Synthesis of Dense Lead Titanate Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense PbTiO₃ ceramics consisting of submicrometer-sized grains were prepared using the spark-plasma-sintering (SPS) method. Hydrothermally prepared PbTiO₃ (0.1 μm) was used as a starting powder. The powder was densified to ≳98% of the theoretical X-ray density by the SPS process. The average grain size of the spark-plasma-sintered ceramics (SPS ceramics) was ≲1 μm, even after sintering at 900°–1100°C, because of the short sintering period (1–3 min). The measured permittivity of the SPS ceramics showed almost no frequency dependence over the range 10¹–10⁶ Hz, mainly because pores were absent from the ceramics. The coercive field of the SPS ceramics was somewhat higher than that of conventionally sintered ceramics, which could be attributed to the small-grained microstructures of the SPS ceramics.     

PMN–PT Ceramics Prepared By Spark Plasma Sintering

(1− x )Pb(Mg_1/2Nb_2/3)O₃− x PbTiO₃ (PMN–PT) ceramics of stoichiometric composition were fabricated by conventional pressureless sintering (CS) and spark plasma sintering (SPS). The CS ceramics exhibited a change from relaxor to normal ferroelectric behavior (FE) with increasing PT content. However, low dielectric constants, frequency dispersion, and diffuse phase transition behavior typical for relaxors were obtained for all SPS ceramics. FE and piezoelectric measurements further demonstrated low remanent polarization and strain, high coercive field, and low electromechanical response from SPS materials. Normal dielectric and enhanced FE performance appeared following high-temperature heat treatment after SPS. The effects of grain size, microstructure, and chemical heterogeneity formed during fast sintering are considered.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Effects of Preparation Conditions on the Structural and Optical Properties of Spark Plasma-Sintered PLZT (8/65/35) Ceramics

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics with high density were fabricated using spark plasma sintering (SPS), a recently developed hot-pressing method. A wet–dry combination method was used to prepare the fine PLZT powders. The average grain size of the PLZT ceramics was less than 1 μm, because of a relatively low sintering temperature and a very short sintering time. The transmittance of PLZT ceramics increased with an increase of calcination temperature up to 700°C and then it slightly decreased with further increase of calcination temperature. The transmittance strongly depended on the SPS temperature and heat-treatment temperature. The pellet sintered at 900°C for 10 min and heat treated at 800°C for 1 h with a thickness of 0.5 mm showed a transmittance of 31% at a wavelength of 700 nm. The relationships between the transmittance and the microstructure were investigated.     

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense BaTiO₃ ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO₃ (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X-ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000-1200°C, because of the short sintering period (5 min). Cubic-phase BaTiO₃ coexisted with tetragonal BaTiO₃ at room temperature in the SPS pellets, even when well-defined tetragonal-phase BaTiO₃ powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent10⁰-10⁶ Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.     

Sintering of titania nanoceramic: Densification and grain growth

Two-step sintering (TSS) has been applied in the current study to suppress the accelerated grain growth of TiO₂ nanopowder compacts in the final sintering stage. While the grain size ranges between 1 and 2 μm in the full dense structures produced by pressureless conventional sintering (CS), application of two-step sintering has led to a remarkable grain size decline to ～250 nm. With regard to the expensive procedure of spark plasma sintering (SPS), similar density and grain size results determine the straightforward TSS method as a desirable rival for SPS.     

Enhanced bipolar fatigue resistance in PMN-PZT ceramics prepared by spark plasma sintering

The bipolar fatigue behaviors of lead magnesium niobate-lead zirconate titanate (PMN-PZT) ceramics sintered by conventional sintering (CS) and spark plasma sintering (SPS) were systematically investigated. Significantly enhanced bipolar fatigue resistance was observed for SPS samples by a comparative analysis of the evolution of both large signal (polarization, strain) and small signal (piezoelectric coefficient, permittivity) hysteresis curves. The enhanced fatigue resistance is not only attributed to the suppressed development of microcracks, which is due to the decrease in grain size and porosity, but is also related to the reduction of domain wall pinning effect induced by migratory point defects, especially oxygen vacancies. Besides, the different phase structure and its evolution upon poling and fatigue are also responsible for the enhanced fatigue resistance.     

