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Transparent Ceramic Spinel

Magnesium aluminate spinel (MgAl2O4) is a transparent ceramic with a cubic crystal structure with an excellent optical transmission from 0.2 to 5.5 microns in its polycrystlalline form. Optical quality transparent spinel has been produced by sinter/HIP, hot pressing, and hot press/HIP operaiotns, and it has been shown that the use of a hot isostatic press can improve its optical and physical properties.

As shown in a field demonstration, a Spinel ceramic plate can survive a direct hit from a .50 BMG round fired at close range. By all indications, this is an important new technology that should find its way into military vehicles in the near future.

Spinel offers some processing advantages over AlONTM, 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 AlONTM, and has been shown to possess superior optical properties within the infrared (IR) region. The improved optical characteristics make spinal attractive in sensor applications where effective communication is impacted by the protective missile dome's absorption characteristics.

Physical and Optical Properties

Chemical formula : MgAl2O4
Thermal conductivity : 17.0 W/m.k @ 20deg. C
Crystal structure : Cubic
Thermal expansion : 7.33 x 10-6/K (20-500deg. C)
Density : 3.58g/cm3
Refractive index : 1.730-1.598(0.4-5.0µm)
Melting point : 2135 deg. C
Transmittance : (Thickness.:4 mm) 0.4-0.8µm >75% 3-5µm
80-87% >98%(Coated antireflection film)
Vickers hardness : >1.26 x 10^4 MPa/mm2
Strength of flexure : 140-200MPa

Application

Transparent ceramic spinel is an attractive material for uses in a wide range of optical, electronic and structural applications including windows and lenses, which require excellent transmission from the visible through to the mid IR. Theoretical transmission is very uniform and approaches 87% between 0.3 to 5 microns. Transmission characteristics rival that of sapphire in the mid-wave IR, making it especially attractive for the ever-increasing performance requirements of current and next-generation IR imaging systems.

• Pressure vessel sight glass, furnace sight glass, boiler water gauge.
• Domes, optical window, lens, prism.
• LED, ceramic substrate material for electronic packaging.
• Valve flap, valve seat, a water-jet cutting flow-shaper, a nozzle for process fluids.
• High-pressure arc lamp and lens, safety goggles.

Specification

Tile and camber : Max Φ280×20mm;
Dome : Max Φ255×8 mm;
Other spec. upon request.

OptoCity now supply transparent spinel ceramic plates to window, lens, and optical system suppliers and end-users.

More information about Transparent Ceramics

From Wikipedia, the free encyclopedia
Large laser elements made from transparent ceramics 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 lasers.Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers. [1] Such devices have found widepread 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 and optical window materials for gas lasers, and infrared (IR) heat seeking devices for missile guidance systems and IR night vision. [2]

While single-crystalline ceramics may be largely defect-free (particularly within the spatial scale of the incident light wave) optical transparency in polycrystalline materials is limited by the amount of light which is scattered by their microstructural features. The amount of light scattering therefore depends on the wavelength of the incident radiation, or light.[3]

For example, since visible light has a wavelength scale on the order of a micrometer, or 'micron' (one millionth of a meter), scattering centers will have dimensions on a similar spatial scale. 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 as opposed to transparent materials. Recent nanoscale technology has, however, made possible the production of (poly)crystalline transparent ceramics such as alumina Al2O3, yttria alumina garnet (YAG), and neodymium-doped Nd:YAG. [4][5][6][7][8][9]

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. [2]

In 1961, GE began selling transparent alumina Lucalox bulbs.[10] 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. [11] [12] [13] [14] [15]

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. [16] [17]

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. [18] 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. [19] 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. [20] In dentistry, Nd:YAG lasers are used for soft tissue surgeries in the oral cavity.

High powered Nd:YAG lasers as large as a football field are currently used used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments.

Glasses

Glasses (non-crystalline ceramics) are also widely used as host materials for lasers. Relative to crystalline lasers, they offer improved flexibility in size and shape and may be readily manufactured as large, homogeneous, isotropic solids with excellent optical properties. The indices of refraction of glass laser hosts can be varied between about 1.5 and 2.0, and both the temperature coefficient of n and hte the strain-optical coefficient can be tailored by altering the chemical composition. Glasses have lower thermal conductivities than the alumina or YAG, however, which imposes limitations on their use in continuous and high repitition-rate applications.[18]

The principal differences between the behavior of glass and crystalline ceramic laser host materials are associated with the greater variation in the local environment of lasing ions in amorphous solids. This leads to a broadening of the fluorescent levels in glasses. For example, the width of the Nd3+ emission in YAG is ~ 10 angstroms as compared to ~ 300 angstroms in typical oxide glasses. The broadened fluorescent lines in glasses make it more difficult to obtain continuous wave laser operation (CW), relative to the same lasing ions in crystalline solid laser hosts.[18]

