全文获取类型
收费全文 | 626篇 |
免费 | 25篇 |
国内免费 | 7篇 |
专业分类
化学 | 493篇 |
晶体学 | 3篇 |
力学 | 23篇 |
数学 | 78篇 |
物理学 | 61篇 |
出版年
2023年 | 3篇 |
2022年 | 10篇 |
2021年 | 14篇 |
2020年 | 16篇 |
2019年 | 17篇 |
2018年 | 6篇 |
2017年 | 12篇 |
2016年 | 25篇 |
2015年 | 19篇 |
2014年 | 16篇 |
2013年 | 36篇 |
2012年 | 28篇 |
2011年 | 55篇 |
2010年 | 29篇 |
2009年 | 35篇 |
2008年 | 47篇 |
2007年 | 40篇 |
2006年 | 36篇 |
2005年 | 18篇 |
2004年 | 25篇 |
2003年 | 23篇 |
2002年 | 12篇 |
2001年 | 7篇 |
2000年 | 9篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1997年 | 12篇 |
1996年 | 5篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1987年 | 3篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1971年 | 2篇 |
1969年 | 2篇 |
1967年 | 3篇 |
1943年 | 2篇 |
1938年 | 2篇 |
排序方式: 共有658条查询结果,搜索用时 500 毫秒
81.
The chemical state of gold in gold ores, mattes and roaster products has been studied by197Au Mössbauer spectroscopy. Gold minerals were studied in order to provide reference data for the ore spectra.57Fe Mössbauer spectroscopy was used to monitor the iron compounds always present in gold ores and to follow their transformations caused by smelting and roasting. 相似文献
82.
Catherine E. Housecroft Ronald Snaith Karen Moss Robert E. Mulvey Marion E. ONeill Kenneth Wade 《Polyhedron》1985,4(11):1875-1881
Cluster bond enthalpies, EL(BB), and orders, n?(BB), for the structurally characterised closo anions, BnHn2? (n = 6 and 8–12), have been estimated using the logarithmic length—enthalpy and enthalpy—order relationships EL(BB) (kJ mol?1) = 1.766 × 1011 [L(BB)]?4.0 and EL(BB) (kJ mol?1) = 318.8[n?(BB)]0.697, respectively. In a parallel study, the molecular-orbital bond index CNDO-based calculation method has been used to give BB and BH bond indices, I(BB) and I(BH), from which bond index based bond enthalpies, EI, have been calculated using the relationships EI(BB) = 297.9 I(BB) and EI(BH) = 374.8I(BH) (enthalpies in kJ mol?1; lengths in pm). From these, total skeletal bond enthalpies Σ E(BB), and total bond enthalpies, Σ E(BB) + Σ E(BH), have been calculated. Although calculated values of EL and Σ EL generally exceed those of EI and Σ EI by some 8% and calculated values of I generally exceed those of n? by a greater amount, the trends in these parameters for the series of BnHn2? anions are very similar, showing the greater efficiency with which the n + 1 skeletal electron pairs are used as n increases. However, the two approaches differ in that, whereas the Σ EI values suggest that the anions are all of comparable stability, the ΣEL values clearly show B6H62?, B10H102? and B12H122? to be more stable than B8H82?, B9H92? and B11H112?. The interatomic distances in B7H72? and in the unknown B5 H52? are estimated and used to assess their relative stabilities. The EL values suggest that B7 H72? is of comparable stability to B8H82? etc., but show B5H52? as relatively unstable. The EI values suggest that both of these anions should be relatively stable members of the series of closo anions. 相似文献
83.
We study the conformational behavior of DNA molecules adsorbed on cationic-lipid membranes that are supported on grooved, one-dimensionally periodic microstructured surfaces. We reveal a striking ability of these periodically structured membranes to stretch DNA coils. We elucidate this DNA stretching phenomenon in terms of surface curvature dependent potential energy attained by the adsorbed DNA molecules. Because of it, DNA molecules undergo a localization transition causing them to stretch by binding to highly curved sections of the supported membranes. 相似文献
84.
85.
Ultrasonic imaging is often used to estimate blood flow velocity. Currently, estimates are carried out using Doppler-based techniques. However, there are a number of shortcomings such as the limited spatial resolution and the inability to estimate longitudinal flows. Thus, alternative methods have been proposed to overcome them. Difficulties are notably encountered with high-frequency imaging systems that use swept-scan techniques. In this article, we propose to compare four vector velocity estimation methods that are complementary to Doppler, focusing on 40 MHz, high-frequency imaging. The goal of this study is to evaluate which method could circumvent the limitations of Doppler methods for evaluation of microcirculation, in the vessels having diameter on the order of 1 mm. We used two region-based approaches, one decorrelation-based approach and one spatiotemporal approach. Each method has been applied to seven flow sequences with various orientations and mean velocities. Four sequences were simulated with a system approach based on a 3D set of moving scatterers. Three experimental sequences were carried out by injecting blood-mimicking fluid within a gelatin phantom and then acquiring images with Visualsonics, Vevo 660 system. From velocity estimates, several performance criteria such as the normalized mean error or the normalized mean standard deviation were defined to compare the performance of the four estimators. The results show that region-based methods are the most accurate exhibiting mean errors less than 10% and mean standard deviation less than 13%. However, region-based approaches are those that require the highest calculative cost compared to the decorrelation-based method, which is the fastest. Finally, the spatiotemporal approach appeared to be a trade-off in terms of computational complexity and accuracy of estimates. It provides estimates with errors less than 10% for mean velocity and the CPU time is approximately 17 s for a ROI of size 40 * 80 pixels. 相似文献
86.
