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1.
The Rayleigh-Bénard theory by Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)] is extended towards very large Prandtl numbers Pr. The Nusselt number Nu is found here to be independent of Pr. However, for fixed Rayleigh numbers Ra a maximum in the Nu(Pr) dependence is predicted. We moreover offer the full functional dependences of Nu(Ra,Pr) and Re(Ra,Pr) within this extended theory, rather than only give the limiting power laws as done in J. Fluid. Mech. 407, 27 (2000). This enables us to more realistically describe the transitions between the various scaling regimes. 相似文献
2.
Persson BN 《The European physical journal. E, Soft matter》2002,8(4):385-401
I have developed a theory of adhesion between an elastic solid and a hard randomly rough substrate. The theory takes into
account that partial contact may occur between the solids on all length scales. I present numerical results for the case where
the substrate surface is self-affine fractal. When the fractal dimension is close to 2, complete contact typically occurs
in the macro-asperity contact areas, while when the fractal dimension is larger than 2.5, the area of (apparent) contact decreases
continuously when the magnification is increased. An important result is that even when the surface roughness is so high that
no adhesion can be detected in a pull-off experiment, the area of real contact (when adhesion is included) may still be several
times larger than when the adhesion is neglected. Since it is the area of real contact which determines the sliding friction
force, the adhesion interaction may strongly affect the friction force even when no adhesion can be detected in a pull-off
experiment.
Received 3 April 2002 相似文献
3.
采用密度泛函理论对M-(Sm、Pr、Ga)掺杂锐钛矿型TiO2能带和电子性质进行了系统的理论研究. 计算结果表明,通过Sm和Pr的掺杂可以降低TiO2的带隙进而使其产生吸收边红移,通过Ga的掺杂能使带隙稍增加. 这主要是由于Sm和Pr的掺杂使Sm和Pr上的4f层电子与原子相邻O原子上的2p层电子相互作用,形成的杂质能级影响了Ti-O的能带结构,从而降低带隙,提高TiO2的可见光吸收性能. 相似文献
4.
Robinson AP Woods PJ Seweryniak D Davids CN Carpenter MP Hecht AA Peterson D Sinha S Walters WB Zhu S 《Physical review letters》2005,95(3):032502
Ground-state proton radioactivity has been identified from 121Pr. A transition with a proton energy of E(p)=882(10) keV [Q(p)=900(10) keV] and half-life t(1/2)=10(+6)(-3) ms has been observed and is assigned to the decay of a highly prolate deformed 3/2(+) or 3/2(-) Nilsson state. The present result is found to be incompatible with a previously reported observation of ground-state proton radioactivity from 121Pr, which would have represented the discovery of this phenomenon. 相似文献
5.
Biswas A Fournier P Qazilbash MM Smolyaninova VN Balci H Greene RL 《Physical review letters》2002,88(20):207004
We present point contact spectroscopy (PCS) data for junctions between a normal metal and the electron-doped cuprate superconductor Pr(2-x)CexCuO4 (PCCO). For the underdoped compositions of this cuprate ( x approximately 0.13) we observe a peak in the conductance-voltage characteristics of the point contact junctions. The shape and magnitude of this peak suggest the presence of Andreev bound states at the surface of underdoped PCCO which is evidence for a d-wave pairing symmetry. For overdoped PCCO ( x approximately 0.17) the PCS data do not show any evidence of Andreev bound states at the surface suggesting an s-wave pairing symmetry. 相似文献
6.
采用改良的固相烧结工艺制备了Bi5-xPrxFe0.5Co0.5Ti3O15(BPFCT-x,x=0.25,0.50,0.75,0.80)陶瓷样品.X射线衍射结构分析表明:镨(Pr)含量对样品微观结构产生了影响,但所有样品均为层状钙钛矿结构;BPFCT-x样品的剩余极化强度(2Pr)随着掺杂量的增加呈现出先增大后减小的变化趋势,当Pr含量为0.75时,样品的2Pr达到最大值,为6.43μC/cm2.样品的磁性与铁电性能具有相同的变化规律,室温下样品的剩余磁化强度(2Mr)也呈现出先增大后减小的趋势,并且也在x=0.75时达到最大为0.097 emu/g.随着Pr掺杂量增大,样品的室温下铁电和铁磁性能得到明显改善,并且当掺杂量为0.75时,样品室温多铁性最好.Pr掺杂降低了样品中的缺陷浓度,从而提高了样品铁电畴动性,这有助于提高样品铁电性能.而样品铁磁性能的改善可能与Pr对样品晶格畸变产生的影响有关. 相似文献
7.
8.
在全相对论理论框架下,研究了类In等电子序(Ba VIII-Nd XII)低激发态组态5s~24f和5s`25p间的光谱性质.系统地研究了Breit相互作用和量子电动力学(QED)效应对能级的影响.离子Pr~(~(10+))附近出现相对论轨道4f-5p能级交叉,窄的能级间隔可以使得跃迁波长在现代激光范围,这可能设计成原子钟.计算得到Pr~(10+)基态和激发态磁偶极和电四极超精细结构常数.目前的计算结果可以为实验测量提供理论依据. 相似文献
9.
