Densification and grain growth of nanocrystalline 3Y-TZP during two-step sintering

Two-step sintering (TSS) was applied on nanocrystalline yttria tetragonal stabilized zirconia (3Y-TZP) to control the grain growth during the final stage of sintering. The process involves firing at a high temperature (T1) followed by rapid cooling to a lower temperature (T2) and soaking for a prolonged time (t). It is shown that for nanocrystalline 3Y-TZP (27 nm) the optimum processing condition is T1 = 1300 °C, T2 = 1150 °C and t = 30 h. Firing at T1 for 1 min yields 0.83 fractional density and renders pores unstable, leading to further densification at the lower temperature (T2) without remarkable grain growth. Consequently, full density zirconia ceramic with an average grain size of 110 nm is obtained. XRD analysis indicated that the ceramic is fully stabilized. Single-step sintering of the ceramic compact yields grain size of 275 nm with approximately 3 wt.% monoclinic phase. This observation indicates that at a critical grain size lower than 275 nm, phase stabilization is induced by the ultrafine grain structure.     

Ultralow dielectric loss in Y-doped SrTiO3 colossal permittivity ceramics via designing defect chemistry

Effects of doping of Y and sintering atmosphere on the dielectric properties of Sr_1-1.5xY_xTiO₃ ceramics (SYT, x = 0-0.014) were systematically investigated. The SYT14 (x = 0.014) ceramic sintered in N₂ attains a colossal permittivity (CP, Ɛ_r = 28 084@ 1kHz, 27 685@ 2MHz) and an ultralow dielectric loss (tanδ = 0.007@ 1kHz, 0.003@ 2MHz) at room temperature. Because of using of the A-site deficient, there are in SYT ceramics. Through the comprehensive analysis of dielectric responses, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and complex impedance data, it is proved that doping of Y promotes the formation of (Y³⁺ are located at Sr²⁺ site), (Y³⁺ are located at Ti⁴⁺ site), and Ti³⁺, and sintering in reducing atmosphere of N₂ results in more (oxygen vacancy) and (strontium vacancy) generating in SYT ceramics. The defect dipoles, , , , , , and formed by introduced defects make charge carriers localized in SYT ceramics. The combined action of the massive defect dipoles is responsible for the ultralow tanδ and CP in SYT14 ceramics sintered in N₂.     

The nonlinear optical properties of silver nanoparticles decorated glass obtained from sintering mesoporous powders

Silver nanoparticles (Ag NPs) have excellent third-order nonlinear optical properties but it is still difficult to obtain silica glass containing Ag NPs with homogenous dispersion and small particle size. Herein, silica glass with homogenously distributed Ag NPs in its matrix was derived from sintering a famous type of mesoporous silica (FDU-12) encapsulated with Ag NPs (Ag NPs/FDU-12) through Spark Plasma Sintering (SPS) technique. Benefited from the low-temperature sintering property of the Ag NPs/FDU-12 powders (~930°C within 5 min), the Ag NPs can be directly trapped in the derived silica glass with small particle size (<3.0 nm) and without mass loss. The as-prepared Ag NPs/glass showed a typical reverse saturable absorption curve, which is measured via the Z-scan method by using a 532 nm nanosecond laser. The nonlinear coefficient and imaginary third-order susceptibility were calculated as 11.46 cm/GW and 2.22 × 10⁻¹² esu, respectively, indicating the excellent third-order nonlinear optical properties of the Ag NPs/glass. This study demonstrates a great potential for preparing silica glasses functionalized with well dispersed ultrafine functional particles, which is appealing in photonic field.     

Electric current effect during the early stages of field-assisted sintering

The linear shrinkage during isothermal treatment in the early stages of conventional and field-assisted sintering of 8YSZ was analyzed. The results reveal that the application of a moderate electric current causes a remarkable acceleration of the self-diffusion kinetics, resulting in an enhanced densification. In addition, when an electric current is applied, two different field-assisted sintering regimes can be identified. The first one, observed at low currents (ie 5 mA/mm²), is associated to sintering shrinkage which follows the conventional trend () although the densification is accelerated. The second regime takes place when larger current are applied (J > 10 mA/mm²) and is described by a different sintering law, , this pointing out that the electric current causes the activation of alternative densification mechanisms when it exceeds a certain threshold value.     

