Ga partial substitution effect in Bi-2212 superconducting matrix on crucial characteristic features

Accepted Manuscript A detailed research for determination of Bi/Ga partial substitution effect in Bi-2212 superconducting matrix on crucial characteristic features S.B. Guner, Y. Zalaoglu, T. Turgay, O. Ozyurt, A.T. Ulgen, M. Dogruer, G. Yildirim PII:

S0925-8388(18)33309-7

DOI:

10.1016/j.jallcom.2018.09.071

Reference:

JALCOM 47490

To appear in:

Journal of Alloys and Compounds

Received Date: 28 June 2018 Revised Date:

4 September 2018

Accepted Date: 8 September 2018

Please cite this article as: S.B. Guner, Y. Zalaoglu, T. Turgay, O. Ozyurt, A.T. Ulgen, M. Dogruer, G. Yildirim, A detailed research for determination of Bi/Ga partial substitution effect in Bi-2212 superconducting matrix on crucial characteristic features, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.09.071. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Line for optimum mobile hole carrier concentration

Beginning of divergence

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Un-substituted Bi/Ga-1 Bi/Ga-2 Bi/Ga-3 Bi/Ga-4 Bi/Ga-5 Bi/Ga-6

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ACCEPTED MANUSCRIPT A Detailed Research for Determination of Bi/Ga Partial Substitution Effect in Bi-2212 Superconducting Matrix on Crucial Characteristic Features

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S.B. Guner1,Y. Zalaoglu2,*, T. Turgay3, O. Ozyurt4, A.T. Ulgen5, M. Dogruer6, G. Yildirim4 1 Recep Tayyip Erdogan University, Department of Physics, Bolu–Turkey, 53100 2 Osmaniye Korkut Ata University, Department of Physics, Osmaniye–Turkey, 80000 3 Sakarya University, Department of Architecture, Sakarya–Turkey, 54187 4 Abant Izzet Baysal University, Department of Mechanical Engineering, Bolu–Turkey, 14280 5 Sirnak University, Department of Electric-Electronic Engineering, Sirnak–Turkey, 73000 6 Abant Izzet Baysal University, Department of Medical Services and Techniques, Bolu–Turkey, 14280

Abstract

This multidisciplinary study paves way to investigate the crucial of fundamental characteristic

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properties including the bulk density, electrical, superconducting, flux pinning mechanism, crystal structure quality and strength quality of interaction between the superconducting

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grains in the Bi2.1Sr2.0Ca1.1Cu2.0Oy (Bi-2212) superconducting materials with the partial replacement of gallium foreign impurity by bismuth nanoparticles in the crystal structure. Characterizations of polycrystalline ceramic materials prepared by standard ceramic route in the atmospheric air are performed by means of conventional experimental measurement methods such as powder X-ray diffraction, Archimedes water displacement, dc electrical resistivity versus temperature and critical current density examinations. All the bulk Bi-site

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Ga partial replaced materials exhibit the Bi-2212 superconducting phase within the different fraction levels (%73.1-94.8), moderate self-field critical current densities 54-96 A/cm2 and wide-ranging offset and onset critical transition temperature range of 45.65 K-84.52 K and 70.06 K-85.00 K. As for the experimental findings of bulk density and related degrees of

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granularity (porosity) parameters, the bulk density parameter is found to be between 5.76 g/cm3 and 6.12 g/cm3 when the corresponding residual porosity value is also obtained to be in

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a range of 8.57 %-2.86 %. Moreover, the mobile hole carrier concentrations in the shortrange-ordered antiferromagnetic Cu-O2 layers are found to be in the range from 0.085 until 0.152. Additionally, the role of Ga/Bi partial substitution in the crystal lattice on the normal state resistivity, residual resistivity, residual resistivity ratio, vibrational mode intensities, texturing, superconducting volume fractions, mobile hole carrier concentrations, average crystallite sizes, Lotgering indices and cell parameters are discussed in details. All the experimental results and theoretical approaches show that the characteristic properties tend to improve regularly with the increment in the Ga foreign impurity level until x=0.05 due to the

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Corresponding author Phone: +90 328 825 18 18, Fax: +90 328 825 00 97 e-mail address: [email protected] (Y. Zalaoglu)

ACCEPTED MANUSCRIPT increment in the crystal structure quality and interaction between the superconducting grains. After the critical Ga/Bi substitution level of x=0.05, every feature degrades considerably. Keywords: Bi-2212 superconducting phase; Ga/Bi partial replacement; Crystal structure quality; Texturing; Porosity. 1. Introduction

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Discovery of superconductivity on the heavy metal element of mercury below such a low temperature of 4.2 K by Heike Kamerlingh Onnes in 1911 at Leiden University [1] is one of the important milestones in the history of application-oriented material science, technology and industrial application fields. After that time, the phenomenon pays much attention of the

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researchers for the energy production, protection and consumption contents in the energy management. However, the development biography of superconductivity encounters the

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serious problems in the potential applications due to lower engineering current and magnetic field carrying abilities. Regardless, the scientists have endeavored to improve the carrying capacities worldwide by discovering new superconducting parents. When the date shows 1986 [2], the high-temperature superconductors (with 35 K critical temperature) founded on the copper oxide (cuprate) consecutively stacked layers are discovered for the first time. In the year of 1987, the yttrium element instead of lanthanum is embedded in the distorted

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and oxygen deficient multi-layered perovskite structure with the Cu-O2 consecutively stacked layers in the crystal structure of cuprates, and the critical temperature is obtained to be higher than the liquid nitrogen temperature of 77 K [3, 4]. Besides, the other important causes of discovery are the low-cost, non-toxic and accessible of nitrogen. At the end of year, the other

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cuprate superconducting materials including the Ga-, Tl-, Bi- and Hg-containing highTc compounds are discovered in 1987. It is noteworthy that the discovery of new parents

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points out the qualitative jump in the development and usages in the novel and feasible application fields. However, the scientists have, of course, met some difficulties in the potential application areas of cuprate superconducting parents due to their inherit main problems as regards the layered anisotropic structure, brittleness behavior, mechanical stabilization, intergranular boundary, randomly oriented microcrystals, structural problems, phase composition, spatial characteristic of superconducting electron, super-short coherence length, extreme-large penetration depth and low charge carrier densities [5, 6]. Likewise, the weak interaction between the grains, lower operating temperature ranges and much more sensitivity to the applied current and external magnetic fields are other main features for the description of cuprate materials. The different techniques are required to overcome the main

ACCEPTED MANUSCRIPT problems given above. Among the methods, the partial substitution of elements into the crystal structure is one of the most common preferred techniques [7–10]. In the present work, we try to improve the peculiarities of the Bi-2212 superconducting materials (from the cuprate parents) with the homovalent Ga/Bi partial substitution within the different concentration levels (0.00 ≤x≤ 0.30) in the superconducting crystal structure so that

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the materials find much more application areas in the application-oriented material science, network, engineering, energy sectors, commercial, metallurgical, electro-optic, medicine, sensitive process control, power transmission, future hydrogen society, refrigeration, spintronics, innovative energy infrastructure, and especially heavy-industrial technology

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(levitated trains, motors, generators, magnetic separation, magnetic energy storage, transformers, particle accelerators, nuclear fusion, nuclear magnetic resonance and magnetic resonance imaging technologies) applications for the novel and feasible market areas for the

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universe economy [11–18]. The influences of partial substitution of Ga3+ impurities by Bi3+ ions on the fundamental characteristic features as regards the bulk density, electrical, superconducting, flux pinning mechanism, crystal structure quality and strength quality of interaction between the superconducting grains in the Bi-2212 superconducting materials are surveyed by the powder X-ray diffraction, Archimedes water displacement, dc electrical

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resistivity versus temperature and critical current density examinations. All the experimental measurement findings show that the characteristic properties can be developed by the optimum Ga/Bi substitution.

