Nanocrystalline semiconducting donor-doped BaTiO₃ ceramics for laminated PTC thermistor

Abstract

This paper proposed a two-stage thermal processing method combined with the reduction-reoxidation procedure. Samples were firstly sintered in a reducing atmosphere by two stages and then reoxidized in air. Such two-stage thermal processing method was used to adjust density and grain size of ceramics separately. Finally, nanocrystalline semiconducting donor-doped BaTiO₃ ceramics were first successfully prepared. Samples with average grain size of 340 nm exhibited unexpectedly low room resistivity of 136 Ω cm and a significant PTCR effect, with a resistance jump of 4 orders of magnitude. In addition, depletion layer thickness of 40 nm and surface acceptor-state densities of 9.52 × 10¹³ cm⁻² were also calculated. Such good properties had not been reported in former publications. It also means that smaller components based on semiconducting BaTiO₃ ceramics could be produced and more extensive application field could be proposed in the trend of miniaturization of electronic devices.

Introduction

Semiconducting ceramics exhibit distinct non-linear electrical properties, which results from different characteristics of grain boundaries and grains, and therefore have broad applications, such as thermistors [1], [2], varistors [3], [4], gas sensors [5], [6] and grain-boundary barrier-layer capacitors [7] and so on. With the miniaturization of electrical components, laminated single-chip components have been gradually developed [8], [9]. Thanks to its high melting point, low cost, excellent ohmic contact with semiconducting ceramics, the base mental, Nickel, was usually employed as inner electrodes for these laminated single-chip components [8], [9]. Laminated semiconducting ceramic components with base metal inner electrodes should be sintered in a reducing or protective atmosphere to avoid oxidation of the base metal. However, semiconducting ceramics sintered in a reducing or protective atmosphere did not have non-linear electrical properties because the difference between grain boundaries and grain did not form. Thus a reoxidation process in air or oxygen is necessary to form potential barriers at grain boundaries to control their electrical properties. It is called the reduction-reoxidation method [7], [8], [9], [10], [11].

Grain size and density are the most concerned factors influencing properties of semiconducting ceramics prepared by the reduction-reoxidation method. Laminated miniaturized components prefer small grains but several electrical properties deteriorate with the decrease of grain size such as room resistivity of thermistors, breakthrough voltage of varistors and so on. In addition, the relative density should be in a certain range. It cannot be too low in order to obtain basic mechanical strength for applications. Meanwhile it cannot be too high in order to obtain good electrical properties through reoxidation. Generally, grain size and density of ceramics are correlated during the sintering. It is necessary to find out a way to control grain size and density separately.

Chen I-Wei [12] first used a two-stage sintering method to prepare dense nanocrystalline ceramics without final-stage grain growth. In the first-stage, samples were normally sintered for a short time. Then the sintering temperature was rapidly decreased and the samples were sintered at a relatively low temperature in the final-stage. The suppression of the final-stage grain growth was achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration [12]. So far two-stage thermal processing method has been widely used to prepare all kinds of dense nanocrystalline ceramics, such as Y₂O₃, ZnO, BaTiO₃, Ni-Cu-Zn ferrite, BNT-BTO [12], [13], [14], [15]. Therefore, such two-stage thermal processing method could be suitable for controlling grain size and density separately.

Based on the above method, this paper proposed a two-stage thermal processing method combined with the reduction-reoxidation procedure to prepare nanocrystalline semiconducting donor-doped BaTiO₃ ceramics for laminated PTC thermistor. Samples were firstly sintered by two stages in a reducing atmosphere to prevent the oxidation of Ni inner electrodes and then reoxidized in air to adjust their electrical properties. Such two-stage thermal processing method can be used to adjust density and grain size of ceramics separately. It can also be applied to preparing other nanocrystalline semiconducting ceramics and integrated passive components.