全文获取类型
收费全文 | 611篇 |
免费 | 40篇 |
国内免费 | 1篇 |
专业分类
化学 | 501篇 |
晶体学 | 3篇 |
力学 | 13篇 |
数学 | 55篇 |
物理学 | 80篇 |
出版年
2023年 | 12篇 |
2022年 | 8篇 |
2021年 | 14篇 |
2020年 | 35篇 |
2019年 | 29篇 |
2018年 | 15篇 |
2017年 | 10篇 |
2016年 | 27篇 |
2015年 | 28篇 |
2014年 | 29篇 |
2013年 | 31篇 |
2012年 | 59篇 |
2011年 | 65篇 |
2010年 | 29篇 |
2009年 | 21篇 |
2008年 | 34篇 |
2007年 | 29篇 |
2006年 | 33篇 |
2005年 | 36篇 |
2004年 | 26篇 |
2003年 | 17篇 |
2002年 | 12篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有652条查询结果,搜索用时 0 毫秒
81.
82.
Lipids and block copolymers can be individually assembled into unsupported, spherical membranes (liposomes or polymersomes), each having their own particular benefits and limitations. Here we demonstrate the preparation of microscale, hybrid "lipopolymersomes" composed of the common lipid POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine) and the commercially available copolymer PBd-b-PEO (polybutadiene-b-poly(ethylene oxide)) with the goal of incorporating the advantageous qualities of the unitary systems into mixed-membrane capsules. We investigate the lipopolymersomes using confocal fluorescence microscopy and demonstrate that these hybrid membranes are well mixed on nanoscopic length scales within the permittable compositional windows for hybrid vesicle formation. We measure the intramembrane dynamics and mechanical properties of these hybrid membranes by fluorescence recovery after photobleaching (FRAP) and micropipet aspiration, respectively. For the first time, we demonstrate the demixing of lipid-rich and polymer-rich membrane domains within the same vesicle membrane. This is achieved by the biotinylation of one of the constituent species and cross linking with the protein NeutrAvidin. The resultant domain patterning is dependent upon which component carries the biotin functionality: cross linking of the copolymer species results in domains that ripen into a single, large, copolymer-rich island, and cross linking of the lipids yields many small, "spot-like", lipid-rich domains within a copolymer-rich matrix. We discuss these morphological differences in terms of the fluidity and mechanical properties of the membrane phases and the possible resultant interdomain interactions within the membrane. These heterogeneous hybrid lipopolymersomes could find applications in fields such as targeted delivery, controlled release, and environmental detection assays where these capsules possess the characteristics of biocompatible lipid membranes combined with enhanced mechanical strength and stability from the copolymer matrix. 相似文献
83.
Sontag SK Sheppard GR Usselman NM Marshall N Locklin J 《Langmuir : the ACS journal of surfaces and colloids》2011,27(19):12033-12041
Kumada catalyst-transfer polycondensation (KCTP) has proven to be an excellent strategy toward the synthesis of well-defined conjugated polymers. In this report, Ni(0) species are reacted with surface-bound aryl bromides to yield KCTP initiators of structure (aryl)Ni(II)-Br. Surface-confined Kumada reactions are carried out with a ferrocene functionalized Grignard reagent to quantify initiator coverage, ligand exchange, and Kumada reaction kinetics. In addition, surface-initiated Kumada catalyst-transfer polycondensation (SI-KCTP) is carried out from the fabricated initiators to modify SiO(2) and ITO surfaces. Uniform poly(3-methylthiophene) films with thicknesses between 40 and 65 nm were characterized using a variety of spectroscopic and electrochemical techniques. 相似文献
84.
Swor CD Hanson KR Zakharov LN Tyler DR 《Dalton transactions (Cambridge, England : 2003)》2011,40(34):8604-8610
Non-coordinated hydroxymethylphosphines react readily with primary and secondary amines by the phosphorus Mannich reaction. To determine if this reactivity can be used to synthesize phosphine macrocycles, trans-Fe(DHMPE)(2)Cl(2) (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane) was prepared and reacted with various amines. However, no phosphorus Mannich reactivity was observed. In order to understand why no reactions occurred, the Mannich reactivity of the borane-coordinated hydroxymethylphosphines DHMPE·2BH(3) and Ph(2)PCH(2)OH·BH(3) was investigated. These borane-coordinated phosphines also did not undergo the phosphorus Mannich reaction. These results suggest that the lone pair of electrons on the phosphorus atom is essential for the phosphorus Mannich reaction to occur, and therefore it is not possible to use this reaction in a templated synthesis of phosphine macrocycles. It is speculated that the mechanism of the phosphorus Mannich reaction may involve a methylenephosphonium intermediate, analogous to an iminium in the standard Mannich reaction. X-ray crystal structures of trans-Fe(DHMPE)(2)Cl(2) and DHMPE·2BH(3) are also presented. Both crystal structures display an extended hydrogen-bonding network in the solid state. 相似文献
85.
In any Coxeter group, the conjugates of elements in the standard minimal generating set are called reflections, and the minimal
number of reflections needed to factor a particular element is called its reflection length. In this article we prove that
the reflection length function on an affine Coxeter group has a uniform upper bound. More precisely, we prove that the reflection
length function on an affine Coxeter group that naturally acts faithfully and cocompactly on ℝ
n
is bounded above by 2n, and we also show that this bound is optimal. Conjecturally, spherical and affine Coxeter groups are the only Coxeter groups
with a uniform bound on reflection length. 相似文献
86.
