Abstract in English:Abstract In recent years there is a high interest in the freeze-casting process because it is a simple, economical, and environmentally friendly method for obtaining highly porous materials. Most of the materials obtained by this technique have directional pore structure and anisotropic properties. However, a wide range of pore size and morphology can be obtained by the control of processing parameters, such as solvent type, solids concentration, particle size, freezing rate, and types of additives. The varied morphologies permit the application of freeze-casted materials in various technological applications. However, despite the high amount of studies about freeze-casting of ceramic materials in the last years, review articles addressing the influence of processing parameters on the pore characteristics of obtained ceramics are still scanty. Therefore, this review addresses the influence of freeze-casting process parameters on the pore characteristics of the ceramic materials.
Abstract in English:Abstract The fracture toughness of 3Y-TZP ceramics obtained from a nanocrystalline powder with an optimized microstructure and highly transformable tetragonal grains was investigated. Samples of ZrO2-3 mol% Y2O3 were sintered at temperatures between 1250 and 1400 °C, with isothermal holding times of up to 16 h. Samples sintered at 1250 °C exhibited relative densities ranging between 92% and 98%, which increased with increasing isothermal duration, while samples sintered at 1300, 1350, or 1400 °C achieved densification higher than 98% for all isothermal treatments. Crystallographic analysis indicated the presence of a highly transformable ZrO2-tetragonal phase (c/a√2=1.0148-1.0154) for all conditions studied. The average grain size ranged from 0.18±0.04 mm (1250 °C-0 h) to 0.64±0.08 mm (1400 °C-16 h), indicating activation energy of 141.3 kJ/mol for grain growth and a growth exponent of 2.8. Both Vickers hardness (1025 to 1300 HV) and fracture toughness (4.0 to 7.8 MPa.m1/2) increased with increasing sintering temperature and time due to increased densification, reduced porosity, and maintenance of potentially high fracture toughness by the t→m phase transformation.
Abstract in English:Abstract Fish scales of the Arapaima gigas (pirarucu) species were heat-treated to obtain natural hydroxyapatite (HAp), and it was doped with niobium (Nb) by physical ultrasound interference to confirm an effective route to the production of Nb-doped HAp powders. The effect of ultrasound application on doping was investigated. The structure of the powders was elucidated using X-ray diffractometry (XRD), scanning electron microscopy, IR spectroscopy, and X-ray fluorescence spectroscopy (XRF). XRD results of Nb-doped HAp powders were compared with undoped natural hydroxyapatite and ICSD-26204 standard as controls. The crystallinity of Nb-doped HAp powders increased from 39% to 51% after the ultrasound application. IR spectroscopy confirmed hydroxyl, carbonate, and phosphate functional groups in the sample. Results obtained by XRF confirmed the ionic substitution of Ca for Nb with an average content of 1%. Therefore, the effectiveness of the synthesis route was confirmed to produce hydroxyapatite-niobium crystalline powder with physicochemical integrity as a potential biomaterial for bone implant application.
Abstract in English:Abstract MnTiO3 is a semiconductor that has relevant dielectric and optical properties. For the synthesis of manganese titanate (MnTiO3) powders, the combustion method via microwave and subsequent calcination at 500, 700, and 900 °C were used. The structural, morphological, and optical properties of the samples were investigated. X-ray diffraction analyzes were performed, where it was possible to index all peaks in the pyrophanite phase with a rhombohedral structure for the 900 °C sample. FTIR spectra corroborate the diffraction results, showing the presence of the vibrational modes characteristic of the MnTiO3 bonds. SEM images revealed the formation of distorted nanobands with an average diameter of 320 nm. The optical spectrum obtained from the UV-vis absorption spectroscopy suggested a bandgap for MnTiO3 of around 3.18 eV. The present work showed that combustion synthesis via microwave is efficient for the production of pure manganese titanate after calcination at 900 °C for 2 h.
