A lumped mass gear-rattling model with backlash is established by considering the time varying mesh stiffness, composite transmission error and the torque fluctuation. Based on the principle that no separation will oc...A lumped mass gear-rattling model with backlash is established by considering the time varying mesh stiffness, composite transmission error and the torque fluctuation. Based on the principle that no separation will occur if the response amplitude is not larger than the static response, the threshold is obtained by theoretical derivation. The validity of the theoretical derivation is verified by the numerical method. The results show that the time-varying mesh stiffness has little effect on the threshold. When the exciting frequency is less than the system natural frequency, the theoretical agrees well with the numerical one. The higher the exciting frequency or the smaller the average input torque is, the easier the separation will occur. When the exciting frequency is larger than the natural frequency by a certain value, the theoretical threshold does not agree with the numerical results. The numerical results show that the motion state will change from the meshing state to the separating state directly. The phenomenon that the gear pair sometimes separates and sometimes meshes occurs only when the exciting frequency is smaller.展开更多
Inspired by the excellent stability exhibited by experimentally synthesized two-dimensional(2D)MoSi_(2)N_(4) layered material,the thermal and electronic transport,and thermoelectric(TE)properties of MgAl2Te4 monolayer...Inspired by the excellent stability exhibited by experimentally synthesized two-dimensional(2D)MoSi_(2)N_(4) layered material,the thermal and electronic transport,and thermoelectric(TE)properties of MgAl2Te4 monolayer are systematically investigated using the First-principles calculations and Boltzmann transport theory.The mechanical stability,dynamic stability,and thermal stability(900 K)of the MgAl_(2)Te_(4) monolayer are demonstrated,respectively.The MgAl_(2)Te_(4) monolayer exhibits a bandgap of 1.35 eV using the HSE06 functional in combination with spin-orbit coupling(SOC)effect.Band convergence in the valence band is favorable to improve the thermoelectric properties.The rattling thermal damping effect caused by the weak bonding of Mgsingle bondTe bonds in MgAl2Te4 monolayer leads to ultra-low lattice thermal conductivity(0.95/0.38 W/(m·K)@300 K along the x-/y-direction),which is further demonstrated by the phonon group velocities,phonon relaxation time,Grüneisen parameters,and scattering mechanisms.The optimal zT of 3.28 at 900 K is achieved for the p-type MgAl_(2)Te_(4) monolayer,showing the great promising prospect for the excellent p-type thermoelectric material.Our current work not only reveals the underlying mechanisms responsible for the excellent TE properties,but also elaborates on the promising thermoelectric application of MgAl_(2)Te_(4) monolayer material at high temperature.展开更多
Dielectric ceramics with low permittivity(ε_(r)),high quality factor(Q×f),and near-zero resonant frequency(τ_(f))in the microwave bands are key materials used in fifth/sixth-generation(5G/6G)telecommunication,w...Dielectric ceramics with low permittivity(ε_(r)),high quality factor(Q×f),and near-zero resonant frequency(τ_(f))in the microwave bands are key materials used in fifth/sixth-generation(5G/6G)telecommunication,whileτ_(f) of most low-εr microwave dielectric ceramics is relatively negative.In this work,the first low-εr Ga-based ceramic SrGa_(12_O_(19) with an anomalous positiveτ_(f) was reported,and the causes of the positiveτ_(f),intrinsic polarization,and loss mechanism were systematically studied.X-ray diffraction(XRD)and transmission electron microscopy(TEM)revealed that the SrGa_(12_O_(19) ceramic formed a pure hexagonal magnetoplumbite structure with spinel blocks and rock-salt blocks stacked along the crystallographic c-axis.When sintered at 1430℃,it possessed the optimal microwave dielectric properties of a lowεr of 14.46,high Q×f of 64,705 GHz,and exceptional positiveτ_(f) of+55.7 ppm/℃,along with a low linear thermal expansion coefficient(αL)of 11.617 ppm/℃.The large positive deviation betweenεr andεr(C–M)of 45.31%resulted from the rattling effect of atoms in the rock-salt block.The unique positiveτ_(f)(+55.7 ppm/℃)was governed by the rattling effect,resulting in a positiveταm(the temperature coefficient of ion polarizability)of 8.489 ppm/℃ and a large negative temperature coefficient of permittivity(τε)of−132.864 ppm/℃.Phillips–Vechten–Levine(P–V–L)chemical bond theory revealed greater contributions of the spinel block to bond ionicity(fi,52.95%),permittivity(ε,55.15%),bond energy(E,56.87%),and lattice energy(U,74.88%)than those of the rock-salt block.The intrinsic dielectric properties were analyzed using infrared(IR)reflectivity spectra.The favorable performance of the SrGa_(12_O_(19) ceramic indicated that it is a novelτ_(f) compensator.This selection of compounds with different structural layer combinations provides a new idea for exploring excellent microwave dielectric ceramics.展开更多
