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Precision Borosilicate Glass Processing

We craft custom glass parts through expert hand-blowing and die-pressing. From prototype to mass production, our streamlined process ensures fast turnaround without compromising quality.

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High-Borosilicate Glass Solutions: Superior Thermal & Chemical Resistance for Demanding Applications

Borosilicate glass is a cornerstone of advanced material science, engineered through the fusion of silica and boron trioxide. This composition grants it unparalleled thermal shock resistance, exceptional chemical durability, and high electrical resistivity. Its inherent stability and optical clarity make it an indispensable solution for the most demanding applications across healthcare, pharmaceuticals, high-performance lighting, and industrial processes, where reliability under extreme conditions is paramount.

Max Part Size (X/Y/Z):

 500 x 500 x 500 mm

Dimensional Tolerance:

 ±0.2 mm

Minimum Wall Thickness:

 2 mm

Borosilicate Glass for Blowing & Pressing

Our high-borosilicate glass is expertly shaped using specialized hot-forming techniques. The process involves either hand-blowing tubes and rods or press-molding softened glass in precision molds. These methods leverage the material’s inherent thermal stability and low coefficient of thermal expansion. This ensures the production of complex components with superior chemical homogeneity, excellent internal quality, and a defect-free finish, guaranteeing optimal performance and durability.
Property Category Specific Parameter Typical Value / Description
Chemical Composition Silicon Dioxide (SiO₂) ~81%
Chemical Composition Boron Oxide (B₂O₃) ~13%
Chemical Composition Other Oxides (Na₂O, Al₂O₃, etc.) ~6%
Mechanical Properties Knoop Hardness ~480 - 540 kg/mm²
Mechanical Properties Modulus of Elasticity 64 - 67 GPa
Mechanical Properties Density 2.23 g/cm³
Thermal Properties Coefficient of Thermal Expansion 3.25 × 10⁻⁶ /K (20-300°C)
Thermal Properties Annealing Point ~560°C
Thermal Properties Softening Point ~820°C
Thermal Properties Maximum Service Temperature ~450°C (short-term up to 500°C)
Optical Properties Refractive Index 1.47 (at 587.6 nm)
Optical Properties Transmittance >90% (across visible spectrum, for standard thickness)
Electrical Properties Dielectric Constant 4.6 (at 20°C, 1 MHz)
Electrical Properties Volume Resistivity 10⁸ Ω·cm (at 350°C)
Chemical Durability Hydrolytic Resistance (ISO 719) Class 1 (extremely low alkali release)
Chemical Durability Acid Resistance (ISO 195) Class 1 (highly resistant)
Chemical Durability Alkali Resistance (ISO 695) Class A2 (moderately resistant)

Urface Finishing for Borosilicate Glass

Enhance the functionality and aesthetics of borosilicate glass with our custom surface finishing options. We offer professional post-processing services including painting and electroplating for durable color, as well as sandblasting and matte coating for a uniform frosted finish. Lenses can also be coated to add specific properties like anti-reflection. These treatments improve visual appeal, provide privacy, and strengthen brand identity, making them ideal for consumer, industrial, and decorative applications.
Painting & Electroplating
Description & Features: Applies durable, opaque color layers. Electroplating offers a metallic finish.
Advantages: Excellent color fastness and scratch resistance for branding and aesthetics.
Applications: Consumer goods, decorative items, and architectural glass.
Sandblasting & Matte Coating
Description & Features: Creates a uniform, frosted surface by abrasion or a coated layer.
Advantages: Provides light diffusion, reduces glare, and ensures visual privacy. Surface roughness (Ra) can be customized from 1.0 to 5.0 µm.
Applications: Lighting shades, privacy screens, and laboratory equipment.
Lens Coating (e.g., AR Coating)
Description & Features: Deposits thin-film layers (e.g., MgF₂) using vacuum deposition.
Advantages: Increases light transmission to >99.5% and reduces reflectivity for enhanced optical performance.
Applications: High-precision lenses, optical sensors, and instrument covers.

Borosilicate Glass: Advantages & Challenges for Demanding Applications

Borosilicate glass offers superior thermal shock resistance, chemical durability, and optical clarity, ideal for harsh environments. However, it presents challenges like higher cost, complex processing, and impact sensitivity, which are key considerations for project selection.

