Multilayer lead‐free piezoceramic composites: Influence of co‐firing on microstructure and electromechanical behavior

In this study lead‐free 2‐2 and 0‐3 ceramic/ceramic composites comprised of the non‐ergodic relaxor 0.93(Bi_1/2Na_1/2)TiO₃–0.07BaTiO₃ and ergodic relaxor 0.94Bi_0.5(Na_0.75K_0.25)_0.5TiO₃–0.06BiAlO₃ were investigated. The macroscopic electromechanical behavior was characterized as a function of continuent content, revealing an enhancement in the unipolar strain from the multilayer composite structure. Systematic evaluation of the effects of co‐sintering on microstructural properties, such as grain size and porosity, revealed potential mechanisms by which the increase in unipolar strain was achieved. In addition, interdiffusion between the constituents was observed, providing evidence for the formation of a functionally graded ceramic by co‐sintering. These data are contrasted with high‐resolution energy dispersive X‐ray microanalysis for measurement of chemical composition across the interface of 2‐2 ceramics. These findings provide insight into how synthesis routes can be optimized for tailoring the enhancement of electromechanical properties of lead‐free electroceramic composite systems.     

In situ shrinkage measurement during pressure‐assisted sintering of low temperature co‐fired ceramic panels

Shrinkage measurements of miniaturized low temperature co‐fired ceramics (LTCC) samples under load typically lead to collapsing of the samples, which hampers the characterization of shrinkage up to full densification. In this paper, a measurement setup is presented, which allows for in situ shrinkage measurements of practical, large LTCC panels during pressure‐assisted sintering in a sintering press. The shrinkage behavior of two commercial LTCC systems (GreenTape 951 and Ceramtape GC) has been measured under loads of up to 1 MPa. No crushing of the specimens was observed and reproducible characterization of shrinkage up to full densification has been performed. Based on comparisons to thermomechanical analyzer measurements in this and other studies, it was found that the in situ approach is much better suited for shrinkage characterization of LTCC under load. Reproducibility and accuracy of the method are discussed and practical as well as more academic applications are proposed.     

Cold sintering of bioglass and bioglass/polymer composites

Bioactive glasses are widely utilized to regenerate bone tissue and aid bonding of orthopedic implants. Forming composites of bioglass with bioactive polymers allow the mechanical properties and biological response to be tailored. Although several methods for creating bioglass–polymer composites exist, they require dissolution of the polymer, controlled phase separation, and appear to have an upper limit of ∼30 vol.% bioglass. Cold sintering is a novel technique for the densification of ceramics and glasses which utilizes a liquid phase and pressure to allow the production of components at reduced temperatures. We demonstrate that cold sintering (100°C) of Bioglass 45S5 powder produced via flame spray pyrolysis and the fabrication of Bioglass 45S5–polymer composites. Assessment of the in vitro response revealed that composites were not cytotoxic. Solid-state ³¹P and ²⁹Si MAS NMR studies of the silicon and phosphorus speciation in the glass powder, as-received, wetted, and sintered samples show similarities to reactions expected when bioglass is implanted in the body which along with Raman spectroscopy data gave insight into the cold sintering densification mechanism.     

Electrical transport and thermoelectric properties of Ca0.8Y0.2−xDyxMnO3−δ (0 ≤ x ≤ 0.2)

Ca_0.8Y_0.2?xDy_xMnO_3?δ (0≤x≤0.2) samples were fabricated by the solid‐state reaction method, and their thermoelectric properties were studied from 500°C to 800°C. Upon the substitution of Dy³⁺ for Y³⁺ in the Ca_0.8Y_0.2?xDy_xMnO_3?δ, the electrical and thermal conductivities gradually decreased with increasing Dy³⁺ concentration, whereas the absolute value of the Seebeck coefficient significantly increased. The Ca_0.8Dy_0.2MnO_3?δ showed the largest value of dimensionless figure of merit (0.180) at 800°C as a result of the combination of the largest absolute value of the Seebeck coefficient and the lowest thermal conductivity. We believe that the Ca_0.8Dy_0.2MnO_3?δ is a promising thermoelectric material at high temperatures.     

