Slow Phase Change Research Articles

Thermal Management Effectiveness and Efficiency of a Fin Surrounded by a Phase Change Material (PCM)

Extended surfaces, also known as fins are used commonly for heat transfer enhancement in energy storage and thermal management problems. A fin may improve the rate of heat transfer by offering greater surface area. While the performance of a fin in a single-phase ambient such as air is well-understood, relatively lesser work is available on fin performance, including parameters such as fin effectiveness and efficiency, when embedded in a phase change material (PCM). A key theoretical challenge in such analysis is the transient nature of the phase change problem that must be combined with transient diffusion and phase change in the fin. This work presents theoretical analysis of phase change heat transfer between a base wall and a PCM in presence of a fin. The processes of thermal diffusion in the fin, thermal diffusion and phase change in the PCM are combined using perturbation analysis of a problem with time-dependent boundary condition. Results are found to be in good agreement with numerical simulations. Expressions for fin effectiveness and fin efficiency as functions of time are derived. The impact of various problem parameters such as fin geometry, material and the Stefan number on fin performance is analyzed. Expressions for efficiency and effectiveness of an array of equally-spaced fins are also derived. It is shown that while a fin provides additional surface area for enhanced melting of the PCM, transient diffusion in the fin may limit the benefit of the fin, especially at small times. On the other hand, fin performance at large times is limited by slow phase change in the PCM. Results presented here improve the fundamental understanding of PCM and fin based thermal management. Expressions for fin effectiveness and efficiency derived in this work offer useful tools for designing and improving the performance of practical fin and PCM based thermal management systems.

A Novel Single Layer Wideband Reflectarray Design Using Two Degrees of Freedom Elements

A broadband linearly polarized single-layer reflectarray antenna constructed with element cells of spiral-dipoles is presented in this communication. The proposed method achieves the phase change requirements by continuously adjusting the two degrees of freedom (TDF) elements. Then, a linear and smooth phase response covering 360° is obtained by adjusting the TDF. Furthermore, by introducing four rectangular rings around the spiral-dipoles, a relatively slow phase change slope can be obtained and the maximum differences between the achieved phase acquired by element and ideal phase at 6.5 and 9.5 GHz are 20° and 58°, respectively, enlarging the operating bandwidth. A reflectarray prototype with a rectangular aperture of 300 mm in diameter is designed, fabricated, and experimentally verified. The results include a measured gain of 26.5 dBi at 8 GHz with an aperture efficiency of 55%; the 1 dB gain bandwidth can reach 35%. The proposed structure can be extrapolated to higher frequencies due to its single-layer performance. Its benefits include a reduction in manufacturing costs and the mitigation of alignment errors.

Linearized Optomechanics Under Time-Dependent Phase Driving

We study the effect of steepness in smooth phase changes in the laser that drives an optomechanical system in the red-detuned, linearized regime. These phase changes take the semi-classical component out of its steady state. Steeper phase changes produce larger amplitudes for the fast oscillations in the mean fields. In contrast, sufficiently slow phase changes keep the system close to its steady state and allow the implementation of a phase driving scheme designed to minimize the variation of the quantum fluctuation mean excitation values.

Simulations of Lithium-Magnetite Electrodes Incorporating Phase Change

The phase changes occurring in magnetite (Fe3O4) during lithiation and voltage recovery experiments are modeled using a model that simulates the electrochemical performance of a Fe3O4 electrode by coupling the lithium transport in the agglomerate and nano-crystal length-scales to thermodynamic and kinetic expressions. Phase changes are described using kinetic expressions based on the Avrami theory for nucleation and growth. Simulated results indicate that the slow, linear voltage change observed at long times during the voltage recovery experiments can be attributed to a slow phase change from α-LixFe3O4 to β-Li4Fe3O4. In addition, the long voltage plateau at ∼1.2V observed during lithiation of electrodes is attributed to conversion from α-LixFe3O4 to γ-(4 Li2O+3 Fe). Simulations for the lithiation of 6 and 32nm Fe3O4 suggest the rate of conversion to γ-(4 Li2O+3 Fe) decreases with decreasing crystal size.

Proof-of-principle experiment of reference-frame-independent quantum key distribution with phase coding.

