In situ growth of one-dimensional carbon-rich SiC nanowires in porous Sc2Si2O7 ceramics with excellent microwave absorption properties

For enhancing the absorption ability of dielectric and electromagnetic wave (EMW), C-rich SiC NWs /Sc₂Si₂O₇ ceramics are successfully fabricated through in-situ growth of SiC nanowires (NWs) into porous Sc₂Si₂O₇ ceramics by precursor infiltration and pyrolysis (PIP) at 1400?°C in Ar. SiC NWs are in-situ formed in the pore channels via a vapor-liquid-solid (VLS) mechanism, the relative complex permittivity increases notably with the content of absorber (C-rich SiC NWs), which tune the microstructure and dielectric property of C-rich SiC NWs/Sc₂Si₂O₇ ceramics. Meanwhile, the minimum reflection coefficient (RC) of C-rich SiC NWs/Sc₂Si₂O₇ ceramic decreases from ?9.5?dB to ??35.5?dB at 11?GHz with a thickness of 2.75?mm, and the effective absorption bandwidth (EAB) covers the whole X band (8.2–12.4?GHz) when the content of absorber is 24.5?wt%. The results indicate that Sc₂Si₂O₇ ceramics decorated with SiC NWs and nanosized carbon have a superior microwave-absorbing ability, which can be contributed to the Debye relaxation, interfacial polarization and conductivity loss enhanced by in-situ formed SiC NWs and nanosized carbon phases. The C-rich SiC NWs /Sc₂Si₂O₇ ceramics can be a promising microwave absorbing materials within a broad bandwidth.     

Si3N4-BN-SiCN ceramics with unique hetero-interfaces for enhancing microwave absorption properties

SiCN-based ceramics with broadband and strong microwave absorption properties are desired for the structural and functional integration of ceramic matrix composites. The elemental composition and thermal expansion coefficients of the ceramics matrix crucially affect its microstructure and electromagnetic wave (EMW) absorption properties. BN layer with lower electrical conductivity and higher specific area, exhibits both the impedance matching characteristic and EMW attenuation in the process of multiple reflections, electrical conductivity loss, dipole polarization and interfacial polarization. Therefore, Si₃N₄-BN-SiCN ceramics, which were synthesized using chemical vapor infiltration (CVI) method, construct unique hetero-interface of Si₃N₄-BN, Si₃N₄–SiCN and BN-SiCN. Therefore, the Si₃N₄-BN-SiCN ceramics have outstanding EMW absorption performance and realize an effective absorption bandwidth (EAB) that covers the whole X band and the minimum reflection coefficient (RC) reaches -18.43 dB at a thickness of 3.37 mm.     

A novel low-firing microwave dielectric ceramic Li2ZnGe3O8 with cubic spinel structure

A Li₂ZnGe₃O₈ ceramic was investigated as a promising microwave dielectric material for low-temperature co-fired ceramics applications. Li₂ZnGe₃O₈ ceramic was prepared via the conventional solid-state method. X-ray diffraction data shows that Li₂ZnGe₃O₈ ceramic crystallized into a cubic spinel structure with a space group of P4₁32. Dense ceramic with a relative densities of 96.3% were obtained when sintered at 945 °C for 4 h and exhibited the optimum microwave properties with a relative permittivity (ε_r) of 10.3, a quality factor (Q × f) of 47,400 GHz (at 13.3 GHz), and a temperature coefficient of resonance frequency (τ_f) of −63.9 ppm/°C. The large negative τ_f of Li₂ZnGe₃O₈ ceramic could be compensated by rutile TiO₂, and 0.9Li₂ZnGe₃O₈–0.1TiO₂0·1TiO₂ ceramic sintered at 950 °C for 4 h exhibited improved microwave dielectric properties with a near-zero τ_f of −1.6 ppm/°C along with ε_r of 11.3 and a Q × f of 35,800 GHz (11.6 GHz). Moreover, Li₂ZnGe₃O₈ was found to be chemically compatible with silver electrode when sintered at 945 °C.     

Fabrication and optical properties of transparent LaErZr2O7 ceramic with high excess contents of La and Er

In this study, transparent LaErZr₂O₇ ceramic with high excess La and Er contents (nominally La_1.28Er_1.28Zr₂O_7.84) was successfully prepared by vacuum sintering at 1850?°C for 6?h using nanosized powder. The XRD, SEM, EDX and TEM results reveal that the single pyrochlore phase in the powder sample transforms to the coexistence of La-rich pyrochlore phase and Er-rich defect fluorite phase after high temperature sintering. The high excess amounts of La and Er favor the formation of pyrochlore structure. Despite the coexistence of two phases, the sample with 1?mm thickness shows excellent in-line transmittance in the visible to mid-infrared region (as high as 81% at 1100?nm). The upconversion and infrared emission under 980?nm exciting were measured and discussed as well.     

