全文获取类型
收费全文 | 671篇 |
免费 | 20篇 |
国内免费 | 10篇 |
专业分类
化学 | 398篇 |
晶体学 | 7篇 |
力学 | 13篇 |
数学 | 165篇 |
物理学 | 118篇 |
出版年
2021年 | 5篇 |
2017年 | 6篇 |
2016年 | 18篇 |
2015年 | 9篇 |
2014年 | 19篇 |
2013年 | 20篇 |
2012年 | 18篇 |
2011年 | 24篇 |
2010年 | 11篇 |
2009年 | 14篇 |
2008年 | 23篇 |
2007年 | 38篇 |
2006年 | 24篇 |
2005年 | 27篇 |
2004年 | 22篇 |
2003年 | 21篇 |
2002年 | 17篇 |
2001年 | 20篇 |
2000年 | 15篇 |
1999年 | 15篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 17篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1973年 | 11篇 |
1972年 | 6篇 |
1967年 | 7篇 |
1958年 | 6篇 |
1956年 | 9篇 |
1928年 | 4篇 |
1927年 | 8篇 |
1924年 | 4篇 |
排序方式: 共有701条查询结果,搜索用时 15 毫秒
31.
32.
33.
Bernhard Post Reinhold Hempfling Horst Klamberg und Hans-Rolf Schulten 《Fresenius' Journal of Analytical Chemistry》1988,331(3-4):273-281
Summary Humic acids prepared from four horizons of a Podzol in the Segeberger Forest as well as different fractions of humic substances (fulvi acid, humic acid, and humin) of the Ahe horizon were characterized by a combination of analytical methods. As basis served the data from elemental analyses, UV/VIS-, FTIR-, and 13C-NMR spectroscopy, gel permeation chromatography using Fractogel TSK, and vapor pressure osmometry. Utilizing pyrolysis in direct coupling with field ionization mass spectrometry, more detailed information about structural features of the humic substances in soils could be obtained. In this manner, the different importance of carbohydrates, aromatic, and lipidic components in the various fractions is observed. The characteristic temperature dependence of the averaged molecular weights of the pyrolyzates indicates a basic structural skeleton, which is quite similar for the humic acids isolated from the different horizons. By degradation of the carbohydrates and increasing incorporation of lipidic components an enhanced biotransformation takes place with increasing depth of the soil profile.
Gewidmet Herrn Professor Dr. W. Fresenius zum 75. Geburtstag 相似文献
Gewidmet Herrn Professor Dr. W. Fresenius zum 75. Geburtstag 相似文献
34.
35.
A novel phenomenon is observed in the dynamics of laser-prepared coherent wave packets, bound by the Coulombic 1/r potential of an ion-pair system. After exciting weakly bound (approximately 3 meV) H(+)(-) wave packets in a Stark field, and permitting them to evolve in time, control of field dissociation via adiabatic and diabatic routes is demonstrated by applying delayed pulsed-electric fields, involving a zero-field crossing. Control manifests itself through the production of ions from each pathway at a different instant in time. This phenomenon is applied to map the oscillatory behavior of an angular momentum wave packet in a heavy Rydberg system. The characteristic frequencies of the observed Stark oscillations verify predicted mass-scaling laws for heavy Rydberg systems. 相似文献
36.
Berardini N Carle R Schieber A 《Rapid communications in mass spectrometry : RCM》2004,18(19):2208-2216
Polyphenolics were extracted from peels, pulp and kernels of mango fruits (Mangifera indica L. cv. 'Tommy Atkins') and characterized by high-performance liquid chromatography/electrospray ionization mass spectrometry. In the peel 18 gallotannins and five benzophenone derivatives were detected which were tentatively identified as galloylated maclurin and iriflophenone glucosides. Twenty-one and eight gallotannins were found in the kernels and pulp, respectively, whereas no evidence for the presence of benzophenone derivatives was obtained. Gallotannins quantified by the rhodanine assay amounted to 1.4 mg/g dm in the peels (expressed as gallic acid), while only small amounts (0.2 mg/g dm) were found in the pulp. In contrast, mango kernels contained 15.5 mg/g dm and thus proved to be a rich source of gallotannins. 相似文献
37.
