Fused quartz glass, synthesized from high-purity silica sand or silicon tetrachloride, is an amorphous, single-component material renowned for its exceptional combination of properties derived from its strong silicon-oxygen bonds and non-crystalline structure.

• Physical Properties: Its most defining characteristic is an extremely low coefficient of thermal expansion (≈0.55 x 10⁻⁶/°C), making it virtually immune to thermal shock. This allows it to withstand rapid temperature changes from over 1000°C to room temperature without cracking. It has a high softening point (~1680°C) and excellent long-term dimensional stability. While hard and rigid, it behaves as a brittle solid.
• Chemical Properties: Fused quartz exhibits superior chemical purity and inertness. It is non-hygroscopic and offers outstanding resistance to most acids, salts, and halogens (except hydrofluoric acid and hot phosphoric acid). Its high purity minimizes contamination in sensitive processes, and it shows very low rates of weathering or leaching in water and atmospheric exposure compared to other glasses.
• Optical Properties: It is a premier optical material with a wide, continuous transmission range from the deep ultraviolet (~170 nm) through the visible spectrum and into the near-infrared (~2.7 μm, with OH-dependent absorption peaks). Its high transparency and low autofluorescence are critical for UV applications. It possesses a low refractive index (~1.458 at 587.6 nm) and exhibits minimal birefringence under stress.

In summary, fused quartz glass is an enabling material where extreme environmental challenges meet the need for precision. Its unique synergy of near-zero thermal expansion, supreme chemical resistance, and broad-spectrum optical transparency makes it indispensable in semiconductor fabrication, precision optics, laser systems, laboratory ware, and aerospace instrumentation.

Sapphire glass, technically single-crystal aluminum oxide (Al₂O₃), is an exceptional synthetic material prized for its extreme durability and high performance across demanding applications.

Physical Properties: Its most renowned attribute is exceptional hardness, ranking 9 on the Mohs scale, second only to diamond. This grants outstanding scratch and wear resistance. It possesses high mechanical strength, excellent stiffness, and good thermal conductivity. While it has a high melting point (2050°C), its thermal expansion coefficient is moderate (~6 x 10⁻⁶/K), requiring careful design for thermal shock management despite its good high-temperature stability.

Chemical Properties: Sapphire exhibits superior chemical inertness. It is highly resistant to most acids, alkalis, and corrosive agents at room temperature, with notable exceptions being hydrofluoric acid and hot phosphoric acid. It demonstrates excellent resistance to weathering, oxidation, and erosion from molten metals and salts, ensuring long-term stability in harsh environments.

Optical Properties: It is a premier wide-band optical material with a broad transmission windowspanning from the deep ultraviolet (~150 nm) to the mid-infrared (~5.5 μm). This makes it ideal for UV, VIS, and IR applications. It has high refractive index (~1.76 at 550 nm) and excellent optical clarity. A key characteristic is its birefringence (double refraction), which must be accounted for in precision optical systems by using specific crystal orientations (like the c-plane).

In summary, sapphire glass combines unparalleled surface hardness, superb chemical resilience, and excellent optical transmission. This unique synergy makes it the material of choice for high-end watch crystals, semiconductor wafer carriers, armored windows, aerospace sensors, and robust laser optics.

Borosilicate glass is a specialized silicate glass distinguished by its significant boron trioxide (B₂O₃) content, typically 12-15%. This composition is the key to its unique and valuable set of characteristics, making it a staple in scientific, industrial, and domestic applications.

