Formation of ferrospinel layer at the corroded interface between Al2O3-spinel refractory and molten steel in RH refining ladle

Refining ladle is generally the last inclusion-removal vessel before the continuous casting of steel, so, it has a vital effect on the final steel quality. In this work, degradation process of a cement-free Al₂O₃-MgAl₂O₄ refractory in contact with molten steel/slag in the metal bath area of a Ruhrstahl Heraeus (RH) refining ladle was investigated. A reaction product layer with the formation of ferrospinel (hercynite) and Fe-rich phases was observed, suggesting that the interactions/reactions between the refractory lining and the molten steel should also be considered to have a better understanding of overall degradation mechanism of the refractory served under the RH refining conditions. The two types of alumina grains in the refractory, sintered and fused alumina, were attacked in an active way and a passive way, respectively. The effect of the crack generation and steel infiltration on degradation of the refractory was also discussed in detail, based on the microstructural characterizations.     

Al2O3-AlON复合材料的烧结性能和显微结构

研究了在保护气氛下1600℃烧成的Al2O3-AlON复合材料的烧结性能、物相组成和显微结构.结果表明在刚玉材料中加入AlON,烧结性能得到改善,物相组成主要为刚玉和尖晶石型构造的氧氮化铝,两者之间形成紧密结合结构,并有少量MgAlON微晶填充在大颗粒间的孔隙中,形成致密结构.     

In situ fabrication and microstructural evolution of Al2O3-ZrO2 nanoeutectic ceramics by a novel pulse discharge plasma assisted melting method

The present work concentrated on a novel one-step pulse discharge plasma melting method for in situ fabricating Al₂O₃-ZrO₂ binary eutectic in a short processing time. Al₂O₃-ZrO₂ eutectic exhibited well-aligned fibrous and colonial structure with a minimum lamellar spacing of ～250?nm at the growth rate of ～176.0?μm/s. The lamellar spacing was well consistent with the inverse-square-root dependence on the growth rate as $\lambda =2.90\times v^{?1/2}$, and the constant of proportionality of 2.90 was close to 3.32 predicted by the JH eutectic theory. The formation mechanism of Al₂O₃-ZrO₂ eutectic on primary faceted-alumina crystals was also proposed. The Al₂O₃-ZrO₂ eutectic originated from the faceted Al₂O₃ primary crystals, which played the leading role in the coupled growth process. The interfacial fracture behavior across the coarse intercolony regions was explored, and the fracture toughness of Al₂O₃-ZrO₂ eutectic ceramics was comparable to those achieved by traditional directional solidification.     

Effects of Cr2O3 addition on property improvement of magnesia-spinel refractories used in RH snorkel

The Ruhrstahl-Heraeus (RH) snorkel is one of the most important parts of the RH furnace, which is the primary facility for producing high-quality steel. The magnesia-spinel refractory is the ideal replacement for the classic magnesia-chrome refractory in the RH snorkel. In this study, the effects of Cr₂O₃ addition (0–3 wt%) on properties of magnesia-spinel refractories were investigated. It was found that the addition of Cr₂O₃ results in the acceleration of the densification process and a decrease in the cold strength as well as a significant increase in the hot modulus of rupture. Thus, the solid solution formed with the spinel significantly improves the resistance to thermal shocks, with the greatest improvement being observed after the addition of 2 wt% Cr₂O₃.     

Dissolution behavior of a novel Al2O3-SiC-SiO2-C composite refractory in blast furnace slag

Al₂O₃-SiC-SiO₂-C composite refractories are interesting potential blast furnace hearth lining materials that feature several advantageous properties. In this study, the corrosion resistance of a novel Al₂O₃-SiC-SiO₂-C composite refractory to blast furnace slag was investigated by adopting a rotating immersion method (25 r/min) at 1450–1550 °C and comparing it against a conventional corundum-based refractory at 1550 °C as a benchmark. The results showed that the apparent activation energy of Al₂O₃-SiC-SiO₂-C composite refractory over the dissolution process in the slag is 150.4 kJ/mol. Dissolution of the Al₂O₃ and 3Al₂O₃·2SiO₂ phases appeared to be the main cause of Al₂O₃-SiC-SiO₂-C composite refractory corrosion. High-melting-point compounds in the slag layer formed a protective layer which mitigated the corrosion. The novel Al₂O₃-SiC-SiO₂-C composite refractory is better suited to blast furnace hearth lining than the conventional corundum-based refractory, because the carbon phase and SiC phase in the material are not readily wetted by the blast furnace slag and therefore are more resistant to slag penetration. Higher melting point phases also may crystallize on the hot face of the hearth lining due to the high thermal conductivity of the Al₂O₃-SiC-SiO₂-C composite refractory, promoting a more stable protective layer.     

