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毫秒-纳秒组合脉冲激光辐照到熔石英表面,熔石英吸收激光光能,光能转化为热能,在宏观上体现熔石英的温度升高及体积的局部膨胀。假设熔石英是各向同性的连续介质,激光能量的吸收采用体吸收,则激光辐照熔石英的二维轴对称模型如图1所示,图中z为对称轴,
$r$ 为径向方向。入射光束为高斯光束,延z轴垂直入射到熔石英上表面,熔石英的厚度d=4 mm,半径l=12 mm[14-16]。 -
毫秒-纳秒组合脉冲激光辐照熔石英进行能量交换的过程中,激光能量被熔石英所吸收,激光与熔石英相互作用的主要过程是热传导过程,忽略熔石英与外界的对流和辐射效应,可理解为熔石英的热传递方式为固体传热,流体传热与辐射电磁波传热不列入计算模型中[17-20]。结合毫秒脉冲激光与纳秒脉冲激光辐照熔石英的热传导特性,建立了毫秒-纳秒组合脉冲激光辐照熔石英的热传导模型。
当毫秒-纳秒组合脉冲激光辐照熔石英致其温度超过自身熔点,靶材会发生熔融相变物理现象,不包含熔融相变和包含熔融相变的热传导方程可分别写为:
$$\begin{split} frac\partial T\left( {r,{\textit{z}},t} \right)\partial t =& \frac{k}{{\rho c}}\left( {\frac{{{\partial ^2}T\left( {r,{\textit{z}},t} \right)}}{{\partial {r^2}}} + \frac{1}{r}\frac{{\partial T\left( {r,{\textit{z}},t} \right)}}{{\partial r}} + } \right.\\ & \left. {\frac{{{\partial ^2}T\left( {r,{\textit{z}},t} \right)}}{{\partial {{\textit{z}}^2}}} + \frac{{q\left( {r,{\textit{z}},t} \right)}}{k}} \right) \end{split} $$ (1) $$ \begin{split} \frac{{\partial T\left( {r,{\textit{z}},t} \right)}}{{\partial t}} = &\frac{k}{{\rho c}}\left( {\frac{{{\partial ^2}T\left( {r,{\textit{z}},t} \right)}}{{\partial {r^2}}} + \frac{1}{r}\frac{{\partial T\left( {r,{\textit{z}},t} \right)}}{{\partial r}} + } \right.\\ & \left. {\frac{{{\partial ^2}T\left( {r,{\textit{z}},t} \right)}}{{\partial {{\textit{z}}^2}}} + \frac{{\rho c}}{k}L\frac{{\partial {f_s}}}{{\partial t}} + \frac{{q\left( {r,{\textit{z}},t} \right)}}{k}} \right) \end{split} $$ (2) 式中:T(r, z, t)表示在时间t时的温度分布;ρ、c和k分别表示熔石英的密度、比热容和热传导系数;fs和L分别表示材料的固相率和熔融相变潜热。其中固相率对时间的导数可表示如下:
$$\frac{{\partial {f_s}}}{{\partial t}} = \frac{{\partial {f_s}}}{{\partial {T_i}}}\frac{{\partial T}}{{\partial t}}$$ (3) 当固相率fs=0时,熔石英的物理状态为液态;当固相率fs=1时,熔石英的物理状态为固态。由公式(3)可知,固相率对时间的导数为δ函数,该函数在熔石英的熔点附近会出现奇异性变化,这将导致材料计算求解过程中出现不收敛的情况,因此,将近似δ函数的e指数代替了公式(3)中固相率对时间的导数,具体表达式如下:
$$\frac{{\partial {f_s}}}{{\partial T}} = \frac{1}{{\sqrt \pi {\rm d}T}}\exp ( - {(T - {T_m})^2}/{\rm d}{T^2})$$ (4) 式中:Tm表示熔石英材料的熔点。当靶材发生熔融相变时,用等效比热容cp代替c,其二者关系可表示为:
$${c_p} = c - L\frac{{\partial {f_s}}}{{\partial T}}$$ (5) 在毫秒-纳秒组合脉冲激光与熔石英相互作用过程中,熔石英吸收激光能量的热源可表示为:
$$q(r,{\textit{z}},t) = {q_{\rm ms}} + {q_{\rm ns}}$$ (6) $$ \begin{split} q(r,{\textit{z}},t) =& \alpha \left( {1 - R} \right)m\left( {\textit{z}} \right)\left[ {{I_{\rm ms}}{f_{\rm ms}}\left( r \right){g_{\rm ms}}\left( t \right) + } \right.\\ & \left. {{I_{\rm ns}}{f_{\rm ns}}\left( r \right){g_{\rm ns}}\left( t \right)} \right] \end{split} $$ (7) $${f_{\rm ms}}(r) = \exp \left( - \frac{{2{r^2}}}{{{r_{^{\rm ms}}}^2}}\right)$$ (8) $${f_{\rm ns}}(r) = \exp \left( - \frac{{2{r^2}}}{{{r_{^{\rm ns}}}^2}}\right)$$ (9) 式中:q(r, z, t)为毫秒-纳秒组合激光热源;qms为毫秒激光热源;qns为纳秒激光热源;α为熔石英的吸收系数;R为熔石英的反射率;Ims和Ins分别为毫秒脉冲激光和纳秒脉冲激光的辐照中心能量密度;rms和rns分别为毫秒脉冲激光和纳秒脉冲激光的光斑半径;fms(r)和fns(r)分别为毫秒脉冲激光和纳秒脉冲激光的空间分布;r,z分别为轴对称坐标系中的径向和轴向位置。
