氧化硼作为电极涂层的研究

以水和磷酸二氢铝为粘结剂,对氧化硼作为电极防氧化涂层进行了研究。通过调整涂层的配比浓度,并采用预先浸泡的方法,使涂层与电极本体的粘结性能提高,石墨炭块试样在900℃的高温下长时间不氧化。     

Progress in pressureless sintering of boron carbide ceramics – a review

ABSTRACT

Boron carbide (B₄C) ceramics has many outstanding performance, such as extremely high hardness, low density, high melting point, high elastic modulus, high thermoelectromotive force, high chemical resistance, high neutron absorption cross section, high impact and excellent wear resistance. Therefore, B₄C ceramics can be used in various industrial applications, such as lightweight ceramic armour, high temperature thermocouples, neutron absorber, reactor control rods in nuclear power engineering, polishing media for hard materials, abrasive media for lapping and grinding, and wear resistant components (blasting nozzles, die tips and grinding wheels). Pressureless sintering is the method with industrialised application value for B₄C ceramics, however, it is impossible to sinter pure B₄C ceramics to high densities without additives by pressureless sintering. So sintering additives must be used to promote the densification of B₄C ceramics. The different sintering additives used to promote the densification of boron carbide will be described in this review, including carbon additives, metallic additives, oxide additives, non-oxide additives, combined additives and rare earth oxide additives. Finally, the recent research trends for sintering methods and sintering additives of B₄C ceramics will also be proposed.     

Analysis of the fluorescence of mechanically processed defect-laden hexagonal boron nitride and the role of oxygen in catalyst deactivation

ABSTRACT

The use of hexagonal boron nitride (h-BN) as a non-metal heterogeneous catalyst has been a popular subject in research since the discovery of its catalytic properties in 2016. Previous work found that an activation step was necessary for producing an effective catalyst. Density functional theory (DFT) calculations indicate defect sites, such as nitrogen (V_N) and boron (V_B) vacancies, bind favourably to olefins, hydrogen, and oxygen. In particular, the visible fluorescence intensity of processed h-BN increased with the length of exposure to air. The fluorescence behaviour of dh-BN powders when exposed to air after exposure to species such as argon, propene, and carbon dioxide is presented. Density of state calculations for molecular and atomic oxygen bound to V_N and V_B show that this increase in fluorescence may be due to atomic oxygen binding to V_N. The fluorescence emission behaviour observed in dh-BN powders and its relationship to DOS of oxygen species bound to catalytically active defect sites provides a better understanding of potential deactivation modes for catalysts based on dh-BN.     

Preparation of submicron boron carbide powder through gas-solid reaction method

Boron carbide submicron powder was synthesized with boron oxide and graphene as starting materials by gas-solid reaction method using two different apparatuses. The effects of calcination temperature and holding time, apparatus type and B₂O₃/C ratio of the starting materials on the phase composition and morphology of the synthesized powders were evaluated. A newly formed residual carbon morphology distinct from original graphene were present in samples synthesized at a higher B₂O₃/C ratio or temperature. The synthesis temperature of ∼1500 °C was found to be more suitable to obtain boron carbide powder without the existence of residual carbon. The new type of apparatus enabled the synthesis of boron carbide phase at a relatively lower temperature, due to its more efficient use of B₂O₃ vapor.     

Effect of the microstructure of graphitic boron nitride on the kinetics of the formation of boron nitride high-pressure phases

Three types of hexagonal boron nitride (h-BN) with graphitic crystal structure having different microstructures were subjected to high pressures (HP) and high temperatures (HT), and the kinetics of the phase transitions to the sp³-hybridized phases (w-BN, c-BN) was studied using in situ synchrotron diffraction. The analysis of the phase transformation kinetics revealed the transformation paths and activation energies E_a of the transformation of h-BN to the high-pressure forms of BN for different microstructures of h-BN. Defect-poor h-BN transforms to metastable wurtzitic BN (w-BN) with E_a ≈ 0.3 eV/at. Defect-rich forms of h-BN transform directly to c-BN, but with a higher activation energy. It was observed that the turbostratic disorder in h-BN retards the phase transition as compared to h-BN containing corrugated basal planes and a low degree of turbostracity. The experimental results are discussed in view of the microstructure changes during the HP/HT treatment and compared to available theoretical phase transition models.     

Fabrication of B4C from Na2B4O7 + Mg + C by SHS method

This paper deals with the formation of boron carbide (B₄C) powders from Na₂B₄O₇ + Mg + C system by self-propagating high-temperature synthesis (SHS) method. B₄C without impurities could be obtained after the acid enrichment and distilled water washing. The reaction mechanism of SHS of B₄C was proposed: the synthesis of B₄C is a process involving the decomposition of Na₂B₄O₇ into the intermediate phase B₂O₃, which reacts with Mg and carbon to form B₄C.     

Interaction of boron suboxide with compacted graphite cast iron

The chemical interaction of boron suboxide (B₆O) with compacted graphite cast iron (CGI) was investigated using static interaction diffusion couples between B₆O and CGI at 700 °C, 900 °C and 1100 °C for 1 h. This interaction offers the possibility to evaluate the potential of B₆O as a cutting tool. The microstructures and phase compositions of the interaction zones were investigated. At 700 °C and 900 °C the chemical interaction was minimal. However, at 1100 °C, Fe₂B and SiO₂ were formed at the interface. Hence, machining at 1100 °C is likely to result in chemical wear.     

载体对镍基催化剂催化氯代硝基苯加氢性能的影响

分别以碳纳米管(CNTs)、SiO<,2>、TiO<,2>、γ-Al<,2>O<,3>、TiO<,2>-SiO<,2>、SiO<,2>-TiO<,2>和活性炭(AC)为载体,以Ni、B为活性组分,采用浸渍-化学还原法制备了一系列负载型Ni-B非晶态合金催化剂,采用了电感耦合等离子体光谱、X-射线衍射、透射电子显微镜、差...     

Synthesis of boron/nitrogen substituted carbons for aqueous asymmetric capacitors

Boron/nitrogen substituted carbons were synthesized by co-pyrolysis of polyborazylene/coal tar pitch blends to yield a carbon with a boron and nitrogen content of 14 at% and 10 at%, respectively. The presence of heteroatoms in these carbons shifted the hydrogen evolution overpotential to −1.4 V vs Ag/AgCl in aqueous electrolytes, providing a large electrochemical potential window (∼2.4 V) as well as a specific capacitance of 0.6 F/m². An asymmetric capacitor was fabricated using the as-prepared low surface area carbon as the negative electrode along with a redox active manganese dioxide as the positive electrode. The energy density of the capacitor exceeded 10 Wh/kg at a power density of 1 kW/kg and had a cycle life greater than 1000 cycles.     

The feasibility of boron removal from water by capacitive deionization

We report on the possibility of removing boron (in the form of boric acid) from water by electrochemical means. We explore capacitive de-ionization (CDI) processes in which local changes in pH near the surface of high-surface-area activated carbon fiber (ACF) electrodes during charging are utilized, in order to dissociate boric acid into borate ions which can be electro-adsorbed onto the positive electrode in the CDI cells. For this purpose, a special flow-through CDI cell was constructed in which the feed solution flows through the electrodes. Local pH changes near the carbon electrode surface were investigated using a MgCl₂ solution probe in three- (with reference) and two-electrode cells, and described qualitatively. We show that, to a certain extent, boron can indeed be removed from water by CDI.