Chemical stability of superhard rhenium diboride at oxygen and moisture ambient environmental conditions prepared by mechanical milling

In this study, rhenium diboride (ReB₂) was obtained by mechanosynthesis at 640 minutes of milling. The obtained ReB₂ was stored at oxygen and moisture ambient environmental conditions to know the chemical stability. The results indicate that ReB₂ is totally decomposed at oxygen and moisture ambient environmental conditions. Furthermore, the X‐ray diffraction (XRD) analysis of ReB₂ samples after 26 months of storage shows that the final products of degradation are HReO₄ (liquid), H₃BO₃, HBO₂, and ReO₃. Finally, a schematic diagram of the degradation sequence of ReB₂ at oxygen and moisture ambient environmental conditions is proposed and validated with a thermodynamic analysis.     

Isopolyoxorhenates observed by positive ion electrospray mass spectrometry

Unknown positive ion isopolyoxorhenates have been observed using electrospray ionization mass spectrometry (ESI+). The ESI+ studies of ammonium and alkali metal (Na⁺ and K⁺) perrhenate salts in aqueous solution at pH 4.5 show the existence of the series [A_x+1Re^VII_xO_4x]⁺ (where x=1–5 and A=NH₄⁺, Na⁺ and K⁺). In the potassium perrhenate system, the series [K_x+2Re^VRe^VII_xO_4x+3]⁺ (x=0–4) has also been characterised. All of these four series have {AReO₄} as the aggregation unit. In the ammonium perrhenate system, the monomeric Re(VII)-containing species, [(NH₄)₂(H₂ReO₅)]⁺, [(NH₄)₃(HReO₅)]⁺ and [(NH₄)₄(ReO₅)]⁺ were also detected.     

Adsorption of Pb2+ and Cd2+ onto a Novel Activated Carbon‐Chitosan Complex

A novel activated carbon‐chitosan complex adsorbent (ACCA) was prepared via the crosslinking of glutaraldehyde and activated carbon‐(NH₂‐protected) chitosan complex under microwave irradiation. The surface morphology of this adsorbent was characterized. The adsorption of ACCA for Pb²⁺ and Cd²⁺ was investigated. The results demonstrate that ACCA has higher adsorption capacity than chitosan. The adsorption follows pseudo first‐order kinetics. The isotherm adsorption equilibria are better described by Freundlich and Dubinin‐Radushkevich isotherms than by the Langmuir isotherm. The adsorbent can be recycled. These results have important implications for the design of low‐cost and effective adsorbents in the removal of heavy metal ions from wastewaters.     

Molecular beam mass spectrometry studies of nitrogen additions to the gas phase during microwave-plasma-assisted chemical vapour deposition of diamond

In-situ molecular beam mass spectrometry has been used to study the effects of nitrogen-containing gases on the gas-phase composition during microwave plasma chemical vapour deposition (CVD) of diamond. The molecular beam mass spectrometer used in this work extracts gas directly from the plasma bulk via a small sampling orifice inserted into the side of the plasma. The plasma composition was examined for a variety of nitrogen-containing gases added to a standard 1% C:H₂ feedstock. Nitrogen was added to this mixture at a carbon-to-nitrogen ratio, C:N1:1, in one of the following forms: N₂, NH₃, CH₃NH₂, or HCN. For N₂ and NH₃, the carbon source was CH₄, whereas CH₃NH₂ and HCN required no added carbon. Mass spectrometer signals from hydrocarbons, nitrogen-containing species, and the methyl radical were recorded. These were calibrated to give absolute mole fractions of the main carbon- and nitrogen-containing species present at detectable levels. The content of N₂ and NH₃ in the feed was also varied while holding the CH₄ content at 1%. Results are compared with those found during hot-filament CVD. The relevance of the measured abundance of various nitrogen-containing species in the plasma to attempts to create p-type diamond is discussed.     