Benefits

Exceptional Thermal Shock Resistance
With a very low coefficient of thermal expansion (typically around 3.3 x 10⁻⁶/K), it withstands rapid and extreme temperature changes without cracking, ideal for laboratory ware and high-temperature lighting.

High Service Temperature & Thermal Stability
It maintains structural integrity and properties at continuous operating temperatures far exceeding those of soda-lime glass, suitable for applications up to 450°C and higher.

Superior Chemical Durability
Highly resistant to water, acids, alkalis, and pharmaceutical substances, ensuring purity and longevity in aggressive chemical environments and medical applications.

Excellent Mechanical Strength
Its inherent hardness provides high resistance to scratching and abrasion, ensuring long-term clarity and durability in demanding use cases.

Outstanding Optical Clarity & Purity
Offers high light transmission with minimal inherent fluorescence, making it perfect for precision optics, sensitive sensors, and high-quality lighting systems.

Cost-Effective for Complex Shapes
While the raw material cost is higher, its excellent hot-workability through blowing and pressing allows for the formation of complex geometries using relatively simple and affordable metal molds, enabling efficient prototyping and small batches.

Limitations & Challenges of Borosilicate Glass

Higher Material & Processing Cost
The raw materials, particularly boron oxide, are more expensive than those for soda-lime glass. Its high melting temperature also requires more energy, increasing production costs.
Increased Processing Difficulty
Its high softening point and working temperature demand specialized, high-temperature furnaces and skilled operators, making forming processes like blowing and pressing more challenging.
Lower Damage Tolerance to Impact
While highly resistant to thermal shock, its hardness can make it more brittle and susceptible to mechanical impact or sharp point loads compared to some other materials.
Challenges in Complex Shape Forming
Its short working range (between softening and melting) can limit the ability to form highly intricate or delicate geometries that are possible with other types of glass.
Difficulty in Secondary Processing
Machining, cutting, or drilling after annealing is difficult due to its high hardness, often requiring diamond tools and specialized techniques, which increases the risk of breakage.
Limited Supplier Availability
Compared to the ubiquitous soda-lime glass, the production and supply chain for high-quality borosilicate glass are more specialized and limited, which can affect lead times and sourcing.

Application Fields of Borosilicate Glass

Borosilicate glass is the material of choice for applications demanding exceptional thermal shock resistance, high chemical durability, and outstanding optical clarity. Its stability under extreme conditions and high purity make it indispensable for laboratory ware, pharmaceutical packaging, high-performance lighting, and specialized industrial equipment.
Laboratory & Chemical Processing Glassware
Borosilicate glass is indispensable in laboratories and chemical plants due to its supreme chemical inertness and resistance to thermal shock. It ensures purity and integrity in demanding processes, from withstanding aggressive acids and alkalis in reactors to enabling clear visibility in sight glasses and durable performance in complex apparatus like bioreactors.

Pharmaceutical Packaging
Chosen for its high chemical durability and thermal stability, borosilicate glass is the premium material for pharmaceutical containers like vials and ampoules. Its inert nature prevents interaction with sensitive drugs, ensuring product purity and stability throughout sterilization (e.g., autoclaving) and the entire shelf life.

High-Performance Lighting
With its excellent light transmission and superior resistance to heat, borosilicate glass is ideal for high-power lighting lenses and covers. It maintains clarity and structural integrity under intense heat from LEDs or HID lamps, preventing deformation or failure where ordinary glass would not survive.

Specialized Industrial Equipment
In demanding industrial settings, borosilicate glass provides critical viewports, sight glasses, and protective covers. Its ability to withstand high pressure, rapid temperature changes, and corrosive environments ensures safe observation and reliable operation in equipment from industrial ovens to chemical processing units.

