| 1. |
- Qiu, Ren, 1993, et al.
(author)
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CVD TiAlN coatings with tunable nanolamella architectures
- 2021
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In: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 413
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Journal article (peer-reviewed)abstract
- In this work, Ti1-xAlxN (TiAlN) coatings were synthesized by low pressure chemical vapour deposition (LPCVD), and the influence of a rotational precursor gas supply on the coating microstructure was studied. The microstructure of the TiAlN coatings were characterized using X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), and electron backscattered diffraction (EBSD). It is shown that a rotational precursor gas supply induces an oscillatory surface reaction, which causes a nanolamella architecture. When the gas beam directly hits the sample, the local gas flow velocity is high, which increases the deposition rate of Ti and a Ti(Al)N lamella is formed. When the gas beam rotates away, the local gas velocity is low, so the deposition rate of Ti decreases, and an Al(Ti)N lamella is formed. As this is repeated a periodic nanolamella architecture is formed. The nanolamellae grow epitaxially on three {001} facets of the 111 textured grains, which leads to a pyramidal surface morphology. Without gas supply rotation, a high Al content cubic phase was still obtained, but no nanolamella was formed. This indicates that Ti-rich lamellae are not necessary to stabilize an Al-rich cubic TiAlN phase. In addition, spinodal decomposition is not likely to be the driving force behind the nanolamella formation in LPCVD TiAlN, as this would also have happened in the sample without a rotational gas supply. Finally, the nanolamella periodicity is found to be tunable via controlling the rotation speed of the precursor supply relative to the coating growth rate.
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| 2. |
- Ben Hassine, Mohamed, 1985, et al.
(author)
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Growth model for high-Al containing CVD TiAlN coatings on cemented carbides using intermediate layers of TiN
- 2021
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In: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 421
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Journal article (peer-reviewed)abstract
- This work concerns high Al-containing TixAl1-xN coatings prepared using low pressure-chemical vapour deposition (LP-CVD). The coatings were examined using electron microscopy techniques, such as scanning transmission electron microscopy (STEM), energy dispersive X-ray analysis (EDX) and transmission Kikuchi diffraction (TKD). An intermediate TiN-layer with a 〈211〉 texture consisting of twinned, needle-shaped grains influences the subsequent growth of the TiAlN layer. The TiAlN grains were columnar with a texture of 〈211〉. As the grains grow along 〈111〉, with {001} facets, this led to a tilted pyramid surface morphology. The grains developed an internal periodic epitaxial nanolamella structure. The thicknesses were 2 nm for the low (x = 0.6) and 6 nm for the high (x = 0.9) Al-containing lamellae. The TiAlN layer growth could be described by a “two-wing” model, where two TiAlN grains with a twin-related orientation grow on a twinned TiN grain, where the two TiAlN grains gradually switch sides, making the appearance of two wings of columnar grains. In general, this work shows that it should be possible to control the growth of TiAlN layers by controlling the texture and morphology of an intermediate layer, such as TiN.
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| 3. |
- Polley, Craig Michael, 1984, et al.
(author)
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Bottom-Up Growth of Monolayer Honeycomb SiC
- 2023
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In: Physical Review Letters. - : AMER PHYSICAL SOC. - 1079-7114 .- 0031-9007. ; 130:7
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Journal article (peer-reviewed)abstract
- The long theorized two-dimensional allotrope of SiC has remained elusive amid the exploration of graphenelike honeycomb structured monolayers. It is anticipated to possess a large direct band gap (2.5 eV), ambient stability, and chemical versatility. While sp2 bonding between silicon and carbon is energetically favorable, only disordered nanoflakes have been reported to date. Here we demonstrate large-area, bottom-up synthesis of monocrystalline, epitaxial monolayer honeycomb SiC atop ultrathin transition metal carbide films on SiC substrates. We find the 2D phase of SiC to be almost planar and stable at high temperatures, up to 1200 °C in vacuum. Interactions between the 2D-SiC and the transition metal carbide surface result in a Dirac-like feature in the electronic band structure, which in the case of a TaC substrate is strongly spin-split. Our findings represent the first step towards routine and tailored synthesis of 2D-SiC monolayers, and this novel heteroepitaxial system may find diverse applications ranging from photovoltaics to topological superconductivity.
