Lower Crystalline Fraction Research Articles

Rigid amorphous fraction in isotactic polystyrene induced by cold crystallization: Characterization by differential scanning calorimetry and ellipsometry

AbstractRigid amorphous fraction (RAF) in bulk and thin‐film states of isotactic polystyrene (iPS) was studied as a function of crystalline fraction (CF). The CF increased with isothermal cold crystallization in initially amorphous, melt‐quenched bulk iPS samples. Assessed by differential scanning calorimetry, RAF in bulk iPS increased and then decreased with increasing crystallization time and CF. Specific RAF, that is, RAF/CF, decreased with increasing CF. Using ellipsometry, similar behaviors of RAF and specific RAF as functions of CF were observed in initially amorphous iPS thin films. These results indicate that specific RAF in iPS exhibits a continuous decrease from slightly below 2 at very low CF to ~0.2 at the maximum CF, independent of sample type and characterization method. This trend of decreasing specific RAF is consistent with a progression of crystal perfection during cold crystallization which is associated with lamellae thickening and decoupling between crystalline and amorphous regions. Qualitatively similar behaviors of RAF and specific RAF have been reported in other semi‐crystalline polymers and in our recent studies of syndiotactic polystyrene, indicating that this trend of decreasing specific RAF with increasing CF is general for many semi‐crystalline polymers. Finally, maximum CF is strongly reduced by nanoscale confinement in thin iPS films.

Development of rigid amorphous fraction in cold‐crystallized syndiotactic polystyrene films confined near the nanoscale: Novel analysis via ellipsometry

AbstractRigid amorphous fraction (RAF) in syndiotactic polystyrene (sPS) thin films was studied as a function of crystalline fraction (CF). In contrast to bulk sPS, confined films yield close‐to‐amorphous sPS, thus allowing for a complete description of RAF development. To study thin‐film samples, we developed an ellipsometry‐based method to quantify CF and RAF. Maximum crystallinity was thickness dependent, indicating a strong effect of near‐nanoscale confinement on CF. ~360‐nm‐thick spin‐coated films were isothermally cold crystallized to tune CF from 1% to a maximum of 12%. Combining RAF characterized at low CF here by ellipsometry with results from an earlier differential scanning calorimetry (DSC)‐based study on bulk sPS with higher CF, a complete picture was obtained of RAF development as a function of CF in cold‐crystallized sPS. RAF increases from ~4% at 1% CF to ~16% for 10% < CF < 25%; above 25% CF, RAF decreases monotonically with increasing CF, reaching ~7% at a maximum 50% CF. Specific RAF (=RAF/CF) decreases with increasing CF, consistent with increasing crystal perfection with isothermal cold crystallization. This ellipsometry method can be an important tool for evaluating RAF, mobile amorphous fraction (MAF), and CF in semi‐crystalline polymer films and for studying confinement effects on RAF, MAF, and CF.

Fe(III) reduction due to low pe+pH contributes to reducing Cd transfer within a soil-rice system

Effect of Fe redox state caused by low soil pe+pH levels on Cd uptake by rice is unclear. Rice grown in pots of Cd-contaminated paddy soil were subjected to different irrigation regimes: flooding, intermittent flooding (Int-FL), and sustained soil moisture at 70% water holding capacity (WHC). Results showed low pe+pH (5.52 and 7.09) in flooding treatment significantly increased relative abundances of Fe-reducing bacteria (FeRB) (6.29% and 4.51%), especially members within the Clostridium, Geobacter and Desulfuromonadia genera. Stimulation of FeRB activity induced Fe(III) reduction and increased Fe2+ content in flooded soils, which promoted Cd sequestration in low-crystalline fraction of IP (IP-Feh-Cd) and Cd bonded to amorphous Fe-oxides (amFeox-Cd). The 24.9–62.4% higher amFeox-Cd content was the important factor for 20.4–44.2% lower CaCl2-extractable Cd content in flooding treatment than those in other treatments. Soil submergence reduced Cd uptake by rice at tillering and booting stages, the critical periods of Cd transport in the soil-rice system, which was attributed to the increases in dissolved Fe2+ and IP-Feh-Cd contents and decrease in CaCl2-Cd content. Therefore, maintaining flooding during the tillering and booting stages may be an effective strategy to reduce Cd uptake by rice cultivated in Cd-contaminated soil.

Melting simulations of poly(ethylene oxide) nanocrystals in amorphous environments

The embedded model has been proposed for simulating melting process of the semicrystalline polymer with a low crystalline fraction. This approach is exemplarily applied to the multiscale coarse-grained (CG) model of poly(ethylene oxide) (PEO) of 14 monomers per chain. When continuously heated to a high temperature, the nanocrystal undergoes great conformational change and disassembly in order. While the specific volume and potential energy of the semicrystalline polymer system capture only glass-rubber transition of amorphous chains, the interaction energy or contact number between the nanocrystal and its amorphous environment allows for precisely identifying glass transition temperature (Tg) and equilibrium melting point (Tm) at the same time. Furthermore, the so-simulated Tg is insensitive to the lateral dimension of the nanocrystal whereas the so-simulated Tm linearly decrease with the inverse of the lateral dimension by the variant of Gibbs-Thomson equation. The extrapolated Tg and Tm for the nanocrystal with infinite lateral dimension compare well with the referenced data. The proposed scheme is quite competitive to the previous ones for a quick and accurate prediction of the Tm of a crystallizing polymer.

