全文获取类型
收费全文 | 277098篇 |
免费 | 2276篇 |
国内免费 | 875篇 |
专业分类
化学 | 133424篇 |
晶体学 | 4393篇 |
力学 | 14535篇 |
综合类 | 11篇 |
数学 | 50327篇 |
物理学 | 77559篇 |
出版年
2020年 | 1724篇 |
2019年 | 1919篇 |
2018年 | 12371篇 |
2017年 | 12308篇 |
2016年 | 9410篇 |
2015年 | 2993篇 |
2014年 | 3816篇 |
2013年 | 9350篇 |
2012年 | 10092篇 |
2011年 | 18067篇 |
2010年 | 11102篇 |
2009年 | 11488篇 |
2008年 | 13464篇 |
2007年 | 15499篇 |
2006年 | 6883篇 |
2005年 | 7277篇 |
2004年 | 6954篇 |
2003年 | 6960篇 |
2002年 | 5811篇 |
2001年 | 6358篇 |
2000年 | 4886篇 |
1999年 | 3652篇 |
1998年 | 2916篇 |
1997年 | 2831篇 |
1996年 | 2778篇 |
1995年 | 2549篇 |
1994年 | 2402篇 |
1993年 | 2250篇 |
1992年 | 2875篇 |
1991年 | 2787篇 |
1990年 | 2720篇 |
1989年 | 2746篇 |
1988年 | 2770篇 |
1987年 | 2775篇 |
1986年 | 2648篇 |
1985年 | 3349篇 |
1984年 | 3352篇 |
1983年 | 2633篇 |
1982年 | 2752篇 |
1981年 | 2799篇 |
1980年 | 2578篇 |
1979年 | 2917篇 |
1978年 | 2892篇 |
1977年 | 2998篇 |
1976年 | 2832篇 |
1975年 | 2568篇 |
1974年 | 2519篇 |
1973年 | 2488篇 |
1972年 | 1718篇 |
1968年 | 1713篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
971.
An automated on-line sample-preparation method using a computer-controlled pretreatment system (Auto-Pret AES system) coupled with ICP-AES was developed. In this work, an iminodiacetate chelating resin, packed in a mini-column and installed in the system was employed for the collection/concentration of 13 trace metals, including such toxic metals as Be, Cd, Cr, Cu and Pb. The limits of detection of the proposed method for trace metals were in the range of 0.001 (Be) -0.18 (Pb) ng mL-1. The enrichment factors for metal ions were about 19 times, when 5 mL of samples were used. The sample throughput was 11 h-1. The accuracy and the precision of the method were evaluated using river-water reference materials, SLRS-4 from NRCC, JSAC 0301-1 and JSAC 0302 from the Japan Society for Analytical Chemistry. The proposed method can be favorably applied to the collection/concentration of trace metals in natural water samples. 相似文献
972.
J. L. Souza A. F. Santos L. Polese Marisa S. Crespi C. A. Ribeiro 《Journal of Thermal Analysis and Calorimetry》2007,87(3):673-677
Poly(3-hydroxybutyrate), PHB has been structurally
modified through reaction with maleic anhydride, MA. Transesterification reaction
was carried out fixing the PHB and MA and besides time and temperature the
concentration of the triethylamine (used as catalyst) was changed. Glass transition,
melting and crystallization temperature obtained from DSC curves and thermal
degradation temperatures obtained from TG traces were used to evaluate the
influence of the reaction conditions on the modification of PHB according
to factorial design. On the base of the results the optimum conditions are
to perform the PHB modification reaction with MA reaction at 110°C for
1 h with 5% v/v triethylamine. 相似文献
973.
Sergiy V. Rosokha Jian Jiang Lu Sergey M. Dibrov Jay K. Kochi 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(8):o464-o466
The title compound, C6H2N6O10·2C2H4Cl2, forms layered stacks of pentanitroaniline molecules, which possess twofold symmetry. The voids between these stacks are occupied by dichloroethane molecules, which reside near a 2/m symmetry element and display pseudo‐inversion symmetry. The C atoms in one of the two solvent molecules are threefold disordered. In the pentanitroaniline molecule, considerable distortion of the benzenoid ring, coupled with the short C—N(H2) bond and out‐of‐plane NO2 twistings, point to significant intramolecular `push–pull' charge transfer at the amino‐ and nitro‐substituted (ortho and para) positions, as theoretically quantified by natural bond orbital analysis of the π‐electron density. 相似文献
974.
