Stress effect on lattice thermal conductivity of anode material NiNB_(2)O_(6)for lithium-ion batteries

The thermal transport properties of NiNB_(2)O_(6)as anode material for lithium-ion battery and the effect of strain were studied by machine learning interatomic potential combined with Boltzmann transport equation.The results show that the lattice thermal conductivity of NiNB_(2)O_(6)along the three crystal directions[100],[010],and[001]are 0.947 W·m^(-1)·K^(-1),0.727 W·m^(-1)·K^(-1),and 0.465 W·m^(-1)·K^(-1),respectively,indicating the anisotropy of the lattice thermal conductivity of NiNB_(2)O_(6).This anisotropy of the lattice thermal conductivity stems from the significant difference of phonon group velocities in different crystal directions of NiNB_(2)O_(6).When the tensile strain is applied along the[001]crystal direction,the lattice thermal conductivity in all three directions decreases.However,when the compressive strain is applied,the lattice thermal conductivity in the[100]and[010]crystal directions is increased,while the lattice thermal conductivity in the[001]crystal direction is abnormally reduced due to the significant inhibition of compressive strain on the group velocity.These indicate that the anisotropy of thermal conductivity of NiNB_(2)O_(6)can be enhanced by the compressive strain,and reduced by the tensile strain.

Prediction of lattice thermal conductivity with two-stage interpretable machine learning

Thermoelectric and thermal materials are essential in achieving carbon neutrality. However, the high cost of lattice thermal conductivity calculations and the limited applicability of classical physical models have led to the inefficient development of thermoelectric materials. In this study, we proposed a two-stage machine learning framework with physical interpretability incorporating domain knowledge to calculate high/low thermal conductivity rapidly. Specifically, crystal graph convolutional neural network(CGCNN) is constructed to predict the fundamental physical parameters related to lattice thermal conductivity. Based on the above physical parameters, an interpretable machine learning model–sure independence screening and sparsifying operator(SISSO), is trained to predict the lattice thermal conductivity. We have predicted the lattice thermal conductivity of all available materials in the open quantum materials database(OQMD)(https://www.oqmd.org/). The proposed approach guides the next step of searching for materials with ultra-high or ultralow lattice thermal conductivity and promotes the development of new thermal insulation materials and thermoelectric materials.

Effects of Rattling Behavior of K and Cd Atoms along Different Directions in Anisotropic KCdAs on Lattice Thermal Transport and Thermoelectric Properties

We employ advanced first principles methodology,merging self-consistent phonon theory and the Boltzmann transport equation,to comprehensively explore the thermal transport and thermoelectric properties of KCdAs.Notably,the study accounts for the impact of quartic anharmonicity on phonon group velocities in the pursuit of lattice thermal conductivity and investigates 3ph and 4ph scattering processes on phonon lifetimes.Through various methodologies,including examining atomic vibrational modes and analyzing 3ph and 4ph scattering processes,the article unveils microphysical mechanisms contributing to the lowκL within KCdAs.Key features include significant anisotropy in Cd atoms,pronounced anharmonicity in K atoms,and relative vibrations in non-equivalent As atomic layers.Cd atoms,situated between As layers,exhibit rattling modes and strong lattice anharmonicity,contributing to the observed lowκL.Remarkably flat bands near the valence band maximum translate into high PF,aligning with ultralowκL for exceptional thermoelectric performance.Under optimal temperature and carrier concentration doping,outstanding ZT values are achieved:4.25(a(b)-axis,p-type,3×10^(19)cm^(−3),500 K),0.90(c-axis,p-type,5×10^(20)cm^(−3),700 K),1.61(a(b)-axis,n-type,2×10^(18)cm^(−3),700 K),and 3.06(c-axis,n-type,9×10^(17)cm^(−3),700 K).