FAQ: Borosilicate Glass Hot-Forming (Blowing & Pressing)

What are the main advantages of using borosilicate glass for hot-formed components?
Its primary advantages are exceptional thermal shock resistance (due to a low coefficient of thermal expansion ~3.3×10⁻⁶/K), high chemical durability, superior optical clarity, and the ability to withstand much higher operating temperatures than soda-lime glass.
How does the hot-forming process for borosilicate glass differ from that of soda-lime glass?
The key difference is the significantly higher working temperature required. Borosilicate glass softens around 820°C, necessitating specialized high-temperature furnaces and torches. Its shorter working range also demands more precise control and skilled operators.
What are the limitations in design for borosilicate glass components made by blowing or pressing?
While versatile, designs with extremely thin walls, very thick cross-sections, or sharp internal corners can be challenging. The material’s viscosity and short working range can limit the feasibility of highly intricate or delicate geometries.
Is borosilicate glass cost-effective for prototyping and small-batch production?
Yes. Although the raw material cost is higher, the blowing and pressing processes use relatively simple and affordable metal molds compared to other glass-forming methods. This makes borosilicate a viable option for prototypes and small to medium production runs.
What level of dimensional tolerance can be achieved with these hot-forming methods?
While not as tight as machined components, skilled artisans and controlled processes can achieve good dimensional tolerances. Typical tolerances are in the range of a few percent of the dimension, but this should be discussed for specific part geometry.
Can the glass be re-worked or repaired after the initial hot-forming process?
Generally, no. Re-heating an annealed component risks introducing thermal stress and causing fracture. Any modifications typically require re-melting and re-forming the glass, which is often not practical.
What post-processing or secondary operations are available for hot-formed borosilicate parts?
 Common secondary operations include annealing (stress relief), precision cutting, grinding, polishing, and sandblasting. It can also be coated (e.g., AR coating) or undergo surface treatments like enameling for color and branding.
How does the lead time for custom borosilicate components compare to other materials?
Lead times are generally longer than for mass-produced soda-lime glass or plastic injection molding. The process is more artisanal, often involving custom mold fabrication and manual forming, which requires more time, especially for complex parts.

What’s the difference between borosilicate glass and soda-lime glass?

Ordinary glass refers to soda-lime glass, which has attracted many companies because of its cheap and easily available properties. It accounts for 90% of the glass manufactured worldwide and is used in furniture, vases, cups, and windows in daily life. Soda-lime glass can only withstand a temperature change of about 100 °F (55 °C), while borosilicate glass can withstand a temperature difference of about 330 °F (180 °C) before breaking. Borosilicate glass has better mechanical properties than soda-lime glass, and borosilicate glass has significantly improved scratch and abrasion resistance. Not only that, borosilicate glass also has high transmittance and optical clarity, which are not available in soda-lime glass. Interestingly, borosilicate glass is very resistant to chemicals and it is even used to store nuclear waste. So in terms of overall performance, borosilicate glass is far superior to ordinary glass.

Why is borosilicate glass more expensive than regular glass?

Composite materials 

Boron trioxide is far less common than sodium and calcium. It makes up just 0.001% of the Earth’s crust and is, therefore, more expensive to use than other elements.

Energy consumption

Borosilicate glass melts only at extremely high temperatures. Therefore, it requires much energy to melt, increasing energy consumption and pushing up production costs.

Higher Quality 

After an intense and complex production process, the final borosilicate glass objects are considered high-quality glassware and cost more than standard annealed glass.

What are the categories of borosilicate glass？

Borosilicate glass can be divided into several types. Low-alkali borosilicate glass has high thermal and chemical stability and is suitable for laboratory equipment, cookware, and electronics. Alkaline earth borosilicate glass, often containing lead or zinc, is used in optical applications and low-temperature enamels. Lanthanum borosilicate glass offers a high refractive index and low dispersion for advanced optics. Rare-earth-doped borosilicate glass is used in lasers, nonlinear optics, and luminescent materials.

Low-alkali borosilicate glass

Low-alkali borosilicate glass is primarily composed of SiO₂-B₂O₃-R₂O, characterized by low alkali content and high silica content. This type of glass features a low thermal expansion coefficient, low dispersion, excellent chemical stability, high thermal stability, and good electrical insulation properties. It is widely used in applications such as laboratory glassware (e.g., Pyrex), heat-resistant cookware, ampoules, neutral glass, vacuum electronics, optical glass, LCD substrates, solar panels, and solar collectors.

