Influence of sintering conditions on structural,thermal, electric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Na_0.5Bi_0.5TiO₃ ceramics were prepared by a conventional solid-state reaction method and by a hot-pressing route. The influence of sintering conditions on structural, thermal, dielectric and ferroelectric properties of these ceramics was investigated. All obtained samples exhibited a single perovskite phase. It was shown that the sintering conditions significantly influence the properties under investigation. This includes changes in the value of the electric permittivity ? and dielectric loss tanδ, a shift of T_m and T_d and change of the ferroelectric properties. These effects are mainly related to volatility of the Na and Bi components during the sintering process along with formation of their compensating charge defects, which leads to local structure change.     

Dielectric,thermal and Raman spectroscopy studies of lead-free (Na0.5Bi0.5)1−xSrxTiO3 (x = 0, 0.04 and 0.06) ceramics

ABSTRACT

Lead-free (Na_0.5Bi_0.5)_1?xSr_xTiO₃ (x = 0, 0.04 and 0.06) ceramics with relative densities above 97% were prepared by solid-state synthesis process. Their dielectric, thermal and Raman properties were studied. X-ray diffraction analysis shows perovskite structure with rhombohedral symmetry at room temperature. Sr doping of Na_0.5Bi_0.5TiO₃ (NBT) results in an increase of the dielectric permittivity, diffusing of the permittivity maximum and its shift toward lower temperatures. The temperature of the rhombohedral–tetragonal phase transition indicated by the differential scanning calorimetry (DSC) peak and relaxational dielectric anomaly near the depolarization temperature are also shifted toward lower temperatures. The observed increase and broadening of the permittivity maximum, enhancement of the dielectric relaxation near the depolarization temperature, broadening of the DSC anomaly related to the rhombohedral–tetragonal phase transition and broadening of the Raman bands with increasing Sr content are attributed to the increase of the degree of cationic disorder and evident enhancement of the relaxor-like features in NBT–xST. This enhancement could play a positive role in the improvement of the piezoelectric performance of NBT-based ceramics.     

Improved ferroelectric,dielectric and magnetic properties with La- and Mn- doped Bi5Ti3FeO15 thin films

A series of La and Mn co-doped Bi₅Ti₃FeO₁₅ (BLTFMO) thin films were prepared by spin-coating deposition route. X-ray diffraction, atomic force microscopy and scanning electron microscopy were used to characterize the structures of these BLTFMO thin films. Ferromagnetic properties are obtained as the La-doping content is 0, 0.1, 0.2, and 0.3 with the transition temperature of 127.2 K, 65.1 K, 48.1 K, and 7.9 K, respectively. Well-defined ferroelectric loops are found in all these BLTFMO films, and a higher remnant polarization of 27.84, 24.21 and 24.02 μC/cm² is obtained in the 0.1, 0.2 and 0.3 La-doped films, respectively. A weak dielectric dispersion for the BLTFMO without La-doping, a strong one in 0.1, 0.2, 0.3 and 0.4 La doped films as indicated by the appearance of a dielectric loss peak, and a weak dispersion in 0.6, 0.8 and 1 La doped ones are demonstrated.     

Structural,optoelectronic and thermoelectric properties of antiperovskite compounds Ae3PbS (Ae = Ca,Sr and Ba): A first principles study

In the last years, alkaline-earth based antiperovskite compounds with small semiconductor band gap have been proven to be promising candidate for optoelectronic and thermoelectric applications. In this work, the structural, electronic, optical and thermoelectric properties of Ae₃PbS (Ae = Ca, Sr and Ba) compounds have been predicted using first principles calculations based on the full-potential linearized augmented plane-wave (FP-LAPW) method and semiclassical Boltzmann transport theory. Exchange-correlation effect is treated with the generalized gradient approximation with Perdew–Burke–Ernzerhof scheme (GGA-PBE) and Tran–Blaha modified Becke–Johnson exchange potential. The lattice constant of considered materials increases as Ae goes in order from Ca to Ba and the hardness slightly decreases in this order. Ca₃PbS and Sr₃PbS are semiconductor with direct band gap of 0.199 eV and 0.116 eV, respectively, while Ba₃PbS is nearly metallic. Important optical responses of studied antiperovskites are found in the visible and ultraviolet energy range. Finally, the thermoelectric properties including Seebeck coefficient, electrical conductivity, thermal conductivity, power factor and figure of merit are calculated. Obtained results show that Ca₃PbS and Sr₃PbS could be candidate for applications in thermoelectric generators at low and moderate temperatures due to their high figure of merit values.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

