Structural and dielectric properties in the (Ba1−xCax)(Ti0.95Zr0.05)O3 ceramics

Lead-free (Ba_1−xCa_x)(Ti_0.95Zr_0.05)O₃ (x = 0.05-0.40) (BCZT) ceramics were prepared by solid-state reaction technique. XRD results show that the samples in the composition range of 0.05 ≤ x ≤ 0.25 exhibit pure perovskite structures and undergo a polymorphic phase transitions from orthorhombic to tetragonal phase around room temperature. The biphasic structures are detected at x ≥ 0.30 and the dielectric peaks become broad and dielectric constants decrease with increasing Ca content. Ca replacement at Ba site leads to diffuseness, whereas Ca occupancy at Ti site leads to decrease of the T_c.     

Effect of La substitution on conductivity and dielectric properties of Bi1−xLaxFeO3 ceramics: An impedance spectroscopy analysis

Bismuth ferrite (BFO) and La-substituted BFO with composition Bi_1−xLa_xFeO₃ (x=0.05, 0.1 and 0.15) (BLFO_x=0.05-0.15) ceramics were prepared using the solid state reaction route. A structural phase transition from rhombohedral phase to triclinic phase was observed for BLFO_x=0.05-0.15 ceramics. Modulus spectroscopy reveals the deviation of dielectric behavior from ideal Debye characteristics and the dependence of conductivity on ion hopping in BFO and BLFO_x=0.05-0.15 ceramics. The conductivity of the BFO ceramics decreases for La content of 5 mol%, followed by a subsequent increase with 10 and 15 mol% of lanthanum doping. The typical values of the activation energies at high temperature reveal the contribution of short range movement of doubly ionized oxygen vacancies to the conduction process in BFO and BLFO_x=0.05 ceramics. Both short range and long range motion of oxygen vacancies are responsible for large conductivity in BLFO_{x=0.1 and 0.15} ceramics.     

Dielectric properties and phase transitions of (Pb0.87La0.02Ba0.1)(Zr0.6Sn0.4−xTix)O3 ceramics with compositions near AFE/RFE phase boundary

The electrical properties and phase transition behavior of (Pb_0.87La_0.02Ba_0.1)(Zr_0.6Sn_0.4−xTi_x)O₃ solid solutions (PLBZST, 0.04≤x0.2) were investigated by the X-ray diffraction, permittivity, pyroelectric current, and P-E electric hysterisis loops. As the composition x increased from 0.04 to 0.2, the antiferroelectric ceramics (x≤0.07, AFE) with tetragonal phase changed to the ferroelectric relaxors (RFE, 0.09≤x). AFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to −100 °C) due to a frustration between AFE and FE state. With an increase in composition x, electrically field-induced AFE-FE switching field (E_AFE-FE) and AFE-paraelectric (PE) phase transition temperature (T_c) are depressed in the temperature (T)-Ti composition (x) phase diagram, a FE-AFE-PE triple phase point (T_tr) with the lowest transition temperature occurred at x=0.09. The pyroelectric currents under an application of various external electric field (E) were measured to identify a T-E phase diagram of the PLBZST compound.     

Na1−xLixNbO3 ceramics studied by X-ray diffraction, dielectric, pyroelectric, piezoelectric and Raman spectroscopy

Na_1−xLi_xNbO₃ ceramics with composition 0.05≤x≤0.30 were prepared by solid-state reaction method and sintered in the temperature range 1100-1150 °C. These ceramics were characterised by X-ray diffraction as well as dielectric permittivity measurements and Raman spectroscopy. Dielectric properties of ceramics belonging to the whole composition domain were investigated in a broad range of temperatures from 300 to 750 K and frequencies from 0.1 to 200 kHz. The Rietveld refinement powder X-ray diffraction analysis showed that these ceramics have a single phase of perovskite structure with orthorhombic symmetry for x≤0.15 and two phases coexistence of rhombohedral and orthorhombic above x=0.20. The evolution of the permittivity as a function of temperature and frequency showed that these ceramics Na_1−xLi_xNbO₃ with composition 0.05≤x≤0.15 present the classical ferroelectric character and the phase transition temperature T_C increases as x content increases. The polarisation state was checked by pyroelectric and piezoelectric measurements. For x=0.05, the piezoelectric coefficient d₃₁ is of 2pC/N. The evolution of the Raman spectra was studied as a function of temperatures and compositions. The results of the Raman spectroscopy study confirm our dielectric measurements, and they indicate clearly the transition from the polar ferroelectric phase to the non-polar paraelectric one.     

