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Xiaopeng Zhao

Publications and source records attributed to Xiaopeng Zhao.

At least 37 records · Page 2Linked to original sources
arXiv2023

Green-light p-n Junction Particle Inhomogeneous Phase Enhancement of MgB2 Smart Meta-Superconductor

Improving the critical temperature (TC), critical magnetic field (HC), and critical current (JC) of superconducting materials has always been one of the most significant challenges in the field of superconductivity, but progress has been slow over the years. Based on the concept of injecting energy to enhance electron pairing states, in this study, we have employed a solid-state sintering method to fabricate a series of smart meta-superconductors (SMSCs) consisting of p-n junction nanostructures with a wavelength of 550 nm, doped within an MgB2 matrix. Experimental results demonstrate that compared to pure MgB2 samples, the critical transition temperature (TC) has increased by 1.2 K, the critical current (JC) has increased by 52.8%, and the Meissner effect (HC) shows significant improvement in its diamagnetic properties. This phenomenon of enhanced superconducting performance can be explained by the coupling between superconducting electrons and evanescent waves.

cond-mat.supr-con↗
arXiv2022

An improved smart meta-superconductor MgB2

Increasing and improving the critical transition temperature (Tc), current density (Jc) and Meissner effect (Hc) of conventional superconductors are the most important problems in superconductivity research, but progress has been slow for many years. In this study, by introducing the p-n junction electroluminescent inhomogeneous phase with red wavelength to realize energy injection, we found the improved property of smart meta-superconductors MgB2, the critical transition temperature Tc increases by 0.8K, the current density Jc increases by 37%, and the diamagnetism of Meissner effect Hc also significantly improved, compared with pure MgB2. Compared with previous yttrium oxide inhomogeneous phase, p-n junction has higher luminescence intensity, longer stable life and simpler external field requirements. The coupling between superconducting electrons and surface plasmon polaritons may be explain this phenomenon. The realization of smart meta-superconductor by this electroluminescent inhomogeneous phase provides a new way to improve the performance of superconductors.

cond-mat.supr-con↗
arXiv2021

Learning-Based Strategy Design for Robot-Assisted Reminiscence Therapy Based on a Developed Model for People with Dementia

In this paper, the robot-assisted Reminiscence Therapy (RT) is studied as a psychosocial intervention to persons with dementia (PwDs). We aim at a conversation strategy for the robot by reinforcement learning to stimulate the PwD to talk. Specifically, to characterize the stochastic reactions of a PwD to the robot's actions, a simulation model of a PwD is developed which features the transition probabilities among different PwD states consisting of the response relevance, emotion levels and confusion conditions. A Q-learning (QL) algorithm is then designed to achieve the best conversation strategy for the robot. The objective is to stimulate the PwD to talk as much as possible while keeping the PwD's states as positive as possible. In certain conditions, the achieved strategy gives the PwD choices to continue or change the topic, or stop the conversation, so that the PwD has a sense of control to mitigate the conversation stress. To achieve this, the standard QL algorithm is revised to deliberately integrate the impact of PwD's choices into the Q-value updates. Finally, the simulation results demonstrate the learning convergence and validate the efficacy of the achieved strategy. Tests show that the strategy is capable to duly adjust the difficulty level of prompt according to the PwD's states, take actions (e.g., repeat or explain the prompt, or comfort) to help the PwD out of bad states, and allow the PwD to control the conversation tendency when bad states continue.

cs.RO↗
arXiv2021

Assessing the Acceptability of a Humanoid Robot for Alzheimer's Disease and Related Dementia Care Using an Online Survey

In this work, an online survey was used to understand the acceptability of humanoid robots and users' needs in using these robots to assist with care among people with Alzheimer's disease and related dementias (ADRD), their family caregivers, health care professionals, and the general public. From November 12, 2020 to March 13, 2021, a total of 631 complete responses were collected, including 80 responses from people with mild cognitive impairment or ADRD, 245 responses from caregivers and health care professionals, and 306 responses from the general public. Overall, people with ADRD, caregivers, and the general public showed positive attitudes towards using the robot to assist with care for people with ADRD. The top three functions of robots required by the group of people with ADRD were reminders to take medicine, emergency call service, and helping contact medical services. Additional comments, suggestions, and concerns provided by caregivers and the general public are also discussed.

cs.RO↗
arXiv2021

A Simulated Experiment to Explore Robotic Dialogue Strategies for People with Dementia

