全文获取类型
收费全文 | 44870篇 |
免费 | 1095篇 |
国内免费 | 1481篇 |
专业分类
化学 | 24335篇 |
晶体学 | 572篇 |
力学 | 1327篇 |
综合类 | 21篇 |
数学 | 5841篇 |
物理学 | 12447篇 |
无线电 | 2903篇 |
出版年
2023年 | 145篇 |
2022年 | 503篇 |
2021年 | 921篇 |
2020年 | 773篇 |
2019年 | 978篇 |
2018年 | 1388篇 |
2017年 | 1483篇 |
2016年 | 2099篇 |
2015年 | 1115篇 |
2014年 | 1843篇 |
2013年 | 3198篇 |
2012年 | 3075篇 |
2011年 | 3238篇 |
2010年 | 2797篇 |
2009年 | 2986篇 |
2008年 | 2940篇 |
2007年 | 2848篇 |
2006年 | 2457篇 |
2005年 | 1940篇 |
2004年 | 1899篇 |
2003年 | 1277篇 |
2002年 | 1180篇 |
2001年 | 759篇 |
2000年 | 1041篇 |
1999年 | 692篇 |
1998年 | 767篇 |
1997年 | 707篇 |
1996年 | 324篇 |
1995年 | 209篇 |
1994年 | 155篇 |
1993年 | 155篇 |
1992年 | 138篇 |
1991年 | 100篇 |
1990年 | 107篇 |
1989年 | 90篇 |
1988年 | 80篇 |
1987年 | 78篇 |
1986年 | 76篇 |
1985年 | 96篇 |
1984年 | 100篇 |
1983年 | 81篇 |
1982年 | 68篇 |
1981年 | 71篇 |
1980年 | 65篇 |
1979年 | 42篇 |
1978年 | 33篇 |
1977年 | 43篇 |
1976年 | 31篇 |
1975年 | 30篇 |
1974年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
81.
Elena Kardash-Strochkova Yakov I. Tur’yan I. Kuselman Naphtali Brodsky 《Accreditation and quality assurance》2002,7(6):250-254
The results of two interlaboratory comparisons of acid number determinations in used motor oils are discussed. It is shown
that the comparability of the measurement results is not as good as that required by known standards for petroleum products.
The problem is motor oil contaminants which accumulated during use, and which are the source of a matrix effect in the acid
number determination. The standard methods’ drawbacks are analyzed and some improvements are proposed. Repeatability and accuracy
of the improved methods are evaluated.
Received: 11 December 2001 Accepted: 15 February 2002 相似文献
82.
We report an experimental observation of the influence of concentration of Er3+ ions in an Erbiumdoped silica fiber on the spectral characteristics of an erbium fiber laser. Our basic conclusion is that an increase of erbium concentration in the fiber results in the laser optical spectrum narrowing. We also discuss possible mechanisms responsible for this effect. 相似文献
83.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
84.
S. N. Solov’ev K. A. Minasyan A. A. Korunov 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2006,80(4):663-664
The enthalpies of reactions of La2CoO4(cr) and CoCl2(cr) with hydrochloric acid were measured with an isothermal-jacket calorimeter. The results obtained and the available literature data were used to calculate the standard enthalpy of formation of La2CoO4(cr) at 298.15 K, Δf H o = ?2179 ± 7 kJ/mol. 相似文献
85.
Discharges with liquid nonmetallic electrodes of much interest for applications are investigated. It is found that a dc discharge between two streams of tap water in air at atmospheric pressure is stable at a currentof 40≤I≤100 mA. The discharge exists in the diffuse (volume) form with a relatively low current density (∼0.2 A/cm2) and a high (above one kilovolt) voltage drop across the air gap (∼1 cm) between the water electrodes. The current density and voltage depend only slightly on the discharge current. Probe measurements show that three regions can be distinguished in the discharge: two electrode regions (1–2 mm in length) and a discharge column with a constant electric field of ≈0.8 kV/cm (i.e., E/N≈20 Td, because the gas in the discharge is heated up to 1500–2000 K). The average electric field strength near the electrodes is E≈2–3×103 V/cm (E/N≈60–80 Td). The charged particle density in the column is n ∼ 1012 cm−3. The probe measurements of n agree with the previous microwave absorption measurements. The water vapor concentration in the column is also estimated from probe measurements. 相似文献
86.
E. V. Kuz’min 《Physics of the Solid State》2002,44(6):1122-1129
The spin system of the Heisenberg model (s=1/2) on a square lattice with antiferromagnetic (AFM) exchange between nearest neighbors (in which there is no long-range magnetic order at any T≠0) is treated as a spatially homogeneous isotropic spin liquid. The double-time temperature Green’s function method is used in the framework of a second-step decoupling scheme. It is shown that, as T → 0, the spin liquid goes over (without any change in symmetry) to a singlet state with energy (per bond) ?0=?0.352 and the correlation length diverges as ξ ∝ T ?1 exp(T 0/T). The spatial spin correlators oscillate in sign with distance, as in the AFM state. The theory allows one to calculate the main characteristics of the system in all temperature ranges. 相似文献
87.
88.
E. N. Sobol’ O. I. Baum A. V. Bol’shunov V. I. Sipliviy N. Yu. Ignat’eva O. L. Zakharkina V. V. Lunin A. I. Omel’chenko V. A. Kamenskiy A. V. Myakov 《Laser Physics》2006,16(5):735-740
A new approach to alterations in eye refraction upon nondestructive laser action on the sclera and cornea is studied. It is demonstrated in in vivo experiments on rabbit eyes that sequential laser irradiation of the sclera and cornea yields a significant alteration in the eye refraction. The collagen structure of the sclera and cornea is studied after the nondestructive laser action with noninvasive polarization-sensitive optical coherence tomography. It is demonstrated that collagen fibers that provide for the cornea tension and applanation partially survive in the zone of the local denaturation of sclera. An irradiation mode that corresponds to an increase in the cornea’s plasticity and does not cause visible structural changes is chosen. The simplest theoretical model for alterations in the eye refraction upon the nonablative laser action on sclera is analyzed. The alteration in the cornea curvature upon stretching resulting from the local sclera coagulation and the corresponding decrease in its volume is calculated. The model makes it possible to approximately estimate the laser irradiation modes that provide the desired alterations in eye refraction. 相似文献
89.
We analyze the dynamics of an AdS5 braneworld with matter fields when gravity is allowed to deviate from the Einstein form on the brane. We consider exact five-dimensional warped solutions which are associated with conformal bulk fields of weight –4 and describe on the brane the following three dynamics: those of inhomogeneous dust, of generalized dark radiation, and of homogeneous polytropic dark energy. We show that, with modified gravity on the brane, the existence of such dynamical geometries requires the presence of non-conformal matter fields confined to the brane. 相似文献
90.