In this study, superconducting samples of type Bi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy,(Bi, Pb))-2212, added with CdO, CdMnO, and CdFeO nanoparticles, were synthesized using a solid state reaction technique. The samples were examined using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) was utilized to evaluate the thermal stability of the pure sample throughout the different stages of the phase formation and the effect of the nanoparticles addition. The weight loss/gain by the three additions is related to the excess of oxygen, confirmed via Iodometric titration analysis and from the findings of oxygen diffusion energy using TGA data. Vickers microhardness measurements were conducted at various applied loads (0.49–9.8 N). All of the prepared samples displayed a typical 'indentation size effect' (ISE) behavior based on the HV measurements. It was found that the optimum addition of nanoparticles, for increasing the microhardness of the (Bi, Pb)-2212 phase, was at x = 0.05 wt. %. Furthermore, various mechanical properties as Elastic modulus (E), yield strength (Y), fracture toughness(K), and brittleness index (B) were calculated for three additions of the samples studied. As for the theoretical examination of HV in the saturation limit regions, the indentation-induced cracking (IIC) model wins the comparison.
Article
Thermal and Mechanical Investigations of (Bi,Pb)-2212 superconductor added with different oxide nanoparticles
https://doi.org/10.21203/rs.3.rs-1642149/v1
This work is licensed under a CC BY 4.0 License
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published 30 Jan, 2024
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Bi-2212 superconductor
TGA
Vickers microhardness
From the application point of view, the (Bi2 − x Pbx)Sr2Ca Cu2Oy or (Bi, Pb)-2212 superconducting phase of the [(Bi2 − x Pbx)Sr2Can1CunO4 + 2n+δ, n = 1, 2, and 3] homologous superconducting series is considered one of the most promising phase for further investigations because of its appealing characteristics [1–5]. Similar to the other cuprates, the (Bi, Pb)-2212 is a hard ceramic with numerous small fractures, vacancies, and dislocations. These are found in the Cu–O2 sheets, making them not flexible and moldable due to the absence of yield point and inelastic nature [6]. Therefore, the porous structure and brittle nature are the most problematic quantities for the restriction of using (Bi, Pb)-221 in applications. However, some applications such as magnetic coils, devices for storing magnetic energy, cables of power transmission, etc. [7–10], materials must have adequate mechanical strength, which usually requires precise microstructure control. As a result, the general behavior of mechanical performance and its properties such as Hv, E, and Y enable materials to be used in their potential application fields and give a broad insight into their microstructure [11].
Vickers hardness is preferred for estimating the mechanical properties of HTSCs due to its inherent benefits, which include non-destructive nature, simple test procedure, low cost, accurate reading, and it is strongly related to their compositions and structures [12]. In this regard, the microhardness measurement results are accurate and valuable in metallurgical, technological, and industrial applications for quality control procedures and materials selection [13]. As a result, many researchers have been working in investigating and improving the mechanical properties as well as structural, electrical, and magnetic properties of HTSCs [14–24], using nano-oxides, nanometals, etc. For example, consider the effect of adding WO3 (40 nm) to the Bi1.8Sr2WxCa1.1Cu2.1Oy superconductor, with x = 0.0, 0.05, 0.1 and 0.25 wt.%, was studied by Özkurt [25] and demonstrated that extra amount of WO3 (x = 0.25 wt.%) raised the Hv of the phase. Furthermore, all samples exhibited indentation size effect behavior. In addition, Bahadir Akkurt et al. [26] investigated the effect of replacement of Ca by Gd nanoparticles in Bi-2212 with a nominal composition Bi2.1Sr2.0Ca1.1−xGdxCu2.0Oy samples with x = 0.0, 0.01, 0.03, 0.05, 0.07, 0.10, and 0.30. It was found that raising the Gd concentration in the Bi-2212 phase, reduced the mechanical performance and favored the reverse indentation size effect (RISE) behavior. This behavior was explained based on many factors that occur in the samples, such as the coupling problems between the grains, defects, and the plastic deformations that became more prominent with increasing Gd content. In addition, the theoretical modeling showed that the HK model is a good descriptor for defining mechanical properties.
CdO nanoparticles are among the transparent conductive oxide (TCO) nanoparticles that the researchers are interested in because of their beneficial properties [27–29]. The effects of adding CdO nanoparticles to high-temperature superconductors on their superconducting properties have been studied [30, 31]. The structural, optical, and magnetic signatures of the host CdO matrix were found to be improved by transition metal doping, allowing for novel applications. CdO nanoparticles doped with Mn2+ [32–34] and Fe3+ [35–37] have gotten a lot of attention. Even so, no prior research has looked experimentally at the effect of the addition of Cd0.95 Mn0.05O (CdMnO) and Cd0.95 Fe0.05O (CdFeO) nanoparticles on the structural, mechanical, and thermal behavior (TGA) of (Bi, Pb)-2212superconducting ceramics. This prompted us to look into the impact of doping (Bi, Pb)-2212 superconducting phase by low concentrations of CdO, CdMnO and CdFeO nanoparticles, having the general formula: (CdO)x Bi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy (Cd0.95 Mn0.05O)xBi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy and (Cd0.95 Fe0.05 O)x Bi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy with x = 0.00, 0.01%, 0.02%, 0.05% and 0.10% synthesized using the conventional solid-state reaction technique. The aim of this study is to present, for the first time, a comparative study of CdO and its two additives, Mn and Fe, was conducted to investigate the structural, morphological, and thermal behavior (TGA) of the (Bi, Pb)-2212 superconducting phase. In addition, to determine which sample is most desirable for improving mechanical properties. Moreover, theoretical models examination in the saturation limit regions are employed to calculate the true microhardness parameters.
