全文获取类型
收费全文 | 31391篇 |
免费 | 4746篇 |
国内免费 | 4540篇 |
专业分类
化学 | 23414篇 |
晶体学 | 574篇 |
力学 | 1856篇 |
综合类 | 402篇 |
数学 | 3356篇 |
物理学 | 11075篇 |
出版年
2024年 | 62篇 |
2023年 | 440篇 |
2022年 | 913篇 |
2021年 | 995篇 |
2020年 | 1054篇 |
2019年 | 1123篇 |
2018年 | 947篇 |
2017年 | 999篇 |
2016年 | 1435篇 |
2015年 | 1484篇 |
2014年 | 1725篇 |
2013年 | 2318篇 |
2012年 | 2768篇 |
2011年 | 2811篇 |
2010年 | 2050篇 |
2009年 | 1958篇 |
2008年 | 2183篇 |
2007年 | 1927篇 |
2006年 | 1905篇 |
2005年 | 1650篇 |
2004年 | 1405篇 |
2003年 | 1205篇 |
2002年 | 1386篇 |
2001年 | 1062篇 |
2000年 | 829篇 |
1999年 | 701篇 |
1998年 | 494篇 |
1997年 | 391篇 |
1996年 | 375篇 |
1995年 | 343篇 |
1994年 | 272篇 |
1993年 | 259篇 |
1992年 | 209篇 |
1991年 | 183篇 |
1990年 | 164篇 |
1989年 | 139篇 |
1988年 | 111篇 |
1987年 | 78篇 |
1986年 | 80篇 |
1985年 | 79篇 |
1984年 | 39篇 |
1983年 | 33篇 |
1982年 | 23篇 |
1981年 | 24篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1974年 | 2篇 |
1971年 | 2篇 |
1957年 | 7篇 |
1936年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
HT-7中性束诊断电源系统的计算机控制系统由PLC、工控机、逻辑控制和定时单元等组成。对起弧、束流引出等实验进行了研究。在多种参数下,通过调节弧压、进气量等观察了其对弧流的影响。初步实验结果表明,在高压35kV、弧流80A的情况下,束电流可以达到3.6A,引出的中性束流功率可以稳定地达到50kW以上。 相似文献
113.
Model and empirical study on some collaboration networks 总被引:8,自引:0,他引:8
Pei-Pei Zhang Kan Chen Yue He Tao Zhou Bei-Bei Su Yingdi Jin Hui Chang Yue-Ping Zhou Li-Cheng Sun Bing-Hong Wang Da-Ren He 《Physica A》2006,360(2):599-616
In this paper we present an empirical study of a few practical systems described by cooperation networks, and propose a model to understand the results obtained. We study four non-social systems, which are the Bus Route Networks of Beijing and Yangzhou, the Travel Route Network of China, Huai-Yang recipes of Chinese cooked food, and a social system, which is the Collaboration Network of Hollywood Actors. In order to explain the results related to the degree distribution, act-degree distribution and act-size distribution (especially about the degree distribution, which may be better fitted using a stretched exponential distribution (SED)), we suggest a simple model to show a possible evolutionary mechanism for the emergence of such networks. The analytic and numerical results obtained from the model are in good agreement with the empirical results. 相似文献
114.
Wang Binglin Ma Shiming Chian Jian Zong Huiqi Tang Quanyong 《Radiation Physics and Chemistry》1998,52(1-6):439-442
The radiation facility was made by Sulzer Company in Switzerland. The designed capacity is 3.7 × 1016Bq( 1MCi) and 12.21 × 1015Bq(330kCi)of Cobalt -60 source was loaded at the first phase. Current cobalt -60 source-loading is 2.4 × 1016Bq(650kCi). The equipment assembling and installation regulating and testing were completed at the end of 1988. The facility was put into commissioning in 1989. It operates 7000–8000 hours per year. The facility has been utilized to carry out many research work and irradiate many kinds of items. Lots of economical benefits have been gained since then. Now it is becoming a radiation processing model base in China. This paper summarized the strong and weak points of the design of the facility through in eight years' practice and analysed the economical benefits. 相似文献
115.
