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The effect of PbF2 doping on the structural, electrical and mechanical properties of (Bi,Pb)–2223 superconductor
Journal Pre-proofs Research paper The Effect of PbF2 Doping on the Structural, Electrical and Mechanical Properties of (Bi,Pb)-2223 Superconductor M. Anas PII: DOI: Reference:
S0009-2614(19)31014-0 https://doi.org/10.1016/j.cplett.2019.137033 CPLETT 137033
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
11 November 2019 9 December 2019 10 December 2019
Please cite this article as: M. Anas, The Effect of PbF2 Doping on the Structural, Electrical and Mechanical Properties of (Bi,Pb)-2223 Superconductor, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett. 2019.137033
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The Effect of PbF2 Doping on the Structural, Electrical and Mechanical Properties of (Bi,Pb)-2223 Superconductor M. Anas Physics Department, faculty of science, Alexandria University, Alexandria, Egypt *E-mail: [email protected] [email protected] Abstract: The influence of PbF2 doping on the electrical and mechanical properties of Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ superconducting phase was studied. (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples were prepared by the solidstate reaction method. The PbF2 content 'x' was varied from 0.0 to 0.3 of the sample’s total mass. The characterization of the prepared samples was established by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and Proton Induced Gamma ray Emission (PIGE). Moreover, the electrical resistivity 'ρ(T)' and the transport current density 'J' measurements were carried out via four-probe technique. Phase examination by XRD indicated that the PbF2 doping enhanced the (Bi, Pb) -2223 phase formation up to x = 0.10. On the other hand, the high concentrations of PbF2 retarded the phase formation. Granular investigation, from scanning electron microscope, showed that the grain size was increased as x increased from 0.05 to 0.20 with more grain orientation and inter-coupling between superconducting grains. The results of electrical and mechanical measurements showed that the superconducting transition temperature (Tc), the transport critical current density (Jc) and the Vickers microhardness Hv were found to have optimal value at x = 0.1.
Key words: superconductivity, PIGE, Vickers microhardness, critical current density, oxygen content.
Page 1 of 30
1. Introduction: From all phases of High Temperature Superconductors (HTSCs), Bi-Sr-Ca-Cu-O (BSCCO) system had attracted a great attention. The attractive points of this phase are its simple preparation techniques, relative high superconducting transition temperature (Tc) [1], relative high transport critical current density (Jc) and high upper critical magnetic field Bc2 = 150 Tesla [2]. Generally, BSCCO has structural formula Bi2Sr2Can-1CunO2n+4+δ where n is the number of CuO planes in the unit cell. According to the value of n, three phases of BSCCO are achieved; Bi-2201, Bi-2212 and Bi2223. The most promising candidate is Bi-2223 due to its high Tc ~105K, prepared in 1988 by Maeda et al. [1]. In 1990, Pierre et al. [3], enhanced the electrical properties of BSCCO by partial substitution of a small amount of Pb in Bi site to approach Tc ~110K, the new phase was known as BPSCCO. Later, many researchers studied the addition or substitution of different elements with various percentages to (Bi,Pb)-2223 and reported the physical properties of samples. Terzioglua et al. [4], found that the diffusion of Au in (Bi,Pb)-2223 enhanced the volume fraction of Bi-2223 at the cost of Bi-2212 phase, Tc and Jc. The volume fraction, Tc , Jc and the Vickers microhardness number (Hv) of (Bi,Pb)-2223 were improved by Abdeen et al.[5] as a result of the substitution with Ho instead of Ca with 0.025 wt% . Awad et al. [6], showed that the addition of nano-sized SnO2 in (Bi,Pb)-2223 phase with x = 0.2 wt.% enhanced Tc up to 109 K, while the sample with x = 0.4 wt% enhanced Jc up to 293.3A/cm2 and improved Hv. An important enhancement of Tc up to 128 k was obtained by Abbas et al. [7] by the potassium substitution in Bi site (Bi2-xKxPb0.3)Sr2Ca2Cu3O10+δ with x = 0.5 wt.% . Kocabas et al. [8], reported that the substitution of Mg with x = 0.10 wt. % in Cu site enhanced the zero resistivity temperature (To) and the transition width ΔT. On the other hand, the MgO doping didn't have a significant effect in the phase formation rate. The effect of iron diffusion on (Bi,Pb)-2223 was studied by Ozturk et al. [9], They found that iron doping, in comparison with the undoped samples, increased Tc, improved the formation of (Bi,Pb)-2223 phase Page 2 of 30
and decreased the number and size of voids. Moreover, both of the micro hardness and the grain size were also enhanced by increasing the amount of the dopant. Sakiro˘glu et al.[10] found that the substitution of Ag of 0.15 % instead of Ca enhanced the super-conducting behaviour of (Bi,Pb)-2223. Mawassi et al.[11] found that addition of nano-sized silver 'Ag' with x = 1.5 wt. % enhanced the Tc of (Bi,Pb)-2223 to 112.8K and with x = 0.6 wt.% enhanced Jc up to 1014 A/cm2. Türk et al[12] showed that the substitution of W instead of Cu has a negative effect on the superconducting behaviour of (Bi,Pb)-2223. Kocabas et al., [13] found that the substitution of Zn instead of Ca in (Bi,Pb)-2223 with x = 0.15% enhanced the Tc from 108K up to 114K. Bilgili et al.[14] reported that no significant changes were observed due to the substitution of Li instead of Cu on (Bi,Pb)-2223. Many researchers studied the effect of F doping on the properties of BSCCO, Gupta et al.[15] were the first group who prepared fluorine doped (Bi,Pb)-2223, they achieved an enhancement in the Tc from 104K into 114K. Horiuchi et al. [16], enhanced the Tc of BSCCO to 120K by doping CaF2 with CaCO3/CaF2 at 9/1. Gao et al. [17, 18] got Tc = 121K for (Bi,Pb)-2223 by doping PbF2 and they attributed this improvement to the high chemical pressure of F-doped samples. Vit et al. [19], studied the effect of fluorine doping on (Bi,Pb)2223 (Bi1.6Pb0.4)Sr2Ca2.5Cu3.5Oy-xFx. They reported that the sample with x = 0.3 slightly increased the Tc, enhanced the Jc and improved the transition width ΔT, While greater amount of fluorine widens the superconducting transition and worsens the transport properties. Amira et al.[20] manifested the effect of Fluorine doping on (Bi1.6Pb0.4)Sr2Ca2Cu3OyFx where (x = 0– 0.6) and they found that doping of fluorine with x = 0.2 increases the Tc, the volume of the unit cell, the grain size and the transition width. Grivel et al. [21, 22] introduced CuF2 and SrF2, respectively, into (Bi,Pb)2223 to prepare superconducting samples of nominal composition Bi1.72Pb0.34Sr1.87Ca1.91Cu3.13O9.83−xF2x (x = 0.17, 0.34 and 0.51). They accomplished this work in two different ways; the first one by adding CuF2 before the first calcination process while the second one between the second calcination and the sintering process, they found that, the sample with x= 0.17 in both ways, the Tc was slightly enhanced. On the other hand, a small increase in volume of unit cell was observed for the samples prepared by the first method. Page 3 of 30