Several glasses are used in transparent armor, such as normal plate glass (soda-lime-silica), borosilicate glass, 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 necessitated the development of new materials. E.G. Chemical or thermal treatmtents can increase the strength of glasses, and the controlled crystallization of certain glass compositions can produce optical quality glass-ceramics. AREVA, Ltd.,currently produces a lithium disilicate based glass-ceramic known as TransArmTM, for use in transparent armor systems. It has all the workability of an amorphous glass, but upon recrystallization it demonstrates properties similar to a crystalline ceramic. VycorTM is 96% fused silica glass, which is crystal clear, lightweight and high strength. One advantage of these type of materials is that they can be produced in large sheets and other curved shapes.[21].[22]

Nanomaterials

It has been shown fairly recently that laser elements (amplifiers, switches, ion hosts, etc.) made from fine-grained ceramic nanomaterials—produced by the low temperature sintering of high purity nanoparticles and powders—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 highlights the use of ceramic nanomaterials as being particularly important for high-energy laser elements and applications.


SEM of hydrated colloidal silica particles with an average particle diameter = 600 nm.Primary scattering centers in polycrystalline nanomaterials—made from the sintering of high purity nanoparticles and powders—include microstructural defects such as residual porosity and grain boundaries (see Transparent materials). 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 ceramic nanomaterials are in the form of grain boundaries which separate nanoscale regions of crystalline order. Moreover, 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 light scattering no longer occurs to any significant extent. [23]

In the processing of high performance ceramic nanomaterials with superior opto-mechanical properties under adverse conditions, 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 µm or 500 nm) eliminates much of the light scattering, resulting in a translucent or even transparent material.

Furthermore, results indicate that microscopic pores in sintered ceramic nanomaterials, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. it has been observed that the total volume fraction of these nanoscale pores (both intergranular and intragranular porosity) must be less than 1% for high-quality optical transmission. I.E. The density has to be 99.99% of the theoretical crystalline density. [11][24]

One example of such a material is that which has ben developed by researchers at the Fraunhofer Institute for Ceramic Technologies and Sintered Materials. This sintered alumina nanomaterial is very hard and virtually transparent over a range of wavelengths. Yet like other sintered materials using larger particles of larger diameter and less sophisticated processing methodologies, it can be produced at temperatures (1000-1200 °C) much lower than its melting point (2070 °C).

Advantages

Tests have clearly shown that the transparent nanomaterials have exceeded all expectations. For example, the amount of scattered light in the nanomaterials is similar to that measured from single crystalline Nd:YAG components. This is due primarily to the fact that grain boundaries, which act as the primary scattering centers in this theoretically dense material, measure something less than 1 nm in average width. In addition, the nanomaterials have exhibited no more wavefront distortion than that expected from surface polishing.

One research team has found that laser amplifier slabs made from nanomaterials offer several advantages over those made from single crystals. Perhaps most basic is that these slabs can be obtained in a regular, timely fashion without unexpected additional costs. Nanomaterials are more easily fabricated into larger sizes for greater pwoer and they can also be made any size and shape—limited only by the size of the sintering furnace. The time required to produce such slabs of optically transparent nanomaterials is much shorter than the time to grow crystal boules of the same chemical composition. E.G. Nanomaterials may be fabricated in several days, whereas it would take several weeks to grow single crystals of similar chemical composition.[25]

Slabs made form nanomaterials are exhibit a higher value of fracture toughness (and stress intensity factor, KIc) than single-seed crystal slabs and musch laess prone to undergo a catastrophic failure or fracture. I.E. When a crstalline slab fractures, the propagation of primary crack is virtually unimpeded, yielding a large (and fairly linear) path of least resistance. Thus, the crack can eaisly "run", extending some distance from the original crack site and often branching or making a random turn towards the center of the crystal in order to relieve internal stress. In fine-grained nanomaterials, because crack propagation is impeded by grain boundaries, resistance to fracture is significant. Nanomaterials also exhibit lower residual stress, as evidenced by the increased distortion of the laser beam in crystalline materials, which can be a contributing factor to failure of the materials.[25]

Nanomaoterials can accommodate higher concentrations of dopaants (rare earth ions such as neodymium Nd3+ or yterbium Yb3+) which could permit pumping at wavelengths that might otherwise be impractical. Dopant concntraitons are quite homogeneous in nanomaterials and be prisely controlled. In crystals, dopants tend to segregate preferentially towards the bottom of the growing boule.[25]

Nanomaterials also provide the possibility of novel composite structures. The different materials would be co-sintered in order to produce a single (monolithic) integral structure. Another possible approach is to embed different powders with the same host before sintering the slab in order to create a progressive and continuous concentration gradient of the dopant ions—a molecular configuration evidenced in some phase transformations in solids which proceed by the mechanism of spinodal decomposition.[25]

