shown that the use of additives influences the degree of phase segregation within the bulk heterojunction (BHJ) blend and also improves ordering within the polymeric domains. In this work, in situ grazing-incidence wide-angle X-ray scattering as a function of time was used to explore the dynamics of the BHJ evolution. These studies showed that a small percentage of ODT in chlorobenzene (CB) induced the nucleation of polymeric crystallites within 2 min of deposition, increased the orientational order of specific polymorphs, and promoted further crystallite nucleation over a period longer than 40 min after casting. Similar structural changes did not occur when the same BHJ blend was cast from pure CB.

The effect of annealing on the properties of Ga2O3 anodic films

The effect of annealing and oxygen plasma on the electrical characteristics of gallium oxide films has been investigated. Ga2O3 films with thicknesses of 200–300 nm were formed by the anodic oxidation of n-GaAs wafers with the donor concentration Nd = (1–2) × 1016 cm−3. It is shown that, after annealing at 900°C for 30 min, the gallium arsenide films contain only the β Ga2O3 phase. The effect of the duration of oxygen plasma treatment prior to annealing on the nucleation of β-phase crystallites with different orientations has been studied. It has been established that the electrical conductivity of the Ga2O3 films can be managed by annealing and variation in the duration of oxygen plasma treatment. It is shown that the response of the V/Ni-GaAs-Ga2O3-V/Ni structure to a mixture exhaled by a person depends on the value and sign of the potential on the control electrode.