Amorphous Volume Research Articles

Structure versus properties relationship of poly(lactic acid). I. Effect of crystallinity on barrier properties

AbstractPoly(lactic acid) is at present the most promising and commercially available bio‐based and biocompostable (bio)plastic. The properties of PLA, however, are rather poor, notably its low glass transition temperature Tg of ∼55 °C, its intrinsic low crystallization rate and the limited barrier properties with respect to water, oxygen and carbon dioxide in comparison with to oil‐based counterparts, notably polyesters (PET). In this manuscript we address the question of what could ultimately be achieved in terms of barrier properties when PLA is crystallized fully. In fact, this question is rather academic in view of the long crystallization/annealing times needed to obtain maximum crystallinity but serves the purpose of better understanding current limits and notably a better understanding of the role of crystallinity on the barrier properties. A correlation is made between crystallization, various morphological parameters and the barrier properties of PLA. Crystallization of PLA causes a decrease of the oxygen permeability, but not in linear proportion with the decrease in amorphous volume. We explain this in terms of influence of the space filling and the inner crystalline structure on gas transport properties. Furthermore we suggest that the presence of a rigid amorphous fraction with lower density can have strong impact on PLA gas permeability, in particular on the gas solubility coefficient of PLA. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2247–2258, 2009

Internal Strain Distribution in Freestanding Porous Silicon

Elastic properties of freestanding porous silicon layers fabricated by electrochemical anodization were studied by Raman scattering. Different anodization currents provided different degrees of porosity in the nanometer scale. Raman lines corresponding to the longitudinal optical phonons of crystalline and amorphous phases were observed. The amorphous volume fraction increased and the phonon frequencies for both phases decreased with increasing porosity. A strain distribution model is proposed whose fit to the experimental results indicates that the increasing nanoscale porosity causes strain relaxation in the amorphous domains and strain buildup in the crystalline ones. The present analysis has significant implications on the estimation of the crystalline Si domain’s characteristic size from Raman scattering data.

Characterization of phase change memory materials using phase change bridge devices

Having become one of the most promising candidates for future nonvolatile memory applications, phase change random access memory is now driving an intensive search for phase change materials with optimized properties. In this paper, phase change bridge devices are utilized as a unique material test vehicle, allowing systematic extraction of crucial material parameters such as resistance contrast, switching speed, threshold voltage, and set and reset power. Bridge devices fabricated from undoped Ge15Sb85 are presented that reproducibly switch between set and reset states with one decade resistance contrast using current pulses as short as 10 ns. Since devices are fabricated in the amorphous-as-deposited phase, an interesting intermediate device state can be produced, which we attribute to a crystalline center region that fails to completely bridge the amorphous active device volume. The subtle differences between the amorphous-as-melt-quenched and amorphous-as-deposited phases are explored using both bridge devices and optical experiments. It is shown that the large difference in crystallization times between these two amorphous states observed in laser experiments is reflected also in device operation.

Magnetocaloric response of Fe75Nb10B15 powders partially amorphized by ball milling

The magnetocaloric response of mechanically alloyed Fe75Nb10B15 powders was studied for samples with different amorphous and nanocrystal volume fractions. Thermomagnetic properties scale following a Ω3 law for different milling processes, where Ω is the milling frequency. Curie temperature of the amorphous phase increases as the amorphous fraction increases due to its progressive enrichment in B. The peak magnetic entropy change, |ΔSMpk|, as well as the refrigerant capacity increase with increasing amorphous fraction. The field dependence of |ΔSMpk| can be explained by the multiphase character of the studied samples.

Correlation between Structure and Vapor Sorption in Semicrystalline Linear Polyethylene: One Dimensional Nano-Swelling Measured Using in Situ Vapor Sorption Small Angle Neutron Scattering (iVSANS)

