Study of three-dimensional nonlinear backward-wave interaction model and numerical simulation for helical traveling wave tube
Effect of degree correlations on controllability of undirected networks
Progress of electrical control magnetization reversal and domain wall motion
Electrical control of spins in magnetic materials and devices is one of the most important research topics in spintronics. We briefly describe the recent progress of electrical manipulations of magnetization reversal and domain wall motion.This review consists of three parts:basic concepts,magnetization manipulation by electrical current and voltage methods,and the future prospects of the field.The basic concepts,including the generation of the spin current,the interaction between the spin current and localized magnetization,and the magnetic dynamic Landau-Lifshitz-Gilbert-Slonczewski equation are introduced first.In the second part,we reviewed the progress of the magnetization controlled by electrical current and voltage. Firstly we review the electrical current control of the magnetization and domain wall motion.Three widely used structures, single-layer magnets,ferromagnet/heavy metal and ferromagnet/nonmagnetic metal/ferromagnet,are reviewed when current is used to induce magnetization reversal or drive domain wall motion.In a single-layer magnetic material structure,domain wall can be effectively driven by electrical current through spin transfer torque.The factors influencing the domain wall trapping and motion are also discussed.The electrical current control of the skyrmions has big potential applications due to much lower current density.Using the Dresselhaus and Rashba spin orbital coupling,the electrical current can also directly reverse the magnetization of single magnetic or antiferromagnetic layer.Then,we review the electrical current switching the magnetization of the ferromagnetic layer in ferromagnetic/heavy metal structures,where both spin Hall effect and Rashba effect can contribute to the current switching magnetization in such device structures. To identify the relative contributions of these two mechanisms,several quantitative studies are carried,concluding that spin Hall effect plays a major role,which is summarized in this review.Finally,we review the current switching magnetization of free layers in spin valve and magnetic tunnel junctions (MTJs) by spin transfer torque.We also discuss the approaches to the decrease of the critical current density in MTJs,which is desired for future applications.Alternatively,the electric field can also be used to manipulate the magnetization,where three methods are reviewed. Applying an electric field to the ferromagnetic/piezoelectric heterostructures,which changes the crystal structure of magnetic film through piezoelectric effects,realizes the change of the magnetic anisotropy of the ferromagnetic layer.In ferromagnetic/ferroelectric heterostructures,electric field changes the spin distribution and orbital hybridization at the surface of magnetic film through the magnet-electric coupling effects,and then controls the magnetization of the ferromagnetic layer.In ferromagnetic metal (semiconductor)/dielectric/metal structure,electric field controls the electron accumulation or depletion at the surface of the ferromagnetic metal or semiconductor,the change of the electron density in the magnetic layer in turn affects the magnetic exchange interaction and magnetic anisotropy.Finally,we present the prospects for the development of electrical control magnetization reversal and domain wall motion for future applications.
Recent research progress of relaxation performances of defects in ZnO-Bi2O3 varistor ceamics
Advances in applications of positron annihilation spectroscopy to investigating semiconductor microstructures
The design and realization of continuous-variable quantum key distribution system based on real-time shot noise variance monitoring
Bursting oscillations as well as the mechanism with codimension-1 non-smooth bifurcation
The coupling of different scales in nonlinear systems may lead to some special dynamical phenomena, which always behaves in the combination between large-amplitude oscillations and small-amplitude oscillations, namely bursting oscillations. Up to now, most of therelevant reports have focused on the smooth dynamical systems. However, the coupling of different scales in non-smooth systems may lead to more complicated forms of bursting oscillations because of the existences of different types of non-conventional bifurcations in non-smooth systems. The main purpose of the paper is to explore the coupling effects of multiple scales in non-smooth dynamical systems with non-conventional bifurcations which may occur at the non-smooth boundaries. According to the typical generalized Chua's electrical circuit which contains two non-smooth boundaries, we establish a four-dimensional piecewise-linear dynamical model with different scales in frequency domain. In the model, we introduce a periodically changed current source as well as a capacity for controlling. We select suitable parameter values such that an order gap exists between the exciting frequency and the natural frequency. The state space is divided into several regions in which different types of equilibrium points of the fast sub-system can be observed. By employing the generalized Clarke derivative, different forms of non-smooth bifurcations as well as the conditions are derived when the trajectory passes across the non-smooth boundaries. The case of codimension-1 non-conventional bifurcation is taken for example to investigate the effects of multiple scales on the dynamics of the system. Periodic bursting oscillations can be observed in which codimension-1 bifurcation causes the transitions between the quiescent states and the spiking states. The structure analysis of the attractor points out that the trajectory can be divided into three segments located in different regions. The theoretical period of the movement as well as the amplitudes of the spiking oscillations is derived accordingly, which agrees well with the numerical result. Based on the envelope analysis, the mechanism of the bursting oscillations is presented, which reveals the characteristics of the quiescent states and the repetitive spiking oscillations. Furthermore, unlike the fold bifurcations which may lead to jumping phenomena between two different equilibrium points of the system, the non-smooth fold bifurcation may cause the jumping phenomenon between two equilibrium points located in two regions divided by the non-smooth boundaries. When the trajectory of the system passes across the non-smooth boundaries, non-smooth fold bifurcations may cause the system to tend to different equilibrium points, corresponding to the transitions between quiescent states and spiking states, which may lead to the bursting oscillations.