Formation of rhenium diboride via mechanochemical–annealing processing of Re and B

Rhenium diboride (ReB₂) powder was prepared by mechanochemical processing of Re–B powder mixtures with subsequent annealing at temperatures of 600 °C to 1200 °C. Reactive evolution during the synthesis was investigated; furthermore, the effects of the amount of excess B on the reactions that occurred during the synthesis were assessed. The substantial reaction of Re with B occurred at 700 °C to form Re₇B₃ with a small amount of ReB₂. At 800 °C, Re₇B₃ converted into ReB₂; this conversion was enhanced with increasing temperature and increasing amount of excess B. At 1000 °C or above, single-phase ReB₂ powder without trace quantities of Re₇B₃ was obtained for compositions with 15 wt% or greater excess B. The synthesized ReB₂ powder particles were submicrometer with the vast majority being ∼500 nm. In addition, the resulting ReB₂ powders were consolidated by hot pressing or spark plasma sintering to examine the sinterability of the powders.     

Rapid Molten Salt Synthesis of Isotropic Negative Thermal Expansion ScF3

With a simple ReO₃‐type structure, ScF₃ exhibits a rare property of isotropic negative thermal expansion over a large temperature range. In this study, a rapid and low‐temperature synthesis route has been developed to prepare pure phase of ScF₃ in which NaNO₃ or KNO₃ as reaction media and Sc(NO₃)₃ and NH₄HF₂ as precursors (i.e., 30 min and 310°C). The sample of ScF₃ has relatively regular morphology and shows high crystallinity as well as single‐crystalline nature. The type of molten salts has obvious impact on morphologies of the particles. Substituting NaNO₃ with KNO₃, cubes of ScF₃ turns to be nanosticks. The thermal stability of the as‐prepared ScF₃ was investigated by thermal analysis. Molten salts play a significant role in eliminating the nonstoichiometric impurity of ScF_2.76 which is a common impurity during the conventional chemical reaction. This study reveals that molten salt is in favor of preparing those fluorides and relatives which is inert with moisture.     

Hydrothermal synthesis of LA-MN-Hexaaluminates for the catalytic combustion of methane

Hydrothermal synthesis by using urea hydrolysis at 1.0-3.0 MPa and 120-130 ‡C was employed to prepare Mn-substituted hexaaluminate catalysts for methane combustion. The results from DTA-MS demonstrated that CO_3- and Off anions co-exist in the hydrothermal reaction. XRD reveals that the components of carbonates and hydroxides in the hydrothermal reaction are more favorable than those in the (NH₄)₂CO₃ co-precipitation for the formation of the Mn-substituted hexaaluminate phase. After calcination at 1,200 ‡C for 2 h, LaMnAl₁₁O₁₉ is the major phase of the catalyst prepared by the hydrothermal synthesis method while LaAlO₃ is the major one of the catalysts prepared by NH₄OH and (NH₄)₂CO₃ co-precipitation. The catalyst prepared by hydrothermal synthesis has higher activity than that prepared by NH₄OH and (NH₄)₂CO₃ co-precipitation. The major reason is that more Mn²⁺ ions have incorporated into the hexaaluminate lattice. The effect of drying methods on the formation of hexaaluminate phase was also discussed.     

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N₂ atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al₂O₃ is more easily nitrided than γ‐, δ‐, and α‐Al₂O₃. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.     

Facile synthesis of amine-functionalized MIL-53(Al) by ultrasound microwave method and application for CO<Subscript>2</Subscript> capture

An amine functional MIL-53(Al) material was prepared through a clean, rapid, energy-efficient method of microwave and ultrasound irradiation. The pure phase NH₂-MIL-53(Al) can be formed in 25 min, utilizing the synergistic effect of microwave and ultrasound irradiation. The dramatic acceleration in reaction rates suggested that the removal of a passivation coating on the substrate particles and the resultant enhancement in mass and heat transfer. The porous MOFs exhibited a high thermal and chemical stability, decomposing at temperatures above 410 °C in air. The NH₂-MIL-53(Al) performed an excellent adsorption for CO₂. The CO₂ capacities up to 33.86 cm³ g^?1 at 298 K at low pressures, which suggests chemisorption between CO₂ and pendan amine groups. Measurement of CO₂ adsorption cycles proved that the functionalized materials show good regenerability and stability.     