2. Experimental Procedures for Preparation of Bi-2212 Ceramic Compounds

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A series of Ga-containing materials with the nominal composition formula of Bi2.1xGaxSr2.0Ca1.1Cu2.0Oy

(0.00 ≤x≤ 0.30) are produced by the conventional solid-state reaction

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technique in the air atmosphere at the room temperature. The high purity (%99.9) powders including the Bi2O3, SrCO3, CaCO3, CuO and Ga2O3 are purchased from an exclusive distributor of Alfa Aesar products. The particle size of Ga2O3 chemical powder is about 44 µm. The chemicals are exactly weighed in the grinding stoichiometric proportion at the normal atmospheric pressure by an electronic balance. The stoichiometric mixture of powders is milled in a grinding machine for 9 hours to minimize the particle sizes and improve the homogeneity of chemicals. The final form of starting materials is straight after grounded manually in an agate mortar with the assistant of a pestle for half an hour without any solvent or solution to obtain the desired particle sizes of chemicals mixture. The resultant mixture is exposed to the preheating process at the constant temperature value of 800 °C in a Protherm

ACCEPTED MANUSCRIPT Model programmable furnace for 36 hours under the air medium condition in the porcelain crucibles (with the inherit non-reactive characteristic feature) so that the nominal compositions of materials become the Bi2.1-xGaxSr2.0Ca1.1Cu2.0Oy as a result of the release of carbon and carbon-containing residuals. Heating and cooling rates are adjusted to be 5 °C/min during the preheating process. Homogenous powder of chemicals is manually pestled in the

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agate mortar for about half an hour under the atmospheric pressure to improve the homogeneity of resulting powder. Final form of powder is pelletized into a rectangular bar with the sizes of 1.5 cm x0.5 cm x0.2 cm by 300 MPa load at the room temperature in the atmospheric air. The solid bars are subjected to the main heating process at the sintering temperature of 850 °C for

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24 h to provide the superconducting phase. The heating and cooling rates are adjusted as the same values. The bulk Bi-site Ga substituted ceramics with the different Ga concentration level of x= 0.00, 0.01, 0.03, 0.05, 0.07, 0.10 and 0.30 will afterward be displayed as un-

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substituted, Ga/Bi-1, Ga/Bi-2, Ga/Bi-3, Ga/Bi-4, Ga/Bi-5, and Ga/Bi-6, respectively. 3. Experimental Characterization of Every Material Produced in This Work In the current work, we identify the fundamental characteristic properties of pure and Bi-site Ga substituted polycrystalline materials by the bulk density, powder X-ray diffraction, dc electrical resistivity versus temperature and critical current density methods. The bulk density

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measurements are experimentally exerted by Archimedes water displacement technique and the relative degrees of granularity (known as the residual porosities) are deduced from the experimental results recorded for the pristine and Ga/Bi substituted Bi-2212 ceramic compounds so that we discuss the differentiation in the strengths of ability and quality of

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crystal structure and interaction between the superconducting grains with the homovalent Ga/Bi substitution. The bulk density findings also enable us to describe the effects of the

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partial replacement of Ga3+ impurity by Bi3+ inclusions on the structural problems in the crystal structure, and the approach (on) or divergence (from) the optimum mobile hole carrier concentration in the short-range-ordered antiferromagnetic active Cu-O2 layers. Furthermore, the variations of dc electrical resistivity versus temperature curves for all the materials are determined by the conventional four-probe method from 105 K down to 35 K with the aid of a closed-cycle cryogenic refrigeration system based on the helium gas as the working medium. The resistivity curves are collected with the Keithley 2700 nano-voltmeter and Keithley 220 programmable current source by employing the dc current of 5 mA. The curves pave way to the determination of room temperature resistivities, residual resistivities, residual resistivity ratios, ρ90K, degree of broadening, offset and onset critical transition temperature parameters

ACCEPTED MANUSCRIPT within the temperature accuracy of ±0.01 K. At the same time, the self-field transport critical current density values are obtained from the I-V measurements performed by the four-probe contact method in the home-made system at the temperature of 77 K without any magnetic field. In the experiments, the value of 1 µV/cm is used as standard criterion. Additionally, the crystal structures of materials prepared in the current work are analyzed with a Rigaku

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Multiflex+XRD 2kW diffractometer that emits X-Rays coming to the Kα line of Cu (1.54 Å wavelength) with a power of 36 kV and 26 mA in the air atmosphere at the room temperature. Diffraction peaks in the XRD diffractograms are recorded in the 2θ angles between 3° and 60° at a scan speed of 2° min-1 with 0.02° step increment. Indexation is conducted to JCPDS index

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cards with the JCPD-ICDD (international center for diffraction data) numbers of 41-0317 and 41-0374. Besides, the diffraction peaks establish the novel links about the determination of fundamental parameters such as the superconducting volume fractions, average crystallite

4. Results and discussion 4.1. Powder XRD experimental results

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sizes, mobile hole carrier concentrations, Lotgering indices and lattice cell parameters.

X-ray diffractograms of un-substituted and Bi-site Ga substituted Bi-2212 superconducting ceramic compounds are graphically recorded in air medium at constant room temperature of

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24 °C as given in Fig. 1. Prior to serious discussions on the diffraction patterns, it is to be emphasized that new appearances and shifts in the graphics confirm the introduction of Ga foreign impurities into the bulk Bi-2212 crystal matrix. All the characteristic diffraction peaks stemming from the diffracted beams indexed as Miller indices of H (hkl), L (hkl) and VL (hkl)

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where the abbreviations of H, L and VL stand for the high-Tc Bi-2223 phase (high phase for Bi-based superconducting parents in the literature), low-Tc Bi-2212 phase (low phase) and

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very low-Tc Bi-2201 phase (very low phase). The diffraction peaks generally assigned to the tetragonal unit cell (negligible orthorhombic distortion) with the space group P4/mmm are indexed with regards to JCPDS index cards with the JCPD-ICDD numbers of 41-0317 and 41-0374. Furthermore, a few peaks for the other phase of Ga2O3 are observed for only the polycrystalline Ga/Bi-6 ceramic material and provided by the notation of “*” in the X-ray diffraction graphic. According to the diffraction peaks in the XRD diffractograms, the existence of only high-Tc Bi-2223 and low-Tc Bi-2212 phases is evident up to the Ga foreign impurity concentration level up to x=0.05. In more details, the high-Tc Bi-2223 phase content tends to enhance mildly for the cationic replacement of homovalent Ga/Bi in the active CuO2 consecutively stacked layers up to the optimum content value of x=0.05, where, in fact the

ACCEPTED MANUSCRIPT high- Tc Bi-2223 superconducting phase seems to more and more stabilize in the crystal system. However, after the certain value of x=0.05 both the superconducting phases (especially high-Tc Bi-2223 phase) tend to degrade monotonously with the increment in the gallium concentration level. Conversely, the other phase (abbreviated as Bi-2201) from the Bi-containing type II superconducting parents appears immediately once from the gallium