$${g_{\rm ms}}(t) = \left\{ \begin{array}{l} 1,\Delta {t_{\rm ms}} \leqslant t \leqslant \Delta {t_{\rm ms}} + {\tau _{\rm ms}}\\ 0,t < \Delta {t_{\rm ms}}t > \Delta {t_{\rm ms}} + {\tau _{\rm ms}} \end{array} \right.$$ (10) $${g_{\rm ns}}(t) = \left\{ \begin{array}{l} 1,\Delta {t_{\rm ns}} \leqslant t \leqslant \Delta {t_{\rm ns}} + {\tau _{\rm ns}}\\ 0,t < \Delta {t_{\rm ns}}t > \Delta {t_{\rm ns}} + {\tau _{\rm ns}} \end{array} \right.$$ (11) 式中:gms(t)和gns(t)分别为毫秒脉冲激光和纳秒脉冲激光的时间分布;
$\Delta $ tms为毫秒脉冲激光相对于初始时间的延时;$\Delta $ tns为纳秒脉冲激光相对于初始时间的延时;τms为毫秒脉冲宽度;τns为纳秒脉冲宽度。上下表面边界条件为:
$$ - k\frac{{\partial T(r,{\textit{z}},t)}}{{\partial {\textit{z}}}}\bigg| {_{{\textit{z}} = 0}} = 0$$ (12) $$ - k\frac{{\partial T(r,{\textit{z}},t)}}{{\partial {\textit{z}}}}\bigg| {_{{\textit{z}} = d}} = 0$$ (13) 侧面边界条件为:
$$k\frac{{\partial T(r,{\textit{z}},t)}}{{\partial r}}\bigg| {_{r = l}} = 0$$ (14) 初始条件为:
$$T\left( {r,{\textit{z}},t} \right)\left| {_{t = 0}} \right. = 300\;{\rm K}$$ (15) -
熔石英热应力的产生与温度变化有关,熔石英温度发生改变,其体内任何一个单元的收缩和膨胀都会受到相邻单元的作用,致使其形变无法自由发生,这种束缚作用称为热应力。熔石英应力损伤模型需要联立热弹性平衡微分方程、几何方程及虎克定律,在轴对称坐标系下,与热传导方程相耦合的平衡微分方程可以表示为[21-22]:
$$ {\nabla ^2}{u_r} - \frac{{{u_r}}}{{{r^2}}} + \frac{1}{{1 - 2\mu }}\frac{{\partial \varepsilon }}{{\partial r}} - \frac{{2(1 + \mu )}}{{1 - 2\mu }}\beta \frac{{\partial T}}{{\partial r}} = 0 $$ (16) $$ {\nabla ^2}{u_{\textit{z}}} + \frac{1}{{1 - 2\mu }}\frac{{\partial \varepsilon }}{{\partial {\textit{z}}}} - \frac{{2(1 + \mu )}}{{1 - 2\mu }}\beta \frac{{\partial T}}{{\partial {\textit{z}}}} = 0 $$ (17) $$ \varepsilon = \frac{{\partial {u_r}}}{{\partial r}} + \frac{{{u_r}}}{r} + \frac{{{u_{\textit{z}}}}}{{\partial {\textit{z}}}} $$ (18) 式中:ur、uz分别表示位移在r、z方向上的分量;ɛ、μ、β分别表示材料的体应变、泊松比和热应力系数[23]。文中针对上述弹塑性力学方程,采用初始时刻位移为0、速度为0的初始条件和除底面边界沿z方向位移为0外其余边界都为自由边界的边界条件。
通过径向位移u和轴向位移w可以确定物体的应变状态,几何方程可表示为:
$$\begin{split} &\left\{ \varepsilon \right\} = {\left[ {{\varepsilon _r},{\varepsilon _{\textit{z}}},{\varepsilon _\theta },{\gamma _{{\textit{z}}r}}} \right]^{\rm T}} =\\ & {\left[ {\dfrac{{\partial w}}{{\partial {\textit{z}}}},\dfrac{{\partial u}}{{\partial r}},\dfrac{u}{r},\dfrac{{\partial w}}{{\partial r}} + \dfrac{{\partial u}}{{\partial {\textit{z}}}}} \right]^{\rm T}} \end{split} $$ (19) $${\varepsilon _r} = \dfrac{{\left( {u + \dfrac{{\partial u}}{{\partial r}}dr - u} \right)}}{{dr}} = \frac{{\partial u}}{{\partial r}}$$ (20) $${\varepsilon _{\textit{z}}} = \frac{{\partial w}}{{\partial {\textit{z}}}}$$ (21) 式中:ɛr和ɛz分别表示材料的径向应变与轴向应变。
依据广义虎克定律可得:
$$\left\{ \begin{array}{l} {\varepsilon _r} = \dfrac{1}{E}\left[ {{\sigma _r} - u\left( {{\sigma _\theta } + {\sigma _{\textit{z}}}} \right)} \right] \\ {\varepsilon _{\textit{z}}} = \dfrac{1}{E}\left[ {{\sigma _{\textit{z}}} - u\left( {{\sigma _\theta } + {\sigma _r}} \right)} \right] \\ \end{array} \right.$$ (22) 由应变分量可得应力分量的函数可表示为:
