Nitrogen‐doped carbon nanofiber webs (CNFWs) with high surface areas are successfully prepared by carbonization‐activation of polypyrrole nanofiber webs with KOH. The as‐obtained CNFWs exhibit a ...superhigh reversible capacity of 943 mAh g−1 at a current density of 2 A g−1 even after 600 cycles, which is ascribed to the novel porous nanostructure and high‐level nitrogen doping.
Mo2C, the newly synthesized MXene with a large lateral size and superconductivity property, has attracted increasing interest in material science. Employing first-principles density functional ...calculations, its intrinsic structural, electrical, thermal, and mechanical properties are investigated in this work. It is found that this MXene is nonmagnetic with a small molar volume. The electrical conductivity is predicted in the order of 106 Ω–1m–1, and its value is significantly influenced by doping. For thermal conductivity, both of the electron and phonon contributions are studied. At room temperature, the Mo2C’s thermal conductivity is determined to be 48.4 Wm–1 K–1, which can be further enhanced by increasing temperature and introducing n-type dopants. The specific heat and thermal expansion coefficient are also assessed, and their values at room temperature are calculated as 290 Jkg–1 K–1 and 2.26 × 10–6 K–1, respectively. Moreover, the thermal contraction of the MXene is found at low temperatures. Under biaxial strains, the elastic modulus is predicted as 312 ± 10 GPa, and the ideal strength is determined to be 20.8 GPa at a critical strain of 0.086. In view of the small molar volume, superhigh electrical conductivity, favorable thermal conductivity, low thermal expansion coefficient, and high mechanical strength, the Mo2C MXene generally merits more widespread applications besides superconductors, such as applying to substrates for other layer materials, and candidate materials for batteries and supercapacitors.
A novel miniaturised dual-band bandpass filter (BPF) based on defected split-ring resonators (SRRs) and irregular stepped-impedance resonators (SIRs) is proposed. By applying defected SRRs embedded ...with eight combinations of three whole or broken grooves, which are formerly used for divination, the passband of 2.35–2.52 GHz with wideband harmonic suppression is achieved for WLAN applications. To create one more passband of 5.09–5.3 GHz, the irregular SIRs are etched inside the defected SRRs. Finally, such a prototype is designed and fabricated with the dimension of only 20 × 20 mm2 (0.27λg × 0.27λg). The measured results of this dual-band BPF agree well with the simulations verifying the proposed idea.
The electrochemical properties of sulfur cathodes based on commercially available sulfur powder (S) and water-soluble binder have been investigated. The mixture of styrene butadiene rubber (SBR) and ...sodium carboxyl methyl cellulose (CMC) is used as the binder. Compared with conventional poly(vinylidene fluoride) (PVDF) binder, the SBR–CMC binder significantly improves cycling performance of the sulfur cathode. A high specific capacity of 580 mA h g–1 after 60 cycles has been achieved. Studies on the electrode slurries show that the SBR–CMC mixture is not only a high adhesion agent but also a strong dispersion medium, which favors the uniform distribution between insulating sulfur and conductive carbon black (CB) and ensures a good electrical contact, leading to a high sulfur utilization. Furthermore, our experiments show that the improvement in cyclability is ascribed to structural stability of the sulfur cathode promoted by the SBR–CMC binder during charge/discharge cycles due to the combined effects of homogeneous distribution of the S and CB particles in the composite cathode, the low electrolyte uptake, and the suppressed agglomeration of Li2S.
MXenes have attracted intensive attention because of their widespread applications. As a well-studied member of the MXene family, Ti2CO2 has been demonstrated to be semiconducting with ultrahigh ...carrier mobility, acting as a candidate material for electronic devices. In this work, the influence of layer thickness on the electrical conductivity of Ti2CO2 is investigated combined with first-principles density functional calculations and the Boltzmann transport theory. Because of the layer interaction-induced band splitting, the band gap of Ti2CO2 generally decreases with increasing layers. Based on the generalized gradient approximation, the band gap in monolayer Ti2CO2 is determined to be 0.260 eV, which decreases to 0.0369 eV in the five-layer configuration. Further, the strain influence on the electronic structure of the multilayer Ti2CO2 is studied. With increasing compression strains perpendicular to the basal plane, the configuration is found to transform from a semiconductor to a semimetal, then to a semiconductor, and at last to a metal. This result implies that the electronic property of the multilayer Ti2CO2 can be efficiently manipulated by strain and that the multilayer configurations could be applied in strain sensors. Moreover, our work may open a door to realize bulk semiconductors through compression of accordion-like multilayer MXenes.
