Fused Silica Capillary Tubes SiO2 Ultra-Fine Biomedical Semiconductor

Introduction

Large bore, thin-wall fused silica capillary tubes are high-precision hollow glass structures made from amorphous silicon dioxide (SiO₂). Unlike standard fused silica tubes or micro-capillaries, these specialty tubes combine a large internal diameter (typically 1 mm to 10 mm or more) with an exceptionally thin wall (as low as 0.1 mm). This unique geometry offers minimal fluid resistance, reduced material weight, and enhanced optical clarity, while preserving the superior chemical and thermal stability of fused silica.

These capillaries are used in high-throughput gas and liquid transfer, laser beam guidance, spectroscopic flow cells, plasma and vacuum transport systems, and other applications where both internal space and wall transparency are critical.

Manufacturing Principle

1. High-Purity Material Source

Production begins with ultra-pure fused silica synthesized via flame hydrolysis or chemical vapor deposition (CVD), ensuring low metallic content and high UV-VIS transmission.

2. Precision Preform Preparation

A large-diameter, thin-walled preform is prepared either by depositing layers of silica onto a rotating mandrel (FHD/CVD) or through mechanical boring of bulk silica glass.

3. Controlled Tube Drawing

The preform is heated in a clean high-temperature furnace (>2000°C) and carefully drawn into capillary tubes. To achieve thin walls with wide bores, engineers regulate:

• drawing tension,

• internal bore pressure (positive or vacuum), and

• drawing speed (very precise, often under 0.5 mm/s).
  Active feedback systems maintain strict ID/OD ratios and concentricity.

4. Stress Relief and Cleaning

Capillaries are annealed at controlled temperatures to remove internal stress caused by thermal gradients. They are then cleaned using acid-based or ultra-pure water systems to remove particulate and ionic contamination.

5. Final Processing

Tubes may be laser-cut, beveled, or polished. Options include fire-polished ends for optical clarity, or chemically etched surfaces for microfluidic compatibility.

Frequently Asked Questions (FAQ)

1. What is the difference between silica and fused silica?

Silica typically refers to silicon dioxide (SiO₂) in general, including crystalline forms like quartz and amorphous forms like glass.
Fused silica, on the other hand, is a specific type of non-crystalline (amorphous) silica made by melting high-purity silicon dioxide. It has extremely low thermal expansion, excellent UV transparency, and superior chemical purity, making it ideal for high-performance optical, semiconductor, and analytical applications.

2. What are the advantages of fused silica capillary columns compared with glass or metal columns?

Fused silica capillary columns offer several key benefits:

• Higher chemical inertness: Resistant to acids, solvents, and reactive gases.

• Low surface activity: Reduces sample adsorption and improves peak shape in chromatography.

• Superior flexibility: Can be coiled without cracking, unlike brittle glass.

• High temperature resistance: Typically tolerates 320–370°C or higher.

• Smaller internal diameter (ID) and thinner walls, allowing for better resolution and faster analysis in GC and CE.

• Transparent to UV and visible light, useful in optical detection.

3. How do you cut fused silica?

Fused silica is cut using a precision scoring and breaking method. The process involves:

• Using a ceramic or diamond-tipped scribe tool to lightly score the surface.

• Gently applying tension until the tube snaps cleanly along the score line.

• For very fine or thin-wall tubing, laser cutting or fiber cleaving tools may be used to achieve a smoother edge without chipping.
  After cutting, polishing or fire-polishing may be performed to prepare the ends for optical or fluidic applications.

4. What is a bare fused silica capillary?

A bare fused silica capillary refers to a fused silica tube that has no internal coating or surface treatment. It is essentially raw, untreated fused silica tubing and is commonly used in:

• Capillary electrophoresis (CE), where the surface silanol groups interact with analytes or buffer ions.

• Custom microfluidic devices and analytical systems where users wish to apply their own coatings or surface modifications.
  Bare capillaries typically come with polyimide coatings on the outside for mechanical protection but have no internal stationary phase or modification.

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