$$\left\{ \begin{array}{l} {\sigma _r} = \dfrac{E}{{\left( {1 + u} \right)\left( {1 - 2u} \right)}}\left[ {\left( {1 - u} \right){\varepsilon _r} + u\left( {{\varepsilon _\theta } + {\varepsilon _{\textit{z}}}} \right)} \right] \\ {\sigma _\theta } = \dfrac{E}{{\left( {1 + u} \right)\left( {1 - 2u} \right)}}\left[ {\left( {1 - u} \right){\varepsilon _\theta } + u\left( {{\varepsilon _r} + {\varepsilon _{\textit{z}}}} \right)} \right] \\ {\sigma _{\textit{z}}} = \dfrac{E}{{\left( {1 + u} \right)\left( {1 - 2u} \right)}}\left[ {\left( {1 - u} \right){\varepsilon _{\textit{z}}} + u\left( {{\varepsilon _\theta } + {\varepsilon _r}} \right)} \right] \\ \end{array} \right.$$ (23) 式中:E表示弹性模量;u表示泊松比。
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基于对毫秒-纳秒组合脉冲激光作用熔石英的理论,采用COMSOL Mutiphysics有限元软件对物理过程进行仿真研究,在仿真模型中,熔石英的材料仿真参数如表1所示,激光参数如表2所示。
表 1 熔石英仿真热力学参数
Table 1. Simulation thermodynamic parameters of fused quartz
Parameter Value Young modulus/GPa 72.60 Poisson ratio 0.17 density/g·cm−3 2.21 Thermal expansion coefficient/K−1 5.4×10−7 Thermal conductivity/W·(m·K)−1 1.38 Heat capacity/J·(kg·K)−1 703 Absorption coefficient/m−1 1.9×10−2 表 2 毫秒-纳秒组合脉冲激光仿真参数
Table 2. Simulation parameters of millisecond-nanosecond pulse laser
Parameter Value Millisecond pulse energy/J 100-150 Millisecond pulse width/ms 1 Millisecond pulse spot radius/mm 1 Nanosecond pulse energy/mJ 80-220 Nanosecond pulse width/ns 10 Nanosecond pulse spot radius/mm 1 $\Delta $t/ms 0.25-1.25
Numerical analysis of temperature field and stress field of fused silica irradiated by millisecond-nanosecond combined pulse laser
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摘要: 为了研究毫秒-纳秒组合脉冲激光辐照熔石英的温度场和应力场特征,基于热传导理论和弹塑性力学理论建立了二维轴对称几何模型,利用有限元分析软件对毫秒-纳秒组合脉冲激光辐照熔石英的过程进行了数值分析,得到了熔石英表面及内部的瞬态温度场和应力场的时空分布与变化规律。结果表明:组合脉冲激光中,毫秒激光脉宽为1 ms、能量为120 J,纳秒激光脉宽为10 ns、能量为80 mJ,Δt=1.0 ms条件下毫秒-纳秒组合脉冲激光辐照熔石英出现温度最佳延时。观察总能量相同的组合脉冲激光与毫秒脉冲激光致熔石英的热损伤结果,得到最佳能量配比。研究结果表明,组合脉冲激光中,毫秒脉冲激光对熔石英产生热效应,纳秒脉冲激光对熔石英产生应力效应。Abstract: In order to study the characteristics of temperature field and stress field of fused silica irradiated by millisecond-nanosecond combined pulse laser, based on the theory of heat conduction and elastic-plastic mechanics, two dimensional axisymmetric geometric model was established, the numerical simulation software was used to analyze the process that fused silica irradiated by millisecond-nanosecond combined pulse laser. The temporal and spatial distribution and variation of the transient temperature field and stress field on the surface and inside of fused silica were obtained. The result shows, in the combined pulse laser, the millisecond pulse width is 1 ms, the energy is 120 J, the nanosecond laser pulse width is 10 ns, the energy is 80 mJ, Δt=1.0 ms, the best time delay for the temperature of fused silica irradiated by millisecond-nanosecond combined pulse laser, according to the different energy ratio of millisecond and nanosecond, the thermal effect of millisecond pulse laser on fused quartz and the stress effect of nanosecond pulse laser on fused quartz are obtained.
