Ultra-sensitive and wide applicable strain sensor enabled by carbon nanofibers with dual alignment for human machine interfaces

Flexible strain sensors with high sensitivity,wide detection range,and low detection limit have continuously attracted great interest due to their tremendous application potential in areas such as health/medical-care,human-machine interface,as well as safety and security.While both of a high sensitivity and a wide working range are desired key parameters for a strain sensor,they are usually contrary to each other to be achieved on the same sensor due to the tightly structure dependence of both of them.Here,a flexible strain sensor with both high sensitivity and wide strain detection range is prepared based on the design of an integrated membrane containing both of parallel aligned and randomly aligned carbon nanofibers(CNFs).The parallel aligned CNF membrane(p-CNF)exhibits a low strain detection limit and high sensitivity,while the random aligned CNF membrane(r-CNF)exhibits a large strain detection range.Taking the advantages of both p-CNF and r-CNF,the strain sensor with stacked p-CNF and r-CNF(p/r-CNF)exhibits both high sensitivity and wide working range.Its gauge factor(GF)is 1,272 for strains under 0.5%and 2,266 for strain from 70%to 100%.At the same time,it can work in a wide strain range of 0.005%to 100%,fulfilling the requirements for accurately detecting full-range human motions.We demonstrated its applications in the recognition of facial expressions and joint movements.Furtherly,we constructed an intelligent lip-language recognition system,which can accurately track phonetic symbols and may help people with language disabilities,proving the potential of this strain sensor in health management and medical assistance.Besides,we foresee that the dual-alignment structure design of the p/r-CNF strain sensor may also be applied in the design of other high performance sensors.

通过给桑蚕喂食稀土离子改性的饲料制备高强高韧的蚕丝纤维

蚕丝纤维具有优异的生物相容性和良好的力学性能,但以简单且大规模的方式生产强韧的蚕丝纤维仍是一项重大挑战.本文报道了一种通过给桑蚕喂食稀土离子改性桑叶饲料以制备强韧蚕丝纤维的方法.所得蚕丝纤维具有显著提高的拉伸强度和韧性,其平均值分别为0.85±0.07 GPa和156±13 MJ m^(-3),且最大值达到0.97±0.04 GPa和188±19 MJ m^(-3),这些性能接近蜘蛛牵引丝(自然界最强的丝纤维之一)的拉伸强度和韧性.研究结果表明,稀土离子(La^(3+)或En^(3+))成功掺入蚕丝纤维并与其形成相互作用.稀土离子减小了蚕丝纤维的直径,降低了其内部结晶组分的含量并提升了取向度.这些相互作用与结构演变同时提升了蚕丝纤维的强度和韧性.该研究提出了一种简单、可扩展且有效的生产超强韧丝纤维的策略,制备的蚕丝纤维有望应用于个人防护和航空航天等需要轻质结构材料的领域.

VLS和VSS生长机制调控的单层TMD中的反向掺杂分布

掺杂是改善过渡金属二硫族化合物(TMD)单层薄膜性能的重要方法.然而,现有的研究大多集中在掺杂能力上,对掺杂分布的研究还处于起步阶段.近年来,基于旋涂化学气相沉积法研究了掺杂原子空间掺杂分布的差异.这些掺杂的TMD大多是通过气-液-固(VLS)生长机制生长的.采用气-固-固(VSS)生长机制生长的TMD尚未被用来深入研究掺杂分布.本文采用Na_(2)WO_(4),NaVO_(3),NaOH(Na-Pre),(NH_(4))_(2)WO_(4),NH_(4)VO_(3)(NH_(4)-Pre)等前驱体合成V–WS_(2).观察到中心和边缘区域完全相反的掺杂分布.还观察到浓度变化较大的界面的出现.通过透射电子显微镜精确地表征了掺杂原子的空间浓度.根据密度泛函理论的计算结果,掺杂分布的差异是由初始态引起的,这与VLS和VSS的生长机制有关.这项工作为控制掺杂原子的分布提供了一种新的方法,对掺杂TMD在微电子和纳米电子器件中的应用具有重要意义.

CVD growth of perovskite/graphene films for high-performance flexible image sensor

Hybrid perovskite possesses excellent photoelectric properties,including large light-absorption capacity and high carrier mobility,and is an ideal light-absorbing material for photoelectric devices.The grain size and compactness of hybrid perovskite are key factors affecting the performance of photoelectric devices.The photocurrent and photoresponsivity of these devices are relatively low because of the rapidly recombined photoexcited electron-hole pairs in hybrid perovskite.Herein,we develop a facile two-step chemical vapor deposition(CVD)method to synthesize a high-quality van der Waals(vd Ws)MAPb I3/graphene heterostructure for high-performance image sensor.We introduced inorganic sources(PbI2)to vd Ws epitaxially grown Pb I2 film on a seamless graphene monolayer film template through CVD.Methylammonium iodide(MAI)was then reintroduced to prepare the vd Ws MAPb I3/graphene heterostructure.The MAPb I3 layer is composed of densely packed,large-size grains and displays a smooth surface.High photoresponsivity of 107A/W is achieved in the corresponding photodetector.Inspired by the human visual system,we designed a flexible photodetector array containing(24?24)pixels,achieving perfect image recognition and color discrimination.Our study may greatly facilitate the construction of high-performance optoelectronic devices in artificial retina,biomedical imaging,remote sensing,and optical communication.

