This study investigated the effect of Nb3+ substitution on the magnetic and structural properties of CoFe2O4 nanoparticles (NPs) synthesized by hydrothermal approach. The formation of a single phase ...of spinel ferrite was confirmed through X-ray powder diffraction, and crystallite sizes in the range 18–30 nm were observed. Moreover, it found that the Fourier transform infrared (FT-IR) spectra of the NPs included the main vibration bands of the spinel structure. The partially cubic structure was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The energy band gaps for CoNbxFe2-xO4 were estimated to be in the range 0.48–0.53 eV for Nb3+ content x = 0.0–0.10. Magnetization measurements at room temperature (RT; 300 K) and at 10 K were performed on spinel CoNbxFe2-xO4 (0.00 ≤ x ≤ 0.10) NPs using a vibrating sample magnetometer (VSM). Nb3+ doping significantly changed the magnetization and coercivity of the Co ferrite samples. RT hysteresis curves indicated well-defined ferrimagnetic behavior for all prepared NPs with saturation magnetization (Ms) in the range 44.45 – 49.40 emu/g and remanent magnetization (Mr) in the range 12.16 – 17.90 emu/g. The coercive field (Hc) is found to be equal 936 Oe and is decreased with Nb3+ substitutions. However, hysteresis curves at 10 K showed finite remanent specific magnetization (1.90–6.70 emu/g) but significant asymmetric coercivity (715–2810 Oe), particularly for the Nb3+-doped samples. At 10 K, the magnetization values were 4–6 times smaller but symmetric coercivity field values were 2–3 times larger compared with the RT-VSM curves. The obtained magnetic parameters indicated the semi-hard magnetic character of the Co ferrite samples at low temperatures.
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This study probes the effect of annealing temperature on electrical, optical and microstructural properties of indium tin oxide (ITO) films deposited onto soda lime glass substrates by conventional ...direct current (DC) magnetron reactive sputtering technique at 100 watt using an ITO ceramic target (In
2
O
3
:SnO
2
, 90:10 wt%) in argon atmosphere at room temperature. The films obtained are exposed to the calcination process at different temperature up to 700 °C. X–ray diffractometer (XRD), ultra violet-visible spectrometer (UV–vis) and atomic force microscopy (AFM) measurements are performed to characterize the samples. Moreover, phase purity, surface morphology, optical and photocatalytic properties of the films are compared with each other. The results obtained show that all the properties depend strongly on the annealing temperature. XRD results indicate that all the samples produced contain the In
2
O
3
phase only and exhibit the polycrystalline and cubic bixbite structure with more intensity of diffraction lines with increasing the annealing temperature until 400 °C; in fact the strongest intensity of (
222
) peak is obtained for the sample annealed at 400 °C, meaning that the sample has the greatest ratio
I
222
/
I
400
and the maximum grain size (54 nm). As for the AFM results, the sample prepared at 400 °C has the best microstructure with the lower surface roughness. Additionally, the transmittance measurements illustrate that the amplitude of interference oscillation is in the range from 78 (for the film annealed at 400 °C) to 93 % (for the film annealed at 100 °C). The refractive index, packing density, porosity and optical band gap of the ITO thin films are also evaluated from the transmittance spectra. According to the results, the film annealed at 400 °C obtains the better optical properties due to the high refractive index while the film produced at 100 °C exhibits much better photoactivity than the other films as a result of the large optical energy band gap.
•The high density of unreacted material and low annealing temperature (630 °C) o leads to high critical parameters.•The lower density of the unreacted material and the low annealing temperature ...(630 °C) o leads to low critical parameters.•The lower density of unreacted material and high isostatic pressure (1.1 GPa) very slightly increases Tc, Birr and Bc2.•The high density of unreacted material and high isostatic pressure (1.1 GPa) significantly increases Tc, Birr and Bc2.•The research shows that the voids are formed in places with the highest density of unreacted material.
