The Influence of Annealing Temperature on the Magnetic Properties of Mn0.5Co0.5Fe2O4 Nanoferrites Synthesized via Mechanical Milling Method

Mn_0.5Co_0.5Fe₂O₄ nanosized ferrites have been made directly from MnFe₂O₄ and CoFe₂O₄ ferrites and from metal oxides by using high-energy ball milling. Single-phase formation and microstructure of the as-milled samples and samples annealed at 100, 200, 300, 400 and 500 ^°C under argon atmosphere were studied using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average grain sizes were estimated from XRD measurements and found to be between 7 and 11 nm. The microstrain for each sample was relieved by annealing due to crystallite growth. Room temperature magnetic properties were investigated by zero-field ⁵⁷Fe Mössbauer spectroscopy and vibrating sample magnetometer (VSM). Saturation magnetizations of the samples were estimated using the empirical law of approach to saturation. The variation of coercive field, saturation magnetization, maximum magnetization and remanent magnetization for each sample was found to depend on the annealing temperature. The coercive fields are observed to increase with increased annealing temperature (from about 300 Oe for the as-milled samples to about 1000 Oe for samples annealed at 500 ^°C) which we attribute to increases in grain sizes.     

The Effect of Silver Inclusion on Superconducting Properties of YBa2Cu3O y Prepared Using Planetary Ball Milling

The effect of Ag inclusion on the structure microstructure and the critical current density of the YBa₂Cu₃O_7?δ sample prepared using the planetary ball milling process has been investigated. YBa₂Cu₃O_7?δ ceramics have been synthesized in air by a solid state reaction method from an oxide precursor powder, which was prepared from the starting powders of Y₂O₃, Ba₂CO₃, and CuO via a one-step annealing process in air at 950 ^°C. After planetary ball milling for 4 h of the oxide precursor powders, it was mixed with an AgNO₃ solution, and then was dried and uniaxially pressed, and subsequently annealed at 950 ^°C in air. Phase analysis by X-ray diffraction (XRD), granular structure examination by scanning electron microscopy (SEM), microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDXS) were carried out. To understand the effects of the ball milling on the pinning behavior, magnetic field and temperature dependences on a critical current density have been studied. Analyses show that Ag-milled YBCO samples exhibit higher values of critical current density in applied magnetic field compared to Ag-unmilled one. The better pinning properties of the Ag-milled samples are believed to be due to the microstructure of more fine and uniform distribution of silver and Y-deficient nanosized generated by ball milling.     

Significant Change in Mechanical Properties of YBa2Cu3O7−x Bulk Superconductors Diffused with Sn Nanoparticles

Mechanical behavior of YBa₂Cu₃O_7?x (Y123) superconductors exposed to Sn nanoparticles diffusion is determined by the way of Vickers microhardness (H _v ) conducted at different applied loads (0.245N≤F≤2.940 N). Load dependent microhardness, load independent microhardness, elastic modulus, and yield strength values are estimated from the microhardness curves. Unpredictably, the findings of the H _v values reveal that the undiffused sample and Sn diffused sample prepared at 500 ^°C exhibit reverse indentation size effect (RISE) behavior while the other samples obey indentation size effect (ISE) nature. Further, we extract the load independent (true) microhardness using the Meyer’s law, proportional specimen resistance (PSR), elastic/plastic deformation (EPD), Hays–Kendall (HK) approach and indentation-induced cracking (IIC) model, and compare the true hardness with the apparent hardness.     

Effect of Sintering on Magnetic Properties of Ni0.55Zn0.45Fe2O4

Bulk Ni_0.55Zn_0.45Fe₂O₄ samples were obtained by sintering their nanopowder at 1100 ^°C, 1200 ^°C, and 1300 ^°C. Improvement in crystallinity on sintering was identified from increase in intensity of the XRD peaks and grain development in SEM micrographs. Saturation magnetization increased from 81.7 emu/g to 85.3 emu/g as the sintering temperature increased from 1100 ^°C to 1300 ^°C. Initial permeability increases whereas the relative loss factor, resonance frequency, and DC resistivity decreases with increasing the sintering temperature. Curie temperatures obtained from low field AC normalized susceptibility and permeability measurements are in good agreement. The DC resistivity of the samples in the present case is two orders higher than the reported values of samples prepared using conventional ceramic method.     

