Although the physical expansion associated with the in situ formation of magnesium–aluminate spinel (MgAl2O4) is well-reported, some questions related to this behavior, such as the different volume ...change values experimentally attained when compared to theoretical one and the pore generation after the reaction, remain open. Thus, the main objective of this work is to shed some light on these questions by evaluating a cement-bonded alumina–magnesia castables, designed using dead-burnt magnesia of different particle size ranges. Microstructural observations suggested that the faster Mg2+ migration during the spinel formation led to vacancy accumulation and, consequently, to pore generation, as a direct result of the Kirkendall effect. Additionally, the overall expansion of alumina–magnesia castables seemed to be ruled by two main factors: its sintering efficiency and the different possibilities of the Al2O3 and MgO interactions in the mixture. Those consequences, however, do not usually affect the castable corrosion behavior in industrial applications, due to the benefits imposed by the structural constraint.
This work revisits the proposed mechanisms presented in the literature for CA6 formation in Al2O3-MgO and Al2O3-MgAl2O4 castables bonded with calcium aluminate cement. New experimental tests, ...thermodynamic simulations and re-evaluation of the chemical composition and microstructural aspects observed for samples fired in the temperature range of 1150°C to 1500°C were carried out. Based on these data, a new interpretation of the CA6 generation process, as well as the features which influence the location and morphology of this phase were proposed. CA6 formation via solid and liquid states are suggested to take place in all evaluated compositions, where the former (solid-state) is the main reaction predicted for the silica-free refractories (AM0MS and AS0MS), whereas the liquid-state one prevails in the AM1MS and AS1MS materials. The CA6 crystal morphology should be affected by these different reaction mechanisms. According to the experimental results, it was also discussed the role of the calcium hexaluminate features in the overall corrosion behavior of the designed refractories when they were placed in contact with molten slag at high temperatures. Such aspects have not been previously reported in published papers related to this subject.
Microsilica addition in Al2O3–MgO and Al2O3–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of ...conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA6 was formed in the matrix, mainly for the Al2O3–spinel composition, which also favored the castable dissolution into the molten slag.
High-emissivity coatings have been pointed out as a promising energy-saving solution for industrial furnaces since their development in the aerospace area. However, due to a lack of in-depth ...knowledge of radiation heat transfer and thermal-optical interactions, as well as the struggle to obtain proper emissivity values, commercial versions of such coatings have not yet been widely established. In this work, the evaluation of some fundamentals on thermal-optical properties, and their potential applications, led to the development of engineered structures of usual inexpensive oxides, such as the titanate and spinel ones, which presented improved emissivity values, even at high temperatures (1500 °C), when compared to commercial references. The findings of these compounds indicated a considerable cost-effective saving potential for large industrial furnaces.
Due to the lack of studies addressing the relationship between chemical and microstructural features and the corrosion resistance of castables containing pre-formed and in situ spinel (MgAl2O4), the ...main aspects related to the different wear rates of these compositions when in contact with steel ladle slags have not been properly stated. Considering this scenario, this work presents the slag resistance analysis of castables designed by different spinel incorporation routes (in situ formation, pre-formed spinel addition and both). A high-iron oxide containing industrial slag was used and the results indicated that the role of the distinct CA6 (CaO·6Al2O3) distribution in the castable's microstructure before the slag attack was more relevant than the spinel ability as an ion trapper. The location of CA6 crystals in the in situ spinel-forming castable led to a suitable physico-chemical protection of both, the tabular alumina aggregates and the matrix, during the experiment. For the pre-formed spinel-containing castable, the former CA6 presence only in the matrix resulted in a high dissolution of alumina from the aggregates into the liquid during the corrosion test and a great amount of CA6 crystals was formed. Cracks were then generated and followed by further cycles of penetration and chemical reactions, which spoiled the refractory's performance.
