Boron and nitrogen co-doped graphene nanosheets for NO and NO2 gas sensing

Using dispersion-corrected density functional theory calculations, the adsorption behavior of NO and NO₂ molecules is studied over B-doped and BN co-doped graphene sheets (BC_mN_n-Gr; $m,n=0,1,2,3$ and $m+n=3$). To examine practical gas sensing application and selectivity, the adsorption of H₂O, CO and CO₂ molecules is also studied on the BC_mN_n-Gr surfaces. It is found that the preferred adsorption site for the adsorption of these molecules is above the B atom due to accumulation of a local positive charge. Meanwhile, the incorporation of nitrogen atoms in BC_mN_n-Gr makes a substantial increase in the adsorption energies of NO and NO₂, mainly due to the shift in the Fermi energy and electron (donor) concentration states of these surfaces. According to our results, the electronic structure of BC₃-Gr, BC₂N-Gr and BCN₂-Gr is sensitive to NO and NO₂ as evidenced by relatively large variation of the electronic structure as well as charge-transfer values. To address the curvature effect of BC_mN_n-Gr nanosheets on the adsorption and sensing properties of NO and NO₂, the adsorption of these molecules is also investigated over B-doped and BN-codoped ($6,6$) carbon nanotubes. The calculations also indicate that BN co-doped graphene sheets can be used as an efficient and promising gas sensing material for detecting NO and NO₂ molecules in the presence of H₂O, CO and CO₂.     

First principle investigation of H2Se,H2Te and PH3 sensing based on graphene oxide

Detecting toxic gases is of great importance to protect our health and preserve the quality of life. In this work, graphene (G) and graphene oxide with three different modifications (G–O, G–OH, and G–O–OH) have been used to detect hydrogen selenide (H₂Se), hydrogen telluride (H₂Te), and phosphine (PH₃) molecules based on Atomistic ToolKit Virtual NanoLab (ATK-VNL) package. The adsorption energy ($E_{\mathrm{ads}}$), adsorption distance (D), charge transfer (ΔQ), density of states (DOS), and band structure have been investigated to confirm the adsorption of H₂Se, H₂Te, and PH₃ on the surface of G, G–O, G–OH, and G–O–OH systems. The results of G revealed highest $E_{\mathrm{ads}}$ for the case of H₂Te with −0.143 eV. After the functionalization of G surface, the adsorption parameters reflected an improvement due to the presence of the functional groups. Particularly, the highest adsorption energy was found between G–O system and H₂Se gas with $E_{\mathrm{ads}}$ of −0.319 eV. The smallest adsorption distance was found between G–OH system and H₂Se gas. The highest charge transfer was found for the case of H₂Se gas adsorbed on G–O–OH system. By thorough comparison of the adsorption energy, adsorption distance, and charge transfer between G, G–O, G–OH, and G–O–OH systems and the three gases, G–O–OH system can be considered as a potential sensor for H₂Se gas.     

Monodromy in non-integrable systems on certain compact classical phase spaces

On the example of bending vibrational polyads of the acetylene molecule (C₂H₂) in the approximation of the $1:1:1:1$ resonant oscillator with axial symmetry, whose geometry is similar to the n-shell approximation of the perturbed hydrogen atom, we show how remaining invariant tori of the underlying classical non-integrable system form a nontrivial continuous family with monodromy. We read this monodromy off the quantum energy spectrum which was observed experimentally by spectroscopists, and we uncover its origins through the particular topology, geometry, and symmetry. We explain how monodromy characterizes the chaotic region surrounded by the tori. We detail the explicit correspondence between the bending polyads of C₂H₂ and the n-shells of the hydrogen atom, and uncover the dynamical SO(3) symmetry of the bending polyads and the corresponding spherically localized vibrational states.     