People with Alzheimer's disease and related dementias (ADRD) often show the problem of repetitive questioning, which brings a great burden on persons with ADRD (PwDs) and their caregivers. Conversational robots hold promise of coping with this problem and hence alleviating the burdens on caregivers. In this paper, we proposed a partially observable markov decision process (POMDP) model for the PwD-robot interaction in the context of repetitive questioning, and used Q-learning to learn an adaptive conversation strategy (i.e., rate of follow-up question and difficulty of follow-up question) towards PwDs with different cognitive capabilities and different engagement levels. The results indicated that Q-learning was helpful for action selection for the robot. This may be a useful step towards the application of conversational social robots to cope with repetitive questioning in PwDs.

cs.RO↗
arXiv2021

Relationship between the TC of smart meta-superconductor Bi(Pb)SrCaCuO and inhomogeneous phase content

A smart meta-superconductor Bi(Pb)SrCaCuO (B(P)SCCO) may increase the critical transition temperature (TC) of B(P)SCCO by electroluminescence (EL) energy injection of inhomogeneous phases. However, the increase amplitude ΔTC (ΔTC=TC-T(C,pure)) of TC is relatively small. In this study, a smart meta-superconductor B(P)SCCO with different matrix sizes was designed. Three kinds of raw materials with different particle sizes were used, and different series of Y2O3:Sm3+, Y2O3, Y2O3:Eu3+, and Y2O3:Eu3++Ag doped samples and pure B(P)SCCO were prepared. Results indicated that the TC of the Y2O3 or Y2O3:Sm3+ non-luminescent dopant doping sample is lower than that of pure B(P)SCCO. However, the TC of the Y2O3:Eu3++Ag or Y2O3:Eu3+ luminescent inhomogeneous phase doping sample is higher than that of pure B(P)SCCO. With the decrease of the raw material particle size from 30 to 5 μm, the particle size of the B(P)SCCO superconducting matrix in the prepared samples decreases, and the doping content of the Y2O3:Eu3++Ag or Y2O3:Eu3+ increases from 0.2% to 0.4%. Meanwhile, the increase of the inhomogeneous phase content enhances the ΔTC. When the particle size of raw material is 5 μm, the doping concentration of the luminescent inhomogeneous phase can be increased to 0.4%. At this time, the zero-resistance temperature and onset transition temperature of the Y2O3:Eu3++Ag doped sample are 4 and 6.3 K higher than those of pure B(P)SCCO, respectively.

cond-mat.supr-con↗
arXiv2021

Reinforcing increase of ΔTC in MgB2 smart meta-superconductors by adjusting the concentration of inhomogeneous phases

Incorporating with inhomogeneous phases with high electroluminescence (EL) intensity to prepare smart meta-superconductors (SMSCs) is an effective method of increasing the superconducting transition temperature (Tc) and has been confirmed in both MgB2 and Bi(Pb)SrCaCuO systems. However, the increase of ΔTc has been quite small because of the low optimal concentrations of inhomogeneous phases. In this work, three kinds of MgB2 raw materials, namely, aMgB2, bMgB2, and cMgB2, were prepared with particle sizes decreasing in order. Inhomogeneous phases, Y2O3:Eu3+ and Y2O3:Eu3+/Ag, were also prepared and doped into MgB2 to study the influence of doping concentration on the ΔTc of MgB2 with different particle sizes. Results show that reducing the MgB2 particle size increases the optimal doping concentration of inhomogeneous phases, thereby increasing ΔTc. The optimal doping concentrations for aMgB2, bMgB2, and cMgB2 are 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔTc values are 0.4, 0.9, and 1.2 K, respectively. This work open a new approach to reinforcing increase of ΔTc in MgB2 SMSCs.

cond-mat.supr-con↗
arXiv2021

Global well-posedness and decay estimates for three-dimensional compressible Navier-Stokes-Allen-Cahn system

We study the small data global well-posedness and time-decay rates of solutions to the Cauchy problem for 3D compressible Navier-Stokes-Allen-Cahn equations via a refined pure energy method. In particular, the optimal decay rates of the higher-order spatial derivatives of the solution are obtained, the $\dot{H}^{-s}$ ($0\leq s<\frac32$) negative Sobolev norms is shown to be preserved along time evolution and enhance the decay rates.

math.AP↗
arXiv2020

On the Cauchy problem of 3D incompressible Navier-Stokes-Cahn-Hilliard system

In this paper, we are concerned with the well-posedness and large time behavior of Cauchy problem for 3D incompressible Navier-Stokes-Cahn-Hilliard equations. First, using Banach fixed point theorem, we establish the local well-posedness of solutions. Second, assuming $\|(u_0,\nablaϕ_0)\|_{\dot{H}^\frac12}$ is sufficiently small, we obtain the global well-posedness of solutions. Moreover, the optimal decay rates of the higher-order spatial derivatives of the solution are also obtained.

