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Glass materials for capillaries

Fig. 1 Glass materials for capillaries manufactured at Nakahara Opto-Electronics Laboratories, Inc. (NOEL).

Nakahara Opto-Electronics Laboratories, Inc. (NOEL) manufactures and distributes capillaries primarily made of vitreous silica and borosilicate glass (Fig. 1).

There are various types of vitreous silica each having different properties. Typically, the softening temperature of vitreous silica is ~1,700 °C, and hence, vitreous silica exhibits a significantly higher thermal stability in comparison with other glass materials. Transparency is significantly improved in vitreous silica when compared with normal glass materials used for windows, allowing ultraviolet rays to pass through easily. Furthermore, vitreous silica is lead-free, and the levels of impurities such as transition metals and alkali elements are tremendously low, typically in the order of ppm or ppb levels. Therefore, any vitreous silica is considered to exhibit excellent properties for various capillary applications.

However, careful selection of the type of vitreous silica may be necessary in semiconductor and high-power laser applications and for elemental analysis at ultrahigh sensitivities (Fig. 1).

Borosilicate glass exhibits superior heat resistance properties second to vitreous silica. With a softening temperature of 820 °C, borosilicate glass also demonstrates a higher thermal stability than normal glass materials. Typically, borosilicate glass is used as a heat resistance glass often found in laboratory glassware. Capillaries made from borosilicate glass are widely used as fiber arrays to connect optical fibers to optical waveguides, and also used in certain medical applications such as high-precision manipulator for artificial insemination.

NOEL also manufactures capillaries derived from special glass materials such as fluorine-doped silica glass, BK-7 and glasses containing rare earth elements. If you are unsure about what glass materials to use to satisfy your requirements, please feel free to contact us at: Contact Form.

General information on the manufacturing method and properties of vitreous silica and borosilicate glasses are described below:

1-1.Vitreous silica

The two major categories of vitreous silica are: fused quartz glass and synthetic silica glass.

1-1-1.Fused quartz glass

Fused quartz glass is made from a crystalline powder, which is fused at high temperatures to transition into the glass form. Fused quartz glass is classified depending on the heat source used for fusing the crystalline powder, typically by use of an electric furnace or an oxyhydrogen flame.

  1. Fused quartz glass derived from an electric furnace (Fig. 2)

    Fig. 2 Conceptual schematic of the fused quartz glass fabrication process using an electric furnace.

    vitreous silica derived from the electric furnace process is the most widely used for semiconductor applications. This process fuses a natural crystalline powder within a specialized electric furnace. As the heat source is dry, less water is contained in the quartz glass. The water content in glass resides in SiO2 as OH radicals, which reduces the softening temperature. However, fused quartz fabricated by the electric furnace process contains a lesser degree of OH radicals and is more thermally stable with a high softening temperature. Thus, fused quartz manufactured in such a manner is widely used for front-end semiconductor processes.

    The resulting materials exhibit high purities, and contamination by Al, Ca, Fe, K, Li, and Na are in the order of 0.1–10 ppm. The OH content is <5 ppm.
    Other properties of quartz glass derived from the electric furnace process are listed below (nominal values not guaranteed):
    Refractive index: 1.4585 (at 589.29nm)
    Linear expansion coefficient: 5.5×10−7/°C
    Softening point: 1,700–1,720 °C

  2. Fused quartz glass derived from an oxyhydrogen flame (Fig. 3)

    Fig. 3 Conceptual schematic of the fused quartz glass fabrication process using an oxyhydrogen flame.

    here, the quartz glass is also fabricated from a natural crystal powder, however, an oxyhydrogen burner is used as the heat source for fusing instead of an electric furnace. This method fuses the powder into droplets as the crystalline powder falls into the oxyhydrogen flame. Thereafter, the droplets accumulate and solidify on a seed rod or a substrate.

    Quartz glasses derived from the oxyhydrogen flame process are typically characterized by a minimal incorporation of bubbles. Furthermore, the quartz glass exhibits a superior uniformity because of fusing in the presence of a high-temperature flame. Although the purity level is equivalent to that of the fused quartz derived from the electric furnace method, a higher degree of OH substrates are present since the fusing process occurs in the presence of a water atmosphere rendered by an oxyhydrogen thermochemical reaction. The oxyhydrogen process typically results in a softening temperature of 1,660 °C, which is slightly lower than the softening temperature observed for fused quartz glasses derived from the electric furnace process.

