MgO-doped Alumina Research Articles

Influence of catalyst composition on NOx storage and reduction characteristics of Ag catalyst supported on MgO-doped alumina

A novel material (Ag/MgO-Al2O3) for storage of NOx under cold-start conditions and its reduction in the high-temperature stabilized phase is synthesized, and the influence of MgO and Ag loading on its NOx removal performance is investigated. A high MgO loading increases the NOx storage capacity but also decreases the stability of adsorbed NOx. Moreover, it decreases the adsorption rate at early times since NOx spillover is inhibited. The catalysts with the lowest Ag and MgO loading exhibit the highest NO oxidation activity. The amount of NOx storage increases with a decrease in Ag loading and is attributed to enhanced spillover of NOx to the support. Increasing the Ag and MgO content leads to an enhanced NH3 and C3H6 oxidation activity, which is explained by the presence of Ag metallic clusters. Further, the NH3-SCR and C3H6-SCR activity decrease with Ag and MgO loading. Thus, the catalyst composition can be tuned to obtain the desired activity for various oxidation and reduction reactions.

Mechanical and optical properties of transparent alumina obtained by rapid vacuum sintering

Transparent polycrystalline alumina with high bending strength and transmission properties has been fabricated using rapid vacuum sintering approach at low temperature. The microstructure, mechanical and optical properties of alumina has been investigated by varying the MgO doping concentration and sintering parameters. Notably, 0.1wt% MgO-doped alumina with fully density and average grain size of ~11µm was obtained by sintering at 1670°C for 5min, exhibiting the real in-line transmission of 36.37–65.36% at a wavelength of 1068nm, microhardness of 18.44–25.29GPa and fracture toughness of 1.61–2.34MPam1/2 when the bending strength ranged from 450 to 611MPa.

Field assisted and flash sintering of alumina and its relationship to conductivity and MgO-doping

We show that flash-sintering in MgO-doped alumina is accompanied by a sharp increase in electrical conductivity. Experiments that measure conductivity in fully dense specimens, prepared by conventional sintering, prove that this is not a cause-and-effect relationship, but instead that the concomitant increase in the sintering rate and the conductivity share a common mechanism. The underlying mechanism, however, is mystifying since electrical conductivity is controlled by the transport of the fastest moving charged species, while sintering, which requires molecular transport or chemical diffusion, is limited by the slow moving charged species. Joule heating of the specimen during flash sintering cannot account for the anomalously high sintering rates. The sintering behavior of MgO-doped alumina is compared to that of nominally pure-alumina: the differences provide insight into the underlying mechanism for flash-sintering. We show that the pre-exponential in the Arrhenius equation for conductivity is enhanced in the non-linear regime, while the activation energy remains unchanged. The nucleation of Frenkel pairs is proposed as a mechanism to explain the coupling between flash-sintering and the non-linear increase in the conductivity.

Optical Properties of Transparent MgO-Doped Alumina Fabricated by Spark Plasma Sintering

The microstructure and optical properties are investigated for MgO-doped alumina fabricated by spark plasma sintering at temperatures between 1100 and 1550 °C. The MgO doping renders the microstructure less sensitive to the sintering temperature by suppressing grain growth, whereas it has no significant effect on the densification of alumina and resultantly no effect in enhancing the total forward transmission. The value of the total forward transmission can be used as an indirect measure of the slight change in density.

Light scattering in MgO-doped alumina fabricated by spark plasma sintering

The microstructure and optical properties of MgO-doped alumina fabricated by spark plasma sintering were investigated. MgO doping to alumina exhibited no noticeable effect on the total forward transmission, but suppressed grain growth and enhanced in-line transmission. The analytical investigation of the scattering coefficients confirmed that MgO-doped alumina has the scattering characteristics of the homogeneous matrix dispersed with spherical scattering grains, and that the existing model of light scattering in an alumina polycrystal is valid within a limited range of the wavelength/grain size ratio. The investigation also led to experimental determination of the effective volume density of spherical scattering grains satisfying the Rayleigh–Gans theory.

