Relative Phase Change Research Articles

Time-resolved X-ray reflection phases of the nearly forbidden Si(222) reflection under laser excitation.

The covalent electron density, which makes Si(222) measurable, is subject to laser excitation. The three-wave Si(222)/(13 {\overline 1}) diffraction at 7.82 keV is used for phase measurements. It is found that laser excitation causes a relative phase change of around 4° in Si(222) in the first 100 ps of excitation and this is gradually recovered over several nanoseconds. This phase change is due to laser excitation of covalent electrons around the silicon atoms in the unit cell and makes the electron density deviate further from the centrosymmetric distribution.

Optimizing the Quality of Food Powder Products: The Challenges of Moisture-Mediated Phase Transformations.

Water is ubiquitous in the environment and is present to varying degrees even within dry powder products and most ingredients. Water migration between the environment and a solid, or between different components of a product, may lead to detrimental physical and chemical changes. In efforts to optimize the quality of dry products, as well as the efficiency of production practices, it is crucial to understand the cause-effect relationships of water interactions with different solids. Therefore, this review addresses the basis of moisture migration in dry products, and the modes of water vapor interactions with crystalline and amorphous solids (e.g., adsorption, capillary condensation, deliquescence, crystal hydrate formation, absorption into amorphous solids) and related moisture-induced phase and state changes, and provides examples of how these moisture-induced changes affect the quality of the dry products.

Monitoring the pendulum between El Niño and La Niña events

The El Niño-Southern Oscillation (ENSO) pendulum swings back and forth between the El Niño and La Niña phenomena. To monitor the relative phase changes of the ENSO pendulum, 13 clusters are obtained based on the pattern of sea surface temperature anomalies (SSTAs) along the equatorial Pacific. The historical ENSO evolution can be effectively described by the alternation among those clusters. The zonal movement of SSTA loading during the ENSO evolution can also be captured by the changes in the clusters. One scheme was designed to monitor the ENSO pendulum based on those clusters as well the SSTAs in the Niño-3.4 region. When applied in practice to monitor historical ENSO events, including the 1997 El Niño and 2015 El Niño phenomena, the scheme tends be suitable for describing the ENSO pendulum between El Niño and La Niña, and it has the advantage of presenting both the ENSO amplitude and SSTA pattern.

The parallel-sequential field subtraction technique for coherent nonlinear ultrasonic imaging

Nonlinear imaging techniques have recently emerged which have the potential to detect cracks at a much earlier stage than was previously possible and have sensitivity to partially closed defects. This study explores a coherent imaging technique based on the subtraction of two modes of focusing: parallel, in which the elements are fired together with a delay law and sequential, in which elements are fired independently. In the parallel focusing a high intensity ultrasonic beam is formed in the specimen at the focal point. However, in sequential focusing only low intensity signals from individual elements enter the sample and the full matrix of transmit-receive signals is recorded and post-processed to form an image. Under linear elastic assumptions, both parallel and sequential images are expected to be identical. Here we measure the difference between these images and use this to characterise the nonlinearity of small closed fatigue cracks. In particular we monitor the change in relative phase and amplitude at the fundamental frequencies for each focal point and use this nonlinear coherent imaging metric to form images of the spatial distribution of nonlinearity. The results suggest the subtracted image can suppress linear features (e.g. back wall or large scatters) effectively when instrumentation noise compensation in applied, thereby allowing damage to be detected at an early stage (c. 15% of fatigue life) and reliably quantified in later fatigue life.

Numerical techniques for solving solidification and melting phase change problems

ABSTRACTThis article reviews several mathematical formulations including the corresponding boundary conditions for numerical predictions of solidification and melting. Emphasis is on techniques that are used in solving solid–liquid interface phenomena. The fixed grid enthalpy method is reviewed based on the solution techniques of conduction and convection related phase change problems. Variable grid methods are categorized and then analyzed based on their accuracy, computational efforts and convergence characteristics. The article concludes with some guidance for selecting the accurate solution techniques for solving solidification and melting problems.

Performance drifts in two-finger cyclical force production tasks performed by one and two actors.

