Impact of microwave sintering on dielectric properties of screen printed Ba0.6Sr0.4TiO3 thick films

The influence of 30 GHz microwave sintering compared to conventional sintering has been investigated on polycrystalline Ba_0.6Sr_0.4TiO₃ (BST60) thick films with respect to an application as tunable dielectrics. The BST thick films were prepared as metal–insulator–metal (MIM) capacitors on alumina substrates. The average grain size (440 nm) and the porosity (approx. 30%) of the sintered films are only little affected by the sintering method. However, permittivity, dielectric loss and tunability have been influenced substantially. The dielectric improvement by microwave sintering is interpreted in terms of an increased crystal quality (ξ_S) and/or a decrease of defect concentrations. It is assumed that microwave sintering preferably heats up parts of the film where an increased defect density exists and therefore causes a selective heating process. This may heal up charged defects, inhomogeneities, and structural defects.     

Magnetic and microwave properties of BaFe12O19 substituted with magnetic,non-magnetic and dielectric ions

Barium hexaferrite particles were synthesized with conventional solid state reaction route. 1% boron (B₂O₃) was added to the initial mixture of oxides to inhibit crystal growth at lower temperatures. Magnetic (Mn²⁺, Co²⁺, Ni²⁺and Cu²⁺), non-magnetic (Zn²⁺) and dielectric (Ti⁴⁺) ions were replaced by one Fe³⁺ ion of barium hexaferrite to shift the ferromagnetic resonance frequency to low frequencies and to increase the magnetic and dielectric losses. The structural and morphological characterization of samples was done by X-ray powder diffractometer and scanning electron microscopy. Magnetic and microwave properties were determined by vibrating sample magnetometer and vector network analyzer, respectively. The maximum saturation magnetization and the highest reflection losses of −34 dB at 10 GHz, with absoption bandwidth of 1.6 GHz at −20 dB, were observed in Cu²⁺–Ti⁴⁺ and Zn²⁺–Ti⁴⁺ substituted samples. The mechanism of microwave energy dissipation is due to the impedance matching at matching thickness. It was also observed that as the sample thickness increases, the resonance frequency decreases exponentially.     

Effects of ZnO and NiO on material properties of microwave absorptive glass-ceramic tile derived from iron ore tailings

Effects of ZnO and NiO on structural, morphological, magnetic properties and microwave absorption behavior of glass-ceramic tiles derived from iron ore tailings (IOTs) were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), magnetization measurements, fourier transform infrared spectroscopy (FTIR) and complex permittivity and permeability measurements. Gradual replacement of NiO by ZnO was found to lower crystallinity of the glass-ceramic and reduce average grain size of the only crystalline phase indexed as spinel Ni–Zn ferrite. Ni²⁺ ion oscillations at different coordination environments inside glass network led to several notable dielectric losses in 3.8–15 GHz. Ferromagnetic resonance frequency is positively correlated with the amount of Ni²⁺. These Ni-related effects enhanced the maximum microwave attenuation of the glass-ceramic to reach −34.35 dB with its frequency being tunable in 6.70–11.20 GHz.     

Low-loss YIG thick films for microwave applications

We report the fabrication, densification and characterization of polycrystalline free-standing yttrium iron garnet Y₃Fe₅O₁₂ (YIG) thick films in this paper. The thick films were fabricated using a double doctor blade technique from co-precipitated nanocrystalline YIG powders. The sintering temperature of YIG thick films was varied from 1000 to 1400°C. A volume diffusion mechanism is seen to govern the densification process. A high relative density of ~99% could be achieved in a YIG thick film, with a thickness of 80?µm, sintered at a relatively low temperature of 1300?°C. The magnetization value ~1750?Oe, near to the YIG bulk magnetization value and an acceptable low ferromagnetic resonance (FMR) linewidth (?H) of 80?Oe resulted in these high density YIG thick films with microwave device possibilities.     

