全文获取类型
收费全文 | 1584篇 |
免费 | 117篇 |
国内免费 | 198篇 |
专业分类
化学 | 1490篇 |
晶体学 | 48篇 |
力学 | 10篇 |
综合类 | 17篇 |
数学 | 50篇 |
物理学 | 284篇 |
出版年
2024年 | 4篇 |
2023年 | 18篇 |
2022年 | 41篇 |
2021年 | 62篇 |
2020年 | 49篇 |
2019年 | 62篇 |
2018年 | 33篇 |
2017年 | 58篇 |
2016年 | 54篇 |
2015年 | 58篇 |
2014年 | 53篇 |
2013年 | 183篇 |
2012年 | 88篇 |
2011年 | 70篇 |
2010年 | 72篇 |
2009年 | 74篇 |
2008年 | 64篇 |
2007年 | 93篇 |
2006年 | 83篇 |
2005年 | 83篇 |
2004年 | 87篇 |
2003年 | 60篇 |
2002年 | 61篇 |
2001年 | 63篇 |
2000年 | 53篇 |
1999年 | 39篇 |
1998年 | 46篇 |
1997年 | 35篇 |
1996年 | 26篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1899条查询结果,搜索用时 93 毫秒
931.
932.
933.
样品湿法消解后用石墨炉原子吸收法测定了消渴丸中铅含量,用原子荧光法测定了砷含量.结果表明,消渴丸中重金属铅含量在0.23~0.55 mg·kg- 1之间;砷含量在0.55~1.09 mg·kg- 1之间;本法测定消渴丸中重金属铅的平均回收率为96.6%,砷的平均回收率为108.4%,说明该法可用于消渴丸中铅和砷的测定,... 相似文献
934.
构建了采用原子荧光法测定食品中铅含量时控制反应介质酸度的数学模型,根据该数学模型可以协调样品及载流中盐酸的体积分数、硼氢化钾的质量浓度、还原剂中氢氧化钾的质量浓度等反应介质的重要参数,保证铅烷发生的效率。采用该数学模型对2010年度国际弗帕斯(FAPAS)考核样品(辣椒粉)和国家标准物质(茶叶GBWl0016和灌木枝叶GBW07602)中的铅含量进行了测试,测定值的回收率介于98.9%~109.4%之间,取得满意结果,表明该模型可以指导实际应用。 相似文献
935.
Recently, resistivity measurements of some liquid metals and metallic alloys have been obtained using an electrode technique. A marked change of the slope of the resistivity versus temperature has been observed following the history of the alloy in the liquid state and the authors conclude that there are “structural transitions in the melt”. It is of interest to examine such effects and to try to understand their origin. In this work, the resistivities of several liquid metals and metallic alloys were measured as a function of temperature. Particular attention was given to the history of the alloy, including solidification and melting conditions. We also observed “anomalies”. Our experiments lead us to conclude that they must very probably be attributed to the release of gas or vapour bubbles in the liquid alloy at the first heating, resulting from the decomposition of compounds like oxides or hydroxides of the metals, but not from a phase transition in the liquid metallic state involving “breaking” Sn–Sn covalent bands. 相似文献
936.
The electrical transport properties were investigated of a glass system of basic composition 50?mol. % Pb3O4–50?mol. % P2O5 containing CoO, Cr2O3 or V2O5 dopanys. The ac conductivity and the thermoelectric power were measured as a function of temperature. Properties such as dielectric constant, loss factor tangent and electrical conductivity are reported in the frequency range 200?Hz–100?kHz and temperature range 300–450?K. The variation in electrical conductivity with temperature was found to depend on the types of transition metal ions involved. The temperature dependence of the frequency exponent, s, was analyzed using different theoretical models. The variation of the thermoelectric power with temperature indicated the presence of more than one conduction mechanism for the investigated samples. This result was confirmed with the results of the dielectric properties at different frequencies. The introduction of cobalt ions in glass formers improves the electrical properties of non-crystalline ionic conductors. 相似文献
937.
