Structural,dielectric and ferroelectric properties of lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics prepared by Plasma Activated Sintering

Lead-free Ba_0.85Ca_0.15Zr_0.10Ti_0.90O₃ (BCZT) ceramics were prepared by Plasma Activated Sintering (PAS). The influence of PAS sintering temperature on the crystalline phase, microstructure, and, dielectric and ferroelectric properties of BCZT ceramics were studied. The phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral and tetragonal phases and then to tetragonal phase as the sintering temperature increased. Microstructural characterization of BCZT ceramics indicated that PAS can obtain a compact microstructure at lower temperatures of 1150–1300 °C compared with that from common pressureless sintering. The BCZT ceramics showed different degrees of diffuseness with increased temperature, and the diffuseness exponents C are all approximately on the order of 10⁵ °C. The dielectric and ferroelectric properties of BCZT ceramics were enhanced with increased sintering temperature. BCZT ceramics sintered at 1250 °C exhibited optimum properties of room-temperature ε_r=2863, ε_m=6650, and 2P_r=25.24 μC/cm², resulting from the relatively higher tetragonal phase content of the MPB between tetragonal and rhombohedral phases together with a compact microstructure.     

Microstructure development and optical properties of Fe:ZnSe transparent ceramics sintered by spark plasma sintering

Fe:ZnSe transparent ceramics were prepared by spark plasma sintering. Fe:ZnSe powders synthesized via co-precipitation yielded well-dispersed particles with an average particle size of 550 nm. These powders were in the cubic phase Fe:ZnSe, indicating the successful substitution of Fe²⁺ for Zn²⁺. The highest relative density, 99.4%, was obtained by increasing the pressure and sintering time. The effects of sintering temperature, pressure, and time on the microstructure of SPS prepared ceramics were presented by micrographs. With increasing sintering temperature, from 600°C to 900°C, the average grain size increased from < 1 to 10 μm. The intergranular fracture indicated no neck formation in the sintering process. High pressure was essential for the densification process. The average grain size deceased from approximately 10 to 5 μm when the pressure was increased. Increasing the sintering time from 10 to 120 minutes lead to a change in the microstructure, from inter- to transgranular fracture, and eliminated the micropores. The as-prepared Fe:ZnSe ceramics were composed of single-phased cubic ZnSe. The sample sintered at 900°C under a pressure of 90 MPa for 120 minutes yielded a transmittance of approximately 60% at 1.4 μm and 68% at 7.5 μm and had residual micropores as its main scattering source. There was a strong characteristic absorption peak of Fe²⁺ ions at around 3 μm, which was red-shifted compared to Fe:ZnS transparent ceramics. Fe:ZnSe transparent ceramics have a reddish-brown color and it could be a promising mid-infrared laser material.     

Optimizing the piezoelectric properties of Ba0·85Ca0·15Zr0·10Ti0·90O3·lead-free ceramics via two-step sintering

The two-step sintering of lead-free Ba₀·₈₅Ca₀·₁₅Zr₀·₁Ti_0·9O₃·(BCZT) ceramics was investigated as a way to enhance its piezoelectric properties. The variations in grain size as a function of the calcination and sintering conditions and its effect on performance is discussed. Results indicate that as the calcination and first-step sintering temperatures increased, grain size became large and was independent of the second sintering step. Large grains were responsible for the enhanced piezoelectric properties by causing lattice distortion, larger domains, and easy motion of domain walls. The BCZT ceramic calcined at 1200 °C and sintered at 1540 °C without holding and then cooled to 1400 °C and held at 1400 °C for 4 h exhibited optimal performance with the highest remnant polarization P_r ∼13.5 μC/cm², the largest piezoelectric constant d₃₃ ∼ 529 pC/N at room temperature, and the highest Curie temperature T_c ∼125 °C. Two-step sintering has been turned out to be an effective method to realize high-performance BCZT ceramics by microstructure optimization.     

Submicron crystalline buildup and size‐dependent energy harvesting characteristic in PZN–PZT ternary ferroelectrics

