Produced by: IEEE Globalspec Media Solutions September 2017 Transparent and Translucent Technical Ceramics: Thinking Outside the Bulb Sponsored by: CoorsTek, Inc. The world of technical ceramics presents itself in many shapes and flavors. Applications using technical ceramics abound, from cutting tools to bearings to kiln linings and Translucent and biomedical implants. All of these examples take advantage of the various properties transparent technical that different types of ceramic offer. A common misconception of technical ceramics is ceramics have a that their physical color is a basic opaque shade of white. This article explores a family of technical ceramics that is optically translucent to transparent. higher thermal resistance than glass What Can Translucent/Transparent Technical Ceramics Offer? or resin and are stronger and harder. Polycrystalline technical ceramics generally consist of opaque matter that absorbs or scatters visible light. However, through highly controlled manufacturing, some technical ceramics can achieve a transparency similar to glass or single crystal sapphire. Technical ceramics are produced by sintering powdered materials that typically have light scattering characteristics. Using highly precise processes, the light scattering attributes can be removed. The resulting transparent technical ceramics have a polycrystalline structure with a transmittance rate that is strong in the visible light to middle infrared spectrum. Two materials that are commonly optimized for transparency/translucency are high purity alumina (including Sapphal™ translucent alumina and polycrystalline alumina) and spinel (MgAl₂O₄). Figure 1: Translucent ceramic arc tube used in high-intensity lighting (left). And, a spinel dome, an Sponsored by: example of transparent ceramics (right). (Source: CoorsTek) Transparent/translucent technical ceramics have been in use for industrial and commercial applications for over 50 years. Commercial production began in 1961 when General Electric began selling translucent alumina Lucalox® lamps. In the years since, CoorsTek of Golden, Colorado, has become a leader in the field of transparent/translucent Produced by: technical ceramics. Coors (yes, the beer folks) started manufacturing pottery and labware in the early 20th century. On a parallel track, Covalent Materials (previously Toshiba Technical Ceramics) began manufacturing and developing new ceramic materials and material processes in Japan. In 2014, the companies combined when CoorsTek acquired the Japan-based group. 2016 saw the addition of Philips Technical Ceramics, underscoring Transparent and Translucent Technical Ceramics: Thinking Outside the Bulb CoorsTek as a world leader in engineered technical ceramics and advanced materials. Translucent and transparent technical ceramics have a higher thermal resistance than glass or resin and are stronger and harder. Their unique combination of wear resistance, high strength and corrosion resistance enable transparent technical ceramics to outperform traditional glass in high- stress environments that require exceptional clarity and stability. Specific attributes of transparent technical ceramics include: • Thermal Resistance, Strength and Hardness—High- temperature sintered transparent technical ceramics have a higher thermal resistance and are stronger and harder than optical glass or resin, and also maintain their Figure 3: Performance comparison of transparent ceramics and transparency at temperatures of up to 1,200° C. other transparent materials. (Source: CoorsTek) • Resistance to Chemicals and Corrosion—Highly Current Applications of Translucent and pure, highly dense transparent technical ceramics hold Transparent Technical Ceramics up extremely well against oxygen and alkalis. They resist highly corrosive halogen plasma and can be used in harsh Translucent and transparent technical ceramics lend environments that glass and resins cannot tolerate. themselves to many applications. Higher thermal resistance than glass or resin and a unique combination of wear • Optical Functions—Through doping, it is possible to resistance, high strength and corrosion resistance enable add optical functions such as wavelength conversion transparent technical ceramics to outperform traditional to transparent technical ceramics. For example, laser glass in high-stress environments, along with maintaining oscillation has been successfully achieved by doping YAG exceptional clarity and stability: and Y₂O₃ transparent technical ceramics. • High-Intensity Discharge (HID) Lighting—Engineered technical ceramics provide superior thermal stability, higher strength and greater translucency for high- performance HID lighting applications. Each year, tens of millions of translucent ceramic parts