Microwave absorption study of carbon nano tubes dispersed hard/soft ferrite nanocomposite

Nickel substituted strontium hexaferrite, SrNi₂Fe₁₀O₁₉·(SrFe₁₂O₁₉/NiFe₂O₄) nanoparticles have been synthesized by low combustion method by citrate precursors using sol to gel (S–G) followed by gel to nano crystalline (G–N) conversion. The resulting ‘as-synthesized’ powder is heat treated (HT) at 800 and 1000 °C for 4 h in nitrogen atmosphere. The hysteresis loops show an increase in saturation magnetization from 27.443 to 63.706 emu/g with increasing HT temperatures. The multiwalled carbon nano tubes (CNTs) were synthesized by thermal decomposition of acetylene gas over iron-catalyst deposited on silicon wafer in the temperature range of 750–800 °C. A microwave absorbing medium is prepared by adding CNTs in the nickel substituted strontium hexaferrite nanoparticles. Addition of certain mass of CNTs improves the microwave absorption properties and wave band of SrFe₁₂O₁₉/NiFe₂O₄ absorbent. When 10 wt% CNTs is mixed with SrFe₁₂O₁₉/NiFe₂O₄ nanoparticles to fabricate a composite with 2 mm thickness, the maximum reflection loss reaches to ?36.817 dB at 9.292 GHz and ?10 dB bandwidth reaches 3.27 GHz.     

Improved magnetic and electromagnetic absorption properties of xSrFe12O19/(1−x)NiFe2O4 composites

xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites (0 ≤ x ≤ 1.0) were synthesized by using a conventional solid-state synthetic route. The results show that magnetic hysteresis loops of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are similar to those of individual component ferrites, except for the 0.1SrFe₁₂O₁₉/0.9NiFe₂O₄ and 0.3SrFe₁₂O₁₉/0.7NiFe₂O₄, suggesting that the hard/soft magnetic phases are well exchange-coupled. The saturation magnetization, coercivity, and remanent magnetization of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are increased with increasing content of SrFe₁₂O₁₉, with maximal values of 42.1 Am² kg⁻¹, 78.7 kA m⁻¹, 17.2 Am² kg⁻¹, respectively, as the content x is about 0.5. They are higher than those of the individual components, implying that interface coupling is present in the magnetic composites. The coercivity and remanent magnetization of the composites are increased initially with increasing sintering temperature and then show a downward tendency. For the component SrFe₁₂O₁₉ and NiFe₂O₄, the minimum reflection losses are −12.5 dB and −18.3 dB at match thicknesses of 2.5 mm and 2 mm, respectively. Compared with those of the component SrFe₁₂O₁₉ and NiFe₂O₄, the microwave absorption performances of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are improved remarkably, especially for the samples of x = 0.3 and x = 0.9. The minimum reflection losses values of the 0.3SrFe₁₂O₁₉/0.7NiFe₂O₄ composite are −31.6 dB (12.7 GHz) and −20.2 dB (13 GHz), while those of the 0.9SrFe₁₂O₁₉/0.1NiFe₂O₄ composites are −23.7 dB (16.3 GHz) and −33.5 dB (15.8 GHz), as the matching thicknesses are 2.5 mm and 2 mm, respectively. Therefore, the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites could be used as potential microwave absorption materials.     

M-hexaferrite–APTES/Pd(0) Magnetically Recyclable Nano Catalysts (MRCs)

Magnetically recovable BaFe₁₂O₁₉–APTES–Pd(0) and SrFe₁₂O₁₉–APTES–Pd(0) catalysts were easily synthesized by immobilizing Pd nanoparticles on the surface of magnetic hexaferrite–NH₂ microspheres. It was found that the combination of BaFe₁₂O₁₉, SrFe₁₂O₁₉ and 3-aminopropyltriethoxysilane (APTES) could give rise to structurally stable catalytic sites. Furthermore, BaFe₁₂O₁₉–APTES–Pd(0) and SrFe₁₂O₁₉–APTES–Pd(0) magnetically recyclable nano catalysts (MRCs) can be recovered by magnet and reused for nine runs for hydrogenation of 4-nitroaniline and dinitribenzene without significant loss in its catalytic activity which shows the indicative of a potential applications of these catalyst in industry.     

