Shaowei Zhang

Melt corrosion of oxide and oxide–carbon refractories

AbstractPenetration and dissolution mechanisms are reviewed for predominantly single phase oxide, two phase oxide, and oxide–carbon composite refractories by liquid silicates. Theoretical models of these processes, as well as static (sessile drop, dipping, and crucible) and dynamic (rotating finger and rotary slag) experimental tests, along with their practical limitations are considered. Direct (congruent or homogeneous) attack involves dissolution of a solid into a liquid with no intermediate solid phase leading to active corrosion. Indirect (incongruent or heterogeneous) attack leads to formation of one or more new solid phases at the original solid/liquid interface. This may lead to passive corrosion. Examples of direct and indirect attack in a range of refractory–liquid systems are described highlighting the critical influence of the composition and hence viscosity of the local liquid adjacent to the solid. Penetration and corrosion can be controlled either through the local liquid composition via th...

Creating high yield water soluble luminescent graphene quantum dots via exfoliating and disintegrating carbon nanotubes and graphite flakes

We have developed an effective method to exfoliate and disintegrate multi-walled carbon nanotubes and graphite flakes. With this technique, high yield production of luminescent graphene quantum dots with high quantum yield and low oxidization can be achieved.

Castable refractory concretes

Abstract Castable refractories containing calcium aluminate cement (CAC) are used ubiquitously in a range of furnace lining applications in the iron and steel, cement, glass, ceramic, and petrochemical industries. This review outlines their development from conventional high cement materials, through low cement and ultra-low cement castables to the present materials which may be entirely free of CAC. Castables are defined in terms of both CaO content and installation procedure. Production routes, compositions, and microstructural evolution on hydration, setting, dehydration, and firing are described for pure CACs and castable refractories. The development of the low cement systems is discussed in terms of particle packing, dispersion, and rheology highlighting the influence of colloidal matrix additions of silica and alumina. Recent developments including cement free, self-flowing, shotcreting, and basic castables are described and the potential for carbon-containing systems evaluated.

Thermochemistry and microstructures of MgO-C refractories containing various antioxidants

Abstract Reactions on firing MgO–C refractories with added Al, Si and B 4 C have been predicted by thermodynamic calculations and observations made of microstructures given equivalent treatments. At 1200 and 1500 o C, addition of Al leads to generation of Al 4 C 3 , AlN, Al 2 O 3 and magnesium aluminate spinel, MA. Gaseous species, such as Al(g), participate in their formation processes as predicted and suggested by their morphologies. For example, AlN occurs as whiskers and MA as fine precipitates in the matrix and as surface layers on MgO aggregates. Additionally, Al 2 O 3 and Al 4 C 3 shells occur surrounding porous cores at the location of the original Al particles. At 1200 o C, addition of Si leads to formation of SiC, Si 3 N 4 , SiO 2 and forsterite, M 2 S. These phases are still present at 1500 o C except for Si 3 N 4 which is not thermodynamically stable. Gaseous species such as SiO(g) were also involved in formation of these product phases as predicted and suggested by their morphologies. M 2 S occurred as fine precipitates in the matrix and as a layer on MgO aggregate surfaces. SiO 2 formed either directly as a layer on Si particles or indirectly as a shell around a SiC core. Thermodynamic calculations predict that at 1200 and 1500 o C B 4 C reacts with N 2 from the atmosphere to form BN and/or with CO from the atmosphere to form B 2 O 3 which further reacts with MgO to form low melting 3MgO.B 2 O 3 , M 3 B. M 3 B becomes liquid >1350 o C and takes up impurities from MgO and/or graphite raw materials, forming more liquid, which will be detrimental to the refractories corrosion resistance. M 3 B was detected in the microstructures but there was no direct evidence of BN formation and its reaction with CO. Gaseous species such as B 2 O 3 (g) were also involved in M 3 B formation.

Fabrication of Luminescent Monolayered Tungsten Dichalcogenides Quantum Dots with Giant Spin-Valley Coupling

A high yield (>36 wt %) method has been developed of preparing monolayered tungsten dichalcogenide (WS2) quantum dots (QDs) with lateral size ∼8-15 nm from multilayered WS2 flakes. The monolayered WS2 QDs are, like monolayered WS2 sheets, direct semiconductors despite the flake precursors being an indirect semiconductor. However, the QDs have a significantly larger direct transition energy (3.16 eV) compared to the sheets (2.1 eV) and enhanced photoluminescence (PL; quantum yield ∼4%) in the blue-green spectral region at room temperature. UV/vis measurements reveal a giant spin-valley coupling of the monolayered WS2 QDs at around 570 meV, which is larger than that of monolayered WS2 sheets (∼400 meV). This spin-valley coupling was further confirmed by PL as direct transitions from the conduction band minimum to split valence band energy levels, leading to multiple luminescence peaks centered at around 369 (3.36 eV) and 461 nm (2.69 eV, also contributed by a new defect level). The discovery of giant spin-valley coupling and the strong luminescence of the monolayered WS2 QDs make them potentially of interests for the applications in semiconductor-based spintronics, conceptual valley-based electronics, quantum information technology and optoelectronic devices. However, we also demonstrate that the fabricated monolayered WS2 QDs can be a nontoxic fluorescent label for high contrast bioimaging application.

