Transparent ceramics

Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms: bulk solid-state components (phone glass), high surface area forms such as thin films, coatings, and fibers. Ceramics have found widespread use for various applications in the electro-optical field including:

optical fibers for guided lightwave transmission optical switches laser amplifiers and lenses hosts for solid-state lasers optical window materials for gas lasers infrared (IR) heat seeking devices for missile guidance systems IR night vision. Optical transparency in materials is limited by the amount of light that is scattered by their microstructural features with the amount of light scattering depending on the wavelength of the incident radiation, or light. For example, since visible light has a wavelength scale on the order of hundreds of nanometers, scattering centers will have dimensions on a similar spatial scale. Most ceramic materials, such as those made of alumina, are formed from fine powders, yielding a fine grained polycrystalline microstructure filled with scattering centers comparable in size to the wavelength of visible light. Thus, they are generally opaque as opposed to transparent materials. In contrast, single-crystalline ceramics may be manufactured largely defect-free (particularly within the spatial scale of the incident light wave), offering nearly 99% optical transparency. Polycrystalline transparent ceramics based on alumina Al2O3, yttrium aluminium garnet (YAG), and neodymium-doped Nd:YAG were made possible by early 2000s nanoscale technology.

Introduction

Transparent ceramics have recently acquired a high degree of interest and notoriety. Basic applications include lasers and cutting tools, transparent armor windows, night vision devices (NVD), and nose cones for heat seeking missiles. Currently available infrared (IR) transparent materials typically exhibit a trade-off between optical performance and mechanical strength. For example, sapphire (crystalline alumina) is very strong, but lacks full transparency throughout the 3–5 micrometer mid-IR range. Yttria is fully transparent from 3–5 micrometers, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. Not surprisingly, a combination of these two materials in the form of the yttria-alumina garnet (YAG) has proven to be one of the top performers in the field. In 1961, General Electric began selling transparent alumina Lucalox bulbs. In 1966, GE announced a ceramic "transparent as glass", called Yttralox. In 2004, Anatoly Rosenflanz and colleagues at 3M used a "flame-spray" technique to alloy aluminium oxide (or alumina) with rare-earth metal oxides in order to produce high strength glass-ceramics with good optical properties. The method avoids many of the problems encountered in conventional glass forming and may be extensible to other oxides. This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol–gel chemistry and nanotechnology. Many ceramic materials, both glassy and crystalline, have found use as hosts for solid-state lasers and as optical window materials for gas lasers. The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, who had the edge on other research teams led by Charles H. Townes at Columbia University, Arthur Schawlow at Bell Labs, and Gould at TRG (Technical Research Group). Maiman used a solid-state light-pumped synthetic ruby to produce red laser light at a wavelength of 694 nanometers (nm). Synthethic ruby lasers are still in use. Both sapphires and rubies are corundum, a crystalline form of aluminium oxide (Al2O3).

Crystals Ruby lasers consist of single-crystal sapphire alumina (Al2O3) rods doped with a small concentration of chromium Cr, typically in the range of 0.05%. The end faces are highly polished with a planar and parallel configuration. Neodymium-doped YAG (Nd:YAG) has proven to be one of the best solid-state laser materials. Its indisputable dominance in a broad variety of laser applications is determined by a combination of high emission cross section with long spontaneous emission lifetime, high damage threshold, mechanical strength, thermal conductivity, and low thermal beam distortion. The fact that the Czochralski crystal growth of Nd:YAG is a matured, highly reproducible and relatively simple technological procedure adds significantly to the value of the material. Nd:YAG lasers are used in manufacturing for engraving, etching, or marking a variety of metals and plastics. They are extensively used in manufacturing for cutting and welding steel and various alloys. For automotive applications (cutting and welding steel) the power levels are typically 1–5 kW. In addition, Nd:YAG lasers are used in ophthalmology to correct posterior capsular opacification, a condition that may occur after cataract surgery, and for peripheral iridotomy in patients with acute angle-closure glaucoma, where it has superseded surgical iridectomy. Frequency-doubled Nd:YAG lasers (wavelength 532 nm) are used for pan-retinal photocoagulation in patients with diabetic retinopathy. In oncology, Nd:YAG lasers can be used to remove skin cancers. These lasers are also used extensively in the field of cosmetic medicine for laser hair removal and the treatment of minor vascular defects such as spider veins on the face and legs. Recently used for dissecting cellulitis, a rare skin disease usually occurring on the scalp. Using hysteroscopy in the field of gynecology, the Nd:YAG laser has been used for removal of uterine septa within the inside of the uterus. In dentistry, Nd:YAG lasers are used for soft tissue surgeries in the oral cavity.

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