全文获取类型
收费全文 | 813篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
化学 | 398篇 |
晶体学 | 6篇 |
力学 | 10篇 |
数学 | 67篇 |
物理学 | 301篇 |
综合类 | 47篇 |
出版年
2021年 | 9篇 |
2020年 | 5篇 |
2019年 | 13篇 |
2018年 | 8篇 |
2017年 | 5篇 |
2016年 | 14篇 |
2015年 | 13篇 |
2014年 | 16篇 |
2013年 | 23篇 |
2012年 | 25篇 |
2011年 | 42篇 |
2010年 | 16篇 |
2009年 | 20篇 |
2008年 | 35篇 |
2007年 | 35篇 |
2006年 | 48篇 |
2005年 | 51篇 |
2004年 | 56篇 |
2003年 | 38篇 |
2002年 | 20篇 |
2001年 | 11篇 |
2000年 | 19篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 9篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 14篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1974年 | 5篇 |
1973年 | 9篇 |
1971年 | 6篇 |
1967年 | 5篇 |
1892年 | 4篇 |
排序方式: 共有829条查询结果,搜索用时 15 毫秒
41.
Monica A. Gill 《合成通讯》2013,43(10):1460-1468
A variety of allylic and propargylic esters were rapidly prepared via microwave heating of their corresponding mixed anhydride derived from pivaloyl chloride. The reaction conditions were modified to account for the sterics of the alcohol and the electronics of the carboxylic acid. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
42.
P. V. Badadhe L. R. Patil S. S. Bhagat A. V. Chate D. W. Shinde M. D. Nikam C. H. Gill 《Journal of heterocyclic chemistry》2013,50(5):999-1004
The title compounds 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h and 6a , 6b , 6c , 6d , 6e , 6f , 6g , 6h have been synthesized from β‐diketones and chromones, respectively, having 5‐methyl‐3‐phenylisoxazole moiety. Substituted 2‐acetylphenyl 5‐methyl‐3‐phenylisoxazole‐4‐carboxylate 3a , 3b , 3c , 3d , 3e , 3f , 3g , 3h were converted into 1‐(2‐hydroxyphenyl)‐3‐(5‐methyl‐3‐phenylisoxazole‐4‐yl)propane‐1,3‐dione 4a , 4b , 4c , 4d , 4e , 4f , 4g , 4h by Baker–Venketaraman transformation. Further, the cyclodehydration of diketone 4a , 4b , 4c , 4d , 4e , 4f , 4g , 4h with glacial acetic acid in conc. HCl at reflux gave corresponding substituted 2‐(5‐methyl‐3‐phenylisoxazole‐4‐yl)‐4H‐chromen‐4‐one 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h . The corresponding 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h react with hydrazine hydrate in presence of glacial acetic acid in ethanol at reflux to furnish 2‐(5‐5(5‐methyl‐3‐phenylisoxazole‐4‐yl)‐1H‐pyrazole‐3‐yl)phenol 6a , 6b , 6c , 6d , 6e , 6f , 6g , 6h . The structures of all newly synthesized compounds have been confirmed by IR, 1H NMR, mass spectral data, as well as elemental analysis. The synthesized compounds have been screened for their antimicrobial activity. Some of the compounds show better antimicrobial activity as compared with the reference drugs Streptomycin, Ampicillin, Gentamycin, Cefixime, and Ketoconazole. 相似文献
43.
Aubert B Barate R Bona M Boutigny D Couderc F Karyotakis Y Lees JP Poireau V Tisserand V Zghiche A Grauges E Palano A Chen JC Qi ND Rong G Wang P Zhu YS Eigen G Ofte I Stugu B Abrams GS Battaglia M Brown DN Button-Shafer J Cahn RN Charles E Gill MS Groysman Y Jacobsen RG Kadyk JA Kerth LT Kolomensky YG Kukartsev G Lynch G Mir LM Orimoto TJ Pripstein M Roe NA Ronan MT Wenzel WA del Amo Sanchez P Barrett M Ford KE Harrison TJ Hart AJ Hawkes CM Morgan SE Watson AT Held T Koch H Lewandowski B 《Physical review letters》2007,98(5):051802
We present an observation of B-->eta'K*. The data sample corresponds to 232x10(6) BB[over ] pairs collected with the BABAR detector at the PEP-II asymmetric-energy B factory at the Stanford Linear Accelerator Center. We measure the branching fractions (in units of 10(-6)) B(B(0)-->eta'K*0)=3.8+/-1.1+/-0.5 and B(B+-->eta'K*+)=4.9(1.7)(+1.9)+/-0.8, where the first error is statistical and the second systematic. A simultaneous fit results in the observation of B-->eta'K* with B(B-->eta'K*)=4.1(-0.9)(+1.0)+/-0.5. We also search for B-->eta'rho and eta'f(0)(980)(f(0)-->pi+pi-) with results and 90% confidence level upper limits B(B+-->eta'rho+)=8.7(-2.8-1.3)(+3.1+2.3) (<14), B(B(0)-->eta'rho0)<3.7, and B(B(0)-->eta'f(0)(980)(f(0)-->pi+pi-))<1.5. Charge asymmetries in the channels with significant yields are consistent with zero. 相似文献
44.
Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans. 相似文献
45.