Thermal shock study of α-alumina doped with 0.2% MgO

Thermal shock behaviour of α-alumina doped with 0.2% MgO is studied. A uniformly mixed powder of α-alumina, magnesia and 2% PVA solution is dried, granulated and uniaxially pressed into circular discs of 30 mm diameter and 3 mm thick. The discs are sintered at a maximum temperature in the range of 1500–1650 °C for 3 h. The thermal shock test is carried out by heating the sample with an oxy-hydrogen flame and by monitoring the crack formation by acoustic emission technique. In general, the samples possess high thermal shock resistance close to their melting temperature. The spinels formed due to MgO doping possess low fusion temperature which hinders the crack growth. The same has been confirmed from scanning electron microscope (SEM) studies. The samples sintered at lower temperature possess higher thermal shock resistance than those sintered at higher temperature. This is attributed to the higher porosity. The abnormal grain growth (AGG) associated with high sintering temperature might have contributed to lowering of thermal shock resistance. To summarize, MgO-doped alumina samples possess high thermal shock resistance by avoiding the catastrophic failure due to the formation of low fusion spinels.

Porous alumina ceramics prepared by slurry infiltration of expanded polystyrene beads

To produce highly porous MgO-doped alumina (Al2O3) ceramics, expanded polystyrene (EPS) beads were packed as a pore former and well-dispersed alumina slurry was used to infiltrate the pore space in the EPS bead compacts. The alumina particle-EPS bead green compacts were then heated to 1550°C in air to burn out the pore former and subsequently densify the MgO-doped alumina struts. The porous Al2O3 ceramics were featured with uniformly distributed open pore structures with porosities ranging from 72 to 78% and a pore interconnectivity of about 96%. The macropore size and the pore window size could be controlled by adjusting the size of the EPS beads and the contacting area between the EPS beads. The compressive strengths of the porous Al2O3 ceramics were in the range of 5.5–7.5 MPa, similar to those of cancellous bones (2–12 MPa). The porous alumina ceramics were further made bioactive after the dip coating of a sol-gel derived 58 S bioglass powder, followed by sintering at 1200°C.

Examination of morphological changes of pore channel network during the intermediate stage of sintering of undoped, MgO-doped and ZrO2-doped alumina compacts by modified statistical theory of sintering

The modified statistical theory of sintering was used to evaluate the microstructural parameters, such as the average pore radius, grain size, and total length per unit volume of pore, of undoped and doped alumina compacts during the intermediate stage of sintering. The present results further justify the validity of the modified statistical theory of sintering in characterizing the morphological changes of pore channel network. The evaluated data of the evolution of these morphological parameters provide deep insight of the understanding of the mechanism of the effects of particle size distributions, MgO and ZrO2 on breakup of pore channel network. The possible roles of MgO and ZrO2 in the intermediate stage of sintering have been discussed.

The effect of grain boundary phase on contact damage resistance of alumina ceramics

The effect of grain boundary phase on contact damage behavior is investigated in alumina ceramics. Four types of aluminas doped with MgO, anorthite (CaO·Al2O3·2SiO2), silica, and with both MgO and anorthite are prepared such that they have similar average grain size by adjusting sintering conditions. MgO-doped alumina composed of equiaxed grains shows brittle fracture behavior, and anorthite-doped alumina composed of elongated grains shows a quasi-plastic response under Hertzian sphere indentation. The co-doped alumina with MgO and anorthite, however, is damage tolerant even with its rounded grains, while silica-doped alumina with similar grain size and shape to anorthite-doped alumina shows abrupt strength degradation with low critical load for cone cracking. The damage behavior is discussed from the viewpoint of residual stress induced by thermal expansion mismatch between the grains and grain boundary phases. The damage tolerant behavior of alumina ceramics is significantly affected by the composition of grain boundary phase.

Investigation and simulation of hot forming of alumina based materials

Hot forming of a MgO-doped alumina and of a alumina-based nanocomposite has been carried out at 1400°C under vacuum. The forming consisted of discs of the materials being punched between graphite parts into hemispheres. The loading data are presented along with a simulation of these data. The MgO-doped alumina disc broke before completion of the test, whereas the alumina based nanocomposite disc was formed into a hemisphere with reduced damage. Calculated loading data fitted monitored loading data satisfactorily. Microstructural investigations were performed on the nanocomposite hemisphere. The microstructural damage consisted of reduced cavitation which occurred essentially in a shallow region of the outer surface of the hemisphere. The reduced damage observed on the nanocomposite microstructure must be associated with the fineness of the microstructure and the reduced alumina grain growth in this material. Considering the severity of this forming process in terms of deformation, this test illustrates very well the superplasticity of the nanocomposite material. These observations confirmed results gathered from compressive testing.

