Design for ultra sensitive oxide semiconductor gas sensor by controlling meso-pore and high order structure

近年来，各种各样的气体从不同的来源排放到我们的生活空间、工作空间或户外。其中许多污染物，如空气污染物和挥发性有机化合物，对人类和环境都是有害的。这些气体的浓度大多很低，因此对它们的监测需要极好的传感特性。在对半导体气体传感器的基本理解的基础上建立传感器设计原则似乎势在必行。本课题基于Knudsen扩散方程和目标气体的一阶表面反应，从理论上研究了气体输运对半导体气体传感器气敏性能的影响。通过理论研究发现，实现这种高灵敏度气体传感器的重要因素是：(1)气体扩散与表面反应发生竞争的介孔的控制。(2)控制涉及介孔的二次颗粒的大小。(3)控制气体快速扩散的大孔隙。在这个研究项目中还发现了以下几点。(1)通过控制锡酸凝胶的制备工艺和水热处理条件，可以使SnO_2晶粒尺寸扩大并控制在6 ~ 12 nm范围内。(2)通过调整ph值，可以使SnO_2在SiO_2球形颗粒上形成较薄的包覆层。(3)通过混合有机试剂，可以使SnO_2薄膜形成较大的孔隙。此外，还可以用离子交换法制备氧化钨脱水水溶胶。通过对溶胶的洗涤、离心处理和/或超声搅拌，可以控制晶体的大小，从而提供了一种控制晶体大小的方法。由氧化钨脱水溶胶制成的厚膜装置对NO_2具有极高的灵敏度。

Recently, numerous kinds of gases are emitted from various origins into our living space, working space or outdoors. Many of them, like air pollutants and VOCs (volatile organic compounds), are hazardous to human beings and environments. Most of these gases are present at low concentrations so that extremely good sensing characteristics are requested for their monitoring. It seems imperative to establish sensor design principles on the basis of fundamental understandings of semiconductor gas sensors. In this research project, the influences of gas transport on the gas sensing properties of semiconductor gas sensor are investigated theoretically, based on the diffusion equation assuming Knudsen diffusion and first order surface reaction of a target gas. The followings are found to be important factors in order to realize such high sensitive gas sensors by theoretical study(1) Control of mesopores where competition between gas diffusion and surface reaction takes place.(2) Control of size of the secondary particles involving mesopores.(3) Control of macropores through which gas diffusion is rapid.The followings are also found in this research project.(1) Crystallite size of SnO_2 could be enlarged and controlled in the range from 6 nm to 12 nm by controlling preparation procedure of stannic acid gel and hydrothermal treatment conditions.(2) SnO_2 could be coated thinly on SiO_2 spherical particles by adjusting pH.(3) Large size pores could be introduced into thin film of SnO_2 by mixing organic reagent.Besides, aqueous sols of tungsten oxide dehydrate could be prepared by an ion-exchange method. The size of the crystallites could be controlled by washing, centrifugal treatments and/or ultrasonic agitation of the sol, providing a method to control the crystallite size. It was also found that thick film device derived from tungsten oxide dehydrate sol gave an extremely high sensitivity to NO_2.