Abstract
Corundum (?-alumina) is a suiTab. material for usage in various industry fields owing to its chemical stability, electrical and mechanical features. It is known that properties of ceramics could be modified by addition of different oxides, as well as by changing the consolidation parameters. In this respect, alumina was doped with 1 wt.% of Cr2O3, Mn2O3 and NiO, followed by 1 hour of mechanical activation in a high-energy planetary ball mill. A sensitive dilatometer was used for sintering of powder mixtures up to 1400 oC and recording the obtained dilatation. The final density varied between cca. 1.9 and 3.3 g/cm3. Microstructural changes were detected by SEM measurements. Changes in electrical permittivity and loss tangent were associated with the preparation conditions (types of additives, duration of mechanical activation). For a given mixture, the sintering increases the relative permittivity and decreases losses, exhibiting the optimal values of 8.32 and 0.027, respectively, for the sample activated 60 minutes and sintered, with the addition of MnO2. Mechanical measurements indicate significant differences in strength with the addition of different transition metal oxides. Samples with Mn and Ni, activated and sintered, with strength of 121 and 86 MPa, respectively, have a significantly higher tensile strength than the other tested samples, due to their more compact microstructures.
Highlights
Alumina (Al2O3) can exist in several metasTab. phases, which irreversibly transform into α-alumina when heated to ≥ 1000 oC
Diffractograms of alumina doped with manganese oxide, AM–1, AM–60, are presented in order to confirm the presence of different phases of alumina and disordered structure
The starting alumina powder was a mix of a few phases (α, κ, γ-alumina, and Al(OH)3), all identified by appropriate JCPDS cards (075-0783 for α-alumina, 052-0803 for κ-alumina, 079-1558 for γ-alumina, 072-0623 for Al(OH)3)
Summary
Alumina (Al2O3) can exist in several metasTab. phases, which irreversibly transform into α-alumina when heated to ≥ 1000 oC. It is well known that these properties do not depend only on the Al2O3–Cr2O3 solubility, and on the relative density, i.e., porosity, which is mainly the consequence of the particle size and phase composition of starting components, method of preparation and sintering conditions This gives a wide range of opportunities for future investigation [12, 13]. Different manganese oxides (MnO2, MnO) were employed in order to accelerate the sintering process and grain growth of alumina, and it was established that Mn2+ shifts the temperature of reaction to lower values [14,15,16,17] These investigations were mostly performed on α-alumina as the starting material. Afterwards, samples were subjected to measurements of mechanical and electrical properties as well as to the analysis of microstructure changes
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