We explored changes in the cyclical two-finger force performance task caused by turning visual feedback off performed either by the index and middle fingers of the dominant hand or by two index fingers of two persons. Based on an earlier study, we expected drifts in finger force amplitude and midpoint without a drift in relative phase. The subjects performed two rhythmical tasks at 1Hz while paced by an auditory metronome. One of the tasks required cyclical changes in total force magnitude without changes in the sharing of the force between the two fingers. The other task required cyclical changes in the force sharing without changing total force magnitude. Subjects were provided with visual feedback, which showed total force magnitude and force sharing via cursor motion along the vertical and horizontal axes, respectively. Further, visual feedback was turned off, first on the variable that was not required to change and then on both variables. Turning visual feedback off led to a mean force drift toward lower magnitudes while force amplitude increased. There was a consistent drift in the relative phase in the one-hand task with the index finger leading the middle finger. No consistent relative phase drift was seen in the two-person tasks. The shape of the force cycle changed without visual feedback reflected in the lower similarity to a perfect cosine shape and in the higher time spent at lower force magnitudes. The data confirm findings of earlier studies regarding force amplitude and midpoint changes, but falsify predictions of an earlier proposed model with respect to the relative phase changes. We discuss factors that could contribute to the observed relative phase drift in the one-hand tasks including the leader-follower pattern generalized for two-effector tasks performed by one person.

Interferometric enhancement for forward-looking synthetic aperture radar target imaging

We present a technique for interferometric processing of extreme high-squint synthetic aperture radar (SAR) target images. The technique is an improvement over conventional SAR interferometry, as it takes into account the change in relative phase between channels that occur as the radar moves through the synthetic aperture, which can be significant for forward-looking data acquisition geometry. We apply the technique to simulate data from a variety of imaging cases and show that, unlike the conventional interferometric technique, the proposed technique is robust to synthetic aperture length, scatterer elevation, and scatterer dimension in forward-looking scenarios. The technique may be used for various applications related to situational awareness, such as remote sensing for collision avoidance or forward-looking reconnaissance.

Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures

Organic phase change materials (PCMs) have many properties that make them desirable for integration in latent-heat solar thermal energy storage (TES) systems operating in the ambient-to-moderate temperature range, but there are significant gaps concerning their material properties, and their reliability. We present a comprehensive study of thermal and related properties (transition temperature, transition enthalpy change, heat capacity, thermal conductivity, thermal diffusivity, density), thermal stability (on cycling through the transition up to 3000 times) and chemical stability (when in contact with 16 common containment materials at 75°C) for six organic phase change materials: decanoic acid (aka capric acid, [CH3(CH2)8COOH]), dodecanoic acid (aka lauric acid, [CH3(CH2)10COOH]), tetradecanoic acid (aka myristic acid, [CH3(CH2)12COOH]), hexadecanoic acid (aka palmitic acid, [CH3(CH2)14COOH]), octadecanoic acid (aka stearic acid, [CH3(CH2)16COOH]) and 1-octadecanol ([CH3(CH2)17OH]). All show melting transitions in the temperature range 30–70°C which is suitable for solar thermal applications, with substantial enthalpy changes (>145Jg-1), and all are thermally stable over 3000 cycles. These materials all have significant potential for solar thermal energy storage applications.

Phase-resolved heterodyne shearographic vibrometer for observation of transient surface motion: theory and model.

A phase-resolved heterodyne shearing interferometer concept is under development for high-rate, whole field observations of transient surface motion. The sensor utilizes frequency and polarization multiplexing with two temporal carrier frequencies to separate each segment of a shearing Mach-Zehnder interferometer. Post-processing routines have been developed to recombine the segments by extracting the scattered object phase from Doppler shifted intermediate carrier frequencies. The processing routines provide quantitative relative phase changes and information required to generate phase resolved shearographic fringe patterns without temporal or spatial phase shifting. Separation of each segment allows for adjustment of shearing distance and direction as well as simultaneous whole field Doppler velocity (LDV) measurements. This paper presents background theory and numerical model results leading to a sensor concept.

Spin dynamical phase and antiresonance in a strongly coupled magnon-photon system

We experimentally studied a strongly coupled magnon-photon system via microwave transmission measurements. An anti-resonance, i.e. the suppression of the microwave transmission, is observed, indicating a relative phase change between the magnon response and the driving microwave field. We show that this anti-resonance feature can be used to interpret the phase evolution of the coupled magnon-microwave system and apply this technique to reveal the phase evolution of magnon dark modes. Our work provides a standard procedure for the phase analysis of strongly coupled systems, enabling the phase characterization of each subsystem, and can be generally applied to other strongly coupled systems.

Competing covalent and ionic bonding in Ge-Sb-Te phase change materials.

