Dielectric and microwave absorption properties of divalent-doped Na3Zr2Si2PO12 ceramics

The work attempted to develop a new kind of high temperature microwave absorption material. Dense Na₃Zr_1.9M_0.1Si₂PO_11.9 (M?=?Ca²⁺, Ni²⁺, Mg²⁺, Co²⁺, Zn²⁺) and Na₃Zr_2-xZn_xSi₂PO_12-x (x?=?0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reactions for phase, microstructure characterization and dielectric properties, microwave absorption properties analysis. Results show that the complex permittivity increases in all the divalent-doped Na₃Zr₂Si₂PO₁₂ ceramics. Na₃Zr_1.8Zn_0.2Si₂PO_11.8 ceramic exhibits the highest complex permittivity and optimum microwave absorption performance. The lowest reflection loss is -28.1?dB at 9.88?GHz and the bandwidth is 4.14?GHz (8.26–12.4?GHz) with a thickness of 2.1?mm. It indicates that Na₃Zr₂Si₂PO₁₂ ceramic can be chosen as a potential candidate of microwave absorption material and the performance can be enhanced by divalent doping strategy.     

Enhanced electromagnetic absorption properties of Fe-doped Sc2Si2O7 ceramics

Doping transition metal elements in a crystal causes distortion and defects in the lattice structure, which change the electronic structure and magnetic moment, thereby adjusting the electrical conductivity and electromagnetic properties of the material. Fe-doped Sc₂Si₂O₇ ceramics were synthesized using the sol-gel method for application to microwave absorption. The effect of Fe-doped content on the electromagnetic (EM) and microwave absorption properties was investigated in the Ku-band (12.4–18 GHz). As expected, the dielectric and magnetic properties improve substantially with increasing Fe content. Fe doping causes defects and impurity levels, which enhance polarization loss and conductance loss, respectively. Fe replaces Sc atoms in the ScO₆ octahedral structure, creating a difference in spin magnetic moments, which increases the magnetic moment. Moreover, the magnetic coupling of Fe and O atoms occurs at the Fermi level, which benefits magnetic loss. In particular, when the Fe content is 6%, the fabricated Fe-doped Sc₂Si₂O₇ ceramics show an absorption property with absorption peaks located at 14.5 GHz and a minimum reflection loss (RL_min) of ?12.8 dB. Therefore, Fe-doped Sc₂Si₂O₇ ceramics with anti-oxidation and good microwave absorption performance have a greater potential for application in high-temperature and water-vapor environments.     

Targeted design and analysis of microwave absorbing properties in iron-doped SiCN/Si3N4 composite ceramics

The SiCN(Fe)/Si₃N₄ composite ceramics containing C, β-SiC and α-Fe were designed by impregnating polysilazane followed by pyrolysis at 1–5 times with porous silicon nitride matrix. After the heat treatment, these new phases gradually formed and filled in the pores of the matrix to form a large whole. They bond together to facilitate the formation of conductive networks, which can improve the storage and transport capacity of the charge, thus improving the dielectric properties of the material. In this way, the highest dielectric and magnetic loss tangent values were achieved as 0.49 and 0.29 after third and fourth cycles, respectively. The electromagnetic wave can be absorbed ~80–90% in the range of 8–18 GHz with the thickness of 2 mm. The minimum reflectivity was ?12.5 dB at 16 GHz with the thickness of 5 mm, showing >90% electromagnetic wave absorption at this frequency.     

Preparation,mechanical, dielectric and microwave absorption properties of hierarchical porous SiCnw-Si3N4 composite ceramics

Herein, hierarchical porous SiC_nw-Si₃N₄ composite ceramics with good electromagnetic absorption properties were prepared. A porous Si₃N₄ matrix with different pore structures was first prepared by gelcasting-pressureless sintering (G-PLS) and gelcasting combined with particle stabilized foam-pressureless sintering (G-PSF-PLS). SiC_nw was then introduced by catalytic chemical vapor deposition (CCVD). An increase in solid loading (25–40 vol%) decreased apparent porosity (47.7–41.3%) and improved flexural strength (142.19–240.36 MPa) and fracture toughness (2.25–3.68 MPa·m^1/2). The addition of foam stabilizer propyl gallate (PG, 0.5–1.0 wt%) significantly increased apparent porosity (73.2–86.4%) and realized large-sized spherical pores, reducing flexural strength (58.23–38.56 MPa) and fracture toughness (0.75–0.41 MPa·m^1/2). High apparent porosity and large-sized pores facilitated the introduction of SiC_nw. The 25 vol% sample exhibited a reflection loss of ? 14.67 dB with an effective absorption bandwidth of 3.47 GHz, suggesting a development potential in the electromagnetic wave absorption field.     

