Sol–Gel Synthesis of Mn–Sn–Ti-Substituted Strontium Hexaferrite Nanoparticles: Structural,Magnetic, and Reflection-Loss Properties

Substituted strontium hexaferrite nanoparticles with the chemical formula SrFe_12?x (MnSn_0.5Ti_0.5) _x/2O₁₉(x = 0–2.5, in a step of 0.5) were prepared by a sol–gel method. Phase identities and crystal structure of the synthesized nanoparticles were investigated by x-ray diffraction. The morphology of the nanopowders was determined by field emission scanning electron microscopy. Results obtained from Fourier-transform infrared spectroscopy revealed the presence of stretching and bending modes in the citrate complex. Mössbauer spectroscopy (M _S) revealed occupancy of the hexagonal lattice structure by non-magnetic Mn²⁺ –Sn⁴⁺ –Ti⁴⁺ cations. Magnetic properties were measured by use of a vibrating sample magnetometer. The results showed that saturation magnetization and coercivity decreased with increasing x content. Microwave absorption properties were investigated by use of a vector network analyzer. It was found that the maximum reflection loss of substituted Sr-ferrite 1.6 mm thick reached ?41.8 dB at a frequency of 4.3 GHz and a bandwidth of 7.5 GHz, with reflection loss being higher than ?25 dB. These results imply that the prepared composites are good candidates for absorbers in the gigahertz frequency range.     

Structural,Magnetic, and Reflection Loss Characteristics of Ni/Co/Sn-Substituted Strontium Ferrite/Functionalized MWCNT Nanocomposites

Ni/Co/Sn-substituted strontium ferrite [SrFe_12?x (Ni_0.5Co_0.5Sn) _x/2O₁₉]/multiwalled carbon nanotube (MWCNT) nanocomposites were produced by assembling ferrite particles on the external surfaces of MWCNTs. Various techniques including x-ray diffraction (XRD) analysis, transmission electron microscopy, field-emission scanning electron microscopy (FE-SEM), and Fourier-transform infrared (FTIR) spectroscopy were used to demonstrate the successful attachment of ferrite particles onto the external surfaces of the MWCNTs. XRD analysis and FTIR spectroscopy confirmed the presence of strontium ferrite and carbon nanotube phases in ferrite and nanocomposite samples, respectively. FE-SEM micrographs indicated the formation of ferrite particles on the outer surfaces of MWCNTs in nanocomposite samples. Furthermore, vibrating-sample magnetometer (VSM) and reflection loss (RL) measurements were performed to assess the magnetic and microwave characteristics of the synthesized samples. VSM loops confirmed a relatively strong dependence of the saturation magnetization and coercivity on the volume percentage of MWCNTs. With the introduction of MWCNTs or an increase in the substitution, the saturation magnetization and coercivity were decreased. The RL properties of the nanocomposites were investigated in the 8 GHz to 12 GHz frequency range. The sample with 80 wt.% nanocomposite content showed a maximum RL of ?35 dB at 8.3 GHz with a 4 GHz absorption bandwidth over the extended frequency range of 8 GHz to 12 GHz for absorber thickness of 1.8 mm. The RL evaluations indicated that these nanocomposites have high potential for application as wide-band electromagnetic wave absorbers at GHz frequencies.     

Structural, Magnetic, and Microwave Properties of SrFe12−x (Ni0.5Co0.5Sn) x/2O19 Particles Synthesized by Sol–Gel Combustion Method

