Herein, Ti
2
CT
x
MXene and its derivatives with various heterogeneous structures were constructed
via
etching and a facile oxidation treatment. The effect of different oxidation conditions on their ...structural evolution and phase composition was studied in detail. Compared with that of pristine Ti
2
CT
x
MXene, the improvement in the electromagnetic wave absorption capability of the as-prepared Ti
2
CT
x
/TiO
2
and C/TiO
2
nanocomposites was attributed to their enhanced polarization loss and stronger conductivity loss. The enhanced polarization loss is caused by the generated heterogeneous interfaces and higher specific surface area, and the stronger conductivity loss is due to the completely exfoliated carbon layers. Additionally, the remaining multilayered structure after exfoliation of the carbon layers favors energy dissipation. The C/TiO
2
nanocomposites attain a minimum reflection coefficient of −50.3 dB at 7.1 and 14.2 GHz, and an effective absorption bandwidth of 4.7 GHz (covering the whole X-band) with a matching thickness of 2.1 mm; this indicates their excellent electromagnetic wave absorption properties. We believe that these nanocomposites with a heterogeneous structure also hold great promise for application in the fields of photocatalysis, lithium batteries, water purification,
etc.
Herein, Ti
2
CT
x
MXene and its derivatives with various heterogeneous structures were constructed
via
etching and a facile oxidation treatment.
Lightweight and mechanically flexible materials that can provide efficient electromagnetic interference (EMI) shielding are highly desirable for protecting portable and smart electronic devices ...against electromagnetic pollution. Here, the authors report a tunable design of a three‐dimensional (3D) porous aerogel structure made of 2D transition metal carbides and a carbonitride (MXene) with a long‐range order of aligned lamellar architecture for EMI shielding. Bidirectional freeze‐casting of MXene colloidal solutions is used to fabricate robust, compressible and lightweight aerogels, and achieve orientational assembly leading to outstanding EMI shielding performance and tunable ratio of reflection to absorption. EMI shielding effectiveness (SE) of three types of MXene aerogels (Ti3C2Tx, Ti2CTx, and Ti3CNTx) reaches 70.5, 69.2, and 54.1 dB, respectively, while keeping the compression thickness at 1 mm and a density of only ≈11.0 mg cm−3. The highest specific SE reaches 8818.2 dB cm3 g−1, which is among the best values reported for EMI shielding materials. More importantly, during the compression process of the MXene aerogels, the ratio of electromagnetic wave reflection to absorption increases without noticeable change of the total EMI SE. Compressible MXene aerogels with aligned layers offer an effective approach to control the electromagnetic wave absorption and reflection in EMI shielding materials.
Anisotropic MXene aerogels (Ti3C2Tx, Ti2CTx, and Ti3CNTx) with a long‐range ordered lamellar architecture are prepared using a bidirectional freeze‐casting technique. The ratio of reflection to absorption can be adjusted simply by compressing MXene aerogels.
Pseudocapacitive charge storage with Ti3C2T x in protic electrolytes has received significant attention. However, other MXene compositions have received less attention so far. Additionally, ...pseudocapacitance of MXenes has only been reported in acidic electrolytes. Herein, we report on the pseudocapacitance of two vanadium carbide MXenes (V2CT x and V4C3T x ) in various basic electrolytes and sulfuric acid, showing distinct redox couples in their cyclic voltammograms. Freestanding V2CT x film electrodes could deliver gravimetric capacitances above 250 F g–1 in different basic electrolytes with the highest capacitance of 386 F g–1 in 1 M LiOH at 2 mV s–1. Moreover, the cycle life performance showed an increasing capacitance over thousands of cycles (121% of initial capacitance after 60,000 cycles at 10 A g–1 in 6 M KOH). Both materials also exhibit higher capacitances than Ti3C2T x in 3 M sulfuric acid, with 475 and 284 F g–1 for V2CT x and V4C3T x , respectively.
Versatility of infrared properties of MXenes Han, Meikang; Zhang, Danzhen; Singh, Akash ...
