Consumption Of Si Research Articles

Effect of cation in sodium/potassium-based geopolymer coatings for the fire protection of steel structures

Common geopolymers (GPs) are activated by alkaline solutions of potassium (K) or sodium (Na). Although both properties have been extensively investigated, comparative studies on their fire resistance are still lacking. In this research, Na/K-based GPs were applied as fire-protective steel coatings. Their fire protection was evaluated by a fire test named “burn-through”. When the Aluminum/Silicon (Al/Si) molar ratio equaled 0, both GPs showed intumescence (expansion upon heating) during the fire test. The final maximum temperatures of the protected steel plate (TS) were as low as 309/335°C for Na/K-GP. However, with Al/Si=0.54, K-based GP (TS=410°C) exhibited intumescence and better fire protection than Na-based GP (TS=587°C) which even cracked. Physico-chemical properties of the GP samples were characterized before and after the fire test, including electron probe micro-analysis, dynamic mechanical analysis, and 29Si solid-state MAS (CP) NMR. Material softening was proved to come from the consumption of Si (Q2) and caused the following intumescence. Such structure was formed in K-based GP regardless of Al/Si ratio while Si (Q2) atoms were only present in Na-based GP at low Al/Si ratio. It is therefore proven that K-based GP is more appropriate to protect steel than Na-based GP, due to its intumescent property, especially when the Al/Si ratio is high.

Observation of dissolution behavior of dredged sediment in NaOH-activated system to evaluate potential activity for precursor usage

Understanding the dissolution mechanism of dredged sediment (DS) is of great importance for the usage of aluminosilicate precursor and sustainable utilization of solid waste. In this paper, a new method was proposed for evaluating the activity of a complicated system with multi-minerals. From the perspective of solid phases, the dissolution of DS detected by TESCAN Integrated Mineral Analyzer (TIMA) was characterized by the reduction of mineral phase area and element variation of individual phases. Meanwhile, the dissolved element in medium was monitored by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) from the aspect of solution. It was found that the dissolution of DS preferentially occurred at the locations with high surface energy and resulted in the reduction of phase area, and the area with low surface energy was also dissolved to a limited extent. Plagioclase and zeolite exhibit higher activity, and phyllosilicates are generally more reactive than framework silicates. The inconsistency between dissolved Al and Si from solid and those into solution reveal that the formation of initial geopolymer unit requires a higher consumption of Si than Al. This study opens up a new way of DS utilization and provides a theoretical basis for the development of a widely obtained and environmentally friendly precursor of geopolymer.

The effect of different carbon reductants on the production of ferrosilicon 75% on an industrial scale in an electric arc furnace

Ferrosilicon production is based on the carbothermal reduction of silica and iron oxide in submerged electric arc furnaces. The reducing process of iron oxide and silicon oxide is carried out by the carbon contained in carbon materials such as coal, charcoal, semi-coke, and all types of coke. Based on its inherent and functional characteristics, the kind of carbon material can be effective in the ferrosilicon production process and furnace energy consumption. Therefore, in this work, which was performed by Iran Ferrosilice company, in a long period of 5 years, the effects of seven combinations of different carbon materials on the electrical and metallurgical performance of the process were investigated. The results showed that the minimum value of energy coefficient per ton (8.46 MWh/ton) was obtained using combination 5 (consisting of 55% coal, 30% semi-coke, 5% charcoal, and wood chips). The use of wood chips reduced the energy consumption by 3.03 MWh/ton. The composition consisting of 50% coal, 35% semi-coke, 15% charcoal, and wood chips had the highest Si% of 73.64% and the lowest Al% of 1.54%. Finally, by evaluating all the results, especially the reduction of energy consumption and recovery of Si, compound 5 was introduced as the optimal compound in the ferrosilicon production process.