Rattling vibration is an important noise source of gear-box. To controlthat noise, it is necessary to elaborate a mathematics-mechanical model on rattlinggears. In this paper, a rattling system modulated by noise was ...Rattling vibration is an important noise source of gear-box. To controlthat noise, it is necessary to elaborate a mathematics-mechanical model on rattlinggears. In this paper, a rattling system modulated by noise was investigated. Insteadof performing the very tedious numerical calculation, a discrete stochastic modeldescribed by three dimensional mean mapping was established by means of the NonGaussian closure technique. Through the example, the chaotic stochastic behaviormay be revealed. In comparison with deterministic model, the model developed inthis paper is more approximate to practice adn more available for acousticinvestigation, so that it is suggested to be applied to modeling on rattling vibration.展开更多
We prepared (1-x)CaTiO3-xLaAlO3 (0 ≤x≤ 1) microwave dielectric ceramics using a conventional two-step solid-state reaction method,and investigated microwave dielectric properties of the ceramics in terms of A/B-site...We prepared (1-x)CaTiO3-xLaAlO3 (0 ≤x≤ 1) microwave dielectric ceramics using a conventional two-step solid-state reaction method,and investigated microwave dielectric properties of the ceramics in terms of A/B-site ionic-parameters.Ionic-parameters such as ionic polarizability,A-site bond valence,and ionic rattling were linked to the microwave dielectric properties.As the LaAlO3 content x in the (1-x)CaTiO3-xLaAlO3 ceramics increased from 0.3 to 0.7,the dielectric constant gradually decreased,which was attributed to the decrease of polarizability deviation and suppression of the cation rattling.The temperature coefficient of the resonant frequency decreased as the content of LaAlO3 increased because of the increase of A-site cation bond valence.The quality factor value (Q × f) increased as LaAlO3 content increased because of the enhancement of the order degree of B-site cation.A significant deterioration of the temperature coefficient of the resonant frequency and Q ×fvalue was observed at the composition x =0.5.These decreases were attributed to a phase transition from orthorhombic crystal (for x ≤ 0.5) to rhombohedral crystal (for x ＞ 0.5).展开更多
基金This project is supported by National Natural Science Foundation of China(No.50075070).
文摘A lumped mass gear-rattling model with backlash is established by considering the time varying mesh stiffness, composite transmission error and the torque fluctuation. Based on the principle that no separation will occur if the response amplitude is not larger than the static response, the threshold is obtained by theoretical derivation. The validity of the theoretical derivation is verified by the numerical method. The results show that the time-varying mesh stiffness has little effect on the threshold. When the exciting frequency is less than the system natural frequency, the theoretical agrees well with the numerical one. The higher the exciting frequency or the smaller the average input torque is, the easier the separation will occur. When the exciting frequency is larger than the natural frequency by a certain value, the theoretical threshold does not agree with the numerical results. The numerical results show that the motion state will change from the meshing state to the separating state directly. The phenomenon that the gear pair sometimes separates and sometimes meshes occurs only when the exciting frequency is smaller.
基金Financial supports from the National Natural Science Foundation of China(21503039)Department of Science and Technology of Liaoning Province(2019MS164)+1 种基金Department of Education of Liaoning Province(LJ2020JCL034)Discipline Innovation Team of Liaoning Technical University(LNTU20TD-16)are greatly acknowledged.
文摘Inspired by the excellent stability exhibited by experimentally synthesized two-dimensional(2D)MoSi_(2)N_(4) layered material,the thermal and electronic transport,and thermoelectric(TE)properties of MgAl2Te4 monolayer are systematically investigated using the First-principles calculations and Boltzmann transport theory.The mechanical stability,dynamic stability,and thermal stability(900 K)of the MgAl_(2)Te_(4) monolayer are demonstrated,respectively.The MgAl_(2)Te_(4) monolayer exhibits a bandgap of 1.35 eV using the HSE06 functional in combination with spin-orbit coupling(SOC)effect.Band convergence in the valence band is favorable to improve the thermoelectric properties.The rattling thermal damping effect caused by the weak bonding of Mgsingle bondTe bonds in MgAl2Te4 monolayer leads to ultra-low lattice thermal conductivity(0.95/0.38 W/(m·K)@300 K along the x-/y-direction),which is further demonstrated by the phonon group velocities,phonon relaxation time,Grüneisen parameters,and scattering mechanisms.The optimal zT of 3.28 at 900 K is achieved for the p-type MgAl_(2)Te_(4) monolayer,showing the great promising prospect for the excellent p-type thermoelectric material.Our current work not only reveals the underlying mechanisms responsible for the excellent TE properties,but also elaborates on the promising thermoelectric application of MgAl_(2)Te_(4) monolayer material at high temperature.
