Preparation and Superconducting Properties of NbN Filaments Formed on Carbon Fibers

NbN的制备通常通过反应溅射进行，并控制N2气体分压（<P_N2>）。在本研究中，除了控制<P_N2>外，还控制了< n2 >气体流量（<Gf_N2>）和Ar气体流量（<Gf_Ar>），并明确了等离子体温度（Tp）与放电电流（<I_p>）的关系。从而获得了b型NbN的制备条件。在<P_N2> =2.4mTorr, <Gf_N2> =2.5cc/min, <Gf_(Ar)> =4.0cc/min, <I_p> =350 ~ 600ma的条件下，利用两个相对的nb板靶，采用直流反应磁控溅射技术将NbN沉积在碳纤维（纤维直径7 <micro> m, Toray, Japan）上。x射线衍射分析表明，这些样品具有b型晶体结构。用扫描电镜观察了nbn涂层碳纤维细丝的形貌。研究了超导转变温度（Tc）与<I_p>的关系。在<P_N2> =2.2mTorr和<Gf_N2> =2.5cc/min条件下，Tc随< p_p >的增加而增加。在<I_p> =500-600mA区域，碳纤维表面形成Tc（中点）=17K的bl型NbN膜。碳纤维表面NbN膜的最高温度为17K。在较高的<I_p>条件下制备的样品，临界电流变大。这一结果与高Tc的bl型NbN在高<I_p>形成相对应。在4.2K下得到了临界电流密度（Jc）与横向外磁场（Hex）的关系。例如，在Hex=0时，Jc=9x <10^4> A/ <cm^2>，或在Hex=10T时，Jc=4x <10^4> A/ <cm^2>。这些Jc值约为沉积在蓝宝石衬底上的NbN薄膜的十分之一。这一结果的来源被认为是由于样品的不均匀性和微观结构。发现b型NbN在等离子体中有N2（激发态N2分子）存在的情况下合成。

Preparation of NbN is usually carried out by reactive sputtering with controlling the N2 gas partial pressure( <P_N2> ). In the present investigation, besides <P_N2> , the <N_2> gas flow-rate( <Gf_N2> ) and the Ar gas flow-rate( <Gf_Ar> ) were controlled, and also the relationship between the plasma temperature(Tp) and the discharge current( <I_p> ) was clarified. In this way, the preparation condition of Bl-type NbN was obtained.NbN was deposited onto carbon fibers(fiber diameter of 7 <micro> m, Toray, Japan) by DC reactive magnetron sputtering with two opposed Nb-plate targets, under <P_N2> =2.4mTorr, <Gf_N2> =2.5cc/min, <Gf_(Ar)> =4.0cc/min, and <I_p> =350-600mA. X-ray diffraction analysis showed that these samples have the Bl-type crystal structure. Morphology of NbN-coated carbon fiber filaments was observed by a scanning electron microscope. The dependence of the superconducting transition temperature(Tc) on <I_p> was investigated. Under <P_N2> =2.2mTorr and <Gf_N2> =2.5cc/min, it was found that Tc increases with increasing <I_p> . In the region of <I_p> =500-600mA, Bl-type NbN films with Tc(middle point)=17K were formed on carbon fibers. A Tc of 17K is the highest Tc in NbN films deposited onto carbon fibers.The critical current became larger for the samples prepared at higher <I_p> . This result corresponds to the fact that Bl-type NbN with higher Tc was formed at higher <I_p> . The dependence of the critical current density(Jc) on the transverse external magnetic field(Hex) was obtained at 4.2K. For example, Jc=9x <10^4> A/ <cm^2> at Hex=0, or Jc=4x <10^4> A/ <cm^2> at Hex=10T. These Jc values are about one tenth of those of NbN films deposited onto sapphire substrates. The origin of this result is thought to be due to inhomogeneity and microstructure in the samples. It was found that Bl-type NbN was synthesized in the presence of N2 (excited N2 molecule) in a plasma.