No abstract

To improve the cycle performance of LiMn2O4 false(normalFdtrue3¯mfalse) as the cathode of 4 V class lithium secondary batteries, the cathode properties of the quaternary cubic spinel phases LiMgyMn2−yO4 synthesized at 750°C were examined. Although the cycle performance of the LiMgyMn2−yO4 was improved by the substitution of Mg2+ for Mn3+ in the octahedral sites, the first discharge capacity was reduced considerably compared with that of the parent LiMn2O4 . In order to compensate for the theoretical capacity reduction in LiMg1/6Mn11/6O4 , nonstoichiometric spinel oxides LiMg1/6Mn11/6O4−δ were prepared under controlled oxygen partial pressures at 750°C. The single‐phase region of δ in LiMg1/6Mn11/6O4−δ was 0 ≤ δ < 0.04, which was larger than that of parent LiMn2O4 (0 ≤ δ < 0.018) in our previous work. From density data, a metal excess model false(normalLi)8afalse[Liδ/false(4−δfalse)Mgδ/6false(4−δfalse)Mn11δ/6false(4−δfalse)]16cfalse[Mg1/6Mn11/6]16dO4 was proposed as the defect structure in the LiMg1/6Mn11/6O4−δ . The chemical diffusion coefficient of lithium ion for nonstoichiometric LiMg1/6Mn11/6O4−δ was smaller than that for the stoichiometric LiMg1/6Mn11/6O4 . This also supported the metal excess model, because excess metals in 16c sites prevented an easier diffusion of lithium in the 8a-16c-8a diffusion path. © 1999 The Electrochemical Society. All rights reserved.

In this paper, room‐temperature continuous‐wave operation of a GaN‐based vertical‐cavity surface‐emitting laser (VCSEL) with a convex structure for optical guiding is demonstrated. The GaN‐based VCSEL is designed and fabricated with the convex structure containing a 15‐nm step between the center area and the peripheral area of the VCSEL with a Nb2O5 spacer layer. In theory the convex structure with a 1.5% relative refractive index difference between the areas could provide more than 10 high‐order modes. Experimental emission spectra and near field pattern images clearly suggested a multi‐mode operation of the VCSEL with the convex structure. In addition, the VCSEL shows a lower threshold current, 2 mA, and a higher maximum light output power, 0.88 mW, which are superior to those of our standard VCSEL fabricated simultaneously without the convex structure. These results indicate that the optical guiding with the convex structure plays an important role in developments of GaN‐based VCSELs.

In organometallic vapor phase epitaxial growth of group III nitrides on sapphire, insertion of a low temperature interlayer is found to improve crystalline quality of AlxGa1−xN layer with x from 0 to 1. Here the effects of the low temperature deposited GaN or AlN interlayers on the structural quality of group III nitrides is discussed.

In organometallic vapor phase epitaxial growth of AlGaN on sapphire, the role of the low-temperature-deposited interlayers on a high-temperature-grown GaN layer was investigated by in-situ stress measurement, X-ray diffraction, and transmission electron microscopy. Crack-free and lowdislocation-density AlGaN with the whole compositional range has been realized on the sapphire substrate.