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1.
We establish a new model based on fractal theory and cubic spline interpolation to study the effective thermal conductivity of isotropic porous silica low-k materials. A 3D fractal model is introduced to describe the structure of the silica xerogel and silica hybrid materials (such as methylsilsesquioxane, MSQ). Combined with fractal structure, a more suitable medium approximation is developed to study the isotropic porous silica xerogel and MSQ materials. Cubic spline interpolation for fitting discrete predictions from the fractal model is used to obtain the continuous function of the effective thermal conductivity versus porosity. Compared with other common models, the effective thermal conductivity predicted by our model presents better agreement with the experimental data for all porosity. These results indicate that the proposed model is valid. 相似文献
2.
Transient thermal gratings at surfaces for thermal characterization of bulk materials and thin films
O. W. Käding H. Skurk A. A. Maznev E. Matthias 《Applied Physics A: Materials Science & Processing》1995,61(3):253-261
Transient Thermal Gratings (TTGs) at surfaces of absorbing materials have been utilized for investigating heat diffusion in bulk materials and thin films. In this report, we describe the theoretical background of the technique and present experimental data. TTGs were excited in the surface plane by interference of two pulsed laser beams and monitored by a cw probe beam, either via temperature dependence of the reflectivity or by deflection from the displacement pattern. A theoretical model describing the thermal and thermoelastic surface response was developed, both for a homogeneous material and a multilayer structure. The potential of the technique will be demonstrated by experimental results on (i) thermal diffusivities of bulk materials, (ii) anisotropic lateral heat transport, and (iii) thermal diffusivities of metal and diamond films. Furthermore, we will show that TTGs allow thermal depth profiling of inhomogeneous materials whenever there is a vertical gradient in thermal conductivity. 相似文献
3.
Based on fractal theory, two types of random Sierpinski carpets (RSCs) and their periodic structures are generated to model the structures of natural porous media, and the heat conduction in these structures is simulated by the finite volume method. The calculated results indicate that in a certain range of length scales, the size and spatial arrangement of pores have significant influence on the effective thermal conductivity, and the heat conduction presents the aeolotropic characteristic. Above the length scale, however, the influence of size and spatial arrangement of pores on the effective thermal conductivity reduces gradually with the increasing characteristic size of porous media, the aeolotropic characteristic is weakened gradually. It is concluded that the periodicity in structures of porous media is not equal to the periodicity in heat conduction. 相似文献
4.
M. Kothandapani 《Physics letters. A》2008,372(25):4586-4591
The effect of elasticity of the flexible walls on the MHD peristaltic flow of a Newtonian fluid in a two-dimensional porous channel with heat transfer has been studied under the assumptions of long-wavelength and low-Reynolds number. The analytical solution has been obtained for the stream function, temperature and heat transfer coefficient. The effect of various emerging parameters on the flow characteristics are shown and discussed with the help of graphs. The numerical results show that the trapped bolus increases in size and more trapped bolus appears with increasing permeability parameter, elastic tension and mass characterizing parameters but decreases for large values of Hartmann number. 相似文献
5.
A fractal model is presented based on the thermal-electrical analogy technique and statistical self-similarity of fractal saturated porous media. A dimensionless effective thermal conductivity of saturated fractal porous media is studied by the relationship between the dimensionless effective thermal conductivity and the geometrical parameters of porous media with no empirical constant. Through this study, it is shown that the dimensionless effective thermal conductivity decreases with the increase of porosity (?) and pore area fractal dimension (Df) when ks/kg>1. The opposite trends is observed when ks/kg<1. In addition, the dimensionless effective thermal conductivity decreases with increasing tortuous fractal dimension (Dt). The model predictions are compared with existing experimental data and the results show that they are in good agreement with existing experimental data. 相似文献
6.
G. W. Milton 《Applied Physics A: Materials Science & Processing》1981,26(2):125-130
The Hashin-Shtrikman and Walpole bounds for the transport properties and bulk modulus of multicomponent composite materials are shown to be attained in a wide range of cases. Thus in these cases the bounds are the best possible bounds that can be given in terms of the properties of the components and the volume fractions. For three-component materials new bounds are conjectured. The conjectured bounds are presumed to apply in the cases where the Hashin-Shtrikman and Walpole bounds are not attained. 相似文献
7.
K. Hyomi 《Journal of luminescence》2009,129(12):1715-1717
We present a micro-photoluminescence (m-PL) study of electron-spin injection under magnetic fields in self-assembled semiconductor quantum dots (QDs) of CdSe. A characteristic band line-up of the CdSe QDs coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of ZnCdMnSe through a ZnSe barrier layer enabled us to inject the electron spins from the DMS-QW into QDs. An experimental evidence of the electron-spin injection was provided by observations of circularly polarized m-PL spectra from individual QDs in this coupled QD structure. We find anti-correlation of PL intensity in between the DMS-QW (spin injector) and the individual QDs (spin receiver). 相似文献
8.
