Pure Amorphous Phase Research Articles

Properties of CO2-cured cement incorporating fly ash and slag subjected to further water curing

In the present work, the synergetic effect of fly ash (FA) and ground granulated blast furnace slag (GGBS) on the compressive strength and microstructure development of mortars subjected to CO2 curing coupled with further water curing was investigated. The calcite content in pure cement was first increased and then decreased, while it decreased gradually in cement incorporating FA and GGBS during further water curing. Ettringite was interlocked with calcium carbonate and MC and was only observed in pure cement paste. Two distinct forms of C–S–H were observed in pure cement with one being interlocked with calcite, while the other one was pure amorphous phase. The incorporation of FA and GGBS showed a negative effect on the polymerization decrease of silica-rich phase. FA showed a negligible effect on the decrease of porosity, while GGBS had a positive effect on it. A positive correlation was found between the increase of compressive strength subsequent to water curing and three parameters, i.e., crystal size of calcite, carbonation degree and capillary pore percentage after CO2 curing, where the carbonation degree had a negative effect, while the other two parameters had a positive effect. The incorporation of GGBS and FA had a positive effect on the strength increase rate during further water curing. Notably, GGBS showed a more obvious effect on the strength development during further water curing and normal curing at later age. When GGBS content was lower than 10 % or cement content was lower than 10 %, the increase of FA content had a positive effect on the strength increase.

The synthesis and application of crystalline-amorphous hybrid materials.

Crystalline-amorphous hybrid materials (CA-HMs) possess the merits of both pure crystalline and amorphous phases. Abundant dangling bonds, unsaturated coordination atoms, and isotropic structural features in the amorphous phase, as well as relatively high electronic conductivity and thermodynamic structural stability of the crystalline phase simultaneously take effect in CA-HMs. Furthermore, the atomic and bandgap mismatch at the CA-HM interface can introduce more defects as extra active sites, reservoirs for promoted catalytic and electrochemical performance, and induce built-in electric field for facile charge carrier transport. Motivated by these intriguing features, herein, we provide a comprehensive overview of CA-HMs on various aspects-from synthetic methods to multiple applications. Typical characteristics of CA-HMs are discussed at the beginning, followed by representative synthetic strategies of CA-HMs, including hydrothermal/solvothermal methods, deposition techniques, thermal adjustment, and templating methods. Diverse applications of CA-HMs, such as electrocatalysis, batteries, supercapacitors, mechanics, optoelectronics, and thermoelectrics along with underlying structure-property mechanisms are carefully elucidated. Finally, challenges and perspectives of CA-HMs are proposed with an aim to provide insights into the future development of CA-HMs.

Evaluation of the Near-Zero Temperature Coefficient of Resistivity (NZ-TCR) of ALD TiSixn Films

Atomic Layer Deposition (ALD) of ternary TiSixN leads to nanocomposites of metallic TiN atomically mixed with insulating Si3N4. Formulating TiSixN films with various Ti:Si ratios lead to the emergence of a temperature regime where resistivity is independent of thermal drift, denoted as near-zero temperature coefficient of resistivity (nz-TCR). Further, the ease with which nanocomposites of TiSixN can be deposited using ALD offer precise tunability in Ti:Si ratio, thickness, mass density, crystallinity and electrical properties.Recently, our group explored TiSixN films deposited using a Eugenus® 300 mm commercial QXP mini-batch system by modulating the ratio of Ti and Si precursors with NH3 as a co-reactant. Si-content was varied from 0 at % (pure TiN) to 24.2 at % Si while maintaining thickness ~ 140 nm. The X-ray reflectivity and grazing incidence X-ray diffraction measurements showed a reduction in film density and transition from nano-crystalline to pure amorphous phase with increase in Si-fraction. Spectroscopic ellipsometry revealed the optical constants, composition, and electrical resistivities and were supported by X-ray photoelectron spectroscopy and electrical measurements. Room-temperature resistivity measurements show an increase in film resistivity with increasing at % Si. Temperature-dependent Van der Pauw measurements found a nz-TCR of -23 ppm K-1 in the temperature range of 298 K – 398 K and at 3.4 at % Si content.We have now discovered that an at % Si = 3.0% induces a nz-TCR of -5.7 ppm K-1 from 80 K – 420 K – one of the best reported nz-TCR values for ALD thin films. Fine tuning the at % Si in TiSixN films, possible only via ALD, significantly elongated the temperature window of nz-TCR behavior. Mapping the local conductivity of individual grains through conductive atomic force microscopy (c-AFM) indicated higher resistance at the grain boundaries. The local composition at the grain boundaries may play a major role in determining the nz-TCR behavior of TiSixN films. In addition, variable temperature Hall effect measurements were performed to provide deeper insights into the nz-TCR mechanism, decoupling carrier concentration from carrier mobility effects while determining film resistivity.Compared to other nz-TCR films, which are deposited using physical vapor deposition techniques, ALD based nz-TCR films presents a unique synthesis platform for interconnect technology in topologically complex, 3D devices, circuits and sensors that undergo large temperature variation during operation but need to maintain stability in their electrical characteristics.