A traditional solid-state reaction method was applied to prepare 20 grams powder for each of the superconducting phase of nominal composition: (CdO)x Bi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy, (Cd0.95 Mn0.05 O)xBi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy, and (Cd0.95 Fe0.05 O)x Bi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy with x = 0.00, 0.01%, 0.02%, 0.05% and 0.10% [38]. Stoichiometric amounts of CuO, Bi2O3, CaO, and SrCO3 of high purity were grinded and sieved to obtain a homogenous powder. Afterward, the powders were subjected to thermal treatment as follows. First, the powders were heated at 800 oC for 24 h with the aid of a Nabertherm muffle furnace, then they were heated at 820 oC for 24 h, with intermediate grinding and sieving. Afterward, the powders were pressed at 0.96 GPa into pellets and sintered at a heating rate of 6°C/min for 50 h at a temperature of 840°C and a cooling rate of 4°C/min. Finally, CdO. Cd0.95Mn0.05O, and Cd0.95Fe0.05O nanoparticles of average grain size 41.24 nm, 39.79 nm, and 40.87 nm, respectively. prepared by the co-precipitation method, as reported in our previous publication [38], were added at the last step, and the mixture was pressed and heated again with a rate of 4 ºC/min at 840 ºC for 48 hours and then cooled at a rate of 2 ºC/min. The X-ray powder diffraction characterization was performed using a Bruker focus diffractometer Cu Kα radiation (λ = 1.5406 Å), with 2θ ranging from 20° to 60°, setting the voltage and the current at 40 kV and 40 mA, respectively. The morphology of the prepared samples was proceeded through a scanning electron microscope (SEM), (JEOL JSM). SEM technique was applied to image the surface with a resolution of 10 µm utilizing an SDD Apollo X with an accelerating voltage of 20 kV and an AIS 2100C microscope operating at 20 kV, respectively. Thermal deterioration and stability were detected by TGA Shimadzu-50H. Each sample was weighed between 5–7 mg, and the weight changes in the samples were sealed in an aluminum pan and monitored in the air while they were heated from room temperature to 900 oC at a rate of 10 oC min− 1. To study the mechanical performance of our samples, the values of HV have been measured at room temperature using a digital microhardness tester (MHVD-1000IS), with a dwell indentation time of 60 sec, and applied loads ranging from 0.49 N up to 9.8 N. The applied loads have been conducted at various parts of the surface samples. To achieve good mean values for each load, an average of five readings at various places on the surface samples were taken.
Several investigations [39, 40] have shown that a sample's Vickers microhardness, Hv, is dependent on the static test applied load, F. The following equation is used to compute numerically the Vickers microhardness values for various applied static test loads.
$${H}_{V}=\frac{2Fsin(\alpha /2)}{{d}^{2}}=1854.4\frac{F}{{d}^{2}} \left(GPa\right)$$
1 ,
where α is the indenter’s face angle of 136°, and d is the adequate mean diagonal length.
Meyer's law is considered a useful model for examining the fundamental mechanical characterization of material under external stress. It predicts one of the two effect, an indentation size effect (ISE) or a reverse indentation size effect (RISE). For ISE a typical decrement in the Vickers microhardness parameters with the applied indentation test loads is observed. For RISE, an unusual increment in the Vickers microhardness parameters with the applied indentation test loads is observed. The following equation can be used to define the behavior of the samples evaluated by Meyer's [41]:
$$F=A{d}^{n}, \left(2\right)$$
where A is the constant microhardness value for the material and n is the Meyer number calculated from the fitted curves between LnF and Lnd. This is a crucial value in determining whether a material exhibits ISE or RISE behavior, if the value of n < 2, ISE nature is observed. In the case of n > 2, RISE behavior is observed.
Based on the elastic recovery mechanism in the vicinity of the indentation track. Elastic/Plastic Deformation (EPD) model is considered a useful one in this range for determining the behavior of the samples (ISE or RISE). When plastic deformation is the dominant feature of the system, the model is much preferable. Because the EPD method emphasizes plastic deformation by adding a new term related to plastic deformation to the theoretical formula as a correlation factor. According to this, the diagonal length on the inside of the plastic deformation, and the elastic/plastic deformation load-independent microhardness (HEPDin) can be calculated using the following relations, respectively [42, 43]:
$$F={A}_{2}{\left(d+{d}_{o}\right)}^{2}, \text{a}\text{n}\text{d} \left(3\right)$$
$${H}_{EPDin}=1854.4\times {A}_{2}. \left(4\right)$$
Where \({A}_{2}\) is constant and do is indentation size value that is formed with elastic deformation
The mechanical identification of the material exhibiting the ISE/RISE nature is described using the proportional specimen resistance (PSR) model for many materials [44]. Moreover, the foundation of the PSR model is the dispersion of energy associated with the occurrence of vacancies, residual cracks, and flaws on the surface sample. The PSR model can be defined using the equations below:
$$F={\alpha }d+{\beta }{d}^{2}, \text{a}\text{n}\text{d} \left(5\right)$$
$${H}_{PSRin}=1854.4\times {\beta }. \left(6\right)$$
Where α represents the surface energy, β is a parameter that is used to calculate the load-independent microhardness value HPSRin.
