Ti2AlC MAX Phase Research Articles

Oxidation Behavior of Ti2AlC MAX Phase Ceramics at 800 and 1200℃ in Air

The MAX phase ceramics represented by Mn+1AXn in the general chemical formula are layered ternary transition metal carbides and nitrides. The MAX phase ceramics have excellent properties, such as good machinability with cemented carbide tools, high thermal and electrical conductivity as well as high thermal resistance, elastic stiffness and oxidation resistance. Ti2AlC ceramics have been studied as potential candidates for use in various mechanical components operating at high temperatures because of their good resistance against high-temperature oxidation. However, there are both reports of good and poor in oxidation resistance of Ti2AlC. Differences in chemical composition may affect the oxidation behavior of the material. In this study, the oxidation behavior of Ti2AlC ceramics with various Al concentrations and phase composition was investigated. Ti2AlC ceramic powder was synthesized using commercial Ti, Al and C powders. These powders were mixed to obtain the nominal chemical compositions of Ti2AlC0.9, Ti2Al1.2C0.9 and Ti2Al1.5C0.9. Almost fully-densified Ti2AlC samples with various Al concentrations were prepared by means of pulsed electric current sintering. The phase composition of the sintered samples was identified via XRD. The main peaks recorded for all samples were attributed to Ti2AlC. Minority phases in Ti2AlC0.9 were identified as Ti3AlC2 and TiC. In the cases of Ti2Al1.2C0.9 and Ti2Al1.5C0.9, Ti3AlC2 and TiAl3 were detected with X-ray diffraction. Samples for oxidation experiments were cut with a diamond saw to obtain a size of either 4x4x3 mm or 7.5x7.5x3 mm and polished with water-resistant papers with a grit size of 2000. The Ti2AlC0.9, Ti2Al1.2C0.9 and Ti2Al1.5C0.9 samples were heated in an electric furnace at 800℃ for 2, 4, 7 and 14 d and at 1200°C for 0.5, 1, 2 and 4 d in laboratory air. The surface of the oxidized samples was analyzed via XRD to identify the phases. The oxidized samples were placed in a resin and cut with a diamond cutting wheel to allow the observation of their cross-sections using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). Ti2AlC0.9 exhibited non-protective oxidation in all samples for different experimental periods. The thickness of the non-protective oxide scale was over 20 μm. The phase composition of the non-protective oxide scale detected via XRD was identified as rutile-type TiO2 and α-Al2O3. In Ti2Al1.2C0.9 and Ti2Al1.5C0.9, all samples exhibited protective oxidation at 800℃. Some samples exhibited protective oxidation at 1200℃, but some did not. The thickness of the protective oxide scale was approximately 5 μm. The phase of the protective oxide scale was identified as α-Al2O3. The non-protective oxide scale near the surface had a stronger Ti intensity than near the scale/MAX phase interface. The results of XRD, SEM and EDX studies indicate that rutile with Al2O3 had formed as the non-protective oxide scale. Oxidation resistance of Ti2AlC ceramics changed with Al concentration. Non-protective oxidation for Ti2AlC ceramics occurred when the supply of Al from the sample interior had been insufficient. The supply of Al from TiAl3 grains might have been the main factor that ensured a continuous Al2O3 scale. The concurrence of Ti2AlC ceramics and TiAl3 lead to protective oxidation at 800℃. However, the Al supply at 1200℃ was insufficient for the formation of a protective Al2O3 scale on Ti2AlC ceramics.

Effects of furnace annealing on in situ reacted Ti2AlC MAX phase composite coatings deposited by laser cladding

The ability to deposit mm-scale coatings that are metallurgically bonded to substrates shows great promise for the fabrication of components coated with expensive materials, for their resistance to heat, radiation damage or oxidation in extreme environments. Although considered excellent candidates, in situ laser clad MAX phase coatings on metal substrates have found limited success due to the low proportion of MAX phase in the products. In this work, low-purity laser clad MAX phase coatings were subjected to post-deposition furnace annealing at 1350 °C to improve the Ti2AlC purity. Aside from a thin oxide layer on the coating surface, a pure Ti2AlC layer was formed immediately below. In total, the coatings were found to exist as seven microstructurally unique sub-layers due to equilibration of supersaturated phases formed during the laser cladding process. No delamination between any layers was observed. The phase identification and microstructure, as determined using XRD, SEM and EDS, are described in detail. Some unique microstructures were observed, including dendritic Ti2AlC MAX phase grains produced from a topochemical reaction between TiCx and TiyAlz regions, and α-Ti supersaturated with up to 33 at.% C. The kinetically trapped phases produced within the coating using this process may offer a strong combination of material properties which could be advantageous for use in extreme environments.