Changes in the nanoscale structure of semicrystalline polyethylene (PE) resulting from the sorption/diffusion of hexane-d14 vapor have been observed and quantified by in situ vapor sorption small-angle neutron scattering (iVSANS). We find a linear correlation between vapor sorption/diffusion and nanoexpansion of the amorphous phase within the lamellae confined in the spherulite domains. The expansion depends on the degree of crystallinity and spherulite perfection. We measured the structure-based sorption kinetics and isotherms of samples with two different crystalline volume fractions (ϕvc). The low crystalline polyethylene shows a larger expansion in the long period, L, than the high crystalline polyethylene. Low crystalline PE shows a Fickian-like diffusion profile with sorption time while the high crystalline PE (ϕvc = 78%) shows a S-shape diffusion profile (non-Fickian) at P/P° = 0.95. The diffusion coefficient increases with relative vapor pressure (P/P°) in both materials. The apparent diffusion coefficient in the low crystalline polymer is approximately a factor of 2 higher than that of the more crystalline polymer. On the other hand, effective diffusion coefficients, Deff’s, normalized with the amorphous volume fractions at each vapor activity are identical regardless of crystallinity. This indicates that amorphous volume fraction is the predominant factor that controls transport properties, at least for small diffusing molecules. Furthermore, the one-dimensional expansion is reversible during cyclic sorption and desorption processes, suggesting that the crystals were not destroyed unlike the inference of a previous study.(1)

Chemical and physical properties of plasma slags containing various amorphous volume fractions

In this study, municipal solid waste incinerator fly ash was vitrified using a plasma torch. The fly ash contained rich Ca, causing a high basicity of 2.43. Pure quartz was used as an additive to adjust the basicity. BET surface area analysis, X-ray diffraction analysis, and a scanning electron microscope were used to examine the physical properties of slags. The chemical stability and the acid resistance of slags were evaluated using the toxicity characteristics leaching procedure and tests of acid bathing. The results indicate that the plasma torch effectively vitrified the fly ash. Anthropogenic metals with low boiling points, such as Cd, Pb, and Zn, were predominately vaporized into flue gas. Most of the metals with high boiling points, such as Cr, Cu, and Mn, remained in the slag. After the vitrification, hazardous metals were noticeably immobilized in all slags. However, the slags with higher amorphous volume fractions were more effective in metal immobilization and in resisting acid corrosion. This indicates that SiO 2 enhanced the formation of the glassy amorphous phase and improved the resistance of acid corrosion and the immobilization of hazardous metals.

Predict the glass transition temperature of glycerol–water binary cryoprotectant by molecular dynamic simulation

Vitrification is proposed to be the best way for the cryopreservation of organs. The glass transition temperature ( T g) of vitrification solutions is a critical parameter of fundamental importance for cryopreservation by vitrification. The instruments that can detect the thermodynamic, mechanical and dielectric changes of a substance may be used to determine the glass transition temperature. T g is usually measured by using differential scanning calorimetry (DSC). In this study, the T g of the glycerol-aqueous solution (60%, wt/%) was determined by isothermal-isobaric molecular dynamic simulation (NPT-MD). The software package Discover in Material Studio with the Polymer Consortium Force Field (PCFF) was used for the simulation. The state parameters of heat capacity at constant pressure ( C p), density ( ρ), amorphous cell volume ( V cell) and specific volume ( V specific) and radial distribution function (rdf) were obtained by NPT-MD in the temperature range of 90–270 K. These parameters showed a discontinuity at a specific temperature in the plot of state parameter versus temperature. The temperature at the discontinuity is taken as the simulated T g value for glycerol–water binary solution. The T g values determined by simulation method were compared with the values in the literatures. The simulation values of T g (160.06–167.51 K) agree well with the DSC results (163.60–167.10 K) and the DMA results (159.00 K). We drew the conclusion that molecular dynamic simulation (MDS) is a potential method for investigating the glass transition temperature ( T g) of glycerol–water binary cryoprotectants and may be used for other vitrification solutions.

Size Dependent Photoluminescence of Si Nano-Crystals Embedded in Amorphous Silicon

This paper presents the results of PL spectrum studies for Si nano-crystallites embedded in amorphous silicon matrix. Investigated layers were deposited by the hot-wire CVD method on glass substrates at the wafer temperature 300°C and different filament temperatures from the range 1650-1950°C. It was shown that variation of temperatures of filament (hot-wire) allows to produce the films with desirable parameters. Using of X-ray diffraction and photoluminescence methods the correlation between some photoluminescence bands and the sizes of Si nano-crystallites as well as the amorphous phase volume was shown. The nature of light emission is discussed.

