Microwave dielectric ceramics (MWDCs) are pivotal to modern wireless communication systems, with their performance governed by three key parameters: relative dielectric constant (ε_r), Q×f value (product of quality factor Q (reciprocal dielectric loss) and frequency f), and temperature coefficient of resonant frequency (τ_f). This review systematically summarizes the recent research progress of MWDCs from five interrelated aspects. In terms of performance characterization, standardized resonant methods achieve ε_r measurement errors below 1% and a tanδ detection limit as low as 10^-5. Theoretically, frameworks from complex crystal chemistry to the recently elucidated cation rattling effect enable quantitative interpretation of dielectric behavior. In processing, the cold sintering process achieves ceramic densification below 300 °C, reducing energy consumption by over 97% in comparison with conventional sintering. For applications, these materials have been widely deployed in high-performance substrates, resonators, and filters for 5G/6G communications, with device insertion loss maintained below 1 dB. Additionally, data-driven approaches, particularly machine learning, can accurately predict key dielectric properties with a coefficient of determination (R²) higher than 0.9, accelerating the exploration and development of novel MWDCs. By integrating these perspectives, this review offers a systematic insight into the state-of-the-art progress and future development directions of MWDCs research.

Aromatic polyimide (PI) with high glass transition temperature (T_g) shows promise as a polymer dielectric for energy storage, but its rigid aromatic structure and electron delocalization cause significant conduction loss, degrading energy storage performance and breakdown strength (E_b) under high temperatures. Herein, we introduce a novel semi-alicyclic fluorinated polyimide (H-FPI) designed via a molecular engineering strategy that synergistically integrates bandgap and topological conformation modulation. Specifically, the alicyclic group elevates the lowest unoccupied molecular orbital (LUMO) while strong electron-withdrawing trifluoromethyl (—CF₃) substitution depresses the highest occupied molecular orbital (HOMO), creating a wide bandgap (4.2 eV). Concurrently, the chair-conformation alicyclic backbone and sterically bulky —CF₃ groups synergistically disrupt molecular planarity, reducing π-orbital overlap to suppress charge transfer while restricting chain mobility to yield a high T_g of 272 ℃. Remarkably, H-FPI film delivers a high energy density of 6.02 J/cm³ with a superior breakdown strength of 626 MV/m at 200 ℃, surpassing commercial PI and fluorinated polyimide (FPI) by 1261% and 55%, respectively. Furthermore, H-FPI film exhibits exceptional capacitor charge-discharge cyclability, enhanced mechanical robustness, and excellent thermal stability. This work establishes a new molecular design paradigm for organic capacitors in electrified transportation and smart grid systems requiring high-temperature working reliability.

Developing high-efficiency sintering technologies with mild conditions is crucial for reducing the energy consumption and manipulating the performance of ceramics. However, sintering ceramics at low temperatures in short times without pressure is challenging because of their high melting points. Inspired by microwave resonance and dissolution‒precipitation phenomena, an energy efficient sintering, microwave cold sintering process (MW-CSP), is proposed here to densify high-performance ceramics with significantly reduced sintering times and temperatures under pressureless conditions during the sintering stage. A range of ceramics, including chlorides, oxides, phosphates, and molybdates, with various applications, have been shown to be well sintered by MW-CSP. The transmission electron microscopy (TEM) and phase-field simulation results demonstrate that the combination of the transient liquid phase and microwave resonance improves the driving force of sintering. Compared with those of other pressureless sintering technologies, the mechanical and dielectric properties of the selected materials are improved by 50%–95%, whereas the energy consumption of MW-CSP is dramatically reduced by more than 97%. These findings highlight the great potential of MW-CSP in efficiently densifying high-performance ceramics, opening up possibilities for energy-saving sintering.

