Pure Spinel Research Articles

Transport properties in spinel ferrite/graphene oxide nanocomposites for electromagnetic shielding

Herein, (Co0.5Ni0.5)Cr0.3Fe1.7O4/graphene oxide nanocomposites were fabricated by ultrasonication technique, using pure spinel ferrite and graphene oxide synthesized by sol-gel method and modified Hummers' method, respectively. The effect of graphene incorporation with ferrite nanoparticles was studied by X-ray diffraction (XRD), electrical and dielectric measurements. XRD analysis revealed the spinel phase for the ferrite sample and confirmed the formation of graphene oxide. The crystallite size was found in the range of 37−43 nm and the porosity increased with the increase in the concentration of graphene oxide in the composites. The DC electrical resistivity of spinel ferrite was found equal to 3.83×109 Ω.cm and it substantially decreased with the increase in the percentage of graphene oxide at room temperature. The real and imaginary part of relative permittivity followed the Maxwell-Wagner type of interfacial polarization. AC conductivity confirmed the conduction by hopping mechanism and increased on increasing the GO content. The coupling of magnetic ferrite with graphene oxide tunes the magneto-electrical properties for potential applications at high frequencies.

Controlled-synthesis of hierarchical NiCo2O4 anchored on carbon nanofibers mat for free-standing and highly-performance supercapacitors

In this work, a versatile one-step hydrothermal technique was used to produce a hybrid standalone electrode of NiCo2O4 hierarchical nanostructures anchored on CNFs mat for highly-performance and substrate-free supercapacitors. The CNFs mat was working as a conductive and a three-dimensional template for the deposition of the hierarchical NiCo2O4 nanostructures at the same time. The morphological and structural data analysis revealed a pure spinel NiCo2O4 nanostructures with a unique surface morphology comprising interconnected ultrathin nanoneedles and nanoflowers were successfully anchored to the CNFs mat. Real supercapacitors consist of two-symmetrical hybrid electrodes with different NiCo2O4 loading ratios were assembled and tested. The electrochemical performances of the assembled devices in terms of specific capacitance, energy, and power densities were systematically evaluated. Increasing the NiCo2O4 loading on the CNFs mat had shown a positive impact on improving the overall electrochemical performance of the assembled supercapacitors. A hybrid electrode loaded with NiCo2O4 twice as much as CNFs possess a specific capacitance value of 540 F g−1 along with an energy density of 30 Wh kg−1 at a power density of 515.6 W kg−1. In addition, the device showed excellent cycling stability and high capacitance retention against 6000 charge–discharge cycles at a charging current of 1.0 A g−1.

Probing electrochemical charge storage of 3D porous hierarchical cobalt oxide decorated rGO in ultra-high-performance supercapacitor

A 3D porous hierarchical Co 3 O 4 -reduced graphene oxide nanosheet (Co 3 O 4 -rGO) heterostructure is synthesized by a facile hydrothermal method . Microstructure analysis confirms the decoration of Co 3 O 4 hierarchical structures with an urchin-like spherical microstructure on rGO nanosheet with an average size of about 50 nm. The structural analysis reveals the formation of phase pure spinel Co 3 O 4 and reduced graphene showing their simultaneous existence in the composite. Raman spectroscopy confirms the successful reduction of graphite oxide to reduced graphene oxide and the effectual interaction between Co 3 O 4 and rGO matrix. The Co 3 O 4 -rGO composite electrode possesses utmost specific capacitance 688 F g −1 , due to effective ion transfer and slight agglomeration of Co 3 O 4 hierarchical structures on the surface of reduced graphene oxide. It is noteworthy that we have demonstrated a flexible solid-state supercapacitor device that exhibits an excellent performance suggesting the possible use of 3D porous hierarchical Co 3 O 4 -rGO composite in supercapacitor devices. Urchin-like spherical Co 3 O 4 decorated on rGO nanosheet delivered high specific energy of 72.6 Wh kg −1 corresponding to a specific capacitance of 688 F g −1 . • Hierarchical urchin-like Co 3 O 4 spherical microstructure decorated on rGO nanosheets is successfully synthesized. • The composite exhibits high capacitance of 688 F g −1 rate with excellent long-term stability. • Composite shows capacitance retention after 10,000 cycles with superb cycling stability (96%).

