ZERODUR®

For several decades, the unique technical properties of ZERODUR® glass-ceramic have made it ideal for a vast range of applications that rely on exceptionally low thermal expansion materials. It's also now the industry standard for ultra-precise mirror substrates in astronomy.

Low thermal expansion glass-ceramic with exceptional properties

Extremely low thermal expansion

When ZERODUR® is exposed to heat, the micro-crystals of its two-phase microscopic system contract, while their surrounding glass matrix expands. This results in an overall near-zero expansion, achieved with an accuracy down to 0 ± 7 ppb/K at ambient temperature.

Strong performance under extreme conditions

As well as its inertness to large temperature differences, ZERODUR® is suitable for applications involving mechanical loads up to 100 MPa. We can help predict its lifetime under a variety of loads using our long-term expertise and data acquisition on breakage stress.

Homogeneous and reproducible

A major asset of ZERODUR® is its exceptional 3D homogeneity, not only in terms of low CTE, but also its remarkably low level of inclusions, striae and bulk stress. SCHOTT offers these properties up to dimensions of 4.25 m.

Customized shapes

ZERODUR® is available in a variety of highly customizable shapes. Advanced grinding and polishing results in a roughness below several nanometers, which provides the perfect substrate for high-tech coatings. Furthermore, it can be light-weighted by up to 80% while keeping its mechanical stability.

Thermal Expansion

CTE tolerances of ZERODUR®

By default, the mean coefficient of thermal expansion (CTE) of ZERODUR® is measured within the temperature range of 0°C to 50°C. Five expansion classes are differentiated as follows:


CTE Grades CTE (0°C; 50°C)*

CTE Grades

CTE (0°C; 50°C)*

ZERODUR®Expansion Class 2 0 ± 0.100 ・ 10-6/K
ZERODUR®Expansion Class 1 0 ± 0.050 ・ 10-6/K
ZERODUR®Expansion Class 0 0 ± 0.020 ・ 10-6/K
ZERODUR®Expansion Class 0 SPECIAL 0 ± 0.010 ・ 10-6/K
ZERODUR®Expansion Class 0 EXTREME 0 ± 0.007 ・ 10-6/K
ZERODUR®TAILORED TAILORED ± 0.020・10-6/K (± 0.010・10-6/K upon request)
Optimized for application temperature profile

*CTE (0°C; 50°C) describes the linear mean coefficient of thermal expansion in the temperature 0°C to 50°C.

Maximum application temperature 600°C

Upon request, ZERODUR® is available for customized temperature ranges. We offer expansion class 0 or better optimized for your individual application.

 

CTE Homogeneity

Homogeneity is evaluated by measuring CTE samples homogeneously distributed throughout the blank and calculating the difference in CTE between the highest and the lowest value measured. The homogeneity of linear expansion can be guaranteed in the following weight classes:

CTE (0°C; 50°C) Homogeneity Tolerances
up to 18 tons < 0.03 ・ 10-6/K
up to 6 tons < 0.02 ・ 10-6/K
up to 0.3 tons < 0.01 ・ 10-6/K

 

CTE distribution within a 1.5 m diameter blank with a measured CTE homogeneity of 0.004 · 10-6/K

CTE distribution within a 1.5 m diameter blank with a measured CTE homogeneity of 0.004 · 10-6/K image

 

CTE tolerances of ZERODUR® K20

ZERODUR® K20, a low thermal expansion version of ZERODUR®, has been optimized to withstand higher application temperatures.

Mean coefficient of linear thermal expansion of ZERODUR®K20
CTE (20°C; 700°C) 2.4 ・ 10-6/K
CTE (20°C; 300°C) 2.2 ・ 10-6/K
CTE (0°C; 50°C) 1.6 ・ 10-6/K

Maximum application temperature 850°C
Internal Quality

If no quality is specified upon receipt of an order, then ZERODUR® will be supplied in standard quality. Individual specifications for internal quality can be fulfilled upon request.

 

Inclusions

Although the defect level is low, the main inclusions found in ZERODUR® are bubbles. During inspection of ZERODUR® parts, all inclusions with a diameter > 0.3 mm are taken into consideration. If an inclusion has a shape other than spherical, the average diameter is reported as the mean of the length and width. ZERODUR® is available in six different inclusion quality levels, which are defined according to the dimension of your part.

 

Quality levels for inclusions in ZERODUR®

Standard Class 4 Class 3 Glass 2 Class 1 Class 0
Average number of inclusions per 100 cm3 5.0 5.0 4.0 3.0 2.0 1.0
Maximum diameter of individual inclusions in mm for different diameters or diagonals of the ZERODUR® part
In the critical volume:
< 500 mm 1.4 1.2 1.0 0.8 0.6 0.4
< 2000 mm 2.0 1.8 1.6 1.5 1.2 1.0
< 4000 mm 3.0 2.5 2.0 1.8 1.6 1.5
Individual specifications upon request
In the uncritical volume:
< 500 mm 3.0 2.0 1.5 1.0 0.8 0.6
< 2000 mm 6.0 5.0 4.0 3.0 3.0 3.0
< 4000 mm 10.0 8.0 6.0 6.0 6.0 6.0
Individual specifications upon request

 

Bulk Stress

The bulk stress birefringence of ZERODUR® is recorded in path difference per thickness in inspection direction. For discs it is measured in axial direction at 5 % of the diameter from the edge. For rectangular plates, the measurement is performed in the middle of the longer side perpendicular to the plate‘s surface.