• Physical Properties: The defining physical feature of borosilicate glass is its very low coefficient of thermal expansion (≈3.3 x 10⁻⁶/K). This property, approximately one-third that of ordinary soda-lime glass, grants it excellent resistance to thermal shock. It can withstand rapid and extreme temperature differentials without fracturing, enabling direct transfer from a hotplate to a bench. It has a higher softening point (~820°C) than common glass, enhancing its thermal durability.
• Chemical Properties: This glass exhibits high chemical durability and inertness. It is highly resistant to water, acids, halogens, and organic solvents, outperforming standard glass in withstanding chemical corrosion. Its low alkali content minimizes leaching or ion exchange, making it ideal for storing pharmaceutical and chemical reagents. However, it is vulnerable to prolonged exposure to hydrofluoric acid, hot concentrated phosphoric acid, and strong alkaline solutions.
• Optical Properties: Standard borosilicate glass offers good transparency in the visible spectrum, though its transmission range is narrower than fused quartz or sapphire. It typically transmits well from around 350-400 nm in the near ultraviolet through the visible and into the near-infrared (~2.5 µm). While not used for high-precision optics, its clarity is sufficient for laboratory glassware, sight glasses, and lighting applications. Special low-iron formulations can achieve higher clarity for enhanced optical performance.

In summary, borosilicate glass’s balanced and practical combination of superior thermal shock resistance, strong chemical stability, and adequate transparency makes it the material of choice for laboratory equipment (e.g., beakers, flasks), high-quality cookware, industrial sight glasses, and technical lighting components, where durability under thermal and chemical stress is paramount.

1. Overview & Classification

K9 (China GB Standard) / BK7 (International Schott Standard) is the most widely used optical crown glass in the world. It belongs to the borosilicate crown family and serves as the industry benchmark for general-purpose optical components.

Key Identifiers:

• China GB: K9

• International (Schott): N-BK7

• Equivalent Names: BSC7, BSC-7, 517642 glass

• Glass Type: Crown Glass (Low dispersion, high Abbe number)

 

2. Chemical Composition (Typical Weight %)

Component	Content (%)	Function
SiO₂ (Silica)	~69%	Glass former, provides structure
B₂O₃ (Boron Oxide)	~10%	Reduces thermal expansion, improves chemical resistance
Na₂O (Sodium Oxide)	~8%	Flux, lowers melting temperature
K₂O (Potassium Oxide)	~8%	Flux, improves chemical durability
BaO (Barium Oxide)	~3%	Increases refractive index
Others (CaO, Al₂O₃)	~2%	Stabilizers, improve durability

 

3. Critical Optical & Physical Properties

A. Optical Constants (@ 587.6 nm, 20°C)

Parameter	Value	Unit	Significance
Refractive Index (nₑ or n₄)	1.51680	–	Primary design parameter
Abbe Number (νₑ or ν₄)	64.17	–	Measures dispersion (higher = less chromatic aberration)
Transmission Range	330 – 2100	nm	Useful spectral window
Internal Transmittance	>99.8% @ 400mm thickness, 400-700nm	–	Exceptional clarity

B. Thermal Properties

Property	Value	Unit
Thermal Expansion Coefficient (α)	7.1 × 10⁻⁶	K⁻¹ (20-300°C)
Thermal Conductivity	1.114	W/(m·K)
Specific Heat Capacity	858	J/(kg·K)
Transformation Temperature (Tg)	557	°C
Annealing Point	560	°C
Strain Point	516	°C

C. Mechanical & Chemical Properties

Property	Value	Unit/Notes
Density	2.51	g/cm³
Knoop Hardness	610	HK₀.₁/₂₀
Young’s Modulus	82	GPa
Poisson’s Ratio	0.206	–
Chemical Resistance	Class 2 (ISO 8424)	Good resistance to acids, water
Climate Resistance	Class 1 (ISO 9022)	Highly resistant to humidity

 

4. Key Performance Characteristics

Advantages:

1. Excellent Optical Homogeneity (Δn ≤ ±2 × 10⁻⁶)

2. High Transmission with minimal absorption across visible spectrum

3. Low Stress Birefringence (< 10 nm/cm)

4. Good Chemical Stability and weathering resistance

5. Excellent Bubble & Inclusion Class (typically Class 0-1)

6. Superior Processability – easy to cut, grind, polish, and coat

7. Cost-Effective – most economical precision optical glass

Limitations:

1. Moderate Thermal Expansion – not suitable for extreme thermal cycling applications

2. Vulnerable to Alkaline Solutions – prolonged exposure can cause surface degradation

3. Not Laser-Grade – contains trace impurities that can cause absorption at high power densities

 

