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1.
  • Aghda, Soheil Karimi, et al. (author)
  • Ion kinetic energy- and ion flux-dependent mechanical properties and thermal stability of (Ti,Al)N thin films
  • 2023
  • In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 250
  • Journal article (peer-reviewed)abstract
    • Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (Ek)>105 eV, an evolution towards (111) orientation is observed in CAD films for Ek>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux-and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy-and orientation -dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from Ek for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of similar to 18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at Ek=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar Ek. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 degrees C.
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2.
  • Alami, Jones, et al. (author)
  • Ion-assisted Physical Vapor Deposition for enhanced film properties on non-flat surfaces
  • 2005
  • In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 23:2, s. 278-280
  • Journal article (peer-reviewed)abstract
    • We have synthesized Ta thin films on Si substrates placed along a wall of a 2-cm-deep and 1-cm-wide trench, using both a mostly neutral Ta flux by conventional dc magnetron sputtering (dcMS) and a mostly ionized Ta flux by high-power pulsed magnetron sputtering (HPPMS). Structure of the grown films was evaluated by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The Ta thin film grown by HPPMS has a smooth surface and a dense crystalline structure with grains oriented perpendicular to the substrate surface, whereas the film grown by dcMS exhibits a rough surface, pores between the grains, and an inclined columnar structure. The improved homogeneity achieved by HPPMS is a direct consequence of the high ion fraction of sputtered species.
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3.
  • Carlsson, Adam (author)
  • Computational prediction of novel MAB phases
  • 2022
  • Licentiate thesis (other academic/artistic)abstract
    • The synthesis procedure of any materials system is often considered a challenging task if performed without any prior knowledge. Theoretical models may thus be used as an external input and guide experimental efforts toward novel exotic materials which are most likely to be synthesizable. The aim of this work is to apply theoretical models and develop frameworks for reliable predictions of thermodynamically stable materials. The material in focus herein is the family of atomic layered boride-based materials referred to as MAB phases.The ground state energy of a material system may be obtained by applying firstprincipal calculations, such as density functional theory (DFT), which has thoroughly been used throughout this thesis. However, performing modern state-of-the-art quantum mechanical calculations, in general, relies on a pre-defined crystal structure which may be constructed based on an a priori known structure or obtained through the use of crystal structure prediction models. In this work, both approaches are explored. We herein perform a thermodynamical screening study to predict novel stable ternary boron-based materials by considering M2AB2, M3AB4, M4AB6, MAB and M4AB4 compositions in orthorhombic and hexagonal symmetries with inspiration from experimentally synthesized MAB phases. The considered atomic elements are M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, A = Al, Ga, In, and B is boron. Among the considered compounds, seven experimentally synthesized phases are verified as stable, and we predict the three hypothetical phases to be stable - Hf2InB2, Zr2InB2, and Mo4AlB4. Additionally, 23 phases of varying symmetries and compositions are predicted as close to stable or to be metastable.However, the assumption of assigning initial crystal structures based on neighbouring compounds may drastically limit the outcome of a screening study. State-of-the-art techniques to generate low energy crystal structures within the considered material phase space is thus explored. More specifically, the Mo-Sc-Al-B system is studied along the ternary joints of (MoxSc1-x)2AlB2 where 0 < x < 1 by using the cluster expansion (CE) and the crystal structure prediction (CSP) codes, CLEASE and USPEX, in analogy. Previous attempts to study the Mo-Sc-Al-B system has been limited by only considering either hexagonal or orthorhombic symmetries. We challenge such approaches by covering larger portions of the phase space efficiently by combining CSP and CE frameworks. The Mo4/3Sc2/3AlB2 (R ̅3m) phase, previously referred to as i-MAB, is verified stable in addition to Mo2/3Sc4/3AlB2 (R3).The suggested approach of combining CE and CSP frameworks for investigating multi-component systems consists of initially performing CSP searches on the systems of smaller order constituting the system in focus. In the pseudo-ternary (MoxSc1-x)2AlB2 system, this refers to performing CSP searches on the ternary Mo2AlB2 and Sc2AlB2 systems. In addition, we also consider the structures of experimentally known phases with similar compositions. The complete set of structures obtained either from CSP or public databases, was later used to design CE models where mixing tendencies in addition to stability determined which model to further study. The predicted low-energy structures of the CE model were relaxed and used as seed structures within a complete CSP search covering the (MoxSc1-x)2AlB2 system for 0 < x < 1. We demonstrate that the use of seed structures, obtained from CE models, efficiently improved the search for low-energy structures within a multi-component system. The suggested approach is yet to be tested on any other system but is applicable to any alternative multi-component system.
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4.
  • Chang, Jui-Che (author)
  • Metastable orthorhombic Ta3N5 thin films grown by magnetron sputter epitaxy
  • 2022
  • Licentiate thesis (other academic/artistic)abstract
    • The semiconductor tritantalum pentanitride (Ta3N5) is a promising green-energy material for photoelectrolyzing water to produce oxygen and hydrogen owing to its proper bandgap of 2.0 ± 0.2 eV and band positions to redox potential of water. Compare with the conventional setup of water splitting, such as TiO2, Fe2O3, Cu2O, and WO3, the Ta3N5 shows a proper band gap, which leads to a theoretical efficiency as high as 15.9%. However, the complexity of the Ta-N system and the metastability of the Ta3N5 result in the limited research of the growth of high quality stoichiometric Ta3N5.Conventionally, the two-step growth of oxidation and nitridation of a metal Ta using thermal annealing in oxygen and ammonia environment is used to produce the Ta3N5. However, the amount of incorporated oxygen in the Ta3N5 samples and film’s thickness and interface are hardly to be controlled, and the use of ammonia as the nitridation gas is harmful to the environment. Hence, in this thesis work, the reactive magnetron sputtering is used to synthesis the Ta3N5, which demonstrates some advantages, such as possibility to grow on a substrate with nanostructure on the surface, a simplification of growth process, usage of environmental-friendly reactive gas, and even scaling up to the industrial application.The thesis presents a successful growth of orthorhombic Ta3N5-type Ta-O-N compound thin films on Si and sapphire substrates, specifically Ta3-xN5-yOy, using reactive magnetron sputtering with a gas mixture of Ar, N2, and O2. In the deposition process, the total working pressure was increasing from 5 to 40 mTorr, while keeping same partial pressure ratio (Ar: N2: O2 = 3: 2: 0.1). When the total pressure in the region between 5-30 mTorr, a low-degree fiber-textural Ta3-xN5-yOy films were grown. In addition, with the characterization of elastic recoil detection analysis (ERDA), the atomic fraction of O, N, and Ta of as-grown Ta3-xN5-yOy films were found varying from 0.02 to 0.15, 0.66 to 0.54, and 0.33 to 0.31, respectively, which leads to a b-lattice constant decrease around 1.3 %, shown in X-ray diffraction (XRD) results. For a total working pressure up to 40 mTorr, an amorphous O-rich Ta-O-N compound film was formed mixed with non-stoichiometric TaON and Ta2O5, which further raised the oxygen atomic fraction to ~0.48. The increasing total working pressure results in an increasing band gap from 2.22 to 2.66 eV of Ta3-xN5-yOy films, and further increasing to around 2.96 eV of O-rich Ta-O-N compound films. The mechanism of increasing oxygen atomic fraction in the film is founded correlated with the forming oxide on the Ta target surface during the deposition process due to the strong reactivity of O to Ta by the characterization of optical emission spectroscopy (OES). Moreover, the sputter yield was reduced due to the target poisoning, and which is evidenced by both plasma analysis and depth profile from ERDA.A further studies with the deposition parameters for nearly pure Ta3N5 films (oxygen atomic fraction ~2%) was performed using c-axis oriented Al2O3 substrate. In this research, it is found that a Ta2O5 seed layer and a small amount of oxygen were necessary for the growth of Ta3N5. Without the help of seed layer and oxygen, only metallic TaN phases, either mixture of ε- and δ- TaN or δ-TaN were grown, evidenced by X-ray photoelectron spectroscopy (XPS). Furthermore, the structure and phase purity of Ta3N5-phase dominated films was found highly correlated with the thickness of the Ta2O5 seed layer. With the increasing thickness of the seed layer from 5, 9, to 17 nm, the composition of grown films was changed from 111-oriented δ-TaN mixed with c-axis oriented Ta3N5, c-axis oriented Ta3N5, to polycrystalline Ta3N5. In addition, the azimuthal φ-scans in grazing incident geometry demonstrates that the c-axis oriented Ta3N5 contained epitaxially three-variant-orientation domains, in which the a and b planes parallel to the m and a planes of c-axis oriented Al2O3. With the simulation of density functional theory (DFT), the growth of thin seed layers of orthorhombic Ta2O5 (β-Ta2O5) was found promoting by introducing a small amount of oxygen, after calculating the interplay between the topological and energy selection criteria. By the co-action of the mentioned criteria, this already grown Ta2O5 seed layer favored the growth of the orthorhombic Ta3N5 phase. Hence, the mechanism of the domain epitaxial growth of c-axis oriented Ta3N5 on c-axis oriented Al2O3 is attributed to the similar atomic arrangement Ta3N5(001) and β-Ta2O5(201) with a small lattice mismatch around of 2.6% and 4.5%, for the interface of film/seed layer and seed layer/substrate, respectively, and a favorable energetic interaction between involved materials.
