Thermally induced changes in the magnetic properties of iron oxide nanoparticles under reducing and oxidizing conditions

Application of iron oxide nanoparticles in the fields of water purification, biomedicine or chemistry often requires controlled magnetic properties that can be modified by changing temperature and redox conditions. Therefore, this work investigates the changes in the magnetic properties of iron oxide nanoparticles in the FeOOH − Fe₂O₃ − Fe₃O₄ system (i.e. hematite, goethite, lepidocrocite, maghemite and magnetite) at heating under reducing and oxidizing conditions. The results show that heat treatment of hematite and goethite in the presence of a reducing agent (5% starch) leads to their conversion into high magnetic magnetite. The starting temperature of transformation is approximately 350 °C for both samples. The magnetization increases to 86 Am²/kg for hematite reduced at 700 °C and to 88 Am²/kg for goethite reduced at 900 °C. An intense reaction occurs within the first 10 min and then the conversion process decelerates. Thermal treatment of lepidocrocite under both oxidizing and reducing conditions leads to an increase in magnetization due to the formation of maghemite and magnetite, respectively. Regardless of the redox conditions, the formation of magnetic phase begins at a temperature of 250 °C and is associated with the formation of maghemite from lepidocrocite. Under oxidizing conditions, the magnetization begins to decrease at 350 °C, which is associated with the conversion of maghemite to hematite. On the contrary, under reducing conditions, the magnetization of lepidocrocite increases up to 900 °C, which is associated with the formation of magnetite. Maximum values of magnetization are 36 Am²/kg for maghemite obtained at 350 °C, and 88 Am²/kg for magnetite obtained at 900 °C from lepidocrocite. With the help of conventional heating, the magnetic properties of IONs can be altered by phase transformations in the FeOOH − Fe₂O₃ − Fe₃O₄ system. Temperature and redox conditions are the two most important factors controlling the transformation pathways and the magnetic properties of the resulting IONs.     

Effect of annealing parameters on the magnetic properties of NiZn ferrite thin films

Ni_0.5Zn_0.5Fe_2.0O_4.0 thin films (NZFs) were deposited on Si (100) substrate by a sol–gel method, and the effects of annealing parameters on the structure and magnetic properties of the proposed films were investigated. Moderate heating rate was beneficial to the nucleation of NZFs. When the heating rate was 2 °C/min the saturation magnetization (M _s) achieved its maximum and the coercivity (H _c) reached its minimum. Both the crystallization and M _s of NZFs enhanced with increasing annealing time; however, H _c changed contrarily. High quenching temperature produced a large stress and consequently deteriorated magnetic properties. The optimal annealing parameters of NZFs were annealed at 700 °C, heating rate 2 °C/min, annealing time 1 h, and gradually cooled to room temperature. Finally, NZFs showed a high magnetization of 320 emu/cm³ and low coercivity of 86 Oe.     

Preparation and studies on surface modifications of calcium-silico-phosphate ferrimagnetic glass-ceramics in simulated body fluid

The structure and magnetic behaviour of 34SiO₂–(45 − x) CaO–16P₂O₅–4.5 MgO–0.5 CaF₂ − x Fe₂O₃ (where x = 5, 10, 15, 20 wt.%) glasses have been investigated. Ferrimagnetic glass-ceramics are prepared by melt quench followed by controlled crystallization. The surface modification and dissolution behaviour of these glass-ceramics in simulated body fluid (SBF) have also been studied. Phase formation and magnetic behaviour have been studied using XRD and SQUID magnetometer. The room temperature Mössbauer study has been done to monitor the local environment around Fe cations and valence state of Fe ions. X-ray photoelectron spectroscopy (XPS) was used to study the surface modification in glass-ceramics when immersed in simulated body fluid. Formation of bioactive layer in SBF has been ascertained using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The SBF solutions were analyzed using an absorption spectrophotometer. The magnetic measurements indicated that all these glasses possess paramagnetic character and the [Fe²⁺/Fe³⁺] ions ratio depends on the composition of glass and varied with Fe₂O₃ concentration in glass matrix. In glass-ceramics saturation magnetization increases with increase in amount of Fe₂O₃. The nanostructure of hematite and magnetite is formed in the glass-ceramics with 15 and 20 wt.% Fe₂O₃, which is responsible for the magnetic property of these glass-ceramics. Introduction of Fe₂O₃ induces several modifications at the glass-ceramics surface when immersed in SBF solution and thereby affecting the surface dissolution and the formation of the bioactive layer.     