Upgrading superplastic deformation performance of fine-grained alumina by graphite particles

The process of ceramic part forming is often accompanied by grain growth, which in turn limits the deformation capabilities. Alumina ceramics are not usually good candidates for superplasticity as the grain growth reduces quickly their deformation performance. In order to impede the grain boundary mobility and to limit the grain growth, zirconia has often been associated with the alumina since the early 80s. In this work, a dense MgO-doped α-alumina containing a small amount of carbon has been produced. The sintering and the deformation behaviour of this material are compared to those of a MgO-doped alumina. The presence of small carbon particles reduces greatly the grain growth during the sintering and during the compression experiments. The superplasticity of the carbon-containing alumina is greatly improved.

Cavity Damage Accumulation in Alumina Doped with Zirconia or Magnesia

For an initial grain size of 1.0 μm, the accumulation of damage volume in a ZrO 2 -dispersed alumina is shown to proceed more slowly than that in a MgO-doped alumina under a given loading condition, irrespective of higher tensile flow stress in the former than in the latter. The slower damage accumulation in the ZrO 2 -dispersed alumina is caused from the slower formation and growth of cavities larger than the initial grain size. Such slower cavitation is also shown to delay the onset of microcracking. The delayed microcracking leads to higher tensile ductility in the ZrO 2 -doped material than in the MgO-doped one.

Cavitation damage during high temperature tensile deformation in fine-grained alumina doped with magnesia or zirconia

The high temperature tensile ductility of alumina doped with MgO or ZrO{sub 2} is limited around 70--110% for initial grain sizes of about 1 {micro}m. Such limitation may be correlated with strain hardening due to insufficiently suppressed grain growth in MgO-doped alumina or the level of flow stress heightened owing to second phase pinning and/or the intergranular segregation of Zr{sup 4+} ions in ZrO{sub 2}-doped one. This is because higher flow stresses can be assumed to accelerate damage process and thereby to limit tensile ductility. In comparison between these materials, however, such an approach based simply on flow rather similar tensile ductilities as above, irrespective of noticeable differences both in strain hardening behavior and in the level of flow stress between them. Although information on intergranular cavitation damage will give an additional basis for explanation, there has been little quantitative work on cavitation in there martials. The present study, therefore, examined the evolution of cavitation damage in a 0.2-wt%-MgO-doped alumina and a 10-vol%-ZrO{sub 2}-doped alumina with the same initial grain size of 1.0 {micro}m. An emphasis was placed on characterizing the difference in cavitation behavior between the materials.

High temperature anelastic and viscoplastic deformation of fine-grained MgO-doped Al 2O 3

Mechanical loss and dynamic shear modulus measurements at high temperature (1200–1550 K) were performed on fine-grained (0.8 μm) MgO-doped alumina in the low frequency range (10 −4–10 Hz), by using a differential inverted torsion pendulum working with forced vibrations. Anelastic and viscoplastic relaxation phenomena were observed, characterized by an activation enthalpy of 840 kJ/mol. These appear as a mechanical loss peak and a viscoplastic damping background. Moreover, a stress-dependent component, activated above a “threshold”-stress amplitude of 2–4 MPa was found. The mechanical loss spectrum was analysed using classical and modified approaches and the possible mechanisms discussed in terms of a simple model invoking grain boundary dislocations.

Superplastic Tensile Ductility Enhanced by Grain Size Refinement in a Zirconia-Dispersed Alumina

High-temperature tensile ductility in fine-grained pure alumina is limited to {approximately}20% in engineering strain owing to rapid dynamic grain growth accompanied by large strain hardening and resultant severe cavitation. Accordingly, many trials have been made to suppress the dynamic grain growth by use of an additive such as MgO or ZrO{sub 2}. The dynamic grain growth of MgO-doped alumina, however, is still active to limit the tensile ductility to {approximately}80% at 1,623--1,773 K. Although some additional improvement is possible by the codoping of CuO or NiO, the maximum tensile elongation has remained 140%. On the other hand, ZrO{sub 2}-particle dispersion is much more effective in suppressing grain growth and hence strain hardening, whereas the resultant tensile ductility stays up to 110% or less at 1,723--1,773 K. It has been attributed to the increment of flow stress caused inherently by ZrO{sub 2}-dispersion through the suppression of grain boundary sliding. Regardless of these preceding studies, the approach by ZrO{sub 2}-particle pinning has not been completed, because the initial grain sizes of alumina reached about 1 {micro}m already in earlier experiments under tension. A possibility remains to enhance the tensile ductility of ZrO{sub 2}-dispersed alumina by such a reduction in grain size asmore » can compensate the increment of flow stress due to ZrO{sub 2}-addition. From this point of view, the present study examined the high-temperature tensile properties of a fine-grained, 10-vol%-ZrO{sub 2}-dispersed alumina prepared by colloidal processing. The results will demonstrate that large tensile elongation exceeding 500% can be obtained when the initial grain size is maintained below 0.5 {micro}m.« less

