Electrical Behavior of TiO2 Grain BoundariesE.C. Dickey (PI), Pennsylvania State University, DMR-0303279

Electroceramics are utilized in a wide variety of electrical, dielectric and sensing applications. Most of these materials are composed of an ensemble of crystals (grains) and it is often the properties of the interfaces between the grains (grain boundaries) that dictate the macroscopic electrical properties. Thorough judicious chemical doping the grain boundaries, and thus the macroscopic properties, can be controlled. Utilizing complementary transmission electron microscopy and impedance spectroscopy, this research has directly correlated the atomic-scale structure and chemistry of TiO2 grain boundaries with their electrical behaviors. These findings provide the experimental foundation for developing predictive, quantitative models for grain boundary electrical behavior for a broad class of ceramics used in electrical applications.

5nm

Eg Ef

EC

EV

VL

Core

Depletion Region

Grain Interior

B

(Figure below) Z-contrast scanning transmission electron micrograph of a grain boundary in TiO2. The bright intensity corresponds to Y segregation to the boundary.

(Figure right) Model of a grain boundary in Y-doped TiO2 that gives rise to a blocking effect for electronic conduction, increasing the resistivity of the material.

First-Principles Calculations of Intrinsic Defects in Bulk TiO2

S.B. Sinnott (Co-PI), University of Florida, DMR-0303279

Point defects play an important role in many applications of metal oxides, including rutile TiO 2. First-

principles and thermodynamic calculations are used to determine defect formation enthalpies (DFEs) in TiO2 in the reduced state (PO2=10-20). The results indicate that at room temperature,

undoped TiO2 exhibits p-type or n-p behavior when EF=1.5 eV, while at 1400 K, the system clearly

exhibits n-type behavior. This theoretical research helps explain why rutile TiO2 exists

experimentally as an oxygen deficient oxide at high temperature, and indicates how its electronic properties may be tailored by controlling the nature and density of point defects. This work is performed in collaboration with Mike Finnis (Queen’s University) and Elizabeth Dickey (Penn. State).

VTi-2

DF

Es

(eV

)

Tii+4

Tii+3

Tii+2 Tii

+1 Tii0

VO+2

VO+1 VO

0VTi

-1

Oi0

VTi-4VTi

-3

Oi-2

T= 300 K

Tii+4

Tii+3

Tii+2 Tii

+1

Tii0

VO+1

VO0

VTi-1

Oi0

VTi-4

VTi-3

VTi-2

Oi-2

VO+2

T= 1400 K

DF

Es

(eV

)

Training of High School Students in Computational Materials Science and Engineering

S.B. Sinnott (Co-PI), University of Florida, DMR-0303279

Rutile

TiO2

Pyrolusite

MnO2

Cassiterite

SnO2

Cation radius (Å) 0.56 0.53 0.69

Cation electron configuration

[Ar]4S23d2

[Ar]4S23d5 [Kr]5S24d105p2

Bond energy (kJ/mol, gaseous diatomic

species)

672.4 402.9 531.8

O vacancy formation energy (eV)

5.56 2.50 5.06

Prof. Sinnott and graduate student, Mr. Jun He (left), mentored and worked with high school student, Mr. Leemen Weaver (right) on the described research. Mr. Weaver was a University of Florida Student Science Training Program participant during the summer of 2005.

Mr. Weaver’s project was to understand oxygen vacancy formation in three rutile, metal oxides using first principles, density functional theory calculations. A sample unit cell is shown in the top, right-most figure. His preliminary results are shown in the table. They indicate that the cation electron configuration and bond energies are dominant factors in the formation of oxygen vacancies.

Oxygen vacancy Cation (Ti,

Mn, Sn)