温度对AZO薄膜红外辐射性能的影响

随着全球资源的减少和环境的恶化，节能减排已成为人们关注的焦点，具有保温隔热功能的低辐射玻璃成为研究的热点。提高玻璃保温隔热性能最有效的方法就是在其表面涂覆低辐射率层。原材料丰富、导电性能好、可见光透过率高等优势使得Al掺杂ZnO (AZO)薄膜成为最具潜力的低辐射率层。系统研究了温度对AZO薄膜红外辐射性能的影响，分析了变化机理。首先研究了在一定的温度下持续一段时间后，AZO薄膜的红外比辐射率的变化情况。然后研究了在变温环境中红外比辐射率的变化情况。采用直流磁控溅射法在室温下玻璃基片上沉积500 nm厚的AZO薄膜，将薄膜放到马弗炉中进行热处理，在100～400 ℃空气气氛下保温1 h，随炉冷却。采用X射线衍射仪对AZO薄膜进行物相分析，采用扫描电子显微镜观察薄膜表面形貌变化。利用四探针测试法测量AZO薄膜的电阻率，采用红外比辐射率测试仪测试薄膜红外比辐射率, 可见分光光度计测量可见光谱。测试的结果表明，薄膜热处理前后均为六角纤锌矿结构，(002)择优取向。300 ℃及以下热处理1 h后，(002)衍射峰增强，半高宽变窄，晶粒尺寸长大。随着热处理温度的升高，薄膜的电阻率先减小后增大，200 ℃热处理后的薄膜具有最小的电阻率(0.9×10-3 Ω·cm)。热处理温度升高，晶粒长大使得薄膜电阻率降低。热处理温度过高，薄膜会从空气中吸收氧，电阻率下降。薄膜的红外比辐射率变化趋势和电阻率的一致，在200 ℃热处理后获得最小值(0.48)。自由电子对红外光子有较强的反射作用，当电阻率低，自由电子浓度高的时候，更多的红外光子被反射，红外辐射作用弱，红外比辐射率小。薄膜的可见光透过率随着热处理温度的升高先减小后增大，200 ℃热处理后的薄膜的可见光透过率最小，但仍高达82%。这种变化是由于自由电子浓度变化引起的，自由电子对可见光有很强的反射作用。选取未热处理和200 ℃热处理后的样品进行变温红外比辐射率的测量，将样品放在可加热的样品台上，位置固定，在室温到350 ℃的升温和降温过程中每隔25 ℃测量一次红外比辐射率，结果表明，在室温到350 ℃的温度范围内，AZO薄膜的红外比辐射率在升温过程中随着温度的上升而增大，在降温过程中减小，经过整个升、降温过程后，薄膜的红外比辐射率增大。

Effects of Temperature on the Infrared Emission Performance of AZO Films

With the decrease of global resources and environmental deterioration, energy saving and emission reduction have become a hot topic and Low-E glass with thermal insulation performance is becoming the research focus. To improve the thermal insulation performance of glass, the simplest and most effective method is plating low-emissivity coatings on the surface of them. . The Al doped ZnO (AZO) film is the most potential low-emissivity layer for Low-E glass, due to rich raw materials, high conductivity and high transparency and so on. Effects of temperature on the infrared emission performance of AZO films were researched and mechanisms of changes were analyzed in this study. The change of infrared emissivity of AZO film kept at a certain temperature for some time was studied firstly, and then the change of infrared emissivity in variable temperature environment was studied. 500 nm-thick AZO films were deposited on glass substrates at room temperature by direct current magnetron sputtering and then were put in muffle furnace for heat treatment. Films were kept at 100～400 ℃ for 1 h in air and then cooled to room temperature in the furnace. The phase of AZO films was analyzed by X-ray diffraction and the surface morphology was observed by scanning electron microscope. The resistivity was measured by four probe method and the infrared emissivity was measured using infrared emissivity measurement instrument. Visible spectrum was measured by visible spectrophotometer. The results showed that AZO film before and after annealed all show hexagonal wurtzite structure and (002) preferred orientation. With the annealing temperature rising to 300 ℃, the intensity of (002) diffraction peak increases, the full width of half maximum (FWHM) narrows down and grain size increases. With the increase of annealing temperature, the resistivity decreases firstly and then increase. The film annealed at 200 ℃ shows the lowest resistivity of 0.9×10-3 Ω·cm. The decrease of resistivity is attributed to the growth of grains. The film annealed at higher temperature in air will absorb oxygen, resulting in the decrease of resistivity. The change of the infrared emissivity with annealing temperature agrees to that of resistivity. The film annealed at 200 ℃ shows the lowest emissivity of 0.48. Infrared photons are strongly reflected by free electrons. When the resistivity is low and the concentration of free electrons is high, more infrared photons are reflected, the infrared radiation weakens, and the infrared emissivity decreases. The transmittance decreases firstly and then increases. It is the lowest at 200 ℃ but still up to 82%. This change is caused by the change of free electron concentration. Free electron strongly reflects visible light. The infrared emissivities of the films as-deposited and annealed at 200 ℃ were measured during the process of heating and cooling between room temperature and 350 ℃. The sample was fixed on the heated stage and its emissivity was recorded every 25 ℃. It was found that the infrared emissivity increases with the increase of temperature during the heating process, and decreases during the cooling process. After the whole process, the infrared emissivity of the AZO film increases.