Linear thermal expansion coefficients of relaxor-ferroelectric 0.57Pb(Sc1/2Nb1/2)O3–0.43PbTiO3 ceramics in a wide temperature range

The objective of this work was to examine linear thermal expansion of virgin and poled 0.57Pb(Sc_1/2Nb_1/2)O₃–0.43PbTiO₃ ceramics between 30 °C and 600 °C by contact dilatometry. The thermal expansion dL/Lo of the virgin ceramic increases with increasing temperature until approximately 260 °C. The physical and technical thermal expansion coefficients were determined. At 260 °C the physical thermal coefficient is 2.08 × 10^?6 K^?1. Between 260.0 °C and 280.0 °C an anomaly in the thermal expansion vs. temperature and an endothermic peak in the differential scanning calorimetry curves correspond to the phase transition region from tetragonal to cubic phase. At temperatures from 280 °C to 600 °C the thermal expansion dL/Lo increases again.In the derivative of the dL/Lo heating curves of the poled ceramics, additionally to the anomaly at 270 °C, also the anomaly at 160 °C is observed, which is associated with the depolarization of the material during heating.     

Field-induced antiferroelectric to ferroelectric transitions in (Pb1–xLax)(Zr0.90Ti0.10)1–x/4O3 investigated by in situ X-ray diffraction

Phase transitions and field-induced preferred orientation in (Pb_1-xLa_x)(Zr_0.90Ti_0.10)_1–x/4O₃ (PLZT x/90/10) ceramics upon electric field cycling using in situ X-ray diffraction were studied. The evolution of the {200}_pc and {111}_pc diffraction line profiles indicate that PLZT 4/90/10 and PLZT 3/90/10 compositions undergo an antiferroelectric (AFE)–ferroelectric (FE) phase switching. Both PLZT 4/90/10 and PLZT 3/90/10 exhibit irreversible preferred orientation after experiencing the field-induced AFE-to-FE phase switching. An electric field-induced structure develops in both compositions which has a reversible character during the field decreasing in PLZT 4/90/10 and an irreversible character in PLZT 3/90/10. In addition, structural analysis of pre-poled PLZT 3/90/10 ceramics show that it is possible to induce consecutive FE-to-AFE and AFE-to-FE transitions when fields of reversed polarity are applied in sequence. The field range required to induce the AFE phase is broad, and the phase transition is kinetically slow. This kind of transition has rarely been reported before.     

Influence of processing on stability,microstructure and thermoelectric properties of Ca3Co4 − xO9 + δ

Due to high figure of merit, Ca₃Co_4 ? xO_9 + δ (CCO) has potential as p-type material for high-temperature thermoelectrics. Here, the influence of processing including solid state sintering, spark plasma sintering and post-calcination on stability, microstructure and thermoelectric properties is reported. By a new post-calcination approach, single-phase materials were obtained from precursors to final dense ceramics in one step. The highest zT of 0.11 was recorded at 800 °C for CCO with 98 and 72% relative densities. In situ high-temperature X-ray diffraction in air and oxygen revealed a higher stability of CCO in oxygen (～970 °C) than in air (～930 °C), with formation of Ca₃Co₂O₆ which also showed high stability in oxygen, even at 1125 °C. Since achievement of phase pure high density CCO by post-calcination method in air is challenging, the phase stability of CCO in oxygen is important for understanding and further improvement of the method.     

Effect of composition and grain size on dielectric,ferroelectric and induced strain behavior of PLZT/ZrO2 composites

Lead lanthanum zirconate titanate ceramics (PLZT) are well known for their excellent dielectric, piezoelectric and ferroelectric properties. In this study, PLZT 9/70/30, 9/65/35 and 9/60/40 ceramics were prepared by vibro-milling mixed-oxide method. All compositions of powders were uniaxial pressed in pellets and sintered at the temperatures of 1200–1275 °C with various soaking times of 2, 4 and 6 h. The X-ray diffraction (XRD) patterns confirmed that all the PLZT samples had perovskite structure with ZrO₂ as a second phase and PLZT/ZrO₂ composite structure was formed. Dielectric behavior at the frequency of 1 kHz showed broad peak indicating relaxor ferroelectric behavior and the difference of the temperature at maximum dielectric at different frequencies increased when Zr:Ti ratio increased. Polarization with electric field (P-E loop) at room temperature showed that when Zr:Ti ratio increased, the coercive field decreased resulting from crystal structure change from tetragonal to rhombohedral. Induced strain with electric field depended on microstructure where the value of S_max/E_max tended to decrease with increasing grain size. It can be concluded that dielectric and ferroelectric behavior predominantly depended on composition of PLZT ceramics and induced strain behavior predominantly depended on grain size of PLZT ceramics.     