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5.
  • Chen, Leilei, et al. (author)
  • High-temperature wear mechanisms of TiNbWN films : Role of nanocrystalline oxides formation
  • 2023
  • In: Friction. - : Springer. - 2223-7690 .- 2223-7704. ; 10
  • Journal article (peer-reviewed)abstract
    • Refractory high/medium entropy nitrides (HENs/MENs) exhibit comprehensive application prospects as protective films on mechanical parts, particularly those subjected to sliding contacts at elevated temperatures. In this study, a new MEN system TiNbWN, forming a single fcc solution, is designed and its wear performance at temperatures ranging from 25 to 750 °C is explored. The wear mechanisms can be rationalized by examining the subsurface microstructural evolutions using the transmission electron microscopy as well as calculating the phase diagrams and interfacial adhesion behavior employing calculation of phase diagram (CALPHAD) and density functional theory (DFT). To be specific, increased wear losses occur in a temperature range of 25–600 °C, being predominantly caused by the thermally-induced hardness degradation; whereas at the ultimate temperature (750 °C), the wear loss is refrained due to the formation of nanocrystalline oxides (WnO3n−2, TiO2, and γTiOx), as synergistically revealed by microscopy and CALPHAD, which not only enhance the mechanical properties of the pristine nitride film, but also act as solid lubricants, reducing the interfacial adhesion. Thus, our work delineates the role of the in situ formed nanocrystalline oxides in the wear mechanism transition of TiNbWN thin films, which could shed light on the high-temperature wear behavior of refractory HEN/MEN films.
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6.
  • Edström, Daniel, 1986- (author)
  • Growth and Mechanical Properties of Transition Metal Nitrides and Carbides
  • 2016
  • Doctoral thesis (other academic/artistic)abstract
    • The atomic-scale dynamical processes at play during film growth cannot be resolved by even the most advanced experimental methods. As such, computational methods, and chiefly classical molecular dynamics, are the only available research tools to study these processes. The investigation of key dynamical processes during thin film growth yields a deeper understanding of the film growth evolution, ultimately allowing for the optimization of experimental parameters and tailoring of film properties. This thesis details the study of fundamental surface dynamics processes, and the role played by primary diffusing species, during TiN film growth, here employed as a model system for transition metal nitrides in general. It is found that Ti adatoms and TiN2 admolecules are the fastest diffusing species, and the species which most rapidly descend from islands onto the growing film. Thus, they are the main contributors and players in driving the layer-by-layer growth mode. TiN3 admolecules, in contrast, are essentially stationary and thereby promote multilayer growth. Large-scale growth simulations reveal that tailoring the incident N/Ti ratio and N kinetic energy significantly affects the growth mode and film microstructure.The mechanical properties of ternary transition metal nitride and carbide alloys, investigated using density functional theory, are also discussed herein, in comparison to recent experimental results. By optimizing the valence electron concentration in these compounds, the occupation of shear-compliant d‑t2g electronic states can be maximized. The investigation of M1M2N alloys, where M1 = Ti or V and M2 = W or Mo, with different structures demonstrates that this optimization leads to enhanced ductility, and thereby toughness, in transition metal nitride alloys regardless of the degree of ordering on the metal sublattice. Estimations based on the calculation of the mechanical properties of the corresponding M1M2C transition metal carbide alloys indicate that these materials remain brittle. However, charge density analysis and calculations of stress/strain curves reveal features commonly associated with ductile materials.
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7.
  • Elalfy, Loay, et al. (author)
  • Metavalent bonding induced abnormal phonon transport in diamondlike structures:Beyond conventional theory
  • 2021
  • In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 103
  • Journal article (peer-reviewed)abstract
    • A phenomenon appears in a few examples of the chalcopyrites (space group I-42 d) where heavier atoms do not necessarily lead to lower lattice thermal conductivity, in contradiction with Keyes expression that formulates an inverse relation of thermal conductivity with mean atomic mass. Herewith, the thermal conductivity of CuInSe2, CuInTe2, AgInSe2, and AgInTe2 was calculated and compared at room temperature from the linearized Boltzmann transport equation using ab initio density functional theory. CuInSe2 and AgInSe2 solids exhibit lower lattice thermal conductivity than that of CuInTe2 and AgInTe2, respectively, despite the fact that Te atoms are significantly heavier than Se. A comparison between dispersion relation, the Grüneisen parameter, and projected density of states leads to the conclusion that anharmonic transverse acoustic modes in the form of anomalous vibrations of Cu and Ag cause the lower values of the thermal conductivity. By analyzing the electronic structure, the compounds under study fit perfectly into a recently defined region of the metavalent bonding well known for its pronounced anharmonicity. The insight gained from the current results deepens our understanding of the unusual heat transfer phenomenon related to the metavalent bonding and sheds light on design and discovery of thermally functional materials that break the prediction by the conventional theory.
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8.
  • Emmerlich, Jens, et al. (author)
  • Thermal stability of Ti3SiC2 thin films
  • 2007
  • In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 55:4, s. 1479-1488
  • Journal article (peer-reviewed)abstract
    • The thermal stability of Ti3SiC2(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to 1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti3SiC2 by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti3C2 slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti3C2 slabs into (1 1 1)-oriented TiC0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.
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9.
  • Gangaprasad Rao, Smita, 1992- (author)
  • Phase formation in multicomponent films based on 3d transition metals
  • 2021
  • Licentiate thesis (other academic/artistic)abstract
    • The need for materials that enhance life span, performance, and sustainability has propelled research in alloy design from binary alloys to more complex systems such as multicomponent alloys. The CoCrFeMnNi alloy, more commonly known as the Cantor alloy, is one of the most studied systems in bulk as well as thin film. The addition of light elements such as boron, carbon, nitrogen, and oxygen is a means to alter the properties of these materials. The challenge lies in understanding the process of phase formation and microstructure evolution on addition of these light elements. To address this challenge, I investigate multicomponent alloys based on a simplified version of the Cantor alloy.My thesis investigates the addition of nitrogen into a Cantor variant system as a step towards understanding the full Cantor alloy. Me1-yNy (Me = Cr + Fe + Co, 0.14 ≤ y ≤0.28 thin films were grown by reactive magnetron sputtering. The films showed a change in structure from fcc to mixed fcc+bcc and finally a bcc-dominant film with increasing nitrogen content. The change in phase and microstructure influenced the mechanical and electrical properties of the films. A maximum hardness of 11 ± 0.7 GPa and lowest electrical resistivity of 28 ± 5 μΩcm were recorded in the film with mixed phase (fcc+bcc) crystal structure.Copper was added as a fourth metallic alloying element into the film with the mixed fcc + bcc structure, resulting in stabilization of the bcc phase even though Cu has been reported to be a fcc stabilizer. The energy brought to the substrate increases on Cu addition which promotes surface diffusion of the ions and leads to small but randomly oriented grains. The maximum hardness recorded by nanoindentation was found to be 13.7 ± 0.2 GPa for the sample Cu0.05. While it is generally believed that large amounts of Cu can be detrimental to thin film properties due to segregation, this study shows that small amounts of Cu in the multicomponent matrix could be beneficial in stabilizing phases as well as for mechanical properties.This thesis thus provides insights into the phase formation of nitrogen-containing multicomponent alloys.