Rare-earth-doped borosilicate glass

Rare-earth-doped borosilicate glass is based on SiO₂-B₂O₃-R₂O compositions and is further doped with rare-earth elements or nanocrystalline materials, creating specialized functional glasses. Examples include Pr³⁺- and Nd³⁺-doped laser glasses, Au-, Ag-, and Cu-doped nonlinear optical glasses, and CdS-, ZnS: Mn²⁺-doped nanocrystalline luminous glasses. These glasses find critical applications in laser technologies, optical devices, and luminescent materials.

Lanthanum borosilicate glass

Lanthanum borosilicate glass incorporates SiO₂-B₂O₃-La₂O₃-R₂O components (such as SiO₂-B₂O₃-La₂O₃-CaO-ZnO-ZrO₂), exhibiting high refractive index, low dispersion, good chemical stability, and reduced tendency to devitrify. This type of glass is mainly used in the production of optical glass for high-end lenses and optical instruments.

Alkali-earth borosilicate glass

Alkali-earth borosilicate glass consists of SiO₂-B₂O₃-RO (such as PbO, ZnO) components, making it suitable for optical and electronic materials. Its typical applications include the production of optical glass, low-temperature enamels, cathode-ray tube (CRT) sealing materials, as well as passivation materials for rectifiers and electronic devices.

What are the advantages of borosilicate glass?

Borosilicate glass is known for its exceptional thermal resistance, low thermal expansion, high chemical durability, and excellent optical clarity. These properties make it a popular choice for laboratory equipment, cookware, and optical applications. Its key principle lies in its chemical composition, which typically includes a high percentage of silica (SiO2) combined with boron trioxide (B2O3).

Low thermal expansion rate

After introducing the unique composition of high borosilicate glass, let’s take a look at the special abilities it deserves! Its first ability is a low thermal expansion rate.

In materials physics, the thermal expansion coefficient of glass usually denoted α, is a numerical value that describes how the dimensions of a glass material change with temperature. The coefficient of thermal expansion is usually expressed in units of one degree Celsius (1/°C) or one degree Fahrenheit (1/°F). For most types of glass, α ranges from about 5×10−6 to 10×10−6/°C.

This property is critical for applications involving significant temperature changes, as it affects the material’s ability to withstand thermal stress without cracking. Glass is a non-crystalline or amorphous material usually made by rapidly cooling a melt of silicate components. This rapid cooling prevents the formation of regular crystal structures, causing the molecules to arrange themselves in a disordered manner. When glass is heated, its molecules begin to vibrate more vigorously, causing the distance between them to increase. This phenomenon causes the material to expand. High borosilicate glass is known for its extremely low thermal expansion rate (approximately 3 × 10 −6 K −1 at 20 °C) compared to other types of glass, such as soda-lime glass. This property is critical for applications that require stability under rapid temperature changes, such as laboratory glassware, cookware, and certain industrial applications.

As mentioned above, we already know that the low thermal expansion rate of high borosilicate glass can be attributed to its unique chemical composition and structure. The backbone of high borosilicate glass is its high content of silicon dioxide (SiO2), which forms a strong and stable silicon-oxygen tetrahedral network. The silicon-oxygen bonds are very strong and contribute to the glass’s rigidity and low thermal expansion. High borosilicate glasses typically have a higher SiO2 content than standard glass types, making the network stiffer and less prone to expansion. The main chemical reaction that forms the glass network involves converting silica and boron trioxide into a glassy matrix:
SiO2​+B2O3​→[SiO4​]4− and [BO4​]5− network

The addition of boron oxide (B2O3) causes B2O3 to interact with the silicon-oxygen network to form borosilicate glass, introducing boron-oxygen bonds into the structure. These bonds can form three- and four-membered rings, which contribute to the flexibility and stability of the glass network. The presence of B2O3 will therefore help reduce the thermal expansion of the glass by creating a more complex network that is less likely to expand under thermal stress. Compared to other types of glass, borosilicate glass contains lower amounts of alkali metals (such as sodium and potassium). This is because alkali metals weaken the glass network, making it more susceptible to thermal expansion. By minimizing these components, borosilicate glass maintains a more stable and rigid structure. The addition of aluminum oxide during the manufacturing process further enhances its thermal stability. Al2O3 is integrated into the glass network and connected with silicon to form a more complex and thermally stable structure. In this way, this big family can develop stably.

