Resumo em Inglês:

Abstract In the slip casting process, the relationship between the amount of plaster and water used in the production of molds influences the production of ceramic materials, because the higher the water content in the mixture, the lower the mechanical strength of the mold and higher the water absorption rate. In order to evaluate the influence of the variation in the plaster/water (P/W) ratio in the production of ceramic pieces, plaster molds with P/W ratios of 2.0, 1.67, 1.43, and 1.25 (100 parts of plaster/50, 60, 70, and 80 parts of water by weight) were manufactured and used in the production of ceramic pieces. It was found that the fluidity, density, flexural and compressive strength of the molds decreased with the increase of water content. For pieces sintered at 1050 °C, higher density and lower porosity were observed for ceramic materials obtained with molds with higher plaster/water ratios.

Resumo em Inglês:

Abstract 3D printing is a disruptive technology that is a key driver of the new industrial revolution. Among the various 3D printing methods, direct-ink-writing (DIW) takes a viscous ink material, extrudes it through a nozzle, and deposits it layer-by-layer to create 3D objects. DIW is a versatile method that can print ceramics, polymers, metals, living cells, etc. Yet, the ink formulation is critical for the success of printing. One major challenge to operate the transition to Industry 4.0 is to educate laypersons on 3D printing, without the need to master physics and chemistry. In this paper, we propose a protocol to familiarize laypersons with ink formulation for DIW. Using this protocol, a clay-based ink was optimized and the best ink composition containing 48 wt% clay and 2.4 wt% bamboo fibers was used for printing. The experimental set-ups and details used in the work are easily available, cheap, sustainable, and safe, enabling its implementation in various settings from classrooms to workshops, without the need for specialized equipment.

Resumo em Inglês:

Abstract The study on the physical-mechanical properties of alkali-activated materials (AAM) produced by the ‘one-part’ method was carried out. For this, residues such as fly ash and rice husk ash (RHA) were used. The alkaline activator (NaOH) was calcined together with RHA for 2 h at 750 °C. Dry mixes were prepared by replacing fly ash with 10%, 20%, and 30% by weight of the calcined material. The samples produced were characterized after 7 and 28 days of curing. The results showed that the investigated variables (calcined material content and curing time at room temperature) were responsible for important variations: compressive strength from 2.40 to 24.92 MPa, modulus of elasticity from 0.90 to 12.32 GPa, water absorption from 2.92% to 13.64%, apparent porosity from 4.82% to 20.02%, and density from 1.48 to 1.71 g/cm3. It was concluded that these variations were due to the high concentration of Na2O and SiO2 in the dry mixtures.

Resumo em Inglês:

Abstract The literature indicates that it is difficult or even impossible to associate with strong statistical confidence the bond strength of a mortar coating with the properties of the ceramic block, such as roughness and water absorption. In this work, it is assumed that the variability in capillary water absorption and roughness, and the use of methods that provide differences in sampling area are key factors that make it difficult to model and obtain a proper correlation between these properties and the tensile bond strength of the render. Blocks with different porosities and roughness were used in the experimental program, and an original methodology was defined. Substrate properties showed high variability between blocks in the same batch, and in different regions of the same block. The proposed methodology allowed to obtain a significant correlation between substrate properties and the render’s tensile bond strength.

Resumo em Inglês:

Abstract The iron-based ceramic NdFeAsO was synthesized by the solid-state reaction method. The tetragonal crystal structure was confirmed at room temperature through the X-ray diffraction technique. The dielectric characteristics i.e. dielectric constant (εr) and loss tangent (tanδ) of the sample were studied at a wide range of temperature and frequencies with an impedance analyzer and revealed that these quantities decreased with an increase in frequency. The dielectric anomalies were observed in the studied sample, which was found to be shifting towards the higher temperature side on increasing the frequency. The analysis of ac conductivity data of the compound obeyed Jonscher’s universal power law as well as the Arrhenius equation. The compound showed a negative temperature coefficient of resistance (NTCR) behavior. The electrical conduction mechanism in the sample can be explained through the correlated barrier hopping (CBH) model.

Resumo em Inglês:

Abstract The present work aimed at evaluating the microstructures and mechanical properties of spark plasma sintered WC-6 wt% Co ceramic composites doped with boron (0.2 and 0.5 wt%). W-50 at% C (W-6.13 wt% C) powders produced under different milling times (20, 60, 180, and 600 min) were used as starting materials and subsequently mixed with Co (6 wt%) and B (0.2 or 0.5 wt%) powders for 5 min. The resultant WC-Co-B powder mixtures were consolidated by spark plasma sintering at 1450 °C for 10 min at 40 MPa under vacuum in order to obtain the samples with 10 mm diameter and 3 mm thickness. The starting powders and the sintered samples were characterized by laser diffraction particle size analysis, X-ray diffraction with Rietveld refinement, relative density, scanning electron microscopy, energy dispersive spectrometry, Vickers hardness test, and diametral compression test. Supersaturated W solid solution (WSS) with a crystallite size of <50 nm was formed in W-50C (at%) powder mixtures. The relative density of the sintered composites increased with the increasing milling time up to 180 min from 79.7%±0.6% to 86.7%±0.5% for WC-6Co-0.2B and from 86.7%±0.6% to 90.9%±0.4% for WC-6Co-0.5B. Furthermore, the WC phase appeared as the matrix in the sintered samples, and W2CoB2 precipitated homogeneously around WC grain boundaries, mainly to those containing 0.5% B. The Vickers hardness values of the sintered WC-6Co-0.2B and WC-6Co-0.5B ranged from 1790±39 to 2158±25 HV and 1858±31 to 2182±28 HV, respectively. The fracture strength (determined by diametral compression test) varied between 144-353 MPa due to the porosity and precipitates in thin-thickness cylinder samples.

