A SnO 2 /AlGaN ultraviolet heterojunction phototransistor, which achieves high gain and fast speed based on longitudinal and lateral confinement of photogenerated holes, was proposed and developed in this work. The orthogonal transport mechanism of holes enhanced the localization effect of photogenerated holes at the base and meanwhile reduced the recombination of holes and electrons there, which thereby lowered the base barrier and promoted electron transport, resulting in high gain and fast response. The fabricated devices showed a small dark current of lower than 10 pA below 20 V bias, an ultra-high gain of 2.2×10 5 under 315 nm illumination, a very large photo-to-dark current ratio of 1.5×10 8 at 20 V, and a prompt impulse response with a rise /fall time of 1.2 μs/91.1 μs.

Poly(L-lactic acid) (PLLA) thin films with a highly oriented structure, successfully prepared by a fast friction transfer technique, were investigated mainly on the basis of synchrotron radiation wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR). The crystalline structure of the highly oriented PLLA film was remarkably affected by friction transfer temperatures, which exhibited various crystal forms in different friction temperature regions. Interestingly, metastable β-form was generated at all friction transfer temperatures (70–140 °C) between Tg and Tm, indicating that fast friction transfer rate was propitious to the formation of β-form. Furthermore, the relative content among β-, α′-, and α-forms at different friction temperatures was estimated by WAXD as well as FTIR spectroscopy. In situ temperature-dependent WAXD was applied to reveal the complicated phase transition behavior of PLLA at a friction transfer temperature of 100 °C. The results illustrated that the contents of β- and α′-forms decreased in turn, whereas the α-form increased in content due to partially melt-recrystallization or crystal perfection. Moreover, by immersing into a solvent of acetone, β-, α′-form were transformed into stable α-crystalline form directly as a consequence. The highly oriented structure was maintained with the chain perfectly parallel to friction transfer direction after acetone treatment, evidenced by polarized FTIR and polarized optical microscopy (POM) measurements.

The vibration of agricultural machinery significantly influences driving comfort and operation reliability. In recent years, research on the vibrational characteristics of agricultural machinery has gradually expanded. To solve the problem of the obvious vibration of a high-speed transplanter under operating conditions, this article examines the method of beam damage diagnosis for high-speed transplanters based on the strain mode. Based on the modal theory, the displacement and strain modal analysis of the beam of the high-speed transplanter is performed. The analysis shows that the relative deviation of the modal frequencies of the same order before and after the beam is damaged is small. There is no obvious abrupt change in the displacement mode shape, while the strain mode shape has a sudden change peak after the beam is damaged. Thus, the strain mode change rate is constructed as a diagnostic index for beam damage. The analysis results show that the strain mode change rate increases with the increase in beam damage degree. Finally, the least squares method is used to fit the corresponding relationship between the two quantities. The results show that the strain mode change rate is sensitive and reliable as an index of damage diagnosis, which can better determine the location and degree of damage to the beam.

In view of the consumption problem caused by the large-scale development of renewable energy in northwest China, an optimization model of matching power source for sending-end system is proposed, which is suitable for wide area renewable energy collected by flexible DC system. The complexity analysis and solution method of the model are also given. Based on bi-level programming, the model combines planning and operation of thermal power system in two different time scales. Finally, simulation results of the modified IEEE 30-bus system have verified the effectiveness of the proposed model. The research shows that the optimization model can realize matching power planning to improve the renewable energy consumption rate, and is suitable for the application scenario of high-voltage DC power grid at sending-end system.

Silicone/phosphor composite is a functional material used in light-emitting diode packages. In this paper, Young’s modulus of silicone/phosphor composite is studied experimentally and investigated with a multi-sphere random unit cell model. Experimental samples of pure silicone and four groups of silicone/phosphor composites are prepared and subjected to tensile loading. Young’s moduli of the samples are calculated from tensile curves. The results indicate that composite modulus increases with increasing content of phosphor, and silicone matrix is stiffened by the phosphor particles. In the simulation part, unit cell model is utilized and serves as the statistical representation of the original composite. The employed three-dimensional random unit cell model consists of silicone matrix and randomly dispersed phosphor particles with various sizes. Modeling results from random unit cell model are compared with experimental results and theoretical predictions. It is found that, with increasing volume fractions of phosphor, random dispersion and size variation of phosphor have increasingly significant influence on homogenized Young's modulus of silicone/phosphor composite.

