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排序方式: 共有609条查询结果,搜索用时 24 毫秒
41.
Gallardo J. Galliano P. Moreno R. Durán A. 《Journal of Sol-Gel Science and Technology》2000,19(1-3):107-111
Sol-gel materials have been proposed in last years for clinical applications. In this work, bioactive sol-gel coatings were prepared from suspensions of up to 25% wt. of bioactive glass (CaO·SiO2·P2O5) particles in a hybrid sol obtained from TEOS and MTES. Thick dip-coatings showed in vitro bioactivity after a few days, but glass-particles dissolution promotes a slight reduction in the corrosion resistance. In order to overpass this problem, a two-stage sol-gel coating system that includes a first SiO2 hybrid film, acting as barrier against corrosion, and an external bioactive layer from glass particle suspension is proposed. The obtained samples revealed significant improvement in their electrochemical behaviour, and showed in vitro bioactivity. 相似文献
42.
S.M. Al-Marzoug 《Optics Communications》2006,265(1):234-240
Designing multilayer optical coatings is a difficult optimization problem because of the huge size of the search space. In the present paper, the Luus-Jaakola (LJ) optimization procedure, a new optimization algorithm, is employed to model multilayer optical coatings in the X-ray domain. With this algorithm it is not necessary to specify initially the number of layers present in a design. Only an upper limit needs to be defined. The algorithm has been used to maximize the reflectivity over a range of incident angles at a fixed wavelength, and over a wavelength range at a fixed incident angle. The results show that the LJ algorithm can be effectively applied to the design of multilayer optical coatings resulting in fewer layers than obtained using alternative optimization methods. 相似文献
43.
The dc field rheological properties and frequency dependent dielectric properties of a set of electrorheological (ER) fluids composed of oxidized polyacrylonitrile or aluminosilicate materials dispersed in silicone oil were examined in this paper. Our experimental investigations show that there is a complicated relationship between the dielectric properties of dispersed particles and the ER effect. The dielectric loss of dispersed particles, which has not attracted much attention in previous work, was found to play a considerable role in ER response. The large dielectric loss tangent, experimentally around 0.10 at 1000 Hz, is found to be needed for a strong ER effect. A good ER solid material should first have large dielectric loss, and then the higher the dielectric constant, the stronger the ER effect. The large dielectric loss would facilitate the turning of dispersed particles, and the high dielectric constant would maintain the fibrillation structure stable and strong. Two processes, the particle turning process and the particle polarization process, are thought to be involved in ER activity. Our findings, in connection with the Wagner model, can better explain why the strongest ER effect occurs at particle conductivity of 10−7S/m; why the shear stress of some ER fluids decreases with frequency while with others the shear stress increases with frequency; and why trace water can enhance the ER effect considerably, which would help in understanding the mechanism of the ER effect. Too large a dielectric loss is thought to be unfavorable for the ER effect, and its suitable range is worth further study. The results also present a method of designing high performance ER fluids, which would significantly promote development of electrorheology and its application in industrial areas. 相似文献
44.
Chengming Wang Toshiyuki Nagahashi Koichi Aoki Jingyuan Chen 《Journal of Electroanalytical Chemistry》2002,530(1-2)
The conductivity, κ, in a suspension of polystyrene sulfonic latex without supporting electrolyte showed a linear dependence on the volume fraction, vf, of the latex for vf<0.03 with a finite intercept. In contrast, this deviated upward from the linear line for vf>0.03. These variations were qualitatively consistent with the dependence of the voltammetric reduction current of H+ on vf without supporting electrolyte. The current values were only a few percent of the theoretical diffusion-controlled current that could be observed in the suspension with supporting electrolyte. This fact indicates the electrostatic immobilization of the hydrogen ions by sulfonic latex particles. A plot of the current against κ at common values of vf showed that the current for vf>0.07 was smaller than the value predicted from the conductivity. This can be explained in terms of a combination of the increase in electrostatically unbounded H+ estimated by conductance measurements and electric migration in which the electrochemical depletion of [H+] also causes the depletion of the latex. 相似文献
45.
