Context. Since 1995, more than 1500 exoplanets have been discovered around a wide variety of host stars (from M- to A-type stars). Tidal dissipation in stellar convective envelopes is an important ...factor that shapes the orbital architecture of short-period systems. Aims. Our objective is to understand and evaluate how tidal dissipation in the convective envelope of low-mass stars (from M to F types) depends on their mass, evolutionary stage, and rotation. Methods. Using a simplified two-layer assumption, we analytically compute the frequency-averaged tidal dissipation in the convective envelope. This dissipation is due to the conversion into heat of the kinetic energy of tidal non-wavelike/equilibrium flow and inertial waves because of the viscous friction applied by turbulent convection. Using grids of stellar models allows us to study the variation of the dissipation as a function of stellar mass and age on the pre-main sequence and on the main sequence for stars with masses ranging from 0.4 to 1.4 M⊙. Results. During their pre-main sequence, all low-mass stars have an increase in the frequency-averaged tidal dissipation for a fixed angular velocity in their convective envelope until they reach a critical aspect and mass ratios (respectively α = Rc/Rs and β = Mc/Ms, where Rs,Ms,Rc, and Mc are the star’s radius and mass and its radiative core’s radius and mass). Next, the dissipation evolves on the main sequence to an asymptotic value that is highest for 0.6 M⊙ K-type stars and that then decreases by several orders of magnitude with increasing stellar mass. Finally, the rotational evolution of low-mass stars strengthens the importance of tidal dissipation during the pre-main sequence for star-planet and multiple star systems. Conclusions. As shown by observations, tidal dissipation in stars’ convection zones varies over several orders of magnitude as a function of stellar mass, age, and rotation. We demonstrate that i) it reaches a maximum value on the pre-main sequence for all stellar masses and ii) on the main sequence and at fixed angular velocity, it is at a maximum for 0.6 M⊙ K-type stars and decreases with increasing mass.
A monomodal model for stellar and planetary convection is derived for the magnitude of the rms velocity, degree of superadiabaticity, and characteristic length scale as a function of rotation rate as ...well as with thermal and viscous diffusivities. The convection model is used as a boundary condition for a linearization of the equations of motion in the transition region between convectively unstable and stably stratified regions, yielding the depth to which convection penetrates into the stable region and establishing a relationship between that depth and the local convective Rossby number, diffusivity, and pressure scale height of those flows. Upward and downward penetrative convection have a similar scaling with rotation rate and diffusivities, but they depend differently upon the pressure scale height, due to the differing energetic processes occurring in convective cores of early-type stars versus convective envelopes of late-type stars.
Context. The traditional approximation of rotation (TAR) is a treatment of the dynamical equations of rotating and stably stratified fluids in which the action of the Coriolis acceleration along the ...direction of the entropy (and chemicals) stratification is neglected, while assuming that the fluid motions are mostly horizontal because of their inhibition in the vertical direction by the buoyancy force. This leads to the neglect of the horizontal projection of the rotation vector in the equations for the dynamics of gravito-inertial waves (GIWs) that become separable, such as in the non-rotating case, while they are not separable in the case in which the full Coriolis acceleration is taken into account. This approximation, first introduced in geophysical fluid dynamics for thin atmospheres and oceans, has been broadly applied in stellar (and planetary) astrophysics to study low-frequency GIWs that have short vertical wavelengths. The appoximation is now being tested thanks to direct 2D oscillation codes, which constrain its domain of validity. The mathematical flexibility of this treatment allows us to explore broad parameter spaces and to perform detailed seismic modelling of stars. Aims. The TAR treatment is built on the assumptions that the star is spherical (i.e. its centrifugal deformation is neglected) and uniformly rotating while an adiabatic treatment of the dynamics of the waves is adopted. In addition, their induced gravitational potential fluctuations is neglected. However, it has been recently generalised with including the effects of a differential rotation. We aim to carry out a new generalisation that takes into account the centrifugal acceleration in the case of deformed stars that are moderately and uniformly rotating. Methods. We construct an analytical expansion of the equations for the dynamics of GIWs in a spheroidal coordinates system by assuming the hierarchies of frequencies and amplitudes of the velocity components adopted within TAR in the spherical case. Results. We derive the complete set of equations that generalises TAR by taking the centrifugal acceleration into account. As in the case of a differentially rotating spherical star, the problem becomes 2D but can be treated analytically if we assume the anelastic and JWKB approximations, which are relevant for low-frequency GIWs. This allows us to derive a generalised Laplace tidal equation for the horizontal eigenfunctions and asymptotic wave periods, which can be used to probe the structure and dynamics of rotating deformed stars thanks to asteroseismology. A first numerical exploration of its eigenvalues and horizontal eigenfunctions shows their variation as a function of the pseudo-radius for different rotation rates and frequencies and the development of avoided crossings.
