Electro-Thermo-Magnetic Effect-Induced Large Thermoelectric Performance of Calcium Cobaltite Nanocomposites.

As attractive thermoelectric oxides, Ca3Co4O9-based materials have been intensively studied for their applications in recent years. However, their thermoelectric performance is enormously limited due to the contradiction of electrical resistivity and thermal conductivity. Herein, BaFe12O19 nanospheres were introduced into the Ca3Co4O9 matrix. The metallic Ag, ferrites, and matrix phase survived together, and a high density of nanoscale BaFe12O19 precipitation was observed. The reduction of work function could lead to band bending and form an interface potential due to the electro-thermo-magnetic effect contributing to the hole migration. As a result, a huge ZT value of 0.51 for the 8 wt % BaFe12O19/Ca3Co4O9 nanocomposites was obtained at 1073 K, accompanied by a low electrical resistivity of 6.7 mΩ·cm and a high Seebeck coefficient of 217.5 µV/K. In addition, a significant reduction of thermal conductivity (1.11 W/(m·K)) occurred, which was due to the nanoscale ferromagnetic phase effectively scattering the mid- and short-wavelength heat-carrying phonons. The synergistic enhancement of thermoelectric performance confirmed that the electro-thermo-magnetic effect is an effective way to solve the challenging problem of performance deterioration in oxide thermoelectric materials.

Influence of calcination temperature and particle size distribution on the physical properties of SrFe12O19 and BaFe12O19 hexaferrite powders.

The paper deals with the economic optimisation of ferrite powder preparation during producing hard ferrite magnets. The magnetic properties of ferrites are investigated by replacing feedstock and reducing calcination temperature and particles in the order of tens of microns. The granulates about 8-10 mm in size were calcined for 2 h in the temperature range from 1100 °C to 1300 °C and additionally crushed and milled to an average particle size of about 80-90 µm. The scanning electron microscopy images confirmed the agglomerates of particles with different shapes and sizes in tens of µm. The X-ray diffraction measurements revealed that, besides the SrFe12O19 and BaFe12O19 phases, there was also the presence of 2-39% hematite. The highest values of maximum energy product (BH)max = 930 J/m3 and remanent magnetic induction Br = 72.8 mT were obtained at a calcination temperature of 1300 °C. The Henkel plots confirmed the presence of exchange-coupling and dipolar magnetic interactions at lower and higher magnetic fields, respectively. The strength of interactions was also dependent on the calcination temperature. Replacing strontium with barium led to a deterioration of the magnetic parameters, which were optimal at a lower calcination temperature (1100 °C). This phenomenon was partly overcome by reducing the mean particle size of Ba-based hexaferrites to 45-50 µm.

Boosting visible light photocatalysis in BiOI/BaFe12O19 magnetic heterojunction.

Many previous studies have underestimated the role of magnetic components in improving photocatalytic performance. It is significance to explore the migration mechanism of photoinduced carriers in magnetic heterojunction. Here, a magnetic heterojunction, BiOI/BaFe12O19, was synthesized by a simple preparation method. The optimal synthesis conditions and photocatalytic reaction conditions were explored. The growth mechanism of bismuth iodide oxide (BiOI) was elaborated by introducing a micromagnetic field stemming from barium ferrite (BaFe12O19). The electrochemical impedance spectroscopy (EIS), Mott-Schottky curve (MS), transient fluorescence spectrometer (PL), and photocurrent response plot (i ~ t) tests indicated that the BiOI/BaFe12O19 possessed a higher transfer capacity of electrons, higher separation efficiency of photoinduced carriers, stronger photocurrent response, and higher carriers density, compared with pure BiOI. The ultraviolet-visible diffuse reflectance spectrophotometer (UV-vis DRS), electron paramagnetic resonance spectrometer (EPR), MS, and quenching experiments revealed band structure configuration and migration mechanism of photoinduced carriers. The enhancement mechanism of photocatalysis and photocatalytic reaction mechanism was clearly proclaimed in BiOI/BaFe12O19 catalytic system.

Insight into (Electro)magnetic Interactions within Facet-Engineered BaFe12O19/TiO2 Magnetic Photocatalysts.