Large E-field induced strain and polar evolution in lead-free Zr-doped 92.5%(Bi0.5Na0.5)TiO3–7.5%BaTiO3 ceramics

Structure, dielectric permittivity, strain, electric (E) polarization, and piezoelectric responses of (Bi_1/2Na_1/2)_0.925Ba_0.075(Ti_1−xZr_x)O₃ (BNT7.5BT-100xZr; x = 0–0.04) ceramics were investigated as functions of poling E field and temperature. The BNT7.5BT ceramic reveals a phase transition from P4bm nanodomains to long-range-ordered P4mm domains. The Zr-doped BNT7.5BT ceramic reveals a reversible change of unit cell with dynamically fluctuating polar nanoregions, which are responsible for the large strain. The poled BNT7.5BT ceramic displays a depolarization temperature of T_d = 90 °C, which correspond to a phase transition from ferroelectric to relaxor states. The Zr-doped BNT7.5BT ceramics have Burns temperatures (T_B) in the region of 400–435 °C, below which polar nanoregions begin to develop. The Zr-doped BNT7.5BT ceramics display wide diffuse phase transitions, suggesting a transition from R + T to T phases. BNT7.5BT-2Zr ceramic shows a temperature dependent linear large strain of 0.482% at 150 °C and can be a potential candidate for lead-free actuator.     

Ferroelectric phase transition and electrical properties of high-TC PMN-PH-PT ceramics prepared by partial oxalate route

The high-Curie temperature (T_C) 0.15Pb(Mg_1/3Nb_2/3)O₃-0.38PbHfO₃-0.47PbTiO₃ (PMN-PH-PT) piezoelectric ceramics were prepared by the partial oxalate route via the B-site oxide mixing method. The obtained uniform nm-sized PMN-PH-PT precursor powders provide high calcining and sintering activity for synthesizing ceramics, based on which the synthesis conditions were tailored as calcining at 775 °C and sintering at 1245 °C. The partial oxalate route synthesized PMN-PH-PT ceramics are far superior to the counterparts synthesized by the columbite precursor method and exhibit excellent thermal stability of the piezoelectric properties under T_C (～292 °C), ensuring the potential application in transducers under elevated environmental temperatures. The temperature dependent Raman spectroscopy not only proves the occurrence of the ferroelectric to paraelectric phase transition around T_C, but also confirms the successive phase symmetry transitions, which correlate with the polar nanoregions (PNRs) and/or the coexistence of multiple ferroelectric phases, revealing the origin of the enhanced electrical properties in the PMN-PH-PT ceramics.     

Composition and poling-induced modulation on photoluminescence properties for NBT-xBT: Pr3+ ceramics

In this work, the modulation of photoluminescence (PL) properties, which was realized by the composition and poling-induced structural evolution, for the Pr³⁺ doped (1-x)(Na_1/2Bi_1/2)TiO₃-xBaTiO₃ (NBT-xBT: Pr³⁺) piezoelectric ceramics was systematically investigated. Based on the Rietveld refinement structural analysis, there were two distinct composition ranges characterized by different structural features for NBT-xBT: Pr³⁺ ceramics at room temperature: (i) rhombohedral R3c + monoclinic Cc phases for the compositions of x ≤ 0.03, and (ii) tetragonal P4bm + monoclinic Cc phases for 0.04 ≤ x ≤ 0.07. It was interesting to notice that the PL emission intensity is positively correlated with the phase fraction of Cc, which is closely related to the crystal symmetry of NBT-xBT: Pr³⁺ ceramics. The compositions with x ≤ 0.06 underwent an irreversible phase transformation on the application of electric field. The dielectric and Raman measurement revealed a transition from a relaxor state to a normal ferroelectric for the x ≤ 0.06 compositions under an applied poling electric field, with not only the reduction in the in-phase octahedral tilting disorder but also the establishment of long-range ordering. These electric field-induced structural changes were responsible for poling-induced PL quenching behaviors as a result of the increased local structure symmetry around doped Pr³⁺ ions in the poled ceramics.     