多组分复合薄膜的光学、电学、机械性能及其应用

复合薄膜因其可具有比单组份薄膜更加优异的性能而得到广泛的应用。通过以膜层的防护、催化、电学、光学以及力学性能等复合思想为切入点，阐述了通过膜层的复合掺杂旨在增强合金的抗腐蚀性，提高润滑摩擦性能，改善膜层的导电性能以及进行光学薄膜折射率和光谱吸收的调控，增强膜层的硬度及拉伸强度等机械性能的方法。对国内外的相关前沿成果进行简要介绍，并对复合薄膜的未来发展进行展望，为相关领域的研究提供参考。     

Pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations, based on norm-conserving nonlocal pseudopotentials and density functional theory (DFT), are performed to investigate the structural, elastic, dielectric, and vibrational properties of aluminum arsenide AlAs with zinc-blende (B3) structure and nickel arsenide (B8₁) structure under hydrostatic pressure. Firstly, the path for the phase transition from B3 to B8₁ is confirmed by analyzing the energies of different structures, which is in good agreement with previous theoretical results. Secondly, we find that the elastic constants, bulk modulus, static dielectric constants, and the optical phonon frequencies are varying in a nearly linear manner under hydrostatic pressure. What is more, the softening mode of transversal acoustic mode at X point supports the phase transition in AlAs.     

The pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations,based on norm-conserving nonlocal pseudopotentials and density functional theory(DFT),are performed to investigate the structural,elastic,dielectric,and vibrational properties of aluminum arsenide(AlAs) with a zinc-blende(B3) structure and a nickel arsenide(B81) structure under hydrostatic pressure.Firstly,the path for the phase transition from B3 to B81 is confirmed by analyzing the energies of different structures,which is in good agreement with previous theoretical results.Secondly,we find that the elastic constants,bulk modulus,static dielectric constants,and the optical phonon frequencies vary in a nearly linear manner under hydrostatic pressure.What is more,the softening mode of the transversal acoustic mode at the X point supports the phase transition in AlAs.     

Electronic,dielectric and mechanical properties of MoS2/SiC hybrid bilayer: A first principle study

The electronic, mechanical and dielectric properties of lateral MoS₂/SiC heterobilayer are investigated using first principles calculations. Among various stacking conformations, the energetically favorable stackings namely AA₂ and AB′₁ have been considered in the present study. The band gap of the heterobilayer shows reduction as compared to constituent monolayers which also remains stacking dependent. The electronic band-gap is further tunable by applying mechanical strain and perpendicular electric field that rendered heterostructures from semiconductor to metal at critical value of applied strain/field. The stacking of heterobilayer strongly influence its mechanical properties e.g. ultimate tensile strength of considered two favorable stacking differ by more than 50%; the ultimate tensile strain of 17% and 21% respectively has been calculated for two different stackings. The static dielectric constant also shows tunability on heterostructuring the constituent monolayers as well as applying strain and field. These tunable properties of MoS₂/SiC may be useful for the device applications at nanoscale.     

Pressure-dependent mechanical and thermodynamic properties of newly discovered cubic Na2He

Recently for the first time, a stable compound of He and Na (Na₂He) is predicted at high pressure. We explore the pressure-dependent elastic, mechanical and thermodynamic properties of this newly discovered Na₂He by using ab initio technique. The calculation presents good accordance between the theoretical and experimental lattice parameters. Though the most stable structure of Na₂He is found at 300?GPa, present study ensures the mechanical stability of this compound up to 500?GPa. The study of Cauchy pressure, Pugh's ratio, and Poisson's ratio implies the ductile manner of Na₂He up to 500?GPa. According to the value of Poisson's ratio the bonding force exists in Na₂He is central. The study of Zener anisotropy factor indicates that Na₂He is an anisotropic material but near at 300?GPa approximately isotropic nature of Na₂He is revealed. The study of the bulk modulus, shear modulus, Young's modulus and Vickers hardness implies that the hardness of Na₂He can be improved by applying external pressure. However, the Debye temperature, melting temperature and minimum thermal conductivity of Na₂He are also calculated and discussed at different pressures.     