Molecular dynamic characteristic temperatures for predicting metallic glass forming ability

We explore the use of characteristic temperatures derived from molecular dynamics to predict aspects of metallic Glass Forming Ability (GFA). Temperatures derived from cooling curves of self-diffusion, viscosity, and energy were used as features for machine learning models of GFA. Multiple target and model combinations with these features were explored. First, we use the logarithm of critical casting thickness, log10(Dmax), as the target and trained regression models on 21 compositions. Application of 3-fold cross-validation on the 21 log10(Dmax) alloys showed only weak correlation between the model predictions and the target values. Second, the GFA of alloys were quantified by melt-spinning or suction casting amorphization behavior, with alloys that showed crystalline phases after synthesis classified as Poor GFA and those with pure amorphous phases as Good GFA. Binary GFA classification was then modeled using decision tree-based methods (random forest and gradient boosting models) and were assessed with nested-cross validation. The maximum F1 score for the precision–recall with Good Glass Forming Ability as the positive class was 0.82±0.01 for the best model type. We also compared using simple functions of characteristic temperatures as features in place of the temperatures themselves and found no statistically significant difference in predictive abilities. Although the predictive ability of the models developed here are modest, this work demonstrates clearly that one can use molecular dynamics simulations and machine learning to predict metal glass forming ability.

Multiple phase transitions in Sc doped Sb2Te3 amorphous nanocomposites under high pressure

Subnanosecond switching speed from an amorphous state with stable crystal precursors to the crystalline state was recently achieved in amorphous Sc-doped Sb2Te3 (a-SST) phase change materials (PCMs), which is about two orders of magnitude faster than that in the well-studied Ge2Sb2Te5 and Ge1Sb2Te4 PCMs. However, the phase change mechanism and phase stability of a-SST remain unknown. Here, we prepared amorphous Sc0.3Sb2Te3 nanocomposites within a minute amount of face-centered-cubic (FCC) type nanograins embedded in the amorphous matrix. Using in situ high-pressure synchrotron X-ray diffraction, we found that nanograins were frustrated under high pressure and gradually dissolved into the matrix around 11.0 GPa. Beyond 11.0 GPa, the a-SST matrix transformed into a uniform high density metallic like amorphous state with a five orders of magnitude drop in electric resistivity compared to the pristine materials. When further compressed to 23.97 GPa, the high density amorphous (HDA) phase switched into a body-centered-cubic (BCC) high-pressure structure, a different phase from the ambient pressure crystalline structure. Upon decompression back to ambient pressure, a pure amorphous phase was sustained. The present study provides additional insight into the phase change mechanism of amorphous nanocomposites.

