Decomposition Formation Research Articles

Influence of Ag replacement on the formation and heating-induced phase decomposition of Zr65Al7.5Co27.5-xAgx (x=5 to 20 at%) glassy alloys

A glass-type alloy with glass transition (GT) and supercooled liquid region was formed at a limited Ag/Co content ratio around 1.2 in melt-spun Zr65Al7.5Co27.5-xAgx (x = 5–20 at%) alloys. This alloy crystallized through two-stage exothermic reactions where an icosahedral (I) phase appears as a primary precipitation phase, followed by the second-stage reaction from glass + I-phase to four crystalline phases. The I-phase has a spherical morphology and its size is about 8 nm. The Zr65Al7.5Co17.5Ag10 alloy without GT keeps a glass-like structure even after heating for 3.6 ks at 750 K above the first exothermic peak, indicating the formation of a cluster-like glassy phase. The Zr65Al7.5Co17.5Ag10 alloy rod keeps a glassy phase in the diameter up to 1.7 mm, though the rod does not show the first-stage exothermic reaction. These features are the same as those for the annealed ribbons, indicating that a clustered glassy phase is also formed for the bulk alloy. This alloy satisfies with the formation criterion of the clustered glassy phase, i.e., large atomic size mismatch and positive heat of mixing for solute elements. The high stability of glassy phase leading to the formation of the clustered glassy phase is due to the resistance to atomic rearrangement of unlike Co and Ag elements. The Zr65Al7.5Co12.5Ag15 alloy exhibits the lowest decomposition temperature (Tx) and the highest hardness, being opposite to the previous relation for hardness to increase with increasing Tx. The relationship between GT and I-phase precipitation suggests that the GT is attributed to the development of I-like medium-range ordered structure.

Formation of a Graphene-Like SiN Layer on the Surface Si(111)

00-The kinetics of the formation and thermal decomposition of a two-dimensional SiN-(8 x 8) nitride layer on a Si(111) surface is studied. The SiN-(8 x 8) structure is a metastable intermediate phase formed during the nitridation of silicon before the formation of a stable amorphous Si3N4 phase. Studying the SiN-(8 x 8) structure by scanning tunneling microscopy shows its complex structure: it consists of an adsorption (8/3 x 8/3) phase, with the lateral period 10.2 angstrom, and a honeycomb structure with a 6 angstrom side of a hexagon that is turned 30 degrees with respect the adsorption phase. The band gap of the SiN-(8 x 8) phase is measured and found to be 2.8 eV, which is smaller compared to the band gap of the -Si3N4 crystal phase 5.3 eV. The interplanar spacings in the (AlN3)/(SiN)(2) structure on the Si(111) surface are measured. The spacings are 3.3 and 2.86 angstrom in SiN and AlN, respectively. Such interplanar spacings are indicative of weak van der Waals interaction between the layers. A model of the SiN-(8 x 8) structure as a flat graphene-like layer is suggested. The model is consistent with the diffraction and microscopy data.

Chlorine-initiated oxidation of <i>n</i>-alkanes under high-NO<sub><i>x</i></sub> conditions: insights into secondary organic aerosol composition and volatility using a FIGAERO–CIMS

Abstract. Chlorine-initiated oxidation of n-alkanes (C8−12) under high-nitrogen oxide conditions was investigated. Observed secondary organic aerosol yields (0.16 to 1.65) are higher than those for OH-initiated oxidation of C8−12 alkanes (0.04 to 0.35). A high-resolution time-of-flight chemical ionization mass spectrometer coupled to a Filter Inlet for Gases and AEROsols (FIGAERO–CIMS) was used to characterize the gas- and particle-phase molecular composition. Chlorinated organics were observed, which likely originated from chlorine addition to the double bond present on the heterogeneously produced dihydrofurans. A two-dimensional thermogram representation was developed to visualize the composition and relative volatility of organic aerosol components using unit-mass resolution data. Evidence of oligomer formation and thermal decomposition was observed. Aerosol yield and oligomer formation were suppressed under humid conditions (35 % to 67 % RH) relative to dry conditions (under 5 % RH). The temperature at peak desorption signal, Tmax, a proxy for aerosol volatility, was shown to change with aerosol filter loading, which should be constrained when evaluating aerosol volatilities using the FIGAERO–CIMS. Results suggest that long-chain anthropogenic alkanes could contribute significantly to ambient aerosol loading over their atmospheric lifetime.