Molten salt synthesis of LiMn 1 . 2 Ni 0 . 3 Cr 0 . 1 Co 0 . 15 Al 0 . 23 La 0 . 02 O 4 as a positive electrode for lithium‐ion batteries

The molten salt synthesis method became an excellent synthesis technique to elaborate nanomaterials because of its meritorious advantages including low cost, easy to scale up, simple to operate, and environmental friendliness. For this reason, the compound LiMn1.2Ni0.3Cr0.1Co0.15Al0.23La0.02O4 was synthesized for the first time by molten salt method using NaCl as the salt and a nonstandard manganese source with metallurgical grade. The obtained cathode material was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transformer infra-red (FT-IR), and electrochemical characterization. XRD and FT-IR results confirm the stability and formation of pure spinel phase without any impurities. TEM images showed that the spinel synthesized phase consists of 77 to 135 nm-sized polyhedral nanoparticles. The cyclic voltammogram results showed that the synthesized spinel cathode material exhibits two-main redox peaks at 4.06/3.9 V and 3.1/2.8 V vs Li+/Li, which indicate the possibility of insertion of extra lithium in the spinel structure. Galvanostatic charge-discharge studies were also carried out showed that the prepared spinel material has two discharge plateaus in the potential window of 1.5 to 5 V and a discharge capacity of 179 mAh g−1 at 0.1 C, which is higher than the theoretical value which confirms the insertion of extra lithium in the spinel structure and enhance the capacity of the cathode material. The electrochemical impedance spectroscopy was performed, and the Li-ion diffusion coefficient was also calculated.

Synthesis, Electronic Structure, and Electrochemical Properties of the Cubic Mg2MnO4 Spinel with Porous-Spongy Structure.

Mg2MnO4 nanoparticles with cubic spinel structure were synthesized by the sol-gel method using polyvinyl alcohol (PVA) as a chelating agent. X-ray powder diffraction, infrared spectrum (IR), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to characterize the crystalline phase and particle size of as-synthesized nanoparticles. The electronic structure of Mg2MnO4 spinel was studied by X-ray photoelectron spectroscopy (XPS). The results showed that pure cubic Mg2MnO4 spinel nanoparticles were obtained when the annealing temperature was 500–700 °C. The samples had a porous-spongy structure assembled by nanoparticles. XPS studies indicated that Mg2MnO4 nanoparticles were mixed spinel structures and the degree of cation inversion decreased with increasing annealing temperature. Furthermore, the performance of Mg2MnO4 as lithium anode material was studied. The results showed that Mg2MnO4 samples had good cycle stability except for the slight decay in the capacity at 50 cycles. The coulombic efficiency (ratio of discharge and charge capacity) in most cycles was near 100%. The sample annealed at 600 °C exhibited good electrochemical properties, the first discharge capacity was 771.5 mAh/g, and the capacity remained 340 mAh/g after 100 cycles. The effect of calcination temperature on the charge–discharge performance of the samples was studied and discussed.

Existence of two spin dynamics in the temperature and magnetic field dependence of the magnetization curves of ferrimagnetic Co1.75Fe1.25O4 and its composite with BaTiO3

The present work describes ferrimagnetic properties of spinel oxide Co1.75Fe1.25O4 (CFO) and its composite (CFO_BTO) with ferroelectric and non-magnetic BaTiO3 (BTO). Rietveld refinement of the synchrotron X-ray diffraction patterns provided detailed structural phase information of the CFO and CFO_BTO samples. The dc magnetization measurements confirmed blocking temperature of the samples around 300 K for applied magnetic field 500 Oe and exchange bias effect at 5 K. On the other hand, both the samples exhibited a strong magnetic memory effect at around 250 K and the memory effect gradually suppressed at low temperatures. Apart from conventional modifications of magnetic moment and coercivity in composite system with reference to pure spinel ferrite system, the present work has shown that ferrimagnetic parameters can be varied by adopting some unconventional measurement protocols, where time dependent magnetization has been recorded during measurements of the temperature and field dependent magnetization curves. The analysis of waiting time dependent magnetization data confirmed the existence of two non-equilibrium spin dynamics, defined by different time constants, both in CFO ferrite and CFO_BTO composite systems. The measurements also confirmed the dilution of magnetic exchange interaction and faster spin relaxation in the composite system.