Xu ZK Dai QW Wu J Huang XJ Yang Q 《Langmuir : the ACS journal of surfaces and colloids》2004,20(4):1481-1488
A novel method for the surface modification of a microporous polypropylene membrane by tethering phospholipid analogous polymers (PAPs) is given, which includes the photoinduced graft polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA) and the ring-opening reaction of grafted poly-(DMAEMA) with 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes. Five 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes, containing octyloxy, dodecyloxy, tetradecyloxy, hexadecyloxy, and octadecyloxy groups in the molecular structure, were used to fabricate the PAP-modified polypropylene membranes. The attenuated total reflectance FT-IR spectra of the original, poly(DMAEMA)-grafted, and PAP-modified membranes confirmed the chemical changes on the membrane surface. Scanning electron microscope pictures showed that, compared with the original membrane, the surface porosities ofpoly(DMAEMA)-grafted and PAP-modified membranes were somewhat reduced. Water contact angles measured by the sessile drop method on PAP-modified membranes were slightly lower than that on the original polypropylene membrane, but higher than those on poly(DMAEMA)-grafted membranes with the exception of octyloxy-containing PAP-modified membranes. However, BSA adsorption experiments indicated that the five PAP-modified membranes had a much better protein-resistant property than the original polypropylene membrane and the poly(DMAEMA)-grafted membranes. For hexadecyloxy- and octadecyloxy-containing PAP-modified membranes, almost no protein adsorption was observed when the grafting degree was above 6 wt %. It was also found that the platelet adhesion was remarkably suppressed on the PAP-modified membranes. All these results demonstrate that the described approach is an effective way to improve the surface biocompatibility for polymeric membranes. 相似文献
116.
微波常压法合成水杨酸酯 总被引:15,自引:0,他引:15
在浓硫酸催化下,采用微波常压法由水杨酸分别与正丁醇、异丁醇、正戊醇和异戊醇反应合成相应的水杨酸酯。结果表明:当水杨酸:醇:H2SO4=1:5.5:0.3(摩尔比)时,采用560W微波辐射22min,水杨酸酯的产率可达88.7% ̄96.4%,反应速度至少是常规反应的14倍。 相似文献
117.
The selectivity of intermolecular force is caused by the special interaction between two adaptable groups on the molecules. π bond and conjugated π bonds such as benzene ring are negative charged groups,which may attract stongly positively charged H groups such as 3H in β-C_6H_6Cl_6, and repulse other ne- gatively charged groups such as-C=O(Q). Our experiments show that the reduced retention time tr of benzene on non- polar β-C_6H_6Cl_6 is much greater than that on polar fixed phases such as CH_3— —NO_2 in gas chromatography and that the het of solvation of β-C_6H_6Cl_6 in benzene is also much greater than that of the polar α-C_6H_6Cl_6 and γ-C_6H_6Cl_6. This can′t be explained by the usual Van de Waals′ force. It results from the selective intermoleccular force.... 相似文献
118.
119.
Five new thiophene imides having the structures C4H3SC(O)N(H)C(O)R (where R=Me, Et, Pr, t-Bu, 1-4) and C4H3SCH2C(O)N(H)C(O)CH35 have been prepared. Control over the orientation of the carbonyl groups was achieved by the introduction of a bulky group in the terminal position of the side chain, and molecular modeling calculations indicated that the energy difference between the ct and tt conformations is small. The acidity of 1-4 decreases from Me to t-Bu, and, although 1-4 will not undergo oxidative polymerization, the introduction of a single spacer CH2 group allows 5 to polymerize. 相似文献
120.
用裂解气相色谱法研究PMR型聚酰亚胺前体在反应中的化学变化过程,以裂解产物醇和环戊二烯的生成率表示酰胺化或酰亚胺化及交联的程度,结果表明,酰胺化或酰亚胺化在50℃以下不发生,在180~220℃完成,降冰片烯端基在150℃以下不发生交联反应,在280℃,10~18小时可完成交联反应。 相似文献