A series of facet-engineered TiO2/BaFe12O19 composites were synthesized through hydrothermal growth of both phases and subsequent deposition of the different, faceted TiO2 nanoparticles onto BaFe12O19 microplates. The well-defined geometry of the composite and uniaxial magnetic anisotropy of the ferrite allowed alternate interfaces between both phases and fixed the orientation between the TiO2 crystal structure and the remanent magnetic field within BaFe12O19. The morphology and crystal structure of the composites were confirmed by a combination of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses together with the detailed study of BaFe12O19 electronic and magnetic properties. The photocatalytic activity and magnetic field effect were studied in the reaction of phenol degradation for TiO2/BaFe12O19 and composites of BaFe12O19 covered with a SiO2 protective layer and TiO2. The observed differences in phenol degradation are associated with electron transfer and the contribution of the magnetic field. All obtained magnetic composite materials can be easily separated in an external magnetic field, with efficiencies exceeding 95%, and recycled without significant loss of photocatalytic activity. The highest activity was observed for the composite of BaFe12O19 with TiO2 exposing {1 0 1} facets. However, to prevent electron transfer within the composite structure, this photocatalyst material was additionally coated with a protective SiO2 layer. Furthermore, TiO2 exposing {1 0 0} facets exhibited significant synergy with the BaFe12O19 magnetic field, leading to 2 times higher photocatalytic activity when ferrite was magnetized before the process. The photoluminescence emission study suggests that for this particular combination, the built-in magnetic field of the ferrite suppressed the recombination of the photogenerated charge carriers. Ultimately, possible effects of complex electro/magnetic interactions within the magnetic photocatalyst are shown and discussed for the first time, including the anisotropic properties of both phases.

Preparation of a Microwave-Absorbing UV Coating Using a BaFe12O19-Polypyrrole Nanocomposite Filler.

BaFe12O19-polypyrrolenanocomposites were prepared via the in situ chemical oxidative polymerization of pyrrole monomers in the presence of BaFe12O19 powder, with ammonium persulfate as an oxidant and sodium dodecyl benzene sulfonate as a dopant. X-ray diffraction measurements and Fourier-transform infrared spectroscopy indicated that there were no chemical interactions between BaFe12O19 and polypyrrole. In addition, scanning electron microscopy showed that the composites exhibited a core-shell structure. Subsequently, the prepared nanocomposite was used as a filler to prepare a coating suitable for ultraviolet curing. The performance of the coating was investigated by evaluating its hardness, adhesion, absorbance, and resistance to acids and alkalis. Importantly, the addition of BaFe12O19-polypyrrole nanocomposites not only improved the coating hardness and adhesion but also produced a coating with a good microwave absorption performance. The results suggested that BaFe12O19/PPy composite has a lower reflection loss peak and a larger effective bandwidth at the X band when the proportion of the absorbent sample is 5-7%, when the absorption performance is the best. The reflection loss is in the range of 8.88-10.92 GHz below -10 dB.

PANI-wrapped BaFe12O19 and SrFe12O19 with rGO composite materials for electromagnetic interference shielding applications.

We report electromagnetic interference (EMI) shielding efficiency in the PANI-wrapped BaFe12O19 and SrFe12O19 with rGO composites. Barium and strontium hexaferrites were synthesized using the nitrate citrate gel combustion method. These hexaferrites were polymerized in situ with aniline. The PANI-coated ferrite-based composite materials were developed along with reduced graphene oxide (rGO) in acrylonitrile butadiene styrene (ABS) polymer, and their shielding effectiveness was assessed in X-band frequency range (8.2-12.4 GHz). The reflection (SER) and absorption (SEA) mechanism of shielding effectiveness was discussed with the different rGO concentrations. The results reveal that 5 wt% of rGO with PANI-coated barium and strontium hexaferrite polymer composites exhibit shielding efficiency of 21.5 dB and 19.5 dB, respectively, for 1 mm thickness composite. These hexaferrite polymer-based composite materials can be used as an attractive candidate for EM shielding materials in various technological applications.

Construction lamellar BaFe12O19/Bi3.64Mo0.36O6.55 photocatalyst for enhanced photocatalytic activity via a photo-Fenton-like Mo6+/Mo4+redox cycle.

