Metal precursor solution of fabricating Zn- and Sn-doped MgO powder was prepared by dissolving 5.0×10^-4 mol zinc acetate dihydrate [Zn(CH₃COO)₂·2H₂O], 2.5×10^-4 mol tin (Ⅱ) chloride [SnCl₂·2H₂O], and 0.025 mol magnesium nitrate hydrate [Mg(NO₃)₂·6H₂O] in absolute ethyl alcohol (100 mL). Then 4 mL ethanol amine and 16 mL polyethylene glycol (PEG 400) were mixed and added to the above solution. Subsequently, 20 mL absolute ethyl alcohol and 20 mL ammonia were mixed and added dropwise to the metal ion solution at 70 ℃, followed by constant magnetic stirring for 2 h and standing for 8 h. The obtained precipitate was washed with ethanol and deionized water repeatedly several times, and annealed at high temperature for 4 h. Next, the powder was mixed with PEG-400 and ethanol in a mass ratio of 1:1:10 and this slurry was spin coated onto AC PDP front panel with speed of 100 r/min.

The schematic diagram of the panel with functional layer was shown in Fig. 1 and specific parameters were reported in our previous work^[14]. XRD and XPS were used to analyze the structure of the functional layer. Moreover, in order to study the effect of functional layers on the morphology and transmittance of AC PDP, SEM and an ultraviolet -visible spectrophotometer were introduced, respectively.

Figure 1.  Structure of AC PDP front panel with Zn- and Sn-doped MgO powdered functional layer.

The XRD patterns of the Zn- and Sn-doped MgO powder annealed at different temperatures are shown in Fig. 2a~Fig. 2c. At 450 ℃ (Fig. 2a), each orientation peak of MgO was low, indicating the poor crystallinity. At 550 ℃ (Fig. 2a), the sample had excellent crystallinity and no feature peaks from zincite or tin oxide were observed. The (200) orientation of MgO presents the strongest intensity. When the annealing temperature was up to 650 ℃, (200), (220) and (222) orientation peaks of MgO decreased and some impurity peaks appeared. Thus, 550 ℃ was accepted as our final annealing temperature. The possible reactions existing in this experiment were discussed as follows. First, ammonia hydrolyzed and released OH^-. In the alkaline conditions, Zn(CH₃COO)₂·2H₂O, SnCl₂·2H₂O and Mg(NO₃)₂·6H₂O hydrolyzed, respectively, i.e.

Figure 2.  XRD patterns of Zn- and Sn-doped MgO powder as a function of annealing temperature

Then, Zn(OH)₂, Sn(OH)₂ and Mg(OH)₂ decomposed in the annealing furnace and the corresponding reaction were:

According to the above reactions, the Zn- and Sn- doped MgO powder was fabricated successfully.

Fig. 3 shows the XPS pattern of fabricated Zn- and Sn-doped MgO powder. The Mg, Zn, Sn and O characteristic peaks could be clearly detected, which confirmed the existence of Zn and Sn element. Compared with the Mg and O feature peaks, the intensities of Zn and Sn peaks were relatively weak, which attributed to the less quantity of the dopant. The XRD and XPS results implied that Zn²⁺ and Sn²⁺ had been incorporated into the lattice of MgO. Moreover, it could be found that the diffraction angles of all diffraction peaks of the Zn- and Sn-doped MgO sample were nearly identical with pure MgO. The possible reason was that the doping amount of Zn and Sn in MgO was very small, although the ionic radius of Zn²⁺ and Sn²⁺ was larger than Mg²⁺ (Mg²⁺: 0.072 nm, Zn²⁺: 0.074 nm, Sn²⁺: 0.112 nm).

Figure 3.  XPS pattern of Zn- and Sn-doped MgO powder

Fig. 4a shows the SEM image of the AC PDP front panel coated with Zn- and Sn-doped MgO powdered functional layer. The particles were evenly distributed on the substrate surface. Fig. 4b shows the visible light transmittances, in which the conventional AC PDP front panel without functional layer was used as a reference. Because of the favorable light transmittance of Zn- and Sn-doped MgO material and the discontinuous distribution of particle, the functional layer made the transmission only decreased by 4%~7% in the visible light wavelength, which was fit for application in AC PDP.

Figure 4.  Effect of functional layers on the morphology and transmittance of AC PDP

Fig. 5 compares the firing and sustaining maximum voltages of AC PDP test panels with Zn- and Sn-doped MgO powdered functional layer and conventional MgO layer in 7% Xe-Ne. Within the gas pressure range of 200~450 torr, the panel with Zn- and Sn-doped MgO powdered functional layer always presented lower discharging voltage (firing and sustaining maximum voltages) than that with the conventional MgO layer.

Figure 5.  Discharge characteristics of AC PDP test cells with Zn- and Sn- doped MgO powdered functional layer and pure MgO protective layer

For example, the firing and sustaining maximum voltages of Zn- and Sn-doped MgO powdered functional layer were 237 V and 249 V at 250 torr, which were about 17.1% and 18.4% lower than the pure MgO protective layer (286 V, 305 V), respectively. According to the Hagstrum theory^[15], Auger neutralization is the main mechanism of secondary electrons emission in AC PDP, and valence electrons of the protective layer in AC PDP can be emitted to vacuum levels when the energy level meets the following condition^[16]:

where $E$, $E_{\text{i}}^0$, ${E_{\text{g}}}$, and $\chi $ are energy of emitted electron, the ionization energy of discharging gas atom, the band gap energy and the electron affinity of the protective layer. It is known that the band gap energy of MgO, ZnO and SnO₂ are 7.8 eV, 3.37 eV and 3.6 eV, respectively. Thus, the protective layer coated Zn- and Sn-doped MgO powder is prone to produce more secondary electrons than pure MgO, which reduces the discharging voltages effectively.

In conclusion, Zn- and Sn-doped MgO powder has been prepared by a chemical coprecipitation method. It was spin coated on the commercial MgO protective layer in the AC PDP front panel. It was found that zinc and tin atoms were incorporated into the lattice of MgO, which was confirmed by XRD pattern and XPS spectra. The test panel with the Zn- and Sn-doped MgO powdered functional layer presented lower firing and sustain maximum voltages than the pure MgO protective layer did. With 250 torr 7% Xe-Ne pressure, the firing and sustain maximum voltages of the test panel with the Zn- and Sn-doped MgO functional layer were about 17.1% and 18.4% lower than that with a conventional MgO protective layer, respectively.