On the redox reactions between allyl radicals and NOx

NO_x mitigation is a central focus of combustion technologies with increasingly stringent emission regulations. NO_x can also enhance the autoignition of hydrocarbon fuels and can promote soot oxidation. The reaction between allyl radical (C₃H₅) and NO_x plays an important role in the oxidation kinetics of propene. In this work, we measured the absolute rate coefficients for the redox reaction between C₃H₅ and NO_x over the temperature range of 1000–1252 K and pressure range of 1.5–5.0 bar using a shock tube and UV laser absorption technique. We produced C₃H₅ by shock heating of C₃H₅I behind reflected shock waves. Using a Ti:Sapphire laser system with frequency quadrupling, we monitored the kinetics of C₃H₅ at 220 nm. Unlike low-temperature chemistry, the two target reactions, C₃H₅ + NO → products (R1) and C₃H₅ + NO₂ → products (R2), exhibited a strong positive temperature dependence for this radical-radical type reaction. However, these reactions did not show any pressure dependence over the pressure range of 1.5–5.0 bar, indicating that the measured rate coefficients are close to the high-pressure limit. The measured values of the rate coefficients resulted in the following Arrhenius expressions (in unit of cm³/molecule/s):$k_1\left({\mathrm{C}}_3{\mathrm{H}}_5+\text{NO}\right)=1.49\times {10}^{?10}\exp \left(?6083.6\frac{\mathrm{K}}{T}\right)\left(1017?1252K\right)$$k_2\left({\mathrm{C}}_3{\mathrm{H}}_5+\mathrm{N}{\mathrm{O}}_2\right)=1.71\times {10}^{?10}\exp \left(?3675.7\frac{\mathrm{K}}{T}\right)\left(1062?1250K\right)$To our knowledge, these are the first high-temperature measurements of allyl + NO_x reactions. The reported data will be highly useful in understanding the interaction of NO_x with resonantly stabilized radicals as well as the mutual sensitization effect of NO_x on hydrocarbon fuels.     

Hydrogen oxidation near the second explosion limit in a flow reactor

This work investigates the oxidation of hydrogen near its second explosion limit in a turbulent flow reactor at pressures of 1 to 8 bar, temperatures of 950 K and an equivalence ratio of 0.035. The concentrations of H₂, O₂ and H₂O are measured along the reactor and simulated using several kinetic models from the literature. These experiments demonstrate evident negative pressure dependence from roughly 1 to 4 bar, with further increases in pressure resuming its positive impact on reaction rates. The simulated and measured species concentrations along the reactor generally agree within a factor of 2.Further investigation is then conducted to measure the rate coefficient of reaction H + O₂ (+ M) = HO₂ (+M) (R2), which is one of the most sensitive reactions in hydrogen's oxidation chemistry at these conditions. This investigation is conducted by using nitric oxide (NO) as a dopant and measuring the resulting, quasi-steady-state concentrations of NO₂. The rate coefficients are obtained at 950 – 1010 K. Combined with literature results, an Arrhenius expression is proposed, $k_{2,0}^{N_2}$ = 4.50 × 10²⁰ (T/K)^?1.73 [cm⁶ mole^?2 s^?1], for the reaction rate at the low-pressure limit over 500 K – 2000 K with N₂ as the bath gas. Simulations using the models from the literature with the proposed Arrhenius expression for this reaction then demonstrate improved agreement with the experiments.     

Effect of ultrasonic irradiation power on sonochemical synthesis of gold nanoparticles

