Dielectric,ferroelectric and induced strain behavior of PLZT 9/65/35 ceramics modified by Bi2O3 and CuO co-doping

PLZT 9/65/35 (Pb_0.91La_0.09(Zr_0.65Ti_0.35)_0.9775O₃) ceramics with addition of 0.25, 0.5 and 1.0 wt% of Bi₂O₃/CuO (where the ratio of Bi₂O₃:CuO=9:1 by mole) were prepared by sintering at the temperatures between 1000 and 1200 °C. It was found that Bi₂O₃/CuO could bring the sintering temperature down ~50 °C to obtain PLZT with no second phase. Dielectric and ferroelectric properties were investigated. Bi₂O₃/CuO decreased both coercive field and remnant polarization, which was caused by an increase of the degree of diffuseness in relaxor ferroelectric materials. Electric field induced strain behavior was also investigated and it was found that the addition of Bi₂O₃/CuO increased the maximum induced strain and maximized electrostrictive effect. Therefore, Bi₂O₃/CuO was useful as a sintering aid, which improved the dielectric and the relaxor ferroelectric properties as well as the electric field induced strain of PLZT ceramics.     

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.     

Preparation and characterization of polymer-derived Zr/Si/C multiphase ceramics and microspheres with electromagnetic wave absorbing capabilities

Precursors for Zr/Si/C multiphase ceramics were synthesized by the reactions of dilithiozirconocene complex with dichlorodimethylsilane, methyltrichlorosilane and dichloromethylvinylsilane, respectively. The precursor-to-ceramic process of the precursor was investigated by TG-GC–MS and TG-FTIR analyses, confirming a complete transformation from organometallic polymers into ceramics below 800 °C. Annealing experiments of the derived ceramics at temperatures from 1000 °C to 2000 °C indicated the crystallization from ZrSiO₄, ZrO₂ to ZrC. Furthermore, micrometer-sized Zr/Si/C ceramic microspheres were successfully fabricated from the precursor at 1000 °C, showing surface morphology like wrinkled pea. According to the XRD, HRTEM and XPS analyses, such multiphase ceramic microspheres consist of ZrSiO₄, ZrO₂, and amorphous SiO_xC_y. Interestingly, the ceramic microspheres performed satisfactory electromagnetic wave absorbing capacity with the RL_max reaching −34 dB, which could be potential candidates for electromagnetic micro-devices.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

In situ preparation of CoFe2O4–Pb(ZrTi)O3 multiferroic composites by gel-combustion technique

Diphase magnetoelectric composites of CoFe₂O₄–Pb(ZrTi)O₃ were prepared by citrate–nitrate combustion technique by using Pb(Zr,Ti)O₃ template powders obtained by the mixed oxide method. Pure diphase powder composites with a good crystallinity were obtained after calcination. The composition and purity were maintained after sintering at temperature of 1100 °C/2 h, which ensured limited reactions at interfaces, while by sintering at 1250 °C/2 h, some small amounts of secondary phases identified as nonstoichiometric ZrO_2?x resulted. The method allowed to produce diphase ceramics with homogeneous microstructures and a very good mixing of the two phases. The dielectric and magnetic investigation at room temperature confirmed the formation of composite ceramics with both dielectric and magnetic properties at room temperature, with permittivity and magnetization resulted as sum properties from the parent Pb(Zr,Ti)O₃ and ferrite phases.     

Controlling microstructure and film growth of relaxor-ferroelectric thin films for high break-down strength and energy-storage performance

The relaxor ferroelectric Pb_0.9La_0.1(Zr_0.52Ti_0.48)O₃ (PLZT) thin films were deposited using pulsed laser deposition, and their microstructures, break-down field strengths and energy storage performances were investigated as a function of the buffer layer and electrode. A large recoverable energy-storage density (U_reco) of 23.2 J/cm³ and high energy-storage efficiency (η) of 91.6% obtained in the epitaxial PLZT film grown on SrRuO₃/SrTiO₃/Si are much higher than those in the textured PLZT film (U_reco = 21.9 J/cm³, η = 87.8%) on SrRuO₃/Ca₂Nb₃O₁₀-nanosheet/Si and the polycrystalline PLZT film (U_reco = 17.6 J/cm³, η = 82.6%) on Pt/Ti/SiO₂/Si, under the same condition of 1500 kV/cm and 1 kHz, due to the slim polarization loop and significant antiferroelectric-like behavior. Owing to the high break-down strength (BDS) of 2500 kV/cm, a giant U_reco value of 40.2 J/cm³ was obtained for the epitaxial PLZT film, in which U_reco values of 28.4 J/cm³ (at BDS of 2000 kV/cm) and 20.2 J/cm³ (at BDS of 1700 kV/cm), respectively, were obtained in the textured and polycrystalline PLZT films. The excellent fatigue-free properties and high thermal stability were also observed in these films.     