Role of exchange coupling between the hard and soft magnetic phases on the absorption of microwaves by SrFe10Al2O19/Co0.6Zn0.4Fe2O4 nanocomposite

(1-x)SrFe₁₀Al₂O₁₉/(x)Co_0.6Zn_0.4Fe₂O₄-(SFAO/CZFO) hard/soft nanocomposite ferrite materials were synthesized by ‘one-pot’ self-propagating combustion route. The co-existence of the two magnetic phases were confirmed by XRD, FESEM, EDS and VSM. The prepared nanocomposite samples were also characterized by TGA/DSC, Raman spectroscopy and VNA. Exchange coupling between the hard and the soft magnetic grains was observed by determining the switching field distribution (SFD) curve. As a result of the competing effects of exchange interaction and dipolar interaction, magnetic parameters were observed to be sensitive to the incorporation of soft magnetic phase into the nanocomposite. Results showed that with the inclusion of soft magnetic phase, exchange coupling behaviour between the hard and the soft ferrite phases had significant influence on the microwave absorption capacity of the samples. Related electromagnetic parameters and impedance matching ratio of the nanocomposite system were discussed. A minimum reflection loss of ?42.9 dB with an absorber thickness of 2.5 mm was attained by the nanocomposite (90 wt%)SrFe₁₀Al₂O₁₉/(10 wt %)Co_0.6Zn_0.4Fe₂O₄ at a matching frequency of 11.45 GHz. This assured the candidacy of SrFe₁₀Al₂O₁₉/Co_0.6Zn_0.4Fe₂O₄ nanocomposite as a promising microwave absorption material in the X-band (8–12 GHz).     

Synthesis,characterization and magnetic properties of polythiophene/SrFe12-x(ZrZn)0.5xO19 nanocomposite as a microwave absorber

Ferrites are an important group of magnetic materials which are used as absorbers. The incorporation of ferrite and conducting polymer achieves great enhancement in microwave absorption properties. The nanocomposites of hexagonal ferrites embedded by conducting polymers such as polypyrrole, polyaniline and polythiophene (PTH) have been paid much attention. In the present study, strontium hexagonal ferrite doped by Zr and Zn with the final formula of SrFe_12-x(ZrZn)_0.5xO19 considering x = 0.9 and embedded by PTH was produced to achieve a nanocomposite with the highest microwave absorbing ability. In this study, after synthesis of SrFe₁₂O₁₉(ZrZn)_0.5xO19 and PTH, the nanocomposite was prepared by in situ polymerization. Wrapping the ferrite particles and PTH chains could form nanocomposite properly, and therefore acceptable interactions were observable between SrFe_12-x(ZrZn)_0.5xO19ferrite particles and PTH polymer chains in the composites. Assessing the X-ray diffraction (XRD) patterns of SrFe_12-x(ZrZn)_0.5xO19, PTH, and PTH/SrFe_12-x(ZrZn)_0.5xO19 nanocomposite indicated that the PTH characteristic peak shifts slightly and its peak intensity reduces, which may be attribute to the coating of PTH polymer chains onto SrFe_12-x(ZrZn)_0.5xO19 particles. We revealed also lower magnetic properties in the obtained nanocomposite. The morphological assessment also suggested that PTH could effectively coat the SrFe_12-x(ZrZn)_0.5xO19 particles. The synergistic effect of SrFe_12-x(ZrZn)_0.5xO19 particle plus PTH leads to microwave absorption percentage higher than 95% by PTH/SrFe_12-x(ZrZn)_0.5xO19 nanocomposite. Overall, nanocomposite creating by coupling interaction between SrFe_12-x(ZrZn)_0.5xO19 particles (x = 0.9) and PTH can effectively lead to achieve the highest rate of absorption of electromagnetic waves.     