Electric Fatigue in Ferroelectric Lead Zirconate Stannate Titanate Ceramics Prepared by Spark Plasma Sintering

Ferroelectric lead zirconate stannate titanate ceramics were prepared by spark plasma sintering (SPS). Compared with its counterpart densified by conventional sintering (CS), the SPS material shows a smaller remanent polarization and maximum strain as well as a higher coercive field. Electric fatigue in both materials was investigated. In contrast to CS samples, the SPS specimens show a lower resistance to bipolar electric cycling, characterized by a faster decrease in remanent polarization and maximum strain at cycle number below 10^6.5 and a subsequent slower reduction of the properties at high cycle numbers up to 10⁸.     

Photoluminescence Properties of Nanocrystalline ZnO Ceramics Prepared by Pressureless Sintering and Spark Plasma Sintering

Nanocrystalline ZnO ceramics with grain sizes of ∼100 nm were prepared by pressureless sintering at 800°C for 2 h and spark plasma sintering (SPS) at 550°C for 2 min, respectively. Excellent green emission properties were obtained in the ZnO ceramic prepared by the SPS process and in the pressureless-sintered ZnO ceramic prepared at 1000°C for 2 h, which are attributed to the vacuum ambience of the SPS process and the sublimeness of the interstitial Zn at >900°C in air, respectively.     

Studies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies

Densification studies of 8 mol% yttria stabilized zirconia ceramics were carried out by employing the sintering techniques of conventional ramp and hold (CRH), spark plasma sintering (SPS), microwave sintering (MWS) and two-stage sintering (TSS). Sintering parameters were optimized for the above techniques to achieve a sintered density of >99% TD. Microstructure evaluation and grain size analysis indicated substantial variation in grain sizes, ranging from 4.67 μm to 1.16 μm, based on the sintering methodologies employed. Further, sample was also sintered by SPS technique at 1425 °C and grains were intentionally grown to 8.8 μm in order to elucidate the effect of grain size on the ionic conductivity. Impedance spectroscopy was used to determine the grain and grain boundary conductivities of the above specimens in the temperature range of RT to 800 °C. Highest conductivity of 0.134 S/cm was exhibited by SPS sample having an average grain size of 1.16 μm and a decrease in conductivity to 0.104 S/cm was observed for SPS sample with a grain size of 8.8 μm. Ionic conductivity of all other samples sintered vide the techniques of TSS, CRH and MWS samples was found to be ∼0.09 S/cm. Highest conductivity irrespective of the grain size of SPS sintered samples, can be attributed to the low densification temperature of 1325 °C as compared to other sintering techniques which necessitated high temperatures of ∼1500 °C. The exposure to high temperatures while sintering with TSS, CRH and MWS resulted into yttria segregation leading to the depletion of yttria content in fully stabilized zirconia stoichiometry as evidenced by Energy Dispersive Spectroscopy (EDS) studies.     

Two-Stage Sintering of Alumina with Submicrometer Grain Size

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.     

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO₂ (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.     

Synthesis and Characterization of Nano-Hetero-Structured Dy Doped CeO2 Solid Electrolytes Using a Combination of Spark Plasma Sintering and Conventional Sintering

Doped ceria (CeO₂) compounds are fluorite-type oxides that show oxide ionic conductivity higher than yttria-stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for "low" (500°–650°C) temperature operation of solid oxide fuel cells (SOFCs). To improve the conductivity in dysprosium (Dy) doped CeO₂, nano-size round shape particles were prepared using a coprecipitation method. The dense sintered bodies with small grain sizes (<300 nm) were fabricated using a combined process of spark plasma sintering (SPS) and conventional sintering (CS). Dy-doped CeO₂ sintered body with large grains (1.1 μm) had large micro-domains. The conductivity in the sintered body was low (−3.2 S/cm at 500°C). On the other hand, the conductivity in the specimens obtained by the combined process was considerably improved. The micro-domain size in the grain was minimized using the present process. It is concluded that the enhancement of conductivity in dense specimens produced by the combined process (SPS+CS) is attributable to the microstructural changes within the grains.     

Rapid pressure-less and spark plasma sintering of (Ba0.85Ca0.15Zr0.1T0.9)O3 lead-free piezoelectric ceramics

This work shows for the first time the possibility to sinter BCZT powder compacts by rapid heating rates within one hour of sintering, while achieving good piezoelectric properties. The sintering was performed by rapid (heating rates 100 and 200 °C/min) pressure-less sintering (PLS) at 1550 °C/5-60 min and by SPS sintering (100 °C/min, 1450 °C/5?60 min and 1500 °C/15?45 min). The rapid PLS samples reached a relative density up to 94 % and grain sizes of 17–36 μm acquiring d₃₃ up to 414 pC/N. Although the SPS samples reached full density at 1450 °C, their piezoelectric properties worsened due to smaller grains (10?15 μm) as well as formation of cracks at dwell times > 30 min. At elevated SPS temperature of 1500 °C/30 min, the d₃₃ increased to 360 pC/N sustaining full density. Even higher increase in d₃₃ (424 pC/N) of SPS samples was achieved by post-rapid PLS at 1550 °C/60 min resulting from further expansion in grain size.     

