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As MXenes become increasingly widespread, approaches to utilize this versatile class of 2D materials are sought. Recently, there has been growing interest in incorporating MXenes into metal or ceramic matrices to create advanced nanocomposites. This study presents a facile approach of mixing MXene with ceramic particles followed by pressure-assisted sintering to produce bulk MXene/ceramic nanocomposites. The effect of MXene addition on the densification behavior and properties of nanocomposites was explored through the Ti

Efficient water splitting for commercial electrolysis devices is predicated on the development of materials, specifically for the catalytic electrodes, that exhibit an optimal balance between activity and stability. Complicating the development of electrocatalytic materials, particularly for oxygen-evolving anodes in acidic polymer electrolyte membrane electrolyzers, is an inverse relationship between activity and stability. Here the development of a nanostructured oxygen evolution reaction (OER) electrocatalyst for low-pH water splitting is demonstrated. Dealloying of mixed RuIr borides is used to form a high aspect ratio electrocatalytic material that exhibits low OER overpotentials matching that of RuOx and electrolytic stability matching that of IrOx.

Summary The first MXene discovered, Ti3C2Tz, was synthesized by etching of the parent MAX phase, Ti3AlC2, in a solution of concentrated hydrofluoric acid. Since then, several other ways of synthesizing MXenes have been reported, such as electrochemical etching in various electrolytes, high-temperature alkali treatment, molten salt synthesis, etc. The majority of these etching methods, however, use water as their main solvent, limiting direct use of MXenes in water-sensitive applications. In this work, we show that it is possible to etch, and delaminate, MXenes in the absence of water, by using organic polar solvents in the presence of ammonium dihydrogen fluoride. We further show that by using this etching method, it is possible to obtain Ti3C2Tz flakes rich in fluorine terminations. We also demonstrate that electrodes made from Ti3C2Tz etched in propylene carbonate resulted in Na-ion battery anodes with double the capacity to those etched in water.

Contrary to the characteristic strengthening of polycrystalline ceramics with a decrease in grain size, extremely fine nanocrystalline ceramics exhibit softening, increased plasticity and an inverse Hall-Petch relation. Despite experimental evidence, questions remain regarding the underlying deformation mechanisms governing this abnormal mechanical behavior. In the present study, an in-depth microstructural examination was performed on nanostructured transparent magnesium aluminate spinel (MgAl2O4) subjected to microhardness tests. Microstructural observations revealed regions strained to various degrees below the point of indentation, containing varying amounts of dislocations and nano-cavities. Furthermore, the residual strain in different areas was estimated by local electron diffraction. These observations and analysis provided evidence for grain boundary (GB) mediated mechanisms (e.g., GB sliding and rotation). Moreover, shear bands formed and were found to be associated with micro-cracking. By combining the microstructural analysis with suitable models, it was concluded that these mechanisms govern plastic deformation. By elucidating how strain is accommodated within nanocrystalline ceramics, a deeper understanding of their unique mechanical behavior is gained.

Herein, we present the fabrication of dispersed, 5.0 wt % (1.74 vol %) Ti3C2Tz MXene epoxy nanocomposites (NCs), and report on their water transport and mechanical properties. To make the composites, Li+ ions between Ti3C2Tz MXene multilayers, MLs, present after the etching step were exchanged with either 12-aminolauric acid, ALA, or di(hydrogenated tallow)benzyl methyl ammonium chloride, DHT. After drying, the resulting ML powders were added at room temperature to the epoxy resin (diglycidyl ether of bisphenol A), followed by the curing agent, triethylenetetramine. The NCs were characterized by X-ray diffraction, thermogravimetric analysis, dynamic vapor sorption, dynamic mechanical analysis, scanning and transmission electron microscopies, and infrared spectroscopy. From XRD, the lack of signature MXene basal peaks, as well as evidence of exfoliation supported by TEM micrographs, we conclude that the MXene ML had indeed been intercalated by the epoxy. The distribution of the exfoliated multilayers, MLs, however, was not uniform. Nevertheless, our relative permeabilities, with a 1.74 vol % loading, are 5 times lower than results obtained in the carbon- or clay-reinforced epoxy NC literature. The lower permeabilities are due to reductions in both solubilities and diffusivities relative to the neat polymer. In the case of DHT, the water solubility at all temperatures was almost halved. The mechanical properties and thermal stability are found to be slightly improved with the addition of DHT-MXene. As far as we are aware, this is the first report of exfoliation of MXene in an epoxy matrix. Additionally, this study is the first to measure the diffusion of water in MXene epoxy NCs. More work on better dispersion of the MLs is indicated and ongoing.

