Milton J. Linevsky

Integrated intensities of N2, CO2, and SF6 vibrational bands from 1800 to 5000 cm−1 as a function of density and temperature

Author Institution: Johns Hopkins University/Applied Physics Laboratory, Johns Hopkins Road

Infrared refractive indices and thermo-optic coefficients for several materials

Two techniques for deriving refractive index from high- resolution interferometric infrared transmission measurements are presented. The general characteristics of these techniques are described. Artificial data are used to explore the sensitivity of these techniques to various error sources. Refractive index is determined from interferometric measurements of several materials, including diamond, cubic silicon carbide, yttrium oxide, and KRS-5. The described analysis techniques are applied in a complimentary way to develop temperature-dependent Sellmeier type refractive index models that give accurate dispersion and thermo-optic coefficients. For several of these materials our results are the first infrared measurements of temperature-dependent dispersion. Our results are compared to published refractive index and thermo-optic coefficient data when it exists.

Neutron irradiation of sapphire for compressive strengthening. II. Physical properties changes

Abstract Irradiation of sapphire with fast neutrons (0.8–10 MeV) at a fluence of 1022/m2 increased the c-axis compressive strength and the c-plane biaxial flexure strength at 600 °C by a factor of ∼2.5. Both effects are attributed to inhibition of r-plane twin propagation by damage clusters resulting from neutron impact. The a-plane biaxial flexure strength and four-point flexure strength in the c- and m-directions decreased by 10–23% at 600 °C after neutron irradiation. Neutron irradiation had little or no effect on thermal conductivity, infrared absorption, elastic constants, hardness, and fracture toughness. A featureless electron paramagnetic resonance signal at g=2.02 was correlated with the strength increase: This signal grew in amplitude with increasing neutron irradiation, which also increased the compressive strength. Annealing conditions that reversed the strengthening also annihilated the g=2.02 signal. A signal associated with a paramagnetic center containing two Al nuclei was not correlated with strength. Ultraviolet and visible color centers also were not correlated with strength in that they could be removed by annealing at temperatures that were too low to reverse the compressive strengthening effect of neutron irradiation.

High-temperature optical properties of oxide dome materials

A vacuum emissometer utilizing a CO2 laser for high temperature sample heating has been designed and built for use with a Fourier Transform spectrometer. A two-color pyrometer technique is used to calculate sample temperatures. Oxides such as sapphire, spinel, yttria, ALON and fused silica are experimentally characterized from 600 to 2000 K and from 500 to 5000 cm-1. A glowing yttria sample has also been characterized over the spectral range of 8500 to 13500 cm-1. Good agreement with a quantum mechanical multiphoton model for the complex index of refraction, also developed at APL, is obtained.

Compressive strengthening of sapphire by neutron irradiation

Neutron irradiation of sapphire with 1 x 1022 neutrons(<EQ MeV)/m2 increases the c-axis compressive strength by a factor of 3 at 600 degree(s)C. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. 1-B and Cd shielding was employed during irradiation to filter our thermal neutrons (<EQ1 eV), thereby reducing residual radioactivity in the sapphire to background levels in a month. Yellow-brown irradiated sapphire is nearly decolorized to pale yellow by annealing at 600 degree(s)C with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200 degree(s)C for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600 degree(s)C by 0-20% in some experiments. However, the c- plane ring-on-ring flexure strength at 600 degree(s)C is doubled by irradiation. Elastic constants of irradiated sapphire are only slightly changed by irradiation. Infrared absorption and emission and thermal conductivity of sapphire are not affected by irradiation at the neutron fluence used in this study. Defects that might be correlated with strengthening were characterized by electron paramagnetic resonance spectroscopy. Color centers observed in the ultraviolet absorption spectrum were not clearly correlated with mechanical response. No radiation-induced changes could be detected by x-ray topography or x-ray diffraction.

Multispectral pyrometry for insulating material emissometry

A multispectral pyrometric technique has been developed for obtaining the temperature of many insulating materials which makes use of infrared spectral measurements in the two phonon region of these materials. In the two phonon region the emissivity is virtually temperature independent and the approach developed here requires only a single calibration point. The technique has been found to be simple, convenient, and accurate. A sensitivity of +/- 2 degrees Kelvin at 1000 K is demonstrated on an emission spectrum of sapphire.

Optical properties of polycrystalline magnesium fluoride

A common window material for midwave IR systems is polycrystalline magnesium fluoride. This material is now only available from a French manufacturer, Ceramiques Techniques Desmarquest. The optical constant database on this material is based on samples produced by Kodak and Bausch and Lomb. Because the optical constants are extrinsically dominated in the midwave IR, a new characterization study is needed. The IR absorption coefficient is presented as a function of temperature and frequency. Also, the scatter properties are determined by the bidirectional scatter distribution function.