Preparation and Superconducting Properties of NbN Filaments Formed on Carbon Fibers

NbN的制备通常采用控制氮气分压(&lt；P_n2&gt；)的反应溅射方法。在本研究中，除了控制&lt；P_n2&gt；外，还控制了&lt；N_2&gt；气体流量(&lt；GF_n2&gt；)和Ar气体流量(&lt；GF_Ar&gt；)，并阐明了等离子体温度(Tp)与放电电流(&lt；i_p&gt；)之间的关系。用两个相对的Nb板靶，用直流反应磁控溅射法在碳纤维(日本东丽)上沉积NbN，其温度为&lt；P_n2&gt；=2.4mTorr，&lt；GF_n2&gt；=2.5cc/min，&lt；GF_(Ar)&gt；=4.0cc/min，&lt；i_p&gt；=350-600 mA。X-射线衍射分析表明，这些样品具有BL型晶体结构。用扫描电子显微镜观察了NBN包覆碳纤维长丝的形貌。研究了超导转变温度(T_C)与&lt；i_p&gt；的关系。在&lt；P_n2&gt；=2.2mTorr和&lt；GF_n2&gt；=2.5cc/min下，T_c随&lt；i_p&gt；的增加而增大。在~lt；i_p&gt；=500-600 mA范围内，在碳纤维上形成了T_c(中点)=17K的BL型NbN薄膜。在碳纤维上沉积的NbN薄膜中，T_c=17K是最高的T_c，随着温度的升高，样品的临界电流变大。这一结果与BL型NBN是在较高的&lt；i_p&gt；形成的事实相对应的。在4.2K下得到了临界电流密度(Jc)与横向外加磁场(Hex)的关系。例如，在十六进制=0时，JC=9x&lt；10^4；Cm^2&gt；，在十六进制=10T时，Jc=4x&lt；10^4；Cm^2&gt；。这些Jc值大约是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中的不均匀和微观结构。结果表明，在等离子体中有N_2(激发的N_2分子)存在的情况下，合成了BL型NBN。

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.