Yuanming Lai

High-entropy Mg(ZrHfTiSnGe)0.2(Nb1–xTax)2O8 (x = 0–1) microwave dielectric ceramics were successfully synthesized via a solid-state reaction method. The effects of Ta5+ substitution and high entropy on sintering behavior, crystal structure, and microwave dielectric properties were systematically investigated. XRD confirms that Hf4+, Ti4+, Sn4+, Ge4+, and Ta5+ successfully entered the crystal lattice, forming a single-phase solid solution with a wolframite structure (P2/c space group). SEM images show that grains grow well, the grain size is uniform, and there are few pores. HRTEM images show that the sample has both structural long-range order and local lattice distortion. The dielectric constant (εr) increases with Ta5+ substitution, which is primarily attributed to the higher ionic polarizability of Ta5+ and the cation “under-bonding” effect. The continuous decline in the quality factor (Q × f) value is closely related to the reductions in both relative density and lattice energy. This is further supported by the broadening of the Nb/Ta–O symmetric stretching vibration peak in Raman spectra, indicating enhanced phonon interactions. Most importantly, the temperature coefficient of resonant frequency (τf) improves to −10 ppm/°C, benefiting from the increase in bond energy and the weakening of [AO6] (A = Mg, Zr, Hf, Ti, Sn, and Ge) octahedral distortion. Compared with single Ta5+ substitution (MgZrTa2O8: τf = −34 ppm/°C) and single high-entropy (Mg(ZrHfTiSnGe)0.2Nb2O8: τf = −31 ppm/°C) ceramics, the τf value has been greatly improved, indicating that the combined action of Ta5+ substitution and high-entropy strategy has improved the temperature stability of MgZrNb2O8 ceramics.

C-axis textured BaZn0.3Ti0.3Fe11.4O19 M-type hexaferrite was synthesized by solid-state reaction under magnetic field alignment, and its crystallographic site occupation, static and dynamic magnetic properties, and self-biased circulator performance were systematically investigated. At this intermediate substitution level, Rietveld refinement and Raman spectroscopy independently reveal that Zn2+–Ti4+ dopants are distributed across all five Fe3+ sublattices, with the 12k site accommodating the largest individual fraction—marking the onset of a multi-site substitution regime distinct from the selective 4f1/4f2 occupation dominant at lower doping levels. The saturation magnetization reaches 70.01 emu/g with a remanence ratio of 0.84, while the anisotropy field is reduced to 9871 Oe, reflecting a balance between the 4f1/4f2-driven anisotropy reduction and partial compensation by the 12k site. A narrow ferromagnetic resonance linewidth of 428 Oe is obtained at 58 GHz, and surface-integral analysis of the position-dependent demagnetizing factor demonstrates that finite-size-induced broadening contributes less than 9% to the measured linewidth. A slotted-structure self-biased circulator designed with the measured parameters exhibits a center frequency of 31.7 GHz and a 15-dB bandwidth of 2.3 GHz at the Ka-band. These results demonstrate that the intermediate-doping regime, wherein the 12k site becomes the primary dopant recipient, provides an effective route for simultaneously achieving a reduced anisotropy field and a narrow linewidth in substituted M-type hexaferrites for self-biased millimeter-wave circulator applications.

In this work, the phase composition, microstructure, and gyromagnetic properties of LiZn ferrites (Li _0.35 Zn _0.3 Y _x Fe _{2.35 −  x} O ₄ , 0.00 ≤ x ≤ 0.05) prepared at low temperature were systematically investigated. XRD analysis confirmed the formation of LiZn ferrite phase for all samples. In addition, a small amount of BiFeO ₃ and Y ₃ Fe ₅ O ₁₂ secondary phases emerged. SEM images revealed that the incorporation of Y ³⁺ significantly refined the grains and densified the microstructure. Regarding the gyromagnetic performances, the saturation magnetization (4π M _s ) increased from 4417 to 4527.2 Gs, while the ferromagnetic resonance linewidth (Δ H ) decreased notably from 341.7 to 210.1 Oe. We demonstrated that Y ³⁺ substitution is of significant importance for optimizing the core performance metrics for LiZn microwave ferrites.