Thermal Expansion of Borosilicate Glass

Coefficient of linear thermal expansion of glass

Thermal shock resistance

The second ability of high borosilicate hydrochloric acid glass also benefits from its unique structure.
The low thermal expansion rate of high borosilicate glass can be attributed to its unique chemical structure of the silicon-oxygen network: the backbone of high borosilicate glass is a three-dimensional network of silicon-oxygen (Si-O) tetrahedrons. Silicon atoms are covalently bonded to four oxygen atoms, forming a rigid and stable matrix. Because the strength of the silicon-oxygen bond is one of the strongest, this Si-O network is inherently less prone to expansion when heated. The incorporation of boron into the glass matrix in the form of boron oxide (B2O3) is a key factor in reducing thermal expansion. Boron can exist in two coordination states in glass: trigonal (three oxygen atoms bonded to boron) and tetrahedral (four oxygen atoms bonded to boron). The presence of boron in both coordination states increases the complexity and rigidity of the glass network. This complexity inhibits the network’s ability to expand under thermal stress, thereby reducing the overall thermal expansion rate of the glass. Although high borosilicate glasses contain network modifiers such as sodium oxide (Na2O) and potassium oxide (K2O), their concentrations are relatively low compared to soda-lime glasses. These modifiers are less damaging to the glass network and help maintain the material’s low thermal expansion properties.

Due to the chemical structure of high borosilicate glass, in particular the dense Si-O network and the incorporation of boron, it is crucial for its low thermal expansion. The stiffness and stability of this network limit the movement of atoms and the expansion of the material when heated. Therefore, borosilicate glass can withstand sudden temperature changes without cracking, a property called thermal shock resistance.

Low thermal expansion rate

Chemical resistance

It is known that corrosion of glass, including borosilicate glass, often occurs when it reacts with water or other chemicals. However, due to its unique composition, the corrosion mechanism of high borosilicate glass is somewhat different and slower than that of ordinary glass. Initial interaction with water: When borosilicate glass comes into contact with water, the first thing that occurs is a surface reaction. The water begins to react with the glass, causing the exchange of certain ions. In this case, the glass surface may lose some alkali ions (such as sodium), which will be replaced by hydrogen ions in the water. As this exchange continues, a silica-rich layer forms on the glass surface. This layer acts somewhat like a shield, slowing down further reactions between the glass and water.

Here we should praise the element boron, which plays a vital role here. It makes the structure of the glass more complex and tangled. This complexity makes it more difficult for water to penetrate the glass, thereby reducing the rate of corrosion that reacts with water.

In response, high borosilicate hydrochloric acid glass has gained durability in various environments. Corrosion resistance makes high borosilicate glass ideal for environments where there may be exposure to harsh chemicals or extreme conditions. This is why it is used in laboratory glassware as it may be exposed to corrosive substances for extended periods. And for it, this is also its unique ability.

 

High borosilicate glass is known for its excellent corrosion resistance, especially in acidic environments such as hydrochloric acid. This property is critical for applications requiring durability and longevity, such as chemical processing equipment and laboratory glassware. The mechanisms behind this resistance are complex and fascinating, with aspects of chemistry and materials science intertwined. What a wonderful fusion.
To sum up, the core of high borosilicate glass’s resistance to hydrochloric acid lies in its unique composition. High borosilicate glass typically contains silicon dioxide (SiO2), boron oxide (B2O3), and small amounts of alkali such as sodium oxide (Na2O) or potassium oxide (K2O). High levels of SiO2 provide a strong, stable network, while B2O₃ reduces the glass’s thermal expansion coefficient and enhances its resistance to thermal shock. The presence of B2O3 is also crucial to its acid resistance, as it forms a protective layer that blocks further reaction with the glass. When high borosilicate glass comes into contact with hydrochloric acid, the initial interaction occurs at the glass surface, where hydrogen ions (H⁺) from the acid attack the silica network. However, the boron oxide in the glass forms a hydration layer of boric acid (H₃BO₃), which significantly slows down the dissolution of the silica network. This layer acts as a barrier, reducing the rate at which the glass matrix is degraded by acids. A simplified chemical reaction of the interaction between high borosilicate glass and hydrochloric acid can be expressed as follows:
Initial acid attack SiO2+2HCl→SiCl2+2H2OSiO2+2HCl→SiCl2+2H2O
Formation of boric acid protective layer B2O3+3H2O→2H3BO3​

