Russian Physics Journal, Vol. 52, No. 2, 2009

HEAT-REGULATING COATINGS BASED ON BaTi (1-y)SnyO3 POWDERS M. M. Mikhailov

UDC 629.7.023.2.535.362

Keywords: heat regulation, coatings, emissivity, phase transitions, barium titanate.

Only one of the three mechanisms of heat transfer (heat conductivity, convection, and radiation), namely, radiation is possible for space vehicles (SV), since they have contact neither with a solid body (there is no contact with other objects), nor with a gas (they operate in deep vacuum). To maintain the thermal mode, active and passive heatregulating systems are used. Passive systems are heat-regulating coatings (HRC) put on heat-regulating radiators or separate units and devices. According to the law of heat emission, the SV temperature is determined by the fourth root of the ratio of the integral solar radiation absorption coefficient (аs) to the emissivity (ε): Т ~ (аs /ε)0.25. During orbital flights, the absorption coefficient аs of the reflecting HRC increases due to the formation of photo-induced and radiative defects that absorb light in the solar range of the spectrum [1–3]. Since the emissivity of all known HRC either remains unchanged or decreases insignificantly (by 5%), the SV temperature increases in flight, which can cause overheating of the equipment and even its failure. This calls for the development of methods of temperature stabilization, one of which can be based on the emissivity increase with the absorption coefficient аs. Another situation in which the emissivity must be changed during an orbital flight is observed for the absorbing coatings. When the SV are in the Earth’s shadow zone or are rotated about their axes even in the zone illuminated by the sun, a fragment of their surface is screened by the airframe and is in the shadow zone, and the surface temperature here decreases because of the lack of the heat source – electromagnetic solar radiation. One of the methods of temperature stabilization at a preset level can be the emissivity decrease with the absorption coefficient аs remaining unchanged. To solve these problems of space materials science, compounds with phase transitions (PT) and general formula A(1-x)BxC(1-y)DyOz in which cations A and (or) C are partly substituted by ions B and D can be used as pigments of reflecting and absorbing coatings. These compounds without substitutes have phase transitions in the temperature dependences of the electric, dielectric, thermophysical, and other properties; after incorporation of substitutes, the phase transitions are displaced along the temperature scale. The displacement magnitude and rate, as well as some other PT characteristics, are determined by the type and concentration of substituting ions and conditions of their incorporation. Works studying the phase transitions in the temperature dependence of the emissivity are few in number, though the importance and practical necessity of such results are obvious. Works with ceramic samples [4] and powders of manganites of rare earth elements [5] should be mentioned here. The present work studies phase transitions in the temperature dependence of the emissivity of synthesized BaTi(1-y)SnyO3 powders to estimate the possibility of their application as pigments of heat-regulating coatings of space vehicles and household and technological enamels. To prepare solid solutions of barium titanate, BaCO3, SnO2, and TiO2 compounds were mixed by a magnetic mixer in bidistilled water; the mixture was evaporated for 1 h at a temperature of 100°C and put in a heat-treatment furnace for synthesis. The compounds with stannum cation concentrations of 10, 15, and 20 mass% were synthesized. The powder so obtained was then mixed with KO-596 lacquer in the 0.4 : 0.6 ratio, and the mixture was put on metal substrates. The

Tomsk State University of Control Systems and Radioelectronics, Tomsk, Russia, e-mail: [email protected]. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 93–95, February, 2009. Original article submitted November 17, 2008. 218

