全文获取类型
收费全文 | 7105篇 |
免费 | 164篇 |
国内免费 | 15篇 |
专业分类
化学 | 3991篇 |
晶体学 | 167篇 |
力学 | 300篇 |
数学 | 861篇 |
物理学 | 1965篇 |
出版年
2023年 | 46篇 |
2022年 | 108篇 |
2021年 | 120篇 |
2020年 | 120篇 |
2019年 | 166篇 |
2018年 | 116篇 |
2017年 | 137篇 |
2016年 | 194篇 |
2015年 | 148篇 |
2014年 | 192篇 |
2013年 | 510篇 |
2012年 | 366篇 |
2011年 | 420篇 |
2010年 | 251篇 |
2009年 | 220篇 |
2008年 | 304篇 |
2007年 | 322篇 |
2006年 | 261篇 |
2005年 | 261篇 |
2004年 | 241篇 |
2003年 | 147篇 |
2002年 | 129篇 |
2001年 | 90篇 |
2000年 | 67篇 |
1999年 | 78篇 |
1998年 | 61篇 |
1996年 | 66篇 |
1995年 | 66篇 |
1994年 | 65篇 |
1993年 | 85篇 |
1992年 | 78篇 |
1991年 | 56篇 |
1990年 | 70篇 |
1989年 | 69篇 |
1988年 | 71篇 |
1987年 | 65篇 |
1986年 | 43篇 |
1985年 | 85篇 |
1984年 | 110篇 |
1983年 | 73篇 |
1982年 | 81篇 |
1981年 | 83篇 |
1980年 | 103篇 |
1979年 | 89篇 |
1978年 | 89篇 |
1977年 | 94篇 |
1976年 | 77篇 |
1975年 | 58篇 |
1974年 | 59篇 |
1973年 | 67篇 |
排序方式: 共有7284条查询结果,搜索用时 15 毫秒
41.
Spherically symmetric static cosmological models filled with black-body radiation are considered. The models are isotropic
about a central observer but inhomogeneous. It is suggested that the energy density of the free gravitational field, which
is coupled to the isotropic radiation energy density, might play an important role in generating sufficient field (vacuum)
energy (when converted into thermal energy) and initiate processes like inflation. On the central world line the energy density
of the free gravitational field vanishes whereas the proper pressure and density of the isotropic black-body radiation are
constants. Further, it is shown that the cosmological constant is no more arbitrary but given in terms of the central pressure
and density. Also, at its maximum value the energy density of the free gravitational field is proved to be equal to one third
of the combined value of radiation pressure and density. 相似文献
42.
Jai Kumar Singhal 《Pramana》2004,62(5):1029-1040
We examine the effects of mixing induced light heavy charged lepton neutral currents on the partial wave amplitude for the
process l+l− →ZZ (withl = e,μ or τ). By imposing the constraints that the amplitude should not exceed the perturbative unitarity limit at high energy (√s
= Λ), we obtain bounds on light heavy charged lepton mixing parameter sin2(2θ
L
a
) where θ
L
a
is the mixing angle of the ordinary charged lepton with its exotic partner. For Λ = 1 TeV, no bound is obtained on sin2 (2θ
L
a
) form
E < 0.69 TeV. However, sin2 (2θ
L
a
) ≤ 1.52×10−5 form
E = 5 TeV, sin2 (2θ
L
a
) ≤ 2.41 ×10−7 form
E = 10 TeV. Similarity for Λ = ∞ no bound is obtained on sin2 (2θ
L
a
) for mE < 1.97 TeV and sin2 (2θ
L
a
) ≤ 0.15 form
E = 5 TeV and sin2 (2θ
L
a
) ≤ 3.88×10-2 form
E = 10 TeV. 相似文献
43.
Mixed thin films of (CdO)1−x(PbO)x and (CdS)1−x(PbS)x (x=0.25) were prepared on glass substrates by spray pyrolysis technique for various substrate temperatures 300, 320 and 340 °C. Structural and optical properties were studied. XRD studies reveal the formation of mixed films. The substrate temperature of 340 °C seems to be critical for the formation of CdO-PbO mixed films. It is observed that (CdS)1−x(PbS)x mixed films were formed at all the three substrate temperatures. The direct band gap value of (CdO)1−x(PbO)x and (CdS)1−x(PbS)x mixed films is about 2.6 and 2.37 eV, respectively. 相似文献
44.
