The Origin Of Electrical Activity At Grain Boundaries In Perovskites

1999 ◽  
Vol 5 (S2) ◽  
pp. 110-111
Author(s):  
Miyoung Kim ◽  
N. D. Browning ◽  
S. J. Pennycook ◽  
K. Sohlberg ◽  
S. T. Pantelides
Keyword(s):  
Grain Boundary ◽  
Electrical Activity ◽  
Grain Boundaries ◽  
Domain Wall Motion ◽  
Positive Temperature Coefficient ◽  
Boundary Structure ◽  
Positive Temperature ◽  
Boundary Misorientation ◽  
Negative Effect ◽  
Set Up

The electrical activity of grain boundaries in perovskites, of which SrTiO3 is a model system, is the basis for their use as capacitors, varistors and positive-temperature coefficient resistors. In related materials this same electrical activity is often detrimental. The outstanding example of a negative effect is the reduction in critical current by several orders of magnitude across grain boundaries in YBa2CU3O7-δ as the boundary misorientation is increased from 0°-45°. Grain boundaries are also likely to profoundly affect properties such as domain wall motion in ferroelectric and magnetic perovskites. The origin of the electrical activity is ubiquitously attributed to the existence of grain boundary donors, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. We show here, that although electrical barriers can doubtless be modified by dopants, the grain boundary structure itself is the intrinsic origin of the socalled grain boundary donors.

scholarly journals Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO3

1999 ◽  
Vol 586 ◽  
Author(s):  
Miyoung Kim ◽  
Nigel D. Browning ◽  
Stephen J. Pennycook ◽  
Karl Sohlberg ◽  
Sokrates T. Pantelides
Keyword(s):  
Grain Boundary ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Theoretical Calculations ◽  
Intrinsic Property ◽  
Dislocation Core ◽  
Positive Temperature Coefficient ◽  
Positive Temperature ◽  
Z Contrast

ABSTRACTThe understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to resolve the dislocation core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the grain boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.

Influence of Grain Boundary Structure on Liquid Metal Penetration Behavior

1999 ◽  
Vol 578 ◽  
Author(s):  
Liping Ren ◽  
D.F. Bahr ◽  
R.G. Hoagland
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Simulation Method ◽  
High Energy ◽  
Boundary Structure ◽  
Boundary Misorientation ◽  
Kikuchi Pattern ◽  
Grain Boundary Structure ◽  
Grain Boundary Misorientation ◽  
Penetration Behavior

AbstractThe penetration of Ga along Al grain boundaries under stress-free conditions is investigated in the present study. In-situ SEM observations indicate that the penetration rate of Ga along Al grain boundaries at room temperature ranged from 6.4 to 9.2 μm/s, which is similar to the rate of diffusion in the liquid state. For a specific high energy grain boundary, the grain boundary misorientation is determined from the TEM diffraction Kikuchi pattern, and a molecular statics simulation method was employed to investigate grain boundary structure. A comparison of the structure of this high energy boundary is made with the Σ11(131)[101] tilt grain boundary that is not penetrated by Ga in the absence of the applied stress. The results indicate that the grain boundary plane void structure in the high energy grain boundary may provide void channels for Ga monolayer penetration. In addition, penetration behavior investigated under different length scales supports this model.

Measurement of solute segregation to grain boundaries in the AEM: A review

1988 ◽  
Vol 46 ◽  
pp. 606-607
Author(s):  
D. B. Williams ◽  
A. D. Romig
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Migration ◽  
Grain Boundary Migration ◽  
Boundary Misorientation ◽  
Collector Plate ◽  
Segregation Process ◽  
Grain Boundary Misorientation ◽  
Structure Boundary ◽  
Equilibrium Segregation

The segregation of solute or imparity elements to grain boundaries can occur by three well-defined processes. The first is Gibbsian segregation in which an element of minimal matrix solubility confines itself to a monolayer at the grain boundary. Classical examples include Bi in Cu and S or P in Fe. The second process involves the depletion of excess matrix solute by volume diffusion to the boundary. In the boundary, the solute atoms diffuse rapidly to precipitates, causing them to grow by the ‘collector-plate mechanism.’ Such grain boundary diffusion is thought to initiate “Diffusion-Induced Grain Boundary Migration,” (DIGM). This process has been proposed as the origin of eutectoid transformations or discontinuous grain boundary reactions. The third segregation process is non-equilibrium segregation which result in a solute build-up around the boundary because of solute-vacancy interactions.All of these segregation phenomena usually occur on a sub-micron scale and are often affected by the nature of the grain boundary (misorientation, defect structure, boundary plane).

