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971.
Andrews KW Schweitzer A Zhao C Holden JM Roseland JM Brandt M Dwyer JT Picciano MF Saldanha LG Fisher KD Yetley E Betz JM Douglass L 《Analytical and bioanalytical chemistry》2007,389(1):231-239
As part of a study initiating the development of an analytically validated Dietary Supplement Ingredient Database (DSID) in
the United States (US), a selection of dietary supplement products were analyzed for their caffeine content. Products sold
as tablets, caplets, or capsules and listing at least one caffeine-containing ingredient (including botanicals such as guarana,
yerba mate, kola nut, and green tea extract) on the label were selected for analysis based on market share information. Two
or three lots of each product were purchased and analyzed using high-pressure liquid chromatography (HPLC). Each analytical
run included one or two National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) and two
products in duplicate. Caffeine intake per serving and per day was calculated using the maximum recommendations on each product
label. Laboratory analysis for 53 products showed product means ranging from 1 to 829 mg caffeine/day. For products with a
label amount for comparison (n = 28), 89% (n = 25) of the products had analytically based caffeine levels/day of between −16% and +16% of the claimed levels. Lot-to-lot
variability (n = 2 or 3) for caffeine in most products (72%) was less than 10%. 相似文献
972.
Quality of botanical products is a great uncertainty that consumers, clinicians, regulators, and researchers face. Definitions
of quality abound, and include specifications for sanitation, adventitious agents (pesticides, metals, weeds), and content
of natural chemicals. Because dietary supplements (DS) are often complex mixtures, they pose analytical challenges and method
validation may be difficult. In response to product quality concerns and the need for validated and publicly available methods
for DS analysis, the US Congress directed the Office of Dietary Supplements (ODS) at the National Institutes of Health (NIH)
to accelerate an ongoing methods validation process, and the Dietary Supplements Methods and Reference Materials Program was
created. The program was constructed from stakeholder input and incorporates several federal procurement and granting mechanisms
in a coordinated and interlocking framework. The framework facilitates validation of analytical methods, analytical standards,
and reference materials. 相似文献
973.
Alexander Kane Ren-Fan Shao Joseph E Maclennan Lixing Wang David M Walba Noel A Clark 《Chemphyschem》2007,8(1):170-174
We demonstrate, both theoretically and experimentally, that it is possible to use an electric field to drive the formation of macroscopic chiral (conglomerate) domains from an initially homogeneous fluid racemate. Field-induced segregation is exhibited in a fluid smectic liquid-crystal phase of a racemic mesogen, wherein enantiomerically-enriched domains are readily identifiable by their chiral electro-optical response. The sharp field-generated boundaries that form between opposite-handed domains broaden by diffusion in the absence of field, but reform rapidly if the field is switched on again, providing unambiguous evidence for the field-driven physical separation of enantiomers. A mean-field model successfully describes the steady-state and the dynamic evolution of conglomerate formation. 相似文献
974.
Lewis M. Broomfield Joseph A. Wright William Clegg Manfred Bochmann 《Journal of organometallic chemistry》2007,692(21):4603-4611
Treatment of MCl3(OC6H3-2-tBu-6-CHNC6F5)(THF) (M = Ti, Zr) with a variety of different potassium iminopyrrolate salts (K+[RNCHC4H3N]−), (R = phenyl, cyclo-hexyl, ethyl) afforded the corresponding titanium and zirconium mixed-ligand complexes MCl2(N-O)(N-N). The molecular structures of TiCl2(OC6H3-2-tBu-6-CHNC6F5)(C2H5NCHC4H3N) (1c), TiCl2(OC6H3-2-tBu-6-CHNC6F5)(C6H11NCHC4H3N) (1b) and ZrCl2(OC6H3-2-tBu-6-CHNC6F5)(C6H11NCHC4H3N) (2b) show distorted octahedral geometries with trans-O−,N−/cis-Cl2 arrangements. On activation with MAO the titanium (iminopyrrolato)(salicylaldiminato) complexes show excellent activities in ethylene polymerisation and are significantly more effective ethylene/propylene copolymerisation catalysts, both in terms of activity and propene incorporation, than either of the parent complexes. The ethylene-propylene copolymers show ca. 80% 1,2 regioselectivity and at high propylene incorporation tend towards an alternating structure. 相似文献
975.
