Crystallization of Polymer-Derived Silicon Carbonitride at 1873 K under Nitrogen Overpressure

The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC·0.43Si₃N₄·0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at p _N2 < 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at ∼10 bar, and is crystalline above ∼10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si₃N₄ are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at ∼10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate α-Si₃N₄ crystals is calculated to be 30 kJ/mol.     

Detailed numerical modeling of PAH formation and growth in non-premixed ethylene and ethane flames

A chemical kinetic mechanism for C₁ and C₂ fuel combustion and PAH growth, previously validated for laminar premixed combustion, has now been modified and applied to opposed flow diffusion flames. Some modifications and extensions have been made to the reaction scheme to take into account recent kinetic investigations, and to reduce the stiffness of the reaction model. Updates have been made to the cyclopentadienyl reactions, indene formation reactions, and aromatic oxidation and decomposition reactions. Reverse reaction rate parameters have been revised to account for numerical stiffness. Opposed flow diffusion flame simulation data for ethylene and ethane flames with the present mechanism are compared to data computed using two other mechanisms from the literature and to experimental data. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. The current mechanism considers a detailed set of PAH growth routes, and includes hydrogen atom migration, possible free radical addition schemes, methyl substitution/acetylene addition pathways, cyclopentadienyl moiety in aromatic ring formation, and numerous reactions between aromatic radicals and molecules. It is shown that while bulk flame properties and major species profiles are the same for the three mechanisms, the enhanced PAH growth routes in the present mechanism are necessary to numerically predict the correct order of magnitude of PAHs that were measured in the experimental studies. In particular, predicting concentrations of naphthalene, phenanthrene, and pyrene, to within the correct order of magnitude with the present mechanism show a significant improvement over predictions obtained using mechanisms in the literature. Sensitivity and production rate analyses show that this improvement is attributable to the enhanced PAH growth pathways and updated reaction rates in the present mechanism. The overreaching goal of this research is to generate and fully validate a detailed chemical kinetic mechanism, with as few fitted rates as possible, that can be applied to premixed or diffusion systems, and used with any type of soot model. To that end, in recently published works, the present mechanism has been used to simulate premixed flames, while coupled to a method of moments to determine soot formation, and to simulate diffusion flames, while coupled to a sectional representation for soot formation. The present work extends the validation of the mechanism by applying it to counterflow diffusion flames, for which measurements of large PAH molecules are uniquely available. The validation of PAH growth predictions are of key interest to soot modeling studies as soot inception from PAH combination and PAH condensation are often major constituents of soot production.     

Photoelectric characterization of Cu(In,Ga)S2 solar cells obtained from rapid thermal processing at different temperatures

CuInS₂-based solar cells have a strong potential of achieving high efficiencies due to their ideal band gap of 1.5 eV. A further increase in the efficiency is expected from doping the absorber film with gallium due to enlargement of the band gap (E_g) and correspondingly the open-circuit voltage (V_OC). We investigated Cu(In,Ga)S₂ solar cells obtained from stacked metal layers sputtered from In and (Cu,Ga) targets followed by rapid thermal processing (RTP) in sulfur vapor. Depending on the actual RTP temperature profile, the films might exhibit CuInS₂/CuGaS₂ (top/bottom) segregation, which is rather detrimental for a large V_OC. We found that only precursors sulfurized at sufficiently high temperatures exhibit the desired interdiffusion of the segregated CuInS₂/CuGaS₂ layers resulting in an increased V_OC. Moreover, temperature dependent current-voltage profiling (suns-V_OC-analysis) yielded strong indications for improved current collection and reduced losses for devices with proper interdiffusion of the CuInS₂/CuGaS₂ layers. A more fundamental question is related to the variation and formation of defect states in differently processed absorber films. The studied samples were thus further investigated by means of admittance spectroscopy, which allowed us to confirm the presence of three individual defect states in both absorber configurations. Two defects exhibit activation energies, which remain unchanged upon varying the RTP temperature whereas a third state exhibits significantly increased activation energy in devices showing interdiffusion of CuInS₂/CuGaS₂ layers. According to the characteristic shift of the conduction band edge upon Ga-doping we conclude that the latter defect level corresponds with the minority carriers in the p-type absorbers.     

