## 3 Results and Discussion
### Evaluation of Electro-Thermal Performance  
Figure 6a shows the resistance of the carbon fibers decreased from 2.4 \(\Omega\) to 1.9 \(\Omega\) as the sintering temperature increased from 1200 \({}^{\circ}\)C to 1350 \({}^{\circ}\)C. The resistance could be related to the graphitization degree of carbon fibers with a turbostatic structure, which may be influenced by the carbonization temperature [36; 37]. In addition, the densification of the ceramic composites may further compact the carbon fibers inside, which could contribute to the increased conductivity of the ceramic composites obtained at higher temperature. The as-prepared composite can be used as an electric heater to achieve electricity and heat conversion. All four composites showed similar curves of temperature vs. time (Figure 6b), where the temperature of the composites increased as the heating time was prolonged. For Composite-1350, the temperature increased quickly within the first minute, and reached 100 \({}^{\circ}\)C after 15 s and 200 \({}^{\circ}\)C after 90 s at 5 V (Figure 6b). Then, the heating rate decreased gradually and reached a stabilized temperature after 600 s (Figure 6b). It was also observed that the composite sintered at a higher temperature showed better joule heating performance. The highest temperature achieved by Composite-1250 was 241 \({}^{\circ}\)C, which increased to 292 \({}^{\circ}\)C for Composite-1350. Higher voltage led to a faster temperature rising rate and higher stabilized temperature. Figure 6c shows that the temperature of Composite-1350 increased from 57 \({}^{\circ}\)C to 292 \({}^{\circ}\)C when the voltage increased from 2 V to 5 V. A good linear correlation (R\({}^{2}\) = 0.99979) between the temperature and U\({}^{2}\) was observed in Figure 6d.  
The heat stability of the ceramic composite was investigated by performing several power on/power off cycles. In each cycle, after the ceramic composite reached the stabilized temperature, the power was turned off until the ceramic composite cooled down to room temperature. As shown in Figure 7a, the ceramic composite reached 56 \({}^{\circ}\)C at 2 V and then reached 292 \({}^{\circ}\)C at 5 V. After three cycles, the temperature rise/drop curve and stabilized temperature was maintained at 5 V, which indicates that the composite had good heating stability performance. The heat distribution of the composites are shown in Figure 7b. When the current passed through the carbon fibers, the composite surface above the carbon fibers showed a higher temperature than the edges. The surface temperature increased with an increase in the power-on time. When the power was turned off, the heat spread from the center to the edges. Finally, during the cooling down process, the heat evenly distributed on the surface of the composite at 26 \({}^{\circ}\)C. The ceramic composite was also used as a heating element to heat 50 mL of water under 12 V (Figure 7c). The water in the beaker boiled after 15 minutes. The ceramic composite showed a good joule heating performance, which could be used as a water heater.  
Figure 5: Scanning electron microscope (SEM) images of the specimen cross sections fired at (**a**,**b**) 1200 \({}^{\circ}\)C and (**c**,**d**) 1300 \({}^{\circ}\)C.