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10.
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11.
  • Gebhardt, Thomas, et al. (author)
  • Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment
  • 2011
  • In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:8, s. 3145-3155
  • Journal article (peer-reviewed)abstract
    • We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Young's modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Young's modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Young's modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Young's modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.
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12.
  • Gebhardt, T, et al. (author)
  • Influence of chemical composition and magnetic effects on the elastic properties of fcc Fe-Mn alloys
  • 2011
  • In: Acta Materialia. - : Elsevier Science B.V., Amsterdam.. - 1359-6454 .- 1873-2453. ; 59:4, s. 1493-1501
  • Journal article (peer-reviewed)abstract
    • The influence of the Mn content on the elastic properties of face centered cubic Fe-Mn alloys was studied using the combinatorial approach. Fe-Mn thin films with a graded chemical composition were synthesized. Nanoindentation experiments were carried out to investigate the elastic properties as a function of the Mn content. As the Mn content increases from similar to 23 to similar to 39 at.%, the average bulk modulus varies from 143 to 105 GPa. Ab initio calculations served to probe the impact of magnetic effects on the elastic properties. The magnetic state description with disordered local moments yields the best agreement with the experimental results, whereas with non-magnetic and antiferromagnetic configurations the bulk modulus is overestimated. The strong impact of the magnetic configuration may be understood based on the differences in the chemical bonding and the magnetovolume effect. It is suggested that, owing to minute energy differences of competing antiferromagnetic configurations, a mixture of these with a "notional magnetic disorder" is present, which is in fact well described by the disordered local moments model. These results show that the combinatorial thin film synthesis with subsequent nanoindentation is an appropriate tool for investigating the elastic properties of Fe-Mn alloys systematically as a function of the chemical composition, to validate theoretical models.
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13.
  • Hallstedt, Bengt, et al. (author)
  • Thermodynamic evaluation of the Al-Cr-C system
  • 2006
  • In: International Journal of Materials Research - Zeitschrift für Metallkunde. - : Walter de Gruyter GmbH. - 1862-5282 .- 2195-8556. ; 97:5, s. 539-542
  • Journal article (peer-reviewed)abstract
    • The system Al-Cr-C contains one ternary phase, Cr2AlC. By combining a Calphad assessment with ab initio calculations and differential thermal analysis, the Gibbs energy of Cr2AlC could be determined and the complete phase diagram calculated. Cr2AIC melts incongruently at about 1773 K to form Cr7C3 (or Cr3C2) and Al4C3 in addition to liquid. According to the ab initio calculations the equilibrium composition of Cr2AIC is very nearly stoichiometric, but can dissolve some Al metastably by replacing Cr. The agreement between the thermodynamic calculation and the various sources of information is excellent.
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14.
  • Hans, Marcus, et al. (author)
  • Spinodal decomposition of reactively sputtered (V0.64Al0.36)(0.49)N-0.51 thin films
  • 2020
  • In: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 389
  • Journal article (peer-reviewed)abstract
    • We investigate the decomposition mechanisms of metastable cubic (c-)(V0.64Al0.36)(0.49)N-0.51 thin films, grown by reactive high power pulsed magnetron sputtering, by combination of structural and compositional characterization at the nanometer scale with density functional theory (DFT) calculations. Based on thermodynamic considerations of partial derivative(2)Delta G/partial derivative x(2) < 0, spinodal decomposition is expected for c-V1-xAlxN with x >= 0.35. While no indications for spinodal decomposition are observable from laboratory and synchroton diffraction data after annealing in Ar atmosphere at 1300 degrees C, the formation of wurtzite (w-)AlN is evident after annealing at 900 degrees C by utilizing high energy synchrotron X-ray diffraction. However, the complementary nature of elemental V and Al maps, obtained by energy dispersive X-ray spectroscopy in scanning transmission electron microscopy mode, imply spinodal decomposition of c-(V0.64Al0.36)(0.49)N-0.51 into V- and Al-rich cubic nitride phases after annealing at 900 degrees C. These chemical modulations are quantified by atom probe tomography and maximum variations of x in V1-xAlxN are in the range of 0.36 to 0.50. The magnitude of the compositional modulations is enhanced after annealing at 1100 degrees C as x varies on average between 0.30 and 0.61, while the modulation wavelength remains unchanged at approximately 8 nm. Based on DFT data, the local x variation from 0.30 to 0.61 would cause lattice parameter variations from 4.111 to 4.099 angstrom. This difference corresponds to a shift of the (200) peak from 44.0 to 44.1 degrees. As the maximum decomposition-induced peak separation magnitude of 0.1 degrees is significantly smaller than the measured full width at half maximum of 0.4 degrees, spinodal decomposition cannot be unravelled by diffraction data. However, consistent with DFT predictions, spinodal decomposition in c-(V0.64Al0.36)(0.49)N-0.51 is revealed by chemical composition characterization at the nanometer scale.
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15.
  • Hans, Marcus, et al. (author)
  • Stress-Dependent Elasticity of TiAlN Coatings
  • 2019
  • In: Coatings. - : MDPI. - 2079-6412. ; 9:1
  • Journal article (peer-reviewed)abstract
    • We investigate the effect of continuous vs. periodically interrupted plasma exposure during cathodic arc evaporation on the elastic modulus as well as the residual stress state of metastable cubic TiAlN coatings. Nanoindentation reveals that the elastic modulus of TiAlN grown at floating potential with continuous plasma exposure is 7%-11% larger than for coatings grown with periodically interrupted plasma exposure due to substrate rotation. In combination with X-ray stress analysis, it is evident that the elastic modulus is governed by the residual stress state. The experimental dependence of the elastic modulus on the stress state is in excellent agreement with ab initio predictions. The macroparticle surface coverage exhibits a strong angular dependence as both density and size of incorporated macroparticles are significantly lower during continuous plasma exposure. Scanning transmission electron microscopy in combination with energy dispersive X-ray spectroscopy reveals the formation of underdense boundary regions between the matrix and TiN-rich macroparticles. The estimated porosity is on the order of 1% and a porosity-induced elastic modulus reduction of 5%-9% may be expected based on effective medium theory. It appears reasonable to assume that these underdense boundary regions enable stress relaxation causing the experimentally determined reduction in elastic modulus as the population of macroparticles is increased.
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16.
  • Holzapfel, Damian M., et al. (author)
  • Enhanced thermal stability of (Ti,Al)N coatings by oxygen incorporation
  • 2021
  • In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 218
  • Journal article (peer-reviewed)abstract
    • Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive arc evaporation. X-ray diffraction of (Ti,Al)N and (Ti,Al)(OxN1-x) coatings reveals that spinodal decomposition is initiated at approximately 800 degrees C, while the subsequent formation of wurtzite solid solution is clearly delayed from 1000 degrees C to 1300 degrees C for (Ti,Al)(OxN1-x) compared to (Ti,Al)N. This thermal stability enhancement can be rationalized based on calculated vacancy formation energies in combination with spatially-resolved composition analysis and calorimetric data: Energy dispersive X-ray spectroscopy and atom probe tomography data indicate a lower O solubility in wurtzite solid solution compared to cubic (Ti,Al)(O,N). Hence, it is evident that for the growth of the wurtzite, AlN-rich phase in (Ti,Al)N, only mobility of Ti and Al is required, while for (Ti,Al)(O,N), in addition to mobile metal atoms, also non-metal mobility is required. Prerequisite for mobility on the non-metal sublattice is the formation of non-metal vacancies which require larger temperatures than for the metal sublattice due to significantly larger magnitudes of formation energies for the non-metal vacancies compared to the metal vacancies. This notion is consistent with calorimetry data which indicate that the combined energy necessary to form and grow the wurtzite phase is larger by a factor of approximately two in (Ti,Al)(O,N) than in (Ti,Al)N, causing the here reported thermal stability increase. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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17.
