全文获取类型
收费全文 | 1034篇 |
免费 | 45篇 |
国内免费 | 2篇 |
专业分类
化学 | 743篇 |
晶体学 | 13篇 |
力学 | 13篇 |
数学 | 57篇 |
物理学 | 255篇 |
出版年
2024年 | 2篇 |
2023年 | 16篇 |
2022年 | 36篇 |
2021年 | 34篇 |
2020年 | 44篇 |
2019年 | 60篇 |
2018年 | 52篇 |
2017年 | 34篇 |
2016年 | 57篇 |
2015年 | 41篇 |
2014年 | 100篇 |
2013年 | 104篇 |
2012年 | 68篇 |
2011年 | 69篇 |
2010年 | 37篇 |
2009年 | 28篇 |
2008年 | 41篇 |
2007年 | 31篇 |
2006年 | 27篇 |
2005年 | 22篇 |
2004年 | 27篇 |
2003年 | 17篇 |
2002年 | 20篇 |
2001年 | 14篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 8篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1939年 | 1篇 |
排序方式: 共有1081条查询结果,搜索用时 15 毫秒
91.
A novel, efficient and protecting group free enantioselective synthetic approach of (?)-(R)-1 and (+)-(S)-rolipram 2 is described employing the organocatalyzed asymmetric Michael addition, Henry condensation, Wittig olefination and reductive lactamization reactions as key steps. 相似文献
92.
Munish Puri Aneet Kaur Ram Sarup Singh Anubhav Singh 《Applied biochemistry and biotechnology》2010,162(1):181-191
Response surface methodology was used to optimize the fermentation medium for enhancing naringinase production by Staphylococcus xylosus. The first step of this process involved the individual adjustment and optimization of various medium components at shake
flask level. Sources of carbon (sucrose) and nitrogen (sodium nitrate), as well as an inducer (naringin) and pH levels were
all found to be the important factors significantly affecting naringinase production. In the second step, a 22 full factorial
central composite design was applied to determine the optimal levels of each of the significant variables. A second-order
polynomial was derived by multiple regression analysis on the experimental data. Using this methodology, the optimum values
for the critical components were obtained as follows: sucrose, 10.0%; sodium nitrate, 10.0%; pH 5.6; biomass concentration,
1.58%; and naringin, 0.50% (w/v), respectively. Under optimal conditions, the experimental naringinase production was 8.45 U/mL. The determination coefficients
(R
2) were 0.9908 and 0.9950 for naringinase activity and biomass production, respectively, indicating an adequate degree of reliability
in the model. 相似文献
93.
Munish Puri Shivani Gupta Parveen Pahuja Aneet Kaur J. R. Kanwar J. F. Kennedy 《Applied biochemistry and biotechnology》2010,160(1):98-108
β-D-galactosidase (EC 3.2.1.23) from Kluyveromyces marxianus YW-1, an isolate from whey, has been studied in terms of cell disruption to liberate the useful enzyme. The enzyme produced
in a bioreactor on a wheat bran medium has been successfully immobilized with a view to developing a commercially usable technology
for lactose hydrolysis in the food industry. Three chemical and three physical methods of cell disruption were tested and
a method of grinding with river sand was found to give highest enzyme activity (720 U). The enzyme was covalently immobilized
on gelatin. Immobilized enzyme had optimum pH and temperature of 7.0 and 40 °C, respectively and was found to give 49% hydrolysis
of lactose in milk after 4 h of incubation. The immobilized enzyme was used for eight hydrolysis batches without appreciable
loss in activity. The retention of high catalytic activity compared with the losses experienced with several previously reported
immobilized versions of the enzyme is significant. The method of immobilization is simple, effective, and can be used for
the immobilization of other enzymes. 相似文献
94.
The samples of natural mica (muscovite and biotite) were irradiated with energetic heavy ions [136Xe (11.56 MeV/n) and 197Au (11.40 MeV/n)] from universal linear accelerator (UNILAC) GSI, Darmstadt, Germany. The simple track etch technique has been used to develop and analyze the morphology of induced heavy ions and natural alpha recoil tracks with the help of atomic force microscopy. Defective structure and lattice disorder have been observed in the environs of the etch pits in these layered crystalline materials. The distribution of defects produced along the tracks, geometrical analysis of tracks shapes and shallowness and their correlations with the structural arrangement of the materials have been discussed in the present investigation. Importance of different ion track geometries in micaceous minerals for the fabrication of nanoterrace material with different stacks having sharp edges with unique properties has been elaborated in the present paper. 相似文献
95.
