## Vol. 65, No. 18 (2016)

##### 2016-09-20

## INVITED REVIEW

2016, 65 (18): 184203.
doi: 10.7498/aps.65.184203

Abstract +

Ultraviolet laser operating at 355 nm has been found to have wide applications in scientific and industrial fields of laser radar, biological fluorescence medicine, micro processing, laser marking and laser ablation, owing to its superior properties of short wavelength, high single-photon energy, and high resolution. In addition, 355 nm laser plays a vital role in promoting the development of RGB full color display because it can be used as an excitation source for investigating the blue light emitting materials.
LiB3O5 (LBO) crystal possesses relatively high nonlinear coefficient and high optical damage threshold. Therefore, it is generally employed to generate 355 nm light through the third harmonic generation (THG) of the Nd:YAG laser (1064 nm). However, the CsB3O5(CBO) crystal, which also belongs to B3O7 group has attracted more attention for its larger nonlinear coefficient.
The temperature sensitivity is another important characteristic of the nonlinear crystal. Temperature fluctuation can cause the variation of refractive index of nonlinear optical crystal, which leads to phase mismatch and thus affects the nonlinear conversion efficiency. The principal refractive index of CBO crystal was accurately measured using the auto-collimation method in a temperature range from 40 to 190 ℃ for the first time by Zhang et al. in 2013 [Zhang G C, et al. 2013 Opt. Lett. 38 1594], while the temperature bandwidth of CBO for 355 nm THG has not been reported.
In the present paper, a high-power 355 nm laser is produced by efficient THG of an acousto-optic Q-switched quasicontinuous wave 1064 nm laser in CBO crystal. The master-oscillation power-amplification (MOPA) system with Nd:YAG crystal which is side pumped by high-power pulsed laser diode (LD) array delivers 210 W of a quasi-continuous Q-switched 1064 nm laser power. The laser operates at a 1 kHz repetition rate, and each pulse train contains five Q-switched pulses each with a duration of 40 ns. The 98 W of 532 nm green light is produced by second-harmonic generated in type-I LBO crystal. The 28.3 W ultraviolet laser is achieved by a 30-mm type-II CBO crystal through the sum frequency of 1064 nm and 532 nm light. The conversion efficiency from the fundamental light to the third harmonic reaches 13.5%, which is 28.6% higher than that obtained with a type-II LBO crystal under the same experimental conditions. The temperature sensitivity of CBO crystal in the 355 nm THG process is studied. Its temperature bandwidth is 25, which is much broader than that of LBO crystal. The experimental results show that the CBO crystal is superior to LBO crystal in the sense of conversion efficiency and temperature sensitivity for THG of 355 nm.

## INVITED REVIEW

2016, 65 (18): 188501.
doi: 10.7498/aps.65.188501

Abstract +

Superconducting nanowire single photon detector (SNSPD) is a competitive candidate in laser ranging at 1064 nm wavelength compared with other single photon detectors such as InGaAs/InP APD for its high sensitivity, high time precision and low dark counts. In this paper, we apply our SNSPD to a laser ranging system measuring target in Qinghai lake area with atmospheric scatter. The echo photons are received by telescope, and transport through the multimode fiber to the SNSPD photon-sensitive area. The SNSPD, integrated in an optical cavity with a resonant wavelength of 1064 nm, is fabricated on a MgF2 substrate. The optical absorption of NbN film goes up to 98% according to FDTD simulation, and the system efficiency is measured to be about 40%. A pulsed laser at 1064 nm, featuring a peak power of 12 MW and a pulse width of 10 ns, is adopted in the laser ranging system. In this experiment, we first measure the system intrinsic noise and the environment noise introduced into the laser ranging system after turning off the laser. After that, we measure the echo rate for the target at 126 km, which increases up to 96% with an attenuator of 10 dB at the receiver side. The maximum distance of the laser ranging system is analyzed based on the experimental results of dark count and echo rate through a theoretical model of laser radar. The analysis indicates that signal-to-noise ratio (SNR) is increased smoothly with the accumulation of time. At the same time, we simulate how the dark counts influence the capability of laser ranging system based on SNSPD, the simulated SNR matches well with the experimental data of target at 126 km. Furthermore, the dark counts, accumulation of time and probability of echo photon affect the SNR according to the simulation results, showing that large dark counts would result in SNR fluctuation and signal annihilation when the probability of echo photon is low. Thus, the maximum distance of laser ranging under the assumption of integration time is estimated through the SNR simulated result, showing that a maximum distance is up to 280 km, 40 km far away from APD detector based system under the same conditions mainly due to the very low dark counts of SNSPD. It should be pointed out that the coupling efficiency between SNSPD and the receiving telescope is low for small view field limited by the 62.5 m fiber of SNSPD. Thus, further work is to fabricate SNSPD with a larger coupling area which is possible to increase the maximum distance with improved coupling settings.

## INVITED REVIEW

2016, 65 (18): 188801.
doi: 10.7498/aps.65.188801

Abstract +

In recent years, perovskite solar cell (PSC) has achieved power conversion efficiency as high as over 20 %, making it competitive with high-efficiency thin film solar cells such as CuInGaSe and CdTe solar cells. However, the critical issue of reliability and stability for PSC should be addressed since a significant degradation of photovoltaic (PV) performance at low temperature has been found regardless of planar mesoporous PSC. To reveal the degradation of PV performance in PSC, the temperature-dependent PV performance of the planar PSC is investigated in detail. A PSC sample is loaded into a cryostat chamber connected to a compressor and illuminated by a halogen lamp. The operating temperature varies from 200 K to 325 K and the current-voltage (J-V) characteristic of planar PSC is measured at different scan rates from 10 V/s to 0.0017 V/s. At a fast scan rate of 10 V/s, the PSC shows a low PV performance at either low temperature or high temperature. The short-circuit current (JSC), open-circuit voltage (VOC) and maximum power point (PMPP) are found to decline with the temperature decrteasing. Moreover, the J-V curve also shows the S-shape characteristic. This suggests that the inefficient transport of photo-generated carriers occurs in the PSC. Ions such as Pb2+, CH3NH3+ and I-vacancies cause the screening effect of built-in field and the photo-generated carriers cannot be separated nor collected efficiently. As a result, JSC and VOC show small values in J-V curves measured at a fast scan rate. However, the degradation in PV performance is temporary. The PV performance gradually reaches a steady state at different operating temperatures with scan rate going down to 0.0017 V/s. The PMPP and VOC increase with temperature decreasing. These results indicate that a long illumination time is necessary for PSC to reach a steady state. After long-time illumination under biased condition (i.e., J-V curves measured at slow scan rate), the built-in field is compensated for by the external bias and the ions piling in the interface regions have enough time to diffuse towards the opposite direction. Thus, the screening effect of built-in field is reduced and the PV performance of PSC reaches a steady state. According to the result of device simulation, the increasing VOC at low temperature is attributed to the enhanced built-in potential difference and the reduced recombination rate of carriers. The temperature-dependent external quantum efficiency measurements of planar PSC before and after light illuminationis are performed to investigate the mechanism of carrier transport. It reveals that the separation and collection efficiencies of photo-generated carriers can be improved significantly after light illumination due to the fact that the screening effect of built-in field is reduced. These findings help understand the carrier transport mechanism in planar PSC.

