Elsevier

Physica C: Superconductivity

Volume 480, October 2012, Pages 102-107
Physica C: Superconductivity

Use of preceramic polymers for magnesium diboride composites

https://doi.org/10.1016/j.physc.2012.03.052Get rights and content

Abstract

We used preceramic polysiloxane polymers to fabricate superconducting MgB2 composites that are doped with carbon and nanosized inclusions to improve the pinning properties. The polysiloxanes were prepared by atom transfer radical polymerization and the composites were fabricated by the short time spark plasma sintering method. We found that the superconducting critical temperatures were higher than expected from the carbon content found from the X-ray diffraction analysis of the (1 1 0) peak of MgB2. To explain this finding we propose that the grains are unevenly doped, with a core–shell distribution. We also found that both, the upper critical fields and the critical current densities are higher in the preceramic-doped samples than in pure MgB2, in agreement with the carbon doping level. When ferrocene-grafted polysiloxane is used, the upper critical field is the largest, while the critical current density is the lowest. We attribute this fact to the fact that the polymer pyrolysis results in iron-based nanostructures which have a pair breaking effect.

Highlights

► We used polysiloxane copolymers for improving transport properties of MgB2. ► Polysiloxane-co-vinyl-ferrocene was used to insert magnetic nanoparticles. ► Cyclic polysiloxane-co-styrene leads to the best critical properties. ► Magnetic pinning is visible at high temperatures in samples with ferrocene.

Introduction

Preceramic polymers are inorganic polymers that can be converted to ceramic by thermolysis. Silicon based polymers are the most used preceramic polymers due to their high processability combined with a good compositional control [1], [2], [3], [4], [5], [6], [7]. Among them, polysiloxanes, i.e., polymeric structures that contains repeating silicon–oxygen groups in the backbone, side chains or cross links are the most common. They have a high versatility allowing changes of the –Si–O– chain lengths, side groups, and cross linkage. For example, the most simple polysiloxane have the repeat unit [R2SiO] with R ranging from hydrophobic alkyl groups to phenyl or vinyl groups. Most interesting, transition metals atoms can be incorporated into the polymer main chain, which can render unique electric and magnetic properties, promising for applications. The most frequently used unit is ferrocene due to the outstanding nucleophilicity of the cyclopentadiene rings. During thermolysis, the Si–R bonds of the preceramic polymer break and the volatile small molecules are released. At even higher temperatures, the carbonaceous and the inorganic residues, depending on the polymer composition, transform into an inorganic amorphous hydrogenated solid a-SiOCH [8], [9] built on tetrahedral structures of the type Si(OaCb) with a + b = 4. Above 800 °C, clusters of excessive carbon nucleate as stack of a few polyaromatic layers (graphene) called structural units [10]. The pyrolysis process occurs simultaneously with the process of densification of the MgB2. In this process, MgB2 seems to reduce the hydrogen content which hinders the formation of the basic structural units which, further, precipitates as clusters of turbostratic carbon. As we mentioned, the process is important in polymers with complex architectures where the release of the volatile species is hindered. The small amount of turbostratic carbon is one of the sources of carbon doping and, what is not consumed for boron substitution can constitute valuable nanosized pinning center [11].

Here we use of the last steps of the pyrolysis process of polysiloxanes and poly(siloxane-co-ferrocenes) to dope MgB2 with carbon and with efficient pinning sites with the goal of improving the self-field critical current density with respect to the depairing current.

In general, one can follow two approaches to increase the critical current density. As it is known, the critical current density depends on both upper critical field Bc2 [12] and the particular pinning energy. Both quantities are rather modest in the case of MgB2. The upper critical field is around 16 T in zero magnetic field, while the pinning is mainly controlled by the grain borders. Fortunately, the Fermi surface of MgB2 consists of two types of almost independent bands, σ and π, that permits the control and improvement of Bc2 by appropriate chemical doping without severe changes in the critical temperature [13], [14]. The dopant which best complies with these requirements is carbon which substitutes boron. It does not induce out of plane distortions, hence, the σ–π interband scattering is not enhanced [15] and only the effect of band filling might be responsible for the slight decrease of Tc [16]. Thus, the search for the best methods to substitute carbon for boron attracted a lot of interest, with hundreds of reports claiming more or less spectacular enhancement of Bc2. The best results have been obtained using either carbides, like SiC [17], [18], [19], [20], [21], or B4C [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], nanodiamond [25], [26], carbon nanotubes [22], [27], [28], [29], [30], and, most recently, some organic compounds like carbohydrates [31], [32].

The second way to improve the transport properties of MgB2 is to enhance the pinning strength by creating more effective pinning centers. Chemical doping, addition of nanosized inorganic compounds, and neutron irradiation [33] were the main methods used for insertion of artificial pinning centers or pinning-effective disorder.

In this work we consider the polymeric route, namely, polysiloxanes, as a way to pursue both routes. The pyrolysis of these polymers provides the carbon for doping while the carbon-based nanosized compounds provide the valuable pinning centers. This method has also the advantage of controlling the ratio C/Si by appropriate copolymerization. Moreover, it allows for insertion of metallic nanoparticles if metallocenes are linked into the polymer structure.

Section snippets

Experimental

We fabricated carbon-doped MgB2 ceramic samples by spark plasma sintering (SPS) technique starting from a mixture of polysiloxanes and reagent grade MgB2. To this goal, we used three types of polysiloxane-based copolymers. Two of them were grafted with styrene in a linear (l-polysiloxane-co-styrene) (L-PSS) and in a cyclic (c-polysiloxane-co-styrene) (C-PSS) architecture. The third polymer was a linear polysiloxane-co-styrene on which vinyl-ferrocene was grafted (PSVF). The three copolymers

Structure and morphology

X-ray diffraction data of all samples, reference sample included, show a dominant MgB2 phase with traces of MgO and MgB4 (data not shown). Fig. 2 shows the (1 1 0) peaks of the samples. Note the interesting shift of the (1 1 0) peaks of the doped samples to higher angles, which means that the a lattice parameter shrinks (Fig. 2). The reduction of a is due to the partial substitution of carbon for boron and it is directly related to the real amount of carbon that enters the boron sublattice. An

Conclusions

A series of carbon-doped MgB2 superconductors were fabricated using spark plasma sintering. The doping process was the result of the pyrolysis of polysiloxane-based copolymers: cyclic polysiloxane-co-styrene, linear polysiloxane-co-styrene, and linear polysiloxane-co-styrene-vinyl ferrocene. In addition to carbon doping, the polymers were intended to provide pinning centers in order to improve the dissipationless transport of current. One of the polymers was tailored to provide magnetic pinning

Acknowledgments

The research was supported by the Romanian NASR under the Grants PII-72-151/2008 and by the US Naval Academy Research Office at USNA.

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