Effects of carbon in MgB2 thin films: Intrinsic or extrinsic

Abstract

First-principles calculations are performed on MgB 2 , carbon doped MgB 2 , and MgB 2 C 2 . The trend of calculated lattice parameters of MgB 2 with increasing carbon content agrees with bulk experiments but not with thin films produced by hybrid physical-chemical vapor deposition. In this work, the authors propose a model to explain this behavior based on the coefficients of thermal expansion of MgB 2 and MgB 2 C 2 as predicted from first principles and of graphite from literature. It is concluded that the effect of carbon on the lattice parameters of MgB 2 thin films is extrinsic and due to differences of the coefficients of thermal expansion of different phases.

High-field properties of carbon-doped MgB2thin films by hybrid physical–chemical vapor deposition using different carbon sources

“…The intensity of the diffraction spots corresponding to MgB 2 C 2 is stronger in the 33.5% nominal carbon sample (figure 3(d)) than in the less doped sample (figure 3(b)), indicating a higher volume percentage of the MgB 2 C 2 phase in the heavily doped sample. The existence of the MgB 2 C 2 foreign phase in the carbon-doped MgB 2 films is consistent with previously reported carbon-doped MgB 2 samples [21,29] and also agrees with the theoretical calculations [26]. The vertical streaks in figure 3(d) are caused by the small stripes as shown in figure 3(c).…”

“…The foreign phase in the grain boundaries, most likely MgB 2 C 2 [21,26], is highly resistive and will block the current transport and reduce the connectivity between the MgB 2 grains. It will cause an increase in the experimentally measured resistivity and critical current density [27].…”

Structural and thermodynamic properties of compounds in the Mg–B–C system from first-principles calculations

“…To better understand the effect of carbon dopant, it is therefore desirable to know the detailed structural and thermodynamic properties of all the important compounds in the Mg-B-C ternary system. In our previous publication [8], we made an attempt to predict the lattice parameters of carbon-doped MgB 2 thin films produced by hybrid physical-chemical vapor deposition (HPCVD). The prediction was based on the assumption that the elastic deformation of MgB 2 had no effect on the MgB 2 C 2 second phase [8], resulting in an overestimated lattice parameter of the MgB 2 thin films.…”

“…In our previous publication [8], we made an attempt to predict the lattice parameters of carbon-doped MgB 2 thin films produced by hybrid physical-chemical vapor deposition (HPCVD). The prediction was based on the assumption that the elastic deformation of MgB 2 had no effect on the MgB 2 C 2 second phase [8], resulting in an overestimated lattice parameter of the MgB 2 thin films. In the present work, first-principles calculations, based on phonon theory [9] within the framework of density functional theory [10], have been carried out to obtain lattice parameters, elastic constants, enthalpies, entropies, and heat capacities of the compound at finite-temperatures.…”

Fabrication and growth mechanism of MgB2 nanowires during reactive sintering

“…In past years, many groups have put efforts into synthesizing various forms of the MgB 2 superconductor, such as bulk [2,3], thin films [4,5], and wires [6,7]. The influence of different morphology of MgB 2 on the corresponding superconductivity has attracted wide interest recently.…”