Lightweight and integrated electronic devices with high performance have become an inevitable trend in the development of power electronic systems. However, it is challenging to obtain lightweight and miniaturized varistors due to the low threshold electric field. Herein, (1 - x)ZnO-xPS composites with a high threshold electric field are prepared through a cold sintering process at 220 degrees C for 50 min, utilizing lightweight PS to modulate the grain boundary structures of ZnO varistors. With PS content <= 1.5 wt %, the relative densities of cold sintered composites exceed 95%. PS thin layers with a thickness of 2-10 nm are distributed at the grain boundaries of ZnO, forming the Schottky barrier, which triggers nonlinear current-voltage responses. With the addition of PS, the threshold electric field is dramatically enhanced from 135 to 2703 Vmm-1, higher than that of commercial ZnO varistors, which is further demonstrated by the electric field simulation of the Finite Element Method. The interfacial resistance obtained from impedance analysis increases with the increase of the PS content, and the activation energy of (1 - x)ZnO-xPS composites is higher than that of pure ZnO ceramics. This study provides a promising approach for the development of lightweight and cost-effective varistor materials.

Despite the attractive thermoelectric properties in single crystals, the fabrication of high-performance polycrystalline SnSe by a cost-effective strategy remains challenging. In this study, we prepare the undoped SnSe ceramic with remarkable thermoelectric efficiency by the combination of a cold sintering process (CSP) and thermal annealing. The high sintering pressure during CSP induces not only highly oriented grains but also a high concentration of lattice dislocations and stacking faults, which leads to large lattice strain that can shorten the phonon relaxation time. Meanwhile, the thermal annealing breaks the highly resistive SnOx layers at grain boundaries, which improves the electrical conductivity and power factor. In addition, the grain growth during annealing further turns the broken SnOx layers into nanoparticles, which further lowers the thermal conductivity by enhanced scattering. As a result, a peak ZT of 1.3 at 890 K and a high average ZT of 0.69 are achieved in the polycrystalline SnSe, suggesting great potential in mid-temperature power generation. This work may pave the way for the mass production of SnSe-based ceramics for thermoelectric devices.

Since electromagnetic interference (EMI), icing, and fire incidents greatly threaten aviation safety, multifunctional EMI shielding materials are urgently required in the aviation industry. However, developing highly efficient EMI shielding materials that combine fire resistance and thermal management remains a significant challenge. Here, a novel fire-resistant SiBCN ceramic fiber paper with excellent EMI shielding and Joule heating performance was fabricated via electrospinning followed by cross-linking, pyrolysis, and annealing. Compared with the SiBCN fiber paper prepared at 800 degrees C with poor EMI shielding performance, the total shielding effectiveness (SET) of the SiBCN fiber paper annealed at 1400 degrees C significantly increased to 21 dB, with specific shielding effectiveness (SSE/t) as high as 3317 dB center dot cm(2)center dot g(-2)center dot g(-1), due to the precipitation of "island-like" conductive turbostratic C, facilitating the synergistic effects of conduction loss, dipole polarization, and interfacial polarization, resulting in high efficiently attenuation of the electromagnetic waves (EMW). Furthermore, the fiber paper demonstrated excellent fire-resistant and Joule-heating performance, with a rapid electrothermal conversion response and a high saturation temperature of 300 degrees C at 12 V. Therefore, this work proposes a novel strategy for designing multifunctional ceramic fiber paper with integrated EMI shielding, fire resistance, and Joule heating performance for aircraft safety.