Magnetic characterization of Bi(Fe1 − xMnx)O3

Bi(Fe_1 − xMn_x)O₃ ceramics (x up to 0.3) were prepared by rapid sintering. Weak ferromagnetism with two magnetic anomalies at low temperatures was observed for Bi(Fe_0.95Mn_0.05)O₃ and Bi(Fe_0.9Mn_0.1)O₃. From temperature-dependent magnetic relaxation measurements, the anomalies at 20 K and 100 K are related to the freezing of cluster spin glass.     

Magnetic study of phase separation and charge ordering in La1−xSrxMnO3 near x=0.5

We have investigated the magnetic phase diagram of polycrystalline and single-crystal La_1−xSr_xMnO₃ near 0.46≤x≤0.50. It turns out that for x<0.48, the polycrystalline material is ferromagnetic (FM), but for x≥0.48, incipient charge ordering takes place along with antiferromagnetism. At x=0.48, the ferromagnetic-antiferromagnetic phase transition in ceramics occurs at less than 85 kOe but requires significantly larger field for increasing x. These observations are in contrast to what is found in the single crystals, which are all FM.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

Synthesis and ferroelectric properties of bismuth layer-structured (Bi7−xSrx)(Fe3−xTi3+x)O21 solid solutions

Bismuth layer-structured (Bi_7−xSr_x)(Fe_3−xTi_3+x)O₂₁ (BSFT) ceramics were synthesized and the ferroelectric properties and crystal structure were investigated. X-ray powder diffraction profiles and refinement of the lattice parameters indicated single phase BSFT was obtained in the composition range 0-1.5. The lattice parameter b of BSFT remained almost constant, while a slight decrease in the lattice parameter a was observed by the Sr and Ti substitution for Bi and Fe, respectively, which indicated an increase in the orthorhombicity. The dependence of the BSFT lattice parameter on temperature implied a phase transition from the orthorhombic to the tetragonal phase, which was in good agreement with the Curie temperature. The remnant polarization P_r, of BSFT was significantly improved by the Sr and Ti substitution for Bi and Fe, and ranged from 9 to 16 μC/cm², although no remarkable variation in the coercive field E_c was observed. As a result, a well-saturated P-E hysteresis loop of BSFT ceramic was obtained at x=0.5 with a P_r of 30 μC/cm at an applied voltage of 280 kV/cm.     

Piezoelectric properties of [Li0.03(K0.48Na0.52)0.97](Nb0.97Sb0.03)O3-(Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 lead-free piezoelectric ceramics

[Li_0.03(K_0.48Na_0.52)_0.97](Nb_0.97Sb_0.03)O₃-(Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ [(1−x)LKNNS-xBCTZ] lead-free piezoelectric ceramics were prepared by the conventional solid state method, and effects of BCTZ content on the piezoelectric properties of LKNNS ceramics were mainly investigated. A stable solid solution has been formed between LKNNS and BCTZ, and a morphotropic phase boundary of (1−x)LKNNS-xBCTZ ceramics is identified in the range of 0 < x ≤ 0.02. The Curie temperature of (1−x)LKNNS-xBCTZ ceramics decreases with increasing BCTZ content. A higher ?_r value and a lower tan δ value are demonstrated for the (1−x)LKNNS-xBCTZ ceramic with x = 0.02. The (1−x)LKNNS-xBCTZ ceramic with x = 0.02 has an enhanced electrical behavior of d₃₃ ∼ 237 pC/N, k_p ∼ 48.6%, ?_r ∼ 1451, tan δ ∼ 0.037, and T_c ∼ 335 °C. As a result, (1−x)LKNNS-xBCTZ ceramics are promising candidate materials for the field of lead-free piezoelectric materials.     

Dielectric and piezoelectric properties of non-stoichiometric (K0.5Na0.5)(Nb0.9+xTa0.1)O3 ceramics

In the study, in order to develop the lead-free piezoelectric ceramics for actuator, transformer and other electronic-devices application, (K_0.5Na_0.5)(Nb_0.9+xTa_0.1)O₃ + 0.5 mol% CuO + 0.2 mol% MnO₂ ceramics were prepared by conventional mixed oxide method. The effects of B-site non-stoichiometry in [(K_0.5Na_0.5)] [(Nb_0.9+xTa_0.1)O₃] ceramics on microstructure and piezoelectric properties were investigated. The density, electromechanical coupling factor (k_p), mechanical quality factor (Q_m), piezoelectric constant (d₃₃), T_C and T_O-T of NKNT ceramics with x = 0.0065 showed the optimum values of 4.58 g/cm³, 0.427, 1554, 109 pC/N, 373 °C and 226 °C, respectively, suitable for piezoelectric motor, and transformer applications.     