Meminductive Wein-bridge chaotic oscillator
An improved direct position determination method based on correlation accumulation of short-time signals with variable velocity receivers
SM4 key scheme algorithm based on chaotic system
Invesitgation and experiments of wavelet thresholding in ensemble-based background error variance
Event mixing constraints for Bose-Einstein correlations in reactions with three particles in the final state
Spectroscopic properties of low-lying excited electronic states for CF- anion based on ab initio calculation
Ultrafast photodissociation dynamics of butanone in 3s Rydberg state
The initiation and subsequent control or exploration study of chemical transformation in real time by using ultrashort laser pulses aim at femtochemistry. The real-time investigations of ultrafast dynamics of excited molecules in gas and condensed phases have attracted a great deal of attention over the last two decades. As a kind of important organic compound, aliphatic ketone is an area of much interest for many research fields, especially for atmospheric photochemistry. Via photodissociation reaction, it can release carbonyl radical whose chemical character is active and can react with hydroxyl easily. As a typical aliphatic ketone, butanone has been a research focus over the past decades. The ultrafast dissociation dynamics of butanone after excitation to the second electronically excited state (S2) with a 195.8 nm pump pulse is studied by the femtosecond pump-probe technique combined with the time-of-flight mass spectrometry (TOF-MS). Time-resolved mass spectrometry (TRMS) has proven to be a powerful technique to study the ultrafast dynamics of excited states in molecules. In this technique, the MCP detector is capable of recording time-resolved ion yield measurements of different cations by monitoring the current output directly from the anode by using an oscilloscope. This enables a time-of-flight mass spectrum to be recorded at each delay time, which is controlled by a delay stage, and the measured total signal is then integrated, yielding a time-resolved ion yield transient, which is conducted by LABVIEW software. The pump wavelength in this work is set to be 195.8 nm and the probe laser wavelength is centered at 800 nm. The complex ultrafast dynamics in butanone with 3s Rydberg state excitation and its possible decay paths and following dissociation mechanism are given. Experimental results show that the Norrish I type dissociation kinetics of butanone exhibit rich features, for it has a methyl group and an ethyl group at α position. The decay time constant of the parent transient is approximately 2.23 ps±0.02 ps. There is only one time constant of 2.15 ps±0.02 ps for the fitting of the propionyl transient. The best fit of acetyltransient is obtained with four time constants:τ1=(2.40±0.15) ps, τ2=(1.10±0.25) ps, τ3=(0.08±0.02) ps, and τ4=(17.72±0.80) ps, corresponding to S2→S1 internal conversion, the primary dissociation of the S1 state generating CH3CO(Ã), Ã→ internal conversion and secondary dissociation of CH3CO() respectively. Two competitive α-CC bond dissociation processes are observed and discussed. They are dissociation channels through intramolecular vibrational energy redistribution (IVR) and/or by getting over the dissociation barrier in α-cleavage of butanone. But hereunder the condition of this experiment, the dissociation is the result of IVR.
