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We have studied the effect of phosphate capping on the high temperature thermal stability and magnetic properties of magnetite (Fe3O4) nanoparticles synthesized through a single-step co-precipitation method. The prepared magnetic nanoparticles are characterized using various techniques. When annealed in air, the phosphate capped nanoparticle undergoes a magnetic to non-magnetic phase transition at a temperature of 689 °C as compared to 580 °C in the uncoated nanoparticle of similar size. The observed high temperature phase stability of phosphate capped nanoparticle is attributed to the formation of a phosphocarbonaceous shell over the nanoparticles, which acts as a covalently attached protective layer and improves the thermal stability of the core material by increasing the activation energy. The phosphocarbonaceous shell prevents the intrusion of heat, oxygen, volatiles, and mass into the magnetic core. At higher temperatures, the coalescence of nanoparticles occurs along with the restructuring of the phosphocarbonaceous shell into a vitreous semisolid layer on the nanoparticles, which is confirmed from the small angle X-ray scattering, Fourier transform infra red spectroscopy, and transmission electron microscopy measurements. The probable mechanism for the enhancement of thermal stability of phosphocarbonaceous capped nanoparticles is discussed. [ABSTRACT FROM AUTHOR]

The thermal stability and magnetic properties of iron oxide nanomaterials under different environmental conditions are important for technological applications. Herein, we report the influence of dynamic capping of iron oxide nanoparticles with phosphate and oleic acid, on their structure, magnetic properties and thermal stability and compare the results with that of uncoated nanoparticles. The composition, structure, morphology, quality of capping, thermal stability and magnetic properties of prepared magnetite nanoparticles are evaluated by using X-Ray Diffractometer (XRD), Small angle X-ray scattering (SAXS), High resolution transmission electron microscopy (HRTEM), Energy dispersive X-ray (EDX) analysis, Fourier transform infrared (FTIR) spectrometer, Thermo gravimetric analyzer (TGA), Differential scanning calorimetry (DSC) and Vibrating Sample Magnetometer (VSM). Our results suggest that the rapid adsorption of phosphate ions onto specific crystal planes of nucleated magnetite enabled the growth of magnetite core into nearly cubic shape while the oleic acid capping enabled a homogeneous growth of magnetite core along all the crystallographic directions, enabling the formation of nearly spherical particles with narrow size distribution. On the contrary, the phosphate adsorbs as multilayer islands on nanoparticles, leading to a broader size distribution, identical to the uncoated particles. The HRTEM analysis showed dark areas with asymmetric contrast in uncoated nanoparticles, which was not observed in phosphate and oleic acid capped magnetite. The FTIR study on phosphate coated magnetite particles confirms that the surface capping is a mixture of mono and non-protonated monodentate phosphate complexes. The DSC studies confirm a 100 °C enhancement in the magnetic phase thermal stability in phosphate-coated particles, as compared to uncoated and oleic acid coated magnetite. The temperature dependant magnetization results showed a lower blocking temperature for phosphate capped nanoparticles due to the denser multilayer islands of phosphate over magnetite as compared to a uniform close packed monolayer coverage of oleic acid. These results are useful for tailoring stable superparamagnetic nanoparticles for technological applications. [ABSTRACT FROM AUTHOR]

We study the magnetic field induced hyperthermia in water based phosphate coated Fe 3 O 4 nanofluids, synthesized by a co-precipitation method using ferrous and ferric salt solutions, ammonia and orthophosphoric acid. The specific absorption rate (SAR) values were measured at a fixed frequency of 126 kHz and at extremely low field amplitudes. The SAR values were determined from the initial rate of temperature rise curves under non-adiabatic conditions. It was observed that the SAR initially increases with sample concentration, attains a maximum at an optimum concentration and beyond which SAR decreases. The decrease in SAR values beyond the optimum concentration was attributed to the enhancement of dipolar interaction and agglomeration of the particles. The system independent intrinsic loss power (ILP) values, obtained by normalizing the SAR values with respect to field amplitude and frequency, were found to vary between 158–125 nHm 2 kg −1 , which were the highest benchmark values reported in the biologically safe experimental limit of 1.03–0.92×10 8 Am −1 s −1 . The very high value of ILP observed in the bio-compatible phosphate coated iron oxide nanofluids may find practical applications for these nanoparticles in tumor targeted hyperthermia treatment. [ABSTRACT FROM AUTHOR]