de la Garza L Saponjic ZV Dimitrijevic NM Thurnauer MC Rajh T 《The journal of physical chemistry. B》2006,110(2):680-686
Control of surface states of titanium dioxide nanoparticles using 2-(3,4-dihydroxyphenyl)ethylamine (dopamine) and 3,4-dihydrophenylacetic acid, which act as ligands to the undercoordinated surface sites (carrier traps), is demonstrated by electrochemical techniques. The deepest traps were found to be most reactive and are selectively removed by the addition of the ligands which enhances the kinetics of electron accumulation in the film. Furthermore, a shift in the Fermi level to more positive potentials was detected for electrodes modified with the negatively charged ligand (3,4-dihydrophenylacetic acid) compared to that of electrodes modified with the positively charged ligand (dopamine). The presence of the negative charge on the ligand also contributed to the underpotential of hydrogen evolution on 3,4-dihydrophenylacetic acid-modified electrodes. 相似文献
87.
Claude Marion 《代数通讯》2013,41(3):853-925
Let p 1, p 2, p 3 be primes. This is the second article in a series of three on the (p 1, p 2, p 3)-generation of the finite projective special unitary and linear groups PSU3(p n ), PSL3(p n ), where we say a noncyclic group is (p 1, p 2, p 3)-generated if it is a homomorphic image of the triangle group T p 1, p 2, p 3 . This paper is concerned with the case where p 1 = 2 and p 2 = p 3. We determine for any prime p 2 the prime powers p n such that PSU3(p n ) (respectively, PSL3(p n )) is a quotient of T = T 2, p 2, p 2 . We also derive the limit of the probability that a randomly chosen homomorphism in Hom(T, PSU3(p n )) (respectively, Hom(T, PSL3(p n ))) is surjective as p n tends to infinity. 相似文献
88.
This paper reports the feasibility of nano-oxide precipitate formation in Fe–Cr alloy by ion implantation synthesis. High contents of Al+ and O+ ions were implanted into thin films of high purity Fe–10%Cr alloy at room temperature and were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). In contrast, to the common two-stage implantation/annealing scheme of precipitate ensemble synthesis by ion beams, cluster formation took place at the implantation stage in our study, requiring no subsequent high-temperature annealing. The post-implantation microstructural examination revealed in the as-implanted thin foil an array of precipitates with diameters in the range of 3–30?nm. The precipitate number density distribution was found to depend on the foil thickness. The precipitate enrichment with both Al and O was confirmed by the energy-filtered TEM analysis. Judging from the electron diffraction pattern and high-resolution TEM analysis, the crystal lattice of precipitates corresponds to some cubic modification of aluminium-rich oxide or pure aluminium oxide. The precipitate lattice alignment with the host matrix was revealed for at least a part of precipitates. The analysis of APT data using cluster detection algorithm indicates the presence of local zones enriched in Al and O, even in those areas of as-implanted samples where no clusters were visible by TEM. 相似文献
89.
Dr. Matthew D. Shannon Dr. Theint Theint Dr. Dwaipayan Mukhopadhyay Dr. Krystyna Surewicz Prof. Witold K. Surewicz Dr. Dominique Marion Dr. Paul Schanda Prof. Christopher P. Jaroniec 《Chemphyschem》2019,20(2):311-317
Microsecond to millisecond timescale backbone dynamics of the amyloid core residues in Y145Stop human prion protein (PrP) fibrils were investigated by using 15N rotating frame (R1ρ) relaxation dispersion solid-state nuclear magnetic resonance spectroscopy over a wide range of spin-lock fields. Numerical simulations enabled the experimental relaxation dispersion profiles for most of the fibril core residues to be modelled by using a two-state exchange process with a common exchange rate of 1000 s−1, corresponding to protein backbone motion on the timescale of 1 ms, and an excited-state population of 2 %. We also found that the relaxation dispersion profiles for several amino acids positioned near the edges of the most structured regions of the amyloid core were better modelled by assuming somewhat higher excited-state populations (∼5–15 %) and faster exchange rate constants, corresponding to protein backbone motions on the timescale of ∼100–300 μs. The slow backbone dynamics of the core residues were evaluated in the context of the structural model of human Y145Stop PrP amyloid. 相似文献
90.
Wen Liang Mohamed N. Rahaman Delbert E. Day Nicholas W. Marion Gwendolen C. Riley Jeremy J. Mao 《Journal of Non》2008,354(15-16):1690-1696
Borate glass particles and microspheres with size distributions in the range of approximately 100–400 μm, were loosely compacted and sintered for 10 min at 600 °C to form a porous, three-dimensional construct (porosity 25–40%). Conversion of the borate glass to hydroxyapatite was investigated by soaking the constructs in a solution of K2HPO4 (0.25 M) at 37 °C and with a pH value of 9.0, and measuring the weight loss of the constructs as a function of time. Almost full conversion of the borate glass to hydroxyapatite was achieved in less than 6 days. X-ray diffraction revealed an initially amorphous product that subsequently crystallized to hydroxyapatite. The biocompatibility of the porous constructs was investigated by in vitro cell culture with human mesenchymal stem cells derived from bone marrow (bMSC) and human mesenchymal stem cell derived osteoblasts (MSC-Ob). The cells adhered to the scaffolds and the MSC-Obs produced alkaline phosphatase which is an indication of osteogenic differentiation. The data suggest strong bioactive characteristics for the borate glass constructs and the potential use of the constructs as scaffolds for tissue engineering of bone. 相似文献