A-group Element Research Articles

Development of Ti3SiC2 MAX phase tubular structures for solar receiver applications

Solar receiver tubes are key components of concentrating solar-thermal power (CSP) systems that harvest solar energy. For better efficiency, the Gen3 CSP receivers, which collect heat into a heat transfer fluid, require a temperature exceeding 700 °C during operation and need to perform under extreme conditions of high temperature and high thermal stress. Operators are seeking CSP designs using new high-temperature structural materials with high thermal conductivity and high creep resistance to achieve a design life of 30 years and thus help recover the plant capital cost sooner. MAX phase materials, which consist of an early transition metal element, an A-group element, and carbon or nitrogen, are expected to exhibit high creep resistance as well as high fracture toughness. In this paper, we describe fabricating both (1) dense Ti3SiC2 MAX phase disks and (2) short-length tubes using field-assisted sintering technology (FAST). First, the disk samples that we fabricated are fully dense and contain ≈90 % Ti3SiC2 MAX phase materials and ≈10 % TiC phase materials. We determined a flexure strength of 519 ± 32 MPa by conducting a four-point bending test at room temperature with rectangular bar samples of ≈100 % density. The thermal conductivity of the Ti3SiC2 MAX phase samples, measured by light-flashing analysis, decreases linearly from a value of 41 W·m−1·K−1 at room temperature to a value of 36 W·m−1·K−1 at 650 °C. A solar reflectance measurement of the Ti3SiC2 MAX phase revealed that, temperature increases from 400 to 1400 °C, thermal emittance increases from 0.39 to 0.49, while selectivity decreases from 1.8 to 1.4, respectively. Whereas the surface oxidized MAX phase samples after 100 h exposure to air at 1000 °C exhibit that of SiC.Next, we discuss fabrication of the crack-free Ti3SiC2 MAX phase tubular structures accomplished by using FAST processing in graphite bedding. A Ti3SiC2 MAX phase content of > 95 % with traceable ≈3% remaining TiC phase and ≈15 % porosity were demonstrated after high-temperature annealing. An average fracture strength of ≈250 MPa was determined with Ti3SiC2 MAX phase tubes of ≈85 % density by diametral compression testing at room temperature. Our work demonstrated that using FAST processing to produce Ti3SiC2 MAX phase tubular structures for CSP receiver applications is a viable approach.

MAX phases – Past, present, and future

The MAX phases are a class of nanolaminated materials composed of an early transition-metal (M), an A-group element (A) and C, N, B and/or P (X). Progress in MAX phase research in recent years has increased their number from the original 50 or so, to more than 300 phases. Since half of the 342 MAX phases have been discovered after 2018, an overview of the progress made in the field is timely. Currently, 28 M elements, 28 A elements, and 6 X elements have been incorporated in the MAX phases, alloys included. We further categorize MAX phases based on the synthesis route used to make them; if made via a one-step approach in bottom-up synthesis or formed through elemental replacement reactions in top-down synthesis. This classification is also correlated to theoretical phase stability predictions, that in turn, can be used to identify novel synthesizable MAX phase compositions as well as to suggest suitable synthesis routes. Furthermore, using phase stability predictions we identify 182 new theoretically stable MAX phases awaiting experimental confirmation. Notably, as MAX phases are precursors for MXenes, the dramatically increased interest in the latter for a large host of potential applications renders the former even more valuable.

Magnetic ordering and magnetocrystalline anisotropy in epitaxial Mn2GaC MAX phase thin films

${\mathrm{Mn}}_{2}\mathrm{GaC}$ is a MAX phase belonging to a family of naturally nanolaminated materials with formula ${M}_{n+1}A{X}_{n}$ ($n=1,2,3$), where $M$ is a transition metal, $A$ is an A-group element, and $X$ is carbon or nitrogen. It has a complex magnetic phase diagram, and there are many open questions regarding its magnetic properties. Here we study epitaxial films of ${\mathrm{Mn}}_{2}\mathrm{GaC}$ with two different crystal orientations on MgO(1 1 1) substrates: a (0 0 0 l) dominated orientation and a mixed (0 0 0 l) and (1 0 $\overline{1}$ 3) orientation. Magnetic measurements between 3 and 320 K are presented for in-plane and out-of-plane magnetic fields on both types of film, which show that ${\mathrm{Mn}}_{2}\mathrm{GaC}$ has a magnetocrystalline anisotropy with (0 0 0 l) as easy planes. This provides clear experimental evidence of the anisotropic properties associated with the nanolaminated structure of a MAX phase. In addition, a close look at the magnetic response at low temperature shows that the noncollinear magnetic state is unchanged below 50 K, contrary to previous results, with a magnetic moment of 0.38 ${\ensuremath{\mu}}_{B}$ per Mn atom at a temperature of 3 K and applied field of 5 T.

