Development of semiconductor gas sensor ultra-sensitive in ppb level

为了开发高灵敏度的气体传感器，人们认识到控制这类传感器灵敏度（传感器响应）的三个关键因素，即受体功能、传感器功能和效用因素。当考虑到目标气体（还原性气体）在扩散到传感器装置的过程中与氧化物表面发生反应时，最后一个因素的重要性就显而易见了。在本研究中，我们从以上三个因素出发，探索了传感材料加工的新途径。由SnO_2整齐的溶胶制成的薄膜具有相当小的孔。然后，在旋转涂层之前，将不同数量的聚乙二醇（PEG）分散到SnO_2溶胶中，试图改变孔隙结构。结果表明，在相同的旋涂条件下，薄膜对H_2的响应随PEG含量的增加而增加，与孔径的增加平行，但膜厚相应增加。显然，较大的孔隙尺寸有利于提高利用系数。我们尝试通过调整水热处理的条件来增大SnO_2在溶胶中的晶粒尺寸，并取得了成功。目前，单分散SnO_2溶胶的晶体尺寸可达16nm左右。结果表明，随着晶粒尺寸的增大和孔径的增大，所制备的薄膜对H_2的响应有明显的增强趋势。此外，传感器对H_2S的响应对晶粒尺寸和薄膜厚度的依赖性更强。这种气体比H_2分子量大，同时在氧化物表面的反应性也更强。这些特征使这种气体对薄膜的微观结构更加敏感。本文研究了Co_3O_4（通常为0.5 wt%）-SnO_2复合粉末在行星球磨机中充分混合制备的厚膜的传感器响应。这里有趣的是，响应，特别是对CO的响应，随着球磨时间的增加而急剧增加。结果表明，响应的提升几乎与孔隙大小的增加并行。由此可见，孔隙的扩大更有利于CO分子在膜中扩散，从而产生极大的响应。比较了目前报道的几种SnO_2基器件对不同浓度CO的传感器响应。当前薄膜的显著大响应似乎保证了传感器响应与微观结构的相关性。我们还研究了用于检测其他气体的气体传感器。少

In order to develop gas sensors for very high sensitivity, three key factors have been recognized to control the sensitivity (sensor response) of the sensors of this type, i.e., receptor function, transducer function and utility factor. The importance of the last factor is made obvious when one considers that the target gas (reducing gas) reacts with the oxide surface on its way of diffusion into the sensor device. In this study, we have been investigating new ways of processing for sensing materials from the viewpoint of the above three factors.The thin films derived from the neat sols of SnO_2 appeared to have pores rather small in size. It was then tried to modify the pore structure by dispersing various amounts of poly ethylene glycol (PEG) into the SnO_2 sol prior to spin-coating. It was observed that, under the same spin-coating conditions, the responses of the resulting thin films to H_2 increased with increasing PEG content, in parallel with the increase of pore size, although … More film thickness increased correspondingly in this case. Obviously larger pore sizes are favorable for increasing utility factor.We have tried to grow the size of SnO_2 crystallites in the sols and succeeded in it by adjusting the conditions of hydrothermal treatments. Now the sols of mono-dispersed SnO_2 with a crystallite size up to about 16 nm are available. We have shown that the thin films derived tend to increase sensor response to H_2 rather sharply as the crystallite size increases and the pore size increases accordingly. In addition, more strong dependence on the crystallite size and film thickness was disclosed for the sensor response to H_2S. This gas has larger molecular weight than H_2 and at the same time it is more reactive on the oxide surface as well. These features make this gas more sensitive to microstructure of the films.We have investigated sensor responses of thick films derived from Co_3O_4 (typically 0.5 wt%)-SnO_2 composite powders, which were prepared by mixing the component oxides thoroughly in a planet ball mill. What is of interest here was that the response, especially to CO, increased rather drastically with an increase in ball-milling time. It turned out that the promotion of response was almost in parallel with an increase in pore size. It follows that enlarged pores would make it more favorable for CO molecules to diffuse through the films, leading to the extremely large responses. The sensor responses to various concentrations of CO are compared among several SnO_2 based devices so far reported. The outstandingly large responses of the present film appear to assure the relevance of sensor response with microstructure.We investigated also gas sensors for detecting other gases. Less