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TABLE 1 CSn, mass% 10 15 20

Тph, °С 80 63 45

Т1–Т2, °С 32–74 35–67 –40–50

Т с, °С 53 51 5

ε1 – ε2 0.56–0.77 0.52–0.71 0.42–0.69

Δε/εс, % 0.21 0.67 31.3 0.19 0.61 31.1 0.27 0.56 48.2 ΔΕ

εс

Δε/ΔТ, deg–1 5.4·10–3 5.9·10–3 3.0·10–3

εmin

εmax

εmax – εmin

0.42 0.36 0.39

0.77 0.76 0.72

0.35 0.30 0.33

Tmin, Tmax, °C °C –67 99 –64 101 –59 82

Fig. 1

temperature dependence of the emissivity ε = f(T) was measured by the calorimetric method at temperatures from –70 to +105°C. From the temperature dependence of the emissivity of the BaTi(1-y)SnyO3 compound (Fig. 1) it can be seen that the phase transition is registered for all examined stannum concentrations. The phase transition in the temperature dependence of the electrical conductivity or dielectric permittivity of unmodified barium titanate is registered at Т = 125°C. After incorporation of cation-substituting elements, it is displaced toward lower temperatures [6, 7]. In this case, the PT temperature for a stannum ion concentration of 10 mass% is Т1 = 80°C; it increases to 63 (Т2) and 45°C (Т3) for stannum ion concentrations of 15 and 20 mass%, respectively. From these experimental results it follows that in the examined temperature interval, the phase transition occurs from the dielectric to quasi-metal state of barium titanate in which titanium cations have been substituted partly by the titanium ions. Among the characteristics of the phase transition are the working temperature Tc determined by the average ε value, emissivity εс at the working temperature, limiting PT temperatures (T1 – T2), limiting values of the PT emissivity (ε1 – ε2), minimum and maximum temperatures Tmin and Tmax registered in a concrete experiment, minimum and maximum emissivity values εmin and εmax observed in the concrete experiment, change of the PT emissivity Δε, rate of the emissivity change Δε/ΔТ characterizing the emissivity change when the temperature changes by one degree, and heat-regulating efficiency Δε/εc that characterises the PT emissivity change normalized by its working value. Numerical values of these PT characteristics for barium titanate modified by 10, 15, and 20 mass% of stannum cations are given in Table 1. From the table it follows that after incorporation of the substituting element, the PT is displaced toward lower temperatures. The working temperature intervals in the PT region for stannum ion concentrations of 10 and 15 mass% involve two subintervals: the first subintervals are from 32 to 74°C and from 35 to 67°C, and the second subintervals are from –67 to 32°C and from –64 to 35°C, respectively. These PT subintervals differ in the rate of ε change;

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moreover, higher rates correspond to higher temperatures. However, the intervals of lower temperatures can also be used as working ones. For a stannum ion concentration of 20 mass%, the PT is described by the straight line whose slope is much smaller in comparison with that of samples with stannum ion concentrations of 10 and 15 mass%. However, this region is wide. In the entire temperature interval, the emissivity changes by 0.35, 0.30, and 0.33 when the stannum ion concentration increases from 10 to 15 and 20 mass%, respectively. In this case, its minimum value is 0.36 and its maximum value is 0.77. Changes in ε in the PT region are much smaller than its total changes; they are 0.21, 0.19, and 0.27 for stannum ion concentrations of 10, 15, and 20 mass%, respectively. The average emissivity values for average temperatures of 53, 51, and 5°C in the PT region amount to 0.67, 0.61 and 0.56 for stannum ion concentrations of 10, 15, and 20 mass%, respectively. The rate of ε change in the PT region determines the corresponding rate of heat regulation for technological processes or objects with the given coatings. Its maximum value is 5.9·10–3 deg–1 for the sample modified by 15 mass% of stannum ions. It is slightly smaller (5.4·10–3 deg–1) for the sample with 10 mass% of stannum ions, and for the sample with 20 mass% of stannum ions, it is almost halved. The heat-regulating efficiency Δε/εc indicates the relative difference (in percent) of the limiting emissivity values in the PT region from its average (working) value. It characterizes the change of the emitted energy normalized by its working value when the temperature of the object or technological process deviates from the working temperature. It amounts to 31.3, 31.1 and 41.8% for samples with 10, 15, and 20 mass% of stannum ions, respectively. These data demonstrate that the maximum change of the emissivity can be as great as 20.9% when the temperature deviates from its working value. It can be obtained using a coating with the BaTi0.8Sn0.2O3 pigment. For other examined pigments, the maximum change of the radiated thermal energy is smaller. Our investigations have demonstrated the possibility of synthesizing powders based on barium titanate with titanium ions partly substituted by stannum ions that possess phase transitions in the temperature dependence of the emissivity. Such transitions have been registered for the first time for coatings based on barium titanate powders with partly substituted ions. The characteristics of these transitions are promising for practical application in heat regulating coatings of various objects. This work was supported in part by the Russian Foundation for Basic Research (grant No. 07-08-13558-ofi_ts).

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