Transparent ceramics

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Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure which is filled with scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque materials, as opposed to transparent materials. Recent nanoscale technology has, however, made possible the production of polycrystalline transparent ceramics such as transparent alumina.

Contents 1 Introduction 2 Transparent ceramics 2.1 High powered lasers 2.2 Armor and IR windows 2.3 IR missile guidance 3 See also 4 References 5 Further reading 6 External links

[edit] Introduction

Optical transparency in polycrystalline materials is limited by the amount of light which is scattered by their microstructural features. Scattering depends on the wavelength of the light. For example, since visible light has a wavelength scale on the order of a micron (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materials (e.g. ceramics) include microstructural defects such as residual porosity and grain boundaries.

Thus, opacity partly results from the incoherent scattering of light at internal surfaces and interfaces. In addition to porosity, most of the interfaces or internal surfaces in a polycrystalline ceramic object are in the form of grain boundaries which separate tiny regions of crystalline order. When the size of the scattering center (or grain boundary) is reduced well below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.

In the processing of crystalline laser materials for guided lightwave applications, the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during the synthesis or formation of the object. Thus a reduction of the original particle size well below the wavelength of visible light (~ 0.5 microns or 500 nm) eliminates much of light scattering, resulting in a translucent or even transparent material.

Furthermore, modeling showed that microscopic pores in ceramic, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. The volume fraction of these microscopic pores had to be less than 1% for high-quality optical transmission, that is the density has to be 99.99% of the theoretical crystalline density.

[edit] Transparent ceramics

For example, a method of sintering submicron particles of alumina has been developed by researchers at the Fraunhofer Institute for Ceramic Technologies and Sintered Materials. This sintered alumina is very hard and nearly transparent. Yet like other sintered materials, it can be produced at temperatures (1000-1200 °C) much lower than its melting point (2070 °C).

In 1961, GE began selling transparent alumina Lucalox bulbs.^[1] In 2004, Anatoly Rosenflanz and colleagues at 3M in Minnesota used a "flame-spray" technique to alloy alumina (aluminium oxide) with rare-earth metal oxides to produce strong glass 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.^{[2][3][4][5][6][7]}

Transparent ceramics have recently acquired a high degree of interest and notoriety, the basic applications being high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications.

The development of transparent materials will have many advanced applications including high strength, impact-resistant materials that can be used as windows in buildings. Construction materials will have improved overall strength, especially for high-shear conditions introduced by earthquakes and high winds. If these expectations bear fruit, the traditional limits seen current building codes could quickly become outdated if the window portion of the structure actually contributes to the shear strength of the wall.

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.

[edit] High powered 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.^[8][9]

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.

Large ceramic laser elements can be produced at a relatively low cost. These components are free of internal stress or intrinsic birefringence, and allow relatively large doping levels or optimized custom-designed doping profiles. This makes ceramic laser elements particularly important for high-energy laser applications. Thus, a 1.46 kW Nd:YAG laser has been demonstrated by Konoshima Chemical Co. in Japan.^{[citation needed]}

Livermore researchers realized that these ceramics might greatly benefit high-powered lasers used in the National Ignition Facility (NIF) Programs Directorate. In particular, a Livermore research team began to acquire advanced transparent ceramics from Konoshima to determine if they could meet the optical requirements needed for Livermore's Solid-State Heat Capacity Laser (SSHCL). Livermore researchers have also been testing applications of these materials for applications such as advanced drivers for laser-driven fusion power plants.^{[citation needed]}

[edit] Armor and IR windows

There is an increasing need in the military sector for high-strength, robust materials which have the capability to transmit light around the visible (0.2 – 0.8 micrometer) and mid-infrared (1 – 5 micrometers) regions of the spectrum. These materials are needed for applications requiring transparent armor. Transparent armor is a material or system of materials designed to be optically transparent, yet protect from fragmentation or ballistic impacts. The primary requirement for a transparent armor system is to not only defeat the designated threat but also provide a multi-hit capability with minimized distortion of surrounding areas. Transparent armor windows must also be compatible with night vision equipment. New materials that are thinner, lightweight, and offer better ballistic performance are being sought.

