Microwave and infrared dielectric response of tunable Ba1−xSrxTiO3 ceramics

The dielectric properties, infrared (IR) dielectric response and Raman spectra of tunable microwave (MW) Ba_1?xSr_xTiO₃ (x = 0.4, 0.5, 0.6 and 0.7) ceramics were studied systematically as functions of composition and temperature. It was found that, with increasing the content of Sr from 40% to 70%, the permittivity of the Ba_1?xSr_xTiO₃ ceramics decreased from 7200 to 640 while the dielectric tunability decreased from 50.1% to 5.4% measured at 10 kHz. It is particularly interesting that the MW dielectric loss tangent was significantly decreased from 1.5 × 10^?2 to 7.4 × 10^?4. Complex permittivity spectra obtained by fitting the infrared data were also extrapolated to MW frequency and compared with the dielectric data near 1 GHz. For the samples with x = 0.4 and x = 0.5, the dielectric loss measured at ～1 GHz was much higher than that calculated at 10 GHz, which is presumably due to the Debye-type dielectric relaxation in GHz region. However, the dielectric loss of the samples with x = 0.6 and x = 0.7, extrapolated from IR range, was in agreement with that measured at MW frequency. Low-temperature Raman scattering showed that the band at 760 cm^?1 assigned to the Ti?O₃ torsional mode markedly sharpens and shifts upward with increasing content of Sr. This is an indication that the bonding between cations and anions was tightened due to the substitution of Ba with Sr, which explains well the decrease in dielectric permittivity, loss and tunability with increasing concentration of Sr.     

Structural evolution and physical properties of Bi1−xGdxFeO3 ceramics

The crystal structure and physical properties of Bi_1?xGd_xFeO₃ ceramics with x ≤ 0.5 have been studied by X-ray diffraction, differential scanning calorimetry, dielectric and magnetic measurements. Bi_1?xGd_xFeO₃ compounds crystallize at room temperature in R3c structure for x < 0.08 and in Pbnm structure for x ≥ 0.3. The R3c phase transforms to the Pbnm phase as temperature increases. Temperature-dependent dielectric response and low-temperature X-ray diffraction indicate a phase transition of the Pbnm phase at low-temperature. Detailed structural analysis indicates that the interaction between Bi and O plays an important role in the evolution of structure and physical properties. A phase diagram is constructed to illustrate the physical property–composition–temperature relations.     

Synthesis,characterization and electrical properties of the system LaMnxFe1−xO3

The system manganese substituted lanthanum ferrite viz. LaMn_xFe_1?xO₃ (1.0 ≤ x ≥ 0) was prepared by sol–gel autocombusion method. The structural characterization of the samples was carried out by X-ray diffraction technique and it is found that, the phase transfer from cubic to orthorhombic perovskite structure. The lattice parameter and crystallite size decrease with increasing Mn content. The phase formation of perovskite was revealed by thermal analysis technique. The surface morphology and elemental analysis of all the samples were carried out by scanning electron microscopy and energy dispersive X-ray spectroscopic technique, respectively. Electrical properties of the compounds show that, they exhibit semiconducting behavior. The substitution of manganese ions plays an important role in changing their structural, electrical and magnetic properties of lanthanum ferrite.     

Preparation and dielectric properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with difference volume fractions of porosity from 34.1% to 59.2% were produced by adding different amount of the pore-forming agent into initial silicon nitride powder. The microwave dielectric property of these ceramics at a frequency of 9.36 GHz was studied. The crystalline phases of the samples were determined by X-ray diffraction analysis. The influence of porosity on the dielectric properties was evaluated. The results show that α-Si3N4 crystalline phase exists in all the samples while the main crystalline phase of the samples is β-Si3N4, indicating that the αβ transformation happens during the preparation of samples and the transformation is incomplete. There is a dense matrix containing large pores and cavities with needle-shaped and flaky β-Si3N4 grains distributing. The dielectric constant of the ceramics reduces with the increase of porosity.     

Piezoelectric and dielectric properties of Cr-doped PSN-PZN-PZT quaternary piezoelectric ceramics

PSN-PZN-PZT x wt.%Cr2O3, x=0.0-0.9, were prepared by conventional mixed oxide techniques at sintering temperatures of 1220℃-1300℃ for 2 h. The effect of sintering temperature on the microstructure and the piezoelectric properties was investigated by XRD, SEM, and other conventional measurement. The result indicated that with temperature increasing, the valence of Cr ion from Cr^5 or Cr^6 changes into Cr^3 , and the piezoelectric properties turn hard. With increasing Cr2O3 content, the amount of rhombohedral phases increases and the morphotropic boundary phase is correspondingly shifts to rhombohedral phase. A uniform microstructure and excellent comprehensive properties were obtained at 1240℃ as the amount of Cr2O3 is 0.5 wt.%.     