Cold sintering process for ZrO2‐based ceramics: significantly enhanced densification evolution in yttria‐doped ZrO2

We recently developed a novel technique of cold sintering process (CSP) to obtain dense ceramics at extraordinarily low temperatures. In this communication, we demonstrate the feasibility of applying CSP to zirconia‐based ceramics. As exemplified by 3Y‐TZP ceramics, a significantly enhanced densification evolution is observed. Water is simply utilized as a sintering aid to assist the ceramic densification under an applied external pressure. The low‐temperature advantage of CSP outstands in contrast to the densification curves compiled from other sintering techniques. A gradual monoclinic‐to‐tetragonal phase transformation is revealed in correspondence to the densification development, as well as contributes to the mechanical hardness evolution. A Vickers Hardness reaches ~10.5 GPa after annealing the cold‐sintered ceramics at 1100°C, which is comparable to those values reported in the previous studies at higher sintering temperatures. Such a sintering methodology is of significant importance as it provides a roadmap for cost‐effective processing of zirconia‐based ceramics and composites that enable broad practical applications.     

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.     

Mechanical properties of 1 mol% CeO2‐10 mol% Sc2O3 co‐doped and stabilized ZrO2 synthesized through hydro/solve‐thermal method

Ultra‐fine 1 mol% CeO₂‐10 mol% Sc₂O₃ co‐doped and stabilized ZrO₂ (1Ce10ScSZ) powders with average grain size less than 10 nm in diameter were prepared by hydro/solve‐thermal method using either deionized water, ethanol, or methanol as solvent. As‐synthesized powders were characterized in terms of phase structure, particle morphology, and chemical composition by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high‐resolution transmission electron microscopy (HRTEM), and inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), respectively. Sintering studying was conducted on pellets of 15 mm in diameter and 3 mm in thickness under uniaxial compaction using 25 MPa at either 600, 800, 1000, 1100, 1200, 1400, or 1500°C for 1 hour. Phase transitions and grain morphologies of those sintered samples were characterized by XRD and field emission scanning electron microscopy (FESEM). Mechanical properties were characterized on dense pellets sintered at 1500°C by nanoindentation. Experimental results showed that ethanol was more effective to synthesize agglomerate‐free 1Ce10ScSZ powders as compared with deionized water and methanol. Choice of solvent affected the environment of hydro/solve‐thermal solution, which led to variation of chemical compositions of powders and porosities of sintered pellets, and therefore, influenced their mechanical performance. Our study showed that solvent was important to make dense, thin, and mechanically robust 1Ce10ScSZ electrolyte for potential applications in electrochemical devices. Absolute values of hardness (H) and Young's modulus (E) measured from our samples are much higher and more consistence than those results obtained from commercial vendors reported in literatures.     

Characterization of Thermoelectric Metal Oxide Elements Prepared by the Pulse Electric-Current Sintering Method

Thermoelectric elements consisting of the layered polycrystalline materials of Al-doped ZnO and NaCo₂O₄ were prepared using the pulse electric-current sintering (PECS) method at 900°C for 3 min. Direct contact between the polycrystalline Al-doped ZnO and the NaCo₂O₄ was obtained in a single-step process for the stacked powders. The electrical conductivities of the polycrystalline materials prepared by PECS were higher than those of materials prepared by conventional sintering, despite their porous structure. The thermoelectric voltage of the 1-mol%-Al-doped ZnO and NaCo₂O₄ polycrystalline element (measuring ∼6 mm × 3 mm × 15 mm) was 83 mV at d T = 500 K, when the junction of the elements was at 800°C.     

