Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Patent
1998-02-18
2000-06-27
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
501 6, 501 8, 501 9, 501 57, 501 59, 501 67, 501 69, 106 35, 4332121, C03C 1008, C03C 1010, C03C 1016
Patent
active
060806923
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to mica glass ceramics which, because of their mechanical properties, are suitable for a wide range of applications, one example of which is a tooth replacement.
Mica glass ceramics are generally known for the fact that they can be processed by cutting. The processibility is made possible by the foliate morphology of micas and the fact that they are easy to cleave.
Like natural micas, the mica crystals in glass ceramics have a broad range of compositions. The intermediate-layer position in the mica structure can be occupied by elements of the alkali and alkaline-earth groups. While occupation by potassium or other heavy alkali elements corresponds more to the frequently occurring natural compositions, it is also possible for micas having alkaline-earth and lithium ions in the intermediate position to crystallize in glass ceramics. However, apart from micas containing Mg, they exhibit the property of appearing to have only limited use for technical applications. On contact with water, the mica structure swells and the glass ceramic inevitably disintegrates [S. N. Hoda, G. H. Beall: "Alkaline earth mica glass-ceramics" in Adv. in Ceramics, Vol. 4, Nucleation and Crystallization in Glasses, Am. Ceram. Soc, Columbus Ohio, 1982]. Full occupation of the tetrahedral position of the mica structure by silicon leads to tetrasilicate micas which likewise crystallize as a mica phase in mica glass ceramics.
The classical properties of mica glass ceramics permit applications in electronics (high resistivity, low dielectric constant) and in vacuum technology (non-porosity). Because mica glass ceramics are biocompatible, they are also used in medicine as a material for bone replacement.
In many cases, however, the comparatively low strength of mica glass ceramic conflicts with the use of this material. Improved strength, in conjunction with good processibility, at least in the desired phases of the production process, would considerably widen the range of application of mica glass ceramic.
The use of mica glass ceramics in restorative dentistry forms part of the background art. EP 0 083 828 B1 describes tetrasilicate mica glass ceramics with increased resistance to discoloration (through the addition of Al.sub.2 O.sub.3 and ZrO.sub.2) for use as dental structure. The corresponding glass ceramic can be used as a prefabricated block in a standard CAD/CAM method in order to make inlays or onlays from it.
The object of the present invention is to provide a mica glass ceramic which has good chemical stability and high mechanical strength, a high degree of translucence and a degree of processibility which is sufficient to allow it to be processed on CAD/CAM machines.
This object is achieved by a glass ceramic in which mica and ZrO.sub.2 are present in crystallized form, containing: about 4% by weight,
The proportion of F.sup.- is preferably 4-10% by weight.
"Essentially free" is in this case intended to mean that said elements should be present at most in such proportions as are caused by the introduction of impurities, typically overall less than 1% by weight, preferably less than 0.5% by weight. Above all, the proportion of lithium should preferably be less than 0.1% by weight, since, because of the low molecular weight of Li, a quantity of, for example, as little as 1% by weight of Li.sub.2 O has a considerable effect on the properties of the glass ceramic, in particular its resistance to hydrolysis.
Further, extra components for imparting color, for fluorescence or for improving the processing properties of the glass may be added, for example: CeO.sub.2, La.sub.2 O.sub.3, MnO.sub.2, Fe.sub.2 O.sub.3, TiO.sub.2, Y.sub.2 O.sub.3 and B.sub.2 O.sub.3 in quantities of up to about 5% of each component, and together no more than about 10%. The mixtures are produced in the corresponding proportions from the raw materials (oxides, fluorides,. carbonates, etc.). The mixtures are melted at 1300-1600.degree. C., preferably 1400-1550.degree. C., for, for example, 1-5 hours in a covered crucible while being
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Mueller Gerd
Reise Michael
Bell Mark L.
Fraunhofer-Gesellschaft
Sample David
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