Mechanical Properties of Optical Materials

Thermal

Units

Fused Quartz

Fused Silica

BK7

Pyrex®

Sapphire

Softening Point

°C

1180

1120

550

565

Annealing Point

°C

1180

1120

550

565

Strain Point

°C

1075

1025

624

515

Maximum Temperature (Continuous)

°C

1000

950

350

230

1100

Maximum Temperature (Momentary)

°C

1300

1200

490

Thermal Conductivity

@ 20° C

cal  cm

cm²sec°C

0.0033

0.0033

0.0026

0.0027

0.0065

@ 200°C

0.0037

0.0037

@ 950° C

0.0064

0.0064

Specific Heat

0°–100° C

cal

g °C

0.18

0.18

0.21

0.2

0°–900° C

0.25

0.25

Coefficient of Thermal Expansion

-50°–0° C

cm
 cm °C

2.7 × 10^-7

2.7 × 10^-7

0°–300° C

5.9 × 10^-7

5.9 × 10^-7

86 × 10^-7

33 × 10^-7

0°–900° C

4.8 × 10^-7

4.8 × 10^-7

90.3 × 10^-7

Strength/Hardness

Moh Hardness

5.5–6.5

5.5–6.5

5

9

Microhardness

kg/cm²

9.4 × 10⁴

9.4 × 10⁴

Knoop Hardness (100g load)

kg/mm²

605

615

520

481

1525–2000

 Density

g/cm³

2.203

2.203

Specific Gravity

2.203

2.201

2.53

2.23

3.98

Compressive Strength

kg/cm²

1.13 × 10⁴

1.13 × 10⁴

2.04 × 10⁴

 Tensile Strength

kg/cm²

500

500

282

7000

Torsional  Strength

kg/cm²

300

300

1.5 × 10⁶

Bending Strength

kg/cm²

680

680

Young's Modulus

kg/cm²

7.45 × 10⁵

7.45 × 10⁵

8.63 × 105

7 × 10⁶

79 × 10⁵

Shear Modulus

kg/cm²

3.2 × 10⁵

3.2 × 10⁵

Bulk Modulus

kg/cm²

3.76 × 10⁵

3.76 × 10⁵

Poisson's Ratio

0.17

0.17

0.208

0.2

0.02

Electrical

Dielectric Strength

@ 20° C

kv/cm

250–400

250–400

480

@ 500° C

150–200

150–200

Dielectric Constant

(0–1Mhz)

3.7

3.7

5.1

8.6–10.6

Dielectric Loss Tangent 

1 MHz

1 × 10^-4

1 × 10^-4

2.6 × 10^-2

10² MHz

1 × 10^-4

1 × 10^-4

3 × 10⁴ MHz

4 × 10^-4

4 × 10^-4

Resistivity

@ 20° C

Ω/cm

1 × 10¹⁸

1 × 10¹⁸

@ 600° C

8 × 10⁷

8 × 10⁷

1 × 10¹¹

@ 1000°C

1 × 10⁶

1 × 10⁶

1 × 10⁶

All properties measured at 20° C unless otherwise specified.

Striae and Homogeneity

Material Code

Type

Striae/MIL-G-174A

Index Variation Δn(x10^-6)

S1-UVA

Fused Silica

A

6

S1-UVB

Fused Silica

A

10

A1

Fused Quartz or Fused Silica

B

10

G1

Fused Quartz or Fused Silica

C-D

20

I2-IR

Fused Quartz

A

4

BK7

Optical Glass

A

10

Bubbles & Inclusions

up to 1.0" dia.

from 1.0" to 2.0" dia.

from 2.0" to 3.0" dia.

Material

Max. Dia.

Max. Qty.

Max. Dia.

Max. Qty.

Max. Dia.

Max. Qty.

(inches)

per pc.

(inches)

per pc.

(inches)

per pc.

S1-UVA

0.006

0

0.008

1

0.01

1

S1-UVB

0.008

1

0.015

1

0.015

1

A1

0.01

1

0.015

1

0.02

2

G1

0.03

3

0.04

3

0.06

3

I2-IR

0.01

3

0.015

3

0.02

3

BK-7

0.006

0

0.006

0

0.01

1

* These specifications are worst case conditions.

* Materials can be selected and guaranteed to customer's requirements.

* Bubbles less than the maximum diameter count as a fraction of the allowed limit.

* Bubbles less than 0.004" are regarded as undetectable.