全文获取类型
收费全文 | 1650篇 |
免费 | 61篇 |
国内免费 | 20篇 |
专业分类
化学 | 1281篇 |
晶体学 | 42篇 |
力学 | 45篇 |
数学 | 84篇 |
物理学 | 279篇 |
出版年
2024年 | 9篇 |
2023年 | 12篇 |
2022年 | 48篇 |
2021年 | 141篇 |
2020年 | 62篇 |
2019年 | 66篇 |
2018年 | 77篇 |
2017年 | 43篇 |
2016年 | 89篇 |
2015年 | 65篇 |
2014年 | 91篇 |
2013年 | 126篇 |
2012年 | 161篇 |
2011年 | 160篇 |
2010年 | 97篇 |
2009年 | 62篇 |
2008年 | 67篇 |
2007年 | 51篇 |
2006年 | 45篇 |
2005年 | 39篇 |
2004年 | 31篇 |
2003年 | 25篇 |
2002年 | 22篇 |
2001年 | 7篇 |
2000年 | 19篇 |
1999年 | 9篇 |
1998年 | 6篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 9篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1981年 | 3篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1973年 | 2篇 |
1936年 | 1篇 |
1935年 | 1篇 |
1933年 | 1篇 |
排序方式: 共有1731条查询结果,搜索用时 15 毫秒
31.
Ali Z. Abu Zuhri 《Monatshefte für Chemie / Chemical Monthly》1987,118(6-7):749-757
The polarographic behaviour of 1,3-bis[(2-pyridyl)methyleneamino]thiourea (PMAT) in solutions of varyingpH has been studied both in the absence and presence of Triton X-100 (T.X-100). The mechanism for the reduction process is discussed. The adsorption effect of electrochemically inactive T.X-100 on the polarographic waves ofPMAT has been investigated. The values of the kinetic parameters for the electrode reaction at differentpH values have been computed. The applicability of the polarographic method for determination ofPMAT is also discussed.
Das polarographische Verhalten von 1,3-Bis[(2-pyridyl)methylenamino]-thioharnstoff (PMAT) in Anwesenheit und Abwesenheit von Triton X-100. Die polarographische Bestimmung vonPMAT
Zusammenfassung Es wurde das polarographische Verhalten vonPMAT in Lösungen mit verschiedenempH sowohl in Gegenwart als auch ohne Triton X-100 (T.X-100) untersucht. Es wird ein Mechanismus für den Reduktionsprozeß diskutiert. Der Adsorptionseffekt des elektrochemisch inaktiven T.X-100 wurde hinsichtlich der polarographischen Wellen vonPMAT untersucht. Die kinetischen Parameter der Elektrodenreaktion wurden für verschiedenepH-Werte ermittelt. Ebenso wird die Anwendbarkeit der polarographischen Methode für die Bestimmung vonPMAT diskutiert.相似文献
32.
Gold in iron ore samples is separated from iron (major matrix cation), antimony and vanadium using anion exchange resin in (0.2 M) HBr, potassium peroxodisulfate and acetone:water:nitric acid media. The exchangeable anion Cl− of the ion exchanger Dowex 1X 4 is replaced by Br− using (6 M) HBr solution. Certified reference material DGP-M1, spiked ferric magnetic oxide, gold radioactive tracer 198Au and gold standard solutions are used to study the adsorption efficiency and the yield recovery of tetrabromoaurate AuBr4− from the resin. Ten eluents have been tried to elute gold from the column, and it has been found that a 10 ml potassium peroxodisulfate and 240 ml acetone:water:nitric acid [125:5:5] solution fulfills the objective. The set up of the separation procedure allows quantitative adsorption of gold by the resin, while the major matrix cation (Fe) and others (Cd, Ag, Cu, V, Sb, Ti) have been passed through the column with the feeding solution (0.2 M) HBr. The resin selectivity coefficient (K) of separating Au from Fe has been found to be KFeAu≈6.4×1011. The eluted Au is treated with K2S2O8 and H2O2 for spectrophotometric determination as rhodamine-B complex at 555.6 nm. The linearity, detection limit, precision, and accuracy of the determination method have been found to be up to 2.0 μg g−1, 0.018 μg g−1, 0.009 μg g−1 and 3%, respectively. 相似文献
33.
The protolytic equilibria of piperazine (C4H10N2) and phosphate have been investigated in the presence of cobalt or nickel chloride or nitrate by potentiometric titrations between pH 2 and 8. Potentiometric titrations suggest the presence of [M2+(H2O)5(C4H11N2+)]3+ and [M2+(H2O)5(C4H10N2)]2+ in solution with stability constants logK of 3.1 and 3.8 for M = Co and 3.1 and 3.6 for M = Ni, respectively. Crystallization experiments were then conducted at selected pH values to isolate desired species from the known solution composition. Crystallization afforded [M(H2O)6]2+(C4H12N22+)(HPO42—)2 at pH 3.5 and 6.2 (M = Co, Ni), and Co3(PO4)2·8H2O at pH 10.5. No crystals with the dihydrogenphosphate anion or a metal‐bound piperazine ligand could be isolated under the reaction conditions. The solid‐state assembly in the isomorphous structures of [M(H2O)6](C4H12N2)(HPO4)2 with M = Co and Ni is based on an extended hydrogen bonded network between the three ionic building blocks. 相似文献
34.
A
modulated dsc study on the in situ polymerization of cyclic butylene terephthalate
oligomers 总被引:1,自引:0,他引:1
Z. A. Mohd Ishak P. P. Shang J. Karger-Kocsis 《Journal of Thermal Analysis and Calorimetry》2006,84(3):637-641
The polymerization of a cyclic butylene terephthalate
(CBT) oligomer was studied as a function of temperature (T=200
and 260°C, respectively) by modulated DSC (MDSC). The first heating was
followed by cooling after various holding times (5, 15 and 30 min) prior to
the second heating which ended always at T=260°C.