Thickness-dependent ultrathin films of poly(3-hydroxybutyrate) (PHB) were investigated by using infrared reflection-absorption spectroscopy (IRRAS). It was first found that the autogenetic β-form of PHB could be easily obtained through confining the film thickness under 20nm at room temperature. Moreover, the interphase of PHB simultaneously exists with the β-form of PHB in the films with a thickness less than 10nm. With the film thickness increasing from several nanometers to about 30nm, the PHB molecules gradually carry out an orientation shift at the b-axis of PHB crystallite; simultaneously, for the α-form of PHB a conformational rearrangement probably takes place for the growth of helical crystal unit cells in the ultrathin films. The molecular structures and crystal evolutions at sub–100nm were explained in detail by analyzing the corresponding IRRAS features.

Volume fraction of phosphor in silicone has strong impact on microstructure and damage evolution of silicone/phosphor composite, which eventually affect the performance and reliability of light-emitting diodes (LEDs) packaging. In this paper, mechanical response of silicone/phosphor composite under exterior loading is investigated experimentally and numerically. Test samples of silicone with various amount of phosphor are prepared and subjected to tensile loading. Micro-scale morphology and fracture surfaces of samples are observed by scanning electron microscopy (SEM) after tensile tests. Interphase debonding can be identified on fracture surface and cracks are initiated around debonded phosphor particles. In order to provide micro-scale illustrations of the macro-scale behavior, a three dimensional multi-sphere random unit cell (RUC) model is introduced. Silicone matrix is modeled as hyperelastic solid and interphase region is modeled with cohesive law. Both experiment and simulation results demonstrate that, increasing the amount of rigid phosphor stiffens the silicone composite. On the other hand, increasing phosphor amount also shortens the average distance between rigid phosphor particles and intensifies strain localization in silicone matrix, which consequently results in interphase debonding and crack growth in the composite. This effect is regarded as the main mechanism that controls mechanical degradation of the silicone/phosphor composite.

To enhance light extraction efficiency and light quality of white light-emitting diodes (WLEDs), a randomly textured phosphor-in-glass (PiG), which can be fabricated by chemical wet-etching and screen-printing methods, is proposed. The performance of WLED with textured PiG is examined by experiments. Measurement results reveal that the luminous flux of WLED package with textured PiG increases by 4.6% compared that with flat PiG package. The increment is attributed to the fact that the roughened surface can reduce the total internal reflection (TIR) at the interface between air and PiG, leading to the enhancement of light extraction efficiency. Meanwhile, the textured PiG package reduces angular correlated color temperature (CCT) deviation from 1759K to 968K in the viewing angles of -75° to 75°, when the average CCT is about 5500K. With simplified process, low cost and superior optical properties, the textured PiG is expected to be a promising candidate for applications in solid-state lighting.

ABSTRACT Silicone/phosphor composite is a functional material used in light emitting diode (LED) packages. In this article, effect of phosphor settling on mechanical properties and microstructure of phosphor/silicone composite is investigated experimentally and numerically. Test samples of silicones with various degrees of phosphor settling were prepared and uniaxial tensile tests were conducted. The results indicate that, for specific volume fraction of phosphor, phosphor sedimentation tends to reduce the strength and elongation of overall composite. And with increasing degree of sedimentation, the weakening effect becomes more significant. The fractographs of the test samples indicate that cracks initiate around the bottom area where phosphor particles settle. Numerical investigations, which were conducted by random unit cell model with graded particle distribution, demonstrate that strain localization and stress concentration are significant where phosphor particles concentrate. It can be concluded that, to reduce mechanical degradation, phosphor sedimentation should be minimized in silicone/phosphor composite for LED packages. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132 , 42006.

Angular color uniformity (ACU) is a key optical property of white light-emitting diodes (WLEDs) and high ACU is strongly demanded in illumination applications. In this paper, a multilayer phosphor-in-glass (PiG), which can be produced by the screen-printing method, is proposed to improve the ACU of LED packages. The screen-printing method provides a feasible scheme to produce multilayer, various-shaped PiG with a controllable pattern. Angular correlated color temperature (CCT) distributions of the LED packages with multilayer PiG are simulated numerically and measured experimentally. Optical performance of the LED package with optimized three-layer cone-shaped PiG is compared with that of packages with two-layer and one-layer PiG. The experimental measurements indicate that the deviation of angular CCT can be reduced from 761 to 171 K by cone-shaped PiG at average CCT of 6000 K. The results demonstrate that higher angular color uniformity can be achieved by LED packages with multilayer cone-shaped PiG.