Physical sensors for skin-inspired electronics

Skin,the largest organ in the human body,is sensitive to external stimuli.In recent years,an increasing number of skin-inspired electronics,including wearable electronics,implantable electronics,and electronic skin,have been developed because of their broad applications in healthcare and robotics.Physical sensors are one of the key building blocks of skin-inspired electronics.Typical physical sensors include mechanical sensors,temperature sensors,humidity sensors,electrophysiological sensors,and so on.In this review,we systematically review the latest advances of skin-inspired mechanical sensors,temperature sensors,and humidity sensors.The working mechanisms,key materials,device structures,and performance of various physical sensors are summarized and discussed in detail.Their applications in health monitoring,human disease diagnosis and treatment,and intelligent robots are reviewed.In addition,several novel properties of skin-inspired physical sensors such as versatility,self-healability,and implantability are introduced.Finally,the existing challenges and future perspectives of physical sensors for practical applications are discussed and proposed.

A new methodology to obtain the fracture toughness of YAG transparent ceramics

A new methodology was proposed for testing the fracture toughness of YAG transparent ceramics depended on temperature. In our model, the fracture toughness, as a material-specific property, can be expressed as tensile strength treated as the physical property or a material constant. Using this method, a suitable size larger than the inherent defects of ceramic samples and the creation of atomically sharp pre-cracks on the surface of ceramic specimens were able to be ignored. Besides, the fracture of ceramic can be described as the equivalence between the release of elastic storage energy and surface energy associated with the new fracture surface. From thermodynamics theory, the system’s internal energy includes the kinetic energy of atomic motion and the potential energy between atoms in the system. Finally, the fracture toughness at different temperatures can also be calculated by this simple quantitative relationship. In order to confirm the validity of this model, the decreasing fracture toughness of YAG transparent ceramic with increasing temperature was predicted, and the result coincided with the experimental results.

Influence of seeding promoters on the properties of CVD grown monolayer molybdenum disulfide

Chemical vapor deposition (CVD) is the most efficient method to grow large-area two dimensional (2D) transition metal dichiacogenides (TMDCs) in high quality.Monolayer molybdenum disulfide (MoS2) and seed-assistant are the mostly selected 2D TMDC and growth strategy for such CVD processes,respectively.Though the advantages of seed catalysts in facilitating the nucleation,achieving higher yield and better repeatability,as well as their effects on the morphologies of as-grown MoS2 have been studied,the influence of seeding promoters on both optical and electrical properties of as-grown monolayer MoS2 is not known comprehensively,which is indeed critical for understanding fundamental physics and developing practical application of such emerging 2D semiconductors.In this report,we systematically investigated the effect of different seeding promoters on the properties of CVD-grown monolayer MoS2.It is found that different seed molecules lead to different impacts on the optical and electrical properties of as-grown monolayer MoS2.Among three different seed catalysts (perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS),copper phthalocyanine (CuPc),and crystal violet (CV)),PTAS performs better in obtaining large area monolayer MoS2 with good optical quality and high electrical mobility than the other two.Our work gives a guide for modifying the properties of as-grown monolayer MoS2 and other 2D transition metal dichalcogenides in seeding promoters-assisted synthesis process.

High-resolution characterization of hexagonal boron nitride coatings exposed to aqueous and air oxidative environments

Hexagonal boron nitride （h-BN） is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occurrence of galvanic reactions. Nevertheless, this may not be the case when an h-BN-protected material is exposed to aqueous environments. In this work, we analyzed the stability of mono and multilayer h-BN stacks exposed to H202 and atmospheric conditions. Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H202. Multilayer h-BN offered a good degree of protection. Monolayer h-BN was found to be ineffective in an air atmosphere as well. Even a 10-15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration. By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques, we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate. Fortunately, because of the diffusive nature of the process, the oxidized area did not increase with time once a saturated state was reached. This makes multflayer （not monolayer） h-BN a suitable long-term oxidation barrier. Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy. This technique cannot assess the chemical composition of the deeper layers of a material. Hence, the previous reports, which relied on XPS to analyze the passivating properties of h-BN and graphene, may have ignored some important subsurface phenomena. The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.

