Technical specifications of Zerodur mechanical engineering

Zerodur has extraordinary properties. Zerodur is a glass ceramic that Schott has developed for optical, optp-electronic and precision engineering applications.Thhrough intensive material research and technology development this glass ceramic has become a material with extraordinary properties that can be supplied in blocks weighing several tons or as small components, both with unequalled precision.

For this reason zerodur has become a performance and quality determining factor in many spectacular applications in modern technology.

Mirror substrates for large astronomical telescopes.
Mirror substrates for x-ray telescopes in satellites.
Optical elements for comet probes, weather satellites and microlithography.
Frames and mirrors for ring laser gyroscopes.
Distance gauges in laser resonators.
Measurement rods as standards for precision measurement technology.

What is Zerodur and how is it made

Zerodur is an inorganic, non-porous material which has a crystalline phase and a glass phase.

Using methods common in glass technology, suitable raw materials are melted, refined, homogenized and then hot formed.

After the cooling and annealing of the glassy blank a temperature treatment follows in which the glass is transformed into a glass ceramic through controlled volume crystallization.

During this temperature treatment nuclei form within the glass, and crystals subsequently grow at a somewhat higher temperature.The crystalline phase and the glass phase together then lend the glass ceramic zerodur its special properties.

Zero expansion

Zerodur contains 70-78 weight percent crystalline phase with a high quartz structure.

This has a negative linear thermal expansion, while that of the glass phase is positive.The special composition of the base glass for the glass ceramic zerodur and defined nucleation and crystallization conditions result in a material with extremely low thermal expansion which in certain temperature ranges can be zero or slightly negative, depending on the ceramization program.

Homogeneity and quality can be seen in zerodur

Zerodur is a material of the highest homogeneity, and even in large pieces with dimensions of several meters displays thermal and mechanical characteristics with nearly unmeasureble deviations.

The very small crystals and the low difference in refractive index between the crystalline and glass phase result in good transparency in the special range from approximately 0,4um to 2,3 um, a prerequisite for being able to test the internal quality of the material.

No bubble, no inclusion and no strain remains undetected.

Thus both tiny precision parts and massive blocks made from zerodur uniformly exhibit the same precise behaviour.

Zerodur, unusual in its properties easy to fabricate

Engineers and scientists are continually discovering the glass ceramic zerodur as an ideal material for their very special tasks which call for the highest precision.

Zerodur has a completely non-directional structure, and it possesses a non-porous surface.

Crystalline and glass phases have chemical characteristics and hardness that are similar to those of optical glasses so that zerodur can be processed using the same machines and tools as optical and technical glasses.

Therefore, zerodur is the ideal mirror substrate for astronomical telescopes and x-ray telescopes where changes in the mirror caused by temperature would influence the quality of the observations.Zerodur assures that better optical performance can be achieved using smaller and lighter mirrors.

In ring laser gyroscopes zerodur forms the temperature-independent frame.The low helium permeability provides for long life. In microlithography mirror substrates and other important optical components from zerodur allow for precise imaging of structures in the manufacturing of microchips.

In laser technology, too, zerodur’s unusual thermal stability is proven. Zerodur rods used as distance standards assure stable, exact calibration of the resonator despite temperature deviations during operation and increase the performance capability of the laser.

Product Information

Zerodur® is a glass ceramic with an extremely low thermal expansion coefficient. The most important properties of Zerodur® are:

Nearly zero thermal expansion with outstanding 3D homogeneity
High internal quality
Good processing behaviour
Can be polished to a very high accuracy
Can be coated easily
Low Helium permeability
Non-porous
Good chemical stability

Applications

Astronomy

For more than 30 years, Zerodur® has been the favored material for mirror substrates of earthbound and orbital telescopes.

Zerodur® can be made in large amounts with reproducible quality and is therefore suited for future extremely large telescopes (ELTs) in the 20 m to 100 m class.

Modern fabrication technologies enable the production of structures with more than 65% weight reduction. Zerodur® is the ideal mandrel material for the shaping of mirror shells in future x-ray telescope projects due to the excellent homogeneity of its coefficient of thermal expansion.