Investigation of Magnetic and Dielectric Properties of Cobalt Cubic Spinel Ferrite Nanoparticles Synthesized by CTAB-Assisted Co-precipitation Method

In this research work, a novel surfactant (CTAB)-assisted co-precipitation method has been developed for controlled synthesis of CoFe2O4 nanoparticles by using cobalt nitrate, iron nitrate, sodium hydroxide, and CTAB (cetyltrimethylammonium bromide) as reactants. Nanoparticles of CoFe2O4 were synthesized with varying molar amounts of CTAB (0.1M, 0.2M, 0.35M, and 0.4M CTAB). A sample without CTAB (0.0M CTAB) was also synthesized for comparison of properties. Crystal structural, morphological, vibrational spectral, magnetic, and dielectric characterization of synthesized CoFe2O4 nanoparticles were carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometry (VSM), and precision impedance analyzer (LCR meter), respectively. XRD results revealed that single-phase pure spinel CoFe2O4 owing to Fd-3m group was synthesized with improved crystallinity for all samples. It was confirmed by SEM analysis that spherical nanoparticles of CoFe2O4 have been synthesized with an average particle size of 25nm. FTIR analysis revealed that absorption peaks observed at 587–583 cm−1and 377–375 cm−1 referred to the tetrahedral complexes and octahedral complexes, respectively. Dielectric properties which were measured between 100Hz and 5MHz using precision impedance analyzer showed that first dielectric constant and dielectric loss decrease rapidly with increase in frequency, and on further increase in frequency, they decrease slowly and become independent of the frequency, which is a characteristic behavior of magnetic spinel ferrites. VSM analysis which was carried out at maximum magnetic field of 8000 Oe at room temperature and pressure showed that all the specimens showed ferrimagnetic behavior with improvement in magnetic properties originating from the use of surfactant in the synthesis procedure.

Preparation of submicron-sized ZnFe2-xCrxO4 (x = 0.4, 0.8) via a two-step method and evaluation of the corresponding ceramic inks

In this work, submicron-sized ZnFe2-xCrxO4 (x = 0.4, 0.8) pigments were synthesized by a combustion assisted two-step method. The samples and corresponding ceramic inks are investigated by XRD, SEM, TEM, colorimeter, and laser particle size analyzer. The grindability of the pigments and the printing performance (rheological properties, surface tension, filterability and storage stability) of the inks are evaluated. Our research reveals that although the calcination temperature in the present method is 300 °C lower than the conventional solid-state reaction route, the as-prepared samples belong to a pure spinel phase and present highly saturated reddish-brown tone. Compared to the samples prepared from solid-state reaction route, the synthesized submicron-sized pigments exhibit superior grindability. The corresponding ceramic inks present excellent coloring ability, filterability and storage stability. Our work confirms that the submicron-sized ZnFe2-xCrxO4 pigments synthesized by the combustion assisted two-step method are quite suitable for ZnFe2-xCrxO4-based reddish-brown ceramic inks.