Piezoelectric energy harvester converts low‐frequency vibrational energy in the environment into electrical energy, enabling the purpose of self‐supplying power for low‐energy consumption devices. The key to miniaturizing energy harvester is the buildup of the submicron‐grained ceramic with a high transduction coefficient (d×g), which is still a big challenge from a technical point of view. In this work, the popular ternary system of Pb(Zn_1/3Nb_2/3)O₃–Pb(Zr_0.5Ti_0.5)O₃ (PZN–PZT) has been selected as objective compound, and the submicron‐grained ceramics were prepared by a combination of high‐energy ball milling and pressureless sintering technology. The results revealed that nanocrystalline PZN–PZT powders can be synthesized by one step mechanochemical route without the calcination stage. Using these nanopowders as precursors, dense ceramics with different grain size have been prepared through tailoring the sintering temperature. The study of size‐dependent energy harvesting characteristic evidenced an optimum transduction coefficient of 7980×10^?15 m²/N was obtained for 950°C sintered specimen, which has uniform microstructure with mean grain size of 0.33 μm. In the mode of the cantilever‐type energy harvester constructed by this material, the output power at low frequency of 89 Hz was as high as 69 μW at an acceleration of 10 m/s², showing the suitability for piezoelectric generators harvesting environmental vibrational energy.     

Influence of β-Si3N4 particle size and heat treatment on microstructural evolution of α:β-SiAlON ceramics

In the present study, we report the effects of starting β-Si₃N₄ particle sizes and post-sintering heat treatment on microstructure evolution and mechanical properties of prepared α-β SiAlON ceramics. Three different β-Si₃N₄ starting powders, with particle sizes of 2, 1 and 0.5 μm were used to prepare α-β SiAlON ceramics by gas-pressure sintering. Elongated β-SiAlON grain morphology was identified in the samples prepared using 0.5 μm particle size β-Si₃N₄ powder. Low-aspect ratio grain morphology was observed in samples prepared from starting powders with coarse particles (2 μm and 1 μm). The sintered samples were further heat treated to develop desired microstructure with elongated grains. The hardness and indentation fracture toughness values of sintered and heat treated samples were found to lie in the range of 12.4-14.2 GPa and 5.1-6.4 MPa m^1/2 respectively. It was revealed that fracture toughness increases with decrease in particle size of starting β-Si₃N₄ powder.     

Processing and microstructure of γ-LiAlO2 ceramics

γ-LiAlO₂ ceramics with different grain sizes were prepared by controlling the sintering process and regulating the size and shape of the precursor powders. It was found that a size gradation of powders promoted the growth of γ-LiAlO₂ grains. Ceramics with an average grain size of 10 μm were prepared from the size-graded powders. It was demonstrated that the shape of the precursor powders greatly affected the grain growth of the ceramics whereas the granulation of the powders restrained the abnormal grain growth. Furthermore nano-sized precursor powders obtained by a sol–gel route made it possible to prepare nano-structured γ-LiAlO₂ ceramics.     

Processing of CaLa2S4 infrared transparent ceramics: A comparative study of HP and FAST/SPS techniques

The densification of CaLa₂S₄ (CLS) powders prepared by combustion method was investigated by the use of Field-Assisted Sintering Technique (FAST) and Hot Pressing (HP). CLS powders were sintered using FAST at 1000°C at different pressures and heating rates and sintered by HP under 120 MPa from 800°C to 1100°C for 6 hours with a heating rate of 10°C/min. Comparison of both techniques was further realized by use of the same conditions of pressure, dwell time, and heating rate. Complementary techniques (XRD, SEM-EDS, density measurements, FTIR spectroscopy) were employed to correlate the sintering processes/parameters to the microstructural/compositional developments and optical transmission of the ceramics. Both sintering techniques produce ceramics with submicrometer grain size and relative density of about 99%. Nevertheless, HP is more suitable to densify CLS ceramics without fragmentation and also reach higher transmission than FAST. Transmission of 40%–45% was measured out of a possible maximum of 69% based on the Fresnel losses in the 8-14 μm window when HP is applied at 1000°C for 6 hours under 120 MPa. In both techniques, ceramics undergo reduction issues that originate from graphitic sintering atmosphere.     

A novel approach of synthesizing nano Y2O3 powders for the fabrication of submicron IR transparent ceramics

A facile methodology of synthesizing highly reactive, round-edged, Sulfur–free nano Y₂O₃ powders to fabricate submicron IR transparent yttria ceramics having a unique combination of superior optical and mechanical properties are reported for the first time. Dispersion of yttrium hydroxide into aqueous sol and addition of seed particles produced near-spherical yttria powders having non – aggregated particles with narrow size distribution. The powder exhibited excellent sinterability reaching near-theoretical density at temperatures around 1400 °C in air. Effective inter-particle coordination and traces of Al additives assisted achieving superior densification. Sintered specimens showed average grain sizes closer to 700 nm. Post-sinter hot isostatic pressing eliminated the residual porosity from the sintered samples leading to exhibit IR transmissions up to 84% in the 2.0–9.0 μm regions, equivalent to single crystal Y₂O₃. Achieving densification through solid-state sintering and retaining the sintered grain sizes in the submicron regions significantly enhanced the mechanical properties. Sintered and HIPed Y₂O₃ specimens were further characterized for their thermal properties at temperature regions between ambient to 950 °C.     