Polymer based organic photovoltaic systems, which can be mass-produced by roll-to-roll printing technology, hold the promise for a cost-effective, lightweight and mechanical flexible solar energy conversion platform. Nevertheless, challenges still remain for large-scale applications of these flexible solar cells. One of the issues of great importance is the long-time reliability of solar cell module. It is demonstrated that organic solar cells can have wide range of applications, and therefore the surrounding temperature, humidity and load conditions vary. Rapidly changing thermo-mechanical loads often produce early failure of certain parts of the solar cell module, or even fracture of overall packaging which may result in the degradation of performance and life of solar cell. Therefore our work mainly focuses on the effect of elevated temperature on mechanical properties of materials in flexible solar cell panel. Temperature induced mechanical degradation and its effect on reliability of the packaging module are investigated. In our test, the samples made from flexible solar cell panel are stretched at a certain strain rate by test machine at 25°C, 75°C and 125°C, respectively. We find that Young's modulus of the materials decrease sharply with increasing temperature from the nominal stress-strain curve of materials and interfacial delamination occurs prior to material fracture at higher temperatures. Test results indicate that high temperature will not only reduce the mechanical stiffness of the materials in flexible solar cell, but also weaken the tensile strength of the material, which eventually undermined the reliability of solar cell packaging.

Vibrational spectroscopic methods have been used to determine the crystallinity of poly(l-lactic acid) (PLLA), which is the most popular bio-plastic today. However, it is found that influence of crystal polymorphism of PLLA on the quantitative method proposed for crystallinity calculation based on IR technique has seldom been considered. Herein, by preparing a set of PLLA specimens with different crystallinity and crystal forms, the absorption coefficient ratios between amorphous and crystal bands for evaluating the absolute crystallinity of PLLA α and α′ crystal form have been derived, respectively. In addition, a comparison of the proposed IR method with other techniques used to analyze crystallinity of PLLA such as X-ray diffraction (XRD) and differential scanning calorimetry (DSC) is presented. The origin of the disagreement on the results of quantitative measurements of crystallinity by different methods is also discussed. This study provides a simple spectral method to determine the crystallinity of PLLA with various crystal modifications.

Based on the rigid body kinematics theory, a mathematical model of a multi-link suspension was established, which was followed by the validation using the corresponding simulation model based on multi-body system dynamics theory. With perturbation method, multiple factors sensitivity analysis was researched about key hard-points of affecting suspension K&C characteristics, which were optimized by applying the PDJI-MOPSO algorithm, and the Pareto optimal solution set was obtained. The results show that the values of camber angle, toe angle and wheel travel track are reduced. The optimized suspension is more conducive to vehicle handling stability, and reduces tire wearing loss as well.

In this study, we studied the phosphor sedimentation effect in white phosphor-converted light-emitting diode packages by modeling the multi-layer phosphors with gradient concentrations. The essence of phosphor sedimentation can attribute to the variation of phosphor concentrations. By modifying the Kubelka-Munk theory, we built a multi-layer phosphor model with considering the light scattering, light absorption, and light conversion process simultaneously. With a brief review of Kubelka-Munk theory, multi-layer phosphors were modeled on the basis of single-layer phosphor model. The phosphor sedimentation effect was characterized by modeling multi-layer phosphors with gradient concentrations, whereas keeping the total amount of phosphors at the same level. It is found from the five calculation cases that phosphor sedimentation will cause the drop of light extraction efficiency (LEE) by 13.04%. Furthermore, the phosphor layer with inverse-gradient concentrations will enhance the LEE 16.56%. To figure out the reasons, the light losses were calculated, and it is proved that the light loss is enhanced when phosphor sedimentation happens.

By cold drawing the amorphous PLLA sample at 65 °C (just above the glass transition temperature of PLLA), a set of uniaxially oriented samples with various draw ratios was prepared. The mesomorphic ordering and chain orientation in PLLA mesophase induced by the cold drawing process was examined by the WAXD and polarized FTIR techniques. Of note, it is interesting to find that the characteristic backbone vibration mode of PLLA mesophase around 918 cm−1, which has been firstly identified by Stoclet et al. in previous study [Macromolecules 2010, 7228–7237], actually demonstrates a symmetrical peak shifting with increasing the draw ratio. By considering the WAXD data, the correlation between the peak positions for this characteristic backbone vibration mode (located around 918–921 cm−1) that related to the chain conformation and the distance of interchain packing in PLLA mesophase obtained at various draw ratios was revealed.

Polymer-based organic solar cells are attractive in that they can be manufactured on plastic substrates by a variety of printing techniques and thus inexpensive large-volume manufacturing should be possible. In order to reach cost-effective, flexible and stable organic photovoltaic device, mechanical properties of package structure and materials should be concerned. The flexibility of the structure and material properties like ultimate tensile strength, are closely connected to long-time reliability of photovoltaic device subjected to mechanical load. For these considerations, we are motivated to quantitatively analyze the mechanical behavior of structure and materials of a solar cell panel. In our research, a five-layered solar cell structure is studied. The functional part, which consists of a photo-active layer and two electrode layers, is protected by cover and substrate layers. Individual layers and the overall structure have been subjected to tensile loads, and tensile curves are recorded during the tests. Test results indicate that polymer-based layers, which include cover, photo-active and substrate layers, play a dominant role on feature of flexibility of organic solar cell. Indium tin oxide (ITO)-based electrode layers, which have elongation of only 0.2%, are the weakest parts of the overall package. Further studies by scanning electron microscope (SEM) reveal that the fracture surface of electrode layers is flat and smooth. This typical brittle feature in the fractograph is consistent with the macro behavior of the electrode layer. As electrode layers turn out to be the short slab of the package; transparent, conductive and flexible material such as carbon nanotube, graphene or nano silver wire can become possible candidates for electrodes of future organic solar cell, which can result in more reliable packaging modules and systems.

