全文获取类型
收费全文 | 491篇 |
免费 | 9篇 |
专业分类
化学 | 289篇 |
晶体学 | 11篇 |
力学 | 18篇 |
数学 | 53篇 |
物理学 | 129篇 |
出版年
2023年 | 4篇 |
2022年 | 17篇 |
2021年 | 11篇 |
2020年 | 11篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 6篇 |
2016年 | 16篇 |
2015年 | 17篇 |
2014年 | 13篇 |
2013年 | 28篇 |
2012年 | 40篇 |
2011年 | 25篇 |
2010年 | 26篇 |
2009年 | 14篇 |
2008年 | 25篇 |
2007年 | 23篇 |
2006年 | 21篇 |
2005年 | 14篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 11篇 |
2001年 | 6篇 |
2000年 | 4篇 |
1999年 | 12篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1967年 | 6篇 |
1964年 | 2篇 |
1891年 | 2篇 |
排序方式: 共有500条查询结果,搜索用时 15 毫秒
411.
Roxana Ghiulai Marius Mioc Roxana Racoviceanu Alexandra Prodea Andreea Milan Dorina Coricovac Cristina Dehelean tefana Avram Alina D. Zamfir Cristian V. A. Munteanu Viviana Ivan Codrua oica 《Molecules (Basel, Switzerland)》2022,27(21)
Betulinic acid (BA) has been extensively studied in recent years mainly for its antiproliferative and antitumor effect in various types of cancers. Limited data are available regarding the pharmacokinetic profile of BA, particularly its metabolic transformation in vivo. In this study, we present the screening and structural investigations by ESI Orbitrap MS in the negative ion mode and CID MS/MS of phase I and phase II metabolites detected in mouse plasma after the intraperitoneal administration of a nanoemulsion containing BA in SKH 1 female mice. Obtained results indicate that the main phase I metabolic reactions that BA undergoes are monohydroxylation, dihydroxylation, oxidation and hydrogenation, while phase II reactions involved sulfation, glucuronidation and methylation. The fragmentation pathway for BA and its plasma metabolites were elucidated by sequencing of the precursor ions by CID MS MS experiments. 相似文献
412.
Fabio Gosetti Viviana Consonni Davide Ballabio Marco Emilio Orlandi Angelo Amodio Maria Valeria Picci Marco Visentin Veronica Termopoli 《Molecules (Basel, Switzerland)》2023,28(1)
According to the 2021 World Drug Report, around 275 million people use drugs of abuse, and 36 million people suffer from addiction, fostering a thriving market for illicit substances. In Italy, 30,083 people were reported to the Judicial Authority for offenses in violation of the Italian Law D.P.R. 309/1990. These offences are sentenced after a qualitative–quantitative analysis of seized materials. Given the large quantity of seized drugs and the need to perform accurate analytical determinations, Italian forensic laboratories struggle to complete analyses in a short time, delaying the entire reporting process needed to achieve sentencing. For this purpose, an UHPLC-MS/MS-based platform was developed at the University of Milano-Bicocca to support law-enforcement authorities. Software was designed to easily manage street seizure acquisition, documentation registration, and sampling. A sensitive UHPLC-MS/MS method was fully validated for the quantification of the traditional illicit substances (cocaine, heroin, 6-MAM, morphine, amphetamine, methamphetamine, MDMA, ketamine, GHB, GBL, LSD, trans-∆9-THC, and THCA) at the ppb level. The final report is relayed to the Prefecture in 3–4 days, even within 24 h for urgent requests. The platform allows for semi-automatic data handling to minimize erroneous results for an accurate report generation by standardized procedures. 相似文献
413.
We present a procedure to solve the inverse Ising problem, that is, to find the interactions between a set of binary variables from the measure of their equilibrium correlations. The method consists in constructing and selecting specific clusters of spins, based on their contributions to the cross-entropy of the Ising model. Small contributions are discarded to avoid overfitting and to make the computation tractable. The properties of the cluster expansion and its performances on synthetic data are studied. To make the implementation easier we give the pseudo-code of the algorithm. 相似文献
414.
Abstract A smectic B (S,) solvent may provide a very highly ordered, and possibly oriented, host medium for polymerization processes. Photochemical polymerizations of two bisacrylic monomers were therefore carried out in a SB medium. The process conditions were chosen on the basis of a preliminary study of the monomer-solvent binary system carried out by differential scanning calorimetry (D.S.C.), optical microscopy and X-ray diffraction. The polymerization was then studied by D.S.C. at different reaction times. These measurements showed clearly that there is a point along the reaction path at which the growing polymer chains start segregating from the solvent and the mesomorphic bulk no longer acts as a reaction solvent but only as a reservoir for the ponomer. The polymer chains from that point keep growing only upon or between the faces of the SB layers. Electron microscopy observations showed that the morphology of the final polymer is affected by this interlayer process. 相似文献
415.
416.
Mario Mariani Giovanni Consolati Fiorenza Quasso Nadia Lotti Andrea Munari Michele Galletta Elena Macerata 《Journal of Radioanalytical and Nuclear Chemistry》2010,286(3):625-629
Radiation methods are largely used for polymerisation and polymer modification, since irradiation induces transformations in the structure of materials which can be exploited to improve their performance. On the other hand, combined action of ionising radiation and oxygen may lead to degradation of the polymer, with worsening of properties such as mechanical strength or electrical insulation resistance. Therefore, the change of the chemical and physical properties of polymers under irradiation is a dynamic topic of research. In this work there are discussed data on the physical features of a polyester, poly(ethylene isophthalate) (PEI), subjected to gamma irradiation up to 1 MGy. PEI is a semicrystalline polymer with a structure similar to polyethylene terephthalate. Viscosity and differential scanning calorimetry measurements were carried out which allowed the monitoring of changes in the structure in terms of variations in the molecular weight, as well as of the percentage crystallinity depending on the dose. Furthermore, positron annihilation lifetime spectroscopy supplied information on the free volume present in the amorphous phase of the irradiated polymer. 相似文献
417.
