Hydrothermal BaxSr1-xTiO3 Powders for Multilayer Capacitors

Matthew M. Slone

Purdue University

Advising Professor: Elliott B. Slamovich

Introduction

Barium titanate is a ceramic material that is commonly used in capacitors, specifically multilayer capacitors, for its dielectric properties. It has a very high dielectric constant, which has a peak value at its Curie temperature. Dopants are often added to stabilize the dielectric constant over a range of temperatures. Another way to alter the temperature dependence of the dielectric constant is to incorporate strontium into the compound to form the solid solution BaxSr1-xTiO3. Because strontium titanate has a different Curie temperature than barium titanate, their combination results in a material that has a unique intermediate curie temperature. The compound is formed via a hydrothermal processing route and the ratio of barium to strontium in the compound can be controlled by varying the ratios of the elements in the reaction solution. Thus, by controlling the stoichiometry of the BaxSr1-xTiO3, the Curie temperature of the material can be controlled.

Multilayer capacitors are essentially a series of individual capacitors made of thin films of a dielectric material. This structure is of interest because it allows a very high capacitance in a very small volume, which is ideal for use in consumer electronics. The thin dielectric layers can be formed from the ceramic powder by tape casting. Tape casting involves spreading a thin layer of a fluid suspension of the powder on a substrate. The tape can be further processed and sintered to form a solid ceramic material. By laminating tapes in a gradient of powder composition, a novel multilayer capacitor could be made. The gradient in composition would be accompanied by a series of Curie temperatures for the full sample that could essentially create a broadened dielectric constant peak. Ideally this would provide a very high dielectric constant over a range of temperatures.

Project Objectives

  • Hydrothermal production of BaxSr1-xTiO3 powders
  • Tape casting of the powders to form thin films
  • Fabrication of compositionally graded multilayer capacitors
  • Determine the dielectric constants of the films and capacitors

Experimental Approach

  • BaxSr1-xTiO3 powders were produced via a hydrothermal process involving the reaction of BaCl2, SrCl2, and TiO2 in an alkaline, aqueous solution.
  • Powder composition was verified using X-ray diffraction
  • A slip composition was developed and the powders were tape cast
  • The tapes were laminated and sintered to form samples for dielectric constant measurements
  • Measurement of the dielectric properties of the samples using an LCR meter

Research Findings

  • Powder stoichiometry was able to be controlled by varying reactant ratios in the hydrothermal solutions as was found in literature
  • The hydrothermal process was adapted to suit the production quantity needed
  • A tape casting slip composition was developed for the hydrothermally produced powders
  • A tape casting and laminating process was created for the BaxSr1-xTiO3 powders


An X-ray diffraction pattern of a sample of hydrothermally produced BaxSr1-xTiO3 powder. The ratio of Ba / (Ba + Sr) was 0.96 in solution.


A photograph of the tape casting equipment that was used.

Final Research Presentation