MXeene-Based Ceramic Nanocomposites Enabled by Field-Assisted Sintering

Please click Additional Files below to see the full abstrac

Near-Atomic Scale Perspective on the Oxidation of Ti3_3C2_2Tx_x MXenes: Insights from Atom Probe Tomography

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties and great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography analysis of as-synthesized Ti$_3$C$_2$T$_x$ MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalies enriched in TiO$_2$ nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of atom probe tomography to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties

Isothermal Oxidation of Ti3Al0.6Ga0.4C2 MAX Phase Solid Solution in Air at 1000 °C to 1300 °C

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 grain

Exploring the capabilities of high-pressure spark plasma sintering (HPSPS): A review of materials processing and properties

Spark plasma sintering (SPS) is an advanced pressure-assisted sintering technology that combines the application of uniaxial pressure with rapid current-induced heating. The so-called high-pressure SPS (HPSPS) approach involves using specialized tooling made of robust materials that can withstand high pressures and temperatures simultaneously. The application of high pressure during the sintering process enhances densification and allows to produce materials with distinctive qualities at relatively low temperatures. This review focuses on the effects of the applied pressure on densification and the resulting functional, mechanical, optical, and physical properties. Exploring the capabilities of HPSPS for a wide range of materials. Including, but not limited to, thermally sensitive phases, nanocrystalline, ionic, bulk metallic glasses, magnetic, transparent ceramic, and composite materials, among others. The HPSPS approach not only offers a promising technique for densification, but also enables the study of fundamental aspects of high-pressure processing and various consequential materials properties

Fabrication of Polycrystalline Transparent Co

Transparent Co2+ doped MgAl2Ob4 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 Mg2+sites in the spinel ionic structure. The experimental results indicate that the fabricated specimens can be used as a passive laser Q-switching material

Fabrication of Polycrystalline Transparent Co+2: MgAl2O4 by a Combination of Spark Plasma Sintering (SPS) and Hot Isostatic Pressing (HIP) Processes

Transparent Co2+ doped MgAl2Ob4 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 Mg2+sites in the spinel ionic structure. The experimental results indicate that the fabricated specimens can be used as a passive laser Q-switching material

Creep of Polycrystalline Magnesium Aluminate Spinel Studied by an SPS Apparatus

A spark plasma sintering (SPS) apparatus was used for the first time as an analytical testing tool for studying creep in ceramics at elevated temperatures. Compression creep experiments on a fine-grained (250 nm) polycrystalline magnesium aluminate spinel were successfully performed in the 1100–1200 °C temperature range, under an applied stress of 120–200 MPa. It was found that the stress exponent and activation energy depended on temperature and applied stress, respectively. The deformed samples were characterized by high resolution scanning electron microscope (HRSEM) and high resolution transmission electron microscope (HRTEM). The results indicate that the creep mechanism was related to grain boundary sliding, accommodated by dislocation slip and climb. The experimental results, extrapolated to higher temperatures and lower stresses, were in good agreement with data reported in the literature