Influence of collision energy on the stereodynamics of the H+CH+→C++H2 reaction
Effect of grating groove density error on the output pulses of the tiled grating compressor and corresponding compensation scheme
The research of δ13CO2 by use of wavelet de-noising at 2.008 μm based on tunable diode laser absorption spectroscopy
Sheared-beam imaging target reconstruction based on all-phase spectrum analysis
Influence of gradient phased interfaces on the laws of light propagation
Low-noise optical field phase-shifting manipulated using a coherently-prepared three-level atomic medium
Experimental study on increasing signal-to-noise ratio of a beat note by cascading an Yb-doped fiber in an Er-fiber comb
Characterization of Brillouin scattering in a few-mode fiber
First-principles study on the electronic structures and the absorption spectra of In: Mn: LiNbO3 crystals
Dual-core terahertz polarization splitter based on porous fibers with near-tie units
Dual-mode large-mode-area multi-core fiber with circularly arranged airhole cores
Inverse Doppler effect of acoustic metamaterial with negative mass density
Effects of poling state and direction on domain switching and phase transformation of Pb(Zr0.95Ti0.05)O3 ferroelectric ceramics under uniaxial compression
Experimental study on the density characteristics of a supersonic turbulent boundary layer
Simulation investigation of two droplets vertically impacting on solid surface simultaneously
Three-dimensional direct numerical simulation of helicon discharge
Equivalence of energy deposition profile in target between electron beam of multi-energy composite spectrum and X-ray
Simulations of the cathode falling characteristics and its influence factors in atmospheric pressure dielectric barrier glow discharge pulse
Irradiation-induced modifications in the mechanical properties of borosilicate glass
Understanding the evolutions of the mechanical properties of borosilicate glasses under irradiation is crucial for evaluating their performances after long-term interaction with the irradiation environment in the disposal of high level nuclear waste.The variations of the mechanical properties of borosilicate glasses,induced by irradiation have been extensively studied.However,the mechanisms of variations in mechanical properties,induced by irradiation have not been clarified yet,especially when considering the effects of electronic and nuclear processes,respectively.To clarify this issue,a commercial borosilicate glass is investigated through an external irradiation of 5 MeV Xe ions and 1.2 MeV electrons in this paper.The nano-indentation test is used to study the changes of the hardness and modulus.The microstructure evolutions of Xe ion irradiated borosilicate glasses are characterized by Fourier transform infrared (FTIR) spectroscopy to discuss the mechanisms in the evolutions of mechanical properties.The nano-indentation results indicate that the hardness is reduced by 24%,and the modulus is lessened by 7.4% after the glass has been irradiated by Xe ions.Both the hardness and modulus variations reach their stable states when the total deposited energy is around 6.6×1021 keV/cm3.Although hardness and modulus are also observed to decrease by about 4.7% and 2.9%,resepectively, when the total deposited energy reaches approximately 1.4×1022 keV/cm3 after the glass has experienced the electron irradiation,the results still emphasize that the nuclear energy deposition is the major factor for the evolutions of the hardness and modulus of the borosilicate glass under ion irradiation.The decreases of hardness and modulus after the glass has experienced ion irradiation can be attributed to the deformation of glass network and volume expansion, which are induced by reducing the average ring size and transforming from[BO4] to[BO3] units.By considering the recovery resistance,it is found that the toughness of the borosilicate glass is significantly strengthened,and therefore the mechanical properties of the borosilicate glass are enhanced after the glass has been irradiated by Xe ions.Compared with the results after ion irradiation,the mechanical properties have negligible changes after electron irradiation.The present work is important for understanding both the irradiation effects on the hardness/modulus and the variations in the mechanical properties during the high level waste disposal.