Excellent electrostrictive properties of low temperature sintered PZT ceramics with high concentration LiBiO2 sintering aid

The effect of LiBiO₂ (LBO) additive on the sintering of Pb_0.97La_0.03(Zr_0.53Ti_0.47)_0.9925O₃ (PLZT) ceramics was carefully investigated. 6.0 wt% LBO added PLZT powders could be fully densified to 98% relative density at a temperature as low as 950 °C. It is worthy to notice that there are distinct enhancements in piezoelectric and electrostrictive properties by increasing the soaking time from 2 h to 7 h, which could mainly originate from the improvement of crystallinity and grain size of PLZT ceramics. By controlling the soaking time and concentration of LBO addition, PLZT ceramics sintered at 950 °C could exhibit high curie temperature of 240 °C and very high S₁₁ of 0.22% under 3.0 kV/mm, which is even better than that of traditionally sintered PZT-5, PMN–PZT, and this is very promising for actuators designed in multilayer structure in high temperature environment.     

A novel low-temperature firable La2Zr3(MoO4)9 microwave dielectric ceramic

A novel low-temperature fired La₂Zr₃(MoO₄)₉ microwave dielectric ceramic was successfully fabricated by a conventional solid-state reaction method. The powder compact was densified in air in the temperature range of 700–800 °C for 4 h. X-ray diffraction analysis indicated that all studied samples presented a single phase structure. Rietveld refinement results further confirmed that La₂Zr₃(MoO₄)₉ belonged to a trigonal system with space group R$\overline{3}$c. Scanning electron microscopy results revealed dense and homogeneous microstructure of La₂Zr₃(MoO₄)₉ ceramics as sintered in the temperature range of 725–800 °C. The La₂Zr₃(MoO₄)₉ ceramic sintered at 775 °C for 4 h possessed excellent microwave dielectric properties of relative permittivity ε_r ～ 10.8, quality factor Qxf ～ 50,628 GHz (at 10.45 GHz), and temperature coefficient of the resonant frequency τ_f ～ ?38.8 ppm/°C, showing great potentials for applications of low temperature co-fired ceramic technology.     

Giant electrocaloric effect of free-standing Pb0.85La0.1(Zr0.65Ti0.35)O3 thick films fabricated by the self-lift-off screen printing method

Free-standing Pb_0.85La_0.1(Zr_0.65Ti_0.35)O₃ (PLZT) ceramic thick films have been prepared via a facile and low-cost self-separating screen printing method for electrocaloric cooling, and the relation among the fabrication processes, phase composition, microstructure, dielectric characteristics, ferroelectric properties and electrocaloric effect (ECE) has been systematically investigated. Compared to the conventional ceramic thick films supported by substrates, the free-standing feature enables the EC cooling of the free-standing PLZT thick films to be fully used for cooling down different thermal loads rather than be futilely absorbed by the substrates. Furthermore, without the mechanical restriction of the substrates, the free-standing PLZT thick films can freely shrink during the high-temperature densification process, leading to their high density and favorable microstructures. Additionally, by introducing an adequate amount of excess PbO, the pyrochlore phase can be removed from the samples to yield high-purity perovskite PLZTs. With the comprehensive improvement in phase composition, microstructure and the elimination of mechanical strain between the active materials and substrates, the free-standing PLZT thick films exhibited an optimized ECE including changes of temperature and entropy of 1.95 °C and 2.09 J kg^?1 K^?1, which are almost 3 times that of the samples deposited on the Al₂O₃ substrates without excess PbO. This work would contribute to the development of ferroelectric ceramics, especially thick films, for practical EC cooling.     

Phase composition,crystal structure and microwave dielectric properties of Mg2−xCuxSiO4 ceramics

In this work, the Mg_2-xCu_xSiO₄(x = 0–0.40) microwave dielectric ceramics were prepared using solid-state reaction method. Compared with the Mg₂SiO₄ sample, the Cu-substituted Mg samples could be sintered at a lower temperature. The Mg_2?xCu_xSiO₄ ceramics exhibit the composite phases of Mg₂SiO₄ and a small quantity of MgSiO₃. The Cu²⁺ ion presented a solid solution with the Mg₂SiO₄ phase and preferentially occupy Mg(1) site. The distortion of MgO₆ octahedron was modified by Cu²⁺ ions, resulting in a positive change in the temperature coefficient of resonance frequency (τ_f) values. Excellent microwave dielectric properties of ε_r = 6.35, high Qf of ～ 188,500 GHz and near zero τ_f = ?2.0 ppm/°C were achieved at x = 0.08 under sintering at 1250 °C for 4 h. Thus, the fabricated ceramics were considered as possible candidates for millimeter-wave device applications.     