Resumo em Inglês:

Abstract Self-levelling underlayments (SLUs) based on Portland cement (PC) are susceptible to cracking due to drying shrinkage. The present study evaluated the influence of binder contents on systems consisting of calcium aluminate cement (CAC), PC, and a source of calcium sulfate (C$), derived from the flue gas desulfurization process (FGD), on the mechanical (flexural and compressive) properties and drying shrinkage of SLUs. The main objective was to analyze the influence of replacing limestone filler (LF) by bottom ash (BA) and red mud (RM) wastes on the properties of SLUs based on the CAC/FGD/PC (56/24/20) binder system. For this purpose, mini-slump flow, setting times, water absorption, mechanical (flexural and compressive) strength, linear shrinkage, and X-ray diffraction essays were performed. The replacement at a level of 0.2 filler with wastes reduced linear shrinkage while replacing 0.3 filler level provided greater initial (1 day) compressive strength.

Resumo em Inglês:

Abstract A heterogeneous Fenton-like process was explored for oxidation of phenazine dye using ceria polishing material (CP-2). XRD, SEM, FTIR, UV-vis DRS, and pHPZC techniques were applied to characterize the catalyst features. The performance of the heterogeneous Fenton-like process was investigated under various parameters (reaction time, pH, H2O2 concentration, catalyst dosage, initial dye concentration, temperature of the medium, and inorganic salt content). Results indicated that the main content of CP-2 catalyst was CeO2 (60-70%) with lower contents of La2O3 (30-40%), Pr6O11 (≤6%), Nd2O3, and CaF2 (≤5%). CP-2 possessed a typical CeO2 cubic fluorite structure with high content of rare earth oxides (REO) and α-Fe2O3. The average crystallite size and band gap energy of CP-2 were found to be 37.2 nm and 3.0 eV, respectively. Over 81.3% of neutral red (NR) oxidation efficiency was achieved in 60 min. Pseudo-first-order kinetic model gave the best fit with an acceptable coefficient of determination R2. The activation energy (Ea) was 16.4 kJ/mol suggesting that the degradation reaction proceeded with a low energy barrier and the removal process was feasible, spontaneous, and endothermic. The synergistic effect of Fe- and/or REO-CP-2 and H2O2 greatly enhanced the reactive oxygen species (ROS) such as •OH, O2 •-, 1O2, and HO2 • for effective oxidation of NR via a heterogeneous Fenton-like process.

Resumo em Inglês:

Abstract The self-healing capacity of a ceramic material produced by uniaxial pressing with alumina (Al2O3), silicon carbide (SiC), and magnesium oxide (MgO) was evaluated. To assess self-healing, a discontinuity was generated on the surface of the specimens. The healing time in the oven, at a temperature of 1000 ºC, was 1, 30, and 60 min. The specimens were analyzed by physical and mechanical tests. The results showed that, for the mixture containing 15% SiC, the specimens healed at 60 min had an increase in flexural strength of 84.9% when compared to the specimens that were only indented while for the specimens containing 20% SiC, this increase was 61.1% at 30 min. For the 30 min healing time, there was a reduction in the size of the discontinuity in the specimens by about 19.1% and 14.8%, for the formulation containing 15% and 20% of SiC, respectively.

Resumo em Inglês:

Abstract The structural, morphological, and optical properties of α-SnWO4 synthesized by microwave-assisted hydrothermal method at 150 °C for different times were investigated. XRD revealed the formation of the orthorhombic phase, without the presence of a secondary phase. The sample processed for 64 min showed a reduction in unit cell volume, suggesting a decrease in the structural defects present in the sample. The Raman spectroscopy confirmed the formation of α-SnWO4, with the presence of vibrational modes attributed to the [WO6] octahedron. FE-SEM images revealed the formation of agglomerate nanosheets in all samples. The band gap (Egap) of the samples presented values in the range from 1.58 to 1.70 eV. The increase in Egap value of the sample processed for 64 min corroborated the decrease in the structural defects suggested by the Rietveld refinement. Finally, the photoluminescence was measured, with maximum emission in the blue region of the electromagnetic spectrum around 454 nm, which was attributed to the [WO6] octahedron.

Resumo em Inglês:

Abstract This work produced glass foams from soda-lime glass bottles, bentonite, and alumina by the replica method. Compositions with different contents of bentonite (5, 10, 15, and 20 wt%) were prepared and heat-treated at 800 °C. In the alumina-added compositions, 50 wt% of the bentonite was replaced by the alumina. The influences of the content of bentonite (binder) and alumina addition were evaluated through the physical-mechanical properties. The results showed that the alumina did not promote a significant effect on physical and mechanical properties in most glass foams; the exception was the sample with 20 wt% bentonite+alumina, which presented better physical properties (lower linear shrinkage and density and greater porosity) when compared to its equivalent sample (without alumina). Foams with 5% of bentonite or bentonite+alumina showed higher linear shrinkage indexes and high densification. Samples with 15% of bentonite or bentonite+alumina showed the best combination of properties: higher porosity (79.2% and 80.2%), water absorption (58.8% and 59.7%), lower shrinkage index (25.5% and 29.4%), and good flexural strength (0.69 and 0.45 MPa).