Formation of magnesium silicate hydrate (M-S-H) at pH 10 and 50°C in open-flow systems

Magnesium silicate hydrate (M-S-H) is a poorly crystalline Mg-silicate phase formed under alkaline conditions at low temperatures (T < 100 °C). Its formation has been studied in closed systems but not in open-flow systems, which better represent natural surface/subsurface environments. Here, MgO powder was used in reactions at pH ∼10 and 50 °C in flow-through experiments to study the formation of M-S-H as a function of aqueous Si concentration (1.5, 0.15, and 0 mM).Consumption of aqueous Si during precipitation of M-S-H resulted in an increase in the dissolution rate of the primary material (Mg hydroxide). Steady-state Si concentrations were used to calculate the dissolution rate of Mg hydroxide and the precipitation rate of M-S-H. Analyses of retrieved solids by electron microscopy and nuclear magnetic resonance spectroscopy confirmed the formation of M-S-H, although X-ray diffraction patterns provided no clear evidence of the presence of M-S-H because of the small amount precipitated and its nano-crystallinity. The chemical composition (Mg/Si ratio) of the M-S-H varied with the aqueous Si concentration of the injected solution. Mg/Si ratios of 1.00 ± 0.09 and 1.59 ± 0.15 were obtained with Si concentrations of 1.5 and 0.15 mM, respectively. Results indicate that the formation of M-S-H is feasible under Earth surface conditions, with dissolved silica coexisting with Mg-bearing minerals at alkaline pH.

Selectivity of dehydrogenative silicone–oxygen bond formation in diphenylsilane by base and base‐activated catalysts

AbstractVarious base catalysts and base‐activated catalysts were evaluated for their steric inhibitions and selectivity in silicon–oxygen bond formation between hydroxyl groups and each of the two active hydrogen sites of diphenylsilane. Three hydroxides, six amines, and an organic‐soluble N‐heterocyclic carbene‐copper alkoxide complex (CuIPr‐NHC) were reviewed using in situ FTIR monitoring of reaction progress for the stepwise consumption of both Si–H bonds in diphenylsilane in the presence of water and various alcohols. Hydroxide catalysts with sodium, potassium, and tetramethylammonium (TMA) cations showed strong dependence of cation identity on reaction rate and observable rate order but did not lead to accumulation of a once‐oxidized silane. Amine catalysts exhibited varying levels of selectivity to the once oxidized silane, leading to accumulation of diphenylsilanol and hydride terminated oligomers with relative reaction rates seemingly dependent on the size of amine's alkyl substituents. CuIPr‐NHC showed very limited reactivity to the second dehydrogenation step in comparison to the first, leading to high accumulation of once‐oxidized silane. Leveraging this, the production of diphenyl(monoalkoxy)silanes has been demonstrated with representative primary, secondary, and tertiary alcohols.

Liquid metal infiltration of silicon based alloys into porous carbonaceous materials. Part II: Experimental verification of modelling approaches by infiltration of Si-Zr alloy into idealized microchannels

In this work, the mechanisms leading to the pore closure in reactive melt infiltration (RMI) of carbon by pure silicon and a near eutectic Si-8 at-pct Zr alloy at 1500 and 1700 °C under vacuum were studied. Various geometrical configurations of microchannels were fabricated via laser ablation of glassy carbon plates. The micron size capillary channels allowed simplifying the complicated porosity distribution in the infiltration of powder or fibres based porous preform while keeping the physical dimensions in the range of where the physical phenomenon of pore closure takes place. The extent of infiltration was analysed by means of X-ray radiography. For RMI of pure Si, the widely accepted decrease in capillary radius by the formation of a solid state SiC layer by the reaction of liquid Si and C was observed, but did not lead to closure and it is hence not the infiltration limiting step in channels as small as 10 μm. However, in the case of the Si-Zr alloy infiltration, another mechanism of pore closure was observed, namely the precipitation of zirconium silicides at the infiltration front, due to Zr enrichment in the alloy by the continuous consumption of Si for the formation of SiC.