The goal of this work is to clarify the effect of F substitution in (Bi,Pb)-2223 through PbF2 addition on Bi-2223 superconducting phase. For this job, Bi1.8Pb0.4Sr2Ca2.1Cu3.2FxO10+δ-x (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared and investigated by XRD, SEM, electrical resistivity, J-E characteristics and Vickers micro hardness measurements. 2. Experimental Procedure: Bi1.8Pb0.4Sr2Ca2.1Cu3.2FxO10+δ-x with (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared using the solid state reaction technique. The high purity oxides (> 99.9%) Bi2O3, PbO, SrCO3, CaO, CuO and PbF2 were used as starting materials. The weighted powders were crushed in an agate mortar, then sieved for two times by 35 μm sieve. The calcination process was performed twice at 820oC for 24 h of each, among them, the samples were crushed and sieved. The obtained powder was pressed into the form of disks (diameter 1.6 cm and thickness 0.2 cm) under a pressure of 15 ton per inch. The sintering process was carried out by heating the samples from room temperature up to 845oC with a rate of 4oC/min, then fixed at this temperature for 96 h, after that, it was cooled with a rate of 2oC/min to 100oC [23, 24]. The disks were again crushed, sieved and pressed. Then the above process was repeated by a heating rate of 2oC/min up to 845oC and held at this temperature for 48 h and then it cooled to room temperature by a rate 1oC/min. The resulting samples were characterized by XRD (shimadzu MAXima_X XRD-7000) in the range 4o ≤ 2θ ≤ 80o. Samples surfaces morphology and grain size were investigated by SEM (Jsm-5300 operated at 30 KV with resolution power 4nm). The electrical resistivity was measured using four probe technique. Simultaneous PIGE and PIXE experiments (proton induced gamma ray emission and proton induced x-ray emission) were carried out by using 3 MeV proton beam delivered by the NEC 1.7 MV 5-SDH tandem accelerator of the Lebanese Atomic Energy Commission [25]. The beam (2-3 mm diameter) hit the target at 0°. The emitted gamma rays were detected, at 45° referring to the target's normal, by a High Purity Germanium (HPGe) detector with 40% relative efficiency and FWHM of approximately 1.9 keV at 1332 keV, properly shielded with lead. The X-rays emitted from the targets were detected by a Silicon Drift Detector (SDD), having 8 µm thick Be window and 130 eV measured FWHM energy resolution at 5.9 keV. The SDD detector is positioned Page 4 of 30
at 135° referring to the beam direction, and an aluminum sheet, 250 µm thick, is placed between the sample and the detector as an X-ray attenuator [26, 27]. In addition, simultaneously RBS spectra (Rutherford backscattering spectrometry) were acquired, with the same proton energy, using a Passivized Implanted Planar Silicon (PIPS) detector situated at 165° referring to the beam direction. The PIXE and RBS spectra were treated using GUPIX [28] and SIMNRA [29] codes respectively. The PIGE spectra were treated using the SPECTR computer code [30]. Granite MA-N external standard, from Geostandards, of the certified value of fluorine concentration (1.7%) has been used for the quantification of fluorine in the studied samples. It is prepared and analysed in a similar way to the superconductor samples (as pelletized thick target). The transport critical current density of the samples was measured using four probe technique at 77K with a homemade current source up to 8A and Keithley digital nano-voltmeter was used to measure voltage across samples. The mechanical properties of samples were investigated by measuring the Vickers micro hardness number Hv at different applied loads (25 – 1000 gf) using INNOVATEST IN-412A. The average value of (Hv) was calculated by taking five readings at different locations of the sample’s surface. And Hv calculated using the equation (1): 𝐻𝑣 = where 𝑑 =
𝑑1 + 𝑑2 2
18544 𝐹(𝑁) 𝑑2(𝜇𝑚)2
,
(1),
is the average diagonal length
3. Results and discussion: 3.1. Samples Characterizations: Page 5 of 30
3.1.1. X-rays powder diffraction (XRD) XRD Patterns of the prepared samples are displayed in Fig.1. The appearance of the characteristic peaks with indices (002), (0010), (0012), (200) and (220) at 2θ = 4.8o, 23.94o, 28.82o, 33.54o, and 47.96o respectively proves the formation of the tetragonal structure of Bi-2223. In addition the characteristic peaks corresponding to Bi-2212 (usually formed as a secondary phase) appeared with indices (002) and (117) at 2θ = 5.58o and 31.44o respectively [31]. It is important to mention that the peaks which correspond to Bi-2212 are designated by letter "L", while that corresponding to Bi-2223 are labelled by the letter "H". There is no shift in the positions of the peaks due to fluorine replacement, which indicates that there is no crystal structure transformation occurred. On the other hand, it is noted that the relative intensity of {0014} peak increases as the x increases. In contrary, the relative intensity of {200} peaks decreases as x increases. In other wards, the reflection from {0014} is strengthened on the coast of the refection from {200} planes. This observation predicts that the reflection planes {0014} could be increased, which may be due to an increase in the number of these planes in unit length along c-axis. Consequently, a reduction in the lattice parameter c is predicted. The Lattice parameters were calculated by applying Bragg's law for each peak at 2θ and the corresponding interplaner distance dhkl of the crystal planes (hkl). The calculated values of the lattice parameters are listed in table (I). A noticeable decrement for the lattice parameter c is observed as fluorine amount increases, in consistent with the above conclusion. This decrement could be attributed to two possible reasons; first, the slight decrease of the ionic radius of fluorine (1.47 Å) than that of oxygen (1.52 Å), and the second reason is the more electronegativity of F (3.98) than that of O (3.44). The relative volume fraction of both phases (Bi,Pb)-2223, (Bi,Pb)-2212 and the impurity phase are calculated using all the peak intensities according to the following relations [32]; ( Bi, Pb) 2223 100
I ( hkl ) 2223
I I
( hkl ) 2223 ( hkl ) 2212
I Ca2 PbO4
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(2)
( Bi, Pb) 2212 100
Ca2 PbO4 100
I
I ( hkl ) 2223
( hkl ) 2223
I I
I I
( hkl ) 2212 ( hkl ) 2212
Ca2 PbO4 ( hkl ) 2212
I Ca2 PbO4
I Ca2 PbO4
(3)
(4)
The calculated values of relative volume fractions are listed in table (I), it is noted that the relative volume fraction of the main phase (Bi,Pb)-2223 increases as x increases up to 0.10 and then decreases, in contrary the relative volume fraction of the secondary phase (Bi,Pb)-2212 decreases as x increases up to 0.10 and then increases. This observation suggests that a small replacement of oxygen by fluorine enhances the phase formation of (Bi,Pb)-2223 while higher contents of fluorine degraded the main phase. This can be explained by considering that the samples were over doped and become optimally doped at x = 0.10 then it becomes under doped for higher concentration of fluorine [20]. 3.1.2. Scanning Electron Microscopy (SEM) The SEM micrographs of the fractured surface of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3, are illustrated in Figures (2a-g). The presence of plate-like grains ensures the formation of the main phase (Bi,Pb)2223. It is noticed that the size of these plate-like grains increases as x increases up to x = 0.15, then it decreases for higher concentrations of fluorine. It is worth mention that, for samples with x = 0.25 and 0.3, the grains most likely to be fused. This may be explained by knowing that the melting point of PbO and PbF2 are 880 oC and 824 oC respectively. Since the sintering temperature was 845 oC (larger than the melting point of PbF2 and smaller than that of PbO), so the high contents of melted fluorine (due to the melting of PbF2) results in the appearance of welded grains. 3.1.3. Proton Induced Gamma rays Emmition (PIGE) The chemical characterization of lightly doped superconductors samples is limited due to the use small examined amounts, especially if the dopant element has a small atomic number. The adaptation of advanced analytical techniques, like PIXE/PIGE can be considered as a step forward, which provides the chemical analysis of a wide variety of materials. Non-destructive characterization is currently possible by means of Page 7 of 30
these spectrometric techniques, not accessible for more conventional techniques (like EDX). Moreover, these techniques provide extra advantages such as : i) the high sensitivity and speed for obtaining analytical data, ii) the absence of any pre-treatment, preparation and handling of samples and iii) the non-destructive character. Although PIXE is able to detect light elements as Na, Mg and Al, the simultaneous use of PIXE and PIGE improves the quantification, since gamma rays are less absorbed by the sample. Therefore, the combination of PIXE/PIGE arises as the optimal technique for the study, within some minutes, of compounds with very low traces of light elements such as F. Figure (3) shows PIGE spectrum overlay of MA-N standard, (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2 O10+δ, with x= 0.1 and 0.3. Due to a better cross section and prevent the interference with other elements, the fluorine quantification was done using the count yield of the 197-keV gamma ray which results from the reaction 19F(p,p’γ)19F.
