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1.
Though low-temperature sintering of MgB2 superconductors below Mg melting point can effectively depress volatility of Mg and increase sintering density, its development was limited in recent years for the reason that it usually took very long time to form complete MgB2 phase with excellent Jc at low temperature. In present work, significantly improved Jc was surprisingly obtained in the MgB2 samples sintered at 575 °C for only 5 h after short-time ball milling, even though formation of MgB2 phase is not completed and lots of residual Mg is still present in these samples. The grain connectivity in prepared samples is obviously improved compared to referred MgB2 sample sintered at high temperature, which is responsible for the improvement of Jc. The method developed in present work seems bring a new opportunity for the development of low-cost practical MgB2 superconductors with improved Jc without using expensive nanometer-size dopants or high-temperature sintering.  相似文献   

2.
A polycrystalline MgB2 sample with 4 at.% Ni powder addition has been synthesized by a solid-state reaction at 750 °C. Two different Ni powders, raw (40 μm) and ball-milled (10 μm) particles were selected to clarify the influence of the Ni particle size on the secondary phase MgNi2.5B2 formation and the critical current density Jc of MgB2 bulks. According to the investigation of the sintering, it was found that the smaller Ni particles may significantly decrease the formation temperature of the MgNi2.5B2 ternary compound to 550 °C. Combined with scanning electron microscopy (SEM) analysis and magnetic measurements, it was suggested that the addition of fine Ni particles led to MgNi2.5B2 inclusion precipitating along the spiral terraces of MgB2 grains and with small particles sizes. The unique microstructure is responsible for the smaller reduction in Jc, which partly eliminates the effect of weak links compared with the coarse Ni particles. Besides, the formation mechanism of the microstructures and the relative location of MgNi2.5B2 varying from cluster at the MgB2 grain boundaries to a screw arrangement around the MgB2 grains with the decrease of the Ni grain sizes were also discussed in detail.  相似文献   

3.
Effects of Na2W2O7 addition on low-temperature sintering, microstructure and microwave dielectric properties of CaWO4 were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microwave dielectric resonator methods in this paper. The CaWO4 material could be sintered into a dense ceramic (~ 96% theoretical density) at 850 °C/2 h without affecting its microwave dielectric properties greatly by adding appropriate amount of Na2W2O7. The XRD results showed that CaWO4 main phase with trace amount of Na2W2O7 second phase was observed in (1 − x) CaWO4-xNa2W2O7 (0 < x ≤ 0.04) ceramics. The chemical compatibility of 0.96CaWO4-0.04Na2W2O7 ceramic with silver (Ag) powders was also investigated. The Ag showed inert behavior with 0.96CaWO4-0.04Na2W2O7 ceramic when co-fired at 875 °C for 2 h.  相似文献   

4.
Solid solution series, (Bi2O3)1−x (Y2O3)x and (Bi2O3)1−x (Gd2O3)x, forx = 0.10, 0.20, 0.30 and 0.40 were synthesized by standard ceramic technique. The structural phase characterization was carried out using X-ray powder diffraction technique. It was found that the solid solution containing 20–40 mole% of Y2O3 had face-centred cubic structure. All samples of the solid solution series, (Bi2O3)1−x (Gd2O3)x, had rhombohedral single phase in the concentration range 0.10 ≤x ≤ 0.40. Lattice parameters offcc phase of Y2O3 doped samples were calculated from the X-ray diffraction data. The lattice constant ‘a’ gradually decreases with increasing content of dopant concentration (x) for the Y2O3 doped system and obeys Vegard’s rule. The unit cell parameters for the (Bi2O3)1−x (Gd2O3)x doped samples showing rhombohedral phase were obtained on hexagonal setting.  相似文献   

