Development Of Polarized Spectral Emissivity Measurement Apparatus

Emissivity is an important property to describe the radiant ability of a target and is indispensable in radiation thermometry,aerospace,remote sensing,stealth technology,medical science,etc.Comparing to the total emissivity,spectral emissivity of material contains more information and has a broader range of applications.For example,spectral emissivity has been successfully applied in molecular recognition,estimate of stealth material’s property,and fundamental research of infrared radiation.In radiation thermometry,accurate emissivity data can determine the reliability of the temperature measured by thermometer.However,emissivity is usually unstable in a real measurement process and changes with temperature,wavelength,material components,and surface topography,which seriously restrain the development of radiation thermometry.Comparing to the unpolarized emissivity,the polarized spectral emissivity of materials possesses some special properties that it has a wide prospective application field covering remote sensing,radiation thermometry,and so on.In recent years,people start to enhance the research in polarized emissivity,however,it is undeniable that measurement apparatus and knowledge of the polarized emissivity are both scanty.Thus,this paper established a polarized spectral emissivity measurement apparatus and studied the radiation properties of metal materials with support from Natural Science Foundation of China.The main works in this thesis as follow:1、A self-designed apparatus was established to measure the directional polarized spectral emissivity of material,the apparatus contains: The heating and temperature controlling system,the blackbody,the optical system,the mechanical system,the electric measuring system and the data-processing system.The heating and temperature controlling system and blackbody belong to radiation signal source of the apparatus;The optical system contains filter,splitting cube,radiating probe,fiber and photoswitch.The mechanical system is used to execute the transformation between the sample and the blackbody,and control the measurement in different directions.The electric measuring system consists two parts:(1)A In Ga As detector;(2)The amplification system contains an amplifier and an optical chopper.The data-processing system,which based on Labview,controls the rotation of motor,moreover,collect andstore the output signal of lock-in amplifier.2、The test evaluation of some important aspects contained surface temperature uniformity of sample,axial temperature uniformity of blackbody and the response factor of system was performed.By comparing the measured polarized spectral emissivity of Si to the emissivity value obtained by evaluation and the measured emissivity data by foreigner,the reliability of the apparatus was verified.3、A simulation method was proposed to calculate the directional emissivity of an oxidized rough metal surface.The good agreement between the simulated results and the experimental results measured by a self-designed apparatus indicates that this method is effective at 0-70° in thermal oxidation process,and that the directional emissivity of rough aluminum increases gradually for angle <70° and changes slightly at 70°.The experiment calibrating the temperature of rough aluminum sample was performed by using the simulated emissivity.The result illustrates that the simulation method is applicable in thermometry and the emissivity at 70° is more suitable to calibrate the real temperature of oxidized rough aluminum surface.4、The normal spectral emissivity of the oxidized Ti-6Al-4V alloy and exploring the possible reasons for the oscillatory behavior in emissivity.Before the oxidation process,the emissivity shows a liner increase between 823 K and 973 K at 1900 nm.When the sample is oxidized in air below 873 K,the emissivity will increase gradually with the oxidation time.When the temperature is above 923 K,strong oscillations in emissivity are observed.The measurement results of surface composition,roughness and the film thickness indicate that variation in surface roughness is one of the factors influencing the amplitude of oscillation.But the real reason of the oscillatory behavior is demonstrated to be the interference effect of the growing oxide film.The oxide film thickness can be estimated by using the theory of radiative effects of films.The measurements carried out by ellipsometer and scanning electron microscopy testify that this estimation method is effective.Based on this method,it is found that the oxide film grows following a parabolic rule when the Ti-6Al-4V alloy is oxidized in air atmosphere.The rate of the oxidation could be approximatively estimated by using the calculated thickness values.The result shows that the oxidation rate increases rapidly at the temperature above 923 K.