We have demonstrated a proof-of-principle experiment of reference-frame-independent phase coding quantum key distribution (RFI-QKD) over an 80-km optical fiber. After considering the finite-key bound, we still achieve a distance of 50 km. In this scenario, the phases of the basis states are related by a slowly time-varying transformation. Furthermore, we developed and realized a new decoy state method for RFI-QKD systems with weak coherent sources to counteract the photon-number-splitting attack. With the help of a reference-frame-independent protocol and a Michelson interferometer with Faraday rotator mirrors, our system is rendered immune to the slow phase changes of the interferometer and the polarization disturbances of the channel, making the procedure very robust.

Phase contrast coherence microscopy based on transverse scanning

We present what we believe to be a novel approach to measuring optical path length differences with a precision of a few nanometers. The instrument is based on transverse scanning or en-face optical coherence tomography. Owing to the fast motion of the scanning beam over the sample, excellent phase stability in the transverse direction is achieved. Hence, phase changes caused by the varying optical path lengths within the sample arm occur with high frequency in the fast scanning direction. These changes are well separated from the rather slow phase changes introduced by jitter within the interferometer and can therefore be measured. The en-face imaging speed of the instrument is 40 fps (520 x 200 pixels). The measured precision of the method to detect small changes in optical path lengths was approximately 3 nm.

Quantitative characterization of cell synchronization in yeast

Metabolic oscillations in baker's yeast serve as a model system for synchronization of biochemical oscillations. Despite widespread interest, the complexity of the phenomenon has been an obstacle for a quantitative understanding of the cell synchronization process. In particular, when two yeast cell populations oscillating 180 degrees out of phase are mixed, it appears as if the synchronization dynamics is too fast to be explained. We have probed the synchronization dynamics by forcing experiments in an open-flow reactor, and we find that acetaldehyde has a very strong synchronization effect that can account quantitatively for the classical mixing experiment. The fast synchronization dynamics is explained by a general synchronization mechanism, which is dominated by a fast amplitude response as opposed to the expected slow phase change. We also show that glucose can mediate this kind of synchronization, provided that the glucose transporter is not saturated. This makes the phenomenon potentially relevant for a broad range of cell types.

Data detection of MPSK in the presence of rapidly changing carrier phase

We present a novel method for data detection of M-ary phase shift keying (MPSK) in the presence of unknown carrier phase and additive white Gaussian noise. Here, the unknown carrier phase is rapidly changing and random. That is, the unknown phase changes from symbol to symbol in a nondeterministic manner. The rate of phase change is such that phase change over two symbol intervals is negligible, but it is not necessarily negligible over a longer interval. Our proposed system outperforms traditional differential phase shift keying (DPSK) by 1.5 dB at error probabilities of 10/sup -3/ and 10/sup -4/. We also compare the performance of our scheme with that of multiple symbol differential detection (MSDD). Except under very slow phase change conditions, our scheme outperforms MSDD. Our decoder is well suited to mobile communication systems. Here, receivers usually move with different speeds at different times.

Chain dynamics of high molecular weight polyethylene as observed from heats of dissolution in slow calorimetry

The Setaram C80 calorimeter with large cells permits accurate measurements of heats in dilute solution. Slow calorimetry is well suited to follow slow phase changes unnoticeable by fast calorimetry. Heats of solution of nascent high molecular weight polyethylene (HMWPE) have been obtained in a slow T-ramp (v=1–12 K-h−1) in a variety of solvents. A new contribution to the enthalpy of fusion of polyolefins has been found. The total heat of dissolution thus contains two contributions due (1) to the melting of orthorhombic crystals and (2) the disordering of a network of entangled chains. An investigation is made of the new contribution (2), which can evolve at low temperature since strain does not build up due to the solvent. Conditions to change the range of temperature at which the second contribution evolves (nature of the solvent, rate of heating, concentration, cycles of dissolution/crystallization) are investigated.