Synthesis of Li4Ti5O12 negative electrode material in a fluidized bed thermogravimetric analyzer

AbstractSpinel lithium titanate (Li4Ti5O12) is the most promising negative electrode material for energy storage which can be applied in lithium‐ion batteries lithium‐sulphur batteries, and supercapacitors. However, the high cost of Li4Ti5O12 powder and its high molecular weight limit its application due to the complicated synthesis process. In this work, a facile solid‐state reaction in a fluidized bed reactor was used for the first time to synthesize spinel Li4Ti5O12 using lithium carbonate and titanium oxide as solid reactants. We investigated the fluidization behaviour of starting materials and studied different synthesis temperature in fluidized and fixed beds. X‐ray diffraction and scanning electron microscopy were employed to characterize the samples. The results demonstrate that the mixture of Li2CO3 and TiO2 had a complete fluidization velocity of 0.88 cm/s and showed good fluidization behaviour. Pure spinel Li4Ti5O12 was obtained via solid‐state reaction in a fluidized bed reactor at 800°C for 30 minutes, which is much lower than a conventional solid‐state reaction in a fixed bed or muffle furnace. By this novel method, the micro‐sized Li4Ti5O12 particles (6‐8 μm) have hollow porous structure composed of nano‐sized particles (<100 nm) synthesized. According to the experimental results, we proposed a different reaction mechanism for the synthesis of Li4Ti5O12 in fluidized beds which explains how the Li2CO3 and TiO2 react to form hollow lithium titanate nano‐structured micro‐particles in fluidized beds. The bulk density, surface area, and bulk conductivity of the synthesized Li4Ti5O12 are 3240 kg/m3, 4.6 m2/g, and 3.90 × 10−6 S/cm, respectively.

Hydrothermal Synthesis of Binder-Free Kapok (<i>Ceiba pentandra</i>) Fiber Paper-NiCo<sub>2</sub>O<sub>4</sub> Supercapacitor Electrode

There is a need to address the gap between the theoretical benefits and cost-efficient production of supercapacitors in the market in order to sway the preference of the industry from the current perishable energy sources and storage. More extensive exploration of sustainable fabrication methods and materials used for renewable energy storage are just some of the factors that would decrease this gap. A binder-free supercapacitor electrode made of NiCo2O4 and carbonized kapok fiber paper (CKFP) was successfully fabricated by hydrothermal process at relatively low temperatures. NiCo2O4 urchin-like structures were deposited on the surface of carbon fiber paper (CFP) and CKFP. XRD analysis confirmed the successful conversion of kapok fiber paper to CKFP after pyrolysis, as well as the growth of pure spinel NiCo2O4 nanostructures on CFP and CKFP. The cyclic voltammetry curves showed that the CFP-NiCo2O4 prepared at 140 °C had the highest specific capacitance of 143.51 Fg-1 at 2 mVs-1. The CKFP-NiCo2O4 synthesized at the same temperature yielded slightly higher specific capacitance of 146.29 Fg-1 at 2 mVs-1, and 508 Fg-1 at 0.5 Ag-1.

Size and Phase Purity–Dependent Microstructural and Magnetic Properties of Spinel Ferrite Nanoparticles

The urea-assisted combustion pathway was selected to synthesize different spinel ferrite nanoparticles (SFNPs). Synthesized spinel ferrite nanoparticles MFe2O4 (M = Zn, Ni, Cu and Co) were investigated using various techniques likely to be used by XRD, SEM, TEM and VSM to study their structural, morphological and magnetic behaviours. The crystalline sizes of the spinel ferrite nanoparticles were calculated from the major XRD peak (311) which confirms that the prepared spinel powders are in nanoscale (21–55 nm) range. Secondary peaks free XRD spectrum of Cu, and Ni ferrite nanoparticles exhibit pure single-phase cubic spinel structures. Few secondary impurities are observed in Zn and Co ferrite nanoparticles due to the presence of α-Fe2O3. Spherical-shaped nanoparticles have been examined by TEM. Room temperature magnetic measurements of different spinel ferrite nanoparticles suggest that the prepared spinel ferrites exhibit different forms of magnetism that are strongly affected by phase purity and particle size.

