全文获取类型
收费全文 | 2736篇 |
免费 | 67篇 |
国内免费 | 9篇 |
专业分类
化学 | 1881篇 |
晶体学 | 23篇 |
力学 | 32篇 |
数学 | 574篇 |
物理学 | 302篇 |
出版年
2022年 | 19篇 |
2020年 | 47篇 |
2019年 | 30篇 |
2018年 | 18篇 |
2017年 | 26篇 |
2016年 | 42篇 |
2015年 | 69篇 |
2014年 | 52篇 |
2013年 | 97篇 |
2012年 | 93篇 |
2011年 | 104篇 |
2010年 | 75篇 |
2009年 | 58篇 |
2008年 | 121篇 |
2007年 | 112篇 |
2006年 | 98篇 |
2005年 | 106篇 |
2004年 | 80篇 |
2003年 | 66篇 |
2002年 | 51篇 |
2001年 | 23篇 |
2000年 | 33篇 |
1999年 | 30篇 |
1998年 | 35篇 |
1997年 | 26篇 |
1996年 | 25篇 |
1995年 | 31篇 |
1994年 | 36篇 |
1993年 | 41篇 |
1992年 | 33篇 |
1991年 | 24篇 |
1989年 | 31篇 |
1988年 | 42篇 |
1987年 | 31篇 |
1986年 | 17篇 |
1985年 | 38篇 |
1984年 | 51篇 |
1983年 | 30篇 |
1982年 | 24篇 |
1981年 | 37篇 |
1980年 | 41篇 |
1979年 | 28篇 |
1978年 | 26篇 |
1977年 | 31篇 |
1976年 | 25篇 |
1975年 | 30篇 |
1974年 | 31篇 |
1973年 | 18篇 |
1972年 | 18篇 |
1956年 | 25篇 |
排序方式: 共有2812条查询结果,搜索用时 0 毫秒
71.
F. Stobiecki T. Stobiecki F. Kellner K. Thoma K. Röll H. Gärtner 《Fresenius' Journal of Analytical Chemistry》1991,341(5-6):365-368
Summary An energy-dispersive X-ray analysis of Ti1–xNx films was performed by using an Si(Li) detector. The Ti1–xNx films were sputtered on polished steel substrates in a mixed Ar+N2 atmosphere by a reactive magnetron sputtering technique. Because the nitrogen X-ray line is absorbed by the Be window, an analysis method was developed based solely on the calibration of the characteristic Ti-K-intensities using a high-purity Ti standard. The film compositions were computed by means of a ZAF procedure. The samples closest to stoichiometric composition show maximum hardness. 相似文献
72.
Vladimir Müller Pavel Pták Josef Tkadlec 《International Journal of Theoretical Physics》1992,31(5):843-854
LetL be a concrete (=set-representable) quantum logic. Letn be a natural number (or, more generally, a cardinal). We say thatL admits intrinsic coverings of the ordern, and writeL
C
n
, if for any pairA, BL we can find a collection {C
i
iI}, where cardI<n andC
i
L for anyiI, such thatA B=
il
C
i
. Thus, in a certain sense, ifLC
n
, then the rate of noncompatibility of an arbitrary pairA,BL is less than a given numbern. In this paper we first consider general and combinatorial properties of logics ofC
n
and exhibit typical examples. In particular, for a givenn we construct examples ofL
C
n+1\C
n
. Further, we discuss the relation of the classesC
n
to other classes of logics important within the quantum theories (e.g., we discover the interesting relation to the class of logics which have an abundance of Jauch-Piron states). We then consider conditions on which a class of concrete logics reduce to Boolean algebras. We conclude with some open questions. 相似文献
73.
1,3-Dithiane-1-N-p-chlorophenylimides (1,4-9) were prepared and their configuration and conformation was determined by 1H and 13C NMR. The compounds were rearranged to the corresponding 2-(2'-amino-5'-chlorophenyl)-1, 3-dithianes (1U,4U,9U). The rearrangement reactions took place with ?95% stereospecifity. The mechanism of the reaction was investigated with the aid of analogs specifically deuterated at C-2. 相似文献
74.
The capabilities of the recently developed multireference, general-model-space (GMS), state-universal (SU) coupled-cluster (CC) method have been extended in order to enable the handling of any excited state that represents a single (S) or a double (D) excitation relative to the ground state. A series of calculations concerning the ground and excited states of the CH(+), HF, F(2), H(2)O, NH(2), and CH(2) molecules were carried out so as to assess the performance of the GMS SU CCSD method. For diatomics we have computed the entire potential energy curves, while for triatomics we have focused on vertical excitation energies. We demonstrate how a systematic enlargement of the model space enables a consideration of a larger and larger number of excited states. A comparison of the CC and full configuration interaction or large-scale CI results enables an assessment of the accuracy and reliability of the GMS SU CCSD method within a given basis set. In all cases very good results have been obtained, including highly excited states and those having a doubly-excited character. 相似文献
75.
