$SnO_2$ is a widely used sensor material exhibiting high sensitivity. The semiconductor type gas sensors using $SnO_2$ have several merits and demerits: simpler in structure and easier in production. However, they show complex sensing behavior by reason of surface chemistry, structure of electrode, filter and microstructure. Among these factors, the microstructure is an important parameter for electrical and mechanical properties. Understanding of microstructural effects would be therefore essential for examining sensing mechanisms and preparing industrial devices. The difficulty for analyzing the effect of microstructure is related to the fact that the change of any process can affect the microstructure. To overcome this difficulty, the microstructural factors should be separated. The present study on the microstructure effect has been made in three ways.
In chapter III, the relationship between microstructure and electrical properties has been investigated without changing the sintering condition in order not to change the defect chemistry. The effect of sensor dimension has first been studied. For cylinder-type specimens, a thickness variation did not change the sensitivity, but an increase in diameter lowered the sensitivity. It was supposed that the gas diffusion from the surface and the combustion of the gas caused a gradient of the gas concentration and conductivity of the sample (reaction-diffusion mechanism). The effect of the green density has also been studied. The green density increased with increasing isostatic pressure. However, the sintered density was identical to the green density. The particle size did not change with the green density because of the same sintering condition. The sensitivity increased as the green density decreased. This result can be attributed to the decrease of coordination number between the particles, the increase of surface area of the specimen, and an increased diffusion depth of the $H_2$ gas. The effect of large pores on electrical properties of $SnO_2$ gas sensors has also been investigated. Large intergranular pores of 10-20 μm diameter were created in $SnO_2$ gas sensors by adding starch powder in concentrations from 0 to 15 wt%. Addition of starch powder did not change sintering behavior and resultant microstructure except for the formation of large pores. Gas sensitivity increased with starch addition. This result was attributed to enhanced diffusion of gases into the bulk $SnO_2$ through the created large pores. Introduction of large pores in a bulk $SnO_2$ matrix by adding starch can be a simple and useful way of microstructure control and sensitivity enhancement.
In chapter IV, the effect of sintering process on the electrical properties has been investigated. Sintering temperature and relative density were chosen as the process variables. The grain size of the specimen increased with increasing sintering temperature, but the relative density did not change because the sintering mechanism was surface diffusion. The apparent activation energy for conduction was measured with green density and sintering condition. The samples with high green density and high sintering temperature showed low activation energy for conduction and low sensitivity at 375℃. This result was attributed to the disappearance of small necks which were sensitive to gas and conduction between the particles. In an attempt to improve the sensitivity, the powder was heat-treated at 1000℃ for 10 h before compacting and sintering. The samples with the heat-treated powder maintained high activation energy during consequent compacting and sintering. During the heat treatment of the very loosely packed powder sintering already occurred and this might suppress additional grain growth and neck growth during sintering of compacted powder. As a result, the samples with heat-treated powder showed high sensitivity.
In chapter V, the thick film type $SnO_2$ gas sensor was prepared using a new processing technique, centrifugal coating method. The $SnO_2$ powder dispersed in a suspension was attached on a substrate using centrifugal force without organic vehicle which is necessary in screen printing method. Big particles were piled mainly close to the substrate and small particles were stacked near the top of the film. The thickness of the film can be varied by the concentration of the $SnO_2$ suspension. The sensitivity of the samples showed a stable plateau value above a specific thickness. A CIP (Cold Isostatic Pressing) process after centrifugal coating changed not only the green and sintered density but also the subsequent electrical properties. The samples prepared by the centrifugal coating method showed a higher sensitivity than those prepared by the conventional screen printing method. It appeared that the small particles in the surface region improved the sensitivity of the thick film. The developed centrifugal coating method would be a new technique for manufacturing gas sensors.
$SnO_2$ 는 높은 감도를 보이기 때문에 널리 쓰이는 센서재료이다. SnO2를 이용한 반도체형 센서는 구조상 간단하고 제조가 쉽기 때문에 널리 쓰이는 반면, 표면 화학, 전극의 구조, 필터, 미세구조의 영향 등으로 복잡한 감지 거동을 보인다. 이중에서 미세구조는 전기적 기계적 성질에 중요한 변수로서 미세구조의 이해는 센서기구를 확인하고 실제 제품을 생산하는데 필수적이다. 미세구조의 영향을 확인하는데 어려움은 하나의 공정이라도 변화시키게 되면, 미세구조가 변화하는데 있다. 이를 극복하기 위하여 미세구조 변수를 3가지로 분리하여 연구하였다. 3장에서는 물리적으로 미세구조를 변화시키며 전기적 특성을 측정하였다. 시편의 크기 효과와 밀도를 변화시켰을 때의 전기적 성질의 변화를 먼저 확인하였다. 감도를 증진시키기 위하여 시편 내에 거대한 기공을 형성하여 미세구조와 전기적 특성의 변화를 확인하였다. 4장에서는 소결 온도를 변화시키며 전기적 성질을 측정하였다. 소결온도와 성형압이 증가할수록 전기전도도와 감도는 낮아 졌다. 이를 방지하기 위하여 분말상태에서 열처리를 하여 전기전도도와 감도의 저하를 줄였다. 5장에서는 새로운 후막센서로서 원심부착법을 도입하여, 후막의 두께와 밀도를 변화시키기고 이에 따른 미세구조와 감도의 변화를 측정하였다.
