Thermogravimetric Investigation of the Formation Electrochromic Layers of Nickel Oxide

Introduction 

Thin layers of nickel oxide have been used for some time as optically active elements in electrochromic components [1]. Electrochromic layers change their optical transmission when a low electrical voltage is applied to the layer. Chemically, the oxidation state of the molecules in the layer changes, which in turn changes their interaction with light and hence their transmission behavior. These types of layer are used mainly for electrochromic windows and mirrors whose transmission or reflexion behavior automatically adapts to the actual brightness. To prepare such layers, a thin film of nickel acetate is applied to the substrate in a dipping process.

This is then heated and held isothermally for a certain time at a particular temperature (i.e. baked). An electrochromic layer of nickel oxide is formed. The properties of the layer are influenced by the temperature and duration of the baking process: if the temperature is too high or the baking time too long, the electrochromic properties degrade to complete inactivity. If the layer is not properly thermally treated, there is the risk that the active layer separates from the substrate. Thermogravimetry can be used to investigate the influence of temperature and time on the formation of the electrochromic layer. The difficulty is that the layers formed on the substrate are very thin. This results in extremely small weight changes that are often close to the detection limit of the thermobalance. The powder form of the starting material used for the layers is therefore often investigated. Compared with the actual layers, there are however large differences in the microstructure of the powdered starting material. This of course makes a comparison of the results more difficult.

Experimental

A nickel hydroxide slurry was prepared from nickel sulfate using a sol-gel process. Nickel acetate was precipitated from this slurry through the addition of acetic acid (for details see [2]). In the baking process, the acetate groups decompose thermally leaving behind nickel oxide. The thin layers were prepared in the same way. Aluminum foil (thickness 0.01 mm) and glass (glass thickness 1 mm) were used as substrates. The samples were analyzed in 150-µl Pt crucibles using a TGA/SDTA851e.

Measurements

TGA curves for the thin layers on the glass and aluminum substrates and for the dried sol are shown in Figure 1. All the samples initially lose moisture. Afterward, thermal decomposition of the acetate groups begins in which the actual nickel oxide layer is formed. With the thin layers, decomposition of the acetate group already begins at about 285 °C, with the powdered samples at a slightly higher temperature. Over the temperature range 200 to 360 °C, the dry powder loses about 34.5% of its mass. Knowledge of this weight loss and the weight change of the thin layer (0.19 mg, see Fig. 1) allow the ratio of the mass of substrate to layer to be estimated. For the glass substrate this ratio is about 185, and for the aluminum foil about 13. This also explains the different relative weight loss steps during the decomposition of the acetate groups. In addition, it was possible to measure an exothermic SDTA signal during the decomposition step for the layer on the aluminum foil and for the sol.

Conclusions

The formation of nickel oxide from nickel acetate on glass and aluminum substrates and in powdered sol material was measured using TGA. It can be seen that in this process the decomposition of the decomposition of the acetate groups in the layers occurs about 15 K lower than in the sol. By carefully choosing a suitable temperature and baking time, it is possible to obtain a layer with a precise nickel oxide content. This allows the electrochromic properties to be determined as a function of the nickel oxide content. As a result, baking conditions are established under which a nickel oxide layer is formed with optimum electrochromic activity and lifetime.

_{Thermogravimetric Investigation of the Formation Electrochromic Layers of Nickel Oxide | Thermal Analysis Application No. UC 176 | Application published in METTLER TOLEDO Thermal Analysis UserCom 17}