| 1. |
- Eriksson, Anders
(author)
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Cathodic Arc Synthesis of Ti-Si-C-N Thin Films from Ternary Cathodes
- 2010
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Licentiate thesis (other academic/artistic)abstract
- Cathodic arc deposition is a powerful technique for thin film synthesis, associated with explosive phase transformations resulting in an energetic and highly ionized plasma. This Thesis presents film growth through arc deposition from compound cathodes of Ti3SiC2, providing source material for plasma and films rich in Si and C. The interest for the resulting Ti-Si-C-N films is inspired by the two ternaries Ti-Si-N and Ti-C-N, both successfully applied as corrosion and wear resistant films, with a potential for synergistic effects in the quarternary system.When using a rotating substrate fixture setup, which is common in high capacity commercial deposition systems, the repeated passage though the plasma zone results in growth layers in the films. This effect has been observed in several coating systems, in deposition of various materials, but has not been explained in detail. The here investigated layers are characterized by a compositional modulation in Si and Ti content, which is attributed primarily to preferential resputtering in segments of rotation when the plasma has high incidence angle towards the substrate normal. For depositions in a non-reactive environment, the films consist of primarily understoichiometric TiCx, Ti, and silicide phases, and display a modest hardness (20-30 GPa) slightly improved by a decreasing layer thickness. Hence, the side effects of artificial layering from substrate rotation in deposition systems should be recognized.Adding N2 to the deposition process results in reactive growth of nitride material, formed in a wide range of compositions, and thereby enabling investigation of films in little explored parts of the Ti-Si-C-N system. The structure and properties of such films, comprising up to 12 at% Si and 16 at% C, is highly dependent on the supply of N2 during deposition. Superhard (45-50 GPa) cubic-phase (Ti,Si)(C,N) films with a nanocrystalline feathered structure is formed at N-content of 25-30 at%. At higher N2 deposition pressure, C and Si segregate to column and grain boundaries and the cubic phase assumes a more pronounced nitride character. This transformation is accompanied by substantially reduced film hardness to 20 GPa. Ti-Si-C-N films thus display a rich variety of structures with favorable mechanical properties, but in the regime of high Si and C content, the amount of N must be carefully controlled to avoid undesirable formation of weak grain boundary phases based on Si, C and N.
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| 2. |
- Flink, Axel, 1979-
(author)
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Growth and Characterization of Ti-Si-N Hard Coatings
- 2006
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Licentiate thesis (other academic/artistic)abstract
- Metastable (Ti,Si)N alloy and TiN/SiNx multilayer thin solid films as well as SiNx/TiN surfaces have been explored. Cubic Ti1-xSixN (0≤x≤0.14) films deposited onto cemented carbide (WC-Co) substrates by arc evaporation exhibited a competitive columnar growth mode where the structure transforms to a feather-like nanostructure with increasing Si content as revealed by x-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy revealed the presence of Ti-N and Si-N bonding, but no amorphous Si3N4. Band structure calculations showed that phase separation of NaClstructure Ti1-xSixN solid solution into cubic SiN and TiN phases is energetically favorable. The metastable microstructure, however, was maintained for the Ti0.86Si0.14N film annealed at 900°C, while recrystallization in the cubic state took place at 1100°C annealing during 2h. The Si content influenced the film hardness close to linearly, by combination of solid-solution hardening in the cubic state and defect hardening. For x=0 and x=0.14, nanoindentation gave a hardness of 29.9±3.4 GPa and 44.7±1.9 GPa, respectively. The hardness was retained during annealing at 900°C.Nanostructured materials, e.g., nanocomposites and nanolaminates, are defined by internal interfaces, of which the nature is still under debate. In this work two-phase model systems were explored by depositing SiNx/TiN nanolaminate films, including superlattices containing cubic SiNx, by dual target reactive magnetron sputtering. It is demonstrated that the interfacial phase of SiNx onto TiN(001) and TiN(111) can be crystalline, and even epitaxial with complex surface reconstructions. Using in situ structural analyses combined with ab initio calculations, it is found that SiNx layers grow epitaxially, giving rise to strong interfacial bonding, on both TiN(001) and TiN(111) surfaces. In addition, TiN overlayers grow epitaxially on SiNx/TiN(001) bilayers in nanolaminate structures. These results provide insight into the development of design rules for novel nanostructured materials.