Microwave dielectric properties of barium substituted screen printed CaBi2Nb2O9 ceramic thick films

In the present study, Aurivillius-structured Ba²⁺ substituted CaBi₂Nb₂O₉ (CBNO) ceramic powder was synthesized by co-precipitation method. The CBNO thick films were delineated by screen printing method on alumina substrates using co-precipitated ceramic powder. The overlay method was adopted to measure the microwave dielectric properties of prepared thick films. Single phase layered perovskite structure of the prepared thick films was confirmed by X-ray Diffraction. The effects of Ba²⁺ substitution on the surface morphology, bonding, and microwave dielectric properties of thick films were systematically presented. The maximum value of microwave dielectric constant for the CBNO thick films at 11.8 GHz is 15.6 for Ba²⁺=0.8 substitution. The shift in the stretching vibration modes of the Nb-O bond of NbO₆ octahedron in the Raman spectra with a substitution of Ba²⁺ in CBNO was observed. The substitution of Ba²⁺ on A-site of CBNO improves the microwave dielectric properties of prepared thick films. This work may provide a new approach to enhance the microwave dielectric performance of Aurivillius-structured ceramic thick films.     

Enhanced Microwave Absorption Properties of Intrinsically Core/shell Structured La0.6Sr0.4MnO3 Nanoparticles

The intrinsically core/shell structured La_0.6Sr_0.4MnO₃nanoparticles with amorphous shells and ferromagnetic cores have been prepared. The magnetic, dielectric and microwave absorption properties are investigated in the frequency range from 1 to 12 GHz. An optimal reflection loss of −41.1 dB is reached at 8.2 GHz with a matching thickness of 2.2 mm, the bandwidth with a reflection loss less than −10 dB is obtained in the 5.5–11.3 GHz range for absorber thicknesses of 1.5–2.5 mm. The excellent microwave absorption properties are a consequence of the better electromagnetic matching due to the existence of the protective amorphous shells, the ferromagnetic cores, as well as the particular core/shell microstructure. As a result, the La_0.6Sr_0.4MnO₃nanoparticles with amorphous shells and ferromagnetic cores may become attractive candidates for the new types of electromagnetic wave absorption materials.     

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.     

Nb5+ ion substitution assisted the magnetic and gyromagnetic properties of NiCuZn ferrite for high frequency LTCC devices

Nowadays, developing nickle zinc ferrites with excellent magnetic and gyromagnetic properties are of great importance for solving the matching problem of 5G communication system. However, much is discussed about soft magnetic properties, but little is reported gyromagnetic properties that is critical for microwave device applications. Herein, Nb⁵⁺ ions substituted Ni_0.29Cu_0.18Zn_0.53Nb_xFe_2-xO₄ (x = 0.00-0.05), possessing high saturation magnetization, approriate initial permeability, high cut-off frequency and low ferromagnetic resonance linewidth (@9.55 GHz), were synthesized by low-temperature firing (900 °C). The phase structure and morphology evolutions were studied in detail. The results of morphology observations revealed that Nb⁵⁺ substitution has significant role in determining produce compact and uniform microstructures of NiCuZn ferrites via suppress the grain growth, which further corresponding enhance the magnetic and gyromagnetic properties. As a result, a uniform and compact grain size can be obtained, corresponding to the change of magnetic and gyromagenetic properties have different trends. Enhanced magnetic and gyromagnetic performance including high initial permeability (μ' = 203 @1 MHz), saturation magnetization (4πM_s = 3966 Gauss) and low ferromagnetic resonance linewidth (ΔH = 203 Oe) of the NiCuZn ferrites is achieved though adjusting Nb⁵⁺ ions substitution. More importantly, this work not only for low temperature co-fired ceramic (LTCC) technology but also for high frequency and microwave frequency devices including phase shifter and radars.     

Optically transparent polymer composites: A study on the influence of filler/dopant on electromagnetic interference shielding mechanism

Composite materials made of polymers and carbon-based ferromagnetic filler are attractive for electromagnetic interference shielding through a combination of reflection and microwave absorption. It is possible to enhance their shielding properties by controlling electrical conductivity, dielectric, and magnetic properties. In this work, the aforementioned properties are tailored to achieve optically transparent films with microwave absorbing properties. Nanocarbon materials, namely carbon nanotubes, graphene nanoribbons (GNR) and their ferromagnetic nanocomposites with Fe₃O₄ and cobalt in PVA-PEDOT:PSS matrix were made and tested in X-band. The highest shielding effectiveness for PVA films with nanocarbon filler was observed for 0.5 wt% GNR − Fe₃O₄ at 16.36 dB (9.7 GHz) with 79.8% transmittance.     