Another theoretical approach for examining the mechanical properties of the ceramic is the modified proportional sample resistance (MPSR) model. By comparison with PSR, it was found that the MPSR model incorporates an additional constant given as α1 for the arrangement of the diagonal length of the indenter on the surface of a specimen. The modified proportional resistance (MPSR) is defined in the literature as [40]:
$$F={{\alpha }_{1}+ {\alpha }}_{2}d+{{\alpha }}_{3}{d}^{2}, \text{a}\text{n}\text{d} \left(7\right)$$
$${H}_{PSRin}=1854.4\times {\alpha }_{3}. \left(8\right)$$
Where α1 corresponds to the minimum applied load to produce an indentation, and it is equivalent to the load-independent constant (W) in the HK approach. α1 relates to the surface residual stresses associated with surface polishing and machining. While, the values α2 and α3 correspond to the constants α and β in Eq. (5), respectively.
According to the Hays and Kendall (HK) model [45], it was found that there is a critical value of the applied load, W, above which plastic deformation occurs due to the indenter's deep penetration into the sample [46]. As a result, the effective load, Feff =F-W, is used to formalize the alteration in indentation size against the applied test load instead of the applied load itself, as seen in the following equation:
$$F-W={A}_{1}{d}^{2}, \left(9\right)$$
where A1 is the load-independent microhardness constant
According to HK model, the HK load-independent microhardness (HHKin) is given by the following equation:
$${H}_{HKin}=1854.4\times {A}_{1}. \left(10\right)$$
Finally, the last theoretical model discussed in this study is the IIC model, which is used to examine the mechanical performance and true microhardness values in the saturation regions [44, 46]. The IIC model is made up of four primary components that are responsible for the resistance to the imprint diagonal length and particularly determine the behavior of the material (ISE or RISE behaviors): The first two components that cause ISE behavior are the indenter/specimen interface frictional components and elastic deformation, while the second two components responsible for RISE behavior are plastic deformation and crack mechanism. According to the IIC model, the independent load Vickers microhardness is obtained from the following relationship:
$${H}_{IIC}={\lambda }_{1}{K}_{1}(F/{d}^{2})+{K}_{2}({F}^{5/3}/{d}^{3})$$ 11,
where λ1 is constant, K1 is constant rely upon the indenter’s geometry, and K2 is constant depending on the applied load. For type-II superconductors, which are characterized by brittle nature (λ1 = 0), thus the above equation is reduced to the following one:
$${H}_{IIC}=K{\left(\frac{{F}^{5/3}}{{d}^{3}}\right)}^{m}.$$
12
Where K and m are load-independent parameter constants. It is important to note that the m value is similar to the n value in Mayer’s law, where both can be used to identify the indentation size effect. The m value has a crucial value of 0.6, if m > 0.6 the material shows ISE behavior, while for m < 0.6 the RISE behavior is exceptional [20].
Additionally, in this study the critical mechanical characteristics regarding the elastic modulus (E), yield strength (Y), and fracture toughness (K) were deduced, which are essential for the use of the Bi-2212 superconducting phase in industrial applications. As a result, the true microhardness values were used to determine the load-independent parameters E, Y, K, and B from the following equations:
$$E=81.9635 {\text{H}}_{\text{v}}$$
13
$$Y={\text{H}}_{\text{v}}/3$$
14
$$K=\sqrt{2E\alpha }, (\alpha , surfce energy)$$
15
$$B=\frac{{\text{H}}_{\text{v}}}{\text{K}}$$
16
4.1 X-ray powder diffraction, XRD:
The plots of Fig. 1-a show the room temperature XRD patterns of (Bi, Pb)-2212 superconducting phase added by nano-CdO, nano-CdMnO, and nano-CdFeO for some selected concentrations with x = 0.00, 0.05, and 0.1 wt%. The diffraction peaks with 2θ between 20° and 60° for all three nano additions are well indexed with the tetragonal structure of Bi-2212 space P4/mmm using the JCPDS card No. 00-082-2278 [47]. All XRD spectra demonstrate a single phased Bi-2212 without the detection of impurities. It is believed that the improved crystallinity and purity are attributed to the heat treatment, in specific the prolonged calcination at T = 840 [48]. Moreover, peaks corresponding to CdO, CdMnO, and CdFeO phases were not detected probably because of their low concentrations.
The lattice parameters a, b and c were determined by using a Reitveld refinement on the XRD patterns using the MAUD software [49], and their values are listed in Table 1. While the plots of Figs. 1-b, 1-c, 1-d and 1-e show the refinements of selected concentrations of CdO, CdMnO and CdFeO nanoparticles addition (x = 0.0 and 0.05 wt.%). The values of a show a slight increase with the increase of CdO, CdMnO, and CdMnFeO contents. While the lattice parameters b and c decrease with increasing the addition of CdO, CdMnO, and CdFeO nanoparticles.
The existence of the metal oxide nanoparticle within the Bi2212 phase unit cell may create extra oxygen ions in the BiO layer. This excess oxygen content might induce the relocation of holes from the charge reservoir (BiO layer) to the Cu-O conducting plane [50, 51]. The effect of oxygen content is also reflected on the ratio c/a and the orthorhombic distortion OD (\(\frac{\text{b}-\text{a}}{\text{a}}\)) which are calculated and presented in Table 1. A reduction of c/a values with the addition of nanoparticles is observed. The ratio c/a is strongly dependent on the oxygen content of the Bi-2212 superconducting phase. In particular, the excessive oxygen content within the bonds of Cu-O in planes of Cu-O and BiO layers [52–54]. The high oxygen content induces a Jahn-Teller distortion around Cu2+ ions, leading to a reduction of the c/a ratio. An increase of OD is demonstrated with the addition of CdO, CdMnO, and CdFeO up to x = 0.02, followed by a decrease. But, all values of OD are less than 1%, approving the stability of the (Bi, Pb)-2212 phase [55].