The modulation of the electrostatic potential barrier at grain boundaries determines the performance of many ceramic-based electronics such as varistors. However, conventional protocols relying on complex doping and annealing processes inevitably increase the inhomogeneity of microstructure, which may jeopardize the performance stability and mechanical reliability in service. Instead of doping, herein we demonstrate an effective strategy to modulate the potential barrier in ZnO-based low-voltage varistors by exploiting internal stress-induced piezoelectric polarization. The local residual stress as large as ~1 GPa can be created in the ZnO matrix by incorporating ultra-stiff nanodiamond particles using a cold sintering process. As a result, the composite with only 2 wt% of nanodiamond exhibits a prominent nonlinear current-voltage response at a low switch voltage of 15.7 V/mm, which is ascribed to the depressed barrier height induced by the distinct effects of positively and negatively charged polarization on grain boundaries. More strikingly, the large internal stress can significantly enhance the strength of the composite by more than 230% compared with the monolith, owing to the highly strengthened grain boundaries and crack-tip bridging from prestressed nanodiamonds. These findings add internal stress as a new dimension to design mechanically robust ceramic electronics with high performance.

With the rapid development of the electronics industry, the demand for dielectric materials with high permittivities, low losses, and excellent electrical breakdown strengths prepared via low-temperature fabrication techniques is increasing. Herein, we propose a one-step cold sintering process route to improve the comprehensive performance of BaTiO3−based ceramics by integrating polyetherimide (PEI). Dense BaTiO3–PEI nanocomposites can be prepared via a cold sintering process at 250 °C using Ba(OH)2∙8H2O and H2TiO3 as the transient liquid phase. The grain growth of BaTiO3 is inhibited, and thin PEI layers less than 10 nm in size are located at the grain boundaries. The dissolution‒precipitation process triggered by the transient liquid phase and viscous flow assisted by PEI dominates the cold sintering mechanism of the (1−x)BaTiO3–xPEI nanocomposites. The dielectric properties are stable over a broad temperature range up to 200 °C. Compared with BaTiO3, 80% BaTiO3–20% PEI has superior performance, with a relative permittivity of 163 and a low dielectric loss of 0.014, and the electrical breakdown strength is increased by 80.65% compared with BaTiO3. Overall, the cold sintering process provides a potential way to develop dielectric nanocomposites with excellent comprehensive performance.

A series of (1−x)(NaBi)0.5MoO4–xBiPO4 microwave dielectric ceramics with ultra-low sintering temperatures (< 700 °C) are introduced in this work. The phase compositions, microwave dielectric properties and temperature dependences were studied. A small amount of BiPO4 occupies the scheelite position forming a solid solution. Tetragonal scheelite phase together with low temperature phase (LTP) and high temperature phase (HTP) coexist in the (1−x)(NaBi)0.5MoO4–xBiPO4 microwave dielectric ceramics. The scheelite phase and HTP dominate the composition sintered at relatively low temperatures of 600–650 °C. With x increasing from 0 to 0.6, the microwave permittivities, Q×f and τf values increase first and then decrease with εr = 37.6 – 26.8, Q×f = 17,500 – 5500 GHz, and τf = (− 3.3) – 109.5 ppm/ °C. When x = 0.5, the microwave dielectric ceramic sintered at 650 °C exhibits optimal temperature stability with τf = − 0.3 ppm/ °C, and the dielectric properties at low frequencies have a good temperature stability in a wide temperature range of − 170 and 200 °C. Meanwhile, the (NaBi)0.5MoO4–BiPO4 ceramic shows a good chemical compatibility with aluminum (Al) electrode material, making it promising for the ULTCC application.

Typically, commercial Mn-Zn ferrites are sintered at high temperatures with prolong times. In this work, commercial Fe2O3-rich ferrite powders with the composition of 0.21Mn(0.8)Zn(0.2)Fe(2)O(4)-0.79Fe(2)O(3) (wt%) are densified by cold sintering at 300 degrees C with the assistance of organic salts, including MnC(2)O4 center dot 2H(2)O, FeC2O4 center dot 2H(2)O, and Zn(C2H3O2)(2)center dot 2H(2)O. Excessive Fe2O3 enters into spinel structure forming a solid solution through annealing in low pO(2) at 1350 degrees C. The sintering behaviors, microstructures, magnetic properties and impedances are investigated. The dehydration of organic salts provides mediate liquid phase to trigger the dissolution-precipitation process, which assists the densification of ceramics. The grains grow from 0.15 mu m to 0.52 mu m and 7.67 mu m after cold sintering at 300 degrees C and annealing at 1350 degrees C, respectively. The initial permeability of cold sintered sample is improved to 11000 with a Curie temperature of 125 degrees C. This work provides a feasible route for cold sintering assisted processing of commercial soft magnetic ferrites.

