Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.Comment: 29 pages, 5 figure

Institute of Ion Beam Physics and Materials Research: Annual Report 2002

Summary of the scientific activities of the institute in 2002 including selected highlight reports, short research contributions and an extended statistics overview

ブリルアン散乱と誘電分光によるリラクサー強誘電体の電場効果の研究

筑波大学 (University of Tsukuba)201

Development of III-nitride-based waveguides for application in all-optical integrated circuits at 1.55 [my]m

El desarrollo de una nueva tecnología todo-óptica para el procesado de datos en las futuras redes de telecomunicación está generando un gran interés desde hace una década. Esta tecnología está encaminada al total aprovechamiento del gran ancho de banda que proporciona la fibra óptica, evitando la conversión entre los dominios óptico y eléctrico necesaria en cada nodo de las redes de comunicaciones actuales. Esta nueva tecnología todo-óptica requiere de diferentes componentes ópticos que puedan ser controlados ópticamente. Estos dispositivos se obtienen a partir de distintos materiales semiconductores y se implementan de forma miniaturizada en un circuito todo-óptico integrado operando a 1.55 [my]m, mejorando de esta forma la fiabilidad del sistema y reduciendo su coste. Teniendo en cuenta que los nitruros del grupo III son materiales que han demostrado un gran potencial para aplicaciones en comunicaciones ópticas a 1.55 [my]m, el objetivo de este trabajo es el desarrollo de nuevos dispositivos todo-ópticos basados en éstos para su futura implementación en circuitos fotónicos integrados ultrarrápidos operando a longitudes de onda de telecomunicación. Durante esta Tesis se han desarrollado varios dispositivos de guía de onda basados en diferentes estructuras de nitruros del grupo III sobre substratos de zafiro y funcionando a 1.55 [my]m. En primer lugar, se han optimizado diferentes guías de onda ópticas basadas en pozos y puntos cuánticos de GaN/AlN para trabajar como absorbentes saturables a través de sus transiciones intersubbanda. Estas guías de onda podrían utilizarse en procesos de conmutación todo-óptica. En segundo lugar, se ha optimizado el crecimiento de AlN por sputtering de radiofrecuencia permitiendo su uso para la fabricación de guías de onda pasivas. El comportamiento óptico lineal de las guías de AlN por sputtering muestra su idoneidad para actuar como interconectores pasivos de bajo coste en un circuito todo-óptico integrado. Por último, se han optimizado dos tipos de guías de onda basadas en InN por sputtering para funcionar como absorbentes saturables inversos mediante procesos de absorción de dos fotones. La respuesta óptica no lineal de ambas guías abre la posibilidad de utilizar estos dispositivos para aplicaciones en limitación todo-óptica a longitudes de onda de telecomunicación

NASA SBIR abstracts of 1991 phase 1 projects

The objectives of 301 projects placed under contract by the Small Business Innovation Research (SBIR) program of the National Aeronautics and Space Administration (NASA) are described. These projects were selected competitively from among proposals submitted to NASA in response to the 1991 SBIR Program Solicitation. The basic document consists of edited, non-proprietary abstracts of the winning proposals submitted by small businesses. The abstracts are presented under the 15 technical topics within which Phase 1 proposals were solicited. Each project was assigned a sequential identifying number from 001 to 301, in order of its appearance in the body of the report. Appendixes to provide additional information about the SBIR program and permit cross-reference of the 1991 Phase 1 projects by company name, location by state, principal investigator, NASA Field Center responsible for management of each project, and NASA contract number are included

Electron Field Emission from Boron Nitride Thin Films.

A systematic study of electron field emission from boron nitride thin films is presented, establishing nanostructured thin film cubic boron nitride (cBN) as a robust and chemically inert material with a low effective workfunction, able to sustain electron emission in a space plasma environment. RHEED data shows the films as polycrystalline, composed of partially oriented crystallites of cBN with predominantly (001) crystallographic texture relative to the Si substrate. FTIR data showed our films to be overwhelmingly cBN, with a volume fraction greater than 75%. AFM images show nanostructures relevant to field enhancement, with a mean feature height of 79 nm, mean RMS roughness of 19 nm, average grain size of 155 nm2 ±84 nm2, and a mean feature radius of ~7 nm. The results are discussed in the light of current theoretical models for electron field emission, including particulars relevant to semiconductors and nanostructured surfaces. Electron emission thresholds were measured from under 1 V/μm up to just under 20 V/μm in vacuum. Voltage sweep measurements were made both in vacuo and in various gas environments relevant to space applications. Repeatability of emission results was demonstrated, albeit with indications of threshold shifts, possibly due to desorption of adsorbate impurities. Time dependence measurements at constant extraction field show stable field emission over periods of extended operation. An effective barrier height Φw of approximately 9.3meV for the as-grown cBN thin films is measured, based on the application of the generalised Fowler-Nordheim theory to the electron field emission measurements, and employing a model of the film surface as an ensemble of self assembled protruberances in the shape of prolate half ellipsoids of revolution on a flat surface. To our knowledge, this is the first experimental determination of this important parameter for cBN films. It appears that the low value of Φw measured for cBN is a direct consequence of the wide gap nature of the band structure, and is evidence in favor of an NEA-type of emission mechanism in cBN. Overall, the results in this study provide ample motivation for further investigations of cubic boron nitride as a promising field emission material.Ph.D.PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58493/1/pedroe_1.pd

