S. R. Murthy

Low temperature sintering of MgCuZn ferrite and its electrical and magnetic properties

The low temperature sintering of MgCuZn ferrite was investigated using the usual ceramic method. The effect of Cu substitution on the properties of MgZn ferrites was also investigated and it was found that the densification of MgCuZn ferrite is dependent upon Cu concentration. The sintered ferrite with a density of 4.93 g/cm3 and electrical resistivity > 1011 Ω-cm was obtained for the ferrite with 12 mol% Cu at relatively low sintering temperature (910° C). The magnetic properties of the ferrites also improved by the Cu substitution. The chip inductors made of the ferrite fired at 910 C with 12 mol% Cu exhibited higher d.c. resistance. From these studies it is concluded that the good quality chip inductor can be obtained using the MgCuZn ferrites.

Development of low-power loss Mn-Zn ferrites using microwave sintering method

Microwave sintering (MS) method has been successfully used for densifying Mn- Zn ferrites used for high frequency applications. This method needs only a short time to obtain high density when compared to conventionally sintered (CS) Mn- Zn ferrites. The lowest power loss was also achieved at 100 kHz and 200 mT condition for the microwave sintered samples. Conductor- embedded ferrite transformers were constructed using CS and MS samples and output power, efficiency, and surface rise of temperature were measured at sinusoidal voltage of 25 V with frequency, 1 MHz. The efficiency and surface rise of temperature of transformer were found to be high and low, respectively.

Effect of pH on structural and magnetic properties of nanocrystalline Y3Fe5O12 by aqueous co-precipitation method

Abstract The Y3Fe5O12 (YIG) nanopowders were synthesised at different pH using co-precipitation method. The effect of pH on the phase formation of YIG is characterised using XRD, TEM, FTIR and TG/DTA. From the Scherer formula, the particle sizes of the powders were found to be 13, 19 and 28 nm for pH = 10, 11 and 12 respectively. It is found that as the pH of the solution increase the particle size is also increases. It is also clear from the TG/DTA curves that as the pH is increasing the weight losses were found to be small. The nanopowders were sintered at 600, 700, 800 and 900°C for 5 h using conventional sintering method. The phase formation is completed at 800°C/5 h which is correlated with TG/DTA. The average grain size of the samples is found to be ∼161 nm. The high values of Ms = 23 emu g−1 and Hc = 22 Oe were recorded for the sample sintered at 900°C.

Development of nanocrystalline Mn–Zn ferrites for forward type DC–DC converter for switching mode power supplies

Abstract Nanocrystalline Mn–Zn ferrites have been successfully synthesised using the microwave–hydrothermal method for high frequency applications. The nanopowders were characterised using X-ray diffraction (XRD) and transmission electron microscopy (TEM). They were annealed using the microwave sintering method at 900°C for 20 min. The frequency dependence of the dielectric constant ϵ′ and and the initial permeability μ i were measured in the range 10 Hz to 1·3 GHz. The saturation magnetisation M s and coercive force H c were obtained using a vibration sample magnetometer (VSM) in the field of 20 kOe. The total power loss was measured in the range of 100 kHz to 1 MHz with a flux density of 50 mT on the annealed samples. Conductor-embedded ferrite transformers were fabricated and the output power P o and efficiency η were measured; 80% efficiency was obtained for a forward-type multilayered transformer.

Structural stability of methane hydrates in porous medium: Raman spectroscopic study

Thermal and temporal stability of the methane hydrates (MH) at ambient pressure, synthesised in a spherical silica (solid and hollow, with average diameter of 70 μm) matrix, are investigated by the Raman spectroscopy. Identical Raman spectroscopic spectral features for all the synthesized hydrate samples indicate structural resemblance irrespective of matrix. It is observed that the growth of hydrates in hollow silica matrix is homogeneous, while that with solid grain is highly heterogeneous. Temporal and thermal stability of MH depends on the silica matrix. Appearance of the Raman signatures characteristic of MH, in hollow silica, indicates that the hydrates are stable over several hours (upon preserving at 153 K and 0.1 MPa) and until ∼273 K at 0.1 MPa. However, MH in solid silica matrix is highly unstable under similar P, T conditions and they are readily dissociated within 2 h. The thermal stability of these samples at 0.1 MPa is also significantly lower.