基金funded by the Natural Science Foundation of Guangxi Zhuang Autonomous Region(No.2024GXNSFFA010013)the Project of Scientific Research and Technical Exploitation Program of the Guangxi Zhuang Autonomous Region(No.AD24010021)+1 种基金the Guilin University of Technology Research Startup Project(No.GUTQDJJ2019183)Guangxi BaGui Scholars Special Funding。
文摘Dielectric ceramics with low permittivity(ε_(r)),high quality factor(Q×f),and near-zero resonant frequency(τ_(f))in the microwave bands are key materials used in fifth/sixth-generation(5G/6G)telecommunication,whileτ_(f) of most low-εr microwave dielectric ceramics is relatively negative.In this work,the first low-εr Ga-based ceramic SrGa_(12_O_(19) with an anomalous positiveτ_(f) was reported,and the causes of the positiveτ_(f),intrinsic polarization,and loss mechanism were systematically studied.X-ray diffraction(XRD)and transmission electron microscopy(TEM)revealed that the SrGa_(12_O_(19) ceramic formed a pure hexagonal magnetoplumbite structure with spinel blocks and rock-salt blocks stacked along the crystallographic c-axis.When sintered at 1430℃,it possessed the optimal microwave dielectric properties of a lowεr of 14.46,high Q×f of 64,705 GHz,and exceptional positiveτ_(f) of+55.7 ppm/℃,along with a low linear thermal expansion coefficient(αL)of 11.617 ppm/℃.The large positive deviation betweenεr andεr(C–M)of 45.31%resulted from the rattling effect of atoms in the rock-salt block.The unique positiveτ_(f)(+55.7 ppm/℃)was governed by the rattling effect,resulting in a positiveταm(the temperature coefficient of ion polarizability)of 8.489 ppm/℃ and a large negative temperature coefficient of permittivity(τε)of−132.864 ppm/℃.Phillips–Vechten–Levine(P–V–L)chemical bond theory revealed greater contributions of the spinel block to bond ionicity(fi,52.95%),permittivity(ε,55.15%),bond energy(E,56.87%),and lattice energy(U,74.88%)than those of the rock-salt block.The intrinsic dielectric properties were analyzed using infrared(IR)reflectivity spectra.The favorable performance of the SrGa_(12_O_(19) ceramic indicated that it is a novelτ_(f) compensator.This selection of compounds with different structural layer combinations provides a new idea for exploring excellent microwave dielectric ceramics.
文摘Rattling vibration is an important noise source of gear-box. To controlthat noise, it is necessary to elaborate a mathematics-mechanical model on rattlinggears. In this paper, a rattling system modulated by noise was investigated. Insteadof performing the very tedious numerical calculation, a discrete stochastic modeldescribed by three dimensional mean mapping was established by means of the NonGaussian closure technique. Through the example, the chaotic stochastic behaviormay be revealed. In comparison with deterministic model, the model developed inthis paper is more approximate to practice adn more available for acousticinvestigation, so that it is suggested to be applied to modeling on rattling vibration.
文摘We prepared (1-x)CaTiO3-xLaAlO3 (0 ≤x≤ 1) microwave dielectric ceramics using a conventional two-step solid-state reaction method,and investigated microwave dielectric properties of the ceramics in terms of A/B-site ionic-parameters.Ionic-parameters such as ionic polarizability,A-site bond valence,and ionic rattling were linked to the microwave dielectric properties.As the LaAlO3 content x in the (1-x)CaTiO3-xLaAlO3 ceramics increased from 0.3 to 0.7,the dielectric constant gradually decreased,which was attributed to the decrease of polarizability deviation and suppression of the cation rattling.The temperature coefficient of the resonant frequency decreased as the content of LaAlO3 increased because of the increase of A-site cation bond valence.The quality factor value (Q × f) increased as LaAlO3 content increased because of the enhancement of the order degree of B-site cation.A significant deterioration of the temperature coefficient of the resonant frequency and Q ×fvalue was observed at the composition x =0.5.These decreases were attributed to a phase transition from orthorhombic crystal (for x ≤ 0.5) to rhombohedral crystal (for x ＞ 0.5).
