全文获取类型
收费全文 | 651篇 |
免费 | 8篇 |
专业分类
化学 | 357篇 |
晶体学 | 5篇 |
力学 | 24篇 |
数学 | 55篇 |
物理学 | 218篇 |
出版年
2022年 | 6篇 |
2021年 | 9篇 |
2019年 | 6篇 |
2016年 | 7篇 |
2015年 | 4篇 |
2014年 | 6篇 |
2013年 | 34篇 |
2012年 | 24篇 |
2011年 | 32篇 |
2010年 | 17篇 |
2009年 | 10篇 |
2008年 | 21篇 |
2007年 | 18篇 |
2006年 | 22篇 |
2005年 | 33篇 |
2004年 | 19篇 |
2003年 | 15篇 |
2002年 | 11篇 |
2001年 | 15篇 |
2000年 | 21篇 |
1999年 | 8篇 |
1998年 | 11篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 9篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 12篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 17篇 |
1976年 | 9篇 |
1975年 | 16篇 |
1974年 | 10篇 |
1973年 | 18篇 |
1972年 | 11篇 |
1967年 | 3篇 |
1966年 | 4篇 |
1942年 | 5篇 |
排序方式: 共有659条查询结果,搜索用时 382 毫秒
261.
262.
263.
Ethan H. McIlhenny Kelly E. Pipkin Leanna J. Standish Hope A. Wechkin Rick Strassman Steven A. Barker 《Journal of chromatography. A》2009,1216(51):8960-8968
A direct injection/liquid chromatography–electrospray ionization-tandem mass spectrometry procedure has been developed for the simultaneous quantitation of 11 compounds potentially found in the increasingly popular Amazonian botanical medicine and religious sacrament ayahuasca. The method utilizes a deuterated internal standard for quantitation and affords rapid detection of the alkaloids by a simple dilution assay, requiring no extraction procedures. Further, the method demonstrates a high degree of specificity for the compounds in question, as well as low limits of detection and quantitation despite using samples for analysis that had been diluted up to 200:1. This approach also appears to eliminate potential matrix effects. Method bias for each compound, examined over a range of concentrations, was also determined as was inter- and intra-assay variation. Its application to the analysis of three different ayahuasca preparations is also described. This method should prove useful in the study of ayahuasca in clinical and ethnobotanical research as well as in forensic examinations of ayahuasca preparations. 相似文献
264.
Abouzaid E Arenton M Barker AR Bellantoni L Bellavance A Blucher E Bock GJ Cheu E Coleman R Corcoran MD Corti G Cox B Erwin AR Escobar CO Glazov A Golossanov A Gomes RA Gouffon P Hanagaki K Hsiung YB Huang H Jensen DA Kessler R Kotera K Ledovskoy A McBride PL Monnier E Nelson KS Nguyen H Niclasen R Ping H Qi XR Ramberg EJ Ray RE Ronquest M Santos E Shields J Slater W Smith D Solomey N Swallow EC Toale PA Tschirhart R Velissaris C Wah YW Wang J White HB Whitmore J Wilking M Winstein B Winston R 《Physical review letters》2006,96(10):101801
Using the complete KTeV data set of 5,241 candidate K(L)--> pi(+) pi(-) e(+) e(-) decays (including an estimated background of 204 +/- 14 events), we have measured the coupling g(CR)= 0.163 +/- 0.0149(stat) +/- 0.023(syst) of the CP conserving charge radius process and from it determined a K(0) charge radius of = [-0.077 +/- 0.007(stat) +/- 0.011(syst)]fm(2). We have determined a first experimental upper limit of 0.04 (90% C.L.) /g(e1)/ / /g(M1)/ of the couplings for the E1 and M1 direct photon emission processes. We also report the measurement of /g(M1)/ including a vector form factor /g(M1)/(1 + (a(1)/a(2))/((M(2)(p)-(M(2)(k))= 2M(K)E(gamma*)), where vector /g(M1)/= 1.11+/- 0.12(stat) +/- 0.08(syst) and a(1)/a(2) = [-0.744 +/- 0.027(stat) +/- 0.032(syst)] GeV(2)/c(2). Finally, a CP-violating asymmetry of [13.6 +/- 1.4(stat) +/- 1.5(syst)]% in the CP and T odd angle phi between the decay planes of the e(+) e(-) and pi(+) pi(-) pairs in the K(L) center of mass is reported. 相似文献
265.