We retrospectively compared the diagnostic accuracy of abdominal ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) studies performed on the same individual to determine the relative performance of these modalities in the evaluation of disease processes, arising from different intra-abdominal organ systems. We retrospectively reviewed all procedure codes accrued by our abdominal imaging section during a 1-year period to determine how many patients underwent all three imaging procedures in our institution within a 2-week interval. These cases were then further evaluated to determine: (1) the primary organ system of disease involvement, (2) the final diagnosis, and (3) the imaging modality that provided the most accurate information upon which appropriate medical management was based. Imaging findings were determined by review of diagnostic reports, and medical management was determined by chart review. Two thousand six hundred-ninety five patients underwent ultrasound, 4,394 patients underwent CT, and 872 patients underwent MRI for the investigation of abdominal disease. Among these 5,126 patients, 26 underwent sequential US, CT, and MRI evaluation within a two-week interval. Appropriate clinical management was based on the US findings in 12/26 cases (46%; 95% CI: 27-67%), CT findings in 16/26 cases (62%; 95% CI: 41-80%), and MRI findings in 24 cases (92%; 95% CI: 75-99%). Significant differences in directing appropriate clinical management were found when comparing the relative diagnostic accuracies of MRI to US (p = 0.0003) and MRI to CT (p = 0.009). However, differences were not significant when comparing CT to US (p = 0.27). All final diagnoses in a given patient were accurately determined by US in 7/26 cases (27%), by CT in 10/26 cases (38%), and by MRI in 23/26 cases (88%). When analyzed according to individual diagnoses (61 total), US detected 33 of 61 (54%) abnormalities, CT detected 41 (67%) abnormalities, and MRI detected 51 (84%) abnormalities. US correctly characterized 32 (52%) abnormalities, CT correctly characterized 36 (59%) abnormalities, and MRI correctly characterized 49 (80%) abnormalities. Ultrasound, CT, and MR imaging correctly diagnosed the disease process in 3 of 12 (25%), 3 of 12 (25%), and 10 of 12 (83%) patients, respectively, with liver disease; 2 of 9 (22%), 6 of 9 (66%), and 9 of 9 (100%) patients, respectively, with pancreatic disease; 17 of 17 (100%), 8 of 17 (47%), and 10 of 17 (58%) patients, respectively, with gallbladder and biliary disease; 2 of 8 (25%), 5 of 8 (63%), and 5 of 8 (63%) patients, respectively, with renal disease; 0 of 3 (0%), 2 of 3 (66%), and 3 of 3 (100%) patients, respectively, with adrenal disease; and 8 of 12 (75%), 12 of 12 (100%), and 12 of 12 (100%) patients with free intraperitoneal fluid. Our results provide new information, regarding the relative benefits of ultrasound, CT, and MRI for the investigation of abdominal diseases at our institution. This initial data suggests that ultrasound provides the most accurate diagnoses in the investigation of gallbladder disease; MRI provides the most accurate diagnoses in the investigation of hepatic, adrenal, and pancreatic disease; and either CT or MRI may be the most appropriate first imaging study for the detection of renal disease. 相似文献
38.
An unprecedented alpha-C-C agostic interaction in a cyclopropyl tris(pyrazolyl)boratoniobium complex
Jaffart J Etienne M Reinhold M McGrady JE Maseras F 《Chemical communications (Cambridge, England)》2003,(7):876-877
Structural, spectroscopic and theoretical evidence indicate that an unusual alpha-C-C agostic interaction is preferred over both alpha- and beta-C-H agostic alternatives in the title compound, TpMe2NbCl(c-C3H5)(MeCCMe). 相似文献
39.
Helmut?HarbrechtEmail author Sergei?Pereverzev Reinhold?Schneider 《Numerische Mathematik》2003,95(1):123-143
It is well known, that pseudodifferential equations of negative order considered in Sobolev spaces with small smoothness indices are ill–posed. On the other hand, it is known that efficient discretization schemes with properly chosen discretization parameters allow to obtain a regularization effect for such equations. The main accomplishment of the present paper is the principle for the adaptive choice of the discretization parameters directly from noisy discrete data. We argue that the combination of this principle with wavelet–based matrix compression techniques leads to algorithms which are order–optimal in the sense of complexity. 相似文献
40.
We explicitly calculate the normal cones of all monomial primes which define the curves of the form , where . All of these normal cones are reduced and Cohen-Macaulay, and their reduction numbers are independent of the reduction. These monomial primes are new examples of integrally closed ideals for which the product with the maximal homogeneous ideal is also integrally closed.
Substantial use was made of the computer algebra packages Maple and Macaulay2.