In this work, optimized size distribution and optical properties in the colloidal synthesis of gold nanoparticles (GNPs) were obtained using a proposed ultrasonic irradiation assisted Turkevich-Frens method. The effect of three nominal ultrasound (20 kHz) irradiation powers: 60, 150, and 210 W have been analyzed as size and shape control parameters. The GNPs colloidal solutions were obtained from chloroauric acid (HAuCl₄) and trisodium citrate (C₆H₅Na₃O₇·2H₂O) under continuous irradiation for 1 h without any additional heat or stirring. The surface plasmon resonance (SPR) was monitored in the UV–Vis spectra every 10 min to found the optimal time for localized SPR wavelength (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$), and the 210 sample procedure has reduced the ${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ localization at 20 min, while 150 and 60 samples have showed ${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ at 60 min. The nucleation and growth of GNPs showed changes in shape and size distribution associated with physical (cavitation, temperature) and chemical (radical generation, pH) conditions in the aqueous solution. The results showed quasi-spherical GNPs as pentakis dodecahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 560 nm), triakis icosahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 535 nm), and tetrakis hexahedron (${\lambda }_{\mathrm{L}\mathrm{S}\mathrm{P}\mathrm{R}}$ = 525 nm) in a size range from 12 to 16 nm. Chemical effects of ultrasound irradiation were suggested in the disproportionation process, electrons of AuCl₂⁻ are rapidly exchanged through the gold surface. After AuCl₄⁻ and Cl⁻ were desorbed, a tetrachloroaurate complex was recycled for the two-electron reduction by citrate, aurophilic interaction between complexes AuCl₂⁻, electrons exchange, and gold seeds, the deposition of new gold atoms on the surface promoting the growth of GNPs. These mechanisms are enhanced by the effects of ultrasound, such as cavitation and transmitted energy into the solution. These results show that the plasmonic response from the reported GNPs can be tuned using a simple methodology with minimum infrastructure requirements. Moreover, the production method could be easily scalable to meet industrial manufacturing needs.     

Observation of field induced anomalous quantum criticality in Ce0.6Y0.4NiGe2 compound

We report the results of our investigation of magnetization and heat capacity on a series of compounds Ce${}_{1?x}$Y_xNiGe₂ ($x=0.1,0.2\text{ and }0.4$) under the influence of external magnetic field. Our studies of the thermodynamic quantity $?\mathrm{d}M/\mathrm{d}T$ on these compounds indicate that magnetic frustration persists in Ce_0.9Y_0.1NiGe₂, as also reported for the parent compound CeNiGe₂. The weak signature of this frustration is also noted in Ce_0.8Y_0.2NiGe₂, whereas, it is suppressed in Ce_0.6Y_0.4NiGe₂. Heat capacity studies on Ce_0.9Y_0.1NiGe₂ and Ce_0.8Y_0.2NiGe₂ indicate the presence of a new magnetic anomaly at high field which indicates that quantum criticality is absent in these compounds. However, for Ce_0.6Y_0.4NiGe₂ such an anomaly is not noted. For this later compound, the magnetic field (H) and temperature (T) dependence of heat capacity and magnetization obey $H/T$ scaling above critical fields. However, the obtained scaling critical parameter (δ) is 1.6, which is away from mean field value of 3. This deviation suggests the presence of unusual fluctuations and anomalous quantum criticality in these compounds. This unusual fluctuation may arise from disorderness induced by Y-substitution.     

Sonochemical conversion of CO2 into hydrocarbons: The Sabatier reaction at ambient conditions

In this study, we investigated an alternative method for the chemical CO₂ reduction reaction in which power ultrasound (488 kHz ultrasonic plate transducer) was applied to CO₂-saturated (up to 3%) pure water, NaCl and synthetic seawater solutions. Under ultrasonic conditions, the converted CO₂ products were found to be mainly CH₄, C₂H₄ and C₂H₆ including large amount of CO which was subsequently converted into CH₄. We have found that introducing molecular H₂ plays a crucial role in the CO₂ conversion process and that increasing hydrogen concentration increased the yields of hydrocarbons. However, it was observed that at higher hydrogen concentrations, the overall conversion decreased since hydrogen, a diatomic gas, is known to decrease cavitational activity in liquids. It was also found that 1.0 M NaCl solutions saturated with 2% CO₂ + 98% H₂ led to maximum hydrocarbon yields (close to 5%) and increasing the salt concentrations further decreased the yield of hydrocarbons due to the combined physical and chemical effects of ultrasound. It was shown that CO₂ present in a synthetic industrial flue gas (86.74% N₂, 13% CO₂, 0.2% O₂ and 600 ppm of CO) could be converted into hydrocarbons through this method by diluting the flue gas with hydrogen. Moreover, it was observed that in addition to pure water, synthetic seawater can also be used as an ultrasonicating media for the sonochemical process where the presence of NaCl improves the yields of hydrocarbons by ca. 40%. We have also shown that by using low frequency high-power ultrasound in the absence of catalysts, it is possible to carry out the conversion process at ambient conditions i.e., at room temperature and pressure. We are postulating that each cavitation bubble formed during ultrasonication act as a “micro-reactor” where the so-called Sabatier reaction -$C{O}_2+4H_2\stackrel{\mathit{Ultrasonication}}{\to }C{H}_4+2H_{2}O$ - takes place upon collapse of the bubble. We are naming this novel approach as the “Islam-Pollet-Hihn process”.     