Study of magnetic and dielectric properties of ZnFe2O4/CoCr2O4 nanocomposites produced using sol-gel and hydrothermal processes

In this work, zinc ferrite (ZnFe2O4) and cobalt chromite (CoCr2O4) nanoparticles were synthesized using sol-gel process and hydrothermal method, respectively. The two different materials were mixed by physical means such that the final ratio (by weight) of the resulting nanocomposites (ZnFe2O4:CoCr2O4, abbreviated as ZF:CC) were 3ZF-CC, ZF-CC and ZF-3CC. The x-ray diffraction patterns of the ZnFe2O4 and CoCr2O4 nanoparticles confirmed the formation of the spinel structure, whereas the average crystallite size values computed using the Scherrer equation were found to be 43 and 57 nm, respectively. Scanning electron microscope (SEM) examination of the nanocomposites confirmed the nonspherical shape of the nanoparticles, homogenous dispersion, and larger grain size. The phase pure spinel structures of ZnFe2O4 and CoCr2O4 was confirmed from the fourier transform infrared spectroscopy (FTIR) analysis. Also, the composites formation did not influence any change in the functional groups. The dielectric properties, determined using LCR meter, exhibited non-repetitive behavior for the ZF-CC composite. The significant increase in the dielectric constant value was observed as the Fe2+↔Fe3+ and Cr2+↔Cr3+ transition due to electron transfer plays vital role in low frequency region, Also, explained by Maxwell-Wagner model. Moreover, the variation in experimental magnetic moments of ZF, CC and their nanocomposites (ZF-3CC, ZF-CC,3ZF-CC) were studied. VSM results revealed the change in behavior of magnetic nature of materials appreciably from pristine as magnetization was increasing and coercivity was decreasing with CC component and finally an optimized magnetic parameter values (Ms =51.20 emu/g and Hc =200 Oe) at ZF-CC were obtained.

Magnetic Properties of Mechano-Thermally Processed Nanocrystalline MgFe2O4 Spinel Materials

This work highlights the results obtained from different characterizations for the Mg-based spinel ferrite, MgFe2O4 nanomaterials prepared by ball milling followed by annealing at different temperatures, 800 °C, 900 °C, 1000 °C, and 1100 °C each for 2 h. From XRD analysis, the structural data such as lattice parameter, average crystalline size, micro-strain, and dislocation density of different MgFe2O4 samples are obtained. While the produced as-milled powder revealed the majority of the peaks are cubic MgFe2O4 phase materials, the annealed samples yielded phase pure nanocrystalline cubic spinel MgFe2O4 materials. The photoluminescence (PL) intensity of emission peak seen at ~ 417 nm (λexcitation = 375 nm) for different nanocrystalline MgFe2O4 materials varies regardless of the annealing temperatures. The FT-IR results imply the intrinsic stretching vibrations of the Fe–O at the tetrahedral site for these spinel materials. A gradual increase of the saturation magnetization was observed for the present MgFe2O4 materials upon increasing the annealing temperature. These nanocrystalline MgFe2O4 materials may have potential applications in magnetic fluid (hyperthermia) and near-infrared (NIR) pigment applications.

Evolution of structure and magnetic properties in MnZn ferrite-silica nanocomposites fabricated by sol–gel method

We explore the influence of the silica matrix on size, spinel phase stability, and magnetic properties of MnZn ferrite. For this purpose, Mn0.6Zn0.4Fe2O4 (MZF) nanoparticles (NPs) dispersed in silica (SiO2) matrix [MZF1−x/(SiO2)x x = 0, 10, 30%] synthesized by one-pot sol–gel process. The evolution of a pure spinel phase is apparent from the as-prepared (AP) MZF/SiO2 (30%) amorphous state on annealing in the temperature range 600–900 °C in air, whereas the AP free-standing MZF NPs exhibit a secondary phase of α-Fe2O3 along with spinel structure in this temperature range. The crystallite size is 5 ± 1 nm for a 600 °C annealed sample and increases to 10 ± 1 nm for a 900 °C annealed sample. From the detailed analysis of the temperature and magnetic field-dependent magnetization data, it is found that the AP sample exhibits weak magnetic behavior. However, annealed samples exhibit a maximum in zero-field-cooled magnetization at a certain temperature (TP), which increases with the increase of annealing temperature. The M–H curves above TP indicate the presence of superparamagnetic (SPM)-like behavior. From the present study, we show that the phase stability and magnetic properties of MZF/SiO2 composites have improved compared to bare MZF NPs.