Dielectric characterization of a novel Bi2O3-Nb2O5-SiO2-Al2O3 glass-ceramic with excellent charge-discharge properties

A newly designed glass-ceramic system consisting of 15Bi₂O₃-15Nb₂O₅-40SiO₂-30Al₂O₃ was successfully prepared, which was followed by its controlled crystallization at different heating temperatures. The effects of crystallization temperature on the microstructure, phase evolution and the energy storage behaviors of the novel material were systematically investigated. A maximum theoretical energy storage density of up to 15.3 J/cm³ was found in the samples heated at 800 °C. The polarization-electric (PE) hysteresis loops of this material exhibited very good linear character and high energy efficiency. In addition, an approximate value of 25 ns for discharged period T has been obtained, which demonstrated that most of the energy stored in dielectric was released over a very short time. The maximum powder density exceeds a high value of 90 MW/cc in a 390 kV/cm electric field. Therefore, the new developed Bi₂O₃-Nb₂O₅-SiO₂-Al₂O₃ glass-ceramic can be used as an alternative, promising high-performance electrostatic capacitor material.     

Effect of substituting Na2O for SiO2 on the non-isothermal crystallization behavior of CaO-BaO-Al2O3 based mold fluxes for casting high Al steels

The non-isothermal crystallization behaviors of CaO-BaO-Al₂O₃-based mold fluxes in which SiO₂ had been replaced by Na₂O were investigated, using an infrared furnace combined with digital microscopy as well as X-ray diffraction. The initial crystallization temperatures, crystalline phases generated at different cooling rates, crystallization rates, Ozawa indices N_O (as determined using Mo's equation) and effective crystallization activation energies (as determined using the Friedman equation) were evaluated. The initial crystallization temperature increased along with the Na₂O content in the flux. In addition, the precipitation of CaF₂ crystals was inhibited while the growth of CaAl₂O₄ crystals was enhanced at Na₂O concentrations over 4?wt%. Increasing the cooling rate inhibited the precipitation of MgAl₆O₁₀, and of CaSiO₃, MgAl₂SiO₆ and NaAlSi₃O₈, respectively, in fluxes containing 0, 4 and 8?wt% Na₂O (NS1, NS2 and NS3 samples). The peak instantaneous and average crystallization rates in these specimens were increased as the Na₂O level was raised, as a direct result of changes in their N_O values. The NS1 flux consistently had the lowest N_O value, while the NS2 had the largest value in the primary stage of crystallization but the central value in the secondary stage, and the N_O values of the NS3 flux exhibited the opposite trend. The effective crystallization activation energies decreased with increasing Na₂O levels, and this was evidently an important factor affecting the initial crystallization temperature.     

炼铜诺兰达炉用镁铬砖损毁机理的探讨

简要介绍了诺兰达炉用耐火材料的现状,并以风口区使用后的镁铬残砖为试样,采用化学分析、SEM和EDAX等手段对其侵蚀机理进行了分析研究。结果表明:铜渣对镁铬耐火材料的侵蚀,在工作面以熔蚀为主;在砖体内部主要以锍渗透为主。锍渗透的主要途径为开口气孔、晶界及微裂纹。     

Electronic states and cation distributions of MgAl2O4 and Mg0.4Al2.4O4 microwave dielectric ceramics

The electronic state and microwave dielectric properties of MgAl₂O₄ prepared using solid-state (MA-S) and molten salt (MA-M) methods and those of Mg_0.4Al_2.4O₄ (M04A24) were investigated. The λ values, which correspond to the fraction of Al³⁺ cations in tetrahedral sites, for MA-S, MA-M, and M04A24 were 0.23, 0.41, and 0.60, respectively. In molecular orbital calculations, a larger overlap was observed between Al-3s or Al-3p in tetrahedral sites and O-2p orbitals for M04A24, and the bond order for AlO at tetrahedral sites of M04A24 (0.241) was higher than those for MA-S (0.178) and MA-M (0.205). The dielectric constant, ε_r, for M04A24 (7.6) was lower than those for MA-S and MA-M (both 7.9), and the highest quality factor, Q·f, was obtained for M04A24 (235, 800 GHz). It was found that the covalency of the AlO bonds in the MO₄ tetrahedra is closely related to the Q?f values of the present ceramics.     