math.AP↗
arXiv2020

Global well-posedness and decay of solutions to the Cauchy problem of convective Cahn-Hilliard equation

In this paper, we consider the global well-posedness and time-decay rates of solution to the Cauchy problem for 3D convective Cahn-Hilliard equation with double-well potential via a refined pure energy method. In particular, the optimal decay rates of the higher-order spatial derivatives of the solution are obtained, the $\dot{H}^{-s}$ ($0<s\leq\frac12$) negative Sobolev norms is shown to be preserved along time evolution and enhance the decay rates.

math.AP↗
arXiv2020

Adaptive Switching Control of Wind Turbine Generators for Necessary Frequency Response

This letter proposes a new control strategy for wind turbine generators to decide the necessity of switches between the normal operation and frequency support modes. The idea is to accurately predict an unsafe frequency response using a differential transformation method right after power imbalance is detected so as to adaptively activate a frequency support mode only when necessary. This control strategy can effectively avoid unnecessary switches with a conventional use of deadband but still ensure adequate frequency response.

eess.SY↗
arXiv2020

Smart metastructure method for increasing TC of Bi(Pb)SrCaCuO high-temperature superconductors

Improving the critical transition temperature (TC) of Bi(Pb)SrCaCuO (B(P)SCCO) high-temperature superconductors is important, however, considerable challenges exist. In this study, on the basis of the metamaterial structure and the idea that the injecting energy will promote the formation of Cooper pairs, a smart meta-superconductor B(P)SCCO consisting of B(P)SCCO microparticles and Y2O3:Eu3++Ag or Y2O3:Eu3+ luminophor was designed. In the applied electric field, the Y2O3:Eu3++Ag or Y2O3:Eu3+ luminophor generates an electroluminescence (EL), thereby promoting the TC via EL energy injection. A series of Y2O3:Eu3++Ag topological luminophor-doped B(P)SCCO samples was prepared. Results showed that Y2O3:Eu3++Ag was dispersed around B(P)SCCO particles, forming a metastructure. Accordingly, the onset transition temperature (T_(C,on)) and zero resistance transition temperature (T_(C,0)) of B(P)SCCO increased. Meanwhile, the B(P)SCCO sample doped with 0.2 wt% Y2O3 or Y2O3:Sm3+ nonluminous inhomogeneous phase was also prepared to further prove the influence of EL on the T_C rather than the rare earth effect. Results indicated that the TC of the Y2O3 or Y2O3:Sm3+ doping sample decreased. However, the TC of the 0.2 wt% Y2O3:Eu3++Ag or Y2O3:Eu3+ luminophor-doped sample improved. This outcome further demonstrated that the smart metastructure method can improve the TC of B(P)SCCO.

cond-mat.supr-con↗
arXiv2019

On the Cauchy problem of a sixth-order Cahn-Hilliard equation arising in oil-water-surfactant mixtures

We study the global well-posedness and asymptotic behavior of solutions for the Cauchy problem of three-dimensional sixth order Cahn-Hilliard equation arising in oil-water-surfactant mixtures. First, by using the pure energy method and a standard continuity argument, we prove that there exists a unique global strong solution provided that the $H^2$-norm of initial data is sufficiently small. Moreover, we also establish the suitable negative Sobolev norm estimates and obtain the time decay rate of strong solutions. It is worth pointing out that although the problem we considered is a sixth-order parabolic equation, the time decay rate is equivalent to the decay rate of fourth-order generalized heat equation, which is better than our expect.

math.AP↗
arXiv2019

Special uniform decay rate of local energy for the wave equation with variable coefficients on an exterior domain

We consider the wave equation with variable coefficients on an exterior domain in $\R^n$($n\ge 2$). We are interested in finding a special uniform decay rate of local energy different from the constant coefficient wave equation. More concretely, if the dimensional $n$ is even, whether the uniform decay rate of local energy for the wave equation with variable coefficients can break through the limit of polynomial and reach exponential; if the dimensional $n$ is odd, whether the uniform decay rate of local energy for the wave equation with variable coefficients can hold exponential as the constant coefficient wave equation . \quad \ \ We propose a cone and establish Morawetz's multipliers in a version of the Riemannian geometry to derive uniform decay of local energy for the wave equation with variable coefficients. We find that the cone with polynomial growth is closely related to the uniform decay rate of the local energy. More concretely, for radial solutions, when the cone has polynomial of degree $\frac{n}{2k-1}$ growth, the uniform decay rate of local energy is exponential; when the cone has polynomial of degree $\frac{n}{2k}$ growth, the uniform decay rate of local energy is polynomial at most. In addition, for general solutions, when the cone has polynomial of degree $n$ growth, we prove that the uniform decay rate of local energy is exponential under suitable Riemannian metric. It is worth pointing out that such results are independent of the parity of the dimension $n$, which is the main difference with the constant coefficient wave equation. Finally, for general solutions, when the cone has polynomial of degree $m$ growth, where $m$ is any positive constant, we prove that the uniform decay rate of the local energy is of primary polynomial under suitable Riemannian metric.