A cluster-formula composition design approach based on the local short-range order in solid solution structure
The composition design is of importance for developing high-performance complex alloys and is also the primary step to realize a new mode for material development via theoretical prediction and experimental verification, in comparison with the traditional experience-oriented experiments. Traditional alloy design approaches, including Hume-Rothery rule, electron theories, equivalent method, computer simulation, etc., are first reviewed from the viewpoints of their theoretical basis and applicability to limitations. Almost all the traditional alloys are based on solid solution structures, in which the typical characteristic is the chemical short-range order (CSRO) of the solute distribution. We propose a cluster-plus-glue-atom model for stable solid solutions in light of CSRO. A cluster-formula composition design approach is presented for developing the multi-component high-performance alloys. The cluster-plus-glue-atom model classifies the solid solution structure into two parts, i.e., the cluster part and the glue atom part, where the clusters are centered by solute atoms, showing the strong interactions of clusters with the solvent base and the weak interactions of clusters with solute atoms. The clusters are the nearest-neighbor polyhedrons, being cuboctahedron with a coordination number of 12 (CN12) in FCC structure and rhombic dodecahedron with a CN14 in BCC structure, respectively. Then a uniform cluster-formula of[CN12/14 cluster](glue atom)x is achieved from the cluster model. Its wide applications in different multi-component alloy systems confirm its universality as a simple and accurate tool for multiple-component complex alloy composition design. Such alloy systems include corrosion-resistant Cu alloys, high-performance Ni-base superalloys, high-strength maraging stainless steels, Ti/Zr alloys with low Young's modulus, high-entropy alloys, amorphous metallic glasses, quasicrystals, etc.. The specific alloy design steps are incarnated in the βup-Ti alloys with low Young's modulus. Firstly, the necessary alloying elements are chosen according to the service requirements (BCC stability and low Young's modulus). Secondly, the local cluster unit to present CSRO and the corresponding cluster formula of[(Mo, Sn)-(Ti, Zr)14](Nb, Ta)x are built, in which the occupations of the alloying elements in the cluster formula are determined by the enthalpy of mixing ΔH between them with the base Ti. Thirdly, these designed alloys are verified experimentally, and the lowest Young's modulus appears at the βup-[(Mo0.5Sn0.5)-(Ti13Zr1)]Nb1. Finally, a new Mo equivalent formula under the guidance of phase diagram features is proposed to characterize the structural stability of Ti alloy. Thus all the Ti alloy compositions with different structural types can be expressed with a uniform cluster formula, in which the structural types of alloys are determined by the Mo equivalent.
Damage effects of proton beam irradiation on single layer graphene
Graphene was first discovered in 2004 (Novoselov K S, et al. 2004 Science 306 666), it is a single atomic layer of sp2-bonded carbon atoms arranged in a honeycomb-like lattice. According to its extraordinary electronic, mechanical, thermal and optical properties, one can expect it to have a variety of applications in nanoscale electronics, composite materials, energy storage, and biomedicine fields. Although many experimental and theoretical studies on graphene have been carried, there still exist many obstacles to its applications. A representative example is nanoscale electronics (e.g., field-effect transistors and optoelectronic devices) that requires non-zero band-gap. Therefore, introducing defects into graphene and leading to band-gap opening are key steps for its technique applications.Recently, ion beam irradiation as a defects introducing technique was performed by Lee et al. (2015 Appl. Surf. Sci. 344 52) and Zeng et al. (2016 Carbon 100 16) through 5, 10, and 15 MeV protons and highly charged ions (HCIs) irradiating the graphene separately. Considering the advantages of simplity for preparing samples and feasibility in atmospheric condition of Raman spectroscopy compared with common characterization techniques (high resolution transmission electron microscopy, scanning electron microscopy, atomic force microscopy) for nano-materials, in both studies, Raman spectroscopy is used to obtain the evolution of ID/IG (ID is the peak intensity excited by defects, IG is the peak intensity origining from lateral vibration of carbon atoms) with different energies and fluences, respectively. In this work, considered are the following points:1) the absence of quantitive characterization for defects in the above two studies; 2) the low displacement energy of 25 eV required for a carbon atom to be knocked out (Zhao S J, et al. 2012 Nanotechnology 23 285703); 3) the complex interaction between HCIs and material. The irradiation effects of single layer graphene on silicon substrate are investigated by 750 keV and 1 MeV proton bombarding. This introduces the defects into graphene and thus leads to band-gap opening. By comparing Raman spectra of the samples before and after irradiation, a quantitive characterization about defects in graphene is achieved. Detailed analysis shows that 1) the value of ID/IG increases with the energy loss of incident proton, which is consistent with the result of SRIM simulation; 2) the average distance of defects LD increases with the incident proton energy; 3) the defect density nD decreases with the incident proton energy. These indicate that the damage effect for MeV protons in single layer graphene with substrate is similar to those in three-dimensional materials. The method presented here may facilitate the understanding of the physical mechanism of MeV proton interaction with two-dimensional materials, and provide a potential way of controlling the electronic structure and band-gap.