Strong anharmonicity-assisted low lattice thermal conductivities and high thermoelectric performance in double-anion Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)semiconductors

The thermoelectric properties of layered Mo_(2)AB_(2)(A=S,Se,Te;B=Cl,Br,I)materials are systematically investigated by first-principles approach.Soft transverse acoustic modes and direct Mo d–Mo d couplings give rise to strong anharmonicities and low lattice thermal conductivities.The double anions with distinctly different electronegativities of Mo_(2)AB_(2)monolayers can reduce the correlation between electron transport and phonon scattering,and further benefit much to their good thermoelectric properties.Thermoelectric properties of these Mo_(2)AB_(2)monolayers exhibit obvious anisotropies due to the direction-dependent chemical bondings and transport properties.Furthermore,their thermoelectric properties strongly depend on carrier type(n-type or p-type),carrier concentration and temperature.It is found that n-type Mo_(2)AB_(2)monolayers can be excellent thermoelectric materials with high electric conductivity,σ,and figures of merit,ZT.Choosing the types of A and B anions of Mo_(2)AB_(2)is an effective strategy to optimize their thermoelectric performance.These results provide rigorous understanding on thermoelectric properties of double-anions compounds and important guidance for achieving high thermoelectric performance in multi-anion compounds.

Study of lattice thermal conductivity of alpha-zirconium by molecular dynamics simulation

The non-equilibrium molecular dynamics method is adapted to calculate the phonon thermal conductivity of alphazirconium. By exchanging velocities of atoms in different regions, the stable heat flux and the temperature gradient are established to calculate the thermal conductivity. The phonon thermal conductivities under different conditions, such as different heat exchange frequencies, different temperatures, different crystallographic orientations, and crossing grain boundary （GB）, are studied in detail with considering the finite size effect. It turns out that the phonon thermal conductivity decreases with the increase of temperature, and displays anisotropies along different crystallographic orientations. The phonon thermal conductivity in [0001] direction （close-packed plane） is largest, while the values in other two directions of [2īī0] and [01ī0] are relatively close. In the region near GB, there is a sharp temperature drop, and the phonon thermal conductivity is about one-tenth of that of the single crystal at 550 K, suggesting that the GB may act as a thermal barrier in the crystal.

Low lattice thermal conductivity and high figure of merit in p-type doped K3IO

Based on first-principles calculations, Boltzmann transport equation and semiclassical analysis, we conduct a detailed study on the lattice thermal conductivity κL, Seebeck coefficient S, electrical conductivity σ, power factor S2σ and dimensionless figure of merit, zT, for K3IO. It is found that K3IO exhibits relatively low lattice thermal conductivity of 0.93 W·m-1·K-1 at 300 K, which is lower than the value 1.26 W·m-1·K-1 of the classical TE material PbTe. This is due to the smaller phonon group velocity νg and smaller relaxation time τλ. The low lattice thermal conductivity can lead to excellent thermoelectric properties. Thus maximum zT of 2.87 is obtained at 700 K, and the zT = 0.41 at 300 K indicate that K3IO is a potential excellent room temperature TE material. Our research on K3IO shows that it has excellent thermoelectric properties, and it is a promising candidate for applications in fields in terms of thermoelectricity.

Two-dimensional square-Au_(2)S monolayer:A promising thermoelectric material with ultralow lattice thermal conductivity and high power factor

The search for new two-dimensional(2 D)harvesting materials that directly convert(waste)heat into electricity has received increasing attention.In this work,thermoelectric(TE)properties of monolayer square-Au_(2)S are accurately predicted using a parameter-free ab initio Boltzmann transport formalism with fully considering the spin–orbit coupling(SOC),electron–phonon interactions(EPIs),and phonon–phonon scattering.It is found that the square-Au_(2)S monolayer is a promising room-temperature TE material with an n-type(p-type)figure of merit ZT=2.2(1.5)and an unexpected high n-type ZT=3.8 can be obtained at 600 K.The excellent TE performance of monolayer square-Au_(2)S can be attributed to the ultralow lattice thermal conductivity originating from the strong anharmonic phonon scattering and high power factor due to the highly dispersive band edges around the Fermi level.Additionally,our analyses demonstrate that the explicit treatments of EPIs and SOC are highly important in predicting the TE properties of monolayer square-Au_(2)S.The present findings will stimulate further the experimental fabrication of monolayer square-Au_(2)S-based TE materials and offer an in-depth insight into the effect of SOC and EPIs on TE transport properties.

