| 1. |
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| 2. |
- Klionsky, Daniel J., et al.
(author)
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Guidelines for the use and interpretation of assays for monitoring autophagy
- 2012
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In: Autophagy. - : Informa UK Limited. - 1554-8635 .- 1554-8627. ; 8:4, s. 445-544
-
Research review (peer-reviewed)abstract
- In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.
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| 3. |
- Reinius, Björn, et al.
(author)
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Female-biased expression of long non-coding RNAs in domains that escape X-inactivation in mouse
- 2010
-
In: BMC Genomics. - : Springer Science and Business Media LLC. - 1471-2164. ; 11:1, s. 614-
-
Journal article (peer-reviewed)abstract
- Background: Sexual dimorphism in brain gene expression has been recognized in several animal species.However, the relevant regulatory mechanisms remain poorly understood. To investigatewhether sex-biased gene expression in mammalian brain is globally regulated or locallyregulated in diverse brain structures, and to study the genomic organisation of brain-expressedsex-biased genes, we performed a large scale gene expression analysis of distinct brainregions in adult male and female mice. Results: This study revealed spatial specificity in sex-biased transcription in the mouse brain, andidentified 173 sex-biased genes in the striatum; 19 in the neocortex; 12 in the hippocampusand 31 in the eye. Genes located on sex chromosomes were consistently over-represented inall brain regions. Analysis on a subset of genes with sex-bias in more than one tissue revealedY-encoded male-biased transcripts and X-encoded female-biased transcripts known to escapeX-inactivation. In addition, we identified novel coding and non-coding X-linked genes withfemale-biased expression in multiple tissues. Interestingly, the chromosomal positions of allof the female-biased non-coding genes are in close proximity to protein-coding genes thatescape X-inactivation. This defines X-chromosome domains each of which contains a codingand a non-coding female-biased gene. Lack of repressive chromatin marks in non-codingtranscribed loci supports the possibility that they escape X-inactivation. Moreover, RNADNAcombined FISH experiments confirmed the biallelic expression of one such noveldomain. Conclusion: This study demonstrated that the amount of genes with sex-biased expression variesbetween individual brain regions in mouse. The sex-biased genes identified are localized onmany chromosomes. At the same time, sexually dimorphic gene expression that is common toseveral parts of the brain is mostly restricted to the sex chromosomes. Moreover, the studyuncovered multiple female-biased non-coding genes that are non-randomly co-localized onthe X-chromosome with protein-coding genes that escape X-inactivation. This raises thepossibility that expression of long non-coding RNAs may play a role in modulating geneexpression in domains that escape X-inactivation in mouse.
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| 4. |
- Vogel, Jacob W., et al.
(author)
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Four distinct trajectories of tau deposition identified in Alzheimer’s disease
- 2021
-
In: Nature Medicine. - : Springer Science and Business Media LLC. - 1078-8956 .- 1546-170X. ; 27:5, s. 871-881
-
Journal article (peer-reviewed)abstract
- Alzheimer’s disease (AD) is characterized by the spread of tau pathology throughout the cerebral cortex. This spreading pattern was thought to be fairly consistent across individuals, although recent work has demonstrated substantial variability in the population with AD. Using tau-positron emission tomography scans from 1,612 individuals, we identified 4 distinct spatiotemporal trajectories of tau pathology, ranging in prevalence from 18 to 33%. We replicated previously described limbic-predominant and medial temporal lobe-sparing patterns, while also discovering posterior and lateral temporal patterns resembling atypical clinical variants of AD. These ‘subtypes’ were stable during longitudinal follow-up and were replicated in a separate sample using a different radiotracer. The subtypes presented with distinct demographic and cognitive profiles and differing longitudinal outcomes. Additionally, network diffusion models implied that pathology originates and spreads through distinct corticolimbic networks in the different subtypes. Together, our results suggest that variation in tau pathology is common and systematic, perhaps warranting a re-examination of the notion of ‘typical AD’ and a revisiting of tau pathological staging. © 2021, The Author(s), under exclusive licence to Springer Nature America, Inc.
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| 5. |
- Zhou, XP, et al.
(author)
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Non-coding variability at the APOE locus contributes to the Alzheimer's risk
- 2019
-
In: Nature communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10:1, s. 3310-
-
Journal article (peer-reviewed)abstract
- Alzheimer’s disease (AD) is a leading cause of mortality in the elderly. While the coding change of APOE-ε4 is a key risk factor for late-onset AD and has been believed to be the only risk factor in the APOE locus, it does not fully explain the risk effect conferred by the locus. Here, we report the identification of AD causal variants in PVRL2 and APOC1 regions in proximity to APOE and define common risk haplotypes independent of APOE-ε4 coding change. These risk haplotypes are associated with changes of AD-related endophenotypes including cognitive performance, and altered expression of APOE and its nearby genes in the human brain and blood. High-throughput genome-wide chromosome conformation capture analysis further supports the roles of these risk haplotypes in modulating chromatin states and gene expression in the brain. Our findings provide compelling evidence for additional risk factors in the APOE locus that contribute to AD pathogenesis.
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| 6. |
- Anasori, Babak, et al.
(author)
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A Tungsten-Based Nanolaminated Ternary Carbide: (W,Ti)(4)C4-x
- 2019
-
In: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 58:2, s. 1100-1106
-
Journal article (peer-reviewed)abstract
- Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their twodimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten based nanolaminated ternary phase, (W,Ti)(4)C4-x, synthesized by an Al catalyzed reaction of W, Ti, and C powders at 1600 degrees C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure (P6(3)/mmc) with lattice parameters, a = 3.00880(7) angstrom, and c = 19.5633(6) angstrom and a nominal chemistry of (W,Ti)(4)C4-x (actual chemistry, W2.1(1)Ti1.6(1)C2.6(1)). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times.
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| 7. |
- Anasori, Babak, et al.
(author)
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Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3
- 2015
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In: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 118:9, s. 094304-
-
Journal article (peer-reviewed)abstract
- Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases-Mo2TiAlC2 and Mo2Ti2AlC3-synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m) Ti:1.1Al:2C with 1.5 less than= m less than= 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 degrees C for 4 h under Ar flow. In general, for m greater than= 2 an ordered 312 phase, (Mo2Ti) AlC2, was the majority phase; for mless than 2, an ordered 413 phase (Mo2Ti2)AlC3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo2TiAlC1.7 and Mo2Ti1.9Al0.9C2.5, respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo2TiAlC2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo2Ti2AlC3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the Mn+1Xn blocks. At 331 GPa and 367 GPa, respectively, the Youngs moduli of the ordered Mo2TiAlC2 and Mo2Ti2AlC3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10K for both phases showed metallic behavior. (C) 2015 AIP Publishing LLC.
