Increase Of Grain Boundary Resistance Research Articles

Gas sensing properties and complex impedance analysis of La 2O 3-added WO 3 nanoparticles to VOC gases

Tungsten-trioxide (WO 3) and lanthanum oxide (La 2O 3)-added WO 3 nanoparticles were successfully prepared by sol–gel method. The structure and morphological characteristics of the nanopowders were studied by means of field-emission SEM (FE-SEM). The results showed that all the nanopowders of spherical shape with average size smaller than 40 nm have good dispersivity. The effect of La 2O 3 on the gas response of the samples was investigated, especially the volatile organic compound (VOC) gases. The highest gas response of La 2O 3-added WO 3 is found to shift to low temperature, compared to that of the pure WO 3. Analysis with FE-SEM revealed that, compared with the pure WO 3, La 2O 3-added WO 3 has a smaller average grain size but greater surface area. Complex impedance spectroscopy analysis indicated an increase in grain boundary resistance.

Increase of grain boundary resistance with time by impedance spectroscopy in La 0.50Ca 0.50MnO 3+δ at 77 K

In order to investigate the effects of grain boundaries on the electrical properties of La 0.50Ca 0.50MnO 3+δ impedance spectroscopic studies have been carried out at 77 K. With time there is irreversible formation of domains, which promotes the insulating behaviour of this charge ordered material. As time passes, the grain boundary resistance increases, showing the canted spin nature of the material. However, after 150 hours the grain boundary resistance becomes saturated, indicating that most of the metastable states have relaxed and transformed into stable charge ordered ones, most probably of antiferromagnetic nature. An equivalent circuit model, R 1(R 2C 2)(R 3C 3), i.e., a resistor–capacitor network, has been proposed to explain the impedance results.

Dielectric properties of CaCu3Ti4O12 ceramics modified by SrTiO3

In this letter, the dielectric properties of CaCu3Ti4O12 ceramics modified by SrTiO3 have been investigated. These ceramics were prepared successfully by conventional ceramic processing. X-ray diffraction results show that the ceramics with SrTiO3 sintered at 1060 °C for 3 h are mixed phase and consist of CaCu3Ti4O12 and SrTiO3. The change of dielectric constant is explained well by Lichtenecker's logarithmic law of composite. And the reduction in dielectric loss is attributed to the increase of grain boundary resistance according to internal boundary layer capacitor model, with the increase of SrTiO3. When CaCu3Ti4O12/SrTiO3=6/4 (volume ratio), an optimum component was obtained, exhibiting a high dielectric constant of about 2000 and a low dielectric loss (<0.03) with a good temperature stability from −55 °C to 150 °C at 1 kHz.

Properties of barium-strontium titanate PTCR ceramics sintered on different powder beds

Barium-strontium titanate (BST) ceramics, co-doped with Sb and Mn oxides, were sintered on different powder beds: Sb-doped BST; Al2O3; or Sb,Mn-codoped BST powder. Phase formation, microstructure and the electrical properties of the samples were analysed. The PTCR behavior depended significantly on the type of powder bed used. The BST ceramic pellet sintered on the Sb-BST powder displayed the largest PTCR effect, with a ρmax/ρRT ratio of ∼106. This was an order of magnitude greater than for samples sintered on the other two powders. Complex impedance analysis confirmed that this was due to a large increase in grain boundary resistance at 250 °C.

Effect of Sc 2O 3 on the electrical properties of (Co, Ta, Cr)-doped SnO 2 varistors

The effects of Sc 2O 3 doping on the electrical properties of (Co, Ta, Cr)-doped SnO 2 varistors were investigated and the maximal nonlinear coefficient ( α = 34 ) of the sample doped with 0.03 mol% Sc 2O 3 was obtained. It was found that the breakdown electrical field of samples increased significantly with increasing Sc 2O 3 concentration and the sample doped with 0.08 mol% Sc 2O 3 has the highest breakdown electrical field of 3336 V/mm. The measurement of grain size and sample impedances reveal that the increase of grain boundary resistance is the substantial reason for the increase of breakdown electrical field. The increase of grain boundary resistance may be caused by the increase of depletion layer thickness and it can be illustrated by the decrease of relative dielectric constant with increasing Sc 2O 3 concentration.