T. Martinez M. Navarrete L. Cabrera F. Juárez A. Ramos K. Vazquez 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(3):569-572
Nine brands of tobacco cigarettes manufactured and distributed in the Mexican market were analyzed by γ-spectrometry to certify
their non-artificial radioactive contamination. Since natural occurring radioactive materials (NORM) 40K, 232Th, 235U, and 239U (and decay products from the latter three nuclides) are the main sources for human radiation exposure, the aim of this work
was to determine the activity of 40K and potassium concentration. Averages of 40K and potassium concentration were of 1.29±0.18 Bq·g−1, and 4.0±0.57%. The annual dose equivalents to the whole body from ingestion and inhalation of 26 Bq 40K were 0.23 μSv and 15.8 μSv, respectively. The corresponding 50 years committed dose equivalents was 0.23 μSv. The total
committed dose to the lungs due to inhalation of 40K in tobacco was 16 μSv. Potassium concentrations obtained in this work were in the same range of those obtained by INAA,
so showing that the used technique is acute, reproducible, and accessible to laboratories equipped with low background scintillation
detectors. 相似文献
975.
G. Halambalakis N. Rousseau O. Briot S. Ruffenach R.L. Aulombard P.R. Edwards K.P. ODonnell T. Wojtowicz P. Ruterana 《Superlattices and Microstructures》2004,36(4-6):721
We have studied the optical properties of Eu doped GaN thin films. We have grown high quality Eu doped GaN thin films by using Gas Source Molecular Beam Epitaxy (GSMBE), with 1.4% Eu concentration. The Full Width at Half Maximum (FWHM) of the X-ray diffraction in an omega scan was found to be 288 arcsecs. Low Eu concentration (0.08%) doped GaN thin films were grown, where Eu-related photoluminescence at 622 and 613 nm was detected using above band-gap excitation at 2 K. For high Eu concentration of 30% GaN:Eu crystal photoluminescence (PL) and cathodoluminescence (CL) spectra show strong and intense transitions at 622 and 664 nm, but also at 593 nm for CL spectra, with a similar transition observed from the low Eu concentration sample. 相似文献
976.
We study the metrical theory of fibred systems, in particular, in the case of continued fraction mixing systems. We get the
limit distribution of the largest value of a continued fraction mixing stationary stochastic process with infinite expectation
and some related results. These are analogous to J. Galambos, W. Philipp, and H. G. Diamond–J. D. Vaaler theorems for the
regular continued fractions. As an application, we see that these theorems hold for Jacobi-Perron algorithm.
Received September 30, 2001; in revised form January 8, 2002 相似文献
977.
Shaidarova L. G. Gedmina A. V. Budnikov G. K. 《Russian Journal of Applied Chemistry》2003,76(5):755-761
A procedure was developed for analysis of ascorbic acid with a carbon glass electrode coated with a palladium-modified poly(2-vinylpyridine) film. 相似文献
978.
Herein we describe the synthesis and characterization of compounds having the formulae R2SnL2 and R3SnL, where R = Me, n-Bu, Ph and n-Oct and L = 2-[N-(2,4,6-tribromophenylamido]propanoic acid. All the complexes have been characterized by various spectroscopic methods (IR and 1H, 13C, 119Sn NMR), elemental analysis, mass spectrometry and physical data. These compounds were also screened for their biological activity and found some encouraging results. 相似文献
979.
F. Mattioni P.M. Weaver K.D. Potter M.I. Friswell 《International Journal of Solids and Structures》2008,45(2):657-675
This paper models the non-linear flexural response of laminates that have piecewise variation of lay-up in the planform, using finite element analysis. Attention is focused on the effects that thermal stresses have on the potential multiple shapes of a composite structure. Unsymmetric laminates may possess more than a single equilibrium configuration, and during the cool-down the solution thus bifurcates at a critical temperature. In static analyses, numerical solutions are often coaxed to converge into one or the other branch of the solution. A methodology to overcome this problem is presented. Such modelling is necessary to allow application of multistable composite within morphing aircraft structures as multistable composites could provide a viable solution for the realisation of shape-adaptable structures. 相似文献
980.
Ashwani K. Padthe Nalin A. Chaturvedi Dennis S. Bernstein Sanjay P. Bhat Anthony M. Waas 《International Journal of Non》2008,43(4):277-291
We study a linearly damped preloaded two-bar linkage that exhibits hysteresis due to the presence of multiple attracting equilibria. The dynamics at the unstable equilibrium, through which a snap-through buckle occurs, are not linearizable due to a solution-dependent singularity. We stabilize the unstable equilibrium using two distinct non-linear controllers. The feedback-linearization controller requires knowledge of the linkage parameters, whereas the robust version of the intrinsic non-linear proportional-derivative controller requires only an upper bound on the stiffness. 相似文献