Powder Prep

Nd:YAG
Initially, Akin prepared YAG powder by Self-Propagating Combustion synthesis. Then Chung used citrate gel methods for YAG powder synthesis. Also Graskite reported that the single-phase Ln:YAG (Ln = Ce, Nd, Ho, Er)is formed after heating of resulting powder by Sol-gel methods at 1000 C.[26][27][28]

One group in Iran used Sol-gel methods for synthesizing Nd:YAG powder which can be sintered to transparency for laser applications. High purity Nd2O3, Y2O3 and Al(NO3)3 · 9 H2O were used with stoichiometric amounts as the starting materials, including 4 atomic percent of Nd dopant. Initially, the neodymia and yttria powders were dissolved separately in acetic acid at 75 degrees C. This was then combined in solution with the aluminum nitrate and some distilled water, followed by stirring for 3 hours at about 65 degrees C. Then 1,2-ethanediol was added and stirred in thoroughly for about 3 hours at 65 C. The transparent gel formed after a final evaporation of fluid components at 65 C. The gel was then dried at 110 C, then grounded nad pre-heated at 800 C for 2 hours in air. The resulting white powder was ground finely and heated to 900 C for 2 hours in air. Microstructure and morphology of the powders were studied by EXDX analysis and Scanning Electron Microscopy.[12]

Another group in India synthesized nanopowders of YAG which can be prepared by solution/ sol-gel methods. This method yields a precursor powder that forms a phase-pure YAG powder at a lower temperature due to better compositional homogeneity than can be achieved in the solution-based processing. A homogeneous co-precipitation technique is a process in which the particle size distribution and morphology can be tailored by appropriately choosing the precursors salt and modifying their concentrations and reaction temperature appropriately. Due to the molecular evolution of one of the reactants (e.g. ammonia from hydrolysis of urea) a minimum degree of local fluctuation of reactant concentration occurs, leading to the formation of a large number of nuclei, followed by orderly growth into compact particles. However, it would result in sequential precipitation in the case of a two component system due to he difference in the chemistry of different metals, but still results in the mixing of components in the nano- dimension, if not in the molecular level. If the precipitate is already gelled upon filtering and drying, then the particles get strongly agglomerated and require extensive grinding in order to form a powder. One way of getting rid of the problem is to form granular precursor powder directly in the desired particle size range by proper calcining at a higher temperature. In this study, the granular precursor powder was prepared by the homogeneous co-precipitation reaction of aqueous aluminum nitrate - yttrium nitrate-urea in the presence of ammonium sulfate was studied at 95 C.

Al(NO3)3 + Y(NO3)3 + (NH4)2SO4 + CO(NH2)2
The chemistry of the Al3+ and the Y3+ are such that precipitation begins at a ph ~ 4.5 and ends at a pH ~ 6.5.[29]

Using the yttria powder produced from the method described here and a commercial ultrafine Al2O3 powder, fully transparent YAG nanomaterials were fabricated by vacuum sintering at 1700 °C for 4 h through a solid-state reaction method. It was found that the addition of 0.5 wt.% tetraethyl orthosilicate (TEOS) is suitable for successful processing and fabrication. If the amount of TEOS is less than 0.05 wt.%, abnormal grain growth occurs, and intergranular pores become entrapped. If the amount of TEOS is more than 3 wt.%, a large amount of liquid phase is yielded, leaving some residual inclusions at grain boundaries which cause some light scattering and attenuation.[30]

Yttria, Y2O3
One group in China describes a precipitation process for synthesizing nanocrystalline yttria powder and sintering transparent yttria ceramics. In this work, a hydroxide precursor of Y2O3 with an approximate composition of [Y2OH · 5 NO3 · H2O] was synthesized by using ammonia water as precipitant (and morphological catalyst) and yttrium nitrate as the starting salt. Employing appropriate striking method and optimum synthetic conditions, yttrium hydroxide with a card-house or spherical structure can be formed. The addition of a small amount of ammonia sulfate in the yttrium nitrate solution reduces the agglomeration and particle size of the produced yttria powders. Yttria nanoparticels of 60 nm dimaeter were obtained by calcining the precursor at 1100 °C for 4 hours. Transparent yttria nanomaterials were fabricated from the nanopowders by vacuum sintering at 1700 °C for 4 hours.[31]

Huang emphasized that pH value at the end of the precipitation process has a significant effect on the size and morphology of the precursor and the yttria powders. Under the same calcination condition, the yttria powders made from the precursor obtained at a pH of 8 are smaller in mean particle size and narrower in size distribution than those made from the precursor obtained at a pH of 10. It was found that the optimum calcination temperature is 1000 °C, and the yttria powder obtained is fine (30 nm) and well dispersed. Using the yttria powder, transparent yttria nanomaterials were produced by vacuum sintering at 1700 °C for 4 hours without any additives.[32]