Cobalt nanoparticles mechanically deposited on α‐Al2O3: a competitive catalyst for the production of hydrogen through ethanol steam reforming

AbstractBACKGROUNDCobalt (Co)‐containing materials are promising catalysts for Ethanol Steam Reforming (ESR) allowing renewable hydrogen production. Usually, complex catalysts are used where Co, produced by impregnation or coprecipitation, is supported on oxide or carbon‐based materials. Here Co nanoparticles (CoNPs) mechanically mixed with α‐Al2O3 are prepared and tested for ESR.RESULTSCoNPs mechanically mixed with α‐Al2O3 are very active catalysts in ESR at 773 K, even when retaining amorphous structure for Co. After annealing at 773 K, CoNPs crystallize into a cubic structure. These materials are even more active in ESR at 773 K with hydrogen yields ≈70% and nearly stable performances after 8 h on‐stream. Hydrogen yield is limited by the formation of some CH4, probably as a result of acetaldehyde formation and successive decomposition. Two kinds of Co particles are found in the used but still active catalysts: the former ones interacting with alumina and embedded in graphitic carbon, that are probably the most active components, whereas the latter are ‘isolated’ and partly aggregated CoNPs embedded in carbon nanotubes, which are probably less active in ESR.CONCLUSIONSMechanical mixing of CoNPs with α‐Al2O3 produced active and stable catalysts for ESR. The ‘chemical’ interaction of CoNPs with the support may not be an important factor for ESR catalysts. © 2018 Society of Chemical Industry

Stability of Hydrogen Hydrates from Second-Order Møller-Plesset Perturbation Theory.

The formation of gas hydrates and clathrates critically depends on the interaction between the host water network and the guest gas species. Density functional calculations can struggle to quantitatively capture these dispersion-type interactions. Here, we report wave function-based calculations on hydrogen hydrates that combine periodic Hartree-Fock with a localized treatment of electronic correlation. We show that local second-order Møller-Plesset perturbation theory (LMP2) reproduces the stability of the different filled-ice-like hydrates in excellent agreement with experimental data. In contrast to various dispersion-corrected density functional theory implementations, LMP2 correctly identifies the pressures needed to stabilize the C0, C1, and C2 hydrates and does not find a spurious region of stability for an ice-Ih-based dihydrate. Our results suggest that LMP2 or similar approaches can provide quantitative insights into the mechanisms of formation and eventual decomposition of molecular host-guest compounds.

(Invited) In Situ XPS and Aes Characterization of Cathode Surface Chemistry Evolution and Its Effect on Cycle Life