МАХ PHASE (MXENE) IN POLYMER MATERIALS

This article is a review of the Mn+1AXn phases (“MAX phases”, where n = 1, 2 or 3), their MXene derivatives and the reinforcement of polymers with these materials. The MAX phases are a class of hexagonal-structure ternary carbides and nitrides ("X") of the transition metal ("M") and the A-group element. The unique combination of chemical, physical, electrical and mechanical properties that combine the characteristics of metals and ceramics is of interest to researchers in the MAX phases. For example, MAX phases are typically resistant to oxidation and corrosion, elastic, but at the same time, they have high thermal and electrical conductivity and are machinable. These properties stem from an inherently nanolaminated crystal structure, with Mn+1Xn slabs intercalated with pure A-element layers. To date, more than 150 MAX phases have been synthesized. In 2011, a new family of 2D materials, called MXene, was synthesized, emphasizing the connection with the MAX phases and their dimension. Several approaches to the synthesis of MXene have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etching solutions, halogens and molten salts, which allows the synthesis of new materials with better control over the chemical composition of their surface. The use of MAX phases and MXene for polymer reinforcement increases their thermal, electrical and mechanical properties. Thus, the addition of fillers increases the glass transition temperature by an average of 10%, bending strength by 30%, compressive strength by 70%, tensile strength up to 200%, microhardness by 40%, reduces friction coefficient and makes the composite material self-lubricating, and 1 % wt. MAX phases increases thermal conductivity by 23%, Young’s modulus increases. The use of composites as components of sensors, electromagnetic protection, wearable technologies, in current sources, in aerospace and military applications, etc. are proposed.

Optical response, lithiation and charge transfer in Sn-based 211 MAX phases with electron localization function

In this study, optical response, lithiation and charge transfer in existing M2SnC MAX phases with electron localization function (ELF) were investigated for the first time using the density functional theory (DFT). Calculations show that the non-zero value of ε1(0) is an indication of the large availability of free charge carriers in these metallic systems. High reflection of light at the low frequencies indicates the high conductivity and low absorption power of the studied materials. In the visible light region, the average reflectivity of M2SnC is more than 40%, making them potential coating materials for reducing solar heating with greater possibility for Nb2SnC. M2SnC phases are optically anisotropic. The static absorption coefficient represents the universal non-zero value for the hexagonal M2SnC phases. The Plasma frequency is found to be slightly larger for ⟨001⟩ polarization. Lithium (Li) incorporation into M2SnC show that the formation energy required for Li incorporation into Lu2SnC is low, and therefore, it should be suitable for use as an anode in battery. The chemical bonds (M–X) between transition metal ions and carbon are expected to be strong localized bonds as predicted from the ELF maps. The bonds (M–Sn) between transition metal M and A-group element Sn are less localized and more spread out, possibly pointing to a weaker bond. The magnitude of the Bader charges is significantly larger compared to the Mulliken and Hirshfeld charges. According to Bader analysis, maximum charge transfer occurs in Hf2SnC and minimum charge transfer occurs in V2SnC.

Pulsed Laser Deposition of Ti-Based MAX Compounds for Next-Generation Wiring Technology

Semiconductor wiring technology with a width of 10 nm or less is going in order to realize high performance and high integration of large-scale integrated circuits. Copper is currently used as the main wiring material, however, a barrier layer of TiN is necessary due to high diffusivity of Cu into Si or SiO2. In addition, the resistivity of TiN is ten times higher than that of Cu, which results in a drastic increase with decreasing line width less than 10 nm. Therefore, the development of a barrier layer free wiring material to replace copper should be necessary.The MAX compounds (M: transition metal element, A: A-group element, X: either carbon or nitrogen) is one of the promising candidates for next generation wiring technology. The stoichiometry of MAX compounds is Mn+1AXn (n = 1~3), and exhibits the properties of both ceramics and metals, and some of the compounds have a low volume resistivity as 10 μΩcm. In this study, pulsed laser deposition of TiSiN and TiGeN were carried out and physical and electrical properties were evaluated.Targets (TiN : Ti : Ge or Si = 3 : 1 : 1) was bought from TOSHIMA Manufacturing. Al2O3 (001) and 1-μm thickness thermal oxide on Si(001) was used as substrate. Substrate temperature was either room temperature or 600℃. N2 was introduced during deposition. Repetition frequency was fixed at 8 Hz, and laser energy was changed from 25 mJ to 75 mJ. Table 1 and 2 show deposition condition for TiGeN and TiSiN, respectively. X-Ray Diffraction and X-Ray Photoelectron Spectroscopy were used to evaluate the crystallographic structure and composition ratio, respectively. Film thickness was measured by using surface profiler. Electronic properties were measured by four point probe method. Figure 1 shows XRD pattern of TiGeN film deposited on Al2O3 substrate. When TiGeN was deposited at 600℃ and B.P., peaks ascribed to Ge(111), TiN(111) and TiN(200) were observed. TiN(111) peak position 36.85° from TiGeN is 0.04° higher than that from TiN (111) (36.81°, Card No. 00-006-0642), which suggests that lattice parameter decreased from 2.440 Å to 2.437Å. On the other hand, the peak intensity of TiN(200) from TiGeN deposited at 600℃ becomes clear by introducing N2 to 2.0 Pa. TiN(200) peak positions from TiGeN at 600℃ with 0.02 Pa and 2.0 Pa were 39.30° and 39.35°, respectively, the difference from TiN(200) (42.61°, Card No. 00-006-0642) were 3.31° and 3.26°. This suggests that lattice parameter becomes larger from 2.120 Å, to 2.291 (0.02 Pa) or 2.288 Å (2.0 Pa), respectively. One possible reason could be the incorporation fo Ge into TiN lattice. Figure 1