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| 8. |
- Bakhit, Babak, 1983-, et al.
(author)
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Dense Ti0.67Hf0.33B1.7 thin films grown by hybrid HfB2-HiPIMS/TiB2-DCMS co-sputtering without external heating
- 2021
-
In: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 186
-
Journal article (peer-reviewed)abstract
- There is a need for developing synthesis techniques that allow the growth of high-quality functional films at low substrate temperatures to minimize energy consumption and enable coating temperature-sensitive substrates. A typical shortcoming of conventional low-temperature growth strategies is insufficient atomic mobility, which leads to porous microstructures with impurity incorporation due to atmosphere exposure, and, in turn, poor mechanical properties. Here, we report the synthesis of dense Ti0.67Hf0.33B1.7 thin films with a hardness of ∼41.0 GPa grown without external heating (substrate temperature below ∼100 °C) by hybrid high-power impulse and dc magnetron co-sputtering (HfB2-HiPIMS/TiB2-DCMS) in pure Ar on Al2O3(0001) substrates. A substrate bias potential of −300 V is synchronized to the target-ion-rich portion of each HiPIMS pulse. The limited atomic mobility inherent to such desired low-temperature deposition is compensated for by heavy-mass ion (Hf+) irradiation promoting the growth of dense Ti0.67Hf0.33B1.7.
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| 9. |
- Bakhit, Babak, 1983-, et al.
(author)
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Multifunctional ZrB2-rich Zr1-xCrxBy thin films with enhanced mechanical, oxidation, and corrosion properties
- 2021
-
In: Vacuum. - : Elsevier BV. - 0042-207X .- 1879-2715. ; 185
-
Journal article (peer-reviewed)abstract
- Refractory transition-metal (TM) diborides have high melting points, excellent hardness, and good chemical stability. However, these properties are not sufficient for applications involving extreme environments that require high mechanical strength as well as oxidation and corrosion resistance. Here, we study the effect of Cr addition on the properties of ZrB2-rich Zr1-xCrxBy thin films grown by hybrid high-power impulse and dc magnetron co-sputtering (Cr-HiPIMS/ZrB2-DCMS) with a 100-V Cr-metal-ion synchronized potential. Cr metal fraction, x = Cr/(Zr+Cr), is increased from 0.23 to 0.44 by decreasing the power Pzrb2 applied to the DCMS ZrB2 target from 4000 to 2000 W, while the average power, pulse width, and frequency applied to the HiPIMS Cr target are maintained constant. In addition, y decreases from 2.18 to 1.11 as a function of Pzrb2, as a result of supplying Cr to the growing film and preferential B resputtering caused by the pulsed Cr-ion flux. ZrB2.18, Zr0.77Cr0.23B1.52, Zr0.71Cr0.29B1.42, and Zr0.68Cr0.32B1.38 2 films have hexagonal AlB2 crystal structure with a columnar nanostructure, while Zr0.64Cr0.36B1.30 and Zr0.56Cr0.44B1.11 are amorphous. All films show hardness above 30 GPa. Zr0.56Cr0.44B1.11 alloys exhibit much better toughness, wear, oxidation, and corrosion resistance than ZrB2.18. This combination of properties makes Zr0.56Cr0.44B1.11 ideal candidates for numerous strategic applications.
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| 10. |
- Bakhit, Babak, et al.
(author)
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Strategy for simultaneously increasing both hardness and toughness in ZrB2-rich Zr1-xTaxBy thin films
- 2019
-
In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : A V S AMER INST PHYSICS. - 0734-2101 .- 1520-8559. ; 37:3
-
Journal article (peer-reviewed)abstract
- Refractory transition-metal diborides exhibit inherent hardness. However, this is not always sufficient to prevent failure in applications involving high mechanical and thermal stress, since hardness is typically accompanied by brittleness leading to crack formation and propagation. Toughness, the combination of hardness and ductility, is required to avoid brittle fracture. Here, the authors demonstrate a strategy for simultaneously enhancing both hardness and ductility of ZrB2-rich thin films grown in pure Ar on Al2O3(0001) and Si(001) substrates at 475 degrees C. ZrB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a ZrB2 target, while Zr1-xTaxBy alloy films are grown, thus varying the B/metal ratio as a function of x, by adding pulsed high-power impulse magnetron sputtering (HiPIMS) from a Ta target to deposit Zr1-xTaxBy alloy films using hybrid Ta-HiPIMS/ZrB2-DCMS sputtering with a substrate bias synchronized to the metal-rich portion of each HiPIMS pulse. The average power P-Ta (and pulse frequency) applied to the HiPIMS Ta target is varied from 0 to 1800W (0 to 300 Hz) in increments of 600W (100 Hz). The resulting boron-to-metal ratio, y = B/(Zr+Ta), in as-deposited Zr1-xTaxBy films decreases from 2.4 to 1.5 as P-Ta is increased from 0 to 1800W, while x increases from 0 to 0.3. A combination of x-ray diffraction (XRD), glancing-angle XRD, transmission electron microscopy (TEM), analytical Z-contrast scanning TEM, electron energy-loss spectroscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and atom-probe tomography reveals that all films have the hexagonal AlB2 crystal structure with a columnar nanostructure, in which the column boundaries of layers with 0 amp;lt;= x amp;lt; 0.2 are B-rich, whereas those with x amp;gt;= 0.2 are Ta-rich. The nanostructural transition, combined with changes in average column widths, results in an similar to 20% increase in hardness, from 35 to 42 GPa, with a simultaneous increase of similar to 30% in nanoindentation toughness, from 4.0 to 5.2MPa root m. Published by the AVS.
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| 11. |
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| 12. |
- Champagne, A., et al.
(author)
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First-order Raman scattering of rare-earth containing i-MAX single crystals (Mo2/3RE1/3)(2)AlC (RE = Nd, Gd, Dy, Ho, Er)
- 2019
-
In: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 3:5
-
Journal article (peer-reviewed)abstract
- Herein, we report on the growth of single crystals of various (Mo2/3RE1/3)(2)AlC (RE = Nd, Gd, Dy, Ho, Er) i-MAX phases and their Raman characterization. Using first principles, the wave numbers of the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing i-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.
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| 13. |
- Chen, Liugang, et al.