It is calculated using the following formula [33]: 𝑌𝑆𝑎𝑚𝑝𝑙𝑒 (𝐸0)
𝐶𝑆𝑎𝑚𝑝𝑙𝑒/𝑆𝑆𝑎𝑚𝑝𝑙𝑒(𝐸1/2)
𝑌𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑 (𝐸 ) = 𝐶𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑/𝑆𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑(𝐸 0
1/2)
(5)
Yields of gamma emission (YSample and YStandard) at E =3MeV are determined using SPECTR software while the stopping power of the samples (Ssample) and the standard (SStandard) are calculated by applying 𝑆𝑡𝑜𝑡𝑎𝑙 = 𝛴 𝐶𝑖 𝑆𝑖 formula where the elementary stopping powers (Si) are calculated by the SRIM software [34] (using E1/2 = 2.5 MeV, which was determined by the excitation function [35]) and the matrix composition (Ci) is determined experimentally by PIXE analyses. The matrix of each sample was calculated by assuming that the major and minor elements are in oxides forms. The percentages of fluorine in the analysed samples are listed in the table (II). As noted, the measured F concentrations are in a good agreement with the nominal concentrations which proves the success of partial replacements of F instead of O in the prepared samples
3.2. The Physical Properties Measurements 3.2.1. The Electrical Resistivity Page 8 of 30
The temperature dependence of the electrical resistivity of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3 is displayed in Figure (4). All samples show a metallic like behaviour in normal state followed by superconducting transition at Tc. The Tc is determined as the temperature corresponding to the crest of the variation of
dρ
dT with T plots. The calculated values of Tc are listed in table (III). It is clear that Tc is enhanced
as x increased from x = 0.00 to x = 0.10 then it decreased for higher contents of fluorine. The enhancement in Tc may be attributed to The combination of several factors, such as the large volume fraction of the (Bi,Pb)2223 main phase (as concluded from XRD), the improvement in the grain connectivity and increasing of grain size with increasing x up to 0.10 (as shown in SEM micrograghs). In addition, the adjustment of the oxygen content due to the replacement of O by F (as proved by PIGE), with small concentrations, converts the samples from over doped to optimally doped at x=0.10 leads to the improvement of Tc in this sample. On contrary, the suppression of Tc (for x > 0.1) is due to the increase in (Bi,Pb)-2212 phase at the coast of (Bi,Pb)-2223, reduction in grain size, the change in oxygen content in CuO chains [36] and the creation of oxygen vacancy which capture the mobile holes [38]. Moreover, the superconducting transition width ΔT = Tc - To (To is the zero resistivity temperature) is calculated and listed in table (III). As indicated, ΔT decreases by 55%, relative to the value of the pure sample, as x increases up to 0.1. Since the small value of ΔT reflects the enhancement of the purity and quality of the investigated samples, therefore the sample with x=0.1 exhibits the most optimum conditions of preparation, which leads to a highly pure sample and behaves most likely as a single phase. This conclusion is confirmed by SEM micrographs in which the highly aligned grains observed is in the sample of x = 0.1. Similar results were obtained by Abou-Aly et. al [39].
3.2.2. The Transport Current Density:
Page 9 of 30
The transport critical current densities of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3 are measured at 77K using the four probe method. The obtained E-J curves are shown in Figure (5). The critical current density (Jc) is calculated for each E-J curve, and listed in table (III). It is clear that Jc increases as x increases from 0.0 to 0.10 by 18.3 %. Then it decreases with further increase in x, with a decrement percentage of about 57.3% at x = 0.30. The increase in Jc could be attributed to the increase of coupling between superconducting grains and their alignment as confirmed from the SEM micrographs. On the other hand, the depression of Jc with higher concentrations of fluorine may be due the deformation, the fusion and the increase in the disordering of the superconducting grains [11]. 3.2.3. The Vickers Microhardness: The poor mechanical properties of HTSCs stand in the way of a wide range of practical applications. This is because of the absence of slip planes in the oxide compounds, which leads to killing the ductility of the HTSCs and raises its brittleness. One of the most important mechanical properties to be improved is the micro hardness Figure (6) shows the variation of Vickers micro hardness number Hv as a function of applied force of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3. The measured values of Hv for all samples show two distinct behaviours as applied load (F) increases; a rapid decrease in Hv number in the applied force range (50 – 200 gf) followed by a saturation (nearly plateau) for higher applied force. This non-linear behaviour was reported in many literatures [40-46]. These behaviours named as normal indentation size effect NISE [47]. The NISE can be explained on the basis of the penetration depth of the indenter as follows: the indenter at low loads acts only on the surface layers, which is characterized by the relatively weak bonding of its granules. Hence the indenter can displace these granules easily as it penetrates through the sample, which leads to a rapid change in the resistance of the sample to the motion of the indenter blunt. While, at higher loads the penetration depth increases and the effect of inner layers becomes more dominant, hence a slight decreasing in values of Hv as the applied load increases. Page 10 of 30
The micro hardness data can be analysed according to many approaches. According to Mayer's law [44, 47], the applied load (F) and the indentation diagonal length (d) are related by the relation:
F Ad n
(6)
where A is a constant, representing the load needed to initiate unit indentation, the exponent n is called Meyer’s index, which describes the ISE. For n < 2, we have normal ISE, while n >2 indicates the reverse ISE. The values of A and n can be estimated from ln(F) versus ln(d) plots as shown in Figure (7). The calculated values of A and n are listed in table (IV), it is found that the values of n are less than 2 which ensure the normal indentation size effect of the examined samples. Hays–Kendall approach (HK approach) Hays–Kendall (HK) approach [48] suggested that there exists a minimum load W needed to initiate the plastic deformation, below which only an elastic deformation takes place. Accordingly, The effective load becomes Feff = F – W, then
F W A1 d 2
(7)
where A1 is the a load independent coefficient. Both A1 and W can be obtained by plotting (F) against (d2) as indicated in figure (8), the extracted values are listed in table (IV). It is clear that, all of the samples give positive values of W, indicating that the applied load was sufficient to create both the elastic and the plastic deformation. On the other hand, no significant change is observed for A1. According to the HK approximation, the Hays–Kendall micro hardness HHK could be calculated by the equation (8): H HK
1854.4 ( F W ) d2
Elastic/plastic deformation model (EPD)
Page 11 of 30
(8)
Bull et al. [49] supposed that due to elastic recovery, there is an immeasurable portion 'do' of the diagonal. Consequently, the dependence of the indentation size on the applied load can be formulated as: F A2 (d d o ) 2
(9)
where A2 is a constant and d is the recorded indentation diagonal. The values of A2 and do can be estimated by plotting the variation of F1/2 versus d as in figure (9). The calculated values are reported in Table (IV). It is clear that, the value of do is positive for all the samples. This means that, for this range of applied loads, the elastic deformation is observed along with plastic deformation, and the elastic relaxation is present. Also, it is found that do decreases as x increases up to x = 0.10. Then, it increases at higher concentration of fluorine. This behaviour predicts that the samples become more elastic as x increases up to 0.1, while its elasticity dies for further increase in x. Similar behaviour was reported by previous studies on BSCCO system [9, 50]. The elastic/plastic deformation micro hardness HEPD is calculated using the equation (10):
