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Effect of Pyrolysis Temperature on the Mechanical Response in Partially Pyrolysed Polysiloxanes
摘要： The pyrolysis process of polysiloxane resin conducted in the temperature range 400 – 700 °C results in hybrid materials owning some polymeric (thermosetting) behaviour. A certain level of elastic recovery and/or viscoelastic flow showed at various steps of pyrolytic transformation was monitored using the instrumented Vickers hardness method. Determined indentation force-indentation depth curves reflect the mechanical response and the level of the transformation; however, the relaxation behaviour is not covered by this method fully. An extensive indentation relaxation was revealed in the material partially pyrolysed at 400 °C, about 16 % and 8 % when the HV 0.1 and the HV 0.2 loading were applied, respectively. Materials pyrolysed from 500 to 650 °C which exhibited the indentation relaxation below 1 % and the mostly elastic response on the loading were observed. Above the pyrolysis temperature of 600 °C a rapid onset of mechanical properties, namely indentation elastic modulus and hardness, was observed. The short-term indentation relaxation was evaluated via the indentation force relaxation method in the regime of constant indentation depth obtained at the moment of reaching an initial force of 0.981 N or 1.962 N. The obtained indentation force relaxation curves were analysed on the basis of a logarithmic function. The significant effect of the pyrolysis temperature as well as the influence of loaded volume was described.
关键词： polysiloxane,relaxation,partial pyrolysis,instrumented hardness
更新于2025-09-10 09:29:36
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Tuning and enhancing the red upconversion emission of Er3+ in LiYF4 nanoparticles
摘要： The tetragonal-phased LiYF4:Yb3+/Er3+ nanoparticles with different core-shell nanostructures have been successfully synthesized by a modified solution-based method. The upconversion emissions of LiYF4: Yb3+/Er3+ nanoparticles were precisely tuned from green to red with the increase in Yb3+ concentrations under 980 nm excitation. The corresponding intensity ratios of the red and green emissions (R/G ratios) were enhanced from 0.06 to 5.33. However, the overall integral upconversion intensity of LiYF4:Yb3+/Er3+ nanoparticles clearly decreased when the Yb3+ doping concentrations were increased. For the enhancement of the red upconversion intensity and expansion of their application, two kinds of core-shell nanostructures, namely, LiYbF4:Er3+@LiYF4 and LiYbF4:Er3+@LiYbF4, were constructed through epitaxial growth at similar experimental conditions. The LiYbF4:Er3+@LiYF4 core-shell nanostructures exhibited stronger upconversion emission and higher R/G ratios than the LiYbF4:Er3+ and LiYbF4:Er3+@LiYbF4 nanoparticles. The highest R/G ratios increased to 6.27 in LiYbF4:Er3+@LiYF4 core-shell nanostructures, increasing 104-fold. The upconversion regulation and enhancing mechanism were also discussed according to the emission spectra and fluorescence lifetimes.
关键词： upconversion,LiYF4 nanoparticles,core/shell nanostructures,cross relaxation
更新于2025-09-10 09:29:36
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Structural, bulk permittivity and impedance spectra of electronic material: Bi(Fe0.5La0.5)O3
摘要： In the present paper, fabrication and characterization (structural, electrical and ferroelectric properties) of a lead-free lanthanum (La) modified bismuth ferrite electroceramic having composition Bi(Fe0.5La0.5)O3 (BFLO) using various experimental techniques have been reported. BFLO has been synthesized by means of a high-temperature inter-diffusion of oxides method. This investigation mainly aims to correlate the phase formation and physical properties while the Fe-site of bismuth ferrite has partially been replaced by La. The surface micrograph shows the formation of the high dense ceramic sample. Analysis of data of various complex spectroscopies related to modulus and impedance exhibits the relaxation mechanism in this sample is of non-Debye type. The combined effect of grains and grain boundaries has been depicted from the Nyquist plot through the least-squares fitting of suitable circuit model along with the experimental data. The remnant polarization and coercive field decreases as temperature rises which was discerned from the ferroelectric loop obtained at different temperatures. The existence of ferroelectric nature in the sample was postulated from the polarization versus electric field plot.
