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Hematite (α-Fe2O3) emerges as an enticing material for visible-light-driven photocatalysis owing to its remarkable stability, low toxicity, and abundance. However, its inherent shortcomings, such as a short hole diffusion length and high recombination rate, hinder its practical application. Recently, oxygen vacancies (Vo) within hematite have been demonstrated to modulate its photocatalytic attributes. The effects of Vo can be broadly categorized into two opposing aspects: (1) acting as electron donors, enhancing carrier conductivity, and improving photocatalytic performance and (2) acting as surface carrier traps, accelerating excited carrier recombination, and deteriorating performance. Critically, the generation rate, distribution, role, and behavior of Vo significantly differ for synthesis methods due to differences in formation mechanisms and oxygen diffusion. This complexity hampers simplified discussions of Vo, necessitating careful investigation and nuanced discussion tailored to the specific method and conditions employed. Among various approaches, hydrothermal synthesis offers a simple and cost-effective route. Here, we demonstrate a hydrothermal synthesis method for Vo introduction to hematite using a carbon source, where variations in the heating rate have not been previously explored in terms of their influence on Vo generation. The analyses revealed that the concentration of Vo was maximized at a heating rate of 16 °C/min, indicative of a high density of surface defects. With regard to photocatalytic performance, elevated heating rates (16 °C/min) fostered the formation of Vo primarily on the hematite surface. The photocatalytic activity was 7.1 times greater than that of the sample prepared at a low heating rate (2 °C/min). These findings highlight the crucial role of surface defects, as opposed to bulk defects, in promoting hematite photocatalysis. Furthermore, the facile control over Vo concentration achievable via manipulating the heating rate underscores the promising potential of this approach for optimizing hematite photocatalysts. [ABSTRACT FROM AUTHOR]

Nano-sized superparamagnetic Y_{3}Fe_{5}O_{12} ferrite having a high heat generation ability in an AC magnetic field was prepared by physical bead milling. The highest heat ability in the AC magnetic field was for the fine Y_{3}Fe_{5}O_{12} powder with a 16-nm crystallite size (the samples were milled for 4 h using 0.1 mmφ beads). The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. For the samples milled for 4 h using 0.1 mmφ beads, the heat generation ability (W·g^{-1}) was estimated using a 3.58 × 10^{-4} · f· H^{2} frequency (f/kHz) and the magnetic field (H/kA·m^{-1}), which is the highest reported value of superparamagnetic materials. Furthermore, Y_{3}Fe_{5}O_{12} microspheres having a 20-32 μm diameter range were prepared by a spray dryer using the bead-milled nano-sized particles. [ABSTRACT FROM AUTHOR]

We found that the nano-sized Y3Fe5O12 powder prepared by bead milling has the highest heat generation ability in an AC magnetic field of reported superparamagnetic materials. The heat generation ability in an AC magnetic field was strongly improved by a decrease in the average crystallite size of the bead milled samples. The highest heat ability in the AC magnetic field was for the fine Y3Fe5O12 powder with ca. 15 nm crystallite size (the samples milled for 4 h using 0.1 mmφ beads). The reasons for the high heat generation properties of the milled samples are ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability (W·g−1) can be estimated using a 3.58 × 10−4 f·H2 frequency (f/kHz) and the magnetic field (H/kA·m−1) for the sample milled for 4 h using 0.1 mmφ beads. [ABSTRACT FROM AUTHOR]

Abstract: Nanosized MgFe2O4-based ferrite powder having heat generation ability in an AC magnetic field was prepared by bead milling and studied for thermal coagulation therapy applications. The crystal size and the particle size significantly decreased by bead milling. The heat generation ability in an AC magnetic field improved with the milling time, i.e. a decrease in crystal size. However, the heat generation ability decreased for excessively milled samples with crystal sizes of less than 5.5nm. The highest heat ability (ΔT=34°C) in the AC magnetic field (370kHz, 1.77kA/m) was obtained for fine MgFe2O4 powder having a ca. 6nm crystal size (the samples were milled for 6–8h using 0.1mmϕ beads). The heat generation of the samples was closely related to hysteresis loss, a B–H magnetic property. The reason for the high heat generation properties of the samples milled for 6–8h using 0.1mmϕ beads was ascribed to the increase in hysteresis loss by the formation of a single domain. Moreover, the improvement in heating ability was obtained by calcination of the bead-milled sample at low temperature. In this case, the maximum heat generation (ΔT=41°C) ability was obtained for a ca. 11nm crystal size sample was prepared by crystal growth during the sample calcination. On the other hand, the ΔT value for Mg0.5Ca0.5Fe2O4 was synthesized using a reverse precipitation method decreased by bead milling. [ABSTRACT FROM AUTHOR]