Effect of electrohydrodynamic technique as a complementary process for cellulose extraction from bagasse: Crystalline to amorphous transition

Electrohydrodynamic technique (EHD) has lately received considerable attention as an important non-thermal method used for food processing. In this work the effect of EHD on physicochemical properties of cellulose extracted from bagasse was investigated. A three-step process was chosen to extract cellulose fibers, which was performed under a high voltage electric field at the beginning of each step. The cellulose fibers were then characterized using Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Thermogravimetric Analysis (TGA). The morphological evaluation revealed that the cellulose fibers have smaller diameter and larger length compared to the microcrystalline cellulose. Moreover, according to the XRD patterns and FTIR spectrum, EHD-treated fibers mainly showed an amorphous structure, and contained a low crystalline fraction. The amorphous structure of cellulose fibers was confirmed by TGA analysis which revealed the presence of higher proportion of amorphous cellulose fraction causes early onset of thermal decomposition.

Thermal transport properties in amorphous/nanocrystalline metallic composites: A microscopic insight

In the last years new composite materials made of a metallic glass matrix with embedded nanocrystals have arisen as a promising alternative to the metallic glasses, due to their higher hardness. Although the effects of a partial nano-crystallization onto mechanical and magnetic properties have been widely investigated, nothing is known about thermal transport properties, interesting for recently proposed novel applications. Here we investigate how thermal transport is modified in presence of nanocrystalline inclusions in a Zr-based bulk metallic glass. By means of electric measurements and inelastic x ray scattering we are able to disentangle the effects of a partial nanocrystallization onto the two different contributions to heat transport, the electronic and the vibrational one. We show that no enhanced electrons or phonons scattering from the interfaces is observed, while the presence of crystalline nanoinclusions leads to an increase of both contributions, via an increased electric conductivity and transverse acoustic speed of sound. Surprisingly, while a gradual modification of the electric conductivity with the crystalline fraction is observed, even a low crystalline fraction is sufficient to modify elastic and vibrational properties. Our results indicate that low crystallinity composites are indeed promising alternative to both amorphous and crystalline alloys, thanks to their unique combination of high ductility, polycrystal-like vibrational properties and amorphous-like electric transport.

Investigation of nanoparticle agglomerates properties using Monte Carlo simulations

By accounting for realistic interparticle interaction energy of fine dry nanoparticles, an off-lattice Monte Carlo (MC) simulation approach is used to gain insight into such properties of agglomerates of nanoparticles with primary sizes ranging from 10nm to 100nm. This novel numerical approach allows for assessment of the mechanical properties and morphological features of the agglomerates. An interesting outcome is that the fractal dimension depends on the material properties represented via interaction energy. The agglomerate porosity increases with increasing agglomerate mass and may approach unity. With increasing Hamaker constant and fixed particle size, the agglomerates are characterized by a lower fractal dimension, higher packing porosity, higher mechanical strength, larger agglomeration size, and lower crystalline fraction. For a fixed Hamaker constant, agglomerates of the smaller primary particles exhibit a more compact packing structure, higher mechanical strength, smaller agglomerate size, and higher crystalline fraction. The local structure analysis indicates that for a fixed Hamaker constant, there are more particles in crystalline structure within the agglomerates constituted by the smaller primary particles. Likewise, for a fixed primary particle size, a lower Hamaker constant allows particles to configure into more stable agglomerate structures, thus providing useful insights into agglomerate morphology.

Studies on the Structural and Electrical Properties of Hexanoyl Chitosan/Polystyrene-based Polymer Electrolytes

Abstract The effect of TiO2 filler on the structure and electrical properties of hexanoyl chitosan/polystyrene-LiCF3SO3 electrolytes has been investigated by X-ray diffraction and impedance spectroscopy. The XRD results reveal that sample with lower crystalline fraction exhibits higher conductivity. The conductivity for filler-free polymer electrolyte is 7.21 x 10-5 S cm-1. The addition of 4 wt.% TiO2 increased the conductivity by one order of magnitude to 2.27 x 10-4 S cm-1. The ac conductivity of both filler-free and filler-added electrolytes follows the Jonscher's universal power law. The conduction mechanism of the filler-free and filler-added hexanoyl chitosan/polystyrene-LiCF3SO3 can be interpreted based on the overlapping large polaron tunneling (OLPT) and correlation barrier hopping (CBH) model, respectively.