Figure 2-a depicts the results of the calculated porosity percentage (P %) for all synthesized samples [56]. The figure indicates that the addition of nano-CdO, nano-CdMnO, and nano-CdFeO up to x = 0.05 wt.% minimizes grain porosity and repairs microcracks. Following that, the porosity increases gradually up to x = 0.1 0wt.%. The porosity decreases by 30%, 34% and 39% with x up to 0.05 wt% for (CdO)-Bi2212, (CdMnO)-Bi2212 and (CdFeO)-Bi2212, respectively.
As for the variation of the superconducting transition temperature Tc, dc-electrical resistivity measurements were conducted using the four probe method and are reported elsewhere [57–59]. The results are depicted in the graph of Fig. 2-b. Tc of the (Bi, Pb)-2212 system increases up to x = 0.5 for both CdO and CdMnO nanoparticles addition. Whereas for CdFeO addition Tc increases up to x = 0.02 and then decreases significantly. The enhancement of Tc is attributed to the enhancement of the grain connectivity due to the addition of the nano-oxides. On the other hand, the deterioration of Tc for higher additions of CdFeO and CdMnO might be attributed to the ferromagnetic nature of these oxides. This nature may cancel the supercurrents and terminate the Cu ion long-range coupling in the CuO layers. Similar behavior was observed by Basma et al. [60], for ferromagnetic CoFe2O4 addition on the Gd-123 system.
4.2 SEM measurements:
Figure 3 depicts the SEM microscopy images of (Bi,Pb)-2212 superconductor with selected concentrations of CdO, CdMnO and CdFeO nanoparticles addition (x = 0.00, 0.05 and 0.10 wt.%). The microstructures of the pure sample show large plate-like grains distributed randomly, which is typical for the Bi-based superconductors family [61]. Whereas, the morphology of samples added with x = 0.05 wt.% CdO, CdMnO, and CdFeO nanoparticles show enhanced texturing and growth, most likely due to the added nanoparticles sticking to grain boundaries [62]. In addition, no detection of secondary phases or impurities was observed, in agreement with the XRD results. Moreover, it is obvious that nanoparticle addition increases the microstructure density and reduces the porosity among the grains by up to x = 0.05 wt.% for the three different nano additions to (Bi, Pb)-2212 phase. This observation is supported by the presence of fine inclusions among the (Bi, Pb)-2212 grains. These inclusions correspond to the added nano- CdO, nano- CdMnO, and nano-CdFeO. This leads to enhanced grain connectivity and the filling of voids and cracks [63]. On the other hand, the most porous structure and softest surface are observed in the 0.10 wt.% sample for three nano additions because the platelet grains linked well in the sample are degraded with the addition of the CdO, CdMnO, and CdFeO nanoparticles due to the random orientation and weak links between the grains [64]. This result was found to be consistent with the porosity percentage calculations.
4 − 3 Iodometric Titration-Oxygen content determination
Iodometric titration was employed for the oxygen content determination. The method consists of a two-step redox titration reaction [65,66]. In the first step, the superconductor sample is dissolved in Potassium Iodide (KI) acidic solution to achieve the reduction of copper to copper Iodide (CuI). In a second step, a back titration of the CuI was performed by using sodium thiosulphate (Na2S2O3) to determine the oxygen content [67]. The endpoint of titration was detected by using Potassium thiocyanate (KCSN) used as an indicator of idiometry.
The oxygen content, y, in cuprates superconductors might be calculated using the equations below in terms of a parameter H [68].
$$y=8+1.5H ; H= \left[ \left({V}_{1}{m}_{2}/{V}_{2}{m}_{1}\right)– 1\right].$$
17
Where V1, V2, m1, and m2 are the volumes of Na2S2O3 solutions and the sample’s masses throughout the titration in the first and the second stages, respectively. An increase of the value of oxygen content with increasing concentrations for three nano additions and their values are listed in Table 1. This behavior indicated that nano-CdO, nano-CdMnO, and nano-CdFeO aid in the addition of more excess oxygen to the pure sample and are compatible with the variation of the lattice parameters discussed in the XRD section. The effective valence of the Cu ions (Cueff) valence can also be calculated by applying equilibrium between the positive and the negative valence states of the composition elements as shown in Eq. 14, assuming the valency of the elements as (Bi3+, Pb2+, Sr2+, Ca2+, Cd2+, Fe2+and O2−):
$${\text{C}\text{u}}^{\text{e}\text{f}\text{f}}=\frac{2\text{y}-[11.7+2\text{x}]}{2}$$
18
Where x and y represent the Cd and O contents.
The values of Cueff are listed in Table 1. The results show an enhancement in the effective valence of Cu for three different nano additions. This enhancement is attributed to the fact that the development of the (Bi, Pb)-2212 phase demands the existence of Cu3+ ions in the form of additional oxygen in/or near the CuO2 planes [69–71]. The excess oxygen atoms increase the Coulomb potential that confines the doped holes in the CuO2 planes, reducing their total energy [72].
