全文获取类型
收费全文 | 451篇 |
免费 | 16篇 |
国内免费 | 5篇 |
专业分类
化学 | 290篇 |
晶体学 | 3篇 |
力学 | 22篇 |
数学 | 60篇 |
物理学 | 97篇 |
出版年
2024年 | 4篇 |
2023年 | 5篇 |
2022年 | 22篇 |
2021年 | 14篇 |
2020年 | 24篇 |
2019年 | 19篇 |
2018年 | 14篇 |
2017年 | 16篇 |
2016年 | 38篇 |
2015年 | 14篇 |
2014年 | 17篇 |
2013年 | 45篇 |
2012年 | 37篇 |
2011年 | 32篇 |
2010年 | 19篇 |
2009年 | 17篇 |
2008年 | 25篇 |
2007年 | 19篇 |
2006年 | 16篇 |
2005年 | 10篇 |
2004年 | 12篇 |
2003年 | 8篇 |
2002年 | 6篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有472条查询结果,搜索用时 15 毫秒
1.
Kamali Ahangar Ehsan Izanlu Morteza Dolati Khakhian Shabnam Mohamad Abdulmajeed A. Bach Quang-Vu 《Journal of Thermal Analysis and Calorimetry》2021,144(6):2525-2541
Journal of Thermal Analysis and Calorimetry - Thermal microscale gas flow was simulated into a coplanar microchannel was simulated at a broad range of Knudsen numbers. Attempts were made to improve... 相似文献
2.
Yousef Bisabr 《International Journal of Theoretical Physics》2004,43(10):2137-2148
We investigate a conformal invariant gravitational model which is taken to hold at early universe. The conformal invariance allows us to make a dynamical distinction between the two unit systems (or conformal frames) usually used in cosmology and elementary particle physics. In this model we argue that when the universe suffers phase transition, the resulting mass scale introduced by particle physics should have a variable contribution to vacuum energy density. This variation is controlled by the conformal factor which is taken as a dynamical field. We then deal with the cosmological consequences of this model. In particular, we shall show that there is an inationary phase at early times. At late times, on the other hand, it provides a mechanism which makes a large effective cosmological constant relax to a sufficiently small value. Moreover, we shall show that the conformal factor acts as a quintessence field that leads the universe to accelerate at late times. 相似文献
3.
The H-point standard addition method (HPSAM) has been applied for the simultaneous determination of nickel and copper in trace levels, using 1-(2-pyridylazo)-2-naphthol (PAN) as a chromogenic reagent in aqueous Tween 80 micellar media. Under the optimum condition, the simultaneous determinations of nickel and copper by HPSAM were performed. The absorbances at one pair of wavelengths, 548 and 579 nm, were monitored with the addition of standard solutions of copper. The method is able to accurately determine copper-to-nickel ratios of 15:1 to 1:10 (Wt/Wt). The effects of diverse ions on the determination of nickel and copper to investigate the selectivity of the method were also studied. The recommended procedure was successfully applied to some water and alloy samples. 相似文献
4.
5.
Brian A. Naughton Benson Sibanda Jory P. Weintraub Julia San Román Vafa Kamali 《Applied biochemistry and biotechnology》1995,54(1-3):65-91
A method of coculturing adult rat hepatic parenchymal cells (PC) and stromal cells in a three-dimensional framework of nylon
filtration screens or biodegradable polymer meshes was developed in our laboratory. Rat liver stroma, which includes vascular
and bile duct endothelial cells, fat-storing cells, fibroblasts, and Kupffer cells, were isolated by gradient centrifugation
afterin situ liver perfusion and expanded in monolayer culture prior to seeding onto nylon screens or bioresorbable polyglycolic acid
(PGA) polymers oriented into a felt-like construct. A second inoculum of freshly isolated PC was applied after the stromal
cells became established. Histological analyses revealed that PC proliferation occurred until all available space for expansion
within the template was exhausted. These cells retained their rounded morphology, and after 4–5 wk 7–9 “layers” of PC filled
the 140-μm deep template. Dioxin-inducible cytochrome P450 activity was detected for up to 58 d in culture, and albumin, fibrinogen,
transferrin, and soluble fibronectin were detected in the medium by enzyme-linked immunosorbent assay (ELISA) for 48 d in
vitro. Immunohistochemical analysis of sections through the cultures confirmed the presence of these proteins as well as cytokeratin
at the cellular level; the extracellular matrix stained for both collagen type III and laminin. Long-term PC proliferation
and function were enhanced by the presence of stromal cells as well as by a meshwork template whose geometry allows the interaction
of PC with stroma and matrix on several different planes. To permit transplantation, co-cultures of hepatic PC and stromal
cells were established on PGA felt constructs instead of nylon screens. After 24 d in vitro, these constructs were grafted
into sites in the mesentery, omentum, and sub-cutaneous tissues of adult Long-Evans rats. The growth of hepatocytes after
30 din situ was evident by histological analysis; grafts of co-cultures regenerated a liver-like architecture consisting of sinusoids
and putative biliary structures. In addition, PC at these extrahepatic graft sites were positive for albumin, transferrin,
and fibrinogen synthesis by immunohistochemistry. Graft survival was enhanced when recipients were subjected to 40% hepatectomy.