(author)
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Theoretical Prediction and Synthesis of (Cr2/3Zr1/3)(2)AIC i-MAX Phase
- 2018
-
In: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:11, s. 6237-6244
-
Journal article (peer-reviewed)abstract
- Guided by predictive theory, a new compound with chemical composition (Cr2/3Zr1/3)(2)AlC was synthesized by hot pressing of Cr, ZrH2, Al, and C mixtures at 1300 degrees C. The crystal structure is monoclinic of space group C2/c and displays in-plane chemical order in the metal layers, a so-called i-MAX phase. Quantitative chemical composition analyses confirmed that the primary phase had a (Cr2/3Zr1/3)(2)AlC stoichiometry, with secondary Cr2AlC, AlZrC2, and ZrC phases and a small amount of Al-Cr intermetallics. A theoretical evaluation of the (Cr2/3Zr1/3)(2)AlC magnetic structure was performed, indicating an antiferromagnetic ground state. Also (Cr2/3Zr1/3)(2)AlC, of the same structure, was predicted to be stable.
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| 14. |
- Dahlqvist, Martin, et al.
(author)
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Origin of Chemically Ordered Atomic Laminates (i-MAX): Expanding the Elemental Space by a Theoretical/Experimental Approach
- 2018
-
In: ACS Nano. - : AMER CHEMICAL SOC. - 1936-0851 .- 1936-086X. ; 12:8, s. 7761-7770
-
Journal article (peer-reviewed)abstract
- With increased chemical diversity and structural complexity comes the opportunities for innovative materials possessing advantageous properties. Herein, we combine predictive first-principles calculations with experimental synthesis, to explore the origin of formation of the atomically laminated i-MAX phases. By probing (Mo2/3M1/32)(2)AC (where M-2 = Sc, Y and A = Al, Ga, In, Si, Ge, In), we predict seven stable i-MAX phases, five of which should have a retained stability at high temperatures. (Mo2/3Sc1/3)(2)GaC and (Mo2/3Y1/3)(2)GaC were experimentally verified, displaying the characteristic in-plane chemical order of Mo and Sc/Y and Kagome-like ordering of the A-element. We suggest that the formation of i-MAX phases requires a significantly different size of the two metals, and a preferable smaller size of the A-element. Furthermore, the population of antibonding orbitals should be minimized, which for the metals herein (Mo and Sc/Y) means that A elements from Group 13 (Al, Ga, In) are favored over Group 14 (Si, Ge, Sn). Using these guidelines, we foresee a widening of elemental space for the family of i-MAX phases and expect more phases to be synthesized, which will realize useful properties. Furthermore, based on i-MAX phases as parent materials for 2D MXenes, we also expect that the range of MXene compositions will be expanded.
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| 15. |
- Dahlqvist, Martin, et al.
(author)
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Out-Of-Plane Ordered Laminate Borides and Their 2D Ti-Based Derivative from Chemical Exfoliation
- 2021
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In: Advanced Materials. - : Wiley-VCH Verlagsgesellschaft. - 0935-9648 .- 1521-4095. ; 33:38
-
Journal article (peer-reviewed)abstract
- Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. Inspired by M5SiB2 (T2) phases, the finding of a family of laminated quaternary metal borides, M M-4 SiB2, with out-of-plane chemical order is reported here. 11 chemically ordered phases as well as 40 solid solutions, introducing four elements previously not observed in these borides are predicted. The predictions are experimentally verified for Ti4MoSiB2, establishing Ti as part of the T2 boride compositional space. Chemical exfoliation of Ti4MoSiB2 and select removal of Si and MoB2 sub-layers is validated by derivation of a 2D material, TiOxCly, of high yield and in the form of delaminated sheets. These sheets have an experimentally determined direct band gap of approximate to 4.1 eV, and display characteristics suitable for supercapacitor applications. The results take the concept of chemical exfoliation beyond currently available 2D materials, and expands the envelope of 3D and 2D candidates, and their applications.
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| 16. |
- Dahlqvist, Martin, et al.
(author)
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Prediction and synthesis of a family of atomic laminate phases with Kagome-like and in-plane chemical ordering
- 2017
-
In: Science Advances. - : AMER ASSOC ADVANCEMENT SCIENCE. - 2375-2548. ; 3:7
-
Journal article (peer-reviewed)abstract
- The enigma of MAX phases and their hybrids prevails. We probe transition metal (M) alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these MAX hybrids, namely, (V2/3Zr1/3)(2)AlC and (Mo2/3Y1/3)(2)AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermodynamically stable phases, with Kagome-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic C2/c. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials.
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| 17. |
- Eklund, Per, et al.
(author)
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Discovery of the Ternary Nanolaminated Compound Nb2GeC by a Systematic Theoretical-Experimental Approach
- 2012
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 109:3, s. 035502-
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Journal article (peer-reviewed)abstract
- Since the advent of theoretical materials science some 60 years ago, there has been a drive to predict and design new materials in silicio. Mathematical optimization procedures to determine phase stability can be generally applicable to complex ternary or higher-order materials systems where the phase diagrams of the binary constituents are sufficiently known. Here, we employ a simplex-optimization procedure to predict new compounds in the ternary Nb-Ge-C system. Our theoretical results show that the hypothetical Nb2GeC is stable, and excludes all reasonably conceivable competing hypothetical phases. We verify the existence of the Nb2GeC phase by thin film synthesis using magnetron sputtering. This hexagonal nanolaminated phase has a and c lattice parameters of similar to 3.24 angstrom and 12.82 angstrom.
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| 18. |
- Eriksson, Anders, et al.
(author)
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Nanocolumnar Epitaxial Ti1-xSixN (0 ≤ x ≤ 0.18) Thin Films Grown by Dual Reactive Magnetron Sputtering on MgO (001), (011), and (111) Substrates
- 2012
-
Other publication (other academic/artistic)abstract
- Ti1-xSixNy thin films and multilayers have been grown on single-crystal TiN-templated MgO (001), (011), and (111) substrates kept at 550 °C. Elemental Ti and Si targets were used in UVH reactive dual magnetron sputtering in a mixed Ar/N2 discharge. Composition analysis by time-of-flight energy elastic recoil detection analysis show that the films are close to stoichiometric (0.95 ≤ y ≤ 1.00) with respect to TiN over the wide range of Si concentrations 0 ≤ x ≤ 0.22. High-resolution transmission electron microscopy (TEM) combined with scanning TEM and energy dispersive Xray analysis show that all films grow epitaxially for x ≤ 0.18 and that as much as every fifth Ti atom can be replaced by Si (~10 at.%) in Ti1-xSixN(001). For the (011) and (111)-oriented films, however, only 1-2 at.% Si substitutes for Ti. Instead, Si segregates to form crystalline-to-amorphous SiNz (z ≈ 1) tissue phases, which promote the formation of epitaxial TiN nanocolumns. The nanocolumns have preferred {110} interfaces and {200} top facets and grow several hundreds of nm in length.
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| 19. |
- Eriksson, Anders, et al.