Stress induced preferred orientation and phase transition for ternary WCxNy thin films

We deposit ternary WC_xN_y thin films on Si (1 0 0) substrates at 500 °C using direct current (DC) reactive magnetron sputtering in a mixture of CH₄/N₂/Ar discharge, and explore the effects of substrate bias (V_b) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of V_b up to 200 V the carbon (x) and nitrogen (y) atom concentrations of WC_xN_y films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC_0.75N_0.25, show our obtained WC_xN_y films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of V_b, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WC_xN_y to hexagonal α-WC_xN_y occurs as V_b is in the range of −40 to −120 V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WC_xN_y and α-WC_xN_y. The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy.     

Relationship between phase structure and electrical properties of (K0.5Na0.5)NbO3-LiTaO3 lead-free ceramics

Lead-free piezoelectric ceramics of (1−x)K_0.5Na_0.5NbO₃-xLiTaO₃ (KNN-LT) system have been investigated in this work. X-ray diffraction, Raman spectra measurements, DSC (Differential Scanning Calorimetric), and dielectric constant versus temperature provide direct evidence that the phase transition temperature between tetragonal and orthorhombic shift to lower temperature with the increasing of LT content. The KNN-0.05LT ceramics exhibit the highest high-field d₃₃ up to 220 pm/V. At the same time, we also investigated the relationship between phase structure and electric properties, showing that the orthorhombic phase presents better piezoelectric temperature stabilities than the tetragonal phase. The result may provide a new way for KNN-based lead-free ceramics.     

The large low-field magnetic entropy changes in Ni43Mn46Sn11−xSbx alloys

A series of Ni₄₃Mn₄₆Sn_11−xSb_x (x=0, 1, and 3) alloys were prepared by an arc melting method. The martensitic transition shifts to higher temperature with the increasing Sb content. The isothermal magnetization curves and Arrott plots around martensitic transition temperatures show a typical metamagnetic behavior. Under a low applied magnetic field of 10 kOe, large magnetic entropy changes around the martensitic transition temperature are 10.4, 8.9, and 7.3 J/kg K, for x=0, 1, and 3, respectively. The origin of the large magnetic entropy changes and potential application for Ni₄₃Mn₄₆Sn_11−xSb_x alloys as working substances in magnetic refrigeration are discussed.     

Magnetocaloric properties of DyCo2−xGax alloys

The influence of the substitution of Ga atoms for Co atoms in DyCo₂ compounds on magnetocaloric properties has been investigated. A series of DyCo_2−xGa_x alloys with x=0, 0.03, 0.06, 0.1, 0.15, and 0.2 was prepared by the arc-melting method for this investigation. Experimental results revealed that the Ga substitution for Co in DyCo₂ can form a single phase with the cubic Laves phase structure up to x=0.2. As the Ga content x increases, the lattice parameter and the Curie temperature T_c increases from 143 to 196 K linearly. The maximum magnetic entropy changes in a low field change of 0-1.5 T, increasing from 8.24 to 10.61 J/K kg when the Ga content x increases from 0 to 0.03, but decreasing gradually to 3.51 J/K kg as the Ga content further increases to x=0.2. All the samples show a relatively large magnetic entropy change with very small hysteresis loss.     

Effect of Si/Ge ratio on resistivity and thermopower in Gd5SixGe4−x magnetocaloric compounds

The effect of Si/Ge ratio on resistivity and thermopower behavior has been investigated in the magnetocaloric ferromagnetic Gd₅Si_xGe_4−x compounds with x=1.7-2.3. Microstructural studies reveal the presence of Gd₅(Si,Ge)₄-matrix phase (5:4-type) along with traces of secondary phases (5:5 or 5:3-type). The x=1.7 and 2.0 samples display the presence of a first order structural transition from orthorhombic to monoclinic phase followed by a magnetic transition of the monoclinic phase. The alloys with x=2.2 and 2.3 display only magnetic transitions of the orthorhombic phase. A low temperature feature apparent in the AC susceptibility and resistivity data below 100 K reflects an antiferromagnetic transition of secondary phase(s) present in these compounds. The resistivity behavior study correlates with microstructural studies. A large change in thermopower of −8 μV/K was obtained at the magneto-structural transition for the x=2 compound.     