###### SPECIAL TOPIC — Progress in Soft Matter Research

2016, 65 (18): 183601.
doi: 10.7498/aps.65.183601

Abstract +

In this brief review, we look back on the conception of nano-atoms and their gradual evolutions into a new class of giant molecules in the context of soft matter science. The structural features and the characteristics about giant molecular self-assembly are summarized. It is found that these giant molecules with high conformational rigidities and precisely-defined shapes and symmetries can exhibit unusual phase structures and phase transition behaviors which are not commonly observed in conventional polymers. Their self-assembly is robust due to collective and cooperative interactions among nano-atoms, forming hierarchical structures that are sensitive to their primary structures. This modular feature is reminiscent to the domain concept in protein science. It is thus proposed that nano-atoms can serve as unique elements for macromolecular science.

2016, 65 (18): 184701.
doi: 10.7498/aps.65.184701

Abstract +

Colloidal suspension is composed of particles with sizes between 1 nm and 1 m, suspended in liquid phase. The interaction between the particles consists of a hard core repulsive interaction and other kinds of repulsive and attractive interacions. Hard interaction forbids the particles from occupying the same places, resulting in a depletion effect. When big colloid particles are immersed in a colloid of small particles, each big particle has a depletion layer where the small particles cannot enter due to the hard interaction. The depletion layers of two big particles overlap when they are close enough so that extra free volume of the small particles increases and therefore the entropy of the small particles increase, thus an effective interaction between big particles is induced. This effective interaction is the so-called depletion interaction. In this review the concepts and an intuitive explanation of depletion interaction of colloidal suspensions are presented. The numerical calculation methods, including the acceptance ratio method, Wang-Landau-type method, and density functional theory method, are briefly reviewed. Several useful analytic approximations are presented. Stating from the depletion interaction between two flat plates, the Derjaguin approximation is introduced through the Asakura- Oosawa model. With this approximation, the approximate formulas of depletion interaction between two hard spheres, between a hard sphere and a hard wall, and between a hard sphere and curved hard walls in a small hard sphere colloid are derived. The depletion interaction between two hard spheres in a thin rod colloid and a thin disk colloid are also derived in the Derjaguin approximation.

2016, 65 (18): 186101.
doi: 10.7498/aps.65.186101

Abstract +

A majority of the physical, biological, chemical and environmental processes relate to the interfacial water. However, for the interfacial water itself, there are still many puzzles unsolved, which have made the interfacial water an important scientific research object for quite a long time. In this paper, we review some recent progress on the dynamics of interfacial water confined in one-dimensional and two- dimensional spaces, and on the surfaces on biomolecules and materials as well.

2016, 65 (18): 186102.
doi: 10.7498/aps.65.186102

Abstract +

This paper summarizes some theories widely used in soft matter systems, such as elastic theory, phase transition theory, scaling law, theory of granular particles, self-consistent field theory, etc. The role entropy plays in softmatter systems is also discussed. Other dynamic theories like adhesion, diffusion, wave motion, etc. are not included here.

2016, 65 (18): 186401.
doi: 10.7498/aps.65.186401

Abstract +

In this paper we shortly review theoretical progress in the field of active matter, focusing on the continuum theory of dry systems, in which momentum of active particles is not conserved due to the interaction between the particles and a substrate or medium. In particular, we review the phenomenological way of deriving hydrodynamic equations for both polar and apolar systems, and the predictions of these theories such as long-ranged orientational order in two-dimensional polar systems and giant number fluctuations. The comparisons among theoretical predictions, numerical results, and experimental evidence are also summarized.

2016, 65 (18): 186801.
doi: 10.7498/aps.65.186801

Abstract +

Nature always supplies inspirations to scientists and engineers. Many newfangled materials have been fabricated by learning from and mimicking nature. In daily life and industrial processes these bioinspired novel materials have been widely used. The special wettability of natural organisms is significant to their life and attractive to researchers, which inspires us to fabricate the functional interfacial materials with high performances. In the last decade, the bioinspired multiscale interfacial materials exhibiting superwettability have emerged as a new type of functional material. Superwettable materials offer great chances to solve numerous issues ranging from fundamental research to practical exploration, and from bionic philosophy to fabricating technology. Inspired by nature's example, researchers developed a series of scientific strategies of new materials and fabricating methods, technologies, and applications. Based on the requirement of developing advanced materials in the fields of energy, environment, healthcare and resource, superwettable materials possessing binary cooperative nanostructure have been widely investigated to solve scientific and technical problems. In this review, we firstly present the development history of bioinspired multiscale interfacial materials with superwettability and the theoretical basis of the wettability of solid surfaces. Secondly, the principles of superwettable functional surfaces in nature is revealed and the bionic designs of bioinspired materials are discussed in detail. Meanwhile the typical applications of superwettable materials such as self-cleaning, oil-water separation and green printing are introduced. Finally, the perspectives of the future development of bioinspired superwettable materials are proposed for further studying the superwettable materials.

2016, 65 (18): 188103.
doi: 10.7498/aps.65.188103

Abstract +

Soft matter has become one of the most active fields since the 1990 s, for it has enormous interesting behaviors and a broad range of applications. Rational continuum mechanics, as a subject mainly dealing with the kinematics and deformation of materials modeled as continuous mass, is a main source of inspiration in the development of soft matter physics. Here we review the development of rational continuum mechanics and soft matter briefly, and focus on the basic mechanical models and constitutive relations relating to soft matter: entropy elasticity, hyperelasticity, viscoelasticity, poroelasticity, non-Newtonian fluid, and the constitutive equations of these models. We simultaneously introduce the applications of these equations in hot issues in recent years, such as brain, blood vessel, cartilage, muscle, gel, cell, three dimensional printing, etc. According to applications and advances in soft matter mechanics, we then propose the key scientific problems and research fronts: mechanics of the solid-liquid interfacial interactions, introducing multiple factors into constitutive equations to describe the complex behaviors of soft matter in coupling multi-physics, and enhancing connections between soft matter mechanics and soft matter physics, chemistry, biology, etc. Finally, we conclude that the rational continuum mechanics in soft matter could be further developed in energy development, fabrication and analysis of diverse soft materials, and biomedicine development areas.

2016, 65 (18): 188201.
doi: 10.7498/aps.65.188201

Abstract +

In this paper, the history and the recent development of polymer crystallization have been reviewed briefly. After introducing the conventional Hoffman-Lauritzen theory, the recent new experimental results, especially on X-ray scattering, have been summarized. Some new models of crystallization have been reviewed, such as Strobl's mesomorphic phase model, Olmsted's spinodal-assisted crystallization theory, and Muthukumar's molecular modeling of polymer crystallization.