Non-isothermal crystallization of As2Se3 glass

The results of non-isothermal crystallization studies performed at different heating rates on batches of As₂Se₃ glasses prepared from melts at 400°C, 600°C and 800°C are reported. The peak temperature of crystallizationT _p, the enthalpy of crystallization ΔH _c and the activation energy for crystallizationE _c are independent of the melt temperature used in the preparation. Bulk nucleation with three-dimensional growth of crystals is indicated for As₂Se₃. The values of ΔH _c andE _c are found to be respectively 23·3 ± 0·9 cal/g and 36·5 ± 0·9 kcal/mol for As₂Se₃.     

High-temperature electrical conductivity and thermal expansion of SmBa2(Cu1-xFex 3O6+δ (x = 0–0.2)

The electrical conductivity and thermal expansion of SmBa₂(Cu_1-xFe_{x 3}O_6+δ (x = 0-0.2) were measured in air in the temperature range from 20 to 900°C. The linear thermal expansion coefficient was determined to be (12.8–13.5) × 10^-6 K^-1 in the temperature range from 20 to 350°C and (16.2–17.8) × 10^-6 K^-1 in the range from 350 to 800°C. Between 400 and 900°C, the conductivity of SmBa₂(Cu_1-xFe_x)₃O_6+δ was found to decrease with increasingx, mainly in the rangex = 0–0.1. This finding was interpreted in terms of the Fe occupancies on the Cu(1) and Cu(2) sites.     

Crystallization characteristic and properties of some zinc containing soda lime silicate glasses

The crystallization behaviour of some soda lime silicate glasses modified by ZnO/CaO replacement to give the composition (Na₂O)₂·CaO_1−x ·(ZnO) _x ·3SiO₂ (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been investigated using differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD). The thermal expansion coefficients and AC electrical properties in the frequency range 40 Hz–5 MHz of the obtained crystalline products were determined. Two forms of sodium calcium silicate (Na₄CaSi₃O₉ & Na₂Ca₂Si₃O₉), sodium metasilicate-Na₂SiO₃, two types of sodium zinc silicate (Na_1.31Zn_0.655Si_1.345O₄ & Na₂ZnSiO₄) and α-quartz phases were mostly developed in the crystallized glasses using various heat-treatment processes. The coefficient of thermal expansion of the obtained glass–ceramic materials are between 120 × 10⁻⁷°K⁻¹ and 168 × 10⁻⁷°K⁻¹ in the 25°–600 °C temperature range. The increase of frequency generally resulted in the increase of the conductivity and decrease the dielectric constant together with the loss tangent of the glass–ceramic materials.     

Containerless processing of a lithium disilicate glass

Glasses of Li₂O · 2SiO₂ (LS₂), and LS₂ doped with 0.001 wt% platinum (LS₂ + 0.001 wt% Pt) compositions were melted, cooled and reheated at controlled rates while levitated (containerless) inside an electrostatic levitator (ESL) furnace at the NASA Marshall Space Flight Center. The experiments were conducted in vacuum using spherical, 2.5–3 mm diameter, glass samples. The measured critical cooling rate for glass formation, R _c, for the LS₂ and LS₂ + 0.001 wt% Pt glasses processed at ESL were 14 ± 2 °C/min and 130 ± 5 °C/min, respectively. The values of R _c for the same LS₂ and LS₂ + 0.001 wt% Pt glasses processed in a container were 62 ± 3 °C/min and 162 ± 5 °C/min, respectively. The effective activation energy for crystallization, E, for this LS₂ glass processed without a container at ESL, was higher than that for an identical glass processed in a container. These results suggest that the glass formation tendency for a containerless LS₂ melt is significantly increased compared to an identical melt in contact with a container. The absence of heterogeneous nucleation sites that are inherently present in all melts held in containers is believed to be the reason for the increased glass forming tendency of this containerless melt.     

