Engineering topics

Cohen, W.

Publications and source records attributed to Cohen, W..

10 records

DOE OSTI2019

Luminescence of Mn 4+ in the Orthorhombic Perovskites, ASnO 3 (A=Ca, Sr)

The aim of this paper is to provide a comparative study of the spectroscopic properties of Mn 4+ (3 d 3 ) ion in the orthorhombic perovskites, ASnO 3 (A=Ca, Sr). The most significant parameter controlling the energy of the zero-phonon line ( 2 E→ 4 A 2 transition) is the deviation of the octahedral O-Mn-O bond angle from the ideal 90 0 value. It is significant that despite the longer Sn 4+ -O 2- bond distance, the crystal field strength (10 D q ) in CaSnO 3 is higher than that of SrSnO 3 . We discuss this result in relation to the perovskite crystal chemistry. Mulliken population analysis and bond order calculation are made to clarify the nature of chemical bonding in the stannate perovskites.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Optical spectrum of Mn 4+ in Y 2 Ti 2-x Sn x O 7 pyrochlore solid solution: R-line energy and intensity

We report we investigated the luminescence of the Mn 4+ ions in Y 2 Ti 2-x Sn x O 7 (x = 0, 0.5, 1.0, 2.0) pyrochlore solid solutions and compared the results with the spectroscopic data of the two end members. The emission spectra in the mixed pyrochlore solid solutions are broadened relative to the end members due to long range disorder between the Sn 4+ and Ti 4+ ions. The R-line intensity is particularly weak in both Y 2 Sn 2 O 7 and Y 2 Ti 2 O 7 . The increase in R-line intensity in the mixed pyrochlore solid solutions is attributed to the random distribution of Sn 4+ and Ti 4+ ions in the second coordination sphere (next nearest neighbors) that removes the inversion symmetry about the Mn 4+ ion. We find a linear relation between the energy position of the R-line and x in Y 2 Ti 2-x Sn x O 7 ( 2 E g (cm –1 ) = 15 010 + 163.0 x; R 2 = 0.98).

36 MATERIALS SCIENCE↗

DOE OSTI2019

On the Mn 4+ R-line emission intensity and its tunability in solids

The spectroscopic properties of Mn 4+ ions in solids are analyzed to establish key parameters that are responsible for the energy and the intensity of the 2 E g → 4 A 2g zero-phonon emission transition (R-line). These two factors of the emission spectrum are critical in determining the phosphor luminosity or brightness. The emission energy is chiefly determined by the “Mn 4+ -ligand” bonding covalence. In fluorides, the Mn-F bonding covalence is weak, and the R-line occur at higher energy. In oxides, the stronger Mn-O bonding covalence reduces the energy of the 2 E g state. In oxides, the R-line shifts to higher energy with increasing local distortion of the MnO 6 octahedral moiety. Those host crystals that locate the Mn 4+ ion at a noncentrosymmetric site (absence of inversion symmetry) relax the parity section rule, which enhances the R-line intensity. Increased R-line intensity in the emission spectrum produces color changes that increases phosphor luminosity. The influence of second coordination sphere (next-nearest neighbors) on the R-line intensity is considered. This article establishes guidelines for the development of host crystals that meet the requirements of high luminosity phosphors for general illumination applications.

36 MATERIALS SCIENCE↗

NASA NTRS1973

New horizons in chemical propulsion

After a review of the work of the late-Fifties on free radicals for propulsion, it is concluded that atomic hydrogen would provide a potentially large increase in specific impulse. Work conducted to find an approach for isolating atomic hydrogen is considered. Other possibilities for obtaining propellants of greatly increased capability might be connected with the technology for the generation of activated states of gases, metallic hydrogen, fuels obtained from other planets, and laser transfer of energy.

Cohen, W.↗

NASA NTRS1969