全文获取类型
收费全文 | 187276篇 |
免费 | 38693篇 |
国内免费 | 27450篇 |
专业分类
化学 | 125673篇 |
晶体学 | 2138篇 |
力学 | 12364篇 |
综合类 | 664篇 |
数学 | 23202篇 |
物理学 | 89378篇 |
出版年
2024年 | 608篇 |
2023年 | 2859篇 |
2022年 | 4811篇 |
2021年 | 4720篇 |
2020年 | 5516篇 |
2019年 | 5702篇 |
2018年 | 5045篇 |
2017年 | 5389篇 |
2016年 | 7378篇 |
2015年 | 7441篇 |
2014年 | 8375篇 |
2013年 | 12025篇 |
2012年 | 14110篇 |
2011年 | 15787篇 |
2010年 | 15761篇 |
2009年 | 15681篇 |
2008年 | 10220篇 |
2007年 | 9144篇 |
2006年 | 8230篇 |
2005年 | 7694篇 |
2004年 | 7268篇 |
2003年 | 5836篇 |
2002年 | 5399篇 |
2001年 | 5312篇 |
2000年 | 4310篇 |
1999年 | 4832篇 |
1998年 | 4025篇 |
1997年 | 3804篇 |
1996年 | 4113篇 |
1995年 | 4238篇 |
1994年 | 4072篇 |
1993年 | 3860篇 |
1992年 | 3355篇 |
1991年 | 2922篇 |
1990年 | 2467篇 |
1989年 | 2378篇 |
1988年 | 2264篇 |
1987年 | 1516篇 |
1986年 | 1548篇 |
1985年 | 1176篇 |
1984年 | 1192篇 |
1983年 | 542篇 |
1982年 | 1011篇 |
1981年 | 831篇 |
1980年 | 858篇 |
1979年 | 573篇 |
1978年 | 566篇 |
1977年 | 690篇 |
1976年 | 1089篇 |
1972年 | 560篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Several extraction procedures are described allowing arsenic speciation in sediments. The extraction of organometallic compounds
such as dimethylarsinic acid or monomethylarsonic acid is quite simple since these compounds are stable in the different extraction
media (HCl/ HNO3, H3PO4, ammonium oxalate) and are easily released independent of the extraction mode (magnetic stirring or microwave solubilization).
Extraction yields are higher than 96% for these two arsenic forms. An HCl/HNO3 microwave solubilization procedure allows the quantitative solubilization of mineral arsenic, but the differentiation between
the two oxidation states is not possible owing to the oxidation of As(III) to As(V). Extractions with orthophosphoric acid
or ammonium oxalate allow the solubilization of mineral arsenic with extraction yields ranging from 90 to 95% and the differentiation
between As(III) and As(V). Nevertheless, the amount of As(III) is underestimated owing to its partial oxidation. The usefulness
and advantages of microwave solubilization compared with conventional extraction procedures are discussed.
Received: 17 May 1996 / Revised: 19 September 1996 / Accepted: 25 September 1996 相似文献
992.
本文描述了用膜吸收法测量激光等离子体辐射温度空间分布的原理和方法给出了柱形缝靶轴向辐射温度随空间位置变化的特征,对测量结果进行了分析讨论。 相似文献
993.
A broad range echelle spectrograph with a CCD matrix detector is developed for different applications in atomic spectroscopy. The compact optical design in combination with a dispersive entrance slit illumination offers a low stray light level and high throughput in the UV spectral region comparable only with spectrographs of higher focal length. Ray tracing calculations for typical wavelengths and the multi-line spectrum of an iron arc show the excellent overall spectrum quality. The subtraction of two partial spectra of different steel alloys demonstrates the spectra evaluation possibilities of the CCD spectrograph. 相似文献
994.
This paper deals with the optimal control of a one-machine two-product manufacturing system with setup changes, operating in a continuous time dynamic environment. The system is deterministic. When production is switched from one product to the other, a known constant setup time and a setup cost are incurred. Each product has specified constant processing time and constant demand rate, as well as an infinite supply of raw material. The problem is formulated as a feedback control problem. The objective is to minimize the total backlog, inventory and setup costs incurred over a finite horizon. The optimal solution provides the optimal production rate and setup switching epochs as a function of the state of the system (backlog and inventory levels). For the steady state, the optimal cyclic schedule is determined. To solve the transient case, the system's state space is partitioned into mutually exclusive regions such that with each region, the optimal control policy is determined analytically. 相似文献
995.
生物小分子与铜锌超氧化物歧化酶相互作用的波谱学研究及其应用 总被引:1,自引:0,他引:1
本工作以ESR和NMR为主要手段,并结合其它生化方法,考察了氨基酸、核酸碱基、糖等生命基本物质,和抗坏血酸等生命必需物质与钢锌超氧化物歧化酶的相互作用,实验观测到氨基酸、核酸碱基和抗坏血酸在水溶液中可以与酶中的Cu2+作用而使其脱离活性部位,以小分子络合物形式游离在溶液中,同时使酶失活,脱离活性部位的Cu2+的比例和酶的失活程度取决于小分子配体的加入量及其与Cu2+的络合能力。此外,首次尝试使用ESR方法,并借助氨基酸与酶的作用,考察了铜锌超氧化物歧化酶在盐酸胍变性和热变性过程中的构象变化行为,结果表明这一方法是直观而有效的。 相似文献
996.
I.IlltroductionWhenanactiveunderwateracousticalpositioningsystemissearchingforanunderwatertarget,itisnecessaryforittocompleteatransmittingandreceivingcycIewithineachworkingperiodinordertofindoutthelocationofthetargetcorrectly.Forexample,whenanunder-wateracoustica1synchronouspositioningsystemistrackinganunderwatertargetwithagivensynchr0nizingperiod,thepropagati0ntimeofthepositioningsignaltravellingfr0mthetaJrgettothearrayofthesystemshouldbelessthanthesynchronizingperiod0therwisetargetdis-tance… 相似文献
997.
998.
Photoluminescence spectra of a series of MBE-grown Hg1-xCdxTe samples with the same mole fraction of about x≈0.39 have been measured at different temperatures from 5 to 100 K. By aid of the temperature and exciting laser power dependence of photoluminescence peak energy and relative intensity, as well as of the comparison with other measurements, four main structures dominating in the photoluminescence spectra are attributed to band to band, DoX, AoX and DoAo transitions. A deep donor state level located at about 8.5meV below the bottom of the conduction band has been observed and is determined to be due to the arsenic-occupied mercury vacancies. Two acceptor levels located at about 14.5 meV and about 31.5 meV above the top of the valance band have also been found from the measuements, which are identified as the mercury vacancies and As in anion site, respectively. 相似文献
999.
Summary Observation of fluorescence reactions on TLC plates is a valuable additional tool within the scope of screening procedures for many toxicologically relevant substances commonly encountered in clinical- and forensic-toxicological analysis. The reactions are based on native fluorescence without any treatment and on reactions obtained with an approved derivatization procedure. Due to the enormous sensitivity of the fluorescence detection, the method is also applicable to very low concentrations and small amounts of biological materials. The procedures described in this article have proven their high pragmatic usefulness in many practical cases.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
1000.
Summary X-ray reflectivity may be used to determine the internal structure of thin polymer films. An electron density difference of 10% for polystyrene and polyisoprene is sufficient to distinguish between a random distribution of lamellae, complete orientation parallel to the substrate surface and a surface induced formation of lamellae. The disappearance of the lamellar Bragg-peaks, with heating of the film, shows the transition into the disordered state. 相似文献