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| 4. |
- Qiu, Ren, 1993, et al.
(author)
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Atom probe tomography investigation of 3D nanoscale compositional variations in CVD TiAlN nanolamella coatings
- 2021
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In: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 426
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Journal article (peer-reviewed)abstract
- The cubic (Ti1−xAlx)Ny (TiAlN) phase with a nanolamella structure, synthesized via low pressure chemical vapour deposition (LPCVD), has been widely used in wear-resistant coatings during the latest years. The nanolamella structured TiAlN coatings contain periodic and epitaxially grown Ti-rich [denoted as Ti(Al)N] and Al-rich [denoted as Al(Ti)N] lamellae. However, the chemical compositions of these nano-structures have not been fully revealed. In this study, the microstructure of the nanolamella TiAlN coating was studied using scanning and transmission electron microscopy (SEM and TEM), and the chemical content was investigated using atom probe tomography (APT) that provides three-dimensional composition data with good accuracy and a spatial resolution down to the nanometer scale. It was found that over- and under-stoichiometries of N exist for the Ti(Al)N and the Al(Ti)N lamellae, respectively. According to the previous simulation results, such over- and under-stoichiometries are due to metal (Al and Ti) and N vacancies, respectively. The Al(Ti)N lamellae have a chemical formula of (Ti0.12Al0.88)N0.90, and have 10% N vacancies. The Ti(Al)N lamellae have a chemical formula of (Ti0.70Al0.30)0.97N, and have 3% metal (Al and Ti) vacancies. In addition to the nanolamella structure, compositional variations on a scale of a few nm were found in both types of lamellae. In the Ti-richest volumes, the composition corresponds to (Ti0.72Al0.28)0.88N so a maximum of 12% of metal vacancies exists. In the Al-richest volumes, the composition corresponds to (Ti0.07Al0.93)N0.64 so a maximum of 36% N vacancies exists. In addition, a small amount of Cl (around 0.1 at.%) was found in the coating, which could originate from the incomplete dissociation of chloride precursors during the CVD surface reaction.
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| 5. |
- Qiu, Ren, 1993, et al.
(author)
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Effects of gas flow on detailed microstructure inhomogeneities in LPCVD TiAlN nanolamella coatings
- 2020
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In: Materialia. - : Elsevier BV. - 2589-1529. ; 9
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Journal article (peer-reviewed)abstract
- Depositing homogeneous TiAlN coatings with a high Al content on cutting tool inserts is a challenging task. In this work, high-Al cubic Ti1-xAlxN coatings (average x = 0.8) with periodic Ti(Al)N (x = 0.5) and Al(Ti)N (x = 0.9) nanolamellae structure were synthesized by low pressure chemical vapour deposition (LPCVD) with different gas flow velocities, and the detailed microstructure was investigated by electron microscopy and simulations. Using a high gas flow rate, the columnar TiAlN grains with regular periodic nanolamella structures disappeared, the coating became enriched in Ti and hexagonal AlN (h-AlN) formed in the coating. The high Ti content is suggested to be caused by the high gas flow rate that increases the mass transport of the reactants. However, this does not influence the Al-deposition much as it is mainly limited by the surface kinetics due to the relatively low deposition temperature. Density functional theory (DFT) modelling and electron microscopy showed that h-AlN tends to form on the Ti(Al)N phase with a specific crystallographic orientation relationship. The Ti enrichment due to high gas flow rate promotes the formation of h-AlN, which therefore deteriorates the nanolamella structure and causes the disappearance of the columnar TiAlN grains. Thus, by designing the CVD process conditions to avoid too high gas flow rates, homogenous TiAlN coatings with high Al content and nanolamella structures can be deposited, which should yield superior cutting performance.
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| 6. |
- Qiu, Ren, 1993, et al.