This study is intended to evaluate the structural, magnetic, and microwave properties of Ni-Co-Sn-doped strontium hexaferrite SrFe_12?x (Ni_0.5Co_0.5Sn) _x/2 O₁₉ particles with x = 0 to 2.5 synthesized by a sol–gel combustion method. These particles were evaluated to characterize the structural, magnetic, and reflection loss properties of prepared samples by use of x-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), Fourier-transform infrared (FTIR) spectroscopy, vibrating-sample magnetometer (VSM), and vector network analyzer. The XRD results confirmed the presence of strontium ferrite phase with magnetoplumbite structure in all synthesized samples. The results of FTIR analysis indicated the formation of functional groups such as metal-oxygen (Sr-O and Fe-O) and carboxylic groups during the sol–gel process. In addition, FE-SEM micrographs indicated that submicron particles with different morphologies such as spherical, pyramidal, irregular, and hexagonal platelet shapes appeared in the structure. According to hysteresis loops, magnetization and coercivity decreased due to occupation of Ni-Co-Sn cations at low levels of substitutions. The microwave absorption characteristics of this ferrite were investigated in the 8 GHz to 12 GHz frequency range. The sample with 80 wt.% ferrite content showed a maximum reflection loss of ?29 dB at 9.6 GHz with 4 GHz bandwidth through the entire frequency range of 8 GHz to 12 GHz for absorber thickness of 1.5 mm. Based on microwave measurements of reflectivity, this material with the expressed chemical composition could be proposed as a good choice for electromagnetic compatibility and other practical applications such as microwave absorption at high frequencies.     

Synthesis, Structures, and Multiferroic Properties of Strontium Hexaferrite Ceramics

Simultaneous occurrence of large ferroelectricity and strong ferromagnetism has been observed in strontium hexaferrite (SrFe₁₂O₁₉) ceramics. Strontium hexaferrite powders with hexagonal crystal structures have been successfully synthesized through the co-precipitation precursor method using strontium nitrate and ferric nitrate as starting materials. The powders were pressed into pellets and then sintered into ceramics at a temperature range of at 1000°C to 1100°C for 1 h. The structure and morphology of the ceramics were determined using x-ray diffraction and field-emission scanning electron microscopy techniques. Clear ferroelectric hysteresis loops demonstrated large spontaneous polarization in the SrFe₁₂O₁₉ ceramics at room temperature. The maximum remnant polarization of the SrFe₁₂O₁₉ ceramic was estimated to be approximately 15 μC/cm². The FeO₆ octahedron in its perovskite-like hexagonal unit cell and the displacement of Fe³⁺ off the center of the octahedron are proposed to be the origin of electric polarization in SrFe₁₂O₁₉. In our experimental observations, the SrFe₁₂O₁₉ ceramic also revealed strong ferromagnetism at room temperature.     

Synthesis and Characterization of SrFe<Subscript>11.2</Subscript>Zn<Subscript>0.8</Subscript>O<Subscript>19</Subscript> Nanoparticles for Enhanced Microwave Absorption

SrFe₁₂O₁₉/ZnFe₂O₄ (SrFe_11.2Zn_0.8O₁₉) nanoparticles having superparamagnetic nature were synthesized by coprecipitation of chloride salts using 7.5 M sodium hydroxide solution. The resulting precursors were heat-treated at 900°C and 1200°C for 4 h in nitrogen atmosphere. During heat treatment (HT), transformation proceeds through instantaneous nucleation and three-dimensional diffusion-controlled growth with an activation energy of 175.9 kJ/mole. The hysteresis loops showed an increase in saturation magnetization from 1.044 emu/g to 61.227 emu/g with increasing HT temperature. As-synthesized particles had sizes in the range of 20 nm to 25 nm with spherical shape. Further, these spherically shaped nanoparticles tended to change their morphology to hexagonal plate and pyramidal shape with increasing HT temperature. The effects of this systematic morphological transformation of nanoparticles on dielectric (complex permittivity and permeability) and microwave absorption properties were estimated in the X-band (8.2 GHz to 12.2 GHz). The maximum reflection loss of the composite powder reached −29.81 dB at 10.37 GHz, making it suitable for application in radar-absorbing materials.     

Fabrication and Electromagnetic Properties of Conjugated NH<Subscript>2</Subscript>-CuPc@Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>

Conjugated amino-phthalocyanine copper containing carboxyl groups/magnetite (NH₂-CuPc@Fe₃O₄) has been fabricated from FeCl₃·6H₂O and NH₂-CuPc via a simple solvothermal method and its electromagnetic properties investigated. Scanning electron microscopy and transmission electron microscopy revealed that the NH₂-CuPc@Fe₃O₄ was a waxberry-like nanomaterial with NH₂-CuPc molecules effectively embedded in the interior of Fe₃O₄ particles in the form of beads. Introduction of NH₂-CuPc effectively improved the complementarity between the dielectric and magnetic losses of the system, resulting in excellent electromagnetic performance. The minimum reflection loss of the as-prepared composite reached ?33.4 dB at 7.0 GHz for coating layer thickness of 4.0 mm and bandwidth below ?10.0 dB (90% absorption) of up to 3.8 GHz. These results indicate that introduction of NH₂-CuPc results in a composite with potential for use as an electromagnetic microwave absorption material.     