Tailoring Carbon Distribution inα/γPhase of Ductile Iron and Its Effects on Thermal Conductivity

The effects of carbon distribution on the microstructure and thermal conductivity of ductile iron were investigated in the present study.The microstructure of as-cast and quenched ductile iron were characterized by OM and SEM.Results showed that the microstructure of as-cast ductile iron was composed of spheroidal graphite,ferrite with the volume of 80%,and a small amount of pearlite,and quenched ductile iron was composed of spheroidal graphite,coarse/fine acicular martensite(α_(M)phase)and high-carbon retained austenite(γphase).The volume fraction of retained austensite and its carbon content for direct quenched ductile iron and tepmered ductile iron were quantitatively analysed by XRD.Results revealed that carbon atoms diffused fromα_(M)phase toγphase during tempering at low temperatures,which resulted in carbon content in retainedγphase increasing from 1.2 wt%for the direct quenched sample to about 1.9 wt%for the tempered samples.Consequently,the lattice distortion was significantly reduced and gave rise to an increase of thermal conductivity for ductile iron.

ZrNiSn-based compounds with high thermoelectric performance and ultralow lattice thermal conductivity via introduction of multiscale scattering centers

The high lattice thermal conductivity of half-Heuslers(HHs)restricts the further enhancement of their thermoelectric figure-of-merit(ZT).In this study,multiscale scattering centers,such as point defects,dislocations,and nanoprecipitates,are synchronously introduced in a n-type ZrNiSn-based HH matrix through Nb doping and Hf substitution.The lattice thermal conductivity is substantially decreased from 4.55(for the pristine ZrNiSn)to 1.8 W·m^(−1)·K^(−1) at 1123 K via phonon scattering over a broad wavelength range through the adjustment of multiscale defects.This value is close to the theoretically estimated lowest thermal conductivity.The power factor(PF)is enhanced from 3.25(for the pristine ZrNiSn)to 5.01 mW·m^(−1)·K^(−2) for Zr_(0.66)Hf_(0.30)Nb_(0.04)NiSn at 1123 K owing to the donor doping and band regulation via Nb doping and Hf substitution.This can be ascribed to the synergistic interaction between the lowering of the lattice thermal conductivity and retention of the high PF.Consequently,a ZT value of as high as 1.06 is achieved for Zr_(0.66)Hf_(0.30)Nb_(0.04)NiSn at 1123 K.This work demonstrates that these actions are effective in jointly manipulating the transport of electrons and phonons,thereby improving the thermoelectric performance through defect engineering.

Achieving high carrier mobility and low lattice thermal conductivity in GeTe-based alloys by cationic/anionic co-doping

TheⅣ-Ⅵcompound GeTe is considered as a promising alternative to the toxic PbTe for high-efficiency mid-temperature thermoelectric applications.However,pristine GeTe suffers from a high concentration of Ge vacancies,resulting in an excessively high hole concentration(>1×10^(21)cm^(-3)),which greatly limits its thermoelectric enhancement.To address this issue,CuBiTe_(2)alloying is introduced to increase the formation energy of Ge vacancies in GeTe,thereby inhibiting the high carrier concentration.The carrier scattering caused by the electronegativity difference between different elements is suppressed due to the similar electronegativity of Cu and Ge atoms.A relatively high hole mobility is obtained,which ultimately leads to a high power factor.Additionally,by introducing Se as an alloying element at the anionic site in GeTe,dense point defects with mass/strainfield fluctuations are induced.This contributes to the strengthening of phonon scattering,thereby reducing the lattice thermal conductivity from 1.44 W·m^(-1)·K^(-1)for pristine GeTe to 0.28 W·m^(-1)·K^(-1)for Ge_(0.95)Cu_(0.05)Bi_(0.05)Te_(0.9)Se_(0.15)compound at 623 K.

Rattling-like behavior and band convergence induced ultra-low lattice thermal conductivity in MgAl_(2)Te_(4) monolayer