Correlation of sp and sp fraction of carbon with electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon films

In the present work the correlation of electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon (Ar-DLC) thin films with sp³ and sp² fractions of carbon have been explored. These Ar-DLC thin films have been deposited, under varying C₂H₂ gas pressures from 25 to 75 mTorr, by radio frequency-plasma enhanced chemical vapor deposition technique. X-ray photoelectron spectroscopy studies are performed to estimate the sp³ and sp² fractions of carbon by deconvoluting C 1s core level spectra. Various electrical, optical and nano-mechanical parameters such as conductivity, I-V characteristics, optical band gap, stress, hardness, elastic modulus, plastic resistance parameter, elastic recovery and plastic deformation energy have been estimated and then correlated with calculated sp³ and sp² fractions of carbon and sp³/sp² ratios. Observed tremendous electrical, optical and nano-mechanical properties in Ar-DLC films deposited under high base pressure conditions made it a cost effective material for not only hard and protective coating applications but also for electronic and optoelectronic applications.     

The investigation of structural,electronic, elastic and thermodynamic properties of Gd1−xYxAuPb alloys: A first principle study

First principle calculations have been employed to investigate the effects of Y concentration, pressure and temperature on various properties of ${\mathrm{Gd}}_{1?x}{\mathrm{Y}}_x\mathrm{AuPb}$ ($x=0,0.25,0.5,0.75,1$) alloys using density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method within a framework of the generalized gradient approximation (GGA) is used to perform the calculated results of this paper. Phase stability of ${\mathrm{Gd}}_{1?x}{\mathrm{Y}}_x\mathrm{AuPb}$ alloys is studied using the total energy versus unit cell volume calculations. The equilibrium lattice parameters of these alloys are in good agreement with the available experimental results. The mechanical stability of ${\mathrm{Gd}}_{1?x}{\mathrm{Y}}_x\mathrm{AuPb}$ alloys is proved using elastic constants calculations. Also, the influence of Y concentration on elastic properties of ${\mathrm{Gd}}_{1?x}{\mathrm{Y}}_x\mathrm{AuPb}$ alloys such as Young's modulus, shear modulus, Poisson's ratio and anisotropy factor are investigated and analyzed. By considering both Pugh's ratio and Poisson's ratio, the ductility and brittleness of these alloys are studied. In addition, the total density of states and orbital's hybridizations of different atoms are investigated and discussed. Moreover, the effect of pressure and temperature on some important thermodynamic properties is investigated.     

Structural,electronic, magnetic and vibrational properties of half-Heusler NaZrZ (Z = P,As, Sb) compounds

The structural, electronic, magnetic and vibrational properties of NaZrP, NaZrAs and NaZrSb half-Heusler alloys have been investigated on the basis of density functional theory and generalized gradient approximation. There are three types of structures for these compounds where type a is the most stable one. It is found that all of these materials are half-metallic ferromagnets with a magnetic moment of $2{\mu }_{\mathrm{B}}$. The half-metallic gaps are estimated to be 0.16, 0.35 and 0.55 eV for Z = P, As and Sb, respectively. The hybridization between s and $t_{2g}$ orbitals of Zr with s and p orbitals of Z leads to half-metallic ferromagnetism in these compounds. The effect of strain on the half-metallic property is also investigated, and we notice that the half-metallicity is conserved up to the lattice compressions of 54.43%, 48.29% and 47.55% for NaZrP, NaZrAs and NaZrSb, respectively. The dynamical stability of these compounds is confirmed using dispersion curves. The Curie temperatures are also estimated to be 501.29 K, 855.49 K and 1348.88 K for NaZrP, NaZrAs and NaZrSb, respectively. Therefore, it seems that NaZrZ (Z = P, As, Sb) could be suitable materials for spin-injector applications.     

The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

Electronic,elastic and dynamical properties of MgSe under pressure: rocksalt and iron silicide phase

The electronic, elastic and dynamical properties of MgSe in the rocksalt (B1) and iron silicide (B28) phase and the effects of pressure on these properties are investigated using first-principles method. The calculated electronic band structure indicates that the B1 phase of MgSe presents an indirect band-gap feature and the band gaps initially increase with pressure and subsequently decrease upon compression. Remarkably, an indirect-to-direct band-gap transition has been observed at the phase transition pressure. The elastic constants, bulk modulus, shear modulus, Young’s modulus, elastic anisotropy and B/G ratio of MgSe in the B1 and B28 phase at high pressure have also been investigated. The bulk modulus, shear modulus and Young’s modulus all increase monotonously with the increasing of pressure for the B1 and B28 phase of MgSe. The calculated phonon frequencies of the B1 phase at zero pressure agree well with available theoretical results. And the transverse acoustic phonon TA(X) mode of this phase completely softening to zero at 82 GPa. The phonon curves of the B28 phase under pressure have also been successfully investigated.     