H EPD 1854.4
F (d d o ) 2
(10)
Proportional sample resistance model (PSR) Proportional sample resistance (PSR) model which was developed by Li and Bradt [51] is successfully used for the analysis of micro hardness of materials showing the ISE behaviour. This model is given by the formula:
F d d 2
(11)
where α is the surface energy, and the change in the α value is associated with the energy dispersion of the surface cracks [52]. β is a parameter used to calculate the real micro-hardness. According to equation (11), the relation between F/d against d yields a straight line with slope corresponding to the parameter β, and the intercept represents the surface energy constant α as shown in figure (10). The calculated values of α and β are listed in table (IV). It is clear that the value of α decreases with the increasing of fluorine content up to x = Page 12 of 30
0.10, then it has a reverse trend. The enhancement can be ascribed to the dissipation of the cracks at indenter facet/specimen interface [53]. According to this model, the PSR micro hardness number could be calculated by equation (12) and the calculated values are listed in table (IV):
H PSR 1854.4
d d2 d2
(12)
In order to test the applicability of the above mentioned models for the examined samples, the micro hardness is calculated according to each model and compared with the measured HV. Figures (11 a-g) show a comparison between the experimental HV and the theoretical values calculated from the different models for (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3. An excellent agreement between the measured values and the calculated values that obtained from PSR model is observed in all samples. This conclusion agrees with previous studies on (Bi,Pb)-2223 [5, 54]. The suitability of PSR model for describing the behaviour of micro hardness of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ could be attributed to the reduction in the spacing between the grains (as confirmed from SEM), consequently increasing the inter-grain contact surfaces, which, in turns, arises a large surface energy resists the motion of the indenter through the sample. 4. Conclusion: (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) was successfully prepared via the solid state reaction technique. The results of XRD and SEM gave the highest volume fraction and the largest grain size at x = 0.10. The PIGE spectra has proved the partial replacement of F instead of O. The measurements of Tc, Jc and Hv showed that the sample with x = 0.10 has the optimum electrical and mechanical properties. Moreover, the analysis of Vickers micro hardness data revealed an excellent agreement with the results of PSR model. Generally, the substitution process of fluorine instead of oxygen with x = 0.10 has enhanced the structural , transport and mechanical properties of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ. Acknowledgements:
Page 13 of 30
This work was executed in the Superconductivity and Metallic Glass Lab, Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt. Special thanks to Professor R. Awad and professor M. Roumié for PIGE measurements.
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Tables: Table (I): The lattice parameters and the relative volume fractions of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. x
a (Å)
0.00
5.32
0.05
5.33
c (Å)
c/a
V(2223) %
V(2212) %
Ca2PbO4 %
37.65
7.077
87.995
11.139
0.866
37.42
7.021
90.922
9.078
0.000
Page 17 of 30
0.10
5.32
37.12
6.977
92.972
5.271
1.757
0.15
5.32
36.96
6.947
89.922
8.527
1.550
0.20
5.32
36.75
6.908
86.391
11.538
2.071
0.25
5.32
36.64
6.887
84.615
13.757
1.627
0.30
5.33
36.41
6.831
83.696
14.614
1.691
Table (II) Fluorine contents in the (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, calculated from PIGE spectra
Sample
Fluorine (wt.%)
x = 0.00
0.006 ± 0.001
x = 0.05
0.037 ± 0.001
x = 0.10
0.042 ± 0.002
x = 0.15
0.076 ± 0.002
x = 0.20
0.219 ± 0.002
x = 0.25
0.294 ± 0.002
x = 0.30
0.332 ± 0.002
Table (III): The superconducting transition temperature (Tc), zero resistivity temperature (To), the transition width (ΔT) and the critical current density (Jc )of the (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30.
x
Tc (K)
To (K)
ΔT (K)
Jc (A/cm2)
0.00
108.5
98.5
10
573.35
Page 18 of 30
0.05
111
106
5
624.62
0.10
114
108.5
4.5
701.62
0.15
111
108.5
7.5
581.24
0.20
108.5
101
7.5
575.39
0.25
103.5
98.5
5.5
326.05
0.30
101
93.25
7.75
244.82
Table (IV) Mechanical parameters calculated according to different models of of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Mayer's parameters x n
A1 (N/μm2)
Elastic/plastic deformation model
Proportional sample resistance model
W (N)
A21/2(N1/2/μm2)
do (μm)
α (N/μm)
β (N/μm2)
0.00326 0.0003
0.4992
0.0003
45.6
0.0131
0.0002
0.05 1.5042 0.003493 0.0003
0.368
0.0003
42.3
0.0113
0.0002
0.10 1.7503 0.001507 0.0004
0.2436
0.0004
26.7
0.0071
0.0004
0.15 1.6062 0.002805 0.0004
0.2976
0.0003
40.0
0.0107
0.0003
0.20 1.6978 0.001836 0.0004
0.2905
0.0003
38.0
0.0089
0.0004
0.25 1.5248 0.003226 0.0003
0.2981
0.0003
41.3
0.0108
0.0002
0.30 1.5679 0.002616 0.0003
0.4734
0.0003
45.3
0.0128
0.0002
0.00 1.5151
A (GPa)
Hays–Kendall approach
Figure caption Fig. (1) XRD patterns of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.3. Fig. (2a-g) SEM micrographs of the fractured surface of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = (a) x= 0.00, (b) x= 0.05, (c) x= 0.10, (d) x= 0.15, (e) x= 0.20, (f) x= 0.25 and (g) x= 0.30. Fig. (3) PIGE spectra of MA-N standard of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2 O10+δ, with x= 0.1 and 0.3 using 3 MeV proton beam. Page 19 of 30
Fig. (4) Temperature dependence of the electrical resistivity of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (5) E–J curves of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (6) The dependence of Vickers microhardness "HV" on the applied load "F" of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (7) Variation of Ln (HV) with Ln (d) of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (8) Variation of the applied load "F" with d2 according to the Hays–Kendall model of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (9) Plots of F1/2 versus d according to the elastic/plastic deformation model of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (10) Plots of F/d versus d according to the proportional sample resistance model of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. 11a-g Variations of the measured HV and calculated HV according to different models with the applied load "F of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with a) x = 0.00, b) x= 0.05, c) x= 0.10, d) x= 0.15, e) x= 0.20, f) x= 0.25 and g) x= 0.30.
Page 20 of 30
Fig. 1
Page 21 of 30
Fig. 2
Page 22 of 30
Fig. 3
Fig. 4 Page 23 of 30
Fig. 5
Fig. 6 Page 24 of 30
Fig. 7
Fig. 8
Page 25 of 30
Fig. 9
Fig. 10 Page 26 of 30
Fig. 11 Page 27 of 30
Graphical abstract
Page 28 of 30
Highlights
Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ superconducting phase doped by PbF2 was prepared by single step solid state reaction technique. The replacement of F instead of O in the final compound was examined by PIGE and confirmed by XRD. An enhancement in the electrical and mechanical performance was observed as a result of PbF2 doping with small concentrations. While as, for higher concentrations the properties showed a retardation. The results indicate that it is possible to control the Oxygen content in high temperature superconductors by PbF2 doping due to partial replacement of O by F.
Page 29 of 30
Author Contribution Statement I, M. Anas, did the whole research, from developing the idea to providing materials, preparing samples, conducting measurements of electrical resistivity, current density, and Vicker's micro hardness, as well as following up on the technicians assigned to conduct X-ray diffraction and scanning electron microscopy. And ended with the analysis and discussion of the results and writing the manuscript.