Structural,dielectric, and piezoelectric properties of fine-grained NBT–BT0.11 ceramic derived from gel precursor

(Na_1/2Bi_1/2)TiO₃ doped in situ with 11 mol% BaTiO₃ (NBT–BT_0.11) powders were synthesized by a sol–gel method, and the electrical properties of the resulting ceramics were investigated. The powders consisting of uniform and fine preliminary particles of about 50 nm were prepared by calcining the gel precursor at 700 °C. (Na_1/2Bi_1/2)_0.89Ba_0.11TiO₃ ceramics, sintered at temperatures up to 1150 °C have a rhombohedral symmetry, while the ceramic sintered at 1200 °C exhibits a tetragonal crystalline structure. The ceramics show high dielectric constant (?_r ～ 5456), dielectric loss of 0.02, depolarization temperature T_d ～ 110 °C and temperature corresponding to the maximum value of dielectric constant T_m ～ 262 °C. The dielectric constant (?₃₃) and the piezoelectric constant (d₃₃) attain the maximum values of 924 and 13 pC/N, respectively, while the electromechanical coupling factor (k_p) value is 0.035. The NBT–BT_0.11 ceramics derived from sol–gel present high mechanical quality factor (Q_m ～ 860). The dielectric and piezoelectric properties values of NBT–BT_0.11 ceramics derived from sol–gel are smaller than those of samples produced by the conventional solid state reaction method, due to the grains size and oxygen vacancies that generate dipolar defects.     

0-3 type magnetoelectric 0.94Na0.5Bi0.5TiO3-0.06BaTiO3:CoFe2O4 composite ceramics with a deferred thermal depolarization

Developing Na_0.5Bi_0.5TiO₃-based magnetoelectric (ME) coupling composites with higher depolarization temperature is highly valuable for the environment-friendly smart electronic devices. We have developed a new kind of 0-3 type 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃:xCoFe₂O₄ (NBTBT:xCFO, x = 0, 0.1, 0.2, 0.3) composite ceramics with a deferred depolarization temperature, together with an additional strong ME coupling of 9.2 mV/cm·Oe for the NBTBT:0.2CFO. The basic structure, ferroelectric/ferromagnetic properties, and the depolarization temperature of the NBTBT:xCFO composite ceramics were investigated. It was found that an enhancement of depolarization temperature (>25 °C) was obtained in these 0-3 type composites relative to the pure NBTBT ones (115 °C vs 90 °C). The mechanism of the enhanced depolarization temperature of the composites is discussed. The present results demonstrate that NBTBT:xCFO composites have great potential for ME devices.     

Sintering behaviour and microwave dielectric properties of BaAl2−2x(ZnSi)xSi2O8 ceramics

BaAl_2?2x(ZnSi)_xSi₂O₈ (x = 0.2–1.0) ceramics were prepared using the conventional solid-state reaction method. The sintering behaviour, phase composition and microwave dielectric properties of the prepared compositions were then investigated. All compositions showed a single phase except for x = 0.8. By substituting (Zn_0.5Si_0.5)³⁺ for Al³⁺ ions, the optimal sintering temperatures of the compositions decreased from 1475 °C (x = 0) to 1000 °C (x = 0.8), which then slightly increased to 1100 °C (x = 1.0). Moreover, the phase stability of BaAl₂Si₂O₈ was improved. A novel BaZnSi₃O₈ microwave dielectric ceramic was obtained at the sintering temperature of 1100 °C. This ceramic possesses good microwave dielectric properties with ε_r = 6.60, Q × f = 52401 GHz (at 15.4 GHz) and τ_f = ?24.5 ppm/°C.     

Structural,thermal, dielectric and ferroelectric properties of K0.5Bi0.5TiO3 ceramics

Dense K_0.5Bi_0.5TiO₃ (KBT) lead-free ceramics were prepared by conventional solid reaction route. Their temperature behavior (up to 600 °C) was investigated by X-ray diffraction, DSC, dielectric spectroscopy and electric field-polarization technique. The first temperature dependent Raman scattering studies were also performed. X-ray and Raman scattering results show that samples exhibit a single perovskite structure with cubic symmetry at temperatures higher than approximately 400 °C and with coexistence of the cubic and tetragonal phases below this temperature. Two structural phase transitions between tetragonal phases in temperature range 200–225 °C and between tetragonal and cubic ones near 400 °C are observed. The content of the tetragonal phase increases with decreasing temperature and at room temperature it reaches more than 70%. Temperature- dependent P-E loops and pyroelectric data revealed a polar behavior in KBT up to about 400 °C, which means that the intermediate phase (～270–380 °C) is rather ferroelectric than antiferroelectric.     