Effect of CuO Addition on Microwave Dielectric Properties of 0.80Sm(Mg0.5Ti0.5)O3-0.20Ca0.8Sr0.2TiO3 Ceramics

The effects of CuO addition on phase composition, microstructure, sintering behavior, and microwave dielectric properties of 0.80Sm(Mg_0.5Ti_0.5)O₃-0.20 Ca_0.8Sr_0.2TiO₃(8SMT-2CST) ceramics prepared by a conventional solid-state ceramic route have been studied. CuO addition shows no obvious influence on the phase of the 8SMT-2CST ceramics and all the samples exhibit pure perovskite structure. Appropriate CuO addition can effectively promote sintering and grain growth, and consequently improve the dielectric properties of the ceramics. The sintering temperature of the ceramics decreases by 50°C by adding 1.00 wt.%CuO. Superior microwave dielectric properties with a ε _r of 29.8, Q × f of 85,500 GHz, and τ _f of 2.4 ppm/°C are obtained for 1.00 wt.%CuO doped 8SMT-2CST ceramics sintered at 1500°C, which shows dense and uniform microstructure as well as well-developed grain growth.     

Effect of Rare Earth Elements on Electromagnetic and Microwave Absorption Properties of Fe-Based Alloys

The RE₂Fe₁₇ (RE = Ce, Pr, Nd, Sm) and La_xPr_2?xFe₁₇ (x = 0.0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by arc smelting and high-energy ball milling methods. The phase structure, morphology, magnetic properties and electromagnetic parameters of the powders were characterized by x-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer, respectively. The results reveal that the lattice parameters a and c and unit-cell volume V of the La_xPr_2?xFe₁₇ alloys increase linearly upon the La content. The minimum absorption peak frequency shifts towards a lower-frequency region upon the La content. And the minimum reflection loss and saturation magnetization of the La_xPr_2?xFe₁₇ alloys decrease upon the La content, while the minimum reflection loss of Pr₂Fe₁₇ and La_0.4Pr_1.6Fe₁₇ alloy of the 2.0 mm coating thickness reaches about ?13.65 dB and ?7.15 dB at 5.92 GHz and 3.6 GHz, respectively.     

Effect of the Addition of B2O3 on the Density, Microstructure, Dielectric, Piezoelectric and Ferroelectric Properties of K0.5Na0.5NbO3 Ceramics

Boron oxide (B₂O₃) addition to pre-reacted K_0.5Na_0.5NbO₃ (KNN) powders facilitated swift densification at relatively low sintering temperatures which was believed to be a key to minimize potassium and sodium loss. The base KNN powder was synthesized via solid-state reaction route. The different amounts (0.1–1 wt%) of B₂O₃ were-added, and ceramics were sintered at different temperatures and durations to optimize the amount of B₂O₃ needed to obtain KNN pellets with highest possible density and grain size. The 0.1 wt% B₂O₃-added KNN ceramics sintered at 1,100 °C for 1 h exhibited higher density (97 %). Scanning electron microscopy studies confirmed an increase in average grain size with increasing B₂O₃ content at appropriate temperature of sintering and duration. The B₂O₃-added KNN ceramics exhibited improved dielectric and piezoelectric properties at room temperature. For instance, 0.1 wt% B₂O₃-added KNN ceramic exhibited d ₃₃ value of 116 pC/N which is much higher than that of pure KNN ceramics. Interestingly, all the B₂O₃-added (0.1–1 wt%) KNN ceramics exhibited polarization–electric field (P vs. E) hysteresis loops at room temperature. The remnant polarization (P _r) and coercive field (E _c) values are dependent on the B₂O₃ content and crystallite size.     

