MgB<Subscript>2</Subscript> with addition of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by spark plasma sintering technique

Superconducting bulks of MgB₂ with addition of Sb₂O₃ and Sb with different stoichiometric compositions ((MgB₂) + (Sb₂O₃) _x , x = 0.0025, 0.005, 0.015, and (MgB₂) + (Sb)_y, y = 0.01) were obtained by the Spark Plasma Sintering (SPS) technique. All added samples have high density, above 95% and critical temperature, T _c, of 38.1–38.6 K. This result and XRD data suggest that Sb does not enter the lattice of MgB₂. Impurity phases are Mg₃Sb₂, MgO, and MgB₄. The optimum addition is Sb₂O₃ for x = 0.005. This sample shows the critical current density, J _c(5 K, 0 T) = 4 × 10⁵ A/cm² and J _c(5 K, 7 T) = 6 × 10² A/cm², while the irreversibility field, H _irr (5 K, 100 A/cm²) = 8.23 T. Indicated values of J _c and H _irr are higher than for the pristine sample. The mechanism of J _c and H _irr increase in the Sb₂O₃ added samples is complex and composed of opposite effects most probably involving morphology elements, the presence of nano metric MgB₄ and the indirect influence of oxygen or oxygen and Sb. Crystallite size of MgB₂ is decreasing when Sb-based additions are introduced and the effect is stronger for the Sb-metal addition. The sample with Sb-metal addition does not improve J _c and H _irr when compared with pristine sample.     

Al-Doping Effects on the Structural Change of MgB<Subscript>2</Subscript>

A series of polycrystalline Al doped Mg_1−x Al _x B₂ (x=0.00, 0.01, 0.03, 0.06, 0.10, 0.15) samples were prepared using the solid-state reaction route. Phase analysis showed that Al is alloyed into the MgB₂ lattice and there were some Al₂O₃, MgAlB₄ particles present in bulk samples of MgB₂. It is shown that the suppression of T _c by doping originates largely from structural changes and the structure properties play an important role in influencing the normal-superconductor transport. The introduction of defects into the Mg layers and other aluminum compound (Al₂O₃, MgAlB₄) impurity phases both influence the polycrystalline structure.     

Effect of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on formation and superconductivity of bulk MgB<Subscript>2</Subscript>

Bulk materials of MgB₂ have been prepared with the stoichiometry of MgB₂(Al₂O₃) _x (x = 0, 2, 5, 10 and 20% nano-Al₂O₃ powders), by using solid-state reaction route. All samples were sintered at 750 °C for 30 min in a calorimeter to monitor the sintering reaction process. It is found that the onset temperatures of reaction between Mg and B powders increase significantly with increasing the amount of Al₂O₃. However, the reaction time is shortened for the nano-Al₂O₃ powders can effectively activate the reaction as a catalyst. The critical transition temperature decreases from 38.5 to 31.6 K, and the corresponding temperature window becomes narrow (less than 2.6 K). Furthermore, the amount of MgO impurity was found to increase with the increase of Al₂O₃, which probably indicates that partial Mg was replaced by Al.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Phase formation sequences and mechanism of MgB<Subscript>2</Subscript> superconductor with minor Sn doping

Combined with thermal analysis and phase identification, the phase formation of Sn-doped MgB₂ superconductor during the sintering process were systematically investigated. As compared to the sintering of MgB₂, the first exothermal peak occurs at a lower temperature, which suggests the accelerated formation of MgB₂ after minor Sn doping. The sintering process of minor Sn-doped MgB₂ orderly underwent the melting of Sn, the reaction between Mg and Sn, the eutectic Mg–Sn reaction, the solid–solid Mg–B reaction, the melting of Mg, the liquid–solid Mg–B reaction and the Sn precipitation. Based on the phase formation mechanism, MgB₂ bulks was successfully synthesized by Sn-activated sintering at 600 °C for only 5 h, exhibiting a dramatic decrease in the sintering time compared to the sintering of undoped MgB₂.     

