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Published in final edited form as: IEEE Trans Appl Supercond. 2019 Mar 15;29(5):6802205. doi: 10.1109/tasc.2019.2905436

Study of Superconducting, Structural, and Thermal Properties of SnO2 Added MgB2 Bulks

Danlu Zhang 1, Mike D Sumption 1, Edward William Collings 1, Chee J Thong 1, Matthew A Rindfleisch 1
PMCID: PMC7265115  NIHMSID: NIHMS1591714  PMID: 32489243

Abstract

A series of SnO2 added MgB2 bulk superconductors were prepared by in situ route to study the effect of oxygen doping on superconducting and structural properties of MgB2. Several (MgB2)1-x(SnO2)x samples were fabricated with x ranging from 0, 3 wt%, 4 wt%, and 6 wt%. Upper critical field (BC2) and irreversible field (Birr) were measured by physical property measurement system. Thermal analysis was performed on the as-received SnO2 powder. Critical current densities (Jcm) were obtained at 4.2 K using magnetic measurement. X-ray diffraction results showed evidence of full SnO2 decomposition in all the doped bulk samples and a shift of a-axis in MgB2 lattice was seen. Oxygen was successfully released during heat treatment, yet no enhancement of BC2 or Birr was seen, indicating that oxygen atoms did not end up in the host lattice. Further exploration of different processing procedures is still needed in order to get oxygen substitution on the host lattice sites.

Index Terms—: Irreversible field, MgB2 bulks, oxygen doping, upper critical field

I. Introduction

DISCOVERED in 2001 as a superconductor with a transition temperature (Tc) of 39 K, magnesium diboride (MgB2) has stimulated interest in its various physical properties, such as: critical current density (Jc) [1]–[3], irreversibility field (Birr) [4] upper critical field [5], [6], heat capacity [7], [8], and band structure [9]–[11].

The practical application of MgB2 has been limited by its low Jc in high magnetic fields and a low BC2. For example, clean MgB2 wires were reported to have a BC2(0 K) of only 16 T [12]. A kind of “self-doping” can be achieved by varying the starting powder stoichiometries. Thus nominally Mg deficient samples have been seen to exhibit higher Bc2 (and Birr) than those prepared with excess Mg [13]. In particular, Chen et al. found BC2 (4.2 K) of ~21 T in Mg-deficient samples compared to ~17 T in near stoichiometric ones [13]. This suggests that Mg deficiency could be taken advantage of in the quest for higher BC2.

Numerous techniques have been employed in attempts to enhance the superconducting properties of MgB2: high pressure sintering [14], [15], proton irradiation [16], and the introduction of dopants. As reported in [17] many kinds of dopant species have been introduced: nitrides, borides, and silicides, carbon and carbon inorganics, metal oxides, metallic elements, and organic compounds. One of the most successful dopants to date is C, which has been introduced as SiC [1] or carbohydrates [18], and compounds [19] that release C under decomposition during reaction heat treatment. Over the 17 years since the discovery of superconductivity in MgB2, the best BC2 (20–30 K) in wires has always been found in C-doped material. For example, the highest BC2 (20 K) seen so far in C-doped bulk samples (14 T) was produced by 3.8% C [12]. But there is a continuing need for further increasing BC2 in the medium temperature range by the introduction of other dopants; oxygen is a prospective candidate.

Oxygen was first seen to increase the superconducting properties (Jc and Birr) of MgB2 in PLD-deposited thin films. Patnaik et al. [20] showed that oxygen incorporation in the thin films led to a 2 T increase in BC2 in the temperature range 20 K-30 K. Haruta et al. [21] (see also Mori et al. [22]) noted increases in both BC2 and Jc after producing films by electron beam evaporation. Singh et al. [23] produced films by electron beam epitaxy followed by in situ post annealing in oxygen. The films were characterized by high J (~4 × 105 A/cm2 at 8 T, 4.2 K), high extrapolated BC2 (>45 T) and high slope of BC2 (T). In response to oxygen doping they discovered a stronger BC2 enhancement in ab plane than in the c plane.

However, the idea of transferring oxygen doping from MgB2 films into MgB2 bulk samples was not applied until 2011 when Dou et al. [24] carried out a systematic study of oxygen doping in MgB2 bulks using Sb2O3 as a carrier. They pointed out that the decomposition temperature of Sb2O3 (655 °C) is close to the melting point of Mg (650 °C) so that during heat treatment of the mixed and compacted powders at 800 °C, a reaction similar to Mg + Sb2O3 → Mg3Sb2 + O2 was expected to take place. Burdusel et al. [25] produced very dense Sb2O3 added MgB2 samples by spark plasma sintering and increases (compared to the pristine material) in both Birr and Jc have been seen. Even though some rare earth oxides [26], [27] have been added to MgB2 bulk samples, none of them showed evidence of oxygen doping. Our group has added 5 wt% SnO2 via ex situ heat treatment approach and significantly lower critical fields were seen [28].