William J. Evans Kevin A. Miller Wes R. Hillman Joseph W. Ziller 《Journal of organometallic chemistry》2007,692(17):3649-3654
The reductive reactivity of the (BPh4)1− ligand in pentamethylcyclopentadienyl [(C5Me5)2U][(μ-η2:η1-Ph)2BPh2] (1) was compared with that of the tetramethyl analog, [(C5Me4H)2U][(μ-η6:η1-Ph)(μ-η1:η1-Ph)BPh2] (2) using PhSSPh as a probe to determine if the mode of (BPh4)1− bonding affected the reduction. Both complexes act as two-electron reductants to form (C5Me4R)2U(SPh)2 [R = Me, 3; H, 4], but only in the R = H case could the product be crystallographically characterized. An improved synthesis of 1 from [(C5Me5)2UH]2 (5) and [Et3NH][BPh4] is also reported as well as its reaction with MeCN that provides another route to the unusual, parallel-ring, uranium metallocene [(C5Me5)2U(NCMe)5][BPh4]2 (6). 相似文献
976.
Combining Biomimetic Block Copolymer Worms with an Ice‐Inhibiting Polymer for the Solvent‐Free Cryopreservation of Red Blood Cells 下载免费PDF全文
Daniel E. Mitchell Joseph R. Lovett Prof. Dr. Steven P. Armes Dr. Matthew I. Gibson 《Angewandte Chemie (International ed. in English)》2016,55(8):2801-2804
The first fully synthetic polymer‐based approach for red‐blood‐cell cryopreservation without the need for any (toxic) organic solvents is reported. Highly hydroxylated block copolymer worms are shown to be a suitable replacement for hydroxyethyl starch as a extracellular matrix for red blood cells. When used alone, the worms are not a particularly effective preservative. However, when combined with poly(vinyl alcohol), a known ice‐recrystallization inhibitor, a remarkable additive cryopreservative effect is observed that matches the performance of hydroxyethyl starch. Moreover, these block copolymer worms enable post‐thaw gelation by simply warming to 20 °C. This approach offers a new solution for both the storage and transport of red blood cells and also a convenient matrix for subsequent 3D cell cultures. 相似文献
977.
A Mixed‐Ligand Chiral Rhodium(II) Catalyst Enables the Enantioselective Total Synthesis of Piperarborenine B 下载免费PDF全文
Robert A. Panish Srinivasa R. Chintala Prof. Joseph M. Fox 《Angewandte Chemie (International ed. in English)》2016,55(16):4983-4987
A novel, mixed‐ligand chiral rhodium(II) catalyst, Rh2(S‐NTTL)3(dCPA), has enabled the first enantioselective total synthesis of the natural product piperarborenine B. A crystal structure of Rh2(S‐NTTL)3(dCPA) reveals a “chiral crown” conformation with a bulky dicyclohexylphenyl acetate ligand and three N‐naphthalimido groups oriented on the same face of the catalyst. The natural product was prepared on large scale using rhodium‐catalyzed bicyclobutanation/ copper‐catalyzed homoconjugate addition chemistry in the key step. The route proceeds in ten steps with an 8 % overall yield and 92 % ee. 相似文献
978.
Gregory W. Peterson John J. Mahle Jared B. DeCoste Wesley O. Gordon Joseph A. Rossin 《Angewandte Chemie (International ed. in English)》2016,55(21):6235-6238
Here we discuss the removal of nitrogen dioxide, an important toxic industrial chemical and pollutant, from air using the MOF UiO‐66‐NH2. The amine group is found to substantially aid in the removal, resulting in unprecedented removal capacities upwards of 1.4 g of NO2 /g of MOF. Furthermore, whereas NO2 typically generates substantial quantities of NO on sorbents, the amount generated by UiO‐66‐NH2 is significantly reduced. Of particular significance is the formation of a diazonium ion on the aromatic ring of the MOF, and the potential reduction of NO2 to molecular nitrogen. 相似文献
979.
980.
Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic,Green, and Sustainable Sources 下载免费PDF全文
Richard M. Laine Joseph C. Furgal Phi Doan David Pan Vera Popova Xingwen Zhang 《Angewandte Chemie (International ed. in English)》2016,55(3):1065-1069
The direct depolymerization of SiO2 to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Simet, followed by treatment with ROH. We report herein the base‐catalyzed depolymerization of SiO2 with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)4. Thus, 2‐methyl‐2,4‐pentanediol, 2,2,4‐trimethyl‐1,3‐pentanediol, or ethylene glycol (EGH2) react with silica sources, such as rice hull ash, in the presence of NaOH (10 %) to form H2O and distillable spirocyclic alkoxysilanes [bis(2‐methyl‐2,4‐pentanediolato) silicate, bis(2,2,4‐trimethyl‐1,3‐pentanediolato) silicate or Si(eg)2 polymer with 5–98 % conversion, as governed by surface area/crystallinity. Si(eg)2 or bis(2‐methyl‐2,4‐pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)4 in 60 % yield, thus providing inexpensive routes to high‐purity precipitated or fumed silica and compounds with single Si?C bonds. 相似文献