Abstract in English:Abstract The refractory materials used in the wall of the furnaces for glass melting can be prepared from mullite-zirconia composite material. The composite of mullite/zirconia was synthesized from Iraqi kaolin, γ-alumina, and zirconia using thermal decomposition with reaction sintering at 1600 °C. Several batches were prepared with various ratios of kaolin, γ-alumina, and zirconia, and the composite compositions were selected from the Al2O3-SiO2-ZrO2 phase diagram. The mullite-zirconia composite was prepared with different steps beginning with milling the starting materials, semi-dry uniaxial pressing, and then reactive sintering at various temperatures (1200, 1400, and 1600 °C). The predicted phases ZrO2 and Al6Si2O13 were identified by X-ray diffraction patterns according to the phase diagram for all the batches. The lower amount of the zirconia added to mullite reduced porosity and improved the bulk density of the mullite/zirconia composite. The thermal expansion coefficient slightly increased with the addition of zirconia. It also enhanced the thermal shock resistance of the composite. Finally, the mechanical properties were improved by increasing the amount of zirconia particles in a matrix of mullite due to the phase transformation of zirconia from tetragonal to monoclinic phase.
Abstract in English:Abstract This paper aims at exploring the possibility of using a great percentage of recycled materials into vitreous china (VC), thus, contributing to urban waste decrease while lowering the industry’s buying costs, previously optimized, for the sanitaryware production. The recycled blend was introduced in the ceramic slip, Sanitser VC, and used in three sanitaryware production plants. This slip let to improve the environmental performances of the overall ceramic production through energy-saving and a decrease in greenhouse gas emissions. In fact, the firing temperature is 80-100 °C lower than traditional production. Therefore, the objective of this paper was to evaluate the firing behavior, technological properties, and microstructure of a typical industrial vitreous china body in comparison with Sanitser VC. The analytical results confirmed the force of this idea and highlighted the advantages of the choice made.
Abstract in English:Abstract The objective was to evaluate the influence of Baosteel slag short flow (BSSF) steel slag, a byproduct of a Brazilian steel mill, as a fine aggregate on the properties of fresh state and on the compressive strength of mortars. The experimental work was to characterize the BSSF steel slag and to produce simple mortars (only cement) and mixed mortars (cement and lime), replacing the natural sand with the slag aggregate in 20%, 40%, and 60% by volume. The fresh state tests were specific mass, incorporated air, water retention, and squeeze-flow, while in the hardened state, compressive strength at 28 days was investigated. It was verified that mortars with higher slag contents required greater amounts of water to obtain the spread (consistency) on the flow table, which was fixed at 260±5 mm. The mortars with slag presented higher specific mass since the slag had a higher density than natural sand and less water retention in the case of simple mortars. In addition, it was verified that the mortars with BSSF steel slag presented lower values of maximum displacement in the squeeze-flow test, which negatively affected its workability in the fresh state, besides the properties of mortar adherence. In addition, it was verified through the analysis of variance that the substitution of natural sand by BBSF steel slag did not significantly influence the compression strength of mortars.
Abstract in English:Abstract This study investigated the feasibility of synthesizing sodium silicate from rice husk ash (RHA) and its application to produce geopolymers. Samples were prepared at different SiO2/Al2O3 molar ratios (3.0, 3.5, and 4.0) for both sodium silicates (commercial and from RHA). They were underwater cured at 60 ºC for 1 h and then at room temperature (25 ºC in the air) until the time of the tests (7 and 28 days). The results showed that the compressive strength significantly increased along with the SiO2/Al2O3 ratio for both silicates. Geopolymers produced with sodium silicate from RHA showed higher compressive strengths for most studied samples. The highest compressive strength (6.33±0.23 MPa) was obtained for samples with SiO2/Al2O3 ratio of 4.0 at 7 days, representing a 40% increase in average values compared to commercial sodium silicate under the same conditions. XRD data and FTIR spectrum confirmed that it was possible to synthesize a geopolymer with sodium silicate using RHA.
Abstract in English:Abstract The modernization of building systems is increasingly demanding products that improve efficiency during their processing. The development of self-leveling mortars for the regularization of flooring is a consequence of this evolution. The high contents of binder and water to obtain self-flowing property, plus the application of thin layers over large areas, make this type of mortar susceptible to high shrinkage levels. Thus, the ternary system Portland cement-calcium aluminate cement-calcium sulfate (OPC-CAC-CaSO4) is a viable solution for shrinkage compensation. The expansion mechanism based on ettringite formation aims to prevent the mortar from cracking. Considering its complexity and that international literature is still limited (while Brazilian is null), this article presents a literature review that covers diverse aspects related to the application of the system OPC-CAC-CaSO4. The influence of raw materials and mix proportions on rheological behavior, reaction kinetics, development of strength, and length change are discussed.