Page 30 of 30
S0009-2614(19)31014-0 https://doi.org/10.1016/j.cplett.2019.137033 CPLETT 137033
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
11 November 2019 9 December 2019 10 December 2019
Please cite this article as: M. Anas, The Effect of PbF2 Doping on the Structural, Electrical and Mechanical Properties of (Bi,Pb)-2223 Superconductor, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett. 2019.137033
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© 2019 Published by Elsevier B.V.
The Effect of PbF2 Doping on the Structural, Electrical and Mechanical Properties of (Bi,Pb)-2223 Superconductor M. Anas Physics Department, faculty of science, Alexandria University, Alexandria, Egypt *E-mail: [email protected] [email protected] Abstract: The influence of PbF2 doping on the electrical and mechanical properties of Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ superconducting phase was studied. (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples were prepared by the solidstate reaction method. The PbF2 content 'x' was varied from 0.0 to 0.3 of the sample’s total mass. The characterization of the prepared samples was established by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and Proton Induced Gamma ray Emission (PIGE). Moreover, the electrical resistivity 'ρ(T)' and the transport current density 'J' measurements were carried out via four-probe technique. Phase examination by XRD indicated that the PbF2 doping enhanced the (Bi, Pb) -2223 phase formation up to x = 0.10. On the other hand, the high concentrations of PbF2 retarded the phase formation. Granular investigation, from scanning electron microscope, showed that the grain size was increased as x increased from 0.05 to 0.20 with more grain orientation and inter-coupling between superconducting grains. The results of electrical and mechanical measurements showed that the superconducting transition temperature (Tc), the transport critical current density (Jc) and the Vickers microhardness Hv were found to have optimal value at x = 0.1.
Key words: superconductivity, PIGE, Vickers microhardness, critical current density, oxygen content.
Page 1 of 30
1. Introduction: From all phases of High Temperature Superconductors (HTSCs), Bi-Sr-Ca-Cu-O (BSCCO) system had attracted a great attention. The attractive points of this phase are its simple preparation techniques, relative high superconducting transition temperature (Tc) [1], relative high transport critical current density (Jc) and high upper critical magnetic field Bc2 = 150 Tesla [2]. Generally, BSCCO has structural formula Bi2Sr2Can-1CunO2n+4+δ where n is the number of CuO planes in the unit cell. According to the value of n, three phases of BSCCO are achieved; Bi-2201, Bi-2212 and Bi2223. The most promising candidate is Bi-2223 due to its high Tc ~105K, prepared in 1988 by Maeda et al. [1]. In 1990, Pierre et al. [3], enhanced the electrical properties of BSCCO by partial substitution of a small amount of Pb in Bi site to approach Tc ~110K, the new phase was known as BPSCCO. Later, many researchers studied the addition or substitution of different elements with various percentages to (Bi,Pb)-2223 and reported the physical properties of samples. Terzioglua et al. [4], found that the diffusion of Au in (Bi,Pb)-2223 enhanced the volume fraction of Bi-2223 at the cost of Bi-2212 phase, Tc and Jc. The volume fraction, Tc , Jc and the Vickers microhardness number (Hv) of (Bi,Pb)-2223 were improved by Abdeen et al.[5] as a result of the substitution with Ho instead of Ca with 0.025 wt% . Awad et al. [6], showed that the addition of nano-sized SnO2 in (Bi,Pb)-2223 phase with x = 0.2 wt.% enhanced Tc up to 109 K, while the sample with x = 0.4 wt% enhanced Jc up to 293.3A/cm2 and improved Hv. An important enhancement of Tc up to 128 k was obtained by Abbas et al. [7] by the potassium substitution in Bi site (Bi2-xKxPb0.3)Sr2Ca2Cu3O10+δ with x = 0.5 wt.% . Kocabas et al. [8], reported that the substitution of Mg with x = 0.10 wt. % in Cu site enhanced the zero resistivity temperature (To) and the transition width ΔT. On the other hand, the MgO doping didn't have a significant effect in the phase formation rate. The effect of iron diffusion on (Bi,Pb)-2223 was studied by Ozturk et al. [9], They found that iron doping, in comparison with the undoped samples, increased Tc, improved the formation of (Bi,Pb)-2223 phase Page 2 of 30
and decreased the number and size of voids. Moreover, both of the micro hardness and the grain size were also enhanced by increasing the amount of the dopant. Sakiro˘glu et al.[10] found that the substitution of Ag of 0.15 % instead of Ca enhanced the super-conducting behaviour of (Bi,Pb)-2223. Mawassi et al.[11] found that addition of nano-sized silver 'Ag' with x = 1.5 wt. % enhanced the Tc of (Bi,Pb)-2223 to 112.8K and with x = 0.6 wt.% enhanced Jc up to 1014 A/cm2. Türk et al[12] showed that the substitution of W instead of Cu has a negative effect on the superconducting behaviour of (Bi,Pb)-2223. Kocabas et al., [13] found that the substitution of Zn instead of Ca in (Bi,Pb)-2223 with x = 0.15% enhanced the Tc from 108K up to 114K. Bilgili et al.[14] reported that no significant changes were observed due to the substitution of Li instead of Cu on (Bi,Pb)-2223. Many researchers studied the effect of F doping on the properties of BSCCO, Gupta et al.[15] were the first group who prepared fluorine doped (Bi,Pb)-2223, they achieved an enhancement in the Tc from 104K into 114K. Horiuchi et al. [16], enhanced the Tc of BSCCO to 120K by doping CaF2 with CaCO3/CaF2 at 9/1. Gao et al. [17, 18] got Tc = 121K for (Bi,Pb)-2223 by doping PbF2 and they attributed this improvement to the high chemical pressure of F-doped samples. Vit et al. [19], studied the effect of fluorine doping on (Bi,Pb)2223 (Bi1.6Pb0.4)Sr2Ca2.5Cu3.5Oy-xFx. They reported that the sample with x = 0.3 slightly increased the Tc, enhanced the Jc and improved the transition width ΔT, While greater amount of fluorine widens the superconducting transition and worsens the transport properties. Amira et al.[20] manifested the effect of Fluorine doping on (Bi1.6Pb0.4)Sr2Ca2Cu3OyFx where (x = 0– 0.6) and they found that doping of fluorine with x = 0.2 increases the Tc, the volume of the unit cell, the grain size and the transition width. Grivel et al. [21, 22] introduced CuF2 and SrF2, respectively, into (Bi,Pb)2223 to prepare superconducting samples of nominal composition Bi1.72Pb0.34Sr1.87Ca1.91Cu3.13O9.83−xF2x (x = 0.17, 0.34 and 0.51). They accomplished this work in two different ways; the first one by adding CuF2 before the first calcination process while the second one between the second calcination and the sintering process, they found that, the sample with x= 0.17 in both ways, the Tc was slightly enhanced. On the other hand, a small increase in volume of unit cell was observed for the samples prepared by the first method. Page 3 of 30
The goal of this work is to clarify the effect of F substitution in (Bi,Pb)-2223 through PbF2 addition on Bi-2223 superconducting phase. For this job, Bi1.8Pb0.4Sr2Ca2.1Cu3.2FxO10+δ-x (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared and investigated by XRD, SEM, electrical resistivity, J-E characteristics and Vickers micro hardness measurements. 2. Experimental Procedure: Bi1.8Pb0.4Sr2Ca2.1Cu3.2FxO10+δ-x with (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared using the solid state reaction technique. The high purity oxides (> 99.9%) Bi2O3, PbO, SrCO3, CaO, CuO and PbF2 were used as starting materials. The weighted powders were crushed in an agate mortar, then sieved for two times by 35 μm sieve. The calcination process was performed twice at 820oC for 24 h of each, among them, the samples were crushed and sieved. The obtained powder was pressed into the form of disks (diameter 1.6 cm and thickness 0.2 cm) under a pressure of 15 ton per inch. The sintering process was carried out by heating the samples from room temperature up to 845oC with a rate of 4oC/min, then fixed at this temperature for 96 h, after that, it was cooled with a rate of 2oC/min to 100oC [23, 24]. The disks were again crushed, sieved and pressed. Then the above process was repeated by a heating rate of 2oC/min up to 845oC and held at this temperature for 48 h and then it cooled to room temperature by a rate 1oC/min. The resulting samples were characterized by XRD (shimadzu MAXima_X XRD-7000) in the range 4o ≤ 2θ ≤ 80o. Samples surfaces morphology and grain size were investigated by SEM (Jsm-5300 operated at 30 KV with resolution power 4nm). The electrical resistivity was measured using four probe technique. Simultaneous PIGE and PIXE experiments (proton induced gamma ray emission and proton induced x-ray emission) were carried out by using 3 MeV proton beam delivered by the NEC 1.7 MV 5-SDH tandem accelerator of the Lebanese Atomic Energy Commission [25]. The beam (2-3 mm diameter) hit the target at 0°. The emitted gamma rays were detected, at 45° referring to the target's normal, by a High Purity Germanium (HPGe) detector with 40% relative efficiency and FWHM of approximately 1.9 keV at 1332 keV, properly shielded with lead. The X-rays emitted from the targets were detected by a Silicon Drift Detector (SDD), having 8 µm thick Be window and 130 eV measured FWHM energy resolution at 5.9 keV. The SDD detector is positioned Page 4 of 30