Alkali resistance

These reactions highlight the initial stages of the interaction, where the silica network is attacked and a protective layer of boronic acid forms. It should be noted that in actual processes the reaction is more complex and involves various intermediate steps and products, again depending on the glass composition and exposure to acid. Here are a few simple steps.
Step 1: Describe the acid attack on the SiO2 network, resulting in the formation of silicon chloride (SiCl2) and water.
Step 2: Explain how boron oxide reacts with water to form a boric acid protective layer.
The above figure illustrates the corrosion resistance mechanism of high borosilicate glass in the presence of hydrochloric acid. It captures the initial acid attack of the silica network, leading to the formation of silicon chloride and water, and then the formation of a protective boric acid layer. This diagram effectively visualizes the chemical processes involved in enhancing the resistance of glass to acidic environments. Help you better understand the manufacturing process.

Acid resistance

Excellent optical transparency

High borosilicate glass is known for its excellent optical clarity. This property is critical for applications that require transparent materials that do not distort light, such as laboratory glassware, optical lenses, and certain types of windows. Optical transparency is a direct result of the material’s composition and its highly ordered structure.

(1) Low iron content
Borosilicate glass has a lower iron content. Iron impurities absorb light at the red end of the spectrum, causing the glass to appear green. High borosilicate glass maintains a clear, nearly colorless appearance by minimizing iron and other color-causing impurities, ensuring minimal light absorption and distortion. This is also a major innovation in our industrial technology.

(2) Uniform structure
The glass’s structure, characterized by a network of silicon-oxygen (Si-O) and boron-oxygen (B-O) bonds, is very homogeneous and free of crystalline inclusions that could scatter light. This uniformity allows light to be transmitted without significant scattering, thereby increasing its optical transparency.

(3) Chemical stability
High borosilicate glass’s resistance to chemical attack, including hydrochloric acid, means its surface remains smooth over time and does not etch. Rough or etched surfaces may scatter light, reducing clarity. The glass’s chemical stability ensures that its optical properties remain consistent even under harsh chemical exposure.

Light transmittance of high borosilicate glass

Why does borosilicate glass have so many advantages?

The advantages of high borosilicate glass arise from its unique chemical composition and structural properties:

High silica content

High borosilicate glass typically contains over 80% silica (SiO₂), forming a highly stable silicon-oxygen (SiO₄) tetrahedral network. This strong silicon-oxygen bonding results in a low coefficient of thermal expansion, around 3.3 × 10⁻⁶ /°C. Such a low expansion rate minimizes size changes under temperature fluctuations, giving the material excellent thermal shock resistance.

Boron oxide contribution

Boron oxide (B₂O₃), making up about 10-15% of the composition, modifies the glass network by introducing trigonal (BO₃) and tetrahedral (BO₄) units. These structures increase the complexity and rigidity of the glass network, further reducing thermal expansion. Additionally, boron oxide helps form a protective layer on the glass surface when exposed to certain environments, enhancing its resistance to chemical corrosion. This layer significantly slows the leaching of the glass matrix in acidic conditions.

Low alkali metal content

Unlike soda-lime glass, which may contain 10-15% alkali oxides like sodium (Na₂O) and potassium (K₂O), high borosilicate glass typically limits these to under 5%. By reducing alkali metal content, the glass network remains more stable and less prone to thermal expansion and chemical attack. This characteristic is a key factor in its superior durability.

Additives and enhancers

The inclusion of small amounts of aluminum oxide (Al₂O₃) often around 2-5%, strengthens the glass network and improves both mechanical and chemical resistance. Al₂O₃ also enhances its thermal stability, making it well-suited for demanding applications. In some cases, other trace additives are introduced to fine-tune optical clarity, adjust color, or improve resistance to environmental stresses.

Estimation of Expansion Coefficient of Borosilicate Glass

The chemical composition of the tungsten and molybdenum series vacuum glasses typically consists of alkali borosilicate glass with a high boron oxide content and minimal alkaline earth metal oxides. In actual production and new composition design, vacuum glass manufacturers must estimate and adjust the glass’s expansion coefficient. The target is to keep the coefficient within ±0.5 × 10⁻⁷/°C. However, the existing additive methods for calculating the expansion coefficient often lead to large errors when applied to high-boron compositions. Some calculation systems rely on molecular composition (%), yet vacuum glass components are often complex polysilicates, and mass fractions are usually used in production. Converting mass fractions into molecular content introduces inaccuracies that can be unacceptable for vacuum glass production.