Computer-aided design of metal borides

Chemical Structure Because of their extreme hardness and electronic conductivity, transition-metal borides find wide industrial applications. Expanding their elemental space with new elements can enhance their range of useful properties. Using density functional theory calculations, Dahlqvist et al. explored chemical ordering upon M-site alloying of M2AlB2 (where M is a transition metal, A is an A-group element, and B is boron) in various symmetries for M from groups 3 to 9. Several previously unknown stable phases were identified, and two of them with in-plane chemical order were successfully synthesized and characterized in agreement with theory. The proposed theoretical approach can serve as a powerful tool to explore the chemical space of metal borides when metal alloying is considered. J. Am. Chem. Soc. 10.1021/jacs.0c08113 (2020).

A Study on the Precursor System for Ti–Al–C MAX Phase

MAX phase materials are nano-layered, hexagonal, machinable, early transition-metal carbides, and nitrides, where M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C and/or N. These phases exhibit a unique combination of both metallic and ceramic properties. The present paper reports a study on the precursor system for preparing Ti–Al–C MAX phases. Reactive fillers incorporated carbonaceous polymer slurries were prepared in varying molar ratios of Ti:Al. The effects of the Ti:Al molar ratio and heat treatment temperature on the Ti–Al–C phase formation were investigated. Phase evolution and morphology of the precursor derived ceramics were analyzed using powder X-ray diffraction and scanning electron microscopy techniques respectively. Laminar nature of the MAX material was identified using Scanning Electron Microscopy and HR-TEM. It is found that the pressure-less, precursor route is capable of producing Ti–Al–C MAX phases. It is also found that titanium rich precursor composition favours the formation of Ti–Al–C MAX phases, whereas an aluminum rich composition favours the formation of titanium aluminides.

Theoretical Prediction and Synthesis of a Family of Atomic Laminate Metal Borides with In-Plane Chemical Ordering.

All atomically laminated MAB phases (M = transition metal, A = A-group element, and B = boron) exhibit orthorhombic or tetragonal symmetry, with the only exception being hexagonal Ti2InB2. Inspired by the recent discovery of chemically ordered hexagonal carbides, i-MAX phases, we perform an extensive first-principles study to explore chemical ordering upon metal alloying of M2AlB2 (M from groups 3 to 9) in orthorhombic and hexagonal symmetry. Fifteen stable novel phases with in-plane chemical ordering are identified, coined i-MAB, along with 16 disordered stable alloys. The predictions are verified through the powder synthesis of Mo4/3Y2/3AlB2 and Mo4/3Sc2/3AlB2 of space group R3̅m (no. 166), displaying the characteristic in-plane chemical order of Mo and Y/Sc and Kagomé ordering of the Al atoms, as evident from X-ray diffraction and electron microscopy. The discovery of i-MAB phases expands the elemental space of these borides with M = Sc, Y, Zr, Hf, and Nb, realizing an increased property tuning potential of these phases as well as their suggested potential two-dimensional derivatives.

The role of Hume-Rothery's rules play in the MAX phases formability

MAX phases are a family of layered, hexagonal-structure ternary carbides or nitrides of a transitional metal and an A-group element. What makes this type of material fascinating and potentially useful is their remarkable combinations of metallic and ceramic characteristics; as well as the indispensable role in 'top-down' synthesis of their 2D counterparts, MXenes. To enhance the efficiency in the successful search for potential novel MAX phases, the main efforts could go toward creating an information-prediction system incorporating all MAX phases’ databases, as well as generally valid principles and the high-quality regularities. In this work, we employ structure mapping methodology, which has shown its merit of being useful guides in materials design, with Hume-Rothery parameters to provide guiding principles in the search of novel MAX phases. The formable/non-formable data on MAX phases can be ordered within a two-dimensional plot by using proposed expression of geometrical and electron concentration factors.