  • Holzapfel, Damian M., et al. (author)
  • Influence of ion irradiation-induced defects on phase formation and thermal stability of Ti0.27Al0.21N0.52 coatings
  • 2022
  • In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 237
  • Journal article (peer-reviewed)abstract
    • The influence of changes induced by ion irradiation on structure and thermal stability of metastable cubic (Ti,Al)N coatings deposited by cathodic arc evaporation is systematically investigated by correlating experiments and theory. Decreasing the nitrogen deposition pressure from 5.0 to 0.5 Pa results in an ion flux-enhancement by a factor of three and an increase of the average ion energy from 15 to 30 eV, causing the stress-free lattice parameter to expand from 4.170 to 4.206 Å, while the chemical composition of Ti0.27Al0.21N0.52 remains unchanged. The 0.9% lattice parameter increase is a consequence of formation of Frenkel pairs induced by ion bombardment, as revealed by density functional theory (DFT) simulations. The influence of the presence of Frenkel pairs on the thermal stability of metastable Ti0.27Al0.21N0.52 is investigated by scanning transmission electron microscopy, differential scanning calorimetry, atom probe tomography and in-situ synchrotron X-ray powder diffraction. It is demonstrated that the ion flux and ion energy induced formation of Frenkel pairs increases the thermal stability as the Al diffusion enabled crystallization of the wurtzite solid solution is retarded. This can be rationalized by DFT predictions since the presence of Frenkel pairs increases the activation energy for Al diffusion by up to 142%. Hence, the thermal stability enhancement is caused by a hitherto unreported mechanism - the Frenkel pair impeded Al mobility and thereby retarded formation of wurtzite solid solution.
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18.
  • Hostert, Carolin, et al. (author)
  • Density, elastic and magnetic properties of Co–Fe–Ta–Si metallic glasses by theory and experiment
  • 2012
  • In: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456.
  • Journal article (peer-reviewed)abstract
    • Ab initio molecular dynamics simulations were employed to study the density, elastic and magnetic properties of Co–Fe–Ta–Si10 metallic glass thin films. A density of 8.03 g cm3, a Young’s modulus of 187 GPa and a volume magnetization of 0.089 lBA ˚ 3 werecalculated and validated by thin film experiments. The extensive stiffness of this glass may be related to the existence of strong(Fe,Co)–Si clusters as revealed by ab initio chemical short-range order data.
  •  
19.
  • Huang, Shuo (author)
  • Theoretical Investigations of High-Entropy Alloys
  • 2017
  • Licentiate thesis (other academic/artistic)abstract
    • High-entropy alloys (HEAs) are composed of multi-principal elements with equal or near-equal concentrations, which open up a vast compositional space for alloy design. Based on first-principle theory, we focus on the fundamental characteristics of the reported HEAs, as well as on the optimization and prediction of alternative HEAs with promising technological applications.The ab initio calculations presented in the thesis confirm and predict the relatively structural stability of different HEAs, and discuss the composition and temperature-induced phase transformations. The elastic behavior of several HEAs are evaluated through the single-crystal and polycrystalline elastic moduli by making use of a series of phenomenological models. The competition between dislocation full slip, twinning, and martensitic transformation during plastic deformation of HEAs with face-centered cubic phase are analyzed by studying the generalized stacking fault energy. The magnetic moments and magnetic exchange interactions for selected HEAs are calculated, and then applied in the Heisenberg Hamiltonian model in connection with Monte-Carlo simulations to get further insight into the magnetic characteristics including Curie point. The Debye-Grüneisen model is used to estimate the temperature variation of the thermal expansion coefficient.This work provides specific theoretical points of view to try to understand the intrinsic physical mechanisms behind the observed complex behavior in multi-component systems, and reveals some opportunities for designing and optimizing the properties of materials
  •  
20.
  • Hunold, Oliver, et al. (author)
  • Correlative theoretical and experimental investigation of the formation of AIYB(14) and competing phases
  • 2016
  • In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 119:8
  • Journal article (peer-reviewed)abstract
    • The phase formation in the boron-rich section of the Al-Y-B system has been explored by a correlative theoretical and experimental research approach. The structure of coatings deposited via high power pulsed magnetron sputtering from a compound target was studied using elastic recoil detection analysis, electron energy loss spectroscopy spectrum imaging, as well as X-ray and electron diffraction data. The formation of AlYB14 together with the (Y,Al)B-6 impurity phase, containing 1.8 at. % less B than AlYB14, was observed at a growth temperature of 800 degrees C and hence 600 degrees C below the bulk synthesis temperature. Based on quantum mechanical calculations, we infer that minute compositional variations within the film may be responsible for the formation of both icosahedrally bonded AlYB14 and cubic (Y,Al)B-6 phases. These findings are relevant for synthesis attempts of all boron rich icosahedrally bonded compounds with the space group: Imma that form ternary phases at similar compositions.
  •  
21.
  • Hunold, Oliver, et al. (author)
  • Elastic properties of amorphous T0.75Y0.75B14 (T = Sc, Ti, V, Y, Zr, Nb) and the effect of O incorporation on bonding, density and elasticity (T ' = Ti, Zr)
  • 2017
  • In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 29:8
  • Journal article (peer-reviewed)abstract
    • We have systematically studied the effect of transition metal valence electron concentration (VEC) of amorphous T0.75Y0.75B14 (a-T0.75Y0.75B14, T = Sc, Ti, V, Y, Zr, Nb) on the elastic properties, bonding, density and electronic structure using ab initio molecular dynamics. As the transition metal VEC is increased in both periods, the bulk modulus increases linearly with molar- and mass density. This trend can be understood by a concomitant decrease in cohesive energy. T' = Ti and Zr were selected to validate the predicted data experimentally. A-Ti0.74Y0.80B14 and a-Zr0.75Y0.75B14 thin films were synthesized by high power pulsed magnetron sputtering. Chemical composition analysis revealed the presence of up to 5 at.% impurities, with O being the largest fraction. The measured Young's modulus values for a-Ti0.74Y0.80B14 (301 +/- 8 GPa) and a-Zr0.75Y0.75B14 (306 +/- 9 GPa) are more than 20% smaller than the predicted ones. The influence of O incorporation on the elastic properties for these selected systems was theoretically studied, exemplarily in a-Ti0.75Y0.75B12.75O1.25. Based on ab initio data, we suggest that a-Ti0.75Y0.75B14 exhibits a very dense B network, which is partly severed in a-Ti0.75Y0.75B12.75O1.25. Upon O incorporation, the average coordination number of B and the molar density decrease by 9% and 8%, respectively. Based on these data the more than 20% reduced Young's modulus obtained experimentally for films containing impurities compared to the calculated Young's modulus for a-Ti0.75Y0.75B14 (without incorporated oxygen) can be rationalized. The presence of oxygen impurities disrupts the strong B network causing a concomitant decrease in molar density and Young's modulus. Very good agreement between the measured and calculated Young's modulus values is obtained if the presence of impurities is considered in the calculations. The implications of these findings are that prediction efforts regarding the elastic properties of amorphous borides containing oxygen impurities on the at.% level are flawed without taking the presence of impurities into account.
  •  
22.
  • Jiang, Kaiyun, et al. (author)
  • Ab initio study of effects of substitutional additives on the phase stability of gamma-alumina
  • 2010
  • In: Journal of Physics. - : Institute of Physics. - 0953-8984 .- 1361-648X. ; 22:50, s. 505502-
  • Journal article (peer-reviewed)abstract
    • Using ab initio calculations, we have evaluated two structural descriptions of gamma-Al(2)O(3), spinel and tetragonal hausmannite, and explored the relative stability of gamma-Al(2)O(3) with respect to alpha-Al(2)O(3) with 2.5 at.% of Si, Cr, Ti, Sc, and Y additives to identify alloying element induced electronic structure changes that impede the gamma to alpha transition. The total energy calculations indicate that Si stabilizes gamma-Al(2)O(3), while Cr stabilizes alpha-Al(2)O(3). As Si is added, a bond length increase in alpha-Al(2)O(3) is observed, while strong and short Si-O bonds are formed in gamma-Al(2)O(3), consequently stabilizing this phase. On the other hand, Cr additions induce a smaller bond length increase in alpha-Al(2)O(3) than in gamma-Al(2)O(3), therefore stabilizing the a-phase. The bulk moduli of gamma-Al(2)O(3) with these additives show no significant changes. The phase stability and elastic property data discussed here underline the application potential of Si alloyed gamma-Al(2)O(3) for applications at elevated temperatures. Furthermore it is evident that the tetragonal hausmannite structure is a suitable description for gamma-Al(2)O(3).