Investigation of Lu effect on YBa2Cu3O7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7?δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.     

Fabrication and properties of YBa2Cu3O7-δAg composite superconducting wires by plastic extrusion technique

YBa₂Cu₃O_7-δ (Y123)-Ag composite superconducting wires were fabricated by the plastic extrusion method which involves plastic paste making, die extrusion, binder burn-out and the firing process. The as-extruded Y123-Ag wires were so flexible that they can be easily fabricated into a desirable shape. The current-carrying properties of the wire are dependent on sample size, sintering temperature and silver content. The critical current density, J _c, of the Y123 wire with a large cross-section was lower than that of the wires with a small cross-section, probably due to the large self-induced magnetic field. J _c of the Y123-Ag wires increased with increasing sintering temperature but abruptly decreased above 910 °C, which is close to the eutectic temperature of the Y–Ba–Cu–O system. A silver addition of 10–20 wt% slightly increased J _c of the Y123 (at 77 K and 0 T, it was 140 and 250 A cm ^-2 for the undoped Y123 wire and the Y123 wire with 20 wt% Ag addition, respectively), but further silver addition had a deleterious effect on J _c (180 A cm ^-2 for 30 wt% Ag addition). The small increment in J _c in the Y123 wire with 10–20 wt% Ag addition appears to be due to the enhanced densification and the associated microstructural variation. The decreased J _c of the Y123 wire with 30 wt% Ag addition is considered to be due to the formation of non-superconducting phase, Y₂BaCuO₅ (Y211), BaCuO₂ and CuO phases via the decomposition of the Y123 phase. This revised version was published online in November 2006 with corrections to the Cover Date.     

Study on low temperature sintering and electrical properties of CuO-doped Ca0.28Ba0.72Nb2O6 ceramics

Ca_0.28Ba_0.72Nb₂O₆ (CBN28) ceramics with different content of CuO were prepared by the conventional ceramic fabrication technique. The effects of CuO content on the phase structure, microstructure, dielectric and ferroelectric properties of obtained CBN28 ceramics were investigated. XRD results showed that pure tungsten bronze structure was obtained in all ceramics and CuO additive could accelerate the phase formation at lower temperatures. The CuO aid was effective for the uniform grain size in CBN28 ceramics, as it could facilitate the sintering behavior and suppress the anisotropic growth behavior obviously. The dielectric and ferroelectric properties of CBN28 ceramics depended greatly on the CuO content. Curie temperature T _c and dielectric loss tanδ both shifted downward, whereas the maximum dielectric constant ε _m and the dielectric constant around room temperature ε _r all increased initially and then decreased as x increased from 0.1 to 0.4 wt%. Normal ferroelectric hysteresis loops could be observed in all compositions, and the remnant polarization P _r decreased gradually. It was found that the comprehensive electric performance was optimized in CBN28-0.2 wt% CuO ceramics: ε _r = 453, ε _m = 3,371, T _c = 226 °C, tanδ = 0.048, P _r = 4.72 μC/cm² and E _c = 13.81 kV/cm, showed that CuO sintering aid could not only ameliorate the sintering behavior but also improve the electrical properties.     

Study of Thermal Behavior and Single Crystal Growth of A 0.8Fe1.81Se2 (A=K, Rb, and Cs)

We have investigated the melting behavior of alkaline iron selenide compounds of composition A _0.80Fe_1.81Se₂ (A=K, Rb, and Cs) by thermogravimetry/differential thermal analysis (TG/DTA). The analysis indicated that all of the compounds melt incongruently and complete melting occurs at 902, 927, and 900 ^°C for A=K, Rb, and Cs, respectively, as confirmed by high temperature optical microscopy (HTOM). Study of DTA shows the structure of the phase separation present in the compounds and both endothermic and exothermic peaks on heating/cooling measurements are clearly observed. The optimum crystallization temperature range is indicated by the significant exothermic peeks on cooling DTA. Large and high quality single crystals can be obtained at a slow cooling/growth rate by the modified Bridgman, flux, or floating zone methods. XRD reflections in the (00l) plane show that all the single crystals represent an intergrowth of two sets of the c-axis characterized by slightly different lattice constants. Magnetization measurements show that the superconducting transition temperature occurs at T _c=31.6 K with shielding fraction volume of ～100 % for K_0.80Fe_1.81Se₂.     

