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 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 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 . 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 oxide (Al2O3) , called 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 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 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 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 on different ceramic materials, mainly due to their lower fabrication properties. So far as optical behaviour is concerned, cost. Additionally, these might offer comparable durability, majority or all of these parameters might act as scatter good mechanical strength and hardness. The first choice centres, thereby prohibiting transmission. 4 in this class goes to the magnesium aluminate (MgAl2O4) The most common and dominant kind of scatter centre which is consisted of classical structure. This material in polycrystalline ceramics are the pores. Porosities in large has nearly similar optical properties with that of sapphire micron- to the finest nano- sizes may remain present in and therefore may be an attractive alternative dome the microstructure in two forms: intragranular (within grain) material. Unfortunately, magnesium aluminate exhibits and intergranular (outside grain, generally along grain relatively much poorer thermal and mechanical properties boundaries, interfaces, etc.). Both forms of porosities can in comparison to the single crystal sapphire; the fact keeps be removed out from the body by changing the fabrication the search for new materials open. parameters, the task for the earlier one being more difficult and needs more stringent control over the processing More recent times witnessed another very attractive factors. The pores basically provide a second phase having alternative material which is a stabilized a different optical property than the matrix phase separated polycrystalline aluminium oxide based ceramic, called 5 through a boundary. Therefore light, while passing through aluminium oxynitride (-AlON) . The material has been the polycrystalline porous ceramic body, gets reflected and under active consideration worldwide. The insertion of refracted in a random way as per the distribution of grains nitrogen converts the rhombohedral crystal structure of and pores in the microstructure. In the process, light is alumina into the defect containing cubic spinel structure directionally changed and the intensity is attenuated, thereby of the -variety and hence the nomenclature -AlON. The producing a translucent or an opaque material. The presence nitrogen solubility extends the homogeneity region to of secondary phases and foreign inclusions produce very Al(64+x)/3O32–xNx (2  x  5). The material displays the similar effects in converting the material to be less required combination of mechanical properties along with transparent as those also act as active scatter centres. Grain transmittance. It has almost the similar optical and boundaries are the high energy paths and usually retain mechanical properties those of sapphire presented in the the impurity phases in it. Hence, large extent of grain table for reader’s ready reference. The -AlON has isotropic boundaries is detrimental. The amount of light scattering crystal structure (cubic), so its mechanical and optical depends on the relative sizes of the impurity centres with properties are crystallographically isotropic. Like respect to the wavelength of light. The dimension of scatter magnesium aluminate spinel, the development of true centres should be less than the wavelength scale. Another transparency is therefore possible in this material in its source of scattering arises out of the crystal structure of polycrystalline form. To fabricate this polycrystalline the material. In case the crystal possesses birefringes, ceramic the common powder metallurgical methods has scattering takes place between the boundaries of two grains been applied. The uniqueness of the material has been with dissimilar crystal orientation. The problem can be observed in the fact that it becomes as transparent as any sorted out either by a change in orientations in the misfit- single crystal of the same material, provided the ceramic

350 SCIENCE AND CULTURE, NOVEMBER-DECEMBER, 2015 is fully dense having no voids, inclusion or grain boundary oxynitride) and could yield prototype samples transparent phases. in region of visible to mid IR- EMR range.

