Efficient production of nanoporous silicon carbide from rice husk at relatively low temperature

Nanoporous silicon carbide with a specific surface area of up to 186.45 m² g⁻¹ has been efficiently synthesized from waste rice husk using a magnesiothermic reduction at 950 °C as a key step. Throughout the entire process, the recovery rates of silicon, potassium and phosphorus from rice husk can reach 88.46, 91.5 and 65.5%, respectively. Turning rice husk waste into a real treasure, this promising method for producing porous SiC protects the environment and brings economic benefits.     

Synthesis of 4‐Arylthio‐3‐hydroxyphthalate by the Diels–Alder Reactionof 4‐Arylthio‐3‐hydroxy‐2‐pyrone

Treatment of 4‐arylthio‐3‐hydroxy‐2‐pyrones with acetylenedicarboxylates gave 4‐arylthio‐3‐hydroxyphthalates by the base‐catalyzed Diels–Alder reaction via a decarboxylation in good yields.     

Twist-bend nematics,liquid crystal dimers,structure–property relations

ABSTRACT

One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (N_TB) and determine its properties. During our earlier hunt for the N_TB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the N_TB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the ²H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.     

Topological conjugacy of constant length substitution dynamical systems

Primitive constant length substitutions generate minimal symbolic dynamical systems. In this article we present an algorithm which can produce the list of injective substitutions of the same length that generate topologically conjugate systems. We show that each conjugacy class contains infinitely many substitutions which are not injective. As examples, the Toeplitz conjugacy class contains three injective substitutions (two on two symbols and one on three symbols), and the length two Thue–Morse conjugacy class contains twelve substitutions, among which are two on six symbols. Together, they constitute a list of all primitive substitutions of length two with infinite minimal systems which are factors of the Thue–Morse system.     

Existence of stationary turbulent flows with variable positive vortex intensity

We prove the existence of stationary turbulent flows with arbitrary positive vortex circulation on non-simply connected domains. Our construction yields solutions for all real values of the inverse temperature with the exception of a quantized set, for which blow-up phenomena may occur. Our results complete the analysis initiated in Ricciardi and Zecca (2016).     

Mesogenic and optical properties of α,α′‐bis(4‐alkoxyphenylethynyl)oligothiophenes

Liquid crystalline α,α′‐bis(4‐alkoxyphenylethynyl)oligothiophenes (bi‐ and ter‐thiophene) have been synthesized and their mesogenic behaviour and optical properties investigated. They all exhibited a nematic mesophase, and compounds with long alkoxy chains also showed lamellar phases. Increasing the number of thiophene units increased both the transition temperatures and the mesophase ranges. As for their optical properties, incorporating more thiophene units results in red‐shifted absorption and emission spectra, slightly enhanced quantum efficiency, and a larger Stoke's shift. Most importantly, in terms of the absorption and emission maxima, the incorporation of one 4‐alkoxyphenylethynyl moiety was found to be equivalent to adding one thiophene ring.     

Dispersive liquid–liquid microextraction and spectrophotometric determination of cobalt in water samples

A new simple and rapid dispersive liquid–liquid microextraction has been applied to preconcentrate trace levels of cobalt as a prior step to its determination by spectrophotometric detection. In this method a small amount of chloroform as the extraction solvent was dissolved in pure ethanol as the disperser solvent, then the binary solution was rapidly injected by a syringe into the water sample containing cobalt ions complexed by 1-(2-pyridylazo)-2-naphthol (PAN). This forms a cloudy solution. The cloudy state was the result of chloroform fine droplets formation, which has been dispersed in bulk aqueous sample. Therefore, Co-PAN complex was extracted into the fine chloroform droplets. After centrifugation (2 min at 5000 rpm) these droplets were sedimented at the bottom of conical test tube (about 100 µL) and then the whole of complex enriched extracted phase was determined by a spectrophotometer at 577 nm. Complex formation and extraction are usually affected by some parameters, such as the types and volumes of extraction solvent and disperser solvent, salt effect, pH and the concentration of chelating agent, which have been optimised for the presented method. Under optimum conditions, the enhancement factor (as the ratio of slope of preconcentrated sample to that obtained without preconcentration) of 125 was obtained from 50 mL of water sample, and the limit of detection (LOD) of the method was 0.5 µg L^?1and the relative standard deviation (RSD, n = 5) for 50 µg L^?1 of cobalt was 2.5%. The method was applied to the determination of cobalt in tap and river water samples.     

Effect of spacer length on mesomorphic behaviours of chiral liquid crystal compounds containing (-)-menthyl

ABSTRACT

A homologous series of new chiral liquid crystal compounds, MnBEB (n = 4–10), was prepared by covalently linking a chiral (–)-menthyl with biphenyl-benzoate via a dicarboxylic spacer of varying length and parity. A combination of analysis methods, such as FT-IR, ¹H NMR spectra, differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction was carried out to systematically investigate their phase structures and phase transition behaviours. The length and parity of the flexible spacers has a profound influence on the T_m and T_c and a modest odd-even effect is observed for the chiral liquid crystal compounds MnBEB. Only compound M4BEB developed an N* phase with selectively reflection on heating and a blue phase on cooling process. In addition, increasing the length of the flexible spacers tends to narrow the temperature range of the N* phase and widen the smectic phase, moreover, the pitch becomes longer with the spacer increases.     

TRIFLUOROMETHYLTHIOLATION OF NORBORNENE

Treatment of norbornene with trifluoromethylsulfenyl chloride at ?80°C furnishes, in addition to trifluoromethylthionortricyclane, four isomeric (chloro) (trifluoromethylthio)-norbornanes and bis-(2, 6-trifluoromethylthio)norbornane. The probable mechanism of the formation of the various compounds via free radical intermediates and their mass spectral characterization are described in this communication.     

Propagation of a pre-existing turbulent fluid fracture

The propagation of rough and smooth wall pre-existing turbulent fluid fractures is investigated. The laminar fluid fracture is included as a special case for comparison. Lubrication theory is assumed to apply in the fracture and turbulence is introduced through the wall shear stress. The Perkins–Kern–Nordgren approximation is made in which the fluid pressure is proportional to the half-width of the fracture. The fracture half-width satisfies a non-linear diffusion equation. By using a linear combination of the Lie point symmetries of the non-linear diffusion equation a group invariant solution for the fracture length, volume and half-width is derived. The evolution of the length, half-width and mean flow velocity is analysed for a range of working conditions at the fracture entry. It is found that the mean flow velocity increases approximately linearly along the fracture.