Effect of Ar+ irradiation of Ti3InC2 at different ion beam fluences

MAX phases are a group of ternary carbides or nitrides with a nanolayered microstructure. The general formula of MAX phases is Mn+1AXn with n = 1 to 3, where M is the transition metal, A is the A-group element (from IIIA to VIA), and X is either carbon or nitrogen. These carbides and nitride have an unusual behavior that combines the characteristics of metals and ceramics in terms chemical, physical, electrical and mechanical properties. These properties can be explained by the anisotropic lamellar microstructures of the MAX phases. Here, we report a study on thin Ti3InC2 (M3AX2) films, synthetized by repeated ion beam sputtering of single (Ti, In and C) elements at the Low Energy Ion Facility (LEIF). Ion beam sputtering was performed using an Ar+ ion beam with energy of 25 keV and a current of 400 μA. The thickness of the Ti3InC2 films (measured by RBS) was determined to be approximately 65 nm. After deposition, the samples were annealed in vacuum at 120 °C for 24 h to induce interphase chemical interactions and form the Ti3InC2 composite. To evaluate the radiation hardness and effects induced by ion radiation, the as-deposited Ti3InC2 film was irradiated by the 100 keV Ar+ ion beam with two different fluences, 1∙1013 cm−2 and 1∙1015 cm−2. It was determined that the low-level fluence of Ar+ ions (1∙1013 cm−2) did not induce any considerable change in surface roughness and that the polycrystalline structure was preserved. However, at higher fluences, the formation of concentrated point defects within the lattice of nanocrystalline Ti3InC2 and a low level of amorphization were registered. The mechanical properties determined by nanoindentation indicate the potential for using irradiated Ti3InC2 thin films under harsh environmental conditions.

Screen the elastic and thermodynamic properties of MAX solid solution using DFT procedue: Case study on (Ti1-xVx)2AlC

As one of Mn+1AXn (MAX) family, where M is an early transition metal, A is an A-group element, and X is either C or N and n = 1, 2, and 3, Ti2AlC is a promising candidate material for nuclear energy industry due to its unique features such as elastically stiff, good electrical conductivity, high thermal conductivity and excellent resistance to high temperature oxidation and corrosion. However, the mechanical and thermal properties of Ti2AlC compound are expected to be improved. For the first time, the effect of V content on the structural, elastic, electronic and thermodynamic properties of the (Ti1-xVx)2AlC (x = 0, 0.25, 0.50, 0.75 and 1.00) solid solutions was examined systematically and their compression behavior under 50 GPa was studied using density functional theory (DFT). The optimized lattice constants agree well with the experimental values and decrease with the increase of V content. The (Ti1-xVx)2AlC solid solutions are compressible along both a and c directions in the pressure range of 0–50 GPa. Calculated elastic constants verified mechanical stability under 50 GPa. The constant C44, directly related to the valence electron concentration, is found to increase with increasing V content. The bulk modulus, shear modulus, Young's modulus and Vickers hardness are found to reach a significant maximum value respectively when V content is 75%, and the reason is studied. The shear anisotropy factors show that (Ti1-xVx)2AlC compounds containing Ti and V are more anisotropic than Ti2AlC, and they have a tendency towards improved ductility. The metallic behaviors of (Ti1-xVx)2AlC solid solutions are confirmed by analyzing the electronic band structures and density of states. The thermal properties imply that the (Ti0.25V0.75)2AlC is expected to be more thermally conductive due to its highest Debye temperature of 771K of the five compounds. In addition, the bulk modulus, shear modulus, Young's modulus and Debye temperature increase with the increase of pressure under 50 GPa. The results reveal that the mechanical properties of Ti2AlC are improved much better than the thermal properties by the inclusion of V. Especially, the (Ti0.25V0.75)2AlC MAX phase in (Ti1-xVx)2AlC solid solutions is probably the most suitable candidate material for mechanical and thermal applications at high temperature.

The Role of Hume-Rothery's Rules Play in the MAX Phases Formability

MAX phases are a family of layered, hexagonal-structure ternary carbides or nitrides of a transitional metal and an A-group element. What makes this type of material fascinating and potentially useful is their remarkable combinations of metallic and ceramic characteristics; as well as the indispensable role in 'top-down' synthesis of their 2D counterparts, MXenes. To enhance the efficiency in the successful search for potential novel MAX phases, the main efforts could go toward creating an information-prediction system incorporating all MAX phases' databases, as well as generally valid principles and the high-quality regularities. In this work, we employ structure mapping methodology, which has shown its merit of being useful guides in materials design, with Hume-Rothery parameters to provide guiding principles in the search of novel MAX phases. The formable/non-formable data on MAX phases can be ordered within a two-dimensional plot by using proposed expression of geometrical and electron concentration factors.