2016, 65 (18): 188301.
doi: 10.7498/aps.65.188301

Abstract +

The field-induced soft smart material is a kind of soft matter whose macroscopic properties (mechanical, or optical) can be significantly and actively controlled and manipulated by external field such as magnetic field, electric field, temperature or light. In this paper, we briefly review the research and application progress of the filed-induced soft smart materials in recent years and discuss the development problems and trend in this research area. In particular, we focus on three typical field-induced soft materials of smart materials: magnetorheological fluid, electrorheological fluid, and temperature and light sensitive polymer gel.

2016, 65 (18): 188701.
doi: 10.7498/aps.65.188701

Abstract +

Lipid membrane is a continuous barrier between cell and organelle, providing relatively separate room for the vital biological reaction to take place and guarantee substance, energy and information exchange between cells and organelles. Helfrich proposed a spontaneous curvature model to describe the free energy of lipid bilayer. This article reviews the equations describing the equilibrium morphologies of closed lipid membranes and lipid membranes with free edge based on the spontaneous model, and some analytic solutions are provided as well. The practicality of proving linking condition for splitting vesicle is also discussed.

2016, 65 (18): 188702.
doi: 10.7498/aps.65.188702

Abstract +

Kinesin is one of the most important linear motors for intracellular transport. It has two main features. One is its persistence: at least one head is attached to the microtubule during stepping, so that it can move a long distance before detaching. Another feature is the tight mechanochemical coupling: it consumes one adenosine-triphosphate for each step. Therefore, there should be a mechanism responsible for the coordination of the two heads to achieve the high persistence and tight coupling. The underlying mechanism is the mechanochemical coupling, which is the basic issue for all chemical-driven molecular motors. Owing to the developments of single-molecule experiments and molecular dynamics simulations, a breakthrough in the coupling mechanism has been made in recent decades. In this article, we review the progress of the relevant researches from the perspective of kinematics, energetics, coordination of two heads and force generating mechanism. We also present a personal perspective on the future studies of kinesin.

2016, 65 (18): 188703.
doi: 10.7498/aps.65.188703

Abstract +

Elastomeric proteins are a special class of proteins with unique mechanical functions. They bear, transduce mechanical forces inside cell, and serve as biomaterials of high elasticities and strengths outside cell. Depending on their functions, the mechanical properties of elastomeric proteins are very diverse. Some of them are of high mechanical stability and the others are of high extensibility and toughness. Although many elastomeric proteins are engineered for the applications in the fields of biomaterials and nanotechnology, the molecular determinant of the mechanical stability remains elusive. In this review, we summarize recent advances in the field of protein mechanics studied by using single molecule force spectroscopy. Force spectroscopy enables people to probe the unfolding properties of protein domains, thus paving the way for building special proteins with characteristic mechanical functions. To begin with, it is necessary to clarify the factors and their relations with the unfolding force, which is deduced based on Bell's expression. It turns out that the unfolding force is proportional to pulling speed when the speed is relatively small, and has a logarithmic relation in the high-speed approximation. After the external determinant of the force probe is clarified, some intrinsic factors are to be discussed. Hydrogen bound and electrostatic force, rather than covalent bond, contribute to the mechanical performances of proteins. Those interactions rely on the topology structures of protein molecules. By changing the structures of proteins, researchers now manage to change the mechanical characteristics of certain proteins. Since single protein is unable to be detected by traditional optic microscope, three devices used to observe and manipulate single protein are introduced in the present paper. These include atomic force microscopy, magnetic tweezers and optical tweezers. Among them, a more detailed explanation of atomic force microscope (AFM) is provided, which briefly describes the basic mechanism and structure of AFM and possible explanation for the formation of force-extension curves. After that, several recent advances for improving the AFM based single molecule force spectroscopy techniques are highlighted. For example, Tom Perkins group [Sullan R M A, Churnside A B, Nguyen D M, Bull M S, Perkins T T 2013 Methods 60 131] has discovered that the gold-stripped tip gives more accurate and reproducible results than a gold-coated one. Matthias Rief group [Schlierf M, Berkemeier F, Rief M 2007 Biophys. J. 93 3989] has managed to increase the resolution of AFM, pushing it in pair with optical tweezers. Hermann Gaub et al. [Otten M, Ott W, Jobst M A, Milles L F, Verdorfer T, Pippig D A, Nash M A, Gaub H E 2014 Nat. Methods 11 1127] combined the microfluidic chip and DNA expression in vitro to increase the yields of interpretable single-molecule interaction traces. Toshio Ando et al. [Ando T, Uchihashi T, Fukuma T 2008 Prog. Surf. Sci. 83 337] have developed methods to increase the imaging speed of AFM. Finally, the rationally designing the mechanical properties of protein-based materials pioneered by Hongbin Li group is highlighted. They have discovered direct relationship between the mechanical properties of individual proteins and those of the protein materials. To sum up, with AFM, scientists now can explore mechanical properties of a wide range of proteins, which enables them to build biomaterials with exceptional mechanical features.

2016, 65 (18): 188704.
doi: 10.7498/aps.65.188704

Abstract +

Investigations of the growth-induced deformations of soft biological tissues may help understand the underlying mechanical mechanisms of their morphogenesis and provide clues for diagnosing some diseases. In the framework of continuum mechanics, we establish a three-dimensional model to analyze the instabilities of cylindrical soft tissues induced by volumetric growth. The different three-dimensional wrinkling patterns under either free or fixed boundary conditions at the outer surface are considered. It is found that Euler buckling, axially symmetrical wrinkling, and checkerboard wrinkling may occur under the traction-free boundary conditions, while axisymmetric pattern and checkerboard pattern often appear under the fixed boundary conditions. Phase diagrams are constructed to predict the morphologies in terms of the geometrical and material parameters of the system. Besides, a pseudo-dynamic numerical method is invoked to simulate the postbuckling evolutions of the wrinkling patterns.

2016, 65 (18): 188705.
doi: 10.7498/aps.65.188705

Abstract +

Traditional cancer researches focus on the analyses of the mice biopsy in order to understand the formation of cancer and the stage of cancer development. In contrast to in vivo experiments, in vitro investigation of cancer cells provides the flexible manipulation of the experimental parameters and the real time observation of the growth and reproduction of cancer cells, thus has been developing rapidly. However, further studies have demonstrated that cells' behavior in a two-dimensional (2D) environment, e.g. Petri dish, is dramatically different from that in a three-dimensional (3D) environment. Therefore, with assistance of bio-microfluidic chips, 3D bio-printing, direct femtosecond laser writing technology and UV curing hydrogel technology, an increasing number of 3D models have been developed to investigate the behaviors of cancer cells in vitro. Nevertheless, the existing technology is also facing the contradiction between accuracy and speed requirements, as well as the biocompatibility and biodegradability of scaffold materials in use.
In this paper, we first summarize and compare present 2D models, e. g. Agar Plate and Boyden Assay, and the developing 3D models in vitro experimental approaches as mentioned above, and discuss the merits of these fabricating technologies. Then we focus on the recent progress and achievements of 3D bio-techniques, especially the successful applications in probing the invasion behaviors of cancer cells. Though significant progress has been made from 2D to 3D approaches and these in vitro experimental models are becoming more flawless in simulating the in vivo environment of cells, the following challenges remain: 1) biocompatible material with the appropriate mechanic properties simulating the environment in vivo; 2) the viability of cells in the complex 3D model with of biomaterial, especially during the laser or UV-assisted gelation of hydrogels; 3) the speed and resolution of the present 3D fabrication technologies; 4) the in situ observation and control of cells. Nevertheless, with the development of 3D bio-technologies, breakthroughs can be expected in solving those problems, and thus will guide the 3D experimental models for the invasion of cancer cells in next few years. This will eventually help people in the war towards cancers, and at the same time provide new experimental approaches for other relevant researches in the interdisciplinary fields of biology, physics, chemistry, materials and engineering.

