Search

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB2–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB2–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m−1 K−1. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB2–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB2–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB2–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.

To improve the properties of high alumina castables containing calcium aluminate cement (CAC) after firing at elevated temperatures, micro-sized basic zinc carbonate (BZC) was introduced as ZnO-based nano fragments into castables. To account for the influence of BZC on the evolution of castable properties, the phase composition and microstructure of castable matrices were examined with X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectroscopy, respectively. Properties of castables having BZC were compared with those with Zn(OH)2. Results show that strength of castables added with BZC after firing at both medium temperatures and 1550 °C was improved. The volume stability and hot modulus of rupture at 1550 °C of fired castables containing BZC were also enhanced.

This study presents a novel strategy for fabricating high-strength mullite using Al3+/Ce4+-doped SiO2 and γ-Al2O3 powders. Doping of Al3+/Ce4+ ions in SiO2 results in the formation of a large amount of low-viscosity liquid phase in a narrow temperature range, which benefited the particle arrangement (closer) and reduced pores formation, thereby accelerating the mullitization rate and promoting the mullite grains growth. At elevated temperatures, almost all-SiO2 liquid phase were consumed due to a relatively high mullitization rate, and cerianite (CeO2) with high melting point gradually precipitated. Thus, the obtained ceramics with highly dense structure, and fully developed elongated columnar slab-shaped mullite grains form an interlocking structure, thereby leading to a high compressive strength (308.0 MPa) and excellent refractoriness under load (1672 °C).

The present paper investigates the influence of electric currents on the formation of Sn/Cu intermetallic compounds. Solidification experiments of Sn droplets on Cu substrate were performed under the impact of direct csurrents (DC) and pulsed electric currents (ECP), respectively. The results show that the precipitation of Cu6Sn5 from the Sn/Cu interfacial reaction is significantly enhanced by both DC and ECP. DC promotes the formation of Cu6Sn5 at the interface, whereas ECP triggers the detachment of Cu6Sn5 at the interface and favors the growth of freely floated Cu6Sn5 particles in the bulk Sn melt. Numerical simulations were conducted to calculate the distribution of electric current, induced magnetic field, Lorentz force and forced flow inside the Sn melt. The numerical results show that a strong downwards Lorentz force is generated adjacent electrode bottom owing to the interaction between electric current and induced magnetic field. As a result, a global forced flow is caused inside the Sn melt. It is possible that the forced flow induces the elements transport to promote the precipitation of intermetallic compounds. Moreover, a strong Lorentz force difference between Sn melt and Cu6Sn5 is generated under the application of ECP compared with that of DC, thereby increasing the potential to initiate the detachment of Cu6Sn5.

To assess the impact of changes in slag type on the degradation of refractory linings of secondary copper smelters, the degradation behavior of direct-bonded and fused grain magnesia–chromite refractories in ZnO-containing ferrous calcium silicate slags was studied via dynamic refractory finger and static refractory crucible tests at 1200 °C under Ar atmosphere. The effect of ZnO level in the slag and refractory type on the degradation behavior was determined. A combination of microstructural and compositional characterizations of tested refractories, and thermodynamic calculations leads to comprehensive understanding of the degradation mechanism of the refractories. Based on the degradation mechanism, conclusions can be given about the prediction of lifetime of refractory lining when the slag type changes from fayalite-based to ferrous calcium silicate-based slags for secondary copper smelting.

Hydratable alumina (HA) is a superior Ca-free refractory binder, but the quick hydration rate restricts the working time of castables bonded with HA. In this work, HA was grounded for 1 h and 6 h by a rotational ball mill to study the effect of grinding on the hydration of HA and properties of HA-bonded castables. HA samples with and without grinding were cured at 30 °C and then terminated by freeze-vacuum drying. The phase composition and microstructure of the dried HA samples were then examined. Moreover, flow ability and mechanical strength of castables containing ungrounded and grounded HA were also investigated. The results indicate that the specific area of HA particles were decreased by grinding as the micro-pores and micro-cracks on the surface of HA particles were blocked by smaller HA particles, thereby decreasing the hydration rate of HA and increasing the flow ability of castables.

In this work, the effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)2 after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)2 can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)2 also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)2 decomposition in castables.