Processing and magnetic properties of metal-containing SiCN ceramic micro- and nano-composites

Owing to their excellent high temperature and oxidation resistance, non-oxide polymer-derived silicon-based ceramics are suitable for applications in hot and corrosive environments. The metal (Fe, Co)-containing pre-ceramic compounds combine the processability of organic polymers with the physical and chemical characteristics of the metallic component. In this study, we will introduce two different routes to embed metal particles in a SiCN ceramic matrix, derived from the commercially available polysilazane Ceraset^®. (1). Mixing and milling of metal powders (Fe, Co) with pre-crosslinked polysilazane followed by pyrolysis at 1100 °C. (2). Chemical reaction between metal carbonyl compounds, namely Fe(CO)₅ and Co₂(CO)₈, with pure polyorganosilazane followed by pyrolysis at 1100 °C. Both synthetic routes will be discussed on two particular examples, iron- and cobalt-containing samples as well as their resulting different microstructures with respect to their magnetic properties. The phases and microstructures of the metal–SiCN composites were investigated in terms of X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with EDX, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and magnetometer. Upon annealing in argon at 1100 °C, the crosslinked polysilazane blended with iron powder possesses a high saturation magnetization of about ～57 emu/g and exhibits good ferromagnetic behaviour in comparison to the one blended with cobalt. The magnetic measurements were performed within the temperature range 65–300 K.     

Micro/nano multiscale reinforcing strategies toward extreme high-temperature applications:Take carbon/carbon composites and their coatings as the examples

Carbon fiber reinforced carbon composites (C/Cs),are the most promising high-temperature materials and could be widely applied in aerospace and nucleation field...     

Disentangling the adult attention-deficit hyperactivity disorder endophenotype: Parametric measurement of attention.

Attention deficit hyperactivity disorder (ADHD) persists frequently into adulthood. The decomposition of endophenotypes by means of experimental neuro-cognitive assessment has the potential to improve diagnostic assessment, evaluation of treatment response, and disentanglement of genetic and environmental influences. We assessed four parameters of attentional capacity and selectivity derived from simple psychophysical tasks (verbal report of briefly presented letter displays) and based on a “theory of visual attention.” These parameters are mathematically independent, quantitative measures, and previous studies have shown that they are highly sensitive for subtle attention deficits. Potential reductions of attentional capacity, that is, of perceptual processing speed and working memory storage capacity, were assessed with a whole report paradigm. Furthermore, possible pathologies of attentional selectivity, that is, selection of task-relevant information and bias in the spatial distribution of attention, were measured with a partial report paradigm. A group of 30 unmedicated adult ADHD patients and a group of 30 demographically matched healthy controls were tested. ADHD patients showed significant reductions of working memory storage capacity of a moderate to large effect size. Perceptual processing speed, task-based, and spatial selection were unaffected. The results imply a working memory deficit as an important source of behavioral impairments. The theory of visual attention parameter working memory storage capacity might constitute a quantifiable and testable endophenotype of ADHD. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Facile Routes to Manganese(II) Triflate Complexes