The stabilization of Al2O3 into cubic structure by nitrogen was pursued in USA, UK & France in the beginning of seventies followed which a completed phase diagram on the system Al2O3–AlN was established. However, a very few work has been done during that period for exploitation of this material as a transparent ceramic. The worldwide attention that time was directed more towards development of materials belonging to the Si-Al- O-N system for different engineering applications as hard and strong ceramics. A major programme on development of this particular oxynitride, however, was initiated by the Army Materials and Mechanics Research Center in Watertown, Massachusetts in association with the Fig. 3. Transparent oxynitride developed at CSIR-CGCRI; micrograph Massachusetts Institute of Technology during mid- showing interference fringes at the clean grain boundaries and some 7 seventies. The results of such investigations started coming bending contours superimposed to the image (right) in next few years conveying the possibility of optical transparency in such materials. Initially, problems were - AlON encountered when full densification of the artifacts was The basic search was related to the synthesis of considered. Because of the difficulty in the formation of aluminium oxynitride powder by solid state reaction method fully dense material, the achievement of good transparency and to the kinetics of the formation reaction. Aluminium in AlON was also difficult. Detailed prolonged studies nitride (AlN) and alumina (-Al2O3) powder were used as through decades together indicated that very stringent the starting materials for this reaction. The reaction formation conditions (very high temperature, long heat temperature and the duration were considered as the treatment time, precise control of the formation temperature, reaction parameters. Their effects on the phase formation application of pressure, etc.) are essential for the formation were scrutinized in details. The formation schedule is of the fully dense variety. Recently, the Army Materials presented in the form of flow diagram. Appropriately and Mechanics Research Center has transferred the weighed powders were mixed in attrition mill. Mixed technology of transparent AlON to a manufacturing concern in the United States for recommended uses as IR- seeking domes as well as vision systems in both personal and vehicular armor.

CSIR-CGCRI’s Contribution Towards the Transparent Ceramics CSIR-CGCRI has initiated research activities based on transparent ceramics longer than a decade back in collaboration with the strategic sector of our country as well as contemporarily with foreign counterparts (Max- Planck-Instituet fuer Metallforschung und Instituet fuer Nichtmetallische Anorganische Materialen der Universitat Stuttgart, Germany). The programme was later continued with support from CSIR’s resources through five year period planned programmes of national missions. The aim of the programmes was to develop the aluminium oxynitride materials. The programme could successfully achieve a detailed knowledgebase on the parameters controlling the formation mechanism of - AlON6 (gama- aluminium oxynitride) and MgAlON7 (magnesium aluminium Fig. 4. Processing techniques for oxynitrides

VOL. 81, NOS. 11–12 351 powder was placed within a BN crucible covered with a 1820oC. It was observed that disappearance of all the - o BN lid and was fired in a graphite resistance furnace under Al2O3 takes place within one minute at 1820 C; whereas nitrogen with varying temperature and reaction times. rest of the reaction continues between -AlON and AlN and completes within 30 min. Nitrogen diffusion in the - With increasing temperature the -AlON phase appears AlON lattice acts as the rate controlling step. with simultaneous gradual disappearance of the starting phases. However, there exists a differential rate of MgAlON disappearance of -Al2O3 from the reaction system than that of AlN. -AlON phase first starts to form in the Lack of thermochemical stability of -AlON below temperature range of 1560o to 1630oC but its yield 1640oC imparts a new direction in research to develop a o increases actively as the temperature exceeds 1670 C. The new product from MgO-Al2O3-AlN system. A nitrogen- o containing magnesium aluminate spinel was synthesized by consumption of total Al2O3 completes at 1720 C due to its preferential solubility in the -AlON lattice to yield an solid state reaction method. The attrition milled powder –rich -AlON phase. The fact was confirmed from was green pressed under isostatic pressure at 400 MPa and the change in the values of lattice parameter which decrease the green pressed pellets were fired in a graphite furnace up to 1720oC. Lattice contraction occurs comparatively in under nitrogen atmospheres of 0.1–100 MPa. The reaction oxygen rich zones. Further reaction occurs with the rest of sintering was investigated at a heating rate of 15 K/min. the unreacted AlN phase to reach its nominal composition. Sintering curves were differentiated to obtain the values The unit cell dimension expands during more nitrogen of sintering rates. The model reaction mechanism sequences diffusion into -AlON crystal lattices. The lattice parameters were developed by following the rate curves in conjunction were governed by the bond lengths of Al-O (1.73 Å) and with the phase analysis. Al-N (1.83 Å), respectively. The complete single phase - AlON was achieved at temperatures 1800°C.

