全文获取类型
收费全文 | 17695篇 |
免费 | 3121篇 |
国内免费 | 2115篇 |
专业分类
化学 | 12874篇 |
晶体学 | 184篇 |
力学 | 1026篇 |
综合类 | 105篇 |
数学 | 1817篇 |
物理学 | 6925篇 |
出版年
2024年 | 51篇 |
2023年 | 400篇 |
2022年 | 639篇 |
2021年 | 661篇 |
2020年 | 739篇 |
2019年 | 715篇 |
2018年 | 630篇 |
2017年 | 572篇 |
2016年 | 858篇 |
2015年 | 827篇 |
2014年 | 999篇 |
2013年 | 1271篇 |
2012年 | 1586篇 |
2011年 | 1519篇 |
2010年 | 1063篇 |
2009年 | 986篇 |
2008年 | 1094篇 |
2007年 | 1027篇 |
2006年 | 943篇 |
2005年 | 851篇 |
2004年 | 611篇 |
2003年 | 519篇 |
2002年 | 514篇 |
2001年 | 400篇 |
2000年 | 358篇 |
1999年 | 426篇 |
1998年 | 346篇 |
1997年 | 340篇 |
1996年 | 324篇 |
1995年 | 276篇 |
1994年 | 228篇 |
1993年 | 200篇 |
1992年 | 156篇 |
1991年 | 136篇 |
1990年 | 157篇 |
1989年 | 112篇 |
1988年 | 78篇 |
1987年 | 55篇 |
1986年 | 61篇 |
1985年 | 59篇 |
1984年 | 30篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 19篇 |
1980年 | 10篇 |
1977年 | 3篇 |
1957年 | 2篇 |
1942年 | 2篇 |
1930年 | 2篇 |
1916年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Sai Venkatesh Pingali Hugh M. O’Neill Yoshiharu Nishiyama Lilin He Yuri B. Melnichenko Volker Urban Loukas Petridis Brian Davison Paul Langan 《Cellulose (London, England)》2014,21(2):873-878
Morphological changes to the different components of lignocellulosic biomass were observed as they occurred during steam pretreatment by placing a pressure reaction cell in a neutron beam and collecting time-resolved neutron scattering data. Changes to cellulose morphology occurred mainly in the heating phase, whereas changes in lignin morphology occurred mainly in the holding and cooling phases. During the heating stage, water is irreversibly expelled from cellulose microfibrils as the elementary fibrils coalesce. During the holding phase lignin aggregates begin to appear and they increase in size most noticeably during the cooling phase. This experiment demonstrates the unique information that in situ small angle neutron scattering studies of pretreatment can provide. This approach could be useful in optimizing the heating, holding and cooling stages of pretreatments to allow the exact size and nature of lignin aggregates to be controlled in order to enhance enzyme accessibility to cellulose and therefore the efficiency of biomass conversion. 相似文献
992.
通过极差分析研究了不同浓度的硝酸镧和生长素组合对长柄扁桃试管苗生根的影响。结果表明:对长柄扁桃生根影响的主次因素为硝酸镧(La(NO3)3)>吲哚丁酸(IBA)>吲哚乙酸(IAA),最优组合为La(NO3)320 mg·L-1+IBA 0.2 mg·L-1+IAA 0.2 mg·L-1。在此条件下培养42 d后,长柄扁桃试管苗根的诱导频率达到89%,平均根长达到9.2 cm,每株根的平均个数为9.7个。对长柄扁桃的根组织活力进行测定,结果表明添加镧的处理组根系还原力是对照(不加镧)组的1.57倍。经过移栽驯化2个月后,长柄扁桃组培苗的移栽成活率达到94%,株高是对照的1.53倍。 相似文献
993.
Manganese‐Catalyzed Dehydrogenative [4+2] Annulation of NH Imines and Alkynes by CH/NH Activation 下载免费PDF全文
Ruoyu He Prof. Dr. Zhi‐Tang Huang Prof. Dr. Qi‐Yu Zheng Prof. Dr. Congyang Wang 《Angewandte Chemie (International ed. in English)》2014,53(19):4950-4953
Described herein is a manganese‐catalyzed dehydrogenative [4+2] annulation of N? H imines and alkynes, a reaction providing highly atom‐economical access to diverse isoquinolines. This transformation represents the first example of manganese‐catalyzed C? H activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H2 as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition‐metal‐catalyzed C? H activation. Mechanistic studies revealed the five‐membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle. 相似文献
994.
An Unusual Dehydratase Acting on Glycerate and a Ketoreducatse Stereoselectively Reducing α‐Ketone in Polyketide Starter Unit Biosynthesis 下载免费PDF全文
Dr. Hai‐Yan He Dr. Hua Yuan Dr. Man‐Cheng Tang Prof. Dr. Gong‐Li Tang 《Angewandte Chemie (International ed. in English)》2014,53(42):11315-11319
Polyketide synthases (PKSs) usually employ a ketoreductase (KR) to catalyze the reduction of a β‐keto group, followed by a dehydratase (DH) that drives the dehydration to form a double bond between the α‐ and β‐carbon atoms. Herein, a DH*‐KR* involved in FR901464 biosynthesis was characterized: DH* acts on glyceryl‐S‐acyl carrier protein (ACP) to yield ACP‐linked pyruvate; subsequently KR* reduces α‐ketone that yields L ‐lactyl‐S‐ACP as starter unit for polyketide biosynthesis. Genetic and biochemical evidence was found to support a similar pathway that is involved in the biosynthesis of lankacidins. These results not only identified new PKS domains acting on different substrates, but also provided additional options for engineering the PKS starter pathway or biocatalysis. 相似文献
995.
