http://hdl.handle.net/20.500.12474/9 2026-04-07T23:44:03Z http://hdl.handle.net/20.500.12474/58 Vacancy Induced Energy Band Gap Changes of Semiconducting Zigzag Single Walled Carbon Nanotubes Dereli, Gülay; Eyecioğlu, Önder; Süngü Mısırlıoğlu, Banu n this work, we have examined how the multi-vacancy defects induced in the horizontal direction change the energetics and the electronic structure of semiconducting Single-Walled Carbon Nanotubes (SWCNTs). The electronic structure of SWCNTs is computed for each deformed configuration by means of real space, Order(N) Tight Binding Molecular Dynamic (O(N) TBMD) simulations. Energy band gap is obtained in real space through the behavior of electronic density of states (eDOS) near the Fermi level. Vacancies can effectively change the energetics and hence the electronic structure of SWCNTs. In this study, we choose three different kinds of semiconducting zigzag SWCNTs and determine the band gap modifications. We have selected (12,0), (13,0) and (14,0) zigzag SWCNTs according to n (mod 3) = 0, n (mod 3) = 1 and n (mod 3) = 2 classification. (12,0) SWCNT is metallic in its pristine state. The application of vacancies opens the electronic band gap and it goes up to 0.13 eV for a di-vacancy defected tube. On the other hand (13,0) and (14,0) SWCNTs are semiconductors with energy band gap values of 0.44 eV and 0.55 eV in their pristine state, respectively. Their energy band gap values decrease to 0.07 eV and 0.09 eV when mono-vacancy defects are induced in their horizontal directions. Then the di-vacancy defects open the band gap again. So in both cases, the semiconducting-metallic - semiconducting transitions occur. It is also shown that the band gap modification exhibits irreversible characteristics, which means that band gap values of the nanotubes do not reach their pristine values with increasing number of vacancies. 2017-01-01T00:00:00Z http://hdl.handle.net/20.500.12474/53 INVESTIGATION OF INDENTATION FRACTURE TOUGHNESS (K-IC) AND WEIBULL PARAMETERS OF 0.25Li(2)O.2SiO(2)-0.75BaO.2SiO(2) GLASS-CERAMIC Ertuğ, Burcu; Çetiner, Burcu Nilgün; Gökçe, Hasan; Erkmen, Ziya Engin; Öveçoğlu, Mustafa Lütfi Fracture of ceramics in general starts from flaws, which are spread throughout. The higher the load, the larger the probability of the failure. Also, the failure probability changes with the sample size. According to Griffith/Irwin equation, when stress intensity factor K = s.Y(p.a)–1/2 exceeds the fracture toughness, KIC one can estimate the formation of critical flaws. This equation shows the relationship between the flaw size and critical load [1]. Weibull modulus (m) is widely used to evaluate the structural reliability of the material. The higher the Weibull modulus, the higher the structural reliability [2]. For technical ceramics, the value of the Weibull modulus is between 5 and 20 [3]. The probability of failure changes with distribution of the weak stress regions. Over these weak stress regions, the crack starts and propagates. These regions are related to the fracture toughness of the material. Thus the Weibull statistics and fracture toughness are closely related [4]. Ceramics in general, possess higher hardnesses than metallic materials. However, they indicate poor toughness and low strength reliability. Low Weibull moduli show highly variable crack length in the material exhibiting broad strength distribution [5]. If the type of the distribution function is known, the number of experiments can be reduced, because only a few parameters and not the distribution function in each detail must be determined. In the case of the Gaussian and Poisson distributions, the parameters are the mean value and the standard deviation. For the Weibull distribution, the parameters are the characteristic strength and the Weibull modulus [6]. In the present paper, it was attempted to characterize a novel glass-ceramic composition. Main properties such as Vickers microhardness and indentation fracture toughness were examined. In addition, structural reliability of the samples was evaluated using Weibull parameters derived from the experimental results. 2017-12-01T00:00:00Z