ARTICLE TRANSPARENT CERAMICS S. BANDYOPADHYAY* AND R. HALDER* Transmission Through Material ight is electromagnetic radiation (EMR) that consists of a broad spectrum of wavelengths ranging from Lultraviolet (UV) to infrared (IR). So far as the sense of sight in human eyes is concerned, light is categorized into two groups- the visible part of EMR with wavelength from 400-700 nm that enables human-being to see. EM spectrum shorter (UV) or longer (IR) than the wave lengths of the visible range is invisible. When light interact any material medium, it may undergo several changes along its path, resulting in several optical phenomena to take Fig. 1. Interaction of light with matter place. The electronic structure, surface property and Transparent Materials in Common Practice microstructure of the medium guide these optical phenomena. The incident energy may partly or fully be The most common experience on transparent medium lost in certain cases. Light may bounce off any object for us is the natural atmosphere (gaseous) or clean water surface (reflection). Photons of appropriate energy may (liquid). Besides the natural resources, artificially made spontaneously be emitted out when electrons return to the silicate-based super cooled liquid with short ranged ground state after being promoted to the excited state by structure, called glass is the most widely used transparent absorbing photons of different incoming frequency materials those pass large quantities of light through them (luminescence). The incident energy may also scatter with little absorption and reflection. Because of its superior because of variation of refractive index within the medium, transmission property in the visible range glass is therefore usually resulted from the presence of impurities, defects, universally used as structural components like door and pores and inhomogeneities in the medium. The incident window materials in houses, schools and office buildings. light energy may also be absorbed when the photon energy The moving structures like automotive and transport is equal to the band gap energy of the object. On the other systems viz., cars, aircrafts, ships, etc. also use glass most hand, the light wave may move all the way through material extensively in the vision-parts like windscreens, windows, (transmission) when the incident light is not lost because backlights, etc. Despite the various applications, the main of the above reasons. During such process, its speed and drawback of glass is its brittleness. Like other ceramic direction may differ from one medium to another materials, the mechanical failure of such materials is (refraction), however, the illuminated object can be seen governed by the surface flaws (Grifith criterion). The clearly through the material medium. combination of easy scratchability (low hardness) and lesser energy requirement for the cracks to propagate (low fracture toughness) subjects the material to catastrophic failures * CSIR-Central Glass & Ceramic Research Institute, Kolkata easily. Although glass is still suitable for uses as mentioned 700032, India earlier from the techno-economic point of view, however, e-mail : [email protected]; [email protected] 348 SCIENCE AND CULTURE, NOVEMBER-DECEMBER, 2015 it is inappropriate in conditions where the vision system demand. The other potential applications of the material faces severe mechanical, thermal and corrosive include infrared domes and efficient NIR-visible up- environmental conditions. To avoid these constraints, converter. crystalline materials with desired optical properties are ideal Owing to the centro- symmetric isometric crystalline replacement for traditional glass. structure, isotropic optical properties in combination with high thermal stability and chemical inertness, YAG2 Ceramics for Optical Transmission ceramics have found wide applications as host material for In the emerging field of modern advance materials, fluorescence application and high power solid-state lasers. the ceramics with optical transmission property is capable Because YAG can host lanthanides in its structure, the of contributing towards a wide range of applications. Some materials display excellent emission properties with engineering grade ceramics possess basic features like high different wavelengths. Nd- and Er- doped YAG are typical hardness, good mechanical strengths associated with high laser materials. Ce-YAG finds application as phosphor in chemical inertness and thermal shock resistance. It has been cathode ray tubes, white light-emitting diodes and as found that some such polycrystalline ceramics are also scintillator. showing capability of transmitting EMR starting from a The modern day demands cruise missiles with much part of UV, covering visible and ranging up to mid IR higher speed as replacements for the subsonic ones. The region. These are nomenclatured as transparent ceramics. air thrust, particle strikes and thermal shock becomes Windows made of such materials are competent to serve alarming when the speed equals to or crosses mach 2 (the the purpose of optical transmission under hostile stressed relative proportion with respect to the speed of sound) in environments. the supersonic region. Although the design of the missile The field of applications for transparent ceramics is counteracts the thrust portion in majority, however, the very wide from military (infra-red guided ballistic missile reaction force of the colliding particles can still be domes to protect the guidance system, vision blocks for equivalent to a bullet- strike. Hence the erosion by the ice tanks, face shields for individual soldier, etc.) to commercial and suspended particles becomes a major concern. In (different kinds of IR transmitting lenses, covers for sodium guided missiles, IR- sensing detectors are acting as eyes vapour lamp, opto-electronic sensors, lasers, etc) purposes. of the system and are protected by a transparent cover that The prerequisites are thermal and mechanical shock faces all these hostile mechanical and thermal challenges. resistant, durable, threat adjustable, oxidation resistant, In the present day, magnesium fluoride (MgF ) and lower density, multifunctional, and cost effective materials. 2 zinc sulfide (ZnS) domes are in practice in the air-to-air infrared guided ballistic missiles as a cover material over the detector component. The material is suitable for transmitting the electromagnetic radiation corresponding to the mid-IR region. Its main difficulty lies in its poor mechanical properties (mainly poor hardness and strength) due to which the MgF2 or ZnS domes cannot withstand high mechanical stress or erossion. Hence the prerequisite becomes the high wall thickness of the artifact which in addition to the high density of the material (MgF2: 4.44 – 4.60 gcm-3) makes the dome heavier in comparison to that of the other materials, which is a disadvantageous feature. Best material that can be recommended to overcome this problem is the single-crystalline variety of transparent 3 Fig. 2. Optical transmission range for different ceramics aluminium oxide (Al2O3) , called sapphire for consideration as an ideal dome material. Majority of the mechanical and Amongst many widespread uses, one important optical properties of sapphire (combination of high strength development is the laser application. As laser power keeps and low scatter) are excellent. It has higher melting point increasing, heat generation is more and the thermal (2040°C), Knoop hardness (22 GPa), flexural strength (400 management dictates the requirement of a material with MPa), Young’s modulus (379 GPa), and fracture strength. higher thermal conductivity and lower thermal expansion The high mechanical strength of sapphire allows the 1 coefficient. Y2O3 is successfully prepared to cater such VOL. 81, NOS. 11–12 349 fabrication of a much thinner walled dome which in crystals by controlling the fabrication parameters or by combination with the lower density of the material might lowering the grain size to the nano scale level. This is the offer a much lighter cover material in comparison to that reason in general, materials with isotropic crystal structure containing MgF2 or ZnS. Inspite of all these excellent primarily constitutes the chosen family of transparent features sapphire is not much favored because of the ceramics, unless very specifically the non-isotropic material difficulty in single crystal formation and of high processing possesses high degree of some other desirable properties, and finishing cost. like mechanical, thermal etc. Characteristically however, For any specific material in general, single crystals transparent materials must possess band gaps corresponding are the best options that display properties to its highest to wavelengths which are shorter than the visible range of levels. Compared to the single crystals, the polycrystalline 380 nm to 750 nm. Since photons with energies below the materials bear a microstructure that is completely different band gap are not collected for transition through the and complex. Although single crystals may contain defect forbidden zone, visible light passes through. that affects properties, the presence of random orientation For the purpose of replacing single crystals, of grains, grain boundaries, pores, secondary phases, polycrystalline aluminium oxide based ceramics are a very inclusions, dislocations and residual stress in polycrystalline important family of advanced structural and transparent materials have far reaching influences