Abstract in Portuguese:Resumo Células solares sensibilizadas por corante são eficientemente desenvolvidas com TiO2 na composição. O isopropóxido de titânio, por possuir uma extensa cadeia alcoxi, evita a aglomeração de núcleos metálicos, gerando partículas com tamanho reduzido, tornando-se um precursor ideal para produção do TiO2. Este trabalho teve como objetivo avaliar a proporção de isopropóxido de titânio na síntese de TiO2 via metodologia Pechini. As partículas foram caracterizadas via difração de raios X, microscopia eletrônica de varredura e espalhamento dinâmico de luz. A célula foi caracterizada por curvas j-V e espectroscopia de impedância eletroquímica. Os resultados demonstraram que nas proporções testadas não houve mudança significativa de cristalinidade e forma das partículas. Verificou-se que a proporção molar cátion metálico:ácido cítrico de 2:1 apresentou partículas de menores diâmetros, facilitando a adsorção do corante e potencializando os parâmetros fotoeletroquímicos. A caracterização da célula solar nesta condição forneceu eficiência em conversão de energia η=4,16%±0,07%, densidade de corrente j=9,10±0,09 mA.cm-2 e tempo de vida eletrônico τe=56,98ms.
Abstract in English:Abstract Dye-sensitized solar cells are efficiently developed with TiO2 in the composition. Titanium isopropoxide, due to its high organic load, prevents agglomeration of the metal nucleus, generating particles of small size, becoming ideal for TiO2 synthesis. This paper aimed to evaluate the proportion of titanium isopropoxide in TiO2 synthesis by Pechini methodology. The particles were characterized by X-ray diffraction, scanning electron microscopy, and dynamic light scattering. The cells were characterized by j-V curves and electrochemical impedance spectroscopy. It was demonstrated that the studied proportions had no significant effects on particle crystallinity and shape. It was also observed that metallic cation:citric acid molar proportion of 2:1 presented particles with a smaller size, facilitating the adsorption of the dye by enhancing the photoelectrochemical parameters. The characterization of the solar cell in this condition provided efficiency of energy conversion η=6.94%±0.07%, current density j=9,10±0,09 mA.cm-2, and electron lifetime τe=56,98ms.
Abstract in English:Abstract Calcium cobaltite (CCO) thermoelectric ceramics in Ca3Co4O9 and Ca2.95Ba0.05Co4O9 compositions were produced by a simplified chemical route developed in this study. Undoped ceramics sintered at 1193 K for 12 h presented a bulk density equivalent to 77% of the theoretical density, one of the highest values ever found for conventionally sintered undoped calcium cobaltite. The presence of barium improved densification, reaching 80% of the theoretical density. Thermoelectric properties were discussed based on X-ray diffraction patterns, scanning electron micrographs, Seebeck coefficient, electrical and thermal conductivity. Electrical characterizations presented values of about 6200 and 7000 S/m at 873 K for undoped and doped ceramics, respectively. However, all ceramics presented almost the same range of activation energy values, indicating the possibility of non-effective substitution through this methodology. The maximum global PF (power factor) was 0.10 mW.m-1.K-2 for the densest undoped sample and an overall ZT (dimensionless figure of merit) of 0.07 for the undoped ceramic at 873 K.
Abstract in English:Abstract This paper proposes a unified comprehension of the isothermal ceramic process kinetics stemming from mesoscopic irreversible thermodynamics. Accordingly, a unified process kinetic equation (UPKE) is derived, which predicts that the global isothermal process rate of any ceramic process is, in general, nonlinearly related both to its activation energy and its affinity. Nevertheless, for a low-affinity ceramic process conducted either in a field-free or resonant wave-field condition, its global isothermal rate, according to the proposed UPKE, is approximately linearly related to its affinity in the spirit of Fick’s diffusion law. Therefore, the rate enhancement of a low-affinity process occurring in any resonant wave-field may be caused either by a reduction in activation energy, as in microwave-enhanced sintering, or by activation energy reduction along with an affinity augmentation, as in microwave-assisted glass-crystallization. Conversely, for a high-affinity ceramic process, e.g., a chemical reaction, the ‘degenerate’ UPKE predicts that its kinetics is exclusively dictated by the activation energy in the spirit of Arrhenius’s rate law. Hence, isothermal rate enhancements of chemical reactions in ceramic processing under resonant wave-field conditions are believed to predominantly result from a field-induced reduction in activation energy.
