Optical properties of ALON (aluminum oxynitride)
Introduction
Aluminum oxynitride or ALON is a polycrystalline ceramic material of high strength and hardness. This material is made by conventional sintering and hot pressing of a powder compact. Powder compacts of near net shape have been made by isostatic pressing, by slip casting, and by injection molding methods of forming. Full density and intrinsic transparency extending from ultraviolet wavelengths (UV) to mid-infrared wavelengths (MID-IR) have been achieved.
The performance of ALON windows in an optical system will depend on the scatter which can cause a reduction in signal-to-noise and degradation in image resolution. ALON has the cubic [spinel] crystal structure. The optical properties of ALON are therefore isotropic and a polycrystalline ALON ceramic will not scatter light due to birefringence. Scattering due to other sources is present in ALON and recent developments have led to lower scatter levels than have previously been achieved.
In applications where the material will become hot due to aerodynamic heating the emittance of ALON becomes important. IN this paper, recent measurements of scatter in ALON at several wavelengths from the UV to the MID-IR and spectral emittance measurements made from room temperature to 1200°C will be presented.
Measurements of the variation of the refractive index with composition and wavelength are also presented. A comparison of the values of the refractive index, dn/dt, scatter and emissivity from the literature is made.
Section snippets
Composition range of cubic ALON
The stability range for the spinel phase, ALON, in the AlN–Al2O3 system is outside the ideal 9AL2O3·5AlN composition (35.7 mol % AlN). The constant anion model of McCauley [1]and Lejus [2]for the spinel crystal structure given in Eq. (1)describes the defect structure of ALON. In this equation Al is aluminum, V is cation lattice vacancy, O is oxygen, and N is nitrogen. The composition range of stability for single-phase ALON was determined to extend from 5.52 at.% nitrogen (23.7 mol % AlN) to
Optical scatter in ALON
The reduction in optical scatter in transparent polycrystalline materials like ALON is very important in for improving the performance of an optical system. The most detrimental effects of scatter are a reduced signal-to-noise ratio and reduced image resolution. Unlike glass materials such as calcium aluminate or single crystal materials such as sapphire ALON contains grain boundaries. Since ALON has cubic symmetry and is optically isotropic, scattering due to the refractive index difference
Emittance measurements
Emittance measurements were made on a polished ALON sample having a 1-in. diameter and a 0.0775-in. thickness. The measurements were performed by Advanced Fuel Research (East Hartford, CT). Spectra were measured at 32 cm−1 resolution using a Bomem M151 FTIR spectrometer. In the method used, the directional–hemispherical transmittance (τλ), the directional–hemispherical reflectance (ρλ) and the sample radiance (R1) are measured. The spectral emittance (ελ) is obtained from the relationship:
Refractive index of ALON
The variation in refractive index with composition and wavelength was determined by measuring the transmission of thin polished samples of ALON of two different thicknesses. Using the measured external transmission (τ) of the thinnest samples (125 μm to 150 μm thick) and calculated extinction coefficient (β′), the refractive index was calculated numerically from , , , , . Transmission measurements were made using a dual beam spectrophotometer (Perkin-Elmer 580B). This method is considered to be
Summary
Polycrystalline aluminum oxynitride with its cubic Spinel crystal structure is a very durable optical material with a high degree of transparency from the UV to the mid-IR wavelengths. Minimization of grain boundary defects reduces optical scatter significantly. Levels of scatter of about 0.1% can be achieved.
Emittance of the material is an important factor in situations where elevated temperatures are encountered. ALON experiences a rapid emittance increase at wavelengths greater than about 4 μ
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2023, Journal of the European Ceramic SocietyCitation Excerpt :Spinel-type aluminum oxynitride (γ-AlON) is an important single-phase ceramic with a stable structure belonging to the Al2O3-AlN system [1,2]. It has been considered a promising candidate for fabricating transparent armors and infrared/visible windows owing to its excellent properties, namely its outstanding optical transmittance, considerable strength, high hardness, good corrosion resistance, and substantial wear resistance [3–8]. AlON ceramics are usually prepared by a two-step method, in which AlON powders are initially synthesized and then consolidated into a dense bulk at high temperature [9–11].
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2023, Journal of Alloys and CompoundsCompositional tailoring effect on crystal structure, mechanical and thermal properties of γ-AlON transparent ceramics
2022, Journal of the European Ceramic SocietyCitation Excerpt :Due to the characteristic of solid solution [10], it was suggested that the crystal structure and intrinsic properties of γ-AlON can be tuned by changing its composition. Although some reports have already focused on the effect of composition on the lattice parameter of γ-AlON [3,7,9,11], the detailed inspection of local crystalline structure, such as anion parameter and bond length, has rarely been clarified. In addition, some works were concentrated on evaluating composition dependence of mechanical properties.
Direct ink writing of aluminium oxynitride (AlON) transparent ceramics from water-based slurries
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