The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition

Thin silicon films were deposited by the plasma-enhanced chemical vapor deposition using standard 13.6 MHz radiofrequency gas discharge in silane diluted by hydrogen. The deposition conditions were kept constant for all samples, with the exception of only one parameter: the degree of dilution was varied from low values that produce amorphous layers up to the high dilution that resulted in a high degree of crystalline fraction. The structural properties of the samples were analyzed by Raman and grazing incidence small angle X-ray scattering (GISAXS) while direct current (DC) dark conductivity was measured by standard methods. The ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as the ratio between crystal and amorphous volume fraction while the shift of the TO peak position was used for the estimation of the crystal size. By increasing the working gas dilution, the crystalline fraction grew from 0% to 40% and the average individual crystal size increased from 2 to 10 nm. The size of the “particles”, estimated by GISAXS using the Guinier approximation, varied from 2 to 4 nm. For a lower working gas dilution, the “particles” detected by GISAXS were spherically symmetric and showed no difference between near surface and “bulk” of the film. For a higher dilution, the particles became asymmetric and larger closer to the surface, which indicates columnar growth. The DC dark conductivity increased exponentially with the crystalline fraction, except for a very low crystal to amorphous volume ratio where the conductivity was larger, probably due to a better ordering of the amorphous phase in the vicinity of the amorphous to crystalline transition.

Polymer property modeling using grid technology for design of structured products

Property prediction for a given polymer structure using group contribution models require that the structure can be fully represented by groups with well-defined contributions for that particular property. Frequently this cannot be accomplished. To overcome this limitation a group contribution + approach is developed here, for polymers. With this approach the missing groups are created and their contributions are predicted using zero- and first-order connectivity indices. To minimize the time spent on computational aspects of model development, grid technology is used in this work. Property prediction methods for linear polymers are presented for the amorphous and crystalline volumes, glass transition temperature, solubility parameter and refractive index.

Stability of microcrystalline silicon solar cells with HWCVD buffer layer

Microcrystalline silicon solar cells deposited by VHF-PECVD with or without HWCVD grown p/i interface buffer layer were investigated. We studied long-term stability under storage in ambient atmosphere and performed light soaking experiments. Cells with i-layers covering a wide range of crystalline volume fractions were studied. All cells were stable or degraded slightly after storage for 2 years in air, regardless of crystalline volume fraction or presence of p/i buffer interface. Upon light soaking all cells show efficiency degradation to more or less extent depending on crystal volume fraction of the i-layer and the presence of the buffer layer: the solar cell with high crystal volume fraction are nearly stable, cells with high amorphous volume fraction degrade by up to 20%. The solar cell with HWCVD buffer layer shows better stability in the high efficiency range of relative efficiency degradation typically less than 10% after 1000 h AM 1.5 light soaking. The efficiency degradation is mainly caused by Voc and FF deterioration while Jsc is almost stable.

Effect of cooling rate and basicity during vitrification of fly ash: Part 2. On the chemical stability and acid resistance of slags

The object of this study is to investigate how the cooling rate and the basicity during the vitrification process govern the chemical stability and acid resistance of slags. In this experiment, the incineration fly ashes with various basicities were vitrified at 1450 °C and cooled down by air cooling and water quenching, respectively. The amorphous volume fractions (AVF) in slags were estimated by X-ray diffraction analysis with the addition of an internal standard. Scanning electron microscopy was applied to qualitatively examine the microstructures of the original and the acid-immersed slags. It was verified that the addition of SiO 2 and water quenching both significantly affected the crystalline characteristics of the slag. When the basicity was >0.990, the AVF of slags was approximately equal regardless of the cooling rate. When the basicity was <0.674, water quenching greatly enhanced the formation of glassy amorphous phase and the immobilization of metals in slags. In contrast, a lower basicity (<0.511) is required if one wants to vitrify fly ash into slags and cool it down by air. As a whole, the glassy amorphous structure presented better chemical resistance than the crystalline structure to decomposition by an acid.

Encapsulation Behaviors of Metals in Slags Containing Various Amorphous Volume Fractions

In this study, a melting process with addition of SiO2was applied to treat incinerator fly ash. To describe the encapsulation behaviors of metals quantitatively, the amorphous volume fraction (AVF) of slags was initially determined. Vitrification appeared to reduce the mobility of Cr, Cu, Mn, and Ni instead of significantly immobilizing Cd, Pb, and Zn. It was verified that SiO2enhanced the formation of an amorphous glassy structure. With the increase of SiO2, the crystalline phases would gradually diminish and transform into a higher silica-connected species. During the formation of slag matrix, Al, Ca, and Mg could modify the glass network, and consequently the encapsulation behaviors of these species would noticeably affect the chemical stability of slags. Significant immobilization of crust metals could be achieved only when a more compact and interconnected amorphous glass network was formed. Hence, it indicated that a higher AVF silica-based slag had a better potential to resist acid attack. In conclusion, for environmental protection, it is important to investigate the correlation between the encapsulation behaviors of metals and the crystalline characteristics of slag structure.