2016, 65 (18): 188706.
doi: 10.7498/aps.65.188706

Abstract +

Biomolecules such as proteins and nucleic acids play critical roles in biological processes. Traditional molecular biological experimental techniques usually measure the properties of an ensemble of molecules. The detected signal originates from the average response of large number of molecules, which often conceals the detailed dynamic information about conformational transitions. In addition, many biomolecules, such as cytoskeleton proteins and molecular motors, are subjected to stretching forces or are able to generate force while playing their biological roles in vivo. It is difficult for traditional experimental methods to be used to study the mechanical response of biomolecules. Single molecule manipulation techniques developed in recent twenty years are capable of manipulating and measuring the property of single molecule. Especially, the force response of single molecule can be measured in high precision. The most popular single molecular manipulation techniques are atomic force microscope, optical tweezers, and magnetic tweezers. Here we introduce the principle, capability of force and extension measurement, spatial and temporal resolutions of these three techniques. Applications of single molecular manipulation techniques in the conformation transitions of DNA, protein, and their interactions, and mechanism of molecular motors will be briefly reviewed. This review will provide a useful reference to biologists to learn and use single molecular manipulation techniques to solve biological problems.

###### GENERAL

2016, 65 (18): 180201.
doi: 10.7498/aps.65.180201

Abstract +

By using the calculus of variations, the conservative mechanical systems can be formulated by Lagrange's equations or Hamilton's equations, which are the basis of establishing, simplifying and integrating the equations of motion. Thus it is important to find the solutions of inverse problems for different dynamical systems so as to construct the most of the Lagrange's equations and Hamilton's equations. However, the Lagrangian or Hamiltonian formulation for a dynamical system, limited by the conditions of self-adjointness, is not directly universal if the physical variables remain without using Darboux transformations.
Fortunately, Refs. [7, 11] show that based on the Cauchy-Kovalevsky theorem of the integrability conditions for partial differential equations and the converse of the Poincar lemma, it can be proved that there exists a direct universality of Birkhoff's equation for local Newtonian system by reducing the Newton's equations to a first-order form, which means that all local, analytic, regular, finite-dimensional, unconstrained or holonomic, conservative or non-conservative forms always admit, in a star-shaped neighborhood of a regular point of their variables, a representation in terms of first-order Birkhoff's equations in the coordinate and time variables of the experiment. The systems whose equations of motion are represented by the first-order Birkhoff's equations on a symplectic or a contact manifold spanned by the physical variables are called Birkhoffian systems.
At present, one of the most important tasks of Birkhoffian mechanics is to study the method of constructing the Birkhoffian and Birkhoffian functions. However, due to the complexity of Birkhoffian system, there exist only a few of results in the literature. Among them, the most famous main methods in this problem are achieved by Santilli[Santilli R M 1983 Foundations of Theoretical Mechanics II (New York: Springer-Verlag) pp12-15]. But the redundant term in Santilli's second method which is used as the classical construction method, is always neglected. As a result, the calculation procedure is tedious and complicated, and the efficiency is not high. Therefore, it is necessary to simplify the Santilli's second method. In Section 2, we will review the first-order standard form of holonomic system in the frame of Cartesian coordinates, which is the starting point of our studying the Birkhoffian systems. In Section 3, the Birkhoff's equations and the key role of Birkhoffian dynamics functions for deriving Birkhoff's equations are introduced. In Section 4, the redundant items are eliminated by using some mathematical operation skills, and then a more simplified constructing method is put forward. In Section 5, the findings in this study are summarized.
Through simplifying the Santilli's second method, we realize that the determining of the Birkhoff's equations by constructing the Birkhoffian functions is equivalent to the determining of its symplectic matrix. This view provides a new perspective for solving the problem of constructing the Birkhoffian functions. Finally, the simplified method is applied to Lagrangian inverse problem, and a simplified method of solving Lagrangian function is obtained.

2016, 65 (18): 180301.
doi: 10.7498/aps.65.180301

Abstract +

Since the discovery of its superconductivity, magnesium diboride (MgB2) has been identified as a promising superconductor to be used in Josephson junction devices due to its high transition temperature, large energy gap, long coherence length, and expected easier fabrication of Josephson junctions as compared with high temperature superconductors. The high-quality MgB2 films and excellent tunnel barrier materials are the core elements for a Josephson junction. Here in this paper, all MgB2 thin film tunnel junctions with B tunnel barriers are fabricated in situ on sapphire substrates and their tunneling characteristics re investigated. The experimental results indicate that the MgB2/B/MgB2 junctions exhibit good tunneling characteristics.
The deposition of the MgB2/B/MgB2 trilayer is carried out in a completely in situ process. The bottom and top MgB2 layers are grown to a thickness of 100 nm by hybrid physical-chemical vapor deposition (HPCVD) technique at about 973 K and in 102 Pa Ar atmosphere on a single crystal Al2O3 (0001) substrate. The 35-nm-thick amorphous B insulator layer is deposited using chemical vapor deposition method at 723 K and in 103 Pa pure Ar. In the process of the top MgB2 layer deposition, the amorphous B reacts with Mg in Mg vapor, leading to its thickness decreasing to 10 nm. Square-shaped junctions each with a size of 4 mm5 mm are determined by the metallic mask method. The resistivity temperature (R-T) curves and the DC current-voltage (I-V) curves of the MgB2/B/MgB2 junctions at different temperatures are measured by the four-point probe method in the physical property measurement system (PPMS). The experimental results show excellent superconducting properties of the top and bottom superconductor with high Tc (above 39.5 K), appreciable Jc values (107-108 A/cm2). In the I-V characteristics of junction at temperatures ranging from 4.2 K to 39.2 K, the junctions exhibit clear Josephson tunneling characteristics with jc～0.52 A/cm2 at 4.2 K, which remains nonzero up to 31.3 K. The hysteresis is pronounced at 4.2 K, becoming smaller as temperature increases, and eventually disappearing at around 19.2 K. By using the differential I-V curves, only gap is observed in differential conductance vs. voltage characteristics (dI/dV-V) curves, because MgB2 layer grown using HPCVD technique is always c-axis oriented and more than 99% contribution to the conduction is from band charge carriers.