Calcium aluminate cement was premixed with hydromagnesite having different particle sizes, as a calcium magnesium aluminate cement precursor, to investigate the influence of particle size of hydromagnesite on the volume stability and thermo-mechanical properties of corundum based castables after firing at 1550 °C. The impact of particle size of hydromagnesite on the phase composition and microstructure evolution of fired castables matrix were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results demonstrate that hydromagnesite with smaller particle size has a better volumetric stability and thermo-mechanical properties of castables, because of the more homogeneously distributed micro-pores, and smaller size MA and CA6 resulted from the more uniformly distributed hydromagnesite in castables.

The formation of CA2 and CA6 in calcium aluminate cement bonded castables lead to a large volume expansion, which probably results in the generation of cracks, thereby increasing slag penetration and decreasing mechanical strength. In this study, the influence of curing time on the dimensional stability of calcium aluminate cement bonded castables is considered to improve the volumetric stability. Phase compositions and morphologies evolution of matrix samples and castables are characterized by XRD and SEM, respectively. The results indicate that the volume stability of castables is greatly improved by increasing curing time from 0.5 days to 6 days after firing at 1600 °C. This can be attributed to two main reasons. First, the improvement in migration of CaO through extending curing time favors more uniform distribution of in situ CA6. Second, more hydrates after dehydration creates more micro-pores, thereby providing space for the generation of in situ CA6.

High temperature melting treatment and cement solidification are technologies currently used to reduce the leaching of heavy metals in municipal solid waste incinerator (MSWI) fly ash. In this paper, to ascertain the feasibility of melting MSWI fly ash with blast furnace (BF) slag, ultra-risk MSWI(U-MSWI) fly ash having high heavy metal (Zn, Pb, Cu, and Cr) contents were blended with BF slag, then melted and quenched into water to prepare reconstructed slag. The melting and solidification behaviors, phase composition and microstructure, and heavy metal leachability of reconstructed slag were studied. In addition, to study the further solidification and utilization of reconstructed slag in cement, the compressive strength and leaching concentration of cement composites with reconstructed slag were also investigated. The results indicate that the presence of heavy metals in the U-MSWI fly ash had a little influence on the microstructure and phase composition of reconstructed slag. The leaching concentration of heavy metals in the reconstructed slag increased with the increasing of U-MSWI fly ash content, and when the content of U-MSWI fly ash was less than 50 wt%, the reconstructed slag could meet the environmental requirements. The reconstructed slag further solidified by cement could be applied to landfill and construction materials. The technology of melting reconstruction treatment with cement solidification was a technical-economical choice for the industrial treatment of U-MSWI fly ash.

The conventional refractory sagger materials, e.g., mullite, cordierite, and magnesium aluminum spinel, employed to calcine Li-ion battery (LIB) cathode materials, are easily attacked by the LIB material and thus have short lifetimes. To improve the corrosion resistances of refractory saggers to LIB materials, andalusite–KAlSi2O6 (KAS4) materials with different contents of in-situ KAS4 were synthesized using andalusite and KAS4 precursor calcined at 1300 °C. The microstructure evolution, phase development, corrosion resistances to Li(NixCoyMnz)O2 (LNCM) cathode materials and thermomechanical properties of the designed andalusite-KAS4 materials were characterized. The results show that the corrosion resistances of the prepared andalusite–KAS4 material were superior to those of the LNCM materials owing to the in–situ KAS4 wrapping andalusite grains. The prepared andalusite–corundum–KAS4 materials also exhibited improved strengths and appropriate thermal shock resistances compared with mullite-based refractories.

Guided by the corrosion mechanism in contact with Li(NixCoyMnz)O2 materials, a method was developed by wrapping refractory grains with a KAlSi2O6 (KAS4) layer for corrosion protection during the calcination of Li-ion battery (LIB) cathode materials. Preformed or in-situ KAS4 surrounds around refractory grains as a corrosion barrier by physically separating their contact with LIB materials. Because KAS4 did not react (or reacted little) with LIB materials, materials having in-situ or preformed KAS4 exhibit superior corrosion resistance, and in-situ KAS4 improves the corrosion resistance more. Additionally, the in-situ or preformed KAS4 also improves the thermomechanical properties of the prepared materials.

To assess the feasibility of potassium aluminosilicates (KAlSiO4, KAS2 and KAlSi2O6, KAS4) as the refractory sagger material for the calcination of Li-ion battery cathode materials, the interaction behavior between KAS2 (KAS4) and the Li(NixCoyMnz)O2 (LNCM) material was investigated at temperatures ranging from 800 to 1100 °C. The phase composition and microstructure of calcined KAS2 (KAS4)-LNCM precursor mixtures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicate that potassium aluminosilicates exhibit superior resistance to corrosion by LNCM materials compared with mullite and MgO-based refractories, and rarely react with LNCM materials during calcination.