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Key words:
- combined laser /
- numerical analysis /
- laser damage /
- fused quartz
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表 1 熔石英仿真热力学参数
Table 1. Simulation thermodynamic parameters of fused quartz
Parameter Value Young modulus/GPa 72.60 Poisson ratio 0.17 density/g·cm−3 2.21 Thermal expansion coefficient/K−1 5.4×10−7 Thermal conductivity/W·(m·K)−1 1.38 Heat capacity/J·(kg·K)−1 703 Absorption coefficient/m−1 1.9×10−2 表 2 毫秒-纳秒组合脉冲激光仿真参数
Table 2. Simulation parameters of millisecond-nanosecond pulse laser
Parameter Value Millisecond pulse energy/J 100-150 Millisecond pulse width/ms 1 Millisecond pulse spot radius/mm 1 Nanosecond pulse energy/mJ 80-220 Nanosecond pulse width/ns 10 Nanosecond pulse spot radius/mm 1 $\Delta $ t/ms0.25-1.25 -
[1] Kasaai M R, Kacham V, Theberge F, et al. The interaction of femtosecond and nanosecond laser pulses with the surface of glass [J]. J Non-Cryst Solids, 2003, 319(1-2): 129-135. doi: 10.1016/S0022-3093(02)01909-9 [2] Gong Hui, Wang Mingli, Cheng Lei, et al. CW CO2 laser strengthening of fused silica surfaces [J]. Chinese Journal of Lasers, 1997, 24(3): 255-258. (in Chinese) [3] Huang Feng, Niu Yanxiong, Wang Yuefeng, et al. Calculation of thermal and mechanical effect induced by laser in optical window materials [J]. Acta Optica Sinica, 2006, 26(4): 576-580. (in Chinese) [4] Hamam K A, Gamal E D. Numerical analysis of breakdown dynamics dependence on pulse width in laser-induced damage in fused silica: Role of optical system [J]. Results in Physics, 2018(9): 725-733. [5] Wu Zhujie, Pan Yunxiang, Zhao Jingyuan, et al. Research on laser-induced damage of K9 glass irradiated by millisecond laser [J]. Infrared and Laser Engineering, 2019, 48(8): 0805005. (in Chinese) doi: 10.3788/IRLA201948.0805005 [6] Feng Guoying, Zhang Qiuhui, Zhou Shouhuan, et al. Study on the morphology of laser induced damage in K9 glass by focused nanosecond pulse [J]. Acta Physica Sinica, 2008, 57(9): 5558-5564. doi: 10.7498/aps.57.5558 [7] Liu Hongjie, Wang Fengrui, Luo Qing, et al. Experimental comparison of damage performance induced by nanosecond 1 laser between K9 and fused silica optics [J]. Acta Physica Sinica, 2012, 61(97): 076103. (in Chinese) doi: 10.7498/aps.61.076103 [8] Luo Fu, Sun Chengwei, Du Xiangwan. Stress relaxation damage in K9 glass plate irradiated by 1.06 m m CW laser [J]. High Power Laser and Particle Beams, 2001, 13(1): 19-23. (in Chinese) [9] Wang Bin, Qin Yuan, Ni Xiaowu, et al. Effect of defects on long-pulse laser-induced damage of two kinds of optical thin films [J]. Appl Opt, 2010, 49(29): 5537-5544. [10] Pan Yunxiang, Lv Xueming, Zhang Hongchao, et al. Millisecond laser machining of transparent materials assisted by a nanosecond laser with different delays [J]. Opt Lett, 