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    nextel velvet coating 811-21 -36度到82度的輻射率研究

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    Abstract. Emissivity measurements on Nextel Velvet Coating 811-21 between ?36 8C and 82 8C are

    presented. The method used is a periodic one. This method involves sample temperature modulation

    and signal processing of the sample temperature and the output signal of an infrared detector facing

    the sample. The Fourier transform at the modulation frequency of the infrared signal is proportional

    to the emissivity and to the Fourier transform of the temperature raised to the fourth power. As

    with ordinary methods, two measurements are necessary to obtain the emissivity: one on the sample

    and the other on a reference. Instead of a cold reference, the hemisphere method has been employed.

    The sample is glued onto a modulated sample holder, the mean temperature of which can be

    chosen between ?36 8C and 82 8C. This sample and the optical detection system are placed in a

    vacuum chamber. The pressure inside the chamber is less than 10?6 mbar. At each temperature,

    ten measurements have been carried out. The accuracy obtained is better than 1%. The emissivity

    of the coating studied remains constant over the temperature range explored. This coating can be

    employed as a reference in this temperature range. The values obtained are consistent with values

    from the literature.

    1 Introduction

    Design and control of thermal systems require modelling of the radiative transfer and

    thus knowledge of radiative parameters of the various elements concerned, particularly

    the total emissivity. Most emissivity measurement methods demand the use of a reliable

    reference. Moreover, highly emissive black paints are used to control the radiative environment.

    One paint widely used is Nextel Velvet Coating 811-21. Thus, its characteristics

    must be known with a high degree of accuracy. We designed and built a device to measure

    the normal total emissivity of this paint in the temperature range from ?36 8C to

    82 8C. As a reference, this method uses a hemispherical mirror placed over an opaque

    coating, the latter being diffusing and highly emissive.

    2 Principle of emissivity measurements at low temperature

    The direct emissivity measurement requires the measurement of the emitted flux. At low

    temperature the flux emitted by a surface S may be hidden by the reflected ambient

    flux. However, an infrared detector can measure only the sum of the emitted and

    reflected fluxes. If the surface temperature is modulated, then only the emitted flux is

    modulated, and by spectral analysis of the infrared signal, it is possible to extract the

    component of the signal proportional to the emission and thus to the surface emissivity

    (Matte|? et al 1994). Indeed, if the surface temperature is modulated, then the emitted

    flux can be written as:

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