Inspired by the excellent stability exhibited by experimentally synthesized two-dimensional(2D)MoSi_(2)N_(4) layered material,the thermal and electronic transport,and thermoelectric(TE)properties of MgAl2Te4 monolayer are systematically investigated using the First-principles calculations and Boltzmann transport theory.The mechanical stability,dynamic stability,and thermal stability(900 K)of the MgAl_(2)Te_(4) monolayer are demonstrated,respectively.The MgAl_(2)Te_(4) monolayer exhibits a bandgap of 1.35 eV using the HSE06 functional in combination with spin-orbit coupling(SOC)effect.Band convergence in the valence band is favorable to improve the thermoelectric properties.The rattling thermal damping effect caused by the weak bonding of Mgsingle bondTe bonds in MgAl2Te4 monolayer leads to ultra-low lattice thermal conductivity(0.95/0.38 W/(m·K)@300 K along the x-/y-direction),which is further demonstrated by the phonon group velocities,phonon relaxation time,Grüneisen parameters,and scattering mechanisms.The optimal zT of 3.28 at 900 K is achieved for the p-type MgAl_(2)Te_(4) monolayer,showing the great promising prospect for the excellent p-type thermoelectric material.Our current work not only reveals the underlying mechanisms responsible for the excellent TE properties,but also elaborates on the promising thermoelectric application of MgAl_(2)Te_(4) monolayer material at high temperature.

Thermoelectric performance of Bi_(2)Sn_(2)Te_(6)monolayer with ultralow lattice thermal conductivity induced by hybrid bonding properties:A theoretical prediction

The crystal structure,mechanical stability,phonon dispersion,electronic transport properties and thermoelectric(TE)performance of the Bi_(2)Sn_(2)Te_(6)monolayer are assessed with the first-principles calculations and the Boltzmann transport theory.The Bi_(2)Sn_(2)Te_(6)monolayer is an indirect semiconductor with a band gap of 0.91 eV using the Heyd-Scuseria-Ernzerhof(HSE06)functional in consideration of the spin-orbit coupling(SOC)effect.The Bi_(2)Sn_(2)Te_(6)monolayer is high thermodynamically and mechanically stable by the assessments of elastic modulus,phonon dispersion curves,and ab initio molecular dynamics(AIMD)simulations.The hybrid bonding characteristics are discovered in Bi_(2)Sn_(2)Te_(6)monolayer,which is advantageous for phonon scattering.The antibonding interactions near the Fermi level weaken the chemical bonding and reduce the phonon vibrational frequency.Due to the short phonon relaxation time,strong anharmonic scattering,large Grüneisen parameter,and small phonon group velocity,an ultralow lattice thermal conductivity(0.27 W/(m·K)@300 K)is achieved for the Bi_(2)Sn_(2)Te_(6)monolayer.The optimal dimensionless figure of merit(ZT)values for the n-type and p-type Bi_(2)Sn_(2)Te_(6)monolayers are 2.68 and 1.63 at 700 K,respectively,associated with a high TE conversion efficiency of 20.01%at the same temperature.Therefore,the Bi_(2)Sn_(2)Te_(6)monolayer emerges as a promising candidate for TE material with high conversion efficiency.

Crystal lattice free volume and thermal decomposition of nitramines

The linear, directly proportional, equations between the Arrhenius parameters(E_α, and log A) of the thermal decomposition and the crystal lattice free space per molecule, △V, of 22 nitramines are described. It is shown that, because of a significant limitation by the molecular structural characteristics of such compounds, they are divided into a number of partial relationships. These partial relationships divide the nitramines into a group of substances relating to dimethyl nitramine and a sub-group related to e-HNIW. These directly proportional equations mean that an increase in the △Vvalues is related to an increase in the thermal stability of the corresponding nitramines. A comparison with similar published dependencies for the impact and friction sensitivities, on the one hand, and with the relationship between the E_a values and the sum of the negative and positive extremes of molecular surface electrostatic potentials, on the other, confirms the well-known fact that intermolecular interaction in the nitramines studied plays a decisive role in the thermal reactivity of such compounds. The crystal lattice free space manifests itself here perhaps only in the solid state thermal decomposition of RDX, HMX and DINGU. This study again confirms a level of disorder in the distribution of the forces in the crystal lattice of the"common" quality of e-HNIW,compared with its "reduced sensitivity(RS)" or pure analogues.