We found in our research that andalusite aggregate fired in a reducing atmosphere exhibits a lower mullitization rate than that fired in an air atmosphere. For investigating the effect of atmosphere on the transformation of andalusite and the properties of andalusite-containing refractories, andalusite powder (≤0.074 mm) and refractories containing andalusite aggregate (3–1 mm) were fired in air and carbon embedding, respectively. The phases and microstructure of the andalusite fired in both atmospheres were characterized by X-ray diffraction and scanning electron microscopy, respectively. The correlations of the properties of the andalusite-bearing refractories with the firing atmospheres were investigated in terms of volume stability, mechanical strength, and thermal shock resistance. The difference in the properties of the refractories was discussed with respect to the varied transformation rates of andalusite, and in terms of the different viscosities of the silica-rich glass caused by the different atmospheres.

The distribution and size of in-situ CA6 is related to CAC particle size in CAC-bonded corundum-based castables and would influence the properties of the castables. In order to investigate the influence of CAC particle size on the properties of CAC-bonded castables, CAC was milled for 2 h to reduce particle sizes. The properties of CAC-bonded castables with different CAC particle sizes were investigated in terms of demoulding strength after curing at 30 °C, drying strength after drying at 110 °C and, in particular, volumetric stability and thermal shock resistance after firing at 1450 °C, respectively. The results indicate the volumetric stability and thermal shock resistance of the CAC-bonded castables were improved by replacing unground CAC (D50 = 12.39 μm) with ground CAC (D50 = 1.36 μm) because the incorporation of ground CAC produced smaller-sized CA6 with more uniform distribution after firing at 1450 °C.

Micro-sized MgCO3 and Al(OH)3 were used as inorganic porogenic agents for increasing porosity in alumina bubble lightweight castables. The influence of 0–4 wt% micro-sized MgCO3 and 0–8 wt% micro-sized Al(OH)3 additions on the porosity, thermal conductivity and strength of castables after firing was investigated. The results indicate that porosities in castables containing micro-sized Al(OH)3 or MgCO3 after heat-treatment were raised by augmenting micro-sized Al(OH)3 or MgCO3 contents. It is found that MgCO3 favors the porosity formation more than Al(OH)3 in castables because of the different formation behavior of calcium aluminates and MgAl2O4 spinel. The dependence of properties of castables on the micro-sized Al(OH) or MgCO3 addition was discussed with respect to the pore evolution.

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO2-rich liquid phase during the sintering process, and the liquid phase is transient in the Al2O3-SiO2 system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105 MPa) have been obtained after fried at 1700 °C.

In this study, the degradation behavior of Al2O3-Ti3AlC2-Si-C refractories and a complete degradation process of Ti3AlC2 under a temperature gradient along the penetration direction were investigated by a dynamic corrosion at approximately 1630 °C. The degradation process of Ti3AlC2 affected the local liquid components by assisting to form CaAl12O19 and restrained CaO penetration via trapping into high-melting phase. On refractories-steel/slag interfaces affected by Marangoni convection, the molten steel penetration was suppressed in the samples with Ti3AlC2. A refined texture and increased viscosity, accompanying Ti3AlC2 oxidation and dissolution in molten steel, blocked the pathways and restrained the penetration of molten steel.

To assess the viability of micro-sized hydromagnesite as a precursor in refractory castables bonded with calcium aluminate cement, the volumetric stability and thermo-mechanical properties of high-alumina castables containing different micro-sized hydromagnesite amounts (0–1.6 wt%) after firing at 1550 °C were investigated. Phase composition and microstructure evolution in fired castable matrices with different micro-sized hydromagnesite contents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Dimensional stability, hot modulus of rupture and thermal shock resistance of castables were improved by adding micro-sized hydromagnesite. The microstructure evolution in castable matrix added with micro-sized hydromagnesite was discussed to understand the mechanism of enhanced volume stability and thermo-mechanical properties.