2016, 41: 2807-2810. doi: 10.1364/OL.41.002807 [11] Zhang Mingxin, Nie Jinsong, Sun Ke, et al. Numerical analysis on thermal function of single crystal silicon irradiated by combined laser [J]. Infrared and Laser Engineering, 2018, 47(11): 1106011. (in Chinese) doi: 10.3788/IRLA201847.1106011 [12] Lv Xueming, Pan Yunxiang, Jia Zhichao, et al. Surface damage induced by a combined millisecond and nanosecond laser [J]. Appl Opt, 2017, 56: 5060-5067. doi: 10.1364/AO.56.005060 [13] Yuan Boshi, Wang Di, Dong Yuan, et al. Experimental study of the morphological evolution of the millisecond–nanosecond combined-pulse laser ablation of aluminum alloy [J]. Appl Opt, 2018, 57: 5743-5748. doi: 10.1364/AO.57.005743 [14] Norton M A, Carr A V, Carr C W, et al. Laser damage growth in fused silica with simultaneous 351 nm and 1053 nm irradiation [C]//Proceedings of SPIE - The International Society for Optical Engineering, 2008, 7132: 71321H. [15] Negres R A, Norton M A, Cross D A, et al. Growth behavior of laser-induced damage on fused silica optics under UV, ns laser irradiation [J]. Optics Express, 2010, 18(19): 19966-19976. doi: 10.1364/OE.18.019966 [16] Karimelahi S, Abolghasemi L, Herman P R. Rapid micromachining of high aspect ratio holes in fused silica glass by high repetition rate picosecond laser [J]. Appl Phys A, 2014, 114(1): 91-111. doi: 10.1007/s00339-013-8155-8 [17] Pan Y, Wang B, Shen Z, et al. Effect of inclusion matrix model on temperature and thermal stress fields of K9-glass damaged by long-pulse laser [J]. Opt Eng, 2013, 52(4): 044302. doi: 10.1117/1.OE.52.4.044302 [18] Dai G, Chen Y B, Shen Z H, et al. Analysis of laser induced thermal mechanical relationship of HfO2/SiO2 high reflective optical thin film at 1064 nm [J]. Chinese Optics Letters, 2009, 7(7): 601-604. doi: 10.3788/COL20090707.0601 [19] Wang B, Zhang H, Qin Y, et al. Temperature field analysis of single layer TiO2 film components induced by long-pulse and short-pulse lasers [J]. Appl Opt, 2011, 50(20): 3435-3441. doi: 10.1364/AO.50.003435 [20] Wilks S C, Kruer W L. Absorption of ultrashort, ultra-Intense laser light by solids and overdense plasmas [J]. IEEE J Quantum Electron, 1997, 33(11): 1954-1968. doi: 10.1109/3.641310 [21] Hughes T P, Bayfield J E. Plasmas and laser light[J]. Physics Today, 1977, 30(4): 54-56. [22] Hidai H, Matsusaka S, Chiba A, et al. Heat accumulation in microdrilled glass from ultraviolet laser ablation [J]. Appl Phys A, 2015, 120: 357-367. doi: 10.1007/s00339-015-9196-y [23] Pan Yunxiang, Zhang Hongchao, Chen Jun, et al. Millisecond laser machining of transparent materials assisted by nanosecond laser [J]. Optics Express, 2015, 23(2): 000765. [24] Kask N E, Radchenko V, Fedorov G M, et al. Temperature dependence of the ability of optical glass to withstand 10-msec laser pulses [J]. Soviet Journal of Quantum Electronics, 1977, 7(2): 264.