(author)
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Si incorporation in Ti1-xSixN films grown on TiN(001) and (001)-faceted TiN(111) columns
- 2014
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In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 257, s. 121-128
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Journal article (peer-reviewed)abstract
- Thin films consisting of TiN nanocrystallites encapsulated in a fully percolated SiNy tissue phase are archetypes for hard and superhard nanocomposites. Here, we investigate metastable SiNy solid solubility in TiN and probe the effects of surface segregation during the growth of TiSiN films onto substrates that are either flat TiN(001)/MgO(001) epitaxial buffer layers or TiN(001) facets of length 1-5 nm terminating epitaxial TiN(111) nanocolumns, separated by voids, deposited on epitaxial TiN(111)/MgO(111) buffer layers. Using reactive magnetron sputter deposition, the TiSiN layers were grown at 550 degrees C and the TiN buffer layers at 900 degrees C On TiN(001), the films are NaCl-structure single-phase metastable Ti1-xSixN(001) with N/(Ti + Si) = 1 and 0 less than= x less than= 0.19. These alloys remain single-crystalline to critical thicknesses h(c) ranging from 100 +/- 30 nm with x = 0.13 to 40 +/- 10 nm with x = 0.19. At thicknesses h greater than h(c), the epitaxial growth front breaks down locally to form V-shaped polycrystalline columns with an underdense feather-like nanostructure. In contrast, the voided epitaxial TiN(111) columnar surfaces, as well as the TiN(001) facets, act as sinks for SiNy. For Ti1-xSixN layers with global average composition values less than x greater than = 0.16, the local x value in the middle of Ti1-xSixN columns increases from 0.08 for columns with radius r similar or equal to 2 nm to x = 0.14 with r similar or equal to 4 nm. The average out-of-plane lattice parameter of epitaxial nanocolumns encapsulated in SiNy decreases monotonically with increasing Si fraction less than x greater than, indicating the formation of metastable (Ti,Si)N solid solutions under growth conditions similar to those of superhard nanocomposites for which the faceted surfaces of nanograins also provide sinks for SiNy.
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| 20. |
- Etman, Ahmed, et al.
(author)
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Computationally Driven Discovery of Quaternary Tantalum-Based MAB-Phases: Ta4M & DPRIME;SiB2 (M & DPRIME; = V, Cr, or Mo): Synthesis, Characterization, and Elastic Properties
- 2023
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In: Crystal Growth & Design. - : AMER CHEMICAL SOC. - 1528-7483 .- 1528-7505. ; 23:6, s. 4442-4447
-
Journal article (peer-reviewed)abstract
- Out-of-plane chemically ordered transitionmetal boride(o-MAB) phases, Ta4M & DPRIME;SiB2 (M & DPRIME; = V, Cr), and a structurally equivalent disordered solidsolution MAB phase, Ta4MoSiB2, are synthesized.DFT calculations are used to examine the dynamic stability, elasticproperties, and electronic density states of the MAB phases. We report on the synthesis of computationally predictedout-of-planechemically ordered transition metal borides labeled o-MAB phases, Ta4M & DPRIME;SiB2 (M & DPRIME; =V, Cr), and a structurally equivalent disordered solid solution MABphase Ta4MoSiB2. The boride phases were preparedusing solid-state reaction sintering of the constituting elements.High-resolution scanning transmission electron microscopy along withRietveld refinement of the powder-X-ray diffraction patterns revealedthat the synthesized o-MAB phases Ta4CrSiB2 (98 wt % purity) and Ta4VSiB2 (81 wt% purity) possess chemical ordering with Ta preferentially residingin the 16l position and Cr and V in the 4c position, whereas Ta4MoSiB2 (46wt % purity) was concluded to form a disordered solid solution. Densityfunctional theory (DFT) calculations were used to investigate thedynamic stability, elastic properties, and electronic density statesfor the MAB phases, confirming the stability and suggesting the boridesbased on Cr and Mo to be stiffer than those based on V and Nb.
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| 21. |
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| 22. |
- Fashandi, Hossein, et al.
(author)
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Synthesis of Ti3AuC2, Ti3Au2C2 and Ti3IrC2 by noble metal substitution reaction in Ti3SiC2 for high-temperature-stable Ohmic contacts to SiC
- 2017
-
In: Nature Materials. - : NATURE PUBLISHING GROUP. - 1476-1122 .- 1476-4660. ; 16:8, s. 814-818
-
Journal article (peer-reviewed)abstract
- The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 degrees C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.
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| 23. |
- Fashandi, Hossein, et al.
(author)
-
Ti2Au2C and Ti3Au2C2 formed by solid state reaction of gold with Ti2AlC and Ti3AlC2
- 2017
-
In: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 53:69, s. 9554-9557
-
Journal article (peer-reviewed)abstract
- Incorporation of layers of noble metals in non-van der Waals layered materials may be used to form novel layered compounds. Recently, we demonstrated a high-temperature-induced exchange process of Au with Si in the layered phase Ti3SiC2, resulting in the formation of Ti3AuC2 and Ti3Au2C2. Here, we generalize this technique showing that Au/Ti2AlC and Au/Ti3AlC2 undergo an exchange reaction at 650 [degree]C to form Ti2Au2C and Ti3Au2C2 and determine their structures by electron microscopy, X-ray diffraction, and ab initio calculations. These results imply that noble-metal-containing layered phases should be possible to synthesize in many systems. The metal to be introduced should be inert to the transition-metal carbide layers, and exhibit negative heat of mixing with the initial A element in a liquid phase or two-phase liquid/solid region at the annealing temperature.
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| 24. |
- Goyenola, Cecilia, et al.
(author)
-
Theoretical prediction and synthesis of CSxFy thin films
- 2016
-
In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:17, s. 9527-9534
-
Journal article (peer-reviewed)abstract
- A new carbon-based compound: CSxFy was addressed by density functional theory calculations and synthesized by reactive magnetron sputtering. Geometry optimizations and energy calculations were performed on graphene-like model systems containing sulfur and fluorine atoms. It is shown that [S+F] concentrations in the range of 0−10 at.%, structural ordered characteristics similar to graphene pieces containing ring defects are energetically feasible. The modeling predicts that CSxFy thin films with graphite and fullerene-like characteristics may be obtained for the mentioned concentration range. Accordingly, thin films were synthesized from a graphite solid target and sulfur hexafluoride as reactive gas. In agreement with the theoretical prediction, transmission electron microscopy characterization and selected area electron diffraction confirmed the presence of small ordered clusters with graphitic features in a sample containing 0.4 at.% of S and 3.4 at.% of F.
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| 25. |
- Griseri, Matteo, et al.