Low sintering temperature for lead-free BiFeO3-BaTiO3 ceramics with high piezoelectric performance

Through modification of the heat-treatment process using a higher heating rate and a lower binder burnout temperature, the piezoelectric performance of water-quenched 0.67Bi_1.05FeO₃-0.33BaTiO₃ (BF33BT) lead-free piezoelectric ceramics was improved. The observed physical properties of BF33BT ceramics were very sensitive to the process temperatures. The sintering temperature (T_S) was changed within a narrow temperature range, and its effects were investigated. The largest rhombohedral distortion (90°-α_R = 0.14°) and tetragonality (c_T/a_T = 1.022) were observed for the ceramic sintered at 980°C, and its Curie temperature was 476°C. This ceramic showed good piezoelectric properties and large grains; the piezoelectric sensor charge coefficient (d₃₃) was 352 pC/N, and the piezoelectric actuator charge coefficient () was 270 pm/V. The high piezoelectric performance and low T_S of BF33BT ceramics indicate their potential as new low-cost eco-friendly lead-free piezoceramics.     

Flash sintering produces eutectic microstructures in Al2O3–LaPO4 versus conventional microstructures in 8YSZ–LaPO4

While monazite (LaPO₄) does not flash sinter even at high fields of 1130 V/cm and temperatures of 1450°C, composite systems of 8YSZ–LaPO₄ and Al₂O₃–LaPO₄ have been found to more readily flash sinter. 8YSZ added to LaPO₄ greatly lowered the furnace temperature for flash to 1100°C using a field of only 250 V/cm. In these experiments, -Al₂O₃ alone also did not flash sinter at 1450°C even with high fields of 1130 V/cm, but composites of Al₂O₃–LaPO₄ powders flash sintered at 900-1080 V/cm at 1450°C. Alumina–monazite (Al₂O₃–LaPO₄) composites with compositions ranging from 25 vol% to 75 vol% Al₂O₃ were flash sintered with current limits from 2 to 25 mA/mm². Microstructures were evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A eutectic microstructure was observed to form in all flash sintered Al₂O₃–LaPO₄ composites. With higher power (higher current limits), eutectic structures with regular lamellar regions were found to coexist in the channeled region (where both the current and the temperature were the highest) with large hexagonal-shaped -Al₂O₃ grains (up to 75 m) and large irregular LaPO₄ grains. With lower power (lower current limits), an irregular eutectic microstructure was dominant, and there was minimal abnormal grain growth. These results indicate that Al₂O₃–LaPO₄ is a eutectic-forming system and the eutectic temperature was reached locally during flash sintering in regions. These eutectic microstructures with lamellar dimensions on the scale of 100 nm offer potential for improved mechanical properties.     

Grain Boundary Curvatures Measurements in Annealed Yttria‐Stabilized Zirconia (3Y‐TZP) and Their Relation to Mean Grain Size

It was determined that the mean grain boundary radius of curvature in 3 mol% yttria‐stabilized zirconia isothermally annealed without and with a DC electric field  = 18 V/cm was uniquely proportional to the mean linear intercept grain size , the proportionality constant α = 3/2 being in accord with the Rios‐Fonseca stereological model.     

Fabrication of textured cerium-doped lutetium oxyorthosilicate ceramics by slip casting in a strong magnetic field

Cerium-doped lutetium oxyorthosilicate (Lu₂SiO₅:Ce, LSO:Ce) slurry with 35 vol.% solid content and viscosity of 70 mPa $·$s (at a shear rate of 4 s^–1) was prepared via a 24-h ball milling for the powder calcined at 1200°C and adding ammonium polyacrylate of 0.2 wt.%. Textured LSO:Ce ceramics were fabricated via slip casting in a vertical magnetic field and subsequently pressureless sintering of 1650°C and hot isostatic pressing. The maximum Lotgering factor f and relative density of the textured ceramics were 0.48% and 98.8%, respectively. Effects of grain size, dispersant, viscosity of slurry, and magnetic flux density on the grain orientation of LSO:Ce ceramics were systematically investigated. Based on the spatial state of grains, we proposed the rotation rule that LSO:Ce grains with or without space constraints in the slurry align along a and c-axis under the induction of a strong magnetic field.     

