全文获取类型
收费全文 | 4063篇 |
免费 | 485篇 |
国内免费 | 487篇 |
专业分类
化学 | 3369篇 |
晶体学 | 49篇 |
力学 | 173篇 |
综合类 | 20篇 |
数学 | 452篇 |
物理学 | 972篇 |
出版年
2024年 | 8篇 |
2023年 | 74篇 |
2022年 | 109篇 |
2021年 | 117篇 |
2020年 | 157篇 |
2019年 | 152篇 |
2018年 | 124篇 |
2017年 | 88篇 |
2016年 | 172篇 |
2015年 | 143篇 |
2014年 | 209篇 |
2013年 | 254篇 |
2012年 | 343篇 |
2011年 | 325篇 |
2010年 | 222篇 |
2009年 | 198篇 |
2008年 | 231篇 |
2007年 | 227篇 |
2006年 | 217篇 |
2005年 | 180篇 |
2004年 | 157篇 |
2003年 | 166篇 |
2002年 | 178篇 |
2001年 | 142篇 |
2000年 | 115篇 |
1999年 | 138篇 |
1998年 | 83篇 |
1997年 | 64篇 |
1996年 | 98篇 |
1995年 | 73篇 |
1994年 | 37篇 |
1993年 | 43篇 |
1992年 | 28篇 |
1991年 | 31篇 |
1990年 | 30篇 |
1989年 | 21篇 |
1988年 | 17篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1970年 | 1篇 |
1963年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有5035条查询结果,搜索用时 62 毫秒
91.
青海柴达木地区植物白刺叶中微量元素特征 总被引:7,自引:0,他引:7
青海柴达木地区分布有丰富的白刺植物资源。利用220FS原子吸收光谱仪分析了该地区三种白刺叶片的Cu,Zn,Fe,Mn,Cr,Ni,Cd等微量元素。结果表明,铬、 锰含量显著提高是青海柴达木地区植物白刺叶资源微量元素特征。这为柴达木地区白刺叶资源的开发利用提供科学依据。 相似文献
92.
Feng‐You Hao Yong‐Fang Zhao Xiao‐Gong Jing Xin‐Ying Li Feng‐Li Liu 《International journal of quantum chemistry》2007,107(6):1502-1507
Ab initio quantum chemical calculations have been performed on X2Cl? and X2Cl (X = C, Si, Ge) clusters. The geometrical structures, vibrational frequencies, electronic properties and dissociation energies are investigated at the Hartree–Fock (HF), Møller–Plesset second‐ and fourth‐order (MP2, MP4), CCSD(T) level with the 6‐311+G(d) basis set. The X2Cl (X = C, Si, Ge) and X2Cl? (X = Si, Ge) take a bent shape obtained at the ground state, while C2Cl? has a linear structure. The impact on internal electron transfer between the X2Cl and the corresponding anional clusters is studied. The three different types of electron affinities (EAs) at the CCSD(T) are reported. The most reliable adiabatic electronic affinities, obtained at the CCSD(T)/cc‐pvqz level of theory, are predicted to be 3.30, 2.62, and 1.98 eV for C2Cl, Si2Cl, and Ge2Cl, respectively. The calculated EAs of C2Cl and Ge2Cl are in good agreement with theoretical results reported. The correlation effects and basis sets effects on the geometrical structures and dissociation energies are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
93.
Single nucleotide polymorphisms (SNPs) are associated with many human diseases, so accurate and efficient SNP detection is of great significance for early diagnosis and clinical prognosis. This report proposes a universal and high-fidelity genotyping method in microfluidic point-of-care equipment based on the clustered regularly interspaced short palindromic repeat (CRISPR) system. Briefly, by systematically inserting the protospacer-adjacent-motif (PAM) sequence, we improved the universality of the CRISPR/Cas12a based SNP detection; by removing the complementary ssDNA and introducing an additional nucleotide mismatch, we improved the sensitivity and specificity. We preloaded the CRISPR/Cas12a reagents into the point-of-care biochip for automating the process, increasing the stability and long-term storage. This biochip enables us to rapidly and conveniently detect the genotypes within 20 min. In a practical application, the CRISPR/Cas12a biochip successfully distinguished three genotypes (homozygous wild type; the homozygous mutant type; and the heterozygous mutant type) of the CYP1A1*2 (A4889G, rs1048943), CYP2C19*2 (G681A, rs4244285), CYP2C9*3 (A1075C, rs1057910), and CYP2C19*3 (G636A, rs4986893) genes related to multiple cancers from 17 clinical blood samples. This CRISPR/Cas12a-based SNP genotyping method, being universal, accurate, and sensitive, will have broad applications in molecular diagnostics and clinical research.A universal and high-fidelity genotyping method based on the clustered regularly interspaced short palindromic repeat (CRISPR) system was performed on the microfluidic point-of-care equipment. 相似文献
94.
