全文获取类型
收费全文 | 333篇 |
免费 | 6篇 |
国内免费 | 1篇 |
专业分类
化学 | 261篇 |
晶体学 | 3篇 |
力学 | 3篇 |
数学 | 43篇 |
物理学 | 30篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 4篇 |
2021年 | 14篇 |
2020年 | 15篇 |
2019年 | 19篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 10篇 |
2015年 | 9篇 |
2014年 | 7篇 |
2013年 | 31篇 |
2012年 | 21篇 |
2011年 | 29篇 |
2010年 | 32篇 |
2009年 | 20篇 |
2008年 | 18篇 |
2007年 | 15篇 |
2006年 | 14篇 |
2005年 | 10篇 |
2004年 | 9篇 |
2003年 | 4篇 |
2002年 | 7篇 |
2001年 | 13篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1997年 | 1篇 |
1994年 | 1篇 |
排序方式: 共有340条查询结果,搜索用时 343 毫秒
81.
Gnül Yenilmez ifti Mahmut Durmu Elif enkuytu Adem Kl 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2009,74(4):881-886
This study dealt with the reactions of hexachlorocyclotriphosphazatriene, N3P3Cl6 (trimer) (1) with phenolphthalein (2) to give the phenolphthalein bridged compounds 3, 4 and 5. The phenolphthalein bridged cyclotriphosphazatriene derivatives are reported for the first time. The new compounds (3–5) are characterized by elemental analysis, mass spectrometry, UV–vis, FT-IR, 1H, 31P NMR and fluorescence spectroscopy. The more bridged phenolphthalein groups show the higher intensity of the absorption bands in the UV–vis spectra. Fluorescence spectrum of compound 3 shows a small band in the lower spectral range, while the spectra of compounds 4 and 5 show more intense and a band in higher spectral range. 相似文献
82.
In this paper, we investigate the problem of statistical approximation to a function by means of positive linear operators
defined on a modular space. Especially, in order to get more powerful results than the classical aspects we mainly use the
concept of statistical convergence. A non-trivial application is also presented. 相似文献
83.
Elif Tokar Erdemir Rajan Batta Peter A. Rogerson Alan Blatt Marie Flanigan 《European Journal of Operational Research》2010
Aeromedical and ground ambulance services often team up in responding to trauma crashes, especially when the emergency helicopter is unable to land at the crash scene. We propose location-coverage models and a greedy heuristic for their solution to simultaneously locate ground and air ambulances, and landing zones (transfer points). We provide a coverage definition based on both response time and total service time, and consider three coverage options; only ground emergency medical services (EMS) coverage, only air EMS coverage, or joint coverage of ground and air EMS in which the patient is transferred from an ambulance into an emergency helicopter at a transfer point. To analyze this complex coverage situation we develop two sets of models, which are variations of the Location Set Covering Problem (LSCP) and the Maximal Covering Location Problem (MCLP). These models address uncertainty in spatial distribution of motor vehicle crash locations by providing coverage to a given set of both crash nodes and paths. The models also consider unavailability of ground ambulances by drawing upon concepts from backup coverage models. We illustrate our results on a case study that uses crash data from the state of New Mexico. The case study shows that crash node and path coverage percentage values decrease when ground ambulances are utilized only within their own jurisdiction. 相似文献
84.
Tabanca N Demirci B Turner JL Pounders C Demirci F Başer KH Wedge DE 《Natural product communications》2010,5(9):1421-1426
Volatile compounds from seven Salvia species and one interspecific hybrid growing at the Dallas Arboretum and Botanical Garden, Texas, US. Salvia coccinea, S. farinacea, S. greggii, S. leucantha, S. longispicata x farinacea, S. madrensis, S. roemeriana and S. splendens were investigated for their chemical compositions using a microdistillation technique. Volatiles were analyzed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC/MS). One hundred and twenty seven compounds were identified representing 94.3-99.7% of the oils. The major components in each of the seven species were as follows: S. coccinea (Z)-3-hexenal (31%), viridiflorol (19%); S. farinacea 1-octen-3-ol (30%) and (Z)-3-hexenal (23%); S. greggii 1,8-cineole (22%), borneol (17%), camphene (11%) and alpha-pinene (10%); S. leucantha limonene (35%) and alpha-pinene (17%); S. longispicata x farinacea 1-octen-3-ol (50%) and (Z)-3-hexenal (24%); S. madrensis (Z)-3-hexenal (53%); S. roemeriana limonene (49%) and alpha-pinene (20%); and S. splendens (Z)-3-hexenal (36%), 2,5-dimethoxy-p-cymene (19%) and linalool (11%). The microdistillation method was fast, practical and a useful technique that enabled the isolation of the volatiles in samples when only limited quantities were available. 相似文献
85.
