博士 教授 博士生导师
1、 美国机械工程师学会（ASME）会刊Journal of Applied Mechanics 副主编；
2、 英国物理学会（IOP）Flexible and Printed Electronics 编辑；
3、 Nature集团 npj Flexible Electronics 编辑；
Experimental Mechanics ，2016，56（）：1123–1132&
Thermal stress induced by elevated temperature causes the refractive index of transparent materials to become non-uniform, influencing the light deflection and thus performance of imaging systems containing such materials. Here, a digital gradient sensing (DGS) method is developed to measure the full-field non-uniform thermal stress in a material with air disturbance. This real-time optical technique can provide the light deflection and distribution of the principal stress gradients in a transparent medium. The light deflection through the transparent medium caused by the thermal stress is obtained using the elasto-optical effect. Air convection at elevated temperatures also affects the light deflection in optical systems, so the DGS method is extended to eliminate air convection. The light deflection resulting from the heated air is separately identified and calculated from the total deflection. The validity of this method is demonstrated using a bilayer transparent film containing layers with known refractive index and different thermal expansion coefficients that is bent by the thermal stress. Application of the DGS method to a zinc sulfide specimen shows that the thermal stress in it is temperature dependent and can be used to characterize refractive index non-uniformity.
The ability to continuously and reversibly tune the band gap and the strain–photonic coupling effect in optoelectronic materials is highly desirable for fundamentally understanding the mechanism of strain engineering and its applications in semiconductors. However, optoelectronic materials (i.e., GaAs) with their natural brittleness cannot be subject to direct mechanical loading processes, such as tension or compression. Here, we report a strategy to induce continuous strain distribution in GaAs nanoribbons by applying structural buckling. Wavy GaAs nanoribbons are fabricated by transfer printing onto a prestrained soft substrate, and then the corresponding photoluminescence is measured to investigate the strain–photonic coupling effect. Theoretical analysis shows the evolution of the band gap due to strain and it is consistent with the experiments. The results demonstrate the potential application of a buckling configuration to delicately measure and tune the band gap and optoelectronic performance.
strain−photonic coupling optoelectronic material band gap buckling nanoribbons
In this article we provide a large-scale and low-cost method, based on metal–organic frameworks, to fabricate nanostructured conformal coatings with the same material as the substrate. Such design can reduce the thermal mismatch between coatings and substrate and massively change the local thermal conductivity and interfacial heat transfer coefficient. Therefore, the thermal shock resistance can be enhanced to about 75%. We describe all aspects of such nanostructured conformal coatings from fabrication to characterization. Moreover, the mechanism of this enhancement due to high porosity and low pore size of nanoporous coating is discussed.
Thermal shock Ceramic Metal–organic frameworks Nano-structure
In this work an experimental technique to simultaneously measure the full-field temperature and deformation of composite material subjected to flame heating at high temperature is developed using the technique of image processing. The testing stage is integrated with an oxy-propane flame torch for flame heating, a CCD camera for image recording, a synchronized blue light source for light compensation and an infrared pyrometer for temperature calibration and comparison. The principle of the synchronous measurement of temperature and deformation field is demonstrated and discussed. Experiment on carbon fiber reinforced silicon carbide (C/SiC) composite was conducted to validate this method. The temperature was calculated using an improved two-color method while the displacement field and strain field were calculated using the digital image processing method. Results show that the proposed method is applicable for synchronous measurement of temperature and displacement by using one camera, and the mutual interference between the radiation and reflected light can also be effectively eliminated.
Journal of the European Ceramic Society，2016，36（3）：451-456
C/SiC (carbon fiber reinforced silicon carbide) composites were subjected to thermal ablation at temperatures up to 1800 °C and the surface evolution of the composites were recorded by using a high speed camera. After the thermal ablation process, observation of the ablated surface by using scanning electron microscope (SEM) was conducted and results revealed three characteristic structures, i.e., “gas holes” structure, surface cracks, and “skeleton structures”. The formation mechanism of these three types of structures was analyzed numerically and theoretically. Analysis provides insight to the failure mechanism of the surface during and after thermal ablation test.