Electrical Characteristics of Atomic Layer Deposited Y-Silicate Dielectrics

Introduction High k dielectric materials are widely used for scaling the equivalent oxide thickness (EOT) of various very large integrated devices. Rare-earth silicates maintain their amorphous structure within the temperature range of typical semiconductor processes [1] , making them suitable for high-k dielectrics. Rare-earth silicates are easily formed by thermal processes, but their formation consumes Si at the interface. The consumption of Si at the interface is disadvantageous for micro-structured devices such as three-dimensional structures. One of the methods to suppress Si consumption at the interface is to supply Si atoms in the deposited film to form silicates without reacting with the channel material. In this work, we focused on the formation of yttrium silicate (Y-silicate) dielectrics, a typical rare-earth silicate [2] , so that the formation process can be easily extended to other rare-earth silicates, including La-silicate [3] .Y-silicate films were formed by cyclic deposition of Y2O3 and SiO2 based on atomic-layer deposition (ALD). Experiments Firstly, Y2O3/SiO2 multi-stacked layer（call it stacked-layer from now on）with a thickness of 16 nm without annealing process was analyzed by x-ray photoelectron spectroscopy (XPS) to confirm forming of Y-silicates. Next, MOS capacitor was fabricated by stacked-layer with a thickness 5 nm were deposited on the n-Si substrates by ALD at 200oC. Fig. 1 shows the schematic structure of the fabricated MOS capacitors. The cycles are determined to have the Y/Si atomic ratio of the stacked-layer to be 1:1. Then, W gate electrodes and TiN layer are deposited by RF sputtering. Al contact layers were deposited on the backside of the samples by thermal evaporation. The capacitors were then annealed at 800oC in forming gas (N2: H2=97%:3%) ambient for 30 min. [4] Finally, the metal-insulator-metal (MIM) capacitor, 10-nm-thick stacked-layer were also fabricated aim to no interfacial layer. Result Fig.2 shows the results of XPS analysis. The shift in the binding energy indicates that most of the Y atoms are in the form of silicate. Suggesting that the stacked layer is readily transformed in the form of Y-silicate during ALD process. The bandgap of the stacked-layer was estimated to be 6.4 eV. Fig. 3 shows the XPS depth profile of the stacked-layer obtained by Ar etching. The atomic ratio of Y, Si and O remained constant throughout the film at almost 2:2:7 until the Si substrate appeared. From the results, it is confirmed that a uniform Y2Si2O7 layer can be obtained by the multi-stacking of Y2O3/SiO2 ALD process. Fig. 4 shows the C-V characteristics of the n-Si capacitors with a 5-nm-thick Y-silicate layer. After annealing, the capacity of Y-silicate layer capacitor doesn’t increase. This result indicating little reaction at Y-silicate/Si interface. Finally, from the C-V and J-V measurements of MIM capacitor, A k-value of the stacked-layer is calculated to be 8.5, break down field of MIM capacitor is 6.6MV/cm. The k-value of the Y-silicate layer is also reasonable considering the k-values of SiO2 of 3.9 and Y2O3 of 13. [5] Conclusion Low temperature formation of Y-silicate layer has been confirmed by cyclic deposition of stacked-layer of Y2O3 and SiO2 ALD layers. The target composition of Y2Si2O7 and the formation of Y-O-Si bonds have been confirmed by XPS. Bandgap, breakdown field and k-value of Y-silicate dielectrics were determined to be 6.4eV, 6.6MV/cm, 8.5 respectively.

Low temperature joining of NiTi shape memory alloy with Au-Si isothermal solidification

Owing to the isothermal solidification of eutectic Au-Si filler, NiTi joint that could endure 700 °C has been successfully manufactured at 430 °C for the first time. The joining temperauture is within the aging temperature range, which is harmless to the properties of the NiTi base metal. A total consumption of the eutectic Si leaves a seam full of Au, whiskers and nanocrystalline layers due to the reaction of Si and NiTi. With the high-density stacking faults, the nanocrystalline layers at the interface exhibits ultra-high mechanical property, which strengthen the interfaces.