The influence of B4C and MgB2 additions on the behavior of MgO–C bricks

Carbon-containing refractory materials have received great attention over the last years due to their importance in the steelmaking process. The oxidation of carbon present in refractory materials at temperatures above 500 °C is usually accompanied by the decrease of their mechanical strength and chemical resistance. Aiming to improve the oxidation resistance of carbon-oxide refractories, the use of materials known as antioxidants has been extensively studied. In this work we evaluated the performance of MgB₂ and B₄C antioxidants when incorporated into MgO–C bricks. We observed that the co-addition of metallic antioxidants and B₄C or MgB₂ leads to refractory bricks with enhanced hot modulus of rupture and resistance against oxidation and slag corrosion. However, the excessive addition of these antioxidants could impair the performance of the obtained bricks. Thus, when determining the optimum concentration of MgB₂ and B₄C to be added into MgO–C refractories, one must take into consideration this behavior.     

Kinetics of microwave synthesis of AlN by carbothermal‐reduction‐nitridation at low temperature

Aluminum nitride (AlN) was synthesized at 1000‐1400°C from a mixture of alumina and carbon powders using 2.45 GHz microwaves in a N₂ atmosphere. High nitridation ratios (>0.90) were obtained in the temperature range 1200‐1400°C. The apparent activation energy of the carbothermal reduction and nitridation (CRN) reaction using 2.45 GHz microwave irradiation was calculated from the nitridation ratio. The value obtained, 79.9 kJ/mol, is 11% of the energy reported for conventional synthesis using α‐Al₂O₃ as the raw material. This result indicates that 2.45 GHz microwave irradiation could promote the kinetics of the CRN reaction, and AlN could be effectively synthesized at low temperature.     

Conversion of Ethene to Propene by a Dual Function NiSO4/Re2O7/γ-Al2O3 Catalyst

A NiSO₄/Re₂O₇/γ-Al₂O₃ catalyst was prepared by co-impregnation using an aqueous solution of NiSO₄ and [NH₄][ReO₄], which catalyzes direct conversion of ethene to propene at mild reaction condition (323 K, 1 atm). Catalyst deactivation may be caused by coke formation. The two catalytic components appear to work independently of each other.     

Uptake and apparent utilization of urea and ammonium nitrate in wheat seedlings

Intact wheat (Triticum aestivum cv. Quern) seedlings that were grown in presence or absence of NH₄NO₃ were exposed to solutions containing CO(NH₂)₂, NH₄NO₃, CO(NH₂)₂ + NH₄NO₃, CO(NH₂)₂ + KNO₃ and CO(NH₂)₂ + (NH₄)₂SO₄ for consecutive periods of 3, 3, 6, 12 and 24h and N uptake determined by solution depletion measurements. Differences in ethanol-soluble N and ethanol-insoluble N content of roots and shoots of control (zero time) seedlings and seedlings exposed to CO(NH₂)₂, NH₄NO₃ and CO(NH₂)₂ + NH₄NO₃ for 48 h were used to characterize N utilization during/following uptake.Regardless of initial N status, uptake of N from CO(NH₂)₂ was less than one-third of that from NH₄NO₃. Relative absorption of the CO(NH₂)₂ and NH₄NO₃ was not substantially altered by acidity control of the uptake solutions. There was a reciprocal antagonism between uptake of CO(NH₂)₂ and uptake of NH₄NO₃. Whereas CO(NH₂)₂ inhibited NH₄ absorption in each set of seedlings, it decreased NO₃ uptake only in seedlings that had been pretreated with N. Simultaneous presence of KNO₃ enhanced CO(NH₂)₂ uptake but presence of (NH₄)₂SO₄ decreased it to the same extent as NH₄NO₃. All absorption processes involving CO(NH₂)₂ and NH₄ were substantially restricted by pretreatment of the seedlings with NH₄NO₃. The results suggest that apparent utilization of ambient N was dependent on initial N status of the seedlings and on the nature of the N species to which they were exposed.     

