Ab Initio Calculation of BN Generation from Boron and Nitrogen Oxides

Boron Nitride (BN) is one of the products produced in the burning of boron‐containing propellant. A possible reaction mechanism for the reactions of boron and nitrogen oxides (NO, NO₂, N₂O) has been studied using the G2MP2 method. The BN product can be formed in the reactions of B(⁴P) with NO, NO₂ and N₂O. Among these three reactions, B(⁴P)+NO₂ and B(⁴P)+N₂O are 181.42 kJ/mol and 160.92 kJ/mol more‐exothermic than the B(⁴P)+NO reaction. The barrier heights from intermediates to transition states are 64.85 kJ/mol and 111.75 kJ/mol for B(⁴P)+NO₂ and B(⁴P)+N₂O, respectively. However, in the reaction B(⁴P)+NO , the transition from intermediate to product (IM3→BN+O) is very endothermic by 420.70 kJ/mol. So B(⁴P)+ N₂O→BN+NO and B(⁴P)+NO₂→BN+O₂ are more likely reactions to generate BN than B(⁴P)+NO→BN+O.     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.     

Response of Sesame (Sesamum indicum L.) Plants to Foliar Spray with Different Concentrations of Boron

Sesame plants were sprayed with different concentrations of boron solution at 20, 30 and 40 ppm at different stages of plant growth (1, 2 and 3 months). Comparing the treated plants with untreated controls, the obtained results showed that spraying sesame plants with boron (B) solutions improves their growth and yields. Treating plants with boron solution at 20 ppm gave the highest results in growth criteria as compared with corresponding control or plants treated with higher boron solutions (30 and 40 ppm). Moisture and oil percentages were nonsignificantly changed by the different boron concentrations. The highest oil viscosity was recorded at a boron concentration of 30 ppm. Fatty acids were decreased by the effect of boron spray. A remarkable increase in the amino acid content of the plants was observed as a result of treatments with boron solutions, especially in the plants treated at 40 ppm. Spraying sesame plants with boron decreased the Fe, K, Mg and P contents, whereas the lowest concentration of boron (20 ppm) increased Ca and the highest concentration (40 ppm) increased Cl and Na.     

Removal of Boron from Aqueous Solutions by Adsorption Using Fly Ash,Zeolite, and Demineralized Lignite

In the present study for the purpose of removal of boron from water by adsorption using adsorbents like fly ash, natural zeolite, and demineralized lignite was investigated. Boron in water was removed with fly ash, zeolite, and demineralized lignite with different capacities. Ninety-four percent boron was removed using fly ash. Batch experiments were conducted to test the removal capacity, to obtain adsorption isotherms, thermodynamic and kinetic parameters. Boron removal by all adsorbents was affected by pH of solution; maximum adsorption was achieved at pH 10. Adsorption of boron on fly ash was investigated by the Langmuir, Freundlich, and the Dubinin-Radushkevich models. Standard entropy and enthalpy changes of adsorption of boron on fly ash were, ΔS ⁰  = ?0.69 kJ/mol K and ΔH ⁰  = ?215.34 kJ/mol, respectively. The negative value of ΔS ⁰ indicated decreased randomness at the solid/solution interface during the adsorption boron on the fly ash sample. Negative values of ΔH ⁰ showed the exothermic nature of the process. The negative values of ΔG ⁰ implied that the adsorption of boron on fly ash samples was spontaneous. Adsorption of boron on fly ash occurred with a pseudo-second order kinetic model, and intraparticle diffusion of boron species had also some effect in adsorption kinetics.     

Influence of boron treatment on oxidation of carbon fiber in air

An investigation is performed to study the influence of boron treatment on carbon fiber: its oxidation behaviors, kinetics, and structure and properties at a high-temperature atmosphere in air. PAN-based carbon fiber is treated with either liquid organoborate or inorganic borate, which elevates the decomposing point above 260°C and doubles the average oxidation activation energy from about 100 kJ/mol of the untreated fiber to above 200 kJ/mol of the treated fiber, with the first order of the fiber oxidation reaction under nonisothermal conditions. The boron coating formed on the fiber surface after the treatment may cap off the specific surface active sites to function as a diffusion barrier that inhibits the oxidation, and its main structure is identified as boron oxide by means of X-ray diffraction and XPS analyses. During the process, the mechanical properties develop slightly; however, the morphology structure observed by SEM changes greatly. © 1996 John Wiley & Sons, Inc.     

