Scholarly Work - Physics
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Item Dielectric, NMR and X-ray diffraction study of Cs1-x(NH4)xH2PO4(1998) Meschia, S. C.; Lanceros-Mendez, S.; Zidansek, Aleksander; Schmidt, V. Hugo; Larson, R.Mixed crystals Cs1−x(NH4)xH2PO4 of the ferroelectric CsH2PO4 (CDP) and the antiferroelectric (NH4)H2PO4 were grown with x=0.2 (CADP0.2) in solution. The structural properties of the crystal were analyzed by means of x-ray diffraction. Dielectric measurements at several temperatures and frequencies have been performed along the three crystallographic axes in this sample and also in the fully deuterated CADP0.2 sample (DCADP0.2). NMR experiments were also performed.Item Phase coexistence in proton glass(1998) Schmidt, V. HugoPhase coexistence results from quenched structural randomness and frustrated interactions. The nature of this randomness and the frustrated interactions in proton glass are explained. The six types of ordered domains and corresponding ferroelectric and antiferroelectric phases are described, as well as the disordered paraelectric and proton glass phases. Experimental evidence for phase coexistence is presented. Parameters are introduced which describe the volume fractions of phases, and the overall amount of coexistence. This last parameter provides the basis for locating the nominal phase boundaries, and for an expression which describes the sharpness of any of the five smeared-out phase transitions which occur in proton glass crystals.Item Optical properties of RbTiOAsO4 single crystal(1998) Tu, Chi-Shun; Yeh, Y.-L.; Katiyar, R. S.; Guo, Ruqian; Schmidt, V. Hugo; Chien, R.-M.; Guo, Ruyan; Bhalla, A.S.Rubidium titanyl arsenate (RbTiOAsO4) belongs to the family of nonlinear optical crystals with the general formula M1+TiOX5+O4, where M = K, Rb, Tl, Cs and X = P, As. [1–6] The high damage threshold and broad angular acceptance have made such crystals attractive materials for frequency doubling of Nd-based lasers at λ=1.064 and 1.32 µm, and for optical parametric oscillators (OPO). In addition, the ion exchange properties also make them one of the best candidates for waveguide applications. Potassium titanyl phosphate, KTiOPO4 (KTP), is the most popular among such materials and has been used successfully in different applications. However, the orthophosphate absorption at ∼4.3 and ∼3.5 µm in KTP severely limits the oscillator output power. In contrast, RTA has a broad infrared transparency (∼0.35-5.3 µm) and exhibits no overtone absorption between 3 and 5 µm. [4] This makes the RTA crystal a potential candidate for nonlinear optical applications. At room temperature, KTP-type crystals have an orthorhombic structure with non- centrosymmetric point group C2v (mm2) and space group Pna2 (Z=8). The crystal framework is a three-dimensional structure made from corner-linked TiO6 octahedra and PO4 tetrahedra. Four oxygen ions of the TiO6 belong to PO4 tetrahedral groups which link the TiO6 groups. In our earlier Raman results, a slight softening was exhibited by several LO and TO vibrational modes of RTA. [1,2] However, there is no typical soft mode observed in the lowfrequency modes of the Raman spectra. This motivated usto carryoutBrillouinscatteringmeasurements tolook for softening in the acoustic modes. We report here both the temperature-dependent acoustic phonon spectra and wavelength- dependent refractive indices. The Cauchy equations [n(λ) = A + B/λ2 + C/λ4] of nx, ny and nz are obtained. In particular, the first direct evidence for acoustic phonon soft mode is presented.Item Dynamic first-principles molecular-scale model for solid oxide fuel cells(2008) Schmidt, V. HugoThis model for voltage V vs. current density i characteristics applies both to the Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolysis Cell (SOEC) modes of operation. It is based on reaction rates calculated from a molecular-scale model for the physical and chemical processes involved. An expression is obtained for i as a function of activation polarization Vact at either interface. For large applied positive or negative voltage it correctly predicts i to be based respectively on the reverse reaction or forward reaction attempt rate. In contrast, the Butler-Volmer i(Vact) expression incorrectly predicts infinite i for infinite applied voltage. The model expression for V(i) takes open- circuit emf, activation polarization, concentration polarization, and ohmic polarization into account. Its predictions agree quite well with experiment results obtained by another group.Item Temperature dependent Raman spectra of Rb1-x(ND4)xD2AsO4 mixed crystals(1998) Tu, Chi-Shun; Gao, S.