Abstract
We present a deep neural network real/bogus classifier that improves classification performance in the Tomo-e Gozen Transient survey by handling label errors in the training data. In the ...wide-field, high-frequency transient survey with Tomo-e Gozen, the performance of conventional convolutional neural network classifiers is not sufficient as about 106 bogus detections appear every night. In need of a better classifier, we have developed a new two-stage training method. In this training method, label errors in the training data are first detected by normal supervised learning classification, and then they are unlabeled and used for training of semi-supervised learning. For actual observed data, the classifier with this method achieves an area under the curve (AUC) of 0.9998 and a false positive rate (FPR) of 0.0002 at a true positive rate (TPR) of 0.9. This training method saves relabeling effort by humans and works better on training data with a high fraction of label errors. By implementing the developed classifier in the Tomo-e Gozen pipeline, the number of transient candidates was reduced to ∼40 objects per night, which is ∼1/130 of the previous version, while maintaining the recovery rate of real transients. This enables more efficient selection of targets for follow-up observations.
Background. Tracheobronchial injuries are rare but potentially life threatening. Their successful diagnosis and treatment often require a high level of suspicion and surgical repairs unique to the ...given injury.
Methods. We reviewed our experience with 32 patients with tracheobronchial injuries treated over the past 28 years.
Results. Forty-one percent (13/32) of the injuries were due to blunt trauma and 59% (19/32), to penetrating trauma. Most penetrating injuries were located in the cervical trachea (74%), whereas blunt injuries were more commonly located close to the carina (62%). Fifty-nine percent of the patients required urgent measures to secure the airway. Penetrating injuries were usually diagnosed by clinical findings or at surgical exploration. The diagnosis of blunt injuries was more difficult and required a high index of suspicion and the liberal use of bronchoscopy. The majority of the injuries were repaired primarily using techniques specific to the injury, and most patients returned to their normal activity soon after discharge.
Conclusions. A high level of suspicion and the liberal use of bronchoscopy are important in the diagnosis of tracheobronchial injury. A tailored surgical approach is often necessary for definitive repair.
Steel ladle refractory lining plays an important role in steelmaking refinement as its thermal properties are strictly related to the ladle ability to keep the molten steel temperature constant. This ...work developed a transient numerical analysis, considering temperature dependent refractory properties. A holistic view of the process was proposed to help the refractory design of steel ladles, according to their saving energy capability and overall costs. The numerical description of the ladle cycle used the finite element method (FEM) via commercial software (Abaqus). The average heat flux in the hot face while holding the liquid steel determined the steel ladle energy losses and was compared to evaluate the lining efficiency. The investigations considered the application of distinct working refractories and the presence of an insulating layer. The developed thermal and energy balance tool provided a more inclusive and integrated approach for investigating the energy consumption and thermal history of the steel ladle process, regarding the refractory lining design.
Alumina–magnesia and alumina–spinel castables present some unique characteristics. Whereas the
in situ spinel (MgAl
2O
4) formation provides an enhanced corrosion and thermal-shock resistance to the ...castable, the composition with pre-formed grains shows higher volumetric stability and do not have the inherent MgO hydration problems. As these distinct ways of spinel incorporation result in particular properties to the refractory material, a castable containing both
in situ and pre-formed spinel might present balanced properties and a suitable performance in steel ladle applications. Considering these aspects, cement-bonded high alumina castables containing both pre-formed and
in situ spinel were developed and evaluated by means of assisted sintering tests, microstructural analyses, thermodynamics simulation and mechanical and thermal-mechanical properties evaluation. The CA
6 formation presented different features according to the content of
in situ spinel generation in the matrix, which affected the castable properties. Additionally, compositions containing both
in situ and pre-formed spinel seemed to be a feasible way of attaining a designed linear dimensional change without affecting the spinel and CA
6 contents.
The addition of nanoscaled alumina and magnesia particles to the matrix of alumina–magnesia refractory castables drastically reduces the residual expansion related to the in situ spinel formation. ...Nonetheless, as their benefits on other relevant properties have not been reported so far, the effectiveness of such nanoengineering design for castables applied in steel ladles is still uncertain. In the present work, not only the expansion level, but also the corrosion resistance, the hot modulus of rupture and the creep deformation of different nanoparticle-containing castables were evaluated and compared with the results attained by refractory materials designed only by micrometric-scaled Al2O3 and MgO. Although the addition of a nanoalumina and nanomagnesia mixture ensured the best results regarding to the expansive behavior, thermo-mechanical and thermo-chemical properties, its performance was only slightly superior to the castable containing micrometric alumina and magnesia particles. Therefore, as the cost–benefit ratio is one of the main requirements for the end users, the nanotechnology use in the refractory production must be previously carefully analyzed.