at 135° referring to the beam direction, and an aluminum sheet, 250 µm thick, is placed between the sample and the detector as an X-ray attenuator [26, 27]. In addition, simultaneously RBS spectra (Rutherford backscattering spectrometry) were acquired, with the same proton energy, using a Passivized Implanted Planar Silicon (PIPS) detector situated at 165° referring to the beam direction. The PIXE and RBS spectra were treated using GUPIX [28] and SIMNRA [29] codes respectively. The PIGE spectra were treated using the SPECTR computer code [30]. Granite MA-N external standard, from Geostandards, of the certified value of fluorine concentration (1.7%) has been used for the quantification of fluorine in the studied samples. It is prepared and analysed in a similar way to the superconductor samples (as pelletized thick target). The transport critical current density of the samples was measured using four probe technique at 77K with a homemade current source up to 8A and Keithley digital nano-voltmeter was used to measure voltage across samples. The mechanical properties of samples were investigated by measuring the Vickers micro hardness number Hv at different applied loads (25 – 1000 gf) using INNOVATEST IN-412A. The average value of (Hv) was calculated by taking five readings at different locations of the sample’s surface. And Hv calculated using the equation (1): 𝐻𝑣 = where 𝑑 =
𝑑1 + 𝑑2 2
18544 𝐹(𝑁) 𝑑2(𝜇𝑚)2
,
(1),
is the average diagonal length
3. Results and discussion: 3.1. Samples Characterizations: Page 5 of 30
3.1.1. X-rays powder diffraction (XRD) XRD Patterns of the prepared samples are displayed in Fig.1. The appearance of the characteristic peaks with indices (002), (0010), (0012), (200) and (220) at 2θ = 4.8o, 23.94o, 28.82o, 33.54o, and 47.96o respectively proves the formation of the tetragonal structure of Bi-2223. In addition the characteristic peaks corresponding to Bi-2212 (usually formed as a secondary phase) appeared with indices (002) and (117) at 2θ = 5.58o and 31.44o respectively [31]. It is important to mention that the peaks which correspond to Bi-2212 are designated by letter "L", while that corresponding to Bi-2223 are labelled by the letter "H". There is no shift in the positions of the peaks due to fluorine replacement, which indicates that there is no crystal structure transformation occurred. On the other hand, it is noted that the relative intensity of {0014} peak increases as the x increases. In contrary, the relative intensity of {200} peaks decreases as x increases. In other wards, the reflection from {0014} is strengthened on the coast of the refection from {200} planes. This observation predicts that the reflection planes {0014} could be increased, which may be due to an increase in the number of these planes in unit length along c-axis. Consequently, a reduction in the lattice parameter c is predicted. The Lattice parameters were calculated by applying Bragg's law for each peak at 2θ and the corresponding interplaner distance dhkl of the crystal planes (hkl). The calculated values of the lattice parameters are listed in table (I). A noticeable decrement for the lattice parameter c is observed as fluorine amount increases, in consistent with the above conclusion. This decrement could be attributed to two possible reasons; first, the slight decrease of the ionic radius of fluorine (1.47 Å) than that of oxygen (1.52 Å), and the second reason is the more electronegativity of F (3.98) than that of O (3.44). The relative volume fraction of both phases (Bi,Pb)-2223, (Bi,Pb)-2212 and the impurity phase are calculated using all the peak intensities according to the following relations [32]; ( Bi, Pb) 2223 100
I ( hkl ) 2223
I I
( hkl ) 2223 ( hkl ) 2212
I Ca2 PbO4
Page 6 of 30
(2)
( Bi, Pb) 2212 100
Ca2 PbO4 100
I
I ( hkl ) 2223
( hkl ) 2223
I I
I I
( hkl ) 2212 ( hkl ) 2212
Ca2 PbO4 ( hkl ) 2212
I Ca2 PbO4
I Ca2 PbO4
(3)
(4)
The calculated values of relative volume fractions are listed in table (I), it is noted that the relative volume fraction of the main phase (Bi,Pb)-2223 increases as x increases up to 0.10 and then decreases, in contrary the relative volume fraction of the secondary phase (Bi,Pb)-2212 decreases as x increases up to 0.10 and then increases. This observation suggests that a small replacement of oxygen by fluorine enhances the phase formation of (Bi,Pb)-2223 while higher contents of fluorine degraded the main phase. This can be explained by considering that the samples were over doped and become optimally doped at x = 0.10 then it becomes under doped for higher concentration of fluorine [20]. 3.1.2. Scanning Electron Microscopy (SEM) The SEM micrographs of the fractured surface of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3, are illustrated in Figures (2a-g). The presence of plate-like grains ensures the formation of the main phase (Bi,Pb)2223. It is noticed that the size of these plate-like grains increases as x increases up to x = 0.15, then it decreases for higher concentrations of fluorine. It is worth mention that, for samples with x = 0.25 and 0.3, the grains most likely to be fused. This may be explained by knowing that the melting point of PbO and PbF2 are 880 oC and 824 oC respectively. Since the sintering temperature was 845 oC (larger than the melting point of PbF2 and smaller than that of PbO), so the high contents of melted fluorine (due to the melting of PbF2) results in the appearance of welded grains. 3.1.3. Proton Induced Gamma rays Emmition (PIGE) The chemical characterization of lightly doped superconductors samples is limited due to the use small examined amounts, especially if the dopant element has a small atomic number. The adaptation of advanced analytical techniques, like PIXE/PIGE can be considered as a step forward, which provides the chemical analysis of a wide variety of materials. Non-destructive characterization is currently possible by means of Page 7 of 30
these spectrometric techniques, not accessible for more conventional techniques (like EDX). Moreover, these techniques provide extra advantages such as : i) the high sensitivity and speed for obtaining analytical data, ii) the absence of any pre-treatment, preparation and handling of samples and iii) the non-destructive character. Although PIXE is able to detect light elements as Na, Mg and Al, the simultaneous use of PIXE and PIGE improves the quantification, since gamma rays are less absorbed by the sample. Therefore, the combination of PIXE/PIGE arises as the optimal technique for the study, within some minutes, of compounds with very low traces of light elements such as F. Figure (3) shows PIGE spectrum overlay of MA-N standard, (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2 O10+δ, with x= 0.1 and 0.3. Due to a better cross section and prevent the interference with other elements, the fluorine quantification was done using the count yield of the 197-keV gamma ray which results from the reaction 19F(p,p’γ)19F.
