Ab-initio elastic tensor of cubic Ti0.5_{0.5}Al0.5_{0.5}N alloy: the dependence of the elastic constants on the size and shape of the supercell model

In this study we discuss the performance of approximate SQS supercell models in describing the cubic elastic properties of B1 (rocksalt) Ti$_{0.5}$Al$_{0.5}$N alloy by using a symmetry based projection technique. We show on the example of Ti$_{0.5}$Al$_{0.5}$N alloy, that this projection technique can be used to align the differently shaped and sized SQS structures for a comparison in modeling elasticity. Moreover, we focus to accurately determine the cubic elastic constants and Zener's type elastic anisotropy of Ti$_{0.5}$Al$_{0.5}$N. Our best supercell model, that captures accurately both the randomness and cubic elastic symmetry, results in $C_{11}=447$ GPa, $C_{12}=158$ GPa and $C_{44}=203$ GPa with 3% of error and $A=1.40$ for Zener's elastic anisotropy with 6% of error. In addition, we establish the general importance of selecting proper approximate SQS supercells with symmetry arguments to reliably model elasticity of alloys. In general, we suggest the calculation of nine elastic tensor elements - $C_{11}$, $C_{22}$, $C_{33}$, $C_{12}$, $C_{13}$, $C_{23}$, $C_{44}$, $C_{55}$ and $C_{66}$, to evaluate and analyze the performance of SQS supercells in predicting elasticity of cubic alloys via projecting out the closest cubic approximate of the elastic tensor. The here described methodology is general enough to be applied in discussing elasticity of substitutional alloys with any symmetry and at arbitrary composition.Comment: Submitted to Physical Review

Elinvar effect in β−\beta-Ti simulated by on-the-fly trained moment tensor potential

A combination of quantum mechanics calculations with machine learning (ML) techniques can lead to a paradigm shift in our ability to predict materials properties from first principles. Here we show that on-the-fly training of an interatomic potential described through moment tensors provides the same accuracy as state-of-the-art {\it ab inito} molecular dynamics in predicting high-temperature elastic properties of materials with two orders of magnitude less computational effort. Using the technique, we investigate high-temperature bcc phase of titanium and predict very weak, Elinvar, temperature dependence of its elastic moduli, similar to the behavior of the so-called GUM Ti-based alloys [T. Sato {\ it et al.}, Science {\bf 300}, 464 (2003)]. Given the fact that GUM alloys have complex chemical compositions and operate at room temperature, Elinvar properties of elemental bcc-Ti observed in the wide temperature interval 1100--1700 K is unique.Comment: 15 pages, 4 figure

Sűrűségfunkcionálok atomokra és molekulákra = Density functionals for atoms and molecules

A sűrűségfunkcionál elmélet az egyik leghatékonyabb módszer, mellyel az atomok, molekulák és szilárdtestek tulajdonságai vizsgálhatók. A kutatás célja az elmélet továbbfejlesztése, új területekre történő kiterjesztése volt. Új, egzakt tételeket, összefüggéseket vezettünk le (pl. spinviriál tétel). A sűrűségskálázást felhasználva általánosított Kohn-Sham-egyenleteket vezettünk le. Bebizonyítottuk, hogy létezik olyan skálafaktor, melyre az új Kohn-Sham-rendszerben a korrelációs energia eltűnik. Új pársűrűségfunkcionál elméletet dolgoztunk ki. Megmutattuk, hogy alapállapotban a pársűrűség meghatározható egy kételektronos probléma megoldásával. Egy tetszőleges, páros számú elektront tartalmazó rendszer tárgyalása kételektron problémára vezethető vissza. | Density functional theory is one of the most effective methods to study the electronic properties of atoms, molecules and solids. The aim of the project was to develop and extent the density functional theory. New, exact theorems and relations were derived (e.g. spin virial theorem). Generalized Kohn-Sham equations were derived using the method of density scaling. It was proved that there exist a scaling factor for which the correlation energy disappears. A new pair density functional theory was worked out. In the ground state the pair density can be determined by solving a single auxiliary equation of a two-particle problem. The problem of an arbitrary system with even electrons can be reduced to a two-particle problem

