Low-Temperature Oxidation, Hydrothermal Corrosion, and Their Effects on Properties of SiC (Tyranno) Fibers

The effects of oxidation in air and corrosion in high-temperature, high-pressure water on the mechanical properties of three commercially available amorphous Si-Ti-C-O (Tyranno) fibers with different oxygen contents (12%–18%) and diameters (8–11 μm) were investigated. The fibers were exposed to isothermal treatments at elevated temperatures and subsequently tested at room temperature. Structural changes in the fibers after oxidation and corrosion were also studied in order to understand better the degradation mechanisms of the fibers. Oxidation resulted in the formation of vitreous silica films and decreases of strength and Young's modulus of the fibers. Hydrothermal corrosion under 100 MPa water pressure started above 300°C and resulted in the formation of a carbon layer on the surface of the fibers. Dissolution of silica in water during the treatment was observed. Corrosion at temperatures above 400°C led to the formation of relatively thick carbon films which delaminated easily. It caused a decrease of strength and Young's modulus of the fibers. The hydrothermal method can be used for producing carbon coatings with thickness up to 2 μm on the surface of silicon carbide fibers. The degrading of the mechanical properties after oxidation and corrosion was controlled by the thickness of the oxide or carbon layer. Based on this fact, it is possible to predict changes in the mechanical properties from the oxidation data.     

The determination of the energy barrier for phase slips in superfluid4He

The velocity dependence of the energy barrier for vortex creation in microscopic apertures is determined. When compared to results from other laboratories, the energy barrier seems to be a universal function of velocity. This universality suggests that the vortex nucleation process is independent of the microscopic surface structure of the aperture. In DC flow experiments, the vortices are nucleated at rates up to 700kHz. In single phase slip experiments, the rate of nucleation is on the order of 10 Hz. Each of these types of experiments gives the energy barrier in a different velocity regime. The energy barrier has more curvature, as a function of velocity, than can be accounted for by the half-ring model of vortex nucleation.     

Stable circulation in superfluid3He-A

We present evidence of stable nonzero circulation in superfluid³He-A around a 16 µm diameter wire. The measurements are made on a rotating cryostat at T0.15T_c. Circulation offects the splitting of the wire's normal modes, which changes the beat frequency in the emf across the wire as it vibrates in a magnetic field. The different beat frequencies at 14 and at 21 bar suggest that circulation is present at 21 bar. Furthermore, at 21 bar the beat frequency is a linear function of rotation velocity, as is expected if circulation is trapped around the wire. At 14 bar no effect of rotation is observed. We propose that at 21 bar the ground state texture has nonzero circulation of order h/2m₃.     

Stamping of Flexible,Coplanar Micro‐Supercapacitors Using MXene Inks

The fast growth of portable smart electronics and internet of things have greatly stimulated the demand for miniaturized energy storage devices. Micro‐supercapacitors (MSCs), which can provide high power density and a long lifetime, are ideal stand‐alone power sources for smart microelectronics. However, relatively few MSCs exhibit both high areal and volumetric capacitance. Here rapid production of flexible MSCs is demonstrated through a scalable, low‐cost stamping strategy. Combining 3D‐printed stamps with arbitrary shapes and 2D titanium carbide or carbonitride inks (Ti₃C₂T_x and Ti₃CNT_x, respectively, known as MXenes), flexible all‐MXene MSCs with controlled architectures are produced. The interdigitated Ti₃C₂T_x MSC exhibits high areal capacitance: 61 mF cm^?2 at 25 µA cm^?2 and 50 mF cm^?2 as the current density increases by 32 fold. The Ti₃C₂T_x MSCs also showcase capacitive charge storage properties, good cycling lifetime, high energy and power densities, etc. The production of such high‐performance Ti₃C₂T_x MSCs can be easily scaled up by designing pad or cylindrical stamps, followed by a cold rolling process. Collectively, the rapid, efficient production of flexible all‐MXene MSCs with state‐of‐the‐art performance opens new exciting opportunities for future applications in wearable and portable electronics.     