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 °C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 °C under 600 MPa.

Spark plasma sintering (SPS) is an effective process for the fabrication of highly transparent oxide ceramics for photonic applications. In the present study, Nd-doped yttrium aluminum garnet (Nd:YAG) ceramics with various dopant concentrations (0.5–5 at.%) were fabricated at 1300–1400 °C using conventional SPS (60 MPa) and high pressure (300 MPa) conditions. The appearance, X-ray diffraction pattern, densification regime, microstructure, mechanical and optical properties were compared; the dependency on Nd concentration and sintering pressure is discussed. The pressure applied during the sintering process seemed to have only a minor effect on the luminescent properties, while the mechanical properties were superior for the samples sintered under high pressure conditions. The known concentration quenching phenomenon was observed and an equation for estimation of the Nd concentration, based on phosphorescence lifetime, is suggested.

Clay-reinforced nylon-6 nanocomposites (NCs)-characterized by the full exfoliation of the nanoreinforcement-were introduced in the marketplace in the 1990s. Herein, we demonstrate, for the first time, that Ti3C2T z MXene can be incorporated into nylon-6 to synthesize melt-processable nanocomposites with excellent water barrier properties (94% reduction in water vapor permeation). To intercalate the e-caprolactam monomer between the MXene multilayers, the latter were first treated with 12-aminolauric acid, a low-cost, nontoxic, biodegradable, and long shelf life compound. Upon heating to 250 °C, in the presence of 6-aminocaproic acid, in situ polymerization occurred, yielding melt-processable nylon-6/MXene NCs that were, in turn, studied by thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, scanning and transmission electron microscopy, infrared spectroscopy, and dynamic vapor sorption analysis. Using the latter, moisture-sorption isotherms of a neat and a 1.9 vol % NC, at 60 °C, were fit to the Guggenheim, Anderson, and de Boer equation. Solubility, permeation, and diffusion coefficients of water through the NCs were measured as a function of temperature and found to be the lowest ever reported for nylon-6, despite the fact that, at ∼1.9 and 5.0 vol %, the MXene loads were relatively low. This record low diffusivity is ascribed to the very large aspect ratios-500 to 1000-of Ti3C2T z flakes and their dispersion. The water permeation rate is a factor of 5 lower than the best reported in the much more mature nylon/clay field, suggesting lower values can be achieved with further optimization. Lastly infrared spectroscopy spectra of neat and NC samples suggest the surface terminations of the 12-Ti3C2T z flakes bind with nylon-6, limiting water adsorption sites, resulting in reduced solubility in the NC films.

The Mn+1AXn, or MAX, phases are nanolayered, hexagonal, machinable, early transition-metal carbides and nitrides, where n = 1, 2, or 3, 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 are characterized by a unique combination of both metallic and ceramic properties. The fact that these phases are precursors for MXenes and the dramatic increase in interest in the latter for a large host of applications render the former even more valuable. Herein we describe the structure of most, if not all, MAX phases known to date. This review covers ≈155 MAX compositions. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both quaternary in- and out-of-plane ordered MAX phases opens the door to the discovery of many more. The chemical diversity of the MAX phases holds the key to eventually optimizing properties for prospective applications. Since many of the newer quaternary (and higher) phases have yet to be characterized, much work remains to be done.

Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.

Applying high pressure during the sintering of ceramic materials is a common practice that allows for a reduction of the sintering temperature and the obtaining of fine-grained microstructures. In this work, we show that the final grain size of submicron alumina increased consistently with applied pressure during low temperature (1000–1050 °C), high pressure (500–800 MPa) spark plasma sintering. Grain size trajectories and microstructural observations indicated that stress-enhanced grain growth occurred during the final stage of the sintering process, whereas thermally controlled grain boundary migration was negligible. We suggest that this dynamic, stress-enhanced grain growth is controlled by grain-boundary sliding, grain rotation and coalescence. A strong correlation was found between calculated creep strain rates and grain growth rates, such as during superplastic deformation.

