全文获取类型
收费全文 | 2225篇 |
免费 | 57篇 |
国内免费 | 6篇 |
专业分类
化学 | 1371篇 |
晶体学 | 10篇 |
力学 | 39篇 |
数学 | 278篇 |
物理学 | 590篇 |
出版年
2022年 | 23篇 |
2021年 | 31篇 |
2020年 | 35篇 |
2019年 | 17篇 |
2018年 | 24篇 |
2016年 | 39篇 |
2015年 | 32篇 |
2014年 | 46篇 |
2013年 | 82篇 |
2012年 | 113篇 |
2011年 | 146篇 |
2010年 | 55篇 |
2009年 | 54篇 |
2008年 | 103篇 |
2007年 | 107篇 |
2006年 | 114篇 |
2005年 | 111篇 |
2004年 | 107篇 |
2003年 | 106篇 |
2002年 | 90篇 |
2001年 | 61篇 |
2000年 | 49篇 |
1999年 | 16篇 |
1998年 | 21篇 |
1997年 | 21篇 |
1996年 | 36篇 |
1995年 | 20篇 |
1994年 | 38篇 |
1993年 | 41篇 |
1992年 | 41篇 |
1991年 | 41篇 |
1990年 | 28篇 |
1989年 | 25篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 22篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 11篇 |
1973年 | 24篇 |
1967年 | 10篇 |
排序方式: 共有2288条查询结果,搜索用时 15 毫秒
931.
Tran M. Phung Jacob M. Jensen David C. Johnson John J. Donovan Brian G. McBurnett 《X射线光谱测定》2008,37(6):608-614
The homogeneous bulk assumption used in traditional electron probe microanalysis (EPMA) can be applied for thin‐layered systems with individual layers as thick as 50 nm provided the penetration depth of the lowest accelerating voltage exceeds the total film thickness. Analysis of an NIST Ni‐Cr thin film standard on Si using the homogeneous model yielded certified compositions and application of the same model to ultra‐thin Ni‐Si layers on GaAs yielded their expected compositions. In cases where the same element is present in multiple layers or in the substrate as well as the film, the homogeneous assumption in EPMA alone is not sufficient to determine composition. By combining x‐ray reflectivity (XRR) thickness and critical angle data and using an iterative approach, quantitative compositional data in EPMA can be achieved. This technique was utilized to determine the composition of Ni‐Si ultra‐thin films grown on silicon. The Ni‐Si composition determined using this multi‐instrumental technique matched that of Ni‐Si films simultaneously deposited on GaAs. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
932.
Collis JM Siegmann WL Jensen FB Zampolli M Küsel ET Collins MD 《The Journal of the Acoustical Society of America》2008,123(1):51-55
Recent improvements in the parabolic equation method are combined to extend this approach to a larger class of seismo-acoustics problems. The variable rotated parabolic equation [J. Acoust. Soc. Am. 120, 3534-3538 (2006)] handles a sloping fluid-solid interface at the ocean bottom. The single-scattering solution [J. Acoust. Soc. Am. 121, 808-813 (2007)] handles range dependence within elastic sediment layers. When these methods are implemented together, the parabolic equation method can be applied to problems involving variations in bathymetry and the thickness of sediment layers. The accuracy of the approach is demonstrated by comparing with finite-element solutions. The approach is applied to a complex scenario in a realistic environment. 相似文献
933.
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
934.
We discuss the three-body decay mechanisms of many-body resonances. Sequential decays proceed via two-body configurations after emission of the third particle. In direct decay all three particles leave their interaction regions simultaneously. The intermediate paths within the interaction regions are not observables and are only accessible through models. The momentum distributions carry, apart from polarization, all possible information about decay modes and resonance structure. In this context we discuss detailed results for the decay of the (9)Be(5/2(-)) resonance. 相似文献
935.