A new design of dual-constituent triangular lattice metamaterial with unbounded thermal expansion

Triangular lattice metamaterials composed of bi-layer curved rib elements (called the Lehman-Lakes lattice) possess unbounded thermal expansion, high stiffness and impossibility of thermal buckling, which are highly desirable in many engineering structural applications subjected to large fluctuations in temperature. However, the requirement of such lattice metamaterial is that it must be a hinged joint in order to achieve the bending deformation upon heating freely, which directly leads to poor manufacturability, especially in small dimensions. In this study, a new design of dual-constituent triangular lattice metamaterial (DTLM) with good manufacturability is proposed to achieve the identical unbounded thermal expansion. In this lattice, a special bi-layer curved rib element where layer one is partially covered by layer two is presented, where the hinge joints are not necessary because the flexural rigidity in the single-layer part is much smaller than that in the bi-layer part, and the desirable thermal bending deformation can be achieved. A sample fabricated by additive manufacturing is given in order to show the good manufacturability;simultaneously, the multifunctional performance of the tailored DTLM with zero, large positive or negative coefficient of thermal expansion (CTE) can remain excellent, as well as the Lehman-Lakes lattice. Examples illustrate that the DTLM with zero CTE has about 34.2% improvement in stiffness and meanwhile has 17% reduction in weight compared with the Lehman-Lakes lattice. The stiffness of the DTLM has a moderate reduction when achieving the same large positive or negative CTE. In addition, the thermomechanical properties of the DTLM are given by the closed-form analytical solution and their effectiveness is verified by the detailed numerical simulation.

Thermal equation of state for lattice Boltzmann gases

The Galilean invariance and the induced thermo-hydrodynamics of the lattice Boltzmann Bhatnagar-Gross-Krook model are proposed together with their rigorous theoretical background. From the viewpoint of group invariance, recovering the Galilean invariance for the isothermal lattice Boltzmann Bhatnagar-Gross-Krook equation （LBGKE） induces a new natural thermal-dynamical system, which is compatible with the elementary statistical thermodynamics.

Effects of Thermal Lattice Vibration on the Effective Potential of Weak-Coupling Bipolaron in a Quantum Dot

Based on the Huybrechts' linear-combination operator,effects of thermal lattice vibration on the effective potential of weak-coupling bipolaron in semiconductor quantum dots are studied by using the LLP variational method and quantum statistical theory.The results show that the absolute value of the induced potential of the bipolaron increases with increasing the electron-phonon coupling strength,but decreases with increasing the temperature and the distance of electrons,respectively;the absolute value of the effective potential increases with increasing the radius of the quantum dot,electron-phonon coupling strength and the distance of electrons,respectively,but decreases with increasing the temperature;the temperature and electron-phonon interaction have the important influence on the formation and state properties of the bipolaron:the bipolarons in the bound state are closer and more stable when the electron-phonon coupling strength is larger or the temperature is lower;the confinement potential and coulomb repulsive potential between electrons are unfavorable to the formation of bipolarons in the bound state.

Numerical Predictions of the Effective Thermal Conductivity of the Rigid Polyurethane Foam

A reconstruction method is proposed for the polyurethane foam and then a complete numerical method is developed to predict the effective thermal conductivity of the polyurethane foam. The finite volume method is applied to solve the 2D heterogeneous pure conduction. The lattice Boltzmann method is adopted to solve the 1D homogenous radiative transfer equation rather than Rosseland approximation equation. The lattice Boltzmann method is then adopted to solve 1D homogeneous conduction-radiation energy transport equation considering the combined effect of conduction and radiation. To validate the accuracy of the present method, the hot disk method is adopted to measure the effective thermal conductivity of the polyurethane foams at different temperature. The numerical results agree well with the experimental data. Then, the influences of temperature, porosity and cell size on the effective thermal conductivity of the polyurethane foam are investigated. The results show that the effective thermal conductivity of the polyurethane foams increases with temperature; and the effective thermal conductivity of the polyurethane foams decreases with increasing porosity while increases with the cell size.

Simulation of natural convection under high magnetic field by means of the thermal lattice Boltzmann method