Ge2Sb2Te5 and related phase change materials are highly unusual in that they can be readily transformed between amorphous and crystalline states using very fast melt, quench, anneal cycles, although the resulting states are extremely long lived at ambient temperature. These states have remarkably different physical properties including very different optical constants in the visible in strong contrast to common glass formers such as silicates or phosphates. This behavior has been described in terms of resonant bonding, but puzzles remain, particularly regarding different physical properties of crystalline and amorphous phases. Here we show that there is a strong competition between ionic and covalent bonding in cubic phase providing a link between the chemical basis of phase change memory property and origins of giant responses of piezoelectric materials (PbTiO3, BiFeO3). This has important consequences for dynamical behavior in particular leading to a simultaneous hardening of acoustic modes and softening of high frequency optic modes in crystalline phase relative to amorphous. This different bonding in amorphous and crystalline phases provides a direct explanation for different physical properties and understanding of the combination of long time stability and rapid switching and may be useful in finding new phase change compositions with superior properties.

Temperature Dependence of Enthalpy and Heat Capacity of Alkanes and Related Phase Change Materials (PCMs) with a Peltier-element-based Adiabatic Scanning Calorimeter

ABSTRACTResearch in the field of phase change materials (PCMs) requires that the temperature dependence of the thermal storage capacity be well known for the selection of PCMs as well as for simulation input. A differential scanning calorimeter (DSC) is often used, but it substantially misrepresents the true heat capacity in the vicinity of large-enthalpy phase transitions. Therefore, other suitable experimental techniques should be applied for the determination of the thermal storage capacity. Peltier-element-based adiabatic scanning calorimetry (pASC) measures the heat capacity and the enthalpy of PCMs in thermodynamic equilibrium, thus removing the rate dependence and deformation that are inherent to DSC. The technique is illustrated here by measurements on the pure alkane tricosane (C23), the commercial alkane mixture RT42 and its bound counterpart PX42.

Investigation of the nonlinear refractive index of single-crystalline thin gold films and plasmonic nanostructures

The nonlinear refractive index of plasmonic materials may be used to obtain nonlinear functionality, e.g., power-dependent switching. Here, we investigate the nonlinear refractive index of single-crystalline gold in thin layers and nanostructures on dielectric substrates. In a first step, we implement a z-scan setup to investigate ~100-µm-sized thin-film samples. We determine the nonlinear refractive index of fused silica, n 2(SiO2) = 2.9 × 10−20 m2/W, in agreement with literature values. Subsequent z-scan measurements of single-crystalline gold films reveal a damage threshold of 0.22 TW/cm2 and approximate upper limits of the real and imaginary parts of the nonlinear refractive index, |n 2′(Au)| < 1.2 × 10−16 m2/W and |n 2″(Au)| < 0.6 × 10−16 m2/W, respectively. To further determine possible effects of a nonlinear refractive index in plasmonic circuitry, interferometry is proposed as a phase-sensitive probe. In corresponding nanostructures, relative phase changes between two propagating near-field modes are converted to amplitude changes by mode interference. Power-dependent experiments using sub-10-fs near-infrared pulses and diffraction-limited resolution (NA = 1.4) reveal linear behavior up to the damage threshold (0.23 times relative to that of a solid single-crystalline gold film). An upper limit for the nonlinear power-dependent phase change between two propagating near-field modes is determined to Δφ < 0.07 rad.

Oxide ion conductors based on niobium-doped bismuth vanadate: conductivity and phase transition features

Oxide ion conductors are technologically important materials, essential for electrochemical devices such as oxygen separation membranes and solid oxide fuel cells (SOFCs). Bi4V2 − xMexO11 − δ (BiMeVOx) is a well-known family of material with high ionic conductivity at low operating temperatures. However, it is often reported to possess three structurally related phase changes that affect the vacancy order/disorder over the oxygen atom positions. This might be overcome by a careful chemical design of an oxide ion conductor by introducing the stabilizing dopants. Here, it is found that niobium (Nb) doping on the V site can increase the stability (tetragonal structure) and ionic conductivity at a temperature range of 200–600 °C. This study demonstrates the effect of the doping amount on the ionic conductivity and characterizes the obtained materials by scanning electron microscopy (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analyses. The oxidation state of vanadium that related to the electronic structure of BiMeVOx is investigated by X-ray photoelectron spectroscopy (XPS) analysis. It is expected to understand the conductivity and phase transition features of this kind of oxide ion conductor.

Experimental demonstration of differential OFDM underwater acoustic communication with acoustic vector sensor

A new Orthogonal Frequency Division Multiplexing (OFDM) based underwater acoustic communication scheme utilizing acoustic vector sensor is presented. The acoustic channel assumed has the slow time-varying characteristic and the neighboring signals can be considered as transmitted through the same channel. The differential modulation encodes information through the relative phase changes between adjacent subcarriers. Therefore, instead of adding pilots to monitor the channel, the differential demodulation is used to reconstructs transmitted information without necessity of channel estimation. This increases the transmission rate and reduce the complexity of processor at the receiver. An acoustic vector sensor is used in the differential OFDM system to further improve the performance of the system with 4.8dB gains that can be obtained by combined processing pressure and velocity signals from the acoustic vector sensor. To validate the capability of the proposed scheme, an experiment with picture and voice source information was conducted in a lake. The results obtained at different distances demonstrate the proposed system offers a significant advantage.