This allowed us to study the crystallization and melting behavior of the resulting
polybutylene terephthalate (PBT), as well. In contrary to the usual belief,
the CBT polymerization is exothermic and the related process is superimposed
to that of the CBT melting. The melting behavior of the PBT was affected by
the polymerization mode (performed below or above the melting temperature
of the PBT product) of the CBT. Annealing above the melting temperature of
PBT yielded a product featuring double melting. This was attributed to the
presence of crystallites with different degrees of perfection. The crystals
perfection which occurred via recrystallization/remelting was manifested by
a pronounced exothermic peak in the non-reversing trace. 相似文献
35.
Mohd Salmi Md. Noorani William Parry 《Bulletin of the Brazilian Mathematical Society》1992,23(1-2):137-151
We derive a Chebotarev Theorem for finite homogeneous extensions of shifts of finite type. These extensions are of the form
:X×G/H→X×G/H where
(x,gH)=(σx, α(x)gH), for some finite groupG and subgroupH. Given a σ-closed orbit τ, the periods of the
-closed orbits covering τ define a partition of the integer |G/H|. The theorem then gives us an asymptotic formula for the number of closed orbits with respect to the various partitions
of the integer |G/H|. We apply our theorem to the case of a finite extension and of an automorphism extension of shifts of finite type. We also
give a further application to ‘automorphism extensions’ of hyperbolic toral automorphisms.
Financially supported by Universiti Kebangsaan Malaysia 相似文献
36.
I. M. Chung Mohd Ali A. Ahmad C. Y. Yu K. H. Ma J. G. Gwag Y. J. Park 《Chemistry of Natural Compounds》2005,41(6):650-653
One new compound 3,7,11,15,19-pentamethyl-9α,10α,11α,17α,18α-pentahydroxy-n-tetracosan-1-oxy-p-hydroxycaffeoate (oryzaterpenyl
caffeoate) (1), together with three known fatty acids linoleic acid, stearic acid and myristic acid were isolated and identified from the
rice grain of Oryza sativa. The structure of the new compound was elucidated by 1D and 2D NMR spectroscopic techniques (1H-1HCOSY, 1H-13C HETCOR) aided by EI-MS, and IR spectra.
__________
Published in Khimiya Prirodnykh Soedinenii, No. 6, pp. 535–537, November–December, 2005. 相似文献
37.
The effect of anion concentration and the dependence of uranyl ascorbate on the nature of anion present is systematically studied for nine different anions over the concentration range (0.2–2.0) × 10−2 M. These anions, commonly encountered in pharmaceutical preparations with ascorbic acid (vitamin C) are nitrate, sulfate, chloride, bromide, fluoride, phosphate, citrate, oxalate, and tartrate. Based on the absorbance data, and on the value of the replacement constant K calculated, the studied anions may be arranged according to their complexing power on uranium as follows: citrate > tartrate > phosphate > oxalate > fluoride > sulfate > nitrate > chloride > bromide.This order is substantiated by the calculated values of the side reaction coefficients αM of the uranyl ligand complex or the conditional stability constant of uranyl-ascorbate calculated at different ligand concentrations. 相似文献
38.
Nur Atiqah Mohd Nasuha 《Natural product research》2016,30(13):1532-1536
Marsdenia tinctoria is an indigo producing plant commonly found in Borneo, Malaysia. In this present study, one new flavone kapitone (1) and three known compounds, that is 3,2’-dihydroxyflavone (2), 1-methylcyclobutene (3) and dimethyl isatoate (4) were isolated from the Malaysia Borneo M. tinctoria R. Br. (Apocynaceae). These compounds were isolated and characterised using extensive chromatographic and spectroscopic methods. 相似文献
39.
40.
Rehan Zahid Masjuki Bin Haji Hassan Mahendra Varman Riaz Ahmad Mufti Md. Abul Kalam Nurin Wahidah Binti Mohd Zulkifli 《固体与材料科学评论》2017,42(4):267-294
Tribological efficiency of industrial applications involving boundary lubrication regime can be improved to an appreciable extent by the deposition of hard coatings on interacting surfaces. Among such coatings, diamond-like carbon (DLC) coatings are considered to be one of the most suitable ones for the said role. DLC coatings possess a unique combination of physical, chemical, and material properties due to which they can help in minimizing friction-induced energy and material losses even under starved lubrication conditions. Since commercial lubricants are optimized for steel surfaces, therefore, a lot of experimental investigations were carried out to analyze the tribological compatibility of these lubricants with various DLC coatings. However, there is still a lack of understanding about how DLC coatings interact with conventional lubricant additives. Some researchers reported tribologically beneficial interactions between DLC coatings and formulated lubricants while others observed no such behavior. To address these inconsistencies, there is a need to rearrange the published data in a more apprehensible and organized manner with a special emphasis on the mechanisms responsible for a particular tribological behavior. In this way, it can be determined whether synergistic or antagonistic correlation exists between a particular DLC-lubricant combination and research on DLC coatings can be continued in a logical way. In this article, most widely investigated non-doped DLC coatings (ta-C, a-C:H, a-C, and ta-C:H) are tribologically analyzed. Average values of friction and wear coefficients are calculated for various DLC-lubricant combinations using already published data and compared to quantify the effectiveness of a particular lubricant additive in enhancing tribological characteristics of symmetrical non-doped DLC contacts. Moreover, tribological performance parameters of non-doped DLC coatings are compared with those of doped-DLC coatings to understand differences in their tribological behavior in combination with additives. 相似文献