Surface Erosion of Plasma-Facing Materials Using an Electrothermal Plasma Source and Ion Beam Micro-Trenches

Erosion characteristics of tungsten-alternative plasma-facing materials (PFMs) were tested under high heat flux conditions in the electrothermal plasma source facility at Oak Ridge National Laboratory. The PFMs of interest are high-purity β-3C chemical vapor deposition silicon carbide (SiC) and the MAX phases Ti3SiC2 and Ti2AlC [MAX = chemical formula Mn+1AXn, where M is an early transition metal (such as Ti or Ta), A is an A-group element (such as Si or Al), and X is carbon or nitrogen]. An erosion analysis method was developed using a combination of focused ion beam microscopy and scanning electron microscopy, carving micro-trench geometries into polished sample surfaces. Samples of SiC, Ti3SiC2, and Ti2AlC were exposed to the electrothermal plasma source alongside tungsten and monocrystalline silicon. Samples were exposed to a Lexan polycarbonate (C16H14O3) electrothermal plasma stream in a He environment, at a specified impact angle, with infrared camera diagnostics. Edge localized mode–relevant heat fluxes of 0.9 to 1 GW/m2 over 1-ms discharges were generated on the target surfaces. Tungsten samples exhibited pronounced melt-layer formation and deformation, with measured molten pits 2 to 10 μm in diameter and melt-layer depths of up to 7 μm deep. Surface erosion rates for Ti3SiC2 and Ti2AlC ranged from 80 to 775 μm/s and 85 to 470 μm/s, respectively. Both MAX phases exhibited extreme surface fracture and material ejection, with damage depths past 4 μm for Ti2AlC and 11 μm for Ti3SiC2. SiC displayed the best performance, in one case surviving 15 consecutive electrothermal plasma exposures with an average erosion rate of about 29 μm/s and no surface fracturing. SiC erosion rates ranged from 23 to 128 μm/s.

On the interactions of Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC with palladium at 900 °C

Herein we report on the reactivity between palladium, Pd, and the MAX phases, Ti2AlC, Ti3AlC2, Ti3SiC2 and Cr2AlC. Diffusion couples of Pd/MAX were heated to 900 °C under uniaxial stress of ∼20 MPa for 2, 4, and 10 h in a vacuum (<1 Pa) hot press. The diffusion couples were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. After heating to 900 °C for 10 h, the diffusion layer thicknesses in the Ti2AlC/Pd, Cr2AlC/Pd, Ti3AlC2/Pd and Ti3SiC2/Pd couples were found to be 35, 45, 105 and 410 μm, respectively. Thus, Ti2AlC is the most resistant to reaction, while Ti3SiC2 is least resistant, with Cr2AlC and Ti3AlC2, in between. In all cases, the reaction occurred by the diffusion of the A-group element into Pd, concomitant with Pd diffusion into the MAX phase. No diffusion of the M and X atoms was detected.

(Invited) Lithium Ions in Layered 2D Transition Metal Carbides: MAX Phases and Mxenes