Control of magnetic contrast with nonlinear magneto-plasmonics.

The interaction between surface plasmons (SP) and magnetic behavior has generated great research interest due to its potential for future magneto-optical devices with ultra-high sensitivity and ultra-fast switching. Here we combine two surface sensitive effects: magnetic second-harmonic generation (MSHG) and SP to enhance the detection sensitivity of the surface magnetization in a single-crystal iron film. We show that the MSHG signal can be significantly enhanced by SP in an attenuated total reflection (ATR) condition, and that the magnetic contrast can be varied over a wide range by the angle-of-incidence. Furthermore, the magnetic contrast of transverse and longitudinal MSHG display opposite trends, which originates from the change of relative phase between MSHG components. This new effect enhances the sensing of magnetic switching, which has potential usage in quaternary magnetic storage systems and bio-chemical sensors due to its very high surface sensitivity and simple structure.

Phase-induced switching in fiber nonlinear directional coupler

A numerical study of soliton switching characteristics and the maximum output coupling by the relative phase change of a control pulse is carried out for input powers, coupled coefficient and input soliton width. It is shown that input powers ratio, coupled coefficient and input soliton width play important roles in this mode of soliton switching and that these effects may lead to useful soliton switching if the relative phase of the control pulse is monitored judiciously.

Switching among graphic patterns is governed by oscillatory coordination dynamics: implications for understanding handwriting

Revisiting an original idea by Hollerbach (1981), previous work has established that the production of graphic shapes, assumed to be the blueprint for handwriting, is governed by the dynamics of orthogonal non-linear coupled oscillators. Such dynamics determines few stable coordination patterns, giving rise to a limited set of preferred graphic shapes, namely, four lines and four ellipsoids independent of orientation. The present study investigates the rules of switching among such graphic coordination patterns. Seven participants were required to voluntarily switch within twelve pairs of shapes presented on a graphic tablet. In line with previous theoretical and experimental work on bimanual coordination, results corroborated our hypothesis that the relative stability of the produced coordination patterns determines the time needed for switching: the transition to a more stable pattern was shorter, and inversely. Moreover, switching between patterns with the same orientation but different eccentricities was faster than with a change in orientation. Nonetheless, the switching time covaried strictly with the change in relative phase effected by the transition between two shapes, whether this implied a change in eccentricity or in orientation. These findings suggest a new operational definition of what the (motor) units or strokes of handwriting are and shed a novel light on how coarticulation and recruitment of degrees of freedom may occur in graphic skills. They also yield some leads for understanding the acquisition and the neural underpinnings of handwriting.

Relative Phase Change of Nearby Resonances in Temporally Delayed Sum Frequency Spectra.

Surface-selective sum frequency generation (SFG) spectroscopy has been previously shown to benefit from a finite time delay between two input laser pulses, which suppresses the nonresonant background and improves spectral resolution. Here we demonstrate another consequence of the time delay in SFG: depending on the magnitude of the delay, nearby resonances (e.g., vibrational modes) can "flip" their relative phase, i.e., appear either in-phase or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up vs down) of the vibrational chromophore. We present results and model calculations for symmetric and asymmetric CH-stretch modes of the methyl-terminated Si(111) surface, showing that the phase flip occurs when the delay matches half-cycle of the difference frequency between the two modes.

A new interferometry-based electron density fluctuation diagnostic on Alcator C-Mod

The two-color interferometry diagnostic on the Alcator C-Mod tokamak has been upgraded to measure fluctuations in the electron density and density gradient for turbulence and transport studies. Diagnostic features and capabilities are described. In differential mode, fast phase demodulation electronics detect the relative phase change between ten adjacent, radially-separated (ΔR = 1.2 cm, adjustable), vertical-viewing chords, which allows for measurement of the line-integrated electron density gradient. The system can be configured to detect the absolute phase shift of each chord by comparison to a local oscillator, measuring the line-integrated density. Each chord is sensitive to density fluctuations with k(R) < 20.3 cm(-1) and is digitized at up to 10 MS/s, resolving aspects of ion temperature gradient-driven modes and other long-wavelength turbulence. Data from C-Mod discharges is presented, including observations of the quasi-coherent mode in enhanced D-alpha H-mode plasmas and the weakly coherent mode in I-mode.