Full strength and toughness recovery after repeated cracking and healing in bone-like high temperature ceramics

Bones of humans and animals combine two unique features, namely: they are brittle yet have a very high fracture toughness linked to the tortuosity of the crack path and they have the ability to repeatedly heal local fissures such that full recovery of overall mechanical properties is obtained even if the local bone structure is irreversibly changed by the healing process. Here it is demonstrated that Ti2AlC MAX phase metallo-ceramics also having a bone-like hierarchical microstructure and also failing along zig-zag fracture surfaces similarly demonstrate repeated full strength and toughness recovery at room temperature, even though the (high temperature) healing reaction involves the local formation of dense and brittle alumina within the crack. Full recovery of the fracture toughness depends on the healed zone thickness and process zone size formed in the alumina reaction product. A 3-dimensional finite element method (FEM) analysis of the data obtained from a newly designed wedge splitting test allowed full extraction of the local fracture properties of the healed cracks.

General Trends of Structure Formation in Graded Composite Materials Based on the Ti3AlC2 MAX Phase on Titanium

This paper considers the main trends of structure formation in graded composite materials based on the Ti3AlC2 MAX phase on titanium under combustion and high-temperature deformation conditions in the unconfined self-propagating high-temperature synthesis compaction method. We demonstrate that a compact graded composite material can be obtained in tens of seconds in one processing step at relatively low pressures (50 MPa). Varying the delay time before applying pressure, we can control the structure and mechanical properties of the resulting composite material. The largest number of carbide grains are formed in the upper part of the material. As the titanium layer is approached, the number of such grains decreases, which is accompanied by an increase in the number of platelike MAX phase grains. We have studied mechanical characteristics of the synthesized materials: variation of microhardness along the height of the composite material and its modulus of elasticity and bending strength.

Ti3C2Tx MXene playing as a strong methylene blue adsorbent in wastewater

Layer-structured MXenes recently attract a great deal of interest due to their intriguing properties. Despite the broad and intensive research, some of their application areas have been scarcely highlighted. In this work, it was demonstrated that the mostly F-terminated Ti3C2Tx MXene could play as a strong adsorbent of methylene blue (MB) in wastewater. For this, the MXene was synthesized by Al-selective etching of Ti3AlC2 MAX phase using HF. At a standard condition, the MXene turned out to be terminated mostly with F functional groups. Remarkably, the mostly F-terminated MXene adsorbed approximately 92% of MB in 20 μM of MB aqueous solution within 5 min. It was found that the surface termination groups could be controlled simply by adjusting cleaning cycles, and the MB removal efficiency had a strong dependence on the pH of MXene suspension. Furthermore, the MXene exhibited relatively good recyclability, enabling its repeated use. The adsorption capability of the MXene was revealed to become weaker for the other organic dyes. The results of this work may give an insight into practical application of MXene for remediation of polluted water.

Local chemical bonding and structural properties in Ti3AlC2 MAX phase and Ti3C2Tx MXene probed by Ti 1s x-ray absorption spectroscopy