关键词： Ferroelectric properties,Non-Debye relaxation,Lanthanum modification,Bismuth ferrite,Electroceramic
更新于2025-09-10 09:29:36
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Study of dielectric relaxation and thermally activated a.c. conduction in multicomponent Ge10-xSe60Te30Inx (0 ≤ x ≤ 6) chalcogenide glasses using CBH model
摘要： Amorphous Ge10-xSe60Te30Inx (0 ≤ x ≤ 6) chalcogenide glasses were prepared by melt quench technique. Surface morphology with the chemical composition of the prepared glass was examined using SEM and EDS analysis respectively. Dielectric properties and a.c. conductivity of the multicomponent Ge10-xSe60Te30Inx (0 ≤ x ≤ 6) chalcogenide glasses have been examined in the frequency range 100Hz to 1MHz and temperature range 303 to 328K. It was noticed that dielectric constant and dielectric loss decreases with the increase of frequency and increases with the increase of temperatures. Frequency and temperature dependence of dielectric constant was explained by orientational polarization. The variation of dielectric loss with frequency and temperature was explained by conduction loss and theory of single polaron hopping of charge carriers suggested by Elliot and Shimakawa for chalcogenide glasses. The experimental results show that a.c. conductivity follows the power law where s<1 and value of s decreases with the increase of temperature. The present findings of a.c. conductivity and variation of s with temperatures are reasonably well interpreted in terms of CBH model.
关键词： a c conductivity,dielectric relaxation,Chalcogenide glasses,CBH model,activation energy.
更新于2025-09-10 09:29:36
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Dynamic relaxation oscillations in a nonlinearly driven quartz crystal
摘要： We demonstrate thermo-mechanical relaxation oscillations in a strongly driven quartz crystal. Dynamic bifurcation leads to two stable oscillation states with a distinct electrical impedance. Slow Joule-heating, which shifts the susceptibility of the crystal, provides a feedback that leads to thermally induced oscillations in which the amplitude of the crystal is modulated by a relaxation cycle. The frequency of the relaxation cycle is roughly a million times lower than the resonance frequency of the crystal, and it can be adjusted by the detuning from the critical point for dynamic bifurcation. The experimental observations are reproduced by a simple model that takes into account the slow dynamics of the system.
关键词： dynamic bifurcation,Joule-heating,feedback,thermo-mechanical relaxation oscillations,quartz crystal
更新于2025-09-10 09:29:36
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Electron exchange energy of neutral donors inside a quantum well
摘要： We calculated the exchange energy of a pair of donor-bound electrons placed in the middle of an in?nite quantum well (QW). In order to obtain this energy for any interdonor distance and for any QW thickness, we have ?rst adapted to a QW the method developed by Gor’kov and Pitaevskii [L. P. Gor’kov and L. P. Pitaevskii, Dokl. Akad. Nauk SSSR 151, 822 (1963)] for a three-dimensional (3D) distribution of donors, and calculated the asymptotic form of the exchange energy. Second we have calculated the exchange energy of a “helium atom” in a QW; and third, inspired by the interpolation procedure proposed by Ponomarev et al. [I.V. Ponomarev et al., Phys. Rev. B 60, 5485 (1999)], we have obtained an interpolated expression for any interdonor distance. The obtained exchange energy is written in units of effective hartree, and the distance between the donors, as well as the width of the QW, are expressed in units of effective Bohr radius. We calculated the exchange energy for some commonly studied semiconductor materials, and discussed also the relationship between the exchange energy and the spin relaxation time for a donor concentration close to the insulator-metal transition.