Under the assumption of a homogeneous density of excess oxygen places in the CuO2 planes, the excess oxygen atoms introduced into the Bi-2212 are found to be turned into a doping distance, d, given as \(d = \sqrt{{\Sigma }}.a\). This was accomplished by expressing one doping element's unit area as a square planar arrangement. The excess doping density Σ−1, whose value gives the number of Bi-2212 unit cells per unit area for one oxygen excess atom, is given as Σ−1 = (1–8y− 1) [73]. Moreover, using the formula, \(z=\sqrt{{\left(2a\right)}^{2}+{a}^{2}}=a.\sqrt{5}\), the distance z between any two nearby Cu atoms in a certain direction may be determined in terms of a parameter. To test the response of nano-CdO, nano-CdMnO, and nano- CdFeO to the pure system, The samples are simply subjected to the above relationships. The values of Σ−1, d, and z are listed in Table 1, and they show that the increasing the concentration of nanoparticles decreases d, while slightly increasing z and Σ−1. This is excellent evidence for raising the hole carrier concentration in the system. This behavior can be explained based on the excess oxygen mechanism in Bi- cuprates as follows: The electrons are transferred from Cu sites to the BiO layer, resulting in the creation of holes on the Cu and electrons on the Bi as Bi3+ + Cu2+ → Bi3−x + Cu2+x [74]. These changes in the Bi valency are due to a change in hole carriers caused by an oxygen excess.
4–4 Thermal analysis
In an attempt to investigate the thermal stability of the pure sample during the phase formation at different temperatures of preparation stages. Moreover, the effect of the addition of CdO, CdMnO, and CdFeO nanoparticles on the phase formation of the (Bi, Pb)-2212 superconducting phase was reported. TGA is a powerful method for examining material’s thermal stability by precisely measuring the weight loss/gain of the material while it is heated at a consistent pace. The material's reaction to thermal stress is ascribed to the change in material weight during thermal treatments. This method is ideal for investigating decomposition temperatures in depth and ensuring that material works well within a specified temperature range.
Figures 4-a, 4-b and 4-c show the percentages of the loss of weight and derivative weight as TG-DTG curves for a pure sample at three different calcination temperatures, 800, 820, and 840°C (denoted by P-800, P-820, and P-840, respectively). The pure sample at three calcination temperatures decomposed completely into a series of steps, where each losing weight represented by a step on the TG curve was accompanied by a similar drop on the DTG curve. TGA was used to evaluate the weight loss, indicating that the samples lost weight gradually from above 40°C to around 450°C and 600°C, followed by a gain of weight, and finally a sudden weight loss at around 750°C. While the duration of weight gain for P-840 increased as the temperature increased from 368°C to 750°C. De-oxidation is responsible for steady weight loss, whereas oxidation is responsible for weight gain. The weight loss at 40–450 ◦C is ₋0.811%, ₋1.170%, and − 1.281% for P-800, P-820, and P-840, respectively, and might be due to the removal of water adsorbed on the surface of the powder, the decomposition of mixed carbonates, and the crystallization of CaO or Cu2O [74], which is indicated by the DTG dips at approximately 169°C, 360°C, and 450°C, respectively. The weight gain is ₋0.6%, 0.65%, and 0.8%, indicating that several transitional phases throughout the formation of (Bi, Pb)-2212 such as Ca2CuO3 and CuO begin to form at about 600°C, while crystallization of the Bi2Sr2CuOy phase occurs at the same temperature [75]. Finally, the weight loss is ∼2%, ∼2%and∼1.6% above 750°C, with steep DTG drop at 807.43, 809.45, and 845.8 oC for P-800, P-820, and P-840 samples, respectively. This is due to the gradual transition of the previously stated phases to Bi-2212 as described by the following equation [76].
2Bi2Sr2CuOy+(Sr, Ca) CuO2 → 2Bi2Sr2CaCu2Oy. (19)
According to this equation, the formation of Bi-2212 occurs due to the addition of Cu–O and CaO layers into the Bi-2201 phase. As a result, Bi-2201 is more stable in terms of thermodynamics and crystallography than Bi-2212. In general, it may be deduced that at a calcination temperature of 800 oC, several transitional intermediate phases developed in the calcined powder.
It is observed that the Bi-2201 phase, which forms from CuO and Bi-rich phase and Bi2Sr3 − xCa1 + xO7 phase, are both observable up to 800°C (P-800). While, the mentioned phases completely transform into the final Bi-2212 phase, between 820 (P-820) and 840°C (P-840), hence 840°C is chosen as the sintering temperature to get the best results without any impurities [48].
Figures 5-a and 5-b show the weight loss percentage and derivative weight percentage for the sintered samples of (Bi, Pb)-2212 with the addition of CdO, CdMnO, and CdFeO nanoparticles, with x = 0.05 and 0.10 wt.% for the three nano particles additions. Firstly, by comparing a pure sample calcined at 840 oC for 50 h (P-840) and after sintering again at the same temperature and time, it was observed that the weight loss before 750°C for P-840°C (₋1.3%) is smaller than the sintered sample (₋1.5%), which means the complete solid-state reaction between the constituent particles occurred to form the (Bi, Pb)-2212 phase which reinforces the purity of the sample. This result is consistent with the XRD patterns in Fig. 1, which showed that all XRD spectra demonstrate a single phased Bi-2212 without the detection of impurities. It is believed that the improved crystallinity and purity are attributed to the heat treatment, in specific the prolonged calcination at T = 840. Secondly, despite the presence of several dopants of different nanoparticles, the TG-DTG curves behave in the same manner for the growing process. It can be inferred from Fig. 5a, that all the samples decomposed completely in a series of steps. The curves revealed that after the initial weight loss of water, large and small dips in the DTG curve (Fig. 5b) occurred in all the samples. Based on the behavior of oxygen out/diffusion, the weight loss/gain with temperature can be explained by TGA analysis. As we know, the B-2212 superconducting phase comprises oxygen deficits in its unit cell. Thus, to develop the superconductivity of Bi-2212, its unit cell must be filled with oxygen ions. The oxygen diffusion activation energy, E, may be computed using the following formula [77]:
$$ln\left[ln\left(\frac{m}{{m}_{o}}\right)\right]=-\frac{E}{R}\left(\frac{1}{T}\right)+constant,$$
18
where m and mo are the masses of the samples at a given temperature and the samples' beginning mass, respectively, and R is the ideal gas constant. The plots of ln[ln(m/mo)] vs 1/T are depicted in Fig. 5c, for three different nano particles additions with x = 0.00, 0.05, and 0.10 wt.%, where the oxygen diffusion activation energy is calculated from the slope of the lines and are listed in Table 2. E was estimated for \(T\ge\)750 oC to be 5.508 kJmol−1 for the pure sample and then increased with increased nano-CdO to be 8.080 kJmol− 1 (0.05 wt.%) and 8.466 kJmol− 1 (0.10 wt.%). While for nano-CdMnO it was 9.798 kJmol− 1 (0.05 wt.%) and 11.729 kJmol− 1 (0.10 wt.%). The E values for nano-CdFeO are7.007 kJmol− 1 (0.05 wt.%) and 9.749 kJmol− 1 (0.10 wt.%). The high activation energy for diffusion indicates that the fast rate of diffusion of oxygen into the Bi-2212 phase occurs to compensate for the deficiency in oxygen. Hence, as the addition of nanoparticles increases, the amount of oxygen deficiency decreases and the diffusion is dependent on oxygen content [78]. This dependence was confirmed by the plots as a deviation from straight lines; because the observed dependence is an increase in the value of the activation energy with oxygen content. The plots would curve downwards to slower values (an increase in the slope of the lines) as temperature decreased. The curves of weight change show a fast variation, which supports the increase in activation energy [79].