The chemical bonding within the transition-metal carbide materials MAX phase Ti3AlC2 and MXene Ti3C2Tx is investigated by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. MAX phases are inherently nanolaminated materials that consist of alternating layers of Mn+1Xn and monolayers of an A-element from the IIIA or IVA group in the periodic table, where M is a transition metal and X is either carbon or nitrogen. Replacing the A-element with surface termination species Tx will separate the Mn+1Xn-layers forming two-dimensional (2D) flakes of Mn+1XnTx. For Ti3C2Tx the Tx corresponds to fluorine (F) and oxygen (O) covering both sides of every single 2D Mn+1Xn-flake. The Ti K-edge (1s) XANES of both Ti3AlC2 and Ti3C2Tx exhibit characteristic pre-edge absorption regions of C 2p - Ti 3d hybridization with clear crystal-field splitting's while the main-edge absorption features originate from the Ti 1s -> 4p excitation, where only the latter shows sensitivity towards the fcc-site occupation of the termination species. The coordination numbers obtained from EXAFS show that Ti3AlC2 and Ti3C2Tx are highly anisotropic with a strong in-plane contribution for Ti and with a dynamic out-of-plane contribution from the Al monolayers and termination species, respectively. As shown in the temperature-dependent measurements, the O contribution shifts to shorter bond length while the F diminishes as the temperature is raised from room temperature up to 750 {\deg}C.

MAX phase Ti3AlC2 embedded in PVA and deposited onto D-shaped fiber as a passive Q-switcher for erbium-doped fiber laser

2D-layered metal carbides/nitrides known as MXene was shown to exhibit a strong optical nonlinearity in the near-infrared regime. However, the potential of its bulk-counterparts, MAX phase as a saturable absorber was rarely investigated. Here, the development of Ti3AlC2 MAX phase-based SA was demonstrated. We prepared two SA devices which were MAX-thin film (MAX-film) and MAX-d-shaped fiber (MAX-DS), we investigated their optical properties and incorporated them in an erbium-doped fiber laser cavity to initiate a Q-switched. MAX-film and MAX-DS generate a Q-switched at the working wavelength of 1560.2 nm and 1558.2 nm, respectively. We compared their Q-switched laser parameters and revealed a significant improvement by using MAX-DS as SA. The as-prepared MAX-DS exhibit a Q-switched EDFL with a minimum pulse width of 2.93 μs, a maximum repetition rate of 96.15 kHz, a maximum output power of 14.6 mW, a maximum pulse energy of 151.8 nJ, an optical-to-optical efficiency of 11 % and a signal-to-noise ratio of 70 dB.

Soliton mode-locked pulse generation with a bulk structured MXene Ti3AlC2 deposited onto a D-shaped fiber.

We propose a bulk structured MXene, Ti3AlC2 deposited onto D-shaped fiber for soliton generation in an erbium-doped fiber laser (EDFL) cavity. Our saturable absorber (SA) device, based on MAX phase, was prepared by using stirring and ultrasonic vibration, which offer easier sample preparation compared with its 2D counterparts. By means of the polishing wheel technique, we fabricated a D-shaped fiber with a controlled polishing depth and incorporated the MAX phase Ti3AlC2 solution onto its polishing region. We obtained a mode-locked soliton pulse with the proposed MAX phase D-shaped (MAX-DS) SA in EDFL cavity. The pulse width, repetition rate, and central wavelength of the pulse train are 2.21 ps, 1.89 MHz, and 1557.63 nm, respectively. The polarization-insensitive EDFL cavity initiated a soliton operation with superior stability as the pump power tuned from 21 to 131 mW; further, the ML laser exhibits an average power of 15.3 mW, peak power of 3.8 kW, and pump efficiency of 12.5%. The MAX-DS SA incorporated inside the EDFL reveals efficient output performance, with a pulse energy of 8.14 nJ, the highest ever reported, to our best knowledge, among D-shaped fiber-based SA.

Development of the phase composition and the properties of Ti2AlC and Ti3AlC2 MAX-phase thin films – A multilayer approach towards high phase purity

MAX phase thin films have been synthesized by thermal treatment of a Ti-Al-C multilayer system. The preparation of the multilayer system was carried out via magnetron sputtering. Based on the thickness ratio among the individual nanoscale monolayers (Ti, Al, C), the resulting MAX phase stoichiometry can be controlled. This paper describes the synthesis of both Ti2AlC and Ti3AlC2 MAX phases from the same precursor multilayer system which is composed of a sequence of Ti/Al/C pure elemental single layers with thicknesses of 14, 6, and 3.5 nm, respectively. This sequence is repeated 22 times with a total thickness of around 500 nm. Rapid thermal treatment tests were performed to study the phase development. The Ti2AlC MAX phase forms in a temperature range below 850 °C, whereas the Ti3AlC2 MAX phase starts to form at temperatures above 850 °C and reaches its highest phase purity at 950 °C. The thin film structures were studied by X-ray diffraction and Raman spectroscopy. Furthermore, the electrical and mechanical properties were investigated to gain more insights regarding the phase transformation and their influence on the thin film properties.