5.
Nanocrystalline Zn1−x Ni x O (x = 0.00, 0.02, 0.04, 0.06, 0.08) powders were synthesized by a simple sol–gel autocombustion method using metal nitrates of zinc, nickel and glycine. Structural and optical properties of the Ni-doped ZnO samples annealed at 800 °C are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV–visible spectroscopy and photoluminescence (PL). X-ray diffraction analysis reveals that the Ni-doped ZnO crystallizes in a hexagonal wurtzite structure and secondary phase (NiO) was observed with the sensitivity of XRD measurement with the increasing nickel concentration (x ≥ 0.04). The lattice constants of Ni-doped ZnO nanoparticles increase slightly when Ni2+ is doped into ZnO lattice. The optical absorption band edge of the nickel doped samples was observed above 387 nm (3.20 eV) along with well-defined absorbance peaks at around 439 (2.82 eV), 615(2.01 eV) and 655 nm (1.89 eV). PL measurements of Ni-doped samples illustrated the strong UV emission band at ~3.02 eV, weak blue emission bands at 2.82 and 2.75 eV, and a strong green emission band at 2.26 eV. The observed red shift in the band gap from UV–visible analysis and near band edge UV emission with Ni doping may be considered to be related to the incorporation of Ni ions into the Zn site of the ZnO lattice.  相似文献   

6.
Nickel oxide (NiO), iron (III) oxide (Fe2O3), and mixed oxide (Ni0.04Zn0.96O and Fe0.03Zn0.97O) nanoparticles were synthesized by modified sol–gel method. The nanoparticle structural and morphological properties were investigated by infrared spectroscopy (FTIR), X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), and Mössbauer spectroscopy. The mixed oxides were characterized by energy-dispersive X-ray spectroscopy (EDX). The oxide precursor powders were analyzed by thermogravimetry (TG) and differential scanning calorimetry (DSC). The average sizes of the obtained NiO and Ni0.04Zn0.96O nanocrystallites were evaluated by X-ray line broadening using Scherrer's equation and were found to be 36 and 23 nm, respectively. Fe2O3 and Fe0.03Zn0.97O nanoparticles presented similar sizes, around 19 nm. EDX spectroscopy indicated that the calculated compositions of the mixed oxides were nearly consistent with their estimated molar ratios.  相似文献   

7.
Polycrystalline samples of Pb[(1 − x)(Zr1/2Ti1/2) − x(Zn1/3Ta2/3)]O 3 , where x = 0.1–0.5 were prepared by the columbite and wolframite methods. The crystal structure, microstructure, and dielectric properties of the sintered ceramics were investigated as a function of composition via X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric spectroscopy. The results indicated that the presence of Pb(Zn1/3Ta2/3)O3 (PZnTa) in the solid solution decreased the structural stability of overall perovskite phase. A transition from tetragonal to pseudo-cubic symmetry was observed as the PZnTa content increased and a co-existence of tetragonal and pseudo-cubic phases was observed at a composition close to x = 0.1. Examination of the dielectric spectra indicated that PZT–PZnTa exhibited an extremely high relative permittivity at the MPB composition. The permittivity showed a ferroelectric to paraelectric phase transition at 330 °C with a maximum value of 19,600 at 100 Hz at the MPB composition.  相似文献   

8.
0.55Pb(Ni1/3Nb2/3)O3-0.45Pb(Zr0.3Ti0.7)O3(PNN-PZT) ceramics with different concentration of xFe2O3 doping (where x = 0.0, 0.8, 1.2, 1.6 mol%) were synthesized by the conventional solid state sintering technique. X-ray diffraction analysis reveals that all specimens are a pure perovskite phase without pyrochlore phase. The density and grain size of Fe-doped ceramics tend to increase slightly with increasing concentration of Fe2O3. Comparing with the undoped ceramics, the piezoelectric, ferroelectric and dielectric properties of the Fe-doped PNN-PZT specimens are significantly improved. Properties of the piezoelectric constant as high as d33 ~ 956 pC/N, the electromechanical coupling factor kp ~ 0.74, and the dielectric constant εr ~ 6095 are achieved for the specimen with 1.2 mol% Fe2O3 doping sintered at 1200 °C for 2 h.  相似文献   