math.AP↗
arXiv2018

Escape Metrics and its Applications

Geodesics escape is widely used to study the scattering of hyperbolic equations. However, there are few progresses except in a simply connected complete Riemannian manifold with nonpositive curvature. We propose a kind of complete Riemannian metrics in $\mathbb{R}^n$, which is called as escape metrics. We expose the relationship between escape metrics and geodesics escape in $\mathbb{R}^n$. Under the escape metric $g$, we prove that each geodesic of $(\mathbb{R}^n,g)$ escapes, that is, $\lim_{t\rightarrow +\infty} |γ(t)|=+\infty$ for any $x\in \mathbb{R}^n$ and any unit-speed geodesic $γ(t)$ starting at $x$. We also obtain the geodesics escape velocity and give the counterexample that if escape metrics are not satisfied, then there exists an unit-speed geodesic $γ(t)$ such that $\overline{\lim}_{t\rightarrow +\infty} |γ(t)|<+\infty$. In addition, we establish Morawetz multipliers in Riemannian geometry to derive dispersive estimates for the wave equation on an exterior domain of $\mathbb{R}^n$ with an escape metric. More concretely, for radial solutions, the uniform decay rate of the local energy is independent of the parity of the dimension $n$. For general solutions, we prove the space-time estimation of the energy and uniform decay rate $t^{-1}$ of the local energy. It is worth pointing out that different from the assumption of an Euclidean metric at infinity in the existing studies, escape metrics are more general Riemannian metrics.

math.AP↗
arXiv2018

Smart meta-superconductor MgB2 constructed by inhomogeneous phase of luminescent nanocomposite

On the basis of the idea that the injecting energy will improve the conditions for the formation of Cooper pairs, a smart meta-superconductor (SMSC) was prepared by doping inhomogeneous phase of luminescent nanocomposite Y2O3:Eu3+/Ag, which has the strong luminescence characteristic, in MgB2 to improve the superconducting transition temperature (TC) of the MgB2-based superconductor. Two types of Y2O3:Eu3+/Ag with different sizes were prepared and marked as m-Y2O3:Eu3+/Ag and nY2O3:Eu3+/Ag. MgB2 SMSC was prepared through an ex situ process. Results show that when the inhomogeneous phase content was fixed at 2.0 wt.%, the TC of MgB2 SMSC increased initially then decreased with the increase in the Ag content in the dopant. When the Ag content accounted for 5 wt.% of the inhomogeneous phase weight, the TC of MgB2 SMSC was 37.2-38.0 K, which was similar to that of pure MgB2. Meanwhile, the TC of MgB2 SMSC doped with n-Y2O3:Eu3+/Ag increased initially then decreased basically with the increase in the content of n-Y2O3:Eu3+/Ag, in which Ag accounted for 5 wt.% of the inhomogeneous phase. The TC of MgB2 SMSC doped with 0.5 wt.% n-Y2O3:Eu3+/Ag was 37.6-38.4 K, which was 0.4 K higher than that of pure MgB2. It is thought that the doping inhomogeneous phase of luminescent nanocomposite into the superconductor is a new means to improve the TC of SMSC.

cond-mat.supr-con↗
arXiv2018

Topological luminophor Y2O3:Eu3++Ag with high electroluminescence performance

Improving luminescent intensity is a significant technical requirement and scientific problem for the luminescent performance of fluorophor materials through the ages. The process control and luminescence performance still limit the developments of luminescent intensity even through it can be improved partly by covering or magnetron sputtering of precious metals on the surface of the fluorophore materials. On the basis of the improvement of luminescence center radiative transition rate by surface plasma resonance and Y2O3:Eu3+ microsheet phosphors, a fundamental model for topological luminophor Y2O3:Eu3++Ag was designed referencing the concepts of topological materials in order to enhance luminescent performance by composite-luminescence, which composed of Eu3+centric electroluminescence and surface plasma-enhanced photoluminescence by Ag. The topological luminophor Y2O3:Eu3++Ag was successfully synthesized with an asymmetric-discrete Ag nanocrystal topological structure on the surface just via illumination. Experiment results suggest that the luminescence performance of topological luminophor Y2O3:Eu3++Ag increased by about 300% compared with that of Y2O3: Eu3+ phosphors on the same conditions. The design of a topological luminophor provides a new approach to further improve the luminescent intensity of phosphors.

cond-mat.mes-hall↗