Investigation of Structural,Morphological and Magnetic Properties of MFe2O4 (M = Co,Ni, Zn,Cu, Mn) Obtained by Thermal Decomposition

The structural, morphological and magnetic properties of MFe₂O₄ (M = Co, Ni, Zn, Cu, Mn) type ferrites produced by thermal decomposition at 700 and 1000 °C were studied. The thermal analysis revealed that the ferrites are formed at up to 350 °C. After heat treatment at 1000 °C, single-phase ferrite nanoparticles were attained, while after heat treatment at 700 °C, the CoFe₂O₄ was accompanied by Co₃O₄ and the MnFe₂O₄ by α-Fe₂O₃. The particle size of the spherical shape in the nanoscale region was confirmed by transmission electron microscopy. The specific surface area below 0.5 m²/g suggested a non–porous structure with particle agglomeration that limits nitrogen absorption. By heat treatment at 1000 °C, superparamagnetic CoFe₂O₄ nanoparticles and paramagnetic NiFe₂O₄, MnFe₂O₄, CuFe₂O₄ and ZnFe₂O₄ nanoparticles were obtained.     

The characteristics of Ni–Co–Mn–O precursor and Li(Ni1/3Co1/3Mn1/3)O2 cathode powders prepared by spray pyrolysis

Ni–Co–Mn–O precursor powders with spherical shape and dense structure were prepared by spray pyrolysis from a spray solution containing a drying control chemical additive (DCCA) and polymeric precursors. In contrast, the Ni–Co–Mn–O precursor powders obtained from a spray solution without additives had a hollow and porous morphology. Ni–Co–Mn–O precursor powders with a spherical shape and dense structure yielded Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathode powders with a spherical shape and fine size by means of a solid-state reaction with lithium hydroxide. The mean size of the spherical cathode powder was 1.1 μm. The discharge capacity of the Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders with spherical shape and filled morphology was 195 mA h g⁻¹ at a current density of 0.1 C. The discharge capacities of the cathode powders with spherical shape and filled morphology at 55 °C decreased from 183 to 154 mA h g⁻¹ by the 30th cycle at a current density of 0.5 C.     

Investigation on mechanochemical synthesis of Al2O3/BN nanocomposite by aluminothermic reaction

Alpha-alumina–boron nitride (α-Al₂O₃–BN) nanocomposite was synthesized using mixtures of aluminum nitride, boron oxide and pure aluminum as raw materials via mechanochemical process under a low pressure of nitrogen gas (0.5 MPa). The phase transformation and structural evaluation during mechanochemical process were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential thermal analysis (DTA) techniques. The results indicated that high exothermic reaction of Al–B₂O₃ systems under the nitrogen pressure produced alumina, aluminum nitride (AlN), and aluminum oxynitride (Al₅O₆N) depending on the Al value and milling time, but no trace of boron nitride (BN) phases could be identified. On the other hand, AlN addition as a solid nitrogen source was effective in fabricating in-situ BN phase after 4 h milling process. In Al–B₂O₃–AlN system, the aluminothermic reaction provided sufficient heat for activating reaction between B₂O₃ and AlN to form BN compound. DTA analysis results showed that by increasing the activation time to 3 h, the temperature of both thermite and synthesis reactions significantly decreased and occurred as a one-step reaction. SEM and TEM observations confirmed that the range of particle size was within 100 nm.     