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content level of x=0.05 onwards. In case of the highest concentration level of x=0.30, the Bi2201 phase becomes predominantly as compared to the Bi-2223 phase throughout the crystal matrix. Similarly, a new phase of Ga2O3 also appears immediately at the highest concentration level. Namely, in case of the highest Ga impurity content in the Bi-2212 crystal system, the

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material (or crystallinity) quality more and more worsens as a consequence of increment in the structural problems as regards the grain misorientations, lattice distortions/strains, defects (point, line, planar or bulk), dislocations, porosity, cracks, voids and omnipresent flaws (stress

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raisers and crack initiation sites) in the Bi-2212 crystal structure. This can be probable result deduced from the present work that the rapid decrement in the offset critical transition temperature value for the Ga/Bi-6 material (on the experimental electrical resistivitytemperature curves) may verify the Bi-2223 phase substitution by the Bi-2201 phase in the hole-induced Bi-2212 crystal structure. The long and short of it is that the XRD diffraction

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patterns for every material mainly show the Bi-2212 superconducting phase. In other words, the low-Tc Bi-2212 phase is evident for all the materials studied in the present work, and no phases (Bi-2201 and Ga2O3) preclude the formation of effective and strong electron-phonon couplings to drive the superconductivity in the active Cu-O2 plane of adjacent

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superconductive layers. All in all, the existence of optimum gallium nanoparticles enhances the formation of Bi-2212 superconducting phase whereas the excess doping damages totally

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the fundamental characteristics on Bi-2212 superconducting matrix.

Further, it is to be mentioned here that the Bi-2223 and Bi-2201 phases as well as the Ga2O3 phase exhibit generally low-intensity characteristic peaks while the strong reflection intensities of diffraction peaks can be preferred for the Bi-2212 phase in especially (00l) crystallographic diffraction planes in the powders [19]. Accordingly the strong anisotropy within the lamellar structure takes place in the Bi-2212 compound, and the alignments of crystal planes arrange throughout the c-axis orientation. It is obvious from the figure that the diffraction peak intensities belonging to low Tc Bi-2212 and especially high-Tc Bi-2223 phase tend to become slightly stronger with the enhancement of the Ga impurity concentration until the level of x=0.05 due to the enhancement in the quality of crystallinity. After the critical

ACCEPTED MANUSCRIPT dopant level, the peak intensities of Bi-2223 phase tend to decrease monotonously and reach to global minimum values. On the other hand, the diffraction peak intensities of very low-Tc Bi-2201 phase appear at the concentration level of x=0.10 and increase constantly with the dopant content. In fact, at the largest dopant level of x=0.30, the peak intensities belonging to very low-Tc Bi-2201 dwell in the global maximum values. The relative phase percentages of

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every phase including Bi-2223, Bi-2212, Bi-2201 and impurity (Ga2O3) are determined by the

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equations given below.

(1) (2)

(3) (4)

where I presents the peak intensity of related phase. All the calculations are numerically tabulated in Table 1. It is apparent from the table that the Ga/Bi-0, Ga/Bi-1, Ga/Bi-2, Ga/Bi-3

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and Ga/Bi-4 ceramic compounds presents only the low-Tc Bi-2212 and high-Tc Bi-2223 superconducting phases, the Ga/Bi-5 and Ga/Bi-6 materials exhibit the very low-Tc Bi-2201 and impurity phases. In numerical findings, the high-Tc Bi-2223 phase increases from the volume fraction value of 17.5% in an un-substituted compound to 29.8% in the material

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prepared by the gallium concentration level of x=0.05. However, the volume fraction values in Table 1 show that the excess Ga inclusions (higher than x=0.05) not only damage seriously

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the Bi-2223 phase but also enable the Bi-2201 phase to predominate. Moreover, the unsubstituted ceramic compound exhibits the relative phase percentages of 82.5 % for the Bi2212 and 17.5% for the Bi-2223 phase. The volume fraction of Bi-2212 degrades regularly from 82.5 % until 70.2 % with the enhancement in the Ga foreign impurities up to x=0.05. Namely, since the gallium dopant level of x=0.05 in the Bi-2212 superconducting crystal system enables the formation of high-Tc Bi-2223 phase, the foreign impurity provides a favorable medium to convert the low-Tc Bi-2212 phase to the high one. At the same time, as the level surpasses 0.05 the Bi-2212 and Bi-2223 phases diminish significantly towards to 79.4 % and 9.0 %, respectively for the Ga/Bi-5 material due to the beginning of new phase Bi-2201. In this respect, the crystal structure of solid Ga/Bi-5 sample has very low-Tc Bi-2201 with the phase volume fraction of 11.6 %. As for the maximum dopant case, the

ACCEPTED MANUSCRIPT polycrystalline Ga/Bi-6 material presents the mix phases: 3.8 % Bi-2223, 77.2 % Bi-2212, 15.5 % Bi-2201 phase and 3.5 % Ga2O3. Once the gallium particle exceeds the optimum level such a value of x=0.05, the damage influence is observed on the formation of high-Tc Bi-2223 phase [20, 21]. All in all, amount of Ga2O3 addition level affects seriously on the reaction and formation of superconducting phases. Besides, it is to be mentioned here that although the Bi-

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2212 phase reaches to the global maximum value of 94.8 % for the bulk Ga/Bi-4 material, the material obtains the increased structural problems in the crystal lattice, being favored by the lattice cell parameters.

As for the occurrence and shift of diffraction peaks in the XRD patterns, the Ga content

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causes the appearance of new peak H (002) at 2θ ~4.50°-4.60° in the Bi-2212 crystal structure; in fact the intensity of peak tends to increases with the dopant level and resides in the highest position for the Ga substitution level of x=0.05 where another new peak ascribed

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as H (2020) appears immediately at the 2θ angle of 48.76°. On the other hand, the peak begins to be suppressed significantly with the enhancement in the Ga impurity level until x=0.10, and in case of the highest substitution level the intensity of H (002) disappears immediately. Likewise, the peak identified as H (0012) disappears for the excess Ga content level of x=0.30. At the same time, the peak abbreviated as H (020) begins to appear at 2θ~32.54° at

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the Ga concentration level of x=0.03 and remains (of course with the shift of angle value and change of intensity) for the Ga content level of x=0.05 (2θ~32.58°). Unfortunately, the peak completely vanishes for the other bulk samples. As to the Ga impurity dopant level of x=0.10 and x=0.30, a new characteristic impurity

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phase assigned as VL (006) with moderate intensity emerges immediately at 2θ angles of 21.89° and 21.87°, respectively. Similarly, at the same Ga concentration levels the peak assigned as VL (0012) appears instantaneously at 2θ~43.57° and 2θ~43.49° and is invisible

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for the XRD patterns of other materials prepared in this work. Besides, a new characteristic diffraction peak of (401) is observed at approximately 2θ=30.48° and assigned for the impurity phase of Ga2O3 compound. The appearance of new peaks identified as very low phase and Ga2O3 phase is a clear clue for rapid decrement in the qualities of crystallinity and superconductivity of Bi-site Ga substituted Bi-2212 materials. Further, the existence of (401) peak in the XRD diffraction patterns confirms that the solubility limit level is about x=0.30 for the gallium particles in the Bi-2212 superconducting crystal system [22, 23]. In other words, in case of excess Ga impurity concentration, the Ga inclusions begin to predominantly aggregate over the grain boundaries in the crystal system instead of penetration into the superconducting grains. This

