Scholarly Work - Physics
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/3458
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Item Water Vapor Profiling using a Widely Tunable, Amplified Diode Laser Based Differential Absorption Lidar (DIAL)(2009-04) Nehrir, Amin R.; Repasky, Kevin S.; Carlsten, John L.; Obland, Michael D.; Shaw, Joseph A.A differential absorption lidar (DIAL) instrument for automated profiling of water vapor in the lower troposphere has been designed, tested, and is in routine operation at Montana State University. The laser transmitter for the DIAL instrument uses a widely tunable external cavity diode laser (ECDL) to injection seed two cascaded semiconductor optical amplifiers (SOAs) to produce a laser transmitter that accesses the 824–841-nm spectral range. The DIAL receiver utilizes a 28-cm-diameter Schmidt–Cassegrain telescope; an avalanche photodiode (APD) detector; and a narrowband optical filter to collect, discriminate, and measure the scattered light. A technique of correcting for the wavelength-dependent incident angle upon the narrowband optical filter as a function of range has been developed to allow accurate water vapor profiles to be measured down to 225 m above the surface. Data comparisons using the DIAL instrument and collocated radiosonde measurements are presented demonstrating the capabilities of the DIAL instrument.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 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.Item Damping and electromechanical energy losses in the piezoelectric polymer PVDF(2004) Vinogradov, A. M.; Schmidt, V. Hugo; Tuthill, G. F.; Bohannan, Gary W.Polyvinylidene fluoride (PVDF) is a piezoelectric polymer that has been used in many applications including microphones, transducers, sensors and actuators. The electromechanical properties of PVDF are commonly defined by the constitutive equations of piezoelectricity. This paper presents experimental evidence that the assumptions underlying the theory of piezoelectricity have certain limitations in terms of representing adequately the electromechanical properties of PVDF. It is shown that PVDF tends to demonstrate time-dependent behavior in the form of viscoelastic creep and dielectric relaxation, and measurable energy losses under cyclic loading conditions. Moreover, the response of PVDF strongly depends on temperature and cyclic frequencies.Item Paraelectric-antiferroelectric phase coexistence in the deuteron glass Rb0.5(ND4)0.5D2AsO4(2004) Lanceros-Mendez, S.; Schmidt, V. Hugo; Shapiro, S. M.Neutron diffraction was used to study the paraelectric (PE) to antiferroelectric (AFE) phase transition in a deuteron glass crystal Rb 0.5 (ND 4 ) 0.5 D 2 AsO 4 (DRADA-50). Coexistence of AFE and PE phases was proven in a temperature range 7-12 K wide.Item Fractal model for dielectric relaxation in deuteron pseudospin glass DRADP(2002-08) Schmidt, V. Hugo; Arbogast, DarinProton and deuteron glasses such as Rb1−x(ND4)xD2PO4 (DRADP) are ideal systems for investigating dynamics of spin‐glass‐type systems because the basic mechanism for their dynamics is well understood. This mechanism consists of three processes; creation, effective diffusion, and annihilation of DPO4 and D3PO4 “Takagi groups.” Each process involves a deuteron transfer from one side of an O–D⋯O bond to the other. The effective diffusion changes the configurational energies of the D2PO4 “Slater groups” traversed by the Takagi groups. Each diffusion step changes this energy by a random amount with magnitude of order εd. This εd is comparable to the basic energy ε0 of the Slater model for RbD2PO4, and considerably smaller than the Takagi DPO4‐D3PO4 pair creation energy 2εc. The Takagi group diffusion path between creation and annihilation on average does not change the configurational energy. Thus the energy landscape along the path has an unbiased fractal nature, with small energy barriers superimposed on larger ones. The Takagi group diffusion path has side branches that are retraced, and loops of six or more steps. We approximate this path by a deterministic fractal path, having shorter side branches superimposed on longer ones, attached to a trunk running from the creation to the annihilation site. The longest dielectric relaxation time constant is governed by the Boltzmann factor for the highest barrier on the entire trunk. Shorter time constants correspond to the highest barriers on shorter trunk or branch segments. The relaxation strength distribution over these time constants depends on the basic fractal path unit, namely the number m of forward steps per side step. It depends also on temperature T and on εc and εd. This model predicts with good qualitative accuracy the T and f (frequency) dependences of the real (ε’) and imaginary (ε”) parts of the dielectric permittivity measured by Courtens in x=0.62 DRADP over wide T and f ranges. No adjustable parameters were used except for m. The static and high‐f permittivities εs, and ε∞f were chosen to fit Courtens’ data, while εc and εd were set at the values εc/k=940 K and εd/k=140 K chosen by the Blinc/Kind groups to fit DRADP NMR data. This consideration of diffusion path topology and prediction of dielectric permittivity are major extensions of our previous work.