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The quality of crystallographic alignment in textured ceramics produced by tape casting and templated grain growth (TGG) has been little studied despite its demonstrated impact on magnetic, piezoelectric, and optical properties. Physical and crystallographic alignment of anisotropic template particles is shown to be directly linked to the casting rate, gap height, and casting viscosity during tape casting. These parameters are shown to affect the shape and magnitude of the shear rate profile under the doctor blade during casting which in turn causes a gradient in the torque acting on anisotropic particles. The magnitude of the torque, the time the slurry is exposed to torque during casting, and the ratio of casting height to template diameter are demonstrated to enable the particle alignment process to be tailored to produce well‐aligned template particles. Crystallographic alignment of the textured ceramic was quantified by grain misalignment angle (full width at half maximum, FWHM) and degree of orientation (r) and is directly correlated with the degree of torque during casting. High‐quality alignment (FWHM = 4.5°; r = 0.13) was demonstrated in the model TGG system consisting of submicrometer alumina and 5 vol% 11 μm diameter template platelet particles. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we investigate the effects of powder chemistry on the sintering of MgO‐doped specialty alumina. The stages at which MgO influences densification of Al2O3 were identified by comparing dilatometry measurements and the sintering kinetics of MgO‐free and MgO‐doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO2 in alumina grains near the boundaries using EDS. First‐principles DFT calculations demonstrate that the co‐dissolution of MgO and SiO2 in alumina is thermodynamically favored over the dissolution of MgO or SiO2 individually in alumina. This study experimentally demonstrates for the first time that removal of SiO2 from the grain boundaries is a key process by which MgO enhances the sintering of alumina. [ABSTRACT FROM AUTHOR]

To determine how grain-boundary composition affects the liquid phase sintering of MgO-free Bayer process aluminas, samples were singly or co-doped with up to 1029 ppm Na2O and 603 ppm SiO2 and heated at 1525°C up to 8 h. Na2O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as-received, MgO-free Al2O3, Na2O-doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO2 concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20−400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain-boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain-boundary phase. For low Na2O/SiO2 ratios, densification of Bayer Al2O3 at 1525°C is controlled by diffusion of Al3+ through the grain-boundary liquid, whereas for high Na2O/SiO2 ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al2O3 or diffusion of Al3+. Increasing Na2O in SiO2-doped samples increases diffusion of Al3+ and Al2O3 solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al2O3 densification can be manipulated by adjusting the Na2O to SiO2 ratio. [ABSTRACT FROM AUTHOR]

To determine how grain-boundary composition affects the liquid phase sintering of MgO-free Bayer process aluminas, samples were singly or co-doped with up to 1029 ppm Na2O and 603 ppm SiO2 and heated at 1525°C up to 8 h. Na2O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as-received, MgO-free Al2O3, Na2O-doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO2 concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20−400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain-boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain-boundary phase. For low Na2O/SiO2 ratios, densification of Bayer Al2O3 at 1525°C is controlled by diffusion of Al3+ through the grain-boundary liquid, whereas for high Na2O/SiO2 ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al2O3 or diffusion of Al3+. Increasing Na2O in SiO2-doped samples increases diffusion of Al3+ and Al2O3 solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al2O3 densification can be manipulated by adjusting the Na2O to SiO2 ratio. [ABSTRACT FROM AUTHOR]

This paper examines the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3-3xAl5O12 (Nd:YAG) ceramics. Nd:YAG powders were doped with 0.035-0.28 wt% SiO2 and vacuum sintered between 1484° and 1750°C. 29Si magic-angle spinning nuclear magnetic resonance showed that Si41 substitutes onto tetrahedrally coordinated Al3+ sites. High-resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600°-1750°C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and transmission electron microscopy energy-dispersive X-ray spectroscopy observations of increased Nd31 concentration near grain boundaries. Increasing SiO2 content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. Fine-grained (< 3 μm), highly transparent (> 82% real in-line transmission) ceramics were produced by sintering 0.035 wt% SiO2-doped ceramics at 1750°C for 8 h. Coarse-grained (18 μm), transparent samples were obtained with 0.28 wt%SiO2-doped Nd:YAG when sintered at 1600°C for 8 h. [ABSTRACT FROM AUTHOR]

This paper demonstrates that fine-grained (2–3 μm), transparent Nd:YAG can be achieved at SiO2 doping levels as low as 0.02 wt% by the sinter plus hot isostatic pressing (HIP) approach. Fine grain size is assured by sintering to 98% density, in order to limit grain growth, followed by HIP. Unlike dry-pressed samples, tape-cast samples were free of large, agglomerate-related pores after sintering, and thus high transparency (i.e., >80% transmission at 1064 nm) could be achieved by HIP at <1750°C along with lower silica levels, thereby avoiding conditions shown to cause exaggerated grain growth. Grain growth was substantially limited at lower SiO2 levels because silica is soluble in the YAG lattice up to ∼0.02–0.1 wt% at 1750°C, thus allowing sintering and grain growth to occur by solid-state diffusional processes. In contrast, liquid phase enhanced densification and grain growth occur at ∼0.08–0.14 wt% SiO2, especially at higher temperatures, because the SiO2 solubility limit is exceeded. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we investigate the effects of powder chemistry on the sintering of MgO‐doped specialty alumina. The stages at which MgO influences densification of Al2O3 were identified by comparing dilatometry measurements and the sintering kinetics of MgO‐free and MgO‐doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO2 in alumina grains near the boundaries using EDS. First‐principles DFT calculations demonstrate that the co‐dissolution of MgO and SiO2 in alumina is thermodynamically favored over the dissolution of MgO or SiO2 individually in alumina. This study experimentally demonstrates for the first time that removal of SiO2 from the grain boundaries is a key process by which MgO enhances the sintering of alumina. [ABSTRACT FROM AUTHOR]