全文获取类型
收费全文 | 24339篇 |
免费 | 785篇 |
国内免费 | 22篇 |
专业分类
化学 | 15496篇 |
晶体学 | 165篇 |
力学 | 394篇 |
数学 | 3974篇 |
物理学 | 4166篇 |
无线电 | 951篇 |
出版年
2023年 | 167篇 |
2022年 | 131篇 |
2021年 | 254篇 |
2020年 | 348篇 |
2019年 | 343篇 |
2018年 | 519篇 |
2017年 | 469篇 |
2016年 | 897篇 |
2015年 | 708篇 |
2014年 | 754篇 |
2013年 | 1570篇 |
2012年 | 1565篇 |
2011年 | 1543篇 |
2010年 | 917篇 |
2009年 | 848篇 |
2008年 | 1377篇 |
2007年 | 1379篇 |
2006年 | 1255篇 |
2005年 | 1165篇 |
2004年 | 1070篇 |
2003年 | 823篇 |
2002年 | 725篇 |
2001年 | 517篇 |
2000年 | 467篇 |
1999年 | 358篇 |
1998年 | 256篇 |
1997年 | 236篇 |
1996年 | 313篇 |
1995年 | 224篇 |
1994年 | 257篇 |
1993年 | 216篇 |
1992年 | 200篇 |
1991年 | 189篇 |
1990年 | 185篇 |
1989年 | 161篇 |
1988年 | 152篇 |
1987年 | 147篇 |
1986年 | 144篇 |
1985年 | 219篇 |
1984年 | 224篇 |
1983年 | 141篇 |
1982年 | 148篇 |
1981年 | 171篇 |
1980年 | 159篇 |
1979年 | 121篇 |
1978年 | 108篇 |
1977年 | 121篇 |
1976年 | 101篇 |
1975年 | 99篇 |
1973年 | 78篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Summary On the example of the previously tested method of nitrogen dioxide determination in ambient air two aspects of wind influence on the badge-type sampler performance were discussed: an appropriate design of the sampler eliminating any open recess over the face of the sampler and so called starvation effect i.e. decrease in the mass of analyte absorbed by the sampler at wind velocities below 20 cm/s. The wind effect on the performance of the passive sampler has been diminished from about 150% to maximum 20%. 相似文献
142.
Kirk Gustafson Raymond J. Andersen Marie H.M. Chen Jon Clardy Jill E. Hochlowski 《Tetrahedron letters》1984,25(1):11-14
Extracts of the dorid nudibranch contain a diterpenoic acid glyceride 1 whose structure has been determined by x-ray diffraction analysis. The structure of a minor metabolite, the sesquiterpenoic acid glyceride 2, was determined by chemical correlation. 相似文献
143.
O. I. Arillo-Flores M. F. Ruiz-López M. I. Bernal-Uruchurtu 《Theoretical chemistry accounts》2007,118(2):425-435
We discuss the failure of commonly used AM1 and PM3 semiempirical methods to correctly describe acid dissociation. We focus
our analysis on HCl because of its physicochemical importance and its relevance in atmospheric chemistry. The structure of
non-dissociated and dissociated HCl – (H2O)
n
clusters is accounted for. The very bad results obtained with PM3 (and also with AM1) are related to large errors in gas-phase
proton affinity of water and gas-phase acidity of HCl. Indeed, estimation of pKa values shows that neither AM1 nor PM3 are able to predict HCl dissociation in liquid water since HCl is found to be a weaker
acid than H3O+. We have proposed in previous works a modified PM3 approach (PM3-MAIS) adapted to intermolecular calculations. It is derived
from PM3 by reparameterization of the core–core functions using ab initio data. Since parameters for H–Cl and O–Cl core–core
interactions were not yet available, we have carried out the corresponding optimization. Application of the PM3-MAIS method
to HCl dissociation in HCl–(H2O)
n
clusters leads to a huge improvement over PM3 results. Though the method predicts a slightly overestimated HCl acidity in
water environment, the overall agreement with ab initio calculations is very satisfying and justifies efforts to develop new
semiempirical methods. 相似文献
144.