Origin of the anomalous piezoelectric response in wurtzite Scx_xAl1−x_{1-x}N alloys

The origin of the anomalous, 400% increase of the piezoelectric coefficient in Sc$_x$Al$_{1-x}$N alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a strong change in the response of the internal atomic coordinates to strain and pronounced softening of C$_{33}$ elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric response.Comment: 10 pages, 4 figures, accepted for publication in Phys. Rev. Let

Eljárások, módszerek kifejlesztése atomok és molekulák elektronszerkezetének vizsgálata = Development of procedures and methods to investigate the electronic structure of atoms and molecules

1. A kémiai Hamilton operátor közelítés (CHA, chemical Hamiltonian approach) keretén belül módszert fejlesztettünk ki molekulák erőállandóinak és rezgési frekvenciáinak számítására. A kifejlesztett módszert alkalmaztuk hidrogénhidas molekulakomplexekre. 2. Elektronkorreláció számítására alkalmas módszer kifejlesztése és alkalmazása a CHA elmélet keretein belül. 3. Új típusú komplex és imaginárius elnyelő potenciálok kifejlesztése és ezek összehasonlítása. 4. Számos molekuláris rendszer nem adiabatikus tulajdonságait vizsgáltuk a nem adiabatikus csatolási mátrixok segítségével. | 1. Applying the chemical Hamiltonian approach a method has been developed to calculate the force constants and harmonical vibrational frequencies in molecules. This method has been used to study hydrogen-bonded molecular complexes. 2. Using the CHA framework electron correlation method has been developed and applied to study the electronic structure of different molecules. 3. New types of complex and imaginary absorbing potentials have been developed and compared. 4. The non-adiabatic behaviour of several molecular systems have been studied using the non-adiabatic coupling matrices

Significant elastic anisotropy in Ti1−x_{1-x}Alx_xN alloys

Strong compositional-dependent elastic properties have been observed theoretically and experimentally in Ti$_{1-x}$Al$_x$ N alloys. The elastic constant, C$_{11}$, changes by more than 50% depending on the Al-content. Increasing the Al-content weakens the average bond strength in the local octahedral arrangements resulting in a more compliant material. On the other hand, it enhances the directional (covalent) nature of the nearest neighbor bonds that results in greater elastic anisotropy and higher sound velocities. The strong dependence of the elastic properties on the Al-content offers new insight into the detailed understanding of the spinodal decomposition and age hardening in Ti$_{1-x}$Al$_x$N alloys.Comment: 3 figures, 3 page

A lovas turizmus szerepe hazánkban, különös tekintettel az ifjúsági vendégkör ágazati motivációira : Nyitottság és felelősség

szabadidő aktív eltöltése fontos lehetőségét jelenti a szervezet regenerálódásának, a munkára való felkészülésnek. Opaschowsky felosztása szerint a rekreációs szükségletek kielégítése az egyik út a testi, lelki, szellemi egyensúly megtartásához. A számos sportlehetőség mellett ide köthető a rekreációs szükséglet alapján jelentkező lovaglási motivációk kielégítése is. Kutatásunk során a Nemzeti Lovas Program megszületésének küszöbén arra a kérdésre kerestük a választ, hogy a fiatal – egyetemista – korosztály mit tud a lovaglási lehetőségekről, milyenek a korábbi tapasztalataik, mennyire illeszkedik ez a lehetőség szabadidős szokásaikba, illetve hogyan értékelik az ágazatot. A kérdések megválaszolásához alkalmazott piackutatás formája kérdőíves megkérdezés, zárt, nyitott és feleletválasztós kérdések kombinációjával. A kapott válaszokat statisztikailag kiértékeltük és a releváns adatokat diagramon ábrázoltuk. Összességében elmondható, hogy jelenleg messze nem tekinthető általánosnak a lovas turizmus kínálatának kiaknázása a fiatalok körében. A nyitottság a potenciális vendégkör esetében azonban fennáll a lovas turizmusban való részvétel felé. Óriási tehát a felelőssége az oktatással foglalkozó intézményeknek az általános iskolától kezdődően a tudatformálás tekintetében. És legalább ekkora felelőssége van az államnak, valamint a vállalkozásoknak a feltételek, a szükséges infrastruktúra biztosítása vonatkozásában, hogy hazánk a célkitűzések szerint ténylegesen lovasbarát turisztikai desztinációvá válhasson