Mechanochemical effect on synthesis and sintering behavior of MgAl2O4 spinel

This research work accomplishes the mechanochemical effect on synthesis and sintering behavior of MgAl2O4 spinel prepared by separate vibrating ball milling (VBM) and planetary micro-milling (PMM-P7) processes. The pure stoichiometric magnesium aluminate spinel (MgAl2O4) was produced from the precursors [Mg(OH)2 & AlOOH], and their densification at an optimum sintering profile. Non-gelation, soft agglomeration, and compressive stress induced by micro-milling in ethanol solvent facilitated the pure phase at low calcination temperature and confirmed through TEM. Eventually, the formation of nanoscale (25 nm) particles, ~98% densification, and grain size (38 μm) in atmospheric sintering are being achieved through planetary micro-milling. Critical discussion on the effect of planetary micro-milling is emphasized to understand the enhanced densification and optimum grain size that may expedite to development of other oxide particles and dense ceramics.

Experimental determination of the effect of Cr on Mg isotope fractionation between spinel and forsterite

We report the results of spinel-magnesite Mg isotope exchange experiments at 600, 700, and 800 °C and 1 GPa to establish the equilibrium Mg isotope partitioning between magnesite (MgCO3) and spinel as a function of Cr substitution for Al in the spinel phase. We used the three-isotope method to obtain equilibrium fractionation factors between MgAlCrO4 and magnesite and MgCr2O4 and magnesite. The experimentally-determined temperature-dependent Mg isotope fractionations are Δ26MgMgAlCrO4- Mgs = 0.96 ± 0.22 × 106/T2 and Δ26MgMgCr2O4- Mgs = 0.55 ± 0.08 × 106/T2 (2 s.e.). When combined with the previous experimentally determined fractionation between forsterite and magnesite (Macris et al., 2013), the corresponding Mg isotope fractionations between these two Cr-bearing spinel compositions and forsterite are Δ26MgMgAlCrO4- Fo = 0.84 ± 0.23 × 106/T2 and Δ26MgMgCr2O4- Fo = 0.43 ± 0.10 × 106/T2. The experimentally determined isotopic fractionation relationship between magnesiochromite and magnesite agrees with theoretical predictions based on the crystal chemical environment of Mg in these minerals. By combining these new results with the existing experimental calibration of equilibrium Mg isotope exchange between pure spinel and forsterite, we arrive at the temperature-dependent Mg isotope fractionation between spinel and forsterite as a function of Cr concentration in the spinel:Δ26MgMgAl1 - xCrx2O4- Fo=-0.67±0.26x+1.10±0.23106/T2When applied to natural samples, the combination of measured Mg isotope fractionation between spinel and forsterite, the Cr concentrations of the spinels, and estimates of temperature from independent geothermometers can be used to identify differences in closure temperatures between Mg isotope exchange and cation exchange, or the presence of disequilibrium in these systems.

Structural, magnetic and electrical properties of electroactive-superparamagnetic PVDF-Sn0.2Fe2.8O4 nanocomposite films

PVDF films embedded with magnetic Sn0.2Fe2.8O4 nanoparticles at selected volume percentages (≤20%) were prepared by using a quick and cost-efficient method of solution casting. Using a facile technique of alternate stirring and ultrasonication of the dispersion, the less investigated electroactive γ-PVDF phase was predominantly obtained in the pure polymer as well as loaded films. X-ray diffraction with Rietveld analysis and TEM imaging established the pure spinel phase and size of nanoparticles, respectively. Additionally, FTIR spectroscopy evidenced the γ-phase crystallinity of all the nanocomposite films with traces of α-PVDF in ≥5 vol% films. The films exhibited superparamagnetic character; the saturation magnetization increased with Sn0.2Fe2.8O4 content (from ≈0.1 to 7 emu/g) with nearly constant coercivity (≈16 Oe). The dielectric constant varied between 1 and 8 with low tangent loss (≤0.11) in the 100 Hz–1 MHz frequency range. These nanocomposite films hold promise in areas like micro-electronics, interlayer applications, magnetic sensors, actuators and microwave devices.

