全文获取类型
收费全文 | 537篇 |
免费 | 17篇 |
国内免费 | 1篇 |
专业分类
化学 | 431篇 |
晶体学 | 4篇 |
力学 | 8篇 |
数学 | 65篇 |
物理学 | 47篇 |
出版年
2023年 | 5篇 |
2022年 | 2篇 |
2021年 | 9篇 |
2020年 | 20篇 |
2019年 | 8篇 |
2018年 | 7篇 |
2017年 | 13篇 |
2016年 | 18篇 |
2015年 | 10篇 |
2014年 | 12篇 |
2013年 | 37篇 |
2012年 | 36篇 |
2011年 | 38篇 |
2010年 | 16篇 |
2009年 | 19篇 |
2008年 | 21篇 |
2007年 | 35篇 |
2006年 | 29篇 |
2005年 | 21篇 |
2004年 | 14篇 |
2003年 | 18篇 |
2002年 | 14篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1997年 | 2篇 |
1996年 | 8篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1987年 | 2篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1978年 | 3篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 5篇 |
1967年 | 2篇 |
1957年 | 4篇 |
1956年 | 26篇 |
1955年 | 18篇 |
1937年 | 2篇 |
1934年 | 2篇 |
1927年 | 2篇 |
1921年 | 3篇 |
排序方式: 共有555条查询结果,搜索用时 31 毫秒
1.
2.
Crisólogo Dolores-Flores Martha Iris Rivera-López Javier García-García 《International Journal of Mathematical Education in Science & Technology》2019,50(3):369-389
This paper reports the results of a research exploring the mathematical connections of pre-university students while they solving tasks which involving rates of change. We assume mathematical connections as a cognitive process through which a person finds real relationships between two or more ideas, concepts, definitions, theorems, procedures, representations or meanings or with other disciplines or the real-world. Four tasks were proposed to the 33 pre-university students that participated in this research; the central concept of the first task is the slope, the last three tasks contain concepts like velocity, speed and acceleration. Task-based interviews were conducted to collect data and later analysed with thematic analysis. Results showed most of the students made mathematical connections of the procedural type, the mathematical connections of the common features type are made in smaller quantities and the mathematical connection of the generalization type is scarcely made. Furthermore, students considered slope as a concept disconnected from velocity, speed and acceleration. 相似文献
3.
Sergio G. Hernandez-Leon Jose Andre-i Sarabia Sainz Gabriela Ramos-Clamont Montfort Jos ngel Huerta-Ocampo Martha Nydia Ballesteros Ana M. Guzman-Partida María del Refugio Robles-Burgueo Luz Vazquez-Moreno 《Molecules (Basel, Switzerland)》2021,26(6)
In this work, previously synthesized and characterized core-shell silica nanoparticles (FCSNP) functionalized with immobilized molecular bait, Cibacron blue, and a porous polymeric bis-acrylamide shell were incubated with pooled urine samples from adult women or men with normal weight, overweight or obesity for the isolation of potential biomarkers. A total of 30 individuals (15 woman and 15 men) were included. FCSNP allowed the capture of a variety of low molecular weight (LMW) proteins as evidenced by mass spectrometry (MS) and the exclusion of high molecular weight (HMW) proteins (>34 kDa) as demonstrated by SDS-PAGE and 2D SDS-PAGE. A total of 36 proteins were successfully identified by MS and homology database searching against the Homo sapiens subset of the Swiss-Prot database. Identified proteins were grouped into different clusters according to their abundance patterns. Four proteins were found only in women and five only in men, whereas 27 proteins were in urine from both genders with different abundance patterns. Based on these results, this new approach represents an alternative tool for isolation and identification of urinary biomarkers. 相似文献
4.
Caetano P. Sabino Martha S. Ribeiro Mark Wainwright Carolina dos Anjos Fábio P. Sellera Milena Dropa Nathalia B. Nunes Guilherme T. P. Brancini Gilberto U. L. Braga Victor E. Arana-Chavez Raul O. Freitas Nilton Lincopan Maurício S. Baptista 《Photochemistry and photobiology》2023,99(2):742-750
The unbridled dissemination of multidrug-resistant pathogens is a major threat to global health and urgently demands novel therapeutic alternatives. Antimicrobial photodynamic therapy (aPDT) has been developed as a promising approach to treat localized infections regardless of drug resistance profile or taxonomy. Even though this technique has been known for more than a century, discussions and speculations regarding the biochemical mechanisms of microbial inactivation have never reached a consensus on what is the primary cause of cell death. Since photochemically generated oxidants promote ubiquitous reactions with various biomolecules, researchers simply assumed that all cellular structures are equally damaged. In this study, biochemical, molecular, biological and advanced microscopy techniques were employed to investigate whether protein, membrane or DNA damage correlates better with dose-dependent microbial inactivation kinetics. We showed that although mild membrane permeabilization and late DNA damage occur, no correlation with inactivation kinetics was found. On the other hand, protein degradation was analyzed by three different methods and showed a dose-dependent trend that matches microbial inactivation kinetics. Our results provide a deeper mechanistic understanding of aPDT that can guide the scientific community toward the development of optimized photosensitizing drugs and also rationally propose synergistic combinations with antimicrobial chemotherapy. 相似文献
5.
Structure elucidation of configurational isomers of nitrile‐substituted spirocyclopropyloxindoles by NMR spectroscopy,molecular modeling,and X‐ray crystallography 下载免费PDF全文
6.
7.
8.
Giovana Ferronato Mariela S. Viera Osmar D. Prestes Martha B. Adaime 《International journal of environmental analytical chemistry》2013,93(11):1005-1016
ABSTRACTThe food chain is the main source of exposure to humans by organochlorine pesticides (OCPs) due to the bioaccumulation. Breast milk can accumulate OCPs, so this matrix is often used as an environmental bioindicator. The currently available methods for the determination of several OCPs and metabolites in breast milk involve, in general, multi-step sample preparation and quantification techniques with low selectivity, high cost and much time and labour. Thus, a fast and efficient method based on sample preparation using the quick, easy, cheap, effective, rugged and safe (QuEChERS) method combined with gas chromatography coupled to mass spectrometry with negative chemical ionisation (GC-NCI-MS) was developed, validated and applied for determination of 16 OCPs and metabolites in breast milk samples. The extract was cleaned by dispersive solid-phase extraction (d-SPE) using MgSO4 and C18, evaporated in a Turbovap® system, redissolved and analysed by GC-NCI-MS. The method was validated showing acceptable recoveries (72–118%) and precision (RSD <19%). Method limits of detection (LODs) and quantification (LOQs) ranged from 0.75 to 7.5 ng g?1 and from 2.5 to 25 ng g?1 lipid, respectively. The method was successfully applied to 20 samples of breast milk from different regions of the Rio Grande do Sul state, Brazil, of which 75% contained residues below the LOQs. 相似文献
9.
Depending on different reaction time and temperature employed, benzylation of 2,3-dichloronaphthazarin (I) using silver oxide as the catalyst can form either exclusively 5,8-dibenzyloxy-6,7-dichloronaphthalene-1,4-dione (II), or exclusively 5-benzyloxy-2,3-dichloro-8-hydroxynaphthalene-1,4-dione (IV); or a mixture of II and 5,8-dibenzyloxy-2,3-dichloronaphthalene-1,4-dione (III). Structures of these compounds were identified by nmr analysis. 相似文献