(author)
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Facet identification in textured polycrystalline coatings by EBSD-aided SEM trace analysis
- 2024
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In: Materials Characterization. - 1044-5803. ; 209
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Journal article (peer-reviewed)abstract
- Competitive growth usually occurs during deposition of polycrystalline coatings and is associated by the formation of crystal facets. Knowing the facet planes in the crystal coordinates is thus essential for understanding the coating growth process and optimizing corresponding experimental parameters. However, the crystal facets of polycrystalline coatings have not been explored enough due to a lack of easy-to-use experimental methods. In this work, we apply an electron backscattered diffraction (EBSD)-aided scanning electron microscopy (SEM) trace analysis for determining the crystal facets of the chemical vapour deposition (CVD) polycrystalline (Ti,Al)N and Ti(C,N) coatings. Using this method, the crystal orientation of an interesting grain relative to the specimen coordinates is first determined by the EBSD point analysis, and the crystal orientations of edges shared by neighbouring facets are then determined using trace analysis. Finally, the facet normals are calculated by the cross-product of the crystal orientations of edges on the corresponding facet. The (Ti,Al)N coating is found to have {100} crystal facets, which is consistent with the results obtained using transmission electron microscopy in previous work. The Ti(C,N) coating is found to have {211} crystal facets. In principle, the method proposed in this work can be applied to any crystals with planar facets and sharp edges. A possible improvement of the method is also discussed.
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| 7. |
- Qiu, Ren, 1993, et al.
(author)
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Schmid factor analysis for chip flow induced plastic deformation of textured cubic carbonitride coatings
- 2022
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In: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 108
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Journal article (peer-reviewed)abstract
- In high-speed metal machining, cutting tools in the form of cemented carbide inserts coated with thin wear-resistant coatings are commonly used. These coatings are often made of metal carbonitrides with cubic rock salt crystal structure and different growth textures. However, the influence of the crystallographic texture of the coatings on their wear by plastic deformation due to the chip flow during machining needs to be revealed further. In this work, in order to analyse the ability of polycrystalline fibre-textured coatings with a rock salt structure to undergo plastic deformation, a method was developed for calculating Schmid factors of such textured coatings as a function of the loading angle of an external force. The Schmid factors were calculated for coatings with 100 and 211 growth textures, and {100} <110>, {110}<110> and {111}<110> as possible slip systems. For the {111}<110> slip systems, the Schmid factors are not much influenced by the force angle and coating texture, which is contrary to the {100}<110> and {110}<110> slip systems. The simulations were compared to wear on the rake face of two textured Ti(C,N) coatings after short longitudinal turning tests. The variation of the degree of plastic deformation of Ti(C,N) coatings with growth texture and external force angle indicates that the dominant activated slip systems are {110}<110> using the machining conditions applied in this work.
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| 8. |
- Shoja, Siamak, 1980, et al.
(author)
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On the influence of varying the crystallographic texture of alumina CVD coatings on cutting performance in steel turning
- 2022
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In: International Journal of Machine Tools and Manufacture. - : Elsevier BV. - 0890-6955. ; 176
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Journal article (peer-reviewed)abstract
- Understanding the mechanisms at the tool/chip interface during metal cutting is crucial in the production of almost every metallic component used in engineering applications. It is critical to have rapid, durable, and reliable machining processes. This work contributes to the understanding of mechanisms occurring on the tool in the secondary shear zone, and it is focusing on the tool side of the contact. Crystallographic textured Chemical Vapor Deposited (CVD) α-Al2O3 coated cutting tools are dominating the steel turning area, as they show an increased performance compared to coatings with randomly oriented grains. In this study, we investigate the effect of three different CVD α-Al2O3 textures on the initial rake crater behavior. This was done using a turning test designed to generate crater wear only in the alumina layer, which was deposited onto an inner Ti(C,N) layer, which in turn was deposited on a cemented carbide insert. With this approach, the influence of the underlying coating layer and substrate was reduced. Pre- and post-machining characterization of the different contact areas on the surfaces of the three textured CVD α-Al2O3 coatings, (0001)(0001), (011‾2) and (112‾0), was performed using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (XEDS). Plastic deformation, micro-rupture, abrasion and chemical reactions with the workpiece material are all identified as mechanisms involved in crater formation during turning. For the (0001)-textured coating, the observed low wear-rate is attributed to homogeneous basal-slip dominating plastic deformation, while for the (011‾2) and (112‾0) textures the main deformation mechanisms are attributed to heterogeneous plastic deformation, causing micro-rupture and abrasion, leading to higher wear-rates. The effect of a larger coating grain size is mainly seen as the formation of wider ridges and valleys, while the effect on wear rate was limited.