Effects of Ni2+, Zr4+, or Ti4+ doping on the electromagnetic and microwave absorption properties of CaMnO3 particles

Although CaMnO₃ has been widely studied and used for its thermoelectric properties and giant magnetoresistance effect, little information exists about its application for microwave absorption. In this study, we synthesized CaMnO₃, CaNi_0.05Mn_0.95O₃ CaTi_0.05Mn_0.95O₃ and CaZr_0.05Mn_0.95O₃ with an orthorhombic system using a simple high-temperature solid-phase method. The minimum reflection loss value and effective absorption bandwidth could be efficiently improved due to the enhanced match complex permittivity produced after the Ni, Ti or Zr ions were substituted for Mn ions in CaMnO₃. The minimum reflection loss value increases to ?39.7 dB from ?14.1 dB and the effective absorption bandwidth increases to 4.9 GHz from 2.7 GHz. The magnetic loss results only in a negligible influence on the microwave absorption. The enhancement of microwave absorption properties was primarily due to the stronger polarization effect. When Ni²⁺, Ti⁴⁺, or Zr⁴⁺ is introduced in the CaMnO₃ lattice, the charge balance is broken, and the crystal lattice distortion increases because of the substitutive ions, interstitial ions, oxygen vacancy and exchange effect of Mn³⁺~Mn⁴⁺. The results indicate that CaMnO₃ with reasonable doping at the Mn-site could achieve excellent microwave properties of wide bandwidth, high-efficiency absorption, and adjustable response frequency.     

Cup-stacked carbon nanotubes hybridized Si3N4/Si3N4 composite ceramics for high-effciency microwave absorption with excellent thermal stability

Hybridization between carbon nanotubes (CNTs) and Si₃N₄ is a promising strategy for developing high-temperature microwave absorption (MA) materials for military application. Toward long-life services, it's important to achieve strong MA at a filler loading as low as possible on account of antioxidant protection against CNTs wastage. Herein, cup-stacked CNTs (CSCNTs) have been prepared in porous Si₃N₄ ceramics by chemical vapor deposition (CVD) and then CVD Si₃N₄ has been coated on them, forming CSCNT-Si₃N₄/Si₃N₄ composite ceramics. Results show that CSCNTs possess abundant exposed atomic edges on the outer surface and in the inner channel. Such unique defects not only benefit the impedance match but also cause considerable conductive loss, which helps CSCNT-Si₃N₄/Si₃N₄ with a filler content of only 0.79 wt% to achieve an effective absorption bandwidth (EAB) of 3.74 GHz in the X band at a thickness of 3.5 mm coupled with a minimum reflection loss of ?43.3 dB and an EAB covering the entire Ku band at a thickness of 2.25 mm. The ultralow filler loading generates a high efficiency of CVD Si₃N₄ in protecting CSCNTs against high-temperature oxidation, leading to a steady MA performance for CSCNT-Si₃N₄/Si₃N₄ during 23–1200 °C thermal shock tests in air.     

The effect of oxidation on the mechanical properties and dielectric properties of porous Si3N4 ceramics

The effect of oxidation temperature and time on the microstructures, phase compositions, mechanical properties, and dielectric properties of porous Si₃N₄ ceramics was investigated in the temperature range from 900 °C to 1300 °C for 1 h, 5 h, and 24 h. The weight gain measured either at lower temperature (900 °C) for long time (24 h) or at higher temperature (1300 °C) for 1 h demonstrated that the porous Si₃N₄ ceramics were easily oxidized under the current test conditions. Results showed that the amount of open pores, flexural strength, compressive strength, and dielectric constant all decreased with the increase of oxidation temperature independent upon the oxidation time. The oxidation product SiO₂ was low-temperature quartz in mild condition (low temperature, short time) and cristobalite in severe condition (high temperature, long time). The existence of cracks on the oxide scale was due to the phase transformation of SiO₂ and thermal expansion coefficient mismatch between SiO₂ and Si₃N₄.     

The dielectric and microwave absorption properties of polymer-derived SiCN ceramics

The polymer-derived SiCN ceramics were synthesized at different annealing temperature (900 ～ 1400 °C). The XRD, SEM, FT-IR, Raman and XPS were used to analyze the phase composition and microstructure. The result indicated that the crystallization degree and content of free carbon gradually improved with the increase of annealing temperature. The resistivity, dielectric and microwave absorption properties of the samples were studied at 2 ～ 18 GHz. The resistivity decreased gradually as the annealing temperature rose. The dielectric constant of sample decreased with the increase of frequency in 1 ～ 5 MHz. The existence of free carbon could improve the dielectric properties of polymer-derived SiCN ceramics at high frequency. The reflectance of the sample synthesized at 1100 °C was below ?10 dB (> 90% absorption) in a wide frequency range of 6 ～ 16 GHz and the maximum value of dielectric loss angle tangent was about 0.6 at 16 GHz.     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