1-1-2.Synthetic silica glass

Synthetic silica glass is fabricated using silicon chlorides as raw materials such as silicon tetrachloride (SiCl4). The term ‘synthetic’ is used because no natural crystal powder is used as the raw material. Synthetic silica glass is categorized based on the OH radical content.

  1. High OH content synthetic silica glass (Fig. 4)

    Fig. 4 Method of making synthetic silica glass.
    Reference: US patent 2,272,342, February 10, 1942 (Method of making a transparent article of silica).

    liquid phase SiCl4 at room temperature is evaporated into the gaseous phase, which thereafter, is transported to the flame of an oxyhydrogen burner. The temperature of the oxyhydrogen flame is >2,000 °C. The flame hydrolysis reaction, which converts SiCl4 into minute droplets of silicon dioxide (SiO2), occurs in the flame. These droplets are accumulated on a rotating seed rod, which is gradually withdrawn. Finally, a rod-shaped synthetic silica glass is obtained.

    As SiCl4 is supplied as a gaseous phase, the final glass products exhibit extremely high purities because of the difference in the SiCl4 vapor pressure to other impurities. The resulting synthetic silica glasses have low levels of impurities in the order of ppb.

    However, as an oxyhydrogen burner is used as the heat source, the OH radical content is high, similar to the case of fused quartz glasses derived from an oxyhydrogen flame. In addition to SiCl4, trichlorosilane (SiHCl3) and silane (SiH4) may also be used as raw materials to synthesize silica glasses. Such synthetic silica glasses demonstrate superior uniformity, and hence, are often employed as photo masks for photo-lithography processes and as capillaries in analytical and optical components.

    Reference: US patent 2,272,342, February 10, 1942 (Method of making a transparent article of silica).

  2. OH-free synthetic silica glass

    SiCl4 can also be used for the manufacture of non-OH-containing silica glass. Similar to synthetic silica glasses with a high OH content, the raw material is transported to the flame of an oxyhydrogen burner in the vapor phase. In this case, however, a low flame temperature is used resulting in SiO2 particles of <1 μm in diameter being formed, which thereafter, accumulate on a rotating seed rod from the upper side. After gradually lowering the seed rod, a white porous cylindrical rod, which is an aggregation of silica glass particles, is formed. The porous rod is also referred to as a soot preform (Fig. 5). A transparent OH-free synthetic silica glass can be obtained by thermal treatment of the soot preform under a helium (He) and chlorine (Cl2) atmosphere.

    The above manufacturing technique to fabricate OH-free synthetic silica glasses is a simplified method of the vapor-phase axial deposition (VAD) process, which is the manufacturing method to obtain optical fibers. The VAD method (Fig. 6) uses GeCl4 in addition to SiCl4 as a dopant material. An optical fiber preform made by the VAD method is shown in Fig. 7.

    The impurity concentration of OH-free synthetic silica glasses is as low as ppb levels. Furthermore, the OH radical contamination is also extremely low in the order of ppb.

    Capillaries derived from OH-free synthetic silica glasses are primarily employed in fiber sensors at infrared wavelengths and in high-power laser applications.

    Fig. 5 Soot preform.

    Fig. 6 The vapor-phase axial deposition (VAD) process

    Fig. 7 VAD preform (right side).

1-1-3.Other silica glass types

In addition to the aforementioned quartz and synthetic silica glass manufacturing methods, there is another method that uses plasma as the heat source. Although the method of adopting plasma as the heat source is advantageous in terms of a low OH content, fine bubbles may be incorporated into the fabricated glass.

Fluorine-doped silica glass capillaries demonstrate superior ultraviolet ray transmission and lower refractive indices than non-doped vitreous silica. In recent years, fluorine-doped silica glass has attracted significant attention in applications such as capillaries for high-power laser combiners and micro UV lamps.

1-1-4.Vitreous silica properties
  1. Thermal properties

    Thermal properties: unlike single crystals, glass has no definite melting temperature. The viscosity (η) of glass gradually decreases as a function of temperature, and the temperature at η = 107.6 dPa· s is defined as the glass softening point. All vitreous silica types exhibit excellent characteristics toward heat resistance because of their high softening point.