C/， SiC ceramic Thermal ablation Micro structures Oxidation
IEEE Electron Device Letters ，2016，37（4）： 496 - 499
Wearable electronics have attracted much attention and are experiencing rapid growth in recent years. Such devices are expected to stay closer to the human body (i.e., attached to the skin) for better performance. Therefore, ultra-flexibility of such devices is necessary in order to make the sensor conform to the human body when the devices are used for healthcare monitoring. Here, we present a biocompatible and ultra-flexible strain sensor for pulse and body motion real-time and long-term measurement. The sensor, fabricated and integrated on a semi-permeable substrate with good biocompatibility and waterproofness, is mechanically invisible for the human. It owns good linearity (r 2 = 0.997), good repeatability, low resistance (350 Ω), and short response time (less than 100 ms). The sensor is designed with the shear lag theory, obtaining greater measuring range but still with good linearity. The liquid transfer printing method is used for thin-film sensing part and soft substrate integration in order to avoid damage. The sensor shows better performance and higher precision in motion and pulse monitoring than other similar sensors. The in vitro experiments demonstrate that the sensor is more suitable for long-term health monitoring at medical grade.
Applied Optics，-0001，54（26）： 8731-8737
In this work, we propose a structural deformation measuring method based on structural feature processing (straight line/edge detection) of the recorded digital images for specimens subjected to a high-temperature environment. Both radiation light and oxidation at high temperatures challenge the optics-based measurements. The images of a rectangular piece of copper specimen are obtained by using a bandpass filtering method at high temperatures, then all the edges are detected by using an edge detection operator, and then a Hough transform is conducted to search the straight edges for the calculation of deformation. Especially, due to the severe oxidation, a special seed strategy is adopted to reduce the oxidation effect and obtain an accurate result. For validation, the structural thermal deformation and the values of coefficients of thermal expansion for the copper specimen are measured and compared with data in the literature. The results reveal that the proposed method is accurate to measure the deformation of the structures at high temperatures.
Scientific Reports ，2015，5（）：16065 (
Power supply for medical implantable devices (i.e. pacemaker) always challenges not only the surgery but also the battery technology. Here, we report a strategy for energy harvesting from the heart motion by using ultra-flexible piezoelectric device based on lead zirconate titanate (PZT) ceramics that has most excellent piezoelectricity in commercial materials, without any burden or damage to hearts. Experimental swine are selected for in vivo test with different settings, i.e. opened chest, close chest and awake from anesthesia, to simulate the scenario of application in body due to their hearts similar to human. The results show the peak-to-peak voltage can reach as high as 3 V when the ultra-flexible piezoelectric device is fixed from left ventricular apex to right ventricle. This demonstrates the possibility and feasibility of fully using the biomechanical energy from heart motion in human body for sustainably driving implantable devices.
Nanoindentation is adopted to study the oxidation evolution of niobium-based alloy at nano-scale at elevated temperatures. An indentation pit at room temperature was created as a “marker” before the temperature was raised to 800 °C. A non-uniform oxide scale on the surface was observed real time by in situ scanning probe microscope. Elastic modulus and hardness obtained at different temperatures exhibit clearly the oxidation effect, which is also demonstrated by creep tests for 600s by using dynamic mechanical analysis of nanoindentation.
Oxidation Nanoindentation In situ scanning probe microscope Creep
Composites Science and Technology，2015，110（）：210-216
The delamination of SiC coating on C/SiC composites leads to severe oxidation of the carbon fibers and causes reliability problems of these composites during service. In this study, flexural test was conducted at room temperature to analyze the delamination behavior of SiC coating deposited on C/SiC substrate and finite element method (FEM) was adopted to simulate the interfacial crack between the SiC coating and the C/SiC substrate based on the experimental results. Results show that the fiber orientation has significant influence on the delamination of surface SiC coating. When the SiC coating is deposited parallel to the fiber orientation and interfacial delamination direction is along fiber direction, once delamination occurs, it inclines to propagate. However, when coating is prepared in other directions, the delamination inclines to be crested. The present analyses show that a proper combination of different fiber orientations can decrease the delamination of coating for better service of the materials in applications.