ACCEPTED MANUSCRIPT may be attributed to the beginning of fundamental structural problems in the crystal lattice. To sum up, all the experimental findings show that the Ga foreign impurity level of x=0.05 is noted to be the breaking point of high and low phase whereas the solubility limit value is determined to be about x=0.30. The shifts of many diffraction peaks with the Ga concentration level enable us to calculate

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lattice cell parameters. The positions of 2θ peaks such as L (002), L (008), L (0010), L (0012), H (0010) and H (0012) shift to the small-angle values with the enhancement of Ga foreign impurity concentration up to the level of x=0.05 beyond which the peaks tend to shift towards the large-angle values as a consequence of increase/decrease in the c-axis length of Bi-2212

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superconducting system. Conversely, the characteristic peak of H (020) presents completely the opposite behavior as compared to the shifts of those. In this regard, the a and b lattice cell constant parameters also exhibit the opposite relation to the c lattice cell parameter. Regarding

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the numerical values of parameters, since all the polycrystalline Bi-2212 cuprates prepared crystallize in the tetragonal structure with space group P4/mmm, the a and c-axis lengths of pure and Bi-site Ga substituted Bi-2212 ceramic materials are determined for the tetragonal unit cell structure using the relation of

with the aid of lattice spacing (d)

values and diffraction (hkl) planes. Every calculations are numerically provided in Table 1. It

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is visible from the table that the a lattice cell constant parameter is computed to be about 5.39 Å when the c-axis length is found to be about 30.48 Å for the un-substituted sample. Until the critical dopant level of x=0.05, the a cell parameter tends to regularly shrink from about 5.39 Å to 5.31 Å; on the other hand the c lattice cell parameter enlarges constantly and falls in the

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global maximum point of 32.62 Å. After the certain dopant level, the scenario indicates exactly opposite direction. Namely, the former lattice parameter begins to expand rapidly with

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increasing the dopant content level and reaches to the highest value of 5.44 Å for the Ga/Bi-6 material whereas the latter one contracts considerably and locates to the smallest value of 30.06 Å. Based on the experimental findings obtained, the superconductivity improves considerably due to the significant increment in the formation of electron-phonon coupling probability and mobile hole carrier concentrations in the antiferromagnetic Cu-O2 in-plane layers. In more details, the contraction in the a lattice cell parameter stems from the degradation of electrons into anti-bonding orbital, basal plane sizes and effective Cu valences [24–26] and hence the interlayer spaces of Cu–Ca–Cu and Ca–Sr sites shrink significantly. Furthermore, the enlargement in the c lattice cell parameter is attributed to the charge neutrality mechanism known as the balance of oxygen level (valency) in the oxygen deficient

ACCEPTED MANUSCRIPT Bi-O double layers. Thus, the active layers in the crystal lattice expand noticeably the interlayer spaces of Sr–Bi, Bi–Bi and Sr–Bi–Bi–Sr sites and the c-axis length enlarges automatically [27]. Different electron configuration of outer shells for the Bi and Ga atoms is another probable result based on the fact that why the c-axis length enlarges in case of optimum dopant. Namely, the configuration of outer atomic electrons for the former atom is

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associated with 6s26p3 whereas 4s24p1 ascribes the configuration of outer electrons for the latter atom. In the hole-induced Bi-2212 crystal structure once any atom encounter the valencies (oxygen atoms) of Bi–O double layers, the atom struggles to integrate with the surrounding oxygen atoms immediately. Under the light of the fundamental information, the Ga+3 ions tend to more easily integrate with the oxygen atoms in the Bi–O double layers by

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forming the bond of p-p as compared to the integration of Bi+3 ions [28]. In other words, the integration of Ga+3 ions with the surrounding oxygen atoms in the oxygen deficient Bi-O

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double layers spends much less energy for the bond formation. Accordingly, the presence of optimum dopant content makes the c-axis phonons much easier form the strong and effective electron-phonon coupling probabilities in the system. Nevertheless, the excess Ga foreign impurity may lead to require the extra energy for the bond formation and coupling formations of cooper-pair probabilities. This is phenomenologically in correspondence to the degradation

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in the low-Tc Bi-2212 phase.

The diffraction peaks in the XRD diffractograms enable the researchers to calculate the average crystallite sizes with the assistant of Scherrer–Warren approach based on the full width at half maximum (FWHM) for the peaks [29]. That is, B cosθ B

(5)

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t = 0.941λ

where t displays the crystal thickness and λ gives the wavelength of incident X-rays when ƟB

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is the Bragg angle and B demonstrates the full width at half maximum of Bragg peak. Every computation is numerically listed in Table 1. It is apparent from the table that there is a systematic enhancement in the average crystallite size parameter. Namely, the pure sample is composed of the average grains of 59.3 nm. The largest value is found to be about 75.4 nm for the Ga/Bi-3 material while the smallest value of 50.9 nm is attributed to the Ga/Bi-6 material. This gives a pictorial representation on the fact that the differentiation of crystallite sizes with the Ga nanoparticle concentration level promotes wholly the other experimental and theoretical findings. In more details,

ACCEPTED MANUSCRIPT Moreover, we try to determine the change (related to the variation of hole trap energy) of mobile hole carrier concentrations (Phole) per Cu ions with the gallium concentration level with the assistant of following equation [30]:  T offset   P = 0.16 − 1 − c max  / 82.6  Tc  

1/ 2

(6)

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in the relation, Tcmax is abbreviated as a value of 85 K for the Bi-2212 superconducting phase when the Tcoffset values are deduced form Table 1. All the calculations are numerically listed in Table 1. It is obvious from the table that the mobile hole carrier concentrations tend to increase parabolically with ascending the Ga/Bi substitution level up to x=0.05 where the

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parameter gathers the highest value of about 0.152. After the critical dopant value of x=0.05, the parameter degrades parabolically towards to the deepest value of about 0.085 (for the

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Ga/Bi-6 material). Moreover, the mobile hole carrier concentration totally diverges from the optimum value (one can observe the divergence of arrow from the optimum levels on the ρ-T curves) and is damaged dramatically (Fig. 2). For the divergence the beginning level of Ga/Bi substitution in the Bi-2212 crystal structure is x=0.01 (See arrow in Fig. 3). Lotgering index (F) constitutes the last part of the powder XRD experimental measurements results. The parameter is useful to quantify the variations in the degrees of c-

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axis orientation in the un-substituted and Bi-site Ga substituted polycrystalline Bi-2212 cuprates [31, 32]. In the present work, the Lotgering indices calculated for all the materials are embedded in Table 1 in detail. It is obvious from the table that the Lotgering indices exhibit the similar trend with average crystallite sizes (initially increase with the Ga dopant up

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to x=0.05, then decrease). In this respect, the index value is calculated to be about 0.404 for the pure sample. The Ga/Bi-3 ceramic material has the maximum F value of 0.532 as against

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the minimum value of 0.376 for the bulk Ga/Bi-6 compound. The practical consequence is based on the fact that the optimum Ga dopant improves dramatically the texturing of superconducting grain couplings while the excess dopant concentration considerably damages the texturing and degrees of c-axis orientations in the crystal matrix. 4.2. Bulk Density Quantities and Relative Residual Porosity for Poly-crystallized Pure and Bi-site Ga substituted Bi2.1-xGaxSr2.0Ca1.1Cu2.0Oy Superconducting Materials Throughout the history, the researchers have endeavored to examine the bulk density parameter of a material to decide whether the material can be used in potential industrial and technology application fields or not. Namely, the definition of bulk density parameter

ACCEPTED MANUSCRIPT becomes a leader to understand the quality of crystallinity, structural, mechanical, surface morphology and basic electrical quantities as well as the ability (related to the strength quality of interaction between the superconducting grains) and strength (associated with the vortex motions) of flux pinning centers [33]. It is well known that the bulk density is directly related to the degree of granularity or residual porosity and grain connectivity [34]. Archimedes

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water displacement method is one of the most ideal (submethod) measurement systems for the determination of bulk density and corresponding residual porosity parameter. In our research for this paper, the influence of partial replacement of Ga inclusions at the Bi-site in the crystal system on the bulk density and degree of granularity is thoroughly studied by the Archimedes

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method. All the experimental findings are numerically given in Table 2. According to the table, the variations of experimental bulk density results stem from the entering of Ga foreign impurities into the hole-induced Bi-2212 crystal system.

