激光烧蚀聚甲醛的热-化学耦合模型及其验证

针对激光辐照聚甲醛的烧蚀现象,建立了一种包含升温、相变、热解及热解产物飞散等过程的热-化学耦合模型.采用无规热解模型描述聚甲醛升温后的热解过程,给出了不同热解率下热解产物的组成.利用基团贡献法计算烧蚀产物组分的热力学性质,并按照混合法则确定烧蚀产物混合物的名义标准沸点和临界温度.当烧蚀产物温度低于临界温度时,以液态蒸发机理表征热解产物的烧蚀,用Knudsen层关系式计算烧蚀质量;反之热解产物飞散由气体动力学机理控制,采用间断守恒关系及Jouguet条件描述烧蚀进程.本模型可给出激光辐照下聚甲醛的烧蚀质量、烧蚀温度、烧蚀产物组成和不同机理的烧蚀比率.与实验结果对比表明,当激光能量密度小于30 J/cm2时本模型能准确地描述烧蚀过程.

Thermal-chemical coupling model of laser induced ablation on polyoxymethylene

Polyoxymethylene (POM) is a good absorber of CO₂ laser, so it is important to study the ablation mechanisms of polymer materials. Because the laser impact phenomena are terribly complex, there is no general understanding of the mechanism of laser induced ablation of POM. An explicit thermal-chemical coupling model is presented in this paper, which takes account of laser heating, phase transition, thermal degradation, and plume emission. Random thermal degradation is adopted to describe the chemical reaction process when POM is heated up, and consequently, the components of the degradation products under different degradation rates are acquired. The group contribution method is used to evaluate the thermodynamic properties of the degradation products, and the normal boiling point and critical temperature of the product mixture are obtained by the mixing law. If the product temperature is lower than the critical temperature, POM is ablated in the manner of liquid evaporation; otherwise the ablation mechanism is gas-dynamics emission. As for the former, Knudsen layer relationship is employed to calculate the ablation mass; and for the latter, the conservation laws associated with the Jouguet condition are used. Based on the model, the quantitative results of ablation mass, ablation temperature, product component and mass rate of different ablation mechanisms vs. laser fluence are achieved and analyzed, which are consistent with the experimental data quite well.
Keywords： laser ablation polyoxymethylene thermal-chemical coupling critical temperature