Thermodynamic calculation of spin scaling functions

Critical phenomena theory centers on the scaled thermodynamic potential per spin $?(\beta ,h)=|t{|}^{p}Y(h|t{|}^{?q})$, with inverse temperature $\beta =1/T$, $h=?\beta H$, ordering field H, reduced temperature $t=t(\beta )$, critical exponents p and q, and function $Y(z)$ of $z=h|t{|}^{?q}$. I discuss calculating $Y(z)$ with the information geometry of thermodynamics. Scaled solutions are found to obtain with three admissible functions $t(\beta )$: 1) $t=e^{?J\beta }$, 2) $t={\beta }^{?1}$, and 3) $t={\beta }_C?\beta$, where J and ${\beta }_C$ are constants. For $p=q$, information geometry yields $Y(z)=\sqrt{1+z^2}$, consistent with the one-dimensional (1D) ferromagnetic Ising model.     

Exchange bias behavior and magnetocaloric effect in Ni2In-type Mn7Sn4 alloy

In this work, the exchange bias behavior and magnetocaloric effect have been studied in Mn₇Sn₄ alloy. The X-ray powder diffraction pattern recorded at room temperature indicates that the sample crystallizes in a single phase with Ni₂In-type hexagonal structure (space group $P{6}_3/\mathit{mmc}$). The maximum magnetic entropy change value across paramagnetic/ferrimagnetic transition is about 3.3 J kg⁻¹ K⁻¹ under the magnetic field change of ${\mu }_0\mathrm{\Delta }H=0\text{-}5\text{T}$. With further cooling, the reentrant spin-glass-like state is obtained below 150 K, for which the exchange bias effect has been observed. The exchange bias field is ∼7.8 mT and ∼6.7 mT at $T=10\text{K}$ when the cooling field is ${\mu }_0{H}_{\mathrm{CF}}=0.1\text{T}$ and 0.5 T, respectively. The magnetic behavior and the origin of exchange bias in Mn₇Sn₄ are discussed.     

Synthesis and microwave dielectric properties of an electronic ceramic Y2WO6 for wireless communications

Y₂WO₆ ceramics were fabricated via a solid-state reaction method and investigated structure stability, densification, microstructure, and dielectric properties at microwave frequency range. Y₂WO₆ crystallized in a monoclinic structure and stabilized to 1500 ^∘C, beyond which the decomposition of Y₆WO₁₂ occurred. Y₂WO₆ ceramic could be sintered into a compact bulk at 1450 ^∘C, which was characterized by a high relative density ∼ 97.6% and a dense microstructure. The favorable dielectric performances were achieved at 1450 ^∘C with a relative permittivity ${\epsilon }_r\sim 11.4$, a quality factor $Q\times f\sim 42,380$ GHz ($f=8.6$ GHz), and a temperature coefficient of resonant frequency ${\tau }_f\sim -49.0$ ppm/^∘C. The MW properties of Y₂WO₆ suggest that it could be useful candidate material for low-loss dielectric resonators.