In Jeong's synthesis by precipitation, three different precipitants such as NH4OH, mixture of NH4OH and NH4HCO3 and NH4HCO3 were used. It was observed that the size of yttria powder is significantly affected by type of precipitant depending on the rate of formation of precipitate. It is seen that NH4HCO3 shows the most significant pH decrease during the formation of Y2CO3, thus leading to the formation of large crystalline nanoparticles. The result also shows that the crystal size depends on calcinations temperature, and that the degree of agglomeration is more significant beyond 800 °C where the morphology has changed from flake to spherical shapes which are connected to each other. Finally, the nano-sized yttria powder of on the average 114 nm shows excellent uniform coating, sufficient transmittance and excellent brightness for lamp.[33]

Dupont utilized a sol–gel method which they adopted to synthesize a precursor gel which, after being heated, leads to high purity Y2O3 nanopowders. In these studies, two main parameters are taken into account: the nature of the chelating agent and the relative concentrations of yttrium to the chelating agent. The influence of these parameters on the composition and crystallization state of the precursor gel has been studied in the first part and on the grain morphologies of the resulting yttria powder in the second part. It has been shown that, depending on the chelating agent, nanometric powders presenting either needles, platelets or spherical shapes can actually be obtained at 800°C or 1100°C.[34]

Comparison has been made of the achievable purity levels and agglomeration characteristics of zirconia powders obtained via six different wet chemical routes where the starting material was an impure Zr-oxychloride solution obtained from Indian zircon sand. For the highest purity (2–15 ppm of other oxides except Hf), two routes were found to be effective: crystallization of the oxychloride via the basic sulphate, or double crystallization of the oxychloride, followed by dissolution and hydroxide precipitation. The average agglomerate size was the largest (10–15 µm) for the powders obtained by direct decomposition of the basic sulphate at 1000°C, though the size distribution was narrow and distinctly monomodal. For smaller agglomerates (1.5 – 2.5 µm), on the other hand, the size distribution was very wide. Calcination at 800°C of powders obtained via hydroxide precipitation from a solution always yielded only tetragonal zirconia as the crystalline phase.[35] Roles of calcination and sintering schedules on the final sintered density and microstructure of yttria powder compacts and nanomaterials were studied by Rasmussen. The studies of his group showed clearly that the optimum calcination procedure appeared to be at 1000°C and for one hour in air atmosphere. Final sintered density of 95% theoretical was achieved around 1450° and nearly pore free (>99% theoretical) pellets were obtained at 1650°C for one hour sintering in vacuum. High heating/cooling rates when coupled with high sintering temperature caused discontinuous grain growth and large internal cracks in the final stages of densification. [36]

La-doped yttria powder was synthesized by a co-precipitation method using ammonia for pH control. After calcinations, finer particles with narrow distribution and large surface area were obtained. After dry pressing, samples were sintered at 1500–1700° for 4 h in vacuum to produce transparent nanomaterials with uniform grain size. Samples with 9 mol% lanthanum were transparent in visible light after being sintered at 1500° for 4 h. The grain sizes increased with lanthanum doping levels, in comparison to those of pure yttria under the same conditions. The relative density of the transparent ceramic was 99.7%. The in-line transmittance was 73% at 580 nm wavelength after milling and polishing.[37]

Lasers

Nd:YAG
For example, a 1.46 kW Nd:YAG laser has been demonstrated by Konoshima Chemical Co. in Japan. In addition, Livermore researchers realized that these fine-grained ceramic nanomaterials 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 nanomaterials 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. [25]

Assisted by several workers from the NIF, the Livermore team has produced 15 mm dimaeter samples of transparent Nd:YAG from nanoscale particles and powders, and determined the most important parameters affecting their quality. In the these objects, the team largely followed the Japanese production and processing methodologies, and used an in house furnace to vacuum sinter the nanopowders. All specimens were then sent out for hot isostatic pressing (HIP). Finally, the components were returned to Livermore for coating and testing, with results indicating exceptional optical quality and properties.[25]

One Japanese/East Indian consortium has focused specifcially on the spectroscopic and stimulated emission characteristics of Nd3+ in transparent YAG nanomaterials for laser applications. Their materials were synthesized using vacuum sintering techniques. The spectroscopic studies suggest overall improvement in absorption and emission and reduction in scattering loss. SEM and TEM observations revealed an excellent optical quality with low pore volume and narrow grain boundary width. Fluorescence and Raman measurements reveal that the Nd3+ doped YAG nanomaterial is comparable in quality to its single-crystal counterpart in both its radiative and non-radiative properties. Individual Stark levels are obtained from the absorption and fluorescence spectra and are analyzed in oredr to identify the stimulated emission channels possible in the material. Laser performance studies favor the use of high dpoant concentration in the design of an efficient microchip laser. With 4 at% dopant, the group obtained a slope efficiency of 40%. High-power laser experiments yield an optical-to-optical conversion efficiency of 30% for Nd (0.6 at%) YAG nanomaterial as compared to 34% for an Nd (0.6 at%) YAG single crystal. Optical gain measurements conducted in these materials also show values comparable to single crystal, supporting the contention that these materials could be suitable substitutes to single crystals in solid-state laser applications.[38]

