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2011 | Buch

Kapitel lesen Erstes Kapitel lesen

Atomic Scale Characterization and First-Principles Studies of Si₃N₄ Interfaces

verfasst von: Weronika Walkosz

Verlag: Springer New York

Buchreihe : Springer Theses

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

This thesis presents results from a combined atomic-resolution Z-contrast and annular bright-field imaging and electron energy loss spectroscopy in the Scanning Transmission Electron Microscopy, as well as first principles studies of the interfaces between crystalline β−Si3N4 and amorphous (i) CeO2-x as well as (ii) SiO2 intergranular film (IGF). These interfaces are of a great fundamental and technological interest because they play an important role in the microstructural evolution and mechanical properties of Si3N4 ceramics used in many high temperature and pressure applications. The main contribution of this work is its detailed description of the bonding characteristics of light atoms, in particular oxygen and nitrogen, at these interfaces, which has not been achieved before. The atomic-scale information on the arrangement of both light and heavy atoms is critical for realistic modeling of interface properties, such as interface strength and ion transport, and will facilitate increased control over the performance of ceramic and semiconductor materials for a wide-range of applications.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Silicon Nitride Ceramics

Abstract

The widespread interest in silicon nitride ceramics stems from their desirable physical and mechanical properties in many high temperature and pressure applications [1‐5]. Good resistance to oxidation and corrosive environments, low coefficient of friction and thermal expansion, negligible creep, and high decomposition temperature are some of these important properties. Because of these, silicon nitride (especially its polymorph) is widely used in gas turbines, engine parts, bearings, dental drills and gauges, and cutting tools. In addition, thin films and coatings have been studied in relation to high-speed memory devices [6‐10] and optical waveguide applications [11].

Weronika Walkosz

Chapter 2. Theoretical Methods and Approximations

Abstract

This chapter describes the theoretical approaches and approximations used in standard first principles calculations. First, the Born Oppenheimer approximation is stated. Next, an overview of the basic formulation of density functional theory (DFT) is given, underlying its merits in describing various materials and their properties, but also pointing out the aspects which need further improvements. The Kohn–Sham ansatz, which provides a means of calculating properties of many-body system using independent-particle methods, is presented along with practical schemes for solving the resulting Kohn–Sham equations. Finally, the DFT+U method and its implementation within a pseudopotential-plane wave framework are introduced.

Weronika Walkosz

Chapter 3. Overview of Experimental Tools

Abstract

This chapter reviews a range of transmission electron microscopy (TEM) techniques, including conventional electron microscopy, High-Resolution Electron Microscopy (HREM), Z-contrast imaging, Electron Energy-Loss Spectroscopy (EELS), and spectrum imaging (SI). The latter three will be discussed in relation to Scanning TEM (STEM). The key aspects of these techniques will be described in detail, emphasizing their strengths and drawbacks. Moreover, various simulation techniques will be discussed in the chapter, and a short summary of advances in electron optics will be provided.

Weronika Walkosz

Chapter 4. Structural Energetics of β- $${\bf{{Si}_3{N}_4(10\overline{1}0)}}$$ Surfaces

Abstract

This chapter summarizes results on and analyses of first-principles calculations for the structural energetics of two terminations of surface, the open-ring and the closed-ring, in the presence of O, Ce, and Lu. I start with a discussion of these terminations and their reconstructions. Next, the results from an extensive set of calculations for the preferred bonding sites and configurations of O on both surface terminations are presented as a function of coverage and surface stoichiometry. This is followed by the discussion of Ce adsorption on bare surfaces of Lastly, I present the computational models for Ce and Lu adsorption sites on the oxygenated surfaces and compare them to the experimental results.

Weronik Walkosz

Chapter 5. Atomic-Resolution Study of the Interfacial Bonding at Si3N4/CeO2−δ Grain Boundaries

Abstract

This paper reviews minimal repair models where information is available about the condition of the system analyzed. In the traditional minimal repair model, the age of the system is not changed by the repair. When additional information is available, the concept of a minimal repair has to be generalized, and in this paper, we look closer at some of the suggestions that have been put forward. A point process framework can be used to describe the generalized minimal repair concepts. Several examples demonstrate the applications of the models for optimal replacement problems.

Weronika Walkosz

Chapter 6. Atomic-Resolution Study of β-Si3N4/SiO2 Interfaces

Abstract

In this chapter I describe results of the experimental study of $\hbox{Si}_3\hbox{N}_4/\hbox{SiO}_2$ interface. This work was carried out in an effort to examine the morphology of $\hbox{Si}_3\hbox{N}_4\,(10\overline{1}0)$ surface in the absence of rare-earth elements to better understand their importance in stabilizing the ceramics open-ring termination discussed in Chap. 4. The chapter commences with an introduction to $\hbox{Si}_3\hbox{N}_4/\hbox{SiO}_2$ interfaces, followed by a short summary of the experimental methods used in this study. Next, the results of the investigation are presented in detail, focusing in particular on the composition and bonding characteristics across the interface.

Weronika Walkosz

Chapter 7. Imaging Bulk α-Si3N4

Abstract

In this chapter I describe results on bulk α-Si₃N₄. In particular, the problem regarding the stability of this polymorph is addressed in detail with aberration-corrected Scanning Transmission Electron Microscopy (STEM) and ab initio methods. I start this chapter with an introduction to α-Si₃N₄ followed by the discussion of the obtained Z-contrast images and the Bloch-wave Z-contrast image simulations. Next, I present my Density Functional Theory (DFT) calculations along with the Electron Energy-Loss Spectroscopy (EELS) results confirming the presence of interstitial O in bulk α-Si₃N₄.

Weronika Walkosz

Chapter 8. Conclusions and Future Work

Abstract

The goals of this Ph.D. study were to integrate quantum modelling using first-principles techniques and atomic scale characterization methods in the aberration-corrected STEM, to characterize Si₃N₄/rare-earth oxides (REO) interfaces, and to address the outstanding problems regarding the role of light elements at these interfaces. The experimental techniques such as Z-contrast imaging and electron energy loss spectroscopy in the STEM were used to provide information on the structure, composition and bonding characteristics at the interface with a sub-Ångstrøm resolution. The theoretical calculations based on Density Functional Theory, on the other hand, served to model the interface focusing on the question of how the RE elements and lighter atoms bond to the grains of Si₃N₄. Since the morphology of the ceramics grains as well as its overall properties depend upon the selection of the RE elements and their bonding characteristics at the Si₃N₄/REO interfaces, this study is of both fundamental and technological interest.

Weronika Walkosz

Backmatter

Titel: Atomic Scale Characterization and First-Principles Studies of Si₃N₄ Interfaces
verfasst von: Weronika Walkosz
Verlag: Springer New York
Electronic ISBN: 978-1-4419-7817-2
Print ISBN: 978-1-4419-7816-5
DOI: https://doi.org/10.1007/978-1-4419-7817-2

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