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| 3. |
- Halim, Joseph, 1985-
(author)
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Synthesis and Characterization of 2D Nanocrystals and Thin Films of Transition Metal Carbides (MXenes)
- 2014
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Licentiate thesis (other academic/artistic)abstract
- Two dimensional (2D) materials have received growing interest because of their unique properties compared to their bulk counterparts. Graphene is the archetype 2D solid, but other materials beyond graphene, such as MoS2 and BN have become potential candidates for several applications. Recently, a new family of 2D materials of early transition metal carbides and carbonitrides (Ti2CTx, Ti3C2Tx, Ti3CNTx, Ta4C3Tx, and more), labelled MXenes, has been discovered, where T stands for the surface-terminating groups.Before the present work, MXenes had only been synthesized in the form of exfoliated and delaminated powders, which is not suitable for electronic applications. In this thesis, I demonstrate the synthesis of MXenes as epitaxial thin films, a more suitable form for electronic and photonic applications. Results show that 2D epitaxial Ti3C2Tx films - produced by HF and NH4HF2 etching of magnetron sputter-grown Ti3AlC2 - exhibit metallic conductive behaviour down to 100 K and are 90% transparent to light in the visible-infrared range. The results from this work may open the door for MXenes as potential candidates for transparent conductive electrodes as well as in electronic, photonic and sensing applications.MXenes have been shown to intercalate cations and molecules between their layers that in turn can alter the surface termination groups. There is therefore a need to study the surface chemistries of synthetized MXenes to be able to study the effect of intercalation as well as altering the surface termination groups on the electronic structure and chemical states of the elements present in MXene layers. X-ray Photoelectron Spectroscopy (XPS) in-depth characterization was used to investigate surface chemistries of Ti3C2Tx and Ti2CTx. This thesis includes the discussion of the effect of Ar+ sputtering and the number of layers on the surface chemistry of MXenes. This study serves as a baseline for chemical modification and tailoring of the surface chemistry groups to potential uses and applications.New MXene phases, Nb2CTx and V2CTx, are shown in this thesis to be produced from HF chemical etching of Nb2AlC and V2AlC powders. Characterization of the produced MXenes was carried out using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Transmission Electron Microscope (TEM) and XPS. Nb2CTx and V2CTx showed promising performance as electrodes for Li-ion batteries.In this thesis, electrochemical etching was used in an attempt to produce 2D metal carbides (MXene) from their ternary metal carbides, Ti3SiC2, Ti3AlC2 and Ti2AlC MAX phases. MAX phases in the form of highly dense bulk produced by Hot Isostatic Press. Several etching solutions were used such as HF, NaCl and HCl. Unlike the HF chemical etching of MAX phases, which results in MXenes, the electrochemical etching resulted in Carbide Derived Carbon (CDC). Here, I show the characterization of the produced CDC using several techniques such as XRD, TEM, Raman spectroscopy, and XPS. Electrochemical characterization was performed in the form of cyclic voltammetry, which sheds light on the etching mechanism.
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| 4. |
- Mockutė, Aurelija
(author)
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Thin Film Synthesis and Characterization of New MAX Phase Alloys
- 2012
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Licentiate thesis (other academic/artistic)abstract
- The objective of this Thesis is synthesis and characterization of new MAX phase alloys (M = early transition metal, A = A-group element, and X = C or N) based on incorporation of M and X elements previously not used in any known MAX phases. This is motivated by a search for optimized and unique materials properties, such as different magnetic states.Two synthesis routes have been used to attain Ti2AlC1-xOx: deposition of Ti2AlCy under high vacuum conditions with residual gas acting as O source, and solid-state reactions following deposition of understoichiometric TiCy on Al2O3. Detailed local quantification by analytical transmission electron microscopy (TEM) including electron energy loss spectroscopy (EELS) shows up to 13 at.% O within high quality MAX phase structure. According to previous theoretical work, the range of experimentally obtained O content is enough to observe drastic changes in the materials anisotropic electronic properties. Calculations on effect of substitutional O on shear deformation have also been performed.In a recent theoretical study by Dahlqvist et al., (Cr,Mn)2AlC has been predicted as a new stable magnetic nanoscale laminate. Inspired by this work, thin films of (Cr,Mn)2AlC, as well as of a neighboring system (Cr,Mn)2GeC, have been synthesized by magnetron sputtering. Incorporation of 8 and 12.5 at.% of Mn, respectively, has been detected by analytical TEM including EELS and energy dispersive X-ray spectroscopy (EDX). The total saturation moment of 0.36μB per Mn atom at 50 K has been measured by vibrating sample magnetometry (VSM) for a (Cr,Mn)2GeC sample, providing the first experimental evidence of a magnetic MAX phase.The experimental results obtained in this Thesis provide a base for expanding the MAX phase definition and materials characteristics into new areas, towards further fundamental understanding and functionalization.