Metastability in hydrogenated nanocrystalline silicon solar cells

Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells was studied systematically. First, we observed no light-induced degradation when the photon energy was lower than the band gap of the amorphous phase; degradation occurred when the energy was higher than the band gap in the amorphous phase. The light-induced degradation could be annealed away at an elevated temperature. We concluded that the light-induced defect generation occurred mainly in the amorphous phase. Second, forward current injection did not degrade the nc-Si:H cell performance. However, a reverse bias during light soaking enhanced the degradation. Third, the nc-Si:H cells made with an optimized hydrogen dilution profile showed minimal degradation although these cells had a high amorphous volume fraction. This indicated that the amorphous volume fraction was not the only factor determining the degradation. Other factors also played important roles in the nc-Si:H stability.

Nanoparticles in low melting amorphous selenite materials

The aim of the present investigation is to establish the appropriate routе of nanoparticles formation during heat treatment of selected selenite glasses. Multicomponent compositions containing SeO2, V2O5, TeO2, MoO3, ZnO and Ag2O have been selected. Different preparation methods of the initial glass samples have been combined with heat treatment to influence the glass microstructure and formation of different types of microheterogeneites. TEM and SEM have been used to prove the formation of nanosized particles, randomly distributed in the amorphous matrix volume. Samples containing above 50 wt% Ag2O show the formation of elementary silver with an average particle size of 50–100 nm. Glass-ceramic materials have been obtained after a long thermal treatment. The main crystal phases detected are Ag2SeO3, Ag2TeO3 and TeO2.

Study of amorphous nanocrystalline thin silicon films by grazing-incidence small-angle X-ray scattering

Thin Si films, with thicknesses between 100 and 400 nm, were deposited by radio frequency plasma enhanced chemical vapour deposition in silane gas (SiH4) highly diluted by hydrogen. The growing conditions were varied to obtain different degrees of crystal fractions and a variety in individual crystal sizes. The crystalline to amorphous volume fraction, as estimated by Raman spectroscopy, varied from 5 to 45% while the individual crystal sizes varied from 2 to 8 nm. The average density of the samples was estimated by using near infrared spectroscopy and the effective medium approximation. All samples were porous and contained void volume fraction between 15 and 25%. Grazing-incidence small-angle X-ray scattering has been performed at the ELETTRA synchrotron radiation source (Trieste, Italy). The scattering patterns of all examined samples indicate the presence of `particles' in the `bulk' of the thin films with gyration radii in the range of 2 to 5 nm. The higher values were found for the samples with a higher crystalline fraction. The size and the size distribution of `particles' depend upon the deposition conditions. The samples which had been deposited with a higher discharge power and a lower silane fraction had larger particles and the roughness of their surface was higher.

Intrinsic Data Retention in Nanoscaled Phase-Change Memories—Part II: Statistical Analysis and Prediction of Failure Time

The statistical spread of intrinsic data retention times in phase-change memory (PCM) cells is studied. Based on the crystallization and percolation model described in part 1, the crystalline grain size in the amorphous volume after data loss is extracted. From the temperature dependence of grain size, the authors calculate the statistical shape factor for the distribution of failure times, allowing a statistical prediction of data retention in PCM large arrays. The scaling and optimization issues with respect to failure time statistical spread are finally addressed