Klaus Angermund Gerhard Fink Vidar R. Jensen Ralph Kleinschmidt 《Macromolecular rapid communications》2000,21(2):91-97
Comparison of pentad distributions obtained from NMR spectra and from a molecular mechanics‐based modeling approach is performed for the catalysts {iPr(3‐X‐CpFlu)}ZrCl2 (X = H, Me, Et, iPr, tBu) at a range of different temperatures. In order to model the temperature dependency of the pentad distributions the variation in steric influence along with the change of the rotational energy level for catalysts with substituents displaying relatively low barriers to rotation is treated approximately by calculating energy profiles of 360° rotation of the alkyl groups. The temperature at which intermediate chain migration (back‐skip) or chain epimerization starts to be important seems to be rather constant (30–50°C) among the five catalysts. Even in the case of X = tBu, back‐skip seems to be unimportant for explaining the formation of isotactic polymer at room temperature. 相似文献
936.
Morten K. Srensen Nicholas M. Balsgart Michael Beyer Ole N. Jensen Niels Chr. Nielsen 《Molecules (Basel, Switzerland)》2022,27(3)
Robust and easy-to-use NMR sensor technology is proposed for accurate, on-site determination of fat and protein contents in milk. The two parameters are determined using fast consecutive 1H and 35Cl low-field NMR experiments on milk samples upon the 1:1 addition of a low-cost contrast solution. Reliable and accurate measurements are obtained without tedious calibrations and the need for extensive database information and may readily be conducted by non-experts in production site environments. This enables on-site application at farms or dairies, or use in laboratories harvesting significant reductions in costs and time per analysis as compared to wet-chemistry analysis. The performance is demonstrated for calibration samples, various supermarket milk products, and raw milk samples, of which some were analyzed directly in the milking room. To illustrate the wide application range, the supermarket milk products included both conventionally/organically produced, lactose-free milk, cow’s, sheep’s and goat’s milk, homogenized and unhomogenized milk, and a broad nutrient range (0.1–9% fat, 1–6% protein). Excellent agreement between NMR measurements and reference values, without corrections or changes in calibration for various products and during extensive periods of experiment conduction (4 months) demonstrates the robustness of the procedure and instrumentation. For the raw milk samples, correlations between NMR and IR, NMR and wet-chemistry, as well as IR and wet-chemistry results, show that NMR, in terms of accuracy, compares favorably with the other methods. 相似文献
937.
Pia Afzelius Malene Kjelin Morsing Ole Lerberg Nielsen Aage Kristian Olsen Alstrup Svend Borup Jensen Lars Jdal 《Molecules (Basel, Switzerland)》2022,27(9)
Background: [18F]FDG and [11C]methionine accumulate in lymph nodes draining S. aureus -infected foci. The lymph nodes were characterized by weight, [11C]methionine- and [18F]FDG-positron emissions tomography (PET)/computed tomography (CT), and immunohistochemical (IHC)-staining. Methods: 20 pigs inoculated with S. aureus into the right femoral artery were PET/CT-scanned with [18F]FDG, and nine of the pigs were additionally scanned with [11C]methionine. Mammary, medial iliac, and popliteal lymph nodes from the left and right hind limbs were weighed. IHC-staining for calculations of area fractions of Ki-67, L1, and IL-8 positive cells was done in mammary and popliteal lymph nodes from the nine pigs. Results: The pigs developed one to six osteomyelitis foci. Some pigs developed contiguous infections of peri-osseous tissue and inoculation-site abscesses. Weights of mammary and medial iliac lymph nodes and their [18F]FDG maximum Standardized Uptake Values (SUVFDGmax) showed a significant increase in the inoculated limb compared to the left limb. Popliteal lymph node weight and their FDG uptake did not differ significantly between hind limbs. Area fractions of Ki-67 and IL-8 in the right mammary lymph nodes and SUVMetmax in the right popliteal lymph nodes were significantly increased compared with the left side. Conclusion: The PET-tracers [18F]FDG and [11C]methionine, and the IHC- markers Ki-67 and IL-8, but not L1, showed increased values in lymph nodes draining soft tissues infected with S. aureus. The increase in [11C]methionine may indicate a more acute lymph node response, whereas an increase in [18F]FDG may indicate a more chronic response. 相似文献
938.
