首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   93篇
  免费   8篇
化学   75篇
晶体学   1篇
力学   5篇
数学   5篇
物理学   15篇
  2023年   7篇
  2022年   3篇
  2021年   7篇
  2020年   5篇
  2019年   4篇
  2018年   3篇
  2017年   3篇
  2016年   1篇
  2015年   4篇
  2014年   1篇
  2013年   6篇
  2012年   6篇
  2011年   9篇
  2010年   1篇
  2009年   2篇
  2008年   6篇
  2007年   5篇
  2006年   5篇
  2005年   3篇
  2004年   2篇
  2002年   2篇
  2001年   1篇
  2000年   1篇
  1999年   1篇
  1997年   1篇
  1996年   1篇
  1993年   1篇
  1992年   1篇
  1990年   1篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
排序方式: 共有101条查询结果,搜索用时 15 毫秒
81.
Palladium membranes have been used for decades for the separation of hydrogen from other gasses. In this letter the use of thin palladium leaves to act as sources of atomic hydrogen for silicon samples is explored. It has been assumed in the past that although hydrogen diffuses through palladium in atomic form, the atoms recombine to form molecular hydrogen at the surface. In this letter it is shown that hydrogen supplied to one surface of a palladium leaf can result in atomic hydrogen being released from the opposite surface at low pressure. This is demonstrated through the use of a palladium leaf in a direct plasma system which allows for atomic hydrogen to be supplied to a sample while avoiding exposure to UV radiation from the plasma and high energy charged particles. This method is shown to provide significant atomic hydrogen to silicon samples and improve passivation of silicon surfaces. Method of Shielded Hydrogen Passivation: Schematic of direct plasma chamber with a shield inserted between the plasma and the silicon sample.  相似文献   
82.
The monoanion of dihydrogenated corannulene isolated in the form of its potassium salt, namely tris(diglyme‐κ3O,O′,O′′)potassium hexacyclo[11.5.2.04,17.07,16.010,15.014,18]icosa‐1,3,5,7(16),8,10(15),11,13,17‐nonaenide, [K(C6H14O3)3](C20H12), has been structurally characterized for the first time. The X‐ray study confirms the previous NMR spectroscopic prediction that the two H atoms are attached to the same six‐membered ring to form 1,2‐dihydrocorannulene, thus destroying the aromaticity of only one arene ring of the corannulene core. The direct comparison of (C20H12) with the parent corannulene anion, (C20H10), is provided to illustrate the geometry perturbations caused by rim hydrogenation.  相似文献   
83.
Time-resolved fluorescence spectra of gas-phase toluene and naphthalene were investigated upon picosecond laser excitation at 266 nm as a function of temperature (toluene 296–1,025 K, naphthalene 374–1,123 K), pressure (1–10 bar), and bath gas composition (varying concentrations of N2, O2, and CO2) with a temporal resolution of 50 ps. In the investigated temperature range, the fluorescence spectra of both toluene and naphthalene show a significant red-shift, whereas the fluorescence lifetime decreases with increasing temperature, more pronounced for toluene than for naphthalene. Increasing the total pressure of either N2 or CO2 from atmospheric to 10 bar leads to an increase by about 20 % (naphthalene at 373 K) and a decrease by 60 % (toluene at 575 K) in fluorescence lifetimes, respectively. As expected, at atmospheric pressure collisions with O2 shorten the fluorescence lifetime of both toluene and naphthalene significantly, e.g., by a factor of 30 and 90 when changing O2 partial pressure at 373 K from 0 to 0.21 bar, respectively. The fluorescence model of Koban et al. (Appl Phys B 80: 777, 2005) for the dependence of the toluene quantum yield on temperature and O2 partial pressure at atmospheric pressure describes toluene fluorescence lifetimes well within its range of validity. The model is modified to satisfactorily predict effective toluene fluorescence lifetimes in N2 at pressures up to 10 bar. However, it still fails to predict the dependence at simultaneously elevated temperatures and pressures in air as bath gas. Similarly, an empirical model is presented for predicting (relative) fluorescence quantum yields and lifetimes of naphthalene. Although the fitting models have their shortcomings this publication presents a data set of great importance for practical LIF applications, e.g., in-cylinder mixture formation diagnostics in internal combustion engines.  相似文献   
84.
(111)- and (100)-oriented Si samples were implanted with Si+ ions at 1 MeV to a dose of 1?×?1016?cm?2 and with 5?×?1016 He+ cm?2 at 10?keV or 50?keV and eventually annealed in the 800–1000°C temperature range. Sample characterisation was carried out by cross-section transmission electron microscopy, positron annihilation spectroscopy and nuclear reaction analysis. In addition to the formation of He bubbles at the projected range of He, bubbles were observed after solid-phase epitaxial growth (SPEG) of the embedded amorphous Si layer. The He threshold concentration required to obtain thermally stable bubbles in amorphised Si is between one and four orders of magnitude lower than in c-Si. Since bubble formation and growth take place in the a-Si phase, the interaction with SPEG during annealing was studied by considering (100) and (111) Si. Both the SPEG velocity and the resulting defects play a role on bubble spatial distribution and size, resulting in bigger bubbles in (111) Si with respect to (100) Si.  相似文献   
85.