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Moreover, it is visible from the table that the bulk density parameter tends to increase monotonously as the homovalent Ga partial replacement at the Bi-sites enhances up to the level of x=0.05 beyond which the parameter begins to decrease regularly. In this respect, the bulk density of un-substituted superconducting compound is found to be about 5.98 g/cm3. The maximum value of 6.12 g/cm3 is attributed to the bulk Ga/Bi-3 sample (the densest)

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whereas the smallest value of 5.75 g/cm3 is observed for the Ga/Bi-6 compound (the most porous). This is coincided with the fact that the optimum Ga impurity concentration strengthens the ability and quality of crystal structure and especially connection between the superconducting grains. We also define the differentiation in the relative degrees of

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granularity parameters in terms of Ga concentration levels by considering as the density of 6.30 g/cm3 for the pure Bi-based superconducting system [35] with the aid of formulas given in Ref. [36, 37], and the calculations each are illustrated in Table 2 clearly. It is obvious from

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the table that unlike bulk density parameters, the relative degrees of granularity parameters (abbreviated as the residual porosities) initially decrease with the dopant level until x=0.05 where the porosity resides in the smallest value of 2.86 %. However, the parameter begins to augment rapidly with the enhancement in the Ga concentration level inserted in the host Bi2212 crystal lattice. In case of the maximum Ga dopant level the residual porosity goes up the top point such a value of 12.86 %. This may be in attribution to the fact that the excess Ga concentration increases the structural problems (discussed in more details in the other parts).

ACCEPTED MANUSCRIPT 4.3. Role of Ga content on Ability and Strength of Flux Pinning Centers The material exhibits highly the resistance to the external magnetic fields for the usages of small and large scale application fields. However, some mechanisms such as the crystallinity and structural problems (misorientations, lattice distortions/strains, defects, dislocations, porosity, cracks, voids and omnipresent flaws) in the crystal system make the resistant to the

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applied fields much more decrease. In the current study, the un-substituted and Bi-site Ga substituted Bi-2212 superconducting ceramic compounds are composed of distorted, oxygen deficient multi-layered perovskite structure with the Cu-O2 consecutively stacked layers. Namely, the materials with their inherit anisotropic nature (different crystalline directions of

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grains) present the weak performances on the ability and strength of vortex lattice period, elasticity and flux pinning centers due to their intrinsic characteristic features. Hence, in the presence of external magnetic fields the thermal fluxon vortices in two-dimension pancake

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vortices begin to move thermally to jump to the near neighboring states with rather higher energy barriers (pinning barriers) due to the cooper pair-breaking mechanism [38, 39], and the fluxons seem to be the discrete pancakelike nature known as the decoupling of adjacent multilayers. Accordingly, the electrical energy converts more easily into the heat energy due to the beginning of ohmic resistance and energy dissipation [40]. In this part of paper, we try

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to improve the flux pinning ability and strength of coupled vortices for the Bi-2212 superconducting materials with the partial substitution of Ga impurity by Bi particles in the poly-crystallized Bi2.1-xGaxSr2.0Ca1.1Cu2.0Oy ceramic materials 0.00 ≤x≤ 0.30). All the measurement results are collected by the transport critical current density experiments

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conducted in self-field at 77 K without any external magnetic field. It is to be mentioned here that we can, of course, perform the magnetic critical current density measurements to determine the Ga impurity effect on the flux pinning mechanism in the host Bi-2212 crystal

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matrix [41]. However, the former experimental measurement system allows the researchers to record the real density value founded on the crystallinity quality or permanent structural problems along with the entire specimen [42, 43]. In this regard, the transport experimental measurements are turn out to provide much more reliable critical current densities at the external magnetic field applications as compared to the magnetic experimental measurements [44]. One can observe every self-field transport critical current density (Jc) value in Table 1. It is apparent from the table, the Jc values are found to strongly depend on the Ga/Bi substitution level in the Bi-2212 crystal lattice. As the Ga impurity concentration enhances until the critical value of x=0.05, the Jc values increase regularly from about 66 A/cm2 (for the pure bulk Bi-2212 compound) to 96 A/cm2 (for the Ga/Bi-3 sample) together with the Ga content.

ACCEPTED MANUSCRIPT On the other hand, beyond the critic dopant level the Jc parameter decreases considerably towards to about 54 A/cm2 (for the Ga/Bi-6 material). It is obvious from the variation of Jc parameters with the Ga concentration level that the optimum Ga inclusions inserted in the host Bi-2212 superconducting crystal structure act as the artificial and effective flux pinning centers to delay or decrease the thermal fluxon motions of 2D pancake vortices [45–47].

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Based on the numerical values, it is obvious that the Ga/Bi-3 sample with the largest Jc value exhibits the highest resistance to the current and external magnetic fields due to the presence of extreme effective nucleation pinning sites in the crystal structure [48].

Moreover, one can expect that the enhancement in the crystallinity and formation of low-

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Tc Bi-2212 superconducting phase may cause the considerable improvement in the artificial flux pinning centers, the self-field transport critical current density as it does. Conversely, the excess Ga impurity level leads to enhance the recoupling linelike/discrete pancakelike nature

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or decoupling of adjacent layers in the Bi-2212 crystal matrix. Hence, the Ga/Bi-6 material shows the worst flux pinning ability, vortex lattice period and grain boundary couplings. It is to be stressed here that the other parts guarantee that from the Ga impurity concentration level of x=0.30 onwards the fundamental characteristics such as the bulk density, electrical, superconducting, crystal structure quality and strength quality of connection between the

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grains in the Bi-2212 superconducting materials as well as the flux pinning mechanism get worse and worse as a consequence of the aggregation of excess gallium impurity over the grain boundaries. That the Ga3+ ions clearly substitute for the Bi3+ inclusions in the crystal structure is another crucial result inferred from the experimental evidences.

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Furthermore, the rapid degradation of flux pinning ability may stem from the augmented permanent and artificial structural problems in the crystal structure. In other words, the microstructural problems induced by the excess Ga concentration damage directly the flux

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pinning ability and strength of coupled vortices. On the other hand, the optimum Ga/Bi substitution leading to the enhancement in the formation of Bi-2223 volume fraction and crystallinity promotes the formation of effective nucleation pinning sites in the Bi-2212 crystal texture.