Rapid sintering protocol produces dense ceria‐based ceramics

We report on a rapid sintering protocol, which optimizes the preparation of 0‐29 mol% Gd‐doped ceria ceramics with density ≥98% of the theoretical crystal lattice value. The starting material is a nanometer grain‐sized powder prepared by carbonate co‐precipitation and calcined with minimal agglomeration and loss of surface area. Slow (5°C/min) heating of the green‐body from 500°C to the optimum temperature of rapid sintering (, dwell time <1 minute) followed by 20°C/min cooling to 1150°C with 6 minutes dwell time, produces maximum pellet density. increases from 1300 to ~1500°C with increase in Gd‐content, while the average grain size in the maximally dense pellets, as determined by scanning electron microscopy, ranges between 600 nm and ~1 μm. For each doping level, the logarithm of the average grain size decreases linearly with 1/T₁. By avoiding extended exposure to sintering temperatures, this protocol is expected to minimize undesirable Gd segregation.     

Ni-YSZ(001) solid-solid interfacial energy and orientation relationships

Equilibrated Ni particles were produced via solid-state dewetting of continuous Ni films deposited on the (001) surface of yttrium stabilized zirconia (YSZ). The solid-solid interface energy of the equilibrated Ni(111)-YSZ(001) interface was determined using Winterbottom analysis. Two low-index orientation relationships (ORs) were found using the selected area electron diffraction patterns in transmission electron microscopy: (OR₁) and (OR₂). However, many particles were found to deviate from these low-index ORs while maintaining the out-of-plane orientation, . The interface energy was measured to be 2.5 ± 0.1 J/m² regardless of the in-plane orientation. The orientation distribution was determined using electron backscattered diffraction for Ni particles on both (111) and (001) YSZ substrates and was found to correlate well with the interface energies measured in a previous study for the Ni-YSZ(111) interface and in the present study for the Ni-YSZ(001) interface.     

Dielectric resonator antenna with Y3Al5O12 transparent dielectric ceramics for 5G millimeter-wave applications

Microwave dielectric ceramics are considered to be one of the key materials for dielectric resonators (DR) and have very broad application prospects in the fifth generation (5G) mobile communication system. Here we have prepared high-quality factor Y₃Al₅O₁₂ (YAG) transparent dielectric ceramics using high-purity α-Al₂O₃ and Y₂O₃ powders by cold isostatic pressing of the vacuum sintered with tungsten meshes as the heating elements. Optimum relative permittivity () ~10.53, quality factor Q × f (Q = 1/dielectric loss, f = resonant frequency) ~95, 270 GHz (at =7.37 GHz), and temperature coefficient of resonant frequency (TCF) ~ −51.7 ppm °C⁻¹ were obtained at a sintering temperature of 1780°C for 12 h. For the first time, YAG transparent ceramic dielectric resonator antenna (DRA) is designed as a dominant mode and a higher-order mode using the aperture coupling feeding configuration excitation. The proposed transparent dielectric ceramic DRA can provide a broad impedance bandwidth of 4.193 GHz (ranging from 21.90 to 26.09 GHz) for S₁₁ < −10 dB, radiation efficiency of 92.1%, and compact DR unit. The proposed DRA can be used potentially as a 5G millimeter (mm)-wave multiple-input-multiple-output (MIMO) antenna unit.     

Li2O-doped MgAl2O4 nanopowders: Energetics of interface segregation

Manufacturing nanoceramics is challenging owing to the instability of the grain size resulting from the high driving force toward growth associated with the interfaces. Nanometric ceramics of some oxides have exceptional mechanical and optical properties, eg, magnesium aluminate spinel (MgAl₂O₄). The production of these fully conformed ceramics requires a precursor powder, which generally contains sintering-promoting additives. Li salts are typically used as sintering promoters for MgAl₂O₄, but the interface stability associated with the segregation of the additive is poorly understood. In this study, MgAl₂O₄ samples containing 0-2.86 mol% Li ions were synthesized via a simultaneous-precipitation method in an ethylic medium and subsequently calcined at 800°C in air. The nanopowders exhibited only the MgAl₂O₄ phase, and the crystallite size was determined by the Li₂O concentration. The crystallite size was changed via the chemical modification of the interfaces by the segregation of Li ions. The solubility in the bulk material was very low at the fabrication temperature, and small amounts of Li ions saturated the bulk material and segregated to the grain boundaries (GBs), significantly stabilizing the grain–grain interface compared with the surface. The resulting powder was then aggregated further owing to the initial stage of sintering. The surface excess obtained via the selective lixiviation method confirmed that the segregation to the GBs was greater than that to the surface. Energetics calculations confirmed these results, indicating a high enthalpy of segregation at the GBs () compared with that at the surfaces (). The enthalpy of segregation together with the interface excess allowed us to estimate the reduction in the interface energy with Li⁺ segregation of 0.8% to the surface and 11.2% to the GBs. The Li⁺ segregation to the surfaces started by Al³⁺ substitution, and for powders with ≥1.8 mol% Li ions, Mg⁺² was preferentially substituted at the surfaces.     

