Terence E. Mitchell

The role of nonstoichiometry in 180° domain switching of LiNbO3 crystals

We show here conclusively that the internal field originates from nonstoichiometric point defects in LiNbO3 crystals. The switching fields required for 180° domain reversal in congruent crystals [C=Li2O/(Li2O+Nb2O5)=0.484] are ∼4–5 times larger than the switching fields for nearstoichiometric crystals (C=0.498). An internal field of ∼2.5 kV/mm observed in congruent crystals disappears in stoichiometric crystals. The concentration of hydrogen incorporated during crystal growth has no effect on the switching or internal fields. The measured spontaneous polarization, Ps=80±5 μC/cm2 is relatively insensitive to the crystal nonstoichiometry and the hydrogen content.

Crystal growth and low coercive field 180° domain switching characteristics of stoichiometric LiTaO3

We grew LiTaO3 single crystals with a composition close to stoichiometry by using a double crucible Czochralski method. The switching field required for 180° ferroelectric domain reversal and the internal fields originating from nonstoichiometric point defects were compared for the stoichiometric and conventional commercially available crystals. The switching fields for the domain reversal in the stoichiometric crystal with a Curie temperature of 685 °C was 1.7 kV/mm. This is about one thirteenth of the switching field required for the conventional LiTaO3  crystals with a Curie temperature near 600 °C. The internal field in the stoichiometric crystal drastically decreased to 0.1 kV/mm.

Nanoscale-twinning-induced strengthening in austenitic stainless steel thin films

Magnetron-sputter-deposited austenitic 330 stainless steel (330 SS) films, several microns thick, were found to have a hardness ∼6.5 GPa, about an order of magnitude higher than bulk 330 SS. High-resolution transmission electron microscopy revealed that sputtered 330 SS coatings are heavily twinned on {111} with nanometer scale twin spacing. Molecular dynamics simulations show that, in the nanometer regime where plasticity is controlled by the motion of single rather than pile-ups of dislocations, twin boundaries are very strong obstacles to slip. These observations provide a new perspective to producing ultrahigh strength monolithic metals by utilizing growth twins with nanometer-scale spacing.

Displacement threshold energy for type IIa diamond

A type IIa natural diamond was irradiated at room temperature with energetic electrons. The threshold energy for displacement of atoms from their lattice sites was determined for three principal crystallographic directions by observing the formation of defect clusters during irradiation in a transmission electron microscope. The displacement‐threshold energies were found to be 37.5±1.2 eV for the electron incident in the [100] direction, 45.0±1.3 eV in the [111] direction, and 47.6±1.3 eV in the [110] direction.

Elastic properties of C40 transition metal disilicides

Abstract Room-temperature and low temperature elastic properties of hexagonal C40 transition metal disilicides, NbSi2 and TaSi2, have been studied using Resonant Ultrasound Spectroscopy (RUS). All five independent elastic stiffness constantscij for NbSi2 and single crystals have been obtained. The temperature dependence of thecij is normal but not large. The orientation dependence of the Youngs and shear moduli was examined in comparison with other transition metal disilicides. The room temperature shear moduli in the {0001} plane, with values of 145.3 and 143.7 GPa for NbSi2 and TaSi2 respectively, are low relative to those in the equivalent pseudo hexagonal {110} close-packed plane for tetragonal C11b MoSi2 and WSi2. The isotropic elastic constants for polycrystalline materials were also calculated. The results show that the various moduli are all much higher than those of the constituent elements. The room temperature Poissons ratios of NbSi2 and TaSi2 are 0.18 and 0.19, respectively, which are smaller than those of the constituent elements and smaller than most materials. The Debye temperatures,θD, were estimated to be 688 K for NbSi2 and 552 K for TaSi2. The elastic properties of C40 VSi2, NbSi2, TaSi2, and CrSi2 and C11b and WSi2 are compared and the possible influence on mechanical behavior discussed.

Wall velocities, switching times, and the stabilization mechanism of 180° domains in congruent LiTaO3 crystals

A systematic study of the kinetics of 180° domains as a function of external electric field is presented for Z-cut LiTaO3 single crystal wafers at room temperature using transient current measurements combined with nondestructive and real-time imaging of 180° domains by light microscopy. The switching time, wall velocity, and nucleation rate follow an exponential behavior with the applied field. A model is proposed which shows that the nucleation and sideways growth of domains play approximately equal parts in determining the switching time. A domain stabilization process occurs on the time scale of a few seconds even at electric fields where the switching time is milliseconds or less. We show that this stabilization process has a strong correlation to the internal fields in the crystal.

In situ video observation of 180° domain switching in LiTaO3 by electro-optic imaging microscopy

We report video observation of 180° domain switching in congruent LiTaO3 under an external electric field by electro-optic imaging microscopy. The sideways wall velocity of independently growing 180° domains in the Z plane of the crystal was measured to be 0.1–0.2 mm/s at a constant field of 21.2 kV/mm. The merging of independently growing domains results in a serrated domain front which displays an order of magnitude increase in the wall velocity to ∼2.3 mm/s. We show that this is directly related to ledge formation upon domain merging where preferential nucleation takes place. These velocities are also an order of magnitude higher than those reported earlier for equivalent switching times that were measured by pulsed voltage application followed by ex situ optical observation.

Integrated electro-optic lens/scanner in a LiTaO3 single crystal.

We report what we believe to be the first stand-alone integrated electro-optic lens and scanner fabricated on a single crystal of Z-cut LiTaO(3). The independently controlled lens and scanner components consist of lithographically defined domain-inverted regions extending through the thickness of the crystal. A lens power of 0.233 cm(-1) kV(-1) and a deflection angle of 12.68 mrad kV(-1) were observed at the output of the device.

Large-angle electro-optic laser scanner on LiTaO 3 fabricated by in situ monitoring of ferroelectric-domain micropatterning

We report on a horn-shaped electro-optic scanner based on a ferroelectric LiTaO(3) wafer that is capable of scanning 632.8-nm light by an unprecedented 14.88 degrees angle for extraordinary polarized light and by 4.05 degrees for ordinary polarized light. The device concept is based on micropatterning ferroelectric domains in the shape of a series of optimized prisms whose refractive index is electric field tunable through the electro-optic effect. We demonstrate what we believe is a novel technique of using electro-optic imaging microscopy for in situ monitoring of the process of domain micropatterning during device fabrication, thus eliminating imperfect process control based on ex situ monitoring of transient currents.

Elastic constants of the C15 laves phase compound NbCr2

Elastic properties of a solid are important because they relate to various fundamental solid-state phenomena such as interatomic potentials, equations of state, and phonon spectra. Elastic properties are also linked thermodynamically with specific heat, thermal expansion, Debye temperature, and Gruneisen parameter. Most important, knowledge of elastic constants is essential for many practical applications related to the mechanical properties of a solid as well: load-deflection, thermoelastic stress, internal strain (residual stress), sound velocities, dislocation core structure, and fracture toughness. In order to understand better the physical properties and deformation behavior of the C15 compound NbCr{sub 2}, the authors have studied its elastic properties in this paper. In Section 2, the experimental methods are described, including the preparation of the sample and the measurement of the elastic constants. In Section 3, the experimental results are presented and the implications of these experimental results are discussed. Conclusions are drawn in Section 4.