In this article, we report on the fabrication and characterization of Ti 2 AlC, Ti 2 AlN, and Ti 2 AlC 0.5 N 0.5 . Reactive hot isostatic pressing (hipping) at Ϸ40 MPa of the appropriate mixtures of Ti, Al 4 C 3 graphite, and/or AlN powders for 15 hours at 1300 ЊC yields predominantly single-phase samples of Ti 2 AlC 0.5 N 0.5 ; 30 hours at 1300 ЊC yields predominantly single-phase samples of Ti 2 AlC. Despite our best efforts, samples of Ti 2 AlN (hot isostatic pressed (hipped) at 1400 ЊC for 48 hours) contain anywhere between 10 and 15 vol pct of ancillary phases. At Ϸ25 m, the average grain sizes of Ti 2 AlC 0.5 N 0.5 and Ti 2 AlC are comparable and are significantly smaller than those of Ti 2 AlN, at Ϸ100 m. All samples are fully dense and readily machinable. The room-temperature deformation under compression of the end-members is noncatastrophic or graceful. At room temperature, solid-solution strengthening is observed; Ti 2 AlC 0.5 N 0.5 is stronger in compression, harder, and more brittle than the end-members. Conversely, at temperatures greater than 1200 ЊC, a solid-solution softening effect is occurring. The thermal-expansion coefficients (CTEs) of Ti 2 AlC, Ti 2 AlN, and Ti 2 AlC 0.5 N 0.5 are, respectively, 8.2 ϫ 10 Ϫ6 , 8.8 ϫ 10 Ϫ6 , and 10.5 ϫ 10 Ϫ6 ЊC Ϫ1 , in the temperature range from 25 ЊC to 1300 ЊC. The former two values are in good agreement with the CTEs determined from hightemperature X-ray diffraction (XRD). The electrical conductivity of the solid solution (3.1 ϫ 10 6 (⍀ m) Ϫ1 ) is in between those of Ti 2 AlC and Ti 2 AlN, which are 2.7 ϫ 10 6 and 4.0 ϫ 10 6 ⍀ Ϫ1 m Ϫ1 , respectively.

In this, Part II of a two-part study, the oxidation kinetics in air of the ternary compounds Ti 2 AlC, Ti 2 AlC 0.5 N 0.5 , Ti 4 AlN 2.9, and Ti 3 AlC 2 are reported. For the first two compounds, in the 1000-1100°C temperature range and for short times ͑Ϸ20 h͒ the oxidation kinetics are parabolic. The parabolic rate constants are k x (m 2 /s) ϭ 2.68 ϫ 10 5 exp Ϫ 491.5 (kJ/mol)/RT for Ti 2 AlC, and 2.55 ϫ 10 5 exp Ϫ 458.7 (kJ/mol)/RT for Ti 2 AlC 0.5 N 0.5 . At 900°C, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protective. For the second pair, at short times ͑Ͻ10 h͒ the oxidation kinetics are parabolic at all temperatures examined ͑800-1100°C͒, but become linear at longer times. The k x values are 3.2 ϫ 10 5 exp Ϫ 429 ͑kJ/mol͒/RT, for Ti 4 AlN 2.9 and 1.15 ϫ 10 5 exp Ϫ 443 ͑kJ/mol͒/RT for Ti 3 AlC 2 . In all cases, the scales that form are comprised mainly of a rutile-based solid solution, (Ti 1Ϫy Al y ͒O 2Ϫy/2 where y Ͻ 0.05, and some Al 2 O 3 . The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of Al and Ti. The C and N atoms are presumed to also diffuse outward through the oxide layer. At the low oxygen partial pressure side, the Al 3ϩ ions dissolve in and diffuse through the (Ti 1Ϫy Al y ͒O 2Ϫy/2 layer and react with oxygen to form Al 2 O 3 at the high oxygen pressure side. This demixing results in the formation of pores that concentrate along planes, especially at longer times and higher temperatures. These layers of porosity impede the diffusion of Al, but not those of Ti and oxygen, which results in the formation of highly striated scales where three layers, an Al 2 O 3 -rich, a TiO 2 -rich, and a porous layer repeat multiple ͑Ͼ10͒ times. The presence of oxygen also reduces the decomposition ͑into TiX x and Al͒ temperatures of Ti 4 AlN 2.9 and Ti 3 AlC 2 from a T Ͼ 1400°C, to one less than 1100°C.In this, Part II of a two-part study, 1 we report on the oxidation in air in the 800-1100°C temperature range, of the ternary compounds Ti 2 AlC, Ti 2 AlC 0.5 N 0.5 , Ti 4 AlN 2.9 , and Ti 3 AlC 2 . Since this is the first report on the oxidation of these compounds, there are no previous results with which to compare; it is thus instructive to review the oxidation behavior of some related solids such as Ti, and some Ti-aluminides such as TiAl, Ti 3 Al, and ''Ti 2 Al,'' which is a twophase mixture of the first two. The oxidation of Ti 3 SiC 2 2 was briefly reviewed in Part I. 1 The oxidation of pure Ti in the 600-1000°C temperature range is parabolic. [3][4][5][6][7] In this temperature range, individual rutile TiO 2 layers form that range in thickness from 1 to 8 m depending inversely on temperature. 3-7 These stratified layers tend to spall off periodically. Simultaneously with the formation of a TiO 2 scale, substantial amounts of oxygen dissolve in the Ti substrate. The same is true for the Ti-aluminides; most, but especially the ones for which the Ti:Al ratio is around 2:1, dissolve substantial amounts of oxygen ͑...

Double-perovskite Bi oxides are a new series of superconducting materials, and their crystal structure and superconducting properties are under investigation. In this paper, we describe the synthesis and characterization of a new double-perovskite material that has an increased superconductive transition temperature of 31.5 K. The structure of the material was examined using powder neutron diffraction (ND), synchrotron X-ray diffraction (SXRD), and transmission electron microscopy (TEM). Rietveld refinement of the sample based on ND and SXRD data confirmed an A-site-ordered (K1.00)­(Ba1.00)3­(Bi0.89Na0.11)4O12 double-perovskite-type structure with the space group Im3̅m (No. 229). This structural analysis revealed the incorporation of Na with Bi in the structure and a bent bond between (Na, Bi)–O–(Na, Bi). TEM analyses also confirmed a cubic double-perovskite structure. This hydrothermally synthesized compound exhibited a large shielding volume fraction, exceeding 100%, with onset of superconductivity at ∼31.5 K. Its electrical resistivity dropped near onset at ∼28 K, and zero resistivity was confirmed below 13 K. The calculated band structure revealed that the metallicity of the compound and the flatness of the conduction bands near the Fermi level (E F) are important for the appearance of superconductivity.

Perovskite-type structures (ABO3) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double-perovskite-type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A-site-ordered double perovskite structure, (Na(0.25)K(0.45))(Ba(1.00))3(Bi(1.00))4O12, with a maximum T(c) of about 27 K.

Perovskite‐type structures (ABO3) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double‐perovskite‐type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A‐site‐ordered double perovskite structure, (Na0.25K0.45)(Ba1.00)3(Bi1.00)4O12, with a maximum Tc  of about 27 K.