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Our research show that the higher packing density of the unreacted material (Mg +2B) by 8%, especially for Mg in the solid state, allows to eliminate the large number of large voids (20 µm), improves the homogeneity and density of the MgB2 superconducting material, allows to obtain a greater number and longer lengths of connections between MgB2 grains, eliminates the formation of the intermetallic phase (iron borides), allows to obtain MgB2 grains of a uniform size and shape, increase irreversible magnetic field (Birr), critical temperature (Tc) and upper magnetic field (Bc2). Research show that spherical grains significantly reduce the number of inter-grain connections in the material with low and high density of unreacted material and allows to obtain the more superconducting phase. Research shows that MgB2 grains with a plate-like and rectangular shape allow to obtain more connections between the grains. Studies show that thermal treatment under high isostatic pressure does not allow to obtain the large amount of superconducting phase for the unreacted material of low density. Moreover, studies show that the high packing density of unreacted material and thermal treatment under high isostatic pressures (0.8 GPa and 1.1 GPa) allow to obtain a large amount of superconducting phase even for Mg in the solid state. Additionally, studies points that heat treatment under medium isostatic pressure (0.3 GPa) significantly reduce Birr and Bc2. Our results are important for long superconducting wires made by using the powder-in-tube (PIT) technique. Because they indicate that the higher packing density of the unreacted material (Mg +2B) will allow to obtain a the superconducting material with greater homogeneity and density, and improve the critical parameters e.g. coils.
In this article, the significant impact of a lamellar (layered) structure and a high isostatic pressure on the normal state resistance (Rn), critical temperature (Tc), irreversible magnetic field ...(Birr), upper magnetic field (Bc2) and critical current densities (Jc) at 4.2 K and 20 K was presented. Our research showed that annealing at temperatures in the range of 630 °C–680 °C (above the melting point of Mg) at atmospheric pressure (0.1 MPa) did not create a lamellar (layered) structure. This led to low Tc, Jc and Birr and high Rn. The analysis, made by using scanning electron microscopy (SEM), showed that the annealing temperature increased up to 700 °C under a pressure of 0.1 MPa, which created a lamellar structure. This led to significant growth of Tc, Jc and Birr and a slight increase of Rn. Moreover, the measurements showed that annealing at temperatures from 630 °C to 700 °C did not change the Bc2 value. In comparison to pressureless heat treatment, annealing under the high isostatic pressure of 1.1 GPa obtained a lamellar structure with layers of lower thickness and higher density. This led to significant increases in Jc and Birr and a visible reduction of Rn. SEM analysis showed that the increase of isostatic pressure up to 0.3 GPa created a lamellar structure with thicker layers and lower density. This microstructure led to lower Jc and Birr and significantly higher Rn. On the other hand, the SEM analysis showed that annealing under 0.8 GPa did not cause the formation of a layered structure, and as a result, it led to significant reductions in Birr and Jc (4.2 K and 20 K) and higher Rn. The increase of the isostatic pressure from 0.1 MPa to 1.1 GPa did not affect Tc (B = 0 T) and Bc2. The results indicated that the layered structure obtained a high density of pinning centers, which were particularly effective at higher magnetic fields. Jc of 100 A/mm2 in 8 T at 4.2 K was obtained in in situ undoped MgB2 wires after annealing at 700 °C for 40 min under an isostatic pressure of 1.1 GPa.
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•The lack of layered structure leads to a reduction of Birr in in situ MgB2 wires.•The layered structure causes a significant increase of Jc in in situ MgB2 wires.•The layered structure causes numerous longitudinal connections between grains.•Longitudinal connections between grains increase Jc in in situ MgB2 wires.•The annealing at temperature of 700 °C creates longitudinal voids in MgB2 wires.
