Papers by Author: Luis Henrique Leme Louro

Abstract: Niobia is used as a sintering additive in alumina processing because it lowers the sintering temperature. This work investigated the influence of sintering dwell time at 1400^oC on the properties of 4 wt% niobia doped alumina. The sintered ceramics were characterized by scanning electron microscopy (SEM), and X rays diffraction (XRD) with Rietveld refinement. Measurements of density, average grain size, hardness, and elastic constants were also performed. The results showed that the porosity depend on the time sintering and this influence on the elastic properties. Longer sintering times improved densification and with this the Young's Modulus.

Abstract: Lithium fluoride as well as calcium oxide were used as sintering additives to magnesium aluminate ceramics in order to provide both transparency and better densification. This work assessed the dynamic behavior of MgAl₂O₄ by performing split Hopkinson bar dynamic tests. The total amount of additives was 1.5 wt% in which the percentage of LiF and CaO varied from 0 to 100 wt% with an increment of 25 wt%. The obtained results indicated that CaO low concentrations induced low strength values to the ceramics of different compositions. By increasing the amount of CaO, the MgAl₂O₄ dynamic strengths were higher. The strain rate was reduced as the amount of CaO increased, indicating a higher trend of energy absorption corroborated by K_Ic measured values. On the other hand, the ceramic strain increased with the increment CaO additions.

Abstract: Niobia has been successfully used as sintering additive to alumina in order to lower its sintering temperature. This effect can also be obtained by reducing the ceramic particle size. This work investigated the effect of the particle size on the ceramic final density of alumina with 4 wt% niobia. For that two milling media were used. The as-received powders were submitted to ball and planetary milling and then sintered at 1450°C. The planetary milling medium was more efficient in reducing particle size when compared to ball milling. However, planetary milling caused significant contamination in the niobia powder, from the alumina balls used as milling agents. It forced composition balance in order to keep the original proposed formulation. The planetary milled sintered samples showed better densification and lower grain size in comparison with ball milled ones. It could be concluded that the milling medium choice directly affected both microstructure and properties of the sintered alumina with 4wt% of niobia. .

Abstract: This work investigated the sintering behavior of alumina doped with 4wt% niobia. Three sintering temperatures were investigated: 1400°C, 1450°C and 1500°C. The first temperature leads to solid-phase assisted sintering (SSS) while the other ones develop liquid-phase assisted sintering (LPS). The presence of liquid phase in the second case is due to an eutectic reaction occurring at 1440°C ± 20°C in the alumina-niobia system. The sintering behavior was assessed by measuring the final densities. The results indicated that the 1400°C solid-state sintering, comparatively, was better. This paper proposes that defects associated to the substitution of Al⁺³ by Nb⁺⁵ in the alumina cation sub-lattice, fostered diffusion and SSS. Sintering at 1500°C presented the lowest density, apparently due to niobate (liquid phase) loss, at this temperature as observed by XRD results.

Abstract: Incomplete binder removal may introduce severe defects during ceramic processing. This work presents a new method to eliminate polyethylene glycol used as a binder in pre-sintering steps of alumina parts. The effectiveness of the proposed method was evaluated by density and pore size measurements, as well as thermogravimetry (TG), differential thermal analysis (DTA), volumetric physical adsorption analyzer (BET), and Fourier transformed infrared spectroscopy (FTIR). The results show that the new method, although relatively slow in comparison with traditional procedures, yields smaller pore sizes and better mechanical properties of the final products.

Abstract: Oxide ceramics show better oxidation resistance at high temperatures than other ceramics; however they are more susceptible to plastic deformation at elevated temperatures [. If their high temperature mechanical properties could be improved, they would be expected to open a wide range of applications as structural material [2, 3]. Several studies have revealed [4, 5] the potential use of YAG oxides as reinforcing component oxide in a ceramic matrix. Both YAG and Al₂O₃ have similar thermal expansion coefficient and they are chemically stable because of their low O₂ vapor pressure. In addition, there is no solid state phase transition as the temperature rises, but the eutectic reaction at 1826°C with Al₂O₃ molar concentration of 81.5% and 18.5% for Y₂O₃ which enable a fusion processing, turning the Al₂O₃-YAG composites very attractive. This eutectic reaction is possible in the restrictive composition from 18.5 to 20.5 mol% Y₂O₃ [6].

Abstract: Glasses with two different compositions were added to yttria-stabilized zirconia TZP powder: a CAS glass and a bioactive glass. These additions allowed liquid phase sintering to occur at temperature as low as 1300 °C. The concentrations of each glass additions were of 1, 3, and 5 wt%. The prepared compositions were uniaxially pressed at 50 MPa and sintered at 1300oC for 2 hours. The sintered samples were characterized for their mechanical properties, by measuring four-point bending mechanical strength, Vickers microhardnesses, and fracture toughness ( K_Ic ). Vickers microhardness measured values ranged from 10 to 12 GPa, while fracture toughness, from 3.8 to 4.4 MPa.m^1/2. The flexural mechanical strength was situated between 302 and 408 MPa. The achieved mechanical properties, from sintered samples were possible due to glassy phase additions. These properties, associated to biocompatibility, enable such materials to be used in different applications, including bioceramics.

Abstract: TZP yttria-stabilized zirconia powder was mixed with two types of glasses as sintering additives: CAS glass and a bioactive glass. These additions were designed toward the material applications as bioceramics. The glassy phase was chosen to promote liquid phase sintering at lower temperature, when compared to pure material. This procedure contributed to reduce the fabrication costs while keeping the material biocompatibility. Each type of glass was added in concentrations of 1, 3, and 5 wt%. The prepared powders were uniaxially pressed at 50 MPa, and then sintered at 1300°C for two hours. The sintering behavior was evaluated by measuring the final sintered densities. It was found that the samples with bioactive glass additions were denser than those with CAS glass. Zirconia TZP powders without glassy additions would not sinter in this temperature. The microstructure of the sintered samples was characterized by SEM and XRD. The sintered ceramics exhibited both submicrometric and uniform grains. The analyzed grain sizes were slightly lower for the samples with CAS additions than for those with bioactive glass additions.

Abstract: In this work, glasses with compositions, close to commercial borosilicate glasses, were developed. The main raw materials were residues of ornamental rock industries ( marbles and granites ). Beach sand was used in order to enrich the compositions with silica. Also, boron oxide was added to reach borosilicate glasses compositions. The obtained glasses were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), and hydrolytic resistance. The measured properties were compared to those of commercial glasses, available in the market, from two different brands. XRD results confirmed amorphous patterns of the produced glasses. The other tests revealed an excellent agreement between the obtained values and those from commercial glasses. Therefore, this study showed the possibility of using ornamental rocks residues as primary sources to produce borosilicate glasses. In addition, it became clear that an economic value could be aggregated to the residues, which otherwise would be discharged into nature causing pollution.

Abstract: This study aimed to produce nanometric powders of alumina by sol-gel route. Six samples were produced by varying the amount of water for dilution of aluminum nitrate and the calcination temperature. The final products were evaluated by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction and particle size. It could be noticed that, beyond the time of gelation and calcination temperature, the addition of water also influenced the average size of the clusters.