Preparation of nanocomposite thin films by sol-gel and photochemical metal–organic deposition of Ba0.5Sr0.5TiO3 and PbZr0.48Ti0.52O3

The process for generating nanocomposite films constructed from alternate thin film layers of Ba_0.5Sr_0.5TiO₃ and PbZr_0.48Ti_0.52O₃ by photochemical metal–organic deposition and sol-gel has been investigated. By spin coating the appropriate metal organic precursors to Ba_0.5 Sr_0.5TiO₃ followed by photolysis a single layer of amorphous Ba_0.5Sr_0.5TiO₃, is produced. Subsequent spin coating of the appropriate metal organic precursors of PbZr_0.48Ti_0.52O₃ and photolysis led to the formation of a layer of amorphous PbZr_0.48Ti_0.52O₃. By repeating this procedure a material composed of alternating layers of BST and PZT was constructed. In an analogous process alternate coating and heating of the appropriate sol-gel precursors was used to make similar structures. These nanocomposite materials were formed as amorphous layered materials but could be made to crystallize by heat treatment. Heat treatment resulted in crystallization of the films although the resultant diffraction pattern was dependent upon the thickness of the layers. X-ray diffraction patterns of both BST and PZT were apparent in crystalline films formed from layers of more than 25 nm in thickness. The crystallization of films formed from layers less than 25 nm in thickness showed only a lattice constant intermediate between that expected for BST and PST consistent with the interdiffusion of these components. Atomic force microscopy indicated that the amorphous nanocomposite films were near featureless while the crystalline films had a much higher surface roughness.     

Preparation and Characterization of Silica-Coated CaCu3Ti4O12

Silica was homogeneously coated on the surface of CaCu₃Ti₄O₁₂ (CCTO) particles via the sol–gel method. The obtained powders were characterized by x-ray diffraction analysis, Fourier-transform infrared spectroscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy, scanning electron microscopy, and zeta potential analysis. The results demonstrate that there were silica layers on the surface of the CCTO particles. Physical and dielectric properties of silica-coated CCTO were also studied. TEM imaging showed that the thickness of the silica layer on the CCTO particles was about 20 nm to 35 nm. The specimen coated with 1.0 wt.% silica showed the maximum relative density of 96.7% with high dielectric constant (12.78 × 10⁴) and low dielectric loss (0.005) at 20°C after sintering at 1000°C for 6 h.     

Thermoelectric Properties of Bi0.5Sb1.5Te3 Prepared by Liquid-Phase Growth Using a Sliding Boat

A liquid-phase growth process using a graphite sliding boat was applied for synthesis of p-type Bi_0.5Sb_1.5Te₃. The process lasted only 60 min, including rapid heating for melting, boat-sliding, and cooling. Thick sheets and bars of 1 mm and 2 mm in thickness having preferable crystal orientation for thermoelectric conversion were successfully prepared by the process. Control of carrier concentration was attempted through addition of excess tellurium (1 mass% to 10 mass%) to optimize the thermoelectric properties of the material. The Hall carrier concentration was found to be decreased by addition of excess tellurium. The electrical resistivity and Seebeck coefficient varied depending on the carrier concentration. As a result, the maximum observed power factor near 300 K was 4.4 × 10^?3 W/K²m, with corresponding Hall carrier concentration of 4.6 × 10²⁵ m^?3. Thus, thermoelectric properties were controllable by addition of excess tellurium, and a large power factor was thus obtained through a simple and short process.     

Microstructure and Electrical Properties of K0.5Na0.5NbO3-LiSbO3-BiFeO3-x %molZnO Lead-Free Piezoelectric Ceramics

Lead-free piezoelectric ceramics {0.996[(0.95(K_0.5Na_0.5)NbO₃-0.05LiSbO₃]-0.004BiFeO₃}-xmol%ZnO were prepared through a conventional ceramics sintering technique. The effect of ZnO content on structure, microstructure, and piezoelectric properties of KNN-LS-BF ceramics was investigated. The results reveal that ZnO as a sintering aid is very effective in promoting sinterability and electrical properties of the ceramics sintered at a low temperature of 1,020 °C. The ceramics show a single-perovskite structure with predominant tetragonal phase, and coexistence of orthorhombic and tetragonal phases is observed for x = 2.5–3.0. The addition of ZnO causes abnormal grain growth. A dense microstructure is also obtained at x = 2.0 because the relative density reaches up to 94.6 %. The morphotropic phase boundary and dense microstructure lead to significant enhancement of the piezoelectric properties. The ceramic with x = 1.5 exhibits optimum electrical properties as follows: d ₃₃ = 280 pC/N, k _p = 46 %, Q _m = 40.8, P _r = 25 μC/cm², E _c = 1.2 kV/mm, and T _c = 340 °C.     