Controlled synthesis of Fe3O4@SnO2/RGO nanocomposite for microwave absorption enhancement

A ternary nanocomposite of Fe₃O₄@SnO₂/reduced graphene oxide (RGO) with different contents of SnO₂ nanoparticles was synthesized by a simple and efficient three-step method. The transmission electron microscopy and field emission scanning electron microscopy characterization display that plenty of Fe₃O₄@SnO₂ core–shell structure nanoparticles are well distributed on the surface of RGO sheets. The X-ray diffractograms show that the products consist of highly crystallized cubic Fe₃O₄, tetragonal SnO₂ and disorderedly stacked RGO sheets. The magnetic hysteresis measurement reveals the ferromagnetic behavior of the products at room temperature. The microwave absorption properties of paraffin containing 50 wt% products were investigated at room temperature in the frequency range of 2–18 GHz by a vector network analyzer. The electromagnetic data show that the maximum reflection loss is −45.5 dB and −29.5 dB for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. Meanwhile, the reflection loss less than −10 dB is up to 14.4 GHz and 13.8 GHz for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. It is believed that such nanocomposite could be used as promising microwave absorbers.     

Synthesis of hierarchical core-shell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance

A ternary functional composite NiFe₂O₄@MnO₂@graphene was synthesized successfully via a facile method. The phase constitution, microstructures, morphologies and chemical compositions of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). It was observed that the NiFe₂O₄ nanoparticles were coated by hierarchically MnO₂ shells and distributed on the surface of graphene. Investigations of EM wave absorption indicated that NiFe₂O₄@MnO₂@ graphene composite has the strongest reflection loss of −47.4 dB at 7.4 GHz at the matching thickness of 3 mm, compared to NiFe₂O₄ and NiFe₂O₄@MnO₂, and its maximum absorption bandwidth (<−10 dB) is 4.3 GHz (from 5.1 to 9.4 GHz). The enhanced microwave absorption performance can be attributed to the hierarchical structure of MnO₂, void space between MnO₂ and graphene, and better impedance matching of ternary composite. The above results indicate that the novel hierarchical NiFe₂O₄@MnO₂@graphene composite, with intense absorption and wide absorption bandwidth, would be a promising absorber with less EM wave interference.     

A novel approach of synthesizing nano Y2O3 powders for the fabrication of submicron IR transparent ceramics

A facile methodology of synthesizing highly reactive, round-edged, Sulfur–free nano Y₂O₃ powders to fabricate submicron IR transparent yttria ceramics having a unique combination of superior optical and mechanical properties are reported for the first time. Dispersion of yttrium hydroxide into aqueous sol and addition of seed particles produced near-spherical yttria powders having non – aggregated particles with narrow size distribution. The powder exhibited excellent sinterability reaching near-theoretical density at temperatures around 1400 °C in air. Effective inter-particle coordination and traces of Al additives assisted achieving superior densification. Sintered specimens showed average grain sizes closer to 700 nm. Post-sinter hot isostatic pressing eliminated the residual porosity from the sintered samples leading to exhibit IR transmissions up to 84% in the 2.0–9.0 μm regions, equivalent to single crystal Y₂O₃. Achieving densification through solid-state sintering and retaining the sintered grain sizes in the submicron regions significantly enhanced the mechanical properties. Sintered and HIPed Y₂O₃ specimens were further characterized for their thermal properties at temperature regions between ambient to 950 °C.     

Embedded SiO2 interface in SiCnws/Ba0.75Sr0.25Al2Si2O8 ceramic with enhanced electromagnetic absorption at elevated temperature

For high-temperature electromagnetic absorption materials, higher polarization loss is needed to balance the impedance mismatch due to greater conduction loss at elevated temperatures. Here, a SiO₂ interface was introduced into a SiCnws/BSAS ceramic based on wide bandgap and low dielectric constant characteristics of SiO₂. The interface structure was tailored by changing the SiO₂ content. When the SiO₂ content reached 15 vol%, three-phase interlaced interfaces were formed, which produced many nano-heterointerfaces that increased the polarization loss by 77.5 %. The optimized SiCnws/SiO₂-BSAS ceramic achieved enhanced electromagnetic absorption from 298 K to 873 K, and its effective absorption bandwidth reached 4.1 GHz at 873 K. The electromagnetic absorption mechanism was analyzed from the perspectives of electron transport and space charges. This heterointerface design strategy provides a new method for the development of high-temperature electromagnetic absorption materials.     