2016, 65 (18): 180302.
doi: 10.7498/aps.65.180302

Abstract +

At zero-temperature and finite-temperature, the thermodynamic properties of finite unitary Fermi gas in a three-dimensional harmonic trap are investigated by using fractional exclusion statistics, and the results are compared with those of the system which satisfies the thermodynamic limit. At zero-temperature, Fermi energy and average energy of per particle increase with the increase of the number of particles for finite unitary Fermi gas, and their limits are the corresponding parameters of the system which satisfy thermodynamic limits. Fermi energy and average energy of per particle each have a maximum value changing with the boundary of the potential well. For the finite-temperature trapped unitary Fermi system, when the number of particles is certain the average energy of per particle, average entropy of per particle, average heat capacity of per particle each have a characteristic temperature, respectively, when the temperature is equal to the characteristic temperature of the physical parameter, the corresponding parameters for the finite system and the thermodynamic limit system are equal, when the temperature is lower (or higher) than the characteristic temperature of parameter, the physical parameter of the finite system will be greater (or less) than the corresponding parameter of the thermodynamic limit system. The characteristic temperature has particle number effect and boundary effect. When the temperature is determined, the average energy of per particle, average entropy of per particle and average heat capacity of per particle each have a characteristic number of particles, respectively, when the number of particles is equal to the characteristic number of particles for physical parameter, the corresponding parameters for the finite system and the thermodynamic limit system are equal, when the number of particles is less (or more) than the characteristic number of particles for corresponding parameter, the corresponding parameter of the finite system will be less (or larger) than the thermodynamic limit of system.

2016, 65 (18): 180501.
doi: 10.7498/aps.65.180501

Abstract +

The classical attractors, defined as self-excited attractors, such as Lorenz attractor, Rssler attractor, Chua's attractor and many other well-known attractors, are all excited from unstable index-2 saddle-foci, namely, an attractor with an attraction basin corresponds to an unstable equilibrium. A new type of attractors, defined as hidden attractors, was first found and reported in 2011, whose attraction basin does not intersect with small neighborhoods of the equilibria of the system. Due to the existences of hidden attractors, some particular dynamical systems associated with line equilibrium, or no equilibrium, or stable equilibrium have attracted much attention recently. Additionally, by introducing memristors into existing oscillating circuits or substituting nonlinear resistors in classical chaotic circuits with memristors, a variety of memristor based chaotic and hyperchaotic circuits are simply established and has been broadly investigated in recent years. Motivated by these two considerations, in this paper, we present a novel memristive system with no equilibrium, from which an interesting and striking phenomenon of coexistence of the behaviors of hidden multiple attractors and the corresponding multistability is perfectly demonstrated by numerical simulations and experimental measurements.
According to a newly proposed circuit realization scheme, a new type of four-dimensional memristive self-oscillated system is easily implemented by directly replacing a linear coupling resistor in an existing three-dimensional self-oscillated system circuit with a voltage-controlled memristor. The proposed system has no equilibrium, but can generate various hidden attractors including periodic limit cycle, quasi-periodic limit cycle, chaotic attractor, and coexisting attractors and so on. Based on bifurcation diagram, Lyapunov exponent spectra, and phase portraits, complex hidden dynamics with respect to a system parameter of the memristive self-oscillated system are studied. Specially, when different initial conditions are used, the system displays the coexistence phenomenon of chaotic attractors with different topological structures or quasi-periodic limit cycle and chaotic attractor, as well as the phenomenon of multiple attractors of quasi-periodic limit cycle and chaotic attractors with multiple topological structures. The results imply that some coexisting hidden multiple attractors reflecting the emergences of multistability can be observed in the proposed memristive self-oscillated system, which are well illustrated by several conventional dynamical analysis tools. Based on PSIM circuit simulation model, the memristive self-oscillated system is easily made in at a hardware level on a breadboard and two kinds of dynamical behaviors of coexisting hidden multiple attractors are captured in hardware experiments. Hardware experimental measurements are consistent with numerical simulations, which demonstrates that the proposed memristive self-oscillated system has very abundant and complex hidden dynamical characteristics.

2016, 65 (18): 180502.
doi: 10.7498/aps.65.180502

Abstract +

In this paper, the crises in a non-autonomous fractional-order Duffing system are investigated. Firstly, based on the short memory principle of fractional derivative, a global numerical method called an extended generalized cell mapping (EGCM), which combines the generalized cell mapping with the improved predictor-corrector algorithm, is proposed for fractional-order nonlinear systems. The one-step transition probability matrix of Markov chain of the EGCM is generated by the improved predictor-corrector approach for fractional-order systems. The one-step mapping time of the proposed method is evaluated with the help of the short memory principle for fractional derivative to deal with its non-local property and to properly define a bound of the truncation error by considering the features of cell mapping. On the basis of the characteristics of the cell state space, the bound of the truncation error is defined to ensure that the truncation error is less than half a cell size. For a fractional-order Duffing system, boundary and interior crises with varying the derivative order and the intensity of external excitation are determined by the EGCM method. A boundary crisis results from the collision of a chaotic (or regular) saddle in the fractal (or smooth) basin boundary with a periodic (or chaotic) attractor. An interior crisis happens when an unstable chaotic set in the basin of attraction collides with a periodic attractor, which causes a chaotic attractor to occur, and simultaneously the previous attractor and the unstable chaotic set are converted into a part of the chaotic attractor. It is found that a crisis can be generally defined as a collision between a chaotic basic set and a basic set, either periodic or chaotic, to cause the chaotic set to have a sudden discontinuous change. Here the chaotic set involves three different kinds of chaotic basic sets: a chaotic attractor, a chaotic saddle on a fractal basin boundary, and a chaotic saddle in the interior of a basin and disjoint from the attractor. The results further reveal that the EGCM is a powerful tool to determine the global dynamics of fractional-order systems.

2016, 65 (18): 180503.
doi: 10.7498/aps.65.180503

Abstract +

The entropy generation minimization is widely used to deal with optimization problems of heat transfer and heat-work conversion. However, it is found that the minimization of entropy generation does not always lead to the optimization of the design objectives in engineering. So, it is necessary to discuss the optimization direction and application preconditions of the entropy generation minimization. In this paper, we study this topic both theoretically and numerically.
Our analyses show that the concept of entropy generation directly measures the exergy loss or the ability loss of doing work, so the optimization objective of the entropy generation minimization is to minimize the exergy loss and maximize the ability to do work for the optimized system. However, we have different design objectives in engineering, such as the maximum heat transfer rate, the maximum heat exchanger effectiveness, the minimum average temperature of the heated domain, the maximum output power, the maximum coefficient of performance of heat pump systems, the homogenization of temperature field, etc. Not all of these objectives are consistent with the optimization direction of the entropy generation minimization. Therefore, it is reasonable that the entropy generation minimization is not always applicable. Furthermore, when the relationship between entropy generation and design objective can be set up, the application preconditions of the entropy generation minimization are also discussed. When the preconditions are not satisfied, the entropy generation minimization does not always lead to the best system performance, either.
Some examples are also presented to verify the theoretical analyses above. For heat transfer, a one-dimensional heat transfer problem and the entropy generation paradox in heat exchanger are analyzed. For the one-dimensional heat transfer problem, the entropy generation minimization leads to the minimum heat transfer rate when the temperature difference between the boundaries is fixed. Therefore, if our design objective is the maximum heat transfer rate, the entropy generation minimization is not applicable. When the heat transfer rate is fixed, smaller entropy generation rate leads to higher boundary temperature. Therefore, if our design objective is to reduce the boundary temperature, the entropy generation minimization is not applicable, either. For the entropy generation paradox, it is shown that the concept of entropy generation cannot describe the heat transfer performance of heat exchangers. Therefore, the paradox still exists and has not been removed to date. This is verified by the theoretical analyses and the numerical simulation for a parallel flow heat exchanger in which the irreversibility from the pressure drop can be ignored. For heat-work conversion, the energy flow and the exergy flow are analyzed. According to the analyses, we discuss the applicability of the entropy generation minimization to the heat-work conversion system in which the output power, the heat-work conversion efficiency and the thermo-economic performance are taken as the optimization objectives. It is also shown that the application of the entropy generation minimization is conditional. In a word, the discussion on the examples verifies the theoretical analyses.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2016, 65 (18): 184101.
doi: 10.7498/aps.65.184101