A novel SiC-based microwave absorption ceramic with Sc2Si2O7 as transparent matrix

For enhancing the dielectric and electromagnetic wave (EMW) absorption ability, SiC-Sc₂Si₂O₇ ceramics were fabricated by introducing SiC into porous Sc₂Si₂O₇ ceramics through precursor infiltration and pyrolysis (PIP). The Sc₂Si₂O₇ powders were synthesized by sol-gel method and Sc₂Si₂O₇ ceramics were prepared by pressure-less sintering. The results indicate that SiC nano-crystalline and turbostratic carbon derived from polycarbosilane distributed uniformly in electrically insulating matrix (Sc₂Si₂O₇ matrix), resulting in tunable dielectric permittivity and EMW absorbing properties. Additionally, the content of C-rich SiC can be efficiently adjusted by PIP times. The SiC-Sc₂Si₂O₇ ceramic showed excellent microwave absorption performance when the content of C-rich SiC was 25.3?wt.%. A minimum reflection coefficient of ?51.3?dB was obtained at 9.56?GHz with the specimen thickness of 3.6?mm. The effective absorption bandwidth covered 3.6?GHz (from 8.7 to 12.3?GHz). The excellent microwave absorption abilities of SiC-Sc₂Si₂O₇ ceramic were mainly attributed to uniform distribution of C-rich SiC in Sc₂Si₂O₇ matrix. The special structure can improve the impedance matching and enhance microwave absorption performance. Moreover, the defects, interfaces and conductive network existed in the materials, which can synergistically improve the EMW absorption ability.     

Li2MoO4‐based composite ceramics fabricated from temperature‐ and atmosphere‐sensitive MnZn ferrite at room temperature

The first magnetic ceramic composites manufactured, using the room‐temperature densification method are reported. The samples were prepared at room temperature using Li₂MoO₄ as a matrix and MnZn ferrite with loading levels of 10‐30 vol‐% followed by postprocessing at 120°C. The method utilizes the water solubility of the dielectric Li₂MoO₄ and compression pressure instead of high temperatures typical of conventional solid‐state sintering. Hence, composite manufacturing using temperature‐ and atmosphere‐sensitive materials is possible without special conditions. This was demonstrated with MnZn ferrite, which is prone to oxidation when heat treated in air. Samples manufactured with room‐temperature densification showed no signs of reactivity during processing, whereas reference samples sintered at 685°C suffered from oxidation and formation of an additional reaction phase. The densities achieved with different loading levels of MnZn ferrite with both methods were very similar. Measurements up to 1 GHz showed relatively high values of relative permittivity (21.7 at 1 GHz) and permeability (2.6 at 1 GHz) with 30 vol‐% loading of MnZn ferrite in the samples manufactured by room‐temperature densification. In addition, pre‐granulation is proposed to improve the processability of the composite powders in room‐temperature densification.     

Co substituted BaFe12O19 ceramics with enhanced magnetic resonance behavior and microwave absorption properties in 2.6 – 18 GHz

BaFe_12-xCo_xO₁₉ (BFCO, x ≤ 0.4) ceramics with superior magnetic and electromagnetic (EM) properties were prepared using Co³⁺ substituted BaFe₁₂O₁₉ (BFO) in 2.6–18 GHz. Compared with the ceramics without Co³⁺, the incorporation of 40 mol% Co³⁺ enhances magnetic properties; the saturation magnetization (M_S) is improved by about 55.6 emu·g⁻¹ due to the variety of magnetocrystalline anisotropy. The resonance behaviors of complex permeability are observed and resonance frequency shifts to lower frequency range from 6 to 3 GHz. The minimum reflection loss (RL) of −32.1 dB (<−10 dB) is obtained at 11.2 GHz in 8.5–13.5 GHz at 2.0 mm thickness in the sample with x = 0.4, which suggests that such ceramics is highly promising as effective microwave absorbers for EM applications.     