Frank partial dislocation in Ti2AlC-MAX phase induced by matrix-Cu diffusion

The control of complex nanostructures is one of the most promising strategy for designing tailored property materials. Ti2AlC, as nanolayered ternary materials combining both ceramics and metals properties, could efficiently reinforce metal matrix composite by its decomposition into nano-Ti2C particles. However, the comprehensive description of the diffusion mechanisms of Cu element into Ti2AlC during its decomposition has not yet been revealed. Herein, we report detailed characterizations of Cu and Ti2AlC interlayers by means of high-resolution electron microscopy and atomic-scale simulations. In particular, we report the formation of Ti2AlxCuyC solid solutions. Furthermore, we demonstrate the formation of Ti2C platelets and evidenced Frank partial dislocations that are not expected to be found in such material. Impacts of such defects are discussed in the framework of mechanical behavior.

Preparation of Ti2AlC and Ti3AlC2 MAX Phases by Self-Propagating High-Temperature Synthesis with the Reduction Stage

This work is devoted to the preparation of powders of Ti2AlC and Ti3AlC2 MAX phases by self-propagating high-temperature synthesis (SHS) using the magnesium-thermal reduction from oxide feedstock. Oxide TiO2 is used as the titanium source, and magnesium is used as the reducing agent. Purification with the removal of magnesium oxide is performed in diluted hydrochloric acid at a temperature of 70°C and concentration of 1 : 3. The target product yield with the magnesium-thermal reduction is 35–40%. It is revealed that the synthesis product with the stoichiometric component ratio after chemical leaching in hydrochloric acid consists of Ti2AlC, MgAl2O4, and TiC. The formation of spinel MgAl2O4 is associated with the deficiency of the magnesium reducing agent in the charge; herewith, aluminum partially enters the reduction reaction of titanium from its oxide with the formation of Al2O3. This leads to the formation of spinel MgO ⋅ Al2O3. An increase in the content of excess magnesium in the charge from 20 to 30 wt % conditions the complete reduction of titanium from its oxide by magnesium with the formation of the Ti2AlC MAX phase and titanium carbide. A decrease of 10 wt % in the carbon content causes a decrease in the titanium carbide fraction to 4%. The product, containing Ti3AlC2 and Ti2AlC MAX phases, as well as TiC, is formed at an excess soot content from 20 to 35%, and the Ti3AlC2 weight fraction increases from 86 to 89%, respectively. These powders comprise agglomerates (87% of them are smaller than 65 μm) consisting of thin plates of MAX phases 70–100 nm in thickness.

Thermostressed State of a Nozzle Vane from Max Phase Ceramics

Numerical simulation results for the unsteady thermal and stressed states of a nozzle vane under unsteady operation conditions of the gas turbine are presented. The vane and nozzle block duct 3D computer model was constructed. The finite element mesh form, its sizes, and calculation time steps were optimized. The vane and duct contained 1.7 and 3.6 mln elements and 2.3 and 0.8 mln nodes, respectively. The mesh smoothing with corresponding nodal displacement near the transition zones and in the vicinity of curvilinear surfaces was used. The simulation consisted in successive numerical solutions of nonstationary flow problems for a moving medium and heat, transfer, thermal conductivity, and thermoelasticity problems for the vane. The parameters of unsteady thermal loading conditions were defined. The thermal properties of the moving medium and vane material were assumed to be time-dependent. For turbine power improvement conditions, time variations of duct flow parameters, temperatures, thermal stresses in different vane zones, their gradients and rates were analyzed. The essential nonuniformity of the thermostressed vane state and zones of thermal stresses, almost reaching the ultimate strength of the material, were noted. The effect of general heat flow components on the disturbance of thermal stresses was shown. Emphasis was placed upon the appearance of tensile stresses on a heated trailing edge of the vane (most critical zone). The conclusion was drawn regarding a potential application of a MAX phase Ti2ALC us a structural material for the vanes of short-life gas turbines.