Interfacial and defect polarization in MXene-like laminated spinel for electromagnetic wave absorption application

Manufacturing advanced absorbers is an effective way to deal with the greater electromagnetic pollution challenges associated with the application of 5G technology. While reasonable morphology design is an efficacious method to improve the absorption performance of the absorber. Herein, a series of Co-based spinel Co3O4/ACo2O4 (A = Ni, Cu, Zn) were successfully synthesized via a facile PVP-assisted hydrothermal method. It is worth mentioning that the Co3O4/ZnCo2O4 with MXene-like laminated structure was synthesized successfully for the first time (to our knowledge) by changing the type of elements in A position. The EAB of MXene-like laminated Co3O4/ZnCo2O4 absorber can reach 6.24 GHz (from 11.6 to 17.84 GHz) with a matching thickness of 2.62 mm. This excellent performance can be attributed to the multiple scattering, interfacial polarization, and polarization induced by lattice defects and oxygen vacancies (the dominant). This work offers a novel pattern for improving the EMW absorption ability of pure spinel by manufacturing MXene-like laminated Co-based spinel and adjusting annealing temperature reasonably.

Crystal structure and microwave dielectric properties of Mg2Ti1-xGa4/3xO4 (0.05 ≤ x ≤ 0.13) ceramics

In this work, the Mg2Ti1-xGa4/3xO4 (x = 0.05, 0.07, 0.09, 0.11, 0.13) microwave dielectric ceramic are prepared through solid-state reaction. XRD refinement patterns confirm that a pure spinel structure phase is formed. Intrinsic dielectric properties of Mg2Ti1-xGa4/3xO4 ceramics are researched by Raman spectra and P–V-L theory. The change of εr is the result of the combined effect of internal factor ([Mg(2)/Ti/Ga]O6 octahedron bond iconicity) and external factor (relative density). Based on Raman spectra, FWHM of A1g and Q × f value have the same changing trend. In addition, τf value decreases due to the enhancement of bond valence and αL. Microwave dielectric properties of Mg2Ti1-xGa4/3xO4(εr = 14.84, Q × f = 205,416 GHz, τf = −57.9 ppm/°C) sintered at 1420 °C for 4 h are obtained when x = 0.09.

Investigating the structural, magnetic, magnetocaloric and critical behavior of Mg0.35Zn0.65Fe2O4 ferrite

In this study, we successfully prepared the Mg0.35Zn0.65Fe2O4 ferrite sample via standard solid-phase reaction and comprehensive studied its structure, magnetism, magnetocaloric effect and critical behavior. The XRD data refined by Rietveld technology confirms that the sample had a pure phase spinel structure. We conducted a magnetic study on the sample using SQUID and indicated that the sample’ ferromagnetic curie temperature Tc = 295 K, the paramagnetic curie temperature TP = 330 K, and the saturation magnetization can reach 125.41emu/g at an ultra-low temperature of 5 K. We tested the initial magnetization curve near the phase transition temperature (Tc) from ferromagnetic (FM) to paramagnetic states (PM) and produced an Arrott plot that confirmed the presence of a secondary phase transition. The sample’ maximum magnetic entropy change |ΔSMmax| was 1.642 J/kg K and relative cooling power RCP was 185.2 J/kg in the external magnetic field H = 50 kOe. The critical parameter β, γ, δ, and TP were calculated using a Modified Arrott plot (MAP), Kouvel-Fisher (KF) plot and critical isotherm (CI) methods to study the type of magnetic order in the ferromagnetic state that corresponded to the magnetic state equation and mean field model. By analyzing the critical parameters, we obtained the ferromagnetic exchange integral constant J(r) ~ r−4.8498, which was between the 3D Heisenberg model and mean field models and very close to the mean field model, confirming that the long-range and short-range ferromagnetic orderly interactions coexisted with interaction phases and the long-range ferromagnetic orderly interaction dominates.