Effects of Li doping on dielectric properties of Ag(Ta0.5Nb0.5)O3 thick films

Low loss ferroelectric materials have been extensively investigated for the high frequency device applications. Especially, weak frequency dispersion materials with high dielectric permittivity and low loss tangent have enormous potential for electronic components including filters, and embedded capacitors. Ag(Ta_0.5Nb_0.5)O₃ thick films have been prepared by low temperature sintering aid Li₂CO₃ (0, 1, 3 and 5 wt%). Ag(Ta_0.5Nb_0.5)O₃ thick films were characterized by X-ray diffraction analysis and scanning electron microscopy. The dielectric and ferroelectric properties were also investigated. We observed very weak frequency dispersion of dielectric permittivity at the microwave frequency range.     

Structural,dielectric and magnetic properties of (ZnFe2O4/Polystyrene) nanocomposites synthesized by micro-emuslion technique

This work aims to study the effect of polymer on the structure, magnetic and dielectric properties of spinel ferrite composite. Nanocomposites based on polystyrene (PST)/ZnFe₂O₄ were synthesized by using the micro-emulsion method. The novel composites with PST to ZnFe₂O₄ ratios (4:0, 4:1, 4:2, 4:3, 4:4, 0:4) were analyzed by X-ray diffractometer (XRD) which confirms the spinel structure of ZnFe₂O₄ with an average crystallite size of 15.3 nm for pure ZnFe₂O₄ and decreases by increasing the polystyrene concentration. Field Emission Scanning Electron Microscopy (FESEM) gave the optimized results of surface morphology and the crystallite size which are in accordance with XRD data. Fourier Transform Infrared (FTIR) spectra show two main broad metal–oxygen bands corresponding to the intrinsic stretching vibrations of the metal at the tetrahedral site (observed between 837.9 and 1034.3 cm⁻¹) and traces of organic materials were observed at 1499.2 and 1766.4 cm⁻¹, which are associated with CO and CC stretching vibration respectively. O–H stretch of COOH weak acid of the carboxyl group was found at 2978.7 cm^-1. The composite with equal ZnFe₂O₄ to PST ratio (4:4) shows that real part of dielectric constant is independent of frequency at lower frequencies of an applied electric field. The resonance type behaviour was observed at higher frequency (2.5 GHz) which shows the material is excellent for dispersion of electric part of microwaves. The magnetization for pure ferrite (ZnFe₂O₄) at 15000 Oe was found to be 1.49 emu/g which decreases to 0.54 emu/g for the composite with the equal ferrite to polystyrene ratio. Based on their dielectric and magnetic characterization, these composites are considered suitable candidates to employ as microwave absorbing materials.     

Combustion synthesis and electromagnetic properties of nanocrystalline Pb(Zr0.52Ti0.48)O3

The properties of nanocrystalline lead zirconate titanate [PZT, Pb(Zr_0.52Ti_0.48)O₃] thick films and pellets synthesized by self-propagating auto combustion route using sucrose as a fuel are reported. The X-ray diffraction reveals the formation of nanocrystalline tetragonal PZT having a particle size of 5.52 nm in thick films and 7.08 nm in bulk material. SEM images show the grain size of 300–400 nm with some agglomeration, as well as modified open grain boundaries and grain growth in the PZT thick film as compared to the bulk material. The DC electric resistivity of PZT shows semiconducting behavior. We also characterized the 8–18 GHz microwave insertion loss, absorption, complex permittivity, and microwave conductivity of synthesized materials. The real part of permittivity and dielectric loss were found to decrease with increasing frequency.     