  •  
23.
  • Jin, P., et al. (author)
  • Low temperature deposition of a-Al2O3 thin films by sputtering using a Cr2O3 template
  • 2002
  • In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 20:6, s. 2134-2136
  • Journal article (peer-reviewed)abstract
    • A description about low temperature deposition of a-Al2O3 thin films by sputtering was presented. Cr2O3 thin layer was used as a template. Nanoindentation was used to study the mechanical properties of the deposited films. Calculations were made to obtain the hardness and Young's modulus of the films.
  •  
24.
  • Junaid, Muhammad, et al. (author)
  • Structural, mechanical, and magnetic properties of GaFe3N thin films
  • 2016
  • In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 34:4
  • Journal article (peer-reviewed)abstract
    • Using the density-functional theory, the structural, mechanical, and magnetic properties were investigated for different GaFe3N configurations: ferromagnetic, ferrimagnetic, paramagnetic, and nonmagnetic. Ferrimagnetic and high-spin ferromagnetic states exhibit the lowest energy and are the competing ground states as the total energy difference is 0.3 meV/atom only. All theoretically predicted values could be fully confirmed by experiments. For this, the authors synthesized phase pure, homogeneous, and continuous GaFe3N films by combinatorial reactive direct current magnetron sputtering. Despite the low melting point of gallium, the authors succeeded in the growth of GaFe3N films at a temperature of 500 degrees C. Those thin films exhibit a lattice parameter of 3.794 angstrom and an elastic modulus of 226620 GPa. Magnetic susceptibility measurements evidence a magnetic phase transitions at 8.060.1 K. The nearly saturated magnetic moment at 65 T is about 1.6 mu B/Fe and is close to the theoretically determined magnetic moment for a ferrimagnetic ordering (1.72 lB/Fe).
  •  
25.
  • Karimi Aghda, Soheil, et al. (author)
  • Unravelling the ion-energy-dependent structure evolution and its implications for the elastic properties of (V,Al)N thin films
  • 2021
  • In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 214
  • Journal article (peer-reviewed)abstract
    • Ion irradiation-induced changes in the structure and mechanical properties of metastable cubic (V,Al)N deposited by reactive high power pulsed magnetron sputtering are systematically investigated by correlating experiments and theory in the ion kinetic energy (Ek) range from 4 to 154 eV. Increasing Ek results in film densification and the evolution from a columnar (111) oriented structure at Ek ≤ 24 eV to a fine-grained structure with (100) preferred orientation for Ek ≥ 104 eV. Furthermore, the compressive intrinsic stress increases by 336 % to -4.8 GPa as Ek is increased from 4 to 104 eV. Higher ion kinetic energy causes stress relaxation to -2.7 GPa at 154 eV. These ion irradiation-induced changes in the thin film stress state are in good agreement with density functional theory simulations. Furthermore, the measured elastic moduli of (V,Al)N thin films exhibit no significant dependence on Ek. The apparent independence of the elastic modulus on Ek can be rationalized by considering the concurrent and balancing effects of bombardment-induced formation of Frenkel pairs (causing a decrease in elastic modulus) and evolution of compressive intrinsic stress (causing an increase in elastic modulus). Hence, the evolution of the film stresses and mechanical properties can be understood based on the complex interplay of ion irradiation-induced defect generation and annihilation.
  •  
26.
  • Kocherbitov, Vitaly, et al. (author)
  • Hydrogen bonding in glassy trehalose-water system : Insights from density functional theory and molecular dynamics simulations
  • 2024
  • In: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 160:8
  • Journal article (peer-reviewed)abstract
    • We report a detailed density functional theory and molecular dynamics study of hydrogen bonding between trehalose and water, with a special emphasis on interactions in the amorphous solid state. For comparison, water-water interactions in water dimers and tetramers are evaluated using quantum calculations. The results show that the hydrogen bonding energy is dependent not only on the geometry (bond length and angle) but also on the local environment of the hydrogen bond. This is seen in quantum calculations of complexes in vacuum as well as in amorphous solid states with periodic boundary conditions. The temperature-induced glass transition in the trehalose-water system was studied using molecular dynamics simulations with varying cooling and heating rates. The obtained parameters of the glass transition are in good agreement with the experiments. Moreover, the dehydration of trehalose in the glassy state was investigated through a gradual dehydration with multiple small steps under isothermal conditions. From these simulations, the values of water sorption energy at different temperatures were obtained. The partial molar enthalpy of mixing of water value of −18 kJ/mol found in calorimetric experiments was accurately reproduced in these simulations. These findings are discussed in light of the hydrogen bonding data in the system. We conclude that the observed exothermic effect is due to different responses of liquid and glassy matrices to perturbations associated with the addition or removal of water molecules.
  •  
27.
  • Krause, Andreas, et al. (author)
  • Interstitial hydrogen diffusion in M7C3 (M = Cr, Mn, Fe)
  • 2023
  • In: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 218
  • Journal article (peer-reviewed)abstract
    • To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensiticsteels, we have performed a density functional theory study on the solution energies and energy barriers forhydrogen diffusion in orthorhombic M7C3 (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice andcause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogenat different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03eV/H-atom for Cr7C3, Mn7C3, and Fe7C3, respectively. Energy barriers for the diffusion of hydrogen atoms havebeen probed with the nudged elastic band method, which shows comparably low barriers for transport viainterstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation betweenlow solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that thepresence of carbon tends to increase the energy barrier. Our results can explain previous experimental findingsof hydrogen located in the bulk of Cr7C3 precipitates and provide a solid basis for future design efforts of steelswith high strength and commensurable ductility
  •  
28.
  • Krause, Andreas M., et al. (author)
  • Interstitial diffusion of hydrogen in M7C3 (M=Cr,Mn,Fe)
  • 2023
  • In: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 218
  • Journal article (peer-reviewed)abstract
    • To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensitic steels, we have performed a density functional theory study on the solution energies and energy barriers for hydrogen diffusion in orthorhombic M7C3 (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice and cause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogen at different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03 eV/H-atom for Cr7C3, Mn7C3, and Fe7C3, respectively. Energy barriers for the diffusion of hydrogen atoms have been probed with the nudged elastic band method, which shows comparably low barriers for transport via interstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation between low solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that the presence of carbon tends to increase the energy barrier. Our results can explain previous experimental findings of hydrogen located in the bulk of CrC precipitates and provide a solid basis for future design efforts of steels with high strength and commensurable ductility.
  •  
29.
  • Kugler, Veronika Mozhdeh, et al. (author)
  • Low temperature growth and characterization of (Na,K)NbOx thin films
  • 2003
  • In: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 254:3-4, s. 400-404
  • Journal article (peer-reviewed)abstract
    • Thin (Na,K)NbOx perovskite films (NKN) have been deposited on SiO2/Si(0 0 1) substrates at low temperatures, from 350°C to 550°C, by RF magnetron sputtering. The effects of substrate temperature on microstructure, electrical-, and mechanical properties of the NKN films have been studied. X-ray diffraction analysis revealed that films deposited at temperatures in the range of 450-550°C were crystalline, growing as a single phase, with a preferred orientation of (0 0 1). Films deposited at 350°C, were shown to be amorphous. The growth temperature had a strong influence on the electrical properties of the NKN films and the relative dielectric constants of the obtained films were in between 38 and 78. Variations of the mechanical properties of the NKN films were observed for different substrate temperatures: The elastic moduli and the hardness values ranged from 205±26 to 93±29 GPa, and from 12±2 to around 2 GPa, for films deposited at 550°C and 450°C, respectively.