To meet the demands of miniaturization and integration in modern electronic packaging, developing materials with low coefficient of thermal expansion (CTE) is essential to reduce thermal stress and enhance device reliability. In this study, the dense negative thermal expansion ceramic ScF₃ was prepared with a CTE of −8.86 × 10⁻⁶/℃. The ScF₃ ceramic was cold sintered at 150 ℃, exhibiting a low permittivity of 5.3 and a high quality factor (Q×f) of 14,700 GHz. By incorporating ScF₃ to the hexagonal boron nitride (BN) ceramic, the CTE of ScF₃—BN composite ceramic was adjusted to 3.36 × 10⁻⁶/℃, establishing compatibility with silicon-based chips. And finite element simulations verified that ScF₃—BN composite significantly reduces thermal stress compared to Li₂MoO₄ or Al₂O₃ ceramics. Furthermore, this work demonstrates the potential of cold-sintered ScF₃ to regulate thermal expansion in packaging substrates, paving the way for improved performance in next-generation electronic devices.

As electronic devices become increasingly miniaturized and demand greater integration, traditional packaging technologies face substantial challenges in meeting the needs for high-frequency performance and system reliability. Ceramic materials, known for their excellent dielectric properties and thermal stability, are promising candidates for advanced packaging applications. However, conventional high-temperature densification processes, which often exceed 1000 ℃, restrict their compatibility with temperature-sensitive components in modern electronic systems. To overcome this limitation, we propose a novel approach to densify Al₂O₃−H₃BO₃ ceramic at room temperature under low uniaxial stress. It is found that a H₃BO₃ facilitates plastic deformation in the medium of deionized water, enhancing the densification of Al₂O₃−H₃BO₃ ceramics even at minimal uniaxial stress. The resulting material exhibits a high relative density of over 96% and possesses excellent microwave dielectric properties (relative permittivity ε_r: 2.84–5.37; Q × f values: 12,924–69,000 GHz; resonant frequency τ_f values: −156.94 10⁻⁶ ℃⁻¹ to −73.42 10⁻⁶ ℃⁻¹) and thermal conductivity (λ values: 1.96–5.96 W·m⁻¹·K⁻¹). After co-firing with a silicon wafer, the ceramic maintains its structural integrity, with no observable atomic diffusion at the ceramic-silicon interface, rendering it a potential candidate for advanced packaging and integration technologies.

Artificial adaptive soft infrared (IR) materials, mimicking the color-changing abilities observed in soft organisms such as cephalopods, hold significant promise in various emerging technologies, including unconventional flexible displays, intelligent camouflage systems, and advanced sensors. In this study, we integrated inherently deformable liquid metal (LM) microdroplets randomly into an elastomer matrix, creating a fully soft material that exhibits elastic compliance akin to soft biological tissue and adaptive IR-reflecting properties in response to compression. Under compressive strains, each LM inclusion behaves as a unit of dynamic IR reflector, transitioning between a contracted droplet with a corrugated surface and an expanded plate-like filler with a relatively smooth surface. These alterations in shape, size, and surface structure allow dynamic modulation of incident IR radiation's reflection, resulting in reversible changes in IR color (i.e., detected temperature). This mechanism replicates the dynamic alterations observed in cephalopod skin, where chromatophores dynamically manipulate visible light reflection by changing their size and morphology. We demonstrate proof-of-concept applications of this material, showing its ability to modify IR appearance through compression for visualization, with its localized color-change mechanism enabling its use as a tactile sensor in vision-based tactile grippers. These illustrate the potential of this material in emerging adaptive flexible electronics.

Dielectric ceramics are essential components in communication systems that operate within the microwave frequency range. In high-density packages, dielectric substrates ceramics must possess high thermal conductivity to efficiently dissipate heat. However, achieving adequate thermal conductivity (κ) in ceramics sintered at low temperatures is challenging. In this study, we employed the cold sintering process (CSP) to fabricate Li₂MoO₄-x%Al₂O₃ (0≤x ≤ 80, in volume) ceramics under 200 MPa pressure at 150 ℃. The Li₂MoO₄–40%Al₂O₃ composite exhibited significantly enhanced κ of 5.4 W·m⁻¹·K⁻¹, an 80% increase compared to pure Li₂MoO₄ ceramic with κ of 3 W·m⁻¹·K⁻¹. At 40% Al₂O₃ content, the Li₂MoO₄–Al₂O₃ ceramic demonstrated notable microwave properties (ε ~ 6.67, Q×f ~ 17,846 GHz, τ_f ~ −105 × 10⁻⁶ ℃^-1). Additionally, simulation of a microstrip patch antenna for 5 GHz applications using Li₂MoO₄–20%Al₂O₃ ceramic as dielectric substrate via Finite Element Simulation software showed excellent performance, with radiation efficiency exceeding 99% and low return loss (S₁₁ < −30 dB) at both 4.9 GHz and 28.0 GHz center frequencies. These findings underscore the suitability of Li₂MoO₄–Al₂O₃ ceramics for microwave dielectric substrate.

