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自苏格兰物理学家詹姆斯·杜瓦(James Dewar)发明了杜瓦瓶以来,其基本功能仍然是真空绝热。在红外技术领域,广泛采用杜瓦真空封装的结构形式,但真空绝热功能的长期保持问题突出,主要原因包括红外芯片的耐受温度不高,只能低温烘烤除气;红外芯片的封装中使用了有机粘接剂,出气量大;杜瓦真空绝热空间内要求高真空环境,要求苛刻。
在杜瓦制造中的排气过程及用户使用过程中,放气率显著影响杜瓦真空绝热空间内所能达到的压强,同时与抽速相关。杜瓦内压强的理论计算方法见公式(1):
$$p = \frac{Q}{S} = \sum\limits_{i = 1}^n {{p_i}} = \sum\limits_{i = 1}^n {\frac{{{Q_i}}}{{{S_i}}}} = \sum\limits_{j = 1}^m {\sum\limits_{i = 1}^n {\frac{{{A_j} \cdot {q_i}}}{{{S_i}}}} } $$ (1) 式中:p为杜瓦内真空绝热空间内的压强,单位为Pa;Q为杜瓦放气率,单位为Pa·m3/s;S为抽气装置对杜瓦的有效抽速,单位为m3/s;Qi为第i类气体成分的放气率,单位为Pa·m3/s;
${A_j}$ 为第j个零件的表面积;Si为抽气装置对杜瓦内第i类气体成分的有效抽速,单位为m3/s;n为气体成分种类数;m为零件总数。受排气管流阻限制,杜瓦组件在排气过程中的有效抽速为2×10−5 m3/s,引起排气中杜瓦内的压强明显高于真空系统主腔室的压强,导致气体再吸附问题严重,制约杜瓦组件的排气效果。若使用0.3 m3/s的主抽气系统排气,并且忽略真空腔体的放气,杜瓦内压强与主室真空腔体内存在4个数量级的压强差。
放气率决定着杜瓦排气中所能达到的最小压强。若放气率降低1个数量级,杜瓦真空绝热空间内所能达到的压强降低1个数量级,能够弱化再吸附问题。另外,设计中尽量增加排气流导,降低放气率大造成杜瓦内压强过大。
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通常杜瓦内没有配置真空规,可采用升温率法间接测试内部压强。升温率法由以色列SCD公司首创,其初衷是实现高工作温度点下杜瓦组件的热负载测试[6]。另外,采用升温率法能在线测试杜瓦的漏热,实现杜瓦内压强的评判。用户通过升温率的变化幅度即可实现保守型的真空维修方案,避免压强过高造成的制冷机故障[3]。基于某型杜瓦产品,采用升温率法测试了不同杜瓦内部压强下的升温率,验证了该方法的有效性,见图8。
文中实验为了更直观的对比改进效果,借鉴主流的放气率测试方法[7-24],采用压强上升法测试钝化处理前后的放气率变化。压强上升法即气体累积法,通过一定时间内的平均压强上升率来测试放气率,放气率的计算方法见公式(2):
$$Q = A \cdot q = \frac{{{\rm d}P}}{{{\rm d}t}} \cdot V \approx \frac{{{P_1} - {P_2}}}{{{t_1} - {t_2}}} \cdot V$$ (2) 式中:Q为杜瓦的总放气率;q为杜瓦单位面积的放气率;A为杜瓦的内表面积;dP/dt为压强上升率;P1为累积前压强;P2为累积终止时压强;V为测试室容积。
将钝化处理的杜瓦样品(见图9(a))、无处理的杜瓦样品(见图9(b)),分别进行低温烘烤过程下的放气率测试及气氛分析,测试结果见图9、和图10。结果表明,杜瓦封装用结构材料的主要气体成分为H2,内表面镀钝化膜后有效抑制了氢气的释放,放气率降低90%。
Effect of part surface passivation on Dewar outgassing rate
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摘要: 红外焦平面探测器杜瓦组件的真空绝热空间内存在多种气体来源,其中最大的气体来源是材料放气。文中针对制冷型红外焦平面探测器杜瓦组件真空绝热空间内表面吸附气体的解析、体扩散、渗透放气污染问题影响制冷启动时间,限制红外探测器组件的使用寿命,设计了一种用于红外探测器杜瓦内表面钝化处理方案,基于某型杜瓦产品实现了杜瓦内壁钝化膜的制备。采用压强上升法进行放气率对比测试, 并通过四极质谱仪进行了气体成分分析。放气率测试试验结果表明,钝化膜有效抑制了氢气的释放,杜瓦放气率降低一半以上。杜瓦内表面钝化膜处理方法简单有效,提升了真空寿命,具备工程化推广价值。Abstract: There are many gas sources in the Dewar’s vacuum insulation space, the biggest gas source is material outgassing. The problem of outgassing pollution caused by the desorption, volume diffusion, and penetration of the adsorbed gas on the inner surface of the vacuum insulated space of the refrigerated infrared focal plane detector Dewar component was focused, which affected the cool down time and restricted the service life of the infrared detector component. A passivation treatment plan for the inner surface of the infrared detector Dewar was designed, and the passivation film on the inner wall of the Dewar was prepared based on a certain type of Dewar product. The pressure rise method was used to compare the outgassing rate, and the gas composition was analyzed by a quadrupole mass spectrometer. The test results of outgassing rate show that the passivation film effectively inhibits the release of hydrogen, and the outgassing rate of Dewar is reduced by more than half. The processing method of the passivation film on the inner surface of the Dewar is simple and effective, which improves the vacuum life and has engineering promotion value.
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Key words:
- Dewar /
- vacuum insulation /
- cool down time /
- service life /
- surface treatment /
- passive film
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