Advanced Material Engineering - Proceedings Of The 2015 International Conference

Proceedings of the 2015 International Conference on Advanced Material Engineering

Yongchang Liu, Yingquan Peng

• Publisher: ICP
• Year: 2015
• ISBN:
  9789814696036
• Topic: Biowissenschaften
• Subtopic: Wissenschaft Allgemein

This book represents a collection of papers presented at the 2015 International Conference on Advanced Material Engineering (AME 2015), held in Guangzhou, China. With the rapid development of industry and information technology, researchers across all fields began to discuss new ideas related to materials science and manufacturing technology. This proceedings provide a valuable insight from researchers and scientists who exchanged their ideas in the conference.

Contents:

• Material Physics and Chemistry:
  • Composites Materials
  • Nanomaterials and Nanocomposites
  • Iron and Steel
  • Ceramic, Films and Glasses
  • Semiconductors Material
  • Chemical Material
  • Biomaterials
  • Optical, Electronic, Magnetic Materials
  • New Energy Materials and Environmental Friendly Materials
  • New Functional Materials
• Materials Process Engineering:
  • Thermal Engineering Theory and Applications
  • Polymer Materials Processing
  • Metallurgy Technology and Application
  • Surface Engineering/Coatings
  • Materials Forming
  • Welding & Joining
  • Laser Processing
  • Severe Plastic Deformation
  • Tribology in Manufacturing Processes
  • Casting and solidification
• Emerging Areas of Materials Science:
  • Atomic Molecular and Laser Physics
  • Spintronics
  • Solid State Ionics (Materials and Devices)
  • Plasma Physics
  • Nanobiomaterials / Drug Delivery

Readership: Graduate students and research professionals in materials engineering keeping up with the latest advancements in the field.
Key Features:

• Latest Research results on Material Engineering
• Cross-disciplinary Research
• Research results come from all over the world
• Some famous professors give the keynote speech on the conference

CHAPTER 1

Material Physics and Chemistry

First-principles Calculation of Silicon Nanowire^*

Pengxian Lu

College of Materials Science and Engineering,
Henan University of Technology, Zhengzhou, 450001, China
E-mail: [email protected]

Xuemei Wang

College of Information Science and Engineering,
Henan University of Technology, Zhengzhou 450001, China

To reveal the relationship between the electronic structure, the lattice dynamics and the thermoelectric properties of silicon nanowire (SiNW) the first-principles calculations of SiNW and silicon bulk (SiB) were conducted in this work by using density functional theory and Boltzmann transport theory. The results indicate that the electrical conductivity of SiNW is increased significantly while its thermal conductivity is reduced sharply as compared to those of SiB, which consequently results in a large enhancement in the figure of merit (ZT) of SiNW at 1200 K. We attribute the increase in the electrical conductivity to the increased density of states at Fermi energy level and the transition from a semiconductor to semimetal behavior. Moreover, the remarkable reduction in the thermal conductivity is resulted from the weakened covalent bonding, the decreased phonon density of states and the shortened mean free path of phonons.

Keywords: First-principles calculation; Electronic structure; Thermoelectric properties; Silicon nanowire.

1. Introduction

SiGe polycrystalline alloys have been regarded as an excellent candidate in the application of a thermoelectric generator in deep space missions and small unattended terrestrial systems. In optimizing the figure of merit (ZT) value of SiGe alloys, more and more attention has been focused on nanostructuring engineering due to the following merits. Firstly, the electrical conductivity of a nanomaterial can be increased by increasing the concentration and the mobility of carriers, the density of states near Fermi energy level or narrowing the band gap. Secondly, the Seebeck coefficient can be optimized by improving the effective mass of carriers. Thirdly, the thermal conductivity can be suppressed by introducing more phonon scattering^[1,2]. Thus, many interests have been attracted to nanowires in order to optimize the thermoelectric figure of merit of a nanomaterial by reducing the thermal conductivity obviously^[3,4]. It has been reported that the optimal ZT value for SiGe nanowire could be increased significantly^[5]. However, the introduced expensive and rare Ge in SiGe nanowire is much disadvantageous to its larger-scale applications.