Structure of NO dimer multilayer on Rh(111)
Molecular self-assembly is the spontaneous organization of molecules under thermodynamic equilibrium conditions into well-defined arrangements via cooperative effects between chemical bonds and weak noncovalent interactions. Molecules undergo self-association without external instruction to form hierarchical structures. Molecular self-assembly is ubiquitous in nature and has recently emerged as a new strategy in chemical biosynthesis, polymer science and engineering. NO monomer is apt to be absorbed on the surfaces of some metals such as Ir(111), Ni(111), Pd(111), Pt(111), Rh(111) and Au(111), and the interactions of NO monomer with the metal surfaces have been extensively studied. When NO monomer is weakly adsorbed on the noble-metal surface, it cannot be reduced completely but forms a stable structure, which is named NO dimer. The first-principle technique is employed to determine the structures of NO dimer ((NO)2) molecular chains and monolayers on virtual Rh(111), as well as (NO)2 monolayer and multilayer on Rh(111). First, (NO)2 monomers are assembled into two stable molecular chains on the virtual Rh(111) surface, whose bind energies are 0.309 and 0.266 eV, respectively. The molecular chains are self-assembly systems, in which (NO)2 monomers are parallel and ordered, and the O atoms and N atoms are shown to be of (100) and (111) structures, respectively. Then, the two molecular chains are assembled into two stable monolayers (denoted as M1 and M2) on the virtual Rh(111)-(1×√3), and the coverage is 1.00 ML. In the M1 monolayer, the angle between the N–N bond of (NO)2 monomer and the substrate is in a range of 70°-90°, and in the M2 monolayer, the N–N bond is parallel to the substrate.In the adsorption system of M2/Rh(111), (NO)2 molecules can be adsorbed on the top as well as the hcp and fcc hollow sites. When (NO)2 molecules are adsorbed on the top site, the adsorption system is best described by the electron structure Rh+0.14–N0=O-0.14, and when (NO)2 molecules are absorbed on the two hollow sites, the adsorption system is described by the electron structure Rh+0.34–N-0.18=O-0.16. Therefore, (NO)2 molecules are more apt to be adsorbed on the two hollow sites than on the top site. In the adsorption systems of M1+M2/Rh(111) and M1+(M1+M2)/Rh(111), (NO)2 molecules are adsorbed vertically on the two hollow sites, the N–N bond is parallel to the substrate in the first monolayer, and the angle between the N–N bond and the substrate is in a range of 70°-90° in the second and third monolayers. The interaction between the neighbor monolayers is about 0.01 eV, and the thickness of the vacuum layer is 0.31 nm±0.02 nm.
Dependences of valence electronic structure on magnetic moment and electrical resistivity of metals
Conventionally, the energy band theory is used to explain the magnetic and electrical transport properties of metals. However, so far, there has been no quantitative explanation of the relations between the average magnetic moment per atom and the resistivity for Fe, nor Ni, nor Co metals. In this paper, a new itinerant electron model for magnetic metal is proposed on the basis of electron distribution theory at the energy level. 1) In the process of free atoms forming the metal solid, most of the 4s electrons of Fe, Ni and Co enter into the 3d orbits subjected to the Pauli repulsive force, and the remaining 4s electrons form free electrons. 2) Since the average number of 3d electrons is not an integer, a part of atoms have one 3d electron more than the other atoms. These excess 3d electrons have a certain probability to itinerate between the 3d orbits of the adjacent atoms as itinerant electrons; and the other 3d electrons are local electrons. 3) The transition probability of itinerant electrons is very low, thus the contribution to metal resistivity from itinerant electrons is far lower than that from free electrons. Resistivity of metal decreases with increasing the number of free electrons. Therefore, using the observed values of average atomic magnetic moments, 2.22, 0.62 and 1.72 μB, the average numbers of free electrons in Fe, Ni and Co can be calculated to be 0.22, 0.62 and 0.72, respectively. This is the reason why the electrical resistivities of Fe, Ni and Co (8.6, 6.14 and 5.57 μΩ-cm) decease successively. In addition, according to this model, the average number of 3d electrons per atom in Ni metal is 9.38. This indicates that 38% of atoms in Ni metal have ten 3d electrons, forming a full 3d sub-shell, as in Cu or Zn atoms. The 3d electrons in these atoms are difficult to itinerate or exchange. This may be the reason why the Curie temperature of Ni metal (631 K) is far lower than those of Fe and Co metals (1043 and 1404 K). On the basis of the energy band theory, the numbers of 3d electrons in Fe, Ni and Co metals are 7.4, 9.4 and 8.3, which are close to our results (7.78, 9.38 and 8.28), respectively. This indicates that our model is consistent with the energy band theory. Compared with the complex energy band theory, a simple and effective method on investigating valence electron structures through the experimental average magnetic moments per atom in a metal is presented based on our model. Therefore, the new itinerant electron model may be a new clue to understanding the electronic structure of metals and alloys.