To assess the viability of an alumina-chromia refractory alternative for secondary copper smelters, the degradation behavior of alumina-chromia refractories in a secondary copper smelting slag and a Cu-CuxO mixture was studied via lab scale refractory finger testing. Microstructural characterization of the worn refractory samples resulted in a comprehensive understanding of the corrosion and penetration resistance and a comparison with the degradation of magnesia-chromite refractories. Alumina grains in the refractory are preferentially corroded by the slag. The alumina-chromia refractory exhibits superior infiltration resistance to the slag and the Cu-CuxO mixture compared to magnesia-chromite refractories.

Dense NiTi alloy was prepared by liquid phase spark plasma sintering from a TiH2 and Ni powder mixture at 1050 °C for 4 min. Dehydrogenation of TiH2 in the powder mixture was rapidly completed after heating to 850 °C. The sintered alloy showed no obvious transformation peaks, whereas the homogenized alloy, thermally treated in Ar at 1150 °C for 2 h, displayed clear transformation peaks with a high transformation enthalpy of 10.6 J/g. The hardness of the sintered and homogenized alloys were statistically the same.

Used direct-bonded magnesia–chromite refractory bricks from a secondary Cu smelter were collected to characterize the degradation occurring during application. Based on this post-mortem analysis, lab scale experiments were designed using the direct-bonded refractory type and a fused grain magnesia–chromite refractory. Firstly, the infiltration behavior of a Cu–CuxO and a Cu–CuxO–PbO mixture was investigated. Secondly, the influence of temperature and the prior infiltration of a Cu–CuxO mixture on the infiltration of the ZnO containing fayalite slag was determined. The combination of the post-mortem study and the lab scale tests allowed a more comprehensive understanding of the evolution of the refractory microstructure during degradation. Likewise, it allowed to evaluate how certain parameters, such as temperature and refractory type, affect the infiltration and degradation behavior. As a consequence, conclusions can be drawn about measures to minimize refractory wear in copper smelter linings.

A ZnO-containing fayalite-based slag can be formed in copper smelting from secondary raw materials and its high viscosity is a common issue that hinders slag tapping. In this work, the crystallization behavior of the industrial slag was observed in situ by confocal laser scanning microscopy. Solid precipitation was found to be the major cause of the poor slag fluidity. The phase relations in the industrial slag system ZnO-“FeO”-SiO2-Al2O3-CaO (CaO/SiO2 = 0.05, Al2O3/SiO2 = 0.15) were investigated by quenching the samples after reaching equilibrium at 1423 K (1150 °C) under iron saturation. The equilibrium composition of the phases was determined with electron probe micro-analysis. The effect of individual components, such as FeO, ZnO, and CaO on the phase equilibrium of the slag system has been quantitatively studied. The relation between the solid-phase fraction and the chemical composition of the slag has been revealed. Suggestions to modify the slag composition toward low viscosity are provided.

To determine the effect of ZnO in ZnO-containing fayalite (ZFS) slags on the degradation of magnesia-chromite refractories, the corrosion behaviour of a direct-bonded magnesia-chromite refractory was studied by rotating refractory finger tests at 1200 °C under reducing atmosphere. In addition, MgO crucible tests at these conditions were performed to investigate in particular the effect of ZnO on the slag/MgO interaction. The dissolution of MgO from periclase into the ZFS slags is not enhanced by increasing the ZnO content in the slags. (Fe,Mg,Zn)2SiO4 olivine and (Fe,Mg,Zn)O solid solution are the only two products in the slag/MgO interfacial reactions, even when the ZnO content in the ZFS slag reaches 16.7 wt%. The diffusion of ZnO into the chromite grains to form the (Fe,Mg,Zn)(Fe,Al,Cr)2O4 spinel was favoured with increasing the ZnO content in ZFS slags.

The corrosion behavior of alumina-silica (Al 2 O 3 –SiO 2 ) refractories by liquid carbon steel was studied. This was performed by a refractory crucible test in a vacuum induction furnace at 1520–1545 °C under reducing atmosphere. The results indicate that Mn, Si and a minor amount of Fe in the steel were oxidized at steel/refractory interface, combing with refractory components to form a MnO–SiO 2 –Al 2 O 3 –based slag. This slag was liquid at the testing temperature, and infiltrated along the open-pore network and the grain boundaries into the Al 2 O 3 –SiO 2 refractories. Both Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the refractories dissolved into the infiltrating slag. As a result, the refractories were structurally weakened and residual Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the hot face of the refractories were washed away into the liquid steel, affecting steel quality.

