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对于金属涂覆光纤,涂覆质量主要受光纤进入熔融铝液的温度T1、铝液的温度T2与光纤和铝液接触时间t的影响。其中铝液的温度T2由系统自由调控,而光纤进入熔融铝液的温度T1与光纤和铝液接触时间t则是由拉丝速度Vf和上下模具的间隙L所决定。
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金属冷凝涂覆技术的基本原理是金属液体遇冷凝固形成膜层,一般情况下光纤进入熔融铝液时的温度T1与液态铝的温度T2的温差越大,冷凝效果越好。因此,实现金属冷凝涂覆技术的关键技术之一是控制光纤进入熔融铝液时的温度T1,确保T1低于铝的熔点660 ℃。
在线涂覆时,由于光纤在高温炉内拉制成型,其起始温度Ts=2300 ℃。高温炉与涂覆装置间的冷却距离为z,光纤经过z距离自然冷却后温度降为T1。T1计算公式如下:
$$ \frac{{{T_1} - {T_0}}}{{{T_{\rm s}} - {T_0}}} = \exp \left( { - \frac{{4hz}}{{{V_{\rm f}}\rho {C_ p}d}}} \right) $$ (1) 式中:T1为光纤进入涂覆装置时的温度;T 0为环境温度,T0=20 ℃;Ts为高温炉拉丝温度,Ts=2300 ℃;h为对流换热系数;z为高温炉与涂覆装置间的自然冷却距离;Vf为拉丝速度;ρ为金属涂层的密度;Cp为石英玻璃光纤的比热容;d为光纤直径。
以400 μm直径光纤为例,根据公式计算出了三种拉丝速度Vf条件下的T1与z的关系,结果如图9所示,可以得知:随着z的增大,光纤温度迅速降低。
为了保证良好的涂覆效果,需控制温差T2−T1≥120 ℃。由于T2最低温度为铝的熔点660 ℃,则T1必须小于等于540 ℃。因此,笔者的重点关注20~560 ℃段T1随z的关系变化规律。
结果表明,当Vf分别为10、30、50 cm/s时,对应的最小冷却距离zmin分别为10、32、53 cm。由此可见,拉丝速度越快,所需的冷却间距z越大。需要指出的是,文中装置独特的U型设计可保证实际工况中光纤在涂覆装置入口的温度T1不受坩埚温度的影响,同时也可确保实验数值与计算数值的一致性。
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以直径为400 μm的光纤为例,实验发现液态铝的温度直接影响金属涂覆层厚度,如图10所示,涂覆厚度与铝液温度呈线性递减趋势。Andrade[11]给出了的黏度μ随温度T变化的半经验公式(2),经许多研究人员证实,该公式与实验值符合相当好。由该公式可知,随着温度的升高,液态金属铝的黏性会下降。因此,在相同拉丝速度的条件下,温度越高,涂覆厚度越小。可以据此调节液态金属铝的温度,进而实现涂覆厚度的精确调控(45~130 μm)。
$$ \mu {v^{\frac{1}{3}}} = A{\rm exp}\left( {\frac{c}{{vT}}} \right) $$ (2) 式中:υ为液态涂层比热容;A,c为常数。
理论上,液态铝温度介于660 ℃与石英玻璃光纤拉丝温度2100 ℃ (铝沸点2327 ℃)之间都可实现冷凝涂覆。但在实际涂覆过程中,若液态铝的温度T2过高会迅速将光纤加热至铝熔点即660 ℃以上,导致冷凝的铝涂层再次熔融脱落。若液态铝温度低至接近熔点,则铝液凝固速度过快,涂覆层会变得不规则甚至凝固过多、过快导致模具堵塞,影响生产。结合实际工况,液态铝温度一般介于630~690 ℃较为合适。
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上模具内孔处采用平头设计,下模具内孔处采用弧形设计,且二者孔径大小一致。为了避免液态铝向上溢出,要保证液体高度差为(H-L)处的压差作用力小于液态铝的表面张力,因此上模具孔径不不宜过大。
针对不同直径的裸纤,上下模具内径的选择有所不同。原则上来说,光纤与模具之间的空隙越小越好,但不应小于预设涂层厚度,否则模具将损伤铝涂层。另外将装置光纤出口处设计为惰性气体氛围有助于避免凝固后的高温铝涂层发生氧化反应。
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由公式
$L = {V_{\rm f}}t$ 得知,当拉丝速度Vf一定,L值直接反映了光纤与液态金属的接触时间t。若L过大,则光纤在液态铝中停留时间过长,冷凝后的铝涂层将再次融化,使得涂层厚度太小或脱落;若L过小,则涂层厚度过大或会产生气泡。因此,L值的大小会直接影响涂覆层厚度以及涂覆质量。一般来说光纤与铝溶液接触时间t介于0.1~ 10 ms之间,假设普通光纤拉丝速度约为50 cm/s,则L值为0.05~5 mm。
L值的实际大小可以通过旋转上下模具螺钉来控制,同时需要确保陶瓷坩埚盖的U型底部与坩埚底部的距离大于5 mm。
Online preparation technology of quartz glass fiber with metal coating
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摘要: 金属涂层光纤较传统有机涂层光纤具有高热稳定性、抗振动干扰等显著优势,但金属涂层光纤的连续在线制备技术在国内仍处于探索研发阶段,这也直接导致金属涂层光纤无法大批量连续生产,一定程度制约了我国高功率光纤激光器的飞速发展。提出并研制了一种基于熔融金属冷凝涂覆法的金属涂层光纤在线制备装置,经与光纤拉丝塔耦合,能够实现边拉丝边涂覆金属,并且涂层厚度可控。基于该工艺成功拉制了涂层均匀、涂层表面质量良好、直径稳定的铝涂层光纤。通过实验和模拟计算,探讨了影响光纤金属涂覆层质量的影响因素,主要包括了光纤入口温度、液铝温度、模具孔径、接触距离等。通过研究,确定了最佳铝液温度为663~690 ℃,且发现涂覆厚度与铝液温度的线性递减关系;计算了拉丝速度与冷却距离的关系;给出了陶瓷上下模具螺丝的孔径大小、光纤与液态铝接触深度的最佳值。研究成果为金属涂覆光纤的批量生产问题提供了解决方案,为打破国际技术垄断奠定了基础。Abstract: Compared with the traditional organic-coated fiber, the metal-coated fiber has significant advantages such as high thermal stability and anti-vibration interference. However, the continuous online preparation technology of metal-coated fiber is still in the research and development stage in China, which directly leads to the failure of large-scale continuous production of metal-coated fiber, and to a certain extent, limits the rapid development of high-power fiber lasers in China. An on-line fabrication device of metal-coated fiber based on molten metal condensation coating method was proposed and developed. Coupling with the fiber drawing tower, it can realize metal coating while drawing, and the coating thickness can be controlled. Aluminum coated fiber with uniform coating, good surface quality and stable diameter had been successfully fabricated. The influences of fiber inlet temperature, liquid aluminum temperature, mold aperture and contact distance on the coating quality of metal coating were discussed. By theoretical analysis and practical experiment, the optimum temperature of aluminum liquid is 663-690 ℃, and the linear decreasing relationship between the thickness of coating and the temperature of aluminum liquid was obtained. The relationship between drawing speed and cooling distance was calculated. The optimal aperture size of screw of ceramic upper and lower die and the contact depth between optical fiber and liquid aluminum were given. The research results provide a solution to the problem of mass production of metal-coated fiber and lay a foundation for breaking the international technology monopoly.
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