50 Mbps optical homodyne communication receiver with a photorefractive optical beam combiner

Performance measurements are reported for a coherent homodyne optical communication receiver that contained an iron-doped indium phosphide photorefractive beam combiner, rather than a conventional optical beam splitter. The system attained a bit error probability of 10/sup -6/ at received signal powers corresponding to about 75 detected photons per bit. No precision beam alignment or mode-matching optics were required, and tracking of slow phase changes between signal and local-oscillator light was performed automatically by the dynamic nature of the photorefractively formed refractive-index grating. The system used phase-modulated Nd:YAG laser light at lambda =1.06 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Differential scanning calorimetric studies of ethanol interactions with distearoylphosphatidylcholine: transition to the interdigitated phase

It is well established that ethanol and other amphipathic molecules induce the formation of a fully interdigitated gel phase in saturated like-chain phosphatidylcholines (PC's). We have previously shown that the induction of interdigitation in PC's by ethanol is dependent upon the alcohol concentration, the lipid chain length, and the temperature [Nambi, P., Rowe, E. S. & McIntosh, T. J. (1988) Biochemistry 27, 9175-9182]. In the present study, we have used high-sensitivity differential scanning calorimetry to investigate the transitions of distearoylphosphatidylcholine between the noninterdigitated and the interdigitated phases. The enthalpy of the L beta' to L beta I transition is approximately half that of the L beta' to P beta' transition which occurs in the absence of ethanol. The reversibility of these transitions has also been investigated by employing both heating and cooling scans in order to establish the most stable phases as a function of temperature and ethanol concentration. It has been demonstrated that the transition to the interdigitated phase is reversible as a function of temperature. Kinetic studies on the reverse transition (L beta I to L beta') demonstrate that this transition can be very slow, requiring weeks to reach completion. The rate depends upon temperature and ethanol concentration. The slow phase changes mean that the lipid can exist for long periods of time in a phase structure which is not the most stable state. The biological significance of this type of lipid behavior is the implication that the phase structure of biological membranes may depend not only on the most stable phase structure of the lipids present but also on the synthetic pathway or other kinetic variables.

Potentiometric titration of Photosystem II fluorescence decay kinetics in spinach chloroplasts

The fluorescence yield of chloroplasts reflects the redox state of the electron acceptor of the Photosystem II reaction center, with increasing yield as the acceptor is reduced. Chemical reductive titrations of fluorescence yield in chloroplasts at room temperature indicate two distinct midpoint potentials, suggesting the possibility of Photosystem II electron acceptor heterogeneity. We have carried out a potentiometric titration of the fluorescence decay kinetics in spinach chloroplasts using a continuous mode-locked dye laser with low-intensity excitation pulses and a picosecond-resolution single-photon timing system. At all potentials the fluorescence decay is best described by three exponential components. As the potential is lowered, the slow phase changes 30-fold in yield with two distinct midpoint potentials, accompanied by a modest (3-fold) increase in the lifetime. The titration curve for the slow component of the fluorescence decay of spinach chloroplasts is best characterized by two single-electron redox reactions with midpoint potentials at pH 8.0 of +119 and −350 mV, with corresponding relative contributions to the fluorescence yield of 49 and 51%, respectively. There is little change in the fast and middle components of the fluorescence decay. We found that the oxidized form of the redox mediator 2-hydroxy-1,4-naphthoquinone preferentially quenches the fluorescence, causing an anomalous decrease in the apparent midpoint of the high-potential transition. This effect accounts for a significant difference between the midpoint potentials that we observe and some of those previously reported. The selective effect of reduction potentials on particular fluorescence decay components provides useful information about the organization and distribution of the Photosystem II electron acceptor.

珪素青銅の恒温過冷変態

The changes in structure during the isothermal transformation at the temperature of 500° and 400° for 4.75%, 4.97%, and 5.50% Si-Cu alloys quenched from 800° were tested microscopically. The transformation proceed very slowly from the initial state, α for 4.75% and 4.97% Si-Cu alloys and α+κ for 5.50% Si-Cu alloy, to the most stable equilibrium state α+γ through the meta-stable equilibrium state between α and κ, as shown in Fig. I extending the boundary lines α⁄α+κ and α+κ⁄κ to the lower ranges of temperature. It was considered that the slow phase change of α to κ on quenching into a region of temperature of about 400° where it whould be stable as κ, i.e., beneath the extension of the line α+κ⁄κ, is of diffusionless “nucleation and growth” type.