Zircon
Zirconium is a chemical element (symbol Zr). Zirconium is a non-toxic heavy metal in the titanium group of the periodic table.
Zircon (also referred to as zirconium silicate), ZrSiO4, is a naturally occurring mineral found in many colours, which is used a gem; the colourless version is sometimes used as a diamond substitute. Chemically it is a mixture of varying ratios of silicon dioxide (SiO2) and zirconium dioxide (ZrO2), generally with hafnium. Zircon sand is used as the raw material for manufacturing zirconium dioxide.
Zirconia (ZrO2) is a transparent diamond imitation gem, manufactured synthetically from zirconium dioxide. More precisely it is CSZ, cubic stabilised zirconium dioxide at very high temperatures (above 2370°C) due to the addition of other metal oxides. When stabilisation involves using yttrium oxide the term is YSZ and with calcium oxide it is CaSZ.
Zirconium dioxide (ZrO2), also zirconium(IV) oxide, zircon oxide, (previously also referred to as zircon acid or zirkonerde [zirconia]) is a high performance oxide-ceramic. The naturally occurring mineral in monoclinic (at room temperature to 1173°C crystallising) form (modification) is called baddeleyite.
All-zirconia bridgework and crowns
Zirconium dioxide free-end bridge
Lingual view of single-winged bridge
Compaction of highly pure zirconium dioxide powder and binding agents without heat treatment produces a press blank, known as a green compact or green body. Shrinkage of 25% at a later stage must be taken into account. A presintering process is then completed at approx. 1000°C, during which the binding agents are eliminated, shrinkage of approx. 5% occurs. This produces a partially sintered blank. In this partially sintered state the zirconium dioxide can be processed using many conventional dental CAD-CAM milling systems. Subsequent shrinkage of 20% at the (post) sinter firing stage must be factored into the calculation by respective enlarged manufacturing.
Alternatively, the zirconium dioxide can also first be densely sintered and then hot isostatically pressed (recompacted). The resultant material undergoes no further shrinkage and can therefore be processed dimensionally accurately (1:1), however, this material is extremely hard and the tool wear exceptionally high.
CeSZ zirconium dioxide partially or fully stabilised with cerium oxide.
CaSZ zirconium dioxide stabilised with calcium, form of zirconia.
CSZ cubic stabilised zirconium dioxide (above 2370°C) TSZ tetragonal stabilised zirconium dioxide (temperature range 1173°C to 2370°C)
YSZ zirconium dioxide partially or fully stabilised with yttrium oxide.
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Wax build-up technique Wax build-up technique The various anatomical structures (such as cusp tips and slopes as well as marginal ridges) are usually built up one after another by adding small portions of wax (often using differently coloured waxes for didactic purposes). The firm, special waxes first have to be melted at room temperature. This can be carried out by warming small portions on differently shaped working tips of hand instruments in an open flame (such as a gas burner) or using electrically heated instruments which provide for more accurate temperature control and avoid contamination (e.g. electric wax-knife, induction heaters, wax dipping units). The wax is applied drop-by-drop to ensure that the warmer molten wax added last fuses seamlessly with the firm, cooler material. After hardening, the wax pattern can be reduced by sculpting, milling guidance surfaces or drilling to add retainers. Modern procedures include flexible, occlusal preforms for adding contours to soft wax. In addition, wax preforms, such as for occlusal surfaces or bridge pontics, are available in various shapes and sizes. Recently, irreversible, light-curing materials have been introduced for use instead of reversible thermoplastic waxes. Wax preforms To ensure that the wax pattern can be released without being damaged, model surfaces, opposing dentition and preparations must be hardened/sealed with special lacquer (applied by spraying, brushing or dipping). These waxes are mostly relatively rigid/elastic after cooling. Attaching wax sprues to a removable framework supported on double crowns using a hand instrument When employing the lost wax technique, prefabricated wax sprues, bars and reservoirs are attached to the patterns. Once the pattern has been released and its sprues waxed onto the crucible former, it is invested in a casting ring with refractory investment material. The wax can then be burnt out residue-free and casting completed. Unlike standard wax build-up techniques, a diagnostic wax-up is not intended for fabricating an indirect restoration, but rather for simulating the appearance and/or external contouring for producing orientation templates. |