The effect of Al content on the wettability between liquid iron and MgOAl2O3 binary substrate

In the present study, the wettability between liquid iron with two different Al contents and MgOAl₂O₃ binary substrates was studied in reducing atmosphere. The contact angles between liquid iron with 18?ppm Al and MgO, MgO·Al₂O₃, Al₂O₃ were 133.5°, 113.7°, 126.9° respectively. With the variation of the substrate composition, the contact angles for the intermediate binary phases of the three components (MgO, MgO·Al₂O₃, Al₂O₃) obeyed the Cassie theory. In the experiment using iron with 370?ppm Al, all the contact angles were higher than that using low Al-containing iron. The surface of the iron drop was covered with an oxide layer, which mainly consisted of many small particles. With the variation of the substrate gradually from MgO to Al₂O₃, the composition of the oxide layer changed from MgO·Al₂O₃ to CaOAl₂O₃. The formation of the oxide layer prevented the spreading of liquid iron, leading to the increase of the contact angle.     

Microstructure control,competitive growth and precipitation rule in faceted Al2O3/Er3Al5O12 eutectic in situ composite ceramics prepared by laser floating zone melting

Microstructure control and competitive growth of Al₂O₃/Er₃Al₅O₁₂ eutectic/off-eutectics are explored over wide ranges of solidification rates and compositions. Gradual transformation phenomenon of microstructure morphology from complete eutectic to eutectic + coarse Er₃Al₅O₁₂ phase and to eutectic + Er₃Al₅O₁₂ dendrite is observed and the corresponding influence factors are evaluated. Competitive growth between single-phase Al₂O₃ (or Er₃Al₅O₁₂) dendrite and eutectic is analyzed and coupled growth zone is mapped through comparing interface temperatures of different patterns of microstructures. The complete eutectic microstructure could be obtained at Al₂O₃/Er₃Al₅O₁₂ hypoeutectic (Al₂O₃-17.5 mol% Er₂O₃) under fast solidification rate and the onset growth rate (?0.94 × 10⁴ μm/s) estimated from the measured eutectic spacing (?150 nm) fits well with the result calculated on the basis of competitive growth (?1.27 × 10⁴ μm/s). Transformation of microstructure from irregular eutectic to regular eutectic and probable adjustment mechanism of eutectic spacings are discussed when the eutectic spacings refined from micron-scale (<10 μm) to nano-scale (?20 nm).     

Erosion mechanism of the postmortem Al2O3-Ti2O3-Al sliding gate containing novel Al2O3-Ti2O3 raw materials

Al₂O₃-Ti₂O₃-Al sliding gates were prepared from Al₂O₃-Ti₂O₃ raw materials, sintered corundum and aluminum, and used in trials at steel works. The sliding gate with 30 wt% Al₂O₃-Ti₂O₃ added was used on an 80 t ladle for 4 cycles without macrocracks. The postmortem sliding gate can be divided into the permeation layer (0–0.1 mm), transition I layer (0.1–10 mm), transition II layer (10–20 mm) and unchanged layer from the hole working face outward. XRD, SEM and industrial CT were used to analyze the postmortem sliding gate. The results show that, in the Al₂O₃-Ti₂O₃-Al sliding gate, both Al and Ti₂O₃ are involved in reactions, Ti₂O₃ transforms into Ti₂O and TiO in the transition I layer, and part of the Ti₂O₃ in the transition II layer transforms into Ti₈O₁₅. Titanium compounds with different densities are dispersed in the matrix and form microcracks to improve the thermal shock resistance of the sliding gate, which improved the performance.     