  •  
30.
  • Kugler, Veronika Mozhdeh, et al. (author)
  • Microstructure/dielectric property relationship of low temperature synthesised (Na,K)NbOx thin films
  • 2004
  • In: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 262:1-4, s. 322-326
  • Journal article (peer-reviewed)abstract
    • Thin films of (Na,K)NbOx (NKN) were grown by reactive RF magnetron sputtering on polycrystalline Pt80Ir20 substrates, at relatively low growth temperatures between 300°C and 450°C. The results show that the electrical performance and the microstructure of the films are a strong function of the substrate temperature. X-ray diffraction of films grown up to 400°C revealed the formation of only one crystalline NKN-phase with a preferred (0 0 2)-orientation. However, a mixed orientation together with a secondary, paraelectric potassium niobate phase, were observed for NKN films deposited at 450°C. The differences in the microstructure explains the variations in the dielectric constants and losses: The single phase NKN films displayed a dielectric constant and a dielectric loss of 506 and 0.011, respectively, while the films with mixed phases exhibited values of 475 and 0.022, respectively. The possibility of fabricating NKN films with relatively high dielectric properties at low growth temperatures, as demonstrated here, is of high technological importance.
  •  
31.
  • Li, Chun-Mei, 1979- (author)
  • Elastic properties and phase stability of shape memory alloys from first-principles theory
  • 2011
  • Doctoral thesis (other academic/artistic)abstract
    • Ni-Mn-Ga and In-Tl are two examples of shape memory alloys. Their shape memory effect is controlled by the martensitic transformation from the high temperature cubic phase to the low temperature tetragonal phase. Experimentally, it was found that the martensitic transformation, related to the elastic properties, is highly composition-dependent.In order to better understand the phase transition and facilitate the design of new materials with improved shape memory properties, the atomic scale description of the thermophysical properties of these alloys is needed. Therefore, in the presen tthesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbitals method in combination with the coherent-potential approximation. We present a theoretical description of the equilibrium properties of pure In and standard stoichiometric Ni2MnGa alloy with both cubic and tetragonal structures. In In-Tl alloys, all the calculated composition-dependent thermophysical properties: lattice parameter c/a, tetragonal shear modulus C" = (C11 - C12)/2, energy difference between the austenitic and martensitic phases, as well as electronic structures are shown to be in line with the experimentally observed lowering of the martensitic transition temperature TM with the addition of Tl. For most of the off-stoichiometric Ni2MnGa, the excess atoms of the rich component prefer to occupy the sublattice of the deficient one, except for the Ga-rich alloys, where the excess Ga atoms have strong tendency to take the Mn sublattice irrespective of the Mn occupation. In Ni-Mn-Ga-X (X=Fe, Co, and Cu) quarternary alloys, Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni-sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the studied Ni-Mn-Ga and Ni-Mn-Ga-X alloys with stable site-occupations, the shear modulus C" can be considered as a predictor of the composition dependence of TM of the alloys: the alloy with larger C" than that of the perfect Ni2MnGa generally possesses lower TM except for Ni2Mn1+xGa1-x and Ni2Mn1-xGaFex. The failure of C" as a factor of TM in these two types of alloys may be ascribed that the compositiondependentmagnetic interactions and the temperature-dependent C0, which also playan important role on the martensitic transformation in these alloys. Furthermore, wedemonstrate that a proper account of the temperature and composition dependence ofC0 gives us reasonable theoretical TM(x) values in Ni2+xMn1-xGa alloys. Also in this type of Ni-rich and Mn-deficient alloys, by using the Heisenberg model in combination with the mean-field approximation, the abnormal trend of experimental magnetic transition temperature TC(x) with respect to the composition x is shown to be well captured by the theory.
  •  
32.
  • Liu, Sida, et al. (author)
  • Modeling of metastable phase formation for sputtered Ti1-xAlxN thin films
  • 2019
  • In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 165, s. 615-625
  • Journal article (peer-reviewed)abstract
    • Metastable titanium aluminum nitride coatings are widely applied in cutting and forming applications. Although it is generally accepted that the phase formation of metastable TiAIN is governed by kinetic factors, modeling attempts today are based solely on energetics. In this work, the metastable phase formation of TiAIN is predicted based on one combinatorial magnetron sputtering experiment, the activation energy for surface diffusion, the critical diffusion distance, as well as thermodynamic calculations. The phase formation data obtained from further combinatorial growth experiments varying chemical composition, deposition temperature, and deposition rate are in good agreement with the model. Furthermore, it is demonstrated that a significant extension of the predicted critical solubility range is enabled by taking kinetic factors into account. Explicit consideration of kinetics extends the Al solubility limit to lower values, previously unobtainable by energetics, but accessible experimentally. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  •  
33.
  • Liu, Sida, et al. (author)
  • Stress-dependent prediction of metastable phase formation for magnetron-sputtered V1-xAlxN and Ti1-xAlxN thin films
  • 2020
  • In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 196, s. 313-324
  • Journal article (peer-reviewed)abstract
    • Metastable transition metal aluminum nitride (TMAlN, TM = Ti, V) thin films are today deposited utilizing ionized vapor phase condensation techniques where variations in ion flux and ion energy cause compressive film stress, in turn affecting Al solubility. While the metastable phase formation of TiAlN has been modeled, the influence of film stresses on phase formation has so far been overlooked. Using combinatorial deposition via magnetron sputtering, thermodynamic modeling and density functional theory calculations, we investigated the phase formation of V1-xAlxN and Ti1-xAlxN thin films at various substrate temperatures and deposition rates. Ab initio calculations indicate that the maximum solid solubility of Al in face-centered cubic (fcc) V1-xAlxN or fcc-Ti1-xAlxN shows a linear trend as a function of the magnitude of compressive stress. Here, we consider the influence of film stresses on the metastable phase formation of fcc-V1-xAlxN and fcc-Ti1-xAlxN for the first time. Specifically, experimental data from a single combinatorial deposition is utilized to predict the stress-dependent formation of metastable phases based on thermodynamic and ab initio data. Explicit consideration of stress extends the Al solubility limit to higher values for both Ti1-xAlxN and V1-xAlxN thin films, previously unobtainable by energetics, but accessible experimentally. These predictions are experimentally verified and thus provide guidance for experimental efforts with the goal of increasing the Al concentration in fcc-TMAlN thin films. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  •  
34.
  • Music, Denis, et al. (author)
  • Ab initio molecular dynamics of Al irradiation-induced processes during Al(2)O(3) growth
  • 2011
  • In: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 98:11, s. 111908-
  • Journal article (peer-reviewed)abstract
    • Al bombardment induced structural changes in alpha-Al(2)O(3) (R-3c) and gamma-Al(2)O(3) (Fd-3m) were studied using ab initio molecular dynamics. Diffusion and irradiation damage occur for both polymorphs in the kinetic energy range from 3.5 to 40 eV. However, for gamma-Al(2)O(3)(001) subplantation of impinging Al causes significantly larger irradiation damage and hence larger mobility as compared to alpha-Al(2)O(3). Consequently, fast diffusion along gamma-Al(2)O(3)(001) gives rise to preferential alpha-Al(2)O(3)(0001) growth, which is consistent with published structure evolution experiments.
  •  
35.