Microstructure, resistivity, and hardness of aged Ag-7wt.%Cu alloy
Ag-Cu alloys are used as both decorative materials because of beautiful appearance, and conductors due to excellent combinations of strength and electrical conductivity. The strength and electrical conductivity of Ag-Cu alloy are closely related to precipitation behavior of Cu-rich phase in Ag matrix. The morphology, size and volume fraction of Cu-rich phase have been highly concerned. In this work, a series of aging temperatures is used in both supersaturated solid-solution and cold-rolled Ag-7wt.%Cu samples to investigate the relationship between the precipitation behavior of Cu-rich phase and property by using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and properties measurements (hardness and resistivity). The DSC results of as-solid-solution Ag-7wt.%Cu alloy show a distinct exothermic precipitation reaction of Cu out of Ag matrix ranging from 300 °C to 350 °C, and the activation energy is estimated to be (111±1.6) kJ/mol according to Kissinger equation. Because of the existence of deformation energy, the DSC results of cold-rolled Ag-7wt.%Cu sample show a distinct exothermic precipitation reaction of Cu from Ag matrix between 290 °C and 330 °C, and the activation energy is (128±12) kJ/mol. XRD analysis indicates that the dissolved Cu in Ag is dependent on ageing temperature, and the change of solubility of Cu in Ag is calculated by XRD curve. Microstructural analysis demonstrates that spherical Cu-rich phases are precipitated from Ag-matrix at 450 °C in both solid-solution and cold-rolled Ag-7wt.%Cu alloys. Moreover, the banded structure of Cu-rich phase is found in the solid-solution sample after being aged at 450 °C. The deformation twinning Ag is found in the cold-rolled sample. The precipitation and dissolution of Cu-rich phase in Ag matrix play important roles in the resistivity and microhardness. With ageing temperature increasing (ageing temperatures range from 200 to 450 °C), the electrical resistivity of as-solid-solution aged sample decreases and the microhardness increases, however, both electrical resistivity and microhardness of as-cold-rolled aged sample decrease. With ageing temperature increasing further (over 450 °C), the electrical resistivity increases and the microhardness decreases in both aged samples. Because of the formations of dislocation and deformation twinning Ag, the microhardness of cold-rolled sample reaches to 217 HV, which is higher than that of solid-solution sample. Strengthening and electrical resistivity models are built based on the microstructural characterization and concentration contributions. These theoretical predictions are in good agreement with experimental values. Our model demonstrates that the precipitation and dissloution of Cu in Ag significantly affect the electrical conductivity, and dislocation and deformation twinning play important roles in microhardess in Ag-Cu alloy. This work clarifies the influencing mechanism of different microstructures on the microhardness and resistivity of Ag-Cu alloy.
