The Effect of MgO on the Mobility of Grain Boundaries and Pores in Aluminum Oxides

Burke, Jan; Lay, K. W.; Procházka, S.

doi:10.1007/978-1-4899-5301-8_33

Cited by 16 publications

(24 citation statements)

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“…For this reason, analyses intended to identify the stage of sintering that a material has achieved will generally show properties characteristic of more than one stage, or of intermediate stages. That situation is shown in Figure 10 where the data from this work are plotted on a graph of surface area vs. fractional density (1 -porosity) in the fashion of Burke et al (1980). If all surface area reduction occurs through grain coarsening the data would appear as a trend parallel to the surface area axis.…”

Section: Diffusionmentioning

confidence: 97%

“…This is also the reason for the smallest pores being filled first. Burke et al (1980) refer to stage I1 as densification because of the decrease in porosity. The transport processes during this stage are both grain boundary and volume diffusion.…”

Section: Diffusionmentioning

confidence: 99%

“…Stage I11 is characterized by the growth of the grains themselves through a process Burke et al (1980) call coarsening. Pore annihilation along grain boundaries ceases if and when grain boundary mobility exceeds pore mobility, a t which point pores are left behind in what becomes the interiors of grains.…”

Section: Diffusionmentioning

confidence: 99%

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Structural changes in surfactant‐modified sorbents during furnace injection

Kirchgessner

Jozewicz

1989

AIChE Journal

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A calcium hydroxide [Ca(OH),I sorbent modified by the addition of calcium lignosulfonate has recently been developed for use in the Environmental Protection Agency's limestone injection multistage burner process. The increased reactivity with sulfur dioxide (SO,) displayed by this modified sorbent has been shown to be caused, in part, by its decreased particle (agglomerate) size compared to conventional Ca(OH),. Subsequent work has shown that surfactant-modified Ca(OH), also undergoes significantly different structural changes during furnace injection. For a given reactor temperature and residence time, the modified sorbent calcines to a greater extent than unmodified sorbent. It also loses surface area more slowly, and retains more of its porosity, suggesting that it sinters more slowly than conventional sorbent. Therefore, in addition to reducing the particle size of Ca(OH), in some applications, calcium lignosulfonate also appears to cause the water of hydration to be bound less tightly, and to inhibit one or more of the diffusion mechanisms responsible for the process of sintering. IntroductionThe Environmental Protection Agency's LIMB (limestone injection multistage burner) process is designed to reduce SO2 emissions from coal-fired boilers through the injection of a dry, calcium-based sorbent into the furnace at about 1,230°C, while reductions in NO, are achieved by delaying the mixing of fuel and air through the multistage burner. A lengthy testing program has shown that, of the commercially available calciumbased sorbents, calcium hydroxide, Ca(OH),, produces the highest levels of SO2 capture (Overmoe et a]., 1985;Beittel et al., 1985;Bortz and Flament, 1985;Slaughter et al., 1985).Recent efforts to improve the capture of SO2 by commercial Ca(OH), involve the addition of calcium lignosulfonate to the water of hydration to produce Ca(OH)2 that has a smaller particle size, and a 15-20% higher reactivity with SO, relative to conventional Ca(OH), (Kirchgessner and Lorrain, 1987). The effect of smaller particle size on reactivity has been widely documented (McCarthy et al., 1986;Borgwardt and Bruce, 1986a; Cole et al., 1986) and is of the order -0.20 to -0.35 (Ishihara et al., 1975;Gullett and Blom, 1987;Gullett and Kramlich, 1987). The enhanced reactivity noted at laboratory scale by Kirchgessner and Lorrain is partially, but not fully, explained by particle size reduction in the modified hydrate.Surface area and pore structure of the calcined sorbent are also important determinants of reactivity. The relationship of surface area of the calcined sorbent to its ultimate level of sulfation has been widely documented Harrison et al., 1985;Cole et al., 1985;Beittel et al., 1985;Slaughter et al., 1985; Borgwardt and Bruce, 1986b). Borgwardt and Bruce (1 986b) have quantified the relationship in noting that the reactivity increases with the square of the BET surface area of the calcine over the range 2-63 m2/g. The importance of porosity in the CaO was noted by Hartman et al. (1978). Aside from the effect of part...