Lithium ion battery cycle life varies significantly with cathode chemistry, structure, and properties. However, it is well known that deconstructing cycled and faded full cells and reassembling them as half cells can retrieve much of the lost capacity. Therefore, cathode chemistry must play a role in exacerbating other side reactions that induce capacity fade without significantly degrading themselves.(1, 2) A well known example in the literature is the effect of Mn(II) dissolution from the cathode in degrading the anode.(3) However, analogous fade mechanisms resulting from interactions between cathode and anode side reactions are not as well understood in other systems. Since such ‘cross-talk’ interactions can have multiple complex steps and inherently have surface reaction contributions, they can be difficult to characterize and identify. The complex nature of degradation reactions motivates the development of in situ and operando characterization techniques. Of particular value for understanding associated mechanisms are surface sensitive in situ characterization techniques such as x-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), etc. Such methods require specialized electrochemical cells that function in vacuum. Ideally, we seek to relate such information to associated changes occurring in the bulk of the electrode particles, in the physical properties of the underlying material, and in the commercially relevant battery environments. We will describe the development of novel Li ion cells suitable for in situ and operando characterization via a variety of electron, optical, and x-ray based methods.(4-7) In situ surface spectroscopy is revealing new insights into the importance of cathode solid electrolyte interphase (SEI) stability during cycling in affecting cycle life. We compare cells based on LiCoO2, LiMn2O4, LiNiO2, Li[Ni1/3Mn1/3Co1/3]O2, and LiFePO4. In particular, we find that gaseous products produced during SEI formation and SEI decomposition drive degradation at the counter electrode. This has motivated ongoing work to investigate the role of gas in affecting capacity fade in traditional battery configurations, as well as model studies employing in situ characterization during cycling in gaseous environments. We find that the presence of different gases commonly evolved during cycling have varying effects on the rate of capacity fade. The effect is non-linear and the gases have different effects in full cells based on different cathode chemistries. This presentation will highlight the relationships between surface reactions observed in situ during model experiments, gas phase evolution in full cells, and the effects of subsequent reactions on capacity fade. The results provide a reasonable basis for understanding capacity fade associated with several commercially relevant cathode systems and can be extended to provide insights into designing next-generation intercalation cathodes. B. Ziv, V. Borgel, D. Aurbach, J.-H. Kim, X. Xiao and B. R. Powell, Journal of The Electrochemical Society, 161, A1672 (2014).J.-H. Kim, N. P. W. Pieczonka, Z. Li, Y. Wu, S. Harris and B. R. Powell, Electrochimica Acta, 90, 556 (2013).D. H. Jang, Y. J. Shin and S. M. Oh, J. Electrochem. Soc., 143, 2204 (1996).C.-Y. Tang and S. J. Dillon, J. Electrochem. Soc., 163, A1660 (2016).C.-Y. Tang, R. T. Haasch and S. J. Dillon, Chem. Commun. (Cambridge, U. K.), 52, 13257 (2016).C.-Y. Tang, K. Leung, R. T. Haasch and S. J. Dillon, ACS Appl. Mater. Interfaces, 9, 33968 (2017).C.-Y. Tang, Y. Ma, R. T. Haasch, J.-H. Ouyang and S. J. Dillon, J. Phys. Chem. Lett., 8, 6226 (2017).

Photocatalytic Water Splitting for Solar Hydrogen Production Using the Carbonate Effect and the Z‐Scheme Reaction

AbstractThe development of innovative technologies for solar energy conversion and storage is important for solving the global warming problem and for establishing a sustainable society. The photocatalytic water‐splitting reaction using semiconductor powders has been intensively studied as a promising technology for direct and simple solar energy conversion. However, the evolution of H2 and O2 gases in a stoichiometric ratio (H2/O2 = 2) is very difficult owing to various issues, such as an unfavorable backward reaction and mismatched band potentials. Two important findings have widened the variety of photocatalysts available for stoichiometric water‐splitting, viz. the carbonate anion effect and the Z‐scheme photocatalytic reaction using a redox mediator. The bicarbonate anion has been found to act as a redox catalyst via preferential peroxide formation and subsequent decomposition to O2. As the Z‐scheme reaction using a redox mediator mitigates band potential mismatches, it is widely applicable for various visible‐light‐active photocatalysts. This review describes the development of photocatalytic water‐splitting for solar hydrogen production using the carbonate anion effect and the Z‐scheme reaction. Moreover, recent developments in photocatalysis–electrolysis hybrid systems, an advanced Z‐scheme reaction concept, are also reviewed for practical and economical hydrogen production.