关键词： insulator-metal transition,exchange energy,donor-bound electrons,spin relaxation time,quantum well
更新于2025-09-10 09:29:36
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Formation of uniaxial strained Ge via control of dislocation alignment in Si/Ge heterostructures
摘要： Uniaxially strained Ge/SiGe heterostructures are fabricated by selective ion implantation technique, where dislocation alignments are highly controlled by the local defect introduction. Firstly, ion-implantation-defects are selectively induced into a Ge substrate, followed by the growth of a SiGe buffer layer. As a result, the SiGe on the implanted region is largely strain-relaxed due to the defects acting as dislocation sources. In contrast, it is demonstrated that anisotropic strain relaxation takes place in the SiGe on the unimplanted region, leading to the uniaxial strained SiGe. A strained Ge layer is pseudomorphically grown on the SiGe buffer and the same strain states are observed for the Ge layer. It is found that mis?t dislocations generated at the interface between the SiGe layer and the Ge substrate are aligned along only one direction. These one-directional dislocations are an origin of the uniaxial strain relaxation. Moreover, effects of ion-implantation stripe-pattern widths on the strain states are investigated. With the implanted line width increasing, the anisotropy of the strain in the unimplanted region is enhanced. From these results, it can be said that this technique opens a route to engineer dislocation alignments and anisotropic strain in semiconductor hetero structures toward high performance novel devices.
关键词： dislocation alignment,uniaxial strained Ge,Si/Ge heterostructures,selective ion implantation,anisotropic strain relaxation
更新于2025-09-10 09:29:36
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Shape-modulated multiple exciton generation and optoelectronic properties in PbSe nanostructures
摘要： Multiple exciton generation (MEG) in semiconductor nanostructures is of great interest for the enhancement of related performances in optoelectronic devices and for the shape dependence of conversion ef?ciency with which absorbed photons are converted into electron-hole pairs. However, theoretical insight into the coupling effects from the size and shape gradient on the MEG and related optoelectronic properties at the atomic level remains unclear. Here, we investigate the MEG and optoelectronic properties in PbSe nanostructures with different morphologies (nanocrystals, nanowires, and nanocones) based on the bond relaxation correlation mechanism, detailed balance principle, and Fermi statistical theory. It is found that size reduction of nanostructures can increase the bandgap, suppress the threshold energy, and enhance the MEG ef?ciency. Moreover, optimal conversion ef?ciency of PbSe nanostructures can be achieved by modulating the geometrical parameters.
关键词： bond relaxation correlation mechanism,PbSe nanostructures,Fermi statistical theory,Multiple exciton generation,optoelectronic properties
更新于2025-09-10 09:29:36
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Negative-temperature-state relaxation and reservoir-assisted quantum entanglement in double-spin-domain systems
摘要： Spin collective phenomena including superradiance are even today being intensively investigated with experimental tests performed based on state-of-the-art quantum technologies. Such attempts are not only for the simple experimental verification of predictions from the last century, but also as a motivation to explore new applications of spin collective phenomena and the coherent control of the coupling between spin ensembles and reservoirs. In this paper, we investigate the open quantum dynamics of two spin ensembles (double-spin domains) coupled to a common bosonic reservoir. We analyze in detail the dynamics of our collective state and its structure by focusing on both the symmetry and asymmetry of this coupled spin system. We find that when the spin size of one of the double domains is larger than that of the other domain, at the steady state this system exhibits two unusual collective behaviors: the negative-temperature-state relaxation in the smaller spin domain and the reservoir-assisted quantum entanglement between the two domains. These results are the consequence of the asymmetry of this system and the decoherence driven by the common reservoir.
关键词： negative-temperature-state relaxation,spin collective phenomena,bosonic reservoir,superradiance,quantum entanglement,quantum dynamics
更新于2025-09-09 09:28:46
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Photon echoes from an atomic frequency comb affected by population relaxation
摘要： The atomic frequency comb (AFC) is a promising approach for quantum memory, yet creating an ideal AFC only in the ground-state population remains a challenge. We studied an imperfect AFC initially created in the population inversion between the excited and ground states and redistributed due to population relaxation in a three-level system where a long-lived metastable state plays a crucial role. A theoretical model has been developed to treat the storage of a weak input pulse and retrieval in the form of a photon echo. By comparing theory with experimental results we have studied the echo efficiency and overall attenuation of an input pulse in the AFC affected by both its initial spectral structure and the population relaxation.
关键词： atomic frequency comb,population relaxation,quantum memory,three-level system,photon echo
更新于2025-09-09 09:28:46