5. Standard Specifications & Quality Grades

Grade	Designation	Typical Applications	Homogeneity
Standard	N-BK7 / K9	General optics, imaging lenses	±2 × 10⁻⁶
Precision	N-BK7 H5	Precision instruments, microscopy	±5 × 10⁻⁷
High Homogeneity	N-BK7 HX	Interferometry, high-end imaging	±1 × 10⁻⁷
Laser Grade	(Special melt)	Low-power laser applications	±2 × 10⁻⁶

Bubble Classes: Typically 0-1 (Schott scale: 0 = none, 5 = many)
Striae: Class 1-2 (virtually free of striations)

 

6. Primary Applications

A. Imaging & Photography

• Camera lens elements (especially single-lens reflex and compact cameras)

• Telescope objective lenses and eyepieces

• Microscope objectives and condensers

• Projection lens systems

• Binocular prisms and lenses

B. Industrial & Measurement

• Windows for sensors and instruments

• Prisms for spectrometers and monochromators

• Beamsplitters and mirrors (with coating)

• Reference flats for interferometry

• Sight glasses and viewports

C. Consumer & Commercial

• Magnifying glasses and loupes

• Eyeglass lenses (especially for high-prescription applications)

• Decorative optical elements

• Light guide plates for displays

 

7. Processing Guidelines

Machining Parameters:

• Grinding: Diamond wheels, 15-25 µm grit for fine grinding

• Polishing: Cerium oxide or zirconium oxide slurry, pH 6.5-7.5

• Annealing: Slow cool from 560°C at 0.5-2°C/hour to 300°C

• Coating Compatibility: Excellent adhesion with MgF₂, AR, mirror coatings

Critical Notes:

1. Thermal Shock Limit: ΔT ≈ 100°C (avoid rapid temperature changes >50°C/min)

2. Acid Resistance: Good against most acids except hydrofluoric and phosphoric

3. Alkali Sensitivity: Avoid prolonged contact with strong alkaline solutions

4. Cleaning: Use mild detergents, avoid abrasive cleaners

 

8. Comparison with Similar Materials

 

Technical Summary: K9/BK7 represents the optimal balance of optical performance, manufacturability, and cost for the majority of precision optical applications. Its consistent quality, wide availability, and extensive processing history make it the default choice for optical designers worldwide when no special properties (e.g., extreme UV transmission, very low thermal expansion) are required.

Feature/Dimension	ITO (Indium Tin Oxide)	FTO (Fluorine-doped Tin Oxide)
Material	Composite of Indium Oxide (In₂O₃) and Tin Oxide (SnO₂) , typically 90% : 10%.	Based on Tin Oxide (SnO₂) , doped with Fluorine (F) .
Conductivity	Extremely low resistivity, approx. 1.8~8 × 10⁻⁴ Ω·cm (better conductivity).	Slightly higher resistivity, approx. 6~8.5 × 10⁻⁴ Ω·cm.
Transparency	Higher, visible light transmittance can reach 85%-96% (excellent optical performance).	Good, visible light transmittance is typically around 80%-87%.
Heat Resistance	Poor. Conductivity decreases significantly in high temperatures above 300°C; structure is easily damaged.	Excellent. Can withstand high-temperature processes up to 500°C with stable performance.
Surface Morphology	Very flat and smooth.	Has slight凹凸 (texture) , which helps increase light scattering.
Chemical Stability	Good, resistant to general acids and alkalis.	Excellent, remains stable in strong acid and strong alkali environments.
Cost & Raw Material	High cost. Uses the rare metal Indium (In) , which is scarce and expensive.	Low cost. Raw materials (Tin, Fluorine) are abundant and cheap.
Main Applications	– LCD/OLED Displays – Mobile phone touch screens – High-end electronics requiring high optical performance	– Perovskite/Dye-sensitized solar cells – Smart glass, Electrochromic devices – Specialized environments requiring high heat and chemical resistance