Recently, a family of layered ternary transition-metal carbides and/or nitrides, known as the MAX phases, received increasing attention as precursors for two-dimensional (2D) transition-metal carbides and/or nitrides known as MXenes. The MAX phases have the general formula Mn+1AXn, where n= 1 to 3 and M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen.1 - 2 By selectively etching the A-element layers from MAX phases, MXenes have been synthesized. Given that the MXene surfaces are terminated with OH, and F groups resulting from the etching process, it is more accurate to refer to them as Mn+1XnTx, where T represents the surface terminating groups, such as O, F and OH. There are more than 70 compounds known in the MAX family to date (plus dozens of solid solutions),3 most of which offer good electronic conductivity, lamellar structure, and temperature and environmental stability. As-prepared MXenes exist in a multilayered structure, which appears as stacked multilayer flakes. The multilayered structures can be delaminated into single-layer flakes by intercalation and/or sonication.4 - 5 Due to their high specific surface areas, metallic conductivity, and hydrophilic surfaces, MXenes have shown promising performance as electrodes for supercapacitors. MXenes are also promising anode materials for LIBs due to their excellent conductivity and cation intercalation capability.6 - 7 Nb4C3 MXene shows stable capacity, also at high rates (Figure 1). A capacity of 410 mAh g−1 for a Ti3C2 MXene “paper” anode at 1C rate without adding any binder has been reported. This Ti3C2 MXene “paper” was fabricated by intercalating Ti3C2 with dimethyl sulfoxide, bath sonication for 6 h, and filtering the Ti3C2 colloidal solution.8 - 9 The experimental procedure is very simple. MXene-based LIB anodes exhibited Li-ion capacity up to 800 mAh g−1, good rate performance, and excellent cycling stability.9 - 10 These results suggest a potential for MXene-supported hybrid electrodes for energy storage applications. By decreasing the particle size, we successfully demonstrated that not only MXenes but also their precursors - MAX phases, such as Ti2SC and Ti3SiC2, exhibit Li-ion storage capacity and show promise as LIB anode materials. Capacities of 180 and 150 mAh g−1 were achieved by submicrometer size Ti2SC and Ti3SiC2, respectively, accompanied by good cycle life and excellent rate performance.11 It opens the door to exploring a large family of potential anode materials. 1. Wang, X., et al. Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors. Nature communications 2015, 6. 2. Mashtalir, O.; Lukatskaya, M. R.; Zhao, M. Q.; Barsoum, M. W.; Gogotsi, Y., Amine-Assisted Delamination of Nb2C MXene for Li-Ion Energy Storage Devices. Advanced Materials 2015, 27 (23), 3501-3506. 3. Hu, C.; Zhang, H.; Li, F.; Huang, Q.; Bao, Y., New phases’ discovery in MAX family. International Journal of Refractory Metals & Hard Materials 2013, 36, 300-312. 4. Kim, S. J.; Naguib, M.; Zhao, M.; Zhang, C.; Jung, H. T.; Barsoum, M. W.; Gogotsi, Y., High mass loading, binder-free MXene anodes for high areal capacity Li-ion batteries. Electrochimica Acta 2015, 163, 246-251. 5. Ling, Z.; Ren, C. E.; Zhao, M. Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y., Flexible and conductive MXene films and nanocomposites with high capacitance. Proceedings of the National Academy of Sciences 2014, 111 (47), 16676-81. 6. Naguib, M.; Come, J.; Dyatkin, B.; Presser, V.; Taberna, P. L.; Simon, P.; Barsoum, M. W.; Gogotsi, Y., MXene: a promising transition metal carbide anode for lithium-ion batteries. Electrochemistry Communications 2012, 16 (1), 61-64. 7. Sun, D.; Wang, M.; Li, Z.; Fan, G.; Fan, L. Z.; Zhou, A., Two-dimensional Ti3C2 as anode material for Li-ion batteries. Electrochemistry Communications 2014, 47 (10), 80-83. 8. Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall’Agnese, Y.; Min, H.; Barsoum, M. W.; Gogotsi, Y., Intercalation and delamination of layered carbides and carbonitrides. Nature communications 2013, 4 (2), 216-219. 9. Luo, J.; Tao, X.; Zhang, J.; Xia, Y.; Huang, H.; Zhang, L.; Gan, Y.; Liang, C.; Zhang, W., Sn4+ Ions Decorated Highly Conductive Ti3C2 MXene: Promising Lithium-Ion Anodes with Enhanced Volumetric Capacity and Cyclic Performance. ACS Nano 2016, 10 (2). 10. Ren, C. E.; Zhao, M. Q.; Makaryan, T.; Halim, J.; Boota, M.; Kota, S.; Anasori, B.; Barsoum, M. W.; Gogotsi, Y., Porous Two-Dimensional Transition Metal Carbide (MXene) Flakes for High-Performance Li-Ion Storage. ChemElectroChem 2016, 3 (5), 689-693. 11. J. Xu, M.-Q. Zhao, Y. Wang, W. Yao, C. Chen, B. Anasori, A. Sarycheva, C. E. Ren, T. Mathis, L. Gomes, Z. Liang, Y. Gogotsi, Demonstration of Li-ion capacity of MAX phases, ACS Energy Letters, 2016, 1, 1094−1099. Figure 1