High-temperature dielectric ceramics of the tetragonal tungsten-bronze structure and the role of aerosol deposition in modulating permittivity-temperature relationships

This work explores the development of high temperature dielectrics based on an unfilled tungsten bronze-structured ceramic, Ca0.28Ba0.72Nb2O6 (CBN28), for potential power electronics applications. CBN28 exhibited ferroelectric P-E loops, a slightly diffuse permittivity peak around TC at 231 °C, and high losses above 250 °C due to ionic conductivity (tan δ > 0.2). Subsequent A site substitution of Y3+ for Ca2+/Ba2+ with assumed vacancy addition, in the nominal composition (Ca0.28Ba0.72)1–3w/2YwNb2O6 [0 ≤ w ≤ 0.20], increased permittivity-temperature stability at the single-phase limit, w = 0.04 (ɛr = 1143 ±15 % from 71 to 210 ⁰C), and narrowed P-E loops (Pr at 40 kV cm-1 = 1.5 µC cm-2, whereas in CBN28 Pr = 2.4 µC cm-2). Doping with Y3+ and Ta5+, in the nominal composition (Ca0.28Ba0.72)0.94Y0.04Nb1.8Ta0.2O6, induced permittivity-frequency dependence, lowered the temperature of ɛr-Max (TM) to 13 °C at 1 kHz and 46 °C at 1000 kHz, further improved temperature stability of permittivity (ɛr = 954 ±15 % from -58 to 110 ⁰C) and substantially narrowed P-E loops (Pr at 40 kV cm-1 = ~0.5 µC cm-2). Decreasing Ca content, in the formulation (Ca0.22Ba0.78)0.94Y0.04Nb1.8Ta0.2O6, produced ɛr = 972 ±15 % from -9 to 166 ⁰C and removed a secondary phase. To develop low-temperature deposition of strained ceramic films, a particle aerosol deposition (PAD) system was commissioned at Leeds. This produced submicron Al2O3 and CBN28 anchor layer films with evidence of severe ceramic deformation in the layers by electron microscopy. Subsequent PAD (on a system in Manchester) of a calcined, high-energy milled and granulated CBN28 powder produced a thick film (10+ µm) with highly supressed polarisation responses (PMax at 40 kV cm-1 = ~0.3 µC cm-2, εr = ~85). Annealing, up to 750 °C, partially recovered permittivity, retained improved temperature stability, and induced relaxor-like frequency dependence (εr = ~225 ±15 % from ~20 to ~240 °C). XRD peak broadening analysis of the CBN film pre- and post- annealing at 750 °C suggested microstrain reduction from 0.14 to 0.11 % strain in combination with crystallite size increases from ~125 to ~270 nm

Materials sciences programs, Fiscal year 1997

The Division of Materials Sciences is responsible for basic research and research facilities in materials science topics important to the mission of the Department of Energy. The programmatic divisions under the Office of Basic Energy Sciences are Chemical Sciences, Engineering and Geosciences, and Energy Biosciences. Materials Science is an enabling technology. The performance parameters, economics, environmental acceptability and safety of all energy generation, conversion, transmission and conservation technologies are limited by the properties and behavior of materials. The Materials Sciences programs develop scientific understanding of the synergistic relationship among synthesis, processing, structure, properties, behavior, performance and other characteristics of materials. Emphasis is placed on the development of the capability to discover technologically, economically, and environmentally desirable new materials and processes, and the instruments and national user facilities necessary for achieving such progress. Materials Sciences subfields include: physical metallurgy, ceramics, polymers, solid state and condensed matter physics, materials chemistry, surface science and related disciplines where the emphasis is on the science of materials. This report includes program descriptions for 517 research programs including 255 at 14 DOE National Laboratories, 262 research grants (233 of which are at universities), and 29 Small Business Innovation Research Grants. Five cross-cutting indices located at the rear of this book identify all 517 programs according to principal investigator(s), materials, techniques, phenomena, and environment