Influence of additives on corrosion resistance and corroded microstructures of MgO-C refractories

Abstract Corrosion resistance and corroded microstructures of MgO–C refractories containing various antioxidants in a model EAF slag (CaO/SiO2 weight ratio=1.38) were investigated after 30 h at 1650°C. Antioxidants influenced their corrosion resistance by affecting both carbon (C) oxidation and MgO dissolution in the slag. Al additions improved C oxidation resistance at 1650°C only a little, but accelerated MgO dissolution, resulting in a minor effect on corrosion resistance. Additions of Si or Al+Si improved C oxidation resistance slightly but accelerated MgO dissolution more than Al additions, resulting in worse corrosion resistance than Al addition and no addition. B4C conferred the worst corrosion resistance, since boron-containing liquid formed in the refractory, greatly accelerating MgO dissolution and resulting in C (mainly graphite) in the matrix being eroded easily by the slag. With double addition of Al+B4C, boron-containing liquid formed, which not only inhibited effectively C (mainly graphite) oxidation, but also accelerated formation and growth of MgAl2O4 spinel (MA) crystals between graphite in the matrix at the test temperature. In this case, even though MgO dissolution was accelerated to some extent, graphite was not easily washed away by the slag because it was effectively protected from oxidation and held in place by MA crystals. This maintained the integrity of the refractory texture, giving the Al+B4C containing refractory the best corrosion resistance.

Carbon containing castables: current status and future prospects

Abstract Measures to overcome the main technical di fficulties hindering the development and application of carbon containing castables are discussed. The aqueous wettability and dispersion properties of graphite can be improved by coating with materials such as carbides (SiC) and oxides (Al2O3, TiO2, SiO2, MgO, ZrO2) or by forming micropellets / briquettes. Thick and dense crack free coatings are needed not only to improve the aqueous wettability and dispersion of graphite, but also its oxidation resistance. Small and dense micropellets or briquettes enable a homogeneous distribution of graphite in the matrix to be achieved, along with acceptable mechanical strength and corrosion resistance. Coating techniques have also been used to improve the hydration resistance of aluminium based antioxidants, but detailed studies in this area are still needed. The main binder systems are currently based on superfine silica fume and hydratable alumina as these do not form low melting phases in castables. As well as developing the existing coating and micropellet/briquette fabrication techniques, future work will continue to seek novel methods of incorporating graphite, and will begin to investigate installation methods and drying and heating schedules.

Fabrication and Luminescence of Monolayered Boron Nitride Quantum Dots

Monolayered boron nitride (BN) quantum dots (QDs; lateral size ≈10 nm) are fabricated using a novel method. Unlike monolayered BN sheets, these BN QDs exhibit blue-green luminescence due to defects formed during preparation. This optical behavior adds significant functionality to a material that is already receiving much attention. It is further shown that the QDs are nontoxic to biological cells and well suited to bio-imaging.

Corrosion of high alumina and near stoichiometric spinels in iron-containing silicate slags

Dissolution of alumina-rich spinel aggregates in iron-containing silicate slag was studied using grain corrosion tests. High alumina spinels (AR90, 90% Al2O3) dissolve indirectly while dissolution of near stoichiometric (AR78, 78% Al2O3) spinel is entirely direct. This behaviour arises because higher alumina spinels can accommodate more cations from the slag due to the greater number of Mg 2+ vacancies in their crystal structures. Removal of these cations increases the viscosity of the remaining silica-rich local slag so that alumina-rich spinels exhibit better penetration resistance. However, thermodynamic calculations predict that near stoichiometric spinel (AR78) is more corrosion resistant since more slag is needed to dissolve it completely. # 2002 Elsevier Science Ltd. All rights reserved.

Use of phase diagrams in studies of refractories corrosion

Abstract Typical applications of phase diagrams to specific problems of refractories corrosion are highlighted. Saturation solubilities of refractories components in molten slags can be estimated using existing phase diagrams. These can then be used to predict the corrosion behaviour, and qualitatively compare the corrosion resistance, of a refractory in different slags or different refractories in the same slag. If the slag is not saturated with refractories components, using relevant phase diagrams, the conditions under which solid reaction product phases form at refractories/slag interfaces can be predicted, which assists understanding of direct and indirect dissolution. By checking the compatibility between impurities or additives and refractories components at high temperatures, their influence on corrosion resistance can be predicted, aiding their selection. Phase diagrams also reveal that atmosphere affects refractories corrosion resistance by altering the valence of some components (in particular, iron oxide) in the refractories and/or slags. The compatibility between refractories and slags indicates the corrosion resistance of different refractories, assisting refractories selection for specific applications. Finally, phase diagrams can be used to assist design of refractories composition. Repeated experimental corrosion test results show good agreement with phase diagram predictions. Therefore, use of the information which is available in existing phase diagrams can reduce the need for expensive and time consuming experiments to evaluate high temperature corrosion of refractories (and other ceramics).