Yttria, Y2O3
The initial work in developing transparent yttrium oxide nanomaterials was carried out by General Electric in the 1970s, motivated by lighting and ceramic laser applications. Greskovich and Woods fabricated transparent yttrium oxide Y203 containing ~ 10% thorium oxide (ThO2) which they called YttraloxTM.[39] The additive served to control grain growth during densificaiotn, so that porosity remained on grian boundaries and not trapped inside grains where it would be quite difficult to eliminate during the initial stages of sintering. Typically, as polycrystalline ceramics densify during heat treatment, grains grow in size while the remaining porosity decreases both in volume fraction and in size. Optically transparent ceramics must be virtually pore-free.

GE's transparent YttraloxTM was folllowed by GTE's lanthana-doped yttria with similar level of additive.[40] Both of these materials required extended firing times at temperatures above 2000 C. La203 - doped Y203 is of interest for infrared (IR) applications because it is one of the longest wavelength transmitting oxides. It is refractory with a melting point of 2430 deg C and has a moderate coefficient of thermal expansion coefficient. The thermal shock and erosion resistance is considered to be intermediate among the oxides, but outstanding compared to non-oxide IR transmitting materials. A major consideration is the low emissivity of yttria, which limits background radiation upon heating. It is also known that the phonon edge gradually moves to shorter wavelengths as a material is heated. [41]

In addition, ytrria itself, Y2O3 has been clearly identified as a prospective solid-state laser material. In particular, lasers with ytterbium as dopant allow the efficient operation both in cw operation [42] and in pulsed regimes. [43] </ref> At high concentration of excitations (of order of 1%) and poor cooling, the quenching of emission at laser frequency and avalanche broadband emission takes place. [44]

Future

The Livermore team is also exploring new ways to chemically synthesize the initial nanopowders. Borrowing on expertise developed in CMS over the past 5 years, the team is synthesizing nanopowders based on sol-gel processing, and then sintering them accordingly in order to obtain the solid-state laser components. Another technique being tested utilizes a combustion process in order to generate the powders by burning an organic solid containing yttrium, aluminum, and neodymium. The smoke is then collected, which consists of spherical nanoparticles.[25]

The Livermore team is also exploring new forming techniques (e.g. extrusion molding) which have the capacity to create more diverse, and possibly more complicated, shapes. These include shells and tubes for improved coupling to the pump light and for more efficient heat transfer. In addition, different materials can be co-extruded and then sintered into a monolithic transparent solid. E.G. An amplifier slab can formed so that part of the structure acts in guided lightwave transmission in order to focus pump light from laser diodes into regions with a high concentration of dopant ions near the slab center.[25]

In general, nanomaterials promise to greatly expand the availability of low-cost, high-end laser components in much larger sizes than would be possible with traditional single crystalline ceramics. E.G. Many classes of laser designs could benefit from nanomaterial-based laser structures such as amplifies with built-in edge claddings. Nanomaterials could also provide more robust and compact designs for high-peak power, fusion-class lasers for stockpile stewardship, as well as high-average-power lasers for global theater ICBM missile defense systems (e.g. Strategic Defense Initiative SDI, or more recently the Missile Defense Agency.[25]

Night vision

Panoramic Night Vision Goggles in testing.A night vision device (NVD) is an optical instrument that allows images to be produced in levels of light approaching total darkness. They are most often used by the military and law enforcement agencies, but are available to civilian users. Night vision devices were first used in World War II, [45] and came into wide use during the Vietnam War. The technology has evolved greatly since their introduction, leading to several "generations" of night vision equipment with performance increasing and price decreasing. The United States Air Force is experimenting with Panoramic Night Vision Goggles (PNVGs) which double the user's field of view to around 95 degrees by using four 16 mm image intensifiers tubes, rather than the more standard two 18 mm tubes. [46] [47]

Thermal images are actually visual displays of the amount of infrared (IR) energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera is capable of performing algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is actually using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature.