Existing transparent armor systems typically have many layers, separated by polymer (e.g. polycarbonate) interlayers. The polymer interlayer is used to mitigate the stresses from thermal expansion mismatches, as well as to stop crack propagation from ceramic to polymer. The polycarbonate is also currently used in applications such as visors, face shields and laser protection goggles. The search for lighter materials has also led to investigations into other polymeric materials such as transparent nylons, polyurethane, and acrylics. The optical properties and durability of transparent plastics limit their use in armor applications. Investigations carried out in the 1970s had shown promise for the use of polyurethane as armor material, but the optical properties were not adequate for transparent armor applications.

Several glasses are utilized in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glasses, and fused silica. Plate glass has been the most common glass used due to its low cost, but greater requirements for the optical properties and ballistic performance have generated the need for new materials. Chemical or thermal treatments can increase the strength of glasses, and the controlled crystallization of certain glass systems can produce transparent glass-ceramics. The AREVA T&D Technology Centre (Stafford, UK), currently produces a lithium disilicate based glass-ceramic known as TransArm, for use in transparent armor systems. The inherent advantages of glasses and glass-ceramics include having lower cost than most other ceramic materials, the ability to be produced in curved shapes, and the ability to be formed into large sheets.

Transparent crystalline ceramics are used to defeat advanced threats. Three major transparent candidates currently exist: aluminum oxynitride (AlON), magnesium aluminate spinel (spinel), and single crystal aluminum oxide (sapphire). Aluminum oxynitride spinel (Al₂₃O₂₇N₅), one of the leading candidates for transparent armor, is produced by Raytheon Corporation as AlON and marketed under the trade name Raytran.

The incorporation of nitrogen into an aluminum oxide stabilizes a spinel phase, which due to its cubic crystal structure, is an isotropic material that can be produced as a transparent polycrystalline material. Polycrystalline materials can be produced in complex geometries using conventional ceramic forming techniques such as pressing, (hot) isostatic pressing, and slip casting.

Raytheon has produced an 11-inch (280 mm) x 11-inch curved AlON window, and is currently investigating the scale-up and cost reduction of aluminum oxynitride. The Air Force Research Laboratory (AFRL) is currently funding Raytheon to investigate cost reduction of AlON to produce larger windows, which will allow Raytheon to scale-up AlON such that it can be produced in large sizes at reasonable costs. The Army Research Laboratory is simultaneously investigating transient liquid phase sintering of aluminum oxynitride to reduce processing costs. A reaction sintering technique using a reactive liquid is the focus of the investigation, producing small samples with transmission of 85% and haze of 14%.

Magnesium aluminate spinel (MgAl₂O₄) is a transparent ceramic with a cubic crystal structure. It has been shown that the use of a hot isostatic press can improve its optical and physical properties. Spinel offers some processing advantages over AlON, such as the fact that spinel powder is available from commercial manufacturers while AlON powders are proprietary to Raytheon. It is also capable of being processed at much lower temperatures than AlON, and has been shown to possess superior optical properties within the infrared region. Spinel shows promise for many applications, but is currently not available in bulk form from any manufacturer. Efforts to commercialize spinel are underway.

Single-crystal aluminum oxide (sapphire - Al₂O₃) is a transparent ceramic. Sapphire’s crystal structure is trigonal and its properties vary with crystallographic orientation. Transparent alumina is currently one of the most mature transparent ceramics, and is available from several manufacturers. But the cost is high due to the processing temperature involved, as well as machining costs to cut parts out of single crystal boules. It also has a very high mechanical strength – but that is dependent on the surface finish.

There are current programs to scale-up sapphire grown by the heat exchanger method or edge defined film-fed growth processes. Its maturity stems from its use as windows and in semiconductor industry. Crystal Systems Inc., which uses single crystal growth techniques, is currently scaling their sapphire boules to 13-inch (330 mm) diameter and larger.

