NOx production and elimination content in the flue gas

Předložená bakalářská práce se zabývá problematikou oxidů dusíku, které vznikají při procesu spalování. Práce je soustředěna především na popis jednotlivých mechanismů vzniku a na jednotlivá opatření, která jsou používaná pro snižování jejich koncentrací. Součástí práce je také zaměření na legislativu v oblasti ochrany ovzduší, s uvedením hodnot emisních limitů, platné pro Českou republiku a Evropskou unii.The Bachelor thesis deals with oxides of nitrogen, resulting from the combustion process. The thesis is concentrated mainly on the description of the emergence mechanisms and on the various measures that are used to reduce their concentration. Part of the thesis is also focusing on legislation in the field of air protection, with the values of the emission limits valid for the Czech Republic and the European Union.

Methods of actual indenter shape determination

Knowledge of the actual indenter shape (area function) is fundamental for the correct evaluation of measured indentation data. Various methods and models were developed for the determination of indenter area function. Nevertheless, these methods usually do not have clear geometrical interpretation and/or their application is based on some severe assumption so they cannot be applied generally. Basically, the shape of the indenter can be determined by two different procedures – indentation into reference materials with known mechanical properties or reconstruction of the tip by direct imaging methods, e.g. AFM. The aim of this study is the comparison of these two procedures applied for characterization of Berkovich and spherical diamond indenters. Several materials were used for reference indentation measurements and it was found that the area function obtained on studied materials significantly differs. The area function determined by this technique in fact does not correspond to the actual indenter shape but it characterizes the convolution of actual indenter shape and material surface and deformation characteristics. It means that this technique is appropriate only while testing the materials with the properties close to the reference sample. Please click Additional Files below to see the full abstract

DEFORMATION OF FE3SI SINGLE-CRYSTALS UNDER NANOINDENTATION

Knowledge of the complex deformation behavior in the anisotropic materials is one of essential issues in materials science and it is crucial for the applications of a given material. In this study, mechanical response of Fe3(wt.%)Si single crystal to nanoindentation with spherical indenter was investigated. Hardness and indentation Young´s modulus were determined experimentally and by finite element modelling. Observed pop-in phenomenon, shape of the residual imprints and origin of the slip lines were explained on the basis of resolved shear stress computed by finite element model

Nanomechanical characterization of high pressure torsion processed HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) are a new material class in which the configurational entropy of a multicomponent solid solution phase is maximized so that the entropy of mixing stabilizes disordered solid solution phases against the possible intermetallic phases development. Generally, to achieve high entropy of mixing, the alloys contain typically five or more major elements in equimolar concentrations. The composition of HEAs is generally based on 3d transition metals, refractory metals, light metals, lanthanide transition metals, precious metals, brasses and bronzes. The HEAs exhibit promising structural and mechanical properties in wide range of applications. Mechanical properties of such alloys can further be improved by grain refinement especially by severe plastic deformation. However, studies of ultrafine grained HEAs are rather scarce in the literature. An increase of strength with decreasing grain size was achieved in the probably most investigated HEA i.e. Cantor alloy (equiatomic CoCrFeMnNi with fcc structure) processed by high pressure torsion (HPT) [1]. Much less attempts were made to process in such a way HEAs with bcc structure. Recently HfNbTaTiZr bcc HEA was successfully nanostructured by HPT straining [2]. It was reported that grain refinement by HPT resulted in a significant enhancement of the strength of this bcc HEA, keeping excellent ductility during room temperature straining. Nevertheless, there is still a lack of information about the development of microstructure and physical properties of this refractory metal HEA subjected to severe plastic deformation processing. Recent investigations [3] revealed that thermodynamically stable system of HfNbTaTiZr alloy at room temperature is a mechanical mixture of Zr, Hf rich hcp phase and Ta, Nb rich bcc phase. The decomposition of the solid solution after long-term annealing obviously leads to the deterioration of mechanical properties (loss of ductility and decrease of strength). The difference in hardness of both phases is relatively small and both are softer than the random solid solution. On the other hand, considerable contribution to the solid solution strengthening can arise from atomic size misfit (phase separation on the nano-meter scale) which is provoked by the high density of vacancies introduced by HPT. This work thus aims on the relationship between phase (de)composition, microstructure, lattice defects, and length-scale-dependent material response of HfNbTaTiZr HEA after different thermal treatment and HPT straining. The microstructure and phase composition evolution were characterized by the electron microscopy and X-ray diffraction. The length-scale-dependent material response was characterized by indentation at various indentation depths. The contributions of different hardening mechanisms were separated and attributed to distance between dislocation pinning defects so that the differences between thermal treatment (diffusion) and HPT (straining) ‑induced hardening could be explained. Acknowledgement: This research was carried out in frame of the project CZ.02.1.01/0.0/0.0/15_003/0000485 (European Regional Development Fund). [1] A. Heczel et al. Defect structure and hardness in nanocrystalline CoCrFeMnNi high-entropy alloy processed by high-pressure torsion, J. Alloy. Comp. 711 (2017) 143-154. [2] J. Čížek et al. Strength enhancement of high entropy alloy HfNbTaTiZr by severe plastic deformation J. Alloy. Comp. 768 (2018) 924-937. [3] B. Schuh et al. Thermodynamic instability of a nanocrystalline, single-phase TiZrNbHfTa alloy and its impact on the mechanical properties, Acta Mater. 142 (2018) 201-212

Influence of the Interface and the microstructural length scale on the grid indentation

