Bismuth ferrite-based lead-free multilayers with a high recoverable energy density of 6.74 J cm−3 and an efficiency of 77%, stable up to 125 °C.

Ternary system, (1Àx)Pb(Hf 1Ày Ti y )O 3 -xPb(Mg 1/3 Nb 2/3 )O 3 (x = 0-0.5, y = 0.45-0.70) was prepared using two-step precursor method. The phase structure, dielectric, piezoelectric, and ferroelectric properties of the ceramics near morphotropic phase boundary (MPB) have been investigated systematically. On the basis of the results of X-ray powder diffraction and dielectric-temperature measurement, the MPB region and isothermal map of Curie temperature (T C ) for the ternary system were obtained. The optimum piezoelectric and electromechanical properties were achieved in the MPB composition 0.8Pb (Hf 0.445 Ti 0.555 )O 3 -0.2Pb(Mg 1/3 Nb 2/3 )O 3 , with piezoelectric d 33 being on the order of 680 pC/N, while the maximum high-field piezoelectric d 33 * of 780 pm/V was observed for 0.82Pb (Hf 0.445 Ti 0.555 )O 3 -0.18Pb(Mg 1/3 Nb 2/3 )O 3 with tetragonal composition, due to the high extrinsic contribution induced by domain wall motion. In addition, the fracture toughness K IC and flexural strength r f were measured and found to be 1.2 MPa·m 1/2 and 71.4 MPa, respectively, comparable to PZT-based ceramics.

A cold sintering process (150 °C, 30 min and 200 MPa) was employed to fabricate Na 0.5 Bi 0.5 MoO 4 −Li 2 MoO 4 (NBMO-LMO) composites with up to 96.4% relative density. X-ray diffraction traces, backscattered electron images and Raman spectra indicated the coexistence of NBMO and LMO phases in all composites with no detectable secondary phases. The pemittivity (ε r ) and temperature coefficient of resonant frequency (TCF) decreased, whereas microwave quality factor (Q × f) increased, with increasing weight % LMO. Near-zero TCF was obtained for NBMO-20 wt %LMO with ε r ∼ 17.4 and Q × f ∼ 7470 GHz. Functionally graded ceramics were also fabricated with 5 ≤ ε r ≤ 24. To illustrate the potential of these cold sintered composites to create new substrates and device architecture, a dielectric graded radial index lens was designed and simulated based on the range of ε r facilitated by the NBMO-LMO system, which suggested a 78% aperture efficiency at 34 GHz.

Dense Li6B4O9 microwave dielectric ceramics were synthesized at low temperature via solid state reaction using Li2CO3 and LiBO2. Optimum permittivity (r) ~ 5.95, quality factor (Qf) ~ 41,800 GHz and temperature coefficient of resonant frequency (TCF) ~ −72 ppm/ o C were obtained in ceramics sintered at 640 o C with a ultra-small bulk density ~ 2.003 g/cm 3 (~ 95% relative density, the smallest among all the reported microwave dielectric ceramics). Li6B4O9 ceramics were shown to be chemically compatible with silver electrodes but reacted with aluminum forming Li3AlB2O6 and Li2AlBO4 secondary phases. A prototype patch antenna was fabricated by tape casting and screen printing. The antenna resonated at 4.255 GHz with a bandwidth ~ 279 εHz at −10 dB transmission loss (S11) in agreement with simulated results. The Li6B4O9 microwave dielectric ceramic possesses similar microwave dielectric properties to the commercial materials but much lower density and could be good candidate for both antenna substrate and low temperature co-fired ceramics (LTCC) technology.

High‐temperature dielectric polymers are in constant demand for the multitude of high‐power electronic devices employed in hybrid vehicles, grid‐connected photovoltaic and wind power generation, to name a few. There is still a lack, however, of dielectric polymers that can work at high temperature (> 150 °C). Herein, a series of all‐organic dielectric polymer composites have been fabricated by blending the n‐type molecular semiconductor 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTCDA) with polyetherimide (PEI). Electron traps are created by the introduction of trace amounts of n‐type small molecule semiconductor NTCDA into PEI, which effectively reduces the leakage current and improves the breakdown strength and energy storage properties of the composite at high temperature. Especially, excellent energy storage performance is achieved in 0.5 vol.% NTCDA/PEI at the high temperatures of 150 and 200 °C, e.g., ultrahigh discharge energy density of 5.1 J cm−3 at 150 °C and 3.2 J cm−3 at 200 °C with high discharge efficiency of 85–90%, which is superior to its state‐of‐the‐art counterparts. This study provides a facile and effective strategy for the design of high‐temperature dielectric polymers for advanced electronic and electrical systems.