Synthesis of Fe:ZnSe nanopowders via the co-precipitation method for processing transparent ceramics

Fe:ZnSe nanopowders were synthesized via the co-precipitation method for fabricating transparent ceramics. Fe_xZn_1−xSe (0.00 ≤ x ≤ 0.06) powders that were calcined at 400°C yielded a single-phased cubic ZnSe, but when the calcination temperature was raised to 500-600°C, ZnO phase was created. Introduction of pressure could avoid appearance of ZnO. XRD Scherrer analysis revealed a monotonic increase in lattice parameter with increasing Fe²⁺ content. The average powder particle size increased with calcination temperature from several nanometers at 80°C to hundreds of nanometers at 600°C. Attempts to pressurelessly sinter ZnSe powders resulted in the partial decomposition of ZnSe, thus spark plasma sintering was employed to sinter Fe_0.01Zn_0.99Se transparent ceramics with pure ZnSe phase composition, which could be well sintered at 950°C for 30 minutes under an applied pressure of 60 MPa. SEM observations of the polished and thermally etched microstructure of the ceramic revealed a dense microstructure with average grain size of approximately 35 μm, and a few micropores were observed at the grain boundaries. The transparent ceramic exhibited good transmittance in the mid-far infrared range, with the highest transmittance 57% at 12 μm. This paper confirmed the scheme of synthesis of Fe:ZnSe nanopowders by liquid-phase co-precipitation method for sintering transparent ceramics.     

Effects of CaO–B2O3–SiO2 glass additive on the microstructure and electrical properties of BCZT lead-free ceramic

Addition of CaO–B₂O₃–SiO₂ (CBS) glass was performed to lower the sintering temperature of lead-free Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) ceramics. Orthorhombic and tetragonal phases coexisted in CBS-free BCZT ceramics. The BCZT ceramics transformed into a pseudo-cubic phase when sintered at 1300 °C with increasing CBS glass content. Additionally, the secondary phase, Ba₂TiSi₂O₈, was observed when CBS glass was added. The density initially increased, reached a maximum value with 2 wt% CBS glass, and then decreased rapidly with further increase in CBS glass content, which was consistent with the microstructure. The ɛ, T_c, P_r, and d₃₃ depend on microstructure, and the results agree with the density. Evident relaxation behavior was observed. Observed results were inferred to be dependent on the microstructure, phase structure, lattice distortion, and secondary phase. The sample with 2 wt% CBS glass showed the excellent performance, which could be a promising substitute to lead-free piezoelectric ceramics for lead-based materials.     

Perovskite‐Structured BiFeO3–Bi(Zn1/2Ti1/2)O3–PbTiO3 Solid Solution Piezoelectric Ceramics with Curie Temperature About 700°C

In this article, perovskite‐structured BiFeO₃–Bi(Zn_1/2Ti_1/2)O₃–PbTiO₃ (BF–BZT–PT) ternary solid solutions were prepared with traditional solid‐state reaction method and demonstrated to exhibit a coexistent phase boundary (CPB) with Curie temperature of T_C~700°C in the form of ceramics with microstructure grain size of several micron. It was found that those CPB ceramics fabricated with conventional electroceramic processing are mechanically and electrically robust and can be poled to set a high piezoelectricity for the ceramics prepared with multiple calcinations and sintering temperature around 750°C. A high piezoelectric property of T_C = 560°C, d₃₃ = 30 pC/N, ε₃₃^T/ε₀ = 302, and tanδ = 0.02 was obtained here for the CPB 0.53BF–0.15BZT–0.32PT ceramics with average grain size of about 0.3 μm. Primary experimental investigations found that the enhanced piezoelectric response and reduced ferroelectric Curie temperature are closely associated with the small grain size of microstructure feature, which induces lattice structural changes of increased amount ratio of rhombohedral‐to‐tetragonal phase accompanying with decreased tetragonality in the CPB ceramics. Taking advantage of structural phase boundary feature like the Pb(Zr,Ti)O₃ systems, through adjusting composition and microstructure grain size, the CPB BF–BZT–PT ceramics is a potential candidate to exhibit better piezoelectric properties than the commercial K‐15 Aurivillius‐type bismuth titanate ceramics. Our essay is anticipated to excite new designs of high–temperature, high–performance, perovskite‐structured, ferroelectric piezoceramics and extend their application fields of piezoelectric transducers.     