The open-circuit voltage in microcrystalline silicon solar cells of different degrees of crystallinity

We have used a detailed electrical-optical computer model (ASDMP) in conjunction with the experimental characterization of microcrystalline silicon thin-film solar cells of different degrees of crystallinity (but having identical P- and N-layers) to understand the observed decrease of the open-circuit voltage with increasing crystalline fraction. In order to model all aspects of the experimental current density–voltage and quantum efficiency characteristics of cells having low (~ 75%) and high (over 90%) crystalline fraction, we had to assume both a higher mobility gap defect density and a lower band gap for the more crystallized material. The former fact is widely known to bring down the open-circuit voltage. Our calculations also reveal that the proximity of the quasi-Fermi levels to the energy bands in the cell based on highly crystallized (and assumed to have a lower band gap) microcrystalline silicon results in higher free and trapped carrier densities in this device. The trapped hole population is particularly high at and close to the P/I interface on account of the higher inherent defect density in this region and the fact that the hole quasi-Fermi level is close to the valence band edge here. This fact results in a strong interface field, a collapse of the field in the volume, and hence a lower open-circuit voltage. Thus a combination of higher mobility gap defects and a lower band gap is probably the reason for the lower open-circuit voltage in cells based on highly crystallized microcrystalline silicon.

Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements

The dielectric functions of amorphous and polymorphous silicon films prepared under various plasma conditions have been deduced from UV-visible spectroscopic ellipsometry measurements. The measured spectra have been firstsimulated by the use of the Tauc-Lorentz dispersion model and then the compositions of the films have been obtained by the use of the tetrahedron model combined with the Bruggeman effective medium approximation. This approach allows us to determine the hydrogen content, the crystalline fraction, and the void fraction of the films. This is particularly important in the case of polymorphous films in which the low crystalline fraction (below 10%) can only be detected when an accurate description of the effects of hydrogen on the dielectric function through the tetrahedron model is considered. The hydrogen content and film porosity deduced from the analysis of the spectroscopic ellipsometry measurements are in excellent agreement with the hydrogen content and film density deduced from combined elastic recoil detection analysis and Rutherford backscattering spectroscopy measurements. Moreover, despite their high hydrogen content (∼15%-20%) with respect to hydrogenated amorphous silicon films deposited at the same temperature (8%), polymorphous silicon films have a high density, which is related to their very low void fraction.

Structural and Electronic Properties of SiCl4-based Microcrystalline Silicon Films

ABSTRACTStructural and electronic properties of SiCl4-based microcrystalline silicon films were studied. A rather dense (non-porous) material structure is obtained near the transition to amorphous material, in particular at substrate temperatures of 250°C and above. Boron doping results in very high conductivity values while for phosphorus doping only lower values are reached. This latter effect is attributed to a different microstructure with lower crystalline fraction, higher hydrogen and chlorine content and increased porosity in highly phosphorus- doped material.

Hysteresis and thermal relaxation in random anisotropy systems

Recent experimental studies shown that relaxation and hysteresis phenomena at elevated temperatures measured on soft nanocrystalline materials (Finemet-type) with low crystalline fraction, cannot be described by a single activation volume. To clarify the nature of the magnetization processes in these materials we studied the behavior of clusters of grains with random anisotropy in which a direct correspondence between zero-temperature switching fields h c, energy barriers Δ F and activation moments Δ m, is involved, namely: Δ F∼Δ m 1/2; h c∼Δ m −1/2. We show that some aspects of the dynamics of field-rate-dependent hysteresis loops observed in real systems can be reproduced by choosing simple distributions of cluster size.

Ultrafast carrier dynamics in undoped microcrystalline silicon

We have studied ultrafast dynamics of photoexcited carriers in μc-Si:H by pump and probe laser spectroscopy. We have found that the dynamics of photoexcited carriers in μc-Si:H depend on the crystallinity of the material: in the samples with low crystalline fraction, the dynamics have a fast decay and resemble those in a-Si:H, while in the samples with high crystallinity the dynamics are slower and similar to those in c-Si. We have identified an intensity dependent bimolecular recombination in the samples with lower crystalline fraction (coefficient B=2×10 −10 cm 3 s −1 for deposition with silane dilution ratio ≈5% at a fixed power of 6 W), and no bimolecular recombination in the samples with high crystallinity.

Electronic Transitions in Mixed Phase Crystalline/Amorphous Silicon in the Low Crystalline Fraction Regime

AbstractAmorphous silicon films were prepared by dc reactive magnetron sputtering under conditions approaching the phase transition to microcrystallinity. Using TEM imaging these films were found to contain clusters of 5 to 50 nm sized Si crystallites embedded in an amorphous silicon matrix. Photocapacitance and transient photocurrent sub-band-gap optical spectra of this material appear to consist of a superposition of a spectrum typical of amorphous silicon together with an optical transition, with a threshold near 1. 1eV, that exhibits a very large optical cross section. This transition arises from valence band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. Using modulated photocurrent methods we have determined that these states also dominate the electron deep trapping in this material. We argue that these states arise from defects at the crystalline-amorphous boundary.