Abstract +

A novel technique to generate precisely size-controlled hollow beams by controlling the diameter of circular slit is proposed. Firstly, a laser beam is transformed into a quasi-monochromatic incoherent annular source by a rotating ground-glass disk and circular slit. Then, after passing through a thin converging lens, a J0-correlated Schell-model beam is synthesized by placing the annular incoherent source in the first focal plane of the thin lens. Finally, a partially coherent hollow beam is generated by focusing the J0-correlated Schell-model beam with an axicon. Based on the diffraction theory and the propagation law of partially coherent beams, the cross-spectral density function is derived to calculate the intensity distribution of the cross section and the radial intensity distribution along the propagation axis behind the axicon. By carrying out the theoretical calculation, the proposed optical system generates a partially hollow beam, and the size of the hollow beam expands continuously as the propagation distance increases. Before further investigating the effect of the diameter of incoherent annular source on the hollow beam behind the axicon, we also calculate the intensity distribution of the cross section and the size of hollow beams along the propagation axis at z=70 mm with the source diameters being 1, 2, 3, 4 and 5 mm, respectively. Results show that the size of the hollow beam also increases with the diameter of incoherent annular source increasing. In this case, the size of the hollow beam can be precisely controlled by tuning the diameter of incoherent annular source through circular slit. We also design and conduct an experimental generation of the hollow beam and investigate the propagation properties. In the experiment, we control the diameter of the annular source by tuning the diameter of the circular slit located before the rotating ground-glass disk. And the diameter of the annular source is equal to that of the circular slits. When the sizes of circular slits are 1, 2, 3, 4 and 5 mm, respectively, the corresponding hollow beams are measured by CCD. Experimental results show that the size of hollow beam can be controlled by the propagation distance and the diameter of the circular slit. The intensity profiles are in good agreement with theoretical predictions. Therefore, the size of hollow beams can be precisely generated and controlled by the proposed system so that the optical system can be flexibly employed in optical trapping and manipulation of particles with different sizes. The results may provide a powerful tool for manipulating the micro- and nano-particles.

2016, 65 (18): 184201.
doi: 10.7498/aps.65.184201

Abstract +

In order to reduce the second rescue injuries and deaths after the mashgas exploding in the mine, a portable imaging interferometer system is designed to detect CO temperature and concentration by the passive and remote measurement. The CO temperature and concentration are detected according to the rotational spectral line of CO gas molecule and the linear relationship between the radiation intensity of gas molecule and the molecule number density, respectively. The optical system is designed, and then its forward is studied in this work. The forward expression is obtained after studying the following four seed models of the optical system: the radiation model of target gas, where CO six emission spectral lines R11-16 are selected from HITRAN08 database; the mine CO gas transmission model in which the absorptions by the water vapor and CO2 molecule, and absorption and scattering by the mine aerosol are calculated by the relevant rules; the filter function model, in which the matched parameters of the band width of 0.5 nm and max transmittance of 0.23 for CO temperature are measured by the method of rotational line of R11-16, and the model of imaging detector CCD in which the infrared CCD of pixel 320320 and the max quantum efficiency of 0.75 are to be used in the optical system. According to the given parameters and MATLAB programming, the forward imaging interference results of CO differentiable six spectrum of R11-16 are obtained. The forward max noise-signal ratio is 268 when the exposure time is 300 s. The max electric count is 1.5105 that is larger than the selected CCD dark noise of 400 e but less than the CCD full charge quantity of 1106 e. The forward result clearly indicates that the optical system can meet the initial design demand. The accuracies of CO temperature and concentration measured by this optical system can reach 2 K and 0.1%, respectively. This portable system can be used to detect not only the mine CO, but also other gases like the pipe smoke, bomb exploding gas, etc. in which the filter and CCD need to be changed.

2016, 65 (18): 184202.
doi: 10.7498/aps.65.184202

Abstract +

As an expansion of coherent diffraction imaging, ptychographic iterative engine (PIE) not only inherits advantages such as ultra-high resolution and compact optical system, but also expands the field of view in quantitative imaging, thus PIE is widely used in short wavelength imaging such as X-ray and electron beam imaging, and then extended to visible light field. However, PIE requires coherent illumination for both phase and amplitude retrieval, while traditional X-ray or electron beam sources often cannot satisfy this strict coherent condition, which leads to poor-quality information retrieval with low signal-to-noise ratio. Though several proposed methods such as multiple wavelength and multi-mode algorithms can eliminate incoherency influence to some extent, various details such as quantitative spectrum of illuminating source should be obtained before information retrieval, which complicates computing procedures. In addition, it is hard to acquire the spectrum of the illuminating source in most cases. In order to acquire high-quality information based on PIE with partially coherent illumination, a newly designed enhanced phase retrieval method for weakly scattering samples in PIE with partially coherent illumination is presented in this paper, in which only the bright field of the diffraction patterns is used in the iterative procedures mimicking the coherent cases especially for weakly scattering samples without any prior illuminating details. The bright field area can be regarded as purely coherent diffraction patterns composed of a strengthened zeroth order beam and a weakened diffracted beam. While the dark-field area generated by interference of diffracted beams cannot satisfy the requirement for coherence, therefore, dark-field diffraction patterns should be excluded in sample information extraction and only the bright field is used for phase retrieval via iterative process. Compared with the proposed multiple wavelength and multi-mode algorithms, the proposed method can simplify sample reconstructing procedures due to needing no prior knowledge. Moreover, in order to enhance the information of weakly scattering samples in retrieval, high order iteration method is also introduced in the paper. Since the bright field can be regarded as purely coherent diffraction patterns composed of a strengthened zeroth order beam and a weakened diffracted beam. For weakly scattering sample, the weakened diffracted beam is much lower than zeroth order beam, thus it is difficult to acquire high-contrast information with classical PIE algorithms. Introducing high order iterative tactic, the contrast of weakly scattering sample is obviously improved and the details of weakly scattering sample can be retrieved clearly. Both theoretical analysis and numerical simulations are illustrated in detail, proving the robustness and availability of the designed method: high-contrast phase information can be obtained with the proposed method, while traditional phase retrieval algorithm almost loses all details of the sample. In order to mimic the real experimental situation, a 30 dB white noise is added into the simulation, the details of weakly scattering sample phase information can also be retrieved clearly by using the bright field of the diffraction patterns with high order iteration method. With the newly designed enhanced phase retrieval method for weakly scattering samples with partially coherent illumination, sample retrieval via PIE can not only use ordinary X-ray source or electron beam as illumination source, thereby avoiding the dependence on complete coherent source, but also obviously improve the retrieval quality of the sample characteristics, which widely expands the application fields of the PIE.