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Section: Diffusionmentioning

confidence: 97%

Section: Diffusionmentioning

confidence: 99%

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Structural changes in surfactant‐modified sorbents during furnace injection

Kirchgessner

Jozewicz

1989

AIChE Journal

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“…It has been reported in literature 6,[31][32][33][34][35][36][37][38] that dopants are usually added to improve the densification and with the selection of some dopants, near-theoretical density can be achieved with enhancement in grain boundary diffusivity at the expense of surface diffusivity and vapour transport. The suppression of exaggerated grain growth in alumina is due to the effect of dopant such that there is increase in surface diffusivity and pore mobility so that pores remain on grain boundaries until these are eliminated.…”

Section: Influence Of Mgo and Sio 2 -Dopants On Grain Growth Grain Smentioning

confidence: 99%

Alumina-based Ceramic Material for High-voltage Ceramic Substrate

Sangawar

Kumar

2006

DSJ

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The paper presents the study of the particle size distribution, surface area and their effect on sintering of alumina (Al 2 O 3 ) using additives such as magnesium oxide (MgO) and silica (SiO 2 ), so that the samples could be sintered to high relative density (~ 97.43 %) with controlled grain growth. However, the use of MgO along with SiO 2 on Al 2 O 3 produced the powder compacts having high Green density, sintered density with minimum porosity to achieve high dielectric strength ceramic material, so that material can be used for high-voltage insulator applications.

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“…Al 2 O 3 -10vol%SiC nanocomposite have been marked by mixing small size of alumina and SiC particles (<100nm), then hot pressed in the range of 1500 to1800°C 1,6 . The important roles of MgO dopant in Al 2 O 3 -SiC nanocomposite are as: (i) lowering the grain boundary mobility 16,17 ; (ii) increasing the surface diffusivity then increasing the pore mobility [18][19][20] ; and (iii) decreasing the grain boundary anisotropy 21 . The objective of this work is to investigate the effects of adding a small amount of MgO (up to1500ppm) to alumina based nanocomposites containing 10vol%SiC particles.…”

Section: Introductionmentioning

confidence: 99%

EFFECT OF MgO NANO PARTICLES ON SINTERING BEHAVIOR OF Al₂O₃-SiC-MgO NANO COMPOSITES

Surani

Abdizadeh

Baharvandi

et al. 2012

Int. J. Mod. Phys. Conf. Ser.

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Small amount of MgO in the range of ppm and distribution of SiC particles in Al2O3-10vol%SiC nanocomposite has an obvious effect on improving of density and mechanical properties such as hardness of sintered nanocomposite. Improving of these properties continues to 1000 ppm of MgO, but decreases above this amount. Transgranularly fractured alumina grains were observed on the fracture surface of this kind of nanocomposites. The effects of such a small amount of MgO on the sintering and microstructural development of Al2O3-10vol%SiC nanocomposite may be explained using the arguments which have been documented for the beneficial effects of a small amount of MgO in alumina ceramics. In Al2O3-10vol%SiC nanocomposite, Al2O3 particles were coated by nano scale SiC particles which inhibited the abnormal grain growth of Al2O3.

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The Effect of MgO on the Mobility of Grain Boundaries and Pores in Aluminum Oxides

Cited by 16 publications

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Structural changes in surfactant‐modified sorbents during furnace injection

Structural changes in surfactant‐modified sorbents during furnace injection

Alumina-based Ceramic Material for High-voltage Ceramic Substrate

EFFECT OF MgO NANO PARTICLES ON SINTERING BEHAVIOR OF Al₂O₃-SiC-MgO NANO COMPOSITES

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The Effect of MgO on the Mobility of Grain Boundaries and Pores in Aluminum Oxides

Cited by 16 publications

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Structural changes in surfactant‐modified sorbents during furnace injection

Structural changes in surfactant‐modified sorbents during furnace injection

Alumina-based Ceramic Material for High-voltage Ceramic Substrate

EFFECT OF MgO NANO PARTICLES ON SINTERING BEHAVIOR OF Al2O3-SiC-MgO NANO COMPOSITES

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EFFECT OF MgO NANO PARTICLES ON SINTERING BEHAVIOR OF Al₂O₃-SiC-MgO NANO COMPOSITES