Thermostable phytase production by Thermoascus aurantiacus in submerged fermentation 总被引:2,自引:0,他引:2
Nampoothiri KM Tomes GJ Roopesh K Szakacs G Nagy V Soccol CR Pandey A 《Applied biochemistry and biotechnology》2004,118(1-3):205-214
Phytases act on phytic acid, an antinutrient factor present in animal feeds, and release inorganic phosphate. We optimized
the production parameters for phytase production using Thermoascus aurantiacus (TUB F 43), a thermophilic fungal culture, by submerged fermentation. A semisynthetic medium containing glucose, starch,
peptone, and minerals supplemented with 3.75% (w/v) wheat bran particles was found to be the best production medium among
the various combinations tried. Further supplementation of this medium with surfactants such as Tween-20 and Tween-80 considerably
enhanced the enzyme yield. A maximum phytase activity (468.22 U/mL) was obtained using this production medium containing 2%
(v/v) Tween-20 after 72 h of fermentation at 45°C in shake-flask cultures with a rotation of 150 rpm. Herein we present details
of a few of the process parameter optimizations. The phytase enzyme was found to be thermostable, and the optimal temperature
for phytase activity was found to be 55°C. However, 80% of the activity still remained when the temperature was shifted to
70°C. 相似文献
418.
Claudia Bonfio David A. Russell Nicholas J. Green Angelica Mariani John D. Sutherland 《Chemical science》2020,11(39):10688
The complexity of the simplest conceivable cell suggests that the chemistry of prebiotic mixtures needs to be explored to understand the intricate network of prebiotic reactions that led to the emergence of life. Early cells probably relied upon compatible and interconnected chemistries to link RNA, peptides and membranes. Here we show that several types of vesicles, composed of prebiotically plausible mixtures of amphiphiles, spontaneously form and sustain the methyl isocyanide-mediated activation of amino acids, peptides and nucleotides. Activation chemistry also drives the advantageous conversion of reactive monoacylglycerol phosphates into inert cyclophospholipids, thus supporting their potential role as major constituents of protocells. Moreover, activation of prebiotic building blocks within fatty acid-based vesicles yields lipidated species capable of localising to and functionalising primitive membranes. Our findings describe a potentially prebiotic scenario in which the components of primitive cells undergo activation and provide new species that might have enabled an increase in the functionality of protocells.The complexity of the simplest conceivable cell suggests that the chemistry of prebiotic mixtures needs to be explored to understand the intricate network of prebiotic reactions that led to the emergence of life. 相似文献
419.
Bazzicalupi C Bencini A Bianchi A Borsari L Danesi A Giorgi C Lodeiro C Mariani P Pina F Santarelli S Tamayo A Valtancoli B 《Dalton transactions (Cambridge, England : 2003)》2006,(33):4000-4010
The synthesis and characterisation of the new macrocyclic ligand 6-methyl-2,6,10-triaza-[11]-12,25-phenathrolinophane (L1), which contains a triamine aliphatic chain linking the 2,9 positions of 1,10-phenanthroline and of its derivative L2, composed by two L1 moieties connected by an ethylenic bridge, are reported. Their basicity and coordination properties toward Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) have been studied by means of potentiometric and spectroscopic (UV-Vis, fluorescence emission) measurements in aqueous solutions. L1 forms 1:1 metal complexes in aqueous solutions, while L2 can give both mono- and dinuclear complexes. In the mononuclear L2 complexes the metal is sandwiched between the two cyclic moieties. The metal complexes with L1 and L2 do not display fluorescence emission, due to the presence of amine groups not involved in metal coordination. These amine groups can quench the excited fluorophore through an electron transfer process. The ability of the Zn(II) complexes with L1 and L2 to cleave the phosphate ester bond in the presence has been investigated by using bis(p-nitrophenyl)phosphate (BNPP) as substrate. The dinuclear complex with L2 shows a remarkable hydrolytic activity, due to the simultaneous presence within this complex of two metals and two hydrophobic units. In fact, the two Zn(II) act cooperatively in substrate binding, probably through a bridging interaction of the phosphate ester; the interaction is further reinforced by pi-stacking pairing and hydrophobic interactions between the phenanthroline unit(s) and the p-nitrophenyl groups of BNPP. 相似文献
420.
Valeria Alzari Daniele Nuvoli Mariano Casu Roberta Sanna Ernesto Rivera Alberto Mariani 《Journal of polymer science. Part A, Polymer chemistry》2013,51(21):4618-4625
Novel polyacrylamide‐based hydrogels containing 3‐(trimethoxysilyl)propyl methacrylate and/or tetraethoxy silane were synthesized by means of frontal polymerization, using ammonium persulfate as initiator, N,N′‐methylene bisacrylamide as crosslinking agent and dimethyl sulfoxide as solvent. The obtained samples were treated at pH of 2 or 5 to induce the sol–gel reaction and evaluate their swelling behavior in the conditions. The occurrence of this reaction was assessed by solid‐state NMR. Moreover, the thermal properties of the dry materials were studied by differential scanning calorimetry and thermal gravimetric analysis, and their water‐contact angles were measured. It was found that the amount of Si affects the extent of swelling and the hydrophilicity of the resulting materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4618–4625 相似文献