Meridional variations in N* and Si* along 57°30′E and 47°E transects in the indian sector of the Southern Ocean during austral summer 2011

The variations in nutrient utilization in the euphotic layer was studied using geochemical tracers N* (NO3- −16PO43−) and Si* (Si(OH)4 – NO3-) along two transect 57°30′E (T1) and 47°E (T2) during austral summer 2011 in the Indian Ocean sector of the Southern Ocean. Seawater samples collected from 8 depths within the euphotic zone (upper 200 m) along two meridional transects were analysed for nitrate, nitrite, silicate, phosphate and chlorophyll a (chl a). Clear meridional differences in nutrient limitation were observed along the two transects. Depletion of silicate at Antarctic Zone (AZ) stations was 1.6 times higher in T1 than in T2. Significant inverse correlations were observed for N* (p < 0.0001) and Si* (p < 0.001) values with temperature and salinity implying that redistribution of preformed nutrients was influenced by hydrography. However, significant variances in Si* (p < 0.001 at SSTF and p < 0.01 at PF-2) along the two transects could be attributed to differential uptake of silica by phytoplankton. Low Si* values were obtained at Polar Front 1 (PF1) and South Subtropical Front (SSTF) along T1 and from Sub Antarctic Front (SAF) to Polar Frontal Zone (PFZ) along T2. Although hydrography did have inverse correlation with tracers, the observed values indicate high consumption of silicate. Relatively higher Si* values in the north of SSTF coincided with lower abundance of diatoms in these waters. Conversely, high Si* values and abundance of diatoms at Antarctic Zone (AZ) indicate less consumption of Si. The biological consumption of nitrate was more clearly discerned in the northern region where nitrogen strongly limits the productivity both along T1 (39°S and the Agulhas Retroflection Front (ARF)) and T2 (40°S). Our observations suggest that much of the tracer distribution in these waters can be explained by the dominant phytoplankton groups, but influence of other biotic components on nutrient tracer distribution cannot be ruled out.

Influence of Mn on the negative natural aging effect in 6082 Al alloy

The microstructural evolution and mechanical properties of the 6082 alloys with different Mn content were systematically investigated to deeply understand the influence of Mn on the negative natural aging (NA) effect. It is found that α-Al(Mn,Fe)Si precipitated during homogenization is larger in size and less in amount in the alloy with lower Mn content, resulting in the less consumption of Si to create a relatively Si-rich environment in the matrix. The true Mg/Si ratio of alloy with higher Mn content is higher so that the alloy possesses a stronger negative natural aging effect. Additionally, four types of aggregates including clusters, GP zones, elongated β'' and β′ are present in the microstructures aged at 175 °C/8 h which is the peak artificial aging (AA) treatment in the two alloys. The number density of β'' and the total number density of the aggregates in alloy with lower Mn content is always higher than that in alloy with higher Mn content at the same heat treatment condition, which lead to the higher hardness and strength.

Near-infrared emission in Er:ZnO thin films with embedded Si nanoparticles synthesized by ion implantation

Er-doped ZnO thin films on a SiO2/Si substrate were fabricated by radio frequency magnetron sputtering, in which embedded Si nanoparticles (NPs) were formed by ion implantation and subsequent thermal annealing. The effects of Si NPs on the Er photoluminescence (PL) at 1.54 μm were investigated. In addition to the typical emission at 1.54 μm from Er3+, a new 1.16-μm emission peak was also observed after a thermal treatment. Further annealing resulted in shift of emission intensity between the 1.16- and 1.54-μm luminescence features. The observed Si nanoparticles (NPs) were ∼4 nm in diameter. The formation of new components Zn2SiO4 and Er2Si2O7 was also presented in this study. The 1.16-μm luminescence is attributed to the Si NPs, and the suppression of Si NPs related emission is caused by consumption of Si in the formation of Er silicate and zinc silicide and the energy transfer between Si NPs and Er3+. The intensity of Er3+ related 1.54-μm PL can be modulated by the Si NPs fabricated by implantation and optimizing the annealing condition.