Until now, MXenes could only be produced from MAX phases containing aluminum, such as Ti3AlC2. Here, we report on the synthesis of Ti3C2 (MXene) through selective etching of silicon from titanium ...silicon carbide—the most common MAX phase. Liters of colloidal solutions of delaminated Ti3SiC2‐derived MXene (0.5–1.3 mg mL−1) were produced and processed into flexible and electrically conductive films, which show higher oxidation resistance than MXene synthesized from Ti3AlC2. This new synthesis method greatly widens the range of precursors for MXene synthesis.
Electrically conductive films: Synthesis of Ti3C2 (MXene) through oxidant‐assisted selective etching of Si from Ti3SiC2 has the potential to enable synthesis of new MXenes from other Si‐containing MAX precursors. This synthesis method greatly widens the range of precursors for MXene synthesis and expands the growing family of 2D materials to include compositions which could not be synthesized previously.
Abstract
Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance. In particular, the kinetics of ion transport ...in organic electrolytes is slow, especially at low operating temperatures. Herein, we report a new type of MXene-carbon nanotube (CNT) composite electrode that maximizes ion accessibility resulting in exceptional rate performance at low temperatures. The improved ion transport at low temperatures is made possible by breaking the conventional horizontal alignment of the two-dimensional layers of the MXene Ti
3
C
2
by using specially designed knotted CNTs. The large, knot-like structures in the knotted CNTs prevent the usual restacking of the Ti
3
C
2
flakes and create fast ion transport pathways. The MXene-knotted CNT composite electrodes achieve high capacitance (up to 130 F g
−1
(276 F cm
−3
)) in organic electrolytes with high capacitance retention over a wide scan rate range of 10 mV s
−1
to 10 V s
−1
. This study is also the first report utilizing MXene-based supercapacitors at low temperatures (down to −60 °C).
MXenes are a class of two‐dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high‐rate pseudocapacitive energy storage. However, the effects that ...irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti3C2Tx, Tx refers to ‐F, =O, ‐Cl and ‐OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s−1 (with respect to the lowest scan rate of 5 mV s−1) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti3C2Tx is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti3C2Tx. This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.
Good made better: Controllable anodic oxidation of 2D Ti3C2Tx improves the rate performance of supercapacitor electrodes. The capacitance retention at 2000 mV s−1 increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation.
Although significant progress has been achieved in understanding of ion-exchange mechanisms in the new family of 2D transition metal carbides and nitrides known as MXenes, direct gravimetric ...assessment of water insertion into the MXene interlayer spaces and mesopores has not been reported so far. Concurrently, the latest research on MXene and Birnessite electrodes shows that nanoconfined water dramatically improves their gravimetric capacity and rate capability. Hence, quantification of the amount of confined water in solvated electrodes is becoming an important goal of energy-related research. Using the recently developed and highly sensitive method of in situ hydrodynamic spectroscopy (based on surface-acoustic probing of solvated interfaces), we provide clear evidence that typical cosmotropic cations (Li+, Mg2+, and Al3+) are inserted into the MXene interspaces in their partially hydrated form, in contrast to the insertion of chaotropic cations (Cs+ and TEA+), which effectively dehydrate the MXene. These new findings provide important information about the charge-storage mechanisms in layered materials by direct quantification and efficient control (management) over the amount of confined fluid in a variety of solvated battery/supercapacitor electrodes. We believe that the proposed monitoring of water content as a function of the nature of ions can be equally applied to solvated biointerfaces, such as the ion channels of membrane proteins.
Context: We examine the dynamics of low-frequency waves in differentially rotating stellar radiation zones, the angular velocity being taken as generally as possible depending both on radius and on ...latitude in stellar interiors. The associated induced transport of angular momentum, which plays a key role in the evolution of rotating stars, is derived. Aims: We focus on the wave-induced transport of angular momentum, taking into account the Coriolis acceleration in the case of strong radial and latitudinal differential rotation. We thus go beyond the ``weak differential rotation'' approximation, where rotation is almost a solid-body one plus a residual radial differential rotation. As has been shown in previous works, the Coriolis acceleration modifies such transport. Methods: We built analytically a complete formalism that allows the study of rotational transport in stellar radiation zones taking into account the wave action modified by a general strong differential rotation. Results: The different approximations possible for low-frequency waves in a differentially rotating stably stratified radiative region, namely the traditional and the JWKB approximations, are examined and discussed. The complete bidimensional structure of regular elliptic gravito-inertial waves, which verify these approximations, is derived and compared to those in the ``weak differential rotation'' case. Next, associated transport of energy and of angular momentum in the vertical and in the horizontal directions and the dynamical equations, respectively for the mean radial differential rotation and the latitudinal one, are obtained. Conclusions: The complete formalism, which takes into account low-frequency wave action, is derived and can be used for the study of secular hydrodynamics of radiative regions and of the associated mixing. The modification of waves due to general strong differential rotation and their feed-back on the angular momentum transport are treated rigourously. In a forthcoming paper (Paper II), this formalism will be applied to the case of solar differential rotation. However, the case of hyperbolic gravito-inertial waves should be carefully studied.
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