Creep and Viscoelasticity of the Ti3AlC2 MAX Phase at Room Temperature

Creep and Viscoelasticity of the Ti3AlC2 MAX Phase at Room Temperature

A novel ZrB2-based composite manufactured with Ti3AlC2 additive

A novel ZrB2–Ti3AlC2 composite was densified using spark plasma sintering at 1900 °C under pressure of 30 MPa for 7 min. The effect of Ti3AlC2 MAX phase on the densification behavior, microstructural evolutions, phase arrangement, and mechanical properties of the composite were investigated. The phase analysis and microstructural studies revealed the decomposition of the MAX phase at the initial steps of the SPS process. The structural characteristics and surface morphology of the in-situ synthesized reinforcements were verified using X-ray diffraction and scanning electron microscopy, respectively. The formation mechanism of each reinforcement phase was also investigated using thermodynamical assessments. The prepared ZrB2–Ti3AlC2 composite not only possessed a near fully-dense characteristic having an excellent hardness of 31 GPa, but also unexpectedly presented high fracture toughness. The indentation fracture toughness of the composite was calculated as 7.8 MPa m1/2, which is unprecedented compared with the same class of hard ZrB2-based composites. Indeed, the superior mechanical properties of the composite achieved in this study was obtained by the homogenous distribution of Al-based reinforcements, formation of hard interfacial ZrC grains, and solid solutions provided by Ti-based phases. The correlations between the phase arrangement, microstructure, and the attained mechanical properties of the composite were comprehensively discussed.

Modifying MXene surfaces

Surface Chemistry Unlike graphene and transition-metal dichalcogenides, two-dimensional transition-metal carbides (MXenes) have many surface sites that can be chemically modified. Etching of the aluminum layer of a parent MAX phase Ti3AlC2 layered material with hydrofluoric acid leads to the MXene Ti3C2 with various surface terminations. Molten salts can achieve uniform chloride terminations, but these are difficult to further modify. Kamysbayev et al. show that etching of MAX phases in molten cadmium bromide leads to bromide-terminated MXenes that can then be substituted with oxygen, sulfur, selenium, tellurium, and NH groups as well as with vacancy sites. The surface groups can alter electronic transport. For example, the Nb2C MXenes exhibit surface group–dependent superconductivity. Science , this issue p. [979][1] [1]: /lookup/doi/10.1126/science.aba8311

Optoelectronic properties of Ti3C2Tx MXene transparent conductive electrodes: Microwave synthesis of parent MAX phase

Herein, we evaluated the structural and optoelectronic properties of Ti3C2Tx MXene transparent conductive electrodes (TCEs) prepared from microwave synthesized Ti3AlC2 MAX phase. A mixture of Ti, Al, and C was heated at 1300 °C for 30 min in a microwave oven. We synthesized MXene by two different etchant concentrations. A 1.6 g LiF in 9 M HCl solution brought about a higher degree of delamination in MXene with an inter-layer space of 1.35 nm Ti3C2Tx MXene prepared from microwave synthesized MAX phase exhibited a lateral size of ~80 nm with crystalline defects. We examined the effect of spinning speed on optoelectronic properties of Ti3C2Tx MXene TCEs. Increasing the spinning speed from 1000 to 4000 resulted in an increase in transparency (T550) (from 72% to around 94%) and sheet resistance (Rs) (from 2010 to 23660Ω/sq). A FoM value of 2.027 ± 0.163 was calculated from data fitting of T550 vs. Rs (R2 > 0.97). It is suggested that small sheet size and formation of defects are responsible for the relatively high Rs.