The thermal lattice Boltzmann method （TLBM）, which was proposed by J. G. M. Eggels and J. A. Somers previously, has been improved in this paper. The improved method has introduced a new equilibrium solution for the temperature distribution function on the assumption that flow is incompressible, and it can correct the effect of compressibility on the macroscopic temperature computed. Compared to the previous method, where the half- way bounce back boundary condition was used for non-slip velocity and temperature, a non-equilibrium extrapolation scheme has been adopted for both velocity and temperature boundary conditions in this paper. Its second-order accuracy coincides with the ensemble accuracy of lattice Boltzmann method. In order to validate the improved thermal scheme, the natural convection of air in a square cavity is simulated by using this method. The results obtained in the simulation agree very well with the data of other numerical methods and benchmark data. It is indicated that the improved TLBM is also successful for the simulations of non-isothermal flows. Moreover, this thermal scheme can be applied to simulate the natural convection in a non-uniform high magnetic field. The simulation has been completed in a square cavity filled with the aqueous solutions of KC1 （llwt%）, which is considered as a diamagnetic fluid with electrically low-conducting, with Grashof number Gr=4.64~104 and Prandtl number Pr----7.0. And three cases, with different cavity locations in the magnetic field, have been studied. In the presence of a high magnetic field, the natural convection is quenched by the body forces exerted on the electrically low-conducting fluids, such as the magnetization force and the Lorentz force. From the results obtained, it can be seen that the quenching efficiencies decrease with the variation of location from left, symmetrical line, to the right. These phenomena originate from the different distributions of the magnetic field strengths in the zones of the symmetrical central line of the magnetic fields. The results are also compared with those without a magnetic field. Finally, we can conclude that the improved TLBM will enable effective simulation of the natural convection under a high magnetic field.

Adverse Pregnancy Outcomes Following Cryotherapy, Thermal Ablation and Loop Electrosurgical Excision Procedure for Cervical Intraepithelial Neoplasia Treatment: A Pilot Study among Zambian Women

Background: Cervical Intraepithelial neoplasia treatments have become essential interventions to manage cervical lesions. Majority of the recipients of these treatments are women within the reproductive age group, who according to literature may be at risk of adverse pregnancy outcomes. This pilot study is part of a study investigating adverse pregnancy outcomes among women who received Cryotherapy, Thermal ablation and Loop Electrosurgical Excision Procedure compared to the untreated women in Zambia. Materials and Methods: This descriptive study analyzed records of 886 (n = 443 treated and n = 443 untreated) women aged 15 - 49 years. The women were either screened with Visual Inspection with Acetic Acid or treated for Cervical Intraepithelial neoplasia at the Adult Infectious Disease Centre between January 2010 and December 2020. Women meeting the criteria were identified using the Visual Inspection with Acetic Acid screening records and telephone interviews to obtain the adverse pregnancy outcome experienced. Data were analysed using STATA version 16 to determine the prevalence and obtain frequency distribution of outcomes of interest. Univariate and multivariable binary logistic regression estimated odds of adverse pregnancy outcomes across the three treatments. Results: The respondents were aged 15 to 49 years. Adverse pregnancy outcomes were observed to be more prevalent in the treatment group (18.5%) compared to the untreated group (5.4%). Normal pregnancy outcomes were lower in the treated (46.3%;n = 443) than the untreated (53.7%;n = 443). The treated group accounted for the majority of abortions (85.2%), prolonged labour (85.7%) and low birth weight (80%), whereas, the untreated accounted for the majority of still births (72.7%). Women treated with cryotherapy (aOR = 2.43, 95% CI = 1.32 - 4.49, p = 0.004), thermal ablation (aOR = 6.37, 95% CI = 0.99 - 41.2, p = 0.052) and Loop Electrosurgical Excision Procedure (aOR = 9.67, 95% CI = 2.17 - 43.1, p = 0.003) had two-, six- and ten-times higher odds of adverse pregnancy outcomes respectively, relative to women who required no treatment. Conclusion: Adverse pregnancy outcomes are prevalent among women who have received treatment in Zambia. The findings indicate that treating Cervical Intraepithelial Neoplasia has been linked to higher chances of experiencing abortion, delivering low birth weight babies and enduring prolonged labor that may result in a caesarean section delivery. Cervical neoplasia treatments, particularly Loop Electrosurgical Excision Procedure, are associated with significantly increased odds of adverse pregnancy outcomes. It is essential to include information about prior Cervical Intraepithelial neoplasia treatment outcomes in obstetric care.

Predicting lattice thermal conductivity via machine learning: a mini review

Over the past few decades,molecular dynamics simulations and first-principles calculations have become two major approaches to predict the lattice thermal conductivity(κ_(L)),which are however limited by insufficient accuracy and high computational cost,respectively.To overcome such inherent disadvantages,machine learning(ML)has been successfully used to accurately predictκL in a high-throughput style.In this review,we give some introductions of recent ML works on the direct and indirect prediction ofκL,where the derivations and applications of data-driven models are discussed in details.A brief summary of current works and future perspectives are given in the end.