Giovanni Occhipinti Daniel L. Nascimento Marco Foscato Deryn E. Fogg Vidar R. Jensen 《Chemical science》2022,13(18):5107
Ruthenium–cyclic(alkyl)(amino)carbene (CAAC) catalysts, used at ppm levels, can enable dramatically higher productivities in olefin metathesis than their N-heterocyclic carbene (NHC) predecessors. A key reason is the reduced susceptibility of the metallacyclobutane (MCB) intermediate to decomposition via β-H elimination. The factors responsible for promoting or inhibiting β-H elimination are explored via density functional theory (DFT) calculations, in metathesis of ethylene or styrene (a representative 1-olefin) by Ru–CAAC and Ru–NHC catalysts. Natural bond orbital analysis of the frontier orbitals confirms the greater strength of the orbital interactions for the CAAC species, and the consequent increase in the carbene trans influence and trans effect. The higher trans effect of the CAAC ligands inhibits β-H elimination by destabilizing the transition state (TS) for decomposition, in which an agostic MCB Cβ–H bond is positioned trans to the carbene. Unproductive cycling with ethylene is also curbed, because ethylene is trans to the carbene ligand in the square pyramidal TS for ethylene metathesis. In contrast, metathesis of styrene proceeds via a ‘late’ TS with approximately trigonal bipyramidal geometry, in which carbene trans effects are reduced. Importantly, however, the positive impact of a strong trans-effect ligand in limiting β-H elimination is offset by its potent accelerating effect on bimolecular coupling, a major competing means of catalyst decomposition. These two decomposition pathways, known for decades to limit productivity in olefin metathesis, are revealed as distinct, antinomic, responses to a single underlying phenomenon. Reconciling these opposing effects emerges as a clear priority for design of robust, high-performing catalysts.In ruthenium catalysts for olefin metathesis, carbene ligands of high trans influence/effect suppress decomposition via β-H elimination, but increase susceptibility to bimolecular decomposition. 相似文献
939.
We consider asymptotic expansions for sums Sn on the form Sn = ƒ0(X0) + ƒ(X1, X0) + … + ƒ(Xn, Xn−1), where Xi is a Markov chain. Under different ergodicity conditions on the Markov chain and certain conditional moment conditions on ƒ(Xi, Xi−1), a simple representation of the characteristic function of Sn is obtained. The representation is in term of the maximal eigenvalue of the linear operator sending a function g(x) into the function x → E(g(Xi)exp[itƒ(Xi, x)]|Xi−1 = x). 相似文献
940.
Joshua Grabeck Tamara Lützenburg Pia Frommelt Ines Neundorf 《Molecules (Basel, Switzerland)》2022,27(19)
Herein, the design and synthesis of peptide-drug conjugates (PDCs) including different variants of the cell-penetrating peptide sC18 is presented. We first generated a series of novel sequence mutants of sC18 having either amino acid deletions and/or substitutions, and then tested their biological activity. The effects of histidine substituents were found to be not meaningful for sC18 uptake and cell selectivity. Moreover, building a nearly perfect amphipathic structure within a shortened sC18 derivative provided a peptide that was highly membrane-active, but also too cytotoxic. As a result, the most promising analog was sC18ΔE, which stands out due to its higher uptake efficacy compared to parent sC18. In the last set of experiments, we let the peptides react with the cytotoxic drug doxorubicin by Thiol–Michael addition to form novel PDCs. Our results indicate that sC18ΔE could be a more efficient drug carrier than parent sC18 for biomedical applications. However, cellular uptake using endocytosis and resulting entrapment of cargo inside vesicles is still a major critical step to overcome in CPP-containing peptide-drug development. 相似文献