Multifunctional magnetic rotator for micro and nanorheological studies

We report on the development of a multifunctional magnetic rotator that has been built and used during the last five years by two groups from Clemson and Drexel Universities studying the rheological properties of microdroplets. This magnetic rotator allows one to generate rotating magnetic fields in a broad frequency band, from hertz to tens kilohertz. We illustrate its flexibility and robustness by conducting the rheological studies of simple and polymeric fluids at the nano and microscale. First we reproduce a temperature-dependent viscosity of a synthetic oil used as a viscosity standard. Magnetic rotational spectroscopy with suspended nickel nanorods was used in these studies. As a second example, we converted the magnetic rotator into a pump with precise controlled flow modulation. Using multiwalled carbon nanotubes, we were able to estimate the shear modulus of sickle hemoglobin polymer. We believe that this multifunctional magnetic system will be useful not only for micro and nanorheological studies, but it will find much broader applications requiring remote controlled manipulation of micro and nanoobjects.     

A matheuristic for the discrete bilevel problem with multiple objectives at the lower level

In this paper, we solve a discrete bilevel problem with multiple objectives at the lower level and constraints at the upper level coupling variables of both levels. In the case of a multiobjective lower level, we deal with a set of Pareto‐efficient solutions rather than a single optimal lower level solution. To calculate the upper level objective function value, we need to select one solution out of a potentially large set of efficient lower level solutions. To avoid the enumeration of the whole set of Pareto solutions, we formulate an auxiliary mixed integer linear programming problem with a large number of constraints. We propose an iterative exact method to solve it. To find a near‐optimal upper level solution, we apply a metaheuristic. The method is tested on the discrete ()‐centroid problem with multiple objectives at the lower level.     

Scalable Synthesis of Ultrathin Mn3N2 Exhibiting Room‐Temperature Antiferromagnetism

Ultrathin and 2D magnetic materials have attracted a great deal of attention recently due to their potential applications in spintronics. Only a handful of stable ultrathin magnetic materials have been reported, but their high‐yield synthesis remains a challenge. Transition metal (e.g., manganese) nitrides are attractive candidates for spintronics due to their predicted high magnetic transition temperatures. Here, a lattice matching synthesis of ultrathin Mn₃N₂ is employed. Taking advantage of the lattice match between a KCl salt template and Mn₃N₂, this method yields the first ultrathin magnetic metal nitride via a solution‐based route. Mn₃N₂ flakes show intrinsic magnetic behavior even at 300 K, enabling potential room‐temperature applications. This synthesis procedure offers an approach to the discovery of other ultrathin or 2D metal nitrides.     

Electrochromic Effect in Titanium Carbide MXene Thin Films Produced by Dip‐Coating

MXenes, a large family of 2D transition metal carbides and nitrides, have shown potential in energy storage and optoelectronic applications. Here, the optoelectronic and pseudocapacitive properties of titanium carbide (Ti₃C₂T_x) are combined to create a MXene electrochromic device, with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V. By probing the electrochromic effect in different electrolytes, it is shown that acidic electrolytes (H₃PO₄ and H₂SO₄) lead to larger absorption peak shifts and a higher change of transmittance than the neutral electrolyte (MgSO₄) (Δλ is 100 nm vs 35 nm and ΔT_{770 nm} is ≈12% vs ≈3%, respectively), hinting at the surface redox mechanism involved. Further investigation of the mechanism by in situ X‐ray diffraction and Raman spectroscopy reveals that the reversible shift of the absorption peak is attributed to protonation/deprotonation of oxide‐like surface functionalities. As a proof of concept, it is shown that Ti₃C₂T_x MXene, dip‐coated on a glass substrate, functions as both transparent conductive coating and active material in an electrochromic device, opening avenues for further research into optoelectronic and photonic applications of MXenes.     

Book Reviews

ADHESION OF LIQUIDS AND WETTING, by A. D. Zimon, Khimiya, Moscow, 1974, 416 pp.

ADHESION OF DUST AND POWDERS, second edition, by A. D. Zimon, Khimiya, Moscow, 1976, 432 pp.

ADHESION OF FILMS AND COATINGS, by A. D. Zimon, Khimiya, Moscow, 1977, 352 pp.

AUTOHESION OF DRY MATERIALS, by A. D. Zimon, E. I. Andrianov, Metallurgiya, Moscow, 1978, 287 pp.