9.
As a positive temperature coefficient of resistivity (PTCR) material, Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramics with donor doping of Nb5+ and acceptor doping of Mn2+ were prepared by a conventional mixed oxide method. The influence of contents of Nb5+ and Mn2+ on the microstructure and PTCR characteristics of Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramics sintered at 1,360°C for 2 h was investigated. The result showed that the Curie temperature (T c) was shifted to a lower temperature with increasing of the content of Nb5+ and the resistance jump (ρmaxmin) was enhanced with doping of Mn2+. The grain size of ceramic sample decreased with increasing of contents of donor Nb5+ and acceptor Mn2+. The Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramic with 0.4 mol%Nb5+ and 0.04 mol%Mn2+ exhibited a low ρRT of 5.0 × 102 Ω cm, a typical PTCR effect of ρmaxmin > 103, and a T c of 158°C.  相似文献   

10.
The ferroelectric, dielectric and piezoelectric properties of compositions Pb1−x Ce x (Zr0.60Ti0.40)O3, (x = 0.0, 0.01, 0.02, 0.04, 0.06 and 0.08) are studied. The above compositions are prepared from their constituent oxides, calcined at 900 °C for 4 h and various phases present are characterized by X-ray diffraction (XRD) technique. The above powders are uniaxially pressed into circular compacts, sintered at 1,250 °C for 2 h, electroded, poled at 2 kV/mm D.C. voltage and their electrical properties are measured. The XRD analysis shows the presence of rhombohedral phase up to 2 mol% ceria while tetragonal phase found at higher concentrations. It is observed that the ferroelectric, dielectric and piezoelectric properties increase with the addition of ceria with a maximum at 2 mol% and then decreases. The higher piezo properties associated with low ceria concentration are attributed to rhombohedral phase.  相似文献   

11.
In an attempt to obtain dense lead metaniobate-based ceramics with improved dielectric and piezoelectric properties, the (Pb1.06−xBax)(Nb0.94Ti0.06)2O6 (x = 0, 0.04, 0.08, 0.12) piezoelectric ceramics were prepared separately from the two kinds of calcined powders, i.e., the powders with the rhombohedral phase and orthorhombic phase. For obtaining the calcined powders with the different phases, two different calcination temperatures of 900 °C and 1250 °C were chosen. The calcined powders were characterized using X-ray diffraction, scanning electron microscope, laser particle size analyzer and differential scanning calorimetry. Effects of the phase structures of the calcined powders on crystallite structure, microstructure, dielectric and piezoelectric properties of the ceramics were studied in detail. The lattice parameters and grain size of the ceramics are related to the phase structures of the calcined powders. The doping of Ba2+ has an influence on the dielectric and piezoelectric properties of the ceramics. The ceramics with x = 0.08 fabricated from the calcined powders with the orthorhombic phase demonstrate the optimum dielectric and piezoelectric properties.  相似文献   

12.
We report a 11B NMR line shape and spin-lattice relaxation rate (1/(T1T))(1/(T_1T)) study of pure and lightly carbon-doped MgB2-xCx_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB2_2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of Hcc2H^c_{c2} with carbon doping with respect to pure MgB2_2. The spin-lattice relaxation rate 1/(T1T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below TcT_{\rm c} in pure MgB2_2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ?\approx 10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T1T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.  相似文献   

13.
Synthesis and optical transmission of MgB2 thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealed ex situ and rendered superconducting with T c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼10 min at 550°C. The optical absorption coefficient, α and photon energy, E followed a Tauc-type behavior, = _T (E - E_g )(\alpha E)^{1/2} = \beta _T (E - E_{\rm g} ). The band gap (E g) was observed to peak at 2.5 eV; but, the slope parameter β T behaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB2 from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.  相似文献   

