Capacitors in which the main dielectric layers are made from sintered nanocrystalline BaTiO3 have been fabricated and tested in an initially successful and continuing effort to increase energy densities, breakdown potentials, and insulation resistances beyond those of prior commercial capacitors that contain coarser-grained sintered BaTiO3. This development effort is based on the premise that the relevant physical properties of BaTiO3 grains vary with their sizes in such a way that smaller grains are better suited for use as dielectrics in capacitors.
The variations in question can be summarized as follows:
• Capacitance and Energy-Storage Density: For reasons too complex to be explained in the limited space available for this article, hysteretic switching of ferroelectric domains in BaTiO3 gives rise to a loss of capacitance and thus a loss of incremental energy-storage density with increasing applied potential. It had been conjectured that this detrimental effect of ferroelectric-domain switching could be minimized by reducing grain sizes to the nanocrystalline range (<100 nm). Thus, it should be possible to store more energy, especially near the upper limit of applied voltage for a given capacitor.
• Breakdown Potential and Energy- Storage Density: The breakdown potential of BaTiO3 or another ceramic dielectric material is related to its mechanical strength, which is approximately inversely proportional to the square root of the size of its smallest internal flaw. Inasmuch as the flaw size cannot be smaller than the grain size, it is expected that, along with mechanical strength, the breakdown potential should increase with decreasing grain size. The expected increase in the breakdown potential would contribute, along with the expected increase in capacitance, to an increase in achievable energy-storage density.
• Insulation Resistance: The insulation resistance of a capacitor is quantified by measuring the direct current that it passes when charged to a steady potential. A simplified electric model of a grainy dielectric material is that of grain-boundary and graininterior elements in series. In a nanocrystalline (grain sizes less than about 100 nm) dielectric, more inherently resistive grain boundaries are present per unit thickness than are present in a coarser-grained version of the same material, and thus one expects the insulation resistance to be greater.
In preparation for testing these concepts, multilayer capacitors that contained sintered nanocrystalline dielectric layers were fabricated. The nanocrystalline dielectric materials were formulated to satisfy an Electronics Industries of America (EIA) standard, called X7R, that specifies acceptable ranges of dielectric properties as functions of temperature. Each grain of the X7R-compliant BaTiO3 has a duplex microstructure comprising a lightly doped ferroelectric core surrounded by a heavily doped paraelectric shell. (The dopants are Bi, Nb, Zn, and Mn).
The table presents results of tests of capacitors made from one of the nanocrystalline-BaTiO3 formulations and of commercially available capacitors made from coarser-grained BaTiO3. These results clearly indicate the superiority of the nanocrystalline BaTiO3 as the dielectric material. On the basis of these results and of other observations made during the tests, it appears that in comparison with capacitors made from coarser-grained BaTiO3, capacitors made from nanocrystalline BaTiO3 can operate more reliably at high temperatures and high voltages, can be made smaller and lighter for a given capacitance value, and can have higher energy- storage densities and higher capacitances for a given case size.
This work was done by John Freim and Yuval Avniel of Nanomaterials Research Corp. for Glenn Research Center.Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16984.