Silver nanoparticles deposited on polysiloxane networks as active catalysts in dye degradation

AbstractThe article proposes a new method of obtaining silver nanoparticles on polysiloxane networks using the reducing properties of Si–H groups. Three types of networks with different architecture and distribution of Si–H groups were studied as reducing agents for silver ions and as matrices for the obtained silver nanoparticles. As established by FTIR spectroscopic studies, the redox reaction between Ag+ ions from the silver heptafluorobutyrate solution in toluene and Si–H groups of the networks occurred, which resulted in the appearance of silver nanoparticles in the systems. The amount of metal introduced into individual polysiloxane networks is closely related to the consumption of Si–H groups in them. The type of polysiloxane networks used affects the size of Ag NPs obtained and their distribution on the carrier. Polysiloxane‐Ag systems are observed to be an effective catalyst on reduction of hazardous dye like methyl red, which is confirmed by a decrease in absorbance maximum values.

Novel Sulfur‐Containing Cross‐Linking Agent for Si‐Based Preceramic Polymers

AbstractCross‐linking polymethylhydrosiloxane (PMHS) with divinylthiophene (DVT) via hydrosilylation in highly dilute conditions and subsequent supercritical drying in CO2 yield a polymeric aerogel containing aromatic sulfur integrally and uniformly distributed throughout the monolith. Fourier‐transform infrared (FT‐IR) spectroscopy indicates almost complete consumption of vinyl groups and SiH bonds in the product. Both FT‐IR and Raman spectroscopic analyses support loss of conjugation of vinyl groups with the retained double bonds of the thiophene ring. Scanning electron microscopy (SEM) indicates a condensed colloidal structure with characteristic particulate diameters of about 165 nm. SEM coupled with energy dispersive X‐ray spectroscopy elemental mapping shows that sulfur is distributed homogeneously in the polymeric aerogel. Porosimetry of the mesoporous aerogel indicates the effective average pore diameters are about 12 nm. Thermogravimetric analysis (TGA) establishes greater thermal stability of the PMHS‐DVT product than either of the pure unreacted components. TGA coupled with mass spectrometric (TG‐MS) identification of the volatiles released during pyrolysis shows that sulfur is driven from the cross‐linked polymer as thiophene and its derivatives. Recorded mass spectra support the hypothesis that cross‐linking DVT bridges between PMHS chains in the polymeric aerogel, and that this results in a more thermally stable monolith.

Development and evaluation of a chemical kinetics reaction mechanism for tetramethylsilane-doped flames

Tetramethysilane (TMS) is a precursor for flame synthesis of silica (SiO2) nanoparticles. A chemical reaction mechanism was developed for the oxidation of TMS in a lean low-pressure (p ≈ 30 mbar) H2/O2/Ar flame using species mole fractions, obtained from molecular-beam mass spectrometry (MBMS) measurements in a matrix-supported flat flame doped with 600 ppm TMS. The thermodynamic data of Si-containing species were determined from quantum-chemical calculations at the G4 level of theory. The formation and subsequent consumption of Si(OH)4, one of the main products of TMS oxidation, and the formation of Si4O10H4 clusters are hypothesized to be the primary pathway in the synthesis of silica nanoparticles. The reaction rate coefficients are either estimated via an algorithmic optimization procedure or are assumed based on analogies to similar reactions in the literature. The mechanism was further validated based on MBMS measurements with the same base flame doped with 400 and 800 ppm TMS.