Microcrystal structure evolution of mesophase pitch-based carbon fibers with enhanced oxidation resistance and tensile strength induced by boron doping

Mesophase pitch-based carbon fibers (MPCFs) with various concentration of boron were fabricated through an easy and effective strategy by treating stabilized fibers with boron acid solution. The microstructural evolution of boron-doped MPCFs was characterized. It was found that boron doping promoted high degree of graphitization with the decrease of interlayer spacing and the growth of crystallite dimension due to catalytic graphitization of boron to carbon. Boron doping significantly improved 5?wt% burn-off temperature of MPCFs increasing by 344?°C, and the tensile strength of MPCFs was also enhanced by 19%. Both of the outstanding oxidation resistance and mechanical performance of MPCFs was obtained due to effect of boron. The results demonstrated boron doping was a feasible way to produce high-performance MPCFs.     

Kinetics of Carbothermal Reduction Synthesis of Boron Carbide

Carbothermal reduction kinetics to synthesize boron carbide are studied under conditions of high carbon/boron oxide precursor heating rates (> 10³K/s) and intermediate temperatures (1803 K < T < 2123 K). For these conditions, fractional carbon conversion, X, can be described by a nucleation–growth rate expression: In (1 – X ) =– ( kt )³, where K _o= 3.86 × 10⁶ s⁻¹ and E _a= 301 ± 55 kJ/mol for the temperature range (1803 < T < 1976 K) and k _o= 2 × 10²⁰ s⁻¹ and E _a= 820 ± 89 kJ/mol for the temperature range (1976 < T < 2123 K), respectively. Boron carbide formation is highly dependent upon the phase change of reactant boron oxide from solid to liquid to gaseous boron suboxides and the effect of reaction environment (i.e., heating rate and ultimate temperature) on the rate at which the phase changes occur.     

Oxidative Stability of Chia (<Emphasis Type="Italic">Salvia hispanica</Emphasis> L.) Seed Oil: Effect of Antioxidants and Storage Conditions

The oxidative stability of chia oil was evaluated by measuring the effectiveness of the addition of rosemary (ROS) and green tea (GT) extracts, tocopherols (TOC), ascorbyl palmitate (AP) and their blends, and studying the influence of storage conditions. The addition of antioxidants increased induction time, depending on their type and concentration. Considering antioxidants individually, AP at 5,000 ppm was the most effective, whereas ROS + GT at 2,500 and 5,000 ppm provided the best protection among the antioxidant blends. Chia oil peroxide values of 10 mequiv/kg was observed for oils stored at 4 °C while values greater than 10 mequiv/kg were observed between 60 and 120 days when stored at 20 °C. Only AP 2,500 ppm protected oil did not reach 10 mequiv/kg during 225 days at 4 and 20 °C. Similar trends were observed with p-anisidine and Totox values. Differential scanning calorimetry further supported the presence of primary and secondary oxidation. Activation energy of chia oil thermoxidation was 71.9 kJ/mol increasing up to 87.5 kJ/mol when AP was added.     

Oxidation of Chemically-Vapor-Deposited Silicon Nitride and Slnglem-Crystal Silicon

Oxidation of {111} single-crystal silicon and dense, chemically-vapor-deposited silicon nitride was done in clean silica tubes at temperatures of 1000° to woo°C. The oxidation rates of silicon nitride under various atmospheres (dry O₂, wet O₂, wet inert gas, and steam) were several orders of magnitude slower than those of silicon under the identical conditions. The activation energy for the oxidation of silicon nitride decreased from 330 to 259 kJ/mol in going from dry O₂ to steam while that for Si decreased from 120 to 94 kJ/mol. The parabolic rate constant for Si increased linearly as the water vapor pressure increased. However, the parabolic rate constant for silicon nitride showed nonlinear dependency on the water vapor pressure in the presence of oxygen. The oxidation kinetics of silicon nitride is explained by the formation of nitrogen compounds (NO and NH₃) at the reaction interface and the counterpermeation of these reaction products.     

