The oriented attachment (OA) mechanism has been investigated as an important process in the formation of anisotropic nanostructures such as depicted. The results showed that the control of a desired phase in this system may be attained by the control of OA mechanism through pH value, obtaining several morphologies.A new synthetic method for TiO(2) nanocrystals starting from metallic Ti and hydrogen peroxide was developed, in order to obtain minimal interferences to evaluate phase transformation in the system. The results revealed that the crystal morphology appeared to be dictated by the pH value, which shows a strong dependence on the surface energy. The involvement of the oriented attachment (OA) mechanism is important to modify the morphology and, hence, the distribution of the surface energy and confirmed that the mechanism can accelerate certain phase transitions, albeit pH dependence in terms of how the mechanism affects the final particle morphology and direction of crystalline growth. The importance of the mechanism was also apparent in extremely basic conditions, which indicates a possible correlation with the formation of hydrogen titanate nanostructures.

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In the present work, electroactive grain boundaries of highly dense metal oxide SnO2-based polycrystalline varistors were determined by electrostatic force microscopy (EFM). The EFM technique was applied to identify electroactive grain boundaries and thus estimate the amount of active grain boundary, which, in the metal oxide SnO2-based varistor, was calculated at around 85%, i.e., much higher than that found in traditional metal oxide ZnO-based varistors. The mean potential barrier height value obtained from the EFM analysis was in complete agreement with the values calculated from the C-V measurements, together with a complex capacitance plane analysis that validates the methodology proposed here.

We performed a comparative study of electrical and thermal properties of ZnO-and SnO 2 -based varistor. The electrical properties of commercial ZnO-based varistor are equivalent to that found in SnO 2 -based varistor system. In spite of this, the SnO 2 showed a thermal conductivity higher than commercial samples of ZnO-based varistor, which allied with its simpler microstructure and lower dopant concentration is a remarkable result that point out to the use of this system to compete commercially with ZnO-based varistor devices. 3-7 The main feature of a varistor is that they act as an insulator below the varistor voltage, called breakdown voltage, and conductor thereafter. The nonlinear I-V characteristics of varistors are generated by many double Schottky barriers at the grain-boundary layers, which are essentially formed through the segregation of varistor-forming oxide. Moreover, they possess excellent surge-withstanding capability. Therefore, they have been used as a core element of surge absorbers in electronic circuits and as surge arresters in electric power systems. It is largely accepted that the nonohmic properties of such devices are controlled by the grain-boundary features. 2,8-10However, there is more than one conduction paths among grain boundary, particularly for ZnO 10 that posses an evident bulk intergranular material. Despite of this possibility of several conduction paths throughout the grain boundary, the main one that control the nonohmic properties is believed to be because of double Schottky barrier contacts formed among region closest to the grain-grain contact, which has been proved to be similar in both type of varistor mentioned. 8,9 The conduction path formed by double Schottky contact is believed to be very sensitive to the oxygen treatment whatever type of varistor involved 9 and is also thermally activated, temperature-sensitive with leakage conduction related to the barrier height. 10 The leakage current further can progressively increase the temperature until device enters in a thermal runway regime. This kind of failure leads to the formation of a hole through the varistor with distinct signs of melting and vaporization and is commonly associated with thermal runway occurring as a consequence of current localization in the dielectric and semiconductor junction among the grains. The conceptual picture is a positive feedback mechanism in which current localization occurs along some path through the microstructure with the higher current density leading to enhanced local Joule heating. Because of resistance of semiconductor decrease with the increase of temperature, the lowered resistance caused by local heating along the localization path favors further current localization. This feedback continues until melting and electrical shorting occurs, causing the failure of the device.For this reason, it is expected that a good device have more capability of thermal dissipation (a good thermal conductivity) to avoid this thermal runway effect. As obvious consequence, the functionality of...

ResumoEste trabalho mostra o avanço de pesquisas realizadas em cerâmicas eletrônicas à base de SnO 2 e TiO 2 . A adição de diferentes dopantes, bem como a realização de tratamentos térmicos em atmosfera oxidante e redutora, influenciam na densificação, tamanho médio de grãos e nas propriedades elétricas da cerâmica varistora à base de SnO 2 . Resultados obtidos por medidas de espectroscopia de impedância mostram as variações de altura e largura da barreira de potencial em função da atmosfera de tratamento térmico. Altos valores de coeficiente de não-linearidade foram obtidos para os sistemas à base de SnO 2 , característicos de varistores de alta tensão tal como os varistores comerciais de ZnO. Neste trabalho, também serão apresentados estudos de cerâmicas policristalinas à base de TiO 2 e (Sn,Ti)O 2 , as quais apresentam resposta elétrica I-V não linear típica de sistemas varistores de baixa tensão. Todos estes sistemas são potencialmente promissores à aplicação como varistor.Palavras-chave: varistores, SnO 2 , TiO 2 , sistemas não lineares, barreira de Schottky. Abstract INTRODUÇÃONos últimos anos, o estudo de materiais cerâmicos vem evoluindo muito rapidamente, principalmente nas pesquisas relacionadas com cerâmicas avançadas. As diversas aplicações envolvendo estes materiais justificam cada vez mais a necessidade de uma melhor compreensão dos mecanismos responsáveis pelos fenômenos de interface, que são os principais responsáveis pelas propriedades destas cerâmicas na maioria das aplicações. Alguns exemplos destas aplicações são: em células solares, catalisadores, termistores, capacitores, sistemas de proteção contra surtos de energia (varistores) e sistemas sensores à gases (tóxicos, poluentes, combustíveis e de umidade). Alguns dos óxidos utilizados são o ZnO, o SnO 2 e o TiO 2 .O óxido de zinco possui uma grande importância na indústria eletrônica assim como nos estudos dos fenômenos de interface, tendo sido um dos primeiros sistemas cerâmicos policristalinos dopados com óxidos de metais de transição, desenvolvido para aplicação em sistemas de proteção de alta e baixa voltagem. Este sistema é conhecido como sistema multicomponente sendo, até o momento, o mais comercializado. Outros sistemas varistores que são apresentados na literatura, são os sistemas à base de SrTiO 3 [2], TiO 2 [3][4][5][6][7] e mais recentemente varistores à base de WO 3 [8]. O coeficiente de não linearidade (α) destes sistemas são muito baixos (2 ≤ α ≤ 12), quando comparados com os dos varistores à base de ZnO.O SnO 2 é particularmente interessante em aplicações como sensor devido aos seus elevados valores de área superficial (mesmo após sinterização), o que permite uma elevada sensibilidade à adsorção de gases, os quais podem ser obtidos em temperaturas de operações relativamente baixas [9][10]. Foi proposta a adição de dopantes ao SnO 2 com a finalidade de aumentar a concentração de defeitos intrínsecos ao dióxido de estanho, como vacâncias de oxigênio, provocando um aumento no coeficiente de difusão dos íons, promovendo assim, a ...