Effect of iron state on crystallization and dissolution in Fe2O3-CaO-SiO2 glasses

The possibility of iron-containing glasses as thermoseeds for hyperthermia of bone tumor was reported previously. There is, however, no report about the effect of iron state on the crystallization of magnetite and the resultant properties. The iron states were determined by Mo¨ssbauer spectroscopy in Fe₂O₃-CaO-SiO₂ system. It was found that the higher CaO content interrupts the crystallization of magnetite crystallites as well as the oxidation of iron, that is, the transformation from Fe³⁺ to Fe²⁺. A sample containing large amounts of Fe²⁺ showed the faster increment of temperature when the alternating magnetic field was applied. In order to use the thermoseed for a hyperthermia, we can say that the composition with low CaO content is most useful.     

Optical and structural investigations on iron-containing phosphate glasses

The article reports the preparation and complex characterization of iron-containing phosphate glasses considered to be ecological materials, as they contain non-toxic compounds related to environment. The oxide system Li₂O?CMgO?C(CaO)?CAl₂O₃?CP₂O₅?C(FeO/Fe₂O₃) was investigated in respect to its structural changes caused by MgO replacement with CaO and by the iron addition. UV?Cvis?CNIR (ultraviolet?Cvisible?Cnear infrared) spectroscopy as well as thermo-gravimetric (TG) measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD) analysis, electronic paramagnetic resonance (EPR), and Mossbauer (nuclear gamma resonance) spectroscopy have been used to investigate redox states and coordination symmetry of iron, together with vitreous network changes during the heat treatment up to 1000 °C. UV?Cvis?CNIR transmission spectroscopy revealed no structural modifications when MgO was substituted by CaO, but noteworthy absorption bands attributed to Fe²⁺/Fe³⁺ species. TG analysis made in the 20?C1000 °C range shows low weight loss accompanied by several thermal effects, as evidenced by DTA. XRD patterns for the glass samples heat treated at about 700 °C revealed the presence of different phosphate crystalline phases containing Mg, Al, and Fe ions. EPR spectroscopy revealed the presence of paramagnetic Fe³⁺ ions and the change of the first coordination symmetry, when the samples are heated below the vitreous transition temperature. Mossbauer spectroscopy has evidenced two paramagnetic species, Fe²⁺ and Fe³⁺, both in octahedral coordination symmetry and a clustering process supported by only Fe³⁺ ions.     

Formation and properties of SnO–MgO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

A new glass system SnO–MgO–P₂O₅ with low viscosity has been developed by a melt-quenching method. Formation, thermal properties, and chemical durability of these glasses have been investigated. For a constant P₂O₅ concentration, the glass formation ability is enhanced with the increasing Sn/(Sn + Mg) ratio. The glasses exhibit low glass transition temperature (T _g = 270–400 °C), low dilatometric softening temperature (T _DS = 290–420 °C), and high thermal expansion coefficient (CTE = 110–160 × 10⁻⁷ K⁻¹). With the increasing Sn/(Sn + Mg) ratio, T _g and T _DS decrease, and CTE increases. When Sn/(Sn + Mg) ratio is varied, the relationship between chemical durability and thermal properties of the present glasses is not consistent with what expected in general cases. It is noted that the glasses with 32–32.5 mol% P₂O₅ exhibit excellent chemical durability and tunable T _g, T _DS, and CTE (by varying Sn/(Sn + Mg) ratio).     

Effect of crystallite size on oxidation and reduction behavior of a manganesesubstituted magnetite of composition Mn0.67Fe2.33O4

Thermogravimetry and electrical conductivity were used to determine the effect of crystallite size on oxidation and reduction behavior of a manganese substituted magnetite Mn_0.67Fe_2.33O₄ containing several oxidizable cations. We have found that within the single-phase region of the spinel (below 550°C) oxidation and reduction temperatures increase with increase of particle size. A quantitative analysis of cations suggests that there exists a grain size of 45 nm above which the oxidation characteristics of three oxidizable cations (Fe²⁺,Mn³⁺,Mn²⁺) change. Above 600°C, the temperature of structural change spinel → corundum increases with decrease in particle size owing to stresses at the crystal lattice level.     