  • Music, Denis (author)
  • Boron Suboxide Based Thin Films
  • 2003
  • Doctoral thesis (other academic/artistic)abstract
    • Boron suboxide (BOx, x< 1.5) is a promising material for a wide range of applications. The elastic modulus (E) of 473 GPa has been reported for crystalline BO0.17, which tends to exhibit ultra-low friction as a direct consequence of boric acid (H3BO3) formation. When thin films are formed, they are reported to be amorphous with E<300 GPa. The objective of this work was to understand the formation of crystalline BOx based films. The methodology applied was to study the correlation between composition, structure, and properties of this material with experimental and theoretical means. The growth technique used was reactive RF magnetron sputtering in an Ar/O2 ambient and the characterization included a wide range of analytical techniques. The theoretical tools employed were classical molecular dynamics and ab initio calculations. Two distinctive approaches were used to find a pathway for crystalline film synthesis: firstly extensive ion bombardment during film growth and secondly control of chemical composition as well as quantum design. When films were bombarded with Ar+ ions, they remained amorphous and E (55-248 GPa) was found to be a strong function of film density (p=1.5-2.3 g/cm3) at constant chemical composition. On the other hand, when the chemical composition was varied, the behavior of E was more complex. Firstly, it was established that the role of O in amorphous films is different than in crystalline BO0.17, where O shortens the chemical bonds. As x in BOx increased from 0.08 to 0.60, the fraction of long B-O bonds increased resulting in E decreasing from 273 to 15 GPa. Secondly, H incorporation (up to 4.7 at.%) reduced E, most likely due to a H3BO3 formation. Thirdly, C incorporation (up to 0.6 at.%) shortened the average bond length, increasing p and E, but decreasing the relative dielectric constant (19.2-0.9). Based on ab initio calculations investigating the effect of alloying metals with BO0.17, the formation of a crystalline Y containing phase was predicted. This so-called BOY phase was calculated to be 0.36 eV/atom more stable than BO0.17 and B-B were 4.9% shorter. Experimentally, this phase was synthesized and properties were determined. The measured E of 316 GPa was consistent with the prediction based on elastic constants. Moreover, the BOY phase was found to be thermally stable at temperatures up to 1000 °C and exhibited a resistivity of 3.8 Ωcm.
  •  
36.
  • Music, Denis, et al. (author)
  • Conflicting primary and secondary properties of thermoelectric devices – A case study on the thermomechanical behavior of ZrNiSn
  • 2023
  • In: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 230
  • Journal article (peer-reviewed)abstract
    • While the primary properties of thermoelectric devices, directly related to the conversion efficiency, are considered in design efforts, the secondary (thermomechanical) properties are often ignored or overlooked even though they can lead to failure. Here, thermomechanical properties of thermoelectric ZrNiSn in the amorphous and crystalline state (space group F-43m), comprising thermal conductivity, thermal expansion, elastic (Young’s) modulus, and thermal shock, are studied using density functional theory and two phonon models. Thermal conductivity is also a key primary property for thermoelectric applications. Amorphous ZrNiSn exhibits a fourfold lower thermal conductivity than the crystalline counterpart due to high phonon–phonon scattering, which is conducive to thermoelectric performance. However, this is conflicting since a high thermal conductivity value is required to attain high resistance to thermal shock. Due to stronger bonds in the crystalline counterpart, facilitated by the stronger Zr 3d – Ni 3d and Sn 5p – Ni 3d hybridization and higher coordination than in the amorphous state, the linear coefficient of thermal expansion is lower, and the elastic modulus is higher. Hence, the crystalline state yields higher resistance to thermal shock. It is suggested that samples entailing both amorphous and crystalline regions can concurrently satisfy the primary and secondary requirements for enhanced efficiency and durability.
  •  
37.
  • Music, Denis, et al. (author)
  • Effect of chemical composition on the elastic and electrical properties of the boron-oxygen-yttrium system studied by ab initio and experimental means
  • 2004
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:9
  • Journal article (peer-reviewed)abstract
    • The effect of chemical composition on the elastic and electrical properties is studied for the BOxYz system with 0.27less than or equal toxless than or equal to1.14 and 0.36less than or equal tozless than or equal to0.08. We use ab initio calculations to obtain the elastic constants and density of states for BO1.5 and the BOY phase (yttrium substituting for oxygen in the boron suboxide structure). For decreasing x values, the elastic modulus is predicted to increase from 11 to 340 GPa, while electronic structure calculations suggest a shift in electrical properties from insulating to metallic. Thin films in the B-O-Y system are grown by reactive rf magnetron sputtering. As x decreases from 1.14 to 0.27, the elastic modulus increases from 12 to 282 GPa, which is a factor of 24, while resistivity decreases from 7.6+/-0.4 to (3.8+/-0.1)x10(-2) Omegam. The observed shifts in elasticity and resistivity are shown to be induced by the associated changes in chemical bonding from van der Waals type in BO1.5 to icosahedral type in the BOY phase.
  •  
38.
  • Music, Denis, et al. (author)
  • Elastic modulus-density relationship for amorphous boron suboxide thin films
  • 2003
  • In: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 76:2, s. 269-271
  • Journal article (peer-reviewed)abstract
    • Boron suboxide thin films nave been deposited on Si(100) substrates by reactive RF magnetron sputtering of a sintered B target in an Ar/O2 atmosphere. Elastic recoil detection analysis was applied to determine the film composition and density. Film structure was studied by X-ray diffraction and transmission electron microscopy. The elastic modulus, measured by nanoindentation, was found to decrease as the film density decreased. The relationship was affected by tuning the negative substrate bias potential and the substrate temperature during film growth. A decrease in film density, by a factor of 1.55, caused an elastic modulus reduction by a factor of 4.5, most likely due to formation of nano-pores containing Ar. It appears evident that the large scattering in the published data on elastic properties of films with identical chemical composition can readily be understood by density variations. These results are important for understanding the elastic properties of boron suboxide, but may also be qualitatively relevant for other B-based material systems.
  •  
39.
  • Music, Denis, et al. (author)
  • Elastic modulus of amorphous boron suboxide thin films studied by theoretical and experimental methods
  • 2003
  • In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 93:2, s. 940-944
  • Journal article (peer-reviewed)abstract
    • Elastic modulus of amorphous boron suboxide thin films was studied by theoretical and experimental methods. It was shown that the increase of x in the a-BOx films from 0.08 to 0.18 decreased the magnitude of the elastic modulus from 273 to 231 GPa. The decrease of the elastic modulus with an increasing amount of O was correlated to the presence of the long B-O bonds with ionic contribution and the reduction of the film density.
  •  
40.
  • Music, Denis, et al. (author)
  • Elastic properties of Fe-Mn random alloys studied by ab initio calculations
  • 2007
  • In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:19
  • Journal article (peer-reviewed)abstract
    • We have studied the influence of the Mn content on the elastic properties of Fe-Mn random alloys (space group of Fm (3) over barm) using ab initio calculations. The magnetic effects in Fe-Mn alloys have a strong influence on the elastic properties, even above the Neel temperature. As the Mn content is increased from 5 to 40 at. %, the C-44 elastic constant is unaffected, while C-11 and C-12 decrease. This behavior can be understood based on the magnetovolume effect which softens the lattice. Since the amplitude of local magnetic moments is less sensitive to volume conserving distortions, the softening is not present during shearing.
  •  
41.
  • Music, Denis, et al. (author)
  • Electrical resistivity modulation of thermoelectric iron based nanocomposites
  • 2018
  • In: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 157, s. 384-390
  • Journal article (peer-reviewed)abstract
    • Iron oxides are promising thermoelectrics, but their high electrical resistivity impedes broader applications. In this work, we have studied Fe oxides with metallic contributions. Pt and Ir additions are also considered to enhance the valence electron concentration and further modify the transport properties. Based on density functional theory explorations, Fe based clusters (Fe-3, Fe-4, and Fe3Pt) are suggested to act as nucleation sites for metallic crystallites, while O leads to formation of an amorphous matrix. This has been validated by transmission electron microscopy and x-ray photoelectron spectroscopy of sputter-grown Fe-Pt-Ir-O thin films. Densely packed bcc Fe grains, approx. 2-3 nm in diameter, are embedded in an amorphous Fe-O matrix in the as-grown state. The Seebeck coefficient reaches even -411 mu V K-1 and the electrical resistivity is up to 5 orders of magnitude lower than that of previously reported literature data on Fe oxides. We suggest that this peculiarity of our films is due to finite states localized at the Fermi level in these nanocomposites.
  •  
42.
