1. Field of the Invention
The invention is generally related to fluoride salt-coated magnesium aluminate particles.
2. Description of the Related Art
Dense magnesium aluminate spinel is important because it is hard, strong, and could be somewhat transparent from 0.2 to 5.5 μm. Its mechanical properties are several times greater than that of glass and make it a leading candidate for use as a transparent armor, dome, and window material. Commercially it can be used as a stronger and thinner window for laptop computers, cell phones, automotive glassing and headlamps, aerospace windshields, and bar code readers.
However, traditional processing of spinel leads to high scattering and absorption losses that are distributed in localized yet random regions. Basically, the article does not posses uniform optical losses, and consequently the yield is poor, costs are high and large size and different shapes are not possible to manufacture. The scattering sites are typified by voids or inclusions that appear white when viewed in reflected light. The absorbing regions are dark when viewed in both transmitted and reflected light.
Reactions between the LiF sintering aid and the spinel matrix contribute to optical scattering. The common reaction products that contribute to optical scattering are lithium aluminum fluoride and oxyfluoride compounds, magnesium rich grains, unreacted LiF, and compounds that resulted from impurities in the starting spinel powder and sintering aid. A by-product of this is the presence of voids that possess very high scattering efficiencies. The dark absorbing regions are mainly due to hydrocarbons that cracked during the sintering operation, carbon that diffused into the spinel from the furnace/die/die liner environment, or carbon present in the starting powders. In addition they can also be due to the presence of reduced state transition metal ions and silicon. This could also be due to planar precipitates which also scatter light.
Sintering, both pressure-less and with pressure (hot pressing), requires a vast amount of material transport to consolidate an aggregate of loose powder particles into a dense shape. In the case of porcelains and clay products secondary phases do melt and “glue” the primary solid particles together with a glassy phase. These types of systems were the first to be used due to their ease of sintering. However, advanced ceramics do not have these intrinsic sintering aids and must therefore be added. For small samples the powdered sintering aids are mixed to the powder to be sintered with a mortar and pestle. In larger samples, mixing is accomplished by ball milling, attritor milling, high shear wet milling, and variations or combinations of these methods. However, due to the nature of particle-particle interactions, the mixture is far from homogeneous. Inhomogeneity results in areas that have too much sintering aid and other areas that have little or no sintering aid. While this is generally not too important in systems that are relatively easy to sinter, it is a major problem in the fabrication of transparent ceramics, electronic ceramics, and in high tech refractory ceramics. It leads to materials that are inhomogeneous and contain regions of opacity. This adds a tremendous amount of cost to the product since yields are low and size is limited to small regions core drilled out of the large sample.