It is calculated using the following formula [33]: 𝑌𝑆𝑎𝑚𝑝𝑙𝑒 (𝐸0)
𝐶𝑆𝑎𝑚𝑝𝑙𝑒/𝑆𝑆𝑎𝑚𝑝𝑙𝑒(𝐸1/2)
𝑌𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑 (𝐸 ) = 𝐶𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑/𝑆𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑(𝐸 0
1/2)
(5)
Yields of gamma emission (YSample and YStandard) at E =3MeV are determined using SPECTR software while the stopping power of the samples (Ssample) and the standard (SStandard) are calculated by applying 𝑆𝑡𝑜𝑡𝑎𝑙 = 𝛴 𝐶𝑖 𝑆𝑖 formula where the elementary stopping powers (Si) are calculated by the SRIM software [34] (using E1/2 = 2.5 MeV, which was determined by the excitation function [35]) and the matrix composition (Ci) is determined experimentally by PIXE analyses. The matrix of each sample was calculated by assuming that the major and minor elements are in oxides forms. The percentages of fluorine in the analysed samples are listed in the table (II). As noted, the measured F concentrations are in a good agreement with the nominal concentrations which proves the success of partial replacements of F instead of O in the prepared samples
3.2. The Physical Properties Measurements 3.2.1. The Electrical Resistivity Page 8 of 30
The temperature dependence of the electrical resistivity of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3 is displayed in Figure (4). All samples show a metallic like behaviour in normal state followed by superconducting transition at Tc. The Tc is determined as the temperature corresponding to the crest of the variation of
dρ
dT with T plots. The calculated values of Tc are listed in table (III). It is clear that Tc is enhanced
as x increased from x = 0.00 to x = 0.10 then it decreased for higher contents of fluorine. The enhancement in Tc may be attributed to The combination of several factors, such as the large volume fraction of the (Bi,Pb)2223 main phase (as concluded from XRD), the improvement in the grain connectivity and increasing of grain size with increasing x up to 0.10 (as shown in SEM micrograghs). In addition, the adjustment of the oxygen content due to the replacement of O by F (as proved by PIGE), with small concentrations, converts the samples from over doped to optimally doped at x=0.10 leads to the improvement of Tc in this sample. On contrary, the suppression of Tc (for x > 0.1) is due to the increase in (Bi,Pb)-2212 phase at the coast of (Bi,Pb)-2223, reduction in grain size, the change in oxygen content in CuO chains [36] and the creation of oxygen vacancy which capture the mobile holes [38]. Moreover, the superconducting transition width ΔT = Tc - To (To is the zero resistivity temperature) is calculated and listed in table (III). As indicated, ΔT decreases by 55%, relative to the value of the pure sample, as x increases up to 0.1. Since the small value of ΔT reflects the enhancement of the purity and quality of the investigated samples, therefore the sample with x=0.1 exhibits the most optimum conditions of preparation, which leads to a highly pure sample and behaves most likely as a single phase. This conclusion is confirmed by SEM micrographs in which the highly aligned grains observed is in the sample of x = 0.1. Similar results were obtained by Abou-Aly et. al [39].
3.2.2. The Transport Current Density:
Page 9 of 30
The transport critical current densities of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3 are measured at 77K using the four probe method. The obtained E-J curves are shown in Figure (5). The critical current density (Jc) is calculated for each E-J curve, and listed in table (III). It is clear that Jc increases as x increases from 0.0 to 0.10 by 18.3 %. Then it decreases with further increase in x, with a decrement percentage of about 57.3% at x = 0.30. The increase in Jc could be attributed to the increase of coupling between superconducting grains and their alignment as confirmed from the SEM micrographs. On the other hand, the depression of Jc with higher concentrations of fluorine may be due the deformation, the fusion and the increase in the disordering of the superconducting grains [11]. 3.2.3. The Vickers Microhardness: The poor mechanical properties of HTSCs stand in the way of a wide range of practical applications. This is because of the absence of slip planes in the oxide compounds, which leads to killing the ductility of the HTSCs and raises its brittleness. One of the most important mechanical properties to be improved is the micro hardness Figure (6) shows the variation of Vickers micro hardness number Hv as a function of applied force of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3. The measured values of Hv for all samples show two distinct behaviours as applied load (F) increases; a rapid decrease in Hv number in the applied force range (50 – 200 gf) followed by a saturation (nearly plateau) for higher applied force. This non-linear behaviour was reported in many literatures [40-46]. These behaviours named as normal indentation size effect NISE [47]. The NISE can be explained on the basis of the penetration depth of the indenter as follows: the indenter at low loads acts only on the surface layers, which is characterized by the relatively weak bonding of its granules. Hence the indenter can displace these granules easily as it penetrates through the sample, which leads to a rapid change in the resistance of the sample to the motion of the indenter blunt. While, at higher loads the penetration depth increases and the effect of inner layers becomes more dominant, hence a slight decreasing in values of Hv as the applied load increases. Page 10 of 30
The micro hardness data can be analysed according to many approaches. According to Mayer's law [44, 47], the applied load (F) and the indentation diagonal length (d) are related by the relation:
F Ad n
(6)
where A is a constant, representing the load needed to initiate unit indentation, the exponent n is called Meyer’s index, which describes the ISE. For n < 2, we have normal ISE, while n >2 indicates the reverse ISE. The values of A and n can be estimated from ln(F) versus ln(d) plots as shown in Figure (7). The calculated values of A and n are listed in table (IV), it is found that the values of n are less than 2 which ensure the normal indentation size effect of the examined samples. Hays–Kendall approach (HK approach) Hays–Kendall (HK) approach [48] suggested that there exists a minimum load W needed to initiate the plastic deformation, below which only an elastic deformation takes place. Accordingly, The effective load becomes Feff = F – W, then
F W A1 d 2
(7)
where A1 is the a load independent coefficient. Both A1 and W can be obtained by plotting (F) against (d2) as indicated in figure (8), the extracted values are listed in table (IV). It is clear that, all of the samples give positive values of W, indicating that the applied load was sufficient to create both the elastic and the plastic deformation. On the other hand, no significant change is observed for A1. According to the HK approximation, the Hays–Kendall micro hardness HHK could be calculated by the equation (8): H HK
1854.4 ( F W ) d2
Elastic/plastic deformation model (EPD)
Page 11 of 30
(8)
Bull et al. [49] supposed that due to elastic recovery, there is an immeasurable portion 'do' of the diagonal. Consequently, the dependence of the indentation size on the applied load can be formulated as: F A2 (d d o ) 2
(9)
where A2 is a constant and d is the recorded indentation diagonal. The values of A2 and do can be estimated by plotting the variation of F1/2 versus d as in figure (9). The calculated values are reported in Table (IV). It is clear that, the value of do is positive for all the samples. This means that, for this range of applied loads, the elastic deformation is observed along with plastic deformation, and the elastic relaxation is present. Also, it is found that do decreases as x increases up to x = 0.10. Then, it increases at higher concentration of fluorine. This behaviour predicts that the samples become more elastic as x increases up to 0.1, while its elasticity dies for further increase in x. Similar behaviour was reported by previous studies on BSCCO system [9, 50]. The elastic/plastic deformation micro hardness HEPD is calculated using the equation (10):