R. I. Altkorn J. C. Andreshak A. Gupta 《Applied Physics A: Materials Science & Processing》1989,48(3):273-276
A phosphor with temperature-dependent lifetime has been used to measure the temperature distribution produced by laser heating of a thin film surface. A gold thin film deposited on a quartz substrate is coated with 40 m film of the phosphor material. A cw argon ion laser (476 nm) beam is split into two beams, with the more intense beam focused to 15 m (1/e2 radius) to heat the film through the quartz substrate. The weaker probe beam is chopped and focused tightly using a microscope objective to excite the phosphor from the other side. The spatial variation in lifetime, and hence the temperature distribution, is obtained by scanning the probe beam over the heated region. The temperature distribution measured for different film thickness's is compared with calculations using a finite element model. The calculated temperatures at the gold surface near the laser beam are higher than the experimentally measured values, and agree only when the heat-sinking effect of the phosphor material is taken into account. The results suggest that a phosphor layer thinner than a micron will be required (for 15 m laser spot size) so as not to perturb the temperature of the gold layer. 相似文献
9.
The macroscopic properties of fractured porous media locally governed by a Laplace equation are determined by several methods. The first one consists in discretizing the porous medium and the fractures and in solving the Laplace equation in the discretized structure. The other methods consist in successive upscalings. The first upscaling replaces the porous medium by a continuum with a given transport property. The second upscaling replaces the fractures by surfaces with equivalent properties. The results of the various methods give very close results. They suggest a simple approximation which is successful when the properties of the fluid and of the continuous porous medium are not too different. 相似文献
10.
Fan Yang 《Solid State Communications》2007,141(10):555-558
Long wavelength optical lattice vibration and dielectric constants of the quaternary mixed crystal Zn1−x−yMgyBexSe are investigated based on the pseudo-unit-cell mode and Born-Huang procedure. It is found that this material shows a three-mode behavior and the oscillator strength of each mode is mainly controlled by only one component. The theoretical results also show that the linear interpretation method for dielectric constants is reliable. The vibrational frequencies and the oscillator strengths of the ternary mixed crystals BexZn1−xSe, BexMg1−xSe and MgxZn1−xSe are also calculated as special cases of the quaternary mixed crystal for comparing with experiments. The calculation shows agreement with the experimental results. 相似文献
11.
J.H. Müller U.D. Schwarz R. Wepf R. Wiesendanger 《Applied Physics A: Materials Science & Processing》2003,76(6):893-898
We present the design of a new scanning force microscope specially suited for the investigation of soft matter, particularly
biological, in an ultrahigh vacuum. The key point is that the samples are immobilized by shock freezing in order to maintain
their native structure before they are introduced into the vacuum system. The vacuum system itself consists of a transfer
chamber, which allows an exchange of the cold sample with cryo-electron microscopes, a preparation chamber including a stage
for in-situ freeze drying, freeze etching, or freeze fracturing, and the analysis chamber with the microscope. Sample cooling
is maintained in all chambers. The microscope is mounted on a commercially available vibration isolation system; a flow cryostat
cools the sample to the temperature of liquid nitrogen, while the tip is scanned. Besides measurements on test samples, which
demonstrate the imaging capabilities of the instrument, first results on T4-bacteriophages (viruses) are shown.
Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003
RID="*"
ID="*"Corresponding author. Fax: +49-40/42838-6188, E-mail: wiesendanger@physnet.uni-hamburg.de 相似文献
12.
J.L. Pichardo E. Marín J.J. Alvarado-Gil J.G. Mendoza-Alvarez A. Cruz-Orea I. Delgadillo G. Torres-Delgado H. Vargas 《Applied Physics A: Materials Science & Processing》1997,65(1):69-72
y Ga1-yAs alloys grown by liquid phase epitaxy on GaAs substrates, by means of the open photoacoustic cell detection technique and
the temperature-rise method under continuous light illumination. The values of the thermal conductivity, diffusivity and specific
heat were obtained in the 0<y<0.5 region, where the AlyGa1-yAs band gap is mainly direct. The technique presented here is based upon an effective sample model which is shown to be suitable
for the determination of the thermal properties of two layer semiconductor specimens.
Received: 15 November 1996/Accepted: 5 March 1997 相似文献
13.
Petra Lommens Celso de Mello Donega Luc Piraux Stefan Mátéfi-Tempfli Zeger Hens 《Journal of luminescence》2006,118(2):245-250
We performed photoluminescence experiments on colloidal, Co2+-doped ZnO nanocrystals in order to study the electronic properties of Co2+ in a ZnO host. Room temperature measurements showed, next to the ZnO exciton and trap emission, an additional emission related to the Co2+ dopant. The spectral position and width of this emission does not depend on particle size or Co2+ concentration. At 8 K, a series of ZnO bulk phonon replicas appear on the Co2+-emission band. We conclude that Co2+ ions are strongly localized in the ZnO host, making the formation of a Co2+d-band unlikely. Magnetic measurements revealed a paramagnetic behaviour. 相似文献
14.