Comparison of hot pressing and spark plasma sintering in the densification behavior of indium tin oxide‐borosilicate glass composites

The goal of this study was to fabricate borosilicate glass matrix composites with high optical transmittance and high conductivity by forming percolated segregated networks of indium tin oxide (ITO) in the microstructure. ITO nanoparticles and borosilicate glass microspheres were mechanically mixed with ITO concentrations varying from 0 to 2.99 vol%. The mixes were then consolidated using either hot pressing (HP) or spark plasma sintering (SPS). The effects of changing sintering methods, along with varying other processing parameters such as heating rate, maximum temperature, and applied pressure, had surprising and unanticipated effects. Ac impedance spectroscopy (IS), SEM, and EDS results indicated the successful formation of a grain‐like microstructure of the sintered glass using both HP and SPS processing, with the ITO particles segregated to the boundary regions in all samples. IS results indicated percolation threshold values between 0.154 and 0.307 vol% ITO in the HP samples and between 0.307 and 0.764 vol% ITO in the SPS samples, with resistivities as low as 29 (Ω·cm) at 2.99 vol% ITO. Optical properties were dominated by impurities and light scattering at defects such as pores. Contrary to conventional belief, it was found that samples made using SPS required far higher temperatures to fully densify, with all other processing conditions being the same, compared with HP. This behavior was confirmed through repeat tests using different SPS equipment and a wide range of processing conditions.     

Considering the possibility of bonding utilizing cold sintering for ceramic adhesives

Here, we introduce a new bonding technique that enables the joining of different materials at low temperatures and provides a bond superior to that of polymer adhesives at high temperatures, in temperature ranges between 250°C and 500°C. This technique involves a low temperature sintering process that is termed the “Cold Sintering Process,” where a dielectric composite powder material is sintered to function as the adhesive between two other materials being bonded. In order to characterize and further discuss the potential of this new bonding methodology, which we call Cold Sintering Ceramic Bonding (CSCB), we demonstrate the initial mechanical characteristics of samples with sandwich structures of mesh/CSCB/mesh, including four‐point bending, micro‐indentation, and adhesion pull tests. Where appropriate, we compare mechanical properties against low and high temperature epoxies and demonstrate that the CSCB matches up competitively with the epoxies at low temperatures and remains strong at temperatures well above those where standard polymer adhesives fail. Transmission electron microscopy show a high quality interface between a stainless steel plate and the ceramic after the CSCB.     

Powder chemistry effects on the sintering of MgO‐doped specialty Al2O3

In this work, we investigate the effects of powder chemistry on the sintering of MgO‐doped specialty alumina. The stages at which MgO influences densification of Al₂O₃ were identified by comparing dilatometry measurements and the sintering kinetics of MgO‐free and MgO‐doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO₂ in alumina grains near the boundaries using EDS. First‐principles DFT calculations demonstrate that the co‐dissolution of MgO and SiO₂ in alumina is thermodynamically favored over the dissolution of MgO or SiO₂ individually in alumina. This study experimentally demonstrates for the first time that removal of SiO₂ from the grain boundaries is a key process by which MgO enhances the sintering of alumina.     

Liquid‐phase sintering,microstructural evolution,and microwave dielectric properties of Li2Mg3SnO6–LiF ceramics

The liquid‐phase sintering behavior and microstructural evolution of x wt% LiF aided Li₂Mg₃SnO₆ ceramics (x = 1‐7) were investigated for the purpose to prepare dense phase‐pure ceramic samples. The grain and pore morphology, density variation, and phase structures were especially correlated with the subsequent microwave dielectric properties. The experimental results demonstrate a typical liquid‐phase sintering in LiF–Li₂Mg₃SnO₆ ceramics, in which LiF proves to be an effective sintering aid for the Li₂Mg₃SnO₆ ceramic and obviously reduces its optimum sintering temperature from ~1200°C to ~850°C. The actual sample density and microstructure (grain and pores) strongly depended on both the amount of LiF additive and the sintering temperature. Higher sintering temperature tended to cause the formation of closed pores in Li₂Mg₃SnO₆‐x wt% LiF ceramics owing to the increase in the migration ability of grain boundary. An obvious transition of fracture modes from transgranular to intergranular ones was observed approximately at x = 4. A single‐phase dense Li₂Mg₃SnO₆ ceramic could be obtained in the temperature range of 875°C‐1100°C, beyond which the secondary phase Li₄MgSn₂O₇ (<850°C) and Mg₂SnO₄ (>1100°C) appeared. Excellent microwave dielectric properties of Q × f = 230 000‐330 000 GHz, ε_r = ~10.5 and τ_f = ~?40 ppm/°C were obtained for Li₂Mg₃SnO₆ ceramics with x = 2‐5 as sintered at ~1150°C. For LTCC applications, a desirable Q × f value of ~133 000 GHz could be achieved in samples with x = 3‐4 as sintered at 875°C.     