The early observations of critical field changes in response to oxygen doping were made somewhat uncontrollably on MgB2 films [19]–[22]. Given that practical applications of MgB2 require it to be in tape or/and wire form, it is important to direct further attention to improving the critical field properties of in situ prepared bulk samples. In this study, we aimed to control the amount of oxygen in MgB2 bulks with the aid of SnO2 addition. Besides, thermal behavior of the as-received SnO2 powder and the subsequent thermal effect on SnO2 added samples were analyzed. Superconducting properties such as BC2, Birr, Jc are also studied in this work.

II. Experimental

A. Materials Processing

Four samples were made via the in situ processing route in order to study the effect of oxygen doping levels on the superconducting and structural properties in SnO2 added MgB2 bulk samples. Four bulk samples were made by first mixing Mg powder (99.8 % pure, 325 mesh, Alfa Aesar) and PVZ-B powder (99 % pure, ≤3 μm) stoichiometrically, the details are listed in Table I. Then, different amounts of SnO2 (Alfa Aesar, <500 nm) powder were added to the mixture per Table I. Mixture of the powder was transferred to a container in a glove box, the container was subsequently sealed and placed into a mixer mill to have the powder mixed homogeneously. The mixed-and-milled powder mixture was placed into a steel pellet press and a load of 27.6 MPa was applied on the press. The pellets were finally heat treated under flowing Ar (99.99% pure) atmosphere in a furnace at 700 °C for 30 minutes, followed by furnace cooling to room temperature.

Table I.

Fabrication Information OF Four Bulk Samples (the Raw Powder Ratio of Mg: B is 1: 2)

Sample name The weight ratio of SnO2 to the total amount Heat treatment temperature (°C) Heat treatment dwelling time (min)
IS0 0 wt% 700 30
IS3 3 wt% 700 30
IS4 4 wt% 700 30
IS6 6 wt% 700 30
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B. Property Measurements

Resistivity (under AC transport mode) measurements were carried out for each sample as a function of temperature in magnetic fields from 0 T to 14 T using Quantum Design Model-6000 physical property measurement system (PPMS). Pellet samples were carefully cut into small cubes and mounted onto the pucks where four point measurements were performed on. BC2 (T) and Birr (T) data was extracted respectively based on 90% and 10% of the normal state resistivity rules for each sample. Magnetic measurements on the bulk samples were taken by the same PPMS system. The pellets were cut into rectangular prisms and critical current densities of these samples were derived magnetically from Bean’s model using magnetization-magnetic field (M-H) data.

Differential scanning calorimetry (DSC) measurement was performed on the as-received nano SnO2 powder. About 20 mg powder was placed into the small container inside the DSC instrument. Heat flow was recorded as the temperature was ramped up from room temperature to 700 °C in an Ar flowing atmosphere.

Phases, peak locations and intensities of the samples were characterized by powder X-ray diffraction (XRD) measurements on Rigaku MiniFlex 600 instrument. XRD patterns were scanned continuously from 20 degrees to 80 degrees with a rate of 5 deg/min. Phases in the pellets were identified and lattice parameters of MgB2 were calculated based on the data.

III. Results and Discussions

A. Thermal Analysis

Bohnenstiehl [29] performed thermal analysis of the MgB2 reaction, and concluded that the formation of MgB2 usually finishes around the melting point of Mg (650 °C) due to the partial pressure created by Mg. The DSC result of the nano SnO2 powder used in the experiment is shown in Fig. 1. Upon heating, the SnO2 powder started to decompose around 630 °C with a peak around 650 °C. Since 630 °C is lower than the melting point of Mg (650 °C), O2 should be released from SnO2 before or during the formation of MgB2. Knowing this, we intended to control the oxygen doping level in the MgB2 bulk samples by tuning the amount of added SnO2.

Fig. 1.

Fig. 1.

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Heat flow curve measured in the DSC device (with a heating rate of 10 K/min) for the nano-SnO2 powder. There is a characteristic change of the maximum position of the peak and the onset temperature of the decomposition of SnO2.

B. Superconducting Properties

Shown in Fig. 2 is the result of Birr and BC2 of all the four samples as a function of temperature. Little to no changes of Birr was seen in the doped samples. As the addition level increases, Birr values of the SnO2 added samples only slightly increased and started to decrease at higher doping levels. As can be seen, the SnO2 addition also had little to no effect on BC2. In fact, the sample with 6 wt% SnO addition has a slightly decreased BC2 value.

Fig. 2.

Fig. 2.

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Temperature dependent Birr and BC2 for SnO2 added samples and pure sample.

Tc was increased moderately by adding more SnO2, which is due to the fact that the starting powder ratio of Mg to B is 1:2. The pristine sample possesses Mg atomic vacancies after getting heat treated at high temperatures. In contrast, the increased atomic occupancy introduced by SnO2 additions in the doped sample leads to a better crystallinity and a higher Tc. To determine exactly where the oxygen atoms or SnO2 additions are in the SnO2 added bulk samples, further structural analysis is needed.