Improved Superconducting Properties in Graphene-Doped MgB<Subscript>2</Subscript> Prepared by Coating Method

The effects of graphene doping on the phase formation and superconductivity of MgB₂ bulks synthesized with different process have been studied systemically. Considering the scattering structure of graphene, coating method was applied to enhance the uniformity of graphene doping. The graphene coated B addition was expected to improve the critical current density of MgB₂ bulks. In our study, several experiments were performed to find out the suitable way for graphene doping. The coating method could enhance the critical current density of MgB₂ from 1.9 × 10⁵ to 2.5 × 10⁵ A/cm² at 20 K and 0 T, compared with that of the undoped sample. And the superconductivity of MgB₂ prepared by coating method got obvious improvement at high field compared with that of pure graphene doping bulk. It can be concluded that the coating method could ensure the uniformity of graphene doping in MgB₂ and refined the grain crystalline effectively.     

Enhanced Critical Current Properties of MgB<Subscript>2</Subscript> Bulks by Doping Amorphous Carbon Containing Magnetic Impurity

The doping effect of amorphous carbon (C) containing magnetic impurity in MgB₂ bulk has been studied. Structural characterization by means of X-ray diffraction and the superconducting transition temperature, T _c , measurement indicate that little C effectively enters the MgB₂ structure. This should be due to the lower sintering temperature. The upper critical field, H _c2, and irreversibility field, H _irr, of samples show no systematic evolution with C doping. However, critical current density J _c (H) performance is greatly improved with C doping at 4, 15, and 28 K, respectively. Corresponding to this case, scanning electron microscope (SEM) image indicates that the grain size in samples becomes very small and grain boundary is developing roundness with the increasing of C content. This should be intimately related with the increase of magnetic impurity along with C doping. The result is discussed.      

Multiple Magnetic Ordering Temperatures in RuSr<Subscript>2</Subscript>Eu<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10−<Emphasis Type="Italic">δ</Emphasis></Subscript> System

We report synthesis and magnetic characterization of variously processed magneto-superconducting (Rutheno-cuprates) RuSr₂Eu_1.5Ce_0.5Cu₂O_10−δ . The compound crystallizes in I4/mmm tetragonal structure. Magnetization measurements showed bulk magnetic ordering and superconductivity at around 100 K and 30 K, respectively. Further, the careful examination of the low field magnetic susceptibility reveals two minor magnetic transitions at around 135 K and 200 K, in addition to the major transition at 100 K. When the samples are processed in different environments of air, O₂ and slightly pressurized O₂, the nature of magnetic transitions and the superconductivity changes dramatically. The highest superconducting transition is achieved for the high pressure O₂ annealed samples and the lowest for the air annealed one. On the other hand, the minor magnetic transitions are more prominent in air-annealed samples. Though the minor magnetic transitions are intrinsic to all variously processed samples, they are not clearly seen for higher O₂ content samples. Basically, the Ru spins order antiferro magnetically at around 200 K (first minor transition), and reorients themselves at 125 K (second minor transition) before finally ordering in canted ferromagnetic state or a spin glass structure. These results can be explained on the basis of fluctuating valance of Ru⁴⁺/Ru⁵⁺.      

Dielectric and piezoelectric properties of MnO<Subscript>2</Subscript>-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>Nb<Subscript>0.92</Subscript>Sb<Subscript>0.08</Subscript>O<Subscript>3</Subscript> lead-free ceramics

Lead-free MnO₂-doped K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramics have been fabricated by a conventional ceramic technique and their dielectric and piezoelectric properties have been studied. Our results show that a small amount of MnO₂ (0.5–1.0 mol%) is enough to improve the densification of the ceramics and decrease the sintering temperature of the ceramics. The co-effects of MnO₂ doping and Sb-substitution lead to significant improvements in the ferroelectric and piezoelectric properties. The K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping possesses optimum propeties: d ₃₃ = 187 pC/N, k _P = 47.2%, ε _r = 980, tanδ = 2.71% and T _c = 287 °C. Due to high tetragonal-orthorhombic phase transition temperature (T _O-T ~ 150 °C), the K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping exhibits a good thermal stability of piezoelectric properties.     

Sintering mechanism of Ag-doped MgB<Subscript>2</Subscript> superconductor from low temperature to high temperature

Combined with thermal analysis and phase identification, the sintering process of Ag-doped MgB₂ superconductor was investigated. It is found that the Ag doping could form Mg–Ag liquid through the eutectic reaction at low temperature (about 470 °C) and then obviously accelerated the formation of MgB₂ phase. Moreover, a sintering model is also proposed to illustrate the liquid activated sintering mechanism present in the sintering process of Ag-doped MgB₂ samples. The sintering model is supposed to provide theoretic guidance for optimizing the sintering condition in the synthesis of doped MgB₂ superconductors.     