A two-stage processing approach combining spark plasma sintering (SPS) and hot isostatic pressing (HIP) was employed for the fabrication of relatively large (30 mm diameter) and thick (up to 8 mm) samples of transparent polycrystalline magnesium aluminate. The effects of sample thickness, heating rate during SPS, and the temperature and duration of HIP treatments were investigated. It was established that the heating rate during SPS had a major influence on discoloration due to carbon contamination, which increased with sample thickness. HIP treatment allowed for the elimination of cloudiness due to samples porosity, although carbon contamination present after the SPS step could not be reduced by HIP treatment, regardless of the temperature and duration applied. Highly transparent specimens with thicknesses of 4 and 8 mm exhibiting an in-line transmittance of 85.2 and 83.2% at 600 nm, respectively, were fabricated.

Quaternary MAX phases, (Ta1−xTix)3AlC2 (x = 0.4, 0.62, 0.75, 0.91 or 0.95), have been synthesised via pressureless sintering of TaC, TiC, Ti and Al powders. Via chemical etching of the Al layers, (Ta0.38Ti0.62)3C2Tz – a new MXene, has also been synthesised. All materials contain an M-layer solid solution of Ta and Ti, with a variable Ta concentration, paving the way for the synthesis of a range of alloyed (Ta,Ti)3C2Tz MXenes with tuneable compositions for a wide range of potential applications.

Neural networks are one of the first major milestones in developing artificial intelligence systems. The utilisation of integrated photonics in neural networks offers a promising alternative approach to microelectronic and hybrid optical-electronic implementations due to improvements in computational speed and low energy consumption in machine-learning tasks. However, at present, most of the neural network hardware systems are still electronic-based due to a lack of optical realisation of the nonlinear activation function. Here, we experimentally demonstrate two novel approaches for implementing an all-optical neural nonlinear activation function based on utilising unique light-matter interactions in 2D Ti$_3$C$_2$T$_x$ (MXene) in the infrared (IR) range in two configurations: 1) a saturable absorber made of MXene thin film, and 2) a silicon waveguide with MXene flakes overlayer. These configurations may serve as nonlinear units in photonic neural networks, while their nonlinear transfer function can be flexibly designed to optimise the performance of different neuromorphic tasks, depending on the operating wavelength. The proposed configurations are reconfigurable and can therefore be adjusted for various applications without the need to modify the physical structure. We confirm the capability and feasibility of the obtained results in machine-learning applications via an Modified National Institute of Standards and Technology (MNIST) handwritten digit classifications task, with near 99% accuracy. Our developed concept for an all-optical neuron is expected to constitute a major step towards the realization of all-optically implemented deep neural networks., Comment: 30 pages (including supplementary information), 5 figures

The atomically laminated Ti2AlC, Ti3AlC2 and Cr2AlC MAX phases, with A = Al, form adherent, passivating α-alumina, Al2O3, oxide scales when heated in air. The effect of solid solutions on the A layers in affecting the oxidation kinetics remains a subject of open research. Herein we synthesize a dense bulk polycrystalline Ti3Al1−xGaxC2 (x ≈ 0.4) solid-solution and investigate its isothermal oxidation in ambient air, in the 1000 °C–1300 °C temperature range, for times varying between 15 and 300 h. At 1000 °C, a passivating dense Al2O3 layer ( ≈ 1–2.6 μm thick) with near cubic kinetics and an overall weight gain that is slightly less than either Ti3AlC2 or Ti2AlC is formed. At 1200 °C, the Al2O3 layer thickens (3.5–12 μm thick) with some scale delamination on the corners initiating at 15 h. At 1300 °C, the Al2O3 layer (7.6–20.7 μm thick) wrinkles and Al2TiO5 forms. Though the Al2O3 grains coarsen at 1200 °C and 1300 °C, the weight gain is higher than that for Ti3AlC2 or Ti2AlC. At around 7 at. %, this is one of the lowest, if not lowest, Al mole fraction in a Ti-based alloy/compound that forms an Al2O3 passivating layer. We further provide compelling microstructural evidence, in the form of a duplex oxide, that at 1000 °C, the outward Al flux, JAl, and the inward O flux, JO, are related such that 2 JAl = 3 JO. A fraction of these fluxes combine, at the duplex oxide interface, to nucleate small grains