X-Ray Pole Figure Analysis and Magnetic Properties of Microwave Sintered Sr-M-type Hexagonal Ferrites

Sr-M-type sintered hexagonal ferrites were prepared by the conventional and microwave sintering method. X-ray diffraction and pole figure analysis, scanning electron microscopy (SEM), and Robograph 2 magnetic properties test instrument were applied to character the structure and magnetic properties of sintered ferrites. The XRD results reveal all the sintered samples are composed of SrFe₁₂O₁₉ single phase with obvious c-axis alignment. The sample with MS treatment at 1050 ^°C exhibits the optimum magnetic alignment, showing the optimum (0?0?8) and (1?0?7) pole figures with almost all circular lines focused on the center and at about 30^° of α, respectively. Few pores and optimal densification characters can be seen from the SEM image of strontium ferrites microwave sintered at 1050 ^°C, resulting in the optical magnetic properties as remanence (B _r) of 410.5 mT, coercivity (H _cj) of 372.2 kA/m, and maximum magnetic energy product ((BH)_max) of 31.53 kJ/m³, respectively.     

Polymer-metal chelate precursor reduced its firing temperature and time for preparing yttrium barium cuprate powder

Bulk YBa₂Cu₃O _x was prepared by a polymer chelate precursor method using poly[(N,Ndicarboxymethyl)allylamine] as a chelating polymer of which molecular weights were 3 × 10⁴ (PDAA-L) and 3 × 10⁵ (PDAA-H), respectively. X-ray diffraction (XRD) analysis of the precursor from PDAA-H shows that YBa₂Cu₃O _x (Y123) phase appeared after being calcined at 750 °C for 5 h and the mixture was completely converted to tetragonal Y123 phase after being calcined at 800 °C for 5 h. The phase evolution of the precursor from PDAA-H during isothermal experiment at 800 °C showed that pure tetragonal Y123 was produced even after the polymer chelate precursor was heated for 2 h in air, although a very small amount of BaCO₃ was recognized. This BaCO₃ phase was hardly recognized after 4 h calcination. The precursor prepared from PDAA-L was fully converted to pure tetragonal Y123 after 3 h calcining at 800 °C. On the other hand, the sample prepared from metal nitrate solution without PDAA was not fully transferred to Y123 phase after heating at 800 °C for 10 h. Large amounts of Y₂O₃, BaCO₃ and CuO were observed. These results indicated that the greater homogeneity in the polymer chelate precursor leads to reduced firing times and temperature compared with the metal nitrate precursor.     

Hydrothermal Synthesis,Structural and Electrical Properties of Antimony (Sb3+) Substituted Nickel Ferrites

Antimony (Sb³⁺) doped nickel ferrites have been synthesized by hydrothermal route using an autoclave at 160 ^°C for 12 hours. Pure spinel phase NiSb _x Fe_2?x O₄ (x=0.0 to 0.1) with step increment of 0.035 has been prepared by sintering the precursor samples at 500 ^°C. Structural studies have been performed using X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Lattice parameter and X-ray density found to increase with increase in the antimony concentration. Average crystallite size lies in the range of 14 to 24 nm?±?2 nm. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) have been used to characterize the morphology and sizes of nanoparticles. Electrical properties were analyzed by measuring DC-electrical resistivity, complex dielectric permittivity, AC conductivity and complex electrical modulus analysis. DC resistivity of nickel ferrites decreases due to the substitution of antimony from 6.7×10⁸ to 3.4×10⁷ Ω-cm. Dielectric permittivity and losses were studied in the frequency range of 20 Hz–5 MHz and found to increase due to addition of Sb³⁺ in nickel nanoferrites at room temperature. High dielectric permittivity and conductivity made this material a compatible option for single-layered and multilayered chip inductors.     