4.4. Dc electrical findings for pure and Bi-site Ga particles substituted superconducting polycrystalline compounds Variation of dc electrical resistivity behaviors of the pure and Bi-site Ga particles substituted superconducting polycrystalline compounds with the Ga impurity content level in the crystal structure. The experimental findings are recorded in the temperatures intervals 55 K-101 K

ACCEPTED MANUSCRIPT and all the data are graphically depicted in Fig. 3. We deduce the crucial electrical and superconducting properties such as room temperature resistivity (ρ300K), residual resistivities (ρres), residual resistivity ratios (RRR), ρ90K, offset ( T coffset ) and onset ( T conset ) critical transition temperatures and degree of broadening ( ∆ T c = T conset - T coffset ) parameters form the experimental curves. According to the experimental measurement results of dc electrical

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resistivity–temperature curves given in the figure, the characteristic features are found to be influenced considerably by the amount of dopant in the crystal structure. In other words, the homovalent Ga/Bi substitution is performed successfully. Prior to the serious discussions on the characteristic features, it is to be mentioned here that all the materials prepared in the

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present work display the truly-metallic transition nature (described by the liquid model) due to the logarithmic distribution of active and dynamic electronic state densities at the vicinity of

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Fermi energy level [49, 50]. At a temperature value higher than the beginning of transition temperature ( T conset ), the metallic character or positively linear temperature-dependent of resistivity tends to improve with the increment in the gallium concentration level up to the value of x=0.05. Hence much more effective and strong electron-phonon coupling probabilities form in the homogeneous regions and superconducting clusters to stabilize the electrical and superconducting properties. Similarly, the improvement in the metallic

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characteristic behavior is directly related to the refined metallic connection between the intergrain regions, mobile hole carrier concentrations and structural problems in the active Cu-O2 consecutively stacked layers [51]. However, after the certain Ga ingredient level of x=0.05 the mechanism begins to reverse directly due to new induced permanent structural

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problems and metallic interaction between the intergrain regions [52–54]. As for the evaluation parameters defined above, the electrical resistivity parameters at the

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room temperature (ρ300K) are observed to decrease systematically from about 8.42 mΩcm to 4.89 mΩcm as the Ga substitution level at the Bi-sites in the crystal matrix increases until the value of x=0.05. Nevertheless, when the substitution level surpasses the critical value, the room temperature parameter tends to ascend constantly and reaches to the global maximum value of 13.97 mΩcm in case of the excess Ga impurity dopant circumstance (the Ga/Bi-6 sample). It is to be emphasized here that like to the discussions on truly-metallic transition nature, the existence of optimum Ga dopant in the host Bi-2212 crystal core leads to develops the fundamental properties as regards the formation of effective and strong electron-phonon coupling probabilities, stabilization of superconducting clusters, mobile hole carrier concentrations and refinement in the structural problems in the crystal lattices [55].

ACCEPTED MANUSCRIPT Conversely, as the Ga content inserted in the crystal system exceeds the optimum dopant level, the fundamental properties suppress rapidly. Regarding the other electrical findings (the ρres, ρ90K and RRR values), the first parameter can be determined form Matthiessen’s rule [56] where the specimen resistivity at any temperature is composed of two main parts: (I) residual resistivity and (II) temperature-

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dependent resistivity. The former feature is directly in accordance to the temperatureindependent resistivity and is extracted from the resistivity–temperature curves by extrapolating the values. In other words, the ρres parameter is related to the crystal structure quality and metallic links between the intergrain regions [57]. In the present study, the ρres

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values are initially found to decrease dramatically from 3.78 mΩcm to 0.71 mΩcm with increasing the Ga concentration level up to the critical value of x=0.05 beyond which the

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parameter begins to expand hastily. In this regard, the ρres value reaches until the top value of 9.29 mΩcm for the bulk Ga/Bi-6 material (Table 2). Based on the experimental findings, it is fair to conclude that the presence of optimum Ga concentration level improves considerably the fundamental features given above. Besides, we discuss the second parameter of ρ90K that is known to point out the impurity scattering and lattice strain along with the distorted and oxygen deficient multi-layered perovskite structures in the Bi-2212 superconducting crystal

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system. According to the experimental measurement results gathered in Table 2, the ρ90K value resents the similar trend (initially enhance with the Ga dopant level up to x=0.05, then degrade) with the ρ300K and ρres parameters. Accordingly, the solid Ga/Bi-3 superconducting

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material (best sample) shows the value of 1.70 mΩcm whereas the worst compound (Ga/Bi-6 material) presents the maximum value of 10.69 mΩcm as against 5.71 mΩcm for the pure sample.

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The RRR parameter is the third characteristic evidence deduced from the resistivity– temperature curves. The characteristic feature is consistent with the sample production quality and can be inferred from the ratio between ρ300K and ρ90K parameters [58]. In our investigation for this paper, we describe the variation of RRR parameter against the Ga concentration level for the solid Bi2.1-xGaxSr2.0Ca1.1Cu2.0Oy superconducting materials prepared with the Ga/Bi substitution ratio of 0.00 ≤x≤ 0.30 in Table 2. One can observe from the table that when the pristine compound has the ρ300K/ρ90K ratio of 1.48, the parameter tends to increase with the augmentation of Ga impurity concentration level up to x=0.05 where the maximum ratio is found to be 2.88 for the poly-crystallized Ga/Bi-3 ceramic sample. However from the Ga content level of x=0.05 onwards, the parameter begins to degrade sharply towards the ratio of

ACCEPTED MANUSCRIPT 1.31 for the Ga/Bi-6 compound. This is attributed to the fact that the Ga/Bi-3 material exhibits the highest sample quality, being favored by the other crucial findings. To sum up, the characteristic curves are found to be strongly dependent upon the Ga impurity concentration level, and among the materials studied in the current work the Ga/Bi-3 material ceramic compound obtains the highest electrical features.

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Moreover, the discussion on the presence of close relation between the improvement in the electrical characteristics with the enhancement of high-Tc 2223 phase can be interesting. Namely, it is obvious that as the Ga/Bi substitution level is in the optimum point of x=0.05, both the electrical (conductive) characteristics and high-Tc 2223 phase reach to the maximum

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top points. In this respect, the increase of high phase in the crystal structure affects positively the electrical features.

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4.5. Variation of superconducting characteristics of poly-crystallized Bi-2212 compounds with Ga/Bi substitution

In this part of paper, we focus sensitively on the differentiation in the superconducting characteristics ( T conset , T coffset and ∆ T c parameters) of bulk Bi-2212 ceramic materials. As well known, the T coffset parameter is directly attributed to the formation of effective and strong

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electron-phonon coupling probabilities and stabilization of superconducting clusters. Thus, the temperature value presents not only the beginning of superconductivity but also the transition of isolated intragrains into the superconducting phase [59]. Determination of T conset parameter for a superconducting compound is important for defining the order

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parameter of super-electrons, hybridization mechanism, metastability (overlapping of Cu-3d and O-2p wave functions), localization of electronic states, formation of electron-phonon

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couplings and mobile hole carrier concentrations in the antiferromagnetic Cu-O2 in-plane layers [60]. At the same time, the T coffset parameter exhibits the features of intergranular component and phase volume fractions in the crystal system [61]. Hence, a material is totally the superconducting at a temperature lower than Tcoffset where the percentage of small homogenous clusters in the superconducting path reach to the maximum point. Moreover, the direct relation between T coffset and T coffset ( T conset - T coffset ) presents the ∆ T c parameter that is responsible for the applications of the material in the industrial, engineering, small and large scale fields. The fundamental superconducting characteristics can be deduced from the resistivity– temperature curves. It is easy to confirm that the T conset , T coffset and ∆ T c parameters are found