(author)
-
Ta-based 413 and 211 MAX phase solid solutions with Hf and Nb
- 2020
-
In: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 40:54, s. 1829-1838
-
Journal article (peer-reviewed)abstract
- New bulk MAX phase-based ceramics were synthesized in the Ta-Hf-Al-C and Ta-Nb-Al-C systems. Specifically, (Ta1-x,Hf-x)(4)AlC3 and (Ta1-x,Nb-x)(4)AlC3 stoichiometries with x = 0.05, 0.1, 0.15, 0.2, 0.25 were targeted by reactive hot pressing of Ta2H, HfH2, NbH0.89, Al and C powder mixtures at 1550 degrees C in vacuum. The produced ceramics were characterized in terms of phase composition and microstructure by X-ray diffraction, scanning electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. The investigation confirmed the existence of such M-site solid solutions with low solute concentrations, as predicted by first-principles calculations. These calculations also predicted a linear trend in lattice parameter evolution with increasing Hf concentration, in agreement with the experimental results. In order to increase the low phase purity of the produced ceramics, Sn was added to form (Ta1-x,Hf-x)(4)(Al-0.5,Sn-0.5)C-3 and (Ta1-x,Nb-x)(4)(Al-0.5,Sn-0.5)C-3 double solid solutions, thus resulting in a higher content of the 413 MAX phase compounds in the produced ceramics.
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| 26. |
- Halim, Joseph, 1985-
(author)
-
Synthesis and Characterization of 2D Nanocrystals and Thin Films of Transition Metal Carbides (MXenes)
- 2014
-
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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| 27. |
- Halim, Joseph, 1985-
(author)
-
Synthesis and transport properties of 2D transition metal carbides (MXenes)
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Since the isolation and characterization of graphene, there has been a growing interest in 2D materials owing to their unique properties compared to their 3D counterparts. Recently, a family of 2D materials of early transition metal carbides and nitrides, labelled MXenes, has been discovered (Ti2CTz, Ti3C2Tz, Mo2TiC2Tz, Ti3CNTz, Ta4C3Tz, Ti4N3Tz among many others), where T stands for surface-terminating groups (O, OH, and F). MXenes are mostly produced by selectively etching A layers (where A stands for group A elements, mostly groups 13 and 14) from the MAX phases. The latter are a family of layered ternary carbides and/or nitrides and have a general formula of Mn+1AXn (n = 1-3), where M is a transition metal and X is carbon and/or nitrogen. The produced MXenes have a conductive carbide core and a non-conductive O-, OH- and/or F-terminated surface, which allows them to work as electrodes for energy storage applications, such as Li-ion batteries and supercapacitors.Prior to this work, MXenes were produced in the form of flakes of lateral dimension of about 1 to 2 microns; such dimensions and form are not suitable for electronic characterization and applications. I have synthesized various MXenes (Ti3C2Tz, Ti2CTz and Nb2CTz) as epitaxial thin films, a more suitable form for electronic and photonic applications. These films were produced by HF, NH4HF2 or LiF + HCl etching of magnetron sputtered epitaxial Ti3AlC2, Ti2AlC, and Nb2AlC thin films. For transport properties of the Ti-based MXenes, Ti2CTz and Ti3C2Tz, changing n from 1 to 2 resulted in an increase in conductivity but had no effect on the transport mechanism (i.e. both Ti3C2Tx and Ti2CTx were metallic). In order to examine whether the electronic properties of MXenes differ when going from a few layers to a single flake, similar to graphene, the electrical characterization of a single Ti3C2Tz flake with a lateral size of about 10 μm was performed. These measurements, the first for MXene, demonstrated its metallic nature, along with determining the nature of the charge carriers and their mobility. This indicates that Ti3C2Tz is inherently of 2D nature independent of the number of stacked layers, unlike graphene, where the electronic properties change based on the number of stacked layers.Changing the transition metal from Ti to Nb, viz. comparing Ti2CTz and Nb2CTz thin films, the electronic properties and electronic conduction mechanism differ. Ti2CTz showed metallic-like behavior (resistivity increases with increasing temperature) unlike Nb2CTz where the conduction occurs via variable range hopping mechanism (VRH) - where resistivity decreases with increasing temperature.Furthermore, these studies show the synthesis of pure Mo2CTz in the form of single flakes and freestanding films made by filtering Mo2CTz colloidal suspensions. Electronic characterization of free-standing films made from delaminated Mo2CTz flakes was investigated, showing that a VRH mechanism prevails at low temperatures (7 to ≈ 60 K). Upon vacuum annealing, the room temperature, RT, conductivity of Mo2CTx increased by two orders of magnitude. The conduction mechanism was concluded to be VRH most likely dominated by hopping within each flake.Other Mo-based MXenes, Mo2TiC2Tz and Mo2Ti2C3Tz, showed VRH mechanism at low temperature. However, at higher temperatures up to RT, the transport mechanism was not clearly understood. Therefore, a part of this thesis was dedicated to further investigating the transport properties of Mo-based MXenes. This includes Mo2CTz, out-of-plane ordered Mo2TiC2Tz and Mo2Ti2C3Tz, and vacancy ordered Mo1.33CTz. Magneto-transport of free-standing thin films of the Mo-based MXenes were studied, showing that all Mo-based MXenes have two transport regimes: a VRH mechanism at lower temperatures and a thermally activated process at higher temperatures. All Mo-based MXenes except Mo1.33CTz show that the electrical transport is dominated by inter-flake transfer. As for Mo1.33CTz, the primary electrical transport mechanism is more likely to be intra-flake.The synthesis of vacancy ordered MXenes (Mo1.33CTz and W1.33CTz) raised the question of possible introduction of vacancies in all MXenes. Vacancy ordered MXenes are produced by selective etching of Al and (Sc or Y) atoms from the parent 3D MAX phases, such as (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering of Mo and Sc. However, not all quaternary parent MAX phases form the in-plane chemical ordering of the two M metals; thus the synthesis of the vacancy-ordered MXenes is restricted to a very limited number of MAX phases. I present a new method to obtain MXene flakes with disordered vacancies that may be generalized to all quaternary MAX phases. As proof of concept, I chose Nb-C MXene, as this 2D material has shown promise in several applications, including energy storage, photothermal cell ablation and photocatalysts for hydrogen evolution. Starting from synthetizing (Nb2/3Sc1/3)2AlC quaternary solid solution and etching both the Sc and Al atoms resulted in Nb1.33C material with a large number of vacancies and vacancy clusters. This method may be applicable to other quaternary or higher MAX phases wherein one of the transition metals is more reactive than the other, and it could be of vital importance in applications such as catalysis and energy storage.
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| 28. |
- Halim, Joseph, 1985-, et al.
