Celal Kursun

Structural, electrical and magnetic properties of Nd – A – CoO 3 (A = Sr, Ca) Perovskite Powders by Mechanical Alloying

In this work, NdCoO3 (NCO), Nd0.8Sr0.2CoO3 (NSCO) and Nd0.9Ca0.1CoO3 (NCCO) perovskite cobaltites were synthesised by mechanical alloying method. Structural evolutions, magnetic and electrical properties of these perovskite were systematically examined through X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray detection (SEM-EDX), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), vibration sample magnetometer (VSM) and impedance analyser (IA). The XRD and SEM results revealed that the microstructure of the perovskite materials changed during mechanical alloying. The average crystallite size of the perovskite materials was calculated by Debye Scherrer equation and was confirmed by TEM, and it was determined ~19 nm. From the VSM results, the all perovskites had soft ferromagnetic properties. IA measurements showed that relatively dielectric constants of the perovskites decreased with increasing frequency. Therefore, for the first time, nanostructured NCO, NSCO and NCCO perovskites exhibiting good properties were produced in only two steps which are milling and heating.

Phase Identification and Size Evaluation of Mechanically Alloyed Cu-Mg-Ni Powders

Ternary mixture of Cu, Mg, and Ni with the nominal composition of nanocrystalline Cu50Mg25Ni25 (in at.%) was milled for 25 hours. Analysis of an X‐ray diffraction pattern (XRD) and transmission electron microscopy (TEM) was used to characterize the chemi‐ cal phases and microstructure of the final product, which is shown to consist of ternary alloy of Cu‐Mg‐Ni with FCC structure along with small amounts of FCC MgO and Mg0.85Cu0.15. The good agreement between the size values obtained by XRD and TEM is attributed to the formation of defect‐free grains with no substructure during ball milling. Dynamic recrystallization may be a possible mechanism for the emergence of such small grains (<20 nm). The particle size distribution and morphological changes of Cu–Mg–Ni powders were also analyzed by scanning electron microscopy (SEM). According to the SEM results, the particle size of the powders decreased with increasing milling time. Lattice parameter of the Cu‐Mg‐Ni ternary FCC alloy formed during mechanical alloying increased with increase in milling time from 3.61 to 3.65 Å after 20 hours milling.

Structure and Mechanical Behaviour of Cu‐Zr‐Ni‐Al Amorphous Alloys Produced by Rapid Solidification

The amorphous ribbons of Cu50Zr40Ni5Al5 alloy were manufactured by rapid solidifica‐ tion. The ribbons were investigated by X‐ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM‐EDX) and differential scanning calorimetry (DSC). The activation energy of the crystallisation in amorphous alloys was determined by Kissenger technique. The mechanical properties of the ribbons were characterized using Vickers microhardness (HV) tester. According to the XRD and SEM results, the Cu50Zr40Ni5Al5 alloys have a fully amorphous structure. The EDX analysis of the ribbons showed that compositional homogeneity of the Cu50Zr40Ni5Al5 alloy was fairly high. From the DSC curves of the amorphous ribbons, it was deter‐ mined that glass transition temperature (Tg) is around 440–442°C and super‐cooled liquid region (ΔTx = Tx Tx) before crystallisation is around 61–64°C. The microhard‐ ness of the as‐quenched ribbons was measured about 550 HV. However, this micro‐ hardness value decreased with increasing annealing temperature and it was calculated about 465 HV after annealing temperature of 800°C.