To the best of our knowledge, there is no method in literature for solving such fully fuzzy linear programming (FLP) problems in which some or all the parameters are represented by unrestricted L-R flat fuzzy numbers. Also, to propose such a method, there is need to find the product of unrestricted L-R flat fuzzy numbers. However, there is no method in the literature to find the product of unrestricted L-R flat fuzzy numbers.In this paper, firstly the product of unrestricted L-R flat fuzzy numbers is proposed and then with the help of proposed product, a new method (named as Mehar’s method) is proposed for solving fully FLP problems. It is also shown that the fully FLP problems which can be solved by the existing methods can also be solved by the Mehar’s method. However, such fully FLP problems in which some or all the parameters are represented by unrestricted L-R flat fuzzy numbers can be solved by Mehar’s method but can not be solved by any of the existing methods. 相似文献
96.
This paper shares results from a secondary analysis of data from the participation of Japanese, Singaporean, and U.S. students in the International Project on Mathematical Attainment (IPMA). IPMA was a longitudinal study to assess the mathematics achievement of primary students from their first year of schooling through the end of fifth grade. Tests were constructed to enable achievement on the same items to be assessed over multiple years, thus permitting the assessment of growth in achievement throughout primary school. Achievement is compared to the grade at which the content is introduced so that achievement can be related to students’ opportunity to learn. 相似文献
97.
Prof. Dr. Lena Keller Dr. Emilia Oueis Dr. Amninder Kaur Dr. Nasim Safaei Dr. Susanne H. Kirsch Dr. Antonia P. Gunesch Dr. Sibylle Haid Dr. Ulfert Rand Prof. Dr. Luka Čičin-Šain Dr. Chengzhang Fu PD Dr. Joachim Wink Prof. Dr. Thomas Pietschmann Prof. Dr. Rolf Müller 《Angewandte Chemie (International ed. in English)》2023,62(6):e202214595
A new family of highly unusual sesquarterpenoids (persicamidines A–E) exhibiting significant antiviral activity was isolated from a newly discovered actinobacterial strain, Kibdelosporangium persicum sp. nov., collected from a hot desert in Iran. Extensive NMR analysis unraveled a hexacyclic terpenoid molecule with a modified sugar moiety on one side and a highly unusual isourea moiety fused to the terpenoid structure. The structures of the five analogues differed only in the aminoalkyl side chain attached to the isourea moiety. Persicamidines A–E showed potent activity against hCoV-229E and SARS-CoV-2 viruses in the nanomolar range together with very good selectivity indices, making persicamidines promising as starting points for drug development. 相似文献
98.
Gurleen Singh Nancy George Riddima Singh Gurjaspreet Singh Sushma Gurpreet Kaur Harminder Singh Jandeep Singh 《应用有机金属化学》2023,37(1):e6897
1,2,3-Triazole-based ligands obtained through copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) have been exploited in vast array of research domains owing to the stitching of simpler molecules through a needle of Cu(I) catalyst. The numerous reports on ion(s) detection capabilities of synthesized 1,4-disubstituted 1,2,3-triazole ligands using absorption and fluorescence spectroscopy are accessible. This review enlists substituted 1,2,3-triazole-based sensor probes, since 2010, synthesized selectively by CuAAC, having the ability to sense either a single ion or multiple ions under specific set of conditions along with their detection limits. The review also apprehends the different techniques and sensing mechanisms involved in the detection of ions by chemosensor probes. 相似文献
99.
The reaction of copper(I) iodide with tri-m-tolylphosphine (m-tolyl(3)P) in acetonitrile yielded the cluster [Cu(6)(mu2-I)(mu3-I)4(mu4-I)(m-tolyl(3)P)4(CH(3)CN)2] (1), with a bicapped adamantoid geometry. In this compound, four Cu atoms are coordinated to four terminally bonded m-tolyl(3)P ligands, two Cu atoms are bonded to two CH(3)CN ligands, and iodide ligands have mu2-I, mu3-I, and mu4-I bonding modes. This compound has four CuI(3)P and two CuI(3)N cores, and geometry around each Cu center is distorted tetrahedral.The polarizable iodide ligand and the position of the methyl group in the phenyl ring attached to the P atom appear to have played the pivotal role in the formation of monomeric bicapped adamantoid geometry, which is unique in copper chemistry. 相似文献
100.
We report on the synthesis of zero valent iron nano particles (nZVI) via chemical reduction method. The large peak visible in the XRD pattern reveals the presence of an amorphous phase of iron. SEM and TEM images signify the dendritic morphology and core-shell-like structure of manufactured nZVI particles respectively. Methylene blue dye (MB) was used as model contaminant to assess the reductive degradation proficiency of nZVI. With complete elimination of MB, the fresh synthesized nZVI exhibited the best performance (97%), while the regenerated nZVI had an 85.1% MB removal efficiency after five regenerations. The equilibrium data of adsorption were fitted to eight different kinetic and isothermal models. The effects of critical operating factors such as pH, varied amounts of nZVI and dye concentration, adsorption temperature, and adsorption time were also studied. A presumptive reaction mechanism and function of core-shell construction in contaminant sequestration has also been investigated. 相似文献