Study on Li ion migration of glass-ceramic in discharge area of space triaxial laser gyroscope

  Objective   Space triaxial laser gyro is a kind of space laser gyro which integrates three sensitive loops orthogonally on a glass-ceramic substrate. It has been widely used in aviation, aerospace, military and other fields. With the increasing requirement of military equipment for the long-term power-on stability of performance of high-precision laser gyro, improving the long-term power-on stability of performance of laser gyro and extending its working life have become an important topic for researchers in the field of laser gyro both at home and abroad. Foreign companies such as Litton, Honeywell, Thales, etc. had mentioned that Li⁺ on the glass-ceramic substrate migrated under the action of electric field, and in turn, it reduced the working life of laser gyro, but no specific research has been made. In order to improve the long-term power-on stability of performance of laser gyro and prolong its working life, Li⁺ migration on the surface of LAS (Li₂O-Al₂O₃-SiO₂) glass-ceramic in the discharge area was researched.

  Methods   The accelerated power-on life test of space triaxial laser gyro was carried out, the intensity curves with the sputtering depth of Li⁺ in the glass-ceramic in the discharge and non-discharge area, mirrors, and the inner and outer surfaces of cathode of this laser gyro was tested with the time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis, the law of Li⁺ migration in the glass-ceramic and the resonator of space triaxial laser gyro was clarified. Meanwhile, the electric field of the laser gyro in the working state was modeled and simulated by COMSOL Multiphysics. Furthermore, the migration mechanism of Li⁺ in the glass-ceramic and the resonator was discussed.

  Results and Discussions   The color of the glass-ceramic surface in the discharge area of the test gyro was changed significantly (Fig.4). The intensity curves of major elements with the sputtering depth in the glass-ceramic in the discharge and non-discharge area, mirrors, and the inner and outer surfaces of the cathode was tested by means of TOF-SIMS. It was found that Li⁺ on the surface of the glass-ceramic in the discharge area had migrated into the resonator (Fig.6-7), and deposited on the inner surface of the cathode uniformly (Fig.8). Meanwhile, no significant Li⁺ deposition on the mirror which located in the discharge area, or even entered into the film (Fig.9). COMSOL Multiphysics was used to simulate the electric field distribution of the laser gyro under the working condition. The Li⁺ migration mechanism in the glass-ceramic was discussed combined with the simulation results. It showed that Li⁺ migrated to the surface of glass-ceramic and entered into the resonator under the action of electric field, which decreased the Li⁺ concentration and changed the refractive index of glass-ceramic. So it showed different light reflection characteristics from the surrounding area. Furthermore, since the electric field intensity in the mirror in the discharge area is relatively small, the flow path of the plasma in the resonator follows the principle of the shortest path, most of the plasma will not directly touch the mirror, Li⁺ mainly moves to the cathode with the plasma and deposits on the inner surface of the cathode, therefore no obvious Li⁺ was detected on the mirror.

  Conclusions   Li⁺ migrated into resonator under the action of the electric field and plasma of glass-ceramic in the discharge area of space triaxial laser gyroscope, and then flowed with the plasma, finally deposited on the inner surface of the cathode. This phenomenon may reduce the temperature-varying dimensional stability of glass-ceramic and the working life of cathode, and then decreased the long-term power-on performance stability of the laser gyro. Some measures and suggestions are proposed to suppress Li⁺ migration based on its migration law, the specific suppression methods of Li⁺ migration will be further studied.