全文获取类型
收费全文 | 103266篇 |
免费 | 17196篇 |
国内免费 | 11816篇 |
专业分类
化学 | 72423篇 |
晶体学 | 1135篇 |
力学 | 5876篇 |
综合类 | 666篇 |
数学 | 12617篇 |
物理学 | 39561篇 |
出版年
2024年 | 351篇 |
2023年 | 2067篇 |
2022年 | 3667篇 |
2021年 | 3768篇 |
2020年 | 4175篇 |
2019年 | 3957篇 |
2018年 | 3713篇 |
2017年 | 3333篇 |
2016年 | 5013篇 |
2015年 | 4923篇 |
2014年 | 6049篇 |
2013年 | 7775篇 |
2012年 | 9275篇 |
2011年 | 9471篇 |
2010年 | 6621篇 |
2009年 | 6354篇 |
2008年 | 6676篇 |
2007年 | 5965篇 |
2006年 | 5578篇 |
2005年 | 4658篇 |
2004年 | 3646篇 |
2003年 | 2928篇 |
2002年 | 2661篇 |
2001年 | 2230篇 |
2000年 | 1945篇 |
1999年 | 2050篇 |
1998年 | 1735篇 |
1997年 | 1559篇 |
1996年 | 1555篇 |
1995年 | 1368篇 |
1994年 | 1239篇 |
1993年 | 1003篇 |
1992年 | 901篇 |
1991年 | 776篇 |
1990年 | 652篇 |
1989年 | 503篇 |
1988年 | 378篇 |
1987年 | 338篇 |
1986年 | 335篇 |
1985年 | 262篇 |
1984年 | 188篇 |
1983年 | 153篇 |
1982年 | 125篇 |
1981年 | 78篇 |
1980年 | 59篇 |
1979年 | 26篇 |
1978年 | 22篇 |
1976年 | 22篇 |
1975年 | 22篇 |
1957年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
In this study, local least squares (LLS) and principal component analysis (PCA) were applied to deal with the disturbances in a data set of chromatographic fingerprints after necessary data transformations. It has been demonstrated that PCA with standard normal variate (SNV) transformation of data led to meaningful classification of 33 different Erigeron breviscapus herbal samples. The result was also corroborated by variance squares discriminant method. The quality of herbal objects was further evaluated, and the causes of this fact have been explained from a chemical point of view. At the same time, it implied an idea for qualitative evaluation of the herbal objects with a common class pattern of chromatographic fingerprints. 相似文献
982.
桑蚕丝素蛋白初始结构对其矿化作用的影响 总被引:4,自引:0,他引:4
以碱金属离子诱导桑蚕丝素蛋白溶液发生构象转变, 研究了蛋白质初始结构对其矿化作用的影响. FT-IR, XRD和SEM等测试结果显示, 未经任何处理的桑蚕丝素蛋白溶液矿化后形成片状复合物, 其无机相以二水磷酸氢钙(DCPD)为主; 而经过K+和Na+金属离子处理后, 桑蚕丝素溶液的结构由无规线团/螺旋构象向β-折叠发生转变, 矿化后成纤维状, 并相互结合呈现纳米级的三维多孔结构, 其无机相以热力学稳定的羟基磷灰石(HA)为主. 可以认为, 丝素蛋白结构转化为较伸展的β-折叠后, 使得更多的亲水基团暴露在外面, 在丝素蛋白分子不断凝聚成纤过程中, HA结晶快速生长并附着在这些微纤上, 最终形成纤维状的丝素蛋白/HA复合物. 该结果为阐明蛋白质的生物矿化过程及其调控机理提供了理论依据, 同时可以从矿化复合物的形成来反映这些微量元素可能对骨组织形成的影响, 为临床骨组织的修复提供一定的参考. 相似文献
983.
The complexes between R3Tr (Tr = B, Al, and Ga; R = H, F, Cl, and Br) and H2X (X = O, S, and Se) were theoretically studied. The interaction energies of R3Al⋯H2X and R3Ga⋯H2X are consistent with the electronegativity of the halogen atom R (R ≠ H), but an opposite dependence is found for R3B⋯H2X. The triel bond of R3Tr⋯H2X is weaker for the heavier chalcogen donor. The dependence of triel bonding strength on the triel atom is complicated, depending on the nature of R and X. The methyl substitution of H2X causes a substantial increase in the interaction energy from −5.74 kcal/mol to −22.88 kcal/mol, and its effect is relevant to the nature of Tr, X, and R groups. For the S and Se donors, the increased percentage of interaction energy is almost the same due to the methyl substitution, which is larger than that of the O analogue. In most triel-bonded complexes, electrostatic dominates and polarization has comparable contribution. However, polarization plays a dominant role in R3B⋯ and R3B⋯ (R = Cl and Br; R′ = H and Me). 相似文献
984.