Abstract in English:Abstract The sintering is an essential step in the processing of ceramics materials. A number of researchers have reported new techniques for sintering. However, in general, the process still demands high temperatures. This paper reports the results of the cold sintering of ZnO-BaTiO3 composites. The microstructural, structural, and electric properties of the composite were studied. We obtained high densities (>95%) for pure ZnO and for the composition with 10 wt% of BaTiO3 at a low temperature of sintering (250 °C/1 h). The increase of BaTiO3 content reduced the grain size and degraded the sinterability of ZnO. The crystalline structure was investigated by X-ray diffraction and confirmed the separation of the phases after the cold sintering. The BaTiO3 added in the composite reduced the values of conductivity, permittivity, and also the dielectric loss.
Abstract in Portuguese:Resumo O objetivo foi estudar o processamento de materiais cerâmicos de alumina com porosidade graduada a partir das técnicas de freeze casting e colagem de barbotina sequencial, utilizando hidróxido de alumínio como fase de sacrifício. Após conformação, a porosidade foi caracterizada por microscopia eletrônica de varredura, medidas de porosidade aparente pelo princípio de Arquimedes e distribuição de tamanho e tamanho médio de poros pela técnica de porosimetria de mercúrio. Foi avaliada a resistência mecânica das amostras a partir de ensaio de compressão. A caracterização permitiu a confirmação da relação da microestrutura entre as camadas das amostras processadas isoladamente e suas respectivas camadas nos materiais com porosidade graduada. Foi observada uma boa interação interfacial entre as camadas das amostras graduadas. As porosidades obtidas pelos dois métodos foram distintas devido às diferentes morfologias obtidas, sendo que pela técnica de colagem de barbotina, os poros apresentaram menores tamanhos, com poucas interconexões e na técnica de freeze casting os poros apresentaram-se intralamelares com formato alongado e interligados entre si. Os valores de porosidade e resistência à compressão apresentaram resultados distintos para cada técnica. A amostra graduada processada por freeze casting apresentou maior porosidade e resistência mecânica quando comparada à amostra processada por colagem. As duas técnicas utilizadas mostraram ser eficientes para produzir materiais com graduação de porosidade.
Abstract in English:Abstract The objective was to study the processing of alumina ceramic materials with graded porosity produced by freeze casting and slip casting with aluminum hydroxide as a porogenic agent. After processing of the samples, porosity was studied by scanning electron microscopy, apparent porosity by the Archimedes’ principle, and size and average pore size by mercury porosimetry. The mechanical strength of the samples was studied through compression testing. The characterization allowed confirmation of the relation between the samples processed individually and their respective layer in materials with graded porosity. The samples also showed good interfacial interaction between the layers of the graded materials. The porosities obtained by both methods were distinct due to different morphologies; by slip casting, the pores showed smaller sizes with few interconnections and by freeze casting, pores were interlayered with elongated and interconnected morphology. Porosity and compressive strength values were different for each technique. The graded sample processed by freeze casting showed higher porosity and compressive strength when compared to the sample processed by slip casting. Both techniques showed to be efficient for the production of graded porosity material.
Abstract in English:Abstract This work proposed to incorporate the sugarcane bagasse ash (SBA) with a high content of organic matter (loss on ignition of 35.87%) in alkali-activated mixtures with blast furnace slag. After grinding SBA, replacement ratios of 10%, 20%, and 30% of the ground ash were used for the flowability evaluation. The mixtures containing 20% and 30% showed a dry consistency, then only the mixture with 10% of slag was used for the other tests. The use of an air-entraining admixture led to an improvement in the flowability of the alkali-activated mixtures. X-ray diffraction results showed that both SBA and the air-entraining admixture did not interfere in the mineralogical composition of the hydrated products. However, the use of SBA led to a decrease in the compressive strength of about 20% to 28%. In conclusion, SBA proved to be viable for use in alkali-activated mixtures considering its effect on mechanical performance.