Light emitting diodes (LEDs) hold the promise for efficient, lightweight and environmental-friendly lighting sources, which is regarded as the next-generation lighting technology. Since so many problems still remain to be solved, this emerging technology has attracted the attention of a large number of researchers. Among several issues frequently discussed in the community, the long-time reliability of LED module is crucial. Since the packaging material and structure play an indispensable role on the reliability of LED, we are motivated to study the material properties of silicone and the interfacial strengths of silicone on various substrates. These properties are closely connected to the reliability of LED packaging, yet they have rarely been investigated. Uniaxial tensile test is adopted to obtain mechanical properties of silicone and interfacial shearing test is used to investigate the interfacial behavior of silicone on three different substrates. The results of tensile tests show that the ultimate tensile strength of tested silicone is around 0.4MPa at room temperature, exceeded stress will result in failure of the material. The fractograph analysis is conducted by scanning electron microscope (SEM) to provide micro-scale explanations of the fracture under tensile load. In the shearing test part, for the interface of silicone on copper substrate, the fracture force is 8.05N, while the fracture forces of silicone-ceramic and silicone-silicon interfaces are 7.34N and 6.87N, respectively. Since interfacial fracture force is an important indicator for interfacial delamination, it can be summarized that the interfacial strength of silicone-copper is the highest among the tested samples.

The bulk and thin film crystallization behaviors of PLLA/CNTs nanocomposites were investigated by in situ transmittance IR and reflection absorption IR spectroscopy, respectively. The results show that CNTs can enhance both the cold and melt crystallization rates of bulk PLLA, which is consistent with the common observation in the literature. However, it is interesting to find that the crystallization rate of PLLA thin film is suppressed by the addition of CNTs. Moreover, it is found that the existence of CNTs could not alter the preferred orientation of PLLA helical chain in the thin film. The different crystallization kinetics of PLLA induced by the addition of CNTs in bulk and thin film were explained by the confined crystallization behavior of polymer thin film.

The phenomenon that phosphor particles tend to settle in silicone is widely known. Recent researchers have discussed the effects of phosphor concentration on luminous efficacy of LED packaging. But to produce reliable products, mechanical and interfacial considerations are also essential. In this paper, the mechanical behaviors and interfacial strength of silicone with different levels of phosphor concentration are studied. In our experiments, four groups of silicone samples with different levels of phosphor concentration are prepared and are subjected to the uniaxial tensile loads. The results of tensile tests indicate that, for the same level of phosphor addition, higher phosphor concentration will result in lower tensile strength of the phosphor-filled silicone. SEM cross-section images show that the phosphor particles concentrate in the bottom area of the silicone layer. The fractograph of the material indicates that the crack initiates among the bottom area where phosphor particles settle. Interfacial tests of phosphor-filled silicone and GaN substrate are also conducted. The results demonstrate that phosphor concentration has negative effect on interfacial strength of materials. Therefore, for LED package, uniformly distributed phosphor in silicone is extremely demanded for mechanical and interfacial considerations.

Among several materials used in LED packaging, phosphor-filled silicone plays an important role on optical performance and long-time reliability of the module. Many researchers have studied optical properties of phosphor, but few have considered mechanical behavior of phosphor-filled silicone. In this paper, mechanical properties of YAG phosphor-filled silicone are obtained. Tensile tests, which are conducted by universal testing machine, are adopted to investigate mechanical behaviors of materials. The results of tests indicate that elevated temperature has negative effects on the mechanical properties of both pure silicone and phosphor-filled silicone. At the temperature of 125°C, the ultimate tensile strength of the material is only about half that of the material at 25°C. Further studies demonstrate that phosphor addition can strengthen phosphor-filled silicone material. Elastic modulus, ultimate tensile strength and elongation of the material increase with the increased phosphor mass fraction. Nevertheless, interfacial tests of phosphor-filled silicone on the GaN substrate show that higher phosphor addition will result in lower interfacial strength of the material. The interfacial strength of silicone on GaN substrate reduces sharply as phosphor added into silicone.