Ultra low expansion glass (ULE) is a registered trademark of

Corning Incorporated Corning Incorporated is an American multinational technology company specializing in glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The company was name ...

. ULE has a very low

coefficient of thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

and contains as components

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

and less than 10%

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...

. Such high resistance to thermal expansion makes it very resistant to high temperature thermal shock. ULE has been made by Corning since the 1960s, but is still very important to current applications.

Applications

There are many applications for ULE, but by far the most common is for mirrors and lenses for telescopes in both space and terrestrial settings. One of the most well known examples of the use of ULE is in the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

's mirror. Another good example of its application is in the

Gemini telescope The Gemini Observatory comprises two 8.1-metre (26.6 ft) telescopes, Gemini North and Gemini South, situated in Hawaii and Chile, respectively. These twin telescopes offer extensive coverage of the northern and southern skies and rank among ...

's mirror bank. This type of material is needed for this application because the mirrors on telescopes, especially very large, high-precision units, cannot bend or lose their shape even slightly. If this were to happen, the telescope would be out of focus. Some other examples and uses of ULE are: * Ultra-low expansion substrates for

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

s and other optics * Length standards * Lightweight honeycomb

mirror mount A mirror mount is a device that holds a mirror. In optics research, these can be quite sophisticated devices, due to the need to be able to tip and tilt the mirror by controlled amounts, while still holding it in a precise position when it is n ...

s * Astronomical

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

s * Precision measurement technology * Laser cavities Another newer use for this material that is showing promise is in the semiconductor industry, this again because of the purity and extreme low expansion of ULE glass.

Structure

The structure of ULE is completely amorphous; because of this it is a glass, not a ceramic. The amorphous structure of the material comes from there being no crystal phases within the structure, so there is no long range order.

Processing

The way that ULE is made is very different from the standard way that glass is made. Instead of mixing powdered materials together into a batch, melting that batch and pouring out sheets of glass, ULE, being such a high temperature glass, has to be made in a flame hydrolysis process. In this process high purity precursors are injected into the flames, which causes them to react and form TiO₂ and SiO₂. The TiO₂ and SiO₂ then fall down and are deposited onto the growing glass.

Properties

Thermal

Ultra low expansion glass has an coefficient of thermal expansion of about 10⁻⁸/K at 5–35 °C. It has a

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

of 1.31 W/(m·°C), thermal diffusion of 0.0079 cm²/s, a mean

specific heat In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...

of 767 J/(kg·°C), a strain point of 890 °C 634 °F and an estimated softening point of 1490 °C 714 °F an annealing point of 1000 °C 832 °F

Mechanical

Ultra low expansion glass has an

ultimate tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate t ...

of , a Poisson’s ratio 0.17, a density of (), a specific stiffness of (), a

shear modulus In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the Elasticity (physics), elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear s ...

of (), a

bulk modulus The bulk modulus (K or B or k) of a substance is a measure of the resistance of a substance to bulk compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume. Other mo ...

of (), and an

elastic modulus An elastic modulus (also known as modulus of elasticity (MOE)) is a quantity that describes an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. Definition The elastic modu ...

of ().

Optical

ULE has a Stress Optical Coefficient of 4.15 (nm/cm)/(kg/cm³) .292(nm/cm) psi and an

Abbe number In optics and lens design, the Abbe number, also known as the Vd-number or constringence of a Transparency (optics), transparent material, is an approximate measure of the material's dispersion (optics), dispersion (change of refractive index versu ...

of 53.1.

Other

ULE has a high resistance to weathering because of its hardness, and is also unaffected by nearly all chemical agents. ULE also shows no changes when quickly cooled from 350 °C.

References

{{Glass science Glass compositions Low thermal expansion materials