Solid State Phenomena Vol. 189

Abstract: Although Heusler alloys have been known for more than a century, but since the last decade there has been a quantum jump in research in this area. Heusler alloys show remarkable properties, such as ferromagnetic shape memory effect, magnetocaloric effect, half metallicity, and most recently it has been shown that it can be used for direct conversion of heat into electricity. Heusler alloys Ni-Mn-Z (Z=Ga, Al, In, Sn, Sb), show a reversible martensitic transformation and unusual magnetic properties. Other classes of intermetallic Heusler alloy families that are half metallic (such as the half Heusler alloys Ni-Mn-Sb and the full Heusler alloy Co₂MnGe) are attractive because of their high Curie temperature and structural similarity to binary semiconductors. Unlike Ni-Mn-Ga, Ni-Mn-In and Ni-Mn-Sn transform from ferromagnetic austenite to non-ferromagnetic martensite. As is consistent with the Clausius-Clapeyron equation, the martensitic phase transformation can be manipulated by a magnetic field, leading to possible applications of these materials enabling the magnetic shape memory effect, energy conversion and solid state refrigeration. In this paper, we summarize the salient features of Heusler alloys, like the structure, magnetic properties and potential application of this family of alloys in industry.

Abstract: This paper attempts to provide a historical survey of structure of various types of hexaferrites. It provides information about synthesis, characterization, structural, magnetic and dielectric properties of Y-type hexagonal ferrites using various chemical routes. We have prepared a series of cobalt doped Sr₂Cu_2-xCo_xFe₁₂O₂₂ (x = 0.0 to 1.0) hexaferrites using a wet chemical co-precipitation technique. The prepared hexaferrite precursors were calcined at 950 °C for 4 hours in a furnace and slowly cooled to room temperature. The crystal structure of Y-type hexaferrites is rather complicated. The chemical and structural changes were examined in detail by X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Scanning electron microscopy (SEM), and Fourier transform infra-red (FTIR) spectroscopy. X-ray diffraction studies showed that sintering temperature as low as 950°C was sufficient to produce a single-phase Y-type hexaferrite material. The dielectric measurements were carried out over the frequency range of 100 Hz to 2 MHz at room temperature using an LCR meter to study the variation of dielectric constant and loss tangent with frequency. The magnetic properties of hexaferrite samples were investigated using a vibration sample magnetometer (VSM), and a superconducting quantum interference device (SQUID) magnetometer in the temperature range 30K to 200K. A change from ferromagnetic state to super paramagnetic state has been observed in Co doped Sr₂Cu_2-xCo_xFe₁₂O₂₂ (x= 0.6 to 1.0) hexaferrite. The novel applications of all types of hexaferrite materials have been described.

Abstract: Relaxors with general formula AA'BB'O₃ with different cationic distributions at A/B-sites have important device applications in capacitors, piezoelectric ultrasonic transducers, electrostrictive actuators, SAW substrates, etc. The doping or compositional changes in these ceramics can control the high electromechanical characteristics. Lead zirconate titanates (PZT) - based solid solutions exhibit excellent electrochemical properties and are widely used as actuators, transducers, ceramic filters, resonators, sensors and other electronic devices, due to their excellent piezoelectric properties. Although, there has been a concerted effort to develop leadfree piezoelectric ceramics, no effective alternative to PZT has yet been found; most other materials that possess high dielectric and piezoelectric coefficients still contain lead ions. Dielectric relaxation in these materials is of fundamental importance. Recently, new relaxor ferroelectric materials (Pb (B_1/3Nb_2/3)O₃ types) have been reported by our group in which the dielectric relaxation character is different from that observed in typical lead based relaxors. In the first part of the review, relaxor characteristics, strategies to synthesize phase pure lead based relaxors and dielectric relaxation phenomena are presented. Deviation of the frequency dependent susceptibility from Curie-Weiss law is analysed both in terms of Gaussian and Lorentzian formalisms. Lead based ferroelectric relaxor materials, due to environmental, health and social reasons are not preferred in devices and attempts are being made to eliminate the lead content from these materials. Sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, abbreviated as NBT), is considered to be one of the excellent candidates for lead-free piezoelectric material. However, pure NBT piezoelectric ceramics are difficult to pole due to its relatively large coercive field and high electrical conductivity. Therefore, many solid solutions of NBT with other ceramics are proposed to improve the relevant properties. In the second part of the review, relaxor behaviour of newly developed solid solutions of NBT is discussed. Finally, the future scope of research on these advance materials is presented.