Suppressed grain growth in highly porous barium titanate foams by two‐step sintering

Porous barium titanate has gained significant attention in recent years for their potential use in applications such as scaffolds for bone tissue engineering, stress sensors, gas sensors, and many others. However, there is very little control over the grain size of the material during the sintering processes specially to achieve little or no growth of the starting powders. Here, using the two‐step sintering method barium titanate foams were shown to be synthesized with controlled grain size of the struts without significant differences in the pore structure of the materials. In order to evaluate the applicability of two‐step sintering for a variety of processing methods, highly porous (>80% porosity) foams synthesized through the direct polyurethane foaming method were used to create conditions furthest from bulk where two‐step sintering has shown success. Two‐step sintering parameters were identified and the processing conditions were confirmed to not alter the mechanical properties of the samples due to expected residual stresses or thermal shock resulting from the rapid heating and cooling rates employed.     

Fabrication of a transversal multilayer thermoelectric generator with substituted calcium manganite

The sintering behavior and thermoelectric performance of Ca_0.99Gd_0.01Mn_0.99W_0.01O₃ was studied, and a multilayer thermoelectric generator was fabricated. The addition of CuO as sintering additive was found to be effective for the reduction in the sintering temperature from 1300°C to about 1000°C‐1050°C. Dense samples were obtained after firing at 1050°C, whereas some porosity remained after firing at 1000°C. Samples sintered at reduced temperature exhibit lower electrical conductivity, whereas the Seebeck coefficient S = ?150 μV/K at 100°C is not affected by lowering the sintering temperature. The figure of merit is ZT = 0.12 at 700°C for samples sintered at 1300°C; ZT = 0.08 and 0.03 were obtained for multilayer laminates sintered at 1050°C and 1000°C, respectively. A transversal multilayer thermoelectric generator (TMLTEG) was built by stacking layers of substituted CaMnO₃ green tapes, and printing AgPd conductor stripes onto the thermoelectric layers at an angle of 30° relative to the direction of the heat flow. The multilayer stack was co‐fired at 1000°C. The TMLTEG has a power output of 2.5 mW at ?T= 200 K in the temperature interval of 25°C‐300°C. A meander‐like generator with larger power output comprising six TMTEGs is also presented.     

Defect suppression in CaZrO3‐modified (K,Na)NbO3‐based lead‐free piezoceramic by sintering atmosphere control

During high‐temperature crystal growth, lattice defects will inevitably form inside piezoelectric materials, which can be a hindrance for performance optimization. Through appropriate atmosphere control during sintering, defect levels inside the piezoelectric material can be regulated. Herein, CaZrO₃‐modified (K, Na)NbO₃‐based lead‐free piezoelectric ceramics with a nominal composition of 0.95(Na_0.49K_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃‐0.05CaZrO₃ are produced by sintering in an oxygen‐rich atmosphere. Compared with an air‐sintered sample, the piezoelectric constant of the oxygen‐sintered sample has greatly improved 15% up to 390 pC/N, which is comparable to commercial lead‐based counterparts. In addition, the planar electromechanical coupling factor k_p is enhanced from 0.46 to 0.52. A qualitative model related to defect engineering is proposed to support the experimental observations. Our results indicate the feasibility of purposely optimizing the piezoelectric performance by sintering atmosphere control.     