贾德-奥菲尔特(Judd-Ofelt,J-O)理论是半定量地研究稀土离子掺杂基质中4f能级间跃迁光学性质的经典理论,但标准的贾德-奥菲尔特理论表达式本身含有一定的简化近似和假设,应用于Pr3 等具有特殊能级的稀土离子时会产生异常结果。制备了掺Pr3 和Tm3 的氟化镓铟玻璃,研究了玻璃的吸收光谱。应用标准的和改进后的贾德-奥菲尔特理论分别拟合得到了不同的贾德-奥菲尔特强度参量和跃迁振子强度。通过分析对比可知,“超敏感跃迁”是产生异常贾德-奥菲尔特理论计算的重要因素;当加入了“超敏感跃迁”所对应奇数项约化矩阵元后,运用改进的贾德-奥菲尔特理论可计算出氟化镓铟玻璃中Pr3 较合理的贾德-奥菲尔特强度参量,并可对Tm3 的振子强度参量进行更好的拟合。 相似文献
10.
The Josephson current is calculated for an SFIFS tunnel contact where I is an insulator), whose banks are formed by a massive superconductor (S) and a thin layer of a normal ferromagnetic metal (F). Within the microscopic theory of the proximity effect for an SF bilayer, it is shown that, even at mesoscopic dimensions of the ferromagnetic layer in an SFIFS contact, a transition to the state characterized by a π shift of the phase difference between the superconducting wave functions at the banks of the contact can be implemented owing to this effect. The amplitude of the critical current as a function of bilayer parameters is found. It is shown that the exchange-field strength at which the Josephson current in an SFIFS contact is reversed (transition to the π state) can be varied within rather broad limits by changing the parameters of the SF bilayer (resistance of the SF interface and proximity-effect magnitude). 相似文献
11.
可溶性钒氧酞菁在不同介质中的吸收光谱研究 总被引:4,自引:0,他引:4
研究了四丙基取代钒氧酞菁(Pr4VOPc)在不同介质中的吸收光谱。Pr4VOPc分子在不同介质中的吸收光谱差别很大:在稀溶液中以分子的单体吸收为主(λmax=707nm),在浓溶液和PMMA薄膜中由于形成平行排列结构的二聚体而使分子单体的吸收峰发生蓝移(λmax=660nm),在蒸发膜中则由于形成斜交结构的二聚体使得分子单体的吸收峰发生分裂形成红移和蓝移峰(λmax=720nm,660nm),在溶液沉积的固态薄膜中由于形成结晶相I而表现出强烈的近红外吸收(λmax=826.9nm)。造成这种差别的原因是四个丙基的引入加大了VOPc分子的空间位阻效应。通过热处理或溶剂蒸汽处理,可以使Pr4VOPc蒸发膜转变成具有强近红外吸收的结晶相I;而只有通过溶剂蒸汽处理才能使PMMA介质中的Pr4VOPc转变成结晶相II。 相似文献
12.
Guided by experiment and band structure, we introduce and study a phenomenological Landau theory for the unusual charge and spin ordering associated with the Mott transition in the perovskite nickelates, with chemical formula RNiO3, where R=Pr, Nd,Sm, Eu, Ho, Y, and Lu. While the Landau theory has general applicability, we show that for the most conducting materials, R=Pr, Nd, both types of order can be understood in terms of a nearly nested spin-density wave. Furthermore, we argue that in this regime, the charge ordering is reliant upon the orthorhombic symmetry of the sample, and therefore proportional to the magnitude of the orthorhombic distortion. The first order nature of the phase transitions is also explained. We briefly show by example how the theory is readily adapted to modified geometries such as nickelate films. 相似文献
13.
We report a series of numerical simulations showing that the critical magnetic Reynolds number Rm(c) for the nonhelical small-scale dynamo depends on the Reynolds number Re. Namely, the dynamo is shut down if the magnetic Prandtl number Pr(m)=Rm/Re is less than some critical value Pr(m,c)< approximately 1 even for Rm for which dynamo exists at Pr(m)> or =1. We argue that, in the limit of Re-->infinity, a finite Pr(m,c) may exist. The second possibility is that Pr(m,c)-->0 as Re--> infinity, while Rm(c) tends to a very large constant value inaccessible at current resolutions. If there is a finite Pr(m,c), the dynamo is sustainable only if magnetic fields can exist at scales smaller than the flow scale, i.e., it is always effectively a large-Pr(m) dynamo. If there is a finite Rm(c), our results provide a lower bound: Rm(c) greater, similar 220 for Pr(m)< or =1/8. This is larger than Rm in many planets and in all liquid-metal experiments. 相似文献
14.
15.
