In this ever-changing high-tech world, engineers developing scientific and industrial instruments and systems are looking for higher performance from components used in these systems. Many component fabricators are turning to new advanced ceramic materials (e.g., inorganic, non-metallic, often crystalline oxide, nitrate or carbonate materials) and composites to meet the performance demands of their customers.

Although necessary in many applications, these new materials often (justifiably) add cost to the end product. In addition, some of the new materials’ characteristics and properties make them difficult to machine, adding more cost to the production process.

With the excitement created by these new crystalline ceramic materials, OEM component manufacturers tend to overlook the possibility of fabricating a newly designed component using a proven, reliable material that has been around for centuries: glass. Manufactured glass, which is likely an outgrowth of the pottery making industry that existed before biblical times, has progressed to the point where it can be formed into shapes and sizes that can be used in a variety of scientific and industrial applications, including X-ray, flow tubes, CO₂ laser components, helium-neon (HeNe) laser components, consumables for inductively coupled plasma (ICP) mass spectrometry and instrumentation, sight glasses, and a host of other products. A comparative review of glass characteristics can help determine whether glass should be seriously considered for a particular application.

Glass Characteristics

Glass is also defined as a “ceramic,” although it is a non-crystalline amorphous solid with different atomic structure than crystalline materials. In its solid state, glass is hard, strong and resistant to thermal conduction, electrical conduction, and chemical corrosion. It is relatively inert and unaffected by the properties of other materials, allowing fluids like chemicals to pass through it without changes taking place.

The transparent nature of glass, with visual capability of up to 99% transparency in the visible wavelengths, allows increased design possibilities. Glass also offers electrical insulating at low temperatures, semi-conductivity at middle-range temperatures, and super-
conductivity at high temperatures, as well as a high heat capacity of up to 1,250ºC (quartz).

Glasses and quartzes can meet a variety of temperature or environmental exposures. After fabrication, glass and quartz tolerances can be better than green or molded ceramics and, in many cases, as good as more expensive machined ceramics. Like ceramics, glass can be machined, but it can also be heated to a point where it takes on a toffee-like viscosity and is pliable enough to be formed into intricate, complex parts.

The amorphous nature of glass and the ability of skilled glass artisans to work with fire on glass creates design possibilities for product designers and engineers. Not only can borosilicate glass be turned into different sizes and shapes to meet design specifications, it can also be fused to other glasses or metals such as Kovar, stainless steel, and tungsten. Because these materials have different coefficients of expansion, the process of sealing them together is highly technical and complex.

Improved Sealing Methods

Many designs call for hermetic seals of metals to glass, so glassblowers have developed different techniques to meet the demands for seals that will hold and not leak. The most common method of sealing has been to create a graded seal. The graded seal method employs intermediate sealing glasses (softer than the hard borosilicate glass or the even harder quartz) that are sealed to the metal. The coefficient of expansion of these intermediate sealing glasses is closer to the materials being sealed than that of borosilicate glass or quartz.

A series of seals using two and sometimes three different sealing glasses is necessary to make a proper graded seal. The glassblower must control the heat by applying it slowly to avoid fracturing the glasses. At the appropriate time, a more intense flame is applied to bring the material to a point where the seal can be made. Once the seal is made, the heat needs to be carefully reduced using a lower flame to ensure the seal will hold.

The intermediate sealing glasses are more expensive than borosilicate, but in many applications, this method continues to be the best way to meet the component specifications. However, these specification requirements still allow product designers and engineers to develop products with less cost than using strictly advanced ceramics or composites.

Glass cannot be a substitute for all ceramic applications. Therefore, glass fabricators continue to look for techniques to seal different ceramic materials together in more cost-effective ways. In recent years, glass fabricators have been able to seal borosilicate glass directly to mullite. Because the coefficient of thermal expansion of the mullite is close enough to that of borosilicate glass, glassblowers are able to make the seal by controlling the heat only and eliminating the need for intermediate sealing glasses.

This ability to create a direct seal between the borosilicate glass and the mullite provides both cost savings and new design possibilities. For example, specifications for one component called for a part whose one end needed to be exposed to a very high heat (much too hot for glass, but just fine for mullite). The other end of the part needed to be connected to the measuring instrument and would not be so exposed. Using glass at the top of the part allowed for a more standard interface to the instrument through a ball and socket joint (or other interface commonly used in scientific applications).

Cost savings were achieved because no intermediate sealing glasses were used, and the glass could be fabricated into the needed interface, eliminating the need to machine the mullite. With the capability of combining dissimilar materials, new design options have been opened for applications involving the sampling of molten materials maintained in high heat environments.

Looking Ahead

Although glass is not a substitute for ceramics or composites, the possibility of combining old, reliable glass with new, advanced materials as they become available may prove to be interesting avenue for engineers and designers to explore. As new specifications are developed for scientific and industrial instruments and systems, finding a component manufacturer who can combine glass with more expensive materials can be the key to a cost-effective end product in the right applications.  

 For more information, visit www.pegglass.com.