C. Magnetic Properties

Critical current density Jcm (A/cm2) analysis has been performed on the bulk samples based on Bean’s model using magnetic measurements. Fig. 3 shows Jcm for a series of magnetic fields at 4.2 K. There is a decrease trend with the increase of added SnO2 in the bulk samples. Effective oxygen doping means the successful substitution of boron atoms by oxygen atoms, which is supposed to increase electron scattering and give rise to an increase of BC2 as well as an increase of Jc. However, due to the fact that the SnO2 added bulk sample has lower Jc values as well as unchanged BC2 values. It is suspected most of oxygen atoms reacted with Mg and formed impurity phase form, which can be confirmed by Fig. 4 (Boron is really light and usually not well shown in EDS maps). From Fig. 4, one can see that the sample measured was a relatively large piece. Sn atoms and O atoms are distributed mostly along the contour of Mg clusters, they are not well distributed over or in Mg clusters. Besides, Sn and O atoms are very closer together. MgO was almost inevitable during MgB2 reaction due to the high driving force for Mg oxidation reaction and great stability of MgO. In the SnO2 added MgB2 samples, O atoms react with Mg readily and formed MgO, which can be confirmed by both the EDS study and the structural analysis below.

Fig. 3.

Fig. 3.

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Magnetic critical current densities of the pure sample as well as SnO2 added MgB2 bulk samples.

Fig. 4.

Fig. 4.

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EDS image of doped sample IS4.

D. Structural Properties

To understand the structural changes with the addition of SnO2, powder XRD measurements were used. Fig. 5 includes the XRD patterns for samples IS0, IS3, IS4 and IS6. The most intense peak (2 Theta = 42 degrees) of MgB2 phase does not shift with the addition of SnO2. MgB2 peak that lies near 60 degrees shifts significantly with the addition of SnO2. This peak shift could be a result of either lattice strain or substitution of foreign atoms on the atomic sites of the host lattice. It is worth noting that no SnO2 was seen in any XRD curve, which means all the added SnO2 was consumed and oxygen was released during decomposition of SnO2. As can be seen from the graph, second phase, Mg2Sn, showed up in all the doped samples. The amount of Mg2Sn increased with the increasing addition levels of SnO2. Normally, the impurity phases tend to segregate along the grain boundaries because of the favored free energy. The segregation of the impurity phases negatively influences connectivity and Jc. Furthermore, the amount of MgO phase enlarges with the increase of SnO2 addition and may act in a similar way.

Fig. 5.

Fig. 5.

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XRD patterns of IS0, IS3, IS4 and IS6.

With the list of peaks derived from PDXL software, the lattice parameters of MgB2 phase were calculated manually based on Bragg’s law. The lattice parameters were listed in Table II, and only a small change in a-axis lattice parameter was seen. Similar to C-doped MgB2 bulk samples [30], c-axis lengths in added MgB2 of samples were barely changed and a-axis was decreased with the incorporation of C atoms on the B atomic sites in MgB2 when foreign atoms diffuse into MgB2 host lattices, the foreign atom can be a substitute or an institute in the host lattice, which can result in lattice expansion or shrinkage. However, in order to truly show a successful atomic substitution, TEM analysis is always needed. From the XRD results combined with the magnetic critical current transport analysis shown above, we can tentatively make the conclusion that shift of a-axis here is more likely due to the strain effect generated by oxygen incorporation in MgB2 materials, accompanied by small lattice distortions of secondary phase formation. Considering the fact that upper critical fields or irreversible fields were barely changed by adding SnO2, we postulate that the oxygen doping in this case primarily generates oxygen atoms that sit on grain boundaries, rather than controls the electron scattering processes that tune BC2 or Birr.

TABLE II.

Lattice Parameter of Samples IS0, IS3, IS4, IS6 as Well as Values for BC2 (20 K) and Birr (20 K)

Sample a(Å) c(Å) Bc2 [T] Birr [T]
IS0 3.091(19) 3.529(5) 11.62 8.66
IS3 3.085(13) 3.522(3) 10.91 8.52
IS4 3.084(13) 3.522(3) 11.18 8.32
IS6 3.086(20) 3.523(5) 10.91 8.31
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IV. Conclusion

SnO2 has been added to MgB2 bulk samples in order to study the effect of oxygen doping on critical fields via in situ synthesis approach. Various addition levels of SnO2 were explored and SnO2 was seen to fully decompose during heat treatment of 700 °C for 30 minutes. No increase of BC2 or Birr has been seen in the doped samples. An MgB2 lattice shift was seen in SnO2 added samples. This study shows the possibility of controlling oxygen injection into MgB2 bulk samples. Although the superconducting properties in this set of measurements were not improved and the majority of oxygen appeared to segregate at grain boundaries in MgB2. Having developed a method to supply O in a controlled way into bulk samples, we need to see if oxygen atoms can be injected into the host lattice, rather than introducing second phase segregation.

Acknowledgments

This work was supported by the National Institute of Biomedical Imaging and Bioengineering under Grant R01EB018363.

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