全文获取类型
收费全文 | 33543篇 |
免费 | 1779篇 |
国内免费 | 1785篇 |
专业分类
化学 | 21397篇 |
晶体学 | 133篇 |
力学 | 467篇 |
综合类 | 609篇 |
数学 | 6602篇 |
物理学 | 7899篇 |
出版年
2023年 | 346篇 |
2022年 | 345篇 |
2021年 | 1323篇 |
2020年 | 649篇 |
2019年 | 794篇 |
2018年 | 597篇 |
2017年 | 569篇 |
2016年 | 703篇 |
2015年 | 785篇 |
2014年 | 1097篇 |
2013年 | 1900篇 |
2012年 | 1214篇 |
2011年 | 1488篇 |
2010年 | 1383篇 |
2009年 | 1796篇 |
2008年 | 1898篇 |
2007年 | 2070篇 |
2006年 | 1580篇 |
2005年 | 1015篇 |
2004年 | 994篇 |
2003年 | 911篇 |
2002年 | 3413篇 |
2001年 | 1146篇 |
2000年 | 704篇 |
1999年 | 620篇 |
1998年 | 477篇 |
1997年 | 343篇 |
1996年 | 418篇 |
1995年 | 365篇 |
1994年 | 383篇 |
1993年 | 432篇 |
1992年 | 428篇 |
1991年 | 282篇 |
1990年 | 233篇 |
1989年 | 192篇 |
1988年 | 219篇 |
1987年 | 185篇 |
1986年 | 201篇 |
1985年 | 309篇 |
1984年 | 228篇 |
1983年 | 139篇 |
1982年 | 286篇 |
1981年 | 463篇 |
1980年 | 423篇 |
1979年 | 463篇 |
1978年 | 368篇 |
1977年 | 278篇 |
1976年 | 237篇 |
1974年 | 76篇 |
1973年 | 150篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
R. Keil 《Fresenius' Journal of Analytical Chemistry》1978,292(1):13-19
Zusammenfassung Die polarographische Spurenbestimmung von Uran durch katalytische Reduktion des NO
3
–
ist eine sehr empfindliche Analysenmethode. Sie kann aber nur unter Ausschluß einer Reihe von störenden Kationen und Anionen durchgeführt werden und erfordert eine genau zusammengesetzte Grundlösung. Darum wurde für das Uran eine einfache und rasche Extraktionsabtrennungsmethode mit Triphenylarsinoxid in CHCl3 ausgearbeitet. Die Rückextraktion aus der organischen Phase mit 0,02 N Na2CO3 und anschließendes definiertes Ansäuern mit HNO3 ermöglichen die Überführung des Urans in eine für die differentialpulspolarographische Bestimmung geeignete, reproduzierbare Grundlösung. Über 60 untersuchte Kationen und 20 Anionen stören die Bestimmung in weiten Grenzen nicht, was eine vielseitige Anwendung erlaubt. Die Nachweisgrenze liegt bei 1 ppb Uran in wäßriger Lösung. Die relative Standardabweichung beträgt für 100 ppb Uran ±2%.
Highly selective trace determination of uranium by differential pulse polarography (catalytic nitrate reduction), following extraction separation
Summary The trace determination of uranium by differential pulse polarography, using the catalytic reduction of nitrate is known to be very sensitive, if certain interfering ions are excluded and if the composition of the base solution remains constant. In order to fulfil these conditions a simple and selective method for extracting uranium with triphenylarsine oxide in chloroform was developed. Back extraction with 0,02 N Na2CO3 and addition of HNO3 lead to an appropriate base solution for the determination of uranium by differential pulse polarography. The influence of 60 cations and 20 anions was found to be negligible in a wide range. The limit of detection is about 1 ppb U and the standard deviation with 100 ppb U is ±2%.
Für die Überprüfung des Manuskripts möchte ich den Herren Dr. P. Baertschi und O. Antonsen herzlich danken. 相似文献
992.
Summary An emission spectrographic method for the estimation of Ga, In and Tl in semiconductor grade selenium at less than a ppm concentration has been developed. The impurities were separated from 1 g of selenium by volatilizing it from its solution in nitric acid and sulphuric acid. The residue containing the impurities was dissolved in small amounts of 6 M HCl and was loaded along with the washings on 10 mg mixture containing graphite (90%) and sodium chloride carrier (10%) in the electrode crater. The spectra were excited in a d.c. arc carrying 13 A current. The detection limits lie in the range of 0.0025–0.005 ppm. The precision of the method ranges from 7–12% for these elements. The recoveries of impurities from selenium were confirmed using radioactive tracers.