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| 5. |
- Petruhins, Andrejs
(author)
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Synthesis and characterization of Ga-containing MAX phase thin films
- 2014
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Licentiate thesis (other academic/artistic)abstract
- The study of magnetic Mn+1AXn (MAX) phases (n = 1 − 3, M – a transition metal, A – an A group element, X – C or N) is a recently established research area, fuelled by theoretical predictions and first confirmed experimentally through alloying of Mn into the well-known Cr2AlC and Cr2GeC. Theoretical phase stability investigations suggested a new magnetic MAX phase, Mn2GaC, containing Ga which is liquid close to room temperature. Hence, alternative routes for MAX phase synthesis were needed, motivating a further development of magnetron sputtering from liquid targets.In this thesis, (Cr1-xMnx)2GaC 0 ≤ x ≤ 1 MAX phase thin films have been synthesized from elemental and/or compound targets, using ultra high vacuum magnetron sputtering. Initial thin film synthesis of Cr2GaC was performed using elemental targets, including liquid Ga. Process optimization ensured optimal target size and crucible geometry for containing the Ga. Films were deposited at 650 °C on MgO(111) substrates. X-ray diffraction and transmission electron microscopy confirms the growth of epitaxial Cr2GaC MAX phase with minor inclusions of Cr3Ga.To explore the magnetic characteristics upon Mn alloying, synthesis of (Cr0.5Mn0.5)2GaC thin films was performed from elemental Ga and C and a composite Cr/Mn target of 1:1 composition. Films were deposited on MgO(111), Al2O3(0001) (with or without NbN seed layer), and 4° off-cut 4H-SiC(0001) substrates. The films are smooth and of high structural quality as confirmed by X-ray diffraction and transmission electron microscopy. The film composition measured by high resolution energy dispersive X-ray spectroscopy confirms a composition corresponding to (Cr0.5Mn0.5)2GaC. The magnetic response, as measured with vibrating sample magnetometry, displays a ferromagnetic component, however, the temperature dependence of the magnetic moments and saturation fields suggests competing magnetic interaction and possible non-collinear magnetic ordering.Finally, inspired by theoretical predictions, a new member of the MAX phase family, Mn2GaC, was synthesized. This is the first MAX phase containing Mn as a sole M element. X-ray diffraction and transmission electron microscopy confirms the characteristic MAX phase structure with a 2:1:1 composition. Theoretical work suggests that the magnetic ground state is almost degenerate between ferromagnetic and anti-ferromagnetic. Vibrating sample magnetometry shows ferromagnetic response with a transition temperature Tc of 230 K. However, also for this phase, complex magnetism is suggested. Altogether, the results indicate a new family of magnetic nanolaminates with a rich variation of magnetic ground states.
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| 6. |
- Thore, Andreas
(author)
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A theoretical investigation of Tin+1AlCn and Mn2GaC MAX phases : phase stability and materials properties
- 2014
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Licentiate thesis (other academic/artistic)abstract
- This thesis presents theoretical research on MAX phases (M=transition metal, A=A-group element, X=carbon and/or nitrogen), with focus on predictions of phase stability as well as of physical properties.The first part is an investigation of the phase stability of the MAX phases Ti2AlC, Ti3AlC2, and Ti4AlC3 at elevated temperatures, where the former two phases have been obtained experimentally. Phase stability calculations of MAX phases usually do not take temperature dependent effects such as electronic excitations and lattice vibrations into consideration due to significantly increased computational cost. The results have nevertheless so far been quite accurate, with good agreement between theory and experiments. Still, the question whether the inclusion of temperature into the calculations could significantly alter the results as compared to previous 0 K calculations needs to be investigated, since this has bearing on the reliability of future predictions of the stability of not yet known MAX phases. However, it is shown that for Tin+1AlCn, the different temperature dependent effects largely cancel each other. The results therefore suggest that to go beyond 0 K calculations for phase stability predictions of MAX phases is motivated only for borderline cases.The second part investigates the Mn2GaC MAX phase, which was recently predicted from theoretical phase stability calculations and subsequently synthesized. As a new member of the MAX phase family as well as being one of the first known MAX phases to exhibit magnetism, it is of interest to explore its physical properties. Here, we have used firstprinciples calculations to determine the electronic, vibrational and elastic properties. Analysis of the electronic band structure indicates anisotropy in transport properties, while the electronic and phonon density of states shows that the relative orientation of the Mn magnetic moments over the Ga layers affects the distribution of the electronic and vibrational states for both Mn and Ga.The Voigt bulk, Voigt shear, and Young's modulus is also investigated, together with the Poisson's ratio, the elastic anisotropy, and the machinability via two machinability indices. In relation to experimental results of the moduli of other M2AC phases, the Voigt bulk and shear moduli are concluded to be fairly low, 157 and 93 GPa, respectively, while the magnitude of the Young's modulus at 233 GPa is intermediate. The Poisson's ratio, which is 0.25, on the other hand, is comparatively high. The phase is shown to be elastically quite isotropic, and, just as other M2GaC phases, also machinable. As all here investigated properties are affected by the choice of magnetic spin configuration, the results show the importance of identifying the correct magnetic ground state in future theoretical work on magnetic MAX phases.
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