Fig. 6. Densification behavior during reaction sintering of the Al2O3– Fig. 5. Dependence of unit-cell dimensions of -AlON on the MgO–AlN starting mixture; rate curve (right scale)7 concentration of starting AlN6; nitrogen anion replacements in place of oxygen governs lattice dimensions First volume expansion observed in the system at >1025oC was due to the oxide spinel formation through The dependence of yield of -AlON on the time function points towards information about the rate reaction between the oxide precursors. This formation o controlling steps. To study the effect of reaction time, the reaction continues up to 1120 C. During this reaction period reaction was carried out at an isothermal temperature of the total MgO is consumed and the rest reacting species, -Al2O3 and AlN are left behind. The second volume expansion is observed at temperature >1350oC when

-Al2O3 and AlN starts to dissolve into the spinel structure converting the spinel to be an oxynitride

352 SCIENCE AND CULTURE, NOVEMBER-DECEMBER, 2015 with temperature. A maximum rate of shrinkage in conjunction with the dissolution of rest of the reacting species occurs in the range of 1650o-1820oC and consequently the total contents of solid solution increases. Beyond 1820oC the rate of shrinkage becomes one. A differential rate of dissolution of -Al2O3 and AlN into the spinel structure was observed, verified by a lattice nil and the density of the sintered body reaches to >94% of theoretical. parameter measurement. Preferentially -Al2O3 dissolves into MgAlON structure and disappears completely forming The overall reaction sequences with compositional an oxynitride sample which then contains less nitrogen changes of MgO–Al2O3–AlN suggested the MgAlON having lattice parameter of 8.0740 Å. Rest of the reacting should be better described as nitrogen containing species MgAlON and AlN undergoes chemical reactions magnesium aluminate spinel rather than the so called MgO- and reaches the nominal composition having lattice stabilized -AlON. parameter of 7.9781 Å. The final reaction reaches Transparent ceramic materials are under active completion after the slower diffusion of nitrogen into the development since last two decades. New types of ceramics MgAlON lattice. This slow nitrogen diffusion ultimately are being made and the existing ones are continuously being dictates the formation reactions and specifies its role as improved. CSIR-CGCRI is involved in development of such the rate controlling step. As the temperature rises >1565oC activities contemporary to the world wide search; especially the shrinkage of the total system becomes dominant where those took place in the developed countries. Looking at the relative density of the sintered body starts increasing the varied application fields, transparent ceramic materials Material Properties Spinel -AlON Sapphire seem to occupy a major share in the forthcoming market of optical grade engineering materials with priority.  Melting Point (°C) 2135 2140 2040 Knoop Hardness (GPa) 16.45 19.50 22 References -3 Density (Kgm ) 3580 3710 3980 1. P. Hogan, T. Stefanik, C. Willingham, R. Gentilman, 10th DoD Crystal structure Cubic Cubic Hexagonal Electromagnetic Windows Symposium, Norfolk, Virginia, May 19, (2004). Flexural Strength (MPa) 184 300 400 2. T. I. Mah, T. A. Parthasarathy, H. D. Lee, Journal of Ceramic Young’s Modulus (GPa) 277 323 379 Processing Research, 5 [4] 369-379 (2004). 3. R. Klement, S. Rolc, R. Mikulikova, J. Krestan, Journal of the Poisson’s Ratio 0.26 0.24 0.27 European Ceramic Society, 28, 1091–1095 (2008). Thermal conductivity 4. I. Reimanis, H. J. Kleebe, Journal American Ceramic Society, (25°C) [WM-1°C-1] 14.6 12.6 24 92 [7] 1472–1480 (2009). Fracture Strength 5. R.J. Xie, H. T. (Bert) Hintzen, Journal American Ceramic Society, 96 [3] 665–687 (2013). (room temperature) [MPa] 193 310 414 6. S. Bandyopadhyay, G. Rixecker, F. Aldinger, S. Pal, K. Refractive Index (4 mm) 1.68 1.77 1.67 Mukherjee, H. S. Maiti, Journal American Ceramic Society, 85 [4] 1010–1012 (2002). Coefficient of Thermal Expansion 8.1 7.8 8.5 7. S. Bandyopadhyay, G. Rixecker, F.Aldinger, H. S. Maiti, Journal -1 -6 (°C x 10 ) [25-1000°C] American Ceramic Society, 87 [3] 480–482 (2004).

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