Fabrication of dc–dc converter using nanocrystalline Mn–Zn ferrites

Abstract The microwave–hydrothermal method has been successfully used for synthesis of nanocrystalline Mn–Zn ferrites which are used for high frequency applications. The nanopowders were characterised using X-ray diffraction and TEM. The nanopowders were annealed using microwaves at 600°C/10 min. The frequency dependence of dielectric constant ϵ′ was measured in the range from 10 Hz to 1·3 GHz, and initial permeability μ i was measured in the range from 10 Hz to 1 MHz. The total power loss was measured at 100 kHz and 200 mT on the annealed samples. Conductor embedded ferrite transformers were fabricated, and output power P o, efficiency η and temperature increase ΔT were measured at sinusoidal voltage of 25 V with frequency 1 MHz. The transformer efficiency η was found to be high, and surface rise of temperature ΔT is very low.

Microwave Hydrothermal Synthesis and Characterization of Nanosize NiCuZn Ferrites

Ni0.5‐xCuxZn0.5Fe2O4 (x=0,4,6,8,12 mol%) nanoferrite powders were synthesized using microwave‐hydrothermal (M‐H) method. Nanophase ferrites with high surface area were synthesized at 160 °C after a treatment time of 1 h. The ferrite formation conditions such as pH, temperature and time were optimized. The phase identification of the prepared samples was done by X‐ray diffraction, characterized by IR spectra and crystal size and morphology were determined by transmission electron microscopy (TEM). The size of the nanoferrite powders synthesized was 20–60 nm. The nanoferrite powders were conventionally sintered at a temperature of 900 °C/4 h. The variations of the sintered density, saturation magnetization and electrical resistivity as a function of copper concentration at room temperature have been investigated. Dielectric properties were measured in the frequency range of 1 MHz–1.8 GHz and obtained results were understood with the help of available theories.

DIELECTRIC AND MAGNETIC PROPERTIES OF BaTiO3+MgCuZnFe2O4 NANOCOMPOSITES

Nanocrystalline MgCuZnFe2O4 and BaTiO3 powders were synthesized using the microwave hydrothermal (M-H) method at 160°C for 45 min for the preparation of xBaTiO3+(1-x)MgCuZnFe2O4. The nanopowders were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The particle size of the powders was found to be ~30 and ~40 nm for MgCuZnFe2O4 and BaTiO3, respectively. The nanopowders were mixed at different weight percentages and densified at 910°C for 30 min using the microwave sintering method. The sintered composites were characterized using XRD and scanning electron microscopy (SEM). The density of the composites varies 93–96% of theoretical density. The density of the present composites increases as the weight percentage of BaTiO3 content increases. The frequency variation of dielectric constant (e), dissipation factor (D), initial permeability (μi) and quality factor (Q) were measured from 1 kHz to 1 MHz.

In-situ FTIR study of dehydration and rehydration mechanisms of natural scolecite

In-situ FTIR spectroscopy was used to investigate the dehydration mechanism of naturally occurring scolecite from the Deccan traps. The dehydration and rehydration processes were studied by monitoring the thermally induced variations of fundamental bending (v 2 ) and stretching (v 3 and v 1 ) modes of the water molecules in the range 1550–1800 cm −1 and 3000–3600 cm −1 , as well as the corresponding second-order modes in the wavenumber region 4000–8000 cm −1 . Completely reversible spectral variations of the fundamental modes indicate that the transformation of scolecite to meta-scolecite at 495 K is reversible. Two-step dehydration behaviour is clearly observed in the thermally induced variations of the second-order modes. A combination mode in scolecite, due to Al-OH bonding around 4609 cm −1 , is seen to disappear in the meta-scolecite phase. Growth of a new mode around 4485 cm −1 is observed in the temperature range 630–675 K, indicating the presence of hydroxyl groups in the amorphous phase, created by the breaking of T-O-T bonds.

A study of ultrasonic velocity and attenuation on polycrystalline Ni-Zn ferrites

Polycrystalline NiZn ferrites with different grain sizes (1.2 (Am to 10.2 (Am) were prepared by the usual ceramic method. The magnetic properties were measured at room temperature. The ultrasonic velocity and attenuation were measured on Ni-Zn ferrite by using the pulse transmission method at 1 MHz, in the temperature range 300–600 K. The velocity was found to be slightly sample dependent at room temperature and decreased with increasing temperature, except near the Curie temperature,Tc,where a small anomaly was observed. The longitudinal attenuation (α1)at room temperature was found to be more sample dependent. The temperature variation of ultrasonic longitudinal attenuation exhibited a broad maximum around 400 K and a sharp maximum just below Curie temperature (Tc).The above observations were carried out in the demagnetized state. The application of a 380 mT magnetic field allowed us to reach the saturated state of the sample at all the measuring temperatures. The anomaly observed in the thermal variation of velocities (longitudinal and transverse) and attenuation has been qualitatively explained with the help of the temperature variation of the magneto-crystalline anisotropy constant.