Saphikon, Inc. produces transparent sapphire using an edge, defined growth technique. Sapphire grown by this technique produces an optically inferior material to that which is grown via single crystal techniques, but is much less expensive. Saphikon is currently capable of producing ¼" thick sapphire, in 12" x 15" sheets. ARL is currently investigating use of this material in a laminate design for transparent armor systems.

The Chinese Type 99 main battle tank incorporates transparent alumina in its armors. The armor suffered no damage after being shot by a T-72C 125 mm armament 7 times or a 105 mm armament 9 times at a range of 1,800 meters.^{[citation needed]}

[edit] IR missile guidance

AIM-9 Sidewinder
Place of origin	United States

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles (electrons and protons) within atoms is converted to electromagnetic radiation.

Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers", since infrared is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.

The current material of choice for high-speed infrared-guided missile domes is single-crystal sapphire. The optical transmission of sapphire does not actually extend to cover the entire mid-infrared range (3-5 µm), but starts to drop off at wavelengths greater than approximately 4.5 µm at room temperature. While the strength of sapphire is better than that of other available mid-range infrared dome materials at room temperature, it weakens above ~600 °C.

Alternative materials, such as yttrium oxide, offer better optical performance, but inferior mechanical durability. Future high-speed infrared-guided missiles will require new domes that are substantially more durable than those in use today, while still retaining maximum transparency across a wide wavelength range. A long standing trade-off exists between optical bandpass and mechanical durability within the current collection of single-phase infrared transmitting materials, forcing missile designers to compromise on system performance. Optical nanocomposites may present the opportunity to engineer new materials that overcome this traditional compromise.

In optical nanocomposites, two or more interpenetrating phases are mixed in a sub-micrometer grain sized, fully dense body. Infrared light scattering can be minimized (or even eliminated) in the material as long as the grain size of the individual phases is significantly smaller than infrared wavelengths. Experimental data suggests that limiting the grain size of the nanocomposite to approximately 1/15th of the wavelength of light is sufficient to limit scattering.

Nanocomposites of yttria and magnesia have been produced with a grain size of approximately 200 nm. These materials have yielded good transmission in the 3-5 µm range and strengths higher than that for single-phase individual constituents. It is anticipated that further development will result in high strength, high transparency materials suitable for application as next generation high speed missile domes.

Enhancement of mechanical properties in nanocomposite ceramic materials has been extensively studied. Significant increases in strength (2-5 times), toughness (1-4 times), and creep resistance have been observed in systems including SiC/Al₂O₃, SiC/Si₃N₄, SiC/MgO, and Al₂O₃/ZrO₂. In addition, excellent optical transparency in the 3-5 µm range has been demonstrated in yttria-magnesia nanocomposite ceramics. Uniform microstructures have been obtained experimentally, with a characteristic grain size of ~200 nm. Increases in strength and hardness were observed over that of pure transparent yttria or magnesia. These results were obtained using a non-optimized processing scheme. Further improvements in performance are expected as processing is optimized.

[edit] See also

• Sol-gel
• Transparent materials
• Ceramics processing
• Aluminum oxynitride
• Physics of glass
• Light scattering
• Optical fiber
• Nanomaterials

[edit] References

[edit] Further reading

• Ceramic Processing Before Firing, Onoda, G.Y., Jr. and Hench, L.L. Eds., (Wiley & Sons, New York, 1979)
• Colloidal Dispersions, Russel, W.B., et al., Eds., Cambridge Univ. Press (1989)
• Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing by C. Jeffrey Brinker and George W. Scherer, Academic Press (1990)
• Sol-Gel Materials: Chemistry and Applications by John D. Wright, Nico A.J.M. Sommerdijk
• Sol-Gel Technologies for Glass Producers and Users by Michel A. Aegerter and M. Mennig
• Sol-Gel Optics: Processing and Applications, Lisa Klein, Springer Verlag (1994)
• Sol-Gel: A Low temperature Process for the Materials of the New Millenium, Jean Phalippou(2000) www.solgel.com/articles
• Silica Glass from Aerogels, Michael Prassas (2008)www.solgel.com/articles/april01/aerog.htm

[edit] External links

• Light-Scattering Model
• Fraunhofer Institute
• Rosenflanz technique
• Laser Advances