ACCEPTED MANUSCRIPT to depend sensitively on the gallium concentration level in the Bi-2212 crystal structure. In this regard, the T coffset values tend to increase regularly from 81.04 K (for the pure sample) to 84.52 K (for the Ga/Bi-3 compound exhibiting the greatest Bi-2223 phase) with the enhancement in the Ga foreign impurity content level until x=0.05. On the other hand, the T coffset value begins to dramatically decrease with the excess dopant level and in fact falls in

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the deepest point of 45.65 K for the Ga/Bi-6 material. Likewise, the T conset parameters are measured to augment continuously with the Ga content up to critical value of x=0.05 beyond which the value diminishes considerably. Numerically, the Ga/Bi-3 compound obtains the maximum T conset value of 85.00 K while the minimum value of 70.06 K is observed for the

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bulk Ga/Bi-6 material as against 83.04 K for the pristine Bi-2212 superconducting material. Hence, the ∆ T c parameter locates into the minimum point of 0.48 K for the Ga/Bi-3

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compound (Table 2). This is attributed to the fact that in case of optimum substitution value of x=0.05;

* The mobile hole carrier concentrations reach to the optimum value in the short-rangeordered antiferromagnetic Cu-O2 consecutively stacked layers,

* Maximum percentage of small homogenous clusters is obtained in the superconducting

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path,

* The homogeneities in the oxidation states and densities of active and dynamic electronic states at Fermi energy level improve considerably, * Amplitude of pair wave function (Ψ= Ψ0e-iφ) more and more strengthens,

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* The effective and strong electron-phonon coupling probabilities are formed remarkably, * The superconducting clusters are stabilized significantly, * The crystal structure quality enhances and the structural problems reduce extensively,

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* Metallic interactions between the intergrain regions improves noticeably, * The over-doped state of bulk Bi-2212 system transits into optimally doped state, * Hole trap energy and overlapping of Cu-3d and O-2p wave functions increase harshly. To sum up, all the developments in the electrical and superconducting characteristics are directly associated with the increment in the high-Tc Bi-2223 phase. 4.6. Application-oriented investigations based on the experimental and theoretical discussions for Bi-2212 ceramic compounds All the experimental and theoretical discussions founded on the fundamental aspects of bulk density, powder X-ray diffraction, dc electrical resistivity versus temperature and critical

ACCEPTED MANUSCRIPT current density researches in the present work show to able to enlarge the application fields of Bi-2212 superconducting ceramic materials in the novel and feasible areas such as the electrooptic, heavy-industrial technology, network, power transmission, innovative energy infrastructure, magnet and energy technology fields only if the crystal structure quality and strength quality of interaction between the superconducting grains improve considerably. In

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this respect, we present the variations of quality factors (residual resistivity, residual resistivity ratios, critical current density and degree of broadening parameters) with regard to the experimental bulk density findings in Fig. 4. It is visible from the figure that the quality factors tend to improve parallel up to the bulk density value of nearly 6 g/cm3 beyond which

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the parabolic development (towards to optimum substitution level up to x=0.05) is observed on all the factors (especially degree of broadening parameters). In more sophisticated interpretations, the parabolic improvement points out the approach on the optimum

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replacement of trivalent Bi3+ by Ga3+ in the Bi-2212 crystal matrix whereas the linear differentiation shows the divergence from the optimum dopant level. 5. Conclusion

In the present work, we develop a strong conclusion founded on the variations of fundamental characteristic features such as the bulk density, electrical, superconducting, flux pinning

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mechanism, crystal structure quality and strength quality of interaction between the superconducting grains with regard to the partial substitution of homovalent Ga inclusions at the Bi-site in the Bi-2212 crystal system. According to the experimental measurement and theoretical findings, the optimum Ga/Bi substitution level for the Bi2.1-xGaxSr2.0Ca1.1Cu2.0Oy

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(0.00 ≤x≤ 0.30) is found to be x=0.05. The vital conclusions obtained are arranged as follows: The XRD patterns present that the bulk Ga/Bi-3 compound exhibits the maximum high-Tc Bi-

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2223 phase volume fraction (29.8 %), average crystallite size (75.4 nm), mobile hole carrier concentration (0.152), degrees of c-axis orientation or Lotgering index (0.532) and c lattice cell constant (32.62 Å) as against the minimum a lattice cell constant (5.34 Å). This is in attribution to the improvement in the crystallinity quality or decrement in the structural problems in the Bi-2212 crystal matrix. At the same time, the material prepared at x=0.05 level exhibits the greatest conductivity, superconducting and flux pinning characteristics due to the increment in the metallic interactions between the intergrain regions, mobile hole carrier concentrations, percentage of small homogenous clusters in the superconducting path, homogeneities in the oxidation states and densities of active and dynamic electronic states at Fermi energy level, transition of intrinsic over-doped state into optimally doped state, hole

ACCEPTED MANUSCRIPT trap energy and overlapping of Cu-3d and O-2p wave functions. In this respect, the maximum Jc, T conset and T coffset values are found to be 96 A.cm-2, 85.00 K and 84.52 K, respectively for the Bi-2212 sample prepared by x=0.05 substitution level. As for the bulk density parameters, the Ga/Bi-3 sample presents the highest value of 6.12 g/cm3 and lowest residual porosity parameter of 12.86 %. To conclude, the characteristic properties can be developed by the

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optimum Ga/Bi substitution for the usages of Bi-2212 superconductors in the novel and feasible market areas for the universe economy so that the permanent and radical solutions can be obtained for the global energy requirements. References

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Fiory, Nonlinear temperature-dependence of the normal-state resistivity in YBa2Cu4O8γ films, Phys. Rev. B 39 (1989) 9611–9613.

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[54] D.M. Newns, P.C. Pattnaik, C.C. Tsuei, Role of vanhove singularity in high-temperature superconductors - Mean field, Phys. Rev. B 43 (1991) 3075–3084.

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[55] Y. Zalaoglu, G. Yildirim, H. Buyukuslu, N.K. Saritekin, A. Varilci, C. Terzioglu, O. Gorur, Important defeats on pinning of 2D pancake vortices in highly anisotropic Bi-2212 superconducting matrix with homovalent Bi/La substitution, J. Alloy. Compd. 631 (2015) 111–119.

[56] J. Ekin, Experimental techniques for low-temperature measurements: cryostat design,

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material properties and superconductor critical-current testing, Oxford University Press, New

[57] M. Li, Y. Zhang, Y. Li, Y. Qi, Granular superconductivity in polycrystalline Bi2Sr2CaCu2O8+γ by homovalent La substitution on Bi sites, J. Non-Cryst. Solids 356 (2010)

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2831–2835.