H EPD 1854.4
F (d d o ) 2
(10)
Proportional sample resistance model (PSR) Proportional sample resistance (PSR) model which was developed by Li and Bradt [51] is successfully used for the analysis of micro hardness of materials showing the ISE behaviour. This model is given by the formula:
F d d 2
(11)
where α is the surface energy, and the change in the α value is associated with the energy dispersion of the surface cracks [52]. β is a parameter used to calculate the real micro-hardness. According to equation (11), the relation between F/d against d yields a straight line with slope corresponding to the parameter β, and the intercept represents the surface energy constant α as shown in figure (10). The calculated values of α and β are listed in table (IV). It is clear that the value of α decreases with the increasing of fluorine content up to x = Page 12 of 30
0.10, then it has a reverse trend. The enhancement can be ascribed to the dissipation of the cracks at indenter facet/specimen interface [53]. According to this model, the PSR micro hardness number could be calculated by equation (12) and the calculated values are listed in table (IV):
H PSR 1854.4
d d2 d2
(12)
In order to test the applicability of the above mentioned models for the examined samples, the micro hardness is calculated according to each model and compared with the measured HV. Figures (11 a-g) show a comparison between the experimental HV and the theoretical values calculated from the different models for (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ, with 0 ≤ x ≤ 0.3. An excellent agreement between the measured values and the calculated values that obtained from PSR model is observed in all samples. This conclusion agrees with previous studies on (Bi,Pb)-2223 [5, 54]. The suitability of PSR model for describing the behaviour of micro hardness of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ could be attributed to the reduction in the spacing between the grains (as confirmed from SEM), consequently increasing the inter-grain contact surfaces, which, in turns, arises a large surface energy resists the motion of the indenter through the sample. 4. Conclusion: (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) was successfully prepared via the solid state reaction technique. The results of XRD and SEM gave the highest volume fraction and the largest grain size at x = 0.10. The PIGE spectra has proved the partial replacement of F instead of O. The measurements of Tc, Jc and Hv showed that the sample with x = 0.10 has the optimum electrical and mechanical properties. Moreover, the analysis of Vickers micro hardness data revealed an excellent agreement with the results of PSR model. Generally, the substitution process of fluorine instead of oxygen with x = 0.10 has enhanced the structural , transport and mechanical properties of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ. Acknowledgements:
Page 13 of 30
This work was executed in the Superconductivity and Metallic Glass Lab, Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt. Special thanks to Professor R. Awad and professor M. Roumié for PIGE measurements.
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Tables: Table (I): The lattice parameters and the relative volume fractions of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. x
a (Å)
0.00
5.32
0.05
5.33
c (Å)
c/a
V(2223) %
V(2212) %
Ca2PbO4 %
37.65
7.077
87.995
11.139
0.866
37.42
7.021
90.922
9.078
0.000
Page 17 of 30
0.10
5.32
37.12
6.977
92.972
5.271
1.757
0.15
5.32
36.96
6.947
89.922
8.527
1.550
0.20
5.32
36.75
6.908
86.391
11.538
2.071
0.25
5.32
36.64
6.887
84.615
13.757
1.627
0.30
5.33
36.41
6.831
83.696
14.614
1.691
Table (II) Fluorine contents in the (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30, calculated from PIGE spectra
Sample
Fluorine (wt.%)
x = 0.00
0.006 ± 0.001
x = 0.05
0.037 ± 0.001
x = 0.10
0.042 ± 0.002
x = 0.15
0.076 ± 0.002
x = 0.20
0.219 ± 0.002
x = 0.25
0.294 ± 0.002
x = 0.30
0.332 ± 0.002
Table (III): The superconducting transition temperature (Tc), zero resistivity temperature (To), the transition width (ΔT) and the critical current density (Jc )of the (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30.
x
Tc (K)
To (K)
ΔT (K)
Jc (A/cm2)
0.00
108.5
98.5
10
573.35
Page 18 of 30
0.05
111
106
5
624.62
0.10
114
108.5
4.5
701.62
0.15
111
108.5
7.5
581.24
0.20
108.5
101
7.5
575.39
0.25
103.5
98.5
5.5
326.05
0.30
101
93.25
7.75
244.82
Table (IV) Mechanical parameters calculated according to different models of of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Mayer's parameters x n
A1 (N/μm2)
Elastic/plastic deformation model
Proportional sample resistance model
W (N)
A21/2(N1/2/μm2)
do (μm)
α (N/μm)
β (N/μm2)
0.00326 0.0003
0.4992
0.0003
45.6
0.0131
0.0002
0.05 1.5042 0.003493 0.0003
0.368
0.0003
42.3
0.0113
0.0002
0.10 1.7503 0.001507 0.0004
0.2436
0.0004
26.7
0.0071
0.0004
0.15 1.6062 0.002805 0.0004
0.2976
0.0003
40.0
0.0107
0.0003
0.20 1.6978 0.001836 0.0004
0.2905
0.0003
38.0
0.0089
0.0004
0.25 1.5248 0.003226 0.0003
0.2981
0.0003
41.3
0.0108
0.0002
0.30 1.5679 0.002616 0.0003
0.4734
0.0003
45.3
0.0128
0.0002
0.00 1.5151
A (GPa)
Hays–Kendall approach
Figure caption Fig. (1) XRD patterns of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.3. Fig. (2a-g) SEM micrographs of the fractured surface of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = (a) x= 0.00, (b) x= 0.05, (c) x= 0.10, (d) x= 0.15, (e) x= 0.20, (f) x= 0.25 and (g) x= 0.30. Fig. (3) PIGE spectra of MA-N standard of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2 O10+δ, with x= 0.1 and 0.3 using 3 MeV proton beam. Page 19 of 30
Fig. (4) Temperature dependence of the electrical resistivity of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (5) E–J curves of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (6) The dependence of Vickers microhardness "HV" on the applied load "F" of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (7) Variation of Ln (HV) with Ln (d) of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (8) Variation of the applied load "F" with d2 according to the Hays–Kendall model of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (9) Plots of F1/2 versus d according to the elastic/plastic deformation model of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. (10) Plots of F/d versus d according to the proportional sample resistance model of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30. Fig. 11a-g Variations of the measured HV and calculated HV according to different models with the applied load "F of (PbF2)x Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ samples, with a) x = 0.00, b) x= 0.05, c) x= 0.10, d) x= 0.15, e) x= 0.20, f) x= 0.25 and g) x= 0.30.
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Fig. 1
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Fig. 2
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Fig. 3
Fig. 4 Page 23 of 30
Fig. 5
Fig. 6 Page 24 of 30
Fig. 7
Fig. 8
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Fig. 9
Fig. 10 Page 26 of 30
Fig. 11 Page 27 of 30
Graphical abstract
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Highlights
Bi1.8Pb0.4Sr2Ca2.1Cu3.2O10+δ superconducting phase doped by PbF2 was prepared by single step solid state reaction technique. The replacement of F instead of O in the final compound was examined by PIGE and confirmed by XRD. An enhancement in the electrical and mechanical performance was observed as a result of PbF2 doping with small concentrations. While as, for higher concentrations the properties showed a retardation. The results indicate that it is possible to control the Oxygen content in high temperature superconductors by PbF2 doping due to partial replacement of O by F.
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Author Contribution Statement I, M. Anas, did the whole research, from developing the idea to providing materials, preparing samples, conducting measurements of electrical resistivity, current density, and Vicker's micro hardness, as well as following up on the technicians assigned to conduct X-ray diffraction and scanning electron microscopy. And ended with the analysis and discussion of the results and writing the manuscript.
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