Positive Temperature Coefficient of Resistivity in Donor-Doped BaTiO3 Ceramics derived from Nanocrystals synthesized at Low Temperature†

Ternary perovskites, such as BaTiO3, exhibit a wide range of technologically important dielectric, ferroelectric, piezoelectric, and pyroelectric properties. Traditionally, BaTiO3 is prepared in a high-temperature (>1100 °C) solid-state reaction between TiO2 and BaCO3 which yields large crystal grains with wide ranges in shape and size. Current state-ofthe-art processes such as metal–organic chemical vapour deposition, pulsed laser ablation, sputtering, and molecular beam epitaxy offer a high level of control but are energy intensive and still rather expensive. Thus, the low-cost, lowtemperature synthesis of BaTiO3 on the nanometer scale remains a formidable challenge for its potential incorporation into mass produced, miniaturized next-generation electronics. Low-temperature solution-based synthesis routes to nanostructured BaTiO3 (e.g., sol–gel and hydrothermal methods) have received increased notice. We recently reported an innovative vapour-diffusion sol–gel method that, for the first time, offers a pathway to monodisperse 6 nm small BaTiO3 nanocrystals at very low temperatures (16 °C). The synthesis is based on the delivery of water in the vapour-phase which promotes kinetically controlled catalytic hydrolysis and subsequent slow growth of highly crystalline nanoparticles, while the synthetic parameters mimic the physiological conditions of biosilicification (i.e., low temperature, ambient pressure, and catalytic hydrolysis at near neutral pH). BaTiO3 is an insulator; however, donor-doping BaTiO3 with trivalent ions (e.g., Y, La, Nd, Sm, etc.) leads to semiconductor-like behaviour at room temperature and causes an anomalous increase in electrical resistivity near the ferroelectric–paraelectric Curie transition temperature (TC) of ∼ 125 °C. This phenomenon is known as the positive temperature coefficient of resistivity (PTCR) effect. As a result of the PTCR effect, donor-doped BaTiO3 has found application in the electronics industry, e.g., in thermistors to limit overcurrent. The conductivity of donor-doped BaTiO3 is heavily dependent on dopant type and concentration, and the microstructure of the material (crystal grain size). Peng and Lu found that for lanthanum-doped BaTiO3 prepared by traditional solid-state routes, the room temperature conductivity increases with decreasing dopant concentrations, reaching a maximum conductivity at 0.15 at% La. They also found that the average grain size increases with decreasing La concentration, reaching a maximum grain size of 25 lm at 0.15 at% La. Based on these results they proposed that the coinciding occurrence of maximum conductivity and maximum grain size at 0.15 at% La indicates a dopant related correlation between grain growth and charge/vacancy compensation. As a result of the difficulties in preparing BaTiO3 ceramics with both low dopant concentration (i.e., good room temperature conductivity) and small grain size, there have been very few reports on the effect of small-grain sizes on PTCR behaviour. Hence, developing new routes to smallgrain BaTiO3 with low donor-dopant concentrations has potential for studying the effect of grain size on PTCR behaviour and ultimately creating sub-micrometer thin films for device miniaturization. Herein, we report the gram-scale, lowtemperature synthesis of monodisperse, 5.5 nm small donordoped Ba1-xLaxTiO3 (0.05 and 0.17 at% La ) nanocrystals. The nanocrystals were subsequently used as precursors for small-grain Ba1-xLaxTiO3 ceramics, and PTCR and dielectric properties of these small-grain ceramics were investigated. Conventionally, donor-doping is performed via high-temperature solid-state reactions in which the dopant metal oxide is introduced by mechanical grinding and sintering with the parent perovskite. Aside from being an energy intensive technique, this method offers little potential for control over the size and structure of the resulting material because of abnormal grain growth taking place during the sintering process. Nanocrystals of lanthanum-doped Ba1-xLaxTiO3 were prepared, for the first time, by kinetically controlled vapour diffusion at low temperature (16 °C) of H2O/HCl into an alcohol solution containing the bimetallic alkoxide BaTi(OCH2C O M M U N IC A IO N