The instrumented grid indentation of structurally heterogeneous materials using Oliver-Pharr method is frequently employed in order to characterize the properties of individual phases as this method is available in practically all commercial devices. However, when applying the statistical evaluation of results, the presence of boundary-affected results leads to the bias of the distribution, i.e. to the softer phase hardness and/or modulus overestimation together with the underestimation of harder phase values. The indentation in proximity of the interface cannot always be completely avoided – in the grid indentation the position of some indents can inevitably coincide with the phase boundary which can, moreover, be hidden below the indented surface. The aim of this paper is to shed some light on the effect of indentation in proximity of the interface on the statistical distribution of the grid indentation data. A case study on material composed of two phases with distinctly different hardness and Young’s modulus is presented. As an experimental material was chosen tungsten-copper composite. Samples were prepared using spark plasma sintering from pure metallic and ceramic powders which resulted in a sharp interface with abrupt change of material properties. The influence of depth of penetration and microstructural length scale on the measured hardness and/or modulus was integrated in the statistical analysis of the grid indentation data. The conditional probability of the indentation in the (affected) interfacial area was incorporated into the statistical distribution function based on the profile estimated by the finite element analysis and by the experimental study on the model material with the sharp interface. The unbiased (intrinsic) properties (including indentation size effect) were subsequently extracted from the experimental grid indentation data. Please click Additional Files below to see the full abstract

Influence of the Interface and the microstructural length scale on the grid indentation

The instrumented grid indentation of structurally heterogeneous materials using Oliver-Pharr method is frequently employed in order to characterize the properties of individual phases as this method is available in practically all commercial devices. However, when applying the statistical evaluation of results, the presence of boundary-affected results leads to the bias of the distribution, i.e. to the softer phase hardness and/or modulus overestimation together with the underestimation of harder phase values. The indentation in proximity of the interface cannot always be completely avoided – in the grid indentation the position of some indents can inevitably coincide with the phase boundary which can, moreover, be hidden below the indented surface. The aim of this paper is to shed some light on the effect of indentation in proximity of the interface on the statistical distribution of the grid indentation data. A case study on material composed of two phases with distinctly different hardness and Young’s modulus is presented. As an experimental material was chosen tungsten-copper composite. Samples were prepared using spark plasma sintering from pure metallic and ceramic powders which resulted in a sharp interface with abrupt change of material properties. The influence of depth of penetration and microstructural length scale on the measured hardness and/or modulus was integrated in the statistical analysis of the grid indentation data. The conditional probability of the indentation in the (affected) interfacial area was incorporated into the statistical distribution function based on the profile estimated by the finite element analysis and by the experimental study on the model material with the sharp interface. The unbiased (intrinsic) properties (including indentation size effect) were subsequently extracted from the experimental grid indentation data. Please click Additional Files below to see the full abstract

INDENTATION SIZE EFFECT IN HIGH PRESSURE TORSION PROCESSED HIGH ENTROPY ALLOY

High entropy alloy HfNbTaTiZr in as cast conditions and after high pressure torsion straining was characterized by nanoindentation. The length-scale dependent material response (indentation size effect) was characterized by indentation at various indentation depths. Hardness dependence on the characteristic length (depth of penetration) indicated decomposition of disordered high entropy alloy in the as cast sample, which probably occurred during slow cooling after casting. Subsequent severe plastic deformation by high pressure torsion led on the other hand to the short-range disorder of (originally partially decomposed as cast) structure. Further hardening was generated during high pressure torsion by the mechanisms of grain refinement and increasing dislocation density

Instrumentovaná indentace částicového kompozitu WHA: experimentální a numerická studie

Instrumentovaná indentace umožňuje dle zvolené hloubky vtisku stanovit jak makroskopické mechanické vlastnosti celého kompozitu, tak vlastnosti jednotlivých fází. V práci jsou prezentovány výsledky měření indentační tvrdosti i indentačního modulu pružnosti wolframové pseudoslitiny WHA se sférickými částicemi wolframu v matrici na bázi niklu. Současně je pomocí metody konečných prvků modelován vliv materiálového rozhraní pod a vedle indentoru na dosažené hodnoty tvrdosti a indentačního modulu pružnosti v závislosti na hloubce vtisku.Autoři práce by rádi poděkovali pracovníkům Ústavu fyziky plazmatu AV ČR, v.v.i. za výrobu materiálu WHA. Experimentální část této práce byla podpořena grantem SGS č. SGS21/168/OHK4/3T/14

MECHANICAL PROPERTIES OF FORGED TUNGSTEN HEAVY ALLOYS

Tungsten heavy alloys are composite materials containing spherical tungsten particles embedded in binder matrix. Their excellent mechanical properties can be further improved by rotary forging. This paper aims to gain deeper understanding of the forging process by investigating the local elastic modulus, hardness, and residual stress of individual phases in W6Ni3Co pseudo-alloy. The resulting global properties of the composite material such as stress-strain behavior, fracture toughness and fatigue crack growth rate behavior are also studied. The results show that sintered and quenched material consists of highly textured matrix containing nearly perfect single crystal spheres of pure W. The rotary forging leads to significant lattice deformations destroying the texture and significantly increasing the hardness of both WNiCo matrix and W particles and making residual stresses in W particles anisotropic with increased compression along the longitudinal axis of the forged part

Characterization of mechanically alloyed FeAlSi intermetallic powders

Powder metallurgy is very promising material production technology which allows to prepare the alloys that could hardly be manufactured by other processing route. Basic prerequisite to obtain the product of desired properties is the high quality of initial primary commodities, i.e. powders in the case of powder metallurgy. One of the available methods of powder preparation is so called mechanical alloying which starts from blended powder mixtures and allows production of homogeneous materials by severe deformation in a high-energy ball charge. This technology is especially suitable for brittle materials such as intermetallic alloys being developed for high-temperature and corrosive environments applications [1]. Please click Additional Files below to see the full abstract