A novel ultralow-loss Sr2CeO4 microwave dielectric ceramic and its property modification

A new ultralow-loss Sr₂CeO₄ microwave dielectric ceramic was prepared via a conventional solid-state method. The X-ray diffraction and Rietveld refinement results demonstrate that pure-phase Sr₂CeO₄ ceramics belong to the orthorhombic structure with a Pbam space group. Scanning electron microscopy analysis reveals dense and homogeneous microstructure. Optimum microwave dielectric properties of ε_r = 14.8, Q × f = 172,600 GHz (9.4 GHz) and τ_f = -62 ppm/°C were obtained as it was sintered at 1270 °C for 4 h. In addition, the substitution of a few amount of Ti⁴⁺ for Ce⁴⁺ was found to have significant influences on the grain morphology, sintering behavior, phase structure and microwave dielectric properties. Among them, the Sr₂Ce_0.65Ti_0.35O₄ ceramic sintered at 1350 °C for 4 h demonstrates near-zero τ_f of -1.8 ppm/°C, ε_r of 20.7 and Q×f of 115,550 GHz (8.1 GHz) because of its two-phase structure, showing large application potentials.     

Investigation on microwave absorption characteristics of ternary MWCNTs/CoFe2O4/FeCo nanocomposite coated with conductive PEDOT-Polyaniline Co-polymers

In this study, ternary MWCNTs/CoFe₂O₄/FeCo nanocomposite coated with conductive PEDOT-polyaniline (PA@MW/F/C) co-polymers were synthesized by microwave-assisted sol-gel followed in-situ polymerization methods. The phases, crystal structures, morphologies, magnetic and electromagnetic features of the as-prepared samples were identified via XRD, SEM, XPS, VSM, and VNA respectively. Absorption characteristics were investigated in the frequency (12–18 GHz) Ku band. XRD, VSM and SEM analysis confirmed the partial reduction process of CoFe₂O₄ and successfully decorated magneto-dielectric particles with co-polymers. By measuring electromagnetic features of the samples, it was found that coating magneto-dielectric particles with conductive co-polymers improved the permittivity and dielectric constant, accordingly affecting the impedance matching characteristic and attenuation constant performance. Moreover, exchange coupling behavior was found significant impacts on the microwave absorption properties. PA@MW/F/C coated nanocomposite revealed the maximum reflection loss of ?90 dB at 13.8 GHz with 4 GHz effective bandwidth and 1.5 mm thickness. Due to the enhanced interfacial polarization, impedance matching and exchange coupling effects of the as-prepared nanocomposite, it owns excellent microwave absorption properties, which can be applied as an absorber with distinguishing features (strong absorption, thin thickness, and broadest effective bandwidth).     

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ ceramics have been obtained via solid-state reaction method for passive integration use. The Na₂Mo₂O₇ and Na_0.5Bi_0.5MoO₄ crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the ε_r increases from 16.0 to 32.0 and the τ_f value varies from ?58 to 47 ppm/°C. At x = 0.75, the 0.25Na₂Mo₂O₇-0.75Na_0.5Bi_0.5MoO₄ ceramic sintered at an ultra-low sintering temperature of 580 °C can be densified (>96%) and possesses good microwave dielectric properties of an ε_r of 24.0, a Q × f value of 13,000 GHz (at 6.2 GHz), and a τ_f value of 3 ppm/°C. The theoretical ε_r and τ_f of the (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ composites were calculated using the mixing law and in accordance with the measured values.     

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.     

A novel high-Q oxyfluoride Li4Mg2NbO6F microwave dielectric ceramic with low sintering temperature

Novel low-fired Li₄Mg₂NbO₆F ceramics were synthesised using a conventional solid-state reaction method. X-ray diffraction and Rietveld refinement confirmed that the Li₄Mg₂NbO₆F compound had a face-centred-cubic rock salt structure [Fm-3 m(225)] above 625 °C. Li₄Mg₂NbO₆F ceramics sintered between 875 °C and 950 °C displayed the optimised density (> 97.5 %). The theoretical ε_theo was calculated based on the refined crystal parameters, closing to the measured ε_r. The ceramic sintered at 900 °C exhibited excellent microwave dielectric properties with ε_r of 15.53 ± 0.03, Q × f value of 93,300 ± 1100 GHz (at 7.7 GHz) and τ_f value of ?39.8 ± 0.8 ppm/°C. The compatibility with Ag powders makes the oxyfluoride a potential candidate for LTCC applications.     

