________________________________________________________________________


Positive Temperature Coefficient of Resistivity Effect in Highly Donor-Doped Barium Titanate: A Review


Department of Chemical and Materials Engineering,


Barium Titanate (BaTiO3) is one of the most important ferroelectric materials studied. This is a material with many remarkable properties, such as a positive temperature coefficient of Resistivity (PTCR) effect. Because of this property many countries have been interested in it for a number of years. BaTiO3 displays piezoelectric properties as well a large set of operating conditions. This material is partially responsible for many important products today. Many computer parts and chemical sensors rely heavily on the technology that uses BaTiO3. The surface of the ceramic has an affinity for oxygen. This affinity allows the ceramic to be used as a sensor for compounds such as CO. New donor doping methods are allowing BaTiO3 to be used at lower temperatures and to sense much lower concentrations with higher accuracy. Though this material has helped make great advances, there is still much that can be learned about it and much more opportunity for advancement.


The knowledge base for BaTiO3 began accumulating in the \'40s. The impetus for this research was World War II. The war effort needed piezoelectric transducers, thermistors, and actuators. Recently there have been some developments that are allowing more precise engineering of specific properties. There is a newly discovered behavior of barium titanate that is called Positive Temperature Coefficient of Resistivity (PTCR). The effect is that there is a predictable positive relationship between temperature and resistivity. The key to using this effect is to be able to control and predict the way that it works.


When doping BaTiO3 the cell must stay electronically neutral. When using lanthanum the valance is different than barium. To keep the end structure electrically neutral there must be another difference to regain neutrality. Lanthanum has a valance of +3, whereas barium is +2. The structure remains neutrality is by creating Ti vacancies.


In the past different methods were used to attempt to control the PTCR effect. Previous research showed that ceramics could be modeled to a first approximation using a resistance capacitor equivalent circuit. Empirical data supported the conclusion that doping with La changes the electrical behavior of the ceramic. This electrical behavior is known to be alterable with temperature, but the extent to which the ceramic reacts also changes with La content. The Curie temperature, the temperature Tc which when above, the material behaves Para magnetically, whereas below Tc spontaneous magnetization sets in, is affected. The Curie temperature decreases linearly as La content increases. This gives insight to the mechanisms that are at work inside the grains, and at the grain surfaces, but there are extraneous variables that make this conclusion unclear. The samples are initially fired in an oxygen environment. This oxygen environment may be affecting the chemistry of the grains. Because of the concession being made as to the changing surface chemistry, the trend of increasing lanthanum related to Curie temperature decrease may be acceptable, but any modeling would be inaccurate.


In 1998 there was research done on lanthanum doping in a pure oxygen environment. The Morrison et al. group ran experiments doping BaTiO3 with La in an O2 environment, formed in a single firing operation. Then electrodes were made from gold paste and attached themselves to the sample. The desired result was to model the PTCR effect. This research was also interested in the change in the Curie temperature true to doping. They found that the Curie temperature decreases linearly with increasing lanthanum content.


There was research to determine the electronic model of the PTCR behavior. This study modeled the behavior as two parallel resistance-capacitor (RC) elements connected in series.


This study had a grasp of the neutrality that was necessary, and saw the need of the Ti vacancy, but gave no consideration to the possibility of a Ti rich phase.


This paper discusses many ideas that have later been proved inaccurate.


Another paper by


Focuses on the lower boundary of donor-doping levels. The study made some interesting measurements on the concentrations between 0.4-0.6-mol% La. They found that when the samples were reoxidized the resistance increased, but in the higher concentrations the PTCR effect remained.


By far the most interesting recent study is a 2001 article by Morrison et al., which reviews the mechanism responsible