A.， Coating B.， Delamination B.， Fracture Flexural test C.， Finite element analysis
Transfer printing by kinetically switchable adhesion to an elastomeric stamp shows promise as a powerful micromanufacturing method to pickup microstructures and microdevices from the donor substrate and to print them to the receiving substrate. This can be viewed as the competing fracture of two interfaces. This paper examines the mechanics of competing fracture in a model transfer printing system composed of three laminates: an elastic substrate, an elastic thin film, and a viscoelastic member (stamp). As the system is peeled apart, either the interface between the substrate and thin film fails or the interface between the thin film and the stamp fails. The speed-dependent nature of the film/stamp interface leads to the prediction of a critical separation velocity above which separation occurs between the film and the substrate (i.e., pickup) and below which separation occurs between the film and the stamp (i.e., printing). Experiments verify this prediction using films of gold adhered to glass, and the theoretical treatment extends to consider the competing fracture as it applies to discrete micro-objects. Temperature plays an important role in kinetically controlled transfer printing with its influences, making it advantageous to pickup printable objects at the reduced temperatures and to print them at the elevated ones.
International Journal of Solids and Structures，2008，45（13）：3688-3698
Current methodologies used for the inference of thin film stress through curvature measurement are strictly restricted to stress and curvature states that are assumed to remain uniform over the entire film/substrate system. These methodologies have recently been extended to a single layer of thin film deposited on a substrate subjected to the non-uniform misfit strain in the thin film. Such methodologies are further extended to multi-layer thin films deposited on a substrate in the present study. Each thin film may have its own non-uniform misfit strain. We derive relations between the stresses in each thin film and the change of system curvatures due to the deposition of each thin film. The interface shear stresses between the adjacent films and between the thin film and the substrate are also obtained from the system curvatures. This provides the basis for the experimental determination of thin film stresses in multi-layer thin films on a substrate.
Multi-layer thin films Non-uniform misfit strain Non-uniform wafer curvatures Non-local stress-curvature relations Interfacial shears
J. Appl. Mech.，2008，75（2）：021022
Current methodologies used for the inference of thin film stress through curvature measurements are strictly restricted to uniform film stress and system curvature states over the entire system of a single thin film on a substrate. By considering a circular multilayer thin film/substrate system subjected to nonuniform temperature distributions, we derive relations between the stresses in each film and temperature, and between the system curvatures and temperature. These relations featured a “local” part that involves a direct dependence of the stress or curvature components on the temperature at the same point, and a “nonlocal” part, which reflects the effect of temperature of other points on the location of scrutiny. We also derive relations between the film stresses in each film and the system curvatures, which allow for the experimental inference of such stresses from full-field curvature measurements in the presence of arbitrary nonuniformities. These relations also feature a “nonlocal” dependence on curvatures making full-field measurements of curvature a necessity for the correct inference of stress. The interfacial shear tractions between the films and between the film and substrate are proportional to the gradient of the first curvature invariant, and can also be inferred experimentally.
multilayers,， substrates,， thermal stresses,， thin films,， multilayer thin films,， nonuniform film temperatures and stresses,， nonuniform system curvatures,， nonlocal stress-curvature relations,， interfacial shears
Optics Express ，-0001，19（14）：3201-13208
Coherent gradient sensing (CGS), a shear interferometry method, is developed to measure the full-field curvatures of a film/substrate system at high temperature. We obtain the relationship between an interferogram phase and specimen topography, accounting for temperature effect. The self-interference of CGS combined with designed setup can reduce the air effect. The full-field phases can be extracted by fast Fourier transform. Both nonuniform thin-film stresses and interfacial stresses are obtained by the extended Stoney’s formula. The evolution of thermo-stresses verifies the feasibility of the proposed interferometry method and implies the “nonlocal” effect featured by the experimental results.
Appl. Phys. Lett. ，2011，99（14）：141903
We report in situ observation of wrinkles formation and evolution of Si nanoribbons with finite length on elastomeric substrate via white light interferometer. The wrinkle originates from the middle of the nanoribbon, propagates symmetrically to the two ends, and finally reaches the stable configuration. The wavelength and amplitude will increase abruptly when the released strain exceeds the critical value. The interface interaction between Si nanoribbons and elastomeric substrate plays the key role for wrinkles formation. We gratefully acknowledge the support from National Natural Science Foundation of China (Grant Nos. 10902059, 90816007, 10820101048, and 10832005) and Foundation for the Author of National Excellent Doctoral Dissertation of China (FANEDD) (No. 2007B30).
