全文获取类型
收费全文 | 38958篇 |
免费 | 2192篇 |
国内免费 | 985篇 |
专业分类
化学 | 7128篇 |
晶体学 | 298篇 |
力学 | 2209篇 |
综合类 | 288篇 |
数学 | 21588篇 |
物理学 | 10624篇 |
出版年
2023年 | 245篇 |
2022年 | 422篇 |
2021年 | 564篇 |
2020年 | 518篇 |
2019年 | 850篇 |
2018年 | 700篇 |
2017年 | 799篇 |
2016年 | 697篇 |
2015年 | 522篇 |
2014年 | 920篇 |
2013年 | 2147篇 |
2012年 | 1372篇 |
2011年 | 1722篇 |
2010年 | 1219篇 |
2009年 | 1678篇 |
2008年 | 1786篇 |
2007年 | 1885篇 |
2006年 | 1728篇 |
2005年 | 1341篇 |
2004年 | 1590篇 |
2003年 | 1718篇 |
2002年 | 2247篇 |
2001年 | 2239篇 |
2000年 | 2119篇 |
1999年 | 2121篇 |
1998年 | 1922篇 |
1997年 | 1285篇 |
1996年 | 695篇 |
1995年 | 492篇 |
1994年 | 420篇 |
1993年 | 270篇 |
1992年 | 256篇 |
1991年 | 219篇 |
1990年 | 289篇 |
1989年 | 246篇 |
1988年 | 184篇 |
1987年 | 155篇 |
1986年 | 190篇 |
1985年 | 293篇 |
1984年 | 266篇 |
1983年 | 106篇 |
1982年 | 191篇 |
1981年 | 282篇 |
1980年 | 266篇 |
1979年 | 161篇 |
1978年 | 210篇 |
1977年 | 157篇 |
1976年 | 103篇 |
1975年 | 87篇 |
1974年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
991.
New equations are derived and implemented for efficient and accurate computation of solvation energy derivatives for the conductor-like polarizable continuum model (C-PCM) and the isotropic integral equation formalism polarizable continuum model (IEF-PCM). Two new molecular surface tessellation procedures GEPOL-RT and GEPOL-AS that generate near continuous potential energy surfaces are proposed for PCM geometry optimization. The combined use of these new techniques leads to efficient and convergent geometry optimizations with the PCMs. 相似文献
992.
Arsenic Speciation in Urine and Blood Reference Materials 总被引:1,自引:0,他引:1
Todor I. Todorov John W. Ejnik Florabel G. Mullick Jose A. Centeno 《Mikrochimica acta》2005,151(3-4):263-268
Acute and chronic exposure to arsenic is a growing problem in the industrialized world. Arsenic is a potent carcinogen and
toxin in humans. In the body, arsenic is metabolized to produce several species, including inorganic forms, such as trivalent
(AsIII) and pentavalent (AsV), and the methylated metabolites such as monomethylarsonic acid, (MMAV), and dimethylarsinic acid (DMAV), in addition to arsenobetaine (AsB) which is ingested and excreted from the body in the same form. Each of these species
has been reported to possess a specific but different degree of toxicity. Thus, not only is the measurement of total As required,
but also quantification of the individual metabolites is necessary to evaluate the toxicity and risk assessment of this element.
There are a large number of reference materials that are used to validate methodology for the analysis of As in blood and
urine, but they are limited to total As concentrations. In this study, the speciation of five arsenic metabolites is reported
in blood and urine from commercial available control materials certified for total arsenic levels. The separation was performed
with an anion exchange column using inductively coupled plasma mass spectrometry as a detector. Baseline separation was achieved
for AsIII, AsV, MMAV, DMAV, and AsB, allowing us to quantify all five species. Excellent agreement between the total arsenic levels and the sum of the
speciated As levels was obtained. 相似文献
993.
Silver(I) and N,N'-bis(2-hydroxyethyl)dithiooxamide (H2X) in acidic solutions, form a series of water-soluble polynuclear complexes. These complexes can be represented by two general formulae: H2qAgpXq and H2q-1AgpXq with q=1,2,3 and p=q, q-1. 相似文献
994.
Prakasham RS Rao ChS Rao RS Rajesham S Sarma PN 《Applied biochemistry and biotechnology》2005,120(2):133-144
Optimization of alkaline protease production parameters by Bacillus sp. was investigated using Taguchi methodology. The pH of the medium was observed to be the most significant factor among
all selected optimization parameters at an individual level. The combinatorial influence of least significant factors, inoculum
level and salt solution concentration (at the individual level), resulted in an interacting severity index of 76%, suggesting
their interactive role in the regulation of protease production in this microbial species. Protease production could be improved
more than 100% with Taguchi’s optimized conditions of the medium composition by this microorganism. 相似文献
995.
