全文获取类型
收费全文 | 57679篇 |
免费 | 7695篇 |
国内免费 | 10038篇 |
专业分类
化学 | 44706篇 |
晶体学 | 2141篇 |
力学 | 3714篇 |
综合类 | 473篇 |
数学 | 3689篇 |
物理学 | 20689篇 |
出版年
2024年 | 147篇 |
2023年 | 556篇 |
2022年 | 1182篇 |
2021年 | 1404篇 |
2020年 | 1881篇 |
2019年 | 1627篇 |
2018年 | 1508篇 |
2017年 | 1797篇 |
2016年 | 2348篇 |
2015年 | 2192篇 |
2014年 | 2835篇 |
2013年 | 4439篇 |
2012年 | 4397篇 |
2011年 | 3469篇 |
2010年 | 2979篇 |
2009年 | 3690篇 |
2008年 | 3855篇 |
2007年 | 3998篇 |
2006年 | 3746篇 |
2005年 | 3308篇 |
2004年 | 3189篇 |
2003年 | 2758篇 |
2002年 | 2790篇 |
2001年 | 1919篇 |
2000年 | 1929篇 |
1999年 | 1607篇 |
1998年 | 1408篇 |
1997年 | 1136篇 |
1996年 | 1094篇 |
1995年 | 956篇 |
1994年 | 896篇 |
1993年 | 668篇 |
1992年 | 670篇 |
1991年 | 452篇 |
1990年 | 352篇 |
1989年 | 287篇 |
1988年 | 274篇 |
1987年 | 201篇 |
1986年 | 183篇 |
1985年 | 193篇 |
1984年 | 153篇 |
1983年 | 109篇 |
1982年 | 119篇 |
1981年 | 109篇 |
1980年 | 98篇 |
1979年 | 105篇 |
1978年 | 66篇 |
1977年 | 77篇 |
1976年 | 55篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 984 毫秒
881.
M. Röder J. Hahn U. Falke S. Schulze F. Richter M. Hietschold 《Mikrochimica acta》1997,125(1-4):283-286
A series of BN films was deposited by means of r.f. magnetron sputtering of a h-BN target onto Si(1OO) surfaces. Hereby, the substrate bias voltage was varied. Special interest is focussed to the influence of the deposition parameters on the orientation of the growing hexagonal BN film with respect to the substrate. For structural investigation, cross section samples were prepared. In addition to HRTEM and diffraction investigations, especially electron energy loss spectroscopy (EELS) was applied successfully for phase identification. For negative bias voltages of U
B
=–300 V and U
B
=–350V, we found a phase system consisting of a first-grown 25 nm thick layer of hexagonal structure with the c axis parallel to the substrate surface followed by the cubic phase.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday 相似文献
882.
883.
利用等温和非等温方法详细研究了芳香族聚酯──热致型聚合物对,对’-联苯二甲酸二辛酯的结晶相和液晶相形成机理,并计算了相变过程中的表面自由能与温度系数,研究结果表明:从介晶相开始的结晶过程是二维异相成核、三维线性增长的,而从各向同性液相开始的液晶相形成过程则是二维异相成核二维线性增长的.对两个晶化过程的表面自由能的研究表明,该聚合物液晶相形成过程的相转变表面自由能比结晶过程小得多,预示了它将具有更大的晶化速率.研究还发现,该聚合物的液晶相形成过程具有比结晶过程大得多的温度敏感性. 相似文献
884.
Neil G. Connelly John G. Crossley A. Guy Orpen Henrietta Salter 《Journal of organometallic chemistry》1994,480(1-2):c12-c13
The reaction of [NBu4n]2Cu(mnt)2] with [Pt(CNMe)4][PF6]2 gives [Pt(mnt)(CNMe)2]·(NC)2C2S2CNMe, an X-ray study of which reveals co-stacking of neutral planar metal and organic molecules. 相似文献
885.
Crystal Structure of Hexamine Cyclotriphosphazene, P3N3(NH2)6 In the presence of KNH2 hexamine cyclotriphosphazene semi ammoniate (molar ratio 12:1) in NH3 gives crystals of solvent free P3N3(NH2)6 within 5 d at 130°C and p(NH3) = 110 bar. The structure was solved by X-rax methods: P3N3(NH2)6: P21/c, Z = 4, a = 10.889(6) Å, b = 5.9531(6) Å, c = 13.744(8) Å, β = 97.83(3)°, Z(Fo) = 1 721 with (Fo)2 ≥ 3σ(Fo)2, Z(var.) = 157, R/Rw = 0,036/0,041 The structure contains columns of molecules P3N3(NH2)6 all in the same orientation. The six-membered rings within one molecule have boat conformation. The columns are stacked together in a way that one is surrounded by four others shifted by half a lattice constant in direction [010]. Strong hydrogen bridge-bonds N? H…?N connect molecules within the columns and between them. 相似文献
886.
