全文获取类型
收费全文 | 446篇 |
免费 | 7篇 |
国内免费 | 2篇 |
专业分类
化学 | 265篇 |
晶体学 | 32篇 |
力学 | 22篇 |
数学 | 35篇 |
物理学 | 101篇 |
出版年
2022年 | 3篇 |
2021年 | 10篇 |
2020年 | 7篇 |
2019年 | 15篇 |
2018年 | 7篇 |
2017年 | 8篇 |
2016年 | 16篇 |
2015年 | 8篇 |
2014年 | 17篇 |
2013年 | 36篇 |
2012年 | 23篇 |
2011年 | 36篇 |
2010年 | 23篇 |
2009年 | 19篇 |
2008年 | 19篇 |
2007年 | 23篇 |
2006年 | 18篇 |
2005年 | 15篇 |
2004年 | 6篇 |
2003年 | 8篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1975年 | 3篇 |
1973年 | 2篇 |
1967年 | 2篇 |
1959年 | 3篇 |
1957年 | 2篇 |
1956年 | 2篇 |
1955年 | 2篇 |
1948年 | 2篇 |
1942年 | 2篇 |
1940年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有455条查询结果,搜索用时 15 毫秒
101.
Using the spectrally resolved white light interferometry we present our experimental results on the measurement of the optical
constants of thin polymer films coated on a transparent substrate. As an extension to our previous work (J. Opt. Soc. Am.
B12, 1559 (1995)) on thick glass plates, we have shown here that this technique can be effectively applied to very thin polymer
films also. We have improved the accuracy of our results by using the Sellmeier dispersion formula for fitting the data. From
the width and position of the zero-order fringe and the frequency of modulations in the white light spectrum, the refractive
indexn(λ) and thicknesst of the thin polymer films are calculated. To study the accuracies involved in the technique, PVA, PMMA and PS films of varied
thicknesses are coated on glass plates and the measured values are compared with ellipsometer studies. 相似文献
102.
In vivo localized proton magnetic resonance spectroscopy (MRS) studies of brain were performed on eighteen normal subjects using the stimulated echo (STE) sequence. The absolute concentrations and proton relaxation times of N-acetyl aspartate (NAA), total creatine (Cr) and choline (Cho) were estimated. The MRS data was quantitatively analyzed for repeatability and intersubject variability. Quantitative analysis indicates excellent spectral repeatability. Significant intersubject variations in [NAA] and [Cr] have been observed while the intersubject variability in [Cho] has been found to be fairly small. Significant intensity distortions have been observed for mixing times longer than 50 msec. 相似文献
103.
This paper reports the investigation of mean and turbulent flow characteristics of a two-dimensional plane diffuser. Both experimental and theoretical details are considered. The experimental investigation consists of the measurement of mean velocity profiles, wall static pressure and turbulence stresses. Theoretical study involves the prediction of downstream velocity profiles and the distribution of turbulence kinetic energy using a well tested finite difference procedure. Two models, viz., Prandtl's mixing length hypothesis and k- model of turbulence, have been used and compared. The nondimensional static pressure distribution, the longitudinal pressure gradient, the pressure recovery coefficient, percentage recovery of static pressure, the variation of U
max/U
bar along the length of the diffuser and the blockage factor have been valuated from the predicted results and compared with the experimental data. Further, the predicted and the measured value of kinetic energy of turbulence have also been compared. It is seen that for the prediction of mean flow characteristics and to evaluate the performance of the diffuser, a simple turbulence model like Prandtl's mixing length hypothesis is quite adequate.List of symbols
C
1
, C
2
,C
turbulence model constants
-
F
x
body force
-
k
kinetic energy of turbulence
-
l
m
mixing length
-
L
length of the diffuser
-
u, v, w
rms value of the fluctuating velocity
-
u, v, w
turbulent component of the velocity
-
mean velocity in the x direction
-
A
average velocity at inlet
-
U
bar
average velocity in any cross section
-
U
max
maximum velocity in any cross section
-
V
mean velocity in the y direction
-
W
local width of the diffuser at any cross section
-
x, y
coordinates
-
dissipation rate of turbulence
-
m
eddy diffusivity
-
Von Karman constant
-
mixing length constant
-
l
laminar viscosity
-
eff
effective viscosity
-
v
kinematic viscosity
-
density
-
k
effective Schmidt number for k
-
effective Schmidt number for
-
stream function
-
non dimensional stream function 相似文献
104.
Mathew Mendalin Narayana Badiadka Sreekumar Naracham Veedu Vipin Kuttamath 《Mikrochimica acta》2004,144(4):291-295
A rapid and sensitive spectrophotometric method is described for the determination of trace amounts of cerium using Variamine Blue (VB) as a chromogenic reagent. The proposed method is based on the reaction of cerium with potassium iodide in acidic medium to liberate iodine, which oxidizes Variamine Blue to give a violet colored species with an absorption maximum at 560nm. Beers law was obeyed in the range 2–10µg mL–1 of cerium. The molar absorptivity and Sandells sensitivity are found to be 1.65×104L mol–1cm–1 and 8.48×10–3µgcm–2, respectively. The proposed method has been successfully applied to the determination of cerium in alloy and synthetic mixtures. 相似文献
105.
In order to understand the differences between the nature of the electron- and the hole-doped compositions of rare earth manganates, properties of single crystals of Nd1−xCaxMnO3 with x=0.6 and 0.4 have been investigated. In the electrical resistivity measurements, the electron-doped (x=0.6) composition shows no effect of magnetic field unlike the hole-doped (x=0.4) composition, but both exhibit similar current-induced changes. Brillouin scattering studies show that the x=0.6 and 0.4 compositions show softening of the modes on cooling the samples towards charge-ordering transitions. The strength of coupling between the eg orbitals and the elastic strain is considerably larger in the hole-doped composition containing a larger population of Mn3+ ions. 相似文献
106.
Rapid, quantitative SERS analysis of nicotine at ppm/ppb levels has been carried out using stable and inexpensive polymer-encapsulated Ag nanoparticles (gel-colls). The strongest nicotine band (1030 cm(-1)) was measured against d(5)-pyridine internal standard (974 cm(-1)) which was introduced during preparation of the stock gel-colls. Calibration plots of I(nic)/I(pyr) against the concentration of nicotine were non-linear but plotting I(nic)/I(pyr) against [nicotine](x)(x = 0.6-0.75, depending on the exact experimental conditions) gave linear calibrations over the range (0.1-10 ppm) with R(2) typically ca. 0.998. The RMS prediction error was found to be 0.10 ppm when the gel-colls were used for quantitative determination of unknown nicotine samples in 1-5 ppm level. The main advantages of the method are that the gel-colls constitute a highly stable and reproducible SERS medium that allows high throughput (50 sample h(-1)) measurements. 相似文献
107.
108.
Reaction of terminal alkynes with ethylmagneslum bromide In THF followed by the addition of lodine In THF gives 1-lodoalkynes In excellent yields (90–96%). 相似文献
109.
N-Substituted pyrroles were synthesized under neat conditions by the reaction of aromatic amines with 2, 5-dimethoxytetrahydrofuran in excellent yields. 相似文献
110.
Dielectric permittivity studies on the nematic liquid crystal p-(ethoxy phenyl azophenyl) butyrate were carried out at 1 MHz. The effect of parallel and perpendicular magnetic fields on the alignment of the molecules was studied. The threshold voltage for domain formation in the presence of a magnetic field is estimated. 相似文献