  • Music, Denis, et al. (author)
  • Electronic structure and lattice dynamics of CaPd3B studied by first-principles methods
  • 2006
  • In: Physics Letters A. - : Elsevier BV. - 0375-9601 .- 1873-2429. ; 356:3, s. 251-254
  • Journal article (peer-reviewed)abstract
    • Using first-principles methods, we have studied the electronic structure and lattice dynamics of CaPd3B and compared them to isostructural MgNi3C- CaPd3B possesses less electronic states at the Fermi level, but more phonon modes at low frequencies, than MgNi3C. According to the phonon density of states, low frequency acoustic modes are dominated by Pd states, corresponding to Ni in MgNi3C. Furthermore, these Pd modes show soft phonons, which may be significant for second-order phase transitions. Based on the comparison to MgNi3C, we suggest that the properties of these two compounds may be similar.
  •  
43.
  • Music, Denis, et al. (author)
  • Electronic structure and shearing in nanolaminated ternary carbides
  • 2006
  • In: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 139:4, s. 139-143
  • Journal article (peer-reviewed)abstract
    • We have studied shearing in M2AlC phases (M = Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) using ab initio calculations. We propose that these phases can be classified into two groups based on the valence electron concentration induced changes in C-44. One group comprises M = VB and VIB, where the C-44 values are approximately 170 GPa and independent of the corresponding MC. The other group includes M = IIIB and IVB, where the C-44 shows a linear dependency with the corresponding MC. This may be understood based on the electronic structure: shear resistant bands are filled in M2AlC phases with M = VB and VIB, while they are not completely filled when M = IIIB and IVB. This notion is also consistent with our stress-strain analysis. These valence electron concentration induced changes in shear behaviour were compared to previously published valence electron concentration induced changes in compression behaviour [Z. Sun, D. Music, R. Ahuja, S. Li, J.M. Schneider, Phys. Rev. B 70 (2004) 092102]. These classification proposals exhibit identical critical valence electron concentration values for the group boundary. However, the physical mechanisms are not identical: the classification proposal, for the bulk modulus is based on MC-A coupling, while shearing is based on MC-MC coupling.
  •  
44.
  • Music, Denis, et al. (author)
  • Quantum design and synthesis of a boron-oxygen-yttrium phase
  • 2003
  • In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:24, s. 4286-4288
  • Journal article (peer-reviewed)abstract
    • A study was performed on quantum design and synthesis of a boron-oxygen-yttrium (BOY) phase. The calculations predicted that the BOY phase was 0.36 eV/atom more stable than crystalline BO0.17. The results showed that films with Y/B ratios ranging from 0.10 to 0.32, as determined via elastic recoil detection analysis, were grown over wide range of temperatures (300-600°C) and found to withstand 1000°C.
  •  
45.
  • Music, Denis, et al. (author)
  • Reaction of hydrogen peroxide with amorphous Ti–O surfaces
  • 2024
  • In: Results in Surfaces and Interfaces. - : Elsevier. - 2666-8459. ; 16
  • Journal article (peer-reviewed)abstract
    • Amorphous Ti–O thin films were synthesized by reactive magnetron sputtering and their affinity to H2O2 was studied electrochemically. They exhibit a pronounced affinity to H2O2, with TiO0.6 outperforming TiO1.5 and TiO2. Cathodic currents in the range of −211 μA/cm2 suggest that TiO0.6 is highly electroactive to H2O2. Molecular dynamics simulations based on density functional theory revealed rapid dissociation of H2O2 into OH on Ti–O surfaces, leading to diffusion of OH, facilitating both oxidation and reduction processes. These OH groups predominantly dock onto bridge and atop sites, oxidizing the surfaces. Notably, stronger interactions observed for lower oxidation states give rise to higher cathodic currents. Additionally, the ability of amorphous Ti–O thin films to generate free H atoms implies a possible reduction mechanism, likely leading to the desorption of H2O. Hence, amorphous TiO0.6 is more suitable for detection and monitoring of H2O2 than higher oxidation states in their crystalline forms, e.g., rutile TiO2, known as a benchmark for such applications. 
  •  
46.
  • Music, Denis, et al. (author)
  • Role of carbon in boron suboxide thin films
  • 2003
  • In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 21:4, s. 1355-1358
  • Journal article (peer-reviewed)abstract
    • X-ray amorphous BO0.02 thin films with the C content from 0 to 0.6 at. % were grown by reactive dual magnetron sputtering in an UHV system. It was shown that the elastic and dielectric properties of the as-deposited films are affected by the amount of the incorporated C and the film density.
  •  
47.
  • Music, Denis, et al. (author)
  • Selective oxidation of thermoelectric TiNiSn
  • 2021
  • In: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 198
  • Journal article (peer-reviewed)abstract
    • Multiscale modelling, involving thermodynamic assessment and molecular dynamics based on density functionaltheory, was employed to unravel oxidation mechanisms pertinent to half-Heusler TiNiSn (space group F-43m), inparticular counterintuitive Ni inertness. O2 molecules dissociate and chemisorb onto TiNiSn(001) and TiNiSn(110), which is followed by ingress of O. Both Ti and Sn egress, while Ni is less mobile. Such diffusion processesyield point defects (vacancies and interstitials) and give rise to Ti and Sn binary oxide formation, while Ni isinert, which may be corelated to its relatively low mobility. Based on the Mulliken analysis and thermodynamicsat 900 K, the Ti oxide formation sequence is suggested to be Ti2O3 → Ti3O5 → TiO2 → TiO. These data explain theexperimental observations on the Ni inertness during oxidation of TiNiSn.
  •  
48.
  • Music, Denis, et al. (author)
  • Synthesis and mechanical properties of boron suboxide thin films
  • 2002
  • In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 20:2, s. 335-337
  • Journal article (peer-reviewed)abstract
    • The synthesis and mechanical properties of boron suboxide thin films deposited on silicon and graphite substrates was discussed. The deposition was performed using reactive magnetron sputtering technique, and amorphous films were obtained. The affect of varying O2 partial pressure on film composition and microstructure was studied using spectroscopic techniques. It was found that variation of partial pressure from 0.02 to 0.21 resulted in a decrease in elastic modulus from 272 to 109 GPa.
  •  
49.
  • Music, Denis, et al. (author)
  • Theoretical and Experimental Aspects of Current and Future Research on NbO2 Thin Film Devices
  • 2021
  • In: Crystals. - : MDPI AG. - 2073-4352. ; 11:2
  • Research review (peer-reviewed)abstract
    • The present research front of NbO2 based memory, energy generation, and storage thin film devices is reviewed. Sputtering plasmas contain NbO, NbO2, and NbO3 clusters, affecting nucleation and growth of NbO2, often leading to a formation of nanorods and nanoslices. NbO2 (I41/a) undergoes the Mott topological transition at 1081 K to rutile (P42/mnm), yielding changes in the electronic structure, which is primarily utilized in memristors. The Seebeck coefficient is a key physical parameter governing the performance of thermoelectric devices, but its temperature behavior is still controversial. Nonetheless, they perform efficiently above 900 K. There is a great potential to improve NbO2 batteries since the theoretical capacity has not been reached, which may be addressed by future diffusion studies. Thermal management of functional materials, comprising thermal stress, thermal fatigue, and thermal shock, is often overlooked even though it can lead to failure. NbO2 exhibits relatively low thermal expansion and high elastic modulus. The future for NbO2 thin film devices looks promising, but there are issues that need to be tackled, such as dependence of properties on strain and grain size, multiple interfaces with point and extended defects, and interaction with various natural and artificial environments, enabling multifunctional applications and durable performance.
  •  
50.
  • Music, Denis, et al. (author)
  • Vacancy filling effect in thermoelectric NbO
  • 2015
  • In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:11
  • Journal article (peer-reviewed)abstract
    • Using density functional theory, we have systematically explored the 1a and 1b vacancy filling in NbO (space group Pm-3m) with Nb and N, respectively, to design compounds with large Seebeck coefficients. The most dominating effect was identified for filling of 1b Wyckoff sites with N giving rise to a fivefold increase in the Seebeck coefficient. This may be understood based on the electronic structure. Nb d-nonmetal p hybridization induces quantum confinement and hence enables the enhancement of the Seebeck coefficient. This was validated by measuring the Seebeck coefficient of reactively sputtered thin films. At 800 degrees C these electrically conductive oxynitrides exhibit the Seebeck coefficient of -70 mu V K-1, which is the largest absolute value ever reported for these compounds.
  •  
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