Serbetçi T Demirci B Güzel CB Kültür S Ergüven M Başer KH 《Natural product communications》2010,5(9):1369-1374
The chemical compositions of the water-distilled essential oil of Stachys cretica ssp. lesbiaca Rech. fil. and S. cretica ssp. trapezuntica Rech. fil. were determined by GC and GC-MS. Altogether 63 compounds were identified. The sesquiterpene hydrocarbon, germacrene D (20.3% and 12.9% respectively) was the main component identified in both oils. Furthermore, ethanol, light petroleum, dichloromethane, ethyl acetate and n-butanol extracts prepared from the aerial parts of the plants were tested for their antimicrobial activities against six bacterial strains and the yeast Candida albicans. The extracts exhibited no antibacterial activity, but the light petroleum and n-butanolic fractions showed low antifungal activities. Crude ethanolic extracts of the two subspecies were tested for their ability to inhibit the growth of HL-60 and Ishikawa human tumor cell lines. The IC50 values were 100 microg/mL for the HL-60 cell line and 200 microg/mL for the Ishikawa cell line. 相似文献
86.
In the present paper, we study a Korovkin type approximation theorem in the setting of \(K_{a}\)-convergence that contains the classical result. We also study the rate of \(K_{a}\)-convergence and afterwards, we give some concluding remarks. 相似文献
87.
88.
Necmi Dege Hasan Ibudak Elif Adyaman 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(9):m401-m403
In the crystal structure of the title compound {systematic name: bis[6‐methyl‐1,2,3‐oxathiazin‐4(3H)‐one 2,2‐dioxide(1−)‐κ2N3,O4]bis(3‐methylpyridine)copper(II)}, [Cu(C4H4NO4S)2(C6H7N)2], the CuII centre resides on a centre of symmetry and has an octahedral geometry that is distorted both by the presence of four‐membered chelate rings and because of the Jahn–Teller effect. The equatorial plane is formed by the N atoms of two methylpyridine ligands and by the more basic O atoms of the acesulfamate ligands, while the weakly basic N atoms of these ligands are in elongated axial positions with a misdirected valence. The crystal is stabilized by two intermolecular C—H⋯O interactions involving the methyl and CH groups, and the sulfonyl O atoms of the acesulfamate group. 相似文献
89.
Synthesis of a Functional Aluminium Alkynide, Me3C‐C≡C‐AlBr2, and its Reactions with the Bulky Lithium Compound LiCH(SiMe3)2 Treatment of aluminium tribromide with the lithium alkynide (Li)C≡C‐CMe3 afforded the aluminium alkynide Me3C‐C≡C‐AlBr2 ( 1 ) in an almost quantitative yield. 1 crystallizes with trimeric formula units possessing Al3C3 heterocycles and the anionic carbon atoms of the alkynido groups in the bridging positions. A dynamic equilibrium was determined in solution which probably comprises trimeric and dimeric formula units. Reaction of 1 with one equivalent of LiCH(SiMe3)2 yielded the compound [Me3C‐C≡C‐Al(Br)‐CH(SiMe3)2]2 ( 2 ), which is a dimer via Al‐C‐Al bridges. Two equivalents of the lithium compound gave a mixture of four main‐products, which could be identified as 2 , Li[Me3C‐C≡C‐Al{CH(SiMe3)2}3] ( 3 ), Me3C‐C≡C‐Al[CH(SiMe3)2]2 ( 4 ), and Al[CH(SiMe3)2]3. The lithium atom of 3 is coordinated by the C≡C triple bond and an inner carbon atom of one bis(trimethylsilyl)methyl group. Further interactions were observed to C‐H bonds of methyl groups. 相似文献
90.
We are investigating the use of capacitive micromachined ultrasonic transducers (cMUT's) for use in medical imaging. We propose an ultrasound probe architecture designed to provide volumetric ultrasound imaging from within an endoscope channel. A complete automated experimental system has been implemented for testing the imaging performance of cMUT arrays. This PC-based system includes custom-designed circuit boards, a software interface, and resolution test phantoms. We have already fabricated 1D and 2D cMUT arrays, and tested the pulse-echo imaging characteristics of 1D arrays. Beamforming and image formation algorithms that aim to reduce the complexity of data acquisition hardware are tested via numerical simulations and using real data acquired from our system. 相似文献