Soldiers pictured during the 2003 Iraq War seen through Night Vision GogglesNight vision infrared devices image in the near-infrared, just beyond the visual spectrum, and can see emitted or reflected near-infrared in complete visual darkness. All objects above the absolute zero temperature (0 K) emit infrared radiation. Hence, an excellent way to measure thermal variations is to use an infrared vision device, usually a focal plane array (FPA) infrared camera capable of detecting radiation in the mid (3 to 5 µm) and long (7 to 14 µm) wave infrared bands, denoted as MWIR and LWIR, corresponding to two of the high transmittance infrared windows. Abnormal temperature profiles at the surface of an object are an indication of a potential problem. [48] Infrared thermography, thermal imaging, and thermal video, are examples of infrared imaging science. Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 900–14,000 nanometers or 0.9–14 µm) and produce images of that radiation, called thermograms.

Since infrared radiation is emitted by all objects near room temperature, according to the black body radiation law, thermography makes it possible to see one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature. Therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and to security services.


Thermography
Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past fifty years. [49] [50] Firefighters use thermography to see through smoke, to find persons, and to localize the base of a fire. Maintenance technicians use thermography to locate overheating joints and sections of power lines, which are a tell-tale sign of impending failure. Building construction technicians can see thermal signatures that indicate heat leaks in faulty thermal insulation and can use the results to improve the efficiency of heating and air-conditioning units. Some physiological changes in human beings and other warm-blooded animals can also be monitored with thermal imaging during clinical diagnostics.

In thermographic imaging, infrared radiation with wavelengths between 8-13 microns strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers utilizing a transparent ceramic detector do not require cooling. Thus, a microbolometer is essentially an uncooled thermal sensor. [51]


Seen through a Night Vision Device (NVD), paratroopers conduct a raid on a suspected terrorist's home in Fallujah, Iraq. The soldiers are assigned to the: 82nd Airborne Division's Company B, 1st Battalion, 505th Parachute Infantry Regiment, U.S. Army.The material used in the detector must demonstrate large changes in resistance as a result of minute changes in temperature. As the material is heated, due to the incoming infrared radiation, the resistance of the material decreases. This is related to the material's temperature coefficient of resistance (TCR) specifically its negative temperature coefficient. Industry currently manufactures microbolometers that contain materials with TCRs near -2%. [52]

VO2 and V2O5
The most commonly used ceramic material in IR radiation microbolometers is vanadium oxide. The various crystalline forms of vanadium oxide include both VO2 and V2O5. Deposition at high temperatures and performing post-annealing allows for the production of thin films of these crystlalline compounds with superior properties, which may be easily integrated into the fabrication process. VO2 has low resistance but undergoes a metal-insulator phase change near 67oC and also has a lower TCR value. On the other hand, V2O5 exhibits high resistance and also high TCR. [51]

Other IR transparent ceramic materials that have been investigated include doped forms of CuO, MnO and SiO.

Place of origin United States

Many ceramic nanomaterials of interest for transparent armor solutions are also used for electromagnetic (EM) windows. These applications include radomes, IR domes, sensor proteciotn, and multi-spectral windows. Optical properties of the materials used for htese applicaiotns are critical, as the transmission window and related cut-offs (UV - IR)control the spectral bandwidth over which the window is operational. Not only must these materials possess abrasion resistance and strength properties common of most armor applications, but due to the extreme temperatures associated with the environment of military aircraft and missiles, the must also possess excellent thermal stability.[22]

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. 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. [53] [54] [55] [56]

Sapphire
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. [57]

Limitations to larger area sapphires are often business related, in that larger induciton furnaces and costly tooling dies are necessary in order to exceed current fabrication limits. However, as an industry, sapphire producers have remained competitive in the face of coating-hardened glass and new ceramic nanomaterials, and still managed to offer high performance and an expanded market.[22]

Yttria, Y2O3
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.

The first full scale missile domes of transparent yttria manufactured from nanoscale ceramic powders were developed in the 1980s unuder Navy funding. Raytheon perfected and characterized its undoped polycrystalline yttria, while lanthana-doped yttria was similarly developed by GTE Labs. The two versions had comparable IR transmittance, fracture toughness, and thermal expansion, while the undoped version exhibited twice the value of thermal conductivity.

Renewed interest in yttria windows and domes has prompted efforts to enhance mechanical properties by using nanoscale materials with submiron or nanosized grains. Inone study, three vendors were slected to provide nanoscale powders for testing and evaluaiotn, and they were compared to a conventioanl (5 µm) yttria powder previously used to prepare transparent yttria. While all of the nanopowders evaluated had impurity levels that were too high to allow processing to full transparency, 2 of them were processed to theoretical density and moderate transparency. Samples were sintered to a closed pore state at temeperatures as allow as 1400 C.[58]

After the relatively short sintering period, the compnent is placed in a hot isostatic press (HIP) and processed for 3 – 10 hours at ~ 30 kpsi(~200 MPa) at a temperature similar to that of the intitial sintering. The applied isostatic pressure provides additional driving force for densificaiotn by substantially increasing the aomtic diffusion coefficients, which promotes additional viscous flow at or near grain boundaries and intergranular pores. Using this method, transparent yttria nanomaterials were produced at lower temperatures, shorter total firing times, and without extra additives which tend to reduce the thermal conductivity.[58]