14.
Microstructure characteristics, phase transition, and electrical properties of (K0.4425Na0.52Li0.0375) (Nb0.8925Sb0.07Ta0.0375)O3 (KNLNST) lead-free piezoelectric ceramics prepared by normal sintering were investigated with an emphasis on the influence of sintering temperature. The microstructure and piezoelectric, ferroelectric, and dielectric properties were investigated, with a special emphasis on the influence of sintering temperature from 1,100 to 1,140 °C. Orthorhombic phases mainly exist in the ceramics sintered at 1,100–1,130 °C, whereas the tetragonal phase becomes dominant when sintering temperature is above 1,130 °C. Because of the existence of MPB-like transitional behavior, the piezoelectric coefficient (d 33), electromechanical coupling coefficient (kp), and dielectric constant (ε) show peak values of 304pC/N, 0.48, and 1,909, respectively, which are obtained in the sample sintered at 1,120 °C, and its Curie temperature (T C) is about 271 °C.  相似文献   

15.
The (1−x) Ni0.92Co0.03Mn0.05Fe2O4 + (x) BaTiO3 magnetoelectric (ME) composite have been prepared using conventional double sintering ceramic process where x varies as 1.00, 0.85, 0.70, 0.55 and 0.00. The presence of both phases has been confirmed by X-ray diffraction and the microstructure study will be carried out by SEM technique. The dc resistivity and thermo-emf of the samples have been studied with variation in temperature. The variation of dielectric constant (έ) and loss tangent (tan δ) will be measured in the frequency range 100 Hz–1 MHz. The ac conductivity has been derived from dielectric constant (έ) and loss tangent (tan δ). The static value of magnetoelectric conversion factor dc (ME)H has been studied as a function of intensity of magnetic field.  相似文献   

16.
An in-situ synthesis method was employed to produce NiTi-Ti2Ni-Ni3Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni-Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni-Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H2SO4 corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti2Ni/Ti4Ni2Ox phases embedded in a Ni3Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H2SO4 solution was comparable to that of wrought NiTi alloy.  相似文献   

17.
The preparation of dense MgB2 bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB2 powder. The superconducting characteristics of the obtained materials, namely critical temperature (Tc) and current (Jc), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB2 matrix. Both the adopted hot pressing techniques gave rise to undoped MgB2 pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB2, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB2 samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB2 and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB2 crystal.  相似文献   

18.
Two-phase sintered products [yEuTiO3 + (1?y)EuNbO3] were investigated as electrical-resistor materials. Electrical resistivities of the products did not depend on y in the range 0 < y < 0.96, and rapidly increased with increasing y over y = 0.96. The sintered products were metallic conductors from y = 0 to y = 0.96, and were semiconductors at y > 0.96. Thermal coefficients of resistivity for sintered products varied continuously with y, and became a maximum of 2.8 × 10?3 K?1 at y = 0.75 and a minimum around y = 1.00. The coefficient was positive at y < 0.96 and negative at y > 0.96. The absolute value of the thermal coefficient of resistivity minimized, namely (1?) (d?dT) = 0, at y = 0.96.  相似文献   

19.
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.  相似文献   

20.
Amorphization and crystallization behaviors of Ti70Ni15Al15 powders during mechanical alloying (MA) and subsequent heat treatments are studied. Amorphous phase that cannot be obtained in the rapidly quenched ribbon is formed in the powders after MA for 60 h. Upon continuous heating of the amorphous powders in DSC, two exothermic events are observed. The first exothermic event corresponds to the crystallization of the amorphous matrix into a supersaturated α-Ti phase of hexagonal close-packed structure. The growth kinetic of the α-Ti phase is sluggish, resulting in the formation of nanostructured α-Ti matrix. The second exothermic event corresponds to the solid state transformation of the meta-stable α-Ti into the equilibrium phases, Ti2Ni and Ti3Al. Using the amorphous powders, Ti-based bulk materials with novel microstructures can be developed for structural applications.  相似文献   

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