We prepared a series of water-soluble aromatic oligoamide sequences all composed of a segment prone to form a single helix and a segment prone to dimerize into a double helix. These sequences exclusively assemble as antiparallel duplexes. The modification of the duplex inner rim by varying the nature of the substituents borne by the aromatic monomers allowed us to identify sequences that can hybridize by combining two chemically different strands, with high affinity and complete selectivity in water. X-ray crystallography confirmed the expected antiparallel configuration of the duplexes whereas NMR spectroscopy and mass spectrometry allowed us to assess precisely the extent of the hybridization. The hybridization kinetics of the aromatic strands was shown to depend on both the nature of the substituents responsible for strand complementarity and the length of the aromatic strand. These results highlight the great potential of aromatic hetero-duplex as a tool to construct non-symmetrical dynamic supramolecular assemblies.  相似文献   
86.
Endothelialization of engineered vascular grafts for replacement of small-diameter coronary arteries remains a critical challenge. The ability for an acellular vascular graft to promote endothelial cell (EC) recruitment in the body would be very beneficial. This study investigated epsins as a target since they are involved in internalization of vascular endothelial growth factor receptor 2. Specifically, epsin-mimetic UPI peptides are delivered locally from vascular grafts to block epsin activity and promote endothelialization. The peptide delivery from fibrin coatings allowed for controlled loading and provided a significant improvement in EC attachment, migration, and growth in vitro. The peptides have even more important impacts after grafting into rat abdominal aortae. The peptides prevented graft thrombosis and failure that is observed with a fibrin coating alone. They also modulated the in vivo remodeling. The grafts are able to remodel without the formation of a thick fibrous capsule on the adventitia with the 100 µg mL−1 peptide-loaded condition, and this condition enabled the formation of a functional EC monolayer in the graft lumen after only 1 week. Overall, this study demonstrated that the local delivery of UPI peptides is a promising strategy to improve the performance of vascular grafts.  相似文献   
87.
A new colloidal stabilization mechanism, known as nanoparticle "haloing" (Tohver, V.; Smay, J. E.; Braem, A.; Braun, P. V.; Lewis, J. A. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, (16), 8950-8954), has been predicted theoretically and inferred experimentally in microsphere-nanoparticle mixtures that possess high charge and size asymmetry. The term "halo" implies the existence of a nonzero separation distance between the highly charged nanoparticles and the negligibly charged microspheres that they surround. By means of ultrasmall-angle X-ray scattering, we have quantified the microsphere-nanoparticle separation distance as well as the number of nanoparticles and their lateral separation distance within the self-organized halos that form in these binary mixtures.  相似文献   
88.
Studies have been carried out to establish the ability of O2-tolerant membrane-bound [NiFe] hydrogenases (MBH) from Ralstonia sp. to catalyze H2 production in addition to H2 oxidation. These hydrogenases are not noted for H2-evolution activity, and this is partly due to strong product inhibition. However, when adsorbed on a rotating disk graphite electrode the enzymes produce H2 efficiently, provided the H2 product is continuously removed by rapidly rotating the electrode and flowing N2 through the gastight electrochemical cell. Electrocatalytic H2 production proceeds with minimal overpotentiala significant observation because lowering the overpotential (the electrochemically responsive activation barrier) is seen as crucial in developing small-molecule catalysts for H2 production. A mutant having a high KM for H2 oxidation did not prove to be a better H2 producer relative to the wild type, thus suggesting that weak binding of H2 does not itself confer a tendency to be a H2 producer. Inhibition by H2 is much stronger than inhibition by CO and, most significantly, even O2. Consequently, H2 can be produced sustainably in the presence of O2 as long as the H2 is removed continuously, thereby proving the feasibility for biological H2 production in air.  相似文献   
89.
Details of the structures of two conformational polymorphs of the title compound, C12H17N2OS+·Cl, are reported. In form (I) (space group P), the two N—H groups of the cation are in a trans conformation, while in form (II) (space group P21/c), they are in a cis arrangement. This results in different packing and hydrogen‐bond arrangements in the two forms, both of which have extended chains lying along the a direction. In form (I), these chains are composed of centrosymmetric R42(18) (N—H...Cl and O—H...Cl) hydrogen‐bonded rings and R22(18) (N—H...O) hydrogen‐bonded rings. In form (II), the chains are formed by centrosymmetric R42(18) (N—H...Cl and O—H...Cl) hydrogen‐bonded rings and by R42(12) (N—H...Cl) hydrogen‐bonded rings.  相似文献   
90.
A highly efficient one-pot, three-component microwave-assisted procedure has been developed for the preparation of 1-(para-substituted-aryl)-4-(2-acetoxyethyl)piperazines and 1-(para-substituted-aryl)-4-(2-acetoxyethyl)piperidines. Microwave-accelerated heating of electron-deficient aryl halides and potassium acetate with either 1,4-diazabicyclo[2.2.2]octane (DABCO) or quinuclidine at 180 °C for 120 min provided the title products in good yields and with general substrate scope. Similarly, subjection of potassium pthalimide instead of potassium acetate to the same conditions provided good yields of 1-arylpiperazines and 1-arylpiperidines containing a 2-phthalimidoethyl substituent at the C-4 position.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号