Recently, a newer method has been devleoped by Mouzon, which relies on the methods of glass-encapsulation, combined with vacuum sintering at 1600 °C followed by hot isostatic pressing (HIP) at 1500 °C of a highly agglomerated commercial powder. The use of evacuated glass capsules to perform HIP treatment allowed samples that showed open porosity after vacuum sintering to be sintered to transparency. The sintering response of the investigated powder was studied by careful microstructural observations using scanning electron microscopy and optical microscopy both in reflection and transmission. The key to this method is to keep porosity intergranular during pre-sintering, so that it can be removed subsequently by HIP treatment. It was found that agglomerates of closely packed particles are helpful to reach that purpose, since they densify fully and leave only intergranular porosity.[59]

Composites
Prior to the work done at Raytheon, optical properties in nanocomposite ceramic materials had received little attention. Thier studies clearly demonstrated near theoretical transmission in nanocomposite optical ceramics for the first time. The yttria/magnesia binary system is an ideal model system for nanocomposite formation. There is limited solid solubility in either one of the constituent phases, permitting a wide range of compositions to be investigated and compared to each other. According to the phase diagram, bi-phase mixtures are stable for all temperatures below ~ 2100 degrees C. In addition, neither yttria nor magnesia shows any absorption in the 3 - 5 µm mid-range IR portion of the EM spectrum.

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.[60][61] 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.[62][63][64]

The strengthening mechanisms observed vary depending on the material system, and there does not appear to be any general consensus regarding strengthening mechanisms, even within a given system. In the SiC/Al2O3 system, for example, it is widely known and accepted that the addition of SiC particles to the Al2O3 matrix results in a change of failure mechanism from intergranular (between grains) to intragranular (within grains) fracture. The explanations for improved strength include:

A simple reduction of processing flaw concentration during nanocomposite fabrication.
Reduction of the critical flaw size in the material—resulting in increased strength as predicted by the Hall-Petch relation)
Crack deflection at nanophase particels due to residual thermal stresses introduced upon cooling form processing temperatures.
Microcracking along stress-induced dislocations in the matrix material.[65]
[edit] Armor
There is an increasing need in the military sector for high-strength, robust materials which have the capability to transmit light around the visible (0.4–0.7 micrometers) 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. [66][67]

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. [21]

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. [68]

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. [21]

AlONR
Aluminum oxynitride spinel (Al23O27N5), one of the leading candidates for tranparent armor, is produced by the Raytheon Corporation as AlONTM and marketed under the trade name RaytranTM. The incorporation of nitrogen into aluminum oxide stbailizes a crystalline spinel phase, which due to its cubic crystal structure and unit cell, is an isotropic material which can be produced as transparent ceramic nanomaterial. Thus, fine-grained polycrystalline nanomaterials can be produced and formed into complex geometries using conventional ceramic forming techniques such as hot pressing and slip casting.

The Surmet Corporation has acquired Raytheon's ALONR business and is currently building a market for this technology in the area of point-of-sale scanner windows and as an alternative to quartz and sapphire in the semiconductor market. The high hardness of AlONTM provides a scratch resistance which exceeds even the most durable coatings for glass scanner windows, such as those used in supermarkets. Surmet has successfully produced a 15" by 18" curved ALONR window and is currently attempting to scale up the technology and reduce the cost. In addition, the U.S. Army and U.S. Air Force are both pushing the envleope for development into next generation applications.[21][69][70]

Spinel
Magnesium aluminate spinel (MgAl2O4) is a transparent ceramic with a cubic crystal structure with an excellent optical transmission from 0.2 to 5.5 microns in its polycrystlalline form. Optical quality transparent spinel has been produced by sinter/HIP, hot pressing, and hot press/HIP operaiotns, and it has been shown that the use of a hot isostatic press can improve its optical and physical properties.[21] [71]

As shown clearly in the field demonstration in this Online video clip a Spinel ceramic plate can survive a direct hit from a .50 BMG round fired at close range. By all indications, this is an important new technology that should find its way into military vehicles in the near future.

Spinel offers some processing advantages over AlONTM, 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 AlONTM, and has been shown to possess superior optical properties within the infrared (IR) region. The improved optical characteristics make spinal attractive in sensor applications where effective communication is impacted by the protective missile dome's absorption characteristics.[21][72][73]

An extensive review of the literature has indicated clearly that attempts to make high-quality spinel have failed to date because the densification dynamics of spinel are poorly understood. They have conducted extensive research into the dynamics involved during the densification of spinel. Their research has shown that LiF, although necessary, also has extremely adverse effects during the final stages of densification. Additionally, its distribution in the presursor spinel powders is of critical importance.

