正溫度電阻係數熱敏電阻器的製程與電性

Abstract

居里點約 70℃之鈦酸鋇鍶正溫度電阻係數熱敏電阻器和居里點 從140℃到280℃之鈦酸鉛鍶正溫度電阻係數熱敏電阻器 ，在本研究中已 成功的製備 。本研究將針對燒結曲線中 ，持溫處理及降溫速率對鈦酸鋇 鍶正溫度電阻係數熱敏電阻器之電阻率的影響加以探討 ﹔並研究在1200 ℃時之持溫時間對鈦酸鋇鍶熱敏電阻器之電阻溫度係數(TCR)的影響 。由 實驗結果得知 ，較慢的降溫速率或在1200℃較長的持溫時間 ，會導致電 阻率與溫度之特性曲線中之室溫電阻率和最大電阻率的增加 。持溫時間 的長短亦會影響鈦酸鋇鍶熱敏電阻器之電阻溫度係數 。從電容與電壓量 測法求得之表面受體濃度 ，可發現到持溫在1200℃或增加在此溫度之持 溫時間 ，會使表面受體濃度增加 。較慢的降溫速率亦會增加表面受體濃 度 。表面受體濃度的增加 ，會使得鈦酸鋇鍶熱敏電阻器之電阻溫度係數 變大 。利用黑王與強克模型可得知 ，增加表面受體濃度會使得最大電阻 率發生時之溫度變低 ，並增加最大電阻率和室溫電阻率 。從複數平面阻 抗分析法 ，可觀察到室溫電阻率隨持溫時間的增加或降溫速率的減少而 增加 。此現象主要是來自於晶界電阻率的增加 。複數平面阻抗分析法亦 被用來探討鈦酸鋇鍶熱敏電阻器之晶界電阻率對溫度的變化 ，並可發現 此項變化, 主要是由晶界電阻率所貢獻 。 本文亦將探討鈦酸鉛鍶熱 敏電阻器之電學性質 。在相同的製程條件下 ，對鈦酸鉛鍶正溫度電阻係 數熱敏電阻器做阿漢尼斯圖 ，可計算出表面受體濃度 ，並可發現到鈦酸 鉛鍶熱敏電阻器中鉛含量的增加 ，會造成表面受體濃度下降 從掃描式電 子顯微鏡可觀察到 ，增加鉛含量會增加晶粒的成長 。在正溫度電阻係數 的區域 ，由阿漢尼斯圖中的斜率和量測到的介電常數 ，可求得晶界的能 障 ，並觀察到對於不同鉛含量之鈦酸鉛鍶熱敏電阻器而言 ，此能障均隨 著溫度的增加而增加 ，且造成熱敏電阻器的正溫度電阻係數效應 ，此現 象和黑王模型相符合 。複數平面阻抗分析法亦被用來研究鈦酸鉛鍶熱敏 電阻器之晶粒及晶界電阻 。利用此方法可明顯地觀察到晶粒電阻遠小於 晶界電阻 ，而且電阻率對溫度的變化 ，主要是來自於晶界電阻的變化 ，此結果和以鈦酸鋇為主之熱敏電阻器所觀察到的現象一樣 。 (Ba,Sr)TiO3 positive temperature coefficient of resistivity (PTCR) thermistors with Curie point around 70℃ and (Pb,Sr)TiO3 PTCR thermistors of compositions PbxSr0.994-xY0.006TiO3±δ ( x=0.5, 0.6 and 0.7 ) with Curie point over 140℃ up to 280℃ were prepared successfully. The influence of sintering profile, with emphasis on the effect of soaking treatment and cooling rate, on the resistivity of (Ba,Sr)TiO3 thermistors has been investigated. The influence of soaking time at 1200℃ on the temperature coefficient of resistivity (TCR) of (Ba,Sr)TiO3 thermistors has also been studied. Soaking treatment at 1200℃ or slower cooling rate was found to result in an increase in the room temperature and the maximum values of resistivity in the resistivity-temperature characteristics. Soaking duration has influence on the room temperature resistivity, the maximum resistivity, the temperature (Tmax) at which the maximum resistivity appears and TCR of the (Ba,Sr)TiO3 thrmistors. The surface acceptor density (Ns), whose value was extracted from the capacitance-voltage measurement, was found to increase with the soaking treatment and soaking duration at 1200℃ or slower cooling rate. The increased surface acceptor density with increasing the soaking time at 1200℃ increases the temperature coefficient of resistivity of (Ba,Sr)TiO3 thermistors. From the derivations in the scheme of Heywang-Jonker model, higher room temperature and maximum resistivity and lower Tmax are expexted to occur as a result of the increasing Ns, as confirmed by the experimental data. The increased room-temperature resistivity with the increase of soaking time or decrease of cooling rate is mainly contributed by the increase of grain boundary resistivity examined by complex-plane impedance method. The variation in grain boundary resistivity with temperature of (Ba,Sr)TiO3 thermistors is also examined by complex-plane impedance analysis and is primarily a grain boundary resistive effect. The electrical properties of the (Pb,Sr)TiO3 thermistors were also studied in this thesis. The surface acceptor density (Ns) calculated by the Arrhenius plot of the (Pb, Sr)TiO3 PTCR thermistors under the same fabrication conditions are found to decrease with increasing Pb content and the grain sizes are increased with increasing Pb content. The potential barrier heights of grain boundaries versus various Pb contents, calculated from the slopes of the Arrhenius plots and the measured apparent dielectric constants, are found to increase with increasing temperature and contribute to the PTC effect of (Pb,Sr)TiO3 thermistors in agreement with the Heywang model. The resistances of grain and grain boundary of the (Pb,Sr)TiO3 thermistors were studied by complex-plane impedance method. It is clearly observed that the grain resistance of the ceramic is much smaller compared to its grain boundary resistance and the change in resistivity with temperature is contributed mainly by the variation of grain boundary resistance.