Relationship between expansion coefficient and borosilicate content

To address these issues, the composition range of commonly used alkali borosilicate vacuum glasses was studied, focusing on Na₂O at 5%–15%, B₂O₃ at 5%–30%, with the remainder being SiO₂ (all percentages by mass). The relationship between expansion coefficient and composition was examined and is illustrated in Figure . When the Na₂O content remains constant, the effect of replacing SiO₂ with B₂O₃ varies depending on the Na₂O level:

  • Low Na₂O Content: At lower Na₂O levels, substituting SiO₂ with B₂O₃ significantly increases the glass expansion coefficient.
  • Medium Na₂O Content: As the Na₂O content rises, the effect of B₂O₃ replacement diminishes. Around 12.5% Na₂O, replacing SiO₂ with B₂O₃ results in little to no change in the expansion coefficient.
  • High Na₂O Content: When Na₂O exceeds 12.5%, substituting SiO₂ with B₂O₃ can actually decrease the expansion coefficient.

Relationship between expansion coefficient and borosilicate content

Calculate the constant value of αB₂o₃

Content (mass fraction)/% B₂O₃
5 a: 0.0064 b: -0.270 c: 2.57
10 a: -0.0032 b: -0.004 c: 0.68
15 a: -0.0008 b: -0.042 c: 0.86
20 a: 0.0050 b: -0.137 c: 1.01
25 a: 0.0008 b: -0.088 c: 1.06
30 a: 0.0008 b: -0.076 c: 0.91

The variation in expansion coefficient can be attributed to changes in the coordination number of boron oxide within the glass structure. Further details on this phenomenon can be found in the works of Professor Gan Fuxi. This observation indicates that the additive coefficient of boron oxide depends on the alkali metal oxide content and should not be treated as a fixed constant.

Based on this understanding, the expansion coefficient of boron oxide can be expressed as:

αB₂O₃ = a[R₂O]² + b[R₂O] + c

Where:

αB₂O₃ is the additive coefficient for boron oxide.
R₂O represents the alkali metal oxide content in the glass composition (%).
a, b, and c are constants, the values of which are listed in Table .

Oxide Expansion Additivity Coefficient (αi) Oxide Expansion Additivity Coefficient (αi)
SiO₂ 0.05 CaO 1.63
ZrO₂ 0.23 BaO 1.40
B₂O₃ a[R₂O]²+b[R₂O]+c PbO 1.06
Al₂O₃ 1.10 Na₂O 4.32
ZnO 0.70 K₂O 3.90
MgO 0.45 Li₂O 4.90
No. Glass Type B₂O₃ Content (% mass) Actual Value (×10⁻⁷/°C)Calculated Expansion Coefficient (×10⁻⁷/°C)
A B C D E F G
1 N51A 9.6 49 56.2 49.8 54.4 46.9 54.0 44.8
2 GS4 10.5 44 51.3 46.7 49.5 43.4 50.6 47.3 44.8
3 Pyrex 12.5 32 42.0 34.5 36 31.2 36.7 33.1
4 Nonex 14.5 36 41.4 41.8 49.6 36.3 44.1 41.3 39.3
5 1646Ⅲ 16.0 43 45.1 45.6 47.6 39.3 49.3 43.5
6 DM-346 17.2 47 48.0 51.4 54.0 42.3 54.8 46.6
7 DW-211 18.0 40 41.7 42.1 45.9 36.2 45.2 42.5 38.4
8 C-9 18.0 36 41.6 42.1 45.7 35.4 38.9 37.5
9 DM-305 20.3 49 42.6 54.9 63.2 49.8 60.0 48.8
10 DM-308 23.0 48 46.8 49.9 55.7 40.3 53.4 45.7
11 C-40 24 48 46.1 51.2 58.1 51.5 40.0 52.0 48.0
12 FCN 24.5 47 44.2 50.1 31.6 39.4 43.0 47.6
13 DW-203 25.0 34 36.0
14 DW-270 27.0 36.5 36.0
15 C-53 30 30 23.5 27.2 41.5 16.3 22.6 28.5 30.5
Maximum Error (Δαmax) (×10⁻⁷/°C) 10.0 7.7 15.4 13.7 11.9 9.0 3.3
Average Error (Δαav) (×10⁻⁷/°C) 4.5 3.5 7.5 4.1 6.3 4.3 1.0

Why annealing is required?

