Aluminum oxide thermal properties

The small (10 -lm) coating particles are typically aluminum oxide [1344-28-1/, Al O. These particles can have BET surface areas of 100 to 300 m /g. The thermal and physical properties of alumina crystalline phases vary according to the starting phase (aluminum hydroxide or hydrate) and thermal treatment (see ALUMINUM COMPOUNDS, ALUMINUM OXIDE). 

Applications of thermal radiation spectroscopy to expins and pyrots are readily apparent. As a consequence of the highly exothermic nature of explns and flares, significant thermal radiation is emitted which can serve to characterize the reaction processes. The photometric properties of pyrots have been treated in Vol 8, P505-R. In practice, thermal radiation characteristics of explns do not always closely approximate black body properties since the system is non-equilibrium in nature and is time dependent. In addition, some pyrotechnically related materials such as aluminum oxide and magnesium oxide behave as gray bodies with emissivities well below unity. For such systems the radiant emission is reduced as shown in Fig 4... 

Alumina is a porous, high-surface-area form of aluminum oxide. The surface has more polar characteristics than silica gel does therefore, it has both acidic and basic characteristics, reflecting the nature of the metal. Alumina has a high melting point, slightly over 2000°C, which is also a desirable property for a support due to its thermal stability. Alumina is composed of aluminum trihydroxides, Al(OH)3 aluminum oxyhydroxides, AIO(OH) and aluminum oxide, Al203n(H20). 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

Further incorporation of thermal processing into the manufacturing process leads to products with improved color properties. In the commercial paint sector, the use of inorganic stabilizers, for example calcium, aluminum or zinc phosphate or oxides like aluminum oxide, improves other pigment properties, e.g. photochromism, weathering and acid resistance. 

The important properties of aluminum oxide ceramics are their high temperature stability (melting point of AI2O3 2050°C), their good thermal conductivity, their high electrical resistivity and their high chemical resistance. Their mediocre thermal shock resistance is a disadvantage. All these properties are dependent upon the chemical purity and particle size distribution of the oxide powder and the density, structure and pore size di.stribution of the ceramic. 

Electrically insulating and thermally conductive qualities are important in computer chips fabrication. One approach taken is based on boron nitride fillers which offers these two properties. There is also a need to develop materials which are thermally conductive but electrically insulating in high humidity conditions. Polyurethane composites filled with aluminum oxide or carbon fiber can be used for this application. Figure 19.15 shows the effect of the amount of filler on thermal... 

CHEMICAL PROPERTIES chemical properties similar to aluminum high thermal conductivity metal resistant to attack by acid due to the formation of a thin oxide film resistant to oxidation at ordinary temperatures attacked by strong bases with evolution of hydrogen finely divided or amalgamated metal reacts with hydrochloric acid, dilute sulfuric acid, and dilute nitric acid HC (-28,000 Btu/lb, -15,560cal/g, -652 x 10 J/kg) LHf (3.5 kcal/mol). 

Reasons for use abrasion resistance, cost reduction, electric conductivity (metal fibers, carbon fibers, carbon black), EMI shielding (metal and carbon fibers), electric resistivity (mica), flame retarding properties (aluminum hydroxide, antimony trioxide, magnesium hydroxide), impact resistance improvement (small particle size calcium carbonate), improvement of radiation stability (zeolite), increase of density, increase of flexural modulus, impact strength, and stiffness (talc), nucleating agent for bubble formation, permeability (mica), smoke suppression (magnesium hydroxide), thermal stabilization (calcium carbonate), wear resistance (aluminum oxide, silica carbide, wollastonite)... 

Functional Fillers. A variety of fillers can be used to add specific properties. Metals, and beryllium and aluminum oxides, can be added to increase thermal conductivity (Table 3.33). Metals can be added to increase electrical conductivity (Table 3.34). Graphite increases lubricity and electrical conductivity. Mica increases elec-... 

Encapsulation is often performed with resins containing fillers such as mica, aluminum oxide, milled glass fibers, and many others. Although epoxies are the resins most generally used, polyesters, filled and unfilled silicones, urethanes, and polysulfides are also used. By the proper choice of fillers it is possible to match expansion rates of the electronic part and the encapsulant, increase the thermal conductivity of the part, and considerably upgrade the electrical and mecharucal properties of the assembly. 

Sialons, as ceramic alloys of silicon nitride and aluminum oxide, were developed as an economically and fimctionally superior alternative to H PSN (see above). This alloying imposes increased high-temperature mechanical (flexural and tensile strengths, fracture toughness, hardness, wear resistance), thermal (thermal shock resistance) and chemical properties (corrosion and oxidation resistance) compared to unalloyed silicon nitride (see Table 11.13). 

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