Dielectric Properties and Defect Chemistry of WO3-Doped K0.5Na0.5NbO3 Ceramics

The dielectric properties and conductivity behavior of WO₃-doped K_0.5Na_0.5 NbO₃ ceramics were investigated as a function of temperature (25°C to 600°C) and frequency (40 Hz to 10⁶ Hz). The dielectric loss and direct-current (DC) conductivity of the ceramics depend strongly on the tungsten content. A high-temperature dielectric relaxation near temperature of 500°C was observed and analyzed using the semiempirical complex Cole–Cole equation. The activation energy of the dielectric relaxation was estimated to be ～2 eV and increased with increasing WO₃. The frequency-dependent conductivity can be well described by the universal dielectric response law. The activation energy obtained from the DC conductivity changes from 0.93 eV to 1.49 eV. A possible mechanism for the high-temperature dielectric relaxation and conductivity is proposed based on the activation energy value and defect compensation.     

Low-Temperature Sintering of Ba0.5Sr0.5TiO3-SrMoO4 Dielectric Tunable Composite Ceramics for LTCC Applications

A sintering-aid system using melting of B-Li glass for barium strontium titanate (BST)-based compositions to be used in low-temperature cofired ceramic (LTCC) layers is introduced. The effects of the sintering aid on the microstructure, dielectric properties, and application in LTCC were investigated. The composition Ba_0.5Sr_0.5TiO₃-SrMoO₄ with 3 wt.% B-Li glass sintered at 950°C exhibits optimized dielectric properties, including low dielectric constant (368), low dielectric loss (0.007), and moderate tunability (13%, 60 kV/cm) at 10 kHz. At 1.44 GHz, it possesses a dielectric constant of 218 and Q value of 230. LTCC multilayer ceramic capacitors fabricated by the tape-casting process have steady relative tunability of 12% at 300 V, suggesting that BST50-SrMoO₄-B-Li glass composite ceramic is a promising candidate for electrically tunable LTCC microwave device applications.     

Improvement of Thermoelectric Properties in (Bi0.5Sb0.5)2Te3 Films of Nanolayered Pillar Arrays

In this work, it is found that unique pillar arrays with nanolayered structure can favorably influence the carrier and phonon transport properties of films. p-(Bi_0.5Sb_0.5)₂Te₃ pillar array film with (0 1 5) orientation was successfully achieved by a simple ion-beam-assisted technique at deposition temperature of 400°C, owing to the enhanced mobility of deposited atoms for more sufficient growth along the in-plane direction. The pillar diameter was about 250 nm, and the layered nanostructure was clear, with each layer in the pillar array being <30 nm. The properties of the oriented (Bi_0.5Sb_0.5)₂Te₃ pillar array were greatly enhanced in comparison with those of ordinary polycrystalline films synthesized at deposition temperature of 350°C and 250°C. The (Bi_0.5Sb_0.5)₂Te₃ pillar array film with (0 1 5) preferred orientation exhibited a thermoelectric dimensionless figure of merit of ZT = 1.25 at room temperature. The unique pillar array with nanolayered structure is the main reason for the observed improvement in the properties of the (Bi_0.5Sb_0.5)₂Te₃ film.     