In this work, we investigate the magnetic structures of ${({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})}_{2}\mathrm{Al}{\mathrm{B}}_{2}$ solid-solution quaternaries in the $x=0$ to 1 range using x-ray and neutron diffraction, magnetization measurements, and mean-field theory calculations. While ${\mathrm{Fe}}_{2}\mathrm{Al}{\mathrm{B}}_{2}$ and ${\mathrm{Mn}}_{2}\mathrm{Al}{\mathrm{B}}_{2}$ are known to be ferromagnetic (FM) and antiferromagnetic (AFM), respectively, herein we focused on the magnetic structure of their solid solutions, which is not well understood. The FM ground state of ${\mathrm{Fe}}_{2}\mathrm{Al}{\mathrm{B}}_{2}$ becomes a canted AFM at $x\ensuremath{\approx}0.2$, with a monotonically diminishing FM component until $x\ensuremath{\approx}0.5$. The FM transition temperature (${T}_{\mathrm{C}}$) decreases linearly with increasing $x$. These changes in magnetic moments and structures are reflected in anomalous expansions of the lattice parameters, indicating a magnetoelastic coupling. Lastly, the magnetocaloric properties of the solid solutions were explored. For $x=0.2$ the isothermal entropy change is smaller by 30% than it is for ${\mathrm{Fe}}_{2}\mathrm{Al}{\mathrm{B}}_{2}$, while the relative cooling power is larger by 6%, due to broadening of the temperature range of the transition.

High-pressure spark plasma sintering of Si 3 N 4 with Y 2 O 3 , Al 2 O 3 and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si 3 N 4 phase and 8% β-Si 3 N 4 phase. The relatively high pressure applied (up to 650 MPa) had a substantial effect on densification by enhancing particle rearrangement, making it possible to obtain dense Si 3 N 4 at a significantly lower sintering temperature (1350 °C). Consequently, virtually no α to β phase transformation transpired during the liquid phase sintering process. The LiF additive had an indispensable influence on the densification process by lowering the viscous glass formation temperature, which also contributed to enhanced particle rearrangement. The nearly fully dense samples (theoretical density ≥99%) obtained displayed a good combination of mechanical properties, namely elastic modulus (304–316 GPa), hardness (1720–1780 HV2) and fracture toughness (6.0 MPa m 1/2 ).

A spark plasma sintering (SPS) apparatus was successfully employed to perform uniaxial compressive creep tests on pure copper under a stress of 30 MPa in the 400–600 °C temperature range. By utilizing two different configurations, the creep experiments were conducted without or with a low-density pulsed direct electric current (~ 6–7 A/mm2) passing through the samples. It was found that under the influence of the applied electric current, the creep rate increased significantly, while the extent of the effect diminished with temperature. The apparent activation energy for creep with applied current decreased from 110 to 66 kJ/mol. This was attributed to the effect of current on the thermally-activated process and dislocations motion. No distinct evidence that the electric current affects the microstructure was observed.

The effects of LiF on the synthesis and reactive sintering of polycrystalline gahnite (zinc aluminate spinel, ZnAl 2 O 4 ) were studied using XRD, high-temperature simultaneous thermal analysis and a spark plasma sintering (SPS) apparatus. It was demonstrated that the LiF reduces the onset of synthesis by about 200 °C and plays an important role in the densification process. SPS consolidation of a LiF-doped ZnO-Al 2 O 3 mixture under an applied pressure of 150 MPa and at a sintering temperature of 1100 °C for 20 min generated fully dense gahnite with adequate transparency and mechanical properties.

Summary Practicality of lithium-sulfur batteries is severely hindered by the notorious polysulfide-shuttle phenomenon, leading to rapid capacity fade. This issue is aggravated with increase in sulfur loading, causing low-coulombic efficiency and cycle life. Herein, we present a facile strategy to combine hydrophobic sulfur and hydrophilic, conductive Ti3C2Tz-MXene via one-step surface functionalization using di(hydrogenated tallow)benzylmethyl ammonium chloride (DHT). The latter renders the Ti3C2Tz surface hydrophobic, making it readily dispersible in sulfur dissolved in a carbon disulfide (CS2) solvent. By evaporating the solvent, we conformally coat the DHT-Ti3C2Tz (DMX) with sulfur. The developed composite, with higher available active area, enables effective trapping of lithium polysulfides (LiPs) on the electroactive sites within the cathode, leading to improvement in electrochemical performance at higher sulfur loadings. The DMX/S cathodes function with high sulfur loading of ∼10.7 mg·cm−2 and deliver a stable areal capacity of ∼7 mAh·cm−2 for 150 cycles. Moreover, a DMX/S cathode in a pouch-cell configuration retains ∼770 mAh·g−1 after ∼200 cycles at 0.2C (85.5% retention). Ex situ studies elucidate the nature of the LiPs-MXene interaction and the effect of surface functionalization towards improved performance.