Prediction of effective elastic properties of a polypropylene component by an enhanced multiscale simulation of the injection molding process

In the injection molding process of semi-crystalline polymers, the melt undergoes a complex deformation and cooling history which results in an inhomogeneous distribution of spherulites in the component. To evaluate these inhomogeneities in an isotactic polypropylene (α-iPP) part, an integrated multi-scale simulation approach has been developed in Laschet et al. (2017a). This approach combines a macroscale mold filling and heat transfer analysis with a crystallization model at the microscale and a two-scale homogenization scheme in order to calculate microstructure dependent elastic properties of the PP component. This approach is extended here by adding some features improving the local Hooke matrix predictions. A link between Molecular Dynamics (MD) simulations and the homogenization scheme is achieved in order to obtain unknown properties of the pure amorphous and crystalline phases. Next, the accuracy of the crystallization model is improved by considering not only isothermal but also athermal nucleation. The spherulite growth is now evaluated by a continuous cellular growth algorithm. Then, several numerical improvements have been integrated in the homogenization tool. The main ones are the implementation of stabilized mixed finite elements to handle accurately the quasi-incompressibility of the amorphous phase and the application of periodic boundary conditions via a master-slave projection algorithm. Eventually, the distribution of effective mechanical properties over the component thickness is compared for different crystallization scenarios with measured elastic moduli.

Predicted amorphous solubility and dissolution rate advantages following moisture sorption: Case studies of indomethacin and felodipine

Water is often readily absorbed by amorphous compounds, lowering their glass transition temperature (Tg) and facilitating their recrystallization (via nucleation-and-growth). At the same time, the increase in moisture content translates to a decrease in both the thermodynamic solubility and intrinsic dissolution rate, as compared to the corresponding dry (pure) amorphous phase, e.g. see [Murdande SB, Pikal MJ, Shanker RM, Bogner RH. 2010. Solubility advantage of amorphous pharmaceuticals: I. A thermodynamic analysis. J Pharm Sci 99:1254–1264.]. In the case of pure indomethacin and felodipine, the solubility advantage of each amorphous phase over its crystalline counterpart were previously determined to be 7.6 and 4.7, respectively, using a new methodology together with basic calorimetric data taken from the literature. Herein, we demonstrate that, theoretically, following the uptake of just ∼0.5% w/w water, the solubility ratios decrease to 6.9 and 4.5, in the same order. Moreover, as the predicted intrinsic dissolution rate (based on the Noyes-Whitney equation) is directly proportional to the solubility advantage of a given amorphous-crystalline pair, it decreases proportionately upon moisture uptake. Applying the methodology presented herein, one can directly predict the extent of Tg-lowering observed at any moisture content, for a given amorphous phase. Knowing that value, it is possible to estimate the relative decrease in the solubility and/or intrinsic dissolution rate of the plasticized phase compared to the pure glass, and vice-versa.

Effect of pressure on the structure of Ti75Al25 alloy during rapid-quenching process

The structures of the Ti75Al25 alloy during rapid-quenching with and without external pressure are investigated by using molecular dynamic techniques. The amorphous phase can be obtained at the cooling rate 10.0 K/ps without pressure. The alloy is composed of crystal and amorphous phase at the cooling rate 0.1 and 1.0 K/ps without pressure, but the pure amorphous phase can be formed when the pressure exceeds the critical value. The critical pressure is about 20 and 30 GPa when the cooling rate is 0.1 and 1.0 K/ps, respectively. H-A indices analysis indicates that high pressure favors the formation of the ideal icosahedral structures in the amorphous Ti75Al25 alloy, and the content of 1551 bond-type can reach near to 50% when the pressure is 30 GPa. The amorphous state can be maintained if the external pressure is removed from the alloy step-by-step. The content of 1551 bond-type decreases with the deceasing of the pressure, but the 1541 and 1431 bond-types increase in this process.