Sintering-driven effects on the band gap of (Pb,La)(Ti,Ni)O3 photovoltaic ceramics

In this work, the influence of the sintering temperature on the physical properties of (Pb_0.8La_0.2)(Ti_0.9Ni_0.1)O₃ (PLT-Ni) ceramics is reported. The experimental data revealed that the energy band gap of PLT-Ni ceramics could be tailored from approximately 2.7 to 2.0 eV by changing the sintering temperature from 1100°C to 1250°C. It is demonstrated that the simple substitution of Ti⁴⁺ by Ni²⁺ cations is effective to decrease the intrinsic band gap while increasing the tetragonality factor and the spontaneous polarization. However, the additional red-shift observed in the absorption edge of the PLT-Ni with increasing the sintering temperature was associated with a continuous increase in the oxygen vacancies () amount. It is believed that the impact of the creation of these thermally induced is manifold. The presence of and Ni²⁺ ions generate the Ni²⁺- defect-pairs that promoted both a decrease in the intrinsic band gap and an additional increase of the tetragonality factor, consequently, increasing the spontaneous polarization. The creation of Ni²⁺- defects also changed the local symmetry of Ni²⁺ ions from octahedral to a square pyramid, thus lifting the degeneracy of the Ni²⁺ 3d orbitals. With the increase in the sintering temperature, lower-energy absorbing intraband states were also formed due to an excess of , being responsible for an add-on shoulder in the absorption edge, extending the light absorption curve to longer wavelengths and leading to an additional absorption in “all investigated” spectrum as well.     

Lattice and Grain‐Boundary Diffusion of Al in Tetragonal Yttria‐Stabilized Zirconia Polycrystalline Ceramics (3Y‐TZP) Analyzed Using SIMS

Aluminum oxide was deposited on the surface of 3 mol% yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP). The samples were annealed at temperatures from 1523 to 1773 K. Diffusion profiles of Al in the form of mean concentration vs. depth in B‐type kinetic region were investigated by secondary ion mass spectroscopy. The experimental results for the lattice diffusion (D_B) and grain boundary diffusion (D_GB) are as follows: and where δ is the grain‐boundary width and s is the segregation factor.     

Tougher zirconia nanoceramics with less yttria

ABSTRACT

Reducing the grain size in zirconia ceramics has shown to decrease its toughness by size-dependent stabilisation of the tetragonal phase that, in turn, hinders the stress-induced phase transformation from tetragonal to monoclinic. The stability of the tetragonal phase increases with the decrease of grain size but decreases with the reduction of the amount of yttria added, implying the need for adjustment of the yttria content when a nano-grained structure is of concern. In this study, low-yttria compositions were investigated. The ceramics were prepared with two sintering methods namely spark plasma sintering (SPS) and pressureless sintering. A clear tendency was noted for the indentation toughness increase with the reduction of yttria content, and a higher toughness achieved in as-SPSed samples in comparison with the annealed samples. The origins of the increased toughness were discussed in terms of yttria content, carbon contamination and increased oxygen vacancies after sintering at reducing atmosphere in SPS.     