Highly dense spinel ceramics with completely suppressed grain growth prepared via SPS with NaF as a sintering additive

For the preparation of transparent spinel ceramics it is common practice to use LiF as a sintering additive to achieve transparency. However, it is well known that in this case the grain size exhibits a significant increase compared to pure spinel ceramics, which can lead to a deterioration in mechanical properties. The findings of this paper indicate that when NaF is used as a sintering additive for the preparation of spinel ceramics via spark plasma sintering (SPS) translucent materials with a high level of densification can be obtained without observable grain growth. It is shown that the grain size after SPS at 1500 °C with 1 h dwell is essentially the same as the primary particle size of the spinel powder, whereas pure spinel ceramics prepared via SPS under the same conditions exhibit grain growth by approximately a factor 5.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Electromechanical properties of Ba(Zr0.2Ti0.8)O3 ceramics prepared by spark plasma sintering

Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared by spark plasma sintering (SPS) and conventional sintering. The dynamic field-induced displacement and small-signal remnant piezoelectric constant measured by a resonant–antiresonant frequency method were evaluated. By normal sintering, the density, grain size, and dielectric constant of the ceramics increased with sintering temperature. The BZT20 ceramics prepared by SPS were characterized by linear field-induced strain. In response to the application of post-annealing at 1300 °C, BZT20 ceramics exhibited linear strain loop and high field-induced strain corresponding to dynamic strain/field d₃₃ at 20 kV/cm of 290 pm/V. The remnant piezoelectric properties of the BZT20 ceramics were found to largely depend on the preparation conditions, including the sintering temperature and annealing temperature. The BZT20 ceramics prepared by SPS and post-annealed at 1300 °C showed Q_m and k_p values of 325 and 25.1 (%), respectively.     

Rapid Preparation of Lead Titanate Sputtering Target Using Spark-Plasma Sintering

PbTiO₃ sputtering targets 8 cm in diameter were prepared using spark-plasma sintering (SPS) for relatively short periods, ∼2 min. Submicrometer-sized PbTiO₃ powders with a relatively large size distribution were densified to ∼86% of the theoretical X-ray density using the SPS process. In contrast, large-sized (8 cm in diameter) ceramics could not be prepared from starting powders with a relatively narrow particle-size distribution. Formation of cracks in the large PbTiO₃ targets was observed when samples were prepared under higher pressures (>50 MPa) or at higher temperatures (>900°C). Crack formation was attributed to unrelaxed internal stress originating from lower pore contents and from an inhomogeneous distribution of cations in the ceramics prepared under these conditions.     

Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline Alumina

The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of γ-Al₂O₃ nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiC_w/γ-Al₂O₃ nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (∼118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of γ-Al₂O₃ to α-Al₂O₃ of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiC_w/HEBMγ-Al₂O₃ nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m^1/2.     

Microstructure-mechanical-tribological property correlation of multistage spark plasma sintered tetragonal ZrO2

We report here the development of dense yttria-stabilized t-ZrO₂ ceramics with more uniform and finer grain sizes and concomitantly better mechanical and tribological properties via multistage spark plasma sintering (SPS). The dense tetragonal ZrO₂ ceramics were obtained by adopting three different SPS heating cycles, designed on the basis of fundamental sintering theory. The suppression of grain growth to nanosize regime (～100–150 nm), along with the development of more uniform grain size distribution was achieved with multistage sintering (MSS), as compared to normal single stage sintering (SSS). Finer microstructural scale, along with superior hardness also led to improved fretting wear resistance for the ZrO₂ samples processed via MSS. Based on the experimental results and analysis, a correlation has been established between the SPS processing schemes, microstructural development and mechanical as well as tribological properties of the tetragonal ZrO₂. The effectiveness of MSS to produce tetragonal ZrO₂ ceramics with better mechanical and tribological properties was confirmed at two different levels of yttria content (3 and 2 mol%).