Evolution of low sigma grain boundaries in PTC thermistors during sintering

BaTiO 3-based positive temperature coefficient (PTC) thermistors undergo a large and rapid increase in grain boundary resistivity at temperatures just above the Curie temperature, T c. Ample evidence exists for variability in the magnitude and form of the resistivity increase between different grain boundaries, and it has been noted that many of the grain boundaries showing a weak PTC effect have low Σ values when indexed using coincidence site lattice notation. It has also been reported in the literature that, in undoped BaTiO 3, there is a strong preference for the formation of Σ = 3 grain boundaries in the microstructure, with many more present than would be expected by chance. In the current study, the formation and retention of low Σ grain boundaries in a PTC thermistor based on doped BaTiO 3 have been characterised through interrupted sintering experiments. Electron backscatter pattern (EBSP) analysis was used to establish grain boundary misorientation distributions in a series of samples prepared during an interrupted sintering study. A significant proportion of Σ = 3, 5 and 9 boundaries was observed, with Σ = 3 boundaries being systematically preferred over other low Σ boundaries. Since Σ = 3, 5 and 9 grain boundaries are believed to be PTC inactive, their presence in significant numbers in the microstructure is likely to be deleterious to the overall performance of a thermistor, particularly during transient loading. An increase in the proportion of Σ = 3 twin boundaries was noted with sintering time, however, the proportion of Σ = 3 grain boundaries remained fairly constant, although occurring with a higher frequency than would be expected in a random population. The proportion of Σ = 5 and 9 boundaries also remained approximately constant during the sintering process, indicating that the density of low Σ boundaries in the microstructure is fixed at an early stage of sintering and is not affected significantly by grain growth.

Conductive mode imaging of thermistor grain boundaries

PTC thermistors undergo a rapid increase in grain boundary resistivity, covering several orders of magnitude, at temperatures just above the Curie temperature, T C, which is associated with a ferroelectric to paraelectric phase transformation. In this study, hot-stage conductive mode scanning electron microscopy has been used to investigate the characteristics of individual thermistor grain boundaries over a range of temperatures around T C. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers for the first time in a commercial thermistor. Not all grain boundaries within the thermistor were found to be EBIC active and the EBIC contrast was only observed at temperatures above T C. EBSP analysis of grain boundary crystallography indicated that the EBIC active grain boundaries were predominantly high angle.

Direct measurement of electrical and compositional inhomogeneities in PTC thermistors

Positive temperature coefficient (PTC) thermistors, based on donor doped barium titanate, show a large, reproducible increase in grain boundary resistance over a small temperature interval just above the Curie temperature ( T C). T C can be increased by additions of lead, and reduced by adding strontium. In recent times thermistors have been prepared containing additions of both lead and strontium along with other dopants to modify device performance. However, the distribution of these dopants is rarely homogeneous, giving rise to local differences in the functional properties of these devices, which affect the overall behaviour. Electron microprobe analysis of a commercial thermistor disc has revealed a depletion of lead, together with calcium and strontium enrichment in a zone ∼300 μm from the surface of the sintered pellet concurrent with a small decrease in grain size. Local resistance-temperature ( R– T) measurements have also demonstrated that T C is decreased and the pre-switching resistance increased in the near surface region.

Bulk and interfacial ionic conduction in LiI/Al 2O 3 mixtures

We have studied physical mixtures of LiI and nanoscale particles of Al 2O 3, prepared in pellet form under pressure, by X-ray diffraction, complex impedance, and wide-line 7Li and 127I and high resolution 7Li NMR spectroscopy. Enhancement in ionic conductivity by the addition of Al 2O 3 is observed, although there is also an increase in grain boundary resistance. The high resolution (MAS) NMR method clearly resolves two or more distinct Li + sites, one characteristic of bulk LiI and the rest associated with surface/interface regions. The site populations depend strongly on mix composition and temperature, and are correlated with the ionic conductivity behavior.