2000-eV argon ion beam induced formation of multiple chemical states in titanium dioxide mesoporous film

Oxygen vacancies were created on the surface of TiO₂ mesoporous film after exposure to a 2000 eV argon ion beam. Confirmation of the presence of the vacancies was conducted through x-ray photoelectron spectroscopy analysis which reveals Ti 2p, O 1s, C 1s, and Ar 2p chemical states at different binding energies. Ti 2p spectra show five distinct peaks corresponding to Ti²⁺ 2p_1/2, Ti³⁺ 2p_3/2, Ti⁴⁺ 2p_3/2, Ti³⁺ 2p_1/2, and Ti⁴⁺ 2p_1/2 states while the Ti⁴⁺ continuously reduced to Ti³⁺ and Ti²⁺ as the irradiation time of Ar⁺ increases. The Ar⁺ irradiation leads to the lower binding energy of the Ti 2p state with an increase in irradiation time while the binding energy of O 1s becomes higher with irradiation time. The peaks correspond to lattice oxygen; oxygen vacancy and adsorbed oxygen were observed in high-resolution O 1s spectra while the peak area ratio of oxygen vacancy with lattice oxygen increases with irradiation time. The atomic percentage of C 1s decreases with irradiation time while that of Ti 2p, O 1s, and Ar 2p increases with irradiation time. An increase in irradiation time leads to insignificantly shifting of the valence band edge towards the conduction band. The observed response of the prepared nano-particle TiO₂ after 2000 eV Ar⁺ exposure which conveniently creates oxygen vacancies in the crystallographic structure of the semiconductor could be a versatile approach to enhancing the photocatalytic activity of the semiconductor for various industrial and technological applications.     

Nano-structured nitrogenated carbon films — morphology and field emission

Crystalline nitrogenated carbon (C:N) films have been successfully synthesised by plasma-enhanced hot filament chemical vapour deposition (PE-HF-CVD). Nitrogen gas (N₂) and ammonia (NH₃) were used as sources of atomic nitrogen whereas methane (CH₄) acted as a carbon precursor. Structure analysis reveals the growth of a new type of C:N film characterised by various polymorphs including worm- and foil-like microstructures at the nanometer scale. The effects of bias voltage and filament temperature on the film morphology are investigated in detail. Using pure Si as a substrate results in growth of homogeneous nanostructured films, whereas arrays of nanotubes were deposited on Ni-coated substrates. Field emission in vacuum was observed on C:N films deposited on pure Si above applied fields of 15 V/μm. The onset field could be decreased below 4 V/μm with Ni-coated substrates due to the presence of well-separated nano-tubular structures.     

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

The mechanism of selective catalytic reduction (SCR) of NO_x with NH₃ over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH₃ then to flowing NO + O₂ or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH₄NO₂. In situ FTIR results in flowing NO + NH₃ + O₂ at different temperatures show that NH₃ is strongly adsorbed and reacts with impinging NO_x. The intensity of the NH₄NO₂ bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N₂ + H₂O. The mechanistic model rationalizes that the consumption ratio of NO and NH₃ is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH₄NO₂. At low temperature, when adsorbed H₂O is abundant it lowers the rate because it competes with NO_x for adsorption sites.     