Understanding methanol synthesis from CO/H2 feeds over Cu/CeO2 catalysts

Understanding copper-based methanol synthesis catalysts as a function of catalyst type and applied reaction conditions is an area of active industrial and academic research. In this work, results of a methanol synthesis study over a Cu/CeO2 catalyst using CO/H2 feeds are presented, using catalyst performance data combined with catalyst characterisation information (DRIFTS, XPS, TPR, XRD) obtained during start-up and under steady state methanol synthesis conditions. The results indicate that the active site and reaction mechanism for methanol synthesis over Cu/CeO2 are different from the conventional Cu/ZnO/Al2O3 catalyst, with CO, rather than CO2, being the carbon source for methanol. Fixed-bed micro reactors were employed to obtain catalyst performance data in discrete bed sectors using experimental spatial discretisation methods. Methanol synthesis activity over Cu/CeO2 from CO/H2 is preceded by transient CO2 formation, with the onset of methanol synthesis activity observed when the CO2 formation reaches its peak. Considerable differences between reactor bed sectors are observed during start-up, with CO2 formation, CO2 re-adsorption (as surface carbonates and formates), methanol formation and transient methanol decomposition occurring to different degrees and at different time scales.The interfaces of defective CeO2−x in contact with highly dispersed copper particles form the active site of the catalyst under steady-state methanol synthesis conditions, with the copper oxidation state being a combination of 0 and +1 states. The CeO2−x defect sites are formed during start-up upon reaction of CeO2 with CO. The CO2 thus formed leads to catalyst deactivation due to build-up of carbonate and formate species on the catalyst surface. A gradient of progressive poisoning/deactivation is observed going further down the catalyst bed, where the level of deactivation can be controlled by modifying the CO content in the feed. CO2 thus acts as a poison for the Cu/CeO2 catalyst, and addition of even low levels of CO2 to the feed leads to an evenly deactivated catalyst. During start-up in CO/H2, transient CO2 and H2O formation is also observed in the case of Cu/ZnO/Al2O3, ascribed to partial reduction of ZnO, similar to the partial reduction of CeO2 as seen in Cu/CeO2. The key difference between the two systems is the absence of CO2 and H2O re-adsorption on Cu/ZnO/Al2O3, resulting in stable methanol production distributed equally over the catalyst bed. Introduction of CO2 to the CO/H2 feed in case of Cu/ZnO/Al2O3 leads to reduction of Cu+ to Cu0 and a change in methanol synthesis mechanism from CO to CO2 hydrogenation.

Deep ALMA photometry of distant X-ray AGN: improvements in star formation rate constraints, and AGN identification

We present the star formation rates (SFRs) of a sample of 109 galaxies with X-ray selected active galactic nuclei (AGN) with moderate to high X-ray luminosities (L(2-8keV)= 10^42-10^45 erg/s), at redshifts 1 < z < 4.7, that were selected to be faint or undetected in the Herschel bands. We combine our deep ALMA continuum observations with deblended 8-500{\mu}m photometry from Spitzer and Herschel, and use infrared (IR) SED fitting and AGN - star formation decomposition methods. The addition of the ALMA photometry results in an order of magnitude more X-ray AGN in our sample with a measured SFR (now 37%). The remaining 63% of the sources have SFR upper limits that are typically a factor of 2-10 times lower than the pre-ALMA constraints. With the improved constraints on the IR SEDs, we can now identify a mid-IR (MIR) AGN component in 50% of our sample, compared to only ~1% previously. We further explore the F870{\mu}m/F24{\mu}m-redshift plane as a tool for the identification of MIR emitting AGN, for three different samples representing AGN dominated, star formation dominated, and composite sources. We demonstrate that the F870{\mu}m/F24{\mu}m-redshift plane can successfully split between AGN and star formation dominated sources, and can be used as an AGN identification method.