Microstructural and magnetic properties of BaFe12-xNbxO19 (0.0 ≤ x ≤ 0.1) nanohexaferrites (NHFs) have been investigated intensively in this work. The Ba NHFs are synthesized through sol-gel ...auto-combustion route. The formation of single phase Ba NHFs, in different samples are confirmed by XRD powder patterns, scanning electron microscopy and Fourier Transform Infrared spectroscopy. The magnetic properties of BaFe12-xNbxO19 (0.0 ≤ x ≤ 0.1) NHFs were performed at room and low temperatures and discussed in detail. The analysis of hysteresis loops divulged that the different products display hard ferromagnetic behavior at different temperatures. The deduced values of saturation magnetization (Ms), remanence (Mr), magneton number (nB) and magneto crystalline anisotropy constant (Keff) are reduced for lower Nb content and then increased with further increasing the Nb content, reaching a maximum values for BaFe11.9Nb0.1O19 (x = 0.1) nanohexaferrite. The coercivity (Hc) and intrinsic coercivity (Hci) are diminished for lower x and are comparable to that of pristine one for higher x. The squareness ratio (Mr/Ms) are fluctuating between 0.50 and 0.55, implying the uniaxial anisotropy for different BaFe12-xNbxO19 nanohexaferrite.
We have investigated the effect of the Au-diffusion on the mechanical and transport properties of the (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) superconducting samples with different annealing times of 10, 20 ...and 50 h. The samples are prepared by the conventional solid-state reaction method in the polycrystalline bulk form. Doping of Bi-2223 was carried out by means of Au diffusion during sintering from an evaporated Au film on pellets. The experimental works in this study consist of dc electrical resistivity and critical current density measurements for electrical and superconducting properties, microhardness measurements for mechanical properties, powder X-ray diffraction (XRD) for phase analyses (phase ratio) and lattice parameters, and scanning electron microscopy (SEM) for microstructure examination. These measurements showed that Au-doping, in comparison with the undoped samples, increased the critical transition temperature, critical current density and enhanced formation of high-Tc phase. Additionally, microhardness and grain size were also improved with increasing amount of diffusion. Moreover, the diffusion-annealing time decreased the number and size of voids and increased the transition temperature. The experimental results of microhardness measurements were analyzed using the Kick's law, modified proportional specimen resistance (MPSR) model and the Hays-Kendall (HK) approach. Kick's law did not give useful knowledge of the origin of the indentation size effect. It was observed that the load independent microhardness values were determined based on the MPSR and HK models, and found to be similar with each other. The possible reasons for the observed enhancement in transport and mechanical properties due to Au diffusion are discussed.
We have investigated the effect of addition of Gd in Bi
1.8Pb
0.35Sr
1.9Ca
2.1Cu
3Gd
x
O
y
superconductor with
x=0, 0.1, 0.2, 0.3, 0.4 and 0.5. The samples were prepared by standard solid-state ...reaction methods. The investigation consisted of X-ray diffraction (XRD), scanning electron microscopy (SEM), dc electrical resistivity, hole concentration and transport critical current density measurements. Transport measurements indicated that the superconducting transition temperature, transport critical current density and hole concentration values of the samples strongly depend on the Gd addition. The values of
T
c, and
J
c of the samples decreased with the increase in Gd addition. When Gd addition ratio was increased, surface morphology and grain connectivity of the samples were observed to degrade from SEM investigations and phase ratio of the high-
T
c (Bi-2223) phase to the low-
T
c (Bi-2212) phase decreased (XRD measurements). The possible reasons for the observed degradation in superconducting and microstructure properties of Bi-2223 due to Gd addition were discussed.
We have fabricated MgB
2
/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB
2
/Fe monofilament tapes were annealed ...at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB
2
phase, activation energy, temperature dependence of irreversibility field
H
irr
(
T
) and upper critical field
H
c2
(
T
), transition temperature (
T
c
), lattice parameters (
a
and
c
), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB
2
/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB
2
/Fe monofilament tape, the slope of the
H
c2
–
T
and
H
irr
–
T
curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that
T
c
,
J
c
and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The
J
c
and
T
c
offset
values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm
2
at 20 and 38.1 K, respectively.