Hepatic PC:stromal cell cocultures may prove useful in the restoration of liver function either by direct transplantation
using PGA or similar templates, or as extracorporeal devices, using nylon screens. 相似文献
6.
催化增强化学蒸气沉积法在聚酰亚胺上沉积钯-铂合金薄层 总被引:4,自引:0,他引:4
以N2,O2作载气,通过催化增强化学蒸气沉积(CECVD)分别制得在聚酰亚胺上的金属铂、钯及其合金薄层。铂、钯配合物的共同沉积可生成Pt-Pd合金薄膜。在Pd-Pt合金的沉积过程中,Pd/Pt的原子数比率随共同沉积的条件改变而变化。O2为载气、300 ℃条件下,用Pd(η3-allyl)(hfac)和Pt(COD)Me2作前驱体共沉积制备Pd-Pt合金,得到含Pd 37.2%,Pt 62.8%且不 相似文献
7.
Givens RS Timberlake GT Conrad PG Yousef AL Weber JF Amslinger S 《Photochemistry and photobiology》2003,78(1):23-29
On the basis of the earlier examples of diazopyruvoyl (DAP) groups reported by Lawton for covalent binding and cross-linking of proteins and oligopeptides and our recent demonstration that a coumaryl diazopyruvamide was used to label Type-I collagen, we have extended our investigations to the synthesis and cross-linking capabilities of a bis-DAP polyethylene glycol to cross-link Type-I collagen. The new photoactivated cross-linking agent, N,N'-bis(3-diazopyruvoyl)-2,2'-(ethylenedioxy)bis(ethylamine) (DPD, 2), has been designed and synthesized specifically to "weld" collagenous tissues by cross-linking Type-I collagen. A working model for the photochemical welding studies of collagenous tissues was developed using gelatin strips (gel strips) composed of denatured Type-I collagen. Gel strips are transparent to near-UV and visible light, uniform in thickness, and have reproducible composition. Furthermore, the availability of nucleophilic amine sites in gel strips was demonstrated by reaction with o-phthalaldehyde, producing a fluorescent derivative of the protein. Gel strips were coated with a solution of DPD in chloroform 7 irradiated at 320-390 nm, and the resulting bonded gel strips were tested for the strength of the weld. The welds were generally brittle and had average tensile strengths that exceeded 100 N/cm2. Welds were not formed in the absence of light or DPD. Scanning electron microscopy studies revealed a pockmarked surface from severed welds. Welds of rabbit Achilles tendon were also obtained using the tethered diazopyruvamide. These welds were much weaker, having an average tensile strength of 11.95 N/cm2 for DPD-2,2'-ethylenedioxy(bis)ethylamine comonomers in the cross-linking reaction. In both studies the welds obtained by this method were significantly stronger than the controls. 相似文献
8.
Rushdi I. Yousef Tobias Rüffer Harry Schmidt Dirk Steinborn 《Journal of organometallic chemistry》2005,690(5):1178-1191
MgMe2 (1) was found to react with 1,4-diazabicyclo[2.2.2]octane (dabco) in tetrahydrofuran (thf) yielding a binuclear complex [{MgMe2(thf)}2(μ-dabco)] (2). Furthermore, from reactions of MgMeBr with diglyme (diethylene glycol dimethyl ether), NEt3, and tmeda (N,N,N′,N′-tetramethylethylenediamine) in etheral solvents compounds MgMeBr(L), (L = diglyme (5); NEt3 (6); tmeda (7)) were obtained as highly air- and moisture-sensitive white powders. From a thf solution of 7 crystals of [MgMeBr(thf)(tmeda)] (8) were obtained. Reactions of MgMeBr with pmdta (N,N,N′,N″,N″-pentamethyldiethylenetriamine) in thf resulted in formation of [MgMeBr(pmdta)] (9) in nearly quantitative yield. On the other hand, the same reaction in diethyl ether gave MgMeBr(pmdta) · MgBr2(pmdta) (10) and [{MgMe2(pmdta)}7{MgMeBr(pmdta)}] (11) in 24% and 2% yield, respectively, as well as [MgMe2(pmdta)] (12) as colorless needle-like crystals in about 26% yield. The synthesized methylmagnesium compounds were characterized by microanalysis and 1H and 13C NMR spectroscopy. The coordination-induced shifts of the 1H and 13C nuclei of the ligands are small; the largest ones were found in the tmeda and pmdta complexes. Single-crystal X-ray diffraction analyses revealed in 2 a tetrahedral environment