Effects of grain boundary amorphous phase on high-temperature ductility in alumina polycrystals

Two types of polycrystalline alumina were prepared by centrifugal compaction; MgO-doped alumina sintered in air and pure alumina HIPed in argon atmosphere with carbon susceptor. Both MgO-doped and HIP alumina had amorphous grain boundaries that were segregated, respectively by Mg and by C and S. The amorphous phase was found to melt near 1650 K, as evidenced by an endothermic peak in differential scanning calorimeter data. Melting of the grain boundary phase resulted in a ductile-to-brittle transition during the compressive deformation of the HIP alumina. In contrast, the MgO-doped alumina showed a ductile behavior up to the highest test temperature of 1773 K. The difference in ductility between two alumina specimens may be related to charge compensation, localization of bonding electrons in the grain boundary phase, and the spatial distribution of the grain boundary phase.

Influence of magnesia on colloidal processing of alumina

The effects of magnesia (MgO) on electrophoretic and rheological properties of aqueous alumina suspensions and on the characteristics of the slip cast bodies were evaluated. The influence of the dispersants and slip preparation procedure on all processing steps and on the ultimate properties of the sintered bodies was also discussed. The electrophoretic measurements showed a significant decrease in zeta potential when MgO was present in Al 2O 3 aqueous suspension. This destabilising effect of MgO was also confirmed by rheological measurements which showed an increase in viscosity and pseudoplastic behaviour of the slips. In slip casting, the destabilising effect was expressed by a decrease in green body density. Nevertheless, the relative density of the sintered bodies initially increased, reaching a maximum (99.7%) for 0.02 wt% MgO but decreased again with further additions. The results obtained showed that the optimal amount of magnesia resulted from its beneficial effect on densification of alumina, and from its detrimental influence in slip stability and packing behaviour of suspended particles. The slip preparation procedure also affected the ultimate properties of MgO-doped alumina. Namely, the dispersant used, and the order in which the powders were added to the dispersant solution determine the degree of uniformity of the MgO distribution in the mixture, resulting in different packing densities in both green and sintered states. The highest sintered density was obtained when magnesia was the first component added to the solution of the most efficient dispersant used. Considering the observed results, an optimised route for the slip preparation was established.

Solute segregation to grain boundaries in MgO-doped alumina

The spatial distribution of Mg solute in polycrystalline, single-phase Al2O3 has been obtained, using a secondary ion imaging technique. Segregation of Mg to the Al2O3 grain boundaries is clearly observed—strong evidence that the principal role of MgO is to reduce the grain boundary mobility via a solute-drag mechanism. In addition, Mg (dopant) and Ca (impurity) are shown to cosegregate, shedding further light on magnesium’s ability to stabilize microstructural evolution in Al2O3.

High-temperature deformation and cavitation in fine-grained alumina

High-temperature deformation and cavitation in fine-grained alumina are investigated in the temperature range between 1300 and 1550 °C. The addition of magnesia into high-purity alumina reduces the strain hardening rate and improves tensile ductility. The stress-strain behavior and tensile elongation in pure and MgO-doped alumina can be described by grain growth during high-temperature deformation. The high-temperature fracture in fine-grained alumina is caused by cavitation and linkage cavities. There is a critical grain size for cavitation, which is a function of deformation temperature, strain rate and flow stress.

Effects of additive gases on radio-frequency plasma sintering of alumina

Pure and 0.25 wt% MgO-doped alumina powder compacts were sintered using a radio-frequency induction-coupled argon plasma. The effects of additive oxygen, hydrogen, nitrogen and water were investigated by injecting varying amounts of each gas during sintering. The addition of diatomic gases or water vapour to argon gas during plasma sintering increased the temperature and sintered density of the specimens. Water vapour showed the strongest effect, followed by hydrogen, nitrogen and then oxygen. The presence of MgO dopant resulted in greater density and a lower sample temperature than those of the pure material.