Ce4+/Ce3+作氧化媒质间接电氧化对氯甲苯的反应动力学研究

在优化的实验条件下分别从产物和反应物2方面进行分析，系统地研究了硫酸浓度为6．425mol/L和7．7mol/L时Ce^4 /Ce^3 作氧化媒质间接电氧化对氯甲苯的有机氧化动力学，确定出总反应为 表观1级反应，并拟合出了相应的表观反应速率常数和表观活化能，当硫酸介质浓度从6．425mol/L增至7．7mol/L时表观活化能显著降低（从122－128kJ/mol降低到84－89kJ/mol）从而大大加速了化学反应速率。     

Oxidation inhibition effects of phosphorus and boron in different carbon fabrics

To improve oxidation inhibition, elemental boron and two phosphorus compounds were doped into an activated carbon cloth and a carbon felt. The hypothesis was that P can block active sites by virtue of the formation of C-P-O or C-O-P bonds at graphene edges while substitutional B can alter the chemical reactivity of the residual free active sites by reducing the electron density in the graphene layer. To increase the final dopant concentration, the carbon felt was activated in nitric acid. The crystallinity of activated carbon cloth was improved by heat treatment and substitutional B; that of carbon felt was also improved, but not necessarily due to substitutional B. In all cases the oxidation reactivity is suppressed by heat treatment and in the presence of dopants. The oxidation inhibition mechanism in P-doped samples appears to be active sites blockage because of a proportional increase of oxidation inhibition with increasing P loading. The results for B-doped samples are consistent with our previous studies in which B was found to exhibit both a catalytic and an inhibiting effect on carbon oxidation. Samples doped with both P and B showed the most effective oxidation inhibition and their oxidation behavior is described in detail.     

Mechanism and kinetics of oxidation during the thermal stabilization of polyacrylonitrile fibers

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N₂ and air were investigated using differential scanning calorimetry and solid‐state ¹³C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C?O) groups could occur at 160°C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and characterization of boron-doped Bi2O3 - La2O3 fiber derived nanocomposite precursor

Boron doped poly(vinyl) alcohol/ bismuth - lanthanum acetate (PVA/Bi-La) nanofibers were prepared by electrospinning using PVA as a precursor. The effect of boron doping was investigated in terms of solution properties, morphological changes and thermal characteristics. The fibers were characterized by FT-IR, XRD, SEM and BET. The addition of boron did not only increase the thermal stability of the fibers, but also their diameters, which yielded stronger fibers. XRD analyses showed that boron doping increased the peak intensities and indicated that the boron doping enhanced the crystallite size. Moreover, no shifts were noticed in diffraction angles for boron doped and undoped samples. Therefore, boron doping did not significantly alter the lattice spacing. The SEM micrograph of the fibers showed that the addition of boron resulted in the formation of cross linked bright surfaced fibers. Also, grain diameters of boron doped and undoped nanocrystalline sintered powders were measured as 170 nm and 120 nm respectively. The BET results show that boron undoped and doped Bi₂O₃-La₂O₃ nanocrystalline powder ceramic structures sintered at 800 °C have surface areas of 20.44 m²/g and 12.93 m²/g, respectively.     

Improved Oxidation Resistance of Silicon Nitride by Aluminum Implantation: I, Kinetics and Oxide Characteristics

Hot-isostatically-pressed, additive-free Si₃N₄ ceramics were implanted with aluminum at multi-energies and multidoses to achieve uniform implant concentrations at 1, 5, and 10 at.% to a depth of about 200 nm. The oxidation behavior of unimplanted and aluminum-implanted Si₃N₄ samples was investigated in 1 atm flowing oxygen entrained with 100 and 220 ppm NaNO₃ vapor at 900–1100°C. Unimplanted Si₃N₄ exhibits a rapid, linear oxidation rate with an apparent activation energy of about 70 kJ/mol, independent of the sodium content in the gas phase. Oxides formed on the unimplanted samples are rough and are populated with cracks and pores. In contrast, aluminum-implanted Si₃N₄ shows a significantly reduced, parabolic oxidation rate with apparent activation energies in the range of 90–140 kJ/mol, depending on the sodium content as well as the implant concentration. The oxides formed on the implanted samples are glassy and mostly free from surface flaws. The alteration of the oxidation kinetics and mechanism of Si₃N₄ in a sodium-containing environment by aluminum implantation is a consequence of the effective modification of the properties of the sodium silicates through aluminum incorporation.     