A lot of nanocomposites containing C-60 have been developed to enable various application opportunities. However, it seems impossible to incorporate C-60 into ceramics without destroying the C-60 molecules, because ceramics are typically sintered at high temperatures that could result in the chemical reaction of C-60. Herein, we report the successful integration of C-60 with ZnO using cold sintering process. (1-x)ZnO-xC(60) composites with x ranging from 0 to 5 wt% are densified into monolithic structures with relative densities over 95%. Clear C-60 molecules are observed in the cold sintered composites, and thin C-60 layers with less than 10 nm are located at the ZnO grain boundaries. With C-60 altering the interfacial structures of ZnO ceramics, the electrical properties are improved significantly, especially, the breakdown electric field at 1 mA cm(-2) is enhanced from similar to 80 V mm(-1) to similar to 2100 V mm(-1). The FEM analysis indicates that the current density at the interfaces of ZnO and C-60 is higher than other regions. This work thus indicates that cold sintering process provides a promising pathway to design new types of functional materials through the integration of C-60 and ceramics. (C) 2022 Elsevier Ltd. All rights reserved.

Cold sintering process (CSP) combines transient liquid phase and/or external pressure to assist the densification of ceramics, the sintering mechanism of which is typically dominated by the "dissolution-precipitation" process. In this work, Na2WO4 center dot 2H(2)O dry powders without liquid water have been used as the "liquid phase" to realize the densification of Na2WO4 ceramics. The effects of sintering parameters on the CSP of Na2WO4 ceramics using Na2WO4 center dot 2H(2)O dry powders are systematically studied, including sintering temperature, external pressure, sintering process time and the amount of Na2WO4 center dot 2H(2)O phase. It is found that the "liquid phase," that is, Na2WO4 center dot 2H(2)O is the key factor for controlling the CSP of Na2WO4 ceramics, and Na2WO4 center dot 2H(2)O hydrate shows an advantage of homogeneous dispersion of liquid water. The sintering temperature affects the densification mechanism involved with plastic deformation and the "dissolution-precipitation," external pressure accelerates the densification process, and enough sintering process time ensures the completion of CSP. Finally, the optimum cold sintering parameters of Na2WO4 ceramics by Na2WO4 center dot 2H(2)O powders are obtained at 150 degrees C and 240 MPa for 60 min with a pretreatment at 150 degrees C for 30 min.

In this work, a sintering route named cold sintering assisted two step sintering process (CSP-TS) is presented to prepare rutile TiO2 ceramics with submicron grain sizes. Cold sintering process at 300 ?C with tetrabutyl titanate and water as the liquid phase yields a ?green body? with a relatively high density of -80 %, and finally dense (98.5?99.8 %) rutile TiO2 ceramics with grain sizes of -600 nm can be obtained in the second sintering process at 950-1000 ?C. The microstructural analysis with SEM and TEM indicates that the CSP-TS samples sintered at 950 ?C have an obvious phenomenon of recrystallization, accompanying by a decrease of amorphous phases and a formation of clear grain boundaries. Besides, the rutile TiO2 ceramics prepared by CSP-TS possess excellent microwave dielectric properties with relative permittivity of 92.0?98.4 and Q x f values of 27,800-31,900 GHz. Therefore, it is feasible to utilize CSP-TS to prepare ceramics with small grain sizes at low sintering temperatures.