2016, 65 (18): 184301.
doi: 10.7498/aps.65.184301

Abstract +

Resonance peaks of spectral function transformed from echoes are the most important characteristics for distinguishing the different targets. So in frequency domain, response function is usually calculated with small interval in a wider frequency band to satisfy the demand of fast and high precision prediction in practical engineering. According to axis-symmetric model, we use 2 dimensional finite element method to solve the acoustic scattering problem efficiently, even when the scattering target has a large size and complex structure. This article focuses on the explanation of scattering characteristics of a special target, namely, a partially solid-filling cylinder with hemispherical cap and thin-shell. Supposing that the receiver and transmitter are in monostatic arrangement, we calculate scattering strength in far field in a frequency range of 50 Hz-10 kHz, and give pseudo-color image represented by frequency-angle to describe influences of shell, filling and the orientation of the incident wave on scattering properties. According to the numerical results, the following conclusions are given: when the transmitter is facing the hemispherical cap (the cap has a vacuum inside, and the incident angle θ is equal to 0°), the main contribution of scattered wave comes from the shell of target. When θ = 180°, the internal filling inhibits the elastic resonance of the shell, and plays an important role in the total scattering field. Because the acoustic impedance of the shell is much larger than that of the water, elastic resonance of the shell is more difficult to excite than that of the solid filling. While the material property of the solid filling is not significantly different from that of the water, so the elastic resonance of the filling fluctuates relatively fast, and the scattering function vibrates approximately with equal amplitude in a wider frequency band. When θ= 90°, the sound wave is perpendicular to the axis of the cylinder, the shell and the filling work together on scattered waves. Once the incident angle deviates from 90° and the sound wave obliquely illuminates target with respective to the axis of the cylinder, the echo of the filling material plays a predominant role in the total scattering field. The frequency-angle spectrum of the solid filling model presents the “bowl” type resonance curve. In order to validate which physical and geometrical structure must be considered in solution of scattered far field, the acoustic scattering experiments are performed in tank with a target suspending in water, which is in monostatic arrangement and satisfies the free field condition. Frequency of incident wave is in a frequency range of 10-40 kHz. For obtaining pseudo-color image of distance-angle, echoes are received and measured when the target is rotated from 0°-360°. The scattered waves are divided into mirror reflection and various components of elastic wave, and the mechanisms of these echoes are explained based on circumferential wave around the surface. Whispering gallery waves are also considered and clearly seen in the experiment. Due to the coupling interaction between the filling and elastic shell, the resonance curve of frequency-angle spectrum splays “bowl” curve outward the sides of normally direction. Experimental and numerical results are in good agreement, which is indicated by comparing the resonance peaks characteristic in spectral domain. The results of this article will be helpful in studying underwater target with more complicated structure.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2016, 65 (18): 185201.
doi: 10.7498/aps.65.185201

Abstract +

The solution of Grad-Shafranov equation in field-reversed configuration (FRC) is a basic problem. The solution of Grad-Shafranov equation can help to understand most of physical processes in FRC plasma, such as magnetohydrodynamic (MHD) instabilities and plasma transport. In the present paper, based on the FRC asymptotic theory by Barnes D C, the code for solving the two-dimensional Grad-Shafranov equation in FRC is developed. By using the code, the equilibriums of FRC with different elongations and separatrix radii are investigated in the present paper. The one-dimensional numerical results show that the plasma density gradient increases linearly with magnetic flux increasing in the FRC center, while, it steepens due to the high magnetic field distribution at the separatrix. The results also show that the plasma density in the closed field region increases with the density at the separatrix increasing, which implies that FRC embodies the strong confinement ability. It is a key problem to choose equations determining the shape of the separatrix in a two-dimensional numerical investigation. In the present paper, the shape equation is described as rs = rs max (1 - z2a), in which a is the shaping parameter. When a=1, the separatrix shape is elliptical, and when a1, the separatrix shape is like a racetrack. The geometry character of the separatrix appears in the one-order equations (in one-order equations: (0)/(z) = (0)/(rs)(rs)/(z), where (0)/(rs) is determined by lead equations and (rs)/(z) is given by separatrix equation). The two-dimensional numerical results show that O-point moves outward as the sparatrix radius increases. The curvature radius of magnetic flux surface increases with the separatrix radius increasing. The O-point of magnetic flux surface is just at the curvature center. Thus O-point moves outward as the sparatrix radius increases.

2016, 65 (18): 185202.
doi: 10.7498/aps.65.185202

Abstract +

High power impulse magnetron sputtering (HiPIMS) is a popular physical vapor deposition (PVD) technology because of the high ionization of the sputtering materials, large coating density, good adhesion, and other favorable properties. However, this technique suffers some disadvantages such as the small deposition rate induced by the high target potential, the metallic droplets produced by the unstable discharge, and different ionizations for different sputtering materials, thereby hampering wider acceptance by the industry. A cylindric HiPIMS source in which the discharge is restricted in the cylinder is described in this paper. By using this source, coatings can be deposited with 100% ions without metallic droplets arising from the unstable discharge, and the unionized sputtered atoms cannot be extracted by the extraction grid with negative potential. Electron oscillation and repetitive sputtering of the unionized atoms occur in the cylinder to enhance collision and ionization. Due to the enlarged discharge area by the cylinder internal surface comparing with the area of the ion outlet (end face of the cylinder), the sputtering ions converge from the inwall to the center of the cylinder target and form an enhanced flow to spray out from the source, which will improve the deposition rate. The structure and discharge characteristics of the novel HiPIMS source are investigated by simulation and experiments. Our results indicate that 8 magnets can provide the reasonable magnetic field and the highest target utilization rate. The distributions of electrons and ions in the target each consist of 8 petals in the optimized magnetic structure, and the highest plasma density happens near the target, which is above 1.31017 m-3. The discharge characteristics confirm that the cylindric sputtering source can be operated under HiPIMS conditions and the evolution of the target currents with target voltage exhibits I-V characteristics typical of HiPIMS. An obvious pre-ionization is observed on the discharge glow and discharge current curves when the extra direct current (DC) is added. The racetrack area is about 60.0% of the target surface. The ion current curves are similar to those of the target currents, but a 40 s delay and about one-tenth current value are observed compared with the target currents. The sputtering is improved by the extra DC, inducing the increased metallic ions and the opposite evolution of gas ions. The results suggest that the cylindric sputtering source can be effectively used to conduct HiPIMS and is a novel way to improve and promote the application of HiPIMS.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2016, 65 (18): 188101.
doi: 10.7498/aps.65.188101