The copolymerization of styrene (St) with maleic anhydride (MAh) performed at 22 °C in the presence of dibenzyl trithiocarbonate exhibits controlled nature evidenced by: narrow molecular weight distribution, controlled molecular weight and first-order polymerization kinetics. The composition analysis of the copolymers obtained by 13C NMR spectra shows the molar fraction of St in obtained copolymers is almost equal to 0.5 throughout the copolymerization. The sequence structure of the copolymer was obtained from DEPT experiments by recording the spectra at π/4 and 3π/4, and then combining them together, the results showed that the copolymers obtained possessed well-defined alternating structure. The experiment shows that charge-transfer-complex formed from St and MAh participates in both initiation and chain growth throughout the copolymerization. 相似文献
887.
Khodayar Gholivand Zahra Hosseini Mehrdad Pourayoubi Zahra Shariatinia 《无机化学与普通化学杂志》2005,631(15):3074-3079
Some new N‐carbonyl, phosphoramidates with formula C6H5C(O)N(H)P(O)R2 (R = NC3H6 ( 1 ), NC6H12 ( 2 ), NHCH2CH=CH2 ( 3 ), N(C3H7)2 ( 4 )) and CCl3C(O)N(H)P(O)R′2 (R′ = NC3H6 ( 5 ), NHCH2CH=CH2 ( 6 )) were synthesized and characterized by 1H, 13C, 31P NMR and IR spectroscopy and elemental analysis. The structures were determined for compounds 1 and 2 . Compound 1 exists as two crystallographically independent molecules in crystal lattice. Both compounds 1 and 2 produced dimeric aggregates via intermolecular ‐P=O…H‐N‐ hydrogen bonds, which in compound 2 is a centrosymmetric dimer. In compounds with four‐membered ring amine groups, 3J(P,C)>2J(P,C), in agreement with our previous studies about five‐membered ring amine groups. Also, 3J(P,C) values in compounds 1 and 5 are greater than in compounds with five‐, six‐ and seven‐membered ring amine groups. 相似文献
888.
The title compound [Mn(phendione)(PDC)(H2O)2]·2H2O (H2PDC=pyridine-2,6-dicarboxylic acid) has been prepared in aqueous solution and characterized by single X-ray diffraction structure determination, elemental analysis, IR spectroscopy, and thermal analyses. The compound crystallizes in Monoclinic system, space group C2/c, a=1.017 51(11) nm, b=1.483 25(11) nm, c=1.461 21(13) nm, β=109.86(10)°, V=2.074 1(3) nm3, Z=4, F(000)=1 028, μ=0.701 mm-1, Dc=1.609 g·cm-3, R1=0.028 9, wR2=0.078 8 [I>2σ(I)]. Crystal structure reveals that complex consists of one-dimensional chain framework bridged by hydrogen bonds that formed by uncoordinated water and oxygen atom of carboxyl group in PDC2-. Furthermore, the complexes form a three-dimensional super-molecular structure through hydrogen bonds. CCDC: 648570. 相似文献
889.
Synthesis and X-ray structural study of octa-coordinated hexafluoroacetylacetone hafnium(IV) complex
K. V. Zherikova N. B. Morozova I. A. Baidina V. I. Alekseev I. K. Igumenov 《Journal of Structural Chemistry》2006,47(1):82-86
Hexafluoroacetylacetonate hafnium(IV) complex has been synthesized and studied by X-ray structural analysis. Crystal data for C30H8F36Hf2O14 are: a = 12.957(3) Å, b = 16.687(3) Å, c = 12.398(3) Å, β = 108.97°, space group P21/c, Z = 2, d calc = 2.137 g/cm3, R = 0.047. The molecular structure is built from discrete binuclear molecules of Hf2(OH)2(hfac)6 composition; the Hf...Hf distance in dimer is 3.533 Å. The structural units are connected by van der Waals interactions in the crystal. 相似文献
890.
Reaction of tBuPLi2 with Carbodiimides The reaction of bis(cyclohexyl)carbodiimide with tBuPLi2 in THF at 20 °C leads to the tetranuclear Li‐complex [Li4(THF)2{tBuP([cHexN]2C)2}2] ( 1 ). No addition on the carbodiimide but a silyl transfer was observed under similar conditions during the treatment of bis(trimethylsilyl)carbodiimide with tBuPLi2 to give the lithium salts tBuP(SiMe3)Li and [Li(THF)(Me3SiN‐C≡N)]n ( 2 ). 1 was characterized by NMR, IR and RE spectroscopy, mass spectrometry and X‐ray analyses. Theoretical calculations were performed for 1 . According to the structural investigations 1 consists ofa central centrosymmetrical twelve‐membered Li2N4C4P2 ring adjacent by two six‐membered LiN2C2P rings. The peripheric Li+ cations posssess coordination number (cn) 3 buildt‐up by two N atoms and a THF ligand, while the two central Li+ cations possess only cn 2. However, the theoretical calculations have shown no relevant bonding Li···Li or Li···P interaction. 相似文献