Materials today (Kidlington, England),
April 2023, 2023-04-00, Letnik:
64
Journal Article
Recenzirano
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Selective and ultrathin films and coatings capable of controlling infrared (IR) emission are crucial for highly integrated thermal management systems, but are challenging produce ...using conventional materials. Here, we report that the MXene family of two-dimensional carbides and carbonitrides offers a broad range of IR emissivity values (∼0.06–0.59) with diverse colors, varying with MXene composition and structure. Specifically, 200 nm thick purple Ti3C2Tx coating has an average IR emissivity of 0.06, while gold Nb2CTx coating is 0.59 at wavelengths from 3-25 μm. We demonstrate that the IR emissivity can be finely tuned by combining different metals in solid-solution MXenes. Furthermore, the IR identification capability at varying temperatures was validated using different MXene coatings and patterned MXene fabrics. The versatility of MXenes at optical and infrared wavelengths provides a platform for developing MXene-based smart, flexible devices and wearables capable of selective and localized thermal management, aiming at radiative heating/cooling, IR identification, photothermal conversion, and thermal imaging.
Nano SiC modified silicon oxycarbide (n-SiC/SiOC) ceramics were prepared through the pyrolysis of a mixture of liquid polysiloxane and n-SiC with an average grain diameter of 30 nm. After adding ...n-SiC, palingenetic SiC nanograins with an average grain diameter smaller than 10 nm, and nanosized free carbon were gradually separated from the amorphous SiOC phase when the annealing temperature increased from 1100 °C to 1450 °C. The various interfaces among n-SiC,
in situ
formed SiC nanograins, nanosized carbon and amorphous SiOC phases can obtain interfacial scattering. Eventually, the electric dipole polarization and interfacial scattering enhanced the absorption properties. The minimal reflection coefficient (RC
min
) of the n-SiC/SiOC ceramics annealed at 1400 °C (n-SiC/SiOC-1400) reached −61 dB at 8.6 GHz. The widest effective absorption bandwidth (EAB) reached 3.5 GHz in the X-band, which indicates that the n-SiC/SiOC ceramics can be considered as high-performance microwave absorbing materials because of the strong absorption capability and wide absorption bandwidth.
Nano SiC modified silicon oxycarbide (n-SiC/SiOC) ceramics had special microstructure and phase composition that can lead to electric dipole polarization and interfacial scattering which eventually resulted in the strong absorption capability of n-SiC/SiOC.
MXenes occupy a leading position among materials capable of providing lightweight shielding against electromagnetic interference (EMI) owing to their outstanding metallic conductivity, low density, ...tunable surface chemistry, and easy solution processing. In this work, we demonstrate that multiple interfaces of segregated structure in MXene composites enhance the absorption of electromagnetic waves. Ti3C2Tx MXene composite films with segregated polystyrene (PS) inclusions were fabricated via a simple procedure that involves mixing conductive MXene flakes and insulative PS beads in an aqueous medium followed by vacuum filtration and hot pressing. As surface area of multiple interfaces increases, total EMI shielding effectiveness significantly increases at the same volume fraction of PS inclusions. Therefore, the improved shielding efficiency can be attributed to enhanced absorption of electromagnetic waves resulting from strong multiple reflections at internal interfaces of the segregated structure. This work highlights future challenges and provides guidelines toward new structural designs for next-generation shielding materials with tunable electromagnetic wave absorption properties.
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Strontium-doped barium aluminosilicate ceramics were prepared by the sol-gel and hot-pressing processes. The experimental results show that doped Sr promotes the hexacelsian-to-celsian phase ...transformation of barium aluminosilicate ceramics. When the mass ratio of Sr increases, the dielectric constant of the samples decreases from 6.03 to 5.37, which is accompanying by the slight increase of tangent loss. These are attributed to the lower polarizability of Sr ion and its oxides. The infrared emissivity presents a tread of first increasing and then decreasing, owing to the effects of doped Sr on crystal structure and phase transformation of barium aluminosilicate ceramics.