Chemical bond characteristics and infrared reflectivity spectrum of a novel microwave dielectric ceramic CaIn2O4 with near-zero τf

Orthorhombic-structured CaIn₂O₄ ceramics with a space group Pca2₁ were synthesized via a solid-state reaction method. A high relative density (95.6 %) and excellent microwave dielectric properties (ε_r ～11.28, Qf = 74,200 GHz, τ_f ～ ?4.6 ppm/°C) were obtained when the ceramics were sintered at 1375 °C for 6 h. The dielectric properties were investigated on the basis of the Phillips–Van Vechten–Levine chemical bond theory. Results indicated that the dielectric properties were mainly determined by the InO bonds in the CaIn₂O₄ ceramics. These bonds contributed more (74.65 %) to the dielectric constant than the CaO bonds (25.35 %). Furthermore, the intrinsic dielectric properties of the CaIn₂O₄ ceramics were investigated via infrared reflectivity spectroscopy. The extrapolated microwave dielectric properties were ε_r ～10.12 and Qf = 112,200 GHz. Results indicated that ion polarization is the main contributor to the dielectric constant in microwave frequency ranges.     

Construction of core-shell BaFe12O19@MnO2 composite for effectively enhancing microwave absorption performance

BaFe₁₂O₁₉, a traditional ferrite, has always been extensively investigated as a microwave absorption material because of the application value. Herein, the core-shell BaFe₁₂O₁₉@MnO₂ composite was designed and constructed successfully by a facile hydrothermal method. By introducing nanostructured MnO₂, a typical dielectric loss medium, the electromagnetic wave absorption performance is effectively enhanced. The core-shell structure contributes to high interface polarization, thereby promoting the attenuation of electromagnetic waves. In addition, the optimal temperature of the hydrothermal reaction was explored through the characterization of the morphology and the analysis of microwave absorption performance. The result exhibits that the maximum reflection loss of the prepared BaFe₁₂O₁₉@MnO₂-170 reaches -54.39 dB and the effective absorbing bandwidth reaches 4.64 GHz. The simple preparation method and attractive performance make BaFe₁₂O₁₉@MnO₂ a promising candidate as microwave absorbers.     

Fabrication and characterization of Yb2Si2O7-based composites as novel abradable sealing coatings

Ceramic matrix composites (CMCs) have gradually replaced superalloys for use in hot sections of aero-engines to meet the requirements for increasingly high engine temperatures. However, the abradable sealing coatings (ASCs) commonly applied to superalloys are not suitable for CMCs owing to factors such as the difference in the coefficient of thermal expansion, therefore it is necessary to develop a new system suitable for CMCs. Yb₂Si₂O₇ exhibits good high-temperature phase stability and relatively low hardness and may be a suitable material for ASCs. In the present study, Yb₂Si₂O₇-based composite coatings with different hexagonal boron nitride (hBN) contents (0, 5, 10, and 15 wt%) were deposited by air plasma spray (APS), and the microstructure, mechanical properties, and abradability of the coatings were characterized. The results showed that the system of Yb₂Si₂O₇ combined with hBN exhibits lower hardness, lower friction coefficient, higher wear rate, and smaller IDR value than the pure Yb₂Si₂O₇ coating and is appropriate for use in ASCs. However, excessive hBN caused severe wear of the coatings. The relevant wear mechanisms of the coatings were analysed, and this study may provide guidelines for the development of ASCs suitable for CMCs.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

Fabrication of practically pure Si2N2O ceramic with high performance from amorphous BN surface modified nano-sized Si3N4 powders

In this study, amorphous nano-sized Si₃N₄ powders were surface modified by BN. Then a stable and dense Si₂N₂O ceramic was fabricated using the BN surface modified powders, rather than Si₂N₂O-Si₃N₄ composites usually prepared from nano-sized Si₃N₄ powders without surface modification. The effect of BN surface modification on phase transformation, microstructure and mechanical properties were also investigated. Si₂N₂O ceramics obtained by means of the present method have no residual Si, crystal SiO₂ and other oxide additives, which are usually produced by other methods and may seriously influence high-temperature structural and functional applications of Si₂N₂O ceramics.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Enhanced dielectric and microwave absorption properties of Cr/Al2O3 coatings deposited by low‐power plasma spraying

Low‐power plasma‐sprayed Cr/Al₂O₃ coatings have been developed for their potential application as broad bandwidth, thin thickness, lightweight, and strong microwave‐absorbing materials. The dielectric and microwave absorption properties of the as‐sprayed coatings were studied in the X‐band (from 8.2 to 12.4 GHz). High complex permittivity of the coatings was obtained because of a large number of internal boundaries and the conductive networks. Meanwhile, a significant enhancement of microwave absorption properties of the coating was achieved due to the enhanced interfacial polarization and conductance loss. The reflection loss (RL) <?10 dB of the Al₂O₃–15Cr coating was obtained from 9.8 to 11.4 GHz by choosing an appropriate coating thickness, and an optimal minimum reflection loss (RL_min) of ?45.35 dB was achieved at 10.3 GHz with a thin thickness of 1.32 mm.