Experimental data were acquired for: (1) the ignition temperatures of nitrogen–diluted ethylene and propylene by counterflowing heated air for various strain rates and system pressures up to 7 atm; (2) the laminar flame speeds of mixtures of air with acetylene, ethylene, ethane, propylene, and propane, deduced from an outwardly propagating spherical flame in a constant-pressure chamber, for extensive ranges of lean-to-rich equivalence ratio and system pressure up to 5 atm. These data, respectively, relevant for low- to intermediate-temperature ignition chemistry and high-temperature flame chemistry, were subsequently compared with calculated results using a literature C1–C3 mechanism and an ethylene mechanism. Noticeable differences were observed in the comparison for both mechanisms, and sensitivity analyses were conducted to identify the reactions of importance. 相似文献
46.
The dynamic flow behavior of polyamide-6 (PA-6) and a nanocomposite (PNC) based on it was studied. The latter resin contained
2 wt% of organoclay. The two materials were blended in proportions of 0, 25, 50, 75, and 100 wt% PNC. The dynamic shear rheological
properties of well-dried specimens were measured under N2 at T=240 °C, frequency ω=0.1–100 rad/s, and strains γ=10 and 40%. At constant T, γ, and ω the time sweeps resulted in significant
increases of the shear moduli. The γ and ω scans showed a complex rheological behavior of all clay-containing specimens. At
γ=10% the linear viscoelasticity was observed for all compositions only at ω>1 rad/s, while at γ=40% only for 0 and 25 wt%
of PNC. However, the effect was moderate, namely decreasing G′ and G′′ (at ω=6.28 rad/s; γ=50%) by 15 and 7.5%, respectively.
For compositions containing >25 wt% PNC two types of non-linearity were detected. At ω≤ωc=1.4 ± 0.2 rad/s yield stress provided evidence of a 3-D structure. At ω > ωc, G′ and G′′ were sensitive to shear history – the effect was reversible. From the frequency scans at ω > ωc the zero-shear relative viscosity vs concentration plot was constructed. The initial slope gave the intrinsic viscosity from
which the aspect ratio of organoclay particles, p=287 ± 9 was calculated, in agreement with the value calculated from the
reduced permeability data, p=286.
Received: 24 May 2001 Accepted: 27 August 2001 相似文献
47.
Dr. Erik A. Toorman 《Rheologica Acta》1994,33(3):193-202
An analytical solution is presented for the calculation of the flow field in a concentric cylinder viscometer of non-ideal Bingham-fluids, described by the Worrall-Tuliani rheological model. The obtained shear rate distribution is a function of the a priori unknown rheological parameters. It is shown that by applying an iterative procedure experimental data can be processed in order to obtain the proper shear rate correction and the four rheological parameters of the Worrall-Tuliani model as well as the yield surface radius. A comparison with Krieger's correction method is made. Rheometrical data for dense cohesive sediment suspensions have been reviewed in the light of this new method. For these suspensions velocity profiles over the gap are computed and the shear layer thicknesses were found to be comparable to visual observations. It can be concluded that at low rotation speeds the actually sheared layer is too narrow to fullfill the gap width requirement for granular suspensions and slip appears to be unavoidable, even when the material is sheared within itself. The only way to obtain meaningfull measurements in a concentric cylinder viscometer at low shear rates seems to be by increasing the radii of the viscometer. Some dimensioning criteria are presented.Notation
A, B
Integration constants
-
C
Dimensionless rotation speed = µ/y
-
c
= 2µ
-
d
= 0
2–2cy
-
f()
= (–0)2+2c(–y)
-
r
Radius
-
r
b
Bob radius
-
r
c
Cup radius
-
r
y
Yield radius
-
r
0
Stationary surface radius
-
r
Rotating Stationary radius
-
Y
0
Shear rate parameter = /µ
Greek letters
Shear rate
-
= (r
y
/r
b
)2– 1
- µ
Bingham viscosity
- µ0
Initial differential viscosity
- µ
µ0-µ
-
Rotation speed
-
Angular velocity
-
Shear stress
-
b
Bob shear stress
-
B
Bingham stress
-
y
(True) yield stress
- 0
Stress parameter =
B
+µ
Y
0
-
B
-
y
相似文献
48.