Porous MnO as efficient catalyst towards the decomposition of Li2CO3 in ambient Li-air batteries

The undesirable formation and low-efficient decomposition of Li2CO3 on cathode of Li-air battery, is an insurmountable obstacle for its practical applications. High efficient cathode catalysts toward Li2CO3 decomposition still have not been thoroughly studied. Here, we report a porous MnO prepared by heating precursor using a facile ethanol refluxing method and employed as high efficient cathode catalyst in ambient Li-air battery for Li2CO3 decomposition. Compared with the pure carbon electrode, it demonstrated an improved electrochemical performance of more than 100 cycles in ambient air and low charging overpotential. At the 100th cycle, the discharge/charge terminal voltage of MnO cathode remains 2.65/4.00 V, which is much higher/lower than that of pure carbon electrode (2.36/4.44 V). These results reveal that the superior catalytic performance of MnO towards the Li2CO3 decomposition during charging. Additionally, we carefully examined the time- and depth-dependent discharge products distributions in the carbon-containing cathode, especially for the formation of Li2CO3, by X-ray photoelectron spectroscopy. The Li2CO3 was initially formed on the surface of Li2O2, and then gradually into the inner part of the Li2O2, which reveals that the Li2O2 is inclined to react with CO2 rather than the carbon based electrode materials. These results present essential information for the development of high efficient cathode catalysts toward the high-efficient decomposition of Li2CO3 for practical Li-air batteries in ambient air.

Silicon Based Anodes and New Electrolytes for Next Generation Lithium-Ion Batteries and Lithium-Sulfur Batteries

State of the art lithium-ion batteries containing a transition metal cathode and a graphite anode have nearly reached their energy density limit. Due to its high theoretical gravimetric capacity (3590 mAh g-1 Li15Si4) and its low working potential, silicon is an attractive candidate to at least partially substitute the graphite anode [1]. Lithium-sulfur batteries are a promising battery system because of the high capacitive sulfur cathode (1672 mAh g-1) and low material costs. But current cells mainly suffer from the instable lithium anode. Lithiated silicon could be a more stable alternative preventing dendrite formation and electrolyte decomposition for that next generation battery cell type [2]. During the lithiation process of silicon a large undesirable volume expansion occurs. This volume change leads to pulverisation of the silicon and a steady reformation of the solid electrolyte interface (SEI). The electrolyte is consumed and the capacity declines rapidly. This is why silicon anodes with high quantities of silicon are hindered so far. To overcome these obstacles nanostructured silicon carbon composite anodes are very promising. The volume change is potentially compensated by the free volume between silicon core and carbon shell and a stable SEI is ensured at the surface of the carbon shell preventing electrolyte consumption during cycling. These void structures are mainly generated using silica templates which deliver a precise control of the void structure, but need to be removed by toxic hydrofluoric acid and laborious washing steps. In this work an easily scalable process without etching treatment is presented. The void structure results from the removal of a sacrificial template layer on the surface of commercially available silicon particles during the carbonization of the silicon carbon composite within a concerted process. For general electrochemical characterization the silicon carbon composite electrodes are galvanostatic cycled vs. metallic lithium in half cells with different electrolyte systems on coin cell as well as on pouch cell level. The silicon carbon composite electrodes reveal a much higher capacity and cycle stability compared to bare silicon nanoparticles and Si-C composites without void structure. High area capacities above 2.5 mAh cm-2 were reached. In sulfur-lithiated-silicon (SLS) full cells the cycle stability with low lithium excess and the enhanced volumetric energy densities using lithiated silicon anodes was analysed. The electrochemical performance of SLS cells with silicon carbon void structures are compared to that with hierarchical columnar silicon anodes, recently introduced by our group [3]. The feasibility of both SLS cell concepts was successfully demonstrated in coin and in pouch cells. A new electrolyte composition is introduced to enhance the cycle life by stabilization of the SEI and prevention of the electrolyte depletion [4]. Finally, prospects, limits and suggestion for further research directions for SLS cells are envisioned, based on the estimation of the expectable energy density. [1] J. K. Lee, C. Oh, N. Kim, J.-Y. Hwang, Y.-K. Sun, J. Mater. Chem. A 2016, DOI: 10.1039/c6ta00265j [2] J. Brückner, S. Thieme, F. Boettger-Hiller, I. Bauer, H. T. Grossmann, P. Strubel, H. Althues, S. Spange, S. Kaskel, Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201302169 [3] M. Piwko, T. Kuntze, S. Winkler, S. Straach, P. Härtel, H. Althues, S. Kaskel, J. Power Sources 2017 DOI: 10.1016/j.jpowsour.2017.03.080 [4] M. Piwko, S. Thieme, C. Weller, H. Althues, S. Kaskel, J. Power Sources 2017 DOI: 10.1016/j.jpowsour.2017.07.046