Low temperature sintering and microwave dielectric properties of Ce2(WO4)3 ceramics

Ce₂(WO₄)₃ ceramics have been synthesized by the conventional solid-state ceramic route. Ce₂(WO₄)₃ ceramics sintered at 1000 °C exhibited ?_r = 12.4, Qxf = 10,500 GHz (at 4.8 GHz) and τ_f = −39 ppm/°C. The effects of B₂O₃, ZnO–B₂O₃, BaO–B₂O₃–SiO₂, ZnO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂ glasses on the sintering temperature and microwave dielectric properties of Ce₂(WO₄)₃ were investigated. The Ce₂(WO₄)₃ + 0.2 wt% ZBS sintered at 900 °C/4 h has ?_r = 13.7, Qxf = 20,200 GHz and τ_f = −25 ppm/°C.     

Fast synthesis of SrFe12O19 hexaferrite in a single-mode microwave cavity

The M-type SrFe₁₂O₁₉ hexaferrite has been synthesized by a microwave solid state reaction process – a fast heating process – in a home-made 2.45 GHz single-mode microwave cavity. Starting from SrCO₃ and Fe₂O₃ mixtures (1:6 ratio), cold pressed samples have been heated up in the microwave electric field without needing any susceptor, demonstrating the good coupling of the precursors with this microwave mode in our experimental setup.After optimization of the experimental conditions, the properties of the obtained ceramics, including structure, microstructure and magnetic properties, are compared with those of ceramics synthesized by conventional solid state reaction. With that microwave process, it is found that SrFe₁₂O₁₉ ceramics prepared in less than 30 min exhibit magnetic properties similar to those of the same compound produced by a conventional process. This highlights the potentialities of the technique to synthesize hexaferrite ceramics.     

Effect of the B2O3 addition on the sintering behavior and microwave dielectric properties of Ba3(VO4)2–Zn1.87SiO3.87 composite ceramics

The effect of the B₂O₃ addition on the low-temperature sintering, microstructure and microwave dielectric properties of the Ba₃(VO₄)₂–Zn_1.87SiO_3.87 composite ceramics was investigated. The results indicate that the addition of B₂O₃ can effectively promote the densification and further improve the microwave dielectric properties of the composite. The low-temperature sintering mechanism was ascribed to the formation of the liquid phase owing to the reaction between the additive B₂O₃ and the residual SiO₂ in the composite. B₂O₃–SiO₂ liquid phases can not only lower the sintering temperature, but also speed up the grain growth of the composite ceramics. The rapid grain growth occurs as the B₂O₃ content is more than 6 wt%. The 3 wt% B₂O₃ doped 0.5Ba₃(VO₄)₂–0.5Zn_1.87SiO_3.87 ceramics can be well sintered at 925 °C and exhibit excellent microwave dielectric properties of Q×f∼40,800 GHz, ε_r∼10 and τ_f∼0.5 ppm/°C.     

Electromagnetic wave reflection loss and magnetic properties of M-type SrFe12−x(Mn0.5Sn0.5)xO19 hexagonal ferrite nanoparticles in the Ku microwave band

Ferrimagnetic nanoparticles of SrFe_12−x(Mn_0.5Sn_0.5)_xO₁₉ was synthesis by controlling the effective processing parameters of sol-gel techniques. Microwave, magnetic properties and structure of nanosize, high purity sol-gel synthesized SrFe_12−x(Mn_0.5Sn_0.5)_xO₁₉ hexaferrite ferrimagnetic nanoparticles were investigated by vector network analysis, vibrating sample magnetometry, EDS and XRD. 57Fe Mössbauer spectroscopy was employed to find the occupancy sites of incorporated cations. The position of manganese and tin in 12k sites caused reduction in coercivity and magnetization saturation. The values of coercivity and magnetization saturation show that superparamagnetism did not occur in the synthesized products. Results demonstrate that additions of manganese and tin to hexagonal ferrite enhance bandwidth and reflection loss. Such composition could be proposed as a suitable electromagnetic wave absorber in microwave frequencies.     