Microlithography

Zerodur® is used as a movable mechanical part in wafer stepper and scanner machines to achieve precise and reproducible wafer positioning. Zerodur® is an ideal substrate material for reflective optics in the forthcoming EUV lithography due to its almost zero thermal expansion, outstanding homogeneity and good processing behavior.

Measurement Technology

The extremely low thermal expansion and proven long-term dimensional stability of Zerodur® make it an ideal reference standard for measurement instruments.

Mechanics

Zerodur® can be produced in increasingly complex geometries using a combination of modern processing technologies.

Further Applications

Zerodur® has very good transmission properties in the optical and infrared spectral range and a high optical homogeneity. Because of these properties Zerodur® is also often used in optical systems.

Forms of Supply

Zerodur® can be supplied in the form of discs, rectangular blocks, prisms, rods and cut pieces measuring from a few cm up to approximately 4m in length. Modern CNC processing equipment and a variety of grinding technologies allow the generation of complex geometries and filigree structures at the customer’s request.

Specifications

Thermal Expansion Coefficient

Individual pieces of Zerodur® (discs, plates, rods) can be supplied with a mean coefficient of linear thermal expansion in the temperature range 0° to 50°C in three expansion classes as follows:

Expansion class 2 0 ± 0.10*10-6 K-1
Expansion class 1 0 ± 0.05*10-6 K-1
Expansion class 0 0 ± 0.02*10-6 K-1

Zerodur® exhibits excellent homogeneity of the linear thermal expansion coefficient. Typical values are between 0.01 and 0.02*10-6 K-1. On request homogeneities < 0.01*10-6 are possible.

Zerodur® may be used as a mechanical component as well as a window at temperatures up to 600 °C. For applications up to 850°C Zerodur® K20 was developed as a new material modification. The material has an expansion coefficient of 2.0*10-6 K-1 between 20° - 700° C and 1.5*10-6 K-1 at room temperature.

Internal Quality

The following applies for the internal quality of “standard” Zerodur® with typical dimensions < 500 mm:

Average number of inclusions 5 / 100 cm3
Maximum diameter of single inclusions 3.0 (1.4 within the critical volume)
Striae (birefringence) < 60 nm / striae
Bulk stress (birefringence) < 6 nm / cm

Further Properties

Density 2.53 g/cm3
Thermal conductivity at 20°C 1.46 W/(m*K)
Thermal diffusivity at 20°C 0.72*10-6 m2/s
Thermal capacity 0.8 J/(g*K)
Young’s modulus at 20°C 90.3 GPa
Poisson’s ratio 0.243
Knoop hardness 0.1/20 620

Refractive Index

Wavelength (nm)	Fraunhofer Designation	Refractive Index n
656.3	C	1.5394
643.8	C'	1.5399
587.6	d	1.5424
546.1	e	1.5447
486.1	F	1.5491
480.0	F'	1.5497
435.8	g	1.5544

Comparison of thermal expansion coefficients of ZERODUR® and fused silica

The figure below shows the variation of expansion coefficient with temperature for a typical sample. The actual performance varies very slightly, batch to batch, with the room temperature expansion coefficient in the range of ± 0.15 x 10-6/ºC. By design, this material exhibits a change in the sign of the coefficient near room temperature. A comparison of the thermal expansion coefficients of ZERODUR and fused silica is shown in the figure. ZERODUR, is markedly superior over a large temperature range, makes ideal mirror substrates for such stringent applications as multiple-exposure holography, holographic and general interferometry, manipulation of moderately powerful laser beams, and space-borne imaging systems.

Zerodur® Constants

Abbé Constant: v_d = 66
Dispersion: (n_f - n_c) = 0.00967)
Density: 2.53 g-cm^-3 at 25°C
Young's Modulus: 9.1 x 10⁹ dynes/mm²
Poisson's Ratio: 0.24 Specific Heat at 25°C: 0.196 cal/g-°C
Coefficient of Linear Expansion (20° to 300°C) : 0.05 ± 0.10 x 10^-6/°C
Maximum Temperature: 600°C