4–5 Vickers microhardness measurements
Using Vickers microhardness measurements, this study reveals a clear correlation between major changes in general mechanical features and mechanical characteristic behaviors of Bi1.6Pb0.4Sr1.9Ca1.1Cu2.1Oy superconducting phase with the addition of (CdO)x, (CdMnO)x, and (CdFeO)x nanoparticles in the crystal structure. In this regard., firstly, the discussion of the influence of the adding mechanism on the changes in the values of Vickers hardness (HV). Figure 6 illustrates graphically the changes in HV against the applied test load F using Eq. (1) for selected concentrations with x = 0.0,0.01, 0.05, and 0.1 wt.% for the three nano particles additions. According to the results of experimental measurement, it is easy to conclude that, increasing the (CdO)x, (CdMnO)x, and (CdFeO)x in the Bi-2212 phase causes a significant variation in Hv values. Evidently, for three different nanoparticles additions, the increase of addition up to x = 0.05 improves the Hv values significantly, after which the Hv decreases at x = 0.10 wt. %. Therefore, the optimum doping impurity (x = 0.05 wt.%) for three nano additions results in structural improvements, and the HV of the (Bi, Pb)-2212 phase is increased significantly by the addition of (CdO)x, (CdMnO)x, and (CdFeO)x nanoparticles.
This improvement can be attributed to the reduction in porosity or resistance to crack propagation among the grains as well as the enhancement of grain connectivity by adding nanoparticles up to 0.05 wt% into the (Bi, Pb)-2212 phase. While, HV reduction for x = 0.10 wt% could be due to a variety of factors including increased grain boundary weak links, specimen cracking/porosity, disorder, and irregular grain orientation distribution [47]. These results are consistent with the results observed from SEM micrographs and the porosity calculations. Another interesting result seen in Fig. 6 is that, the HV parameters values for (CdO)x(Bi,Pb)-2212, (CdMnO)x(Bi,Pb)-2212 and(CdFeO)x(Bi,Pb)-2212 at a static load 0.49 N are found to be within 1.777–1.291, 1.956–1.396 and 2.066–1.573 GpPa, respectively, while at a static load 9.80 N they are to 0.592 − 0.388, 0.632 − 0.408 and 0.686 − 0.481, respectively. In this respect, the bulk (CdFeO)x(Bi, Pb)-2212 phase demonstrates the highest key mechanical design features, thus the CdFeO nanoparticles have the greatest enhancement on the Vicker hardness values Hv for the (Bi, Pb)-2212 phase. Moreover. Table 4 shows that the addition of 0.05 wt.% of CdO, CdMnO and CdFeO enhanced the load-independent HV by 77.38%, 104.13%, and 113.83%, respectively. Additionally, the enhancement caused by CdFeO is 20.44% and 4.75% as compared to the two additives, CdO and CdMn, respectively. For now, the Hv values decrease consistently up to an applied load of 2.94 N, and the hardness parameters values nearly remain constant above this applied load value, as can be shown in Fig. 6. The value of 2.94 N might thus be considered the saturation limit region. Meaning that the samples in the entire doping range exhibit the typical ISE nature (simultaneous elastic and plastic deformations in the material due to recovery of the system), as explained based on the indenter penetration depth [80,81]. These results are supported by the values of Meyer’s number n (Table.3) (n < 2, for all samples) obtained from the fitted curves of the experimental data between ln F versus ln d as shown in Fig. 7. According to Table 3, all of the prepared samples are considered hard materials because the values of n are between 1 and 1.6 (1 < n < 1.6) [82,83]. which confirms the high-temperature superconductor's ceramic behavior.
