## 3 Results and Discussion  
### The Effect of Formaldehyde/Phenol Molar Ratio on the Char Yield of B-Containing PR  
Previous studies have shown that the formaldehyde/phenol molar ratio affects the crosslinking density of PR after curing, and then leads to a different char yield. Generally, the larger the crosslinking density in the curing process of PR is, the higher the char yield is [20; 21]. The formaldehyde/phenol molar ratio ranges from 1.0 to 2.0 in traditional preparation of PR, and the largest cross-linking density in the curing process is achieved when the formaldehyde/phenol molar ratio is proper, resulting in the highest char yield [20; 21; 22]. Thus, the influence of formaldehyde/phenol molar ratio within 1.0-2.0 on the char yield of B-containing PR was investigated, while the molar ratio of potassium borate/phenol was kept at 0.2 and reaction time was 13 h. The char yields of B-containing PR obtained with different formaldehyde/phenol molar ratios were evaluated; the results are shown in Figure 1. In addition, as a comparison, the char yields of common PR achieved with different formaldehyde/phenol molar ratios were also fabricated under the same conditions, just without potassium borate. As per Figure 1, the char yields of common PR exhibit an increasing trend, followed by a decrease, as the formaldehyde/phenol molar ratio increases; the values of char yield range from 18% to 25%, and the highest value is about 25% when the formaldehyde/phenol molar ratio is 1.6. On introduction of potassium borate, the char yields of B-containing PR are significantly improved, while the values of char yield increase at first and then decrease, similar to the results of common PR. Notably, the char yields of B-containing PR are between 60% and 71%, and maximum char yield is acquired when the formaldehyde/phenol molar ratio is 1.8. The reason for introduction of the B element in char yield can be concluded in the following sentences: Some of the pristine carbon-carbon bonds in PR  
Figure 1: The effect of formaldehyde/phenol molar ratio on the char yield of B-containing PR and common PR. are replaced by boron-oxygen and boron-carbon bonds, and the chemical binding energies of both (515 kJ/mol for B-O and 393 kJ/mol for B-C) are much higher than that of the carbon-carbon bond (332 kJ/mol for C-C), resulting in higher thermal properties [13; 14; 15; 23]. In addition, the formation of boron-oxygen and boron-carbon bonds further affects the carbon-forming structure and property of PR at high temperatures, shortens the interatomic spacing, increases the cross-linking degree of PR, and improves the compactness of carbon. The Effects of Potassium Borate/Phenol Molar Ratio and Reaction Time on the Char Yield of B-Containing PR  
According to the above results, a higher char yield of B-containing PR was achieved when the formaldehyde/phenol molar ratio was 1.8. We further investigated the effect of potassium borate/phenol molar ratio on the char yield of B-containing PR under this optimal condition. Figure 2 reveals the char yields of B-containing PR obtained with different potassium borate/phenol molar ratios and reaction times. From the results, it is obvious that the char yield of B-containing PR clearly increases first and is then relatively constant with increase of reaction time when the potassium borate/phenol molar ratio is determinate, which should be ascribed to the saturated cross-linking reaction in the system with sufficient reaction time. Moreover, the char yield of B-containing PR under the condition of formaldehyde/phenol = 0.1 is distinctly lower than those of products prepared when the formaldehyde/phenol = 0.2 or 0.4, suggesting that the B element content should not reach the optimal value in the reaction system. When the molar ratio of potassium borate/phenol is 0.2 or 0.4, the B-containing PR possesses a high char yield ranging from 60% to 71%, and char yield increases first and then tends to stabilize with the extension of reaction time.