Investigation of the magnetic properties of nanometric SrSmCoNi ferrite/PST matrix

Y-type hexa-ferrite Sr_1.8Sm_0.2Co₂Ni_1.50Fe_10.50O₂₂ was synthesized via micro-emulsion route. Ferrite/PST composites were obtained by mixing the different ferrite ratio in the pure PST matrix. The microstructure was examined by scanning electron microscopy (SEM) and exhibited heterogonous distribution of grains. A keen observation of these SEM images revealed that the grain morphology changes noticeably with increasing ferrite filler contents. The electrical modulus, Cole–Cole plots and quality factor of ferrite polymer composites have been investigated in the frequency range (1 MHz to 3 GHz). The field dependent magnetic properties of the prepared samples were investigated at room temperature by using vibrating sample magnetometer (VSM). The shape of hysteresis loops and linearity of M_s, M_r, H_c values vs. ferrite contents unfold that the ferrite nanoparticles are evenly dispersed within the composite. The occurrence of resonance at high frequency suggests that the present investigated composite samples are best candidate for multilayer chip inductors.     

Target grain size dependence of the morphology,crystallinity and magnetic properties of yttrium iron garnet films

In this work, yttrium iron garnet films were prepared on a Ga₃Gd₅O₁₂ substrate by pulsed laser deposition. The dependence of target grain size on the morphology, crystallinity, and magnetism of films was investigated. Firstly, X-ray diffraction revealed that the fine grain size of the target was beneficial for increasing the crystallinity of films. Secondly, scanning electron microscopy and roughness measurements displayed the film prepared by the small grain size of the target has better morphology and lower roughness. Lastly, magnetic hysteresis loops and ferromagnetic resonance measurements further shown that a good microstructure and fine grain size of target facilitated the growth of film with greater magnetic properties. Consequently, YIG film deposited by 1.0?μm-target has magnetic properties with a high saturation magnetization of 155?emu/cm³, a low saturation magnetic field of 1077?Oe, and low ferromagnetic resonance linewidth of 42?Oe.     

Preferred-oriented polycrystalline Y3Fe5O12 films grown on quartz with low microwave loss

Yttrium iron garnet (YIG) is an ideal magnetic insulator for room temperature microwave and spintronic applications. The growth of high-quality YIG films on substrates other than isostructural Gadolinium gallium garnet (GGG) is a primary requirement to integrate with current semiconductor electronics. Here, we are presenting the growth of highly oriented YIG films on amorphous quartz substrates using pulsed laser deposition. Structural characterization suggests the growth of pure polycrystalline garnet phase with preferred (100) orientation. The 430-nm thick YIG film shows a saturation magnetization (4πM_S) of 1620 (92% of the bulk value of YIG) with a very small coercive field of 6 Oe and a minimum value for ferromagnetic resonance linewidth of 78 Oe for the magnetic field applied perpendicular to the film plane. Estimated damping constant shows reasonably low value, beneficial for realizing the applications integrated with semiconductor technology. Importantly, the formation of YIG with preferred orientation could unveil enormous opportunities in the preparation of textured or single crystal materials on amorphous substrates.     

甚高频段应用的Co2-Z型铁氧体材料研究

采用普通陶瓷工艺，制备了适用于甚高频段（UHF）的Co2-Z型六角晶系铁氧体材料。通过测量这种材料的复数磁导率μ的频散特性，发现其共振频率可达3GHz以上。测试不同烧结温度下的样品的磁谱表明，随着烧结温度的上升，复数磁导率μ的实部μ'上升，但共振频率fr和截止频率fc均下降。SEM照片显示，这是由于晶粒随烧结温度升高而长大所致。     

Microwave enhanced sintering of tape-cast ferroelectric films

Porous Pb(Zr_xTi_1−x)O₃(PZT) thick films that had been prepared by tape casting were densified by microwave energy. The microwave absorption effect is substantially correlated with the film thickness. In microwave-processed PZT thick films, rapid particle necking causes densification with no grain growth nearly in a short treatment time of 20 min at 820 °C. The same porous PZT thick films are difficult to densify in a conventional process. A 30-μm-thick PZT thick film has a pure perovskite structure. Self-supporting PZT thick films with a crack-free and uniform microstructure formed in a microwave process have larger coercive field than conventionally processed bulk PZT. The polarization, 14 μC/cm², of PZT thick films in a microwave process exceeds that, 7 μC/cm², of PZT bulk formed in a conventional process.