    (Note: dPa· s = 1 poise).

    However, because quartz glass manufacturers use different raw materials, different manufacturing equipment and different manufacturing conditions, even the same classification of quartz glasses can exhibit slightly different softening points depending on the company.

    The softening point of vitreous silica is typically ~1,700 °C, however, the temperature varies between 1,600 °C and 1,720 °C, and principally depends on the OH contamination level. That is, vitreous silica possessing a high level of OH contamination exhibits a low softening point, whereas, a low degree of OH contamination results in a high softening point.

    In any case, any vitreous silica type is considered as a super heat-resistant glass that only softens at extremely high temperatures when compared with ordinary glasses.

  2. Optical properties

    Optical properties: vitreous silica is a highly transparent material under ultraviolet, visible, and near-infrared light (160–4,000 nm). For OH-containing vitreous silica, the OH fundamental absorption appears at 27,300 nm and is prominent at 850 and 1,390 nm as overtones (no such absorption is observed in OH-free vitreous silica).

    Typically, the refractive index of vitreous silica is 1.4585, at a sodium D line wavelength of 589.29 nm. The refractive index of vitreous silica depends on wavelength. The refractive index is generally not relevant to the capillary manufacturing process and to its use, however, the optical fiber properties and the lens aberration may be influenced.

  3. Mechanical properties

    Mechanical properties: the Young`s modulus of vitreous silica reaches as high as 7.2×1,010 Pa (10.5×106 psi). Therefore, vitreous silica stretches elastically, and silica glass optical fibers with 125 μm diameters exhibit superior strength compared with piano wires. Vitreous silica capillaries having dimensions of tens of millimeters in length and several millimeters in diameter exhibit superior mechanical strength and are highly unlikely to break or crack under normal conditions of use. However, capillaries are also glass materials, which are brittle by nature. Thus, vitreous silica capillaries can be easily cut by scratching their surface using a glass cutter and applying a tensile force.

1-2.Borosilicate glass

The objective of developing borosilicate glass was to achieve a lower softening point than that of vitreous silica for ease of glass working processing. Borosilicate glass contains boron (B2O5) and SiO2, and is primarily composed of Na2O-B2O5-Al2O3-SiO2. The softening point of borosilicate glass is 820 °C.

Borosilicate glass materials are typically formed as tubes, rods and plates. Previously, Corning distributed borosilicate glass under the Pyrex Glass brand; currently, Schott AG internationally distributes Tempax Float or DURAN borosilicate glass products. Capillaries are manufactured using these materials. Borosilicate glass is considered to be cheaper than vitreous silica, however, the cost of the capillaries is not only determined by the material cost, but is also greatly influenced by the shape, size and accuracy of the capillaries. For details, please feel free to contact us at: Contact Form.

1-3.Dimensional accuracy of capillaries

  1. Uniformity of outer and inner diameters

    The dimensional accuracy of the standard capillaries manufactured by Nakahara Opto-Electronics Labs., Inc. (NOEL) is controlled within ±1% of the nominal value (Fig. 8). For example, a ±10 μm deviation is allowed for φ1 mm O.D., and a 5 μm deviation is allowed for φ0.5 mm I.D. NOEL also offers capillaries with an internal diameter allowance of ±1–3 μm upon request. The accuracy with respect to length is as low as ±1–2%, for lengths up to 1,200 mm. Thus, NOEL offers capillaries having extremely uniform dimensions.

    Fig. 8 Capillary diameter uniformity

  2. Internal diameter circularity

    For some applications, circularity of the internal diameter rather than the average diameter of the minor and major axes is the main concern. For applications that require superior circularity, NOEL offers capillaries with near perfect circularity, whose ratio between the minor and major axes is close to 1.001.

    Figure 9 shows a typical cross-section of the capillaries offered by NOEL, which exhibit significantly less distortion than those of other competitors (Fig.10). This property is critical in secondary processing of capillaries for component manufacturing. For details, please feel free to contact us at:Contact Form

    Fig. 9 major/minor<0.001

    Fig. 10 Major/minor axes <0.01.