Kinetics on the oxidation of biodiesel stabilized with antioxidant

Oxidation stability of safflower biodiesel stabilized with propyl gallate whose concentration spreads from 0 to 5000 ppm was studied by Rancimat method at temperatures from 100 °C to 120 °C. It was consequently demonstrated that the induction period of biodiesel increases with the increase of antioxidant concentration and decreases with increase of temperature. Kinetics on its oxidation was described by the first order rate law with an accuracy higher than 0.98. The reaction rate of propyl gallate consumed in safflower biodiesel obtained from the experiment fits well with Arrhenius equation and the activation energy obtained from Arrhenius equation was 97.02 kJ/mol. Logarithm of induction periods determined by Rancimat method with various antioxidant concentrations shows a linear relation with temperatures. It was, consequently, found that the Rancimat method for the oxidation stability determination shows an approximate correlation between storage stability and Rancimat induction period. The Rancimat method cannot directly measure the overall storage stability of fuels, since other conditions such as presence of water, microbial contamination and storage conditions would affect fuel quality during storage.     

Preferential distribution and oxidation inhibiting/catalytic effects of boron in carbon fiber reinforced carbon (CFRC) composites

Two different batches of CFRC composites were prepared in the absence/presence of B with the expectation of increasing oxidation stability and improving the processing compatibility of CFRC composites in commercial applications. The composites were examined to reveal the nature of substitutional B in oxidation, crystallinity and distribution preference in the composites. Substitutional B acts both a catalyst and an inhibitor in carbon oxidation, depending on the content and the extent of carbon burn-off reaction. Crystallinity increases with the incorporation of B, as expected; d₀₀₂ decreases, and L_c and L_a increase. Boron prefers to be distributed in the less ordered structure; non-graphitizable PAN-based carbon fibers have higher B contents than graphitizable coal-tar pitch, but processing conditions can change this preference. The incorporation of B in CFRC composites seems to be beneficial for improving the potential ability of the composites in applications by increasing crystallinity and oxidation stability.     

Breath Figure Patterns in the Oxidation of Boron Nitride

Liquids condensing on solids can form a pattern known as "breath figures." Here we report similar patterns for liquid boron oxide droplets formed by high temperature oxidation reaction of boron nitride. Boron oxide does not wet boron nitride, so the liquid B₂O₃ oxide forms small droplets. As oxidation proceeds at 1200°C, the oxide created by reaction on the surface migrates to existing droplets or forms new droplets. The average diameter and maximum diameter of the boron oxide droplets increase with time following the kinetics of breath figures, similar to water condensation breath figures for dew drops.     

Structure of coke powder heat-treated with boron

Coal tar pitch-based coke powder with a fine mosaic texture was heat-treated with various concentrations of boron powder at 2900°C. Increasing the boron amount led to smaller d₀₀₂ and larger d₁₁₀, and made the original fine texture coarser. Some small particles showed specific structures of polyhedrons, of which surfaces are 002 planes of graphite lattice, after heat treatment with boron. The size of the polyhedron increased with boron content. Boron concentration was lower at the surface than at the inner portions of particles for a powder heat-treated with a higher amount of boron, while it depended less on the depth for that heat-treated with a lower amount of boron. The formation mechanism of the polyhedron particle is discussed.     

Calcium Segregation to a Magnesium Oxide (100) Surface

The segregation of calcium to a (100) cleavage surface of an MgO crystal, with bulk calcium concentration of 200 ppm was measured in situ at T =900° to 1450°C in ultrahigh vacuum, using Auger and low-energy ion-scattering spectroscopies. A measured heat of segregation of approximately -50.3 kJ/mol (-12 kcal/mol) is in favorable agreement with a value of -58.7 kJ/mol (-14 kcal/mol) determined using solute strain energy and surface free energy criteria. The equilibrium value for the calcium segregation between 950° and 1000°C is estimated to correspond to a 20% occupation of the surface cation sites.     

Oxidation behavior and protection of carbon/carbon composites prepared using rapid directional diffused CVI techniques

The oxidation kinetics of carbon/carbon (C/C) composites prepared using a rapid directional diffused (RDD) CVI process were studied. The results showed that the Arrhenius curve for the RDD CVI C/C composites consists of two straight lines, the intercept of which is at about 700 °C at the linear oxidation stage. The oxidation rates are controlled by the surface reaction at 600-700 °C, and the corresponding activation energy is 121 kJ/mol. Between 700 and 800 °C, the oxidation rates are dominated by chemical reaction and diffusion, and the relevant activation energy is 80 kJ/mol. SEM investigation showed that the oxidation starts with original pores on the C/C composite surface with the carbon fiber and matrix oxidized simultaneously. An inexpensive and easily pasted coating containing epoxy organic silicon resin, borates, refractory particulates, etc. was developed. After isothermal temperature, thermal cycle and immersion water oxidation tests, the coating was demonstrated to exhibit good oxidation-resistance properties. The oxidation-resistant mechanism of the coating is discussed.