-S.; Jaw, R.-J.; Hwa, L.-G.; Schmidt, V. Hugo; Brandt, Dan; Chien, R.-M.In the mixed ferroelectric (FE)-antiferroelectric (AFE) systemA1−x(ND4)xD2BO4 [A=Rb(orK,Cs)andB=As (or P)], there is competition between the FE and the AFE orderings, each characterized by specific configurations of the acid deuterons. [1–8] The random distribution of the Rb and ND4 ions is the main source to produce frustration which can increase local structural competition such that the long-range order of electric dipole disappears. Instead of a typical sharp FE or AFE phase transition, the phase coexistence becomes a characteristic in this type of mixed compounds. By a group theoretical analysis for the KDP-type structure (which contains two molecular units in a primitive unit cell); at zero wavevector, the vibrational modes in the tetragonal symmetry (space group I¯ 42d − D12 2d) can be decomposed into the following irreducible representations: Γvib = 4A1(R) + 5A2(Silent) + 6B1(R) + 6B2(R,IR) + 12E(R,IR). [9] The symmetry species A1, B1, B2 and E are Raman active. The situation in the mixed system D*RADA-x is more complicated than one in the parent crystals, because some Rb (or ND4) ions have been substituted by ND4 (or Rb) ions. In this case, the selection rule of the free AsO4 group is expected to be broken much easily than in the pure crystal. In the recent years, many measurements in D*RADAx system have been achieved on ferroelectric-side crystals x = 0.1, 0.10 and 0.28. [2–5] However, only a few experiments were done on antiferroelectric-side compounds (x ≥0.50).[6,7]A complete understanding for this mixed system is still lacking. This motivated us to carry out the polarized Raman scattering on D*RADA-0.55, 0.69 and 1.0. Here, we pay special attention to the stretching mode ν1 (near 755 cm−1) and the in-plane bending mode δ(O-D) (near 825 cm−1).Item Brillouin light scattering anomalies and new phase transition in Cs5H3(SO4)4 crystals(2000) Lushnikov, Sergey G.; Shuvalov, L. A.; Dolbinina, V. V.; Schmidt, V. HugoThe behavior of hypersonic longitudinal acoustic phonons at temperatures ranging from 290 to 370 K in a crystal of Cs5H3(SO4)4·nH2O was studied by Brillouin light scattering. Anomalies in the temperature dependences of the frequency shift and spectral width of the Brillouin components in the vicinity of T=360K were observed. We found a “point of isotropization” near 360 K where C11=C33=2.6X10 10 N/m2 We attribute it to an isostructural phase transition from P63/mmc⇔P63/mmc where the apparent acoustic symmetry changes from hexagonal to cubic. Possible models for the phase transition have been discussed.Item Anode-pore tortuosity in solid oxide fuel cells found from gas and current flow rates(2008) Schmidt, V. Hugo; Tsai, Chih-LongThe effect of solid oxide fuel cell (SOFC) anode thickness, porosity, pore size, and pore tortuosity on fuel and exhaust gas flow is calculated. Also determined is the concentration of these gases and of diluent gases as a function of position across the anode. The calculation is based on the dusty-gas model which includes a Knudsen (molecule–wall) collision term in the Stefan–Maxwell equation which is based on unlike-molecule collisions. Commonly made approximations are avoided in order to obtain more exact results. One such approximation is the assumption of uniform total gas pressure across the anode. Another such approximation is the assumption of zero fuel gas concentration at the anode–electrolyte interface under the anode saturation condition for which the SOFC output voltage goes to zero. Elimination of this approximation requires use of a model we developed (published elsewhere) for terminal voltage V as a function of electrolyte current density i. Key formulae from this model are presented. The formulae