High emissivity coatings for high temperature application: Progress and prospect
Introduction
High emissivity coatings are widely used in many high temperature applications to effectively transfer the heat by radiation [1], [2], [3], [4]. For example, the surface of Ni-, Fe-, Co-based alloys applied in metal thermal protection systems (MTPS) [5], [6] in third generation of reused space vehicles involves high friction heat from acute friction between space vehicle surface and atmosphere, which causes obvious increase of surface temperature up to 1000 °C during hypervelocity flight [7], [8], [9]. As a result, the lifetime and performance of MTPS will be seriously degraded. Therefore, the high emissivity coating is intentionally deposited on MTPS to decrease the surface temperature by radiation.
When a radiation falls on a body it may be partially reflected, transmitted, or absorbed, which is associated with reflection (R), absorption (A), transmission (T), and emission (ε). The principle of conservation of energy ties together these radiation characteristics as A + R + T = 1. According to Kirchhoff's law, at equilibrium for a given wavelength λ and temperature T, the emissivity of any body is equal to its absorption [10], i.e. ε(ν, T, θ) = A(ν,T,θ), where ε is the emissivity, ν the frequency, and the angle θ specifies the observer angle. This indicates that the emissivity of an object can be indirectly obtained by measuring its absorptivity. For a uniform and isotropic opaque surface in thermal equilibrium, the transmission is zero, the relationship between the emissivity and the reflectivity is: ε(ν,T, θ) = A(ν,T,θ) = 1 − R(ν,T,θ). Many materials can be considered as opaque body. The emissivity here is characterized to describe the surface radiative property which involves the transfer of heat by electromagnetic radiation arising due to the temperature of a body. The emissivity is defined as the ratio of energy radiated by the material to energy radiated by a black body (A body that emits the maximum amount of heat for its absolute temperature is called a black body, meaning that a blackbody completely absorbs all radiation incident upon it, and at the same time it emits all the energy that it absorbs with the same absorbing spectrum. That is to say, ε = 1) at the same temperature, which is described by Eq. (1):
Where E is radiant heat of gray body to its surroundings, ε hemispherical emissivity of the gray body (dimensionless), σ Stefan–Boltzman constant (5.67 × 10− 8 W/(m2×K4)), T temperature (K). The emittance is dependent on direction and wavelength, thus the radiance ability can also be evaluated by the spectral emissivity and directional emissivity, described as Eqs. (2), (3) respectively:
A real object does not radiate as much as a perfect black body, which radiates less heat than a black body and is called gray body (ε < 1).
Thermally emitted radiance from any surface mainly depends on two factors. (1) the surface temperature, which is an indication of the equilibrium thermodynamic state resulting from the energy balance of the fluxes between the gray body surface and its surroundings; and (2) the surface emissivity, which is the efficiency of the surface for transmitting the radiant energy generated in the surface into its surroundings. The latter depends on the temperature (but the relationship between emissivity and temperature is not definite, depending on material nature, surface parameters and wavelength), composition, surface roughness, coating thickness, wavelength, and physical parameters of the surface. Here we review recent progress with emphasis on the discussion of the design methodology, followed by briefing the emissivity characterization. Lastly, the prospects for research and technology in high emissivity coatings will be summarized.
Section snippets
Doping
According to Wien's displacement law and Planck's law [11], most energy of the black body at high temperature is radiated in the wavelength range of 1–5 μm, however it happens that many materials have the weak intrinsic absorption within this range. As a result, the emissivity in this region is quite low since the object's absorptivity and emissivity spectra are identical in thermal equilibrium state. Extensive work proves that total emissivity or spectral emissivity could be effectively
Emissivity characterization
In a classical definition, emissivity measures the ability of a material to radiate the heat through electromagnetic wavelength. Emissivity measurements are difficult or tedious in practice because it is a function of several variables and an extrinsic parameter of a material. Although many methods have been proposed and developed so far, a standard procedure or a universally accepted methodology does not exist yet. Qualitatively, the emissivity can be estimated by four developed basic methods:
Prospect
Absorption coefficient α and scattering coefficient S are important factors to be considered when designing a high emissivity coating; increasing α and decreasing S can enhance the emissivity. In principle, all emissivity enhancement mechanisms can be applied to the enhancement of ceramic coatings, the real challenge lies in the implementation. However, for real engineering applications, pursuing high emissivity is not enough, while a combination of high emissivity and other comprehensive
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