of the Mg atoms with a bridging dabco ligand and in 8 a trigonal-bipyramidal coordination of the Mg atom. The single-crystal X-ray diffraction analyses of [MgMe2(pmdta)] (12) and [MgBr2(pmdta)] (13) showed them to be monomeric with five-coordinate Mg atoms. The square-pyramidal coordination polyhedra are built up of three N and two C atoms in 12 and three N and two Br atoms in 13. The apical positions are occupied by methyl and bromo ligands, respectively. Temperature-dependent 1H NMR spectroscopic measurements (from 27 to −80 °C) of methylmagnesium bromide complexes MgMeBr(L) (L = thf (4); diglyme (5); NEt3 (6); tmeda (7)) in thf-d8 solutions indicated that the deeper the temperature the more the Schlenk equilibria are shifted to the dimethylmagnesium/dibromomagnesium species. Furthermore, at −80 °C the dimethylmagnesium compounds are predominant in the solutions of Grignard compounds 4-6 whereas in the case of the tmeda complex7 the equilibrium constant was roughly estimated to be 0.25. In contrast, [MgMeBr(pmdta)] (9) in thf-d8 revealed no dismutation into [MgMe2(pmdta)] (12) and [MgBr2(pmdta)] (13) even up to −100 °C. In accordance with this unexpected behavior, 1:1 mixtures of 12 and 13 were found to react in thf at room temperature yielding quantitatively the corresponding Grignard compound 9. Moreover, the structures of [MgMeBr(pmdta)] (9c), [MgMe2(pmdta)] (12c), and [MgBr2(pmdta)] (13c) were calculated on the DFT level of theory. The calculated structures 12c and 13c are in a good agreement with the experimentally observed structures 12 and 13. The equilibrium constant of the Schlenk equilibrium (2 9c ? 12c + 13c) was calculated to be Kgas = 2.0 × 10−3 (298 K) in the gas phase. Considering the solvent effects of both thf and diethyl ether using a polarized continuum model (PCM) the corresponding equilibrium constants were calculated to be Kthf = 1.2 × 10−3 and Kether = 3.2 × 10−3 (298 K), respectively. 相似文献
9.
F. Firoozi K. Javidnia M. Kamali A. Fooladi A. Foroumadi A. Shafiee 《Journal of heterocyclic chemistry》1995,32(1):123-128
Starting from readily available ethyl-4-nitropyrrole-2-carboxylate ( 1 ), substituted 1-methyl-2-(1,3,4-thiadiazol-2-yl)-4-nitropyrroles and 1-methyl-2-(1,3,4-oxadiazol-2-yl)-4-nitropyrroles were prepared. The reaction of 1 with diazomethane gave ethyl 1-methyl-4-nitropyrrole-2-carboxylate ( 2 ). Reaction of compound 2 with hydrazine hydrate afforded the corresponding hydrazide 3 . The reaction of 3 with formic acid yielded 1-(1-methyl-4-nitropyrrole-2-carboxyl)-2-(formyl)hydrazine ( 7 ). Refluxing of the latter with phosphorus pentasulfide in xylene yielded compound 6 in 40% yield. Reaction of compound 7 with phosphorus pentoxide afforded compound 9 . Reaction of compound 3 with 1,1′-carboxyldiimidazole in the presence of triethylamine yielded 2-(1-methyl-4-nitro-2-pyrrolyl)-1,3,4-oxadiazoline-4(H)-5-one ( 11 ). Refluxing compound 3 with cyanogen bromide in methanol gave compound 12 . Compound 13 could be obtained through the reaction of compound 3 with carbon disulfide in basic medium. Alkylation of compound 13 afforded the correspanding alkylthio derivative 14 . Reaction of 1-methyl-4-nitropyrrole-2-carboxylic acid ( 15 ) with thiosemicarbazide and phosphorus oxychloride gave 2-amino-5-(1-methyl-4-nitro-2-pyrrolyl)-1,3,4-thiadiazole ( 16 ). Sandmeyer reaction of compound 16 yielded 2-chloro-5-(1-methyl-4-nitro-2-pyrrolyl)-1,3,4-thiadiazole ( 17 ). Refluxing of the latter with thiourea afforded 2-(1-methyl-4-nitro-2-pyrrolyl)-1,3,4-thiadiazoline-4(H)-5-thione ( 18 ). Alkylation of compound 18 gave the corresponding alkylthio derivative 19 . Oxidation of the latter with hydrogen peroxide in acetic acid yielded 2-(1-methyl-4-nitro-2-pyrrolyl)-5-methylsulfonyl-1,3,4-thiadiazole ( 20 ). 相似文献
10.