The novel BaFe12O19/Bi3.64Mo0.36O6.55 composite materials were constructed as magnetically recyclable photo-Fenton-like degradation systems. The composite catalyst not only promoted the effective transfer of photo-generated electrons and improved the Mo6+/Mo4+ cycle consequent, but also activated hydrogen peroxide to generate oxidizing free radicals. BaFe12O19/Bi3.64Mo0.36O6.55-0.25 exhibited an outstanding degradation performance for tetracycline hydrochloride it is 1.3 times to Bi3.64Mo0.36O6.55. The thermal catalytic performance of the Bi3.64Mo0.36O6.55 monomer is similar to that of the BaFe12O19/Bi3.64Mo0.36O6.55 material without light. However, the removal rate of BaFe12O19/Bi3.64Mo0.36O6.55 material reaches 84.5% after 60 min with light, far exceeding that of Bi3.64Mo0.36O6.55 material. By way of the contrast experiment with light and without light, it is further demonstrated that interfacial interaction between BaFe12O19 and Bi3.64Mo0.36O6.55 acted a key role in the photocatalytic reaction system. It is also a good advantage that pollutants can be efficiently degraded without adjusting the pH. The characterization of photocurrent and X-ray photoelectron spectroscopy (XPS) also further proved the synergy between the two materials, which is useful to the separation of electrons and holes. The synergy ultimately improves the degradation performance. Besides, BaFe12O19/Bi3.64Mo0.36O6.55 can be easily separated by an external magnetic field after the photocatalytic activity reaction owing to BaFe12O19's magnetic properties. It provides a new research idea for the construction and iron-based heterogeneous Fenton-like system for magnetic degradation of antibiotics.

Structural and magnetic properties of hard-soft BaFe12O19/(Zn0.5Co0.5)Fe2O4ferrites.

Hard-soft nanocomposites of (1 -x) BaFe12O19/x(Zn0.5Co0.5)Fe2O4, forx= 0.00, 0.25, 0.50, 0.75 and 1.00, were prepared via co-precipitation and high-speed ball milling techniques, respectively. The synthesized samples were characterized via x-ray diffraction, transmission electron microscope, Fourier transform infrared (FTIR), and vibrating sample magnetometer. XRD revealed the formation of hard-soft nanocomposites. TEM indicated that the two phases are well distributed and the particle size distribution is narrower for low content of soft phase, leading to better exchange coupling between the grains. Magnetic measurements were performed at 300 K and 77 K. The results showed a good single-phase magnetic behavior, verifying the good exchange coupling between hard and soft phases. For low (Zn0.5Co0.5)Fe2O4content, the dipolar interactions were dominated by the exchange-coupling interactions. Additionally, the optimum values of saturation and remanent magnetizations, coercivity, and squareness ratio were obtained forx= 0.5. This was attributed to the dominance of exchange-coupling interaction. The enhancement of magnetic properties and energy product (BH)maxfor nanocomposites at low temperature is skilled in the reduction of the thermal fluxes of magnetic moments at the surface. The maximum energy product (BH)maxwas observed in C2 at both temperatures with a smaller value than that of pure BaFe12O19.

Candidate for Magnetic Doping Agent and High-Temperature Thermoelectric Performance Enhancer: Hard Magnetic M-type BaFe12O19 Nanometer Suspension.

How to prevent the agglomeration of nanoparticles in nanocomposites remains a key challenge. Using nanometer suspension as a doping agent provides an effective approach to solve this challenge. A new technique that consists of chemical coprecipitation, ball milling and sedimentation separation metheds was developed for preparing hard magnetic M-type BaFe12O19 nanometer suspension. The single-phase BaFe12O19 nanoparticles dispersed uniformly in alcohol have been prepared by this new technique. Magnetic nanocomposite thermoelectric materials with a homogeneous dispersion of BaFe12O19 nanoparticles were prepared through a combination process of an ultrasonic mixing of BaFe12O19 nanometer suspension and In-filled CoSb3 thermoelectric matrix material and spark plasma sintering. The microstructure analysis of magnetic nanocomposite thermoelectric materials confirmed that using the nanometer suspension as a doping agent is an effective way to solve the agglomeration phenomenon of nanoparticles in nanocomposites. In addition, the decline of thermoelectric performance in the high-temperature intrinsic excitation region of In-filled CoSb3 can be effectively suppressed by the magnetic phase transition of BaFe12O19 nanoparticles dried by nanometer suspension from ferromagnetism to paramagnetism. It is also confirmed that using the BaFe12O19 nanometer suspension as a thermoelectric performance enhancer is an effective way to solve the challenging problem of performance deterioration of thermoelectric materials at high temperature.