The steady laminar flow and thermal characteristics of a continuously moving vertical sheet of extruded material are studied close to and far downstream from the extrusion slot. The velocity and temperature variations, obtained by a finite volume method, are used to map out the entire forced, mixed and natural convection regimes. The effects of the Prandtl number (Pr) and the buoyancy force parameter (B) on the friction and heat transfer coefficients are investigated. Comparisons with experimental measurements and solutions by others in the pure forced and pure natural convection regions are made. In the mixed convection region, the results are compared with available finite-difference solutions of the boundary layer equations showing excellent agreement. The region close to the extrusion slot is characterized as a non-similar forced-convection dominated region in which NuxRex−1/2 drops sharply with increasing Richardson number (Rix). This is followed by a self-similar forced-convection dominated region in which NuxRex−1/2 levels off with increasing Rix until the buoyancy effect sets in. The existence and extent of the latter region depend upon the value of B. A non-similar mixed convection region where increasing buoyancy effect enhances the heat transfer rate follows. Finally, this region is followed downstream by a self-similar natural-convection dominated region in which NuxRex−1/2 approaches the pure natural convection asymptote at large Rix. Critical values of Rix to distinguish the various convection regimes are determined for different Pr and B. 相似文献
49.
对槽道纤维悬浮流进行染色线流动显示和流场PIV实验测量,实验中选用的是直径为20μm、长径比为20~100的尼龙纤维。PIV2100处理器被用来加工处理采集的实验数据。槽道长度1.5m,横截面为矩形,尺寸为105×19mm。实验结果说明在Reynolds数相同的情况下,纤维悬浮流比对应的牛顿流更不容易失稳,悬浮流中的纤维起着抑制流场失稳的作用,而且随着纤维体积分数和长径比的增大,抑制失稳的程度也提高。扰动衰减率的最小值随纤维体积分数和长径比的增加而增大,这一效果在大Re数时更明显。 相似文献
50.
An experimental investigation of heat transfer to pulsating pipe air flow with different amplitudes 总被引:1,自引:0,他引:1
Heat transfer characteristics to both laminar and turbulent pulsating pipe flows under different conditions of Reynolds number,
pulsation frequency, pulsator location and tube diameter were experimentally investigated. The tube wall of uniform heat flux
condition was considered for both cases. Reynolds number varied from 750 to 12,320 while the frequency of pulsation ranged
from 1 to 10 Hz. With locating the pulsator upstream of the inlet of the test section tube, results showed an increase in
heat transfer rate due to pulsation by as much as 30% with flow Reynolds number of 1,643 and pulsation frequency of 1 Hz,
depending on the upstream location of the pulsator valve. Closer the valve to the tested section inlet, the better improvement
in the heat transfer coefficient is achieved. Upon comparing the heat transfer results of the upstream and the downstream
pulsation, at Reynolds number of 1,366 and 1,643, low values of the relative mean Nusselt number were obtained with the upstream
pulsation. Comparing the heat transfer results of the two studied test sections tubes for Reynolds number range from 8,000
to 12,000 and pulsation frequency range from 1.0 to 10 Hz showed that more improvement in heat transfer rate was observed
with a larger tube diameter. For Reynolds number ranging from 8,000 to 12,000 and pulsation frequency of 10 Hz, an improvement
in the relative mean Nusselt number of about 50% was obtained at Reynolds number of 8,000 for the large test section diameter
of 50 mm. While, for the small test section diameter of 15 mm, at same conditions of Reynolds number and frequency, a reduction
in the relative mean Nusselt number of up to 10% was obtained. 相似文献