Controllable dielectric properties and strong electromagnetic wave absorption properties of SiC/spherical graphite-AlN microwave-attenuating composite ceramics

Fully dense SiC/spherical graphite-AlN microwave-attenuating composite ceramics were manufactured via hot-pressing sintering, in which, apart from the primary SG (spherical graphite) attenuating agent, 5–30 wt% semiconductive α-SiC was employed as the second attenuating agent. The incorporation of SiC contributed to a slightly decreasing electrical conductivity and enhanced polarization relaxation. Controllable complex permittivities were obtained, namely, both the real and imaginary permittivities exhibit first a decrease and then an increase with the SiC addition, and which delivers an optimized impedance matching of the composites. RL_min values below ?10 dB (more than 90% absorption) were achieved by all the composites containing 5–20 wt% SiC with the sample thickness of 1–1.4 mm, and the absorption performance characteristics were significantly tunable by controlling the of SiC content at 8.2–12.4 GHz. Impressively, a superior reflection loss of ?46 dB (1.1 mm) and wide effective absorption bandwidth of 2.1 GHz were achieved at a 5 wt% SiC content, respectively, rendering SiC/SG–AlN composites a potential ultra-thin and highly efficient microwave-attenuating ceramic candidate.     

Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

Microstructure and mechanical properties of Si3N4f/Si3N4 composites with different coatings

The Si₃N₄ coating and Si₃N₄ coating with Si₃N₄ whiskers as reinforcement (Si₃N_4w-Si₃N₄) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si₃N_4f/Si₃N₄ composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si₃N_4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si₃N₄ coating, Si₃N_4w-Si₃N₄ coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed.     

Enhanced bending strength and microwave dielectric properties of Li2MgTi3O8 ceramics by adding Si3N4 reinforcing phase

In the 5G era, the dielectric materials used in microwave electronic components must have not only have good microwave dielectric characteristics but also excellent structural characteristics. Li₂MgTi₃O₈ (LMT) ceramics have excellent microwave dielectric properties; however, their low bending strength limits their further applications in the 5G era. In this work, the dielectric properties and bending strength of LMT ceramics were optimized by the addition of Si₃N₄ reinforcing phase using a solid-phase method, and the effects of Si₃N₄ addition on the sintering properties, microscopic structure, crystalline phase, dielectric properties and bending strength of ceramics were investigated. The X-ray diffraction pattern indicates that all ceramics exhibit spinel structure. Combined with the phenomenon of grain reduction in the SEM graph, it indicates that the addition of Si₃N₄ can inhibit the grain growth and achieve the purpose of fine-grain strengthening. The dispersion enhancement of second phase particles is also one of the reasons for the increase of bending strength. LMT ceramics doped with 0.5 wt% Si₃N₄ exhibited the maximum bending strength after sintering at 1050 °C for 4 h, which was 76.97% higher than that of pure LMT ceramics. In addition, the ceramics exhibited outstanding dielectric properties: a dielectric constant of 23.20, quality factor of 49344 GHz, and temperature coefficient of ?5.90 ppm/°C. The high bending strength and good microwave dielectric properties indicate that Si₃N₄-added LMT ceramics can be effectively applied in the 5G era.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Mechanical and dielectric properties of spark plasma sintered Si3N4/SiO2/Eu2O3 composite

In this work, effect of Eu₂O₃ (by 0, 3, 5, and 7 wt%) on mechanical and dielectric properties of Si₃N₄–SiO₂ composites (denoted by E0, E3, E5, E7) was studied. Samples were sintered by spark plasma at 1750 °C - 15 min by 100 °C/min heating rate and 40 MPa pressure under a vacuum atmosphere. The SEM micrographs confirmed the β-Si₃N₄ rod-shaped grains growth. The α to β phase transformation was completed in samples. The E5 and E7 samples have about 180.8 MPa flexural strength due to β-Si₃N₄ rod-shaped grains and increased grain aspect ratio compared to other samples. With the increase of Eu₂O₃ additive, due to an increase in grain size and β phase concentration the average dielectric constant and dielectric loss at 10 GHz frequency growth from 6.6 to 8 and from 0.125 to 0.160, respectively. High hardness values due to the dissolution, diffusion, and precipitation mechanism, and the α phase concentration was developed in these samples.     

