Huili Gao

Does aluminum play well with others? Intrinsic Al-A alloying behavior in 211/312 MAX phases

ABSTRACT The search for further control over the properties of MAX phases as well as the promise of discovering compounds with new functionalities has resulted in an increased interest in MAX solid solutions resulting from mixing in either the M, A, or X sublattices. The possibility of alloying MAX compounds not only enables finer tuning of their properties but can also be used to stabilize compounds that may otherwise be metastable in their pure state. In this letter, we present an ab initio-based investigation of the intrinsic alloying behavior in the A sublattice of Ti(Al,A)C, Zr(Al,A)C and Ti(Al,A)C MAX compounds. GRAPHICAL ABSTRACT IMPACT STATEMENT In this work, we present for the first time a comprehensive study of the intrinsic alloying tendencies in MAX solid solutions with (Al,A) mixing in the A sublattice.

Electrochemical etching of Ti2AlC to Ti2CTx (MXene) in low-concentration hydrochloric acid solution

In this study, we successfully demonstrate the electrochemical etching of Al from porous Ti2AlC electrodes in dilute hydrochloric acid to form a layer of Ti2CTx MXene on Ti2AlC. This is the first report on etching of the A layer from the MAX phase in a fluoride-free solution as a less hazardous method to process and handle MXenes. In addition, these MXenes possess only –Cl terminal groups, as well as the common ones, such as –O and –OH. However, electrochemical etching can also result in subsequent over-etching of parent MAX phases to carbide-derived carbon (CDC). We propose a core–shell model to explain electrochemical etching of Ti2AlC to Ti2CTx and CDC. The proposed model suggests that a careful balance in etching parameters is needed to produce MXenes while avoiding over-etching. Our electrochemical approach expands the possible range of both etching techniques and resulting MXene compositions.

Surface-agnostic highly stretchable and bendable conductive MXene multilayers

Bendable, stretchable MXene multilayer coatings are deposited onto a wide variety of surfaces, rendering them conductive and responsive. Stretchable, bendable, and foldable conductive coatings are crucial for wearable electronics and biometric sensors. These coatings should maintain functionality while simultaneously interfacing with different types of surfaces undergoing mechanical deformation. MXene sheets as conductive two-dimensional nanomaterials are promising for this purpose, but it is still extremely difficult to form surface-agnostic MXene coatings that can withstand extreme mechanical deformation. We report on conductive and conformal MXene multilayer coatings that can undergo large-scale mechanical deformation while maintaining a conductivity as high as 2000 S/m. MXene multilayers are successfully deposited onto flexible polymer sheets, stretchable poly(dimethylsiloxane), nylon fiber, glass, and silicon. The coating shows a recoverable resistance response to bending (up to 2.5-mm bending radius) and stretching (up to 40% tensile strain), which was leveraged for detecting human motion and topographical scanning. We anticipate that this discovery will allow for the implementation of MXene-based coatings onto mechanically deformable objects.

Static and Dynamic Thermo-Mechanical Behavior of Ti2AlC MAX Phase and Fiber Reinforced Ti2AlC Composites

Ti2AlC MAX phase samples were processed by using Spark Plasma Sintering from commercially available Ti2AlC powder. Static and dynamic loading was performed by Universal Testing Machine and Split Hopkinson Pressure Bar (SHPB) respectively. The SHPB apparatus was modified to investigate the dynamic fracture initiation toughness. High speed photography was used to determine the fracture initiation time and the associated failure load. To widen applications, 20 vol% fiber of NextelTM-610 and NextelTM-720 have been added for the reinforcement of Ti2AlC, respectively. The results reveal that the peak compressive failure stress in dynamic conditions decreases with increasing temperatures, from 1,645 MPa at 25 °C to 1,210 MPa at 1,200 °C. The fracture experiments show that the dynamic fracture toughness is higher than the quasi-static value by approximately 35 %. The fracture toughness decreases with increase in temperature. The post mortem analysis of the fracture surfaces conducted using Scanning Electron Microscopy revealed that kinking along with intergranular cracking and delamination play important role in deformation of Ti2AlC. Compared to pure Ti2AlC, compressive fracture strength of 20 vol% Ti2AlC/720f and Ti2AlC/610f composites were enhanced by 39.7 and 32.6 % under static loading.