Structural and photoelectrical properties of AZO thin films improved by Ag buffer layers
In order to obtain more excellent photoelectric properties of transparent conductive film, a series of high-quality AZO thin films and AZO/Ag/AZO thin films with various thickness values of Ag buffer layers are prepared on glass substrates by the radio frequency magnetron sputtering method at room temperature. The phase and surface morphologies of films are characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively. The technology of Hall effect measurement and ultraviolet, visible spectrophotometer are employed to investigate the photoelectric properties of films. The electrical properties (including sheet resistance, sheet concentration and mobility) of films are also determined by using non isothermal technique to explore their thermal stability performances. The results indicate that the thickness values of Ag buffer layers have a large influence on the crystalline structures and photoelectric properties of AZO thin films. The XRD results show that with the increase of the thickness of Ag, the diffraction peak of Ag (111) is gradually enhanced, the ZnO (002) diffraction peak is gradually weakened, and the preferred orientation of ZnO (002) crystal plane is weakened. AFM test indicates that the change of Ag layer thickness has a great influence on the surface growth mode of the upper layer AZO thin film. When the Ag layer thickness is less than 5 nm, AZO thin film surface is rough and the grain size is smaller. When the Ag layer thickness is larger than 10 nm, the continuous surfaces of multilayer films begin to be shaped, directly affecte the photoelectric properties of the films. Hall effect measurement and transmittance test show that with the increase of Ag layer thickness, the transmission of AZO/Ag/AZO multilayer film gradually decreases, and also the resistance gradually decreases. When the thickness of Ag layer is 10 nm, AZO(30 nm)/Ag(10 nm)/AZO(30 nm) thin film gains a best figure of merit of 1.59×10-1 Ω-1 an average transmittance of 84.2% and a sheet resistance of 0.75 Ω/sq. Hall effect measurement versus temperature indicates that AZO film without an Ag layer proves to be subjecte to the regular change of semiconductor resistance with temperature. When adding an Ag layer, the trend of the relationship of resistance with temperature presentes the characteristic of that metal resistance relating to temperature. Moreover, the sheet concentration of AZO with Ag layer is higher than that of AZO. The highest sheet concentration and the excellent thermal stability are obtained on AZO/Ag (10 nm)/AZO. The changes of the mobility of AZO under different temperatures turn out to be poorly stable. However, when adding an Ag layer, the better stability of AZO/Ag/AZO can be obtained. In conclusion, the photoelectric properties of films own excellent thermal stabilities with optimum thickness of Ag layer.
Mode identification via temperature variation in resonant ultrasonic spectroscopy technique for piezoelectric material
The full matrix material constants of piezoelectric materials should be characterized first before they have been used to make actuators or sensors. Up to now, they are usually determined by the ultrasonic pulse-echo and electric impedance resonance techniques through using multiple samples with drastically different sizes. However, the constants determined by the aforementioned techniques are probably inconsistent because the sample-to-sample variation cannot be eliminated. The technique of resonant ultrasonic spectroscopy (RUS) only needs one sample to determine the full matrix constants of piezoelectric material. Therefore, the consistency of the constants is guaranteed. During the implementation of the RUS technique, the elastic stiffness cijE and piezoelectric constants cij can be determined from the resonance modes identified from the resonant ultrasonic spectrum. The free and clamped dielectric constants cannot be determined by the RUS technique because they have very weak influence on resonance frequency. However, they can be directly measured from the same sample by using an impedance analyzer. To ensure the reliable inversion of material constants, enough resonance modes should be identified from the measured resonant ultrasonic spectrum. However, there are many missing and overlapped modes in the spectrum, which makes mode identification become a biggest obstacle to the implementation of the RUS technique. The adjacent modes may overlap if the resonance frequencies corresponding to them have a very small difference. In addition, the lower the mechanical quality factor QM, the more likely to overlap the adjacent modes are. During the RUS measurement, the rectangular parallelepiped sample is placed between the transmitting and receiving transducers with contacts only at the opposite corners of the sample. Resonance modes would not be detected if the receiving point, i.e., one corner of the sample, is the node of these modes. Therefore, there are missing modes in the resonant ultrasonic spectrum. To overcome the difficulty in identifying the modes, caused by modes missing and overlapping, the mode identifying method via temperature variation is presented in this study. Note that a change of temperature may change the material properties of a piezoelectric sample. The material properties have a great influence on the resonance frequency of the sample. Moreover, the influences corresponding to resonance modes are different. Therefore, the variation of temperature may make the overlapped modes separated from each other and the missing modes appear, namely, the missing and overlapped modes may be identified by comparing the resonant ultrasonic spectra measured at different temperatures. The experimental results of piezoelectric ceramics (PZT-8) show that this method can effectively improve the accuracy of mode identification and guarantee the reliability of inversion in the RUS technique.