Bestimmung der Festigkeit von PTC-Elektrokeramiken in Abhängigkeit von der Temperatur

Translated title of the contribution: Strength of PTC-electroceramics with respect to temperature

Yusuf Ziya Salik

Research output: ThesisMaster's Thesis

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Abstract

Electroceramic positive temperature coefficient resistors (PTC) are nowadays produced with Barium titanate-based ceramics. These electroceramics are polycrystalline, ferroelectric ceramic materials and are used for electronic components. The Barium titanate lattice (Perowskit structure) shows two polymorphic types of crystal systems depending on the temperature. At a temperature above the so-called Curie temperature (120 °C) the crystal structure is the cubic and below this temperature it is tetragonal. In this project, the mechanical strength of PTC components, i.e. cylindrical discs, is investigated studied above and below the Curie temperature. Since the components are relatively small (i.e. diameter: 8 mm, height: 1.5 mm), the so-called “Ball on Three Balls Test” has been choosen as strength testing method. The benefits of this method are described in the theoretical section of this work. To obtain also strength values at temperatures above room temperature (i.e. 100 ° C, 120 ° C, 150 ° C and 200 ° C) the test has been carried out in an oven. To interpret the strength results, microstructural and fractographical analysis of the specimens has been carried out. It has been found, that the strength of the PTC-ceramic is about 15 % higher at temperatures above the Curie temperature compared to the strength at room temperature. Generally tensile strength of brittle materials is related to the defect distribution and the fracture toughness. The observed change in strength can only be caused due to a change in the fracture toughness, since the defect distribution does statistically not change due to the temperature change. The fracture toughness itself is dependent on the Young`s modulus and the fracture energy release rate. Measurements of the Young`s modulus show an increase of about 80 % at temperatures above the Curie temperature compared to room temperature. This effect itself can not explain the observed result quantitatively. It can be concluded that some contributions to the change in strength have to be related with a temperature dependence of the specific energy release rate, which is probably caused by non-elastic domain wall movement in the tetragonal (low-temperature) phase.
Translated title of the contributionStrength of PTC-electroceramics with respect to temperature
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Danzer, Robert, Assessor A (internal)
  • Supancic, Peter Hans, Assessor B (internal)
Publication statusPublished - Sept 2011

Keywords

  • PTC
  • Bariumtitanate
  • Strength
  • Ball on Three Balls Test

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