Effect of Sintering Temperature on the Microstructure,Electrical, and Magnetic Properties of Bi 0 . 8 5 Eu 0 . 1 5 FeO 3 Ceramics

Polycrystalline Bi _0.85Eu _0.15FeO ₃ ceramics were synthesized by solid-state reaction method with rapid liquid phase sintering process at various sintering temperatures. The dependence of structural, microstructural, electrical, and magnetic properties on sintering temperature was systematically investigated. X-ray diffraction measurements reveal that single perovskite phase is developed in Bi _0.85Eu _0.15FeO ₃ ceramics sintered at 850 and 870^° C, while secondary phases can be detected in the samples sintered at 890^° C due to the volatilization of Bi ³⁺ ions, and the crystallinity increases with increasing sintering temperature from 850 to 890 °C. The scanning electron microscopy investigation has suggested that the grain size increases with increasing sintering temperature from 855 t o 890^° C; while the pore size decreases with increasing sintering temperature from 850 to 870^° C and then increases with a further increase of sintering temperature. The electrical and magnetic measurements show that the leakage current, dielectric constant, dielectric loss, and magnetic properties are strongly dependent on the sintering temperature. The Bi _0.85Eu _0.15FeO ₃ ceramics sintered at 870^° C have the lower leakage current, higher dielectric constant, and lower dielectric loss. The room temperature magnetization increases with increasing sintering temperature from 850 to 890 °C. The possible reason for all the above observations was discussed.     

Sn diffusion coefficient and activation energy determined by way of XRD measurement and evaluation of micromechanical properties of Sn diffused YBa2Cu3O7−x superconducting ceramics

This study manifests not only the effect of Sn diffusion on physical, electrical, mechanical, structural and superconducting properties of the bulk YBa₂Cu₃O_7?x (Y123) superconductors prepared by the conventional solid-state reaction route by use of electrical resistance, X-ray diffraction analysis (XRD), electron dispersive X-ray, scanning electron microscopy, transport critical current density (J _c ) and Vickers microhardness (H _v ) measurements but also the diffusion coefficient and the activation energy of tin (Sn) in the Y123 material for the first time. The diffusion coefficient and the activation energy of Sn are investigated in the temperature range 500–945 °C using the change of the lattice parameters extracted from the XRD patterns. The resistance (at room temperature), critical (onset and offset) temperature, variation of critical temperature, hole-carrier concentration, crystallinity, lattice parameter, texturing, surface morphology, lotgering index, element distribution, critical current density, oxygen content, load dependent microhardness, elastic modulus and yield strength values are obtained for the pure and Sn diffused samples and compared with each other. One can see that all superconducting parameters given above depend sensitively on the Sn diffusion on Y123 system. The obtained results exhibit that the room temperature resistance enhances with the Sn diffusion because of the hole filling when the onset $ (T_{c}^{onset} ) $ ( T c onset ) and offset $ (T_{c}^{offset} ) $ ( T c offset ) critical temperatures are obtained to be about 93.4 and 89.6 K for the pure sample as against 92.2 and 88.1 K for the Sn diffused sample, respectively. This may be attributed to the fact that the decrement in the critical temperatures is due to the deterioration of crystallinity and descend in the grain size. As for the critical current density measurements, J _c values are obtained to be about 125.4–65.3 A/cm², respectively, for the undiffused and Sn diffused materials. This may be led to the decrease of the flux pinning mechanism stemming from the stacking faults, planar and micro-defects. At the same time, XRD measurements display that the samples produced in this work exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. Besides, the peak intensities belonging to major phase (Y123) decrease monotonously with Sn diffusion in the system; however, new peaks belonging to the minor (BaCuO₂) phases start to appear for Sn diffused sample confirming both the reduction of the grain size and degradation of the critical temperature. Moreover, the pure sample is confirmed by both enhancement of a and b lattice constants and the decrement of the cell parameter c of the sample in comparison with that of the Sn diffused sample. According to SEM examination, the crystallinity and grain connectivity suppress with the Sn diffusion. EDX measurements illustrate that not only do the elements used for the preparation of the Y123 superconductors with and without Sn diffusion distribute homogeneously but also the level of Cu element reduces with the Sn diffusion, presenting that the Cu²⁺ ions may partly be diffused by tetravalent tin (Sn⁴⁺) ions. Further, surprising results of the Vickers microhardness values demonstrate that the pure sample visualizes Indentation Size Effect (ISE) feature; however, the Sn diffused sample reports Reverse Indentation Size Effect (RISE) nature. Additionally, the diffusion coefficient is observed to increase from 1.11 × 10^?9 to 2.82 × 10^?8 cm² s^?1 as the diffusion-annealing temperature increases, verifying that the Sn diffusion at lower temperatures is much less significant as compared to the higher ones. Temperature dependence of the Sn diffusion coefficient and activation energy in the range of 500–945 °C is defined with the aid of the following equation: $$ D = 7.78 \times 10^{ - 6} { \exp }\left[ {\left( {( - 0.590 \pm 0.005){\raise0.7ex\hbox{${\text{eV}}$} \!\mathord{\left/ {\vphantom {{\text{eV}} {k_{B} T}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${k_{B} T}$}}} \right)} \right] $$ D = 7.78 × 10 ? 6 exp [ ( ( ? 0.590 ± 0.005 ) eV / eV k B T k B T ) ] .     