Enhanced microwave absorption properties of Zn-substituted SrW-type hexaferrite composites in the Ku-band

Enhanced microwave absorption properties were successfully achievable from SrFe_2-xZn_xFe₁₆O₂₇ (SrFe_2-xZn_xW; x = 0.0, 0.5, 1.0, and 2.0) hexaferrite filler-epoxy resin matrix composites. The composite samples were fabricated with the filler volume fractions (V_f) of 30, 50, 70, and 90%. Compared with fully Zn-substituted SrZn₂W composite (x = 2.0), unsubstituted and partially Zn-substituted SrFe_2-xZn_xW (x = 0.0, 0.5, and 1.0) composites exhibited much higher real and imaginary parts of complex permittivity (ε_r), which is attributable to higher electron hopping between Fe²⁺ and Fe³⁺ ions, and also slightly higher real and imaginary parts of complex permeability (μ_r) due to higher saturation magnetization (M_s). Among all samples, a 2.8 mm-thick SrFe_1·5Zn_0·5W (x = 0.5) composite with the V_f of 90% exhibited the most appropriate for application in the region of 3.4–3.8 GHz, having the minimum reflection loss (RL_min) of ?46 dB at 3.6 GHz with the bandwidth of 0.43 GHz (3.38–3.81 GHz) below ?10 dB, while a 2.15 mm-thick SrFeZnW (x = 1.0) composite with the V_f of 70% showed the most appropriate for application in the region of 5.9–7.1 GHz, possessing the RL_min value of ?23.7 dB at 6.6 GHz with the bandwidth of 1.38 GHz (5.85–7.23 GHz) below ?10 dB. Consequently, partially Zn-substituted SrW-type hexaferrites are very promising microwave absorbers for 5G mobile communications in the Ku band (0.5–18 GHz).     

Cr2O3 doping in ZnO–0.5 mol% V2O5 varistor ceramics

The effects of the amount of Cr₂O₃ (0.5–4 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with ZnCr₂O₄ and α-Zn₃(VO₄)₂ as the minority secondary phases. The addition of Cr₂O₃ is found to be effective in controlling the abnormal ZnO grain growth often found in V₂O₅-doped ZnO ceramic system, and a more uniform microstructure can be obtained. The varistor performance is also improved as observed from the increase in the non-linear coefficient α of the Cr₂O₃-doped ZnO–V₂O₅ samples. The α value is found to increase with the amount of Cr₂O₃ for up to 3 mol% Cr₂O₃ content. Further increase in Cr₂O₃ is found to cause a decrease in the α value. The highest α value of 28.9 is obtained for the ZnO–0.5 mol% V₂O₅–3 mol% Cr₂O₃ sample.     

Preparation and characterization of core–shell structure Fe3O4/C nanoparticles with unique stability and high electrochemical performance for lithium-ion battery anode material

Core–shell structure carbon coating Fe₃O₄ nanoparticles are prepared by a two-step method. The crystalline structure and the electrochemical performance of the prepared samples are investigated. The results indicate that a uniform and continuous carbon layer is formed on the surface of Fe₃O₄ nanoparticles. The core–shell structure Fe₃O₄/C nanoparticles show a high initial discharge capacity of 1546 mAh g⁻¹ and a specific stable discharge capacity of about 800 mAh g⁻¹ at 0.5 C with no noticeable capacity fading up to 100 cycles.     

The effect of antioxidants on the oxidation behaviour of magnesia–carbon refractory bricks

Oxidation of carbon is the main problem in magnesia–carbon refractories. The effects of various antioxidants, Al, Si, SiC and B₄C on the oxidation resistance of magnesia–carbon bricks were investigated at temperatures of 1300 °C and 1500 °C. Carbon losses as wt.% of the bricks were calculated and oxidized areas of the bricks were examined by XRD, SEM and EDS. B₄C was found to be the most effective antioxidant at both temperatures. Magnesium–borate (Mg₃B₂O₆) compound was determined to be present by characterization studies on B₄C added specimens. Magnesium–borate, which is in liquid state above 1360 °C, had an excellent effect on the oxidation resistance of the bricks by filling up the open pores and forming a protective layer on the surface. Forsterite (Mg₂SiO₄) and spinel (MgAl₂O₄) provided similar effects on the Si and Al added specimens respectively at both temperatures. The SiC added specimens had similar phases with Si added specimens, but SiC was the least effective antioxidant at both temperatures.     