Recently, a new family of MAX phases with in-plane chemical order, i-MAX, have been discovered which incorporate new elements expanding the family of MAX phases. i-MAX phases remain to be synthesized in single-phase bulk form for characterization. Herein, we show that by reactively hot pressing an intermetallic precursor, Y2.23Al, instead of elemental Y, in combination with excess overall Al and a sub-stoichiometric fraction of carbon, we enhance the yield of (Cr2/3Y1/3)2AlC to 85±3 wt% (86±3 mol%). Both the fractions of the impurity Y2O3 phase and the undesirable ternary Cr2AlC are reduced. Subsequent isothermal oxidation of (Cr2/3Y1/3)2AlC in natural air in the 1000–1400 °C temperature range reveals the formation of Y3Al2(AlO4)3 (YAG), Cr2O3 and Y2O3, without a continuous Cr7C3 sub-layer. Additionally, we show that by starting with Y2.23Al reagent we synthesize dense bulk (W2/3Y1/3)2AlC and (Mo2/3Y1/3)2AlC samples with enhanced i-MAX yields of 72±3 wt% (59±2 mol%) and 91±3 wt% (72±2 mol%), respectively. Oxidation of (Mo2/3Y1/3)2AlC at 1300 °C for 12 h leads to formation of a thick, porous oxide of Y2Mo3O12.

Lithium-Sulfur batteries are of great interest owing to sulfur's high theoretical capacity, low-cost and environmental benignity. However, the practicality of these batteries has been impeded due to the infamous “polysulfide-shuttle” effect. Although several research articles in the past decade have focused to mitigate this effect, capacity fade and low coulombic efficiency still prevail. 2D MXenes are early transition metal carbides known for their high metallic conductivity with tunable surface functionalities. MXenes with formula Mn+1XnTx are usually derived from the parental MAX phases by selectively etching the A layer (where M represents a transition metal, A represents the group 13 or 14 elements in the periodic table and X represents carbon or nitrogen, Tx stands for the surface terminations). Thus, the obtained MXenes possess a unique layered structure. After etching, the surface of MXenes is terminated with various functional groups such as hydroxyl (-OH), oxygen (-O), chlorine (-Cl), and fluorine (-F). Rich surface polar sites in Ti3C2Tx enhance electrochemical performance via suppression of polysulfide shuttle. However, past synthesis procedures result in the development of hydrophilic MXene nanosheets unable to be coupled with hydrophobic sulfur. Herein, we propose a new design based on the functionalization of MXene nanosheets with an organic molecule for the effective deposition of sulfur nanoparticles on the surface. This strategy enables the dispersion of MXene sheets in sulfur dissolved solvents enabling the conformal deposition of sulfur on MXene sheets with negligible water in the composite. The metallic sheet-like conductivity (~103 S.cm-1) also aids in faster reaction kinetics during the conversion of polysulfides to Li2S, mitigating the shuttle effect. Consequently, the developed composite enables the efficient utilization of active surface sites to bind polysulfides and alleviate the shuttle effect. This novel cathode delivers a high initial reversible capacity of 1220 mAh·g-1 at 0.1 C and at 0.5C, these cathodes stabilize at 880mAh·g-1 after the first 5 cycles, 85% of which is retained post 500 cycles. Additionally, we developed cathodes with high sulfur loading of ~10.7 mg·cm-2 that delivers a stable areal capacity of ~7 mAh·cm-2 for 150 cycles. Moreover, these cathodes in a pouch cell configuration retained ~770 mAh·g-1 after ~200 cycles at 0.2C (85.5% retention). Ex-situ studies elucidated the nature of the LiPs–MXene interaction and the effect of surface functionalization towards improved performance +RP and VN have equal contribution

Samples of transparent polycrystalline spinel with average grain size varying from 0.14 to 170 μm were prepared by different sintering approaches. The effect of grain size on the flexural strength, hardness and Hugoniot elastic limit (impact loading) was investigated. It was found that values of hardness divided by three for samples with grain size in the 0.14–15 μm range were almost equal to the dynamic yield strength values, estimated based on the Hugoniot elastic limit. This led to the assumption that the onset of inelastic deformation at the Hugoniot elastic limit was brittle rather than ductile. The observed departure of the dynamic yield strength from the hardness divided by three value for ceramics with grain size >15 μm was associated with either impact-induced shear banding or twinning. The feasibility of such banding/twinning intervention in initiating inelastic deformation in the spinel is supported by the values of apparent Hall-Petch coefficients in the corresponding grain size domains.

