The white titanium dioxide, or titania, of the composition TiO2, is an important paint pigment. The best quality is produced from ilmenite, and is higher in price than many white pigments but has great hiding power and durability. Off-color pigments, with a light buff tone, are made by grinding rutile ore. The pigments have fine physical qualities and may be used wherever the color is not important. Titania is also substituted for zinc oxide and lithopone in the manufacture of white rubber goods, and for paper filler. The specific gravity is about 4.
Titania crystals are produced in the form of pale-yellow, single-crystal boules for making optical prisms and lenses for applications where the high refractive index is needed. The crystals are also used as electric semiconductors, and for gemstones. They have a higher refractive index than the diamond, and the cut stones are more brilliant but are much softer. The hardness is about 925 Knoop, and the melting point is 1825°C. The refractive index of the rutile form is 2.7 and that of the anatase is 2.5; the synthetic crystals have a refractive index of 2.616 vertically and 2.903 horizontally.
Titanium oxide is a good refractory and electrical insulator. The finely ground material gives good plasticity without binders, and is molded to make resistors for electronic use. A micro sheet is titanium oxide in sheets as thin as 0.008 cm for use as a substitute for mica for electrical insulation where brittleness is not important. Titania-magnesia ceramics have been made in the form of extruded rods and plates and pressed parts.
Uses
Titanium dioxide is a most important ceramic finish coat for sheet metal products. The opacity of this enamel imparted by titanium dioxide has lowered film thickness of these finishes to the range of organic coatings while retaining the durability of porcelain. These enamels are self-opacified. That is, titanium dioxide is not dispersed as an insoluble suspension during smelting nor is it added at the mill. Rather, titanium dioxide is taken into solution during smelting of the batch and is held in supersaturated solution through fritting. Upon firing the enamel, titanium dioxide crystallizes or precipitates from the glassy matrix.
Trimmers or trimmer condensers employing TiB2 bodies are used for minute adjustments of capacitance. Normally, the rotor consists of a TiO2 body. Parts are made with extreme accuracy, and are usually supplied in one of three temperature coefficient types. The base is a low-loss ceramic composition.
Mechanical and physical properties of TiO2 include relatively low strength (MOR 123.5 to 150.9 MPa; tensile strength 40.8 to 54.4 MPa, low thermal conductivity (0.14 cal/cm/s/°C), and a coefficient of thermal expansion (for rutile) of 7 to 9 x 10-6/°C.