A. Bugayev H. Kalt J. Kuhl M. Rinker 《Applied Physics A: Materials Science & Processing》1991,53(1):75-80
Picosecond time-resolved spectroscopy of the edge luminescence band of CdS
x
Se1–x
quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday 相似文献
15.
The N-Ag codoped ZnO films are deposited on quartz glass substrates by ultrasonic spray pyrolysis technology. The results indicate that the p-type conductivity in ZnO films is greatly enhanced by the double acceptor codoping of N and Ag compared with that of Ag- and N-monodoped ZnO films, and the NAg codoped low-resistivity p-type ZnO films with the resistivity of 1.05 Ω·cm, relatively high carrier concentration of 5.43×10^17 cm^-3, and Hall mobility of 10.09 cm^2 V^-1s^-1 are obtained under optimized conditions. This achievement confirms that p- type ZnO with acceptable properties for optoelectronic applications could be realized by simultaneous codoping with two potential acceptors. 相似文献
16.
A. V. Scherbakov A. V. Akimov D. R. Yakovlev I. A. Merkulov M. Bayer A. Waag L. W. Molenkamp 《physica status solidi b》2006,243(4):934-938
The magnetization kinetics in (Zn,Mn)Se/(Zn,Be)Se quantum wells is studied on a ps‐time scale after pulsed laser excitation. The magnetization induced by an external magnetic field is reduced by up to 30% within about 100 ps due to spin and energy transfer from photocarriers to magnetic Mn ions. A model based on spin‐momentum coupling in the valence band is suggested for explaining this transfer. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
G. H. Li Z. L. Fang F. H. Su B. S. Ma K. Ding H. X. Han I. K. Sou W. K. Ge 《physica status solidi b》2003,235(2):401-406
The photoluminescence from ZnS1–xTex alloy with 0 < x < 0.3 was investigated under hydrostatic pressure up to 7 GPa. Two peaks were observed in the alloys with x < 0.01, which are related to excitons bound to isolated Te isoelectronic impurities (Te1 centers) and Te pairs (Te2 centers), respectively. Only the Te2 related emissions were observed in the alloys with 0.01 < x < 0.03. The emissions in the alloys with 0.03 < x < 0.3 are attributed to the excitons bound to the Ten (n ≥ 3) cluster centers. The pressure coefficient of the Te1 related peak is 89(4) meV/GPa, about 40% larger than that of the band gap of ZnS. On the other hand, the pressure coefficient of the Te2 related emissions is only 52(4) meV/GPa, about 15% smaller than that of the ZnS band gap. A simple Koster–Slater model has been used to explain the different pressure behavior of the Te1 and Te2 centers. The pressure coefficient of the Te3 centers is 62(2) meV/GPa. Then the pressure coefficients of the Ten centers decrease rapidly with further increasing Te composition. 相似文献
18.
Phosphorus-doped ZnO (ZnO:P) thin films are deposited on a c-plane sapphire in oxygen at 350℃, 450℃, 550℃ and 650℃, respectively, by pulsed laser deposition (PLD), then all the ZnO:P samples are annealed at 650℃ in oxygen with a pressure of 1 × 10^5 Pa. X-ray diffraction measurements indicate that the crystalline quality of the ZnO:P thin films is improved with the increasing substrate temperature from 350℃ to 550℃. With a further increase of the deposition temperature, the crystalline quality of the ZnO:P sample is degraded. The measurements of low-temperature photoluminescence spectra demonstrate that the samples deposited at the substrate temperatures of 350℃ and 450℃ show a strong acceptor-bound exciton (A^0X) emission. The electrical properties of ZnO:P films strongly depend on the deposition temperature. The ZnO:P samples deposited at 350℃ and 450℃ exhibit p-type conductivity. The p-type ZnO:P film deposited at 450℃ shows a resistivity of 1.846Ω·cm and a relatively high hole concentration of 5.100 × 10^17 cm^-3 at room temperature. 相似文献
19.
The optical properties of single ZnO nanowires with diameters <200 nm are analyzed. A comparison of the photoluminescence properties of the as‐grown ensemble and of an individual nanowire is given. The temperature dependence of the individual nanowire photoluminescence shows the same behavior as that of bulk material due to the wire diameter being very large compared to the exciton Bohr radius of ZnO. Furthermore, high excitation‐density measurements performed on an individual nanowire are presented in which a sharp line is observed resulting from resonator effects inside the wire. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
20.
Lucien Besombes Yoan Lger Julien Bernos Claire Le‐gall Herve Boukari Jean Philippe Poizat Henri Mariette 《physica status solidi b》2009,246(4):756-761
We analyzed the photoluminescence intermittency generated by a single paramagnetic spin localized in an individual semiconductor quantum dot (QD). The statistics of the photons emitted by the QD reflect the quantum fluctuations of the localized spin. We report photon correlation measurements that reveal unique signatures of these fluctuations. A phenomenological model is proposed to quantitatively describe these observations, allowing a measurement of the spin dynamics of an individual magnetic atom at zero magnetic field. The data demonstrate the existence of a spin relaxation channel arising from a spin‐exchange with individual free carriers surrounding the QD. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献