(author)
-
Synthesis of Two-Dimensional Nb1.33C (MXene) with Randomly Distributed Vacancies by Etching of the Quaternary Solid Solution (Nb2/3Sc1/3)2AlC MAX Phase
- 2018
-
In: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 1:6, s. 2455-2460
-
Journal article (peer-reviewed)abstract
- Introducing point defects in two-dimensional (2D) materials can alter or enhance their properties. Here, we demonstrate how etching a laminated (Nb2/3Sc1/3)2AlC MAX phase (solid solution) of both the Sc and Al atoms results in a 2D Nb1.33C material (MXene) with a large number of vacancies and vacancy clusters. This method is applicable to any quaternary, or higher, MAX phase, wherein one of the transition metals is more reactive than the other and could be of vital importance in applications such as catalysis and energy storage. We also report, for the first time, on the existence of solid solution (Nb2/3Sc1/3)3AlC2 and (Nb2/3Sc1/3)4AlC3 phases.
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| 29. |
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| 30. |
- Hu, C., et al.
(author)
-
Mo2Ga2C: a new ternary nanolaminated carbide
- 2015
-
In: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 51:30, s. 6560-6563
-
Journal article (peer-reviewed)abstract
- We report the discovery of a new hexagonal Mo2Ga2C phase, wherein two Ga layers - instead of one - are stacked in a simple hexagonal arrangement in between Mo2C layers. It is reasonable to assume this compound is the first of a larger family.
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| 31. |
- Kashiwaya, Shun, et al.
(author)
-
Formation of Ti2AuN from Au-Covered Ti2AlN Thin Films: A General Strategy to Thermally Induce Intercalation of Noble Metals into MAX Phases
- 2020
-
In: Crystal Growth & Design. - : AMER CHEMICAL SOC. - 1528-7483 .- 1528-7505. ; 20:6, s. 4077-4081
-
Journal article (peer-reviewed)abstract
- Thermally induced intercalation of noble metals into non-van der Waals ceramic compounds presents a method to produce a new class of layered materials. We recently demonstrated an exchange reaction of Au with A layers of MAX phase carbides with plentiful combinations of A and M elements. Here, we report the first substitution of Al with Au in a Ti2AlN MAX phase nitride at an elevated temperature without destroying the original layered structure. These results bolster the generalization of the Au intercalation for the A elements in MAX phases with diverse combinations of M, A, and X elements. Furthermore, we propose crucial factors to achieve the exchange reaction: there should be a chemical potential for the A element to dissolve in or react with noble metals to intercalate; the noble metals should be inert to the initial metal carbides/nitrides; and it is necessary to choose the reaction temperature that allows balanced interdiffusion of the noble metals and A elements.
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| 32. |
- Kashiwaya, Shun, et al.
(author)
-
Synthesis of goldene comprising single-atom layer gold
- 2024
-
In: Nature Synthesis. - : Nature Publishing Group. - 2731-0582.
-
Journal article (peer-reviewed)abstract
- The synthesis of monolayer gold has so far been limited to free-standingseveral-atoms-thick layers, or monolayers confned on or inside templates.Here we report the exfoliation of single-atom-thick gold achieved throughwet-chemically etching away Ti3C2 from nanolaminated Ti3AuC2, initiallyformed by substituting Si in Ti3SiC2 with Au. Ti3SiC2 is a renown MAX phase,where M is a transition metal, A is a group A element, and X is C or N. Ourdeveloped synthetic route is by a facile, scalable and hydrofuoric acid-freemethod. The two-dimensional layers are termed goldene. Goldene layerswith roughly 9% lattice contraction compared to bulk gold are observedby electron microscopy. While ab initio molecular dynamics simulationsshow that two-dimensional goldene is inherently stable, experiments showsome curling and agglomeration, which can be mitigated by surfactants.X-ray photoelectron spectroscopy reveals an Au 4f binding energy increaseof 0.88 eV. Prospects for preparing goldene from other non-van der WaalsAu-intercalated phases, including developing etching schemes,are presented.
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| 33. |
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| 34. |
- Lai, Chung-Chuan, et al.
(author)
-
Magnetic properties and structural characterization of layered (Cr0.5Mn0.5)(2)AuC synthesized by thermally induced substitutional reaction in (Cr0.5Mn0.5)(2)GaC
- 2018
-
In: APL Materials. - : AMER INST PHYSICS. - 2166-532X. ; 6:2
-
Journal article (peer-reviewed)abstract
- The magnetic properties of the new phase (Cr0.5Mn0.5)(2)AuC are compared to the known MAX-phase (Cr0.5Mn0.5)(2)GaC, where the former was synthesized by thermally induced substitution reaction of Au for Ga in (Cr0.5Mn0.5)(2)GaC. The reaction introduced a lattice expansion of similar to 3% along the c-axis, an enhancement of the coercive field from 30 mT to 140 mT, and a reduction of the Curie temperature and the saturation magnetization. Still, (Cr0.5Mn0.5)(2)AuC displays similar features in the magnetic field-and temperature-dependent magnetization curves as previously reported magnetic MAX phases, e.g., (Cr0.5Mn0.5)(2)GaC and (Mo0.5Mn0.5)(2)GaC. Thework suggests a pathway for tuning the magnetic properties of MAX phases. (c) 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.
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| 35. |
- Lai, Chung-Chuan, et al.
(author)
-
Phase formation of nanolaminated Mo2AuC and Mo-2(Au1-xGax)(2)C by a substitutional reaction within Au-capped Mo2GaC and Mo2Ga2C thin films
- 2017
-
In: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 9:45, s. 17681-17687
-
Journal article (peer-reviewed)abstract
- Au-containing nanolaminated carbides Mo2AuC and Mo-2(Au1-xGax)(2)C were synthesized by a thermally induced substitutional reaction in Mo2GaC and Mo2Ga2C, respectively. The Au substitution of the Ga layers in the structures was observed using cross-sectional high-resolution scanning transmission electron microscopy. Expansion of c lattice parameters was also observed in the Au-containing phases compared to the original phases. Energy dispersive spectroscopy detected residual Ga in Au-substituted layers of both phases with a peculiar Ga in-plane ordering for Au : Ga = 9 : 1 ratio along the Au-Ga layers in Mo-2(Au1-xGax)(2)C. These results indicate a generalization of the Au substitution reaction for the A elements in MAX phases.
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| 36. |
- Lai, Chung-Chuan, et al.