We reported a simple and universal strategy for DNA-mediated assembly of CdTe quantum dots (QDs) and lanthanide-doped upconversion nanoparticles (UCNPs). Such DNA-QD/UCNPs heterostructures not only maintains both fluorescent properties of QDs and upconversion luminescence behaviors of UCNPs, but also offers a polyvalent DNA surface, allowing for targeted dual-modality imaging of cancer cells using an aptamer 相似文献
985.
A reversed-phase liquid chromatographic (RPLC) method has been developed for determination of arabinosylcytosin (Ara-C) and
its metabolite 1-β-D-arabinofuranosyluracil (Ara-U) on a 250 mm × 4.6 mm i.d., 5 μm particle, Diamonsil C18 column. The mobile phase was a mixture of 5% methanol and 95% 10 mmol L−1 phosphate buffer adjusted to pH 5.5. The flow-rate was 1.0 mL min−1 and the injection volume 20 μL. Eluting compounds were detected at 270 nm by use of an ultraviolet detector. Under these
LC conditions cyclophosphamide (CTX) and pirarubicin (THP), two other medicines given with Ara-C in clinical treatment, do
not interfere with measurement of Ara-C and Ara-U. Individual calibration plots of peak area against concentration generated
from analysis of standard solutions were used to calculate the concentrations of Ara-C and Ara-U in sample solutions. The
calibration plot was linear in the range 2.5–100 μg mL−1, the average recovery of Ara-C and Ara-U was more than 98% (RSD < 2.5%), and between-day and within-day precision, expressed as relative standard deviation (RSD), were below 4.0%. LOQ for both Ara-C and Ara-U was 2 μg mL−1. The method is rapid, simple, accurate and reproducible, and especially useful for application to patient samples. 相似文献
986.
本文报道了一种新型三维亚磷酸锌[HO(CH2)2NH3]2•[Zn3(HPO3)4]的合成和晶体结构。在它的结构中,ZnO4和HPO3严格按照顶点连接的方式交替相连。骨架结构存在两个沿着不同方向无限延伸的由ZnO4和HPO3组成的四元环链,在[0 1 0]、[0 4 15]和[0 -4 15]三个方向形成了三个交叉8元环孔道,有机胺阳离子起着平衡电荷和稳定骨架的结构。它的晶体数据为:[HO(CH2)2NH3]2•[Zn3(HPO3)4],M=640.21, 正交晶系, Fdd2空间群, a=2.8528, b=0.8426, c=1.6159nm, Z=8, V=3.884nm3, R1=0.0219, wR2=0.0544。 相似文献
987.
Xiang Ming He Jian Jun Li Yan Cai Yaowu Wang Jierong Ying Changyin Jiang Chunrong Wan 《Journal of Solid State Electrochemistry》2005,9(6):438-444
A novel process is proposed for synthesis of spinel LiMn2O4 with spherical particles from the inexpensive materials MnSO4, NH4HCO3, and NH3H2O. The successful preparation started with carefully controlled crystallization of MnCO3, leading to particles of spherical shape and high tap density. Thermal decomposition of MnCO3 was investigated by both DTA and TG analysis and XRD analysis of products. A precursor of product, spherical Mn2O3, was then obtained by heating MnCO3. A mixture of Mn2O3 and Li2CO3 was then sintered to produce LiMn2O4 with retention of spherical particle shape. It was found that if lithium was in stoichiometric excess of 5% in the calcination of spinel LiMn2O4, the product had the largest initial specific capacity. In this way spherical particles of spinel LiMn2O4 were of excellent fluidity and dispersivity, and had a tap density as high as 1.9 g cm–3 and an initial discharge capacity reaching 125 mAh g–1. When surface-doped with cobalt in a 0.01 Co/Mn mole ratio, although the initial discharge capacity decreased to 118 mAh g–1, the 100th cycle capacity retention reached 92.4% at 25°C. Even at 55°C the initial discharge capacity reached 113 mAh g–1 and the 50th cycle capacity retention was in excess of 83.8%. 相似文献
988.
989.
Stanley Bruckenstein Alex Fensore Zhufen Li A.Robert Hillman 《Journal of Electroanalytical Chemistry》1994,370(1-2)
Absolute resonant frequency measurements were made on gold-coated AT-cut quartz crystals with one face in contact with a series of liquids. The effect of surface roughness and liquid properties (viscosity and density) was analyzed in terms of a “trapped liquid” model. In this model, liquid present in surface imperfections is viewed as rigidly coupled mass. In some of the literature this density-dependent, but not viscosity-dependent, term is viewed as being additive to the hydrodynamic shift seen for a smooth surface. Data obtained using 1 μm and 5 μm surface finish crystals are inconsistent with the predictions of the trapped liquid model. This suggests hydrodynamic coupling between liquid internal and external to the crevices. Despite the lack of a theoretical model for the liquid motion, it is possible to compensate for frequency variations resulting from changing liquid properties and for roughness effects by making direct measurements of the resonant frequency difference between two crystals exposed to the same solution. This novel procedure works to the extent that the two crystals have similar surface topographies. Compensation is excellent for 1 μm finish crystals and good for 5 μm finish crystals. 相似文献
990.