KNbTeO6 transparent ceramics prepared by the combination of pressure-less sintering and pseudo hot isostatic pressing

KNbTeO₆ transparent ceramics were prepared by combining pressure-less sintering and pseudo-hot isostatic pressing (PHIP) of the synthesized submicron single-phase powder. The PHIP was conducted by wrapping coarse magnesium aluminate powders around the pre-sintered body in the spark plasma sintering (SPS) furnace. With an average grain size of 412 ± 23 nm, the in-line transmittance of transparent KNbTeO₆ ceramics reaches 80.25% at 2677 nm. By contrast, the density of the samples prepared by conventional SPS with the same sintering procedure is only 98.73%, and the highest in-line transmittance 64.25% occurs at 4976 nm. In particular, by investigating the sintering mechanism of PHIP, the improvement of microstructure and optical transmittance could be attributed to the plastic deformation caused by shear stress. The obtained ceramics exhibited excellent mechanical and dielectric properties, which was benefited from the novel sintering technology.     

Grain Size Effects on Piezoelectric Properties and Domain Structure of BaTiO3 Ceramics Prepared by Two‐Step Sintering

A series of highly dense barium titanate (BaTiO₃) ceramics with the average grain size (GS) from 0.29 to 8.61 μm are successfully prepared by two‐step sintering, and the GS effect on piezoelectric coefficient (d₃₃) is systematically discussed in this work. It is found that when GS above 1 μm, d₃₃ can be enhanced with decreasing GS, reaching a maximum value of 519 pC/N around 1 μm due to the high activity of domain wall mobility. Subsequently, d₃₃ rapidly drops with a further decrease in GS owing to the reduced domain density. The results suggest that it is possible to prepare high‐performance BaTiO₃ ceramics by controlling the GS and domain configuration properly, which brings great revitalization to the BaTiO₃‐based piezoceramics.     

Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment

We succeeded in the optimization of highly transparent Y₂O₃ ceramics with a submicrometer grain size approximately 0.6?μm by hot pressing (1300–1550?°C) and a subsequent HIP (1450?°C) treatment using commercial Y₂O₃ powders as starting powders and ZrO₂ as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400?°C for 3?h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2?mm thick) ever reported of 83.4% and 78.3% at 1100 and 400?nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15?nm in size.     

Study on robust additive of CaCO3 for fast fabrication of highly transparent AlON ceramics by pressureless sintering

Fabrication of transparent AlON ceramics is extra sensitive to both particle size of starting powder and sintering additive due to shuttling transformation between AlON and Al₂O₃ + AlN during heating. One possible solution is to select robust additive to suppress the shuttling transformation. In this work, three AlON powders with different median particle sizes of 0.6, 0.9, and 1.1 μm were prepared. After studying the effect of CaCO₃ on densification process, AlON ceramics with the maximum transmittance of ≥81.1% were successfully fast prepared by pressureless sintering (PS) at 1880°C for only 2.5 h by using three AlON powders doped with different CaCO₃ amount. Specifically, AlON ceramics prepared from 1.1 μm with 0.5–0.8 wt.% CaCO₃ doping consistently showed the maximum transmittance of ≥85.3%, which indicates that CaCO₃ can serve as a robust additive to enable fast fabrication of highly transparent AlON ceramics even by PS.     

Enhancement of mechanical properties of Os0.9Re0.1B2 ceramics via buried boron powder assisted sintering

As a new type of hard/super-hard materials, the consolidation of transition metal borides is very critical for obtaining bulk ceramics with excellent properties. In the present work, buried boron powder assisted pressures-less sintering was applied for preparation of Os_0.9Re_0.1B₂ ceramics with the aim for mechanical properties improvement. Os_0.9Re_0.1B₂ powders were firstly synthesized via mechanochemical technique with moral ratios of (Os + Re):B = 1:2.5 and 1:2.25, respectively. Bulk samples were then consolidated using buried powder sintering and exposed sintering, respectively, for comparison. The influence of buried boron powder sintering on the phase composition, microstructure, and mechanical properties (micro-hardness, nano-hardness, and Young's modulus) of Os_0.9Re_0.1B₂ ceramic samples were investigated. The results show that by employing buried powder sintering, B powders surrounded the sample during the sintering process, which on the one hand, inhibited decomposition of Os_0.9Re_0.1B₂ to (Os_0.9Re_0.1)₂B₃, while on the other hand, decreased the grain size of the sample. Further, a columnar to equiaxial transition for the grains was found with grain size decreased when (Os + Re):B = 1:2.25. The samples prepared with buried powder sintering have higher mechanical properties as compared with those prepared with exposed sintering. The sample prepared from (Os + Re): B = 1:2.25 by buried powder sintering had the best mechanical properties among the four studied samples, along with the smallest grain size. The mechanical properties of the samples were greatly influenced by the grain size and relative density.     

