全文获取类型
收费全文 | 3308篇 |
免费 | 553篇 |
国内免费 | 354篇 |
专业分类
化学 | 2499篇 |
晶体学 | 41篇 |
力学 | 135篇 |
综合类 | 4篇 |
数学 | 340篇 |
物理学 | 1196篇 |
出版年
2024年 | 2篇 |
2023年 | 66篇 |
2022年 | 72篇 |
2021年 | 129篇 |
2020年 | 141篇 |
2019年 | 125篇 |
2018年 | 114篇 |
2017年 | 104篇 |
2016年 | 159篇 |
2015年 | 169篇 |
2014年 | 191篇 |
2013年 | 273篇 |
2012年 | 346篇 |
2011年 | 336篇 |
2010年 | 195篇 |
2009年 | 236篇 |
2008年 | 191篇 |
2007年 | 203篇 |
2006年 | 185篇 |
2005年 | 128篇 |
2004年 | 122篇 |
2003年 | 84篇 |
2002年 | 75篇 |
2001年 | 47篇 |
2000年 | 57篇 |
1999年 | 70篇 |
1998年 | 48篇 |
1997年 | 37篇 |
1996年 | 58篇 |
1995年 | 24篇 |
1994年 | 28篇 |
1993年 | 29篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 21篇 |
1989年 | 22篇 |
1988年 | 14篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有4215条查询结果,搜索用时 31 毫秒
961.
962.
The thermochemical sulfur-iodine cycle is a potential method for hydrogen production, and the hydrogen iodide (HI) decomposition is the key step to determine the efficiency of hydrogen production in the cycle. To further reduce the irreversibility of various transmission processes in the HI decomposition reaction, a one-dimensional plug flow model of HI decomposition tubular reactor is established, and performance optimization with entropy generate rate minimization (EGRM) in the decomposition reaction system as an optimization goal based on finite-time thermodynamics is carried out. The reference reactor is heated counter-currently by high-temperature helium gas, the optimal reactor and the modified reactor are designed based on the reference reactor design parameters. With the EGRM as the optimization goal, the optimal control method is used to solve the optimal configuration of the reactor under the condition that both the reactant inlet state and hydrogen production rate are fixed, and the optimal value of total EGR in the reactor is reduced by 13.3% compared with the reference value. The reference reactor is improved on the basis of the total EGR in the optimal reactor, two modified reactors with increased length are designed under the condition of changing the helium inlet state. The total EGR of the two modified reactors are the same as that of the optimal reactor, which are realized by decreasing the helium inlet temperature and helium inlet flow rate, respectively. The results show that the EGR of heat transfer accounts for a large proportion, and the decrease of total EGR is mainly caused by reducing heat transfer irreversibility. The local total EGR of the optimal reactor distribution is more uniform, which approximately confirms the principle of equipartition of entropy production. The EGR distributions of the modified reactors are similar to that of the reference reactor, but the reactor length increases significantly, bringing a relatively large pressure drop. The research results have certain guiding significance to the optimum design of HI decomposition reactors. 相似文献
963.
Chenchen Xing Dr. Jianlin Deng Wenlong Fu Dr. Jichao Li Lijie Xu Ruihao Sun Dan Wang Chengwen Li Prof. Kang Liang Meng Gao Prof. Biao Kong 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(6):e202103642
Mesoporous silica nanoparticles (MSNs) functionalized with benzimidazole-derived fluorescent molecules (DHBM) are fabricated via a feasible interfacial superassembly strategy for the highly sensitive and selective detection of Cu2+. DHBM-MSN exhibits an obvious quenching effect on Cu2+ in aqueous solutions, and the detection limit can be as low as 7.69×10−8 M. The DHBM-MSN solid-state sensor has good recyclability, and the silica framework can simultaneously improve the photostability of DHBM. Two mesoporous silica nanoparticles with different morphologies were specially designed to verify that nanocarriers with different morphologies do not affect the specific detectionability. The detection mechanism of the fluorescent probe was systematically elucidated by combining experimental results and density function theory calculations. Moreover, the detection system was successfully applied to detect Cu2+ in bovine serum, juice, and live cells. These results indicate that the DHBM-MSN fluorescent sensor holds great potential in practical and biomedical applications. 相似文献
964.
Computational Optimization and Applications - This paper presents two inexact composite gradient methods, one inner accelerated and another doubly accelerated, for solving a class of nonconvex... 相似文献
965.
This is the part II of a two-part work started in [18]. In part I, Cardy algebras were studied, a notion which arises from the classification of genus-0, 1 open–closed rational conformal field theories. In this part, we prove that a Cardy algebra also satisfies the higher genus factorisation and modular-invariance properties formulated in [7] in terms of the notion of a solution to the sewing constraints. We present the proof by showing that the latter notion, which is defined as a monoidal natural transformation, can be expressed in terms of generators and relations, which correspond exactly to the defining data and axioms of a Cardy algebra. 相似文献
966.
