Aluminum oxide, commonly referred to as alumina, has the chemical formula Al2O3. Sapphire is the single crystal form of alumina, and is not only a gemstone of various colors, but a very durable and robust material for optical components. Corundum or α-alumina is the most common natural crystalline form of alumina.
Typical of many ceramics, alumina has low electrical conductivity and is considered an electrical insulator. Unlike many ceramics, alumina has a fairly high thermal conductivity and can be used as a heat sink. Alumina has high temperature capability and is chemically inert, which enables use in corrosive environments even at elevated temperature.
CeraNova has developed a high hardness, high strength, high temperature form of alumina called CeraLumina™ that is transparent in the infrared portion of the spectrum. CeraLumina is being used in applications where infrared transparency is required, and its superior mechanical properties are essential. In optical applications, high strength CeraLumina can be thinner than sapphire reducing adsorption losses. The high hardness of CeraLumina lends itself well to applications where scratch resistance and surface finish durability are vital. A non-transparent form of CeraLumina is also produced for applications where the mechanical properties are needed, but transparency is not required.
Spinel is a naturally occurring mineral that is popular in the jewelry industry, but also has many commercial applications. The cubic crystal structure produces a very transparent single crystal, thus making a wonderful quality gemstone. In the polycrystalline form, spinel is still highly transparent from the near ultraviolet (UV) region of the spectrum, through the visible (VIS) and well into the mid-wave infrared region (IR).
CeraNova uses a transparent polycrystalline form of magnesium aluminate spinel (MgAl2O4) in several optical applications. Unique to CeraNova’s spinel material is its high optical quality combined with high strength. CeraNova’s transparent polycrystalline spinel has strength two to three times higher than the strength of other spinels on the market. Combined with CeraNova’s near net shape forming manufacturing and free form fabrication capabilities, optics that were previously not available are readily produced. Flat plates from millimeters up to a meter are available, as well as spherical and aspherical optical components.
Yttria, or yttrium oxide, is represented by the chemical formula Y2O3. Polycrystalline yttria has a cubic crystal structure and can be produced with very high transparency. Yttria is not a very strong ceramic, but as a single crystal or when produced in a transparent polycrystalline form, yttria transmits energy over one of the broadest bands of any oxide. Good transmission levels are possible from the UV, through the visible, and well into the IR regions of the spectrum.
One growing application for polycrystalline yttria is as a solid-state laser host. The polycrystalline form of transparent yttria allows for higher doping levels and better distribution of dopant elements leading to improved lasing. A number of wave-length specific lasers have been created based on yttria as the host material. Yttria is also used as color emitting phosphors.
CeraNova’s transparent yttria windows and domes have a very fine grain microstructure which produces an optical surface superior to large grain yttria. As with other ceramic components made by CeraNova’s process, both simple and complex transparent yttria parts can be readily produced.
Zirconium dioxide (ZrO2), also termed zirconia, is a chemically inert, high strength and low thermally conductive ceramic that can be used for many applications in many markets. Naturally occurring in a monoclinic crystalline form, other crystal structures are produced using stabilizing dopants. One of the most popular forms in the consumer market is stabilized cubic zirconia found in jewelry as a synthetic diamond or gemstone. The stabilized tetragonal form of zirconia also has very high toughness and wear resistance. This allows zirconia to be used as ball bearings in applications where heat and chemical resistance would limit the use of metal bearings.
Zirconia has high ionic oxygen conductivity at high temperatures allowing its use as an electroceramic, e.g., in oxygen sensors and fuel cell membranes. Other applications of zirconia that take advantage of the material’s high strength, toughness and wear resistance include dental restorations and ceramic knives and cutting blades. Zirconia is used as an optical coating due to its high index of refraction from the near-UV to the mid-IR. CeraNova has produced a transparent form of zirconia that transmits in the same region that could be used in window and dome applications.
The design, production and application of composites as optical materials can be challenging due to the degradation in optical properties that can occur when two dissimilar materials are combined. There are polymer/glass fibers composites used as optical components where improved mechanical properties are realized. Recently, a new class of optical composites has been developed where the microstructure is tailored to produce properties not possible in the single component materials. Properties can be varied through compositional gradients or by combining individual, immiscible materials on very fine scale.
CeraNova was a member of Raytheon’s Nano-Composite Optical Ceramic (NCOC) material team where one such unique composite was developed under a four-year DARPA (Defense Advanced Research Projects Agency) project. This new optical composite provides a revolutionary improvement in infrared transmitting window performance combining a distinctive balance of optical and mechanical performance.
Ceramic high temperature superconductors (HTS) can conduct electricity with zero electrical resistance. The unique electrical and magnetic properties of these materials enable a number of developing technologies. CeraNova has manufactured HTS fiber that has been successfully used in a number of ground-based and space satellite antenna applications.
Piezoelectric materials can provide a force when an electrical field is applied to them due to a small but significant displacement that results. Alternatively, piezoelectric materials will generate an electrical field upon the application of a force.
Piezoelectric ceramics including lead zirconate titanates (PZTs) are the most widely utilized materials for actuators, sensors and transducers due to their excellent piezoelectric properties. The global market for piezoelectric actuators and motors is greater than $10 billion. Piezoelectric ceramics are used in many applications, but are often not seen in devices used every day. For example, piezoelectrics are used in ultrasound imaging in the medical field. They also convert electronic signals to sound in ear buds. When attached to a moving or vibrating object, electrical signals can charge batteries or power devices by energy harvesting.
During normal use electronic devices with components made from PZT materials do not present a health threat. However, when PZT containing devices are disposed, their lead content poses possible health and environment issues if not processed properly. As a result, proper disposal and recycling of devices containing lead-based piezoelectric materials is of high importance, especially those used in consumer products. As the need for materials that are environmentally safe and biocompatible increases throughout the world, some ceramics have been assessed. An increasing amount of government regulations have been enacted such as the European Restriction of Hazardous Substances (RoHS) directives. For almost a decade, restricts hazardous substances used in electrical and electronic equipment have included lead and ceramics that contain lead.
The move toward lead-free piezoelectric materials is understandable. But another reason for using lead-free alternatives is the growing need for piezoelectric ceramics for use at higher temperatures. Lead free piezoelectric ceramics have a much lower specific gravity than lead containing ceramics and can potentially reduce the weight of components in which they are used. In many markets, applications for piezoelectric materials that are lighter than PZTs are desirable. Though no lead-free piezoelectric material has replaced PZT-based ceramics, their development has shown great improvements in all properties. Please contact CeraNova if you wish assistance in processing lead-free piezoelectric ceramics and components made from these materials.