Experimental and DFT simulation study of a novel felodipine cocrystal: Characterization, dissolving properties and thermal decomposition kinetics

In this study, a new cocrystal of felodipine (Fel) and glutaric acid (Glu) with a high dissolution rate was developed using the solvent ultrasonic method. The prepared cocrystal was characterized using X-ray powder diffraction, differential scanning calorimetry, thermogravimetric (TG) analysis, and infrared (IR) spectroscopy. To provide basic information about the optimization of pharmaceutical preparations of Fel-based cocrystals, this work investigated the thermal decomposition kinetics of the Fel–Glu cocrystal through non-isothermal thermogravimetry. Density functional theory (DFT) simulations were also performed on the Fel monomer and the trimolecular cocrystal compound for exploring the mechanisms underlying hydrogen bonding formation and thermal decomposition. Combined results of IR spectroscopy and DFT simulation verified that the Fel–Glu cocrystal formed via the NH⋯OC and CO⋯HO hydrogen bonds between Fel and Glu at the ratio of 1:2. The TG/derivative TG curves indicated that the thermal decomposition of the Fel–Glu cocrystal underwent a two-step process. The apparent activation energy (Ea) and pre-exponential factor (A) of the thermal decomposition for the first stage were 84.90 kJ mol−1 and 7.03 × 107 min−1, respectively. The mechanism underlying thermal decomposition possibly involved nucleation and growth, with the integral mechanism function G(α) of α3/2. DFT calculation revealed that the hydrogen bonding between Fel and Glu weakened the terminal methoxyl, methyl, and ethyl groups in the Fel molecule. As a result, these groups were lost along with the Glu molecule in the first thermal decomposition. In conclusion, the formed cocrystal exhibited different thermal decomposition kinetics and showed different Ea, A, and shelf life from the intact active pharmaceutical ingredient.

An Inorganic Electrolyte Li–O2 Battery with High Rate and Improved Performance

As an alternative to air intolerant nonaqueous electrolytes, a LiNO3–KNO2–CsNO3 (37:39:24) eutectic salt mixture at 140 °C is investigated as a molten electrolyte for Li–O2 batteries. The inorganic eutectic promotes the highly reversible formation–decomposition mechanism of Li2O2, as a robust electrolyte in oxidative conditions. An in situ formed shielding layer composed of Li2O and Li3N keeps the Li anode intact during vigorous cell conditions, providing faster Li-ion diffusion kinetics and stable cycling performance. We report an inorganic electrolyte Li–O2 battery that exhibits the lowest charge–discharge overpotentials of 40 mV and high rate capabilities at 3 mA/cm2 and is cycleable up to 200 times at a restricted capacity of 500 mAh/gcarbon. The performance is achieved on a bare carbon electrode with a round-trip energy efficiency close to 98%. This work emphasizes that the use of an inorganic electrolyte would significantly eliminate the side reactions associated with charge–discharge reactions and ...