Understanding the influence of electrolyte additives on the electrochemical performance and morphology evolution of silicon nanowire based lithium-ion battery anodes

Abstract Here we report new insights into the effect various electrolyte additives have on the cycling stability and rate capability of Si nanowire (NW) Li-ion battery anodes. The additives tested were vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate and lithium bis(oxalato)borate. All four significantly improve the capacity retention of the electrodes over 250 cycles compared to the additive-free electrolyte, with vinylene carbonate being the outstanding performer. The results provide a new understanding of the cycling behaviour of Si in the presence of electrolyte additives, revealing that not only is the stability of the SEI layer affected but that this consequently has a profound influence on the morphology evolution and chemical composition of the Si active material. Ex-situ characterisation of the electrodes post-cycling demonstrates that the improvement in cycling stability arises as the additives minimise irreversible decomposition reactions at the surface and facilitate a transformation from a NW morphology into a porous sponge-like network. This transformation process does not occur in the absence of any stable SEI forming additives as instability in the passivating layer leads to the continuous and irreversible consumption of Si to form Li silicates.

A Study on the Optimization of CP Based Low-temperature Tabbing Process for Fabrication of Thin c-Si Solar Cell Module

Thin crystalline silicon (C-Si) solar cell is expected to be a low price energy source by decreasing the consumption of Si. However, thin c-Si solar cell entails the bowing and crack issues in high temperature manufacturing process. Thus, the conventional tabbing process, based on high temperature soldering (> 250°C), has difficulties for applying to thin c-Si solar cell modules. In this paper, a conductive paste (CP) based interconnection process has been proposed to fabricate thin c-Si solar cell modules with high production yield, instead of existing soldering materials. To optimize the process condition for CP based interconnection, we compared the performance and stability of modules fabricated under various lamination temperature (120, 150, and 175°C). The power from CP based module is similar to that with conventional tabbing process, as modules are fabricated. However, the output of CP based module laminated at 120°C decreases significantly (14.1% for Damp heat and 6.1% for thermal cycle) in harsh condition, while the output drops only in 3% in the samples process at 150°C, 175°C. The peel test indicates that the unstable performance of sample laminated at 120°C is attributed to weak adhesion strength (1.7 N) between cell and ribbon compared to other cases (2.7 N). As a result, optimized lamination temperature for CP based module process is 150°C, considering stability and energy consumption during the fabrication.

Discharge Behavior of Si-Air Batteries with Aqueous Electrolyte

A recent concept of metal-air batteries, Si-air, has a good potential to play a major role in the primary battery market due to possessing the second most abundant element in Earth’s crust as an anode. The main drawback of this system is the low practical discharge capacities both in aqueous alkaline and room temperature ionic liquids. In the previous studies, limitation of discharge is suggested to originate from a formation of an oxide layer on the Si surface, which prevents further oxidation of the active material.1,2,3 Additionally, there is a high corrosion rate of Si in aqueous systems that results in low anodic mass utilization efficiencies. The main objective of current work is to find an approach to overcome the discharge limitations in aqueous alkaline systems. Electrochemical characterizations of Si-air cells were carried out by galvanostatic discharge experiments with Si wafers under controlled ambient conditions. The complete consumption of the active Si in the cell was achieved when a suitable setup, i.e. sufficient electrolyte, was employed. Discharge behavior of the Si-air cells was also investigated along with variations of temperature and molarity of alkaline electrolyte. Since large amounts of the Si mass were consumed in the corrosion reaction during the experiments, the current work focuses also on corrosion studies. From the data on Si anode mass and discharge capacities, the corrosion rate and anodic mass utilization efficiencies were calculated. To have a better insight into the corrosion mechanism, nuclear magnetic resonance experiments were performed on the Si dissolution in alkaline electrolyte. From these experiments, formation of different Si-O-H silicate species as products of the corrosion reaction was identified. Moreover, the effect of corrosion products on the discharge performance and corrosion rate was studied as well. Based on the experimental results, specific discharge capacities of Si-air batteries are discussed while taking the corrosion process into account. In summary, in spite of the corrosion process, the main goal to overcome the discharge limitations was accomplished by complete consumption of the active Si during the discharge. The effect of corrosion products on the discharge performance was investigated and the results will be reported.

Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation

AbstractWe present the first set of dissolved silicon isotope data in seawater (δ30SiSi(OH)4) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface δ30SiSi(OH)4 signatures of +2.1‰ corresponding to the highest Si(OH)4 concentrations (∼30.0 µmol L−1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface δ30SiSi(OH)4 increased rapidly to +3.2‰ while Si(OH)4 became depleted, indicating increasing biological utilization of the Si(OH)4 originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of δ30SiSi(OH)4 and an increase of Si(OH)4 with depth on the East China Sea mid‐shelf and slope. In winter (December 2009–January 2010), however, the δ30SiSi(OH)4 was nearly constant at +1.9‰ throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in δ30SiSi(OH)4 were detected in the surface waters beyond the Changjiang Diluted Water‐influenced region on the East China Sea shelf, where the winter values were ∼1.0‰ lower than those in summer, despite the same primary Si(OH)4 supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)4 in summer than in winter.

Synthesis, characterization, and thermal properties of new polysiloxanes containing 1,3-bis(silyl)-2,4-dimethyl-2,4-diphenylcyclodisilazane

New polysiloxane derivatives (P1 and P2) having mixed phenyl-substituted cyclodisilazane in the main-chain were synthesized. Their thermal properties have been characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The glass transition temperature (Tg) of P1 and P2 would be dependent on the polymer structure and were in the range of −27∼−80 °C. The Tgs decreased considerably with an increase in the content of the siloxane main-chain groups and increased with higher content of phenyl groups. All of the copolymers showed good thermal stability, with their temperatures at 5% weight loss (Td5) being higher than 490 °C and char yields over 50% under nitrogen. The chemistry of pyrolytic conversion was investigated by FT-IR, solid NMR and mass spectrometry. The results demonstrate that the cyclodisilazane group significantly alters the degradation behavior of the PDMS, modifying the profile of the thermal degradation and reducing the overall rate of volatiles evolution. The thermal stability and degradation behavior also indicate that the inter-molecular rearrangement occurs prior to the intra-molecular backbiting reactions by these OH-terminated copolymers. The thermolysis of the Si4N2 groups in the backbone could bring the greater thermal stability to form the silicon-nitrogen networks via the consumption of Si–OH bonds. Retardation of the degradation rate is resulted from this cross-linking that prevents rearrangement of the siloxane to allow further depolymerization.

Structural and optical characterization of two-dimensional arrays of Si nanocrystals embedded in SiO2 for photovoltaic applications

We report on the structural and optical characterization of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. Single and multiple SiNC layers were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. The single SiNC layers consisted of one SiNC layer embedded in two silicon dioxide (SiO2) layers, whereas the multi-layered structure consisted of five SiNC layers of equal thickness separated by SiO2 layers. SiNC layers with thicknesses ranging from 2 to 25 nm were investigated. A thorough structural characterization of the films was carried out by combining grazing incidence x-ray diffraction, x-ray reflectivity, and transmission electron microscopy (TEM). Both XRD and TEM measurements revealed that the SiNC layers were polycrystalline in nature and composed of SiNCs, separated by grain boundaries, with their vertical size equal to the SiNC layer and their lateral size characterized by a narrow size distribution. The high resolution TEM (HRTEM) images showed that oxidation of the SiNC layers proceeded by consumption of Si from their top surface, without any detectable oxidation at the grain boundaries. Only in the case of the thinnest investigated SiNC layer (2 nm), the SiNCs were well separated by SiO2 tunnel barriers. From transmission and reflection optical measurements, energy band gaps of the SiNCs were estimated. These results were correlated with the sizes of the SiNCs obtained by HRTEM. A shift of the estimated band gaps with decreasing SiNC size was observed. This was consistent with quantum size effects in the SiNCs. The film containing the smallest SiNCs (2 nm in the growth direction), besides a significant shift of the absorption edge to higher energies, showed light emission at room temperature which is due to radiative recombination of photo-generated carriers in localized SiNCs separated by SiO2 tunnel barriers.