Element-specific amorphization of vacancy-ordered GeSbTe for ternary-state phase change memory

GeSbTe alloys have the ability of rapidly transforming between amorphous and crystalline phases. Therefore, they can be used in the non-volatile phase change memory. Recently, a vacancy-ordered cubic Ge2Sb2Te5 (VOC GST) phase change material where the vacancies are highly ordered in the (111) plane, has been experimentally demonstrated by STEM. However, studies are mainly on the structural characterization, rather than on the phase change behavior and possible applications of the VOC GST. Here, using first-principles molecular dynamic simulations, we study the melt-quenched amorphization process and its possible applications. We find that the VOC GST exhibits a quasi-two-dimensional amorphization process that is triggered by the diffusion of Ge atoms but not others. A partial amorphous (P-amor) phase is obtained, which can act as an intermediate state between the pure amorphous and pure crystalline phases for possible ternary-state data storage.

Raman and spectroscopic ellipsometry studies of a-Si:H thin films on low-cost photo paper substrate

Photo papers are very light weight, cheapest easily available and compatible with the roll-to-roll printing process. Here we report hydrogenated amorphous silicon (a-Si:H) thin films deposited on photo paper by plasma enhanced chemical vapor deposition (rf-PECVD) technique. A series of intrinsic a-Si-H thin films were deposited by varying the substrate temperature (Ts) between 70 °C – 150 °C. Raman measurement atdifferent positions on the film corresponds to multiphase (amorphous as well as nanocrystalline phase) nature of films. The film having nanocrystalline phase, exhibit higher bandgap and higher photosensitivity (∼ 6 orders of magnitude) as compared to films with pure amorphous phase. The optoelectronic properties of these films are comparable to those on conventional corning 1737 glass substrate.

The amorphous state: first-principles derivation of the Gordon-Taylor equation for direct prediction of the glass transition temperature of mixtures; estimation of the crossover temperature of fragile glass formers; physical basis of the "Rule of 2/3".

Predicting the glass transition temperature (Tg) of mixtures has applications that span across industries and scientific disciplines. By plotting experimentally determined Tg values as a function of the glass composition, one can usually apply the Gordon-Taylor (G-T) equation to determine the slope, k, which subsequently can be used in Tg predictions. Traditionally viewed as a phenomenological/empirical model, this work proposes a physical basis for the G-T equation. The proposed equations allow for the calculation of k directly and, hence, they determine/predict the Tg values of mixtures algebraically. Two derivations for k are provided, one for strong glass-formers and the other for fragile mixtures, with the modeled trehalose-water and naproxen-indomethacin systems serving as examples of each. Separately, a new equation is described for the first time that allows for the direct determination of the crossover temperature, Tx, for fragile glass-formers. Lastly, the so-called "Rule of 2/3", which is commonly used to estimate the Tg of a pure amorphous phase based solely on the fusion/melting temperature, Tf, of the corresponding crystalline phase, is shown to be underpinned by the heat capacity ratio of the two phases referenced to a common temperature, as evidenced by the calculations put forth for indomethacin and felodipine.

Practical Estimation of Amorphous Solubility Enhancement Using Thermoanalytical Data: Determination of the Amorphous/Crystalline Solubility Ratio for Pure Indomethacin and Felodipine

Use of amorphous phases can mitigate the low in vivo exposures of poorly soluble, crystalline active pharmaceutical ingredients. However, it remains challenging to accurately predict the solubility enhancement offered even by a pure amorphous phase relative to the crystalline form. In this work, a methodology is presented that allows estimation of the amorphous:crystalline solubility ratio, α, using only measured thermodynamic quantities for each of the pure phases. With this approach, α values of 7.6 and 4.7 were calculated for indomethacin and felodipine, respectively, correlating more closely than previous predictions with the experimentally measured values of 4.9 and 4.7 reported in the literature. There are 3 key benefits to this approach. First, it uses simple mathematical functions to more precisely relate the temperature variations in the heat capacity (Cp) to allow a more accurate estimation of the configurational energy difference between the 2 phases, whereas traditional models typically assume that Cp of both phases are constant(s). Second, the Hoffman equation is leveraged in translating the free energy of crystal lattice formation to the actual temperature of interest (selected to be 25°C/298K in this work), again, for better accuracy. Finally, as only 2 modulated differential scanning calorimetry scans are required (one for each phase), it is attractive from an experimental simplicity standpoint.