Low-temperature sintering and microwave dielectric properties of 3Z2B glass–Al2O3 composites

A potential low temperature co-fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and Al₂O₃ filler was investigated with regard to phase development and microwave dielectric properties as functions of the glass content and sintering temperature. The densification mechanism for 3Z2B–Al₂O₃ composites was reported. The linear shrinkage of 3Z2B glass–Al₂O₃ composites exhibited a typical one-stage densification behavior. XRD patterns showed that a new crystalline phase, ZnAl₂O₄ spinel, formed during densification, indicating that certain chemical reaction took place between the 3Z2B glass matrix and the alumina filler. Meanwhile, several zinc borate phases, including 4ZnO·3B₂O₃, crystallized from the glass matrix. Both of the reaction product phase and crystallization phases played an important role in improving the microwave dielectric properties of composites. The optimal composition sintered at 850–950 °C showed excellent microwave dielectric properties: ?_r = ∼5.0, Q·f₀ = ∼8000 GHz, and τ_f = ∼−32 ppm/°C at ∼7.0 GHz.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.     

Effect of carbon shell thickness on the microwave absorption of magnetite-carbon core-shell nanoparticles

Carbon coated magnetite nanoparticles with two different shell thicknesses were synthesized by a facile two-step method using glucose as a source of carbon. At first, hematite nanoparticles were synthesized by hydrothermal process. Carbon shells were coated on hematite nanoparticles by hydrothermal carbonization process, and the carbon coat thickness on particles was controlled by the amount of glucose in the second step. The hematite-carbon core-shell nanoparticles were then heat treated under argon gas flow in order to produce magnetite-carbon nanocapsules. Phase transformation during the heat treatment was studied by X-ray diffraction (XRD). The existence of carbon shell on nanoparticles was investigated by transmission electron microscopy (TEM) and Raman spectroscopy. The effect of carbon shell thickness variation on the relative complex permittivity (ε_r=ε′+iε″) and permeability (μ_r=μ′+iμ″) was studied in a frequency range of 1–18 GHz. The effect of carbon shell thickness on the Fe₃O₄-C nanoparticles reflection loss was also studied. The results showed that the microwave properties of the carbon coated magnetite nanoparticles can be controlled effectively by adjusting carbon shell thickness.     

Mg0.5Ni0.5Fe2O4 nanoparticles as heating agents for hyperthermia treatment

In the present work, Mg_0.5Ni_0.5Fe₂O₄ nanopowders were prepared by a sol-gel combustion method. The magnetic properties, heat generation ability in an AC magnetic field and cytotoxicity of the mixed ferrite nanopowders were investigated. The results showed that the powders have crystalline spinel structure with a particle size in the range of 20-90 nm. Maximum saturation magnetization (Ms) of 51 emu/g was obtained for the Mg_0.5Ni_0.5Fe₂O₄ nanoparticles calcined at 900°C. The results showed that, the coercivity (Hc) of the Mg_0.5Ni_0.5Fe₂O₄ initially increases up to 700°C and then decreases with increasing temperature, whereas the Ms of the samples continuously increases. The Mg_0.5Ni_0.5Fe₂O₄ sample exhibited a temperature increase up to 45°C during 10 minutes in the exposure of magnetic field of 200 Oe. By increasing the viscosity of ferrofluid, the heat generation ability of nanoparticles reduced up to 9% at magnetic field of 200 Oe. Cell compatibility of the ferrite powders was studied by MTT assay using MG63 cell line proliferation. MTT results showed that calcination temperature of the Mg_0.5Ni_0.5Fe₂O₄ nanoparticles significantly affects the cell compatibility.