Low-temperature sintering of Ti1−xCux/3Nb2x/3O2 (x = 0.23) microwave dielectric ceramics with CuO and B2O3 addition

The influence of CuO and B₂O₃ addition on the sintering behavior, microstructure and microwave dielectric properties of Ti_1?xCu_x/3Nb_2x/3O₂ (TCN, x = 0.23) ceramic have been investigated. It was found that the addition of CuO and B₂O₃ successfully reduced the sintering temperature of TCN ceramics from 950 to 875 °C. X-ray diffraction studies showed that addition of CuO-B₂O₃ has no effect on the phase composition. The TCN ceramics with 0.5 wt% CuO-B₂O₃ addition showed a high dielectric constant of 95.63, τ_f value of + 329 ppm/°C and a good Q × f value of 8700 GHz after sintered at 875 °C for 5 h, cofirable with silver electrode.     

Structural evolution,grain growth kinetics and microwave dielectric properties of Li2Ti1-x(Mg1/3Nb2/3)xO3

Li₂Ti_1-x(Mg_1/3Nb_2/3)_xO₃ ceramics were prepared by conventional solid state process. Their structural evolution, grain growth kinetics and microwave dielectric properties have been studied in this paper. The results show that continuous solid solution could be formed within the experiment compositional range. The structure changed from long range ordered monoclinic into short range ordered cubic phase as the increase in x. Small levels of substitution for Ti⁴⁺by (Mg_1/3Nb_2/3)⁴⁺ slightly decreased the dielectric permittivity, while considerably improved the Q × f value. The temperature coefficient of resonant frequency changed from positive into negative value. The grain growth kinetics during sintering process and Q × f value of the sintered body were affected by different calcining temperature of mixed powders. Excellent combined microwave dielectric properties with ε_r ～21.0, Q × f ～ 200 000 GHz and τ_f value of ?1 ppm/ °C could be obtained after optimizing calcining temperature for the x = 0.24 composition after sintering at 1250 °C/2 h.     

Effect of Ba0.85Ca0.15Ti0.90Zr0.10O3 content on the microstructure and electrical properties of Bi0.51(Na0.82K0.18) 0.50TiO3 ceramics

(1?x)Bi_0.51(Na_0.82K_0.18)_0.50TiO₃–xBa_0.85Ca_0.15Ti_0.90Zr_0.10O₃ [(1?x)BNKT–xBCTZ] ceramics were prepared by the conventional solid-state method, and the effect of BCTZ content on their microstructure and electrical properties was investigated. A stable solid solution with a pure perovskite phase is formed between BNKT and BCTZ, and these ceramics have a coexistence of rhombohedral and tetragonal phases in the range of 0 ≤ x < 0.15. Their T_m and T_d values are strongly independent on the BCTZ content. Moreover, the sintering temperature strongly affects the ferroelectric and piezoelectric properties of these ceramics with x = 0.02. These ceramics with x = 0.02 exhibit an optimum electrical behavior of d₃₃ ～ 205, k_p ～ 0.25, P_r ～ 31.8 μC/cm², and E_c ～ 19.1 kV/cm together with a high T_d value of ～91 °C when sintered at 1180 °C and poled at an optimum condition. As a result, the (1?x)BNKT–xBCTZ ceramic is a promising candidate material for lead-free piezoelectric ceramics.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

The design,synthesis and nonlinear optical properties of a novel,Y-type polyurethane containing tricyanovinylthiophene of high thermal stability

A novel, Y-type polyurethane containing 1-(2,4-dioxyethoxy)phenyl-2-{5-(2,2-dicyanovinyl)-2-thiophenyl}ethenes as nonlinear optical chromophores present within the polymer backbone, was prepared and characterized. The compound was soluble in common organic solvents and displayed thermal stability up to 260 °C and a glass transition temperature (T_g) of 163 °C. The second harmonic generation coefficient (d₃₃) of poled polymer films at the compound's fundamental wavelength of 1560 nm was ～3.72 × 10^?9 esu. Dipole alignment exhibited high thermal stability up to the T_g, and there was no decay in d₃₃ below 148 °C owing to the partial main-chain character of the polymer structure.