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Inelastic-neutron-scattering and electron-microscopy studies were performed on single crystals of the cubic phase of Ni, A1, 00, alloys for x=50, 58, 62.5, and 63.9 at. %. The [110]-TAz phonon branch, corresponding to atomic displacements along the [110]direction, is shown to exhibit anomalous behavior in that the entire branch is very sensitive to composition and exhibits a "kink" whose position in q space is also sensitive to x. A peak in the elastic diffuse scattering is also present at the same q as the kink. Amplitude-contrast electron-microscope images display a "tweed" pattern characteristic of many martensitically transforming materials. The high-resolution (phase-contrast) electron image reveals rnicroscopic deviations of the cubic structure and can be viewed as embryos of the low-temperature (5,2) martensitic phase. The displacement fields associated with these distortions are shown to be the origin of the elastic central peak observed in the experiments. Temperature-dependence studies show a substantial softening of the phonon branch in the vicinity of the kink and the elastic diffuse scattering increases as T approaches TM, the martensitic-transformation onset temperature. The mode never becomes completely soft. Furthermore, the softening is narrowly restricted to the [110]direction, which is perpendicular to [110]. Studies of the phonon dispersion curve under uniaxial stress show a softening of the phonon mode near the q value of the kink, which can be interpreted in terms of Clapp's proposed localized-soft-mode theory of nucleation of materials undergoing martensitic transformation. I. INTRQDUCTK)NThe study of martensitic transformations (MT's) have interested metallurgists for nearly a century. ' The use of MT's in physical metallurgy has been prominent in the development of alloys with high strength (e.g. , steels) and others with exotic properties (e.g. , shape-memory alloys).Various applications of MT s have evolved, despite incomplete understanding of the mechanism. Metallurgists have a precise, albeit phenomenological, definition of MT's, whereas physicists have come to use the term loosely to define many first-order phase transitions with acoustic anomalies. It is generally agreed by both disciplines, however, that MT's are first-order, displacive (meaning a coordinated displacement of atoms over distances much less than ihe atomic spacing), and diffusionless (meaning no atom by atom jumping within the cell as in order-disorder transitions).Shear strains are known to play an important role. Furthermore, nucleation and growth is important because of the firstorder nature of the transformation; the system passes through a two-phase regime with distinct interfaces between the two phases. The role of atomic displacements in the nucleation process is still not completely understood.In this paper we describe the study of one system, NiAl, over a range of compositions with the aim at understanding the role of the atomic displacements in the MT of the ordered phase. The crystal structure of the hig...

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While numerous types of gas sensors have been developed for various industries and applications such as the automotive industry, environmental monitoring, and personal safety, nanoscale chemiresistive gas sensors have gained significant research interest due to several advantages such as high sensitivity, low power consumption, and portability. An essential component of these gas sensors is the sensing material where metal oxide semiconductor (MOS) materials are the most prevalent sensing material. Since the adoption of nanoscale synthesis methods for sensing materials development, such as electrospinning and hydrothermal synthesis, many novel 1D MOS‐based nanostructured sensing materials have been demonstrated to enhance gas sensing performance. Overall, nanoengineering approaches and mechanisms for enhancement of gas sensing performance of 1D metal oxide‐based sensing materials are systematically discussed and categorized into several overarching strategies, such as tuning of materials dimension, morphology, and composition. Furthermore, integration of 1D sensing nanomaterials into sensor devices are discussed from the perspective of different chemiresistive sensor architectures and device fabrication methods. Finally, this review also discusses use of 1D MOS materials for emerging and novel electronic nose applications.

The recently built electron-energy-loss spectrometer (EELS) (typical FWHM 60 meV) was employed to measure the EELS spectra of argon in the discrete and autoionization resonance regions at a 2.5-keV impact energy and a mean scattering angle of O'. Relative differential optical oscillator strength spectra were established by multiplying the EELS spectra by the known Bethe-Born conversion factor of the spectrometer, and were normalized at a single point in the smooth continuum using the absolute values reported by Chan et al. [Phys. Rev. A 46, 149 (1992)]. The absolute oscillator strengths corresponding to these energy regions are reported and compared with previously published experimental and theoretical values. The values of parameters q, p, E", and I for autoionization resonances involving states 3s3p np 'P, (n =4, 5, and 6) have been determined.

A high-resolution neutron scattering study of Ni62.5Al37.5 shows that the martensitic transformation is associated with a homogeneous distortion and a nearly sevenfold modulation of the cubic lattice which is predicted by an observed phonon anomaly above TM-In the low-temperature IR phase a nonuniform spacing of superlattice reflections and unusual line shapes are observed which are believed to arise from stacking faults.

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