Thermodynamic properties of CexTh1−xO2 solid solution from first-principles calculations

A systematic study based on first-principles calculations along with a quasi-harmonic approximation has been conducted to calculate the thermodynamic properties of the Ce_xTh_1?xO₂ solid solution. The predicted density, thermal expansion coefficients, heat capacity and thermal conductivity for the Ce_xTh_1?xO₂ solid solution all agree well with the available experimental data. The thermal expansion coefficient for ThO₂ increases with CeO₂ substitution, and complete substitution shows the highest expansion coefficient. On the other hand, the mixed Ce_xTh_1?xO₂ (0 < x < 1) solid solution generally exhibits lower heat capacity and thermal conductivity than the ThO₂ and CeO₂ end members. Our calculations indicate a strong effect of Ce concentration on the thermodynamic properties of the Ce_xTh_1?xO₂ solid solution.     

Phase field simulations of ferroelectrics domain structures in PbZrxTi1−xO3 bilayers

Domain stability and structures in Pb(Zr_0.3Ti_0.7)O₃/Pb(Zr_0.7Ti_0.3)O₃ bilayer films under different substrate strains are studied using the phase field method. It is demonstrated that the domain structure of the bilayer film is very different from those of the corresponding single layer films grown on the same silicon substrate with an incoherent interface. Moreover, the predicted rhombohedral domains in the Pb(Zr_0.7Ti_0.3)O₃ layer of the bilayer film have smaller sizes than those in the single layer case. These results are compared with experimental observations and previous thermodynamic analyses. The polarization distributions of the ferroelectric–paraelectric bilayer are analyzed as a function of the thickness of the bilayer film, where there is a “ferroelectric proximity effect” due to dipole–dipole interactions. The phase diagrams for both the bilayer and single layer films as a function of temperature and effective in-plane substrate strain are constructed.     

Effect of microalloying on the structure and properties of alloys in the Al−Si−Cu system

Conclusions  

Effect of Y on microstructure evolution and mechanical properties of Mg−4Li−3Al alloys

To obtain magnesium alloys with a low density and improved mechanical properties, Y element was added into Mg−4Li−3Al (wt.%) alloys, and the effect of Y content on microstructure evolution and mechanical properties was investigated by using optical microscopy, scanning electron microscopy and tensile tests. The results show that mechanical properties of as-cast Mg−4Li−3Al alloys with Y addition are significantly improved as a result of hot extrusion. The best comprehensive mechanical properties are obtained in hot-extruded Mg−4Li−3Al−1.5Y alloy, which possesses high ultimate tensile strength (UTS=248 MPa) and elongation (δ=27%). The improvement of mechanical properties of hot-extruded Mg−4Li−3Al−1.5Y alloy was mainly attributed to combined effects of grain refinement, solid solution strengthening and precipitation strengthening.     

Effect of poling condition on piezoelectric properties of （K0.5Na0.5）NbO3 ceramics

The lead-free piezoelectric ceramics （K0.5Na0.5）NbO3 （abbreviated as KNN） with the relative density of 97.6% were synthesized by press-less sintering owing to the careful control of processing conditions. The phase structure of KNN ceramics was analyzed. The results show that the pure perovskite phase with orthorhombic symmetry is in all ceramics specimens. The effect of poling conditions on the piezoelectric properties of KNN ceramics was investigated. The results show that the piezoelectric constant d33 and electromechanical coupling factor kp increase with poling field, poling temperature and poling time increasing, then decrease because of electric broken. Take into account of poling conditions and piezoelectric properties of pure KNN ceramics, the optimum poling conditions for pure KNN ceramics are poling field of 4 kV/mm, poling temperature of 140 ℃ and poling time of 20-25 min.     

Effect of sintering temperature and composition on microstructure and properties of PMS-PZT ceramics

The piezoelectric ceramics xPb（Mn1/3Sb2/3）O3-（1-x）Pb（Zr1/2Ti1/2）O3 （abbreviated as PMS-PZT） were synthesized by traditional ceramics process. The effect of sintering temperature and the amount of Pb（Mn1/3Sb2/3）O3 （abbreviated as PMS） on phase structure, microstructure, piezoelectric and dielectric properties of PMS-PZT ceramics was investigated. The results show that the pure perovskite phase is in all ceramics specimens, the phase structure of PMS-PZT ceramics changes from tetragonal phase to single rhombohedral phase with the increasing amount of PMS. The dielectric constant εr, Curie temperature Tc, electromechanical coupling factor kp and piezoelectric constant d33 decrease, whereas the mechanical quality factor Qm and dielectric loss tanδ increase with the increasing amount of PMS in system. The optimum sintering temperature is 1 200-1 250 ℃. It is concluded that the PMS-PZT （x=0.07） ceramics sintered at 1 250 ℃ is suitable for high-power piezoelectric transformer. These properties include εr=674.8, tanδ =0.005 25, kp=0.658, Qm= 1 520, d33=230 pC/N, Tc=275 ℃.     