Wolstenholme DJ Titah JT Che FN Traboulsee KT Flogeras J McGrady GS 《Journal of the American Chemical Society》2011,133(41):16598-16604
The solid-state structures of LiNH(2)BH(3) and NaNH(2)BH(3) have been shown recently to exhibit intricate M(δ+)···(δ-)H-B and N-H(δ+)···(δ-)H-B interactions. However, closer inspection of these structures reveals additional homopolar H···H interactions, viz., B-H(δ-)···(δ-)H-B and N-H(δ+)···(δ+)H-N, which contribute to the relative stability of the extended structures of these crystalline materials. In addition, an NMR study of the isotopomer LiND(2)BH(3) shows that a significant quantity of H(2) is desorbed thermally along with HD, which can only arise from hydride-hydride interactions, either directly from B-H(δ-)···(δ-)H-B moieties or indirectly through the participation of Li-H intermediates. 相似文献
87.
Lebedeva NV Schmidt RD Concepcion JJ Brennaman MK Stanton IN Therien MJ Meyer TJ Forbes MD 《The journal of physical chemistry. A》2011,115(15):3346-3356
The proton-coupled electron transfer (PCET) reaction between the bpz-based photoexcited (3)MLCT state of [Ru(II)(bpy)(2)(bpz)](2+) (bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine) and a series of substituted hydroquinones (H(2)Q) has been studied by transient absorption (TA) and time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band. When the reaction is carried out in a CH(3)CN/H(2)O mixed solvent system with unsubstituted hydroquinone, the neutral semiquinone radical (4a) and its conjugate base, the semiquinone radical anion (4b), are both observed. Variation of the acid strength in the solvent mixture allows the acid/base dependence of the PCET reaction to be investigated. In solutions with very low acid concentrations, TREPR spectra exclusively derived from radical anion 4b are observed, while at very high acid concentrations, the spectrum is assigned to the protonated structure 4a. At intermediate acid concentrations, either a superposition of spectra is observed (slow exchange between 4a and 4b) or substantial broadening in the TREPR spectrum is observed (fast exchange between 4a and 4b). Variation of substituents on the H(2)Q ring substantially alter this acid/base dependence and provide a means to investigate electronic effects on both the ET and PT components of the PCET process. The TA results suggest a change in mechanism from PCET to direct ET quenching in strongly basic solutions and with electron withdrawing groups on the H(2)Q ring system. Changing the ligand on the Ru complex also alters the acid/base dependence of the TREPR spectra through a series of complex equilibria between protonated and deprotonated hydroquinone radicals and anions. The relative intensities of the signals from radical 4a versus 4b can be rationalized quantitatively in terms of these equilibria and the relevant pK(a) values. An observed equilibrium deuterium isotope effect supports the conclusion that the post-PCET HQ(?)/Q(?-) equilibrium is the most important in determining the 4a/4b ratio at early delay times. 相似文献
88.
Thin films of SnSb2S4 have been prepared on glass substrate by using thermal evaporation techniques. The films were annealed in argon gas at low pressure in sealed glass ampoules at 85 °C, 150 °C, 275 °C and 325 °C. XRD of the films reveal that the low temperature annealed films are poly crystalline while the as deposited films and high annealed films are in amorphous states. There is no adequate variation in the photoconductivity response of the amorphous and crystalline phases. The transmittance of the films is low and having no transmittance below 740 nm. The band gap calculated by ellipsometry technique is in the range of 1.82–3.1 eV. The films have n-type conductivity but the film annealed at 325 °C show p-type conductivity. 相似文献
89.
Bendikov M Duong HM Starkey K Houk KN Carter EA Wudl F 《Journal of the American Chemical Society》2004,126(24):7416-7417
A series of oligoacenes from benzene to decacene were studied computationally with DFT and CASSCF methods. In contrast to the common view that acenes are closed-shell systems or may have a triplet ground state, these results offer the first theoretical predictions for the singlet ground state and diradical character for oligoacenes. The nature of the ground states of these molecules arises from the disjoint nature of the NBMOs that are singly occupied in the diradical. 相似文献
90.
B. Kotliński D. Androić G. Backenstoss D. Bosnar H. Breuer H. Döbbeling T. Dooling M. Furić P.A.M. Gram N.K. Gregory A. Hoffart C.H.Q. Ingram A. Klein K. Koch J. Köhler M. Kroedel G. Kyle A. Lehmann A.O. Mateos K. Michaelian T. Petković M. Planinić R.P. Redwine D. Rowntree U. Sennhauser N. Šimičević R. Trezeciak H. Ullrich H.J. Weyer M. Wildi K.E. Wilson 《The European Physical Journal A - Hadrons and Nuclei》1998,1(4):435-445
The pion absorption reaction (π+,3
p
) on Ar was studied at pion energies of 70, 118, 162 and 239 MeV, and on N and Xe at 239 MeV. The 3p cross section with a 50 MeV cut on the missing energy is presented. The existence of an initial state interaction (ISI) component
to the reaction was investigated. The portion of the 3p cross section assigned to the ISI process is less than one half and at lower energies significantly so.
Received: 11 December 1997 相似文献