•Doped Sr promotes the phase transformation from hexagonal phase to monoclinic phase.•Doped Sr decreases the dielectric constant of BSAS from 6.03 to 5.37.•Doped Sr decreases the infrared emissivity of BSAS from 0.78 to 0.54.
The porous alumina ceramics with closed pores were fabricated using C@Al2O3 microspheres as pore-forming agent. The closed pores with a diameter about 5μm were successfully obtained in the dense ...matrix, owing to the additive C@Al2O3 microspheres. Different amount of C@Al2O3 microspheres was added to investigate the effects of closed pores on thermal, dielectric and mechanical properties. With the increasing microspheres amount, the closed porosity of alumina ceramics ranges from 5.82 to 14.5%, the thermal conductivity decreases continuously at the temperature ranging from 30 to 1200°C, and the flexural strength and fracture roughness decrease. As the closed porosity increases, the real permittivity decreases, while the tangent loss has a slight increase. When the mass ratio of C@Al2O3 microspheres is 10wt.%, the porous alumina ceramics with the thermal conductivity (13.07–3.8W·m−1·K−1), the flexural strength (97.05±18MPa), and the fracture roughness (2.65±0.13MPa·m1/2) can be obtained.
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•The closed pores in alumina ceramics were achieved using core-shell structured C@Al2O3 microspheres as pore-forming agent.•The closed pores significantly decreased the thermal conductivity to 3.8W·m−1·K−1 at 1200°C.•The closed pores decreased the real permittivity from 9.74 to 7.48 with a slight increase of the imaginary permittivity.
Achieving pseudocapacitive intercalation in MXenes with neutral aqueous electrolytes and driving reversible redox reactions is scientifically appealing and practically useful. Here, we report that ...the partial oxidation of MXene intensifies pseudocapacitive Li+ intercalation into Ti3C2T x MXene from neutral water-in-salt electrolytes. An in situ X-ray absorption near-edge structure analysis shows that the Ti oxidation state changes during the Li+ intercalation, indicating the presence of a surface redox reaction. The Ti oxidation/reduction is further confirmed by an in situ extended X-ray absorption fine structure analysis, which shows a reversible contraction/expansion of the Ti–C interatomic distance. The intensified Li+ pseudocapacitive intercalation can be explained by the higher oxidation state of Ti at the open circuit potential. This work demonstrates the possibility of tuning the pseudocapacitive intercalation by adjusting the initial oxidation state of the transition metal on the MXene and offers a facile way to enhance the pseudocapacitance of various MXenes.
Two-dimensional transition metal carbides, nitrides and carbonitrides, popular by the name MXenes, are a promising class of materials as they exhibit intriguing optical, optoelectronic and ...electrochemical properties. Taking advantage of their metallic conductivity and hydrophilicity, titanium carbide MXenes (Ti3C2Tx and others) are used to fabricate solution processable transparent conducting electrodes (TCEs) for the design of three-electrode electrochromic cells. However, the tunable electrochromic behavior of various titanium-based MXene compositions across the entire visible spectrum has not yet been demonstrated. Here, we investigate the intrinsic electrochromic properties of titanium-based MXenes, Ti3C2Tx, Ti3CNTx, Ti2CTx, and Ti1.6Nb0.4CTx, where individual MXenes serve as a transparent conducting, electrochromic, and plasmonic material layer. Plasmonic extinction bands for Ti3C2Tx, Ti2CTx and Ti1.6Nb0.4CTx are centered at 800, 550 and 480 nm, which are electrochemically tunable to 630, 470 and 410 nm, respectively, whereas Ti3CNTx shows a reversible change in transmittance in the wide visible range. Additionally, the switching rates of MXene electrodes with no additional transparent conductor electrodes are estimated and correlated with the respective electrical figure of merit values. This work demonstrates that MXene-based electrochromic cells are tunable in the entire visible spectrum and suggests the potential of the MXene family of materials in optoelectronic, plasmonic, and photonic applications, such as tunable visible optical filters and modulators, to name a few.
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