Efficient Solar Cells Sensitized by Porphyrins with an Extended Conjugation Framework and a Carbazole Donor: From Molecular Design to Cosensitization 下载免费PDF全文
Yueqiang Wang Bin Chen Dr. Wenjun Wu Dr. Xin Li Prof. Dr. Weihong Zhu Prof. Dr. He Tian Prof. Dr. Yongshu Xie 《Angewandte Chemie (International ed. in English)》2014,53(40):10779-10783
Porphyrin dyes containing the carbazole electron donor have been designed and optimized by wrapping the porphyrin framework, introducing an additional ethynylene bridge to extend the wavelength range of light absorption, and further suppression of the dye aggregation by introducing additional alkoxy chains. Application of a cosensitization approach results in improved current density (Jsc) and open‐circuit voltage (Voc) values, thus achieving the highest cell efficiency of 10.45 %. This work provides an effective combined strategy of molecular design and cosensitization for developing efficient dye‐sensitized solar cells (DSSCs). In addition, carbazole has been demonstrated to be a promising donor for porphyrin sensitizers. 相似文献
996.
Lin Yang Guoli He Ruifeng Yin Dr. Lili Zhu Prof. Dr. Xiaoxia Wang Prof. Dr. Ran Hong 《Angewandte Chemie (International ed. in English)》2014,53(43):11600-11604
Polyketide antibiotics bearing skipped polyols represent a synthetic challenge. A SiCl4‐promoted oxonia‐Cope rearrangement of syn,syn‐2‐vinyl‐1,3‐diols was developed to forge an array of 1,5‐pentenediols, thus providing versatile motifs for the preparation of 1,2,3,5‐stereoarrays in a highly stereoselective manner. Further exploration with Sn(OTf)2 realized the rearrangement of a cross‐aldehyde which tactically warrants the utility of the current approach to access complex polyketides. The origin of high stereoselectivity is attributed to a chairlike anti‐conformation of the oxonium ion intermediate. 相似文献
997.
Lihong Liu Prof. Bin He Dr. Qian Liu Dr. Zhaojun Yun Xueting Yan Dr. Yanmin Long Prof. Guibin Jiang 《Angewandte Chemie (International ed. in English)》2014,53(52):14476-14479
We have developed a new method for the identification and accurate size characterization of nanoparticles (NPs) in complex media based on capillary electrokinetic (CE) separation coupled to inductively coupled plasma mass spectrometry (ICP‐MS). Through mass scanning and Gaussian fitting of electropherogram peaks, we can obtain multidimensional information on chemical compositions, size distributions, and ionic species of multiple NPs in a single run. The results are more accurate than those obtained by using conventional methods. This method provides a powerful tool for investigating polydisperse NP systems and rapid screening of NP‐containing products. 相似文献
998.
Two‐Component Supramolecular Gels Derived from Amphiphilic Shape‐Persistent Cyclo[6]aramides for Specific Recognition of Native Arginine 下载免费PDF全文
Youzhou He Min Xu Rongzhao Gao Xiaowei Li Fengxue Li Xuedan Wu Prof. Dingguo Xu Dr. Huaqiang Zeng Prof. Lihua Yuan 《Angewandte Chemie (International ed. in English)》2014,53(44):11834-11839
A unique supramolecular two‐component gelation system was constructed from amphiphilic shape‐persistent cyclo[6]aramides and diethylammonium chloride (or triethylammonium chloride). This system has the ability to discriminate native arginine from 19 other amino acids in a specific fashion. Cyclo[6]aramides show preferential binding for the guanidinium residue over ammonium groups. This specificity was confirmed by both experimental results and theoretical simulations. These results demonstrated a new modular displacement strategy, exploring the use of species‐binding hydrogen‐bonded macrocyclic foldamers for the construction of two‐component gelation systems for selective recognition of native amino acids by competitive host–guest interactions. This strategy may be amenable to developing a variety of functional two‐component gelators for specific recognition of various targeted organic molecular species. 相似文献
999.
1000.
Self‐Propelled Janus Mesoporous Silica Nanomotors with Sub‐100 nm Diameters for Drug Encapsulation and Delivery 下载免费PDF全文
Mingjun Xuan Jingxin Shao Dr. Xiankun Lin Dr. Luru Dai Prof. Qiang He 《Chemphyschem》2014,15(11):2255-2260
The synthesis of an innovative self‐propelled Janus nanomotor with a diameter of about 75 nm that can be used as a drug carrier is described. The Janus nanomotor is based on mesoporous silica nanoparticles (MSNs) with chromium/platinum metallic caps and propelled by decomposing hydrogen peroxide to generate oxygen as a driving force with speeds up to 20.2 μm s?1 (about 267 body lengths per second). The diffusion coefficient (D) of nanomotors with different H2O2 concentrations is calculated by tracking the movement of individual particles recorded by means of a self‐assembled fluorescence microscope and is significantly larger than free Brownian motion. The traction of a single Janus MSN nanomotor is estimated to be about 13.47×10?15 N. Finally, intracellular localization and drug release in vitro shows that the amount of Janus MSN nanomotors entering the cells is more than MSNs with same culture time and particle concentrations, meanwhile anticancer drug doxorubicin hydrochloride loaded in Janus MSNs can be slowly released by biodegradation of lipid bilayers in cells. 相似文献