The effect of gas flow rate on the evolution of the surface oxide on a molten low carbon Al killed steel

The oxide phase formation on a molten Al killed low carbon steel surface under a flowing Ar atmosphere with an oxygen partial pressure of Po ₂ = 1–5 × 10^?5 atm has been visualized with a Confocal Scanning Laser Microscope (CSLM) equipped with a gold image furnace. In this study, the effect of gas flow rate variation (170–300 cm³/min) on the oxide evolution under isothermal conditions of 1600^°C was investigated. Al₂O₃, rather than the thermodynamically stable phase FeAl₂O₄, was found to precipitate under all the experimental conditions studied and the apparent rate of evolution was found to increase with increasing gas flow rate. The oxide evolved as a network that started from the container wall and grew towards the crucible center. At low flow rates the growth was a result of primarily crystal growth resulting in distinctly dendritic crystals. As the flow rate was increased, growth due to the attachment of discrete inclusions to the advancing front was observed which resulted in a final oxide network that constituted of smaller facetted particles. In the latter case, the transport of the individual inclusions to the advancing front could be caused by surface Marangoni flow due to gradients in both temperature and dissolved oxygen concentration.     

X-Ray and High-Resolution Neutron Diffraction Studies on Nd x Y1−x Ba2Cu3O7−δ Superconductors

A synthesis of neodymium-substituted YBCO superconductor Nd _x Y_1?x Ba₂Cu₃O_7?δ (x=0,0.25,0.5,0.75,1) has been done using a dissolved method in order to obtain homogeneous crystals and higher critical current density. The effects of the substitutions on the structural and magnetic properties of the superconductors after sintering at 970 ^°C have been examined. Crystallinity of the synthesized powders was confirmed using X-ray and high-resolution neutron diffraction (XRD and HRPD) techniques. Rietveld analyses for both diffraction data sets gave increasing lattice parameters with addition of Nd content and decreasing orthorhombicity. Such addition also caused a decrease in occupancy of the oxygen in the O(4) site. Further investigation using SQUID showed critical temperature of the superconductors between 90.9 and 92.0 K. The critical current density (J _c ) was calculated from the magnetic hysteretic loop at 5 K as 40 kA?cm^?2 for Nd_0.25Y_0.75Ba₂Cu₃O_7?δ sample and 100 kA?cm^?2 for Nd_0.5Y_0.5Ba₂Cu₃O_7?δ sample. We also found that increasing Nd content on the Nd _x Y_1?x Ba₂Cu₃O_7?δ superconductor samples can improve their resilience of superconductivity and critical current density.     

Flux Pinning in YBCO Single Crystal Grown on Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Layer

In this study, single-crystal Y123 samples were grown by a cold top-seeding method by using Nd123 seed, and the effect of Y₂O₃ buffer layer was investigated. The upper critical magnetic field and coherence length were established as 110 T and 17.3 ?, respectively. The dependence of the effective activation energy U of the flux pinning on the magnetic field and temperature of the sample were determined using the Arrhenius activation energy law from the resistivity curves. It was found that the deduced value of the activation energy for a Y123 sample is in good agreement with the corresponding values in YBCO samples. The maximum activation energy value was approximately 0.9 eV in the zero magnetic field. In order to examine the homogeneity of the pinning properties of different layers, rectangular specimens were cut from the sample. AC susceptibility measurement was performed, and it was found that the shifting of the peak temperature (T _p) with an AC magnetic field is small, indicating good pinning properties. The normalized pinning force density versus the reduced field was examined at different temperatures to determine the pinning mechanism. It was found that normal core-type pinning was effective, and in low fields, pinning was only due to Y211 particles.     

