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高超声速飞行器由于飞行速度高、飞行时间长,光学窗口外表面的空气动力热造成窗口产生温度梯度,在材料上产生热张应力,从而造成窗口损坏以及热辐射影响正常成像,这是高超声速红外窗口面临的最主要的问题[8, 12-13]。为了满足实际的飞行要求,人们对整流罩和红外窗口的结构[14]和材料[2, 15]等方面进行了研究。
应用于高速飞行下的红外光学窗口材料主要有氟化镁、蓝宝石、尖晶石、氧化钇、 氧化锆、硫化锌、金刚石等。其中氟化镁的应用最为成熟,但其硬度和抗热冲击性较低,多用于2 Ma以下的飞行器中[16]。蓝宝石在硬度和抗热冲击性上具有很好的性能表现,但其制作成本高[4, 13]。金刚石相比于其他材料除了满足基本需求外,具有更好的热力学性能,在高马赫数飞行状态下的长波红外制导系统中金刚石不可或缺,但因其制备成本高和制备技术难度大的问题制约了其发展。硫化锌是目前唯一可以实现应用的长波红外透射窗口材料,其具有较高的透过率和较宽的透射波段,在机械和热性能方面表现也比较不错[15]。
为了验证不同材料光学窗口在高温条件下的热辐射对光学成像的影响,对比验证了中波窗口(尖晶石、氧化钇、氧化锆、硫化锌)、长波窗口(硫化锌)分别对中、长波热像仪的影响,如图9所示。
Figure 9. Different material windows under test (From left to right: Spinel, Zinc sulfide, Zirconia, Yttrium oxide)
分别将中、长波热像仪开机,调整相对应平行光管的位置,使十字靶标在热像仪视场中心,将窗口加热后放置热像仪与平行光管之间,观察显示器上平行光管的十字靶标图像变化,不同温度的高温窗口对成像影响如表1~2所示。
20 ℃ 150 ℃ 250 ℃ 300 ℃ Spinel Yttrium oxide Zirconia Zinc sulfide Table 1. Thermal imaging effect of the medium wave window in various temperature environments
20 ℃ 150 ℃ 250 ℃ 300 ℃ Zinc sulfide Table 2. Thermal imaging effect of the long wave window in various temperature environments
以上试验表明窗口在高温条件下存在不同程度透过率下降情况,材料的高温透过率的下降意味着材料对该红外波长处吸收的增强,吸收增强必然导致红外辐射增强。为了验证高温条件下不同窗口透过率的衰减情况,测量窗口辐射量的相对增幅情况。
进行试验验证如图10所示。测试步骤如下:
(1)将黑体设置为50 ℃,20 ℃窗口置于黑体与热像仪中间,读取热像仪的AD值为A1;
(2)将黑体设置为25 ℃,20 ℃窗口置于黑体与热像仪中间,读取热像仪的AD值为A2;
(3)将黑体设置为50 ℃,高温窗口置于黑体与热像仪中间,读取热像仪的AD值为A3;
(4)将黑体设置为25 ℃,高温窗口置于黑体与热像仪中间,读取热像仪的AD值为A4。
不同温度下窗口的透过率相对于20 ℃透过率的衰减率Y可根据公式(1)得出,试验结果如表3所示。
20 ℃ 150 ℃ 250 ℃ 300 ℃ Spinel 100% 86% 75% 71.8% Yttrium oxide 100% 94.6% 90.2% 88% Zirconia 100% 93% 87.7% 85.2% Zinc
sulfide
(Medium)100% 97.4% 96% 95% Zinc
sulfide
(Long)100% 96.3% 93.5% 93.2% Table 3. Attenuation of window transmittance at different temperatures
对比中波窗口热辐射对中波热像仪成像影响时,尖晶石窗口在150 ℃时热像仪图像饱和,高温250 ℃和350 ℃时,热像仪图像出现饱和,无法正常成像,相对透过率降28%左右;氧化钇和氧化锆窗口在150 ℃时成像效果较差,高温250 ℃和350 ℃时,热像仪图像均出现饱和,无法正常成像,相对透过率下降12%和15%左右;硫化锌窗口热辐射的影响较小,高温时仍可观察到十字靶标。对比硫化锌窗口热辐射对中、长波热像仪成像影响时,硫化锌窗口在高温时中、长波热像仪图像成像质量影响均较小,相对透过率衰减均较小,且硫化锌材料折射率均匀一致性好,可以作为双色红外光学系统材料。
Infrared waveband and window selection for hypersonic vehicle (Invited)
doi: 10.3788/IRLA20220161
- Received Date: 2022-01-10
- Rev Recd Date: 2022-02-20
- Accepted Date: 2022-04-15
- Publish Date: 2022-05-06
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Key words:
- hypersonic /
- waveband selection /
- optical windows /
- thermal radiation /
- zinc sulfide
Abstract: With the development of technology, modern warfare puts forward higher requirements for new weapons, and the development of hypersonic vehicles has attracted much attention. Infrared imaging equipment plays an important role in the terminal guidance field of hypersonic vehicles. Infrared imaging equipment is susceptible to interference from background radiation and thermal radiation from windows, and the background noise generated by the interference can easily cause imaging saturation. The imaging effects of mid- and long-wave thermal imagers were compared by experiments, high-temperature objects, the sun, clouds, sea surface, jamming bombs and rotating, high speed, high dynamics conditions. And the effects of spinel, yttrium oxide, zirconia and zinc sulfide materials' own thermal radiation on the imaging of medium and long wave thermal imaging instrument were compared, the relative attenuation rate of transmittance of windows at high temperature was obtained by testing. The comparative analysis show that the long wave thermal imager has advantages in anti-interference, and the zinc sulfide material has the advantages of low radiation, high transmittance and good pressure resistance. Mid- and long-wave comparison tests provide reference and support for the selection of working wavebands and window materials, and have reference value for the subsequent design and research of mid-long wavelength dual-band systems.