The effect of heat treatment on the mechanical and structural properties of one-part geopolymer-zeolite composites

This contribution presents the results of structural and compressive strength investigations on cured and high-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens were synthesized from two different silica sources, sodium aluminate and water. The phase content as well as the compressive strength of the cured composites varied depending on the starting mix-design and the silica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reaction products. One of the silica feedstocks yielded significantly higher compressive strength (19MPa), while the other one appears to cause less variation in phase content. Strength testing indicated an improvement on heating up to 200–400°C (28MPa) followed by a moderate decrease up to 700°C. Above 700°C the systems underwent new phase formation and shrinkage (volume decrease) deformations. After exposure at 1000°C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexagonal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the high temperature phase transformations varied.

Structure-dependent size effects in CuTa/Cu nanolaminates

CuTa monolayers with different Ta contents (34at% and 37at%) and CuTa/Cu multilayers with varying nanoscale Cu layer thickness (3–24nm) were prepared by magnetron sputtering. Their microstructure and mechanical properties were investigated by means of high resolution transmission electron microscopy (HRTEM) and nanoindentation testing. The microstructure of CuTa34 was pure amorphous phase while that of CuTa37 was amorphous phase embedded with nanoparticles. Adding Cu layers into the two different CuTa monolayers led to CuTa-Cu interfaces with different microstructures. For CuTa34/Cu, the interface was relatively straight and the Cu layers exhibited textured growth. For CuTa37/Cu, the interface was wavy and GBs (grain boundaries) were formed in the Cu layers. To investigate the influence of the two different microstructures on multilayer deformation, the residue indentation morphologies of CuTa/Cu were observed under scanning electronic microscope (SEM). For both CuTa34/Cu and CuTa37/Cu, SBs (shear bands) could be effectively inhibited if the Cu layers had a proper thickness. Dominant deformation mechanisms and size effects were proposed and discussed for CuTa/Cu thin films possessing different interfaces and Cu layer structures.

Determination of the Physical Properties of Oil Sands Components using Scanning Probe Microscopy

ABSTRACTAtomic force microscopy is employed to study the structural changes in the morphology and physical characteristics of asphaltene aggregates as a function of temperature. The exotic fractal structure obtained by evaporation-driven asphaltene aggregates shows an interesting dynamics for a large range of temperatures from 25°C to 80°C. The changes in the topography, surface potential and adhesion are unnoticeable until 70°C. However, a significant change in the dynamics and material properties is displayed in the range of 70°C - 80°C, during which the aspahltene aggregates acquire ‘liquid-like’ mobility and fuse together. This behaviour is attributed to the transition from the pure amorphous phase to a crystalline liquid phase which occurs at approximately 70°C as shown by using Differential Scanning Calorimetry (DSC). Additionally, the charged nature of asphaltenes and bitumen is also explored using kelvin probe microscopy. Such observations can lead to the development of a rational approach to the fundamental understanding of asphaltene aggregation dynamics and may help in devising novel techniques for the handling and separation of asphaltene aggregates using dielectrophoretic methods.

All-Atom Molecular Dynamics Simulation of Temperature Effects on the Structural, Thermodynamic, and Packing Properties of the Pure Amorphous and Pure Crystalline Phases of Regioregular P3HT

Molecular dynamics simulations have been carried out to probe chain self-organization and structure in regioregular poly-3-hexylthiophene (Rr-P3HT). Taking into account recent experimental findings that provide evidence for a semicrystalline ordering in P3HT where crystalline lamellae are periodically separated by interlamellar amorphous zones, we have separately simulated the pure crystalline and the pure amorphous phases of Rr-P3HT, and studied the dependence of their conformational and configurational properties on temperature (from 225 to 600 K). All simulations have been carried out with the very accurate, all-atom Dreiding force-field. In the pure crystalline phase, the system is found to adopt a noninterdigitated and tilted structure irrespective of temperature and chain molecular weight (Mw) consistent with the results of XRD measurements and other theoretical works. Increasing the temperature did not seem to dramatically affect the interchain distance inside the crystal. Temperature therefore sho...