Effect of PbO<Subscript>2</Subscript> on the Formation of YBCO Films Fabricated Via Sedimentation Deposition and Partial Melting Technique

PbO₂ addition in sedimentation deposition prepared films was investigated by SEM, EDX, and RT measurements to determine its effect on the formation of YBCO films. YBCO films on SrTiO₃ (STO) (100) substrates of varying amounts of PbO₂ were partially melted at 980 ^°C, annealed, and oxygenated in flowing oxygen atmosphere. In addition, a YBCO sample partially melted at higher temperature was also done for comparison. From the SEM, EDX, and RT analyses, it was found that the film having 8 wt% PbO₂ produces a film with the highest superconducting transition temperature comparable to that of a YBCO sample partially melted at peritectic point. Moreover, it was observed that addition of PbO₂ enhances matter transport between Y123 particles and lowers the processing temperature for the fabrication of YBCO films via the sedimentation deposition method.     

Optimized sintering properties and temperature stability of MgZrTa<Subscript>2</Subscript>O<Subscript>8</Subscript> ceramics with CuO addition for microwave application

Microwave dielectric ceramics CuO–modified MgZrTa₂O₈ were synthesized by the conventional solid-state reaction method. The effects of CuO additives on the sintering characteristics and microwave dielectric properties have been investigated. With CuO addition, the sintering temperature of MgZrTa₂O₈ ceramics can be effectively lowered from 1475 to 1375 °C without decreasing its dielectric properties obviously and the temperature coefficient of the resonant frequency of MgZrTa₂O₈ ceramics have been optimized to near-zero. The crystalline phase exhibited a wolframite crystal structure and no second phase was detected at low addition levels. The grain growth of CuO–modified MgZrTa₂O₈ ceramics was accelerated due to liquid phase effect. The relative dielectric constants (ε_r) were correlated with apparent density and were not significantly different for all levels of CuO concentration. The quality factors (Q?×??) and temperature coefficient of resonant frequency (τ_?), which were strongly dependent on the CuO concentration, were analyzed by the grain size and the dielectric constant respectively. A best Q?×?? value of 116400 GHz and τ_? value of ?6.19 ppm/℃ were obtained for specimen with 0.05 wt% CuO addition at 1375 °C.     

Optical and magnetic properties of CuO/CuFe2O4 nanocomposites

CuO/c-CuFe₂O₄ nanocomposites have been synthesized via the oxalate precursor route. Effect of synthesis conditions on the crystal structure, microstructure, magnetic and optical properties of the formed powders was studied. The results indicated that pure CuO nanoparticles were obtained from the oxalate precursor annealed at 600 °C for 2 h. However, substitution of Cu²⁺ ion by Fe³⁺ ion (Cu_1?X Fe _X O, where X = 0, 0.05, 0.1 and 0.2) led to form of CuO/CuFe₂O₄ nanocomposites. The microstructures of the powders appeared as a monoclinic like shape. Furthermore, the band gap energy of the obtained CuO nanopowders was 1.41 eV and the value was slightly decreased by Fe³⁺ ion substitution. In addition, the formed CuO particles had weak ferromagnetic characteristics. However, the substitution Cu²⁺ ion by Fe³⁺ ion enhanced the magnetic properties of the formed composite as the result of increasing the CuFe₂O₄ phase formation. Hence, the saturation magnetization was increased from 0.13 to 9.8 emu/g by increasing the Fe³⁺ ion from 0 to 0.2.     

Growth Mechanism in Infiltration Growth Processed YBCO Composites Through Quench Studies

The mechanism of YBa₂Cu₃O_7?x (Y123/YBCO) growth and the dispersion of Y₂BaCuO₅ (Y211) particles in YBCO composites were investigated at various stages during infiltration growth process by quenching the samples in liquid nitrogen from different temperatures (1005, 1010 and 1045 °C) through peritectic reaction. Structural details of the samples were investigated using X-ray diffraction, microstructural and elemental analysis was carried out to study the distribution and composition of the matrix and Y211 inclusions at different stages. The present studies reveal that breaking down of sintered Y211 particles on application of pressure and slow cooling to 1005 °C allow sufficient infiltration of liquid phases. Peritectic reaction of liquids with Y211 particles caused rounding up of the broken sharp edges and their refinement leading to distribution of very fine Y211 precipitates in Y123 matrix.