Crystal water is a kind of water molecules that are incorporated into the crystal structure of hydrated salts, which are generally decomposed at very low temperatures (usually 100 similar to 200 degrees C). Na2WO4 is a compound that could react with H2O forming the Na2WO4 center dot 2H(2)O hydrated crystal salt under certain conditions. In this work, the cold sintering of Na2WO4 ceramics is studied using a Na2WO4-2H2O chemistry at a low sintering temperature of 120 degrees C. Their sintering mechanism has been investigated using SEM, TEM, DSC, FTIR, together with the shrinkage rates of the samples. During the sintering process, the intentionally added liquid water reacts with Na2WO4 powders to produce a Na2WO4 center dot 2H(2)O and Na2WO4 composite, and the crystal water can be decomposed from the Na2WO4 center dot 2H(2)O phase under a certain sintering condition. The water in both liquid and solid forms contributes to the densification of Na2WO4 ceramics. Finally, dense Na2WO4 ceramics with excellent properties (permittivity: 5.7; Q x f: 70 000 GHz; TCF: -70 ppm/degrees C) have been obtained by removing the residual water after a heat treatment. Such sintering process can be generalized to the densification of other inorganic materials that easily react with liquid water to form hydrated salts, which provides a strategy for the cold sintering with crystal water.

The manipulation of interfacial structures offers an effective route to improve the physical and chemical properties of materials. However, it is challenging to design ceramic-based composites with hybrid interfaces involved with organics and inorganics through the conventional sintering technique, due to the incompatibility of these materials at high temperatures. Here, we propose a strategy to integrate poly-ether-ether-ketone together with several metal-oxide additives into zinc oxide (ZnO) to form composite varistors via cold sintering process. Nanoscale layers of hybrid additives are dispersed between densified ZnO grain structures forming Schottky barriers, which dramatically improves the electrical properties of the resulted composites. Compared with pure ZnO, the breakdown electric field at 0.1 mA mm(-2) reaches over 13 kV mm(-1). Particularly, the composite shows a switch-like effect similar with switching devices, with an extraordinarily high nonlinear coefficient of 375. In addition, the elastic module decreases with the addition of PEEK. Given the flexibility in the dopants of polymers and metal oxides, this work provides a unique route to design composite materials with superior performances.

Appropriate preparation routes allow designing and improving the performances of ceramic-based functional composites. In this work, two powder-preparation methods enabled by the cold sintering process are utilized to fabricate unique ZnO-based varistor composites. The thermoplastic polymer, poly-ether-ether-ketone (PEEK), has been successfully integrated with ZnO to form dense ZnO-PEEK composites. With a dissolution method, PEEK particles can be homogeneously dissolved by the mixed solution of tetrahydrofuran and toluene and then form nanoscale thin grain boundaries after cold sintering process. In the direct mixing method, large PEEK particles are observed in the cold sintered samples. A Finite Element Method (FEM) analysis indicates that von-mises stresses concentrate at the grain boundaries of ZnO and at the interfaces of ZnO and PEEK, and these can be increased with the increase of PEEK particle sizes. The electrical properties have been improved with the addition of PEEK, and the cold sintered 95ZnO-5PEEK shows a high breakdown electric field (0.1 mA/mm(2)) of 3070 V/mm, and a nonlinear coefficient of 5. In addition, the conduction mechanism of the composites has been investigated using impedance spectroscopy. Overall, our work provides a strategy for the development of high-performance ceramic-polymer composites via cold sintering process. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In this work, cold sintering was adopted to prepare Na2WO4 ceramics with different grain sizes ranging from 0.632 mu m to 17.825 mu m. Their microstructures, complex impedance, and microwave dielectric properties were studied in-depth. It was found that samples with relative densities higher than 92% can be successfully synthesized by cold sintering process at a low temperature of 240 degrees C. However, their electrical properties have strong dependence on the grain size. Specifically, the resistance of grain boundaries decreases dramatically with the increase of grain sizes, while the quality factor has a positive correlation with the grain sizes of Na2WO4 ceramics. Excellent microwave dielectric properties, including permittivity = 5.80, Q x f = 22,000 GHz, and TCF = -70 ppm/degrees C, are obtained for Na2WO4 ceramics with a grain size of 4.477 mu m prepared by cold sintering process.