So, it is of significance to replace SiGe nanowires with abundant and low-cost silicon semiconductor. However, the ZT value of SiB is only 0.001 at room temperature due to its huge thermal conductivity. Very recently, it has been demonstrated that Si nanowires have a Seebeck coefficient and an electrical conductivity as same as those of SiB, but exhibit a 100-fold reduction in thermal conductivity, which yields a large ZT value of 0.6^[6,7]. Meanwhile, many theoretical calculations have been focused on SiNW to investigate the mechanism of the electron and phonon transport. In addition to the interesting scattering mechanisms observed in nanowires, Roh et al.^[8] reported that there is strong anisotropy in the thermal conductivity of nanowires. Moreover, there are many more factors to be explored such as surface roughness scattering. The ZT value of Si nanowires is expected to be increased as much as 20 times as compared to that of SiB for its large reduction in phonon thermal conductivity^[9]. The cross section shape will play an important role in the thermoelectric properties for Si nanowires since the extremely small size is accompanied by a strong quantum confinement effects. The ZT value of Si nanowires can be enhanced to 1.5 and 0.85 for n-doped and p-doped triangular Si nanowires^[10,11]. So it is undoubted that the large enhancement in ZT value of SiNW is attributed to the significant reduction in its phonon thermal conductivity.

To the best of our knowledge, however, there have been no reports demonstrating the relationship between the electronic structure, the dynamics and the increased thermoelectric properties of a SiNW up to now. Therefore, our work is aimed to provide a complete understanding on the mechanism of the electron and phonon transport in SiNW by using a first-principles calculation.

2. Calculation Methods

A SiNW model was fabricated by following the four steps. Firstly, a 4×4×2 supercell of SiB crystal was built. Secondly, a circle surface along [001] plane was cleaved. Thirdly, the atoms outside the cleaved surface of the SiNW were all deleted. Lastly, a vacuum slab of 10 Å was added perpendicular to the grow direction of SiNW. Calculations were performed using CASTEP (Cambridge Serial Total Energy Package), a first-principle pseudopotential method based on the density-functional theory (DFT)^[12,13]. The used pseudopotential was the Norm-conserving pseudopotential. As for the approximation of the exchange correlation term of the DFT, the generalized gradient approximation (GGA) of Perdew was adopted with Perdew-Burke-Ernzerhof parameters^[14]. The cutoff energy of atomic wave functions was set at 390 eV. Sampling of the irreducible wedge of Brillouin zone was performed with a regular Monkhorst-Pack grid of special k-points, which was 30 × 30 × 30 and 6 × 6 × 1 for SiB and SiNW, respectively. The high symmetry points in Brillouin zone were selected as X (0.500, 0.000, 0.000), R (0.500, 0.500, 0.500), A (0.000, 0.000, 0.500), H (-0.333, 0.667, 0.500), K (-0.333, 0.667, 0.000), M (0.000, 0.500, 0.000) for SiB, and L (0.000, 0.500, 0.500)G (0.000, 0.000, 0.000), F (0.000, 0.500, 0.000), Q (0.000, 0.500, 0.500), Z (0.000, 0.000, 0.500), G (0.000, 0.000, 0.000) for SiNW. The geometry optimization of the models was done with the cell optimization criterion (RMs force of 0.01 eV/nm, displacement of 0.0005 Å, SCF tolerance of 5.0×10⁻⁷ eV).

The temperature dependence of the heat capacity C and the Debye temperature Θ_D is obtained by calculating the phonon dispersion. The mean free path λ of phonons is calculated in terms of Eq. 1:

where Θ_D is the Debye temperature, T is the absolute temperature and η is a constant depending on the scattering limitation of phonons^[15,16]. The effective mass of carriers m* is obtained by differentiating the energy at Fermi energy level for twice with respect to the wave vector of the high symmetric points in Brillouin zone.

where

is the Plank constant, E is the energy band near Fermi level, k is the wave vector of the high symmetric points at Brillouin zone. The temperature dependence of the Fermi energy is calculated by Eq. 3:

where

is the Fermi energy at 0 K, T is the absolute temperature and k_B is the Boltzmann constant^[17]. The Fermi-Dirac distribution function

writt...