Abouzaid E Arenton M Barker AR Bellantoni L Blucher E Bock GJ Cheu E Coleman R Corcoran MD Cox B Erwin AR Escobar CO Glazov A Golossanov A Gomes RA Gouffon P Hsiung YB Jensen DA Kessler R Kotera K Ledovskoy A McBride PL Monnier E Nguyen H Niclasen R Phillips DG Ramberg EJ Ray RE Ronquest M Santos E Slater W Smith D Solomey N Swallow EC Toale PA Tschirhart R Wah YW Wang J White HB Whitmore J Wilking MJ Winstein B Winston R Worcester ET Yamanaka T Zimmerman ED Zukanovich RF;KTeV Collaboration 《Physical review letters》2008,100(18):182001
We present a new determination of the parity of the neutral pion via the double Dalitz decay pi0-->e+e-e+e-. Our sample, which consists of 30,511 candidate decays, was collected from KL-->pi0pi0pi0 decays in flight at the KTeV-E799 experiment at Fermi National Accelerator Laboratory. We confirm the negative pi0 parity and place a limit on scalar contributions to the pi0-->e+e-e+e- decay amplitude of less than 3.3% assuming CPT conservation. The pi0gamma*gamma* form factor is well described by a momentum-dependent model with a slope parameter fit to the final state phase-space distribution. Additionally, we have measured the branching ratio of this mode to be B(pi0-->e+e-e+e-)=(3.26+/-0.18)x10(-5). 相似文献
266.
The fracture toughness of CIP-HIP Beryllium was determined using the short bar fracture toughness (KIcSB) method. The KIcSB value measured was 10.96 MPa√m at room temperature. This falls well within the expected range of 9 to 12 MPa√m as observed from previous fracture toughness measurements of beryllium. Toughness increased rapidly between 400°F and 500°F reaching a value of 16.7 MPa√m at 500°F. 相似文献
267.
268.
Dieter Seebach H. Michael Bürger Hans-Martin Müller Urs D. Lengweiler Albert K. Beck Kathy E. Sykes Phil A. Barker Peter J. Barham 《Helvetica chimica acta》1994,77(4):1099-1123
Repetitive treatment of the biopolymer P(3-HB) (molecular weight > 105 Dalton, storage or s-P(3-HB)), with lithium hexamethyl disilazanid (LHMDS) at ?70° in THF leads to a mixture of oligomers with increasingly sharp distribution around a 15-, 30-, and 45mer. Discrete fragments are also isolated when P(3-HB) is heated under reflux (89°) in neat Et3N. Linear oligo(3-HB) derivatives ( 3-7 ) containing up to 96 3-HB units are synthesized using an exponential segment-coupling strategy. These oligomers are used to calibrate size-exclusion chromatography columns for the analysis of oligo(3-HB) samples from the different sources. The linear oligo-(3-HB) derivatives also served as a model with respect to the physical properties of high molecular weight P(3-HB) and were investigated as such by transmission electron microscopy (TEM) and by small- and wide-angle X-ray scattering (SAXS and WAXS). The thicknesses of the lamellar crystallites (long periods) formed by the 8mer, 16mer, and 32mer, are ca. 26, 52, and 53 Å, respectively, indicating that the 32mer molecules are folded once, very tightly, into a ‘hair-pin’-type conformation. High-molecular-weight P(3-HB), which was crystallized in a similar way, also has a lamellar crystallite thickness of ca. 50–65 Å. Thus, the treatment of P(3-HB) with LHMDS at low temperature causes etching of the amorphous regions, an effect well known in polymer science for studying the regularity of chain folding. The ca. 50-Å packing within the tight folds of P(3-HB) is discussed in view of its possible function in ion transport through cell membranes. 相似文献
269.
Summary A moving column-bundle counter-current chromatographic system consisting of twelve 0.9 cm id×72.5 cm long glass columns has been used for the separation of glucose-starch and glucose-maltose mixtures. The system was packed with Spherosil XOBO75 for the glucose-starch separation and with Dowex-5OW-X4K+ for the glucose-maltose separation.Purities of up to 100% were obtained and the operating throughputs were up to 4.4 kg sugar solids/m3 resin/hr. 相似文献
270.
Absolute rate constants at room temperature for the metathesis reaction have been measured under VLPP conditions: k1 = (2.0 ± 0.5) × 108M?1·s?1, k2 = (3.0 ± 0.7) × 108M?1·s?1. The radicals were generated through collisionless infrared-multiphoton decomposition of the corresponding iodides by irradiation from a high-power CO2-TEA laser. The reaction of ?2F5 and ?3F7 with \documentclass{article}\pagestyle{empty}\begin{document}$$\mathop {\rm N}\limits^{\rm .} {\rm O}_{\rm 2} $$\end{document} are briefly discussed in relation to the reaction of ?3 with \documentclass{article}\pagestyle{empty}\begin{document}$$\mathop {\rm N}\limits^{\rm .} {\rm O}_{\rm 2} $$\end{document}, which had been measured previously. 相似文献