 

Quality levels for bulk stress in ZERODUR®

Bulk stress birefringence [nm/cm] for parts with diameters or diagonals Standard Class 4
< 500 mm 6 4
< 2000 mm 12 10
< 4000 mm 15 12
Individual specifications upon request

 

Striae

Additionally to the bulk stress birefringence, the stress birefringence induced by local striae is classified as a function of part diameter as follows:

Stress birefringence caused by striae [nm/striae] for parts with diameters or diagonals Standard Class 4 Class 3 Class 2 Class 1
< 500 mm 60 45 30 5
< 2000 mm 60 45 30 30 5
< 4000 mm 60 45 30 30 30
Processing

ZERODUR® is processed into complex geometries based on technical drawings and specifications from our customers. Our application and process engineers support you during the design phase of your product to get the most out of the ZERODUR® properties for your individual application. We also provide finite element modelling and special quality requirements upon request.

The 5-axis CNC grinding machines allow for precise fabrication of parts up to 4.25 m in diameter. The highlight of ZERODUR® processing is its light weighting by grinding, challenging aspect ratios of pocket heights to rib thickness used for parts with strict weight requirements.

By single-side and double-side polishing, we offer different surface quality grades for dimensions up to 500 mm. Depending on the size of your part, a roughness down to the sub-nanometer range can be achieved.

As ZERODUR® acts as a very good substrate for coating, several coatings from standard aluminum to complex customized coatings are available for parts smaller than 300 mm. Our coating experts are ready to support you in choosing the coating that matches your specifications best.

 

Proposed CNC grinding tolerances for dimensions and shapes

*tighter tolerances depend on the size and geometry. They cannot be combined freely.
** according ISO 1101
Dimension < 2000 mm Dimension≤ 4000 mm
Tolerances [mm] Tighter tolerances [mm]* Tolerances [mm] Tighter tolerances [mm]*
Length, width, height ± 0.3 ± 0.1 ± 0.4 ± 0.2
Diameter ± 0.3 ± 0.1 ± 0.4 ± 0.2
Angle ± 5’ ± 1’ ± 5’ ± 1’
Flatness ** 0.1 - 0.2 0.05 0.2 0.1
Cylindricity ** 0.1 0.05 0.2 0.1
Profile ** 0.2 0.1 0.4 0.2
Parallelism ** 0.1 - 0.2 0.05 0.2 0.1
Position ** 0.1 0.05 0.2 0.1
Concentricity ** 0.1 0.05 0.2 0.1
Run-out ** 0.1 0.05 0.2 0.1
Physical Properties

Bending stress and lifetime calculation

ZERODUR® is the material of choice when it comes to excellent thermal properties and precision in high-tech applications. Often these applications also require to withstand certain mechanical loads, such as continuously as in telescope mirror holders or short-term during rocket launch.

The key factor to be evaluated to quantify the breakage stress of ZERODUR® is the surface quality, especially the occurrence of microcracks. In general, applying loads below 10 MPa tensile stress does not demand any special breakage analysis of ZERODUR®.

SCHOTT’s exhaustive data on the breakage events of ground ZERODUR® samples has shown that it can withstand long-term (tens of years) mechanical loads of 30 to 100 MPa. This is much higher than previously predicted. Using a three-parameter Weibull distribution, we are happy to discuss the ZERODUR® lifetime under your individual long-term mechanical loads.

Typical mechanical and optical properties
ZERODUR® ZERODUR® K20

Thermal Conductivity λ at 20°C [W/(m・K)]

1.46

1.63

Thermal diffusivity index a at 20°C [10-6m2/s]

0.72

-

Heat capacity cat 20°C [J/(g · K)]

0.80

0.90

Young's modulus E at 20°C [GPa]-mean value

90.3

84.7

Poisson‘s ratio

0.24

0.25

Density [g/cm3]

2.53

2.53

Knoop Hardness HK 0,1/20 (ISO9385)

0.1 - 0.2

0.05

Refractive index nd

0.1

0.05

Abbe number νd

0.1

0.05

Internal transmittance at 580 nm

 0.1  0.05

5 mm thickness

0.95

-

10 mm thickness

0.90

-

Stress optical coefficient K at λ = 589.3 nm [10-6MPa-1]

3.0

-

Electrical resistivity ρ at 20°C [Ω · cm]

2.6 · 1013

-

Tk100 [°C], Temperature for ρ = 10[Ω · cm]

178

-

Chemical Properties

At room temperature, most acids, alkalis, salts and dye solutions leave no residual traces on ZERODUR® surfaces. It can be etched by hydrofluoric acid as well as concentrated sulfuric acid at elevated temperatures. Furthermore, construction materials such as mica, chamotte, MgO and SiO2 do not react noticeably with ZERODUR® (up to 600°C for 5h). By contrast, enamel reacts above 560°C by having its surface destroyed. 

Based on the good chemical resistance of the material, coatings such as mirrors are removable in a reproducible manner. The polished surface is simply cleaned and recoated by an optimized protocol.

Typical chemical properties
ZERODUR® ZERODUR®K20
Hydrolytic resistance class (ISO 719) HGB 1 -
Acid resistance class (ISO 8424) 1.0 -
Alkali resistance class (ISO 10629) 1.0 -
Climate resistance Class 1 -
Stain resistance Class 0 -
Helium permeability [Atoms/(cm · s · bar)]
at 20°C 1.6 · 106 -
at 100°C 5.0 · 107 -
at 200°C 7.2 · 108 -

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