Temperature-dependent elastic properties of Ti_(1−x)Al_xN alloys

Ti_(1−x)Al_xN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the symmetry imposed force constant temperature dependent effective potential method, which include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C_(11) decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy

Machine-learning potentials for nanoscale simulations of deformation and fracture: example of TiB2_2 ceramic

Machine-learning interatomic potentials (MLIPs) offer a powerful avenue for simulations beyond length and timescales of ab initio methods. Their development for investigation of mechanical properties and fracture, however, is far from trivial since extended defects -- governing plasticity and crack nucleation in most materials -- are too large to be included in the training set. Using TiB$_2$ as a model ceramic material, we propose a strategy for fitting MLIPs suitable to simulate mechanical response of monocrystals until fracture. Our MLIP accurately reproduces ab initio stresses and failure mechanisms during room-temperature uniaxial tensile deformation of TiB$_2$ at the atomic scale ($\approx{10}^3$ atoms). More realistic tensile tests (low strain rate, Poisson's contraction) at the nanoscale ($\approx{10}^4$--10$^6$ atoms) require MLIP up-fitting, i.e. learning from additional ab initio configurations. Consequently, we elucidate trends in theoretical strength, toughness, and crack initiation patterns under different loading directions. To identify useful environments for further up-fitting, i.e., making the MLIP applicable to a wider spectrum of simulations, we asses transferability to other deformation conditions and phases not explicitly trained on

Developing information literacy programmes for public university libraries in Tanzania : a case study of the University of Dar es Salaam

The aim of the research was to develop a training course that inculcated infonnation literacy that could be implemented by staff at the University of Dar es Salaam library (Tanzania), in order to provide students with appropriate infonnation literacy skills to meet their educational goals. The course was developed from an integration of knowledge from infonnation behaviour research and educational theory with current perspectives of infonnation literacy from Infonnation and Library Science (ILS). The ultimate goal of the research was to create a framework that would be used by public university libraries in Tanzania to teach infonnation literacy courses. The study was carried out in two parts. In the first implementation "pilot" programme, the course was tested by involving librarians who took the entire course, in order to see whether the same course structure could be used to implement to the Masters of Education students. Following adaptations made on the first course, a second course programme was implemented to Masters of Education students by two librarians who attended the first implementation "pilot" programme. Therefore, the success of the course was partly judged on whether it effectively enabled knowledge transfer from the librarians to students. Data collection methods were predominantly qualitative, although quantitative methods in tenns of diagnostic tests were also used. The tests were used to evaluate trainees' knowledge ofinfonnation literacy before and after the course to provide an indication of changes in knowledge. Qualitative methods used included semistructured interviews with librarians and academic staff at the University of Dar es Salaam in Tanzania in order to make sure that assumptions made about students' knowledge of infonnation literacy and the kind of problems experienced by students were correct. Other methods included quizzes, exercises, group reflection and presentations that related to each stage in the course. These methods served to indicate trainees' understanding of what was taught, reflections on the leaming process and provided feedback for improvements on the course. The major findings showed that there was a recognized need for infonnation literacy and that problems such as unfamiliarity with categories of infonnation sources, analysis, synthesis, evaluation and use, were experienced in Tanzania as was the case in other "Western" countries. In addition, librarians were able to transfer skills learned to students, who in turn used the same course materials to teach fellow students who did not participate in the course. Furthermore, the design of the course was facilitated by the integration of Information and library science (ILS) approaches to information literacy with the knowledge of information behaviour and pedagogic theory. The thesis provides recommendations for the library and information curricula to introduce information literacy, teaching information literacy in a holistic way and with librarians participating in teaching and research. In addition, the study recommended that librarians should facilitate the development of information literacy in primary and secondary schools.EThOS - Electronic Theses Online ServiceGBUnited Kingdo