According to the EPD model, Fig. 8 depicts the variation of F 0.5 against d for the (Bi, Pb)-2212 phase added by nano-CdO, nano-CdMnO, and nano-CdFeO, with x = 0.00, 0.05, and 0.10 wt.%. The values of A2 and do are listed in Table 3. The calculated values of do are positive with a decrement trend in their values with the enhancement of the addition level to x = 0.05 wt.% for three nano additions. This statement confirms that elastic as well as plastic deformations occur side by side for all our samples [84], confirming that, the addition of CdO, CdMnO, and CdFeO into the (Bi, Pb)-2212 crystal enhanced the mechanical features. The reduction in A2 at x = 0.10 wt. % is due to an increase in disorders and weak links between the grains, which results in a reduction in observed microhardness and fast crack propagation [20]. Table 4 shows each HEPDin value for each sample of the three different nanoparticles additions to (Bi,Pb)-2212 phase. Due to the far expectation values, it is logical to conclude that this model is insufficient to define the original load-independent values for all prepared samples of the three nano additions to (Bi,Pb)-2212 phase in the saturation region.
The variation of F/d against d for each phase (x = 0.00, 0.05, and 0.10 wt.%) is pictured in Fig. 9 using the PSR model. Table 3 lists all of the extrapolated parameters (α and β) from the graph. According to Table 3, every sample exhibits the standard ISE feature due to the positive values of the α parameter, confirming the immediate production of both elastic and plastic deformations. While, the β parameter is found to have a similar variation trend as HV with x for three nano additions, confirming the enhancement of grain connectivity and local structure between the grains. Furthermore, the HPSRin values are lower than the measured load-independent microhardness values in the saturation regions. This means the PSR model is not practically sufficient to determine the load-independent values., HV, and is considered the worst theoretical model among the other theoretical models applied to nano-CdO, nano-CdMnO, and nano-CdFeO/(Bi, Pb)-2212 superconducting samples
Variations of F against d are depicted in Fig. 10, and the obtained fitting parameters of the MPSR model (α1, α2, and α3) are listed in Table 3. Two major conclusions are drawn from the values of α3 parameters: Because of the positive α3values, the samples have an ISE nature, and the ISE characteristic tends to grow systematically with the addition of CdO, CdMnO, and CdFeO nanoparticles until the value of x = 0.05 wt.%. Table 4 shows the calculated values of the load-independent microhardness HMPSRin. It's easy to notice that the MPSR approach's Vickers hardness, HMPSRin, findings are substantially lower than the experimental microhardness values in the plateau region. As a result, this model is ineffective in computing the original load-independent values.
The linear plotting of F versus d2 is given in Fig. 11 for the (Bi,Pb)-2212, added by CdO, CdMnO and CdFeO nanoparticles,with x = 0.00, 0.05 and0.10 wt.%. Table 3 shows the values of the HK approach's fitting parameters (A1 and W). It is visible from the table that the W values are positive in each of the three nanoparticles additions showing the ISE nature. indicating that under the applied indentation test load, every sample has both elastic and plastic deformations. Moreover, all of the load-independent microhardness HHKin values (Table 4) for three phases are observed to be close to the values of the Vickers hardness in the saturation region. But at the same time, they are far from the values of the IIC model in the saturation regions with respect to the values of the Vickers hardness in the saturation region. Hence, the HK model is inadequate for determining the initial microhardness values for the samples prepared for this study.
According to IIC, the values of K and m parameters are calculated from the variation of ln (Hv) versus indentation test load, ln(F5/3/d3) curves as depicted in Fig. 12. All the calculation results are represented in Table 3 in detail, which show that all the values of m for all the samples are higher than the crucial value of 0.6, i.e., m > 0.6. These findings show that all our samples exhibit ISE behavior and are consistent with the findings from Meyer’s law. Moreover, Table 4, reveals that HIICin microhardness values are much closer to the load-independent microhardness values in the plateau region in comparison to the other approximation models in this study. As a result, the IIC is an adequate methodology for the mechanical identification of nano-CdO, nano-CdMnO, and nano-CdFeO added to Bi-2212, according to the shreds of evidence produced using this method.
Moreover, there is a link between HV and the mechanical parameters E, Y, K, and B, which mention the elastic deformation ability of a material when subjected to a force, the point of transition between elastic and plastic deformation, and, very crucially, from the viewpoint of an industrial application, the material's capacity to withstand cracks, propagation, and fracture. The values of E, Y, K, and B for three nano additions can be calculated using (13), (14), (15), and (16), respectively, and their values are represented in Table 4. For three phases, Table 4 shows that E, Y, K, and B rise consistently as x increases up to 0.05 wt.% before displaying a substantial decline at x = 0.10%. where the increase in their values can be explained based on the occupation of nanoparticle inter-grains and voids of the (Bi, Pb)-2212 phase [16]. However, the optimum added concentrations of CdO, CdMnO, and CdFeO nanoparticles in three phases should not exceed 0.05 wt.%, respectively, to obtain a maximal increase in the mechanical characteristics of the (Bi, Pb)-2212 phase. Moreover, CdFeO addition outperformed CdO and CdMnO addition in improving the parameters of E, Y, K, and B, which display better ductility and an enhanced capacity to resist indentation fractures.
The effect of nano-CdO, nano-CdMnO, and nano-CdFeO additions on the phase formation, microstructure, thermal stability (TGA), and mechanical characteristics of (Bi, Pb)-2212 superconducting phase was studied. The formation of (Bi, Pb)-2212 completely occurred at a temperature of 840 oC, where above it there is thermal stability as shown by TGA analysis. The weight loss/gain by the addition of CdO, CdMnO, and CdFeO nanoparticles was related to the excess of oxygen calculated through Iodometric titration analysis. The high values of oxygen diffusion activation energy estimated by TGA analysis show that oxygen deficits were reduced throughout the formation of (Bi, Pb)-2212 superconducting phase confirming the high stability of the samples and consistency with XRD findings. Microhardness values of (Bi, Pb)-2212 added by nano-CdO, nano-CdMnO, and nano-CdFeO were found to increase gradually up to x = 0.05 wt.%, verifying the strengthening of the grain-to-grain linkages. CdFeO addition outperformed CdO and CdMnO addition in improving HV in the plateau region, with an enhancement of 113.83%, 20.44%, and 4.75% as compared to the pure sample and its two additives, CdO and CdMn, respectively. In the prepared samples, normal ISE behavior was found since HV dropped as the applied load increased. Six theoretical models explored in this extensive study are effective in mechanical modeling of the acquired HV values, allowing us to detect the influence of the addition process of nanoparticles on the mechanical features and stability in the (Bi, Pb)-2212 phase. when the results of the models are compared, the IIC model provided considerably closer findings in terms of load-independent microhardness. In addition, the mechanical parameters E, Y, K, and B for three nanoparticle additions appeared to follow the same pattern as the variation of HV values with the content of nanoparticle additions.