Spektrographische Bestimmung von Gallium, Indium und Thallium in Halbleiterselen
Zusammenfassung Eine 1 g-Probe wird in Salpeter- und Schwefelsäure gelöst und die Verunreinigungen durch Verflüchtigung des Selens isoliert. Sie werden in 6 M HCl gelöst und zusammen mit einem Graphit/NaCl-Gemisch (91) in die Elektrode eingebracht. Zur Anregung dient ein Gleichstrombogen (13 A). Die Nachweisgrenzen liegen im Bereich von 0.0025–0.005 ppm, der Variationskoeffizient bei 7–12%. Die Wiedergewinnung der Verunreinigungen wurde mit Hilfe von Radioindicatoren geprüft.相似文献
993.
Jutta Weisel 《Numerische Mathematik》1980,35(2):201-222
Summary In this note we consider so called p-analytic mappings of simply or doubly connected domains on rectangles or circular rings. Real and imaginary parts of the mappings can be described by minimal-principles. By minimizing the corresponding functionals in a class of linear or bilinear finite elements we obtain an approximation of the mapping and also upper and lower bounds for the p-module of a domain with polygonal boundary. Error bounds are given for smooth and for piecewise constant functionsp. We present numerical experiments. 相似文献
994.
Shie-Shien Yang 《Annals of the Institute of Statistical Mathematics》1981,33(1):463-470
Summary Let (X
1,Y
1), (X
2,Y
2),…, (X
n,Y
n) be i.i.d. as (X, Y). TheY-variate paired with therth orderedX-variateX
rn is denoted byY
rn and terms the concomitant of therth order statistic. Statistics of the form
are considered. The asymptotic normality ofT
n is established. The asymptotic results are used to test univariate and bivariate normality, to test independence and linearity
ofX andY, and to estimate regression coefficient based on complete and censored samples. 相似文献
995.
996.
Osvaldo Marrero 《Aequationes Mathematicae》1977,16(3):195-220
This work is an attempt to give a complete survey of all known results about pseudo (v, k, )-designs. In doing this, the author hopes to bring more attention to his conjecture given in Section 6; an affirmative answer to this conjecture would settle completely the existence and construction problem for a pseudo (v, k, )-design in terms of the existence of an appropriate (v, k, )-design. 相似文献
997.
Herbert Spohn 《Journal of statistical physics》1977,17(6):385-412
A single (nonrelativistic, spinless) electron subject to a constant external electric field interacts with impurities located on an infinitely extended lattice by a potential of random strength. The random strength is given by a field of Gaussian random variables. We show the existence of the averaged dynamics and prove that in the weak coupling limit, 0, 2
t= fixed, one obtains the usual transport equation for the velocity distribution.Work supported by a Max Kade Foundation fellowship.On leave of absence of the Fachbereich Physik der Universität München. 相似文献
998.
Frank J.S. Wang 《Stochastic Processes and their Applications》1977,5(2):173-193
Consider a population consisting of one type of individual living in a fixed region with area A. In [8], we constructed a stochastic population model in which the death rate is affected by the age of the individual and the birth rate is affected by the population density PA(t), i.e., the population size divided by the area A of the given region. In [8], we proposed a continuous deterministic model which in general is a nonlinear Volterra type integral equation and proved that under appropriate conditions the sequence PA(t) would converge to the solution P(t) of our integral equation in the sense that .In this paper, we obtain a “central limit theorem” for the random element √A(PA(t)?P(t)). We prove that under appropriate conditions √A(PA(t)?P(t)) will converge to a Gaussian process. (See Theorem 3.4 for the explicit formula of this Gaussian process.) 相似文献
999.
Toshinobu Korenaga 《Journal of fluorine chemistry》2007,128(5):557-561
A pentafluorophenyl group can act as a stereo-controlling group in oxazaborolidine-catalyzed asymmetric borane reduction through intramolecular π-π stacking interaction with a phenyl group. The intramolecular π-π interaction in oxazaborolidine bearing pentafluorophenyl group is confirmed by calculations and 1H NMR study. The interaction affects the enantioselectivity of the asymmetric reduction of acetophenone while the extent is small. 相似文献
1000.
CeO2和Pd在Ni/γ-Al2O3催化剂中的助剂作用 总被引:6,自引:0,他引:6
采用脉冲微反技术研究了添加n型半导体氧化物CeO2及贵金属Pd对Ni/γ Al2O3催化剂上CH4积炭/CO2消炭反应性能的影响,并运用BET、TPR、CO2 TPSR及氢吸附等技术对催化剂进行了表征.结果表明, n型半导体氧化物CeO2的添加可以降低Ni/γ Al2O3催化剂上CH4裂解积炭活性,提高CO2消炭活性,添加少量贵金属Pd可以进一步改变载体Al2O3、助剂CeO2和活性组分Ni之间的相互作用,从而改善Ni/γ Al2O3催化剂的抗积炭性能.通过Ni Ce Pd/γ Al2O3催化剂上CH4积炭/CO2消炭模型对上述作用机制作出了新的解释. 相似文献