Porozite yapıcıların La-Ba$TiO _3$seramiklerde mikroyapıya etkisi

Özet

Yüksek porozite ve elektriksel direnç, seramik esaslı sensör malzemelerin ana karakteristiklerinden birisi olup sinterleme parametrelerinin değişimi ve La+3 dop etme ile Ba$TiO _3$ yapısına söz konusu özellikler kazandırılabilmektedir. Ba$TiO _3$ esaslı seramiklerin elektriksel özellikleri, söz konusu seramiklerin çeşitli uygulama alanları ve elektriksel davranışının açıklanmasındaki güçlükler nedeniyle, günümüzde ilgi çekici bir araştırma konusu olmaya devam etmektedir. Ba$TiO _3$ esaslı seramiklerde elektriksel direnç, perovskit kafeste baryum veya titanyum konumlarına nadir toprak elementleri dop edilerek etkin şekilde kontrol edilebilmektedir. Porozite yapıcılar ile katkılandırma yönteminin de, sensör uygulamalarında kullanılacak poroz seramiklerin üretiminde etkili bir yöntem olduğu bilinmektedir. Bu çalışmada, grafit ve PMMA (Polimetil metakrilat) katkılarının La+3 katkılı Ba$TiO _3$ esaslı seramiklerin porozite özellikleri üzerine etkileri incelenmiştir. Farklı bileşimlerde grafit ve PMMA katkıları içeren örnekler için porozite çalışmaları ve mikroyapısal karakterizasyon yapılmıştır. Ba$TiO _3$ esaslı seramiklerde porozite yapıcı katkı türünün porozite ve mikroyapısal özellikler üzerine etkisi incelenmiştir. Buna göre, porozite yapıcı katkılar veya donör katkısının yapıda ikincil bir faz oluşturmadığı belirlenmiştir. PMMA katkısının, grafit katkısına göre daha yüksek porozite yüzdesi ve büyük por boyutları oluşturduğu gözlenmiştir. Grafit katkısı, ekzotermik reaksiyonlar nedeniyle tane büyümesine neden olmuştur ancak tane morfolojisinde belirgin herhangi bir değişim gözlenmemiştir. PMMA ile katkılandırılmış örneklerde ise tane boyutu incelmiştir. Üretilen Ba$TiO _3$ esaslı poroz seramiklerin, gaz veya nem sensörü uygulamalarında kullanılmaya elverişli olduğu belirlenmiştir.

Abstract

High porosity and electrical resistance are among the main characteristics of ceramic based sensor materials, which are obtained by the variation of sintering parameters and $La^{+3}$ doping for Ba$TiO _3$. Addition of pore-forming agents (PFA) is also an effective method to fabricate porous ceramics for sensor applications. Several approaches have been done recently for manufacturing porous ceramics. It has been reported that porous Ba$TiO _3$ can be prepared by the incorporation of graphite, polyvinylalcohol (PVA), polyvinylbutyral (PVB), borides, silicides, carbides, corn-starch, potato-starch and partially oxidized titanium powder addition to Ba$TiO _3$ or by the thermal decomposition of barium titanyl oxalate BaTiO($C _2O _4) _2· 4H _2O$. Oxygen can be adsorbed at the grain boundaries due to the presence of pores in the porous ceramics, which are more favorable to form surface acceptor states compared with ordinary dense ceramics. The addition of organic particles, which vaporize at relatively low temperatures forming small and homogeneously distributed pores, is one of these methods. Porous structures having gas / liquid permeability, which can serve as gas or humidity sensors, could be fabricated via this method. The electrical properties of Ba$TiO _3$ based ceramics are still a matter of considerable interest due to their various applications and the difficulty in explaining the electrical behaviour. The electrical resistance of Ba$TiO _3$ based ceramics can be controlled effectively by rare earth doping into the barium or titanium sites in the perovskite lattice. In the present study, the effect of low amount of donor doping on the microstructural and electrical properties of Ba$TiO _3$ based ceramics was investigated. Instead of applying lower sintering temperatures and/or compaction pressures, using low amounts of additives, Ba$TiO _3$-based ceramics were fabricated with sufficient mechanical stability to fit long-term room temperature humidity environment applications. On the other hand, low amount of additives provided the porosity percentages, required for humidity sensing process, which is mainly a surface reaction including the adsorption of water vapour to the ceramic surface. In the present study, the effects of graphite and PMMA (Polymethyl metacrylate) additions on the porosity characteristics of $La^{+3}$ doped Ba$TiO _3$ based ceramics were observed. Porosity and microstructural studies were carried out for PFA containing samples. The effects of the type and PFA content on the porosity and microstructural features of Ba$TiO _3$ based ceramics were examined. It was observed that there were not any secondary phases due to the existance of PFAs and donor dopant. PMMA additions caused a more pronounced effect in porosity percent and pore size values than graphite. Donor doping and variable graphite addition concerning BT, BTL and BTLC compositions, did not change the overall microstructural features and grain morphology was similar to BT compositions. However, graphite addition enhanced grain growth due to exothermic reactions. On the other hand, PMMA caused grain refinement. Besides, organic based additive affected the grain morphology and microstructural features. Grain morphology became more homogeneous, regular and globular due to PMMA addition. This result is of great importance from the view that porous ceramics with low density could be manufactured easily by organic addition. SEM images showed that spaces between the pores and intergranular pore morphology was present. Since intergranular pores were connected by larger pores, pore network can be regarded as interconnected spaces forming a type of capillary tube. One of the main advantage of employing additives with low percentages is that it provides economical benefit for the porous ceramics. Stable microstructures related to high temperature processes were obtained by organic addition and porosity requirements were satisfied with homogeneous distribution of high porosity percentages throughout the microstructures of the porous ceramics. The effects of the type of PFA on the porosity and microstructure of Ba$TiO _3$ ceramics were examined. It was observed that there is no evolution of a secondary phase due to the existance of PFAs and $La^{+3}$ dopant. PMMA additions caused a more pronounced effect in porosity and pore size than graphite addition. It was concluded that based on electrical conductivity results, porous Ba$TiO _3$ ceramics are effective candidate materials for humidity sensing applications.