Preparation and physical properties of CuxWO3

We report on the study of WO₃ doped with Cu using sol-gel (Cu_xWO₃^d) and impregnation (Cu_xWO₃ⁱ) methods. All materials are well crystallized and exhibit single phases whose crystallite size ranges from 17 to 100 nm depending on Cu amount and the preparation technique. The conductivity dependence on temperature demonstrates semiconductor behavior and follows the Arrhenius model, with activation energies, E_σ, commonly in the range 0.4-0.6 eV. Moreover, the thermopower study shows that Cu_xWO₃^d is mainly of p-type conductivity, whereas Cu_xWO₃ⁱ is n-type. The mechanism of conduction is attributed to a small polaron hopping. The doping process is found to decrease the interband transition down to 520 nm depending on the preparation conditions. The photoelectrochemical characterization confirms the conductivity type and demonstrates that the photocurrent J_ph increases with Cu-doping. Taking into consideration the activation energy, the flat band potential and the band gap energy, the band positions of each material are proposed according to the preparation method and Cu amount.     

Dielectric and ferroelectric properties of NBT-BT systems

ABSTRACT

The (1-x)Na_0.5Bi_0.5TiO₃- xBaTiO₃ ceramics (x = 0.1, 0.135 and 0.17) were fabricated by a conventional solid phase sintering process. The bulk density of the obtained samples exceeded 95% of the theoretical relative density as determined by Archimedes method. Dielectric and ferroelectric measurements of these ceramics were performed. Measurements of the ferroelectric properties show that, above the depolarization temperature T_d, the shape of the hysteresis loops approaches that of linear dielectrics. The dielectric study results correlate with the hysteresis loops measurements. The relaxor-like behavior of the investigated materials was revealed.     

Theoretical investigations of phases of AlAs by first-principles

Five potential novel phases of AlAs (Pmn2₁-AlAs, Pbam-AlAs, Pbca-AlAs, bct-AlAs and wz-AlAs) are proposed and the stabilities of them are verified by the enthalpy, independent elastic constants and phonon dispersion spectra. The mechanical, electronic and thermodynamic properties of ten AlAs phases are studied and the electronic properties are calculated by PBE0 hybrid functional. Pmn2₁-AlAs and Pbam-AlAs own direct band gaps, so they have electronic advantages over other phases of AlAs at 0?GPa. Our calculated band gaps are 3.02, 3.07, 3.21, 2.76, 3.12, 1.15, 1.88, 2.53, 2.75 and 2.53?eV for Pmn2₁-AlAs, Pbam-AlAs, Pbca-AlAs, bct-AlAs, wz-AlAs, oC12-AlAs, hP6-AlAs, cI24-AlAs, zb-AlAs and cmcm-AlAs, respectively. Additionally, these phases: Pmn2₁-AlAs, Pbam-AlAs, Pbca-AlAs, bct-AlAs, cI24-AlAs, and cmcm-AlAs behave in a ductile manner, and wz-AlAs, oC12-AlAs, hP6-AlAs and zb-AlAs behave in a brittle manner. Otherwise, cmcm-AlAs has the greatest anisotropy, and hP6-AlAs has the least anisotropy.     

Dielectric and piezoelectric properties of manganese‐modified PbHfO3–PbTiO3–Pb(Mg1/3Nb2/3)O3 ternary ceramics with morphotropic phase boundary compositions

High piezoelectric and electromechanical properties were reported in the PbHfO₃–PbTiO₃–Pb(Mg_1/3Nb_2/3)O₃ ternary system with morphotropic phase boundary (MPB) compositions. This work focuses on the effect of MnO₂ addition on 0.8Pb(Hf_0.443Ti_0.557)O₃–0.2PMN (0.8PHT–0.2PMN) ceramics. It was observed that the Mn acceptor modification induced a “hardening” effect in 0.8PHT–0.2PMN, with decreased piezoelectric coefficients d₃₃ and dielectric loss tan δ and a significantly increased mechanical quality factor Q_m. Moreover, the 0.2 wt% MnO₂‐doped 0.8PHT–0.2PMN ceramics exhibited good piezoelectric and electromechanical properties with d₃₃, planar electromechanical coupling k_p and Q_m being on the order of 360 pC/N, 61% and 700, respectively, showing advantages compared to those of commercial hard PZT4 ceramics, which is attractive for high power applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)