Spray forming of glass former Fe 63Nb 10Al 4Si 3B 20 alloy

In this study the Fe 63Nb 10Al 4Si 3B 20 (at.%) alloy was processed by spray forming with the aim of investigate the formation of amorphous phases and novel microstructures by the high cooling rate involved in this process. The gas/metal mass flow rate used was 0.25, and nitrogen was used as the atomization gas. The resulting deposit of a mass of about 1.2 kg, as well as the overspray powder, with a median particle diameter about 120 μm, were characterized by using X-ray diffractometry, differential scanning calorimetry and scanning and transmission electron microscopy with energy of dispersive spectroscopy. Magnetic properties were measured through hysteresis loop tracer equipment. The overspray powder presented a mass median diameter of d 50 ≅ 120 μm and an increasing of amorphous phase volume fraction with decreasing of particle size. The microstructure of the deposit was heterogeneous varying with the thickness, presenting at center region 20 mm and at border 1 mm with different porosities 5 and 10%, respectively. The thicker region presented fully crystalline microstructure whereas the thinner presented about 75% amorphous phase. The phases identified in both deposit and overspray powder were: amorphous, α-Fe, Fe 23B 6, FeB and FeNbB. Amorphous phase formation in the thinner region of the deposit and the absence in the thicker region indicates that the resulting amorphous phase depend on the maintenance of amorphous phase formed during atomization that might be controlled by process parameters. Spray forming of Fe-based amorphous alloys followed by hot consolidation in T < T x for elimination of porosity opens the possibility of obtaining a bulk amorphous or nanocrystalline deposit (millimeters thick) with good soft magnetic properties and enlarging the applications range, which is limited by the reduced (micrometric) dimensions of common amorphous products.

Free Volume, Glass Transition and Degree of Branching in Ethylene/α‐Olefin Copolymers: Positron Lifetime, Differential Scanning Calorimetry, Wide‐Angle X‐Ray Scattering, and Density Studies

AbstractSummary: Positron annihilation lifetime spectroscopy, differential scanning calorimetry, wide‐angle X‐ray scattering, and density measurements were used to systematically study the variation of the glass transition temperature Tg and the mean size vh of holes (local free volumes) in n‐alkyl branched polyethylenes. The samples were commercial ethylene‐rich copolymers with 1‐propene, 1‐butene, and 1‐octene comonomers. From the total specific volume V and the crystallinity Xc the specific volume of the amorphous phase Va was estimated and used to calculate the specific hole‐free volume Vf. It was found that Tg and Xc decrease and V, Va, Vf, and vh increase with increasing weight fraction of comonomers. Both the real crystalline and amorphous specific volumes decrease with increasing crystallinity Xc. For not too high contents of comonomers, Tg decreases and vh increases linearly with the number and with the length of n‐alkyl branches. This behavior was attributed to an increased segmental mobility caused by branching. Both Tg and vh follow linear master curves as a function of the degree of branching if this is defined as the fractional number of carbon atoms in the side chains compared with the total number of carbon atoms. A method for estimating Tg from vh measured at room temperature is shown. The number density of holes Nh shows a slight variation from 0.6 to 0.8 (±0.1) nm−3 with increasing branching. magnified image

Metastability in Hydrogenated Nanocrystalline Silicon Solar Cells

AbstractWe have recently reported that hydrogenated nanocrystalline silicon (nc-Si:H) solar cells exhibit metastability behavior that is significantly different from hydrogenated amorphous silicon (a-Si:H) solar cells. First, we studied the light-induced change of nc-Si:H cells under different light spectra, and observed that no light-induced degradation occurs when the photon energy is lower than the bandgap of a-Si:H, while light-induced degradation indeed occurs when the photon energy is higher than the bandgap of a-Si:H. Based on this observation, we conclude that the light-induced defect generation occurs mainly in the amorphous and/or grain boundary regions. Secondly, we found that forward-bias current injection in the dark does not cause any degradation in the performance of nc-Si:H cells, in contrast to observations in a-Si:H cells. This phenomenon can be explained by assuming transport percolation through crystallites, where excess carrier recombination does not cause degradation. Third, we found that a reverse electrical bias does not reduce, but rather enhances the light-induced degradation in the nc-Si:H cell performance. This enhancement increases with the magnitude of the reverse bias. We carried out a systematic study including measurements of quantum efficiency losses and fill factors under different color filters. In view of the heterogeneity of the material structure, we proposed a “back-to-back” diode model which explains most of the experimental results. Finally, we present results of the improvement of stability of nc-Si:H cells made using hydrogen dilution profiling. We find that the nc-Si:H cells with an optimized hydrogen dilution profile show very little light-induced degradation though these cells have a high amorphous volume fraction. This indicates that the amorphous volume fraction is not the only factor determining the degradation. Other factors, such as the structure and distribution of the amorphous phase, as well as the properties of the grain boundaries, apparently play important roles in the overall stability of nc-Si:H cells.