Fe3O4 nanoparticles impregnated eggshell as a novel catalyst for enhanced biodiesel production

Biodiesel is a green fuel which can replace diesel while addressing various issues such as scarcity of hydrocarbon fuels and environmental pollution to an extent. The high production cost of biodiesel and the recovery of the catalyst after the transesterification process are the major challenges to be addressed in biodiesel production. In the present work, a cheap and promising solid base oxide catalyst was synthesized from chicken eggshell by calcination at 900 °C forming catalyst eggshells (CES) and was impregnated with the nanomagnetic material (Fe₃O₄) to obtain Fe₃O₄ loaded catalytic eggshell (CES–Fe₃O₄). Fe₃O₄ nanomaterials were synthesized by co-precipitation method and were loaded in catalytic eggshell by sonication, for better recovery of the catalyst after transesterification process. CES–Fe₃O₄ material was characterized by Thermogravimetric analysis, X-ray diffraction, Fourier transform infrared spectroscopy, a vibrating-sample magnetometer, Brunauer-Emmett-Teller, Dynamic light scattering, and Scanning electron microscopy. Biodiesel was synthesized by transesterification of Pongamia pinnata raw oil with 1:12 oil to methanol molar ratio and 2 wt% catalyst loading for 2 h at a temperature of 65 °C and yields were compared. The reusability of the catalyst was studied by the transesterification of the raw oil and its catalytic activity was found to be retained up to 7 cycles with a yield of 98%.     

Chemical attack of Al2O3-MgAl2O4 refractory castables in the non-slag-tapping side of refining ladle

In the present work, chemical attacks and corrosion behaviors of a used Al₂O₃-MgAl₂O₄ refractory castable in the non-tapping slag side of a refining ladle were investigated and also compared with that of the counterpart (in the tapping slag side). Corrosion microstructures revealed the stress cracks occurred on the corroded interface and just along the boundary between the infiltration layer and original layer, indicating the thermo-mechanical properties’ mismatch above those two layers was the main reason for the formation of cracks on the corroded interface. Furthermore, the corrosion process of Al₂O₃ aggregates on the interface was studied in detail, and an intergranular mechanism was proposed based on observed microscopic characteristics.     

Electronic structure and luminescent properties of Ce3+-doped Ba3Lu2B6O15, a high-efficient blue-emitting phosphor

A high-efficient blue-emitting phosphor Ba₃Lu₂B₆O₁₅: Ce³⁺ was synthesized by three-step solid-state method. XRD Rietveld refinement analysis reveals that Ba₃Lu₂B₆O₁₅ is of cubic Ia$3(——)$ system, owning a high-symmetric layer structure with two regular LuO₆ octahedrons and one BaO₉ polyhedron. Density functional theory (DFT) calculation result indicates BLB is a kind of direct band-gap material with broad band-gap of ~5.10?eV. Ba₃Lu₂B₆O₁₅:Ce³⁺ phosphor exhibits a bright blue emission under UV excitation with high quantum efficiency of >?90%, and the possible reasons are discussed basing on structural and spectral data. Because of its excellent luminescent properties, Ba₃Lu₂B₆O₁₅:Ce₃⁺ could be a promising candidate as a blue phosphor for NUV LED devices.     

Microwave assisted synthesis and antimicrobial activity of Fe3O4-doped ZrO2 nanoparticles

Composites play important role in dental filling by controlling shrinkage along with correction in teeth's shape and position. Rehabilitation of severely worn dentition can be achieved using mechanically strong composites. This study aims to synthesize zirconia-based composites to be used as dental fillers. Effect of microwave powers (100–900?W) along with Fe₃O₄ doping are studied on the structural, mechanical and magnetic properties of stabilized zirconia. SEM and TEM reveal formation of spherical nanoparticles with diameter of ～30?nm. XRD results shows phase pure tetragonal zirconia (t-ZrO₂) at microwave power of 500?W without any post heat treatment. Crystallite size calculated from XRD data (～23?nm) matches well with the previously reported value for stabilization of t-ZrO₂. Microwave energy dissipation results in stresses causing volume shrinkage leading to monoclinic to tetragonal phase transformation with higher X-ray density and hardness of ～1347HV. VSM results show ferromagnetic response with low coercivity (600Oe) value and saturation magnetization (～2emu/g). It is worth mentioning here that this is one of its kind study reporting synthesis of room temperature stabilized Fe₃O₄ doped zirconia composites at microwave power of 500?W. Antibacterial studies reveal inhibition zone of ～32?mm against bacillus bacteria suggesting their potential use as dental filler.     