Low-temperature-sintered Bi0.5Na0.5TiO3-based lead-free ferroelectric ceramics with good piezoelectric properties

Li₂CO₃ has been used as a sintering aid for fabricating lead-free ferroelectric ceramic 0.93(Bi_0.5Na_0.5TiO₃)-0.07BaTiO₃. A small amount (0.5 wt%) of it can effectively lower the sintering temperature of the ceramic from 1200 °C to 980 °C. Unlike other low temperature-sintered ferroelectric ceramics, the ceramic retains its good dielectric and piezoelectric properties, giving a high dielectric constant (1570), low dielectric loss (4.8%) and large piezoelectric coefficient (180 pC/N). The “depolarization” temperature is also increased to 100 °C and the thermal stability of piezoelectricity is improved. Our results reveal that oxygen vacancies generated from the diffusion of the sintering aid into the lattices are crucial for realizing the low temperature sintering. Owing to the low sintering temperature and good dielectric and piezoelectric properties, the ceramics, especially of multilayered structure, should have great potential for practical applications.     

Effect of Li and Bi co-doping and sintering temperature on dielectric properties of PLZT 9/65/35 ceramics

In the present study, lead-lanthanum-zirconate-titanate (PLZT) ceramics were prepared by a solid-state mixed oxide method. Different amount of lithium carbonate and bismuth oxide (0.15 mol%, 0.45 mol% and 0.75 mol%), where the ratio of Li:Bi =1:1 by mole, was added to PLZT to investigate the effect of Li and Bi co-doping. The ceramic samples were sintered at the temperatures of 1000, 1050, 1100, 1150 and 1200 °C for 4 h. After that, all samples were subjected to phase identification, physical property determination (sintered density and microstructure) and dielectric property measurement. It was found that doping of 0.15 mol% Li and Bi resulted in maximum dielectric constant (ε_r =7819) when sintered at 1200 °C. Grain size of PLZT ceramics was dependent on sintering temperature and dielectric properties were affected by the chemical composition rather than the grain size of the ceramics. Therefore, co-doping of Li and Bi was useful as it could improve the dielectric properties of PLZT ceramics.     

Dielectric relaxation properties of SrTiO3 ceramics modulated by stoichiometry

SrTiO₃ has received increasing attention duo to the fact that small perturbation can produce outstanding physical properties. Nonstoichiometric SrTiO₃ ceramics have been prepared in the range of Sr/Ti = 1.064–0.943. Their dielectric behaviors are investigated in the temperature range of 30–500 °C and in the frequency range 100 Hz to 2 MHz. It is observed that the two types of dielectric relaxation processes are caused by polar nanoregions associated with Sr non-stoichiometry and variable range hopping behavior associated with the dipolar effects for Sr/Ti>1 and Sr/Ti<1 ceramics samples, respectively. Our findings imply that the cation vacancy may be responsible for the adjusted properties in nonstoichiometric SrTiO₃.     

Field induced metastable ferroelectric phase in Pb0.97La0.03(Zr0.90Ti0.10)0.9925O3 ceramics

Pb_0.97La_0.03(Zr_0.9Ti_0.1)_0.9925O₃ (PLZT 3/90/10) ceramics prepared by solid-state reaction with the compositions near the antiferroelectric/ferroelectric (FE/AFE) phase boundary were studied. From the polarization–electric field P(E) dependence and ex situ X-ray study, an irreversible electric field induced AFE-to-FE phase transition is verified at room temperature. Dielectric and in situ temperature dependent X-ray analysis evidence that the phase transition sequence in PLZT 3/90/10-based ceramics can be readily altered by poling. A first order antiferroelectric-paraelectric (AFE-to-PE) transition occurred at?～190 °C in virgin sample and at?～180 °C in poled sample. In addition, a FE-to-AFE transition occurs in the poled ceramic at much lower temperatures (～120 °C) with respect to the Curie range (～190 °C). The temperature-induced FE-to-AFE transition is diffuse and takes place in a broad temperature range of 72–135 °C. The recovery of AFE is accompanied by an enhancement in the piezoelectric properties.     