developed herein for gas flow and tortuosity are applied to the results of a series of careful experiments performed by another group, who used binary and ternary gas mixtures on the anode side of an SOFC. Our values for tortuosity are in a physically reasonable low range, from 1.7 to 3.3. They are in fair agreement with those obtained by the other group, once a difference in nomenclature is taken into account. This difference consists in their definition of tortuosity being what some call tortuosity factor, which is the square of what we and some others call tortuosity. The results emphasize the need for careful design of anode pore structures, especially in anode-supported SOFCs which require thicker anodes.Item Electric-field poling effect on thermal stability of monoclinic phase in Pb(Mg1/3Nb2/3)0.74Ti0.26O3 single crystal(2006) Chien, R. R.; Schmidt, V. Hugo; Tu, Chi-ShunPhases and domains in a (1 1 0)-cut Pb(Mg1/3Nb2/3)0.74Ti0.26O3 (PMNT26%) single crystal have been investigated as functions of temperature and direct current (DC) electric (E) field by dielectric permittivity, polarizing microscopy, and electric polarization. The unpoled sample has a dominant rhombohedral (R) phase coexisting with monoclinic (M) phase domains, i.e. R/M at room temperature (RT). With 45 kV/cm DC poling applied along [1 1 0] at RT, a single domain of R phase with polar orientation perpendicular to the poling field, i.e. R, was obtained. No microcracking was observed under such high DC field poling. After the poling was removed, the poled sample has R/M microdomains, where the M distortion is close to the R phase. The zero-field-heating domain patterns in the unpoled and poled samples exhibit continuous polarization rotation via an intrinsic M phase in the regions of 355–373 and 365–378 K, respectively. Orthohombic (O) and tetragonal (T) phases were not observed in the temperature-dependent study. The whole crystal becomes cubic (C) phase near 393 and 399 K in the unpoled and poled sample, respectively. In brief, an R/M→M→C transition sequence takes place upon heating for both unpoled and poled samples.Item Direct observation of ferroelectric domains and phases in (001)-cut Pb(Mg1/3Nb2/3)1-xTixO3 single crystals(2006) Chien, R. R.; Schmidt, V. Hugo; Tu, Chi-Shun; Wang, F.-T.Real-time direct observation of ferroelectric domains and phases under electric-field poling along [0 0 1] at room temperature in Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal has been performed by polarizing microscopy. A hysteresis loop of polarization vs. electric field at room temperature was also measured for comparison. By using relations of crystallographic symmetry and optical extinction, polarizing microscopy reveals orientations of the domain polarizations and their corresponding phases. It also provides direct real-time observation of microcracking phenomena. It was found that the monoclinic phase domains play a crucial role in bridging higher symmetry (tetragonal and rhombohedral) phases while field-induced phase transitions take place.Item Electric-field-induced and temperature‑induced phase transitions in high-strain ferroelectric Pb(Mg1/3Nb2/3)0.67Ti0.33O3 single crystal(2006) Chien, R. R.; Tu, Chi-Shun; Schmidt, V. Hugo; Wang, F.-T.This work is to study electric (E)-field-induced and temperature-induced phase transitions in (001)-cut Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal, which are critical concerns for piezoelectric applications. Dielectric properties and domain structures (by polarizing microscope) are measured as functions of temperature and E field. The hysteresis loop of the polarization versus E field at room temperature is also measured. Without any E-field application, upon heating a first-order-type phase transition sequence rhombohedral (R) → rhombohedral/monoclinic/[001]tetragonal (R/M/T001) → cubic (C) takes place near 350 and 430 K, respectively. Under a dc E-field application along [001] at room temperature, [001] tetragonal (T001) phase domains are induced by various phase transition sequences, i.e. R → T001,R → M → T001, R → T → T001,andR→ M → T → T001,asthe E-field strength increases. In addition, E-field-induced microcracking is observed in this work.