Contents I. Introduction
Since carbon nanotubes (CNTs) were discovered, their magnificent properties have made them recognized as valuable carbon materials for electronics, nanotechnology and other fields of material sciences and technologies. Some emerging applications of CNTs, however, require improvement of their properties to whole new levels. For instance, their applications to electric heaters and field emitters need CNTs with good oxidation endurance at high temperature [1], [2]. Rather than finding a way to synthesize CNTs with perfect crystalline structure, the other convenient ways have been pursued to improve their oxidation resistance. The first is to cover CNTs with oxidation-resistant materials, e.g., SiC, and the second is to chemically poison active sites of CNTs with suitable inhibitors. Other previous studies have reported that boron and phosphorus compounds worked as excellent inhibitors in the case of oxidation of graphite or carbon/carbon composites [3], [4]. Their uses for CNTs to improve their oxidation endurance have not yet been studied. Here, we not only studied the effect of boron and phosphorous compounds on the thermal stability of CNTs but also applied the CNTs treated with these compounds to the CNT pastes for electron sources of x-ray tubes. As of now, x-ray tubes are often fabricated by brazing all components including CNT emitters under vacuum at high temperature. Our conventional CNT paste emitters showed serious degradation of field emission properties probably due to reaction of CNTs with fillers during vacuum brazing at 830°C [5]. Addition of boron and phosphorus compounds, however, was expected to improve thermal stability of CNTs and to reduce degradation of field emission characteristics of CNT paste emitters upon exposure to vacuum brazing.