Aubert B Boutigny D Gaillard JM Hicheur A Karyotakis Y Lees JP Robbe P Tisserand V Palano A Chen GP Chen JC Qi ND Rong G Wang P Zhu YS Eigen G Reinertsen PL Stugu B Abbott B Abrams GS Borgland AW Breon AB Brown DN Button-Shafer J Cahn RN Clark AR Gill MS Gritsan AV Groysman Y Jacobsen RG Kadel RW Kadyk J Kerth LT Kluth S Kolomensky YG Kral JF LeClerc C Levi ME Liu T Lynch G Meyer AB Momayezi M Oddone PJ Perazzo A Pripstein M Roe NA Romosan A Ronan MT Shelkov VG Telnov AV Wenzel WA Zisman MS 《Physical review letters》2002,88(10):101805
The branching fractions of the exclusive decays B0-->K(*0)gamma and B+-->K(*+)gamma are measured from a sample of (22.74+/-0.36)x10(6) BB decays collected with the BABAR detector at the PEP-II asymmetric e(+)e(-) collider. We find B (B0-->K(*0)gamma) = [4.23+/-0.40(stat)+/-0.22(syst)]x10(-5), B(B+-->K(*+)gamma) = [3.83+/-0.62(stat)+/-0.22(syst)]x10(-5) and constrain the CP-violating charge asymmetry to be -0.170K(*)gamma)<0.082 at 90% C.L. 相似文献
46.
Let N denote the set of natural numbers and let P =(N
k
, ) be a countably infinite poset on the k-dimensional lattice N
k
. Given x N
k
, we write max(x) (min(x)) for the maximum (minimum) coordinate of x. Let
be the directed-incomparability graph of P which is defined to be the graph with vertex set equal to N
k
and edge set equal to the set of all (x, y) such that max(x) max(y) and x and y not comparable in P. For any subset D N
k
, we let P
D
and
D
denote the restrictions of P and
to D. Points x N
k
with min(x) = 0 will be called boundary points. We define a geometrically natural notion of when a point is interior to P or
relative to the lattice N
k
, and an analogous notion of monotone interior with respect to
or
D
. We wish to identify situations where most of these interior points are exposed to the boundary of the lattice or, in the case of monotone interior points, not concealed very much from the boundary. All of these ideas restrict to finite sublattices F
k
and/or infinite sublattices E
k
of N
k
. Our main result shows that for any poset P and any arbitarily large integer M > 0, there is an F E with F = M where, relative to the sublattices F
k
E
k
, the ideal situation of total exposure of interior points and very little concealment of monotone interior points must occur. Precisely, we prove that for any P =(N
k
, ) and any integer M > 0, there is an infinite E N and a finite D F
k
with F E and F = M such that (1) every interior vertex of P
E
k
or
E
k
is exposed and (2) there is a fixed set C E, C k
k
, such that every monotone-interior point of
D
belonging to F
k
has its monotone concealment in the set C. In addition, we show that if P
1 =(N
k
, 1),..., P
r
=(N
k
,
r
) is any sequence of posets, then we can find E,D, and F so that the properties (1) and (2) described above hold simultaneously for each P
i
. We note that the main point of (2) is that the bound k
k
depends only on the dimension of the lattice and not on the poset P. Statement (1) is derived from classical Ramsey theory while (2) is derived from a recent powerful extension of Ramsey theory due to H. Friedman and shown by Friedman to be independent of ZFC, the usual axioms of set theory. The fact that our result is proved as a corollary to a combinatorial theorem that is known to be independent of the usual axioms of mathematics does not, of course, mean that it cannot be proved using ZFC (we just couldn"t find such a proof). This puts our geometrically natural combinatorial result in a somewhat unusual position with regard to the axioms of mathematics. 相似文献
47.
Fritzsch like mass matrices with non-zero 22-elements both in U sector and D sector have been investigated in the context
of latest data regardingm
t
phys
, |V
ub|, |V
cb|, |V
td| and |V
ts|. Unlike several other phenomenological models, the present model not only accommodates the value ofm
t
phys
in the range 150–240 GeV, encompassing the CDF and D0 values, but is also able to reproduce |V
cb| ≊0.040 and |V
ub/Vcb| = 0.08±0.02 and |V
td| is predicted to lie in the range 0.005–0.014. Further, the angles of the unitarity triangle, related to the CP-violating
asymmetries, are calculated to be in the ranges −1.0⩽sin2α⩽−0.1, 0.6 ⩽sin2α⩽1.0 and 0.48⩽sin2β⩽0.56, which are in agreement
with other recent calculations. 相似文献
48.
Repeated freeze-thaw cycles in cryosurgery 总被引:2,自引:0,他引:2
49.
Manganese(II) iodide, iron(II) iodide and copper(I) iodide each react with tetramethylammonium disulphite to form anhydrous manganese(II) sulphite, iron(II) sulphite and copper(I) disulphite respectively. Iron(II) sulphite and copper(I) disulphite react with dimethylsulphoxide-sulphur dioxide to form iron(II) disulphate and copper(II) disulphate respectively. Hydrated sulphites of manganese(II), iron(II), magnesium(II) and calcium(II) were also prepared. The properties of the sulphites were investigated using thermogravimetric and IR measurements. 相似文献
50.
R. Bailey E. Belau T. Böhringer M. Bosman V. Chabaud C. Damerell C. Daum C. De Rijk H. Dijkstra S. Gill A. Gillman R. Gilmore Z. Hajduk C. Hardwick W. Hoogland B.D. Hyams R. Klanner T. Zeludziewicz 《Physics letters. [Part B]》1984,139(4):320-326
We have observed three unique decays of F mesons into KKπ and five unique decays of D mesons into KKπ using the NA11 spectrometer together with a telescope of high-resolution silicon microstrip detectors. The mass value obtained for the F meson is (1975 ± 4) MeV, the lifetime (3.2±1.33..0)×10?13 s. 相似文献