Submicrometer aluminas

Fig. 3. Sedigraph particle size distribution for superground submicrometer alumina, (a) Partially dispersed (b) fully dispersed.

A dispersion of solids can be prepared by dispersing a powder in a liquid or by synthesising the solid particles in situ in a liquid. Examples of the first method are suspensions of submicrometer alumina or zirconia powders in water. Examples of the second method are boehmite sols and titania sols prepared from organo-metallic precursors. ... 

With Eq. (3/3a), this deereasing influence of the load in more fine-grained microstructures is expressed by smaller values of the ratio (5e/5i)o- Table 2 displays fitting parameters for the experimental data of Fig. 4 for smaller grain sizes, increasing asymptotic hardness values H o are associated with decreasing parameters ( e/ i)o- la submicrometer alumina microstructures, the increase of the microplastic deformability (the decrease of the hardness) becomes smaller and smaller and approaches zero already at small indent sizes of 10-20 pm (see Fig. 4), and which characterizes the extension of the plastic zone approaches the initial deformability at rather small loads of about 1N. 

A. Krell and P. Blank, Grain size dependence of hardness in dense submicrometer alumina, /. Am. Ceram. Soc. 1995, 78, 1118-1120. 

Figure 4. Microstructure of a single phase submicrometer alumina grinding material manufactured by powder processing (average crystallite size 0.4pm, hardness HVIO (ground) = 22.4 0.7GPa).

Maleksaeedi S, Paydar MH, Saadat S, Ahmadi H (2008) In situ vibration enhanced pressure slip casting of submicrometer alumina powders, J Eur Ceram Soc 28 3059-3064... 

A higher density sol—gel abrasive, produced by the introduction of seed crystaUites formed by wet-milling with high alumina media or by introduction of submicrometer a-alumina particles, was patented (28) and designated Norton SG. The microstmcture of this abrasive consists of submicrometer a-alumina crystals (Fig. 1) and its bulk density approaches that of fused alumina. Norton SG has proven to be an exceptional performer in coated and bonded abrasive products it was awarded the 1989 ASM Engineering Materials Achievement Award (29). 

Alkali metal haHdes can be volatile at incineration temperatures. Rapid quenching of volatile salts results in the formation of a submicrometer aerosol which must be removed or else exhaust stack opacity is likely to exceed allowed limits. Sulfates have low volatiHty and should end up in the ash. Alkaline earths also form basic oxides. Calcium is the most common and sulfates are formed ahead of haHdes. Calcium carbonate is not stable at incineration temperatures (see Calcium compounds). Transition metals are more likely to form an oxide ash. Iron (qv), for example, forms ferric oxide in preference to haHdes, sulfates, or carbonates. SiHca and alumina form complexes with the basic oxides, eg, alkaH metals, alkaline earths, and some transition-metal oxidation states, in the ash. 

SiC, titania, alumina and polymer particles Cu, Ni Submicrometer ceramic powders were coated by electroless deposition 77... 

A representative comparison of the effect of the catalyst bed geometry on methane conversion and product selectivity over a range of methane/air ratios is shown in Fig 4 Unlike typical supported catalysts, where the catalyst is well-dispersed and submicrometer-sized, the noble-metal catalysts in these methane oxidation reactions were basically films with micrometer-sized surface features (Other tests on both extruded cordiente and alumina foam monoliths with lower catalyst loading resulted in similar carbon monoxide production but lower hydrogen yields than those illustrated in the figure, which provided evidence that the reaction is catalyst-dependent and not initiated by the monoliths or gas... 

Fig. 6.44. Electrophoretic mobility of submicrometer a-alumina as a function of the (diluted) suspension pH no electrolyte added. Above 1 mg/g ammonium polycarboxylate (unsaturated adsorption). Below 5 mg/g ammonium polycarboxylate (saturated adsorption).

In Fig. 6.51 the mean layer thickness after drying is plotted as a function of the withdrawal speed for a colloidally stable concentrated (cp = 0.42) submicrometer a-alumina suspension. Three curves can be seen ... 

Table I. MSA-Coated Submicrometer a-Alumina Particle-Size Analysis...

Table II. MSA Coating of Submicrometer a-Alumina Particles, First Stage Hydrous Silica on Hydroxylated Alumina Surface...

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