Low‐temperature sintering and enhanced piezoelectric properties of random and textured PIN–PMN–PT ceramics with Li2CO3

Low‐temperature sintered random and textured 36PIN–30PMN–34PT piezoelectric ceramics were successfully synthesized at a temperature as low as 950°C using Li₂CO₃ as sintering aids. The effects of Li₂CO₃ addition on microstructure, dielectric, ferroelectric, and piezoelectric properties in 36PIN–30PMN–34PT ternary system were systematically investigated. The results showed that the grain size of the specimens increased with the addition of sintering aids. The optimum properties for the random samples were obtained at 0.5 wt% Li₂CO₃ addition, with piezoelectric constant d₃₃ of 450 pC/N, planar electromechanical coupling coefficient k_p of 49%, peak permittivity ε_max of 25 612, remanent polarization P_r of 36.3 μC/cm². Moreover, the low‐temperature‐sintered textured samples at 0.5 wt% Li₂CO₃ addition exhibited a higher piezoelectric constant d₃₃ of 560 pC/N. These results indicated that the low‐temperature‐sintered 36PIN–30PMN–34PT piezoelectric ceramics were very promising candidates for the multilayer piezoelectric applications.     

Reactive Hot Pressing and Properties of Zr1−xTixB2–ZrC Composites

Reactive hot pressing was used to prepare Zr_1?xTi_xB₂–ZrC composites with advantageous microstructure and mechanical properties from ZrB₂–TiC powders. The reaction mechanisms and the effects of different levels of TiC on the physical and mechanical properties of the resulting composite were explored in detail and compared to conventionally hot‐pressed ZrB₂ and ZrB₂–ZrC. Incorporation of 10 to 30 vol% TiC enabled full densification and restrained grain growth, reducing the final average grain size from 5.6 μm in pure ZrB₂ to a minimum of 1.4 μm in samples with 30 vol% TiC. The flexural strengths and hardnesses of the composites sintered with TiC were consequently greater than the conventionally processed ZrB₂–ZrC materials, increasing from 440 MPa and 17.4 GPa to a maximum of 670 MPa and 24.2 GPa at 10 vol% TiC. However, despite a decrease in the total average grain size, the flexural strength at higher TiC levels was limited by an increase in ZrC grain growth, which was observed to determine the flexural strength of the reaction sintered composites similar to the case of ZrB₂–SiC.     

Enhancement of piezoelectric properties of BF–BT ceramics by using ZrO2 powder bed during the sintering process

ZrO₂ powders of various particle sizes (0.15, 0.7, 500 µm) were used to simulate loose powder bed sintering to prepare BF–BT piezoelectric ceramics. The phase structure, dielectric properties, ferroelectric properties, and piezoelectric properties were compared with the samples sintered by the conventional powder bed method (i.e., powder of the same composition as the sample). Results showed that the use of loose ZrO₂ powder bed could improve the heat conduction rate and the sintering quality of bulk BF–BT piezoelectric ceramics. The XPS results showed that the samples sintered with 500 µm ZrO₂ powder beds had the lowest concentration of Fe²⁺, exhibited the largest piezoelectric coefficients (d₃₃ = 201 pC/N). In contrast, the sample sintered with a conventional powder bed under the same sintering conditions had a piezoelectric coefficient d₃₃ of 156 pC/N.     

Fabrication and characterization of highly transparent ZrO2-doped Tm2O3 ceramics

Highly transparent polycrystalline Tm₂O₃ ceramics were successfully fabricated by vacuum sintering at temperatures from 1650 to 1850 °C for 8 h using commercial Tm₂O₃ and ZrO₂ (1 at%) powders as starting materials. It is the first time that ZrO₂ was reported as a sintering additive to prepare Tm₂O₃ transparent ceramics. The effects of sintering temperature on the optical transmittance and microstructure of Tm₂O₃ transparent ceramics were studied. The desired Tm₂O₃ ceramics with relative density of 99.8% and an average grain size of approximately 9.7 μm were obtained at 1800 °C and the in-line transmittance reached 75% at 880 nm and fluctuated around 80% from 2100 to 2400 nm, respectively. This study demonstrated that Tm₂O₃ transparent ceramics with higher in-line transmittance and smaller grain size could be prepared by using ZrO₂ as sintering additive at a relatively lower vacuum sintering temperature compared to those already reported in open literatures.