Microstructural design of BaTiO3-based ceramics for temperature-stable multilayer ceramic capacitors

In order to satisfy EIA X8R specification, a new type of BaTiO₃-based ceramic with hierarchical structure in a formula scheme “a ferroelectric ABO₃ + another ferroelectric ABO₃”, was designed. There were (Ba, Bi)TiO₃ and Ba(Ti, Zr)O₃ phases with different Curie temperatures coexisting in the grains from inside to outside, prepared by wet chemical method under 100 °C. The hierarchical structure of the ceramic grains was proved by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The dielectric constant of (Ba, Bi)TiO₃–Ba(Ti, Zr)O₃ ceramic was ～6000, the ΔC/C_20 °C was ?12.0%, 14.1%, and ?8.3% at ?55 °C, 130 °C, and 160 °C, respectively, and the dielectric loss is less than 0.1, which is obviously superior to (Ba, Bi)TiO₃ and Ba(Ti, Zr)O₃. The results of this work showed that the formula scheme “a ferroelectric ABO₃ + another ferroelectric ABO₃” for solid solutions is a promising approach to prepare high performance temperature-stable capacitor materials.     

Investigation of optical,mechanical, and thermal properties of ZrO2-doped Y2O3 transparent ceramics fabricated by HIP

Highly transparent ZrO₂-doped Y₂O₃ ceramics were successfully synthesized using the hot isostatic pressing (HIP) process. The effects of the ZrO₂ content on the sintering behaviors, optical transmission spectra, Vickers hardness, grain size, size distribution, and Raman spectra were determined. The results indicated that decreased ZrO₂ content could promote increased transmittance, red-shifted infrared cutoff wavelength, increased thermal conductivity, decreased Vickers hardness, and increased lattice ordering. According to the optical transmission spectra, the optimized ZrO₂ content was 0.50 at%, at which point the ceramic exhibited a larger pre-sintering temperature range of 1650–1750 °C and the average grain size of 3.35 µm at 1750 °C. The grain size was significantly decreased at lower pre-sintering temperatures. Furthermore, a moderate Vickers hardness of 8.42 GPa and high thermal conductivity of 10.85 W/m K at room temperature were obtained for the optimized ceramic.     

Synthesis and electrical properties of lead-free piezoelectric Bi0.5Na0.5TiO3 thin films prepared by Sol-Gel method

Lead-free Bi_0.5Na_0.5TiO₃ (BNT) piezoelectric thin films were deposited on Pt/TiO_x/SiO₂/Si substrates by Sol-Gel method. A dense and well crystallized thin film with a perovskite phase was obtained by annealing the film at 700 °C in a rapid thermal processing system. The relative dielectric constant and loss tangent at 12 kHz, of BNT thin film with 350 nm thickness, were 425 and 0.07, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 9 μC/cm² and a coercive field of 90 kV/cm. Piezoelectric measurements at the macroscopic level were also performed: a piezoelectric coefficient (d_33effmax) of 47 pm/V at E = 190 kV/cm was obtained. The piezoresponse force microscopy data confirmed that BNT thin films present ferroelectric and piezoelectric behavior at the nanoscale level.     

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.     