(author)
-
Structural and chemical determination of the new nanolaminated carbide Mo2Ga2C from first principles and materials analysis
- 2015
-
In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 99, s. 157-164
-
Journal article (peer-reviewed)abstract
- Following our recent discovery of a new nanolaminated carbide, Mo2Ga2C, we herein present a detailed structural and chemical analysis of this phase based on ab initio calculations, X-ray photoelectron spectroscopy, high resolution scanning transmission electron microscopy, and neutron powder diffraction. Calculations suggest an energetically and dynamically stable structure for C in the octahedral sites between the Mo layers, with Ga bilayers - stacked in a simple hexagonal arrangement - between the Mo2C layers. The predicted elastic properties are below those of the related nanolaminate Mo2GaC. The predicted structure, including lattice parameters and atomic positions, is experimentally confirmed. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 37. |
- Lai, Chung-Chuan, et al.
(author)
-
Synthesis and characterization of Zr2Al3C4 thin films
- 2015
-
In: Thin Solid Films. - : ELSEVIER SCIENCE SA. - 0040-6090 .- 1879-2731. ; 595, s. 142-147
-
Journal article (peer-reviewed)abstract
- Zr2Al3C4 is an inherently nanolaminated carbide where layers of ZrC alternate with layers of Al3C2. Characterization of bulk samples has shown it has improved damage tolerance and oxidation resistance compared to its binary counterpart ZrC. Though a potential candidate for coatings applied for use in harsh environments, thin films of Zr2Al3C4 have not been reported. We have synthesized epitaxial Zr2Al3C4 thin films by pulsed cathodic arc deposition from three elemental cathodes, and have studied the effect of incident atomic flux ratio, deposition temperature, and choice of substrate on material quality. X-ray diffraction analysis showed that Zr2Al3C4 of the highest structural quality was obtained for growth on 4 H-SiC(001) substrate at 800 degrees C. Also, suppression of competing phases could be achieved on alpha-Al2O3(001) at elevated substrate temperatures. Very similar growth behavior to that of the well-known M(n+1)AX(n) phases - Al supersaturation, binary carbide intergrowth and high sensitivity to choice of substrate - indicates a strong connection between the two families of materials, despite their differences in structure and in chemistry. (C) 2015 Elsevier B.V. All rights reserved.
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| 38. |
- Lai, Chung-Chuan, et al.
(author)
-
Thermally induced substitutional reaction of Fe into Mo2GaC thin films
- 2017
-
In: Materials Research Letters. - : TAYLOR & FRANCIS INC. - 2166-3831. ; 5:8, s. 533-539
-
Journal article (peer-reviewed)abstract
- The first Fe-based MAX phase is realized by solid-state substitution reaction of an Fe/Au/Mo2GaC thin-film diffusion couple, as determined by X-ray diffraction and scanning transmission electron microscopy. Chemical analysis together with elemental mapping reveals that as much as 50 at.% Fe on the A site can be obtained by thermally induced Au and Fe substitution for Ga atomic layers in Mo2GaC. One-sixth of the original Ga is also replaced by Au atoms. When annealing Mo2GaC thin films covered with Fe only, the Mo2GaC phase remains intact, that is, Au acts as a catalyst for the substitution reaction. [GRAPHICS] .
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| 39. |
- Lapauw, T., et al.
(author)
-
Synthesis of the new MAX phase Zr2AlC
- 2016
-
In: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 36:8, s. 1847-1853
-
Journal article (peer-reviewed)abstract
- This study reports on the first experimental evidence of the existence of the Zr2AlC MAX phase, synthesised by means of reactive hot pressing of a ZrH2, Al and C powder mixture. The crystal structure of this compound was investigated by X-ray and neutron diffraction. The lattice parameters were determined and confirmed by high-resolution transmission electron microscopy. The effect of varying the synthesis temperature was investigated, indicating a relatively narrow temperature window for the synthesis of Zr2AlC. ZrC was always present as a secondary phase by hot pressing in the 1475-1575 degrees C range.
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| 40. |
- Li, Mian, et al.
(author)
-
Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes
- 2019
-
In: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 141:11, s. 4730-4737
-
Journal article (peer-reviewed)abstract
- Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesizing a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition-metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between the Zn element from molten ZnCl2 and the Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl-terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition-metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to preparing MXenes through an HF-free chemical approach.
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| 41. |
- Lu, Jun, et al.
(author)
-
Theoretical and Experimental Exploration of a Novel In-Plane Chemically Ordered (Cr2/3M1/3)(2)AIC i-MAX Phase with M = Sc and Y
- 2017
-
In: Crystal Growth & Design. - : AMER CHEMICAL SOC. - 1528-7483 .- 1528-7505. ; 17:11, s. 5704-5711
-
Journal article (peer-reviewed)abstract
- We have uncovered two inherently laminated transition metal carbides, (Cr2/3Sc1/3)(2)A1C and (Cr2/3Y1/3)(2)A1C, which display in-plane chemical order in the carbide sheet and a Kagome pattern in the Al layer. The phases belong to the most recently discovered family of so-called i-MAX phases. The materials were synthesized and the crystal structures were evaluated by means of analytical high resolution scanning transmission electron microscopy, selected area electron diffraction, and X-ray diffraction Rietveld refinement. An orthorhombic structure of space group Cmcm (#63) and a monoclinic structure of space group C2/c (#15) are solved. The compounds were investigated by first-principles calculations based on density functional theory, suggesting close to degenerate anti-ferro- and ferromagnetic spin states, dynamical and mechanical stability, and a Voigt bulk modulus in the range 134-152 GPa.
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| 42. |
- Lu, Jun, 1962-, et al.
(author)
-
Tin+1Cn MXenes with fully saturated and thermally stable Cl terminations
- 2019
-
In: Nanoscale Advances. - : Royal Society of Chemistry. - 2516-0230. ; 1:9, s. 3680-3685
-
Journal article (peer-reviewed)abstract
- MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the materials properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 degrees C. Thus, we present an attractive new termination that widely expands the MXenes functionalization space and enables new applications.
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| 43. |
- Lu, Jun, et al.
(author)
-
Transmorphic epitaxial growth of AlN nucleation layers on SiC substrates for high-breakdown thin GaN transistors
- 2019
-
In: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 115:22
-
Journal article (peer-reviewed)abstract
- Interfaces containing misfit dislocations deteriorate electronic properties of heteroepitaxial wide bandgap III-nitride semiconductors grown on foreign substrates, as a result of lattice and thermal expansion mismatches and incompatible chemical bonding. We report grain-boundary-free AlN nucleation layers (NLs) grown by metalorganic chemical vapor deposition on SiC (0001) substrates mediated by an interface extending over two atomic layers L1 and L2 with composition (Al1/3Si2/3)(2/3)N and (Al2/3Si1/3)N, respectively. It is remarkable that the interfaces have ordered vacancies on one-third of the Al/Si position in L1, as shown here by analytical scanning transmission electron microscopy and ab initio calculations. This unique interface is coined the out-of-plane compositional-gradient with in-plane vacancy-ordering and can perfectly transform the in-plane lattice atomic configuration from the SiC substrate to the AlN NL within 1 nm thick transition. This transmorphic epitaxial scheme enables a critical breakdown field of similar to 2 MV/cm achieved in thin GaN-based transistor heterostructures grown on top. Lateral breakdown voltages of 900 V and 1800 V are demonstrated at contact distances of 5 and 20 mu m, respectively, and the vertical breakdown voltage is amp;gt;= 3 kV. These results suggest that the transmorphic epitaxially grown AlN layer on SiC may become the next paradigm for GaN power electronics. (C) 2019 Author(s).