ACS Nano ，2011，5（4）：3326–3332
Applications of ferroelectric ceramics, ranging from components for sensors, memory devices, microelectromechanical systems, and energy convertors, all involve planar and rigid layouts. The brittle nature of such materials and their high-temperature processing requirements limit applications to devices that involve only very small mechanical deformations and narrow classes of substrates. Here, we report a strategy for integrating nanoribbons of one of the most widely used ferroelectric ceramics, lead zirconate titanate, in “wavy” geometries, on soft, elastomeric supports to achieve reversible, linear elastic responses to large strain deformations (i.e., stretchable properties), without any loss in ferroelectric or piezoelectric properties. Theoretical and computational analysis of the mechanics account for these characteristics and also show that the amplitudes of the waves can be continuously tuned with an applied electric field, to achieve a vertical (normal) displacement range that is near 1000 times larger than is possible in conventional planar layouts. The results suggest new design and application possibilities in piezoelectric devices.
stretchable electronics ferroelectrics nanoribbons piezoelectrics energy harvesting
Soft Matter，2011，8（）： Soft Matt
This paper describes the mechanism of the rate-dependent adhesion for a transfer printing system. A cohesive element model accounting for the viscoelastic effect of the stamp reveals the evolution of the stress and the stress distributions as the peeling velocity increases. Experiments using Si ribbons separating from a wafer verify this prediction and demonstrate the influence of the rate-dependent effect on the transfer efficiency. The simulations show the relaxation time of the stamp has little effect on the transfer efficiency, and implies that it can be improved by adjusting the viscoelastic modulus.
Appl. Phys. Lett.，2013，103（15）：151607
Kinetically controlled transfer printing based on rate-dependent adhesion is widely used to heterogeneously integrate micro/nano-devices. Through analysis of peeling stamps with grating-like micropatterns from the rigid substrate in different directions, the directionally dependent adhesion strength is investigated in detail. Experiments of peel test and picking up silicon ribbons from silicon-on-insulator wafer were conducted and consistent with the analytical prediction. The method and analytical results proposed in this Letter can guide the design of the micropatterns on stamp to realize a more effective transfer printing approach. We gratefully acknowledge the support from National Natural Science Foundation of China (Grant Nos. 11222220, 90816007, 91116006, 10902059, and 11320101001) and Tsinghua University Initiative Scientific Research Program (Grant No. 2011Z02173).
Journal of the Mechanics and Physics of Solids，2013，61（8）：1737-1752
Transfer printing is an exceptionally sophisticated approach to assembly and micro-/nanofabrication that relies on a soft, elastomeric ‘stamp’ to transfer solid, micro-/nanoscale materials or device components from one substrate to another, in a large-scale, parallel fashion. The most critical control parameter in transfer printing is the strength of adhesion between the stamp and materials/devices. Conventional peel tests provide effective and robust means for determining the interfacial adhesion strength, or equivalently the energy release rate, and its dependence on peel speed. The results presented here provide analytic solutions for tests of this type, performed using viscoelastic strips with and without patterns of relief on their surfaces, and validated by systematic experiments. For a flat strip, a simple method enables determination of the energy release rate as a function of the peel speed. Patterned strips can be designed to achieve desired interfacial properties, with either stronger or weaker adhesion than that for a flat strip. The pattern spacing influences the energy release rate, to give values that initially decrease to levels smaller than those for a corresponding flat strip, as the pattern spacing increases. Once the spacing reaches a critical value, the relief self-collapses onto the substrate, thereby significantly increasing the contact area and the strength of adhesion. Analytic solutions capture not only these behaviors, as confirmed by experiment, but also extend to strips with nearly any pattern geometry of cylindrical pillars.
Viscoelasticity Peel test Patterned strips Energy release rate
Coherent gradient sensing (CGS) method, a real time, full-field optical technique, is insensitive to vibrations and able to provide slope and curvature maps and surface topographies, to investigate non-uniform deformations. In this paper, we analyze the thermal effects on the optical path in CGS due to air convection, and the influence of grating thickness and refractive index on the measurement accuracy. A modified governing equation is derived considering the grating thickness, which is demonstrated by testing a standard sample. Finally, we apply CGS method to measure the full-field deformation of a specimen at high temperature.