Introduction The Stone Age and

Introduction The Stone Age and Bronze Age were named for the materials that dominated these major historical periods. Consequently, the modern era, which is encountering an ever-increasing chemicals from this source of ceramic materials for industrial or biomedical use, could be characterized as the “Ceramic Age”. Ceramic materials that specially developed for medical and dental use are termed bioceramics. Over the last decade, zirconia technology has encouraged a rapid development of metal free dentistry that may provide high biocompatibility, enhanced esthetics and improved strength [1]. The name “Zirconium” comes from Arabic word “Zargon” which means “golden in color” which in turn comes from the two Persian words Zar (gold) and Gun (color). Zirconium dioxide (ZrO2) was accidentally discovered by a German chemist his name is Martin Heinrich Klaproth in 1789 while he was working with certain procedures that involved the heating of some gems [2]. Subsequently, Zirconium dioxide was used as rare pigment for a long time. In late sixties the progress of research, the use of zirconium as biomaterials was refined. The first use of zirconium oxide (ZrO2) for medical purposes was made in 1969 in orthopedic as a new material for hip head replacement instead of titanium or alumina prostheses [3]. Zirconium (symbol Zr) is a transition metal element, atomic number 40, atomic weight 91.22, density of 6.49 g/cm3, a melting point of 2,128 K (1855 °C or 3371 °F) and a boiling of 4,682 K (4409 °C or 7968 °F) and lustrous with exceptional corrosion resistance. Pure zirconium exists in crystalline form as a white and ductile metal and in an amorphous form as a blue black powder. Zirconium is ranked 18th in abundance among the element in earth\'s crust, however, this element does not occur in nature in a pure state but only in conjugation with silicate oxides (ZrO2 XSiO2) or as a free oxide (ZrO2) [4-7]. Zirconium Dioxide (ZrO2) is a white crystalline oxide of zirconium found in the minerals baddeleyite (ZrO2) and zircon (ZrO2). Zirconium oxide crystals can be categorized into three crystallographic phases: the cubic phase (C) in the form of straight prism with square side, the tetragonal phase (T) in the form of a straight prism with rectangular sides, and the monoclinic phase (M) in the form of a deformed prism with parallelepiped sides. The cubic phase is stable above 2370 °C and with moderate mechanical properties, the tetragonal phase is stable between 1170 °C and 2370 °C with improved mechanical properties, and the monoclinic phase, which is stable at room temperature up to 1170 °C, with lower mechanical properties and may contribute to a reduction of the ceramic particles cohesion [8-10]. These lattice transformations are martensitic, characterized by being diffusionless (i.e. involving only coordinated shifts in lattice positions versus transport of atoms), occurring athermally implying the need for a temperature change over a range rather than at a specific temperature and, involving a shape deformation [4].This transformation range is bounded by the martensitic start (Ms) and martensitic finish temperatures. Volume changes on cooling associated with these transformations are very crucial and substantial so it results in a pure material that is unsuitable for any applications requiring an intact solid structure. This change is about 2.31% in case of C→T transformation and approximately 4.5% on cooling from T to M [5].
Different types of zirconia ceramics materials available for dental applications
Conclusions
Introduction The apparent color of natural teeth is the result of the reflectance from dentin modified by absorption, scattering and thickness of enamel. Therefore, understanding optical properties of teeth is imperative for accurate and consistent color reproduction [1]. Color and its elements such as Hue, Chroma, value, opacity, translucency, light transmission, scattering, metamerism and fluorescence influence the esthetics of a dental restoration. The eye is able to distinguish between artificial and natural teeth, based on minute differences in color and translucency [1,2].