Manganese(II) chloride reacts with trimethylsilyl triflate (TMS(OTf) where OTf = (-)OSO(2)CF(3)) in a 1:1 mixture of acetonitrile and tetrahydrofuran, and after recrystallization affords the linear coordination polymer [Mn(II)(CH(3)CN)(2)(OTf)(2)](n). Each distorted octahedral manganese(II) center in the polymeric chain has trans-acetonitriles and the remaining equatorial coordination positions are occupied by the bridging triflate anions. Dissolving [Mn(II)(CH(3)CN)(2)(OTf)(2)](n) in equal volumes of acetonitrile and pyridine followed by recrystallization with diethyl ether yields trans-[Mn(II)(C(5)H(5)N)(4)(OTf)(2)]. The distorted octahedral geometry of the manganese center features monodentate trans-triflate anions and four equatorial pyridines. Exposure of either [Mn(II)(CH(3)CN)(2)(OTf)(2)](n) or [Mn(II)(C(5)H(5)N)(4)(OTf)(2)] to water readily gives [Mn(II)(H(2)O)(6)](OTf)(2). XRD reveals hydrogen-bonding interactions between the [Mn(II)(H(2)O)(6)](2+) cation and the triflate anion. All three of these species are easily crystallized and provide convenient sources of manganese(II) for further synthetic elaboration.     

Host Sex Discrimination by an Egg Parasitoid on Brassica Leaves

Egg parasitoids are able to find their hosts by exploiting their chemical footprints as host location cues. In nature, the apolar epicuticular wax layer of plants that consists of several classes of hydrocarbons serves as the substrate that retains these contact kairomones. However, experiments on chemical footprints generally have used filter paper as substrate to study insect behavior. Here, we explored the ability of Trissolcus basalis (Scelionidae) females to discriminate between footprint cues left by male and female Nezara viridula (Pentatomidae) on leaves of their host plant Brassica oleracea (broccoli). Furthermore, we analyzed the chemical composition of the outermost wax layer of broccoli leaves to evaluate the degree of overlap in insect and plant cuticular hydrocarbons that could lead to masking effects in the detection of footprint cues. Our results showed that B. oleracea epicuticular wax retains the chemical footprints of adult bugs and allows T. basalis females to differentiate hosts of different sex. Traces of female bugs elicited more extensive searching behavior in egg parasitoids than traces of males. The application of n-nonadecane, a compound specific to male N. viridula, on the tarsi of female bugs prevented parasitoid females from distinguishing between host male and host female footprints. Analyses of B. oleracea leaves revealed that epicuticular waxes were mainly composed of linear alkanes, ketones, and secondary alcohols. Alkanes were dominated by n-nonacosane (nC29) and n-hentriacontane (nC31), while male-specific n-nonadecane (nC19) was absent. The ecological significance of these results for parasitoid host location behavior is discussed.     

Thermodynamic Control of Phase Composition and Crystallization of Metal‐Modified Silicon Oxycarbides

Silicon oxycarbides modified with main group or transition metals (SiMOC) are usually synthesized via pyrolysis of sol‐gel precursors from suitable metal‐modified orthosilicates or polysiloxanes. In this study, the phase composition of different SiMOC systems (M = Sn, Fe, Mn, V, and Lu) was investigated. Depending on the metal, different ceramic phases formed. For M = Mn and Lu, MO_x/SiOC ceramic nanocomposites were formed, whereas other compositions revealed the formation of M/SiOC (M = Sn), MSi_x/SiOC (M = Fe) or MC_x/SiOC (M = V) upon pyrolysis. The different phase compositions of the SiMOC materials are rationalized by a simple thermodynamic approach which generally correctly predicts which type of ceramic nanocomposite is expected upon ceramization of the metal‐modified precursors. Calculations show that the thermodynamic stability of the MO_x phase with respect to that of the C–O system is the most important factor to predict phase formation in polymer‐derived SiMOC ceramic systems. A secondary factor is the relative stability of metal oxides, silicates, carbides, and silicides.     

High mobility electron heterostructure wafer fused onto LiNbO3

An (Al,Ga)As heterostructure consisting of a 10 nm wide GaAs single quantum well and an optimized AlAs/GaAs type-II-superlattice barrier is fused onto a new LiNbO₃ substrate by epitaxial lift-off and subsequent wafer bonding. X-electrons formed in the superlattice barrier effectively screen the high mobility electrons in the single quantum well from electronic defects arising at the new hybrid interface. Thus, the electron density as well as its high electron mobility can be preserved in the hybrid system.