45.
Transverse flow transversely excited (TFTE) CO2 lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow
and discharge volume. It was observed in a TFTE CW CO2 laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge
volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant.
The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity.
This necessitated revision of the scaling laws for designing this type of high power CO2 laser. Experimental results of laser performance for different electrode separations are discussed and the modifications
in the scaling laws are presented. 相似文献
46.
Chae Kyu Kim Eun Seok Song Hyung Joo Kim Chiyoung Park Young Chul Kim Jai Kyeong Kim Jae Woong Yu Chulhee Kim 《Journal of polymer science. Part A, Polymer chemistry》2006,44(1):254-263
We have synthesized conjugated dendrimer with triazine peripheries, and their luminescence properties were investigated. The dendrimers consist of dendritic triazine wedges for electron transport, distyrylbenzene core as an emitting moiety, and t‐butyl peripheral groups for good processing properties. The dendrimers have LUMO values of about ?2.7 eV possibly because of the triazine moiety with high electron affinity. Photoluminescence study indicates that energy transfer occurs from the triazine wedges to the stilbene bridge, and finally to the core chromophore units due to a cascade decrease of bandgap from the peripheral wedge to core moiety. Therefore, the emission wavelength was determined by the structure of the core unit. The energy transfer efficiency of distyrylbenzene‐cored dendrimers was about 75 and 55% for Trz‐1GD‐DSB and Trz‐2GD‐DSB, respectively. A preliminary electroluminescence property also was investigated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 254–263, 2006 相似文献
47.
Kishore K Tiwari Girdhar L Mundhara Manish K Rai Vinay K Gupta 《Analytical sciences》2006,22(2):259-262
The results of a study and application of leucocrystal violet for the determination of antimony in parts per million levels is described here. The proposed method is based on the reaction of antimony(III) with acidified potassium iodate to liberate iodine. The liberated iodine selectively oxidizes leucocrystal violet to crystal violet dye. The formed dye shows maximum absorbance at 590 nm. The color system obeys Beer's law in the concentration range from 0.4 - 3.6 microg antimony per 25 ml of final solution. The molar absorptivity and Sandell's sensitivity were found to be 7.32 x 10(5) l mol(-1) cm(-1) and 0.0016 microg cm(-2), respectively. All variables were studied in order to optimize the reaction. The proposed method is satisfactorily applicable for the analysis of antimony in various environmental and biological samples. The method is simple, highly sensitive, accurate and reliable. 相似文献
48.
A Jayanth Kumar N M Gowri R Venkateswara Raju G Nirmala B S Bellubbi T Radha Krishna 《Pramana》2006,67(2):383-387
Over the last two decades, the fiber optic technology has passed through many analytical stages. Some commercially available
fiber optic sensors, though in a small way, are being used for automation in mechanical and industrial environments. They
are also used for instrumentation and controls.
In the present work, an intensity-modulated intrinsic fiber optic sugar sensor is presented. This type of sensor, with slight
modification, can be used for on-line determination of the concentration of sugar content in sugarcane juice in sugar industry.
In the present set-up, a plastic fiber made of polymethylmethacrylate is used. A portion of the cladding (1 cm, 2 cm, 3 cm)
at the mid-point along the length of the fiber is removed. This portion is immersed in sugar solution of known concentration
and refractive index. At one end of the fiber an 850 nm source is used and at the other end a power meter is connected. By
varying the concentration of sugar solution, the output power is noted. These studies are made due to the change in refractive
index of the fluid. The device was found to be very sensitive which is free from EMI and shock hazards, stable and repeatable
and they can be remotely interfaced with a computer to give on-line measurements and thus become useful for application in
sugar industries. 相似文献
49.
Josephson effects have been observed in bulk samples of Y-Ba-Cu-O. The magnitude of the zero-voltage current is found to change
systematically with externally applied small magnetic fields of a few mG. It is also found to vary when samples are irradiated
with microwaves. These observations suggest the presence of inter-grain Josephson junctions. 相似文献
50.