High-temperature polymer nanocomposites with high energy storage density (U_e) are promising dielectrics for capacitors used in electric vehicles, aerospace, etc. However, filler agglomeration and interface defects at high filler loadings significantly limit the enhancement of U_e and hamper the large-scale production of the nanocomposites. Here, polyetherimide (PEI) nanocomposites with nanoscale alumina (AO) at ultra-low contents were prepared via in situ polymerization from PEI monomers. We compared two composite dielectric preparation methods (in situ polymerization and ordinary solution blending) under the same conditions. In contrast to the nanocomposites obtained by blending PEI polymers with AO, the in situ nanocomposites exhibit substantially improved filler dispersion, together with largely suppressed conduction loss at high fields and high temperatures, leading to comprehensive enhancements of breakdown strength (E_b), charge-discharge efficiency (η) and U_e, simultaneously. The 0.3% (in volume) AO filled PEI nanocomposite film exhibits a superior U_e of 4.8 J/cm³ with η of 90% at 150 ℃, which is 128% and 218% higher than those of pristine PEI and the ex situ PEI/AO nanocomposite film under the same conditions, respectively. This work provides a scalable strategy for the preparation of dielectrics with both good processability and excellent high-temperature energy storage performance.

A series of high-k [(Na_0.5Bi_0.5)_xBi_1−x](W_xV_1−x)O₄ (abbreviated as NBWV(x value)) solid solution ceramics with a scheelite-like structure are synthesized by a modified solid-state reaction method at the temperature range of 680–760 ℃. A monoclinic (0 ≤ x < 0.09) to tetragonal scheelite (0.09 ≤ x ≤ 1.0) structural phase transition is confirmed by X-ray diffraction (XRD), Raman, and infrared (IR) analyses. The effect of structural deformation and order–disorder caused by Na⁺/Bi³⁺/W⁶⁺ complex substitution on microwave dielectric properties is investigated in detail. The compositional series possess a wide range of variable relative permittivity (ε_r = 24.8–80) and temperature coefficient of resonant frequency (TCF value, −271.9–188.9 ppm/℃). The maximum permittivity of 80 and a high Q×f value of ~10,000 GHz are obtained near the phase boundary at x = 0.09. Furthermore, the temperature-stable dielectric ceramics sintered at 680 ℃ with excellent microwave dielectric properties of ε_r = 80.7, Q×f = 9400 GHz (at 4.1 GHz), and TCF value = −3.8 ppm/℃ are designed by mixing the components of x = 0.07 and 0.08. In summary, similar sinterability and structural compatibility of scheelite-like solid solution systems make it potential for low-temperature co-fired ceramic (LTCC) applications.

Although many dielectric polymers exhibit high energy storage density (U_e) with enhanced dipolar polarization at room temperature, the substantially increased electric conduction loss at high applied electric fields and high temperatures remains a great challenge. Here, we report a strategy that high contents of medium-polar ester group and end-group (St) modification are introduced into a biodegradable polymer polylactic acid (PLA) to synergistically reduce the loss and enhance U_e and charge-discharge efficiency (η). The resultant St-modified PLA polymer (PLA-St) exhibits an U_e of 6.5 J/cm³ with an ultra-high η (95.4%), far outperforming the best reported dielectric polymers. It is worth noting that the modified molecular structures can generate deep trap centers and restrict the local dipole motions in the polymer, which are responsible for the reduction of conduction loss and improvements in high-temperature capacitive performance. In addition, the PLA-St polymer shows intrinsically excellent self-healing ability and cyclic stability surviving over 500 000 charge-discharge cycles. This work offers an efficient route to next-generation eco-friendly dielectric polymers with high energy density, low loss, and long-term stability.