of various sizes, shapes and geometries for HID lamps are produced. In recent years, ceramics have improved solid state lighting by taking advantage of the wavelength conversion properties of phosphors. Automotive LED headlights are a prevalent example of leveraging this transparent ceramic capability. • Polycrystalline Translucent Ceramic Crucibles—Ideal for TGA (thermogravimetry) and DSC (differential scanning calorimeter) analysis, ceramic translucent crucibles are highly valued for their purity, inertness and chemical Figure 2: Doping can modify ceramic wavelength transmittance. resistance. They are temperature resistant up to 1,800° (Source: CoorsTek) C, translucent and non-porous, meeting specifications of sapphire crucibles at a fraction of the cost. • Mass Productivity—Technical ceramics make it possible to create and sinter a part from material that • Inspection Windows—Transparent and translucent has been manufactured close to the shape of the final technical ceramics are an ideal material for inspection product (near net shape). Ceramic injection molding windows. They can be used as a process window in (CIM) is an important production technology that is chemical processing equipment due to resistance to both suitable for scaling to high-volume mass production. acids and bases. Ceramic windows are wear resistant and are optimal for observing high-temperature Transparent technical ceramics stack up well against other environments while maintaining high strength. transparent materials such as optical glass, optical resin and single crystal sapphire. The chart to the right illustrates a comparison of certain properties of these materials. 2 Transparent and Translucent Technical Ceramics: Thinking Outside the Bulb • Sapphire Replacement—Optical sapphire is currently transmit light that enters at one wavelength and exits used in many applications, including cryogenic at another. Ceramic scintillators, which emit light view ports, UV and IR windows and lenses, detector when excited by radiation, are finding their way into cells, photodiodes and barcode readers. The light medical detection systems and scanners such as PET, transmission and non-porous properties of transparent CAT and others. These highly-sensitive scintillators are technical ceramics meet the specifications of 100 also excellent for use in inspection equipment such as percent sapphire components at a fraction of the cost. non-destructive testing (NDT) equipment. • Infrared Windows—IR windows constructed from • Laser Components—Transparent and translucent spinel (MgAl₂O₄) are used in sensor and armor technical ceramics are designed and engineered as high- windows. Durable, scratch-resistant IR windows are performance, long-life laser equipment components. especially key in aircraft applications where guidance They are advantageous in laser applications and and similar systems rely on IR transmittance as are preferred because of their electrical resistivity, opposed to visible light. Industrial and laboratory corrosion resistance, thermal conductivity, high applications using sensors also realize the advantage strength and hardness, and dimensional stability. of ceramic properties over glass and other traditional transparent and translucent materials. • Transparent Armor—The military has many applications for transparent armor, including vehicle windows, missile domes and face shields. Bulletproof windows today have layers of plastic and glass totaling many inches of thickness. A transparent ceramic replacement could reduce the glass-plastic window’s total weight by a factor of two and still provide critical impact and scratch resistance. Emerging Applications for Translucent and Transparent Technical Ceramics Translucent and transparent ceramic properties and uses continue to expand. For example, using a highly Figure 4: Lasers can also utilize transparent technical ceramics. controlled process, light scattering attributes have been (Source: CoorsTek) removed to develop transparent ceramics, YAG (Y₃Al₅O₁₂; yttrium aluminum garnet) and Y₂O₃ (yttrium oxide). These Transparent and translucent technical ceramics have long transparent ceramics have a polycrystalline structure with been used as lamp encasements in severe duty applications. a transmittance that has been measured in the visible light As LED lighting takes over incandescent in these areas, to middle infrared spectrum at approximately 84 percent other practical and safety-oriented uses of this material for YAG and 81 percent for Y₂O₃. New transparent and have moved into the spotlight. Favorable properties include translucent technical ceramic capabilities are finding their chemical inertness with good thermal stability, high way into new designs and new applications. Emerging temperature