Molecular Dynamics Simulation of Methane Hydrate Formation and Dissociation in the Clay Pores with Fatty Acids

Natural gas hydrate is a promising energy resource, but it is challenging to recover methane from the clay pores rich with sediment organic matter due to the inadequate information on hydrate evolution in the organoclay complex. Molecular dynamics simulations were conducted to investigate the methane hydrate formation and dissociation in the sodium montmorillonite interlayer (Na-MMT) with fatty acids by characterizing the four-body structural order parameter, radial distribution functions, and cage types. Results demonstrated the slight inhibition of fatty acid types (butyric and isovaleric acid) at molar concentrations (0.6% and 1.5%) on the methane hydrate formation in the context of this study. This finding was a result of several main processes including the formation of “quasihydrate” water structures around the hydrophobic carbon chains of fatty acids, the disruption of hydrogen bond networks by the hydrophilic carboxyl of fatty acids, and the restriction on the thermal motion of the sodium ions by coordination with the carboxyl of fatty acids. Results also demonstrated the promotion of fatty acids on the methane hydrate dissociation in the Na-MMT, which highlighted the role of fatty acid accumulation in the accelerated breakdown of hydrates at relatively low decomposing temperature. Overall results provided theoretical support for better understanding the formation mechanisms and decomposition strategies of methane hydrates in the heterogeneous sediment environments since fatty acids are microbial intermediates of special importance in the hydrate-bearing sediment.

Формирование графеноподобного слоя SiN на поверхности (111)Si

Abstract00-The kinetics of the formation and thermal decomposition of a two-dimensional SiN-(8 × 8) nitride layer on a Si(111) surface is studied. The SiN-(8 × 8) structure is a metastable intermediate phase formed during the nitridation of silicon before the formation of a stable amorphous Si_3N_4 phase. Studying the SiN-(8 × 8) structure by scanning tunneling microscopy shows its complex structure: it consists of an adsorption (8/3 × 8/3) phase, with the lateral period 10.2 Å, and a honeycomb structure with a ~6 Å side of a hexagon that is turned 30° with respect the adsorption phase. The band gap of the SiN-(8 × 8) phase is measured and found to be ~2.8 eV, which is smaller compared to the band gap of the β-Si_3N_4 crystal phase 5.3 eV. The interplanar spacings in the (AlN_3)/(SiN)_2 structure on the Si(111) surface are measured. The spacings are 3.3 and 2.86 Å in SiN and AlN, respectively. Such interplanar spacings are indicative of weak van der Waals interaction between the layers. A model of the SiN-(8 × 8) structure as a flat graphene-like layer is suggested. The model is consistent with the diffraction and microscopy data.

Simultaneous grain refinement and nanoscale spinodal decomposition of β phase in Ti-Nb-Ta-Zr alloy induced by ultrasonic mechanical impacts

It was found that ultrasonic nanocrystal surface modification (UNSM) treatment performed on Ti-Nb-Ta-Zr (TNTZ) alloy surface to produce a gradient nanostructured surface layer also resulted in the nano-scale spinodal decomposition of β phase. For the first time, nano-scale spinodal decomposition of β phase induced by ultrasonic mechanical impacts was observed in a short time without any age treatment. The peak shift of XRD to lower angles, (110) β XRD peak splitting, overlapped and asymmetric XRD peaks, satellite reflections in the SAED pattern, and lattice straining in HR-TEM images confirmed nano-scale spinodal decomposition of β phase in the TNTZ alloy induced by UNSM treatment. Nano-scale EDS line-scan probe analysis revealed the β phase separation into nano-scale domains of Ti-rich (β1) and Ti-depleted (β2) phases. HR-TEM images showed the semi-coherent arrangement of Ti-rich (β1) and Ti-depleted (β2) regions. Nanosized grain formation and spinodal decomposition of β phase were induced simultaneously by UNSM treatment. The nano-scale grains, high dislocation density, and semi-coherent interface between the spinodally separated phases contributed to the high hardness of the UNSM-treated TNTZ alloy specimen.

Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals.