Abstract +

Graphene has been a superstar in the fields ranging from materials science to condensed-matter physics since 2004. Graphene possesses good thermal and mechanical properties, high electron transfer capability and relatively low production cost. As a consequence, graphene has been used in the areas of multi-functional advanced materials and electronics. A direct disperse method has been widely applied to polymers to improve their dielectric properties. Recently, graphene/polymer composites have received much attention. Graphene nanosheets can significantly improve the physical properties of the host polymer at a very low content of conductive filler loading. Poly vinylidene fluoride (PVDF) is a semicrystalline thermoplastic polymer with remarkably high piezo-/pyroelectric coefficient, and excellent thermal stability and chemical resistance. More efforts have been recently devoted to the preparations of high-' composites based on PVDF. In this work, a graphene/PVA/PVDF nanocomposite film composed of poly(vinyl alcohol) (PVA), reduced graphene oxide (RGO), and poly (vinylidene fluoride) (PVDF) is fabricated. First of all, graphene oxide (GO) is prepared by the modified Hummers method. GO and PVA are successively dissolved in the dimethyl sulfoxide (DMSO) solution, in order to obtain PVA functionalized GO which is formed via non-covalent bonds. Then PVDF is added into this solution to form a homogeneous three-phase aqueous mixture. According to the solution-casting and thermal reduction processes, the three-phase nanocomposite films are formed. The thickness values of the films are in a range of 0.3-0.4 mm. The square specimens are coated with a silver paste prior to electrical measurements. The obtained products are characterized using X-ray diffraction, UV Vis absorption spectrum, Fourier transform infrared absorption spectrum, and atomic force microscopy. The morphologies of PVDF and RGO/PVA/PVDF films are investigated by a scanning electron microscope. Electrical measurements are conducted in a frequency range from 102 to 104 Hz. Results suggest that GO can be reduced to RGO and phase transition of PVDF from to phases is effectively promoted at 120 ℃. The dielectric properties of the polymer matrix are improved. Furthermore, PVA modified RGO is easier to disperse in the PVDF substrate than the original one, which strongly reduces the spherulite size of PVDF and improves the dielectric property of the composite film. The percolation threshold (fvol*) of RGO/PVA/PVDF film is estimated to be 8.45 vol.%, and the dielectric constant of the film is 238 times as large as that of the pure PVDF films at 102 Hz. In addition, the dielectric constant increases rapidly near the percolation threshold and depends on frequency, which is mainly ascribed to the Maxwell-Wagner-Sillars polarization in the low frequency range. This study provides a low-cost and simple method of preparing polymer nanocomposites with high dielectric properties.

2016, 65 (18): 188102.
doi: 10.7498/aps.65.188102

Abstract +

In directional solidification, two characteristic parameters determine the dendritic growth: the thermal gradient and the pulling velocity. To achieve the suitable microstructure and improve the performance of casting, they are usually used to resize the pulling velocity or temperature gradient in directional solidification process. The structures obtained under different directional solidification conditions, and their associated properties both have been hot research points. It is difficult to observe the microstructure, which is usually on a micrometer scale, directly in experiment, and the phase-field method becomes a strong tool to understand the dendrite growth pattern. We mainly study the liquid channel formed after Fe-C alloy dendrite tip splitting under the specific condition of directional solidification and analyze the influence on liquid channel of pulling velocity in this paper. We choose the fixed thermal gradient G =20 K/mm which is on the order of the experimental value, and pulling velocity VP no more than 10 mm/s to keep the cooling rate in the range of low speed in dendrite growth, so that the interface kinetic effect can be neglected. Recent experimental results show the different interfacial energies in various compositions of Al-Zn alloy and Fe-C alloy, then we can investigate a series of directional solidification microstructures with fixed alloy Fe-0.5 wt.%C composition at different interfacial energies in our simulations. We find that the liquid channel is formed as a result of anisotropy competition between system and materials, the length and C concentration of liquid channel increase with the pulling velocity increasing, while the diameter of liquid channel is constant. It is interesting to find that there is a minimum of pulling velocity almost equal to 1 mm/s, the tip will not split and no liquid channel forms in the following steps either when the velocity is smaller than the minimum. We also compare the segregation caused by solute enrichment in liquid channel and solute segregation between dendrite arms in a series of simulations: the former is more serious than the latter. Then we point out the way to reduce the segregation caused by liquid phase channel by reducing the pulling velocity properly. It will be more practical to couple the flow field with other external field, such as magnetic field, in the simulation.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2016, 65 (18): 189401.
doi: 10.7498/aps.65.189401

Abstract +

China seismo-electromagnetic satellite (CSES) is launched to detect the electromagnetic environment in space for the study of seismic early warning. Langmuir probe is one of the payloads of the CSES satellite, and it is the first time that the Langmuir probe technique has been used in the Chinese satellite. The use of the Langmuir probe is to measure the space plasma parameters, such as electron density (Ne), electron temperature (Te), and to identify the instantaneous change of the space plasma.
The Langmuir probe payload is composed of three parts, i.e., two sensors, two rods, and one electronics box. The sensor is installed at the top of the rod to extend out of the satellite surface, and is parallel to the direction of the satellite orbit. The electronics box is installed inside the satellite which includes the sweep voltage circuit, sensor signal circuit, DPU control and processing circuit, the satellite interface circuit, power supply circuit, etc.
The sensor is spherical. Its upper hemisphere is a collecting electrode, and its lower hemisphere is a protective electrode. The same sweep voltage is applied to the upper hemisphere and the lower one which can eliminate the terminal effect of the connecting point between the traditional spherical structure and the rod. The diameters of the two sensors are respectively 50 and 10 mm, and the surface areas of the two sensors are respectively 1/2000 and 1/13000 times the satellite surface area. The stability of the satellite ground potential is not affected by the sweep voltages on the sensors. In addition, TiN material is coated on the sensor surface to ensure a uniform surface work function, and to prevent the space atomic oxygen erosion. The decontamination function is designed for the Langmuir probe to eliminate the possible pollution on the orbit. A positive 100 V voltage is applied to the sensor to accelerate electrons to bombard the sensor surface, thereby removing the contamination from the sensor surface. The advantage of the electron bombardment effect is that the TiN film is not damaged, meanwhile the positive 100 V voltage has high reliability and safety on orbit. The decontamination function has been proved to be effective by the test in Italy National Institute for Astrophysics-Institute for Space Astrophysics and Planetology (INAF-IAPS).
The plasma environment calibration test of the Langmuir probe is carried out in INAF-IAPS. We measure the electron density and temperature at three different distances from the plasma source, and compare the results with the measured results of the INAF-IAPS reference Langmuir probe. Results show that the test data of our Langmuir probe are consistent with the INAF-IAPS reference data. Our Langmuir probe design is proved to be feasible to achieve the missions of the satellite.