An effective strategy for preparing transparent ceramics using nanorod powders based on pressure-assisted particle fracture and rearrangement

Achieving full densification of some ceramic materials,such as Y_(2)O_(3),without sintering aids by spark plasma sintering(SPS)is a great challenge when plastic deformation contributes limitedly to the densification as the yield stress of the material at an elevated temperature is higher than the applied sintering pressure.Herein,we demonstrate that particle fracture and rearrangement is an effective strategy to promote the densification during the pressure-assisted sintering process.Specifically,Y_(2)O_(3) nanocrystalline powders composed of nanorod and near-spherical particles were synthesized and sintered at various temperatures by the SPS.The results show that the relative density of the ceramics prepared by the nanorod powders is higher than the density of the ceramics from the near-spherical powders after 600℃ due to the fracture and rearrangement of the nanorods at low temperatures,which leads to the decrease of particle size and the increase of density and homogeneity.Based on this novel densification mechanism,ultrafine-grained Y_(2)O_(3) transparent ceramics with good optical and mechanical properties were fabricated successfully from the nanorod powders.

Mechanically Reinforced Silkworm Silk Fiber by Hot Stretching

Silkworm silk,which is obtained from domesticated Bombyx mori(B.mori),can be produced in a large scale.However,the mechanical properties of silkworm silk are inferior to its counterpart,spider dragline silk.Therefore,researchers are continuously exploring approaches to reinforce silkworm silk.Herein,we report a facile and scalable hot stretching process to reinforce natural silk fibers obtained from silkworm cocoons.Experimental results show that the obtained hot-stretched silk fibers(HSSFs)retain the chemical components of the original silk fibers while being endowed with increasedβ-sheet nanocrystal content and crystalline orientation,leading to enhanced mechanical properties.Significantly,the average modulus of the HSSFs reaches 21:6±2:8 GPa,which is about twice that of pristine silkworm silk fibers(11:0±1:7 GPa).Besides,the tensile strength of the HSSFs reaches 0:77±0:13 GPa,which is also obviously higher than that of the pristine silk(0:56±0:08 GPa).The results show that the hot stretching treatment is effective and efficient for producing superstiff,strong,and tough silkworm silk fibers.We anticipate this approach may be also effective for reinforcing other natural or artificial polymer fibers or films containing abundant hydrogen bonds.

Micromechanical Modeling of Elastic-Viscoplastic Behavior of Armco-Fe at High Strain Rate

Heterogeneous dynamical stress-strain response of Armco-Fe was investigated at high strain rates through the Split Hopkinson Pressure Bar （SHPB） testing. It was found that the viscoplastic deformation in BCC ferrite grains is affected by the strain rate. Thermal softening and variation in crystal orientations under high-strain-rate loading were used in the elastic-viscoplastic modeling. The micromechanical analysis with self-consistent transition and homogenization was used for estimation of the global impact response of the material. The results from modeling were found in good agreement with the experimental data.

Electrical field tuning of magneto-Raman scattering in monolayer graphene

In this work, we report the electrical field tuning of magneto-phonon resonance in monolayer graphene under magnetic fields up to 9 T. It is found that the carrier concentration can drastically affect the G （E2g） phonon response to a varying magnetic field through a pronounced magneto-phonon resonance （MPR）. In charge neutral or slightly doped monolayer graphene, both the energy and the line width of the E2g phonon show clear variation with magnetic fields. This is attributed to magneto-phonon resonance between magnetoexcitations and the E2g phonons. In contrast, when the Fermi level of the monolayer graphene is far away from the Dirac point, the G band shows weak magnetic dependence and exhibits a symmetric line-shape. This suggests that the magneto-phonon coupling around 4 T has been switched off due to the Pauli blocking of the inter-Landau level excitations. Moreover, the G band asymmetry caused by Fano resonance between excitonic many-body states and the E2g phonons is observed. This work offers a way to study the magnetoexcitation phonon interaction of materials through magneto-Raman spectroscopy with an external electrical field.

Carbothermal shock enabled facile and fast growth of carbon nanotubes in a second

Carbon nanotubes(CNTs)hold great promise in many fields because of their unique structures and properties.However,the preparation of CNTs generally involves cumbersome equipment and time-consuming processes.Here,we report an ultra-fast carbothermal shock(CTS)approach for synthesizing CNTs with a simple homemade setup by employing Joule heating of a carbon substrate.Carbonized silk fabric(CSF)loaded with transition metal salts in ethanol solution was used as the substrate,which was treated with a pulse voltage of 40 V for only 50 ms and then covered with uniform CNTs grown with bimetallic alloy catalyst nanoparticles(diameter:~9 nm).The temperature ramp rate is as high as 105 K/s.The as-obtained sample has a unique fluffy structure similar to the trichobothrium of spiders,endowing it versatile applications such as airflow sensors or air filters.The CTS technique presents an easy-accessible and highly efficient approach for synthesizing CNTs,which may be also applied in synthesizing other nanomaterials.