Characterization Of Radiation Damaged Polyimide Materials By Cantilever Enhanced Photoacoustic Spectroscopy

With the excellent properties of wide applicable temperature,high chemical stability,and high mechanical strength,etc,polyimide(PI)materials have been widely used in the aerospace field.Space PI materials are exposed to the complex environment during use,and the space charged particles continuously collide with molecular chains containing C,H,O,N and other atoms,resulting in atomic displacement and energy deposition.Radiation of charged particles will cause the PI material to complicated effects by introducing amorphous carbon,free radicals,unsaturated groups and molecular cross-linking,which will consequently cause irreversible damage,performance degrading or device failure.Therefore,an accurate,fast and efficient method to detect and reasonably evaluate the radiation characteristics of PI materials is urgently needed,to fully understand the damage mechanism,accurately predict on-orbit decay behavior and effectively guide the improvement of radiation resistance for the PI materials.By the coupling resonance of the cantilever and air,cantilever-enhanced photoacoustic spectroscopy(CE-PAS)sensitively collects the sound pressure generated by the absorbed light of the PI samples,can directly and quantitatively measure the light absorption characteristics which accurately reflect the degree of radiation damage.Firstly,the principles of CE-PAS were studied.According to PI samples with different optical,thermal and dimensional properties,a one-dimensional photo-induced acoustic pressure analytical model was established to quantitatively describe the relationship between material characteristics and photoacoustic pressure,and the analytic calculation based on the adopted PI samples were performed.The lumped parameter method was used to analyse the resonance characteristics of the photoacoustic cavity and the natural frequency of the cantilever.The beam bending theory was used to obtain the high-order mode shape and frequency of the cantilever.The coupling theory of sound field and cantilever was established to clarify the quantitative relationship between the the sound pressure gradient and the force loaded on the cantilever.Secondly,finite element method(FEM)was utilized to study the acoustic properties of the photoacoustic cavity and its coupling effects with the cantilever.The numerical solution of the sound field distribution for different forms of photoacoustic cavity was calculated.The influence of the cavity size,tube size and tube orientation,on the sound field were analyzed.The modal analysis of the cantilever was carried out,and the characteristics of mode shape,modal frequency and quality factor(Q)were obtained.The frequency response characteristics of the cantilever and the cavity were studied,and the influence of the relative position and orientation of the cantilever on its vibration were analyzed.Thirdly,a CE-PAS system was developed and tested.The photoacoustic cavity,cantilever and optical interferometry vibration extraction system were designed.The xenon lamp with monochromator and the laser source were used as the excitation,to establish the CE-PAS system with the operation modes of wavelength-scanning excitation and laser excitation.In addition,the all-optical method for the dynamic characterization of the cantilever was investigated to accurately test the frequency response,mode shape and Q of the cantilever.The frequency response properties,sensitivity and signal-to-noise ratio(SNR)of the established CE-PAS system were experimentally researched.Finally,CE-PAS measurement was carried out for the characterization of the charged particle radiation damage in PI materials.The quantitative relationship between the radiation damage parameters(normalized radiation damaged fraction f_a,the damage area of single-particleσand the radiation fluenceφ)and the photoacoustic signal was established.The wavelength-scanning CE-PAS characterization of the radiation damage in PI films,aerogels and non-woven verified the sensitivity of photoacoustic signals to the radiation damage.The method of the dual-wavelength laser CE-PAS was utilized to test the irradiated PI samples and the radiation damage parameters(f_a,σ,φ)were characterized by the photoacoustic signal according to the established theory.