Preparation and performance of MAX phase Ti3AlC2 by in-situ reaction of Ti-Al-C system

In order to clarify the effects of the proportion of raw powders, heating temperature and holding time on the purity and properties of MAX phase Ti3AlC2, powder mixtures of Ti, Al and C powder with different ratio were prepared by planetary ball mill and heated under different conditions by spark plasma sintering. The microstructures and phase structures of the as-synthesized samples were characterized, the correlation between the mechanical properties and microstructure and fracture mechanism were investigated systematically. The results show that with the proportion of raw powders Ti:Al:C = 3:1.2:2, the sample heated at 1300 °C for 60 min has the highest purity 97.23 wt% of MAX phase. It has a compact and uniform lath-like structure with the length-thickness ratios of 3–5 and excellent comprehensive mechanical properties: the Vickers hardness, bending strength and fracture toughness are 5.26 GPa, 500 MPa and 7.35 MPa∙m1/2, respectively. The experimental results show that among the three factors, the proportion of raw powders has the greatest influence on purity of Ti3AlC2 phase, followed by heating temperature and holding time. The fracture morphologies of the tested samples demonstrate that under the action of external force, extrusion and kink occurred in the layered structures of Ti3AlC2 phase. These two forms of energy dissipation lead to the bending strength and fracture toughness of Ti3AlC2 are higher than that of traditional ceramics.

In situ synthesis of exfoliation TiO2@C hybrids with enhanced photocatalytic hydrogen evolution activity

TiO2 is an ideal semiconductor material for photocatalytic applications. However, inadequate visible light response and fast recombination of charge carriers restrict its photocatalytic performance. Constructing a junction structure between TiO2 and carbon nanomaterials can solve the above shortcomings. In the present work, two-dimensional (2D) transition metal carbides (Ti3C2) were synthesized from the Ti3AlC2 MAX phase firstly. Then the porous carbon-supported TiO2 (TiO2@C) hybrids were in-situ synthesized by calcination in air. Also, the pristine Ti3C2 was exfoliated by intercalation with dimethyl sulfoxide (DMSO). The exfoliation TiO2@C catalyst was in-situ prepared by freeze-drying and calcining, and it delivered an enhanced visible-light-driven photocatalytic H2 evolution rate of 160.42 µmol·h−1·g−1, 13 times higher than the primary TiO2@C catalyst. The enhanced photocatalytic activity results from the increased specific surface area, facilitating charge separation and elevated conduction band. The paper indicates that MXene oxidation can further apply in the visible light photocatalysis.

Mechanical and oxidation behavior of textured Ti2AlC and Ti3AlC2 MAX phase materials

Highly textured Ti2AlC and Ti3AlC2 ceramics were successfully fabricated by a two-step fabrication process, and the Lotgering orientation factors for {00l} planes of textured Ti2AlC and Ti3AlC2 were calculated as 0.82 and 0.71, respectively. The effect of texturing was evaluated in terms of elastic modulus and hardness by macro- and micro-indentation. Moreover, the oxidation behavior of the MAX phases was investigated at 1300 °C in air, revealing that the oxidation was markedly anisotropic, where the textured side surface exhibited much better oxidation resistance, resulting from the rapid diffusion of Al element within its basal planes to form a protective Al2O3 scale on it. Furthermore, Ti2AlC had larger difference regarding oxidation behavior between the top and side surface than Ti3AlC2, correlated to its higher Al ratio, leading to higher texturing degree and more diffusion pathways to the outer surface to produce an Al2O3 layer already at the initial oxidation stage.

Thermodynamic Description of Chemical Reactions in a Titanium–Aluminum–Hexamethylenetetramine System under Conditions of Self-Propagating, High Temperature Synthesis

Based on experimental and theoretical data, thermodynamic analysis of possible chemical reactions occurring in the titanium–aluminum–hexamethylenetetramine system under conditions of self-propagating, high temperature synthesis (SPHTS) was performed. It was shown that, under the above conditions, the MAX-phase Ti3AlC2 can be predominantly formed at cooling down the material up to temperatures lower than 1100 K. The highest negative values at T = 298 K are those of enthalpy and Gibbs energy of the combustion reaction of hexamethylenetetramine combustion in the air, which allows for the assumption that this is the starting reaction for the synthesis process under study.