ZnO:Zn低能电子发光的亮度饱和

本文讨论了ZnO:Zn低能电子发光亮度饱和的物理原因。指出,在Vg=Va(通常数字显示的情况)的条件下,亮度饱和的主要原因是ZnO:Zn次级电子发射和阴极发射疲劳;若取Vg=Va,则饱和亮度成倍提高,这时热猝灭将是亮度饱和的主要因素。同时指出,对于一定的输入功率,存在着一个与最佳效率相对应的最佳阳极电流密度;低阳极电流密度,高阳极电压往往是低效率的。作者根据本文的研究结果,制成了饱和亮度高达37,000cd/m2的发光管和饱和亮度为22,000cd/m2、能在5,000至10,000cd/m2稳定工作的高亮度数字显示管。后者配上适当的滤光片、在阳光直接照射或照度高达100,000lux的条件下仍能清晰地阅读。

BRIGHTNESS SATURATION OF THE LOW ENERGY ELECTRON LUMINESCENCE OF ZnO:Zn

The relationship of the brightness B and the anode voltage Va for the low energy electron luminescence(LEEL) may be expressed byβ= QVanwhere Q and n are constants. This equation shows that the brightness is proprotional to Van. But at higher anode voltages,the brightness gees through a maximum and then decreases. That is so-called "brightness saturation" of LEEL. In this paper,the basic physical reasons of the brightness saturation have been discussed. It was pointed out that when Va=Vg(Vg is the grid voltage) the brightness saturation is not due to thermal quenching or dynamic saturation of the luminescent centres,but mainly due to the following processes:first,the electron emitting from the cathode is fatigued,resulting in a decrease of anode current density,and second,the ratio cf the anode and the grid current densities is decreased,since the secondary electron emission of ZnO:Zn increases with increasing applied anode voltage. For a flat triode luminescent device the ratio Ia/Ig may be given bywhere u is a constant,o(Va)is the effective cross section of the grid,d. and dg are the distances from the anode and from the grid,to the cathode respectively. When a voltage much less than that of the anode is applied to the grid,the secondary electrons emitted from the znO:zn will return to the anode,so the above expression can be simplified as As can be seen,the ratio of the anode and grid current densities increases with increasing (Va)1/2. In this case,the saturation brightness will obviously increase,e.g.,4230cd/m2 for Va=Vg and 8918cd/m2 for Vg<>Va,the principal factor for the brightness saturation will be thermal quenching. The brightness is related to the input power W0 bywhere a and b are constants,B10 is the brightness at 10℃,P the power efficiency,the thermal conductivity,d the thickness of the anode substrate,and TO the temperature of the external surface of the substrate. For a given anode currect density,the optimal input power Wm corresponding to the efficiency maximum can be written asFinally,according to the results mentioned above,a luminescent device with saturation brightness of 37,000cd/m2 as well as a high brightness vacuum fluorescent numerical display tube havebeen manufactured. The brightness of the display tube increases with increasing anode voltage and reaches a maximum(22,000cd/m2) at about 157V. At still higher voltages,the temperature exceeds the quenching temperature of the ZnO:Zn phosphor and the brightness decreases. The display tube can be steadily operated at the brightness of 5,000-10,000cd/m2 (at Va=40-65V),and a good legibility in a high ambient illumination (e.g.,bright sunlight falling directly or an ambient illumination of 100,000lux) can be achieved.