Low-Temperature Sintering and Dielectric Properties of Ag(Nb,Ta)O3 Composite Ceramics

The sintering behavior and dielectric properties of perovskite Ag(Nb_{1− x} Ta _x )O₃ (0 < x < 1) solid solutions and two-phase composite assemblages were explored. A small amount of CuO (1 wt%) was used for liquid-phase sintering and led to high densification at temperatures <950°C. The temperature coefficient of capacitance, TCC, was adjusted by varying the Nb:Ta ratio within the solid-solution series and by creating composite microstructures. Two-phase assemblages consisting of Ag(Nb_3/4Ta_1/4)O₃ and Ag(Nb_1/4Ta_3/4)O₃ were synthesized to achieve a temperature-stable dielectric material for high-frequency applications. The composite dielectric with CuO addition had an average dielectric constant of 390 and a Q × f factor of 410 GHz at 2 GHz, with a stable TCC (0 to −180 ppm/°C) in the temperature range from −20° to +60°C. In addition, process compatibility with a silver conductor was confirmed by high-frequency ring-resonator measurements and microstructural characterization. The Ag(Nb_{1− x} Ta _x )O₃ solid solutions and composites are promising candidates as embedded capacitors for radio-frequency/microwave applications.     

Mechanochemical Synthesis and Pressureless Sintering of TiB2–AlN Composites

TiB₂-AlN composites have been fabricated by the pressureless sintering of a mechanochemically processed Ti, Al, and BN powder mixture. TiB₂-AlN powder was obtained from the mixture of Ti, Al, and BN, which had a composition corresponding to 45.7 wt% TiB₂-54.3 wt% AlN, after mechanochemical processing for longer than 24 h. X-ray diffraction and transmission electron microscopy analysis showed that the powder subjected to mechanochemical processing for 60 h consisted of crystallites less than 300 nm in size with a disordered crystal structure. TiB₂-AlN composites with 95% relative density, a flexural strength of 172 MPa, a fracture toughness of 4.6 MPa·m^1/2, a hardness of 12.0 GPa, and an electrical resistivity of 1488 μΩ·cm were obtained by pressureless sintering at 1700°C for 2 h of the powder subjected to mechanochemical processing for 60 h.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Application of a Tough Surface Layer to a Fiber-Bonded Composite

The feasibility of creating "tough surface material" using oxide-fiber-reinforced oxide matrix ceramics was studied. Al₂O₃ fiber/(ZrO₂, Al₂O₃) matrix composite was used as the surface material of a Si–Ti–C–O-fiber-bonded composite. The sintering of the matrix (ZrO₂ and Al₂O₃) of the surface composite layer (SCL) and its bonding to the fiber-bonded composite (FBC) were done simultaneously by vacuum hot pressing. A spherical indentation test demonstrated the advantage of the SCL in reducing the damage of the base FBC from an indenter, because the high fracture resistance of the surface composite layer could reduce the stress concentration by the cumulative microfracture process.     

High-Q and temperature-stable microwave dielectrics in layer cofired Zn1.01Nb2O6/TiO2/Zn1.01Nb2O6 ceramic architectures

A multilayer cofired architecture was proposed and demonstrated to achieve high-Q and temperature-stable microwave dielectrics in a derived system, Zn_1.01Nb₂O₆-TiO₂. This approach could effectively allow the chemical reactions between Zn_1.01Nb₂O₆ and TiO₂ occur at a rather narrow area (~12 μm), the interfaces of heterogeneous layers, where the diffusion of Zn, Nb, and Ti could be observed. Such interfaces could act as the in situ “glues” to connect each layer well. The effects of stacking scheme and TiO₂ content on the microwave dielectric properties of layered architectures were investigated systematically. The resonant frequency, Q-factor, and electric field distribution were reported using the eigenmode solver of high-frequency structure simulator. Among the available layer architectures, the optimized microwave dielectric characteristic was observed in Zn_1.01Nb₂O₆/TiO₂/Zn_1.01Nb₂O₆ stacked with 0.058 mol TiO₂ (~1.84 vol%). The τ_f can be effectively tuned to approximately +0.53 ppm/°C, and importantly, a high Q × f value ~99 500 GHz together with ε_r ~26.8 was achieved. This design could be beneficial for opening up new ways to develop high-performance microwave dielectrics based on current material systems and therefore to meet with the high requirements for 5G wireless communication components and multilayer packing technology.