Grain‐Boundary Characteristics and Their Influence on the Electrical Resistance of Barium Titanate Ceramics

Abstract

The integral and spectral cathodoluminescence (CL) properties of grain-boundary zones in positive-temperaturecoefficient-type barium titanate ceramics were investigated with respect to typical sintering parameters in the spectral range 300 to 1800 nm at room temperature. Spatially resolved CL micrographs for the wavelength range 300 to 850 nm show dark grain-boundary zones and light grain interiors. Corresponding micrographs for the infrared wavelength range 800 to 1800 nm show just the opposite CL contrast. The CL properties of the grain-boundary zone can be correlated to doubly ionized barium vacancies. Therefore, grain-boundary zones which are visible in integral CL images of both wavelength ranges represent the theoretically predicted grain-boundary zone of high barium vacancy concentrations. The width of these grain-boundary zones varies characteristically with sintering time and cooling rate. Additional measurements of the electrical conductivity of the specimens confirmed the theoretically assumed correlation between grain-boundary-zone width and electrical resistance of the samples.

In-Gap States in Wide-Band-Gap SrTiO3 Analyzed by Cathodoluminescence

“…Figure 1(c) presents the whole‐spectrum CL image (CLI), where emission from TiO 2 grains showing bright contrast (indicated by filled arrows) is comparatively more pronounced than that of the SrTiO 3 matrix grains. A residual pore in the framed region exhibiting edges of bright contrast suggesting surface acceptor states 10–12 is also indicated.…”

“…Carrier‐doping, by donor or acceptor, induces spectral weight, or “gap‐states” within the band gap of the host compound 4 . Often, these in‐gap energy levels are investigated by photoluminescence (PL), 3,5–8 and cathodoluminescence (CL) 3,7–11 spectroscopy, where optical transition mechanisms are derived 5,10–12 from the spectra.…”

“…Not restricted to SrTiO 3 , several alternative mechanisms have been suggested for the origin of CL emission in titanate perovskites as follows: (i) self‐trapped excitons (STE's) 14,15 from donor–acceptor recombination, 3,9,11 (ii) recombination of electron and hole in STE states, 5,15 (iii) charge‐transfer vibronic excitons (CTVE's), 16–19 and (iv) transitions associated with oxygen vacancy in the TiO 6 complexes 13,20,21 …”

“…Not restricted to SrTiO 3 , several alternative mechanisms have been suggested for the origin of CL emission in titanate perovskites as follows: (i) self-trapped excitons (STE's) 14,15 from donor-acceptor recombination, 3,9,11 (ii) recombination of electron and hole in STE states, 5,15 (iii) charge-transfer vibronic excitons (CTVE's), [16][17][18][19] and (iv) transitions associated with oxygen vacancy in the TiO 6 complexes. 13,20,21 The conclusion 10 from CL studies of BaTiO 3 , 11,12 SrTiO 3, and TiO 2 is that the emission process was independent of impurities, sample preparation, and ferroelectricity, but dependent on the electronic structure. Unlike in BaTiO 3 , [10][11][12]22 however, research 5,6 into SrTiO 3 for the correspondence among the luminescence, the in-gap states, and the emission mechanisms were limited to work with thin films of Ar 1 -irradiated SrTiO 3 , 5 and Ba 0.6 Sr 0.4 TiO 3 .…”

Ba(Ti 1 − 5/4 x Nb x )O 3 Relaxor Ferroelectrics

“…Previous investigations of higher valent modified BT have focused on a positive temperature coefficient of resistance (PTCR) effect [17][18][19]. In spite of the importance of defects and their distributions in PTCR, relaxor ferroelectric behavior has never been reported in higher valent modified BT.…”