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| 44. |
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| 45. |
- Meshkian, Rahele, et al.
(author)
-
A magnetic atomic laminate from thin film synthesis: (Mo0.5Mn0.5)2GaC
- 2015
-
In: APL Materials. - : American Institute of Physics (AIP): Open Access Journals / AIP Publishing LLC. - 2166-532X. ; 3:7
-
Journal article (peer-reviewed)abstract
- We present synthesis and characterization of a new magnetic atomic laminate: (Mo0.5Mn0.5)(2)GaC. High quality crystalline films were synthesized on MgO(111) substrates at a temperature of similar to 530 degrees C. The films display a magnetic response, evaluated by vibrating sample magnetometry, in a temperature range 3-300 K and in a field up to 5 T. The response ranges from ferromagnetic to paramagnetic with change in temperature, with an acquired 5T-moment and remanent moment at 3 K of 0.66 and 0.35 mu(B) per metal atom (Mo and Mn), respectively. The remanent moment and the coercive field (0.06 T) exceed all values reported to date for the family of magnetic laminates based on so called MAX phases.
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| 46. |
- Meshkian, Rahele, et al.
(author)
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Synthesis of two-dimensional molybdenum carbide, Mo2C, from the gallium based atomic laminate Mo2Ga2C
- 2015
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In: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 108, s. 147-150
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Journal article (peer-reviewed)abstract
- We report on the synthesis of a two-dimensional transition metal carbide, Mo2C, (MXene) obtained by immersing Mo2Ga2C thin films in hydrofluoric acid. Experimental evidences for neither synthesis of a Mo-based MXene nor selective etching of Ga from an atomic nanolaminate have previously been presented. MXene formation is verified through X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. This discovery unlocks new potential applications for Mo-based MXenes in a host of applications, from thermoelectrics to catalysis and energy storage.
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| 47. |
- Meshkian, Rahele, et al.
(author)
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Theoretical stability and materials synthesis of a chemically ordered MAX phase, Mo2ScAlC2, and its two-dimensional derivate Mo2ScC2 MXene
- 2017
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In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 125, s. 476-480
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Journal article (peer-reviewed)abstract
- We present theoretical prediction and experimental evidence of a new MAX phase alloy, Mo2ScAlC2, with out-of-plane chemical order. Evaluation of phase stability was performed by ab initio calculations based on Density Functional Theory, suggesting that chemical order in the alloy promotes a stable phase, with a formation enthalpy of -24 meV/atom, as opposed to the predicted unstable Mo3AlC2 and Sc3AlC2. Bulk synthesis of Mo2ScAlC2 is achieved by mixing elemental powders of Mo, Sc, Al and graphite which are heated to 1700 degrees C. High resolution transmission electron microscopy reveals a chemically ordered structure consistent with theoretical predictions with one Sc layer sandwiched between two Mo-C layers. The two-dimensional derivative, the MXene, is produced by selective etching of the Al-layers in hydrofluoric acid, resulting in the corresponding chemically ordered Mo2ScC2, i.e. the first Sc-containing MXene. The here presented results expands the attainable range of MXene compositions and widens the prospects for property tuning. (C)2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 48. |
- Meshkian, Rahele, et al.
(author)
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Theoretical stability, thin film synthesis and transport properties of the Mon+1GaCn MAX phase
- 2015
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In: Physica Status Solidi. Rapid Research Letters. - : Wiley. - 1862-6254 .- 1862-6270. ; 9:3, s. 197-201
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Journal article (peer-reviewed)abstract
- The phase stability of Mon +1GaCn has been investigated using ab-initio calculations. The results indicate stability for the Mo2GaC phase only, with a formation enthalpy of 0.4 meV per atom. Subsequent thin film synthesis of Mo2GaC was performed through magnetron sputtering from elemental targets onto Al2O3 [0001], 6H-SiC [0001] and MgO [111] substrates within the temperature range of 500 degrees C and 750 degrees C. High structural quality films were obtained for synthesis on MgO [111] substrates at 590 degrees C. Evaluation of transport properties showed a superconducting behavior with a critical temperature of approximately 7 K, reducing upon the application of an external magnetic field. The results point towards the first superconducting MAX phase in thin film form.
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| 49. |
- Meshkian, Rahele, et al.
(author)
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W-Based Atomic Laminates and Their 2D Derivative W1.33C MXene with Vacancy Ordering
- 2018
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In: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 30:21
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Journal article (peer-reviewed)abstract
- Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W 2/3 M 2 1/3 ) 2 AC, where M 2 = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two—with a monoclinic C2/c structure—are predicted to be stable: (W 2/3 Sc 1/3 ) 2 AlC and (W 2/3 Y 1/3 ) 2 AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W 1.33 C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.
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| 50. |
- Mian, Li, et al.
(author)
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Synthesis of Novel MAX Phase Ti3ZnC2 via A-site-element-substitution Approach
- 2019
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In: Journal of Inorganic Materials. - : SCIENCE PRESS. - 1000-324X. ; 34:1, s. 60-64
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Journal article (peer-reviewed)abstract
- Using Ti3AlC2 as the precursor, a new MAX phase Ti3ZnC2 was synthesized via an A-elemental substitution reaction in a molten salts bath. Composition and crystal structure of Ti3ZnC2 were confirmed by XRD, SEM and TEM analysis. Its structure stability and lattice parameter of Ti3ZnC2 were further proved by a theoretical calculation based on density function theory (DFT). Moreover, thermodynamics of A-elemental substitution reactions based on Fe, Co, Ni, and Cu were investigated. All results indicated that the similar substitution reactions are feasible to form series of MAX phases whose A sites are Fe, Co, Ni, and Cu elements. The substitution reaction was achieved by diffusion of Zn atoms into A-layers of Ti3AlC2, which requires Al-Zn eutectic formation at high temperatures. The molten salts provided a moderate environment for substitution reaction and accelerated reaction dynamics. The major advantage of this substitution reaction is that MAX phase keeps individual metal carbide layers intact, thus the formation of competitive phases, such as MA alloys, was avoided. The proposed A-elemental substitution reactions approach opens a new door to design and synthesize novel MAX phases which could not be synthesized by the traditional methods.
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