Temperature-stable and low loss Fe-containing dielectrics in BaO-Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Polycrystalline samples of Ba₄Ln₂Fe₂Ta₈O₃₀ (Ln = La and Nd) were prepared by a high temperature solid-state reaction technique. The formation, structure, dielectric and ferroelectric properties of the compounds were studied. Both compounds are found to be paraelectrics with filled tetragonal tungsten bronze (TB) structure at room temperature. Dielectric measurements revealed that the present ceramics have exceptional temperature stability, a relatively small temperature coefficient of dielectric constant (τ _ε ) of −25 and −58 ppm/°C, with a high dielectric constant of 118 and 96 together with a low dielectric loss of 1.2 × 10⁻³ and 2.8 × 10⁻³ (at 1 MHz) for Ba₄La₂Fe₂Ta₈O₃₀ and Ba₄Nd₂Fe₂Ta₈O₃₀, respectively. The measured dielectric properties indicate that both materials are possible candidates for the fabrication of discrete multilayer capacitors in microelectronic technology.     

Synthesis,microstructure and microwave dielectric properties of Ca<Subscript>4-x</Subscript>Mg<Subscript>x</Subscript>La<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript> ceramics

Ca_4-xMg_xLa₂Ti₅O₁₇ ceramics were prepared by a solid state ceramic route for x = 0, 0.5, 1, 2, 3 and 4. The structure and microstructure of the ceramics were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction results show that the Ca_4-x Mg _x La₂Ti₅O₁₇ adopts an orthorhombic crystal structure with no secondary phase observed for x from 0 to 0.5. Secondary phase, MgTiO₃ occurs with further increasing doping level (1 ≤ x ≤ 3). When x = 4, mixture phases La_0.66TiO_2.993, MgTiO₃ and a trace of unknown phase coexist. Ca₄La₂Ti₅O₁₇ ceramic exhibits a relative permittivity (ε_r) ~ 65, quality factor (Q × f) ~13,338 GHz (at ~4.75 GHz), and temperature coefficient of resonant frequency (τ _f ) ~ 165 ppm/°C. The sintering temperature was distinctly reduced from 1,580 °C for x = 0 to 1,350 °C for x = 4. With increasing Mg content, ε_r and τ_f obviously decrease, while Q × f value initially decreases and then increases. The ceramic for x = 2 shows ε_r ~ 50, Q × f ~ 9,451 and τ _f  ~ 62.5 ppm/°C. By the complete replacement of Ca with Mg, Mg₄La₂Ti₅O₁₇ ceramic sintered at 1,350 °C for 4 h combines a high dielectric permittivity (ε _r  = 31), high quality factor (Q × f ~ 15,021) and near-zero temperature coefficient of resonant frequency (τ _f  ~ 4.0 ppm/°C). The materials are suitable for microwave applications.     

Kinetics analysis for the sintering of bulk MgB<Subscript>2</Subscript> superconductor

The pertaining kinetic characteristics during the sintering of bulk polycrystalline MgB₂ superconductors is essentially important for the improvement of properties. Here Differential Thermal Analysis was adopted to record the heat effect during the preparation of bulk MgB₂ samples. The reaction between Mg and B powders starts before the melting point of pure Mg and the evolution for the fractions of MgB₂ were determined as a function of sintering temperatures. After fitting with different kinetic mechanism functions assumed, the sintering process of bulk MgB₂ superconductors was attributed to a solid-state interface-reaction controlled mechanism with an apparent activation energy of 4.54 × 10⁵ J mol⁻¹. Combined with microstructural observations by scanning electron microscopy and phase identification by X-ray diffraction, the formation process of MgB₂ phase was classified into two different stages: (i) solid-solid reaction stage, in which Mg and B powder starts to react and the growth of MgB₂ grain is restricted by the pinning effects of pores; (ii) solid–liquid reaction stage, in which the molten Mg melt promotes the reaction process and the regular hexagon bulk MgB₂ grain forms in a solution-reprecipitation and growth mode.     

