## 1 Introduction  
Rapid developments in space science and technology have resulted in the introduction of a large variety of spacecraft. Thermal protection systems (TPSs) that offer excellent performance are, therefore, required to shield spacecraft from high temperatures generated during atmospheric entry in the stagnation region [1; 2; 3]. TPSs can be of two types--non-ablative and ablative systems [1; 4]. Non-ablating TPSs are usually employed in reusable hypersonic vehicles that are not subjected to intense entry conditions. Ablative TPSs, on the other hand, are used in hypersonic vehicles under harsh environmental conditions due to numerous heat dissipation mechanisms, such as phase transition and chemical reaction [1; 5]. Among ablative TPSs, carbon/phenolic systems have attracted much interest, particularly with the closure of the Space Shuttle program and renewed focus on Apollo-style spacecraft [6; 7]. The Phenolic Impregnated Carbon Ablator (PICA) developed at NASA Ames Research Center is a state-of- the-art carbon/phenolic ablator, which exhibits low mass per unit volume and superior ablation property. Research results have confirmed that PICA offers outstanding anti-ablation performance, along with a low-density property (about 0.2-0.4 g/cm\({}^{3}\)) [7; 8; 9; 10].  
PICA is a highly porous fibrous carbon substrate partially impregnated with phenolic resin (PR) that has high thermal stability, excellent ablation resistance, and low cost; this phenolic resin is used as an ablation-resistant matrix [11; 12]. With rapid developments in aerospace technology, PRs with better heat resistance and stronger mechanical properties are required to create advanced ablation-resistant composite materials [13]. The literature reports that introducing boron (B) into the PR molecular structure is an effective way to enhance its thermal and mechanical properties. Various approacheshave been adopted to synthesize B-containing PR, including methods using polyformaldehydehyde and salicyl alcohol, and copolymerization blending [13; 14; 15; 16]. Several process parameters, including reaction time and raw material ratio, influence the properties of final products, especially for organic materials. [PERSON] et al. and co-workers have confirmed that boric acid content plays an important role in the properties of B-containing phenolic fibers prepared by melt-spinning a mixture of novolak resin and boron acid, followed by curing the filaments with formaldehyde solution in the presence of an acid catalyst [17; 18; 19]. Thus, it is reasonable to believe that the critical process parameters strongly affect the properties of B-containing PR before the composites are created. In addition, there is very little research on the surface tension of PR solutions, thus limiting guidance on impregnation of B-containing PR into carbon fiber braids and further preparation of PICA materials.  
The work presented here builds on the creation of B-containing PR through a polyformaldehydehyde method using phenol and formaldehyde with catalyst potassium borate to investigate the effects of the formaldehyde/phenol and potassium borate/phenol molar ratios on the char yield of the B-containing PR. Measurements of wettability and surface energy between carbon fibers and B-containing PR were also performed for the first time. The analysis results can provide useful experimental data for the creation of high-performance PICA materials.