Development Of Bionic Material Simulating Vegetation And Research On Formation Mechanism Of Its Spectrum Characteristics

Hyperspectral imaging is becoming an important detection method,which can identify the subtle differences between the reflection spectra of a target and its background.In addition,the infrared detection technology is also developing rapidly.At present,the traditional camouflage materials are not capable of countering the above detections.Considering that vegetation is one of the most important backgrounds where a target locates,a novel bionic material was prepared by a solution-casting method to simulate the solar spectrum reflection and infrared thermal effect characteristics of natural leaves.The reflection mechanism of the bionic material was studied emphatically and its optical bionic performance was optimized based on a four flux model.To understand the characteristics of the solar spectrum reflection of natural leaves,the reflectances of several kinds of green leaves were measured and they all exhibit four common features.The reflectance in the visible region is maintained at a low level and a small reflection peak called the "green apex" appears around 550nm.The reflectance increases sharply between 680 and 780 nm,forming the "red edge".The reflectance between 780 and 1300nm is maintained at a high level among 40%?60%,which is called the "near infrared plateau".The reflectance above 1300nm is mainly determined by water,and two absorbing bands of water appear in this range at 1450 and 1930nm,respectively.The reflection spectra of the above green leaves are highly similar,whereas there are significant differences among their reflectance values.To clarify the key factors affecting the characteristics of the reflection spectra of natural leaves,the Omanthus fragrans leaves with different colors?green and yellow?and with different water contents were selected as the research objects.The reflectances and component contents of the above leaves were measured,and the structure parameter,N,which means a leaf is composed of a pile of N homogeneous layers separated by N-1 air spaces was calculated based on a PROSPECT model.It is found that the reflectance of yellow leaf in the visible region increases significantly and the "green peak" tends to shift toward the long wavelength compared with that of green leaf due to the degradation of chlorophyll occurred during the senescence process.The reflectance of dry leaf between 400 and 2400nm increases significantly compared with that of fresh leaf,which can be attributed to the increase of N and the decrease of water content.In conclusion,the results reveal that the variations of the chlorophyll content,water content and structure parameter significantly affect the characteristics and intensities of the reflection spectra of natural leaves.Based on the key factors affecting the solar spectrum reflection characteristics of natural leaves,and considering the important effect of transpiration on the infrared thermal characteristic of natural leaves,we developed a kind of bionic material composed of polyvinyl alcohol?PVA?,lithium chloride?LiCl?and chromium sesquioxide?Cr₂O₃?.The bionic material can simulate the infrared thermal effect of natural leaves' transpiration because that the hygroscopic PVA and LiCl can absorb and desorb water due to the high and low humidity of the ambient air at night and day,respectively.The water stored in hygroscopic material with water retention property and Cr₂O₃ help the bionic material simulate the solar spectrum reflection characteristics of natural leaves.The measured diurnal maximum radiation temperature difference between the bionic material containing 12.66 wt%LiCl and osmanthus fragrans leaf is only 0.55 ? and the correlation coefficient between the spectral reflectances of them is above 0.953.Considering that the PVA membrane tends to swell or even dissolve in water,it was modified by crosslinking with L-malic acid to improve its stability in water.In order to determine a suitable crosslinking process,the effects of the heat-treatment temperature?100,110 and 120??and crosslinker concentration?10.00 wt%?18.18 wt%?25.00 wt%and 30.77 wt%?on their swelling and hygroscopic performances were investigated.Both the swelling and hygroscopic performances of the PVA membranes decrease with increasing heat-treatment temperature;while the former decreases,the latter increases with increasing crosslinker concentration.In addition,among the above PVA membranes treated with different crosslinking process,the PVA membrane containing 30.77 wt%L-malic acid and heat-treated at 100? for 1 h has the highest moisture adsorption capacity equal to 0.163 g/g,and only tends to swell rather than dissolve in water.To elucidate the reflection mechanism,the bionic materials containing different Cr₂O₃ content were prepared and the absorption and scattering coefficients of the materials in dry state were calculated via a four flux model.An absorption valley in the range of 460?600 nm leads to a reflection peak of the bionic material similar to the"green peak" of natural leaves.The characteristics of weak absorption and strong scattering in the range of 800?1400 nm make the bionic material form the "red edge"and "near infrared plateau" of natural leaves.Moreover,the absorption coefficient of the PVA membrane in wet state without Cr₂O₃ was derived from a ray tracing method and the absorption coefficient curve shows two absorbing bands of water at 1450 and 1930 nm,respectively.Therefore,the bionic material can simulate the "water absorption bands" of natural leaves.To optimize the optical bionic performance,the reflectances of the bionic materials with different content of Cr₂O₃ were simulated via the four flux model.It is found that when the volume fraction of Cr₂O₃ is 1.61%,the reflection spectra of the materials with the volume fraction of water in the range from 9.75%to 51.92%are in the spectra channel of USA military.The correlation coefficients between the spectral reflectances of the above bionic materials and an Omanthus fragrans leaf could be above 0.945.