Although borosilicate glass has a relatively low expansion coefficient compared to soda-lime glass, it can still accumulate thermal stress during rapid cooling or thermal processing. This stress can impact the glass’s mechanical properties, thermal stability, and optical clarity. Annealing is employed to minimize or remove such residual stresses, enhancing the glass’s durability and performance.

Mechanism of Thermal Stress Formation

Thermal stress in borosilicate glass can be divided into:

  • Temporary Stress:

    As the glass cools, the surface layers contract more quickly than the interior, creating tensile stress on the surface and compressive stress inside. Once the glass reaches a uniform temperature, these temporary stresses naturally dissipate.

  • Permanent Stress:

    If cooling occurs too rapidly and the glass does not have time to equalize, residual stress remains locked in. This permanent stress can weaken the glass and reduce its reliability under thermal or mechanical loads.

Annealing Process

The annealing process for borosilicate glass involves heating the material to a temperature above its strain point and below its softening point—typically within the range of 500–600°C. This temperature is held long enough to allow stress relaxation at a molecular level. Following this, the glass is cooled slowly and uniformly to prevent reintroducing new stresses. Through annealing, borosilicate glass achieves a more uniform internal structure, improved thermal shock resistance, and better overall durability.

How is the anti-crystallization performance of borosilicate glass?

Borosilicate glass exhibits remarkable anti-crystallization performance, largely due to its unique composition and processing conditions. Composed of approximately 80% silica (SiO₂) and 12–13% boron oxide (B₂O₃), it maintains a highly amorphous state. Silica forms a stable, continuous network that resists crystallization, while boron oxide acts as a network modifier, disrupting the silica structure and reducing crystallization tendencies. This combination ensures that the glass remains chemically and thermally stable under various operating conditions.
Additionally, the controlled inclusion of alkali oxides further enhances the material’s anti-crystallization properties. These oxides are carefully managed to maintain a low thermal expansion coefficient, minimizing internal stresses that can lead to crystallization. The manufacturing process, including precisely chosen annealing and softening points, also plays a critical role. By maintaining processing temperatures below the onset of crystallization (typically above 900°C), borosilicate glass remains amorphous and free from crystalline inclusions, even under demanding thermal conditions.
In terms of real-world data, the crystallization temperature of borosilicate glass far exceeds its typical working temperature range. For instance, strain points are generally around 520–560°C, annealing points are about 550–600°C, and softening points reach 820–850°C. Crystallization generally begins at temperatures higher than these, making it virtually non-existent under normal usage conditions. This allows borosilicate glass to maintain its optical and mechanical properties without degradation from crystallization.
Property Value Range Notes
SiO₂ Content (%) 75–81 High silica content provides network stability.
B₂O₃ Content (%) 12–13 Disrupts silica network, reduces crystallization tendency.
Alkali Oxides (e.g., Na₂O, K₂O) (%) 2–5 Minor amounts to adjust thermal properties, minimal crystallization effect.
Strain Point (°C) 520–560 Temperature below which residual stress is negligible.
Annealing Point (°C) 550–600 Temperature at which stress relaxation occurs.
Softening Point (°C) 820–850 Indicates the temperature range for forming.
Crystallization Onset (°C) >900 Above normal service and annealing temperatures, ensuring stability.
Thermal Expansion Coefficient (×10⁻⁷/°C) ~32–36 Low expansion helps prevent thermal shock, contributing to stability.
Furthermore, research has shown that the presence of boron oxide increases the viscosity of the glass melt at lower temperatures. This enhanced viscosity provides manufacturers with greater control during production, reducing the likelihood of crystal nucleation. As a result, borosilicate glass maintains high stability through repeated thermal cycling, a critical factor in its widespread use in laboratory equipment, cookware, and other high-performance applications.

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