MnO2-Modified Ba(Ti,Zr)O3 Ceramics with High Q m and Good Thermal Stability

MnO₂-modified Ba(Ti_0.9625Zr_0.0375)O₃ ceramics have been prepared by the conventional solid-state reaction technique at different sintering temperatures. Room-temperature piezoelectric properties, thermal stability, and crystalline structures were investigated. It was found that both the MnO₂ additive and sintering temperature significantly influence the piezoelectric properties of the MnO₂-modified Ba(Ti_0.9625Zr_0.0375)O₃ ceramics. The sample sintered at 1400°C exhibited the best room-temperature piezoelectric properties of Q _m = 1907, d ₃₃ = 205 pC/N, and k _p = 40.5% with tan δ = 0.46%, and its k _p remains larger than 35% in the broad temperature range from ?38°C to 65°C. The results indicate that MnO₂-modified Ba(Ti_0.9625Zr_0.0375)O₃ ceramics are promising lead-free materials for frequency device and power device applications.     

Synthesis of Size-Controlled SrFe12O19 Using Modified Spray Pyrolysis–Calcination Method and Their Magnetic Properties

Strontium hexaferrite (SrFe₁₂O₁₉, SrM) suitable for high-performance permanent magnet applications was synthesized by salt-assisted ultrasonic spray pyrolysis (SA-USP) and subsequent calcination. To control the particle size, the intermediate phase of SrM was collected by SA-USP and various sizes of SrM were obtained by calcining the as-prepared sample at temperatures ranging from 800°C to 1050°C. The synthesized SrM was magnetically aligned by using an external magnetic field to improve remanence. The synthesized particles were of nano- to submicron scale and nonagglomerated. The magnetic properties and squareness of the material depended on the particle size and distribution. Additionally, the NaCl added during synthesis facilitated the formation of nonagglomerated particles, while enhancing and controlling particle growth. The optimum magnetic properties were achieved at calcination temperature of 1000°C, resulting in coercivity of 5646 Oe, saturation magnetization of 73.3 emu/g, and remanence of 59.1 emu/g (80.6% of M _s).     

Synthesis of Bi0.5Sb1.5Te3 Thermoelectric Powder Using an Oxide-Reduction Process

The present study focused on synthesis of Bi_0.5Sb_1.5Te₃ thermoelectric powder using an oxide-reduction process. The phase structure and particle size of the synthesized powders were analyzed using x-ray diffractometry and scanning electron microscopy. The synthesized powder was sintered using the spark plasma sintering method. The thermoelectric properties of the sintered body were evaluated by measuring the Seebeck coefficient, electrical resistivity, and thermal conductivity. Bi_0.5Sb_1.5Te₃ powder was synthesized using a combination of mechanical milling, calcination, and reduction processes, using a mixture of Bi₂O₃, Sb₂O₃, and TeO₂ powders. The sintered body of the oxide-reduction-synthesized Bi_0.5Sb_1.5Te₃ powder showed p-type thermoelectric characteristics. The thermoelectric properties of the sintered bodies depended on the reduction time. After being reduced for 2 h at 663 K, the sintered body of the Bi_0.5Sb_1.5Te₃ powder showed a figure of merit of approximately 1.0 at room temperature.     

Electronic Materials Based on Co0.5Zn0.5Fe2O4/Pb(Zr0.52Ti0.48)O3 Nanocomposites

The reduction of the radar cross-sectional area achieved in stealth technology has been a major challenge since the Second World War, being accomplished by covering the metallic surfaces of aircraft, ships, tanks, etc. with radar-absorbing materials. Nowadays, the development of lightweight microwave-absorbing materials with reduced thickness has a greater impact due to their excellent microwave-absorbing properties. In this study, the microwave-absorbing properties of nanocomposites based on Zn-substituted cobalt ferrite and lead zirconium titanate have been investigated in the X-band (8.2 GHz to 12.4 GHz) region. Zn-substituted cobalt ferrite (CZF) and lead zirconium titanate (PZT) nanoparticles were prepared by the coprecipitation and homogeneous precipitation method, respectively. Nanocomposites were developed by dispersing these nanoparticles with different compositions into an epoxy resin matrix. All the composite materials showed more than 90% microwave absorption in the X-band region. The nanocomposite containing CZF/PZT (3:1) with 2 mm thickness displayed maximum return loss of ?47.87 dB at 12.23 GHz. The microwave absorbers based on epoxy resin polymeric matrix exhibited better absorbing properties when the dielectric contribution matched the magnetic contribution, and the loss mechanisms were mainly due to the dielectric loss.