A key uncertainty in the heterogeneous oxidation of carboxylic acids by hydroxyl radicals (OH) in aqueous-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chemistry. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimolecular decomposition of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of organic carbon between the gas and aqueous phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidation of citric acid aerosol at high relative humidity. The reaction scheme, containing both free-radical and acid-base elementary reaction steps with physically validated rate coefficients, accurately predicts the experimentally observed molecular composition, particle size, and average elemental composition of the aerosol upon oxidation. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimolecular fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equilibria activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low molecular weight, highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and volume growth that accompany oxidation. These results frame the oxidative lifecycle of atmospheric aerosol: it is governed by feedbacks between reactions that first increase the particle oxidation state, then eventually promote water uptake and acid-base chemistry. When coupled to free-radical reactions, acid-base channels lead to formation of low molecular weight gas-phase reaction products and decreasing particle size.

The sequestration and turnover of soil organic carbon in subtropical leucaena-grass pastures

Abstract The sequestration of soil organic carbon (SOC) in agricultural systems has the potential to partially mitigate climate change by removing carbon dioxide (CO2) from the atmosphere. Furthermore, incorporating forage legumes into a grazing system may aid in C storage, as mature unfertilized pasture soils are typically nitrogen (N) limited. Leucaena leucocephala, a leguminous shrub, is often incorporated into grazing systems to provide a source of protein for cattle production. Additionally, leucaena improves the N fertility of grazing lands. Despite this, there is limited research on the C and N dynamics beneath leucaena-grass pastures, particularly at depth in the soil profile (1 m). We assessed the potential of leucaena to sequester SOC by estimating the origin (C3 or C4), quantity and vertical distribution of SOC stocks. A chronosequence of seasonally grazed leucaena stands (0, 9, 22, 34, 40 years) were sampled for organic C and natural δ13C abundance. Pasture type affected SOC stocks within the upper 0.2 m (grass C signature. Hence, SOC sequestration primarily occurred due to N accretion by leucaena (36 kg N ha−1 year−1) contributing to the increased C4 grass productivity, humus formation and slowed SOC decomposition. Therefore, there is a need to maintain soil fertility in grazed pastures to ensure SOC stocks are accreted and conserved.

Time-Varying Output Formation for Linear Multiagent Systems via Dynamic Output Feedback Control

Time-varying output formation control problems for linear multiagent systems with directed interaction topologies are studied using a dynamic output feedback approach. First, a dynamic output formation protocol is constructed using the outputs of neighboring agents. Based on the output decomposition, observability decomposition and output formation decomposition, necessary and sufficient conditions for linear multiagent systems to achieve time-varying output formations are presented. An explicit expression of the output formation reference function is derived to describe the macroscopic movement of the entire output formation. Then, an algorithm with three steps is proposed to design the dynamic output formation protocol, where the gain matrices of the protocol can be determined by solving an algebraic Riccati equation. It is shown that if the dynamics of each agent is stabilizable, the solvability of the algorithm can be guaranteed. Moreover, a feasible time-varying output formation set is derived, and the stability of the proposed algorithm is proven based on the separation principle and Lyapunov stability theory. Finally, numerical simulation is presented to demonstrate the effectiveness of the theoretical results.

Influence of cryochemical and ultrasonic processing on the texture and thermal decomposition of xerogels and properties of nanoceramics in the ZrO2〈Y2O3〉–Al2O3 system

We have studied the influence of cryochemical and ultrasonic processing on the formation, structure, particle size, and thermal decomposition of xerogels in the ZrO2〈Y2O3〉–Al2O3 (20 wt %) system. Nanopowders of tetragonal-zirconia-based solid solutions with a high degree of tetragonality (c/a = 1.4366) have been synthesized. Al2O3 has been shown to slow down t-ZrO2 crystallite growth in the temperature range 600–1400°C. We have optimized nanopowder consolidation conditions, obtained nanoceramics stable to low-temperature “aging” in a humid medium, and investigated their physicochemical and mechanical properties.