Persson BN Prodanov N Krick BA Rodriguez N Mulakaluri N Sawyer WG Mangiagalli P 《The European physical journal. E, Soft matter》2012,35(1):5
The dynamics of fluid flow at the interface between elastic solids with rough surfaces depends sensitively on the area of
real contact, in particular close to the percolation threshold, where an irregular network of narrow flow channels prevails.
In this paper, numerical simulation and experimental results for the contact between elastic solids with isotropic and anisotropic
surface roughness are compared with the predictions of a theory based on the Persson contact mechanics theory and the Bruggeman
effective medium theory. The theory predictions are in good agreement with the experimental and numerical simulation results
and the (small) deviation can be understood as a finite-size effect. The fluid squeeze-out at the interface between elastic
solids with randomly rough surfaces is studied. We present results for such high contact pressures that the area of real contact
percolates, giving rise to sealed-off domains with pressurized fluid at the interface. The theoretical predictions are compared
to experimental data for a simple model system (a rubber block squeezed against a flat glass plate), and for prefilled syringes,
where the rubber plunger stopper is lubricated by a high-viscosity silicon oil to ensure functionality of the delivery device.
For the latter system we compare the breakloose (or static) friction, as a function of the time of stationary contact, to
the theory prediction. 相似文献
16.
多电子原子能量的相对论修正 总被引:3,自引:1,他引:2
以Breit-Pauli哈密顿的球张量形式为基础,借助不可约张量理论,建立了计算多电子原子能量的相对论修正的一种解析理论形式,导出了多电子原子相对论修正项(包括相对论质量修正项、单体和双体达尔文修正项、自旋-自旋接触相互作用项和轨道-轨道相互作用项)在斯莱特表象中的矩阵元的解析表达式,完成了所有角向积分和自旋求和计算.利用所建立的理论,对类锂体系(1s)2(2p)2P态能量的相对论修正进行了具体计算. 相似文献
17.
E. Holland-Moritz 《Zeitschrift für Physik B Condensed Matter》1992,89(3):285-288
Inelastic scattering experiments were performed with thermal neutrons on Pr6O11 in order to investigate the magnetic excitation spectrum. Two different kinds of behaviour were found in the response: first a rather broad feature, which is purely quasielastic for high temperatures and becomes inelastic at low temperatures, and second very narrow inelastic excitations which are interpreted as crystal-field (CF) splitting. This different behaviour can be explained as a consequence of vacancies on the oxygen sites in the cubic CaF2 structure. This yields Pr sites with at least two different coordination numbers. On sites with the full cubic coordination number of eight the Pr ion is intermediate valent (broad feature) and on sites with a lower coordination number the Pr ion is trivalent (CF excitations). 相似文献
18.
Judd-Ofelt光谱分析理论 总被引:10,自引:1,他引:9
对Judd-Ofelt光谱分析理论(J-O理论)的发展和应用进行了综述。J-O理论用于分析固体中的稀土离子的吸收、发射光谱,可计算它们的跃迁几率、谱线强度、能级寿命、发射截面等。在J-O理论中考虑J混合后,可使理论和实验结果更好地吻合。采用改进的J-O理论,可较好地对Pr3+光谱进行分析。由TPM方法,可用非极化光透射光谱来分析各向异性晶体的光谱性质,并且对样品的加工无特殊定向要求。对于重叠吸收光谱带,拟合J-O参数时可以近似为一个带进行处理。利用荧光寿命也可对J-O参数进行计算。 相似文献
19.
作为一种稳定性好、抗辐射能力强、原材料丰富的宽禁带半导体, ZnO在光催化的研究领域中成为热点材料,但是其仅能吸收可见光中的紫光,因此如何扩大ZnO对可见光的响应范围是一个值得研究的问题.掺杂改性是解决这个问题的常用方法.基于以上考量,本文应用第一性原理计算方法研究了N与Pr掺杂对ZnO的电子结构和光学性质的影响.研究结果表明:共掺体系比单掺体系更容易形成,且共掺体系的稳定性随Pr浓度的增加先增强后变弱;同一体系的最短Zn—O键与最长Zn—O键的布居数比例随杂质浓度的增大先增大后减小,说明杂质的掺入对体系的晶格畸变有很大的影响,有利于光生空穴-电子对的分离,从而提高材料的光催化活性. N 2p态与Pr 4f态发生杂化对晶体的完整性产生了破坏,在杂质原子周围形成晶场,造成能级劈裂,带隙减小;介电函数虚部的主峰位均向低能区域移动,吸收光谱中各掺杂体系发生红移,各共掺体系随着杂质原子Pr浓度的增加,在可见光区的响应范围依次扩大,吸收能力也依次增加,说明N与Pr的共掺杂对提高ZnO的光催化性是有利的. 相似文献
20.
The instability of metal point contacts under voltage bias is calculated based on scattering theory. When the bias is applied, the transport channels will be closed and the chemical bonds will be broken, which modify the cohesive force of the point contact. If the cross section of the contact is narrow enough, an applied bias will break the point contact, which is in agreement with the experimental results. The effect of bias-dependent force can play an important role in the stability of a nanosize device. 相似文献