Dielectric and impedance measurements on (1−x)Ba(Fe1/2Ta1/2)O3-xBa(Zn1/3Ta2/3)O3 ceramics

In this work, ((1−x)Ba(Fe_1/2Ta_1/2)O₃-xBa(Zn_1/3Ta_2/3)O₃), ((1−x)BFT-xBZT) ceramics with x = 0.00–0.12 were synthesized by the solid–state reaction method. X-ray diffraction data revealed that both the powders and ceramics were of a pure-phase cubic perovskite structure. All ceramics showed large dielectric constants. For the x = 0.12 sample, a very high dielectric constant (>20,600) was observed. A lowering in the dielectric loss compared to pure BFT ceramics was observed with the BZT addition. The impedance measurements indicated that BZT has a strong effect on the bulk grain and grain boundary resistance of BFT ceramics. These results are in agreement with the measured dielectric properties. Based on dielectric and impedance results, (1−x)BFT-xBZT ceramics could be of great interest for high performance dielectric materials applications due their giant dielectric constant behavior.     

A study on magnetoelastic properties of Tb3 (Fe28−xCox) V1.0 (x=0, 3, 6) compounds

In this work, The magnetoelastic properties of polycrystalline samples of Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) intermetallic compounds are investigated by means of linear thermal expansion and magnetostriction measurements in the temperature range of 77–515 K under applied magnetic fields up to 1.5 T. The linear thermal expansion increases with the Co content. The well-defined anomalies observed in the linear thermal expansion coefficients for Tb₃ (Fe_28−xCo_x) V_1.0 (x=0, 3, 6) compounds are associated with the magnetic ordering temperature for x=0 and spin reorientation temperatures for x=3, 6. Below transition temperatures, the value of the longitudinal magnetostriction (λ_Pa) at 1.6 T increases with Co content.     

Electroresistance effect in La1−xCaxMnO3 (0<x<1) ceramics

Electroresistance (ER) effects were investigated for a full series of manganite ceramics La_1−xCa_xMnO₃ (0<x<1), synthesized by solid state reaction. The results indicate that while the ER effects are large only in the presence of electrically active, high E-field boundaries, the equilibrium or metastable electronic-magnetic states in the adjoining domains are also significant, as a large ER occurs only at x=0.51 and x=0.17; those compositions are both near a two-phase coexistence region, i.e. close to a compositional regime where equilibrium insulating/metallic phase domains and interfaces would occur spontaneously.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Phase transition, structural and electrical properties of Pb(Zn1/3Nb2/3)O3-doped Pb(Ni1/3Nb2/3)O3-PbTiO3 ceramics prepared by solid-state reaction method

Phase pure perovskite (1−x−y)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zn_1/3Nb_2/3)O₃-yPbTiO₃ (PNN-PZN-PT) ferroelectric ceramics were prepared by conventional solid-state reaction method via a B-site oxide mixing route. The PNN-PZN-PT ceramics sintered at the optimized condition of 1185 °C for 2 h exhibit high relative density and rather homogenous microstructure. With the increase of PbTiO₃ (PT) content, crystal structure and electrical properties of the synthesized PNN-PZN-PT ceramics exhibit successive phase transformation. A morphotropic phase boundary (MPB) is supposed to form in (0.9−x)PNN-0.1PZN-xPT at a region of x=32-36 mol% confirmed by X-ray diffraction (XRD) measurement and dielectric measurement. The MPB composition can be pictured as providing a “bridge” connecting rhombohedral ferroelectric (FE) phase and tetragonal one since crystal structure of the MPB composition is similar to both the rhombohedral and tetragonal lattices. Dielectric response of the sintered PNN-PZN-PT ceramics also exhibits successive phase-transition character. 0.64PNN-0.1PZN-0.26PT exhibits broad, diffused and frequency dependent dielectric peaks indicating a character of diffused FE-paraelectric (PE) phase transition of relaxor ferroelectrics and 0.40PNN-0.1PZN-0.50PT exhibits narrow, sharp and frequency independent dielectric peaks indicating a character of first-order FE-PE phase transition of normal ferroelectrics. The FE-PE phase transition of 0.56PNN-0.1PZN-0.34PT is nearly first-order with some diffused character, which also exhibits the largest value of piezoelectric constant d₃₃ of 462pC/N.