[58] X. Xu, J.H. Kim, S. X. Dou, S. Choi, J.H. Lee, H. W. Park, M. Rindfleish, M. Tomsic, A

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correlation between transport current density and grain connectivity in MgB2/Fe wire made from ball-milled boron, J. Appl. Phys. 105 (2009) 103913. [59] A. Ianculescu, M. Gartner, B. Despax, V. Bley, Th Lebey, R. Gavrila, M. Modreanu, Optical characterization and microstructure of BaTiO(3) thin films obtained by RF-magnetron sputtering, Appl. Surf. Sci. 253 (2006) 344–348. [60] N.K. Sartekin, M. Pakdil, G. Yildirim, M. Oz, T. Turgay, Decrement in metastability with Zr nanoparticles inserted in Bi-2223 superconducting system and working principle of hybridization mechanism, J. Mater. Sci: Mater. El. 27 (2016) 956–965. [61] R. Awad, A.I. Abou-Aly, M.M.H. Abdel Gawad, I. G-Eldeen, The influence of SnO2 nano-particles addition on the vickers microhardness of (Bi, Pb)-2223 superconducting phase, J. Supercond. Nov. Magn. 25 (2012) 739–745.

ACCEPTED MANUSCRIPT Table Captions Table 1. Powder XRD experimental results including Lotgering indices, phase volume fractions, average grain sizes, lattice cell parameters (a and c), and in-self transport critical current densities for the pure and Bi-site Ga substituted polycrystalline materials. Table 2. Dc electrical resistivity measurement results such as T conset , T coffset , ρ300K, ρ90K, ρres,

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Phole and RRR parameters and Archimedes water displacement findings as regards ρ and P.

ACCEPTED MANUSCRIPT Table 1 Materials

a (Å)

c (Å)

Volume fraction ( ≈%) 2212 - 2223 - 2201

Ga2O3

Average grain size (nm)

Lotgering index, F

Jc (A.cm-2)

5.39

30.48

82.5

17.5

0

0

59.3

0.404

66

Ga/Bi-1

5.36

31.01

80.1

19.9

0

0

62.6

0.430

71

Ga/Bi-2

5.34

31.44

73.1

26.9

0

0

68.7

0.463

80

Ga/Bi-3

5.31

32.62

70.2

29.8

0

0

75.4

0.532

96

Ga/Bi-4

5.41

30.37

94.8

5.2

0

0

55.3

0.391

60

Ga/Bi-5

5.43

30.28

79.4

9.0

11.6

0

52.5

0.387

56

Ga/Bi-6

5.44

30.06

77.2

3.8

15.5

3.5

50.9

0.376

54

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Ga/Bi-0

ACCEPTED MANUSCRIPT Table 2 ρ300K (mΩcm)

ρres (mΩcm)

RRR (ρ300K/ρ90K)

ρ90K (mΩcm)

Tcoffset (K)

Tconset (K)

∆Tc (K)

Phole

ρ (g/cm3)

P (%)

Ga/Bi-0

8.42

3.78

1.48

5.71

81.04

83.04

2.00

0.13625

5.89

6.51

Ga/Bi-1

6.58

2.87

1.63

4.05

82.72

83.81

1.09

0.14198

5.94

5.71

Ga/Bi-2

5.84

1.66

2.11

2.77

83.12

84.15

1.03

0.14338

6.01

4.60

Ga/Bi-3

4.89

0.71

2.88

1.70

84.52

85.00

Ga/Bi-4

10.60

6.34

1.39

7.65

73.24

81.66

Ga/Bi-5

11.42

7.88

1.34

8.55

60.71

79.29

Ga/Bi-6

13.97

9.29

1.31

10.69

45.65

70.06

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Samples

0.15173

6.12

2.86

8.42

0.11907

5.82

7.62

18.58

0.10118

5.78

8.25

24.41

0.08514

5.76

8.57

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0.48

ACCEPTED MANUSCRIPT Figure Captions Figure 1 Powder X-ray partial diffraction peaks for virgin and Bi-site Ga substituted superconducting ceramic materials. Notations of “H”, “L”, “VL” and “*” are related to the phases of high, low, very low and Ga2O3, respectively. Figure 2 Variation of mobile hole concentration against Ga/Bi substitution level

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Figure 3 Experimental dc electrical resistivity measurement results over temperatures interval 35K-105 K for un-substituted and Bi-site Ga substituted ceramic materials. Lines are related to the divergence of optimum mobile hole concentration in the short-range-ordered antiferromagnetic active Cu-O2 layers

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Figure 4 Changes of quality factors as regards a- critical current densities, b- residual

resistivity ratios, c- degree of broadening and d- residual resistivities according to bulk

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densities.

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20 30

2Θ (Degree) 40

Figure 1

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10 (0012)VL

* (401)

(006)VL

Bi/Ga-4

Bi/Ga-5

50 Bi/Ga-6

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(0010)H

(2012)L (2020)H (1115)L

(028)L

(0012)L (1111)L

(117)L (020)H (202)L

(0010)L (0012)H

(113)L

(008)L

(002)H (002)L

ACCEPTED MANUSCRIPT

Bi/Ga-3

Bi/Ga-2

Bi/Ga-1

Un-Substituted

60

ACCEPTED MANUSCRIPT Bi/Ga-0

0,15

0,14

Bi/Ga-4 Bi/Ga-6

0,13 Bi/Ga-1

0,12

0,10

Bi/Ga-2

0,09

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0,11

Bi/Ga-3

0,00

0,05

0,10

0,15

0,20

Bi/Ga Substitiution Concentration Level

0,30

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Figure 2

0,25

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Mobile Hole Carrier Concentration

Bi/Ga-5

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10

Un-substituted Bi/Ga-1 Bi/Ga-2 Bi/Ga-3 Bi/Ga-4 Bi/Ga-5 Bi/Ga-6

*

Line for optimum mobile hole carrier concentration

Beginning of divergence

8

*

6

4

2

0 60

80

Temperature (K)

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Figure 3

100

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40

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Dc Electrical Resistivity (mΩcm)

12

ACCEPTED MANUSCRIPT 90

Bi/Ga-3

(b)

2,7

Residual Resistivity Ratio

2

Critical Current Densities (A/cm )

Bi/Ga-3

(a)

2,4

Bi/Ga-2

80

2,1

Bi/Ga-1

*Critical Point

70 Bi/Ga-0

60

Bi/Ga-2

1,8

Bi/Ga-4

Bi/Ga-1

Bi/Ga-0

1,5

Bi/Ga-4 Bi/Ga-6

Bi/Ga-5 Bi/Ga-6

Bi/Ga-5 5.961

5,90

5,95

6,00

3

Bulk Densities (gr/cm )

6,05

5,80

5,85

5,90

(c)

Bi/Ga-4

6

15

10

5 Bi/Ga-1

Bi/Ga-2

Bi/Ga-3

0 5,80

5,85

5,90

5,95

6,00 3

Bulk Densities (gr/cm )

6,05

6,10

5,75

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6,10

Bi/Ga-1

2

Bi/Ga-0

6,05

Bi/Ga-0

4

Bi/Ga-4

6,00

3

(d)

Bi/Ga-5

Residual Resistivity (mΩ cm)

8

5,95

Bulk Densities (gr/cm )

Bi/Ga-6

Bi/Ga-5

5,75

5,75

Bi/Ga-6

20

Degree of Broadening (K)

6,10

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5,85

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5,80

Bi/Ga-2

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5,80

5,85

5,90

5,95

Bi/Ga-3

6,00

3

Bulk Densities (gr/cm )

6,05

6,10

ACCEPTED MANUSCRIPT *Defining optimum trivalent Bi/Ga replacement level for Bi-2212 ceramic materials * Transition of intrinsic over-doped state into optimally doped state with Ga impurity * Formation of more effective and strong electron-phonon couplings at Fermi level * Role of Bi/Ga substitution on strengths of ability and quality of crystal structure

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* Change in ability and strength of artificial flux pinning centers with Ga content