Luminescence of europium-doped BaO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

We have prepared europium-doped BaO-Bi2O₃-B2O₃ glasses and investigated the doping effect on the main physicochemical properties and local structure of the glasses. Using Judd-Ofelt analysis, we calculated intensity parameters (Ω₂, Ω₄, and Ω₆), spontaneous emission probabilities, the radiative lifetime, luminescence branching factors, the quantum yield of luminescence, and the stimulated emission cross sections for ⁵ D ₀ → ⁷ F _J transitions.     

Properties of neodymium-doped Ca<Subscript>0.15</Subscript>Sr<Subscript>1.85</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>18</Subscript> ferroelectric ceramics

Bismuth-layered compound Ca_0.15Sr_1.85Bi_4−xNd_xTi₅O₁₈ (CSBNT, x = 0–0.25) ferroelectric ceramics samples were prepared by solid-state reaction method. The effects of Nd³⁺ doping on their ferroelectric and dielectric properties were investigated. The remnant polarization Pr of CSBNT ceramics increases at beginning then decreases with increasing of Nd³⁺ doping level, and a maximum Pr value of 9.6 μC/cm² at x = 0.05 was detected with a coercive field Ec = 80.2 kV/cm. Nd³⁺ dopant not only decreases the Curie temperature linearly, but also the dielectric constant (ε_r) and dielectric loss tangent (tan δ). The magnitudes of ε_r and tan δ at the frequency of 100 kHz are estimated to be 164 and 0.0083 at room temperature, respectively.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/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 MgB₂ 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 MgB₂/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 MgB₂/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/cm² at 20 and 38.1 K, respectively.     

Magnetic and Thermal Behavior of Ru<Subscript>0.9</Subscript>Sr<Subscript>2</Subscript>YCu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript> Magneto-Superconductor Synthesized by High-Pressure High-Temperature Technique

The structural, physical, and thermal property details of Ru_0.9Sr₂YCu_2.1O_7.9 (Y/Ru-1212) superconducting material synthesized through high pressure (6 GPa) and a high temperature (1400 °C) (HPHT) route are reported here. (Y/Ru-1212) crystallizes in P4/mmm tetragonal structure and is found free of any detectable impurities through X-ray diffractometry. Ru-spins are ordered magnetically above 145 K, with a sizeable ferromagnetic component at 5 K. Further clear diamagnetic transitions are seen in both zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility measurements and exhibiting superconductivity below 50 K. Both the thermoelectric power (S) and thermal conductivity (κ) measurements show superconductivity onset below 50 K with S=0 at 30 K and a broad hump in heat capacity C _p (T) below 30 K. Heat capacity (C _p ) measurements also exhibit the magnetic ordering temperature as a hump below 145 K. The appearance of a hump in C _p (T), instead of a clear transition, is indicative of short range magnetic correlations like spin glass (SG). Neither the high (145 K) nor the low (30 K) temperature humps of C _p (T) could be analyzed quantitatively because of short magnetic correlations in former and mixing of both superconductivity and FM components in a later case. The observed data is compared with various reported Ru-1212 systems synthesized under normal pressure conditions. It is concluded that HPHT synthesized Y/Ru-1212 is a bulk superconductor below 30 K with a substantial FM component.     

Critical Temperature of Smart Meta-superconducting MgB<Subscript>2</Subscript>

Enhancing the critical temperature (T _C ) is important not only to widen the practical applications but also to expand the theories of superconductivity. Inspired by the meta-material structure, we designed a smart meta-superconductor consisting of MgB₂ microparticles and Y₂O₃/Eu³⁺ nanorods. In the local electric field, Y₂O₃/Eu³⁺ nanorods generate an electroluminescence (EL) that can excite MgB₂ particles, thereby improving the T _C by strengthening the electron–phonon interaction. An MgB₂-based superconductor doped with one of four dopants of different EL intensities was prepared by an ex situ process. Results showed that the T _C of MgB₂ doped with 2 wt% Y₂O₃, which is not an EL material, is 33.1 K. However, replacing Y₂O₃ with Y₂O₃/Eu³⁺II, which displays a strong EL intensity, can improve the T _C by 2.8 to 35.9 K, which is even higher than that of pure MgB₂. The significant increment in T _C results from the EL exciting effect. Apart from EL intensity, the micromorphology and degree of dispersion of the dopants also affected the T _C . This smart meta-superconductor provides a new method to increase T _C .