Plasticity of semicrystalline polyethylenes viewed through the prism of thermodynamics

AbstractThermodynamic characteristics such as mechanical work Wdef and heat Qdef of plastic deformation were measured at room temperature for several non‐oriented linear high and ultrahigh molecular mass polyethylenes (PEs). The characteristics were registered simultaneously at room temperature active uniaxial compressive loading in the strain interval εdef = 0–50% and rate 4 × 10−2 min−1. An isothermal Calvet‐type deformation calorimeter was used for the measurements. Changes of the internal energy ΔUdef stored by deformed samples were calculated from Wdef and Qdef according the first law of thermodynamics. It appears that all thermodynamic quantities linearly depends on degree of crystallinity χ = 0.5–0.9 (DSC) at conditions of the study. Such behavior of Wdef, Qdef, and ΔUdef had permitted an extrapolation of measured quantities to crystallinities χ = 0.0 (pure amorphous phase) and χ = 1.0 (pure crystalline phase) and determination of deformation thermodynamic characteristics for each of them. Both phases participate into Wdef. It appears that the work W, necessary to deform PE crystallites is considerably higher than W, the work necessary to deform the amorphous phase. At εdef ≤ 30% W is 3–4 times higher than W and about two times higher at higher strains. From W and W stress–strain curves for both phases of PE were withdrawn. Deformation heat of the amorphous phase Q is orders of magnitude lower than Q. It reflects the entropic nature of deformation of rubbery amorphous phase of PEs at low εdef. The Q originates from a friction during glide of dislocations trough crystallites. Interesting behavior shows the stored energy of cold work ΔUdef = f(χ). At strains εdef ≤ 30%, the stored energy ΔU is a little lower than ΔU. However, ΔU becomes higher than ΔU at εdef >30%. The ratio ΔUdef/Wdef = f(εdef) was constructed also. The ratio gives the fraction of Wdef, which is transformed into the stored energy of cold work ΔUdef at loading. Behavior of several materials: glassy polymers, PE, and crystalline metals were compared in terms of the ratio. At elastic process, the ratio ΔUdef/Wdef tends to unity for all the materials. Whole Wdef in this case is converted into ΔUdef. With εdef growth dissipative processes appear and deformation heat is evolved. The ratio tends to ΔUdef/Wdef < 1. Comparison of three mentioned above materials show, that critical stage of their deformation kinetics is nucleation of the inelastic strain carriers (dislocations in crystals, for example). Initiation is completed very early (at εdef ≤ εy, the yield strain) for crystalline metals and about 92–98% of the expended Wdef becomes converted into deformation heat at εdef ≥ εy. Plasticity proceeds differently in glassy polymers. Initiation stage continues in glasses for a high strain εdef level, usually higher than εy. The curve ΔUdef/Wdef = f(εdef) for PEs is located between curves characteristic for metals and glassy polymers. Initiation of PE plasticity becomes completed at εdef ≈ 10–20%. At εdef >30% the ratio does not depends on εdef and stay constant on the level 0.35–0.55 for different PEs. The nature of such saturation is not understood yet. Thermally stimulated recovery of residual strains εres stored in deformed and unloaded PEs at different εdef was measured also. The rate recovery curves dεres/dT show two separate peaks, one with maximum at Tm and the other with maximum much below Tm. Integration of both gives amount of εres accumulated in amorphous phase and crystallites of PE at different applied strains. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Photoluminescence from SiOx layers containing amorphous silicon nanoparticles