Traditional bulk mixing processes used to mix LiF sintering aid into a powder leave failry inhomogeneous distribution of Lif that must be homogenized by extended heat treatment times at elevated temperatures. The homogenizing temperature for Lif/Spinel occurs at he temperatureof fast reaction between the LiF and the Al2O3. In order to avoid this detrimental reaction, they have developed a new process that uniformly coats the spinel particles with the sintering aid. This allows them to reduce the amount of Lif necessary for densificaiotn and to rapdily heat through the temperature of maximum reactivity. These developments have allowed to fabricate MgAl2O4 spinel to high transparency with extremly high reproducibility that should enable military as well as commercial use of spinel.[74]

Sapphire

Synthetic sapphireSingle-crystal aluminum oxide (sapphire – Al2O3) is a transparent ceramic. Sapphire’s crystal structure is rhombohedral and thus its properties are anisotropic vary with crystallographic orientation. Transparent alumina is currently one of the most mature transparent ceramics from a production and application perspective, 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.[21]

The high level of maturity of sapphire from a production and application standpoint can be atrributed to two areas of business: EM windows for missiles and domes, and electronic/semiconductor industries and applications.

There are current programs to scale-up sapphire grown by the heat exchanger method or edge defined film-fed growth (EFG) 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. Another producer, the Saint-Gobain Group 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, and retains much of the hardness, transmission, and scratch-resistant characteristics. Saint-Gobain is currently capable of producing 0.43" thick (as grown)sapphire, in 12" × 18.5" sheets, as well as thick, single-curved sheets.[21]ARL is currently investigating use of this material in a laminate design for transparent armor systems. The Saint Gobain Group have commercialized the capability to meet flight requirements on the F-35 Joint Strike Fighter and F-22 Raptor next generation fighter aircraft.[22]

Composites
Future high-speed infrared-guided missiles willl require new dome materials that are substantially more durable than those inuse today, while retinaing maximum transparency across the entire operational spectrum or bandwidth. A long-standing compromise exists between optical bandpass and mechanical durability within the current group of single-phase (crystalline or glassy) IR transmitting ceramic materials, forcing missile designers to accept substandard overall system performance. Optical nanocomposites may provide the opportunity to engineer new materials that may overcome these traditional limitations.[22]

For example, transparent ceramic armor consisting of a light weight composite has been formed by tilizing a face plate of transparent alumina Al2O3 (or magnesia MgO) with a back-up plate of transparent plastic. The two plates (bonded together with a transparent adhesive) afford complete ballistic protection against 0.30 AP M2 projectiles at 0° obliquity with a muzzle velocity of 2770 ft. per second. [75] Another transparent composite armor provided complete protection for small arms projectiles up to and including caliber .50 AP M2 projectiles consisting of two or more layers of transparent ceramic material.[76][77]

Nanocomposites of yttria and magnesia have been produced with an average grain size of ~ 200 nm. These materials have exhibited near theoretical transmission in the 3 - 5 µm IR band. Additionally, such composites have yielded higher strengths than those observed for single phase solid-state components. Despite a lack of agreement regarding mechanism of failure, it is widely accpted that nanocomposite ceramic materials can and do offer improved mechanical properties over those of single phase materials or nanomaterials of uniform chemical composition.[60]

It should also be noted here that nanocomposite ceramic materials also offer interesting mechanical properties not achievable in other materials, such as superplastic flow and metal-like machinabality. It is anticipated that further devleopment will result in high strength, high transparency nanomaterials which are suitable for application as next generation armor.[21].

Future
With regard to future developments, a European consortium of companies and institutions has been working to develop a transparent alumina Al2O3. The main objective is the development of significantly improved alumina ceramic nanomaterials with an extremly reduced grain size. These components must exhibit the following physical properties: exceptional mechanical strength ~700 MPa; high optical transparency; improved corrosion resistance (e.g. against metal halides); and a complex hollow shape (vs. cylindrical). The improved material should solve problems in existing applications (e.g. metal halide lamps) and may lead to new applications such as scratch-resistant windows for bar-code scanners (now sapphire). Improvements in metal halide lamps could lead to substantial energy savings up to 7 billion kWh in Europe, which corresponds to one big power station or 4.5 million tons of CO2 production.[78]

Researchers at OSU have made significant advances in the emerging science of transparent electronics, creating transparent copper oxide (CuO) based semiconductors which exhibit more than 200 times the conductivity of previously competitive materials. This basic research is opening the door to new types of electronic circuits which, when deposited onto glass, are literally invisible. The studies are so cutting edge that the products which could emerge from them have not yet been invented—although they may find applications ranging everywhere from flat-panel displays (e.g. automobile dashboard displays) to invisible circuits on screens and visors.[79]

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