Tetragonal Er3+-doped (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)O3: Lead-free ferroelectric transparent ceramics with electrical and optical multifunctional performances

The lead-free Er³⁺-doped (K_0.48Na_0.48Li_0.04)(Nb_0.96Bi_0.04)O₃ (KNLNB-Er-x) ceramics were fabricated by conventional pressureless sintering. They possess a tetragonal perovskite phase with dense microstructure. High transmittances of the ceramics are obtained both in the visible and infrared regions. The optical band gap energies of them are 3.07–3.11 eV, close to other KNN-based materials. The relaxor-like characteristics, good dielectric, ferroelectric and piezoelectric properties of the ceramics have been obtained. The up-conversion photoluminescence spectra have been studied and obvious color-tunable emissions have been observed by modulating temperature. The fluorescence intensity ratio (FIR) value based on green emissions at 533 and 555 nm in the temperature ranging from 300 to 750 K have been investigated, giving the maximum sensitivity of ~ 0.0038 K^?1. Furthermore, the FIR technique opens up a method to detect the Curie temperature of the ceramics. Owing to both optical and electrical properties, the KNLNB-Er-x transparent ceramics could be a promising candidate in the application of color-tunable solid-state lightings, optical temperature sensors, and electrical-optical coupling devices.     

Enhanced energy-storage properties of (1-x)(0.7Bi0.5Na0.5TiO3-0.3Bi0.2Sr0.7TiO3)-xNaNbO3 lead-free ceramics

A series of (1-x)(0.7Bi_0.5Na_0.5TiO₃-0.3Bi_0.2Sr_0.7TiO₃)-xNaNbO₃ (BNT-BST-100xNN) lead-free ceramics were fabricated using conventional solid-state reaction technique. The phase behavior, microstructure, dielectric, ac impedance and energy-storage properties of the sintered ceramics were systematically investigated. XRD patterns and surface SEM micrographs revealed the introduction of NaNbO₃ didn't change the perovskite structure of BNT-BST at low doping level. The NaNbO₃ doping gave rise to slimmer P-E loops and thus gained enhanced energy storage properties. Therefore, a maximum energy storage density of 1.03 J/cm³ was achieved at 85 kV/cm at x = 0.01 via increasing the dielectric breakdown strength (DBS). Temperature-dependent dielectric permittivity illustrated the enhanced relaxor characteristics, implying the long-rang ferroelectric order was further damaged due to the introduction of NaNbO₃. The results above indicate the sintered ternary ceramics can be a promising lead-free candidate for energy storage capacitors.     

Incorporating Zr to achieve self-protecting and enhancement of silica sol bonded SiC castables at active oxidation condition

SiC castables exhibit degraded properties in static air at 1700 °C, due to the formation of gaseous products. The efficiency of different contents of Zr in SiC castables was evaluated by considering sintered properties, mechanical performance, isothermal oxidation behavior, and microstructural analysis of the SiC castables. Specimens with more Zr exhibited enhanced mechanical behavior and anti-oxidation capability. The addition of Zr decreased the evaporation of SiO₂ by reducing its equilibrium partial pressure (g), and formed a dense ZrO₂-SiO₂ protective layer (e.g., the sample with 0.9 wt% Zr) to prevent further degradation of the SiC castable. The Zr that was preferentially oxidized to ZrO₂ reduced the partial pressure of the oxidizing gases (O₂ and CO₂) in the matrix, and increased SiO (g) content, which facilitates formation of SiC fibers, which, in turn, improves the anti-oxidation capability and mechanical behavior of SiC castables, preventing their degradation in static air at 1700 °C. The addition of Zr created a ZrO₂-SiO₂ protective layer on the surface and prevented the decrease in SiC content, by forming SiC fibers. This made the silica sol bonded SiC castable a self-protecting refractory.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

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.     

Study of aging behavior of 9/70/30 and 9/65/35 PLZT by optical interferometric technique

In this work Pb_0.91La_0.09(Zr_0.70Ti_0.30)_0.9775O₃ PLZT (9/70/30) and Pb_0.91La_0.09(Zr_0.65Ti_0.35)_0.9775O₃ PLZT (9/65/35) ceramics were prepared at four different sintering temperatures by solid solution method. The samples were aged by keeping at room temperature for 19 days. After the aging process, structural and electrical properties of the samples were studied using Modified Michelson Interferometer, LCR meter and X-ray diffraction. The effect of the Zr/Ti ratios on the dielectric and ferroelectric properties was also investigated. The aged PLZT (9/70/30) showed pinched P–E loop and remnant strain in rhombohedral phase, while PLZT (9/65/35) showed distort s–E shape implying non-180° domain in tetragonal phase.