# Temperature Comparison of Looped and Vertical Carbon Nanotube Fibers during Field Emission
## 2 Experimental Methods  
The CNT fibers were fabricated from concentrated dispersions of CNTs via a wet spinning technique [5]. Prefabricated CNTs were dissolved in a superacid at a concentration of 2-8 wt % and were filtered to remove particles in order to form a spinnable liquid crystal dope. This ordered CNT dope was extruded through a spinneret (65-130 \(\upmu\)m diameter) into a coagulant (acetone or water) to remove the acid in a controlled fashion to produce continuous lengths of macroscopic neat CNT fibers. The spun fibers exhibited very high nanotube packing density and alignment [5]. The fibers were comprised of primarily double walled CNTs with an uncontrolled distribution of chiralities and types (metallic versus semiconductors), with a diameter of \(\sim\)2.4 nm and an aspect ratio of \(\sim\)2600-2800 [29]. [PERSON] et al. have shown that CNT aspect ratio and purity are important for obtaining CNT fiber with good electrical and mechanical properties [29]. Additionally, improved CNT alignment and packing density in the CNT fibers has shown to lead to improved field emission performance and cathode lifetime [3]. The 3 Omega method was used to measure both the thermal (\(\kappa\)) and electrical (\(\sigma\)) conductivities of the fiber samples. This is detailed in Reference [3], however it is described here briefly for completeness. The sample was prepared by placing four silver epoxy contacts (EPO-TEK H20) in-line on a sapphire substrate (approximately 1 mm apart and 1 mm high). The substrate was mounted in a vacuum micro probe station with temperature control (MMR Technology Micro Probe chamber, 1 \(\times\) 10\({}^{-8}\) Torr). The fiber was placed on top of the electrodes to form a 4-point probe electrical conductivity configuration. Due to the temperature coefficient of the resistivity of the fiber, a voltage change is induced when the temperature of the fiber varies with the frequency and the amplitude of the current. Electrical conductivity measurements utilize the 1 st harmonic with the current that is low enough to prevent Joule heating, and thermal conductivity measurement utilizes the 3 rd harmonic with a high current to induce Joule heating [30]. FE measurements were performed in a custom designed ultra-high vacuum (UHV) chamber in a background pressure of 5.0 \(\times\) 10\({}^{-9}\) Torr in a diode configuration, as shown in Figure 3. The CNT fibers were mounted with silver paint between 20 mm \(\times\) 5 mm \(\times\) 1 mm Cu tabs stacked together to make a block sample base. The CNT fiber emitters are prepared in two configurations: a single fiber standing vertically on the substrate, and a fiber loop with both ends attached to the substrate, as shown in Figure 2. To maintain the same height from the Cu cathode base, the CNT fiber used for the looped emitter is cut slightly longer than twice the length of that for the vertical emitter. To form the looped emitter, one end of the longer CNT fiber was mounted on the Cu base with silver paint first, the other end was then secured on the Cu base to have about 2 mm separation between the two ends. In both cases, the average diameter of the fiber is 21.3 \(\upmu\)m, and the height of the emitter is approximately 4 mm. A stainless steel anode probe tip (7 mm diameter) was aligned with the CNT fiber emitter. The anode-cathode substrate gap distance is adjusted for each configuration to keep 3 mA emission current. The single and loop fiber has the gap (from the emitter apex to anode) at 1.5 mm and 2 mm, respectively. A Keithley 2410 sourcemeter was used for providing voltages up to 1 kV and measuring the current. During FE measurements, the voltage was ramped up from 0-1 kV at 1 s per volt.