Direct Observation of Magnetocrystalline Anisotropy Tuning Magnetization Configurations in Uniaxial Magnetic Nanomaterials.

Discovering the effect of magnetic anisotropy on the magnetization configurations of magnetic nanomaterials is essential and significant for not only enriching the fundamental knowledge of magnetics but also facilitating the designs of desired magnetic nanostructures for diverse technological applications, such as data storage devices, spintronic devices, and magnetic nanosensors. Herein, we present a direct observation of magnetocrystalline anisotropy tuning magnetization configurations in uniaxial magnetic nanomaterials with hexagonal structure by means of three modeled samples. The magnetic configuration in polycrystalline BaFe12O19 nanoslice is a curling structure, revealing that the effect of magnetocrystalline anisotropy in uniaxial magnetic nanomaterials can be broken by forming an amorphous structure or polycrystalline structure with tiny grains. Both single crystalline BaFe12O19 nanoslice and individual particles of single-particle-chain BaFe12O19 nanowire appear in a single domain state, revealing a dominant role of magnetocrystalline anisotropy in the magnetization configuration of uniaxial magnetic nanomaterials. These observations are further verified by micromagnetic computational simulations.

Effect of Ratio in Ammonium Nitrate on the Structural, Microstructural, Magnetic, and AC Conductivity Properties of BaFe12O19.

This paper investigates the effect of the ratio of ammonium nitrate (AN) on the structural, microstructural, magnetic, and alternating current (AC) conductivity properties of barium hexaferrite (BaFe12O19). The BaFe12O19 were prepared by using the salt melt method. The samples were synthesized using different powder-to-salt weight ratio variations (1:3, 1:4, 1:5, 1:6 and 1:7) of BaCO3 + Fe2O3 and ammonium nitrate salt. The NH4NO3 was melted on a hot plate at 170 °C. A mixture of BaCO3 and Fe2O3 were added into the NH4NO3 melt solution and stirred for several hours using a magnetic stirrer under a controlled temperature of 170 °C. The heating temperature was then increased up to 260 °C for 24 hr to produce an ash powder. The x-ray diffraction (XRD) results show the intense peak of BaFe12O19 for all the samples and the presence of a small amount of the impurity Fe2O3 in the samples, at a ratio of 1:5 and 1:6. From the Fourier transform infra-red (FTIR) spectra, the band appears at 542.71 cm - 1 and 432.48 cm - 1 , which corresponding to metal⁻oxygen bending and the vibration of the octahedral sites of BaFe12O19. The field emission scanning electron microscope (FESEM) images show that the grains of the samples appear to stick each other and agglomerate at different masses throughout the image with the grain size 5.26, 5.88, 6.14, 6.22, and 6.18 µm for the ratios 1:3, 1:4, 1:5, 1:6, and 1:7 respectively. From the vibrating sample magnetometer (VSM) analysis, the magnetic properties of the sample ratio at 1:3 show the highest value of coercivity Hc of 1317 Oe, a saturation magnetization Ms of 91 emu/g, and a remnant Mr of 44 emu/g, respectively. As the temperature rises, the AC conductivity is increases with an increase in frequency.

Self-propagating Combustion Triggered Synthesis of 3D Lamellar Graphene/BaFe12O19 Composite and Its Electromagnetic Wave Absorption Properties.

The synthesis of 3D lamellar graphene/BaFe12O19 composites was performed by oxidizing graphite and sequentially self-propagating combustion triggered process. The 3D lamellar graphene structures were formed due to the synergistic effect of the tremendous heat induced gasification as well as huge volume expansion. The 3D lamellar graphene/BaFe12O19 composites bearing 30 wt % graphene present the reflection loss peak at -27.23 dB as well as the frequency bandwidth at 2.28 GHz (< -10 dB). The 3D lamellar graphene structures could consume the incident waves through multiple Reflection and scattering within the layered structures, Prolonging the propagation path of electromagnetic waves in the absorbers.