On the performance of pyrolytic MnO 2 /tantalum capacitors: Columnar vs. nanocrystalline cathodic layers D. Dias a,b , P.A. Carvalho a , W. Lohwasser b , A.C. Ferro a, * a Departamento Engenharia Materiais, Instituto Superior Te ´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b KEMET, R. Werner Von Siemens, 7002-504 E ´ vora, Portugal Received 11 August 2006; accepted 17 February 2007 Available online 12 April 2007 Abstract MnO 2 cathodic layers of solid tantalum capacitors have been manufactured under standard and low convective flow conditions. MnO 2 layers formed by low convective flow pyrolysis display lower resistance in the 1 kHz–10 MHz frequency range. The cathodic mate- rials present both highly faulted pyrolusite with a columnar morphology and nanocrystalline structures. The proportion of the latter regions is higher for pyrolyses carried out under lower convective flow. It is proposed that the improved performance of capacitors pro- duced under lower convective flow is related to the electrical conductivity anisotropy of semiconductors with rutile-type structures and results from the absence of the columnar texture as well as from the formation of an essentially nanocrystalline cathodic layer. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Manganese dioxide; Compound semiconductors; Transmission electron microscopy; Nanostructure; Electrical properties 1. Introduction Solid tantalum capacitors (STC) using MnO 2 as counter electrode were developed at Bell laboratories in 1955 [1]. This technology owes its long-lasting success to low equiv- alent series resistance (ESR), high volumetric efficiency, reliability and suitable temperature stability. However, due to new market demands and also to competition with emerging technologies (e.g., [2,3]), the ESR of MnO 2 -based capacitors has been stringently required to improve. An MnO 2 solid tantalum capacitor is an electronic device built from a high open-porosity body (capacitor anode) produced by pressing and sintering Ta powder into a pellet. The pellet is subsequently dipped in an electrolyte to anodically form a tantalum pentoxide dielectric film. The counter-electrode (capacitor cathode) is produced by soaking the porous oxidized pellet in an Mn(NO 3 ) 2 Æ nH 2 O solution, followed by pyrolysis to decompose the nitrate into manganese dioxide. The cathode-forming pyrolytic process is repeated several times to produce an adequately thick MnO 2 layer. Graphite and silver are deposited to establish contact with a lead frame and the capacitor is finally encapsulated in epoxy resin. Capacitor performance is strongly affected by the con- ductivity of the MnO 2 counter-electrode [4–6]. Pyrolusite (b-MnO 2 ), ramsdellite (R-MnO 2 ) and their intergrown mixtures (c-MnO 2 ) are n-type semiconductors: the effects of temperature, applied electrical field, frequency, degree of reduction and water content on their electrical conduc- tivity have been the subject of several studies since the early work by Das [5,7–16]. However, no values have been firmly established and published results span over various orders of magnitude (e.g., [10,12,13]). Manganese dioxide exhibits polymorphism [17,18] and the forms adopted during crystallization are determined by processing conditions [5,6,19,20]. The thermal decom- position of nitrate into MnO 2 and Mn 2 O 3 in unconstrained environments has been the subject of several studies [19–23]. A proposed decomposition path involves melting around 328 K, dehydration by means of several endother- mic steps between 400 and 450 K, onset of MnO 2 forma- 1359-6454/$30.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.02.024 * Corresponding author. Tel.: +351 218418119; fax: +351 218499242. E-mail address: alberto.ferro@ist.utl.pt (A.C. Ferro). www.elsevier.com/locate/actamat Acta Materialia 55 (2007) 3757–3763