The CGC analysis of 25 organophosphorus and organonitrogen pesticides was optimized on two different stationary phases, namely a 5% diphenyl dimethyl silicone (SE-54 type) and a 50% diphenyl dimethyl silicone (OV-17 type) by selecting the best temperature program conditions, using computer simulation. 相似文献
996.
Galal Elmanfe Hosna Benbouzid Nawal Derkaoui Mireille Privat 《Comptes Rendus Chimie》2006,9(11-12):1476
Despite its extremely weak adsorption at the water/silica interface, carbofuran can, however, induce the coadsorption of metallic salts, like, for example, lead nitrate, and thus enhance its adsorption. We quantitatively studied this phenomenon with a depletion method and under concentration conditions close to the environmental ones. Heavy metal salt was found to positively adsorb, whereas carbofuran relative adsorption is generally slightly negative. This study provides evidence that considering these facts is of paramount importance in environmental prospects. To cite this article: G. Elmanfe et al., C. R. Chimie 9 (2006). 相似文献
997.
This paper restudies the method of direct chelatometric titration of V(IV) with N-benzoyl-N-phenylhydroxylamine (BPHA) as indicator. The indicator functions entirely differently from indicators hitherto used in chelatometry. The solution contains two independent systems – the indicator and the titrand-titrant system – without any color changing species. The BPHA-V(V) chelate once formed as the result of aerial oxidation constitutes the indicator system and does not react with EDTA. Its color therefore remains unchanged throughout titration. It is not a metallochromic indicator. Its function is that of color-screening. The color of the titrant system deepens during titration, but does not change. The end-point is indicated by one color completely covering up the other. Hence, this method is of purely empirical nature.Received July 5, 2002; accepted February 11, 2003
Published online July 16, 2003 相似文献
998.
Neutron activation offers some important advantages for the determination of selected trace elements in shale oil products. This paper gives techniques and results of a study of crude shale oil and naphtha, heavy distillate, and wax products of shale oil. The elements determined were Al, As, Au, Br, Ce, Cl, Co, Cr, Fe, Hg, I, K, Mn. Mo, Na, S, Sb, Se, V, and Zn. Some elements (Mn, Na, As) tend to accumulate in heavier fractions, whereas chlorine and iodine are concentrated in the more volatile fractions. The volatility of sulphur compounds in the shale oil products appears to be essentially uniform, with some tendency toward accumulation in distillation residues. The tendency for the trace elements to accumulate in the waxes that precipitated from cooled heavy distillates was very low. 相似文献
999.
Fabio E. Penotti 《International journal of quantum chemistry》2006,106(5):1153-1159
Nonorthogonal single‐configuration and multiconfiguration ab initio calculations are carried out on the BeH2 molecule in its equilibrium (D∞h) geometry, using a highly optimized even‐tempered Slater‐type orbital (STO) basis set. The results are used as a basis for a discussion of the electronic structure of the molecule in modern‐VB terms. A value of Re = 1.329 Å is obtained. Values are also obtained for the symmetric‐stretch harmonic frequency (2053 cm?1), electric quadrupole moment (?5.60 Buckingham), and nuclear quadrupole coupling constants (?3.59 MHz for 9Be and 0.0915 MHz for 2H). © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
1000.
Mohamed A. F. Elmosallamy Mohamed M. Ghoneim Hamada M. A. Killa Amr L. Saber 《Mikrochimica acta》2005,151(1-2):109-113
A potentiometric poly(vinyl chloride) membrane sensor for determination of saccharin is described. It is based on the use of Aliquat 336S-saccharinateion-pair as an electroactive material in plasticized PVC membranes with o-nitrophenyloctylether or dioctylphthalate. The sensor is conditioned for at least two days in 0.1 mol L−1 sodium saccharinate before use. It exhibits fast, stable and Nernstian response for saccharinate ions over the concentration range of 1.0 × 10−1–5.0 × 10−5 mol L−1 and pH range of 4.5–11. The sensor is used for determination of saccharin in some dosage forms. Results with an average recovery of 101% and a mean standard deviation of 0.2% are obtained which is compared favourably with data obtained using the British pharmacopoeia method. The sensor shows reasonable selectivity towards saccharin in presence of many anions and natural sweeteners. 相似文献