Evaluation of CaO–SiO<Subscript>2</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>–Na<Subscript>2</Subscript>O–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> bioglass-ceramics for hyperthermia application

Magnetic bioglass ceramics (MBC) are being considered for use as thermoseeds in hyperthermia treatment of cancer. While the bioactivity in MBCs is attributed to the formation of the bone minerals such as crystalline apatite, wollastonite, etc. in a physiological environment, the magnetic property arises from the magnetite [Fe₃O₄] present in these implant materials. A new set of bioglasses with compositions 41CaO · (52 ? x)SiO₂ · 4P₂O₅  · xFe₂O₃ · 3Na₂O (2 ≤ x ≤ 10 mol% Fe₂O₃) have been prepared by melt quenching method. The as-quenched glasses were then heat treated at 1050°C for 3 h to obtain the glass-ceramics. The structure and microstructure of the samples were characterized using X-ray diffraction and microscopy techniques. X-ray diffraction data revealed the presence of magnetite in the heat treated samples with x ≥ 2 mol% Fe₂O₃. Room temperature magnetic property of the heat treated samples was investigated using a Vibrating Sample Magnetometer. Field scans up to 20 kOe revealed that the glass ceramic samples had a high saturation magnetization and low coercivity. Room temperature hysteresis cycles were also recorded at 500 Oe to ascertain the magnetic properties at clinically amenable field strengths. The area under the magnetic hysteresis loop is a measure of the heat generated by the MBC. The coercivity of the samples is another important factor for hyperthermia applications. The area under the loop increases with an increase in Fe₂O₃ molar concentration and the. coercivity decreases with an increase in Fe₂O₃ molar concentration The evolution of magnetic properties in these MBCs as a function of Fe₂O₃ molar concentration is discussed and correlated with the amount of magnetite present in them.     

Infrared study of the effect of heat treatment and irradiation on barium borate glass containing iron

A barium borate glass system was prepared containing different amounts of iron. The prepared glasses were heat treated at 550° C for 2, 6, 12, 18 and 24 h. Also the glasses were irradiated usingψ-ray at a dose of 4.805 × 10₄rad h⁻¹ for 12, 18 and 24 h. The infrared spectra were recorded for the untreated and heat treated samples. It was found that, when the Fe₂O₃ was introduced in the glass the triangle BO₃ groups were transferred to BO₄ groups. The formation of non-bridging oxygen with high concentration was also observed as a result of introducing Fe₂O₃ in the glass. The absorption bands of the IR spectra of the irradiated samples indicated no significant variations, and only a transfer of some BO₄ groups to BO₃ groups could be observed.     

Microstructures formed during devitrification of Na2O·Al2O3·B2O3·SiO2·Fe2O3 glasses

Soda alumina borosilicate glasses of composition (24-y)Na₂O·yAl₂O₃·14B₂O₃·37SiO₂·25Fe₂O₃, y = 8, 12, 14, 16 mol%, were melted using Fe₂O₃ as raw material. Besides, samples with y = 12 and Fe₂O₃ concentrations of 14.32, 17.8, and 25.0 mol% were prepared from FeC₂O₄·2H₂O as raw material. The X-ray diffraction analyses showed the presence of magnetite for the samples from all the investigated compositions. Transmission electron microscopy (TEM) evidenced that all the samples are phase separated and droplets in the diameter range 100–1000 nm, enriched in iron, are formed. Inside these droplets, numerous small magnetite particles, with size in the 25–40 nm interval, are crystallized.     

Mössbauer spectroscopy and X-ray diffraction of oxide precipitates formed from FeSO4 solution

Chemical and structural properties of oxide precipitates formed from FeSO₄ solution were investigated using X-ray diffraction and⁵⁷Fe Mössbauer spectroscopy. The hydrolysis of urea at elevated temperature was used for the generation of OH^– ions during the precipitation process. The formation of particular oxide phase in the precipitate is strongly dependent on the concentrations of FeSO₄ and urea, as well as on the rate of oxygenation. The phase analysis of precipitates showed the presence of different oxide phases, such as goethite, lepidocrocite, hematite and magnetite, and in one sample of a small amount of siderite. Only substoichiometric magnetite, Fe_3–x O₄, was detected. Significant differences in the Mössbauer spectrum of goethite were observed, due to a very small particle size, the degree of crystallinity and/or different content of structurally bonded water. The correlation between the Mössbauer spectra of precipitated goethite and goethite formed during the atmospheric corrosion of steel is discussed.     