Aluminum nitride (AlN) is an important piezoelectric material for a wide range of applications, but its low piezoelectric coefficient is the main problem it faces. Many efforts are devoted to ...improving its piezoelectric response by alloying transition metals (TMs). On the basis of the density functional theory, the influence of the type and distribution of TM on the piezoelectric response is discussed in this paper. TM0.0625Al0.9375N with twenty-eight different TMs are investigated, and most show higher values of piezoelectric strain modulus d33 than that of AlN. This is because the TM introduces weaker TM-N bonds and locates closer to the centre of three neighbouring N atoms. The location of TM is determined to be significantly correlated with its group number. Alloys of TMxAl1−xN (TM=Sc, Cr, Sr, Mo, Ru and Rh) with varying x are further studied. On the basis of the cost of the TMs and piezoelectric performances, the alloy with Mo is more effective in enhancing d33. A high d33 of 12.3 times that of pure AlN is realized in a metastable configuration of Mo0.167Al0.833N. The distribution of Mo plays a key role in the piezoelectric performance. A higher d33 is more likely to appear in MoxAl1−xN with more Al sublayers containing Mo atoms and with fewer dimers of Mo atoms along the z-axis.
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•Piezoelectric response of AlN is enhanced via alloying of transitional metals (TMs).•Influence of type and distribution of TMs on d33 is discussed for the first time.•A high d33 of 12.3 times that of pure AlN is realized in a metastable Mo-alloy AlN.
HotMAIL! A microwave‐assisted ionic‐liquid (MAIL) method has been developed for controlled production of tellurium nanorods or nanowires (see pictures). The MAIL method is a fast, high‐yield, and may ...be extended to synthesize a variety of other elemental and compound nanostructures.
MXenes have attracted intensive attention in chemistry and material science for their special structures and properties. In order to understand the basic physical properties of the M3C2/M3C2T2 (MSc, ...Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; TF, O, OH) MXenes, first‐principles calculations are carried out to investigate the structural, vibrational, and mechanical properties in this work. Both the metal atoms and surface groups can significantly influence the configurations or mechanical behaviors of the MXenes. The dehydrogenation tendency is calculated to evaluate the possible forms of the M3C2(OH)2 toward M3C2O2. The work functions of MXenes functionalized by different groups are compared, and the lower work functions for the OH functionalized ones, which can be as low as 1.358 eV for the Sc3C2(OH)2, suggest potential good performance in electron emission. In addition, the stability, mechanical properties, and the Raman and infrared (IR) activity modes of the MXenes are reported. Generally, functionalized MXenes would present smaller lattice parameters, lower free energies, and stronger mechanical strength compared to their counterparts. The data obtained may provide important theoretical ground for the investigations of the applications of MXenes.
The structures and dynamically stabilities of the M3C2/M3C2T2 (MSc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; TF, O, OH) MXenes are analyzed and compared. From the energy differences ΔEf of M3C2O2 + H2 and M3C2(OH)2 MXenes, dehydrogenation is more likely to occur for the fifth and sixth period element OH‐terminated MXenes.
Ti
2
CO
2
, as the representative MXene with semiconducting characteristics and ultrahigh carrier mobility, has attracted increasing attention in material science. Herein, various Ti
2
CO
2
alloys ...with Ti displaced by neighboring elements Sc and V, or C by B and N are investigated in this paper based on the first-principles density functional calculations. The structures and mechanical and electronic properties are thoroughly studied for the configurations with varying alloying atom concentrations. The choices of alloying elements play a critical role in determining the lattice parameters and layer thickness. The Sc substitutions generally increase the lattice parameter but decrease the layer thickness. In contrast, the introduction of N presents slight influence on the structural parameters. The mechanical strength shows remarkable variations by introducing the alloying elements. The maximum elastic constant
c
11
is determined to be 425 GPa in (Ti
0.25
V
0.75
)
2
CO
2
, and the corresponding minimum value is only 104 GPa found in (Ti
0.125
Sc
0.875
)
2
CO
2
. With respect to the electronic properties, although B and Sc both present one less valance electron compared to their replaced elements C and Ti, it is easier to realize the
p
-type semiconductor in the configurations containing Sc. Both the V and N substitutions are capable of generating
n
-type semiconductors, but their optimal stoichiometric compositions are quite different. Among all the configurations investigated, only (Ti
0.5
V
0.5
)
2
CO
2
and (Ti
0.375
V
0.625
)
2
CO
2
are magnetic, with their magnetism determined to be 2.61 uB/cell and 1.52 uB/cell, respectively. Thus, the method of alloying neighboring elements provides an effective approach in manipulating the physical properties of the Ti
2
CO
2
, which might widen the possible applications of MXene materials.