Structural and Mechanical Properties of Nanophase Ni: A Molecular-Dynamics Study of the Influence of Grain-Boundary Structure on Elastic and Plastic Behavior

1997 ◽  
Vol 492 ◽  
Author(s):  
H. Van Swygenhoven ◽  
M. Spaczér ◽  
A. Caro
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Diffusion Mechanism ◽  
Distribution Functions ◽  
Boundary Structure ◽  
Plastic Behavior ◽  
Self Diffusion ◽  
Common Neighbour ◽  
Grain Boundary Structure

ABSTRACTMolecular dynamics computer simulations of high load plastic deformation at temperatures up to 500K of Ni nanophase samples with mean grain size of 5 nm are reported. Two types of samples are considered: a polycrystal nucleated from different seeds, each having random location and random orientation, representing a sample with mainly high angle grain boundaries, and polycrystals with seeds located at the same places as before, but with a limited missorientation representing samples with mainly low angle grain boundaries. The structure of the grain boundaries is studied by means of pair distribution functions, coordination number, atom energetics, and common neighbour analysis. Plastic behaviour is interpreted in terms of grain-boundary viscosity, controlled by a self diffusion mechanism at the disordered interface activated by thermal energy and stress.

Systematic Study of Grain Boundary Structure and Chemical Analysis of [100] and [111] Tilt Grain Boundaries in Yttria-Stabilized Zirconia

2004 ◽  
Vol 10 (S02) ◽  
pp. 304-305 ◽  
Author(s):  
James P Buban ◽  
Katsuyuki Matsunaga ◽  
Takahisa Yamamoto ◽  
Yuichi Ikuhara
Keyword(s):  
Chemical Analysis ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Systematic Study ◽  
Yttria Stabilized Zirconia ◽  
Boundary Structure ◽  
Stabilized Zirconia ◽  
Grain Boundary Structure

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

GRAIN BOUNDARIES: PHASE TRANSITIONS AND CRITICAL PHENOMENA

1991 ◽  
Vol 05 (19) ◽  
pp. 2989-3028 ◽  
Author(s):  
E.I. RABKIN ◽  
L.S. SHVINDLERMAN ◽  
B.B. STRAUMAL
Keyword(s):  
Phase Transitions ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Critical Phenomena ◽  
External Surface ◽  
Theoretical Models ◽  
Boundary Structure ◽  
Boundary Phase ◽  
Recent Experimental Data ◽  
Grain Boundary Structure

Recent theories of grain boundary structure have been reviewed briefly. The possibility of existence of the same variety of phase transitions on grain boundaries as that on the crystal external surface has been demonstrated. Recent experimental data and theoretical models concerning grain boundary phase transitions are critically analysed. Grain boundary phase transitions connected with the formation of thin disordered layers on the boundary (prewetting, premelting) are particularly distinguished. Results of recent indirect experiments, which may be treated in terms of prewetting and premelting, have been reviewed. Experimentally observed critical phenomena in the vicinity of the prewetting transition on the tin-germanium interphase boundary have been discussed in terms of the critical exponents theory. Some ideas regarding directions of further research are presented.

Semiconducting BaTiO3 ceramic prepared by low temperature liquid phase sintering

1998 ◽  
Vol 13 (3) ◽  
pp. 660-664 ◽  
Author(s):  
I. Zajc ◽  
M. Drofenik
Keyword(s):  
Liquid Phase ◽  
Grain Boundary ◽  
Sintering Temperature ◽  
Positive Temperature Coefficient ◽  
Liquid Phase Sintering ◽  
Positive Temperature ◽  
Sintering Aid ◽  
Ptcr Effect ◽  
Temperature Coefficient Of Resistivity ◽  
Temperature Liquid

Donor-doped BaTiO3 ceramics were prepared by adding PbO B2O3 SiO2 as a sintering aid. Semiconducting BaTiO3 was obtained at a sintering temperature of 1100 °C. The sintered samples exhibit the Positive Temperature Coefficient of Resistivity (PTCR) effect, which depends on the amount of liquid phase, the concentration of the donor-dopant, and the sintering temperature. The cold resistivity of the samples decreases when the sintering temperature increases. The increase of the grain boundary resistivity and hence of the cold resistivity at lower sintering temperatures was explained by applying the diffusion grain boundary layer model.

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