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| 9. |
- Tao, Qiang, 1987, et al.
(author)
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D-A(1)-D-A(2) Copolymers with Extended Donor Segments for Efficient Polymer Solar Cells
- 2015
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In: Macromolecules. - : American Chemical Society (ACS). - 1520-5835 .- 0024-9297. ; 48:4, s. 1009-1016
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Journal article (peer-reviewed)abstract
- Typically a donor-acceptor (D-A) design strategy is used for engineering the bandgap of polymers for solar cells. However, in this work, a series of alternating D-A(1-)D-A(2) copolymers PnTQTI(F) were synthesized and characterized with oligothiophenes (nT, n = 1, 2, 3) as the donor and two electron-deficient moieties, quinoxaline and isoindigo, as the acceptors in the repeating unit. We have studied the influence of the donor segments with different numbers of thiophene units and the effect of the addition of fluorine to the quinoxaline unit of the D-A(1)-D-A(2) polymers. The photophysical, electrochemical, and photovoltaic properties of the polymers were examined via a range of techniques and related to theoretical simulations. On increasing the length of the donor thiophene units, broader absorption spectra were observed in addition to a sequential increase in HOMO levels, while the LUMO levels displayed very small variations. The addition of fluorine to the quinoxaline unit not only decreased the HOMO levels of the resulting polymers but also enhanced the absorption coefficients. A superior photovoltaic performance was observed for the P3TQTI-F-based device with a power conversion efficiency (PCE) of 7.0%, which is the highest efficiency for alternating D-A(1)-D-A(2) polymers reported to date. The structureproperty correlations of the PnTQTI(F) polymers demonstrate that varying of the length of the donor segments is a valuable method for designing high-performance D-A(1)-D-A(2) copolymers and highlight the promising nature of D-A(1)-D-A(2) copolymers for efficient bulk-heterojunction solar cells.
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| 10. |
- Xu, Xiaofeng, 1984, et al.
(author)
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Effects of side chain isomerism on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene–quinoxaline copolymers: toward a side chain design for enhanced photovoltaic performance
- 2014
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In: Journal of Materials Chemistry. - 1364-5501 .- 0959-9428. ; 2:44, s. 18988-18997
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Journal article (peer-reviewed)abstract
- Four new D–A polymers PIDTT-Q-p, PIDTT-Q-m, PIDTT-QF-p and PIDTT-QF-m, using indacenodithieno[3,2-b]thiophene (IDTT) as an electron-rich unit and quinoxaline (Q) as an electron-deficient unit, were synthesized via a Pd-catalyzed Stille polymerization. The side chains on the pendant phenyl rings of IDTT were varied from the para- to the meta-position, and the effect of the inclusion of fluorine on the quinoxaline unit was simultaneously investigated. The influence on the optical and electrochemical properties, film topography and photovoltaic properties of the four copolymers were thoroughly examined via a range of techniques. The inductively electron-withdrawing properties of the fluorine atoms result in a decrease of the highest occupied molecular orbital (HOMO) energy levels. The effect of meta-substitution on the PIDTT-Q-m polymer leads to good solubility and in turn higher molecular weight. More importantly, it exhibits optimal morphological properties in the PIDTT-Q-m/PC71BM blends. As a result, the corresponding solar cells (ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al) attain the best power conversion efficiency (PCE) of 6.8%. The structure–property correlations demonstrate that the meta-alkyl-phenyl substituted IDTT unit is a promising building block for efficient organic photovoltaic materials. This result also extends our strategy with regards to side chain isomerism of IDTT-based copolymers for enhanced photovoltaic performance.
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