全文获取类型
收费全文 | 51325篇 |
免费 | 7495篇 |
国内免费 | 6610篇 |
专业分类
化学 | 37566篇 |
晶体学 | 494篇 |
力学 | 3048篇 |
综合类 | 366篇 |
数学 | 5850篇 |
物理学 | 18106篇 |
出版年
2024年 | 147篇 |
2023年 | 933篇 |
2022年 | 1570篇 |
2021年 | 1774篇 |
2020年 | 1828篇 |
2019年 | 1748篇 |
2018年 | 1475篇 |
2017年 | 1387篇 |
2016年 | 2173篇 |
2015年 | 2180篇 |
2014年 | 2729篇 |
2013年 | 3692篇 |
2012年 | 4484篇 |
2011年 | 4469篇 |
2010年 | 3016篇 |
2009年 | 3037篇 |
2008年 | 3165篇 |
2007年 | 2751篇 |
2006年 | 2590篇 |
2005年 | 2346篇 |
2004年 | 2293篇 |
2003年 | 1818篇 |
2002年 | 1821篇 |
2001年 | 1335篇 |
2000年 | 1076篇 |
1999年 | 1152篇 |
1998年 | 997篇 |
1997年 | 872篇 |
1996年 | 921篇 |
1995年 | 742篇 |
1994年 | 714篇 |
1993年 | 581篇 |
1992年 | 581篇 |
1991年 | 482篇 |
1990年 | 418篇 |
1989年 | 318篇 |
1988年 | 267篇 |
1987年 | 232篇 |
1986年 | 190篇 |
1985年 | 194篇 |
1984年 | 158篇 |
1983年 | 141篇 |
1982年 | 100篇 |
1981年 | 76篇 |
1980年 | 63篇 |
1979年 | 42篇 |
1978年 | 40篇 |
1976年 | 43篇 |
1975年 | 40篇 |
1974年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
61.
Ju-Chun Huang Chien Hong Cheng Shyang Roeng Sheen Zhi Ling Lee Chien Ming Lei Maw Kuen Wu 《中国化学会会志》1996,43(2):139-143
The synthesis of bulk Y2Ba4Cu7O15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y2Ba4Cu7O15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y2Ba4Cu7O15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described. 相似文献
62.
A tetrasubstituted derivative of 1,4,7,10-tetraazacyclododecane with amide coordinating groups and extended noncoordinating phosphonate groups forms a complex with gadolinium(III) (shown in the picture) which contains one slowly exchanging inner-sphere water molecule (tau(M)=21 μs). The 20-MHz water proton relaxivity of the complex was found to be highly pH dependent. Protonation of the noncoordinating phosphonate groups appears to catalyze prototropic exchange of the bound water protons, thereby providing a mechanism for enhanced water contrast below pH 7. 相似文献
63.
Synthesis of six new triorganosiloxybenzaldehydes, two m-methoxy-(triorganosiloxy)benzaldehydes, and seven methoxy(triorganosiloxy)benzenes are reported. From comparison of the stability of these compounds in the atmosphere, it is concluded that formyl group shows unfavorable influence and methoxy group shows favorable influence on the stability of these compounds. The oposite influences of these two groups on phenoxysilane linkage seems to connect with the oposite directiones of inductive effect of formyl group and resonance effect of methoxy group on the benzene ring. 相似文献
64.
Yuh-Wern Wu 《中国化学会会志》1996,43(6):507-509
The relative rates for the addition reactions of tert-butyl radical to 2-substituted ally chlorides I have een determined. The correlation of log k/k0, vs. σm gives a ρ mvalue of 3.59 with correlation coefficient of 0.930. When the substituent CH2Cl is excluded, correlation coefficient raises to 0.990 and ρ value becomes 3.39. The large p value and the deviation of relative rate of substituent CH2C1 are discussed. 相似文献
65.
The cyclization of 2-(1-alkynyl)-2-alken-1-ones 1 proceeded very smoothly in the presence of alcohols 2 with a catalytic amount of Cu(I)Br in DMF at 80 degrees C, leading to the formation of highly substituted furans 3. The catalytic system reported herein is easy to handle, compared to the previously known system wherein the reaction between 1 and 2 needed to use moisture sensitive gold(III) chloride. 相似文献
66.
用火焰原子吸收法检测了深圳市1408例13岁以下儿童发中钙,铁,镁的元素含量,并探讨了元素含量与年龄,性别的关系,结果显示,性别差异有显著意义,钙,镁含量随年龄增长有相似变化。 相似文献
67.
The native oxide layers on Si(100) surface were irradiated under UHV conditions by an ArF excimer laser pulses with energy density varied between melting and evaporating thresholds. The resulting changes were studied by LEED, AES and UPS. The increase of the energy density up to evaporation threshold results in the recrystallisation of native oxide layer. The pulses with energy densities just above the evaporation threshold ablate the top layer leaving an ordered and atomicaly clean surface. The observed (1x1) surface reconstruction is probably stabilised by strains introduced during rapid melting and quenching of the topmost layers. The surface electronic structure is dominated by random satisfaction of dangling bonds swearing a well defined surface states observed on (2x1)Si(100) surface. 相似文献
68.
The photochemistry of diazomethane in toluene and p-xylene solutions was investigated. The reactions of methylene with toluene gave eight products. In the diazomethane p-xylene solution, p-ethyltoluene, 1,2,4-trimethylbenzene, 1,4-dimethylcyclohepatriene-1,3,5 and three unidentified compounds were found as products of the reaction of methylene with p-xylene. The relative rates of addition and insertion reaction of methylene with toluene and p-xylene have been calculated. 相似文献
69.
New anti-HIV prodrugs, conjugates of AZT and d4T with boranophosphates, were prepared by the H-phosphonate method. Their structures were determined by negative ion electrospray ionization mass spectrometry (ESI-MS) in conjunction with tandem mass spectrometry (MS/MS). The fragmentation pathways were investigated, and most of the fragment ions contained the boranophosphate or phosphinate group. 相似文献
70.
The addition of iodine azide to chiral conjugated N-enoyl-sultam or α,β-unsaturated N-acyloxazolidinones generated two asymmetric centers at C(α) and C(β) with high π-face differentiation and regioselectivity. The diastereomerically pure product was easily obtained by crystallization with purity up to 94% de. The structure of 2a was determined by X-ray diffraction analysis which also indicated that B and 4 are reactive conformations. 相似文献