Dielectric permittivity and microwave absorption properties of transition metal Ni and Mn doped SiC nanowires

In order to enhance the microwave absorption properties of SiC nanowires, two transition metals Ni and Mn were selected as doping elements to improve their electromagnetic parameters. The experimental results indicate that Ni and Mn as catalysts reduce the stacking defect density of SiC nanowires, which will weaken the interfacial polarization loss induced by stacking defects. However, they can increase the electrical conductivity of SiC nanowires and generate new impurity defects, thereby effectively improving the conductance loss and dipole polarization loss. Therefore, the dielectric loss of SiC nanowires is significantly enhanced, but they still do not have considerable magnetic loss capability. In addition, Ni and Mn doping also improves the impedance matching characteristics of SiC nanowires. Therefore, the microwave absorption ability of SiC nanowires is effectively enhanced. As the nanowire filling ratio is 20 wt%, the minimum reflection loss of the Ni_0.01Si_0.99C nanowire is −11.1 dB and the effective absorption bandwidth is 1.1 GHz (9.3–12.4 GHz) at a thickness of 2.8 mm; Mn_0.01Si_0.99C nanowires have a minimum reflection loss of −16.8 dB and an effective absorption bandwidth of 3.1 GHz (9.3–12.4 GHz) at a thickness of 2.8 mm.     

Structure,magnetic properties and microwave absorption properties of NdFe1-xNixO3

In this paper, a high-purity NdFe_1-xNi_xO₃ perovskite-type material was prepared by a simple sol method. At the same time, adjust the substitution content of nickel to achieve the purpose of adjusting the dielectric properties and magnetic properties. According to the respective instruments, as Ni is substituted into the NdFeO₃, the crystal microstructure will change to a certain extent, and there is a certain causal relationship between the magnetic properties and the bonding. Therefore, by adding a certain amount of nickel, the dielectric properties and magnetic properties can be adjusted to a certain balance point. NdFe_1-xNi_xO₃ material has excellent microwave absorption performance. When x = 0.2, the minimum reflection loss value is ?49.32, and the corresponding impedance matching value is 1, and the effective bandwidth is 2.2 GHz when the thickness is 5.0 mm. The material that adjusts the perovskite structure by Ni element is beneficial to make the microwave absorption peak move from high frequency to low frequency, which has a wider application range and is closer to civil, commercial, military and aerospace.     

Fabrication and properties of Si2N2O-Si3N4 ceramics via direct ink writing and low-temperature sintering

Direct ink writing (DIW) and low-temperature sintering methods were applied to prepare Si₂N₂O-Si₃N₄ ceramics for radome materials. Lattices of Si-SiO₂ green body were printed by DIW with 78 wt % solid portion of water-based Si-SiO₂ slurry, in which silicon particles and silica fume were used as the solid portion and Methylcellulose (HPMC) was used as the dispersant. Effects of HPMC addition on stability and silica fume content on rheological properties of the slurry were studied, respectively. The pseudoplastic mechanism of the slurry was analyzed. The Si-SiO₂ green bodies were sintered at 1250 °C–1400 °C in nitrogen. The effect of temperature on phase composition, microstructure, mechanical and dielectric properties of samples was investigated. With the HPMC addition of 0.12 wt% and the silica fume proportion of 30 wt% in solid portion, a stable and pseudoplastic slurry with the yield stress of 110.9 Pa was obtained, which is suitable for DIW. With the decrease of initial holding temperature, more N₂ enters the sample and reacts with silicon and silica fume, promoting the generation of Si₂N₂O and Si₃N₄. The optimal condition yields Si₂N₂O-Si₃N₄ ceramics with apparent porosity of 42.73%, compressive strength of 24.7 MPa, dielectric constant of 4.89 and loss tangent of 0.0054. It is found that columnar Si₃N₄ comes from a direct reaction between silicon and N₂, and fibrous Si₂N₂O is mainly generated by the reaction between silicon, SiO(g), and N₂ through the chemical vapor deposition mechanism. Good dielectric properties are achieved due to high porosity, high proportion of Si₂N₂O phase and no residual silicon.     

Dielectric properties of conventional and microwave sintered Lanthanum doped CaCu3Ti4O12 ceramics for high-frequency applications

The colossal dielectric response of La-doped CaCu₃Ti₄O₁₂ ceramics has been probed at room temperature for a frequency of 1Hz–20 MHz. In this work, the La-doped (CaCu₃Ti₄O₁₂)_x samples for x = 0.1, 0.2, and 0.3 have been sintered at 1100 °C using two different heating modes. SEM and EDS analysis investigated the microstructural chrysalis, grain size distribution, and the inhibitions of Cu-rich phase segregation into grain boundaries by the effect of La³⁺. The presence of main cubic single-phase of CCTO and the diminutive Bragg peak shift due to ion size effect of La³⁺ and Ca²⁺ have been identified by XRD for both conventional (CS) and microwave sintered (MWS) samples. XPS study revealed the effect of La³⁺ on the binding energies of Cu and Ti in CCTO. The dielectric properties namely dielectric constant (?), tan δ, and dielectric relaxation peaks were measured using BDS in which CS and MWS La-doped samples demonstrated (?) ～ >10⁴ and ～ >10³ along with low tan δ for x ≥ 0.1 at medium and high frequency (10⁴–10⁷Hz) than pure CCTO.