Guided by predictive theory, a new compound with chemical composition (Cr

This study evaluates commercially available magnesia–chromite, magnesia–carbon and magnesia–doloma bricks for their use in a Vacuum Oxygen Decarburisation ladle. The corrosion behaviour of these bricks by stainless steelmaking slags is, therefore, investigated through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (5.3 and 3.0×10 −11 atm). The results reveal that magnesia–carbon bricks are severely corroded due to the high dissolution of MgO, while magnesia–chromite and magnesia–doloma refractories exhibit an excellent corrosion resistance. The MgO enrichment in the slag is believed to be the reason of the low wear rate of the MgO–doloma refractories. Rebonded and direct-bonded MgO–chromite refractories show similar corrosion resistance against the slags because of the ‘secondary chromite inactivation’. Decreasing the slag basicity enhances the dissolution of MgO into the slag, thereby increasing the corrosion of the magnesia-based refractories.

Brucite-fumed silica and hydromagnesite-fumed silica mixtures were used to investigate the influence of MgO precursors on mechanically activated forsterite synthesis. The changes in morphology, chemical bond and phase composition of the ground and calcined mixtures were examined with scanning electron microscopy (SEM), Si 2p X-ray photoelectron spectroscopy (XPS) and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR), and X-ray diffraction (XRD), respectively. The XPS and MAS-NMR analyses show that high-energy milling generates more Mg–O–Si chemical bonds in the brucite-fumed silica mixture than in the hydromagnesite-fumed silica sample. This is because brucite has a higher concentration of Mg–OH bonds than hydromagnesite. However, single-phase forsterite forms at a higher temperature of 1000 °C in the milled brucite-fumed silica mixture than that of 800 °C in the ground hydromagnesite-fumed silica sample after the same grinding. The different forsterite completion temperature is probably due to the longer Mg 2+ and Si 4+ diffusion distance of over 500 nm in the former milled mixture than that of less than 300 nm in the latter ground sample.

In order to study the role of ZnO in the degradation of magnesia-chromite refractories, the chemical corrosion behaviour in a ZnO-containing fayalite slag (FeO–SiO 2 –ZnO–Al 2 O 3 ) is investigated using a rotating magnesia-chromite refractory finger corrosion test under a reducing atmosphere at 1200 °C. The results show that, likewise to ZnO-free fayalite slag, periclase is predominantly corroded. Both ZnO and FeO diffused into the original periclase and chromite grains thereby forming, respectively, (Zn,Fe,Mg)O solid solution and (Zn,Fe,Mg)[Cr,Al,Fe] 2 O 4 spinel. Concurrently, ZnO and FeO were also incorporated into the (Zn,Fe,Mg) 2 SiO 4 olivine during its formation. None of these phase formations resulted in new crack generation in the refractory. Considering the infiltrated slag is almost completely depleted in ZnO at 4 mm from the hot face, the severe slag penetration up to the centre of the refractory sample is not caused by the ZnO presence in the initial slag.

Forsterite was prepared via solid-state reaction by using hydromagnesite (basic magnesia carbonate) and fumed silica, aiming at completing the reaction 2MgO+SiO 2 →Mg 2 SiO 4 at low calcination temperature. The phase development and morphology evolution of the hydromagnesite and hydromagnesite–fumed silica mixture during heat treatment were characterized by means of scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) nitrogen-gas adsorption method, and X-ray powder diffraction (XRD). A diffusion distance of less than 300 nm for Mg 2+ and Si 4+ for forsterite formation was obtained because the formed high reactivity MgO with the nanosheet structure of its parental hydromagnesite and the fumed silica with particle size of less than 100 nm were homogeneously mixed. Monolithic forsterite was synthesized after calcination of the hydromagnesite–fumed silica mixture at 1100 °C.

Chemical bond changes of gibbsite and fumed silica mixture during mechanical activation were investigated by X-ray powder diffraction (XRD), magic angle spinning nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron spectroscopy (XPS). Crystallinity and intrinsic structural change of the mixture during high-energy milling were examined by XRD. Formation of new AlOSi chemical bonds was detected by the changes in Al coordination number of gibbsite in the mixture during milling, and the appearance of new resonance in 29Si NMR spectrum of the ground mixture. The presence of AlO units in silica framework was demonstrated by the increase of Al 2p binding energy and the decrease of Si 2p binding energy. © 2012 Elsevier B.V. All rights reserved. ispartof: MATERIALS LETTERS vol:85 pages:91-94 status: published