Abthagir PS Ha YG You EA Jeong SH Seo HS Choi JH 《The journal of physical chemistry. B》2005,109(50):23918-23924
The neutral cluster beam deposition (NCBD) method has been applied to produce and characterize organic thin-film transistors (OTFTs) based upon tetracene and pentacene molecules as active layers. Organic thin films were prepared by the NCBD method on hexamethyldisilazane (HMDS)-untreated and -pretreated silicon dioxide (SiO2) substrates at room temperature. The surface morphology and structures for the tetracene and pentacene thin films were examined by atomic force microscopy (AFM) and X-ray diffraction (XRD). The measurements demonstrate that the weakly bound and highly directional neutral cluster beams are efficient in producing high-quality single-crystalline thin films with uniform, smooth surfaces and that SiO2 surface treatment with HMDS enhances the crystallinity of the pentacene thin-film phase. Tetracene- and pentacene-based OTFTs with the top-contact structure showed typical source-drain current modulation behavior with different gate voltages. Device parameters such as hole carrier mobility, current on/off ratio, threshold voltage, and subthreshold slope have been derived from the current-voltage characteristics together with the effects of surface treatment with HMDS. In particular, the high field-effect room-temperature mobilities for the HMDS-untreated OTFTs are found to be comparable to the most widely reported values for the respective untreated tetracene and pentacene thin-film transistors. The device performance strongly correlates with the surface morphology, and the structural properties of the organic thin films are discussed. 相似文献
95.
In this paper, the authors study the multiplicity of solutions to the weighted p-Laplacian with isolated singularity and di?usion suppressed by convection ?div(|x|α|?u|p?2?u) + λ 1/|x|β |?u|p?2?u · x = |x|γg(|x|) in B \ {0} subject to nonlinear Robin boundary value condition |x|α|?u|p?2?u · ?n = A ? ρu on ?B,where λ > 0, B ? RN(N ≥ 2) is the unit ball centered at the origin, α > 0, p > 1, β ∈ R, γ > ?N, g ∈ C([0,1]) with g(0) > 0, A ∈ R, ρ > 0 and ?n is the unit outward normal. The same problem with di?usion promoted by convection, namely λ ≤ 0, has already been discussed by the last two authors (Song-Yin (2012)), where the existence, nonexistence and classi?cation of singularities for solutions are presented. Completely di?erent from [Song, H. J. and Yin, J. X., Removable isolated singularities of solutions to the weighted p-Laplacian with singular convection, Math. Meth. Appl. Sci., 35, 2012, 1089–1100], in the present case λ > 0, namely the di?usion is suppressed by the convection, non-singular solutions are not only existent but also may be in?nite which vary according only to the values of solutions at the isolated singular point. At the same time, the singular solutions may exist only if the di?usion dominates the convection. 相似文献
96.
As a continuation of part I of the paper under the same title, we developgeneral monotonic enclosure methods for the couple systems of the splitting equations {x=G([x]a,[x]b,[y]c) y=G([y]a,[y]b,[x]c),which models the system of equations associated with hybrid and aaynchronotts monotonicity as well as convexity. The resulting algorithms and convergence theorems generalize and unify various known methods and monotonic enclosure theorents established by other authors. 相似文献
97.
X. H. Yan J. R. Yan J. Q. You Y. P. Mei 《Zeitschrift für Physik B Condensed Matter》1994,94(4):439-446
Electronic properties of a general class of one-dimensional two-tile systems are calculated exactly. The systems containing periodic crystals, generalized Fibonacci quasicrystals, generalized Thue-Morse aperiodic lattices and even other two-tile aperiodic lattices, can be divided into two different families which are constructed by the inflation rules: {A, B}{A
m11
B
m12,A
m21
B
m22} and {A, B}{A
n11
B
n12,B
n21
A
n22}, respectively. As typical examples, global spectra of bands and density of states in some two-tile aperiodic systems are numerically calculated. Some interesting properties are obtained. 相似文献
98.
99.
In a previous paper, a scheme of integral approximation was proposed, in which a large number of two-electron integrals containing a certain kind of orbital pairs, i.e., weakly related pairs, can be neglected. A new criterion for partitioning orbital pairs into two groups, i.e., strongly and weakly related pairs, is introduced in order to allow for neglect of more integrals without sacrificing the accuracy of a computation. Test calculations on the BH3? C2H4 complex show that by the use of the new version, results of roughly the same accuracy as was obtained by the old version is obtained by retaining 80% of the integrals used in the old version. This kind of saving is more significant for larger molecules. For example, in a calculation on Cu-porphine with 203 CGTO 'S , the revised version will require about 2.0 × 106 integrals, while the original version will require about 4.5 × 106. And these two approximate calculations are expected to give the results of roughly the same accuracy. 相似文献
100.
You WK So SH Lee H Park SY Yoon MR Chang SI Kim HK Joe YA Hong YK Chung SI 《Experimental & molecular medicine》1999,31(4):197-202
Endostatin, a carboxyl-terminal fragment of collagen XVIII is known as an anti-angiogenic agent, that specifically inhibits the proliferation of endothelial cell and the growth of several primary tumor. We report here the purification and characterization of the recombinant murine endostatin (rmEndostatin) which was expressed in a prokaryotic expression system. This rmEndostatin has similar physiochemical properties of yeast-produced recombinant endostatin, and it also specifically inhibits the proliferation and migration of bovine capillary endothelial cells stimulated by basic fibroblast growth factor. The biological activity of rmEndostatin was also shown by its anti-angiogenic ability on the chorioallantoic membrane of chick embryo in vivo. In this article, we demonstrate the refolding and purification of rmEndostatin, expressed using E. coli system, to a biologically active and soluble form. In addition, these results confirm the activity of endostatin as a potent anti-angiogenic agent. 相似文献