Anodized Ti3SiC2 As a Potential Anode Material for Li-Ion Microbatteries

To date, Li-ion microbatteries (µLIBs) are the most in demand storage systems for many microelectronic devices due to their high energy densities and cycling performances1. To enhance their performances beyond the current state of the art, there is an ongoing search to find alternative electrode materials. Recently, the synthesis and electrochemical performance of a new anode material based on MXene2-4, which are fabricated by chemical etching of A element from MAX phases5 (Mn+1AXn: where n = 1, 2, or 3; M is an early transition metal; A is an A-group element; and the X is carbon and/or nitrogen), has been reported. In this work, a fully dense Ti3SiC2 was electrochemically anodized in fluoride containing aqueous electrolyte to produce a thin oxide film based on titania, some silica and carbon. The anodization takes place in a two electrode electrochemical cell in the presence of aqueous electrolyte, 1 M NaOH, 1 M H3PO4, containing 0.1 v/v HF at an applied potential of 10 and 60 V for 3 h. Fig 1. a and b, shows the SEM image of Ti3SiC2 anodized (A-312)at potential of 10 and 60 V, respectively. The morphology and thickness of the porous film formed by the anodization is dependent on the competition between electrochemical oxide formation and chemical dissolution which is strongly affected by the applied potential. At lower applied potential the oxide layer adopt a layered morphology which might due to the selective etching of one of its elements. When the anodizing potential is 60 V, the oxide formation is a dominant process leading to formation of pores. Fig. 1 SEM images of anodized Ti3SiC2 for 3 h at potential of 10 (a) and 60 V (b). Fig. 2a shows the cyclic voltammgram curves obtained for pristine and Ti3SiC2 anodized at 10 and 60 V, recorded at a scan rate of 1 mV.s-1 between 1 – 3 V vs Li/Li+. Unlike the pristine sample, the A-312 exhibits the presence of single cathodic and anodic peaks at 1.67 and 2.17 V vs Li/Li+ corresponding to the insertion and extraction of Li+ in/from anatase titania6. The cycle life performance tests were performed in two electrode Swagelok cell. The half-cells were assembled by using A-312 as a working electrode, Li foil as a counter electrode, and a Whatman glass microfiber soaked in 1 M LiPF6 in EC:DEC electrolyte as separator. The cell was cycled at 200, 300, 500 A.cm-2 in a potential window of 1 – 3 V vs Li/Li+. It delivers a specific capacity of 460, 350, 380 and mAh.cm-2 after 1st, 2nd, and 140th cycles, respectively, Fig2b. The cycle life performance shows a good capacity recovery after being cycled at faster rates. Furthermore, the capacity values are much higher than that reported for amorphous titania nanotubes7, and comparable to that for self-supported titania nanotubes fabricated by Wei and co-workers8. Fig. 2 (a) CVs of pristine material and Ti3SiC2 anodized at 10 and 60 V. (b) Cycling performance of Ti3SiC2 anodized at 60 V at various current density. Reference 1. Ellis, B. L.; Knauth, P.; Djenizian, T., Three-Dimensional Self-Supported Metal Oxides for Advanced Energy Storage. Adv. Mater. 2014, 26 (21), 3368-3397. 2. Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W., Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23 (37), 4248-4253. 3. Come, J.; Naguib, M.; Rozier, P.; Barsoum, M.; Gogotsi, Y.; Taberna, P.-L.; Morcrette, M.; Simon, P., A Non-Aqueous Asymmetric Cell with a Ti2C-Based Two-Dimensional Negative Electrode. J. Electrochem. Soc. 2012, 159 (8), A1368-A1373. 4. Naguib, M.; Halim, J.; Lu, J.; Cook, K. M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W., New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-ion Batteries. J. Am. Chem. Soc. 2013, 135 (43), 15966-15969. 5. Barsoum, M. W.; El-Raghy, T., The MAX Phases: Unique New Carbide and Nitride Materials-Ternary Ceramics Turn out to Be Surprisingly Soft and Machinable, yet Also Heat-Tolerant, Strong and Lightweight. Am. Sci. 2001, 89 (4), 334-343. 6. Kavan, L.; Grätzel, M.; Rathouský, J.; Zukalb, A., Nanocrystalline TiO2 (Anatase) Electrodes: Surface Morphology, Adsorption, and Electrochemical Properties. J. Electrochem. Soc. 1996, 143 (2), 394-400. 7. Ortiz, G. F.; Hanzu, I.; Djenizian, T.; Lavela, P.; Tirado, J. L.; Knauth, P., Alternative Li-ion Battery Electrode based on Self-Organized Titania Nanotubes. Chem. Mater. 2009, 21 (1), 63-67. 8. Wei, W.; Oltean, G.; Tai, C.-W.; Edstrom, K.; Bjorefors, F.; Nyholm, L., High Energy and Power Density TiO2 Nanotube Electrodes for 3D Li-ion Microbatteries. J. Mater. Chem. A 2013, 1 (28), 8160-8169. Figure 1

Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings.

MAX phases (M = transition metal, A = A-group element, and X = C/N) are of special interest because they possess a unique combination of the advantages of both metals and ceramics. Most attention is attracted to the ternary carbide Cr2AlC because of its excellent high-temperature oxidation, as well as hot corrosion resistance. Despite lots of publications, up to now the influence of bias voltage on the chemical bonding structure, surface morphology, and mechanical properties of the film is still not well understood. In the current study, Cr-Al-C films were deposited on silicon wafers (100) and Inconel 718 super alloy by dc magnetron sputtering with different substrate bias voltages and investigated using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), and nanoindentation. Transmission Electron Microscopy (TEM) was used to analyze the correlation between the growth of the films and the coating microstructure. The XPS results confirm the presence of Cr2AlC MAX phase due to a negative shift of 0.6–0.9 eV of the Al2p to pure aluminum carbide peak. The XRD results reveal the presence of Cr2AlC MAX Phase and carbide phases, as well as intermetallic AlCr2. The film thickness decreases from 8.95 to 6.98 µm with increasing bias voltage. The coatings deposited at 90 V exhibit the lowest roughness (33 nm) and granular size (76 nm) combined with the highest hardness (15.9 GPa). The ratio of Al carbide to carbide-like carbon state changes from 0.12 to 0.22 and correlates with the mechanical properties of the coatings. TEM confirms the columnar structure, with a nanocrystalline substructure, of the films.