Jandera P Skeifíková V Rehová L Hájek T Baldriánová L Skopová G Kellner V Horna A 《Journal of separation science》2005,28(9-10):1005-1022
Methods were developed for the analysis of natural antioxidants including phenolic compounds and flavonoids in beverages and plant extracts using gradient HPLC with multi-channel electrochemical coulometric detection. Suitability of various reversed-phase columns for this purpose was compared; pH and mobile phase gradients were optimized with respect to the separation selectivity and sensitivity of detection. Because of different target compounds in various sample types, the overlapping resolution maps and the normalized resolution product approaches described earlier were used to select optimum columns and gradients to suit the analysis of the individual sample types. The methods were applied to the analysis of phenolic compounds and flavonoids in beer, wine, tea, and yacon extracts. 32 phenolic compounds were identified and determined, including derivatives of benzoic and cinnamic acids, flavones, and a few related glycosides. Eight-channel CoulArray detection offers high selectivity and sensitivity with limits of detection in the low microg L(-1) range, at least an order of magnitude lower than single-channel coulometric detection using the Coulochem detector. No special sample pretreatment is necessary and, because of the compatibility of the CoulArray detector with gradient elution, phenolic antioxidants of different polarities can be determined in a single run. In addition to the retention times, the ratios of the areas of the pre-dominant and post-dominant peaks to the area of the dominant peak can be used for improved identification of natural antioxidants. 相似文献
76.
Antunes MA Ferrence GM Domingos A McDonald R Burns CJ Takats J Marques N 《Inorganic chemistry》2004,43(21):6640-6643
Reaction of [(Tp(Me)2)(2)UI] with KNR(2) (R = C(6)H(5), SiMe(3)) in tetrahydrofuran (THF) afforded the monomeric trivalent actinide amide complexes [(Tp(Me)2)(2)U[N(C(6)H(5))(2)]], 1, and [(Tp(Me)2)(2)U[N(SiMe(3))(2)]], 2. The complexes have been fully characterized by spectroscopic methods and their structures were confirmed by X-ray crystallographic studies. In the solid state 1 and 2 exhibit distorted pentagonal bipyramidal geometries. The U-NR(2) bond lengths in both complexes are the same but in complex 2 the greater steric demands of the N(SiMe(3))(2) ligand led to elongated U-N(pz) bonds, especially those opposite the amido ligand. 相似文献
77.
Stíbr B Holub J Bakardjiev M Pavlík I Tok OL Císarová I Wrackmeyer B Herberhold M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(10):2239-2244
The reaction of nido-[7,8,9-PC(2)B(8)H(11)] (1) with [[CpFe(CO)(2)](2)] (Cp=eta(5)-C(5)H(5) (-)) in benzene (reflux, 3 days) gave an eta(1)-bonded complex [7-Fp-(eta(1)-nido-7,8,9,-PC(2)B(8)H(10))] (2; Fp=CpFe(CO)(2); yield 38 %). A similar reaction at elevated temperatures (xylene, reflux 24 h) gave the isomeric complex [7-Fp-(eta(1)-nido-7,9,10-PC(2)B(8)H(10))] (3; yield 28 %) together with the fully sandwiched complexes [1-Cp-closo-1,2,4,5-FePC(2)B(8)H(10)] 4 a (yield 30%) and [1-Cp-closo-1,2,4,8-FePC(2)B(8)H(10)] 4 b (yield 5%). Compounds 2 and 3 are isolable intermediates along the full eta(5)-complexation pathway of the phosphadicarbaborane cage; their heating (xylene, reflux, 24 h) leads finally to the isolation of compounds 4 a (yields 46 and 52%, respectively) and 4 b (yields 4 and 5%, respectively). Moreover, compound 3 is isolated as a side product from the heating of 2 (yield 10%). The structure of compound 4 a was determined by an X-ray structural analysis and the constitution of all compounds is consistent with the results of mass spectrometry and IR spectroscopy. Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B]-COSY, and (1)H[(11)B(selective)] magnetic resonance measurements led to complete assignments of all resonances and are in excellent agreement with the structures proposed. 相似文献
78.
Jií Davídek Miloslava Neméthová Josef Seifert 《Fresenius' Journal of Analytical Chemistry》1977,287(4-5):286-287
Summary An indirect polarographic method for the determination of phosmet was elaborated. It is based on the fact that N-hydroxymethylphthalimide formed by the reaction of phosmet with sulphuric acid is polarographically active and exhibits a behaviour during electrolysis at a Hg dropping-electrode similar to that of phthalimide. The method has been applied to the determination of phosmet in apples. The sensitivity is 0.1 ppm.
Indirekte polarographische Bestimmung von Phosmet O,O-Dimethyl-S-(phthalimidomethyl)dithiophosphat
Zusammenfassung Das Verfahren beruht darauf, daß N-Hydroxymethylphthalimid, das bei der Reaktion von Phosmet mit Schwefelsäure gebildet wird, polarographisch aktiv ist und sich bei der Elektrolyse mit der Quecksilber-Tropfelektrode ähnlich wie Phthalimid verhält. Die Methode wurde zur Bestimmung von Phosmet in Äpfeln angewandt. Die Empfindlichkeit beträgt 0,1 ppm.相似文献
79.
Leyla Bitiş Sevda Süzgeç Ulaş Sözer Hasan Özçelik Josef Zapp Alexandra K. Kiemer Filiz Meriçli Ali H. Meriçli 《Helvetica chimica acta》2007,90(11):2217-2221
From the aerial parts of Delphinium buschianum Grossh ., collected in Turkey, a new diterpenoid alkaloid 1 , named budelphine, was isolated along with the known diterpenoid alkaloids karakoline ( 2 ), 18‐hydroxy‐14‐O‐methylgadesine ( 3 ), delsoline ( 4 ), lapaconidine ( 5 ), columbianine ( 6 ), 14‐benzoylneoline ( 7 ), and hetisine ( 9 ). The structure of 1 was established on the basis of 1H‐, 13C‐, DEPT, 1H,1H‐COSY, NOESY, HSQC, and HMBC NMR studies. 相似文献
80.