Verschiebbare und fahrbare Strahlenschutzwände für radiochemische Laboratorien werden von verschiedenen Firmen hergestellt. Für unscr Laboratoriumsgebäude ließen wir in den eigenen Werkstätten vierrädrige Gestellezur Aufnahmevon Strahlenschutzwänden aus Bleibausteinen herstellen (vgl. Abb. 1), um folgende Vorteile zu erreichen: 1. Bei Versuchen, die einen häufigen Wechsel von Prüflingen oder Anderungen des Aufbaues erfordern, ist eine bewegliche Wand zu bevorzugen. Bewegliche Bleiwände auf Schienen oder hängend, aber raumgebunden, sind finanziell sehr aufwendig und meist nur in einer Richtung veränderlich. Die von uns gebaute Schutzwand ist mit 2 festen und 2 beweglichen Rollenböcken versehen, so daß alle Richtungen befahren werden können. 2. Da Arbeiten mit γ-Strahlern häufig in den verschiedenen Raumen zeitlich verschoben ausgeführt werden, kann eine fahrbare Schutzwand für mehrere Versuchsaufbauten eingesetzt werden, so daß Abschirmmaterial eingespart wird. 3. Viele unserer Versuchseinrichtungen würden den Bau starrer Gestelle für die Aufnahme von Bleiwänden erfordern, was bei nur für kurze Zeit benotigten Versuchsaufbauten unrentabel wäre; die fahrbare Bleiwand ermoglicht auch hier Einsparungen. 4. Der Umbau einer Wand (Einsetzen von Fenstern, Manipulatoren, Öffnungen für Stulpenhandschuhe) läßt sich leicht ausführen, weil man sie in eine allseitig zugängliche Position fahren kann. 5. Durch die Stützung auf 4 Räder mit verhältnismäßig großem Abstand wird eine günstige Lastverteilung hinsichtlich Bodenbelastung erreicht. 相似文献
938.
A method for the determination of Pb isotope ratios (IR) in cigarette tobacco by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS) has been developed and applied to tobacco samples from genuine and counterfeit cigarettes obtained in the USA. The IR 207Pb/206Pb, 208Pb/206Pb, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb were measured using a Q-ICP-MS instrument. Two certified reference materials, grown in the USA and Bulgaria, were also analysed for comparison with the tobacco samples, as were tobacco samples from cigarettes obtained in Pakistan and China. The precision of the results was sufficient to distinguish between the counterfeit and genuine USA cigarettes. All of the genuine cigarettes and both reference materials, grown in different regions, were statistically distinct based on the measured ratios. The counterfeit cigarettes were indistinguishable from the reference material grown in Bulgaria. The technique shows promise as a method for identifying counterfeit cigarettes, possibly determining the source region. 相似文献
939.
P. Horn S. Hölzl W. Todt D. Matthies 《Isotopes in environmental and health studies》2013,49(1-2):31-42
Abstract In this contribution, the various and fundamentally different uses and applications of isotope signatures (of both heavy and light elements) are discussed. Examples are given for the successful use of 87Sr/86Sr in uncovering fraud in wine trade. Also, in an experiment related to “Waldsterben”, 87Sr/86Sr analyses reveal rather unexpected responses of spruces and maple-trees to mechanical damage of their roots. In another study, from 206Pb/207Pb (and 208Pb/207Pb) analyses of tree growth-rings it is demonstrated that they do not accurately record lead burdens in the environment. This is contrary to current views on the subject of tree-rings as reliable banks of past heavy metal pollutions of the biosphere. Furthermore, new perspectives of applications of isotopes in biological tissues, including those of cosmogenic and nucleogenic origin will be shortly outlined. 相似文献
940.
The local structure of the Cu2+ centers in alkali lead tetraborate glasses was theoretically studied based on the optical spectra data and high-order perturbation formulas of the spin Hamiltonian parameters (electron paramagnetic resonance g factors g∥, g⊥ and hyperfine structure constants A∥, A⊥) for a 3d9 ion in a tetragonally elongated octahedron. In these formulas, the relative axial elongation of the ligand O2? octahedron around the Cu2+ due to the Jahn–Teller effect is taken into account by considering the contributions to the g factors from the tetragonal distortion which is characterized by the tetragonal crystal-field parameters Ds and Dt. From the calculations, the ligand O2? octahedral around Cu2+ is determined to suffer about 19.2% relative elongation along the C4 axis of the alkali lead tetraborate glass system, and a negative sign for A∥ and a positive sign for A⊥ for these Cu2+ centers are suggested in the discussion. 相似文献