The authors consider a fourth order two-point boundary value problem. Some a priori estimates to positive solutions for the problem are obtained. Explicit sufficient conditions for existence and nonexistence of positive solutions to the problem are established. An example to illustrate the results is included. 相似文献
967.
968.
Jonas Verhoeven Freija De Vleeschouwer Hanchu Kong Prof. Kristof Van Hecke Vineet Pande Weimei Sun Ann Vos Tongfei Wu Lieven Meerpoel Jan Willem Thuring Guido Verniest 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(67):15419-15423
Despite the large variety of modified nucleosides that have been reported, the preparation of constrained 4′-spirocyclic adenosine analogues has received very little attention. We discovered that the [2+2]-cycloaddition of dichloroketene on readily available 4′-exo-methylene furanose sugars efficiently results in the diastereoselective formation of novel 4′-spirocyclobutanones. The reaction mechanism was investigated via density functional theory (DFT) and found to proceed either via a non-synchronous or stepwise reaction sequence, controlled by the stereochemistry at the 3′-position of the sugar substrate. The obtained dichlorocyclobutanones were converted into nucleoside analogues, providing access to a novel class of chiral 4′-spirocyclobutyl adenosine mimetics in eight steps from commercially available sugars. Assessment of the biological activity of designed 4′-spirocyclic adenosine analogues identified potent inhibitors for protein methyltransferase target PRMT5. 相似文献
969.
Ya‐Jun Yang Xi‐Wang Liu Xiao‐Jun Kong Zhe Qin Shi‐Hong Li Zeng‐Hua Jiao Jian‐Yong Li 《Biomedical chromatography : BMC》2019,33(7)
An LC–MS/MS method with internal standard tolfenamic acid for determining diclofenac sodium (DCF) in dairy cow plasma was developed and validated. Samples were processed with protein precipitation by cold formic acid–acetonitrile. Determination of DCF was performed using LC–ESI+–MS/MS with the matrix‐matched calibration curve. The results showed that the method was sensitive (LOD 2 ng mL?1, LOQ 5 ng mL?1), accurate (97.60 ± 5.64%), precise (<10%) and linear in the range of 5–10,000 ng mL?1. A single intravenous (i.v.) or intramuscular (i.m.) administration of 5% diclofenac sodium injection at a dose of 2.2 mg kg?1 was performed in six healthy dairy cows according to a two‐period crossover design. The main pharmacokinetic (PK) parameters after a single i.v. administration were as follows: t1/2β, 4.52 ± 1.71 h; AUC, 77.79 ± 16.76 h μg mL?1; mean residence time, 5.16 ± 1.11 h. The main PK parameters after a single i.m. administration were as follows: Tmax, 2.38 ± 1.19 h; Cmax, 7.46 ± 1.85 μg mL?1; t1/2β, 9.46 ± 2.86 h; AUC 67.57 ± 13.07 h μg mL?1. The absolute bioavailability was 87.37 ± 5.96%. The results showed that the diclofenac sodium injection had PK characteristics of rapid absorption and slow elimination, and high peak concentration and bioavailability in dairy cows, and that the recommended clinical dosage of diclofenac sodium injection is 2.2 mg kg?1. 相似文献
970.
1H NMR‐Guided Isolation of Formyl‐Phloroglucinol Meroterpenoids from the Leaves of Eucalyptus robusta 下载免费PDF全文
Zhi‐Chun Shang Dr. Ming‐Hua Yang Kai‐Li Jian Dr. Xiao‐Bing Wang Prof. Dr. Ling‐Yi Kong 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(33):11778-11784
Nine formyl‐phloroglucinolmeroterpenoids (FPMs), namely, eucalrobusones A–I ( 1 – 9 ), were isolated from the leaves of Eucalyptus robusta by tracking the phenolic hydroxyl 1H NMR peaks. The Snatzke helicity rules for the Cotton effects of twisted benzene rings were applied to elucidate the absolute configurations of the FPMs. These findings, along with NMR spectroscopy, the circular dichroism (CD) exciton chirality method, and CD calculations, allowed complete structures for the FPMs to be assigned. Eucalrobusones A–F ( 1 – 6 ) are novel adducts formed between a formyl‐derived carbon atom on the phloroglucinol ring and monoterpene and sesquiterpene components. Eucalrobusones G–I ( 7 – 9 ) are the first examples of FPMs with cubebane part structures connected by an unusual 1‐oxaspiro[5.5]undecane subunit. Among these isolates, eucalrobusone C ( 3 ) showed significant cytotoxicity against HepG2, MCF‐7, and U2OS cancer cell lines, with IC50 values less than 10 μm . Compound 3 significantly blocks cell proliferation in MCF‐7 cells and induces MCF‐7 cell death through apoptosis. 相似文献