A spark plasma sintering (SPS) apparatus was used to perform uniaxial compressive creep tests on dense SPS-processed fine-grained alumina. Experiments were carried out in the 1125–1250 °C temperature range under an applied stress of 80–120 MPa. Creep rates, stress exponent and apparent activation energy were determined. The microstructure of deformed samples was characterized by HRSEM. The creep rates, stress exponent (1.9–2.1) and apparent activation energy (454 kJ/mol) values obtained are in a good agreement with data reported in the literature regarding creep of fine-grained polycrystalline alumina. These results, together with microstructural observations, suggest that the creep mechanism involved was grain boundary sliding (GBS), accommodated by dislocation climb, controlled by diffusion along the grain boundaries. It was thus demonstrated that an SPS apparatus can be employed as an accurate high-temperature creep testing system.

The authors of this paper have recently shown that deformation in layered solids is not mediated by basal dislocations, as conventional wisdom would have it, but rather by a new mechanism labeled ripplocations. The latter are defects that form in layered materials as a consequence of confined buckling. Here, using shock wave compression---at extremely high strain rates---of Ti${}_{3}$SiC${}_{2}$, a member of the nanolayered MAX-phase family, the authors show that in some ways Ti${}_{3}$SiC${}_{2}$ behaves like a ductile metal but, in others, it behaves more like a hard, brittle ceramic. In other words, the unique dual-nature behavior of Ti${}_{3}$SiC${}_{2}$ is like nothing seen before. The authors attribute this behavior to its nanolayered structure and believe it reflects how ripplocations respond to very high strain rates. This work not only provides new results on the dynamic mechanical properties of Ti${}_{3}$SiC${}_{2}$, but is a critical first step towards understanding the response of ripplocations in layered solids to high strain rates.

MXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti3 C2 Tz and V2 CTz , by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.

Clay-reinforced nylon-6 nanocomposites (NCs)-characterized by the full exfoliation of the nanoreinforcement-were introduced in the marketplace in the 1990s. Herein, we demonstrate, for the first time, that Ti

Contrary to the characteristic strengthening of polycrystalline ceramics with a decrease in grain size, extremely fine nanocrystalline ceramics exhibit softening, increased plasticity and sometimes an inverse Hall-Petch relation. Despite experimental evidence, questions remain regarding the underlying deformation mechanisms governing this abnormal mechanical behavior. In the present study, an in-depth microstructural examination was performed on nanostructured transparent magnesium aluminate spinel (MgAl2O4) subjected to microhardness tests. Microstructural observations revealed regions strained to various degrees below the point of indentation, containing varying amounts of dislocations and nano-cavities. Furthermore, the residual strain in different areas was estimated by local electron diffraction. This provided evidence supporting the roles of both dislocation motion and grain boundary sliding (GBS) mechanisms. Moreover, shear bands formed and were found to be associated with micro-cracking. By combining the microstructural analysis with suitable models, it was concluded that these mechanisms govern plastic deformation. By elucidating how strain is accommodated within nanocrystalline ceramics, a deeper understanding of their unique mechanical behavior is gained.

Herein we report on the reactivity between palladium, Pd, and the MAX phases, Ti${_2}$AlC, Ti${_3}$AlC${_2}$, Ti${_3}$SiC${_2}$ and Cr${_2}$AlC. Diffusion couples of Pd/MAX were heated to 900 {\deg}C under uniaxial stress of ~20 MPa for 2, 4, and 10 h in a vacuum (, Comment: 8 pages, 11 figures

Alumina forming, oxidation and thermal shock resistant MAX phases are of a high interest for high temperature applications. Herein we report, on bonding and resulting interactions between a Ni-based superalloy, NSA, and two alumina forming MAX phases. The diffusion couples Cr2AlC/Inconel-718/Ti2AlC were assembled and heated to 1000 or 1100 °C in a vacuum hot press under loads corresponding to stresses of either 2 MPa or 20 MPa. The resulting interfaces were examined using X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Good bonding between Cr2AlC and NSA was achieved after hot pressing at 1000 °C and a contact pressure of only 2 MPa; in the case of Ti2AlC a higher temperature (1100 °C) and pressure (20 MPa) were needed. In both cases, a diffusion bond, in which mainly Ni and Cr out diffused from the NSA into the MAX phase and a concomittant out diffusion of Al from the latter, was realized with no evidence of interfacial damage or cracking after cooling to room temperature. The reactions paths were determined to be: Cr2AlC/Cr7C3/Cr7C3,β-NiAl/α-Cr(Mo)/NSA and Ti2AlC/Ti2AlC,Ti3NiAl2C/β-NiAl/α-Cr(Mo)/NSA. Twenty thermal cycles from room temperature to 1000 °C showed that Ti2AlC is a poor oxidation barrier for Inconel-718. However, in the case of Cr2AlC no cracks, delamination nor surface degradation was observed, suggesting that this material could be used to protect Inconel-718 from oxidation.