Microwave irradiation graft copolymerization of hydroxyethyl methacrylate onto wool fabrics

Hydroxyethyl methacrylate was grafted onto woolen fabrics by microwave irradiation in the presence of catalyst (NH₄)₂S₂O₈. Various parameters of the graft copolymerization reaction, namely, time, microwave intensity, catalyst, and monomer concentration, were optimized. The graft copolymerization was also compared with conventional heating graft copolymerization at the same condition. Microwave irradiation was shown to improve the reactivity of the monomer. The moisture regain decreased as graft add-on increased. The Max load and the strain at Max load increased as graft add-on increased. The infrared spectra showed an additional peak at 1700 cm⁻¹, confirming ester carbonyl groups of the monomer. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2343–2347, 1998     

Low Activation Energy Pathway for the Catalyzed Reduction of Nitrogen Oxides to N<Subscript>2</Subscript> by Ammonia

A low activation energy pathway for the catalytic reduction of nitrogen oxides to N₂, with reductants other than ammonia, consists of two sets of reaction steps. In the first set, part of the NO _x is reduced to NH₃; in the second set ammonium nitrite, NH₄NO₂ is formed from this NH₃ and NO + NO₂. The NH₄NO₂ thus formed decomposes at ~100 °C to N₂ + H₂O, even on an inert support, whereas ammonium nitrate, NH₄NO₃, which is also formed from NH₃ and NO₂ + O₂, (or HNO₃), decomposes only at 312 °C yielding mainly N₂O. Upon applying Redhead's equations for a first order desorption to the decomposition of ammonium nitrite, an activation energiy of 22.4 is calculated which is consistent with literature data. For the reaction path via ammonium nitrite a consumption ratio of 1/1 for NO and NO₂ is predicted and confirmed experimentally by injecting NO into a mixture of NH₃ + NO₂ flowing over a BaNa/Y catalyst. This leads to a yield increase of one N₂ molecule per added molecule of NO. Little N₂ is produced from NH₃ + NO in the absence of NO₂.     

Structural characterization and superconducting properties of MgB<Subscript>2</Subscript> prepared by SHS-method

Self-propagating high-temperature synthesis (SHS) of bulk MgB₂ superconductor from Mg-2B powder blend is reported. This reaction proceeds violently at 100 A under a protective atmosphere. Since the heat of reaction of Mg and B was not enough for chain reaction, then (Ti + C) mixed powders were used as the ignition agent to assist the reaction (Mg + 2B). In this case, the combustion front moved without any difficulty. The diffraction lines of the product can be indexed to a hexagonal MgB₂ phase, with lattice constants a = 3.0845 Å, and c = 3.5259 Å. For comparison, the direct synthesis of (Mg + 2B) mixture was carried out at (800°C–1000°C). It can be found, that high-temperature sintering (1000°C) will induce the formation of impurities. The MgB₂ grains are fine, well compacted and more homogenous. The structure of materials was studied using XRD, FESEM and EDX. M-H curvatures were measured under the magnetic fields between ?80 kOe and 80 kOe. J _c was calculated from width of magnetization hysteresis loops based on the extended Bean Model.     

Rapid Microwave Annealing of Amorphous Lead Zirconate Titanate Thin Films Deposited by Sol‐Gel Method on LaNiO3/SiO2/Si Substrates

The heating behavior of LaNiO₃ (LNO) films on SiO₂/Si substrate heated by 2.45 GHz microwave irradiation in the microwave magnetic field was first investigated, and then amorphous Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films were deposited on LNO‐coated SiO₂/Si substrates by a sol‐gel method and crystallized in the microwave magnetic field. The crystalline phases and microstructures as well as the electrical properties of the PZT films were investigated as a function of the elevated temperature generated by microwave irradiation. The perovskite PZT films with a highly (100)‐preferred orientation can be obtained by microwave annealing at 700°C for only 180 s of total processing time, and have good electrical properties. The results demonstrated that conductive metal oxide LNO as a bottom electrode layer is an advantage for the crystallization of PZT thin films by microwave irradiation in the microwave magnetic field.