Structural properties of lead vanadate glasses containing La3+ or Fe3+ ions

Structural properties of lead vanadate glasses containing La³⁺ or Fe³⁺ ions were investigated using X-ray diffraction, Fourier transform infrared spectroscopy and laser Raman spectroscopy. Crystalline Pb₂V₂O₇ was formed for the molar composition 66.7PbO-33.3V₂O₅. Incorporation of greater quantities of La³⁺ into lead metavanadate glass caused the crystallization of Pb₂V₂O₇. Fourier transform infrared and laser Raman spectra also suggested the presence of LaVO₄. Incorporation of Fe³⁺ ions into lead metavanadate glass, up to 20 wt% Fe₂O₃, did not cause crystallization inside the glass matrix. Changes in the vibrational spectra are discussed.     

Compositional dependence of the structural and dielectric properties of Li<Subscript>2</Subscript>O–GeO<Subscript>2</Subscript>–ZnO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

(10Li₂O–20GeO₂–30ZnO–(40-x)Bi₂O₃–xFe₂O₃ where x = 0.0, 3, 6, and 9 mol%) glasses were prepared. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, DC and AC conductivities, and dielectric properties (constant ε′, loss tan δ, AC conductivity, σ _ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of iron ion concentration. The analysis of the results indicate that, the density and molar volume decrease with an increasing of iron content indicates structural changes of the glass matrix. The glass transition temperature T _g and onset of crystallization temperature T _x increase with the variation of concentration of Fe₂O₃ referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter ΔT decrease with increase Fe₂O₃ content, indicates an increasing concentration of iron ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BiO₃, BiO₆, ZnO₄, GeO₄, and GeO₆. The structural changes observed by varying the Fe₂O₃ content in these glasses and evidenced by FTIR investigation suggest that the iron ions play a network modifier role in these glasses while Bi₂O₃, GeO₂, and ZnO play the role of network formers. The temperature dependence of DC and AC conductivities at different frequencies was analyzed using Mott’s small polaron hopping model and, the high temperature activation energies have been estimated and discussed. The dielectric constant and dielectric loss increased with increase in temperature and Fe₂O₃ content.     

Enhancement of structural stability of nanosized amorphous Fe2O3 powders by surface modification

The surface of low-dimensional solids plays a key role in their phase transition. In the present study, to enhance the structural stability of nanosized amorphous Fe₂O₃ powders their surfaces were modified by employing NaOH solution, which leads to an increase in both the crystallization temperature from 364 °C to 411 °C and the crystallization activation energy from 81.5 kJ/mol to 156.8 kJ/mol. The surface-modified amorphous Fe₂O₃ powders show an entirely different crystallization behavior as compared with the as-prepared amorphous powders. The enhanced structural stability is attributed to the increase of the amount of hydroxide groupings at the surfaces of amorphous powders, which lowers their surface energy.     

Crystallization of glasses obtained by recycling goethite industrial wastes to produce glass-ceramic materials

Goethite waste, originated in the hydrometallurgy of zinc, was characterized and recycled, in combination with raw materials, for producing glass-ceramic (GC) materials. Four base compositions were prepared with an Fe₂O₃ content ranging from 15 to 25 wt%. The mixtures were melted at 1400–1450 °C and quenched to obtain the glass. The nucleation and crystallization temperatures and the activation energy of the crystallization process were determined by differential thermal analysis. The glass samples were nucleated at 660 °C for times ranging from 1 to 4 h and crystallized at 800–900 °C for 1 to 4 h. X-ray diffraction and fluorescence analysis were performed on the glasses and GC samples and the crystalline phases characterized. The percentage of crystallinity was determined as a function of the temperature and duration of the thermal treatment. The fractional factorial experimental approach was carried out on the 20 wt% Fe₂O₃ composition with the aim of evaluating the influence of the nucleation temperature and time, the crystallization temperature and time and the presence of a reducing agent on the glass devitrification process.