Solid-phase spectrophotometric microdetermination of iron with ascorbic acid and ferrozine 总被引:1,自引:0,他引:1
M. L. Fernández-de Córdova A. Ruíz-Medina A. Molina-Díaz 《Analytical and bioanalytical chemistry》1997,357(1):44-49
A very sensitive and selective method for the determination of trace amounts of iron has been developed, based on the reduction
of Fe(III) to Fe(II) by ascorbic acid, followed by chromogenic chelation of Fe(II) with ferrozine. The complex Fe(II)-ferrozine
is easily sorbed on a dextran-type anion-exchange gel packed in a 1 mm cell, and the absorbance of the gel is measured directly
at 569 and 800 nm. The extended linear range of the determination is 0.5–10 ng ml-1 of iron (apparent molar absorptivity=4.4×107 l mol-1 cm-1) and the precision (RSD) 1.3% for a concentration of 5 ng ml-1 of iron (n=10). The detection limit for a sample volume of 1000 ml, using 0.040 g of anion-exchanger, corresponds to 0.12 ng ml-1. The method has been successfully applied to the determination of iron in natural and waste waters, wine, soil extract and
previously digested vegetal tissues, drugs and human hair.
Received: 20 November 1995/Revised: 23 January 1996/Accepted: 26 January 1996 相似文献
145.
The cross-sections for the production of the short-lived nuclides:26mAl,197mAu,136mBa,79mBr,139mCe,34Cl,167mEr,114m3In,114m2In,38mK,38mCl,26Na,20F,192Re,207mPb,203mPb,46mSc,183mW,90mZr obtained from (n, 2n), (n, n′), (n, alpha), (n,p) reactions using 14 MeV neutrons, were evaluated by the activation method.
The experimental results of this work were compared with those obtained by other authors. 相似文献
146.
The behaviour of iron in the Bayer process has been investigated by neutron activation analysis and Ge(Li) spectrometry, using
bauxite samples of various origin. The amount of iron impurities in alumina was found to be independent of the origin of the
bauxites. Alumina is contaminated by iron in the process of decomposition of aluminate liquors; regardless whether iron is
present in ‘dissolved’ form or as a floating impurity, it will pass into the alumina almost completely. 相似文献
147.
The behaviour of oxazepam in adsorptive stripping voltammetry was studied taking into account those conditions which have an influence on the accumulation step (electrolyte, pH, time, potential, drop size and stirring rate), rest time and stripping step (pulse amplitude and scan rate). Oxazepam can be determined at a hanging mercury drop electrode by differential-pulse voltammetry in 0.008 M Britton-Robinson buffer at pH 2.0 with a ?0.50 V accumulation potential. Its detection limit was found to be 3.6 × 10?10 M (30-s accumulation) and the relative standard deviation for oxazepam concentrations in the range 2.8 × 10?8?4.0 × 10?7 M is lower than 2.8% (80-s accumulation). In addition, a procedure using adsorptive stripping voltammetry was developed to study the interactions occurring between human albumin and the camazepam metabolic series (camazepam, temazepam and oxazepam). The interactions decreased in the order temazepam ? oxazepam ? camazepam and the groups and structural modifications favouring interaction were determined. 相似文献
148.
A room-temperature flow injection spectrofluorimetric method is presented for the determination of Zn(II), based on the use of salicylaldehyde thiocarbohydrazone in the presence of Triton X-100 and sodium acetate-acetic acid buffer. Various physical and chemical variables affecting the reaction in the flow system were evaluated. The proposed method is very selective. The calibration graph is linear over the range 10-1000 ng/ml, with a detection limit of 5 ng/ml and a relative standard deviation at the 50 ng/ml level of 1.8 %. The method was successfully applied to the determination of Zn(II) in drinking waters and biological samples. 相似文献
149.
L. L. Stachó 《Annali di Matematica Pura ed Applicata》1981,128(1):207-225
Summary Banach M-lattices are studied from the view point whether all the biholomorphic automorphisms of their unit balls admit fixed points when continuously extended to the closure of the unit ball. A characterization of compact topologieal F-spaces is found in terms of the fixed points of the elements of AutB(C()) which enables to establish some particular properties also of the topological automorphisms of compact F-spaces. Finally it is shown that if the M- lattice E admits a predual then each member of Aut B(E) has fixed point if and only if E is isometrically isomorphic with some l-space. 相似文献
150.
Let X = {x1, x2,…} be a finite set and associate to every xi a real number αi. Let f(n) [g (n)] be the least value such that given any family of subsets of X having maximum degree n [cardinality n], one can find integers αi, i=1,2,… so that αi ? αi|<1 and for all . We prove . 相似文献