Acknowledgments:
This research was accomplished in the Specialized Materials Science Lab and Advanced Nanomaterials Research Lab, Physics Department, Faculty of Science, Beirut Arab University, Lebanon, in cooperation with the superconductivity and metallic-glass lab, Faculty of Science, Alexandria University, Egypt.
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Table.1 XRD parameters, oxygen content (y), and effective valence of the Cu ions (Cueff) of (Bi, Pb)-2212 added by nano-CdO, nano-CdMnO, and nano-CdFeO
|
Table.1 |
||||||||||
|
x |
a (Å) |
b (Å) |
c (Å) |
c/a |
OD% |
y |
Cueff |
|
d |
z |
|
0.00 |
5.416 |
5.388 |
30.861 |
5.698 |
0.521 |
8.088 |
2.238 |
0.011 |
51.923 |
12.111 |
|
CdO |
|
|
|
|
|
|
|
|
|
|
|
0.01 0.02 0.05 0.10 |
5.417 5.418 5.418 5.419 |
5.383 5.382 5.378 5.374 |
30.845 30.847 30.807 30.844 |
5.694 5.693 5.692 5.691 |
0.623 0.672 0.631 0.645 |
8.113 8.194 8.213 8.273 |
2.262 2.343 2.363 2.423 |
0.014 0.024 0.026 0.033 |
45.900 35.212 33.643 29.831 |
12.113 12.115 12.115 12.117 |
|
CdMnO |
|
|
|
|
|
|
|
|
|
|
|
0.01 0.02 0.05 0.10 |
5.417 5.417 5.418 5.419 |
5.384 5.382 5.381 5.378 |
30.842 30.840 30.821 30.804 |
5.693 5.693 5.688 5.684 |
0.612 0.669 0.818 0.856 |
8.172 8.207 8.265 8.308 |
2.321 2.337 2.414 2.457 |
0.021 0.025 0.032 0.037 |
37.339 34.109 30.258 28.144 |
12.113 12.113 12.115 12.117 |
|
CdFeO |
|
|
|
|
|
|
|
|
|
|
|
0.01 0.02 0.05 0.10 |
5.417 5.419 5.419 5.421 |
5.386 5.382 5.381 5.378 |
30.839 30.812 30.779 30.772 |
5.693 5.686 5.680 5.676 |
0.572 0.683 0.701 0.793 |
8.142 8.213 8.238 8.331 |
2.291 2.361 2.387 2.481 |
0.017 0.026 0.029 0.040 |
41.019 33.650 31.882 27.197 |
12.113 12.117 12.117 12.122 |
Table 2 weight loss (%), weight gain (%) and the activation energies for oxygen diffusion of (Bi, Pb)-2212 added by nano-CdO, nano-CdMnO, and nano-CdFeO with x = 0.00, 0.05 and 0.10 wt.%
|
Table 2 |
|||||
|
x |
ΔT (oC) |
weight loss (w%) |
ΔT (oC) |
weight gain (w%) |
E ( kJ mol-1 ) (750-890 oC) |
|
0.00 |
40-568 |
1.515 |
568-593 |
-0.0679 |
5.508 |
|
750-890 |
1.6303 |
||||
|
CdO |
|
|
|
|
|
|
0.05 |
42-568 |
1.154 |
568-592 |
-0.0680 |
8.080 |
|
750-890 |
1.6405 |
||||
|
|
|
|
|
|
|
|
0.10 |
44-568 |
2.514 |
568-592 |
-0.0682 |
8.466 |
|
750-890 |
1.6407 |
||||
|
CdMnO |
|
|
|
|
|
|
0.05 |
52-570 |
1.722 |
568-592 |
-0.0685 |
9.798 |
|
750-890 |
1.6409 |
||||
|
|
|
|
|
|
|
|
0.10 |
56-567 |
2.790 |
567-592 |
-0.0687 |
11.729 |
|
750-890 |
1.6501 |
||||
|
CdFeO |
|
|
|
|
|
|
0.05 |
44-570 |
0.731 |
570-592 |
-0.0688 |
7.007 |
|
750-890 |
1.6502 |
||||
|
|
|
|
|
|
|
|
0.10 |
44-567 |
1.015 |
567-592 |
-0.0689 |
9.749 |
|
750-890 |
1.6504 |
||||
Table 3 Fitted parameters estimated from several theoretical models for (Bi, Pb)-2212 added by CdO, CdMnO, and CdFeO nanoparticles, with x = 0.00, 0.01, 0.02, 0.05, and 0.10 wt.%
Table 4 Load-independent microhardness parameters in saturation regions were compared using five theoretical models and related mechanical parameters E, Y, K, and B for (Bi, Pb)-2212 added by CdO, CdMnO, and CdFeO nanoparticles with x = 0.00, 0.01, 0.05, and 0.10 wt.%
No competing interests reported.