Giant electro-strain in textured Li+-doped 0.852BNT–0.11BKT–0.038BT ternary lead-free piezoelectric ceramics

Texturing is an effective approach to improving the piezoelectricity of piezoelectric ceramics. In this work, <001> textured Li⁺-doped 0.852Bi_0.5Na_0.5TiO₃–0.11Bi_0.5K_0.5TiO₃–0.038BaTiO₃ ternary lead-free piezoelectric ceramics are prepared by the reactive templates grain growth (RTGG) method. X-ray diffraction (XRD) results demonstrate a high orientation degree of 77% along the <001> direction. Outstanding electro-strain response, which is higher than most of reported BNT-based textured ceramics, is achieved due to the contribution of oriented-grains along the <001> direction. A large electro-strain of 0.55% with a relatively low hysteresis is obtained at 6.5 kV/mm with corresponding large signal piezoelectric coefficient () of 846 pm/V in the textured ceramics, which is 49% higher than that of the random ceramics. Besides, the electro-strain could reach as high as 0.52%@5.5 kV/mm ( = 945 pm/V) at 100°C. These results indicate that the RTGG is an effective way to design high performance lead-free piezoelectric materials.     

Sintering Behavior of Nanocrystalline Zirconia Doped with Alumina Prepared by Chemical Vapor Synthesis

Powders of nanocrystalline zirconia doped with 3–30 mol% alumina have been synthesized using chemical vapor synthesis (CVS). Dense or mesoporous ceramics of small and narrowly distributed grain and pore sizes in the nanometer range are obtained via pressureless vacuum sintering. The microstructural development of the doped samples is strongly dependent on the alumina content. Sintering of zirconia samples with 3 and 5 mol% alumina at temperatures of 1000°C for 1 h results in fully dense, transparent ceramics with grain sizes of 40–45 nm and homogeneous microstructures.     

Eight mole percent yttria stabilized zirconia powders by organic precursor route

An organic precursor-mixing route has been developed for preparation of 8 mol% yttria stabilized zirconia (8YSZ) ceramics. Polymeric salt of succinic acid with yttrium and zirconium has been prepared separately by treating sodium succinate with yttrium chloride and zirconyl chloride followed by washing with water and drying at 120 °C. Thorough mixing of the two salts in stoichiometric proportions by planetary ball milling followed by calcination at 850 °C resulted in a precursor powder containing nanocrystalline (∼40 nm) monoclinic zirconia, tetragonal YSZ, cubic YSZ and yttria. Compacts prepared after deagglomeration of powder by planetary ball milling produce 8YSZ ceramics having density 99.3% TD on sintering at 1550 °C for 2 h.     

Design of p-type NKN-based piezoelectric ceramics sintered in low oxygen partial pressure by defect engineering

The reduction-resistant properties of piezoelectric ceramics are of great importance for multilayer monolithic structures based on base metal inner electrodes, particularly for recently reported niobate-based lead-free perovskites. In this letter, the Hall-effect measurement and impedance analysis indicate that conventional (K,Na)NbO₃ (NKN)-based ceramics exhibit an n-type electronic conduction. Rapid increase in the concentrations of oxygen vacancies and electrons is responsible for severe degradation of resistivity and piezoelectric properties as sintered in N₂. By comparison, p-type NKN-based ceramics by Mn doping exhibit excellent electrical properties (d₃₃ = 368 pC/N,  = 643 pm/V, tanδ = 0.019, and IR = 39.9 GΩ · cm) in N₂ sintering atmosphere, as well interpreted by a series of proposed defect chemistry equations. The experimental results suggest that an introduction of the p-type conduction behavior should be an effective strategy to enabling NKN-based ceramics and Cu/Ni electrodes to well co-fire in a weakly reducing atmosphere.     

Transparent high-strength nanosized yttria stabilized zirconia obtained by pressure-less sintering

This work describes the development of transparent high-strength Yttria-Stabilized Zirconia (YSZ) ceramics with ultra-fine grain size utilizing conventional pressure-less densification. Starting with nanoparticles with diameter < 10 nm, it was possible to achieve full densification (>99.5% of theoretical density) at a sintering temperature of 1100–1200 °C. The average grain size of the resulting dense ceramics was 75 nm in 3 mol. % YSZ and 85 nm in 8 mol. % YSZ, showing in-line light transmission of 38% and 51% at a wavelength of 800 nm and average biaxial strength (piston on three balls test on samples of diameter 12 mm and thickness 1 mm) of 1980 MPa and 680 MPa, respectively. The nano-grained structure, absence of color centers, and miniaturization of residual pores enable the excellent light transmission. The high biaxial strength is attributed to the refined microstructure, but also to the martensitic tetragonal-to-monoclinic phase transformation that remains active even in nano-sized zirconia grains.