A series of MgxNi0.25-xCu0.25Zn0.5Fe1.96O3.94 ferrites were successfully synthesized via the conventional solid-state reaction method using 2 wt.% Bi2O3 as sintering aid. The effects of Mg2+ substitution on phase composition, microstructure, gyromagnetic properties, and dielectric performances of the samples were systematically investigated. Structural characterization reveals that no secondary phase is detected in all specimens, and the lattice constant increases with the rise of Mg2+ content. According to the magnetic hysteresis loops, the saturation magnetization (Ms) decreases gradually from 65.74 emu/g to 55.07 emu/g due to the substitution of non-magnetic Mg2+ for Ni2+. In addition, the sample with x = 0.06 exhibits significantly improved density and grain uniformity, with the maximum average grain size of 6.56 μm. Meanwhile, this composition possesses excellent gyromagnetic properties, with the minimum coercivity (Hc) of 119.15 A/m and ferromagnetic resonance linewidth (ΔH) of 208.03 Oe. Dielectric characterization tests demonstrate that the maximum dielectric constant (εᵣ) is 15.95, which decreases continuously with increasing Mg2+ substitution content. The dielectric loss factor (tanδε) reaches the lowest value of 0.54×10-2 at x = 0.06. In conclusion, Mg2+ substitution can effectively optimize the comprehensive performances of NiCuZn ferrites.

Enhanced microwave dielectric properties achieved via configurational entropy engineering.

This study synthesizes a series of nominal composition (Mg_1/2Zn_1/2)_0.4+xLi_0.4(Co_1/2Ni_1/2)_0.4-xTiO₃ (x = 0, 0.1, 0.2, 0.3, and 0.35) novel high-entropy ceramics via the solid-state reaction method. The spinel-structured (Mg, Zn, Li, Co, Ni)₂TiO₄ phase is identified as primary phase, with MgTi₂O₅ as a minor phase. As the x value increases, the ceramic grains gradually evolve towards a layered structure, resulting in an increase of density. The dielectric constant (ε_r) of the samples is primarily influenced by relative density and the degree of dielectric polarization. The quality factor (Qf) exhibits a monotonic increase with the enhancement of A-site covalency, which is further influenced by the relative density. Moreover, the temperature coefficient of resonant frequency (Tf) displays a strong correlation with A-site bond strength and the secondary phase. Notably, the (Mg_1/2Zn_1/2)_0.4+xLi_0.4(Co_1/2Ni_1/2)_0.4-xTiO₃ ceramic with x = 0.2 achieves high densification after sintering at 1200 °C for 4 hours, exhibiting promising microwave dielectric properties (εr = 19.3, Qf = 32,000 GHz (at 8.19 GHz), and Tf = -16 ppm/°C (a 68% improvement)). Therefore, the (Mg_1/2Zn_1/2)_0.4+xLi_0.4(Co_1/2Ni_1/2)_0.4-xTiO₃ ceramics hold potential applications in 5G wireless communications.

V₂O₅ effectively regulates the microstructure, magnetic properties, and gyromagnetic response of Ba0.9Sm0.1Fe11.9Cu0.1O19 by inducing liquid-phase sintering. Moderate V₂O₅ addition (x ≤ 1.0 wt%) contributes to microstructural refinement, promoting uniform grain growth and densification, as evidenced by an increase in density from 4.14 g/cm³ to 4.59 g/cm³. Static magnetic characterization demonstrates that V2O5 doping significantly enhances the coercivity and remanence ratio. Particularly, sample doped with 1.0 wt% V₂O₅ exhibits a distinct c-axis texture, obtaining a remanence ratio Mr/Ms as high as 0.88 (compared to 0.83 of the sample without V2O5), and a maximum remanence of 55.2 emu/g, which is highly beneficial for minimizing the low-field loss of circulator. The ferromagnetic resonance linewidth ΔH measurements are performed through perturbational method at V band (40–44 GHz). Circulators with single-Y and double-Y structures are designed based on the performance evaluation of the BaM sample containing 0.5 wt% V₂O₅. Both structures exhibite good circulator functionality.

This study employs ion radius matching technology to co-dope Si ⁴⁺ sites in Mg ₂ Al ₄ Si ₅ O ₁₈ with (Mn _0.5 Ti _0.5 ) ⁴⁺ , enhancing the [Si ₄ Al ₂ ] and [O ₆ ] hexagonal ring symmetry, synergistically optimizing Q × f (159,681 GHz) and τ _f (−21.075 ppm °C ⁻¹ ).