In Situ Neutron Diffraction Study of Phase Transformation of High Mn Steel with Different Carbon Content

In situ neutron diffraction was employed to examine the phase transformation behavior of high-Mn steels with different carbon contents (0.1, 0.3, and 0.5 wt.%C). With increasing carbon contents from 0.1 C to 0.5 C, the austenite phase fraction among the constituent phases increased from ~66% to ~98%, and stacking fault energy (SFE) increased from ~0.65 to ~16.5 mJ/m2. The 0.1 C and 0.3 C steels underwent phase transformation from γ-austenite to ε-martensite or α’-martensite during tensile deformation. On the other hand, the 0.5 C steel underwent phase transformation only from γ-austenite to ε-martensite. The 0.3 C steel exhibited a low yield strength, a high strain hardening rate, and the smallest elongation. The high strain hardening of the 0.3 C alloy was due to a rapid phase transformation rate from γ-austenite to ε-martensite. The austenite of 0.5 C steel was strengthened by mechanical twinning during loading process, and the twinning-induced plasticity (TWIP) effect resulted in a large ductility. The 0.5 wt.% carbon addition stabilized the austenite phase by delaying the onset of the ε-martensite phase transformation

MXene-enhanced beta-phase crystallization in ferroelectric porous composites for highly-sensitive dynamic force sensors

Piezoelectric polyvinylidene fluoride (PVDF) has been widely utilized in flexible and self-powered tactile sensors, which require high ferroelectricity of polar phase PVDF. Herein, we demonstrate self-powered piezoelectric e-skins with high sensitivity and broad sensing range based on 3D porous structures of MXene (Ti3C2Tx)/PVDF. MXene was used as a nucleation agent to increase the ferroelectric properties of PVDF. This was carried out considering its 2D geometry and abundant surface functional groups that facilitate intermolecular hydrogen bonding between the surface functional groups of MXene and the CH2 group of PVDF. In addition, porous structures can increase the variation in contact area and localized stress concentration in response to applied pressure. This further enhances the piezoelectric sensitivity. Owing to structural deformation and localized stress concentration, the piezoelectric sensitivity of porous MXene/PVDF e-skin is 11.9 and 1.4 nA kPa(-1) for low (< 2.5 kPa) and high (2.5-100 kPa) pressure ranges, respectively. These are 31 and 3.7 times higher, respectively, than that of planar MXene/PVDF e-skin (0.4 nA kPa(-1) for <100 kPa). In addition, porous MXene/PVDF e-skin exhibits a broad sensing range of up to 100 kPa, and stable sensing performance (5000 repetitions). Our piezoelectric porous MXene/PVDF e-skins enable the monitoring of high-frequency dynamic signals such as acoustic sound waves as well as low-frequency radial artery pulses. In particular, the detection of high-frequency vibrations from sliding friction enables our sensor array to perceive various surface textures with different roughness and moduli, as well as the spatial distribution of words embossed on surfaces. This demonstrates its substantial potential for application in wearable devices, prosthetic limbs, robotics, and healthcare monitoring devices

Broadband omnidirectional light detection in flexible and hierarchical ZnO/Si heterojunction photodiodes

The development of flexible photodetectors has received great attention for future optoelectronic applications including flexible image sensors, biomedical imaging, and smart, wearable systems. Previously, omnidirectional photodetectors were only achievable by integration of a hemispherical microlens assembly on multiple photodetectors. Herein, a hierarchical photodiode design of ZnO nanowires (NWs) on honeycomb-structured Si (H-Si) membranes is demonstrated to exhibit excellent omnidirectional light-absorption ability and thus maintain high photocurrents over broad spectral ranges (365 to 1,100 nm) for wide incident angles (0?? to 70??), which enabled broadband omnidirectional light detection in flexible photodetectors. Furthermore, the stress-relieving honeycomb pattern within the photodiode micromembranes provided photodetectors with excellent mechanical flexibility (10% decrease in photocurrent at a bending radius of 3 mm) and durability (minimal change in photocurrent over 10,000 bending cycles). When employed in semiconductor thin films, the hierarchical NW/honeycomb heterostructure design acts as an efficient platform for various optoelectronic devices requiring mechanical flexibility and broadband omnidirectional light detection.[Figure not available: see fulltext.] © 2016 Tsinghua University Press and Springer-Verlag Berlin Heidelberclose