The MAX phases comprise of a group of layered ternary carbides that exhibit unique mechanical properties which bridge the gap between their metal and ceramic constituents. To study the effects of the global grain orientation, Ti, Si and TiC powders were hot pressed to synthesize highly oriented bulk Ti3SiC2. X-ray diffraction (XRD) was used to verify the grain orientation and a Lotgering factor of 0.87 with respect to the c-axis was obtained. Prepared Ti3SiC2 samples have been compressed in two orientations, loading along the c-axis ( ∥ c-axis) and perpendicular to the c-axis ( ⊥ c-axis) at 10 − 3 s − 1 using a standard load frame and at 10 2 s − 1 using a Kolsky (split-Hopkinson) bar. The average compressive strength along the ⊥ c-axis orientation was 761 MPa under quasi-static conditions and 987 MPa under dynamic loading, exhibiting a 30% increase on average. The ∥ c-axis orientation exhibited no rate dependence in compressive strength; however both orientations exhibited an increase of strain at failure under dynamic conditions by over 0.5%, on average. The orientation-dependent failure behavior at different strain rates were examined using high-speed imaging and 2D digital image correlation (DIC) during loading and via scanning electron microscopy (SEM) post-mortem. Results indicate that the ⊥ c-axis fracture surface exhibited a mixture of transgranular and intergranular cracks, kink bands and delaminations, whereas ∥ c-axis was limited to a combination of intergranular and transgranular cracks. Such fracture distinctions due to the availability (or lack thereof) for kink band formation appear to be responsible for the anisotropic compressive behavior.

The present study focused on the fabrication of bulk materials from Ti 1− x Al x N nano-powders using a spark plasma sintering (SPS) apparatus. Super-saturated Ti 1− x Al x N solid solutions containing differing fractions of AlN (10, 20, 30 and 50 mol%) were synthesized by high-energy ball milling (HEBM) of pure nitrides. The complete dissolution of AlN in TiN was achieved after 100 h of milling. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-filtered transmission electron microscopy spectra imaging and energy dispersive X-ray spectroscopy. The crystalline size of the mechanically alloyed powders after 100 h of milling was about 12–14 nm. Ti 1− x Al x N powders of various compositions were sintered by SPS under pressure of 63 MPa at 1673 K. Maximal hardness and bending strength values (610 MPa and 18.6 GPa, respectively) were obtained for composites containing 20 mol%AlN. Powder with 20% mole%AlN was consolidated under pressure of 500 MPa in the 1273–1423K temperature range by high pressure SPS (HPSPS). A fully dense nano-structured specimen, processed at 1423 K, displayed a Young modulus of 420 GPa, hardness of 20.5 GPa, bending strength of 670 MPa and fracture toughness of 7.1 MPa m 0.5 .

High-pressure spark plasma sintering (HPSPS) was employed to fabricate polycrystalline Nd:YAG specimens with desired functional properties. Specimens fabricated under a uniaxial pressure of 300 MPa at 1300°C at a heating rate of 50°C/min and holding time of 60 min displayed submicrometer microstructure and elevated mechanical properties, including resistance to thermal shock. Optical properties (i.e., spectral transmittance, fluorescence emission spectra and fluorescence lifetime) of the HPSPS-processed specimens were close to those obtained with specimens fabricated by conventional sintering procedure. Specifically, remarkable differences in threshold power and laser slope efficiency were found and attributed to the variance in Nd concentration in the specimens tested. The results of this study indicate that the low cost and timesaving HPSPS process allows for the fabrication of polycrystalline Nd:YAG specimens with optical properties suitable for laser applications.

Inverse Hall-Petch behavior was observed in a transparent nanocrystalline magnesium aluminate spinel ceramic with grain size lower than 30 nm fabricated by high pressure spark plasma sintering (HPSPS). The ceramic was consolidated from 12 nm powder, synthesized by a solution combustion method and displayed an optical transmission of 80% at 550 nm wavelength. The hardness values of the specimens followed the Hall-Petch relation down to a grain size of about 30 nm, reaching a maximal hardness of 20 GPa. The hardness value decreased from 20 to 17.6 GPa when grain size decreased to 17 nm.