Abstract: To study surface behaviors, MgFe2O4 ferrite materials having different grain sizes were synthesized by two different chemical methods, i.e., a polymerization method and a reverse coprecipitation method. The single phase of the cubic MgFe2O4 was confirmed by the X-ray diffraction method for both the precursors decomposed at 600–1000°C except for a very small peak of Fe2O3 was detected for the samples calcined at 600 and 700°C by the polymerization method. The crystal size and particle size increased with an increase in the sintering temperature using both methods. The conductance of the MgFe2O4 decreased when the atmosphere was changed from ambient air to air containing 10.0ppm NO2. The conductance change, C = G(air)/G(10ppm NO2), was reduced with an increase in the operating temperature. For the polymerization method, the maximum C-value was ca. 40 at 300°C for the samples sintered at 900°C. However, the samples sintered at 1000°C showed a low conductance change in the 10ppm NO2 gas, because the ratio of the O2 gas adsorption sites on the particle surface is smaller than those of the samples having a high C-value. The low Mg content on the surface affects the low ratio of the gas adsorption sites. For the reverse coprecipitation method, the particle size was smaller than that of the polymerization method. Although a stable conductance was obtained for the sample sintered at 900 and 1000°C, its conductance change was less than that of the polymerization method. [Copyright &y& Elsevier]

Fine Mg1-xCaxFe2O4 ferrite powders for local thermal coagulation therapy were synthesized by a reverse coprecipitation method. The enhancement of temperature under an AC magnetic field was improved by the partial Ca2+ substitution. A high heat ability was obtained for the samples calcined at 300 and 800 °C. [ABSTRACT FROM AUTHOR]

Abstract: Nano-sized magnetic Y3Fe5O12 ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y3Fe5O12 ferrites. The highest heat ability in the AC magnetic field was for the fine Y3Fe5O12 powder with a 15-nm crystallite size (the samples were milled for 4h using 0.1mmϕ beads). The heat generation ability of the excessively milled Y3Fe5O12 samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4h using 0.1mmϕ beads, the heat generation ability (Wg−1) was estimated using a 3.58×10−4 fH 2 frequency (f/kHz) and the magnetic field (H/kAm−1), which is the highest reported value of superparamagnetic materials. [Copyright &y& Elsevier]

Abstract: Nano-sized FeFe2O4 ferrite powder having a heat generation ability in an AC magnetic field was prepared by bead milling for a thermal coagulation therapy application. A commercial powder sample (non-milled sample) of ca. 2.0μm in particle size showed a temperature enhancement (ΔT) of 3°C in an AC magnetic field (powder weight 1.0g, 370kHz, 1.77kAm−1) in ambient air. The heat generation ability in the AC magnetic field improved with the milling time, i.e., due to a decrease in the average crystallite size for all the examined ferrites. The highest heat ability (ΔT =26°C) in the AC magnetic field in ambient air was for the fine FeFe2O4 powder with a 4.7nm crystallite size (the samples were milled for 6h using 0.1mmϕ beads). However, the heat generation ability decreased for the excessively milled FeFe2O4 samples having average crystallite sizes of less than ca. 4.0nm. The heat generation of the samples showed some dependence on the hysteresis loss for the B–H magnetic property. The reasons for the high heat generation properties of the milled samples would be ascribed to an increase in the Néel relaxation of the superparamagnetic material. The hysteresis loss in the B–H magnetic curve would be generated as the magnetic moment rotates (Néel relaxation) within the crystal. The heat generation ability (Wg−1) can be estimated using a 1.07×10−4 fH 2 frequency (f, kHz) and the magnetic field (H, kAm−1) for the samples milled for 6h using 0.1mmϕ beads. Moreover, an improvement in the heating ability was obtained by calcination of the bead-milled sample at low temperature. The maximum heat generation (ΔT =59°C) ability in the AC magnetic field in ambient air was obtained at ca. 5.6nm for the sample calcined at 500°C. The heat generation ability (Wg−1) for this heat treated sample was 2.54×10−4 fH 2. [Copyright &y& Elsevier]

Abstract: We found the most promising powder material for the application of the thermal coagulation therapy for the treatment of cancerous tissues. The maximum heat generation ability (ΔT=40–77°C, 370kHz, 1.77kA·m−1) was obtained for the powder materials by the calcination at 1100°C for the Y3−XGdXFe5O12 system. This ΔT value is higher than ca. ΔT=30°C in same magnetic field for fine FeFe2O4 particles as the candidate material for this type of therapy. The particle growth with the formation of the cubic single phase might influence to the high heat generation. As an unexpected result, the Gd3Fe5O12 (X=3) has no heat generation ability in an AC magnetic field. [Copyright &y& Elsevier]