全文获取类型
收费全文 | 414065篇 |
免费 | 3895篇 |
国内免费 | 1668篇 |
专业分类
化学 | 207387篇 |
晶体学 | 6056篇 |
力学 | 20378篇 |
综合类 | 7篇 |
数学 | 68311篇 |
物理学 | 117489篇 |
出版年
2019年 | 2556篇 |
2018年 | 12958篇 |
2017年 | 12674篇 |
2016年 | 10708篇 |
2015年 | 4224篇 |
2014年 | 5254篇 |
2013年 | 15291篇 |
2012年 | 14289篇 |
2011年 | 23342篇 |
2010年 | 14059篇 |
2009年 | 14214篇 |
2008年 | 18275篇 |
2007年 | 20386篇 |
2006年 | 11721篇 |
2005年 | 12102篇 |
2004年 | 11076篇 |
2003年 | 10545篇 |
2002年 | 9483篇 |
2001年 | 10023篇 |
2000年 | 7619篇 |
1999年 | 6011篇 |
1998年 | 5128篇 |
1997年 | 5120篇 |
1996年 | 4870篇 |
1995年 | 4636篇 |
1994年 | 4264篇 |
1993年 | 4292篇 |
1992年 | 4697篇 |
1991年 | 4729篇 |
1990年 | 4415篇 |
1989年 | 4407篇 |
1988年 | 4450篇 |
1987年 | 4386篇 |
1986年 | 4155篇 |
1985年 | 5681篇 |
1984年 | 5903篇 |
1983年 | 4918篇 |
1982年 | 5470篇 |
1981年 | 5228篇 |
1980年 | 5171篇 |
1979年 | 5168篇 |
1978年 | 5516篇 |
1977年 | 5257篇 |
1976年 | 5408篇 |
1975年 | 4992篇 |
1974年 | 4960篇 |
1973年 | 5440篇 |
1972年 | 3345篇 |
1971年 | 2567篇 |
1968年 | 2440篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
261.
Proton magnetic resonance imaging was performed on rats before induction of diabetes with streptozotocin (STZ) and at 2 and 12 days postinduction. Images revealed an increase in maximal longitudinal and axial dimensions of the kidneys at 2 days and a further increase at 12 days. Similarly, an increase in the size of the remaining kidney was seen in a rat which underwent uninephrectomy as a positive control. Two major differences were observed between the kidney undergoing compensatory hypertrophy and those developing diabetic nephropathy: (i) Expansion of the renal vasculature was seen only in images of the diabetic rat; (ii) A loss in conspicuity of the normal corticomedullary junction was seen in the T2-weighted images of the diabetic rat but not in the uninephrectomized rat. Histologic examination revealed that the medulla increased to a size greater than the cortex during diabetic nephropathy whereas the medullary volume was less than that of the cortex during compensatory hypertrophy. In vitro T1 relaxation times in cortex, outer medulla and inner medulla of kidneys from control rats were measured and compared with the same respective regions in diabetic rats. When these values were correlated with tissue water content, a linear increase in relaxation rate versus percent water content from cortex to inner medulla was found in the control kidneys, but this correlation was absent in diabetic nephropathy. These studies demonstrate that MRI is an effective noninvasive tool for studying the course of renal hypertrophy and hydration changes in the development of renal disease in STZ-induced diabetes in the rat. 相似文献
262.
Joseph B. Schlenoff W. Jack Rink Lawrence Seger 《Physica C: Superconductivity and its Applications》1991,180(5-6):387-393
Single crystals and polycrystalline pellets of the high-temperature cuprate superconductor Bi2Sr2Ca1Cu2O8 were doped at room temperature by electrochemical reduction at > 95% Coulombic efficiency using lithium dopant ions in propylene carbonate electrolyte. Cyclic voltammetry and potential step measurements on single crystals suggest an unusual reduction mechanism, with a diffusion coefficient for Li+ in the c-axis direction of bulk superconductor of ca. 3 × 10−11 cm2s−1. Sintered pellets of polycrystalline powder could be doped more rapidly, with an apparent diffusion coefficient of 7 × 10−8 cm2s−1. X-ray susceptibility analysis show extensive disordering occurs on heavy Li doping, with a first-order transition from a crystalline/superconducting to an amorphous/non-superconducting phase. Single, crystals of Bi2Sr2Ca1Cu2O8 exhibited a color change on reduction from metallic gray to golden bronze. The reduced material was highly air-sensitive, forming a hydroxide surface film on exposure to ambient atmosphere. 相似文献
263.