Nanoscale toughening of low carbon Al2O3-C refractories by means of SiCnw/Al2O3 composite reinforcement

To improve the thermal shock resistance of low carbon Al₂O₃-C refractories, SiC nanowires (SiC_nw) containing SiC_nw/Al₂O₃ composite reinforcement were introduced. The specific fracture energy of the Al₂O₃-C refractory matrix was obtained by statistical grid nano-indentation. The reinforcement mechanism of SiC_nw/Al₂O₃ on thermal shock resistance of refractories was investigated. The results revealed that the matrix-specific fracture energy of A6 (6 wt% SiC_nw/Al₂O₃ added) was 217 N/m, which was 58.4% higher than reference sample A0 (137 N/m) and 18.6% higher than MA6 (183 N/m, 6 wt% SiC/Al₂O₃ added). A6 showed the highest residual strength ratio of 49.8%, which was 114.7 % higher than A0 (23.2%) and 82.4 % higher than MA6 (27.3%). The components with different morphology in SiC_nw/Al₂O₃ cluster, especially SiC nanowires, promote the generation of microcracks, crack multi-deflection, and branching, which toughen the matrix and improve the thermal shock resistance of refractories. In comparison to the literature, A6 showed a higher rising in residual strength ratio than those with higher graphite content (4 wt% and 20 wt%), which will greatly reduce the consumption of carbon-containing refractories and contribute to the reduction of CO₂ emission.     

Al2O3-Ce:GdYAG composite ceramic phosphors for high-power white light-emitting-diode applications

In order to meet the increasing demand of high-power light-emitting-diode (LED) lighting, state-of-the-art white light-emitting diode technology needs phosphors with high thermal conductivity and high luminous efficacy as color converters. In this work, translucent Al₂O₃-Ce:GdYAG composite phosphors were prepared by solid-state reactive sintering. The microstructure shows that the Al₂O₃ particles are uniformly dispersed in the Ce:GdYAG matrix. These particles can not only improve the thermal conductivity of the ceramics, but also promote the extraction efficacy. The luminous characteristics of the Ce:GdYAG and Al₂O₃-Ce:GdYAG ceramics were analyzed after being packaged with blue LED. When the molar ratio of Al₂O₃/Ce:GdYAG is 0.8, a high luminous efficacy value of 112.6 lm/W is achieved by the Al₂O₃-Ce:GdYAG composite ceramic phosphor with the thickness of 0.4 mm, as well as the highest CRI valve of 71.4. The appropriate photoelectric properties of this kind of ceramic phosphor make it a promising candidate for high-power LED device.     

Friction and wear properties of MoAlB against Al2O3 and 100Cr6 steel counterparts

The present work investigates, for the first time, the dry sliding friction and wear behaviour of fully dense, predominantly single-phase MoAlB ceramics against alumina (Al₂O₃) and 100Cr6 steel counterparts. Against Al₂O₃, the friction coefficient (μ) increased with increasing load and the wear was highly dependent on the load applied. A transition from mild wear under 1 N and 4 N to severe wear at 10 N occurred. Scanning electron microscopy revealed that abrasion is the dominant wear mechanism. Against steel, μ decreased with increasing load and the wear rates were low, under all applied loads. The morphologies of the worn surfaces against steel were characterized by the appearance of a rippled layers. Atomic force microscopy and Raman spectroscopy were used to propose a possible formation mechanism of such patterns. X-ray photoelectron spectroscopy revealed the rippled surfaces to be composed of Fe₂O₃ and a mixture of MoO_x.     

Stimulation of densification during the reduction of U3O8 to UO2 by atmosphere control

A reduction process in the head-end for pyroprocessing has been adopted to avoid oxidation attack on the molybdenum crucible during sintering. The reduction process is employed to reduce U₃O₈ pellets to UO₂ prior to sintering. This allows elimination of the oxygen source, which causes oxidation attack during sintering, thereby permitting the use of a metallic crucible. However, little densification occurs due to the low reduction temperature limited by the INCONEL crucible. Consequently, the amount of material scraps from the pellets increases, thus creating an additional processing burden due to its high radioactivity. To reduce the amount of scraps, densification should be enhanced. This study suggests a simple atmospheric control strategy and clarifies its effects. With the atmospheric control, a higher bulk density and better attrition resistance were obtained in comparison to without this strategy. This can be explained in terms of O/U ratio dependent diffusion kinetics during the reduction of U₃O₈ to UO₂.