Mxene: Novel Negative Electrode Materials for Sodium-Ion Batteries

Recently, urgent concerns over lithium resources have focused attention on Na-ion batteries owing to the abundance and low cost of sodium. However, the presently-considered negative-electrode materials for Na-ion batteries are Na-ion intercalation (carbon compounds, metal oxides and metal sulfides), alloying (phosphorus and tin) or conversion materials (metal oxides and metal sulfides), the energy and power densities of which are not satisfactory if the Na-ion batteries were to outperform the state-of-the-art Li-ion batteries. Hence, exploration for novel negative electrode materials of sodium system is primarily important. We have targeted MXenes as potential electrodes for sodium system. MXenes have been developed as a novel family of nanosheet compounds, which are chemically derived from layered Mn +1 AXn or MAX phases (M: early transition metal, A: A-group element, and X: C and/or N). As the MXene has both a high electrical conductivity and interlayer porous space, its application to Na-ion battery negative electrodes holds great promise. In this presentation, we report on Ti n +1C n T x as negative electrode materials for Na-ion batteries. Ti n +1C n T x consists of abundant and low-cost elements, enabling fabrication of sustainable energy sources. We demonstrate that Ti2CT x exhibits a reversible capacity of 175 mAh/g at the average potential of 1.3 V vs. Na/Na+. Furthermore, the prototype full cell consisting of alluaudite Na2Fe2(SO4)3-Ti2CT x delivers a high specific energy of 260 Wh/kg at a high specific power of 1.4 kW/kg based on the weight of Ti2CT x . Using various experimental and theoretical analyses, we clarify that the Ti n +1C n T x electrodes allow Na-ion batteries to be liberated from the trade-off between high energy and high power. [Reference]X. Wang, et al., M. Okubo, & A. Yamada, Nat. Commun., 2015, 6, 6544. Figure 1

Investigation of the Charge Storage Mechanism of Mxenes As Anode Materials for Na-Ion Batteries

Two dimensional (2D) materials such as graphene have attracted tremendous interest and have shown promising potential in various applications due to their unique physical structure and properties. Another new class of 2D materials, a group of early transition metal carbides and/or carbonitrides, called MXene, have been studied and reported recently. MXenes are chemically derived from layered Mn+1AXn or MAX phases, where M is early transition metal, A is an A-group element (mainly groups 13 and 14), X is C and/or N, and n=1,2, and 3. Since discovery of the first MXene (Ti3C2) in 2011, more than 10 new MXenes have been synthesized and demonstrated its interesting and unique properties in many applications. It has been shown that MXenes have promising potential in electrochemical energy storage applications such as lithium ion batteries (LIBs) and electrochemical capacitors (ECs), due to their high electrical conductivity, good structural/chemical stability, and large surface areas. Recently, MXene application studies are expanding to other battery systems such as Na-ion batteries (NIBs). The growing concerns on the cost and resource limit of lithium for large scale applications have triggered the development of NIBs as an alternative battery system to LIBs for large scale energy storage applications due to their potentially low-cost and natural abundance. Moreover, the accumulated knowledge and technology of LIBs enables fast-advancements of NIBs research since the operating principles of the NIBs are similar to that of “rocking-chair” mechanism in LIBs. Although many concepts can be adopted from LIB in NIB research, the electrochemistry of NIBs turns out to be different in many aspects requiring new design of electrode materials and electrolyte for improving and/or optimizing performance of NIBs. In particular, Na+ has a larger radius than Li+, which directly affects the mass transport and storage in the electrochemical reaction. This makes many of current LIB anode materials, such as graphite, unsuitable for NIBs. In this regards, the laminar nature of MXenes bonded by weak van der walls forces and its inherent interlayer spacing are very attractive as anode materials for NIBs. However, most of studies on the electrochemical behavior of MXenes as anode materials for NIBs are just at the theoretical estimation only without much experimental support. In this study, we present a systematic study on the electrochemical properties and their charge storage mechanisms of V2C and Nb2C MXenes as anode materials for NIBs. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were used in both ex situ and in situ way to look at the structural changes during electrochemical cycling. By utilizing such combined X-ray techniques, we are able to get better understanding of structural/electronic structure changes of MXenes and their related charge storage behavior during sodiation/desodiation process. A systematic comparison of electrochemical properties and charge storage mechanism for MXenes between NIBs and LIBs as well as the approaches to improve them will also be discussed. Acknowledgement The work done at Brookhaven National Lab was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, under Contract Number DE-SC0012704. The work performed at Drexel University was supported by the Office of Electricity Delivery and Energy Reliability, Energy Storage Systems Program, through Sandia National Laboratories. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.