Advanced Ceramics / Glass

Success by Design

Custom glass formulations can fulfill specific functions in a range of applications.

Mirrors on the Hubble Space Telescope are made of ultra-low-expansion glass. (Image credit: NASA, 2009.)


Matching the thermal expansion coefficient of glass to metal in these hermetic seals is critical to successful performance.

Glass has two main defining characteristics: it is an amorphous (non-crystalline) solid-meaning that there is no long-range order of the positioning of its molecules-and it exhibits a reversible transition from a hard, brittle state to a molten state when heated, without a pronounced change in material structure. These two characteristics provide the basis for the development of innovative glass formulations that can result in novel solutions to high-tech challenges.

Successful glass formulation requires a thorough knowledge of the raw materials to be used, an understanding of the dynamic interaction of these raw materials, and a great deal of experience with the formulation process itself. Complementing those skills is another critical element of glass formulation: a mindset that embraces the prospect of creating something new and unique that functions like no other material.

Specially formulated glass is fundamental to the fiber optic industry.

Meeting the Application Challenge

The decision by a researcher or manufacturer to investigate the feasibility of a custom glass formulation for a technical application usually follows a thorough review of the scientific literature and an in-house evaluation of existing materials. Given the desired characteristics and performance, is there a familiar or readily obtainable material that will do the job? Only after all contenders are eliminated will a custom glass formulation be considered.

Today, specially formulated technical glass is found in a growing list of scientific fields (see sidebar). More important than the particular field, however, is the performance that can be crafted into or onto the glass. A specific formulation can potentially find application in any number of fields. The following represent just a few examples of how formulation can influence function.

Tailoring Thermal Properties

With careful selection of the type and proportions of raw material and diligent adjustments to the formulation process itself, it is possible to achieve the specific melt temperature required for a particular application. For example, durable glass has been made to melt at 400°C while other glass has been produced with a transition temperature below 150°C. Even existing non-custom glass can be altered to melt at 100°C lower than conventional container glasses.

Other thermal properties can be modified in the technical glass development process. This is important because matching the thermal stability and/or thermal expansion of the glass to other materials in a component or to the operational requirements of the application is critical to structural integrity and performance. A practical, high-tech example of the control of thermal properties during formulation is found in the mirrors of the Hubble Space Telescope, which are made of ultra-low-expansion glass and kept at a nearly constant room temperature (21øC) to prevent warping.

Blocking or Channeling Electromagnetic Energy

As a coating or as a component, glass can be formulated to block or channel certain wavelengths of light (e.g., ultraviolet and infrared). Glass can also be formulated to block radiation. Although glass coatings to block ultraviolet (UV) rays have long been available, glass is now being formulated with UV protection built right into the glass.

A particularly sophisticated glass-based band pass filter has also been developed for tight infrared transmission. A less complex (but perhaps more widely appreciated) application of specially formulated glass is seen in the production of tinted beer bottles, which are created to protect their contents from flavor-killing "light strike."

Lead glass, though not as widely available as in years past, is still the most effective way to shield aerospace electronics from radiation during use. As for radiation detection, the addition of specific oxide dopants during glass formulation has resulted in specialized glass that is used in setting up and calibrating standard sets for X-ray fluorescence (XRF) spectroscopy.

Maximizing the Power of Light and Color

For the exacting field of fiber optic communication, glass has been formulated to make dense wavelength division multiplexing (DWDM) substrates, which are used for increased functional capacity of fiber optic strands. Specialists in technical glass formulation have also developed an innovative dichroic (Greek for "two colors") glass that shifts from red to green, depending on lighting and the environment. This is accomplished through in-glass oxide additions rather than the more typical (but less reliable) multi-layer coatings. And for many applications, both aesthetic and functional, selective rare earth doping of glass formulations can yield tight control of the desired final color.

Several different glass compositions are often produced and subsequently machined before an ideal formulation is achieved.

Balanced Properties

Virtually every technical glass application requires a balance of properties that meet specific performance objectives. This balance is far more delicate than that which is found in most commercially available types of glass.

For example, the custom yellow glass shown on this page needed to have specific properties for its use in a new product, and it had to be easily machinable so that it could be manufactured in commercial quantities. Several different glass compositions were produced and subsequently machined before the ideal formulation was achieved. This highly refined process of trial and error is typical of custom glass development; it is not until the science of formulation meets the real world of application testing that the winning combination emerges.

The development of any new material or component in a commercial setting is highly confidential. However, the process of development through partnerships with outside experts is by nature intensely collaborative. Mutual trust and respect, energized by the thrill of developing something new that uniquely meets a need, can make the process a shared voyage of discovery that pays big dividends in performance and commercial success.

For more information, email the author at ian.doggett@technicalglass.co.uk.

SIDEBAR: Technical Glass Applications

Aerospace--instrumentation, radiation-shielding devices, connectors

Automotive--light controls, sensors, air bag detonation systems

Electronics--sensors, capacitors, thermostats, component coatings

Telecommunications--fiber optic apparatus

Optics--specialty lenses, mirrors, infrared and UV filters

Fuel cells and microwaves--hermetic connectors, sealing components

Medical--bio-active coatings for orthopedic implants




SIDEBAR: Alterable Properties

Thermal properties--melt temperature, stability, expansion for compatibility with other components

Physical durability--extreme hardness, abrasion resistance for wear-resistant coatings and components

Color--aesthetic and functional

Chemical durability--resistance to acids and bases

Refractive index matching and dispersion--optical and fiber optic applications

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