W.A. Mangum 《Natural Resource Modeling》1992,6(4):373-384
This paper presents a discrete age structured SI model of tracheal mite (Acarapis woodi (Rennie)) infestation of honey bees (Apis mellifera L.) at the colony level. The model describes the elevated honey bee attrition during winter and predicts that winter brood rearing should promote colony survival by decreasing the colony's infestation and increasing the colony's population. The model also predicts that swarming and the artificial removal of part of the colony's forager honey bees should reduce the colony's infestation level. 相似文献
264.
Chrysostomos Wesdemiotis Aberra Fura Fred W. McLafferty 《Journal of the American Society for Mass Spectrometry》1991,2(6):459-463
Collisionally activated dissociation and neutralization-reionization experiments reveal that protonation of ethanol leads to two distinct isomers, the classical ion CH3CH2OH+2 and the proton-bound complex C2H4…H+…OH2. The neutral counterpart of the latter is unstable, whereas that of the former can be produced in a bound state if the CH3CH2OH+2 precursor ion is formed under low ion source pressure conditions and, thus, with higher internal energies. This suggests that there are substantial differences in the geometries of CH3CH2OH+2 and the hypervalent CH3CH2OH2 ·. This provides only a partial explanation for unusual isotope effects; C2H5OD2 ·, CH3CD2OD2 ·, and CD3CH2OD2 · are substantially more stable than C2D5OD2 · and C2H5OH2 ·. 相似文献
265.
266.
New concepts for the study of incompressible plane or axisymmetric flows are analysed by the stream tube method. Flows without eddies and pure vortex flows are considered in a transformed domain where the mapped streamlines are rectilinear or circular. The transformation between the physical domain and the computational domain is an unknown of the problem. In order to solve the non-linear set of relevant equations, we present a new algorithm based on a trust region technique which is effective for non-convex optimization problems. Experimental results show that the new algorithm is more robust compared to the Newton-Raphson method. 相似文献
267.
268.
M.N. Kotby S.R. El-Sady S.E. Basiouny Y.A. Abou-Rass M.A. Hegazi 《Journal of voice》1991,5(4):316-320
The management of voice disorders may follow one of four approaches: surgical, pharmacological, technical aid, and/or voice therapy. The latter approach offers a wide variety of techniques, each based on certain theoretical beliefs. One of these techniques is the Smith Accent Method. The principles and goals of the Accent Method are presented. The results of its application are discussed. This technique proved effective for cases of functional voice disorders, vocal nodules, and some degrees of vocal fold paralysis. 相似文献
269.
A 480 L evacuable reaction chamber, equipped with FT-IR spectroscopy on-line and ion chromatography off-line, has been used to study the gas phase reaction between the nitrate radical, NO3, and the reduced organic sulphur compounds CH3CH2SH, (CH3CH2)2S, (CH3CH2)2S2, and CH3CH2SCH3 in air. The products CH3CH2SO3H, SO2, H2SO4, CH3CHO, and CH3CH2ONO2 were identified and quantified in the reactions of the first three compounds, CH3CH2SH, (CH3CH2)2S, and (CH3CH2)2S2. The reaction products were CH3CH2SO3H, CH3SO3H, SO2, H2SO4, CH3CHO, and CH2O in the reaction of CH3CH2SCH3. On the basis of identified reaction products and intermediates observed in the infrared spectra, mechanisms are proposed for the reactions between the NO3 radical and the four reduced organic sulphur compounds. The results of this study, together with those from previous experiments performed in this laboratory on CH3SCH3, CH3SH, and CH3SSCH3 lead to the conclusion that all these species, in the reaction with the NO3 radical, follow a similar degradation mechanism producing SO2, H2SO4, R? SO3H, R? CHO, and R? CH2ONO2, as the main reaction products. The inital step of the reaction of NO3 with R? S? R and R? S? H type (R = CH3, CH2CH3) reduced organic sulphur compounds was found to be H-atom abstraction, probably after the formation of an initial adduct. For the reaction between NO3 and R? S? S? R type compounds, evidence for an addition-decomposition reaction, as the initial steps, was obtained. R? S·, R? S(O)·, and R? S(O)2· appear to be formed as intermediates in all the reactions. © John Wiley & Sons, Inc. 相似文献
270.
To assess the historical and sociological significance of the cold fusion saga, researchers need accurate information about the dates of various events associated with the saga. Based on materials in the Cornell Cold Fusion Archive, this article provides both a chronology and citations to documentary evidence for cold fusion events from 1926 to the end of 1990. 相似文献