Cenk Kocer

Ab initio calculation of the crystal structure of the lanthanide Ln2O3 sesquioxides

The Ln2O3 rare-earth oxides, sesquioxides, in the lanthanide series are a group of compounds of particular importance, which are the most widespread lanthanide compounds generally used as a catalyst for the synthesis of many other 4f-materials. In this study, density functional theory was employed to calculate the equilibrium crystal lattice dimensions for most of the rare-earth sesquioxides. The results were found to be in reasonable agreement with experimental data given in the literature.

Simulating temperature effects in the growth of tetrahedral amorphous carbon: The importance of infrequent events

This thin-film deposition study of tetrahedral amorphous carbon shows that including infrequent processes on the millisecond scale substantially improves the accuracy of molecular dynamics simulations. Elevated temperature between energetic impacts is used to activate processes which are typically ignored. In agreement with experiment, the simulations show an abrupt transition in which diamondlike carbon transforms into vertically oriented graphitic sheets. The simulations also highlight the importance of infrequent events in combination with energetic impact. In the absence of the latter, the transition temperature is significantly higher, in good correlation with experiment.

An ab initio study of the ideal tensile and shear strength of single-crystal β–Si3N4

In this study, the ideal tensile and shear strength of single-crystal β-Si 3 N 4 was calculated using an ab initio density functional technique. The stress-strain curve of the silicon nitride polymorph was calculated from simulations of uniaxial strain deformation. In particular, the ideal strength calculated for an applied ∈ 1 1 tensile strain was estimated to be approximately 57 GPa. Recently, a good correlation was reported between the shear modulus of high-strength materials and the experimentally determined Vickers indentation hardness value. Using the reported correlation an estimate was made of the Vickers indentation hardness of single-crystal β-Si 3 N 4 : approximately 20.4 GPa.

An automated incremental finite element study of Hertzian cone crack growth

In this study a novel finite element method is presented for modeling Hertzian cone crack growth. The method involves the automation of an existing incremental finite element technique. The local stress fields in the vicinity of the crack tip at each stage of growth of the crack are determined, and the crack is advanced by a small amount along the direction of maximum strain energy release. The method is used to investigate crack growth behavior in two well-known fracture systems: the simple edge crack system and the Hertzian indentation system. It is shown that the automated incremental finite element method is accurate and reliable, and generates a crack trajectory that agrees closely with experimentally measured trajectories. It is found that a typical trajectory involving 30 incremental steps can be generated in approximately 3 h using a computer with a processor having a clock rate of 200 MHz.

A thermodynamic description of the Al-Ir system

The thermodynamic assessment of the Al-Ir binary system, one of the key sub-systems of the Ir-based alloys, was performed using the CALPHAD technique. The AlIr(B2) phase was described using the two sublattice model with the formula (Al,Ir)0.5(Ir,Va)0.5, while other intermetallic phases were treated as stoichiometric compounds. The calculated data of the phases in the Al-Ir system can be used to accurately reproduce experimental data, such as phase equilibria, invariant reactions, and formation enthalpies of the intermetallic phases.

Impact of a composite metal-glass system; a study using high-speed imaging

In many large and small scale devices metal and glass are used side-by-side. In general, metal components are coupled directly to glass components to provide extra strength. However, in certain configurations the metal-glass interface is a structural weak point. This is particularly the case when the composite metal-glass systems are subjected to impact loading. In this work the impact, and subsequent failure, process of a simply layered glass-metal-glass composite structure was investigated. The structure consisted of a core array of cylindrically shaped metal separators sandwiched between two flat sheets of soda-lime glass. High speed photography was used to capture the impact process, and the subsequent failure, of the composite. Even though significant damage was sustained at the impact point, the high speed photography showed that the initial failure point was not at the impact point.

The importance of rare events in thin film deposition: a molecular dynamics study of tetrahedral amorphous carbon

The transition of diamond-like materials from an sp3 to sp2 rich state is of particular interest because of the desirable properties of tetrahedral amorphous carbon (ta-C). Previous works indicate that infrequent processes may dominate this transition, but simulation of these processes presents significant difficulties, since the infrequent processes are activated on the millisecond scale. In this molecular dynamics study the environment dependent interaction potential is used to simulate the thin film deposition of ta-C. Infrequent processes occurring between energetic impacts were activated on the picosecond scale using elevated temperatures. The simulations reveal an abrupt transition in which the ta-C films transform into graphite-like sheets. A similar transformation, albeit at much higher temperatures, is also observed when the films are heated without energetic impacts. These results are found to be in good agreement with published experimental data.

Conception and design of a thermal valveless micropump

This paper describes the conception and design of a micropump for gas, with no moving or deformable parts, for use on the future sensor network system, which includes environment monitoring. This design is original because of the complete absence of moving parts for the pumping of gas. In fact, fluid movement was obtained by means of repetitive heating and cooling cycles of the gas, in the pump chamber. The prototyping work is on going and the first results are presented with particular emphasis on thermal characterization.

Gibbs energy functions with the vacancy complexes in the Al-Cu binary system

The Gibbs energy functions of the phases in the Al-Cu binary system are taken from the CALPHAD-type thermodynamic assessment (Witusiewicz et al., 2004; Ansara et al., 1998) [1], [2], where the effect of the monovacancy (Va), divacancy (VaVa) and Va-solute atom pair are taken into account based on the formulation (Abe et al., In press). The divacancy is modeled as an associate, VaVa, in the FCC solid solution. The contributions from the Va-solute pair are included through the ternary excess Gibbs energy term. Using the Gibbs energy functions provided in this data article, the fractions of the monovacancies and divacancies, even in various metastable conditions, can be calculated. Since the Gibbs energy functions and phase descriptions are written in the TDB (Thermodynamic DataBase) format, one can use this file with various thermodynamic software packages, such as OpenCalphad [3] etc.