Effect of Gd content on microstructure and dynamic mechanical properties of solution-treated Mg−xGd−3Y−0.5Zr alloy

The effect of Gd content ranging from 6.5 wt.% to 8.5 wt.% on microstructure evolution and dynamic mechanical behavior of Mg?xGd?3Y?0.5Zr alloys was investigated by optical microscopy, X-ray diffraction, scanning electron microscopy and split Hopkinson pressure bar. The microstructure of as-cast Mg?xGd?3Y?0.5Zr alloys indicates that the addition of Gd can promote grain refinement in the casting. Due to the rapid cooling rate during solidification, a large amount of non-equilibrium eutectic phase Mg₂₄(Gd,Y)₅ appears at the grain boundary of as-cast Mg?xGd?3Y?0.5Zr alloys. After solution treatment at 520 °C for 6 h, the Mg₂₄(Gd,Y)₅ phase dissolves into the matrix, and the rare earth hydrides (REH) phase appears. The stress?strain curves validate that the solution-treated Mg?xGd?3Y?0.5Zr alloys with optimal Gd contents maintain excellent dynamic properties at different strain rates. It was concluded that the variation of Gd content and the agglomeration of residual REH particles and dynamically precipitated fine particles are key factors affecting dynamic mechanical properties of Mg?xGd?3Y?0.5Zr alloys.     

Effect of NiAl phase on the mechanical properties of Cr−Mn−Ni steels

Metal Science and Heat Treatment -     

Effect of Al addition on microstructure and mechanical properties of Mg−Zn−Sn−Mn alloy

The microstructure and properties of the as-cast, as-homogenized and as-extruded Mg−6Zn−4Sn−1Mn (ZTM641) alloy with various Al contents (0, 0.5, 1, 2, 3 and 4 wt.%) were investigated by OM, XRD, DSC, SEM, TEM and uniaxial tensile tests. The results show that when the Al content is not higher than 0.5%, the alloys are mainly composed of α-Mg, Mg₂Sn, Al₈Mn₅ and Mg₇Zn₃ phases_. When the Al content is higher than 0.5%, the alloys mainly consist of α-Mg, Mg₂Sn, MgZn, Mg₃₂(Al,Zn)₄₉, Al₂Mg₅Zn₂, Al₁₁Mn₄ and Al₈Mn₅ phases. A small amount of Al (≤1%) can increase the proportion of fine dynamic recrystallized (DRXed) grains during hot-extrusion process. The room- temperature tensile test results show that the ZTM641−1Al alloy has the best comprehensive mechanical properties, in which the ultimate tensile strength is 332 MPa, yield strength is 221 MPa and the elongation is 15%. Elevated- temperature tensile test results at 150 and 200 °C show that ZTM641−2Al alloy has the best comprehensive mechanical properties.     

Phase transition behavior and electrical properties of [(K0.50Na0.50)1−xAgx](Nb1−xTax)O3 lead-free ceramics

The effect of AgTaO₃ on the electrical properties of (K_0.5Na_0.5)NbO₃ lead-free ceramics was systematically investigated, and the phase transition behavior of the ceramics was also studied in terms of high temperature X-ray diffraction. The experimental results show that Ag⁺ and Ta⁵⁺ ions diffuse into the (K_0.5Na_0.5)NbO₃ lattices to form a stable solid solution with orthorhombic structure, and also lead to the decrease in the orthorhombic to the tetragonal phase transition temperature and the Curie temperature. The 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramics exhibit optimum electrical properties (d₃₃ = 183 pC/N, k_p = 41%, T_c = 356 °C, T_o–t = 158 °C, ?_r ～ 683, and tan δ ～ 3.3%) and good thermal-depoling behavior. The piezoelectric properties of (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramics are much superior to pure (K_0.5Na_0.5)NbO₃ ceramics. X-ray diffraction patterns for the 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramic at different temperatures indicated a pure perovskite phase with an orthorhombic structure at below 160 °C, a tetragonal structure at 160–350 °C, and a cubic structure at above 360 °C. As a result, the (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramic is one of the promising candidate materials for lead-free piezoelectric ceramics.