Herein we report on the phases and microstructures obtained when Ni and commercial Ti2AlC powder mixtures - with Ti2AlC molar fractions of 0.1, 0.5 and 0.64 – are hot pressed (HPed) at 1050 °C for 2 h at a stress of 36 MPa in a vacuum hot press and then further annealed either at 1200 °C for 16 h or at 1350 °C for 16 h under the protection of high purity argon, Ar. After HPing, the 0.1 Ti2AlC composite sample was comprised of a Ni-based solid-solution with some dissolved Ti, fine TiCx particles and γ′ precipitates (Ni3(Al,Ti)). Heat treating at 1200 °C for 16 h did not change the phase content. However, after annealing at 1350 °C for 16 h, the TiCx phase substantially coarsened and become the major phase. HPing of the 0.5 Ti2AlC mixture, resulted in the formation of sub-stoichiometric TiCx, NiAl, Ti3NiAl2C, Ni2AlTi, and a small amount of Ti3AlC2. After annealing at 1200 °C for 16 h, the Ti3AlC2 phase disappeared. After the 1350 °C anneal, the Ni2AlTi phase was no longer detected and the microstructure was comprised of coarsened TiCx particles dispersed in a Ti-solutioned NiAl matrix with a small fraction of Ti3NiAl2C. Lastly, after HPing, the 0.64 Ti2AlC mixture, the phases present were TiCx, Ti3NiAl2C, NiAl, small amounts of Ti3AlC2 and traces of unreacted Ti2AlC. After annealing at 1200 or 1350 °C for 16 h, the Ti3AlC2 and Ti2AlC phases disappear, and a new ternary Al-rich aluminide, Ti6Al16Ni7 phase emerges. The final microstructure was thus comprised mostly of Ti6Al16Ni7 and TiCx. The reactions resulting in these compositions are developed.

Summary To date, MXene dispersions have been mostly limited to polar solvents. Here, we show that when the lithium cations present between MXene multilayers after etching are exchanged with di(hydrogenated tallow)benzyl methyl ammonium chloride (DHT), they become organophilic and form highly stable colloidal suspensions in nonpolar solvents. The rapid cation exchange occurs under ambient conditions and the resulting two-dimensional flakes are well dispersed and remain oxide free. A 142 ± 0.7-μm thick film (≈10 mg ⋅ cm−2) made from a DHT-Ti3C2Tz suspension in toluene exhibits pseudocapacitive behavior, with a capacitance of 305 F ⋅ g−1 at a scan rate 2 mV ⋅ s−1. The moduli and tensile strengths of solution-processed polyethylene nanocomposites are increased by ≈11% and 32% with a loading of ≈1 vol% DHT-Ti3C2Tz. Our approach may enable the use of MXenes in multiple research and industrial fields.

A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200 °C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200 °C at a heating rate of 5°/min, a holding time of 15 min and an applied pressure of 350–400 MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).

The present study deals with a comprehensive comparison of the mechanical and functional properties of Nd:YAG single crystals with those of the polycrystalline ceramics (PCs), undoped and LiF-doped, processed by Spark Plasma Sintering (SPS). The polycrystalline ceramics have higher mechanical properties (hardness, bending strength and thermal shock resistance) than the single crystals. The optical transmittance of the LiF-doped PC Nd:YAG is significantly higher than that of the undoped one and is close to that of the single crystal. With respect to other optical and thermal properties, i.e. refraction index, absorption coefficient, extinction ratio, thermo-optic coefficient, fluorescence and thermal conductivity, no significant differences were observed between the single crystals and the polycrystalline ceramic.

Transparent Co 2+ doped MgAl 2 O b4 spinel was fabricated by SPS consolidation followed by and HIP treatment. It was established that HIP treatment significantly improved transparency of the ceramic in a wide range of wavelengths, especially, in a range, which is relevant for Q-switching. Nonlinear absorption was demonstrated and the ground and excited state absorption cross sections were estimated. The positive effect of the HIP treatment on the optical properties is related to an elimination of extremely fine porosity and to the location of Co ions at Mg 2+ sites in the spinel ionic structure. The experimental results indicate that the fabricated specimens can be used as a passive laser Q-switching material.