AbstractThe article summarizes the main results of a systematic study on the preparation of amorphous SiOx thin films with various contents (1.1≤ x ≤1.7) by thermal evaporation of SiO in vacuum, growth of amorphous Si nanoparticles by furnace annealing at 973 K for various times (5–200 min) and investigation of photoluminescence (PL) from the annealed films. Infrared (IR) transmission measurements prove the annealing‐induced phase separation. Raman scattering data indicate the formation of pure amorphous Si phase. Intense room‐temperature PL is detected from films with x ≥ 1.5, visible with a naked eye. Two PL bands are resolved, peaked at around 2.3 eV and in the range 1.6–2.2 eV. The former does not shift appreciably with varying annealing time and initial film composition. It is related to radiative recombination via defect states at the a‐Si/SiOx interface. The maximum of the low‐energy band shows a gradual red shift with decreasing x and increasing annealing time, i.e., with increasing nanoparticle size. This is related to carrier recombination in amorphous Si nanoparticles. Investigations of thermal quenching of the PL confirm the last assumption.magnified image Room‐temperature PL spectra of SiOx films with four different compositions annealed in an Ar atmosphere at 973 K for 60 min. The dash‐dot spectrum was measured on a film with x = 1.15 annealed for 30 min at 523 K.

The characterization of hydrogenated amorphous silicon and epitaxial silicon thin films grown on crystalline silicon substrates by using spectroscopic ellipsometry

Two series of silicon films on c-Si substrates with different thicknesses are deposited by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD). The structures of samples are investigated by spectroscopic ellipsometry (SE) with fixed angle. The results show that the films are all amorphous for the first series of samples. In such a case an abrupt a-Si:H/c-Si heterojunction is formed which is beneficial for the passivation of the interface of HIT solar cell. For amorphous silicon films, the fitting result is acceptable by using Tauc-Lorentz Genosc model. For the second series, the films are of epitaxial Si at the initial deposition stages, the amorphous fraction increases with the increase of thickness. When the film thickness reaches a critical value of 46 nm, a transition to pure amorphous phase occurs. The epitaxial film shows excellent fitting by using the effective medium approximation (EMA) model under the assumption of the mixture of c-Si phases and a-Si:H. However, we obtain better fitting result by using a three-layer model, whose bulk layer is divided into EMA layer and a-Si layer after the transition to pure amorphous phase. This study indicates that the SE analysis, operated in different models, is effective to characterize different structures of silicon films on c-Si substrate.

Influence of crystallization-induced amorphous phase confinement onα- andβ-relaxation molecular mobility in parylene F

The molecular mobility of cooperative segmental (α-process) and local (β-process) motions in semicrystalline fluorinated parylene (PA-F) films has been studied using broadband dielectric spectroscopy in a wide temperature range. Particularly, the α-relaxation is, for the first time in a semicrystalline polymer, probed well above the glass transition temperature (∼10Tg) based on the PA-F strong difference between Tg and the crystallization temperature (Tc ∼ 16Tg). The influence of the amorphous phase confinement on the chain dynamics, induced by increasing crystallinity, is also explored. Thus, in the range of Tg, the α-relaxation is described by two crossover Vogel-Fulcher-Tamman characteristics, and the high temperature one presents an exacerbated low fragility. The space confinement of the amorphous regions, as characterized by x-ray diffraction, shows an important mobility restriction of both the α- and β-relaxations. The β-process, which has been related to CF2 group local motions, does not present a modification of its activation energy (Ea ∼ 30.8 kJ mol−1) with confinement, showing that it happens in the pure amorphous regions. The dielectric strength analysis of each process, through the Onsager-Kirkwood-Fröhlich (OKF) theory, has demonstrated that a rigid amorphous phase is strongly involved in the very high temperature range well above Tg. In the range around Tg, a peculiar behavior of the low temperature α-relaxation dielectric strength is reported, in agreement with the OKF temperature decreasing dependency that has been related to cooperative rearranging regions in the pure amorphous phase. The disappearance of the α-relaxation with the amorphous phase confinement leads to a transformation from 2D to 3D crystallite arrangements of the PA-F chains in correlation with the formation of spherulitic structures.